Steel plates and parts

Abstract

This hot-rolled steel sheet has a predetermined chemical composition, and in its metal structure at the 1 / 4 position of the sheet thickness, the area percentages are as follows: tempered martensite: more than 30% but not more than 80%, granular bainite: 10 to 50%, one or more of ferrite and pearlite: less than 5% in total, one or more of fresh martensite and retained austenite: not more than 10% in total, and the average aspect ratio of the prior austenite grains is 3.0 to 6.0.

Description

[Technical Field]

[0001] The present disclosure relates to steel sheets and components. This application claims priority based on Japanese Patent Application No. 2024-123403, 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] 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] Patent Document 1 discloses that bainite transformation is promoted by coiling in a temperature range of 300 to 530°C. This temperature range is the so-called transition boiling temperature range, and therefore temperature variations are likely to occur within the manufactured steel sheet. Fresh martensite is formed in areas where the amount of cooling is large, such as areas where cooling water falls on areas where no steam is generated on the surface of the steel sheet, and workability is significantly reduced in these areas. In other words, a steel sheet is manufactured that includes areas with locally low workability. When such a steel sheet is press-formed, press cracks unexpected from the strength of the steel sheet occur, causing a decrease in yield.

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

[0008] The gist of the present disclosure is as follows. [1] Chemical composition, in mass%, C: 0.050~0.180%, Si: 0.40 to 1.30% Mn: 1.60~2.80%, P: 0.100% or less, S: 0.0500% or less, Al: 0.020~0.100%, N: 0.0100% or less, O: 0.0060% or less, Ti: 0.030~0.150%, B: 0.0005~0.0050%, Cr: 0~1.00%, 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 0.0400%, REM: 0 to 0.0400%, and The balance is Fe and impurities. In the metal structure at 1 / 4 of the plate thickness from the surface in the plate thickness direction, Area % Tempered martensite: over 30% and up to 80% Granular bainite: 10-50% One or more of ferrite and pearlite: less than 5% in total, One or more of fresh martensite and retained austenite: 10% or less in total; A steel sheet characterized in that the average aspect ratio of prior austenite grains is 3.0 to 6.0. [2] The chemical composition is, in mass%, Cr: 0.01 to 1.00%, Mo: 0.001 to 0.500%, W: 0.001 to 0.500%, Co: 0.001 to 0.500%, Ni: 0.001 to 1.000%, Cu: 0.001 to 1.000%, V: 0.001~0.500%, 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 0.0400%, 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 0.0400%. [3] In the texture at 1 / 2 position of the plate thickness from the surface in the plate thickness direction, {100} <011> ~{223} <110> The average value of the X-ray random intensity ratio of the orientation group is 5.0 to 11.0, {332} <113> The steel sheet according to [1] or [2], characterized in that the X-ray random intensity ratio in the above orientation is 5.0 to 9.0. [4] The chemical composition is B: The steel sheet according to any one of [1] to [3], characterized in that it contains 0.0016 to 0.0050% of B. [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, excellent ductility and hole expandability, and reduced material variation within the steel plate, 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 reduce the variation in material properties within steel sheets, it is effective to promote the formation of martensite rather than bainite. If bainite forms first during the cooling process, carbon will concentrate in the untransformed austenite, making it difficult to subsequently form martensite. Therefore, suppressing the formation of bainite can promote the formation of martensite. One method for suppressing the formation of bainite is (a) to create the shape of the austenite grains by imparting an appropriate amount of strain to the austenite grains during hot rolling. This slows the rate of bainite formation. Furthermore, (b) during cooling after finish rolling, shortening the residence time in the temperature range where bainite is likely to form can suppress the formation of bainite first and promote the formation of martensite.

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

[0013] The steel sheet according to this embodiment will be described in detail below. First, the reasons for limiting the chemical composition of the steel sheet according to this embodiment will be described.

[0014] The steel sheet according to this embodiment has the following chemical composition. Note that the numerical ranges described below, separated by "to", include the lower and upper limits. Numerical values ​​indicated as "less than" and "greater than" do not include the numerical range. All % in the chemical composition indicates mass %.

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

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

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

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

[0019] P:0.100% or less P is an element that segregates at grain boundaries and reduces the ductility and hole expandability of steel sheets. If the P content exceeds 0.100%, the ductility and hole expandability of steel sheets are significantly reduced. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.080% or less, 0.050% or less, 0.030% or less, or 0.020% or less. The lower the P content, the better, so it may be 0%. However, if the P content is reduced too much, the cost of dephosphorization increases significantly. Therefore, the P content may be set to 0.001% or more, or 0.005% or more.

[0020] S: 0.0500% or less S is an element that forms sulfides such as MnS, thereby reducing the ductility and hole expandability of 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 0.0300% or less, 0.0200% or less, 0.0100% or less, or 0.0050% 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.

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

[0022] N: 0.0100% or less N is an element that forms coarse nitrides in steel and reduces the hole expandability of 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.

[0023] 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 0.0050% or less or 0.0040% 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.

[0024] Ti: 0.030 to 0.150% Ti is an element that precipitates in steel as Ti carbides such as TiC and increases the strength of the steel sheet through precipitation strengthening. If the Ti content is less than 0.030%, the strength of the steel sheet decreases. Therefore, the Ti content is set to 0.030% or more. The Ti content is preferably 0.050% or more, 0.080% or more, or 0.100% or more. On the other hand, if the Ti content exceeds 0.150%, coarse carbides are formed in the steel, which causes slab cracking during hot rolling and reduces the ductility and hole expandability of the steel sheet. Therefore, the Ti content is set to 0.150% or less. The Ti content is preferably 0.130% or less or 0.110% or less.

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

[0026] The balance of the chemical composition of the steel sheet according to this embodiment may be Fe and impurities. In this embodiment, the impurities refer to substances that are mixed in from raw materials such as ore 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] Cr: 0 to 1.00% Cr is an element that improves the hardenability of steel and increases the strength of steel sheets. Cr also promotes the formation of granular bainite. To ensure these effects, the Cr content is preferably 0.01% or more, or 0.05% or more. On the other hand, if the Cr content exceeds 1.00%, the ductility of the steel sheet decreases. Therefore, the Cr content is set to 1.00% or less. The Cr content is preferably 0.85% or less, 0.70% or less, 0.65% or less, 0.50% or less, 0.35% or less, or 0.20% or less.

[0029] Mo: 0 to 0.500% Mo is an element that increases the strength of steel sheet by forming fine carbides in 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, 0.300% or less, 0.150% or less, or 0.110% or less.

[0030] W: 0 to 0.500% W is an element that increases the strength of steel sheet through solid solution strengthening. To 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 0.400% or less, 0.300% or less, 0.200% or less, 0.100% or less, 0.050% or less, or 0.040% or less.

[0031] Co: 0 to 0.500% Co is an element that increases the strength of steel sheet through solid solution strengthening, 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, 0.300% or less, 0.200% or less, 0.100% or less, 0.050% or less, or 0.040% or less.

[0032] Ni: 0 to 1.000% Ni is an element that improves the hardenability of steel sheet and increases its strength. Furthermore, when Cu is contained, Ni has the effect of effectively suppressing 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 0.200% or less.

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

[0034] V: 0 to 0.500% V is an element that increases the strength of 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, 0.150% or less, or 0.110% or less.

[0035] Nb: 0 to 0.150% Nb is an element that increases the strength of steel sheet by refining the metal structure and strengthening the precipitation of NbC. To reliably obtain this effect, the Nb content is preferably 0.001% or more, or 0.010% or more. On the other hand, if the Nb content exceeds 0.150%, the above effect saturates. 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 0.025% or less.

[0036] As: 0 to 0.050% As is an element that reduces the austenite single-phase temperature, thereby refining prior austenite grains and improving the hole expandability of 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 0.010% or less or 0.005% or less.

[0037] Zr: 0 to 0.050% Zr is an element that increases the strength of steel sheet through solid solution strengthening, 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 0.010% or less or 0.005% or less.

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

[0039] Sb: 0 to 0.050% Sb is an element that suppresses the generation of oxides that serve as fracture initiation sites, thereby improving the ductility and hole expandability of steel sheets. To reliably obtain this effect, the Sb content is preferably 0.001% or more or 0.010% or more. On the other hand, even if 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.

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

[0041] 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 0.0100% or less or 0.0015% or less.

[0042] 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 0.0100% or less or 0.0015% or less.

[0043] Mg: 0 to 0.0400% Like Ca, Mg is an element that controls the morphology of 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 0.0400%, 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 0.0400% or less. The Mg content is preferably 0.0100% or less or 0.0015% or less.

[0044] REM: 0 to 0.0400% Like Ca, REM is an element that controls the morphology of 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 0.0400%, excessive inclusions are formed in the steel, which reduces the ductility and hole expandability of the steel sheet. Therefore, the REM content is set to 0.0400% or less. REM refers to a total of 17 elements consisting of Sc, Y, and lanthanoids, and the above REM content refers to the total content of these elements. The REM content is preferably 0.0250% or less, or 0.0200% or less.

[0045] The chemical composition of the 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.

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

[0047] In this embodiment, the 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.

[0048] The surface of the steel sheet referred to here refers to the surface of the steel sheet when the steel sheet does not have a coating on its surface, and refers to the interface between the coating and the steel sheet when the steel sheet has a coating on its surface. The interface between the coating and the steel sheet is identified by a BSE COMPO image (BSE Compositional Image) which will be described later.

[0049] <Plate thickness 1 / 4 position> Tempered martensite: over 30% and under 80% Tempered martensite is a structure that improves strength, ductility, and hole expandability, and further reduces the material variability within the steel sheet. If the area fraction of tempered martensite is 30% or less, the material variability within the steel sheet increases. Therefore, the area fraction of tempered martensite is set to more than 30%. The area fraction of tempered martensite is preferably 35% or more, 40% or more, 45% or more, or 50% or more. On the other hand, if the area ratio of tempered martensite exceeds 80%, the ductility of the steel sheet decreases. Therefore, the area ratio of tempered martensite is set to 80% or less. The area ratio of tempered martensite is preferably 75% or less, 70% or less, or 65% or less.

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

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

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

[0053] The area ratio of the metal structure is measured by the following method. First, a method for measuring the area ratios of fresh martensite and retained austenite will be described. A test piece is taken from 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.

[0054] In Repela corrosion, fresh martensite and retained austenite are not corroded, so by calculating the area ratio of the uncorroded region, the total area ratio of fresh martensite and retained austenite is obtained. It is sometimes thought that fresh martensite, tempered martensite, and retained austenite are not corroded by Repela corrosion. However, in this embodiment, for convenience, the structure that is not corroded by Repela corrosion is considered to be fresh martensite or retained austenite. Furthermore, in order to observe the same region in the area ratio measurement (excluding X-ray diffraction) described below, it is preferable to make Vickers indentations at three of the four corners of the observation region with the FE-SEM within 100 μm of each corner. By using these Vickers indentations as markers, it is possible to observe the same region as the observation region with the FE-SEM.

[0055] 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.

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

[0057] The area ratio of pearlite is obtained by the following method. For the same area (200 μm × 600 μm) as that used to determine the area ratio of fresh martensite by FE-SEM observation, only the corroded layer is removed by polishing and the specimen is given a mirror finish, then etched using nital solution, and observed using FE-SEM, followed by image analysis. 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, tempered martensite, and granular bainite are obtained by the following method. The same area (200 μm × 600 μm) as that used to determine the area ratio of fresh 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 (registered trademark) 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 2The 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 fresh martensite and tempered martensite, and the area ratio (the total measured area including those other than the BCC crystal structure is used as the denominator) is used. 2 The total area ratio of "fresh martensite and tempered martensite" is obtained by calculating the ratio (the ratio where the numerator is the area of ​​crystal grains that are greater than 100%). The area ratio of tempered martensite is obtained by subtracting the area ratio of fresh martensite obtained by the above-mentioned methods using Lepera corrosion and X-ray diffraction from the total area ratio of "fresh martensite and tempered martensite".

[0061] The metal structure determined to be tempered martensite by the above method (i.e., a metal structure other than a structure that is not corroded by Repellant corrosion (i.e., fresh martensite and retained austenite)) and among the grains with a GAM value of more than 0.5° (crystal grains with a BCC crystal structure other than crystal grains determined to be ferrite), the 5° boundary density is 0.4 μm / μm 2 The crystal grains (thicker than 10 ... The structure known as "martensite" is a structure formed by an aggregate of laths, and satisfies both (1) and (2), where (1) needle-like cementite is present within the laths that make up the structure, and the cementite elongates in two or more directions, or (2) the average lath width is 1.0 μm or less. The lath width can be measured by drawing the shortest straight line that penetrates an aggregate of three or more laths that have the same long axis direction, and dividing the length of that line by the number of laths that penetrate. This is done for 10 fields of view, and the average value is taken as the average lath width. In FE-SEM observation, needle-like precipitates present inside the laths are considered to be cementite. Strictly speaking, needle-like precipitates include η carbide (Fe2C), ε carbide (Fe 2+x C), χ carbide (Fe5C2), etc., which are interpreted as precursor carbides of cementite, are collectively referred to as cementite. In other words, when the cementite is corroded by Repellant corrosion, the GAM value is greater than 0.5° and the 5° boundary density is 0.4 μm / μm 2 Grains of a BCC crystal structure that are not determined to be "martensite" by the above-mentioned method A are "bainite." Metal structures determined as tempered martensite by the above-mentioned Method A using Repela corrosion, GAM value, 5° boundary density, etc., are not determined as "martensite" but as bainite by the above-mentioned Method A when the B content is 0.0015% or less. When the B content is 0.0016% or more, these metal structures are determined as "martensite" by the above-mentioned Method A, and when the B content is 0.0020% or more, most of them are determined as "martensite" by the above-mentioned Method A. In this embodiment, for a structure determined to be tempered martensite by the above-mentioned area fraction measurement method using RePella corrosion, GAM value, 5° boundary density, etc., there is no need to confirm by the above-mentioned method A. The area fraction of tempered martensite measured by the above-mentioned area fraction measurement method using RePella corrosion, GAM value, 5° boundary density, etc. is not corrected by the above-mentioned method A.

[0062] In this embodiment, the area ratio of the metallographic structure is calculated by image analysis using 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".

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

[0064] The aspect ratio and grain size of prior austenite grains at the 1 / 4 position 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 / 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."

[0065] 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.

[0066] <Plate thickness 1 / 2 position> {100} <011> ~{223} <110> Average value of X-ray random intensity ratio of the orientation group: 5.0 to 11.0 {332} <113> X-ray random intensity ratio in the direction of: 5.0~9.0 In the texture at the 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 {100} <011> ~{223} <110> The average value of the X-ray random intensity ratio of the orientation group is 5.0 or more, and the {332} <113> By making the X-ray random intensity ratio of the orientation 5.0 or more, the strength of the steel sheet can be further increased. <011> ~{223} <110> The average value of the X-ray random intensity ratio of the orientation group is 11.0 or less, and the {332} <113> By making the X-ray random intensity ratio of the orientation 9.0 or less, the hole expandability of the steel sheet can be further improved.

[0067] {100} of the board surface <011> ~{223} <110> The average value of the X-ray random intensity ratio of the orientation group is {110} <001> , {116} <110> , {114} <110> , {113} <110> , {112} <110> , {335} <110> and {223} <110> The average value of each X-ray random intensity ratio is {hkl} <uvw>The crystal orientation is expressed as <hkl>Parallel to the rolling direction <uvw>This indicates that the line is parallel to the

[0068] In this embodiment, the 1 / 2 position in the plate thickness refers to the range from the surface of the steel plate in the plate thickness direction, from the position 3 / 8 of the plate thickness to the position 5 / 8 of the plate thickness, which can be rephrased as the range starting from the position 3 / 8 of the plate thickness from the surface of the steel plate and ending at the position 5 / 8 of the plate thickness.

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

[0070] Next, this sample is subjected to X-ray diffraction using a Rigaku RINT-2500 with Mo-Kα. Using Rigaku's Pole Figure Analysis Version 7.0 software, the crystal orientation distribution function is calculated using the series expansion method, and pole figures for {110}, {100}, {211}, and {310} are obtained. The intensity is normalized using the measurement results of a standard sample with random orientation. A uniform BG mode is also selected. Smoothing is also selected, and the number of points for the α and β directions is set to five. Using multiple pole figures (preferably three or more) from {110}, {100}, {211}, and {310} obtained in this way, the three-dimensional texture (three-dimensional distribution function of the X-ray random intensity ratio) is calculated using the series expansion method using Rigaku's ODF Data Processing Version 1.4 software. In this case, the expansion times for even terms are set to 22, those for odd terms to 19, and the intensity at which the X-ray random intensity ratio is set to 0.0 is set to 0.2. <001> , {116} <110> , {114} <110> , {113} <110> , {112} <110> , {335} <110> and {223} <110> By calculating the X-ray random intensity ratio and calculating the average value, {100} <011> ~{223} <110> The average value of the X-ray random intensity ratio of the orientation group is obtained. <113> By calculating the X-ray random intensity ratio of {332} <113> Obtain the X-ray random intensity ratio in the direction of

[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. 2 The 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 1300 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] Material variation In this embodiment, the material variation is preferably such that the variation in total elongation and hole expansion ratio measured for test specimens taken from different positions on the steel sheet is small. Specifically, tensile test specimens and hole expansion test specimens are first taken from the 1 / 4, 1 / 2, and 3 / 4 portions from the end of the steel sheet in the sheet width direction. Tensile tests and hole expansion tests are performed on each test specimen using the above-mentioned methods, and representative values ​​for each portion are obtained. Note that for the 1 / 4 portion from the end of the steel sheet in the sheet width direction, no new tests are performed, and the representative values ​​obtained by the above-mentioned tests are used.

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

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

[0086] The steel sheet according to this embodiment has high strength, excellent ductility and hole expandability, and reduced material variation within the steel sheet. Therefore, it can be suitably used for parts, particularly automobile parts. Among automobile parts, it can be suitably used for automobile suspension parts such as lower arms and trailing arms.

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

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

[0089] Next, a preferred method for manufacturing the steel sheet according to this embodiment will be described. According to the manufacturing method described below, the steel sheet according to this embodiment can be stably manufactured. The steel sheet according to this embodiment 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 at a temperature range of 1040°C or less with a total reduction ratio of 40 to 70%, The finish rolling temperature is in the range of 850 to 980°C. After finish rolling, the material is cooled to 550°C in 20 seconds or less. Coiling is carried out in the temperature range of 350 to 500°C.

[0091] In addition, in a more preferable method for manufacturing a steel sheet according to this embodiment, After being wound into a coil, it is more preferable to cool the coil so that the temperature at the end face at the midpoint between the outermost and innermost peripheries is maintained in the temperature range of 350 to 500° C. for 3600 to 18000 seconds. 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, 1150°C or higher. From the viewpoint of energy cost, the heating temperature of the slab is preferably 1350°C or lower.

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

[0094] The conditions for rough rolling in 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.

[0095] The area fraction of granular bainite at the 1 / 4 thickness position can be preferably controlled by performing a first cooling step in which the surface temperature drops by 30°C or more 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, by performing a first cooling step in which the surface temperature drops by more than 100°C but not more than 200°C, followed by rolling to a total reduction of 30% or more within 5 seconds, the range of thickness positions where the preferred area fraction of granular bainite is achieved is expanded, and a high balance of strength, elongation, and hole expandability can be achieved.

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

[0097] 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.

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

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

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

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

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

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

[0104] Next, the effects of one embodiment of the present disclosure will be explained in more detail using examples, but the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present disclosure, and the present disclosure is not limited to these examples. Various conditions may be adopted in the present disclosure as long as they do not deviate from the gist of the present disclosure and the object of the present disclosure is achieved.

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

[0106] The obtained steel sheets were evaluated for metallographic structure, tensile strength, total elongation, hole expansion ratio, and material variation by the above-mentioned methods. 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.

[0107] 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.

[0108] When the total elongation was 10.0% or more, the specimen was judged to have excellent ductility and to have passed the test, whereas when the total elongation was less than 10.0%, the specimen was judged to have poor ductility and to have passed the test.

[0109] When the hole expansion ratio was 30% or more, the specimen was judged to have excellent hole expandability and to have passed the test. On the other hand, when the hole expansion ratio was less than 30%, the specimen was judged to have poor hole expandability and to have passed the test.

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

[0111] [Table 1]

[0112] [Table 2]

[0113] [Table 3A]

[0114] [Table 3B]

[0115] [Table 4A]

[0116] [Table 4B]

[0117] [Table 5A]

[0118] [Table 5B]

[0119] It can be seen from Tables 4A to 5B that the steel sheets according to the present invention have high strength, excellent ductility and hole expandability, and also have reduced material variation within the steel sheets. 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.

[0120] 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]

[0121] According to the above aspects of the present disclosure, it is possible to provide a steel plate having high strength, excellent ductility and hole expandability, and reduced material variation within the steel plate, and a part using this steel plate.< / uvw> < / hkl> < / uvw>

Claims

1. The chemical composition is expressed in mass percent. C: 0.050-0.180%, Si: 0.40-1.30%, Mn: 1.60-2.80%, P: 0.100% or less, S: 0.0500% or less, Al: 0.020-0.100%, N: 0.0100% or less, O: 0.0060% or less, Ti: 0.030 to 0.150%, B: 0.0005 to 0.0050%, Cr: 0-1.00%, 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 0.0400%, REM: 0-0.0400%, and, The remainder consists of Fe and impurities. In the metallographic structure at a position 1 / 4 of the plate thickness in the direction of plate thickness from the surface, In area percentage, Tempered martensite: over 30%, 80% or less. Granular bainite: 10-50%, One or more types of ferrite and pearlite: less than 5% in total. Fresh martensite and one or more types of residual austenite: totaling 10% or less. A steel sheet characterized by having an average aspect ratio of 3.0 to 6.0 for the prior austenite grains.

2. The aforementioned chemical composition is, in mass%, Cr: 0.01-1.00%, 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 to 0.100%, Bi: 0.0001-0.0400%, Ca: 0.0001-0.0400%, Mg: 0.0001 to 0.0400%, 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 0.0400%.

3. In the texture at the 1 / 2 position of the plate thickness in the direction of the plate thickness from the surface, The average value of the X-ray random intensity ratio for the orientation groups {100}<011> to {223}<110> is 5.0 to 11.

0. The steel plate according to claim 1 or 2, characterized in that the X-ray random intensity ratio in the direction {332}<113> is 5.0 to 9.

0.

4. The aforementioned chemical composition is The steel plate according to claim 1 or 2, characterized in that it contains B: 0.0016 to 0.0050%.

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

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