Steel plates and parts

Abstract

This steel plate has a predetermined chemical composition, and in the metallographic structure at the 1 / 4 position of the plate thickness, it contains, by area percentage, 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 precipitates / μm 2 At the 1 / 2 position of the plate thickness, the metal structure contains granular bainite in an area percentage of 30 to 65%.

Description

[Technical Field]

[0001] The present disclosure relates to steel sheets and components. This application claims priority based on Japanese Patent Application No. 2024-123402, filed on July 30, 2024, the contents of which are incorporated herein by reference. [Background technology]

[0002] In recent years, in order to address environmental issues, there has been a demand for lighter automotive parts with the aim of reducing CO2 gas emissions and improving fuel efficiency. At the same time, societal demands for improved collision safety are also increasing. Increasing the strength of steel is an effective means of achieving both weight reduction and improved collision safety in automotive parts. However, increasing the strength of steel reduces its workability, so steel that combines strength and workability is 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 the main phase is 85% or more by area of ​​bainite, the second phase is 15% or less by area of ​​martensite or a martensite-austenite mixed phase, and the remainder is ferrite, 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 hot-rolled steel sheet containing 0.10% or less by mass of precipitates with a diameter of less than 20 nm, and the tensile strength TS of 980 MPa or more. Patent Document 1 also discloses that the above configuration results in a high-strength hot-rolled steel sheet having a tensile strength TS of 980 MPa or more and excellent punchability and hole expandability. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2017 / 017933 Summary of the Invention [Problem to be solved by the invention]

[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. [Means for solving the problem]

[0008] The gist of the present disclosure is as follows. [1] Chemical composition, in mass%, C: 0.040~0.180%, Si: 0.20 to 2.00% Mn: 1.00-3.00%, 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: 0.50~1.00%, B: 0.00003% or more, less than 0.00150% Mo: 0 to 0.500%, W: 0~0.500%, Co: 0 to 0.500%, Ni: 0 to 1.000%, Cu: 0-1.000%, V: 0~0.500%, Nb: 0 to 0.150%, As: 0~0.050%, 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 is Fe and impurities. At 1 / 4 of the plate thickness from the surface in the plate thickness direction, The metal structure is, in area%, Granular bainite: 30-50% Acicular bainite: 20-50% 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; The number density of precipitates containing B is 0.0050 particles / μm 2 is as follows: At a position halfway along the thickness from the surface to the thickness direction, A steel plate characterized in that the metal structure contains, by area %, 30 to 65% granular bainite. [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: 0.001~0.500%, 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%, Ca: 0.0001 to 0.0400%, Mg: 0.0001 to 0.1000%, and The steel sheet according to [1], characterized in that it contains at least one selected from the group consisting of REM: 0.0001 to 0.1000%. [3] At a position halfway through the thickness from the surface to the thickness direction, The steel sheet according to [1] or [2], wherein the maximum aspect ratio of the prior austenite grains is 3.5 to 5.0. [4] At a position halfway through the thickness from the surface to the thickness direction, The steel sheet according to any one of [1] to [3], wherein the maximum value of the prior austenite grain size is 200 μm or less. [5] A part made of the steel sheet according to any one of [1] to [4]. [Effects of the Invention]

[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. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram for explaining a method for approximating prior austenite grains to ellipsoids. DETAILED DESCRIPTION OF THE INVENTION

[0011] As a result of investigations conducted by the present inventors to solve the above problems, the present inventors have come to the following findings. 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, if the B content is high, B-containing precipitates precipitate on the prior austenite grain boundaries, which become the starting point for fracture. Therefore, by suppressing the formation of B-containing precipitates, it is possible to improve the impact resistance of the steel sheet while maintaining 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 % in the chemical composition indicates mass %.

[0014] The steel sheet according to this embodiment contains 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 but less than 0.00150%, and the balance: 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 a 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 sheets. 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 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, resulting in a decrease in the ductility of the steel sheet. 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 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. The lower the P content, the better, so it may be 0%. However, if the P content is reduced too much, the dephosphorization cost will increase significantly. 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 sheets. If the S content exceeds 0.0500%, the ductility and hole expandability of steel sheets 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. The lower the S content, the better, so it may be 0%. However, if the S content is reduced too much, the desulfurization cost increases 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. Furthermore, Al is an element effective in increasing the area ratio of granular bainite. From the viewpoint of further increasing the area ratio 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 the steel sheet. If the N content exceeds 0.0100%, the hole expandability of the steel sheet will be significantly reduced. Furthermore, if a large amount of N is contained, the risk of slab cracking will increase. 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. The lower the N content, the better, so it may be 0%. However, if the N content is reduced too much, the cost of denitrification will increase significantly. 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 O is contained in a large amount 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. The lower the O content, the better, so 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 0.0005% or more or 0.0010% or more.

[0023] Ti: 0.020 to 0.150% Ti is an element that precipitates in steel as Ti carbides such as TiC, enhancing the strength of steel sheets through precipitation strengthening. Furthermore, Ti also enhances the hole expandability of steel sheets 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 causes slab cracking during hot rolling and reduces 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: 0.50~1.00% 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 are 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 and scrap, or from 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 the steel, and to ensure 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 ensure 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, and in order 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 grain boundary cracking of slabs caused by Cu. To more reliably obtain the above effect, the Ni content is preferably 0.001% or more or 0.010% or more. On the other hand, Ni is an expensive element, so it is not economically preferable to include a large amount of it. 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 acts to improve the hardenability of steel sheet and precipitates as carbides in steel at low temperatures to increase the strength of the steel sheet. To more reliably obtain the effects of these actions, 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 may occur in the slab. 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 the steel sheet by forming fine carbides in the steel. To ensure 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. Also, 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 the steel sheet. To more reliably obtain this effect, the As content is preferably 0.001% or more or 0.010% or more. On the other hand, since the above effects are saturated even when As is contained in a large amount, the As content is set to 0.050% or less, and 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, and in order 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 formation 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. On the other hand, even if Sn is contained in a large amount, the above effect saturates, so the Sn content is set to 0.050% or less, and 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 Sb is contained in a large amount, the above effect saturates, so the Sb content is set to 0.050% or less, and 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 sheets by forming fine carbides in the steel. To ensure 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 will decrease. Therefore, the Ta content is set to 0.100% or less. The Ta content is preferably 0.080% or less, 0.050% or less, or 0.025% or less, or 0.020% or less.

[0040] Bi: 0 to 0.0400% Bi is an element that refines the solidification structure and thereby improves the ductility and hole expandability of the steel sheet. To more reliably obtain this effect, the Bi content is preferably 0.0001% or more, or 0.0010% or more. On the other hand, if the Bi content exceeds 0.020%, the above-mentioned effects will saturate, which is not economically preferable. 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 improves the ductility and hole expandability of steel sheets by controlling the morphology of nonmetallic inclusions that act as fracture initiation sites and cause a decrease in 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 formed in the steel, which reduces the ductility and hole expandability of the steel sheet. 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 nonmetallic inclusions to improve 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 formed in the steel, which reduces 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 nonmetallic inclusions to improve the ductility and hole expandability of steel sheets. To ensure this effect, the REM content is preferably 0.0001% or more, or 0.0010% or more. On the other hand, if the REM content exceeds 0.1000%, excessive inclusions are formed in the steel, reducing the ductility and hole expandability of the steel sheet. Therefore, the REM content is set to 0.1000% or less. The REM content is preferably 0.0900% or less, 0.0850% or less, 0.0500% or less, or 0.0100% or less. REM refers to a total of 17 elements consisting of Sc, Y and lanthanoids, and the content of the REM refers to the total content of these elements.

[0044] The chemical composition of the steel sheet described above can be determined by the following method. Test pieces are taken from the area from 1 / 8 to 3 / 8 of the thickness from the surface of the steel plate in the plate thickness direction, and the chemical composition of these test pieces is measured using a common method such as ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Note that 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 the surface, the coating is removed by mechanical grinding, and then the chemical composition is analyzed in the same manner. In addition, when the molten steel analysis value, the slab analysis value, or the steel plate analysis value of another steel plate manufactured 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 regarded as the chemical composition of the steel plate.

[0045] Next, the metal structure of the steel plate according to this embodiment will be described. The steel sheet according to this embodiment has a metal structure at a 1 / 4 position of the sheet thickness from the surface in the sheet thickness direction, in area %, of 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 at a position halfway through the plate thickness from the surface in the plate thickness direction contains, in area %, granular bainite: 45 to 65%.

[0046] In this embodiment, the metal structure is defined at a 1 / 4 position of the plate thickness from the surface in the plate thickness direction (hereinafter, sometimes referred to as the 1 / 4 position of the plate thickness) and at a 1 / 2 position of the plate thickness from the surface in the plate thickness direction (hereinafter, sometimes referred to as the 1 / 2 position of the plate thickness). The 1 / 4 position in the plate thickness refers to the range from the surface of the steel plate to the position of 1 / 8 of the plate thickness in the plate thickness direction, and can be stated as the range starting from the position of 1 / 8 of the plate thickness from the surface of the steel plate and ending at the position of 3 / 8 of the plate thickness. The 1 / 2 position in the plate thickness refers to the range from the surface of the steel plate to the position of 3 / 8 of the plate thickness in the plate thickness direction, and can be stated as the range starting from the position of 3 / 8 of the plate thickness from the surface of the steel plate and ending at the position of 5 / 8 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 identified by a BSE COMPO image (BSE Compositional Image) which will be described later.

[0048] <Plate thickness 1 / 4 position> Granular Bainite: 30~50% Granular bainite improves the ductility and hole expandability of a steel sheet. If the area fraction of granular bainite at the 1 / 4 position in the sheet thickness direction 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 the sheet thickness direction 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 the plate 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 plate decreases. Therefore, the area fraction of granular bainite at the 1 / 4 position in the plate 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: 20~50% Acicular bainite increases the strength of the steel plate. If the area fraction of acicular bainite at the 1 / 4 position in the plate thickness direction is less than 20%, the strength of the steel plate will decrease. Therefore, the area fraction of acicular bainite at the 1 / 4 position in the plate thickness direction 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 position in the plate thickness exceeds 50%, the ductility of the steel plate decreases. Therefore, the area fraction of acicular bainite at the 1 / 4 position in the plate thickness is set to 50% or less. The area fraction of acicular bainite is preferably 47% or less or 45% or less. In addition, at 1 / 4 of the plate thickness, the total area ratio of granular bainite and acicular bainite may be 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] Fresh martensite, tempered martensite, and / or retained austenite: 10-30% in total Fresh martensite, tempered martensite, and retained austenite increase the strength of the steel sheet. If the total area ratio of fresh martensite, tempered martensite, and retained austenite is less than 10%, the strength of the steel sheet decreases. Therefore, the total area ratio of fresh martensite, tempered martensite, and retained austenite is set to 10% or more. The total area ratio of fresh martensite, tempered martensite, and retained austenite is preferably 13% or more, 15% or more, or 20% or more. It is not necessary to include all of fresh martensite, tempered martensite, and retained austenite. When any one of them is included, it is sufficient that the area ratio of that one kind is 10% or more. On the other hand, if the total area ratio of fresh martensite, tempered martensite, and retained austenite exceeds 30%, the hole expandability of the steel sheet decreases. Therefore, the total area ratio of fresh martensite, tempered martensite, and retained austenite is set to 30% or less. The total area ratio of fresh martensite, tempered martensite, and retained austenite may be 27% or less, 25% or less, 23% or less, 20% or less, or 17% or less.

[0051] One or more of ferrite and pearlite: Less than 5% in total Excessive ferrite and pearlite reduce the strength of the steel sheet. If the total area ratio of ferrite and pearlite is 5% or more, the strength of the steel sheet will be significantly reduced. 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 4% or less, 3% or less, 2% or less, or 1% or less. The total area ratio of ferrite and pearlite may be 0%.

[0052] The area ratio of the metal 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 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). The cross section of the test piece is mirror-polished and etched with LePera. After that, a 200 μm (in the plate thickness direction) × 600 μm (in the direction perpendicular to the plate thickness direction) area at the 1 / 4 position in the plate thickness direction is observed using a thermal field emission scanning electron microscope (FE-SEM) (JEOL JSM-7200F), and image analysis is performed.

[0053] In Repela corrosion, tempered martensite, fresh martensite, and retained austenite are not corroded, so by calculating the area ratio of the uncorroded area, the total area ratio of tempered martensite, fresh martensite, and retained austenite is obtained. 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 a range of 100 μm from each of the four corners. 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 the steel plate is milled from the plate surface to the 1 / 4 position of the plate thickness (from the surface to the position 1 / 8 of the plate thickness in the plate thickness direction), and the exposed surface is used as the observation surface. This observation surface is mirror-polished and then finished by electrolytic polishing. For the observation surface, the integrated intensity of a total of five peaks, α(200), α(211), γ(200), γ(220), and γ(311), is determined using a Rigaku RINT-2500 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 described above. 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 considered to be 0%.

[0056] The area ratio of pearlite is obtained by the following method. The same area (200 μm × 600 μm) as that used for determining the area ratios of fresh martensite, tempered martensite, and retained austenite by FE-SEM observation is polished to remove only the corroded layer, and the specimen is then mirror-finished. It is then etched using nital solution, observed using 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 ratios of ferrite, granular bainite, and acicular bainite are determined by the following method. The following operation is performed on regions other than those determined to be pearlite by the above method. The same area (200 μm × 600 μm) as that used to determine the area ratios of fresh martensite, tempered martensite, and retained austenite by FE-SEM observation was subjected to colloidal polishing or electrolytic polishing, and then crystal orientation information was obtained by electron backscatter diffraction at a measurement interval of 0.2 μm. For the measurement, an EBSD analysis device consisting of a thermal field emission scanning electron microscope (JEOL JSM-7200F) and an EBSD detector (EDAX Velocity® ultra-high speed EBSD detector) was used. The degree of vacuum inside the device was 9.6 × 10 -5 The acceleration voltage is 25 kV and the probe current level is 16.

[0059] The following analysis is performed on the obtained crystal orientation information of the BCC crystal structure using version 7 or later of OIM Analysis (registered trademark) manufactured by EDAX / TSL Solutions. Measurement points with a crystal orientation misorientation of 15° or more are considered to be crystal grain boundaries, and the area surrounded by these crystal grain boundaries is considered to be crystal grains. Next, the difference in crystal orientation between all measurement points within a crystal grain is calculated, and the average of these differences is calculated to obtain the grain average misorientation (GAM) value of the crystal grain. Crystal grains with a BCC crystal structure with a GAM value of 0.5° or less are considered to be ferrite, and their area fraction (the ratio of the total measured area, including areas other than the BCC crystal structure, to the area of ​​ferrite as the numerator) is calculated to obtain the ferrite area fraction.

[0060] Next, for grains with a GAM value of more than 0.5° (crystal grains with a BCC crystal structure other than those identified as 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 area ratio of granular bainite is calculated by determining that crystal grains with a 5° boundary density of 0.4 μm / μm are granular bainite and calculating the area ratio (the ratio of the total measured area, including those other than BCC crystal structures, as the denominator and the area of ​​granular bainite as the numerator). 2 The crystal grains with a 5° boundary density of 0.4 μm / μm are classified as acicular bainite, fresh martensite, and tempered martensite, and the area ratio (the total measured area including those other than BCC crystal structure is used as the denominator) is used. 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 FE-SEM, X-ray diffraction, and EBSD analysis, so the total of each structure may not be 100%. In such cases, the area ratio of each structure is corrected so that the total is 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 below Precipitates containing B precipitate on prior austenite grain boundaries and act as fracture initiation points, reducing the impact resistance of steel sheets. 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 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 precipitates containing B is calculated using time-of-flight secondary ion mass spectrometry (ToF-SIMS). Test specimens are taken from a quarter of the steel plate so that they can be analyzed. The taken test specimens are mirror-polished and then buffed, and then the contaminant layer on the surface of the test specimen is sputtered and removed using a Cs ion beam. The TOF-SIMS manufactured by ION-TOF is used. 5 As the primary ion, Bi + 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, BO 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 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 originating 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 originating from B-containing precipitates. To eliminate the effects of noise, if 25 or more adjacent pixels show a signal intensity of five times or more the overall average value consecutively 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-mentioned operation is carried out for four fields of view, and the number of precipitates containing B is divided by the area of ​​the four fields of view to obtain the number density of precipitates containing B.

[0065] <Plate thickness 1 / 2 position> Granular bainite: 30-65% At the half-thickness position, granular bainite contributes to improving the impact resistance of the steel plate. If the area fraction of granular bainite at the half-thickness position is less than 30%, the impact resistance of the steel plate will deteriorate. 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 decreases. 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 surface in the plate thickness direction to the 3 / 8 position to the 5 / 8 position in the plate thickness direction using the same method as for the 1 / 4 position in the plate thickness direction.

[0067] <Plate thickness 1 / 2 position> Maximum aspect ratio of prior austenite grains: 3.5 to 5.0 Maximum prior austenite grain size: 200 μm or less The present inventors have investigated methods for further improving the impact resistance of high-strength steel sheets and have obtained the following findings. In order to further improve the strength and impact resistance of steel sheets, it is effective to utilize the strength-enhancing effect of ausforming (dislocation strengthening) by flattening austenite grains. However, if the flat austenite grains are coarse, cracks tend to propagate in a specific direction, making parts more susceptible to fracture when impacted in a specific direction. This reduces the impact resistance of the steel sheet. Therefore, the impact resistance of steel sheets can be further improved by flattening the austenite grains or by reducing the grain size 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 of the steel sheet 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. The lower limit of the maximum value of the prior austenite grain size is not particularly limited, but may be 50 μm or more, 70 μm or more, or 100 μm or more.

[0071] The aspect ratio and grain size of the prior austenite grains at the 1 / 2 position of the plate thickness are measured by the following method. A test piece is taken from the steel plate so that the metal structure can be observed at the 1 / 2 thickness position (from the surface to the 3 / 8 position of the thickness in the thickness direction). After mirror-polishing the cross section of the plate parallel to the rolling direction, the prior austenite grain boundaries are revealed using an etchant (as specified in JIS G 0551:2020, Appendix JA, JA.2). Using an optical microscope, the prior austenite grains are identified in an area of ​​200 μm (thickness direction) × 600 μm (perpendicular to the thickness direction) at the 1 / 2 thickness position. Next, the prior austenite grains are approximated as 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. Furthermore, the aspect ratio is obtained by calculating the ratio of the major axis to the minor axis using the major axis and the minor axis of the five prior austenite grains (the five largest prior austenite grains). The "maximum aspect ratio of the prior austenite grains" is obtained by calculating the average value of the aspect ratios of the five prior austenite grains. 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." The method for determining the rolling direction of a steel sheet will be described later.

[0072] Prior austenite grains are approximated to ellipsoids by the following method. As shown in Figure 1, for the identified prior austenite grain G, the area S of the grain region not included in the ellipsoid is out and the area of ​​the non-grain region within the ellipsoid, S in Approximate it as an ellipsoid g so that the sum of a and b is minimized. By approximating it as an ellipsoid g in this way, (x0, y0): the center of ellipsoid g, a: the major axis of ellipsoid g, and b: the minor axis of ellipsoid g are found.

[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 alloy) is a coating containing 70% or more by mass of Fe and Al in total. A Zn-based coating (a coating mainly made of an Fe-Zn alloy) is a coating containing 70% or more by mass of Fe and Zn in total.

[0075] The Al-based coating (a coating mainly composed 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 remainder being impurities. The Zn-based coating (a coating mainly composed 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 remainder 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 10 mm or more 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 can be seen between the coating and the base steel (steel plate). Therefore, the thickness of the coating can be determined 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. 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 spot elemental analysis (beam diameter 1 μm or less) using an electron probe microanalyzer (EPMA) on the same observation area as above 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 value is taken as the concentration of Fe, Al, and Zn contained in the coating. The same method can be used to determine the concentrations even if elements other than Fe, Al, and Zn are present.

[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 sheet 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. The upper limit of the tensile strength is preferably set to 1200 MPa or less from the viewpoint of suppressing die wear and ensuring the ductility of the steel sheet.

[0081] Ductility: Total elongation (El) is 10.0% or more The total elongation 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 sheet 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 30° around this Z direction. The thickness cross section of the taken test specimens is polished, and the prior austenite grain boundaries are revealed using the above-mentioned etching solution. 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 expansion: 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 conducting a hole expansion test in accordance with JIS Z 2256:2020. The hole expansion test specimen is preferably taken from a quarter of the width of the steel plate, similar to 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 the separation cracks is 5.0 mm or less, and the Charpy impact value at -40°C is 50 J / cm2 End In this 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 2 Therefore, the steel plate according to this embodiment is a steel plate having a total separation crack length of 5.0 mm or less and a Charpy impact value of 50 J / cm at -40°C. 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 of 75 J / cm at -40°C. 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 carried out 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 shape of the test specimen, including the notch shape described below, conforms to JIS Z 2242:2023 except for the thickness. Even if the steel plate has a coating, the test specimen is taken without removing the coating. A 2 mm deep V-notch 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 longitudinal end of the test specimen. Three test specimens are stacked and secured with screws, and a Charpy impact test is performed. However, if the test specimen thickness (steel plate thickness) is less than 2.0 mm, the test is performed using three stacked specimens. However, if the test specimen thickness is 2.0 mm or more, the test is performed using a single specimen without stacking.

[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 method described above, 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 and at 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 (thickness of the steel plate) is taken as the length of the separation crack. If multiple separation cracks occur 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 (or the total thickness if 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 can also be called a hot-rolled steel sheet because it is manufactured by hot-rolling a slab. In the following description, the temperature refers to the surface temperature of the steel sheet.

[0095] In a preferred method for manufacturing a steel sheet according to this embodiment, In the finishing process of hot rolling, After the first cooling step, in which the surface temperature is reduced by 30 to 200°C in the temperature range of 1050 to 1150°C, the steel is rolled to a total reduction rate of 30% or more within 10 seconds. Rolling is performed with a total reduction of 30 to 80% at the stand one before the final stand (the n-1 stand) and the final stand (the n stand), The rolling in the final stand (nth stand) is carried out so that the finish rolling completion temperature is in the temperature range of 860 to 950°C, After finish rolling, cooling is performed at an average cooling rate of 50°C / s or more to a temperature range from the finish rolling completion temperature to 700°C, and the residence time in the temperature range of 580 to 700°C is 10.0 seconds or less, Coiling is performed in the temperature range of 450 to 530°C. Furthermore, in finish rolling, It is more preferable to carry out second cooling to reduce the surface temperature by 30 to 60°C before rolling in the stand immediately before 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-mentioned 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 the hot rolling are not particularly limited. In the finish rolling, it is preferable to carry out a first cooling step in which the surface temperature is reduced by 30 to 200°C in a temperature range of 1050 to 1150°C, followed by rolling to a total reduction of 30% or more within 10 seconds. An example of the first cooling step in which the surface temperature is reduced by 30°C or more is water cooling. The first cooling step may be started when the surface temperature before cooling is in the temperature range of 1050 to 1150°C, and cooling may be carried out so that the surface temperature is reduced by 30 to 200°C.

[0099] The area fraction of granular bainite at the 1 / 4 and 1 / 2 thickness positions can be preferably controlled by performing a first cooling step 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 recuperation).Furthermore, the maximum value of the prior austenite grain size at the 1 / 2 thickness position can be preferably controlled by performing a first cooling step in which 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.

[0100] The total rolling reduction in this embodiment can be expressed as (1-t1 / t0) x 100 (%), where t0 is the initial plate thickness before rolling in the set range and t1 is the final plate thickness after rolling in the set range.

[0101] Furthermore, in finish rolling, it is preferable that the total reduction in the stand immediately before the final stand (the n-1th stand) and the final stand (the nth stand) is 30 to 80%, and that the rolling in the final stand is performed 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 to reduce the surface temperature 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, the maximum aspect ratio of the prior austenite grains at the half-thickness position can be preferably controlled.

[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 700°C, and to perform cooling such 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, it is possible to suppress the formation of ferrite and pearlite and to preferably control other structures. Furthermore, by performing cooling such that the residence time in the temperature range of 580 to 700°C is 10.0 seconds or less, it is possible to preferably control the number density of precipitates containing B at the 1 / 4 position in the plate thickness. After cooling is performed so that the residence time in the temperature range of 580 to 700°C is 10.0 seconds or less, the material may be cooled to the coiling temperature by air cooling, for example.

[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 cooling, the sheet is preferably coiled in a temperature range of 450 to 530° C. By coiling in this temperature range, the area ratio of granular bainite at the 1 / 4 thickness position and the 1 / 2 thickness position can be preferably controlled.

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

[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 with 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 slab was set to 1200°C or higher, and the holding time in this temperature range was set to 3500 seconds. In addition, in the examples where the "temperature drop 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 table.

[0108] The obtained steel sheets were evaluated for their metallographic structure, tensile strength, total elongation, hole expansion ratio, and impact resistance properties by the methods described above. Crystal orientation information of the metallographic structure was analyzed using OIM Analysis (registered trademark) version 7.3.1 manufactured by EDAX / TSL solution. The results obtained are shown in Tables 4A to 5. The underlines in the table indicate that the material is outside the scope of the present disclosure, that the manufacturing conditions are not preferable, or that the characteristic values ​​are not preferable.

[0109] When the tensile strength was 980 MPa or more, the specimen was judged to have high strength and pass the test, whereas when the tensile strength was less than 980 MPa, the specimen was judged to have insufficient strength and fail the test.

[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 poor 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 2If the result was equal to or greater than this, the sample was judged to have excellent impact resistance properties and passed the test, and the result was recorded as "Good" in the table. On the other hand, if the total length of the separation cracks exceeds 5.0 mm, and / or the Charpy impact value at -40°C is 50 J / cm 2 If the impact resistance was less than 100%, the sample was judged as not having excellent impact resistance properties and was rejected, and the rating was entered as "Poor" in the table. In addition, 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 2 If the test piece was found to have the above-mentioned properties, it was judged to have better impact resistance properties and was recorded as "Excellent" in the table.

[0113] [Table 1]

[0114] [Table 2]

[0115] [Table 3A]

[0116] [Table 3B]

[0117] [Table 4A]

[0118] [Table 4B]

[0119] [Table 5]

[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. On the other hand, it is clear that the steel sheets according to the comparative examples are inferior in one or more of the above properties.

[0121] Furthermore, 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. [Industrial Applicability]

[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. The chemical composition is expressed in mass percent. C: 0.040-0.180%, Si: 0.20-2.00%, Mn: 1.00-3.00%, 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: 0.50-1.00%, B: 0.00003% or more, less than 0.00150% Mo: 0-0.500%, W: 0 to 0.500%, Co: 0 to 0.500%, Ni: 0-1.000%, 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: 0-0.100%, Bi: 0 to 0.0400%, Ca: 0-0.0400%, Mg: 0 to 0.1000%, REM: 0-0.1000%, and, The remainder consists of Fe and impurities. At a position 1 / 4 of the plate thickness in the direction of plate thickness from the surface, The metallic structure, in area percentage, Granular bainite: 30-50%, Acicular bainite: 20-50%, Fresh martensite, tempered martensite, and one or more retained austenites: 10-30% in total. One or more ferrites and pearlites: less than 5% in total. The number density of precipitates containing B is 0.0050 particles / μm ２ The following: From the surface, in the direction of the plate thickness, at a position that is half the thickness of the plate, A steel sheet characterized by its metallic structure containing granular bainite: 30-65% by area percentage.

2. The aforementioned chemical composition, in mass%, Mo: 0.001-0.500%, W: 0.001-0.500%, Co: 0.001 to 0.500%, Ni: 0.001 to 1.000%, Cu: 0.001 to 1.000%, V: 0.001-0.500%, 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: 0.001-0.100%, Bi: 0.0001-0.0400%, Ca: 0.0001-0.0400%, Mg: 0.0001 to 0.1000%, and The steel sheet according to claim 1, characterized in that it contains one or more substances from the group consisting of REM: 0.0001 to 0.1000%.

3. From the surface, in the direction of the plate thickness, at a position that is half the thickness of the plate, The steel sheet according to claim 1 or 2, characterized in that the maximum aspect ratio of the prior austenite grains is 3.5 to 5.

0.

4. From the surface, in the direction of the plate thickness, at a position that is half the thickness of the plate, The steel sheet according to claim 1 or 2, characterized in that the maximum value of the prior austenite particle size is 200 μm or less.

5. A component characterized by being made of the steel plate described in claim 1 or 2.

Images