Steel sheet and component

Abstract

This hot-rolled steel sheet has a prescribed chemical composition, wherein in the metal structure at a 1 / 4 sheet thickness location, in terms of area%, more than 30% and not more than 80% is tempered martensite, 10% to 50% is granular bainite, less than 5% total is one or more types among ferrite and pearlite, and not more than 10% total is one or more types among fresh martensite and residual austenite, and the average value of the aspect ratio of prior austenite grains is 3.0 to 6.0.

Description

Steel plates and parts

[0001] This application claims priority to Japanese Patent Application No. 2024-123403, 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] 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.

[0005] International Publication No. 2017 / 017933

[0006] Patent Document 1 discloses that bainite transformation is promoted by coiling in a temperature range of 300 to 530°C. This temperature range is the so-called transition boiling temperature range, and therefore temperature variations are likely to occur within the manufactured steel sheet. Fresh martensite is formed in areas where the amount of cooling is large, such as areas where cooling water is applied to areas where no steam is generated on the surface of the steel sheet, and workability is significantly reduced in these areas. In other words, a steel sheet is manufactured that includes areas with locally low workability. When such a steel sheet is press-formed, press cracks and the like unexpected from the strength of the steel sheet occur, causing a decrease in yield.

[0007] The present disclosure has been made in view of the above-mentioned circumstances, and aims to provide a steel sheet having high strength, excellent ductility and hole expandability, and reduced material variation within the steel sheet, and a part using the steel sheet.

[0008] The gist of the present disclosure is as follows. [1] Chemical composition, in mass%, is: C: 0.050 to 0.180%, Si: 0.40 to 1.30%, Mn: 1.60 to 2.80%, P: 0.100% or less, S: 0.0500% or less, Al: 0.020 to 0.100%, N: 0.0100% or less, O: 0.0060% or less, Ti: 0.030 to 0.150%, B: 0.0005 to 0.0050%, Cr: 0 to 1.00%, 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 0.0400%, REM: 0 to 0.0400%, and the balance: Fe and impurities, wherein the metallographic structure at a position of 1 / 4 of the sheet thickness from the surface in the sheet thickness direction contains, in area %, tempered martensite: more than 30% but not more than 80%, granular bainite: 10 to 50%, one or more of ferrite and pearlite: less than 5% in total, A steel plate characterized in that the steel sheet contains at least one of fresh martensite and retained austenite in a total amount of 10% or less, and the average aspect ratio of prior austenite grains is 3.0 to 6.0. [2] The chemical composition is, in mass%, Cr: 0.01 to 1.00%, 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%, The steel sheet according to [1], characterized in that it contains one or more selected from the group consisting of Bi: 0.0001 to 0.0400%, Ca: 0.0001 to 0.0400%, Mg: 0.0001 to 0.0400%, and REM: 0.0001 to 0.0400%.[3] The steel sheet according to [1] or [2], characterized in that, in the texture at a position halfway through the sheet thickness from the surface in the sheet thickness direction, the average value of the X-ray random intensity ratio of the orientation group of {100}<011> to {223}<110> is 5.0 to 11.0, and the X-ray random intensity ratio of the orientation of {332}<113> is 5.0 to 9.0. [4] The steel sheet according to any one of [1] to [3], characterized in that the chemical composition includes B: 0.0016 to 0.0050%. [5] A part characterized in that it is made of the steel sheet according to any one of [1] to [4].

[0009] 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 reduced material variation within the steel plate, and a part using this steel plate.

[0010] FIG. 1 is a diagram for explaining a method for approximating prior austenite grains to ellipsoids.

[0011] As a result of investigations conducted by the present inventors to solve the above problems, the present inventors have discovered the following. In order to reduce the variation in material properties within a steel sheet, it is effective to promote the formation of martensite rather than bainite. If bainite is formed first during the cooling process, C will concentrate in the untransformed austenite, making it difficult to subsequently form martensite. Therefore, by suppressing the formation of bainite, the formation of martensite can be promoted. As a method for suppressing the formation of bainite, (a) the shape of the austenite grains can be engineered by imparting an appropriate amount of strain to the austenite grains during hot rolling. This can slow the rate of bainite formation. Furthermore, (b) during cooling after finish rolling, by shortening the residence time in a temperature range where bainite is likely to form, the initial formation of bainite can be suppressed and the formation of martensite can be promoted.

[0012] Furthermore, it is also effective to utilize the heat generated by martensitic transformation. Martensite is hard as it is, which causes a decrease in the workability of the steel sheet. However, by utilizing the heat generated by martensitic transformation and performing "tempering," it is possible to adjust the hardness of the martensite and favorably control the balance between strength and workability. Furthermore, when the temperature increases due to the heat generation, granular bainite is generated. Since this granular bainite is a relatively soft structure, adding it to the steel sheet can improve the workability of the steel sheet. It is important that the shape of the austenite grains in the above (a) is controlled within a range that does not excessively suppress the generation of granular bainite.

[0013] 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.

[0014] 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 %.

[0015] The steel sheet according to this embodiment contains 0.050 to 0.180% C, 0.40 to 1.30% Si, 1.60 to 2.80% Mn, 0.100% or less P, 0.0500% or less S, 0.020 to 0.100% Al, 0.0100% or less N, 0.0060% or less O, 0.030 to 0.150% Ti, 0.0005 to 0.0050% B, and the balance being Fe and impurities. Each element will be described in detail below.

[0016] C: 0.050 to 0.180% C is an element that increases the strength of steel sheet. If the C content is less than 0.050%, the strength of the steel sheet decreases. Therefore, the C content is set to 0.050% or more. The C content is preferably 0.060% or more, 0.070% or more, or 0.100% or more. On the other hand, if the C content exceeds 0.180%, the strength becomes too high, and the ductility and hole expandability of the steel sheet decrease. Therefore, the C content is set to 0.180% or less. The C content is preferably 0.160% or less, 0.150% or less, or 0.140% or less.

[0017] Si: 0.40 to 1.30% Si is an element that promotes the formation of granular bainite and increases the ductility of the steel sheet. If the Si content is less than 0.40%, this effect cannot be achieved. Therefore, the Si content is set to 0.40% or more. The Si content is preferably 0.50% or more, 0.60% or more, or 0.70% or more. On the other hand, if the Si content exceeds 1.30%, a large amount of ferrite is formed prior to martensite, making it impossible to ensure the desired amount of tempered martensite, resulting in greater variation in the material properties of the steel sheet. Therefore, the Si content is set to 1.30% or less. The Si content is preferably 1.20% or less, 1.10% or less, 1.00% or less, or 0.80% or less.

[0018] Mn: 1.60 to 2.80% Mn is an element that increases the strength of steel sheet by improving hardenability and solid solution strengthening. If the Mn content is less than 1.60%, the strength of the steel sheet decreases. In addition, ferrite and bainite are more likely to form before martensite, resulting in greater variation in the material properties of the steel sheet. Therefore, the Mn content is set to 1.60% or more. The Mn content is preferably 1.80% or more, 2.00% or more, or 2.10% or more. On the other hand, if the Mn content exceeds 2.80%, the amount of granular bainite becomes insufficient and the strength becomes too high, resulting in reduced ductility and hole expandability of the steel sheet. Therefore, the Mn content is set to 2.80% or less. The Mn content is preferably 2.70% or less, 2.60% or less, 2.50% or less, or 2.40% or less.

[0019] P: 0.100% 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 0.100%, the ductility and hole expandability of steel sheets are significantly reduced. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.080% or less, 0.050% or less, 0.030% or less, or 0.020% or less. Since a lower P content is more preferable, it may be 0%. However, if the P content is reduced excessively, the dephosphorization cost increases significantly. Therefore, the P content may be set to 0.001% or more or 0.005% or more.

[0020] S: 0.0500% 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 0.0500%, the ductility and hole expandability of the steel sheet will be significantly reduced. Therefore, the S content is set to 0.0500% or less. The S content is preferably 0.0300% or less, 0.0200% or less, 0.0100% or less, or 0.0050% or less. Since a lower S content is 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.

[0021] Al: 0.020 to 0.100% Al is an element contained as a deoxidizer for molten steel. Al also controls ferrite transformation. If the Al content is less than 0.020%, the hole expandability of the steel sheet will decrease. Therefore, the Al content is set to 0.020% or more. The Al content is preferably set to 0.025% or more, 0.030% or more, or 0.035% or more. On the other hand, if the Al content exceeds 0.100%, the amount of ferrite will increase, and the hole expandability of the steel sheet will decrease. Therefore, the Al content is set to 0.100% or less. The Al content is preferably set to 0.080% or less, 0.050% or less, or 0.030% or less.

[0022] 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.

[0023] 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 0.0050% 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.

[0024] Ti: 0.030 to 0.150% 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. If the Ti content is less than 0.030%, the strength of the steel sheet decreases. Therefore, the Ti content is set to 0.030% or more. The Ti content is preferably 0.050% or more, 0.080% or more, or 0.100% or more. On the other hand, if the Ti content exceeds 0.150%, coarse carbides are formed in the steel, which may cause slab cracking during hot rolling or reduce the ductility and hole expandability of the steel sheet. Therefore, the Ti content is set to 0.150% or less. The Ti content is preferably 0.130% or less or 0.110% or less.

[0025] B: 0.0005 to 0.0050% B is an element that improves the hardenability of steel and increases the strength of steel sheet. If the B content is less than 0.0005%, the strength of the steel sheet decreases. In addition, ferrite and bainite tend to form before martensite, resulting in greater variation in the material properties of the steel sheet. Therefore, the B content is set to 0.0005% or more. The B content is preferably 0.0007% or more, 0.0010% or more, 0.0015% or more, 0.0016% or more, 0.0018% or more, or 0.0020% or more. If the B content exceeds 0.0050%, a large amount of precipitates containing B is formed, reducing the hole expandability of the steel sheet. Therefore, the B content is set to 0.0050% or less. The B content is preferably 0.0040% or less, 0.0030% or less, or 0.0025% or less.

[0026] 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.

[0027] 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.

[0028] Cr: 0 to 1.00% Cr is an element that improves the hardenability of steel and increases the strength of the steel sheet. Cr also promotes the formation of granular bainite. To reliably obtain these effects, the Cr content is preferably 0.01% or more or 0.05% or more. On the other hand, if the Cr content exceeds 1.00%, the ductility of the steel sheet decreases. Therefore, the Cr content is set to 1.00% or less. The Cr content is preferably 0.85% or less, 0.70% or less, 0.65% or less, 0.50% or less, 0.35% or less, or 0.20% or less.

[0029] 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, 0.300% or less, 0.150% or less, or 0.110% or less.

[0030] 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 0.400% or less, 0.300% or less, 0.200% or less, 0.100% or less, 0.050% or less, or 0.040% or less.

[0031] 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, 0.300% or less, 0.200% or less, 0.100% or less, 0.050% or less, or 0.040% or less.

[0032] 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-described 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 0.200% or less.

[0033] 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, 0.600% or less, 0.400% or less, 0.250% or less, 0.150% or less, 0.100% or less, or 0.090% or less.

[0034] 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, 0.320% or less, 0.200% or less, 0.150% or less, or 0.110% or less.

[0035] 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 0.025% or less.

[0036] 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 0.010% or less or 0.005% or less.

[0037] 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 0.010% or less or 0.005% or less.

[0038] 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 0.010% or less or 0.005% or less.

[0039] 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.

[0040] 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 0.010% or less or 0.005% or less.

[0041] Bi: 0 to 0.0400% 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 0.0001% 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 0.0100% or less or 0.0015% or less.

[0042] 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, which act as fracture initiation sites and reduce the ductility and hole expandability of steel sheets. To ensure 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, reducing 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 0.0100% or less or 0.0015% or less.

[0043] Mg: 0 to 0.0400% 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 more reliably obtain 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 0.0400%, excessive inclusions are generated in the steel, reducing the ductility and hole expandability of the steel sheet. Therefore, the Mg content is set to 0.0400% or less. The Mg content is preferably 0.0100% or less or 0.0015% or less.

[0044] REM: 0 to 0.0400% 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 0.0400%, excessive inclusions are generated in the steel, reducing the ductility and hole expandability of the steel sheet. Therefore, the REM content is set to 0.0400% 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 0.0250% or less or 0.0200% or less.

[0045] 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.

[0046] Next, the metallographic structure of the steel sheet according to this embodiment will be described. In the steel sheet according to this embodiment, the metallographic structure at a position of 1 / 4 of the sheet thickness from the surface in the sheet thickness direction contains, in area %, tempered martensite: more than 30% but not more than 80%, granular bainite: 10 to 50%, one or more of ferrite and pearlite: less than 5% in total, one or more of fresh martensite and retained austenite: 10% or less in total, and the average aspect ratio of the prior austenite grains is 3.0 to 6.0.

[0047] In this embodiment, the metallographic structure is defined at a ¼ position of the plate thickness from the surface in the plate thickness direction (hereinafter, sometimes referred to as the ¼ position of the plate thickness) and a ½ position of the plate thickness from the surface in the plate thickness direction (hereinafter, sometimes referred to as the ½ position of the plate thickness). The ¼ position of the plate thickness refers to a range from a position of ⅛ of the plate thickness to a position of ⅜ 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 ⅛ of the plate thickness from the surface of the steel plate and ending at a position of ⅜ of the plate thickness.

[0048] 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.

[0049] <1 / 4 position in sheet thickness> Tempered martensite: more than 30% and not more than 80% Tempered martensite is a structure that improves strength, ductility, and hole expandability, and further reduces the material variability within the steel sheet. If the area fraction of tempered martensite is 30% or less, the material variability within the steel sheet increases. Therefore, the area fraction of tempered martensite is set to more than 30%. The area fraction of tempered martensite is preferably 35% or more, 40% or more, 45% or more, or 50% or more. On the other hand, if the area fraction of tempered martensite exceeds 80%, the ductility of the steel sheet decreases. Therefore, the area fraction of tempered martensite is set to 80% or less. The area fraction of tempered martensite is preferably 75% or less, 70% or less, or 65% or less.

[0050] Granular Bainite: 10 to 50% Granular bainite is a structure that increases the ductility of a steel plate. If the area fraction of granular bainite is less than 10%, the ductility of the steel plate decreases. Therefore, the area fraction of granular bainite is set to 10% or more. The area fraction of granular bainite is preferably 15% or more, 20% or more, or 25% or more. On the other hand, if the area fraction of granular bainite exceeds 50%, the material variation within the steel plate increases. Therefore, the area fraction of granular bainite is set to 50% or less. The area fraction of granular bainite is preferably 47% or less, 45% or less, or 40% or less. Note that, in 1 / 4 plate thickness, the total area fraction of tempered martensite and granular bainite may be 85% or more. The area ratio of tempered martensite and granular bainite in total may be 88% or more, 90% or more, or 92% or more, and may be 100% or less, or 98% or less.

[0051] One or more of ferrite and pearlite: less than 5% in total If the total area ratio of ferrite and pearlite is 5% or more, it is not possible to obtain sufficient tempered martensite, resulting in large variations within the steel sheet. Therefore, the total area ratio of ferrite and pearlite is set to less than 5%. The smaller the total area ratio of ferrite and pearlite, the better, so it is preferably set to 3% or less, 2% or less, or 1% or less. The total area ratio of ferrite and pearlite may be 0%.

[0052] Fresh martensite and / or one or more of retained austenite: 10% or less in total Fresh martensite and / or retained austenite increase the strength of the steel sheet. However, if the total area fraction of fresh martensite and / or retained austenite exceeds 10%, the hole expandability of the steel sheet decreases. Therefore, the total area fraction of one or more of fresh martensite and / or retained austenite is set to 10% or less. The total area fraction of one or more of fresh martensite and / or retained austenite is preferably 7% or less, 5% or less, or 3% or less. Fresh martensite and / or retained austenite may not be present, so it may be set to 0%.

[0053] The area ratio of the metallographic structure is measured by the following method. First, a method for measuring the area ratios of fresh martensite and retained austenite will be described. A test piece is taken from a 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, and then a 200 μm (plate thickness direction) × 600 μm (direction perpendicular to the plate thickness direction) area 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.

[0054] In R.P.E. corrosion, fresh martensite and retained austenite are not corroded. Therefore, by calculating the area ratio of the uncorroded region, the sum of the area ratios of fresh martensite and retained austenite is obtained. It is sometimes considered that fresh martensite, tempered martensite, and retained austenite are not corroded in R.P.E. corrosion. However, in this embodiment, for convenience, the structure that is not corroded by R.P.E. corrosion is considered to be fresh martensite or retained austenite. Furthermore, 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. Using these Vickers indentations as markers, the same region as the observation region with the FE-SEM can be observed.

[0055] 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.

[0056] The area fraction of fresh martensite is obtained by subtracting the area fraction of retained austenite obtained by X-ray diffraction from the sum of the area fractions of "fresh martensite and retained austenite" obtained by the observation using the FE-SEM. If the area fraction of fresh martensite is calculated to be a negative value, the area fraction of fresh martensite is taken to be 0%.

[0057] The area fraction of pearlite is obtained by the following method. For the same region (200 μm × 600 μm) as that used to determine the area fraction of fresh martensite by FE-SEM observation, only the corroded layer is removed by polishing and mirror-finished, and then the region is etched with a nital solution, observed using FE-SEM, and image analysis is performed. The region where cementite and ferrite are arranged in a lamellar pattern is determined to be pearlite, and the area fraction of that region is calculated to obtain the area fraction of pearlite.

[0058] The area fractions of ferrite, tempered martensite, and granular bainite are obtained by the following method. The same region (200 μm × 600 μm) as that used to determine the area fractions of fresh 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 measurement intervals of 0.2 μm. For the measurement, an EBSD analysis device consisting of a thermal field emission scanning electron microscope (JSM-7200F manufactured by JEOL) and an EBSD detector (EDAX Velocity (registered trademark) ultra-high speed EBSD detector) is used. At this time, the degree of vacuum inside the device is 9.6 × 10 －５ The pressure is set to 0.2 Pa or less, the acceleration voltage is set to 25 kV, and the irradiation current level is set to 16.

[0059] 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.

[0060] 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 0.4 μm / μm or more were determined to be fresh martensite and tempered martensite, and the area ratio (the total measured area including those other than the BCC crystal structure was used as the denominator, and the 5° boundary density was 0.4 μm / μm or less) ２ The total area ratio of "fresh martensite and tempered martensite" is obtained by calculating the ratio (the ratio where the numerator is the area of ​​crystal grains that are greater than 100%). The area ratio of tempered martensite is obtained by subtracting the area ratio of fresh martensite obtained by the above-mentioned method using R.P.A. corrosion and X-ray diffraction from the total area ratio of "fresh martensite and tempered martensite".

[0061] The metal structure determined to be tempered martensite by the above-mentioned method (i.e., a metal structure other than a structure that is not corroded by Repellant corrosion (i.e., fresh martensite and retained austenite) and has a GAM value of more than 0.5° (crystal grains of a BCC crystal structure other than crystal grains determined to be ferrite) has a 5° boundary density of 0.4 μm / μm ２(crystal grains exceeding 100 μm) may not be considered to be a structure normally determined to be "martensite." Specifically, by FE-SEM structural observation (referred to as Method A) of a sample etched with a nital solution used to determine pearlite, it is possible to determine whether or not the structure corresponds to a structure called "martensite" based on the following criteria. A structure called "martensite" is a structure formed by an aggregate of laths, and satisfies both (1) the laths that make up the structure contain acicular cementite, and the cementite elongates in two or more directions, or (2) the average lath width is 1.0 μm or less. The lath width can be measured by drawing the shortest straight line penetrating an aggregate of three or more laths with the same long axis direction and dividing the length of that line by the number of laths that penetrate. This is performed for 10 fields of view, and the average value is taken as the average lath width. During FE-SEM observation, acicular precipitates present inside the laths are considered to be cementite. Strictly speaking, the needle-like precipitates include η carbides (Fe ２ C), ε carbide (Fe ２＋ｘ C), χ carbide (Fe ５ C ２ ) and the like, but these are interpreted as precursor carbides of cementite, and are therefore collectively referred to as cementite. In other words, when the carbides are corroded by Repellant corrosion, the GAM value is greater than 0.5° and the 5° boundary density is 0.4 μm / μm ２A structure having crystal grains of a BCC crystal structure of more than 1000 nm and not determined as "martensite" by the above-mentioned method A is "bainite." A metal structure determined as tempered martensite by the above-mentioned method A using LePella corrosion, the GAM value, the 5° boundary density, etc., is not determined as "martensite" by the above-mentioned method A but is determined as bainite when the B content is 0.0015% or less. Such metal structures are determined as "martensite" by the above-mentioned method A when the B content is 0.0016% or more, and most of them are determined as "martensite" by the above-mentioned method A when the B content is 0.0020% or more. Note that in this embodiment, confirmation by the above-mentioned method A is not required for a structure determined as tempered martensite by the above-mentioned method for measuring the area fraction using LePella corrosion, the GAM value, the 5° boundary density, etc. The area fraction of tempered martensite measured by the above-mentioned area fraction measurement methods using Repela corrosion, GAM value, 5° boundary density, etc. is not corrected by the above-mentioned method A.

[0062] 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".

[0063] <1 / 4 thickness position> Average aspect ratio of prior austenite grains: 3.0 to 6.0 If the average aspect ratio of prior austenite grains at the 1 / 4 thickness position is less than 3.0, the effect of increasing strength through work hardening cannot be fully achieved, resulting in a decrease in the strength of the steel sheet. Furthermore, bainite is more likely to form before martensite, resulting in increased material variability. Therefore, the average aspect ratio of prior austenite grains at the 1 / 4 thickness position is set to 3.0 or more. The average aspect ratio of prior austenite grains is more preferably 3.2 or more or 3.4 or more. On the other hand, if the average aspect ratio of prior austenite grains at the 1 / 4 thickness position exceeds 6.0, excessive work hardening occurs, and the in-plane anisotropy increases, resulting in a decrease in the hole expandability of the steel sheet. Therefore, the average aspect ratio of prior austenite grains at the 1 / 4 thickness position is set to 6.0 or less. The average aspect ratio of the prior austenite grains is more preferably 5.5 or less, or 5.0 or less.

[0064] The aspect ratio and grain size of prior austenite grains at the 1 / 4 position in the plate thickness direction are measured using the following method. A test specimen 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 (the etchant described in JA.2 of Appendix JA of JIS G 0551:2020). Using an optical microscope, the 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 ratios 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".

[0065] 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.

[0066] <1 / 2 position in sheet thickness> Average value of X-ray random intensity ratios of orientation group of {100}<011> to {223}<110>: 5.0 to 11.0 X-ray random intensity ratio of orientation of {332}<113>: 5.0 to 9.0 In the texture at the 1 / 2 position in the sheet thickness direction from the surface (hereinafter sometimes referred to as the 1 / 2 position in the sheet thickness), the strength of the steel sheet can be further increased by setting the average value of the X-ray random intensity ratios of orientation group of {100}<011> to {223}<110> on the sheet surface to 5.0 or more and setting the X-ray random intensity ratio of the orientation of {332}<113> on the sheet surface to 5.0 or more. Furthermore, in the texture at the 1 / 2 position in the sheet thickness, the average value of the X-ray random intensity ratio of the orientation group of {100}<011> to {223}<110> on the sheet surface is set to 11.0 or less, and the X-ray random intensity ratio of the orientation of {332}<113> on the sheet surface is set to 9.0 or less, thereby further improving the hole expandability of the steel sheet.

[0067] The average value of the X-ray random intensity ratio of the {100}<011> to {223}<110> orientation group on the sheet surface is the average value of the X-ray random intensity ratios of {110}<001>, {116}<110>, {114}<110>, {113}<110>, {112}<110>, {335}<110>, and {223}<110>. Note that the crystal orientation represented by {hkl}<uvw> indicates that the normal direction to the sheet surface is parallel to <hkl> and the rolling direction is parallel to <uvw>.

[0068] In this embodiment, the 1 / 2 plate thickness position refers to a range from a position 3 / 8 of the plate thickness to a position 5 / 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 3 / 8 of the plate thickness from the surface of the steel plate and ending at a position 5 / 8 of the plate thickness.

[0069] The texture at the 1 / 2 thickness position is measured by the following method. A sample is prepared from a steel plate so that the plate surface at the 1 / 2 thickness position (ranging from the surface to the 3 / 8 position to the 5 / 8 position of the thickness in the plate thickness direction) serves as the measurement surface. At this time, the steel plate is thinned from the surface to a predetermined thickness by mechanical polishing or the like, and then strain due to mechanical polishing is removed by chemical polishing or electrolytic polishing. By setting the amount of thinning by chemical polishing or electrolytic polishing at this time to 50 μm or more, strain due to mechanical polishing is sufficiently removed and measurement errors caused by strain are minimized.

[0070] Next, this sample is subjected to X-ray diffraction using a Rigaku RINT-2500 with Mo-Kα. Using Rigaku's Pole Figure Analysis Version 7.0 software, the crystal orientation distribution function is calculated using the series expansion method, and pole figures for {110}, {100}, {211}, and {310} are obtained. At this time, the intensity is normalized using the measurement results of a standard sample with random orientation. Furthermore, a uniform BG mode is selected. Furthermore, smoothing is selected, and the number of points in the α and β directions is set to 5. Using a plurality of pole figures (preferably three or more) of {110}, {100}, {211}, and {310} thus obtained, a three-dimensional texture (a three-dimensional distribution function of the X-ray random intensity ratio) is calculated by the series expansion method using ODF Data Processing Version 1.4, a software manufactured by Rigaku Co., Ltd. At this time, the expansion times of the even-numbered terms are set to 22, the expansion times of the odd-numbered terms to 19, and the intensity for making the X-ray random intensity ratio 0.0 to 0.2. The X-ray random intensity ratios of {110}<001>, {116}<110>, {114}<110>, {113}<110>, {112}<110>, {335}<110>, and {223}<110> present in the φ2=45° cross section in the obtained three-dimensional texture are calculated, and the average of these X-ray random intensity ratios is obtained to obtain the average X-ray random intensity ratio of the orientation group of {100}<011> to {223}<110>. In addition, the X-ray random intensity ratio of {332}<113> present in the same φ2=45° cross section is calculated to obtain the X-ray random intensity ratio of the {332}<113> orientation.

[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 vehicle body weight 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 1300 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] In this embodiment, the material variation is preferably small in the total elongation and hole expansion ratio measured for test specimens taken from different positions on the steel sheet. Specifically, tensile test specimens and hole expansion test specimens are first taken from the ¼, ½, and ¾ portions from the end of the steel sheet in the sheet width direction. Tensile tests and hole expansion tests are performed on each test specimen using the above-described methods, and representative values ​​for each portion are obtained. Note that for the ¼ portion from the end of the steel sheet in the sheet width direction, no new tests are performed, and the representative values ​​obtained by the above-described tests are used.

[0084] The average values ​​of the total elongation and hole expansion ratio obtained for the 1 / 4, 1 / 2, and 3 / 4 portions are calculated, respectively, to obtain the average value of the total elongation and the average value of the hole expansion ratio for the three portions (1 / 4, 1 / 2, and 3 / 4 portions). If the difference between the average value of the total elongation for the three portions and the minimum value of the total elongation for each portion is 2.5% or less, it can be determined that the variation in ductility within the steel sheet is small. Furthermore, if the difference between the average value of the hole expansion ratio for the three portions and the minimum value of the hole expansion ratio for each portion is 15% or less, it can be determined that the variation in hole expandability within the steel sheet is small.

[0085] Therefore, in the steel sheet according to this embodiment, the difference between the average total elongation and the minimum total elongation is preferably 2.5% or less, and the difference between the average hole expansion ratio and the minimum hole expansion ratio is preferably 15% or less. More preferably, the difference between the average total elongation and the minimum total elongation is 2.0% or less or 1.8% or less, and the difference between the average hole expansion ratio and the minimum hole expansion ratio is 10% or less or 8% or less.

[0086] The steel sheet according to this embodiment has high strength, excellent ductility and hole expandability, and reduced material variation within the steel sheet. 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 a steel sheet according to this embodiment, in finish rolling in hot rolling, a first cooling step is performed 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, rolling to a total reduction of 40 to 70% in a temperature range of 1040°C or less, setting the finish rolling completion temperature to a temperature range of 850 to 980°C, cooling is performed after finish rolling so that the time required to reach 550°C is 20 seconds or less, and coiling is performed in a temperature range of 350 to 500°C.

[0091] In a more preferred method for producing the steel sheet according to this embodiment, after winding into a coil, it is more preferred to cool the coil so that the temperature at the end face at a position midway between the outermost and innermost peripheries is maintained in a temperature range of 350 to 500°C for 3,600 to 18,000 seconds. 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, 1150° 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 first cooling in a temperature range of 1050 to 1150°C, in which the surface temperature is reduced by 30 to 200°C, and then perform 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 the temperature range of 1050 to 1150°C, and cooling may be performed so that the surface temperature is reduced by 30 to 200°C.

[0095] In the temperature range of 1050 to 1150°C, the first cooling is performed to reduce the surface temperature by 30°C or more, and then rolling is performed within 10 seconds to achieve a total reduction of 30% or more (i.e., rolling is performed during reheating), thereby making it possible to preferably control the area ratio of granular bainite at the 1 / 4 thickness position. Furthermore, by performing the first cooling so that the surface temperature drops by more than 100°C and not more than 200°C, and then rolling within 5 seconds to achieve a total reduction of 30% or more, the range of thickness positions that satisfy the preferred area ratio of granular bainite is expanded, and a high balance of strength, elongation, and hole expandability can be achieved.

[0096] In the finish rolling, it is preferable to perform rolling at a total reduction rate of 40 to 70% in a temperature range of 1040° C. or less, and to set the finish rolling completion temperature in a temperature range of 850 to 980° C. In the finish rolling, it is preferable to perform rolling at a total reduction rate of 40 to 70% in a temperature range of 1040° C. or less, and to set the finish rolling completion temperature in a temperature range of 850 to 980° C., thereby making it possible to preferably control the fraction of each structure at the 1 / 4 position in the plate thickness and the aspect ratio of the prior austenite grains.

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

[0098] The finish rolling start temperature may be in the temperature range of 1020 to 1200° C. The finish rolling start temperature is the entry temperature of the first pass of finish rolling. The finish rolling completion temperature is the exit temperature of the final pass of finish rolling.

[0099] After finish rolling, it is preferable to perform cooling so that the time required to reach 550°C is 20 seconds or less (i.e., cooling at an average cooling rate of 15°C / s or more). The temperature range from the finish rolling completion temperature to 550°C is the temperature range in which ferrite and bainite are formed, and shortening the residence time in this temperature range suppresses the formation of ferrite and bainite that precede martensite.

[0100] After cooling to 550°C, it is preferable to coil the steel sheet in a temperature range of 350 to 500°C. By coiling the steel sheet in this temperature range, a desired amount of tempered martensite can be obtained. Note that cooling between 550°C and the coiling temperature may be performed by air cooling, for example.

[0101] After coiling, it is more preferable to cool the coil so that the temperature at the end face at the midpoint between the outermost and innermost peripheries is maintained in a temperature range of 350 to 500°C for 3,600 to 18,000 seconds. Although the temperature inside the coil is higher than the temperature at the end face, the temperature range of 350 to 500°C at the end face is a suitable temperature range for tempering martensite inside the coil. By cooling the coil under these conditions, it becomes easier to obtain the desired amount of granular bainite and tempered martensite. In addition, the texture at the half-thickness position can be favorably controlled.

[0102] The means for achieving the coil cooling is not particularly limited. For example, a method of spraying water onto the coil during or after winding, or a method of covering the coil after winding for the purpose of keeping it warm, may be employed. Note that the time before being wound into a coil is short, so the above-mentioned holding time does not include the time before being wound into a coil.

[0103] The manufacturing method described above allows the steel sheet according to this embodiment to be manufactured stably.

[0104] 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.

[0105] Slabs having the chemical compositions shown in Tables 1 and 2 were obtained by converter melting and continuous casting. Steel plates having thicknesses of 2.6 to 3.2 mm were obtained from the obtained slabs under the conditions shown in Tables 3A and 3B. The heating temperature of the slab was 1200°C or higher, and the holding time in that temperature range was 3500 seconds or longer. The start temperature of finish rolling was in the temperature range of 1020 to 1200°C. Furthermore, for No. 1, in which the "temperature decrease due to the first cooling" exceeded 100°C, the total reduction within 5 seconds after the cooling was 33%.

[0106] The obtained steel sheets were evaluated for metallographic structure, tensile strength, total elongation, hole expansion ratio, and material variation 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 were unfavorable, or that the characteristic values ​​were unfavorable.

[0107] 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.

[0108] 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.

[0109] 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.

[0110] If the difference between the average total elongation and the minimum total elongation was 2.5% or less and the difference between the average hole expansion ratio and the minimum hole expansion ratio was 15% or less, it was determined that the material variation within the steel sheet was reduced and the steel sheet was judged to have passed. On the other hand, if either of the conditions was not met, it was determined that the material variation within the steel sheet was not reduced and the steel sheet was judged to have failed.

[0111]

[0112]

[0113]

[0114]

[0115]

[0116]

[0117]

[0118]

[0119] From Tables 4A to 5B, it can be seen that the steel sheets according to the present invention have high strength, excellent ductility and hole expandability, and have reduced material variability within the steel sheets, whereas the steel sheets according to the comparative examples are inferior in one or more of the above properties.

[0120] 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.

[0121] 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 reduced material variation within the steel plate, and a part using this steel plate.

Claims

1. Chemical composition, in mass%, is: C: 0.050 to 0.180%, Si: 0.40 to 1.30%, Mn: 1.60 to 2.80%, P: 0.100% or less, S: 0.0500% or less, Al: 0.020 to 0.100%, N: 0.0100% or less, O: 0.0060% or less, Ti: 0.030 to 0.150%, B: 0.0005 to 0.0050%, Cr: 0 to 1.00%, 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 0.0400%, REM: 0 to 0.0400%, and the balance: Fe and impurities, wherein the metallographic structure at a position of 1 / 4 of the sheet thickness from the surface in the sheet thickness direction contains, in area %, tempered martensite: more than 30% but not more than 80%, granular bainite: 10 to 50%, one or more of ferrite and pearlite: less than 5% in total, A steel plate characterized in that the steel sheet contains at least one of fresh martensite and retained austenite in a total amount of 10% or less, and the average aspect ratio of prior austenite grains is 3.0 to 6.

0.

2. The chemical composition is, in mass%, Cr: 0.01 to 1.00%, 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%, The steel sheet according to claim 1, further comprising at least one selected from the group consisting of Bi: 0.0001 to 0.0400%, Ca: 0.0001 to 0.0400%, Mg: 0.0001 to 0.0400%, and REM: 0.0001 to 0.0400%.

3. The steel sheet according to claim 1 or 2, characterized in that in the texture at a position halfway through the sheet thickness from the surface in the sheet thickness direction, the average value of the X-ray random intensity ratio of the orientation group {100}<011> to {223}<110> is 5.0 to 11.0, and the X-ray random intensity ratio of the orientation {332}<113> is 5.0 to 9.

0.

4. A steel sheet according to any one of claims 1 to 3, characterized in that the chemical composition contains B: 0.0016 to 0.0050%.

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

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