Steel sheet, component including same, and method for manufacturing steel sheet

Abstract

This steel sheet has a predetermined chemical composition and has, at a position corresponding to 1 / 2 of the sheet thickness, a metal structure in which when the total length of grain boundaries of prior austenite grains is Lt in units of μm and the total circumferential length of the prior austenite grains approximate to an ellipse is Le in units of μm, the unevenness degree of the grain boundaries of the prior austenite grains, as represented by expression (1), is at most 3.20.　Expression (1): 2×(Lt / Le)

Description

Steel plate, part including same, and method of manufacturing steel plate

[0001] This disclosure relates to a steel sheet, a component including the steel sheet, and a method for manufacturing the steel sheet. This application claims priority to Japanese Patent Application No. 2024-153422, filed on September 5, 2024, the contents of which are incorporated herein by reference.

[0002] In recent years, the application of high-strength steel sheets to automotive parts has been expanding with the aim of reducing the weight of automobile bodies and contributing to improved fuel efficiency. Among automotive parts, steel sheets used as materials for suspension parts and the like are subjected to repeated loads during use after being formed into parts. Therefore, steel sheets used as materials for suspension parts and the like are required to have excellent fatigue properties, particularly high fatigue strength.

[0003] It is generally known that the fatigue strength of steel increases as its strength increases. However, the increase in fatigue strength is smaller than the increase in the strength of the steel, so the fatigue limit ratio (fatigue strength / strength) decreases as the strength of the steel increases. Therefore, it is more difficult to improve the fatigue strength of high-strength steel plates than low-strength steel plates.

[0004] For example, Patent Document 1 discloses a steel material with excellent fatigue properties, characterized in that the base material structure has a bainite structure and / or a martensite structure, the total structure fraction of the bainite structure and the martensite structure is 10% or more, and further, with respect to the prior austenite grain boundaries of the hardened layer after induction hardening, the grain boundary irregularity A calculated from the total grain boundary length Le (μm) and the total length L (μm) of the straight lines connecting adjacent grain boundary triple junctions by equation (1) (A = Le / L) is 1.05 or more. Patent Document 1 also discloses that the above steel material can stably provide a steel material with excellent fatigue properties, including torsional fatigue properties, bending fatigue properties, rolling fatigue properties, and sliding rolling fatigue properties.

[0005] Japanese Patent Application Publication No. 2007-231348

[0006] However, the steel material disclosed in Patent Document 1 has room for improvement in terms of fatigue properties.

[0007] If the surface roughness of a steel sheet is high after strain is imparted by forming it into a product or by applying cyclic stress at the early stage of a fatigue test, stress concentration on the surface will cause early formation of fatigue fracture initiation sites. This will lead to fracture at a lower cyclic stress, and it is presumed that the fatigue properties of high-strength steel sheets where this phenomenon is evident will be impaired. The present disclosure has been made based on the above background and presumption, and aims to provide a steel sheet that has low surface roughness after strain is imparted by forming it into a product or by applying cyclic stress equivalent to that at the early stage of a fatigue test, a part manufactured using the steel sheet, and a method for manufacturing the steel sheet.

[0008] The gist of the present disclosure is as follows: [1] A steel sheet having a chemical composition, in mass %, of C: 0.020 to 0.300%, Si: 0 to 3.00%, Mn: 0.10 to 4.00%, sol. Al: 0 to 2.000%, P: 0.100% or less, S: 0.0300% or less, N: 0.0200% or less, O: 0.0100% or less, Nb: 0 to 0.200%, Ti: 0 to 0.200%, V: 0 to 2.000%, Cu: 0 to 2.00%, Cr: 0-2.00%, Mo: 0-2.00%, Ni: 0-2.00%, B: 0-0.0100%, Sb: 0-0.100%, Ca: 0-0.0500%, Mg: 0-0.0500%, Bi: 0-0.010%, Zr: 0 to 0.500%, Co: 0 to 0.010%, Zn: 0 to 0.010%, W: 0 to 0.100%, Sn: 0 to 0.050%, As: 0 to 0.100%, REM: 0 to 0.0100%, and the balance: Fe and impurities, wherein in a metallographic structure at a position half the thickness from the surface in the thickness direction, the degree of irregularity of the grain boundaries of the prior austenite grains, as expressed by the following formula (1), is 3.20 or less, where Lt is the sum of the lengths of the grain boundaries of prior austenite grains in μm, and Le is the sum of the perimeters of the prior austenite grains approximated to ellipses in μm: 2×(Lt / Le) (1) [2] The steel sheet according to [1], wherein in a metallographic structure at a position one-quarter the thickness from the surface in the thickness direction, the average grain size of the prior austenite grains is 30.0 μm or less. [3] The steel sheet according to [1] or [2], characterized in that the metallographic structure at a position of 1 / 4 of the sheet thickness from the surface in the sheet thickness direction contains, in area %, martensite: 90.0% or more and retained austenite: 0.0 to 3.0%. [4] The steel sheet according to any one of [1] to [3], characterized in that the degree of irregularity of the grain boundaries of the prior austenite grains is 3.20 or less in the metallographic structure at all positions of 1 / 8, 1 / 4, 3 / 4 and 7 / 8 of the sheet thickness from the surface in the sheet thickness direction.[5] The chemical composition, in mass%, is: Nb: 0.001 to 0.200%, Ti: 0.001 to 0.200%, V: 0.001 to 2.000%, Cu: 0.01 to 2.00%, Cr: 0.01 to 2.00%, Mo: 0.01 to 2.00%, Ni: 0.01 to 2.00%, B: 0.0001 to 0.0100%, Sb: 0.001 to 0.100%, Ca: 0.0001 to 0.0500%, Mg: 0.0001 to 0.0500%, Bi: 0.001 to 0.010%, Zr: 0.001 to 0.500%, [4] The steel sheet according to any one of [1] to [4], comprising one or more elements selected from the group consisting of Co: 0.001 to 0.010%, Zn: 0.001 to 0.010%, W: 0.001 to 0.100%, Sn: 0.001 to 0.050%, As: 0.001 to 0.100%, and REM: 0.0001 to 0.0100%. [6] The steel sheet according to any one of [1] to [5], wherein the sheet thickness is 1.0 to 8.0 mm. [7] A part comprising the steel sheet according to any one of [1] to [6]. [8] A method for producing a steel sheet, characterized in that the heating temperature of a slab having the chemical composition described in [1] is in a temperature range of 1100°C or more, and the holding time in said temperature range is 6000 seconds or more, the average value of the top and bottom surface cooling ratios in rough rolling and finish rolling is 0.8 to 1.2, and in said finish rolling, the rolling reductions in the rolling one stage before the final stage and the final stage rolling are 20 to 50%, the final rolling temperature is in a temperature range of 960 to 1100°C, and cooling is performed so that the time from the completion of said finish rolling to the start of cooling is 0.5 seconds or less and the time from the start of cooling to reach a temperature range of 400°C or less is 20.0 seconds or less, and the coiling temperature is 400°C or less.

[0009] According to the above aspects of the present disclosure, it is possible to provide a steel sheet having a small surface roughness after strain is imparted by forming into a product or by applying repeated stress equivalent to the initial stage of a fatigue test, a part manufactured using the same, and a method for manufacturing the steel sheet. According to a preferred aspect of the present disclosure, it is possible to provide a high-strength steel sheet having the above characteristics, and a part manufactured using the same.

[0010] The present inventors have investigated methods for reducing the surface roughness of a steel sheet after strain has been imparted by forming into a product or by applying cyclic stress equivalent to the initial stage of a fatigue test (hereinafter, sometimes referred to as a "formed steel sheet") in order to obtain excellent fatigue properties, and have discovered the following. After being formed into a part, a small surface roughness on the part surface leads to a good fatigue limit ratio. Reducing the surface roughness of a formed part can be achieved by reducing the irregularity of the grain boundaries of prior austenite grains (prior austenite grain boundaries) at a position halfway through the thickness of the steel sheet. Furthermore, it is also effective to reduce the average grain size of prior austenite grains at a position one-quarter through the thickness of the steel sheet. In order to produce such a steel sheet, it is effective to strictly control the top and bottom surface cooling ratios of the slab or steel sheet during rough rolling and finish rolling.

[0011] The steel sheet according to this embodiment 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.

[0012] The steel sheet according to this embodiment has the following chemical composition. Note that the numerical ranges described below, separated by "to", include the lower and upper limits. Numerical values ​​indicated as "less than" and "greater than" do not include the numerical range. All percentages regarding the chemical composition represent mass %.

[0013] The steel sheet according to this embodiment has a chemical composition, in mass%, of C: 0.020 to 0.300%, Si: 0 to 3.00%, Mn: 0.10 to 4.00%, sol. Al: 0 to 2.000%, P: 0.100% or less, S: 0.0300% or less, N: 0.0200% or less, O: 0.0100% or less, as well as Fe and impurities. Each element will be described in detail below.

[0014] C: 0.020 to 0.300% C is an element effective in increasing the strength of steel sheet. Furthermore, C forms carbides and / or carbonitrides with Nb in steel, and the pinning effect of the formed precipitates contributes to refining the metal structure. If the C content is less than 0.020%, the strength of the steel sheet decreases and the irregularity of the prior austenite grain boundary at the half-thickness position increases. Therefore, the C content is set to 0.020% or more. The C content is preferably 0.040% or more, 0.060% or more, 0.080% or more, 0.100% or more, or 0.120% or more. On the other hand, excessive C content reduces the workability of the steel sheet. Furthermore, the irregularity of the prior austenite grain boundary at the half-thickness position increases. Therefore, the C content is set to 0.300% or less. The C content is preferably 0.280% or less, 0.240% or less, 0.200% or less, or 0.180% or less.

[0015] Si: 0 to 3.00% Si is an element effective in increasing the strength of steel sheet through solid solution strengthening. Si is not necessarily contained, so the Si content may be 0%. However, to fully obtain the above effect, the Si content is preferably 0.05% or more. The Si content is more preferably 0.20% or more, 0.30% or more, 0.40% or more, 0.50% or more, or 0.60% or more. On the other hand, excessive Si content reduces the chemical conversion treatability and workability of the steel sheet and may cause slab cracking during hot rolling. In addition, the irregularity of the prior austenite grain boundary at the half-thickness position increases. Therefore, the Si content is set to 3.00% or less. The Si content is preferably 2.80% or less, 2.60% or less, 2.40% or less, or 2.20% or less.

[0016] Mn: 0.10 to 4.00% Mn is an element effective in increasing the strength of steel sheet by improving hardenability and solid solution strengthening. If the Mn content is less than 0.10%, the strength of the steel sheet decreases and the irregularity of the prior austenite grain boundary at the half-thickness position increases. Therefore, the Mn content is set to 0.10% or more. The Mn content is preferably 0.50% or more, 0.80% or more, 1.00% or more, 1.50% or more, or 2.00% or more. On the other hand, if Mn is contained in excess, the workability of the steel sheet decreases. Furthermore, the irregularity of the prior austenite grain boundary at the half-thickness position increases. Therefore, the Mn content is set to 4.00% or less. The Mn content is preferably 3.80% or less, 3.50% or less, 3.20% or less, 3.00% or less, or 2.80% or less.

[0017] Sol. Al: 0 to 2.000% Sol. Al is an element that acts as a deoxidizer for molten steel. Sol. Al also suppresses the precipitation of cementite, which is harmful to the hole expandability of steel sheets. Sol. Al may not be present, so the sol. Al content may be 0%. However, to fully obtain the above effects, the sol. Al content is preferably 0.001% or more. The sol. Al content is more preferably 0.010% or more, 0.050% or more, or 0.080% or more. On the other hand, excessive sol. Al content saturates the effect and increases manufacturing costs. Furthermore, the degree of irregularity of the prior austenite grain boundary at the 1 / 2 sheet thickness position increases. Therefore, the sol. Al content is set to 2.000% or less. The Al content is preferably 1.500% or less, 1.000% or less, 0.500% or less, or 0.100% or less. In this embodiment, sol. Al means acid-soluble Al, and indicates solute Al present in the steel in a solid solution state.

[0018] P: 0.100% or less Excessive P content reduces the workability of the steel sheet due to grain boundary segregation, etc. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.070% or less, 0.050% or less, 0.030% or less, or 0.020% or less. The lower the P content, the better, so it may be 0%. However, since excessive reduction of the P content causes a significant increase in dephosphorization costs, the P content is preferably set to 0.001% or more or 0.005% or more.

[0019] S: 0.0300% or less Excessive S content generates large amounts of sulfides such as MnS, which reduces the workability of the steel sheet. Therefore, the S content is set to 0.0300% or less. The S content is preferably 0.0200% or less, 0.0100% or less, or 0.0050% or less. The lower the S content, the better, so it may be 0%. However, excessive reduction of the S content causes a significant increase in desulfurization costs, so the S content is preferably 0.001% or more or 0.005% or more.

[0020] N: 0.0200% or less Excessive N content forms coarse nitrides, reducing the workability of the steel sheet. Therefore, the N content is set to 0.0200% or less. The N content is preferably 0.0150% or less, 0.0100% or less, 0.0070% or less, 0.0050% or less, 0.0040% or less, or 0.0030% or less. The lower the N content, the better, so it may be 0%. However, excessive reduction of the N content causes a significant increase in denitrification costs, so the N content is preferably 0.0005% or more or 0.0010% or more.

[0021] O: 0.0100% or less Excessive O content forms coarse inclusions, reducing the workability of the steel sheet. Therefore, the O content is set to 0.0100% or less. The O content is preferably 0.0080% or less, 0.0060% or less, or 0.0040% or less. Since a lower O content is more preferable, it may be 0%. However, in order to disperse a large number of fine oxides during deoxidation of molten steel, the O content may be set to 0.0005% or more or 0.0010% or more.

[0022] The balance of the chemical composition of the steel sheet according to this embodiment may be 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 are acceptable within a range that does not adversely affect the properties of the steel sheet according to this embodiment.

[0023] The steel sheet according to this embodiment may contain the following optional elements instead of part of Fe. When no optional elements are contained, the lower limit of the content is 0%. Each optional element will be described below.

[0024] Nb: 0 to 0.200% Nb is an element that forms carbides, nitrides, and / or carbonitrides in steel, thereby refining prior austenite grains through a pinning effect and increasing the strength of the steel sheet. To fully obtain these effects, the Nb content is preferably 0.001% or more. The Nb content is more preferably 0.005% or more, 0.010% or more, or 0.050% or more. On the other hand, excessive Nb content generates coarse carbides and the like in the steel, reducing the workability of the steel sheet. In addition, the irregularity of the prior austenite grain boundaries at the half-thickness position increases. Therefore, the Nb content is set to 0.200% or less. The Nb content is preferably 0.180% or less, 0.150% or less, 0.120% or less, 0.100% or less, or 0.080% or less.

[0025] Ti: 0 to 0.200% Ti is an element that precipitates in steel as Ti carbides such as TiC and increases the strength of the steel sheet through precipitation strengthening. To ensure this effect, the Ti content is preferably 0.001% or more. The Ti content is more preferably 0.010% or more or 0.050% or more. On the other hand, if the Ti content exceeds 0.200%, coarse carbides may form in the steel, which may cause slab cracking during hot rolling. Therefore, the Ti content is set to 0.200% or less. The Ti content is preferably 0.180% or less, 0.150% or less, or 0.130% or less.

[0026] V: 0 to 2.000% V is an element that increases the strength of steel sheet by forming fine carbides in steel. To reliably obtain this effect, the V content is preferably 0.001% or more. The V content is more preferably 0.010% or more or 0.050% or more. On the other hand, excessive V content increases alloy costs. Therefore, the V content is set to 2.000% or less. The V content is preferably 0.180% or less, 0.150% or less, or 0.130% or less.

[0027] Cu: 0 to 2.00% Cu is an element that improves the hardenability of steel sheets and, by precipitating as carbides in steel at low temperatures, increases the strength of the steel sheets. To reliably obtain these effects, the Cu content is preferably 0.01% or more. The Cu content is more preferably 0.05% or more or 0.10% or more. On the other hand, if the Cu content exceeds 2.00%, grain boundary cracking of the slab may occur. Therefore, the Cu content is set to 2.00% or less. The Cu content is preferably 1.80% or less or 1.50% or less.

[0028] Cr: 0 to 2.00% Cr is an element that improves the hardenability of steel and contributes to improving the strength and / or corrosion resistance of the steel sheet. To reliably obtain these effects, the Cr content is preferably 0.01% or more. The Cr content is more preferably 0.05% or more or 0.10% or more. On the other hand, even if Cr is contained in excess, the above effects saturate, so the Cr content is set to 2.00% or less. The Cr content is preferably 1.80% or less or 1.50% or less.

[0029] Mo: 0 to 2.00% Mo is an element that increases the strength of steel sheet by forming fine carbides in steel. To reliably obtain this effect, the Mo content is preferably 0.01% or more. The Mo content is more preferably 0.05% or more or 0.10% or more. On the other hand, even if Mo is contained in excess, the above effect saturates, so the Mo content is set to 2.00% or less. The Mo content is preferably 1.80% or less or 1.50% or less.

[0030] Ni: 0 to 2.00% Ni is an element that improves the hardenability of the steel sheet and increases its strength. Furthermore, when Cu is contained, Ni is an element that effectively suppresses intergranular cracking of the slab caused by Cu. To reliably obtain these effects, the Ni content is preferably 0.01% or more. The Ni content is more preferably 0.05% or more or 0.10% or more. On the other hand, since Ni is an expensive element, it is economically undesirable to include a large amount of Ni. Therefore, the Ni content is set to 2.00% or less. The Ni content is preferably 1.80% or less or 1.50% or less.

[0031] B: 0 to 0.0100% B is an element that improves the hardenability of steel and increases the strength of 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, even if B is contained in excess, the above effect saturates, so the B content is set to 0.0100% or less. The B content is preferably 0.0080% or less, 0.0050% or less, or 0.0030% or less.

[0032] Sb: 0 to 0.100% Sb is an element that suppresses the generation of oxides that serve as fracture initiation sites, thereby improving the ductility and hole expandability of steel sheets. To reliably obtain these effects, 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 effects saturate, so the Sb content is set to 0.100% or less. The Sb content is preferably 0.080% or less or 0.050% or less.

[0033] Ca: 0 to 0.0500% Ca is an element that acts as a fracture initiation point and controls the morphology of non-metallic inclusions, thereby improving the ductility and hole expandability of the steel sheet. To reliably obtain this effect, 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, since the above effect saturates even when a large amount of Ca is added, the Ca content is set to 0.0500% or less. The Ca content is preferably 0.0300% or less, 0.0100% or less, 0.0080% or less, or 0.0050% or less.

[0034] Mg: 0 to 0.0500% Like Ca, Mg is an element that controls the morphology of non-metallic inclusions, thereby improving the ductility and hole expandability of steel sheets. To reliably obtain this effect, the Mg content is preferably 0.0001% or more. The Mg content is more preferably 0.0005% or more or 0.0010% or more. On the other hand, even if a large amount of Mg is added, the above effect saturates, so the Mg content is set to 0.0500% or less. The Mg content is preferably 0.0300% or less, 0.0100% or less, 0.0080% or less, or 0.0050% or less.

[0035] Bi: 0 to 0.010% Bi is an element that refines the solidification structure, thereby improving the ductility and hole expandability of the steel sheet. To ensure this effect, the Bi content is preferably 0.001% or more. The Bi content is more preferably 0.003% or more or 0.005% or more. On the other hand, even if Bi is contained in excess, the above effect saturates, so the Bi content is set to 0.010% or less. The Bi content is preferably 0.008% or less or 0.007% or less.

[0036] Zr: 0 to 0.500% Zr is an element that increases the strength of steel sheet through solid solution strengthening. To reliably obtain this effect, the Zr content is preferably 0.001% or more. The Zr content is more preferably 0.005% or more or 0.010% or more. On the other hand, even if Zr is contained in excess, the above effect saturates, so the Zr content is set to 0.500% or less. The Zr content is preferably 0.300% or less, 0.100% or less, 0.050% or less, 0.030% or less, or 0.020% or less.

[0037] Co: 0 to 0.010% Co is an element that increases the strength of steel sheet through solid solution strengthening. To more reliably obtain this effect, the Co content is preferably 0.001% or more. The Co content is more preferably 0.003% or more or 0.005% or more. On the other hand, even if Co is contained in excess, the above effect saturates, so the Co content is set to 0.010% or less. The Co content is preferably 0.008% or less or 0.007% or less.

[0038] Zn: 0 to 0.010% Zn is an element that increases 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 0.003% or more or 0.005% or more. On the other hand, even if Zn is contained in excess, the above effect saturates, so the Zn content is set to 0.010% or less. The Zn content is preferably 0.008% or less or 0.007% or less.

[0039] W: 0 to 0.100% W is an element that increases the strength of steel sheet through solid solution strengthening. To more reliably obtain this effect, the W content is preferably 0.001% or more. The W content is more preferably 0.005% or more or 0.010% or more. On the other hand, even if W is contained in excess, the above effect saturates, so the W content is set to 0.100% or less. The W content is preferably 0.080% or less or 0.050% or less.

[0040] Sn: 0 to 0.050% Sn is an element that suppresses the generation of oxides that serve as fracture initiation sites, thereby improving the hole expandability of steel sheets. To reliably obtain this effect, the Sn content is preferably 0.001% or more. The Sn content is more preferably 0.005% or more or 0.010% or more. On the other hand, even if a large amount of Sn is added, the above effect saturates, so the Sn content is set to 0.050% or less. The Sn content is preferably 0.040% or less or 0.030% or less.

[0041] As: 0 to 0.100% As is an element that refines prior austenite grains by lowering the austenite single-phase transformation temperature. To reliably obtain this effect, the As content is preferably 0.001% or more. The As content is more preferably 0.005% or more or 0.010% or more. On the other hand, even if a large amount of As is contained, the above effect saturates, so the As content is set to 0.100% or less. The As content is preferably 0.070% or less or 0.050% or less.

[0042] REM: 0 to 0.0100% Like Ca, REM is an element that controls the morphology of non-metallic inclusions, thereby improving the ductility and hole expandability of steel sheets. To reliably obtain 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, since the above effect saturates even when a large amount of REM is contained, the REM content is set to 0.0100% or less. The REM content is preferably 0.0070% or less or 0.0050% or less. REM refers to a total of 17 elements consisting of Sc, Y, and lanthanides (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu), and the REM content refers to the total content of these elements.

[0043] The chemical composition of the above-mentioned steel plate is determined by the following method. Test specimens are taken from the region extending from the surface of the steel plate to a position 1 / 8 of the plate thickness to a position 3 / 8 of the plate thickness in the plate thickness direction, and the chemical composition of these test specimens is measured by a common method such as ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). C and S are measured 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. Sol. Al is measured by ICP-AES using the filtrate obtained after thermal decomposition of the sample with acid. If the steel plate has a coating on its surface, the coating is removed by mechanical grinding, and then the chemical composition is analyzed in the same manner.

[0044] Next, the metallographic structure of the steel sheet according to this embodiment will be described. In the steel sheet according to this embodiment, in the metallographic structure at a position halfway through the sheet thickness from the surface in the sheet thickness direction, when the total length of the grain boundaries of the prior austenite grains is Lt (unit: μm) and the total perimeter of the prior austenite grains approximated as an ellipse is Le (unit: μm), the degree of irregularity of the grain boundaries of the prior austenite grains, expressed by the following formula (1), is 3.20 or less: 2×(Lt / Le) (1)

[0045] The surface of the steel sheet referred to here refers to the surface of the steel sheet when the steel sheet does not have a coating on its surface, and refers to the interface between the coating and the steel sheet when the steel sheet has a coating on its surface.

[0046] <1 / 2 position of plate thickness> Irregularity of prior austenite grain boundaries: 3.20 or less The inventors have found that when the irregularities of the prior austenite grain boundaries (prior austenite grain boundaries) are large, stress concentrates at the recesses of the prior austenite grains, resulting in increased surface roughness after forming and making it difficult to obtain excellent fatigue properties. Therefore, in the steel plate according to this embodiment, the irregularity is specified as an index of the degree of irregularity of the prior austenite grain boundaries, and by setting the irregularity within a desired range, excellent fatigue properties can be obtained in the steel plate.

[0047] In the steel sheet according to this embodiment, when the total length of the grain boundaries of the prior austenite grains at the half-thickness position is Lt (unit: μm) and the total perimeter of the elliptical prior austenite grains is Le (unit: μm), the irregularity of the prior austenite grain boundaries expressed by the above formula (1) is 3.20 or less. If the irregularity of the prior austenite grain boundaries exceeds 3.20, the fatigue properties of the steel sheet after forming deteriorate. The irregularity of the prior austenite grain boundaries is preferably 3.00 or less, 2.80 or less, 2.50 or less, or 2.00 or less. There is no particular limitation on the lower limit of the irregularity of the prior austenite grain boundaries, but it may be 0.10 or more or 0.30 or more.

[0048] The length of the prior austenite grain boundary and the perimeter of the elliptical prior austenite grain, used to calculate the degree of irregularity of the prior austenite grain boundary, are measured using the following method. A test specimen is cut from any position at least 50 mm away from the end face of the steel plate so that a cross section of the plate thickness parallel to the rolling direction can be observed. If a test specimen cannot be obtained from this position, a test specimen is cut from a position avoiding the end. The size of the test specimen, although depending on the measuring device, should be large enough to allow observation of approximately 10 mm in the rolling direction. The cross section of the test specimen is polished using #600 to #1500 silicon carbide paper, and then finished to a mirror finish using a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in a diluted solution such as alcohol or pure water, and then electrolytic polishing is performed.

[0049] On the observation surface of the test piece, at the half-thickness position (center of the sheet thickness), crystal orientation information is obtained by electron backscatter diffraction in a region of 100 μm in the rolling direction and 100 μm in the sheet thickness direction at measurement intervals of 0.1 μm. For the measurement, an EBSD analyzer consisting of a thermal field emission scanning electron microscope and an EBSD detector is used. For example, an EBSD analyzer 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 EBSD analyzer is 9.6 × 10 －５The pressure is set to 25 Pa or less, the acceleration voltage is 25 kV, and the probe current level is 16. Using the obtained crystal orientation information, the crystal orientation of the prior austenite grains is calculated from the crystal orientation relationship between the prior austenite grains and the crystal grains having a body-centered structure after transformation.

[0050] The crystal orientation of the prior austenite grains is calculated by the following method. A crystal orientation map of the prior austenite grains, displaying the prior austenite grain boundaries, is created by the method described in Acta Materialia, 58 (2010), 6393-6403. To display the crystal orientation map, OIM Analysis (registered trademark) version 7 or later manufactured by EDAX / TSL Solution is used, and grain boundaries with a crystal orientation difference of 15° or more are displayed.

[0051] In the crystal orientation map, the length (μm) of the prior austenite grain boundary is measured for one prior austenite grain included in the observation field. The total length of the prior austenite grain boundaries is obtained by performing the above operation on all prior austenite grains, excluding prior austenite grains whose entire crystal grains are not included in the observation field, such as those at the edges of the observation field, and prior austenite grains with a circle-equivalent diameter of less than 2 μm. At this time, the length of the adjacent grain boundary is measured twice for adjacent prior austenite grains. Next, all prior austenite grains included in the observation field are approximated as ellipses, and the major axis length and minor axis length are obtained. Using the above-mentioned software (OIM Analysis (registered trademark)), the major axis length and minor axis length of the prior austenite grains when approximated as ellipses can be obtained. In the ellipse approximation here, after loading data into Project Tree in the above-mentioned software (OIM Analysis (registered trademark)), right-click on "All data" and check the "Ellipse minor axis (radius) in microns" and "Ellipse major axis (radius) in microns" checkboxes in "export grain file," thereby entering the major axis length and minor axis length in the case of ellipse approximation into the output file. The perimeter L of the ellipse-approximated prior austenite grain is calculated using Ramanujan's approximation formula (L = π × [3 (a + b) - √{(3a + b) × (a + 3b)}]), where a is the major axis length and b is the minor axis length. The above-mentioned operation is performed for five regions (each region is 100 μm × 100 μm), and the average value of the irregularities of the prior austenite grain boundaries obtained for each region is calculated to obtain the "irregularity of the grain boundaries of the prior austenite grains."

[0052] The rolling direction of the steel plate is determined by the following method. Test specimens are taken so that the thickness cross section of the steel plate can be observed. The direction perpendicular to the plate surface is defined as the Z direction, and a total of 12 test specimens are taken by rotating the plate every 30° around this Z direction as an axis. The thickness cross sections of the taken test specimens are polished, and the prior austenite grains are identified by the above-mentioned method. The average aspect ratio (major axis / minor axis) of the prior austenite grains is calculated by the intercept method. The test specimen with the largest average aspect ratio of the prior austenite grains is identified, and the direction from which the test specimen was taken is determined to be the rolling direction of the steel plate. In other words, the direction parallel to the thickness cross section of the test specimen and perpendicular to the thickness direction is determined to be the rolling direction of the steel plate.

[0053] <1 / 4 thickness position> Average grain size of prior austenite grains: 30.0 μm or less In the metal structure at a position 1 / 4 of the thickness from the surface in the thickness direction (1 / 4 thickness position), by setting the average grain size of the prior austenite grains to 30.0 μm or less, the density of the prior austenite grain boundaries can be increased, and the fatigue properties of the steel sheet after forming can be further improved. Therefore, the average grain size of the prior austenite grains at the 1 / 4 thickness position is preferably 30.0 μm or less. The average grain size of the prior austenite grains at the 1 / 4 thickness position is more preferably 25.0 μm or less, 23.0 μm or less, or 20.0 μm or less. The lower limit of the average grain size of the prior austenite grains at the 1 / 4 thickness position is not particularly limited, but may be 5.0 μm or more, 7.0 μm or more, or 10.0 μm or more.

[0054] The average grain size of the prior austenite grains is obtained by the following method. A crystal orientation map of the prior austenite grains at the 1 / 4 position in the plate thickness is created by the same method as when creating the crystal orientation map of the prior austenite grains at the 1 / 2 position in the plate thickness. In the crystal orientation map, the average value of the shortest diameter and the longest diameter of one of the prior austenite grains included in the observation field is calculated, and this average value is used as the grain size of that prior austenite grain. The above operation is performed for all prior austenite grains, excluding prior austenite grains whose entirety is not included in the observation field, such as those at the edges of the observation field, and prior austenite grains with a circle equivalent diameter of less than 2 μm, to determine the grain sizes of all prior austenite grains in the observation field. The average grain size of the prior austenite grains is obtained by calculating the average value of the grain sizes of all prior austenite grains.

[0055] In the steel sheet according to this embodiment, the metal structure at 1 / 4 of the sheet thickness preferably contains, in area percentages, 90.0% or more of martensite and 0.0 to 3.0% of retained austenite. By setting the area fraction of martensite to 90.0% or more, high strength can be obtained in the steel sheet. Furthermore, the metal structure becomes more uniform, and surface roughness after forming can be reduced. As a result, better fatigue properties can be obtained. The area fraction of martensite is preferably 93.0% or more, 95.0% or more, or 97.0% or more. Since a higher area fraction of martensite is preferable, it is more preferable to set it to 100.0%.

[0056] The retained austenite may become the starting point of fracture during forming, etc. Furthermore, by reducing the amount of retained austenite, the surface roughness after forming can be further reduced. Therefore, the area fraction of retained austenite is preferably 3.0% or less. The area fraction of retained austenite is 2.5% or less, 2.0% or less, 1.5% or less, 1.0% or less, or 0.5% or less. Since the area fraction of retained austenite is preferably as low as possible, it is preferably 0.0%.

[0057] In the steel plate according to this embodiment, the metal structure at 1 / 4 of the plate thickness may contain, as the remaining structure, ferrite: 10.0% or less, bainite: 10.0% or less, and pearlite: 10.0% or less.

[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 the 1 / 4 position of the plate thickness (in the range from the surface to the 1 / 8 position of the plate thickness in the plate thickness direction). The plate thickness cross section of the test piece is mirror-polished and LePera etched, and then an FE-SEM (thermal field emission scanning electron microscope, JSM-7001F, manufactured by JEOL) is used to observe eight fields of view in a 200 μm (plate thickness direction) × 600 μm (direction perpendicular to the plate thickness direction) region centered at the 1 / 4 position of the plate thickness, and image analysis is performed.

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

[0060] The area fraction of retained austenite is obtained by X-ray diffraction. A test specimen taken from a steel plate is face-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.

[0061] The area ratio of martensite is obtained by subtracting the area ratio of retained austenite obtained by X-ray diffraction from the sum of the area ratios of "martensite and retained austenite" obtained by the observation using the FE-SEM described above.

[0062] Next, for the same region (200 μm × 600 μm) as used to determine the area ratios of martensite and retained austenite, only the corroded layer is removed by polishing, and the region is mirror-finished. The region is then etched using a nital solution, and observed using an FE-SEM for image analysis. In the image analysis, each structure is identified using the following method, and its area ratio is calculated to obtain the area ratio of each structure.

[0063] Bainite is defined as a structure that is a collection of lath-shaped crystal grains and does not contain Fe-based carbides with a major axis of 20 nm or more within the structure, and is not martensite, or a structure that contains Fe-based carbides with a major axis of 20 nm or more within the structure, where the Fe-based carbides have a single variant, i.e., elongated in the same direction. Here, Fe-based carbides elongated in the same direction refer to Fe-based carbides whose elongation directions differ by no more than 5°. A structure that is composed of massive crystal grains and does not contain a substructure such as lath within the structure is defined as ferrite. A region in which cementite and ferrite are arranged in a lamellar shape is defined as pearlite.

[0064] In this embodiment, the area ratio of the metallographic structure is calculated by image analysis using an FE-SEM and X-ray diffraction, so the total of the individual structures may not be 100%. In such cases, the area ratio of each structure is corrected so that the total becomes 100%. For example, if the total of the area ratios of each structure is 103%, the area ratio of each structure is corrected by multiplying it by "100 / 103".

[0065] <1 / 8 Sheet Thickness Position, 1 / 4 Sheet Thickness Position, 1 / 2 Sheet Thickness Position, 3 / 4 Sheet Thickness Position, and 7 / 8 Sheet Thickness Position> In the steel sheet according to this embodiment, it is preferable that the irregularity of the prior austenite grain boundary be 3.20 or less not only at the 1 / 2 sheet thickness position but also throughout the entire thickness in the sheet thickness direction. Specifically, it is preferable that the irregularity of the prior austenite grain boundary be 3.20 or less in the metallographic structure at all positions, including the 1 / 8 sheet thickness position from the surface in the sheet thickness direction (1 / 8 sheet thickness position), the 1 / 4 sheet thickness position from the surface in the sheet thickness direction (1 / 4 sheet thickness position), the 1 / 2 sheet thickness position from the surface in the sheet thickness direction (1 / 2 sheet thickness position), the 3 / 4 sheet thickness position from the surface in the sheet thickness direction (3 / 4 sheet thickness position), and the 7 / 8 sheet thickness position from the surface in the sheet thickness direction (7 / 8 sheet thickness position). By setting the irregularity of the prior austenite grain boundary to 3.20 or less in the metallographic structure at all of the above positions, the fatigue properties of the steel sheet after forming can be further improved.

[0066] The irregularity degree of the prior austenite grain boundary at the above positions is obtained by measuring the irregularity degree of the prior austenite grain boundary at the 1 / 8, 1 / 4, 3 / 4 and 7 / 8 thickness positions using the same method as when measuring the irregularity degree of the prior austenite grain boundary at the 1 / 2 thickness position.

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

[0068] The tensile strength is evaluated by conducting a tensile test in accordance with JIS Z 2241:2022. The test piece is a No. 5 test piece of JIS Z 2241:2022. The tensile test piece is taken from a quarter portion from the end in the plate width direction, with the direction perpendicular to the rolling direction as the longitudinal direction. If a No. 5 test piece cannot be taken because the steel plate or part is small in size or has a complex shape, a small strip-shaped piece having a parallel portion of any width may be taken, and the tensile strength may be determined by conducting a tensile test using the small strip. The direction perpendicular to the rolling direction is the longitudinal direction of the small strip-shaped piece.

[0069] Arithmetic mean roughness Ra after 10% tension and 10% compression: 7.0 μm or less. The inventors have found that if the arithmetic mean roughness Ra of the surface is 7.0 μm or less after applying a 10% tensile strain and a 10% compressive strain, excellent fatigue properties, specifically a high fatigue limit ratio, can be obtained even after forming. Therefore, in the steel sheet according to this embodiment, the target arithmetic mean roughness Ra of the surface is 7.0 μm or less after applying a 10% tensile strain and a 10% compressive strain. The smaller the arithmetic mean roughness Ra of the surface, the more preferable it is, and the target may be 5.0 μm or less, 4.0 μm or less, or 3.0 μm or less.

[0070] The arithmetic mean roughness Ra of the surface is measured by the following method. A tensile test is performed on a test piece taken from a steel plate using the above-mentioned method, imparting a 10% tensile strain followed by a 10% compressive strain. To prevent buckling of the test piece, buckling prevention measures such as restraining the parallel portions of the test piece with a jig are taken before imparting tensile and compressive strain. A 1000 mm x 1000 mm sample is taken from the longitudinal center of the test piece to which tensile and compressive strains have been imparted. Measurement points on the sample surface are set at 200 mm intervals in the rolling direction and the plate width direction (directions perpendicular to the rolling direction and plate thickness direction), and the surface roughness is measured at each measurement point. Five measurement points are set in each of the rolling direction and the plate width direction. The measurement length at each measurement point is 5 mm. A roughness curve is obtained by sequentially applying profile filters with cutoff values ​​λc and λs to the measured cross-sectional curve obtained by the measurement. Specifically, a roughness curve is obtained by removing components with a wavelength λc of 0.8 mm or less and components with a wavelength λs of 2.5 μm or more from the obtained measurement results. Based on the obtained roughness curve, the arithmetic mean roughness Ra of each measurement point is calculated in accordance with JIS B 0601:2013. The arithmetic mean roughness Ra of the surface is obtained by calculating the average value of the measurements obtained at each measurement point.

[0071] When the steel sheet has a surface treatment coating such as plating or painting on its surface, the surface treatment coating is removed from the steel sheet before preparing the tensile test specimen. The method for removing the surface treatment coating can be selected appropriately depending on the type of surface treatment coating, as long as it does not affect the surface roughness of the steel substrate. For example, when the surface treatment coating is a zinc-based plating layer, the zinc-based plating layer can be dissolved using dilute hydrochloric acid containing an inhibitor. This allows only the zinc-based plating layer to be peeled off from the steel sheet. The inhibitor is an additive used to suppress changes in roughness due to excessive dissolution of the steel substrate. For example, a solution prepared by adding "IBIT No. 700BK," a corrosion inhibitor for hydrochloric acid pickling manufactured by Asahi Chemical Industry Co., Ltd., to hydrochloric acid diluted 10 to 100 times to a concentration of 0.6 g / L can be used.

[0072] The thickness of the steel plate according to this embodiment is not particularly limited, but may be 1.0 to 8.0 mm. The thickness may be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more. The thickness may also be 7.0 mm or less, 6.0 mm or less, 5.5 mm or less, 5.0 mm or less, 4.4 mm or less, 4.2 mm or less, or 4.0 mm or less.

[0073] 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).

[0074] The steel sheet according to the present embodiment has excellent fatigue properties even after forming, and therefore can be suitably used for parts, particularly automobile parts. Among automobile parts, the steel sheet according to the present embodiment 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.

[0075] 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. However, areas that have undergone heavy processing or welding, the edges of the part, or areas where red rust has formed may not have the above-described metallurgical structure, or it may be difficult to determine whether they have the same structure. Therefore, when measuring the metallurgical structure and mechanical properties of a part, these areas are avoided and measurements are made on unprocessed parts. If there are no unprocessed parts, measurements are made on unprocessed parts. An unprocessed or unprocessed part refers to, for example, a flat part of the part, a part where the thickness change due to processing is small, and a part that avoids parts that have undergone punching, hole expansion, bending, etc. As an example, in the case of the above-described part, a test piece is taken from the flat part with the largest area near the center of gravity and examined. Specific locations to avoid measurement include the following (i) to (iv). If the part has a paint film or chemical conversion coating on its surface, the part is measured after undergoing the following paint film removal process and chemical conversion coating removal process. (i) Welded parts: Locations within 20 mm from the toe of spot welds, and locations within 20 mm from the toe of arc / laser welds. (ii) Processed parts: Processed parts with a curvature radius of less than 15 mm, and locations within 5 mm from the processed parts. (iii) Edges: Edges within 5 mm from the cut end surface of the part. (iv) Red rust: Locations within 5 mm from locations where red rust is visible to the naked eye.

[0076] (Coating Removal Process) A coating remover (Neo River #160, manufactured by Sansai Kako Co., Ltd.) is applied to the surface of a sample cut out from a part at room temperature and allowed to stand for 5 minutes. Thereafter, the surface of the sample coated with the coating remover is rubbed with a hard sponge (for example, "Kanefeel", manufactured by Aion Co., Ltd.) to remove the coating from the surface of the sample.

[0077] Next, the surface of the sample after the coating film removal is washed with water and dried. At this time, the state of the remaining coating film is confirmed by SEM-EPMA measurement of the surface of the sample after washing and drying (100 μm square, 5 fields of view).

[0078] In the element distribution image obtained by EPMA, a region where the C concentration is 10 mass % or more is identified, and if the area ratio of this region is 5% or more, it is determined that the peeling of the coating film is insufficient.

[0079] To measure the area ratio of regions where the C concentration is 10% by mass or more, first, an elemental distribution image of C is obtained using EPMA with the C concentration range set to 10 to 30%. Specific measurement conditions for EPMA are as follows: Apparatus: JXA-8230 Electron Probe Microanalyzer manufactured by JEOL Ltd. Acceleration voltage: 15 kV Probe current: 0.05 μA Area analysis: WDS Analysis interval: 300 μm or more Area ratio: Average value of 5 fields of view Next, the obtained elemental distribution image of C is subjected to image processing to measure the area ratio. Image analysis software "ImageJ" is used for image processing. Specifically, the above elemental distribution image of C is loaded into ImageJ, and then binarized using "Make Binary" in "Binary" under "Process" so that regions where the C concentration is 10% by mass or more are displayed as black and regions where the C concentration is less than 10% by mass are displayed as white. After binarization, use "Measure" under "Analyze" to read the value of "Area fraction" under "Results." This value is determined as the area fraction of the region where the C concentration is 10% by mass or more.

[0080] If the coating film is not sufficiently removed, the removal of the coating film is repeated until the area ratio of the region where the C concentration is 10% by mass or more becomes less than 5%.

[0081] (Chemical conversion coating removal step) A sample cut out from a part and having the coating removed is subjected to a method in accordance with JIS K 3151: 1996 to remove the chemical conversion coating from the surface of the sample. Specifically, the sample after the coating removal is immersed in a 5% by mass aqueous solution of chromic acid heated to 75°C for 15 minutes to remove the chemical conversion coating from the surface of the sample.

[0082] Next, the surface of the sample after the removal of the chemical conversion coating is washed with water and dried. At this time, the state of remaining chemical crystals is confirmed by SEM-EPMA measurement of the surface of the sample after washing and drying (100 μm square, 5 fields of view).

[0083] In the element distribution image obtained by EPMA, a region where the P concentration is 5 mass % or more is identified, and if the area ratio of this region is 5% or more, it is determined that the chemical conversion coating has not been sufficiently peeled off.

[0084] To measure the area fraction of regions with a P concentration of 5% by mass or more, first, an elemental distribution image of P is obtained using an EPMA with a P concentration range set to 5-10%. The obtained elemental distribution image of P is then subjected to image processing to measure the area fraction. Image analysis software "ImageJ" is used for image processing. Specifically, the elemental distribution image of P is loaded into ImageJ, and then binarized using "Make Binary" in "Binary" under "Process" so that regions with a P concentration of 5% by mass or more are displayed as black and regions with a P concentration of less than 5% by mass are displayed as white. After binarization, "Measure" under "Analyze" is used to read the value of "Area fraction" in "Results." This read value is determined as the area fraction of regions with a P concentration of 5% by mass or more.

[0085] If the chemical conversion coating is not sufficiently removed, removal of the chemical conversion coating is repeated until the area ratio of the region having a P concentration of 5 mass % or more becomes less than 5%.

[0086] Next, a method for manufacturing a steel sheet according to this embodiment will be described. According to the manufacturing method described below, the steel sheet according to this embodiment can be stably manufactured. Note that the temperature of the slab and the temperature of the steel sheet in this embodiment refer to the surface temperature of the slab and the surface temperature of the steel sheet.

[0087] In the method for manufacturing a steel sheet according to this embodiment, the heating temperature of a slab having the above-described chemical composition is in a temperature range of 1100°C or higher, and the holding time in this temperature range is 6000 seconds or longer, the average top and bottom surface cooling ratios in rough rolling and finish rolling are 0.8 to 1.2, and in the finish rolling, the rolling reductions in the rolling one stage before the final stage and the final stage are 20 to 50%, the final rolling temperature is in a temperature range of 960 to 1100°C, cooling is performed so that the time from the end of the finish rolling to the start of cooling is 0.5 seconds or less and the time from the start of cooling to reach 400°C is 20.0 seconds or less, and the coiling temperature is 400°C or lower. Each step will be described in detail below.

[0088] Heating Temperature and Holding Time in Slab Heating: The heating temperature of a slab having the above-described chemical composition is set to a temperature range of 1100°C or higher, and the holding time in this temperature range is set to 6000 seconds or longer. Holding the temperature range of 1100°C or higher for 6000 seconds or longer allows the coarse carbides, which serve as crack initiation points, to be completely dissolved. Holding the temperature below 1100°C or the holding time below 6000 seconds prevents the coarse carbides from being fully dissolved. As a result, during cooling (described below), ferrite and bainite transformations occur starting from the carbides, resulting in an insufficient area fraction of martensite. Furthermore, the irregularity of the prior austenite grain boundary at the half-thickness position increases. Therefore, the heating temperature in slab heating is set to 1100°C or higher, and the holding time is set to 6000 seconds or longer. The holding time at 1100°C or higher is preferably set to 6500 seconds or longer or 7000 seconds or longer. The upper limit of the heating temperature in slab heating is not particularly limited, but may be 1300°C or less or 1200°C or less from the viewpoint of energy costs. The upper limit of the holding time is not particularly limited, but may be 10000 seconds or less or 9500 seconds or less. Furthermore, in the temperature range of 1100°C or more, the holding temperature may be constant or may be varied.

[0089] The slab to be heated is not particularly limited except that it has the above-mentioned chemical composition. For example, a slab produced by melting molten steel having the above-mentioned chemical composition using a converter or electric furnace, etc. and then by continuous casting can be used. Instead of continuous casting, an ingot casting method, thin slab casting method, etc. may also be used.

[0090] The average value of the top and bottom surface cooling ratios in rough rolling and finish rolling is set to 0.8 to 1.2. By controlling the top and bottom surface cooling ratios in rough rolling and finish rolling, which are performed in a relatively high temperature range in a series of manufacturing methods, the cooling gradient in the thickness direction of the slab or steel plate can be preferably controlled. As a result, the irregularity of the prior austenite grain boundary at the half thickness position can be preferably controlled. In order to preferably control the irregularity of the prior austenite grain boundary throughout the entire thickness in the thickness direction, the average value of the top and bottom surface cooling ratios in rough rolling and finish rolling is preferably set to 0.9 to 1.1.

[0091] The top and bottom surface cooling ratio can be controlled by adjusting the amount of cooling water sprayed onto the top and bottom surfaces of the slab or steel plate between passes of rough rolling or finish rolling. The top and bottom surface cooling ratio is determined by the amount of cooling water (m ３ / m ２ s) is the amount of cooling water (m ３ / m ２ s). One value of this top and bottom surface cooling ratio is obtained for each pass during which cooling with cooling water is performed. The "average value of top and bottom surface cooling ratios" can be obtained by calculating the average value of the top and bottom surface cooling ratios over all passes during which cooling with cooling water is performed. In this embodiment, as described above, the "average value" of the top and bottom surface cooling ratios in rough rolling and finish rolling is set to 0.8 to 1.2, but it is preferable that the top and bottom surface cooling ratio over all passes during which cooling with cooling water is performed is set to 0.8 to 1.2. By setting the top and bottom surface cooling ratios over all passes during which cooling with cooling water is performed within the above range, the irregularity of the prior austenite grain boundary at the 1 / 2 position in the plate thickness can be more preferably controlled.

[0092] Rough Rolling The rough rolling may be performed under any conditions as long as the desired sheet bar dimensions are ensured, except for controlling the top and bottom surface cooling ratio.

[0093] The slab after rough rolling is subjected to finish rolling. The finish rolling is performed using a tandem rolling mill having multiple rolling stands. The reduction ratios in the rolling from the last stage to the last stage and in the final stage of finish rolling are 20 to 50%.

[0094] By performing rolling with a relatively high reduction rate in the rolling one stage before the final stage and the final stage, recrystallization can be promoted and prior austenite grains can be refined. If the reduction rate in the rolling one stage before the final stage and / or the final stage is less than 20%, the irregularity of the prior austenite grain boundaries at the 1 / 2 position in the plate thickness increases. Furthermore, the average grain size of prior austenite grains at the 1 / 4 position in the plate thickness cannot be favorably controlled. Therefore, the reduction rate in each of the rolling one stage before the final stage and the final stage is set to 20% or more. The reduction rate in each of the rolling one stage before the final stage and the final stage is preferably 25% or more or 30% or more. On the other hand, if the reduction rate in the rolling one stage before the final stage and / or the final stage is greater than 50%, the rolling load becomes excessive, increasing the load on equipment such as a rolling mill. Therefore, the reduction rate in each of the rolling one stage before the final stage and the final stage is set to 50% or less. The rolling reduction in the rolling one stage before the final stage and the final stage is preferably 45% or less or 40% or less.

[0095] The rolling one stage before the final stage refers to, for example, the F6 pass when finish rolling is performed in passes F1, F2, ... F6, F7.

[0096] Final Rolling Temperature of Finish Rolling The final rolling temperature of finish rolling is set to a temperature range of 960 to 1100°C. The final rolling temperature of finish rolling refers to the delivery temperature of the final stage of finish rolling. If the final rolling temperature of finish rolling is less than 960°C, the irregularity of the prior austenite grain boundaries at the 1 / 2 thickness position will increase. Furthermore, recrystallization will not be completed or will not be sufficiently promoted, making it impossible to preferably control the average grain size of the prior austenite grains at the 1 / 4 thickness position. Therefore, the final rolling temperature of finish rolling is set to 960°C or higher. The final rolling temperature of finish rolling is preferably 980°C or higher or 1000°C or higher. On the other hand, if the final rolling temperature exceeds 1100°C, the irregularity of the prior austenite grain boundaries at the 1 / 2 thickness position will increase. Therefore, the final rolling temperature of finish rolling is set to 1100°C or lower. The final rolling temperature of finish rolling is preferably 1080°C or lower or 1050°C or lower. In order to preferably control the average grain size of the prior austenite grains at the 1 / 4 position of the plate thickness, it is more preferable that the final rolling temperature of the finish rolling be 1050°C or less.

[0097] Cooling after finish rolling: Cooling is carried out so that the time from the completion of finish rolling to the start of cooling is 0.5 seconds or less, and the time from the start of cooling to reach 400°C is 20.0 seconds or less. Cooling here means water cooling and does not include air cooling at a cooling rate of 1°C / s or less.

[0098] If the time from the completion of finish rolling to the start of cooling exceeds 0.5 seconds, grain growth progresses, and the irregularity of the prior austenite grain boundaries at the half-thickness position increases. Therefore, cooling is started within 0.5 seconds after the completion of finish rolling. The time until the start of cooling is preferably 0.4 seconds or less or 0.3 seconds or less. Since a shorter time until the start of cooling is preferable, the lower limit may be 0.1 seconds or 0.0 seconds. When the time until the start of cooling is 0.0 seconds, cooling water may be sprayed on the outlet side of the final stage of finish rolling.

[0099] If the time from the start of cooling to reaching 400°C exceeds 20.0 seconds, the degree of irregularity of the prior austenite grain boundaries at the half-thickness position increases. Furthermore, the amount of martensite becomes insufficient. Therefore, the time from the start of cooling to reaching 400°C is set to 20.0 seconds or less. The time from the start of cooling to reaching a temperature range of 400°C or less is preferably 15.0 seconds or less, 13.0 seconds or less, or 10.0 seconds or less. In view of limitations of the cooling equipment, the time from the start of cooling to reaching 400°C may be set to 3.0 seconds or more or 5.0 seconds or more.

[0100] Coiling After cooling to a temperature range of 400°C or less, the steel sheet is coiled in this temperature range. If the coiling temperature exceeds 400°C, the irregularity of the prior austenite grain boundaries at the half-thickness position increases. Furthermore, the amount of martensite becomes insufficient, resulting in a decrease in the strength of the steel sheet. Therefore, the coiling temperature is set to 400°C or less. The coiling temperature is preferably 200°C or less, 100°C or less, or 50°C or less. The lower limit of the coiling temperature is not particularly limited, but may be 5°C or more, or 10°C or more.

[0101] 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.

[0102] Steels having the chemical compositions shown in Table 1 were melted and continuously cast into slabs with thicknesses of 240 to 300 mm. The resulting slabs were used to obtain the steel plates shown in Tables 3A and 3B under the production conditions shown in Tables 2A and 2B. In Nos. 7, 15, 16, and 23, the top and bottom cooling ratios were set to 0.8 to 1.2 throughout all passes where cooling with cooling water was performed.

[0103] The heating temperature for the slab heating was in the range of 1100 to 1200° C. Also, underlines in the table indicate that the property value is outside the range of the present disclosure or is not preferable.

[0104] The obtained steel sheets were measured for the following: prior austenite grain boundary irregularity at the 1 / 2 thickness position, the average grain size and area ratio of each structure of the prior austenite grains at the 1 / 4 thickness position, prior austenite grain boundary irregularity across the entire thickness (1 / 8 thickness position, 1 / 4 thickness position, 1 / 2 thickness position, 3 / 4 thickness position, and 7 / 8 thickness position), tensile strength, and arithmetic mean roughness Ra after 10% tension and 10% compression. The metallographic structure at the 1 / 4 thickness position of the steel sheet contained, in addition to martensite and retained austenite, ferrite: 10.0% or less, bainite: 10.0% or less, and pearlite: 10.0% or less as the remaining structure. Furthermore, the arithmetic mean roughness of the surface of the steel sheet before 10% tension and 10% compression was less than 1.5 μm. The arithmetic mean roughness here was measured by the same method as above, except that the test specimen was used before the tensile strain and compressive strain were applied.

[0105] If the arithmetic mean roughness Ra after 10% tension and 10% compression was 7.0 μm or less, the specimen was judged to have excellent fatigue properties and pass. On the other hand, if the arithmetic mean roughness Ra after 10% tension and 10% compression was more than 7.0 μm, the specimen was judged to have poor fatigue properties and fail. Furthermore, if the arithmetic mean roughness Ra after 10% tension and 10% compression was 5.0 μm or less, the specimen was judged to have even better fatigue properties.

[0106] When the tensile strength was 980 MPa or more, it was determined that the strength was high.

[0107]

[0108]

[0109]

[0110]

[0111]

[0112] From Tables 2A to 3B, it can be seen that the steel sheets according to the present invention have excellent fatigue properties after forming. On the other hand, it can be seen that the steel sheets according to the comparative examples do not have excellent fatigue properties after forming. In addition, for all examples, lower arms (components) were manufactured by press working. The flat portion of the lower arm was evaluated in the same manner as described above. The measurement results and evaluation results were the same as those shown in Tables 3A and 3B.

[0113] According to the above aspects of the present disclosure, it is possible to provide a steel sheet having a small surface roughness after strain is applied by forming into a product or by applying repeated stress equivalent to the initial stage of a fatigue test, a part manufactured using the same, and a method for manufacturing the steel sheet. According to a preferred aspect of the present disclosure, it is possible to provide a steel sheet having the above properties and high strength, and a part manufactured using the same.

Claims

1. The chemical composition is, in mass %, C: 0.020 to 0.300%, Si: 0 to 3.00%, Mn: 0.10 to 4.00%, sol. Al: 0 to 2.000%, P: 0.100% or less, S: 0.0300% or less, N: 0.0200% or less, O: 0.0100% or less, Nb: 0 to 0.200%, Ti: 0 to 0.200%, V: 0 to 2.000%, Cu: 0 to 2.00%, Cr: 0-2.00%, Mo: 0-2.00%, Ni: 0-2.00%, B: 0-0.0100%, Sb: 0-0.100%, Ca: 0-0.0500%, Mg: 0-0.0500%, Bi: 0-0.010%, Zr: 0 to 0.500%, Co: 0 to 0.010%, Zn: 0 to 0.010%, W: 0 to 0.100%, Sn: 0 to 0.050%, As: 0 to 0.100%, REM: 0 to 0.0100%, and the balance: Fe and impurities, wherein in a metallographic structure at a position halfway through the plate thickness from the surface in the plate thickness direction, when the total length of the grain boundaries of prior austenite grains is Lt (unit: μm), and the total perimeter of the prior austenite grains approximated as an ellipse is Le (unit: μm), the degree of irregularity of the grain boundaries of the prior austenite grains, as expressed by the following formula (1), is 3.20 or less: 2×(Lt / Le) (1) 2. The steel plate according to claim 1, characterized in that in the metal structure at a position 1 / 4 of the plate thickness from the surface in the plate thickness direction, the average grain size of prior austenite grains is 30.0 μm or less.

3. The steel plate according to claim 1, characterized in that the metal structure at a position 1 / 4 of the plate thickness from the surface in the plate thickness direction contains, in area percentages, martensite: 90.0% or more and retained austenite: 0.0 to 3.0%.

4. The steel plate according to claim 2, characterized in that the metal structure at a position 1 / 4 of the plate thickness from the surface in the plate thickness direction contains, in area percentages, martensite: 90.0% or more and retained austenite: 0.0 to 3.0%.

5. A steel plate according to any one of claims 1 to 4, characterized in that in the metal structure at all positions from the surface in the plate thickness direction at 1 / 8, 1 / 4, 3 / 4 and 7 / 8 of the plate thickness, the irregularity degree of the grain boundaries of the prior austenite grains is 3.20 or less.

6. The chemical composition is, in mass%, Nb: 0.001 to 0.200%, Ti: 0.001 to 0.200%, V: 0.001 to 2.000%, Cu: 0.01 to 2.00%, Cr: 0.01 to 2.00%, Mo: 0.01 to 2.00%, Ni: 0.01 to 2.00%, B: 0.0001 to 0.0100%, Sb: 0.001 to 0.100%, Ca: 0.0001 to 0.0500%, Mg: 0.0001 to 0.0500%, Bi: 0.001 to 0.010%, Zr: 0.001 to 0.500%, The steel sheet according to any one of claims 1 to 4, characterized in that it contains one or more selected from the group consisting of Co: 0.001 to 0.010%, Zn: 0.001 to 0.010%, W: 0.001 to 0.100%, Sn: 0.001 to 0.050%, As: 0.001 to 0.100%, and REM: 0.0001 to 0.0100%.

7. The chemical composition is, in mass%, Nb: 0.001 to 0.200%, Ti: 0.001 to 0.200%, V: 0.001 to 2.000%, Cu: 0.01 to 2.00%, Cr: 0.01 to 2.00%, Mo: 0.01 to 2.00%, Ni: 0.01 to 2.00%, B: 0.0001 to 0.0100%, Sb: 0.001 to 0.100%, Ca: 0.0001 to 0.0500%, Mg: 0.0001 to 0.0500%, Bi: 0.001 to 0.010%, Zr: 0.001 to 0.500%, The steel sheet according to claim 5, further comprising at least one selected from the group consisting of Co: 0.001 to 0.010%, Zn: 0.001 to 0.010%, W: 0.001 to 0.100%, Sn: 0.001 to 0.050%, As: 0.001 to 0.100%, and REM: 0.0001 to 0.0100%.

8. The steel plate according to any one of claims 1 to 4, characterized in that the plate thickness is 1.0 to 8.0 mm.

9. The steel plate according to claim 5, characterized in that the plate thickness is 1.0 to 8.0 mm.

10. The steel plate according to claim 6, characterized in that the plate thickness is 1.0 to 8.0 mm.

11. The steel plate according to claim 7, characterized in that the plate thickness is 1.0 to 8.0 mm.

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

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

14. A component comprising the steel sheet according to claim 6.

15. A component comprising the steel sheet according to claim 7.

16. A component comprising the steel sheet according to claim 8.

17. A component comprising the steel sheet according to claim 9.

18. A component comprising the steel sheet according to claim 10.

19. A component comprising the steel sheet according to claim 11.

20. A method for manufacturing a steel sheet, characterized in that the heating temperature of a slab having the chemical composition defined in claim 1 is in a temperature range of 1100°C or higher, the holding time in said temperature range is 6000 seconds or longer, the average top and bottom surface cooling ratios in rough rolling and finish rolling are 0.8 to 1.2, and in said finish rolling, the rolling reductions in the rolling one stage before the final stage and the final stage rolling are 20 to 50%, the final rolling temperature is in a temperature range of 960 to 1100°C, cooling is performed so that the time from the completion of said finish rolling to the start of cooling is 0.5 seconds or less and the time from the start of cooling to reach a temperature range of 400°C or lower is 20.0 seconds or less, and the coiling temperature is 400°C or lower.

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