Steel plates and parts

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

[Technical Field]

[0001] The present disclosure relates to a steel plate and a part, and more particularly 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. [Background technology]

[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 automotive suspension parts, minute cracks (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 at an area ratio of 95% or more, has a structure in which the average aspect ratio of prior austenite grains is 3.0 or more, has a 5-minute relaxation stress value of 20 MPa or less when a stress of 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 with excellent delayed fracture resistance that has a high strength of tensile strength TS of 1180 MPa or more and significantly improved 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 with excellent fatigue crack growth resistance, which contains 1 to 60% acicular ferrite in terms of area fraction, and in which the proportion of acicular ferrite particles with major axes in the range of 5 to 100 μm is 80% or more. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] International Publication No. 2021 / 193310 [Patent Document 2] Japanese Patent Application Publication No. 2021-116476 [Patent Document 3] Japanese Patent Application Publication No. 2007-321220 Summary of the Invention [Problem to be solved by the invention]

[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. [Means for solving the problem]

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

[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. DETAILED DESCRIPTION OF THE INVENTION

[0014] A steel sheet and a part according to an embodiment of the present disclosure (hereinafter, sometimes referred to as the steel sheet 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] The individual constituent elements of the present disclosure will be described in detail below. First, the reasons for limiting the chemical composition of the steel sheet according to the present embodiment will be described. Below, numerical ranges indicated with "to" include the lower and upper limits. Numerical values ​​indicated as "less than" or "greater than" are not included in the numerical range. In the following description, percentages relating to chemical compositions are mass % unless otherwise specified.

[0016] The chemical composition of the steel sheet according to this embodiment contains, in mass%, 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%, 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, more preferably 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 sheet. 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: 1.20~3.00% Mn is an element that is effective in improving the hardenability and strength of steel sheet 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: 0.001~0.500% Al has the effect of improving the quality of steel by deoxidizing it and also has the effect of controlling the ferrite transformation. To obtain these effects, the sol. Al content is set to 0.001% or more. The sol. Al content is preferably 0.010% or more, and more preferably 0.020% or more. On the other hand, if the Al content exceeds 0.500%, ferrite tends to be excessively formed, resulting in a decrease in 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 sheets. In particular, if the P content exceeds 0.0800%, the weldability of the steel sheets 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 0.0010% or more.

[0022] S: 0 to 0.0100% S is an element that affects the ductility and hole expandability of steel sheets. 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 steel sheets, are generated. Therefore, the S content is set to 0.0100% or less. The S content is preferably 0.0080% or less, or 0.0060% or less. The S content may be 0%. From the viewpoint of refining costs, the S content may be 0.0001% or more.

[0023] N: 0 to 0.0050% N is an element that combines 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 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 the starting point of fracture, 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 may not be 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 0.0005% or more, or 0.0010% or more.

[0025] The steel sheet according to this embodiment may contain the above chemical components, with the balance being Fe and impurities. In this 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 this 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 ensure these effects, the Ti content is preferably 0.001% or more. The Ti content is more preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, if the Ti content exceeds 0.180%, excessive precipitation of TiC will deteriorate 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, and more preferably 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, and more preferably 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, strengthening through grain refinement by inhibiting the growth of ferrite crystal grains, and strengthening through dislocations 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, and more preferably 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 the form of fine particles in steel 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, and more preferably 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, and more preferably 0.600% or less.

[0031] Cr: 0.001 to 2.000% Cr is an element effective in improving the strength of steel sheets. To reliably obtain this effect, the Cr content is preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 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 ferrite precipitation. To ensure this effect, the Mo content is preferably 0.001% or more. The Mo content is more preferably 0.005% or more, and even more preferably 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, and more preferably 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, and more preferably 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, and more preferably 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, and more preferably 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 ensure 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, if the Ca content exceeds 0.0500%, the above effect saturates. Therefore, the Ca content is set to 0.0500% or less. The Ca content is preferably 0.0300% or less, and more preferably 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, and more preferably 0.010% or more. On the other hand, if the Mg content exceeds 0.050%, excessive inclusions are formed in the steel, resulting in a decrease in 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, and more preferably 0.010% or more. On the other hand, if the REM content exceeds 0.100%, excessive inclusions are generated in the steel, resulting in a decrease in the yield ratio of the steel sheet. Therefore, the REM content is set to 0.100% or less. The REM content is preferably 0.080% or less, and more preferably 0.060% or less. Here, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanides, and the REM content refers to the total content of these elements. In the case of lanthanides, they are industrially added in the form of misch metal.

[0038] Bi: 0.001 to 0.100% Bi has the effect of increasing the yield ratio of steel sheets by refining the solidification structure. To ensure this effect, the Bi content is preferably 0.001% or more. The Bi content is more preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, if the Bi content exceeds 0.100%, the above effects become saturated, 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. To ensure this effect, the Ta content is preferably 0.001% or more. The Ta content is more preferably 0.005% or more, and more preferably 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, and more preferably 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, more preferably 0.005% or more, and even more preferably 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, and more preferably 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, more preferably 0.005% or more, and even more preferably 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, and more preferably 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, more preferably 0.005% or more, and even more preferably 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, and more preferably 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, more preferably 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 ensure this effect, the Sb content is preferably 0.001% or more. The Sb content is more preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, even if Sb is contained in a large amount, the above effect saturates, so the Sb content is set to 0.500% or less, preferably 0.300% or less, and more preferably 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, and more preferably 0.010% or more. On the other hand, since the above effects saturate even when As is contained in a large amount, the As content is set to 0.050% or less, preferably 0.040% or less, and more preferably 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, and more preferably 0.010% or more. On the other hand, even if Sn is contained in a large amount, the above effect saturates, so the Sn content is set to 0.050% or less, preferably 0.040% or less, and more preferably 0.030% or less.

[0047] The chemical composition of the steel sheet described above can be analyzed using a spark discharge optical emission spectrometer or the like. Values ​​of C and S are determined by burning the steel sheet in an oxygen stream using a gas composition analyzer or the like and measuring the values ​​by infrared absorption. Values ​​of O and N are determined by melting a test piece taken from the steel sheet in a helium stream and measuring the values ​​by thermal conductivity. When the steel sheet has a plating layer or a coating film on the surface, the plating layer or the coating film is removed by mechanical grinding or the like as necessary before analyzing the chemical composition.

[0048] Next, the metal structure of the steel sheet according to this embodiment will be described. In the steel sheet according to this embodiment, in the metal structure at a position 1 / 4 of the sheet thickness from the surface, 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 plate. When the steel sheet has a plating layer, a coating film, or the like on its surface, the surface here refers to the interface between the steel sheet and the plating layer, the 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% by area Prior austenite grains with an aspect ratio of 2.0 or more and less than 6.0: 40% or more by area Prior austenite grains with an aspect ratio of 6.0 or more: Less than 30% by area If the area ratio of prior austenite grains having a predetermined aspect ratio is outside the above range, it is not possible to suppress the unevenness that occurs within the bent portion during forming. Therefore, the area ratio of prior austenite grains having an aspect ratio of 1.0 or more and less than 2.0 is set to less than 20 area %, the area ratio of prior austenite grains having an aspect ratio of 2.0 or more and less than 6.0 is set to 40 area % or more, and the area ratio of prior austenite grains having an aspect ratio of 6.0 or more is set to less than 30 area %.

[0051] The more prior austenite grains with an aspect ratio of 2.0 or more and less than 6.0, the better, so they may be 100% by area. The prior austenite grains with 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 preferably account for 15% by area or less, and more preferably 10% by area or less. The fewer prior austenite grains having an aspect ratio of 1.0 or more and less than 2.0 the better, so they may account for 0% by area. Furthermore, the area percentage of prior austenite grains having an aspect ratio of 6.0 or more is preferably 25% or less, more preferably 20% or less, and even more preferably 15% or less, most preferably 10% or less. The fewer prior austenite grains having an aspect ratio of 6.0 or more, the better, so they may be 0%.

[0053] Prior austenite grains with an aspect ratio of 2.0 or more and less than 4.0: 30% by area or more Prior austenite grains with an aspect ratio of 4.0 or more and less than 6.0: 10% by 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 making 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 making the prior austenite grains having an aspect ratio of 4.0 or more and less than 6.0 account for 10 area% or more, the occurrence of irregularities within the bend 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 the 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 full 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 turn black due to corrosion are identified as prior austenite grains. The observation surface revealing the prior austenite grains is observed using an optical microscope, and eight fields of view, 200 μm in the thickness direction × 600 μm in the rolling direction, are photographed at 500x magnification from the surface to a quarter-way point in the thickness direction (the region from 1 / 8 of the thickness depth from the surface to 3 / 8 of the thickness depth from the surface). The aspect ratio of each prior austenite grain is obtained by calculating the value of the major axis / minor axis for each prior austenite grain from the photographed structural image. The area ratios of prior austenite grains having each aspect ratio are obtained by calculating the area ratios of prior austenite grains having an aspect ratio of 1.0 or more and less than 2.0, prior austenite grains having an aspect ratio of 2.0 or more and less than 4.0, prior austenite grains having an aspect ratio of 4.0 or more and less than 6.0, and prior austenite grains having an aspect ratio of 6.0 or more.

[0056] If the prior austenite grains cannot be sufficiently revealed by the above-mentioned method, the prior austenite grains can be identified and their aspect ratios determined by the reconstruction method described in "Studies on High-Precision Reconstruction Methods for Austenite Structures in Steel" (Hata Kengo, Wakita Masayuki, Fujiwara Tomoya, Kono Kaori, 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 (field of view of 200 μm in the sheet 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 measurements, an EBSD analysis system consisting of a thermal field emission scanning electron microscope (JEOL JSM-7200F) and an EBSD detector (EDAX Velocity (registered trademark) ultra-high speed EBSD detector) was used. The vacuum level inside the system was 9.6 × 10 -5 The acceleration voltage is set to 15 kV and the probe current to 36 nA. After the acceleration voltage is determined, the probe current may be adjusted to obtain a clearer pattern. To display the EBSD map, version 7 or later of OIM Analysis (registered trademark) manufactured by EDAX / TSL Solution is used for the obtained crystal orientation information.

[0057] When prior austenite grains having an equivalent circle diameter of less than 2 μm are included, they are excluded from the above measurement because prior austenite grains having an equivalent circle diameter of less than 2 μm do not adversely affect the properties of the steel sheet according to this embodiment. Furthermore, when only a portion of the prior austenite grains is included in the edge of the structural photograph, the prior austenite grains are excluded from the measurement. These exclusion conditions also apply to the measurement of the major axis length of prior austenite grains, which will be described later. 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 plate can be observed. The direction perpendicular to the plate surface is the Z direction, and a total of 12 test specimens are taken by rotating the plate 30° around this Z direction. The thickness cross section of the taken test specimens 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 using the intercept method. The test specimen with 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 plate. 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 plate.

[0058] Maximum length of the major axis 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 value of the major axis length of the prior austenite grains is preferably 200 μm or less. By setting the maximum value of the major axis length of the prior austenite grains to 200 μm or less, the occurrence of irregularities in the bent portion during forming can be further suppressed.

[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] Bainite area ratio: 70-95% 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 steel plate cannot obtain the desired strength. 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] Martensite area ratio: 5 to 30% Martensite is a structure that increases the strength of steel sheet. 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 exceeds 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, more preferably 20% or less.

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

[0063] The area ratios of bainite, martensite and the remaining structure are measured by the following method. A sample is taken from a steel sheet at a position 1 / 4 from the end face in the sheet width direction so that the metal structure of a cross section (sheet thickness direction × rolling direction cross section) normal to the sheet 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 sheet thickness direction, 15 mm in the rolling direction, and 10 mm in the sheet width direction. Next, the observation surface is mirror-polished and then etched with nital. After etching, eight visual fields of 200 μm in the sheet 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 sheet thickness (the region from 1 / 8 depth of the sheet thickness to 3 / 8 depth of the sheet thickness from the surface). The obtained microstructure photographs are subjected to image analysis to obtain the area fractions of ferrite, pearlite, and bainite. 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 structure 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 using electron channeling contrast images taken with a scanning electron microscope.

[0065] Bainite is 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, and 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., the structure contains Fe-based carbides elongated in the same direction. Here, the Fe-based carbides elongated in the same direction refer to Fe-based carbides whose elongation directions differ by 5° or less.

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

[0067] Tensile strength (TS): 980 MPa or more The steel sheet 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 making the tensile strength 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 to the tensile strength, but from the viewpoint of suppressing die 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] 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 plate width direction, with the direction perpendicular to the rolling direction as the longitudinal direction. If a No. 5 test piece cannot be taken because the steel plate or part is small or has a complex shape, a small rectangular piece with a parallel section of any width may be taken and used for a tensile test to determine the tensile strength and total elongation. The direction perpendicular to the rolling direction is the longitudinal direction of the rectangular piece. The hole expansion ratio is measured by performing a hole expansion test in accordance with JIS Z 2256:2020.

[0070] Evaluation of unevenness inside bending In the steel sheet 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 property is excellent.

[0071] The bending test is carried out by the following method. A 100mm x 30mm rectangular test piece is cut from the steel plate at half the width. 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-axis bending).

[0072] Next, a section is cut 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, 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. Specifically, components with a wavelength λc of 0.8 mm or less are removed from the measurement results to obtain the 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, the bending test is performed with R / t set to 1.3, 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. As an example, if 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. It is preferably 1.4 mm or more. On the other hand, if the plate thickness exceeds 8.0 mm, it may be difficult to obtain the above-mentioned metal structure after hot rolling. Therefore, the plate thickness may be 8.0 mm or less, and is 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 the 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 coating and electrolytic Zn-Ni alloy coating. Examples of hot-dip plated layers include hot-dip galvanized coating, alloyed hot-dip galvanized coating, hot-dip aluminum coating, hot-dip Zn-Al alloy coating, hot-dip Zn-Al-Mg alloy coating, and hot-dip Zn-Al-Mg-Si alloy coating. The coating weight is not particularly limited and may be the same as conventional coatings. Furthermore, it is possible to further enhance corrosion resistance by carrying out an appropriate chemical conversion treatment after plating (for example, applying a silicate-based chromium-free chemical conversion treatment solution and drying it).

[0076] The steel sheet according to 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 the 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 preferable. (2) Finish rolling is carried out under the following conditions: The finish rolling start temperature is set to 1020 to 1100°C. The reduction rate in the first stage is 40% or more. The total reduction in the final two stages is less than 40%. The temperature at the end of finish rolling is set to 900°C or higher and less than 980°C. In the finish rolling, it is more preferable to perform rolling at a temperature range of 980°C or higher and lower than 1020°C at a reduction rate of 30% or higher two or more times. (3) After the completion of finish rolling, cooling is started within 1 second and cooled 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 the rough rolling, it is more preferable that the total reduction rate is 50% or more and the rough rolling completion temperature is 1100° C. or more. By performing the 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 the chemical composition described above. For example, a slab can be produced by melting molten steel having the above chemical composition using a converter or electric furnace, and then by continuous casting. Instead of continuous casting, an ingot casting method, thin slab casting method, or the like may be used. When heating the slab before rough rolling, the heating temperature may be maintained at 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 rate of 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, in the finish rolling, it is more preferable to perform rolling at least twice at a reduction ratio of 30% in a temperature range of 980° C. or higher and lower than 1020° C. 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 higher and lower than 4.0 and prior austenite grains having an aspect ratio of 4.0 or higher and lower than 6.0.

[0083] The reduction ratio here can be expressed as (1-t1 / t0) x 100(%), where t0 is the thickness before rolling and t1 is the thickness after rolling. Furthermore, the total reduction ratio of the final two stages can be expressed as (1-t3 / t2) x 100(%), where t2 is the entry thickness one stage before the final stage and t3 is the delivery thickness of the final stage.

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

[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 with thicknesses of 240 to 300 mm. The resulting slabs were used to obtain 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. After the finish rolling was completed, cooling was started within 1 second.

[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 by the methods described above, at a position 1 / 4 of the sheet thickness from the surface. In addition, the tensile strength, total elongation, and hole expansion ratio were measured, and the unevenness inside the bend was evaluated by the methods described above. In some cases, the metal structure at the 1 / 4 position from the surface of the steel plate contained ferrite and pearlite as the remaining structure in addition to bainite and martensite. The measurement results are shown in Table 5.

[0089] Evaluation method for steel plate properties If the tensile strength was 980 MPa or more, the steel sheet was judged to have high strength and passed the test, whereas if the tensile strength was less than 980 MPa, the steel sheet was judged to have low strength and failed the test.

[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 failed 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. If the evaluation was A or higher, the steel sheet was judged to be pass, as it was deemed that the occurrence of unevenness within the bend during forming was suppressed. On the other hand, if the evaluation was B, the steel sheet was judged to be fail, as it was deemed that the occurrence of unevenness within the bend during forming was not suppressed. Furthermore, if the evaluation was AA, it was determined that the steel sheet was one in which the occurrence of unevenness inside the bend during forming was further suppressed. AA: The maximum valley depth of the roughness curve is 15 μm or less A: The maximum valley depth of the roughness curve is over 15 μm and 20 μm or less B: The maximum valley depth of the roughness curve is more than 20 μm

[0093] [Table 1]

[0094] [Table 2]

[0095] [Table 3A]

[0096] [Table 3B]

[0097] [Table 4A]

[0098] [Table 4B]

[0099] [Table 5]

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

[0101] 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 4A, 4B, and 5. [Industrial Applicability]

[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 to 0.500%, P: 0 to 0.0800%, S: 0 to 0.0100%, N: 0 to 0.0050%, O: 0 to 0.0100%, Ti: 0 to 0.180%, Nb: 0 to 0.100%, V: 0 to 1.000%, Cu: 0 to 1.000%, Cr: 0-2.000%, Mo: 0-3.000%, Ni: 0 to 0.500%, B: 0 to 0.0100%, Ca: 0-0.0500%, Mg: 0 to 0.050%, REM: 0-0.100%, Bi: 0-0.100%, Ta: 0-0.100%, Zr: 0 to 0.500%, Co: 0-3.000%, Zn: 0-0.200%, W: 0-0.200%, Sb: 0 to 0.500%, As: 0 to 0.050%, and Sn: 0 to 0.050%; the balance being Fe and impurities; In the metal structure at 1 / 4 of the plate thickness from the surface, in area%, 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 account for 40% or more, Prior austenite grains having an aspect ratio of 6.0 or more are less than 30%; Bainite is 70 to 95%; A steel plate characterized in that martensite is 5 to 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: 0.001-2.000%, Mo: 0.001-3.000%, Ni: 0.001 to 0.500%, B: 0.0001 to 0.0100%, Ca: 0.0001-0.0500%, Mg: 0.001-0.050%, REM: 0.001-0.100%, Bi: 0.001-0.100%, Ta: 0.001 to 0.100%, Zr: 0.001 to 0.500%, Co: 0.001 to 3.000%, Zn: 0.001-0.200%, W: 0.001-0.200%, Sb: 0.001 to 0.500%, As: 0.001 to 0.050%, and The steel sheet according to claim 1, further comprising at least one element selected from the group consisting of Sn: 0.001 to 0.050%.

3. In the metal structure at a position of 1 / 4 of the plate thickness from the surface, in terms of area%, Prior austenite grains having an aspect ratio of 2.0 or more and less than 4.0 account for 30% or more, 3. The steel sheet according to claim 1, wherein the content of prior austenite grains having an aspect ratio of 4.0 or more and less than 6.0 is 10% or more.

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

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

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

7. A component comprising the steel sheet of claim 3.

8. A component comprising the steel sheet of claim 4.

9. A component comprising the steel sheet of claim 5.

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