CN112154222B

CN112154222B - High-strength steel sheet and method for producing same

Info

Publication number: CN112154222B
Application number: CN201980033936.5A
Authority: CN
Inventors: 胁田昌幸; 畑显吾; 伊藤一真; 藤原知哉; 河野佳织; 林宏太郎
Original assignee: Nippon Steel and Sumitomo Metal Corp
Current assignee: Nippon Steel Corp
Priority date: 2018-06-29
Filing date: 2019-06-28
Publication date: 2022-04-01
Anticipated expiration: 2039-06-28
Also published as: JPWO2020004661A1; TW202010852A; WO2020004661A1; CN112154222A; JP6690793B1

Abstract

The present invention relates to a high-strength steel sheet having a predetermined chemical composition, wherein the metal structure is martensite in terms of area%: 70.0-95.0%, retained austenite: 5.0-30.0%, the remainder: 0 to 10.0%, and the average grain size of retained austenite: 0.2 to 2.0 μm, and the average Mn concentration [ Mn ] in the retained austenite]_γSatisfies the condition that [ Mn ] is not less than 7.0]_γNot more than 20.0(i), the content of C [ C%]And the average C concentration [ C ] in the martensite]_MSatisfies the condition that [ C ] is more than or equal to 0.6]_M/[C](ii) Formula (II) is shown.

Description

High-strength steel sheet and method for producing same

Technical Field

The present invention relates to a high-strength steel sheet and a method for manufacturing the same.

The present application claims priority based on Japanese application laid-open No. 2018-124974 on 29/06/2018, and the contents thereof are incorporated herein by reference.

Background

In recent years, from the viewpoint of global environmental conservation, studies on reduction of carbon dioxide emission have been started in many fields. In automobile manufacturers, development of technologies for reducing the weight of automobile bodies for the purpose of low fuel consumption is also actively performed. However, in order to ensure the safety of passengers, emphasis is also placed on the improvement of collision resistance, and therefore, it is not easy to reduce the weight of the vehicle body.

In order to achieve both weight reduction of the vehicle body and improvement of the collision resistance, it is studied to use a high-strength steel sheet to reduce the thickness of the member. Therefore, a steel sheet having both high strength and excellent formability is strongly desired.

In recent years, a steel sheet used for automobile parts such as structural members and reinforcing members is desired to have a high tensile strength of 1180MPa or more and excellent formability. In addition, in order to ensure the rigidity of a member that is thinned for the purpose of reducing the weight of the vehicle body and using a high-strength steel sheet, and to improve the collision energy absorption characteristics from the viewpoint of ensuring the safety of the passengers, it is required to have a high yield stress in addition to a high tensile strength.

Generally, a steel sheet containing retained austenite in a metal structure exhibits a large elongation due to the effect of transformation induced plasticity (TRIP) caused by martensite transformation of the retained austenite during working. Therefore, in order to achieve excellent formability in a high-strength steel sheet, it is effective to use the residual austenite.

Conventionally, several techniques for improving the strength and elongation by effectively utilizing such retained austenite have been proposed.

For example, patent document 1 discloses a high-strength steel sheet for automobiles, which is excellent in collision safety resistance and formability, and in which the volume fraction (fill-in ratio) of retained austenite is 5% to 50%, the average grain size of the retained austenite is 5 μm or less, and the tensile strength × total elongation is 20000MPa ·% or more.

Patent document 2 discloses a high-strength steel sheet having excellent stretch flangeability and elongation, wherein 3 to 20% of retained austenite and/or martensite having an average grain diameter of 500nm or less is contained as a phase 2 structure in grains in terms of a volume fraction relative to the entire structure.

In recent years, medium Mn steels containing about 3 to 10 mass% of Mn and containing a large amount of retained austenite have attracted attention as materials having an excellent balance between strength and ductility.

For example, patent document 3 discloses a high-strength steel sheet having excellent workability, which is characterized by containing, in mass%, C: 0.03% -0.35%, Si: 0.5% -3.0%, Mn: 3.5% -10.0%, P: 0.1% or less, S: 0.01% or less, N: 0.008% or less, the remainder including Fe and inevitable impurities, 30.0% or more ferrite in terms of area ratio, a value obtained by dividing an amount (mass%) of Mn in the ferrite by an amount (mass%) of Mn in the steel sheet being 0.80 or less, 10.0% or more retained austenite in terms of volume ratio, 6.0% or more Mn in the retained austenite, and 2.0 μm or less average crystal grain size of the retained austenite.

Further, patent document 4 discloses a high-strength hot-pressed member characterized by containing, in mass%, C: 0.090% or more and less than 0.30%, Mn: 3.5% or more and less than 11.0%, Si: 0.01-2.5%, P: 0.05% or less, S: 0.05% or less, Al: 0.005 to 0.1%, N: a composition of 0.01% or less, the balance comprising Fe and unavoidable impurities, and a structure comprising 80% or more by volume of a martensite phase and 3.0 to 20.0% by volume of a retained austenite phase, and having a tensile strength TS: 1500MPa or more and uniform elongation uEl: a tensile property of 6.0% or more.

However, the steel sheet described in patent document 1 is considered to have improved ductility due to the refinement of retained austenite, but in order to improve the work hardening index and to improve collision safety resistance, the main phase needs to be a soft ferrite phase, and it is difficult to obtain high tensile strength.

In addition, the steel sheet described in patent document 2 has an excellent strength-ductility balance obtained by dispersing fine retained austenite of submicron size in the crystal grains of tempered martensite or bainite. However, in the technique of patent document 2, expensive elements such as Cu and Ni are contained in large amounts as austenite stabilizing elements, and further, solution treatment at a high temperature for a long time is required, so that the increase in production cost and the reduction in productivity are remarkable.

Further, according to patent document 3, it is considered that a high-strength steel sheet having a tensile strength of 980MPa or more and a strength-ductility balance of 24000MPa ·% or more can be obtained by annealing steel containing 3.5% to 10.0% of Mn in a dual phase region of austenite and ferrite to stabilize the retained austenite by the enrichment of Mn. However, the steel sheet of patent document 3 needs to contain 30.0% or more of soft ferrite in order to ensure good ductility, and thus further enhancement of strength is difficult.

According to patent document 4, by containing martensite as a main phase and 3.0 to 20.0% by volume of a retained austenite phase, a balance between a high strength such as a tensile strength of 1500MPa or more and a uniform elongation of 6.0% or more, which is excellent, and a uniform elongation, can be obtained. However, patent document 4 does not consider improvement of yield stress effective for ensuring improvement of rigidity and collision energy absorption characteristics of the member.

Documents of the prior art

Patent document

Patent document 1: japanese laid-open patent publication No. 11-61326

Patent document 2: japanese patent laid-open publication No. 2005-179703

Patent document 3: japanese patent laid-open publication No. 2013-76162

Patent document 4: international publication No. 2016/063467

Disclosure of Invention

Problems to be solved by the invention

The present invention has been made to solve the above problems, and an object thereof is to provide a steel sheet having high tensile strength and yield stress and excellent ductility (elongation) and a method for producing the same. Specifically, the object is to provide a high-strength steel sheet having a tensile strength of 1180MPa or more, a yield stress of 800MPa or more, and a strength-ductility balance of 18000 MPa% or more, and a method for manufacturing the same.

Means for solving the problems

The gist of the present invention is the following high-strength steel sheet and a method for producing the same.

(1) One aspect of the present invention relates to a high-strength steel sheet having a chemical composition, in mass%, of C: 0.08-0.45%, Si: 0.05-3.0%, Mn: 3.5-10.0%, P: 0.10% or less, S: 0.030% or less, sol.al: 0.01-2.0%, N: 0.010% or less, Ti: 0 to 0.20%, Nb: 0-0.10%, V: 0-0.50%, Cr: 0% or more and less than 1.0%, Mo: 0-0.50%, Ni: 0-1.0%, B: 0-0.0050%, Ca: 0-0.020%, Mg: 0 to 0.020%, REM: 0-0.020%, Cu: 0 to 1.0%, Bi: 0-0.020%, remainder: fe and impurities, wherein the metal structure is martensite in area percent: 70.0-95.0%, retained austenite: 5.0-30.0%, the remainder: 0 to 10.0%, and the average grain size of retained austenite: 0.2 to 2.0 μm, the average Mn concentration in the retained austenite satisfying the following expression (i), and the C content and the average C concentration in the martensite satisfying the expression (ii);

7.0≤[Mn]_γ≤20.0 (i)

0.6≤[C]_M/[C] (ii)

wherein the symbols in the above formula have the following meanings:

[Mn]_γ: the average Mn concentration (mass%) in the retained austenite

[C] The method comprises the following steps The content of C in the steel sheet (% by mass)

[C]_M: the average C concentration (mass%) in the martensite is described above.

(2) The high-strength steel sheet according to the item (1), wherein the chemical composition may contain, in mass%, a chemical composition selected from the group consisting of Ti: 0.005-0.20%, Nb: 0.002 to 0.10% and V: 0.005-0.50% of more than 1.

(3) The high-strength steel sheet according to the item (1) or (2), wherein the chemical composition may contain, in mass%, a chemical composition selected from the group consisting of Cr: 0.05% or more and less than 1.0%, Mo: 0.02 to 0.50%, Ni: 0.05 to 1.0% and B: 0.0002 to 0.0050% of at least 1 type.

(4) The high-strength steel sheet according to any one of the above (1) to (3), wherein the chemical composition may contain, in mass%, a chemical component selected from the group consisting of Ca: 0.0005 to 0.020%, Mg: 0.0005 to 0.020% and REM: more than 1 of 0.0005-0.020%.

(5) The high-strength steel sheet according to any one of the above (1) to (4), wherein the chemical composition may contain, in mass%, Cu: 0.05 to 1.0 percent.

(6) The high-strength steel sheet according to any one of the above (1) to (5), wherein the chemical composition may contain, in mass%, Bi: 0.0005 to 0.020%.

(7) Another aspect of the present invention relates to a method for producing a high-strength steel sheet by sequentially performing a hot rolling step, a cooling step, a coiling step, a primary annealing step, an optional cold rolling step, and a secondary annealing step on a slab having a chemical composition according to any one of (1) to (6), wherein in the hot rolling step, the reduction ratios in the final pass and the preceding pass in the final pass are set to 15 to 60%, respectively, the inter-pass time from the end of rolling in the preceding pass to the start of rolling in the final pass satisfies the following expression (v), and the rolling end temperature in the final pass is set to Ar₃A temperature range of point-1100 ℃; in the cooling step, the hot-rolled steel sheet after the hot rolling step is air-cooled for 1 to 10 seconds and then cooled at an average cooling rate of 10 ℃/second or more; in the winding step, winding is performed in a temperature range of 550 ℃ or lower; in the primary annealing step, the hot-rolled steel sheet is annealed at an annealing temperature satisfying (Ac)₁Point-80 ℃ to (Ac)₃Point-55 ℃ C.) and a holding time satisfying the following(vi) The formula (I) is maintained; in the secondary annealing step, the hot-rolled steel sheet is annealed at an annealing temperature of (Ac)₃At a temperature of +30 ℃ or higher and lower than (Ac)₃Point +200 ℃ C.) and holding time is less than 150 seconds, and then the sample is held in accordance with Ac₃Cooling to a temperature of 500 ℃ or lower so that the average cooling rate in the temperature range of 500 ℃ or lower is 15 ℃/sec or higher;

0.002/exp(-6080/(T₁+273))≤t₁≤2.0 (v)

2.3×10^-8×exp{23500/(T₂+273)}≤t₂≤4.0×10⁵ (vi)

wherein, the meaning of each symbol in the above formula is as follows:

t₁: the time (seconds) between the end of rolling preceding the final rolling pass and the start of rolling in the final rolling pass

T₁: the temperature (. degree.C.) of the end of rolling in the preceding rolling pass to the final rolling pass

t₂: the holding time (second) at the annealing temperature of the primary annealing

T₂: the annealing temperature (. degree. C.) of the primary annealing.

(8) The method for producing a high-strength steel sheet according to item (7), wherein the total reduction ratio in the cold rolling step may be set to 30% or more and less than 80%.

Effects of the invention

According to the aspect of the present invention, a steel sheet having high tensile strength and yield stress and excellent ductility can be obtained.

Drawings

FIG. 1 is a schematic view showing a metal structure in secondary annealing holding.

Detailed Description

The present inventors have conducted extensive studies on the relationship between the steel structure and mechanical properties of medium Mn steels containing about 3 to 10 mass% Mn. As a result, the following findings were obtained.

(a) In order to obtain retained austenite in medium Mn steel, the two-phase region (A) of ferrite and austenite is generally used₁～A₃Temperature region) for a long time. In this case, a large amount of retained austenite can be obtained by thickening austenite with C and Mn as austenite stabilizing elements. As a result, ductility is improved. On the other hand, C, which is excellent in solid solution strengthening, is consumed for stabilizing austenite, and thus it is difficult to increase the strength. Therefore, in order to achieve both high strength and excellent ductility, it is important to form a structure that contains martensite mainly solid-solution-strengthened by C and contains retained austenite stabilized by Mn concentration.

(b) The steel structure mainly composed of martensite contains retained austenite, thereby achieving high strength and improving ductility by the TRIP effect. However, when coarse austenite remains, coarse voids are formed due to martensite generated by the work-induced transformation, and therefore local deformation occurs at an early stage, and the effect of improving ductility is not obtained. On the other hand, if the retained austenite is too fine, the stability of the retained austenite against deformation is excessively improved due to three-dimensional constraint of the surrounding crystal grains, and therefore, there is a possibility that local deformation occurs before the TRIP effect is sufficiently exhibited in the high-strength steel sheet, and improvement of ductility cannot be achieved. Therefore, the average grain size of the retained austenite contained in the steel structure is controlled to be within an appropriate range, whereby the steel structure can have high strength and excellent ductility.

(c) When the steel structure mainly containing martensite that is solid-solution-strengthened by C contains retained austenite that is stabilized by Mn concentration, the Mn concentration in the retained austenite has a large influence on ductility and yield stress. If the Mn concentration in the retained austenite is too low, the retained austenite has low stability against deformation, and the TRIP effect occurs during elastic deformation or at the initial stage of plastic deformation, so that the yield stress is reduced and the ductility cannot be improved. On the other hand, in the case where the Mn concentration in the residual austenite is too high, not the TRIP effect but the effect of twin induced plasticity (TWIP) becomes remarkable, and the yield stress is lowered. Therefore, by controlling the Mn concentration in the retained austenite to be in an appropriate range, a steel sheet having both high tensile strength and high yield stress and excellent ductility can be obtained.

(d) In order to produce a steel structure mainly composed of martensite which is solid-solution-strengthened by C and containing retained austenite which is stabilized by Mn, it is effective to divide the annealing process performed on the hot-rolled steel sheet into two steps. Specifically, first, the hot-rolled steel sheet is annealed at a temperature in a two-phase region of ferrite and austenite, or ferrite and cementite, or a three-phase region of ferrite, austenite and cementite, so that C and Mn are distributed in austenite or cementite. After that, by annealing in the austenite single-phase region, the diffusion of Mn is suppressed, and only C is diffused. This annealing allows C to partially remain in the austenite region having a high Mn concentration, and allows C to diffuse into the surroundings. As a result, by such annealing and cooling after annealing, the austenite region having a high Mn concentration becomes retained austenite, and the austenite region having a low Mn concentration becomes martensite having a constant C concentration, and a desired metal structure can be obtained.

The present invention has been made based on the above-described knowledge. Hereinafter, each requirement of the high-strength steel sheet according to an embodiment of the present invention (steel sheet according to the present embodiment) will be described in detail.

(A) Chemical composition

The reasons for limiting the elements are as follows. In the following description, "%" as to the content means "% by mass". Unless otherwise specified, a numerical range represented by the term "to" means a range including numerical values described before and after the term "to" as a lower limit value and an upper limit value. That is, 0.08 to 0.45% means 0.08% to 0.45%.

C：0.08～0.45％

C has a function of improving the strength of the steel sheet by solid solution strengthening by solid solution in martensite. When the C content is less than 0.08%, the amount of solid solution strengthening is small, and it becomes difficult to secure a desired strength of the steel sheet. Therefore, the C content is set to 0.08% or more. The C content is preferably 0.10% or more, more preferably 0.12% or more.

On the other hand, if C is contained in an amount exceeding 0.45%, pearlite is preferentially produced, making it difficult to ensure a desired steel sheet strength. Therefore, the C content is set to 0.45% or less. The C content is preferably 0.40% or less.

Si：0.05～3.0％

Si has a function of improving the strength of the steel sheet by solid solution strengthening and a function of strengthening the steel by deoxidation. Si is an element that delays the precipitation of cementite, increases the area fraction of retained austenite, and contributes to the improvement of ductility. When the Si content is less than 0.05%, the effects of the above-described effects are difficult to obtain. Therefore, the Si content is set to 0.05% or more.

On the other hand, if the Si content exceeds 3.0%, the surface properties, chemical conversion treatability, and weldability of the steel sheet deteriorate significantly. Therefore, the Si content is set to 3.0% or less. The Si content is preferably 2.5% or less, more preferably 2.0% or less.

Mn：3.5～10.0％

Mn has an action of enhancing hardenability of steel to promote formation of martensite, and an action of stabilizing retained austenite by being dissolved in austenite. When the Mn content is less than 3.5%, it is difficult to secure the target martensite amount and retained austenite amount and Mn concentration in the retained austenite. Therefore, the Mn content is set to 3.5% or more. The Mn content is preferably 4.0% or more, more preferably 4.5% or more.

On the other hand, if the Mn content exceeds 10.0%, the amount of produced retained austenite becomes large, and it becomes difficult to secure a desired martensite amount. Further, the Mn concentration in austenite is excessively increased, and the TWIP effect, not the TRIP effect, becomes remarkable, and the yield stress is reduced. Therefore, the Mn content is set to 10.0% or less. The Mn content is preferably 8.0% or less.

P: less than 0.10%

P is an element generally contained as an impurity. P is an element that is easily segregated, and if the content thereof exceeds 0.10%, the formability and toughness are significantly reduced due to grain boundary segregation. Therefore, the P content is set to 0.10% or less. The P content is preferably 0.050% or less, more preferably 0.030% or less, and further preferably 0.020% or less.

On the other hand, P is also an element having an effect of improving strength by solid solution strengthening. Therefore, P may be positively contained. The lower limit of the P content is not particularly limited, but is preferably set to 0.001% or more in order to obtain the above-described effects.

S: less than 0.030%

S is an element contained as an impurity, and forms sulfide-based inclusions in steel, thereby reducing the formability of steel sheet. If the S content exceeds 0.030%, the moldability is remarkably lowered. Therefore, the S content is set to 0.030% or less. The S content is preferably 0.010% or less, more preferably 0.005% or less, and further preferably 0.001% or less. The lower limit of the S content is not particularly limited, and is preferably set to 0.0001% or more from the viewpoint of suppressing an increase in refining cost.

sol.Al：0.01～2.0％

Al has the function of deoxidizing the steel and strengthening the steel sheet, similarly to Si. Further, Al is an element contributing to improvement of ductility by an action of delaying precipitation of cementite to increase the area ratio of retained austenite. When the Al (acid-soluble Al) content is less than 0.01%, the effects due to the above-described effects are difficult to obtain. Therefore, the sol.al content is set to 0.01% or more. The al content is preferably 0.03% or more.

On the other hand, if the sol.Al content exceeds 2.0%, A is₃The transformation point is remarkably increased, and stable hot rolling is difficult. Therefore, the sol.al content is set to 2.0% or less. The al content is preferably 1.5% or less, more preferably 1.0% or less.

N: 0.010% or less

N is an element contained as an impurity, and has an effect of reducing the formability of the steel sheet. If the N content exceeds 0.010%, the moldability is remarkably lowered. Therefore, the N content is set to 0.010% or less. The N content is preferably 0.0080% or less, more preferably 0.0070% or less. The lower limit of the N content is not particularly limited, but when considering a case where the steel structure is miniaturized by containing 1 or more of Ti, Nb, and V as described later, the N content is preferably set to 0.0010% or more, and more preferably 0.0020% or more, in order to promote the precipitation of carbonitrides.

The steel sheet of the present embodiment may further contain 1 or more elements selected from Ti, Nb, V, Cr, Mo, Ni, B, Ca, Mg, REM, Cu, and Bi in the amounts shown below in addition to the above elements. The lower limit of these elements is 0% because they may not be contained.

Ti：0～0.20％

Nb：0～0.10％

V：0～0.50％

Ti, Nb, and V have the effect of precipitating as carbides or nitrides in steel and refining the steel structure by the pinning effect. Therefore, 1 or more selected from these elements may be contained. In order to more reliably obtain the effects due to the above-described actions of these elements, it is preferable to contain Ti: 0.005% or more, Nb: 0.002% or more and V: 0.005% or more.

On the other hand, even if these elements are contained excessively, the effects by the above-described actions are saturated and become uneconomical. Therefore, even when it is contained, the Ti content is set to 0.20% or less, the Nb content is set to 0.10% or less, and the V content is set to 0.50% or less.

Cr: more than 0% and less than 1.0%

Mo：0～0.50％

Ni：0～1.0％

B：0～0.0050％

Cr, Mo, Ni and B have the function of improving hardenability. In addition, Mo has the effect of precipitating carbides in the steel to improve the strength. Further, when Cu is contained as described later, Ni effectively suppresses grain boundary cracking of the slab caused by Cu. Therefore, 1 or more selected from these elements may be contained.

In order to more reliably obtain the effects of the above-described actions, it is preferable to set the Ni content to 0.05% or more, the Cr content to 0.05% or more, the Mo content to 0.02% or more, and/or the B content to 0.0002% or more.

On the other hand, when the Cr content is 1.0% or more, the chemical conversion treatability is remarkably lowered. Therefore, even when it is contained, the Cr content is set to less than 1.0%.

If the Mo content exceeds 0.50%, the effects of the above-described actions are saturated, which is disadvantageous in terms of cost. Therefore, even when contained, the Mo content is set to 0.50% or less. Preferably 0.20% or less.

In addition, since Ni is an expensive element, it is disadvantageous in terms of cost to contain a large amount of Ni. Therefore, even when contained, the Ni content is set to 1.0% or less.

When the B content exceeds 0.0050%, the moldability is remarkably reduced. Therefore, even when contained, the B content is set to 0.0050% or less.

Ca：0～0.020％

Mg：0～0.020％

REM：0～0.020％

Ca. Mg and REM have an effect of improving formability by adjusting the shape of inclusions. Therefore, 1 or more selected from these elements may be contained. In order to more reliably obtain the effects due to the above-described actions, it is preferable to contain at least any one of the above-described elements by 0.0005% or more.

On the other hand, if the contents of these elements exceed the above upper limit values, the inclusions in the steel become excessive, and the formability may be rather lowered. Therefore, even when contained, the Ca content is set to 0.020% or less, the Mg content is set to 0.020% or less, and the REM content is set to 0.020% or less. Each element is preferably 0.010% or less, more preferably 0.005% or less.

Here, REM means a total of 17 elements of Sc, Y and lanthanoid, and the content of REM means a total content of these elements. In the case of lanthanides, the addition is industrially in the form of mixed rare earth alloys.

Cu：0～1.0％

Cu has a function of increasing strength by precipitating at a low temperature, and therefore may be contained in the steel. In order to more reliably obtain the effects of the above-described actions, the Cu content is preferably set to 0.05% or more.

On the other hand, if the Cu content exceeds 1.0%, grain boundary cracking of the slab may occur. Therefore, the Cu content is set to 1.0% or less even when contained. The Cu content is preferably less than 0.5%, more preferably less than 0.3%.

Bi：0～0.020％

Bi has an effect of improving formability by refining a solidification structure, and therefore Bi may be contained in steel. In order to more reliably obtain the effects of the above-described actions, the Bi content is preferably set to 0.0005% or more.

On the other hand, if the Bi content exceeds 0.020%, the effects of the above-described actions are saturated, which is disadvantageous in terms of cost. Therefore, even when contained, the Bi content is set to 0.020% or less. The Bi content is preferably 0.010% or less.

In the chemical composition of the steel sheet of the present embodiment, the balance is Fe and impurities.

The "impurities" are components mixed by raw materials such as ores and scraps and various factors of a manufacturing process in the industrial production of steel materials, and are components that are allowed within a range that does not adversely affect the present invention.

(B) Metallic structure of steel plate

The metal structure of the steel sheet of the present embodiment will be described below. In the following description, "%" relating to the proportion (area ratio) of the tissue means "% by area".

Martensite: 70.0 to 95.0 percent

Martensite is a hard and homogeneous structure and is a structure suitable for obtaining high tensile strength. When the area ratio of martensite is less than 70.0%, it becomes difficult to obtain a desired tensile strength. Therefore, the area ratio of martensite is set to 70.0% or more. The area ratio of martensite is preferably 75.0% or more. On the other hand, if the area ratio of martensite exceeds 95.0%, ductility is significantly reduced. Therefore, the area ratio of martensite is set to 95.0% or less. The martensite of the steel sheet of the present embodiment is a general term including tempered martensite in addition to so-called primary martensite which is generated by a mechanism of no diffusion and no shearing.

Retained austenite: 5.0 to 30.0 percent

The retained austenite has an effect of increasing ductility by the TRIP effect. If the area ratio of the retained austenite is less than 5.0%, the effect of the above-described action is difficult to obtain. Therefore, the area ratio of the retained austenite is set to 5.0% or more. The retained austenite area ratio is preferably 8.0% or more, and more preferably 10.0% or more. On the other hand, if the area ratio of the retained austenite exceeds 30.0%, the amount of martensite is relatively reduced, and it becomes difficult to obtain a desired tensile strength. Therefore, the area ratio of the retained austenite is set to 30.0% or less.

The method of quantifying retained austenite includes methods using X-ray diffraction, electron back scattering diffraction pattern (EBSP) analysis, magnetic measurement, and the like, and the quantitative value may vary depending on the method. The area ratio of the retained austenite defined in the steel sheet of the present embodiment is a measured value obtained by X-ray diffraction.

The rest is as follows: 0 to 10.0%

The steel sheet of the present embodiment may contain polygonal ferrite, bainitic ferrite, bainite, cementite, pearlite, and the like as the remainder in addition to the above-described structure. If the total area ratio of the structures of the remaining portions excluding martensite and retained austenite exceeds 10.0%, it becomes difficult to obtain desired strength and ductility. Therefore, the area ratio of the remaining portion is set to 10.0% or less. The area ratio of the remaining portion is preferably 8.0% or less, more preferably 6.0% or less. The area ratio of the remaining portion may be 0%.

Average grain size of retained austenite: 0.2 to 2.0 μm

When the average grain size of the retained austenite exceeds 2.0 μm, coarse voids are formed by martensite generated by the work-induced transformation. In this case, local deformation occurs at an early stage, and the effect of improving ductility is not obtained. Therefore, the average grain size of the retained austenite is set to 2.0 μm or less. The average grain size of the retained austenite is preferably 1.5 μm or less, and more preferably 1.0 μm or less.

On the other hand, if the average grain size of the retained austenite is less than 0.2 μm, the stability of the retained austenite against deformation is excessively improved by three-dimensional constraint from the surrounding crystal grains, and local deformation may occur before the TRIP effect is sufficiently exhibited in the high-strength steel sheet, and improvement of ductility may not be achieved. Therefore, the average grain size of the retained austenite is set to 0.2 μm or more.

The identification of the metal structure and the calculation of the area ratio were performed by the following methods. First, using a sample in which a cross section perpendicular to the rolling direction of a steel sheet was mirror-polished and subjected to Lepera corrosion, an area of 200 μm in the width direction x 50 μm in the normal direction of the rolling surface at a position of 1/4 depths from the surface of the steel sheet was photographed with an optical microscope, and the total area ratio of retained austenite and martensite was calculated by binarization using commercially available Image processing software "Image-Pro". Here, the retained austenite and martensite become the bright portions. A histogram of the brightness is created, and a region having a brightness in a range exceeding the brightness of the peak appearing on the dark portion side and not more than 255 is set as retained austenite and martensite, and binarization processing is performed to separate the region from the other regions.

Next, the area ratio of the retained austenite was calculated by X-ray diffraction measurement using a Co radiation source. The integrated intensities of the (110), (200) and (211) planes of ferrite and the (111), (200) and (220) planes of retained austenite were obtained by X-ray diffraction measurement using a sample whose thickness was reduced to 1/4 depths from the surface of the steel sheet by a facing work (machining work) and chemical grinding. Then, the volume fraction of the retained austenite was determined by the strength averaging method, and the value was set as the area fraction of the retained austenite. The martensite area ratio is set as a value obtained by subtracting the retained austenite area ratio obtained by X-ray diffraction measurement from the total area ratio of retained austenite and martensite obtained by the above method.

Then, the area ratio of the remaining portion structure was set to a value obtained by subtracting the sum of the area ratios of martensite and retained austenite obtained in the above-described manner from 100%.

The average grain size of the retained austenite was calculated by EBSP analysis and Transmission Electron Microscope (TEM) observation of a thin film test piece taken from a position 1/4 deep from the surface of the steel sheet. At this time, the observation magnification of the TEM was set to 50000 times, the austenite grain sizes of 10 crystal grains were measured from a dark field image imaged by an electron diffraction pattern of austenite, and the average value of these was set as the average grain size of the retained austenite.

In the steel sheet of the present embodiment, it is necessary to control the chemical composition and the metal structure as described above, and further control the average Mn concentration [ Mn ] in the retained austenite as described below]_γAnd the ratio of the C concentration in martensite to the C content in the steel sheet [ C]_M/[C]。

7.0≤[Mn]_γ≤20.0 (i)

Wherein the symbols in the above formula have the following meanings:

[Mn]_γ: average Mn concentration in retained austenite (% by mass)

The Mn concentration in the retained austenite has a large influence on ductility and yield stress. In order to obtain the desired yield stress and ductility, the average Mn concentration in the retained austenite needs to satisfy the above formula (i).

When the average Mn concentration in the retained austenite is less than 7.0 mass%, the retained austenite has low stability against deformation, and TRIP effect occurs during elastic deformation or at the initial stage of plastic deformation. In this case, the yield stress is reduced, and the ductility cannot be improved. On the other hand, in the case where the average Mn concentration in the retained austenite exceeds 20.0 mass%, not the TRIP effect but the TWIP effect becomes remarkable, and the yield stress is lowered. Therefore, the average Mn concentration in the retained austenite is set to 7.0 to 20.0% by mass%.

The average Mn concentration in the retained austenite was determined by the following method using an electron probe microanalyzer (FE-EPMA) equipped with a field emission electron gun. A mapping analysis (mapping analysis) of Mn concentration was performed by using FE-EPMA at intervals of 0.1 μm for a region of 50 μm × 50 μm of the sample used in the X-ray diffraction measurement, i.e., the sample which had been subjected to the planar cutting to a depth of 1/4 mm from the surface of the steel sheet. Next, the retained austenite in the metal structure of the same region was identified by EBSP analysis. For the identified residual austenite, the average of the measured values of the Mn concentration at arbitrary 10 points within 1 crystal grain was set as the Mn concentration in the 1 crystal grain. Then, the average value of the Mn concentrations measured from each of the 10 crystal grains was calculated, and the value was set as the average Mn concentration of the retained austenite.

0.6≤[C]_M/[C] (ii)

Wherein the symbols in the above formula have the following meanings:

[C] the method comprises the following steps C content in Steel sheet (% by mass)

[C]_M: average C concentration (mass%) in martensite

The C concentration in martensite has a large influence on the tensile strength. In order to obtain a desired tensile strength, the C concentration in martensite needs to satisfy the above-described expression (ii) in relation to the C content in the steel sheet.

When the ratio of the C concentration in martensite to the C content in the steel sheet is less than 0.6, the amount of solid solution strengthening by C is insufficient, and the tensile strength of the steel sheet is lowered. The upper limit is not particularly limited since the higher the C concentration in martensite, the higher the tensile strength.

In the steel sheet of the present embodiment, the C content in the steel sheet is measured by a high-frequency combustion method.

The average C concentration in martensite was determined by the following method using a vickers hardness tester. First, the vickers hardness of 10 points of the martensite identified by the above-described method was measured with a test force of 1kgf, and the average value thereof was obtained. Then, the average C concentration in the martensite was calculated based on the average value of the vickers hardness of the martensite and the following expressions (iii) and (iv).

[C]_M＝{1573-(1573²-4096×A)^0.5}/2048 (iii)

Wherein A in the above formula (iii) is a value calculated by the following formula (iv), and the symbols in the formula (iv) have the following meanings:

A＝HV-14×[Mn]-27×[Si]-200 (iv)

HV: average value of Vickers hardness of martensite

[ Mn ]: mn content in Steel sheet (% by mass)

[ Si ]: si content (mass%) in steel sheet

(C) Mechanical characteristics

In the steel sheet of the present embodiment, it is preferable that the yield stress is 800MPa or more, the tensile strength is 1180MPa or more, and the value of TS × EL is 18000MPa ·% or more. When the above value is used, the steel sheet has high yield stress and tensile strength, and is excellent in strength-ductility balance, so that the vehicle body weight reduction and the improvement of collision resistance can be achieved at the same time.

(D) Coating layer

The surface of the high-strength steel sheet of the present invention having the above chemical composition and steel structure may be plated to improve corrosion resistance, to form a surface-treated steel sheet. The plating layer may be an electrolytic plating layer or a hot-dip plating layer.

Examples of the plating layer include a zinc plating layer and a Zn — Ni alloy plating layer. Examples of the hot-dip coating layer include a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, a hot-dip aluminum layer, a hot-dip Zn — Al alloy layer, a hot-dip Zn — Al — Mg alloy layer, and a hot-dip Zn — Al — Mg — Si alloy layer. The amount of plating deposited is not particularly limited, and is as good as conventional. After plating, appropriate chemical conversion treatment (for example, coating and drying of a silicate-based chromium-free chemical conversion treatment liquid) may be performed to further improve the corrosion resistance.

(E) Production conditions

The high-strength steel sheet according to an embodiment of the present invention can be obtained by a manufacturing method including the following steps, for example.

< casting step >

The steel having the above chemical composition is melted by a known method and then made into an ingot by a continuous casting method, or made into a billet by a method of making an ingot by an arbitrary casting method and then cogging-rolling the ingot, or the like. In the continuous casting step, in order to suppress the occurrence of surface defects due to inclusions, it is preferable to generate an externally added flow such as electromagnetic stirring in the molten steel in the mold (mold).

The ingot or slab may be once cooled and then reheated for hot rolling, or the ingot or slab in a high-temperature state after continuous casting or the slab in a high-temperature state after cogging may be directly or while being kept warm, or may be subjected to auxiliary heating for hot rolling. In the present embodiment, such a steel ingot and a steel slab are collectively referred to as a "slab" as a material for hot rolling.

< Hot Rolling Process >

Heating temperature of the plate blank: below 1350 ℃

The heating temperature of the slab subjected to hot rolling is set to 1350 ℃ or lower from the viewpoint of suppressing the loss of scale. The heating temperature is preferably set to 1280 ℃ or lower. The lower limit of the heating temperature of the slab to be subjected to hot rolling is not particularly limited as long as it can be heated in Ar as described below₃The temperature above the point of finishing hot rolling.

Rolling finishing temperature: ar (Ar)₃Point 1100 deg.C

Hot-rolled into Ar to refine the metal structure of the hot-rolled steel sheet by transforming austenite after rolling is completed₃Ending in a temperature region above the point. The rolling end temperature (temperature after the end of the final pass) is lower than Ar₃In this case, ferrite transformation occurs during hot rolling, and a coarse metal structure extending in the rolling direction is formed in the hot-rolled steel sheet. This causes coarse retained austenite to be formed in the metal structure after annealing, and the ductility is easily deteriorated. Therefore, the finishing temperature of hot rolling was set to Ar₃The point is above. The slab is hot-rolled to obtain a hot-rolled steel sheet.

Here, in the present embodiment, Ar₃The point is calculated by the following formula (I).

Ar₃＝622.2+149.4×[C]-24.2×[Si]-13.7×[Mn] (I)

Wherein each [ element symbol ] in the formula represents the content (mass%) of each element contained in the steel.

On the other hand, when the rolling completion temperature exceeds 1100 ℃, the microstructure of the hot-rolled steel sheet becomes coarse, and the Mn concentration in the retained austenite becomes insufficient after annealing, and the yield ratio decreases. Therefore, the finishing temperature of hot rolling is set to 1100 ℃ or lower. The finishing temperature of hot rolling is preferably set to 1050 ℃ or lower.

The reduction ratios in the final rolling pass and the rolling pass immediately before the final rolling pass are as follows: 15 to 60 percent

The reduction ratios in the final rolling pass and the preceding rolling pass are preferably set to 15 to 60%, respectively. By setting the reduction ratios in the final pass and the previous pass to 15% or more, the recrystallized austenite grains are mainly refined, and the equiaxed and fine martensite structure is easily obtained by cooling thereafter. Thereby fine retained austenite can be obtained after annealing.

If the reduction ratio in the final pass and/or the pass immediately preceding the final pass is less than 15%, recrystallization of austenite may become insufficient, and the ductility may deteriorate after annealing. The reduction ratios in the final rolling pass and the preceding rolling pass are preferably 20% or more, and more preferably 25% or more, respectively. On the other hand, from the viewpoint of flatness of the steel sheet and suppression of relaxation of the introduced strain due to heat generation during working, the reduction ratios in the final pass and the pass immediately before the final pass are set to 60% or less, respectively. The reduction ratios in the final rolling pass and the preceding rolling pass are preferably 50% or less, respectively.

Time (inter-pass time) from the end of rolling in the preceding rolling pass to the start of rolling in the final rolling pass: satisfies the following formula (v)

0.002/exp(-6080/(T₁+273))≤t₁≤2.0 (v)

Wherein the meanings of the symbols are as follows:

t₁: time (seconds) between pass before the end of the final pass and the start of the final pass

T₁: rolling completion temperature (. degree. C.) of the rolling pass immediately preceding the final rolling pass

By satisfying the above expression (v), recrystallization of austenite is promoted and grain growth of austenite is suppressed during the pass from the end of rolling in the preceding rolling pass to the start of rolling in the final rolling pass. Therefore, the recrystallized austenite grains can be made finer during rolling. Thereby, it becomes further easy to obtain a steel structure suitable for high tensile strength and yield stress and having excellent ductility after annealing.

In the case where the hot rolling includes rough rolling and finish rolling, the rough rolled material may be heated between the rough rolling and the finish rolling in order to finish the finish rolling at the above temperature. In this case, it is preferable to suppress the variation in temperature of the rough rolled material over the entire length at the start of finish rolling to 140 ℃ or less by heating the rough rolled material so that the rear end becomes higher in temperature than the front end. This improves the uniformity of product characteristics in the web.

The heating method of the rough rolled material may be performed by a known method. For example, a solenoid type induction heating device may be provided between the roughing mill and the finishing mill, and the heating temperature rise amount may be controlled based on the temperature distribution in the longitudinal direction of the rough rolled material on the upstream side of the induction heating device.

The hot rolling is preferably performed by a reversing mill or a tandem mill in which a plurality of passes are set. In particular, from the viewpoint of industrial productivity, at least the final stages are more preferably set to rolling using a tandem mill.

< Cooling Process >

After the hot rolling is finished, air cooling is carried out for 1-10 seconds, and then cooling is carried out to the coiling temperature at an average cooling speed of more than 10 ℃/second

The hot-rolled steel sheet obtained by the hot rolling step is air-cooled for 1 to 10 seconds after the hot rolling is completed. Thereby promoting recrystallization of austenite grains processed by hot rolling while suppressing grain growth of austenite. If the air cooling time after hot rolling is less than 1 second, recrystallization of austenite becomes insufficient, and a coarse metal structure extending in the rolling direction is formed. As a result, coarse retained austenite is formed after the secondary annealing, and the workability of the steel sheet is deteriorated. On the other hand, when air-cooling is performed for more than 10 seconds, the microstructure of the hot-rolled steel sheet becomes coarse, and the Mn concentration in the retained austenite becomes insufficient after annealing, and the yield ratio decreases.

In addition, when the average cooling rate to the coiling temperature after the air cooling is less than 10 ℃/sec, pearlite is easily generated, Mn is distributed between ferrite and pearlite during coiling, and it becomes difficult to obtain a structure suitable for high strength and ductility. The upper limit of the average cooling rate is not particularly limited, but it is preferably set to 300 ℃/sec or less because control of the sheet shape may become difficult if the cooling rate is too high.

< winding Process >

Coiling temperature: below 550 deg.C

As described later, the distribution of Mn among ferrite, austenite, and cementite is promoted by annealing (primary annealing) performed after hot rolling. Further, by the subsequent annealing (secondary annealing), it is possible to control the steel structure which mainly includes martensite that is solid-solution-strengthened by C and which contains retained austenite that is stabilized by Mn concentration.

To obtain this effect, the coiling temperature after hot rolling is set to 550 ℃ or lower. When the coiling temperature exceeds 550 ℃, pearlite is easily produced, Mn is distributed between ferrite and pearlite during coiling, and it becomes difficult to obtain a structure suitable for high strength and ductility. The coiling temperature is preferably set to less than 400 ℃ and more preferably to less than 300 ℃.

< Primary annealing step >

The hot-rolled steel sheet having undergone the hot rolling step, the cooling step, and the coiling step described above is annealed at a temperature in a two-phase region of ferrite and austenite or ferrite and cementite or in a three-phase region of ferrite, austenite, and cementite. This annealing is referred to as "primary annealing". The descaling of the hot-rolled steel sheet may be performed by pickling or the like before the primary annealing. By promoting the distribution of Mn among ferrite, austenite, and cementite by the primary annealing, it becomes easy to obtain a microstructure suitable for high strength and ductility by the secondary annealing performed later. The primary annealing condition is performed under a condition satisfying the following range.

Annealing temperature: (Ac)₁Point-80 ℃ to (Ac)₃Point-55 deg.C)

The annealing temperature (primary annealing temperature) was set to (Ac)₁Point-80 ℃ to (Ac)₃Point-55 deg.C. By annealing at a temperature in this range, Mn can be efficiently concentrated in austenite or cementite, or both, and by secondary annealing performed thereafter, the area ratio and average grain size of retained austenite, and the Mn concentration in retained austenite can be controlled to a desired range.

Primary annealing temperature lower than (Ac)₁At-80 c), it may become difficult to secure a desired amount of retained austenite after the secondary annealing, and ductility may deteriorate. On the other hand, if the primary annealing temperature exceeds (Ac)₃Point-55 c), coarse retained austenite with low Mn concentration is easily formed after the secondary annealing, and it becomes difficult to obtain desired yield stress and ductility. When the primary annealing temperature further becomes high, it reaches Ac₃As described above, although coarse retained austenite is not formed, it is difficult to concentrate Mn in the retained austenite.

Here, in the present invention, Ac₁Point and Ac₃The points are respectively set to values calculated by the following formulas (II) and (III).

Ac₁＝631.3+235.4×[C]+10.5×[Si]-9.4×[Mn] (II)

Ac₃＝781.7+3.7×[C]^0.5-7.2×[Si]-9.7×[Mn] (III)

Holding time at annealing temperature of primary annealing: satisfies the following formula (vi)

The retention time of the primary annealing preferably satisfies the following expression (vi) in relation to the primary annealing temperature.

2.3×10^-8×exp{23500/(T₂+273)}≤t₂≤4.0×10⁵ (vi)

Wherein each symbol in the above formula (vi) has the following meaning:

t₂: hold time at Primary annealing temperature (seconds)

T₂: temperature of Primary annealing (. degree.C.)

Since the diffusion rate of Mn is very slow, the Mn distribution from ferrite to austenite or cementite is promoted by holding at the above-described primary annealing temperature for a predetermined time. By performing secondary annealing on this annealed steel sheet as described later, the Mn concentration in the retained austenite can be controlled to a desired range. The retention time is lower than that of the left side [ 2.3X 10 ] of the formula (vi)^-8× exp{23500/(T₂+273)}]If the calculated value is obtained, the distribution of Mn becomes insufficient, the Mn concentration in the residual austenite becomes low, the yield strength decreases, and the ductility tends to deteriorate. On the other hand, the ratio of Mn concentration approaches the equilibrium state when the holding is performed for a long time, and therefore, the holding is performed in excess of 4.0X 10⁵The second annealing is also saturated in the effect, and only the heat treatment cost is increased.

< Cold Rolling Process >

Cold pressing rate: more than 30 percent and less than 80 percent

The annealed steel sheet after the primary annealing step may be cold-rolled by a conventional method to obtain a cold-rolled steel sheet. Further, the annealed steel sheet may be descaled by pickling or the like before the cold rolling. In the case of cold rolling, in order to promote recrystallization at the time of secondary annealing, to uniformize the microstructure after annealing and to improve ductility, the cold reduction (total reduction (cumulative reduction) in cold rolling) is preferably set to 30% or more. The cold pressing ratio is more preferably set to 40% or more. This further refines the microstructure after the secondary annealing, improves the texture, and improves the ductility. From this viewpoint, the cold reduction ratio is more preferably set to a value exceeding 50%, particularly preferably to a value exceeding 60%. On the other hand, if the cold reduction ratio is too high, the rolling load increases, making rolling difficult. Therefore, the cold reduction ratio is preferably set to less than 80%, more preferably to less than 70%.

Further, when cold rolling is performed after the secondary annealing, a steel sheet containing martensite that is solid-solution strengthened by C is cold rolled. In this case, the yield point becomes easy to occur. When the yield point is present, uneven deformation is likely to occur, and surface unevenness after press molding is likely to occur, which is not preferable.

< Secondary annealing Process >

The hot-rolled steel sheet (annealed steel sheet) after the primary annealing step or the cold-rolled steel sheet obtained by the cold rolling step is further annealed. This annealing is referred to as "secondary annealing". By performing the secondary annealing, a metal structure mainly composed of martensite solid-solution-strengthened by C and containing Mn-enriched retained austenite is easily obtained. The secondary annealing condition preferably satisfies the following range.

Annealing temperature: (Ac)₃At a temperature of +30 ℃ or higher and lower than (Ac)₃Point +200 deg.C)

The annealing temperature (secondary annealing temperature) was set to (Ac)₃Point +30 ℃ C. or higher. This is to generate retained austenite of a desired volume fraction, and as shown in fig. 1, C present in cementite and/or retained austenite generated by primary annealing is diffused into the entire steel structure (particularly, austenite region having a low Mn concentration), thereby obtaining a metal structure in which the main phase is martensite solid-solution-strengthened by C and the retained austenite is included in the second phase. However, when the secondary annealing temperature is too high, the austenite during heating is excessively coarsened, and the metal structure after cooling is inducedWhile the grain size is increased, Mn distribution promoted by the primary annealing is reduced by diffusion, and the ductility is easily deteriorated. Therefore, the secondary annealing temperature is preferably set to be lower than (Ac)₃Point +200 ℃ C.). The secondary annealing temperature is more preferably set to be lower than (Ac)₃Point +150 ℃ C.).

Retention time at annealing temperature of secondary annealing: less than 150 seconds

If the holding time at the secondary annealing temperature becomes too long, Mn distributed by the primary annealing diffuses, the yield stress decreases, and the ductility easily deteriorates. Therefore, the holding time is set to less than 150 seconds, preferably less than 120 seconds. The lower limit of the retention time at the secondary annealing temperature is not particularly limited from the viewpoint of suppressing the diffusion of Mn distributed by the primary annealing, and may be more than 0 second.

In the heating process in the secondary annealing, in order to suppress the diffusion of Mn distributed by the primary annealing, the average heating rate is preferably set to 5 ℃/sec or more, more preferably 20 ℃/sec or more, and still more preferably 100 ℃/sec or more.

In order to obtain a metal structure having martensite as a main phase, Ac was added in the cooling process after soaking at the secondary annealing temperature₃The average cooling rate in the temperature range of point-500 ℃ is set to be not less than 15 ℃/sec and the temperature is cooled to be not more than 500 ℃. At Ac₃When the average cooling rate in the temperature range of from point to 500 ℃ is less than 15 ℃/sec, ferrite is easily generated, it becomes difficult to obtain a desired tensile strength, and the average C concentration [ C ] in martensite is obtained by precipitation of cementite]_MDecrease, [ C ]]_M/[C]Becomes lower than 0.6. Since the area ratio of martensite increases as the cooling rate increases, the above-mentioned average cooling rate is preferably set to a rate exceeding 30 ℃/sec, more preferably to a rate exceeding 50 ℃/sec.

On the other hand, if Ac₃If the cooling rate at a temperature of from 500 ℃ C. is too high, the shape of the steel sheet is impaired, so that Ac₃The average cooling rate in the temperature range of point-500 ℃ is preferably set to 200 ℃/sec or less, more preferably less than 150 ℃/sec, and still more preferably less than 130 ℃/sec. When the cooling stop temperature exceeds 500 ℃, cementite is formed, it becomes difficult to secure desired tensile strength and ductility, and the average C concentration [ C ] in martensite]_MDecrease, [ C ]]_M/[C]Becomes lower than 0.6. Therefore, the cooling stop temperature is set to 500 ℃ or lower.

< plating Process >

In the case of producing a plated steel sheet, the annealed steel sheet produced by the above method may be subjected to a known pretreatment for cleaning and conditioning the surface as needed, and then plated according to a conventional method, and the chemical composition and the amount of deposit of the plating film are not limited. Examples of the kind of plating include zinc plating, Zn — Ni alloy plating, and the like.

In the case of manufacturing a hot dip plated steel sheet, in the cooling process of the secondary annealing described above, the steel sheet is cooled to 450 ℃ or lower, then heated as necessary, and then immersed in a plating bath to perform hot dip plating. Further, the alloy may be alloyed by reheating after the hot dip plating. The chemical composition and the amount of adhesion of the plating film are not limited. Examples of the type of hot dip plating include hot dip galvanizing, alloy hot dip galvanizing, hot dip aluminizing, hot dip Zn — Al alloy, hot dip Zn — Al — Mg — Si alloy, and the like.

In order to further improve the corrosion resistance of the plated steel sheet, an appropriate chemical conversion treatment may be performed after plating. The chemical conversion treatment is preferably carried out using a chromium-free chemical conversion treatment liquid (for example, silicate-based, phosphate-based, etc.) in place of the conventional chromate treatment.

The annealed steel sheet and the plated steel sheet obtained in this manner may be temper-rolled by a conventional method. However, since high elongation in temper rolling leads to deterioration in ductility, the elongation in temper rolling is preferably set to 1.0% or less, more preferably 0.5% or less.

Examples

The present invention will be described more specifically with reference to the following examples, but the present invention is not limited to these examples.

A steel ingot of 180kg having a chemical composition shown in table 1 was melted by a high-frequency vacuum melting furnace and hot forged to form a billet 30mm thick. The slabs were hot-rolled by a small-sized tandem mill for testing under the conditions shown in Table 2-1 to obtain hot-rolled steel sheets having a thickness of 2 to 4 mm. The hot-rolled steel sheet was subjected to a heat treatment at various primary annealing temperatures shown in Table 2-2 for a predetermined period of time, and then cooled to room temperature.

Next, the obtained primary annealed steel sheets were heated to various secondary annealing temperatures shown in table 2-2 using a continuous annealing simulator and held for a predetermined period of time. Thereafter, the steel sheets were cooled under various conditions shown in Table 2-2 to obtain annealed steel sheets.

The above-described several types of primary annealed steel sheets were annealed after pickling and cold rolling.

TABLE 2-1

Tables 2 to 2

The obtained steel sheet was subjected to mirror polishing on a cross section perpendicular to the rolling direction of the steel sheet and then subjected to Lepera corrosion, and a region of 200 μm in the sheet width direction x 50 μm in the normal direction of the rolling surface at a position of 1/4 depths from the surface of the steel sheet was photographed with an optical microscope, and the total area ratio of retained austenite and martensite was calculated by binarization processing using commercially available Image processing software "Image-Pro". In the binarization process, a histogram of the brightness is created, and the region having a brightness in a range of 255 or less over the brightness of the peak appearing on the dark portion side is set as retained austenite and martensite, and is separated from the other regions.

Next, the area ratio of retained austenite was measured. First, the thickness was reduced to 1/4 degrees from the surface of the steel sheet by a flat cutting process and chemical polishing, and a sample for measurement was prepared. Then, the above samples were subjected to X-ray diffraction measurement using a Co radiation source (RINT 2500HL manufactured by Rigaku Corporation) to obtain integrated intensities of the ferrite (110), (200) and (211) planes and the retained austenite (111), (200) and (220) planes. Then, the volume fraction of the retained austenite was determined by the strength averaging method, and the value was set as the area fraction of the retained austenite.

The martensite area ratio is set as a value obtained by subtracting the retained austenite area ratio obtained by X-ray diffraction measurement from the total area ratio of retained austenite and martensite obtained in the above. Then, the area ratio of the remaining portion structure was set to a value obtained by subtracting the sum of the area ratios of martensite and retained austenite obtained in the above-described manner from 100%.

Further, using a sample prepared by electropolishing after mirror polishing, the presence of austenite was confirmed by measuring and analyzing the crystal orientation by the EBSP method at a position 1/4 depth from the surface of the steel sheet to the thickness. Then, a sheet for TEM observation was taken from a position where the presence of austenite was confirmed, and the vicinity of the hole opened by the double-jet electrolysis was observed by TEM. At this time, the observation magnification of the TEM was set to 50000 times, the austenite grain diameters of 10 crystal grains were measured from a dark field image imaged by an electron diffraction pattern of austenite, and the average value of these was set as the average grain diameter of retained austenite. The average grain size of the retained austenite was determined by these measurements.

Then, a mapping analysis of Mn concentration was performed by FE-EPMA at intervals of 0.1 μm for a sample used in the X-ray diffraction measurement, that is, a sample subjected to planar cutting to a depth of 1/4 mm from the surface of the steel sheet. Next, the retained austenite in the metal structure of the same region was identified by EBSP analysis. Then, for the identified retained austenite, the average of the measured values of Mn concentration at arbitrary 10 points within 1 grain is set as the Mn concentration in the 1 grain, the average of the Mn concentrations measured from each of the 10 grains is calculated, and this value is set as the average Mn concentration of the retained austenite.

Furthermore, vickers hardnesses of 10 points of martensite identified by the above-described method were measured with a test force of 1kgf, and the average values thereof were obtained, and the average C concentration in martensite was calculated based on the above-described formulas (iii) and (iv).

As the mechanical properties, tensile properties were evaluated. Tensile properties were measured according to JIS Z2241: 2011 tensile test is performed, and Yield Stress (YS), Tensile Strength (TS) and total elongation (El) are measured. Specifically, JIS Z2241: 2011, the test piece No. 5 was subjected to a tensile test. At this time, the rolling direction of the steel sheet was set to the longitudinal direction of the tensile test piece.

The microstructure and mechanical properties of the obtained steel sheet are shown in tables 3-1 and 3-2. In the present invention, a steel sheet having a yield stress of 800MPa or more, a tensile strength of 1180MPa or more, and a TS × EL value of 18000MPa · or more is judged to have high yield stress and tensile strength and to have an excellent strength-ductility balance.

Referring to tables 1 to 3-2, test numbers 1 to 20 and 43, which are examples of the present invention, have high yield stress and tensile strength and excellent strength-ductility balance. On the other hand, test numbers 21 to 42 as comparative examples which do not satisfy the specification of the present invention were inferior in at least one of YS, TS, and TS × El in characteristics.

Industrial applicability

According to the present invention, a steel sheet having high tensile strength and high yield stress and having excellent ductility can be obtained. Therefore, the high-strength steel sheet of the present invention is suitable for use as a material for automobile members, machine structural members, building members, and the like.

Claims

1. A high-strength steel sheet having a chemical composition, in mass%:

C：0.08～0.45％、

Si：0.05～3.0％、

Mn：3.5～10.0％、

p: less than 0.10 percent,

S: less than 0.030%,

sol.Al：0.01～2.0％、

N: less than 0.010%,

Ti：0～0.20％、

Nb：0～0.10％、

V：0～0.50％、

Cr: more than 0% and less than 1.0%,

Mo：0～0.50％、

Ni：0～1.0％、

B：0～0.0050％、

Ca：0～0.020％、

Mg：0～0.020％、

REM：0～0.020％、

Cu：0～1.0％、

Bi：0～0.020％、

The rest is as follows: fe and impurities in the iron-based alloy, and the impurities,

the metal structure is calculated by area percent:

martensite: 70.0 to 95.0 percent,

Retained austenite: 5.0 to 30.0 percent,

The rest is as follows: 0 to 10.0%, and

average grain size of retained austenite: 0.2 to 2.0 μm,

the average Mn concentration in the retained austenite satisfies the following expression (i), the C content and the average C concentration in the martensite satisfy the expression (ii),

7.0≤[Mn]_γ≤20.0 (i)

0.6≤[C]_M/[C] (ii)

wherein the symbols in the formula have the following meanings:

[Mn]_γ: the average Mn concentration in the retained austenite by mass%,

[C] the method comprises the following steps The content of C in mass% in the steel sheet,

[C]_M: the average C concentration in the martensite in mass%.

2. The high-strength steel sheet according to claim 1, wherein the chemical composition contains, in mass%, a chemical component selected from the group consisting of

Ti：0.005～0.20％、

Nb: 0.002 to 0.10%, and

v: 0.005-0.50% of more than 1.

3. The high-strength steel sheet according to claim 1 or claim 2, wherein the chemical composition contains, in mass%, a chemical component selected from the group consisting of

Cr: more than 0.05 percent and less than 1.0 percent,

Mo：0.02～0.50％、

Ni: 0.05 to 1.0%, and

b: 0.0002 to 0.0050% of at least 1 type.

4. The high-strength steel sheet according to any one of claims 1 to 3, wherein the chemical composition contains, in mass%, a chemical component selected from the group consisting of

Ca：0.0005～0.020％、

Mg: 0.0005 to 0.020%, and

REM: more than 1 of 0.0005-0.020%.

5. The high-strength steel sheet according to any one of claims 1 to 4, wherein the chemical composition contains, in mass%, Cu: 0.05 to 1.0 percent.

6. The high-strength steel sheet according to any one of claims 1 to 5, wherein the chemical composition contains, in mass%, Bi: 0.0005 to 0.020%.

7. A method for producing a high-strength steel sheet, comprising subjecting a slab having a chemical composition according to any one of claims 1 to 6 to a hot rolling step, a cooling step, a coiling step, a primary annealing step, an optional cold rolling step, and a secondary annealing step in this order,

in the hot-rolling step, the hot-rolling step is carried out,

setting the reduction ratios in the final rolling pass and the rolling pass before the final rolling pass to 15-60%,

(iv) the time between passes from the end of rolling in the preceding rolling pass to the start of rolling in the final rolling pass satisfies the following expression (v),

setting the rolling end temperature of the final rolling pass to Ar₃A temperature range of point-1100 ℃;

in the cooling step, the hot-rolled steel sheet after the hot-rolling step is air-cooled for 1 to 10 seconds and then cooled at an average cooling rate of 10 ℃/second or more;

in the above-mentioned winding-up step,

coiling in a temperature region below 550 ℃;

in the primary annealing process, the primary annealing process is carried out,

the hot rolled steel sheet satisfies (Ac) at an annealing temperature₁Point-80 ℃ to (Ac)₃A temperature range of-55 ℃ and a holding time satisfying the following formula (vi);

in the secondary annealing process, the annealing process is carried out,

the hot rolled steel sheet is annealed at an annealing temperature of (Ac)₃At a temperature of +30 ℃ or higher and lower than (Ac)₃Point +200 ℃) and a holding time ofAfter holding for less than 150 seconds, according to Ac₃Cooling to a temperature of 500 ℃ or lower so that the average cooling rate in the temperature range of 500 ℃ or lower is 15 ℃/sec or higher;

0.002/exp(-6080/(T₁+273))≤t₁≤2.0 (v)

2.3×10^-8×exp{23500/(T₂+273)}≤t₂≤4.0×10⁵ (vi)

wherein the symbols in the formula have the following meanings:

t₁: the inter-pass time/sec from the end of rolling immediately before the final rolling pass to the start of rolling at the final rolling pass,

T₁: the rolling end temperature/DEG C of the rolling pass preceding the final rolling pass,

t₂: the holding time/sec at the annealing temperature of the primary annealing,

T₂: the annealing temperature/° C of the primary annealing.

8. The method for producing a high-strength steel sheet according to claim 7, wherein a total reduction ratio is set to 30% or more and less than 80% in the cold rolling step.