Steel sheet and component

Abstract

This steel sheet has a chemical composition that contains, in mass%, at least 0.045% C, 0.27 to 3.00% Si, 1.20 to 3.00% Mn, 0 to 0.180% Ti, and 0 to 0.100% Nb. The area ratio of bainite in the 1 / 4 region is 70 to 95%; the area ratio of martensite in the 1 / 2 region is 35 to 100%; and the sheet thickness is 1.2 to 8.0 mm.

Description

Steel plates and parts

[0001] This application claims priority to Japanese Patent Application No. 2024-016822, filed on February 7, 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, if one attempts to maintain the strength properties of parts, such as static fracture strength and yield strength, while reducing the plate thickness, it becomes necessary to use high-strength materials. In particular, the application of higher-strength steel sheets has begun to be considered for automobile suspension parts, such as lower arms, trail links, and knuckles. These automobile suspension parts are manufactured by subjecting steel sheets to processes such as punching, burring, stretch flange forming, and bending.

[0004] When a high-strength steel sheet is punched, minute cracks may occur on the end surface after punching. Therefore, steel sheets used for the above-mentioned parts are required to be less susceptible to cracks on the end surface after punching, i.e., to have excellent punching workability.

[0005] For example, Patent Document 1 discloses a metal structure in the center of the plate thickness that is composed of 60% or more by volume of tempered martensite, less than 30% each of ferrite, bainite, pearlite, and retained austenite, and less than 5% of as-quenched martensite, the thickness of the surface soft portion is more than 10 μm per side and 15% or less of the thickness of the center of the plate thickness, the average hardness of the surface soft portion is 0.90 times or less the average hardness of the center of the plate thickness, and the surface soft portion contains carbides in a number density of 1×10 ４ / mm ２The steel sheet disclosed includes the above, wherein the average particle size of the carbide is 0.250 μm or less, the standard deviation of the logarithm of the particle size is 0.05 or less, and the tensile strength is 1180 MPa or more. Patent Document 1 discloses that the technology disclosed therein can provide a steel sheet having both excellent bending workability and hydrogen embrittlement resistance.

[0006] International Publication No. 2021 / 186510

[0007] However, in Patent Document 1, punching workability is not taken into consideration, and there is room for improvement in terms of improving punching workability.

[0008] The present disclosure has been made in view of the above-mentioned problems, and aims to provide a steel sheet having excellent punching workability and a part obtained using the same.

[0009] Furthermore, since steel sheets used in the above-described automobile suspension parts are subjected to various processes, they are required to have high strength and excellent formability, particularly ductility and bendability. Therefore, in a preferred embodiment of the present disclosure, an object is to provide a steel sheet having excellent punching workability, high strength, and excellent ductility and bendability, and a part obtained using this steel sheet.

[0010] The gist of the present disclosure is as follows. [1] A steel sheet characterized in that it has a chemical composition, in mass%, of C: 0.045% or more, Si: 0.27 to 3.00%, Mn: 1.20 to 3.00%, Al: 0.400% or less, P: 0 to 0.080%, N: 0 to 0.0050%, Ti: 0 to 0.180%, and Nb: 0 to 0.100%, with the balance being Fe and impurities, and in a 1 / 4 region that is a 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, an area ratio of bainite is 70 to 95%, and in a 1 / 2 region that is a region from the surface to a depth of 3 / 8 of the plate thickness to a depth of 1 / 2 of the plate thickness from the surface, an area ratio of martensite is 35 to 100%, and the steel sheet has a thickness of 1.2 to 8.0 mm. [2] The chemical composition, in mass%, is: C: 0.45 to 0.120%, Al: 0.010 to 0.400%, S: 0 to 0.100%, O: 0 to 0.0100%, V: 0 to 1.000%, Cu: 0 to 1.000%, Cr: 0 to 2.000%, Mo: 0 to 3.000%, Ni: 0 to 0.500%, 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%, The steel sheet according to [1], comprising 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%, and an area ratio of bainite is 70 to 100% in a surface layer region that is a region from the surface to a depth of 1 / 15 of the sheet thickness from the surface.[3] The chemical composition, in mass%, is: 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: 0.0001 to 0.1000%, Bi: 0.001 to 0.100%, Ta: 0.001 to 0.100%, The steel sheet according to [2], characterized in that it contains one or more elements selected from the group consisting of Zr: 0.001 to 0.500%, Co: 0.001 to 3.000%, Zn: 0.001 to 0.200%, W: 0.001 to 0.200%, Sb: 0.001 to 0.500%, As: 0.001 to 0.050%, and Sn: 0.001 to 0.050%. [4] A part comprising the steel sheet according to any one of [1] to [3].

[0011] According to the above aspects of the present disclosure, it is possible to obtain a steel sheet having excellent punching workability and a part obtained using the same. Furthermore, according to a preferred aspect of the present disclosure, it is possible to provide a steel sheet having excellent punching workability, high strength, and excellent ductility and bendability, and a part obtained using the steel sheet.

[0012] The present inventors have investigated methods for improving the punching workability of steel sheets and have discovered the following: Punching workability is affected by the relationship between the area fraction of bainite in a quarter region of the steel sheet and the area fraction of martensite in a half region of the steel sheet. By simultaneously controlling the area fraction of bainite in a quarter region of the steel sheet and the area fraction of martensite in a half region of the steel sheet, the punching workability of the steel sheet can be improved.

[0013] In order to obtain a metal structure having the above-mentioned composition, it is effective to preferably control the finish rolling conditions, the slow cooling conditions after the finish rolling, and the cooling conditions after the slow cooling.

[0014] Hereinafter, a steel sheet according to an embodiment of the present disclosure (hereinafter, sometimes referred to as a 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.

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

[0016] The chemical composition of the steel sheet according to this embodiment includes, in mass%, C: 0.045% or more, Si: 0.27 to 3.00%, Mn: 1.20 to 3.00%, Al: 0.400% or less, P: 0 to 0.080%, N: 0 to 0.0050%, Ti: 0 to 0.180%, Nb: 0 to 0.100%, and the balance: Fe and impurities. Each element will be described below.

[0017] C: 0.045% or more C is an element that improves the strength of the steel sheet. It is also an element that improves punching workability by strengthening the grain boundaries. In order to ensure high strength and a sufficient amount of segregation to the grain boundaries, 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 0.070% or more. On the other hand, if the C content exceeds 0.120%, the ductility and bendability of the steel sheet deteriorate. Therefore, in order to obtain excellent ductility and bendability, the C content is preferably 0.120% or less. The C content is more preferably 0.110% or less, 0.100% or less.

[0018] Si: 0.27 to 3.00% Si is an element that suppresses the formation of carbides during ferrite transformation and improves the toughness of the steel sheet. Si also enhances punching workability. If the Si content is less than 0.27%, the desired punching workability cannot be obtained. Therefore, the Si content is set to 0.27% or more. From the viewpoint of increasing the martensite fraction in the surface layer region and further improving punching workability, the Si content is preferably set to 0.35% or more. The Si content is more preferably 0.50% or more, 0.70% or more. On the other hand, if the Si content exceeds 3.00%, the cracking susceptibility of the slab increases, making it more susceptible to slab cracking and making the slab difficult to handle. Therefore, the Si content is preferably set to 3.00% or less. The Si content is more preferably 2.00% or less, 1.50% or less, or 1.20% or less.

[0019] Mn: 1.20 to 3.00% Mn is an element that increases the area fraction of martensite in the 1 / 2 region by improving the hardenability of steel. If the Mn content is less than 1.20%, the area fraction of martensite in the 1 / 2 region decreases, and the punching workability of the steel sheet deteriorates. Therefore, the Mn content is set to 1.20% or more. The Mn content is preferably 1.50% or more, 1.70% or more, or 2.00% or more. On the other hand, if the Mn content exceeds 3.00%, the amount of bainite in the 1 / 4 region becomes insufficient, and the punching workability of the steel sheet deteriorates. Therefore, the Mn content is set to 3.00% or less. The Mn content is preferably 2.70% or less, 2.50% or less, or 2.20% or less.

[0020] Al: 0.400% or less Al has the effect of improving the soundness of steel by deoxidization and also has the effect of controlling ferrite transformation. Since there is no need to set a lower limit for the Al content, the Al content may be 0%. However, if the Al content is less than 0.010%, the bendability of the steel sheet deteriorates. Therefore, from the viewpoint of improving the bendability of the steel sheet, the Al content is preferably 0.010% or more. The Al content is more preferably 0.015% or more or 0.020% or more. On the other hand, if the Al content exceeds 0.400%, alumina precipitates in clusters, which increases the cracking susceptibility of the slab, making it more susceptible to slab cracking and making the slab 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.

[0021] P: 0 to 0.080% P is an element that affects the weldability of steel sheets. In particular, if the P content exceeds 0.080%, the weldability of the steel sheets deteriorates significantly. Furthermore, the cracking sensitivity of the slab increases, making the slab more susceptible to cracking and making the slab difficult to handle. Therefore, the P content is preferably 0.080% or less. The P content is more 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 0.001% or more.

[0022] 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 more susceptible to cracking and making the slab difficult to handle. Therefore, the N content is preferably 0.0050% or less. The N content is more 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.

[0023] Ti: 0 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. Since Ti is not necessarily contained, the Ti content may be 0%. To reliably obtain the above effects, the Ti content is preferably 0.001% or more. The Ti content is more preferably 0.005% or more, or 0.010% or more. On the other hand, if the Ti content exceeds 0.180%, the amount of unrecrystallized austenite increases, deteriorating the punching workability 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.

[0024] 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. Nb is not necessarily contained, so the Nb content may be 0%. To reliably obtain the above effect, the Nb content is preferably 0.001% or more. The Nb content is more preferably 0.005% or more or 0.010% or more. On the other hand, if the Nb content exceeds 0.100%, the amount of unrecrystallized austenite increases, deteriorating the punching workability of the steel sheet. Therefore, the Nb content is set to 0.100% or less. The Nb content is preferably 0.080% or less, 0.050% or less, or 0.030% or less.

[0025] In a preferred embodiment of the steel sheet according to this embodiment, in order to obtain high strength, excellent ductility and bendability in addition to excellent punching workability, it is preferable to contain Al: 0.010 to 0.400%, S: 0 to 0.0100%, and O: 0 to 0.0100%. Each element will be described below.

[0026] S: 0 to 0.0100% S is an element that affects the bendability 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 bendability of steel sheet, are generated. Therefore, the S content is preferably 0.0100% or less. The S content is more 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.

[0027] 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 bendability of the steel sheet deteriorates. Therefore, the O content is preferably 0.0100% or less. The O content is more preferably 0.0080% or less, 0.0060% or less, 0.0040% or less, 0.0030% or less, or 0.0020% 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 0.0005% or more, or 0.0010% or more.

[0028] In a preferred embodiment of the steel sheet according to the present embodiment, the steel sheet contains the above chemical components, with the balance being Fe and impurities. In the present embodiment, the term "impurities" refers to substances that are mixed in from raw materials such as ore and scrap, or from the manufacturing environment, and / or substances that are allowed to exist within a range that does not adversely affect the steel sheet according to the present embodiment.

[0029] Although not essential for providing the desired properties, the steel sheet according to the present embodiment may contain the following optional elements 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%.

[0030] V: 0.001 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. To reliably obtain these effects, the V content is preferably 0.001% or more. The V content is more preferably 0.005% or more, or 0.010% or more. On the other hand, if the V content is excessive, a large amount of carbonitrides precipitates, deteriorating the bendability of the steel sheet. Therefore, the V content is preferably 1.000% or less. The V content is more preferably 0.800% or less, or 0.600% or less.

[0031] Cu: 0.001 to 1.000% Cu exists in steel in the form of fine particles and has the effect of increasing the strength of the steel sheet. To reliably obtain this effect, the Cu content is preferably 0.001% or more. The Cu content is more preferably 0.005% or more, or 0.010% or more. On the other hand, if the Cu content is excessive, the weldability of the steel sheet deteriorates. Therefore, the Cu content is preferably 1.000% or less. The Cu content is more preferably 0.800% or less, or 0.600% or less.

[0032] Cr: 0.001 to 2.000% Cr is an element effective in improving the strength of steel sheet. To reliably obtain this effect, the Cr content is preferably 0.001% or more. The Cr content is more preferably 0.005% or more, or 0.010% or more. On the other hand, if the Cr content is excessive, the bendability of the steel sheet deteriorates. Therefore, the Cr content is preferably 2.000% or less. The Cr content is more preferably 1.500% or less, 1.200% or less, or 1.000% or less.

[0033] Mo: 0.001 to 3.000% Mo is an element effective in strengthening ferrite precipitation. To reliably obtain this effect, the Mo content is preferably 0.001% or more. The Mo content is more preferably 0.005% or more, or 0.010% or more. On the other hand, an excessive Mo content increases the cracking sensitivity of the slab, making it difficult to handle. Therefore, the Mo content is preferably 3.000% or less. The Mo content is more preferably 2.500% or less, 2.000% or less, or 1.500% or less.

[0034] Ni: 0.001 to 0.500% Ni has the effect of suppressing phase transformation at high temperatures and increasing the strength of the steel sheet. To reliably obtain this effect, the Ni content is preferably 0.001% or more. The Ni content is more preferably 0.005% or more, or 0.010% or more. On the other hand, if the Ni content is excessive, the weldability of the steel sheet deteriorates. Therefore, the Ni content is preferably 0.500% or less. The Ni content is more preferably 0.300% or less, or 0.150% or less.

[0035] B: 0.0001 to 0.0100% B has the effect of suppressing phase transformation at high temperatures and increasing the strength of the steel sheet. To reliably obtain this effect, the B content is preferably 0.0001% or more. The B content is more preferably 0.0005% or more, or 0.0010% or more. On the other hand, if the B content is excessive, B precipitates are formed, reducing the strength of the steel sheet. Therefore, the B content is preferably 0.0100% or less. The B content is more preferably 0.0080% or less, or 0.0050% or less.

[0036] Ca: 0.0001 to 0.0500% Ca has the effect of dispersing a large number of 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, suppressing the formation of elongated inclusions such as MnS, and improving the bendability of the steel sheet. To reliably obtain these effects, the Ca content is preferably 0.0001% or more. The Ca content is more preferably 0.0005% or more, or 0.0010% or more. On the other hand, even if the Ca content exceeds 0.0500%, the above effects saturate. Therefore, the Ca content is preferably 0.0500% or less. The Ca content is more preferably 0.0300% or less, or 0.0200% or less.

[0037] Mg: 0.001 to 0.050% Mg has the effect of adjusting the shape of inclusions in steel to a preferred shape, thereby increasing the yield ratio of the steel sheet. To ensure this effect, the Mg content is preferably 0.001% or more. The Mg content is more preferably 0.005% or more, or 0.010% or more. On the other hand, if the Mg content exceeds 0.050%, excessive inclusions are formed in the steel, reducing the yield ratio of the steel sheet. Therefore, the Mg content is preferably 0.050% or less. The Mg content is more preferably 0.040% or less, or 0.030% or less.

[0038] REM: 0.0001 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. To ensure 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, reducing the yield ratio of the steel sheet. Therefore, the REM content is preferably 0.1000% or less. The REM content is more preferably 0.0800% or less, or 0.0600% 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: 0.001 to 0.100% Bi has the effect of increasing the yield ratio of steel sheet by refining the solidification structure. To reliably obtain this effect, the Bi content is preferably 0.001% or more. The Bi content is more preferably 0.005% or more, or 0.010% or more. On the other hand, if the Bi content exceeds 0.100%, the effect of the above action saturates, which is economically undesirable. Therefore, the Bi content is preferably 0.100% or less. The Bi content is more preferably 0.080% or less, 0.060% or less, or 0.040% or less.

[0040] Ta: 0.001 to 0.100% Like V, Ta has the effect of increasing the strength of the steel sheet by forming fine carbides in the steel. To reliably obtain this effect, the Ta content is preferably 0.001% or more. The Ta content is more preferably 0.005% or more, or 0.010% or more. On the other hand, if the Ta content exceeds 0.100%, the bendability of the steel sheet deteriorates. Therefore, the Ta content is preferably 0.100% or less. The Ta content is more preferably 0.080% or less, or 0.050% or less.

[0041] Zr: 0.001 to 0.500% Zr has the effect of increasing the strength of the steel sheet through solid solution strengthening. To reliably obtain this effect, the Zr content is preferably 0.001% or more. The Zr content is more preferably 0.005% or more, or 0.010% or more. On the other hand, if the Zr content exceeds 0.500%, the bendability of the steel sheet deteriorates. Therefore, the Zr content is preferably 0.500% or less. The Zr content is more preferably 0.300% or less, or 0.100% or less.

[0042] Co: 0.001 to 3.000% Co has the effect of increasing the strength of steel sheet through solid solution strengthening. To reliably obtain this effect, the Co content is preferably 0.001% or more. The Co content is more preferably 0.005% or more, or 0.010% or more. On the other hand, if the Co content exceeds 3.000%, the bendability of the steel sheet deteriorates. Therefore, the Co content is preferably 3.000% or less. The Co content is more preferably 1.000% or less, or 0.500% or less.

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

[0044] W: 0.001 to 0.200% W has the effect of increasing the strength of the 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 bendability of the steel sheet deteriorates. Therefore, the W content is preferably 0.200% or less. The W content is more preferably 0.150% or less, 0.100% or less.

[0045] 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 bendability of the steel sheet. To reliably obtain this effect, the Sb content is preferably 0.001% or more. The Sb content is more preferably 0.005% or more, or 0.010% or more. On the other hand, since the above effect saturates even when a large amount of Sb is added, the Sb content is preferably 0.500% or less. The Sb content is more preferably 0.300% or less, or 0.100% or less.

[0046] 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 bendability of the steel sheet. To reliably obtain this effect, the As content is preferably 0.001% or more. The As content is more preferably 0.005% or more, or 0.010% or more. On the other hand, since the above effect saturates even when a large amount of As is contained, the As content is preferably 0.050% or less. The As content is more preferably 0.040% or less, or 0.030% or less.

[0047] 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 bendability of the steel sheet. To reliably obtain this effect, the Sn content is preferably 0.001% or more. The Sn content is more preferably 0.005% or more, or 0.010% or more. On the other hand, since the above effect saturates even when a large amount of Sn is added, the Sn content is preferably 0.050% or less. The Sn content is more preferably 0.040% or less, or 0.030% or less.

[0048] The chemical composition of the above-mentioned steel sheet is analyzed by optical emission spectroscopy using a spark discharge optical emission spectroscopy analyzer or the like. Values ​​for C and S are determined by burning the steel sheet in an oxygen stream using a gas component analyzer or the like and measuring by infrared absorption. Values ​​for O and N are determined by melting a test piece taken from the steel sheet in a helium stream and measuring by thermal conductivity. If the steel sheet has a plating layer or a coating film on its surface, the plating layer or coating film is removed by mechanical grinding or the like as necessary before analyzing the chemical composition.

[0049] Next, the metal structure of the steel plate according to this embodiment will be described. In the steel plate according to this embodiment, the area ratio of bainite is 70 to 95% in a 1 / 4 region, which is a 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, and the area ratio of martensite is 35 to 100% in a 1 / 2 region, which is a region from the surface to a depth of 3 / 8 of the plate thickness to a depth of 1 / 2 of the plate thickness from the surface.

[0050] As described above, in this embodiment, the 1 / 4 region refers to 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. In other words, it is a region that has a thickness from 1 / 8 to 3 / 8 in the plate thickness direction and extends parallel to the direction in which the plate surface extends. Furthermore, the 1 / 2 region refers to the region from the surface to a depth of 3 / 8 of the plate thickness to a depth of 1 / 2 of the plate thickness. In other words, it is a region that has a thickness from 3 / 8 to 1 / 2 in the plate thickness direction and extends parallel to the direction in which the plate surface extends.

[0051] When a steel sheet has a plating layer or a coating film on its surface, the surface here refers to the interface between the steel sheet and the plating layer or coating film. The interface between the steel sheet and the plating layer or coating film is identified using a BSE image (or COMPO image) obtained by the following method. A sample is cut out so that the cross section of the steel sheet through its thickness can be observed. The cut sample is mechanically polished and then mirror-finished. Using a scanning electron 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 (or COMPO image), a clear difference in contrast can be 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 can be identified using a similar method.

[0052] By setting the area ratio of bainite in the 1 / 4 region to 70 to 95% and the area ratio of martensite in the 1 / 2 region to 35 to 100%, the punching workability of the steel sheet can be improved. If the desired metal structure is not present in either region, the punching workability of the steel sheet will deteriorate. Therefore, in this embodiment, the area ratio of bainite in the 1 / 4 region and the area ratio of martensite in the 1 / 2 region are simultaneously controlled.

[0053] The area fraction of bainite in the 1 / 4 region is preferably 75% or more. The area fraction of bainite in the 1 / 4 region is preferably 90% or less or 85% or less. The area fraction of martensite in the 1 / 2 region is preferably 40% or more, 45% or more, or 50% or more. The area fraction of martensite in the 1 / 2 region is preferably 80% or less, 70% or less, or 65% or less, and more preferably 60% or less, less than 60%, or 55% or less.

[0054] The metallographic structure in the 1 / 4 region may contain one or more of martensite, ferrite, and pearlite with a total area ratio of 5 to 30% as the remaining structure other than bainite.The metallographic structure in the 1 / 2 region may contain one or more of bainite, ferrite, and pearlite with a total area ratio of 0 to 65% as the remaining structure other than martensite.

[0055] In the present embodiment, martensite refers to both fresh martensite and tempered martensite, but there is no need to distinguish between them. In the measurement method described below, both fresh martensite and tempered martensite are identified as martensite.

[0056] In a preferred aspect of the steel plate according to this embodiment, the area ratio of bainite is preferably 70 to 100% in the surface to surface layer region, which is a region from the surface to a depth of 1 / 15 of the plate thickness. By making the area ratio of bainite in the surface layer region 70% or more, the bendability of the steel plate can be improved. The area ratio of bainite in the surface layer region is preferably 75% or more, 80% or more, or 85% or more. Furthermore, the area ratio of bainite in the surface layer region is preferably 95% or less, or 90% or less.

[0057] The metal structure in the surface layer region may contain one or more of martensite, ferrite, and pearlite, with a total area ratio of 0 to 30%, as the remaining structure other than bainite. The area ratio of martensite in the surface layer region is preferably more than 2% and not more than 30%. By making the area ratio of martensite in the surface layer region more than 2%, the composition ratio of the sheared edge surface can be appropriately controlled, and the punching workability of the steel sheet can be further improved. The area ratio of martensite in the surface layer region is more preferably 3% or more, 4% or more, or 5% or more.

[0058] The area ratio of each structure is measured by the following method. A test piece is taken from the steel plate so that the metal structure can be observed at a 1 / 4 position in the plate thickness (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). The plate thickness 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 region of t / 15 μm (plate thickness direction, t is plate thickness) × 600 μm (rolling direction) at a 1 / 4 position in the plate thickness, and image analysis is performed.

[0059] In Repelle corrosion, martensite and retained austenite are not corroded, so by calculating the area ratio of the uncorroded area, the total area ratio of martensite and retained austenite is obtained.

[0060] The area fraction of retained austenite is obtained by X-ray diffraction. A test piece taken from a steel plate is ground to a position of 1 / 4 of the plate thickness (a range from a position of 1 / 8 of the plate thickness to a position of 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 intensity of a total of five peaks, α(200), α(211), γ(200), γ(220), and γ(311), is 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.

[0061] The area fraction of martensite is obtained by subtracting the area fraction of retained austenite obtained by X-ray diffraction from the total area fraction of martensite and retained austenite obtained by observation using the FE-SEM. When the total area fraction of martensite is a negative value in the calculation, the area fraction of martensite is taken to be 0%.

[0062] The area fraction of pearlite is obtained by the following method. For the same region ("t / 15 μm" × 600 μm region) as that used to determine the area fractions of martensite and retained austenite, only the corroded layer is removed by polishing and mirror-finished, followed by etching with a nital solution, observation using an FE-SEM, and image analysis. 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.

[0063] The area fraction of ferrite is obtained by the following method. Note that the following operation is performed on regions other than the region determined to be pearlite by the above method. The same region (region of "t / 15 μm" × 600 μm) as that 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. For the measurement, an EBSD analysis device consisting of a thermal field emission scanning electron microscope (JSM-7200F manufactured by JEOL) and an EBSD detector (EDAX Velocity (registered trademark) ultra-high speed EBSD detector) is used. At this time, the degree of vacuum inside the device is 9.6 × 10 －５ The pressure is set to 0.2 Pa or less, the acceleration voltage is set to 25 kV, and the irradiation current level is set to 16.

[0064] 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 0.6° or less are considered to be ferrite, and their area ratio is calculated to obtain the ferrite area ratio.

[0065] The area fraction of bainite is obtained by subtracting the area fractions of martensite, retained austenite, pearlite, and ferrite obtained by the above-mentioned method 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 the individual structures 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".

[0066] The observation conditions for the FE-SEM are as follows: Electron gun type: thermal emission type WD (working distance): 10 mm Acceleration voltage: 20 kV Number of pixels: 5120 x 3840 Obtained image: secondary electron image

[0067] By performing the above-described structural observation for each of the 1 / 4 region, the 1 / 2 region, and the surface layer region, the area fraction of bainite in the 1 / 4 region, the area fraction of martensite in the 1 / 2 region, and the area fraction of bainite in the surface layer region are obtained.

[0068] The rolling direction of a steel sheet is determined using the following method. A test piece is taken from any position at least 50 mm away from the end of the steel sheet so that the thickness cross section can be observed. The thickness cross section of the taken test piece is mirror-polished and then observed using an optical microscope at magnifications of 100x, 200x, 500x, and 1000x. An appropriate magnification at which the dimensions of the inclusions can be measured is selected depending on the size of the inclusions. The observation range is 500 μm or more in width and across the entire thickness of the sheet, and dark regions extending perpendicular to the thickness direction are determined to be inclusions. Observation may be performed using multiple fields of view. Next, using the thickness cross section initially observed using the above method as a reference, planes parallel to the planes rotated in 5° increments in the range of 0° to 180° around the thickness direction are observed using the same method. The average length of the major axes of all inclusions in each cross section is calculated for each cross section. The cross section where the average value of the long axis length of the inclusions obtained is greatest is identified. The direction parallel to the long axis of the inclusions in that cross section is determined to be the rolling direction. The rolling direction of the part is also determined using the same method.

[0069] In the metal structure in the 1 / 4 region, the average aspect ratio of the prior austenite grains is preferably 3.0 to 6.0. By making the average aspect ratio of the prior austenite grains 3.0 or more, the ductility of the steel sheet can be further improved. Therefore, the average aspect ratio of the prior austenite grains is preferably 3.0 or more. The average aspect ratio of the prior austenite grains is more preferably 3.5 or more, 4.0 or more, or 4.2 or more. Furthermore, by making the average aspect ratio of the prior austenite grains in the 1 / 4 region 6.0 or less, the hole expandability of the steel sheet can be further improved. Therefore, the average aspect ratio of the prior austenite grains is preferably 6.0 or less. The average aspect ratio of the prior austenite grains is more preferably 5.5 or less, 5.0 or less, or 4.5 or less.

[0070] The average aspect ratio of prior austenite grains is measured using the following method. A test specimen is taken 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 1 / 4 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 test specimen depends on the measuring device, but for example, it may be a rectangular parallelepiped measuring the full thickness in the thickness direction, 15 mm in the rolling direction, and 10 mm in the width direction. The width direction is defined as the direction perpendicular to the rolling and thickness directions of the steel plate. Prior austenite grains are revealed using 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 t / 15 μm (in the thickness direction, where t is the thickness) × 600 μm (in the rolling direction), are photographed in 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. The ratio is then 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 by the following formula: 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.

[0071]

[0072] 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.

[0073] 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.

[0074] 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.

[0075] Total elongation (El): 10.0% or more The steel sheet according to this embodiment may have a total elongation (total elongation at break) of 10.0% or more. The total elongation is preferably 12.0% or more or 13.0% or more.

[0076] The tensile strength and total elongation are evaluated by conducting a tensile test in accordance with JIS Z 2241:2022. The test specimen is a No. 5 test specimen of JIS Z 2241:2022. The tensile test specimen is taken from a quarter portion from the end in the plate width direction, with the direction perpendicular to the rolling direction as 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 the part's small size or complex shape, a small rectangular piece having a parallel portion of any width may be taken, and a tensile test may be performed to determine the tensile strength from the maximum test force and the original cross-sectional area of ​​the parallel portion.

[0077] Maximum bending angle (α): 80° or more In the steel sheet according to this embodiment, the maximum bending angle obtained by a bending test based on the VDA standard described later may be 80° or more. The maximum bending angle is preferably 85° or more or 90° or more.

[0078] The test specimens used for the bending test are 60 mm (rolling direction) x 30 mm (width direction) specimens taken from the steel plate. Using these test specimens, a bending test is performed under the following conditions based on the VDA standard (VDA238-100:2017-04) specified by the German Association of the Automotive Industry.

[0079] If the thickness of the test piece exceeds 1.6 mm, the punch-side surface is ground to a thickness of 1.6 mm before the bending test. If the thickness of the test piece is 1.6 mm or less, the maximum bending angle obtained by the following formula is used. However, in the following formula, α ｔ indicates the maximum bending angle obtained in the bending test, t indicates the plate thickness, and uEL indicates the uniform elongation (total elongation at the maximum test force). The uniform elongation is a value obtained by conducting a tensile test using the method described above. The maximum bending angle when the plate thickness is 1.6 mm or less = α ｔ -13.852×(1-t / 1.6)×(uEL+0.22) ０．２９２

[0080] Test piece dimensions: 60 mm (rolling direction) x 30 mm (width direction) Bending ridge: Parallel to the width direction Test method: Roll support, punch pressing Roll diameter: φ30 mm Punch shape: Tip R = 0.4 mm Distance between rolls: 2.0 x plate thickness (mm) + 0.5 mm Pressing speed: 20 mm / min Testing machine: SHIMADZU AUTOGRAPH 20 kN

[0081] The steel sheet according to this embodiment may have a damage incidence rate, which is an index of punching workability and is obtained by the following method, of 0 to 25%. If the damage incidence rate is 25% or less, it can be determined that the steel sheet has excellent punching workability.

[0082] Nine punched holes were created by performing a hole expansion test on a steel plate with hole diameters of 10 mm, 20 mm, and 30 mm, varying the clearance to 5%, 10%, and 20%, respectively, with other conditions conforming to JIS Z 2256:2020. Next, the angle of the range where damage was observed visually around the entire circumference of the punched hole (the central angle formed by the center and the area where damage was observed on the hole wall, when the center of the punched hole is the center) was measured. The total angle of the damage range around the entire circumference of the punched hole was divided by 360° and multiplied by 100 to determine the damage occurrence rate for one punched hole. For example, if the 0° point (and the 360° point) are defined around the entire circumference of one punched hole, and damage is present at the 50° point to the 75° point and the 90° point to the 110° point, the damage occurrence rate is (25 + 20) / 360 x 100 = 12.5%. The damage occurrence rate, which is an index of punching workability, is obtained by averaging the damage occurrence rates of the nine punched holes. Regarding the presence or absence of damage around the entire circumference of a punched hole, damage is considered to have occurred if there is a crack with a length of 1 mm or more.

[0083] Hole expansion ratio: 50% or more In the steel sheet according to this embodiment, the hole expansion ratio may be 50% or more. The hole expansion ratio is measured by performing a hole expansion test in accordance with JIS Z 2256:2020.

[0084] The thickness of the steel plate according to this embodiment is set to 1.2 to 8.0 mm. If the thickness of the steel plate is less than 1.2 mm, it becomes difficult to ensure the rolling completion temperature and the rolling load becomes excessive, making hot rolling difficult. Therefore, the thickness of the steel plate according to this embodiment is set to 1.2 mm or more, preferably 1.4 mm or more. On the other hand, if the thickness exceeds 8.0 mm, it becomes difficult to obtain the above-mentioned metal structure after hot rolling. In addition, punching workability deteriorates. Therefore, the thickness is set to 8.0 mm or less, preferably 6.0 mm or less.

[0085] The steel sheet according to this embodiment may have a plating layer on its surface. The plating layer may be formed on both sides or one side of the steel sheet. By having a plating layer on the surface of the steel sheet, the corrosion resistance of the part can be improved. Examples of types of plating layers include hot-dip aluminum plating, hot-dip aluminum-zinc alloy plating, hot-dip aluminum-silicon alloy plating, hot-dip galvanizing, electrogalvanizing, alloyed hot-dip galvanizing, and electrogalvan-nickel alloy plating. Furthermore, corrosion resistance can be further improved by performing an appropriate chemical conversion treatment after plating (for example, applying and drying a silicate-based chromium-free chemical conversion treatment solution).

[0086] The steel sheet according to the present embodiment has excellent punching workability, and is therefore considered to have excellent durability when processed into parts, and therefore can be suitably used for parts, particularly automobile parts. Among automobile parts, the steel sheet can be suitably used for automobile suspension parts such as lower arms, trail links, and knuckles. These automobile parts may be made solely of the steel sheet according to the present embodiment, or may be formed by joining the steel sheet according to the present embodiment with other steel sheets.

[0087] 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 a part other than the edge that has not been processed. If there is no unprocessed part, the measurement is performed on a 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.

[0088] 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.

[0089] A preferred method for producing a steel sheet according to this embodiment includes the steps of: performing finish rolling so that the sum of R×t (R×t of the first pass + R×t of the second pass), which is the product of the roll contact length R expressed by the following formula (A) and the roll contact time t expressed by the following formula (B) in the first and second passes, is 100 or less, and the total reduction in a temperature range of less than 1,050°C is 40 to 60%; after finish rolling, cooling to a temperature range of 500 to 680°C at an average cooling rate of 30°C / s or more, 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, after completion of the slow cooling, cooling to 200°C at an average cooling rate of 30°C / s or more.

[0090]

[0091]

[0092]

[0093] In the above formulas (A) and (B), n represents the number of passes in the finish rolling, and R ｎ indicates the roll radius (mm) for the nth pass, and h ｎ indicates the plate thickness (mm) before passing through the nth roll, and h ｎ＋１ indicates the plate thickness (mm) after passing through the nth roll, and v ｎ indicates the inlet speed (mm / sec) of the nth pass, and is expressed by the above formula (C). In the above formula (C), v ｆ indicates the exit speed of the final pass (mm / sec), and h ｆ indicates the delivery thickness (mm) of the final pass, and h ｎ indicates the plate thickness (mm) before passing through the nth roll.

[0094] The slab to be subjected to rough rolling is not particularly limited except that it has the above-mentioned chemical composition. For example, a slab produced by melting molten steel having the above-mentioned chemical composition using a converter or 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 be used. In the slab heating before rough rolling, the heating temperature may be in the range of 1100 to 1300°C. Furthermore, the conditions for rough rolling are not particularly limited.

[0095] After rough rolling, it is preferable to perform finish rolling so that the sum of R×t (R×t of the first pass + R×t of the second pass), which is the product of the roll contact length R expressed by the above formula (A) and the roll contact time t expressed by the above formula (B) in the first and second passes, is 100 or less, and the total reduction in the temperature range below 1050°C is 40 to 60%.

[0096] By setting the sum of R×t, which is the product of the roll contact length R expressed by the above formula (A) and the roll contact time t expressed by the above formula (B), to 100 or less, it is possible to prevent excessive strain from being introduced into the ¼ region and to prevent the amount of bainite from becoming excessive in the ¼ region. Furthermore, by setting the sum of R×t, which is the product of the roll contact length R expressed by the above formula (A) and the roll contact time t expressed by the above formula (B), to 5 or more, it is possible to preferably introduce strain into the surface layer region and to increase the amount of bainite in the surface layer region. As a result, the bendability of the steel sheet can be improved.

[0097] The total of R×t is obtained by the following method. R and t are calculated for the first pass of finish rolling using the above formulas (A) and (B), and R×t (mm sec) for the first pass is obtained from the obtained R and t. Note that the above formula (B) is used to calculate t. Similarly, for the second pass of finish rolling, R×t (mm sec) for the second pass is obtained using the above formulas (A) and (B). The total of R×t (mm sec) is obtained by calculating the sum of the two obtained R×t.

[0098] In the finish rolling, the total reduction in the temperature range below 1050°C is preferably set to 40 to 60%. By setting the total reduction in the temperature range below 1050°C to 40 to 60%, a preferred amount of strain can be introduced, and the amount of bainite in the 1 / 4 region and the amount of martensite in the 1 / 2 region can be preferably controlled. Furthermore, by setting the total reduction in the temperature range below 1050°C to 56% or less, the aspect ratio of prior austenite grains in the 1 / 4 region can be preferably controlled. Note that the total reduction in the temperature range below 1050°C is set by adjusting the entry thickness of the first rolling in the temperature range below 1050°C to t ０ The delivery thickness of the final rolling (rolling in the final stand of finish rolling) in the temperature range below 1050°C is t １ When this is done, (1-t １ / t ０ ) × 100 (%).

[0099] After finish rolling, it is preferable to cool 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 for 2.0 seconds or more at an average cooling rate of 20°C / s or less. By starting slow cooling in the temperature range of 500 to 680°C and performing slow cooling in this temperature range for 2.0 seconds or more, the desired amount of bainite can be obtained in the 1 / 4 region. From the viewpoint of productivity, the slow cooling time may be 10.0 seconds or less.

[0100] 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.

[0101] 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, it is possible to obtain a desired amount of bainite in the 1 / 4 region and a desired amount of martensite in the 1 / 2 region. Cooling only needs to be performed to a temperature range of 200°C or less, and may be, for example, 100°C or less. After cooling, the steel sheet is wound into a coil.

[0102] 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.

[0103] 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 element in question was below the detection limit. The resulting slabs were used to obtain the steel plates shown in Tables 4A and 4B under the manufacturing conditions shown in Tables 3A and 3B. The steel plates were cooled to the slow cooling start temperature at an average cooling rate of 30°C / s or more, and the average cooling rate of the slow cooling was 20°C / s or less.

[0104] The items in the table represent the following: Sum of R×t: Sum of R×t, which is the product of the roll contact length R expressed by the above formula (A) and the roll contact time t expressed by the above formula (B) in the first and second passes (R×t of the first pass + R×t of the second pass) Total reduction: Total reduction below 1050°C in finish rolling Average cooling rate: Average cooling rate in the temperature range from the slow cooling completion temperature to 200°C

[0105] The metal structure, tensile strength (TS), total elongation (EL), and maximum bending angle (α) of each region of the obtained steel sheet were determined, and the punching workability was evaluated.

[0106] The metallographic structure in the 1 / 4 region contained one or more of martensite, ferrite, and pearlite with a total area ratio of 5 to 30% as the remaining structure other than bainite. The metallographic structure in the 1 / 2 region contained one or more of bainite, ferrite, and pearlite with a total area ratio of 0 to 65% as the remaining structure other than martensite. The metallographic structure in the surface layer region contained one or more of martensite, ferrite, and pearlite with a total area ratio of 0 to 30% as the remaining structure other than bainite. The area ratio of martensite was as shown in the table. The measurement results obtained are shown in Table 4A and Table 4B.

[0107] When the tensile strength was 980 MPa or more, the steel sheet was judged to have high strength, whereas when the tensile strength was less than 980 MPa, the steel sheet was judged not to have high strength.

[0108] When the total elongation was 10.0% or more, the steel sheet was judged to have excellent ductility, whereas when the total elongation was less than 10.0%, the steel sheet was judged not to have excellent ductility.

[0109] When the maximum bending angle was 80° or more, the steel sheet was judged to have excellent bendability. On the other hand, when the maximum bending angle was less than 80°, the steel sheet was judged not to have excellent bendability.

[0110] When the damage incidence rate, which is an index of punching workability, was more than 15% and 25% or less, the steel plate was judged to have excellent punching workability and passed, and was marked with "Good" in the table. When the damage incidence rate was 15% or less, the steel plate was judged to have even better punching workability and passed, and was marked with "Excellent" in the table. On the other hand, when the damage incidence rate was more than 25%, the steel plate was judged to have poor punching workability and failed, and was marked with "Bad" in the table.

[0111] When the hole expansion ratio was 50% or more, the steel sheet was judged to have excellent hole expandability. On the other hand, when the hole expansion ratio was less than 50%, the steel sheet was judged not to have excellent hole expandability.

[0112]

[0113]

[0114]

[0115]

[0116]

[0117]

[0118] It can be seen from Tables 1 to 4B that the steel sheets according to the present invention have excellent punching workability. Furthermore, it can be seen that the steel sheets according to the preferred embodiments of the steel sheets according to the present invention have excellent punching workability, high strength, and excellent ductility and bendability.

[0119] 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.

[0120] According to the above aspects of the present disclosure, it is possible to obtain a steel sheet having excellent punching workability and a part obtained using the same. Furthermore, according to a preferred aspect of the present disclosure, it is possible to provide a steel sheet having excellent punching workability, high strength, and excellent ductility and bendability, and a part obtained using the steel sheet.

Claims

1. A steel sheet having a chemical composition, in mass%, of C: 0.045% or more, Si: 0.27 to 3.00%, Mn: 1.20 to 3.00%, Al: 0.400% or less, P: 0 to 0.080%, N: 0 to 0.0050%, Ti: 0 to 0.180%, and Nb: 0 to 0.100%, with the balance being Fe and impurities; in a 1 / 4 region that is a 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, an area ratio of bainite is 70 to 95%; and in a 1 / 2 region that is a region from the surface to a depth of 3 / 8 of the plate thickness to a depth of 1 / 2 of the plate thickness from the surface, an area ratio of martensite is 35 to 100%; and the sheet thickness is 1.2 to 8.0 mm.

2. The chemical composition is, in mass%, C: 0.45 to 0.120%, Al: 0.010 to 0.400%, S: 0 to 0.100%, O: 0 to 0.0100%, V: 0 to 1.000%, Cu: 0 to 1.000%, Cr: 0 to 2.000%, Mo: 0 to 3.000%, Ni: 0 to 0.500%, 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%, 2. The steel plate according to claim 1, comprising 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%, wherein an area ratio of bainite is 70 to 100% in a surface layer region that is a region from the surface to a depth of 1 / 15 of the plate thickness from the surface.

3. 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: 0.0001 to 0.1000%, Bi: 0.001 to 0.100%, Ta: 0.001 to 0.100%, The steel sheet according to claim 2, characterized in that it contains one or more selected from the group consisting of Zr: 0.001 to 0.500%, Co: 0.001 to 3.000%, Zn: 0.001 to 0.200%, W: 0.001 to 0.200%, Sb: 0.001 to 0.500%, As: 0.001 to 0.050%, and Sn: 0.001 to 0.050%.

4. A part comprising the steel sheet according to any one of claims 1 to 3.

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