Steel sheet and component

Abstract

This steel sheet has a prescribed chemical composition. In the metal structure in a position at 1 / 4 of the sheet thickness from the surface, bainite is 60.0% to less than 90.0%, martensite is more than 10.0% to 40.0%, and the total of ferrite, pearlite, and retained austenite is 10.0% or less by area. 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.

Description

Steel plates and parts

[0001] This application claims priority to Japanese Patent Application No. 2023-193550, filed on November 14, 2023, the contents of which are incorporated herein by reference.

[0002] In recent years, CO ２ To reduce emissions, automobile bodies are being made lighter. 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 modulus of over 980 MPa is being 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 in which 90 area % or more of the steel structure is bainite, the remainder being 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 in 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 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: 1180 MPa or more and significantly improved delayed fracture resistance, and is 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.

[0008] Japanese Patent Publication No. 2011-246768 International Publication No. 2021 / 193310 Japanese Patent Publication No. 2021-116476

[0009] However, Patent Documents 1 to 3 do not take into consideration 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.

[0011] The gist of the present disclosure is as follows: [1] A steel sheet having 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%, Cr: 0-2.00%, 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 to 0.0200%, 1. A steel sheet comprising: 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: Fe and impurities; wherein a metallographic 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: 10.0% or less in total; a value obtained by dividing the standard deviation of 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. [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 Zn: 0.001 to 1.000%, and REM: 0.0001 to 0.1000%.[3] A part made of the steel sheet according to [1] or [2] above.

[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.

[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, the numerical ranges indicated 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 compositions 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%, Cr: 0 to 2.00%, Mo: 0 to 1.00%, Cu: 0 to 2.00%, Ni: 0 to 2.00%, V : 0 to 0.50%, Sn: 0 to 0.050%, As: 0 to 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: 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 forms on the steel sheet surface, impairing the surface properties of the steel sheet. Furthermore, the chemical conversion treatability of the steel sheet deteriorates. 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 the 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, and the ductility of the steel sheet deteriorates. 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 to 0.500% Sol. Al acts as a deoxidizer and is an element that 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 this embodiment, sol. Al refers to acid-soluble Al, which refers to 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 a lower P content is preferable, it is preferably set to 0%. However, since excessive reduction of 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 embrittles slabs when present as sulfides in steel. 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 a lower S content is preferable, it is preferably set to 0%. However, excessive reduction of the S content significantly increases desulfurization costs. 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, a large amount of N increases the risk of slab cracking. 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 a lower N content is preferable, it is preferably set to 0%. However, excessive reduction of the N content significantly increases the cost of denitrification. Therefore, the N content may be set to 0.0005% or more or 0.0010% or more.

[0023] O: 0.0100% or less When a large amount of O is contained 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. Since a lower O content is more preferable, it is preferably set to 0%. However, in order to disperse a large number of fine oxides during deoxidation of molten steel, the O content may be set to 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 also increases the strength of steel sheets by forming fine carbides. If the Nb content is less than 0.001%, the prior austenite grains cannot be made small in the metal structure of the steel sheet, and the crash 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 increases the strength of steel sheet by forming fine nitrides in steel. 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. Furthermore, if Ti is contained in a large amount, 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 described 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 more reliably obtain 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 reliably obtain the effect of increasing the strength of the steel sheet due to the inclusion of Cr, 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. 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 steel sheet and precipitating as carbides in steel at low temperatures to increase the strength of the steel sheet. To more reliably obtain the effects of these functions, the Cu content is preferably 0.01% or more. However, if the Cu content exceeds 2.00%, intergranular cracking of the slab may occur. 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 contained, Ni has the effect of effectively suppressing intergranular cracking of the slab caused by Cu. In order to more reliably obtain the above-mentioned effect, the Ni content is preferably 0.01% or more. Since Ni is an expensive element, it is economically undesirable to include a large amount of Ni. 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 sheet by forming fine carbides in steel. To reliably obtain 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 suppresses the generation of oxides that serve as fracture initiation sites, thereby improving the hole expandability of steel sheets. To ensure this effect, the Sn content is preferably 0.001% or more. However, since the above effect saturates even when a large amount of Sn is added, 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 steel sheets. To ensure this effect, the As content is preferably 0.001% or more. However, since the above effect saturates even when a large amount of As is added, 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 sheet through solid solution strengthening. 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 non-metallic inclusions, which act as fracture initiation points 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 generated 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 non-metallic inclusions, thereby improving the ductility and hole expandability of steel sheets. To ensure 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 generated in the steel, deteriorating 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 more reliably obtain 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 the above action 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 sheet through solid solution strengthening. 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 sheet through solid solution strengthening. 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 sheet through solid solution strengthening. To ensure 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 non-metallic inclusions, thereby improving the ductility and hole expandability of steel sheets. To ensure 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 generated 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. Note that 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 added industrially 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 above-mentioned steel sheet may be analyzed using a spark discharge optical emission spectrometer or the like. Values ​​for C and S are determined by burning the steel sheet in an oxygen stream using a gas component analyzer or the like and measuring the values ​​using an infrared absorption method. Values ​​for N are determined by melting a test piece taken from the steel sheet in a helium stream and measuring the value 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. If the steel sheet has a plating layer or a coating on its surface, the plating layer or coating may be removed by mechanical grinding or the like, as necessary, before analyzing the chemical composition.

[0045] Next, the metallographic structure of the steel sheet according to this embodiment will be described. In the steel sheet according to this embodiment, in the metallographic structure at a position from the surface to ¼ of the sheet thickness, the steel sheet has, 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 will be described below.

[0046] In this embodiment, the metallographic structure is defined at a position 1 / 4 of the sheet thickness from the surface (a region from 1 / 8 of the sheet thickness depth from the surface to 3 / 8 of the sheet thickness depth from the surface). In other words, this position is a region starting from 1 / 8 of the sheet thickness depth from the surface and ending at 3 / 8 of the sheet thickness depth from the surface. The reason for defining the metallographic structure at this position is that the metallographic structure at this position represents a typical metallographic structure of a steel sheet. In addition, when a steel sheet has a plating layer, a coating film, or the like on its surface, the surface referred to here refers to the interface between the steel sheet and the plating layer, 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 desired ductility cannot be obtained in the steel sheet. 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: More than 10.0% and 40.0% or less Martensite is stronger than bainite and is a structure that increases the strength of the steel sheet. 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 is more than 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 be 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. This sum of the area fractions 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%.

[0049] Ferrite, pearlite, and retained austenite: 10.0% or less in total The metal structure of the steel sheet according to this embodiment may contain ferrite, pearlite, and retained austenite as a residual structure other than bainite and martensite. If the area ratio of these residual structures exceeds 10.0%, the steel sheet cannot obtain the desired strength. Therefore, the area ratio of the residual structures is set to 10.0% or less in total. The area ratio of the residual 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 a 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 test specimen was polished using #600 to #1500 silicon carbide paper, and then polished to a mirror finish using a liquid in which diamond powder with a particle 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 particle size of 0.25 μm and no alkaline solution to remove the strain introduced into the surface layer. At any position along the longitudinal direction of the cross section of the test specimen, a region 200 μm in the longitudinal direction of the test specimen, extending from 1 / 8 of the plate thickness depth to 3 / 8 of the plate thickness depth from the surface, was measured at 400x magnification using electron backscatter diffraction 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 (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL). At this time, the degree of vacuum in the EBSD device is 9.6×10 －５ It is preferable that the pressure be 100 Pa or less, the acceleration voltage be 15 kV, the irradiation current level be 13, and the electron beam irradiation level be 62. Other observation conditions are preferably as follows: Electron gun type: Schottky WD (working distance): 15 mm 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 software "OIM Analysis (registered trademark)" attached to the EBSD analyzer is used to identify regions with an fcc crystal structure, and the area ratio of these regions is calculated, thereby obtaining the area ratio of retained austenite.

[0054] Next, regions with a bcc crystal structure are determined to be "bainite, ferrite, pearlite, and martensite." For these regions, the "Grain Orientation Spread" function included 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 calculated to obtain the area ratio of ferrite.

[0055] Next, within the remaining region (the region where "Grain Orientation Spread" exceeds 1°), under the condition that the boundary where the crystal orientation difference is 15° is regarded as a grain boundary, when the maximum value of "Grain Average IQ" of the ferrite region is Iα, the region where it exceeds Iα / 2 is extracted as bainite, and the region where it is Iα / 2 or less is 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 in the observation field of view, the GAM "Grain Average Misorientation" function is used for the same field of view, under the condition that boundaries with a crystal orientation misorientation of 5° are considered to be grain boundaries, and regions where "Grain Average Misorientation" is greater than 0.50° and less than 0.75° are extracted as bainite, and regions 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 performed on the region determined to be "pearlite and martensite" in the EBSD measurement. To perform SEM observation on the same region as the EBSD measurement region, a Vickers indentation is stamped near the observation position. Surface contamination is polished away, leaving the structure of the observation surface, and the specimen is then etched with nital. The etched observation surface is then subjected to SEM observation in the same field of view as the EBSD observation region. The magnification is, for example, 170x. Of the region determined to be "pearlite and martensite" in the EBSD measurement, a structure in which plate-like ferrite and Fe-based carbides are layered together in the 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 by the above-mentioned method from 100%.

[0059] The rolling direction of a steel plate is determined using the following method. A test specimen is taken so that the thickness cross section of the steel plate can be observed. The thickness cross section of the taken test specimen is mirror-polished and then observed using an optical microscope at magnifications of 100x, 200x, 500x, and 1000x. 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 using 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 long axis length of the multiple inclusions in each cross section is calculated for each cross section. The cross section with the largest average value of the long axis length of the inclusions obtained is identified. The direction parallel to the longitudinal axis of the inclusions in the cross section is determined to be the rolling direction.

[0060] 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 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. There is no particular lower limit for the average grain size of the prior austenite grains, but 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 using the following method. A test piece is taken from the steel plate so that the plate thickness cross section perpendicular to the rolling direction can be observed. The observation surface of the test piece is exposed to the plate thickness cross section structure using an etchant. The etchant used is a saturated aqueous solution of picric acid (100 cc) to which the following additives are added: hydrochloric acid (0.5 to 2.0 cc), sodium dodecylbenzenesulfonate (0.3 to 1.0 g), iron chloride (0.1 to 0.3 g), calcium chloride (0.1 to 0.3 g), and ethyl alcohol (0.5 to 2.0 cc). This etchant is heated to a temperature range of 40 to 60°C before use. A secondary electron image of an area of ​​200 μm in the rolling direction and 200 μm in the thickness direction is taken at a magnification of 500x using a scanning electron microscope at a position 1 / 4 of the sheet thickness from the surface of the observation surface (a region from 1 / 8 of the sheet thickness depth from the surface to 3 / 8 of the sheet thickness depth from the surface). The obtained secondary electron image is used to measure the circle equivalent diameter and area of ​​the prior austenite grains. The scanning electron microscope is equipped with a two-electron detector, and a 9.6 x 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 (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL). 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): 15 mm Objective aperture number: 4 Number of pixels: 4096 x 5120 pix

[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 Here, 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 based on the following formula using the obtained aspect ratio of the prior austenite grains.

[0066] Average aspect ratio of prior austenite grains = Σ ｉ (A ｉ ×E ｉ ) / Σ ｉ A ｉ Here, 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 ratios of the prior austenite grains is obtained by calculating the standard deviation of the aspect ratios of the prior austenite grains based on the following formula:

[0067] Standard deviation of aspect ratio of prior austenite grains = √{Σa ｉ × (E ｉ E' is the average aspect ratio of the prior austenite grains determined by the above method. 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 / ΣAｉ ) ) 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 to 1200 MPa The steel plate 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 vehicle body weight 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 plate. 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 for 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 for the hole expansion ratio, but it may be 80% or less, 75% or less, or 70% or less. Note that total elongation refers to the "total elongation at break" 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 portion 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 properties after pre-straining Impact resistance properties after pre-straining are evaluated by performing a Charpy impact test on a test specimen after 5% pre-straining in a tensile test. A tensile test is performed in the same manner as described above, and a 5% tensile pre-deformation is applied to the test specimen. A 2.5 mm subsize V-notch test specimen is then 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, it can be determined that the impact resistance properties after pre-straining are excellent. Note that for steel plates with a plate thickness of less than 2.5 mm, the measurement is performed across 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 making the thickness 8.0 mm or less, it is possible to stably obtain the above-mentioned metal structure after hot rolling. The thickness may be 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, etc., to form 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 weight. Furthermore, corrosion resistance can be further improved by performing 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. Note that the temperature of the slab and the temperature of the steel sheet in this embodiment refer to the surface temperature of the slab and the surface temperature of the steel sheet.

[0080] In a preferred method for manufacturing a steel plate according to this embodiment, the slab is heated in a temperature range of 1200°C or higher for 3000 seconds or longer; in the hot rolling, rolling is performed two or more times in a temperature range of 1010 to 1180°C with a reduction of 40% or more, with a total interpass time in the temperature range of 1010 to 1180°C of 20 seconds or less; the total reduction in the temperature range of 960 to 1010°C is 0% or more but less than 5%; the total reduction in the temperature range of 960°C or lower is 20% or more but less than 80%; after the hot rolling, the slab 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 in the temperature range of 500 to 650°C for 3 to 10 seconds; and after the air-cooling, cooling to 100°C at an average cooling rate of 50°C / s or more. Each step will be described below.

[0081] In order to sufficiently 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 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, and the inter-pass time is set to 20 seconds or less. By performing two or more times of rolling at a high reduction rate in the fully recrystallized region of 1010 to 1180°C and setting the inter-pass time within a desired time, recrystallization can be promoted and prior austenite grains can be refined. Note that the inter-pass time here is the total time between all passes in the temperature range of 1010 to 1180°C. In view of equipment constraints, the inter-pass 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 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. Note that the reduction rate here refers to the ratio of the thickness of the plate before rolling to ０ The thickness of the plate after rolling is t １ When this is the case, (1-t １ / t ０ ) × 100 (%).

[0084] The total reduction in the temperature range of 960 to 1010°C is 0% or more and less than 5%, and the total reduction in the temperature range below 960°C is 20% or more and 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. In order 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, in order 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 and less than 5%.

[0085] The total reduction in the temperature range of 960°C or less is set to 20% or more and less than 80%. By setting the total reduction in the non-recrystallization 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 reduction is the thickness of the sheet before the first rolling in the set temperature range. ２ The thickness of the plate after the final rolling in the set temperature range is t ３ When this is the case, (1-t ３ / t ２ ) × 100 (%).

[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, the desired amount of martensite can be obtained. The average cooling rate here is the value obtained by dividing the temperature difference between the start and end points of the set range 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. Note that, in this embodiment, air cooling refers to cooling at an average cooling rate of less than 10°C / s.

[0088] After air-cooling, the steel sheet 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, a desired amount of martensite can be obtained. For example, even if the steel sheet 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 that the average cooling rate to 100°C after air-cooling is 50°C / s or more. After the cooling, the steel sheet can be wound into a coil. The steel sheet according to this embodiment can be stably produced by the manufacturing method described above.

[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 slabs was 1250°C or higher, and the holding time in this temperature range was 3500 seconds. After hot rolling, the slabs were 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 using the methods described above. The obtained results are shown in Tables 3A and 3B. Note that the "standard deviation / average value" in Tables 3A and 3B indicates the value obtained by dividing the standard deviation of the aspect ratios of prior austenite grains by the average aspect ratio of the prior austenite grains. The "remaining structure" in Tables 3A and 3B indicates the sum of the area fraction of ferrite, the area fraction of pearlite, and the area fraction of retained austenite.

[0092] If the tensile strength was 980 MPa or more, the specimen was judged to have high strength and passed the test. On the other hand, if the tensile strength was less than 980 MPa, the specimen was judged to have insufficient strength and failed the test. Furthermore, if the tensile strength was more than 1200 MPa, the specimen was judged to have too high a strength to ensure the desired ductility and failed the test.

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

[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 no excellent 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 crash resistance properties after pre-straining and to have passed the test. On the other hand, if the ductile-brittle transition temperature was higher than −40° C., the specimen was judged to have poor crash resistance properties after pre-straining and to have passed the test.

[0096]

[0097]

[0098]

[0099]

[0100]

[0101]

[0102]

[0103]

[0104] From Tables 3A and 3B, it can be seen that the steel sheets according to the examples of the present invention have high strength, as well as excellent ductility and hole expandability, and also have excellent crash resistance properties after pre-straining, whereas 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.

[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 to 1.000%, Ti: 0.070 to 0.200%, B: 0 to 0.0100%, Cr: 0-2.00%, Mo: 0-1.00%, Cu: 0-2.00%, Ni: 0-2.00%, V: 0-0.50%, Sn: 0-0.050%, As: 0-0.100%, Zr: 0-1.000%, Ca: 0-0.0200%, Mg: 0 to 0.0200%, 1. A steel sheet comprising: 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; and a metal structure at a position from the surface to 1 / 4 of the sheet thickness having, in area percentages, Bainite: 60.0% or more and less than 90.0%, Martensite: more than 10.0% and 40.0% or less, Ferrite, Pearlite and Retained Austenite: 10.0% or less in total; a value obtained by dividing the standard deviation of 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 an 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 2. The steel sheet according to claim 1, further comprising at least one selected from the group consisting of: Mn: 0.001 to 1.000%, Zn: 0.001 to 1.000%, and REM: 0.0001 to 0.1000%.

3. A part comprising the steel plate according to claim 1 or 2.

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