Steel sheet and component

Abstract

This steel sheet has a predetermined chemical composition. In the metal structure at the 1 / 4 position of the sheet thickness, in the area percentages, granular bainite takes 30-50%, acicular bainite takes 20-50%, one or more of fresh martensite, tempered martensite, and retained austenite takes 10-30% in total, ferrite and / or pearlite takes less than 5% in total, and the number density of precipitates containing B is 0.0050 precipitates / μm2 or less, and the metal structure at the 1 / 2 position of the sheet thickness includes, in the area percentages, 30-65% of granular bainite.

Description

Steel plates and parts

[0001] This application claims priority to Japanese Patent Application No. 2024-123402, filed on July 30, 2024, the contents of which are incorporated herein by reference.

[0002] In recent years, in response to environmental issues, ２ There is a demand for lighter automotive parts to reduce gas emissions and improve fuel efficiency. At the same time, social demands for improved collision safety are also increasing. Increasing the strength of steel materials is an effective way to achieve both weight reduction and improved collision safety of automotive parts. However, increasing the strength of steel materials reduces their workability, so steel materials that combine strength and workability are needed.

[0003] Among automobile parts, chassis and other undercarriage parts have complex shapes, so steel materials used for these automobile parts are particularly required to have excellent ductility and hole expandability. Furthermore, from the viewpoint of improving the collision safety of such automobile parts, the steel materials are required to have excellent impact resistance properties.

[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] 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 impact resistance properties.

[0007] The present disclosure has been made in view of the above circumstances, and an object of the present disclosure is to provide a steel plate having high strength, as well as excellent ductility, hole expandability, and impact resistance, and a part using the steel plate.

[0008] 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, One or more of ferrite and pearlite: less than 5% in total, and the number density of precipitates containing B is 0.0050 particles / μm ２and the metal structure contains, in area %, 30 to 65% granular bainite at a position halfway through the plate thickness from the surface in the plate thickness direction. [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, at a position halfway through the thickness from the surface in the thickness direction, the maximum aspect ratio of prior austenite grains is 3.5 to 5.0. [4] The steel sheet according to any one of [1] to [3], characterized in that, at a position halfway through the thickness from the surface in the thickness direction, the maximum value of prior austenite grain size is 200 μm or less. [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, as well as excellent ductility, hole expandability, and impact resistance properties, 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 obtain high strength, it is effective to include B in the chemical composition to improve the hardenability of the steel and increase the amount of bainite formed at low temperatures. However, when the B content is high, precipitates containing B precipitate on prior austenite grain boundaries, which become the starting point of fracture. Therefore, by suppressing the formation of precipitates containing B, it is possible to improve the impact resistance of the steel sheet while ensuring strength and workability.

[0012] 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.

[0013] 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 %.

[0014] 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.

[0015] 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.

[0016] 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, or 0.70% 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.

[0017] 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.

[0018] 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 and hole expandability of steel sheets are significantly reduced. Therefore, the P content is set to 0.0150% or less. The P content is preferably 0.0120% or less, 0.0100% or less, 0.0080% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less. Since a lower P content is preferable, it may be 0%. However, excessive reduction of the P content significantly increases the dephosphorization cost. Therefore, the P content may be set to 0.0001% or more or 0.0005% or more.

[0019] S: 0.0100% or less S is an element that forms sulfides such as MnS, thereby reducing the ductility and hole expandability of 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 0.0080% or less, 0.0060% or less, or 0.0030% 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.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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 0.130% or less, 0.120% or less, or 0.110% or less.

[0024] 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 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.

[0025] 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 of the steel sheet. 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.

[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] 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 0.200% or less.

[0029] 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, 0.350% or less, or 0.300% or less.

[0030] 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.

[0031] 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, or 0.200% or less.

[0032] 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.

[0033] 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, or 0.100% or less.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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 0.0200% or less or 0.0100% or less.

[0041] 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.

[0042] 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.

[0043] 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 generated in the steel, reducing the ductility and hole expandability of the steel sheet. Therefore, the REM content is set to 0.1000% or less. The REM content is preferably 0.0900% or less, 0.0850% or less, 0.0500% or less, or 0.0100% or less. Note that 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.

[0044] 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.

[0045] Next, the metallographic structure of the steel sheet according to this embodiment will be described. The steel sheet according to this embodiment has a metallographic structure at a quarter position of the sheet thickness from the surface in the sheet thickness direction, which is composed of, 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, one or more of ferrite and pearlite: less than 5% in total, and a number density of precipitates containing B of 0.0050 precipitates / μm ２ or less, and the metal structure contains, in area %, granular bainite: 45 to 65% at a position half the thickness of the plate from the surface in the plate thickness direction.

[0046] 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 the surface of the steel plate to a position of ⅛ of the plate thickness in the plate thickness direction, and can be rephrased as a range starting from the ⅛ position of the plate thickness from the surface of the steel plate and ending at the ⅛ position of the plate thickness. The ½ position of the plate thickness refers to a range from the surface of the steel plate to a position of ⅛ of the plate thickness in the plate thickness direction, and can be rephrased as a range starting from the ⅛ position of the plate thickness from the surface of the steel plate and ending at the ⅔ position of the plate thickness.

[0047] 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.

[0048] <1 / 4 position in thickness> Granular bainite: 30 to 50% Granular bainite improves the ductility and hole expandability of the steel sheet. If the area fraction of granular bainite at the 1 / 4 position in thickness is less than 30%, the ductility and / or hole expandability of the steel sheet will decrease. Therefore, the area fraction of granular bainite at the 1 / 4 position in thickness is set to 30% or more. The area fraction of granular bainite is preferably 33% or more, 35% or more, or 40% or more. On the other hand, if the area fraction of granular bainite at the 1 / 4 position in thickness exceeds 50%, the desired amounts of acicular bainite, fresh martensite, tempered martensite, and retained austenite cannot be obtained, and the strength of the steel sheet will decrease. Therefore, the area fraction of granular bainite at the 1 / 4 position in thickness is set to 50% or less. The area fraction of granular bainite is preferably 47% or less or 45% or less.

[0049] 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 or 45% 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.

[0050] One or more of fresh martensite, tempered martensite, and retained austenite: 10 to 30% in total. Fresh martensite, tempered martensite, and retained austenite increase the strength of the steel sheet. If the total area fraction of fresh martensite, tempered martensite, and retained austenite is less than 10%, the strength of the steel sheet decreases. Therefore, the total area fraction of fresh martensite, tempered martensite, and retained austenite is set to 10% or more. The total area fraction of fresh martensite, tempered martensite, and retained austenite is preferably 13% or more, 15% or more, or 20% or more. Note that it is not necessary for all of fresh martensite, tempered martensite, and retained austenite to be included. If any one of them is included, it is sufficient that the area fraction of that one type is 10% or more. On the other hand, if the total area fraction 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.

[0051] 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%.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] 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%.

[0056] 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.

[0057] 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.

[0058] 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.

[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 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."

[0061] 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".

[0062] Number density of precipitates containing B: 0.0050 particles / μm ２ Precipitates containing B precipitate on prior austenite grain boundaries and act as fracture origins, thereby reducing the impact resistance of the steel sheet. At the 1 / 4 position in the sheet thickness, the number density of precipitates containing B is 0.0050 particles / μm ２ If the number density of precipitates containing B is more than 0.0050 particles / μm, the impact resistance of the steel sheet will be significantly reduced. ２ The number density of precipitates containing B is preferably 0.0030 particles / μm ２ Less than or equal to 0.0020 pieces / μm ２ The lower the number density of the precipitates containing B, the better. Therefore, the lower limit is set to 0.0000 particles / μm ２ , 0.0001 pieces / μm ２ , 0.0005 pieces / μm ２ or 0.0010 pieces / μm ２ It may also be possible to use the following.

[0063] The number density of B-containing precipitates is obtained by the following method. The number density of B-containing precipitates is calculated by time-of-flight secondary ion mass spectrometry (ToF-SIMS). A test specimen is taken from a quarter position of the steel plate so that it can be analyzed. The taken test specimen is mirror-polished and then buffed, and then a contaminated layer on the surface of the test specimen is sputtered and removed using a Cs ion beam. A TOF-SIMS manufactured by ION-TOF is used. ５ As the primary ion, Bi ＋ was used, and BO was used as the secondary ion containing B. ２－(Mass-to-charge ratio = 43 m / z) is detected. The beam diameter of the primary ions is 0.1 μm. A 50 μm × 50 μm field of view is measured with 2048 × 2048 pixels, and the obtained map data is pixel binned to 512 × 512.

[0064] In the map data after pixel binning, ２－ The number of precipitates containing B is calculated by analyzing the numerical data of the ion signal intensity. First, the number of B precipitates in the entire measurement area (all 256 × 256 pixels) is calculated. ２－ The average value of the signal intensity of ions is calculated. Pixels showing a signal intensity less than five times the average value of the entire measurement area (overall average value) are pixels derived from the background or from solute B segregated at grain boundaries. To exclude these pixels, pixels showing a signal intensity of five times or more the overall average value are determined to be derived from B-containing precipitates. To eliminate the influence of noise, if 25 or more adjacent pixels continuously show a signal intensity of five times or more the overall average value in at least either the vertical or horizontal direction, the consecutive pixels are determined to be one "B-containing precipitate." The number of B-containing precipitates in one field of view (50 μm × 50 μm) is counted to obtain the number of B-containing precipitates. The above operation is performed for four fields of view, and the number of B-containing precipitates is divided by the area of ​​the four fields of view to obtain the number density of B-containing precipitates.

[0065] <Half thickness position> Granular bainite: 30 to 65% At the half thickness position, granular bainite contributes to improving the impact resistance properties of the steel plate. If the area fraction of granular bainite at the half thickness position is less than 30%, the impact resistance properties of the steel plate will decrease. Therefore, the area fraction of granular bainite at the half thickness position is set to 30% or more. The area fraction of granular bainite is preferably 34% or more or 53% or more. On the other hand, if the area fraction of granular bainite at the half thickness position exceeds 65%, the strength of the steel plate will decrease. Therefore, the area fraction of granular bainite at the half thickness position is set to 65% or less. The area fraction of granular bainite is preferably 62% or less or 60% or less.

[0066] The area ratio of granular bainite at the 1 / 2 position in the plate thickness direction is obtained by measuring in the range from the 3 / 8 position to the 5 / 8 position in the plate thickness direction from the surface in the same manner as at the 1 / 4 position in the plate thickness direction.

[0067] <1 / 2 position in plate thickness> Maximum aspect ratio of prior austenite grains: 3.5 to 5.0 Maximum size of prior austenite grain diameter: 200 μm or less The present inventors have investigated methods for further improving the impact resistance of high-strength steel plates, and have obtained the following findings. In order to further improve the strength and impact resistance of steel plates, it is effective to flatten the austenite grains and utilize the strength-improving effect of ausforming (dislocation strengthening). However, when the flat austenite grains are coarse, cracks tend to propagate in a specific direction, making parts more susceptible to fracture when subjected to an impact in a specific direction. This reduces the impact resistance of the steel plate. Therefore, the impact resistance of steel plates can be further improved by flattening the austenite grains or by reducing the grain diameter of the flat austenite grains.

[0068] By setting the maximum aspect ratio of the prior austenite grains at the half-thickness position to 3.5 or more, the impact resistance properties of the steel plate can be further improved. Specifically, the Charpy impact value obtained by the Charpy impact test described below can be increased. Therefore, it is preferable that the maximum aspect ratio of the prior austenite grains at the half-thickness position be 3.5 or more. The maximum aspect ratio of the prior austenite grains at the half-thickness position is more preferably 3.8 or more or 4.0 or more.

[0069] Furthermore, by setting the maximum aspect ratio of the prior austenite grains at the half-thickness position to 5.0 or less, the impact resistance properties of the steel plate can be further improved. Specifically, the total length of separation obtained in a Charpy impact test, which will be described later, can be shortened. Therefore, it is preferable that the maximum aspect ratio of the prior austenite grains at the half-thickness position be 5.0 or less. The maximum aspect ratio of the prior austenite grains at the half-thickness position is more preferably 4.5 or less or 4.4 or less.

[0070] By setting the maximum value of the prior austenite grain size at the half-thickness position to 200 μm or less, the impact resistance properties of the steel plate can be further improved. Specifically, the Charpy impact value obtained by the Charpy impact test described below can be increased. Therefore, the maximum value of the prior austenite grain size is preferably set to 200 μm or less. The maximum value of the prior austenite grain size is more preferably set to 180 μm or less, 150 μm or less, or 130 μm or less. There is no particular limitation on the lower limit of the maximum value of the prior austenite grain size, but it may be set to 50 μm or more, 70 μm or more, or 100 μm or more.

[0071] The aspect ratio and grain size of prior austenite grains at the half-thickness position 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 half-thickness position (ranging from 3 / 8 of the thickness to 5 / 8 of the thickness from the surface in the thickness direction). After mirror-polishing the thickness cross section parallel to the rolling direction, the prior austenite grain boundaries are revealed using an etchant (as specified 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 (thickness direction) × 600 μm (perpendicular to the thickness direction) at the half-thickness position. Next, the prior austenite grains are approximated to ellipses using the method described below, and their major and minor axes are determined. The value (major axis + minor axis) / 2 is considered to be the prior austenite grain size. The "maximum prior austenite grain size" is obtained by calculating the average value of the five largest prior austenite grains. The aspect ratio is obtained by calculating the major axis / minor axis ratio using the major axis and minor axis of the five prior austenite grains (the five largest prior austenite grains). The "maximum prior austenite grain aspect ratio" is obtained by calculating the average value of the aspect ratios of the five prior austenite grains. If the above-described method fails to reveal the prior austenite grains sufficiently, 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." The method for determining the rolling direction of a steel sheet will be described later.

[0072] 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 ｉｎ By 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.

[0073] 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.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] 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.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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.

[0085] Impact resistance: The total length of separation cracks is 5.0 mm or less, and the Charpy impact value at -40°C is 50 J / cm ２ In the present embodiment, the impact resistance is evaluated by the length of separation cracks generated by a Charpy impact test performed by the method described below and the Charpy impact value at -40°C. If the total length of separation cracks is 5.0 mm or less and the Charpy impact value at -40°C is 50 J / cm ２Therefore, the steel plate according to this embodiment has a total separation crack length of 5.0 mm or less and a Charpy impact value at -40°C of 50 J / cm or less. ２ It is preferable that this is equal to or greater than this.

[0086] Furthermore, the steel plate according to this embodiment has a total separation crack length of 3.0 mm or less and a Charpy impact value at −40° C. of 75 J / cm ２ It is more preferable that the total length of the separation cracks is 3.0 mm or less and the Charpy impact value at −40° C. is 75 J / cm or more. ２ If it is equal to or greater than this, it can be determined that the impact resistance is superior.

[0087] The Charpy impact test is performed at -40°C in accordance with JIS Z 2242:2023. First, a test specimen is taken from the steel plate. The test specimen is 10 mm wide and 55 mm long, with the same thickness as the steel plate. The test specimen has a shape conforming to JIS Z 2242:2023 except for the thickness, including the notch shape described below. Even if the steel plate has a coating, the test specimen is taken without removing the coating. A V-notch with a depth of 2 mm is formed in the taken test specimen. The V-notch has a notch angle of 45°, a notch bottom radius of 0.25 mm, a notch bottom width of 8 mm, and a notch position (center): 27.5 mm from the end of the test specimen in the longitudinal direction. Three test specimens are stacked and fixed with screws, and the Charpy impact test is performed. However, when the thickness of the test piece (steel plate thickness) is less than 2.0 mm, the test is conducted by stacking three test pieces, but when the thickness is 2.0 mm or more, the test is conducted by using one test piece without stacking the test pieces.

[0088] The Charpy impact value varies depending on the relationship between the depth direction of the notch and the rolling direction of the steel plate, but when taking test specimens from the members, the test specimens should be taken so that their longitudinal direction coincides with the rolling direction of the steel plate.

[0089] After conducting a Charpy impact test using the above-mentioned method, the length of separation cracks that occur in the region from the notch bottom to a depth of 3 mm of the test piece (region from the notch bottom to a depth of 3 mm) is measured. By observing the fracture surface using an SEM, cracks in a direction parallel to the plate surface of the steel plate to an angle of 45° are determined to be separation cracks. The length of this separation crack in the direction perpendicular to the thickness of the test piece (plate thickness of the steel plate) is defined as the length of the separation crack. If multiple separation cracks are formed in the region from the notch bottom to a depth of 3 mm, the lengths of all separation cracks in that region are measured. The total length of the separation cracks is obtained by calculating the sum of the lengths of the separation cracks.

[0090] The Charpy impact value is obtained by dividing the Charpy impact absorbed energy at −40° C. by the cross-sectional area of ​​the test specimen. The cross-sectional area of ​​the test specimen is calculated by multiplying the width under the notch (8 mm) by the thickness of the test specimen (the thickness of the steel plate (the total thickness when the test specimens are stacked)).

[0091] The steel sheet according to this embodiment has high strength, excellent ductility and hole expandability, and excellent impact resistance. Therefore, it can be suitably used for parts, particularly automobile parts. Among automobile parts, it can be suitably used for automobile underbody parts such as chassis.

[0092] 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.

[0093] For example, the lower arm of the chassis can be manufactured by performing drawing, bending, and trimming of excess material on the steel plate according to the present embodiment, followed by punching and hole expanding, while the trailing arm can be manufactured by performing burring, bending, and cutting on the steel plate according to the present embodiment.

[0094] 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.

[0095] 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 is performed in the stand immediately preceding the final stand (the n-1th stand) and the final stand (the nth stand) so that the total reduction is 30 to 80%; rolling in the final stand (the nth stand) is performed so that the finish rolling completion temperature is in a temperature range of 860 to 950°C; after finish rolling, cooling is performed from the finish rolling completion temperature to a temperature range of 700°C at an average cooling rate of 50°C / s or more, and the residence time in the temperature range of 580 to 700°C is 10.0 seconds or less; and coiling is performed in a temperature range of 450 to 530°C. Furthermore, in the finish rolling, it is more preferable to carry out a second cooling step in which the surface temperature is reduced by 30 to 60° C. before rolling in the stand immediately preceding the final stand (the n-1th stand) or the final stand (the nth stand). Each step will be described in detail below.

[0096] 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.

[0097] 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.

[0098] 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.

[0099] The area ratio of granular bainite at the 1 / 4 thickness position and the 1 / 2 thickness position can be preferably controlled by performing a first cooling in which the surface temperature is reduced by 30 to 200°C in the temperature range of 1050 to 1150°C, followed by rolling to a total reduction of 30% or more within 10 seconds (i.e., rolling during reheating). Furthermore, by performing cooling in the first cooling so that the surface temperature is reduced by more than 100°C but not more than 200°C, followed by rolling to a total reduction of 30% or more within 5 seconds, the maximum value of the prior austenite grain size at the 1 / 2 thickness position can be preferably controlled.

[0100] 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 (%).

[0101] Furthermore, in the finish rolling, it is preferable to perform rolling such that the total reduction ratio in the stand immediately preceding the final stand (the n-1th stand) and the final stand (the nth stand) is 30 to 80%, and to perform rolling in the final stand so that the finish rolling completion temperature is in the temperature range of 860 to 950°C. By performing finish rolling in this manner, the area ratio of granular bainite at the 1 / 4 thickness position and / or the 1 / 2 thickness position can be preferably controlled. Furthermore, it is more preferable to perform second cooling, in which the surface temperature is reduced by 30 to 60°C, before rolling in the stand immediately preceding the final stand (the n-1th stand) or before rolling in the final stand. By performing this second cooling, it is possible to preferably control the maximum aspect ratio of prior austenite grains at the 1 / 2 thickness position.

[0102] 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 a temperature range of 700 ° C., and to perform cooling so that the residence time in the temperature range of 580 to 700 ° C. is 10.0 seconds or less (i.e., cooling at an average cooling rate of 12.0 ° C. / s or more). By performing cooling at an average cooling rate of 50 ° C. / s or more from the finish rolling completion temperature to 700 ° C., the formation of ferrite and pearlite can be suppressed and other structures can be favorably controlled. Furthermore, by performing cooling so that the residence time in the temperature range of 580 to 700 ° C. is 10.0 seconds or less, the number density of precipitates containing B at the quarter thickness position can be favorably controlled. Note that after performing cooling so that the residence time in the temperature range of 580 to 700 ° C. is 10.0 seconds or less, it is sufficient to cool to the coiling temperature, for example, by air cooling.

[0103] 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.

[0104] After the above cooling, it is preferable to coil the sheet in a temperature range of 450 to 530° C. By coiling the sheet in this temperature range, it is possible to preferably control the area ratio of granular bainite at the 1 / 4 sheet thickness position and the 1 / 2 sheet thickness position.

[0105] The manufacturing method described above allows the steel sheet according to this embodiment to be manufactured stably.

[0106] 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.

[0107] Slabs were obtained by converter melting and continuous casting, and steel plates having thicknesses of 2.2 to 4.0 mm were obtained under the conditions shown in Tables 3A and 3B. Tables 1 and 2 show the chemical compositions obtained by analyzing test pieces 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. In addition, in cases where the "temperature decrease due to the first cooling" exceeded 100°C, rolling was performed within 5 seconds after the cooling at the "total reduction within 10 seconds after the first cooling" shown in the tables.

[0108] The obtained steel sheets were evaluated for metallographic structure, tensile strength, total elongation, hole expansion ratio, and impact resistance 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 5. 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.

[0109] 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.

[0110] 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.

[0111] 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.

[0112] The total length of the separation cracks is 5.0 mm or less, and the Charpy impact value at -40°C is 50 J / cm ２ If the total length of the separation cracks was more than 5.0 mm, and / or the Charpy impact value at -40°C was less than 50 J / cm, the specimen was judged to have excellent impact resistance and was judged to have passed the test, and was marked "Good" in the table. ２If the total length of the separation cracks was less than 3.0 mm and the Charpy impact value at -40°C was 75 J / cm or less, the specimen was judged to have no excellent impact resistance and was rejected, and this was recorded as "Poor" in the table. ２ If the value was equal to or greater than this, it was determined that the sample had better impact resistance properties, and was recorded as "Excellent" in the table.

[0113]

[0114]

[0115]

[0116]

[0117]

[0118]

[0119]

[0120] From Tables 4A to 5, it can be seen that the steel sheets according to the examples of the present invention have high strength as well as excellent ductility, hole expandability, and impact resistance properties, while 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 5.

[0122] According to the above aspects of the present disclosure, it is possible to provide a steel plate having high strength, as well as excellent ductility, hole expandability, and impact resistance properties, 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, One or more of ferrite and pearlite: less than 5% in total, and the number density of precipitates containing B is 0.0050 particles / μm ２ and the metal structure contains, in area %, 30 to 65% granular bainite at a position halfway through the plate thickness from the surface in the plate thickness direction.

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 the maximum aspect ratio of prior austenite grains is 3.5 to 5.0 at a position halfway through the plate thickness from the surface in the plate thickness direction.

4. A steel plate according to any one of claims 1 to 3, characterized in that the maximum value of the prior austenite grain size is 200 µm or less at a position halfway through the plate thickness from the surface in the plate thickness direction.

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

Images