STEEL SHEET AND METHOD FOR MANUFACTURING STEEL SHEET

Abstract

This steel sheet includes, in mass % C: 0.15% to 0.50%, Si: 0.01% to 1.00%, Mn: 1.00% to 3.00%, P: 0% to 0.0200%, S: 0.0001% to 0.0200%, Al: 0.001% to 0.100%, and N: 0% to 0.0200%, with the remainder being Fe and impurities, in which a metallorgraphic structure has an area fraction of 0% to 10.0% of retained austenite and 0% to 5.0% of pearlite, ferrite, and bainite in total, with the remaining structure being martensite and tempered martensite, a maximum diameter of MnS predicted by extreme value statistics is 30 μm or less, a surface roughness Ra is 5 μm or less, and a surface layer has a Vickers hardness of greater than or equal to a tensile strength TS (MPa) of the steel sheet×0.25.

Description

TECHNICAL FIELD

The present invention relates to a steel sheet and a method for manufacturing a steel sheet.

Priority is claimed on Japanese Patent Application No. 2022-028110, filed Feb. 25, 2022, the content of which is incorporated herein by reference.

BACKGROUND ART

To reduce the amount of greenhouse gases emitted from automobiles, attempts have been in progress to use high-strength steel sheets to reduce the weight of automobile bodies while ensuring safety.

For example, Patent Document 1 discloses, as a steel sheet with excellent elongation, hole expandability, bending workability, and delayed fracture resistance, a high-strength TRIP steel sheet having a component composition including, 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: 0.0005% or less, Al: 0.001% to 0.50%, N: 0.010% or less, and B: 0.0003% to 0.0050% and contains one or more selected from the group consisting of 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%, with the remainder being Fe and unavoidable impurities, in which the microstructure includes 15% or less (including 0%) of the ferrite in volume fraction with an average crystal grain size of 2 μm or less, 2% to 15% of retained austenite in volume fraction with an average crystal grain size of 2 μm or less, and 10% of less (including 0%) of martensite in volume fraction with an average crystal grain size of 3 μm or less, with the remainder being bainite and tempered martensite with an average crystal grain size of 6 μm or less, and an average of 10 or more cementite grains with a grain size of 0.04 μm or more are contained in the bainite and tempered martensite grains.

Patent Document 2 discloses, as a steel sheet having both high tensile strength (TS): 980 MPa or more and excellent bendability, a high-strength cold-rolled steel sheet having a tensile strength of 980 MPa, a specific component composition, and a specific steel structure in which the area fraction of ferrite is 30% to 70%, the area fraction of martensite is 30% to 70%, the average grain size of ferrite grains is 3.5 μm or less, the standard deviation of the grain sizes 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 martensite grains is 3.0 μm or less, and the average aspect ratio of the martensite grains is 2.5 or less.

Patent Document 3 discloses, as a steel sheet having a yield strength (YS) of 780 MPa or more, a tensile strength (TS) of 1,180 MPa or more, and excellent spot weldability, ductility, and bending workability, a high-strength steel sheet in which a 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 only to ferrite in the whole 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 having a crystal grain size of 10 μm or more in ferrite present in a range from a depth of 20 μm to a depth of 100 μm from the surface of the steel sheet is less than 5%.

Patent Document 4 discloses, as a steel sheet with less variation in mechanical properties (especially strength and ductility), a high-strength cold-rolled steel sheet which has a component composition including, in mass %, C: 0.10% to 0.25%, Si: 0.5% to 2.00%, Mn: 1.0% to 3.0%, P: 0.1% or less, S: 0.01% or less, Al: 0.01% to 0.5%, and N: 0.01% or less, with the remainder being iron and unavoidable impurities, and has a structure containing 20% to 50% of ferrite in area fraction as a soft first phase, with the remainder being tempered martensite and/or tempered bainite as hard second phases, in which the total area of grains having an average grain size of 10 to 25 μm in all grains of the ferrite accounts for 80% or more of the total area of all the grains of the ferrite, the dispersion state of cementite grains with a circle-equivalent diameter of 0.3 μm or more present in all the grains of the ferrite is greater than 0.15 and 1.0 or less per μm² of the ferrite, and the tensile strength is 980 MPa or more.

CITATION LIST

Patent Documents

Patent Document 1

PCT International Publication No. WO2017/179372

Patent Document 2

PCT International Publication No. WO2016/194272

Patent Document 3

Japanese Unexamined Patent Application, First Publication No. 2015-117404

Patent Document 4

Japanese Unexamined Patent Application, First Publication No. 2013-245397

SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

In addition to structural members such as members, there has been an increased demand for higher strength in automobile steel sheets for exterior sheet members such as doors and roofs. To meet such a demand, materials have been developed to achieve both strength and elongation (moldability).

On the other hand, the shapes of both the above-described structural members and exterior sheet members of automobiles tend to become more complex. When a steel sheet is made thinner by increasing their strength to reduce weight, the surface of the steel sheet is likely to become irregular when molded into complex shape, and when the surface is irregular, the appearance after molding is degraded. In other words, in addition to properties such as strength, the surface properties of steel sheets for automobiles are also important. Especially for exterior sheet members of automobiles, design and surface properties are also important, and therefore, it is required for the appearance after molding to be excellent.

However, with conventional technology, it is difficult to achieve both strength and surface properties (appearance) after molding at a high level in steel sheets (especially high-strength steel sheets).

An object of the present invention is to provide a steel sheet in which generation of surface irregularities during molding can be suppressed and which has a high strength, and a method for manufacturing the same.

Means for Solving the Problem

The present inventors have found that the generation of surface irregularities during molding can be suppressed by controlling dimensions of MnS in steel, which is an inclusion, and by ensuring that the Vickers hardness of a surface layer of a steel sheet is above a predetermined level, while ensuring strength by using martensite and tempered martensite as a main structure of the steel sheet.

The present invention has been made based on the above-described findings, and the gist thereof is as follows.

• • (1) A steel sheet according to one aspect of the present invention has a component composition including, in mass %, C: 0.15% to 0.50%, Si: 0.01% to 1.00%, Mn: 1.00% to 3.00%, P: 0% to 0.0200%, S: 0.0001% to 0.0200%, Al: 0.001% to 0.100%, and N: 0% to 0.0200%, Co: 0% to 0.500%, Ni: 0% to 1.000%, Mo: 0% to 1.000%, Cr: 0% to 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% to 0.500%, W: 0% to 0.1000%, Ta: 0% to 0.1000%, Sn: 0% to 0.500%, 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%, with the remainder being Fe and impurities, in which a metallographic structure has an area fraction of 0% to 10.0% of retained austenite and 0% to 5.0% of pearlite, ferrite, and bainite in total, with the remaining structure being martensite and tempered martensite, a maximum diameter of MnS predicted by extreme value statistics is 30.0 μm or less, a surface roughness Ra is 5.0 μm or less, and a surface layer has a Vickers hardness of greater than or equal to a tensile strength TS (MPa) of the steel sheet×0.25.
  • (2) In the steel sheet according to one aspect described above, the component composition may include, in mass %, one or more of Co: 0.010% to 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% to 0.0100%, Nb: 0.001% to 0.500%, V: 0.001% to 0.500%, Cu: 0.001% to 0.500%, W: 0.0010% to 0.1000%, Ta: 0.0010% to 0.1000%, Sn: 0.0010% to 0.0500%, Sb: 0.0010% to 0.0500%, As: 0.0010% to 0.0500%, Mg: 0.0001% to 0.0500%, Ca: 0.0010% to 0.0500%, Zr: 0.0010% to 0.0500%, and REM: 0.0010% to 0.1000%
  • (3) In the steel sheet according to one aspect described above, the component composition may include, in mass %, Mn: 1.00% to 2.00% and Si: 0.30% to 1.00%.
  • (4) In the steel sheet according to one aspect described above, the tensile strength may be 1,470 MPa or more.
  • (5) In the steel sheet according to one aspect described above, a coating film layer containing at least one of zinc, aluminum, magnesium, and their alloys may be provided on a single surface or both surfaces of the steel sheet.
  • (6) A method for manufacturing the steel sheet according to one aspect of the present invention is a method for manufacturing a steel sheet according to one aspect described above, including: a refining process in which molten steel subjected to a vacuum degassing treatment to adjust a component composition of the molten steel to have an Al concentration of 0.0500 mass % or less and to include, in mass %, C: 0.15% to 0.50%, Si: 0.01% to 1.00%, Mn: 1.00% to 3.00%, P: 0% to 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% to 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% 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%, with the remainder being Fe and impurities; a casting process in which a slab is manufactured using the molten steel after the refining process; a hot rolling process in which slab is heated directly or after temporary cooling and hot-rolled to obtain a hot-rolled steel sheet; a coiling process in which hot-rolled steel sheet is coiled in a temperature range of 700° C. or lower; a pickling process in which the hot-rolled steel sheet after the coiling process is pickled; a cold rolling process in which 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; and an annealing process in which the cold-rolled steel sheet is annealed in an atmosphere with a dew point of −80° C. and −15° C. in a temperature range of 820° C. to 900° C.
  • (7) In the method for manufacturing a steel sheet according to one aspect described above, in the refining process, a deoxidation time may be shorter than 5 minutes.
  • (8) In the method for manufacturing a steel sheet according to one aspect described above, the component composition may include, in mass %, Mn: 1.00% to 2.00% and Si: 0.030% to 1.00%.
  • (9) In the method for manufacturing a steel sheet according to one aspect described above, the annealing process may include a coating film layer forming process in which a coating film containing at least one of zinc, aluminum, magnesium, and their alloys is formed on a single surface or both surfaces of the cold-rolled steel sheet.

Effects of the Invention

According to the present invention, it is possible to provide a steel sheet in which generation of surface irregularities during molding can be suppressed and which has a high strength, and a method for manufacturing the same. In addition, according to the present invention, it is possible to provide, as an automobile steel sheet, a steel sheet having suitable surface properties and a high tensile strength.

EMBODIMENT FOR IMPLEMENTING THE INVENTION

In one embodiment of the present invention, it is possible to obtain a high-strength steel sheet in which generation of surface irregularities during molding can be suppressed by controlling the maximum grain size of an inclusion (MnS), the surface roughness Ra of the surface of the steel sheet, and the Vickers hardness of the surface of the steel sheet in addition to the area fraction of a metallographic structure of the steel sheet.

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

First, a metallographic structure of the steel sheet according to the present embodiment will be described. Hereafter, since the structure fraction will be expressed as an area fraction , the “%” for the structure fraction means area %.

<Metallographic Structure>

(Area Fraction of Retained Austenite: 0% to 10.0%)

Retained austenite is a structure that contributes to improvement in elongation through transformation induced plasticity (TRIP). However, martensite formed through transformation induced plasticity of retained austenite is significantly hard and can be a starting point of formation of voids, which may degrade surface irregularities after prestraining. Therefore, the area fraction of retained austenite is set to 10.0% or less. The area fraction thereof is preferably 5.0% or less. In the present embodiment, retained austenite may not be generated, and the area fraction of retained austenite may be 0%.

(Total Area Fraction of Ferrite, Bainite, and Pearlite: 0% to 5.0%)

Ferrite and bainite are relatively soft structures. Therefore, if the area fraction of ferrite and bainite is excessive, it may not be possible to obtain a desired tensile strength. In addition, pearlite is also a structure that has a low strength and reduces ductility. In addition, if the area fraction of these soft structures increases while the main structure is martensite and tempered martensite, the soft structures are preferentially deformed during pre-deformation, and this local deformation is transmitted to the surface, causing irregularities on the surface of the steel sheet after pre-deformation. Therefore, from the viewpoint of ensuring strength and suppressing surface irregularities after pre-formation, the smaller the area fractions of ferrite, bainite, and pearlite may be 0%. In other words, the steel sheet according to the present embodiment preferably does not contain ferrite, bainite, and pearlite. In addition, even when ferrite, bainite, and pearlite are incorporated, the total area fraction of ferrite, bainite, and pearlite is set to 5.0% or less from the viewpoint of ensuring strength. The total area fraction thereof is preferably 4.0% or less and more preferably 3.0% or less.

(Remaining Structure: Martensite and Tempered Martensite)

In the metallographic structure of the steel sheet according to the present embodiment, the remaining structure other than retained austenite, ferrite, bainite, and pearlite described above includes martensite and tempered martensite. In other words, the structure of the steel sheet according to the present embodiment has martensite and tempered martensite as its main structure.

Martensite and tempered martensite are hard structures that contribute to an increase in tensile strength. Therefore, the total area fraction of martensite and tempered martensite is preferably set to 90.0% or more, and more preferably set to 95.0% or more. This makes it easier to ensure high tensile strength (for example, a tensile strength of 1,300 MPa or more). From a strength perspective, the total area fraction of martensite and tempered martensite may be 100%.

From the viewpoint of securing toughness, the area fraction of martensite is preferably set to 0.0% to 75.0%, and the area fraction of tempered martensite is preferably set to 20.0% to 99.9%.

The term “martensite” in the present embodiment refers to fresh martensite. Fresh martensite is martensite containing no carbides. In addition, the term “tempered martensite” is martensite containing carbides.

Next, an identification method and an area fraction calculation method of each metallographic structure will be described.

Identification of each metallographic structure and calculation of its area and area fraction can be performed through electron back scattering diffraction (EBSD), X-ray measurement, corrosion with Nital reagent or LePera solution, and observation of a 100 μm×100 μm area of a cross section of the steel sheet perpendicular to the sheet surface and parallel to the rolling direction using a scanning electron microscope at 1000× to 50000× magnification. In measuring the area fraction of any structure, three measurement points will be used and an average value thereof will be calculated.

The area and area fraction of ferrite can be measured through the following method. In other words, EBSD attached to the scanning electron microscope is used to measure an area of ⅛ to ⅜ thickness centered at the ¼ position of the sheet thickness at intervals (pitch) of 0.2 μm. A grain average misorientation (GAM) value is calculated from the measured data. Then, a region where the grain average misorientation value is less than 0.5° is defined as ferrite, and the area and area fraction are measured. Here, the grain average misorientation is a value obtained by calculating the orientation difference between adjacent measurement points in a region surrounded by grain boundaries with a crystal orientation difference of 5° or more and averaging it over all measurement points within the crystal groin.

The area and area fraction of bainite is calculated using well-known image analysis software such that a sample of a sheet thickness cross section parallel to the rolling direction of the steel sheet as an observation surface is collected, the observation surface is polished and etched with Nital solution, and an area of ⅛ to ⅜ thickness centered at ¼ of the sheet thickness is observed with a filed emission scanning electron microscope (FE-SEM). The area fraction can be calculated using image analysis software such “ImageJ”. Here, “ImageJ” is open-source, public-domain image processing software that is widely used by those skilled in the art.

In the FE-SEM observation, for example, the structure on the observation surface, which is a square with one side of 30 μm, is distinguished as follows. Bainite is a lath-like crystal grain aggregate that does not contain iron-based carbides with an internal long diameter of 20 nm or more, or that contains iron-based carbides with an internal long diameter of 20 nm or more and the carbides belong to a single variant, that is, an iron-based carbide group elongated in the same direction. Here, the iron-based carbide group elongated in the same direction means that the difference in the direction of elongation of iron-based carbide groups in within 5°. Bainite surrounded by grain boundaries with an orientation difference of 15° or more is counted as one bainite grain.

The area fraction of martensite and tempered martensite can be calculated by performing etching with LePera solution, observing and imaging an area of ⅛ to ⅜ thickness centered at ¼ of the sheet thickness with FE-SEM, and subtracting the area fraction (whose details will be described below) of retained austenite measured using X-rays from the area fraction of an uncorroded region.

The area fraction of retained austenite can be calculated from the integrated intensity ratio of diffraction peaks of (200) and (211) for the bcc phase and (200), (22), and (311) for the fcc phase using a sample with a 100 μm region removed in the sheet thickness direction from the surface through electrolytic polishing or chemical polishing and using MoKα-rays as characteristic X-rays.

The area fraction of pearlite can be calculated by corroding with Nital reagent and observing an area of ⅛ to ⅜ thickness centered at the ¼ position of the sheet thickness from the surface of the steel sheet using a secondary electron image from a scanning electron microscope. A region imaged in bright contrast in the secondary electron image is regarded as pearlite, and the area fraction is calculated using the above-described image analysis software “ImageJ”.

(Maximum Diameter of MnS: 30.0 μm or Less)

In the inclusion MnS, coarse MnS becomes a starting point of microcracks of cast slabs after casting.

Furthermore, the microcracks in the cast slabs become cracks during hot rolling, resulting in degradation of the surface properties of the steel sheet (increased surface roughness Ra). Furthermore, as a result of investigations of the present inventors, it has been found that formation of surface irregularities due to cold molding (for example, 5% prestraining) becomes significant in a location where MnS above a certain size is present. From these facts, the maximum diameter of MnS in the steel is 30.0 μm or less in the present embodiment. Specifically, in the present embodiment, the predicted value of the maximum diameter of MnS by extreme value statistics measured in the rolling direction cross section of the steel sheet under the conditions of inspection standard area: 9.58 mm², number of inspections: 40 views, and area to be predicted: 383.39 mm² is set to 30.0 μm or less. When coarse MnS is present in the steel sheet, as described above, wavy surface irregularities are created on the surface of parts after molding when the parts are made through cold molding. When the maximum diameter of MnS predicted by extreme value statistics under the above-described conditions exceeds 30.0 μm, the surface irregularities become particularly significant. For the above reasons, the maximum diameter of MnS predicted by extreme value statistics under the above-described conditions is set to 30.0 μm or less. The maximum diameter thereof is preferably set to 25.0 μm or less. The smaller the maximum diameter of MnS predicted by extreme value statistics under the above-described conditions, the more preferable it is. Therefore, the lower limit value of the maximum diameter of the MnS is not particularly limited. The lower limit value of the maximum diameter of the MnS may be substantially 1.0 μm or more.

In the present embodiment, the method described on pages 233 to 239 of “Metal Fatigue: Effect of Small Defects and Inclusions” published by Yokendo Publishing Co., Ltd. on Mar. 8, 1993 is used as a method of measuring and predicting the maximum diameter of precipitates by extreme value statistics. A two-dimensional inspection method in which maximum precipitates observed within a certain area (area to be predicted: 383.39 mm²) are estimated by two-dimensional inspection is used in the present embodiment. The area to be predicted may be set by considering the hazardous volume of a typical component.

Next, a method for specifying MnS in steel and a specific method for predicting the maximum diameter of MnS through an extreme value statistical method will be described.

(Method for Specifying MnS)

MnS can be evaluated by observing the structure in the cross section (full thickness) of the steel sheet. When the polished surface of a sample obtained by finishing the cross section of the steel sheet to a mirror surface through mechanical polishing is observed using an optical microscope, MnS is recognized as a slight black contrast (gray) against mirror-finished ferrite. Examples of alternative methods to the observation with an optical microscope include a method of performing composition analysis using an energy dispersive X-ray detector (energy dispersive X-ray spectrometry: EDX). The above-described inspection standard area may be subjected to surface analysis at intervals of 0.1 μm, and a region with high concentration of both Mn and S may be obtained and regarded as MnS.

(Prediction of Maximum Diameter of MnS by Extreme Value Statistical Method)

First, a test piece is collected from a steel sheet, and a region of an inspection standard area of 9.58 mm² (an area of 3.57 mm×2.68 mm; if the sheet thickness (t) is less than 2.68 mm, an area of t (mm)×9.58/t (mm) is used) in the rolling direction section of the test piece is prepared for 40 views in advance. Then, MnS with the largest area (maximum MnS) in each 9.58 mm² inspection standard area is detected and photographed with an optical microscope at 400×. Such photography is repeated 40 times for the view of each 9.58 mm² inspection standard area (that is, the number of inspections being 40 views). The diameter of MnS in each inspection standard area is measured from the obtained photographs. Since most MnS has an elliptical shape, when measuring the diameter of MnS, the geometric mean of the long and short diameters is determined and used as the diameter of MnS. The resulting data of 40 maximum MnS diameters are plotted on extreme-value probability paper through the method described on pages 233 to 239 of “Metal Fatigue: Effects of Small Defects and Inclusions” (Yokendo Publishing Co., Ltd.) to determine a maximum MnS distribution line (linear function of the maximum MnS diameter and extreme value statistical standardized variable). Then, by extrapolating the maximum MnS distribution line, the maximum diameter of MnS in the area: 383.39 mm² to be predicted is predicted.

<Surface Roughness Ra>

The surface roughness Ra of the steel sheet of the present embodiment is 5.0 μm or less.

As described above, coarse MnS becomes a starting point of microcracking of cast slabs after casting. Furthermore, the microcracks in the cast slabs become cracks during hot rolling, resulting in increased surface roughness Ra. If the surface roughness Ra of the steel sheet increased, the appearance (design) of the steel sheet is impaired, and it may also cause degradation of bendability and fatigue properties since it becomes a starting point during bending deformation. From these facts, the surface roughness Ra of the steel sheet in the present embodiment is set to 5.0 μm or less. The surface roughness Ra is preferably 4.5 μm or less and more preferably 4.0 μm or less. The smaller the surface roughness Ra of the steel sheet, the more preferable it is, and therefore, the lower limit value of the surface roughness Ra is not particularly limited. The lower limit value of the surface roughness may be substantially 0.5 μm or more. The surface roughness Ra of the steel sheet of the present embodiment is specifically measured in accordance with JIS B 0601:2013.

The surface roughness may be measured using a contact measurement device that presses a diamond stylus against the steel sheet surface to measure the change in height of the steel sheet surface. Alternatively, a non-contact measurement device that measures the height of the steel sheet surface using laser may be used. The measurement area is set to at least 9 mm², preferably 16 mm² or more, and still more preferably 25 mm² or more.

When checking surface irregularities after 55 prestraining as an indicator of the presence of absence of irregularities on the surface of the steel sheet after cold molding, the surface roughness Ra is measured through the above-described method for the steel sheet after 5% prestraining. As long as the measurement area requirements are met, it is sufficient to measure either one or the surfaces of a steel sheet subjected to 5%±1% prestraining to evaluate the degree of irregularities of the surface of the steel sheet after cold molding. Alternatively, both surfaces of the steel plate may be subject to measurement.

<Surface Layer Hardness>

In the present embodiment, a surface layer of the steel sheet is relatively hardened, and the Vickers hardness of the surface layer of the steel sheet (surface layer hardness) is above a predetermined value. In other words, in the present embodiment, the surface layer hardness of the steel sheet is greater than or equal to (tensile strength TS of the steel sheet)×0.25. This prevents the generation of surface irregularities during molding because the absence of a soft layer on the surface of the steel sheet distributes a strain applied during molding evenly within the sheet thickness and suppresses excessive concentration of strain on the surface layer. To better achieve such effects, the surface layer hardness of the steel sheet is preferably greater than or equal to TS×0.28 and more preferably greater than or equal to TS×0.30. It does not usually occur that the surface layer of the steel sheet hardens significantly more than the interior. Therefore, the upper limit of the surface layer hardness of the steel sheet is practically TS×0.35.

The surface layer hardness of the steel sheet can be measured by the following procedure.

First, the sheet thickness cross section of the steel sheet is finished to a mirror surface through mechanical polishing. The Vickers hardness (HV) is measured at 12points on the polished surface at a distance (depth) of 50 μm from the sheet surface to the inside of the sheet thickness and on a straight line parallel to the rolling direction, with a pushing load of 20 gf in accordance with JIS Z 2244-1 (2020). The average value of the 10 Vickers hardness values, excluding the lowest and highest of these 12 measured Vickers hardness values, is regarded as a surface hardness of the steel sheet. The distance between measurement points is preferably a distance of four times or more the indentation. The distance of four times or more the indentation mentioned here is a distance multiplied by a number of four times or more the length of a diagonal line of the indentation created by the diamond indenter during measurement of the Vickers hardness.

Next, the reasons for limiting the component composition of the steel sheet according to the present embodiment will be explained. Hereinafter, % for the component composition means mass %.

(C: 0.15% to 0.50%)

C is an element that ensures sufficient amount of martensite and tempered martensite and increases the strength of the steel sheet. If C is less than 0.15%, the area fraction of martensite and tempered martensite becomes insufficient, making it difficult to ensure a high tensile strength (for example, a tensile strength of 1,300 MPa or more). Accordingly, the C content is set to 0.15% or more. The C content is preferably 0.20% or more and more preferably 0.25% or more. On the other hand, if the C content exceeds 0.50%, the volume change accompanying martensitic transformation becomes large, which may lead to the formation of irregularities on the surface of the steel sheet and extremely deteriorate moldability. Therefore, the C content is set to 0.50% or less. The C content is preferably 0.40% or less and more preferably 0.35% or less.

(Si: 0.01% to 1.00%)

Si acts as a solid-solution strengthening element to increase strength. In addition, Si is an effective element for obtaining a structure containing martensite and tempered martensite. From these facts, the Si content is adjusted according to the target strength level. If the Si content exceeds 1.00%, the area fraction of retained austenite may increase too much, which may degrade surface irregularities after prestraining and cause a decrease in press moldability and chemical conversion treatability. Therefore, the upper limit of the Si content is set to 1.00% or less. The Si content is preferably 0.95% or less and more preferably 0.90% or less. On the other hand, excessive reduction is Si content may result in higher production costs. Accordingly, the Si content is set to 0.01% or more. The Sn content is preferably 0.05% or more, more preferably 0.10% or more, and still more preferably 0.30% or more.

(Mn: 1.00% to 3.00%)

Mn is an element that contributes to improvement in strength and also has an action of suppressing ferrite transformation that occurs during heat treatment in continuous annealing or continuous hot-dip galvanizing facilities. If Mn is less than 1.00%, these effects are not sufficiently expressed, leading to ferrite transformation, and as a result, it is difficult to obtain a high tensile strength (for example, a tensile strength of 1,300 MPa or more). Therefore, the Mn content is set to 1.00% or more. 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%, moldability may deteriorate. Furthermore, the 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 still more preferably 2.00% or less.

(P: 0% to 0.00200%)

P is an impurity element which is segregated in the sheet thickness center of the steel sheet and inhibits toughness. In addition, P is an element that makes a welded part embrittle when the steel sheet is welded. If the P content exceeds 0.0200%, the strength of the welded part and the ductility of hole expandability significantly deteriorate. In addition, if the P content exceeds 0.0200%, the steel sheet may become brittle and the surface layer of the steel sheet may delaminate during cold rolling, resulting in degradation of the surface properties (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. On the other hand, the lower the P content, the more desirable it is, and the lower limit is not particularly limited. The P content may be 0%. On the other hand, reducing the P content to less than 0.0001% in practical steel sheets is economically unfavorable because it significantly increases production costs. Therefore, the lower limit value of the P content may be set to 0.0001% or more.

(S: 0.0001% to 0.0200%)

S is an impurity element that inhibits weldability and also manufacturability during casting and hot rolling. In addition, S is also an element which forms coarse MnS, inhibits hole expandability, and also causes surface irregularities on the surface of the steel sheet during cold molding. When the S content exceeds 0.0200%, these effects become significant. Therefore, the S content is set to 0.0200% or less. The S contents is preferably 0.0100% or less and more preferably 0.0050% or less. On the other hand, reducing the S content to less than 0.00001% in practical steel sheets is economically unfavorable because it significantly increases production costs. Therefore, the lower limit value of the S content is set to 0.0001% or more.

(Al: 0.001% to 0.100%)

Al is an element that acts as a deoxidizing material for steel. If the Al content is less than 0.001%, this effect is not sufficiently obtained, and therefore, the lower limit is set to 0.001%, or more. The lower limit is preferably 0.005% or more. On the other hand, in the present embodiment, the Al concentration in molten steel during vacuum degassing treatment is reduced to suppress formation of coarse MnS. IN other words, the reduction of Al content in the present embodiment promotes formation of MnO and Ti₂O₃, thereby suppressing the formation of coarse MnS and preventing the generation of surface irregularities after cold molding. However, if the Al content in the steel sheet exceeds 0.100%, the formation of coarse MnS cannot be sufficiently suppressed. In addition, coarse Al oxides may form, causing degradation of ductility. Therefore, the Al content is set to 0.100% or less. The Al content is preferably 0.080% or less and more preferably 0.060% or less.

As described below, in a method for manufacturing a steel sheet according to the present embodiment, the Al concentration in molten steel is adjusted to 0.0500 mass % or less. In other words, the Al concentration in the steel sheet, which is a product sheet is also basically 0.0500 mass % or less, but if, for example, Al is to be actively incorporated, a separate process (between the completion of the vacuum degassing treatment and injection of molten steel into a tundish) may be provided to add additional Al in the performing of the vacuum degassing treatment. However, even in such a case, the Al content in the steel obtained as a final product is set to 0.100% or less

(N: 0% to 0.0200%)

N is an element that forms coarse nitrides, inhibits bendability and hole expandability, and causes formation of blowholes during welding. If the N content exceeds 0.0200%, coarse nitrides are formed, resulting in degradation of moldability. toughness and significant formation of blowholes, In addition, if the N content exceeds 0.0200%, the surface layer of the steel sheet may become brittle and the surface layer of the steel sheet may delaminate during cold rolling, resulting in degradation of the surface properties (surface roughness Ra) of the steel sheet. Therefore, the N content is set to 0.0200% or less. The N content is preferably 0.0170% or less and more preferably 0.0150% or less. The lower the N content, the more preferable it is, and the lower limit is not particularly limited. 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 unfavorable because it significantly increases production costs. Therefore, the lower limit value of the N content may be set to0.0005% or more.

(Co: 0% to 0.500%)

Co is an effective element for improving the strength of the steel sheet. Co content may be 0%, but to obtain the above-described 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, resulting in degradation of moldability. In addition, if the Co content is too high, the surface layer of the steel sheet may become brittle and the surface layer of the steel sheet may delaminate during cold rolling, resulting in degradation of the surface properties (surface roughness Ra) of the steel sheet. Therefore, the Co content is set to 0.500% or less and preferably 0.300% or less.

(Ni: 0% to 1.000%)

Ni, like Co, is an effective element for improving the strength of the steel sheet. Ni content may be0%, but to obtain the above-described 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, resulting in degradation of moldability. In addition, if the Ni content is too high, the surface layer of the steel sheet may become brittle and the surface layer of the steel sheet may delaminate during cold rolling, resulting in degradation of the surface properties (surface roughness Ra) of the steel sheet. Therefore, the Ni content is set to 1.00% or less and preferably 0.800% or less.

(Mo: 0% to 1.000%)

Mo, like Mn, is an element that contributes to high strength of the steel sheet. This effect can be obtained even with a trace Mn content. The Mo content may be 0%, but to obtain the above-described effect, the Mo content is preferably 0.010% or more. On the other hand, if the Mo content exceeds 1.000%, coarse Mo carbides may be formed, resulting in degradation of cold moldability of the steel sheet. Excessive Mo content leads to embrittlement of the steel sheet surface layer. If the surface layer of the steel sheet becomes brittle, the surface layer of the steel sheet may delaminate during cold rolling, resulting in degradation for the surface properties (surface roughness Ra) of the steel sheet. Therefore, the Mo content is set to 1.000% or less and preferably 0.800% or less.

(Cr: 0% to 2.000%)

Cr, like Mn or Mo, is an element that contributes to high strength of the steel sheet. This effect can be obtained even with a trace Cr content. The Cr content may be 0%, but to obtain the above-described 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 may be formed in the steel, and dissolution of carbides into austenite is suppressed when holding at a temperature range of 820° C. to 900° C. during annealing after cold rolling (a so-called heat equalization process), leading to ferrite transformation, and a high tensile strength (for example, tensile a strength of 1,300 MPa or more) may not be obtained. Therefore, the Cr content is set to 2.000% or less and preferably 1.500% or less.

(O: 0% to 0.020%)

O is an element that forms coarse oxides, degrades moldability and rupture resistance, and causes formation of blowholes during welding. If the O content exceeds 0.020%, formation of blowholes and degradation of moldability and ductility of the punched edge surface due to coarse oxides become significant. In addition, the formation of coarse oxides also leads to embrittlement of the surface layer of the steel sheet. If the surface layer of the steel sheet becomes brittle, the surface layer of the steel sheet may delaminate during cold rolling, resulting in degradation 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 the lower limit is not particularly limited. 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 unfavorable because it significantly increases production costs. Therefore, the lower limit value of the O content may be set to 0.0001% or more.

(Ti: 0% to 0.5000%)

Ti is an element that may form coarse Ti oxides or TiN, which may degrade moldability of the steel sheet. In addition, the formation of coarse Ti oxides also leads to embrittlement of the surface layer of the steel sheet. If the surface layer of the steel sheet becomes brittle, the surface layer of the steel sheet may delaminate during cold rolling, resulting in degradation of the surface properties (surface roughness Ra) of the steel sheet. Therefore, from the viewpoint of ensuring moldability and favorable surface properties of the steel sheet, the lower the Ti content, the more preferable it is, and it 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 unfavorable because it leads to an excessive increase in refining costs. Therefore, the lower limit of the Ti content may be set to 0.0010% or more.

(B: 0% to 0.0100%)

B is an element that suppresses formation of ferrite and pearlite and promotes formation of martensite during a cooling process from austenite. In addition, B is also a beneficial element for increasing the strength of the steel sheet. These effects can be obtained even with a trace B content. The B content may be 0%, but to obtain the above-described effects, the B content is preferably 0.0001% or more. However, if the B content is too high, coarse B oxides may be formed. These B oxides become a starting point of formation of voids during press molding, and the formation of such voids may degrade moldability of the steel sheet. In addition, the formation of B oxides also leads to embrittlement of the surface layer of the steel sheet. If the surface layer of the steel sheet becomes brittle, the surface layer of the steel sheet may delaminate during cold rolling, resulting in degradation of the surface properties (surface roughness Ra) of the steel sheet. Therefore, the B content is set to 0.0100% or less. The B content is preferably set to 0.0090% or less. In a case where less than 0.0001% of B is identified, it is necessary to take great care in the analysis. If the B content is below the detection limit of an analyzer, the B content may be considered as 0%.

(Nb: 0% to 0.500%)

Nb is an effective element for controlling the morphology of carbides and for improving toughness of the steel sheet by refining the structure. These effects can be obtained even with a trace Nb content. Nb content may be 0%, but to obtain the above-described effects, 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 may precipitate, resulting n significant degradation of ductility of the steel sheet and degradation of moldability of the steel sheet. In addition, if the Nb content is too high, the surface layer of the steel sheet may become brittle and the surface layer of the steel sheet may delaminate during cold rolling, resulting in degradation of the surface properties (surface roughness Ra) of the steel sheet. Therefore, the Nb content is set to 0.500% or less. The Nb content is preferably set to 0.450% or less.

(V: 0% to 0.500%)

V, like Nb, is an effective element for controlling the morphology of carbides and for improving toughness of the steel sheet by refining the structure. The V content may be 0%, but to obtain the above-described effects, the V content is preferably 0.001% or more. However, if the V content is too high, a large number of hard V carbides may precipitate, resulting in significant degradation of ductility of the steel sheet and degradation of moldability of the steel sheet. In addition, if the V content is too high, the surface layer of the steel sheet may become brittle and the surface layer of the steel sheet may delaminate during cold rolling, resulting in degradation of the surface properties (surface roughness Ra) of the steel sheet. Therefore, the V content is set to 0.500% or less. The V content is preferably set to 0.450% or less.

(Cu: 0% to 0.500%)

Cu is an element that contributes to improvement in strength of the steel sheet. This effect can be obtained even with a trace Cu content. The Cu content may be 0%, but to obtain the above-described effect, the Cu content is preferably 0.001% or more. However, too high Cu content may lead to red brittleness and lower productivity in hot rolling. In addition, if the Cu content is too high, the surface layer of the steel sheet may become brittle and the surface layer of the steel sheet may delaminate during cold rolling, resulting in degradation of the surface properties (surface roughness Ra) of the steel sheet. Therefore, the Cu content is set to 0.500% or less. The Cu content is preferably set to 0.450% or less.

(W: 0% to 0.1000%)

W, like Nb and V, is also an effective element for controlling the morphology of carbides and improving the strength of the steel sheet.

The W content may be 0%, but to obtain the above-described effects, the W content is preferably 0.0010% or more. On the other hand, if the W content is too high, a large number of W carbides may precipitate, resulting in degradation of ductility of the steel sheet and degradation of cold workability of the steel sheet. In addition, if the W content is too high, the surface layer of the steel sheet may become brittle and the surface layer of the steel may delaminate during cold rolling, resulting in degradation of the surface properties (surface roughness Ra) of the steel sheet. Therefore, the W content is set to 0.1000% or less. The W content is preferably set to 0.900% or less.

(Ta: 0% to 0.1000%)

Ta, like Nb, V, and W, is also an effective element for controlling the morphology of carbides and improving the strength of the steel sheet. The Ta content may be 0%, but to obtain the above-described effects, the Ta content is preferably 0.0010% or more. On the other hand, if the Ta content is too high, a large number of Ta carbides may precipitate, resulting in degradation of ductility of the steel sheet and degradation of cold workability of the steel sheet. In addition, if the Ta content is too high, the surface layer of the steel sheet may become brittle and the surface layer of the steel sheet may delaminate during cold rolling, resulting in degradation of the surface properties (surface roughness Ra) of the steel sheet. Therefore, the Ta content is set to 0.1000% or less, preferably 0.0300% or less, and more preferably 0.0100% or less.

(Sn: 0% to 0.0500%)

Sn is an element that can be contained in the steel sheet when scrap is used as a raw material for the steel sheet. In addition, Sn may cause degradation the cold moldability of the steel sheet due to embrittlement of ferrite. In addition, if the Sn content is too high, the surface layer of the steel sheet may become brittle and the surface layer of the steel sheet may delaminate during cold rolling, resulting in degradation of the surface properties (surface roughness Ra) of the steel sheet. Therefore, the Sn content is preferably as low as possible. 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 unfavorable because it leads to an excessive increase in refining cost.

Therefore, the Sn content may be set to 0.0010% or more.

(Sb: 0% to 0.0500%)

Sb, like Sn, is an element that can be contained in the steel sheet when scrap is used as a raw material for the steel sheet. Sb may be segregated strongly at grain boundaries, resulting in embrittlement in the grain boundaries, degradation of ductility, and degradation of cold moldability. In addition, if the Sb content is too high, the surface layer of the steel sheet may become brittle and the surface layer of the steel sheet may delaminate during cold rolling, resulting in degradation of the surface properties (surface roughness Ra) of the steel sheet. Therefore, the Sb content is preferably as low as possible. 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 unfavorable because it leads to an excessive increase in refining costs. Therefore, the Sb content may be set to 0.0010% or more.

(As: 0% to 0.0500%)

As, like Sn and Sb, is an element that can be contained in the steel sheet when scrap is used as raw material for the steel sheet. As may be segregated strongly at grain boundaries, resulting in degradation of ductility and degradation of cold moldability. In addition, if the As content is too high, the surface layer of the steel sheet may become brittle and the surface layer of the steel sheet may delaminate during cold rolling, resulting in degradation of the surface properties (surface roughness Ra) of the steel sheet. Therefore, the As content is preferably as low as possible. The As content is se 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 unfavorable because it leads to an excessive increase in refining costs. Therefore, the As content may be set to 0.0010% or more.

(Mg: 0% to 0.0500%)

Mg controls the morphology of sulfides and oxides and contributes to improvement in bendability of the steel sheet. These effects can be obtained even with a trace Mg content. The Mg content may be 0%, but to obtain the above-described effects, the Mg content is preferably 0.0001% or more. However, too high Mg content may cause degradation of cold moldability due to formation of coarse inclusions and Mg oxides. In addition, the formation of Mg oxides also leads to embrittlement of the surface layer of the steel sheet. If the surface layer of the steel sheet becomes brittle, the surface layer of the steel sheet may delaminate during cold rolling, resulting in degradation of the surface properties (surface roughness Ra) of the steel sheet. Therefore, the Mg content is set to 0.0500% or less and preferably 0.0400% or less.

(Ca: 0% to 0.0500%)

Ca, like Mg, is an element that can control the morphology of sulfides in trace amounts. The Ca content may be 0%, but to obtain the above-described effect, the Ca content is preferably 0.0010% or more. However, if the Ca content is too high, coarse Ca oxides may be formed, and these Ca oxides may become a starting point of occurrence of cracks during cold molding. IN addition, the formation of Ca oxides also leads to embrittlement of the surface layer of the steel sheet. If the surface layer of the steel sheet becomes brittle, the surface layer of the steel sheet may delaminate during cold rolling, resulting in degradation 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.

(Zr: 0% to 0.0500%)

Zr, like Mg and Ca, is an element that can control the morphology of sulfides in trace amounts. The Zr content may be 0%, but to obtain the above-described effect, the Zr content is preferably 0.0010% or more. However, if the Zr content is too high, coarse Zr oxides may be formed, resulting in degradation of cold moldability. In addition, the formation of Zr oxides also leads to embrittlement of the surface layer of the steel sheet. If the surface layer of the steel sheet becomes brittle, the surface layer of the steel sheet may delaminate during cold rolling, resulting in degradation 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.

(REM: 0% to 01000%)

REM is rare earth metal (rate earth element). REM is an element that is effective in controlling the morphology of sulfides even in trace amounts. The REM content may be 0%, but to obtain the above-described effect, the REM content is preferably 0.0010% or more. However, if the REM content is too high, coarse REM oxides may be formed, resulting in degradation of workability and rupture resistance. In addition, the formation of REM oxides also leads to embrittlement of the surface layer of the steel sheet. If the surface layer of the steel sheet becomes brittle, the surface layer of the steel sheet may delaminate during cold rolling, resulting in degradation of the surface properties (surface roughness Ra) of the steel sheet. Therefore, the REM content is set to 0.1000% or less and preferably 0.0500% or less.

Here, REM is a generic term for two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) ranging from lantbanum (La) to lutetium (Lu). In addition, the “REM” referred to in the present embodiment is composed of one of more selected from the group consisting of these rare earth elements, and the “REM content” is the total amount of rare earth elements.

In the component composition of the steel sheet according to the present embodiment, the remainder, excluding that are mixed in during the industrial manufacture of steel due to various factors in raw materials such as ore and scrap and in the manufacturing process, and their presence is acceptable within the scope not inhibiting the properties of the steel sheet according to the present embodiment. In addition, they also include elements that are not components intentionally added to the steel sheet.

The sheet thickness of the steel sheet according to the present embodiment is not limited to a specific range, but considering strength, versatility, and manufacturability, 0.3 to 6.0 mm is preferable.

Next, a method for manufacturing a steel sheet according to the present embodiment will be described. The method for manufacturing a steel sheet of the present embodiment includes processes: refining process—casting process—hot rolling process—coiling process—pickling process—cold rolling process—annealing process (continuous annealing process). Although the manufacturing conditions for each process may be determined as appropriate within the scope not impairing the effect of the present invention, it is particularly important to appropriately control the conditions of the refining process and continuous annealing process, respectively, from the viewpoints of suppressing formation of coarse MnS and controlling hardness of a surface layer.

Hereinafter, each process and condition of the manufacturing method will be described in detail.

• • (a) Molten steel is subjected to vacuum degassing treatment, the Al concentration of the molten steel is adjusted to 0.0500 mass % or less, and the component composition of the molten steel is adjusted to the component composition (where except Al) described above (refining process).
  • (b) A slab is heated directly or after temporary cooling and hot-rolled to obtain (casting process).
  • (c) The slab is heated directly or after temporary cooling and hot-rolled to obtain a hot-rolled steel sheet (hot rolling process).
  • (d) The hot-rolled steel sheet is coiled in a temperature range of 700° C. or lower (coiling process).
  • (e) The hot-rolled steel sheet is pickled after the coiling process (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. to −15° in a temperature range of 820° C. to 900° C. (annealing process).

In the annealing process described in (g) above, a coating film layer forming process may be performed to form a coating film layer containing at least one of zinc, aluminum, magnesium, and their alloys on a single surface or both surfaces (front surface and/or rear surface) of the cold-rolled steel sheet.

The steel sheet according to the present embodiment suppresses formation of coarse MnS to suppress degradation of surface properties (surface roughness Ra) of the steel sheet and to suppress development of surface irregularities after cold molding. The coarse MnS becomes a starting point of microcracks of cast slabs after casting, which leads to cracks during hot rolling. Furthermore, in the areas where this coarse MnS is present, the development of surface irregularities after cold molding becomes significant. Therefore, in the present embodiment, the components of molten steel are adjusted in the refining process prior to the casting process so that the formation of coarse MnS is suppressed. Specifically, as described below, the Al concentration of molten steel is controlled to be below a certain level during the refining process.

(a) Refining Process

In the refining process, molten iron manufactured through a well-known method is first refined in a converter furnace (primary refining). The molten steel tapped from the converter furnace is subjected to secondary refining, that is, vacuum degassing treatment using a vacuum degassing device (for example, RH). IN the present embodiment, the Al concentration in the molten steel in this vacuum degassing treatment is adjusted to 0.0500 mass % or less, and the components are adjusted so that the components except Al have the component composition described above. Specifically, it is necessary for the Al concentration in the molten steel during the vacuum degassing process to be monitored to promote the formation of MnS with MnO or Ti₂O₃ as a nucleus and its fine dispersion. However, when the Al concentration in the molten steel is high, the amount of dissolved oxygen in the molten steel is low, and therefore, formation of oxides such as MnO and Ti₂O₃ is suppressed. As a result, it is difficult to promote formation and dispersion of fine MnS. In addition, when the Al concentration in the molten steel is high, MnS is difficult to form with Al₂O₃ as a nucleus, although oxides of Al₂O₃ are formed. In other words, the nucleation site of MnS is reduced when the Al concentration in the molten steel is high. As a result, coarse MnS exceeding 30 μm may be formed in the steel sheet obtained by subjecting a slab formed after solidification to hot rolling and cold rolling processes. From these facts, in the present embodiment, the Al concentration in the molten steel in this vacuum degassing treatment is adjusted to 0.0500 mass % or less. The Al concentration is preferably 0.0400 mass % or less and more preferably 0.0350 mass % or less.

In addition, in the present embodiment, the deoxidation time in the vacuum degassing process is preferably set to be shorter than 5 minutes. Excessively long deoxidization time may result in low amount of dissolved oxygen in the molten steel, which reduces the amount of MnO or Ti₂O₃ formed as a nucleus of MnS, resulting in the coarsening of MnS. Therefore, the deoxidation time is preferably shorter than 5 minutes. More preferably, the deoxidation time is 4 minutes or shorter. The “deoxidation time” referred to herein is the time required from the start of deoxidation, that is, from the addition of Al as a deoxidant, to the completion of secondary refining.

In addition, in the present embodiment, when adjusting the components except Al in the vacuum degassing treatment to achieve the component composition described above, the time required from the completion secondary refining to the start of the casting process is preferably set to be less than 3 minutes to prevent a decrease in number density due to aggregation and coarsening of MnO and Ti₂O₃.

(b) Casting Process

Next, a slab is manufactured using the molten steel whose Al concentration has been adjusted by the above-described refining process (casting process). Specifically, a slab can be manufactured using the above-described molten steel, for example, through a continuous casting method.

(c) Hot Rolling Process

Next, the manufactured slab is heated directly or after temporary cooling and hot-rolled to obtain a hot-rolled steel sheet (hot rolling process). In the present embodiment, each condition of the hot rolling process is not particularly limited, but from the viewpoint of ensuring the shape of a product sheet, the finish temperature may be set to 800° C. to 1,000° C., and the rolling reduction in the final stage of a finishing stand may be set to 10% to 80%.

(d) Coiling Process

Next, the steel sheet after hot rolling (hot-rolled steel sheet) is coiled in a temperature range of 700° C. or lower. When the coiling temperature exceeds 700° C., a relatively thick film of an oxide (oxide scale) is formed on the surface of the hot-rolled steel sheet, and this oxide is formed in a wedge shape at crystal grain boundaries in the steel. Thereafter, if the oxide scale is removed in the pickling process, the surface of the steel sheet may appear to have slight cracks, resulting in the appearance of numerous irregularities on the surface of the steel sheet after the annealing process, which may degrade the surface properties (surface roughness Ra) of a final steel sheet. Therefore, the coiling temperature is set to 700° C. or lower and preferably 680° C. or lower. On the other hand, by controlling the coiling temperature in a range that is not excessively low, the strength of the hot-rolled sheet can be prevented from becoming excessively high and an increase in cold rolling load can be controlled, thereby increasing productivity. Therefore, the coiling temperature is preferably 500° C. or higher.

(e) Pickling Process

The hot-rolled steel sheet after the coiling process is pickled (pickling process). There are no particular restrictions on the conditions of the pickling process. For example, pickling ma be done once or divided into several times if necessary.

(f) Cold Rolling Process

The hot-rolled steel sheet after the pickling process is subjected to cold rolling at a rolling reduction of 30% to 90% to manufacture a cold-rolled steel sheet (cold rolling process). If the rolling reduction is less than 30%, the surface roughness and the sheet shape may deteriorate. On the other hand, if the rolling reduction of the cold rolling process exceeds 90%, the cold-rolling load becomes excessive and productivity deteriorates. Therefore, the rolling reduction in the cold rolling process is set to 30% to 90%. The rolling reduction is preferably 40% to 80%. There are no restrictions on the method of cold rolling, and the number of rolling passes and the rolling reduction per pass may be set as appropriate.

(g) Annealing Process (Continuous Annealing Process)

The cold-rolled steel sheet is annealed in an atmosphere with a dew point of −80° C. and −15° C. in a temperature range of 820° C. to 900° C. (continuous annealing).

The dew point in the furnace during continuous annealing is set to −80° C. to −15° C. When the dew point is lower than −80° C., a high degree of control of the in-furnace atmosphere is required, causing a decrease in manufacturability and an increase in costs. On the other hand, when the dew point is higher than −15° C., the surface layer of the steel sheet softens, resulting in a decrease in tensile strength. Preferred dew points are −70° C. to −20° C.

The heating temperature (holding temperature) in the annealing process affects the area fraction of the metallographic structure. When the heating temperature is less than 820° C., the amount of austenite during heating is low and the total area fraction of ferrite, bainite, and pearlite after annealing is high, making it difficult to achieve a high tensile strength (for example, a tensile strength of 1,300 MPa or more). On the other hand, when the heating temperature exceeds 900° C., the surface properties (surface roughness Ra) deteriorate due to progress of shape changes caused by recesses, called thermal grooves, occurring at crystal grain boundaries while high temperatures are maintained. Therefore, the heating temperature in continuous annealing is set to 820° C. to 900° C. The heating temperature is preferably 830° C. to 880° C.

Although the holding time (stopping time) during continuous annealing is not particularly limited, from the viewpoints of increasing the strength and ensuring a sufficient area fraction of martensite and tempered martensite after annealing, the holding time is preferably 10 seconds or longer and more preferably 100 seconds or longer.

In the annealing process of the present embodiment, a coating film layer forming process may be performed to form a coating film layer (for example, a plating layer or an alloyed plating layer) containing at least one of zinc, aluminum, magnesium, and their alloys on a single surface or both surfaces (front surface and/or rear surface) of the cold-rolled steel sheet. In addition, the coating film layer may be formed through a method such as electroplating after the annealing process.

(Cooling Rate After Annealing Process)

In cooling after the above-described annealing process, it is preferable to cool from 750° C. to 550° C. or lower at an average cooling rate of 100° C./s or slower. The lower limit value 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 value of the average cooling rate at 2.5° C./s is to suppress the occurrence of ferrite transformation from austenite and softening of a 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 faster, still more preferably 10° C./s or faster, and still more preferably 20° C./s or faster. The cooling rate is not limited because ferrite transformation does not occur significantly at temperatures above 750° C. In addition, the cooling rate is not limited at temperatures below 550° C. because a low-temperature transformation structure is obtained. The average cooling rate fro 750° C. to 550° C. or lower is preferably 100° C./s or slower, more preferably 50° C./s or slower, and still more preferably 20° C./s or slower. In the present embodiment, as described above, the temperature range where speed should be controlled in cooling after the annealing process is at least from 750° C. to 550° C. The cooling may be performed at an average cooling rate of 100° C./s or slower even in a range other than the temperature range.

(Cooling Stop Temperature and Reheating After Annealing Process)

In addition, after the above-described cooling, cooling may be further performed at a temperature of 25° C. or higher and lower than 550° C., and the cooling may be stopped, followed by reheating to a temperature range between 150° C. and 550° C. for retention. Martensite is formed from untransformed austenite during cooling when cooling is performed in the temperature (cooling stop temperature) range described above. Subsequent reheating will temper the martensite and improve the strength-ductility balance of the steel sheet. The lower limit of the cooling stop temperature is set to 25° C. because excessive cooling not only requires significant capital investment but also saturates the effect.

(Retention Temperature)

As described above, after the cooling is stopped, the steel sheet may be reheated to 150° C. to 550° C. and then allowed to remain within that temperature range. The temperature (retention temperature) of this reheating may further be 350° C. to 55° C. Retention in this temperature range contributes to tempering of martensite. If the cooling stop temperature is 150° C. to 550° C., the retention may be performed as it is without reheating.

(Retention Time)

The duration of retention in the temperature range of 150° C. to 550° C. may be 30 seconds to 500 seconds to achieve such an effect, and is desirably 30 seconds to 300 seconds.

(Tempering)

In a series of annealing processes, the steel sheet may be allowed to remain at the retention temperature and further cooled to room temperature, or reheating may be started in the middle of cooling to room temperature (but below Ms) and the steel sheet may be held in the temperature range of 150° C. to 400° C. for 2 seconds or longer (tempering process). According to this tempering process, the martensite formed during cooling after reheating can be tempered to become tempered martensite, which further improves the strength-ductility balance. When the tempering process is performed, a holding temperature of 150° C. or higher and holding time of 2 seconds or longer will sufficiently temper the martensite and cause changes in microstructure and mechanical properties. On the other hand, a holding temperature of 400° C. or lower suppresses the decrease in dislocation density in tempered martensite and increases the tensile strength. Therefore, when tempering, it is preferable to hold the steel sheet in the temperature range of 150° C. to 400° C. for 2 seconds or longer. Tempering may be performed in a continuous annealing facility or offline in a separate facility after continuous annealing. At this time, the tempering time varies depending on the tempering temperature. In other words, the lower the temperature, the longer the time, and the higher the temperature, the shorter the time.

(Plating)

Hot-dip galvanizing may be performed on the steel sheet as necessary. In that case, hot-dip galvanizing may be performed through heating or cooling to (galvanizing bath temperature −40)° C. to (galvanizing bath temperature +50)° C. before or after the process of retention (that is, the reheating). The hot-dip galvanizing process forms a hot-dip galvanized layer on the surface of the steel sheet. This is preferable because it improves the corrosion resistance of a cold-rolled steel sheet. In the present embodiment, the type of plating layer is not limited to hot-dip galvanized layer, and various types of coating film layers can be employed. In addition, the timing of performing plating on the surface of the steel sheet is not particularly limited. For example, in the manufacturing method according to the present embodiments, a coating film layer consisting of zinc, aluminum, magnesium, or alloys thereof may be formed on the front and rear surfaces of the sheet in the process of cooling to room temperature after being held in an austenite single phase region in annealing. Alternatively, the coating film layer may be formed on the front and rear surfaces of the annealed sheet.

(Temperature of Steel Sheet When Immersed in Plating Bath)

The temperature range of the steel sheet when immersed in a hot-dip galvanizing bath is preferably in a temperature range from 40° C. below the hot-dip galvanizing bath temperature (hot-dip galvanizing bath temperature −40° C.) to 50° C. above the hot-dip galvanizing bath temperature (hot-dip galvanizing bath temperature +50° C. ). If this temperature is higher than or equal to the hot-dip galvanizing bath temperature −40° C., heat release during immersion in the plating bath is not too large, and partial solidification of hot-dip zinc is suppressed, thereby preventing degradation of plating appearance. If the sheet temperature before immersion is below the hot-dip galvanizing bath temperature −40° C., the sheet may be further heated before immersion in the plating bath through an arbitrary method to control the sheet temperature to be higher than or equal to the hot-dip galvanizing bath temperature −40° C. before immersion in the plating bath. In addition, if the steel sheet temperature at the time of immersion in the plating bath is lower than or equal to the hot-dip galvanizing bath temperature +50° C., operational problems associated with the increase in the plating bath temperature can be suppressed.

(Composition of Plating Bath)

It is preferable that the composition of the plating bath be mainly Zn with an effective Al content (value obtained by subtracting the total Fe content from the total Al content in the plating bath) of 0.050 to 0.250 mass %. An effective Al content of 0.050 mass % or more in the plating bath suppresses intrusion of Fe into the plating layer and enhances plating adhesion. ON the other hand, when the effective Al content in the plating bath is 0.250 mass % or less, formation of Al-based oxides that inhibit the migration of Fe and Zn atoms at a boundary between the steel sheet and the plating layer can be suppressed and plating adhesion is enhanced. The effective Al content in the plating bath is preferably 0.065 mass % or more and more preferably 0.180 mass % or less.

(Temperature of Steel Sheet After Immersion in Plating Bath)

When performing alloying treatment on the hot-dip galvanized layer, the steel sheet with the hot-dip galvanized layer formed is preferably heated to a temperature range 9alloying temperature) of 450° C. to 600° C. If the alloying temperature is 450° C. or higher, alloying is sufficiently advanced. On the other hand, an alloying temperature of 600° C. or lower prevents alloying from progressing too far, suppressing the formation of a Γ phase and increasing the Fe concentration in the plating layer (for example, exceeding 15%), thereby enhancing corrosion resistance. The alloying temperature is more preferably 470° C. to 550° C. The alloying temperature may be set while checking the Fe concentration in the plating layer, since it needs to be changed depending on the component composition of the steel sheet and the degree of formation of an internal oxide layer.

(Pre-Treatment)

To further improve plating adhesion, the base steel sheet may be plated with Ni, Cu, Co, or Fe, alone or in combination prior to annealing or the like in a continuous hot-dip galvanizing line.

(Post-Treatment)

The surface of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet can also be subjected to top-layer plating or various treatments, such as chromate treatment, phosphate treatment, lubricity improvement treatment, and weldability improvement treatment, to improve coating properties and weldability.

(Skin-Pass Rolling)

Furthermore, skin-pass rolling may be performed to improve ductility by correcting the steel sheet shape and introducing movable dislocations. The rolling reduction of skin-pass rolling after heart treatment is preferably in a range of 0.1% to 1.5%. If the rolling reduction is 0.1% or larger, a sufficient effect is obtained, and control is also easy.

If the rolling reduction is 1.5% or smaller, productivity is enhanced. Skin-pass rolling may be performed inline or offline.

EXAMPLES

Hereinafter, the present invention will be described in more detail using examples, but is not limited to these examples.

Example 1

Various slabs having the component compositions listed in Tables 1A to 1F were used a s materials, a hot rolling process at a finish temperature of 910° C. and a coiling temperature of 550° C. was performed thereon to obtain a hot-rolled steel sheet, followed by performing a cold rolling process at a cold rolling rate of 45% to obtain cold-rolled steel sheets. Next, the obtained cold-rolled steel sheets were subjected to continuous annealing under the conditions of a dew point of −35° C., a holding temperature of 860° C., a holding time of 130 seconds, an average cooling rate of 35° C./second, and a cooling stop temperature of 180° C. Thereafter, the steel sheets after the continuous annealing process were then reheated to 260° C. and allowed to stay for 140 seconds to obtain steel sheets. Furthermore, the obtained steel sheets were skin-pass rolled at a rolling rate of 0.2% to produce final steel sheets (thickness: 1.4 mm). Pickling was performed between the coiling process and cold rolling process. In addition, the deoxidation time in a refining process was set to 4 minutes, and the time required from the completion of secondary refining to the start of casting was set to 2 minutes.

In Tables 1A to 1F, “-” means that the corresponding elemental content is 0% in significant figures (numerical values to the smallest digit) specified in the present embodiment. The units of components of each slab are mass %, and the remained was iron and impurities. In Table 1A to 1F and 2A to 2C, values outside the scope of the present invention are underlined.

The “molten steel Al” columns in Table 1A to 1F are the Al concentration (mass %) of the molten steel in the refining process.

A metallographic structure (ferrite, pearlite, bainite, retained austenite (retained γ), martensite (fresh martensite), and tempered martensite) in an area of ⅛ to ⅜ thickness centered at a ¼ position of the sheet thickness from the surface of each of these steel sheets (¼ sheet thickness section), the maximum diameter (μm) of MnS, the surface roughness Ra, and Vickers hardness (HV) of a surface layer were evaluated. The evaluation results are shown in Table 2A to 2C. These evaluations were conducted according to the methods described above.

Furthermore, the yield stress (YS), tensile strength (TS), elongation (t-El), and hole expansion rate (λ) of these steel sheets were evaluated and are listed in Table 2B. Evaluation methods thereof are as follows.

The yield stress (YS) and tensile strength (TS) of each steel sheet were evaluated by collecting a JIS No. 5 test piece from each steel sheet so that the longitudinal direction is perpendicular to the rolling direction of each steel sheet, and conducting a tensile test in accordance with JIS Z 2241:2011. Steel sheets with a tensile strength (TS) of 1,300 MPa or more were determined to be acceptable with respect to tensile strength. In addition, steel sheets with a tensile strength (TS) of 1,470 MPa or more were determined to be superior with respect to tensile strength.

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

The hole expansion rate (λ) was measured in accordance with JIS Z 2256:2010 using a No. 5 test piece of JIS Z 2241:2011. A hole expansion test piece was taken at ¼ portion from an end portion of each steel sheet in the sheet width direction.

The presence or absence of irregularities on the surface of the steel sheets after cold molding (in the tables, “surface irregularities after 5% prestraining”) was evaluated as follows. The surface roughness Ra was measured on the surface of steel sheets subjected to 5%±1% prestraining through the method described above in accordance with JIS B 0601:2013. Those with a surface roughness Ra of 5.0 μm or less were considered acceptable (OK), and those with a surface roughness Ra of greater than 5.0 μm were considered unacceptable (NG).

	TABLE 1A
	Component composition (mass %), Remainder: Fe and impurities
No.	C	Si	Mn	P	S	Al	N	Co	Ni	Mo	Cr	O	Ti	B	Nb
A	0.20	0.94	1.21	0.0012	0.0015	0.083	0.0130	—	—	—	—	—	0.0244	0.0033	—
B	0.22	0.43	2.26	0.0146	0.0016	0.008	0.0010	—	—	—	—	—	—	—	—
C	0.29	0.64	2.54	0.0028	0.0023	0.011	0.0160	—	—	—	—	—	—	—	—
D	0.43	0.17	1.03	0.0114	0.0018	0.007	0.0030	—	—	—	—	—	—	—	—
E	0.40	0.88	1.68	0.0141	0.0010	0.011	0.0060	—	—	—	—	—	—	—	—
F	0.49	0.71	1.55	0.0032	0.0014	0.014	0.0010	—	—	—	—	—	—	—	—
G	0.38	0.15	1.42	0.0021	0.0030	0.028	0.0030	—	—	—	—	—	—	—	—
H	0.31	0.64	2.65	0.0022	0.0166	0.069	0.0010	—	—	—	—	—	—	—	—
I	0.47	0.75	2.41	0.0009	0.0018	0.012	0.0020	—	—	—	—	—	—	—	—
J	0.42	0.56	2.46	0.0041	0.0035	0.038	0.0150	—	—	—	—	—	—	—	—
K	0.44	0.29	2.17	0.0017	0.0019	0.017	0.0020	—	—	—	—	—	—	—	—
L	0.46	0.55	2.18	0.0019	0.0013	0.005	0.0020	0.148	—	—	0.238	—	—	—	0.261
M	0.19	0.68	1.79	0.0163	0.0034	0.078	0.0020	0.432	0.100	—	—	0.006	0.1010	0.0008	—
N	0.25	0.04	1.10	0.0041	0.0157	0.008	0.0030	—	—	—	1.099	0.002	—	0.0004	0.074
O	0.29	0.31	1.71	0.0010	0.0041	0.008	0.0020	0.040	0.300	—	—	—	—	0.0010	—
P	0.28	0.91	2.00	0.0023	0.0065	0.018	0.0030	0.038	—	0.041	0.183	0.015	—	—	0.415
Q	0.40	0.83	1.44	0.0015	0.0015	0.009	0.0080	—	—	—	—	0.001	0.2130	0.0007	—
R	0.17	0.52	1.74	0.0013	0.0086	0.019	0.0010	—	0.106	—	—	—	0.0650	—	—
S	0.35	0.75	2.74	0.0019	0.0018	0.012	0.0020	0.031	—	—	—	0.016	—	0.0012	—
T	0.31	0.38	2.70	0.0084	0.0019	0.011	0.0040	0.036	—	—	1.731	—	—	0.0010	—
* The underlines mean that the results are outside the scope of the present invention.

	TABLE 1B
	Component composition (mass %), Remainder: Fe and impurities	Molten
No.	V	Cu	W	Ta	Sn	Sb	As	Mg	Ca	Zr	REM	steel Al	Remarks
A	—	—	—	—	—	—	—	—	—	—	—	0.0030	Invented
													steel
B	—	—	—	—	—	—	—	—	—	—	—	0.0030	Invented
													steel
C	—	—	—	—	—	—	—	—	—	—	—	0.0160	Invented
													steel
D	—	—	—	—	—	—	—	—	—	—	—	0.0120	Invented
													steel
E	—	—	—	—	—	—	—	—	—	—	—	0.0440	Invented
													steel
F	—	—	—	—	—	—	—	—	—	—	—	0.0030	Invented
													steel
G	—	—	—	—	—	—	—	—	—	—	—	0.0050	Invented
													steel
H	—	—	—	—	—	—	—	—	—	—	—	0.0050	Invented
													steel
I	—	—	—	—	—	—	—	—	—	—	—	0.0070	Invented
													steel
J	—	—	—	—	—	—	—	—	—	—	—	0.0050	Invented
													steel
K	—	—	—	—	—	—	—	—	—	—	—	0.0380	Invented
													steel
L	—	—	—	0.0070	—	—	—	0.0153	—	—	0.0050	0.0040	Invented
													steel
M	—	—	—	0.0090	0.0200	—	0.0030	—	0.0040	—	—	0.0350	Invented
													steel
N	—	0.063	—	0.0110	0.0040	0.0150	—	—	—	0.0200	0.0140	0.0030	Invented
													steel
O	0.078	0.051	0.0730	—	—	0.0070	—	0.066	—	—	—	0.0030	Invented
													steel
P	0.056	0.127	—	0.0120	—	—	0.0190	—	—	0.0100	0.0050	0.0050	Invented
													steel
Q	—	—	0.0100	0.0080	0.0080	0.0050	—	—	—	0.0140	—	0.0080	Invented
													steel
R	—	0.038	0.0860	—	0.0030	0.0020	—	—	—	—	0.0080	0.0060	Invented
													steel
S	—	—	—	—	—	0.0030	—	—	—	0.0040	0.0740	0.0030	Invented
													steel
T	—	0.051	0.0410	—	—	—	0.0070	—	0.0350	—	0.0380	0.0420	Invented
													steel
* The underlines mean that the results are outside the scope of the present invention.

	TABLE 1C
	Component composition (mass(%), Remainder: Fe and impurities
No.	C	Si	Mn	P	S	Al	N	Co	Ni	Mo	Cr	O	Ti	B	Nb
U	0.39	0.95	1.87	0.0017	0.0110	0.010	0.0020	—	—	—	—	—	—	—	0.043
V	0.37	0.31	2.85	0.0062	0.0131	0.080	0.0030	0.080	0.064	0.090	0.135	—	0.0280	0.0009	—
W	0.16	0.10	1.28	0.0012	0.0016	0.058	0.0020	—	—	0.826	0.179	—	0.4170	—	—
X	0.21	0.49	2.32	0.0160	0.0015	0.005	0.0010	—	—	0.117	—	0.017	0.660	—	—
Y	0.34	0.07	1.28	0.0018	0.0010	0.009	0.0170	—	0.062	—	—	—	0.4040	—	—
Z	0.24	0.23	2.13	0.0018	0.0017	0.006	0.0010	0.377	—	—	—	—	0.0450	—	0.191
AA	0.24	0.19	1.92	0.0021	0.0154	0.022	0.0010	—	0.053	—	—	—	—	—	0.352
AB	0.35	0.41	2.71	0.0014	0.0013	0.007	0.0010	—	0.695	0.388	0.157	—	—	—	—
AC	0.26	0.45	1.59	0.0009	0.0026	0.009	0.0020	—	—	—	0.184	—	—	—	—
AD	0.46	0.81	2.87	0.0013	0.0020	0.009	0.0110	—	—	0.664	—	0.001	—	—	0.394
AE	0.14	0.22	1.69	0.0008	0.0022	0.016	0.0010	—	0.822	0.784	0.117	—	0.3860	—	—
AF	0.51	0.74	2.46	0.0010	0.0166	0.012	0.0050	0.038	—	0.097	0.183	0.002	—	0.0007	—
AG	0.17	1.03	2.11	0.0072	0.0022	0.023	0.0020	0.045	0.728	—	—	—	—	—	0.058
AH	0.38	0.09	0.93	0.0022	0.0022	0.005	0.0020	—	—	—	—	0.007	—	—	—
AI	0.30	0.49	3.05	0.0022	0.0021	0.008	0.0010	—	—	—	—	—	0.0480	0.0009	—
AJ	0.28	0.20	2.67	0.0207	0.0018	0.008	0.0030	—	—	0.598	—	0.001	—	—	—
AK	0.32	0.18	2.96	0.0013	0.0206	0.007	0.0150	—	0.576	—	—	—	—	—	0.177
AL	0.46	0.16	1.21	0.0033	0.0011	0.103	0.0020	—	—	—	0.091	—	—	—	—
AM	0.27	0.40	1.37	0.0012	0.0162	0.034	0.0210	—	0.116	—	—	—	—	0.0005	0.037
AN	0.18	0.74	1.47	0.0011	0.0040	0.005	0.0040	0.519	—	—	0.184	—	—	—	—
* The underlines mean that the results are outside the scope of the present invention.

	TABLE 1D
	Component composition (mass %), Remainder: Fe and impurities	Molten
No.	V	Cu	W	Ta	Sn	Sb	As	Mg	Ca	Zr	REM	steel Al	Remarks
U	0.046	—	—	0.0050	0.0040	0.0050	0.0030	0.0034	0.0050	—	—	0.0030	Invented
													steel
V	0.026	—	—	0.0060	—	0.0210	—	0.0427	0.0410	0.0330	0.0090	0.0050	Invented
													steel
W	—	0.049	—	—	0.0060	—	—	—	—	—	—	0.0410	Invented
													steel
X	—	—	—	—	—	—	—	—	0.0040	0.0040	—	0.0290	Invented
													steel
Y	0.102	0.050	0.0090	0.0100	—	—	0.0020	—	—	—	—	0.0080	Invented
													steel
Z	—	—	—	0.0660	—	0.0070	—	—	—	—	—	0.0040	Invented
													steel
AA	—	—	0.0090	0.0820	—	—	—	—	—	—	—	0.0040	Invented
													steel
AB	—	—	—	—	0.0380	—	0.0040	0.0077	—	—	—	0.0040	Invented
													steel
AC	—	—	—	—	0.0090	—	0.0030	0.0098	0.0030	—	0.0040	0.0210	Invented
													steel
AD	0.066	0.059	—	—	0.0020	—	—	—	0.0030	—	—	0.0050	Invented
													steel
AE	0.044	—	0.0320	—	—	—	—	0.0046	0.0040	0.0040	—	0.0030	Comparative
													steel
AF	0.098	0.391	0.0140	—	—	0.0280	0.0070	0.0124	0.0030	—	—	0.0050	Comparative
													steel
AG	—	—	—	—	—	—	0.0030	—	0.0050	0.0020	—	0.0040	Comparative
													steel
AH	—	0.037	—	—	—	—	—	—	—	—	—	0.0060	Comparative
													steel
AI	—	—	0.0730	—	—	—	—	—	0.0060	0.0050	—	0.0110	Comparative
													steel
AJ	0.038	—	—	—	0.0160	—	—	—	—	—	0.0190	0.0420	Comparative
													steel
AK	—	—	—	—	—	0.0060	—	—	0.0090	—	0.0040	0.0050	Comparative
													steel
AL	—	—	—	0.0720	—	—	0.0030	0.0055	—	0.0050	0.0110	0.0030	Comparative
													steel
AM	—	0.045	0.0090	—	0.0030	—	—	—	0.0410	—	—	0.0030	Comparative
													steel
AN	—	—	0.0090	—	0.0030	—	—	—	—	0.0140	—	0.0390	Comparative
													steel
* The underlines mean that the results are outside the scope of the present invention.

	TABLE 1E
	Component composition (mass %), Reminder: Fe and impurities
No.	C	Si	Mn	P	S	Al	N	Ca	Ni	Mo	Cr	O	Ti	B	Nb
AO	0.47	0.51	2.12	0.0154	0.0068	0.084	0.0020	0.432	1.028	0.269	1.342	0.001	0.0460	—	—
AP	0.29	0.92	2.57	0.0013	0.0145	0.011	0.0020	—	—	1.031	—	0.002	0.0340	—	0.038
AQ	0.19	0.67	2.37	0.0148	0.0016	0.008	0.0010	—	0.215	0.122	2.065	0.017	—	—	0.377
AR	0.39	0.41	1.12	0.0041	0.0011	0.005	0.0010	—	—	0.109	—	0.021	—	—	0.033
AS	0.20	0.09	1.52	0.0163	0.0019	0.050	0.0080	—	0.190	—	—	0.002	0.5140	—	—
AT	0.24	0.28	2.24	0.0009	0.0019	0.011	0.0160	—	—	—	—	0.012	—	0.0103	—
AU	0.25	0.37	1.97	0.0062	0.0056	0.008	0.0010	0.144	0.516	—	—	0.004	—	—	0.515
AV	0.35	0.17	1.79	0.0026	0.0024	0.009	0.0020	—	0.065	0.047	—	—	0.0620	—	—
AW	0.24	0.62	2.21	0.0012	0.0055	0.010	0.0010	—	—	—	0.159	—	0.0680	—	0.036
AX	0.18	0.81	2.16	0.0016	0.0016	0.085	0.0020	—	—	0.061	—	0.002	0.0480	—	—
AY	0.37	0.55	2.42	0.0019	0.0088	0.029	0.0160	0.029	—	—	0.162	—	—	0.0007	0.336
AZ	0.35	0.45	1.38	0.0017	0.0135	0.018	0.0010	0.074	—	—	—	0.001	—	0.0007	0.028
BA	0.29	0.34	1.18	0.0023	0.0010	0.011	0.0020	—	0.271	—	1.702	—	0.0410	0.0017	0.028
BB	0.31	0.68	2.43	0.0131	0.0144	0.010	0.0150	—	—	0.044	—	—	—	0.0009	—
BC	0.37	0.81	1.81	0.0012	0.0033	0.064	0.0140	0.045	—	—	0.216	—	—	0.0009	—
BD	0.31	0.48	2.07	0.0021	0.0016	0.008	0.0020	0.067	—	0.133	—	—	0.0370	—	—
BE	0.37	0.96	1.52	0.0013	0.0010	0.012	0.0130	0.023	—	0.004	—	—	0.4130	—	0.101
BF	0.38	0.12	2.18	0.0014	0.0043	0.044	0.0020	—	0.162	—	—	—	—	0.0006	—
BG	0.33	0.89	2.87	0.0083	0.0019	0.008	0.0010	0.053	0.104	—	—	—	—	0.0009	—
* The underlines mean that the results are outside the scope of the present invention.

	TABLE 1F
	Component composition (mass %), Remainder: Fe and impurities	Molten
No.	V	Cu	W	Ta	Sn	Sb	As	Mg	Ca	Zr	REM	steel Al	Remarks
AO	0.025	0.080	0.0040	0.0070	0.0070	—	—	0.0049	—	0.0050	—	0.0030	Comparative
													steel
AP	0.062	—	—	—	—	—	—	0.0049	—	—	—	0.0180	Comparative
													steel
AQ	—	0.075	0.0130	—	—	—	0.0080	0.0070	—	0.0040	—	0.0020	Comparative
													steel
AR	0.061	—	—	—	0.0380	0.0410	0.0070	—	—	—	0.0560	0.0020	Comparative
													steel
AS	—	—	—	—	0.0120	—	—	0.0049	0.0350	—	—	0.0290	Comparative
													steel
AT	—	—	0.0080	0.0750	0.0030	—	—	0.0400	0.0060	0.0030	0.0110	0.0030	Comparative
													steel
AU	—	—	—	—	—	—	—	0.0256	—	—	—	0.0040	Comparative
													steel
AV	0.520	—	0.0130	—	—	—	—	—	—	0.0030	0.0080	0.0030	Comparative
													steel
AW	—	0.518	—	—	0.0310	—	0.0410	—	—	—	—	0.0040	Comparative
													steel
AX	—	0.053	0.1020	0.0070	—	0.0040	0.0050	—	0.0030	0.0060	0.0690	0.0040	Comparative
													steel
AY	—	—	0.0140	0.1030	—	0.0080	—	—	0.0070	—	—	0.0030	Comparative
													steel
AZ	0.162	—	—	0.0220	0.0520	0.0040	—	—	—	—	—	0.0020	Comparative
													steel
BA	—	0.339	0.0120	—	—	0.0510	0.0400	—	—	0.0030	—	0.0090	Comparative
													steel
BB	—	—	—	—	—	0.0050	0.0520	—	—	0.0080	0.0790	0.0040	Comparative
													steel
BC	0.030	—	—	0.0810	—	—	—	0.0519	—	—	0.0100	0.0040	Comparative
													steel
BD	0.155	0.043	—	—	—	0.0090	—	0.0025	0.0520	—	0.0140	0.0060	Comparative
													steel
BE	0.389	—	—	—	—	—	—	—	—	0.0510	—	0.0240	Comparative
													steel
BF	0.029	0.199	0.0420	0.0100	0.0080	—	0.0180	0.0046	0.0020	—	0.1040	0.0370	Comparative
													steel
BG	—	0.034	—	0.0110	—	0.0030	0.0380	—	—	—	—	0.0520	Comparative
													steel
* The underlines mean that the results are outside the scope of the present invention.

	TABLE 2A
	Area fraction (%)
		Sum of Ferrite,
	Steel	pearlite, and	Retained	Tempered	Fresh	YS	TS	#Z,899
No.	No.	bainite	γ	martensite	martensite	(MPa)	(MPa)	(%)
A-1	A	3.3	0.0	89.4	7.3	920	1349	11.8
B-1	B	0.9	0.1	86.8	1 .2	10 9	14 9	10.7
C-1	C	0.1	0.2	80.2	19.5	12 5	1658	10.2
D-1	D	3.9	0.6	73.1	22.4	1486	2015	9.4
E-1	E	0.7	0.6	73.6	23.1	421		9.9
F-1	F	0.3	1.4	60.2	38.			9.7
G-1	G	3.7	0.4	7 .9	2 .6	1275	18	9.3
H-1	H	0.5	0.3	76.7	72.	1292	1711	10.2
I-1	I	0.8	1.9	50.9	46.4	1847	2138	10.0
J-1	J	0.3	1.1	1.7	6.9	1629	2 6	9.8
K-1	K	0.5	1.	61.6	36.8	1721	2096	9.4
L-1	L	0.6	1.5	5.8	42.1	1950	2143	9.7
M-1	M	3.6	0.0	7.8	8.6	937	1	11.
N-1	N	3.2	0.1	86.1	1	1083	1523	10.4
O-1	O	2.4	0.1	83.4	14.1	1076	1639	10.0
P-1	P	0.	0.2	83.		1120	44	10.7
Q-1	Q	0.1	0.3	76.2	23.2	1499	2007	9.8
R-1	R	3.8	0.0	88.6	7.6	8	0	11.3
S-1	S	0.2	0.6	70.6	28.6	1379	1820	1 .3
T-1	T	0.6	0.	70.2	2	0	1	10.
			Maximum	Surface	Vickers		Surface
			diameter	roughness	hardness		irregularities
		λ	of Mos	Ra	of surface		after 5%
	No.	(%)	(μm)	(μm)	layer (HV)	HV/TS	prestraining	Remarks
	A-1	48.5	4.6	1.4	375	0.28	OK	Invention
								example
	B-1	52.8	3.2	3.0	474	0.32	OK	Invention
								example
	C-1	41.6	28.8	1.4	516	0.31	OK	Invention
								example
	D-1	22.0	9.6	1.8	671	0.33	OK	Invention
								example
	E-1	23.6	5.4	1.3	571	0.29	OK	Invention
								example
	F-1	10.7	5.9	2.0	792	0.36	OK	Invention
								example
	G-1	27.5	3.3	1.6	630	0.33	OK	Invention
								example
	H-1	36.2	6.1	3.0	4 6	0.27	OK	Invention
								example
	I-1	1 .8	6.8	1.5	689	0.32	OK	Invention
								example
	J-1	22.3	7.2	2.3	543	0.2	OK	Invention
								example
	K-1	18.9	3.3	4.6	7 2	0.35	OK	Invention
								example
	L-1	10.4	5.3	1.5	759	0.35	OK	Invention
								example
	M-1	2.4	3.	.4		0.28	OK	Invention
								example
	N-1	48.5	3.4	1.		0.27	OK	Invention
								example
	O-1	36.1	3.4	4.3	46.3	0.28	OK	Invention
								example
	P-1	3.4	4.5	2.1	428	0.26	OK	Invention
								example
	Q-1	23.4	28.1	6.5	6 2	0.30	OK	Invention
								example
	R-1	.2	4.9	.5	339	0.26	OK	Invention
								example
	S-1	31.0	5.6	1.3		0.35	OK	Invention
								example
	T-1	39.1	8.4	1.	447	0.26	OK	Invention
								example
	indicates data missing or illegible when filed

	TABLE 2B
	Area fraction (%)
		Sum of Ferrite,
	Steel	pearlite, and	Retained	Tempered	Fresh	YS	TS	#Z,899
No.	No.	bainite	γ	martensite	martensite	(MPa)	(MPa)	(%)
U-1	U	0.1	0.6	73.6	26.7	1 28	1	10.0
V-1	V	0.3	0.8	65.5	33.6	1 5	1 0	.7
W-1	W	3.3	0.0		6.2	817	1308	10.9
X-1	X	0.4	0.1	87.6	11.9		1450	10.9
Y-1	Y	3.7	0.2	8 .6	15.5	1263
Z-1	Z	0.1	0.1	86.8	13.0		1 3	10.
AA-1	AA	3.3	0.1	87.9	11.7	9	1496	10.4
AB-1	AB	0.8	0.7	69.8	28.7	1478	1817	10.0
AC-1	AC	.7	0.1	84.5	11.7	1040	1547	10.
AD-1	AD	0.3	2.	45.1	31.9	1993	20 0	10.
AE-1	AE	5.5	0.	88.0	6.2	892		11.4
AF-1	AF	0.2		38.9	57.9	2239	2192	3.8
AG-1	AG	4.3	11.3	.0	7.	844	53	.5
AH-1	AH		0.3	76.8	16.3	766	1238	9.6
AI-1	AI	0.4	0.4	74.5	24.7	1163	1688	10.1
AJ-1	AJ	0.1	0.2	80.2	19.5		1629	9.8
AK-1	AK	0.9	0.4	72.4	26.	125		9.7
AL-1	AL	2.6	0.8	67.9	28.7	1702	2132	0.2
AM-1	AM	3.2	0.1	85.5	11.2			10.6
AN-1	AN	3.0		89.5	7.3	84	1 5	.6
			Maximum	Surface	Vickers		Surface
			diameter	roughness	hardness		irregularities
		λ	of Mos	Ra	of surface		after 5%
	No.	(%)	(μm)	(μm)	layer (HV)	HV/TS	prestraining	Remarks
	U-1	24.6	28.	1.	544	0.28	OK	Invention
								example
	V-1	20.6	3.8	5.0	611	0.33	OK	Invention
								example
	W-1	53.3	28.3	3.4	397	0.30	OK	Invention
								example
	X-1	46.4	3.8	1.7	371	0.26	OK	Invention
								example
	Y-1	32.2	4.0			0.32	OK	Invention
								example
	Z-1	46.1	3.9	1.4	390	0.26	OK	Invention
								example
	AA-1	43.7	3.9	1.6	425	0.28	OK	Invention
								example
	AB-1	33.4	29.1	1.4	512	0.28	OK	Invention
								example
	AC-1	40.8	6.1	1.4	539	0.35	OK	Invention
								example
	AD-1	20.3	2.8	2.4	371	0.	OK	Invention
								example
	AE-1	60.6	5.6	1.4	42	0.33	OK	Invention
								example
	AF-1	14.9	5.7	1.		0.37	NG	Comparative
								example
	AG-1	52.5	5.6	4.9	367	0.2	NG	Comparative
								example
	AH-1	27.8	4.1	3.4		0.33	NG	Comparative
								example
	AI-1	37.2	34.7	1.7	590	0.	NG	Comparative
								example
	AJ-1	38.2	5.	5.3	405	0.30	NG	Comparative
								example
	AK-1	38.4	32.6	1.5	387	0.34	NG	Comparative
								example
	AL-1	13.8	37.8	4.4	750	0.35	NG	Comparative
								example
	AM-1	37.1	29.5	5.7	531	0.34	NG	Comparative
								example
	AN-1	43.5	3.1	6.4		0.29	NG	Comparative
								example
	indicates data missing or illegible when filed

	TABLE 2C
	Area fraction (%)
		Sum of Ferrite,
	Steel	pearlite, and	Retained	Tempered	Fresh	YS	TS	#Z,899
No.	No.	bainite	γ	martensite	martensite	(MPa)	(MPa)	(%)
AO-1	AO	0.8	2.5	43.3	53.4	2249	2067	10.4
AP-1	AP	0.9	0.4	79.1	19.6	1172	1661	10.8
AQ-1	AQ	6.2	0.1	80.2	13.5	719	1208	11.
AR-1	AR	3.1	0.4	77.5	19.0	1437	1916	9.7
AS-1	AS	3.0	0.0		2	890	1346	10.6
AT-1	AT	0.5	0.1	85.9	13.5	979	1533	10.2
AU-1	AU	0.1	0.1	86.9	12.9	1047	1557	10.3
AV-1	AV	0.3	0.3	79.1	20.3	1224	1834	9.4
AW-1	AW	0.6	0.1	85.6	13.7	1061	1528	10.7
AX-1	AX	1.0	0.0	89.5	9.5	882	1380	11.1
AY-1	AY	0.8	0.6	69.9	28.7	1417	1897	9.8
AZ-1	AZ	3.9	0.3	79.0	16.8	1262	1793	10.0
BA-1	BA	0.8	0.1	84.2	14.9	1176	1656	10.1
BB-1	BB	0.2	0.3	78.7	20.8	1175	1719	10.2
BC-1	BC	0.3	0.5	75.9	23.3	1322	1899	10.1
BD-1	BD	0.7	0.2	80.9	18.2	1174	1724	9.9
BE-1	BE	0.6	0.4	78.7	20.3	1429	1905	10.1
BF-1	BF	0.3	0.5	71.9	27.3	1484	1030	0.2
BG-1	BG	1.0	0.5	71.7	26.8	1306	1769	10.4
			Maximum	Surface	Vickers		Surface
			diameter	roughness	hardness		irregularities
		λ	of Mos	Ra	of surface		after 5%
	No.	(%)	(μm)	(μm)	layer (HV)	HV/TS	prestraining	Remarks
	AO-1	20.4	6.3	5.5	724	0.35	NG	Comparative
								example
	AP-1	40.6	14.3	5.9	544	0.33	NG	Comparative
								example
	AQ-1	48.4	3.8	1.7	4 6	0.38	NG	Comparative
								example
	AR-1	25.9	9.3	5.9	528	0.28	NG	Comparative
								example
	AS-1	41.5	2.6		439	0.33	NG	Comparative
								example
	AT-1	44.8	3.8	5.7	470	0.31	NG	Comparative
								example
	AU-1	43.5	2.6	5.4	327	0.34	NG	Comparative
								example
	AV-1	32.2	6.8	5.6	495	0.2	NG	Comparative
								example
	AW-1	43.3	4.3	5.6	509	0.33	NG	Comparative
								example
	AX-1	54.8	27.4	5.5	471	0.34	NG	Comparative
								example
	AY-1	29.8	6.4	5.4	572	0.30	NG	Comparative
								example
	AZ-1	33.2	3.5	6.0	536	0.30	NG	Comparative
								example
	BA-1	36.6	26.3	5.6	581	0.35	NG	Comparative
								example
	BB-1	38.8	2.5	5.5	604	0.35	NG	Comparative
								example
	BC-1	27.3	8.8	5.5	626	0.28	NG	Comparative
								example
	BD-1	35.8	5.5	5.1	550	0.32	NG	Comparative
								example
	BE-1	2 .5	3.4	5.4	672	0.35	NG	Comparative
								example
	BF-1	26.6	2.6	5.9	556	0.29	NG	Comparative
								example
	BG-1	34.2	35.4	1.4	598	0.34	NG	Comparative
								example
	indicates data missing or illegible when filed

It was found that the invention examples satisfying both the component composition and the manufacturing conditions were within the scope of the invention in terms of all of the structure proportion of the metallographic structure and the characteristics and properties of the structure, and that a steel sheet capable of achieving both surface irregularities during molding (surface irregularities after 5% prestraining) and a high strength at a high level could be obtained.

On the other hand, in the comparative examples where the component composition did not meet the scope of the invention, at least any of the structure proportion or the characteristics of the structure fell outside the scope of the invention, resulting in degradation of any of the properties. In the comparative example No. AF-1, the volume of the steel sheet expanded rapidly during martensitic transformation caused by rapid cooling in the annealing process because the C content was too high, causing large irregularities on the surface due to relaxation of transformation plasticity.

Example 2

Next, various steel sheets (thickness: 1.4 mm) were produced using various slabs having the component compositions listed in Tables 1A to 1F as materials according to various manufacturing conditions listed 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 acceptance criteria are underlined.

The same evaluations as in Example 1 above were performed on these steel sheets.

	TABLE 3A
	Cold rolling	Continuous annealing process (cold-rolled sheet annealing process)
	process	process		Average
		Finish	Coiling	Rolling
Example	Steel	temperature	temperature	reduction		temperature	time	rate
No.	No.	(° C.)	(° C.)	(° C.)	(° C.)	(° C.)	(second)	(° C.)
A-2	A	8	459	67	−46	850	49	6
B-2	B	6.3	57	47	−3	87	83	92
C-2	C	93.2	531	42	−36	83.	82	55
D-2	D	94.3	467	79	−45	830	235
E-2	E	94.9	571	5	−53		193	70
F-2	F	901	708	69	−40	8	84	34
G-2	G	910	48	61	−62	892	93	38
H-2	H	924		53	−75	864	212	93
I-2	I	88	558	81	−39	856	48	35
J-2	J	928	492	57	−44	905	58	40
K-2	K	840	526	82	−67	869	258
L-2	L	902	586	51	−56	840	36	65
M-2	M	927		42	−	854	32	59
N-2	N	03	620	72	−42	835	21	33
O-2	O	873	481	58	−47	893	20	41
P-2	P	818	578	45	−73	881	153	84
Q-2	Q	865	5	81	−72	866	84
R-2	R	936	41	87	−43	889	223	44
S-2	S	930	10	30	−79	829	18	5
T-2	T	923	732	86	− 1	891	106	3
U-2	U	91.3	42	70	−44	8	14	89
V-2	V		4	40	−77	870	79	66
W-2	W	845	468	57	−29	831	262	45
X-2	X	48	416	61	−63	835	125	71
Y-2	Y	899	94		−35	864	71	38
Z-2	Z	19		67	−71	872	82	25
AA-2	AA	29	545	80	−74	850	248	60
AB-2	AB		525	73	−65	857	72	65
AC-2	AC	906	51.5	31	−41	88	265	33
AD-2	AD	576	411		−	86.3	200	13
	Continuous annealing process (cold-rolled sheet annealing process)
						Temperature	Holding
						of steel	temperature
		Cooling				sheet during	after
		stop	Retention	Retention	Tempering	immersion	immersion	rolling
	Example	temperature	temperature	time	temperature	in plating	in plating	reduction
	No.	(° C.)	(° C.)	(sec)	(° C.)	bath (° C.)	bath (° C.)	( )
	A-2	222	204	56	237	—	—	.8
	B-2	34	229	118	—	479	513	.0
	C-2	153	530	54	363	469	333	1.3
	D-2	159	207	241	—	—	—	1
	E-2	43	247	74	—	470	476	1.2
	F-2	2	391	178	278	461	466	1.4
	G-2	63	485	231	—	482	469	0.6
	H-2	3	533	95	—	454	465	1.4
	I-2	302	489	117	387	—	—	0.
	J-2	318	28	46	234	472	481	0.
	K-2	98	323	181	—	469	474
	L-2	170	4	379	—	477	4
	M-2	134	478	91	—	436	530	0.2
	N-2	276	424	237	—	479	517	0.8
	O-2	230	387	34	—	445	5 6	0.5
	P-2	399	462	1 0	—	—	—	0.
	Q-2	430	381	57	—	—	—	0.
	R-2	454	465	68	—	—	—	0.3
	S-2	59		148	326	4 4	482	1.0
	T-2	87	324	292	275	4 9	4	1.4
	U-2	198	365	84	—	453	464	0.2
	V-2	237	206	167	—	448	44	1.0
	W-2	134	448	61	—	—	—	1.0
	X-2	1	426		—	459	510	1.4
	Y-2	419	393	104	308	436	483	0.7
	Z-2	135	4	128	—	—	—	0.7
	AA-2	244	231	139	—	—	—	0.9
	AB-2	248	507	163	—	450	5 7	0.8
	AC-2	316			—	4 6	480	0.5
	AD-2	287	450	201	—	—	—	0.
	indicates data missing or illegible when filed

	TABLE 3B
	Cold rolling	Continuous annealing process (cold-rolled sheet annealing process)
	process	process		Average
		Finish	Coiling	Rolling
Example	Steel	temperature	temperature	reduction		temperature	time	rate
No.	No.	(° C.)	(° C.)	(° C.)	(° C.)	(° C.)	(second)	(° C.)
A-3	A	923	603	69	−55	844	200	55
B-3	B	825		65	−59	843	47	66
C-3	C	901		38	−60	884	172	87
D-3	D	904		44	−23	846	279	35
E-3	E	953	414	31	−30	853	106	16
F-3	F	921		55	−36	846	43	80
G-3	G	817	602	45	−71	850	243	31
H-3	H	905	5 6	86	− 0	898	141	51
I-3	I	895		60	−24	890	189	61
J-3	J	935	580	75	−44	836	31	43
K-3	K	920	435	50	−23	883	37	12
L-3	L	914	631	53	−73	865	63	90
M-3	M	904	492	73	−70	850	81	46
N-3	N	931	548	70	−19	816	165	64
O-3	O	815	654	34	−48	897	164	32
P-3	P	851	553	40	−53	836	1	81
Q-3	Q	939	653	56	−52	846	250	93
R-3	R	874	6 3	78	−36	884	216	60
S-3	S	889	578	883	−59	865	213	11
T-3	T	863	599	46	−80	853	217	69
U-3	U	894	588	48	−80	851	149	76
V-3	V	843	574	68	−33	837	84	32
W-3	W	834	617	65	−70	843	47	39
X-3	X	936	450	47	−33	835	24	63
Y-3	Y	824	48	36	−65	848	50	33
Z-3	Z	885	456	67	−45	874	120	26
AA-3	AA	88	536	88	−38	846	271	45
AB-3	AB	921	648	59	−50	833	74	40
AC-3	AC		53	53	−37	855	210	33
AD-3	AD		491	41	−47	864	244	89
	Continuous annealing process (cold-rolled sheet annealing process)
						Temperature	Holding
						of steel	temperature
		Cooling				sheet during	after
		stop	Retention	Retention	Tempering	immersion	immersion	rolling
	Example	temperature	temperature	time	temperature	in plating	in plating	reduction
	No.	(° C.)	(° C.)	(sec)	(° C.)	bath (° C.)	bath (° C.)	( )
	A-3	339	40	138	—	—	—	0.
	B-3	255	220	50	—	466	454	1.4
	C-3	236	252	58	155	—	—	1.4
	D-3	341	493	178	—	—	—	0.9
	E-3	360	356	54	—	846	456	0.8
	F-3	253	401	34	208	444	465	1.3
	G-3	187	375	195	—	449	447	1.3
	H-3	126	239	288	—	—	—	0.2
	I-3	308	399	33	—	463	502	1.0
	J-3	345	467	162	—	462	460	0.8
	K-3	69	268	38	—	—	—	0.
	L-3	80	528	127	—	468	463	1.3
	M-3	72	454	211	—	444	300	0.4
	N-3	96	233	2	—	439	462	0.3
	O-3	216	349	126	—	—	—	1.0
	P-3	116	471	98	335	456		0.3
	Q-3	125	480	36	—	—	—	0.2
	R-3	178	317	244	196	—	—	0.2
	S-3	410	88	55	—	—	—	0.
	T-3	233	471	227	—	445	305	0.3
	U-3	2 3	340	135	342	—	—	0.5
	V-3	3 3	470	233	—	—	—	0.
	W-3	262	224	64	—	464	486	1.3
	X-3	230	223	9	—	—	—	1.1
	Y-3	379	449	0	—	478	446	0.3
	Z-3	377	261	130	220	—	—	0.2
	AA-3	174	297	126	351	—	—	1.3
	AB-3	150	540	141	—	467	461	0.5
	AC-3	338	318	251	—	450	455	0.9
	AD-3	188	313	216	—	463	323	0.2
	indicates data missing or illegible when filed

	TABLE 3C
	Area fraction (%)
	Sum of Ferrite,
Example	pearlite, and	Retained	Tempered	Fresh	YS	TS		λ
No.	bainite	γ	martensite	martensite	(MPa)	(MPa)	(%)	(%)
A-2	4.3	0.0	25.7	0.0	953	1 88	11.	47.4
B-2	0.5	0.1	96.9	2.5	972	14	10.7	49.1
C-2	0.2	0.2	99.	0.0	1	1	10.2	42.1
D-2	0.0	0.5	81.0	18.5	1435	212	8.9	17.3
E-2	0.4	0.6	93.4	5.6	12	2	9.8	21.9
F-2	0.8	1.4	7.8	0.0	1329	2 33	9.3	5.9
G-2	.3	0.4	0.6	5.7	1338	1	9.4	76.2
H-2	0.6	0.3	94.2	4.5	12 0	1730	10.1	37.7
I-2	0.4	1.9	97.3	0.0	15 3	22	9.6	12.4
J-2	0.2	1.1	98.7	0.0		2	9.5	18.6
K-2	0.9	1.1	83.1	14.9		2132	9.3	17.9
L-2	0.	1.5	0.1	37.7	1872	2 9	9.9	20.3
M-2	0.3	0.0	94.4	5.3	932	140	10.9	47.3
N-2	3.5	0.1	66.1	30.3	1175	14	10.	43.6
O-2	1.0	0.1	74.0	24.8	1258	1	10.0	44.4
P-2	0.3	0.	50.7	43.8	1038	164	10.7	39.9
Q-2	0.5	0.5	31.7	67.3	1368	2023	9.7
R-2	4.3	0.		10.	897	1317	11.2	48.9
S-2	0.3	0.8	99.2	0.0	213	1856	10.0	31.3
T-2	0.3	0.3	98.8	0.0	07	1728	10.1	37.9
U-2	0.3	0.6	67.6	31.1	130	1927	10.7	26.4
V-2	0.3	0.8	36.3	62.7	1904	1831	9.9	34.3
W-2	0.9	0.0	95.2	3.9	05	1324	10.8	34.4
X-2	1.0	0.1	83.3	13.6	956	1444	10.9	48.4
Y-2	4.9	0.2	94.9	0.0	1195	1757	9.6	2 .5
Z-2	4.9	0.1	87.3	7.5	252	1878	10.5	4 .6
AA-2	0.6	0.1	75.1	24.7	1130	1532	10.1	49.6
AB-2	1.0	0.7	37.9	60.4	1845	1538	11.2	49.5
AC-2	4.8	0.	43.4	31.	1373	1437	12.1	4 .6
AD-2	0.	2.8	22.5	74.5	1404	223	10.1	11.4
		Maximum	Surface	Vickers		Surface
		diameter	roughness	hardness		irregularities
	Example	of Mos	Ra	of surface		after 5%
	No.	(μm)	(μm)	layer (HV)	HV/TS	prestraining	Remarks
	A-2	3 2	1.5	448	0.3	OK	Invention
							example
	B-2		2.0	466	0.3	OK	Invention
							example
	C-2	29	1.5	448	0.27	OK	Invention
							example
	D-2	2.	1.6	699	0.33	OK	Invention
							example
	E-2		2.0	571	0.28	OK	Invention
							example
	F-2	28.8	5.4	630	0.28	NG	Compar-
							ative
							example
	G-2	2.8	1.5	580	0.30	OK	Invention
							example
	H-2	28.8	1.	48	0.28	OK	Invention
							example
	I-2	.6	4.9		0.26	OK	Invention
							example
	J-2	29.0	5.1		0.30	NG	Comparative
							example
	K-2	28.7	2.1		0.28	OK	Invention
							example
	L-2	29.3	4	74	0.36	OK	Invention
							example
	M-2	6.9	4.5	95	0.28	OK	Invention
							example
	N-2	8.3	1.9	450	0.31	OK	Invention
							example
	O-2	11.4	1.6	568	0.35	OK	Invention
							example
	P-2	2	1.9	486	0.30	OK	Invention
							example
	Q-2	2.6	1.4	604	0.30	OK	Invention
							example
	R-2	2.1	4.8	384	0.29	OK	Invention
							example
	S-2	4.3	1.8	470	0.23	OK	Invention
							example
	T-2	.3	2.	582	0.34	NG	Compar-
							ative
							example
	U-2	.3	1.5	583	0.30	OK	Invention
							example
	V-2	28.4	4.5	603	0.35	OK	Invention
							example
	W-2		4.4	433	0.33	OK	Invention
							example
	X-2	26.7	1.7	442	0.31	OK	Invention
							example
	Y-2	.1	1.9	451	0.26	OK	Invention
							example
	Z-2	8.5	1.3	4	0.29	OK	Invention
							example
	AA-2	3.1	1.4	416	0.27	OK	Invention
							example
	AB-2	3.4	1.6	585	0.38	OK	Invention
							example
	AC-2	76.8	1 7	441	0.30	OK	Invention
							example
	AD-2	8.1	1 4	570	0.25	OK	Invention
							example
	indicates data missing or illegible when filed

	TABLE 3D
	Area fraction (%)
	Sum of Ferrite,
Example	pearlite, and	Retained	Tempered	Fresh	YS	TS		λ
No.	bainite	γ	martensite	martensite	(MPa)	(MPa)	(%)	(%)
A-3	4.4	0.0	54.0	51.	1141	130	12.1	53.2
B-3	0.5	0.1	41.7	57.7	1	14	10.7	4.0
C-3	1.0	0.2	94.8	4.0	1355	38	10.3	.2
D-3	1.0	0.5	56.2	43.2	1	212	9.0	17.9
E-3	4.7	0.7	48.2	.4	125	1964	10.1	24.1
F-3	0.1	1.4	98.5	0.0	2265	177	13.3	33.6
G-3	4.8	0.3	7	.4	13	1858	9.6	28.3
H-3	1.0	0.3		12.3	121	1727	10.1	5.6
I-3	0.3	1.9	4	5	139	2267	9.6	10.4
J-3	0.6	1.3	47.2	51	1415	2 94	9.6	19.
K-3	0.	1.4	87.8	1	1522	21 4	9.2	15.2
L-3	0.7	1.5	83.8	14.0	1473	2191	9.5	15.3
M-3	1.2	0.0	96.1	2.7		1198	12.0	48.5
N-3	5.5	0.1	90.3	4.1	1005	1283	10.4	31.5
O-3	3.	0.1	75.4	20.7	1102	1603	10.1	42.
P-3	0.2	0.2	1.9	1.7	1121	1642	10.8	37.6
Q-3	0.2	0.5	85.	14.1	1285	1995	9.8	23.2
R-3	0.5	0.0	91.9		18	1355	11.0	39.7
S-3	0.6	0.	7.2	31.6	12 1	1852	10.1	23.5
T-3	0.9	0.5	47.	51.3	1492	1589	10.8	41.2
U-3	0.3	0.6	99.1	0.0	1773	1394	12.9	32.7
V-3	0.	0.8	66.8	31.5	1208	1925		2 .8
W-3	1.7	0.0	52.6	15.7	840		10.9	38.8
X-3	0.2	0.3	79.3	20.6		1439	10.9	48.1
Y-3	1.6	0.2	62.8	35.4	1241	1804	9.4	30.
Z-3	4.3	0.1	95.6	0.0	1173	1457	10.	41.7
AA-3	0.	0.1	99.3	0.0	1013	152	10.3	43.2
AB-3	0.3	0.6	78.4	20.	1383	1838	9.9	31.3
AC-3	4.3	0.1	30.7	44.9	1 63		11.4	44.1
AD-3	0.5	2.8	49.2	56.5	2030	1986	11.0	23.8
		Maximum	Surface	Vickers		Surface
		diameter	roughness	hardness		irregularities
	Example	of Mos	Ra	of surface		after 5%
	No.	(μm)	(μm)	layer (HV)	HV/TS	prestraining	Remarks
	A-3	11.2	1.4	403	0.31	OK	Invention
							example
	B-3	27.6	1.7	445	0.30	OK	Invention
							example
	C-3	3.0	1	487	0.30	OK	Invention
							example
	D-3	6.	1.7	494	0.33	OK	Invention
							example
	E-3	28.3	4.5	603	0.32	OK	Invention
							example
	F-3	2.8	2.3	591	0.32	OK	Invention
							example
	G-3	28.4	2.3	604	0.32	OK	Invention
							example
	H-3	3.1	2.1	489	0.28	OK	Invention
							example
	I-3	4.0	1.6	749	0.33	OK	Invention
							example
	J-3	26.0	1.6	648	0.31	OK	Invention
							example
	K-3	3.5	1.5	547	0.25	OK	Invention
							example
	L-3	4.4	1.7	628	0.29	OK	Invention
							example
	M-3	3.	4	422	0.30	OK	Invention
							example
	N-3	29.6	4.2	431	0. 4	NG	Compar-
							ative
							example
	O-3	4.7	1.7	824	0.26	OK	Invention
							example
	P-3	4.3	1.8	530	0.32	OK	Invention
							example
	Q-3		1.8	643	0.32	OK	Invention
							example
	R-3	8.0	1.9	439	0.37	OK	Invention
							example
	S-3		.4	532	0.29	OK	Invention
							example
	T-3	3.7	1 4	523	0.34	OK	Invention
							example
	U-3	3.2	4.4	574	0.	OK	Invention
							example
	V-3	3.1	1.4	495	0.26	OK	Invention
							example
	W-3	3.1	4.6	360	0.27	OK	Invention
							example
	X-3	3.3	1.4	484	0.33	OK	Invention
							example
	Y-3	6.1	1.4	526	0.79	OK	Invention
							example
	Z-3	4.0	1.5	406	0.28	OK	Invention
							example
	AA-3	4.6	1.4	398	0.2	OK	Invention
							example
	AB-3	3.	1	63	0.33	OK	Invention
							example
	AC-3	5.	1	481	0.55	OK	Invention
							example
	AD-3	28.0	4.4	779	0.39	OK	Invention
							example
	indicates data missing or illegible when filed

It was found that the invention examples satisfying both the component composition and the manufacturing conditions were within the scope of the invention in terms of all of the structure proportion of the metallographic structure and the characteristics and properties of the structure, and that a steel sheet capable of achieving both surface irregularities during molding (surface irregularities after 5% prestraining) and a high strength at a high level could be obtained.

On the other hand, in the comparative examples where either the component composition or the manufacturing conditions did not meet the scope of the invention, at least any of the structure proportion or the structural characteristics fell outside the scope of the invention, resulting in degradation of any of the properties.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to provide a steel sheet in which generation of surface irregularities during molding can be suppressed and which has a high strength.

Claims

1. A steel sheet having a component composition including, in mass %,

C: 0.15% to 0.50%,

Si: 0.01% to 1.00%,

Mn: 1.00% to 3.00%,

P: 0% to 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% to 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% 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%,

with the remainder being Fe and impurities,

wherein a metallographic structure has an area fraction of

0% to 10.0% of retained austenite and

0% to 5.0% of pearlite, ferrite, and bainite in total, with the remaining structure being martensite and tempered martensite,

wherein a maximum diameter of MnS predicted by extreme value statistics is 30.0 μm or less,

wherein a surface roughness Ra is 5.0 μm or less, and

wherein a surface layer has a Vickers hardness of greater than or equal to a tensile strength TS (MPa) of the steel sheet×0.25.

2. The steel sheet according to claim 1,

wherein the component composition includes, in mass %, one or more of

Co: 0.010% to 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% to 0.0100%,

Nb: 0.001% to 0.500%,

V: 0.001% to 0.500%,

Cu: 0.001% to 0.500%,

W: 0.0010% to 0.1000%,

Ta: 0.0010% to 0.1000%,

Sn: 0.0010% to 0.0500%,

Sb: 0.0010% to 0.0500%,

As: 0.0010% to 0.0500%,

Mg: 0.0001% to 0.0500%,

Ca: 0.0010% to 0.0500%,

Zr: 0.0010% to 0.0500%, and

REM: 0.0010% to 0.1000%.

3. The steel sheet according to claim 1,

wherein the component composition includes, in mass %,

Mn: 1.00% to 2.00% and

Si: 0.30% to 1.00%.

4. The steel sheet according to claim 1, wherein a tensile strength is 1,470 MPa or more.

5. The steel sheet according to claim 1,

wherein a coating film layer containing at least one of zinc, aluminum, magnesium, and their alloys is provided on a single surface or both surfaces of the steel sheet.

6. A method for manufacturing the steel sheet according to claim 1, comprising:

a refining process in which molten steel subjected to a vacuum degassing treatment to adjust a component composition of the molten steel to have an Al concentration of 0.0500 mass % or less and to include, in mass %,

C: 0.15% to 0.50%,

Si: 0.01% to 1.00%,

Mn: 1.00% to 3.00%,

P: 0% to 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% to 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% 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%,

with the remainder being Fe and impurities;

a casting process in which a slab is manufactured using the molten steel after the refining process;

a hot rolling process in which slab is heated directly or after temporary cooling and hot-rolled to obtain a hot-rolled steel sheet;

a coiling process in which hot-rolled steel sheet is coiled in a temperature range of 700° C. or lower;

a pickling process in which the hot-rolled steel sheet after the coiling process is pickled;

a cold rolling process in which 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; and

an annealing process in which the cold-rolled steel sheet is annealed in an atmosphere with a dew point of −80° C. and −15° C. in a temperature range of 820° C. to 900° C.

7. The method for manufacturing a steel sheet according to claim 6,

wherein, in the refining process, a deoxidation time is shorter than 5 minutes.

8. The method for manufacturing a steel sheet according to claim 6, Mn: 1.00% to 2.00% and Si: 0.30% to 1.00%.

wherein the component composition includes, in mass %,

9. The method for manufacturing a steel sheet according to claim 6, comprising:

a coating film layer forming process in which a coating film layer containing at least one of zinc, aluminum, magnesium, and their alloys is formed on a single surface or both surfaces of the cold-rolled steel sheet in the annealing process.

10. The steel sheet according to claim 2, wherein a tensile strength is 1,470 MPa or more.

11. The steel sheet according to claim 2,

wherein a coating film layer containing at least one of zinc, aluminum, magnesium, and their alloys is provided on a single surface or both surfaces of the steel sheet.

12. The method for manufacturing a steel sheet according to claim 7, comprising:

a coating film layer forming process in which a coating film layer containing at least one of zinc, aluminum, magnesium, and their alloys is formed on a single surface or both surfaces of the cold-rolled steel sheet in the annealing process.