Steel sheet and component

Abstract

This steel sheet has a predetermined chemical composition, and has a metallographic structure, at a position 1 / 4 of the sheet thickness, comprising, in terms of area%, granular bainite at 30-50%, acicular bainite at 20-50%, at least one selected from fresh martensite, tempered martensite, and retained austenite at 10-30% in total, and at least one selected from ferrite and pearlite at less than 5% in total. The metallographic structure in a surface region of the steel sheet comprises, in terms of area%, at least one selected from fresh martensite, tempered martensite, and retained austenite at 10% or less in total. MA phases, which are composed of the fresh martensite and the retained austenite, and the tempered martensite each have an equivalent circle diameter of 5.0 μm or less.

Description

Steel plates and parts

[0001] This application claims priority to Japanese Patent Application No. 2024-123619, filed on July 30, 2024, the contents of which are incorporated herein by reference.

[0002] In recent years, CO ２ To reduce emissions, automobile bodies are being made lighter. For blank-formed parts such as press-formed parts, weight can be reduced by reducing the plate thickness of the part material. In particular, for automobile suspension parts such as lower arms and trailing arms, the use of steel plates with a modulus of over 980 MPa is being considered in order to achieve weight reduction in automobile bodies.

[0003] The above-mentioned parts have complex shapes. As the strength of steel sheets increases, their formability decreases. Therefore, when high-strength steel sheets are used for such parts, necking or fracture may occur in the parts due to insufficient formability. Therefore, steel sheets used for such parts are required to have excellent formability, particularly excellent ductility and hole expandability.

[0004] Furthermore, as the strength of steel sheets increases, their ductility after pre-straining decreases. As a result, the impact resistance of parts that have been processed and pre-strained decreases. Therefore, steel sheets used for such parts are required to have excellent impact resistance after pre-straining.

[0005] Patent Document 1 discloses a high-strength hot-rolled steel sheet having a structure in which a bainite phase accounts for 85% or more of the area of ​​the main phase, a martensite phase or a martensite-austenite mixed phase accounts for 15% or less of the area of ​​the second phase, and the remainder is a ferrite phase, the second phase having an average grain size of 3.0 μm or less, the prior austenite grains having an average aspect ratio of 1.3 to 5.0, the area ratio of recrystallized prior austenite grains to unrecrystallized prior austenite grains being 15% or less, the heat-rolled steel sheet containing 0.10% or less by mass of precipitates having a diameter of less than 20 nm, and the tensile strength TS of 980 MPa or more. Patent Document 1 discloses that the above configuration allows for the production of a high-strength hot-rolled steel sheet having a tensile strength TS of 980 MPa or more and excellent punchability and hole expandability.

[0006] International Publication No. 2017 / 017933

[0007] However, it is necessary to further improve the workability of the hot-rolled steel sheet disclosed in Patent Document 1. Furthermore, Patent Document 1 does not take into consideration the impact resistance properties after pre-straining.

[0008] The present disclosure has been made in view of the above circumstances, and an object of the present disclosure is to provide a steel sheet having high strength, excellent ductility and hole expandability, and excellent impact resistance after pre-straining, and a part using the steel sheet.

[0009] The gist of the present disclosure is as follows. [1] Chemical composition, in mass%, C: 0.040 to 0.180%, Si: 0.20 to 2.00%, Mn: 1.00 to 3.00%, P: 0.0150% or less, S: 0.0100% or less, Al: 0.100% or less, N: 0.0100% or less, O: 0.0060% or less, Ti: 0.020 to 0.150%, Cr: 0.50 to 1.00%, B: 0.00003% or more, less than 0.00150%, Mo: 0 to 0.500%, W: 0 to 0.500%, Co: 0 to 0.500%, Ni: 0 to 1.000%, Cu: 0 to 1.000%, V : 0 to 0.500%, Nb: 0 to 0.150%, As: 0 to 0.050%, Zr: 0 to 0.050%, Sn: 0 to 0.050%, Sb: 0 to 0.050%, Ta: 0 to 0.100%, Bi: 0 to 0.0400%, Ca: 0 to 0.0400%, Mg: 0 to 0.1000%, REM: 0 to 0.1000%, and the balance: Fe and impurities, and at a position of 1 / 4 of the plate thickness from the surface in the plate thickness direction, the metallographic structure is, in area %, granular bainite: 30 to 50%, acicular bainite: 20 to 50%, one or more of fresh martensite, tempered martensite, and retained austenite: 10 to 30% in total, a metal structure in a surface layer region that is a region from the surface to a position 120 μm from the surface in the plate thickness direction, in terms of area percentage, of one or more of fresh martensite, tempered martensite, and retained austenite: a total of 10.0% or less, and an MA phase consisting of the fresh martensite and the retained austenite, and the tempered martensite have an average circle equivalent diameter of 5.0 μm or less.[2] The chemical composition is, in mass%, Mo: 0.001 to 0.500%, W: 0.001 to 0.500%, Co: 0.001 to 0.500%, Ni: 0.001 to 1.000%, Cu: 0.001 to 1.000%, V: 0.001 to 0.500%, Nb: 0.001 to 0.150%, As: 0.001 to 0.050%, Zr: 0.001 to 0.050%, Sn: 0.001 to 0.050%, Sb: 0.001 to 0.050%, Ta: 0.001 to 0.100%, Bi: 0.0001 to 0.0400%, [1] The steel sheet according to [1], characterized in that it contains one or more elements selected from the group consisting of Ca: 0.0001 to 0.0400%, Mg: 0.0001 to 0.1000%, and REM: 0.0001 to 0.1000%. [3] The steel sheet according to [1] or [2], characterized in that in the metallographic structure at a position 1 / 4 of the plate thickness from the surface in the plate thickness direction, the average aspect ratio of prior austenite grains is 3.0 or more. [4] The steel sheet according to any one of [1] to [3], characterized in that in the metallographic structure at a position 1 / 4 of the plate thickness from the surface in the plate thickness direction, the average aspect ratio of prior austenite grains is 6.0 or less. [5] A part characterized in that it is made of the steel sheet according to any one of [1] to [4].

[0010] According to the above aspects of the present disclosure, it is possible to provide a steel plate having high strength, excellent ductility and hole expandability, and excellent impact resistance after pre-straining, and a part using this steel plate.

[0011] 1 is a diagram for explaining a method for approximating prior austenite grains to ellipsoids, and FIG. 2 is a diagram for explaining a drop weight test.

[0012] As a result of investigations conducted by the present inventors to solve the above-mentioned problems, the present inventors have discovered the following: In the metallographic structure in the ¼ position of the sheet thickness from the surface of the steel sheet in the sheet thickness direction and in the surface layer region, the present inventors have discovered that the impact resistance after pre-strain can be improved by controlling the total area ratio of one or more of fresh martensite, tempered martensite, and retained austenite, and the average value of the circle equivalent diameter of the fresh martensite, tempered martensite, and retained austenite in the surface layer region.

[0013] Specifically, it was found that by reducing the area ratio and the circle equivalent diameter of the phase consisting of one or more of fresh martensite, tempered martensite, and retained austenite in the surface layer region, it is possible to suppress the occurrence of cracks in the thickness direction of the plate, and as a result, it is possible to improve the impact resistance properties after pre-straining.

[0014] 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.

[0015] 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 %.

[0016] The steel sheet according to this embodiment contains 0.040 to 0.180% C, 0.20 to 2.00% Si, 1.00 to 3.00% Mn, 0.0150% or less P, 0.0100% or less S, 0.100% or less Al, 0.0100% or less N, 0.0060% or less O, 0.020 to 0.150% Ti, 0.50 to 1.00% Cr, 0.00003% or more but less than 0.00150% B, and the balance being Fe and impurities. Each element will be described in detail below.

[0017] C: 0.040 to 0.180% C is an element that increases the strength of the steel sheet. If the C content is less than 0.040%, the strength of the steel sheet decreases. Therefore, the C content is set to 0.040% or more. The C content is preferably 0.050% or more, 0.060% or more, 0.070% or more, or 0.080% or more. On the other hand, if the C content exceeds 0.180%, the hole expandability of the steel sheet decreases. Therefore, the C content is set to 0.180% or less. The C content is preferably 0.160% or less, 0.150% or less, 0.120% or less, or 0.100% or less.

[0018] Si: 0.20 to 2.00% Si is an element that suppresses the formation of iron carbides and improves the strength, ductility, and hole expandability of steel sheet. If the Si content is less than 0.20%, these effects cannot be obtained. Therefore, the Si content is set to 0.20% or more. The Si content is preferably 0.50% or more, 0.60% or more, 0.70% or more, or 0.80% or more. On the other hand, if the Si content exceeds 2.00%, the amount of ferrite increases, and the hole expandability of the steel sheet decreases. Therefore, the Si content is set to 2.00% or less. The Si content is preferably 1.60% or less, 1.30% or less, or 1.00% or less.

[0019] Mn: 1.00 to 3.00% Mn is an element that increases the strength of a steel sheet by improving hardenability and solid solution strengthening. If the Mn content is less than 1.00%, the strength of the steel sheet decreases. Therefore, the Mn content is set to 1.00% or more. The Mn content is preferably 1.10% or more, 1.30% or more, 1.50% or more, or 1.70% or more. On the other hand, if the Mn content exceeds 3.00%, the amount of granular bainite becomes insufficient, and the ductility of the steel sheet decreases. Therefore, the Mn content is set to 3.00% or less. The Mn content is preferably 2.70% or less, 2.60% or less, 2.40% or less, or 2.00% or less.

[0020] P: 0.0150% or less P is an element that segregates at grain boundaries and thereby reduces the ductility and hole expandability of steel sheets. If the P content exceeds 0.0150%, the ductility of the steel sheets is significantly reduced. Therefore, the P content is set to 0.0150% or less. The P content is preferably 0.0130% or less, 0.0100% or less, 0.0080% or less, or 0.0070% or less. Since a lower P content is more preferable, it may be 0%. However, if the P content is reduced excessively, the cost of dephosphorization increases significantly. Therefore, the P content may be set to 0.0001% or more or 0.0005% or more.

[0021] S: 0.0100% or less S is an element that forms sulfides such as MnS, thereby reducing the ductility and hole expandability of the steel sheet. If the S content exceeds 0.0100%, the ductility and hole expandability of the steel sheet will be significantly reduced. Therefore, the S content is set to 0.0100% or less. The S content is preferably 0.0080% or less, 0.0060% or less, or 0.0040% or less. Since a lower S content is more preferable, it may be 0%. However, if the S content is reduced excessively, the desulfurization cost will increase significantly. Therefore, the S content may be set to 0.0001% or more, 0.0005% or more, or 0.0010% or more.

[0022] Al: 0.100% or less Al is an element contained as a deoxidizer for molten steel. If the Al content exceeds 0.100%, the amount of ferrite increases, and the hole expandability of the steel sheet decreases. Therefore, the Al content is set to 0.100% or less. The Al content is preferably 0.080% or less, 0.050% or less, or 0.035% or less. Al is also an element effective in increasing the area fraction of granular bainite. From the viewpoint of further increasing the area fraction of granular bainite, the Al content is preferably 0.001% or more, 0.005% or more, 0.010% or more, or 0.015% or more.

[0023] N: 0.0100% or less N is an element that forms coarse nitrides in steel and reduces the hole expandability of steel sheets. If the N content exceeds 0.0100%, the hole expandability of steel sheets is significantly reduced. Furthermore, if a large amount of N is contained, the risk of slab cracking increases. Therefore, the N content is set to 0.0100% or less. The N content is preferably 0.0070% or less or 0.0050% or less. Since a lower N content is more preferable, it may be 0%. However, excessive reduction of the N content significantly increases the cost of denitrification. Therefore, the N content may be set to 0.0001% or more, 0.0005% or more, or 0.0010% or more.

[0024] O: 0.0060% or less When a large amount of O is contained in steel, it forms coarse oxides. If the O content exceeds 0.0060%, the hole expandability of the steel sheet is significantly reduced. Therefore, the O content is set to 0.0060% or less. The O content is preferably 0.0040% or less or 0.0020% 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.

[0025] Ti: 0.020 to 0.150% Ti precipitates in steel as Ti carbides such as TiC, enhancing the strength of the steel sheet through precipitation strengthening. Furthermore, Ti also enhances the hole expandability of the steel sheet by reducing the hardness difference between phases in the metal structure due to precipitation strengthening. If the Ti content is less than 0.020%, these effects cannot be achieved. Therefore, the Ti content is set to 0.020% or more. The Ti content is preferably 0.030% or more, 0.040% or more, or 0.060% or more. On the other hand, if the Ti content exceeds 0.150%, coarse carbides are formed in the steel, which can cause slab cracking during hot rolling or reduce the hole expandability of the steel sheet. Therefore, the Ti content is set to 0.150% or less. The Ti content is preferably 0.120% or less, 0.110% or less, or 0.100% or less.

[0026] Cr: 0.50 to 1.00% Cr is an element that promotes the formation of granular bainite. If the Cr content is less than 0.50%, the desired amount of granular bainite cannot be obtained, and the ductility of the steel sheet decreases. Therefore, the Cr content is set to 0.50% or more. The Cr content is preferably 0.55% or more or 0.60% or more. On the other hand, if the Cr content exceeds 1.00%, the hardenability becomes excessive and the ductility of the steel sheet decreases. Therefore, the Cr content is set to 1.00% or less. The Cr content is preferably 0.90% or less or 0.80% or less.

[0027] B: 0.00003% or more, less than 0.00150% B is an element that improves the hardenability of steel and increases the strength of steel sheet. If the B content is less than 0.00003%, the strength of the steel sheet decreases. Therefore, the B content is set to 0.00003% or more. The B content is preferably 0.00010% or more, 0.00050% or more, or 0.00100% or more. If the B content is 0.00150% or more, a large amount of precipitates containing B is formed, which reduces the impact resistance properties after pre-straining. Therefore, the B content is set to less than 0.00150%. The B content is preferably 0.00140% or less, or 0.00130% or less.

[0028] 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.

[0029] 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.

[0030] Mo: 0 to 0.500% 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 0.001% or more or 0.010% or more. On the other hand, if the Mo content exceeds 0.500%, the hole expandability of the steel sheet decreases. Therefore, the Mo content is set to 0.500% or less. The Mo content is preferably 0.400% or less, 0.320% or less, 0.250% or less, or 0.210% or less.

[0031] W: 0 to 0.500% 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 or 0.010% or more. On the other hand, if the W content exceeds 0.500%, the hole expandability of the steel sheet decreases. Therefore, the W content is set to 0.500% or less. The W content is preferably 0.450% or less, 0.400% or less, or 0.300% or less.

[0032] Co: 0 to 0.500% 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 or 0.010% or more. On the other hand, if the Co content exceeds 0.500%, the hole expandability of the steel sheet decreases. Therefore, the Co content is set to 0.500% or less. The Co content is preferably 0.400% or less, 0.320% or less, 0.250% or less, or 0.200% or less.

[0033] Ni: 0 to 1.000% Ni is an element that improves the hardenability of steel sheet and increases its strength. Furthermore, when Cu is contained, Ni has the effect of effectively suppressing intergranular cracking of slabs caused by Cu. To more reliably obtain the above-mentioned effect, the Ni content is preferably 0.001% or more or 0.010% 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 1.000% or less. The Ni content is preferably 0.800% or less, 0.700% or less, 0.500% or less, 0.250% or less, or 0.210% or less.

[0034] Cu: 0 to 1.000% Cu is an element that improves the hardenability of steel sheets and precipitates as carbides in steel at low temperatures to increase the strength of the steel sheets. To more reliably obtain the effects of the above-mentioned functions, the Cu content is preferably 0.001% or more or 0.010% or more. On the other hand, if the Cu content exceeds 1.000%, intergranular cracking of the slab may occur. Therefore, the Cu content is set to 1.000% or less. The Cu content is preferably 0.800% or less, 0.700% or less, 0.500% or less, 0.300% or less, or 0.250% or less.

[0035] V: 0 to 0.500% 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 or 0.010% or more. On the other hand, if the V content exceeds 0.500%, the hole expandability of the steel sheet decreases. Therefore, the V content is set to 0.500% or less. The V content is preferably 0.400% or less, 0.250% or less, 0.100% or less, or 0.070% or less.

[0036] Nb: 0 to 0.150% Nb is an element that increases the strength of steel sheet by refining the metal structure and strengthening the precipitation of NbC. To reliably obtain this effect, the Nb content is preferably 0.001% or more or 0.010% or more. On the other hand, if the Nb content exceeds 0.150%, the above effect saturates. In addition, the hole expandability of the steel sheet decreases. Therefore, the Nb content is set to 0.150% or less. The Nb content is preferably 0.100% or less, 0.080% or less, 0.050% or less, or 0.030% or less.

[0037] As: 0 to 0.050% As is an element that reduces the austenite single-phase temperature, thereby refining prior austenite grains and improving the hole expandability of steel sheets. To more reliably obtain this effect, the As content is preferably 0.001% or more or 0.010% or more. However, since the above effect saturates even when a large amount of As is contained, the As content is set to 0.050% or less. The As content is preferably 0.020% or less or 0.015% or less.

[0038] Zr: 0 to 0.050% Zr is an element that increases the strength of steel sheet through solid solution strengthening. To more reliably obtain this effect, the Zr content is preferably 0.001% or more or 0.010% or more. On the other hand, if the Zr content exceeds 0.050%, the hole expandability of the steel sheet decreases. Therefore, the Zr content is set to 0.050% or less. The Zr content is preferably 0.045% or less or 0.040% or less.

[0039] 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 ensure this effect, the Sn content is preferably 0.001% or more or 0.010% or more. However, since the above effect saturates even when a large amount of Sn is added, the Sn content is set to 0.050% or less. The Sn content is preferably 0.045% or less or 0.040% or less.

[0040] Sb: 0 to 0.050% 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 this effect, the Sb content is preferably 0.001% or more or 0.010% or more. On the other hand, even if a large amount of Sb is added, the above effect saturates, so the Sb content is set to 0.050% or less. The Sb content is preferably 0.010% or less or 0.005% or less.

[0041] Ta: 0 to 0.100% Ta is an element that increases the strength of steel sheet by forming fine carbides in steel. To reliably obtain this effect, the Ta content is preferably 0.001% or more or 0.010% or more. On the other hand, if the Ta content exceeds 0.100%, the ductility and hole expandability of the steel sheet decrease. Therefore, the Ta content is set to 0.100% or less. The Ta content is preferably 0.080% or less or 0.050% or less. The Ta content is preferably 0.025% or less or 0.020% or less.

[0042] Bi: 0 to 0.0400% Bi is an element that refines the solidification structure, thereby improving the ductility and hole expandability of steel sheets. To ensure this effect, the Bi content is preferably 0.0001% or more or 0.0010% or more. On the other hand, even if the Bi content exceeds 0.020%, the effect of the above action saturates, which is economically undesirable. Therefore, the Bi content is set to 0.0400% or less. The Bi content is preferably 0.0150% or less or 0.0110% or less.

[0043] Ca: 0 to 0.0400% Ca is an element that enhances the ductility and hole expandability of steel sheets by controlling the morphology of non-metallic inclusions that become fracture initiation sites and cause a decrease in the ductility and hole expandability of steel sheets. To more reliably obtain this effect, the Ca content is preferably 0.0001% or more or 0.0010% or more. On the other hand, if the Ca content exceeds 0.0400%, excessive inclusions are generated in the steel, resulting in a decrease in the ductility and hole expandability of the steel sheets. Therefore, the Ca content is set to 0.0400% or less. The Ca content is preferably 0.0200% or less or 0.0100% or less.

[0044] Mg: 0 to 0.1000% 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 ensure this effect, the Mg content is preferably 0.0001% or more or 0.0010% or more. On the other hand, if the Mg content exceeds 0.1000%, excessive inclusions are generated in the steel, reducing the ductility and hole expandability of the steel sheet. Therefore, the Mg content is set to 0.1000% or less. The Mg content is preferably 0.0500% or less or 0.0300% or less.

[0045] REM: 0 to 0.1000% 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 ensure this effect, the REM content is preferably 0.0001% 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 ductility and hole expandability of the steel sheet. Therefore, the REM content is set to 0.1000% or less. 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. The REM content is preferably 0.0950% or less, 0.0900% or less, 0.0500% or less, or 0.0100% or less.

[0046] The chemical composition of the above-mentioned steel plate can be determined by the following method. A test piece is taken from a region extending from the surface of the steel plate to a position ⅛ of the plate thickness to a position ⅜ of the plate thickness in the plate thickness direction, and the chemical composition of this test piece 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. 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. If the molten steel analysis values, slab analysis values, or steel plate analysis values ​​of other steel plates produced from the same molten steel can be confirmed, the analysis of the test piece taken from the steel plate may be omitted, and these analysis values ​​may be considered to be the chemical composition of the steel plate.

[0047] Next, the metallographic structure of the steel sheet according to this embodiment will be described. In the steel sheet according to this embodiment, at a position 1 / 4 of the sheet thickness from the surface in the sheet thickness direction, the metallographic structure is, in area percentages, 30 to 50% granular bainite, 20 to 50% acicular bainite, 10 to 30% in total of one or more of fresh martensite, tempered martensite, and retained austenite, and less than 5% in total, and in a surface layer region that is a region from the surface to a position 120 μm from the surface in the sheet thickness direction, the metallographic structure is, in area percentages, 10% or less in total of one or more of fresh martensite, tempered martensite, and retained austenite, and the MA phase consisting of the fresh martensite and the retained austenite, and the tempered martensite have an average equivalent circle diameter of 5.0 μm or less.

[0048] In this embodiment, the metallographic structure of a 1 / 4 position of the plate thickness from the surface in the plate thickness direction (hereinafter, sometimes referred to as the 1 / 4 position of the plate thickness) and a surface layer region is defined. The 1 / 4 position of the plate thickness refers to 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 of the steel plate in the plate thickness direction, and can be rephrased as a range starting from a position of 1 / 8 of the plate thickness from the surface of the steel plate and ending at a position of 3 / 8 of the plate thickness. The surface layer region can be rephrased as a range starting from the surface of the steel plate and ending at a position 120 μm from the surface of the steel plate in the plate thickness direction.

[0049] 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. The interface between the coating and the steel sheet is determined by a BSE COMPO image (BSE Compositional Image) described below.

[0050] <1 / 4 position in plate thickness> Granular bainite: 30 to 50% Granular bainite increases the ductility of the steel plate. If the area fraction of granular bainite at the 1 / 4 position in plate thickness is less than 30%, the ductility of the steel plate decreases. Therefore, the area fraction of granular bainite at the 1 / 4 position in plate thickness is set to 30% or more. The area fraction of granular bainite is preferably 33% or more or 35% or more. On the other hand, if the area fraction of granular bainite at the 1 / 4 position in plate thickness is more than 50%, the desired amounts of acicular bainite, fresh martensite, tempered martensite, and retained austenite cannot be obtained, and the strength of the steel plate decreases. Therefore, the area fraction of granular bainite at the 1 / 4 position in plate thickness is set to 50% or less. The area fraction of granular bainite is preferably 47% or less or 45% or less.

[0051] Acicular Bainite: 20 to 50% Acicular bainite increases the strength of the steel plate. If the area fraction of acicular bainite at the 1 / 4 plate thickness position is less than 20%, the strength of the steel plate decreases. Therefore, the area fraction of acicular bainite at the 1 / 4 plate thickness position is set to 20% or more. The area fraction of acicular bainite is preferably 25% or more, 30% or more, or 35% or more. On the other hand, if the area fraction of acicular bainite at the 1 / 4 plate thickness position exceeds 50%, the ductility of the steel plate decreases. Therefore, the area fraction of acicular bainite at the 1 / 4 plate thickness position is set to 50% or less. The area fraction of acicular bainite is preferably 47% or less, 45% or less, or 40% or less. Note that the area fractions of granular bainite and acicular bainite at the 1 / 4 plate thickness position may be a total of 65 to 90%. The total area ratio of granular bainite and acicular bainite may be 70% or more, or 72% or more, or may be 89% or less, 88% or less, 83% or less, or 80% or less.

[0052] One or more of fresh martensite, tempered martensite, and retained austenite: 10 to 30% in total. Fresh martensite, tempered martensite, and retained austenite at the 1 / 4 thickness position improve impact resistance properties after pre-straining. If the total area fraction of fresh martensite, tempered martensite, and retained austenite at the 1 / 4 thickness position is less than 10%, impact resistance properties after pre-straining will decrease. Therefore, the total area fraction of fresh martensite, tempered martensite, and retained austenite at the 1 / 4 thickness position is set to 10% or more. The total area fraction of fresh martensite, tempered martensite, and retained austenite is preferably 15% or more, 18% or more, or 20% or more. Note that it is not necessary for all of fresh martensite, tempered martensite, and retained austenite to be present. If any one of them is present, it is sufficient that the area fraction of that one is 10% or more. On the other hand, if the total area ratio of fresh martensite, tempered martensite, and retained austenite exceeds 30%, the hole expandability of the steel sheet decreases. Therefore, the total area ratio of fresh martensite, tempered martensite, and retained austenite is set to 30% or less. The total area ratio of fresh martensite, tempered martensite, and retained austenite may be 27% or less, 25% or less, 23% or less, 20% or less, or 17% or less.

[0053] One or more of ferrite and pearlite: less than 5% in total If ferrite and pearlite are contained in excess, the strength of the steel sheet will decrease. If the total area fraction of ferrite and pearlite is 5% or more, the strength of the steel sheet will decrease significantly. Therefore, the total area fraction of ferrite and pearlite is set to less than 5%. The smaller the total area fraction of ferrite and pearlite, the better, so it is preferably 4% or less, 3% or less, 2% or less, or 1% or less. The total area fraction of ferrite and pearlite may be 0%.

[0054] The area ratio of the metallographic structure is measured by the following method. First, a method for measuring the area ratios of tempered martensite, fresh martensite, and retained austenite will be described. A test piece is taken from the steel plate so that the metallographic structure can be observed at the 1 / 4 position in the plate thickness direction (in the range from the surface to the 1 / 8 position in the plate thickness direction to the 3 / 8 position in the plate thickness direction). The plate thickness cross section of the test piece is mirror-polished and etched with LePera. Then, a 200 μm (plate thickness direction) × 600 μm (direction perpendicular to the plate thickness direction) region at the 1 / 4 position in the plate thickness direction is observed using a FE-SEM (Field Emission-Scanning Electron Microscope): a thermal field emission scanning electron microscope (JSM-7200F manufactured by JEOL), and image analysis is performed.

[0055] In the case of Répéra corrosion, tempered martensite, fresh martensite, and retained austenite are not corroded. Therefore, by calculating the area ratio of the non-corroded region, the total area ratio of tempered martensite, fresh martensite, and retained austenite is obtained. In addition, in order to observe the same region in the area ratio measurement (excluding X-ray diffraction) described below, it is preferable to make Vickers indentations at three of the four corners of the observation region with the FE-SEM within 100 μm of each corner. By using these Vickers indentations as markers, it is possible to observe the same region as the observation region with the FE-SEM.

[0056] 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.

[0057] The total area fraction of fresh martensite and tempered martensite is obtained by subtracting the area fraction of retained austenite obtained by X-ray diffraction from the total area fraction of "fresh martensite, tempered martensite, and retained austenite" obtained by observation using the FE-SEM. If the total area fraction of fresh martensite and tempered martensite is calculated to be a negative value, the total area fraction of fresh martensite and tempered martensite is set to 0%.

[0058] The area ratio of pearlite is obtained by the following method: for the same region (200 μm × 600 μm) as that used to determine the area ratios of fresh martensite, tempered martensite, and retained austenite by FE-SEM observation, only the corroded layer is removed by polishing, followed by mirror finishing, and then etching with a nital solution, followed by observation with FE-SEM, and image analysis is performed.

[0059] The area where cementite and ferrite are arranged in a lamellar shape is determined as pearlite, and the area ratio of this area is calculated to obtain the area ratio of pearlite.

[0060] The area fractions of ferrite, granular bainite, and acicular bainite are obtained by the following method. The following procedure is performed on regions other than those identified as pearlite by the above method. The same region (200 μm × 600 μm) as that used to determine the area fractions of fresh martensite, tempered martensite, and retained austenite by FE-SEM observation is subjected to colloidal polishing or electrolytic polishing, and then crystal orientation information is obtained by electron backscatter diffraction at a measurement interval of 0.2 μm. 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 for the measurement. The degree of vacuum in the apparatus was 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.

[0061] The following analysis is performed using the obtained crystal orientation information of the BCC crystal structure using version 7 or later of OIM Analysis (registered trademark) manufactured by EDAX / TSL Solution. The measurement points between which the crystal orientation difference is 15° or more are considered to be crystal grain boundaries, and the areas surrounded by the crystal grain boundaries are considered to be crystal grains. Next, the difference in crystal orientation between all measurement points present within the crystal grain is calculated, and the average value of this difference is calculated to obtain the GAM value (Grain Average Misorientation value) of the crystal grain. Crystal grains of the BCC crystal structure with a GAM value of 0.5° or less are considered to be ferrite, and the area ratio of ferrite is calculated by calculating the ratio (the ratio of all measured areas, including those other than the BCC crystal structure, as the denominator and the area of ​​ferrite as the numerator) to obtain the area ratio of ferrite.

[0062] Next, for crystal grains with a GAM value of more than 0.5° (crystal grains with a BCC crystal structure other than crystal grains determined to be ferrite), the boundaries with a crystal misorientation of more than 5° are displayed. The density of boundaries within a crystal grain with a crystal misorientation of more than 5° (the length of grain boundaries per unit area with a crystal misorientation of more than 5°) is calculated to obtain the 5° boundary density of that crystal grain. When the 5° boundary density is 0.4 μm / μm ２ The crystal grains with a 5° boundary density of 0.4 μm / μm or less are judged to be granular bainite, and the area ratio of the granular bainite is calculated by calculating the area ratio (the ratio of the area of ​​the granular bainite as the numerator to the total measured area including the BCC crystal structure as the denominator). ２ The crystal grains with an area ratio of more than 0.4 μm / μm are classified as acicular bainite, fresh martensite, and tempered martensite. The area ratio (the total measured area including those other than the BCC crystal structure is used as the denominator, and the 5° boundary density is 0.4 μm / μm) ２ The total area fraction of "acicular bainite, fresh martensite, and tempered martensite" is obtained by calculating the area fraction of "acicular bainite, fresh martensite, and tempered martensite" (the ratio where the numerator is the area of ​​crystal grains that are greater than 100%). The area fraction of acicular bainite is obtained by subtracting the area fractions of fresh martensite and tempered martensite obtained by the above method from the total area fraction of "acicular bainite, fresh martensite, and tempered martensite."

[0063] In this embodiment, the area ratio of the metallographic structure is calculated by image analysis using an FE-SEM, X-ray diffraction, and EBSD analysis, and therefore 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".

[0064] <Surface Layer Region> One or more of martensite and retained austenite: 10.0% or less in total. If the fresh martensite, tempered martensite, and retained austenite in the surface layer region are excessive, the impact resistance properties after pre-straining will be reduced. If the total area ratio of fresh martensite, tempered martensite, and retained austenite exceeds 10.0%, cracking in the thickness direction cannot be suppressed, and the impact resistance properties after pre-straining will be reduced. Therefore, the total area ratio of fresh martensite, tempered martensite, and retained austenite in the surface layer region is set to 10.0% or less. The total area ratio of fresh martensite, tempered martensite, and retained austenite in the surface layer region is preferably 8.0% or less, 5.0% or less, or 4.0% or less. When any one of these is included, it is sufficient that the area ratio of that one type is 10.0% or less. The smaller the total area ratio of fresh martensite, tempered martensite, and retained austenite in the surface layer region, the better. Therefore, the fresh martensite, tempered martensite and retained austenite in the surface layer region may be 0.0% or more.

[0065] Average circle-equivalent diameter of the MA phase consisting of fresh martensite and retained austenite and the tempered martensite: 5.0 μm or less. If the sizes of the MA phase consisting of fresh martensite and retained austenite and the tempered martensite in the surface region are large, it is impossible to suppress cracking in the sheet thickness direction, and it is impossible to improve the impact resistance properties after pre-strain. Therefore, the average circle-equivalent diameter of the MA phase consisting of fresh martensite and retained austenite and the tempered martensite in the surface region is set to 5.0 μm or less. The average circle-equivalent diameter of the MA phase consisting of fresh martensite and retained austenite and the tempered martensite in the surface region is preferably 4.0 μm or less, 3.5 μm or less, 3.0 μm or less, or 2.5 μm or less. The smaller the average circle-equivalent diameter of the MA phase consisting of fresh martensite and retained austenite and the tempered martensite in the surface region, the better, but it may be 1.0 μm or more or 2.0 μm or more.

[0066] The area ratios of fresh martensite, tempered martensite, and retained austenite in the surface region are obtained by measuring the region from the surface to a position 120 μm from the surface in the thickness direction using the same method as for the 1 / 4 position in the plate thickness direction. Furthermore, the microstructure of the surface region is determined using the same method as for the 1 / 4 position in the plate thickness direction, and the circle-equivalent diameters of the MA phase consisting of fresh martensite and retained austenite and the tempered martensite are calculated to obtain the circle-equivalent diameters of each microstructure. The average of the obtained circle-equivalent diameters is calculated to obtain the average circle-equivalent diameters of the MA phase consisting of fresh martensite and retained austenite, and the tempered martensite. Note that fresh martensite and retained austenite are identified as regions that have not been corroded by Repelle corrosion. The circle-equivalent diameter of the MA phase consisting of fresh martensite and retained austenite is obtained by calculating the circle-equivalent diameter of this uncorroded region. Note that regions with a circle-equivalent diameter of 0.3 μm or less are excluded from the measurement.

[0067] <1 / 4 position in sheet thickness> Average value of aspect ratio of prior austenite grains: 3.0 or more By setting the average value of the aspect ratio of the prior austenite grains at 1 / 4 position in sheet thickness to 3.0 or more, it is possible to further improve the impact resistance properties after pre-straining. Therefore, it is preferable that the average value of the aspect ratio of the prior austenite grains at the 1 / 4 position in sheet thickness is 3.0 or more. The average value of the aspect ratio of the prior austenite grains at the 1 / 4 position in sheet thickness is more preferably 3.5 or more or 4.0 or more.

[0068] Average value of aspect ratio of prior austenite grains: 6.0 or less By setting the average value of the aspect ratio of the prior austenite grains at 1 / 4 of the plate thickness to 6.0 or less, the hole expandability of the steel plate can be further improved. Therefore, the average value of the aspect ratio of the prior austenite grains at 1 / 4 of the plate thickness is preferably set to 6.0 or less. The average value of the aspect ratio of the prior austenite grains at 1 / 4 of the plate thickness is more preferably 5.5 or less, or 5.0 or less.

[0069] The aspect ratio of prior austenite grains at the 1 / 4 position in the plate thickness direction 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 in the plate thickness direction (from the surface to the 1 / 8 position in the plate thickness direction to the 3 / 8 position in the plate thickness direction). After mirror polishing the plate thickness cross section parallel to the rolling direction, the prior austenite grain boundaries are revealed using an etchant (etchant described in JA.2 of Appendix JA of JIS G 0551:2020). Using an optical microscope, prior austenite grains are identified in an area of ​​200 μm (in the plate thickness direction) × 600 μm (perpendicular to the plate thickness direction) at the 1 / 4 position in the plate thickness direction. Next, the prior austenite grains are approximated to ellipses using the method described below, and their major and minor axes are determined. The ratio of the major axis to the minor axis (aspect ratio) is calculated for all prior austenite grains in the above region, and the ratio is weighted by the area of ​​each prior austenite grain to calculate the average value, thereby obtaining the average value of the aspect ratios of the prior austenite grains. Prior austenite grains with a major axis of 2 μm or less are excluded from the measurement. The average value of the aspect ratios of the prior austenite grains can be generally expressed by the following formula: where Ai is the area of ​​the i-th prior austenite grain, and ri is the aspect ratio of the i-th prior austenite grain. Average value of aspect ratio of prior austenite grains = Σi (Ai × ri) / ΣiAi If 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 "Kengo Hata, Masayuki Wakita, Kazuki Fujiwara, Kaori Kawano, Nippon Steel & Sumitomo Metal Technical Report, No. 114 (2017), pp. 26-31". Note that a method for determining the rolling direction of a steel plate will be described later.

[0070] The prior austenite grains are approximated to ellipsoids by the following method. As shown in FIG. 1, for the identified prior austenite grains G, the area S of the crystal grain region not included in the ellipsoid is ｏｕｔ and the area S of the non-grain region within the ellipsoid ｉｎBy approximating it as an ellipsoid g in this way, (x0, y0): the center of the ellipsoid g, a: the major axis of the ellipsoid g, and b: the minor axis of the ellipsoid g are obtained.

[0071] The steel sheet according to this embodiment may have a coating on a part or all of its surface. The coating may be an Al-based coating (a coating mainly made of an Fe-Al-based alloy), a Zn-based coating (a coating mainly made of an Fe-Zn-based alloy), or may contain an epoxy resin applied by electrodeposition coating. The coating is also called a film, an alloyed plating layer, or an intermetallic compound layer. The presence of the coating can improve corrosion resistance. The thickness of the coating is preferably 5 to 100 μm.

[0072] An Al-based coating (a coating mainly made of an Fe—Al-based alloy) is a coating containing 70 mass % or more of Fe and Al in total, and a Zn-based coating (a coating mainly made of an Fe—Zn-based alloy) is a coating containing 70 mass % or more of Fe and Zn in total.

[0073] The Al-based coating (a coating mainly made of an Fe—Al-based alloy) may contain, in addition to Fe and Al, one or more of Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Zn, Co, In, Bi, Zr, Se, As, and REM, with the balance being impurities. The Zn-based coating (a coating mainly made of an Fe—Zn-based alloy) may contain, in addition to Fe and Zn, one or more of Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Al, Co, In, Bi, Zr, Se, As, and REM, with the balance being impurities.

[0074] The chemical composition and thickness of the coating can be determined by cross-sectional observation using a scanning electron microscope. A sample is cut out from an arbitrary position at least 10 mm away from the end face. The cross-section of the cut sample is mechanically polished and then mirror-finished. The observation range using a scanning electron microscope is, for example, 400 times magnification and 40,000 μm in area. ２ The above range applies.

[0075] When observing a cross section using a BSE COMPO image, a clear difference in contrast is observed between the coating and the base steel (steel sheet). Therefore, the thickness of the coating can be measured by measuring the thickness from the outermost surface to the point where the contrast changes. Measurements are taken at 20 equally spaced locations within the observation photograph, with the distance between measurement locations being 6.5 μm. Furthermore, when measuring, five fields of view are observed in the same manner as above, and the average value is used to determine the coating thickness.

[0076] The chemical composition of the coating can be determined by performing spot elemental analysis (beam diameter 1 μm or less) on the same observation range as above using an electron probe microanalyzer (EPMA) to determine the concentrations of Fe, Al, and Zn contained in the coating. A total of 10 points are analyzed on the coating in any 10 fields of view, and the average values ​​are used as the concentrations of Fe, Al, and Zn contained in the coating. The same method can be used even when elements other than Fe, Al, and Zn are contained in the coating.

[0077] The thickness of the steel plate according to this embodiment is not particularly limited, but may be 0.4 to 5.0 mm. The thickness may be 0.8 mm or more, 1.0 mm or more, 1.6 mm or more, or 2.0 mm or more, or 4.8 mm or less, 4.2 mm or less, 3.8 mm or less, or 3.6 mm or less.

[0078] Strength: Tensile strength (TS) of 980 MPa or more The steel plate according to this embodiment preferably has a tensile strength of 980 MPa or more. By making the tensile strength 980 MPa or more, the effect of reducing the weight of the vehicle body can be increased. The tensile strength is more preferably 1000 MPa or more or 1050 MPa or more. From the viewpoint of suppressing die wear and ensuring the ductility of the steel plate, the upper limit of the tensile strength is preferably 1200 MPa or less.

[0079] Ductility: Total elongation (El) is preferably 10.0% or more. If the total elongation is 10.0% or more, it can be determined that the ductility is excellent. The total elongation is the "total elongation at break" as defined in JIS Z 2241:2022.

[0080] The tensile strength and total elongation are measured by preparing a No. 5 test piece in accordance with JIS Z 2241:2022 and conducting a tensile test in accordance with JIS Z 2241:2022. The longitudinal direction of the tensile test piece is perpendicular to the rolling direction. The tensile test piece is preferably prepared from a quarter of the width of the steel sheet from the end. The tensile test is performed twice, and the average value is used as the representative value.

[0081] 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 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 grain boundaries are revealed using the above-mentioned etching solution, and the average aspect ratio of the prior austenite grains is calculated using the intercept method. The test specimen with the largest average aspect ratio of the prior austenite grains is identified, and the direction from which the test specimen was taken is determined to be the rolling direction of the steel plate. In other words, the direction parallel to the thickness cross section of the test specimen and perpendicular to the thickness direction is determined to be the rolling direction of the steel plate.

[0082] Hole expandability: hole expansion ratio (λ) is 30% or more. The hole expansion ratio is preferably 30% or more. If the hole expansion ratio is 30% or more, it can be determined that the hole expandability is excellent. The hole expansion ratio is obtained by performing a hole expansion test in accordance with JIS Z 2256:2020. The hole expansion test specimen is preferably taken from a quarter portion from the end of the steel plate in the plate width direction, as with the tensile test specimen. The test is performed at least twice, and the average value is used as the representative value.

[0083] Impact resistance after pre-straining: In this embodiment, the impact resistance after pre-straining is evaluated by a drop weight test using a test piece after a bending test using the V-block method in accordance with JIS Z 2248:2020. A test piece measuring 50 mm in length in the direction perpendicular to the rolling direction and 100 mm in length in the rolling direction is taken from the steel plate. As shown in Figure 2, the taken test piece is subjected to a bending test using a presser and a V-block so that the bending ridge is perpendicular to the rolling direction. The radius R of the presser tip is 2.0 mm and 3.0 mm, and the bending test is performed using presser tips having the respective radii R.

[0084] Next, a test piece measuring 10 mm in length in the direction perpendicular to the rolling direction and 50 mm in the rolling direction is cut from the bend apex position of the test piece after the bending test, and a drop weight test is performed. The test piece after the bending test is placed on a base with the outer side of the bend facing up, and a 15 kg weight is allowed to fall freely, impacting the test piece at a speed of 15 km / h. After this drop weight test, the bent portion of the test piece is observed. If no fracture occurs in the bent portion of the test piece after the bending test in which the radius R of the pusher tip is 3.0 mm, it can be determined that the test piece has excellent impact resistance properties after pre-straining. The presence or absence of fracture is determined by the following method. The test piece after the drop weight test is cut at the center of the bent portion (center in the rolling direction) and a 10 mm wide thickness cross section is observed. If a crack penetrating in the thickness direction is observed in the thickness cross section, it is determined that a fracture has occurred. Even if a crack occurs in the thickness cross section, if the crack does not penetrate in the thickness direction, it is determined that no fracture has occurred.

[0085] In the steel plate according to this embodiment, it is preferable that no fracture occurs in the bent portion of the test piece after the bending test and the drop weight test using a press fitting with a tip radius R of 2.0 mm, or if a fracture occurs, the fracture does not penetrate through the plate thickness direction.

[0086] The steel sheet according to this embodiment has high strength, excellent ductility and hole expandability, and excellent impact resistance after pre-straining. Therefore, it can be suitably used for parts, particularly automobile parts. Among automobile parts, it can be suitably used for automobile suspension parts such as lower arms and trailing arms.

[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, or it may be difficult to determine whether it has. Therefore, when measuring the metallurgical structure of a part, the measurement is performed on the unprocessed part. If there is no unprocessed part, the measurement is performed on the part that has not been heavily processed. An unprocessed or heavily processed part refers to, for example, a flat part of the part, a part where the thickness change due to processing is small, and a part that avoids parts that have been subjected to 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 measured.

[0088] For example, a lower arm can be manufactured by drawing, bending, and trimming the excess material from the steel plate according to this embodiment, followed by punching and hole expanding, while a trailing arm can be manufactured by burring, bending, and cutting the steel plate according to this embodiment.

[0089] 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. The steel sheet according to this embodiment is manufactured by hot rolling a slab, and therefore can also be called a hot-rolled steel sheet. Note that the temperature in the following description refers to the surface temperature of the steel sheet.

[0090] In a preferred method for manufacturing the steel sheet according to this embodiment, in the finish rolling in the hot rolling, a first cooling is performed in a temperature range of 1050 to 1150°C, in which the surface temperature is reduced by 30 to 200°C, and then rolling is performed within 10 seconds to a total reduction of 30% or more, in a temperature range of 1000°C or higher, three or more rounds of rolling with a reduction of 30% or more, in a temperature range of less than 1000°C, in which a total reduction of 20 to 50% is performed to achieve a shape ratio of 3.0 to 10.0, after the finish rolling, cooling is performed from the finish rolling completion temperature to 700°C at an average cooling rate of 50°C / s or more, and coiling is performed in a temperature range of 450 to 530°C.

[0091] In a more preferred method for producing a steel sheet according to this embodiment, in the finish rolling of the hot rolling, rolling is performed in a temperature range of less than 1050° C. with a total reduction of 30 to 60%. Each step will be described in detail below.

[0092] The slab having the above-described chemical composition is heated and hot-rolled. The heating temperature of the slab may be, for example, 1100° C. or higher. From the viewpoint of energy cost, the heating temperature of the slab is preferably 1350° C. or lower.

[0093] 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.

[0094] The conditions for rough rolling in hot rolling are not particularly limited. In finish rolling, it is preferable to perform a first cooling in a temperature range of 1050 to 1150 ° C., in which the surface temperature is reduced by 30 to 200 ° C., followed by rolling to a total reduction of 30% or more within 10 seconds. An example of the first cooling in which the surface temperature is reduced by 30 ° C. or more is water cooling. The first cooling may be started when the surface temperature before cooling is in a temperature range of 1050 to 1150 ° C., and the cooling may be performed so that the surface temperature is reduced by 30 to 200 ° C. In the temperature range of 1050 to 1150 ° C., after performing a first cooling in which the surface temperature is reduced by 30 to 200 ° C., rolling to a total reduction of 30% or more within 10 seconds (i.e., rolling is performed during recuperation) can preferably control the area ratio of granular bainite at the 1 / 4 position in the plate thickness.

[0095] Furthermore, in the finish rolling, it is preferable to perform rolling at a temperature of 1000° C. or higher and a reduction ratio of 30% or more three times or more. By performing rolling at a temperature of 1000° C. or higher and a reduction ratio of 30% or more three times or more, it is possible to refine the prior austenite grains in the surface layer region. As a result, it is possible to reduce the average value of the circle-equivalent diameter of the MA phase consisting of fresh martensite and retained austenite, and of the tempered martensite in the surface layer region.

[0096] Furthermore, it is preferable that the finish rolling be performed at a temperature below 1000°C, with a total reduction ratio of 20 to 50% and an area ratio of 3.0 to 10.0. By performing rolling under these conditions, it is possible to flatten the prior austenite grains in the surface region. By refining and flattening the prior austenite grains in the surface region, it is possible to promote the transformation into granular bainite and acicular bainite. As a result, it is possible to reduce the area ratios of fresh martensite, tempered martensite, and retained austenite in the surface region.

[0097] The rolling reduction in this embodiment is the reduction of the plate thickness before rolling by t ０ The thickness of the plate after rolling is t １ When this is done, (1-t １ / t ０) × 100 (%). In this embodiment, the total rolling reduction is the thickness of the plate before the first rolling in the set range. ２ The final thickness of the rolled plate in the set range is t ３ When this is done, (1-t ３ / t ２ ) × 100 (%).

[0098] The shape ratio γ can be expressed by the following formula (1): γ=l ｄ / h ｍ (1)

[0099] l in the above formula (1) ｄ is the projected contact arc length, and h ｍ is the average plate thickness. ｄ and h ｍ can be expressed by the following formulas (2) and (3), respectively. ｄ = √{Dr / 2 × (h ｉｎ +h ｏｕｔ ) (2) h ｍ = (h ｉｎ +2 x h ｏｕｔ ) / 3 (3) where Dr is the roll radius and h ｉｎ is the entry plate thickness, and h ｏｕｔ is the delivery thickness.

[0100] Furthermore, in the finish rolling, it is more preferable to perform rolling at a total reduction rate of 30 to 60% in a temperature range below 1050°C. By performing rolling at a total reduction rate of 30% or more in a temperature range below 1050°C, the average aspect ratio of prior austenite grains at the 1 / 4 position in the plate thickness can be set to 3.0 or more. Also, by performing rolling at a total reduction rate of 60% or less in a temperature range below 1050°C, the average aspect ratio of prior austenite grains at the 1 / 4 position in the plate thickness can be set to 6.0 or less.

[0101] After finish rolling, it is preferable to perform cooling at an average cooling rate of 50°C / s or more from the finish rolling completion temperature to 700°C. By performing cooling at an average cooling rate of 50°C / s or more from the finish rolling completion temperature to 700°C, it is possible to suppress the generation of ferrite at the 1 / 4 plate thickness position. As long as the average cooling rate is 50°C / s or more, the cooling method is not particularly limited. After the above cooling, cooling to the coiling temperature described below may be performed, for example, by air cooling.

[0102] The average cooling rate here 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.

[0103] After the cooling, the steel sheet is preferably coiled in a temperature range of 450 to 530°C. Coiling in this temperature range allows desired amounts of granular bainite, acicular bainite, fresh martensite, tempered martensite, and retained austenite to be obtained at the 1 / 4 position in the sheet thickness direction. Furthermore, by controlling the prior austenite grains favorably under the above-mentioned manufacturing conditions, particularly in finish rolling, and then coiling under desired conditions, the area ratios of fresh martensite, tempered martensite, and retained austenite at the 1 / 4 position in the sheet thickness direction and in the surface region can be favorably controlled, and the average circle-equivalent diameter of the MA phase consisting of fresh martensite and retained austenite in the surface region can be favorably controlled.

[0104] The manufacturing method described above allows the steel sheet according to this embodiment to be manufactured stably.

[0105] 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 merely 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.

[0106] Slabs were obtained by converter melting and continuous casting, and steel plates having thicknesses of 2.5 to 3.5 mm were obtained under the conditions shown in Tables 3A and 3B. Tables 1 and 2 show the chemical compositions obtained by analyzing test specimens taken from the steel plates. The heating temperature of the slabs was 1200°C or higher, and the holding time in that temperature range was 3500 seconds.

[0107] The obtained steel sheets were evaluated for metallographic structure, tensile strength, total elongation, hole expansion ratio, and impact resistance after pre-straining using the methods described above. Crystal orientation information of the metallographic structure was analyzed using version 7.3.1 of OIM Analysis (registered trademark) manufactured by EDAX / TSL Solution. The results are shown in Tables 4A to 5B. Note that underlines in the tables indicate that the results are outside the scope of the present disclosure, that the manufacturing conditions are unfavorable, or that the characteristic values ​​are unfavorable.

[0108] When the tensile strength was 980 MPa or more, it was judged as having high strength and passing, whereas when the tensile strength was less than 980 MPa, it was judged as not having high strength and failing.

[0109] When the total elongation was 10.0% or more, the specimen was judged to have excellent ductility and to have passed the test, whereas when the total elongation was less than 10.0%, the specimen was judged to have poor ductility and to have passed the test.

[0110] When the hole expansion ratio was 30% or more, the specimen was judged to have excellent hole expandability and to have passed the test. On the other hand, when the hole expansion ratio was less than 30%, the specimen was judged to have no excellent hole expandability and to have passed the test.

[0111] If no breaks occurred in the bent portion of the test piece after the bending test using a presser with a tip radius R of 3.0 mm and the drop weight test, the test piece was judged to have excellent impact resistance properties after pre-straining and was judged to have passed, and this was marked "Good" in the table. Furthermore, if no breaks occurred in the bent portion of the test piece after the bending test using a presser with a tip radius R of 2.0 mm and the drop weight test, the test piece was judged to have even better impact resistance properties after pre-straining and was marked "Excellent" in the table. On the other hand, if breaks occurred in the bent portion of the test piece after the bending test using a presser with a tip radius R of 3.0 mm and the drop weight test, the test piece was judged to have poor impact resistance properties after pre-straining and was marked "Poor" in the table.

[0112]

[0113]

[0114]

[0115]

[0116]

[0117]

[0118]

[0119]

[0120] From Tables 4A to 5B, it can be seen that the steel sheets according to the examples of the present invention have high strength, as well as excellent ductility and hole expandability, and also have excellent impact resistance properties after pre-straining, whereas the steel sheets according to the comparative examples are inferior in one or more of the above properties.

[0121] 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 to 5B.

[0122] According to the above aspects of the present disclosure, it is possible to provide a steel plate having high strength, excellent ductility and hole expandability, and excellent impact resistance after pre-straining, and a part using this steel plate.

Claims

1. Chemical composition, in mass%, is: C: 0.040 to 0.180%, Si: 0.20 to 2.00%, Mn: 1.00 to 3.00%, P: 0.0150% or less, S: 0.0100% or less, Al: 0.100% or less, N: 0.0100% or less, O: 0.0060% or less, Ti: 0.020 to 0.150%, Cr: 0.50 to 1.00%, B: 0.00003% or more, less than 0.00150%, Mo: 0 to 0.500%, W: 0 to 0.500%, Co: 0 to 0.500%, Ni: 0 to 1.000%, Cu: 0 to 1.000%, V : 0 to 0.500%, Nb: 0 to 0.150%, As: 0 to 0.050%, Zr: 0 to 0.050%, Sn: 0 to 0.050%, Sb: 0 to 0.050%, Ta: 0 to 0.100%, Bi: 0 to 0.0400%, Ca: 0 to 0.0400%, Mg: 0 to 0.1000%, REM: 0 to 0.1000%, and the balance: Fe and impurities, and at a position of 1 / 4 of the plate thickness from the surface in the plate thickness direction, the metallographic structure is, in area %, granular bainite: 30 to 50%, acicular bainite: 20 to 50%, one or more of fresh martensite, tempered martensite, and retained austenite: 10 to 30% in total, a metal structure in a surface layer region that is a region from the surface to a position 120 μm from the surface in the plate thickness direction, in terms of area percentage, of one or more of fresh martensite, tempered martensite, and retained austenite: a total of 10.0% or less, and an MA phase consisting of the fresh martensite and the retained austenite, and the tempered martensite have an average circle equivalent diameter of 5.0 μm or less.

2. The chemical composition is, in mass%, Mo: 0.001 to 0.500%, W: 0.001 to 0.500%, Co: 0.001 to 0.500%, Ni: 0.001 to 1.000%, Cu: 0.001 to 1.000%, V: 0.001 to 0.500%, Nb: 0.001 to 0.150%, As: 0.001 to 0.050%, Zr: 0.001 to 0.050%, Sn: 0.001 to 0.050%, Sb: 0.001 to 0.050%, Ta: 0.001 to 0.100%, Bi: 0.0001 to 0.0400%, The steel sheet according to claim 1, further comprising at least one selected from the group consisting of Ca: 0.0001 to 0.0400%, Mg: 0.0001 to 0.1000%, and REM: 0.0001 to 0.1000%.

3. A steel plate according to claim 1 or 2, characterized in that in the metal structure at a position 1 / 4 of the plate thickness from the surface in the plate thickness direction, the average aspect ratio of prior austenite grains is 3.0 or more.

4. A steel plate according to any one of claims 1 to 3, 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 aspect ratio of prior austenite grains is 6.0 or less.

5. A part made of the steel sheet according to any one of claims 1 to 4.

Images