Steel plates and parts

Description

[Technical Field]

[0001] The present disclosure relates to steel sheets and components. This application claims priority based on Japanese Patent Application No. 2023-193550, filed on November 14, 2023, the contents of which are incorporated herein by reference. [Background technology]

[0002] In recent years, efforts have been made to reduce the weight of automobile bodies in order to reduce CO2 emissions. For blank formed parts such as press-formed parts, weight can be reduced by reducing the plate thickness of the part material. In particular, for automobile suspension parts such as lower arms and trailing arms, the use of steel plates with a strength of over 980 MPa has begun to be considered in order to achieve weight reduction in automobile bodies.

[0003] The above-mentioned parts have complex shapes. As the strength of steel sheets increases, their formability deteriorates. Therefore, when high-strength steel sheets are used for such parts, necking may occur in the parts due to insufficient formability. Therefore, steel sheets used for such parts are required to have excellent formability, particularly excellent ductility and hole expandability.

[0004] Furthermore, as the strength of steel sheets increases, pre-strain embrittlement deteriorates. As a result, the crashworthiness of parts that have been processed and pre-strained deteriorates. Therefore, steel sheets used in such parts are required to have excellent crashworthiness after pre-straining.

[0005] For example, Patent Document 1 discloses a high-tensile steel plate characterized in that 90 area % or more of the steel structure is bainite, the remainder is a mixed structure (MA structure) consisting of martensite and austenite, ferrite, and pseudo-polygonal ferrite, the bainite structure has an average crystal grain size of 5 to 20 μm, and the prior austenite grains have an average aspect ratio of 5.0 or more. Patent Document 1 discloses that the above configuration makes it possible to achieve weldability (HAZ toughness and weld crack resistance) and stable base material performance (tensile strength and toughness).

[0006] Patent Document 2 discloses a high-strength hot-rolled steel sheet that contains a martensite phase at an area ratio of 95% or more, has a structure in which the average aspect ratio of prior austenite grains is 3.0 or more, has a 5-minute relaxation stress value of 20 MPa or less when a pressure of 400 MPa is applied in a stress relaxation test, and has a tensile strength of 1180 MPa or more. Patent Document 2 discloses that the above configuration enables the production of a high-strength hot-rolled steel sheet that has a high strength of tensile strength TS of 1180 MPa or more, while having significantly improved delayed fracture resistance, and is therefore suitable as a material for automobile parts.

[0007] Patent Document 3 discloses a method for producing a high-strength hot-rolled steel sheet characterized in that the average aspect ratio of prior austenite grains is 1.3 or more and 5.0 or less, and the area ratio of the bainite phase is 80% or more. Patent Document 3 also discloses that the above configuration makes it possible to obtain a high-strength hot-rolled steel sheet with excellent hole expandability. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 2011-246768 [Patent Document 2] International Publication No. 2021 / 193310 [Patent Document 3] Japanese Patent Application Publication No. 2021-116476 Summary of the Invention [Problem to be solved by the invention]

[0009] However, Patent Documents 1 to 3 do not consider the crash resistance characteristics after pre-straining.

[0010] The present disclosure has been made in view of the above circumstances, and an object of the present disclosure is to provide a steel sheet having high strength, excellent ductility and hole expandability, and also having excellent crash resistance after pre-straining, and a part using the steel sheet. [Means for solving the problem]

[0011] The gist of the present disclosure is as follows. [1] Chemical composition, in mass%, C: 0.050~0.200%, Si: 0.05 to 3.00%, Mn: 1.00-3.00%, sol.Al: 0.001~0.500%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0.0100% or less, Nb: 0.001 to 1.000%, Ti: 0.070 to 0.200%, B: 0~0.0100%, Cr: 0~2.00%, Mo: 0-1.00%, Cu: 0-2.00% Ni: 0-2.00% V: 0~0.50%, Sn: 0 to 0.050% As: 0~0.100%, Zr: 0 to 1.000%, Ca: 0 to 0.0200%, Mg: 0 to 0.0200%, Bi: 0 to 0.0200%, Co: 0 to 1.000%, W: 0 to 1.000%, Zn: 0 to 1.000%, REM: 0 to 0.1000%, and The balance is Fe and impurities. In the metal structure at 1 / 4 of the plate thickness from the surface, Area % Bainite: 60.0% or more, less than 90.0% Martensite: over 10.0% and not more than 40.0% Ferrite, pearlite and retained austenite: a total of 10.0% or less; a value obtained by dividing the standard deviation of the aspect ratios of the prior austenite grains by the average value of the aspect ratios of the prior austenite grains is 0.50 or less, The steel plate is characterized in that the average grain size of the prior austenite grains is 30 μm or less. [2] The chemical composition is, in mass%, B: 0.0001 to 0.0100%, Cr: 0.01 to 2.00%, Mo: 0.01 to 1.00%, Cu: 0.01 to 2.00%, Ni: 0.01 to 2.00% V: 0.01 to 0.50%, Sn: 0.001 to 0.050%, As: 0.001 to 0.100%, Zr: 0.001 to 1.000%, Ca: 0.0001 to 0.0200%, Mg: 0.0001 to 0.0200%, Bi: 0.0001 to 0.0200%, Co: 0.001 to 1.000%, W: 0.001 to 1.000%, Zn: 0.001 to 1.000%, and The steel sheet according to the above [1], characterized in that it contains one or more selected from the group consisting of REM: 0.0001 to 0.1000%. [3] A part made of the steel plate according to [1] or [2] above. [Effects of the Invention]

[0012] According to the above aspects of the present disclosure, it is possible to provide a steel plate having high strength, excellent ductility and hole expandability, and excellent crash resistance after pre-straining, and a part using the same. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, the steel sheet and the part according to the present embodiment will be described in detail. However, the present disclosure is not limited to the configuration disclosed in the present embodiment, and various modifications are possible within the scope of the present disclosure.

[0014] In the following, numerical ranges separated by "to" include the lower and upper limits. Numerical values ​​indicated as "less than" and "greater than" are not included in the numerical range. All "%" in chemical composition refers to "mass %."

[0015] The steel sheet according to this embodiment has a chemical composition, in mass%, of C: 0.050 to 0.200%, Si: 0.05 to 3.00%, Mn: 1.00 to 3.00%, sol. Al: 0.001 to 0.500%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0.0100% or less, Nb: 0.001 to 1.000%, Ti: 0.070 to 0.200%, B: 0 to 0.0100%, and Cr: 0 to 2.00%. , Mo: 0-1.00%, Cu: 0-2.00%, Ni: 0-2.00%, V: 0-0.50%, Sn: 0-0.050%, As: 0-0.100%, Zr: 0-1.000%, Ca: 0-0.0200%, Mg: 0-0.0200%, Bi: 0-0.0200%, Co: 0-1.000%, W: 0-1.000%, Zn: 0-1.000%, REM: 0-0.1000%, and the balance: Fe and impurities. Each element will be described in detail below.

[0016] C: 0.050 to 0.200% C is an element necessary for obtaining a desired tensile strength in a steel sheet. If the C content is less than 0.050%, the desired tensile strength cannot be obtained in the steel sheet. Therefore, the C content is set to 0.050% or more. The C content is preferably 0.060% or more, 0.070% or more, 0.080% or more, 0.090% or more, or 0.100% or more. On the other hand, if the C content exceeds 0.200%, the strength of the steel sheet becomes too high and the ductility deteriorates. Therefore, the C content is set to 0.200% or less. The C content is preferably 0.190% or less, 0.170% or less, 0.150% or less, 0.130% or less, or 0.120% or less.

[0017] Si: 0.05 to 3.00% Si is an element effective as a deoxidizer. If the Si content is less than 0.05%, it is not possible to improve the soundness of the steel by deoxidation (to prevent defects such as blowholes from occurring in the steel). Therefore, the Si content is set to 0.05% or more. The Si content is preferably 0.10% or more, 0.30% or more, 0.50% or more, 0.60% or more, or 0.70% or more. On the other hand, if the Si content exceeds 3.00%, striped Si scale will form on the steel sheet surface, impairing the surface properties of the steel sheet. Furthermore, the chemical conversion treatability of the steel sheet will deteriorate. Therefore, the Si content is set to 3.00% or less. The Si content is preferably 2.50% or less, 2.00% or less, 1.50% or less, or 1.20% or less.

[0018] Mn: 1.00 to 3.00% Mn is an element necessary for improving the strength of steel sheet. If the Mn content is less than 1.00%, the desired tensile strength cannot be obtained in the steel sheet. Therefore, the Mn content is set to 1.00% or more. The Mn content is preferably 1.30% or more, 1.50% or more, 1.80% or more, or 2.00% or more. On the other hand, if the Mn content exceeds 3.00%, the strength of the steel sheet becomes too high, resulting in deterioration of the ductility of the steel sheet. Therefore, the Mn content is set to 3.00% or less. The Mn content is preferably 2.70% or less, 2.50% or less, 2.30% or less, or 2.20% or less.

[0019] sol.Al: 0.001~0.500% Sol-Al is an element that acts as a deoxidizer and improves the cleanliness of steel. To achieve this effect, the sol-Al content is set to 0.001% or more. The sol-Al content is preferably 0.005% or more, 0.010% or more, or 0.020% or more. On the other hand, if the sol.Al content exceeds 0.500%, casting becomes difficult. Therefore, the sol.Al content is set to 0.500% or less. The sol.Al content is preferably 0.400% or less, 0.300% or less, 0.200% or less, 0.100% or less, 0.050% or less, 0.040% or less, or 0.035% or less. In the present embodiment, sol. Al means acid-soluble Al, which indicates solute Al present in the steel in a solid solution state.

[0020] P:0.100% or less P is an element that segregates in the center of the steel plate thickness. If the P content exceeds 0.100%, slab cracking is likely to occur, making casting difficult. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.080% or less, 0.050% or less, 0.040% or less, 0.030% or less, 0.020% or less, or 0.015% or less. Since the lower the P content, the better, it is preferable to set it to 0%. However, since an excessive reduction in the P content significantly increases the dephosphorization cost, the P content may be set to 0.001% or more or 0.005% or more.

[0021] S: 0.0300% or less S is an element that exists in steel as sulfides and embrittles slabs. S also deteriorates the formability of steel sheets. If the S content exceeds 0.0300%, the hole expandability of the steel sheet deteriorates. Therefore, the S content is set to 0.0300% or less. The S content is preferably 0.0200% or less, 0.0100% or less, 0.0080% or less, 0.0050% or less, 0.0040% or less, or 0.0030% or less. Since the lower the S content, the better, it is preferable to set it to 0%. However, if the S content is reduced too much, the cost of desulfurization increases significantly. Therefore, the S content may be set to 0.0001% or more, 0.0005% or more, or 0.0010% or more.

[0022] N: 0.1000% or less N is an element that forms coarse nitrides in steel and deteriorates the hole expandability of steel sheets. If the N content exceeds 0.1000%, the hole expandability of steel sheets deteriorates. Furthermore, if a large amount of N is contained, the risk of slab cracking increases. Therefore, the N content is set to 0.1000% or less. The N content is preferably 0.0500% or less, 0.0100% or less, 0.0080% or less, 0.0060% or less, 0.0050% or less, 0.0040% or less, or 0.0030% or less. Since the lower the N content, the better, it is preferable to set it to 0%. However, if the N content is reduced too much, the cost of denitrification will increase significantly. Therefore, the N content may be set to 0.0005% or more, or 0.0010% or more.

[0023] O: 0.0100% or less When O is contained in a large amount in steel, it forms coarse oxides. If the O content exceeds 0.0100%, the hole expandability of the steel sheet deteriorates significantly. Therefore, the O content is set to 0.0100% or less. The O content is preferably 0.0080% or less, 0.0050% or less, 0.0040% or less, or 0.0030% or less. The lower the O content, the better, so it is preferably 0%. However, in order to disperse a large number of fine oxides during deoxidation of molten steel, the O content may be 0.0005% or more or 0.0010% or more.

[0024] Nb: 0.001 to 1.000% Nb is an element that suppresses abnormal grain growth of austenite grains during hot rolling. Nb is also an element that increases the strength of steel sheet by forming fine carbides. If the Nb content is less than 0.001%, it is not possible to reduce the size of prior austenite grains in the metal structure of the steel sheet, and the impact resistance properties deteriorate after pre-straining. Therefore, the Nb content is set to 0.001% or more. The Nb content is preferably 0.010% or more, 0.015% or more, or 0.020% or more. On the other hand, if the Nb content exceeds 1.000%, the hole expandability of the steel sheet deteriorates. Therefore, the Nb content is set to 1.000% or less. The Nb content is preferably 0.900% or less, 0.500% or less, 0.100% or less, or 0.050% or less.

[0025] Ti: 0.070 to 0.200% Ti is an element that forms fine nitrides in steel, thereby increasing the strength of the steel sheet. If the Ti content is less than 0.070%, the strength of the steel sheet decreases. Therefore, the Ti content is set to 0.070% or more. The Ti content is preferably 0.080% or more, 0.090% or more, or 0.100% or more. On the other hand, if the Ti content exceeds 0.200%, the hole expandability of the steel sheet deteriorates. Also, if a large amount of Ti is contained, the risk of slab cracking increases. Therefore, the Ti content is set to 0.200% or less. The Ti content is preferably 0.180% or less, 0.150% or less, 0.130% or less, or 0.120% or less.

[0026] The following elements are optional elements, and the inclusion of these optional elements is not essential, with the lower limit of the content of each optional element being 0%. Each optional element will be explained below.

[0027] B: 0 to 0.0100% B is an element that suppresses phase transformation at high temperatures and increases the strength of the steel sheet. To ensure this effect, the B content is preferably 0.0001% or more. On the other hand, if the B content is excessive, B precipitates are formed, reducing the strength of the steel sheet. Therefore, the B content is set to 0.0100% or less. If necessary, the B content may be set to 0.0080% or less, 0.0050% or less, or 0.0030% or less.

[0028] Cr: 0 to 2.00% Cr is an element that exhibits an effect similar to that of Mn. To ensure that the effect of adding Cr to increase the strength of the steel sheet is obtained, the Cr content is preferably 0.01% or more. On the other hand, if the Cr content exceeds 2.00%, the martensite fraction increases and the ductility of the steel sheet deteriorates. Therefore, the Cr content is set to 2.00% or less. If necessary, the Cr content may be set to 1.50% or less, 1.00% or less, or 0.60% or less.

[0029] Mo: 0 to 1.00% Mo is an element that increases the strength of steel sheets by forming fine carbides in the steel, and to ensure this effect, the Mo content is preferably 0.01% or more. On the other hand, if the Mo content exceeds 1.00%, the hole expandability of the steel sheet deteriorates. Therefore, the Mo content is set to 1.00% or less. If necessary, the Mo content may be set to 0.80% or less, 0.50% or less, or 0.25% or less.

[0030] Cu: 0 to 2.00% Cu has the effect of improving the hardenability of the steel sheet and the effect of precipitating as carbides in the steel at low temperatures to increase the strength of the steel sheet. To more reliably obtain the effects of these actions, the Cu content is preferably 0.01% or more. However, if the Cu content exceeds 2.00%, intergranular cracking may occur in the slab. Therefore, the Cu content is set to 2.00% or less. If necessary, the Cu content may be set to 1.50% or less, 1.00% or less, or 0.60% or less.

[0031] Ni: 0 to 2.00% Ni has the effect of improving the hardenability of the steel sheet and increasing its strength. Furthermore, when Cu is added, Ni has the effect of effectively suppressing grain boundary cracking in the slab caused by Cu. To ensure the above effects, the Ni content is preferably 0.01% or more. Since Ni is an expensive element, it is not economically preferable to include a large amount of it. Therefore, the Ni content is set to 2.00% or less. If necessary, the Ni content may be set to 1.50% or less, 1.00% or less, or 0.60% or less.

[0032] V: 0 to 0.50% V is an element that increases the strength of steel sheets by forming fine carbides in the steel. To ensure this effect, the V content is preferably 0.01% or more. On the other hand, if the V content exceeds 0.50%, the hole expandability of the steel sheet deteriorates. Therefore, the V content is set to 0.50% or less. If necessary, the V content may be set to 0.30% or less, 0.10% or less, or 0.06% or less.

[0033] Sn: 0 to 0.050% Sn is an element that improves the hole expandability of steel sheets by suppressing the formation of oxides that serve as fracture initiation sites. To ensure this effect, the Sn content is preferably 0.001% or more. On the other hand, since the above effects are saturated even when Sn is contained in a large amount, the Sn content is set to 0.050% or less. If necessary, the Sn content may be set to 0.030% or less, 0.010% or less, or 0.006% or less.

[0034] As: 0 to 0.100% As is an element that reduces the austenite single-phase temperature, thereby refining prior austenite grains and improving the hole expandability of the steel sheet. To more reliably obtain this effect, the As content is preferably 0.001% or more. On the other hand, since the above effects are saturated even when As is contained in a large amount, the As content is set to 0.100% or less. If necessary, the As content may be set to 0.050% or less, 0.010% or less, or 0.006% or less.

[0035] Zr: 0 to 1.000% Zr is an element that increases the strength of steel sheets through solid solution strengthening, and in order to more reliably obtain this effect, the Zr content is preferably 0.001% or more. On the other hand, if the Zr content exceeds 1.000%, the hole expandability of the steel sheet deteriorates. Therefore, the Zr content is set to 1.000% or less. If necessary, the Zr content may be set to 0.050% or less, 0.010% or less, or 0.006% or less.

[0036] Ca: 0 to 0.0200% Ca is an element that improves the ductility and hole expandability of steel sheets by controlling the morphology of nonmetallic inclusions that act as fracture initiation sites and cause deterioration of formability. To ensure this effect, the Ca content is preferably 0.0001% or more. On the other hand, if the Ca content exceeds 0.1000%, excessive inclusions are formed in the steel, deteriorating the ductility and hole expandability of the steel sheet. Therefore, the Ca content is set to 0.1000% or less. If necessary, the Ca content may be set to 0.0500% or less, 0.0100% or less, or 0.0060% or less.

[0037] Mg: 0 to 0.0200% Like Ca, Mg is an element that controls the morphology of nonmetallic inclusions and thereby improves the ductility and hole expandability of steel sheets. To more reliably obtain this effect, the Mg content is preferably 0.0001% or more. On the other hand, if the Mg content exceeds 0.0200%, excessive inclusions are formed in the steel, which deteriorates the ductility and hole expandability of the steel sheet. Therefore, the Mg content is set to 0.0200% or less. If necessary, the Mg content may be set to 0.0100% or less, 0.0060% or less, or 0.0040% or less.

[0038] Bi: 0 to 0.0200% Bi has the effect of improving the formability of steel sheet by refining the solidification structure. To ensure the effect of this effect, the Bi content is preferably 0.0005% or more. However, even if the Bi content exceeds 0.020%, the effect of this effect saturates, which is economically undesirable. Therefore, the Bi content is set to 0.020% or less. The Bi content is preferably 0.010% or less. If necessary, the Bi content may be set to 0.0100% or less, 0.0060% or less, or 0.0040% or less.

[0039] Co: 0 to 1.000% Co is an element that increases the strength of steel sheets through solid solution strengthening, and in order to more reliably obtain this effect, the Co content is preferably 0.001% or more. On the other hand, if the Co content exceeds 1.000%, the hole expandability of the steel sheet deteriorates. Therefore, the Co content is set to 1.000% or less. If necessary, the Co content may be set to 0.500% or less, 0.100% or less, or 0.060% or less.

[0040] W: 0 to 1.000% W is an element that increases the strength of steel sheets through solid solution strengthening, and in order to more reliably obtain this effect, the W content is preferably 0.001% or more. On the other hand, if the W content exceeds 1.000%, the hole expandability of the steel sheet deteriorates. Therefore, the W content is set to 1.000% or less. If necessary, the W content may be set to 0.500% or less, 0.100% or less, or 0.060% or less.

[0041] Zn: 0 to 1.000% Zn is an element that increases the strength of steel sheets through solid solution strengthening, and in order to more reliably obtain this effect, the Zn content is preferably 0.001% or more. On the other hand, if the Zn content exceeds 1.000%, the hole expandability of the steel sheet deteriorates. Therefore, the Zn content is set to 1.000% or less. If necessary, the Zn content may be set to 0.500% or less, 0.100% or less, or 0.060% or less.

[0042] REM: 0 to 0.1000% Like Ca, REM is an element that controls the morphology of nonmetallic inclusions and thereby improves the ductility and hole expandability of steel sheets. To more reliably obtain this effect, the REM content is preferably 0.0001% or more. On the other hand, if the REM content exceeds 0.1000%, excessive inclusions are formed in the steel, deteriorating the ductility and hole expandability of the steel sheet. Therefore, the REM content is set to 0.1000% or less. If necessary, the REM content may be set to 0.0500% or less, 0.0100% or less, or 0.0060% or less. REM refers to a total of 17 elements consisting of Sc, Y, and lanthanides, and the REM content refers to the total content of these elements. In the case of lanthanides, they are industrially added in the form of misch metal.

[0043] The balance of the chemical composition of the steel sheet according to this embodiment is Fe and impurities. In this embodiment, the impurities refer to substances that are mixed in from raw materials such as ore and scrap, or the environment during production, or substances that are allowed to exist within a range that does not adversely affect the properties of the steel sheet according to this embodiment.

[0044] The chemical composition of the steel sheet described above may be analyzed using a spark discharge optical emission spectrometer or the like. Values ​​of C and S are determined by burning the steel sheet in an oxygen stream using a gas composition analyzer or the like and measuring the values ​​using an infrared absorption method. Values ​​of N are determined by melting a test piece taken from the steel sheet in a helium stream and measuring the values ​​using a thermal conductivity method. O is measured using an inert gas fusion-non-dispersive infrared absorption method. Sol. Al is measured, for example, according to JIS G 1257-10-2:2013. When the steel sheet has a plating layer or a coating film on the surface, the plating layer or the coating film is removed by mechanical grinding or the like as necessary before analyzing the chemical composition.

[0045] Next, the metal structure of the steel sheet according to this embodiment will be described. In the steel sheet according to this embodiment, the metal structure at a position from the surface to 1 / 4 of the sheet thickness contains, in area percentages, bainite: 60.0% or more and less than 90.0%, martensite: more than 10.0% and 40.0% or less, and ferrite, pearlite, and retained austenite: a total of 10.0% or less, the value obtained by dividing the standard deviation of the aspect ratios of prior austenite grains by the average value of the aspect ratios of the prior austenite grains is 0.50 or less, and the average grain size of the prior austenite grains is 30 μm or less. Each requirement is explained below.

[0046] In this embodiment, the metal structure is defined at a position 1 / 4 of the plate thickness from the surface (a region from 1 / 8 of the plate thickness depth from the surface to 3 / 8 of the plate thickness depth from the surface). In other words, this position is a region starting from 1 / 8 of the plate thickness depth from the surface and ending at 3 / 8 of the plate thickness depth from the surface. The reason for specifying the metal structure at the above positions is that the metal structure at the above positions represents a typical metal structure of the steel plate. In addition, when the steel sheet has a plating layer, a coating film, or the like on the surface, the surface here refers to the interface between the steel sheet and the plating layer, the coating film, or the like.

[0047] Bainite: 60.0% or more, less than 90.0% Bainite is a structure that increases the strength, ductility, and hole expandability of a steel sheet. If the area fraction of bainite is less than 60.0%, the steel sheet cannot achieve the desired ductility. Therefore, the area fraction of bainite is set to 60.0% or more. The area fraction of bainite is preferably 65.0% or more, 70.0% or more, 75.0% or more, or 80.0% or more. On the other hand, if the area fraction of bainite is 90.0% or more, the desired amount of martensite cannot be obtained, and the strength of the steel sheet decreases. Therefore, the area fraction of bainite is set to less than 90.0%. The area fraction of bainite is preferably 88.0% or less or 85.0% or less.

[0048] Martensite: Over 10.0% and up to 40.0% Martensite is stronger than bainite and is a structure that increases the strength of steel sheets. If the area fraction of martensite is 10.0% or less, the desired strength cannot be obtained in the steel sheet. Therefore, the area fraction of martensite is set to be more than 10.0%. The area fraction of martensite is preferably 12.0% or more or 15.0% or more. On the other hand, if the area fraction of martensite exceeds 40.0%, the desired amount of bainite cannot be obtained, and the desired ductility cannot be obtained in the steel sheet. Therefore, the area fraction of martensite is set to 40.0% or less. The area fraction of martensite is preferably 35.0% or less, 30.0% or less, or 25.0% or less. The sum of the area fractions of bainite and martensite is preferably 90.0% or more, and is preferably 92.0% or more, 95.0% or more, or 97.0% or more, and may be 98.5% or more, or 100.0% or more.

[0049] Ferrite, pearlite and retained austenite: total of 10.0% or less The metal structure of the steel sheet according to this embodiment may contain ferrite, pearlite, and retained austenite as remaining structures other than bainite and martensite. If the area ratio of these remaining structures exceeds 10.0%, the steel sheet will not be able to obtain the desired strength. Therefore, the total area ratio of the remaining structures is set to 10.0% or less. The area ratio of the remaining structures is preferably 8.0% or less, 5.0% or less, 3.0% or less, 1.5% or less, or 0.0%.

[0050] The area ratio of each structure is measured by the following method. A test piece is taken from the steel plate in a thickness cross section parallel to the rolling direction so that the metal structure can be observed at a position 1 / 4 of the plate thickness from the surface (a region from 1 / 8 depth of the plate thickness to 3 / 8 depth of the plate thickness from the surface) and at the center position in the plate width direction.

[0051] The cross section of the specimen was polished using #600 to #1500 silicon carbide paper, then polished to a mirror finish using a liquid in which diamond powder with a grain size of 1 to 6 μm was dispersed in a diluted solution such as alcohol or pure water. Next, the specimen was polished for 8 minutes at room temperature using colloidal silica with a grain size of 0.25 μm and no alkaline solution to remove the strain induced in the surface layer of the specimen. At any position along the longitudinal direction of the cross section of the specimen, measurements were made using electron backscatter diffraction at 400x magnification, covering a region 200 μm in the longitudinal direction of the specimen, from 1 / 8 of the surface to 3 / 8 of the surface, at 0.1 μm measurement intervals, to obtain crystal orientation information.

[0052] For the measurement, it is preferable to use an EBSD device consisting of a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL DVC5 detector). In this case, the degree of vacuum inside the EBSD device is 9.6 × 10 -5 It is preferable that the observation conditions are as follows: 1 Pa or less, accelerating voltage is 15 kV, probe current level is 13, and electron beam irradiation level is 62. Electron gun type: Schottky WD (Working Distance): 15mm Objective aperture number: 4 Number of pixels: 4096 x 5120 pixels

[0053] From the obtained crystal orientation information, the "Phase Map" function installed in the "OIM Analysis (registered trademark)" software attached to the EBSD analyzer is used to identify regions with an fcc crystal structure and calculate the area ratio of these regions, thereby obtaining the area ratio of retained austenite.

[0054] Next, those with a bcc crystal structure are judged to be "bainite, ferrite, pearlite, and martensite." For these regions, the "Grain Orientation Spread" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer is used to extract regions with a "Grain Orientation Spread" of 1° or less as ferrite, under the condition that boundaries with a crystal orientation difference of 15° are considered to be grain boundaries. The area ratio of the extracted ferrite is obtained by calculating the area ratio of ferrite.

[0055] Next, within the remaining region (region where "Grain Orientation Spread" exceeds 1°), boundaries with a crystal orientation difference of 15° are considered to be grain boundaries, and when the maximum value of "Grain Average IQ" in the ferrite region is Iα, regions exceeding Iα / 2 are extracted as bainite, and regions where Iα / 2 or less are extracted as "pearlite and martensite." The area fraction of bainite is obtained by calculating the area fraction of the extracted bainite.

[0056] If ferrite is not extracted from the observation field, the GAM "Grain Average Misorientation" function is used for the same field, and under the condition that boundaries with a crystal orientation misorientation of 5° are considered to be grain boundaries, areas where "Grain Average Misorientation" is greater than 0.50° and less than 0.75° are extracted as bainite, and areas where it is greater than 0.75° are extracted as "pearlite and martensite." The area fraction of bainite is obtained by calculating the area fraction of the extracted bainite.

[0057] Contamination on the surface of the observation surface can be removed by buffing using alumina particles with a particle size of 0.1 μm or less, or by Ar ion sputtering.

[0058] The following SEM observation is carried out for the region determined to be "pearlite and martensite" in the EBSD measurement. To perform SEM observation of the same area as the EBSD measurement area, a Vickers indentation is made near the observation position. Surface contamination is polished away, leaving the structure of the observation surface, and the specimen is then etched with nital. After etching, the observation surface is observed under SEM in the same field of view as the EBSD observation area. The magnification is, for example, 170x. Of the areas identified as "pearlite and martensite" in the EBSD measurement, the structure in which plate-like ferrite and Fe-based carbides are layered together in SEM observation is considered to be pearlite, and its area fraction is calculated to obtain the area fraction of pearlite. The area fraction of martensite is obtained by subtracting the area fractions of retained austenite, ferrite, bainite, and pearlite obtained using the above method from 100%.

[0059] The rolling direction of the steel sheet is determined by the following method. A test piece is taken so that the thickness cross section of the steel plate can be observed. The thickness cross section of the taken test piece is mirror-polished and then observed at magnifications of 100x, 200x, 500x, and 1000x using an optical microscope. An appropriate magnification at which the dimensions of the inclusions can be measured is selected depending on the size of the inclusions. The observation range is 500 μm or more in width and across the entire thickness of the plate, and areas with dark brightness are determined to be inclusions. Observation may be performed from multiple fields of view. Next, using the thickness cross section initially observed using the above method as a reference, a plane parallel to the plane rotated in 5° increments in the range of 0° to 180° around the thickness direction is observed using the same method as above. The average value of the length of the major axes of the multiple inclusions in each cross section is calculated for each cross section. The cross section with the largest average value of the length of the major axes of the inclusions obtained is identified. The direction parallel to the major axis of the inclusions in that cross section is determined to be the rolling direction.

[0060] The value obtained by dividing the standard deviation of the aspect ratio of the prior austenite grains by the average value of the aspect ratio of the prior austenite grains is 0.50 or less. If the value obtained by dividing the standard deviation of the aspect ratios of the prior austenite grains by the average value of the aspect ratios of the prior austenite grains (standard deviation / average value) exceeds 0.50, the uniformity of the prior austenite grains is low, and the hole expandability of the steel sheet deteriorates. Therefore, the value obtained by dividing the standard deviation of the aspect ratios of the prior austenite grains by the average value of the aspect ratios of the prior austenite grains is set to 0.50 or less. The value obtained by dividing the standard deviation of the aspect ratios of the prior austenite grains by the average value of the aspect ratios of the prior austenite grains is preferably 0.45 or less, 0.40 or less, or 0.35 or less. The lower limit of the value obtained by dividing the standard deviation of the aspect ratios of the prior austenite grains by the average value of the aspect ratios of the prior austenite grains is not particularly limited, but may be, for example, 0.20 or more or 0.25 or more.

[0061] Average grain size of prior austenite grains: 30 μm or less If the average grain size of the prior austenite grains exceeds 30 μm, the crash resistance properties of the steel sheet after pre-straining deteriorate. Therefore, the average grain size of the prior austenite grains is set to 30 μm or less. The average grain size of the prior austenite grains is preferably 28 μm or less, 25 μm or less, 23 μm or less, or 20 μm or less. Although there is no particular lower limit for the average grain size of the prior austenite grains, if the average grain size of the prior austenite grains is too small, the number of nucleation sites for bainite increases, resulting in a large amount of bainite and a decrease in the strength of the steel sheet. Therefore, from the viewpoint of reliably increasing the strength of the steel sheet, the average grain size of the prior austenite grains may be set to 5 μm or more or 10 μm or more.

[0062] The aspect ratio and average grain size of prior austenite are measured by the following method. A test piece is taken from the steel plate so that the thickness cross section perpendicular to the rolling direction can be observed. The observation surface of the test piece is exposed to a corrosive solution to reveal the cross-sectional structure of the thickness cross section. The corrosive solution used is 100 cc of saturated aqueous picric acid, to which 0.5-2.0 cc of hydrochloric acid, 0.3-1.0 g of sodium dodecylbenzenesulfonate, 0.1-0.3 g of iron chloride, 0.1-0.3 g of calcium chloride, and 0.5-2.0 cc of ethyl alcohol are added. The corrosive solution is heated to a temperature range of 40-60°C. A secondary electron image of an area measuring 200 μm in the rolling direction and 200 μm in the thickness direction is taken at 500x magnification using a scanning electron microscope at a position 1 / 4 of the thickness from the surface of the observation surface (a region from 1 / 8 of the thickness depth from the surface to 3 / 8 of the thickness depth from the surface). Using the obtained secondary electron image, the circle equivalent diameter and area of ​​the prior austenite grains are measured. The scanning electron microscope is equipped with a two-electron detector and has a resolution of 9.6 × 10 -5 In a vacuum of 100 Pa or less, an electron beam is irradiated onto the observation surface at an acceleration voltage of 15 kV and an irradiation current level of 13, and a secondary electron image is taken. As the scanning electron microscope, it is preferable to use a device consisting of a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL DVC5 detector). The observation conditions are preferably as follows: The number of fields of view taken is 10 or more. In the taken secondary electron image, the prior austenite grain boundaries are imaged as bright contrast. Electron gun type: Schottky WD (Working Distance): 15mm Objective aperture number: 4 Number of pixels: 4096 x 5120 pixels

[0063] The circle-equivalent diameter is calculated for one of the prior austenite grains included in the observation field. The above operation is performed for all prior austenite grains included in the observation field, excluding prior austenite grains whose entire crystal grains are not included in the observation field, such as those at the edges of the observation field, to determine the circle-equivalent diameters of all prior austenite grains in the observation field. The circle-equivalent diameters of the prior austenite grains obtained in each observation field are used for calculation based on the following formula to obtain the average grain size of the prior austenite grains.

[0064] Average grain size of prior austenite grains = Σ i (A i ×d i ) / Σ i A i where d i is the circle equivalent diameter of the i-th prior austenite grain, and A i is the area of ​​the i-th prior austenite grain. The circle-equivalent diameter is the diameter of a circle having the same area as the area of ​​the identified crystal grain.

[0065] Next, the major and minor axes of prior austenite grains with a circle equivalent diameter of 2 μm or more contained in the secondary electron image are measured. The aspect ratio of each prior austenite grain is obtained by calculating the ratio of the major axis to the minor axis (major axis / minor axis) obtained by measuring each prior austenite grain. The average aspect ratio of the prior austenite grains is obtained by performing a calculation using the obtained aspect ratio of the prior austenite grains based on the following formula.

[0066] Average aspect ratio of prior austenite grains = Σ i (A i ×E i ) / Σ i A i where E i is the aspect ratio of the i-th prior austenite grain, and A i is the area of ​​the i-th prior austenite grain. In addition, the standard deviation of the aspect ratio of the prior austenite grains is calculated based on the following formula to obtain the standard deviation of the aspect ratio of the prior austenite grains.

[0067] Standard deviation of aspect ratio of prior austenite grains = √{Σa i ×(E i -E')} E' is the average aspect ratio of the prior austenite grains obtained by the above method. E i is the aspect ratio of the i-th prior austenite grain, as above. i is the formula (A i / ΣA i ) (i.e., a i =A i / ΣA i ) , A i is the area of ​​the i-th prior austenite grain, as above.

[0068] In addition, when prior austenite grains having an equivalent circle diameter of less than 2 μm are contained, these are excluded from the above measurement because prior austenite grains having an equivalent circle diameter of less than 2 μm are considered not to affect the properties of the steel sheet according to this embodiment.

[0069] Tensile strength (TS): 980-1200MPa The steel sheet according to this embodiment preferably has a tensile strength of 980 MPa or more. By making the tensile strength 980 MPa or more, the effect of reducing the weight of the vehicle body can be increased. The tensile strength is more preferably 1000 MPa or more or 1050 MPa or more. The upper limit of the tensile strength is set to 1200 MPa or less from the viewpoint of suppressing die wear and ensuring the ductility of the steel sheet, and the tensile strength is more preferably 1150 MPa or more or 1100 MPa or more.

[0070] Total elongation (El): 12% or more Hole expansion ratio (λ): 50% or more The total elongation may be 12% or more and the hole expansion ratio may be 50% or more. If the total elongation is 12% or more, it can be determined that the ductility is excellent, and if the hole expansion ratio is 50% or more, it can be determined that the hole expandability is excellent. If necessary, the total elongation may be 13% or more, or 14% or more. There is no need to set an upper limit to the total elongation, but it may be 20% or less, 18% or less, or 17% or less. If necessary, the hole expansion ratio may be 53% or more, 55% or more, or 57% or more. There is no need to set an upper limit to the hole expansion ratio, but it may be 80% or less, 75% or less, or 70% or less. The total elongation is the "total elongation at break" as defined in JIS Z 2241:2022.

[0071] The tensile strength and total elongation are obtained by taking a No. 5 test piece in accordance with JIS Z 2241: 2022 and conducting a tensile test in accordance with JIS Z 2241: 2022. The tensile test piece is taken from a quarter section from the end in the plate width direction, with the direction perpendicular to the rolling direction as the longitudinal direction.

[0072] The hole expansion ratio is measured by performing a hole expansion test in accordance with JIS Z 2256:2020.

[0073] Impact resistance after pre-straining The crashworthiness after pre-straining is evaluated by performing a Charpy impact test on a test specimen after 5% pre-straining in a tensile test. A tensile test is performed using the same method as described above, with a 5% tensile pre-strain applied to the test specimen. Then, a 2.5 mm subsize V-notch test specimen is taken from the pre-strained test specimen in accordance with JIS Z 2242:2023. A Charpy impact test is performed using this V-notch test specimen in accordance with JIS Z 2242:2023. The temperature at which the brittle fracture surface ratio is 50% is defined as the ductile-brittle transition temperature (DBTT). If the obtained ductile-brittle transition temperature is -40°C or lower, the crashworthiness after pre-straining is deemed to be excellent. For steel plates with a thickness of less than 2.5 mm, the measurement is carried out over the entire thickness.

[0074] The thickness of the steel plate according to this embodiment is not particularly limited, but may be 1.2 to 8.0 mm. By making the thickness of the steel plate 1.2 mm or more, it is possible to prevent the rolling load from becoming excessive and making hot rolling difficult. The thickness may be 1.6 mm or more, 2.0 mm or more, or 2.4 mm or more. Furthermore, by setting the sheet thickness to 8.0 mm or less, the above-mentioned metal structure can be stably obtained after hot rolling. The sheet thickness may also be set to 7.0 mm or less, 6.0 mm or less, or 4.8 mm or less.

[0075] The steel sheet according to this embodiment may be provided with a plating layer on the surface to improve corrosion resistance or the like, thereby forming a surface-treated steel sheet. The plating layer may be an electroplated layer or a hot-dip plated layer. Examples of electroplated layers include electrogalvanized plating and electrolytic Zn-Ni alloy plating. Examples of hot-dip plated layers include hot-dip galvanized plating, alloyed hot-dip galvanized plating, hot-dip aluminum plating, hot-dip Zn-Al alloy plating, hot-dip Zn-Al-Mg alloy plating, and hot-dip Zn-Al-Mg-Si alloy plating. The coating weight is not particularly limited and may be the same as conventional coating weights. Furthermore, it is possible to further enhance corrosion resistance by carrying out an appropriate chemical conversion treatment after plating (for example, applying a silicate-based chromium-free chemical conversion treatment solution and drying it).

[0076] The steel sheet according to the present embodiment has high strength, excellent ductility and hole expandability, and is considered to have excellent crash resistance after pre-straining, and therefore can be suitably used for parts, particularly automobile parts, and among automobile parts, it can be suitably used for automobile suspension parts such as lower arms and trailing arms.

[0077] A part manufactured using the steel plate according to this embodiment has the same chemical composition as the above-described steel plate. Furthermore, the part may have a mixture of processed and unprocessed parts. The unprocessed part has the same metallurgical structure as the above-described steel plate. The processed part basically has the same metallurgical structure as the above-described steel plate, but if heavily processed, it may not have the above-described metallurgical structure. Therefore, when measuring the metallurgical structure of a part, the measurement is performed on the unprocessed part. If there is no unprocessed part, the measurement is performed on the part that has not been heavily processed. An unprocessed or heavily processed part refers to, for example, a flat part of the part, and a part that avoids parts that have been punched, hole-expanded, bent, or the like. As an example, in the case of the above-described part, a test piece is taken from the flat part with the largest area near the center of gravity and measured.

[0078] For example, a lower arm can be manufactured by drawing, bending, and trimming the excess material from the steel plate according to this embodiment, followed by punching and hole expanding, while a trailing arm can be manufactured by burring, bending, and cutting the steel plate according to this embodiment.

[0079] Next, a preferred method for manufacturing the steel sheet according to this embodiment will be described. According to the manufacturing method described below, the steel sheet according to this embodiment can be stably manufactured. The steel sheet according to this embodiment is a steel sheet manufactured by hot rolling a slab, and is not subsequently subjected to cold rolling, so it can also be called a hot-rolled steel sheet. In this embodiment, the temperature of the slab and the temperature of the steel plate refer to the surface temperature of the slab and the surface temperature of the steel plate.

[0080] In a preferred method for manufacturing a steel sheet according to this embodiment, The slab is heated to a temperature range of 1200°C or higher for 3000 seconds or more. In hot rolling, Rolling is performed at least twice at a reduction rate of 40% or more in the temperature range of 1010 to 1180 ° C, and the total interpass time in the temperature range of 1010 to 1180 ° C is 20 seconds or less, The total reduction rate in the temperature range of 960 to 1010 ° C is 0% or more and less than 5%, The total reduction rate in the temperature range of 960°C or less is 20% or more and less than 80%. After the hot rolling, the steel sheet is cooled to a temperature range of 500 to 650°C at an average cooling rate of 50°C / s or more, Air cooling is performed for 3 to 10 seconds in the temperature range of 500 to 650 ° C. After the air cooling, the material is cooled to 100°C at an average cooling rate of 50°C / s or more. Each step will be described below.

[0081] In order to fully dissolve the carbides and obtain the desired strength in the steel sheet, the heating temperature of the slab is set to 1200° C. or higher. From the viewpoint of energy costs, the heating temperature of the slab is preferably set to 1350° C. or lower.

[0082] The slab to be heated is not particularly limited except that it has the above-mentioned chemical composition. For example, a slab produced by melting molten steel having the above-mentioned chemical composition using a converter or an electric furnace and then continuous casting the slab can be used. Instead of continuous casting, an ingot casting method, a thin slab casting method, or the like may also be used.

[0083] In the hot rolling, rolling is performed two or more times at a reduction rate of 40% or more in the temperature range of 1010 to 1180°C, with an interpass time of 20 seconds or less. By performing two or more times of rolling at a high reduction rate in the fully recrystallized range of 1010 to 1180°C and setting the interpass time within a desired time, recrystallization can be promoted and prior austenite grains can be refined. Note that the interpass time here refers to the total time between all passes in the temperature range of 1010 to 1180°C. In view of equipment constraints, the interpass time may be set to 3 seconds or more. In order to further reduce the value obtained by dividing the standard deviation of the aspect ratio of the prior austenite grains by the average aspect ratio of the prior austenite grains, it is preferable to increase the reduction rate of two or more times of rolling in the temperature range of 1010 to 1180°C. The rolling reduction here can be expressed as (1-t1 / t0) x 100 (%), where t0 is the thickness before rolling and t1 is the thickness after rolling.

[0084] The total reduction in the temperature range of 960 to 1010°C is 0% or more but less than 5%, and the total reduction in the temperature range below 960°C is 20% or more but less than 80%. By reducing the reduction in the partial recrystallization region at 960 to 1010°C or by not performing rolling, the uniformity of the austenite grain morphology (aspect ratio) can be improved. As a result, the value obtained by dividing the standard deviation of the aspect ratios of the prior austenite grains by the average aspect ratio of the prior austenite grains can be preferably controlled. To further reduce the value obtained by dividing the standard deviation of the aspect ratios of the prior austenite grains by the average aspect ratio of the prior austenite grains, it is preferable to set the total reduction in the temperature range of 960 to 1010°C to 3% or less. Alternatively, to reduce the reduction in the expanded partial recrystallization region, it is preferable to set the total reduction in the temperature range of 940 to 1010°C to 0% or more but less than 5%.

[0085] The total rolling reduction in the temperature range of 960°C or less is set to 20% or more and less than 80%. By setting the total rolling reduction in the non-recrystallized range of 960°C or less to 20% or more and less than 80%, dislocations can be preferably introduced into austenite, and as a result, desired amounts of bainite and martensite can be obtained. The total rolling reduction can be expressed as (1-t3 / t2) x 100 (%), where t2 is the plate thickness before the first rolling in the set temperature range, and t3 is the plate thickness after the last rolling in the set temperature range.

[0086] After hot rolling, the steel is cooled to a temperature range of 500 to 650° C. at an average cooling rate of 50° C. / s or more. By setting the average cooling rate to 50° C. / s or more, a desired amount of martensite can be obtained. The average cooling rate here is the temperature difference between the start point and the end point of the set range divided by the elapsed time from the start point to the end point.

[0087] In the temperature range of 500 to 650°C, air cooling is performed for 3 to 10 seconds. By setting the temperature range for air cooling to 500°C or higher, the desired amount of bainite can be obtained. Furthermore, by setting the temperature range for air cooling to 650°C or lower, the area ratio of the remaining structure can be reduced. Furthermore, by setting the air cooling time to 3 to 10 seconds, the desired amount of bainite can be obtained. In this embodiment, air cooling refers to cooling at an average cooling rate of less than 10° C. / s.

[0088] After air-cooling, the material is cooled to 100°C at an average cooling rate of 50°C / s or more. By setting the average cooling rate to 100°C to 50°C / s or more, the desired amount of martensite can be obtained. For example, even if the material is cooled to a certain temperature at a high cooling rate after air-cooling and then cooled from that temperature to 100°C at a low cooling rate, it is sufficient as long as the average cooling rate to 100°C after air-cooling is 50°C / s or more. After cooling, the material may be wound into a coil. The manufacturing method described above allows the steel sheet according to this embodiment to be manufactured stably. [Example]

[0089] Next, the effects of one embodiment of the present disclosure will be explained in more detail using examples, but the conditions in the examples are examples adopted to confirm the feasibility and effects of the present disclosure, and the present disclosure is not limited to these examples. Various conditions may be adopted in the present disclosure as long as they do not deviate from the gist of the present disclosure and the object of the present disclosure is achieved.

[0090] Slabs having the chemical compositions shown in Tables 1A to 1D were obtained by converter melting and continuous casting. Steel plates having thicknesses of 2.1 to 6.8 mm were obtained from the obtained slabs under the conditions shown in Tables 2A and 2B. The heating temperature of the slab was 1250°C or higher, and the holding time in that temperature range was 3500 seconds. After hot rolling, the slab was cooled to the air-cooling start temperature at the average cooling rate shown in Tables 2A and 2B, and then air-cooled for 5 seconds.

[0091] The obtained steel sheets were evaluated for metal structure, tensile strength, total elongation, hole expansion ratio, and crash resistance after pre-straining by the methods described above. The results are shown in Tables 3A and 3B. The "standard deviation / average value" in Tables 3A and 3B indicates the standard deviation of the aspect ratios of the prior austenite grains divided by the average value of the aspect ratios of the prior austenite grains. The "remaining structure" in Tables 3A and 3B indicates the sum of the area ratios of ferrite, pearlite, and retained austenite.

[0092] When the tensile strength was 980 MPa or more, the specimen was judged to have high strength and pass the test, whereas when the tensile strength was less than 980 MPa, the specimen was judged to have insufficient strength and fail the test. Furthermore, if the tensile strength was more than 1200 MPa, the strength was too high to ensure the desired ductility, and the specimen was therefore judged to be unacceptable.

[0093] If the total elongation was 12% or more, the specimen was judged to have excellent ductility and pass, whereas if the total elongation was less than 12%, the specimen was judged to have poor ductility and fail.

[0094] When the hole expansion ratio was 50% or more, the specimen was judged to have excellent hole expandability and to have passed the test. On the other hand, when the hole expansion ratio was less than 50%, the specimen was judged to have poor hole expandability and to have passed the test.

[0095] In the Charpy impact test after pre-straining, if the ductile-brittle transition temperature (DBTT) was -40°C or lower, the specimen was judged to have excellent crashworthiness after pre-straining and to have passed the test. On the other hand, if the ductile-brittle transition temperature was above -40°C, the specimen was judged to have poor crashworthiness after pre-straining and to have failed the test.

[0096] [Table 1A]

[0097] [Table 1B]

[0098] [Table 1C]

[0099] [Table 1D]

[0100] [Table 2A]

[0101] [Table 2B]

[0102] [Table 3A]

[0103] [Table 3B]

[0104] It can be seen from Tables 3A and 3B that the steel plates according to the examples of the present invention have high strength, excellent ductility and hole expandability, and also have excellent crash resistance properties after being pre-strained. On the other hand, it is clear that the steel sheets according to the comparative examples are inferior in one or more of the above properties.

[0105] In addition, for all examples, lower arms (components) were manufactured by press working. The flat portion of the lower arm was evaluated in the same manner as described above. The measurement results and evaluation results were the same as those shown in Tables 3A and 3B. [Industrial Applicability]

[0106] According to the above aspects of the present disclosure, it is possible to provide a steel plate having high strength, excellent ductility and hole expandability, and excellent crash resistance after pre-straining, and a part using the same.

Claims

1. The chemical composition, in mass%, is C: 0.050-0.200%, Si: 0.05-3.00%, Mn: 1.00-3.00%, sol. Al: 0.001 to 0.500%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0.0100% or less, Nb: 0.001-1.000%, Ti: 0.070-0.200%, B: 0 to 0.0100%, Cr: 0-2.00%, Mo: 0-1.00%, Cu: 0-2.00%, Ni: 0-2.00%, V: 0 to 0.50%, Sn: 0 to 0.050%, As: 0 to 0.100%, Zr: 0 to 1.000%, Ca: 0-0.0200%, Mg: 0 to 0.0200%, Bi: 0 to 0.0200%, Co: 0-1.000%, W: 0-1.000%, Zn: 0 to 1.000%, REM: 0 to 0.1000%, and The balance is Fe and impurities. In the metal structure at 1 / 4 of the plate thickness from the surface, In area %, Bainite: 60.0% or more and less than 90.0% Martensite: more than 10.0% and not more than 40.0%; Ferrite, pearlite and retained austenite: a total of 10.0% or less; a value obtained by dividing the standard deviation of the aspect ratios of the prior austenite grains by the average value of the aspect ratios of the prior austenite grains is 0.50 or less, The steel plate is characterized in that the average grain size of the prior austenite grains is 30 μm or less.

2. The chemical composition is, in mass %, B: 0.0001 to 0.0100%, Cr: 0.01-2.00%, Mo: 0.01-1.00%, Cu: 0.01-2.00%, Ni: 0.01-2.00%, V: 0.01 to 0.50%, Sn: 0.001 to 0.050%, As: 0.001 to 0.100%, Zr: 0.001 to 1.000%, Ca: 0.0001-0.0200%, Mg: 0.0001-0.0200%, Bi: 0.0001-0.0200%, Co: 0.001 to 1.000%, W: 0.001-1.000%, Zn: 0.001 to 1.000%, and 2. The steel sheet according to claim 1, further comprising at least one selected from the group consisting of REM: 0.0001 to 0.1000%.

3. A component comprising the steel sheet according to claim 1 or 2.