FIELD OF INVENTION
[0001] The present invention relates to a high-hardness steel strip product exhibiting excellent
resistance to climatic corrosion, a good balance of high hardness and excellent mechanical
properties such as impact strength and bendability. The present invention further
relates to a method of manufacturing the high-hardness steel strip product.
BACKGROUND
[0002] High hardness has a direct effect on wear resistance of a steel product, the higher
hardness the better wear resistance. By high hardness it is meant that the Brinell
hardness is at least 450 HBW and especially in the range of 500 HBW to 650 HBW.
[0003] Wear resistant steels are also known as abrasion resistant steels. They are used
in applications in which high resistance against abrasive and shock wear is required.
Such applications can be found in e.g. mining and earth moving industry, and waste
transportation. Wear resistant steels are used for instance in gravel truck's bodies
and excavator buckets, whereby longer service time of the vehicle components are achieved
due to the high hardness provided by the wear resistant steels. The benefits of wear
resistant steels are even more crucial when the paint layer on a machine's outer surface
is frequently exposed to mechanical stresses such as impacts which can cause scratch
to paint layers.
[0004] Such high hardness in steel product is typically obtained by martensitic microstructure
produced by quench hardening steel alloy having high content of carbon (0.41-0.50
wt. %) after austenitization in the furnace. In this process steel plates are first
hot-rolled, slowly cooled to room temperature from the hot-rolling heat, reheated
to austenitization temperature, equalized and finally quench hardened. This process
is hereinafter referred to as the reheating and quenching (RHQ) process. Examples
of steels produced in this way are wear resistant steels disclosed in
CN102199737 or some commercial wear resistant steels. Due to the relatively high content of carbon,
which is required to achieve the desired hardness, the resulting martensite reaction
causes significant internal residual stresses to the steel. This is because the higher
the carbon content the higher the lattice distortion. Therefore, this type of steel
is very brittle and can even crack during the quench hardening. To overcome these
drawbacks related to brittleness, a tempering step after quench hardening is usually
introduced, which however increases the processing efforts and costs.
[0005] Due to the high carbon content these steels have deteriorated impact strength, poor
formability or bendability, and low resistance to stress corrosion cracking (SCC).
Stress corrosion cracking is the cracking induced from the combined influence of tensile
stress and a corrosive environment. Usually, stress corrosion cracking starts as a
pitting corrosion with hard-to-detect fine cracks penetrating into the material while
most of the material surface appears intact. Stress corrosion cracking is classified
as a catastrophic form of corrosion, as the detection of such fine cracks can be very
difficult and the damage not easily predicted. There is a need of better approaches
to decrease the carbon content without compromising the hardness or any of the other
mechanical properties, such as impact strength, formability/bendability or resistance
to stress corrosion cracking.
[0006] CN102392186 and
CN103820717 relate to RHQ steel plates having relatively low carbon content (0.25-0.30 wt. %
in
CN102392186; 0.22-0.29 wt. % in
CN103820717) and also relatively low manganese content. A tempering step after quench hardening
is required for making such RHQ steel plates, which inevitably increases the processing
efforts and costs.
[0007] EP2695960 relates to an abrasion-resistant steel product exhibiting excellent resistance to
stress corrosion cracking, which steel sheet can be made in a process where direct
quenching (DQ) may be performed immediately after hot rolling, without the reheating
treatment after hot rolling as in the RHQ process. The steel sheet of
EP2695960 has a relatively low carbon content (0.20-0.30 wt. %) and a relatively high manganese
content (0.40-1.20 wt. %). In order to increase the resistance to stress corrosion
cracking, the base phase or main phase of the microstructure of the steel product
of
EP2695960 must be made of tempered martensite. On the other hand, the area fraction of untempered
martensite is restricted to 10% or less because the resistance to stress corrosion
cracking is reduced in the presence of untempered martensite. In balancing abrasion
resistance and resistance to stress corrosion cracking, the steel product of
EP2695960 has a surface hardness of 520 HBW or less.
[0008] JP 2009030093 relates to a wear-resistance steel plate excellent in the low-temperature-resistance
tempering and embrittlement crack characteristics, and in which the surface hardness
is more than 400HBW10/3000 with a Brinell hardness.
[0009] AU 2013204206 relates to a wear resistant quenched and tempered steel plate having a hardness of
600 Brinell and a yield stress of at least 1800 MPa and a tensile strength of at least
2000 MPa.
[0010] The present invention extends the utilization of the cost-effective thermomechanically
controlled processing (TMCP) in conjunction with direct quenching (DQ) to produce
a high-hardness steel strip product exhibiting improved resistance to climatic corrosion,
guaranteed impact strength values and excellent formability/bendability.
SUMMARY OF INVENTION
[0011] In view of the state of art, the object of the present invention is to solve the
problem of providing a high-hardness steel strip product exhibiting excellent resistance
to climatic corrosion, guaranteed impact strength values and excellent formability/bendability.
The problem is solved by the combination of specific alloy designs with cost-efficient
TMCP procedures which produces a metallographic microstructure comprising mainly martensite.
[0012] In a first aspect, according to the independent claim 1, the present invention provides
a hot-rolled steel strip product comprising a composition consisting of, in terms
of weight percentages (wt. %):
C |
0.17 - 0.38, preferably 0.21 - 0.35, more preferably 0.22 - 0.28 |
Si |
0.01 - 0.5, more preferably 0.03 - 0.25 |
Mn |
0.1 - 0.4, preferably 0.15 - 0.3 |
Al |
0.015 - 0.15 |
Cu |
0.1 - 0.6, preferably 0.1 - 0.5, more preferably 0.1 - 0.35 |
Ni |
0.2-0.8 |
Cr |
0.1 - 1, preferably 0.3 - 1, more preferably 0.35 - 1, even more preferably 0.35 -
0.8 |
Mo |
0.01 - 0.3, preferably 0.03 - 0.3, more preferably 0.05 - 0.3 |
Nb |
0 - 0.005 |
Ti |
0 - 0.05, preferably 0 - 0.035, more preferably 0 - 0.02 |
V |
0-0.06 |
B |
0.0005 - 0.005, preferably 0.0008 - 0.005 |
P |
0 - 0.025, preferably 0.001 - 0.025, more preferably 0.001 - 0.012 |
S |
0 - 0.008, preferably 0 - 0.005 |
N |
0 - 0.01, preferably 0 - 0.005, more preferably 0 - 0.004 |
Ca |
0.0008 - 0.003 |
remainder Fe and inevitable impurities,
wherein the steel product has
a Brinell hardness in the range of 420 - 580 HBW, and
a corrosion index (ASTM G101-04) of at least 5, and
wherein the steel product has a microstructure consisting of, in terms of volume percentages
(vol. %),
martensite |
≥ 90, |
residual austenite |
0-1, |
remainder bainite, ferrite and/or pearlite, and
wherein, the steel strip product has a thickness of 10 mm or less, and
the amount of Ti is in the range of 0 - 0.005 wt. % when the amount of N is in the
range of 0 - 0.003 wt. %, the amount of Ti is more than 0.005 wt. % and not more than
0.05 wt. % when the amount of N is more than 0.003 wt. % and not more than 0.01 wt.
%, and wherein the steel product has a Charpy-V impact toughness of at least 34 J/cm
2 at a temperature of -20 °C or -40 °C in transversal and/or longitudinal direction.
[0013] Preferably, [Ni] > [Cu]/3, and more preferably [Ni] > [Cu]/2, wherein
[Ni] is the amount of Ni in the composition,
[Cu] is the amount of Cu in the composition.
[0014] The steel product is alloyed with the essential alloying elements Si, Cu, Ni and
Cr, which provides good resistance against climatic corrosion and increases durability
of a paint layer.
[0015] The steel product has a low content of Mn, which is important for improving impact
toughness and bendability.
[0016] The Ca/S ratio is adjusted such that CaS cannot form thereby improving impact toughness
and bendability. The Ca/S ratio is preferably in the range of 1 - 2, more preferably
1.1 - 1.7, and even more preferably 1.2 - 1.6.
[0017] The level of Nb should be restricted to the lowest possible to increase formability
or bendability of the steel product. Elements such as Nb may be present as residual
contents that are not purposefully added.
[0018] The difference between residual contents and unavoidable impurities is that residual
contents are controlled quantities of alloying elements, which are not considered
to be impurities. A residual content as normally controlled by an industrial process
does not have an essential effect upon the alloy.
[0019] In a second aspect, according to method claim 11, the present invention provides
a method for manufacturing hot-rolled steel strip product comprising the following
steps of
- providing a steel slab consisting of the chemical composition of the claims 1 to 3;
- heating the steel slab to the austenitizing temperature of 1200 - 1350 °C;
- equalizing the temperature for 30 to 150 minutes;
- hot-rolling to the desired thickness at a temperature in the range of Ar3 to 1300 °C, wherein the finish rolling temperature is in the range of 800 °C to 960
°C, preferably 870°C - 930°C, more preferably 885°C - 930°C; and
- direct quenching the hot-rolled steel strip product to a cooling end and coiling temperature
of 450 °C or less, preferably 250 °C or less, more preferably 150 °C or less, and
even more preferably 100 °C or less.
[0020] Optionally, a step of temper annealing is performed on the direct quenched and coiled
strip product at a temperature in the range of 150 °C - 250 °C. However, the step
of temper annealing is not required according to the present invention.
[0021] The steel product is a steel strip having a thickness of 10 mm or less, preferably
8 mm or less, and more preferably 7 mm or less.
[0022] The obtained steel product has a microstructure comprising, in terms of volume percentages
(vol. %), at least 90 vol. % martensite, preferably at least 95 vol. % martensite,
and more preferably at least 98 vol. % martensite, measured from ¼ thickness of the
steel strip product. The martensitic structure may be untempered, autotempered and/or
tempered. Preferably, the martensitic structure is not tempered. More preferably,
the aforementioned microstructure comprises more than 10 vol. % untempered martensite.
Preferably, the microstructure comprises 0 - 1 vol. % residual austenite, and more
preferably 0 - 0.5 vol. % residual austenite. Typically, the microstructure also comprises
bainite, ferrite and/or pearlite.
[0023] The obtained steel product has a prior austenite grain size of 50 µm or less, preferably
30 µm or less, more preferably 20 µm or less, measured from ¼ thickness of the steel
strip product.
[0024] The aspect ratio of a prior austenite grain structure is one of the factors affecting
a steel product's impact toughness and bendability. In order to improve impact toughness,
the prior austenite grain structure should have an aspect ratio of at least 1.5, preferably
at least 2, and more preferably at least 3. In order to improve bendability, the prior
austenite grain structure should have an aspect ratio of 7 or less, preferably 5 or
less, and more preferably 1.5 or less. The obtained steel product according to the
present invention has a prior austenite grain structure with an aspect ratio in the
range of 1.5 - 7, preferably 1.5 - 5, and more preferably 2-5, which ensures that
a good balance of excellent impact toughness and excellent bendability can be achieved.
[0025] The obtained steel product has a good balance of hardness and other mechanical properties
such as improved resistance to climatic corrosion and excellent impact strength. The
steel product has at least one of the following mechanical properties:
a Brinell hardness in the range of 420 - 580 HBW, preferably 450 - 550 HBW, and more
preferably 470 - 530 HBW;
a corrosion index (ASTM G101-04) ≥ 5, preferably ≥ 5.5, and more preferably ≥ 6;
a Charpy-V impact toughness of at least 34 J/cm2 at a temperature of -20 °C or -40 °C.
[0026] The steel product exhibits excellent bendability or formability. The steel product
has a minimum bending radius of 3.4 t or less in a measurement direction longitudinal
to the rolling direction wherein the bending axis is longitudinal to rolling direction;
a minimum bending radius of 2.7 t or less in a measurement direction transversal to
the rolling direction wherein the bending axis is transversal to rolling direction;
and wherein t is the thickness of the steel strip product.
[0027] The steel product has a good balance of high hardness and excellent mechanical properties
such as impact strength and formability/bendability. Consequently, the steel product
exhibits excellent resistance to climatic corrosion.
BRIEF DESCRIPTION OF DRAWINGS
[0028]
- Figure 1
- illustrates the microstructures.
DETAILED DESCRIPTION OF THE INVENTION
[0029] The term "steel" is defined as an iron alloy containing carbon (C).
[0030] The term climatic corrosion (a.k.a. atmospheric corrosion) refers to outdoor corrosion
caused by local environmental conditions. Environmental conditions are formed from
weather phenomena like rain and sunshine. They are also affected by different impurities
in the air like chlorides from sea water and sulfur compounds coming from volcanic
activity and industry or mining.
[0031] The term "Brinell hardness (HBW)" is a designation of hardness of steel. The Brinell
hardness test is performed by pressing a 10 mm spherical tungsten carbide ball against
a clean prepared surface using a 3000 kilogram force, producing an impression, measured
and given a special numerical value.
[0032] The term "corrosion index (ASTM G101-04)" refers to the American Society for Testing
and Materials (ASTM) standard G101 which is currently the only available guide to
quantify the atmospheric corrosion resistance of weathering steels as a function of
their composition.
[0033] The term "accelerated continuous cooling (ACC)" refers to a process of accelerated
cooling at a cooling rate down to a temperature without interruption.
[0034] The term "ultimate tensile strength (UTS, Rm)" refers to the limit, at which the
steel fractures under tension, thus the maximum tensile stress.
[0035] The term "yield strength (YS, Rp
0.2)" refers to 0.2 % offset yield strength defined as the amount of stress that will
result in a plastic strain of 0.2 %.
[0036] The term "total elongation (TEL)" refers to the percentage by which the material
can be stretched before it breaks; a rough indicator of formability, usually expressed
as a percentage over a fixed gauge length of the measuring extensometer. Two common
gauge lengths are 50 mm (A
50) and 80 mm (A
80).
[0037] The term "minimum bending radius (Ri)" is used to refer to the minimum radius of
bending that can be applied to a test sheet without occurrence of cracks.
[0038] The term "bendability" refers to the ratio of Ri and the sheet thickness (t).
[0039] The alloying content of steel together with the processing parameters determines
the microstructure which in turn determines the mechanical properties of the steel.
[0040] Alloy design is one of the first issues to be considered when developing a steel
product with targeted mechanical properties. Next the chemical composition according
to the present invention is described in more details, wherein % of each component
refers to weight percentage.
[0041] Carbon C is used in the range of 0.17 % to 0.38 %.
[0042] C alloying increases strength of steel by solid solution strengthening, and hence
C content determines the strength level. C is used in the range of 0.17 % to 0.38%
depending on targeted hardness. If the carbon content is less than 0.17%, it is difficult
to achieve a Brinell hardness of more than 420 HBW. However, C has detrimental effects
on weldability, impact toughness, formability or bendability, and resistance to stress
corrosion cracking. Therefore, C content is set to not more than 0.38 %.
[0043] Preferably, C is used in the range of 0.21 % to 0.35 %, and more preferably 0.22
% to 0.28 %.
[0044] Silicon Si is used in an amount of 0.01 to 0.5 %.
[0045] Si is added to the composition to facilitate formation of a protective oxide layer
under corrosive climate conditions, which provides good resistance against climatic
corrosion and increases the durability of a paint layer that is easily damaged or
removed from machines surfaces due to wear. Si is effective as a deoxidizing or killing
agent that can remove oxygen from the melt during a steelmaking process. Si alloying
enhances strength by solid solution strengthening, and enhances hardness by increasing
austenite hardenability. Also the presence of Si can stabilize residual austenite.
However, silicon content of higher than 0.5 % may unnecessarily increase carbon equivalent
(CE) value thereby weakening the weldability. Furthermore, surface quality may be
deteriorated if Si is present in excess.
[0046] As previously mentioned, Si is an important alloying element for providing sufficient
hardness and good resistance to climatic corrosion, and for increasing durability
of a paint layer. Si is used in the range of 0.01 % to 0.5 %, and more preferably
0.03 % to 0.25 %.
[0047] Manganese Mn is used in the range of 0.1 % to 0.4 %.
[0048] Mn alloying lowers martensite start temperature (Ms) and martensite finish temperature
(Mf), which can suppress autotempering of martensite during quenching. Reduced autotempering
of martensite leads to higher internal stresses that enhance the risk for quench-induced
cracking or distortion of shape. Although a lower degree of autotempered martensitic
microstructures is beneficial to higher hardness, its negative effects on impact strength
should not be underestimated.
[0049] Mn alloying also enhances strength by solid solution strengthening, and enhances
hardness by increasing austenite hardenability. However, if the Mn content is too
high, hardenability of the steel will increase at the expense of impact toughness.
Excessive Mn alloying may also lead to C-Mn segregation and formation of MnS, which
could induce formation of initiation sites for pitting corrosion and stress corrosion
cracking.
[0050] Thus, Mn is used in an amount of at least 0.1 % to ensure hardenability, but not
more than 0.4 % to avoid the harmful effects as described above and to ensure excellent
mechanical properties such as impact strength and bendability. Preferably, a low level
of Mn is used in the range of 0.15 % to 0.3 %.
[0051] Aluminum Al is used in the range of 0.015 % to 0.15 %.
[0052] Al is effective as a deoxidizing or killing agent that can remove oxygen from the
melt during a steelmaking process. Al also removes N by forming stable AIN particles
and provides grain refinement, which is beneficial to high toughness, especially at
low temperatures. Also Al stabilizes residual austenite. However, an excess of Al
may increase non-metallic inclusions thereby deteriorating cleanliness.
[0053] Copper Cu is used in the range of 0.1 % to 0.6 %.
[0054] Cu is added to the composition to facilitate formation of a protective oxide layer
under corrosive climate conditions, which provides good resistance against climatic
corrosion and increases the durability of a paint layer that is easily damaged or
removed from machines surfaces due to wear. Cu may promote formation of low carbon
bainitic structures, cause solid solution strengthening and contribute to precipitation
strengthening. Cu may also have beneficial effects of inhibiting stress corrosion
cracking. When added in excessive amounts, Cu deteriorates field weldability and the
heat affected zone (HAZ) toughness. Therefore, the upper limit of Cu is set to 0.6%.
[0055] As previously mentioned, Cu is an important alloying element for providing sufficient
hardness and good resistance to climatic corrosion, and for increasing durability
of a paint layer. Preferably, Cu is used in the range of 0.1 % to 0.5 %, and more
preferably 0.1 % to 0.35 %.
[0056] Nickel Ni is used in in an amount of 0.2- to 0.8 %.
[0057] Ni is used to avoid quench induced cracking and also to improve low temperature toughness.
Ni is an alloying element that improves austenite hardenability thereby increasing
strength with no or marginal loss of impact toughness and/or HAZ toughness. Ni also
improves surface quality thereby preventing pitting corrosion,
i.e. initiation site for stress corrosion cracking. Ni is added to the composition to
facilitate formation of a protective oxide layer under corrosive climate conditions,
which provides good resistance against climatic corrosion and increases the durability
of a paint layer that is easily damaged or removed from machines surfaces due to wear.
However, nickel contents of above 0.8 % would increase alloying costs too much without
significant technical improvement. An excess of Ni may produce high viscosity iron
oxide scales which deteriorate surface quality of the steel product. Higher Ni contents
also have negative impacts on weldability due to increased CE value and cracking sensitivity
coefficient.
[0058] As previously mentioned, Ni is an important alloying element for providing sufficient
hardness and good resistance to climatic corrosion with no or marginal loss of impact
toughness, and for increasing durability of a paint layer. Ni is used in the range
of 0.2 % to 0.8 %.
[0059] Chromium Cr is used in the range of 0.1 % to 1 %.
[0060] Cr is added to the composition to facilitate formation of a protective oxide layer
under corrosive climate conditions, which provides good resistance against climatic
corrosion and increases the durability of a paint layer that is easily damaged or
removed from machines surfaces due to wear. Cr alloying provides better resistance
against pitting corrosion thereby preventing stress corrosion cracking at an early
stage. As mid-strength carbide forming element Cr increases the strength of both the
base steel and weld with marginal expense of impact toughness. Cr alloying also enhances
strength and hardness by increasing austenite hardenability. However, if Cr is used
in an amount above 1 % the HAZ toughness as well as field weldability may be adversely
affected.
[0061] As previously mentioned, Cr is an important alloying element for providing sufficient
hardness and good resistance to climatic corrosion with no or marginal loss of impact
toughness, and for increasing durability of a paint layer. Preferably, Cr is used
in the range of 0.3 % to 1 %, more preferably 0.35 % to 1 %, and even more preferably
0.35 % to 0.8 %.
[0062] Molybdenum Mo is used in the range of 0.01 % to 0.3 %.
[0063] Mo alloying improves impact strength, low-temperature toughness and tempering resistance.
The presence of Mo enhances strength and hardness by increasing austenite hardenability.
Mo can be added to the composition to provide hardenability in place of Mn. In the
case of B alloying, Mo is usually required to ensure the effectiveness of B. However,
Mo is not an economically acceptable alloying element. If Mo is used in an amount
of above 0.3 % toughness may be deteriorated thereby increasing the risk of brittleness.
An excessive amount of Mo may also reduce the effect of B. Furthermore, the inventors
have noticed that Mo alloying retards recrystallization of austenite thereby increasing
the aspect ratio of a prior austenite grain structure. Therefore, the level of Mo
content should be carefully controlled to prevent excessive elongation of the prior
austenite grains which may deteriorate bendability of the steel product.
[0064] Preferably, Mo is used in the range of 0.03 % to 0.3 %, and more preferably 0.05
% to 0.3 %.
[0065] Niobium Nb is used in an amount of 0.005 % or less.
[0066] Nb forms carbides NbC and carbonitrides Nb(C,N). Nb is considered to be the major
grain refining element. Nb contributes to strengthening and toughening of steels.
Yet, Nb addition should be limited to 0.005 % since an excess of Nb deteriorates bendability,
in particular when direct quenching is applied and/or when Mo is present in the composition.
Furthermore, Nb can be harmful for HAZ toughness since Nb may promote the formation
of coarse upper bainite structure by forming relatively unstable TiNbN or TiNb(C,N)
precipitates. The level of Nb should be restricted to the lowest possible to increase
formability or bendability of the steel product.
[0067] Titanium Ti is used in an amount of 0.05 % or less.
[0068] TiC precipitates are able to deeply trap a significant amount of hydrogen H, which
decreases the H diffusivity in the materials and removes some of the detrimental H
from the microstructure to prevent stress corrosion cracking. Ti is also added to
bind free N that is harmful to toughness by forming stable TiN that together with
NbC can efficiently prevent austenite grain growth in the reheating stage at high
temperatures. TiN precipitates can further prevent grain coarsening in the HAZ during
welding thereby improving toughness. TiN formation suppresses BN precipitation, thereby
leaving B free to make its contribution to hardenability. For this purpose, the ratio
of Ti/N is at least 3.4. However, if Ti content is too high, coarsening of TiN and
precipitation hardening due to TiC develop and the low-temperature toughness may be
deteriorated. Therefore, it is necessary to restrict titanium so that it is less than
0.05%.
[0069] Preferably, Ti is used in an amount of 0.035 % or less, and more preferably 0.02
% or less. If the steel product has a low nitrogen content of 0.003 % or less, it
is unnecessary to add Ti to ensure the boron hardenability effect, and the Ti content
can be as low as 0.005 % or less. If the nitrogen content is more than 0.003 % but
no more than 0.01 %, the Ti content can be more than 0.005 % but no more than 0.05%.
[0070] Vanadium V is used in an amount of 0 to 0.06 %.
[0071] V has substantially the same but smaller effects as Nb. V
4C
3 precipitates are able to deeply trap a significant amount of hydrogen H, which decreases
the H diffusivity in the materials and removes some of the detrimental H from the
microstructure to prevent HIC. V is a strong carbide and nitride former, but V(C,N)
can also form and its solubility in austenite is higher than that of Nb or Ti. Thus,
V alloying has potential for dispersion and precipitation strengthening, because large
quantities of V are dissolved and available for precipitation in ferrite. However,
an addition of more than 0.2 % V has negative effects on weldability and hardenability.
[0072] V is used in an amount of 0.06 % or less.
[0073] Boron B is used in the range of 0.0005 % to 0.005 %.
[0074] B is a well-established microalloying element to increase hardenability. The most
effective B alloying would preferably require the presence of Ti in an amount of at
least 3.42 N to prevent formation of BN. In the presence of an amount of 0.003 % or
less nitrogen, the Ti content can be lowered to 0.005 % or less, which is beneficial
to low-temperature toughness. Hardenability deteriorates if the B content exceeds
0.005 %.
[0075] Preferably, B is used in the range of 0.0008 % to 0.005 %.
[0076] Calcium Ca is used in an amount of 0.0008 to 0.003 %.
[0077] Ca addition during a steelmaking process is for refining, deoxidation, desulphurization,
and control of shape, size and distribution of oxide and sulphide inclusions. Ca is
usually added to improve subsequent coating. However, an excessive amount of Ca should
be avoided to achieve clean steel thereby preventing the formation of calcium sulfide
(CaS) or calcium oxide (CaO) or mixture of these (CaOS) that may deteriorate the mechanical
properties such as bendability and SCC resistance.
[0078] Ca is used in an amount of 0.0008 % to 0.003 % to ensure excellent mechanical properties
such as impact strength and bendability.
[0079] The Ca/S ratio is adjusted such that CaS cannot form thereby improving impact toughness
and bendability. The inventors have noticed that, in general, during the steelmaking
process the optimal Ca/S ratio is in the range of 1 - 2, preferably 1.1 - 1.7, and
more preferably 1.2 - 1.6 for clean steel.
[0080] Unavoidable impurities can be phosphor P, sulfur S, nitrogen N. Their content in
terms of weight percentages (wt. %) is preferably defined as follows:
P |
0 - 0.025, preferably 0.001 - 0.025, more preferably 0.001 - 0.012 |
S |
0 - 0.008, preferably 0 - 0.005, more preferably 0 - 0.002 |
N |
0 - 0.01, preferably 0 - 0.005, more preferably 0 - 0.004 |
[0081] Other inevitable impurities may be hydrogen H, oxygen O and rare earth metals (REM)
or the like. Their contents are limited in order to ensure excellent mechanical properties,
such as impact toughness.
[0082] The steel product with the targeted mechanical properties is produced in a process
that determines a specific microstructure which in turn dictates the mechanical properties
of the steel product.
[0083] The first step is to provide a steel slab by means of, for instance a process of
continuous casting, also known as strand casting.
[0084] In the reheating stage, the steel slab is heated to the austenitizing temperature
of 1200 - 1350 °C, and thereafter subjected to a temperature equalizing step that
may take 30 to 150 minutes. The reheating and equalizing steps are important for controlling
the austenite grain growth. An increase in the heating temperature can cause dissolution
and coarsening of alloy precipitates, which may result in abnormal grain growth.
[0085] The final steel product has a prior austenite grain size of 50 µm or less, preferably
30 µm or less, more preferably 20 µm or less, measured from ¼ thickness of the steel
strip product.
[0086] In the hot rolling stage the slab is hot rolled to the desired thickness at a temperature
in the range of Ar3 to 1300°C, wherein the finish rolling temperature (FRT) is in
the range of 800 °C to 960 °C, preferably 870°C - 930°C, more preferably 885°C - 930°C.
[0087] The aspect ratio of a prior austenite grain structure is one of the factors affecting
a steel product's impact toughness and bendability. In order to improve impact toughness,
the prior austenite grain structure should have an aspect ratio of at least 1.5, preferably
at least 2, and more preferably at least 3. In order to improve bendability, the prior
austenite grain structure should have an aspect ratio of 7 or less, preferably 5 or
less, and more preferably 1.5 or less. A desired aspect ratio of prior austenite grains
can be achieved by adjusting a number of parameters such as finish rolling temperature,
strain/deformation, strain rate, and/or alloying with the elements such as Mo that
retard recrystallization of austenite.
[0088] The obtained steel product according to the present invention has a prior austenite
grain structure with an aspect ratio in the range of 1.5 - 7, preferably 1.5 - 5,
and more preferably 2-5, which ensures that a good balance of excellent impact toughness
and excellent bendability can be achieved.
[0089] The obtained steel strip product has a thickness of 10 mm or less, preferably 8 mm
or less, more preferably 7 mm or less.
[0090] The hot-rolled steel strip product is direct quenched to a cooling end and coiling
temperature of 450 °C or less, preferably 250 °C or less, more preferably 150 °C or
less, and even more preferably 100 °C or less. The cooling rate is at least 30 °C/s.
[0091] The direct quenched steel strip product is coiled at temperature of 450 °C or less,
preferably 250 °C or less, more preferably 150 °C or less, and even more preferably
100 °C or less.
[0092] The obtained steel strip product has a microstructure comprising, in terms of volume
percentages (vol. %), at least 90 vol. % martensite, preferably at least 95 vol. %
martensite, and more preferably at least 98 vol. % martensite, measured from ¼ thickness
of the steel strip product. The martensitic structure may be untempered, autotempered
and/or tempered. Preferably, the martensitic structure is not tempered. More preferably,
the aforementioned microstructure comprises more than 10 vol. % untempered martensite.
Preferably, the microstructure comprises 0 - 1 vol. % residual austenite, and more
preferably 0 - 0.5 vol. % residual austenite. Typically, the microstructure also comprises
bainite, ferrite and/or pearlite.
[0093] Optionally, an extra step of temper annealing is performed at a temperature in the
range of 150 °C - 250 °C.
[0094] The steel strip product has a good balance of hardness and other mechanical properties
such as excellent impact strength, improved resistance to climatic corrosion and excellent
formability/bendability.
[0095] The steel strip product has a high Brinell hardness in the range of 420 - 580 HBW,
preferably 450 - 550 HBW, and more preferably 470 - 530 HBW.
[0096] The steel strip product has a corrosion index (ASTM G101-04) of at least 5, preferably
at least 5.5, and more preferably at least 6, which indicates improved resistance
against climatic corrosion. The durability of a paint layer is increased and the repainting
interval can be 1.5 - 2 times longer by using the steel product of the invention.
[0097] The corrosion index (ASTM G101-04) is used for estimating long term atmospheric corrosion
of low alloy steels in various environments. The corrosion index (ASTM G101-04) equation
is formed with a statistical method from long term outdoor corrosion exposure tests,
which equation is represented as follows.

[0098] The steel strip product with high hardness has a Charpy-V impact toughness of at
least 34 J/cm
2 at a temperature of -20 °C or -40 °C thereby fulfilling the conventional impact strength
requirements.
[0099] The steel strip product exhibits excellent bendability or formability. The steel
product has a minimum bending radius of 3.4 t or less in a measurement direction longitudinal
to the rolling direction wherein the bending axis is longitudinal to rolling direction;
a minimum bending radius of 2.7 t or less in a measurement direction transversal to
the rolling direction wherein the bending axis is transversal to rolling direction;
and wherein t is the thickness of the steel strip product.
[0100] The following examples further describe and demonstrate embodiments within the scope
of the present invention. The examples are given solely for the purpose of illustration
and are not to be construed as limitations of the present invention, as many variations
thereof are possible without departing from the scope of the invention.
[0101] The chemical compositions used for producing the tested steel strip products are
presented in Table 1.
[0102] The manufacturing conditions for producing the tested steel strip products are presented
in Table 2.
[0103] The mechanical properties of the tested steel strip products are presented in Table
3.
Microstructure
[0104] Microstructure can be characterized from SEM micrographs and the volume fraction
can be determined using point counting or image analysis method. The microstructures
of the tested inventive examples no. 1 - 4 all have a main phase of at least 90 vol.
% martensite. Figure 1 is an SEM image on the RD-ND plane from ¼ thickness of the
steel strip no. 1, where the prior austenite grain boundaries are visualized. The
prior austenite grain structure of the steel strip no. 1 has an aspect ratio of 3.4.
Brinell hardness HBW
[0105] The Brinell hardness test is performed by pressing a 10 mm spherical tungsten carbide
ball against a clean prepared surface using a 3000 kilogram force, producing an impression,
measured and given a special numerical value. The measurement is done perpendicular
to the upper surface of the steel sheet at 10 - 15 % depth from the steel surface.
As shown in Table 3, each one of the inventive examples no. 1 - 4 exhibits a Brinell
harness in the range of 475 - 491 HBW. The comparative example no. 5 exhibits a Brinell
harness of 486 HBW while the comparative example no. 6 exhibits a Brinell harness
of 469 HBW.
Corrosion index (ASTM G101-04)
[0106] The corrosion index (ASTM G101-04) is calculated based on the American Society for
Testing and Materials (ASTM) standard G101. As shown in Table 3, each one of the inventive
examples no. 1 - 4 has a corrosion index (ASTM G101-04) of at least 5.28. On the other
hand, the comparative examples no. 5 and 6 have a much lower corrosion index (ASTM
G101-04) of 3.4 and 1.04 respectively.
Charpy-V impact toughness
[0107] The impact toughness values at -20 °C or -40 °C were obtained by Charpy V-notch tests
according to the ASME (American Society of Mechanical Engineers) Standards. The inventive
examples no. 1 and 2 have a Charpy-V impact toughness of 63 J/cm
2 and 45 J/cm
2 respectively at a temperature of -20 °C (Table 3). Each one of the inventive examples
no. 1 - 4 has a Charpy-V impact toughness in the range of 38 - 120 J/cm
2 at a temperature of -40 °C if the measurement direction is longitudinal to the rolling
direction. Each one of the inventive examples no. 1 - 4 has a Charpy-V impact toughness
in the range of 58 - 105 J/cm
2 at a temperature of -40 °C if the measurement direction is transversal to the rolling
direction. The impact toughness of the inventive examples no. 1 - 4 is improved compared
to the comparative example no. 6. The comparative example no. 5 has a better Charpy-V
impact toughness values than the inventive examples no. 1 and 2 at the expense of
bendability.
Elongation
[0108] Elongation was determined according ASTM E8 standard using transverse specimens of
a produced batch of 2000 ton of plates. The mean value of total elongation (A
50) of the inventive examples no. 1 and 2 is 11.6 and 11.3 respectively (Table 3), which
is better than the comparative examples no. 5 and 6 having a mean A
50 value of 10.1 and 9.1 respectively. The comparative examples no. 5 and 6 have better
A
50 values than the inventive examples no. 3 and 4 at the expense of Charpy-V impact
toughness.
Bendability
[0109] The bend test consists of subjecting a test piece to plastic deformation by three-point
bending, with one single stroke, until a specified angle 90° of the bend is reached
after unloading. The inspection and assessment of the bends is a continuous process
during the whole test series. This is to be able to decide if the punch radius (R)
should be increased, maintained or decreased. The limit of bendability (R/t) for a
material can be identified in a test series if a minimum of 3 m bending length, without
any defects, is fulfilled with the same punch radius (R) both longitudinally and transversally.
Cracks, surface necking marks and flat bends (significant necking) are registered
as defects.
[0110] According to the bend tests, each one of the inventive examples no. 1 - 4 has a minimum
bending radius of 3.3 t or less in a measurement direction longitudinal to the rolling
direction; a minimum bending radius of 2.6 t or less in a measurement direction transversal
to the rolling direction; and wherein t is the thickness of the steel strip product
(Table 3). The comparative example no. 5 exhibits lower bendability with a minimum
bending radius of 3.7 t in a measurement direction longitudinal to the rolling direction
and a minimum bending radius of 2.2 t in a measurement direction transversal to the
rolling direction.
Yield strength
[0111] Yield strength was determined according ASTM E8 standard using transverse specimens
of a produced batch of 2000 ton of plates. Each one of the inventive examples no.
1 - 4 has a mean value of yield strength (Rp
0.2) in the range of 1302 MPa to 1399 MPa, measured in the longitudinal direction (Table
3). The comparative examples no. 5 and 6 have a mean value of yield strength (Rp
0.2) of 1262 MPa and 1338 MPa respectively, measured in the longitudinal direction (Table
3).
Tensile strength
[0112] Tensile strength was determined according ASTM E8 standard using transverse specimens
of a produced batch of 2000 ton of plates. Each one of the inventive examples no.
1 - 4 has a mean value of ultimate tensile strength (Rm) in the range of 1509 MPa
to 1566 MPa, measured in the longitudinal direction (Table 3). The comparative examples
no. 5 and 6 have a mean value of ultimate tensile strength (Rm) of 1550 MPa and 1552
MPa respectively, measured in the longitudinal direction (Table 3).
Table 1. Chemical compositions (wt. %).
Steel type |
C |
Si |
Mn |
P |
S |
Al |
Cu |
Ni |
Cr |
Mo |
Nb |
Ti |
V |
B |
Ca (ppm) |
N (ppm) |
Remarks |
A |
0,251 |
0,098 |
0,246 |
0,008 |
0,0016 |
0,094 |
0,30 |
0,493 |
0,718 |
0,098 |
0 |
0,016 |
0,04 |
0,0018 |
23 |
39 |
Inventive example |
B |
0,23 |
0,179 |
0,200 |
0,007 |
-0,0006 |
0,051 |
0,16 |
0,51 |
0,39 |
0,05 |
0,001 |
0,002 |
0,01 |
0,0011 |
8 |
24 |
Inventive example |
C |
0,233 |
0,179 |
0,714 |
0,009 |
0,0006 |
0,035 |
0,009 |
0,506 |
0,713 |
0,067 |
0 |
0,017 |
0,008 |
0,0017 |
21 |
31 |
Comparative example |
D |
0,262 |
0,175 |
1,19 |
0,008 |
0,0002 |
0,048 |
0,01 |
0,035 |
0,212 |
0,005 |
0 |
0,015 |
0,008 |
0,0014 |
30 |
21 |
Comparative example |
Table 2. Manufacturing conditions
Steel strip no. |
Steel type |
Strip thickness (mm) |
Hot rolling |
Cooling rate (°C/s) |
Coiling temperature (°C) |
Temper annealing |
Remarks |
Heating temperature (°C) |
FRT (°C) |
Heating temperature (°C) |
Holding time (h) |
1 |
A |
6 |
1280 |
895 |
70 |
50 |
- |
- |
Inventive example |
2 |
A |
6 |
1280 |
925 |
70 |
50 |
- |
- |
Inventive example |
3 |
B |
6 |
1280 |
900 |
- |
50 |
200 |
8 |
Inventive example |
4 |
B |
3 |
1280 |
905 |
- |
50 |
200 |
8 |
Inventive example |
5 |
C |
6 |
1280 |
870 |
55 |
50 |
- |
- |
Comparative example |
6 |
D |
6 |
1280 |
915 |
55 |
50 |
- |
- |
Comparative example |
Table 3. Mechanical properties
Steel strip no. |
Steel type |
Corr. Index |
HBW |
Rp0.2 (L) (MPa) |
Rm (L) (MPa) |
A50 |
ChV (-20) T (J/cm2) |
ChV (-40) L (J/cm2) |
ChV (-40) T (J/cm2) |
Bending r/t |
Remarks |
longit. |
transv. |
1 |
A |
6.74 |
487 |
1399 |
1566 |
11,6 |
63 |
63 |
80 |
3,3 |
2,0 |
Inventive example |
2 |
A |
6.74 |
491 |
1337 |
1529 |
11,3 |
45 |
38 |
58 |
3,0 |
2,0 |
Inventive example |
3 |
B |
5.28 |
475 |
1355 |
1509 |
6,9 |
- |
120 |
83 |
2,3 |
1,3 |
Inventive example |
4 |
B |
5.28 |
487 |
1302 |
1549 |
8,8 |
- |
120 |
105 |
2,6 |
2,6 |
Inventive example |
5 |
C |
3.40 |
486 |
1262 |
1550 |
9,4 |
73 |
68 |
83 |
3,7 |
2,2 |
Comparative example |
6 |
D |
1.04 |
469 |
1338 |
1552 |
10,0 |
32 |
30 |
42 |
- |
- |
Comparative example |
1. A hot-rolled steel strip product comprising a composition consisting of, in terms
of weight percentages (wt. %):
C |
0.17 - 0.38, preferably 0.21 - 0.35, more preferably 0.22 - 0.28 |
Si |
0.01 - 0.5, more preferably 0.03 - 0.25 |
Mn |
0.1 - 0.4, preferably 0.15 - 0.3 |
Al |
0.015 - 0.15 |
Cu |
0.1 - 0.6, preferably 0.1 - 0.5, more preferably 0.1 - 0.35 |
Ni |
0.2-0.8 |
Cr |
0.1 - 1, preferably 0.3 - 1, more preferably 0.35 - 1, even more preferably 0.35 -
0.8 |
Mo |
0.01 - 0.3, preferably 0.03 - 0.3, more preferably 0.05 - 0.3 |
Nb |
0 - 0.005 |
Ti |
0 - 0.05, preferably 0 - 0.035, more preferably 0 - 0.02 |
V |
0-0.06 |
B |
0.0005 - 0.005, preferably 0.0008 - 0.005 |
P |
0 - 0.025, preferably 0.001 - 0.025, more preferably 0.001 - 0.012 |
S |
0 - 0.008, preferably 0 - 0.005, more preferably 0 - 0.002 |
N |
0 - 0.01, preferably 0 - 0.005, more preferably 0 - 0.004 |
Ca |
0.0008 - 0.003 |
remainder Fe and inevitable impurities, wherein the steel product has a Brinell hardness
in the range of 420 - 580 HBW, and
a corrosion index (ASTM G101-04) of at least 5, and
wherein the steel product has a microstructure consisting of, in terms of volume percentages
(vol. %),
martensite |
≥ 90, |
residual austenite |
0-1, |
remainder bainite, ferrite and/or pearlite, and
wherein, the steel strip product has a thickness of 10 mm or less, and
the amount of Ti is in the range of 0 - 0.005 wt. % when the amount of N is in the
range of 0 - 0.003 wt. %, the amount of Ti is more than 0.005 wt. % and not more than
0.05 wt. % when the amount of N is more than 0.003 wt. % and not more than 0.01 wt.
%, and wherein the steel product has a Charpy-V impact toughness of at least 34 J/cm2 at a temperature of - 20 °C or -40 °C in transversal and/or longitudinal direction.
2. The steel product according to claim 1 wherein
[Ni] > [Cu]/3, preferably [Ni] > [Cu]/2, and wherein
[Ni] is the amount of Ni in the composition,
[Cu] is the amount of Cu in the composition.
3. The steel product according to any one of the preceding claims, wherein the Ca/S ratio
is in the range of 1 - 2, preferably 1.1 - 1.7, and more preferably 1.2 - 1.6.
4. The steel product according to any one of the preceding claims, wherein the steel
product has a Brinell hardness in the range of 450 - 550 HBW, preferably 470 - 530
HBW.
5. The steel product according to any one of the preceding claims, wherein the steel
product has a corrosion index (ASTM G101-04) of at least 5.5, preferably at least
6.
6. The steel product according to any one of the preceding claims, wherein the steel
product has a minimum bending radius of 3.4 t or less in a measurement direction longitudinal
to the rolling direction; a minimum bending radius of 2.7 t or less in a measurement
direction transversal to the rolling direction; and wherein t is the thickness of
the steel strip product.
7. The steel product according to any one of the preceding claims, wherein the steel
product has a microstructure consisting of, in terms of volume percentages (vol. %),
martensite |
preferably ≥ 95, more preferably ≥ 98 |
residual austenite |
preferably 0 - 0.5, |
remainder bainite, ferrite and/or pearlite.
8. The steel product according to any one of the preceding claims, wherein the steel
product has a prior austenite grain size of 50 µm or less, preferably 30 µm or less,
more preferably 20 µm or less.
9. The steel product according to any one of the preceding claims, wherein the steel
product has a prior austenite grain structure with an aspect ratio in the range of
1.5 - 7, preferably 1.5 - 5, more preferably 2-5.
10. The steel product according to any one of the preceding claims, preferably 8 mm or
less, and more preferably 7 mm or less.
11. A method for manufacturing the steel product according to any one of the preceding
claims comprising the following steps of
- providing a steel slab consisting of the chemical composition according to any one
of the claims 1 to 3;
- heating the steel slab to the austenitizing temperature of 1200 - 1350 °C;
- equalizing the temperature for 30 to 150 minutes;
- hot-rolling to the desired thickness at a temperature in the range of Ar3 to 1300°C,
wherein the finish rolling temperature is in the range of 800 °C to 960 °C, preferably
870 °C - 930 °C, more preferably 885 °C - 930 °C;
- direct quenching the hot-rolled steel strip product to a cooling end and coiling
temperature of 450 °C or less, preferably 250 °C or less, more preferably 150 °C or
less, and even more preferably 100 °C or less; and
- optionally, temper annealing at a temperature in the range of 150 °C - 250 °C.
1. Warmgewalztes Stahlbanderzeugnis, umfassend eine Zusammensetzung, bestehend aus, in
Form von Gewichtsprozenten (Gew.-%):
C |
0,17 bis 0,38, vorzugsweise 0,21 bis 0,35, mehr bevorzugt 0,22 bis 0,28 |
Si |
0,01 bis 0,5, mehr bevorzugt 0,03 bis 0,25 |
Mn |
0,1 bis 0,4, vorzugsweise 0,15 bis 0,3 |
Al |
0,015 bis 0,15 |
Cu |
0,1 bis 0,6, vorzugsweise 0,1 bis 0,5, mehr bevorzugt 0,1 bis 0,35 |
Ni |
0,2 bis 0,8 |
Cr |
0,1 bis 1, vorzugsweise 0,3 bis 1, mehr bevorzugt 0,35 bis 1, noch mehr bevorzugt
0,35 bis 0,8 |
Mo |
0,01 bis 0,3, vorzugsweise 0,03 bis 0,3, mehr bevorzugt 0,05 bis 0,3 |
Nb |
0 bis 0,005 |
Ti |
0 bis 0,05, vorzugsweise 0 bis 0,035, mehr bevorzugt 0 bis 0,02 |
V |
0 bis 0,06 |
B |
0,0005 bis 0,005, vorzugsweise 0,0008 bis 0,005 |
P |
0 bis 0,025, vorzugsweise 0,001 bis 0,025, mehr bevorzugt 0,001 bis 0,012 |
S |
0 bis 0,008, vorzugsweise 0 bis 0,005, mehr bevorzugt 0 bis 0,002 |
N |
0 bis 0,01, vorzugsweise 0 bis 0,005, mehr bevorzugt 0 bis 0,004 |
Ca |
0,0008 bis 0,003 |
Rest Fe und unvermeidliche Verunreinigungen, wobei das Stahlerzeugnis aufweist eine
Brinell-Härte in dem Bereich von 420 bis 580 HBW und
einen Korrosionsindex (ASTM G101-04) von mindestens 5 und
wobei das Stahlerzeugnis eine Mikrostruktur aufweist, bestehend aus, in Form von Volumenprozenten
(Vol.-%),
verbleibender Austenit 0 bis 1,
Rest Bainit, Ferrit und/oder Perlit, und
wobei das Stahlbanderzeugnis eine Dicke von höchstens 10 mm aufweist, und
die Menge an Ti in dem Bereich von 0 bis 0,005 Gew.-% liegt, wenn die Menge an N in
dem Bereich von 0 bis 0,003 Gew.-% liegt, die Menge an Ti mehr als 0,005 Gew.-% und
nicht mehr als 0,05 Gew.-% beträgt, wenn die Menge an N mehr als 0,003 Gew.-% und
nicht mehr als 0,01 Gew.-% beträgt, und wobei das Stahlerzeugnis eine Charpy-V-Schlagzähigkeit
von mindestens 34 J/cm2 bei einer Temperatur von -20 °C oder -40 °C in Quer- und/oder Längsrichtung aufweist.
2. Stahlerzeugnis nach Anspruch 1, wobei
[Ni] > [Cu]/3, vorzugsweise [Ni] > [Cu]/2 und wobei
[Ni] die Menge an Ni in der Zusammensetzung ist,
[Cu] die Menge an Cu in der Zusammensetzung ist.
3. Stahlerzeugnis nach einem der vorstehenden Ansprüche, wobei das Ca/S-Verhältnis in
dem Bereich von 1 bis 2, vorzugsweise 1,1 bis 1,7 und mehr bevorzugt 1,2 bis 1,6 liegt.
4. Stahlerzeugnis nach einem der vorstehenden Ansprüche, wobei das Stahlerzeugnis eine
Brinell-Härte in dem Bereich von 450 bis 550 HBW, vorzugsweise 470 bis 530 HBW, aufweist.
5. Stahlerzeugnis nach einem der vorstehenden Ansprüche, wobei das Stahlerzeugnis einen
Korrosionsindex (ASTM G101-04) von mindestens 5,5, vorzugsweise von mindestens 6,
aufweist.
6. Stahlerzeugnis nach einem der vorstehenden Ansprüche, wobei das Stahlerzeugnis einen
minimalen Biegeradius von höchstens 3,4 t in einer Messrichtung längs zu der Walzrichtung;
einen minimalen Biegeradius von höchstens 2,7 t in einer Messrichtung quer zu der
Walzrichtung aufweist; und wobei t die Dicke des Stahlbanderzeugnisses ist.
7. Stahlerzeugnis nach einem der vorstehenden Ansprüche, wobei das Stahlerzeugnis eine
Mikrostruktur aufweist, bestehend aus, in Form von Volumenprozenten (Vol.-%), Martensit
vorzugsweise ≥ 95, mehr bevorzugt ≥ 98
verbleibender Austenit vorzugsweise 0 bis 0,5,
Rest Bainit, Ferrit und/oder Perlit.
8. Stahlerzeugnis nach einem der vorstehenden Ansprüche, wobei das Stahlerzeugnis eine
vorherige Austenitkorngröße von höchstens 50 µm, vorzugsweise höchstens 30 µm, mehr
bevorzugt höchstens 20 µm aufweist.
9. Stahlerzeugnis nach einem der vorstehenden Ansprüche, wobei das Stahlerzeugnis eine
vorherige Austenitkornstruktur mit einem Aspektverhältnis in dem Bereich von 1,5 bis
7, vorzugsweise 1,5 bis 5, mehr bevorzugt 2 bis 5 aufweist.
10. Stahlerzeugnis nach einem der vorstehenden Ansprüche, vorzugsweise höchstens 8 mm
und mehr bevorzugt höchstens 7 mm.
11. Verfahren zum Herstellen des Stahlerzeugnisses nach einem der vorstehenden Ansprüche,
umfassend die folgenden Schritte
- Bereitstellen einer Stahlbramme, bestehend aus der chemischen Zusammensetzung nach
einem der Ansprüche 1 bis 3;
- Erhitzen der Stahlbramme auf die Austenitisierungstemperatur von 1200 bis 1350 °C;
- Ausgleichen der Temperatur für 30 bis 150 Minuten;
- Warmwalzen auf die gewünschte Dicke bei einer Temperatur in dem Bereich von Ar3
bis 1300 °C, wobei die Fertigwalztemperatur in dem Bereich von 800 °C bis 960 °C,
vorzugsweise 870 °C bis 930 °C, mehr bevorzugt 885 °C bis 930 °C liegt;
- direktes Abschrecken des warmgewalzten Stahlbanderzeugnisses auf eine Kühlende-
und Aufspultemperatur von höchstens 450 °C, vorzugsweise höchstens 250 °C, mehr bevorzugt
höchstens 150 °C und noch mehr bevorzugt höchstens 100 °C; und
- optional Anlassglühen bei einer Temperatur in dem Bereich von 150 °C bis 250 °C.
1. Produit de bande d'acier laminées à chaud comprenant une composition constituée, en
termes de pourcentages en poids (% en poids), de :
C |
de 0,17 à 0,38, de préférence de 0,21 à 0,35, plus préférablement de 0,22 à 0,28 |
Si |
de 0,01 à 0,5, plus préférablement de 0,03 à 0,25 |
Mn |
de 0,1 à 0,4, de préférence de 0,15 à 0,3 |
Al |
de 0,015 à 0,15 |
Cu |
de 0,1 à 0,6, de préférence de 0,1 à 0,5, plus préférablement de 0,1 à 0,35 |
Ni |
de 0,2 à 0,8 |
Cr |
de 0,1 à 1, de préférence de 0,3 à 1, plus préférablement de 0,35 à 1, encore plus
préférablement de 0,35 à 0,8 |
Mo |
de 0,01 à 0,3, de préférence de 0,03 à 0,3, plus préférablement de 0,05 à 0,3 |
Nb |
de 0 à 0,005 |
Ti |
de 0 à 0,05, de préférence de 0 à 0,035, plus préférablement de 0 à 0,02 |
V |
de 0 à 0,06 |
B |
de 0,0005 à 0,005, de préférence de 0,0008 à 0,005 |
P |
de 0 à 0,025, de préférence de 0,001 à 0,025, plus préférablement de 0,001 à 0,012 |
S |
de 0 à 0,008, de préférence de 0 à 0,005, plus préférablement de 0 à 0,002 |
N |
de 0 à 0,01, de préférence de 0 à 0,005, plus préférablement de 0 à 0,004 |
Ca |
de 0,0008 à 0,003 |
du reste de Fe et des impuretés inévitables, le produit d'acier ayant une dureté Brinell
dans la plage comprise entre 420 et 580 HBW, et
un indice de corrosion (ASTM G101-04) d'au moins 5, et
le produit d'acier ayant une microstructure constituée, en termes de pourcentage en
volume (% en volume), de :
martensite |
≥ 90, |
austénite résiduelle |
0 à 1, |
le reste de la bainite, de la ferrite et/ou de la perlite, et
le produit de bande d'acier ayant une épaisseur de 10 mm ou moins, et
la quantité de Ti étant dans la plage comprise entre 0 et 0,005 % en poids lorsque
la quantité de N est dans la plage comprise entre 0 et 0,003 % en poids, la quantité
de Ti étant supérieure à 0,005 % en poids et pas plus de 0,05 % en poids lorsque la
quantité de N est supérieure à 0,003 % en poids et pas plus de 0,01 % en poids, et
le produit d'acier ayant une ténacité Charpy-V d'au moins 34 J/cm2 à une température de -20 °C ou -40 °C dans le sens transversal et/ou longitudinal.
2. Produit d'acier selon la revendication 1, dans lequel
[Ni] > [Cu]/3, de préférence [Ni] > [Cu]/2, et
[Ni] étant la quantité de Ni dans la composition,
[Cu] étant la quantité de Cu dans la composition.
3. Produit d'acier selon l'une quelconque des revendications précédentes, dans lequel
le rapport Ca/S est dans la plage comprise entre 1 et 2, de préférence entre 1,1 et
1,7, et plus préférablement entre 1,2 et 1,6.
4. Produit d'acier selon l'une quelconque des revendications précédentes, dans lequel
le produit d'acier a une dureté Brinell dans la plage de 450 à 550 HBW, de préférence
de 470 à 530 HBW.
5. Produit d'acier selon l'une quelconque des revendications précédentes, dans lequel
le produit d'acier a un indice de corrosion (ASTM G101-04) d'au moins 5,5, de préférence
d'au moins 6.
6. Produit d'acier selon l'une quelconque des revendications précédentes, dans lequel
le produit d'acier a un rayon de courbure minimum de 3,4 t ou moins dans une direction
de mesure longitudinale à la direction de laminage ; un rayon de courbure minimal
de 2,7 t ou moins dans une direction de mesure transversale à la direction de laminage
; et t étant l'épaisseur du produit de bande d'acier.
7. Produit d'acier selon l'une quelconque des revendications précédentes, dans lequel
le produit d'acier a une microstructure constituée, en termes de pourcentages en volume
(% en volume), de :
martensite de préférence ≥ 95, plus préférablement ≥ 98 austénite résiduelle de préférence
entre 0 et 0,5,
le reste de la bainite, de la ferrite et/ou de la perlite.
8. Produit d'acier selon l'une quelconque des revendications précédentes, dans lequel
le produit d'acier a une taille de grain d'austénite antérieure de 50 µm ou moins,
de préférence de 30 µm ou moins, plus préférablement de 20 µm ou moins.
9. Produit d'acier selon l'une quelconque des revendications précédentes, dans lequel
le produit d'acier a une structure antérieure de grains d'austénite avec un rapport
d'aspect dans la plage comprise entre 1,5 et 7, de préférence entre 1,5 et 5, plus
préférablement entre 2 et 5.
10. Produit d'acier selon l'une quelconque des revendications précédentes, de préférence
de 8 mm ou moins, et plus préférablement de 7 mm ou moins.
11. Procédé de fabrication du produit d'acier selon l'une quelconque des revendications
précédentes, comprenant les étapes suivantes
- la fourniture d'une dalle d'acier constituée de la composition chimique selon l'une
quelconque des revendications 1 à 3 ;
- le chauffage de la dalle d'acier à la température d'austénitisation entre 1200 et
1350 °C ;
- l'égalisation de la température pendant 30 à 150 minutes ;
- le laminage à chaud à l'épaisseur souhaitée à une température dans la plage comprise
entre Ar3 et 1300 °C, la température de laminage de finition étant dans la plage comprise
entre 800 °C et 960 °C, de préférence entre 870 °C et 930 °C, plus préférablement
entre 885 °C et 930 °C ;
- la trempe directe du produit de bande d'acier laminé à chaud à une extrémité de
refroidissement et à une température d'enroulement de 450 °C ou moins, de préférence
de 250 °C ou moins, plus préférablement de 150 °C ou moins, et encore plus préférablement
de 100 °C ou moins ; et
- éventuellement, un recuit de revenu à une température dans la plage comprise entre
150 °C et 250 °C.