Steel plate and component

Abstract

This component is formed from a steel plate which has a predetermined chemical composition and in which, in a region extending from a depth of 1 / 8 the plate thickness from a surface to a depth of 3 / 8 the plate thickness from the surface, prior-austenite grains have an average aspect ratio of 3.00-5.50, the areal proportion of bainite is 70-95%, and the areal proportion of martensite is 5-30%, wherein the ratio of the total amount of Ti and Nb analyzed as electroextraction residues at a depth of 1 / 60 the plate thickness from the surface to the total amount of Ti and Nb analyzed as electroextraction residues at a depth of 1 / 4 the plate thickness from the surface is 1.5 or greater.

Description

Steel plates and parts

[0001] This application claims priority to Japanese Patent Application No. 2024-006002, filed on January 18, 2024, 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] It is also known that the stronger the steel plate, the more likely it is that cracks will occur on the inside of the bend during bending (hereinafter, cracks that occur on the inside of the bend during bending will be referred to as inside-bend cracks). The mechanism of inside-bend cracks is presumed to be as follows: During bending, compressive stress is generated on the inside of the bend. Initially, the entire inside of the bend deforms uniformly as the bending progresses, but as the amount of deformation increases, the uniform deformation alone is no longer able to support the deformation, and strain concentrates in localized areas, causing the deformation to progress (the generation of shear deformation bands). As these shear deformation bands continue to grow, cracks initiate and grow from the inside surface of the bend along the shear bands.

[0005] The reason why internal cracks tend to occur more easily as the strength of steel sheets increases is presumably because the work hardening ability decreases with increasing strength, making it difficult for uniform deformation to occur and making it easier for deformation to become uneven, resulting in the formation of shear deformation bands early in processing (or under loose processing conditions). Automotive suspension parts are manufactured in various ways, and steel sheets are often subjected to bending. Internal cracks during processing of steel sheets are undesirable because they reduce the durability and crashworthiness of the parts. Therefore, steel sheets, particularly those used in automotive suspension parts, are required to be able to suppress the occurrence of internal cracks, i.e., to have excellent resistance to internal cracks during bending.

[0006] For example, Patent Document 1 discloses a high-strength hot-rolled steel sheet with excellent punchability, which has a bainite phase that accounts for more than 95% in terms of area ratio throughout the entire thickness direction, and in which the average grain size of the bainite phase in a region from the surface to a position 1 / 4 of the thickness in the thickness direction is 5 μm or less in a thickness cross section parallel to the rolling direction and 4 μm or less in a thickness cross section perpendicular to the rolling direction, and further has a structure in which there are 7 or less crystal grains that are elongated in the rolling direction and have an average aspect ratio of 5 or more in a region whose width in the thickness direction is 1 / 10 of the thickness centered at the center position of the thickness, and which has a tensile strength TS of 780 MPa or more.

[0007] Japanese Patent Application Publication No. 2012-62562

[0008] However, Patent Document 1 does not take into consideration cracks inside the bend.

[0009] The present disclosure has been made in consideration of the above-mentioned problems, and aims to provide a steel plate having high strength, as well as excellent ductility, hole expandability, and resistance to internal bending cracking, and a part using the same.

[0010] The gist of the present invention 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%, Ti: 0.020 to 0.180%, Al: 0.010 to 0.400%, P: 0 to 0.080%, S: 0 to 0.0100%, N: 0 to 0.0050%, O: 0 to 0.0100%, Nb: 0 to 0.100%, V: 0 to 1.000%, Cu: 0 to 1.000%, Cr: 0 to 2.000%, Mo: 0 to 3.000%, Ni: 0 to 1.000%, B: 0 to 0.0100%, Ca: 0 to 0.0500%, Mg: 0 to 0.050%, REM: 0 to 0.1000%, 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%, Sn: 0 to 0.050%, and the balance: Fe and impurities, wherein in a region from 1 / 8 of the plate thickness depth from the surface to 3 / 8 of the plate thickness depth from the surface, the average aspect ratio of prior austenite grains is 3.00 to 5.50, and the area fraction of bainite is 70 to 95%, a steel sheet having an area ratio of martensite of 5 to 30%, and a ratio of a total amount of Ti and Nb analyzed as an electrolytic extraction residue at a depth of 1 / 60 of the sheet thickness from the surface to a total amount of Ti and Nb analyzed as an electrolytic extraction residue at a depth of 1 / 4 of the sheet thickness from the surface to 1.5 or more.[2] The chemical composition, in mass%, is: 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 1.000%, B: 0.0001 to 0.0100%, Ca: 0.0001 to 0.0500%, Mg: 0.001 to 0.050%, REM: 0.0001 to 0.1000%, Bi: 0.001 to 0.100%, Ta: 0.001 to 0.100%, [1] The steel sheet according to [1], characterized by containing one or more 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] The steel sheet according to [1] or [2], characterized by having a sheet thickness of 1.2 to 8.0 mm. [4] The steel sheet according to any one of [1] to [3], characterized by having a tensile strength of 980 MPa or more. [5] A part, characterized by comprising the steel sheet according to any one of [1] to [4].

[0011] According to the above aspects of the present disclosure, it is possible to provide a steel plate having high strength, as well as excellent ductility, hole expandability, and resistance to internal bending cracking, and a part using the same.

[0012] FIG. 10 is a diagram for explaining a bending test.

[0013] The present inventors have discovered that by controlling the average aspect ratio of prior austenite grains in the inner region, it is possible to achieve both excellent ductility and hole expandability in a high-strength steel plate.

[0014] Furthermore, the present inventors have found that the resistance to internal bending cracking of a steel sheet can be improved by increasing the total amount of Ti and Nb in the surface layer of the steel sheet to be greater than the total amount of Ti and Nb in the interior of the steel sheet. The present inventors speculate that this is due to the following reasons.

[0015] The C and Ti (and Nb, if Nb is contained) contained in the steel sheet are consumed as coarse precipitates. It has been found that consuming C can increase the ductility of the metal structure of the surface layer. It has also been found that consuming C, Ti, and Nb as coarse precipitates in the surface layer of the steel sheet can reduce the strength of the surface layer of the steel sheet and increase the ductility. In this way, it has been found that increasing the ductility of the surface layer of the steel sheet can improve the internal bending crack resistance of the steel sheet.

[0016] The present inventors have also found that the above-described steel sheet can be produced by strictly controlling the temperature history, particularly from rough rolling to finish rolling.

[0017] A steel sheet according to an embodiment of the present disclosure (hereinafter, sometimes referred to as the steel sheet 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.

[0018] 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.

[0019] 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%, Ti: 0.020 to 0.180%, Al: 0.010 to 0.400%, P: 0 to 0.080%, S: 0 to 0.0100%, N: 0 to 0.0050%, O: 0 to 0.0100%, Nb: 0 to 0.100%, V: 0 to 1.000%, Cu: 0 to 1.000%, Cr: 0 to 2.000%, Mo: 0 to 3.000%, Ni: 0-1.000%, B: 0-0.0100%, Ca: 0-0.0500%, Mg: 0-0.050%, REM: 0-0.1000%, Bi: 0-0.100%, Ta: 0-0.100%, Zr: 0-0.500%, Co: 0-3.000%, Zn: 0-0.200%, W: 0-0.200%, Sb: 0-0.500%, As: 0-0.050%, Sn: 0-0.050%, and the balance: Fe and impurities. Each element will be described in detail below.

[0020] 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, 0.055% or more, 0.060% or more, or 0.065% or more. On the other hand, if the C content exceeds 0.120%, the ductility and hole expandability of the steel sheet deteriorate. Therefore, the C content is set to 0.120% or less. The C content is preferably 0.100% or less, 0.900% or less.

[0021] 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, 1.50% or less, or 1.00% or less.

[0022] Mn: 1.20 to 3.00% Mn is an element effective in improving hardenability and the 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%, MnS, which has an adverse effect on the ductility of the steel sheet, is likely to be formed. 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.

[0023] Ti: 0.020 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. If the Ti content is less than 0.020%, the strength of the steel sheet decreases. Therefore, the Ti content is set to 0.020% or more. The Ti content is preferably 0.050% or more, 0.080% or more, or 0.100% or more. On the other hand, if the Ti content exceeds 0.180%, recrystallization is significantly inhibited, making it impossible to favorably control the average aspect ratio of prior austenite grains, and the hole expandability of the steel sheet deteriorates. 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.

[0024] Al: 0.010 to 0.400% Al has the effect of improving the soundness of steel by deoxidizing and also has the effect of controlling ferrite transformation. Furthermore, if the Al content is less than 0.010%, the hole expandability of the steel sheet deteriorates. Therefore, the Al content is set to 0.010% or more. The Al content is preferably 0.020% or more, 0.050% or more, or 0.100% or more. On the other hand, if the Al content exceeds 0.400%, alumina precipitates in the form of clusters, increasing the cracking sensitivity of the slab and making it difficult to handle. Therefore, the Al content is set to 0.400% or less. The Al content is preferably 0.300% or less, 0.250% or less, or 0.200% or less.

[0025] P: 0 to 0.080% P is an element that affects the weldability of steel plate. In particular, if the P content exceeds 0.080%, 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 0.080% or less. The P content is preferably 0.040% or less, 0.020% or less, or 0.010% or less. The P content may be 0%. From the viewpoint of refining costs, the P content may be set to 0.001% or more.

[0026] S: 0 to 0.0100% S is an element that affects the 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 hole expandability of 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, or 0.0050% or less. The S content may be 0%. From the viewpoint of refining costs, the S content may be 0.0001% or more, or 0.0010% or more.

[0027] 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, or 0.0010% or more.

[0028] O: 0 to 0.0100% O is an element that, when contained in large amounts in steel, forms coarse oxides that serve as 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. Furthermore, the hole expandability of the steel sheet deteriorates. Therefore, the O content is set to 0.0100% or less. The O content is preferably 0.0080% or less, 0.0060% or less, 0.0040% or less, or 0.0035% 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.

[0029] Nb: 0 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 precipitation strengthening of NbC. The Nb content may be 0%, but to obtain this effect, the Nb content is preferably 0.001% or more. The Nb content is more preferably 0.005% or more, 0.010% or more. On the other hand, if the Nb content exceeds 0.100%, the above effect saturates. In addition, the hole expandability of the steel sheet deteriorates. 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, 0.060% or less.

[0030] V: 0 to 1.000% V has the effect of increasing the strength of the steel sheet by strengthening through precipitation, grain refinement strengthening by inhibiting ferrite grain growth, and dislocation strengthening by inhibiting recrystallization. The V content may be 0%, but to 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, 0.600% or less, 0.400% or less, or 0.200% or less.

[0031] Cu: 0 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. The Cu content may be 0%, but to 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, 0.600% or less, 0.400% or less, or 0.200% or less.

[0032] Cr: 0 to 2.000% Cr is an element effective in improving the strength of steel sheet. The Cr content may be 0%, but to 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, 1.000% or less, 0.600% or less, or 0.300% or less.

[0033] Mo: 0 to 3.000% Mo is an element effective in strengthening ferrite precipitation. The Mo content may be 0%, but to 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, an excessive Mo content increases the cracking sensitivity of the slab, making it 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, 1.500% or less, 1.000% or less, 0.500% or less, or 0.300% or less.

[0034] Ni: 0 to 1.000% Ni has the effect of suppressing phase transformation at high temperatures and increasing the strength of the steel sheet. The Ni content may be 0%, but to 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 1.000% or less. The Ni content is preferably 0.500% or less, 0.300% or less, or 0.150% or less.

[0035] B: 0 to 0.0100% B has the effect of suppressing phase transformation at high temperatures and increasing the strength of the steel sheet. The B content may be 0%, but to 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.

[0036] Ca: 0 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. The Ca content may be 0%, but to 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, 0.0200% or less, 0.0150% or less, or 0.0100% or less.

[0037] Mg: 0 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. The Mg content may be 0%, but to achieve 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.

[0038] REM: 0 to 0.1000% REM has the effect of increasing the yield ratio of steel sheets by adjusting the shape of inclusions in steel to a preferred shape. While the REM content may be 0%, to achieve this effect, the REM content is preferably 0.0001% or more. The REM content is more preferably 0.0005% or more, or 0.0010% or more. On the other hand, if the REM content exceeds 0.1000%, excessive inclusions are formed in the steel, resulting in a decrease in the yield ratio of the steel sheet. Therefore, the REM content is set to 0.1000% or less. The REM content is preferably 0.0800% or less, 0.0600% or less, 0.0300% or less, or 0.0100% 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 added industrially in the form of misch metal.

[0039] Bi: 0 to 0.100% Bi has the effect of refining the solidification structure and thereby increasing the yield ratio of the steel sheet. The Bi content may be 0%, but to 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.

[0040] Ta: 0 to 0.100% Ta, like V, has the effect of increasing the strength of the steel sheet by forming fine carbides in the steel. The Ta content may be 0%, but to 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.

[0041] Zr: 0 to 0.500% Zr has the effect of increasing the strength of steel sheet by solid solution strengthening. The Zr content may be 0%, but to 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.

[0042] Co: 0 to 3.000% Co has the effect of increasing the strength of steel sheet through solid solution strengthening. The Co content may be 0%, but to obtain this effect, the Co content is preferably 0.001% or more. The Co content is more preferably 0.005% or more, 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, 0.500% or less.

[0043] Zn: 0 to 0.200% Zn has the effect of increasing the strength of steel sheet through solid solution strengthening. The Zn content may be 0%, but to obtain this effect, the Zn content is preferably 0.001% or more. The Zn content is more preferably 0.005% or more, 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, 0.100% or less.

[0044] W: 0 to 0.200% W has the effect of increasing the strength of steel sheet through solid solution strengthening. The W content may be 0%, but to obtain this effect, the W content is preferably 0.001% or more. The W content is more preferably 0.005% or more, 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.

[0045] Sb: 0 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. The Sb content may be 0%, but to 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.

[0046] As: 0 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 the steel sheet. The As content may be 0%, but to 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.

[0047] Sn: 0 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 the steel sheet. The Sn content may be 0%, but to 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 contained, 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.

[0048] The steel sheet according to this embodiment contains the above chemical components, with the balance being Fe and impurities. In this embodiment, the impurities refer to substances that are mixed in from raw materials such as ore, scrap, or the manufacturing environment, and / or substances that are allowed to exist within a range that does not adversely affect the properties of the steel sheet according to this embodiment.

[0049] The chemical composition of the above-mentioned steel sheet is analyzed using a spark discharge optical emission spectrometer or the like. C and S are analyzed using the combustion-infrared absorption method, N using the inert gas fusion-thermal conductivity method, and O using the inert gas fusion-non-dispersive infrared absorption method. If the steel sheet has a plating layer or a coating film on the surface, the plating layer or coating film is removed by mechanical grinding or the like as necessary before analyzing the chemical composition.

[0050] Next, the metallographic structure of the steel sheet according to this embodiment will be described. In the steel sheet according to this embodiment, in a region from the surface to a depth of ⅛ of the plate thickness from the surface to a depth of ⅜ of the plate thickness from the surface, the average aspect ratio of prior austenite grains is 3.00 to 5.50, the area fraction of bainite is 70 to 95%, and the area fraction of martensite is 5 to 30%, and the ratio of the sum of the amounts of Ti and Nb analyzed as electrolytic extraction residue at a depth of 1 / 60 of the plate thickness from the surface to the sum of the amounts of Ti and Nb analyzed as electrolytic extraction residue at a depth of ¼ of the plate thickness from the surface is 1.5 or more.

[0051] In this embodiment, the region from the surface to a depth of ⅛ of the plate thickness from the surface is, in other words, a region starting from a depth of ⅛ of the plate thickness from the surface and ending at a depth of ⅜ of the plate thickness from the surface. The reason for specifying the metallographic structure in this region is that the metallographic structure in this region represents a typical metallographic structure of a steel plate.

[0052] When a 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, coating film, or the like. The interface between the steel sheet and the plating layer, coating film, or the like is identified by a BSE image (specifically, a COMPO image (BSE compositional image)) obtained by the following method. A sample is cut out so that the cross section of the steel sheet thickness can be observed. The cut sample is mechanically polished and then mirror-finished. Using a scanning microscope, an area of ​​40,000 μm is observed at a magnification of 400 times. ２ The above range is observed. When a cross section is observed using a BSE image (specifically, a COMPO image), a clear difference in contrast is confirmed between the plating layer, coating film, etc. and the base steel (steel sheet). Therefore, the position where the contrast changes from the outermost surface can be identified as the interface between the steel sheet and the plating layer, coating film, etc. When the part described below has a plating layer, coating film, etc., the interface is identified using a similar method.

[0053] Average aspect ratio of prior austenite grains: 3.00 to 5.50 If the average aspect ratio of prior austenite grains in the inner region is less than 3.00, the ductility of the steel sheet will deteriorate. Therefore, the average aspect ratio of prior austenite grains in the region from 1 / 8 depth of the sheet thickness from the surface to 3 / 8 depth of the sheet thickness from the surface is set to 3.00 or more. The average aspect ratio of prior austenite grains in the above region is preferably 3.40 or more, 3.60 or more, or 3.80 or more. On the other hand, if the average aspect ratio of prior austenite grains in the above region exceeds 5.50, the hole expandability of the steel sheet will deteriorate. Therefore, the average aspect ratio of prior austenite grains in the inner region is set to 5.50 or less. The average aspect ratio of prior austenite grains in the inner region is preferably 5.30 or less, 5.00 or less, or 4.50 or less.

[0054] The aspect ratio of a prior austenite grain is a value obtained by dividing the major axis of the prior austenite grain by the minor axis, and is a value of 1.00 or more.

[0055] The average aspect ratio of prior austenite grains is measured using the following method. It is preferable to collect a sample at a position sufficiently distant from the widthwise end of the steel plate, for example, at a quarter-width position from the end face in the widthwise direction of the steel plate (a position ¼ of the width from the end face in the widthwise direction), so that the metal structure of the thickness cross section (thickness direction x rolling direction cross section) 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. The method for revealing prior austenite grains is the "Bechet-Beaujard method using corrosion with a saturated aqueous solution of picric acid" in Appendix JA.2 of 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 using an optical microscope, and eight fields of view, each measuring 200 μm in the thickness direction and 600 μm in the rolling direction, are photographed within the region from ⅛ of the plate thickness depth from the surface to ⅜ of the plate thickness depth from the surface. The ratio of the major axis to the minor axis measured for each prior austenite grain is calculated from the photographed structure photographs, and the ratio is weighted by the area of ​​each prior austenite grain to calculate an average value, thereby obtaining the average aspect ratio of the prior austenite grains. For example, if the "major axis / minor axis" of a certain prior austenite grain G1 is r1 and its area is A1, and the "major axis / minor axis" of another prior austenite grain G2 is r2 and its area is A2, the average aspect ratio of the two prior austenite grains is calculated as "(A1 × r1 + A2 × r2) / (A1 + A2)." The general formula can be expressed as follows: where ri is the "major axis / minor axis" of the i-th prior austenite grain, and Ai is the area of ​​the i-th prior austenite grain.

[0056]

[0057] When the prior austenite grains cannot be sufficiently revealed by the above-mentioned method, the prior austenite grains are identified by the reconstruction method described in "Study on High-Precision Reconstruction Method of Austenite Structure of Steel" (Hata Kengo, Wakita Masayuki, Fujiwara Tomoya, Kono Kaori, Nippon Steel & Sumitomo Metal Technical Report No. 404 (2016), pp. 24-30), and the average value of the aspect ratio of the prior austenite grains is determined.

[0058] In addition, when prior austenite grains having an equivalent circle diameter of less than 2 μm are contained, these are excluded from the above measurement because prior austenite grains having an equivalent circle diameter of less than 2 μm do not adversely affect the properties of the steel sheet according to this embodiment.

[0059] Unless the rolling direction is known in advance, the rolling direction of a steel plate is determined using the following method. A test specimen is taken from any position at least 50 mm away from the end of the steel plate so that the plate thickness cross section can be observed. The plate thickness cross section of the taken test specimen is mirror-polished and then observed using an optical microscope at magnifications of 100x, 200x, 500x, and 1000x. An appropriate magnification at which the inclusion dimensions can be measured is selected depending on the dimensions of the inclusions. The observation range is 500 μm or more in width and across the entire plate thickness, and areas with dark brightness are determined to be inclusions. Observation may be performed using multiple fields of view. Next, using the plate thickness cross section initially observed using the above method as a reference, a plane parallel to the plane rotated in 5° increments in the range of 0° to 180° around the plate thickness direction is observed using the same method as above. The average length of the major axes of the multiple inclusions in each cross section is calculated for each cross section. The cross section with the largest average value of the major axis length of the inclusions obtained is identified. The direction parallel to the longitudinal axis of the inclusion in the cross section is determined as the rolling direction. The same method is used to determine the rolling direction for parts.

[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 7% or more, 10% or more, or 15% 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, or 20% or less.

[0062] Remaining structure: 0 to 5% The metal structure of the internal 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%. The area ratio of the remaining structure may be 3% or less, 2% or less, or 1% or less, or there may be no remaining structure, i.e., the area ratio of the remaining structure may be 0%. The total area ratio of bainite and martensite may be 95 to 100%. The total area ratio of bainite and martensite may be 97% or more, 98% or more, 99% or more, or 100%.

[0063] The area ratios of bainite, martensite, and the remaining structure are measured by the following method. A test piece is taken from the steel plate so that the metal structure can be observed at the 1 / 4 position in the plate thickness direction (from the surface to the 1 / 8 position in the plate thickness direction to the 3 / 8 position in the plate thickness direction). The cross section of the test piece is mirror-polished and etched with LePera, and then an FE-SEM (thermal field emission scanning electron microscope, JSM-7001F, manufactured by JEOL) is used to observe a 200 μm (in the plate thickness direction) × 600 μm (in the direction perpendicular to the plate thickness direction) region at the 1 / 4 position in the plate thickness direction, and image analysis is performed.

[0064] In Repelle corrosion, martensite and retained austenite are not corroded, so the total area fraction of martensite and retained austenite is obtained by calculating the area fraction of the uncorroded area (i.e., the white area).

[0065] The area fraction of retained austenite is obtained by X-ray diffraction. A test specimen taken from a steel plate is ground from the plate surface to a 1 / 4 position in the plate thickness direction (a range from a position 1 / 8 of the plate thickness to a position 3 / 8 of the plate thickness from the surface in the plate thickness direction), and the exposed surface is used as the observation surface. This observation surface is mirror-polished and then finished by electrolytic polishing. For the observation surface, the integrated intensities of a total of five peaks, α(200), α(211), γ(200), γ(220), and γ(311), are determined using a Rigaku RINT-2500 and Mo-Kα, and the volume fraction of retained austenite is calculated using the intensity averaging method. This volume fraction of retained austenite is considered to be the area fraction of retained austenite.

[0066] The total area fraction of martensite is obtained by subtracting the area fraction of retained austenite obtained by X-ray diffraction from the sum of the area fractions of martensite and retained austenite obtained by observation using the Fe-SEM. If the total area fraction of martensite is a negative value in the calculation, the total area fraction of martensite is set to 0%.

[0067] The area fraction of pearlite is obtained by the following method. For the same region (200 μm × 600 μm) as that used to determine the area fractions of martensite and retained austenite, only the corroded layer is removed by polishing and the surface is mirror-finished, and then the region is etched using a nital solution, observed using an FE-SEM, and image analysis is performed. The region where cementite and ferrite are arranged in a lamellar pattern is determined to be pearlite, and the area fraction of that region is calculated to obtain the area fraction of pearlite.

[0068] The ferrite area fraction is obtained by the following method. The following operation is performed on regions other than those identified as pearlite by the above method. The same region (200 μm × 600 μm) as used to determine the area fractions of martensite and retained austenite is subjected to colloidal polishing or electrolytic polishing, and then crystal orientation information is obtained by electron backscatter diffraction at measurement intervals of 0.2 μm. 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 for the measurement. The vacuum level inside the apparatus is 9.6 × 10 Pa or less, the acceleration voltage is 25 kV, and the probe current level is 16.

[0069] The following analysis is performed using the obtained crystal orientation information using version 7 or later of OIM Analysis (registered trademark) manufactured by EDAX / TSL Solution. Measurement points with a crystal orientation difference of 15° or more are considered to be crystal grain boundaries, and the area surrounded by the crystal grain boundaries is considered to be crystal grains. Next, the difference in crystal orientation between all measurement points within a crystal grain is calculated, and the average value of these differences is calculated to obtain the GAM value (Grain Average Misorientation value) of the crystal grain. Crystal grains with a GAM value of 0.5° or less are considered to be ferrite, and their area ratio is calculated to obtain the ferrite area ratio.

[0070] The area fraction of bainite is obtained by subtracting the area fractions of martensite, retained austenite, pearlite, and ferrite obtained above from 100%. If the area fraction of bainite is a negative value in the calculation, the area fraction of bainite is set to 0%. In this embodiment, the area fraction of the metallographic structure is calculated by image analysis using an FE-SEM, X-ray diffraction, and EBSD analysis, so the total of each structure may not be 100%. In such cases, the area fraction of each structure is corrected so that the total becomes 100%. For example, if the total of the area fractions of each structure is 103%, the area fraction of each structure is corrected by multiplying the area fraction of each structure by "100 / 103".

[0071] The observation conditions for the FE-SEM are as follows: Electron gun type: thermal emission type Current irradiation number: 9 WD (working distance): 10 mm Acceleration voltage: 20 kV Objective aperture number: 4 Number of pixels: 5120 x 3840

[0072] The ratio of the sum of the Ti and Nb amounts analyzed as electrolytic extraction residue at a depth of 1 / 60 of the plate thickness from the surface to the sum of the Ti and Nb amounts analyzed as electrolytic extraction residue at a depth of 1 / 4 of the plate thickness from the surface: 1.5 or more The present inventors have investigated a method for improving internal bending crack resistance while ensuring high strength and excellent ductility and hole expandability in a steel plate. As a result of focusing on the Ti and Nb amounts in the surface layer and the interior of the steel plate, the present inventors have found that the internal bending crack resistance of the steel plate can be improved by increasing the Ti and Nb amounts in the surface layer of the steel plate compared to the Ti and Nb amounts in the interior of the steel plate. Specifically, the ratio of the sum of the Ti and Nb amounts analyzed as electrolytic extraction residue at a depth of 1 / 60 of the plate thickness from the surface (ER １／６０ ) and the total amount of Ti and Nb analyzed as electrolytic extraction residue at a depth of 1 / 4 of the plate thickness from the surface (ER １／４ ) and the ratio (ER １／６０ / ER １／４ ) to 1.5 or more, the present inventors have found that the resistance to internal bending cracking of the steel sheet can be improved while ensuring the above-mentioned properties.

[0073] ER １／６０ / ER １／４ If ER is less than 1.5, the resistance to internal cracking due to bending of the steel sheet cannot be improved. １／６０ / ER １／４ is 1.5 or more. １／６０ / ER １／４ is preferably 1.8 or more or 2.0 or more. １／６０ / ER １／４ There is no particular upper limit for ER １／６０ / ER １／４ may be 5.0 or less, 4.0 or less, or 3.5 or less.

[0074] The total amount of Ti and Nb analyzed as electrolytic extraction residue at a depth of 1 / 60 of the plate thickness from the surface (ER１／６０ ) and the total amount of Ti and Nb analyzed as electrolytic extraction residue at a depth of 1 / 4 of the plate thickness from the surface (ER １／４ ) is measured by the following method. A test piece of a predetermined size is taken from the steel plate. The test piece is mechanically polished to expose a position at a depth of 1 / 60 of the plate thickness - 2.5 μm from the surface of the steel plate, and a position at a depth of 1 / 4 of the plate thickness - 2.5 μm from the surface of the steel plate. If necessary, the surfaces other than the exposed surface are protected with a resin or the like. The exposed surface of the test piece is subjected to constant current electrolysis using a 10% by volume acetylacetone-1% by mass tetramethylammonium chloride methanol solution as the electrolyte at a current density of 20 mA / cm. ２ The specimen is subjected to anodic dissolution at a depth of 5.0 μm using a tungsten carbide (TCE) ion beam, and carbonitrides and nitrides are extracted as residue. The extracted residue is then subjected to acid decomposition, followed by ICP (inductively coupled plasma) emission spectrometry to measure the masses of Ti and Nb in the residue. The masses of Ti and Nb in the residue are divided by the dissolved amounts of the test piece to determine the total amount of Ti and Nb present as carbonitrides and nitrides. This gives the total amount of Ti and Nb analyzed as electrolytic extraction residue.

[0075] 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, 1300 MPa or less, or 1250 MPa or less.

[0076] 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 break) of 10.0% or more, and a hole expansion ratio of 50% or more. The total elongation is preferably 12.0% or more or 13.0% or more. The hole expansion ratio is preferably 55% or more, 60% or more, or 70% or more.

[0077] 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 preferably taken at a quarter-width position from the end in the width direction (a position 1 / 4 of the width from the end in the width direction), with the direction perpendicular to the rolling direction being the longitudinal direction. When measuring the tensile strength of a part, if a No. 5 test specimen cannot be taken from the part due to its small size or complex shape, a small rectangular piece having a parallel portion of any width may be taken and subjected to a tensile test, and the tensile strength may be determined from the maximum test force and the original cross-sectional area of ​​the parallel portion. The hole expansion ratio is measured by conducting a hole expansion test in accordance with JIS Z 2256:2020.

[0078] Resistance to internal cracking in bending: crack length in bending is 50 μm or less. The steel plate according to this embodiment may have a crack length in bending of 50 μm or less, as determined by the following evaluation. For example, a rectangular test piece measuring 100 mm (rolling direction) x 50 mm (perpendicular to the rolling direction) is cut from a half-width position in the width direction of the steel plate (a position half the width away from the end in the width direction) to obtain a bending test piece. In accordance with the V-block method of JIS Z 2248:2020, as shown in FIG. 1, the obtained test piece is subjected to a bending test using a presser and a V-block with an interior angle of 60°, with the bending axis perpendicular to the rolling direction (C-axis bending, i.e., L-bending). The radius R of the tip of the presser is set such that the ratio R / t of the radius R to the plate thickness t is 0.517. The length of the crack generated in the bending in a cross section perpendicular to the bending axis is measured to obtain the internal crack length. If the crack length inside the bend is 50 μm or less, the steel plate can be judged to have excellent resistance to internal bending cracking. However, the crack length is measured at the center of the plate width, on the inside of the bend at the center of the bent part, on a plane perpendicular to the bending axis and perpendicular to the plate surface. Specifically, after bending the test piece, the test piece is cut at the above plane, the cross section is mirror-polished, and then the cracks are observed with an optical microscope, and the length of the cracks observed on the inside of the bend of the test piece is measured.

[0079] 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, it is 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, it is 6.0 mm or less, 4.8 mm or less, or 3.6 mm or less.

[0080] 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).

[0081] The steel sheet according to this embodiment has high strength, excellent ductility and hole expandability, and is thought to have excellent crash resistance properties due to suppressed occurrence of internal bending cracks. 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. Note that the steel sheet according to this embodiment may be a hot-rolled steel sheet.

[0082] A part manufactured using the steel plate according to this embodiment has the same chemical composition as the above-described steel plate. Furthermore, the part may contain both processed and unprocessed parts. The unprocessed part has the same metallurgical structure as the above-described steel plate. The processed part basically has the same metallurgical structure as the above-described steel plate, but if heavily processed, it may not have the above-described metallurgical structure. Therefore, when measuring the metallurgical structure of a part, the measurement is performed on the unprocessed part. If there is no unprocessed part, the measurement is performed on the part that has not been heavily processed. An unprocessed or heavily processed part refers to, for example, a flat part of the part, and a part that avoids parts that have been punched, hole-expanded, bent, or the like. As an example, in the case of the above-described part, a test piece is taken from the flat part with the largest area near the center of gravity and examined.

[0083] 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 temperatures of the slab and the steel sheet in this embodiment refer to the surface temperatures of the slab and the steel sheet, and are measured using a radiation thermometer.

[0084] A preferred method for manufacturing a steel sheet according to this embodiment involves starting rough rolling in a temperature range of 1050 to 1200°C, setting the surface temperature rise ΔT from 1 second after the end of rough rolling to 10 seconds after the end of rough rolling to 150°C or more, setting the total reduction below 1050°C to 30 to 50% in finish rolling, cooling to a temperature range of 500 to 680°C at an average cooling rate of 30°C / s or more after the completion of finish rolling, and slow cooling in this temperature range at an average cooling rate of 20°C / s or less for 2.0 seconds or more, and cooling at an average cooling rate of 30°C / s or more in a temperature range from the slow cooling completion temperature to 200°C. Each step will be described below.

[0085] The slab to be subjected to rough rolling is not particularly limited except for the chemical composition described above. For example, a slab produced by melting molten steel having the above chemical composition using a converter or electric furnace, etc. and then by continuous casting can be used. Instead of continuous casting, an ingot casting method, a thin slab casting method, etc. may also be used. When heating the slab before rough rolling, the heating temperature may be in the range of 1100 to 1300°C.

[0086] Rough rolling is started in a temperature range of 1050 to 1200°C, and it is preferable that the temperature rise width ΔT of the surface temperature from 1 second to 10 seconds after the end of rough rolling (= "surface temperature at 10 seconds after the end of rough rolling" - "surface temperature at 1 second after the end of rough rolling") is 150°C or more. The inventors have calculated the ratio (ER) of the total amount of Ti and Nb analyzed as electrolytic extraction residue at a depth of 1 / 60 of the plate thickness from the surface to the total amount of Ti and Nb analyzed as electrolytic extraction residue at a depth of 1 / 4 of the plate thickness from the surface. １／６０ / ER １／４ In order to favorably control the temperature history from the start of rough rolling to the start of finish rolling, it has been found that it is effective to strictly control the temperature history from the start of rough rolling to the start of finish rolling. Specifically, the present inventors have found that it is effective to increase the temperature difference between the surface layer and the interior after the start of rough rolling.

[0087] After rough rolling begins in the temperature range of 1050 to 1200 ° C., when the surface of the rough bar (slab) during rough rolling is cooled by contact with the rolling roll, cooling water from the rolling roll, or descaling (injection of high-pressure water), the surface temperature of the steel plate rises due to reheating from the inside after rough rolling is completed. Therefore, in this embodiment, the temperature rise width ΔT of the surface temperature from 1 second to 10 seconds after the end of rough rolling is set to 150 ° C. or more. In order to achieve a temperature rise width ΔT of 150 ° C. or more, a person skilled in the art can achieve a ΔT of 150 ° C. or more, which is a condition that is normally unattainable, by shortening the interpass time and performing multiple rough rollings in a short period of time, or by injecting as much cooling water as possible. The fact that the temperature 10 seconds after the end of rough rolling is 150 ° C. or more higher than the temperature 1 second after the end of rough rolling indicates that the temperature has risen sufficiently due to reheating from the inside, and that the temperature difference between the surface and the inside has been increased by cooling. By setting the temperature rise width ΔT of the surface temperature from 1 second after the end of rough rolling to 10 seconds after the end of rough rolling to 150 ° C. or more, ER １／６０ / ER １／４ can be preferably controlled.

[0088] If the rough bar (slab) is not cooled sufficiently during rough rolling, the temperature 10 seconds after the end of rough rolling may not be increased by 150°C or more compared to the temperature 1 second after the end of rough rolling. As a result, a large amount of coarse precipitates are precipitated inside, causing ER. １／６０ / ER １／４ In addition, if the bar is cooled excessively, the temperature of the rough bar (slab) may be lowered too much to allow sufficient finish rolling (for example, the total reduction rate in finish rolling at less than 1050°C may not be 30 to 50%).

[0089] The temperatures 1 second and 10 seconds after the end of rough rolling are measured by a radiation thermometer installed between the roughing mill and the finishing mill.

[0090] In the finish rolling, the total reduction rate below 1050°C is set to 30 to 50%. By setting the total reduction rate in the temperature range below 1050°C to 30 to 50%, it is possible to preferably control the average aspect ratio of prior austenite grains in the region from the surface to a depth of 1 / 8 of the plate thickness to a depth of 3 / 8 of the plate thickness.

[0091] The total reduction in the temperature range below 1050 ° C is the entry thickness of the first rolling in the temperature range below 1050 ° C. ０ The delivery thickness of the final rolling in the temperature range below 1050°C is t １ When this is done, (1-t １ / t ０ ) × 100 (%).

[0092] After the finish rolling is completed, it is preferable to cool the steel sheet to a temperature range of 500 to 680°C at an average cooling rate of 30°C / s or more, and then perform slow cooling in this temperature range at an average cooling rate of 20°C / s or less for 2.0 seconds or more. By starting slow cooling in the temperature range of 500 to 680°C and continuing slow cooling in this temperature range for 2.0 seconds or more, a desired amount of bainite can be obtained.

[0093] 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.

[0094] After the slow cooling is completed, it is preferable to cool the steel sheet so that the average cooling rate in the temperature range from the slow cooling completion temperature to 200°C is 30°C / s or more. By cooling under these conditions, the desired amount of martensite can be obtained. Cooling may be performed to a temperature range of 200°C or less, for example, 100°C or less. After cooling, the steel sheet is wound into a coil.

[0095] 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.

[0096] 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. Blank spaces in the tables indicate that the corresponding elements were below the detection limit. Using the resulting slabs, steel plates shown in Tables 4A and 4B were obtained under the manufacturing conditions shown in Tables 3A and 3B. The thicknesses of the resulting steel plates were 1.2 to 8.0 mm. Finish rolling was initiated in a temperature range of 1000 to 1150°C, and cooling was initiated in a temperature range of 850 to 1050°C. Cooling was performed at an average cooling rate of 30°C / s or more up to the slow cooling start temperature, and the average cooling rate of the slow cooling was 20°C / s or less.

[0097] The items in the table indicate the following: ΔT: The surface temperature rise from 1 second to 10 seconds after the end of rough rolling ("surface temperature 10 seconds after the end of rough rolling" - "surface temperature 1 second after the end of rough rolling") Total reduction: The total reduction of less than 1050°C in finish rolling Average cooling rate: The average cooling rate in the temperature range from the temperature at the end of slow cooling to 200°C

[0098] The obtained steel plate was analyzed by the above-mentioned method to determine the average value of the aspect ratio of the prior austenite grains and the metal structure in the region from the surface to a depth of 1 / 8 of the plate thickness to a depth of 3 / 8 of the plate thickness, and the ratio (ER) of the sum of the amounts of Ti and Nb analyzed as electrolytic extraction residue at a depth of 1 / 60 of the plate thickness from the surface to the sum of the amounts of Ti and Nb analyzed as electrolytic extraction residue at a depth of 1 / 4 of the plate thickness from the surface. １／６０ / ER １／４ ), tensile strength (TS), total elongation (EL), hole expansion ratio (λ), and crack length (L) in the bent portion were determined. In the examples, the rolling direction was known in advance, so the above-mentioned determination of the rolling direction was not performed.

[0099] In some cases, the metal structure in the region from the surface of the steel plate to a depth of 1 / 8 of the plate thickness to a depth of 3 / 8 of the plate thickness from the surface contained ferrite and pearlite as the remaining structure in addition to the bainite and martensite shown in the table. The measurement results obtained are shown in Tables 4A and 4B.

[0100] When the tensile strength was 980 MPa or more, the steel sheet was judged to have high strength and to have passed the test, whereas when the tensile strength was less than 980 MPa, the steel sheet was judged to have low strength and to have failed the test.

[0101] 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.

[0102] 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.

[0103] When the crack length inside the bend was 50 μm or less, the steel sheet was judged to have excellent resistance to internal cracking under bending and to have passed the test. On the other hand, when the crack length inside the bend was more than 50 μm, the steel sheet was judged to have poor resistance to internal cracking under bending and to have passed the test.

[0104]

[0105]

[0106]

[0107]

[0108]

[0109]

[0110] It can be seen from Tables 1 to 4B that the steel plates according to the examples of the present invention have high strength as well as excellent ductility, hole expandability and resistance to internal cracking in bending.

[0111] 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 and 4B.

[0112] According to the above aspects of the present disclosure, it is possible to provide a steel plate having high strength, as well as excellent ductility, hole expandability, and resistance to internal bending cracking, and a part 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%, Ti: 0.020 to 0.180%, Al: 0.010-0.400%, P: 0 to 0.080%, S: 0 to 0.0100%, N: 0 to 0.0050%, O: 0 to 0.0100%, Nb: 0 to 0.100%, V: 0 to 1.000%, Cu: 0 to 1.000%, Cr: 0-2.000%, Mo: 0-3.000%, Ni: 0-1.000%, B: 0 to 0.0100%, Ca: 0-0.0500%, Mg: 0 to 0.050%, REM: 0-0.1000%, 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%, Sn: 0 to 0.050%, and The balance is Fe and impurities. In the region from the surface to a depth of 1 / 8 of the plate thickness to a depth of 3 / 8 of the plate thickness from the surface, The average aspect ratio of the prior austenite grains is 3.00 to 5.50, The area ratio of bainite is 70 to 95%, The area ratio of martensite is 5 to 30%, a ratio of the sum of the amount of Ti and the amount of Nb analyzed as an electrolytic extraction residue at a depth of 1 / 60 of the sheet thickness from the surface to the sum of the amount of Ti and the amount of Nb analyzed as an electrolytic extraction residue at a depth of 1 / 4 of the sheet thickness from the surface to 1.5 or more.

2. The chemical composition is, in mass %, 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 1.000%, B: 0.0001 to 0.0100%, Ca: 0.0001-0.0500%, Mg: 0.001-0.050%, REM: 0.0001-0.1000%, 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, characterized in that it contains at least one of Sn: 0.001 to 0.050%.

3. 3. The steel plate according to claim 1, wherein the thickness is 1.2 to 8.0 mm.

4. 3. The steel plate according to claim 1, wherein the tensile strength is 980 MPa or more.

5. An automobile part, comprising the steel sheet according to claim 1 or 2.