Steel plates and parts

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

[Technical Field]

[0001] The present disclosure relates to steel plates and components. This application claims priority based on Japanese Patent Application No. 2024-005797, filed on January 18, 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. In particular, the application of steel sheets with a strength of over 780 MPa has begun to be considered for automobile suspension parts such as lower arms, trail links, and knuckles. These automobile suspension parts are manufactured by subjecting steel sheets to burring, stretch flange forming, bending, and other processes. Therefore, steel sheets used for 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 describes 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 The hot-dip galvanized steel sheet is characterized by having a steel structure as follows: and having the following mechanical properties: tensile strength: 780 MPa or more, total elongation: 10% or more, hole expansion ratio: 35% or more, and the minimum inner radius at which cracks do not occur in a bending test at a bending angle of 180°: 3.5 times or less of the sheet thickness.

[0004] Patent Document 2 describes a microstructure containing, by volume fraction, 70% or more of martensite, tempered martensite, and bainite in total, and containing 5 to 20% of retained austenite, and in a surface layer region ranging from the surface to a position 1 / 10 of the plate thickness, <111> ~{111} <112> The average pole density of the orientation group consisting of {110} <001> and the pole density of the crystal orientations is 6.0 or less, the concentration of solute carbon in the retained austenite is 0.5 mass % or more, and the tensile strength is 980 MPa or more. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent No. 5910396 [Patent Document 2] International Publication No. 2021 / 167079 Summary of the Invention [Problem to be solved by the invention]

[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 discovered that when high-strength steel sheets are used in, for example, the lower arm of an automobile suspension part, cracks occur in the curved protruding portion near the link, as shown in Figure 1. These cracks occur in the portion that undergoes tensile deformation perpendicular to the crack after undergoing protruding deformation.

[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 reduction rate of sheet thickness at fracture 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. [Means for solving the problem]

[0012] The gist of the present invention is as follows. [1] Chemical composition, in mass%, C: 0.090~0.210%, Si: 0.20 to 1.00%, Mn: 1.95-2.55% 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: 0.10-0.18% Nb: 0.01 to 0.04%, B: 0.0001 to 0.0030%, Cr: 0~0.47%, Mo: 0-0.12%, Cu: 0-0.40% Ni: 0 to 0.30% V: 0~0.30%, Sn: 0 to 0.040% As: 0~0.100%, 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: 0~0.010%, Sb: 0 to 0.010%, and Ta: 0 to 0.010% the balance being Fe and impurities, 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 metal structure is, in area%, Tempered martensite: 80.0~99.0%, MA: 1.0~20.0%, Ferrite and bainite: not more than 10.0% in total; and Pearlite: 2.0% or less, The average grain size of prior austenite grains is 25 μm or less, In the region, a structural photograph is obtained by continuously photographing five fields of view of 100 μm in the rolling direction × 100 μm in the plate thickness direction in the rolling direction, and 10 or more line segments in a 45° direction with respect to the plate thickness direction are drawn at equal intervals of 10 μm or more in the structural photograph. A steel sheet characterized in that, when the difference between the maximum and minimum numbers of MAs present on a line segment per 100 μm in each of the structural photographs is calculated and the 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%, REM: 0.001~0.010%, Sb: 0.001 to 0.010%, and The steel sheet according to [1], characterized in that it contains at least one of Ta: 0.001 to 0.010%. [3] A part characterized by including the steel plate according to [1] or [2]. [Effects of the Invention]

[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. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a view showing a cracked portion of a lower arm manufactured from a high-strength steel plate. [Figure 2] FIG. 10 is a diagram for explaining a method for measuring the difference between the maximum and minimum numbers of MAs. [Figure 3]FIG. 1 is a diagram showing the relationship between the average value of the difference between the maximum and minimum numbers of MA in structural photographs of five fields of view and the critical reduction rate in sheet thickness at fracture after pre-straining in Examples. [Figure 4] 10A to 10C are diagrams for explaining a manufacturing method of the hat part. DETAILED DESCRIPTION OF THE INVENTION

[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 favorably control the average grain size of prior austenite grains and to favorably 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 a 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] The individual constituent elements of the present disclosure will be described in detail below. First, the reasons for limiting the chemical composition of the steel sheet according to the present embodiment will be described. Below, numerical ranges indicated with "to" include the lower and upper limits. Numerical values ​​indicated as "less than" or "greater than" are not included in the numerical range. In the following description, percentages relating to chemical compositions are mass % unless otherwise specified.

[0019] The steel sheet according to this embodiment has a chemical composition, in mass%, of C: 0.090-0.210%, Si: 0.20-1.00%, Mn: 1.95-2.55%, P: 0.060% or less, S: 0.005% or less, Al: 0.01-0.26%, N: 0.0070% or less, O: 0-0.010%, Ti: 0.10-0.18%, Nb: 0.01-0.04%, B: 0.0001-0.0030%, 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 a 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 through 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 also embrittles welded joints. 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 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.

[0024] S: 0.005% or less S is an element that embrittles slabs when present as sulfides. S also 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 better, and 0% is preferable. However, if the S content is reduced too much, the cost of desulfurization 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 acts as a deoxidizer and is an element that improves the cleanliness of steel. If the Al content is less than 0.01%, a sufficient deoxidizing effect cannot be 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 the 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, if the N content is reduced excessively, the cost of denitrification increases significantly. 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 point of fracture when contained in large amounts in steel. If the O content exceeds 0.010%, cracks are likely 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. O may not be contained, and the O content may be 0%.

[0028] Ti: 0.10 to 0.18% Ti is an element that forms fine nitrides in steel, thereby increasing the strength of the steel sheet. 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, and 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 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. To ensure that the effect of adding Cr to increase the strength of the steel sheet is obtained, the Cr content is preferably 0.01% or more. On the other hand, if the Cr content exceeds 0.47%, the above effect saturates, so 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 sheets by forming fine carbides in the steel, and to ensure this effect, the Mo content is preferably 0.01% or more. On the other hand, if the Mo content exceeds 0.12%, the above effects saturate, so 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 the effect of 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%, intergranular cracking of the slab may occur, so 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 added, Ni has the effect of effectively suppressing grain boundary cracking in the slab caused by Cu. To reliably obtain the above effects, it is preferable that the Ni content be 0.01% or more. Since Ni is an expensive element, it is not economically preferable to add a large amount of Ni. Furthermore, even if the Ni content exceeds 0.30%, the above effects saturate. 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 sheets by forming fine carbides in the steel. To ensure this effect, the V content is preferably 0.01% or more. On the other hand, if the V content exceeds 0.30%, the above effect saturates, so 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, if the Sn content exceeds 0.040%, the above effect saturates, so 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 the steel sheet. 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, and 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 increasing hole expandability, and in order to reliably obtain this effect, the Zr content is preferably 0.001% or more. On the other hand, if the Zr content exceeds 0.050%, the above effect saturates, so 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, if the Ca content exceeds 0.010%, the above effect saturates, so 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, if the Mg content exceeds 0.010%, the above effect saturates, so 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, and to reliably obtain this effect, the Bi content is preferably 0.001% or more. On the other hand, if the Bi content exceeds 0.010%, the above effect saturates, so 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 by solid solution strengthening, and in order to reliably obtain this effect, the Co content is preferably 0.001% or more. On the other hand, if the Co content exceeds 0.010%, the above effect saturates, so 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, and in order to reliably obtain this effect, the W content is preferably 0.001% or more. On the other hand, if the W content exceeds 0.100%, the above effect saturates, so 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, and in order 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, so 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 reliably obtain 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, so the REM content is set to 0.010% or less. Here, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanides, and the REM content refers to the total content of these elements. In the case of lanthanides, they are industrially added 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 the steel sheet. To reliably obtain this effect, the Sb content is preferably 0.001% 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 0.010% or less.

[0049] Ta: 0.001 to 0.010% Ta has the effect of increasing the strength of the steel sheet by forming fine carbides in the steel, similar to V. 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, so 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, etc. 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. When the steel sheet has a plating layer or a coating film on the surface, the plating layer or the 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 1 / 8 of the plate thickness from the surface to a depth of 3 / 8 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 at 45° to the plate thickness direction. The difference between the maximum and minimum numbers of MA particles present on each 100 μm line segment is determined for each of the structural photographs, and the average value of the differences between the maximum and minimum numbers of MA particles in the five fields of view of the structural photographs 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 of, in area %, only 80.0 to 99.0% tempered martensite, 1.0 to 20.0% MA, 10.0% or less in total of ferrite and bainite, and 2.0% or less pearlite 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.

[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 the steel sheet has a plating layer, a coating film, or the like on its surface, the surface here refers to the interface between the steel sheet and the plating layer, the coating film, or the like. The interface between the steel sheet and the plating layer or paint film is identified by 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 400x, an area of ​​40,000 μm 2 The above range is observed. When cross-sections are observed using BSE images (or COMPO images), a clear difference in contrast can be seen between the plating layer, paint 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, paint film, etc. The same method is used to identify the interface when the parts described below have plating layers, paint films, etc.

[0054] Tempered martensite: 80.0~99.0% If the area fraction of tempered martensite is less than 80.0%, the yield ratio of the steel sheet will decrease. 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: 1.0~20.0% 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, resulting in a decrease in the yield ratio of the steel sheet. 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 total area fraction of ferrite and bainite exceeds 10.0%, the hole expandability of the steel sheet deteriorates. Therefore, the total area fraction of ferrite and bainite is set to 10.0% or less. 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 ratio of ferrite and bainite may be 0.0%.

[0057] Perlite: 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 by the following method. First, a method for measuring the area ratio of ferrite will be described. A test specimen was taken from the 1 / 4 position in the width direction of the steel plate so that a thickness cross-section could be observed from 1 / 8 of the thickness depth from the surface to 3 / 8 of the thickness depth from the surface. The thickness cross-section of the test specimen was polished using silicon carbide paper, graded from #600 to #1500, and then polished to a mirror finish using a liquid in which diamond powder with a grain size of 1 to 6 μm was dispersed in a diluted solution such as alcohol or pure water. The specimen was then polished for 8 minutes at room temperature using colloidal silica with a grain size of 0.25 μm and no alkaline solution to remove the strain induced in the surface layer of the test specimen. Crystal orientation information was obtained by measuring the area of ​​the test specimen from 1 / 8 of the thickness depth from the surface to 3 / 8 of the thickness depth, with a field of view of 200 μm in the rolling direction and 100 μm in the thickness direction centered at 1 / 4 of the thickness depth from the surface, at 0.1 μm measurement intervals using electron backscatter diffraction.

[0059] For the measurements, an EBSD device consisting of a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL DVC5 detector) was used. The vacuum level inside the EBSD device was 9.6 × 10 -5 The conditions are: 1. Vapor pressure (MPa) or less, 2. Acceleration voltage (kV), probe current level (level 13), and electron beam irradiation level (level 62). From the obtained crystal orientation information, the "Phase Map" function installed in the "OIM Analysis (registered trademark)" software 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. Regions with a GAM value of 0.5° or less are identified as ferrite. The same procedure is performed for five fields of view to calculate the area ratio of ferrite. Here, the "GAM value" is the average misorientation between adjacent pixels in regions 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 with EBSD, Vickers indentations are engraved at three of the four corners of the field of view measured with EBSD, within 100 μm of each corner, so that the observation position can be identified. Surface contamination is then polished away, leaving the metal structure of the observation surface. Using the Vickers indentations as landmarks, it is possible to observe the same field of view as that measured with EBSD. Contamination can be removed by buffing using alumina particles with a particle size of 0.1 μm or less, polishing using colloidal silica that does not contain alkaline solution at room temperature, or Ar ion sputtering.

[0062] The observation surface, including the field of view measured by EBSD, is subjected to nital etching to reveal the microstructure. Then, using a scanning electron microscope at 500x magnification, photographs are taken of the same five fields of view as those measured by EBSD. In the microstructure photographs, lath-like 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 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 then subtracted from this area fraction to obtain the area fraction of tempered martensite. Furthermore, in each microstructure photograph, regions containing lamellar carbides are identified as pearlite, and the average 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 and then mirror-finished, after which it is etched with LePera to reveal the microstructure. Then, using a scanning electron microscope at 500x magnification, photographs are taken of the same five fields of view as those measured by EBSD. In each microstructure photograph, areas with white contrast are identified as MA, and the area ratio of MA is determined by calculating the average of these area ratios.

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

[0065] The rolling direction of the steel sheet is determined by the following method. A test piece is taken from any position at least 50 mm away from the end of the steel plate 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 plate, and areas with dark brightness are determined to be inclusions. Observation may be performed from multiple fields of view. Next, using the thickness cross section initially observed using the above method as a reference, the plate is rotated in 5° increments between 0° and 180° around the thickness direction, and the cross sections parallel to the plane are 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 major axis of the inclusions in that cross section is determined to be the rolling direction. The rolling direction of the part is also determined in a similar manner.

[0066] Average grain size of prior austenite grains: 25 μm or less The inventors have found that even when the distribution of MA is favorably controlled as described below, if the average grain size of 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 prior austenite grains.

[0067] MA is generated from the grain boundaries of prior austenite. If the average grain size of the prior austenite grains is too large, the distance between MA generated from the grain size becomes large, and the difference between areas with a large amount of MA and areas with a small amount of MA becomes large. As a result, shear bands are localized in areas with a small amount of MA, causing strain concentration. If the average grain size of the 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 the prior austenite grains is set to 25 μm or less. The average grain size of the prior austenite grains is preferably 23 μm or less, 20 μm or less, or 15 μm or less. There is no particular lower limit to the average grain size of the prior austenite grains. However, since setting the average grain size of the prior austenite grains to less than 4 μm places a heavy burden on the rolling equipment, the average grain size of the prior austenite grains is preferably 4 μm or more. The average grain size of the prior austenite grains may be 5 μm or more or 10 μm or more.

[0068] The average grain size of prior austenite is measured by the following method. A test piece is taken from a quarter position in the width direction of the steel plate so that a thickness cross section can be observed from 1 / 8 of the thickness depth from the surface to 3 / 8 of the thickness depth from the surface. The observation surface of the test piece is etched using an etching solution made by adding a saturated aqueous solution of picric acid and a sodium dodecylbenzenesulfonate etching solution to reveal the structure of the thickness cross section. Using a scanning electron microscope, microstructure photographs are taken of the test piece in a region from 1 / 8 of the thickness depth from the surface of the steel plate to 3 / 8 of the thickness depth from the surface, with a field of view of 200 μm in the rolling direction and 100 μm in the thickness direction centered at a depth of 1 / 4 of the thickness from the surface of the steel plate. Five microstructure photographs are taken. The grain size of prior austenite grains is measured using the photographed 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] MA distribution 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 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 parts that have been bent and unbent by press working, strain is localized in a band-like shape in the 45° direction relative to the thickness direction. This strain band (shear band) develops in a way that avoids MA. Therefore, if there is a large difference between areas with high and low MA on the line at 45° to the thickness direction, the shear band will localize in areas with low MA, causing strain concentration. Therefore, it is important to reduce the difference between areas with high and low MA on the line at 45° to the thickness direction.

[0071] In order to reduce the difference between areas with a high level of MA and areas with a low level of MA on a line at an angle of 45° to the thickness direction of the steel plate, the following definitions are used in this embodiment. In a region from ⅛ the depth of the sheet thickness from the surface to ⅜ the depth of the sheet thickness from the surface, five fields of view, each 100 μm in the rolling direction and 100 μm in the sheet thickness direction, were photographed consecutively in the rolling direction to obtain structural photographs, and 10 or more line segments were drawn in the structural photographs at a 45° angle to the sheet thickness direction at equal intervals of 10 μm or more. For each of the structural photographs, the difference between the maximum and minimum numbers of MAs present on each 100 μm line segment was determined, and the average value of the differences between the maximum and minimum numbers of MAs in the structural photographs for the five fields of view was calculated; the average value was 8 to 16. Hereinafter, this will be described in detail 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 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 observation surface is etched with nital 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 the method for measuring the difference between the maximum and minimum numbers of MA. Note that MA is not shown in Figure 2. In taking the structural photograph, as shown in Figure 2, a scanning electron microscope is used to photograph five fields of view 100 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, in succession in the rolling direction to obtain a structural photograph.

[0073] Next, on the 100 μm x 100 μm photograph, a line segment L1 is drawn diagonally at a 45° angle to the plate thickness direction. At least 10 lines, such as line segments L2, L3, etc., are drawn from line segment L1 at equal intervals of 10 μm or more. In Figure 2, 11 lines are drawn. Next, the number of MAs present on the line segment per 100 μm is counted. Specifically, first, the number of MAs present on the line segment from endpoint A of line segment L1 to point B, 100 μm away, is counted. The obtained number of MAs is designated as the number of MAs q1 on line segment AB. Next, the number of MAs present on the line segment from point B to a position 100 μm away is counted. If no line segments exist up to 100 μm, the number of MAs present on another line segment is counted so that the total length is 100 μm. In the case of Figure 2, count the number of MAs on the line segment from point B to point C, then count the number of MAs 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 the number of MAs q2 on line segment BCDE. Repeat the same operation for line segment L2, line segment L3, ... line segment L 11 The number of particles per 100 μm of each line segment q1, q2...q n get.

[0074] The numbers obtained are q1, q2...q n From the maximum number q Max and the minimum number q min The maximum number q Max and the minimum number q min The difference D1 between the maximum and minimum numbers of MAs for the first structural photograph is obtained by calculating the difference between the maximum and minimum numbers of MAs for the first structural photograph. The same operation is performed for the remaining four fields of view that are consecutive in the rolling direction to obtain differences D2, D3, D4, and D5 between the maximum and minimum numbers of MAs. The average value of the obtained differences D1 to D5 is calculated to obtain the average value of the differences between the maximum and minimum numbers of MAs for the five fields of view of the structural photographs. When drawing lines on the structure photograph, avoid drawing lines that are too thick, and use lines that are within the range of reasonable thickness.

[0075] Fig. 3 is a diagram showing 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 Fig. 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 also preferably 15 or less, or 14 or less.

[0076] Tensile strength (TS): 1180 MPa or more The steel sheet 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 to the tensile strength, but from the viewpoint of suppressing die 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, 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] 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 section 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 it is not possible to take a No. 5 test piece from the part because the part is small in size or has a complex shape, a small rectangular piece with a parallel part of any width can be taken and a tensile test performed, and the tensile strength can be calculated from the maximum test force and the original cross-sectional area of ​​the parallel part.

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

[0081] Critical fracture thickness reduction rate after pre-straining: 0.25 or more In this embodiment, pre-straining by bending and unbending deformation is performed by draw-bending a steel sheet using the method shown in FIG. 4. In the formation of a hat part using the draw-bending process shown in FIG. 4, the steel sheet comes into contact with a punch while undergoing bending and unbending deformation when the vertical walls of the hat part are formed. A test piece for a tensile test is taken from the vertical wall of the hat part so that the height direction of the vertical wall is the longitudinal direction. The obtained test piece is used to perform a tensile test using the method described above. The critical fracture thickness reduction rate is obtained by calculating the value (t0-t1) / t0, where t0 is the thickness of the test piece before the tensile test and t1 is the minimum thickness at the center of the width direction (short direction) of the test piece after fracture. Five tensile tests are performed, and the maximum and minimum values ​​of the critical fracture thickness reduction rate are excluded to calculate the average value of three tests, thereby obtaining 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. It is preferably 1.4 mm or more. On the other hand, if the plate thickness exceeds 8.0 mm, it may be difficult to obtain the above-mentioned metal structure after hot rolling. Therefore, the plate thickness may be 8.0 mm or less, and is 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 the 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 coating and electrolytic Zn-Ni alloy coating. Examples of hot-dip plated layers include hot-dip galvanized coating, alloyed hot-dip galvanized coating, hot-dip aluminum coating, hot-dip Zn-Al alloy coating, hot-dip Zn-Al-Mg alloy coating, and hot-dip Zn-Al-Mg-Si alloy coating. The coating weight is not particularly limited and may be the same as conventional coatings. In addition, it is possible to further improve corrosion resistance by carrying out an appropriate chemical conversion treatment after plating (for example, 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 the preferred manufacturing method for 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 prior austenite grains. Furthermore, the distribution of MA generated from the grain boundaries of the prior austenite grains during subsequent reheating can be preferably controlled. In this embodiment, the temperature of the slab and the temperature of the steel sheet refer to the surface temperature of the slab and the surface temperature of the steel sheet, and are measured with a radiation thermometer. Also, Tf refers to the rough rolling completion temperature (°C).

[0088] A preferred method for manufacturing a steel sheet according to this embodiment is as follows: Rough rolling is performed so that the total reduction rate is 60 to 85% and the rough rolling completion temperature is over 1220°C. After rough rolling is completed, the material is cooled to 1220°C within 1.00 seconds, and then held in the temperature range of 1100 to 1220°C for 3 to 25 seconds, after which finish rolling begins. Finish rolling is performed so that the temperature at the end of finish rolling is in the temperature range of 940 to 1020°C, and the final reduction rate and the reduction rate in the rolling stage one stage before the final one are 20 to 43%, respectively. Cooling is started within 2 seconds after the completion of finish rolling, and is cooled to reach 25°C within 17 seconds after the completion of finish rolling. Winding is carried out, Reheat to the temperature range of Ac1 to Ac3 and hold at that temperature range for 10 seconds or more. Air-cool to a temperature range of 200°C or less. Each step will be described below.

[0089] The slab to be subjected to rough rolling 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. 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 rate of rough rolling can be expressed as (1-t3 / t2) x 100 (%), where t2 is the entry thickness of the first roll of rough rolling and t3 is the exit thickness of the last roll of rough rolling. The rough rolling completion temperature (the temperature at the outlet of the final stage of rough rolling) is preferably set to exceed 1220° C. By setting the rough rolling completion temperature to exceed 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 to reach 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 limiting the time required to reach 1220°C after completion of rough rolling to within 1.00 seconds, 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 next-to-last rolling 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 next to last rolling stage to 20% or more, it is possible to prevent prior austenite grains from becoming coarse. By setting the final rolling reduction and the rolling reduction in the next to last rolling stage to 43% or less, it is possible to preferably control the distribution of MA.

[0093] After the completion of finish rolling, cooling is preferably started within 2 seconds, and the steel sheet is cooled to reach 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 to reach 25°C within 17 seconds after the completion of finish rolling, it is possible to prevent a large amount of bainite from being generated. After cooling, the steel sheet is wound into a coil.

[0094] After winding, it is preferable to reheat the film to a temperature range between Ac1 and Ac3 points and hold the film at that temperature for 10 seconds or more. Holding the film at that temperature for 10 seconds or more allows the desired amount of MA to be produced. Furthermore, the holding time at that temperature is preferably 200 seconds or less. Holding the film at that temperature for 200 seconds or less prevents excessive production of MA.

[0095] The Ac1 point and Ac3 point can be obtained by the following formulas. Ac1(℃)=727-32.7×C+14.9×Si+2×Mn-17×Cu-14.2×Ni+17.8×Cr+25.6×Mo Ac3(℃)=937.2-436.5×C+56×Si-19.7×Mn-16.3×Cu-26.6×Ni-4.9×Cr+38.1×Mo+124.8×V+136.3×Ti-19.1×Nb+198.4×Al+3315×B The element symbols in the above formula indicate the content of each element in mass %, and 0 is substituted when the element is not contained.

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

[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 corresponding elements were below the detection limit. The resulting slabs were used to obtain steel plates shown in Tables 4A and 4B under the production conditions shown in Tables 3A and 3B. The thickness of the obtained steel plates was 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 began within 2.0 seconds, and the slab was reheated to a temperature range of Ac1 to Ac3, and then 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 amount after completion of rough rolling was too large, so it was not possible to maintain the temperature in the desired range. In No. 6, the cooling amount 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 desired 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 of finish rolling Reduction rate of the rolling one before the final stage: Reduction rate of the rolling one before the final stage of finishing rolling Time to reach 25°C: The time elapsed from the completion of finish rolling until the temperature reaches 25°C Reheating time: Holding time in the temperature range from Ac1 to Ac3

[0101] For the obtained steel plates, the metal structure 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, 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 using the methods described above. The measurement results obtained are shown in Tables 4A and 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] If the tensile strength was 1180 MPa or more, the steel sheet was judged to have high strength and passed the test, whereas if the tensile strength was less than 1180 MPa, the steel sheet was judged to have low strength and 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 failed 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 fracture thickness reduction rate after pre-straining was 0.25 or more, the critical fracture thickness reduction rate after pre-straining was high and the specimen was judged to have excellent formability even after pre-straining, and was therefore judged to have passed. On the other hand, when the critical fracture thickness reduction rate after pre-straining was less than 0.25, the specimen was judged to have poor formability after pre-straining, and was therefore judged to have passed.

[0107] [Table 1A]

[0108] [Table 1B]

[0109] [Table 2A]

[0110] [Table 2B]

[0111] [Table 3A]

[0112] [Table 3B]

[0113] [Table 4A]

[0114] [Table 4B]

[0115] It can be seen from Tables 1A to 4B that the steel sheets according to the present invention have high strength and yield ratio, as well as excellent ductility and hole expandability, and also have a high critical reduction in thickness 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. [Industrial Applicability]

[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, in mass%, is C: 0.090-0.210%, Si: 0.20-1.00%, Mn: 1.95-2.55%, P: 0.060% or less, S: 0.005% or less, Al: 0.01-0.26%, N: 0.0070% or less, O: 0 to 0.010%, Ti: 0.10-0.18%, Nb: 0.01-0.04%, 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: 0-0.010%, 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: 0-0.010%, Sb: 0 to 0.010%, and Ta: 0 to 0.010%; the balance being Fe and impurities; 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 metal structure is, in area%, Tempered martensite: 80.0 to 99.0%, MA: 1.0-20.0%, Ferrite and bainite: 10.0% or less in total; and Pearlite: 2.0% or less, The average grain size of prior austenite grains is 25 μm or less, In the region, a structural photograph is obtained by continuously photographing five fields of view of 100 μm in the rolling direction × 100 μm in the plate thickness direction in the rolling direction, and 10 or more line segments in a 45° direction with respect to the plate thickness direction are drawn at equal intervals of 10 μm or more in the structural photograph. A steel sheet characterized in that, when the difference between the maximum number and the minimum number of MAs present on a line segment per 100 μm in each of the structural photographs is determined and the average value of the differences between the maximum number and the minimum number 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: 0.01-0.47%, Mo: 0.01-0.12%, Cu: 0.01-0.40%, 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: 0.001-0.010%, Mg: 0.001-0.010%, Bi: 0.001 to 0.010%, Co: 0.001 to 0.010%, W: 0.001-0.100%, Zn: 0.001 to 0.010%, REM: 0.001-0.010%, Sb: 0.001 to 0.010%, and The steel sheet according to claim 1, characterized in that it contains at least one of Ta: 0.001 to 0.010%.

3. A component comprising the steel sheet according to claim 1 or 2.

Images