Steel plate and method for manufacturing steel plate

Abstract

This steel sheet contains, in % by mass, 0.15 to 0.50% of C, 0.01 to 1.00% of Si, 1.00 to 3.00% of Mn, 0 to 0.0200% of P, 0.0001 to 0.0200% of S, 0.001 to 0.100% of Al, 0 to 0.0200% of N and a remainder comprising Fe and impurities, in which, with respect to surface area ratios of metal structures, the surface area ratio of retained austenite is 0% to 10% inclusive and the total surface area of pearlite, ferrite and bainite is 0% to 5% inclusive, and in which the remaining structure comprises martensite and tempered martensite, the maximum diameter of MnS is 30 μm or less when determined by extreme value statistics, the surface roughness Ra is 5 μm or less, and the Vickers hardness of a surface layer is a value determined by the formula: (the tensile strength TS (MPa) of the steel sheet)×0.25 or more.

Description

[Technical Field]

[0001] The present invention relates to a steel sheet and a method for manufacturing the steel sheet. This application claims priority based on Japanese Patent Application No. 2022-028110, filed on February 25, 2022, the contents of which are incorporated herein by reference. [Background technology]

[0002] In order to reduce greenhouse gas emissions from automobiles, attempts are being made to reduce the weight of automobile bodies while ensuring safety by using high-strength steel sheets.

[0003] For example, Patent Document 1 describes a steel sheet having excellent elongation, hole expandability, bending workability, and delayed fracture resistance, which contains, in mass%, C: 0.15 to 0.25%, Si: 1.00 to 2.20%, Mn: 2.00 to 3.50%, P: 0.05% or less, S: 0.005% or less, Al: 0.01 to 0.50%, N: 0.010% or less, and B: 0.0003 to 0.0050%, and also contains one or more selected from Ti: 0.005 to 0.05%, Cu: 0.003 to 0.50%, Ni: 0.003 to 0.50%, Sn: 0.003 to 0.50%, Co: 0.003 to 0.05%, and Mo: 0.003 to 0.50%. The high-strength TRIP steel sheet is characterized by a component composition consisting of ferrite having an average grain size of 2 μm or less at a volume fraction of 15% or less (including 0%), retained austenite having an average grain size of 2 μm or less at a volume fraction of 2 to 15%, martensite having an average grain size of 3 μm or less at a volume fraction of 10% or less (including 0%), and the balance being bainite and tempered martensite having an average grain size of 6 μm or less, and the bainite and tempered martensite grains containing an average of 10 or more cementite particles having a grain size of 0.04 μm or more.

[0004] Patent Document 2 discloses a high-strength cold-rolled steel sheet having a tensile strength (TS) of 980 MPa or more and excellent bendability, the steel sheet having a specific chemical composition and a specific steel structure in which the area ratio of the ferrite phase is 30% to 70%, the area ratio of the martensite phase is 30% to 70%, the average grain size of the ferrite grains is 3.5 μm or less, the standard deviation of the grain size of the ferrite grains is 1.5 μm or less, the average aspect ratio of the ferrite grains is 1.8 or less, the average grain size of the martensite grains is 3.0 μm or less, and the average aspect ratio of the martensite grains is 2.5 or less, and the high-strength cold-rolled steel sheet has a tensile strength of 980 MPa.

[0005] Patent Document 3 discloses a high-strength steel sheet having a yield strength (YS) of 780 MPa or more, a tensile strength (TS) of 1180 MPa or more, and excellent spot weldability, ductility, and bending workability, in which the C content is 0.15% or less, the area fraction of ferrite is 8 to 45%, the area fraction of martensite is 55 to 85%, the proportion of martensite adjacent to ferrite alone in the total structure is 15% or less, the average crystal grain size of ferrite and martensite is 10 μm or less, and the area fraction of ferrite with a crystal grain size of 10 μm or more among ferrite present in a depth range from 20 μm to 100 μm from the steel sheet surface is less than 5%.

[0006] Patent Document 4 describes a steel sheet with little variation in mechanical properties (particularly strength and ductility), which has a composition containing, by mass%, C: 0.10 to 0.25%, Si: 0.5 to 2.0%, Mn: 1.0 to 3.0%, P: 0.1% or less, S: 0.01% or less, Al: 0.01 to 0.05%, N: 0.01% or less, with the balance being iron and unavoidable impurities, and has a structure consisting of tempered martensite and / or tempered bainite as a hard second phase and containing ferrite as a soft first phase in an area ratio of 20 to 50%, with the balance being hard second phase, and in which the total area of ​​particles of the ferrite having an average particle diameter of 10 to 25 μm accounts for 80% or more of the total area of ​​all the ferrite particles, and the dispersion state of cementite particles present in all the ferrite particles and having a circle equivalent diameter of 0.3 μm or more is such that the area ratio of the ferrite particles is 20 to 50% of the total area of ​​the ferrite particles. 2Disclosed is a high-strength cold-rolled steel sheet having a density of more than 0.15 and not more than 1.0 per square inch and a tensile strength of 980 MPa or more. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] International Publication No. 2017 / 179372 [Patent Document 2] International Publication No. 2016 / 194272 [Patent Document 3] Japanese Patent Publication No. 2015-117404 [Patent Document 4] Japanese Patent Application Publication No. 2013-245397 Summary of the Invention [Problem to be solved by the invention]

[0008] Demand for higher strength in automotive steel sheets is increasing not only for structural components such as members, but also for exterior panel components such as doors and roofs. To meet these demands, material development has been carried out with the aim of achieving both strength and elongation (formability).

[0009] On the other hand, the shapes of both the structural components and exterior panel components of automobiles as described above tend to become more complex. When steel sheets are made stronger and thinner to reduce weight, unevenness tends to occur on the surface of the steel sheet when it is formed into a complex shape, and the occurrence of surface unevenness results in a poor appearance after forming. That is, for automotive steel sheets, in addition to properties such as strength, the surface properties of the steel sheet are also important. In particular, for exterior panel components of automobiles, design and surface properties are also important, so excellent appearance after forming is required.

[0010] However, with conventional techniques, it has been difficult to achieve high levels of both strength and surface quality (appearance) after forming in steel sheets (particularly high-strength steel sheets).

[0011] An object of the present invention is to provide a steel sheet that can suppress the occurrence of surface irregularities during forming and has high strength, and a method for manufacturing the same. [Means for solving the problem]

[0012] The inventors have discovered that the occurrence of surface irregularities during forming can be suppressed by ensuring strength by making the main structure of the steel sheet martensite and tempered martensite, while controlling the size of MnS in the steel as an inclusion, and ensuring that the Vickers hardness of the steel sheet surface is at a predetermined level or higher.

[0013] The present invention has been made based on the above findings, and the gist of the present invention is as follows.

[0014] (1) A steel sheet according to one aspect of the present invention has a chemical composition, in mass%, C: 0.15~0.50%, Si: 0.01 to 1.00%, Mn: 1.00-3.00%, P: 0~0.0200%, S: 0.0001 to 0.0200%, Al: 0.001 to 0.100%, N: 0 to 0.0200%, Co: 0 to 0.500%, Ni: 0 to 1.000%, Mo: 0 to 1.000%, Cr: 0~2.000%, O: 0 to 0.020%, Ti: 0 to 0.5000%, B: 0~0.0100%, Nb: 0 to 0.500%, V: 0 to 0.500%, Cu: 0 to 0.500% W: 0 to 0.1000%, Ta: 0 to 0.1000%, Sn: 0 to 0.0500%, Sb: 0 to 0.0500%, As: 0 to 0.0500%, Mg: 0 to 0.0500%, Ca: 0 to 0.0500%, Zr: 0 to 0.0500%, and REM: 0 to 0.1000% and the balance being Fe and impurities, In the metal structure, the area ratio is Retained austenite: 0% or more, 10.0% or less, The total content of pearlite, ferrite, and bainite is 0% or more and 5.0% or less, and the remaining structure is martensite and tempered martensite, The maximum diameter of MnS according to extreme value statistics is 30.0 μm or less, The surface roughness Ra is 5.0 μm or less, The Vickers hardness of the surface layer is equal to or greater than 0.25 times the tensile strength TS (MPa) of the steel plate. (2) In the steel sheet according to the above aspect, the composition is, in mass%, Co: 0.010~0.500%, Ni: 0.010 to 1.000%, Mo: 0.010 to 1.000%, Cr: 0.001 to 2.000%, O: 0.0001 to 0.020%, Ti: 0.0010 to 0.5000%, B: 0.0001~0.0100%, Nb: 0.001 to 0.500%, V: 0.001 to 0.500%, Cu: 0.001 to 0.500%, W: 0.0010~0.1000%, Ta: 0.0010 to 0.1000%, Sn: 0.0010~0.0500%, Sb: 0.0010 to 0.0500%, As: 0.0010~0.0500%, Mg: 0.0001 to 0.0500%, Ca: 0.0010~0.0500%, Zr: 0.0010 to 0.0500%, and REM: 0.0010 to 0.1000% One or more of these may be contained. (3) In the steel sheet according to the above aspect, the composition is, in mass%, Mn: 1.00 to 2.00% Si: 0.30 to 1.00% may be. (4) The steel sheet according to the above aspect may have a tensile strength of 1470 MPa or more. (5) The steel sheet according to the above embodiment may have a coating layer containing at least one of zinc, aluminum, magnesium, and alloys thereof on one or both sides of the steel sheet.

[0015] (6) A method for producing a steel sheet according to an aspect of the present invention is the method for producing a steel sheet according to the above aspect, wherein molten steel is subjected to a vacuum degassing treatment, and the Al concentration of the molten steel is adjusted to 0.0500 mass% or less in the chemical composition of the molten steel, and the following is added in mass%: C: 0.15~0.50%, Si: 0.01 to 1.00%, Mn: 1.00-3.00%, P: 0~0.0200%, S: 0.0001 to 0.0200%, N: 0 to 0.0200%, Co: 0 to 0.500%, Ni: 0 to 1.000%, Mo: 0 to 1.000%, Cr: 0~2.000%, O: 0 to 0.020%, Ti: 0 to 0.5000%, B: 0~0.0100%, Nb: 0 to 0.500%, V: 0 to 0.500%, Cu: 0 to 0.500% W: 0 to 0.1000%, Ta: 0 to 0.1000%, Sn: 0 to 0.0500%, Sb: 0 to 0.0500%, As: 0 to 0.0500%, Mg: 0 to 0.0500%, Ca: 0 to 0.0500%, Zr: 0 to 0.0500%, and REM: 0 to 0.1000% a refining step of adjusting the composition of the sintered ... a casting step of producing a slab using the molten steel after the refining step; a hot rolling step in which the slab is heated directly or after being cooled once, and hot rolled to obtain a hot-rolled steel sheet; a winding step of winding the hot-rolled steel sheet in a temperature range of 700°C or less; a pickling step of pickling the hot-rolled steel sheet after the coiling step; a cold rolling step of cold rolling the hot-rolled steel sheet after the pickling step at a rolling reduction of 30 to 90% to obtain a cold-rolled steel sheet; An annealing process in which the cold-rolled steel sheet is annealed in an atmosphere having a dew point of -80°C or higher and -15°C or lower at a temperature range of 820°C to 900°C; It has. (7) In the method for producing a steel sheet according to the above aspect, the deoxidation time in the refining step may be set to less than 5 minutes. (8) In the method for producing a steel sheet according to the above aspect, the component composition is, in mass%, Mn: 1.00 to 2.00% Si: 0.30 to 1.00% may be. (9) In the method for producing a steel sheet according to the above-described embodiment, the annealing step may include a coating layer formation step of forming a coating layer containing at least one of zinc, aluminum, magnesium, and alloys thereof on one or both sides of the cold-rolled steel sheet. [Effects of the Invention]

[0016] According to the present invention, it is possible to provide a steel sheet that can suppress the occurrence of surface irregularities during forming and has high strength, and a manufacturing method thereof. Furthermore, according to the present invention, it is possible to provide a steel sheet that has surface properties suitable for automotive steel sheets and has high tensile strength. DETAILED DESCRIPTION OF THE INVENTION

[0017] In one embodiment of the present invention, by controlling the area ratio of the metal structure in the steel sheet, as well as the maximum particle size of inclusions (MnS), the surface roughness Ra of the steel sheet surface, and the Vickers hardness of the steel sheet surface, it is possible to obtain a high-strength steel sheet that can suppress the occurrence of surface irregularities during forming.

[0018] Hereinafter, a steel sheet according to one embodiment of the present invention will be described.

[0019] First, the metal structure of the steel sheet according to this embodiment will be described. Hereinafter, the structure fraction will be expressed as an area fraction, and therefore the unit "%" of the structure fraction means area %.

[0020] <Metal structure> (Area ratio of retained austenite: 0% or more, 10.0% or less) Retained austenite is a structure that contributes to improving elongation through transformation-induced plasticity (TRIP). However, the martensite generated by the transformation of retained austenite is very hard and may become the starting point for void generation, deteriorating the surface roughness after pre-strain. Therefore, the area fraction of retained austenite is set to 10.0% or less, and preferably 5.0% or less. In this embodiment, retained austenite does not have to be generated, and the area fraction of retained austenite may be 0%.

[0021] (Total area ratio of ferrite, bainite and pearlite: 0% or more, 5.0% or less) Ferrite and bainite are relatively soft structures. Therefore, excessive area fractions of ferrite and bainite may prevent the desired tensile strength from being obtained. Pearlite also has low strength and reduces ductility. Furthermore, when the area fraction of these soft structures increases in a steel sheet composed primarily of martensite and tempered martensite, the soft structures deform preferentially during pre-deformation. This local deformation propagates to the surface, resulting in unevenness on the steel sheet surface after pre-deformation. Therefore, from the perspective of ensuring strength and suppressing surface unevenness after pre-deformation, it is preferable that the area fractions of ferrite, bainite, and pearlite are as small as possible. The total area fraction of ferrite, bainite, and pearlite may be 0%. In other words, it is preferable that the steel sheet according to this embodiment does not contain ferrite, bainite, and pearlite. Even when ferrite, bainite, and pearlite are contained, the total area fraction of ferrite, bainite, and pearlite is 5.0% or less from the perspective of ensuring strength. It is preferably 4.0% or less, and more preferably 3.0% or less.

[0022] (Remaining structure: martensite and tempered martensite) In the metal structure of the steel sheet according to this embodiment, the remaining structure other than the above-mentioned retained austenite, ferrite, bainite, and pearlite is martensite and tempered martensite. That is, the structure of the steel sheet according to this embodiment is mainly composed of martensite and tempered martensite. Martensite and tempered martensite are hard structures, which contribute to improving tensile strength. Therefore, the total area ratio of martensite and tempered martensite is preferably 90.0% or more, and more preferably 95.0% or more. This makes it easier to ensure high tensile strength (for example, tensile strength of 1300 MPa or more). From the viewpoint of strength, the total area ratio of martensite and tempered martensite may be 100%. From the viewpoint of ensuring toughness, the martensite content is preferably 0.0% to 75.0%, and the tempered martensite content is preferably 20.0% to 99.9%.

[0023] In this embodiment, "martensite" refers to fresh martensite. Fresh martensite is martensite that does not contain carbides. Furthermore, "tempered martensite" is martensite that contains carbides.

[0024] Next, the method for identifying each metal structure and calculating the area ratio will be described.

[0025] Identification of each metal structure and calculation of its area and area ratio can be performed by EBSD (Electron Backscattering Diffraction), X-ray measurement, corrosion using Nital reagent or Lepera solution, and by observing a 100 μm × 100 μm region of the steel sheet cross section parallel to the rolling direction and perpendicular to the sheet surface at a magnification of 1,000 to 50,000 times using a scanning electron microscope. When measuring the area ratio of each structure, three measurement locations are used and the average value is calculated.

[0026] The area and area fraction of ferrite can be measured using the following method. Specifically, using an EBSD attached to a scanning electron microscope, measurements are taken at 0.2 μm intervals (pitch) over a range of 1 / 8 to 3 / 8 of the thickness, centered at the 1 / 4 position of the sheet thickness. The value of the grain average misorientation (GAM) is calculated from the measurement data. The area with an average local misorientation value of less than 0.5° is then defined as ferrite, and its area and area fraction are measured. Here, the average local misorientation is the misorientation between adjacent measurement points in a region surrounded by grain boundaries with a crystal misorientation of 5° or more, calculated, and averaged over all measurement points within the grain.

[0027] The area and area fraction of bainite are measured by taking a sample from a cross-section of the steel sheet parallel to the rolling direction, polishing the sample, etching it with nital, and observing the area from 1 / 8 to 3 / 8 of the thickness centered at 1 / 4 of the sheet thickness using a field emission scanning electron microscope (FE-SEM). The area fraction can be calculated using known image analysis software, such as "ImageJ." ImageJ is open-source, public domain image processing software widely used by those skilled in the art.

[0028] In FE-SEM observations, the structure of an observation surface, for example, a square with sides of 30 μm, is classified as follows: Bainite is a collection of lath-shaped crystal grains that either does not contain iron-based carbides with a major axis of 20 nm or more, or contains iron-based carbides with a major axis of 20 nm or more, and the carbides belong to a single variant, i.e., a group of iron-based carbides elongated in the same direction. Here, a group of iron-based carbides elongated in the same direction refers to iron-based carbides whose elongation directions differ by within 5°. Bainite grains surrounded by grain boundaries with a misorientation of 15° or more are counted as one bainite grain.

[0029] The area fraction of martensite and tempered martensite is etched with a repeller solution, and the area between 1 / 8 and 3 / 8 of the thickness, centered at 1 / 4 of the plate thickness, is observed and photographed using an FE-SEM. The area fraction of the retained austenite measured using X-rays (details will be described later) can be calculated from the area fraction of the uncorroded area.

[0030] The area fraction of retained austenite can be calculated from the integrated intensity ratio of the diffraction peaks of (200), (211) of the bcc phase and (200), (220), and (311) of the fcc phase using MoKα characteristic X-rays, using a sample in which a 100 μm region in the thickness direction from the surface has been removed by electrolytic polishing or chemical polishing.

[0031] The pearlite area ratio can be determined by corroding the steel plate with Nital reagent and observing the secondary electron images with a scanning electron microscope in the area from 1 / 8 to 3 / 8 of the thickness, centered at 1 / 4 of the thickness from the surface. Areas with bright contrast in the secondary electron images are considered pearlite, and the area ratio is calculated using the image analysis software "ImageJ" mentioned above.

[0032] (Maximum diameter of MnS: 30.0 μm or less) Among the MnS inclusions, coarse MnS becomes the starting point for microcracks in the cast slab after casting. Furthermore, these microcracks in the slab turn into cracks during hot rolling, which results in deterioration of the surface properties of the steel sheet (increase in surface roughness Ra). Furthermore, as a result of research by the present inventors, it was found that where MnS of a certain size or larger is present, the formation of surface irregularities due to cold forming (for example, application of 5% pre-strain) becomes significant. For these reasons, in this embodiment, the maximum diameter of MnS in the steel is set to 30.0 μm or less. Specifically, in this embodiment, in the rolling direction cross section of the steel sheet, the inspection reference area: 9.58 mm 2 ,Number of inspections: 40 fields of view,Prediction area: 383.39mm 2 The predicted value of the maximum diameter of MnS measured by the statistics of extremes under the above conditions is set to 30.0 μm or less. If coarse MnS particles are present in a steel sheet, as described above, when a part is produced by cold forming, wavy surface irregularities will occur on the surface of the formed part. If the maximum diameter of MnS predicted by the statistics of extremes under the above conditions exceeds 30.0 μm, the surface irregularities will be particularly noticeable. For these reasons, the maximum diameter of MnS predicted by the statistics of extremes under the above conditions is set to 30.0 μm or less, preferably 25.0 μm or less. Since the smaller the maximum diameter of MnS predicted by the statistics of extremes under the above conditions, the more preferable it is, the no particular lower limit is imposed on the maximum diameter of MnS. Substantially, the lower limit of the maximum diameter of MnS may be set to 1.0 μm or more.

[0033] In this embodiment, the method for measuring and predicting the maximum diameter of precipitates using extreme value statistics is the method described on pages 233 to 239 of "Metal Fatigue: The Effects of Minute Defects and Inclusions" published by Yokendo on March 8, 1993. In this embodiment, the method used is a method for measuring and predicting the maximum diameter of precipitates using extreme value statistics within a certain area (area for prediction: 383.39 mm) by two-dimensional inspection. 2 ) is a two-dimensional inspection method that estimates the maximum precipitate observed in the area. The area for the prediction may be set by taking into account the critical volume of a typical part.

[0034] Next, a method for identifying MnS in steel and a specific method for predicting the maximum diameter of MnS using the extreme value statistics method will be described.

[0035] (Method for identifying MnS) MnS can be evaluated by observing the structure of the cross section (full thickness) of the steel sheet. When the cross section of a steel sheet is mechanically polished to a mirror finish and the polished surface is observed using an optical microscope, MnS appears as a slight black contrast (gray) against the mirror-finished base steel. An alternative observation method to optical microscopy is to perform composition analysis using an energy dispersive X-ray detector (EDX: Energy Dispersive X-ray Spectrometry). Area analysis of the aforementioned inspection reference area is performed at 0.1 μm intervals to identify areas with high concentrations of both Mn and S, and these areas can be considered to be MnS.

[0036] (Prediction of maximum diameter of MnS using extreme value statistics) First, a test piece was taken from the steel plate, and the test area was 9.58 mm 2 (3.57mm x 2.68mm area. If the plate thickness (t) is less than 2.68mm, the area is t (mm) x 9.58 / t (mm)) for 40 fields of view. 2 The MnS with the largest area (maximum MnS) was detected and photographed at 400x magnification using an optical microscope. 2This is repeated 40 times for each field of view (i.e., 40 fields of view are inspected). The diameter of MnS in each inspection area is measured from the obtained photograph. Since most MnS are elliptical, when measuring the diameter of MnS, the geometric mean of the major and minor axes is calculated and this is used as the MnS diameter. The data for the 40 largest MnS diameters obtained are plotted on an extreme value probability sheet using the method described on pages 233-239 of "Metal Fatigue: The Effects of Minute Defects and Inclusions" (Yokendo), and the maximum MnS distribution line (a linear function of the maximum MnS diameter and the extreme value statistical normalization variable) is obtained. The maximum MnS distribution line is then extrapolated to obtain the prediction area: 383.39 mm 2 Predict the maximum diameter of MnS in

[0037] <Surface roughness Ra> The surface roughness Ra of the steel plate of this embodiment is 5.0 μm or less. As described above, coarse MnS particles become the starting points for microcracks in the cast slab after casting. Furthermore, these microcracks in the slab become cracks during hot rolling, resulting in an increase in the surface roughness Ra. An increase in the surface roughness Ra of the steel sheet not only impairs the appearance (design) of the steel sheet, but also serves as the starting point for cracks during bending deformation, which may lead to deterioration in bendability and fatigue properties. For these reasons, in this embodiment, the surface roughness Ra of the steel sheet is set to 5.0 μm or less. It is preferably 4.5 μm or less, and more preferably 4.0 μm or less. Since a smaller surface roughness Ra of the steel sheet is preferable, the lower limit of the surface roughness Ra is not particularly limited. The lower limit of the surface roughness may essentially be 0.5 μm or more. Specifically, the surface roughness Ra of the steel sheet in this embodiment is measured in accordance with JIS B 0601:2013. The surface roughness can be measured using a contact-type measuring device that presses a diamond stylus against the steel plate surface to measure the change in height of the steel plate surface. Alternatively, a non-contact-type measuring device can be used that measures the height of the steel plate surface using a laser. The measurement area must be at least 9 mm 2 More than 16mm 2 It is preferable that it is 25 mm or more. 2 More preferably, it is equal to or greater than this. Furthermore, when checking the surface roughness after 5% pre-strain as an indicator of the presence or absence of unevenness on the steel sheet surface after cold forming, the surface roughness Ra of the steel sheet after 5% pre-strain is measured by the above method. As long as the above-mentioned measurement area requirement is met, it is sufficient to measure one side of the steel sheet pre-strained by 5%±1% in order to evaluate the degree of unevenness on the steel sheet surface after cold forming. Alternatively, both sides of the steel sheet may be measured.

[0038] <Surface hardness> In this embodiment, the surface layer of the steel sheet is relatively hardened, and the Vickers hardness (surface layer hardness) of the steel sheet surface layer is set to a predetermined value or higher. That is, in this embodiment, the surface layer hardness of the steel sheet is set to (tensile strength of the steel sheet, TS) × 0.25 or higher. As a result, by not providing a soft layer on the steel sheet surface, strain applied during forming is uniformly distributed throughout the sheet thickness, and excessive strain concentration in the surface layer is suppressed, thereby suppressing the occurrence of surface irregularities during forming. To further demonstrate this effect, the surface layer hardness of the steel sheet is preferably set to TS × 0.28 or higher, and more preferably TS × 0.30 or higher. It is not common for the surface layer of a steel sheet to be significantly harder than the interior. Therefore, the practical upper limit of the surface layer hardness of the steel sheet is TS × 0.35.

[0039] The surface hardness of the steel sheet can be measured by the following procedure. First, the thickness cross section of the steel plate is mechanically polished to a mirror finish. On this polished surface, Vickers hardness (HV) is measured at 12 points on a line parallel to the rolling direction, at a distance (depth) of 50 μm from the plate surface toward the inside of the plate thickness, in accordance with JIS Z 2244-1 (2020), with an indentation load of 20 gf. The Vickers hardness values ​​measured at these 12 points are averaged to obtain the surface hardness of the steel plate, excluding the lowest and highest values. The distance between each measurement point is preferably at least four times the distance of the indentation. The distance at least four times the distance of the indentation referred to here refers to the distance obtained by multiplying the diagonal length of the indentation made by the diamond indenter when measuring Vickers hardness by a value at least four times the distance.

[0040] Next, the reasons for limiting the chemical composition of the steel sheet according to this embodiment will be explained. Hereinafter, % in the chemical composition means % by mass.

[0041] (C: 0.15% or more, 0.50% or less) C is an element that ensures sufficient martensite and tempered martensite and improves the strength of the steel sheet. If the C content is less than 0.15%, the area ratio of martensite and tempered martensite will be insufficient, making it difficult to ensure high tensile strength (for example, tensile strength of 1300 MPa or more). Therefore, the C content is set to 0.15% or more. The C content is preferably 0.20% or more, more preferably 0.25% or more. On the other hand, if the C content exceeds 0.50%, the volume change associated with martensitic transformation will be large, which may lead to the generation of irregularities on the surface of the steel sheet and significantly deteriorate formability. Therefore, the C content is set to 0.50% or less. The C content is preferably 0.40% or less, more preferably 0.35% or less.

[0042] (Si: 0.01% or more, 1.00% or less) Si acts as a solution strengthening element to increase strength. Si is also effective in obtaining a structure containing martensite and tempered martensite. For these reasons, the Si content is adjusted according to the target strength level. If the Si content exceeds 1.00%, the area ratio of retained austenite increases excessively, which may deteriorate the surface roughness after pre-straining and may also lead to deterioration in press formability and phosphatability. Therefore, the upper limit of the Si content is set to 1.00%. The Si content is preferably 0.95% or less, more preferably 0.90% or less. On the other hand, excessively reducing the Si content may increase manufacturing costs. Therefore, the Si content is set to 0.01% or more. The Si content is preferably 0.05% or more, more preferably 0.10% or more, and even more preferably 0.30% or more.

[0043] (Mn: 1.00% or more, 3.00% or less) Mn is an element that contributes to improving strength and also has the effect of suppressing ferrite transformation that occurs during heat treatment in continuous annealing equipment or continuous hot-dip galvanizing equipment. If the Mn content is less than 1.00%, these effects are not fully exerted, leading to ferrite transformation, and as a result, it becomes difficult to obtain high tensile strength (for example, tensile strength of 1300 MPa or more). Therefore, the Mn content is set to 1.00% or more. The Mn content is preferably 1.70% or more, and more preferably 1.90% or more. On the other hand, if the Mn content exceeds 3.00%, formability may deteriorate. Furthermore, excessive Mn content may cause coarsening of MnS. Therefore, the Mn content is set to 3.00% or less. The Mn content is preferably 2.70% or less, more preferably 2.65% or less, and even more preferably 2.00% or less.

[0044] (P: 0% or more, 0.0200% or less) P is an impurity element that segregates in the central portion of the steel sheet's thickness and impairs its toughness. Furthermore, P embrittles the welded portion when the steel sheet is welded. If the P content exceeds 0.0200%, the strength of the welded portion and the ductility of the hole expandability are significantly reduced. Furthermore, if the P content exceeds 0.0200%, the steel sheet becomes brittle, causing the surface layer of the steel sheet to peel off during cold rolling, which may result in deterioration of the surface quality (surface roughness Ra) of the steel sheet. Therefore, the P content is set to 0.0200% or less. The P content is preferably 0.0100% or less. Meanwhile, the lower the P content, the better, and there is no particular lower limit. The P content may be 0%. Meanwhile, reducing the P content to less than 0.0001% in practical steel sheets is economically undesirable because it significantly increases manufacturing costs. Therefore, the lower limit of the P content may be set to 0.0001% or more.

[0045] (S: 0.0001% or more, 0.0200% or less) S is an impurity element that impairs weldability and manufacturability during casting and hot rolling. Furthermore, S forms coarse MnS, impairing hole expandability, and also causes surface irregularities on the steel sheet surface during cold forming. If the S content exceeds 0.0200%, these effects become significant. Therefore, the S content is set to 0.0200% or less. The S content is preferably 0.0100% or less, and more preferably 0.0050% or less. On the other hand, reducing the S content to less than 0.0001% in practical steel sheets is economically undesirable because it significantly increases manufacturing costs. Therefore, the lower limit of the S content is set to 0.0001% or more.

[0046] (Al: 0.001% or more, 0.100% or less) Al is an element that acts as a deoxidizer for steel. If the Al content is less than 0.001%, this effect cannot be fully achieved, so the lower limit is set to 0.001% or more, preferably 0.005% or more. On the other hand, in this embodiment, the Al concentration in the molten steel during vacuum degassing is reduced to suppress the formation of coarse MnS. That is, in this embodiment, reducing the Al content promotes the formation of MnO and Ti2O3, thereby suppressing the formation of coarse MnS and preventing the occurrence of surface irregularities after cold forming. However, if the Al content in the steel sheet exceeds 0.100%, the formation of coarse MnS cannot be sufficiently suppressed. Furthermore, coarse Al oxides may be formed, which may cause a decrease in ductility. For this reason, the Al content is set to 0.100% or less. The Al content is preferably 0.080% or less, more preferably 0.060% or less. As will be described later, in the method for producing a steel sheet according to this embodiment, the Al concentration in the molten steel is adjusted to 0.0500% by mass or less. In other words, the Al concentration in the steel sheet product is also basically 0.0500% by mass or less. However, if it is desired to actively add Al, for example, a process of additionally adding Al in the refining process (between the completion of the vacuum degassing process and the pouring of the molten steel into the tundish) may be separately provided. However, even in such a case, the Al content in the steel obtained as the final product is set to 0.100% or less.

[0047] (N: 0% or more, 0.0200% or less) N is an element that forms coarse nitrides, impairs bendability and hole expandability, and causes blowholes during welding. If the N content exceeds 0.0200%, coarse nitrides are formed, resulting in a decrease in formability and toughness, and the occurrence of blowholes becomes significant. Furthermore, if the N content exceeds 0.0200%, the surface layer of the steel sheet becomes embrittled, causing the surface layer to peel off during cold rolling, which may result in deterioration of the surface properties (surface roughness Ra) of the steel sheet. Therefore, the N content is set to 0.0200% or less. It is preferably 0.0170% or less, and more preferably 0.0150% or less. The lower the N content, the better, and there is no particular lower limit. The N content may be 0%. On the other hand, reducing the N content to less than 0.0005% in practical steel sheets is economically undesirable because it significantly increases production costs. Therefore, the lower limit of the N content may be set to 0.0005% or more.

[0048] (Co: 0 to 0.500%) Co is an element effective in improving the strength of steel sheet. The Co content may be 0%, but to obtain the above effect, the Co content is preferably 0.001% or more, and more preferably 0.010% or more. On the other hand, if the Co content is too high, the ductility of the steel sheet may decrease, leading to a decrease in formability. Furthermore, if the Co content is too high, the surface layer of the steel sheet may become embrittled, causing the surface layer to peel off during cold rolling, which may result in a deterioration in the surface properties (surface roughness Ra) of the steel sheet. For this reason, the Co content is set to 0.500% or less, and preferably 0.300% or less.

[0049] (Ni: 0 to 1.000%) Like Co, Ni is an element effective in improving the strength of steel sheet. The Ni content may be 0%, but to obtain the above effect, the Ni content is preferably 0.001% or more, and more preferably 0.010% or more. On the other hand, if the Ni content is too high, the ductility of the steel sheet may decrease, leading to a decrease in formability. Furthermore, if the Ni content is too high, the surface layer of the steel sheet may become embrittled, causing the surface layer to peel off during cold rolling, which may result in a deterioration in the surface properties (surface roughness Ra) of the steel sheet. For this reason, the Ni content is set to 1.000% or less, and preferably 0.800% or less.

[0050] (Mo: 0 to 1.000%) Like Mn, Mo is an element that contributes to increasing the strength of steel sheets. This effect can be achieved even with a trace amount of Mn. The Mo content may be 0%, but to achieve the above effect, the Mo content is preferably 0.010% or more. On the other hand, if the Mo content exceeds 1.000%, coarse Mo carbides are formed, which may reduce the cold formability of the steel sheet. Furthermore, an excessive Mo content may lead to embrittlement of the surface layer of the steel sheet. If the surface layer of the steel sheet becomes embrittled, the surface layer of the steel sheet may peel off during cold rolling, which may result in deterioration of the surface properties (surface roughness Ra) of the steel sheet. For this reason, the Mo content is set to 1.000% or less, and preferably 0.800% or less.

[0051] (Cr:0~2.000%) Like Mn and Mo, Cr is an element that contributes to increasing the strength of steel sheets. This effect can be achieved even with a trace amount of Cr. Although the Cr content may be 0%, to achieve the above effect, the Cr content is preferably 0.001% or more, and more preferably 0.100% or more. On the other hand, if the Cr content exceeds 2.000%, Cr carbides are formed in the steel, and dissolution of the carbides into austenite is suppressed during annealing at a temperature range of 820 to 900°C after cold rolling (the so-called soaking step), which may lead to ferrite transformation and prevent high tensile strength (e.g., tensile strength of 1300 MPa or more). For this reason, the Cr content is set to 2.000% or less, and preferably 1.500% or less.

[0052] (O: 0% or more, 0.020% or less) O forms coarse oxides, which deteriorate formability and fracture resistance, and is an element that causes blowholes during welding. If the O content exceeds 0.020%, the coarse oxides cause deterioration in formability and ductility of punched edges, and the occurrence of blowholes becomes significant. Furthermore, the formation of coarse oxides leads to embrittlement of the steel sheet surface. If the steel sheet surface becomes embrittled, the surface layer of the steel sheet may peel off during cold rolling, resulting in deterioration of the surface properties (surface roughness Ra) of the steel sheet. Therefore, the O content is set to 0.020% or less. The lower the O content, the more preferable it is, and there is no particular lower limit. The O content may be 0%. On the other hand, reducing the O content to less than 0.0001% in practical steel sheets is economically undesirable because it significantly increases production costs. Therefore, the lower limit of the O content may be set to 0.0001% or more.

[0053] (Ti: 0 to 0.5000%) Ti is an element that may form coarse Ti oxides or TiN, thereby reducing the formability of the steel sheet. Furthermore, the formation of coarse Ti oxides leads to embrittlement of the surface layer of the steel sheet. Embrittlement of the surface layer of the steel sheet may cause the surface layer of the steel sheet to peel off during cold rolling, resulting in deterioration of the surface quality (surface roughness Ra) of the steel sheet. Therefore, from the viewpoint of ensuring the formability and good surface quality of the steel sheet, the lower the Ti content, the better, and the Ti content is set to 0.5000% or less. The Ti content may be 0%. However, reducing the Ti content to less than 0.0010% is economically undesirable because it results in an excessive increase in refining costs. Therefore, the lower limit of the Ti content may be set to 0.0010% or more.

[0054] (B: 0 to 0.0100%) B is an element that suppresses the formation of ferrite and pearlite during the cooling process from austenite and promotes the formation of martensite. B is also an element beneficial for increasing the strength of steel sheets. These effects can be achieved even with trace amounts of B. Although the B content may be 0%, to achieve the above effects, it is preferable that the B content be 0.0001% or more. However, if the B content is too high, coarse B oxides may be formed. These B oxides serve as the starting points for voids during press forming, and the formation of such voids may reduce the formability of the steel sheet. Furthermore, the formation of B oxides leads to embrittlement of the steel sheet surface. Embrittlement of the steel sheet surface layer may cause peeling of the steel sheet surface during cold rolling, resulting in deterioration of the surface properties (surface roughness Ra) of the steel sheet. For this reason, the B content is set to 0.0100% or less, preferably 0.0090% or less. Note that, when identifying B at less than 0.0001%, careful attention must be paid to analysis. If the B content is below the detection limit of the analytical instrument, the B content may be considered to be 0%.

[0055] (Nb: 0 to 0.500%) Nb is an element effective in controlling the morphology of carbides and is also effective in refining the structure and improving the toughness of steel sheets. This effect can be achieved even with a trace amount of Nb. The Nb content may be 0%, but to achieve the above effect, the Nb content is preferably 0.0001% or more, and more preferably 0.001% or more. However, if the Nb content is too high, a large number of hard Nb carbides will precipitate, which will significantly deteriorate the ductility of the steel sheet and may reduce the formability of the steel sheet. Furthermore, if the Nb content is too high, the surface layer of the steel sheet will become embrittled, causing the surface layer to peel off during cold rolling, which may result in deterioration of the surface properties (surface roughness Ra) of the steel sheet. For this reason, the Nb content is set to 0.500% or less, preferably 0.450% or less.

[0056] (V:0~0.500%) Like Nb, V is an element effective in controlling the morphology of carbides and is also effective in refining the structure to improve the toughness of steel sheets. The V content may be 0%, but to achieve the above effects, the V content is preferably 0.001% or more. However, if the V content is too high, a large number of V carbides will precipitate, which will significantly deteriorate the ductility of the steel sheet and may reduce the formability of the steel sheet. Furthermore, if the V content is too high, the surface layer of the steel sheet will become embrittled, causing the surface layer to peel off during cold rolling, which may result in deterioration of the surface properties (surface roughness Ra) of the steel sheet. Therefore, the V content is set to 0.500% or less, preferably 0.450% or less.

[0057] (Cu: 0 to 0.500%) Cu is an element that contributes to improving the strength of steel sheets. This effect can be achieved even with a trace amount of Cu. The Cu content may be 0%, but to achieve the above effect, the Cu content is preferably 0.001% or more. However, if the Cu content is too high, red shortness may occur, which may reduce productivity in hot rolling. Furthermore, if the Cu content is too high, the surface layer of the steel sheet may become embrittled, causing the surface layer to peel off during cold rolling, which may result in deterioration of the surface properties (surface roughness Ra) of the steel sheet. For this reason, the Cu content is set to 0.500% or less, preferably 0.450% or less.

[0058] (W:0~0.1000%) Like Nb and V, W is also an element that is effective in controlling the morphology of carbides and improving the strength of steel sheets. The W content may be 0%, but to obtain the above effects, the W content is preferably 0.0010% or more. On the other hand, if the W content is too high, a large amount of W carbides will precipitate, which may reduce the ductility of the steel sheet and reduce the cold workability of the steel sheet. Furthermore, if the W content is too high, the surface layer of the steel sheet will become embrittled, causing the surface layer to peel off during cold rolling, which may result in deterioration of the surface properties (surface roughness Ra) of the steel sheet. For this reason, the W content is set to 0.1000% or less, and preferably 0.0900% or less.

[0059] (Ta: 0 to 0.1000%) Ta, like Nb, V, and W, is an element effective in controlling the morphology of carbides and improving the strength of steel sheets. While the Ta content may be 0%, to obtain the above effects, it is preferable that the Ta content be 0.0010% or more. On the other hand, if the Ta content is too high, a large number of Ta carbides precipitate, which may reduce the ductility of the steel sheet and reduce the cold workability of the steel sheet. Furthermore, if the Ta content is too high, the surface layer of the steel sheet may become embrittled, causing the surface layer to peel off during cold rolling, which may result in deterioration of the surface properties (surface roughness Ra) of the steel sheet. Therefore, the Ta content is set to 0.1000% or less, preferably 0.0200% or less, and more preferably 0.0100% or less.

[0060] (Sn: 0 to 0.0500%) Sn is an element that can be contained in steel sheets when scrap is used as the raw material for the steel sheets. Sn may also cause a decrease in the cold formability of the steel sheets due to the embrittlement of ferrite. If the Sn content is too high, the surface layer of the steel sheet may become embrittled, causing the surface layer to peel off during cold rolling, which may result in a deterioration in the surface properties (surface roughness Ra) of the steel sheet. Therefore, the lower the Sn content, the better. The Sn content is set to 0.0500% or less, and preferably 0.0400% or less. The Sn content may be 0%. However, reducing the Sn content to less than 0.0010% is not preferable because it results in an excessive increase in refining costs. Therefore, the Sn content may be set to 0.0010% or more.

[0061] (Sb: 0 to 0.0500%) Like Sn, Sb is an element that can be contained in steel sheets when scrap is used as the raw material for the steel sheet. Sb strongly segregates at grain boundaries, which may lead to embrittlement of the grain boundaries, reduced ductility, and even reduced cold formability. Furthermore, if the Sb content is too high, the surface layer of the steel sheet may become embrittled, causing the surface layer to peel off during cold rolling, which may result in deterioration of the surface properties (surface roughness Ra) of the steel sheet. Therefore, the lower the Sb content, the better. The Sb content is set to 0.0500% or less, and preferably 0.0400% or less. The Sb content may be 0%. However, reducing the Sb content to less than 0.0010% is not preferable because it results in an excessive increase in refining costs. Therefore, the Sb content may be set to 0.0010% or more.

[0062] (As: 0 to 0.0500%) Like Sn and Sb, As is an element that can be contained in steel sheets when scrap is used as the raw material for the steel sheet. As strongly segregates at grain boundaries, which may result in reduced ductility and reduced cold formability. Furthermore, if the As content is too high, the surface layer of the steel sheet may become embrittled, causing the surface layer to peel off during cold rolling, which may result in a deterioration in the surface properties (surface roughness Ra) of the steel sheet. Therefore, the lower the As content, the better. The As content is set to 0.0500% or less, and preferably 0.0400% or less. The As content may be 0%. However, reducing the As content to less than 0.0010% is not preferable because it results in an excessive increase in refining costs. Therefore, the As content may be set to 0.0010% or more.

[0063] (Mg: 0 to 0.0500%) Mg controls the morphology of sulfides and oxides, contributing to improving the bendability of steel sheets. This effect can be achieved even with a trace amount of Mg. While the Mg content may be 0%, to achieve the above effect, a Mg content of 0.0001% or more is preferred. However, if the Mg content is too high, there is a risk of deterioration in cold formability due to the formation of coarse inclusions and Mg oxides. Furthermore, the formation of Mg oxides leads to embrittlement of the steel sheet surface. If the steel sheet surface becomes embrittled, the surface layer may peel off during cold rolling, which may result in deterioration of the surface properties (surface roughness Ra) of the steel sheet. For this reason, the Mg content is set to 0.0500% or less, and preferably 0.0400% or less.

[0064] (Ca: 0 to 0.0500%) Like Mg, Ca is an element that can control the morphology of sulfides with a small amount. The Ca content may be 0%, but to obtain the above-mentioned effects, the Ca content is preferably 0.0010% or more. However, if the Ca content is too high, coarse Ca oxides may be formed, which can become the starting point for cracks during cold forming. Furthermore, the formation of Ca oxides leads to embrittlement of the steel sheet surface. Embrittlement of the steel sheet surface layer can cause the steel sheet surface to peel off during cold rolling, which can result in deterioration of the surface properties (surface roughness Ra) of the steel sheet. Therefore, the Ca content is set to 0.0500% or less, and preferably 0.0300% or less.

[0065] (Zr: 0 to 0.0500%) Like Mg and Ca, Zr is an element that can control the morphology of sulfides with a small amount. The Zr content may be 0%, but to obtain the above-mentioned effects, the Zr content is preferably 0.0010% or more. However, if the Zr content is too high, coarse Zr oxides may be formed, which may reduce cold formability. Furthermore, the formation of Zr oxides may lead to embrittlement of the steel sheet surface. Embrittlement of the steel sheet surface layer may cause the steel sheet surface to peel off during cold rolling, resulting in deterioration of the surface properties (surface roughness Ra) of the steel sheet. Therefore, the Zr content is set to 0.0500% or less, and preferably 0.0400% or less.

[0066] (REM: 0 to 0.1000%) REM stands for rare earth metal. Even a trace amount of REM effectively controls the morphology of sulfides. The REM content may be 0%, but to achieve the above-mentioned effects, the REM content is preferably 0.0010% or more. However, if the REM content is too high, coarse REM oxides may be formed, which may reduce workability and fracture resistance. Furthermore, the formation of REM oxides may lead to embrittlement of the steel sheet surface. If the steel sheet surface becomes embrittled, the surface layer may peel off during cold rolling, which may result in deterioration of the surface properties (surface roughness Ra) of the steel sheet. For this reason, the REM content is set to 0.1000% or less, and preferably 0.0500% or less. Here, REM refers to two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu). In this embodiment, "REM" refers to one or more elements selected from these rare earth elements, and the "REM content" refers to the total amount of rare earth elements.

[0067] In the chemical composition of the steel sheet according to this embodiment, the balance excluding the above elements is Fe and impurities. Impurities are elements that are mixed in during industrial steel production due to raw materials such as ore and scrap, and various factors in the manufacturing process, and are elements whose presence is permitted to the extent that they do not impair the properties of the steel sheet according to this embodiment. Impurities also include elements that are not intentionally added to the steel sheet.

[0068] The thickness of the steel plate according to this embodiment is not limited to a particular range, but is preferably 0.3 to 6.0 mm in consideration of strength, versatility, and manufacturability.

[0069] Next, a method for manufacturing a steel sheet according to this embodiment will be described. The method for manufacturing a steel sheet according to this embodiment includes the following steps: refining, casting, hot rolling, coiling, pickling, cold rolling, and annealing (continuous annealing). The manufacturing conditions for each step may be determined appropriately within a range that does not impair the effects of the present invention. However, from the viewpoint of suppressing the formation of coarse MnS and controlling the hardness of the surface layer, it is particularly important to appropriately control the conditions for each of the refining and continuous annealing steps. Each step and condition of the production method will be described in detail below.

[0070] (a) subjecting molten steel to a vacuum degassing treatment to adjust the Al concentration of the molten steel to 0.0500 mass% or less, and adjusting the chemical composition of the molten steel to the chemical composition described above (excluding Al) (refining step); (b) Slabs are produced using the molten steel after the refining process (casting process), (c) The slab is heated directly or after being cooled once, and then hot-rolled to obtain a hot-rolled steel sheet (hot rolling process); (d) The hot-rolled steel sheet is coiled at a temperature of 700°C or less (coiling process); (e) The hot-rolled steel sheet after the coiling process is pickled (pickling process), (f) The hot-rolled steel sheet after the pickling process is cold-rolled at a rolling reduction of 30 to 90% to obtain a cold-rolled steel sheet (cold rolling process); (g) The cold-rolled steel sheet is annealed in an atmosphere with a dew point of −80° C. or higher and −15° C. or lower in a temperature range of 820° C. to 900° C. (annealing step).

[0071] In the annealing step described in (g) above, a coating layer formation step may be carried out to form a coating layer containing at least one of zinc, aluminum, magnesium, and alloys thereof on one or both sides (front and / or back sides) of the cold-rolled steel sheet.

[0072] The steel sheet according to this embodiment suppresses the deterioration of the surface quality (surface roughness Ra) of the steel sheet and reduces the generation of coarse MnS in order to suppress the appearance of surface irregularities after cold forming. The coarse MnS becomes the starting point for microcracks in the cast slab after casting, and these microcracks in the slab cause cracks during hot rolling. Furthermore, the appearance of surface irregularities after cold forming becomes more pronounced in areas where this coarse MnS is present. Therefore, in this embodiment, the composition of the molten steel is adjusted in the refining process before the casting process so as to suppress the generation of coarse MnS. Specifically, as described below, the Al concentration of the molten steel is controlled to be below a certain level in the refining process.

[0073] (a) Refining process In the refining process, molten pig iron produced by a known method is first refined in a converter (primary refining). The molten steel tapped from the converter is then subjected to secondary refining, i.e., vacuum degassing treatment using a vacuum degassing apparatus (e.g., RH). In this embodiment, during this vacuum degassing treatment, the Al concentration in the molten steel is adjusted to 0.0500 mass% or less, and the components other than Al are adjusted to the composition described above. Specifically, it is necessary to monitor the Al concentration in the molten steel during the vacuum degassing treatment and promote the formation and fine dispersion of MnS using MnO or TiO as nuclei. However, when the Al concentration in the molten steel is high, the amount of dissolved oxygen in the molten steel is low, suppressing the formation of oxides such as MnO and TiO. As a result, it becomes difficult to promote the formation and dispersion of fine MnS. Furthermore, when the Al concentration in the molten steel is high, AlO oxides are formed, but MnS is not easily formed using AlO as nuclei. That is, when the Al concentration in the molten steel is high, the number of nucleation sites for MnS decreases, and as a result, coarse MnS particles exceeding 30 μm may be generated in the steel sheet obtained after solidification into a slab and further through the hot rolling and cold rolling processes. For these reasons, in this embodiment, the Al concentration in the molten steel is adjusted to 0.0500 mass% or less in this vacuum degassing treatment, preferably 0.0400 mass% or less, and more preferably 0.0350 mass% or less.

[0074] Furthermore, in this embodiment, the deoxidation time in the vacuum degassing treatment is preferably less than 5 minutes. If the deoxidation time is too long, the amount of dissolved oxygen in the molten steel decreases, reducing the amount of MnO and Ti2O3 produced, which serve as nuclei for MnS, and as a result, there is a risk of the MnS becoming coarse. Therefore, the deoxidation time is preferably less than 5 minutes. More preferably, the deoxidation time is 4 minutes or less. Note that the "deoxidation time" referred to here refers to the time required from the start of deoxidation, i.e., after adding Al as a deoxidizer, to the completion of secondary refining.

[0075] In addition, in this embodiment, when adjusting the components in the vacuum degassing treatment so that the components other than Al have the composition described above, it is preferable to set the time required from the completion of secondary refining to the start of the casting process to less than 3 minutes in order to prevent a decrease in number density due to aggregation and coarsening of MnO and TiO.

[0076] (b) Casting process Next, a slab is produced using the molten steel whose Al concentration has been adjusted by the refining process (casting process). Specifically, the molten steel can be used to produce a slab by, for example, a continuous casting method.

[0077] (c) Hot rolling process Next, the produced slab is heated directly or after being cooled once, and then hot-rolled to obtain a hot-rolled steel sheet (hot rolling process). In this embodiment, the conditions of the hot rolling process are not particularly limited, but from the viewpoint of ensuring the shape of the product sheet, the finishing temperature may be 800 to 1000°C, and the rolling reduction in the final stage of the finishing stand may be 10 to 80%.

[0078] (d) Winding process Next, the hot-rolled steel sheet (hot-rolled steel sheet) is coiled in a temperature range of 700°C or less. If the coiling temperature exceeds 700°C, a relatively thick oxide film (oxide scale) is formed on the surface of the hot-rolled steel sheet, and this oxide is generated in a wedge shape at the grain boundaries in the steel. If this oxide scale is subsequently removed in a pickling process, the steel sheet surface may have properties similar to those of microcracks, and many irregularities may appear on the surface of the steel sheet after the annealing process, which may deteriorate the surface properties (surface roughness Ra) of the final steel sheet. Therefore, the coiling temperature is set to 700°C or less, preferably 680°C or less. On the other hand, by controlling the coiling temperature within a range that is not excessively low, excessive strength of the hot-rolled sheet is prevented, an increase in the cold-rolling load can be suppressed, and productivity can be improved. Therefore, the coiling temperature is preferably 500°C or more.

[0079] (e) Pickling process The hot-rolled steel sheet after the coiling step is pickled (pickling step). There are no particular restrictions on the conditions for the pickling step. For example, pickling may be carried out once, or may be carried out multiple times as necessary.

[0080] (f) Cold rolling process The hot-rolled steel sheet after the pickling process is subjected to cold rolling at a reduction of 30 to 90% to produce a cold-rolled steel sheet (cold rolling process). If the reduction is less than 30%, the surface roughness may decrease and the sheet shape may deteriorate. On the other hand, if the reduction in the cold rolling process exceeds 90%, the cold rolling load becomes excessive, resulting in a decrease in productivity. Therefore, the reduction in the cold rolling process is set to 30% or more and 90% or less. Preferably, it is 40% or more and 80% or less. There are no restrictions on the cold rolling method, and the number of rolling passes and the reduction per pass may be set appropriately.

[0081] (g) Annealing process (continuous annealing process) The cold-rolled steel sheet is annealed in an atmosphere with a dew point of -80°C or higher and -15°C or lower in a temperature range of 820 to 900°C (continuous annealing). The dew point inside the furnace during continuous annealing is -80°C or higher and -15°C or lower. If the dew point is lower than -80°C, advanced control of the atmosphere inside the furnace is required, which reduces manufacturability and increases costs. On the other hand, if the dew point is higher than -15°C, the surface layer of the steel sheet softens, resulting in a decrease in tensile strength. A preferred dew point is -70°C or higher and -20°C or lower.

[0082] The heating temperature (holding temperature) in the annealing process affects the area ratio of the metal structure. If the heating temperature is less than 820°C, the amount of austenite during heating is small, and the total area ratio of ferrite, bainite, and pearlite after annealing is high, making it difficult to achieve high tensile strength (for example, tensile strength of 1300 MPa or more). On the other hand, if the heating temperature is more than 900°C, shape changes due to depressions that occur at grain boundaries called thermal grooves progress while the material is held at high temperatures, and the surface properties (surface roughness Ra) deteriorate. Therefore, the heating temperature in continuous annealing is set to 820°C or higher and 900°C or lower. Preferably, it is 830°C or higher and 880°C or lower.

[0083] The holding time (dwell time) during continuous annealing is not particularly limited, but from the viewpoint of ensuring a sufficient area ratio of martensite and tempered martensite after annealing and improving strength, the holding time is preferably 10 seconds or more, and more preferably 100 seconds or more.

[0084] In the annealing step of this embodiment, a coating layer formation step may be performed in which a coating layer (e.g., a plating layer, an alloy plating layer) containing at least one of zinc, aluminum, magnesium, and alloys thereof is formed on one or both sides (front and / or back sides) of the cold-rolled steel sheet. Furthermore, a coating layer may be formed by a method such as electroplating after the annealing step.

[0085] (Cooling rate after annealing process) In the cooling after the annealing step, it is preferable to cool from 750°C to 550°C at an average cooling rate of 100°C / s or less. The lower limit of the average cooling rate is not particularly limited, but may be, for example, 2.5°C / s. The reason for setting the lower limit of the average cooling rate at 2.5°C / s is to prevent austenite from transforming into ferrite and softening the base steel sheet. If the average cooling rate is not too slow, a decrease in strength can be suppressed. The average cooling rate is more preferably 5°C / s or more, even more preferably 10°C / s or more, and even more preferably 20°C / s or more. Note that at temperatures above 750°C, ferrite transformation is significantly less likely to occur, so the cooling rate is not limited. Furthermore, at temperatures below 550°C, a low-temperature transformed structure is obtained, so the cooling rate is not limited. The average cooling rate from 750°C to 550°C is preferably 100°C / s or less, more preferably 50°C / s or less, and even more preferably 20°C / s or less. In this embodiment, as described above, the temperature range in which it is better to control the cooling rate after the annealing step is at least the range from 750°C to 550°C, but cooling may also be performed at an average cooling rate of 100°C / s or less outside this temperature range.

[0086] (Cooling stop temperature and reheating after annealing process) Alternatively, after the above cooling, the steel may be further cooled to a temperature of 25°C or higher but lower than 550°C, and then reheated to a temperature range of 150°C to 550°C and retained there. Cooling to the above temperature range (cooling stop temperature) produces martensite from untransformed austenite during cooling. Subsequent reheating tempers the martensite, improving the strength-ductility balance of the steel sheet. The reason for setting the lower limit of the cooling stop temperature at 25°C is that excessive cooling not only requires significant capital investment but also saturates its effectiveness.

[0087] (residence temperature) As described above, after cooling is stopped, the steel sheet may be reheated to 150°C to 550°C and then allowed to dwell within that temperature range. This reheating temperature (dwelling temperature) may further be set to 350°C to 550°C. Dwelling within this temperature range contributes to tempering of martensite. Note that when the cooling stop temperature is 150°C to 550°C, the steel sheet may be allowed to dwell as is without reheating.

[0088] (residence time) The retention time in the temperature range of 150 to 550°C may be 30 seconds or more and 500 seconds or less in order to obtain this effect, and is preferably 30 seconds or more and 300 seconds or less.

[0089] (tempering) In the series of annealing steps, the steel sheet may be retained at the retention temperature and further cooled to room temperature, or may be reheated during cooling to room temperature (but below Ms) and held at a temperature range of 150°C to 400°C for 2 seconds or more (tempering step). This tempering step tempers martensite formed during cooling after reheating to form tempered martensite, thereby further improving the strength-ductility balance. When performing the tempering step, a holding temperature of 150°C or higher and a holding time of 2 seconds or longer allows the martensite to be sufficiently tempered, resulting in changes in the microstructure and mechanical properties. On the other hand, a holding temperature of 400°C or lower suppresses a decrease in dislocation density in the tempered martensite, thereby increasing tensile strength. Therefore, when tempering is performed, it is preferable to hold the steel sheet at a temperature range of 150°C to 400°C for 2 seconds or more. Tempering may be performed in a continuous annealing facility or, alternatively, offline after continuous annealing in a separate facility. In this case, the tempering time varies depending on the tempering temperature. That is, the lower the temperature, the longer the tempering time, and the higher the temperature, the shorter the tempering time.

[0090] (plating) The steel sheet may be subjected to hot-dip galvanization, if necessary. In this case, the steel sheet may be heated or cooled to (galvanization bath temperature -40)°C to (galvanization bath temperature +50)°C before or after the dwelling (i.e., reheating) step, and then hot-dip galvanization may be performed. A hot-dip galvanized layer is formed on the surface of the steel sheet by the hot-dip galvanization step. This is preferable because it improves the corrosion resistance of the cold-rolled steel sheet. In this embodiment, the type of coating layer is not limited to a hot-dip galvanized layer, and various coating layers can be used. Furthermore, the timing of coating the surface of the steel sheet is not particularly limited. For example, in the manufacturing method according to this embodiment, coating layers made of zinc, aluminum, magnesium, or an alloy thereof may be formed on the front and back surfaces of the sheet during annealing, after holding in the austenite single-phase region and cooling to room temperature. Alternatively, such coating layers may be formed on the front and back surfaces of the sheet after annealing.

[0091] (Steel sheet temperature when immersed in plating bath) The temperature of the steel sheet when immersed in the hot-dip galvanizing bath is preferably in the range from 40°C lower than the hot-dip galvanizing bath temperature (hot-dip galvanizing bath temperature -40°C) to 50°C higher than the hot-dip galvanizing bath temperature (hot-dip galvanizing bath temperature +50°C). By keeping this temperature at or above the hot-dip galvanizing bath temperature -40°C, excessive heat removal during immersion in the galvanizing bath is prevented, which inhibits partial solidification of the molten zinc and reduces deterioration of the coating appearance. If the sheet temperature before immersion is below the hot-dip galvanizing bath temperature -40°C, the sheet may be further heated by any method before immersion in the galvanizing bath to control the sheet temperature to at least the hot-dip galvanizing bath temperature -40°C before immersion in the galvanizing bath. Furthermore, by keeping the steel sheet temperature during immersion in the galvanizing bath at or below the hot-dip galvanizing bath temperature +50°C, operational problems associated with elevated bath temperatures can be reduced.

[0092] (Plating bath composition) The coating bath preferably contains Zn as a main component, with an effective Al content (the total Al content minus the total Fe content in the coating bath) of 0.050 to 0.250 mass%. When the effective Al content in the coating bath is 0.050 mass% or more, the penetration of Fe into the coating layer is suppressed, improving coating adhesion. On the other hand, when the effective Al content in the coating bath is 0.250 mass% or less, the formation of Al-based oxides that inhibit the migration of Fe and Zn atoms at the boundary between the steel sheet and the coating layer is suppressed, improving coating adhesion. The effective Al content in the coating bath is more preferably 0.065 mass% or more, and more preferably 0.180 mass% or less.

[0093] (Steel sheet temperature after immersion in plating bath) When alloying treatment is performed on the hot-dip galvanized layer, the steel sheet on which the hot-dip galvanized layer has been formed is preferably heated to a temperature range of 450 to 600°C (alloying temperature). An alloying temperature of 450°C or higher allows for sufficient alloying. On the other hand, an alloying temperature of 600°C or lower prevents excessive alloying, suppresses the formation of the Γ phase, and prevents the Fe concentration in the coating layer from increasing (for example, exceeding 15%), thereby improving corrosion resistance. The alloying temperature is more preferably 470°C or higher, and more preferably 550°C or lower. The alloying temperature needs to be changed depending on the component composition of the steel sheet and the degree of formation of an internal oxide layer, so it can be set while checking the Fe concentration in the coating layer.

[0094] (Pre-processing) In order to further improve the coating adhesion, the base steel sheet may be coated with a coating of one or more of Ni, Cu, Co and Fe before annealing in a continuous hot-dip galvanizing line.

[0095] (Post-processing) For the purpose of improving paintability and weldability, the surface of the hot-dip galvanized steel sheet and the galvannealed steel sheet may be coated with an upper plating layer or subjected to various treatments such as chromate treatment, phosphate treatment, lubricity improvement treatment, weldability improvement treatment, etc.

[0096] (skin pass rolling) Furthermore, skin-pass rolling may be performed for the purpose of correcting the steel sheet shape or introducing mobile dislocations to improve ductility. The reduction ratio of the skin-pass rolling after heat treatment is preferably in the range of 0.1 to 1.5%. A reduction ratio of 0.1% or more provides sufficient effect and is easy to control. When the thickness is 1.5% or less, productivity can be improved. Skin pass rolling may be carried out inline or off-line. [Example]

[0097] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples in any way.

[0098] Example 1 Various slabs having the chemical compositions listed in Tables 1A to 1F were used as materials. Hot-rolling was performed at a finishing temperature of 910°C and a coiling temperature of 550°C to obtain hot-rolled steel sheets. This was followed by a cold-rolling process at a cold rolling reduction of 45% to obtain cold-rolled steel sheets. The cold-rolled steel sheets were then subjected to continuous annealing under the following conditions: dew point: -35°C, holding temperature: 860°C, holding time: 130 seconds, average cooling rate: 35°C / second, and cooling stop temperature: 180°C. The steel sheets after the continuous annealing process were then reheated to 260°C and held for 140 seconds to obtain steel sheets. Furthermore, the resulting steel sheets were subjected to skin-pass rolling at a rolling reduction of 0.2% to produce final steel sheets (sheet thickness: 1.4 mm). Pickling was performed between the coiling and cold-rolling processes. The deoxidation time in the refining process was 4 minutes, and the time required from the completion of secondary refining to the start of casting was 2 minutes. In Tables 1A to 1F, "-" means that the content of the corresponding element is 0% in significant figures (numbers down to the least significant digit) as defined in this embodiment. The units of the components of each slab are mass %, with the remainder being iron and impurities. In Tables 1A to 1F and Tables 2A to 2C, values ​​outside the range of the present invention are underlined. The column "Al in molten steel" in Tables 1A to 1F indicates the Al concentration (mass %) of the molten steel in the refining process.

[0099] The metal structure (ferrite, pearlite, bainite, retained austenite (residual γ), martensite (fresh martensite), tempered martensite), maximum diameter (μm) of MnS, surface roughness Ra, and Vickers hardness (HV) of the surface layer were evaluated in the range of 1 / 8 to 3 / 8 thickness (1 / 4 thickness part) from the surface of these steel plates, centered at the 1 / 4 position of the plate thickness. The evaluation results are shown in Tables 2A to 2C. These evaluations were performed according to the methods described above.

[0100] Furthermore, the yield stress (YS), tensile strength (TS), elongation (t-El) and hole expansion ratio (λ) of these steel sheets were evaluated and are shown in Table 2B. These evaluation methods are as follows.

[0101] The yield stress (YS) and tensile strength (TS) of steel sheets were evaluated by taking JIS No. 5 test pieces from the steel sheets with the longitudinal direction perpendicular to the rolling direction of the steel sheets and conducting tensile tests in accordance with JIS Z 2241:2011. Steel sheets with a tensile strength (TS) of 1300 MPa or more were judged to have passed the test. Steel sheets with a tensile strength (TS) of 1470 MPa or more were judged to have superior tensile strength.

[0102] The elongation (t-El) of the steel sheet was also evaluated by taking a JIS No. 5 test piece from the steel sheet so that the longitudinal direction was perpendicular to the rolling direction of the steel sheet, and conducting a tensile test in accordance with JIS Z 2241: 2011. From the viewpoint of ensuring formability, the elongation (t-El) is preferably 5.5% or more.

[0103] The hole expansion ratio (λ) was measured in accordance with JIS Z 2256: 2010 using a No. 5 test piece of JIS Z 2241: 2011. The hole expansion test piece was taken from a quarter portion from the end of the steel plate in the plate width direction.

[0104] The presence or absence of unevenness on the steel sheet surface after cold forming (referred to in the table as "surface unevenness after 5% pre-strain") was evaluated as follows. The surface of the steel sheet that had been pre-strained by 5%±1% was measured for surface roughness Ra by the method described above in accordance with JIS B 0601:2013. Steel sheets with a surface roughness Ra of 5.0 μm or less were rated as passing (OK), and steel sheets with a surface roughness Ra of more than 5.0 μm were rated as failing (NG).

[0105] [Table 1A]

[0106] [Table 1B]

[0107] [Table 1C]

[0108] [Table 1D]

[0109] [Table 1E]

[0110] [Table 1F]

[0111] [Table 2A]

[0112] [Table 2B]

[0113] [Table 2C]

[0114] In the examples of the invention that satisfy both the component composition and manufacturing conditions, the structural ratio of the metal structure, the structural characteristics and properties all fall within the scope of the invention, and it was found that it is possible to obtain steel sheets that can achieve both high levels of surface unevenness during forming (surface unevenness after 5% pre-strain) and high strength. On the other hand, in the comparative examples whose composition did not satisfy the scope of the invention, at least one of the structural ratio and structural characteristics was outside the scope of the invention, resulting in a deterioration of one of the properties.In addition, in the comparative example No. AF-1, the C content was too high, so the volume of the steel sheet rapidly expanded during the martensitic transformation associated with the rapid cooling treatment in the annealing process, and large irregularities occurred on the surface due to the relaxation of transformation plasticity.

[0115] Example 2 Next, various steel sheets (thickness: 1.4 mm) were manufactured using various slabs having the chemical compositions shown in Tables 1A to 1F under various manufacturing conditions shown in Tables 3A and 3B. Pickling was performed between the coiling process and the cold rolling process. In Tables 3A to 3D, values ​​outside the scope of the invention and values ​​that did not meet the pass / fail criteria are underlined. These steel sheets were evaluated in the same manner as in Example 1 above.

[0116] [Table 3A]

[0117] [Table 3B]

[0118] [Table 3C]

[0119] [Table 3D]

[0120] In the examples of the invention that satisfy both the component composition and manufacturing conditions, the structural ratio of the metal structure, the structural characteristics and properties all fall within the scope of the invention, and it was found that it is possible to obtain steel sheets that can achieve both high levels of surface unevenness during forming (surface unevenness after 5% pre-strain) and high strength. On the other hand, in the comparative examples that did not satisfy the scope of the invention in either the component composition or the manufacturing conditions, at least one of the texture ratio and texture characteristics was outside the scope of the invention, resulting in a deterioration of one of the properties. [Industrial Applicability]

[0121] According to the present invention, it is possible to suppress the occurrence of surface irregularities during forming and to obtain a high-strength steel sheet.

Claims

1. The component composition is, in mass%, C: 0.15-0.50%, Si: 0.01-1.00%, Mn: 1.00-3.00%, P: 0-0.0200%, S: 0.0001-0.0200%, Al: 0.001-0.100%, N: 0 to 0.0200%, Co: 0 to 0.500%, Ni: 0-1.000%, Mo: 0-1.000%, Cr: 0-2.000%, O: 0 to 0.020%, Ti: 0 to 0.5000%, B: 0 to 0.0100%, Nb: 0 to 0.500%, V: 0 to 0.500%, Cu: 0-0.500%, W: 0-0.1000%, Ta: 0-0.1000%, Sn: 0-0.0500%, Sb: 0 to 0.0500%, As: 0 to 0.0500%, Mg: 0 to 0.0500%, Ca: 0-0.0500%, Zr: 0 to 0.0500%, and REM: 0~0.1000% and the balance being Fe and impurities, In the metal structure, the area ratio is The retained austenite is 0% or more and 10.0% or less. The total content of pearlite, ferrite, and bainite is 0% or more and 5.0% or less, and the remainder structure is martensite and tempered martensite, The maximum diameter of MnS according to extreme value statistics is 30.0 μm or less; The surface roughness Ra is 5.0 μm or less, A steel plate characterized in that the Vickers hardness of the surface layer is equal to or greater than the tensile strength TS (MPa) of the steel plate × 0.

25.

2. The component composition is, in mass%, Co: 0.010-0.500%, Ni: 0.010 to 1.000%, Mo: 0.010-1.000%, Cr: 0.001-2.000%, O: 0.0001-0.020%, Ti: 0.0010 to 0.5000%, B: 0.0001 to 0.0100%, Nb: 0.001-0.500%, V: 0.001-0.500%, Cu: 0.001 to 0.500%, W: 0.0010-0.1000%, Ta: 0.0010-0.1000%, Sn: 0.0010-0.0500%, Sb: 0.0010 to 0.0500%, As: 0.0010 to 0.0500%, Mg: 0.0001-0.0500%, Ca: 0.0010-0.0500%, Zr: 0.0010 to 0.0500%, and REM: 0.0010-0.1000% The steel sheet according to claim 1, characterized in that it contains one or more of the following:

3. The component composition is, in mass%, Mn: 1.00-2.00% Si: 0.30~1.00% The steel sheet according to claim 1 or 2, characterized in that:

4. The steel plate according to any one of claims 1 to 3, characterized in that it has a tensile strength of 1470 MPa or more.

5. The steel sheet according to any one of claims 1 to 4, characterized in that one or both sides of the steel sheet have a coating layer containing at least one of zinc, aluminum, magnesium and alloys thereof.

6. The method for producing a steel sheet according to any one of claims 1 to 5, The molten steel is subjected to a vacuum degassing treatment, and the composition of the molten steel is adjusted to have an Al concentration of 0.0500 mass% or less, and to contain, in mass%, C: 0.15-0.50%, Si: 0.01-1.00%, Mn: 1.00-3.00%, P: 0-0.0200%, S: 0.0001-0.0200%, N: 0 to 0.0200%, Co: 0 to 0.500%, Ni: 0-1.000%, Mo: 0-1.000%, Cr: 0-2.000%, O: 0 to 0.020%, Ti: 0 to 0.5000%, B: 0 to 0.0100%, Nb: 0 to 0.500%, V: 0 to 0.500%, Cu: 0-0.500%, W: 0-0.1000%, Ta: 0-0.1000%, Sn: 0-0.0500%, Sb: 0 to 0.0500%, As: 0 to 0.0500%, Mg: 0 to 0.0500%, Ca: 0-0.0500%, Zr: 0 to 0.0500%, and REM: 0~0.1000% a refining step of adjusting the composition of the sintered steel to a composition containing the above-mentioned elements, with the remainder being Fe and impurities; a casting step of producing a slab using the molten steel after the refining step; a hot rolling step in which the slab is heated directly or after being cooled once, and hot rolled to obtain a hot-rolled steel sheet; a winding step of winding the hot-rolled steel sheet in a temperature range of 700°C or less; a pickling step of pickling the hot-rolled steel sheet after the coiling step; a cold rolling step of cold rolling the hot-rolled steel sheet after the pickling step at a rolling reduction of 30 to 90% to obtain a cold-rolled steel sheet; An annealing step of annealing the cold-rolled steel sheet in an atmosphere having a dew point of −80° C. or higher and −15° C. or lower at a temperature range of 820° C. to 900° C.; A method for manufacturing a steel sheet, comprising:

7. In the refining step, The method for producing a steel sheet according to claim 6, wherein the deoxidation time is less than 5 minutes.

8. The component composition is, in mass%, Mn: 1.00-2.00% Si: 0.30~1.00% The method for producing a steel sheet according to claim 6 or 7, characterized in that

9. In the annealing step, The method for producing a steel sheet according to any one of claims 6 to 8, further comprising a coating layer forming step of forming a coating layer containing at least one of zinc, aluminum, magnesium, and an alloy thereof on one or both surfaces of the cold-rolled steel sheet.

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