Steel sheet and component

Abstract

This steel sheet has a desired chemical composition and is such that in a metal structure at a position that is 1 / 4 of the sheet thickness from the surface, prior austenite grains having an aspect ratio of less than 2.0 constitute less than 20% in terms of area%, prior austenite grains having an aspect ratio of greater than or equal to 2.0 and less than 6.0 constitute greater than or equal to 40%, prior austenite grains having an aspect ratio of greater than or equal to 6.0 constitute less than 30%, bainite constitutes 70-95%, and martensite constitutes 5-30%.

Description

Steel plates and parts

[0001] The present disclosure relates to a steel plate and a part. Specifically, the present disclosure relates to a steel plate having high strength, excellent ductility, and hole expandability, and suppressing the occurrence of unevenness in bending, and a part manufactured using the same. This application claims priority based on Japanese Patent Application No. 2023-171497 filed on October 2, 2023, the contents of which are incorporated herein by reference.

[0002] In recent years, efforts have been made to reduce the weight of automobiles and machine parts. By ensuring rigidity through optimal part design, it is possible to reduce the weight of automobiles and machine parts. Furthermore, for blank-formed parts such as press-formed parts, weight can be reduced by reducing the thickness of the part material.

[0003] However, when trying to ensure the strength properties of parts, such as static fracture strength and yield strength, while reducing the plate thickness, it is necessary to use high-strength materials. In particular, the application of higher-strength steel sheets to automobile suspension parts, such as lower arms, trail links, and knuckles, has begun to be considered. These automobile suspension parts are manufactured by subjecting steel sheets to burring, stretch flange forming, bending, and the like. Therefore, steel sheets used for these automobile suspension parts are required to have excellent formability, particularly excellent ductility and hole expandability.

[0004] Furthermore, when high-strength steel sheets are used in the manufacture of the above-mentioned automobile suspension parts, microcracks (irregularities) tend to occur within the bends during forming, which deteriorates the crashworthiness of the parts.

[0005] For example, Patent Document 1 discloses a high-strength 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 1 discloses that the above configuration enables the production of a high-strength steel sheet that has a high strength of tensile strength TS: 1180 MPa or more and is excellent in delayed fracture resistance, making it suitable as a material for automobile parts.

[0006] Patent Document 2 discloses a method for manufacturing a high-strength 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 2 also discloses that the above configuration allows for the production of a high-strength steel sheet with excellent hole expandability.

[0007] Patent Document 3 discloses a steel sheet having excellent fatigue crack growth resistance, which contains 1 to 60% acicular ferrite in terms of area fraction, and is characterized in that the proportion of acicular ferrite particles having a major axis in the range of 5 to 100 μm is 80% or more.

[0008] International Publication No. 2021 / 193310 Japanese Patent Application Publication No. 2021-116476 Japanese Patent Application Publication No. 2007-321220

[0009] However, Patent Documents 1 to 3 do not take into consideration the occurrence of unevenness within the bend.

[0010] The present disclosure has been made in consideration of the above-mentioned problems, and aims to provide a steel sheet that has high strength, excellent ductility and hole expandability, and that suppresses the occurrence of unevenness within the bend during forming, and a part using the steel sheet.

[0011] The present inventors have found that by controlling the aspect ratio distribution of prior austenite grains, it is possible to ensure high strength, as well as excellent ductility and hole expandability, while suppressing the occurrence of irregularities within the bent portion during forming. Furthermore, the present inventors have found that suppressing the occurrence of irregularities within the bent portion during forming can improve the crash resistance of the steel sheet.

[0012] The gist of the present disclosure made based on the above findings is as follows: (1) A steel sheet having a chemical composition, in mass %, of C: 0.045 to 0.120%, Si: 0 to 3.00%, Mn: 1.20 to 3.00%, sol. Al: 0.001-0.500%, P: 0-0.0800%, S: 0-0.0100%, N: 0-0.0050%, O: 0-0.0100%, Ti: 0-0.180%, Nb: 0-0.100%, V: 0-1.000%, Cu: 0-1.000%, Cr: 0-2.000%, Mo: 0-3.000%, Ni: 0-0.500%, B: 0-0.0100%, Ca: 0-0.0500%, Mg: 0-0.050%, REM: 0-0.100%, Bi: 0 to 0.100%, Ta: 0 to 0.100%, 1. A steel sheet comprising: Zr: 0-0.500%, Co: 0-3.000%, Zn: 0-0.200%, W: 0-0.200%, Sb: 0-0.500%, As: 0-0.050%, and Sn: 0-0.050%, with the balance being Fe and impurities; and 2.0. In a metallographic structure at a position 1 / 4 of the sheet thickness from the surface, the steel sheet is characterized in that, in area percentages, prior austenite grains having an aspect ratio of 1.0 or more and less than 2.0 are less than 20%, prior austenite grains having an aspect ratio of 2.0 or more and less than 6.0 are 40% or more, prior austenite grains having an aspect ratio of 6.0 or more are less than 30%, bainite is 70-95%, and martensite is 5-30%.(2) The chemical composition is, in mass%, Ti: 0.001 to 0.180%, Nb: 0.001 to 0.100%, V: 0.001 to 1.000%, Cu: 0.001 to 1.000%, Cr: 0.001 to 2.000%, Mo: 0.001 to 3.000%, Ni: 0.001 to 0.500%, B: 0.0001 to 0.0100%, Ca: 0.0001 to 0.0500%, Mg: 0.001 to 0.050%, REM: 0.001 to 0.100%, Bi: 0.001 to 0.100%, Ta: 0.001 to 0.100%, (3) The steel sheet according to (1), characterized in that the steel sheet contains one or more elements selected from the group consisting of Zr: 0.001 to 0.500%, Co: 0.001 to 3.000%, Zn: 0.001 to 0.200%, W: 0.001 to 0.200%, Sb: 0.001 to 0.500%, As: 0.001 to 0.050%, and Sn: 0.001 to 0.050%. (4) The steel sheet according to (1) or (2), characterized in that in the metallographic structure at a position 1 / 4 of the sheet thickness from the surface, prior austenite grains having an aspect ratio of 2.0 or more and less than 4.0 account for 30% or more, in terms of area percentage, and prior austenite grains having an aspect ratio of 4.0 or more and less than 6.0 account for 10% or more. (4) The steel sheet according to any one of (1) to (3), characterized in that in the metal structure at a position 1 / 4 of the sheet thickness from the surface, the maximum value of the length of the major axis of prior austenite grains is 200 μm or less. (5) A part characterized in that it includes the steel sheet according to any one of (1) to (4).

[0013] 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 suppressing the occurrence of unevenness within the bend during forming, and a part using the steel plate.

[0014] A steel plate and a part according to an embodiment of the present disclosure (hereinafter, sometimes referred to as the steel plate and the part according to the present embodiment) will be described. 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.

[0015] Each constituent element of the present disclosure will be described in detail below. First, the reasons for limiting the chemical composition of the steel sheet according to this embodiment will be described. Below, the numerical ranges described with "to" include the lower and upper limits. Numerical values ​​indicated as "less than" or "greater than" do not include the numerical range. In the following description, percentages related to the chemical composition are mass% unless otherwise specified.

[0016] The steel sheet according to this embodiment has a chemical composition, in mass%, of C: 0.045 to 0.120%, Si: 0 to 3.00%, Mn: 1.20 to 3.00%, sol. Al: 0.001 to 0.500%, P: 0 to 0.0800%, S: 0 to 0.0100%, N: 0 to 0.0050%, O: 0 to 0.0100%, and the balance: Fe and impurities. Each element will be described in detail below.

[0017] C: 0.045 to 0.120% C is an important element for improving the strength of steel sheet. To obtain the desired strength, the C content is set to 0.045% or more. The C content is preferably 0.050% or more, 0.55% or more, or 0.60% or more. On the other hand, if the C content exceeds 0.120%, the hole expandability of the steel sheet deteriorates. Therefore, the C content is set to 0.120% or less. The C content is preferably 0.110% or less, or 0.100% or less.

[0018] Si: 0 to 3.00% Si is an element that has the effect of suppressing the formation of carbides during ferrite transformation and improving the toughness of the steel plate. Since Si is not necessarily contained, the Si content may be 0%. To reliably obtain the above effect, the Si content is preferably 0.10% or more. The Si content is more preferably 0.20% or more, or 0.50% or more. On the other hand, if the Si content exceeds 3.00%, the cracking sensitivity of the slab increases, making the slab difficult to handle. Therefore, the Si content is set to 3.00% or less. The Si content is preferably 2.50% or less, 2.00% or less, or 1.50% or less.

[0019] Mn: 1.20 to 3.00% Mn is an element effective in improving hardenability and the strength of steel plate by solid solution strengthening. To obtain the desired strength, the Mn content is set to 1.20% or more. The Mn content is preferably 1.30% or more, 1.50% or more, or 1.70% or more. On the other hand, if the Mn content exceeds 3.00%, the bainite transformation is delayed, making it difficult to obtain the desired amount of bainite. Therefore, the Mn content is set to 3.00% or less. The Mn content is preferably 2.70% or less, 2.50% or less, or 2.30% or less.

[0020] Sol. Al: 0.001 to 0.500% Al has the effect of improving the soundness of steel by deoxidation and also has the effect of controlling ferrite transformation. To achieve these effects, the sol. Al content is set to 0.001% or more. The sol. Al content is preferably 0.010% or more, 0.020% or more. On the other hand, if the Al content exceeds 0.500%, excessive ferrite tends to be formed, reducing the strength of the hot-rolled steel sheet. Therefore, the Al content is set to 0.500% or less. The Al content is preferably 0.400% or less, 0.300% or less, or 0.200% or less.

[0021] P: 0 to 0.0800% P is an element that affects the weldability of steel plate. In particular, if the P content exceeds 0.0800%, the weldability of the steel plate deteriorates significantly. Furthermore, the cracking sensitivity of the slab increases, making the slab difficult to handle. Therefore, the P content is set to 0.0800% or less. The P content is preferably 0.0600% or less, or 0.0400% or less. The P content may be 0%. From the viewpoint of refining costs, the P content may be set to 0.0010% or more.

[0022] S: 0 to 0.0100% S is an element that affects the ductility and hole expandability of steel sheet. In particular, if the S content exceeds 0.0100%, a large amount of inclusions such as MnS, which are harmful to the ductility and hole expandability of the steel sheet, are generated. Therefore, the S content is set to 0.0100% or less. The S content is preferably 0.0080% or less, 0.0060% or less. The S content may be 0%. From the viewpoint of refining costs, the S content may be set to 0.0001% or more.

[0023] N: 0 to 0.0050% N is an element that bonds with Ti to form Ti nitrides. In particular, if the N content exceeds 0.0050%, the cracking sensitivity of the slab increases, making the slab difficult to handle. Therefore, the N content is set to 0.0050% or less. The N content is preferably 0.0040% or less, or 0.0030% or less. The N content may be 0%. From the viewpoint of refining costs, the N content may be set to 0.0001% or more.

[0024] O: 0 to 0.0100% O is an element that, when contained in large amounts in steel, forms coarse oxides that become fracture initiation sites, causing brittle fracture and hydrogen-induced cracking. If the O content exceeds 0.0100%, brittle fracture and hydrogen-induced cracking are more likely to occur. Therefore, the O content is set to 0.0100% or less. The O content is preferably 0.0080% or less, 0.0060% or less, or 0.0040% or less. Since O is not necessarily contained, the O content may be 0%. 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.

[0025] The steel sheet according to the present embodiment may contain the above chemical components, with the balance being Fe and impurities. In the present embodiment, the term "impurities" refers to substances that are mixed in from raw materials such as ore, scrap, or the manufacturing environment, and / or substances that are acceptable within a range that does not adversely affect the properties of the steel sheet according to the present embodiment, for example, properties related to the problem to be solved by the present disclosure.

[0026] Although not essential for providing the desired properties, the following optional elements may be contained in order to reduce manufacturing variations and further improve the strength of the steel sheet. However, since the inclusion of these elements is not essential, the lower limit of the content of these elements is 0%.

[0027] Ti: 0.001 to 0.180% Ti precipitates in steel as carbides or nitrides, refining the metal structure through a pinning effect and increasing the strength and yield ratio of the steel sheet through precipitation strengthening. To reliably obtain these effects, the Ti content is preferably 0.001% or more. The Ti content is more preferably 0.005% or more, or 0.010% or more. On the other hand, if the Ti content exceeds 0.180%, excessive precipitation of TiC deteriorates the ductility and hole expandability of the steel sheet. Therefore, the Ti content is set to 0.180% or less. The Ti content is preferably 0.160% or less, 0.150% or less, or 0.130% or less.

[0028] Nb: 0.001 to 0.100% Nb has the effect of increasing the strength of the steel sheet by refining the crystal grain size of the steel sheet and by precipitation strengthening of NbC. To reliably obtain this effect, the Nb content is preferably 0.001% or more. The Nb content is more preferably 0.005% or more, or 0.010% or more. On the other hand, if the Nb content exceeds 0.100%, the above effect saturates. Therefore, even when Nb is contained, the Nb content is set to 0.100% or less. The Nb content is preferably 0.080% or less, or 0.060% or less.

[0029] V: 0.001 to 1.000% V has the effect of increasing the strength of steel sheet by strengthening through precipitation, grain refinement strengthening by inhibiting ferrite grain growth, and dislocation strengthening by inhibiting recrystallization. To reliably obtain these effects, the V content is preferably 0.001% or more. The V content is more preferably 0.005% or more, or 0.010% or more. On the other hand, if the V content is excessive, a large amount of carbonitrides will precipitate, deteriorating the ductility and hole expandability of the steel sheet. Therefore, the V content is set to 1.000% or less. The V content is preferably 0.800% or less, or 0.600% or less.

[0030] Cu: 0.001 to 1.000% Cu exists in steel in the form of fine particles and has the effect of increasing the strength of the steel sheet. To reliably obtain this effect, the Cu content is preferably 0.001% or more. The Cu content is more preferably 0.005% or more, or 0.010% or more. On the other hand, if the Cu content is excessive, the weldability of the steel sheet deteriorates. Therefore, the Cu content is set to 1.000% or less. The Cu content is preferably 0.800% or less, or 0.600% or less.

[0031] Cr: 0.001 to 2.000% Cr is an element effective in improving the strength of steel sheet. To reliably obtain this effect, the Cr content is preferably 0.001% or more. The Cr content is more preferably 0.005% or more, or 0.010% or more. On the other hand, if the Cr content is excessive, the ductility and hole expandability of the steel sheet deteriorate. Therefore, the Cr content is set to 2.000% or less. The Cr content is preferably 1.500% or less, 1.200% or less, or 1.000% or less.

[0032] Mo: 0.001 to 3.000% Mo is an element effective in strengthening the precipitation of ferrite. To reliably obtain this effect, the Mo content is preferably 0.001% or more. The Mo content is more preferably 0.005% or more, or 0.010% or more. On the other hand, if the Mo content is excessive, the cracking sensitivity of the slab increases, making the slab difficult to handle. Therefore, the Mo content is set to 3.000% or less. The Mo content is preferably 2.500% or less, 2.000% or less, or 1.500% or less.

[0033] Ni: 0.001 to 0.500% Ni has the effect of suppressing phase transformation at high temperatures and increasing the strength of the steel sheet. To reliably obtain this effect, the Ni content is preferably 0.001% or more. The Ni content is more preferably 0.005% or more, or 0.010% or more. On the other hand, if the Ni content is excessive, the weldability of the steel sheet deteriorates. Therefore, the Ni content is set to 0.500% or less. The Ni content is preferably 0.300% or less, or 0.150% or less.

[0034] B: 0.0001 to 0.0100% B has the effect of suppressing phase transformation at high temperatures and increasing the strength of the steel sheet. To reliably obtain this effect, the B content is preferably 0.0001% or more. The B content is more preferably 0.0005% or more, or 0.0010% 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. The B content is preferably 0.0080% or less, or 0.0050% or less.

[0035] Ca: 0.0001 to 0.0500% Ca disperses many fine oxides during deoxidation of molten steel, thereby refining the structure of the steel sheet. Ca also fixes S in the steel as spherical CaS, suppresses the formation of elongated inclusions such as MnS, and improves the hole expandability of the steel sheet. To reliably obtain these effects, the Ca content is preferably 0.0001% or more. The Ca content is more preferably 0.0005% or more, or 0.0010% or more. On the other hand, even if the Ca content exceeds 0.0500%, the above effects saturate. Therefore, the Ca content is set to 0.0500% or less. The Ca content is preferably 0.0300% or less, or 0.0200% or less.

[0036] Mg: 0.001 to 0.050% Mg has the effect of adjusting the shape of inclusions in steel to a preferred shape, thereby increasing the yield ratio of the steel sheet. To ensure this effect, the Mg content is preferably 0.001% or more. The Mg content is more preferably 0.005% or more, or 0.010% or more. On the other hand, if the Mg content exceeds 0.050%, excessive inclusions are formed in the steel, reducing the yield ratio of the steel sheet. Therefore, the Mg content is set to 0.050% or less. The Mg content is preferably 0.040% or less, or 0.030% or less.

[0037] REM: 0.001 to 0.100% REM has the effect of increasing the yield ratio of steel sheets by adjusting the shape of inclusions in steel to a preferred shape. To reliably obtain this effect, the REM content is preferably 0.001% or more. The REM content is more preferably 0.005% or more, or 0.010% or more. On the other hand, if the REM content exceeds 0.100%, excessive inclusions are formed in the steel, reducing the yield ratio of the steel sheets. Therefore, the REM content is set to 0.100% or less. The REM content is preferably 0.080% or less, or 0.060% or less. Here, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanoids, and the REM content refers to the total content of these elements. In the case of lanthanoids, they are added industrially in the form of misch metal.

[0038] Bi: 0.001 to 0.100% Bi has the effect of increasing the yield ratio of steel sheet by refining the solidification structure. To reliably obtain this effect, the Bi content is preferably 0.001% or more. The Bi content is more preferably 0.005% or more, or 0.010% or more. On the other hand, if the Bi content exceeds 0.100%, the effect of the above action saturates, which is economically undesirable. Therefore, the Bi content is set to 0.100% or less. The Bi content is preferably 0.080% or less, 0.060% or less, or 0.040% or less.

[0039] Ta: 0.001 to 0.100% Ta, like V, has the effect of increasing the strength of the steel sheet by forming fine carbides in the steel. In order to reliably obtain this effect, the Ta content is preferably 0.001% or more. The Ta content is more preferably 0.005% or more, or 0.010% or more. On the other hand, if the Ta content exceeds 0.100%, the ductility and hole expandability of the steel sheet deteriorate. Therefore, the Ta content is set to 0.100% or less. The Ta content is preferably 0.080% or less, or 0.050% or less.

[0040] Zr: 0.001 to 0.500% Zr has the effect of increasing the strength of steel sheet through solid solution strengthening. To reliably obtain this effect, the Zr content is preferably 0.001% or more. The Zr content is more preferably 0.005% or more, or 0.010% or more. On the other hand, if the Zr content exceeds 0.500%, the ductility and hole expandability of the steel sheet deteriorate. Therefore, the Zr content is set to 0.500% or less. The Zr content is preferably 0.300% or less, or 0.100% or less.

[0041] Co: 0.001 to 3.000% Co has the effect of increasing the strength of steel sheet through solid solution strengthening. To reliably obtain this effect, the Co content is preferably 0.001% or more. The Co content is more preferably 0.005% or more, or 0.010% or more. On the other hand, if the Co content exceeds 3.000%, the ductility and hole expandability of the steel sheet deteriorate. Therefore, the Co content is set to 3.000% or less. The Co content is preferably 1.000% or less, or 0.500% or less.

[0042] Zn: 0.001 to 0.200% Zn has the effect of increasing the strength of steel sheet through solid solution strengthening. To reliably obtain this effect, the Zn content is preferably 0.001% or more. The Zn content is more preferably 0.005% or more, or 0.010% or more. On the other hand, if the Zn content exceeds 0.200%, the ductility and hole expandability of the steel sheet deteriorate. Therefore, the Zn content is set to 0.200% or less. The Zn content is preferably 0.150% or less, or 0.100% or less.

[0043] W: 0.001 to 0.200% W has the effect of increasing the strength of steel sheet through solid solution strengthening. To reliably obtain this effect, the W content is preferably 0.001% or more. The W content is more preferably 0.005%, 0.010%, or more. On the other hand, if the W content exceeds 0.200%, the ductility and hole expandability of the steel sheet deteriorate. Therefore, the W content is set to 0.200% or less. The W content is preferably 0.150% or less, 0.100% or less.

[0044] Sb: 0.001 to 0.500% Sb has the effect of suppressing the generation of oxides that serve as fracture initiation sites, thereby improving the ductility and hole expandability of steel sheets. To reliably obtain this effect, the Sb content is preferably 0.001% or more. The Sb content is more preferably 0.005% or more, or 0.010% or more. On the other hand, even if a large amount of Sb is added, the above effect saturates, so the Sb content is set to 0.500% or less. The Sb content is preferably 0.300% or less, or 0.100% or less.

[0045] As: 0.001 to 0.050% As has the effect of reducing the austenite single-phase temperature, thereby refining prior austenite grains and improving the hole expandability of steel sheets. To reliably obtain this effect, the As content is preferably 0.001% or more. The As content is more preferably 0.005% or more, or 0.010% or more. On the other hand, even if a large amount of As is contained, the above effect saturates, so the As content is set to 0.050% or less. The As content is preferably 0.040% or less, or 0.030% or less.

[0046] Sn: 0.001 to 0.050% Sn has the effect of suppressing the generation of oxides that serve as fracture initiation sites, thereby improving the ductility and hole expandability of steel sheets. To reliably obtain this effect, the Sn content is preferably 0.001% or more. The Sn content is more preferably 0.005% or more, or 0.010% or more. On the other hand, even if a large amount of Sn is added, the above effect saturates, so the Sn content is set to 0.050% or less. The Sn content is preferably 0.040% or less, or 0.030% or less.

[0047] The chemical composition of the above-mentioned steel sheet 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 component analyzer or the like and measuring by an infrared absorption method. Values ​​of O and N are determined by melting a test piece taken from the steel sheet in a helium stream and measuring by a thermal conductivity method. If the steel sheet has a plating layer or a coating film on its surface, the plating layer or coating film may be removed by mechanical grinding or the like as necessary before analyzing the chemical composition.

[0048] 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 1 / 4 of the sheet thickness, prior austenite grains having an aspect ratio of 1.0 or more and less than 2.0 account for less than 20%, prior austenite grains having an aspect ratio of 2.0 or more and less than 6.0 account for 40%, prior austenite grains having an aspect ratio of 6.0 or more account for less than 30%, bainite accounts for 70 to 95%, and martensite accounts for 5 to 30%.

[0049] In this embodiment, the "position 1 / 4 of the plate thickness from the surface" refers to 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, it refers to 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 metallographic structure at this position is that the metallographic structure at this position represents a typical metallographic structure of a steel sheet. 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.

[0050] Prior austenite grains with an aspect ratio of 1.0 or more and less than 2.0: less than 20 area% Prior austenite grains with an aspect ratio of 2.0 or more and less than 6.0: 40 area% or more Prior austenite grains with an aspect ratio of 6.0 or more: less than 30 area% If the area ratio of prior austenite grains having a specified aspect ratio is outside the above range, it is not possible to suppress unevenness that occurs within the bend during forming. Therefore, the prior austenite grains with an aspect ratio of 1.0 or more and less than 2.0 are set to less than 20 area%, the prior austenite grains with an aspect ratio of 2.0 or more and less than 6.0 are set to 40 area% or more, and the prior austenite grains with an aspect ratio of 6.0 or more are set to less than 30 area%.

[0051] The amount of prior austenite grains having an aspect ratio of 2.0 or more and less than 6.0 is preferably as large as possible, and therefore may be 100% by area. The amount of prior austenite grains having an aspect ratio of 2.0 or more and less than 6.0 may be 95% by area or less, or 90% by area or less.

[0052] Prior austenite grains having an aspect ratio of 1.0 or more and less than 2.0 are preferably 15 area % or less, and more preferably 10 area % or less. Since the fewer prior austenite grains having an aspect ratio of 1.0 or more and less than 2.0, the better, they may be 0 area %. Furthermore, prior austenite grains having an aspect ratio of 6.0 or more are preferably 25 area % or less, and more preferably 20 area %, and more preferably 15 area % or less, and 10 area % or less. Since the fewer prior austenite grains having an aspect ratio of 6.0 or more, the better, they may be 0 area %.

[0053] Prior austenite grains having an aspect ratio of 2.0 or more and less than 4.0: 30 area % or more Prior austenite grains having an aspect ratio of 4.0 or more and less than 6.0: 10 area % or more It is preferable that the prior austenite grains having an aspect ratio of 2.0 or more and less than 6.0 account for 40 area % or more, the prior austenite grains having an aspect ratio of 2.0 or more and less than 4.0 account for 30 area % or more, and the prior austenite grains having an aspect ratio of 4.0 or more and less than 6.0 account for 10 area % or more. By accounting for 30 area % or more of the prior austenite grains having an aspect ratio of 2.0 or more and less than 4.0 and 10 area % or more of the prior austenite grains having an aspect ratio of 4.0 or more and less than 6.0, the occurrence of irregularities in the bent portion can be further suppressed.

[0054] The aspect ratio of a prior austenite grain is the value obtained by dividing the major axis of the prior austenite grain by the minor axis, and is a value of 1.0 or more.

[0055] The aspect ratio of prior austenite grains is measured by the following method. A sample is taken from a quarter-way point from the end face in the width direction of the steel plate so that the metal structure of the cross section (thickness direction × rolling direction cross section) normal to the width direction can be observed. The size of the sample depends on the measuring device, but it may be, for example, a rectangular parallelepiped measuring the entire thickness in the thickness direction, 15 mm in the rolling direction, and 10 mm in the width direction. Next, the observation surface is mirror-polished and then corroded using a saturated aqueous solution of picric acid using the Bechet-Beaujard method in accordance with JIS G 0551:2020. Grains that appear black due to corrosion are identified as prior austenite grains. The observation surface revealing the prior austenite grains is observed with an optical microscope, and eight fields of view, each 200 μm in the thickness direction and 600 μm in the rolling direction, are photographed at a magnification of 500x from 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). From the photographed structural photographs, the major axis / minor axis value for each prior austenite grain is calculated to obtain the aspect ratio of each prior austenite grain. The area ratios of prior austenite grains having an aspect ratio of 1.0 or more but less than 2.0, prior austenite grains having an aspect ratio of 2.0 or more but less than 4.0, prior austenite grains having an aspect ratio of 4.0 or more but less than 6.0, and prior austenite grains having an aspect ratio of 6.0 or more are calculated to obtain the area ratios of prior austenite grains having each aspect ratio.

[0056] If the above-mentioned method fails to reveal the prior austenite grains sufficiently, the prior austenite grains are identified and their aspect ratios are determined using the reconstruction method described in "Study on High-Precision Reconstruction Methods for Austenite Structures in Steel" (Hata, Kengo, Wakita, Masayuki, Fujiwara, Tomoya, Kono, Nippon Steel & Sumitomo Metal Technical Report No. 404 (2016), pp. 24-30). The EBSD measurement data used in the reconstruction method is obtained as follows. After colloidal polishing or electrolytic polishing is performed on the above-mentioned observation field (200 μm in the thickness direction × 600 μm in the rolling direction), crystal orientation information is obtained by electron backscatter diffraction at measurement intervals of 0.1 μm. For the measurement, an EBSD analyzer consisting of a thermal field emission scanning electron microscope (JEOL JSM-7200F) and an EBSD detector (EDAX Velocity® ultra-high-speed EBSD detector) is used. At this time, the degree of vacuum inside the device was 9.6×10 －５ The pressure is set to 10 Pa or less, the acceleration voltage is 15 kV, and the probe current is 36 nA. After the acceleration voltage is determined, the probe current may be adjusted to obtain a clearer pattern. To display the EBSD map, OIM Analysis (registered trademark) version 7 or later manufactured by EDAX / TSL Solution is used for the obtained crystal orientation information.

[0057] Note that, if prior austenite grains with a circle equivalent diameter of less than 2 μm are included, these are excluded from the above-mentioned measurements. This is because prior austenite grains with a circle equivalent diameter of less than 2 μm do not adversely affect the properties of the steel sheet according to this embodiment. Furthermore, if only a portion of the prior austenite grains is included in the edge of the structural photograph, these are excluded from the measurements. These exclusion conditions also apply to the measurement of the major axis length of the prior austenite grains described below. The rolling direction of the steel sheet is determined by the following method. Test specimens are taken so that the thickness cross section of the steel sheet can be observed. The direction perpendicular to the sheet surface is the Z direction, and a total of 12 test specimens are taken by rotating the specimen 30° around this Z direction. The thickness cross section of the taken test specimen is polished, and the prior austenite grain boundaries are revealed using the above-mentioned etching solution. The average aspect ratio of the prior austenite grains is calculated by the intercept method. The test specimen having the largest average aspect ratio of the prior austenite grains is identified, and the direction from which the test specimen was taken is determined to be the rolling direction of the steel sheet. In other words, the direction parallel to the thickness cross section of the test specimen and perpendicular to the thickness direction is determined to be the rolling direction of the steel sheet.

[0058] Maximum long axis length of prior austenite grains: 200 μm or less In the metal structure at a position 1 / 4 of the plate thickness from the surface, the maximum long axis length of prior austenite grains is preferably 200 μm or less. By setting the maximum long axis length of prior austenite grains to 200 μm or less, it is possible to further suppress the occurrence of irregularities within the bent portion during forming.

[0059] The maximum value of the major axis length of the prior austenite grains is obtained by measuring the major axis length of each prior austenite grain in the structural photograph taken by the above-mentioned method and calculating the maximum value.

[0060] Area fraction of bainite: 70 to 95% Bainite is a structure consisting of fine crystal grains and carbides. If the area fraction of bainite is less than 70%, the desired ductility cannot be obtained in the steel plate. Therefore, the area fraction of bainite is set to 70% or more. The area fraction of bainite is preferably 75% or more, or 80% or more. On the other hand, if the area fraction of bainite exceeds 95%, the desired strength cannot be obtained in the steel plate. Therefore, the area fraction of bainite is set to 95% or less. The area fraction of bainite is preferably 93% or less, 90% or less, or 87% or less.

[0061] Area fraction of martensite: 5 to 30% Martensite is a structure that increases the strength of steel plate. If the area fraction of martensite is less than 5%, the desired strength cannot be obtained. Therefore, the area fraction of martensite is set to 5% or more. The area fraction of martensite is preferably more than 5%, 6% or more, 7% or more, 10% or more, or 13% or more. On the other hand, if the area fraction of martensite is more than 30%, the desired ductility cannot be obtained. Therefore, the area fraction of martensite is set to 30% or less. The area fraction of martensite is preferably 25% or less, 20% or less.

[0062] Remaining structure: 0 to 5% The metal structure of the inner region of the steel plate according to this embodiment may contain ferrite and pearlite as the remaining structure in addition to bainite and martensite. The area ratio of the remaining structure may be 0 to 5%.

[0063] The area fractions of bainite, martensite, and the remaining structure are measured by the following method. A sample is taken at a position 1 / 4 from the end face in the width direction of the steel plate so that the metal structure of a cross section (thickness direction × rolling direction cross section) normal to the width direction can be observed. The size of the sample depends on the measuring device, but may be, for example, a rectangular parallelepiped measuring the entire thickness in the thickness direction, 15 mm in the rolling direction, and 10 mm in the width direction. Next, the observation surface is mirror-polished and then etched with nital. After etching, eight visual fields of 200 μm in the thickness direction × 600 μm in the rolling direction are photographed at a magnification of 1000 times or more using an optical microscope and a scanning electron microscope (SEM) from the surface to a position 1 / 4 of the thickness (the region from 1 / 8 of the thickness depth from the surface to 3 / 8 of the thickness depth from the surface). The area fractions of ferrite, pearlite, and bainite are obtained by performing image analysis on the obtained microstructure photographs. Then, at the same observation position, only the corroded layer is removed by polishing, followed by a mirror finish, and after REPELLER corrosion, the structure is observed using an optical microscope and a scanning electron microscope, and the area ratio of martensite is obtained by performing image analysis on the obtained structure photograph.

[0064] In the above-mentioned structural observation, each structure is identified by the following method: Martensite has a high dislocation density and a substructure such as blocks and packets within the grains, so it can be distinguished from other metal structures by electron channeling contrast images taken using a scanning electron microscope.

[0065] Bainite is defined as a structure that is an aggregate of lath-shaped crystal grains and does not contain Fe-based carbides with a major axis of 20 nm or more within the structure, but is not martensite, or a structure that contains Fe-based carbides with a major axis of 20 nm or more within the structure, and the Fe-based carbides have a single variant, i.e., Fe-based carbides elongated in the same direction. Here, Fe-based carbides elongated in the same direction refer to Fe-based carbides whose elongation directions differ by no more than 5°.

[0066] A structure consisting of massive crystal grains that does not contain substructures such as laths within the structure is considered to be ferrite, and a structure consisting of plate-like ferrite and Fe-based carbides layered together is considered to be pearlite.

[0067] Tensile strength (TS): 980 MPa or more The steel plate according to this embodiment may have a tensile strength of 980 MPa or more. The tensile strength is more preferably 1000 MPa or more. By setting the tensile strength to 980 MPa or more, the applicable parts are not limited, and the contribution to vehicle body weight reduction can be increased. There is no particular need to set an upper limit for the tensile strength, but from the viewpoint of suppressing mold wear, it may be set to 1500 MPa or less or 1300 MPa or less.

[0068] Total elongation (El): 10.0% or more Hole expansion ratio (λ): 50% or more The steel sheet according to this embodiment may have a total elongation (total elongation at maximum test force) of 10.0% or more and a hole expansion ratio of 50% or more. The total elongation is preferably 12.0% or more, or 14.0% or more. The hole expansion ratio is preferably 55.0% or more, or 60.0% or more.

[0069] The tensile strength and total elongation are evaluated by conducting a tensile test in accordance with JIS Z 2241:2022. The test specimen is a No. 5 test specimen of JIS Z 2241:2022. The tensile test specimen is taken from a quarter section from the end in the sheet width direction, with the direction perpendicular to the rolling direction as the longitudinal direction. If a No. 5 test specimen cannot be taken due to the small size or complex shape of the steel sheet or part, a small strip having a parallel section of any width may be taken, and the tensile strength and total elongation may be determined by conducting a tensile test using the small strip. The direction of the small strip is perpendicular to the rolling direction as the longitudinal direction. The hole expansion ratio is measured by conducting a hole expansion test in accordance with JIS Z 2256:2020.

[0070] Evaluation of unevenness within bend In the steel plate according to this embodiment, when a bending test described later is performed and the surface roughness within the bend is measured, the maximum valley depth (Rv) of the roughness curve may be 20 μm or less. If the maximum valley depth of the roughness curve within the bend after the bending test described later is 20 μm or less, it can be determined that crack generation within the bend is suppressed and the impact resistance characteristics are excellent.

[0071] The bending test is performed as follows: A 100 mm × 30 mm rectangular test piece is cut from the steel plate at half the width direction. The test is performed in accordance with the V-block method (bending angle θ is 90°) of JIS Z 2248:2022, where the bending ridgeline is parallel to the rolling direction (L direction) (L-axis bending).

[0072] Next, a section is cut out in a direction parallel to the bend ridgeline so that the apex of the bend can be observed, and the roughness is measured using a contact roughness meter. Within the bend at the apex of the bend, the surface roughness is measured at two points 2.0 mm apart, along a direction perpendicular to the ridgeline, using a contact roughness meter. A roughness curve is obtained by applying a profile curve filter with a cutoff value λc to the measured cross-sectional curve obtained by the measurement. Specifically, components with a wavelength λc of 0.8 mm or less are removed from the obtained measurement results to obtain a roughness curve. Based on the obtained roughness curve, the maximum valley depth Rv of the roughness curve is calculated in accordance with JIS B 0601:2013.

[0073] In addition, since the effect is considered to be equivalent if the value obtained by dividing the bending radius R by the plate thickness t is the same, the bending test is performed with R / t set to 1.3. As an example, when the plate thickness t is 3.0 mm, R is 3.9 mm.

[0074] The thickness of the steel plate according to this embodiment is not particularly limited, but may be 1.2 to 8.0 mm. If the thickness of the steel plate is less than 1.2 mm, it may be difficult to ensure the rolling completion temperature and the rolling load may become excessive, making hot rolling difficult. Therefore, the thickness of the steel plate according to this embodiment may be 1.2 mm or more, preferably 1.4 mm or more. On the other hand, if the thickness exceeds 8.0 mm, it may be difficult to obtain the above-mentioned metal structure after hot rolling. Therefore, the thickness may be 8.0 mm or less, preferably 6.0 mm or less.

[0075] The steel sheet according to this embodiment, having the above-described chemical composition and metallographic structure, may be provided with a plating layer on its surface to provide a surface-treated steel sheet for the purpose of improving corrosion resistance, etc. The plating layer may be an electroplated layer or a hot-dip plated layer. Examples of electroplated layers include electrogalvanized galvanized and electrolytic Zn—Ni alloy plating. Examples of hot-dip plated layers include hot-dip galvanized, alloyed hot-dip galvanized, 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, corrosion resistance can be further improved by performing an appropriate chemical conversion treatment after plating (e.g., applying a silicate-based chromium-free chemical conversion treatment solution and drying it).

[0076] The steel sheet according to this embodiment has high strength, excellent ductility, and hole expandability, and is thought to have excellent crash resistance because it suppresses the occurrence of unevenness in the bend during forming. Therefore, it can be suitably used for parts, particularly automobile parts. Among automobile parts, it can be suitably used for automobile suspension parts such as lower arms, trail links, and knuckles. These automobile parts may be made only of the steel sheet according to this embodiment, or may be formed by joining the steel sheet according to this embodiment with other steel sheets.

[0077] Parts manufactured using the steel plate according to this embodiment have the same chemical composition as the above-described steel plate. Furthermore, parts may contain both processed and unprocessed portions. The unprocessed portions have the same metallurgical structure as the above-described steel plate. Processed portions generally have the same metallurgical structure as the above-described steel plate, but parts that have undergone heavy processing or the edges of parts may not have the above-described metallurgical structure. Therefore, when measuring the metallurgical structure of a part, the edges are avoided and the measurement is performed on unprocessed parts. If there are no unprocessed parts, the measurement is performed on parts that have not undergone heavy processing. An unprocessed or heavily processed part refers to, for example, a part of the part that avoids flat parts and parts that have undergone punching, hole expansion, bending, etc. As an example, in the case of the above-described part, a test piece is taken from the flat, largest-area part near the center of gravity and inspected. However, for the surface roughness on the inner periphery of a bent part, which is thought to have a significant impact on crashworthiness, the roughness is measured on the part of the part that has been processed with the smallest bending radius.

[0078] 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. 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.

[0079] A preferred method for manufacturing a steel sheet according to this embodiment is as follows. (1) Rough rolling is performed with a total reduction of 50% or more and a rough rolling completion temperature of 1100°C or more. However, these rough rolling conditions are more preferred. (2) Finish rolling is performed under the following conditions. The finish rolling start temperature is 1020 to 1100°C. The first stage reduction is 40% or more. The total reduction of the final two stages is less than 40%. The finish rolling completion temperature is 900°C or more and less than 980°C. In finish rolling, it is more preferred to perform rolling with a reduction of 30% or more two or more times in a temperature range of 980°C or more and less than 1020°C. (3) After the completion of finish rolling, cooling is started within 1 second, and cooling is performed down to 200°C at an average cooling rate of 30°C / s or more. Each step will be described below.

[0080] (1) Rough Rolling In rough rolling, it is more preferable to set the total reduction rate to 50% or more and the rough rolling completion temperature to 1100° C. or more. By performing rough rolling under these conditions, the maximum value of the major axis length of the prior austenite grains can be preferably controlled.

[0081] The slab is not particularly limited except for having the above-mentioned chemical composition. For example, a slab can be produced by melting molten steel having the above-mentioned chemical composition using a converter or an electric furnace, and then by continuous casting. Instead of continuous casting, an ingot casting method, a thin slab casting method, or the like may be used. In the slab heating before rough rolling, the heating temperature may be maintained in a temperature range of 1100°C or higher for 3000 seconds or more.

[0082] (2) Finish Rolling In finish rolling, it is preferable that the finish rolling start temperature (first-stage entry temperature) be 1020 to 1100°C, the first-stage reduction be 40% or more, the total reduction in the final two stages be less than 40%, and the finish rolling completion temperature (final-stage exit temperature) be 900°C or more and less than 980°C. By performing finish rolling under these conditions, the distribution of the aspect ratio of the prior austenite grains can be preferably controlled. Furthermore, it is more preferable that finish rolling be performed at a temperature range of 980°C or more and less than 1020°C with a reduction of 30% or more, two or more times. By also satisfying this condition, it is possible to preferably control the area ratio of prior austenite grains having an aspect ratio of 2.0 or more and less than 4.0 and prior austenite grains having an aspect ratio of 4.0 or more and less than 6.0.

[0083] The reduction rate here is the thickness of the plate before rolling. ０ The thickness of the plate after rolling is t １ When this is the case, (1-t １ / t ０ ) × 100 (%). The total reduction ratio of the final two stages can be expressed as t ２ The final delivery thickness is t ３ When this is the case, (1-t ３ / t ２ ) × 100 (%).

[0084] (3) Cooling After the finish rolling is completed, it is preferable to start cooling within 1 second and cool down to 200°C at an average cooling rate of 30°C / s or more. By cooling under these conditions, the desired amount of bainite can be obtained. After cooling down to 200°C, the steel sheet can be wound into a coil.

[0085] The average cooling rate in this embodiment 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.

[0086] 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.

[0087] Steels having the chemical compositions shown in Tables 1 and 2 were melted and continuously cast into slabs having thicknesses of 240 to 300 mm. The resulting slabs were used to obtain the steel plates shown in Tables 4A and 4B under the production conditions shown in Tables 3A and 3B. The thicknesses of the resulting steel plates were 1.2 to 8.0 mm. Cooling was initiated within 1 second after the completion of finish rolling.

[0088] The area ratios of the prior austenite grains having an aspect ratio of 1.0 or more and less than 2.0, the area ratios of the prior austenite grains having an aspect ratio of 2.0 or more and less than 4.0, the area ratios of the prior austenite grains having an aspect ratio of 4.0 or more and less than 6.0, the area ratios of the prior austenite grains having an aspect ratio of 6.0 or more, the maximum major axis length of the prior austenite grains, and the area ratios of bainite and martensite were measured for the steel sheets obtained using the above-described methods at a quarter-thickness position from the surface. Furthermore, the tensile strength, total elongation, and hole expansion ratio were measured, and the unevenness within the bend was evaluated using the above-described methods. In some cases, the metallographic structure at a quarter-thickness position from the surface of the steel sheets included ferrite and pearlite as the remaining structure in addition to bainite and martensite. The measurement results are shown in Table 5.

[0089] Method for Evaluating Steel Sheet Properties When the tensile strength was 980 MPa or more, the steel sheet was judged to be high strength and passed, whereas when the tensile strength was less than 980 MPa, the steel sheet was judged not to be high strength and passed.

[0090] When the total elongation was 10.0% or more, the steel sheet was judged to have excellent ductility and to have passed the test, whereas when the total elongation was less than 10.0%, the steel sheet was judged to have poor ductility and to have passed the test.

[0091] When the hole expansion ratio was 50% or more, the steel sheet 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 steel sheet was judged to have poor hole expandability and to have passed the test.

[0092] After the bending test, the maximum valley depth of the roughness curve within the bend was evaluated according to the following criteria. When the evaluation was A or higher, the steel sheet was judged to be pass, as it was deemed that the occurrence of irregularities within the bend during forming was suppressed. On the other hand, when the evaluation was B, the steel sheet was judged to be fail, as it was deemed that the occurrence of irregularities within the bend during forming was not suppressed. Furthermore, when the evaluation was AA, it was judged that the occurrence of irregularities within the bend during forming was further suppressed. AA: The maximum valley depth of the roughness curve was 15 μm or less. A: The maximum valley depth of the roughness curve was more than 15 μm and 20 μm or less. B: The maximum valley depth of the roughness curve was more than 20 μm.

[0093]

[0094]

[0095]

[0096]

[0097]

[0098]

[0099]

[0100] From Tables 1 to 5, it can be seen that the steel sheets according to the examples of the present invention have high strength, excellent ductility and hole expandability, and suppress the occurrence of unevenness inside the bend. On the other hand, it can be seen that the steel sheets according to the comparative examples are inferior in one or more of the above properties.

[0101] Furthermore, for all examples, lower arms (components) were manufactured by press working. The flat portions of the lower arms were evaluated in the same manner as described above. The measurement results and evaluation results were the same as those shown in Tables 4A, 4B, and 5.

[0102] According to the above aspects of the present disclosure, it is possible to provide a steel plate that has high strength, excellent ductility and hole expandability, and that suppresses the occurrence of unevenness within a bend, and a part manufactured using the same.

Claims

1. The chemical composition, in mass%, is: C: 0.045-0.120%, Si: 0-3.00%, Mn: 1.20-3.00%, sol. Al: 0.001-0.500%, P: 0-0.0800%, S: 0-0.0100%, N: 0-0.0050%, O: 0-0.0100%, Ti: 0-0.180%, Nb: 0-0.100%, V: 0-1.000%, Cu: 0-1.000%, Cr: 0-2.000%, Mo: 0-3.000%, Ni: 0-0.500%, B: 0-0.0100%, Ca: 0-0.0500%, Mg: 0-0.050%, REM: 0-0.100%, Bi: 0 to 0.100%, Ta: 0 to 0.100%, 1. A steel plate comprising: Zr: 0-0.500%, Co: 0-3.000%, Zn: 0-0.200%, W: 0-0.200%, Sb: 0-0.500%, As: 0-0.050%, and Sn: 0-0.050%, with the balance being Fe and impurities; and in a metal structure at a position 1 / 4 of the plate thickness from the surface, the metal structure has, in area %, less than 20% of prior austenite grains having an aspect ratio of 1.0 or more and less than 2.0, 40% or more of prior austenite grains having an aspect ratio of 2.0 or more and less than 6.0, less than 30% of prior austenite grains having an aspect ratio of 6.0 or more, 70-95% bainite, and 5-30% martensite.

2. The chemical composition is, in mass%, Ti: 0.001 to 0.180%, Nb: 0.001 to 0.100%, V: 0.001 to 1.000%, Cu: 0.001 to 1.000%, Cr: 0.001 to 2.000%, Mo: 0.001 to 3.000%, Ni: 0.001 to 0.500%, B: 0.0001 to 0.0100%, Ca: 0.0001 to 0.0500%, Mg: 0.001 to 0.050%, REM: 0.001 to 0.100%, Bi: 0.001 to 0.100%, Ta: 0.001 to 0.100%, The steel plate according to claim 1, characterized in that it contains one or more selected from the group consisting of Zr: 0.001 to 0.500%, Co: 0.001 to 3.000%, Zn: 0.001 to 0.200%, W: 0.001 to 0.200%, Sb: 0.001 to 0.500%, As: 0.001 to 0.050%, and Sn: 0.001 to 0.050%.

3. A steel plate according to claim 1 or 2, characterized in that in the metal structure at a position 1 / 4 of the plate thickness from the surface, prior austenite grains having an aspect ratio of 2.0 or more and less than 4.0 account for 30% or more, and prior austenite grains having an aspect ratio of 4.0 or more and less than 6.0 account for 10% or more, in area percentage.

4. A steel plate according to claim 1 or 2, characterized in that in the metal structure at a position 1 / 4 of the plate thickness from the surface, the maximum value of the major axis length of prior austenite grains is 200 μm or less.

5. The steel plate according to claim 3, characterized in that in the metal structure at a position 1 / 4 of the plate thickness from the surface, the maximum value of the major axis length of prior austenite grains is 200 μm or less.

6. A part comprising the steel sheet according to claim 1 or 2.

7. A part comprising the steel sheet according to claim 3.

8. A part comprising the steel plate according to claim 4.

9. A part comprising the steel sheet according to claim 5.

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