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(11) | EP 4 682 280 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
| published in accordance with Art. 153(4) EPC |
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| (54) | STEEL MEMBER AND STEEL SHEET |
| (57) This steel member has a predetermined chemical composition, in which, when a range
between a 1/8 position of a thickness and a 3/8 position of the thickness in a thickness
direction from a surface of the steel member, with respect to a 1/4 position of the
thickness from the surface as a center, is defined as a 1/4 depth position, a microstructure
at the 1/4 depth position includes, by area ratio, martensite, bainite, and tempered
martensite: 90% or more in total, and at the 1/4 depth position, when a random intensity
ratio of {111}<011> is denoted by I1, a random intensity ratio of {111}<112> is denoted
by I2, a random intensity ratio of {100}<011> is denoted by I3, and a random intensity
ratio of {100}<001> is denoted by I4, the steel member has a texture in which I1,
12, 13, and I4 satisfy (I1 + I3)/(I2 + 14) ≤ 1.20. |
TECHNICAL FIELD
[0002] Priority is claimed on Japanese Patent Application No. 2023-038696, filed March 13, 2023, the content of which is incorporated herein by reference.
BACKGROUND ART
[0003] In the field of steel sheets for a vehicle, against the background of tightening of recent environmental regulations and collision safety standards, the application of steel sheets having high tensile strength (high strength steel sheets) has expanded in order to improve both fuel efficiency and collision safety. However, press formability of a steel sheet decreases with high-strengthening, which makes it difficult to manufacture a product having a complex shape.
[0004] Specifically, ductility of the steel sheet decreases with the high-strengthening, and there is a problem in that the steel sheet is fractured at a highly processed portion in the case of being processed into a complex shape. In addition, with the high-strengthening of the steel sheet, residual stress after processing causes springback and wall curvature, which also causes a problem that dimensional accuracy is deteriorated. Therefore, it is not easy to perform press forming on a steel sheet having high strength, particularly a tensile strength of 780 MPa or more, into a product having a complex shape. Roll forming rather than press forming makes it easier to process a high strength steel sheet, but the application thereof is limited to components having a uniform cross section in a longitudinal direction thereof.
[0005] Therefore, in recent years, for example, as disclosed in Patent Documents 1 to 3, hot stamping has been adopted as a technology of press-forming a material that is difficult to form, such as a high strength steel sheet. The hot stamping is a hot forming technology of heating a material to be subjected to forming and then forming the material.
[0006] In this technology, the material is formed after being heated. Therefore, the steel is soft at the time of forming and has good formability. Accordingly, even a high strength steel sheet can be accurately formed into a complex shape. Furthermore, in the hot stamping, since quenching is performed simultaneously with forming by a press die, steel (steel member) after the forming has sufficient strength.
[0007] For example, according to Patent Document 1, it is disclosed that it is possible to impart a tensile strength of 1,400 MPa or more to a steel member obtained by forming a steel sheet through the hot stamping.
[0008] In recent years, countries around the world have set higher CO2 reduction targets, and each vehicle manufacturer has progressed in reducing fuel consumption in consideration of collision safety. Not only gasoline vehicles but also rapidly developing electric vehicles require, in terms of materials, higher strength materials to protect not only passengers but also batteries from collision and to cancel out the amount of an increase in weight. For example, in a steel member used in vehicles and the like, there is a need for a higher strength (higher than 1.5 GPa) steel that exceeds the strength described above in Patent Document 1 or a strength generally used as a steel member currently formed by hot stamping.
[0009] Regarding a high strength steel having a tensile strength of more than 1.5 GPa, for example, Patent Document 2 discloses a press-formed article that has excellent toughness and a tensile strength of 1.8 GPa or more and is hot press-formed. Patent Document 3 discloses a steel having a tensile strength as extremely high as 2.0 GPa or more, and further having good toughness and ductility. Patent Document 4 discloses a steel having a tensile strength as high as 1.8 GPa or more and further having good toughness. Patent Document 5 discloses a steel having a tensile strength as extremely high as 2.0 GPa or more and further having good toughness.
Citation List
Patent Documents
[0010]
Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2002-102980
Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2012-180594
Patent Document 3: Japanese Unexamined Patent Application, First Publication No. 2012-1802
Patent Document 4: PCT International Publication No. WO2015/182596
Patent Document 5: PCT International Publication No. WO2015/182591
SUMMARY OF INVENTION
Technical Problem
[0011] Many metal materials deteriorate in various properties with high-strengthening, and particularly increase in susceptibility to hydrogen embrittlement. It is known that a steel member has an increased susceptibility to hydrogen embrittlement when a tensile strength thereof is 1.2 GPa or more, and there are concerns that hot stamped members having a tensile strength of more than 1.5 GPa have an even greater susceptibility to hydrogen embrittlement. In order to apply a hot stamped member of more than 1.5 GPa to a vehicle body for a further reduction in weight of the vehicle body, it is desirable to further improve hydrogen embrittlement resistance.
[0012] An object of the present invention is to provide a steel member having high strength and excellent hydrogen embrittlement resistance, and a steel sheet suitable as a material for the steel member.
Solution to problem
[0013] In order to obtain a steel member having a high tensile strength and excellent hydrogen embrittlement resistance, the present inventors investigated influences of a steel sheet that serves as a material and a microstructure on these properties. As a result, the following findings were obtained.
[0014]
- (a) Most of commonly used steel sheets showing a tensile strength of about 1.5 GPa (1,500 MPa) after a heat treatment including quenching such as hot stamping contain about 0.200 mass% of C, and strength after the heat treatment is secured due to this C.
[0015] In order to achieve a further reduction in the weight of the vehicle body, the present inventors conducted a detailed examination to obtain a steel member having a 3 strength as high as more than 1.5 GPa after a heat treatment by increasing a C content. As a result, it was found that by setting the C content to 0.260 mass% or more, an ultrahigh strength of more than 1.5 GPa in terms of tensile strength can be obtained after a heat treatment including quenching such as hot stamping.
[0016] On the other hand, there were concerns that susceptibility to hydrogen embrittlement increases with ultrahigh-strengthening to a tensile strength of more than 1.5 GPa and hydrogen embrittlement cracking is caused by hydrogen generated in a corrosive environment while a vehicle is in operation.
[0017] (b) The present inventors examined a method for improving the hydrogen embrittlement resistance in a high strength steel member having a tensile strength of more than 1.5 GPa. As a result, it was found that the hydrogen embrittlement resistance can be improved by developing a specific texture.
[0018] The present invention has been made in view of the above findings. The gist of the present invention is as follows.
- [1] A steel member according to an aspect of the present invention includes, as a
chemical composition, by mass%: C: 0.260% to 0.700%; Si: 0% to 2.000%; Mn: 0% to 3.00%;
Al: 0% to 1.000%; Nb: 0% to 0.100%; Ti: 0% to 0.200%; Cr: 0% to 1.00%; B: 0% to 0.0200%;
Mo: 0% to 1.00%; W: 0% to 2.00%; Co: 0% to 1.00%; Ni: 0% to 2.00%; Cu: 0% to 2.00%;
V: 0% to 1.00%; Ca: 0% to 0.200%; Mg: 0% to 0.20%; REM: 0% to 0.300%; Sb: 0% to 1.00%;
Sn: 0% to 1.00%; Zr: 0% to 1.00%; As: 0% to 1.00%; Se: 0% to 1.00%; Bi: 0% to 1.00%;
Ta: 0% to 1.00%; Re: 0% to 1.00%; Os: 0% to 1.00%; Ir: 0% to 1.00%; Tc: 0% to 1.00%;
P: 0.100% or less; S: 0.0100% or less; N: 0.020% or less; O: 0.010% or less; and a
remainder: Fe and impurities, in which, when a range between a 1/8 position of a thickness
and a 3/8 position of the thickness in a thickness direction from a surface of the
steel member, with respect to a 1/4 position of the thickness from the surface as
a center, is defined as a 1/4 depth position, a microstructure at the 1/4 depth position
includes, by area ratio, martensite, bainite, and tempered martensite: 90% or more
in total, and at the 1/4 depth position, when a random intensity ratio of {111}<011>
is denoted by I1, a random intensity ratio of {111}<112> is denoted by 12, a random
intensity ratio of {100}<011> is denoted by I3, and a random intensity ratio of {100}<001>
is denoted by I4, the steel member has a texture in which the I1, the I2, the I3,
and the I4 satisfy Expression (1),
- [2] In the steel member according to [1], the chemical composition may contain one
or more selected from the group consisting of, by mass%, Nb: 0.005% to 0.100%, Ti:
0.005% to 0.200%, Cr: 0.01% to 1.00%, B: 0.0010% to 0.0200%, Mo: 0.01% to 1.00%, W:
0.001% to 2.00%, Co: 0.01% to 1.00%, Ni: 0.01% to 2.00%, Cu: 0.01% to 2.00%, V: 0.01%
to 1.00%, Ca: 0.001% to 0.200%, Mg: 0.01% to 0.20%, REM: 0.001% to 0.300%, Sb: 0.01%
to 1.00%, Sn: 0.01% to 1.00%, Zr: 0.01% to 1.00%, As: 0.01% to 1.00%, Se: 0.01% to
1.00%, Bi: 0.01% to 1.00%, Ta: 0.01% to 1.00%, Re: 0.01% to 1.00%, Os: 0.01% to 1.00%,
Ir: 0.01% to 1.00%, and
Tc: 0.01% to 1.00%. - [3] In the steel member according to [1] or [2], a Vickers hardness at the 1/4 depth position may be 450 or more.
- [4] In the steel member according to any one of [1] to [3], the surface of the steel member may have a coating.
- [5] In the steel member according to [4], the coating may be an Fe-Al-based coating or an Fe-Zn-based coating.
- [6] A steel sheet according to another aspect of the present invention includes, as
a chemical composition, by mass%: C: 0.260% to 0.700%; Si: 0% to 2.000%; Mn: 0% to
3.00%; Al: 0% to 1.000%; Nb: 0% to 0.100%; Ti: 0% to 0.200%; Cr: 0% to 1.00%; B: 0%
to 0.0200%; Mo: 0% to 1.00%; W: 0% to 2.00%; Co: 0% to 1.00%; Ni: 0% to 2.00%; Cu:
0% to 2.00%; V: 0% to 1.00%; Ca: 0% to 0.200%; Mg: 0% to 0.20%; REM: 0% to 0.300%;
Sb: 0% to 1.00%; Sn: 0% to 1.00%; Zr: 0% to 1.00%; As: 0% to 1.00%; Se: 0% to 1.00%;
Bi: 0% to 1.00%; Ta: 0% to 1.00%; Re: 0% to 1.00%; Os: 0% to 1.00%; Ir: 0% to 1.00%;
Tc: 0% to 1.00%; P: 0.100% or less; S: 0.0100% or less; N: 0.020% or less; O: 0.010%
or less; and a remainder: Fe and impurities, in which, when a range between a 1/8
position of a sheet thickness and a 3/8 position of the sheet thickness in a sheet
thickness direction from a surface of the steel sheet, with respect to a 1/4 position
of the sheet thickness from the surface as a center, is defined as a 1/4 depth position,
at the 1/4 depth position, when a random intensity ratio of {111}<011> is denoted
by 11, a random intensity ratio of {111}<112> is denoted by 12, a random intensity
ratio of {100}<011> is denoted by I3, and a random intensity ratio of {100}<001> is
denoted by I4, the steel sheet has a texture in which the I1, the 12, the I3, and
the I4 satisfy Expression (1),
- [7] In the steel sheet according to [6], the chemical composition may contain one or more selected from the group consisting of, by mass%, Nb: 0.005% to 0.100%, Ti: 0.005% to 0.200%, Cr: 0.01% to 1.00%, B: 0.0010% to 0.0200%, Mo: 0.01% to 1.00%, W: 0.001% to 2.00%, Co: 0.01% to 1.00%, Ni: 0.01% to 2.00%, Cu: 0.01% to 2.00%, V: 0.01% to 1.00%, Ca: 0.001% to 0.200%, Mg: 0.01% to 0.20%, REM: 0.001% to 0.300%, Sb: 0.01% to 1.00%, Sn: 0.01% to 1.00%, Zr: 0.01% to 1.00%, As: 0.01% to 1.00%, Se: 0.01% to 1.00%, Bi: 0.01% to 1.00%, Ta: 0.01% to 1.00%, Re: 0.01% to 1.00%, Os: 0.01% to 1.00%, Ir: 0.01% to 1.00%, and Tc: 0.01% to 1.00%.
- [8] In the steel sheet according to [6] or [7], the surface of the steel sheet may have a coating.
- [9] In the steel sheet according to [8], the coating may be an Al-based coating or a Zn-based coating.
Advantageous Effects of Invention
[0019] According to the above aspects of the present invention, it is possible to provide a steel member having high tensile strength and excellent hydrogen embrittlement resistance, and a steel sheet that is a material for the steel member.
DESCRIPTION OF EMBODIMENTS
[0020] A steel member according to an embodiment of the present invention (a steel member according to the present embodiment), a steel sheet according to an embodiment of the present invention suitable as a material thereof (a steel sheet according to the present embodiment), and manufacturing methods thereof will be described.
[Steel Member]
[0021] The steel member according to the present embodiment has a chemical composition, which will be described later, and a microstructure at a 1/4 depth position (a range between a 1/8 position of a thickness and a 3/8 position of the thickness in a thickness direction (in a case of including a steel sheet, a sheet thickness direction of the steel sheet) from a surface of the steel member, with respect to a 1/4 position of the thickness from the surface as a center, hereinafter, the same applies) includes, by area ratio, martensite, bainite, and tempered martensite: 90% or more in total.
[0022] In addition, in the steel member according to the present embodiment, at the 1/4 depth position, when a random intensity ratio of { 111 }<011> is denoted by I1, a random intensity ratio of { 111 }<112> is denoted by I2, a random intensity ratio of {100}<011> is denoted by I3, and a random intensity ratio of {100}<001> is denoted by I4, the steel member has a texture in which the I1, the I2, the I3, and the I4 satisfy (I1 + I3)/(I2 + I4) ≤ 1.20.
[0023] The steel member according to the present embodiment may be coated on the surface. Even in that case, in the present embodiment, since the coating itself is not a steel member, the chemical composition, microstructure, texture, and the like of the steel member are a chemical composition, a microstructure, a texture, and the like of a part excluding the coating (this part is sometimes referred to as a "base steel member" or "base metal steel member"). In a case where the surface of the steel member according to the present embodiment is coated, that is, the surface of the steel member according to the present embodiment serving as a reference for the 1/4 depth position is a surface of the part excluding the coating (base steel member), that is, a boundary between the base steel member and the coating. Therefore, for example, in a case where front and back surfaces of the steel member are coated, a thickness t1 of the steel member including the coating is measured, a thickness t2 of the coating is then measured by a method described later, and a thickness (thickness of the base steel member) t3 = t1 - 2 × t2 of the steel member is calculated. Thereafter, a range separated by t3/8 to (3 × t3)/8 in the thickness direction from a center of the thickness t1 of the steel member including the coating can be set as the 1/4 depth position.
[0025] A shape of the steel member according to the present embodiment is not particularly limited. That is, the steel member may be a flat sheet, or may be a formed body obtained by forming a steel sheet into a predetermined shape. A hot-formed steel member is often a formed body, for example, a hot-stamping formed body. However, in the present embodiment, a case of a formed body and a case of a flat sheet are collectively referred to as a "steel member".
<Chemical Composition>
[0026] Specifically, the chemical composition of the steel member according to the present embodiment includes, by mass%: C: 0.260% to 0.700%; Si: 0% to 2.000%; Mn: 0% to 3.00%; Al: 0% to 1.000%; Nb: 0% to 0.100%; Ti: 0% to 0.200%; Cr: 0% to 1.00%; B: 0% to 0.0200%; Mo: 0% to 1.00%; W: 0% to 2.00%; Co: 0% to 1.00%; Ni: 0% to 2.00%; Cu: 0% to 2.00%; V: 0% to 1.00%; Ca: 0% to 0.200%; Mg: 0% to 0.20%; REM: 0% to 0.300%; Sb: 0% to 1.00%; Sn: 0% to 1.00%; Zr: 0% to 1.00%; As: 0% to 1.00%; Se: 0% to 1.00%; Bi: 0% to 1.00%; Ta: 0% to 1.00%; Re: 0% to 1.00%; Os: 0% to 1.00%; Ir: 0% to 1.00%; Tc: 0% to 1.00%; P: 0.100% or less; S: 0.0100% or less; N: 0.020% or less; O: 0.010% or less; and a remainder: Fe and impurities.
[0027] The reasons for limiting the amount of each element are as follows. In the following description, % regarding the amount of the element is mass% unless otherwise specified.
C: 0.260% to 0.700%
[0028] C is an element that enhances hardenability of steel and improves strength of a steel member obtained after a steel sheet is subjected to a heat treatment including quenching such as hot stamping (post-quenching). When a C content is less than 0.260%, it becomes difficult to secure sufficient strength (more than 1.5 GPa (1,500 MPa)) in the steel member after quenching (obtained after being subjected to quenching). Therefore, the C content is set to 0.260% or more. The C content is set to preferably 0.280% or more, and more preferably 0.310% or more. In addition, in a case of obtaining a higher tensile strength, for example, 2,300 MPa or more, the C content is preferably 0.450% or more.
[0029] On the other hand, when the C content is more than 0.700%, the strength of the steel member after quenching becomes excessively high, and a decrease in hydrogen embrittlement resistance becomes significant. Therefore, the C content is set to 0.700% or less. The C content is set to preferably 0.650% or less, and more preferably 0.600% or less.
Si: 0% to 2.000%
[0030] Si does not have to be contained (may be 0%), but is an effective element for enhancing the hardenability of the steel and stably securing the strength of the steel member after quenching. Therefore, Si may be contained. In a case where the above effects are obtained, a Si content is set to preferably 0.100% or more, and more preferably 0.350% or more.
[0031] On the other hand, when the Si content in steel is more than 2.000%, a heating temperature required for austenitic transformation becomes significantly high during the heat treatment (quenching). Accordingly, there are cases where the cost required for the heat treatment increases, or ferrite remains during heating, resulting in a decrease in the strength of the steel member. Therefore, the Si content is set to 2.000% or less. The Si content is preferably set to 1.500% or less.
Mn: 0% to 3.00%
[0032] Mn does not have to be contained (may be 0%), but is a very effective element for enhancing the hardenability of the steel and stably securing the strength after quenching. Further, Mn is an element that lowers an Ac3 point and promotes lowering of a quenching treatment temperature. Therefore, Mn may be contained. In a case of obtaining the above effect, a Mn content is set to preferably 0.05% or more, and more preferably 0.15% or more or 0.40% or more.
[0033] On the other hand, when the Mn content is more than 3.00%, the hydrogen embrittlement resistance of the steel member after quenching deteriorates. Therefore, the Mn content is set to 3.00% or less. The Mn content is set to preferably 2.50% or less, and more preferably 1.50% or less.
Al: 0% to 1.000%
[0034] Al is an element generally used as a steel deoxidizing agent. Therefore, Al may be contained. An Al content may be 0%, but in order to obtain the above effects, the Al content is preferably set to 0.010% or more. The Al content may be set to 0.020% or more or 0.030% or more as necessary.
[0035] On the other hand, when the Al content is more than 1.000%, the above effect is saturated and the economic efficiency is lowered. Therefore, in a case where A1 is contained, the Al content is set to 1.000% or less. The Al content may be set to 0.300% or less, 0.100% or less, or 0.075% or less as necessary.
Nb: 0% to 0.100%
[0037] Nb is an element that forms fine carbides, nitrides, or carbonitrides in steel and suppresses Cu hot embrittlement cracking in a hot rolling step through a grain refining effect of these precipitates. In addition, in the steel member according to the present embodiment, the hydrogen embrittlement resistance of the steel member is improved by concentrating W of Nb-based precipitates (making a W concentration higher than a W concentration of a base steel material). Therefore, Nb may be contained.
[0038] A Nb content may be 0%, but in a case where the above effects are obtained, the Nb content is preferably set to 0.005% or more. The Nb content is more preferably 0.010% or more.
[0039] On the other hand, when the Nb content is more than 0.100%, the carbonitrides become coarse and bending straightening cracking in a continuous casting step is promoted. In addition, solute Nb inhibits the development of grain boundaries having a specific rotation angle in the steel member, which will be described later, resulting in a decrease in the hydrogen embrittlement resistance of the steel member. Therefore, the Nb content is set to 0.100% or less. The Nb content is preferably 0.080% or less. The Nb content may be set to 0.060% or less or 0.040% or less as necessary.
Ti: 0% to 0.200%
[0040] Ti is an element that forms fine carbides, carbonitrides, and the like together with Nb in steel, suppresses Cu hot embrittlement cracking in the hot rolling step through the grain refining effect thereof, and has an action of improving the hydrogen embrittlement resistance of the steel member. In addition, Ti is an element that also forms nitrides by being preferentially bonded to N in the steel, suppresses the consumption of solute B due to precipitation of BN, and promotes an effect of improving the hardenability by B, which will be described later. Therefore, Ti may be contained.
[0041] A Ti content may be 0%, but in a case where the above effects are obtained, the Ti content is preferably set to 0.005% or more. The Ti content is set to more preferably 0.010% or more, and even more preferably 0.015% or more.
[0042] On the other hand, when the Ti content is more than 0.200%, the carbonitrides and the like become coarse and bending straightening cracking in the continuous casting step is promoted. In addition, solute Ti inhibits the development of grain boundaries having a specific rotation angle in the steel member, which will be described later, resulting in a decrease in the hydrogen embrittlement resistance of the steel member. In addition to the carbonitrides with Nb and TiN, the amount of TiC precipitated increases and C is consumed, so that the strength of the steel member after quenching decreases.
[0043] Accordingly, the Ti content is set to 0.200% or less. The Ti content is preferably set to 0.080% or less. The Ti content may be set to 0.060% or less or 0.040% or less as necessary.
Cr: 0% to 1.00%
[0044] Cr is an effective element for enhancing the hardenability of the steel and stably securing the strength of the steel member after quenching. Therefore, Cr may be contained. A Cr content may be 0%, but in a case where the above effects are obtained, the Cr content is set to preferably 0.01% or more, and more preferably 0.03% or more.
[0045] On the other hand, when the Cr content is more than 1.00%, the above effects are saturated and the cost increases. Furthermore, since Cr has an action of stabilizing iron carbides, when the Cr content is more than 1.00%, there are cases where coarse iron carbides remain undissolved during the heat treatment of the steel sheet, and the hydrogen embrittlement resistance of the steel member decreases. Therefore, the Cr content is set to 1.00% or less. The Cr content is set to preferably 0.50% or less, more preferably 0.20% or less, and even more preferably 0.15% or less.
B: 0% to 0.0200%
[0046] B is an element having an action of enhancing the hardenability of the steel even in a small amount. In addition, B is an element that strengthens grain boundaries and improves the hydrogen embrittlement resistance by being segregated at the grain boundaries, and is an element that suppresses the growth of austenite grains when the steel sheet is heated. Therefore, B may be contained. The B content may be 0%, but in a case where the above effects are obtained, the B content is set to preferably 0.0005% or more, and more preferably 0.0010% or more or 0.0020% or more.
[0047] On the other hand, when the B content is more than 0.0200%, a large amount of coarse compounds are precipitated, and the hydrogen embrittlement resistance of the steel member decreases. Accordingly, in a case where B is to be contained, the B content is set to 0.0200% or less. The B content is set to preferably 0.0100% or less or 0.0050% or less.
Mo: 0% to 1.00%
[0048] Mo is a very effective element for enhancing the hardenability of the steel and stably securing the strength of the steel member after quenching. In particular, a synergistic effect on the improvement in the hardenability can be obtained by including a compound of Mo and B. Therefore, Mo may be contained. A Mo content may be 0%, but in a case where the above effects are obtained, the Mo content is set to preferably 0.01% or more, and more preferably 0.03% or more.
[0049] On the other hand, when the Mo content is more than 1.00%, the above effect is saturated and the cost increase is significant. Therefore, in a case where Mo is contained, the Mo content is set to 1.00% or less. In order to reduce the cost, the Mo content is preferably set to 0.80% or less, 0.50% or less, or 0.25% or less.
W: 0% to 2.00%
[0050] W is an effective element for enhancing the hardenability of steel and stably securing the strength of the steel member after quenching. In addition, W is an element that improves corrosion resistance in a corrosive environment. In addition, W is an element that segregates to grain boundaries and is also an element that contributes to the development of the above-described texture. Therefore, W may be contained.
[0051] In a case where a W content is less than 0.01%, it is not possible to sufficiently obtain the effect. The W content may be 0%, but in a case where the above effects are obtained, the W content is set to preferably 0.01% or more, more preferably 0.05% or more, even more preferably 0.10% or more, and still more preferably 0.20% or more.
[0052] On the other hand, in a case where the W content is more than 2.00%, the above-described effects are saturated and economic efficiency is lowered. Therefore, the W content is set to 2.00% or less. In order to reduce an alloying cost, the W content is set to preferably 1.50% or less, and more preferably 1.00% or less, 0.50% or less, or 0.20% or less.
Co: 0% to 1.00%
[0053] Co is a very effective element for enhancing the hardenability of the steel and stably securing the strength of the steel member after quenching. In particular, a synergistic effect on the improvement in the hardenability can be obtained by including a compound of Co and B. Therefore, Co may be contained. A Co content may be 0%, but in a case where the above effects are obtained, the Co content is set to preferably 0.10% or more, and more preferably 0.20% or more.
[0054] On the other hand, when the Co content is more than 1.00%, the above effect is saturated and the cost increase is significant. Therefore, in a case where Co is contained, the Co content is set to 1.00% or less. In order to reduce the alloying cost, the Co content is preferably set to 0.80% or less, 0.50% or less, or 0.25% or less.
Ni: 0% to 2.00%
[0055] Ni is an effective element for enhancing the hardenability of the steel and stably securing the strength of the steel member after quenching. In addition, Ni is an element having an action of suppressing Cu hot embrittlement cracking in the manufacturing of a steel sheet. Therefore, Ni may be contained. A Ni content may be 0%, but in a case where the above effects are obtained, the Ni content is set to preferably 0.01% or more, and more preferably 0.03% or more.
[0056] On the other hand, when the Ni content is more than 2.00%, the above effect is saturated and the cost increases. Therefore, the Ni content is set to 2.00% or less. The Ni content is set to preferably 1.00% or less, more preferably 0.50% or less, and even more preferably 0.20% or less.
Cu: 0% to 2.00%
[0057] Cu is an effective element for enhancing the hardenability of steel and stably securing the strength of the steel member after quenching. In addition, Cu is an element that improves corrosion resistance in a corrosive environment. Therefore, Cu may be contained. A Cu content may be 0%, but in a case where the above effects are obtained, the Cu content is preferably set to 0.01% or more. The Cu content is more preferably 0.03% or more.
[0058] On the other hand, in a case where the Cu content is more than 2.00%, the above-described effects are saturated and the cost increases. Therefore, the Cu content is set to 2.00% or less. In order to reduce the alloying cost, the Cu content is set to preferably 1.50% or less, and more preferably 1.00% or less, 0.80% or less, or 0.50% or less.
V: 0% to 1.00%
[0059] V is an element that forms fine carbides in steel and improves the hydrogen embrittlement resistance of the steel member through a refining effect or hydrogen trapping effect of carbides thereof. Therefore, V may be contained. A V content may be 0%, but in a case where the above effects are obtained, the V content is preferably set to 0.01% or more, and more preferably set to 0.05% or more.
[0060] On the other hand, when the V content is more than 1.00%, the above effect is saturated and the economic efficiency is lowered. Therefore, in a case where V is contained, the V content is set to 1.00% or less. In order to reduce the alloying cost, the V content is set to preferably 0.80% or less, and more preferably 0.50% or less, 0.30% or less, or 0.10% or less.
Ca: 0% to 0.200%
[0061] Ca is an element having an effect of refining inclusions in steel and enhancing the hydrogen embrittlement resistance of the steel member after quenching. Therefore, Ca may be contained. A Ca content may be 0%, but in a case where the above effects are obtained, the Ca content is preferably set to 0.001 % or more, and more preferably set to 0.010% or more or 0.020% or more.
[0062] On the other hand, in a case where the Ca content is more than 0.200%, the effects are saturated and the cost increases. Accordingly, in a case where Ca is to be contained, the Ca content is set to 0.200% or less. The Ca content is set to preferably 0.100% or less, and more preferably 0.050% or less.
Mg: 0% to 0.20%
[0063] Mg is an element having an effect of refining inclusions in steel and enhancing the hydrogen embrittlement resistance after the heat treatment. Therefore, Mg may be contained. A Mg content may be 0%, but in a case where the above effects are obtained, the Mg content is preferably set to 0.01% or more. The Mg content is more preferably 0.02% or more.
[0064] On the other hand, when the Mg content is more than 0.20%, the effect is saturated and the cost increases. Therefore, in a case where Mg is contained, the Mg content is set to 0.20% or less. The Mg content is preferably 0.10% or less, and more preferably 0.05% or less.
REM: 0% to 0.300%
[0065] Similar to Ca, REM is an element having an effect of refining inclusions in steel and improving the hydrogen embrittlement resistance of the steel member after quenching. Therefore, REM may be contained. A REM content may be 0%, but in a case where the above effects are obtained, the REM content is preferably set to 0.001% or more, and more preferably set to 0.010% or more or 0.020% or more.
[0066] On the other hand, when the REM content is more than 0.300%, the effect is saturated and the cost increases. Therefore, in a case where REM is contained, the REM content is set to 0.300% or less. In order to reduce the alloying cost, the REM content is preferably set to 0.200% or less, 0.100% or less, or 0.050% or less.
[0067] Here, REM refers to a total of 17 elements including Sc, Y, and lanthanoids such as La, Ce, and Nd, and the REM content means the total amount of these elements. REM is added to molten steel using, for example, an Fe-Si-REM alloy, and this alloy contains, for example, Sc, Y, La, Ce, Pr, and Nd.
Sb: 0% to 1.00%
[0068] Sb is an element that improves corrosion resistance in a corrosive environment. Therefore, Sb may be contained. A Sb content may be 0%, but in a case where the above effects are obtained, the Sb content is preferably set to 0.01% or more.
[0069] On the other hand, when the Sb content is more than 1.00%, the effect is saturated and the cost increases. Therefore, in a case where Sb is contained, the Sb content is set to 1.00% or less. The Sb content may be set to 0.50% or less, 0.20% or less, 0.10% or less, or 0.05% or less as necessary.
Sn: 0% to 1.00%
[0070] Sn is an element that contributes to an increase in the strength of the steel member. When a Sn content is less than 0.01%, these effects are not sufficient. Therefore, the Sn content may be 0%, but in a case where Sn is contained, the Sn content is preferably set to 0.01% or more. The Sn content is set to more preferably 0.03% or more, and even more preferably 0.05% or more.
[0071] On the other hand, when the Sn content is more than 1.00%, the effects are saturated and the cost increases. Therefore, in a case where Sn is contained, the Sn content is set to 1.00% or less. The Sn content may be set to 0.50% or less, 0.20% or less, 0.10% or less, or 0.05% or less as necessary.
Zr: 0% to 1.00%
[0072] Zr is an element that improves corrosion resistance in a corrosive environment. Therefore, Zr may be contained. A Zr content may be 0%, but in a case where the above effects are obtained, the Zr content is preferably set to 0.01% or more.
[0073] On the other hand, when the Zr content is more than 1.00%, the effect is saturated and the cost increases. Therefore, in a case where Zr is contained, the Zr content is set to 1.00% or less. The Zr content may be set to 0.50% or less, 0.20% or less, 0.10% or less, or 0.05% or less as necessary.
As: 0% to 1.00%
[0074] As is an element that improves the hydrogen embrittlement resistance. Therefore, As may be contained. The As content may be 0%, but in a case where the above effects are obtained, the As content is preferably set to 0.01% or more.
[0075] On the other hand, when the As content is more than 1.00%, the effect is saturated and the cost increases. Therefore, in a case where As is contained, the As content is set to 1.00% or less. The As content may be set to 0.50% or less, 0.20% or less, 0.10% or less, or 0.05% or less as necessary.
Se: 0% to 1.00%
[0076] Se is an element that improves the hydrogen embrittlement resistance. Therefore, Se may be contained. A Se content may be 0%, but in a case where the above effects are obtained, the Se content is preferably set to 0.01% or more.
[0077] On the other hand, when the Se content is more than 1.00%, the effect is saturated and the cost increases. Therefore, in a case where Se is contained, the Se content is set to 1.00% or less. The Se content may be set to 0.50% or less, 0.20% or less, 0.10% or less, or 0.05% or less as necessary.
Bi: 0% to 1.00%
[0078] Bi is an element that improves the hydrogen embrittlement resistance. Therefore, Bi may be contained. The Bi content may be 0%, but in a case where the above effects are obtained, the Bi content is preferably set to 0.01% or more.
[0079] On the other hand, when the Bi content is more than 1.00%, the effect is saturated and the cost increases. Therefore, in a case where Bi is contained, the Bi content is set to 1.00% or less. The Bi content may be set to 0.50% or less, 0.20% or less, 0.10% or less, or 0.05% or less as necessary.
Ta: 0% to 1.00%
[0080] Ta is an element that improves the hydrogen embrittlement resistance. Therefore, Ta may be contained. A Ta content may be 0%, but in a case where the above effects are obtained, the Ta content is preferably set to 0.01% or more.
[0081] On the other hand, when the Ta content is more than 1.00%, the effect is saturated and the cost increases. Therefore, in a case where Ta is contained, the Ta content is set to 1.00% or less. The Ta content may be set to 0.50% or less, 0.20% or less, 0.10% or less, or 0.05% or less as necessary.
Re: 0% to 1.00%
[0082] Re is an element that improves the hydrogen embrittlement resistance. Therefore, Re may be contained. A Re content may be 0%, but in a case where the above effects are obtained, the Re content is preferably set to 0.01% or more.
[0083] On the other hand, when the Re content is more than 1.00%, the effect is saturated and the cost increases. Therefore, in a case where Re is contained, the Re content is set to 1.00% or less. The Re content may be set to 0.50% or less, 0.20% or less, 0.10% or less, or 0.05% or less as necessary.
Os: 0% to 1.00%
[0084] Os is an element that improves the hydrogen embrittlement resistance. Therefore, Os may be contained. An Os content may be 0%, but in a case where the above effects are obtained, the Os content is preferably set to 0.01% or more.
[0085] On the other hand, when the Os content is more than 1.00%, the effect is saturated and the cost increases. Therefore, in a case where Os is contained, the Os content is set to 1.00% or less. The Os content may be set to 0.50% or less, 0.20% or less, 0.10% or less, or 0.05% or less as necessary.
Ir: 0% to 1.00%
[0086] Ir is an element that improves the hydrogen embrittlement resistance. Therefore, Ir may be contained. An Ir content may be 0%, but in a case where the above effects are obtained, the Ir content is preferably set to 0.01% or more.
[0087] On the other hand, when the Ir content is more than 1.00%, the effect is saturated and the cost increases. Therefore, in a case where Ir is contained, the Ir content is set to 1.00% or less. The Ir content may be set to 0.50% or less, 0.20% or less, 0.10% or less, or 0.05% or less as necessary.
Tc: 0% to 1.00%
[0088] Tc is an element that improves the hydrogen embrittlement resistance. Therefore, Tc may be contained. The Tc content may be 0%, but in a case where the above effects are obtained, the Tc content is preferably set to 0.01% or more.
[0089] On the other hand, when the Tc content is more than 1.00%, the effect is saturated and the cost increases. Therefore, in a case where Tc is contained, the Tc content is set to 1.00% or less. The Tc content may be set to 0.50% or less, 0.20% or less, 0.10% or less, or 0.05% or less as necessary.
P: 0.100% or Less
[0090] P is an element that decreases the hydrogen embrittlement resistance of the steel member after quenching. In particular, when a P content is more than 0.100%, the decrease in the hydrogen embrittlement resistance becomes significant. Therefore, the P content is limited to 0.100% or less. The P content is preferably limited to 0.050% or less or 0.020% or less.
[0091] Since the P content is preferably as small as possible, the P content may be 0%. However, from the viewpoint of cost, the P content may be set to 0.001% or more.
S: 0.0100% or Less
[0092] S is an element that decreases the hydrogen embrittlement resistance of the steel member after quenching. In particular, when a S content is more than 0.0100%, the decrease in the hydrogen embrittlement resistance becomes significant. Therefore, the S content is limited to 0.0100% or less. The S content is preferably limited to 0.0050% or less. Since the S content is preferably as small as possible, the S content may be 0%. However, from the viewpoint of cost, the S content may be set to 0.0001% or more.
N: 0.020% or Less
[0093] N is an element that decreases the hydrogen embrittlement resistance of the steel member after quenching. In particular, when a N content is more than 0.020%, coarse nitrides are formed in steel, and the hydrogen embrittlement resistance significantly decreases. Accordingly, the N content is set to 0.020% or less. A lower limit of the N content does not need to be particularly limited and may be 0%. However, setting the N content to less than 0.001% leads to an increase in steelmaking cost and is economically undesirable. Therefore, the N content may be set to 0.001 % or more, 0.002% or more, 0.008% or more, or 0.010% or more.
O: 0.010% or Less
[0094] O is an element that decreases the hydrogen embrittlement resistance of the steel member after quenching. In particular, when an O content is more than 0.010%, coarse nitrides are formed in steel, and the hydrogen embrittlement resistance significantly decreases. Therefore, the O content is set to 0.010% or less.
[0095] A lower limit of the O content does not need to be particularly limited and may be 0%. However, setting the O content to less than 0.001% leads to an increase in steelmaking cost and is economically undesirable. Therefore, the O content may be set to 0.0001% or more, 0.002% or more, 0.0008% or more, or 0.001% or more.
(Remainder: Fe and Impurities)
[0096] In the chemical composition of the steel member according to the present embodiment, the addition or inclusion of elements other than the above-described elements is not excluded as long as the effects of the present embodiment are achieved. However, the addition or inclusion of elements other than the above-described elements may not be permitted as necessary. In the chemical composition of the steel member according to the present embodiment, the remainder including elements other than the above-described elements includes at least Fe and impurities. The remainder may include only Fe and impurities.
[0097] Here, the "impurities" are elements that are incorporated due to various factors including raw materials such as ore and scrap and a manufacturing process when the steel sheet is industrially manufactured, and are acceptable in a range without adversely affecting the properties of the steel member according to the present embodiment. An industrial manufacturing method is a blast furnace steelmaking method or an electric furnace steelmaking method, and includes a level (impurity level) incorporated during manufacturing by any of the methods. Examples of the impurities include Pb and Zn.
[0098] Since the total of the amounts of the impurities is usually 1.0% or less, the total of the amounts of the impurities may be set to 1.0% or less. The total of the amounts of the impurities may be set to 0.5% or less, 0.2% or less, 0.1 % or less, or 0.05% or less as necessary. For various reasons such as reducing raw material costs, raw materials that contain relatively large amounts of elements other than the above-mentioned elements may be intentionally used. Therefore, in the present embodiment, these elements are all regarded as impurity elements, regardless of whether these elements are mixed in or intentionally used. Therefore, the total of the concentrations of these elements may be set to 1.0% or less as described above.
[0100] The chemical composition can be obtained by performing elemental analysis on the 1/4 depth position of the steel member (a range of 1/8 to 3/8 of the thickness from the surface in the thickness direction) using a general method such as ICP-AES. For elements, which are difficult to measure using ICP-AES, C and S may be measured using a combustion-infrared absorption method, N may be measured using an inert gas fusion-thermal conductivity method, and O may be measured using an inert gas fusion-non-dispersive infrared absorption method. In a case where a chemical composition of the steel sheet that is a material of the steel member or ladle analysis values of molten steel are known, the chemical composition of the steel sheet or the ladle analysis values of the molten steel may be used as the chemical composition of the steel member.
<Microstructure>
[0101] In the steel member according to the present embodiment, the microstructure at the 1/4 depth position includes, by area ratio, martensite, bainite, and tempered martensite: 90% or more in total.
[0102] Martensite, bainite, and tempered martensite are structures (phases) that contribute to the high-strengthening of the steel member, and it is difficult to obtain sufficient strength in the steel member when the total of the area ratios of these structures is less than 90%. Hereinafter, martensite, bainite, and tempered martensite may be collectively referred to as a hard structure.
[0103] The total area ratio of martensite, bainite, and tempered martensite may be 95% or more, 98% or more, or 100% (an area ratio of a remainder in microstructure excluding the hard structure may be 0%), but the remainder in microstructure may include one or more of pearlite, bainite, ferrite, cementite, and residual austenite.
[0105] A test piece is collected from any position (a position avoiding an end portion in a case where the sample cannot be collected from this position) 50 mm or more away from an end surface of the steel member so that the microstructure at the 1/4 depth position can be observed in a cross section parallel to a rolling direction and parallel to the sheet thickness direction.
[0106] The cross section of the test piece is polished using #600 to #1500 silicon carbide paper and is thereafter mirror-finished using a liquid obtained by dispersing a diamond powder having a particle size of 1 to 6 µm in a diluted solution such as alcohol or in pure water. Next, the cross section of the test piece is polished at room temperature using colloidal silica containing no alkaline solution to remove strain introduced into a surface layer of the sample.
[0107] At any position in a longitudinal direction (rolling direction) of the cross section of the test piece after the polishing, a region extending 200 µm in the longitudinal direction and extending from a 1/8 position of a thickness from a surface to a 3/8 position of the thickness from the surface with respect to a 1/4 position of the thickness from the surface as a center is set as an observation region. Then, the observation region is measured by an electron backscatter diffraction method at a measurement interval of 0.1 µm to obtain crystal orientation information. For the measurement, an apparatus including a thermal field-emission scanning electron microscope (JSM-7001F manufactured by JEOL Ltd.) and an EBSD detector (DVC5 type detector manufactured by TSL) is used. In this case, the degree of vacuum in the apparatus is set to 9.6 × 10-5 Pa or less, an accelerating voltage is set to 15 kv, an irradiation current level is set to 13, and an irradiation level of an electron beam is set to 62. In addition, during the measurement, "iron-α" and "iron-y" are set as Phases, and the measurement is performed.
[0108] In addition, the same region as the measurement region in EBSD (EBSD measurement region) is observed at a magnification of 1,000-fold or more using the thermal field-emission scanning electron microscope (JSM-7001F manufactured by JEOL Ltd.)
[0109] In order to observe the same region, Vickers indentations are stamped at three points among four corners of the EBSD measurement region in a range of 100 µm or less from the four corners of the EBSD measurement region so that observation positions can be specified. Thereafter, foreign matter or the like adhering to a surface layer is polished away to leave a structure of an observed section, followed by nital etching. When the Vickers indentation is used as a mark, the same region as the EBSD measurement region can be observed. In a case where foreign matter adheres to the surface of the sample, the foreign matter is removed as necessary by a method such as buffing using alumina particles having a secondary particle size of 0.1 µm or less, polishing using colloidal silica not containing an alkaline solution at room temperature, or Ar ion sputtering.
[0110] An image obtained by the observation with the thermal field-emission scanning electron microscope (FE-SEM) is subjected to image analysis, and particles having a higher brightness than that of a primary phase structure, a particle size (circle equivalent diameter) of 0.3 µm or more and 2.0 µm or less, and an aspect ratio RI/Rs, which is a ratio between a minor axis Rs and a major axis Rl of a particle, of less than 2.5 (spherical), or plate-like particles having an aspect ratio RI/Rs of 2.5 or more are determined to be cementite. A region in which these spherical cementite particles are present at a density of 3.0 (/µm2) or more per unit area or a region in which plate-like cementite particles are distributed in a lamellar form, is determined to be pearlite. The plate-like cementite may have a deformed and curved shape due to rolling. In addition, the lamellar form means a form in which, for three or more of the above plate-like cementite particles, an interior angle at an intersection point where long sides of adjacent particles intersect is within 15° (including cases where the particles are parallel and do not intersect), and the closest distance between the adjacent plate-like cementite particles is 2 µm or less.
[0111] Next, the crystal orientation information obtained by the EBSD measurement is used to calculate the area ratio of the residual austenite by using the "Phase Map" function provided in the software "OIM Analysis (registered trademark)" included in the EBSD analysis apparatus. In the calculation of the area ratio, a region having an fcc crystal structure is determined to be residual austenite.
[0112] In addition, for a region having a bcc crystal structure, the "Grain Average Misorientation" function provided in the software "OIM Analysis (registered trademark)" included in the EBSD analysis apparatus is used to determine whether the region is cementite, pearlite, ferrite, bainite, martensite, or tempered martensite, and an area ratio of the region is measured.
[0113] Specifically, under the condition in which a boundary having a crystal misorientation of 15° or more is defined as a grain boundary (15° grain boundary), a region in which a Grain Average Misorientation value (GAM value) is 3.0° or less is determined to be ferrite.
[0114] In addition, a region where the GAM value is more than 3.0° is determined to be martensite, bainite, or tempered martensite. Furthermore, the EBSD measurement results and the structure image obtained by the FE-SEM observation are superimposed using the Vickers indentation as a mark, and for regions determined to be cementite and pearlite from the structure image of the FE-SEM observation, these regions are determined to be cementite and pearlite regardless of the classification of the structure determined by the "Phase Map" function and the "Grain Average Misorientation" function. At this time, positions of cementite and pearlite may be specified by comparison between a grain boundary MAP using the 15° grain boundary and a position of the Vickers indentation.
[0115] In a case where the rolling direction of the steel member is unknown, the rolling direction can be determined by the following method.
[0116] A test piece is collected from any position 50 mm or more away from the end portion of the steel member (a position avoiding the end portion in a case where the test piece cannot be collected from this position) so that a cross section in the sheet thickness direction can be observed.
[0117] The cross section of the collected test piece is finished by mirror polishing, and then observed at each of magnifications of 100-fold, 200-fold, 500-fold, and 1,000-fold using an optical microscope. An observation result at an appropriate magnification at which dimensions of inclusions can be measured is selected according to the dimensions of the inclusions. The observation range is set to a range of 500 µm or more in width and across the overall sheet thickness, and a region having a low brightness is determined to be an inclusion. The observation may be performed in a plurality of visual fields.
[0118] Next, using a cross section that is first observed by the above method as a reference, a cross section parallel to a cross section obtained by rotating the test piece around the sheet thickness direction as an axis in 5° increments within a range of 0° to 180° is observed. An average value of lengths of major axes of a plurality of inclusions in each of the obtained cross sections is calculated for each cross section, and a direction parallel to a major axis direction of the inclusions in a cross section in which the average value of the lengths of the major axes of the inclusions is maximum is determined to be the rolling direction.
[0119] In a case where the rolling direction of the steel member can be identified, for example, in a case of a steel member that is manufactured using a steel sheet directly collected from a steel strip or the like, and in which a lengthwise direction (that is, rolling direction) of the steel strip in the steel member can be identified, the above method does not need to be used.
<Texture>
[0120] As a result of the studies by the present inventors, it was found that the resistance to hydrogen embrittlement differs depending on the crystal orientation of the crystal grains. In particular, it was found that the crystal orientations of {111}<011>, {111}<112>, {100}<011>, and {100}<001> have a significant influence on the hydrogen embrittlement resistance. Therefore, in the steel member according to the present embodiment, a probability of the presence of the crystal grains having the above-described crystal orientations is controlled (the development degree of the crystal grains having the specific crystal orientation is controlled) to improve the hydrogen embrittlement resistance.
[0121] Crystal grains having the crystal orientations of {111}<011> and {100}<011> have a particularly large adverse effect on the hydrogen embrittlement resistance, and crystal grains having the crystal orientations of { 111 }<112> and { 100 }<001> have a particularly large effect on improving the hydrogen embrittlement resistance.
[0122] Therefore, in the steel member according to the present embodiment, at the 1/4 depth position, when a random intensity ratio of {111}<011> is denoted by I1, a random intensity ratio of { 111 }<112> is denoted by I2, a random intensity ratio of {100}<011> is denoted by I3, and a random intensity ratio of { 100}<001> is denoted by I4, a texture is provided in which I1, I2, I3, and I4 satisfy Expression (1).
[0123] In a case where (I1 + I3)/(I2 + I4) is more than 1.20, the hydrogen embrittlement resistance decreases. Preferably, (I1 + I3)/(I2 + I4) is 1.15 or less, 1.00 or less, or 0.90 or less.
[0124] The random intensity ratios (values of I1, I2, I3, and I4) are measured by the following method.
[0125] A test piece is collected from any position (a position avoiding an end portion in a case where the test piece cannot be collected from this position) 50 mm or more away from an end surface of the steel member so that the microstructure at the 1/4 depth position can be observed in a cross section parallel to the rolling direction and parallel to the sheet thickness direction.
[0126] The cross section of the test piece is polished using #600 to #1500 silicon carbide paper and is thereafter mirror-finished using a liquid obtained by dispersing a diamond powder having a particle size of 1 to 6 µm in a diluted solution such as alcohol or in pure water. Next, the cross section of the test piece is polished at room temperature using colloidal silica containing no alkaline solution to remove strain introduced into a surface layer of the test piece.
[0127] At any position in the longitudinal direction (rolling direction) of the cross section of the obtained test piece, measurement is performed by the electron backscatter diffraction (EBSD) method in a range of 300 µm (an entire range of 1/8 to 3/8 of the thickness from the surface in a case where the thickness is less than 1.2 mm) in the thickness direction at the 1/4 depth position, with a measurement interval of 4 µm and a measurement area of 150,000 µm2 or more to obtain crystal orientation information. For the measurement, an apparatus including a thermal field-emission scanning electron microscope (JSM-7001F manufactured by JEOL Ltd.) and an EBSD detector (DVC5 type detector manufactured by TSL) is used. In this case, the degree of vacuum in the apparatus is set to 9.6 × 10-5 Pa or less, an accelerating voltage is set to 15 kv, an irradiation current level is set to 13, and an irradiation level of an electron beam is set to 62.
[0128] The obtained crystal orientation information is calculated using spherical harmonic functions using OIM Analysis (registered trademark) manufactured by TSL, and the random intensity ratio of each orientation is obtained from the crystal orientation distribution function (ODF) that displays the calculated three-dimensional texture.
[0129] In the analysis, "sample symmetry" is set to "orthohombic", the ODF is created for a range of phi1 (φ1) = 0° to 90° and PHI (Φ) = 0° to 90° in a cross section of phi2 (φ2) = 45° at "Bunge Euler angles" and the random intensity ratio of each orientation is calculated. Since there is a measurement error due to the processing of the test piece and the setting of the sample, maximum values are adopted for the random intensity ratio I1 of { 111 }<011> (in a range of Φ = 50° to 60° and φ1 = 0° to 10°), the random intensity ratio I2 of {111}<112> (in a range of Φ = 50° to 60° and φ1 = 25° to 35°), the random intensity ratio 13 of {100}<011> (in a range of Φ = 0° to 10° and φ1 = 0° to 10°), and the random intensity ratio I4 of { 100}<001> (in a range of Φ = 0° to 10° and φ1 = 40° to 50°). The above-described phi1 (φ1) and PHI (Φ) are defined in the ODF drawing function of the analysis software (OIM Analysis).
<Properties>
[0130] A tensile (maximum) strength TS of the steel member according to the present embodiment is preferably more than 1,500 MPa. The tensile strength is more preferably 1,800 MPa or more, and even more preferably 2,300 MPa or more. The tensile strength may be set to 3,000 MPa or less or 2,700 MPa or less as necessary. There is a correlation between the tensile strength and a Vickers hardness, and in the present embodiment, a value obtained by multiplying the Vickers hardness by 3.33 can be regarded as the tensile (maximum) strength TS. Therefore, in the steel member according to the present embodiment, the Vickers hardness (HV1) at a test force of 9.807 N (load 1 kgf) is preferably 450 or more. The Vickers hardness (HV1) is more preferably 470 or more, 510 or more, or 540 or more, and even more preferably 600 or more or 690 or more. The Vickers hardness (HV1) may be set to 900 or less, 860 or less, or 820 or less as necessary.
[0132] A sample is cut out from any position 50 mm or more away from the end surface of the steel member so that a cross section (thickness direction cross section) perpendicular to the surface can be observed. A size of the sample depends on a measurement device, but may be set so that a size of about 10 mm can be observed in the rolling direction. The cross section of the sample is polished using #600 to #1500 silicon carbide paper and thereafter mirror-finished using a liquid obtained by dispersing a diamond powder having a particle size of 1 to 6 µm in a diluted solution such as alcohol or pure water. For the cross section mirror-finished, the Vickers hardness (HV1) is obtained by measuring the hardness at 20 points in total at intervals of three or more times the indentation with a test force of 9.807 N in accordance with JIS Z 2244-1:2020 in a direction parallel to a sheet surface at the 1/4 depth position of the base steel sheet using a micro-Vickers hardness tester, and calculating an average value thereof.
[0133] In addition, in the steel member according to the present embodiment, the chemical composition, the microstructure, and the texture are controlled as described above, so that the hydrogen embrittlement resistance is excellent.
[Coating]
[0134] A part or the entirety of the surface of the steel member according to the present embodiment may have a coating.
[0135] The coating may be a coating primarily containing an Fe-Al-based alloy (Fe-Al-based coating) or a coating primarily containing an Fe-Zn-based alloy (Fe-Zn-based coating). The coating is also referred to as a film, an alloyed plating layer, or an intermetallic compound layer.
[0136] The coating primarily containing an Fe-Al-based alloy is a coating containing 70 mass% or more of Fe and Al in total, and the coating primarily containing an Fe-Zn-based alloy is a coating containing 70 mass% or more of Fe and Zn in total. The coating primarily containing an Fe-Al-based alloy may further contain, in addition to Fe and Al, Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Zn, Co, In, Bi, Zr, Se, As, and REM, and a remainder including impurities. The coating primarily containing an Fe-Zn-based alloy may further contain, in addition to Fe and Zn, Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Al, Co, In, Bi, Zr, Se, As, and REM, and a remainder including impurities.
[0137] By including the coating, corrosion resistance is obtained, so that an effect of improving the hydrogen embrittlement resistance in use in a vehicle can be obtained.
[0139] The chemical composition and the thickness of the coating can be obtained by observing a cross section with a scanning electron microscope.
[0140] Specifically, a measurement sample is cut out from a 1/2 portion of the steel member in a longitudinal direction (a 1/2 position of a length in the longitudinal direction from a longitudinal end portion) and a 1/4 width portion (a 1/4 position of the width in the width direction from the width-directional end portion) and is observed. An observation range of the microscope is set to, for example, a range of 40,000 µm2 or more in terms of area at a magnification of 400-fold. The cut sample is mechanically polished and subsequently mirror-finished. Next, the thickness of the coating is measured in any 10 visual fields, and an average value thereof is used as the thickness of the coating.
[0141] By observation with a BSE image (or a COMPO image) a clear difference in contrast between the coating and the base metal (steel sheet substrate) is confirmed. Therefore, the thickness of the coating can be measured by measuring a thickness from an outermost surface to a position where the contrast changes. Measurement is performed at 20 points at equal intervals in an observation photograph, and a distance between the measurement points is set to 6.5 µm. During the measurement, observation is performed in five visual fields in the above-described manner, and an average value thereof is used as the thickness of the coating.
[0142] In addition, as the chemical composition of the coating, the amounts of Fe, Al, and Zn contained in the coating can be obtained by performing spot elemental analysis (beam diameter: 1.0 µm or less) on the observation range described above using an electron probe micro-analyzer (EPMA). A total of 10 points are analyzed in the coating in 10 random visual fields, and average values thereof are regarded as the amounts of Fe, Al, and Zn contained in coating. Even in a case where an element other than Fe, Al, and Zn is contained, the amount thereof is obtained using the same method.
[Steel Sheet]
[0143] Next, the steel sheet according to the present embodiment will be described. The steel sheet according to the present embodiment can be used as a material for the steel member according to the present embodiment, since the steel member according to the present embodiment can be obtained by performing a heat treatment such as hot stamping on the steel sheet.
[0144] Hereinafter, when a range between a 1/8 position of a sheet thickness in a sheet thickness direction from a surface of the steel sheet and a 3/8 position of the sheet thickness in the sheet thickness direction from the surface is defined as a 1/4 depth position.
[0145] The steel sheet according to the present embodiment has a predetermined chemical composition, and, at the 1/4 depth position, when a random intensity ratio of {111}<011> is denoted by I1, a random intensity ratio of { 111 }<112> is denoted by 12, a random intensity ratio of {100}<011> is denoted by I3, and a random intensity ratio of {100}<001> is denoted by I4, has a texture in which I1, I2, I3, and I4 satisfy Expression (1).
[0146] The steel sheet according to the present embodiment may be coated on the surface. Even in that case, since the coating is not a steel sheet, the chemical composition, a microstructure, the texture, and the like of the steel sheet are a chemical composition, a microstructure, a texture, and the like of a part excluding the coating (this part is sometimes referred to as a "base steel sheet" or "base metal steel sheet").
[0147] In a case where the surface of the steel sheet according to the present embodiment is coated, that is, the surface of the steel sheet according to the present embodiment serving as a reference for the 1/4 depth position is a surface of the part excluding the coating (base steel sheet), that is, a boundary between the base steel sheet and the coating. Therefore, for example, in a case where front and back surfaces of the steel sheet are coated, a thickness t1' of the steel sheet including the coating is measured, a thickness t2' of the coating is then measured by a method described later, and a thickness (thickness of the base steel sheet) t3' = t1' - 2 × t2' of the steel sheet is calculated. Thereafter, a range separated by t3'/8 to (3 × t3')/8 in the thickness direction from a center of the thickness t1' of the steel member including the coating can be set as the 1/4 depth position.
<Chemical Composition>
[0149] The chemical composition of the steel sheet according to the present embodiment needs to be set to obtain preferable properties for the steel member after the heat treatment. However, since the chemical composition does not substantially change due to the heat treatment, the chemical composition of the steel sheet according to the present embodiment may be the same as the chemical composition of the steel member according to the present embodiment.
[0151] The chemical composition can be obtained by performing elemental analysis on the 1/4 depth position of the steel sheet (a range of 1/8 to 3/8 of the thickness from the surface in the thickness direction) using a general method such as ICP-AES. For elements, which are difficult to measure using ICP-AES, C and S may be measured using a combustion-infrared absorption method, N may be measured using an inert gas fusionthermal conductivity method, and O may be measured using an inert gas fusion-non-dispersive infrared absorption method. In a case where ladle analysis values of molten steel or a chemical composition of a slab at the 1/4 depth position is known, the ladle analysis values of molten steel or the chemical composition of the slab at the 1/4 depth position may be used as the chemical composition of the steel sheet.
<Microstructure>
[0152] The microstructure of the steel sheet according to the present embodiment is not limited, but in terms of workability, the microstructure at the 1/4 depth position preferably contains, by area ratio, ferrite: 5% or more and less than 90%, pearlite: more than 10% and 95% or less, and a remainder in microstructure including one or more of bainite, martensite, cementite, and residual austenite.
[0153] Ferrite is a structure having low strength and excellent ductility. When ferrite is contained in less than 5%, cold rolling properties are poor, and there is a concern that defects in a shape of the steel sheet are incurred. Therefore, the area ratio of ferrite is preferably 5% or more. An upper limit of the area ratio of ferrite is not particularly limited. However, in a case where the C content is 0.260% or more, it is difficult to control ferrite to 90% or more. Therefore, the area ratio of ferrite may be set to less than 90%. The area ratio of ferrite may be set to 85% or less as necessary.
[0154] Pearlite contains fine lamellar cementite within a structure, and is an important structure that shares austenite nucleation sites during heating. The area ratio is preferably more than 10% from the viewpoint of suppressing coarsening of prior γ grains of the steel member. On the other hand, when the area ratio of pearlite is more than 95%, the cold rolling properties become poor, and defects in the shape of the steel sheet are incurred. Therefore, an upper limit thereof is preferably set to 95% or less.
[0155] The remainder includes one or more of bainite, martensite, cementite, and residual austenite. It is preferable that the total of the area ratios of ferrite and pearlite is 50% or more, and the area ratio of the remainder is 50% or less. The total of ferrite and pearlite is more preferably 80% or more.
[0156] Microstructural fractions in the microstructure of the steel sheet at the 1/4 depth position can be measured in the same manner as for the steel member. In a case where a rolling direction of the steel sheet is unknown, the rolling direction can be determined in the same manner as for the steel member.
<Texture>
[0157] In the steel sheet according to the present embodiment, at the 1/4 depth position, when a random intensity ratio of { 111 }<011 > is denoted by I1, a random intensity ratio of { 111 }<112> is denoted by I2, a random intensity ratio of {100}<011> is denoted by I3, and a random intensity ratio of {100}<001> is denoted by I4, a texture is provided in which I1, I2, I3, and I4 satisfy Expression (1).
[0158] This texture is retained even after a heat treatment such as hot stamping under predetermined conditions is performed. Therefore, by controlling the steel sheet to have a texture satisfying Expression (1) at a stage of the steel sheet, a steel member obtained by performing a heat treatment including quenching on this steel sheet also has a texture satisfying Expression (1).
[0159] The random intensity ratios (values of I1, I2, I3, and I4) of the steel sheet can be measured in the same manner as for the steel member.
[0160] The sheet thickness of the steel sheet in the present embodiment is not limited, but may be set to 0.4 to 5.0 mm from the viewpoint of forming into a component.
[Coating]
[0161] A part of the surface of the steel sheet according to the present embodiment may have a coating. The coating may be a coating primarily containing Al (Al-based coating) or a coating primarily containing Zn (Zn-based coating). The coating is also referred to as a film or a plating layer. The coating primarily containing Al is a coating containing 70 mass% or more of Al, and the coating primarily containing Zn is a coating containing 70 mass% or more of Zn. The coating primarily containing Al may further contain, in addition to Al, Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Zn, Co, In, Bi, Zr, Se, As, and REM, and a remainder including impurities. The coating primarily containing Zn may further contain, in addition to Zn, Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Al, Co, In, Bi, Zr, Se, As, and REM, and a remainder including impurities. A chemical composition and a thickness of the coating can be measured in the same manner as for the steel member.
[Manufacturing Method]
[0162] Details of manufacturing methods of the steel member according to the present embodiment and the steel sheet according to the present embodiment will be described below, but the steel member and the steel sheet can be manufactured according to a manufacturing method including the following steps.
[0163] Specifically, the steel sheet according to the present embodiment can be manufactured by a manufacturing method including the following steps:
- (I) a casting step of obtaining a slab having a predetermined chemical composition by casting;
- (II) a heating step of heating the slab to a temperature range of 1,200°C or higher and holding the slab for 20 minutes or longer;
- (III) a hot rolling step of performing hot rolling on the slab after the heating step to obtain a hot-rolled steel sheet;
- (IV) a cooling step of cooling the hot-rolled steel sheet to T1 - 150°C or lower;
- (V) a coiling step of coiling the hot-rolled steel sheet after the cooling step at a coiling temperature of T1 - 150°C to 450°C; and
- (VI) a cold rolling step of performing cold rolling on the hot-rolled steel sheet after the coiling step at a rolling reduction of 15% to 60% to obtain a steel sheet (cold-rolled steel sheet).
[0164] In addition, the manufacturing method of the steel sheet according to the present embodiment may further include any or both of the following steps:
(VII) an annealing step of heating the steel sheet after the cold rolling step to an annealing temperature (maximum attainment temperature) of 680°C to 950°C and holding the steel sheet in a temperature range of 680°C to 950°C for 5 to 1,200 seconds; and
(VIII) a coating forming step of forming a coating on a surface of the steel sheet.
[0165] In addition, the steel member according to the present embodiment can be manufactured by a manufacturing method including the following step using the steel sheet according to the present embodiment obtained through the above steps:
(IX) a heat treatment step of heating the steel sheet according to the present embodiment to a temperature range of Ac3 to Ac3 + 300°C at an average heating rate of 1 to 1,000 °C/s, holding the steel sheet in the temperature range for 60 to 600 seconds, and then cooling the steel sheet to a temperature range of 300°C or lower at an average cooling rate of 20 °C/s or faster.
[0167] The temperature of the slab and the temperature of the steel sheet in the present embodiment refer to a center temperature of the slab and a surface temperature of the steel sheet.
[Manufacturing Method of Steel Sheet]
<Casting Step>
[0168] In the casting step, a slab having a predetermined chemical composition is obtained by casting. The chemical composition of the slab may be the same as the steel sheet according to the present embodiment.
[0169] Molten steel having the same chemical composition as the steel sheet according to the present embodiment is melted by a normal melting method such as a converter or an electric furnace, and cast into a slab by casting such as a continuous casting method. In addition to the continuous casting method, an ingot-making method, a thin slab casting method, or the like may also be adopted to manufacture the steel slab.
<Heating Step>
[0170] In the heating step, the slab is heated such that a center portion of the slab reaches 1,200°C or higher prior to the hot rolling, and held at 1,200°C or higher for 20 minutes or longer. When a heating temperature (the temperature of the center portion of the slab) is lower than 1,200°C or a holding time is shorter than 20 minutes, sufficient uniformity of crystal grains cannot be obtained in the subsequent steps, and an effect of sufficient uniformity of crystal grains cannot be obtained in the subsequent rough rolling step.
[0171] The temperature of the center portion of the slab can be obtained by actually measuring a surface temperature of the slab with a radiation-type thermometer and performing heat transfer calculations.
<Hot Rolling Step>
[0172] In the hot rolling step, the heated slab is hot-rolled to obtain a hot-rolled steel sheet, and the hot-rolled steel sheet is cooled to a coiling temperature. It is preferable that the hot rolling includes rough rolling and finish rolling, and conditions of each of the rough rolling and the finish rolling are preferably as follows. The temperatures controlled in the following steps are all surface temperatures of the steel sheet.
<<Rough Rolling>>
[0173] In the rough rolling, it is preferable to perform rolling two or more times at a rolling reduction of 30% or more and complete the rough rolling in a temperature range of T1 + 50°C or higher. By performing the rolling two or more times at a rolling reduction of 30% or more, uniformity of the microstructure can be increased, and a predetermined texture can be obtained.
[0174] In addition, when the temperature (the temperature on an outlet side of a final pass of the rough rolling) at which the rough rolling is completed is denoted by Tr, in a case where Tr is lower than T1 + 50°C, recrystallization before the start of the finish rolling becomes non-uniform, austenite grain sizes become non-uniform, and the microstructure during the finish rolling becomes non-uniform, so that a predetermined texture cannot be obtained. Therefore, the rough rolling completion temperature Tr is set to T1 + 50°C or higher.
[0175] In addition, in the rough rolling, the time from the final pass where rolling is performed at a rolling reduction of 30% or more to the start of the finish rolling is denoted by trs (sec), and trs, Tr, and T1 (°C) obtained by Expression (3) are set to satisfy Expression (2).
<<Finish Rolling>>
[0176] After the rough rolling, finish rolling is performed. In the finish rolling, a finish rolling start temperature Ts is set to be equal to or higher than T1 (°C) obtained by Expression (3), and a finish rolling completion temperature is set to T1 - 20°C or lower.
[0177] Here, the element symbols in the expression are the amounts of the elements in the steel sheet by mass%.
[0178] By setting a total rolling reduction (cumulative rolling reduction) at T1 to T1 + 150°C to 80% or more and the total rolling reduction at lower than T1 °C to 10% to 50%, a predetermined texture is developed.
[0179] In a case where the total rolling reduction (cumulative rolling reduction) at T1 to T1 + 150°C is less than 80%, or the total rolling reduction at lower than T1 °C is less than 10% or more than 50%, a predetermined texture does not develop, or a texture in an undesirable orientation develops.
[0180] When the finish rolling completion temperature is higher than T1 - 20°C, the uniformity of the microstructure decreases, and a predetermined texture cannot be obtained.
[0181] On the other hand, when the finish rolling completion temperature is lower than T1 - 100°C, a predetermined texture cannot be obtained, which is not preferable. Therefore, the finish rolling completion temperature is preferably set to T1 - 100°C or higher.
[0182] Here, the total rolling reduction (%) in each temperature range is calculated by (start thickness - final thickness) / start thickness × 100, and is calculated for each rolling in each temperature range. Therefore, the sum of the total rolling reductions in each temperature range may simply exceed 100%.
<Cooling Step>
[0183] In the cooling step, the hot-rolled steel sheet after the finish rolling is cooled. The time until the start of the cooling step after the finish rolling and the average cooling rate in a predetermined temperature range affect the formation of the texture of the steel sheet. Therefore, the elapsed time from the completion of the finish rolling to the start of cooling is set to shorter than 2.5 seconds, and the steel sheet is cooled to a temperature of T1 - 150°C or lower at an average cooling rate of faster than 50 °C/s and 150 °C/s or slower.
[0184] In a case where the elapsed time from the completion of the finish rolling to the start of the cooling is 2.5 seconds or longer, or the average cooling rate up to T1 - 150°C or lower is 50 °C/s or slower, the predetermined texture does not develop.
[0185] On the other hand, when the average cooling rate to T1 - 150°C or lower is faster than 150 °C/s, temperature control becomes difficult, the material becomes non-uniform, and cold rolling properties deteriorate, which causes defects in the shape of the steel sheet. Therefore, the average cooling rate to T1 - 150°C or lower is preferably set to 150 °C/s or slower.
[0186] A cooling stop temperature is preferably set to 450°C or higher in order to secure the coiling temperature.
<Coiling Step>
[0187] In the coiling step, the hot-rolled steel sheet after the cooling step is coiled at a temperature of T1 - 150°C to 450°C.
[0188] When the coiling temperature is higher than T1 - 150°C, the steel sheet is coiled while transformation hardly progresses, and transformation progresses in the coil, so that defects in a shape of a coil are incurred, which is not preferable. The coiling temperature is more preferably T1 - 150°C or lower and 800°C or lower.
[0189] On the other hand, when the coiling temperature is lower than 450°C, bainite is excessively generated, the cold rolling properties of the steel sheet deteriorate, and defects in the shape of the steel sheet are incurred. Therefore, the coiling temperature is preferably 450°C or higher.
<Cold Rolling Step>
[0190] In the cold rolling step, the hot-rolled steel sheet after the coiling step is cold-rolled at a rolling reduction (cumulative rolling reduction) of 15% to 60% to obtain a steel sheet (cold-rolled steel sheet).
[0191] A predetermined texture can be developed at a rolling reduction of 15% or more. When the rolling reduction is less than 15%, the development of the texture cannot be sufficiently achieved.
[0192] On the other hand, when the rolling reduction is more than 60%, a texture in an undesirable orientation develops.
<Annealing Step>
[0193] The steel sheet after the cold rolling step may be annealed for the purpose of softening. This annealing may be a heat treatment for the purpose of softening by reducing a carbon concentration in the surface layer in addition to softening by microstructure control of a sheet thickness center portion of the steel sheet.
[0194] The annealing is preferably performed under the conditions in which the annealing temperature (maximum attainment temperature) is 680°C to 950°C and the holding time in a temperature range of 680°C to 950°C is 5 to 1,200 seconds. The holding time referred to here means the time from when the steel sheet temperature rises and reaches 680°C to when the steel sheet temperature decreases and reaches 680°C after being held at 680°C to 950°C.
<Coating Formation Step>
[0195] In a case where a coating is formed on the surface, the coating is formed on the surface of the steel sheet (the hot-rolled steel sheet after the coiling step, the hot-rolled steel sheet after the hot-rolled sheet annealing step, the cold-rolled steel sheet after the cold rolling step, or the cold-rolled steel sheet after the annealing step) to obtain a coated steel sheet. A method for forming the coating is not particularly limited, and a hot-dip plating method, an electro plating method, a vacuum vapor deposition method, a cladding method, a thermal spraying method, and the like can be used. The hot-dip plating method is the most popular in the industry.
[0196] Examples of the coating may include an Al-based coating containing Al and a Zn-based coating containing Zn.
[0197] In a case where the Al-based coating is formed by hot-dip plating, in addition to Al, Fe is mixed in a plating bath as an impurity in many cases. Furthermore, in addition to the above elements, Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Zn, Co, In, Bi, Zr, Se, As, and mischmetal may be contained in the plating bath as long as 70 mass% or more of Al is contained.
[0198] In the case of performing hot-dip plating, plating may be performed after the steel sheet after the annealing step is cooled to room temperature and is then heated again, or hot-dip plating may be performed after performing cooling to 450°C to 750°C, which is close to a plating bath temperature, after annealing without temporarily performing cooling to room temperature.
[0200] Pretreatments and post-treatments of the coating are not particularly limited, and precoating, solvent coating, an alloying treatment, temper rolling, or the like can be performed. As the alloying treatment, for example, annealing at 450°C to 800°C can be performed. Furthermore, as a post-treatment, temper rolling is useful for shape adjustment and the like, and can achieve, for example, a rolling reduction of 0.1% to 0.5%.
[Manufacturing Method of Steel Member]
<Heat Treatment Step>
[0201] In the heat treatment step, the steel sheet according to the present embodiment is heated to a temperature range of Ac3 to Ac3 + 300°C at an average heating rate of 1.0 to 1,000 °C/s, held in the temperature range for 60 to 600 seconds, and then cooled to a temperature range of 300°C or lower at an average cooling rate of 20 °C/s or faster.
[0202] When the temperature rising rate is slower than 1.0 °C/s, productivity of the heat treatment decreases, which is not preferable. On the other hand, when the temperature rising rate is faster than 1,000 °C/s, a duplex grain structure is formed and the limit hydrogen amount decreases, which is not preferable. The average heating rate referred to here is a value obtained by dividing a temperature difference between the surface temperature of the steel sheet at the time of the start of the heating and the holding temperature by a time difference from the start of the heating to the time when the holding temperature is reached.
[0203] Furthermore, when the heat treatment temperature is lower than Ac3 (°C), ferrite remains after cooling and the strength is insufficient, which is not preferable. On the other hand, when the heat treatment temperature is higher than Ac3 + 300°C, grains become coarse in the structure, and the limit hydrogen amount decreases, which is not preferable.
[0204] When the average cooling rate to 300°C or lower is slower than 20 °C/s, the area ratio of ferrite or pearlite becomes 10% or more, resulting in insufficient strength.
[0205] During heating, holding may be performed for 1 to 300 seconds within a range of the heating temperature ± 10°C.
[0206] In addition, after cooling to a temperature equal to or lower than 300°C, a tempering treatment may be performed in a temperature range of about 150°C to 600°C in order to adjust the strength of the steel member. In addition, a part of the hotstamping formed body may be tempered by, for example, laser irradiation to partially provide a softened region.
[0208] Ac3 (°C) is calculated from Expression (4) using the amount (mass%) of each element in the chemical composition of the steel sheet (steel sheet for hot stamping).
[0209] Here, the element symbols in the expression are the amounts of the elements in the steel sheet by mass%.
Examples
[0210] Slabs having the chemical compositions (ladle analysis values) shown in Tables 1-1 to 1-10 were obtained by casting.
[0211] The slabs were heated until a temperature of a slab center portion reached the temperature shown in "Heating temperature (°C)" in Tables 2-1 to 2-6, and were controlled to be held at a temperature of 1,200°C or higher for "Holding time (min)" shown in Tables 2-1 to 2-6.
[0212] The slabs after the heating were subjected to hot rolling (rough rolling and finish rolling) to obtain hot-rolled steel sheets. In the rough rolling, rough rolling was performed at a rolling reduction of 30% or more in rolling passes including a final stage of the rough rolling for the number of times shown in "Number of rolling passes at rolling reduction of 30% or more (times) " in Tables 2-1 to 2-6, and the rolling was performed so that the rolling was completed at the temperature of "Rough rolling completion temperature (°C)". A sheet thickness of the rough-rolled sheet was controlled in a range of 25 to 50 mm.
[0213] After the completion of the rough rolling, no reduction was performed until the start of finish rolling for the time shown in "Time from completion of rough rolling to start of finish rolling trs (sec)" in Tables 2-1 to 2-6.
[0214] In the finish rolling, rolling was started at "Finish rolling start temperature T1 (°C)", and rolling was performed so that "Total rolling reduction at T1 to T1 + 150°C (%) " and "Total rolling reduction at lower than T1°C (%)" were the values in Tables 2-1 to 2-6, and the rolling was completed at "Finish rolling completion temperature (°C)".
[0216] After the finish rolling was completed, cooling was started after "Time until start of cooling (sec) " shown in Tables 2-7 to 2-12 had elapsed. Cooling to "Cooling stop temperature (°C)" was performed at "Average cooling rate (°C/s)" in Tables 2-7 to 2-12, followed by coiling into coils at "Coiling temperature (°C)", and air cooling to room temperature.
[0217] Next, the obtained hot-rolled steel sheets were cold-rolled at "Rolling reduction (%)" in Tables 2-7 to 2-12 to obtain cold-rolled steel sheets. In addition, some of the cold-rolled steel sheets were further annealed at "Annealing temperature (°C)" for "Annealing holding time (sec)" shown in Tables 2-7 to 2-12. In addition, in some examples, after the annealing, coating (plating) was performed by hot-dip galvanizing (GI), hot-dip galvannealing (GA), or Al plating (Al). The plating was performed by a known method. For GA, heating to a temperature range of 500°C to 570°C was performed after hot-dip galvanizing to promote alloying. In addition, some of the examples were subjected to temper rolling at an elongation rate of 0.2% after the annealing.
[0218] The microstructures at the 1/4 depth positions of the obtained steel sheets were observed in the above-described manner. The area ratios of ferrite and pearlite are shown in Tables 2-7 to 2-12. The remainder of the microstructure was not shown in the table, but included one or more of bainite, martensite, cementite, and residual austenite.
[0219] In addition, for the obtained steel sheets, a ratio (11 + I3)/(I2 + 14) between the random intensity ratio I1 of {111}<011>, the random intensity ratio I2 of {111}<112>, the random intensity ratio I3 of {100}<011>, and the random intensity ratio I4 of {100}<001> was measured.
[0221] In addition, the steel sheets were subjected to a heat treatment of heating to "Heating temperature (°C)" shown in Tables 3-1 to 3-6 at "Average heating rate (°C/s)", holding for "Heating holding time (sec)", and then cooling to "Cooling completion temperature (°C)" at "Average cooling rate (°C/s)" to obtain steel members.
[0222] In some examples, after the heat treatment, tempering was further performed at 150°C for 25 minutes.
[0223] The microstructure at the 1/4 depth position was observed in the above-described manner for the obtained steel members. The area ratio of the hard structure (the total area ratio of martensite, bainite, and tempered martensite) is shown in Tables 3-1 to 3-6.
[0224] In addition, for the obtained steel members, the Vickers hardness was measured as an alternative index of tensile strength.
[0225] Specifically, a sample is cut out from a position 50 mm or more away from the end surface of the steel member so that a cross section (sheet thickness cross section) perpendicular to the surface could be observed. The sample was set to a size that allowed observation of 10 mm in the rolling direction. The cross section of the sample was polished using #600 to #1500 silicon carbide paper and thereafter mirror-finished using a liquid obtained by dispersing a diamond powder having a particle size of 1 to 6 µm in a diluted solution such as alcohol or pure water. For the cross section mirror-finished, the hardness was measured in a direction parallel to the sheet surface at the 1/4 depth position of the sheet thickness from the surface of the base steel sheet using a micro-Vickers hardness tester with a test force of 9.807 N at intervals of three or more times the indentation. A total of 20 points were measured, and the average value thereof was taken as the Vickers hardness (MHV) of the steel member.
[0227] In a case where the Vickers hardness (HV1) was 450 or more, it was determined that the steel sheet had sufficient strength (tensile strength).
[0228] In addition, the hydrogen embrittlement resistance of the obtained steel members was evaluated by a slow strain rate tensile test (slow strain rate technique (SSRT)).
[0229] Specifically, a sample was cut out from a flat part 50 mm or more away from the end surface of the steel member, front and back surfaces were reduced in thickness by the same amount by polishing up to a thickness of 1.2 mm, and a test piece having a width of 9.0 mm and a length of 120 mm was collected. In this test piece, a length of a parallel portion was set to 20 mm, a width of the parallel portion was set to 2.0 mm, and U-notches having a notch depth of 0.35 mm and a notch bottom radius of 0.1 mm were provided on both sides of the parallel portion.
[0230] The test piece provided with the U-notches was immersed in a 3% NaCl solution, and using a galvanostat as a power source, a current density at the immersed part of the surface of the test piece was controlled to 0.1 mA/cm2 to perform hydrogen charging for 24 hours. Next, a low strain rate tensile test was conducted on the test piece subjected to the hydrogen charging at a tensile rate of 0.0060 mm/min, and a stress (SSRT TS (MPa)) at the time of fracture was investigated. The same test was conducted three times for the same manufacturing No., and an average value of the three fracture loads in such a hydrogen environment was obtained.
[0231] In a case where a relationship between the SSRT TS and the Vickers hardness (MHV) of the steel member satisfied the following expression, the steel member was determined to have excellent hydrogen embrittlement resistance (GOOD in the table).
[0233] As can be seen from Tables 1-1 to 3-6, in the steel members of the present invention examples, the chemical composition, the microstructure (area ratio of each phase), and the texture were within predetermined ranges, and as a result, the Vickers hardness was high (that is, the tensile strength was high) and the hydrogen embrittlement resistance was excellent.
[0234] Contrary to this, in the steel members of the comparative examples, one or more of the chemical composition, the microstructure (area ratio of each phase), and the texture were outside the ranges of the present invention. As a result, the Vickers hardness and the hydrogen embrittlement resistance of the steel member were both or either inferior.
INDUSTRIAL APPLICABILITY
1. A steel member comprising, as a chemical composition, by mass%:
C: 0.260% to 0.700%;
Si: 0% to 2.000%;
Mn: 0% to 3.00%;
Al: 0% to 1.000%;
Nb: 0% to 0.100%;
Ti: 0% to 0.200%;
Cr: 0% to 1.00%;
B: 0% to 0.0200%;
Mo: 0% to 1.00%;
W: 0% to 2.00%;
Co: 0% to 1.00%;
Ni: 0% to 2.00%;
Cu: 0% to 2.00%;
V: 0% to 1.00%;
Ca: 0% to 0.200%;
Mg: 0% to 0.20%;
REM: 0% to 0.300%;
Sb: 0% to 1.00%;
Sn: 0% to 1.00%;
Zr: 0% to 1.00%;
As: 0% to 1.00%;
Se: 0% to 1.00%;
Bi: 0% to 1.00%;
Ta: 0% to 1.00%;
Re: 0% to 1.00%;
Os: 0% to 1.00%;
Ir: 0% to 1.00%;
Tc: 0% to 1.00%;
P: 0.100% or less;
S: 0.0100% or less;
N: 0.020% or less;
O: 0.010% or less; and
a remainder: Fe and impurities,
wherein, when a range between a 1/8 position of a thickness and a 3/8 position of the thickness in a thickness direction from a surface of the steel member, with respect to a 1/4 position of the thickness from the surface as a center, is defined as a 1/4 depth position,
a microstructure at the 1/4 depth position includes, by area ratio,
martensite, bainite, and tempered martensite: 90% or more in total, and
at the 1/4 depth position, when a random intensity ratio of {111}<011> is denoted
by 11, a random intensity ratio of {111}<112> is denoted by 12, a random intensity
ratio of {100}<011> is denoted by I3, and a random intensity ratio of {100}<001> is
denoted by I4, the steel member has a texture in which the I1, the I2, the 13, and
the I4 satisfy Expression (1),
2. The steel member according to Claim 1,
wherein the chemical composition contains one or more selected from the group consisting of, by mass%,
Nb: 0.005% to 0.100%,
Ti: 0.005% to 0.200%,
Cr: 0.01% to 1.00%,
B: 0.0010% to 0.0200%,
Mo: 0.01% to 1.00%,
W: 0.001% to 2.00%,
Co: 0.01% to 1.00%,
Ni: 0.01% to 2.00%,
Cu: 0.01% to 2.00%,
V: 0.01% to 1.00%,
Ca: 0.001% to 0.200%,
Mg: 0.01% to 0.20%,
REM: 0.001% to 0.300%,
Sb: 0.01% to 1.00%,
Sn: 0.01% to 1.00%,
Zr: 0.01% to 1.00%,
As: 0.01% to 1.00%,
Se: 0.01% to 1.00%,
Bi: 0.01% to 1.00%,
Ta: 0.01% to 1.00%,
Re: 0.01% to 1.00%,
Os: 0.01% to 1.00%,
Ir: 0.01% to 1.00%, and
Tc: 0.01% to 1.00%.
3. The steel member according to Claim 1 or 2, wherein a Vickers hardness at the 1/4
depth position is 450 or more.
4. The steel member according to any one of Claims 1 to 3, wherein the surface of the
steel member has a coating.
5. The steel member according to Claim 4, wherein the coating is an Fe-Al-based coating
or an Fe-Zn-based coating.
6. A steel sheet comprising, as a chemical composition, by mass%:
C: 0.260% to 0.700%;
Si: 0% to 2.000%;
Mn: 0% to 3.00%;
Al: 0% to 1.000%;
Nb: 0% to 0.100%;
Ti: 0% to 0.200%;
Cr: 0% to 1.00%;
B: 0% to 0.0200%;
Mo: 0% to 1.00%;
W: 0% to 2.00%;
Co: 0% to 1.00%;
Ni: 0% to 2.00%;
Cu: 0% to 2.00%;
V: 0% to 1.00%;
Ca: 0% to 0.200%;
Mg: 0% to 0.20%;
REM: 0% to 0.300%;
Sb: 0% to 1.00%;
Sn: 0% to 1.00%;
Zr: 0% to 1.00%;
As: 0% to 1.00%;
Se: 0% to 1.00%;
Bi: 0% to 1.00%;
Ta: 0% to 1.00%;
Re: 0% to 1.00%;
Os: 0% to 1.00%;
Ir: 0% to 1.00%;
Tc: 0% to 1.00%;
P: 0.100% or less;
S: 0.0100% or less;
N: 0.020% or less;
O: 0.010% or less; and
a remainder: Fe and impurities,
wherein, when a range between a 1/8 position of a sheet thickness and a 3/8 position of the sheet thickness in a sheet thickness direction from a surface of the steel sheet, with respect to a 1/4 position of the sheet thickness from the surface as a center, is defined as a 1/4 depth position,
at the 1/4 depth position, when a random intensity ratio of {111}<011> is denoted
by I1, a random intensity ratio of {111}<112> is denoted by I2, a random intensity
ratio of {100}<011> is denoted by 13, and a random intensity ratio of {100}<001> is
denoted by I4, the steel sheet has a texture in which the I1, the I2, the I3, and
the I4 satisfy Expression (1),
7. The steel sheet according to Claim 6,
wherein the chemical composition contains one or more selected from the group consisting of, by mass%,
Nb: 0.005% to 0.100%,
Ti: 0.005% to 0.200%,
Cr: 0.01% to 1.00%,
B: 0.0010% to 0.0200%,
Mo: 0.01% to 1.00%,
W: 0.001% to 2.00%,
Co: 0.01% to 1.00%,
Ni: 0.01% to 2.00%,
Cu: 0.01% to 2.00%,
V: 0.01% to 1.00%,
Ca: 0.001% to 0.200%,
Mg: 0.01% to 0.20%,
REM: 0.001% to 0.300%,
Sb: 0.01% to 1.00%,
Sn: 0.01% to 1.00%,
Zr: 0.01% to 1.00%,
As: 0.01% to 1.00%,
Se: 0.01% to 1.00%,
Bi: 0.01% to 1.00%,
Ta: 0.01% to 1.00%,
Re: 0.01% to 1.00%,
Os: 0.01% to 1.00%,
Ir: 0.01% to 1.00%, and
Tc: 0.01% to 1.00%.
8. The steel sheet according to Claim 6 or 7, wherein the surface of the steel sheet
has a coating.
9. The steel sheet according to Claim 8, wherein the coating is an Al-based coating or
a Zn-based coating.
REFERENCES CITED IN THE DESCRIPTION
This list of references cited by the applicant is for the reader's convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disclaims all liability in this regard.
Patent documents cited in the description
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