WO2023032339A1

WO2023032339A1 - Steel sheet and method for producing same

Info

Publication number: WO2023032339A1
Application number: PCT/JP2022/016848
Authority: WO
Inventors: 健悟竹田; 克哉中野
Original assignee: 日本製鉄株式会社
Priority date: 2021-08-31
Filing date: 2022-03-31
Publication date: 2023-03-09
Also published as: CN117836453A; KR20240040094A; MX2024002489A; JPWO2023032339A1

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

Steel plate and its manufacturing method

This application discloses a steel sheet and a method for manufacturing the same.

In recent years, in order to improve the fuel efficiency of automobiles, efforts have been made to reduce the weight of automobile bodies by applying high-strength steel sheets. Also, in order to ensure the safety of passengers, high-strength steel sheets are being used more often than mild steel sheets for automobile bodies. 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.

In addition, automotive parts are required to exhibit the function of suppressing deformation in the event of an automobile collision. In order to increase the resistance of automobile parts to deformation in automobile collisions, it is desirable to increase the bending resistance of automobile parts. In addition, steel sheets are required to have high formability in order to increase bending resistance from a structural standpoint by optimizing the shape of parts. Therefore, steel sheets that are applied to automobile parts are required to have high strength, excellent bending resistance, and high elongation. However, in the prior art, although the workability of high-strength steel sheets has been studied (for example, Patent Documents 1 to 3 below), sufficient bending resistance against bending deformation from both the front and back sides has not been ensured. not considered.

In Patent Document 1, as a high-strength steel sheet with excellent workability, the main phase is ferrite, the average amount of retained austenite is 5% by volume or more, and the thickness between 0.1 mm from the steel plate surface and 0.1 mm from the steel plate back surface is A steel sheet is disclosed in which the difference ΔVγ between the maximum and minimum retained austenite contents at each position in the sheet thickness direction is 3.0% by volume or less.

In Patent Document 2, C, Si, Mn, and Al are contained as a steel plate for hulls with excellent shock absorption ability that can minimize the destruction of the hull at the time of a tanker collision, and if necessary, a strengthening element A steel sheet having a thickness of 8 mm or more, the balance being Fe and inevitable impurities, wherein the front and back layers of at least 1/8 of the thickness of the steel sheet contain 1.0 to 20% residual γ in terms of area ratio. A steel plate characterized by

In Patent Document 3, as a structural thick steel plate that can dramatically improve brittle crack arrestability and Charpy characteristics at the same time without relying on the addition of expensive alloying elements such as Ni, C: 0.04 to 0.30%, Si: ≤0.5%, Mn: ≤2.0%, Al: ≤0.1%, Ti: 0.001 to 0.10%, N: 0. 001 to 0.01%, the balance being Fe and unavoidable impurities, the average grain size d of the structure in a predetermined region of the front and back layers of the plate thickness is 3 μm or less, and the Vickers hardness of the structure is A steel sheet is disclosed that satisfies certain requirements.

Japanese Patent No. 3546266 Japanese Patent No. 3499126 Japanese Patent No. 3845113

In view of the above circumstances, the present application discloses a steel sheet that is excellent in mechanical properties such as strength and elongation, as well as excellent bending resistance, and a method for manufacturing the same.

The present inventors have made intensive research on methods for solving the above problems, optimized the ratio of the steel sheet structure including retained austenite, and reduced the difference in the number density of precipitates on the front and back surfaces, thereby improving the strength and elongation. It was clarified that a steel sheet having excellent mechanical properties such as high bending strength can be obtained. In addition, it was also confirmed that, in steel sheets with a difference of more than 10% in the number density of precipitates on the front and back surfaces, the bending yield strength changes depending on the bending direction, and the deformation resistance of the part at the time of collision accidentally decreases.

In addition, the present inventors perform two annealing processes on the cold-rolled sheet, and perform predetermined aging treatment by winding and unwinding the sheet between the two annealing processes. It was found that a steel sheet with an optimized structure and a small difference in the number density of precipitates on the front and back surfaces can be manufactured by an integrated manufacturing method.

In addition, the inventors of the present invention have found that the steel sheet with improved bending yield strength by reducing the difference in number density of precipitates on the front and back surfaces as described above can be obtained by simply devising hot rolling conditions, annealing conditions, etc. Through extensive research, we also found that manufacturing is difficult, and that manufacturing can only be achieved by achieving optimization in a so-called integrated process such as hot rolling and annealing.

The gist of the present invention is as follows.
(1)
in % by mass,
C: 0.10 to 0.30%,
Si: 0.60 to 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: 0-0.500%,
Cr: 0 to 2.000%,
O: 0 to 0.0100%,
B: 0 to 0.0100%,
Nb: 0 to 0.500%,
V: 0 to 0.500%,
Cu: 0 to 0.500%,
W: 0 to 0.1000%,
Ta: 0 to 0.1000%,
Sn: 0 to 0.0500%,
Sb: 0 to 0.0500%,
As: 0 to 0.0500%,
Mg: 0-0.0500%,
Ca: 0 to 0.0500%,
Y: 0 to 0.0500%,
Zr: 0 to 0.0500%,
La: 0 to 0.0500%, and
Ce: 0 to 0.0500%,
and has a chemical composition with the balance consisting of Fe and impurities,
area ratio,
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,
and having a steel structure with the remainder consisting of fresh martensite and tempered martensite,
The difference between the number density of precipitates in the tempered martensite on the first surface on the front side of the steel sheet and the number density of precipitates in the tempered martensite on the second surface on the back side of the steel sheet is within 10.0%,
Yield strength is 600 MPa or more,
steel plate.
(2)
in % by 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 to 2.000%
O: 0.0001 to 0.0100%
B: 0.0001 to 0.0100%,
Nb: 0.001 to 0.500%,
V: 0.001 to 0.500%,
Cu: 0.001 to 0.500%,
W: 0.0001 to 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 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%,
having the chemical composition containing one or more of
The steel plate according to (1) above.
(3)
The steel structure contains the needle-shaped retained austenite,
The steel plate according to (1) or (2) above.
(4)
A method for manufacturing a steel plate,
Obtaining a hot-rolled sheet by hot-rolling a steel slab having the chemical composition described in (1) or (2) above;
winding the hot-rolled sheet;
pickling the hot-rolled sheet;
obtaining a cold-rolled sheet by cold-rolling the hot-rolled sheet;
performing Q annealing (Q: Quenching) on the cold-rolled sheet;
Performing IA annealing (IA: Intercritical Annealing) on the cold-rolled sheet that has been subjected to the Q annealing, and
performing an aging treatment between the Q annealing and the IA annealing;
including
The Q annealing is a step of heating the cold-rolled sheet to a single austenite phase region and a temperature of 1000 ° C. or less and cooling to obtain a martensite structure with an area ratio of 90.0% or more,
The IA annealing is a step of holding the cold-rolled sheet in a two-phase region of ferrite and austenite to obtain retained austenite,
In the aging treatment, 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 is held at 0 to 40 ° C. for 20 hours or more. 1 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. ,including,
A method of manufacturing a steel plate.
(5)
In the IA annealing, after the cold-rolled sheet is held in a two-phase region of ferrite and austenite, in the process of cooling to room temperature, a coating of zinc, aluminum, magnesium, or an alloy thereof on the front and back surfaces of the cold-rolled sheet to form layers,
The production method according to (4) above.
(6)
Obtaining the needle-shaped retained austenite in the IA annealing,
(4) or the production method according to (5).

The steel sheet of the present disclosure has excellent mechanical properties such as strength and elongation, as well as excellent bending strength.

Embodiments of the present invention will be described below. It should be noted that these descriptions are intended to be merely examples of embodiments of the present invention, and the present invention is not limited to the following embodiments.

<Steel plate>
The steel plate according to the present embodiment is mass %,
C: 0.10 to 0.30%,
Si: 0.60 to 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: 0-0.500%,
Cr: 0 to 2.000%,
O: 0 to 0.0100%,
B: 0 to 0.0100%,
Nb: 0 to 0.500%,
V: 0 to 0.500%,
Cu: 0 to 0.500%,
W: 0 to 0.1000%,
Ta: 0 to 0.1000%,
Sn: 0 to 0.0500%,
Sb: 0 to 0.0500%,
As: 0 to 0.0500%,
Mg: 0-0.0500%,
Ca: 0 to 0.0500%,
Y: 0 to 0.0500%,
Zr: 0 to 0.0500%,
La: 0 to 0.0500%, and
Ce: 0 to 0.0500%,
and has a chemical composition with the balance consisting of Fe and impurities,
area ratio,
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,
and having a steel structure with the remainder consisting of fresh martensite and tempered martensite,
The difference between the number density of precipitates in the tempered martensite on the first surface on the front side of the steel sheet and the number density of precipitates in the tempered martensite on 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.

First, the reason for limiting the chemical composition of the steel sheet according to the embodiment of the present invention will be explained. Here, "%" for components means % by weight. Furthermore, in this specification, the term "to" indicating a numerical range is used to include the numerical values before and after it as lower and upper limits, unless otherwise specified.

(C: 0.10-0.30%)
C is an element that increases the tensile strength at low cost, and is an extremely important element for controlling the strength of steel. Such an effect is easily obtained when the C content is 0.10% or more. The C content may be 0.12% or more. On the other hand, if C is contained excessively, the elongation is lowered, and brittle fracture of the steel is caused, which may promote a decrease in bending resistance when the part is deformed. Such problems are easily avoided when the C content is 0.30% or less. The C content may be 0.28% or less.

(Si: 0.60 to 1.20%)
Si is an element that acts as a deoxidizing agent, increases the stability of the retained austenite structure against working, and suppresses the precipitation of carbides in the martensite structure during aging. Such an effect is easily obtained when the Si content is 0.60% or more. The Si content may be 0.70% or more. On the other hand, if Si is excessively contained, the formation of ε carbide is suppressed in the aging treatment, and the bending yield strength may be lowered. Such problems are easily avoided when the Si content is 1.20% or less. The Si content may be 1.00% or less.

(Mn: 1.00-3.50%)
Mn is a factor that affects ferrite transformation of steel, suppresses ferrite transformation in the cooling process of Q annealing described later, increases the martensite structure ratio after Q annealing, and is an element effective in increasing strength. . Such an effect is easily obtained when the Mn content is 1.00% or more. The Mn content may be 1.30% or more. On the other hand, when Mn is excessively contained, a Mn-enriched layer due to micro-segregation and center segregation becomes conspicuous in the steel sheet. Since there is a difference in the distribution state of the oxide layer, the difference in the formation of the Mn segregation band may cause a difference in bending strength between the front and back surfaces. Such problems are easily avoided when the Mn content is 3.50% or less. The Mn content may be 3.00% or less.

(P: 0.0200% or less)
P is an element that strongly segregates at ferrite grain boundaries and promotes embrittlement of the grain boundaries, and is preferably as small as possible. Also, an excessive P content may lead to brittle fracture of the steel, promoting a decrease in bending resistance when parts are deformed. 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, may be 0.0001% or more, or may be 0.0010% or more.

(S: 0.0200% or less)
S is an element that forms non-metallic inclusions such as MnS in steel and causes a decrease in ductility of steel material parts. In addition, if the S content is excessive, voids originating from non-metallic inclusions may be generated when the part is deformed, and the bending strength may be lowered. 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, may be 0.0001% or more, or may be 0.0005% or more.

(Al: 0.001 to 1.000%)
Al is an element that acts as a deoxidizing agent for steel and stabilizes ferrite, and is added as necessary. Such an effect is easily obtained when the Al content is 0.001% or more. Al content may be 0.010% or more. On the other hand, an excessive Al content may excessively promote ferrite transformation and bainite transformation during the cooling process during annealing, resulting in a decrease in the strength of the steel sheet. Such problems are easily avoided when the Al content is 1.000% or less. The Al content may be 0.800% or less.

(N: 0.0200% or less)
N is an element that forms coarse nitrides in the steel sheet and reduces the workability of the steel sheet. Also, N is an element that causes blowholes during welding. Also, when N is excessively contained, it combines with Al and Ti to form a large amount of AlN or TiN, and these nitrides become starting points for void generation during part deformation, which may lead to 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 N content is not particularly limited. The N content is 0% or more, may be 0.0001% or more, or may be 0.0010% or more.

The basic chemical composition of the steel sheet in this embodiment is as described above. Furthermore, the steel sheet in the present embodiment may contain at least one of the following optional elements, if necessary. Since these elements do not have to be contained, the lower limit is 0%.

(Ti: 0 to 0.500%)
Ti is a strengthening element. It contributes to increasing the strength of the steel sheet by strengthening precipitates, strengthening fine grains by suppressing grain growth, and strengthening dislocations by suppressing recrystallization. On the other hand, if Ti is contained excessively, the precipitation of coarse carbides increases, and these carbides become starting points for the generation of voids when parts are deformed, which may lead to 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 is 0.500% or less and 0.400% or less. good too.

(Co: 0 to 0.500%)
Co is an element effective for controlling the morphology of carbides and increasing the strength, and is added as necessary for controlling the strength. On the other hand, when Co is excessively contained, a large number of fine Co carbides are precipitated, and these carbides become starting points for void generation during part deformation, which may lead to 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, and may be 0.400% or less.

(Ni: 0 to 0.500%)
Ni is a strengthening element and effective in improving hardenability. In addition, it 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 affects the peelability of oxide scale during hot rolling and promotes the generation of scratches on the surface of the steel sheet, 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, and may be 0.400% or less.

(Mo: 0-0.500%)
Mo is an element effective in improving the strength of the steel sheet. In addition, Mo is 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, when Mo is excessively contained, a large number of fine Mo carbides precipitate, and these carbides become starting points for void generation during part deformation, which may lead to 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, and may be 0.400% or less.

(Cr: 0 to 2.000%)
Cr, like Mn, suppresses pearlite transformation and is an element effective in increasing the strength of steel, and is added as necessary. On the other hand, an excessive Cr content promotes the formation of retained austenite, and the presence of excessive 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.

(O: 0 to 0.0100%)
Since O forms an oxide and deteriorates workability, it is necessary to suppress the addition amount. In particular, oxides often exist as inclusions, and if they exist on the punched edge or cut surface, they form notch-like scratches or coarse dimples on the edge, resulting in stress concentration when the part is deformed. , it may become a starting point for crack formation and lead to 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 production cost, the O content may be 0.0001% or more, or 0.0010% or more.

(B: 0 to 0.0100%)
B is an element that suppresses the formation of ferrite and pearlite in the cooling process from austenite and promotes the formation of a low temperature transformation structure such as bainite or martensite. Moreover, B is an element useful for increasing the strength of steel, and is added as necessary. On the other hand, an excessive B content leads to the formation of coarse B inclusions in the steel, and these inclusions act as starting points for the generation of voids, which may lead to a decrease in bending strength when parts are deformed. The B content is 0% or more, may be 0.0001% or more, may be 0.0010% or more, and is 0.0100% or less and 0.0080% or less. good too.

(Nb: 0 to 0.500%)
Nb is an element that is effective for controlling the morphology of carbides, and is an element that is also effective for improving toughness because its addition refines the structure. On the other hand, if Nb is contained excessively, a large number of fine and hard Nb carbides are precipitated, and these carbides become starting points for void generation, which may lead to a decrease in bending resistance when parts are deformed. The Nb content is 0% or more, and may be 0.001% or more, and may be 0.500% or less, and may be 0.400% or less.

(V: 0-0.500%)
V is a strengthening element. It contributes to increasing the strength of the steel sheet by strengthening precipitates, strengthening fine grains by suppressing the growth of ferrite grains, and strengthening dislocations by suppressing recrystallization. On the other hand, if V is excessively contained, the precipitation of carbonitrides increases, and these carbonitrides become starting points for the generation of voids, which may lead to a decrease in bending strength when parts are deformed. The V content is 0% or more, and may be 0.001% or more, and may be 0.500% or less, and may be 0.400% or less.

(Cu: 0 to 0.500%)
Cu is an element effective in improving the strength of the steel sheet. On the other hand, if Cu is contained excessively, the steel becomes embrittled during hot rolling, making hot rolling difficult. Furthermore, as the strength of steel increases, ductility decreases, which may lead to a decrease in bending resistance when parts are deformed. The Cu content is 0% or more, and may be 0.001% or more, and may be 0.500% or less, and may be 0.400% or less.

(W: 0 to 0.1000%)
W is effective in increasing the strength of steel sheets, and precipitates and crystallized substances containing W serve as hydrogen trap sites. On the other hand, an excessive W content facilitates the formation of voids originating from coarse carbides, which may lead to a decrease in bending resistance when parts are deformed. The W content is 0% or more, may be 0.0001% or more, may be 0.0010% or more, and is 0.1000% or less and 0.0800% or less. good too.

(Ta: 0 to 0.1000%)
Ta, like Nb, V, and W, is an element effective in controlling the morphology of carbides and increasing the strength, and is added as necessary. On the other hand, when Ta is excessively contained, a large number of fine Ta carbides are precipitated, and voids are easily generated starting from these carbides, which may lead to a decrease in bending strength when the part is deformed. The Ta content is 0% or more, may be 0.0001% or more, may be 0.0010% or more, and is 0.1000% or less and 0.0800% or less. good too.

(Sn: 0 to 0.0500%)
Sn is an element contained in steel when scrap is used as a raw material, and is preferably as small as possible. Excessive Sn content may cause embrittlement of the steel sheet, which may lead to a decrease in bending strength when parts are deformed. The Sn content is 0.0500% or less, and may be 0.0400% or less. Although the Sn content may be 0%, controlling the Sn content to less than 0.0001% may increase the refining time and increase the manufacturing cost. In order to prevent an increase in manufacturing cost, the Sn content may be 0.0001% or more, or may be 0.0010% or more.

(Sb: 0 to 0.0500%)
Sb, like Sn, is an element contained when scrap is used as a raw material for steel. Since Sb strongly segregates at grain boundaries and causes embrittlement of grain boundaries and deterioration of ductility, the smaller the amount, the better. In addition, excessive Sb may cause embrittlement of the steel sheet, which may lead to a decrease in bending resistance when parts are deformed. 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. The Sb content may be 0.0001% or more, or may be 0.0010% or more for the purpose of preventing an increase in manufacturing cost.

(As: 0 to 0.0500%)
Like Sn and Sb, As is contained when scrap is used as a raw material for steel, and is an element that strongly segregates at grain boundaries. In addition, excessive As content may cause embrittlement of the steel sheet, which may lead to a decrease in bending resistance when parts are deformed. The As content is 0.0500% or less, and may be 0.0400% or less. Although the As content may be 0%, controlling the As content to less than 0.0001% may increase the refining time and increase the manufacturing cost. In order to prevent an increase in manufacturing cost, the As content may be 0.0001% or more, or 0.0010% or more.

(Mg: 0 to 0.0500%)
Mg is an element capable of controlling the morphology of sulfides by adding a very small amount, and is added as necessary. On the other hand, when Mg is contained excessively, coarse inclusions are formed, and these inclusions become starting points for the generation of voids, which may lead to a decrease in bending resistance when parts are deformed. The Mg content is 0% or more, may be 0.0001% or more, may be 0.0010% or more, and is 0.0500% or less and 0.0400% or less. good too.

(Ca: 0 to 0.0500%)
Ca is useful as a deoxidizing element, and is also effective in controlling the morphology of sulfides. On the other hand, an excessive Ca content may cause embrittlement of the steel sheet, which may lead to a decrease in bending resistance when parts are deformed. The Ca content is 0% or more, may be 0.0001% or more, may be 0.0010% or more, and is 0.0500% or less and 0.0400% or less. good too.

(Y: 0 to 0.0500%)
Y, like Mg and Ca, is an element capable of controlling the morphology of sulfides by adding a very small amount, and is added as necessary. On the other hand, if Y is contained excessively, coarse Y inclusions are formed, and these inclusions become starting points for the generation of voids, which may lead to a decrease in bending resistance when parts are deformed. Y content is 0% or more, may be 0.0001% or more, may be 0.0010% or more, and is 0.0500% or less and 0.0400% or less good too.

(Zr: 0 to 0.0500%)
Zr, like Mg, Ca, and Y, is an element capable of controlling the morphology of sulfides by adding a very small amount, and is added as necessary. On the other hand, if Zr is contained excessively, coarse Zr inclusions are formed, and these inclusions become starting points for the generation of voids, which may lead to a decrease in bending resistance when parts are deformed. The Zr content is 0% or more, may be 0.0001% or more, may be 0.0010% or more, and is 0.0500% or less and 0.0400% or less. good too.

(La: 0 to 0.0500%)
La is an element effective in controlling the morphology of sulfides when added in a very small amount, and is added as necessary. On the other hand, if La is contained excessively, La inclusions are formed, and these inclusions become starting points for the generation of voids, which may lead to a decrease in bending resistance when parts are deformed. The La content is 0% or more, may be 0.0001% or more, may be 0.0010% or more, and is 0.0500% or less and 0.0400% or less. good too.

(Ce: 0 to 0.0500%)
Ce, like La, is an element capable of controlling the morphology of sulfides by adding a very small amount, and is added as necessary. On the other hand, when Ce is contained excessively, Ce inclusions are formed, and these inclusions become starting points for the generation of voids, which may lead to a decrease in bending resistance when parts are deformed. The Ce content is 0% or more, may be 0.0001% or more, may be 0.0010% or more, and is 0.0500% or less and 0.0400% or less. good too.

In the steel sheet of the present embodiment, the rest of the components mentioned above are Fe and impurities. Impurities are components and the like that are mixed 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.

Next, the characteristics of the structure and properties of the steel sheet according to the embodiment of the present invention will be described.

(Total area ratio of ferrite, pearlite and bainite: 0 to 30.0%)
Ferrite, pearlite and bainite are structures that are effective in improving the strength-ductility balance of steel sheets, but if they are contained in a large amount, they may cause local ductility to decrease. Also, from the viewpoint of efficiently increasing the strength of steel, the smaller the area ratios of ferrite, pearlite, and bainite, the better. The total area ratio of ferrite, pearlite and bainite may be 0%, may be 1.0% or more, may be 30.0% or less, and may be 25.0% or less. may be 20.0% or less. It should be noted that although the productivity is slightly lowered, it is possible to reduce the total area ratio of ferrite, pearlite and bainite to 0% by controlling the integrated manufacturing conditions with high accuracy.

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

In the steel sheet according to the present embodiment, the steel structure of the steel sheet preferably contains acicular retained austenite. The following effects can be expected because the form of retained austenite is "acicular". That is, if the shape of retained austenite is spherical (massive), deformation-induced transformation easily occurs with deformation of the steel sheet, and bending deformation may start at low stress. On the other hand, if the retained austenite has a needle-like shape, deformation-induced transformation is less likely to occur, and the bending yield strength is further increased. In the steel sheet according to the present embodiment, the effect of needle-like retained austenite and the effect of the difference in the number density of precipitates are combined to significantly improve the bending strength of the steel sheet. The area ratio of needle-shaped retained austenite may be 30% or more or 50% or more, or may be 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 one having a ratio of major axis to minor axis (major axis/minor axis) of 3.0 or more. The "major axis" and "minor axis" of retained austenite can be specified by structural observation by EBSD. Specifically, in the structure observation, one retained austenite crystal grain is specified, and the minimum Feret diameter of the crystal grain is specified as the minor axis, and the maximum Feret diameter is specified as the major axis.

(Remainder: fresh martensite and tempered martensite)
Fresh martensite and tempered martensite are microstructures that are extremely effective in increasing the strength of steel sheets, and the higher their area ratios, the better. In the steel sheet according to the present embodiment, fresh martensite and tempered martensite constitute the remainder of the ferrite, pearlite, bainite, and retained austenite. The total area ratio of fresh martensite and tempered martensite may be 40.0% or more, 45.0% or more, 50.0% or more, or 90%. 0% or less, or 85.0% or less. In addition, 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, and 50% or less. Or it may be 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 it may be 40% or less.

(Difference in number density of precipitates in tempered martensite between the first surface on the front side and the second surface on the back side of the steel sheet: 0 to 10.0%)
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 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 plate, the higher the bending yield strength. Yield occurs on the surface with a low number density, and the bending strength decreases. Therefore, it is preferable that the difference between the number density of precipitates in tempered martensite on the front side first surface of the steel sheet and the number density of precipitates in tempered martensite on the back side second surface 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 the present embodiment, the ratio A1/A2 between the number density A1 of the precipitates on the front first surface of the steel sheet and the number density A2 of the precipitates on the second surface of the back side of the steel sheet is 0. .90 or more and 1.10 or less, may be 0.92 or more, 0.94 or more, 0.96 or more, or 0.98 or more, 1.08 or less, 1.06 or less, 1.04 or less, or It may be 1.02 or less. It should be noted that setting the difference in number density to 0% causes an increase in the manufacturing load and the manufacturing cost due to precise control of the steel sheet structure. In this regard, 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 are iron-based carbides or alloy carbides in which alloy elements such as Cr, Ti, and V are combined with carbon in place of iron, in addition to cementite. may 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 plate is, for example, 1/μm ² or more, 5/μm ² or more, or 10 It may be 300/μm ² or less, 100/μm ² ^or less, or 30/μm ² or less.

In the present application, for convenience of explanation, the "front" and "back" of the steel plate are distinguished, but which of the steel plates is the front and which is the back is not particularly limited.

(Yield strength YS)
In order to reduce the weight of a structure using steel as a material and to improve the yield strength for starting plastic deformation, 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 working, so-called springback, may increase. The yield strength of the steel sheet according to the present embodiment may be 600 MPa or more, or may be 650 MPa or more. Although the upper limit of the yield strength is not particularly limited, it may be 1100 MPa or less or 1050 MPa or less from the viewpoint of suppressing the influence of the springback.

(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, fracture is likely to occur at low energy during plastic deformation, and formability may deteriorate. The tensile strength of the steel sheet is not particularly limited, but may be 900 MPa or more, 980 MPa or more, 2000 MPa or less, or 1800 MPa or less.

(Total elongation t-El)
Elongation is necessary in order to finish a complicated shape when a structure is manufactured by cold forming a steel plate as a raw material. If the total elongation is too low, the 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. good too.

(Hole expandability)
When a structure is manufactured by cold-forming a steel plate as a material, not only elongation but also hole expansibility is required in order to finish it into a complicated shape. If the hole expansibility is too small, the material may crack during cold forming. On the other hand, although a higher hole expansibility is preferable, excessively increasing the hole expansibility requires a large amount of retained austenite in the steel structure, which may reduce the yield strength during bending deformation. The hole expansion ratio λ of the steel plate is not particularly limited, but may be 20% or more, 25% or more, 90% or less, or 80% or less. There may be.

(bendability)
When a structure is manufactured by cold-forming a steel plate as a raw material, bendability is also required in order to finish it into a complicated shape. If the VDA bend 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 may be 50° or more.

(Thickness)
The plate thickness is a factor that affects the rigidity of the steel member after molding, and the greater the plate thickness, the higher the rigidity of the member. If the plate thickness is too small, the rigidity may be lowered, and the press formability may be lowered 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 increases, causing wear on the mold and a decrease in productivity. The plate thickness of the steel plate is not particularly limited, but may be 0.2 mm or more and may be 6.0 mm or less.

Next, we will describe the observation and measurement methods for the structure specified above, and the measurement and evaluation methods for the characteristics specified above.

(Method for measuring total area ratio of ferrite, pearlite, and bainite)
Texture observation is performed with a scanning electron microscope. Prior to observation, the sample for structure observation was wet-polished with emery paper and polished with diamond abrasive grains having an average particle size of 1 μm, and after finishing the observation surface to a mirror surface, the structure was etched with a 3% nitric acid alcohol solution. Keep The observation magnification is set to 3000 times, and 10 images of a field of view of 30 μm×40 μm at each 1/4 thickness position from the surface side of the steel plate are randomly photographed. Tissue ratios are determined by the point counting method. A total of 100 lattice points arranged at intervals of 3 μm in length and 4 μm in width are determined on the obtained structure image, the structure existing under the lattice points is determined, and the structure ratio contained in the steel plate is calculated from the average value of 10 sheets. Ask for Ferrite is a massive crystal grain that does not contain iron-based carbide having a major axis of 100 nm or more. Bainite is an aggregate of lath-shaped crystal grains that does not contain iron-based carbides with a major axis of 20 nm or more inside, or contains iron-based carbides with a major axis of 20 nm or more inside, and the carbide is a single variant, That is, they belong to a group of iron-based carbides elongated in the same direction. Here, the iron-based carbide group extending in the same direction means that the difference in the extending direction of the iron-based carbide group is within 5°. A bainite surrounded by grain boundaries with an orientation difference of 15° or more is counted as one bainite grain. Pearlite is a structure containing cementite that is precipitated in rows, and the area ratio is calculated using pearlite as a region photographed with bright contrast in a secondary electron image.

(Method for distinguishing between fresh martensite and tempered martensite)
Fresh martensite and tempered martensite are observed with scanning and transmission electron microscopes, and those containing Fe-based carbides inside (having 1 Fe-based carbide/μm ² or more) are classified as tempered martensite, A martensite containing almost no Fe-based carbide (less than 1 Fe-based carbide/μm ² ) is identified as fresh martensite. Regarding Fe-based carbides, those having various crystal structures have been reported, but any Fe-based carbide may be contained. A plurality of types of Fe-based carbides may exist depending on the heat treatment conditions. In the present application, the total area ratio A1 of ferrite, pearlite, and bainite is measured by the above method, the area ratio A2 of retained austenite is measured by the method described later, and the total value of the area ratios A1 and A2 is subtracted from 100%. The remainder is considered to be the total area ratio of fresh martensite and tempered martensite.

(Method for measuring area ratio of retained austenite)
The area fraction of retained austenite is determined by X-ray measurement as follows. First, a 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, from the integrated intensity ratio of the diffraction peaks (200) and (211) of the body-centered cubic (bcc) phase and (200), (220) and (311) of the face-centered cubic (fcc) phase, Calculate the area fraction of retained austenite at the center of the sheet thickness using the following formula.
Sγ=(I200f+I220f+I311f)/(I200b+I211b)×100
(Sγ is the area fraction of retained austenite in the center of the sheet thickness, I200f, I220f and I311f indicate the intensity 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.)

Samples subjected to X-ray diffraction are obtained by reducing the thickness of a steel plate from the surface to a predetermined thickness by mechanical polishing or the like, and then removing strain by chemical polishing or electrolytic polishing, etc., and reducing the thickness to 1/8 to 3/8. In the range of , the sample may be adjusted and measured according to the above-described method so that a suitable surface becomes the measurement surface. As a matter of course, the material anisotropy is further reduced by satisfying the above limitation of the X-ray intensity not only in the vicinity of 1/4 plate thickness but also in as many thicknesses as possible. However, by measuring within the range of 1/8 to 3/8 from the surface of the steel sheet, it is possible to generally represent the material properties of the entire steel sheet. Therefore, the measurement range is set to 1/8 to 3/8 of the plate thickness.

Among the retained austenite, the area ratio of needle-shaped retained austenite can be measured by EBSD, for example.

(Method for measuring 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)
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 measured as follows. First, the surface or back surface of the steel sheet (meaning the surface or back surface of the base steel sheet. For example, in the case of a surface-treated steel sheet having a surface treatment layer such as plating, the surface or back surface of the base steel sheet excluding the surface treatment layer ) is sampled at a depth of 1/8 of the thickness in the plate thickness direction, and adjusted to a thin film or extraction replica observation test piece. The test piece was observed with a transmission electron microscope at a magnification of 10,000 times, captured images in at least 30 fields of view were obtained, and the number density of precipitates per unit area was measured in each observed image. , and the value obtained by arithmetically averaging the number density for 30 fields of view is defined 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 with a transmission electron microscope at a magnification of 10,000 times is a rectangular region with a side of about 600 nm, and the area of 30 fields of view for measurement of the number density of precipitates is about 10.8 μm ^{2 .} is the size of

(Method for measuring yield strength YP, tensile strength TS, total elongation t-El)
A tensile test for measuring yield strength, tensile strength and total elongation conforms to JIS Z 2241: 2011, and a JIS No. 5 test piece is taken from a direction in which the longitudinal direction of the test piece is parallel to the rolling direction of the steel strip. do.

(Method for measuring hole expansibility)
The hole expansibility was measured by punching out a circular hole with a diameter of 10 mm under the conditions of a clearance of 12.5%, placing the burr on the die side, forming with a 60° conical punch, and measuring the hole expansion ratio λ (%). evaluate. A hole expansion test is carried out five times, and the average value is taken as the hole expansion ratio.

(Method for measuring bendability)
Bendability is performed using a test piece with a width of 60 mm according to the provisions of Standard 238-100 of the German Automobile Manufacturers Association (Verband der Automobilindustrie: VDA), and the VDA bending angle is measured by measuring the maximum bending angle α. evaluate. The bending strength is evaluated by dividing the load at a bending angle of 5° by the plate thickness.

<Manufacturing method of steel plate>
The steel sheet manufacturing method according to the present embodiment is characterized by consistently managing hot rolling, cold rolling, and annealing using the material having the chemical composition described above. Specifically, in the method for manufacturing a steel sheet according to the present embodiment, a steel slab (steel slab) having the same chemical composition as the chemical composition described above for the steel sheet is hot-rolled, coiled, and the obtained hot-rolled It is characterized by including the steps of pickling, cold-rolling, annealing, aging and then re-annealing the sheet. More specifically, the steel sheet manufacturing method according to the present embodiment includes:
Obtaining a hot-rolled sheet by hot-rolling the steel slab having the above chemical composition;
winding the hot-rolled sheet;
pickling the hot-rolled sheet;
obtaining a cold-rolled sheet by cold-rolling the hot-rolled sheet;
performing a first annealing (Q annealing) on the cold-rolled sheet;
Performing second annealing (IA annealing) on the cold-rolled sheet that has been subjected to the first annealing, and
performing an aging treatment between the first annealing and the second annealing;
including
The first annealing is a step of heating the cold-rolled sheet to a single austenite phase region and a temperature of 1000 ° C. or less and cooling to obtain a martensite structure with an area ratio of 90.0% or more,
The second annealing is a step of holding the cold-rolled sheet in a two-phase region of ferrite and austenite to obtain retained austenite,
In the aging treatment, 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 is held at 0 to 40 ° C. for 20 hours or more. 1 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. , is characterized by including Each step will be described in detail below, focusing on the point of this embodiment.

(Finish rolling temperature of hot rolling)
In this embodiment, a hot-rolled sheet is obtained by performing hot rolling on a steel slab obtained by a known method such as continuous casting. Here, the finish rolling temperature of hot rolling is a factor that exerts an effect on the control of the texture of the prior austenite grain size. The finish rolling temperature is preferably 650 ° C. or higher from the viewpoint that the austenite rolling texture develops and causes the anisotropy of the steel material properties. The rolling temperature is desirably 950° C. or lower.

(coil winding temperature)
The temperature at which the hot-rolled sheet is coiled (the coiling temperature of the hot-rolled coil) is a factor that controls the state of oxide scale formation in the hot-rolled sheet and affects the strength of the hot-rolled sheet. It is preferable that the scale formed on the surface of the hot-rolled sheet is thin, and therefore the coiling temperature is preferably low. In addition, if the winding temperature is extremely lowered, special equipment is required. Also, if the coiling temperature is too high, as described above, the oxide scale formed on the surface of the hot-rolled sheet becomes extremely thick. From the above point of view, the temperature at which the hot-rolled sheet is wound may be 700° C. or lower, 680° C. or lower, 0° C. or higher, or 20° C. or higher. good too.

(Pickling of hot-rolled sheet)
The hot-rolled sheet is pickled for the purpose of removing scale, etc., and may be pickled under known pickling conditions.

(Draft reduction in cold rolling)
In cold rolling, if the total rolling reduction is too large, the ductility of the base steel plate is lost, increasing the risk of breakage of the base steel plate during cold rolling. In this regard, the total rolling reduction in cold rolling is preferably 85% or less. On the other hand, in order to sufficiently promote recrystallization in the annealing step, the total rolling reduction is preferably 20% or more, more preferably 30% or more. Annealing may be performed at a temperature of 700° C. or less for the purpose of reducing the cold rolling load before cold rolling.

(Holding temperature in the first annealing)
In the first annealing (Q annealing), the cold-rolled steel sheet, which is the base material steel sheet, is heated to a temperature of Ac 3 or more and 1000° C. or less (that is, austenite single phase region and 1000° C. or less). The reason why the maximum heating temperature is set to Ac3 or more is that the base material steel plate is heated to the austenite single phase region, and then rapidly cooled to obtain a martensite structure with an area ratio of 90% or more, and ε due to aging. This is for promoting the precipitation of carbides. If the steel is held at a lower temperature than this, a martensite-based structure cannot be obtained, and the flexural strength is remarkably lowered. On the other hand, if the steel sheet is heated to over 1000° C., the surface layer of the steel sheet is decarburized and the strength is lowered, so that the bending strength may be lowered.

(Holding time in the first annealing)
In the first annealing (Q annealing), it is preferable to hold at a heating temperature of Ac3 point or more and 1000° C. or less for 5 seconds or more. If the holding time is too short, the progress of the austenite transformation of the base material steel sheet will be insufficient, and in addition, the enrichment of substitutional elements that stabilize austenite, such as Mn, in austenite will be insufficient, so retained austenite will be inadequate. This is because there are cases where the ductility of the steel sheet decreases significantly due to the stability. From these points of view, the holding time is more preferably 10 seconds or longer. More preferably, it is 20 seconds or longer.

(Atmosphere in first annealing)
In the first annealing (Q annealing), the oxygen potential in one or both of the heating zone and soaking zone during annealing may be controlled in order to provide a decarburized layer on the surface layer of the steel sheet and improve bendability. Specifically, the annealing is preferably performed in an atmosphere containing 0.1 to 30% by volume of hydrogen and H ₂ O with a dew point of −40 to 20° C., the balance being nitrogen and impurities. More preferably, an atmosphere containing 0.5 to 20% by volume of hydrogen and H ₂ O with a dew point of −30 to 15° C., still more preferably 1 to 10% by volume of hydrogen and H ₂ O with a dew point of −20 to 10° C. It is an atmosphere that includes

(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 martensitic structure with an area ratio of 90% or more can be obtained, but may be, for example, 3°C/s. The reason why the lower limit of the average cooling rate is set 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 from becoming less than 90%. . It is more preferably 10° C./s or higher, still more preferably 15° C./s or higher, still more preferably 20° C./s or higher. On the other hand, if the cooling rate from 750° C. to 550° C. is too fast, a low-temperature transformation structure occurs in the surface layer of the steel sheet, causing variations in hardness. 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. At 750° C. or higher, ferrite transformation hardly occurs, so the cooling rate is not limited. At a temperature of 550° C. or less, a low-temperature transformed structure is obtained, so the cooling rate is not limited.

(Cooling stop temperature and reheating in the first annealing)
Further, after cooling as described above, it may be further cooled to a temperature of 25°C to 550°C and then retained 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 equipment investment but also the effect is saturated. The residence time is not particularly limited, but may be, for example, 30 seconds to 500 seconds.

(Aging treatment 1)
Maintaining a steel plate controlled to a martensite-based structure by the first annealing at 0 to 40 ° C. for 20 hours or more in a state in which bending deformation with a bending radius R of 2.0 m or less is applied is the bending of the steel plate. It is an important factor in increasing the yield strength. Carbon atoms dissolved in the martensite during this treatment form clusters or transition carbides, which nucleate carbide precipitation during the subsequent second annealing at elevated temperatures. In order to finely disperse the carbides and increase the bending strength, it is important to allow clusters or transition carbides, which serve as nuclei for the precipitation of carbides, to exist finely and at a high density. Utilization of tensile strain is extremely effective in promoting the formation of clusters or transition carbides, and this effect is likely to be 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, by winding the steel sheet (steel strip) after the first annealing to form a coil, the steel sheet can be subjected to the bending deformation described above.

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 coarsely formed (the number of nuclei is reduced. ), it is difficult to obtain fine carbides in the second annealing, and the bending yield strength may decrease. If the holding temperature in aging treatment 1 and aging treatment 2 described later is within the range of 0 to 40° C., the difference in the number of precipitates between aging treatment 1 and aging treatment 2 becomes small, and the first The difference between the number density of precipitates on the surface 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 higher, 10° C. or higher, 35° C. or lower, or 30° C. or lower.

Furthermore, when 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. It may be difficult to keep the difference from the number density of precipitates on the second surface within 10%. The holding time is preferably longer, and may be 30 hours or longer, 40 hours or longer, or 50 hours or longer. If the holding time exceeds 300 hr, the clustering of carbon atoms or the formation of transition carbides is saturated, and if the holding time is longer than that, the morphology (size) of the precipitates hardly changes significantly, so the holding time is 300 hr. It may be below. When the holding time is long, the precipitates become larger, but the number of precipitates does not change significantly. That is, if the holding time of aging treatment 1 and aging treatment 2 described later is 20 hours or more, the number of nuclei generated is stable, and the number density of precipitates on the first surface on the front side of the steel sheet and the second surface on the back side of the steel sheet The difference from the number density of precipitates on the surface is within 10%.

(Aging treatment 2)
When bending deformation is applied to the steel sheet to promote aging, clustering of carbon atoms and precipitation of transition carbides occur remarkably in the region subjected to tensile deformation. If the steel sheet is only cracked, the precipitates are finely dispersed only on one side of the steel sheet. Therefore, in the manufacturing method according to the present embodiment, after aging treatment 1 for precipitating and dispersing precipitates on one of the front side and the back side of the steel sheet, precipitates on the other side of the front side and the back side of the steel sheet. Aging treatment 2 is performed to precipitate and disperse substances. For example, after the first annealing, the sheet is wound up in a coil shape so that the front side of the sheet is outside and the back side is inside, and the front side of the sheet is subjected to tensile deformation with a bending radius R of 2.0 m or less for aging. After processing 1, the coil is unwound, and the plate is wound again in a coil shape so that the back side of the plate is outside and the front side is inside, and the bending radius R is 2.0 m on the back side of the plate. Aging treatment 2 may be performed by giving the following tensile deformation. 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, the number density of precipitates on the first surface of the front side of the steel sheet and the number density of precipitates on the back side of the steel sheet The difference from the number density of precipitates on the second surface can be suppressed 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 the number density of precipitates within 10%.

Also in aging treatment 2, similarly to aging treatment 1, if the holding temperature is less than 0°C, the clustering of carbon atoms or the formation of transition carbides is suppressed, and if the holding temperature exceeds 40°C, transition carbides are formed. Since the grains are coarsely formed (the number of nuclei is reduced), it is difficult to obtain fine carbides in the second annealing, and the bending yield strength may be lowered. As described above, if the holding temperature in aging treatment 1 and aging treatment 2 described later is within the range of 0 to 40 ° C., the number density of precipitates on the first surface of the front side of the steel sheet and the number density of The difference from the number density of precipitates is within 10%. The holding temperature may be 5° C. or higher, 10° C. or higher, 35° C. or lower, or 30° C. or lower.

Furthermore, in aging treatment 2, similarly to aging treatment 1, when the holding time is less than 20 hr, the number of nuclei generated is not stable, It may be difficult to suppress the difference from the number density of the precipitates on the second surface of the second surface to within 10%. The holding time is preferably longer, and may be 30 hours or longer, 40 hours or longer, or 50 hours or longer. As in aging treatment 1, when 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, the morphology (size) of the precipitates hardly changes significantly. Therefore, the retention time may be 300 hr or less.

(Holding temperature in the second annealing)
In the second annealing (IA annealing), the holding temperature is the temperature at which the two-phase region of ferrite and austenite occurs. For example, it is preferably 720° C. or higher and 860° C. or lower. If the annealing temperature is less than 720°C, austenite will not be generated sufficiently. In this case, the martensite obtained by the first annealing (Q annealing) is tempered, causing the precipitation of carbides, which may make it impossible to satisfy the predetermined area ratio of retained austenite. In addition, since the area ratio of austenite at the maximum heating temperature (annealing temperature) is also reduced, it becomes impossible to concentrate the carbon necessary to obtain retained austenite in austenite, ensuring retained austenite of 10.0% or more. may become impossible. On the other hand, if the annealing temperature exceeds 860°C, austenite is excessively generated, and the tempered martensite containing precipitates is reduced, resulting in a decrease in bending strength, and in addition, it is difficult to ensure a retained austenite of 10.0% or more. may not be possible. Therefore, the upper limit of the holding temperature in the second annealing is preferably 860°C. Annealing may be performed in the atmosphere, or may be performed in an atmosphere in which the hydrogen concentration and dew point are controlled for the purpose of improving the adhesion of the plating.

(Holding time in second annealing)
In the second annealing (IA annealing), it is preferable to hold the heating temperature at 720° C. or higher and 860° C. or lower for 5 seconds or longer. If the holding time is too short, the progress of the austenite transformation of the base material steel sheet will be insufficient, and in addition, the enrichment of substitutional elements that stabilize austenite, such as Mn, in austenite will be insufficient, so retained austenite will be inadequate. This is because there are cases where the ductility of the steel sheet decreases significantly due to the stability. From these points of view, the holding time is more preferably 10 seconds or longer. More preferably, it is 20 seconds or longer.

(Atmosphere in second annealing)
In the second annealing (IA annealing), in the same way as in the first annealing (Q annealing), one of the heating zone and the soaking zone during annealing is used to provide a decarburized layer on the surface layer of the steel sheet to improve bendability. Or you may control the oxygen potential in both. Specifically, the annealing is preferably performed in an atmosphere containing 0.1 to 30% by volume of hydrogen and H ₂ O with a dew point of −40 to 20° C., the balance being nitrogen and impurities. More preferably, an atmosphere containing 0.5 to 20% by volume of hydrogen and H ₂ O with a dew point of −30 to 15° C., still more preferably 1 to 10% by volume of hydrogen and H ₂ O with a dew point of −20 to 10° C. It is an atmosphere that includes

(Cooling rate in 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 why the lower limit of the average cooling rate is 2.5° C./s is to suppress softening of the base steel sheet due to ferrite transformation from needle-shaped austenite in which alloying elements are concentrated in the base steel sheet. be. If the average cooling rate is too slow, the strength tends to decrease. It is more preferably 5° C./s or more, still more preferably 10° C./s or more, still 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 occurs in the surface layer of the steel sheet, causing variations in hardness. 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. At 750° C. or higher, ferrite transformation hardly occurs, so the cooling rate is not limited. At a temperature of 550° C. or less, a low-temperature transformed structure is obtained, so the cooling rate is not limited.

(Cooling stop temperature and reheating in second annealing)
Further, after cooling as described above, it may be further cooled to a temperature of 25° C. to 550° C. and then reheated to a temperature range of 150° C. to 550° C. for retention. When cooling is performed in the above temperature range, martensite is formed from untransformed austenite during cooling. After that, by performing reheating, carbon is concentrated from martensite to untransformed austenite, and the strength and ductility balance of the steel sheet is improved. 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 equipment investment but also the effect is saturated. The residence time is not particularly limited, but may be, for example, 30 seconds to 500 seconds.

(Conditions for obtaining acicular retained austenite in the second annealing)
In the steel sheet manufacturing method according to the present embodiment, it is preferable to obtain acicular retained austenite in the second annealing (IA annealing). For example, the area ratio of retained austenite obtained through IA annealing is 10 to 50%, and the temperature is controlled so that the temperature change per second of the steel sheet is within ± 3 ° C in the holding process of IA annealing. By segregating an alloying element at the interface between ferrite and austenite while the two-phase region is maintained, and lowering the mobility of the interface, needle-like retained austenite can be obtained even at room temperature.

(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 the tempering of martensite, but also eliminates the temperature unevenness in the width direction of the sheet and improves the appearance after plating. In addition, when the cooling stop temperature is 350° C. to 550° C., the residence may be performed without reheating.

(Residence time)
The residence time is preferably 30 seconds or more and 300 seconds or less in order to obtain the effect.

(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 (however, Ms or less), reheating is started, and 150 C. to 400.degree. C. for 2 seconds or more. According to this step, the hydrogen embrittlement resistance can be improved by tempering the martensite generated during cooling after reheating to obtain tempered martensite. When performing a tempering process, if the holding temperature is too low or the holding time is too short, the martensite will not be sufficiently tempered and there will be little change in microstructure and mechanical properties. On the other hand, if the holding temperature is too high, the dislocation density in the tempered martensite will decrease, resulting in a decrease in tensile strength. Therefore, when tempering, it is preferable to hold the temperature in the temperature range of 150° C. or higher and 400° C. or lower for 2 seconds or longer. Tempering may be performed in a continuous annealing facility, or off-line after continuous annealing in a separate facility. At this time, 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.

(Plating)
If necessary, the steel sheet may be heated or cooled to (galvanizing bath temperature -40)°C to (galvanizing bath temperature +50)°C to be hot-dip galvanized. A hot-dip galvanized layer is formed on the surface of the steel sheet by the hot-dip galvanizing process. In this case, the corrosion resistance of the cold-rolled sheet is improved, which is preferable. In this embodiment, the type of plating layer is not limited to the hot-dip galvanized layer, and various coating layers can be employed. Also, the timing of plating the surface of the steel sheet is not particularly limited. For example, in the manufacturing method according to the present embodiment, in the IA annealing, after holding the cold-rolled sheet in the two-phase region of ferrite and austenite, in the process of cooling to room temperature, zinc, aluminum, and magnesium are added to the front and back surfaces of the steel sheet. Or you may form the coating layer which consists of these alloys. Alternatively, the coating layer may be formed on the front and back surfaces of the steel sheet after annealing.

(Steel plate temperature when immersed in plating bath)
The temperature of the steel sheet when immersed in the hot-dip galvanizing bath ranges from 40°C lower than the hot-dip galvanizing bath temperature (hot-dip galvanizing bath temperature -40°C) to 50°C higher than the hot-dip galvanizing bath temperature (hot-dip galvanizing bath temperature +50° C.) is preferred. If this temperature is lower than the hot-dip galvanizing bath temperature of −40° C., a large amount of heat is removed during immersion in the galvanizing bath, and part of the molten zinc solidifies, which may deteriorate the appearance of the coating. If the plate temperature before immersion is lower than the hot-dip galvanizing bath temperature of -40°C, heat the plate further before immersion in the galvanizing bath by any method to control the plate temperature to the hot-dip galvanizing bath temperature of -40°C or higher. It may be immersed in the plating bath. Moreover, if the steel sheet temperature during immersion in the galvanizing bath exceeds +50° C. of the hot-dip galvanizing bath temperature, problems in operation may be induced due to the increase in the galvanizing bath temperature.

(Composition of plating bath)
The composition of the plating bath is preferably composed mainly of Zn and has an effective Al content (a value obtained by subtracting the total Fe content from the total Al content in the plating bath) of 0.050 to 0.250% by mass. If the effective amount of Al in the plating bath is too small, Fe may excessively penetrate into the plating layer and the adhesion of the plating may deteriorate. On the other hand, if the effective amount of Al in the plating bath is too large, an Al-based oxide that inhibits the movement of Fe atoms and Zn atoms is generated at the boundary between the steel sheet and the plating layer, which may reduce the adhesion of the plating. . The effective Al content in the plating bath is more preferably 0.065% by mass or more, and more preferably 0.180% by mass or less.

(Steel sheet temperature after entering the plating bath)
When the hot-dip galvanized layer is alloyed, 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 proceeds too much, and the Fe concentration in the coating layer exceeds 15% due to the generation of the 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. Since the alloying temperature needs to be changed according to the composition of the steel sheet, it can be set while checking the Fe concentration in the coating layer.

(pre-treatment)
In order to further improve plating adhesion, the steel sheet may be plated with one or more of Ni, Cu, Co, and Fe before annealing or the like in the continuous hot-dip galvanizing line.

(post-processing)
On the coated surface of a hot-dip galvanized steel sheet having a steel sheet and a hot-dip galvanized coating formed on the surface of the steel sheet, or on the coated surface of a hot-dip galvannealed steel sheet having a steel sheet and an alloyed hot-dip galvanized coating formed on the surface of the steel sheet For the purpose of improving paintability and weldability, it is also possible to apply an upper layer plating and various treatments such as chromate treatment, phosphate treatment, lubricity improvement treatment, weldability improvement treatment, and the like.

(Skin pass rolling rate)
Further, skin pass rolling may be performed for the purpose of improving ductility by correcting the shape of the steel sheet or introducing mobile dislocations. The rolling reduction of 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%, the productivity drops significantly, so this is made the upper limit. A skin pass may be performed inline or offline.

<Supplement>
The difference in hardness between the front and back surfaces of the steel sheet is substantially unrelated to the difference in number density of precipitates on the front and back surfaces of the steel sheet. That is, even if the difference in hardness between the front and back sides of the steel sheet is reduced, the difference in the number density of precipitates on the front and back sides of the steel sheet cannot be reduced, and the bending strength of the steel sheet does not necessarily improve. cannot be made smaller. As in the steel sheet according to the present embodiment, by reducing the difference in the number density of precipitates on the front and back of the steel sheet, the bending strength of the steel sheet can be improved, and the difference in bending strength on the front and back of the steel sheet can be reduced. In order to reduce the difference in the number density of precipitates on the front and back of the steel sheet, the first annealing (Q annealing) and the second annealing (IA annealing) of the cold-rolled sheet are performed as in the steel sheet manufacturing method according to the present embodiment. It is effective to perform aging treatments 1 and 2 between. Conventionally, no study has been made to reduce the difference in the number density of precipitates on the front and back of the steel sheet, and Q annealing, aging treatments 1 and 2, and IA annealing are performed as in the manufacturing method according to the present embodiment. I didn't make any assumptions about.

Examples according to the present invention are shown below. The present invention is not limited to this one conditional example. The present invention can adopt various conditions as long as it achieves its purpose without departing from the gist thereof.

(Example 1)
Billets were produced by melting steels with various chemical compositions. These steel slabs were placed in a furnace heated to 1220° C., held for 60 minutes for homogenization, taken out into the atmosphere, and hot rolled to obtain a steel plate having a thickness of 2.8 mm. In hot rolling, the completion temperature of finish rolling was 910°C, and the coil was cooled to 550°C. Subsequently, the oxide scale of this hot-rolled sheet was removed by pickling, and the sheet was cold-rolled at a rolling reduction of 45.0% to finish the sheet to a thickness of 1.54 mm. Further, the cold-rolled sheet was Q-annealed, specifically, the temperature was raised to 930° C., and the holding time in that temperature range was 90 seconds. The cold-rolled sheet was then cooled and held at 280° C. and wound into a coil shape with a maximum radius of 1.4 m. The area ratio of martensite in the steel sheet after coiling was 90% or more for any steel composition. The coil after winding is subjected to aging treatment 1 in which it is held in a temperature range of 6 ° C. to 22 ° C. for 38 hours, and then the coil is paid out and coiled to form a coil with a maximum radius of 1.4 m again. The steel plate was again subjected to aging treatment 2 in which the steel plate was subjected to bending in the opposite direction to that of aging treatment 1 and maintained at a temperature range of 6° C. to 22° C. for 38 hours. Next, the aged steel sheet subjected to two aging treatments was 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., held at that temperature for 140 seconds, cooled to room temperature, and skin-pass rolled. Tables 1 to 3 show the chemical compositions obtained by analyzing the samples taken from each of the obtained steel sheets. In Tables 1 to 3, "-" means 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 subjected to the above heat treatment.

In Table 4, the "area ratio", "yield strength YS", "tensile strength TS", "total elongation t-El", "hole expansion ratio λ" and "tempered martensite" of each structure/phase in the steel structure The measurement method for each of the "difference in the number density of precipitates in the medium" is as described above. Regarding the "yield strength YS", those of 600 MPa or more were determined as "acceptable". As for the "bending strength (bending resistance)", (1) the value of the bending strength of the steel sheet itself, and (2) the difference in bending strength between the front and back surfaces of the steel sheet were used as evaluation indices. Of these values, (1) the value of "bending yield strength" was determined based on the load during VDA bending described above. Specifically, a bending test is performed from both the front and back surfaces of the steel plate, and "A" indicates that the load is 1400 N or more per 1 mm of the plate thickness when a bending angle of 5° is applied, and 900 N or more and less than 1400 N. Those with a N of less than 900 N were judged as "B", those with A or B were judged as "pass". Furthermore, (2) the difference in bending yield strength between the front and back surfaces of the steel sheet was also determined based on the load during VDA bending. Specifically, "A" indicates that the difference in load between the front and back surfaces of the steel plate is within 3% when a predetermined bending is applied to each of the front and back surfaces, and "A" is more than 3% and 8% or less. was judged to be "B", those exceeding 8% were judged to be "C", and those of A or B were judged to be "pass".

The results shown in Tables 1 to 4 reveal the following.

BA-1 had too little C content in the steel plate, so the yield strength YS and tensile strength TS of the steel were lowered, and sufficient bending resistance (bending strength) could not be secured.

BB-1 had too much C content in the steel sheet, so the elongation decreased and the steel brittle fracture occurred, resulting in a decrease in bending resistance.

With BC-1, the Si content in the steel sheet was too low, so it is thought that the stability of the retained austenite structure against working decreased and the precipitation of carbides in the martensite structure during aging could not be suppressed. As a result, when the total area ratio of ferrite, pearlite, and bainite increases, the yield strength YS and tensile strength TS of the steel sheet decrease, and when the area ratio of retained austenite decreases and bending deformation is applied to the steel sheet, It was not possible to obtain the effect of strength increase due to deformation-induced transformation from retained austenite to martensite, and it was also not possible to ensure sufficient bending resistance.

It is believed that BD-1 has an excessively high Si content in the steel sheet, so the formation of ε carbides was suppressed during the aging treatment. As a result, sufficient bending resistance could not be ensured.

BE-1 has too little Mn content in the steel sheet, so ferrite transformation easily occurs in the cooling process of Q annealing, the structure ratio of martensite after Q annealing decreases, and retained austenite in the finally obtained steel sheet area ratio also decreased. As a result, the yield strength YS and tensile strength TS of the steel sheet are lowered, and when bending deformation is applied to the steel sheet, the effect of increasing the strength due to the deformation-induced transformation from retained austenite to martensite cannot be obtained. It was not possible to ensure sufficient bending resistance.

In BF-1, since the Mn content in the steel sheet was too high, a Mn-enriched layer due to micro-segregation and center segregation appeared prominently in the steel sheet, and due to the difference in solidification speed between the front and back surfaces of the slab, Since there is a difference in the distribution state of the Mn-enriched layer on the front and back surfaces of the steel sheet, the difference in the formation of the Mn segregation bands causes a difference in the number density of precipitates in the tempered martensite, resulting in a difference in the bending strength of the front and back surfaces. It is considered to be As a result, sufficient bending resistance could not be ensured.

It is believed that BG-1 caused brittle fracture of the steel sheet because the P content in the steel sheet was too high, promoting a decrease in bending yield strength during bending deformation. As a result, sufficient bending resistance could not be ensured.

In BH-1, since the S content in the steel sheet was too large, non-metallic inclusions were formed, leading to a decrease in ductility of the steel sheet, and voids originating from the non-metallic inclusions during bending deformation. it is conceivable that. As a result, sufficient bending resistance could not be ensured.

It is believed that BI-1 has an excessively high Al content in the steel sheet, which excessively promotes ferrite transformation and bainite transformation during the cooling process during annealing. As a result, the yield strength YS and tensile strength TS of the steel sheet were lowered, and sufficient bending resistance could not be ensured.

In BJ-1, the N content in the steel sheet was too high, so it is believed that a large amount of AlN was formed by combining with Al, and these nitrides became the starting points for the generation of voids during bending deformation. As a result, sufficient bending resistance could not be ensured.

In BK-1, since the Ti content in the steel plate was too high, a large amount of coarse carbide precipitated, and it is believed that these carbides became the starting points for the generation of voids during bending deformation. As a result, sufficient bending resistance could not be ensured.

In BL-1, since the Co content in the steel sheet was too high, a large number of fine Co carbides were precipitated, and it is believed that these carbides became the starting points for the generation of voids during bending deformation. As a result, sufficient bending resistance could not be ensured.

It is believed that BM-1 had an excessive Ni content in the steel sheet, which affected the exfoliation of oxide scale during hot rolling and promoted the occurrence of scratches on the steel sheet surface. As a result, sufficient bending resistance could not be ensured.

With BN-1, the Mo content in the steel sheet was too high, so a large number of fine Mo carbides were precipitated, and it is believed that these carbides became the starting points for the generation of voids during bending deformation. As a result, sufficient bending resistance could not be ensured.

BO-1 had too much Cr content in the steel sheet, which promoted the formation of retained austenite, and due to the presence of excessive retained austenite, sufficient bending resistance could not be secured.

BP-1 has too much O content in the steel sheet, so a large amount of oxides are generated as inclusions, and notch-like scratches and coarse dimples are formed on the end faces due to the oxides on the punched end faces and cut faces. It is thought that stress concentration was caused during bending deformation and became the starting point of crack formation. As a result, sufficient bending resistance could not be ensured.

It is believed that BQ-1 caused the formation of coarse B inclusions in the steel because the B content in the steel sheet was too high, and these inclusions became the starting points for the generation of voids. As a result, sufficient bending resistance could not be ensured.

In BR-1, since the Nb content in the steel sheet was too high, a large number of fine, hard Nb carbides were precipitated, and it is believed that these carbides became the starting points for the generation of voids. As a result, sufficient bending resistance could not be ensured.

　Because BS-1 contained too much V in the steel sheet, it is believed that precipitation of carbonitrides increased, and these carbonitrides became the starting points for the generation of voids. As a result, sufficient bending resistance could not be ensured.

It is thought that BT-1 has an excessively high Cu content in the steel sheet, resulting in a decrease in ductility as the strength of the steel sheet increases. As a result, the bending resistance during bending deformation was lowered, and sufficient bending resistance could not be secured.

BU-1 is thought to be because the W content in the steel sheet was too high, which facilitated the development of voids starting from coarse carbides. As a result, sufficient bending resistance could not be ensured.

In BV-1, the Ta content in the steel sheet was too high, so many fine Ta carbides were precipitated, and it is believed that these carbides facilitated the generation of voids starting from these carbides. As a result, sufficient bending resistance could not be ensured.

With BW-1, the steel sheet contained too much Sn, so it was considered that the steel sheet became embrittled and the bending resistance at the time of bending deformation decreased, and sufficient bending resistance could not be secured.

With BX-1, the Sb content in the steel sheet was too high, so it is thought that the steel sheet became embrittled, resulting in a decrease in bending resistance during bending deformation, and sufficient bending resistance could not be secured.

For BY-1, the As content in the steel sheet was too high, so it was considered that the steel sheet became embrittled and the bending resistance at the time of bending deformation decreased, and sufficient bending resistance could not be secured.

In BZ-1, since the Mg content in the steel sheet was too high, coarse inclusions were formed, and it is believed that these inclusions became the starting points for the generation of voids. As a result, sufficient bending resistance could not be ensured.

With CA-1, the steel sheet had too much Ca content, so it was considered that the steel sheet became embrittled and the bending resistance at the time of bending deformation decreased, and sufficient bending resistance could not be secured.

In CB-1, since the Y content in the steel sheet was too high, coarse Y inclusions were generated, and it is believed that these inclusions became the starting points for the generation of voids. As a result, sufficient bending resistance could not be ensured.

In CC-1, since the Zr content in the steel sheet was too high, coarse Zr inclusions were formed, and it is believed that these inclusions became the starting points for the generation of voids. As a result, sufficient bending resistance could not be ensured.

In CD-1, the La content in the steel sheet was too high, so La inclusions were generated, and it is believed that these inclusions became the starting points for the generation of voids. As a result, sufficient bending resistance could not be ensured.

In CE-1, since the Ce content in the steel sheet was too high, Ce inclusions were formed, and it is believed that these inclusions became the starting points for the generation of voids. As a result, sufficient bending resistance could not be ensured.

On the other hand, for A-1 to AZ-1, a steel sheet having a predetermined chemical composition was manufactured under predetermined conditions, so that a predetermined metal structure was obtained in the steel sheet, and the mechanical properties and bending resistance of the steel sheet were obtained. It was excellent in quality. Further, among the steel sheets obtained in Example 1, the number density of precipitates was within the range of 1/μm ² or more and 300/μm ² or less in the steel sheets in which tempered martensite was present.

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

The results shown in Tables 5-7 reveal the following.

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 increased, and as a result, tempered marten was formed on the front and back of the steel sheet. The difference in the number density of precipitates in the site increased, and the bending resistance of the steel sheet decreased.

For C-2 and S-3, it is considered that the surface layer of the steel sheet decarburized because the annealing holding temperature in Q annealing was too high. As a result, the strength of the steel sheet was lowered, and sufficient bending resistance could not be ensured.

For I-2 and W-2, the annealing holding temperature in IA annealing was too high (because the annealing holding temperature was outside the range of the two-phase region of ferrite and austenite), so austenite was excessively generated. Therefore, the bending yield strength decreased due to the decrease in tempered martensite containing precipitates. Moreover, 10.0% or more of retained austenite could not be ensured, and the elongation also decreased.

For L-2 and Z-3, the annealing holding temperature in IA annealing was too low (because the annealing holding temperature was outside the range of the two-phase region of ferrite and austenite), so austenite was not sufficiently formed. However, since the martensite obtained by the Q annealing is tempered, the retained austenite of 10.0% or more could not be ensured. As a result, when bending deformation is applied to the steel sheet, the effect of increasing strength due to deformation-induced transformation from retained austenite to martensite cannot be obtained, and sufficient bending resistance cannot be ensured.

For E-3 and AX-2, the annealing holding temperature in Q annealing was low, and the base material steel plate could not be heated to the austenite single phase region, so the martensite area ratio after Q annealing decreased. 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, when the area ratio of retained austenite decreases and bending deformation is applied to the steel sheet, the effect of increasing strength due to deformation-induced transformation from retained austenite to martensite cannot be obtained. Bendability could not be ensured.

For G-3 and U-2, the bending radius in aging treatment 1 was too large, so it is thought that sufficient tensile strain was not generated to promote the formation of clusters or transition carbides. As a result, a sufficient amount of ε carbide cannot be precipitated in aging treatment 1, and the difference in the number density of precipitates in the tempered martensite increases between the front and back sides of the steel sheet, ensuring sufficient bending resistance. I couldn't do it.

For M-3, the bending radius in aging treatment 2 was too large, so it is thought that sufficient tensile strain was not generated to promote the formation of clusters or transition carbides. As a result, a sufficient amount of ε carbide cannot be precipitated in aging treatment 2, and the difference in the number density of precipitates in the tempered martensite increases between the front and back sides of the steel sheet, ensuring sufficient bending resistance. I couldn't do it.

For N-3 and AF-2, the aging time in aging treatment 2 was too short, so the difference in the amount of precipitates between aging treatment 1 and aging treatment 2 increased, and as a result, tempered marten was formed on the front and back of the steel sheet. The difference in the number density of precipitates in the site increased, and the bending resistance of the steel sheet decreased.

For K-4 and AW-4, at least one of the aging treatments 1 and 2 was omitted, so 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 increased. As a result, the bending resistance of the steel sheet decreased.

On the other hand, in the examples other than the above, a steel sheet having a predetermined chemical composition is manufactured under predetermined conditions, so that the steel sheet has a predetermined metal structure and is excellent in formability and bending resistance. It was something. Among the steel sheets obtained in Example 2, those that were subjected to aging treatment and contained tempered martensite had a number density of precipitates of 1/μm ² or more and 300/μm ² or less. was within range.

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

(I) % by mass, C: 0.10 to 0.30%, Si: 0.60 to 1.20%, Mn: 1.00 to 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: 0-0.500%, 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- It has a chemical composition containing 0.0500% and Ce: 0 to 0.0500%, with the balance being Fe and impurities.
(II) 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, with the balance being martensite and tempered marten It has a steel structure consisting of sites.
(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% be.
(IV) Yield strength is 600 MPa or more.

In addition, steel sheets that satisfy the above requirements (I) to (IV) can be manufactured by the following method.

Obtaining a hot-rolled sheet by hot-rolling a steel slab having the chemical composition of (I) above;
winding the hot-rolled sheet;
pickling the hot-rolled sheet;
obtaining a cold-rolled sheet by cold-rolling the hot-rolled sheet;
Q-annealing the cold-rolled sheet;
performing IA annealing on the cold-rolled sheet that has been subjected to the Q annealing;
performing an aging treatment between the Q annealing and the IA annealing;
including
The Q annealing is a step of heating the cold-rolled sheet to a single austenite phase region and a temperature of 1000 ° C. or less and cooling to obtain a martensite structure with an area ratio of 90.0% or more,
The IA annealing is a step of holding the cold-rolled sheet in a two-phase region of ferrite and austenite to obtain retained austenite,
In the aging treatment, 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 is held at 0 to 40 ° C. for 20 hours or more. 1 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. ,including,
A method of manufacturing a steel plate.

Claims

in % by mass,
C: 0.10 to 0.30%,
Si: 0.60 to 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: 0-0.500%,
Cr: 0 to 2.000%,
O: 0 to 0.0100%,
B: 0 to 0.0100%,
Nb: 0 to 0.500%,
V: 0 to 0.500%,
Cu: 0 to 0.500%,
W: 0 to 0.1000%,
Ta: 0 to 0.1000%,
Sn: 0 to 0.0500%,
Sb: 0 to 0.0500%,
As: 0 to 0.0500%,
Mg: 0-0.0500%,
Ca: 0 to 0.0500%,
Y: 0 to 0.0500%,
Zr: 0 to 0.0500%,
La: 0 to 0.0500%, and
Ce: 0 to 0.0500%,
and has a chemical composition with the balance consisting of Fe and impurities,
area ratio,
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,
and having a steel structure with the remainder consisting of fresh martensite and tempered martensite,
The difference between the number density of precipitates in the tempered martensite on the first surface on the front side of the steel sheet and the number density of precipitates in the tempered martensite on the second surface on the back side of the steel sheet is within 10.0%,
Yield strength is 600 MPa or more,
steel plate.
in % by 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 to 2.000%
O: 0.0001 to 0.0100%
B: 0.0001 to 0.0100%,
Nb: 0.001 to 0.500%,
V: 0.001 to 0.500%,
Cu: 0.001 to 0.500%,
W: 0.0001 to 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 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%,
having the chemical composition containing one or more of
The steel plate according to claim 1.
The steel structure contains the needle-shaped retained austenite,
The steel plate according to claim 1 or 2.
A method for manufacturing a steel plate,
Hot-rolling a steel slab having the chemical composition according to claim 1 or 2 to obtain a hot-rolled sheet;
winding the hot-rolled sheet;
pickling the hot-rolled sheet;
obtaining a cold-rolled sheet by cold-rolling the hot-rolled sheet;
Q-annealing the cold-rolled sheet;
performing IA annealing on the cold-rolled sheet that has been subjected to the Q annealing;
performing an aging treatment between the Q annealing and the IA annealing;
including
The Q annealing is a step of heating the cold-rolled sheet to a single austenite phase region and a temperature of 1000 ° C. or less and cooling to obtain a martensite structure with an area ratio of 90.0% or more,
The IA annealing is a step of holding the cold-rolled sheet in a two-phase region of ferrite and austenite to obtain retained austenite,
In the aging treatment, 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 is held at 0 to 40 ° C. for 20 hours or more. 1 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. ,including,
A method of manufacturing a steel plate.
In the IA annealing, after the cold-rolled sheet is held in a two-phase region of ferrite and austenite, in the process of cooling to room temperature, a coating of zinc, aluminum, magnesium, or an alloy thereof on the front and back surfaces of the cold-rolled sheet to form layers,
The manufacturing method according to claim 4.
Obtaining the needle-shaped retained austenite in the IA annealing,
The manufacturing method according to claim 4 or 5.