Steel sheet and component

Abstract

This steel sheet has a specific chemical composition and has a specific metal structure in which the average grain size of old austenite grains is 25 μm or less, the area% of tempered martensite is 80.0-99.0%, and the MA area ratio is 1.0-20.0% in a region from a depth of 1 / 8th of the sheet thickness from the surface to 3 / 8th of the sheet thickness from the surface. When the difference between the maximum number and minimum number of MA present on a line segment per 100 μm line segment is determined when 10 or more line segments are drawn at equal intervals of 10 μm or more at a 45° direction to the sheet thickness direction in structure photographs obtained by continuously photographing five fields of view in the rolling direction, each field of view being 100 μm in the rolling direction×100 μm in the thickness direction, in this region, and the average of the difference between the maximum number and minimum number of MA in the structure photographs of the five fields of view is calculated, the average is 8-16.

Description

Steel plates and parts

[0001] This application claims priority to Japanese Patent Application No. 2024-005797, filed on January 18, 2024, the contents of which are incorporated herein by reference.

[0002] In recent years, CO ２ In order to reduce emissions, weight reduction of automobile bodies has been attempted. In particular, the application of steel sheets of 780 MPa class or more to automobile suspension parts such as lower arms, trail links, and knuckles has begun to be considered. These automobile suspension parts are manufactured by subjecting steel sheets to burring, stretch flange forming, bending forming, and the like. Therefore, steel sheets used in these automobile suspension parts are required to have formability, in particular, a high yield ratio, as well as excellent ductility and hole expandability.

[0003] For example, Patent Document 1 discloses a steel sheet containing, in area %, more than 40% but not more than 95% ferrite and a total of 3% or more and 20% or less of one or more types selected from a group of ultra-hard phases consisting of martensite, retained austenite, and cementite, and wherein the average grain size of the ferrite is 2.5 μm or less, the average grain size of the group of ultra-hard phases is 2.0 μm or less, the average spacing of ultra-hard phases, which is the average value of the nearest neighbor distances of the group of ultra-hard phases, is 2.0 μm or less, and the number density of Ti—B-based precipitates having a circle equivalent diameter of 1 μm or more is 500 / mm ２ and mechanical properties including a tensile strength of 780 MPa or more, a total elongation of 10% or more, a hole expansion ratio of 35% or more, and a minimum inner radius at which cracks do not occur in a bending test at a bending angle of 180° of 3.5 times or less of the sheet thickness.

[0004] Patent Document 2 discloses a hot-rolled steel sheet in which the microstructure contains, by volume fraction, 70% or more of martensite, tempered martensite, and bainite in total, and 5 to 20% of retained austenite, and in a surface layer region ranging from the surface to a position that is 1 / 10 of the sheet thickness, the sum of the average pole density of an orientation group consisting of {211}<111> to {111}<112> and the pole density of the crystal orientation {110}<001> is 6.0 or less, the solute carbon concentration in the retained austenite is 0.5% by mass or more, and the tensile strength is 980 MPa or more.

[0005] Japanese Patent No. 5910396 International Publication No. 2021 / 167079

[0006] When manufacturing lightweight parts using high-strength steel sheets, it is necessary to compensate for the loss of rigidity caused by thinning the parts by strengthening the cross-sectional shape or material. The inventors have discovered that when high-strength steel sheets are used for, for example, the lower arm of an automobile suspension part, as shown in Figure 1, cracks occur in the curved protruding portion near the link. These cracks occur in the portion that, after undergoing protruding deformation, is subjected to tensile deformation perpendicular to the crack.

[0007] The present inventors have found that when high-strength steel sheets are used in parts having complex cross-sectional shapes to ensure high rigidity, as in the above example, cracks occur. The present inventors have also found that the cracks occur in locations other than the flanges and burred end faces, which have been problems in the past.

[0008] As a result of analyzing the cracked portion, the inventors found that the cracked portion had been subjected to bending and unbending deformation by pressing, and that after this bending and unbending deformation, deformation in a direction perpendicular to the bending strain caused local contraction, resulting in fracture. The inventors found that in order to prevent this fracture, it is necessary for the sheet to have excellent formability, in particular a high critical thickness reduction rate at fracture, after being pre-strained by bending and unbending deformation.

[0009] The critical thickness reduction rate at fracture is a value calculated from the thickness of the tensile test piece before fracture and the minimum thickness of the tensile test piece after fracture. A low critical thickness reduction rate at fracture after pre-straining is undesirable because it may cause early fracture in subsequent processes or deteriorate durability and crash resistance when used as a part.

[0010] However, Patent Documents 1 and 2 do not take into consideration the critical fracture thickness reduction rate after pre-straining.

[0011] The present disclosure has been made in view of the above-mentioned problems, and aims to provide a steel sheet having high strength and yield ratio, as well as excellent ductility and hole expandability, and a high critical fracture thickness reduction rate after pre-straining, and a part using the same.

[0012] The gist of the present invention is as follows. [1] Chemical composition, in mass%, C: 0.090 to 0.210%, Si: 0.20 to 1.00%, Mn: 1.95 to 2.55%, P: 0.060% or less, S: 0.005% or less, Al: 0.01 to 0.26%, N: 0.0070% or less, O: 0 to 0.010%, Ti: 0.10 to 0.18%, Nb: 0.01 to 0.04%, B: 0.0001 to 0.0030%, Cr: 0 to 0.47%, Mo: 0 to 0.12%, Cu: 0 to 0.40%, Ni: 0 to 0.30%, V: 0 to 0.30%, Sn: 0 to 0.040%, The steel sheet has a metal structure comprising, in area %, As: 0 to 0.100%, Zr: 0 to 0.050%, Ca: 0 to 0.010%, Mg: 0 to 0.010%, Bi: 0 to 0.010%, Co: 0 to 0.010%, W: 0 to 0.100%, Zn: 0 to 0.010%, REM: 0 to 0.010%, Sb: 0 to 0.010%, and Ta: 0 to 0.010%, with the balance being Fe and impurities, in a region from the surface to a depth of 1 / 8 of the plate thickness to a depth of 3 / 8 of the plate thickness from the surface, the metal structure comprises, in area %, tempered martensite: 80.0 to 99.0%, MA: 1.0 to 20.0%, ferrite and bainite: 10.0% or less in total, and a pearlite content of 2.0% or less, an average grain size of prior austenite grains of 25 μm or less, and in the region, a structural photograph is obtained by photographing five fields of view of 100 μm in the rolling direction x 100 μm in the plate thickness direction consecutively in the rolling direction, and when 10 or more line segments are drawn in a direction of 45° to the plate thickness direction at equal intervals of 10 μm or more in the structural photograph, a difference between a maximum and a minimum number of MAs present on each 100 μm line segment is determined for each of the structural photographs, and an average value of the differences between the maximum and minimum numbers of MAs in the structural photographs of the five fields of view is calculated, the average value is 8 to 16.[2] The chemical composition is, in mass%, Cr: 0.01 to 0.47%, Mo: 0.01 to 0.12%, Cu: 0.01 to 0.40%, Ni: 0.01 to 0.30%, V: 0.01 to 0.30%, Sn: 0.001 to 0.040%, As: 0.001 to 0.100%, Zr: 0.001 to 0.050%, Ca: 0.001 to 0.010%, Mg: 0.001 to 0.010%, Bi: 0.001 to 0.010%, Co: 0.001 to 0.010%, W: 0.001 to 0.100%, Zn: 0.001 to 0.010%, The steel sheet according to [1], characterized in that it contains one or more of REM: 0.001 to 0.010%, Sb: 0.001 to 0.010%, and Ta: 0.001 to 0.010%. [3] A part comprising the steel sheet according to [1] or [2].

[0013] According to the above aspects of the present disclosure, it is possible to provide a steel sheet having high strength and yield ratio, as well as excellent ductility and hole expandability, and a high critical fracture thickness reduction rate after pre-straining, and a part using the steel sheet.

[0014] Fig. 1 is a diagram showing a cracked portion of a lower arm manufactured from a high-strength steel plate. Fig. 2 is a diagram for explaining a method for measuring the difference between the maximum and minimum numbers of MAs. Fig. 3 is a diagram showing the relationship between the average value of the difference between the maximum and minimum numbers of MAs in structural photographs of five fields of view and the critical fracture thickness reduction rate after pre-straining in an example. Fig. 4 is a diagram for explaining a manufacturing method of a hat part.

[0015] The present inventors have found that in order to achieve a high critical fracture thickness reduction rate after pre-straining while ensuring high strength and yield ratio, as well as excellent ductility and hole expandability by having a desired metal structure, it is important to preferably control the average grain size of prior austenite grains and to preferably control the distribution of MA, which is a hard phase.

[0016] Furthermore, the present inventors have found that in order to obtain a steel sheet having the above-described metal structure, it is effective to more strictly control the temperature history from the completion of rough rolling to the start of finish rolling, and then control the finish rolling conditions.

[0017] A steel sheet according to an embodiment of the present disclosure (hereinafter, sometimes referred to as the steel sheet according to the present embodiment) will be described. However, the present disclosure is not limited to the configuration disclosed in the present embodiment, and various modifications are possible within the scope of the present disclosure.

[0018] Each constituent element of the present disclosure 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. Below, the numerical ranges described with "to" include the lower and upper limits. Numerical values ​​indicated as "less than" or "greater than" do not include the numerical range. In the following description, percentages related to the chemical composition are mass% unless otherwise specified.

[0019] The steel sheet according to this embodiment has a chemical composition, in mass%, of C: 0.090 to 0.210%, Si: 0.20 to 1.00%, Mn: 1.95 to 2.55%, P: 0.060% or less, S: 0.005% or less, Al: 0.01 to 0.26%, N: 0.0070% or less, O: 0 to 0.010%, Ti: 0.10 to 0.18%, Nb: 0.01 to 0.04%, B: 0.0001 to 0.0030%, and the balance: Fe and impurities. Each element will be described in detail below.

[0020] C: 0.090 to 0.210% C is an element necessary for obtaining the desired strength of the steel sheet. If the C content is less than 0.090%, the desired strength cannot be obtained. Therefore, the C content is set to 0.090% or more. The C content is preferably 0.100% or more, 0.120% or more, or 0.150% or more. On the other hand, if the C content exceeds 0.210%, the hole expandability of the steel sheet deteriorates. Therefore, the C content is set to 0.210% or less. The C content is preferably 0.200% or less, or 0.180% or less.

[0021] Si: 0.20 to 1.00% Si is an element that improves the strength of steel sheet by solid solution strengthening. If the Si content is less than 0.20%, the desired strength or hole expandability cannot be obtained. Therefore, the Si content is set to 0.20% or more. The Si content is preferably 0.40% or more or 0.50% or more. On the other hand, if the Si content exceeds 1.00%, the hole expandability of the steel sheet deteriorates. Therefore, the Si content is set to 1.00% or less. The Si content is preferably 0.90% or less, 0.70% or less, or 0.60% or less.

[0022] Mn: 1.95 to 2.55% Mn is an element necessary for improving the strength of steel sheet. If the Mn content is less than 1.95%, the area ratio of ferrite becomes too high, making it impossible to obtain the desired strength and hole expandability. Therefore, the Mn content is set to 1.95% or more. The Mn content is preferably 2.00% or more or 2.10% or more. On the other hand, if the Mn content exceeds 2.55%, the hole expandability of the steel sheet deteriorates. Therefore, the Mn content is set to 2.55% or less. The Mn content is preferably 2.50% or less or 2.40% or less.

[0023] P: 0.060% or less P is an element that segregates in the center of the steel plate thickness. P is also an element that embrittles welds. If the P content exceeds 0.060%, the hole expandability of the steel plate deteriorates. Therefore, the P content is set to 0.060% or less. The P content is preferably 0.030% or less or 0.020% or less. The lower the P content, the better, and 0% is preferable. However, if the P content is reduced excessively, the dephosphorization cost increases significantly. Therefore, the P content may be set to 0.001% or more or 0.005% or more.

[0024] S: 0.005% or less S is an element that embrittles slabs when present as sulfides. S is also an element that deteriorates the formability of steel sheets. If the S content exceeds 0.005%, the hole expandability of the steel sheet deteriorates. Therefore, the S content is set to 0.005% or less. The S content is preferably 0.004% or less or 0.003% or less. The lower the S content, the more preferable it is, and 0% is preferable. However, if the S content is reduced excessively, the desulfurization cost increases significantly. Therefore, the S content may be set to 0.0005% or more or 0.001% or more.

[0025] Al: 0.01 to 0.26% Al is an element that acts as a deoxidizer and improves the cleanliness of steel. If the Al content is less than 0.01%, a sufficient deoxidizing effect is not obtained, and a large amount of inclusions (oxides) is formed in the steel sheet. Such inclusions deteriorate the formability of the steel sheet. Therefore, the Al content is set to 0.01% or more. The Al content is preferably 0.02% or more, 0.03% or more, or 0.05% or more. On the other hand, if the Al content exceeds 0.26%, the hole expandability of the steel sheet deteriorates. Therefore, the Al content is set to 0.26% or less. The Al content is preferably 0.25% or less, 0.20% or less, or 0.15% or less.

[0026] N: 0.0070% or less N is an element that forms coarse nitrides in steel and deteriorates the hole expandability of steel sheet. If the N content exceeds 0.0070%, the hole expandability of the steel sheet deteriorates. Therefore, the N content is set to 0.0070% or less. The N content is preferably 0.0060% or less, 0.0050% or less, 0.0040% or less, or 0.0030% or less. The lower the N content, the better, and 0% is preferable. However, excessive reduction of the N content significantly increases the cost of denitrification. Therefore, the N content may be set to 0.0005% or more or 0.0010% or more.

[0027] O: 0 to 0.010% O is an element that forms coarse oxides that become the starting points of fracture when contained in large amounts in steel. If the O content exceeds 0.010%, cracks tend to occur in the slab. Therefore, the O content is set to 0.010% or less. The O content is preferably 0.005% or less or 0.001% or less. Since O may not be contained, the O content may be 0%.

[0028] Ti: 0.10 to 0.18% Ti is an element that increases the strength of steel sheet by forming fine nitrides in steel. If the Ti content is less than 0.10%, the desired strength cannot be obtained. Therefore, the Ti content is set to 0.10% or more. The Ti content is preferably 0.12% or more or 0.13% or more. On the other hand, if the Ti content exceeds 0.18%, the hole expandability of the steel sheet deteriorates. Therefore, the Ti content is set to 0.18% or less. The Ti content is preferably 0.16% or less or 0.15% or less.

[0029] Nb: 0.01 to 0.04% Nb is an element that suppresses abnormal grain growth of austenite grains during hot rolling. Nb also forms fine carbides, thereby increasing the yield ratio of the steel sheet. If the Nb content is less than 0.01%, the desired yield ratio cannot be obtained. Therefore, the Nb content is set to 0.01% or more. The Nb content is preferably 0.02% or more. On the other hand, if the Nb content exceeds 0.04%, the hole expandability of the steel sheet deteriorates. Therefore, the Nb content is set to 0.04% or less. The Nb content is preferably 0.03% or less.

[0030] B: 0.0001 to 0.0030% B is an element that suppresses the formation of ferrite during the cooling process and increases the strength of the steel sheet. If the B content is less than 0.0001%, the desired strength cannot be obtained. Therefore, the B content is set to 0.0001% or more. The B content is preferably 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, if the B content exceeds 0.0030%, the hole expandability of the steel sheet deteriorates. Therefore, the B content is set to 0.0030% or less. The B content is preferably 0.0025% or less, or 0.0020% or less.

[0031] The balance of the chemical composition of the steel sheet according to this embodiment is composed of Fe and impurities. In this embodiment, the impurities refer to substances mixed in from raw materials such as ore and scrap, or the manufacturing environment.

[0032] 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.

[0033] Cr: 0.01 to 0.47% Cr is an element that exhibits an effect similar to that of Mn. In order to reliably obtain the effect of increasing the strength of the steel sheet due to the inclusion of Cr, the Cr content is preferably 0.01% or more. On the other hand, even if the Cr content exceeds 0.47%, the above effect saturates. Therefore, the Cr content is set to 0.47% or less.

[0034] Mo: 0.01 to 0.12% Mo is an element that increases the strength of steel sheet by forming fine carbides in steel. To ensure this effect, the Mo content is preferably 0.01% or more. On the other hand, even if the Mo content exceeds 0.12%, the above effect saturates. Therefore, the Mo content is set to 0.12% or less.

[0035] Cu: 0.01 to 0.40% Cu has the effect of improving the hardenability of steel sheet and precipitating as carbides in steel at low temperatures to increase the strength of the steel sheet. To reliably obtain the effects of these actions, the Cu content is preferably 0.01% or more. However, if the Cu content exceeds 0.40%, grain boundary cracking of the slab may occur. Therefore, the Cu content is set to 0.40% or less.

[0036] Ni: 0.01 to 0.30% Ni has the effect of improving the hardenability of the steel sheet and increasing its strength. Furthermore, when Cu is contained, Ni has the effect of effectively suppressing intergranular cracking of the slab caused by Cu. In order to reliably obtain the above-mentioned effect, the Ni content is preferably 0.01% or more. Since Ni is an expensive element, it is economically undesirable to include a large amount of Ni. Furthermore, even if the Ni content exceeds 0.30%, the above-mentioned effect saturates. Therefore, the Ni content is set to 0.30% or less.

[0037] V: 0.01 to 0.30% V is an element that increases the strength of steel sheet by forming fine carbides in steel. To reliably obtain this effect, the V content is preferably 0.01% or more. On the other hand, even if the V content exceeds 0.30%, the above effect saturates. Therefore, the V content is set to 0.30% or less.

[0038] Sn: 0.001 to 0.040% Sn has the effect of suppressing the generation of oxides that serve as fracture initiation sites, thereby improving the hole expandability of steel sheets. To reliably obtain this effect, the Sn content is preferably 0.001% or more. On the other hand, even if the Sn content exceeds 0.040%, the above effect saturates. Therefore, the Sn content is set to 0.040% or less.

[0039] As: 0.001 to 0.100% As has the effect of reducing the austenite single-phase temperature, thereby refining prior austenite grains and improving the hole expandability of steel sheets. To reliably obtain this effect, the As content is preferably 0.001% or more. On the other hand, excessive As content may cause slab cracking. This effect becomes significant when the As content exceeds 0.100%, so the As content is set to 0.100% or less.

[0040] Zr: 0.001 to 0.050% Zr has the effect of improving hole expandability. To reliably obtain this effect, the Zr content is preferably 0.001% or more. On the other hand, even if the Zr content exceeds 0.050%, the above effect saturates. Therefore, the Zr content is set to 0.050% or less.

[0041] Ca: 0.001 to 0.010% Ca disperses many fine oxides during deoxidation of molten steel, thereby refining the structure of the steel sheet. Ca also fixes S in the steel as spherical CaS, suppresses the formation of elongated inclusions such as MnS, and improves the hole expandability of the steel sheet. To ensure these effects, the Ca content is preferably 0.001% or more. On the other hand, even if the Ca content exceeds 0.010%, the above effects saturate. Therefore, the Ca content is set to 0.010% or less.

[0042] Mg: 0.001 to 0.010% Mg has the effect of improving the hole expandability of steel sheets by adjusting the shape of inclusions in steel to a preferred shape. To reliably obtain this effect, the Mg content is preferably 0.001% or more. On the other hand, even if the Mg content exceeds 0.010%, the above effect saturates. Therefore, the Mg content is set to 0.010% or less.

[0043] Bi: 0.001 to 0.010% Bi has the effect of improving the hole expandability of steel sheets by refining the solidification structure. To ensure this effect, the Bi content is preferably 0.001% or more. On the other hand, even if the Bi content exceeds 0.010%, the above effect saturates. Therefore, the Bi content is set to 0.010% or less.

[0044] Co: 0.001 to 0.010% Co has the effect of increasing the strength of the steel sheet through solid solution strengthening. To reliably obtain this effect, the Co content is preferably 0.001% or more. On the other hand, even if the Co content exceeds 0.010%, the above effect saturates. Therefore, the Co content is set to 0.010% or less.

[0045] W: 0.001 to 0.100% W has the effect of increasing the strength of the steel sheet through solid solution strengthening. To reliably obtain this effect, the W content is preferably 0.001% or more. On the other hand, even if the W content exceeds 0.100%, the above effect saturates. Therefore, the W content is set to 0.100% or less.

[0046] Zn: 0.001 to 0.010% Zn has the effect of increasing the strength of steel sheet through solid solution strengthening. To reliably obtain this effect, the Zn content is preferably 0.001% or more. On the other hand, if the Zn content exceeds 0.010%, slab cracking may occur. Therefore, the Zn content is set to 0.010% or less.

[0047] REM: 0.001 to 0.010% REM has the effect of increasing the yield ratio of steel sheets by adjusting the shape of inclusions in steel to a preferred shape. To ensure this effect, the REM content is preferably 0.001% or more. On the other hand, if the REM content exceeds 0.010%, slab cracking may occur. Therefore, the REM content is set to 0.010% or less. Here, 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. In the case of lanthanoids, they are added industrially in the form of misch metal.

[0048] Sb: 0.001 to 0.010% Sb has the effect of suppressing 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. However, even if a large amount of Sb is added, the above effect saturates, so the Sb content is set to 0.010% or less.

[0049] Ta: 0.001 to 0.010% Ta, like V, has the effect of increasing the strength of the steel sheet by forming fine carbides in the steel. To reliably obtain this effect, the Ta content is preferably 0.001% or more. On the other hand, if the Ta content exceeds 0.010%, the ductility and hole expandability of the steel sheet deteriorate. Therefore, the Ta content is set to 0.010% or less.

[0050] The chemical composition of the above-mentioned steel sheet is analyzed using a spark discharge optical emission spectrometer or the like. C and S are analyzed 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 sheet has a plating layer or a coating film on the surface, the plating layer or coating film is removed by mechanical grinding or the like as necessary before analyzing the chemical composition.

[0051] Next, the metal structure of the steel sheet according to this embodiment will be described. In the steel sheet according to this embodiment, in a region from the surface to a depth of ⅛ of the plate thickness from the surface to a depth of ⅜ of the plate thickness, the metal structure is, in area %, 80.0 to 99.0% tempered martensite, 1.0 to 20.0% MA, 10.0% or less ferrite and bainite in total, and 2.0% or less pearlite, and the average grain size of prior austenite grains is 25 μm or less. In the region, a structural photograph is obtained by photographing five fields of view, each 100 μm in the rolling direction and 100 μm in the plate thickness direction, consecutively in the rolling direction, and ten or more line segments are drawn in the structural photograph at equal intervals of 10 μm or more in a 45° direction relative to the plate thickness direction. The difference between the maximum and minimum numbers of MA present on each 100 μm line segment is determined for each structural photograph, and the average value of the differences between the maximum and minimum numbers of MA in the five fields of view is calculated. The steel sheet according to this embodiment does not include any structure other than the structure described above in its metallographic structure. Therefore, in other words, the steel sheet according to this embodiment has a metallographic structure consisting, in area percentages, of only tempered martensite: 80.0 to 99.0%, MA: 1.0 to 20.0%, ferrite and bainite: 10.0% or less in total, and pearlite: 2.0% or less in a region from the surface to a depth of ⅛ of the plate thickness to a depth of ⅜ of the plate thickness from the surface.

[0052] In this embodiment, the region from the surface to a depth of ⅛ of the plate thickness from the surface refers to a region starting from a depth of ⅛ of the plate thickness from the surface and ending at a depth of ⅜ of the plate thickness from the surface. The reason for specifying the metallographic structure in this region is that the metallographic structure in this region represents a typical metallographic structure of a steel plate.

[0053] When a steel sheet has a plating layer or a coating film on its surface, the surface here refers to the interface between the steel sheet and the plating layer or coating film. The interface between the steel sheet and the plating layer or coating film is identified using a BSE image (or COMPO image) obtained by the following method. A test piece is cut out so that the cross section of the steel sheet thickness can be observed. The cut-out test piece is mechanically polished and then mirror-finished. Using a scanning electron microscope, for example, at a magnification of 400 times, an area of ​​40,000 μm ２ The above range is observed. When a cross section is observed using a BSE image (or COMPO image), a clear difference in contrast can be confirmed between the plating layer, coating film, etc. and the base steel (steel sheet). Therefore, the position where the contrast changes from the outermost surface can be identified as the interface between the steel sheet and the plating layer, coating film, etc. When the part described below has a plating layer, coating film, etc., the interface can be identified using a similar method.

[0054] Tempered martensite: 80.0 to 99.0% If the area fraction of tempered martensite is less than 80.0%, the yield ratio of the steel sheet decreases. Therefore, the area fraction of tempered martensite is set to 80.0% or more. The area fraction of tempered martensite is preferably 83.0% or more or 85.0% or more. On the other hand, if the area fraction of tempered martensite exceeds 99.0%, the ductility of the steel sheet deteriorates. Therefore, the area fraction of tempered martensite is set to 99.0% or less. The area fraction of tempered martensite is preferably 95.0% or less or 90.0% or less.

[0055] MA: 1.0 to 20.0% MA is called island martensite and refers to a mixed structure of fresh martensite and retained austenite. If the area ratio of MA is less than 1.0%, the ductility of the steel sheet deteriorates. Therefore, the area ratio of MA is set to 1.0% or more. The area ratio of MA is preferably 3.0% or more, 5.0% or more, or 10.0% or more. On the other hand, if the area ratio of MA exceeds 20.0%, the area ratio of tempered martensite decreases, and the yield ratio of the steel sheet decreases. Therefore, the area ratio of MA is set to 20.0% or less. The area ratio of MA is preferably 15.0% or less, 13.0% or less, or 10.0% or less.

[0056] Ferrite and bainite: 10.0% or less in total If the area fraction of ferrite and bainite exceeds 10.0% in total, the hole expandability of the steel sheet deteriorates. Therefore, the area fraction of ferrite and bainite is set to 10.0% or less in total. The area fraction of ferrite and bainite is preferably 5.0% or less, 3.0% or less, or 2.5% or less. Since ferrite and bainite do not necessarily need to be contained, the total area fraction of ferrite and bainite may be 0.0%.

[0057] Pearlite: 2.0% or less If the area fraction of pearlite exceeds 2.0%, the hole expandability of the steel sheet deteriorates. Therefore, the area fraction of pearlite is set to 2.0% or less. The area fraction of pearlite may be set to 1.5% or less, 1.0% or less, or 0.0%.

[0058] The area ratio of each structure is measured using the following method. First, the method for measuring the ferrite area ratio will be described. A test specimen is taken from the 1 / 4 position in the width direction of the steel plate so that the thickness cross section can be observed from 1 / 8 of the plate thickness depth from the surface to 3 / 8 of the plate thickness depth from the surface. The thickness cross section of the test specimen is polished using silicon carbide paper ranging from #600 to #1500 in stages, and then finished to a mirror finish using a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in a diluted solution such as alcohol or pure water. Next, the specimen is polished for 8 minutes at room temperature using colloidal silica with a particle size of 0.25 μm that does not contain an alkaline solution to remove the strain introduced into the surface layer of the specimen. In the region of the test piece from a depth of 1 / 8 of the plate thickness from the surface of the steel plate to a depth of 3 / 8 of the plate thickness from the surface, crystal orientation information is obtained by measuring a field of view of 200 μm in the rolling direction and 100 μm in the plate thickness direction centered at a depth of 1 / 4 of the plate thickness from the surface of the steel plate at measurement intervals of 0.1 μm using electron backscatter diffraction.

[0059] For the measurement, an EBSD device consisting of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL) is used. At this time, the degree of vacuum in the EBSD device is 9.6 × 10 －５ The pressure is set to 100 Pa or less, the acceleration voltage is 15 kV, the irradiation current level is 13, and the electron beam irradiation level is 62. From the obtained crystal orientation information, the "Phase Map" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer is used to identify regions with an fcc crystal structure and regions with a bcc crystal structure.

[0060] For regions with a bcc crystal structure, the grain average misorientation (GAM value: Grain Average Misorientation) is calculated using the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer. Here, regions with a GAM value of 0.5° or less are determined to be ferrite. The same operation is performed for five fields of view to calculate the area ratio of ferrite. Here, the "GAM value" is the average value of the misorientation between adjacent pixels in a region surrounded by grain boundaries with a misorientation of 15° or more.

[0061] Next, in order to observe the same field of view as that measured by EBSD, Vickers indentations are stamped at three of the four corners of the field of view measured by EBSD within a range of 100 μm from each of the four corners so that the observation position can be identified. Then, surface contamination is polished away, leaving the metal structure of the observation surface. Using the Vickers indentations as a guide, it is possible to observe the same field of view as that measured by EBSD. Contamination can be removed by buffing using alumina particles with a particle diameter of 0.1 μm or less, polishing using colloidal silica that does not contain an alkaline solution at room temperature, or Ar ion sputtering, among other methods.

[0062] The observation surface, including the field of view measured by EBSD, is subjected to nital etching to reveal the structure. Then, using a scanning electron microscope, photographs are taken of the same five fields of view as those measured by EBSD at 500x magnification. In the microstructure photographs, lath-shaped regions are identified as "tempered martensite and MA." The area fractions of "tempered martensite and MA" in each microstructure photograph are calculated, and the average value of these is calculated to obtain the area fraction of "tempered martensite and MA." The area fraction of MA, obtained by the method described below, is subtracted from this area fraction to obtain the area fraction of tempered martensite. Furthermore, in each microstructure photograph, regions having lamellar carbides are identified as pearlite, and the average value of these area fractions is calculated to obtain the area fraction of pearlite.

[0063] Next, a method for measuring the area ratio of MA will be described. The observation surface including the field of view measured by EBSD is polished to remove only the corroded layer, then mirror-finished, and then subjected to LePera corrosion to reveal the structure. Then, using a scanning electron microscope at a magnification of 500x, photographs are taken of the same five fields of view as those measured by EBSD. In each microstructure photograph, areas with a white contrast are identified as MA, and the average of these area ratios is calculated to obtain the area ratio of MA.

[0064] The area fraction of bainite is obtained by subtracting the area fractions of tempered martensite, MA, ferrite, and pearlite obtained by the above-mentioned method from 100.0%. If the area fraction of bainite is a negative value, the area fraction of bainite is set to 0%. If the area fraction of bainite is a negative value, that is, if the sum of the area fractions of tempered martensite, MA, ferrite, and pearlite exceeds 100%, the area fractions of each structure are corrected so that their sum becomes 100%. For example, if the sum of the area fractions of tempered martensite, MA, ferrite, and pearlite is 103.0%, the area fraction of each structure is corrected by multiplying the area fraction of each structure by "100.0 / 103.0".

[0065] The rolling direction of a steel sheet is determined using the following method. A test piece is taken from any position at least 50 mm away from the end of the steel sheet so that the thickness cross section can be observed. The thickness cross section of the taken test piece is mirror-polished and then observed using an optical microscope at magnifications of 100x, 200x, 500x, and 1000x. An appropriate magnification at which the dimensions of the inclusions can be measured is selected depending on the size of the inclusions. The observation range is 500 μm or more in width and across the entire thickness of the sheet, and areas with dark brightness are determined to be inclusions. Observation may be performed using multiple fields of view. Next, using the thickness cross section initially observed using the above method as a reference, a plane parallel to the plane rotated in 5° increments in the range of 0° to 180° around the thickness direction is observed using the same method as above. The average length of the major axes of the multiple inclusions in each cross section is calculated for each cross section. The cross section with the largest average value of the major axis length of the inclusions obtained is identified. The direction parallel to the longitudinal axis of the inclusion in the cross section is determined as the rolling direction. The same method is used to determine the rolling direction for parts.

[0066] Average grain size of prior austenite grains: 25 μm or less The present inventors have found that even when the distribution of MA is favorably controlled as described below, if the average grain size of the prior austenite grains is too large, a high critical reduction in thickness at fracture cannot be obtained after pre-straining. Therefore, in order to obtain a high critical reduction in thickness at fracture after pre-straining, it is important to control not only the distribution of MA but also the average grain size of the prior austenite grains.

[0067] MAs are generated from the grain boundaries of prior austenite. If the average grain size of prior austenite grains is too large, the spacing between MAs generated from the grain size becomes large, increasing the difference between areas with a high concentration of MAs and areas with a low concentration of MAs. As a result, shear bands are localized in areas with a low concentration of MAs, causing strain concentration. If the average grain size of prior austenite grains exceeds 25 μm, a high critical fracture thickness reduction rate cannot be obtained after pre-straining. Therefore, the average grain size of prior austenite grains is set to 25 μm or less. The average grain size of prior austenite grains is preferably 23 μm or less, 20 μm or less, or 15 μm or less. There is no particular lower limit for the average grain size of prior austenite grains. However, because setting the average grain size of prior austenite grains to less than 4 μm places a heavy burden on the rolling equipment, the average grain size of prior austenite grains is preferably 4 μm or more. The average grain size of prior austenite grains may be 5 μm or more or 10 μm or more.

[0068] The average grain size of prior austenite is measured using the following method. A test specimen is taken from a quarter position in the width direction of the steel plate so that a thickness cross section can be observed from a depth of 1 / 8 of the plate thickness from the surface to a depth of 3 / 8 of the plate thickness from the surface. The observation surface of the test specimen is etched using an etchant made by adding a saturated aqueous solution of picric acid and a sodium dodecylbenzenesulfonate etchant to reveal the structure of the thickness cross section. Using a scanning electron microscope, a microstructure photograph is taken of the test specimen in a region from a depth of 1 / 8 of the plate thickness from the surface to a depth of 3 / 8 of the plate thickness from the surface, covering a field of view of 200 μm in the rolling direction and 100 μm in the plate thickness direction, centered at a depth of 1 / 4 of the plate thickness from the surface of the steel plate. Five microstructure photographs are taken. The grain size of prior austenite grains is measured using the taken microstructure photographs. The circle equivalent diameter of one of the prior austenite grains included in each observation field is calculated. The above operation is performed for all prior austenite grains included in each observation field, excluding prior austenite grains that are not entirely included in the field of view, such as those at the edges of the field of view, to determine the circle-equivalent diameters of all prior austenite grains in each field of view. The average circle-equivalent diameters of the prior austenite grains obtained in each field of view are calculated to obtain the average grain size of the prior austenite grains.

[0069] The present inventors have found that in order to achieve a high critical reduction in thickness at fracture after pre-straining, it is important to favorably control the average grain size of prior austenite grains and to favorably control the distribution of MA, which is a hard phase. As a result of investigating the relationship between the distribution of MA and the critical reduction in thickness at fracture after pre-straining, the present inventors have found the following.

[0070] In the strain distribution of the part that has been subjected to bending and unbending deformation by press working, the strain is localized in a band-like shape in the 45° direction to the plate thickness direction. Because this strain band (shear band) develops to avoid MA, if there is a large difference between areas with a high level of MA and areas with a low level of MA on the line in the 45° direction to the plate thickness direction, the shear band will localize in areas with a low level of MA, causing strain concentration. Therefore, it is important to reduce the difference between areas with a high level of MA and areas with a low level of MA on the line in the 45° direction to the plate thickness direction.

[0071] In order to reduce the difference between locations with a high number of MAs and locations with a low number of MAs on a line at a 45° angle to the thickness direction of the steel sheet, the present embodiment specifies the following: A structural photograph is obtained by photographing five consecutive fields of view, each 100 μm in the rolling direction and 100 μm in the thickness direction, in the region from ⅛ depth of the sheet thickness from the surface to ⅜ depth of the sheet thickness from the surface, and ten or more line segments at 45° to the thickness direction are drawn at equal intervals of 10 μm or more in each structural photograph. The difference between the maximum and minimum numbers of MAs present on each 100 μm line segment is determined for each structural photograph, and the average value of the differences between the maximum and minimum numbers of MAs in the five fields of view in the structural photographs is calculated; the average value is 8 to 16. This will be described in detail below with reference to the drawings.

[0072] First, a test piece is taken from a 1 / 4 position in the width direction of the steel plate so that a thickness cross section from 1 / 8 of the plate thickness depth from the surface to 3 / 8 of the plate thickness depth from the surface can be observed. The observation surface is subjected to nital etching to reveal the structure, and then a structural photograph is taken at 1000x magnification using a scanning electron microscope. Figure 2 is a diagram for explaining a method for measuring the difference between the maximum and minimum numbers of MAs. Note that MAs are not shown in Figure 2. In taking the structural photographs, as shown in Figure 2, a scanning electron microscope is used to photograph five consecutive fields of view in the rolling direction, each 100 μm in width and centered at a 1 / 4 depth from the surface of the steel plate.

[0073] Next, in the photograph of 100 μm × 100 μm, a line segment L 45° to the plate thickness direction is drawn on the diagonal. １ Draw the line segment L １ At equal intervals of 10 μm or more, the line segment L ２ , line segment L ３ ...and draw 10 or more line segments. In Figure 2, 11 line segments are drawn. Next, count the number of MAs present on the line segments per 100 μm. Specifically, first, １ Count the number of MAs on the line segment from the end point A to point B, 100 μm away. The number of MAs obtained is called the number of MAs on the line segment A-B, q １Next, count the number of MAs present on the line segment from point B to a position 100 μm away. If there is no line segment until 100 μm is reached, count the number of MAs present on another line segment so that the total length is 100 μm. In the case of FIG. 2, count the number of MAs present on the line segment from point B to point C, and then count the number of MAs present on the line segment from point D to point E so that the total length of the line segment is 100 μm. The number of MAs obtained is called the number q of MAs on the line segment B-C-D-E. ２ The same operation is performed on the line segment L ２ , line segment L ３ ...Line segment L １１ The number q per 100 μm of each line segment was calculated. １ , q ２ …q ｎ get.

[0074] The number obtained q １ , q ２ …q ｎ From the above, the maximum number q Ｍａｘ and the minimum number q ｍｉｎ The maximum number q Ｍａｘ and the minimum number q ｍｉｎ The difference between the maximum and minimum numbers of MAs for the first structural photograph is calculated by calculating the difference D １ The same operation is performed for the remaining four fields of view that are consecutive in the rolling direction, and the difference D between the maximum and minimum numbers of MA is obtained. ２ , difference D ３ , difference D ４ , difference D ５ The obtained difference D １ ~difference D ５ The average value of the difference between the maximum and minimum numbers of MAs in the five fields of view is calculated by calculating the average value of the difference between the maximum and minimum numbers of MAs in the five fields of view. Note that the line segments drawn on the structural photographs should not be too thick, and should be within the range of reasonable thickness for a line segment.

[0075] Figure 3 shows the relationship between the average value of the difference between the maximum and minimum numbers of MAs in five visual fields of the microstructure photographs and the critical reduction in thickness at fracture after pre-straining in the examples described below. As shown in Figure 3, if the average value of the difference between the maximum and minimum numbers of MAs in five visual fields of the microstructure photographs is less than 8 or more than 16, a high critical reduction in thickness at fracture after pre-straining cannot be obtained. Therefore, the average value of the difference between the maximum and minimum numbers of MAs in five visual fields of the microstructure photographs is set to 8 to 16. The average value is preferably 10 or more, or 12 or more. The average value is preferably 15 or less, or 14 or less.

[0076] Tensile strength (TS): 1180 MPa or more The steel plate according to this embodiment may have a tensile strength of 1180 MPa or more. The tensile strength is more preferably 1200 MPa or more or 1250 MPa or more. By setting the tensile strength to 1180 MPa or more, the applicable parts are not limited, and the contribution to vehicle body weight reduction can be increased. There is no particular need to set an upper limit for the tensile strength, but from the viewpoint of suppressing mold wear, it may be set to 1500 MPa or less or 1400 MPa or less.

[0077] Yield ratio (YR): 70% or more The steel sheet according to this embodiment may have a yield ratio of 70% or more. The yield ratio is preferably 80% or more or 85% or more. The yield ratio can be determined by dividing the yield stress by the tensile strength and multiplying the result by 100 ({yield stress / tensile strength} × 100).

[0078] Total elongation (El): 4.0% or more Hole expansion ratio (λ): 45% or more The steel sheet according to this embodiment may have a total elongation (total elongation at break) of 4.0% or more, and a hole expansion ratio of 45% or more. The total elongation is preferably 5.0% or more or 6.0% or more. The hole expansion ratio is preferably 50% or more or 60% or more.

[0079] The tensile strength and total elongation are evaluated by conducting a tensile test in accordance with JIS Z 2241:2022. The test specimen is a No. 5 test specimen of JIS Z 2241:2022. The tensile test specimen is taken from a quarter portion from the end in the plate width direction, with the direction perpendicular to the rolling direction as the longitudinal direction. When measuring the tensile strength of a part, if a No. 5 test specimen cannot be taken from the part due to the part's small size or complex shape, a small rectangular piece having a parallel portion of any width may be taken, and a tensile test may be performed to determine the tensile strength from the maximum test force and the original cross-sectional area of ​​the parallel portion.

[0080] The yield stress used to calculate the yield ratio is obtained by conducting a tensile test using the method described above. When the steel plate undergoes discontinuous yielding, the upper yield point is regarded as the yield stress, and when the steel plate undergoes continuous yielding, the 0.2% proof stress is regarded as the yield stress. The hole expansion ratio is measured by conducting a hole expansion test in accordance with JIS Z 2256:2020.

[0081] Critical thickness reduction rate at break after pre-straining: 0.25 or more In this embodiment, pre-straining by bending and unbending deformation is performed by draw-bending the steel sheet using the method shown in Figure 4. In forming the hat part using the draw-bending process shown in Figure 4, when the vertical wall of the hat part is formed, the steel sheet comes into contact with the punch while undergoing bending and unbending deformation. A test piece for a tensile test is taken from the vertical wall part of the hat part so that the height direction of the vertical wall part is the longitudinal direction. A tensile test is performed using the obtained test piece using the method described above. The thickness of the test piece before the tensile test is t ０ The minimum thickness at the center of the width direction (short direction) of the test piece after fracture is t １ When this is done, (t ０ -t １ ) / t ０ The critical fracture thickness reduction rate is obtained by calculating the value of . The tensile test is performed five times, and the average value of three times excluding the maximum and minimum values ​​of the critical fracture thickness reduction rate is calculated to obtain the critical fracture thickness reduction rate after pre-straining.

[0082] If the critical fracture thickness reduction rate after pre-straining is 0.25 or more, it can be determined that the steel sheet has excellent formability even after pre-straining. Therefore, the steel sheet according to this embodiment may have a critical fracture thickness reduction rate after pre-straining of 0.25 or more. The critical fracture thickness reduction rate after pre-straining is preferably 0.30 or more or 0.35 or more.

[0083] The thickness of the steel plate according to this embodiment is not particularly limited, but may be 1.2 to 8.0 mm. If the thickness of the steel plate is less than 1.2 mm, it may be difficult to ensure the rolling completion temperature and the rolling load may become excessive, making hot rolling difficult. Therefore, the thickness of the steel plate according to this embodiment may be 1.2 mm or more, preferably 1.4 mm or more. On the other hand, if the thickness exceeds 8.0 mm, it may be difficult to obtain the above-mentioned metal structure after hot rolling. Therefore, the thickness may be 8.0 mm or less, preferably 6.0 mm or less.

[0084] The steel sheet according to this embodiment, having the above-described chemical composition and metallographic structure, may be provided with a plating layer on its surface to provide a surface-treated steel sheet for the purpose of improving corrosion resistance, etc. The plating layer may be an electroplated layer or a hot-dip plated layer. Examples of electroplated layers include electrogalvanized galvanized and electrolytic Zn—Ni alloy plating. Examples of hot-dip plated layers include hot-dip galvanized, alloyed hot-dip galvanized, hot-dip aluminum plating, hot-dip Zn—Al alloy plating, hot-dip Zn—Al—Mg alloy plating, and hot-dip Zn—Al—Mg—Si alloy plating. The coating weight is not particularly limited and may be the same as conventional coating weights. Furthermore, corrosion resistance can be further improved by performing an appropriate chemical conversion treatment after plating (e.g., applying a silicate-based chromium-free chemical conversion treatment solution and drying it).

[0085] The steel sheet according to this embodiment has high strength and yield ratio, as well as excellent ductility and hole expandability, and also has a high critical thickness reduction rate at fracture after pre-straining, and is therefore considered to have excellent crashworthiness, making it suitable for use in parts, particularly automobile parts. Among automobile parts, it can be suitable for use in automobile suspension parts such as lower arms, trail links, and knuckles. These automobile parts may consist solely of the steel sheet according to this embodiment, or may be formed by joining the steel sheet according to this embodiment with other steel sheets.

[0086] 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. 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, and a part that avoids parts that have been punched, hole-expanded, bent, or the like. 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 examined.

[0087] 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. In a preferred method for manufacturing the steel sheet according to this embodiment, austenite grains are preferably formed by holding the steel sheet in a desired temperature range after completion of rough rolling, and then finish rolling is performed under preferred conditions, thereby making it possible to preferably control the average grain size of the prior austenite grains. Furthermore, it is possible to preferably control the distribution of MA generated from the grain boundaries of the prior austenite grains during subsequent reheating. Note that the temperature of the slab and the temperature of the steel sheet in this embodiment refer to the surface temperature of the slab and the surface temperature of the steel sheet, and are measured using a radiation thermometer. Furthermore, Tf refers to the temperature (°C) at the end of rough rolling.

[0088] A preferred method for producing the steel sheet according to this embodiment comprises: performing rough rolling so that the total reduction is 60 to 85% and the rough rolling completion temperature exceeds 1220°C; cooling after completion of rough rolling so that the temperature reaches 1220°C within 1.00 seconds, and holding in a temperature range of 1100 to 1220°C for 3 to 25 seconds, and then starting finish rolling; performing finish rolling so that the finish rolling completion temperature is in a temperature range of 940 to 1020°C, and the final reduction and the reduction in the penultimate rolling stage are each 20 to 43%; starting cooling within 2 seconds after completion of finish rolling, and cooling so that the temperature reaches 25°C within 17 seconds after completion of finish rolling; and coiling; １ Point ~ Ac ３ The material is reheated to the temperature range of the point, held at that temperature range for 10 seconds or more, and then air-cooled to a temperature range of 200° C. or less. Each step is explained below.

[0089] The slab to be subjected to rough rolling is not particularly limited except for the chemical composition described above. For example, a slab produced by melting molten steel having the above 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, a thin slab casting method, etc. may also be used. In the slab heating before rough rolling, the heating temperature may be in the range of 1220 to 1300°C, and the holding time may be 40 minutes or more.

[0090] Rough rolling is preferably carried out so that the total reduction is 60 to 85%. By setting the total reduction in rough rolling to 60% or more, it is possible to prevent prior austenite grains from becoming coarse. Furthermore, by setting the total reduction in rough rolling to 85% or less, it is possible to preferably control the distribution of MA. The total reduction in rough rolling is determined by setting the inlet thickness of the first roll in rough rolling to t ２ The thickness of the final rolled strip at the delivery side of the rough rolling is t ３ When this is done, (1-t ３ / t ２ ) × 100 (%). The rough rolling completion temperature (the temperature at the outlet of the final stage of rough rolling) is preferably set to more than 1220°C. By setting the rough rolling completion temperature to more than 1220°C, recrystallization can be promoted, and as a result, the average grain size of prior austenite grains can be preferably controlled.

[0091] After completion of rough rolling, it is preferable to cool the steel so that the temperature reaches 1220°C within 1.00 seconds, hold the steel in a temperature range of 1100 to 1220°C for 3 to 25 seconds, and then start finish rolling. By setting the time required to reach 1220°C within 1.00 seconds after completion of rough rolling, it is possible to prevent prior austenite grains from becoming coarse. Furthermore, by setting the holding temperature to 1220°C or less and the holding time to 25 seconds or less, it is possible to prevent prior austenite grains from becoming coarse. By setting the holding time to 1100°C or more, it is possible to prevent prior austenite grains from becoming flattened, and as a result, it is possible to preferably control the distribution of MA.

[0092] Finish rolling is preferably carried out so that the finish rolling completion temperature (final stage delivery temperature) is in the temperature range of 940 to 1020°C, and the final rolling reduction and the rolling reduction in the rolling step before the final stage are each 20 to 43%. By setting the finish rolling completion temperature to 940°C or higher, the distribution of MA can be favorably controlled. By setting the finish rolling completion temperature to 1020°C or lower, coarsening of prior austenite grains can be suppressed. By setting the final rolling reduction and the rolling reduction in the rolling step before the final stage to 20% or higher, coarsening of prior austenite grains can be suppressed. By setting the final rolling reduction and the rolling reduction in the rolling step before the final stage to 43% or lower, the distribution of MA can be favorably controlled.

[0093] After the completion of finish rolling, cooling is preferably started within 2 seconds, and the steel sheet is cooled so that the temperature reaches 25°C within 17 seconds after the completion of finish rolling. By starting cooling within 2 seconds after the completion of finish rolling, and cooling so that the temperature reaches 25°C within 17 seconds after the completion of finish rolling, it is possible to suppress the formation of a large amount of bainite. After cooling, the steel sheet is wound into a coil.

[0094] After winding, Ac １ Point ~ Ac ３ It is preferable to reheat the material to the temperature range of the point and hold it at that temperature range for 10 seconds or more. By holding it at that temperature range for 10 seconds or more, it is possible to generate a desired amount of MA. Furthermore, it is preferable that the holding time at that temperature range is 200 seconds or less. By holding it at that temperature range for 200 seconds or less, it is possible to prevent excessive generation of MA.

[0095] In addition, Ac １ Points and Ac ３ The points can be obtained by the following formula: １ (℃)=727-32.7×C+14.9×Si+2×Mn-17×Cu-14.2×Ni+17.8×Cr+25.6×Mo Ac ３(°C) = 937.2 - 436.5 x C + 56 x Si - 19.7 x Mn - 16.3 x Cu - 26.6 x Ni - 4.9 x Cr + 38.1 x Mo + 124.8 x V + 136.3 x Ti - 19.1 x Nb + 198.4 x Al + 3315 x B. The element symbols in the above formula indicate the content of each element in mass%, and if the element is not contained, 0 is substituted.

[0096] After reheating and holding, it is preferable to air-cool the material to a temperature range of 200°C or less. Air-cooling refers to cooling at an average cooling rate of 10°C / s or less. Note that the average cooling rate in this embodiment is the value obtained by dividing the temperature difference between the start point and the end point of a set range by the elapsed time from the start point to the end point.

[0097] 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 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.

[0098] Steels having the chemical compositions shown in Tables 1A to 2B were melted and continuously cast into slabs with thicknesses of 240 to 300 mm. Blank cells in the tables indicate that the element in question was below the detection limit. The resulting slabs were used to obtain steel plates shown in Tables 4A and 4B under the manufacturing conditions shown in Tables 3A and 3B. The thicknesses of the resulting steel plates were 1.2 to 8.0 mm.

[0099] The slab heating temperature was in the range of 1220 to 1300°C, and the holding time in this temperature range was 40 minutes or more. After the completion of finish rolling, cooling was started within 2.0 seconds, and reheating was performed using AcOEt. １ Point ~ Ac ３ The samples were heated to a temperature range of 100°C or less, and after reheating, air-cooled to a temperature range of 200°C or less (average cooling rate of 10°C / s or less). In No. 5, the cooling rate after completion of rough rolling was too large, so it was not possible to maintain the temperature in the preferred range. In No. 6, the cooling rate after completion of rough rolling was too small, so the temperature rose due to reheating, and it was not possible to maintain the temperature in the preferred range.

[0100] The items in the table indicate the following: Holding time: Holding time at the "holding temperature" in the table Final stage reduction: Final stage reduction in finish rolling Rolling reduction before the final stage: Rolling reduction before the final stage in finish rolling Time to reach 25°C: Elapsed time from the completion of finish rolling until the temperature reaches 25°C Reheating time: Ac １ Point ~ Ac ３ Holding time in the temperature range of the point

[0101] The metal structure of the obtained steel sheets in the region from the surface to a depth of 1 / 8 of the plate thickness to a depth of 3 / 8 of the plate thickness from the surface, the average grain size of prior austenite grains, the average value of the difference between the maximum and minimum numbers of MA grains, the yield ratio (YR), the tensile strength (TS), the total elongation (EL), the hole expansion ratio (λ), and the critical fracture thickness reduction rate after pre-straining were determined by the above-mentioned methods. The obtained measurement results are shown in Table 4A and Table 4B.

[0102] When the yield ratio was 70% or more, the steel sheet was judged to have a high yield ratio and to have passed the test, whereas when the yield ratio was less than 70%, the steel sheet was judged to have an unsatisfactory yield ratio and to have failed the test.

[0103] When the tensile strength was 1180 MPa or more, the steel sheet was judged to have high strength and to have passed the test, whereas when the tensile strength was less than 1180 MPa, the steel sheet was judged to have low strength and to have failed the test.

[0104] When the total elongation was 4.0% or more, the steel sheet was judged to have excellent ductility and to have passed the test, whereas when the total elongation was less than 4.0%, the steel sheet was judged to have poor ductility and to have passed the test.

[0105] When the hole expansion ratio was 45% or more, the steel sheet 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 45%, the steel sheet was judged to have poor hole expandability and to have passed the test.

[0106] When the critical reduction in thickness at break after pre-straining was 0.25 or more, the specimen was judged to have a high critical reduction in thickness at break after pre-straining and to have excellent formability even after pre-straining, and was therefore judged to have passed. On the other hand, when the critical reduction in thickness at break after pre-straining was less than 0.25, the specimen was judged to have no excellent formability after pre-straining, and was therefore judged to have passed.

[0107]

[0108]

[0109]

[0110]

[0111]

[0112]

[0113]

[0114]

[0115] From Tables 1A to 4B, it can be seen that the steel sheets according to the present invention have high strength and yield ratio, as well as excellent ductility and hole expandability, and have a high critical thickness reduction rate at fracture after pre-straining.

[0116] In addition, for all examples, lower arms (components) were manufactured by press working. The flat portion of the lower arm was evaluated in the same manner as described above. The measurement results and evaluation results were the same as those shown in Tables 4A and 4B.

[0117] According to the above aspects of the present disclosure, it is possible to provide a steel sheet having high strength and yield ratio, as well as excellent ductility and hole expandability, and a high critical fracture thickness reduction rate after pre-straining, and a part using the steel sheet.

Claims

1. The chemical composition is, by mass%, C: 0.090 to 0.210%, Si: 0.20 to 1.00%, Mn: 1.95 to 2.55%, P: 0.060% or less, S: 0.005% or less, Al: 0.01 to 0.26%, N: 0.0070% or less, O: 0 to 0.010%, Ti: 0.10 to 0.18%, Nb: 0.01 to 0.04%, B: 0.0001 to 0.0030%, Cr: 0 to 0.47%, Mo: 0 to 0.12%, Cu: 0 to 0.40%, Ni: 0 to 0.30%, V: 0 to 0.30%, Sn: 0 to 0.040%, As: 0 to 0.100%, Zr: 0 to 0.050%, Ca: 0 to 0.010%, Mg: 0 to 0.010%, Bi: 0 to 0.010%, Co: 0 to 0.010%, W: 0 to 0.100%, Zn: 0 to 0.010%, REM: 0 to 0.010%, Sb: 0 to 0.010%, and Ta: 0 to 0.010%, with the balance being Fe and impurities. In the region from 1 / 8 of the plate thickness from the surface to 3 / 8 of the plate thickness from the surface, the metallographic structure is, by area%, tempered martensite: 80.0 to 99.0%, MA: 1.0 to 20.0%, ferrite and bainite: 10.0% or less in total, and pearlite: 2.0% or less. The average grain size of the prior austenite grains is 25 μm or less. In the said region, a microstructure photograph is obtained by continuously taking 5 photographs in the rolling direction with a field of view of 100 μm in the rolling direction × 100 μm in the plate thickness direction. When 10 or more line segments in the direction of 45° with respect to the plate thickness direction are drawn at equal intervals of 10 μm or more in the microstructure photograph, the difference between the maximum number and the minimum number of MA present on the line segment per 100 μm of the line segment for each microstructure photograph is obtained. When the average value of the difference between the maximum number and the minimum number of MA in the 5-field microstructure photographs is calculated, the average value is 8 to 16, characterized steel plate.

2. The steel sheet according to claim 1, wherein the chemical composition contains, by mass%, one or more of Cr: 0.01 to 0.47%, Mo: 0.01 to 0.12%, Cu: 0.01 to 0.40%, Ni: 0.01 to 0.30%, V: 0.01 to 0.30%, Sn: 0.001 to 0.040%, As: 0.001 to 0.100%, Zr: 0.001 to 0.050%, Ca: 0.001 to 0.010%, Mg: 0.001 to 0.010%, Bi: 0.001 to 0.010%, Co: 0.001 to 0.010%, W: 0.001 to 0.100%, Zn: 0.001 to 0.010%, REM: 0.001 to 0.010%, Sb: 0.001 to 0.010%, and Ta: 0.001 to 0.010%.

3. A component characterized by including the steel sheet according to claim 1 or 2.

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