Steel plates and parts

Abstract

This steel plate has a predetermined chemical composition, and in its metal structure at a 1 / 4 position in the plate thickness, it contains, in area percentages, 30 to 50% granular bainite, 20 to 50% acicular bainite, 10 to 30% in total of one or more of fresh martensite, tempered martensite, and retained austenite, and less than 5% in total of one or more of ferrite and pearlite; and in the surface layer region, it contains, in area percentages, one or more of fresh martensite, tempered martensite, and retained austenite, and less than 10% in total of one or more of fresh martensite, tempered martensite, and retained austenite, and the average circle equivalent diameter of the MA phase consisting of the fresh martensite and the retained austenite, and the tempered martensite is 5.0 μm or less.

Description

[Technical Field]

[0001] The present disclosure relates to steel sheets and components. This application claims priority based on Japanese Patent Application No. 2024-123619, filed on July 30, 2024, the contents of which are incorporated herein by reference. [Background technology]

[0002] In recent years, efforts have been made to reduce the weight of automobile bodies in order to reduce CO2 emissions. For blank formed parts such as press-formed parts, weight can be reduced by reducing the plate thickness of the part material. In particular, for automobile suspension parts such as lower arms and trailing arms, the use of steel plates with a strength of over 980 MPa has begun to be considered in order to achieve weight reduction in automobile bodies.

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

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

[0005] Patent Document 1 discloses a high-strength hot-rolled steel sheet having a structure in which 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]

[0006] [Patent Document 1] International Publication No. 2017 / 017933 Summary of the Invention [Problem to be solved by the invention]

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

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

[0009] 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: 0.020~0.150%, 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; In the surface region which is a region from the surface to a position 120 μm from the surface in the plate thickness direction, The metal structure is, in area percentage, One or more of fresh martensite, tempered martensite, and retained austenite: a total of 10.0% or less; The steel sheet is characterized in that the average circle equivalent diameter of the MA phase consisting of the fresh martensite and the retained austenite, and the tempered martensite is 5.0 μm or less. [2] The chemical composition is, in mass%, Mo: 0.001 to 0.500%, W: 0.001 to 0.500%, Co: 0.001 to 0.500%, Ni: 0.001 to 1.000%, Cu: 0.001 to 1.000%, V: 0.001~0.500%, 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] In the metal structure at a 1 / 4 position of the plate thickness from the surface in the plate thickness direction, The steel sheet according to [1] or [2], characterized in that the average aspect ratio of the prior austenite grains is 3.0 or more. [4] In the metal structure at a 1 / 4 position of the plate thickness from the surface in the plate thickness direction, The steel sheet according to any one of [1] to [3], wherein the average aspect ratio of prior austenite grains is 6.0 or less. [5] A part made of the steel sheet according to any one of [1] to [4]. [Effects of the Invention]

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

[0011] [Figure 1] FIG. 1 is a diagram for explaining a method for approximating prior austenite grains to ellipsoids. [Figure 2] FIG. 10 is a diagram for explaining a drop weight test. DETAILED DESCRIPTION OF THE INVENTION

[0012] As a result of investigations conducted by the present inventors to solve the above problems, the present inventors have come to the following findings. It has been discovered that the impact resistance properties after pre-strain can be improved by controlling the total area ratio of one or more of fresh martensite, tempered martensite, and retained austenite in the metal structure at a position 1 / 4 of the plate thickness from the surface of the steel plate in the plate thickness direction and in the surface layer region, as well as the average value of the circle equivalent diameter of fresh martensite, tempered martensite, and retained austenite in the surface layer region.

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

[0014] The steel sheet according to this embodiment will be described in detail below. First, the reasons for limiting the chemical composition of the steel sheet according to this embodiment will be described.

[0015] The steel sheet according to this embodiment has the following chemical composition. Note that the numerical ranges described below, separated by "to", include the lower and upper limits. Numerical values ​​indicated as "less than" and "greater than" do not include the numerical range. All % in the chemical composition indicates mass %.

[0016] 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.

[0017] 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.

[0018] Si: 0.20 to 2.00% Si is an element that suppresses the formation of iron carbides and improves the strength, ductility, and hole expandability of steel 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, 0.70% or more, or 0.80% or more. On the other hand, if the Si content exceeds 2.00%, the amount of ferrite increases and the hole expandability of the steel sheet decreases. Therefore, the Si content is set to 2.00% or less. The Si content is preferably 1.60% or less, 1.30% or less, or 1.00% or less.

[0019] Mn: 1.00 to 3.00% Mn is an element that increases the strength of 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.

[0020] 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 of the steel sheet is significantly reduced. Therefore, the P content is set to 0.0150% or less. The P content is preferably 0.0130% or less, 0.0100% or less, 0.0080% or less, or 0.0070% or less. 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.

[0021] 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 0.0100%, the ductility and hole expandability of steel sheets will be significantly reduced. Therefore, the S content is set to 0.0100% or less. The S content is preferably 0.0080% or less, 0.0060% or less, or 0.0040% or less. 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.

[0022] Al: 0.100% or less Al is an element contained as a deoxidizer for molten steel. If the Al content exceeds 0.100%, the amount of ferrite increases, and the hole expandability of the steel sheet decreases. Therefore, the Al content is set to 0.100% or less. The Al content is preferably 0.080% or less, 0.050% or less, or 0.035% or less. 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.

[0023] 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.

[0024] 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.

[0025] 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 0.120% or less, 0.110% or less, or 0.100% or less.

[0026] 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 hardenability becomes excessive and the ductility of the steel sheet decreases. Therefore, the Cr content is set to 1.00% or less. The Cr content is preferably 0.90% or less or 0.80% or less.

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

[0028] The balance of the chemical composition of the steel sheet according to this embodiment may be Fe and impurities. In this embodiment, the impurities refer to substances that are mixed in from raw materials such as ore 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.

[0029] The steel sheet according to this embodiment may contain the following optional elements instead of part of Fe. When no optional elements are contained, the lower limit of the content is 0%. Each optional element will be described below.

[0030] Mo: 0 to 0.500% Mo is an element that increases the strength of steel sheet by forming fine carbides in 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 0.210% or less.

[0031] W: 0 to 0.500% W is an element that increases the strength of steel sheet through solid solution strengthening. To 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, or 0.300% or less.

[0032] Co: 0 to 0.500% Co is an element that increases the strength of steel sheet through solid solution strengthening, 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.

[0033] Ni: 0 to 1.000% Ni is an element that improves the hardenability of steel sheet and increases its strength. Furthermore, when Cu is contained, Ni has the effect of effectively suppressing 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, since Ni is an expensive element, it is not economically preferable to include a large amount of Ni. Therefore, the Ni content is set to 1.000% or less. The Ni content is preferably 0.800% or less, 0.700% or less, 0.500% or less, 0.250% or less, or 0.210% or less.

[0034] Cu: 0 to 1.000% Cu is an element that 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.

[0035] 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, 0.250% or less, 0.100% or less, or 0.070% or less.

[0036] Nb: 0 to 0.150% Nb is an element that increases the strength of steel sheet by refining the metal structure and strengthening the precipitation of NbC. To reliably obtain this effect, the Nb content is preferably 0.001% or more, or 0.010% or more. On the other hand, if the Nb content exceeds 0.150%, the above effect saturates. 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.

[0037] As: 0 to 0.050% As is an element that reduces the austenite single-phase temperature, thereby refining prior austenite grains and improving the hole expandability of 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.

[0038] 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.

[0039] 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.

[0040] Sb: 0 to 0.050% Sb is an element that suppresses the generation of oxides that serve as fracture initiation sites, thereby improving the ductility and hole expandability of steel sheets. To reliably obtain this effect, the Sb content is preferably 0.001% or more or 0.010% or more. On the other hand, even if 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.

[0041] 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.

[0042] 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 0.0150% or less or 0.0110% or less.

[0043] 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.

[0044] 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.

[0045] 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, which reduces the ductility and hole expandability of the steel sheet, so the REM content is set to 0.1000% or less. REM refers to a total of 17 elements consisting of Sc, Y, and lanthanoids, and the REM content refers to the total content of these elements. The REM content is preferably 0.0950% or less, 0.0900% or less, 0.0500% or less, or 0.0100% or less.

[0046] The chemical composition of the 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.

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

[0048] In this embodiment, the metal structure is defined at a quarter position of the plate thickness from the surface in the plate thickness direction (hereinafter, sometimes referred to as the quarter position of the plate thickness) and in the surface layer region. 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 surface layer region can be expressed as a range starting from the surface of the steel plate and ending at a position 120 μm from the surface of the steel plate in the plate thickness direction.

[0049] The surface of the steel sheet referred to here refers to the surface of the steel sheet when the steel sheet does not have a coating on its surface, and refers to the interface between the coating and the steel sheet when the steel sheet has a coating on its surface. The interface between the coating and the steel sheet is identified by a BSE COMPO image (BSE Compositional Image) which will be described later.

[0050] <Plate thickness 1 / 4 position> Granular Bainite: 30~50% Granular bainite increases the ductility of the steel plate. If the area fraction of granular bainite at the 1 / 4 position in the plate thickness direction is less than 30%, the ductility of the steel plate decreases. Therefore, the area fraction of granular bainite at the 1 / 4 position in the plate thickness direction is set to 30% or more. The area fraction of granular bainite is preferably 33% or more or 35% or more. On the other hand, if the area fraction of granular bainite at the 1 / 4 position in 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.

[0051] 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 sheet thickness exceeds 50%, the ductility of the steel sheet decreases. Therefore, the area fraction of acicular bainite at the 1 / 4 position in the sheet thickness is set to 50% or less. The area fraction of acicular bainite is preferably 47% or less, 45% or less, or 40% 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.

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

[0053] One or more of ferrite and pearlite: Less than 5% in total 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%.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] The total area fraction of fresh martensite and tempered martensite is obtained by subtracting the area fraction of retained austenite obtained by X-ray diffraction from the total area fraction of "fresh martensite, tempered martensite, and retained austenite" obtained by observation using the FE-SEM 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%.

[0058] 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.

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

[0060] The area 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.

[0061] 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.

[0062] 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."

[0063] 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".

[0064] <Surface area> One or more of martensite and retained austenite: 10.0% or less in total If there is an excess of fresh martensite, tempered martensite, and retained austenite in the surface layer region, the impact resistance properties after pre-straining will decrease. If the total area ratio of fresh martensite, tempered martensite, and retained austenite exceeds 10.0%, crack generation in the plate thickness direction cannot be suppressed, and the impact resistance properties after pre-straining will decrease. Therefore, the total area ratio of fresh martensite, tempered martensite, and retained austenite in the surface layer region is set to 10.0% or less. The total area ratio of fresh martensite, tempered martensite, and retained austenite in the surface layer region is preferably 8.0% or less, 5.0% or less, or 4.0% or less. When any one of them is contained, it is sufficient that the area ratio of that one kind is 10.0% or less. The smaller the total area ratio of fresh martensite, tempered martensite, and retained austenite in the surface layer region, the better, and therefore the total area ratio of fresh martensite, tempered martensite, and retained austenite in the surface layer region may be 0.0% or more.

[0065] Average circle equivalent diameter of MA phase consisting of fresh martensite and retained austenite, and tempered martensite: 5.0 μm or less If the size of the MA phase consisting of fresh martensite and retained austenite in the surface layer region and the tempered martensite are large, it is not possible to suppress cracking in the sheet thickness direction, and it is not possible to improve the impact resistance properties after pre-strain. Therefore, the average circle equivalent diameter of the MA phase consisting of fresh martensite and retained austenite and the tempered martensite in the surface layer region is set to 5.0 μm or less. The average circle equivalent diameter of the MA phase consisting of fresh martensite and retained austenite and the tempered martensite in the surface layer region is preferably 4.0 μm or less, 3.5 μm or less, 3.0 μm or less, or 2.5 μm or less. The smaller the average circle equivalent diameter of the MA phase consisting of fresh martensite and retained austenite in the surface layer region, and the tempered martensite, the better, but it may be 1.0 μm or more, or 2.0 μm or more.

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

[0067] <Plate thickness 1 / 4 position> Average aspect ratio of prior austenite grains: 3.0 or more By setting the average aspect ratio of the prior austenite grains at 1 / 4 of the plate thickness to 3.0 or more, the impact resistance after pre-straining can be further improved. Therefore, the average aspect ratio of the prior austenite grains at the 1 / 4 position of the plate thickness is preferably set to 3.0 or more. The average aspect ratio of the prior austenite grains at the 1 / 4 position of the plate thickness is more preferably 3.5 or more or 4.0 or more.

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

[0069] The aspect ratio of the prior austenite grains at the 1 / 4 position of the plate thickness is measured by the following method. A test piece is taken from the steel plate so that the metal structure can be observed at the 1 / 4 position in the thickness direction (from the surface to the 1 / 8 position in the thickness direction to the 3 / 8 position 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 (the etchant described in JA.2 of Appendix JA of JIS G 0551:2020). Using an optical microscope, the prior austenite grains are identified in a 200 μm (thickness direction) × 600 μm (perpendicular to the thickness direction) region at the 1 / 4 position in the thickness direction. Next, the prior austenite grains are approximated as ellipses using the method described below, and their major and minor axes are determined. The ratio of the major and minor axes (aspect ratio) is calculated for all prior austenite grains in the region, and the average value is calculated by weighting the area of ​​each prior austenite grain to obtain the average aspect ratio of the prior austenite grains. Prior austenite grains with a major axis of 2 μm or less are excluded from the measurement. The average aspect ratio of prior austenite grains can be generally expressed by the following formula: where Ai is the area of ​​the i-th prior austenite grain, and ri is the aspect ratio of the i-th prior austenite grain. Average aspect ratio of prior austenite grains = Σi(Ai×ri) / ΣiAi If the prior austenite grains cannot be sufficiently revealed by the above-mentioned method, the prior austenite grains are identified by the reconstruction method described in "Kengo Hata, Masayuki Wakita, Kazuki Fujiwara, Kaori Kawano, Nippon Steel & Sumitomo Metal Technical Report, No. 114 (2017), pp. 26-31." The method for determining the rolling direction of a steel sheet will be described later.

[0070] 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 inApproximate 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.

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

[0072] An Al-based coating (a coating mainly made of an Fe-Al 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.

[0073] 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.

[0074] The chemical composition and thickness of the coating can be determined by cross-sectional observation using a scanning electron microscope. A sample is cut out from an arbitrary position 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. 2The above range applies.

[0075] 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.

[0076] 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.

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

[0078] Strength: Tensile strength (TS) of 980 MPa or more The steel 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.

[0079] 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.

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

[0081] The rolling direction of the steel 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.

[0082] 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.

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

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

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

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

[0087] A part manufactured using the steel plate according to this embodiment has the same chemical composition as the above-described steel plate. Furthermore, the part may contain both processed and unprocessed parts. The unprocessed part has the same metallurgical structure as the above-described steel plate. The processed part basically has the same metallurgical structure as the above-described steel plate, but if heavily processed, it may not have the above-described metallurgical structure, or it may be difficult to determine whether it has. Therefore, when measuring the metallurgical structure of a part, the measurement is performed on the unprocessed part. If there is no unprocessed part, the measurement is performed on the part that has not been heavily processed. An unprocessed or heavily processed part refers to, for example, a flat part of the part, a part where the thickness change due to processing is small, and a part that avoids parts that have been subjected to punching, hole expansion, bending, etc. As an example, in the case of the above-described part, a test piece is taken from the flat part with the largest area near the center of gravity and measured.

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

[0089] Next, a preferred method for manufacturing the steel sheet according to this embodiment will be described. According to the manufacturing method described below, the steel sheet according to this embodiment can be stably manufactured. The steel sheet according to this embodiment 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.

[0090] 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 three or more times at a temperature of 1000°C or higher with a reduction rate of 30% or more, Rolling is performed in a temperature range of less than 1000°C with a total reduction of 20 to 50% and a shape ratio of 3.0 to 10.0, After finish rolling, the material is cooled from the finish rolling completion temperature to 700°C at an average cooling rate of 50°C / s or more. Coiling is performed in the temperature range of 450 to 530°C.

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

[0092] 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.

[0093] The slab to be heated is not particularly limited except that it has the above-mentioned chemical composition. For example, a slab produced by melting molten steel having the above-mentioned chemical composition using a converter or electric furnace, etc. and then by continuous casting can be used. Instead of continuous casting, an ingot casting method, thin slab casting method, etc. may also be used.

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

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

[0096] Furthermore, in the finish rolling, it is preferable to perform rolling at a temperature range of less than 1000° C., with a total reduction of 20 to 50% and an area ratio of 3.0 to 10.0. By performing rolling under these conditions, it is possible to flatten the prior austenite grains in the surface layer region. By refining and flattening the prior austenite grains in the surface layer region, the transformation into granular bainite and acicular bainite can be promoted, and as a result, the area ratios of fresh martensite, tempered martensite, and retained austenite in the surface layer region can be reduced.

[0097] The rolling reduction in this embodiment can be expressed as (1-t1 / t0) x 100 (%), where t0 is the thickness before rolling and t1 is the thickness after rolling. Furthermore, the total rolling reduction in this embodiment can be expressed as (1-t3 / t2) x 100 (%), where t2 is the initial plate thickness before rolling in the set range and t3 is the final plate thickness after rolling in the set range.

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

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

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

[0101] After finish rolling, it is preferable to perform cooling at an average cooling rate of 50°C / s or more from the finish rolling completion temperature to 700°C. By performing cooling at an average cooling rate of 50°C / s or more from the finish rolling completion temperature to 700°C, it is possible to suppress the generation of ferrite at the 1 / 4 plate thickness position. There are no particular restrictions on the cooling method as long as the average cooling rate is 50°C / s or more. After the above cooling, the sheet may be cooled to a coiling temperature, which will be described later, by air cooling, for example.

[0102] The average cooling rate here is the temperature difference between the start point and the end point of the set range divided by the elapsed time from the start point to the end point.

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

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

[0105] Next, the effects of one embodiment of the present disclosure will be explained in more detail using examples, but the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present disclosure, and the present disclosure is not limited to these examples. Various conditions may be adopted in the present disclosure as long as they do not deviate from the gist of the present disclosure and the object of the present disclosure is achieved.

[0106] Slabs were obtained by converter melting and continuous casting, and steel plates with thicknesses of 2.5 to 3.5 mm were obtained under the conditions shown in Tables 3A and 3B. Tables 1 and 2 show the chemical compositions obtained by analyzing test 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.

[0107] The obtained steel sheets were evaluated for their metallographic structure, tensile strength, total elongation, hole expansion ratio, and impact resistance after pre-straining using the methods described above. Crystal orientation information of the metallographic structure was analyzed using OIM Analysis (registered trademark) version 7.3.1 manufactured by EDAX / TSL solution. The results obtained are shown in Tables 4A to 5B. 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.

[0108] 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.

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

[0110] When the hole expansion ratio was 30% or more, the specimen was judged to have excellent hole expandability and to have passed the test. On the other hand, when the hole expansion ratio was less than 30%, the specimen was judged to have poor hole expandability and to have passed the test.

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

[0112] [Table 1]

[0113] [Table 2]

[0114] [Table 3A]

[0115] [Table 3B]

[0116] [Table 4A]

[0117] [Table 4B]

[0118] [Table 5A]

[0119] [Table 5B]

[0120] It can be seen from Tables 4A to 5B that the steel sheets according to the examples of the present invention have high strength, excellent ductility and hole expandability, and also have excellent impact resistance properties after being pre-strained. 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 5B. [Industrial Applicability]

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

Claims

1. 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 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: 0-0.100%, 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. In the surface region, which is the area from the surface to a location 120 μm from the surface in the thickness direction of the plate, The metallic structure, expressed as an area percentage, Fresh martensite, tempered martensite, and retained austenite: total of 10.0% or less. A steel sheet characterized by having an MA phase consisting of the fresh martensite and the retained austenite, and an average value of the equivalent circular diameter of the tempered martensite being 5.0 μm or less.

2. The aforementioned chemical composition is, 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. In the metal structure at a position 1 / 4 of the plate thickness in the direction of the plate thickness from the surface, The steel sheet according to claim 1 or 2, characterized in that the average aspect ratio of the prior austenite grains is 3.0 or greater.

4. In the metal structure at a position 1 / 4 of the plate thickness in the direction of the plate thickness from the surface, The steel sheet according to claim 1 or 2, characterized in that the average aspect ratio of the prior austenite grains is 6.0 or less.

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

Images