[Technical Field]
[0001] The present invention relates to niobium (Nb) and titanium (Ti)-added interstitial
free (IF) cold rolled steel sheets that are used as materials for automobiles, household
electronic appliances, etc. More specifically, the present invention relates to highly
formable IF cold rolled steel sheets whose yield strength is enhanced due to the distribution
of fine precipitates, and a process for producing the IF cold rolled steel sheets.
[Background Art]
[0002] In general, cold rolled steel sheets for use in automobiles and household electronic
appliances are required to have excellent room-temperature aging resistance and bake
hardenability, together with high strength and superior formability.
[0003] Aging is a strain aging phenomenon that arises from hardening caused by dissolved
elements, such as C and N, fixed to dislocations. Since aging causes defect, called
"stretcher strain", it is important to secure excellent room-temperature aging resistance.
[0004] Bake hardenability means increase in strength due to the presence of dissolved carbon
after press formation, followed by painting and drying, by leaving a slight small
amount of carbon in a solid solution state. Steel sheets with excellent bake hardenability
can overcome the difficulties of press formability resulting from high strength.
[0005] Room-temperature aging resistance and bake hardenability can be imparted to aluminum
(Al)-killed steels by batch annealing of the Al-killed steels. However, extended time
of the batch annealing causes low productivity of the Al-killed steels and severe
variation in steel materials at different sites. In addition, Al-killed steels have
a bake hardening (BH) value (a difference in yield strength before and after painting)
of 10-20 MPa, which demonstrates that an increase in yield strength is low.
[0006] Under such circumstances, interstitial free (IF) steels with excellent room-temperature
aging resistance and bake hardenability have been developed by adding carbide and
nitride-forming elements, such as Ti and Nb, followed by continuous annealing.
[0007] For example, Japanese Unexamined Patent Publication No.
Sho 57-041349 describes an enhancement in the strength of a Ti-based IF steel by adding 0.4-0.8%
of manganese (Mn) and 0.04-0.12% of phosphorus (P). In very low carbon IF steels,
however, P causes the problem of secondary working embrittlement due to segregation
in grain boundaries.
[0008] Japanese Unexamined Patent Publication No.
Hei 5-078784 describes an enhancement in strength by the addition of Mn as a solid solution strengthening
element in an amount exceeding 0.9% and not exceeding 3.0%.
[0009] Korean Patent Laid-open No.
2003-0052248 describes an improvement in secondary working embrittlement resistance as well as
strength and workability by the addition of 0.5-2.0% of Mn instead of P, together
with aluminum (Al) and boron (B).
[0010] Japanese Unexamined Patent Publication No.
Hei 10-158783 describes an enhancement in strength by reducing the content of P and using Mn and
Si as solid solution strengthening elements. According to this publication, Mn is
used in an amount of up to 0.5%, Al as a deoxidizing agent is used in an amount of
0.1%, and nitrogen (N) as an impurity is limited to 0.01% or less. If the Mn content
is increased, the plating characteristics are worsened.
[0011] Japanese Unexamined Patent Publication No.
Hei 6-057336 discloses an enhancement in the strength of an IF steel by adding 0.5-2.5% of copper
(Cu) to form ε-Cu precipitates. High strength of the IF steel is achieved due to the
presence of the ε-Cu precipitates, but the workability of the IF steel is worsened.
[0012] Japanese Unexamined Patent Publication Nos.
Hei 9-227951 and
Hei 10-265900 suggest technologies associated with improvement in workability or surface defects
due to carbides by the use of Cu as a nucleus for precipitation of the carbides. According
to the former publication, 0.005-0.1% of Cu is added to precipitate CuS during temper
rolling of an IF steel, and the CuS precipitates are used as nuclei to form Cu-Ti-C-S
precipitates during hot rolling. In addition, the former publication states that the
number of nuclei forming a {111} plane parallel to the surface of a plate increases
in the vicinity of the Cu-Ti-C-S precipitates during recrystallization, which contributes
to an improvement in workability. According to the latter publication, 0.01-0.05%
of Cu is added to an IF steel to obtain CuS precipitates and then the CuS precipitates
are used as nuclei for precipitation of carbides to reduce the amount of dissolved
carbon (C), leading to an improvement in surface defects. According to the prior art,
since coarse CuS precipitates are used during production of cold rolled steel sheets,
carbides remain in the final products. Further, since emulsion-forming elements, such
as Ti and Zr, are added in an amount greater than the amount of sulfur (S) in an atomic
weight ratio, a main portion of the sulfur (S) reacts with Ti or Zr rather than Cu.
[0013] On the other hand, Japanese Unexamined Patent Publication Nos.
Hei 6-240365 and
Hei 7-216340 describe the addition of a combination of Cu and P to improve the corrosion resistance
of baking hardening type IF steels. According to these publications, Cu is added in
an amount of 0.05-1.0% to ensure improved corrosion resistance. However, in actuality,
Cu is added in an excessively large amount of 0.2% or more.
[0014] Japanese Unexamined Patent Publication Nos.
Hei 10-280048 and
Hei 10-287954 suggest the dissolution of carbosulfide (Ti-C-S based) in a carbide at the time of
reheating and annealing to obtain a solid solution in crystal grain boundaries, thereby
achieving a bake hardening (BH) value (a difference in yield strength before and after
baking) of 30 MPa or more.
[0015] According to the aforementioned publications, strength is enhanced by strengthening
solid solution or using ε-Cu precipitates. Cu is used to form ε-Cu precipitates and
improve corrosion resistance. In addition, Cu is used as a nucleus for precipitation
of carbides. No mention is made in these publications about an increase in high yield
ratio (
i.e. yield strength/tensile strength) and a reduction in in-plane anisotropy index. If
the tensile strength-to-yield strength ratio (
i.e. yield ratio) of an IF steel sheet is high, the thickness of the IF steel sheet can
be reduced, which is effective in weight reduction. In addition, if the in-plane anisotropy
index of an IF steel sheet is low, fewer wrinkles and ears occur during processing
and after processing, respectively.
[0016] EP-A-1136575 discloses a method for manufacturing cold-rolled steel sheet which comprises the
steps of: rough-rolling a slab using a rough-rolling unit; finish-rolling the sheet
bar using a continuous hot finishing-rolling mill; cooling the hot-rolled steel strip
on a runout table; coiling thus cooled hot-rolled steel strip; and applying picking,
cold-rolling the hot-rolled steel strip, and final annealing to the cold-rolled steel
strip.
[Disclosure]
[Technical Problem]
[0017] It is one object of certain embodiments of the present invention to provide Nb and
Ti-added IF cold rolled steel sheets that are capable of achieving a high yield ratio
and a low in-plane anisotropy index.
[0018] It is another object of certain embodiments of the present invention to provide a
process for producing the IF cold rolled steel sheets.
[Technical Solution]
[0019] According to the present invention, there is provided a colled rolled sheet steel
of claim 1, 2, 3 or 4.
[0020] When the cold rolled steel sheets of the present invention satisfy the following
relationships between the C, Ti, Nb, N and S contents: 0.8 ≤ (Ti*/48 + Nb/93)/(C/12)
≤ 5.0 and Ti* = Ti - 0.8 x ((48/14) x N + (48/32) x S), they show room-temperature
non-aging properties. In addition, when solute carbon (Cs) [Cs = (C - Nb x 12/93 -
Ti* x 12/48) x 10000 wherein Ti* = Ti - 0.8 x ((48/14) x N + (48/32) x S), provided
that when Ti* is less than 0, Ti* is defined as 0], which is determined by the C and
Ti contents, is from 5 to 30, the cold rolled steel sheets of the present invention
show bake hardenability.
[0021] Depending on the design of the compositions, the cold rolled steel sheets of the
present invention have characteristics of soft cold rolled steel sheets of the order
of 280 MPa and high-strength cold rolled steel sheets of the order of 340 MPa or more.
[0022] When the content of P in the compositions of the present invention is 0.015% or less,
soft cold rolled steel sheets of the order of 280 MPa are produced. When the soft
cold rolled steel sheets further contain at least one solid solution strengthening
element selected from Si and Cr, or the P content is in the range of 0.015-0.2%, a
high strength of 340 MPa or more is attained. The P content in the high-strength steels
containing P alone is preferably in the range of 0.03% to 0.2%. The Si content in
the high-strength steels is preferably in the range of 0.1 to 0.8%. The Cr content
in the high-strength steels is preferably in the range of 0.2 to 1.2. In the case
where the cold rolled steel sheets of the present invention contain at least one element
selected from Si and Cr, the P content may be freely designed in an amount of 0.2%
or less.
[0023] For better workability, the cold rolled steel sheets of the present invention may
further contain 0.01-0.2 wt% of Mo.
[0024] According to the present invention, there is provided a process for producing the
cold rolled steel sheet of claim 17, 18, 19 or 20.
[Best Mode]
[0025] The present invention will be described in detail below.
[0026] Fine precipitates having a size of 0.2 µm or less are distributed in the cold rolled
steel sheets of the present invention. Examples of such precipitates include MnS precipitates,
CuS precipitates, and composite precipitates of MnS and CuS. These precipitates are
referred to simply as "(Mn,Cu)S".
[0027] The present inventors have found that when fine precipitates are distributed in Nb
and Ti-added IF steels (also referred to simply as "Nb-Ti composite IF steels"), the
yield strength of the IF steels is enhanced and the in-plane anisotropy index of the
IF steels is lowered, thus leading to an improvement in workability. The present invention
has been achieved based on this finding. The precipitates used in the present invention
have drawn little attention in conventional IF steels. Particularly, the precipitates
have not been actively used from the viewpoint of yield strength and in-plane anisotropy
index.
[0028] Regulation of the components in the Nb-Ti composite IF steels is required to obtain
(Mn,Cu)S precipitates and/or AlN precipitates. If the IF steels contain Ti, Zr and
other elements, S and N preferentially react with Ti and Zr. Since the cold rolled
steel sheets of the present invention are Nb-Ti composite IF steels, Ti reacts with
C, N and S. Accordingly, it is necessary to regulate the components so that S and
N are precipitated into (Mn,Cu)S and AlN forms, respectively.
[0029] The fine precipitates thus obtained allow the formation of minute crystal grains.
Minuteness in the size of crystal grains relatively increases the proportion of crystal
grain boundaries. Accordingly, the dissolved carbon is present in a larger amount
in the crystal grain boundaries than within the crystal grains, thus achieving excellent
room-temperature non-aging properties. Since the dissolved carbon present within the
crystal grains can more freely migrate, it binds to movable dislocations, thus affecting
the room-temperature aging properties. In contrast, the dissolved carbon segregated
in stable positions, such as in the crystal grain boundaries and in the vicinity of
the precipitates, is activated at a high temperature, for example, a temperature for
painting/baking treatment, thus affecting the bake hardenability.
[0030] The fine precipitates distributed in the steel sheets of the present invention have
a positive influence on the increase of yield strength arising from precipitation
enhancement, improvement in strength-ductility balance, in-plane anisotropy index,
and plasticity anisotropy. To this end, the fine (Mn,Cu)S precipitates and AlN precipitates
must be uniformly distributed. According to the cold rolled steel sheets of the present
invention, contents of components affecting the precipitation, composition between
the components, production conditions, and particularly cooling rate after hot rolling,
have a great influence on the distribution of the fine precipitates.
[0031] The constituent components of the cold rolled steel sheets according to the present
invention will be explained.
[0032] The content of carbon (C) is preferably limited to 0.01% or less.
[0033] Carbon (C) affects the room-temperature aging resistance and bake hardenability of
the cold rolled steel sheets. When the carbon content exceeds 0.01%, the addition
of the expensive agents Nb and Ti is required to remove the remaining carbon, which
is economically disadvantageous and is undesirable in terms of formability. When it
is intended to achieve room-temperature aging resistance only, it is preferred to
maintain the carbon content at a low level, which enables the reduction of the amount
of the expensive agents Nb and Ti added. When it is intended to ensure desired bake
hardenability, the carbon is preferably added in an amount of 0.001% or more, and
more preferably 0.005% to 0.01%. When the carbon content is less than 0.005%, room-temperature
aging resistance can be ensured without increasing the amounts of Nb and Ti.
[0034] The content of copper (Cu) is preferably in the range of 0.01-0.2%.
[0035] Copper serves to form fine CuS precipitates, which make the crystal grains fine.
Copper lowers the in-plane anisotropy index of the cold rolled steel sheets and enhances
the yield strength of the cold rolled steel sheets by precipitation promotion. In
order to form fine precipitates, the Cu content must be 0.01% or more. When the Cu
content is more than 0.2%, coarse precipitates are obtained. The Cu content is more
preferably in the range of 0.03 to 0.2%.
[0036] The content of manganese (Mn) is preferably in the range of 0.01-0.3%.
[0037] Manganese serves to precipitate sulfur in a solid solution state in the steels as
MnS precipitates, thereby preventing occurrence of hot shortness caused by the dissolved
sulfur, or is known as a solid solution strengthening element. From such a technical
standpoint, manganese is generally added in a large amount. The present inventors
have found that when the manganese content is reduced and the sulfur content is optimized,
very fine MnS precipitates are obtained. Based on this finding, the manganese content
is limited to 0.3% or less. In order to ensure this characteristic, the manganese
content must be 0.01% or more. When the manganese content is less than 0.01%,
i.e. the sulfur content remaining in a solid solution state is high, hot shortness may
occur. When the manganese content is greater than 0.3%, coarse MnS precipitates are
formed, thus making it difficult to achieve desired strength. A more preferable Mn
content is within the range of 0.01 to 0.12%.
[0038] The content of sulfur (S) is preferably limited to 0.08% or less.
[0039] Sulfur (S) reacts with Cu and/or Mn to form CuS and MnS precipitates, respectively.
When the sulfur content is greater than 0.08%, the proportion of dissolved sulfur
is increased. This increase of dissolved sulfur greatly deteriorates the ductility
and formability of the steel sheets and increases the risk of hot shortness. In order
to obtain as many CuS and/or MnS precipitates as possible, a sulfur content of 0.005%
or more is preferred.
[0040] The content of aluminum (Al) is preferably limited to 0.1% or less.
[0041] Aluminum reacts with nitrogen (N) to form fine AlN precipitates, thereby completely
preventing aging by dissolved nitrogen. When the nitrogen content is 0.004% or more,
AlN precipitates are sufficiently formed. The distribution of the fine AlN precipitates
in the steel sheets allows the formation of minute crystal grains and enhances the
yield strength of the steel sheets by precipitation enhancement. A more preferable
Al content is in the range of 0.01 to 0.1%.
[0042] The content of nitrogen (N) is preferably limited to 0.02% or less.
[0043] When it is intended to use AlN precipitates, nitrogen is added in an amount of up
to 0.02%. Otherwise, the nitrogen content is controlled to 0.004% or less. When the
nitrogen content is less than 0.004%, the number of the AlN precipitates is small,
and therefore, the minuteness effects of crystal grains and the precipitation enhancement
effects are negligible. In contrast, when the nitrogen content is greater than 0.02%,
it is difficult to guarantee aging properties by use of dissolved nitrogen.
[0044] The content of phosphorus (P) is preferably limited to 0.2% or less.
[0045] Phosphorus is an element that has excellent solid solution strengthening effects
while allowing a slight reduction in r-value. Phosphorus guarantees high strength
of the steel sheets of the present invention in which the precipitates are controlled.
It is desirable that the phosphorus content in steels requiring a strength of the
order of 280 MPa be defined to 0.015% or less. It is desirable that the phosphorus
content in high-strength steels of the order of 340 MPa be limited to a range exceeding
0.015% and not exceeding 0.2%. A phosphorus content exceeding 0.2% can lead to a reduction
in ductility of the steel sheets. Accordingly, the phosphorus content is preferably
limited to a maximum of 0.2%. When Si and Cr are added in the present invention, the
phosphorus content can be appropriately controlled to be 0.2% or less to achieve the
desired strength.
[0046] The content of boron (B) is preferably in the range of 0.0001 to 0.002%.
[0047] Boron is added to prevent occurrence of secondary working embrittlement. To this
end, a preferable boron content is 0.0001% or more. When the boron content exceeds
0.002%, the deep drawability of the steel sheets may be markedly deteriorated.
[0048] The content of niobium (Nb) is preferably in the range of 0.002 to 0.04%.
[0049] Nb is added for the purpose of ensuring the non-aging properties and improving the
formability of the steel sheets. Nb, which is a potent carbide-forming element, is
added to steels to form NbC precipitates in the steels. In addition, the NbC precipitates
permit the steel sheets to be well textured during annealing, thus greatly improving
the deep drawability of the steel sheets. When the content of Nb added is not greater
than 0.002%, the NbC precipitates are obtained in very small amounts. Accordingly,
the steel sheets are not well textured and thus there is little improvement in the
deep drawability of the steel sheets. In contrast, when the Nb content exceeds 0.04%,
the NbC precipitates are obtained in very large amounts. Accordingly, the deep drawability
and elongation of the steel sheets are lowered, and thus the formability of the steel
sheets may be markedly deteriorated.
[0050] The content of titanium (Ti) is preferably in the range of 0.005 to 0.15%.
[0051] Titanium is added for the purpose of ensuring the non-aging properties and improving
the formability of the steel sheets. Ti, which is a potent carbide-forming element,
is added to steels to form TiC precipitates in the steels. The TiC precipitates allow
the precipitation of dissolved carbon to ensure non-aging properties. When the content
of Ti added is less than 0.005%, the TiC precipitates are obtained in very small amounts.
Accordingly, the steel sheets are not well textured and thus there is little improvement
in the deep drawability of the steel sheets. In contrast, when the titanium is added
in an amount exceeding 0.15%, very large TiC precipitates are formed. Accordingly,
minuteness effects of crystal grains are reduced, resulting in high in-plane anisotropy
index, reduction of yield strength and marked worsening of plating characteristics.
[0052] To obtain (Mn,Cu)S and AlN precipitates, the Mn, Cu, S, Nb, Ti, Al, N and C contents
are adjusted within the ranges defined by the following relationships. The respective
components indicated in the following relationships are expressed as percentages by
weight.
[0053] In Relationship 1, S*, which is determined by Relationship 2, represents the content
of sulfur that does not react with Ti and thereafter reacts with Cu. To obtain fine
CuS precipitates, it is preferred that the value of (Cu/63.5)/(S*/32) be equal to
or greater than 1. If the value of (Cu/63.5)/(S*/32) is greater than 30, coarse CuS
precipitates are distributed, which is undesirable. To stably obtain CuS precipitates
having a size of 0.2 µm or less, the value of (Cu/63.5)/(S*/32) is preferably in the
range of 1 to 20, more preferably 1 to 9, and most preferably 1 to 6.
[0054] Relationship 3 is associated with the formation of (Mn,Cu)S precipitates, and is
obtained by adding a Mn content to Relationship 1. To obtain effective (Mn,Cu)S precipitates,
the value of (Mn/55 + Cu/63.5)/(S*/32) must be 1 or greater. When the value of Relationship
3 is greater than 30, coarse (Mn,Cu)S precipitates are obtained. To stably obtain
(Mn,Cu)S precipitates having a size of 0.2 µm or less, a more preferable value of
(Cu/63.5)/(S*/32) is preferably in the range of 1 to 20, more preferably 1 to 9, and
most preferably 1 to 6. When Mn and Cu are added together, the sum of Mn and Cu is
more preferably 0.05-0.4%. The reason for this limitation to the sum of Mn and Cu
is to obtain fine (Mn,Cu)S precipitates.
[0055] Relationship 4 is associated with the formation of fine (Mn,Cu)S precipitates. In
Relationship 4, N*, which is determined by Relationship 5, represents the content
of nitrogen that does not react with Ti and thereafter reacts with Al. To obtain fine
AlN precipitates, it is preferred that the value of (Al/27)/(N*/14) be in the range
of 1-10. To obtain effective AlN precipitates, the value of (Al/27)/(N*/14) must be
1 or greater. If the value of (Al/27)/(N*/14) is greater than 10, coarse AlN precipitates
are obtained and thus poor workability and low yield strength are caused. It is preferred
that the value of (Al/27)/(N*/14) be in the range of 1 to 6.
[0056] The components of the cold rolled steel sheets according to the present invention
may be combined in various ways according to the kind of precipitates to be obtained.
For example, the present invention provides a cold rolled steel sheet which has a
composition comprising 0.01% or less of C, 0.08% or less of S, 0.1% or less of Al,
0.004% or less of N, 0.2% or less of P, 0.0001-0.002% of B, 0.002-0.04% of Nb, 0.005-0.15%
of Ti, at least one kind selected from 0.01-0.2% of Cu, 0.01-0.3% of Mn and 0.004-0.2%
of N, by weight, and the balance of Fe and other unavoidable impurities, wherein the
composition satisfies the following relationships: 1 ≤ (Mn/55 + Cu/63.5)/(S*/32) ≤
30, 1 ≤ (Al/27)/(N*/14) ≤ 10 (with the proviso that the N content is 0.004% or more),
S* = S - 0.8 x (Ti - 0.8 x (48/14) x N) x (32/48) and N* = N - 0.8 x (Ti - 0.8 x (48/32)
x S)) x (14/48), and the steel sheet comprises at least one kind selected from MnS,
CuS, MnS and AlN precipitates having an average size of 0.2 µm or less. That is, one
or more kinds selected from the group consisting of 0.01-0.2% of Cu, 0.01-0.3% of
Mn and 0.004-0.2% of N lead to various combinations of (Mn,Cu)S and AlN precipitates
having a size not greater than 0.2 µm.
[0057] In the steel sheets of the present invention, carbon is precipitated into NbC and
TiC forms. Accordingly, the room-temperature aging resistance and bake hardenability
of the steel sheets are affected depending on the conditions of dissolved carbon under
which NbC and TiC precipitates are not obtained. Taking into account these requirements,
it is most preferred that the Nb, Ti and C contents satisfy the following relationships.
[0058] Relationship 6 is associated with the formation of NbC and TiC precipitates to remove
the carbon in a solid solution state, thereby achieving room-temperature non-aging
properties. In Relationship 6, Ti*, which is determined by Relationship 7, represents
the content of titanium that reacts with N and S and thereafter reacts with C.
[0059] When the value of (Ti*/48 + Nb/93)/(C/12) is less than 0.8, it is difficult to ensure
room-temperature non-aging properties. In contrast, when the value of (Ti*/48 + Nb/93)/(C/12)
is greater than 5, the amounts of Nb and Ti remaining in a solid solution state in
the steels are large, which deteriorates the ductility of the steels. When it is intended
to achieve room-temperature non-aging properties without securing bake hardenability,
it is preferred to limit the carbon content to 0.005% or less. Although the carbon
content is more than 0.005%, room-temperature non-aging properties can be achieved
when Relationship 6 is satisfied but the amounts of Nb and TiC precipitates are increased,
thus deteriorating the workability of the steel sheets.
(provided that when Ti* is less than 0, Ti* is defined as 0.)
[0060] Relationship 8 is associated with the achievement of bake hardenability. Cs, which
is expressed in ppm by Relationship 8, represents the content of dissolved carbon
that is not precipitated into NbC and TiC forms. In order to achieve a high bake hardening
value, the Cs value must be 5 ppm or more. If the Cs value exceeds 30 ppm, the content
of dissolved carbon is increased, making it difficult to attain room-temperature non-aging
properties.
[0061] It is advantageous that the fine precipitates are uniformly distributed in the compositions
of the present invention. It is preferable that the precipitates have an average size
of 0.2 µm or less. According to a study conducted by the present inventors, when the
precipitates have an average size greater than 0.2 µm, the steel sheets have poor
strength and low in-plane anisotropy index. Further, large amounts of precipitates
having a size of 0.2 µm or less are distributed in the compositions of the present
invention. While the number of the distributed precipitates is not particularly limited,
it is more advantageous with higher number of the precipitates. The number of the
distributed precipitates is preferably 1 x 10
5/mm
2 or more, more preferably 1 x 10
6/mm
2 or more, and most preferably 1 x 10
7/mm
2 or more. The plasticity-anisotropy index is increased and the in-plane anisotropy
index is lowered with increasing number of the precipitates, and as a result, the
workability is greatly improved. It is commonly known that there is a limitation in
increasing the workability because the in-plane anisotropy index is increased with
increasing plasticity-anisotropy index. It is worth noting that as the number of the
precipitates distributed in the steel sheets of the present invention increases, the
plasticity-anisotropy index of the steel sheets is increased and the in-plane anisotropy
index of the steel sheets is lowered. The steel sheets of the present invention in
which the fine precipitates are formed satisfy a yield ratio (yield strength/tensile
strength) of 0.58 or higher.
[0062] @ When the steel sheets of the present invention are applied to high-strength steel
sheets, they may further contain at least one solid solution strengthening element
selected from P, Si and Cr. The addition effects of P have been previously described,
and thus their explanation is omitted.
[0063] The content of silicon (Si) is preferably in the range of 0.1 to 0.8%.
[0064] Si is an element that has solid solution strengthening effects and shows a slight
reduction in elongation. Si guarantees high strength of the steel sheets of the present
invention in which the precipitates are controlled. Only when the Si content is 0.1%
or more, high strength can be ensured. However, when the Si content is more than 0.8%,
the ductility of the steel sheets is deteriorated.
[0065] The content of chromium (Cr) is preferably in the range of 0.2 to 1.2%.
[0066] Cr is an element that has solid solution strengthening effects, lowers the secondary
working embrittlement temperature, and lowers the aging index due to the formation
of Cr carbides. Cr guarantees high strength of the steel sheets of the present invention
in which the precipitates are controlled and serves to lower the in-plane anisotropy
index of the steel sheets. Only when the Cr content is 0.2% or more, high strength
can be ensured. However, when the Cr content exceeds 1.2%, the ductility of the steel
sheets is deteriorated.
[0067] The cold rolled steel sheets of the present invention may further contain molybdenum
(Mo).
[0068] The content of molybdenum (Mo) in the cold rolled steel sheets of the present invention
is preferably in the range of 0.01 to 0.2%.
[0069] Mo is added as an element that increases the plasticity-anisotropy index of the steel
sheets. Only when the molybdenum content is not lower than 0.01%, the plasticity-anisotropy
index of the steel sheets is increased. However, when the molybdenum content exceeds
0.2%, the plasticity-anisotropy index is not further increased and there is a danger
of hot shortness.
Production of cold rolled steel sheets
[0070] Hereinafter, a process for producing the cold rolled steel sheets of the present
invention will be explained with reference to the preferred embodiments that follow.
Various modifications of the embodiments of the present invention can be made, and
such modifications are within the scope of the present invention.
[0071] The process of the present invention is characterized in that a steel satisfying
one of the steel compositions defined above is processed through hot rolling and cold
rolling to form precipitates having an average size of 0.2 µm or less in a cold rolled
sheet. The average size of the precipitates in the cold rolled plate is affected by
the design of the steel composition and the processing conditions, such as reheating
temperature and winding temperature. Particularly, cooling rate after hot rolling
has a direct influence on the average size of the precipitates.
Hot rolling conditions
[0072] In the present invention, a steel satisfying one of the compositions defined above
is reheated, and is then subjected to hot rolling. The reheating temperature is preferably
1,100°C or higher. When the steel is reheated to a temperature lower than 1,100°C,
coarse precipitates formed during continuous casting are not completely dissolved
and remain. The coarse precipitates still remain even after hot rolling.
[0073] It is preferred that the hot rolling is performed at a finish rolling temperature
not lower than the Ar
3 transformation point. When the finish rolling temperature is lower than the Ar
3 transformation point, rolled grains are created, which deteriorates the workability
and causes poor strength.
[0074] The cooling is preferably performed at a rate of 300 °C/min or higher before winding
and after hot rolling. Although the composition of the components is controlled to
obtain fine precipitates, the precipitates may have an average size greater than 0.2
µm at a cooling rate of less than 300 °C/min. That is, as the cooling rate is increased,
many nuclei are created and thus the size of the precipitates becomes finer and finer.
Since the size of the precipitates is decreased with increasing cooling rate, it is
not necessary to define the upper limit of the cooling rate. When the cooling rate
is higher than 1,000 °C/min., however, a significant improvement in the size reduction
effects of the precipitates is not further shown. Therefore, the cooling rate is preferably
in the range of 300-1000 °C/min.
Winding conditions
[0075] After the hot rolling, winding is performed at a temperature not higher than 700°C.
When the winding temperature is higher than 700°C, the precipitates are grown too
coarsely, thus making it difficult to ensure high strength.
Cold rolling conditions
[0076] The steel is cold rolled at a reduction rate of 50-90%. Since a cold reduction rate
lower than 50 % leads to creation of a small amount of nuclei upon annealing recrystallization,
the crystal grains are grown excessively upon annealing, thereby coarsening of the
crystal grains recrystallized through annealing, which results in reduction of the
strength and formability. A cold reduction rate higher than 90 % leads to enhanced
formability, while creating an excessively large amount of nuclei, so that the crystal
grains recrystallized through annealing become too fine, thus deteriorating the ductility
of the steel.
Continuous annealing
[0077] Continuous annealing temperature plays an important role in determining the mechanical
properties of the final product. According to the present invention, the continuous
annealing is preferably performed at a temperature of 700 to 900°C. When the continuous
annealing is performed at a temperature lower than 700°C, the recrystallization is
not completed and thus a desired ductility cannot be ensured. In contrast, when the
continuous annealing is performed at a temperature higher than 900°C, the recrystallized
grains become coarse and thus the strength of the steel is deteriorated. The continuous
annealing is maintained until the steel is completely recrystallized. The recrystallization
of the steel can be completed for about 10 seconds or more. The continuous annealing
is preferably performed for 10 seconds to 30 minutes.
[Mode for Invention]
[0078] The present invention will now be described in more detail with reference to the
following examples.
[0079] The mechanical properties of steel sheets produced in the following examples were
evaluated according to the ASTM E-8 standard test methods. Specifically, each of the
steel sheets was machined to obtain standard samples. The yield strength, tensile
strength, elongation, plasticity-anisotropy index (r
m value) and in-plane anisotropy index (Δr value), and the aging index were measured
using a tensile strength tester (available from INSTRON Company, Model 6025). The
plasticity-anisotropy index r
m and in-plane anisotropy index (Δr value) were calculated by the following equations:
r
m = (r
0 + 2r
45 + r
90)/4 and Δr = (r
0 - 2r
45 + r
90)/2, respectively.
[0080] The aging index of the steel sheets is defined as a yield point elongation measured
by annealing each of the samples, followed by 1.0% skin pass rolling and thermally
processing at 100°C for 2 hours. The bake hardening (BH) value of the standard samples
was measured by the following procedure. After a 2% strain was applied to each of
the samples, the strained sample was annealed at 170°C for 20 minutes. The yield strength
of the annealed sample was measured. The BH value was calculated by subtracting the
yield strength measured before annealing from the yield strength value measured after
annealing.
Example 1
[0081] First, steel slabs were prepared in accordance with the compositions shown in the
following tables. The steel slabs were reheated and finish hot-rolled to provide hot
rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400 °C/min.,
wound at 650°C, cold-rolled at a reduction rate of 75%, followed by continuous annealing
to produce cold rolled steel sheets. At this time, the finish hot rolling was performed
at 910°C, which is above the Ar
3 transformation point, and the continuous annealing was performed by heating the hot
rolled steel sheets at a rate of 10 °C/second to 830°C for 40 seconds to produce the
final cold rolled steel sheets.
TABLE 1
Sample No. |
Chemical Components (wt%) |
C |
Cu |
S |
Al |
N |
P |
B |
Nb |
Ti |
Others |
A11 |
0.0006 |
0.14 |
0.008 |
0.032 |
0.0012 |
0.048 |
0.0003 |
0.014 |
0.008 |
|
A12 |
0.0017 |
0.12 |
0.012 |
0.043 |
0.0026 |
0.082 |
0.0006 |
0.02 |
0.022 |
Si:0.17 |
A13 |
0.0031 |
0.09 |
0.012 |
0.028 |
0.0016 |
0.106 |
0.0012 |
0.028 |
0.02 |
Si:0.28 |
A14 |
0.0012 |
0.118 |
0.02 |
0.042 |
0.0015 |
0.078 |
0.0011 |
0.033 |
0.033 |
Si:0.15 |
Mo:0.09 |
A15 |
0.0018 |
0.1 |
0.018 |
0.036 |
0.0019 |
0.085 |
0.0009 |
0.04 |
0.018 |
Si:0.15 |
Cr:0.15 |
A16 |
0.0022 |
0.11 |
0.01 |
0.038 |
0.0015 |
0.059 |
0 |
0 |
0 |
|
A17 |
0.0012 |
0 |
0.011 |
0.034 |
0.0027 |
0.12 |
0.0008 |
0.03 |
0.16 |
|
TABLE 2
Sample No. |
S* |
(Cu/63.5)/(S*/32) |
(Ti*/48+Nb/93)/ (C/12) |
Average size of CuS precipitates (µm) |
Number of CuS precipitates (mm-2) |
A11 |
0.0055 |
12.854 |
0.97 |
0.04 |
1.5 x 107 |
A12 |
0.0041 |
14.858 |
1.59 |
0.05 |
2.5 x 107 |
A13 |
0.0037 |
12.345 |
1.26 |
0.05 |
3.8 x 107 |
A14 |
0.0046 |
12.943 |
4.57 |
0.05 |
4.1 x 107 |
A15 |
0.0112 |
4.5077 |
1.64 |
0.04 |
5.2 x 107 |
A16 |
0.0122 |
4.5458 |
-1.8 |
0.08 |
4.5 x 106 |
A17 |
-0.07 |
0 |
32.3 |
0.08 |
6.7 x 104 |
S*=S-0.8x(Ti-0.8x(48/14)xN)x(32/48) |
Ti*=Ti-0.8x((48/14)xN+(48/32)xS) |
TABLE 3
Sample No. |
Mechanical Properties |
Remarks |
YS (MPa) |
TS (MPa) |
El (%) |
rm |
Δr |
AI (%) |
SWE (DBTT-°C) |
A11 |
208 |
345 |
46 |
2.32 |
0.14 |
0 |
-70 |
IS |
A12 |
263 |
402 |
39 |
1.88 |
0.18 |
0 |
-60 |
IS |
A13 |
332 |
448 |
36 |
1.73 |
0.12 |
0 |
-50 |
IS |
A14 |
329 |
452 |
36 |
1.84 |
0.18 |
0 |
-50 |
IS |
A15 |
334 |
450 |
37 |
1.74 |
0.13 |
0 |
-60 |
IS |
A16 |
232 |
348 |
43 |
1.12 |
0.29 |
0.62 |
-70 |
CS |
A17 |
270 |
445 |
28 |
1.82 |
0.48 |
0 |
-50 |
CS |
* Note:
YS = Yield strength, TS = Tensile Strength, El = Elongation, rm = Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = Aging Index,
SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS = Comparative steel |
Example 2
[0082] First, steel slabs were prepared in accordance with the compositions shown in the
following tables. The steel slabs were reheated and finish hot-rolled to provide hot
rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400 °C/min.,
wound at 650°C, cold-rolled at a reduction rate of 75%, followed by continuous annealing
to produce cold rolled steel sheets. At this time, the finish hot rolling was performed
at 910°C, which is above the Ar
3 transformation point, and the continuous annealing was performed by heating the hot
rolled steel sheets at a rate of 10 °C/second to 830°C for 40 seconds to produce the
final cold rolled steel sheets.
TABLE 4
Sample No. |
Chemical Components (wt%) |
C |
mn |
Cu |
S |
Al |
N |
P |
B |
Nb |
Ti |
Others |
A21 |
0.0007 |
0.06 |
0.08 |
0.007 |
0.038 |
0.0011 |
0.05 |
0.0008 |
0.01 |
0.009 |
|
A22 |
0.0014 |
0.15 |
0.15 |
0.013 |
0.027 |
0.0018 |
0.082 |
0.0009 |
0.02 |
0.019 |
Si:0.22 |
A23 |
0.0029 |
0.18 |
0.12 |
0.02 |
0.041 |
0.0025 |
0.12 |
0.0011 |
0.029 |
0.028 |
Si:0.33 |
A24 |
0.0015 |
0.08 |
0.11 |
0.018 |
0.028 |
0.0026 |
0.085 |
0.0009 |
0.039 |
0.015 |
Si:0.22 |
Mo:0.11 |
A25 |
0.0012 |
0.13 |
0.15 |
0.022 |
0.032 |
0.0011 |
0.073 |
0.0009 |
0.005 |
0.032 |
Si:0.3 |
Cr:0.24 |
A26 |
0.0036 |
0.45 |
0.14 |
0.009 |
0.033 |
0.0024 |
0.048 |
0.005 |
0 |
0 |
Si:0.05 |
A27 |
0.0015 |
0.13 |
0 |
0.008 |
0.038 |
0.0021 |
0.118 |
0 |
0.04 |
0.02 |
Si:0.35 |
TABLE 5
Sample No. |
Cu+Mn |
S* |
(Mn/55+Cu/63.5)/ (S*/32) |
(Ti*/48+Nb/93)/ (C/12) |
Average size of (Mn,Cu)S precipitates (pm) |
Number of (Mn,Cu)S precipitates (mm-2) |
A21 |
0.14 |
0.0038 |
19.748 |
0.98 |
0.04 |
3.3 x 107 |
A22 |
0.3 |
0.0055 |
29.613 |
1.57 |
0.04 |
4.2 x 107 |
A23 |
0.3 |
0.0087 |
18.937 |
1.04 |
0.03 |
5.0 x 107 |
A24 |
0.19 |
0.0138 |
7.3879 |
1.07 |
0.04 |
4.5 x 107 |
A25 |
0.28 |
0.0065 |
23.115 |
1.08 |
0.04 |
4.9 x 107 |
A26 |
0.59 |
0.0125 |
26.566 |
-1.2 |
0.25 |
5.5 x 106 |
A27 |
0.13 |
0.0004 |
186.6 |
4.21 |
0.16 |
4.3 x 104 |
S*=S-0.8x(Ti-0.8x(48/14)xN)x(32/48) |
Ti*=Ti-0.8x((48/14)xN+(48/32)xS) |
TABLE 6
Sample No. |
Mechanical Properties |
Remarks |
YS (MPa) |
TS (MPa) |
El (%) |
rm |
Δr |
SWE (DBTT-°C) |
AI (%) |
A21 |
218 |
348 |
45 |
2.1 |
0.19 |
-40 |
0 |
IS |
A22 |
262 |
410 |
36 |
1.94 |
0.17 |
-40 |
0 |
IS |
A23 |
328 |
455 |
33 |
1.89 |
0.17 |
-40 |
0 |
IS |
A24 |
247 |
401 |
35 |
1.89 |
0.19 |
-50 |
0 |
IS |
A25 |
229 |
392 |
38 |
1.79 |
0.17 |
-40 |
0 |
IS |
A26 |
233 |
359 |
37 |
1.11 |
0.62 |
-60 |
1.56 |
CS |
A27 |
283 |
425 |
33 |
1.81 |
0.57 |
-40 |
0 |
CS |
* Note:
YS = Yield strength, TS = Tensile Strength, El = Elongation, rm = Plasticity-anisotropy index, Δr = In-plane anisotropy index, SWE = Secondary Working
Embrittlement, AI = Aging Index, IS = Inventive Steel, CS = Comparative steel |
Example 3
[0083] First, steel slabs were prepared in accordance with the compositions shown in the
following tables. The steel slabs were reheated and finish hot-rolled to provide hot
rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400 °C/min.,
wound at 650°C, cold-rolled at a reduction rate of 75%, followed by continuous annealing
to produce cold rolled steel sheets. At this time, the finish hot rolling was performed
at 910°C, which is above the Ar
3 transformation point, and the continuous annealing was performed by heating the hot
rolled steel sheets at a rate of 10 °C/second to 830°C for 40 seconds to produce the
final cold rolled steel sheets.
TABLE 7
Sample No. |
Chemical Components (wt%) |
C |
Cu |
S |
Al |
N |
P |
B |
Nb |
Ti |
Others |
A31 |
0.0005 |
0.08 |
0.007 |
0.029 |
0.0139 |
0.044 |
0.0008 |
0.025 |
0.038 |
|
A32 |
0.0012 |
0.13 |
0.016 |
0.026 |
0.011 |
0.08 |
0.0008 |
0.05 |
0.028 |
Si:0.11 |
A33 |
0.0025 |
0.14 |
0.011 |
0.04 |
0.0148 |
0.116 |
0.0009 |
0.032 |
0.051 |
Si:0.26 |
A34 |
0.0013 |
0.17 |
0.012 |
0.031 |
0.0088 |
0.047 |
0.0011 |
0.043 |
0.029 |
Si:0.09 |
Mo:0.12 |
A35 |
0.0005 |
0.15 |
0.015 |
0.03 |
0.0089 |
0.043 |
0.0009 |
0.009 |
0.04 |
Si:0.11 |
Cr:0.22 |
A36 |
0.0038 |
0.09 |
0.013 |
0.032 |
0.0012 |
0.042 |
0.0005 |
0 |
0 |
|
A37 |
0.0014 |
0 |
0.009 |
0.055 |
0.012 |
0.12 |
0.0005 |
0 |
0.14 |
Si:0.13 |
TABLE 8
Sample No. |
S* |
(Cu/63.5)/ (S*/32) |
(Ti*/48+Nb/93) /(C/12) |
N* |
(Al/27)/ (N*14) |
Average size of precipitates (µm) |
Number of precipitates (mm-2) |
A31 |
0.0071 |
5.7046 |
2.19 |
0.007 |
2.15 |
0.04 |
3.3 x 107 |
A32 |
0.0172 |
3.8181 |
0.92 |
0.0089 |
1.51 |
0.04 |
4.2 x 107 |
A33 |
0.0055 |
12.944 |
1.37 |
0.006 |
3.47 |
0.03 |
5.0 x 107 |
A34 |
0.0094 |
9.1075 |
2.43 |
0.0054 |
2.98 |
0.04 |
4.5 x 107 |
A35 |
0.0067 |
11.306 |
1.12 |
0.0038 |
4.13 |
0.04 |
4.9 x 107 |
A36 |
0.0148 |
3.0737 |
0 |
0.0048 |
3.43 |
0.25 |
5.5 x 106 |
A37 |
0 |
0 |
17.2 |
0 |
-1.6 |
0.16 |
4.3 x 104 |
S*=S-0.8x(Ti-0.8x(48/14)xN)x(32/48) |
Ti*=Ti-0.8x((48/14)xN+(48/32)xS) |
N*=N-0.8x(Ti-0.8x(48/32)xS))x(14/48) |
TABLE 9
Sample No. |
Mechanical Properties |
Remarks |
YS (MPa) |
Ts (MPa) |
El (%) |
rm |
Δr |
SWE (DBTT-°C) |
AI (%) |
A31 |
218 |
348 |
45 |
2.1 |
0.19 |
-40 |
0 |
IS |
A32 |
262 |
410 |
36 |
1.94 |
0.17 |
-40 |
0 |
IS |
A33 |
328 |
455 |
33 |
1.89 |
0.17 |
-40 |
0 |
IS |
A34 |
247 |
401 |
35 |
1.89 |
0.19 |
-50 |
0 |
IS |
A35 |
229 |
392 |
38 |
1.79 |
0.17 |
-40 |
0 |
IS |
A36 |
233 |
359 |
37 |
1.11 |
0.62 |
-60 |
1.56 |
CS |
A37 |
283 |
425 |
33 |
1.81 |
0.57 |
-40 |
0 |
CS |
* Note:
YS = Yield strength, TS = Tensile Strength, El = Elongation, rm = Plasticity-anisotropy index, Δr = In-plane anisotropy index, SWE = Secondary Working
Embrittlement, AI = Aging Index, IS = Inventive Steel, CS = Comparative steel |
Example 4
[0084] First, steel slabs were prepared in accordance with the compositions shown in the
following tables. The steel slabs were reheated and finish hot-rolled to provide hot
rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400 °C/min.,
wound at 650°C, cold-rolled at a reduction rate of 75%, followed by continuous annealing
to produce cold rolled steel sheets. At this time, the finish hot rolling was performed
at 910°C, which is above the Ar
3 transformation point, and the continuous annealing was performed by heating the hot
rolled steel sheets at a rate of 10 °C/second to 830°C for 40 seconds to produce the
final cold rolled steel sheets.
TABLE 10
Sample No. |
Chemical Components (wt%) |
C |
Mn |
Cu |
S |
Al |
N |
P |
B |
Nb |
Ti |
Others |
A41 |
0.0008 |
0.07 |
0.15 |
0.009 |
0.025 |
0.0089 |
0.045 |
0.0009 |
0.018 |
0.03 |
Si:0.03 |
A42 |
0.0015 |
0.15 |
0.12 |
0.014 |
0.034 |
0.011 |
0.082 |
0.001 |
0.039 |
0.039 |
Si:0.12 |
A43 |
0.0028 |
0.12 |
0.16 |
0.011 |
0.029 |
0.0109 |
0.118 |
0.0007 |
0.03 |
0.038 |
Si:0.09 |
A44 |
0.0012 |
0.15 |
0.1 |
0.02 |
0.03 |
0.013 |
0.035 |
0.0011 |
0.012 |
0.063 |
Si:0.12 |
Mo:0.09 |
A45 |
0.0019 |
0.13 |
0.14 |
0.017 |
0.053 |
0.0132 |
0.034 |
0.0008 |
0.045 |
0.05 |
Si:0.09 |
Cr:0.22 |
A46 |
0.0034 |
0.45 |
0.1 |
0.0083 |
0.038 |
0.0015 |
0.048 |
0.0005 |
0 |
0 |
|
A47 |
0.0038 |
0.07 |
0 |
0.012 |
0.035 |
0.0024 |
0.13 |
0.0005 |
0 |
0.17 |
Si:0.08 |
TABLE 11
Sample No. |
Cu+Mn |
S* |
(Mn/55+C u/63.5)/ (S*/32) |
(Ti*/48+Nb/ 93)/(C/12) |
N* |
(Al/27) /(N*14) |
Average size of precipitates (µm) |
Number of precipitates (mm-2) |
A41 |
0.22 |
0.006 |
19.324 |
1.27 |
0.0044 |
2.93 |
0.04 |
9.4 x 107 |
A42 |
0.27 |
0.0093 |
15.901 |
2.03 |
0.0058 |
3.03 |
0.03 |
9.0 x 107 |
A43 |
0.28 |
0.0067 |
22.527 |
0.93 |
0.0051 |
2.94 |
0.04 |
8.2 x 107 |
A44 |
0.25 |
0.0054 |
25.413 |
1.99 |
0.0039 |
3.99 |
0.04 |
7.9 x 107 |
A45 |
0.27 |
0.0096 |
15.16 |
2.19 |
0.0063 |
4.37 |
0.03 |
9.6 x 107 |
A46 |
0.55 |
0.0105 |
29.751 |
-1 |
0.0038 |
5.15 |
0.25 |
1.5 x 104 |
A47 |
0.07 |
0 |
0 |
9.8 |
0 |
0 |
0.04 |
3.5 x 105 |
S*=S-0.8x(Ti-0.8x(48/14)xN)x(32/48) |
Ti*=Ti-0.8x((48/14)xN+(48/32)xS) |
N*=N-0.8x(Ti-0.8x(48/32)xS))x(14/48) |
TABLE 12
Sample No. |
Mechanical Properties |
Remarks |
YS (MPa) |
TS (MPa) |
El (%) |
rm |
Δr |
AI (%) |
SWE (DBTT-°C) |
A41 |
222 |
357 |
43 |
2.22 |
0.09 |
0 |
-70 |
IS |
A42 |
260 |
409 |
35 |
1.93 |
0.06 |
0 |
-60 |
IS |
A43 |
332 |
453 |
34 |
1.73 |
0.06 |
0 |
-60 |
IS |
A44 |
229 |
367 |
40 |
2.18 |
0.08 |
0 |
-60 |
IS |
A45 |
231 |
359 |
45 |
1.89 |
0.07 |
0 |
-50 |
IS |
A46 |
202 |
355 |
38 |
1.59 |
0.39 |
0 |
-60 |
CS |
A47 |
338 |
458 |
24 |
1.31 |
0.58 |
0.55 |
-70 |
CS |
* Note:
YS = Yield strength, TS = Tensile Strength, El = Elongation, rm = Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = Aging Index,
SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS = Comparative steel |
Example 5
[0085] First, steel slabs were prepared in accordance with the compositions shown in the
following tables. The steel slabs were reheated and finish hot-rolled to provide hot
rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400 °C/min.,
wound at 650°C, cold-rolled at a reduction rate of 75%, followed by continuous annealing
to produce cold rolled steel sheets. At this time, the finish hot rolling was performed
at 910°C, which is above the Ar
3 transformation point, and the continuous annealing was performed by heating the hot
rolled steel sheets at a rate of 10 °C/second to 830°C for 40 seconds to produce the
final cold rolled steel sheets.
TABLE 13
Sample No. |
Chemical Components (wt%) |
C |
Mn |
P |
S |
Al |
Ti |
Nb |
B |
N |
Others |
A51 |
0.0009 |
0.08 |
0.008 |
0.006 |
0.042 |
0.008 |
0.017 |
0.0005 |
0.0018 |
|
A52 |
0.0016 |
0.12 |
0.059 |
0.012 |
0.033 |
0.016 |
0.025 |
0.0008 |
0.0012 |
Si:0.11 |
A53 |
0.0026 |
0.12 |
0.094 |
0.021 |
0.043 |
0.026 |
0.039 |
0.0011 |
0.0033 |
Si:0.31 |
A54 |
0.0012 |
0.11 |
0.129 |
0.013 |
0.033 |
0.016 |
0.038 |
0.0009 |
0.0012 |
Si:0.26 |
Mo:0.14 |
A55 |
0.0015 |
0.13 |
0.053 |
0.021 |
0.039 |
0.032 |
0.011 |
0.0009 |
0.0011 |
Si:0.33 |
Cr:0.24 |
A56 |
0.0028 |
0.48 |
0.052 |
0.009 |
0.033 |
0.022 |
0.021 |
0.0005 |
0.0024 |
Si:0.05 |
A57 |
0.0015 |
0.13 |
0.118 |
0.008 |
0.038 |
0 |
0 |
0 |
0.0021 |
Si:0.35 |
TABLE 14
Sample No. |
S* |
(Mn/55) / (S*/32) |
(Ti*/48+Nb/93) / (C/12) |
Average size of precipitates (µm) |
Number of precipitates (mm-2) |
A51 |
0.0044 |
10.66 |
1.29 |
0.06 |
3.8 x 106 |
A52 |
0.0052 |
13.37 |
1.75 |
0.06 |
4.6 x 106 |
A53 |
0.012 |
5.8373 |
1.14 |
0.05 |
5.2 x 106 |
A54 |
0.0062 |
10.286 |
3.48 |
0.06 |
4.1 x 106 |
A55 |
0.0055 |
13.647 |
1.58 |
0.06 |
3.9 x 106 |
A56 |
0.0008 |
359.18 |
1.38 |
0.28 |
1.2 x 104 |
A57 |
0.0111 |
6.8313 |
-2.6 |
0.18 |
6.3 x 105 |
S*=S-0.8x(Ti-0.8x(48/14)xN)x(32/48) |
Ti*=Ti-0.8x((48/14)xN+(48/32)xS) |
TABLE 15
Sample No. |
Mechanical Properties |
Remarks |
YS (MPa) |
TS (MPa) |
El (%) |
rm |
(Δr) |
AI (%) |
SWE (DBTT-°C) |
A51 |
188 |
302 |
49 |
2.09 |
0.25 |
0 |
-60 |
IS |
A52 |
221 |
352 |
42 |
1.93 |
0.22 |
0 |
-50 |
IS |
A53 |
256 |
409 |
38 |
1.73 |
0.19 |
0 |
-40 |
IS |
A54 |
270 |
444 |
34 |
1.69 |
0.21 |
0 |
-40 |
IS |
A55 |
231 |
362 |
43 |
1.87 |
0.21 |
0 |
-50 |
IS |
A56 |
202 |
356 |
41 |
1.85 |
0.29 |
0 |
-40 |
CS |
A57 |
254 |
401 |
37 |
1.28 |
0.54 |
2.33 |
-40 |
CS |
* Note:
YS = Yield strength, TS = Tensile Strength, El = Elongation, rm = Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = Aging Index,
SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS = Comparative steel |
Example 6
[0086] First, steel slabs were prepared in accordance with the compositions shown in the
following tables. The steel slabs were reheated and finish hot-rolled to provide hot
rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400 °C/min.,
wound at 650°C, cold-rolled at a reduction rate of 75%, followed by continuous annealing
to produce cold rolled steel sheets. At this time, the finish hot rolling was performed
at 910°C, which is above the Ar
3 transformation point, and the continuous annealing was performed by heating the hot
rolled steel sheets at a rate of 10 °C/second to 830°C for 40 seconds to produce the
final cold rolled steel sheets.
TABLE 16
Sample No. |
Chemical Components (wt%) |
C |
P |
S |
Al |
Ti |
Nb |
B |
N |
Others |
A61 |
0.0007 |
0.01 |
0.009 |
0.042 |
0.039 |
0.03 |
0.0008 |
0.011 |
|
A62 |
0.0015 |
0.037 |
0.017 |
0.053 |
0.029 |
0.042 |
0.0009 |
0.0074 |
Si:0.11 |
A63 |
0.0028 |
0.073 |
0.012 |
0.049 |
0.048 |
0.039 |
0.0009 |
0.0123 |
Si:0.22 |
A64 |
0.0014 |
0.119 |
0.011 |
0.038 |
0.029 |
0.044 |
0.0011 |
0.0085 |
Si:0.12 |
Mo:0.12 |
A65 |
0.0007 |
0.035 |
0.016 |
0.037 |
0.04 |
0.022 |
0.0009 |
0.0089 |
Si:0.11 |
Cr:0.27 |
A66 |
0.0025 |
0.074 |
0.011 |
0.039 |
0.025 |
0.022 |
0.0005 |
0.0018 |
Si:0.05 |
A67 |
0.0015 |
0.053 |
0.012 |
0.038 |
0 |
0 |
0 |
0.0026 |
Si:0.35 |
TABLE 17
Sample No. |
(Ti*/48+Nb/93) /(C/12) |
N* |
(Al/27)/(N* /14) |
Average size of precipitates (µm) |
Number of precipitates |
A61 |
4.83 |
0.0044 |
4.93 |
0.05 |
6.8 x 105 |
A62 |
1.66 |
0.0054 |
5.1 |
0.05 |
5.3 x 105 |
A63 |
1.78 |
0.0045 |
5.7 |
0.05 |
7.2 x 105 |
A64 |
2.71 |
0.0048 |
4.09 |
0.05 |
5.5 x 105 |
A65 |
2.77 |
0.004 |
4.74 |
0.05 |
6.3 x 105 |
A66 |
1.82 |
-0.001 |
-21 |
0.05 |
2.8 x 104 |
A67 |
0 |
0.006 |
3.31 |
0.05 |
3.3 x 104 |
Ti*=Ti-0.8x((48/14)xN+(48/32)xS) |
N*=N-0.8x(Ti-0.8x(48/32)xS))x(14/48) |
TABLE 18
Sample |
Mechanical Properties |
Remarks |
No. |
YS (MPa) |
TS (MPa) |
El (%) |
rm |
Δr |
SWE (DBTT-°C) |
AI (%) |
|
A61 |
218 |
352 |
43 |
2.03 |
0.25 |
-60 |
0 |
IS |
A62 |
228 |
369 |
42 |
1.82 |
0.24 |
-50 |
0 |
IS |
A63 |
269 |
417 |
36 |
1.73 |
0.27 |
-50 |
0 |
IS |
A64 |
289 |
452 |
33 |
1.71 |
0.29 |
-50 |
0 |
IS |
A65 |
222 |
358 |
40 |
1.83 |
0.21 |
-60 |
0 |
IS |
A66 |
202 |
356 |
40 |
1.92 |
0.34 |
-40 |
0 |
CS |
A67 |
254 |
401 |
37 |
1.28 |
0.54 |
-40 |
2.33 |
CS |
* Note:
YS = Yield strength, TS = Tensile Strength, El = Elongation, rm = Plasticity-anisotropy index, Δr = In-plane anisotropy index, SWE = Secondary Working
Embrittlement, AI = Aging Index, IS = Inventive Steel, CS = Comparative steel |
Example 7
[0087] First, steel slabs were prepared in accordance with the compositions shown in the
following tables. The steel slabs were reheated and finish hot-rolled to provide hot
rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400 °C/min.,
wound at 650°C, cold-rolled at a reduction rate of 75%, followed by continuous annealing
to produce cold rolled steel sheets. At this time, the finish hot rolling was performed
at 910°C, which is above the Ar
3 transformation point, and the continuous annealing was performed by heating the hot
rolled steel sheets at a rate of 10 °C/second to 830°C for 40 seconds to produce the
final cold rolled steel sheets.
TABLE 19
Sample No. |
Chemical Components (wt%) |
C |
mn |
P |
S |
Al |
Ti |
Nb |
B |
N |
Others |
A71 |
0.0007 |
0.09 |
0.009 |
0.012 |
0.039 |
0.028 |
0.025 |
0.0006 |
0.0084 |
|
A72 |
0.0017 |
0.14 |
0.038 |
0.015 |
0.039 |
0.041 |
0.047 |
0.0005 |
0.0109 |
Si:0.14 |
A73 |
0.0025 |
0.16 |
0.078 |
0.012 |
0.047 |
0.039 |
0.029 |
0.0007 |
0.011 |
|
A74 |
0.0014 |
0.09 |
0.12 |
0.021 |
0.043 |
0.063 |
0.013 |
0.0011 |
0.0128 |
Si:0.12 |
Mo:0.11 |
A75 |
0.0015 |
0.13 |
0.042 |
0.016 |
0.059 |
0.052 |
0.048 |
0.0009 |
0.013 |
Si:0.1 |
Cr:0.29 |
A76 |
0.0028 |
0.48 |
0.052 |
0.009 |
0.033 |
0.022 |
0.021 |
0.0005 |
0.0024 |
Si:0.05 |
A77 |
0.0015 |
0.13 |
0.118 |
0.008 |
0.038 |
0 |
0 |
0 |
0.0021 |
Si:0.35 |
TABLE 20
Sample No. |
S* |
(Mn/55) /(S*/32) |
(Ti*/48+N b/93)/(C/ 12) |
N* |
(Al/27) /(N*/14) |
Average size of precipitates (µm) |
Number of precipitates (mm-2) |
A71 |
0.0094 |
5.5976 |
1.24 |
0.0052 |
3.87 |
0.05 |
2.9 x 107 |
A72 |
0.0091 |
8.9723 |
2.55 |
0.0055 |
3.65 |
0.05 |
3.5 x 107 |
A73 |
0.0073 |
12.767 |
0.94 |
0.0053 |
4.63 |
0.05 |
3.3 x 107 |
A74 |
0.0061 |
8.5498 |
1.68 |
0.004 |
5.6 |
0.04 |
4.5 x 107 |
A75 |
0.0073 |
10.384 |
3.65 |
0.0053 |
5.72 |
0.04 |
4.2 x 107 |
A76 |
0.0008 |
359.18 |
1.38 |
-2E-04 |
-80 |
0.28 |
1.2 x 104 |
A77 |
0.0111 |
6.8313 |
0 |
0.0043 |
4.54 |
0.18 |
6.3 x 105 |
S*=S-0.8x(Ti-0.8x(48/14)xN)x(32/48) |
Ti*=Ti-0.8x((48/14)xN+(48/32)xS) |
N*=N-0.8x(Ti-0.8x(48/32)xS))x(14/48) |
TABLE 21
Sample No. |
Mechanical Properties |
Remarks |
YS (MPa) |
TS (MPa) |
El (%) |
rm |
Δr |
SWE (DBTT-°C) |
AI (%) |
A71 |
215 |
347 |
46 |
2.1 |
0.25 |
-60 |
0 |
IS |
A72 |
254 |
404 |
38 |
1.91 |
0.27 |
-40 |
0 |
IS |
A73 |
265 |
411 |
36 |
1.61 |
0.22 |
-50 |
0 |
IS |
A74 |
292 |
450 |
32 |
1.65 |
0.24 |
-50 |
0 |
IS |
A75 |
246 |
395 |
37 |
1.67 |
0.22 |
-50 |
0 |
IS |
A76 |
202 |
356 |
41 |
1.85 |
0.29 |
-40 |
0 |
CS |
A77 |
254 |
401 |
37 |
1.28 |
0.54 |
-40 |
2.33 |
CS |
* Note:
YS = Yield strength, TS = Tensile Strength, El = Elongation, rm = Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = Aging Index,
SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS = Comparative steel |
Example 8
[0088] First, steel slabs were prepared in accordance with the compositions shown in the
following tables. The steel slabs were reheated and finish hot-rolled to provide hot
rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400 °C/min.,
wound at 650°C, cold-rolled at a reduction rate of 75%, followed by continuous annealing
to produce cold rolled steel sheets. At this time, the finish hot rolling was performed
at 910°C, which is above the Ar
3 transformation point, and the continuous annealing was performed by heating the hot
rolled steel sheets at a rate of 10 °C/second to 830°C for 40 seconds to produce the
final cold rolled steel sheets.
TABLE 22
Sample No. |
Chemical components (wt%) |
C |
P |
S |
Al |
Cu |
Ti |
Nb |
B |
N |
Others |
B81 |
0.0017 |
0.009 |
0.009 |
0.039 |
0.11 |
0.005 |
0.005 |
0.0005 |
0.0012 |
|
B82 |
0.0016 |
0.032 |
0.007 |
0.042 |
0.09 |
0.004 |
0.004 |
0.0007 |
0.0021 |
|
B83 |
0.0018 |
0.048 |
0.01 |
0.034 |
0.09 |
0.005 |
0.004 |
0.0004 |
0.0005 |
Si:0.05 |
B84 |
0.0026 |
0.083 |
0.011 |
0.038 |
0.09 |
0.012 |
0.005 |
0.0008 |
0.0021 |
Si:0.15 |
B85 |
0.0028 |
0.11 |
0.012 |
0.042 |
0.14 |
0.02 |
0.006 |
0.001 |
0.0022 |
Si:0.26 |
B86 |
0.0025 |
0.086 |
0.008 |
0.042 |
0.1 |
0.01 |
0.005 |
0.0007 |
0.0016 |
Si:0.19 |
Mo:0.071 |
B87 |
0.0025 |
0.084 |
0.01 |
0.033 |
0.15 |
0.009 |
0.004 |
0.0006 |
0.0016 |
Si:0.21 |
Cr:0.21 |
B88 |
0.0017 |
0.065 |
0.012 |
0.035 |
0.11 |
0.033 |
0.02 |
0.0009 |
0.0012 |
|
B89 |
0.0039 |
0.123 |
0.011 |
0.035 |
0 |
0 |
0 |
0.0008 |
0.0025 |
|
TABLE 23
Sample No. |
(Cu/63.5)/(S*/32) |
Cs |
Average size of precipitates (µm) |
Number of precipitates (mm-2) |
B81 |
6.85 |
17 |
0.06 |
3.3 x 106 |
B82 |
5.71 |
16 |
0.06 |
3.5 x 106 |
B83 |
5.62 |
18 |
0.06 |
3.1 x 106 |
B84 |
5.91 |
26 |
0.05 |
4.5 x 106 |
B85 |
15.5 |
21.3 |
0.05 |
4.8 x 106 |
B86 |
10.1 |
25 |
0.05 |
5.2 x 106 |
B87 |
10 |
25 |
0.05 |
4.1 x 106 |
B88 |
-14 |
-36 |
0.08 |
2.5 x 106 |
B89 |
0 |
39 |
0.08 |
6.2 x 104 |
S*=S-0.8x(Ti-0.8x(48/14)xN)x(32/48) |
Cs=(C-Nbx12/93-Ti*x12/48)x10000, |
Ti*=Ti-0.8x((48/14)xN+(48/32)xS) |
TABLE 24
Sample No. |
Mechanical Properties |
Remarks |
YS (MPa) |
TS (MPa) |
El (%) |
rm |
Δr |
AI (%) |
BH value (MPa) |
SWE (DBTT-°C) |
B81 |
189 |
308 |
49 |
2.04 |
0.31 |
0 |
42 |
-40 |
IS |
B82 |
193 |
320 |
45 |
2.01 |
0.34 |
0 |
44 |
-50 |
IS |
B83 |
209 |
352 |
43 |
1.93 |
0.28 |
0 |
37 |
-40 |
IS |
B84 |
276 |
406 |
39 |
1.78 |
0.25 |
0 |
58 |
-50 |
IS |
B85 |
335 |
450 |
35 |
1.62 |
0.19 |
0 |
55 |
-60 |
IS |
B86 |
329 |
452 |
36 |
1.55 |
0.21 |
0 |
49 |
-50 |
IS |
B87 |
333 |
449 |
34 |
1.66 |
0.24 |
0 |
45 |
-50 |
IS |
B88 |
210 |
346 |
42 |
1.98 |
0.22 |
0 |
0 |
-50 |
CS |
B89 |
285 |
463 |
29 |
1.22 |
0.28 |
3.9 |
89 |
-70 |
CS |
* Note:
YS = Yield strength, TS = Tensile Strength, El = Elongation, rm = Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = Aging Index,
SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS = Comparative steel |
Example 9
[0089] First, steel slabs were prepared in accordance with the compositions shown in the
following tables. The steel slabs were reheated and finish hot-rolled to provide hot
rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400 °C/min.,
wound at 650°C, cold-rolled at a reduction rate of 75%, followed by continuous annealing
to produce cold rolled steel sheets. At this time, the finish hot rolling was performed
at 910°C, which is above the Ar
3 transformation point, and the continuous annealing was performed by heating the hot
rolled steel sheets at a rate of 10 °C/second to 830°C for 40 seconds to produce the
final cold rolled steel sheets.
TABLE 25
Sample No. |
Chemical Components (wt%) |
C |
Mn |
P |
S |
Al |
Cu |
Ti |
Nb |
B |
N |
Others |
B91 |
0.0018 |
0.11 |
0.009 |
0.008 |
0.038 |
0.06 |
0.007 |
0.004 |
0.0005 |
0.0019 |
|
B92 |
0.0016 |
0.1 |
0.023 |
0.01 |
0.042 |
0.11 |
0.009 |
0.005 |
0.0008 |
0.0022 |
|
B93 |
0.0015 |
0.09 |
0.042 |
0.011 |
0.028 |
0.08 |
0.006 |
0.005 |
0.0006 |
0.0005 |
Si:0.1 |
B94 |
0.0021 |
0.11 |
0.08 |
0.009 |
0.041 |
0.11 |
0.011 |
0.012 |
0.0008 |
0.0012 |
Si:0.22 |
B95 |
0.0028 |
0.12 |
0.1 |
0.008 |
0.031 |
0.16 |
0.011 |
0.009 |
0.0005 |
0.002 |
Si:0.31 |
B96 |
0.0019 |
0.09 |
0.081 |
0.011 |
0.042 |
0.11 |
0.005 |
0.008 |
0.0011 |
0.0029 |
Si:0.25 |
Mo:0.15 |
B97 |
0.0023 |
0.1 |
0.078 |
0.008 |
0.035 |
0.13 |
0.007 |
0.005 |
0.0008 |
0.002 |
Si:0.3 |
Cr:0.27 |
B98 |
0.0025 |
0.55 |
0.05 |
0.009 |
0.037 |
0 |
0.022 |
0.018 |
0.0009 |
0.0028 |
|
B99 |
0.0041 |
0.11 |
0.116 |
0.017 |
0.038 |
0.08 |
0 |
0 |
0.009 |
0.0021 |
Si:0.33 |
TABLE 26
Sample No. |
Cu+Mn |
(Mn/55+Cu/63.5)/(S*/32) |
Cs |
Average size of precipitates (µm) |
Number of precipitates (mm-2) |
B91 |
0.17 |
13.4 |
18 |
0.05 |
3.5 x 106 |
B92 |
0.21 |
13.5 |
16 |
0.05 |
3.7 x 106 |
B93 |
0.17 |
10.9 |
15 |
0.05 |
3.3 x 106 |
B94 |
0.22 |
24.4 |
13.2 |
0.05 |
5.2 x 106 |
B95 |
0.28 |
29.7 |
26.6 |
0.05 |
5.5 x 106 |
B96 |
0.2 |
8.57 |
19 |
0.05 |
4.3 x 106 |
B97 |
0.23 |
17.2 |
23 |
0.04 |
5.9 x 106 |
B98 |
0.55 |
235 |
4.52 |
0.29 |
2.5 x 104 |
B99 |
0.19 |
5.2 |
41 |
0.06 |
2.7 x 105 |
S*=S-0.8x(Ti-0.8x(48/14)xN)x(32/48) |
Cs=(C-Nbx12/93-Ti*x12/48)x10000, |
Ti*=Ti-0.8x((48/14)xN+(48/32)xS) |
TABLE 27
Sample No. |
Mechanical Properties |
Remarks |
YS (MPa) |
TS (MPa) |
El (%) |
rm |
Δr |
AI |
BH value (MPa) |
SWE (DBTT-°C) |
B91 |
197 |
308 |
47 |
1.95 |
0.31 |
0 |
42 |
-40 |
IS |
B92 |
210 |
332 |
47 |
1.92 |
0.29 |
0 |
35 |
-50 |
IS |
B93 |
222 |
350 |
45 |
1.92 |
0.27 |
0 |
35 |
-50 |
IS |
B94 |
292 |
405 |
39 |
1.71 |
0.22 |
0 |
44 |
-60 |
IS |
B95 |
341 |
456 |
35 |
1.65 |
0.2 |
0 |
61 |
-50 |
IS |
B96 |
338 |
452 |
34 |
1.62 |
0.23 |
0 |
42 |
-60 |
IS |
B97 |
333 |
457 |
35 |
1.61 |
0.22 |
0 |
46 |
-50 |
IS |
B98 |
193 |
347 |
41 |
1.99 |
0.35 |
0 |
0 |
-50 |
CS |
B99 |
327 |
446 |
32 |
1.17 |
0.2 |
5.3 |
93 |
-60 |
CS |
* Note:
YS = Yield strength, TS = Tensile Strength, El = Elongation, rm = Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = Aging Index,
SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS = Comparative steel |
Example 10
[0090] First, steel slabs were prepared in accordance with the compositions shown in the
following tables. The steel slabs were reheated and finish hot-rolled to provide hot
rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400 °C/min.,
wound at 650°C, cold-rolled at a reduction rate of 75%, followed by continuous annealing
to produce cold rolled steel sheets. At this time, the finish hot rolling was performed
at 910°C, which is above the Ar
3 transformation point, and the continuous annealing was performed by heating the hot
rolled steel sheets at a rate of 10 °C/second to 830°C for 40 seconds to produce the
final cold rolled steel sheets.
TABLE 28
Sample No. |
Chemical Components (wt%) |
C |
P |
S |
Al |
Cu |
Ti |
Nb |
B |
N |
Others |
B01 |
0.0015 |
0.008 |
0.008 |
0.052 |
0.09 |
0.009 |
0.004 |
0.0006 |
0.0073 |
|
B02 |
0.0017 |
0.022 |
0.01 |
0.038 |
0.11 |
0.01 |
0.004 |
0.0009 |
0.011 |
|
B03 |
0.0018 |
0.045 |
0.008 |
0.032 |
0.12 |
0.005 |
0.003 |
0.0005 |
0.0075 |
Si:0.07 |
B04 |
0.0023 |
0.081 |
0.011 |
0.052 |
0.13 |
0.011 |
0.004 |
0.001 |
0.0103 |
Si:0.14 |
B05 |
0.0026 |
0.118 |
0.011 |
0.028 |
0.16 |
0.021 |
0.005 |
0.0009 |
0.012 |
Si:0.2 |
B06 |
0.0021 |
0.046 |
0.021 |
0.052 |
0.09 |
0.038 |
0.004 |
0.0009 |
0.0118 |
Mo:0.082 |
B07 |
0.0015 |
0.045 |
0.008 |
0.067 |
0.12 |
0.011 |
0.003 |
0.0007 |
0.0071 |
Cr:0.23 |
B08 |
0.0022 |
0.044 |
0.01 |
0.028 |
0 |
0.021 |
0.022 |
0.0009 |
0.0015 |
|
B09 |
0.0042 |
0.12 |
0.009 |
0.052 |
0.13 |
0 |
0 |
0.0008 |
0.0073 |
Si:0.15 |
TABLE 29
Sample No. |
(Cu/63.5)/(S*/32) |
(Al/27)/(N*/14) |
Cs |
Average size of precipitates (µm) |
Number of precipitates (mm-2) |
B01 |
3.27 |
3.624 |
15 |
0.06 |
2.7 x 106 |
B02 |
2.67 |
1.718 |
17 |
0.06 |
3.9 x 106 |
B03 |
3.71 |
1.935 |
18 |
0.06 |
3.6 x 106 |
B04 |
3.24 |
2.493 |
23 |
0.05 |
6.4 x 106 |
B05 |
4.65 |
1.426 |
26 |
0.05 |
8.3 x 106 |
B06 |
2.52 |
3.059 |
21 |
0.05 |
8.5 x 106 |
B07 |
4.83 |
5.129 |
15 |
0.04 |
7.9 x 106 |
B08 |
0 |
-24.2 |
-8.5 |
0.2 |
3.7 x 104 |
B09 |
3.33 |
2.746 |
42 |
0.05 |
5.1 x 106 |
S*=S-0.8x(Ti-0.8x(48/14)xN)x(32/48) |
Cs=(C-Nbx12/93-Ti*x12/48)x10000, |
Ti*=Ti-0.8x((48/14)xN+(48/32)xS) |
N*=N-0.8x(Ti-0.8x(48/32)xS))x(14/48) |
TABLE 30
Sample No. |
Mechanical Properties |
Remarks |
YS (MPa) |
TS (MPa) |
El (%) |
rm |
Δr |
AI |
BH value (MPa) |
SWE (DBTT-°C) |
B01 |
201 |
315 |
48 |
2.05 |
0.29 |
0 |
38 |
-40 |
IS |
B02 |
213 |
347 |
46 |
1.96 |
0.27 |
0 |
42 |
-50 |
IS |
B03 |
212 |
353 |
42 |
1.93 |
0.27 |
0 |
38 |
-40 |
IS |
B04 |
294 |
418 |
35 |
1.79 |
0.24 |
0 |
53 |
-50 |
IS |
B05 |
323 |
451 |
34 |
1.69 |
0.21 |
0 |
48 |
-40 |
IS |
B06 |
254 |
394 |
38 |
1.79 |
0.28 |
0 |
55 |
-50 |
IS |
B07 |
231 |
387 |
37 |
1.71 |
0.27 |
0 |
35 |
-40 |
IS |
B08 |
205 |
348 |
40 |
2.03 |
0.46 |
0 |
0 |
-50 |
CS |
B09 |
299 |
452 |
31 |
1.21 |
0.17 |
4.4 |
84 |
-40 |
CS |
* Note:
YS = Yield strength, TS = Tensile Strength, El = Elongation, rm = Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = Aging Index,
SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS = Comparative steel |
Example 11
[0091] First, steel slabs were prepared in accordance with the compositions shown in the
following tables. The steel slabs were reheated and finish hot-rolled to provide hot
rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400 °C/min.,
wound at 650°C, cold-rolled at a reduction rate of 75%, followed by continuous annealing
to produce cold rolled steel sheets. At this time, the finish hot rolling was performed
at 910°C, which is above the Ar
3 transformation point, and the continuous annealing was performed by heating the hot
rolled steel sheets at a rate of 10 °C/second to 830°C for 40 seconds to produce the
final cold rolled steel sheets.
TABLE 31
Sample No. |
Chemical Components (wt%) |
C |
Mn |
P |
S |
Al |
Cu |
Ti |
Nb |
B |
N |
Others |
B11 |
0.0014 |
0.11 |
0.008 |
0.008 |
0.042 |
0.08 |
0.009 |
0.004 |
0.0007 |
0.0072 |
|
B12 |
0.0019 |
0.08 |
0.0028 |
0.008 |
0.036 |
0.11 |
0.011 |
0.004 |
0.0005 |
0.011 |
|
B13 |
0.0015 |
0.09 |
0.043 |
0.007 |
0.034 |
0.09 |
0.01 |
0.003 |
0.0009 |
0.011 |
Si:0.09 |
B14 |
0.0024 |
0.11 |
0.082 |
0.009 |
0.042 |
0.13 |
0.01 |
0.004 |
0.0011 |
0.012 |
Si:0.12 |
B15 |
0.0027 |
0.08 |
0.11 |
0.008 |
0.067 |
0.12 |
0.025 |
0.006 |
0.0009 |
0.0087 |
Si:0.1 |
B16 |
0.0025 |
0.15 |
0.037 |
0.012 |
0.073 |
0.14 |
0.02 |
0.005 |
0.0009 |
0.0072 |
Si:0.11 |
Mo:0.087 |
B17 |
0.0022 |
0.1 |
0.037 |
0.012 |
0.041 |
0.13 |
0.009 |
0.004 |
0.0007 |
0.014 |
Si:0.13 |
Cr:0.31 |
B18 |
0.0013 |
0.55 |
0.044 |
0.007 |
0.03 |
0 |
0.03 |
0.012 |
0.0005 |
0.0027 |
|
B19 |
0.0045 |
0.08 |
0.121 |
0.013 |
0.04 |
0.15 |
0 |
0 |
0.0008 |
0.0018 |
|
TABLE 32
Sample No. |
Cu+Mn |
(Mn/55+Cu/63. 5)/(S*/32) |
(Al/27)/(N*/ 14) |
Cs |
Average size of precipitates (µm) |
Number of precipitates (mm-2) |
B11 |
0.19 |
7.6 |
2.967 |
14 |
0.06 |
2.3 x 107 |
B12 |
0.19 |
5.6 |
1.749 |
19 |
0.06 |
2.9 x 107 |
B13 |
0.18 |
5.5 |
1.659 |
15 |
0.06 |
2.5 x 107 |
B14 |
0.24 |
6.1 |
1.787 |
24 |
0.05 |
4.2 x 107 |
B15 |
0.2 |
14.5 |
6.803 |
27 |
0.05 |
2.9 x 107 |
B16 |
0.29 |
13.3 |
6.423 |
25 |
0.05 |
3.1 x 107 |
B17 |
0.23 |
4.47 |
1.393 |
22 |
0.04 |
3.4 x 107 |
B18 |
0.55 |
-63 |
-6.65 |
-28 |
0.27 |
1.2 x 104 |
B19 |
0.23 |
7.81 |
3.813 |
45 |
0.06 |
9.5 x 105 |
S*=S-0.8x(Ti-0.8x(48/14)xN)x(32/48) |
Cs=(C-Nbx12/93-Ti*x12/48)x10000, |
Ti*=Ti-0.8x((48/14)xN+(48/32)xS) |
N*=N-0.8x(Ti-0.8x(48/32)xS))x(14/48) |
TABLE 33
Sample No. |
Mechanical Properties |
Remarks |
YS (MPa) |
TS (MPa) |
El (%) |
rm |
Δr |
AI |
BH value (MPa) |
SWE (DBTT-°C) |
B11 |
192 |
320 |
48 |
2.06 |
0.31 |
0 |
37 |
-40 |
IS |
B12 |
211 |
349 |
46 |
1.98 |
0.29 |
0 |
48 |
-60 |
IS |
B13 |
221 |
359 |
42 |
1.93 |
0.27 |
0 |
33 |
-60 |
IS |
B14 |
252 |
403 |
37 |
1.78 |
0.27 |
0 |
45 |
-50 |
IS |
B15 |
321 |
457 |
34 |
1.62 |
0.31 |
0 |
58 |
-60 |
IS |
B16 |
234 |
355 |
41 |
1.88 |
0.27 |
0 |
51 |
-60 |
IS |
B17 |
222 |
351 |
42 |
1.87 |
0.3 |
0 |
51 |
-50 |
IS |
B18 |
189 |
359 |
42 |
1.95 |
0.38 |
0 |
0 |
-50 |
CS |
B19 |
336 |
461 |
27 |
1.27 |
0.21 |
3.5 |
96 |
-60 |
CS |
* Note:
YS = Yield strength, TS = Tensile Strength, El = Elongation, rm = Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = Aging Index,
SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS = Comparative steel |
Example 12
[0092] First, steel slabs were prepared in accordance with the compositions shown in the
following tables. The steel slabs were reheated and finish hot-rolled to provide hot
rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400 °C/min.,
wound at 650°C, cold-rolled at a reduction rate of 75%, followed by continuous annealing
to produce cold rolled steel sheets. At this time, the finish hot rolling was performed
at 910°C, which is above the Ar
3 transformation point, and the continuous annealing was performed by heating the hot
rolled steel sheets at a rate of 10 °C/second to 830°C for 40 seconds to produce the
final cold rolled steel sheets.
TABLE 34
Sample No. |
Chemical Components (wt%) |
C |
Mn |
P |
S |
Al |
Ti |
Nb |
B |
N |
Others |
B21 |
0.0018 |
0.09 |
0.011 |
0.009 |
0.036 |
0.011 |
0.004 |
0.0005 |
0.0019 |
|
B22 |
0.0015 |
0.07 |
0.054 |
0.012 |
0.042 |
0.012 |
0.003 |
0.0007 |
0.0016 |
|
B23 |
0.0023 |
0.1 |
0.064 |
0.009 |
0.023 |
0.01 |
0.004 |
0.0008 |
0.0021 |
Si:0.15 |
B24 |
0.0025 |
0.07 |
0.11 |
0.009 |
0.037 |
0.008 |
0.005 |
0.0005 |
0.003 |
|
B25 |
0.0028 |
0.12 |
0.09 |
0.01 |
0.024 |
0.015 |
0.006 |
0.0011 |
0.0015 |
Mo:0.1 |
B26 |
0.0024 |
0.12 |
0.095 |
0.008 |
0.031 |
0.014 |
0.004 |
0.001 |
0.0016 |
Cr:0.19 |
B27 |
0.0019 |
0.47 |
0.042 |
0.011 |
0.03 |
0.028 |
0.016 |
0.0007 |
0.002 |
|
B28 |
0.0042 |
0.32 |
0.12 |
0.01 |
0.024 |
0 |
0 |
0.0014 |
0.0013 |
|
TABLE 35
Sample No. |
(Mn/55)/(S*/32) |
Cs |
Average size of precipitates (µm) |
Number of precipitates (mm-2) |
B21 |
8.86 |
18 |
0.06 |
1.9 x 105 |
B22 |
5.13 |
15 |
0.06 |
1.8 x 105 |
B23 |
8.63 |
23 |
0.05 |
2.7 x 105 |
B24 |
4.46 |
25 |
0.05 |
3.3 x 106 |
B25 |
16.6 |
23 |
0.05 |
2.9 x 106 |
B26 |
24.3 |
18.8 |
0.04 |
4.1 x 106 |
B27 |
-271 |
-13 |
0.28 |
1.1 x 104 |
B28 |
15.6 |
42 |
0.22 |
7.4 x 103 |
S*=S-0.8x(Ti-0.8x(48/14)xN)x(32/48) |
Cs=(C-Nbx12/93-Ti*x12/48)x10000, |
Ti*=Ti-0.8x((48/14)xN+(48/32)xS) |
TABLE 36
Sample No. |
Mechanical Properties |
Remarks |
YS (MPa) |
TS (MPa) |
El (%) |
rm |
Δr |
AI (%) |
BH value (MPa) |
SWE (DBTT-°C) |
B21 |
188 |
305 |
49 |
2.18 |
0.34 |
0 |
48 |
-40 |
IS |
B22 |
221 |
350 |
43 |
2.09 |
0.3 |
0 |
33 |
-50 |
IS |
B23 |
245 |
397 |
37 |
1.88 |
0.29 |
0 |
43 |
-50 |
IS |
B24 |
316 |
444 |
33 |
1.62 |
0.26 |
0 |
54 |
-50 |
IS |
B25 |
275 |
452 |
33 |
1.55 |
0.24 |
0 |
55 |
-40 |
IS |
B26 |
319 |
446 |
31 |
1.5 |
0.21 |
0 |
49 |
-50 |
IS |
B27 |
219 |
362 |
38 |
2.09 |
0.37 |
0 |
0 |
-50 |
CS |
B28 |
251 |
466 |
26 |
1.2 |
0.19 |
4.2 |
87 |
-60 |
CS |
* Note:
YS = Yield strength, TS = Tensile Strength, El = Elongation, rm = Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = Aging Index,
SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS = Comparative steel |
Example 13
[0093] First, steel slabs were prepared in accordance with the compositions shown in the
following tables. The steel slabs were reheated and finish hot-rolled to provide hot
rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400 °C/min.,
wound at 650°C, cold-rolled at a reduction rate of 75%, followed by continuous annealing
to produce cold rolled steel sheets. At this time, the finish hot rolling was performed
at 910°C, which is above the Ar
3 transformation point, and the continuous annealing was performed by heating the hot
rolled steel sheets at a rate of 10 °C/second to 830°C for 40 seconds to produce the
final cold rolled steel sheets.
TABLE 37
Sample No. |
Chemical Components (wt%) |
C |
P |
S |
Al |
Ti |
Nb |
B |
N |
Others |
B31 |
0.0019 |
0.009 |
0.01 |
0.047 |
0.008 |
0.004 |
0.0005 |
0.0094 |
|
B32 |
0.0017 |
0.047 |
0.01 |
0.059 |
0.009 |
0.003 |
0.0008 |
0.0072 |
Si:0.03 |
B33 |
0.0024 |
0.086 |
0.008 |
0.067 |
0.016 |
0.003 |
0.001 |
0.0068 |
Si:0.11 |
B34 |
0.0026 |
0.118 |
0.012 |
0.047 |
0.027 |
0.005 |
0.0009 |
0.0125 |
Si:0.25 |
B35 |
0.0024 |
0.037 |
0.01 |
0.051 |
0.036 |
0.003 |
0.0007 |
0.011 |
Si:0.26 |
Mo:0.074 |
B36 |
0.0026 |
0.115 |
0.009 |
0.039 |
0.01 |
0.005 |
0.0011 |
0.01 |
Si:0.22 |
Cr:0.23 |
B37 |
0.0022 |
0.057 |
0.011 |
0.035 |
0.02 |
0.024 |
0.0007 |
0.0011 |
|
B38 |
0.0045 |
0.125 |
0.015 |
0.042 |
0 |
0 |
0.0008 |
0.012 |
|
TABLE 38
Sample No. |
(Al/27)/(N*/14) |
Cs |
Average size of precipitates (pm) |
Number of precipitates (mm-2) |
B31 |
2.358 |
19 |
0.06 |
5.1 x 106 |
B32 |
3.872 |
17 |
0.06 |
4.3 x 106 |
B33 |
6.547 |
24 |
0.05 |
4.4 x 106 |
B34 |
2.549 |
26 |
0.05 |
6.3 x 106 |
B35 |
4.897 |
24 |
0.05 |
5.2 x 106 |
B36 |
1.985 |
26 |
0.04 |
7.4 x 106 |
B37 |
19.87 |
8.3 |
0.29 |
1.1 x 104 |
B38 |
1.344 |
45 |
0.06 |
2.8 x 106 |
Cs=(C-Nbx12/93-Ti*x12/48)x10000, |
Ti*=Ti-0.8x((48/14)xN+(48/32)xS) |
N*=N-0.8x(Ti-0.8x(48/32)xS))x(14/48) |
TABLE 39
Sample No. |
Mechanical Properties |
Remarks |
YS (MPa) |
TS (MPa) |
El (%) |
rm |
Δr |
AI (%) |
BH value (MPa) |
SWE (DBTT-°C) |
B31 |
221 |
325 |
47 |
2.02 |
0.31 |
0 |
43 |
-40 |
IS |
B32 |
232 |
352 |
44 |
1.87 |
0.27 |
0 |
35 |
-50 |
IS |
B33 |
263 |
409 |
37 |
1.76 |
0.26 |
0 |
58 |
-50 |
IS |
B34 |
325 |
450 |
31 |
1.7 |
0.28 |
0 |
58 |
-50 |
IS |
B35 |
232 |
358 |
42 |
1.81 |
0.29 |
0 |
49 |
-50 |
IS |
B36 |
334 |
463 |
31 |
1.55 |
0.28 |
0 |
58 |
-50 |
IS |
B37 |
205 |
369 |
38 |
2.11 |
0.33 |
0 |
0 |
-40 |
CS |
B38 |
343 |
461 |
29 |
1.19 |
0.22 |
4.3 |
109 |
-40 |
CS |
* Note:
YS = Yield strength, TS = Tensile Strength, El = Elongation, rm = Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = Aging Index,
SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS = Comparative steel |
Example 14
[0094] First, steel slabs were prepared in accordance with the compositions shown in the
following tables. The steel slabs were reheated and finish hot-rolled to provide hot
rolled steel sheets. The hot rolled steel sheets were cooled at a rate of 400 °C/min.,
wound at 650°C, cold-rolled at a reduction rate of 75%, followed by continuous annealing
to produce cold rolled steel sheets. At this time, the finish hot rolling was performed
at 910°C, which is above the Ar
3 transformation point, and the continuous annealing was performed by heating the hot
rolled steel sheets at a rate of 10 °C/second to 830°C for 40 seconds to produce the
final cold rolled steel sheets.
TABLE 40
Sample No. |
Chemical Components (wt%) |
C |
Mn |
P |
S |
Al |
Ti |
Nb |
B |
N |
Others |
B41 |
0.0012 |
0.11 |
0.009 |
0.008 |
0.052 |
0.009 |
0.004 |
0.0005 |
0.0072 |
|
B42 |
0.0017 |
0.09 |
0.024 |
0.011 |
0.039 |
0.011 |
0.003 |
0.0009 |
0.013 |
|
B43 |
0.0014 |
0.07 |
0.046 |
0.006 |
0.067 |
0.019 |
0.003 |
0.0007 |
0.0069 |
Si:0.04 |
B44 |
0.0022 |
0.12 |
0.073 |
0.01 |
0.039 |
0.013 |
0.005 |
0.0005 |
0.0103 |
Si:0.1 |
B45 |
0.0022 |
0.08 |
0.113 |
0.01 |
0.05 |
0.029 |
0.004 |
0.001 |
0.012 |
Si:0.11 |
B46 |
0.0027 |
0.09 |
0.038 |
0.012 |
0.048 |
0.035 |
0.005 |
0.0008 |
0.0093 |
Si:0.21 |
Mo:0.083 |
B47 |
0.0025 |
0.13 |
0.04 |
0.011 |
0.048 |
0.018 |
0.003 |
0.0011 |
0.011 |
Cr:0.26 |
B48 |
0.0028 |
0.68 |
0.043 |
0.013 |
0.038 |
0.03 |
0.02 |
0.0005 |
0.0021 |
|
B49 |
0.0044 |
0.08 |
0.12 |
0.009 |
0.025 |
0 |
0 |
0.0011 |
0.0067 |
Si:0.05 |
TABLE 41
Sample No. |
(Mn/55)/(S*/32) |
(Al/27)/(N*/14) |
Cs |
Average size of precipitates (pm) |
Number of precipitates (mm-2) |
B41 |
4.66 |
3.673 |
12 |
0.05 |
5.2 x 106 |
B42 |
2.17 |
1.496 |
17 |
0.05 |
7.5 x 106 |
B43 |
6.83 |
8.378 |
14 |
0.05 |
6.7 x 106 |
B44 |
3.85 |
2.009 |
22 |
0.05 |
6.9 x 106 |
B45 |
3.85 |
3.227 |
22 |
0.04 |
9.6 x 106 |
B46 |
7.55 |
5.539 |
27 |
0.05 |
5.9 x 106 |
B47 |
4.32 |
2.519 |
25 |
0.05 |
7.8 x 106 |
B48 |
5495 |
-15.6 |
-6.1 |
0.21 |
1.2 x 104 |
B49 |
2.48 |
1.406 |
44 |
0.06 |
8.7 x 105 |
S*=S-0.8x(Ti-0.8x(48/14)xN)x(32/48) |
Cs=(C-Nbx12/93-Ti*x12/48)x10000, |
Ti*=Ti-0.8x((48/14)xN+(48/32)xS) |
N*=N-0.8x(Ti-0.8x(48/32)xS))x(14/48) |
TABLE 42
Sample No. |
Mechanical Properties |
Remarks |
YS (MPa) |
TS (MPa) |
El (%) |
rm |
Δr |
AI (%) |
BH value (MPa) |
SWE (DBTT-°C) |
B41 |
196 |
308 |
48 |
2.03 |
0.3 |
0 |
37 |
-40 |
IS |
B42 |
211 |
349 |
47 |
1.92 |
0.29 |
0 |
47 |
-50 |
IS |
B43 |
220 |
362 |
43 |
1.87 |
0.31 |
0 |
38 |
-50 |
IS |
B44 |
263 |
390 |
37 |
1.7 |
0.28 |
0 |
44 |
-40 |
IS |
B45 |
320 |
457 |
32 |
1.62 |
0.21 |
0 |
51 |
-60 |
IS |
B46 |
231 |
364 |
43 |
1.73 |
0.31 |
0 |
57 |
-50 |
IS |
B47 |
218 |
360 |
44 |
1.61 |
0.28 |
0 |
53 |
-50 |
IS |
B48 |
209 |
359 |
39 |
1.92 |
0.37 |
0 |
0 |
-40 |
CS |
B49 |
356 |
471 |
28 |
1.25 |
0.18 |
5.6 |
93 |
-60 |
CS |
* Note:
YS = Yield strength, TS = Tensile Strength, El = Elongation, rm = Plasticity-anisotropy index, Δr = In-plane anisotropy index, AI = Aging Index,
SWE = Secondary Working Embrittlement, IS = Inventive Steel, CS = Comparative steel |
[0095] The preferred embodiments illustrated in the present invention do not serve to limit
the present invention, but are set forth for illustrative purposes. Any embodiment
having substantially the same constitution and the same operational effects thereof
as the technical spirit of the present invention as defined in the appended claims
is encompassed within the technical scope of the present invention.
[Industrial Applicability]
[0096] As apparent from the above description, according to the cold rolled steel sheets
of the present invention, the distribution of fine precipitates in Nb-Ti composite
IF steels allows the formation of minute crystal grains, and as a result, the in-plane
anisotropy index is lowered and the yield strength is enhanced by precipitation enhancement.
1. A cold rolled steel sheet with superior formability and high yield ratio, the cold
rolled steel sheet having a composition comprising 0.01% or less of C, 0.01-0.2% of
Cu, 0.005-0.08% of S, 0.1% or less of Al, 0.004% or less of N, 0.2% or less of P,
0.0001-0.002% of B, 0.002-0.04% of Nb, 0.005-0.15% of Ti, optionally at least one
of 0.1-0.8% of Si and 0.2-1.2% of Cr, optionally 0.01-0.2% of Mo, by weight, and the
balance of Fe and other unavoidable impurities,
wherein the composition satisfies the following relationships: 1 ≤ (Cu/63.5)/(S*/32)
≤ 30 and S* = S - 0.8 x (Ti - 0.8 x (48/14) x N) x (32/48), and
wherein the steel sheet comprises CuS precipitates having an average size of 0.2 µm
or less.
2. A cold rolled steel sheet with superior formability and high yield ratio, the cold
rolled steel sheet having a composition comprising 0.01% or less of C, 0.01-0.2% of
Cu, 0.01-0.3% of Mn, 0.005-0.08% of S, 0.1% or less of Al, 0.004% or less of N, 0.2%
or less of P, 0.0001-0.002% of B, 0.002-0.04% of Nb, 0.005-0.15% of Ti, by weight,
and the balance of Fe and other unavoidable impurities,
wherein the composition satisfies the following relationships: 1 ≤ (Mn/55 + Cu/63.5)/(S*/32)
≤ 30 and S* = S - 0.8 x (Ti - 0.8 x (48/14) x N) x (32/48), and the steel sheet comprises
(Mn,Cu)S precipitates having an average size of 0.2 µm or less.
3. A cold rolled steel sheet with superior formability and high yield ratio, the cold
rolled steel sheet having a composition comprising 0.01% or less of C, 0.01-0.2% of
Cu, 0.005-0.08% of S, 0.1% or less of Al, 0.004-0.02% of N, 0.2% or less of P, 0.0001-0.002%
of B, 0.002-0.04% of Nb, 0.005-0.15% of Ti, by weight, and the balance of Fe and other
unavoidable impurities, wherein the composition satisfies the following relationships:
1 ≤ (Cu/63.5)/(S*/32) ≤ 30, 1 ≤ (Al/27)/(N*/14) ≤ 10, S* = S - 0.8 x (Ti - 0.8 x (48/14)
x N) x (32/48) and N* = N - 0.8 x (Ti - 0.8 x (48/32) x S) x (14/48), and the steel
sheet comprises CuS precipitates and AlN precipitates having an average size of 0.2
µm or less.
4. A cold rolled steel sheet with superior formability and high yield ratio, the cold
rolled steel sheet having a composition comprising 0.01% or less of C, 0.01-0.2% of
Cu, 0.01-0.3% of Mn, 0.005-0.08% of S, 0.1% or less of Al, 0.004-0.02% of N, 0.2%
or less of P, 0.0001-0.002% of B, 0.002-0.04% of Nb, 0.005-0.15% of Ti, by weight,
and the balance of Fe and other unavoidable impurities,
wherein the composition satisfies the following relationships: 1 ≤ (Mn/55 + Cu/63.5)/(S*/32)
≤ 30, 1 ≤ (Al/27)/(N*/14) ≤ 10, S* = S - 0.8 x (Ti - 0.8 x (48/14) x N) x (32/48)
and N* = N - 0.8 x (Ti - 0.8 x (48/32) x S) x (14/48), and the steel sheet comprises
(Mn,Cu)S precipitates and AlN precipitates having an average size of 0.2 µm or less.
5. The cold rolled steel sheet according to claim 1, wherein the C, Ti, Nb, N and S contents
satisfy the following relationships: 0.8 ≤ (Ti*/48 + Nb/93)/(C/12) ≤ 5.0 and Ti* =
Ti - 0.8 x ((48/14) x N + (48/32) x S).
6. The cold rolled steel sheet according to claim 5, wherein the C content is 0.005%
or less.
7. The cold rolled steel sheet according to claim 1, wherein solute carbon (Cs) [Cs =
(C - Nb x 12/93 - Ti* x 12/48) x 10000 in which Ti* = Ti - 0.8 x ((48/14) x N + (48/32)
x S), provided that when Ti* is less than 0, Ti* is defined as 0], which is determined
by the C and Ti contents, is from 5 to 30.
8. The cold rolled steel sheet according to claim 7, wherein the C content is 0.001-0.01%.
9. The cold rolled steel sheet according to any of claims 1 to 4, wherein the cold rolled
steel sheet satisfies a yield ratio (yield strength/tensile strength) of 0.58 or higher.
10. The cold rolled steel sheet according to any of claims 1 to 4, wherein the number
of the precipitates is 1 x 106/mm2 or more.
11. The cold rolled steel sheet according to claim 1, wherein the P content is 0.015%
or less.
12. The cold rolled steel sheet according to claim 1, wherein the P content is from 0.03%
to 0.2%.
13. The cold rolled steel sheet according to claim 2 or 4, wherein the sum of Mn and Cu
is from 0.05% to 0.4%.
14. The cold rolled steel sheet according to claim 2 or 4, wherein the Mn content is 0.01-0.12%.
15. The cold rolled steel sheet according to claim 2 or 4, wherein the value of (Mn/55
+ Cu/63.5)/(S*/32) is in the range of 1 to 9.
16. The cold rolled steel sheet according to claim 3 or 4, wherein the value of (Al/27)/(N*/14)
is in the range of 1 to 6.
17. A method for producing a cold rolled steel sheet with superior formability and high
yield ratio, the method comprising the steps of:
reheating a slab to a temperature of 1,100°C or higher, the slab having a composition
comprising 0.01% or less of C, 0.01-0.2% of Cu, 0.005-0.08% of S, 0.1% or less of
Al, 0.004% or less of N, 0.2% or less of P, 0.0001-0.002% of B, 0.002-0.04% of Nb,
0.005-0.15% of Ti, optionally at least one of 0.1-0.8% of Si and 0.2-1.2% of Cr, optionally
0.01-0.2% of Mo, by weight, and the balance of Fe and other unavoidable impurities,
and the composition satisfying the following relationships: 1 ≤ (Cu/63.5)/(S*/32)
≤ 30 and S* = S - 0.8 x (Ti - 0.8 x (48/14) x N) x (32/48) ;
hot rolling the reheated slab at a finish rolling temperature of the Ar3 transformation point or higher to provide a hot rolled steel sheet;
cooling the hot rolled steel sheet at a rate of 300 °C/min or higher;
winding the cooled steel sheet at 700°C or lower;
cold rolling the wound steel sheet; and
continuously annealing the cold rolled steel sheet, the cold rolled steel sheet comprising
CuS precipitates having an average size of 0.2 µm or less.
18. A method for producing a cold rolled steel sheet with superior formability and high
yield ratio, the method comprising the steps of:
reheating a slab to a temperature of 1,100°C or higher, the slab having a composition
comprising 0.01% or less of C, 0.01-0.2% of Cu, 0.01-0.3% of Mn, 0.005-0.08% of S,
0.1% or less of Al, 0.004% or less of N, 0.2% or less of P, 0.0001-0.002% of B, 0.002-0.04%
of Nb, 0.005-0.15% of Ti, by weight, and the balance of Fe and other unavoidable impurities,
and the composition satisfying the following relationships: 1 ≤ (Mn/55 + Cu/63.5)/(S*/32)
≤ 30 and S* = S - 0.8 x (Ti - 0.8 x (48/14) x N) x (32/48);
hot rolling the reheated slab at a finish rolling temperature of the Ar3 transformation point or higher to provide a hot rolled steel sheet;
cooling the hot rolled steel sheet at a rate of 300 °C/min or higher;
winding the cooled steel sheet at 700°C or lower;
cold rolling the wound steel sheet; and
continuously annealing the cold rolled steel sheet, the cold rolled steel sheet comprising
(Mn,Cu)S precipitates having an average size of 0.2 µm or less.
19. A method for producing a cold rolled steel sheet with superior formability and high
yield ratio, the method comprising the steps of:
reheating a slab to a temperature of 1,100°C or higher, the slab having a composition
comprising 0.01% or less of C, 0.01-0.2% of Cu, 0.005-0.08% of S, 0.1% or less of
Al, 0.004-0.02% of N, 0.2% or less of P, 0.0001-0.002% of B, 0.002-0.04% of Nb, 0.005-0.15%
of Ti, by weight, and the balance of Fe and other unavoidable impurities, and the
composition satisfying the following relationships: 1 ≤ (Cu/63.5)/(S*/32) ≤ 30, 1
≤ (Al/27)/(N*/14) ≤ 10, S* = S - 0.8 x (Ti - 0.8 x (48/14) x N) x (32/48) and N* =
N - 0.8 x (Ti - 0.8 x (48/32) x S) x (14/48);
hot rolling the reheated slab at a finish rolling temperature of the Ar3 transformation point or higher to provide a hot rolled steel sheet;
cooling the hot rolled steel sheet at a rate of 300 °C/min or higher;
winding the cooled steel sheet at 700°C or lower;
cold rolling the wound steel sheet; and
continuously annealing the cold rolled steel sheet, the cold rolled steel sheet comprising
CuS precipitates and AlN precipitates having an average size of 0.2 µm or less.
20. A method for producing a cold rolled steel sheet with superior formability and high
yield ratio, the method comprising the steps of:
reheating a slab to a temperature of 1,100°C or higher, the slab having a composition
comprising 0.01% or less of C, 0.01-0.2% of Cu, 0.01-0.3% of Mn, 0.005-0.08% of S,
0.1% or less of Al, 0.004-0.02% of N, 0.2% or less of P, 0.0001-0.002% of B, 0.002-0.04%
of Nb, 0.005-0.15% of Ti, by weight, and the balance of Fe and other unavoidable impurities,
and the composition satisfying the following relationships: 1 ≤ (Mn/55 + Cu/63.5)/(S*/32)
≤ 30, 1 ≤ (Al/27)/(N*/14) ≤ 10, S* = S - 0.8 x (Ti - 0.8 x (48/14) x N) x (32/48)
and N* = N - 0.8 x (Ti - 0.8 x (48/32) x S) x (14/48) ;
hot rolling the reheated slab at a finish rolling temperature of the Ar3 transformation point or higher to provide a hot rolled steel sheet;
cooling the hot rolled steel sheet at a rate of 300 °C/min or higher;
winding the cooled steel sheet at 700°C or lower;
cold rolling the wound steel sheet; and
continuously annealing the cold rolled steel sheet, the cold rolled steel sheet comprising
(Mn,Cu)S precipitates and AlN precipitates having an average size of 0.2 µm or less.
21. The method according to claim 17, wherein the C, Ti, Nb, N and S contents satisfy
the following relationships: 0.8 ≤ (Ti*/48 + Nb/93)/(C/12) ≤ 5.0 and Ti* = Ti - 0.8
x ((48/14) x N + (48/32) x S).
22. The method according to claim 21, wherein the C content is 0.005% or less.
23. The method according to claim 17, wherein solute carbon (Cs) [Cs = (C - Nb x 12/93
- Ti* x 12/48) x 10000 in which Ti* = Ti - 0.8 x ((48/14) x N + (48/32) x S), provided
that when Ti* is less than 0, Ti* is defined as 0], which is determined by the C and
Ti contents, is from 5 to 30.
24. The method according to claim 23, wherein the C content is 0.001-0.01%.
25. The method according to any of claims 17 to 20, wherein the cold rolled steel sheet
satisfies a yield ratio (yield strength/tensile strength) of 0.58 or higher.
26. The method according to any of claims 17 to 20, wherein the number of the precipitates
is 1 x 106/mm2 or more.
27. The method according to claim 17, wherein the P content is 0.015% or less.
28. The method according to claim 17, wherein the P content is from 0.03% to 0.2%.
29. The method according to claim 18 or 20, wherein the sum of Mn and Cu is from 0.08%
to 0.4%.
30. The method according to claim 18 or 20, wherein the Mn content is 0.01-0.12%.
31. The method according to claim 18 or 20, wherein the value of (Mn/55 + Cu/63.5)/(S*/32)
is in the range of 1 to 9.
32. The method according to claim 19 or 20, wherein the value of (Al/27)/(N*/14) is in
the range of 1 to 6.
1. Kaltgewalztes Blech mit verbesserter Formbarkeit und hohem Streckgrenzenverhältnis,
worin das kaltgewalzte Blech eine Zusammensetzung aufweist, umfassend 0,01% oder weniger
von C, 0,01-0,2% von Cu, 0,005-0,08% von S, 0,1% oder weniger von Al, 0,004% oder
weniger von N, 0,2% oder weniger von P, 0,0001-0,002% von B, 0,002-0,04% von Nb, 0,005-0,15%
von Ti, wahlweise mindestens eines von 0,1-0,8% von Si und 0,2-1,2% von Cr, wahlweise
0,01-0,2% von Mo, bezogen auf das Gewicht,
worin der Rest Fe und andere nicht-vermeidbare Verunreinigungen sind, worin die Zusammensetzung
den folgenden Verhältnissen genügt: 1 ≤ (Cu/63,5)/(S*/32) ≤ 30 und S* = S - 0,8 x
(Ti - 0,8 x (48/14) x N) x (32/48), und worin das Blech CuS-Ausfällungen mit einer
durchschnittlichen Größe von 0,2 µm oder weniger umfasst.
2. Kaltgewalztes Blech mit verbesserter Formbarkeit und hohem Streckgrenzenverhältnis,
worin das kaltgewalzte Blech eine Zusammensetzung aufweist, umfassend 0,01% oder weniger
von C, 0,01-0,2% von Cu, 0,01-0,3% von Mn, 0,005-0,08% von S, 0,1% oder weniger von
Al, 0,004% oder weniger von N, 0,2% oder weniger von P, 0,0001-0,002 % von B, 0,002-0,04
% von Nb, 0,005-0,15% von Ti, bezogen auf das Gewicht, worin der Rest Fe und andere
nicht-vermeidbare Verunreinigungen sind,
worin die Zusammensetzung den folgenden Verhältnissen genügt: 1 ≤ (Mn/55 + Cu/63,5)/(S*/32)
≤ 30 und S* = S - 0,8 x (Ti - 0,8 x (48/14) x N) x (32/48), und worin das Blech (Mn,Cu)S-Ausfällungen
mit einer durchschnittlichen Größe von 0,2 µm oder weniger umfasst.
3. Kaltgewalztes Blech mit verbesserter Formbarkeit und hohem Streckgrenzenverhältnis,
worin das kaltgewalzte Blech eine Zusammensetzung aufweist, umfassend 0,01% oder weniger
von C, 0,01-0,2% von Cu, 0,005-0,08% von S, 0,1% oder weniger von Al, 0,004-0,02%
von N, 0,2% oder weniger von P, 0,0001-0,002% von B, 0,002-0,04% von Nb, 0,005-0,15%
von Ti, bezogen auf das Gewicht, worin der Rest Fe und andere nicht-vermeidbare Verunreinigungen
sind, worin die Zusammensetzung den folgenden Verhältnissen genügt: 1 ≤ (Cu/63,5)/(S*/32)
≤30, 1 ≤ (Al/27)/(N*/14) ≤ 10, S* = S - 0,8 x (Ti - 0,8 x (48/14) x N) x (32/48) und
N* = N - 0,8 x (Ti - 0,8 x (48/32) x S) x (14/48), und das Blech CuS-Ausfällungen
und AlN-Ausfällungen mit einer durchschnittlichen Größe von 0,2 µm oder weniger umfasst.
4. Kaltgewalztes Blech mit verbesserter Formbarkeit und hohem Streckgrenzenverhältnis,
worin das kaltgewalzte Blech eine Zusammensetzung aufweist, umfassend 0,01% oder weniger
von C, 0,01-0,2 % von Cu, 0,01-0,3 % von Mn, 0,005-0,08 % von S, 0,1% oder weniger
von Al, 0,004-0,02% von N, 0,2% oder weniger von P, 0,0001-0,002 % von B, 0,002-0,04
% von Nb, 0,005 -0,15% von Ti, bezogen auf das Gewicht, worin der Rest Fe und andere
nicht-vermeidbare Verunreinigungen sind,
worin die Zusammensetzung den folgenden Verhältnissen genügt: 1 ≤ (Mn/55 + Cu/63,5)/(S*/32)
≤ 30, 1 ≤ (Al/27)/(N*/14) ≤ 10, S* = S - 0,8 x (Ti - 0,8 x (48/14) x N) x (32/48)
und N* = N - 0,8 x (Ti - 0,8 x (48/32) x S) x (14/48), und worin das Blech (Mn,Cu)S-Ausfällungen
und AlN-Ausfällungen mit einer durchschnittlichen Größe von 0,2 µm oder weniger umfasst.
5. Kaltgewalztes Blech nach Anspruch 1, worin der Gehalt an C, Ti, Nb, N und S den folgenden
Verhältnissen genügt: 0,8 ≤ (Ti*/48 + Nb/93)/(C/12) ≤ 5,0 und Ti* = Ti - 0,8 x ((48/14)
x N + (48/32) x S).
6. Kaltgewalztes Blech nach Anspruch 5, worin der Gehalt an C 0,005% ist oder weniger.
7. Kaltgewalztes Blech nach Anspruch 1, worin gelöster Kohlenstoff (Cs) [Cs = (C - Nb
x 12/93 - Ti* x 12/48) x 10000 worin Ti* = Ti - 0,8 x ((48/14) x N + (48/32) x S)
von 5 bis 30 ist, mit der Maßgabe, dass wenn Ti* weniger ist als 0, Ti* als 0 definiert
wird, bestimmt anhand des Gehalts von C und Ti.
8. Kaltgewalztes Blech nach Anspruch 7, worin der Gehalt an C 0,001-0,01% ist.
9. Kaltgewalztes Blech nach einem der Ansprüche 1 bis 4, worin das kaltgewalzte Blech
einem Streckgrenzenverhältnis (Formänderungsfestigkeit/Bruchkraft) von 0,58 oder höher
genügt.
10. Kaltgewalztes Blech nach einem der Ansprüche 1 bis 4, worin die Anzahl an Ausfällungen
1 x 106/mm2 oder mehr beträgt.
11. Kaltgewalztes Blech nach Anspruch 1, worin der Gehalt an P 0,015% oder weniger ist.
12. Kaltgewalztes Blech nach Anspruch 1, worin der Gehalt an P von 0,03% bis 0,2% ist.
13. Kaltgewalztes Blech nach Anspruch 2 oder 4, worin die Summe an Mn und Cu von 0,05%
bis 0,4% ist.
14. Kaltgewalztes Blech nach Anspruch 2 oder 4, worin der Gehalt an Mn 0,01-0,12% ist.
15. Kaltgewalztes Blech nach Anspruch 2 oder 4, worin der Wert von (Mn/55 + Cu/63,5)/(S*/32)
im Bereich von 1 bis 9 ist.
16. Kaltgewalztes Blech nach Anspruch 3 oder 4, worin der Wert von (Al/27)/(N*/14) im
Bereich von 1 bis 6 ist.
17. Verfahren zur Herstellung von kaltgewalztem Blech mit verbesserter Formbarkeit und
hohem Streckgrenzenverhältnis, wobei das Verfahren die Schritte umfasst:
erneutes Erhitzen einer Platte auf eine Temperatur von 1.100°C oder höher, worin die
Platte eine Zusammensetzung aufweist, umfassend 0,01% oder weniger von C, 0,01-0,2%
von Cu, 0,005-0,08% von S, 0,1% oder weniger von Al, 0,004% oder weniger von N, 0,2%
oder weniger von P, 0,0001-0,002 % von B, 0,002-0,04% von Nb, 0,005-0.15% von Ti,
wahlweise mindestens eines von 0,1-0,8% von Si und 0,2-1,2% von Cr, wahlweise 0,01-0,2%
von Mo, bezogen auf das Gewicht, worin der Rest Fe und andere nicht-vermeidbare Verunreinigungen
sind, worin die Zusammensetzung den folgenden Verhältnissen genügt: 1 ≤ (Cu/63,5)/(S*/32)
≤ 30 und S* = S - 0,8 x (Ti - 0,8 x (48/14) x N) x (32/48);
Heißwalzen der wiedererhitzten Platte bei einer endgültigen Walztemperatur des Ar3 Transformationspunkt oder höher um heißgewalztes Blech zu liefern;
Kühlen des heißgewalzten Blechs bei einer Rate von 300 °C/min oder höher;
Winden des gekühlten Blechs bei 700 ° C oder darunter;
Kaltwalzen des gewundenen Blechs; und
kontinuierliches Annealen des kaltgewalzten Blechs, worin das kaltgewalzte Blech CuS-Ausfällungen
mit einer durchschnittlichen Größe von 0,2 µm oder weniger enthält.
18. Verfahren zur Herstellung von kaltgewalztem Blech mit verbesserter Formbarkeit und
hohem Streckgrenzenverhältnis, wobei das Verfahren die Schritte umfasst:
erneutes Erhitzen einer Platte auf eine Temperatur von 1.100° C oder höher, worin
die Platte eine Zusammensetzung aufweist, umfassend 0,01% oder weniger von C, 0,01-0,2%
von Cu, 0,01-0,3% von Mn, 0,005-0,08 % von S, 0,1% oder weniger von Al, 0,004% oder
weniger von N, 0,2% oder weniger von P, 0,0001-0,002% von B, 0,002- 0,04% von Nb,
0,005-0,15% von Ti, bezogen auf das Gewicht, worin der Rest Fe und andere nicht-vermeidbare
Verunreinigungen sind, worin die Zusammensetzung den folgenden Verhältnissen genügt:
1 ≤ (Mn/55 + Cu/63,5)/(S*/32) ≤ 30 und S* = S - 0,8 X (Ti - 0,8 X (48/14) X N) X (32/48);
Heißwalzen der wiedererhitzten Platte auf eine Endwalz-Temperatur des Ar3 Transformationspunkts oder höher, um ein heißgewalztes Blech zu liefern;
Kühlen des heißgewalzten Blechs bei einer Rate von 300 °C/min oder höher;
Winden des gekühlten Blechs bei 700° C oder darunter;
Kaltwalzen des gewundenen Blechs; und
kontinuierliches Annealen des kaltgewalzten Blechs, worin das kaltgewalzte Blech (Mn,Cu)S-Ausfällungen
mit einer durchschnittlichen Größe von 0,2 µm oder weniger umfasst.
19. Verfahren zur Herstellung eines kaltgewalzten Blechs mit verbesserter Verformbarkeit
und hohem Streckgrenzenverhältnis, wobei das Verfahren die Schritte umfasst:
Wiedererhitzen einer Platte auf eine Temperatur von 1.100° C oder höher, worin die
Platte eine Zusammensetzung aufweist, umfassend 0,01% oder weniger von C, 0,01-0,2%
von Cu, 0,005-0,08% von S, 0,1% oder weniger von Al, 0,004-0,02% von N, 0,2% oder
weniger von P, 0,0001-0,002 % von B, 0,002-0,04 % von Nb, 0,005-0,15 % von Ti, bezogen
auf das Gewicht, worin der Rest Fe und andere nicht-vermeidbare Verunreinigungen sind,
worin die Zusammensetzung den folgenden Verhältnissen genügt: 1 ≤ (Cu/63,5)/(S*/32)
≤ 30, 1 ≤ (Al/27)/(N*/14) ≤ 10, S* = S - 0,8 x (Ti - 0,8 x (48/14) x N) x (32/48)
und N* = N - 0,8 X (Ti - 0,8 X (48/32) X S) X (14/48);
Heißwalzen der wiedererhitzten Platte bei einer End-Walz-Temperatur des Ar3 Transformations-Punkts oder höher, um ein heißgewalztes Blech zu liefern;
Kühlen des heißgewalzten Blechs bei einer Rate von 300 °C/min oder höher;
Winden des gekühlten Blechs bei 700 °C oder tiefer;
Kaltwalzen des gewundenen Blechs; und
kontinuierliches Annealen des kaltgewalzten Blechs, worin das das kaltgewalzte Blech
CuS-Ausfällungen und AlN-Ausfällungen mit einer durchschnittlichen Größe von 0,2 µm
oder weniger umfasst.
20. Verfahren zur Herstellung von kaltgewalztem Blech mit verbesserter Formbarkeit und
hohem Streckgrenzenverhältnis, wobei das Verfahren die Schritte umfasst:
erneutes Erhitzen einer Platte auf eine Temperatur von 1.100 °C oder höher, worin
die Platte eine Zusammensetzung aufweist, umfassend 0,01% oder weniger von C, 0,01-0,2%
von Cu, 0,01-0,3% von Mn, 0,005-0,08% von S, 0,1% oder weniger von Al, 0,004-0,02%
von N, 0,2% oder weniger von P, 0,0001-0,002% von B, 0,002-0,04% of Nb, 0,005-0,15%
von Ti, bezogen auf das Gewicht, worin der Rest Fe und andere nicht-vermeidbare Verunreinigungen
sind, worin die Zusammensetzung den folgenden Verhältnissen genügt: 1 ≤ (Mn/55 + Cu/63,5)/(S*/32)
≤ 30, 1 ≤ (Al/27)/(N*/14) ≤ 10, S* = S - 0,8 X (Ti - 0,8 x (48/14) x N) x (32/48)
und N* = N - 0,8 x (Ti - 0,8 x (48/32) x S) x (14/48);
Heißwalzen der wiedererhitzten Platte bei einer End-Walz-Temperatur des Ar3 Transformationspunkts oder höher, um ein heißgewalztes Blech zu liefern;
Kühlen der heißgewalzten Blechs bei einer Rate bei 300 °C/min oder darüber;
Winden des gekühlten Blechs bei 700 °C oder darunter;
Kaltwalzen des gewundenen Blechs; und
kontinuierliches Annealing des kaltgewalzten Blechs, worin das kaltgewalzte Blech
(Mn,Cu)S-Ausfällungen und AlN-Ausfällungen mit einer durchschnittlichen Größe von
0,2 µm oder weniger umfasst.
21. Verfahren nach Anspruch 17, wobei der Gehalt an C, Ti, Nb, N und S den folgenden Verhältnissen
genügt: 0,8 ≤ (Ti*/48 + Nb/93) / (C/12) ≤ 5,0 und Ti* = Ti - 0,8 x ((48/14) x N +
(48/32) x S).
22. Verfahren nach Anspruch 21, wobei der Gehalt an C 0,005% oder weniger ist.
23. Verfahren nach Anspruch 17, wobei gelöster Kohlenstoff (Cs) [Cs = (C - Nb x 12/93
- Ti* x 12/48) x 10000 worin Ti* = Ti - 0,8 x ((48/14) x N + (48/32) x S) von 5 bis
30 ist, mit der Maßgabe, dass wenn Ti* weniger als 0 ist, Ti* als 0 definiert wird,
bestimmt durch den Gehalt an C und Ti.
24. Verfahren nach Anspruch 23, wobei der Gehalt an C 0,001-0,01% ist.
25. Verfahren nach einem der Ansprüche 17 bis 20,
wobei das kaltgewalzte Blech einem Streckgrenzenverhältnis (Formänderungsfestigkeit/
Bruchkraft) von 0,58 oder höher genügt.
26. Verfahren nach einem der Ansprüche 17 bis 20, wobei die Anzahl an Ausfällungen 1 x
106/mm2 oder mehr ist.
27. Verfahren nach Anspruch 17, wobei der Gehalt an P 0,015% oder weniger ist.
28. Verfahren nach Anspruch 17, wobei der Gehalt an P von 0,03% bis 0,2% ist.
29. Verfahren nach Anspruch 18 oder 20, wobei die Summe von Mn und Cu von 0,08% bis 0,4%
ist.
30. Verfahren nach Anspruch 18 oder 20, wobei der Gehalt an Mn 0,01-0,12% ist.
31. Verfahren nach Anspruch 18 oder 20, wobei der Wert von (Mn/55 + Cu/63,5)/(S*/32) im
Bereich von 1 bis 9 ist.
32. Verfahren nach Anspruch 19 oder 20, wobei der Wert von (Al/27)/(N*/14) im Bereich
von 1 bis 6 ist.
1. Feuille d'acier laminée à froid ayant une formabilité supérieure et un rapport d'élasticité
élevé, la feuille d'acier laminée à froid ayant une composition comprenant 0,01 %
de C ou moins, 0,01 à 0,2 % de Cu, 0,005 à 0,08 % de S, 0,1 % de Al ou moins, 0,004
% de N ou moins, 0,2 % de P ou moins, 0,0001 à 0,002 % de B, 0,002 à 0,04 % de Nb,
0,005 à 0,15 % de Ti, facultativement au moins un parmi 0,1 à 0,8 % de Si et 0,2 à
1,2 % de Cr, facultativement 0,01 à 0,2 % de Mo, en poids, et le reste étant Fe et
autres impuretés inévitables,
la composition satisfaisant les relations suivantes : 1 ≤ (Cu/63,5)/(S*/32) ≤ 30 et
S* = S-0,8 x (Ti-0,8 x (48/14) x N) x (32/48), et
la feuille d'acier comprenant des précipités de CuS ayant une taille moyenne de 0,2
µm ou moins.
2. Feuille d'acier laminée à froid ayant une formabilité supérieure et un rapport d'élasticité
élevé, la feuille d'acier laminée à froid ayant une composition comprenant 0,01 %
de C ou moins, 0,01 à 0,2 % de Cu, 0,01 à 0,3 % de Mn, 0,005 à 0,08 % de S, 0,1 %
de Al ou moins, 0,004 % de N ou moins, 0,2 % de P ou moins, 0,0001 à 0,002 % de B,
0,002 à 0,04 % de Nb, 0,005 à 0,15 % de Ti, en poids, et le reste étant Fe et autres
impuretés inévitables,
la composition satisfaisant les relations suivantes : 1 ≤ (Mn/55 + Cu/63,5)/(S*/32)
≤ 30 et S* = S-0,8 x (Ti-0,8 x (48/14) x N) x (32/48), et la feuille d'acier comprenant
des précipités de (Mn,Cu)S ayant une taille moyenne de 0,2 µm ou moins.
3. Feuille d'acier laminée à froid ayant une formabilité supérieure et un rapport d'élasticité
élevé, la feuille d'acier laminée à froid ayant une composition comprenant 0,01 %
de C ou moins, 0,01 à 0,2 % de Cu, 0,005 à 0,08 % de S, 0,1 % de Al ou moins, 0,004
à 0,02 % de N, 0,2 % de P ou moins, 0,0001 à 0,002 % de B, 0,002 à 0,04 % de Nb, 0,005
à 0,15 % de Ti, en poids, et le reste étant Fe et autres impuretés inévitables,
la composition satisfaisant les relations suivantes : 1 ≤ (Cu/63,5)/(S*/32) ≤ 30,
1 ≤ (Al/27)/(N*/14) ≤ 10, S* = S-0,8 x (Ti-0,8 x (48/14) x N) x (32/48) et N* = N-0,8
x (Ti-0,8 x (48/32) x S) x (14/48), et la feuille d'acier comprenant des précipités
de CuS et des précipités de AlN ayant une taille moyenne de 0,2 µm ou moins.
4. Feuille d'acier laminée à froid ayant une formabilité supérieure et un rapport d'élasticité
élevé, la feuille d'acier laminée à froid ayant une composition comprenant 0,01 %
de C ou moins, 0,01 à 0,2 % de Cu, 0,01 à 0,3 % de Mn, 0,005 à 0,08 % de S, 0,1 %
de Al ou moins, 0,004 à 0,02 % de N, 0,2 % de P ou moins, 0,0001 à 0,002 % de B, 0,002
à 0,04 % de Nb, 0,005 à 0,15 % de Ti, en poids, et le reste étant Fe et autres impuretés
inévitables,
la composition satisfaisant les relations suivantes : 1 ≤ (Mn/55 + Cu/63,5)/(S*/32)
≤ 30, 1 ≤ (Al/27)/(N*/14) ≤ 10, S* = S-0,8 x (Ti-0,8 x (48/14) x N) x (32/48) et N*
= N-0,8 x (Ti-0,8 x (48/32) x S) x (14/48), et la feuille d'acier comprenant des précipités
de (Mn,Cu)S et des précipités de AlN ayant une taille moyenne de 0,2 µm ou moins.
5. Feuille d'acier laminée à froid selon la revendication 1, dans laquelle les teneurs
en C, Ti, Nb et S satisfont les relations suivantes : 0,8 ≤ (Ti*/48 + Nb/93)/(C/12)
≤ 5,0 et Ti* = Ti-0,8 x ((48/14) x N + (48/32) x S).
6. Feuille d'acier laminée à froid selon la revendication 5, dans laquelle la teneur
en C est de 0,005 % ou moins.
7. Feuille d'acier laminée à froid selon la revendication 1, dans laquelle du soluté
de carbone (Cs) [Cs = (C - Nb x 12/93 - Ti* x 12/48) x 1000 où Ti* = Ti-0,8 x (((48/14)
x N + (48/32) x S), à condition que, lorsque Ti* est inférieur à 0, Ti* soit défini
comme étant 0], qui est déterminé par les teneurs en C et Ti, est compris entre 5
et 30.
8. Feuille d'acier laminée à froid selon la revendication 7, dans laquelle la teneur
en C est comprise entre 0,001 et 0,01 %.
9. Feuille d'acier laminée à froid selon l'une quelconque des revendications 1 à 4, dans
laquelle la feuille d'acier laminée à froid satisfait un rapport d'élasticité (limite
d'élasticité/résistance à la traction) de 0,58 ou plus.
10. Feuille d'acier laminée à froid selon l'une quelconque des revendications 1 à 4, dans
laquelle le nombre des précipités est 1 x 106/mm2 ou plus.
11. Feuille d'acier laminée à froid selon la revendication 1, dans laquelle la teneur
en P est de 0,015 % ou moins.
12. Feuille d'acier laminée à froid selon la revendication 1, dans laquelle la teneur
en P est comprise entre 0,03 % et 0,2 %.
13. Feuille d'acier laminée à froid selon la revendication 2 ou 4, dans laquelle la somme
de Mn et Cu est comprise entre 0,05 % et 0,4 %.
14. Feuille d'acier laminée à froid selon la revendication 2 ou 4, dans laquelle la teneur
en Mn est comprise entre 0,01 et 0,12 %.
15. Feuille d'acier laminée à froid selon la revendication 2 ou 4, dans laquelle la valeur
de (Mn/55 + Cu/63,5)/(S*/32) est comprise dans la plage allant de 1 à 9.
16. Feuille d'acier laminée à froid selon la revendication 3 ou 4, dans laquelle la valeur
de (Al/27)/(N*/14) est comprise dans la plage allant de 1 à 6.
17. Procédé de production d'une feuille d'acier laminée à froid ayant une formabilité
supérieure et un rapport d'élasticité élevé, le procédé comprenant les étapes :
réchauffer une plaque à une température de 1100 °C ou plus, la plaque ayant une composition
comprenant 0,01 % de C ou moins, 0,01 à 0,2 % de Cu, 0,005 à 0,08 % de S, 0,1 % de
Al ou moins, 0,004 % de N ou moins, 0,2 % de P ou moins, 0,0001 à 0,002 % de B, 0,002
à 0,04 % de Nb, 0,005 à 0,15 % de Ti, facultativement au moins un parmi 0,1 à 0,8
% de Si et 0,2 à 1,2 % de Cr, facultativement 0,01 à 0,2 % de Mo, en poids, et le
reste étant Fe et autres impuretés inévitables, et la composition satisfaisant les
relations suivantes : 1 ≤ (Cu/63,5)/(S*/32) ≤ 30 et S* = S-0,8 x (Ti-0,8 x (48/14)
x N) x (32/48) ;
laminer à chaud la plaque réchauffée à une température de laminage de finition du
point de transformation Ar3 ou plus, pour fournir une feuille d'acier laminée à chaud ;
refroidir la feuille d'acier laminée à chaud à une vitesse de 300°C/min ou plus ;
enrouler la feuille d'acier refroidie à 700°C ou moins ;
laminer à froid la feuille d'acier enroulée ; et
recuire de manière continue la feuille d'acier laminée à froid, la feuille d'acier
laminée à froid comprenant des précipités de CuS ayant une taille moyenne de 0,2 µm
ou moins.
18. Procédé de production d'une feuille d'acier laminée à froid ayant une formabilité
supérieure et un rapport d'élasticité élevé, le procédé comprenant les étapes :
réchauffer une plaque à une température de 1100°C ou plus, la plaque ayant une composition
comprenant 0,01 % de C ou moins, 0,01 à 0,2 % de Cu, 0,01 à 0,3 % de Mn, 0,005 à 0,08
% de S, 0,1 % de Al ou moins, 0,004 % de N ou moins, 0,2 % de P ou moins, 0,0001 à
0,002 % de B, 0,002 à 0,04 % de Nb, 0,005 à 0,15 % de Ti, en poids, et le reste étant
Fe et autres impuretés inévitables, et la composition satisfaisant les relations suivantes
: 1 ≤ (Mn/55 + Cu/63,5)/(S*/32) ≤ 30 et S* = S-0,8 x (Ti-0,8 x (48/14) x N) x (32/48)
;
laminer à chaud la plaque réchauffée à une température de laminage de finition du
point de transformation Ar3 ou plus, pour fournir une feuille d'acier laminée à chaud
;
refroidir la feuille d'acier laminée à chaud à une vitesse de 300°C/min ou plus ;
enrouler la feuille d'acier refroidie à 700°C ou moins ;
laminer à froid la feuille d'acier enroulée ; et
recuire de manière continue la feuille d'acier laminée à froid, la feuille d'acier
laminée à froid comprenant des précipités de (Mn,Cu)S ayant une taille moyenne de
0,2 µm ou moins.
19. Procédé de production d'une feuille d'acier laminée à froid ayant une formabilité
supérieure et un rapport d'élasticité élevé, le procédé comprenant les étapes :
réchauffer une plaque à une température de 1100°C ou plus, la plaque ayant une composition
comprenant 0,01 % de C ou moins, 0,01 à 0,2 % de Cu, 0,005 à 0,08 % de S, 0,1 % de
Al ou moins, 0,004 à 0,02 % de N, 0,2 % de P ou moins, 0,0001 à 0,002 % de B, 0,002
à 0,04 % de Nb, 0,005 à 0,15 % de Ti, en poids, et le reste étant Fe et autres impuretés
inévitables, et la composition satisfaisant les relations suivantes : 1 ≤ (Cu/63,5)/(S*/32)
≤ 30, 1 ≤ (Al/27)/(N*/14) ≤ 10, S* = S-0,8 x (Ti-0,8 x (48/14) x N) x (32/48) et N*
= N-0,8 x (Ti-0,8 x (48/32) x S) x (14/48) ;
laminer à chaud la plaque réchauffée à une température de laminage de finition du
point de transformation Ar3 ou plus, pour fournir une feuille d'acier laminée à chaud
;
refroidir la feuille d'acier laminée à chaud à une vitesse de 300°C/min ou plus ;
enrouler la feuille d'acier refroidie à 700°C ou moins ;
laminer à froid la feuille d'acier enroulée ; et
recuire de manière continue la feuille d'acier laminée à froid, la feuille d'acier
laminée à froid comprenant des précipités de CuS et des précipités de AlN ayant une
taille moyenne de 0,2 µm ou moins.
20. Procédé de production d'une feuille d'acier laminée à froid ayant une formabilité
supérieure et un rapport d'élasticité élevé, le procédé comprenant les étapes :
réchauffer une plaque à une température de 1100°C ou plus, la plaque ayant une composition
comprenant 0,01 % de C ou moins, 0,01 à 0,2 % de Cu, 0,01 à 0,3 % de Mn, 0,005 à 0,08
% de S, 0,1 % de Al ou moins, 0,004 à 0,02 % de N, 0,2 % de P ou moins, 0,0001 à 0,002
% de B, 0,002 à 0,04 % de Nb, 0,005 à 0,15 % de Ti, en poids, et le reste étant Fe
et autres impuretés inévitables, et la composition satisfaisant les relations suivantes
: 1 ≤ (Mn/55 + Cu/63,5)/(S*/32) ≤ 30, 1 ≤ (Al/27)/(N*/14) ≤ 10, S* = S-0,8 x (Ti-0,8
x (48/14) x N) x (32/48) et N* = N-0,8 x (Ti-0,8 x (48/32) x S) x (14/48) ;
laminer à chaud la plaque réchauffée à une température de laminage de finition du
point de transformation Ar3 ou plus, pour fournir une feuille d'acier laminée à chaud
;
refroidir la feuille d'acier laminée à chaud à une vitesse de 300°C/min ou plus ;
enrouler la feuille d'acier refroidie à 700°C ou moins ;
laminer à froid la feuille d'acier enroulée ; et
recuire de manière continue la feuille d'acier laminée à froid, la feuille d'acier
laminée à froid comprenant des précipités de (Mn,Cu)S et des précipités de AlN ayant
une taille moyenne de 0,2 µm ou moins.
21. Procédé selon la revendication 17, dans lequel les teneurs en C, Ti, Nb, N et S satisfont
les relations suivantes : 0,8 ≤ (Ti*/48 + Nb/93)/(C/12) ≤ 5,0 et Ti* = Ti-0,8 x ((48/14)
x N + (48/32) x S).
22. Procédé selon la revendication 21, dans lequel la teneur en C est de 0,005 % ou moins.
23. Procédé selon la revendication 17, dans lequel du soluté de carbone (Cs) [Cs = (C
- Nb x 12/93 - Ti* x 12/48) x 1000 où Ti* = Ti-0,8 x (((48/14) x N + (48/32) x S),
à condition que, lorsque Ti* est inférieur à 0, Ti* soit défini comme étant 0], qui
est déterminé par les teneurs en C et Ti, est compris entre 5 et 30.
24. Procédé selon la revendication 23, dans lequel la teneur en C est comprise entre 0,001
et 0,01 %.
25. Procédé selon l'une quelconque des revendications 17 à 20, dans lequel la feuille
d'acier laminée à froid satisfait un rapport d'élasticité (limite d'élasticité/résistance
à la traction) de 0,58 ou plus.
26. Procédé selon l'une quelconque des revendications 17 à 20, dans lequel le nombre des
précipités est 1 x 106/mm2 ou plus.
27. Procédé selon la revendication 17, dans lequel la teneur en P est de 0,015 % ou moins.
28. Procédé selon la revendication 17, dans lequel la teneur en P est comprise entre 0,03
% et 0,2 %.
29. Procédé selon la revendication 18 ou 20, dans lequel la somme de Mn et Cu est comprise
entre 0,08 % et 0,4 %.
30. Procédé selon la revendication 18 ou 20, dans lequel la teneur en Mn est comprise
entre 0,01 et 0,12 %.
31. Procédé selon la revendication 18 ou 20, dans lequel la valeur de (Mn/55 + Cu/63,5)/(S*/32)
est comprise dans la plage allant de 1 à 9.
32. Procédé selon la revendication 19 ou 20, dans lequel la valeur de (Al/27)/(N*/14)
est comprise dans la plage allant de 1 à 6.