JP7253479B2 - high strength steel plate - Google Patents

Description

TECHNICAL FIELD The present invention relates to a high-strength steel sheet that can be used for various applications including automobile parts.

Steel plates used for automobile parts, etc. have high strength in order to maintain the functions of the steel products themselves against impacts and loads from the outside, and to protect people and things inside structures made of steel. is required. In recent years, in particular, there has been a demand for weight reduction of steel sheets from the viewpoint of reducing environmental load and reducing costs. Therefore, it is necessary to maintain the durability even if the steel sheet is made thinner, and the demand for higher strength is increasing more and more.

However, since increasing the strength of steel generally leads to deterioration of formability, maintaining high formability as well as increasing strength is an issue. As an index for achieving both strength and formability, it is important to have a high strength-ductility balance represented by the product TS×El of tensile strength (TS) and elongation (El) in a tensile test.

Until now, as a material that meets such applications, transformation induced plasticity (TRIP: Transformation Induced Plasticity), which improves elongation by making part or all of the microstructure metastable austenite and transforming this into strain-induced martensite, has been proposed. TRIP steel has been proposed that utilizes the plasticity effect.

For example, Patent Document 1 discloses a high-strength steel sheet having a TS of 980 MPa or more and a TS×EL (total elongation) of 24000 MPa·% or more. The steel sheet described in Patent Document 1 has a predetermined chemical composition, the steel structure has ferrite in an area ratio of 30.0% or more, and the Mn amount in the ferrite is The divided value is 0.80 or less, the volume fraction of retained austenite is 10.0% or more, the amount of Mn in the retained austenite is 6.0% by mass or more, and the average crystal of retained austenite Desired mechanical properties are realized by satisfying the particle size of 2.0 μm or less.

However, in recent years, there has been a demand for steel sheets that achieve even higher levels of strength and strength-ductility balance. In order to meet these demands, TWIP steel is developed using the twin induced plasticity (TWIP) effect, in which austenite is stabilized at room temperature and then strain-induced twinning transformation occurs to improve ductility. is proposed. For example, in Non-Patent Documents 1 to 3, attempts have been made to improve the strength and strength-ductility balance by expressing the TWIP effect in addition to the TRIP effect.

JP 2013-076162 A

Sangwon Lee, Bruno C. De Cooman, "Tensile Behavior of Intercritically Annealed 10 pct Mn Multi-phase Steel", Metallurgical and Materials Transactions A, February 2014, Volume 45, Issue 2, pp 709-716 Sangwon Lee, Bruno C. De Cooman, "Tensile Behavior of Intercritically Annealed Ultra-Fine Grained 8% Mn Multi-Phase Steel", steel research int., 86, 2015, No.10 Yun-bo Xu, Zhi-ping Hu, Ying Zou, Xiao-dong Tan, Ding-ting Han, Shu-qing Chen, De-gang Ma, R.D.K. Misra, "Effect of two-step intercritical annealing on microstructure and mechanical properties of hot-rolled medium manganese TRIP steel containing δ-ferrite", Materials Science and Engineering, 2017, 688, C, pp 40-55

However, in Patent Document 1, the amount of retained austenite described in the examples is at most about 42.1%, and the strength-ductility balance is insufficient.

Also, in Non-Patent Documents 1 and 2, a large amount of Mn is added in order to secure the amount of retained austenite. Therefore, the retained austenite is excessively stabilized and the strength or strength-ductility balance is insufficient. Also, in Non-Patent Document 3, the strength is insufficient because the amount of C added is insufficient.

Thus, in the conventional techniques, either strength or strength-ductility balance was insufficient.

The present invention has been made in view of such circumstances, and an object thereof is to provide a steel sheet having high strength and excellent strength-ductility balance.

Aspect 1 of the present invention is
C: 0.32% by mass or more and 0.50% by mass or less,
Si: more than 0% by mass, 3.0% by mass or less,
Mn: 5.0% by mass or more and 7.8% by mass or less,
Al: 1.60% by mass or more and 3.50% by mass or less,
N: more than 0% by mass, 0.01% by mass or less,
P: more than 0% by mass, 0.1% by mass or less, and S: more than 0% by mass, 0.01% by mass or less, the balance being Fe and inevitable impurities,
The metal structure contains austenite in an area ratio of 40% or more to 80% or less with respect to the total metal structure, and the balance is at least one of ferrite and martensite,
The fraction of ferrite grains having an area of 20 μm ² or more is 10% or less in area ratio with respect to the entire metal structure,
The austenite composition is a high-strength steel sheet, in which the X value represented by the following formula (1) satisfies 15 or more and 45 or less.

X=−25.9+50.8×[Cγwt. %]+1.03×[Siγwt. %]+0.881×[Mnγwt. %]+4.94×[Alγwt. %]−0.469×[Crγwt. %]+1.28×[Cuγwt. %] (1)
However, [Cγwt. %], [Siγwt. %], [Mnγwt. %], [Alγwt. %], [Crγwt. %] and [Cuγwt. %] respectively represent the concentrations of C, Si, Mn, Al, Cr and Cu contained in the austenite, and the concentration of elements not contained is assumed to be zero.

Aspect 2 of the present invention is
The high-strength steel sheet according to aspect 1, further containing Cr: more than 0% by mass and 3.0% by mass or less.

Aspect 3 of the present invention is
The high-strength steel sheet according to aspect 1 or 2, further containing B: more than 0% by mass and 0.01% by mass or less.

Aspect 4 of the present invention is
Cu: more than 0% by mass, 3.0% by mass or less, and Ni: more than 0% by mass, 3.0% by mass or less, further containing one or two selected from the group consisting of Embodiments 1 to 3 A high-strength steel sheet according to any one of the above.

Aspect 5 of the present invention is
V: more than 0% by mass and 0.5% by mass or less,
Nb: more than 0% by mass, 0.5% by mass or less,
Mo: more than 0% by mass, 0.5% by mass or less, and Ti: more than 0% by mass, 0.5% by mass or less, further containing one or more selected from the group, any of aspects 1 to 4 It is a high-strength steel sheet according to.

Aspect 6 of the present invention is
Ca: more than 0% by mass and 0.01% by mass or less,
Any of aspects 1 to 5, further containing one or more selected from the group consisting of Mg: more than 0% by mass and 0.01% by mass or less, and REM: more than 0% by mass and 0.01% by mass or less It is a high-strength steel sheet according to.

According to the present invention, it is possible to provide a steel sheet with high strength and excellent strength-ductility balance.

FIG. 1 is a schematic diagram of a micro-tensile test piece in an example.

As a result of intensive studies, the inventors of the present invention have found that the TRIP effect is exhibited by increasing the fraction of retained austenite (hereinafter sometimes referred to as "retained γ"), and the chemical composition of the retained γ is controlled. As a result, the inventors have found that by making the TWIP effect appear more easily than before, it is possible to ensure higher elongation than before. Furthermore, it was found that by controlling the ferrite size, it is possible to suppress the decrease in strength accompanying the improvement in elongation, and to achieve an excellent balance between strength and ductility at a high strength level. In addition, the inventors have found that the desired metallographic structure can be obtained by controlling the chemical composition of steel to an appropriate range that has not been possible in the past.

Specifically, the present inventors controlled the residual γ fraction to 40% or more and 80% or less in terms of area ratio in order to ensure high elongation. In addition, the present inventors have made it easier to express the TWIP effect by controlling the chemical component composition in the residual γ so that the X value represented by the formula (1) described later is 15 or more and 45 or less. . Furthermore, the present inventors suppressed the area ratio of coarse ferrite grains having an area of 20 μm ² or more to 10% or less in order to ensure strength. From these, the inventors have found that an excellent strength-ductility balance can be achieved at high strength levels.

1. Metallographic Structure Details of the metallographic structure of the high-strength steel sheet according to the embodiment of the present invention will be described below.
The following description of the metal structure may explain the mechanism by which various properties can be improved by having such a structure. These are mechanisms considered by the present inventors based on the knowledge currently available, but it should be noted that they do not limit the technical scope of the present invention.

The metal structure of the high-strength steel sheet according to the embodiment of the present invention contains austenite (that is, retained γ) in an area ratio of 40% to 80% with respect to the total metal structure, and the balance is at least one of ferrite and martensite. The fraction of ferrite grains having an area of 20 μm ² or more is 10% or less in terms of area ratio with respect to the total metal structure, and the austenite composition has an X value of 15, which is represented by the formula (1) described later. Above 45 below is satisfied.

[Fraction of residual γ: 40% or more and 80% or less]
Retained γ is a structure capable of exhibiting a TRIP phenomenon of transforming into martensite due to work-induced transformation during working such as press working, thereby obtaining a large elongation. Also, the formed martensite has a high hardness. Therefore, the retained γ contributes to improving the strength-ductility balance. In order to effectively exhibit such effects, the area ratio of residual γ to the entire metal structure should be 40% or more, preferably 45% or more, and more preferably 50% or more. On the other hand, if the retained γ content is excessive, there is retained γ that does not transform into martensite during working, resulting in insufficient strength. Therefore, the area ratio of residual γ should be 80% or less, preferably 70% or less, and more preferably 60% or less.

The fraction of residual γ can be measured using known methods such as EBSD (Electron Backscatter Diffraction) and X-ray diffraction (XRD). The details of how to measure the fraction of residual γ using EBSD will be described later. When measuring the fraction of residual γ using the X-ray diffraction method (XRD), for example, after grinding the steel plate to a thickness of 1/4, it can be measured by the X-ray diffraction method after chemically polishing ( ISIJ Int. Vol.33, (1993), No. 7, p.776). Incident X-rays can be Co-Kα rays.

[X value represented by formula (1): 15 or more and 45 or less]
In the embodiment of the present invention, the chemical component composition in the residual γ is controlled so that the X value represented by the following formula (1) is 15 or more and 45 or less.
X=−25.9+50.8×[Cγwt. %]+1.03×[Siγwt. %]+0.881×[Mnγwt. %]+4.94×[Alγwt. %]−0.469×[Crγwt. %]+1.28×[Cuγwt. %] (1)
However, [Cγwt. %], [Siγwt. %], [Mnγwt. %], [Alγwt. %], [Crγwt. %] and [Cuγwt. %] respectively represent the concentrations of C, Si, Mn, Al, Cr and Cu contained in the austenite, and the concentration of elements not contained is assumed to be zero.

The X value represented by the above formula (1) is a formula corresponding to the stacking fault energy (SFE). When stacking fault energy decreases and dislocations tend to expand, stress-induced transformation to ε-martensite, which is a close-packed hexagonal crystal (hcp), and/or face-centered cubic lattice (fcc) twinning deformation tends to occur. Become. When fine deformation twins are introduced during working, work hardening increases and high ductility is realized by the TWIP effect. Here, the stacking fault energy is said to be determined by chemical components and is known to be calculated or experimentally determined, but the calculation is very complicated. Therefore, the present inventors created a regression equation for chemical components in order to simply obtain the stacking fault energy. The result is equation (1). When the X value represented by formula (1) exceeds 45, the TWIP effect is not sufficiently exhibited. Therefore, the X value should be 45 or less, preferably 40 or less. On the other hand, if the X value is less than 15, which is too low, the TWIP effect is not sufficiently exhibited. Therefore, the X value should be 15 or more, preferably 20 or more.

The chemical component composition in the residual γ can be determined by known methods such as field emission electron probe microanalyzer (FE-EPMA), X-ray diffraction method (XRD), and three-dimensional atom probe field ion microscope (Three Dimensional Atom Probe, 3DAP). can be measured using The details of the measurement method using FE-EPMA will be described later.

[The fraction of ferrite grains having an area of 20 μm ² or more: 10% or less]
Controlling the residual γ fraction and the chemical component composition in the residual γ (hereinafter sometimes referred to as “residual γ composition”) can contribute to high ductility, but may reduce strength. . Here, ferrite generally has excellent workability but low strength. Also, coarsening of the ferrite can reduce the grain boundaries that restrict the movement of dislocations, thus reducing the strength. Therefore, in the present invention, the strength is improved by controlling the fraction of coarse ferrite grains having an area of 20 μm ² or more to 10% or less. As a result, it is possible to suppress the decrease in strength caused by controlling the residual γ fraction and the residual γ composition, and improve the strength more than before. The fraction of ferrite grains having an area of 20 μm ² or more is preferably 7% or less, more preferably 5% or less, most preferably 0%.

The fraction of ferrite grains with an area greater than or equal to 20 μm ² can be measured using EBSD. Details of the measurement method will be described later.

Except for the residual γ fraction, the chemical component composition in the residual γ, and the fraction of coarse ferrite grains, there are no particular limitations. For example, the total fraction of ferrite grains and martensite having an area of less than 20 μm ² is preferably 10% or more and 50% or less. When the total fraction of ferrite grains having an area of less than 20 μm ² and martensite is low, the residual γ fraction is relatively too high, so the martensite transformation may not be completed during working, and the strength may be insufficient. be. Therefore, the total fraction is preferably 10% or more, more preferably 20% or more. On the other hand, when the above total fraction is high, the residual γ fraction is relatively insufficient, and the elongation may decrease. Also, the strength may be insufficient. Therefore, the total fraction is preferably 50% or less, more preferably 40% or less.

Note that martensite in the embodiment of the present invention includes both tempered martensite and as-quenched martensite. It may also contain inclusions and/or carbides in retained gamma, ferrite and martensite. When inclusions and/or carbides are included in the retained γ, ferrite and martensite, the area of each of the retained γ, ferrite and martensite is the area of the region including the inclusion and/or carbide region. For example, if the ferrite grain contains inclusions and/or carbides, the area of the ferrite grain is the area of the region surrounded by the grain boundaries of the ferrite grain. Moreover, in the chemical composition according to the embodiment of the present invention, which will be described later, bainite and the like are not generated, and the entire metal structure is composed of retained γ and at least one of ferrite and martensite.

2. Chemical Component Composition The chemical component composition of the high-strength steel sheet according to the embodiment of the present invention will be described below. First, the basic elements, C, Si, Mn, Al, N, P, and S, will be explained, and then the elements that can be selectively added will be explained.

[C: 0.32% by mass or more and 0.50% by mass or less]
C (carbon), together with Mn, contributes to an increase in the fraction of retained γ as an austenite stabilizing element and improvement in the stability of retained γ against working. Also, C has the effect of suppressing the formation of coarse ferrite. In order to exhibit such action effectively, C must be contained in an amount of 0.32% by mass or more. Preferably, it is contained in an amount of 0.34% by mass or more. However, if the C content exceeds 0.50%, the weldability deteriorates. Therefore, the C content should be 0.50% by mass or less, preferably 0.45% by mass or less.

[Si: more than 0% by mass, 3.0% by mass or less]
Si (silicon) is useful as a solid-solution strengthening element for ferrite, and contributes to high YS (yield stress) and high TS while minimizing the decrease in elongation. Therefore, Si is contained in an amount exceeding 0% by mass, preferably 0.5% by mass or more, and more preferably 1.0% by mass or more. However, if Si is excessively contained, the local ductility is lowered, and crack formation is accelerated particularly at the shear end face, thereby lowering the bendability. Therefore, the Si content should be 3.0% by mass or less, preferably 2.5% by mass or less, and more preferably 2.0% by mass or less.

[Mn: 5.0% by mass or more and 7.8% by mass or less]
Mn (manganese), as an austenite stabilizing element, contributes to increasing the fraction of retained γ and improving the stability of retained γ against working. Moreover, Mn has the effect of suppressing the formation of coarse ferrite. In order to effectively exhibit such action, Mn must be contained in an amount of 5.0% by mass or more. The content is preferably 6.0% by mass or more, more preferably 6.5% by mass or more. However, if the Mn content exceeds 7.8% by mass, recovery of ferrite is suppressed, and a structure with poor ductility affected by working remains. In addition, the residual γ becomes excessively stable, which may make it difficult to ensure strength and elongation. Therefore, the Mn content should be 7.8% by mass or less, preferably 7.5% by mass or less.

[P: more than 0% by mass, 0.1% by mass or less]
P (phosphorus) is inevitably present as an impurity element, and if contained in excess of 0.1% by mass, the elongation deteriorates. Therefore, the P content is limited to 0.1% by mass or less, preferably 0.02% by mass or less.

[S: more than 0% by mass, 0.01% by mass or less]
S (sulfur) also inevitably exists as an impurity element, forms sulfide-based inclusions such as MnS, and is an element that acts as a starting point for cracks and reduces elongation. Therefore, the S content is limited to 0.01% by mass or less, preferably 0.005% by mass or less.

[Al: 1.60% by mass or more and 3.50% by mass or less]
Al (aluminum) is used as a deoxidizer, and if its content is less than 0.001% by mass, a sufficient cleaning action for steel cannot be obtained. In addition, Al has the effect of increasing the stacking fault energy (that is, SFE) of retained γ, and is an element that contributes to the manifestation of the TWIP effect and is effective in improving ductility. In order to effectively exhibit such effects, Al is contained in an amount of 1.60% by mass or more, preferably 1.80% by mass or more, and more preferably 2.00% by mass or more. On the other hand, if the Al content exceeds 3.50% by mass, the steel becomes embrittled, causing cracking of steel chips during casting, and coarse ferrite is formed during solidification, which remains in the final product, resulting in a decrease in strength. . Therefore, the Al content should be 3.50% by mass or less, preferably 3.00% by mass, and more preferably 2.50% by mass or less.

[N: more than 0% by mass, 0.01% by mass or less]
N (nitrogen) is also inevitably present as an impurity element, and reduces elongation due to strain aging. In addition, since N combines with Al and precipitates as coarse nitrides, it causes breakage at the shear end face. Therefore, the N content is preferably as low as possible, and its upper limit is 0.01% by mass or less, preferably 0.006% by mass or less.

[Remainder]
The balance is iron and unavoidable impurities. As unavoidable impurities, trace elements (for example, As, Sb, Sn, etc.) brought in depending on the conditions of raw materials, materials, manufacturing equipment, etc. are allowed. For example, there are elements, such as P and S, whose content is generally preferably as low as possible and thus are unavoidable impurities, but whose composition range is separately defined as described above. For this reason, in this specification, the term "inevitable impurities" constituting the balance is a concept excluding elements whose composition range is separately defined.

However, it is not limited to this embodiment. Any other element may be further included as long as the properties of the high-strength steel sheet according to the embodiment of the present invention can be maintained. Other elements that can be so selectively included are exemplified below.

[One or more of the following (i) to (iv). (i) Cr: more than 0% by mass, 3.0% by mass or less, (ii) B: more than 0% by mass, 0.01% by mass or less, (iii) Cu: more than 0% by mass, 3.0% by mass or less , and Ni: more than 0% by mass, one or two selected from the group consisting of 3.0% by mass or less, (iv) V: more than 0% by mass, 0.5% by mass or less, Nb: 0% by mass One or more selected from the group consisting of more than 0.5 mass% or less, Mo: more than 0 mass% and 0.5 mass% or less, and Ti: more than 0 mass% and 0.5 mass% or less]
Cr (chromium), B (boron), Cu (copper), Ni (nickel), V (vanadium), Nb (niobium), Mo (molybdenum) and Ti (titanium) are elements useful as steel strengthening elements. be. Cr and Cu can also be used to control the stacking fault energy (ie, SFE) of retained γ. In order to effectively exhibit these actions, Cr, B, Cu, Ni, V, Nb, Mo and Ti are preferably contained in an amount of 0.01% by mass or more, more preferably 0.05% by mass or more. good. However, Cr, B, Cu, Ni, V, Nb, Mo and Ti are economically wasteful because even if they are excessively contained, the above effect is saturated. Therefore, Cr, Cu and Ni are each 3.0% by mass or less (more preferably 2.0% by mass or less), B is 0.01% by mass or less (more preferably 0.005% by mass or less), V, Nb , Mo and Ti are recommended to be limited to 0.5 mass % or less (more preferably 0.3 mass % or less).

[Ca: more than 0% by mass, 0.01% by mass or less, Mg: more than 0% by mass, 0.01% by mass or less, and REM: more than 0% by mass, selected from the group consisting of 0.01% by mass or less 1 or more]
Ca (calcium), Mg (magnesium) and REM (rare earth elements) are elements that control the morphology of sulfides in steel and are effective in improving workability. Therefore, Ca, Mg and REM may be contained in an amount of preferably 0.0005% by mass or more, more preferably 0.001% by mass or more. In addition, Sc (scandium), Y (yttrium), lanthanide, etc. are mentioned as REM used for embodiment of this invention. However, even if Ca, Mg and REM are contained excessively, the above effect is saturated, which is economically wasteful. Therefore, it is recommended to limit each of Ca, Mg and REM to 0.01 mass % or less (more preferably Ca and Mg to 0.003 mass % or less and REM to 0.006 mass % or less).

3. Mechanical Properties As described above, the high-strength steel sheets according to the embodiments of the present invention have high levels of both TS and TS×uEL (uniform elongation). These mechanical properties of high-strength steel sheets according to embodiments of the present invention are described in detail below.

(1) Tensile strength (TS)
The TS in the rolling direction of the steel sheet is 1140 MPa or more, preferably 1180 MPa or more. A higher tensile strength is more preferable, but the upper limit of the tensile strength is about 1470 MPa, considering the chemical composition, manufacturing conditions, and the like of the steel sheet according to the embodiment of the present invention.

(2) Product of TS and uEL (uniform elongation) (TS x uEL)
TS×uEL in the rolling direction of the steel sheet is 48000 MPa·% or more. It is preferably 52000 MPa·% or more, more preferably 56000 MPa·% or more. Having a high TS×uEL makes it possible to obtain a steel sheet with a high level of strength-ductility balance that simultaneously has high strength and high elongation.

The object of the present invention is a thin steel sheet having a thickness of about 1 to 3 mm, but the form of the product is not particularly limited. For example, the product form is a steel sheet that has been hot-rolled, cold-rolled, or both, and then subjected to two-phase annealing, which will be described later. It also includes surface-treated steel sheets that have been subjected to various coating treatments, coating base treatments, organic film treatments, and the like.

4. Manufacturing Method Next, a method for manufacturing a high-strength steel sheet according to an embodiment of the present invention will be described.

The inventors of the present invention produced a steel sheet using a steel having a predetermined chemical composition, which has not been found in the prior art, by adopting the following production method, thereby having the above-described desired metal structure and its As a result, the inventors have found that a high-strength steel sheet having the desired properties described above can be obtained. The details are described below.

(casting, hot rolling)
A steel having the above-mentioned chemical composition is melted, made into a slab (steel material) by ingot casting or continuous casting, and then hot rolled. The slab heating temperature is preferably 1100°C to 1300°C. If the temperature is less than 1100°C, the rolling load may increase, and if it exceeds 1300°C, the energy required for heating will increase and the production cost will increase. Also, the finish rolling temperature is preferably 800°C to 1000°C. If it is less than 800°C, the rolling load may increase, and if it exceeds 1000°C, the crystal grains may become coarse and the ductility may decrease. After that, winding is performed at a temperature of 700° C. or less, and the film is cooled to room temperature. After this, pickling and cold rolling may be performed. Also, hot forging may be performed instead of hot rolling.

(Dual-phase annealing)
Subsequently, the hot-rolled steel sheet thus produced is subjected to dual-phase annealing. As a result, a steel sheet having an excellent balance between strength and ductility can be produced. The two-phase region annealing is performed by holding at a heating temperature of Ac1 point +15°C or higher and Ac1 point +150°C or lower for 180 seconds or longer. When the heating temperature exceeds Ac1 point +150°C, the amount of residual γ becomes excessive, and the strength of the steel sheet becomes insufficient. Also, when the heating temperature is less than Ac1 point +15°C, the amount of ferrite becomes excessive and the amount of residual γ decreases. As a result, strength, elongation, or both of the steel sheet cannot be ensured. On the other hand, if the holding time is less than 180 seconds, distribution of the alloying elements to the residual γ becomes insufficient, resulting in insufficient elongation of the steel sheet.

The lower limit of the heating temperature is preferably Ac1 point +20°C or higher, more preferably Ac1 point +25°C or higher. The upper limit of the heating temperature is preferably Ac1 point +130°C or less, more preferably Ac1 point +100°C or less. Also, the retention time is preferably 360 seconds or longer, more preferably 1800 seconds or longer. Although the upper limit of the retention time is not particularly limited, it is about 86400 seconds from the viewpoint of productivity. In addition, the Ac1 point is obtained from the chemical composition of the steel sheet, by Leslie, "Iron and Steel Material Science", translated by Shigeyasu Koda, Maruzen Co., Ltd., 1985, p. 273 using the following formula (2). After the two-phase region annealing, plating treatments such as chemical conversion treatment, hot-dip plating, electroplating, and vapor deposition, various coating treatments, coating base treatments, organic film treatments, and the like may be performed.

Ac1 (° C.)=723−10.7×[Mn]−16.9×[Ni]+29.1×[Si]+16.9×[Cr] (2)
Here, [ ] in the above formula represents the content (% by mass) of each element.

1. Sample preparation No. shown in Table 1. Steels having chemical compositions of 1 to 3 were melted by VIF in a laboratory, and then hot forged into steel materials having a thickness of 50 mm and a width of 150 mm. After that, it was subjected to hot rough rolling, further heated at 1200° C. for 30 minutes, and then subjected to finish rolling. Then, after being held in an atmospheric furnace at 500° C. for 30 minutes (simulating winding), the steel sheet was cooled in the furnace to produce a hot-rolled steel sheet having a thickness of 3 to 4 mm. After that, softening annealing was performed between 500° C. and 575° C. for 2 hours. Also, No. The steel plate of No. 3 was further subjected to cold rolling after pickling to a thickness of 1.4 mmt. The above softening annealing and cold rolling do not essentially affect the metallographic structure specified in the present invention. After that, two-phase region annealing was performed at the heating temperature and holding time shown in Table 1. In addition, in Table 1 and Tables 2 and 3, which will be described later, the underlined numerical values indicate values outside the scope of the embodiment of the present invention.

2. Evaluation of structure Using the obtained samples, the residual γ fraction, the coarse ferrite fraction and the residual γ composition were evaluated by the following methods. The evaluation results are shown in Table 2.

(1) Residual γ fraction The residual γ fraction was measured by mirror-polishing each sample and using an FE-SEM (manufactured by JEOL Ltd.) to examine the structure of a 1/4 sheet thickness of the cross section perpendicular to the rolling direction. observed. Then, an area of approximately 20 μm×20 μm or 30 μm×30 μm was measured by EBSD (manufactured by EDAX, OIM Data Collection) under the condition of step size: 0.05 μm. Then, the fractions of iron-α and iron-γ were determined using analysis software TSL OIM Analysis 7×64. A region determined to be iron-α was defined as ferrite and martensite, and a region determined to be iron-γ was defined as residual γ, and the residual γ fraction was measured.

(2) Coarse ferrite fraction 20 μm The measurement of the coarse ferrite fraction having an area of ² or more is performed by mirror-polishing each sample and using an FE-SEM (manufactured by JEOL Ltd.) to obtain a plate with a cross section perpendicular to the rolling direction. A 1/4 thick tissue was observed. Then, an EBSD analysis (manufactured by EDAX, OIM Data Collection) was performed on a region of approximately 300 μm×300 μm under the condition of a step size of 0.5 μm. Then, when a misorientation of 15° or more is defined as a grain boundary using the analysis software TSL OIM Analysis 7 x 64, an α-Fe grain having a grain area of 20 µm ² or more is defined as coarse ferrite, and the area The ratio was taken as the coarse ferrite fraction. In addition, in the component composition according to the embodiment of the present invention, the residual structure other than the residual γ and coarse ferrite is fine ferrite and/or martensite having a crystal grain area of less than 20 μm ² .

(3) Residual γ composition The residual γ composition was measured by mirror-polishing each sample and using an FE-SEM (manufactured by JEOL Ltd.) to observe the structure of a 1/4th part of the plate thickness of the cross section perpendicular to the rolling direction. bottom. A field emission electron probe microanalyzer (FE-EPMA) was used to perform quantitative analysis on an area of approximately 4.5 μm×6 μm. A measurement point where the Mn concentration is 1.2 times or more the average value of the entire field of view is defined as residual γ, 50 arbitrary measurement points on the residual γ are extracted, and the alloy element composition (mass%) at each point is obtained. rice field. Then, the average value was calculated as the residual γ composition (% by mass).

Here, the amount of C in the residual γ [Cγwt. %] is determined by the following formula (3) using the residual γ fraction Vγ (%) and the C content [C] (wt. It can be calculated by In this example, each sample was mirror-polished, the surface was corroded with a 3% nital solution to expose the metal structure, and then an FE-SEM (manufactured by JEOL Ltd.) was used to examine the surface perpendicular to the rolling direction. The structure of 1/4 plate thickness of the cross section was observed at 5000 times (observation field: about 18 μm×24 μm). As a result, no precipitate was observed, so the amount of C in the residual γ was calculated using the following formula (3). When precipitates are observed, the amount of C in the residual γ can be measured using 3DAP (three-dimensional atom probe field ion microscope) or EPMA.

[Cwt. %]=[C]/Vγ×100 (3)

3. Characterization Tensile tests were performed to measure tensile strength (TS) and uniform elongation (uEL). Sample no. For 3, a No. 5 test piece described in JIS Z 2201 was produced. The cutting direction of the test piece was such that the longitudinal direction of the test piece was parallel to the rolling direction of the sample. Then, tensile strength (TS) and uniform elongation (uEL) were measured by performing a tensile test according to JIS Z 2241.

On the other hand, sample no. 1 and no. For No. 2, the front and back surfaces were evenly ground and processed to a plate thickness of 1 mm, and then a micro-tensile test piece shown in FIG. 1 was produced. The cutting direction of the test piece was such that the longitudinal direction of the test piece was parallel to the rolling direction of the sample. Then, a tensile test was performed at a stroke speed of 2 mm/min to measure tensile strength (TS) and uniform elongation (uEL). In addition, the same result as the test described in JIS is obtained in the test using the micro-tensile test piece. Table 3 shows the above measurement results. Samples with a TS of 1140 MPa or more and a TS×uEL of 48000 MPa·% or more were judged to be excellent in both strength and strength-ductility balance and passed.

Consider the results in Table 3.
Sample no. 1 is an invention example that satisfies all the requirements according to the embodiments of the present invention. Since it had a predetermined component composition, and had a specified residual γ fraction, coarse ferrite fraction, and residual γ composition, it had high strength and an excellent balance between strength and ductility.

On the other hand, sample no. 2 and No. 3 is a comparative example that did not satisfy any of the requirements according to the embodiments of the present invention.
Sample no. In No. 2, since the C content was low, the coarse ferrite fraction was large and the tensile strength TS was low.
Sample no. In No. 3, since the C content and the Al content were low, the X value of the residual γ composition was lowered, and both the tensile strength TS and the strength-ductility balance were inferior.

Claims (1)

C: 0.32% by mass or more and 0.50% by mass or less,
Si: more than 0% by mass, 3.0% by mass or less,
Mn: 5.0% by mass or more and 7.8% by mass or less,
Al: 1.60% by mass or more and 3.50% by mass or less,
N: more than 0% by mass, 0.01% by mass or less,
P: more than 0% by mass, 0.1% by mass or less, and S: more than 0% by mass, 0.01% by mass or less, the balance being Fe and inevitable impurities,
The metal structure contains austenite in an area ratio of 40% or more to 80% or less with respect to the total metal structure, and the balance is at least one of ferrite and martensite,
The fraction of ferrite grains having an area of 20 μm ² or more is 10% or less in area ratio with respect to the entire metal structure,
A high-strength steel sheet, wherein the austenite composition satisfies an X value represented by the following formula (1) of 15 or more and 45 or less.

X=−25.9+50.8×[Cγwt. %]+1.03×[Siγwt. %]+0.881×[Mnγwt. %]+4.94×[Alγwt. %]−0.469×[Crγwt. %]+1.28×[Cuγwt. %] (1)
However, [Cγwt. %], [Siγwt. %], [Mnγwt. %], [Alγwt. %], [Crγwt. %] and [Cuγwt. %] respectively represent the concentrations of C, Si, Mn, Al, Cr and Cu contained in the austenite, and the concentration of elements not contained is assumed to be zero.

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