WO2017208763A1

WO2017208763A1 - High-strength steel sheet and method for producing same

Info

Publication number: WO2017208763A1
Application number: PCT/JP2017/017753
Authority: WO
Inventors: 俊夫村上; 茂生大谷; 裕一二村; エライジャ柿内; 忠夫村田
Original assignee: 株式会社神戸製鋼所
Priority date: 2016-05-30
Filing date: 2017-05-10
Publication date: 2017-12-07

Abstract

Disclosed is a high-strength steel sheet comprising 0.15-0.35 mass% C, 0.5-3.0 mass% in total of Si and Al, 1.0-4.0 mass% Mn, 0.05 mass% or less of P, and 0.01 mass% or less of S, with the remainder comprising Fe and inevitable impurities, wherein: the steel structure comprises a ferrite fraction of 5% or less, a total fraction of tempered martensite and tempered bainite of 60% or greater, and a retained austenite amount of 10% or greater; the average size of MA is 1.0 µm or less; and the half width in Mn concentration distribution in a carbon concentrated region of an amount equivalent to said retained austenite amount is 0.3 mass% or greater.

Description

High strength steel plate and manufacturing method thereof

This disclosure relates to a high-strength steel sheet that can be used for various applications including automobile parts.

Steel sheets used for automobile parts and the like are required to be thin in order to improve fuel efficiency, and high strength of the steel sheets is required to achieve thinning and securing of component strength. Patent Document 1 discloses a high-strength steel sheet having a tensile strength of 980 to 1180 MPa and showing good deep drawability.

JP 2009-203548 A

However, in many applications, including automotive parts, not only has high tensile strength and excellent deep drawability, but also has excellent strength-ductility balance, high yield ratio, and excellent stretch formability. It has been.
Specifically, the following is required for each of tensile strength, strength ductility balance, yield ratio, deep drawing characteristics, and stretch formability.
The tensile strength is required to be 980 MPa or more. Further, the tensile strength is required to have a sufficient value even in the welded portion. Specifically, the cross tensile strength of the spot weld is required to be 6 kN or more.
In order to increase the stress that can be applied during use, it is necessary to have a high yield strength (YS) in addition to a high tensile strength (TS). Moreover, it is also necessary to increase the yield strength of the steel sheet from the viewpoint of ensuring collision safety and the like. Therefore, specifically, a yield ratio (YR = YS / TS) of 0.70 or more is required.

Regarding the strength ductility balance, the product of TS and total elongation (EL) (TS × EL) is required to be 21000 MPa% or more. Furthermore, in order to ensure the moldability at the time of component molding, the hole expansion ratio λ is required to be 20% or more, and the limit overhang height (overhang height) indicating the overhang moldability is also required to be 16 mm or more. Yes. Moreover, the joint strength of a spot welded part is also calculated | required as basic performance of the steel plate for cars.

However, the high-strength steel sheet disclosed in Patent Document 1 is difficult to satisfy all these requirements, and a high-strength steel sheet that can satisfy all these requirements has been demanded.

Embodiments of the present invention have been made in order to meet such requirements, and include tensile strength (TS), cross tensile strength of spot welds (SW cross tension), yield ratio (YR), and (TS). The object is to provide a high-strength steel sheet having a high product of all elongation (EL) (TS × EL), hole expansion ratio (λ), and limit overhang height, and a method for producing the same.

Aspect 1 of the present invention
C: 0.15% by mass to 0.35% by mass,
Total of Si and Al: 0.5% by mass to 3.0% by mass,
Mn: 1.0% by mass to 4.0% by mass,
P: 0.05 mass% or less,
S: 0.01% by mass or less,
And the balance consists of Fe and inevitable impurities,
Steel structure
The ferrite fraction is 5% or less,
The total fraction of tempered martensite and tempered bainite is 60% or more,
The amount of retained austenite is 10% or more,
This is a high-strength steel sheet having an average size of MA of 1.0 μm or less and a half-value width of Mn concentration distribution in the carbon concentration region that is equal to the amount of retained austenite being 0.3% by mass or more.

Aspect 2 of the present invention is the high-strength steel sheet according to Aspect 2 in which the C content is 0.30 mass% or less.

Aspect 3 of the present invention is the high-strength steel sheet according to

Aspect

1 or 2, wherein the Al content is less than 0.10% by mass.

In aspect 4 of the present invention, Cu: 0.50% by mass or less, Ni: 0.50% by mass or less, Cr: 0.50% by mass or less, Mo: 0.50% by mass or less, B: 0.01 % By mass or less, V: 0.05% by mass or less, Nb: 0.05% by mass or less, Ti: 0.05% by mass or less, Ca: 0.05% by mass or less, REM: 0.01% by mass or less The high-strength steel sheet according to any one of aspects 1 to 3, including one or more kinds.

Aspect 5 of the present invention provides a rolled material having the component composition described in any one of the aspects 1 to 4, and
The rolled material is held at a temperature between Ac ₁ point and 0.2 × Ac ₁ point + 0.8 × Ac ₃ point for 5 seconds or more, then heated to Ac ₃ point or more and held for 5 to 600 seconds. Austenite,
After the austenitization, cooling to a cooling stop temperature between 650 ° C. and 100 ° C. to 300 ° C. at a cooling rate of 10 ° C./second or more;
And heating to a reheating temperature in the range of 300 to 500 ° C. from the cooling stop temperature.

Aspect 6 of the present invention is that cooling to the cooling stop temperature is performed at an average cooling rate of 0.1 ° C./second or more and less than 10 ° C./second to a rapid cooling start temperature that is 650 ° C. or higher, It is a manufacturing method of aspect 5 including cooling at an average cooling rate of 10 degrees C / sec or more from rapid cooling start temperature to the said cooling stop temperature.

According to the embodiment of the present invention, the tensile strength (TS), the cross tensile strength (SW cross tension) of the spot weld, the yield ratio (YR), the product of (TS) and total elongation (EL) (TS × EL ), A high-strength steel sheet having a high hole expansion ratio (λ) and a limit overhang height, and a method for producing the same.

FIG. 1 is a diagram for explaining a method for producing a high-strength steel sheet according to an embodiment of the present invention, particularly heat treatment.

As a result of intensive studies, the present inventors have found that in steel having a predetermined component, the steel structure (metal structure) has a ferrite fraction of 5% or less, a total fraction of tempered martensite and tempered bainite: 60% or more, and remains. The amount of γ: 10% or more, the average size of MA: 1.0 μm or less, and the half-value width of the Mn concentration distribution in the carbon enriched region corresponding to the retained austenite is 0.3% by mass or more. Thus, tensile strength (TS), yield ratio (YR), product of (TS) and total elongation (EL) (TS × EL), hole expansion ratio, limit overhang height, and cross tensile strength (SW It was found that a high strength steel plate having a high level of (cross tension) can be obtained.

Although details will be described later, high-strength steel sheet according to an embodiment of the present invention, in the austenitizing process of the heat treatment during manufacture, the intermediate two-phase coexisting region of _one point and the Ac ₃ point Ac, and more Ac ₁ point It has a Mn-enriched region formed by holding for a predetermined time at a temperature of at least ₃ points of Ac after holding for a predetermined time at a temperature between ~ 0.2 × Ac ₁ point + 0.8 × Ac ₃ points. Yes. Further, a carbon enriched region corresponding to the retained austenite (the same amount as the retained austenite amount) is formed during the heat treatment. And this carbon concentration area | region forms both a Mn concentration area | region and the area | region where Mn is not concentrated. That is, in the carbon concentration region (residual austenite), there are those that contain more Mn and those that do not. For this reason, when the distribution of the Mn concentration is measured over the entire carbon enriched region (that is, corresponding to the entire retained austenite), the Mn concentration has a certain degree of variation. Specifically, the half width of the Mn concentration distribution is 0.3% by mass or more.

Thus, varying the amount of Mn contained in retained austenite means that retained austenite having various stability can be provided. Low-stability residual austenite that causes processing-induced transformation with a relatively small amount of strain and high-stability residual austenite that causes processing-induced transformation with a large amount of strain are mixed, and processing-induced transformation occurs in various strain regions. It becomes possible. As a result, the n value can be increased in a wide strain region, and the strain dispersibility can be improved to achieve high overhang workability.
Details of the high-strength steel sheet and the manufacturing method thereof according to the present invention will be described below.

1. Composition The composition of the high-strength steel sheet according to the embodiment of the present invention will be described below. First, basic elements C, Si, Al, Mn, P and S will be described, and further elements that may be selectively added will be described.
In addition, unit% display of a component composition means the mass% altogether.

(1) C: 0.15 to 0.35%
C is an essential element for obtaining a desired structure and ensuring high characteristics (TS × EL) and the like, and it is necessary to add 0.15% or more in order to exert such an action effectively. is there. However, more than 0.35% is not suitable for welding, and sufficient welding strength cannot be obtained. Preferably it is 0.18% or more, More preferably, it is 0.20% or more. Further, it is preferably 0.30% or less. If the C content is 0.30% or less, welding can be performed more easily.

(2) Total of Si and Al: 0.5 to 3.0%
Si and Al each have a function of suppressing the precipitation of cementite and promoting the formation of retained austenite. In order to exhibit such an action effectively, it is necessary to add Si and Al in total of 0.5% or more. However, if the total of Si and aluminum exceeds 3.0%, coarse MA is formed. Preferably it is 0.7% or more, More preferably, it is 1.0% or more. Moreover, it is preferably 2.0% or less.
Al may be added in an amount that functions as a deoxidizing element, that is, less than 0.10% by mass, and is 0 for the purpose of, for example, suppressing the formation of cementite and increasing the amount of retained austenite. A larger amount such as 7% by mass or more may be added.

(3) Mn: 1.0 to 4.0%
Manganese suppresses the formation of ferrite. Further, Mn is an indispensable element for forming a Mn-concentrated region, forming retained austenite having different stability, and improving the stretch workability. In order to exhibit such an action effectively, it is necessary to add 1.0% or more. However, if it exceeds 4.0%, the temperature range of the two-phase region heating is narrow and difficult to control, and the temperature becomes too low, so Ac ₁ point to 0.2 × Ac ₁ point + 0.8 × Ac ₃ points Even if the temperature is maintained for a predetermined time, the transformation may not proceed and the Mn-enriched region may not be formed. Preferably it is 1.5% or more, More preferably, it is 2.0% or more. Moreover, Preferably it is 3.5% or less.

(4) P: 0.05% or less P is unavoidably present as an impurity element. If P exceeds 0.05%, EL and λ deteriorate. Therefore, the P content is 0.05% or less (including 0%). Preferably, it is 0.03% (including 0%) or less.

(5) S: 0.01% or less S is unavoidably present as an impurity element. If S exceeding 0.01% is present, sulfide inclusions such as MnS are formed, which becomes a starting point of cracking and lowers λ. Therefore, the S content is 0.01% or less (including 0%). Preferably, it is 0.005% (including 0%) or less.

(6) Balance In one preferred embodiment, the balance is iron and inevitable impurities. As inevitable impurities, mixing of trace elements (for example, As, Sb, Sn, etc.) brought in depending on the situation of raw materials, materials, manufacturing equipment, etc. is allowed. In addition, for example, like P and S, it is usually preferable that the content is small. Therefore, although it is an unavoidable impurity, there is an element that separately defines the composition range as described above. For this reason, in this specification, the term “inevitable impurities” constituting the balance is a concept that excludes elements whose composition ranges are separately defined.

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

(7) Other elements Cu: 0.50 mass% or less, Ni: 0.50 mass% or less, Cr: 0.50 mass% or less, Mo: 0.50 mass% or less, B: 0.01 mass% or less V: 0.05% by mass or less, Nb: 0.05% by mass or less, Ti: 0.05% by mass or less, Ca: 0.05% by mass or less, REM: 0.01% by mass or less, or 2 or more types of Cu, Ni, Cr, Mo and B prevent the formation of ferrite by enhancing the hardenability, and contribute to the stabilization of austenite and the refinement of bainite, thereby improving the strength-ductility balance. improves.
V, Nb, and Ti improve the strength-ductility balance by precipitation strengthening the parent phase and increasing the strength without significantly degrading the ductility.
Ca and REM contribute to the improvement of strength-ductility balance and hole expandability by finely dispersing inclusions typified by MnS. Here, examples of the REM (rare earth element) used in the embodiment of the present invention include Sc, Y, and lanthanoid.
However, even if these elements are contained excessively, the above effects are saturated and it is economically wasteful. Therefore, it is preferable that these elements have amounts not more than the above upper limit values.

2. Steel Structure Details of the steel structure of the high-strength steel sheet according to the embodiment of the present invention will be described below.
In the following description of the steel structure, a mechanism that can improve various properties by having such a structure may be described. It should be noted that these are the mechanisms considered by the present inventors based on the knowledge obtained at the present time, but do not limit the technical scope of the present invention.

(1) Ferrite fraction: 5% or less Although ferrite is generally excellent in workability, it has a problem of low strength. As a result, the yield ratio decreases when the amount of ferrite is large. Therefore, the ferrite fraction is set to 5% or less (5% by volume or less).
The ferrite fraction is preferably 3% or less.
The ferrite fraction can be obtained by observing with a light microscope and measuring a white region by a point calculation method. That is, the ferrite fraction can be obtained by an area ratio (area%) by such a method. And the value calculated | required by area ratio may be used as a value of volume ratio (volume%) as it is.

(2) Total fraction of tempered martensite and tempered bainite: 60% or more Both high strength and high hole expansibility are achieved by setting the total fraction of tempered martensite and tempered bainite to 60% or more (60% by volume or more). it can. The total fraction of tempered martensite and tempered bainite is preferably 70% or more.
The amount of tempered martensite and tempered bainite (total fraction) is measured by SEM observation of the cross-section subjected to nital corrosion, and the fraction of MA (that is, the sum of residual austenite and as-quenched martensite) is measured. It can be obtained by subtracting the above-mentioned ferrite fraction and MA fraction from the entire structure.

(3) Residual austenite amount: 10% or more Residual austenite causes a TRIP phenomenon that transforms into martensite by processing-induced transformation during processing such as press processing, and can obtain a large elongation. Further, the formed martensite has a high hardness. Therefore, an excellent strength-ductility balance can be obtained. By setting the amount of retained austenite to 10% or more (10% by volume or more), TS × EL is 21000 MPa% or more and an excellent strength-ductility balance can be realized.
The amount of retained austenite is preferably 15% or more.

In the high-strength steel plate according to the embodiment of the present invention, most of retained austenite exists in the form of MA. MA is an abbreviation for martensite-austenite constituent and is a composite (composite structure) of martensite and austenite.
The amount of retained austenite can be obtained by calculating by calculating the diffraction intensity ratio of austenite when ferrite (including tempered martensite and untempered martensite in X-ray diffraction is included) by X-ray diffraction. -Kα radiation can be used.

(4) Average size of MA: 1.0 μm or less MA is a hard phase, and the vicinity of the interface between the mother phase and the hard phase acts as a void formation site during deformation. The coarser the MA size, the more concentrated the strain on the matrix / hard phase interface, and the more likely the fracture starts from voids formed in the vicinity of the matrix / hard phase interface.
For this reason, the hole expansion ratio λ can be improved by making the MA size, particularly the MA average size as fine as 1.0 μm or less, and suppressing breakage.
The average size of MA is preferably 0.8 μm or less.

The average size of MA is observed by observing three or more fields of view at 3000 times or more by SEM with a SEM, drawing a straight line of 200 μm or more at an arbitrary position in the photograph, and measuring a section length where the straight line and the MA intersect, It can be obtained by calculating an average value of the intercept lengths.

(5) The half-value width of the Mn concentration distribution in the carbon-enriched region, which is equal to the amount of retained austenite, is 0.3 mass% or more. As described above, most of retained austenite exists in the form of MA. It is difficult to identify only retained austenite with a microscope or SEM. Residual austenite has a carbon solid solubility limit larger than that of ferrite or the like, so that the heat treatment described later causes carbon to concentrate in the retained austenite. Therefore, the element mapping of carbon is performed using EPMA, and the measurement points having the same amount as the amount of retained austenite obtained by the above-mentioned X-ray diffraction in order from the measurement points having the highest carbon concentration are defined as the carbon concentration region. The region can be determined as retained austenite. That is, for example, when the amount of retained austenite is 15% by volume, by selecting 15% from the higher carbon concentration of the measurement points where the carbon content was measured by element mapping, these high carbon concentration measurement points (carbon concentration It can be determined that the crystallization region is retained austenite.
Therefore, the “carbon enriched region having an amount equal to the amount of retained austenite” means a region corresponding to (corresponding to) retained austenite.

Further, the concentration distribution of Mn in the carbon enriched region, which is equal to the amount of retained austenite, particularly the half width of the Mn concentration distribution, can also be measured using EPMA. The distribution of the Mn amount at the measurement point determined to be the carbon enrichment region is graphed, and the half width can be obtained therefrom.

The larger the half width of this Mn concentration distribution, the larger the variation in Mn concentration in the retained austenite (the wider the range of Mn concentration distribution). In the high-strength steel sheet according to the embodiment of the present invention, the half width of the Mn concentration distribution is 0.3% by mass or more, preferably 0.5% by mass or more, more preferably 0.6% by mass or more. And more preferably 0.75% by mass or more.

Thus, by varying the amount of Mn contained in the retained austenite (carbon-enriched region), it is possible to form retained austenite having a wide range of stability, from low-stability residual austenite to high-stability residual austenite. Residual austenite with low stability causes a work-induced transformation with a small amount of strain and becomes martensite. Residual austenite having high stability does not cause a processing-induced transformation at a small strain amount, and does not cause a processing-induced transformation until a large strain amount is imparted to become martensite. Accordingly, if there is residual austenite having a wide range of stability, the machining-induced transformation will occur continuously from when the amount of strain immediately after the start of processing is small to when processing is advanced and the amount of strain is large. As a result, the n value can be increased over a wide strain range, and the strain dispersibility can be enhanced to achieve high overhang processability.

(6) Other steel structures:
In the present specification, the steel structure other than the above-described ferrite, tempered martensite, tempered bainite and retained austenite is not particularly defined. However, in addition to steel structures such as ferrite, pearlite, untempered bainite, untempered martensite, and the like may exist. If the steel structure such as ferrite satisfies the above-described structure condition, the effect of the embodiment of the present invention is exhibited even if pearlite or the like is present.

3. Characteristics As described above, the high-strength steel plate according to the embodiment of the present invention has TS, YR, TS × EL, hole expansion ratio (λ), cross-tension strength (SW cross-tension) of spot welds, and limit overhang height. Both are at a high level. These characteristics of the high-strength steel sheet according to the embodiment of the present invention will be described in detail below.

(1) Tensile strength (TS)
It has a TS of 980 MPa or more. Thereby, sufficient strength can be secured.

(2) Yield ratio (YR)
It has a yield ratio of 0.70 or more. Thereby, combined with the above-described high tensile strength, high yield strength can be realized, and the final product obtained by processing such as deep drawing can be used under high stress. Preferably, it has a yield ratio of 0.75 or more.

(3) Product of TS and total elongation (EL) (TS x EL)
TS × EL is 21000 MPa% or more. By having TS × EL of 21000 MPa% or more, it is possible to obtain a high level of strength and ductility balance having both high strength and high ductility. Preferably, TS × EL is 23000 MPa% or more.

(4) Overhang formability (limit overhang height)
The limit overhang height is an index used for evaluating the overhang formability. The limit overhang height is the punch stroke at the time of breakage where the load decreases rapidly in the load-stroke diagram.
More specifically, a test piece having a diameter of 120 mm is used, a die having a diameter of 53.6 mm and a shoulder radius of 8 mm and a ball head punch having a diameter of 50 mm are sandwiched between the punch and the steel plate, and a blanking force is 1000 kgf. As a result, the limit overhang height is obtained by measuring the height at break (punch stroke).

The high strength steel plate according to the embodiment of the present invention has a limit overhang height of 16 mm or more, preferably 17 mm or more.

(5) Hole expansion rate (λ)
The hole expansion rate λ is obtained according to JIS Z 2256. A punched hole having a diameter d ₀ (d ₀ = 10 mm) is formed in the test piece, a punch having a tip angle of 60 ° is pushed into the punched hole, and the diameter of the punched hole when the generated crack penetrates the plate thickness of the test piece. d is measured and obtained from the following equation.
λ (%) = {(d−d ₀ ) / d ₀ } × 100

The high-strength steel plate according to the embodiment of the present invention has a hole expansion ratio λ of 20% or more, preferably 30% or more. Thereby, excellent workability such as press formability can be obtained.

(6) Spot welded cross tensile strength (SW cross tension)
The cross tensile strength of the spot weld is evaluated according to JIS Z 3137. The spot welding conditions were two steel plates (1.4 mm thick in the examples described later), a dome radius type electrode with a pressure of 4 kN and a current of 6 kA to 12 kA at a 0.5 kA pitch. Perform spot welding. As a result, the minimum current at which dust is generated is obtained. And then. The cross tensile strength of the joint spot-welded at a current 0.5 kA lower than the lowest current at which the dust has occurred is measured.

The high strength steel plate according to the embodiment of the present invention has a cross weld strength (SW cross tension) of a spot welded portion of 6 kN or more, preferably 8 kN or more, more preferably 10 kN or more.

4). Manufacturing Method Next, a manufacturing method of the high strength steel plate according to the embodiment of the present invention will be described.
The inventors have the above-mentioned desired steel structure by performing the heat treatment (austempering treatment) described below on the rolled material having a predetermined composition, and as a result, have the above-mentioned desired characteristics. It was found that a high strength steel plate was obtained.
Details will be described below.

FIG. 1 is a diagram for explaining a method for producing a high-strength steel sheet according to an embodiment of the present invention, particularly heat treatment.
The rolled material to be heat-treated is usually produced by hot rolling followed by cold rolling. However, the present invention is not limited to this, and either one of hot rolling and cold rolling may be performed. The conditions for hot rolling and cold rolling are not particularly limited.

(1) Austenitization As shown in [2] in FIG. 1, the austenitization step is a two-phase coexistence region between Ac ₁ point and Ac ₃ point, more specifically Ac ₁ point and 0.2 × Ac _1. after holding for more than 5 seconds at a temperature _T 1 of between point + 0.8 × Ac ₃ point _{_{(Ac 1 ≦ T 1 ≦ 0.2}} × Ac 1 point + 0.8 × Ac _3), further in FIG. 1 [3 As shown in [4], austenite is obtained by holding at the heating temperature T ₂ for 5 to 600 seconds up to a temperature T ₂ (Ac ₃ ≦ T ₂ ) of Ac ₃ points or more.

By heating to temperatures T ₁ holding at least 5 seconds. Preferably, the holding time is 900 seconds or less. The holding temperature T ₁ may be held at a constant temperature between Ac ₁ point and 0.2 × Ac ₁ point + 0.8 × Ac ₃ point as shown in [2] in FIG. Between Ac ₁ point and 0.2 × Ac ₁ point + 0.8 × Ac ₃ points, such as gradually heating between Ac ₁ point and 0.2 × Ac ₁ point + 0.8 × Ac ₃ points. It may be varied. Thus, by maintaining at a relatively low temperature range in the two-phase coexistence region of ferrite and austenite, a large amount of Mn is distributed to the austenite side among the coexisting ferrite and austenite. Obtainable. And since the Mn concentration of the austenite formed in this Mn-concentrated region and remaining as austenite after heat treatment is high, it becomes possible to increase the variation in the Mn concentration in the carbon-concentrated region, and to realize high stretch formability. .

When the temperature T ₁ is lower than the Ac ₁ point, the amount of Mn-enriched austenite becomes small, variation in Mn concentration in the retained austenite (carbon-enriched region) becomes small, and sufficient stretch formability can be obtained. Can not.
When the temperature T ₁ is higher than 0.2 × Ac ₁ point + 0.8 × Ac ₃ points, the Mn concentration of austenite becomes low, and the variation of Mn concentration in the retained austenite (carbon enriched region) becomes small, which is sufficient. The stretch formability cannot be obtained.
The holding time at the temperature T ₁ is from time shorter 5 seconds, insufficient time for Mn is diffused, Mn enrichment of the austenite is insufficient, variation in Mn in residual austenite (carbon concentrated region) It becomes small and sufficient stretch formability cannot be obtained.
Although preferably longer holding time at the temperature T _1, preferably 900 seconds or less from the viewpoint of productivity.

Preferably, the temperature T ₁ is between 0.9 × Ac ₁ point + 0.1 × Ac ₃ points and 0.3 × Ac ₁ point + 0.7 × Ac ₃ points, and the holding time at the temperature T ₁ Is 10 seconds or more and 800 seconds or less. More preferably, the temperature T ₁ is between 0.8 × Ac ₁ point + 0.2 × Ac ₃ points and 0.4 × Ac ₁ point + 0.6 × Ac ₃ points, and is maintained at the temperature T _1. The time is 30 seconds or more and 600 seconds or less.
The heating rate up to the temperature T ₁ shown as [1] in FIG. 1 is preferably 5 to 20 ° C./second.

Next, as shown in [3] and [4] 1, austenitizing held in Ac ₃ point or more temperature _{_{_{T 2 (Ac 3 ≦ T 2}}} ) until the temperature was raised temperature _{T 2.} The holding time at the temperature T ₂ is 5 to 600 seconds.

By heating the Ac ₃ point or more temperature T _2, when heated to temperatures T ₁ also becomes austenite in a part ferrite. In this newly transformed part to austenite, Mn is not concentrated. For this reason, in the austenite, there is a region where Mn is not concentrated together with the above-described Mn concentration region, and in the high-strength steel sheet after the heat treatment, there is a variation in Mn concentration in the retained austenite (carbon concentrated region). It becomes possible to increase the size, and high stretch formability can be realized.

If the temperature T ₂ is lower than the Ac ₃ point or the holding time at the temperature T ₂ is shorter than 5 seconds, the ferrite fraction of the obtained high-strength steel sheet exceeds 5%, and YR decreases.
When temperature T ₂ is too high, diffused Mn of Mn concentrated region formed earlier, there is a possibility that variation in Mn concentration becomes too small. Therefore, temperature _{T 2} is preferably less Ac ₃ point + 50 ° C..
If the holding time at the temperature T ₂ is longer than 600 seconds, Mn concentration of Mn concentrated region by diffusion is low, the variation of the Mn concentration in the retained austenite is reduced, bulging formability is lowered.

Preferably, the temperature T ₂ is Ac ₃ point + 10 ° C. or higher, and the holding time at the temperature T ₂ is 10 to 450 seconds. More preferably, the temperature T ₂ is Ac ₃ point + 20 ° C. or more, and the holding time at the temperature T ₂ is 20 to 300 seconds.
Shown in [3] in FIG. 1, the heating from temperature _{T 1} of the temperature _{T 2} is, 0.1 ° C. / sec or more, is preferably carried out at a heating rate of less than 10 ° C. / sec.
In addition, about Ac ₁ point and Ac ₃ point, although you may obtain | require by a measurement, you may calculate by the calculation formula generally known using the composition.
For example, Ac ₁ point and Ac ₃ point can be calculated by using the following formulas (1) and 2 (formulas) (see, for example, “Leslie Steel Material Science” Maruzen, (1985)).

Ac ₁ point (° C.) = 723 + 29.1 × [Si] −10.7 × [Mn] + 16.9 × [Cr] −16.9 × [Ni] (1)
Ac ₃ points (° C.) = 910−203 × [C] ^1/2 + 44.7 × [Si] −30 × [Mn] + 700 × [P] + 400 × [Al] + 400 × [Ti] + 104 × [V] −11 × [Cr] + 31.5 × [Mo] −20 × [Cu] −15.2 × [Ni] (2)
Here, [] shows content shown by the mass% of the element described in it.

(2) Cooling to a cooling stop temperature between 100 ° C. and less than 300 ° C. After the above-mentioned austenitization, as shown in [7] in FIG. 1, to a cooling stop temperature T ₃ between 100 ° C. and 300 ° C. Cooling. This cooling causes martensitic transformation while leaving some austenite. Cooling stop temperature T ₃ of 100 ° C. or higher, by controlling within a temperature range below 300 ° C., by adjusting the amount of austenite remaining without transformation to martensite, to control the final amount of retained austenite. As shown in [7] in FIG. 1, it may be held at a cooling stop temperature T _3. A preferable holding time in the case of holding can be 1 to 600 seconds. Even if the holding time is increased, there is almost no influence on the characteristics, but if the holding time exceeds 600 seconds, the productivity is lowered.

Regarding the cooling from the temperature T ₂ to the cooling stop temperature T ₃ , the cooling from the temperature T ₂ to the rapid cooling start temperature T _4, which is a temperature of 650 ° C. or more, shown in FIG. As described above, it is preferable to cool relatively slowly at less than 10 ° C./second.
Then, it is shown in [6] in FIG. 1, the cooling from the quenching start temperature T ₄ to a cooling stop temperature T ₃ is cooled at an average cooling rate of 10 ° C. / sec or more. This is because the formation of ferrite during cooling can be suppressed.

If the cooling stop temperature _{T 3} is lower than 100 ° C., the amount of retained austenite is insufficient, EL is lowered although TS increases, TS × EL is lowered.
If the cooling stop temperature T ₃ is higher than 300 ° C., more coarse untransformed austenite, that also remains in the subsequent cooling, finally MA size becomes coarse, hole expansion ratio becomes low.

Cooling stop temperature _{T 3} is preferably 120 ° C. or more and 280 ° C. or less, more preferably,
140 ° C. or higher and 260 ° C. or lower.
The average cooling rate from the quench initiation temperature T ₄ to a cooling stop temperature T ₃ is preferably not 15 ° C. / s or higher, more preferably 20 ° C. / s or higher.

(3) Reheating to a temperature range of 300 to 500 ° C. As shown in [8] of FIG. 1, heating is performed to the reheating temperature T _{5 in} the range of 300 to 500 ° C. from the cooling stop temperature T ₃ described above. The heating rate is not particularly limited. After reaching the reheating temperature T _5, it is preferably maintained at that temperature as shown in the FIG. 1 [9]. A preferred holding time is 50 to 1200 seconds.

This reheating causes carbon in the martensite to be expelled, promoting carbon concentration to the surrounding austenite, and forming a carbon concentrated region. Thereby, the amount of retained austenite finally obtained (carbon enrichment region) can be increased.
When the reheating temperature T ₅ is lower than 300 ° C., it decreases the TS × EL is not obtained sufficient amount of residual austenite is insufficient diffusion of carbon. Further, if the holding is not performed or the holding time is shorter than 50 seconds, there is a possibility that the carbon diffusion is similarly insufficient. For this reason, it is preferable to hold for 50 seconds or more at the reheating temperature.
Reheating temperature T ₅ is carbon and higher than 500 ° C. precipitated as cementite, residual austenite sufficient amount for not be obtained, TS × EL is lowered. Further, if the holding time is longer than 1200 seconds, similarly, carbon may be precipitated as cementite. For this reason, the holding time is preferably 1200 seconds or less.
A preferred reheating temperature T ₅ is 320 to 480 ° C. In this case, the upper limit of the holding time is preferably 900 seconds or less. The reheating temperature is more preferably 340 to 460 ° C. In this case, the upper limit of the holding time is preferably 600 seconds or less.

After the reheating, as shown in [10] in FIG. 1, it may be cooled to a temperature of 200 ° C. or lower such as room temperature. A preferable average cooling rate up to 200 ° C. or less can be 50 to 2 ° C./second.
By the above heat treatment, the high-strength steel plate according to the embodiment of the present invention can be obtained.

If it is those skilled in the art who contacted the manufacturing method of the high-strength steel plate which concerns on embodiment of this invention demonstrated above, the high-strength steel plate which concerns on embodiment of this invention by the manufacturing method different from the manufacturing method mentioned above by trial and error is demonstrated. There is a possibility that can be obtained.

1. Sample Production After a cast material having the chemical composition described in Table 1 was manufactured by vacuum melting, the cast material was hot forged into a steel plate having a thickness of 30 mm and subjected to hot rolling. Table 1 also shows the Ac ₁ point and Ac ₃ point calculated using the above-described equations (1) and (2). In addition, the value of 0.2 × Ac ₁ point + 0.8 × Ac ₃ points calculated from the Ac ₁ point and Ac ₃ points obtained in this way is also described.
The hot rolling conditions do not substantially affect the final structure and characteristics of this patent, but after heating to 1200 ° C., the sheet thickness is 2.5 mm by multi-stage rolling. At this time, the end temperature of the hot rolling was 880 ° C. Then, it cooled to 600 degreeC at 30 degree-C / sec, stopped cooling, and after inserting into the furnace heated at 600 degreeC, it hold | maintained for 30 minutes, and then cooled in the furnace, and it was set as the hot-rolled steel plate.
The hot-rolled steel sheet was pickled to remove the surface scale, and then cold-rolled to 1.4 mm. The cold-rolled plate was heat-treated to obtain a sample. The heat treatment conditions are shown in Table 2. In Table 2, for example, a number indicated in [] as in [2] corresponds to a process having the same number indicated in [] in FIG.

In Table 2, sample no. In No. 1, austenitization was not performed in two stages of temperature T ₁ and temperature T ₂ , but was held only at a temperature equal to or higher than Ac ₃ points corresponding to temperature T ₂ .
Sample No. 9 is a sample (steps corresponding to [7] to [8] in FIG. 1) that is cooled to the reheating temperature and held at that temperature instead of cooling to a cooling stop temperature between 100 ° C. and less than 300 ° C. Sample skipped).
Samples 15 and 31-36 are sample heating temperature _{T 2} and quench start temperature _{T 4} to the same. That is, after austenitization, a sample cooled in one step to a cooling stop temperature T _3.
In Tables 1 to 4, underlined numerical values indicate that they are out of the scope of the embodiment of the present invention. However, it should be noted that “-” is not underlined even if it falls outside the scope of the embodiment of the present invention.

2. Steel structure By the method described above for each sample, the ferrite fraction, the total fraction of tempered martensite and tempered bainite (described as “tempered M / B” in Table 3), the amount of retained austenite (residual γ amount), MA The half-value width of the Mn concentration distribution in the carbon-enriched region was determined. For the measurement of the amount of retained austenite, a two-dimensional micro part X-ray diffractometer (RINT-RAPIDII) manufactured by Rigaku Corporation was used. The obtained results are shown in Table 3.

3. Mechanical properties About the obtained sample, YS, TS, and EL were measured using the tensile tester, and YR and TSxEL were computed. Further, the hole expansion ratio λ, the limit overhang height, and the cross tensile strength (SW cross tension) of the spot welded portion were determined by the above-described method. Table 4 shows the obtained results.

4). Summary Sample No., which is an example sample that satisfies the conditions in the embodiment of the present invention. 11 to 15, 17, 18, 21, and 28 to 46 all have a tensile strength (TS) of 980 MPa or more, a yield ratio (YR) of 0.70 or more, and the product of (TS) and total elongation (EL) ( (TS × EL) is 21000 MPa% or more, the hole expansion ratio (λ) is 20% or more, the limit overhang height is 16 mm or more, and the SW cross tension is 6 kN or more.

On the other hand, sample no. In No. 1, since austenitization is not performed in two stages of temperature T ₁ and temperature T ₂ , but is held only at a temperature of _three or more points corresponding to temperature T ₂ , the Mn concentration in the carbon concentration region The value of the half width of the distribution is small, and the limit overhang height is low.
Sample No. In _No. 2, since the holding temperature T1 is low, the value of the half-value width of the Mn concentration distribution in the carbon concentration region is small, and the limit overhang height is low.
Sample No. 3, because of the high holding temperature T _1, the value of the half width of the density distribution of Mn in the carbon concentrated region is small, the limit projecting height is lower.

Sample No. Since 4 and 5 were heated to the heating temperature T ₁ and held, and the same temperature as T ₁ was selected as the heating temperature T ₂ , austenitization at a sufficiently high temperature could not be performed. For this reason, the amount of ferrite is large, the total fraction of tempered martensite and tempered bainite is low, and the value of the half-value width of the Mn concentration distribution in the carbon enriched region is small. As a result, the yield ratio and the limit overhang height are low.

Sample No. 6, the heating temperature T ₂ is low, it often Feraido amount, as a result, the yield ratio is low.
Sample No. 7, because of the high cooling stop temperature T _3, tempered martensite and tempered bainite total fraction has become 0%, and the average size of the MA is increased. As a result, the yield ratio and the hole expansion rate are low.
Sample No. In _No. 8, since the holding time at the heating temperature T1 is short, the value of the half width of the Mn concentration distribution in the carbon concentration region is small, and as a result, the limit overhang height is low.

Sample No. No. 9 has a long holding time at the heating temperature T ₂ and a high cooling stop temperature T ₃ . For this reason, the total fraction of tempered martensite and tempered bainite is 0%, the average size of MA is large, and the value of the half-value width of the concentration distribution of Mn is small. As a result, the hole expansion rate and the limit overhang height are low.
Sample No. 10, the cooling stop temperature T ₃ is low, less the amount of retained austenite, as a result, the values and limitations projecting height of TS × EL is low.

Sample No. No. 16 has a low rapid quenching start temperature T ₄ , so the amount of ferride is large, and the total fraction of tempered martensite and tempered bainite is low. As a result, the yield ratio is low.
Sample No. 19, the reheating temperature T ₅ is high, the amount of retained austenite is low. As a result, the tensile strength, the value of TS × EL, and the limit overhang height are low.
Sample No. 20, the reheating temperature T ₅ is low, the amount of retained austenite is low. As a result, the value of TS × EL and the limit overhang height are low.

Sample No. No. 22 has a low amount of C and a small amount of retained austenite. As a result, the value of TS × EL and the limit overhang height are low.
Sample No. No. 23 has a large amount of Mn, a low total fraction of tempered martensite and tempered bainite, and a large average size of MA. As a result, the hole expansion rate is low.

Sample No. No. 24 has a small amount of Mn, a large amount of ferrite, and a low total fraction of tempered martensite and tempered bainite. As a result, the yield ratio and the value of TS × EL are low.
Sample No. No. 25 has a low Si + Al content, a low total fraction of tempered martensite and tempered bainite and a residual austenite content, and a large MA average size. As a result, the value of TS × EL, the hole expansion rate, and the limit overhang height are low.

Sample No. No. 26 has a large amount of C, and as a result, the SW cross tensile strength is low.
Sample No. In No. 27, the amount of Si + Al is large and the average size of MA is large. As a result, the value of TS × EL, the hole expansion ratio, and the limit overhang height are low.

The present application is a Japanese patent application filed on May 30, 2016, Japanese Patent Application No. 2016-107598, and a Japanese patent application filed on March 31, 2017, Japanese Patent Application No. 2017-072109. Accompanied by claiming priority as a basic application. Japanese Patent Application No. 2016-107598 and Japanese Patent Application No. 2017-072109 are incorporated herein by reference.

Claims

C: 0.15% by mass to 0.35% by mass,
Total of Si and Al: 0.5% by mass to 3.0% by mass,
Mn: 1.0% by mass to 4.0% by mass,
P: 0.05 mass% or less,
S: 0.01% by mass or less,
And the balance consists of Fe and inevitable impurities,
Steel structure
The ferrite fraction is 5% or less,
The total fraction of tempered martensite and tempered bainite is 60% or more,
The amount of retained austenite is 10% or more,
The average size of MA is 1.0 μm or less,
A high-strength steel sheet having a half-value width of 0.3% by mass or more of the concentration distribution of Mn in the carbon enriched region that is equal to the amount of retained austenite.
The high-strength steel sheet according to claim 1, wherein the C content is 0.30 mass% or less.
The high-strength steel sheet according to claim 1, wherein the Al content is less than 0.10% by mass.
Cu: 0.50 mass% or less,
Ni: 0.50 mass% or less,
Cr: 0.50 mass% or less,
Mo: 0.50 mass% or less,
B: 0.01% by mass or less,
V: 0.05 mass% or less,
Nb: 0.05 mass% or less,
Ti: 0.05 mass% or less,
Ca: 0.05 mass% or less,
REM: 0.01% by mass or less,
The high-strength steel sheet according to claim 1, comprising one or more of the following.
Preparing a rolled material having the component composition according to any one of claims 1 to 4,
The rolled material is held at a temperature between Ac 1 point and 0.2 × Ac 1 point + 0.8 × Ac 3 point for 5 seconds or more, then heated to Ac 3 point or more and held for 5 to 600 seconds. Austenite,
After the austenitization, cooling to a cooling stop temperature between 650 ° C. and 100 ° C. to 300 ° C. at a cooling rate of 10 ° C./second or more;
Heating from a cooling stop temperature to a reheating temperature in the range of 300-500 ° C .;
A method for producing a high-strength steel sheet, comprising:
Cooling to the cooling stop temperature is performed at an average cooling rate of 0.1 ° C./second or more and less than 10 ° C./second to a rapid cooling start temperature that is a temperature of 650 ° C. or higher, and the cooling is stopped from the rapid cooling start temperature. The method for producing a high-strength steel sheet according to claim 5, comprising cooling to a temperature at an average cooling rate of 10 ° C./second or more.