Steel plate and its manufacturing method

Abstract

Disclosed is a steel sheet having both excellent workability and flexural strength. This steel sheet has a predetermined chemical composition and a predetermined steel composition. The difference between the number density of precipitates in tempered martensite of a first face on the front side of the steel sheet and the number density of precipitates in tempered martensite of a second face on the back side of the steel sheet is within 10.0%. The yield strength is 600 MPa or more.

Description

[Technical field]

[0001] The present application discloses a steel sheet and a method for manufacturing the same.

[0002] In recent years, efforts to reduce the weight of automobile bodies have been made by applying high-strength steel sheets in order to improve the fuel efficiency of automobiles. Also, to ensure the safety of passengers, high-strength steel sheets are increasingly being used in automobile bodies instead of mild steel sheets. In order to further reduce the weight of automobile bodies in the future, the strength level of high-strength steel sheets must be increased more than ever before.

[0003] Moreover, automobile parts are required to exhibit a function of suppressing deformation during an automobile collision. In order to increase the resistance of automobile parts to deformation during an automobile collision, it is desirable to increase the bending strength of the automobile parts. In addition, in order to increase the bending strength from a structural standpoint by optimizing the part shape, high formability is required for the steel plate. For this reason, the steel plate used in automobile parts is required to have high strength, excellent bending strength, and also exhibit high elongation. However, in the prior art, although the workability of high-strength steel plate has been studied (for example, Patent Documents 1 to 3 below), sufficient study has not been made on ensuring bending strength against bending deformation from both the front and back sides.

[0004] Patent Document 1 discloses, as a high-strength steel sheet with excellent workability, a steel sheet which contains ferrite as a main phase, an average of 5 volume% or more of retained austenite, and in which the difference ΔVγ between the maximum and minimum retained austenite content at each position in the sheet thickness direction between 0.1 mm from the front surface of the steel sheet and 0.1 mm from the back surface of the steel sheet is 3.0 volume% or less.

[0005] Patent Document 2 discloses a steel plate for ship hulls that has excellent impact absorbing ability and can minimize destruction of the hull in the event of a collision with a tanker. The steel plate has a thickness of 8 mm or more and contains C, Si, Mn, Al, and further contains strengthening elements as necessary, with the balance being Fe and unavoidable impurities, and is characterized in that the steel plate contains 1.0 to 20% residual γ by area in front and back layers that are at least 1 / 8 of the thickness of the steel plate.

[0006] Patent Document 3 discloses a thick structural steel plate that can dramatically improve both brittle crack propagation arrestability and Charpy properties at the same time without relying on the addition of expensive alloy elements such as Ni, and that is composed of, by weight, 0.04-0.30% C, ≦0.5% Si, ≦2.0% Mn, ≦0.1% Al, 0.001-0.10% Ti, 0.001-0.01% N, with the balance being Fe and unavoidable impurities, in which the average crystal grain size d of the structure in specified regions of the front and back layers of the plate thickness is 3 μm or less, and the Vickers hardness of the structure satisfies specified requirements. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 3546266 [Patent Document 2] Patent No. 3499126 [Patent Document 3] Patent No. 3845113 Summary of the Invention [Problem to be solved by the invention]

[0008] In view of the above circumstances, the present application discloses a steel sheet having excellent mechanical properties such as strength and elongation as well as excellent bending strength, and a method for manufacturing the same. [Means for solving the problem]

[0009] The inventors have conducted extensive research into methods for solving the above problems, and have found that by optimizing the ratio of the steel sheet structure, including retained austenite, and reducing the difference in number density of precipitates on the front and back surfaces, a steel sheet can be obtained that has excellent mechanical properties such as strength and elongation as well as high bending strength. Additionally, they have confirmed that in steel sheets with a difference in number density of precipitates on the front and back surfaces of more than 10%, the bending strength changes depending on the bending direction, and the deformation resistance of parts during a collision accidentally decreases.

[0010] The inventors have also discovered that an integrated manufacturing method, which is characterized by subjecting a cold-rolled sheet to two annealing processes and then coiling and uncoiling the sheet between the two annealing processes to a specified aging treatment, can produce a steel sheet with an optimized structure and a small difference in number density of precipitates between the front and back surfaces.

[0011] Furthermore, the inventors have discovered through accumulating various studies that a steel sheet having increased bending strength by reducing the difference in number density of precipitates on the front and back surfaces as described above is difficult to manufacture by simply devising a single hot rolling condition or annealing condition, etc., and can only be manufactured by achieving optimization in a so-called integrated process, including the hot rolling and annealing steps.

[0012] The gist of the present invention is as follows. (1) In mass%, C: 0.10-0.30%, Si: 0.60-1.20%, Mn: 1.00-3.50%, P:0.0200% or less, S: 0.0200% or less, Al: 0.001 to 1.000%, N: 0.0200% or less, Ti: 0 to 0.500%, Co: 0 to 0.500%, Ni: 0 to 0.500%, Mo: 0 to 0.500%, Cr: 0~2.000%, O: 0 to 0.0100%, B: 0~0.0100%, Nb: 0 to 0.500%, V: 0 to 0.500%, Cu: 0 to 0.500%, W: 0~0.1000%, Ta: 0 to 0.1000%, Sn: 0 to 0.0500%, Sb: 0 to 0.0500%, As: 0~0.0500%, Mg: 0 to 0.0500%, Ca: 0 to 0.0500%, Y: 0~0.0500%, Zr: 0 to 0.0500%, La: 0 to 0.0500%, and Ce: 0 to 0.0500%, and the balance being Fe and impurities, In terms of area ratio, The sum of ferrite, pearlite and bainite: 0% or more and 30.0% or less, and Retained austenite: 10.0% to 30.0% The balance has a steel structure consisting of fresh martensite and tempered martensite, the difference between the number density of precipitates in the tempered martensite in the first surface on the front side of the steel sheet and the number density of precipitates in the tempered martensite in the second surface on the back side of the steel sheet is within 10.0%; The yield strength is 600 MPa or more. steel plate. (2) In mass%, Ti: 0.001 to 0.500%, Co: 0.001 to 0.500%, Ni: 0.001 to 0.500%, Mo: 0.001 to 0.500%, Cr: 0.001~2.000% O: 0.0001 to 0.0100% B: 0.0001~0.0100%, Nb: 0.001 to 0.500%, V: 0.001 to 0.500%, Cu: 0.001 to 0.500%, W: 0.0001 to 0.1000%, Ta: 0.0001 to 0.1000%, Sn: 0.0001~0.0500%, Sb: 0.0001 to 0.0500%, As: 0.0001 to 0.0500%, Mg: 0.0001 to 0.0500%, Ca: 0.0001 to 0.0500%, Y: 0.0001 to 0.0500%, Zr: 0.0001 to 0.0500%, La: 0.0001 to 0.0500%, and Ce: 0.0001 to 0.0500%, The chemical composition contains one or more of the following: The steel plate according to (1) above. (3) The steel structure contains the acicular retained austenite. The steel sheet according to (1) or (2) above. (4) A method for manufacturing a steel sheet, comprising the steps of: hot rolling a steel slab having the chemical composition described in (1) or (2) above to obtain a hot rolled sheet; coiling the hot-rolled sheet; pickling the hot-rolled sheet; cold rolling the hot rolled sheet to obtain a cold rolled sheet; Q annealing (Q: Quenching) is performed on the cold rolled sheet; The cold-rolled sheet subjected to the Q annealing is subjected to IA annealing (IA: Intercritical Annealing); and performing an aging treatment between the Q annealing and the IA annealing; Including, The Q annealing is a process of heating the cold-rolled sheet to a temperature in the austenite single phase region and 1000° C. or less, and then cooling the sheet to obtain a martensite structure having an area ratio of 90.0% or more. The IA annealing is a process of holding the cold-rolled sheet in a two-phase region of ferrite and austenite to obtain retained austenite, The aging treatment includes aging treatment 1 in which one of the front side and the back side of the cold-rolled sheet is subjected to tensile deformation with a bending R of 2.0 m or less and held at 0 to 40 ° C. for 20 hours or more, and aging treatment 2 in which the other of the front side and the back side of the cold-rolled sheet is subjected to tensile deformation with a bending R of 2.0 m or less and held at 0 to 40 ° C. for 20 hours or more. Manufacturing method of steel plate. (5) In the IA annealing, the cold-rolled sheet is held in a two-phase region of ferrite and austenite, and then cooled to room temperature, forming a coating layer made of zinc, aluminum, magnesium, or an alloy thereof on the front and back surfaces of the cold-rolled sheet. The manufacturing method described in (4) above. (6) In the IA annealing, the retained austenite is obtained in an acicular shape. The manufacturing method according to (4) or (5). Effect of the Invention

[0013] The steel sheet of the present disclosure has excellent mechanical properties such as strength and elongation, as well as excellent bending strength. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Hereinafter, embodiments of the present invention will be described. Note that these descriptions are intended to be merely examples of the embodiments of the present invention, and the present invention is not limited to the following embodiments.

[0015] <Steel plate> The steel sheet according to this embodiment has, in mass%, C: 0.10-0.30%, Si: 0.60-1.20%, Mn: 1.00-3.50%, P:0.0200% or less, S: 0.0200% or less, Al: 0.001 to 1.000%, N: 0.0200% or less, Ti: 0 to 0.500%, Co: 0 to 0.500%, Ni: 0 to 0.500%, Mo: 0 to 0.500%, Cr: 0~2.000%, O: 0 to 0.0100%, B: 0~0.0100%, Nb: 0 to 0.500%, V: 0 to 0.500%, Cu: 0 to 0.500%, W: 0~0.1000%, Ta: 0 to 0.1000%, Sn: 0 to 0.0500%, Sb: 0 to 0.0500%, As: 0~0.0500%, Mg: 0 to 0.0500%, Ca: 0 to 0.0500%, Y: 0~0.0500%, Zr: 0 to 0.0500%, La: 0 to 0.0500%, and Ce: 0 to 0.0500%, and the balance being Fe and impurities, In terms of area ratio, The sum of ferrite, pearlite and bainite: 0% or more and 30.0% or less, and Retained austenite: 10.0% to 30.0% The balance has a steel structure consisting of fresh martensite and tempered martensite, the difference between the number density of precipitates in the tempered martensite in the first surface on the front side of the steel sheet and the number density of precipitates in the tempered martensite in the second surface on the back side of the steel sheet is within 10.0%; It is characterized by a yield strength of 600 MPa or more.

[0016] First, the reason for limiting the chemical composition of the steel sheet according to the embodiment of the present invention will be described. Here, "%" for the components means mass%. Furthermore, in this specification, "to" indicating a numerical range is used to mean that the numerical values ​​before and after it are included as the lower and upper limits, unless otherwise specified.

[0017] (C: 0.10~0.30%) C is an element that inexpensively increases tensile strength and is an extremely important element for controlling the strength of steel. When the C content is 0.10% or more, such an effect is easily obtained. The C content may be 0.12% or more. On the other hand, excessive C content may lead to a decrease in elongation and brittle fracture of the steel, which may promote a decrease in bending strength when a part is deformed. When the C content is 0.30% or less, such problems are easily avoided. The C content may be 0.28% or less.

[0018] (Si: 0.60~1.20%) Silicon acts as a deoxidizer, and is an element that increases the stability of the retained austenite structure against processing, and suppresses the precipitation of carbides in the martensite structure during aging. When the Si content is 0.60% or more, such effects are easily obtained. The Si content may be 0.70% or more. On the other hand, when excessive Si is contained, the generation of ε carbides is suppressed during aging treatment, and bending strength may be reduced. When the Si content is 1.20% or less, such problems are easily avoided. The Si content may be 1.00% or less.

[0019] (Mn: 1.00~3.50%) Mn is a factor that affects the ferrite transformation of steel, and is an element that suppresses the ferrite transformation during the cooling process of Q annealing described later, increases the structure ratio of martensite after Q annealing, and is effective in increasing strength. When the Mn content is 1.00% or more, such effects are easily obtained. The Mn content may be 1.30% or more. On the other hand, when Mn is excessively contained, a Mn-enriched layer due to microsegregation and center segregation becomes prominent in the steel sheet, and due to the difference in solidification rate on the front and back sides of the slab, there is a difference in the distribution state of the Mn-enriched layer on the front and back sides of the steel sheet, which may lead to a difference in bending strength on the front and back sides due to the difference in formation of Mn segregation bands. When the Mn content is 3.50% or less, such problems are easily avoided. The Mn content may be 3.00% or less.

[0020] (P:0.0200% or less) P is an element that strongly segregates at ferrite grain boundaries and promotes embrittlement of the grain boundaries, and the less the better. Excessive P content may lead to brittle fracture of the steel and promote a decrease in bending strength during deformation of parts. In this regard, the P content is 0.0200% or less. The P content may be 0.0180% or less. On the other hand, the lower limit of the P content is not particularly limited. The P content is 0% or more, and may be 0.0001% or more, or may be 0.0010% or more.

[0021] (S:0.0200% or less) S is an element that generates nonmetallic inclusions such as MnS in steel and reduces the ductility of steel parts, so the less S, the better. Excessive S content can lead to the generation of voids originating from the nonmetallic inclusions during part deformation and can also reduce bending strength. In this regard, the S content is 0.0200% or less. The S content may be 0.0180% or less. On the other hand, the lower limit of the S content is not particularly limited. The S content is 0% or more, and may be 0.0001% or more, or may be 0.0005% or more.

[0022] (Al: 0.001 to 1.000%) Al is an element that acts as a deoxidizer for steel and stabilizes ferrite, and is added as necessary. When the Al content is 0.001% or more, such an effect is easily obtained. The Al content may be 0.010% or more. On the other hand, when an excessive amount of Al is contained, the ferrite transformation and bainite transformation during the cooling process in annealing are excessively promoted, and the strength of the steel sheet may decrease. When the Al content is 1.000% or less, such a problem is easily avoided. The Al content may be 0.800% or less.

[0023] (N:0.0200% or less) N is an element that forms coarse nitrides in steel sheets and reduces the workability of the steel sheets. N is also an element that causes blowholes during welding. Excessive N content combines with Al and Ti to generate a large amount of AlN or TiN, and these nitrides may become the starting point for void generation during part deformation, resulting in a decrease in bending strength. In this regard, the N content is 0.0200% or less. The N content may be 0.0160% or less. On the other hand, the lower limit of the N content is not particularly limited. The N content is 0% or more, and may be 0.0001% or more, or may be 0.0010% or more.

[0024] The basic chemical composition of the steel sheet in this embodiment is as described above. Furthermore, the steel sheet in this embodiment may contain at least one of the following optional elements as necessary. These elements do not necessarily have to be contained, so the lower limit is 0%.

[0025] (Ti: 0 to 0.500%) Ti is a strengthening element. It contributes to increasing the strength of steel sheets through precipitation strengthening, fine grain strengthening by inhibiting crystal grain growth, and dislocation strengthening through inhibiting recrystallization. On the other hand, excessive Ti content increases the precipitation of coarse carbides, which may become the starting point for void generation during part deformation, resulting in a decrease in bending strength. The Ti content is 0% or more, may be 0.001% or more, may be 0.005% or more, and may be 0.500% or less, or may be 0.400% or less.

[0026] (Co: 0~0.500%) Co is an element effective in controlling the morphology of carbides and increasing strength, and is added as necessary to control strength. On the other hand, excessive Co content causes a large number of fine Co carbides to precipitate, which may become the starting point for void generation during part deformation, resulting in a decrease in bending strength. The Co content is 0% or more, and may be 0.001% or more, and may be 0.500% or less, or 0.400% or less.

[0027] (Ni: 0~0.500%) Ni is a strengthening element and is effective in improving hardenability. In addition, Ni may be added because it improves the wettability between the steel sheet and the plating and promotes the alloying reaction. On the other hand, excessive Ni content affects the peelability of oxide scale during hot rolling and promotes the generation of scratches on the steel sheet surface, which may reduce the yield strength during bending deformation. The Ni content is 0% or more, and may be 0.001% or more, and may be 0.500% or less, or 0.400% or less.

[0028] (Mo:0~0.500%) Mo is an element effective in improving the strength of steel sheets. Mo is also an element that has the effect of suppressing ferrite transformation that occurs during heat treatment in continuous annealing equipment or continuous hot-dip galvanizing equipment. On the other hand, excessive Mo content causes a large number of fine Mo carbides to precipitate, and these carbides may become the starting points for void generation during part deformation, resulting in a decrease in bending strength. The Mo content is 0% or more, and may be 0.001% or more, and may be 0.500% or less, or 0.400% or less.

[0029] (Cr:0~2.000%) Cr, like Mn, is an element that suppresses pearlite transformation and is effective in increasing the strength of steel, and is added as necessary. On the other hand, excessive Cr content promotes the formation of retained austenite, and the presence of excess retained austenite may lead to a decrease in bending strength. The Cr content is 0% or more, and may be 0.001% or more, and may be 2.000% or less, and may be 1.500% or less.

[0030] (O:0~0.0100%) Since O forms oxides and deteriorates workability, it is necessary to suppress the amount of addition. In particular, oxides often exist as inclusions, and when they exist on punched end surfaces or cut surfaces, they form notch-shaped scratches or coarse dimples on the end surfaces, which may lead to stress concentration during part deformation and may become the starting point for crack formation, resulting in a decrease in bending strength. The O content is 0.0100% or less, and may be 0.0080% or less. Although the O content is 0% or more, controlling the O content to less than 0.0001% may increase the refining time and increase the manufacturing cost. In order to prevent an increase in manufacturing costs, the O content may be 0.0001% or more, or may be 0.0010% or more.

[0031] (B: 0~0.0100%) B is an element that suppresses the formation of ferrite and pearlite during the cooling process from austenite and promotes the formation of low-temperature transformation structures such as bainite or martensite. In addition, B is an element that is beneficial for increasing the strength of steel and is added as necessary. On the other hand, excessive B content leads to the formation of coarse B inclusions in the steel, and these inclusions become the starting points for void generation, which may lead to a decrease in bending strength during part deformation. The B content is 0% or more, and may be 0.0001% or more, 0.0010% or more, and may be 0.0100% or less, or 0.0080% or less.

[0032] (Nb: 0~0.500%) Nb is an element effective in controlling the morphology of carbides, and its addition refines the structure, so it is also effective in improving toughness. On the other hand, excessive Nb content causes a large number of fine, hard Nb carbides to precipitate, and these carbides become the starting points for void generation, which may lead to a decrease in bending strength during part deformation. The Nb content is 0% or more, and may be 0.001% or more, and may be 0.500% or less, or 0.400% or less.

[0033] (V:0~0.500%) V is a strengthening element. It contributes to increasing the strength of steel sheets through precipitation strengthening, fine grain strengthening by inhibiting the growth of ferrite crystal grains, and dislocation strengthening through inhibiting recrystallization. On the other hand, excessive V content increases the precipitation of carbonitrides, which become the starting point for void generation, and may lead to a decrease in bending strength during part deformation. The V content is 0% or more, and may be 0.001% or more, and may be 0.500% or less, or 0.400% or less.

[0034] (Cu: 0~0.500%) Cu is an element effective in improving the strength of steel sheet. On the other hand, if Cu is contained in an excessive amount, the steel material becomes embrittled during hot rolling, making hot rolling difficult. Furthermore, as the strength of the steel increases, the ductility decreases, which may lead to a decrease in bending strength during part deformation. The Cu content is 0% or more, and may be 0.001% or more, and may be 0.500% or less, or 0.400% or less.

[0035] (W:0~0.1000%) W is effective in increasing the strength of steel sheets, and precipitates and crystallized materials containing W act as hydrogen trapping sites. On the other hand, excessive W content facilitates the progression of void generation from coarse carbides, which may lead to a decrease in bending strength during part deformation. The W content is 0% or more, and may be 0.0001% or more, 0.0010% or more, and may be 0.1000% or less, or 0.0800% or less.

[0036] (Ta: 0 to 0.1000%) Ta, like Nb, V, and W, is an effective element for controlling the morphology of carbides and increasing strength, and is added as necessary. On the other hand, excessive Ta content causes a large number of fine Ta carbides to precipitate, which makes it easy for voids to occur starting from these carbides, and may result in a decrease in bending strength during deformation of parts. The Ta content is 0% or more, and may be 0.0001% or more, 0.0010% or more, and may be 0.1000% or less, or 0.0800% or less.

[0037] (Sn: 0~0.0500%) Sn is an element contained in steel when scrap is used as a raw material, and the less the better. Excessive Sn content may lead to a decrease in bending strength during part deformation due to embrittlement of the steel sheet. The Sn content is 0.0500% or less, and may be 0.0400% or less. The Sn content may be 0%, but controlling the Sn content to less than 0.0001% may increase the refining time and may increase the manufacturing cost. In order to prevent an increase in manufacturing costs, the Sn content may be 0.0001% or more, or may be 0.0010% or more.

[0038] (Sb: 0~0.0500%) Sb, like Sn, is an element contained when scrap is used as a steel raw material. Sb strongly segregates at grain boundaries, which leads to embrittlement of the grain boundaries and a decrease in ductility, so the less Sb, the better. In addition, excessive Sb content may lead to a decrease in bending strength during part deformation due to embrittlement of the steel sheet. The Sb content is 0.0500% or less, and may be 0.0400% or less. The Sb content may be 0%, but controlling the Sb content to less than 0.0001% may increase the refining time and increase the manufacturing cost. In order to prevent an increase in manufacturing costs, the Sb content may be 0.0001% or more, or may be 0.0010% or more.

[0039] (As:0~0.0500%) Like Sn and Sb, As is an element that is contained when scrap is used as a steel raw material and strongly segregates at grain boundaries, and the less the better. In addition, excessive As content may lead to a decrease in bending strength during part deformation due to embrittlement of the steel sheet. The As content is 0.0500% or less, and may be 0.0400% or less. The As content may be 0%, but controlling the As content to less than 0.0001% may increase the refining time and may increase manufacturing costs. In order to prevent an increase in manufacturing costs, the As content may be 0.0001% or more, or may be 0.0010% or more.

[0040] (Mg: 0~0.0500%) Mg is an element that can control the morphology of sulfides by adding a small amount, and is added as necessary. On the other hand, excessive Mg content forms coarse inclusions, which become the starting point for void generation, and may lead to a decrease in bending strength during part deformation. The Mg content is 0% or more, may be 0.0001% or more, may be 0.0010% or more, and may be 0.0500% or less, or may be 0.0400% or less.

[0041] (Ca: 0~0.0500%) Ca is useful as a deoxidizing element and is also effective in controlling the morphology of sulfides. On the other hand, excessive Ca content may lead to a decrease in bending strength during part deformation due to embrittlement of the steel sheet. The Ca content is 0% or more, may be 0.0001% or more, may be 0.0010% or more, and may be 0.0500% or less, or may be 0.0400% or less.

[0042] (Y:0~0.0500%) Y, like Mg and Ca, is an element that can control the morphology of sulfides by adding a small amount, and is added as necessary. On the other hand, excessive Y content generates coarse Y inclusions, which become the starting point for void generation, and may lead to a decrease in bending strength during part deformation. The Y content is 0% or more, may be 0.0001% or more, may be 0.0010% or more, and may be 0.0500% or less, or may be 0.0400% or less.

[0043] (Zr: 0~0.0500%) Zr is an element that can control the morphology of sulfides by adding a small amount, similar to Mg, Ca, and Y, and is added as necessary. On the other hand, excessive Zr content generates coarse Zr inclusions, which become the starting point for void generation and may lead to a decrease in bending strength during part deformation. The Zr content is 0% or more, may be 0.0001% or more, may be 0.0010% or more, and may be 0.0500% or less, or may be 0.0400% or less.

[0044] (La:0~0.0500%) La is an element that is effective in controlling the morphology of sulfides when added in small amounts, and is added as necessary. On the other hand, excessive La content generates La inclusions, which become the starting point for void generation, and may lead to a decrease in bending strength during part deformation. The La content is 0% or more, may be 0.0001% or more, may be 0.0010% or more, and may be 0.0500% or less, or may be 0.0400% or less.

[0045] (Ce: 0~0.0500%) Ce, like La, is an element that can control the morphology of sulfides by adding a small amount, and is added as necessary. On the other hand, excessive Ce content generates Ce inclusions, which become the starting point for void generation, and may lead to a decrease in bending strength during part deformation. The Ce content is 0% or more, may be 0.0001% or more, may be 0.0010% or more, and may be 0.0500% or less, or may be 0.0400% or less.

[0046] In the steel sheet according to the present embodiment, the balance of the above-mentioned components is Fe and impurities. The impurities are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ores and scraps, when the steel sheet according to the present embodiment is industrially manufactured.

[0047] Next, the characteristics of the structure and properties of the steel plate according to the embodiment of the present invention will be described.

[0048] (Total area ratio of ferrite, pearlite and bainite: 0~30.0%) Although ferrite, pearlite and bainite are effective structures for improving the strength-ductility balance of steel sheets, their inclusion in large amounts may lead to a decrease in local ductility. In addition, from the viewpoint of efficiently increasing the strength of steel, it is preferable that the area ratios of ferrite, pearlite and bainite are as small as possible. The total area ratio of ferrite, pearlite and bainite may be 0%, 1.0% or more, 30.0% or less, 25.0% or less, or 20.0% or less. Although the productivity is somewhat reduced, it is possible to make the total area ratio of ferrite, pearlite and bainite 0% by controlling the integrated manufacturing conditions with high precision.

[0049] (Area ratio of retained austenite: 10.0~30.0%) Retained austenite is an effective structure for improving the strength-ductility balance of a steel sheet. If the area ratio of retained austenite is too small, the effect of increasing strength due to the processing-induced transformation from retained austenite to martensite cannot be obtained when bending deformation is applied to the steel sheet, which may result in a decrease in bending strength. On the other hand, if the area ratio of retained austenite is too large, it may result in a decrease in bending strength as well as a decrease in yield strength. The area ratio of retained austenite is 10.0% or more, and may be 13.0% or more, and may be 30.0% or less, or may be 25.0% or less.

[0050] In the steel plate according to the present embodiment, the steel structure of the steel plate preferably contains needle-shaped retained austenite. The "needle-shaped" form of the retained austenite is expected to have the following effects. That is, if the shape of the retained austenite is spherical (lumpy), deformation-induced transformation easily occurs with deformation of the steel plate, and bending deformation may begin at low stress. In contrast, if the shape of the retained austenite is needle-shaped, deformation-induced transformation is less likely to occur, and bending strength is further increased. In the steel plate according to the present embodiment, the effect of the needle-shaped retained austenite and the effect of the difference in number density of precipitates are combined to significantly improve the bending strength of the steel plate. The area ratio of the needle-shaped retained austenite may be 30% or more or 50% or more, or 95% or less or 90% or less, when the area ratio of the entire retained austenite is 100%. In the present application, "needle-shaped retained austenite" refers to a ratio of the major axis to the minor axis (major axis / minor axis) of 3.0 or more. The "long diameter" and "short diameter" of the retained austenite can be determined by observing the structure using EBSD. Specifically, in the observation of the structure, one retained austenite grain is identified, and the smallest Feret diameter of the grain is determined as the short diameter, and the largest Feret diameter is determined as the long diameter.

[0051] (balance: fresh martensite and tempered martensite) Fresh martensite and tempered martensite are extremely effective structures for increasing the strength of the steel sheet, and the higher the area ratio, the more preferable. In the steel sheet according to the present embodiment, the remainder other than the above-mentioned ferrite, pearlite, bainite, and retained austenite is composed of fresh martensite and tempered martensite. The total area ratio of fresh martensite and tempered martensite may be 40.0% or more, 45.0% or more, 50.0% or more, and 90.0% or less, or 85.0% or less. The area ratio of fresh martensite may be 5% or more, 10% or more, 20% or more, 30% or more, or 40% or more, and may be 80% or less, 70% or less, 60% or less, 50% or less, or 40% or less. Furthermore, the area ratio of tempered martensite may be 5% or more, 10% or more, 20% or more, 30% or more, or 40% or more, and may be 80% or less, 70% or less, 60% or less, 50% or less, or 40% or less.

[0052] (Difference in number density of precipitates in tempered martensite on the first surface on the front side of the steel sheet and the second surface on the back side: 0 to 10.0%) The number density of precipitates in tempered martensite on the first surface on the front side and the second surface on the back side of the steel sheet is an important factor for increasing resistance to bending deformation. The higher the number density of both the first surface on the front side and the second surface on the back side of the steel sheet, the higher the bending strength. If the number density of one surface is high, strain is biased during bending deformation, and yielding occurs on the other surface with a low number density, decreasing the bending strength. Therefore, it is preferable that the difference between the number density of precipitates in tempered martensite on the first surface on the front side of the steel sheet and the number density of precipitates in tempered martensite on the second surface on the back side of the steel sheet is as small as possible. Specifically, it is important that the difference in number density of the precipitates is 10.0% or less. This difference in number density may be 8.0% or less, 6.0% or less, 4.0% or less, or 2.0% or less. In other words, in this embodiment, the ratio A1 / A2 of the number density A1 of the precipitates on the first surface on the front side of the steel sheet to the number density A2 of the precipitates on the second surface on the back side of the steel sheet is 0.90 or more and 1.10 or less, and may be 0.92 or more, 0.94 or more, 0.96 or more, or 0.98 or more, or 1.08 or less, 1.06 or less, 1.04 or less, or 1.02 or less. In addition, making the difference in number density 0% is a factor that increases the manufacturing load and the manufacturing cost due to precise control of the steel sheet structure. In this respect, the difference in number density may be 0.1% or more. In addition, the precipitates are mainly carbides generated by tempering martensite, and the carbides may be cementite, or iron-based carbides or alloy carbides in which alloy elements such as Cr, Ti, and V are bonded to carbon instead of iron. The specific value of the number density of precipitates in the tempered martensite on the first surface on the front side and the second surface on the back side of the steel sheet is, for example, 1 precipitate / μm 2 More than 5 pieces / μm 2 More than 10 / μm 2 More than 300 pieces / μm 2 Below, 100 pieces / μm 2 Less than or 30 pieces / μm 2 It may be the following.

[0053] In this application, for convenience of explanation, a distinction is made between the "front" and "back" of the steel plate, but which side of the steel plate is the front and which side is the back is not particularly limited.

[0054] (yield strength YS) In order to reduce the weight of a structure using steel as a material and to improve the strength at which plastic deformation begins, it is preferable that the yield strength of the steel material is high. On the other hand, if the yield strength is too high, the effect of shape change due to elastic deformation after plastic processing, so-called springback, may become large. The yield strength of the steel plate according to this embodiment may be 600 MPa or more, or 650 MPa or more. The upper limit of the yield strength is not particularly limited, but from the viewpoint of suppressing the effect of the above-mentioned springback, it may be 1100 MPa or less, or 1050 MPa or less.

[0055] (Tensile strength TS) In order to reduce the weight of a structure using steel as a material and to improve the resistance of the structure to plastic deformation, it is preferable that the steel material has a large work hardening ability and exhibits maximum strength. On the other hand, if the tensile strength is too high, it may be prone to fracture with low energy during plastic deformation, and formability may be reduced. The tensile strength of the steel plate is not particularly limited, but may be 900 MPa or more, 980 MPa or more, and 2000 MPa or less, or 1800 MPa or less.

[0056] (Total elongation t-El) When manufacturing a structure by cold forming a steel sheet as a raw material, elongation is required to finish the structure into a complex shape. If the total elongation is too low, the raw material may crack during cold forming. On the other hand, the higher the total elongation, the better, but if the total elongation is excessively increased, a large amount of retained austenite is required in the steel structure, which may reduce the yield strength during bending deformation. The total elongation of the steel sheet is not particularly limited, but may be 13% or more, 20% or more, 35% or less, or 30% or less.

[0057] (Hole expandability) When manufacturing a structure by cold forming a steel sheet as a material, hole expandability is required as well as elongation in order to finish the structure into a complex shape. If the hole expandability is too small, the material may crack during cold forming. On the other hand, although the higher the hole expandability, the more preferable it is, if the hole expandability is excessively increased, a large amount of retained austenite is required in the steel structure, which may reduce the yield strength during bending deformation. The hole expansion ratio λ of the steel sheet is not particularly limited, but may be 20% or more, 25% or more, 90% or less, or 80% or less.

[0058] (bendability) When manufacturing a structure by cold forming the steel plate as a material, bendability is also required to finish it into a complex shape. If the VDA bending angle is too small, the material may crack during cold forming. The higher the bendability, the better. The VDA bending angle of the steel plate is not particularly limited, but may be 45° or more, or 50° or more.

[0059] (Thickness) Plate thickness is a factor that affects the rigidity of a steel member after forming, and the greater the plate thickness, the higher the rigidity of the member. If the plate thickness is too small, the rigidity will decrease, and press formability may decrease due to the influence of unavoidable non-ferrous inclusions present inside the steel plate. On the other hand, if the plate thickness is too large, the press forming load will increase, leading to wear of the mold and reduced productivity. The plate thickness of the steel plate is not particularly limited, but may be 0.2 mm or more and 6.0 mm or less.

[0060] Next, the methods for observing and measuring the above-specified structure, and the methods for measuring and evaluating the above-specified characteristics will be described.

[0061] (Method for measuring the total area ratio of ferrite, pearlite, and bainite) The structure observation is performed using a scanning electron microscope. Prior to the observation, the sample for structure observation is wet-polished with emery paper and polished with diamond abrasives with an average particle size of 1 μm, the observation surface is mirror-finished, and the structure is etched with a 3% nitric acid alcohol solution. The magnification of the observation is 3000 times, and 10 random images are taken of a 30 μm x 40 μm field of view at each 1 / 4 thickness position from the surface side of the steel plate. The structure ratio is determined by the point count method. A total of 100 lattice points are set on the obtained structure image at intervals of 3 μm vertically and 4 μm horizontally, and the structure present under the lattice points is identified, and the structure ratio contained in the steel plate is calculated from the average value of the 10 images. Ferrite is a blocky crystal grain that does not contain iron-based carbides with a major axis of 100 nm or more inside. Bainite is a collection of lath-shaped crystal grains that does not contain iron-based carbides with a major axis of 20 nm or more, or contains iron-based carbides with a major axis of 20 nm or more, and the carbides belong to a single variant, i.e., a group of iron-based carbides elongated in the same direction. Here, a group of iron-based carbides elongated in the same direction refers to iron-based carbides whose elongation directions differ by 5° or less. Bainite grains are counted as bainite grains that are surrounded by grain boundaries with an orientation difference of 15° or more. Pearlite is a structure that contains cementite precipitated in rows, and the area ratio is calculated by counting the areas photographed with bright contrast in the secondary electron image as pearlite.

[0062] (How to distinguish between fresh martensite and tempered martensite) Fresh martensite and tempered martensite were observed using scanning and transmission electron microscopes, and those containing Fe-based carbides inside (Fe-based carbides 1 particle / μm) were 2 or more) are tempered martensite, which contains almost no Fe-based carbides (Fe-based carbides are 1 particle / μm 2The area ratio A1 of the total of ferrite, pearlite, and bainite is measured by the above method, and the area ratio A2 of the retained austenite is measured by the method described later. The remainder obtained by subtracting the total value of the area ratios A1 and A2 from 100% is regarded as the total area ratio of fresh martensite and tempered martensite. Various crystal structures have been reported for Fe-based carbides, and any Fe-based carbide may be contained. Depending on the heat treatment conditions, multiple types of Fe-based carbides may be present. In the present application, the total area ratio A1 of ferrite, pearlite, and bainite is measured by the above method, and the area ratio A2 of the retained austenite is measured by the method described later. The remainder obtained by subtracting the total value of the area ratios A1 and A2 from 100% is regarded as the total area ratio of fresh martensite and tempered martensite.

[0063] (Method for measuring area ratio of retained austenite) The area fraction of retained austenite is determined by X-ray measurement as follows. First, the portion from the surface of the steel plate to 1 / 4 of the thickness of the steel plate is removed by mechanical polishing and chemical polishing, and the chemically polished surface is measured using MoKα rays as characteristic X-rays. Then, the area fraction of retained austenite in the center of the plate thickness is calculated using the following formula from the integrated intensity ratio of the diffraction peaks of (200) and (211) of the body-centered cubic lattice (bcc) phase and (200), (220), and (311) of the face-centered cubic lattice (fcc) phase. Sγ=(I200f+I220f+I311f) / (I200b+I211b)×100 (Sγ is the area fraction of retained austenite at the center of the sheet thickness, I200f, I220f, and I311f indicate the intensities of the (200), (220), and (311) diffraction peaks of the fcc phase, respectively, and I200b and I211b indicate the intensities of the (200) and (211) diffraction peaks of the bcc phase, respectively.)

[0064] The sample to be subjected to X-ray diffraction is prepared by reducing the thickness of the steel plate from the surface to a predetermined thickness by mechanical polishing, etc., and then removing distortion by chemical polishing or electrolytic polishing, etc., and adjusting the sample according to the above-mentioned method so that an appropriate surface becomes the measurement surface within the range of 1 / 8 to 3 / 8 of the plate thickness. Naturally, the material anisotropy is further reduced by satisfying the above-mentioned X-ray intensity limit not only in the vicinity of 1 / 4 of the plate thickness but also for as many thicknesses as possible. However, by performing the measurement within the range of 1 / 8 to 3 / 8 of the plate surface, the material properties of the entire steel plate can be roughly represented. Therefore, the measurement range is set to 1 / 8 to 3 / 8 of the plate thickness.

[0065] The area ratio of acicular retained austenite among the retained austenite can be measured by, for example, EBSD.

[0066] (Method for measuring the number density of precipitates in tempered martensite on the first surface (front side) and the second surface (back side) of a steel sheet) The number density of precipitates in tempered martensite on the first surface on the front side and the second surface on the back side of a steel sheet is measured as follows. First, a material for observation is taken from the front or back side of the steel sheet (meaning the front or back side of the base steel sheet. For example, in the case of a surface-treated steel sheet having a surface treatment layer such as plating, it means the front or back side of the base steel sheet excluding the surface treatment layer) at a depth position of 1 / 8 of the thickness in the sheet thickness direction, and prepared into a test piece for observation of a thin film or extraction replica. The test piece is observed at a magnification of 10,000 times with a transmission electron microscope to obtain photographed images in at least 30 fields of view, the number density of precipitates per unit area is measured in each observation image, and the value obtained by arithmetically averaging the number densities for the 30 fields of view is taken as the number density of precipitates on the first surface on the front side or the second surface on the back side. The field of view observed at a magnification of 10,000 times with a transmission electron microscope is a rectangular region with one side of about 600 nm, and the area of ​​the 30 fields of view to be used for measuring the number density of precipitates is about 10.8 μm 2 The area will be as follows.

[0067] (Methods for measuring yield strength YP, tensile strength TS, and total elongation t-El) The tensile test for measuring the yield strength, tensile strength and total elongation is performed in accordance with JIS Z 2241:2011, using a JIS No. 5 test piece taken in such a way that the longitudinal direction of the test piece is parallel to the rolling direction perpendicular to the steel strip.

[0068] (Method of measuring hole expandability) The hole expandability is evaluated by punching a circular hole with a diameter of 10 mm under the condition of a clearance of 12.5%, forming it with a 60° conical punch so that the burr is on the die side, and evaluating the hole expansion ratio λ (%). Five hole expansion tests are performed, and the average value is the hole expansion ratio.

[0069] (Method of measuring bendability) The bendability is evaluated by testing 60mm wide test pieces in accordance with the Verband der Automobilindustrie (VDA) standard 238-100, and the VDA bending angle is evaluated by measuring the maximum bending angle α. The bending strength is evaluated as the load at a bending angle of 5° divided by the plate thickness.

[0070] <Steel sheet manufacturing method> The method for producing a steel sheet according to this embodiment is characterized by using a material having the above-mentioned chemical composition and consistently managing hot rolling, cold rolling, and annealing. Specifically, the method for producing a steel sheet according to this embodiment is characterized by including the steps of hot rolling a steel billet (steel slab) having the same chemical composition as that described above for the steel sheet, coiling it, pickling the obtained hot-rolled sheet, cold rolling it, annealing it, performing aging treatment, and then annealing it again. More specifically, the method for producing a steel sheet according to this embodiment includes the steps of: hot rolling the steel slab having the above chemical composition to obtain a hot rolled sheet; coiling the hot-rolled sheet; pickling the hot-rolled sheet; cold rolling the hot rolled sheet to obtain a cold rolled sheet; The cold-rolled sheet is subjected to a first annealing (Q annealing); The cold-rolled sheet subjected to the first annealing is subjected to a second annealing (IA annealing); and performing an aging treatment between the first annealing and the second annealing; Including, The first annealing is a process of heating the cold-rolled sheet to a temperature in the austenite single phase region and equal to or lower than 1000° C., and then cooling the sheet to obtain a martensite structure having an area ratio of 90.0% or more. The second annealing is a process of holding the cold-rolled sheet in a two-phase region of ferrite and austenite to obtain retained austenite, The aging treatment is characterized by including aging treatment 1 in which one of the front side and the back side of the cold-rolled sheet is subjected to tensile deformation with a bending radius of 2.0 m or less and held at 0 to 40° C. for 20 hours or more, and aging treatment 2 in which the other of the front side and the back side of the cold-rolled sheet is subjected to tensile deformation with a bending radius of 2.0 m or less and held at 0 to 40° C. for 20 hours or more. Each step will be described in detail below, focusing on the key points of this embodiment.

[0071] (Hot rolling finishing temperature) In this embodiment, a hot-rolled sheet is obtained by hot rolling a steel slab obtained by a known method such as a continuous casting method. Here, the finish rolling temperature of the hot rolling is a factor that has an effect on controlling the texture of the prior austenite grain size. From the viewpoint that the rolling texture of austenite develops and causes anisotropy in the steel material properties, the finish rolling temperature is preferably 650°C or higher, and from the aim of suppressing the deviation of the texture due to abnormal grain growth of austenite, the finish rolling temperature is preferably 950°C or lower.

[0072] (Coil winding temperature) The temperature at which the hot-rolled sheet is coiled (the coiling temperature of the hot-rolled coil) controls the state of oxide scale formation in the hot-rolled sheet, and is a factor that affects the strength of the hot-rolled sheet. The thinner the thickness of the scale formed on the surface of the hot-rolled sheet, the better, and therefore the lower the coiling temperature is. In addition, when the coiling temperature is extremely reduced, special equipment is required. In addition, if the coiling temperature is too high, as described above, the oxide scale formed on the surface of the hot-rolled sheet becomes significantly thick. From the above viewpoints, the temperature at which the hot-rolled sheet is coiled may be 700°C or less, 680°C or less, 0°C or more, or 20°C or more.

[0073] (Pickling of hot-rolled sheet) The pickling of the hot-rolled sheet is aimed at removing scales and the like, and may be carried out under known pickling conditions.

[0074] (Reduction in cold rolling) In cold rolling, if the total reduction is too large, the ductility of the base steel sheet is lost, and the risk of the base steel sheet breaking during cold rolling increases. In this respect, the total reduction in cold rolling is preferably 85% or less. On the other hand, in order to sufficiently promote recrystallization in the annealing process, the total reduction is preferably 20% or more, and more preferably 30% or more. Annealing may be performed at a temperature of 700°C or less in order to reduce the cold rolling load before cold rolling.

[0075] (Holding temperature for the first annealing) In the first annealing (Q annealing), the cold-rolled steel sheet, which is the base steel sheet, is heated to a temperature above the Ac3 point and below 1000°C (i.e., in the austenite single-phase region and below 1000°C). The reason for setting the maximum heating temperature at or above the Ac3 point is that by heating the base steel sheet to the austenite single-phase region, a martensite structure with an area ratio of 90% or more can be obtained by subsequent rapid cooling, which promotes the precipitation of epsilon carbide by aging. Holding at a lower temperature than this does not result in a martensite-based structure, and the bending strength decreases significantly. On the other hand, heating to above 1000°C may result in decarburization of the surface layer of the steel sheet, reducing its strength and thus reducing its bending strength.

[0076] (Holding time for the first annealing) In the first annealing (Q annealing), it is preferable to hold the heating temperature at the Ac3 point or higher and 1000°C or lower for 5 seconds or more. If the holding time is too short, the austenite transformation of the base steel sheet will not proceed sufficiently, and in addition, the concentration of substitutional elements such as Mn that stabilize austenite will not be sufficient in the austenite, so that the retained austenite will become unstable and the ductility of the steel sheet may decrease significantly. From these viewpoints, the holding time is more preferably 10 seconds or more. Further preferably, it is 20 seconds or more.

[0077] (Atmosphere during the first annealing) In the first annealing (Q annealing), the oxygen potential in one or both of the heating zone and the soaking zone during annealing may be controlled in order to provide a decarburized layer on the surface layer of the steel sheet to improve bendability. Specifically, the annealing is preferably performed in an atmosphere containing 0.1 to 30% by volume of hydrogen and H2O with a dew point of -40 to 20°C, with the balance being nitrogen and impurities. More preferably, the atmosphere contains 0.5 to 20% by volume of hydrogen and H2O with a dew point of -30 to 15°C, and even more preferably, the atmosphere contains 1 to 10% by volume of hydrogen and H2O with a dew point of -20 to 10°C.

[0078] (Cooling rate in the first annealing) In the first annealing (Q annealing), when cooling after heating and holding, it is preferable to cool from 750 ° C. to 550 ° C. at an average cooling rate of 100 ° C. / s or less. The lower limit of the average cooling rate is not particularly limited as long as a martensite structure with an area ratio of 90% or more can be obtained, but it may be, for example, 3 ° C. / s. The reason for setting the lower limit of the average cooling rate to 3 ° C. / s is to suppress the occurrence of ferrite transformation in the base steel sheet and the area ratio of martensite in the steel structure after Q annealing becoming less than 90%. More preferably, it is 10 ° C. / s or more, more preferably 15 ° C. / s or more, and even more preferably 20 ° C. / s or more. On the other hand, if the cooling rate from 750 ° C. to 550 ° C. is too fast, a low-temperature transformation structure will also occur in the steel sheet surface layer, causing hardness variations. In this regard, the average cooling rate is preferably 100 ° C. / s or less, more preferably 80 ° C. / s or less, and even more preferably 50 ° C. / s or less. The cooling rate is not limited at temperatures above 750°C because ferrite transformation is unlikely to occur. Also, the cooling rate is not limited at temperatures below 550°C because a low-temperature transformed structure is obtained.

[0079] (Cooling stop temperature in the first annealing and reheating) After the above cooling, the material may be further cooled to a temperature of 25°C to 550°C, and then allowed to remain in a temperature range of 150°C to 550°C. The reason why the lower limit of the cooling stop temperature is set to 25°C is that excessive cooling not only requires a large investment in equipment, but also has a saturating effect. The residence time is not particularly limited, and may be, for example, 30 seconds to 500 seconds.

[0080] (Prescription Processing 1) The steel sheet, which has been controlled to have a martensite-based structure by the first annealing, is subjected to bending deformation with a bending radius R of 2.0 m or less and is held at 0 to 40 ° C for 20 hours or more in this state, which is an important factor in increasing the bending strength of the steel sheet. During this treatment, the carbon atoms dissolved in the martensite form clusters or transition carbides, which become nuclei for carbide precipitation when the temperature is increased in the subsequent second annealing. In order to finely disperse carbides and increase the bending strength, it is important to make the clusters or transition carbides, which become the nuclei for carbide precipitation, exist finely and densely. In order to promote the formation of clusters or transition carbides, the use of tensile strain is extremely effective, and this effect is easily obtained in bending deformation with a bending radius R of 2.0 m or less. The bending radius R may be 1.8 m or less, 1.5 m or less, or 1.3 m or less. On the other hand, if the bending radius R exceeds 2.0 m, it becomes difficult to obtain this effect. For example, the steel sheet (steel strip) after the first annealing is wound into a coil, whereby the above bending deformation can be imparted to the steel sheet.

[0081] In addition, when the holding temperature is less than 0°C, the clustering of carbon atoms or the formation of transition carbides is suppressed, and when the holding temperature exceeds 40°C, the transition carbides are formed coarsely (the number of nuclei decreases), so that it is difficult to obtain fine carbides in the second annealing, and bending strength may decrease. If the holding temperature in the aging treatment 1 and the aging treatment 2 described below is within the range of 0 to 40°C, the difference in the number of precipitates between the aging treatment 1 and the aging treatment 2 becomes small, and the difference between the number density of precipitates on the first surface on the front side of the steel sheet and the number density of precipitates on the second surface on the back side of the steel sheet is within 10%. The holding temperature may be 5°C or more, 10°C or more, 35°C or less, or 30°C or less.

[0082] Furthermore, if the holding time is less than 20 hours, the number of nuclei generated is not stable, and a sufficient amount of nuclei is not generated, and it may be difficult to suppress the difference between the number density of precipitates on the first surface on the front side of the steel sheet and the number density of precipitates on the second surface on the back side of the steel sheet to within 10%. A longer holding time is preferable, and it may be 30 hours or more, 40 hours or more, or 50 hours or more. Note that, if the holding time exceeds 300 hours, the clustering of carbon atoms or the formation of transition carbides is saturated, and if the holding time is longer than that, a significant change in the form (size) of the precipitates is unlikely to occur, so the holding time may be 300 hours or less. Note that, although the precipitates become larger when the holding time is long, there is no significant change in the number of precipitates. In other words, if the holding time of the aging treatment 1 and the aging treatment 2 described below is 20 hours or more, the number of nuclei generated is stable, and the difference between the number density of precipitates on the first surface on the front side of the steel sheet and the number density of precipitates on the second surface on the back side of the steel sheet is within 10%.

[0083] (Prescription Processing 2) When bending deformation is applied to a steel sheet to promote aging, clustering of carbon atoms and precipitation of transition carbides occur significantly in the region subjected to tensile deformation, so that if the winding and unwinding process is performed only once by the above-mentioned aging treatment 1, precipitates will be finely dispersed only on one side of the steel sheet. For this reason, in the manufacturing method according to this embodiment, after the aging treatment 1 in which precipitates are precipitated and dispersed on one side of the front side and the back side of the steel sheet, aging treatment 2 in which precipitates are precipitated and dispersed on the other side of the front side and the back side of the steel sheet is performed. For example, after the first annealing, the sheet is coiled so that the front side of the sheet is on the outside and the back side is on the inside, and tensile deformation with a bending radius R of 2.0 m or less is applied to the front side of the sheet to perform aging treatment 1, and then the coil is uncoiled, and the sheet is coiled again so that the back side of the sheet is on the outside and the front side is on the inside, and tensile deformation with a bending radius R of 2.0 m or less is applied to the back side of the sheet to perform aging treatment 2. In aging treatment 2, similarly to the holding conditions in aging treatment 1, by performing bending deformation with a bending radius R of 2.0 m or less, it is possible to suppress the difference in number density of precipitates on the first surface, which is the front side of the steel sheet, and the difference in number density of precipitates on the second surface, which is the back side of the steel sheet, to within 10%. The bending radius R may be 1.8 m or less, 1.5 m or less, or 1.3 m or less. On the other hand, if the bending radius R exceeds 2 m, it becomes difficult to suppress the difference in number density of precipitates to within 10%.

[0084] In the aging treatment 2, similarly to the aging treatment 1, when the holding temperature is less than 0°C, the clustering of carbon atoms or the formation of transition carbides is suppressed, and when the holding temperature exceeds 40°C, the transition carbides are formed coarsely (the number of nuclei decreases), so that it is difficult to obtain fine carbides in the second annealing, and bending strength may decrease. As described above, when the holding temperature in the aging treatment 1 and the aging treatment 2 described later is within the range of 0 to 40°C, the difference between the number density of precipitates on the first surface on the front side of the steel sheet and the number density of precipitates on the second surface on the back side of the steel sheet is within 10%. The holding temperature may be 5°C or more, 10°C or more, 35°C or less, or 30°C or less.

[0085] Furthermore, in the aging treatment 2, similarly to the aging treatment 1, if the holding time is less than 20 hours, the number of nuclei generated is not stable, and it may be difficult to suppress the difference between the number density of precipitates on the first surface on the front side of the steel sheet and the number density of precipitates on the second surface on the back side of the steel sheet to within 10%. A longer holding time is preferable, and it may be 30 hours or more, 40 hours or more, or 50 hours or more. Note that, similarly to the aging treatment 1, if the holding time exceeds 300 hours, the clustering of carbon atoms or the formation of transition carbides becomes saturated, and if the holding time is longer than that, it becomes difficult to cause a significant change in the morphology (size) of the precipitates, so the holding time may be 300 hours or less.

[0086] (Holding temperature for second annealing) In the second annealing (IA annealing), the holding temperature is a temperature at which the two-phase region of ferrite and austenite is formed. For example, it is preferably 720°C or more and 860°C or less. If the annealing temperature is less than 720°C, austenite is not sufficiently generated. In this case, the martensite obtained in the first annealing (Q annealing) is tempered, which leads to the precipitation of carbides, and the area ratio of the predetermined retained austenite may not be satisfied. In addition, since the area ratio of austenite at the maximum heating temperature (annealing temperature) also decreases, it becomes impossible to concentrate carbon necessary for obtaining retained austenite in austenite, and it may become impossible to secure 10.0% or more of retained austenite. On the other hand, if the annealing temperature exceeds 860°C, austenite is excessively generated, and the bending strength decreases due to the decrease in tempered martensite containing precipitates, and it may become impossible to secure 10.0% or more of retained austenite. Therefore, it is preferable that the upper limit of the holding temperature in the second annealing is 860°C. The annealing may be carried out in air, or in order to improve the adhesion of the plating, it may be carried out in an atmosphere in which the hydrogen concentration and dew point are controlled.

[0087] (Holding time for second annealing) In the second annealing (IA annealing), it is preferable to hold the heating temperature at 720°C or more and 860°C or less for 5 seconds or more. If the holding time is too short, the austenite transformation of the base steel sheet will not proceed sufficiently, and in addition, the concentration of substitutional elements such as Mn that stabilize austenite will not be sufficient in the austenite, so that the retained austenite will become unstable and the ductility of the steel sheet may decrease significantly. From these viewpoints, the holding time is more preferably 10 seconds or more. Further preferably, it is 20 seconds or more.

[0088] (Atmosphere during second annealing) In the second annealing (IA annealing), as in the first annealing (Q annealing), the oxygen potential in one or both of the heating zone and the soaking zone during annealing may be controlled in order to provide a decarburized layer in the surface layer of the steel sheet to improve bendability. Specifically, the annealing is preferably performed in an atmosphere containing 0.1 to 30% by volume of hydrogen and H2O with a dew point of -40 to 20°C, with the balance being nitrogen and impurities. More preferably, the atmosphere contains 0.5 to 20% by volume of hydrogen and H2O with a dew point of -30 to 15°C, and even more preferably, the atmosphere contains 1 to 10% by volume of hydrogen and H2O with a dew point of -20 to 10°C.

[0089] (Cooling rate in the second annealing) In the second annealing (IA annealing), when cooling after heating and holding, it is preferable to cool from 750 ° C. to 550 ° C. at an average cooling rate of 100 ° C. / s or less. The lower limit of the average cooling rate is not particularly limited, but may be, for example, 2.5 ° C. / s. The reason for setting the lower limit of the average cooling rate to 2.5 ° C. / s is to suppress the softening of the base steel sheet caused by the occurrence of ferrite transformation from acicular austenite in which alloy elements are concentrated in the base steel sheet. If the average cooling rate is too slow, the strength is likely to decrease. More preferably, it is 5 ° C. / s or more, more preferably 10 ° C. / s or more, and even more preferably 20 ° C. / s or more. On the other hand, if the cooling rate from 750 ° C. to 550 ° C. is too fast, a low-temperature transformation structure is also generated in the surface layer of the steel sheet, which causes hardness variation. In this respect, the average cooling rate is preferably 100 ° C. / s or less, more preferably 80 ° C. / s or less, and even more preferably 50 ° C. / s or less. The cooling rate is not limited at temperatures above 750°C because ferrite transformation is unlikely to occur. Also, the cooling rate is not limited at temperatures below 550°C because a low-temperature transformed structure is obtained.

[0090] (Cooling stop temperature and reheating in the second annealing) After the above cooling, the steel sheet may be further cooled to a temperature of 25°C to 550°C, and then reheated and retained in a temperature range of 150°C to 550°C. When the steel sheet is cooled in the above temperature range, martensite is generated from untransformed austenite during cooling. By subsequently reheating the steel sheet, carbon is concentrated from martensite to untransformed austenite, improving the strength-ductility balance of the steel sheet. The reason why the lower limit of the cooling stop temperature is set to 25°C is that excessive cooling not only requires a significant investment in equipment, but also has a saturation effect. The retention time is not particularly limited, and may be, for example, 30 seconds to 500 seconds.

[0091] (Conditions for obtaining needle-shaped retained austenite in the second annealing) In the method for producing a steel sheet according to the present embodiment, it is preferable to obtain needle-shaped retained austenite in the second annealing (IA annealing). For example, by controlling the area ratio of the retained austenite obtained through IA annealing to 10 to 50% and controlling the temperature change of the steel sheet per second within ±3°C in the holding step of IA annealing, alloy elements are segregated at the interface between ferrite and austenite during holding in the two-phase region to reduce the mobility of the interface, thereby obtaining needle-shaped retained austenite even at room temperature.

[0092] (residence temperature) Furthermore, after reheating and before immersion in the plating bath, the steel sheet may be retained in a temperature range of 350 to 550°C. Retention in this temperature range not only contributes to tempering of martensite, but also eliminates temperature unevenness in the width direction of the sheet, improving the appearance after plating. Note that when the cooling stop temperature is 350 to 550°C, retention can be performed without reheating.

[0093] (Residence time) The retention time is preferably 30 seconds or more and 300 seconds or less in order to obtain the desired effect.

[0094] (Tempering) In a series of annealing steps, after cooling the cold-rolled sheet or the plated steel sheet obtained by plating the cold-rolled sheet to room temperature, or during cooling to room temperature (but below Ms), reheating may be started, and the temperature may be held in a temperature range of 150°C to 400°C for 2 seconds or more. According to this process, the martensite generated during cooling after reheating is tempered to form tempered martensite, thereby improving hydrogen embrittlement resistance. When the tempering process is performed, if the holding temperature is too low or the holding time is too short, the martensite is not sufficiently tempered, and there is almost no change in the microstructure and mechanical properties. On the other hand, if the holding temperature is too high, the dislocation density in the tempered martensite decreases, leading to a decrease in tensile strength. Therefore, when tempering is performed, it is preferable to hold the temperature range of 150°C to 400°C for 2 seconds or more. Tempering may be performed in a continuous annealing facility, or may be performed offline after continuous annealing in a separate facility. In this case, the tempering time varies depending on the tempering temperature. That is, the lower the temperature, the longer the time, and the higher the temperature, the shorter the time.

[0095] (Plating) The steel sheet may be heated or cooled to (galvanizing bath temperature -40) ° C. to (galvanizing bath temperature +50) ° C. as necessary to perform hot-dip galvanization. A hot-dip galvanized layer is formed on the surface of the steel sheet by the hot-dip galvanization process. In this case, the corrosion resistance of the cold-rolled sheet is improved, which is preferable. In this embodiment, the type of the plating layer is not limited to a hot-dip galvanized layer, and various coating layers can be adopted. In addition, the timing of plating the surface of the steel sheet is not particularly limited. For example, in the manufacturing method according to this embodiment, in the process of cooling the cold-rolled sheet to room temperature after holding it in the two-phase region of ferrite and austenite in IA annealing, a coating layer made of zinc, aluminum, magnesium, or an alloy thereof may be formed on the front and back surfaces of the steel sheet. Alternatively, the coating layer may be formed on the front and back surfaces of the steel sheet after annealing.

[0096] (Steel sheet temperature when immersed in plating bath) The temperature of the steel sheet when immersed in the hot-dip galvanizing bath is preferably in the range from a temperature 40°C lower than the hot-dip galvanizing bath temperature (hot-dip galvanizing bath temperature -40°C) to a temperature 50°C higher than the hot-dip galvanizing bath temperature (hot-dip galvanizing bath temperature +50°C). If this temperature is lower than the hot-dip galvanizing bath temperature -40°C, heat removal during immersion in the galvanizing bath is large, and some of the molten zinc may solidify, deteriorating the appearance of the coating. If the temperature of the sheet before immersion is lower than the hot-dip galvanizing bath temperature -40°C, it is acceptable to further heat the sheet before immersion in the galvanizing bath by any method, and control the sheet temperature to the hot-dip galvanizing bath temperature -40°C or higher before immersing it in the galvanizing bath. In addition, if the temperature of the steel sheet when immersed in the galvanizing bath exceeds the hot-dip galvanizing bath temperature +50°C, operational problems due to the rise in the galvanizing bath temperature may be induced.

[0097] (Plating bath composition) The composition of the plating bath is preferably Zn-based, with an effective Al content (the total Al content minus the total Fe content in the plating bath) of 0.050 to 0.250 mass%. If the effective Al content in the plating bath is too small, Fe may penetrate excessively into the plating layer, resulting in a decrease in plating adhesion. On the other hand, if the effective Al content in the plating bath is too large, Al-based oxides that inhibit the movement of Fe and Zn atoms may be formed at the boundary between the steel sheet and the plating layer, resulting in a decrease in plating adhesion. The effective Al content in the plating bath is more preferably 0.065 mass% or more, and more preferably 0.180 mass% or less.

[0098] (Steel sheet temperature after immersion in plating bath) When alloying treatment is performed on the hot-dip galvanized layer, the steel sheet on which the hot-dip galvanized layer is formed is heated to a temperature range of 450 to 600°C. If the alloying temperature is too low, the alloying may not proceed sufficiently. On the other hand, if the alloying temperature is too high, the alloying may proceed too much, and the Fe concentration in the coating layer may exceed 15% due to the formation of Γ phase, which may deteriorate the corrosion resistance. The alloying temperature is more preferably 470°C or higher, and more preferably 550°C or lower. The alloying temperature needs to be changed depending on the component composition of the steel sheet, so it may be set while checking the Fe concentration in the coating layer.

[0099] (Pre-processing) In order to further improve the coating adhesion, the steel sheet may be coated with a coating of one or more of Ni, Cu, Co and Fe before annealing in a continuous hot-dip galvanizing line or the like.

[0100] (Post-processing) For the purpose of improving paintability and weldability, a top layer plating may be applied to the plating surface of a hot-dip galvanized steel sheet having a steel sheet and a hot-dip galvanizing coating formed on the surface of the steel sheet, or a plating surface of a galvannealed steel sheet having a steel sheet and a hot-dip galvanizing coating formed on the surface of the steel sheet, or various treatments such as chromate treatment, phosphate treatment, lubricity improvement treatment, weldability improvement treatment, etc.

[0101] (Skin pass rolling ratio) Furthermore, skin pass rolling may be performed for the purpose of correcting the steel sheet shape and introducing mobile dislocations to improve ductility. The reduction ratio of the skin pass rolling after heat treatment is preferably in the range of 0.1 to 1.5%. If it is less than 0.1%, the effect is small and control is difficult, so this is the lower limit. If it exceeds 1.5%, productivity decreases significantly, so this is the upper limit. Skin pass rolling may be performed inline or offline.

[0102] <Supplementary Information> The difference in hardness between the front and back of the steel sheet is not substantially related to the difference in number density of precipitates between the front and back of the steel sheet. That is, even if the difference in hardness between the front and back of the steel sheet is reduced, the difference in number density of precipitates between the front and back of the steel sheet cannot be reduced, the bending strength of the steel sheet is not necessarily improved, and the bending strength difference between the front and back of the steel sheet cannot be reduced. By reducing the difference in number density of precipitates between the front and back of the steel sheet as in the steel sheet according to this embodiment, the bending strength of the steel sheet can be improved and the bending strength difference between the front and back of the steel sheet can be reduced. In order to reduce the difference in number density of precipitates between the front and back of the steel sheet, it is effective to perform aging treatments 1 and 2 between the first annealing (Q annealing) and the second annealing (IA annealing) for the cold-rolled sheet as in the manufacturing method of the steel sheet according to this embodiment. Conventionally, there has been no consideration on reducing the difference in number density of precipitates between the front and back of the steel sheet, and there was no expectation of performing Q annealing, aging treatments 1 and 2, and IA annealing as in the manufacturing method according to this embodiment. EXAMPLES

[0103] An embodiment of the present invention will be described below. The present invention is not limited to this example of conditions. The present invention can adopt various conditions as long as it does not deviate from the gist of the invention and achieves its object.

[0104] (Example 1) Steels having various chemical compositions were melted to produce steel billets. These billets were inserted into a furnace heated to 1220°C, and after a homogenization treatment of holding them for 60 minutes, they were taken out into the air and hot rolled to obtain steel plates with a thickness of 2.8 mm. In the hot rolling, the finishing rolling was completed at a temperature of 910°C, and the plate was cooled to 550°C and coiled. Subsequently, the oxide scale of the hot rolled plate was removed by pickling, and the plate was cold rolled at a rolling reduction of 45.0% to a thickness of 1.54 mm. Furthermore, the cold rolled plate was Q-annealed, specifically, the temperature was raised to 930°C, and the holding time in that temperature range was 90 seconds. Next, the cold rolled plate was cooled and held at 280°C, and coiled into a coil shape with a maximum radius of 1.4 m. The area ratio of martensite in the steel plate after coiling was 90% or more for all steel components. The coil after winding was subjected to aging treatment 1 in which the coil was held at a temperature range of 6°C to 22°C for 38 hours, and then the coil was unwound and wound up to form a coil with a maximum radius of 1.4 m again. The steel sheet was bent in the opposite direction to that of aging treatment 1 and was again held at a temperature range of 6°C to 22°C for 38 hours in aging treatment 2. The aged steel sheet subjected to these two aging treatments was then subjected to IA annealing, specifically, the temperature was raised to 785°C and the holding time in that temperature range was 130 seconds. Next, the aged steel sheet was cooled to 270°C, reheated to 390°C, and held at that temperature for 140 seconds, and then cooled to room temperature, after which skin pass rolling was performed. The chemical compositions of the samples obtained by analyzing each steel sheet are shown in Tables 1 to 3. In Tables 1 to 3, "-" means that the content was below the detection limit. The balance other than the components shown in Tables 1 to 3 is Fe and impurities. Table 4 shows the evaluation results of the properties of the steel sheets that were subjected to the above-mentioned thermomechanical treatment.

[0105] In Table 4, the methods for measuring the "area ratio," "yield strength YS," "tensile strength TS," "total elongation t-El," "hole expansion ratio λ," and "difference in number density of precipitates in tempered martensite" of each structure and phase in the steel structure are as described above. For "yield strength YS," a value of 600 MPa or more was judged to be "passed." For "bending strength (bending resistance)," two evaluation indices were used: (1) the bending strength value of the steel sheet itself, and (2) the bending strength difference between the front and back surfaces of the steel sheet. Of these, the value of (1) "bending strength" was judged based on the load during VDA bending described above. Specifically, bending tests were performed on both the front and back sides of the steel plate, and the load when a 5° bend angle was applied was rated as "A" if it was 1400N or more per 1mm of plate thickness on both sides, "B" if it was 900N or more but less than 1400N, and "C" if it was less than 900N. Those that were A or B were rated as "passed." Furthermore, (2) "Difference in bending strength between the front and back sides of the steel plate" was also judged based on the load during VDA bending. Specifically, those that were given a specified bend on each of the front and back sides of the steel plate and the load difference between the front and back sides was within 3% were rated as "A," those that were more than 3% but not more than 8% were rated as "B," and those that were more than 8% were rated as "C." Those that were A or B were rated as "passed."

[0106] [Table 1-1]

[0107] [Table 1-2]

[0108] [Table 2-1]

[0109] [Table 2-2]

[0110] [Table 3-1]

[0111] [Table 3-2]

[0112] [Table 4-1]

[0113] [Table 4-2]

[0114] [Table 4-3]

[0115] The results shown in Tables 1 to 4 reveal the following:

[0116] In BA-1, the C content in the steel plate was too low, which reduced the yield strength YS and tensile strength TS of the steel, and also made it impossible to ensure sufficient bending resistance (bending strength).

[0117] BB-1 had too much C content in the steel plate, which reduced elongation and led to brittle fracture of the steel, resulting in reduced bending resistance.

[0118] In BC-1, the Si content in the steel sheet was too low, which is thought to have reduced the stability of the retained austenite structure against processing and made it impossible to suppress the precipitation of carbides in the martensite structure during aging. As a result, the total area ratio of ferrite, pearlite, and bainite increased, reducing the yield strength YS and tensile strength TS of the steel sheet. In addition, the area ratio of the retained austenite decreased, making it impossible to obtain the effect of increasing strength due to processing-induced transformation from retained austenite to martensite when the steel sheet is subjected to bending deformation, and it was also impossible to ensure sufficient bending resistance.

[0119] It is believed that the Si content in the steel sheet of BD-1 was too high, which suppressed the formation of ε carbide during aging treatment. As a result, sufficient bending resistance could not be secured.

[0120] In BE-1, the Mn content in the steel sheet was too low, which made it easier for ferrite transformation to occur during the cooling process of Q-annealing, reducing the structure ratio of martensite after Q-annealing and the area ratio of retained austenite in the final steel sheet. As a result, the yield strength YS and tensile strength TS of the steel sheet decreased, and the effect of increasing strength due to the deformation-induced transformation from retained austenite to martensite when the steel sheet was subjected to bending deformation could not be obtained, and sufficient bending resistance could not be ensured.

[0121] In BF-1, the Mn content in the steel plate was too high, so the Mn-rich layer due to micro-segregation and central segregation became prominent in the steel plate, and due to the difference in solidification speed on the front and back sides of the slab, the distribution state of the Mn-rich layer differed on the front and back sides of the steel plate, and the difference in the formation of the Mn segregation bands caused a difference in the number density of precipitates in the tempered martensite, which is thought to have caused a difference in the bending strength on the front and back sides. As a result, sufficient bending resistance could not be secured.

[0122] It is believed that the P content in the steel plate of BG-1 was too high, which led to brittle fracture of the steel plate and promoted a decrease in bending strength during bending deformation. As a result, sufficient bending resistance could not be secured.

[0123] The S content in the steel plate of BH-1 was too high, which is thought to have led to the formation of nonmetallic inclusions, which reduced the ductility of the steel plate, and also led to the formation of voids originating from the nonmetallic inclusions during bending deformation. As a result, sufficient bending resistance could not be secured.

[0124] In the case of BI-1, the Al content in the steel sheet was too high, which is believed to have excessively promoted the ferrite and bainite transformation during the cooling process during annealing. As a result, the yield strength (YS) and tensile strength (TS) of the steel sheet decreased, and sufficient bending resistance could not be secured.

[0125] The steel plate BJ-1 had too much N, which combined with Al to form a large amount of AlN, and these nitrides were thought to be the starting points for voids during bending deformation. As a result, sufficient bending resistance could not be secured.

[0126] The Ti content of the steel plate in BK-1 was too high, which resulted in the precipitation of large carbides, and these carbides are thought to be the starting points for the generation of voids during bending deformation. As a result, sufficient bending resistance could not be secured.

[0127] In BL-1, the Co content in the steel sheet was too high, which caused many fine Co carbides to precipitate, and these carbides are thought to have become the starting points for void generation during bending deformation. As a result, sufficient bending resistance could not be secured.

[0128] The Ni content of the BM-1 steel sheet was too high, which affected the peeling of the oxide scale during hot rolling and promoted the generation of scratches on the steel sheet surface. As a result, sufficient bending resistance could not be secured.

[0129] The Mo content in the steel plate of BN-1 was too high, which caused many fine Mo carbides to precipitate, and these carbides are thought to be the starting points for void generation during bending deformation. As a result, sufficient bending resistance could not be secured.

[0130] In BO-1, the Cr content in the steel plate was too high, which promoted the formation of retained austenite, and the presence of excessive retained austenite made it impossible to ensure sufficient bending resistance.

[0131] The BP-1 steel sheet had too much O content, which resulted in the formation of a large amount of oxides as inclusions, and these oxides on the punched and cut edges formed notch-like scratches and large dimples on the edges, which led to stress concentration during bending deformation and became the starting point for crack formation. As a result, sufficient bending resistance could not be secured.

[0132] The B content in the steel plate of BQ-1 was too high, which led to the formation of coarse B inclusions in the steel, and these inclusions are thought to have become the starting points for voids. As a result, sufficient bending resistance could not be secured.

[0133] The BR-1 steel sheet contained too much Nb, which caused many fine, hard Nb carbides to precipitate, and these carbides were thought to be the starting points for void formation. As a result, sufficient bending resistance could not be secured.

[0134] In BS-1, the V content in the steel plate was too high, which caused a lot of carbonitrides to precipitate, and it is believed that these carbonitrides became the starting points for void generation. As a result, sufficient bending resistance could not be secured.

[0135] It is believed that the Cu content in the steel plate of BT-1 was too high, which increased the strength of the steel plate while decreasing its ductility. As a result, the bending strength during bending deformation decreased, and sufficient bending resistance could not be secured.

[0136] In BU-1, the W content in the steel plate was too high, which is thought to have facilitated the development of voids starting from coarse carbides. As a result, sufficient bending resistance could not be secured.

[0137] In BV-1, the Ta content in the steel sheet was too high, which caused many fine Ta carbides to precipitate, and it is believed that these carbides became the starting point for voids to form. As a result, sufficient bending resistance could not be secured.

[0138] In BW-1, the Sn content in the steel plate was too high, which is thought to have caused the steel plate to become embrittled, reducing its bending strength during bending deformation, and sufficient bending resistance could not be secured.

[0139] In BX-1, the Sb content in the steel plate was too high, which is thought to have caused the steel plate to become embrittled, resulting in a decrease in bending strength during bending deformation, and sufficient bending resistance could not be ensured.

[0140] In the case of BY-1, the As content in the steel plate was too high, which is thought to have caused the steel plate to become embrittled, resulting in a decrease in bending strength during bending deformation, and sufficient bending resistance could not be secured.

[0141] In BZ-1, the Mg content in the steel sheet was too high, which resulted in the formation of coarse inclusions, which are believed to have become the starting points for void formation. As a result, sufficient bending resistance could not be secured.

[0142] In CA-1, the Ca content in the steel plate was too high, which is thought to have caused the steel plate to become embrittled, resulting in a decrease in bending strength during bending deformation, and sufficient bending resistance could not be secured.

[0143] In CB-1, the Y content in the steel sheet was too high, which resulted in the formation of large Y inclusions, which are believed to have become the starting points for void formation. As a result, sufficient bending resistance could not be secured.

[0144] In CC-1, the Zr content in the steel plate was too high, which resulted in the formation of coarse Zr inclusions, which are believed to have become the starting points for void formation. As a result, sufficient bending resistance could not be secured.

[0145] CD-1 steel sheet contained too much La, which resulted in the formation of La inclusions, which became the starting points for void formation. As a result, sufficient bending resistance could not be secured.

[0146] The cerium content in the steel plate of CE-1 was too high, which resulted in the formation of cerium inclusions, which are believed to have become the starting points for the generation of voids. As a result, sufficient bending resistance could not be secured.

[0147] In contrast, for A-1 to AZ-1, the steel sheets having the specified chemical composition were manufactured under the specified conditions, and thus the specified metal structure was obtained in the steel sheets, and the mechanical properties and bending resistance of the steel sheets were excellent. In addition, among the steel sheets obtained in Example 1, those having tempered martensite had a precipitate density of 1 precipitate / μm 2 More than 300 pieces / μm 2 It was within the following range.

[0148] (Example 2) Furthermore, in order to investigate the influence of manufacturing conditions, hot-rolled sheets having a thickness of 2.8 mm were prepared at the hot-rolling finishing temperatures shown in Tables 5 to 7 for the steel types A to AZ, which were recognized to have excellent properties in Table 1. The hot-rolled sheets were wound, pickled, and cold-rolled to prepare cold-rolled sheets. The cold-rolled sheets were annealed and aged, and further optionally plated to obtain steel sheets for property evaluation. Here, the plated steel sheets were immersed in a hot-dip galvanizing bath and then held at the temperatures shown in Tables 5 to 7 to produce alloyed hot-dip galvanized steel sheets in which an alloy plating layer of iron and zinc was provided on the surface of the steel sheets. In addition, for some steel sheets, a tempering treatment was performed in which the steel sheets were once cooled to 150 ° C. and reheated to a predetermined temperature and held for 2 seconds or more before the steel sheets were cooled to room temperature after being held at each residence temperature in the cold-rolled sheet annealing. The obtained results are shown in Tables 5 to 7. The evaluation method of the properties was the same as in Example 1.

[0149] [Table 5-1]

[0150] [Table 5-2]

[0151] [Table 5-3]

[0152] [Table 6-1]

[0153] [Table 6-2]

[0154] [Table 6-3]

[0155] [Table 7-1]

[0156] [Table 7-2]

[0157] [Table 7-3]

[0158] The results shown in Tables 5 to 7 reveal the following:

[0159] For A-2 and X-2, the aging time in aging treatment 1 was too short, so the difference in the amount of precipitates between aging treatment 1 and aging treatment 2 became large. As a result, the difference in the number density of precipitates in tempered martensite on the front and back of the steel sheet became large, and the bending resistance of the steel sheet decreased.

[0160] For C-2 and S-3, the annealing temperature in Q-annealing was too high, which is believed to have caused the surface layer of the steel sheets to be decarburized. As a result, the strength of the steel sheets was reduced and sufficient bending resistance could not be secured.

[0161] For I-2 and W-2, the annealing temperature in IA annealing was too high (the annealing temperature was outside the two-phase region of ferrite and austenite), which resulted in the excessive generation of austenite, and the reduction of tempered martensite containing precipitates, which reduced the bending strength. In addition, the residual austenite could not be secured at 10.0% or more, and the elongation also decreased.

[0162] For L-2 and Z-3, the annealing temperature in IA annealing was too low (the annealing temperature was outside the range of the two-phase region of ferrite and austenite), so austenite was not sufficiently generated and the martensite obtained in Q annealing was tempered, making it impossible to ensure 10.0% or more of retained austenite. As a result, the effect of increasing strength due to the processing-induced transformation from retained austenite to martensite when bending deformation is applied to the steel plate cannot be obtained, and sufficient bending resistance cannot be ensured.

[0163] For E-3 and AX-2, the annealing holding temperature in Q-annealing was low, and the base steel sheet could not be heated to the austenite single phase region, so the martensite area ratio after Q-annealing was reduced. As a result, a sufficient amount of ε carbide could not be precipitated in the aging treatment, and sufficient bending resistance could not be ensured. In addition, the area ratio of retained austenite was reduced, and the effect of increasing strength due to the deformation-induced transformation from retained austenite to martensite when bending deformation was applied to the steel sheet could not be obtained, and from this point of view, sufficient bending resistance could not be ensured.

[0164] For G-3 and U-2, it is considered that the bending radius in aging treatment 1 was too large, and therefore sufficient tensile strain was not generated to promote the formation of clusters or transition carbides. As a result, a sufficient amount of ε carbide could not be precipitated in aging treatment 1, and the difference in number density of precipitates in tempered martensite on the front and back of the steel sheet became large, making it impossible to ensure sufficient bending resistance.

[0165] For M-3, it is considered that the bending radius in aging treatment 2 was too large, and therefore sufficient tensile strain was not generated to promote the formation of clusters or transition carbides. As a result, a sufficient amount of ε carbide could not be precipitated in aging treatment 2, and the difference in number density of precipitates in tempered martensite on the front and back of the steel sheet became large, making it impossible to ensure sufficient bending resistance.

[0166] For N-3 and AF-2, the aging time in Aging Treatment 2 was too short, which resulted in a large difference in the amount of precipitates between Aging Treatment 1 and Aging Treatment 2. As a result, the difference in the number density of precipitates in tempered martensite on the front and back of the steel sheet became large, and the bending resistance of the steel sheet decreased.

[0167] For K-4 and AW-4, at least one of the aging treatments 1 and 2 was omitted, so that the number density of precipitates in the tempered martensite on at least one of the front and back sides of the steel sheet was not controlled, and the difference in the number density of precipitates in the tempered martensite on the front and back sides of the steel sheet became large, resulting in a decrease in bending resistance of the steel sheet.

[0168] In contrast, in the other examples, the steel sheets having the specified chemical composition were produced under the specified conditions, and the specified metal structure was obtained in the steel sheets, and the steel sheets had excellent formability and bending resistance. In addition, among the steel sheets obtained in Example 2, those that were subjected to aging treatment and contained tempered martensite had a precipitate density of 1 precipitate / μm 2 More than 300 pieces / μm 2 It was within the following range.

[0169] From the above results, it can be said that a steel sheet satisfying the following requirements (I) to (IV) has excellent mechanical properties such as strength and elongation as well as excellent bending strength.

[0170] (I) By mass%, C: 0.10-0.30%, Si: 0.60-1.20%, Mn: 1.00-3.50%, P: 0.0200% or less, S: 0.0200% or less, Al: 0.001-1.000%, N: 0.0200% or less, Ti: 0-0.500%, Co: 0-0.500%, Ni: 0-0.500%, Mo: 0-0.500%, Cr: 0-2.000%, O: 0-0.0100%, B: 0-0.0100%, Nb: 0-0.500% , V: 0-0.500%, Cu: 0-0.500%, W: 0-0.1000%, Ta: 0-0.1000%, Sn: 0-0.0500%, Sb: 0-0.0500%, As: 0-0.0500%, Mg: 0-0.0500%, Ca: 0-0.0500%, Y: 0-0.0500%, Zr: 0-0.0500%, La: 0-0.0500%, and Ce: 0-0.0500%, with the balance being Fe and impurities. (II) The steel structure includes, in terms of area ratio, a total of ferrite, pearlite and bainite: 0% or more and 30.0% or less, and retained austenite: 10.0% or more and 30.0% or less, with the remainder being martensite and tempered martensite. (III) The difference between the number density of precipitates in tempered martensite on the first surface on the front side of the steel sheet and the number density of precipitates in tempered martensite on the second surface on the back side of the steel sheet is within 10.0%. (IV) The yield strength is 600 MPa or more.

[0171] Moreover, a steel sheet satisfying the above requirements (I) to (IV) can be produced by the following method.

[0172] hot rolling the steel slab having the chemical composition of (I) to obtain a hot rolled sheet; coiling the hot-rolled sheet; pickling the hot-rolled sheet; cold rolling the hot rolled sheet to obtain a cold rolled sheet; Q annealing is performed on the cold rolled sheet; The cold-rolled sheet subjected to the Q annealing is subjected to IA annealing; and performing an aging treatment between the Q annealing and the IA annealing; Including, The Q annealing is a process of heating the cold-rolled sheet to a temperature in the austenite single phase region and 1000° C. or less, and then cooling the sheet to obtain a martensite structure having an area ratio of 90.0% or more. The IA annealing is a process of holding the cold-rolled sheet in a two-phase region of ferrite and austenite to obtain retained austenite, The aging treatment includes aging treatment 1 in which one of the front side and the back side of the cold-rolled sheet is subjected to tensile deformation with a bending R of 2.0 m or less and held at 0 to 40 ° C. for 20 hours or more, and aging treatment 2 in which the other of the front side and the back side of the cold-rolled sheet is subjected to tensile deformation with a bending R of 2.0 m or less and held at 0 to 40 ° C. for 20 hours or more. Manufacturing method of steel plate.

Claims

1. In mass percent, C: 0.10-0.30%, Si: 0.60-1.20%, Mn: 1.00-3.50%, P: 0.0200% or less, S: 0.0200% or less, Al: 0.001-1.000%, N: 0.0200% or less, Ti: 0 to 0.500%, Co: 0 to 0.500%, Ni: 0 to 0.500%, Mo: 0-0.500%, Cr: 0-2.000%, O: 0 to 0.0100%, B: 0 to 0.0100%, Nb: 0 to 0.500%, V: 0 to 0.500%, Cu: 0-0.500%, W: 0-0.1000%, Ta: 0-0.1000%, Sn: 0-0.0500%, Sb: 0 to 0.0500%, As: 0 to 0.0500%, Mg: 0 to 0.0500%, Ca: 0-0.0500%, Y: 0 to 0.0500%, Zr: 0 to 0.0500%, La: 0 to 0.0500%, and Ce: 0 to 0.0500%, and the balance being Fe and impurities, In terms of area ratio, The sum of ferrite, pearlite and bainite: 0% or more and 30.0% or less, and Retained austenite: 10.0% or more and 30.0% or less, The balance has a steel structure consisting of fresh martensite and tempered martensite, a difference between the number density of precipitates in the tempered martensite in the first surface on the front side of the steel sheet and the number density of precipitates in the tempered martensite in the second surface on the back side of the steel sheet is within 10.0%; The yield strength is 600 MPa or more. steel plate.

2. In mass percent, Ti: 0.001 to 0.500%, Co: 0.001 to 0.500%, Ni: 0.001 to 0.500%, Mo: 0.001-0.500%, Cr:0.001~2.000% O: 0.0001-0.0100% B: 0.0001 to 0.0100%, Nb: 0.001-0.500%, V: 0.001-0.500%, Cu: 0.001 to 0.500%, W: 0.0001-0.1000%, Ta: 0.0001 to 0.1000%, Sn: 0.0001 to 0.0500%, Sb: 0.0001 to 0.0500%, As: 0.0001 to 0.0500%, Mg: 0.0001-0.0500%, Ca: 0.0001-0.0500%, Y: 0.0001-0.0500%, Zr: 0.0001 to 0.0500%, La: 0.0001 to 0.0500%, and Ce: 0.0001 to 0.0500%, The chemical composition contains one or more of the following: The steel sheet according to claim 1.

3. The steel structure contains the acicular retained austenite. The steel sheet according to claim 1 or 2.

4. A method for manufacturing a steel sheet according to claim 1 or 2, hot rolling a steel slab having the chemical composition according to claim 1 or 2 to obtain a hot rolled sheet; coiling the hot-rolled sheet; pickling the hot-rolled sheet; cold rolling the hot rolled sheet to obtain a cold rolled sheet; Q annealing is performed on the cold-rolled sheet; The cold-rolled sheet subjected to the Q annealing is subjected to IA annealing; and performing an aging treatment between the Q annealing and the IA annealing; Including, The Q annealing is a process of heating the cold-rolled sheet to a temperature in the austenite single phase region and 1000° C. or less, and then cooling the sheet to obtain a martensite structure having an area ratio of 90.0% or more. The IA annealing is a process of holding the cold-rolled sheet in a two-phase region of ferrite and austenite to obtain retained austenite, The aging treatment includes aging treatment 1 in which one of the front and back sides of the cold-rolled sheet is subjected to tensile deformation with a bending R of 2.0 m or less and held at 0 to 40 ° C. for 20 hr or more, and aging treatment 2 in which the other of the front and back sides of the cold-rolled sheet is subjected to tensile deformation with a bending R of 2.0 m or less and held at 0 to 40 ° C. for 20 hr or more. Manufacturing method of steel plate.

5. In the IA annealing, the cold-rolled sheet is held in a two-phase region of ferrite and austenite, and then cooled to room temperature. In the process, a coating layer made of zinc, aluminum, magnesium, or an alloy thereof is formed on the front and back surfaces of the cold-rolled sheet. The method according to claim 4.

6. In the IA annealing, needle-shaped retained austenite is obtained. The method according to claim 4 or 5.

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