[Technical Field of the Invention]
[0001] The present invention relates to a hot-rolled steel sheet.
[Background Art]
[0003] In recent years, a weight of a vehicle body has been reduced by applying a high strength
steel sheet for the purpose of improving fuel efficiency and collision safety of a
vehicle. However, high-strengthening of a steel sheet generally causes deterioration
of toughness. Therefore, in the development of the high strength steel sheet, it is
an important issue to achieve high-strengthening without deteriorating the toughness.
[0004] In general, in order to improve toughness, a method of improving toughness by performing
rolling at a low temperature and applying a high cumulative strain in a state of unrecrystallized
austenite is known. However, when a rolling reduction in the state of unrecrystallized
austenite is increased, there is a problem that an aspect ratio of prior austenite
grains increases and anisotropy in toughness increases.
[0005] For example, Patent Document 1 discloses a hot-rolled steel sheet which has a texture
in which, in a thickness middle portion which is a steel sheet portion partitioned
between a 3/8 thickness position and a 5/8 thickness position of a sheet thickness
from a surface of a steel sheet, an average value of X-ray random intensity ratios
of {100}<011> to {223}<110> orientation groups on a sheet surface is 6.5 or less and
an X-ray random intensity ratio of a {332}<113> crystal orientation is 5.0 or less,
and has a microstructure in which a total area ratio of tempered martensite, martensite,
and lower bainite is more than 85% and an average grain size is 12.0 µm or less.
[Prior Art Document]
[Patent Document]
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0007] However, in the technique disclosed in Patent Document 1, from the viewpoint of improving
the fuel efficiency of a vehicle and improving collision safety, there is room for
further improvement in a reduction in anisotropy in toughness in a high strength steel
sheet.
[0008] The present invention has been made in view of the above circumstances, and an object
of the present invention is to provide a hot-rolled steel sheet having high strength,
excellent toughness, and reduced anisotropy in toughness.
[Means for Solving the Problem]
[0009] The present inventors investigated a relationship between a texture and mechanical
properties of a hot-rolled steel sheet, and as a result, found that anisotropy in
toughness can be further reduced even in a hot-rolled steel sheet having a tensile
strength of 1,180 MPa or more. The present inventors found that in a rolled steel
sheet, different textures develop on a surface and an inside. In addition, the present
inventors found that, in order to reduce the anisotropy in toughness, it is more effective
to control a texture in an austenite region than a texture of martensite after rapid
cooling. Furthermore, the present inventors found that it is effective to preferably
control hot rolling conditions in order to obtain a texture having a desired crystal
orientation.
[0010] The gist of the present invention made on the basis of the above-mentioned findings
is as follows.
[0011]
- (1) A hot-rolled steel sheet according to an aspect of the present invention includes,
as a chemical composition, by mass%:
C: 0.100% to 0.500%;
Si: 0.100% to 3.000%;
Mn: 0.50% to 3.00%;
P: 0.100% or less;
S: 0.0100% or less;
Al: 1.000% or less;
N: 0.0100% or less;
Ti: 0% to 0.20%;
Nb: 0% to 0.100%;
Ca: 0% to 0.0060%;
Mo: 0% to 0.50%;
Cr: 0% to 1.00%;
V: 0% to 0.50%;
Cu: 0% to 0.50%;
Ni: 0% to 0.50%;
Sn: 0% to 0.050%; and
a remainder comprising Fe and impurities,
in which a microstructure in a region from a depth of 1/8 of a sheet thickness from
a surface to a depth of 3/8 of the sheet thickness from the surface includes, by area%,
90% to 100% of martensite, and
0% to 10% of a remainder in microstructure,
in a texture of a region from the surface to the depth of 1/8 of the sheet thickness
from the surface,
pole densities of {001}<110>, {111}<110>, and {112}<110> orientation groups are 2.0
or more,
in a texture of a region from the depth of 1/8 of the sheet thickness from the surface
to a depth of 1/2 of the sheet thickness from the surface,
a pole density of a { 110 } <112> orientation is 5.0 or less, and
a tensile strength of the hot-rolled steel sheet is 1,180 MPa or more.
- (2) In the hot-rolled steel sheet according to (1), the chemical composition may contain,
by mass%, one or two or more selected from the group consisting of
Ti: 0.02% to 0.20%,
Nb: 0.010% to 0.100%,
Ca: 0.0001 % to 0.0060%,
Mo: 0.01% to 0.50%,
Cr: 0.01% to 1.00%,
V: 0.01% to 0.50%,
Cu: 0.01% to 0.50%,
Ni: 0.01% to 0.50%, and
Sn: 0.001% to 0.050%.
[Effects of the Invention]
[0012] According to the above-described aspect according to the present invention, it is
possible to provide a hot-rolled steel sheet having high strength, excellent toughness,
and reduced anisotropy in toughness.
[Embodiments of the Invention]
[0013] Hereinafter, a hot-rolled steel sheet according to the present embodiment will be
described in detail.
[0014] First, reasons for limiting a chemical composition of the hot-rolled steel sheet
according to the present embodiment will be described. In addition, numerical limiting
ranges described below using "to" include the lower limit and the upper limit in the
ranges. Numerical values indicated as "less than" or "more than" do not fall within
the numerical range. In addition, all % regarding the chemical composition means mass%.
[0015] The hot-rolled steel sheet according to the present embodiment includes, as a chemical
composition, by mass%: C: 0.100% to 0.500%; Si: 0.100% to 3.000%; Mn: 0.50% to 3.00%;
P: 0.100% or less; S: 0.0100% or less; Al: 1.000% or less; N: 0.0100% or less; and
a remainder: Fe and impurities. Hereinafter, each element will be described in detail.
C: 0.100% to 0.500%
[0016] C is an important element for improving strength of the hot-rolled steel sheet. When
a C content is less than 0.100%, the strength of the hot-rolled steel sheet decreases.
Therefore, the C content is set to 0.100% or more. The C content is preferably 0.150%
or more, 0.170% or more, 0.200% or more, or 0.220% or more.
[0017] On the other hand, when the C content is more than 0.500%, toughness of the hot-rolled
steel sheet deteriorates. Therefore, the C content is set to 0.500% or less. The C
content is preferably 0.450% or less, 0.400% or less, or 0.370% or less.
Si: 0.100% to 3.000%
[0018] Si is an element having an effect of improving the strength of the hot-rolled steel
sheet. When a Si content is less than 0.100%, the strength of the hot-rolled steel
sheet deteriorates. Therefore, the Si content is set to 0.100% or more. The Si content
is preferably 0.200% or more, 0.300% or more, 0.400% or more, or 0.500% or more. The
Si content is more preferably more than 1.000%, and still more preferably 1 .100%
or more.
[0019] However, when the Si content is more than 3.000%, the toughness of the hot-rolled
steel sheet deteriorates. Therefore, the Si content is set to 3.000% or less. The
Si content is preferably 2.700% or less, 2.500% or less, or 2.300% or less.
Mn: 0.50% to 3.00%
[0020] Mn is an element effective for improving the strength of the hot-rolled steel sheet
by improving hardenability and solid solution strengthening. When a Mn content is
less than 0.50%, the strength of the hot-rolled steel sheet decreases. Therefore,
the Mn content is set to 0.50% or more. The Mn content is preferably 1.00% or more,
1.20% or more, or 1.50% or more.
[0021] On the other hand, when the Mn content is more than 3.00%, MnS that increases anisotropy
in the toughness of the hot-rolled steel sheet is generated. Therefore, the Mn content
is set to 3.00% or less. The Mn content is preferably 2.50% or less, 2.30% or less,
or 2.00% or less.
P: 0.100% or Less
[0022] P is an impurity element, and the lower a P content is, the more preferable it is.
When the P content is more than 0.100%, deterioration of workability and weldability
of the hot-rolled steel sheet becomes significant, and fatigue properties also deteriorate.
Therefore, the P content is set to 0.100% or less. The P content is preferably 0.070%
or less, 0.050% or less, or 0.030% or less.
[0023] A lower limit of the P content is not particularly specified. However, since an excessive
reduction in the P content causes an increase in manufacturing cost, the P content
may be set to 0.001% or more or 0.005% or more.
S: 0.0100% or Less
[0024] S is an impurity element, and the lower a S content is, the more preferable it is.
When the S content is more than 0.0100%, a large amount of inclusions such as MnS
that increase the anisotropy in the toughness of the hot-rolled steel sheet are generated.
Therefore, the S content is set to 0.0100% or less. The S content is preferably 0.0080%
or less, 0.0060% or less, or 0.0040% or less.
[0025] A lower limit of the S content is not particularly specified. However, since an excessive
reduction in the S content causes an increase in the manufacturing cost, the S content
may be set to 0.0005% or more, or 0.0010 or more.
Al: 1.000% or Less
[0026] Al is an element that acts as a deoxidizing agent in a steelmaking stage and is effective
for improving cleanliness of steel. However, when an Al content is more than 1.000%,
alumina precipitated in the form of clusters is generated, and the toughness of the
hot-rolled steel sheet deteriorates. Therefore, the Al content is set to be 1.000%
or less. The Al content is preferably 0.700% or less, 0.500% or less, or 0.400% or
less.
[0027] A lower limit of the Al content is not particularly specified. However, since an
excessive reduction in the Al content causes an increase in the manufacturing cost,
the Al content may be set to 0.001% or more, or 0.005% or more.
N: 0.0100% or Less
[0028] N is an impurity element. When a N content is more than 0.0100%, a coarse Ti nitride
is formed at a high temperature, and the toughness of the hot-rolled steel sheet deteriorates.
Therefore, the N content is set to 0.0100% or less. The N content is preferably 0.0080%
or less, 0.0060% or less, or 0.0040% or less.
[0029] A lower limit of the N content is not particularly specified. However, since an excessive
reduction in the N content causes an increase in the manufacturing cost, the N content
may be set to 0.0010% or more.
[0030] The hot-rolled steel sheet according to the present embodiment may contain the above
elements, and the remainder of Fe and impurities. Examples of the impurities include
those that are unavoidably incorporated from steel raw materials or scrap and/or in
a steelmaking process or elements that are allowed in a range in which the properties
of the hot-rolled steel sheet according to the present embodiment are not inhibited.
[0031] In order to improve various properties, the hot-rolled steel sheet according to the
present embodiment may contain optional elements shown below instead of a portion
of Fe. In order to reduce an alloy cost, it is not necessary to intentionally include
these optional elements in steel. Therefore, lower limits of the amounts of these
optional elements are all 0%.
Ti: 0.02% to 0.20%
[0032] Ti is an element effective for suppressing austenite recrystallization and grain
growth between stands of hot rolling. By suppressing the recrystallization of austenite
between the stands, strain can be further accumulated. As a result, the texture of
the hot-rolled steel sheet can be preferably controlled. In a case of reliably obtaining
the effect, a Ti content is preferably set to 0.02% or more.
[0033] On the other hand, when the Ti content is more than 0.20%, inclusions attributed
to TiN are generated, and the toughness of the hot-rolled steel sheet deteriorates.
Therefore, the Ti content is set to 0.20% or less.
Nb: 0.010% to 0.100%
[0034] Nb is an element effective for suppressing austenite recrystallization and grain
growth between the stands of hot rolling. By suppressing the recrystallization of
austenite between the stands, strain can be further accumulated. As a result, the
texture of the hot-rolled steel sheet can be preferably controlled. In a case of reliably
obtaining the effect, a Nb content is preferably set to 0.010% or more.
[0035] On the other hand, when the Nb content is more than 0.100%, the effect is saturated.
Therefore, the Nb content is set to 0.100% or less.
Ca: 0.0001% to 0.0060%
[0036] Ca is an element having an effect of refining the structure of the hot-rolled steel
sheet by dispersing a number of fine oxides during deoxidation of molten steel. In
addition, Ca is also an element that fixes S in steel as spherical CaS, suppresses
the generation of elongated inclusions such as MnS, and reduces the anisotropy in
the toughness of the hot-rolled steel sheet. In a case of reliably obtaining these
effects, a Ca content is preferably set to 0.0001% or more.
[0037] On the other hand, when the Ca content is more than 0.0060%, the above effects are
saturated. Therefore, the Ca content is set to 0.0060% or less.
Mo: 0.01 % to 0.50%
[0038] Mo is an element effective for precipitation hardening of ferrite. In a case of reliably
obtaining this effect, a Mo content is preferably set to 0.01 % or more.
[0039] On the other hand, when the Mo content is more than 0.50%, cracking susceptibility
of a slab increases, and it becomes difficult to handle the slab. Therefore, the Mo
content is set to 0.50% or less.
Cr: 0.01% to 1.00%
[0040] Cr is an effective element for improving the strength of the hot-rolled steel sheet.
In a case of reliably obtaining this effect, a Cr content is preferably set to 0.01%
or more.
[0041] However, when the Cr content is more than 1.00%, ductility of the hot-rolled steel
sheet deteriorates. Therefore, the Cr content is set to 1.00% or less.
V: 0.01% to 0.50%
[0042] V improves the strength of the hot-rolled steel sheet through strengthening with
precipitates and refinement of ferrite grains. In a case of reliably obtaining this
effect, a V content is preferably set to 0.01% or more.
[0043] On the other hand, when the V content is more than 0.50%, a large amount of carbonitrides
is precipitated, and formability of the hot-rolled steel sheet deteriorates. Therefore,
the V content is set to be 0.50% or less.
Cu: 0.01% to 0.50%
[0044] Cu is an element that is solid-solubilized in steel and contributes to an improvement
in the strength of steel. Cu is also an element that improves hardenability. In a
case of reliably obtaining these effects, a Cu content is preferably set to 0.01%
or more.
[0045] On the other hand, when the Cu content is more than 0.50%, surface properties of
the hot-rolled steel sheet deteriorate, and there are cases where chemical convertibility
and corrosion resistance deteriorate. Therefore, the Cu content is set to 0.50% or
less.
Ni: 0.01% to 0.50%
[0046] Ni is an element that is solid-solubilized in steel and contributes to an increase
in the strength of the steel. Ni is also an element that improves hardenability. In
a case of reliably obtaining these effects, a Ni content is preferably set to 0.01%
or more.
[0047] On the other hand, since an alloy cost of Ni is high, including a large amount of
Ni causes an increase in the cost. In addition, when the Ni content is more than 0.50%,
there are cases where the weldability of the hot-rolled steel sheet deteriorates.
Therefore, the Ni content is set to 0.50% or less.
Sn: 0.001% to 0.050%
[0048] Sn has an effect of suppressing internal oxidation and an effect of improving the
strength. In a case of reliably obtaining the effects, a Sn content is preferably
set to 0.001% or more.
[0049] On the other hand, when a large amount of Sn is contained, there are cases where
defects occur during hot rolling. Therefore, the Sn content is set to 0.050% or less.
[0050] The chemical composition described above may be measured by a general analysis method.
For example, the chemical composition may be measured using inductively coupled plasma-atomic
emission spectrometry (ICP-AES). C and S may be measured using a combustion-infrared
absorption method, and N may be measured using an inert gas fusion-thermal conductivity
method.
[0051] In a case where the hot-rolled steel sheet includes a plating layer on a surface,
the chemical composition may be analyzed after removing the plating layer from the
surface by mechanical grinding.
[0052] Next, a microstructure of the hot-rolled steel sheet according to the present embodiment
will be described.
[0053] In the hot-rolled steel sheet according to the present embodiment, a microstructure
in a region from a depth of 1/8 of a sheet thickness from a surface to a depth of
3/8 of the sheet thickness from the surface includes, by area%, 90% to 100% of martensite,
and 0% to 10% of a remainder in microstructure, in a texture of a region from the
surface to the depth of 1/8 of the sheet thickness from the surface, pole densities
of {001}<110>, {111}<110>, and {112}<110> orientation groups are 2.0 or more, and
in a texture of a region from the depth of 1/8 of the sheet thickness from the surface
to a depth of 1/2 of the sheet thickness from the surface, a pole density of a { 110}<112>
orientation is 5.0 or less.
[0054] In the present embodiment, area% of martensite and the remainder in microstructure
in the region from the depth of 1/8 of the sheet thickness from the surface to the
depth of 3/8 of sheet thickness from the surface is specified because the microstructure
at this position represents a typical microstructure of the hot-rolled steel sheet.
Hereinafter, each specifying will be described in detail.
Area Ratio of Martensite: 90% to 100%
[0055] When an area ratio of martensite is less than 90%, the strength of the hot-rolled
steel sheet deteriorates, and a desired strength cannot be obtained. Therefore, the
area ratio of martensite is set to 90% or more. The area ratio of martensite is preferably
92% or more, 95% or more, or 97% or more, and more preferably 100%.
[0056] In the present embodiment, martensite refers to fresh martensite and tempered martensite.
In the present embodiment, it is not necessary to distinguish between fresh martensite
and tempered martensite, and thus both will be collectively referred to as martensite.
[0057] Tempered martensite is obtained by tempering fresh martensite and has a lower dislocation
density than fresh martensite. A preferred manufacturing method of the hot-rolled
steel sheet according to the present embodiment, which will be described later, does
not include a heat treatment for the purpose of tempering after rapid cooling. However,
there are cases where tempered martensite is generated during cooling after hot rolling
or by reheating after coiling.
Area Ratio of Remainder in Microstructure: 0% to 10%
[0058] The microstructure of the hot-rolled steel sheet according to the present embodiment
may contain bainite as the remainder in microstructure. When an area ratio of the
remainder in microstructure is more than 10%, the strength of the hot-rolled steel
sheet decreases, and a desired strength cannot be obtained. Therefore, the area ratio
of the remainder in microstructure is set to 10% or less. The area ratio of the remainder
in microstructure is preferably 8% or less, 5% or less, or 3% or less, and more preferably
0%.
[0059] The area ratio of each structure is obtained by the following method.
[0060] A test piece for structure observation is collected from a 1/4 thickness position
of the hot-rolled steel sheet (a region from the depth of 1/8 of the sheet thickness
from the surface to the depth of 3/8 of the sheet thickness from the surface) and
from a sheet width center position so that a sheet thickness cross section parallel
to a rolling direction serves an observed section. The observed section is mirror-polished
and then corroded with a 3 volume% Nital solution. Three visual fields of the observed
section after the corrosion were photographed at a magnification of 2,000-fold using
an optical microscope and a scanning electron microscope (SEM). Each photographed
visual field is set to 500 µm × 500 µm. Image analysis is performed on the photographs,
and the area ratio of each structure is calculated. The area ratio of each structure
is obtained by calculating an average value of the area ratios obtained for the three
visual fields.
[0061] Since martensite is a structure having a substructure called a block or a packet
in grains, it is possible to distinguish martensite from other microstructures in
an electron channeling contrast image for which the scanning electron microscope is
used.
[0062] Among structures that are each an aggregate of lath-shaped crystal grains and do
not contain any Fe-based carbides having a major axis of 20 nm or more in the structure,
a structure that is not martensite or a structure in which Fe-based carbides having
a major axis of 20 nm or more are contained in the structure and the Fe-based carbides
have a single variant, that is, the Fe-based carbides extend in the same direction,
is regarded as the bainite. Here, the Fe-based carbides extending in the same direction
refer to Fe-based carbides for which a difference in extending direction between the
Fe-based carbides is 5° or less.
Average Grain Size of Prior Austenite Grains: More Than 5.0 µm and 30.0 µm or Less
[0063] In the hot-rolled steel sheet according to the present embodiment, at the 1/4 thickness
position (the region from the depth of 1/8 of the sheet thickness from the surface
to the depth of 3/8 of the sheet thickness from the surface), an average grain size
of prior austenite grains may be more than 5.0 µm and 30.0 µm or less. By setting
the average grain size of the prior austenite grains to more than 5.0 µm, the predetermined
texture required in the present embodiment can be stably obtained, and the anisotropy
in the toughness of the hot-rolled steel sheet can be further reduced. The average
grain size of the prior austenite grains is preferably 6.0 µm or more, 7.0 µm or more,
8.0 µm or more, or 9.0 µm or more.
[0064] On the other hand, when the average grain size of the prior austenite grains is more
than 30.0 µm, there are cases where a desired strength cannot be obtained. Therefore,
the average grain size of the prior austenite grains is preferably set to 30.0 µm
or less.
[0065] The average grain size of the prior austenite grains is obtained by the following
method.
[0066] A test piece for structure observation is collected from the 114 thickness position
of the hot-rolled steel sheet (the region from the depth of 1/8 of the sheet thickness
from the surface to the depth of 3/8 of the sheet thickness from the surface) and
from the sheet width center position so that the sheet thickness cross section parallel
to the rolling direction serves an observed section. The observed section is mirror-polished
and then corroded with a 3 volume% Nital solution, and the microstructure is observed
with a scanning electron microscope (SEM). A range in which approximately 10,000 crystal
grains are observed in one visual field is photographed in three visual fields by
SEM observation. The obtained photograph is subjected to image analysis using image
analysis software (WinROOF) to calculate the average grain size of the prior austenite
grains. For one of the prior austenite grains included in the observed visual field,
an average value of a shortest diameter and a longest diameter is calculated, and
the average value is used as the grain size of the prior austenite grains. The above
operation is performed on all the prior austenite grains except for the prior austenite
grains which are not entirely included in the photographed visual fields, such as
crystal grains in an end portion of the photographed visual field, and the grain sizes
of all the prior austenite grains in the photographed visual fields are obtained.
The average grain size of the prior austenite grains in the photographed visual fields
is obtained by calculating a value obtained by dividing the sum of the obtained grain
sizes of the prior austenite grains by the total number of prior austenite grains
of which grain sizes are measured. This operation is performed for each of all the
photographed visual fields, and the average grain size of the prior austenite grains
of all the photographed visual fields is calculated, thereby obtaining the average
grain size of the prior austenite grains.
[0067] Pole Densities of {001}<110>, {111}<110>, and {112}<110> Orientation Group in Texture
of Region from Surface to Depth of 1/8 of Sheet Thickness from Surface: 2.0 or More
[0068] When the pole densities of the {001}<11 0>, {111}<110>, and {112}<110> orientation
groups in the texture of the region (hereinafter, sometimes referred to as a surface
layer region) from the surface to the depth of 1/8 of the sheet thickness from the
surface are less than 2.0, the occurrence of fine cracks in the surface layer region
cannot be suppressed. As a result, the anisotropy in the toughness of the hot-rolled
steel sheet increases. Therefore, the pole densities of the {001}<110>, {111 }<110>,
and {112 } <110> orientation groups in the texture of the surface layer region are
set to 2.0 or more. The pole densities of the {001 }<110>, {111}<110>, and {112}<110>
orientation groups in the texture of the surface layer region are preferably 2.2 or
more, 2.5 or more, or 2.7 or more.
[0069] An upper limit of the pole densities of the {001 }<110>, { 111 }<110>, and { 112}<110>
orientation groups in the texture of the surface layer region is not particularly
specified, but may be set to 9.0 or less, 8.0 or less, 7.0 or less, or 5.0 or less
from the viewpoint of suppressing the deterioration of ductility.
[0070] Pole Density of { 110 }<112> Orientation in Texture of Region from Depth of 1/8 of
Sheet Thickness from Surface to Depth of 1/2 of Sheet Thickness from Surface: 5.0
or Less
[0071] The pole density of the { 110 }<112> orientation in the texture of the region (hereinafter,
sometimes referred to as an internal region) from the depth of 1/8 of the sheet thickness
from the surface to the depth of 1/2 of the sheet thickness from the surface is more
than 5.0, the anisotropy in the toughness of the hot-rolled steel sheet increases.
Therefore, the pole density of the { 110}<112> orientation in the texture of the internal
region is set to 5.0 or less. The pole density of the { 110}<112> orientation in the
texture of the internal region is preferably 4.6 or less, 4.2 or less, or 4.0 or less.
[0072] A lower limit of the pole density of the { 110}<112> orientation in the texture of
the internal region is not particularly specified, but may be set to 2.0 or more or
2.5 or more from the viewpoint of suppressing the deterioration of the strength.
[0073] For the pole densities, a device in which a scanning electron microscope and an EBSD
analyzer are combined and OIM analysis (registered trademark) manufactured by AMETEK
Inc. are used. From an orientation distribution function (ODF) that is calculated
by using orientation data measured by an electron backscattering diffraction (EBSD)
method and a spherical harmonic function and displays a three-dimensional texture,
the pole densities of the {001 }<110>, { 111 }<110>, and { 112}<110> orientation groups
in the texture of the surface layer region and the pole density of the {110}<112>
in the texture of the internal region are obtained.
[0074] A measurement range is set to, for the surface layer region, the region from the
surface to the depth of 1/8 of the sheet thickness from the surface and for the internal
region, the region from the depth of 1/8 of the sheet thickness from the surface to
the depth of 1/2 of the sheet thickness from the surface. Measurement pitches are
set to 5 µm/step.
[0075] It should be noted that {hkl) indicates a crystal plane parallel to a rolled surface
and <uvw> indicates a crystal direction parallel to the rolling direction. That is,
{hkl}<uvw> indicates a crystal in which {hkl} is oriented in a sheet surface normal
direction and <uvw> is oriented in the rolling direction.
[0076] The rolling direction of the hot-rolled steel sheet can be determined by the following
method.
[0077] First, a test piece is collected so that a sheet thickness cross section of the hot-rolled
steel sheet can be observed. The sheet thickness cross section of the collected test
piece is finished by mirror polishing and then observed using an optical microscope.
An observation range is set to an overall thickness of the sheet thickness, and a
region with dark brightness is determined to be an inclusion. Among inclusions, in
inclusions having a major axis length of 40 µm or more, a direction parallel to a
direction in which the inclusion extends is determined to be the rolling direction.
Tensile Strength: 1,180 MPa or More
[0078] A tensile strength of the hot-rolled steel sheet according to the present embodiment
is set to 1,180 MPa or more from the viewpoint of improving collision safety of a
vehicle or the like or reducing a weight of a vehicle body. The tensile strength is
preferably 1,250 MPa or more, 1,300 MPa or more, 1,350 MPa or more, or 1,400 MPa or
more.
[0079] An upper limit of the tensile strength is not particularly specified, but is preferably
2,000 MPa or less, 1,600 MPa or less, 1,500 MPa or less, or 1,400 MPa or less.
[0080] The tensile strength is measured according to JIS Z 2241: 2011. A No. 5 test piece
of JIS Z 2241: 2011 is used as a test piece, and a test direction is set to a direction
perpendicular to the rolling direction.
[0081] The sheet thickness of the hot-rolled steel sheet according to the present embodiment
is not particularly limited and may be set to 1.2 to 8.0 mm. When the sheet thickness
of the hot-rolled steel sheet is less than 1.2 mm, it is difficult to secure a rolling
completion temperature, a rolling force becomes excessive, and there are cases where
it is difficult to perform hot rolling.
[0082] When the sheet thickness is more than 8.0 mm, it becomes difficult to control the
texture, and there are cases where it is difficult to obtain the above-described texture.
Therefore, the sheet thickness may be set to 8.0 mm or less.
[0083] The hot-rolled steel sheet according to the present embodiment may have a plating
layer on the surface. As the plating layer, an aluminum plating layer, an aluminum-zinc
plating layer, an aluminum-silicon plating layer, a hot-dip galvanized layer, an electrogalvanized
layer, a hot-dip galvannealed layer, or the like is an exemplary example.
[0084] Next, a preferred manufacturing method of the hot-rolled steel sheet according to
the present embodiment will be described. The preferred manufacturing method of the
hot-rolled steel sheet according to the present embodiment includes the following
steps (a) to (d). Unless otherwise specified, a temperature in the following description
refers to a surface temperature of the steel sheet.
[0085]
- (a) A heating step of heating a slab having the above-described chemical composition
to a temperature range of 1,100°C or higher and lower than 1,350°C.
- (b) A finish rolling step of performing finish rolling on the heated slab using a
rolling mill having a plurality of stands, in which the following conditions (1) to
(V) are satisfied.
- (I) A finish rolling start temperature is set to 800°C or higher.
- (II) In each of the last four stands among the plurality of stands, rolling is performed
so that σ represented by Expression (1) becomes 40 to 80.

Here, T is a temperature (°C) immediately before entering each stand, ε is an equivalent
plastic strain, and ε' is a strain rate.
- (III) Interpass times between the last four stands are set to 0.2 to 10.0 seconds.
- (IV) A cumulative rolling reduction of the last four stands is set to 60% or larger.
- (V) A finishing temperature is set to 800°C to 950°C.
- (c) A cooling step of starting cooling within 1.0 second after completion of the finish
rolling, and performing cooling to a temperature range of 300°C or lower so that an
average cooling rate in a temperature range of the finishing temperature to 300°C
is 100 °C/s or faster.
- (d) A coiling step of performing coiling after the cooling.
[0086] Hereinafter, each step will be described.
(a) Heating Step
[0087] In the heating step, it is preferable to heat the slab having the above-mentioned
chemical composition to a temperature range of 1,100°C or higher and lower than 1,350°C.
A method of manufacturing the slab does not need to be particularly limited, and a
commonly used method can be applied in which molten steel having the above-described
chemical composition is melted in a converter or the like and is cast into a slab
by a casting method such as continuous casting. In addition, an ingot-making and blooming
method may be used.
[0088] In the slab, most of carbonitride-forming elements such as Ti are present in the
slab as coarse carbonitrides in a non-uniform distribution. The coarse precipitates
(carbonitrides) present in a non-uniform distribution deteriorate various properties
(for example, tensile strength, toughness, and hole expansibility) of the hot-rolled
steel sheet. Therefore, the slab before hot rolling is heated to solid-solubilize
the coarse precipitates. In order to sufficiently solid-solubilize these coarse precipitates
before hot rolling, a heating temperature of the slab is preferably set to 1,100°C
or higher. However, an excessively high heating temperature for the slab causes the
generation of surface defects and a decrease in yield due to scale removal. Therefore,
the heating temperature of the steel material is preferably set to lower than 1,350°C.
[0089] The slab is heated to the temperature range of 1,100°C or higher and lower than 1,350°C
and held for a predetermined time. However, when a holding time is longer than 4,800
seconds, the amount of scale generated increases. As a result, a rolled-in scale or
the like is likely to occur in the subsequent finish rolling step, and there are cases
where surface quality of the hot-rolled steel sheet deteriorates. Therefore, the holding
time in the temperature range of 1,100°C or higher and lower than 1,350°C is preferably
set to 4,800 seconds or shorter.
Rough Rolling Step
[0090] Rough rolling may be performed on the slab between the heating step and the finish
rolling step. Conditions of the rough rolling are not particularly limited as long
as desired sheet bar dimensions can be obtained.
(b) Finish Rolling Step
[0091] In the finish rolling step, the heated slab is subjected to finish rolling using
the rolling mill having the plurality of stands. Here, it is preferable to satisfy
the conditions (I) to (V) to be described below.
[0092] It is preferable to perform descaling before the finish rolling or during rolling
between the rolling stands during the finish rolling.
(I) Finish Rolling Start Temperature: 800°C or Higher
[0093] The finish rolling start temperature (an entry-side temperature of a first pass of
the finish rolling) is preferably set to 800°C or higher. When the finish rolling
start temperature is lower than 800°C, rolling in some of the plurality of rolling
stands (particularly the stands in the first half) is performed at a temperature in
a ferrite/austenite dual phase region. As a result, a worked structure remains after
the finish rolling, and there are cases where the strength and toughness of the hot-rolled
steel sheet deteriorate. Therefore, the finish rolling start temperature is preferably
set to 800°C or higher.
[0094] The finish rolling start temperature is preferably set to 1,100°C or lower in order
to suppress coarsening of austenite and to preferably control the texture in the surface
layer region and in the internal region.
(II) In Each of Last Four Stands, σ Represented by Expression (1): 40 to 80
[0095] 
[0096] Here, T is the temperature (°C) immediately before entering each stand (that is,
the entry-side temperature), ε is the equivalent plastic strain, and ε' is the strain
rate.
[0097] The fact that σ in each of the last four stands is 40 to 80 can be rephrased as follows:
σ of the fourth stand from the last stand, σ of the third stand from the last stand,
σ of the second stand from the last stand, and σ of the last stand are all 40 to 80.
[0098] When there is even one stand in which σ is less than 40, there are cases where strain
necessary for the development of the texture in the surface layer region is not suitably
applied in each of the last four stands. As a result, there are cases where in the
texture of the region from the surface to the depth of 1/8 of the sheet thickness
from the surface, the pole densities of the {001}<110>, { 111 }<110>, and {112}<110>
orientation groups cannot be preferably controlled. Therefore, σ in each of the last
four stands is preferably set to 40 or more.
[0099] In addition, when there is even one stand in which σ is more than 80, the texture
of the internal region cannot be preferably controlled, and there are cases where
the anisotropy in the toughness of the hot-rolled steel sheet increases. Therefore,
σ in each of the last four stands is preferably set to 80 or less.
[0100] In addition, ε, which is the equivalent plastic strain, can be obtained by ε = (2/√3)
× (h/H) when an entry-side sheet thickness is represented by h and an exit-side sheet
thickness is represented by H. In addition, the strain rate ε' can be obtained by
ε' = ε/t when a rolling time is t (s). In addition, the rolling time t refers to a
time during which strain is applied to the steel sheet when the steel sheet and the
rolling roll come into contact with each other.
(111) Interpass Times between Last Four Stands: 0.2 to 10.0 Seconds
[0101] When there is even one pass in which the interpass time is longer than 10.0 seconds
between the last four stands, recovery and recrystallization between the passes progresses.
As a result, it becomes difficult to accumulate strain, and there are cases where
a desired structure cannot be obtained in the hot-rolled steel sheet. Therefore, the
interpass times between the last four stands are preferably set to 10.0 seconds or
shorter.
[0102] The interpass times between the last four stands are preferably short, but a reduction
in the interpass times are limited in terms of an installation space of each stand
and a rolling rate. In addition, when the interpass times between the last four stands
become shorter than 0.2 seconds, the number of unrecrystallized grains significantly
increases, and there are cases where a desired texture cannot be obtained. Therefore,
the interpass times between the last four stands are preferably set to 0.2 seconds
or longer.
[0103] The interpass times between the last four stands are 0.2 to 10.0 seconds can be rephrased
as follows: interpass time between the fourth stand from the last stand and the third
stand from the last stand, the interpass time between the third stand from the last
stand and the second stand from the last stand, and the interpass time between the
second stand from the last stand and the last stand are all 0.2 to 10.0 seconds.
(IV) Cumulative Rolling Reduction of Last Four Stands: 60% or Larger
[0104] When the cumulative rolling reduction of the last four stands is smaller than 60%,
there are cases where a dislocation density introduced into unrecrystallized austenite
decreases. When the dislocation density introduced into unrecrystallized austenite
decreases, it becomes difficult to obtain a desired structure, and there are cases
where the strength and toughness of the hot-rolled steel sheet deteriorate. Therefore,
the cumulative rolling reduction of the last four stands is preferably set to 60%
or larger.
[0105] When the cumulative rolling reduction of the last four stands is larger than 97%,
there are cases where a shape of the hot-rolled steel sheet deteriorates. Therefore,
the cumulative rolling reduction of the last four stands may be set to 97% or smaller.
[0106] The cumulative rolling reduction of the last four stands can be represented by {1
- (t1/t0)} × 100 (%) when an inlet sheet thickness of the fourth stand from the last
stand is represented by t0 and an outlet sheet thickness of the last stand is represented
by t1.
(V) Finishing temperature: 800°C to 950°C
[0107] When the finish rolling finishing temperature (exit-side temperature of the last
stand) is lower than 800°C, the rolling is performed at a temperature in the ferrite/austenite
dual phase region. Therefore, there are cases where the worked structure remains after
the rolling and the strength and toughness of the hot-rolled steel sheet decrease.
Therefore, the finishing temperature is preferably set to 800°C or higher.
[0108] In addition, in the slab having the chemical composition according to the present
embodiment, an unrecrystallized austenite region is a temperature range of approximately
950°C or lower. Therefore, when the finishing temperature is higher than 950°C, austenite
grains grow, and a grain length of martensite in the hot-rolled steel sheet obtained
after the cooling increases. As a result, it becomes difficult to obtain a desired
texture, and there are cases where the strength and toughness of the hot-rolled steel
sheet decrease. Therefore, the finishing temperature is preferably set to 950°C or
lower.
(c) Cooling Step
[0109] In the cooling step, it is preferable that cooling is started within 1.0 second after
the completion of the finish rolling, and cooling to a temperature range of 300°C
or lower is performed so that an average cooling rate in a temperature range of the
finishing temperature to 300°C is 100 °C/s or faster.
[0110] In the present embodiment, it is preferable that cooling equipment is installed at
a rear stage of finish rolling equipment, and the cooling is performed while the steel
sheet after the finish rolling passes through the cooling equipment. The cooling equipment
is preferably equipment that can cool the steel sheet at an average cooling rate of
100 °C/s or faster. Examples of the cooling equipment include water cooling equipment
using water as a cooling medium.
[0111] The average cooling rate in the cooling step is a value obtained by dividing a temperature
drop width of the steel sheet from when the cooling is started to when the cooling
is ended by a time required from when the cooling is started to when the cooling is
ended. When the cooling is started refers to a time when the steel sheet is introduced
into the cooling equipment, and when the cooling is ended refers to a time when the
steel sheet is taken out of the cooling equipment.
[0112] Examples of the cooling equipment include equipment having no intermediate air cooling
section and equipment having at least one intermediate air cooling section. In the
present embodiment, any cooling equipment may be used. Even in a case where cooling
equipment having an air cooling section is used, the average cooling rate from the
start of cooling to the end of cooling may be 100 °C/s or faster.
[0113] Hereinafter, the reasons for limiting cooling conditions will be described. A cooling
stop temperature is 300°C or lower, and this condition will be described in the coiling
step.
Cooling Start Time: Within 1.0 Second after Completion of Finish Rolling
[0114] It is preferable to start cooling immediately after the completion of the finish
rolling. When the cooling start time is longer than 1.0 second, recrystallization
proceeds, cooling is performed in a state where the strain is released, and there
are cases where a desired texture cannot be obtained in the hot-rolled steel sheet.
Therefore, it is preferable to start the cooling within 1.0 second after the completion
of the finish rolling.
Average Cooling Rate in Temperature Range of Finishing temperature to 300°C: 100 °C/s
or Faster
[0115] When the average cooling rate in the temperature range of the finishing temperature
to 300°C is slower than 100 °C/s, bainite or ferrite is likely to be formed, and there
are cases where a desired amount of martensite cannot be obtained. Therefore, the
average cooling rate in the temperature range of the finishing temperature to 300°C
is preferably set to 100 °C/s or faster.
(d) Coiling Step
[0116] In the coiling step, the steel sheet cooled to a temperature range of 300°C or lower
is preferably coiled. Since the steel sheet is coiled immediately after the cooling,
a coiling temperature is almost equal to the cooling stop temperature. When the coiling
temperature is higher than 300°C, polygonal ferrite or bainite is generated, and there
are cases where the strength of the hot-rolled steel sheet decreases. Therefore, the
coiling temperature is preferably set to a temperature range of 300°C or lower.
[0117] After the coiling, the hot-rolled steel sheet may be subjected to temper rolling
according to a conventional method, or subjected to pickling to remove the scale formed
on the surface. Alternatively, a plating treatment such as aluminum plating, aluminum-zinc
plating, aluminum-silicon plating, hot-dip galvanizing, electrogalvanizing, and hot-dip
galvannealing, or a chemical conversion treatment may be performed.
[0118] The hot-rolled steel sheet according to the present embodiment can be stably manufactured
by the preferred manufacturing method described above.
[Examples]
[0119] Next, examples of the present invention will be described. Conditions in the examples
are one example of conditions adopted to confirm the feasibility and effects of the
present invention, and the present invention is not limited to this example of conditions.
The present invention may adopt various conditions to achieve the object of the present
invention without departing from the scope of the present invention.
[0120] Molten steels having the chemical compositions shown in Table 1 were melted in a
converter and slabs were obtained by a continuous casting method. Next, these slabs
were heated under the conditions shown in Tables 2A and 2B, subjected to rough rolling,
and then subjected to finish rolling under the conditions shown in Tables 2A and 2B.
After the finish rolling was completed, the slabs were cooled and coiled under the
conditions shown in Tables 3A and 3B to obtain hot-rolled steel sheets having the
sheet thicknesses shown in Tables 3A and 3B.
[0121] In the heating step, holding times at the heating temperatures shown in Tables 2A
and 2B were set to 4,800 seconds or shorter.
[0122] In addition, as cooling after the finish rolling, water cooling was performed in
which the steel sheet was passed through water cooling equipment having no intermediate
air cooling section. An average cooling rate in Tables 3A and 3B is a value obtained
by dividing a temperature drop width of the steel sheet from when the steel sheet
was introduced into the water cooling equipment to when the steel sheet was taken
out of the water cooling equipment by a time required for the steel sheet to be passed
through the water cooling equipment.
[0123] A test piece was collected from the obtained hot-rolled steel sheet, an area ratio
of each structure and pole densities of textures were measured and a tensile test
was conducted by the above-described methods.
The obtained results are shown in Tables 4A and 4B.
[0124] In a case where an obtained tensile strength was 1,180 MPa or more, the hot-rolled
steel sheet was determined to be acceptable as having high strength. On the other
hand, in a case where the obtained tensile strength was less than 1,180 MPa, the hot-rolled
steel sheet was determined to be unacceptable as not having high strength.
[0125] A Charpy impact test was conducted to evaluate the toughness of the hot-rolled steel
sheets, and a ductile-brittle transition temperature was measured. For the measurement
of the ductile-brittle transition temperature, a C-direction notch Charpy impact test
was conducted using a V-notch test piece having a subsize of 2.5 mm according to JIS
Z 2242: 2018. A temperature at which a brittle fracture surface ratio became 50% was
defined as the ductile-brittle transition temperature. In addition, for hot-rolled
steel sheets having a final sheet thickness of less than 2.5 mm, an overall thickness
was measured.
[0126] In a case where the obtained ductile-brittle transition temperature was -50°C or
lower, the hot-rolled steel sheet was determined to be acceptable as having excellent
toughness. On the other hand, in a case where the obtained ductile-brittle transition
temperature was higher than -50°C, the hot-rolled steel sheet was determined to be
unacceptable as being inferior in toughness.
[0127] Furthermore, the anisotropy in toughness was evaluated by the following method. According
to JIS Z 2242: 2018, an absorbed energy of a C-direction notch and an absorbed energy
of an L-direction notch were measured by a Charpy impact test using a V-notch test
piece having a subsize of 2.5 mm. The Charpy impact test was conducted at -60°C. A
difference between the absorbed energy of the L-direction notch and the absorbed energy
of the C-direction notch was calculated, and in a case where the difference was ±15
J or less, the hot-rolled steel sheet was determined to be acceptable as having reduced
anisotropy in toughness. On the other hand, in a case where the difference between
the absorbed energy of the L-direction notch and the absorbed energy of the C-direction
notch was more than ±15 J, the hot-rolled steel sheet was determined to be unacceptable
as not having reduced anisotropy in toughness.
[Table 1]
Kind of steel |
Chemical composition (mass%), remainder: Fe and impurities |
Note |
C |
Si |
Mn |
P |
S |
Al |
N |
Others |
A |
0.210 |
1.100 |
1.50 |
0.013 |
0.0010 |
0.010 |
0.0030 |
- |
Present Invention Steel |
B |
0.180 |
1.100 |
1.60 |
0.014 |
0.0020 |
0.020 |
0.0030 |
Ti: 0.03, Nb: 0.020 |
Present Invention Steel |
C |
0.150 |
1.400 |
1.40 |
0.014 |
0.0020 |
0.010 |
0.0030 |
Nb: 0.015, Mo: 0.40 |
Present Invention Steel |
D |
0.150 |
1.400 |
1.40 |
0.014 |
0.0020 |
0.010 |
0.0030 |
Ti: 0.02, Ca: 0.0030, Cr: 0.70 |
Present Invention Steel |
E |
0.150 |
1.800 |
1.20 |
0.014 |
0.0020 |
0.010 |
0.0030 |
Ti: 0.02, V: 0.20 |
Present Invention Steel |
F |
0.240 |
1.400 |
1.40 |
0.015 |
0.0020 |
0.010 |
0.0030 |
Ti: 0.03, Cu: 0.40 |
Present Invention Steel |
G |
0.130 |
1.400 |
1.40 |
0.014 |
0.0020 |
0.010 |
0.0030 |
Ti: 0.02, Nb: 0.020, Ni: 0.02, Sn: 0.001 |
Present Invention Steel |
H |
0.130 |
3.100 |
1.20 |
0.014 |
0.0020 |
0.010 |
0.0030 |
- |
Comparative Example |
I |
0.080 |
1.000 |
1.10 |
0.014 |
0.0020 |
0.010 |
0.0030 |
- |
Comparative Example |
J |
0.100 |
1.000 |
1.10 |
0.014 |
0.0020 |
0.010 |
0.0030 |
|
Present Invention Steel |
K |
0.490 |
1.400 |
1.40 |
0.015 |
0.0020 |
0.010 |
0.0030 |
Ti: 0.03 |
Present Invention Steel |
L |
0.180 |
0.200 |
1.60 |
0.014 |
0.0020 |
0.020 |
0.0030 |
|
Present Invention Steel |
M |
0.150 |
0.800 |
1.40 |
0.014 |
0.0020 |
0.010 |
0.0030 |
|
Present Invention Steel |
N |
0.150 |
2.900 |
1.40 |
0.014 |
0.0020 |
0.010 |
0.0030 |
Ti: 0.03 |
Present Invention Steel |
O |
0.150 |
0.900 |
0.60 |
0.014 |
0.0020 |
0.010 |
0.0030 |
|
Present Invention Steel |
P |
0.150 |
0.900 |
2.80 |
0.014 |
0.0020 |
0.010 |
0.0030 |
|
Present Invention Steel |
Q |
0.150 |
0.900 |
1.40 |
0.090 |
0.0020 |
0.010 |
0.0030 |
|
Present Invention Steel |
R |
0.150 |
0.900 |
1.40 |
0.014 |
0.0100 |
0.010 |
0.0030 |
Ti: 0.03 |
Present Invention Steel |
s |
0.150 |
0.900 |
1.40 |
0.014 |
0.0020 |
1.000 |
0.0030 |
|
Present Invention Steel |
T |
0.150 |
0.900 |
1.40 |
0.014 |
0.0020 |
0.010 |
0.0100 |
|
Present Invention Steel |
U |
0.600 |
0.900 |
1.40 |
0.014 |
0.0020 |
0.010 |
0.0030 |
Ti: 0.03 |
Comparative Steel |
V |
0.150 |
0.050 |
1.40 |
0.014 |
0.0020 |
0.010 |
0.0030 |
|
Comparative Steel |
W |
0.150 |
0.900 |
0.30 |
0.014 |
0.0020 |
0.010 |
0.0030 |
|
Comparative Steel |
X |
0.150 |
0.900 |
3.10 |
0.014 |
0.0020 |
0.010 |
0.0030 |
|
Comparative Steel |
[0128] The underlined value indicates outside of the range of the present invention.
[Table 2A]
Test No. |
Kind of steel |
Heating temperature of slab °C |
Finish rolling |
Note |
(I) Finish rolling start temperature °C |
(II) σ of fourth stand from last stand - |
(II) σ of third stand from last stand - |
(II) σ of second stand from last stand - |
(II) σ of last stand - |
(III) Interpass time between fourth stand from last stand and third stand from last
stand s |
(III) Interpass time between third stand from last stand and second stand from last
stand s |
(III) Interpass time between second stand from last stand and last stand s |
(IV) Cumulative rolling reduction of last four stands % |
(V) Finishing temperature °C |
1 |
A |
1,241 |
1,094 |
50 |
52 |
56 |
58 |
8.0 |
3.0 |
0.7 |
94 |
919 |
Present Invention Example |
2 |
A |
1.263 |
1,077 |
51 |
59 |
56 |
58 |
6.0 |
4.0 |
0.9 |
90 |
862 |
Present Invention Example |
3 |
A |
1,208 |
965 |
52 |
52 |
50 |
58 |
5.0 |
5.0 |
0.9 |
93 |
833 |
Present Invention Example |
4 |
A |
1,206 |
962 |
53 |
52 |
44 |
62 |
7.0 |
6.0 |
1.0 |
96 |
907 |
Present Invention Example |
5 |
A |
1,251 |
966 |
49 |
42 |
41 |
42 |
9.0 |
4.0 |
0.8 |
57 |
824 |
Comparative Example |
6 |
B |
1,207 |
1,006 |
51 |
61 |
52 |
46 |
6.0 |
7.0 |
3.8 |
95 |
904 |
Present Invention Example |
7 |
B |
1,221 |
1,039 |
50 |
44 |
57 |
65 |
6.0 |
4.0 |
0.7 |
89 |
942 |
Present Invention Example |
8 |
B |
1,275 |
1,000 |
41 |
70 |
52 |
38 |
8.0 |
7.0 |
0.6 |
88 |
823 |
Comparative Example |
9 |
C |
1,300 |
1,003 |
51 |
55 |
59 |
68 |
9.0 |
3.0 |
0.3 |
92 |
842 |
Present Invention Example |
10 |
C |
1,249 |
1,006 |
43 |
69 |
42 |
55 |
8.0 |
6.0 |
0.3 |
92 |
905 |
Present Invention Example |
11 |
C |
1,246 |
1,060 |
69 |
42 |
54 |
47 |
5.0 |
5.0 |
1.0 |
92 |
888 |
Present Invention Example |
12 |
D |
1,233 |
1,066 |
40 |
63 |
54 |
50 |
9.0 |
3.0 |
0.9 |
97 |
920 |
Present Invention Example |
13 |
D |
1.220 |
1,038 |
66 |
67 |
53 |
61 |
6.0 |
5.0 |
0.3 |
94 |
917 |
Present Invention Example |
14 |
D |
1,263 |
966 |
56 |
64 |
42 |
67 |
11.0 |
7.0 |
5 |
96 |
929 |
Comparative Example |
15 |
E |
1.264 |
1,014 |
47 |
49 |
54 |
66 |
9.0 |
3.0 |
0.7 |
93 |
897 |
Present Invention Example |
16 |
E |
1,264 |
993 |
55 |
42 |
41 |
42 |
7.0 |
5.0 |
0.9 |
93 |
885 |
Present Invention Example |
17 |
E |
1,282 |
1,060 |
45 |
42 |
43 |
82 |
5.0 |
5.0 |
0.4 |
93 |
805 |
Comparative Example |
18 |
F |
1,262 |
1,044 |
40 |
49 |
42 |
63 |
9.0 |
6.0 |
0.5 |
94 |
920 |
Present Invention Example |
19 |
F |
1,215 |
1,048 |
47 |
70 |
58 |
52 |
7.0 |
7.0 |
0.2 |
92 |
855 |
Present Invention Example |
20 |
F |
1,296 |
953 |
54 |
58 |
59 |
62 |
5.0 |
4.0 |
1.2 |
89 |
780 |
Comparative Example |
21 |
G |
1,212 |
1,091 |
68 |
53 |
59 |
51 |
9.0 |
4.0 |
0.2 |
95 |
912 |
Present Invention Example |
22 |
G |
1,246 |
987 |
60 |
52 |
69 |
44 |
8.0 |
3.0 |
2.0 |
94 |
897 |
Present Invention Example |
23 |
G |
1,240 |
1,220 |
76 |
40 |
41 |
33 |
6.0 |
7.0 |
0.9 |
91 |
832 |
Comparative Example |
24 |
H |
1,286 |
1,075 |
64 |
49 |
67 |
51 |
6.0 |
4.0 |
0.8 |
98 |
960 |
Comparative Example |
25 |
I |
1,245 |
974 |
43 |
68 |
62 |
59 |
8.0 |
7.0 |
0.1 |
76 |
818 |
Comparative Example |
[0129] The underlined value indicates undesirable manufacturing conditions.
[Table 2B]
Test No. |
Kind of steel |
Heating temperature of slab °C |
Finish rolling |
Note |
(1) Finish rolling start temperature °C |
(II) α of fourth stand from last stand- |
(II) σ of (II) third stand from last stand - |
(II) σ of second stand from last stand- |
(II) σ of last stand- |
(III) interpass time between fourth stand from last stand and third stand from last
stand s |
(III) Interpass time between third stand from last stand and second stand from last
stand s |
(III) Interpass time between second stand from last stand and last stand s |
(IV) Cumulative rolling reduction of last four stands % |
(V) Finishing temperature °C |
26 |
J |
1,201 |
1,048 |
48 |
75 |
51 |
41 |
3.0 |
6.0 |
8.0 |
77 |
810 |
Present Invention Example |
27 |
K |
1,137 |
903 |
65 |
58 |
61 |
58 |
2.0 |
4.0 |
7.0 |
79 |
808 |
Present Invention Example |
28 |
L |
1,235 |
981 |
60 |
43 |
53 |
52 |
4.0 |
6.0 |
5.0 |
96 |
905 |
Present Invention Example |
29 |
M |
1,266 |
1,011 |
77 |
45 |
70 |
55 |
5.0 |
2.0 |
7.0 |
70 |
803 |
Present Invention Example |
30 |
N |
1,102 |
951 |
78 |
74 |
59 |
75 |
5.0 |
8.0 |
8.0 |
86 |
868 |
Present Invention Example |
31 |
O |
1,134 |
953 |
58 |
47 |
47 |
60 |
6.0 |
9.0 |
9.0 |
79 |
849 |
Present Invention Example |
32 |
P |
1,255 |
875 |
45 |
54 |
65 |
79 |
6.0 |
5.0 |
7.0 |
74 |
809 |
Present Invention Example |
33 |
Q |
1,157 |
1,040 |
56 |
47 |
78 |
51 |
5.0 |
7.0 |
6.0 |
92 |
826 |
Present Invention Example |
34 |
R |
1,307 |
1,012 |
55 |
62 |
71 |
64 |
5.0 |
7.0 |
2.0 |
70 |
850 |
Present Invention Example |
35 |
S |
1,188 |
860 |
42 |
77 |
47 |
63 |
3.0 |
7.0 |
3.0 |
95 |
919 |
Present Invention Example |
36 |
T |
1,346 |
948 |
46 |
44 |
42 |
70 |
3.0 |
8.0 |
8.0 |
65 |
921 |
Present Invention Example |
37 |
U |
1,298 |
863 |
57 |
68 |
75 |
54 |
4.0 |
5.0 |
3.0 |
78 |
884 |
Comparative Example |
38 |
V |
1.122 |
972 |
63 |
67 |
72 |
62 |
5.0 |
2.0 |
3.0 |
68 |
826 |
Comparative Example |
39 |
W |
1.138 |
917 |
73 |
77 |
65 |
69 |
2.0 |
2.0 |
3.0 |
68 |
891 |
Comparative Example |
40 |
X |
1,283 |
1,002 |
65 |
75 |
73 |
52 |
2.0 |
3.0 |
6.0 |
67 |
937 |
Comparative Example |
41 |
C |
1,228 |
910 |
78 |
71 |
45 |
66 |
5.0 |
5.0 |
9.0 |
89 |
852 |
Present Invention Example |
42 |
A |
1.314 |
893 |
45 |
65 |
51 |
42 |
6.0 |
4.0 |
4.0 |
86 |
863 |
Present Invention Example |
43 |
C |
1,231 |
899 |
56 |
65 |
46 |
75 |
5.0 |
9.0 |
3.0 |
87 |
934 |
Comparative Example |
44 |
B |
1,200 |
1.120 |
70 |
72 |
63 |
68 |
4.0 |
9.0 |
5.0 |
73 |
900 |
Comparative Example |
45 |
B |
1,145 |
790 |
71 |
51 |
47 |
61 |
3.0 |
3.0 |
3.0 |
77 |
809 |
Comparative Example |
46 |
C |
1.118 |
1,150 |
57 |
63 |
60 |
42 |
5.0 |
5.0 |
7.0 |
81 |
837 |
Comparative Example |
47 |
J |
1,299 |
895 |
32 |
58 |
67 |
61 |
5.0 |
5.0 |
3.0 |
82 |
882 |
Comparative Example |
48 |
O |
1,335 |
1,045 |
82 |
41 |
66 |
64 |
3.0 |
5.0 |
2.0 |
81 |
824 |
Comparative Example |
49 |
B |
1.144 |
869 |
58 |
35 |
68 |
71 |
4.0 |
8.0 |
8.0 |
71 |
922 |
Comparative Example |
50 |
A |
1,140 |
863 |
46 |
86 |
64 |
75 |
2.0 |
8.0 |
7.0 |
70 |
909 |
Comparative Example |
51 |
C |
1,236 |
924 |
69 |
45 |
32 |
64 |
6.0 |
8.0 |
6.0 |
82 |
813 |
Comparative Example |
52 |
A |
1,312 |
918 |
69 |
69 |
86 |
64 |
6.0 |
9.0 |
7.0 |
81 |
854 |
Comparative Example |
53 |
F |
1,255 |
873 |
79 |
72 |
64 |
68 |
2.0 |
11.0 |
9.0 |
93 |
885 |
Comparative Example |
54 |
D |
1,240 |
1,014 |
42 |
60 |
78 |
52 |
2.0 |
3.0 |
11.0 |
87 |
926 |
Comparative Example |
55 |
E |
1,305 |
970 |
56 |
71 |
56 |
71 |
6.0 |
6.0 |
8.0 |
67 |
886 |
Comparative Example |
56 |
B |
1,114 |
978 |
46 |
42 |
60 |
78 |
4.0 |
2.0 |
7.0 |
63 |
888 |
Comparative Example |
57 |
A |
1,119 |
970 |
70 |
45 |
64 |
51 |
3.0 |
2.0 |
5.0 |
86 |
936 |
Comparative Example |
[0130] The underlined value indicates undesirable manufacturing conditions.
[Table 3A]
Test No. |
Kind of steel |
Time until cooling is started s |
Average cooling rate in temperature range of finishing temperature to 300°C °C/s |
Coiling temperature °C |
Sheet thickness mm |
Note |
1 |
A |
0.5 |
180 |
244 |
3.0 |
Present Invention Example |
2 |
A |
0.6 |
114 |
219 |
4.0 |
Present Invention Example |
3 |
A |
0.7 |
118 |
233 |
5.0 |
Present Invention Example |
4 |
A |
0.5 |
102 |
235 |
5.0 |
Present Invention Example |
5 |
A |
0.6 |
169 |
253 |
7.0 |
Comparative Example |
6 |
B |
0.6 |
168 |
228 |
6.0 |
Present Invention Example |
7 |
B |
0.7 |
115 |
214 |
6.0 |
Present Invention Example |
8 |
B |
0.1 |
182 |
207 |
3.0 |
Comparative Example |
9 |
C |
0.6 |
174 |
220 |
6.0 |
Present Invention Example |
10 |
C |
0.3 |
152 |
224 |
2.0 |
Present Invention Example |
11 |
C |
0.9 |
170 |
247 |
4.0 |
Present Invention Example |
12 |
D |
0.2 |
116 |
300 |
2.0 |
Present Invention Example |
13 |
D |
0.3 |
125 |
280 |
5.0 |
Present Invention Example |
14 |
D |
0.4 |
104 |
204 |
3.0 |
Comparative Example |
15 |
E |
0.2 |
139 |
212 |
5.0 |
Present Invention Example |
16 |
E |
0.2 |
142 |
223 |
4.0 |
Present Invention Example |
17 |
E |
0.3 |
137 |
217 |
4.0 |
Comparative Example |
18 |
F |
0.8 |
190 |
256 |
6.0 |
Present Invention Example |
19 |
F |
1.0 |
118 |
278 |
6.0 |
Present Invention Example |
20 |
F |
0.7 |
134 |
259 |
5.0 |
Comparative Example |
21 |
G |
0.4 |
176 |
219 |
7.0 |
Present Invention Example |
22 |
G |
0.6 |
151 |
247 |
7.0 |
Present Invention Example |
23 |
G |
0.6 |
145 |
300 |
4.0 |
Comparative Example |
24 |
H |
0.3 |
80 |
400 |
6.0 |
Comparative Example |
25 |
I |
2.0 |
111 |
229 |
6.0 |
Comparative Example |
[0131] The underlined value indicates undesirable manufacturing conditions.
[Table 3B]
Test No. |
Kind of steel |
Time until cooling is started s |
Average cooling rate in temperature range of finishing temperature to 300°C °C/s |
Coiling temperature °C |
Sheet thickness mm |
Note |
26 |
J |
0.6 |
249 |
258 |
3.0 |
Present Invention Example |
27 |
K |
0.2 |
145 |
298 |
4.0 |
Present Invention Example |
28 |
L |
0.6 |
173 |
264 |
5.0 |
Present Invention Example |
29 |
M |
1.0 |
159 |
293 |
5.0 |
Present Invention Example |
30 |
N |
0.2 |
138 |
298 |
7.0 |
Present Invention Example |
31 |
O |
0.2 |
208 |
297 |
6.0 |
Present Invention Example |
32 |
P |
0.8 |
146 |
245 |
6.0 |
Present Invention Example |
33 |
Q |
0.1 |
142 |
215 |
3.0 |
Present Invention Example |
34 |
R |
0.7 |
234 |
279 |
3.0 |
Present Invention Example |
35 |
S |
0.5 |
164 |
261 |
3.0 |
Present Invention Example |
36 |
T |
0.7 |
187 |
202 |
4.0 |
Present Invention Example |
37 |
U |
0.6 |
216 |
205 |
5.0 |
Comparative Example |
38 |
V |
0.2 |
160 |
225 |
5.0 |
Comparative Example |
39 |
W |
0.7 |
126 |
237 |
7.0 |
Comparative Example |
40 |
X |
0.8 |
230 |
243 |
6.0 |
Comparative Example |
41 |
C |
0.6 |
260 |
228 |
6.0 |
Present Invention Example |
42 |
A |
0.1 |
151 |
233 |
3.0 |
Present Invention Example |
43 |
B |
0.2 |
217 |
273 |
6.0 |
Comparative Example |
44 |
B |
0.4 |
106 |
283 |
2.0 |
Comparative Example |
45 |
B |
0.7 |
213 |
214 |
4.0 |
Comparative Example |
46 |
C |
0.9 |
218 |
244 |
2.0 |
Comparative Example |
47 |
J |
1.0 |
127 |
239 |
5.0 |
Comparative Example |
48 |
O |
0.6 |
226 |
258 |
3.0 |
Comparative Example |
49 |
B |
0.3 |
194 |
247 |
5.0 |
Comparative Example |
50 |
A |
0.4 |
151 |
295 |
4.0 |
Comparative Example |
51 |
C |
0.9 |
137 |
264 |
4.0 |
Comparative Example |
52 |
A |
0.4 |
109 |
219 |
6.0 |
Comparative Example |
53 |
F |
0.6 |
219 |
206 |
6.0 |
Comparative Example |
54 |
D |
0.8 |
148 |
236 |
5.0 |
Comparative Example |
55 |
E |
1.2 |
197 |
230 |
7.0 |
Comparative Example |
56 |
B |
0.2 |
88 |
220 |
7.0 |
Comparative Example |
57 |
A |
0.0 |
201 |
350 |
4.0 |
Comparative Example |
[0132] The underlined value indicates undesirable manufacturing conditions.
[Table 4A]
Test No. |
Kind of steel |
Microstructure |
Surface layer region |
Internal region |
Tensile strength MPa |
Ductile-brittle transition temperature °C |
Difference between absorbed energy of L-direction notch and absorbed energy of C-direction
notch (L-direction - C-direction) J |
Note |
Martensite area% |
Remainder in microstructure area% |
Average grain size of prior austenite grains µm |
Pole densities of {001}<110>, {111}<110>, and {112}<110> orientation groups- |
Pole density of {110}<112> orientation- |
1 |
A |
92 |
8 |
19.0 |
3.2 |
3.1 |
1,262 |
-70 |
-8 |
Present Invention Example |
2 |
A |
93 |
7 |
10.0 |
3.4 |
3.4 |
1.205 |
-80 |
11 |
Present Invention Example |
3 |
A |
95 |
5 |
12.0 |
2.8 |
3.9 |
1,319 |
-56 |
-13 |
Present Invention Example |
4 |
A |
97 |
3 |
15.0 |
3.7 |
4.2 |
1,397 |
-62 |
10 |
Present Invention Example |
5 |
A |
92 |
8 |
7.0 |
6.6 |
6.0 |
1,351 |
-68 |
21 |
Comparative Example |
6 |
B |
91 |
9 |
16.0 |
2.3 |
4.6 |
1,364 |
-63 |
6 |
Present Invention Example |
7 |
B |
93 |
7 |
7.0 |
2.7 |
4.7 |
1,286 |
-77 |
2 |
Present Invention Example |
8 |
B |
99 |
1 |
8.0 |
1.7 |
43 |
1,227 |
-73 |
-16 |
Comparative Example |
9 |
C |
91 |
9 |
7.0 |
3.2 |
3.8 |
1,237 |
-69 |
1 |
Present Invention Example |
10 |
C |
95 |
5 |
8.0 |
3.4 |
3.1 |
1.287 |
-74 |
-12 |
Present Invention Example |
11 |
c |
97 |
3 |
21.0 |
3.5 |
2.9 |
1,231 |
-70 |
-2 |
Present Invention Example |
12 |
D |
97 |
3 |
21.0 |
3.2 |
4.6 |
1,256 |
-61 |
4 |
Present Invention Example |
13 |
D |
98 |
2 |
10.0 |
2.7 |
4.7 |
1,282 |
-81 |
-8 |
Present Invention Example |
14 |
D |
92 |
8 |
7.0 |
1.6 |
43 |
1,239 |
-54 |
-17 |
Comparative Example |
15 |
E |
100 |
0 |
8.0 |
4.0 |
3.8 |
1.279 |
-71 |
6 |
Present Invention Example |
16 |
E |
98 |
2 |
21.0 |
3.2 |
3.1 |
1,227 |
-51 |
4 |
Present Invention Example |
17 |
E |
97 |
3 |
21.0 |
5.8 |
6.4 |
1.346 |
-79 |
22 |
Comparative Example |
18 |
F |
93 |
7 |
10.0 |
3.2 |
4.6 |
1,393 |
-63 |
-6 |
Present Invention Example |
19 |
F |
99 |
1 |
21.0 |
3.2 |
4.7 |
1,252 |
-58 |
-8 |
Present Invention Example |
20 |
F |
91 |
9 |
21.0 |
7.0 |
6.2 |
1,358 |
-52 |
18 |
Comparative Example |
21 |
G |
95 |
5 |
10.0 |
3.2 |
4.6 |
1,396 |
-77 |
4 |
Present Invention Example |
22 |
G |
97 |
3 |
10.0 |
3.4 |
4.7 |
1,393 |
-76 |
-11 |
Present Invention Example |
23 |
c |
97 |
3 |
38.0 |
1.8 |
4.3 |
1,349 |
42 |
-22 |
Comparative Example |
24 |
H |
81 |
19 |
10.0 |
3.1 |
3.8 |
1,150 |
-80 |
-19 |
Comparative Example |
25 |
I |
93 |
7 |
10.0 |
4.1 |
1.9 |
1020 |
-55 |
18 |
Comparative Example |
[0133] The underlined value indicates outside of the range of the present invention, or
undesirable properties.
[Table 4B]
Test No. |
Kind of steel |
Microstructure |
Surface layer region |
internal region |
Tensile strength MPa |
Ductile-brittle transition temperature °C |
Difference between absorbed energy of L-direction notch and absorbed energy of C-direction
notch (L-direction - C-direction) J |
Note |
Martensite area% |
Remainder in microstructure area% |
Average grain size of prior austenite grains µm |
Pole densities of {001}<110>, {1111}<110>, and {112}<110> orientation groups |
Pole density of {110}<112> orientation |
26 |
J |
92 |
8 |
19.0 |
3.2 |
4.7 |
1,262 |
-70 |
1 |
Present Invention Example |
27 |
K |
93 |
7 |
10.0 |
3.4 |
4.3 |
1.205 |
-80 |
-12 |
Present Invention Example |
28 |
L |
95 |
5 |
12.0 |
2.8 |
3.8 |
1,319 |
-56 |
-2 |
Present Invention Example |
29 |
M |
97 |
3 |
15.0 |
3.7 |
3.1 |
1,397 |
-62 |
4 |
Present Invention Example |
30 |
N |
92 |
8 |
7.0 |
6.6 |
2.9 |
1,351 |
-68 |
-8 |
Present Invention Example |
31 |
O |
91 |
9 |
16.0 |
2.3 |
4.7 |
1.364 |
-63 |
9 |
Present Invention Example |
32 |
P |
93 |
7 |
7.0 |
2.7 |
4.3 |
1,286 |
-77 |
2 |
Present Invention Example |
33 |
Q |
99 |
1 |
8.0 |
3.2 |
3.8 |
1,227 |
-73 |
10 |
Present Invention Example |
34 |
R |
91 |
9 |
7.0 |
3.4 |
3.1 |
1,262 |
-70 |
8 |
Present Invention Example |
35 |
S |
95 |
5 |
8.0 |
2.8 |
2.9 |
1,205 |
-80 |
-2 |
Present Invention Example |
36 |
T |
97 |
3 |
21.0 |
3.7 |
4.6 |
1,262 |
-70 |
6 |
Present Invention Example |
37 |
U |
92 |
8 |
21.0 |
7.8 |
4.7 |
1,262 |
-70 |
29 |
Comparative Example |
38 |
V |
93 |
7 |
10.0 |
2.3 |
4.3 |
1.262 |
-70 |
-16 |
Comparative Example |
39 |
W |
95 |
5 |
7.0 |
2.7 |
3.8 |
1.170 |
-80 |
18 |
Comparative Example |
40 |
X |
97 |
3 |
8.0 |
3.2 |
1.9 |
1,319 |
-56 |
21 |
Comparative Example |
41 |
C |
92 |
8 |
21.0 |
3.4 |
2.0 |
1.397 |
-62 |
-4 |
Present Invention Example |
42 |
A |
91 |
9 |
21.0 |
5.2 |
4.7 |
1,180 |
-68 |
2 |
Present Invention Example |
43 |
B |
88 |
12 |
8.0 |
3.7 |
4.3 |
1,150 |
-63 |
18 |
Comparative Example |
44 |
B |
99 |
1 |
21.0 |
7.3 |
3.8 |
1,286 |
-77 |
16 |
Comparative Example |
45 |
B |
91 |
9 |
21.0 |
2.3 |
1.9 |
1.090 |
-73 |
16 |
Comparative Example |
46 |
C |
95 |
5 |
10.0 |
2.7 |
1.8 |
1,237 |
-69 |
18 |
Comparative Example |
47 |
J |
97 |
3 |
7.0 |
3.2 |
1.9 |
1.287 |
-74 |
16 |
Comparative Example |
48 |
O |
97 |
3 |
8.0 |
8.0 |
4.6 |
1,231 |
-70 |
23 |
Comparative Example |
49 |
B |
98 |
2 |
21.0 |
2.8 |
5.2 |
1,256 |
-61 |
-19 |
Comparative Example |
50 |
A |
92 |
8 |
21.0 |
7.6 |
4.3 |
1,282 |
-81 |
22 |
Comparative Example |
51 |
C |
100 |
0 |
10.0 |
6.6 |
5.2 |
1,239 |
-54 |
-19 |
Comparative Example |
52 |
A |
98 |
2 |
21.0 |
7.3 |
3.1 |
1,279 |
-71 |
12 |
Comparative Example |
53 |
F |
97 |
3 |
21.0 |
3.2 |
5.9 |
1,227 |
-51 |
-18 |
Comparative Example |
54 |
D |
93 |
7 |
10.0 |
7.3 |
4.6 |
1,346 |
-79 |
17 |
Comparative Example |
55 |
E |
99 |
1 |
21.0 |
7.9 |
4.7 |
1,393 |
-63 |
-19 |
Comparative Example |
56 |
B |
69 |
31 |
21.0 |
3.7 |
1.8 |
0.881 |
-58 |
18 |
Comparative Example |
57 |
A |
67 |
33 |
21.0 |
6.6 |
2.9 |
1.100 |
-52 |
-16 |
Comparative Example |
[0134] The underlined value indicates outside of the range of the present invention, or
undesirable properties.
[0135] From Tables 4A and 4B, it can be seen that the hot-rolled steel sheets according
to the present invention examples had high strength, excellent toughness, and reduced
anisotropy in toughness. On the other hand, it can be seen that in the hot-rolled
steel sheets according to the comparative examples, any of the properties deteriorated.