JP6875914B2 - High-strength steel plate and its manufacturing method - Google Patents

Description

The present invention relates to a high-strength steel plate that can be used for various purposes including automobile parts.

Steel sheets used for automobile parts and the like are required to be thinned in order to improve fuel efficiency, and to be thinned and to secure the strength of parts, the strength of the steel sheet is required to be increased. Patent Document 1 discloses a high-strength steel sheet having a tensile strength of 980 to 1180 MPa and exhibiting good deep drawing property.

Japanese Unexamined Patent Publication No. 2009-203548

However, in many applications including automobile parts, it is required to have not only high tensile strength and excellent deep drawing property, but also excellent strength ductility balance, high yield ratio and excellent overhang formability. Has been done.
Specifically, the following are required for each of the tensile strength, the strength ductility balance, the yield ratio, the deep drawing characteristics, and the overhang 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 welded portion 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). 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 (TS × EL) of TS and total elongation (EL) is required to be 21000 MPa% or more. Furthermore, in order to ensure moldability during component molding, it is also required that the hole expansion ratio λ is 20% or more, and that the limit overhang height (overhang height) indicating overhang formability is 16 mm or more. There is. In addition, the joint strength of spot welds is also required as the basic performance of steel sheets for automobiles.

However, it is difficult for the high-strength steel plate disclosed in Patent Document 1 to satisfy all of these requirements, and a high-strength steel plate that can satisfy all of these requirements has been required.

The present invention has been made to meet such demands, and includes tensile strength (TS), cross-tensile strength of spot welds (SW cross-strength), yield ratio (YR), (TS) and total elongation (TS). It is an object of the present invention to provide a high-strength steel plate having a high level of product (TS × EL) with EL), a hole expansion ratio (λ), and a limit overhang height, and a method for manufacturing the same.

Aspect 1 of the present invention is
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% by mass or less,
S: 0.01% by mass or less,
Containing, the balance consisting of Fe and unavoidable impurities,
The steel structure is
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,
It is a high-strength steel plate having an average size of MA of 1.0 μm or less and a half-value range of Mn concentration distribution of 0.3% by mass or more in a carbon-enriched region which is equal to the amount of retained austenite.

Aspect 2 of the present invention is the high-strength steel plate according to Aspect 2, wherein the amount of C is 0.30% by mass or less.

Aspect 3 of the present invention is the high-strength steel plate according to Aspect 1 or 2, wherein the Al content is less than 0.10% by mass.

Aspect 4 of the present invention further describes 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. 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 mass% or less. The high-strength steel plate according to any one of aspects 1 to 3, which includes one type or two or more types.

In aspect 5 of the present invention, a rolled material having the component composition according to any one of the above aspects 1 to 4 is prepared.
After holding the rolled material at a temperature of between ₁ point and 0.2 × Ac ₁ point + 0.8 × Ac Ac ₃ point 5 seconds or more, then heated to a temperature of Ac ₃ point or more holding 5-600 seconds To make it austenite
After the austenitization, cooling is performed at a cooling rate of 10 ° C./sec or more from 650 ° C. to a cooling shutdown temperature between 100 ° C. and 300 ° C.
A method for producing a high-strength steel sheet, which comprises heating from a cooling shutdown temperature to a reheating temperature in the range of 300 to 500 ° C.

Aspect 6 of the present invention comprises cooling to the cooling stop temperature at an average cooling rate of 0.1 ° C./sec or more and less than 10 ° C./sec to a quenching start temperature which is a temperature of 650 ° C. or higher. The manufacturing method according to the fifth aspect, which comprises cooling from the quenching start temperature to the cooling stop temperature at an average cooling rate of 10 ° C./sec or more.

According to the present invention, the tensile strength (TS), the cross tensile strength of the spot weld (SW cross tension), the yield ratio (YR), the product of (TS) and the total elongation (EL) (TS × EL), and the hole. It is possible to provide a high-strength steel plate having a high spread ratio (λ) and a critical overhang height, and a method for producing the same.

FIG. 1 is a diagram illustrating a method for manufacturing a high-strength steel sheet according to the present invention, particularly heat treatment.

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

The details of the high-strength steel sheet according to the present invention will be described later, but in the austenitizing step of the heat treatment at the time of manufacture, the two-phase coexistence region between the _{Ac 1} point and the Ac _{3 point, more specifically, the Ac 1} point to 0. It has a Mn-enriched region formed by holding at a temperature between 2 × Ac ₁ point + 0.8 × Ac ₃ points for a predetermined time and then holding at a _{temperature of 3 points or more for a predetermined time.} Further, during the heat treatment, a carbon-enriched region corresponding to the retained austenite (the same amount as the amount of retained austenite) is formed. Then, this carbon-enriched region forms both a Mn-enriched region and a Mn-unconcentrated region. That is, some carbon-enriched regions (retained austenite) contain more Mn and some do not. Therefore, 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 or more. Specifically, the half-value range of the Mn concentration distribution is 0.3% by mass or more.

As described above, varying the amount of Mn contained in the retained austenite means that the retained austenite having various stability can be provided. Retained austenite with low stability that causes machining-induced transformation with a relatively small amount of strain and highly stable retained austenite that causes machining-induced transformation with a large strain amount are mixed, causing machining-induced transformation in various strain regions. It becomes possible. As a result, the n value can be increased in a wide strain region, the strain dispersibility can be enhanced, and high overhang workability can be realized.
The details of the high-strength steel plate of the present invention and the manufacturing method thereof are shown below.

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

(1) C: 0.15 to 0.35%
C is an essential element for obtaining a desired structure and ensuring properties such as high (TS × EL), and it is necessary to add 0.15% or more in order to effectively exert such an action. is there. However, if it exceeds 0.35%, it is not suitable for welding, and sufficient welding strength cannot be obtained. It is preferably 0.18% or more, more preferably 0.20% or more. Further, it is preferably 0.30% or less. Welding can be performed more easily when the amount of C is 0.30% or less.

(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 effectively exert such an action, it is necessary to add 0.5% or more of Si and Al in total. However, when the total of Si and aluminum exceeds 3.0%, coarse MA is formed. It is preferably 0.7% or more, more preferably 1.0% or more. Further, it is preferably 2.0% or less.
The amount of Al added may be less than 0.10% by mass, that is, an amount that functions as a deoxidizing element, and is 0 for the purpose of suppressing the formation of cementite and increasing the amount of retained austenite, for example. More amounts may be added, such as 7.7% by weight or more.

(3) Mn: 1.0 to 4.0%
Manganese suppresses the formation of ferrite. Further, Mn is an element indispensable for forming a Mn-enriched region, forming retained austenite having different stability, and improving overhang workability. In order to effectively exert such an action, 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 x Ac ₁ point + 0.8 x Ac ₃ points. Even if the temperature is maintained between the two for a predetermined time, the transformation may not proceed and the Mn-concentrated region may not be formed. It is preferably 1.5% or more, more preferably 2.0% or more. Further, it is preferably 3.5% or less.

(4) P: 0.05% or less P is inevitably present as an impurity element. The presence of P in excess of 0.05% deteriorates EL and λ. Therefore, the content of P is set to 0.05% or less (including 0%). Preferably, it is 0.03% (including 0%) or less.

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

(6) Residue In one preferred embodiment, the balance is iron and unavoidable impurities. As unavoidable impurities, it is permissible to mix trace elements (for example, As, Sb, Sn, etc.) brought in depending on the conditions of raw materials, materials, manufacturing equipment, and the like. In addition, for example, there are elements such as P and S, which are usually preferable as the content is smaller, and therefore are unavoidable impurities, but the composition range thereof is separately specified as described above. Therefore, in the present specification, the term "unavoidable 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 contained as long as the characteristics of the high-strength steel sheet of the present invention can be maintained. Other elements that can be selectively contained in this way are illustrated below.

(7) Other elements 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. Two or more types of Cu, Ni, Cr, Mo and B improve the hardenability to prevent the formation of ferrite and contribute to the stabilization of austenite and the miniaturization of bainite to achieve a strength-dextency balance. improves.
V, Nb and Ti improve the strength-ductility balance by increasing the strength without significantly deteriorating the ductility by precipitating and strengthening the matrix phase.
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 present invention include Sc, Y, and lanthanoids.
However, even if these elements are excessively contained, the above-mentioned effects are saturated and it is economically wasteful. Therefore, it is preferable that the amount of these elements is equal to or less than the above upper limit values.

2. Steel structure The details of the steel structure of the high-strength steel sheet 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 findings obtained at present, but do not limit the technical scope of the present invention.

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

(2) Total fraction of tempered martensite and tempered bainite: 60% or more By setting the total fraction of tempered martensite and tempered bainite to 60% or more (60% by volume or more), both high strength and high hole-spreading property are achieved. 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 determined by performing SEM observation of the cross section with nital corrosion, measuring the fraction of MA (ie, the sum of retained austenite and as-quenched martensite), and steel. It can be obtained by subtracting the above-mentioned ferrite fraction and MA fraction from the entire structure.

(3) Amount of retained austenite: 10% or more The retained austenite causes a TRIP phenomenon in which it transforms into martesite by process-induced transformation during processing such as press working, and a large elongation can be obtained. In addition, the martensite formed 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), an excellent strength-ductility balance of 21000 MPa% or more in TS × EL can be realized.
The amount of retained austenite is preferably 15% or more.

In the high-strength steel plate of the present invention, most of the retained austenite exists in the form of MA. MA is an abbreviation for martensite-austenite constituent, which is a complex of martensite and austenite.
The amount of retained austenite can be obtained by calculating ferrite by X-ray diffraction (when X-ray diffraction includes tempered martensite and untempered martensite, the diffraction intensity ratio of austenite is calculated and calculated. The X-ray source is Co. -Kα rays can be used.

(4) Average size of MA: 1.0 μm or less MA is a hard phase, and the vicinity of the matrix / hard phase interface acts as a void forming site during deformation. The coarser the MA size, the more strain is concentrated on the matrix / hard phase interface, and the more likely it is that fractures originate from voids formed in the vicinity of the matrix / hard phase interface.
Therefore, the MA size, particularly the MA average size, can be made as fine as 1.0 μm or less to suppress fracture, and the hole expansion rate λ can be improved.
The average size of MA is preferably 0.8 μm or less.

For the average size of MA, observe the cross section of nital corrosion by SEM at 3000 times or more in 3 fields or more, draw a straight line of 200 μm or more in total at an arbitrary position in the photograph, and measure the intercept length at which the straight line and MA intersect. It can be obtained by calculating the average value of these intercept lengths.

(5) The half-value range of the Mn concentration distribution in the carbon-enriched region, which is equal to the amount of retained austenite, is 0.3% by mass or more. It is difficult to identify only retained austenite by microscope or SEM. Since the solid solution limit of carbon in retained austenite is larger than that in ferrite and the like, carbon is concentrated in retained austenite by performing the heat treatment described later. Therefore, elemental mapping of carbon is performed using EPMA, and the measurement points of the amount equal to the amount of retained austenite obtained by the above-mentioned X-ray diffraction are set as the carbon enrichment region in order from the measurement points having the highest carbon concentration, and this carbon enrichment is performed. The region can be judged as retained austenite. That is, for example, when the amount of retained austenite is 15% by volume, these measurement points having a high carbon concentration (carbon concentration) can be selected from the measurement points having the highest carbon concentration for the measurement points whose carbon amount is measured by element mapping. It can be determined that the chemical 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) the retained austenite.

Then, the concentration distribution of Mn in the carbon-enriched region, which is equal to the amount of retained austenite, particularly the half-value range of the Mn concentration distribution can also be measured using EPMA. The distribution of the amount of Mn at the measurement points in the carbon-enriched region can be graphed, and the half-value range can be obtained from the graph.

The larger the half-value range of the Mn concentration distribution, the larger the variation in the Mn concentration in the retained austenite (the wider the range of the Mn concentration distribution). In the high-strength steel plate of the present invention, the half-value range 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 further. More preferably, it is 0.75% by mass or more.

By varying the amount of Mn contained in the retained austenite (carbon-enriched region) in this way, it is possible to form retained austenite having a wide range of stability, from retained austenite having low stability to retained austenite having high stability. Retained austenite with low stability undergoes process-induced transformation with a small amount of strain to become martensite. Retained austenite with high stability does not undergo work-induced transformation with a small amount of strain, but undergoes process-induced transformation only when a large amount of strain is applied to become martensite. Therefore, in the presence of retained austenite having a wide range of stability, machining-induced transformation will continuously occur from the time when the strain amount immediately after the start of machining is small to the time when the machining progresses and the strain amount is large. As a result, the n value can be increased over a wide strain range, the strain dispersibility can be enhanced, and high overhang workability can be realized.

(6) Other steel structure:
In this specification, steel structures other than the above-mentioned ferrite, tempered martensite, tempered bainite and retained austenite are not particularly specified. However, in addition to steel structures such as ferrite, pearlite, untempered bainite, and untempered martensite may be present. As long as the steel structure such as ferrite satisfies the above-mentioned structure conditions, the effect of the present invention can be exhibited even in the presence of pearlite or the like.

3. 3. Characteristics As described above, the high-strength steel plate of the present invention has high levels of TS, YR, TS × EL, hole expansion ratio (λ), cross-strength strength of spot welds (SW cross-strength), and limit overhang height. It is in. These characteristics of the high-strength steel sheet of the present invention will be described in detail below.

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

(2) Yield ratio (YR)
It has a yield ratio of 0.70 or higher. As a result, a high yield strength can be realized in combination with the above-mentioned high tensile strength, 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 ductility balance having both high strength and high ductility at the same time. Preferably, TS × EL is 23000 MPa% or more.

(4) Overhang moldability (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 when the load decreases sharply in the load-stroke diagram.
More specifically, using a test piece of Φ120 mm, using a die of Φ53.6 mm and a shoulder radius of 8 mm and a ball head punch of Φ50 mm, a poly sheet for lubrication is sandwiched between the punch and the steel plate, and a blank holding force of 1000 kgf is used. The limit overhang height is obtained by measuring the height at break (punch stroke).

The high-strength steel plate 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 ratio λ is obtained according to JIS Z 2256. A punch hole with a diameter of d ₀ (d ₀ = 10 mm) is made in the test piece, a punch with a tip angle of 60 ° is pushed into this punch hole, and the diameter of the punch hole when the generated crack penetrates the plate thickness of the test piece. d is measured and calculated from the following formula.
λ (%) = {(d−d ₀ ) / d ₀ } × 100

The high-strength steel plate of the present invention has a hole expansion ratio λ of 20% or more, preferably 30% or more. As a result, excellent processability such as press moldability can be obtained.

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

In the high-strength steel plate of the present invention, the cross tensile strength (SW cross tension) of the spot welded portion is 6 kN or more, preferably 8 kN or more, and more preferably 10 kN or more.

4. Manufacturing Method Next, a manufacturing method for a high-strength steel sheet according to the present invention will be described.
The present inventors have the above-mentioned desired steel structure by subjecting a rolled material having a predetermined composition to the heat treatment (austempering treatment) described below, and as a result, have the above-mentioned desired properties. We found that we could obtain high-strength steel sheets.
The details will be described below.

FIG. 1 is a diagram illustrating a method for manufacturing a high-strength steel sheet according to the present invention, particularly heat treatment.
The rolled material to be heat-treated is usually produced by hot rolling and then cold rolling. However, the production is not limited to this, and either hot rolling or cold rolling may be performed for production. The conditions for hot rolling and cold rolling are not particularly limited.

(1) Austenitization As shown in [2] of FIG. 1, the austenitization step is a two-phase coexistence region between the _{Ac 1} point and the Ac _{3 point, and more specifically, the 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], the austenite is formed by holding at the heating temperature T _{2 for 5 to 600 seconds until the temperature T 2} (Ac ₃ ≤ T _{2 ) at 3} points or more of Ac.

Heat to temperature T ₁ and hold for 5 seconds or longer. Preferably, the holding time is 900 seconds or less. The holding temperature T ₁ may be held at a constant temperature between _{1 point of Ac and 1} point of 0.2 × Ac _{+ 3} points of 0.8 × Ac as shown in [2] of FIG. 1, for example. , Ac ₁ point and 0.2 × Ac ₁ point + 0.8 × Ac ₃ points, such as slowly heating between Ac ₁ point and 0.2 × Ac ₁ point + 0.8 × Ac ₃ points It may be varied. In this way, by holding in a relatively low temperature range within the two-phase coexistence region of ferrite and austenite, more Mn is distributed to the austenite side of the coexisting ferrite and austenite, thereby forming the Mn-enriched region. Obtainable. Since the Mn concentration of austenite formed in the Mn-enriched region and remaining as retained austenite even after the heat treatment is high, it is possible to increase the variation in the Mn concentration in the carbon-enriched region, and high overhang formability can be realized. ..

When the temperature T ₁ is _{lower than the Ac 1} point, the amount of Mn-enriched austenite becomes small, the variation in Mn concentration in the retained austenite (carbon-enriched region) becomes small, and sufficient overhang moldability 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 in Mn concentration in the retained austenite (carbon-enriched region) becomes small, which is sufficient. Overhang moldability cannot be obtained.
If the holding time at the temperature T ₁ is shorter than 5 seconds, the time for Mn diffusion is insufficient, the Mn concentration to austenite is insufficient, and the Mn variation in the retained austenite (carbon-enriched region) varies. It becomes small, and sufficient overhang moldability cannot be obtained.
The holding time at the temperature T ₁ is preferably long, but it is 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.
Incidentally, it is shown as [1] in Figure 1, heating rate of up to temperatures _{T 1} is preferably 5 to 20 ° C. / sec.

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 to _{a temperature T 2 of 3} points or more, _{when heated to a temperature T 1} , the portion that was ferrite also becomes austenite. Mn is not concentrated in this newly transformed portion to austenite. Therefore, in the austenite, there is a region where Mn is not concentrated together with the above-mentioned region where Mn is concentrated, and in the high-strength steel sheet after heat treatment, the variation in Mn concentration in the retained austenite (carbon concentrated region) is dispersed. It can be made larger, and high overhang formability can be realized.

When 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 the YR decreases.
If the temperature T ₂ is too high, the Mn in the previously formed Mn-enriched region may diffuse, and the variation in the Mn concentration may become too small. Therefore, the temperature T ₂ is _{preferably Ac 3} points + 50 ° C. or lower.
When the holding time at the temperature T ₂ is longer than 600 seconds, the Mn concentration in the Mn-concentrated region becomes low due to diffusion, the variation in the Mn concentration in the retained austenite becomes small, and the overhang formability deteriorates.

Preferably, the temperature T ₂ is Ac ₃ points + 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 points + 20 ° C. or higher, and the holding time at the _{temperature T 2 is 20 to 300 seconds.
The heating from the temperature T 1} to the temperature T ₂ shown in [3] of FIG. 1 is preferably performed at a heating rate of 0.1 ° C./sec or more and less than 10 ° C./sec.
It should be _{noted that 1} point of Ac and ₃ points of Ac may be obtained by measurement, but may be calculated by a commonly known calculation formula using the composition.
For example, by using the following equations (1) and 2 (formula), ₁ point of Ac and ₃ points of Ac can be calculated (see, for example, "Leslie Steel Materials Science" Maruzen, (1985)).

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

(2) 100 ° C. or more, after the austenitization cooling above to the cooling stop temperature between less than 300 ° C., as shown in [7] in FIG. 1, to a cooling stop temperature _{T 3} of between 100 ° C. to 300 ° C. Cooling. This cooling causes martensitic transformation while leaving some austenite. By controlling the cooling shutdown temperature T ₃ within a temperature range of 100 ° C. or higher and lower than 300 ° C., the amount of austenite remaining without being transformed into martensite is adjusted, and the final amount of retained austenite is controlled. As shown in [7] in FIG. 1, it may be held at a cooling stop temperature T _3. A preferable holding time for holding is 1 to 600 seconds. A longer holding time has little effect on the characteristics, but a holding time of more than 600 seconds reduces productivity.

Regarding the cooling from the temperature T ₂ to the cooling stop temperature T _{3 , the cooling from the temperature T 2 to the quenching start temperature T 4} which is a temperature of 650 ° C. or higher shown in [5] of FIG. 1 has an average cooling rate of 0.1. As mentioned above, it is preferable to cool the mixture relatively slowly at less than 10 ° C./sec.
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.

When the cooling shutdown temperature T ₃ is lower than 100 ° C., the amount of retained austenite is insufficient, TS increases but EL decreases, and TS × EL decreases.
When the cooling shutdown temperature T ₃ is higher than 300 ° C., coarse untransformed austenite increases and remains even after subsequent cooling, so that the MA size is finally coarse and the hole expansion rate is low.

The cooling shutdown temperature T ₃ is preferably 120 ° C. or higher and 280 ° C. or lower, and more preferably.
It is 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, the heating is performed to a reheating temperature T _{5 in the range of the above-mentioned cooling shutdown temperature T 3 to 300 to 500 ° C.} The heating rate is not particularly limited. After reaching the reheating temperature T ₅ , it is preferable to keep the temperature at that temperature as shown in [9] of FIG. A preferred retention time can be 50 to 1200 seconds.

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

After reheating, as shown in [10] of FIG. 1, it may be cooled to a temperature of 200 ° C. or lower, for example, room temperature. A preferred average cooling rate up to 200 ° C. or lower can be 50 to 2 ° C./sec.
The high-strength steel plate of the present invention can be obtained by the above heat treatment.

A person skilled in the art who has come into contact with the method for producing a high-strength steel sheet according to the embodiment of the present invention described above can obtain the high-strength steel sheet according to the present invention by a production method different from the above-mentioned production method by trial and error. There is a possibility that it can be done.

1. 1. Sample Preparation After the cast material having the chemical composition shown in Table 1 was produced by vacuum melting, the cast material was hot-forged into a steel sheet having a thickness of 30 mm and hot-rolled. _{In addition, Table 1 also shows 1} point of Ac and ₃ points of Ac calculated by using the above formulas (1) and (2). In addition, the values of 0.2 × Ac ₁ point + 0.8 × Ac _{3 points calculated from the Ac 1} point and the Ac ₃ point obtained in this way are also described.
The conditions of hot rolling do not have an essential effect on the final structure and characteristics of this patent, but after heating to 1200 ° C., the plate thickness was adjusted to 2.5 mm by multi-stage rolling. At this time, the end temperature of hot rolling was set to 880 ° C. Then, it cooled to 600 ° C. at 30 ° C./sec, stopped cooling, inserted into a furnace heated to 600 ° C., held for 30 minutes, and then cooled in a furnace to obtain a hot-rolled steel sheet.
The hot-rolled steel sheet was pickled to remove surface scale, and then cold-rolled to 1.4 mm. This cold-rolled plate was heat-treated to obtain a sample. The heat treatment conditions are shown in Table 2. For example, the numbers shown in [] in Table 2 correspond to the processes having the same numbers shown in [] in FIG. 1.

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 maintained only at a temperature of ₃ points or more of Ac corresponding to _{temperature T 2.}
Sample No. In FIG. 9, instead of cooling to a cooling shutdown temperature between 100 ° C. and lower than 300 ° C., a sample held at that temperature after being cooled to a reheating temperature (steps corresponding to [7] to [8] in FIG. 1). This is a sample that skips.
Samples 15 and 31 to 36 are samples in which the heating temperature T ₂ and the quenching start temperature T ₄ are the same. That is, after austenitization, a sample cooled in one step to a cooling stop temperature T _3.
In Tables 1 to 4, the underlined values indicate that the values are outside the scope of the present invention. However, it should be noted that "-" is not underlined even if it is out of the scope of the present invention.

2. Steel structure For each sample by the method described above, ferrite fraction, total fraction of tempered martensite and tempered bainite (listed as "tempered M / B" in Table 3), retained austenite amount (residual γ amount), MA The half-value range of the Mn concentration distribution in the carbon-enriched region was determined. A two-dimensional microscopic X-ray diffractometer (RINT-RAPIDII) manufactured by Rigaku Co., Ltd. was used for measuring the amount of retained austenite. The results obtained are shown in Table 3.

3. 3. Mechanical properties For the obtained sample, YS, TS and EL were measured using a tensile tester, and YR and TS × EL were calculated. 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 method. The results obtained are shown in Table 4.

4. Summary Sample No. which is an example sample satisfying the conditions of the present invention. 11 to 13, 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 (TS) and total elongation (EL). The product (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, austenitization was not performed in two stages of _{temperature T 1} and temperature T _{2 , but was maintained only at a temperature of 3} points or more of Ac corresponding to _{temperature T 2} , so that the concentration of Mn in the carbon-enriched region was maintained. The value of the half-value range of the distribution is small, and the limit overhang height is low.
Sample No. _{In No.} 2, since the holding temperature T 1 is low, the value of the half-value range of the Mn concentration distribution in the carbon-enriched region is small, and the limit overhang height is low.
Sample No. _{In No.} 3, since the holding temperature T 1 is high, the value of the half-value range of the Mn concentration distribution in the carbon-enriched region is small, and the limit overhang height is low.

Sample No. Since 4 and 5 were heated to the heating temperature T ₁ and held, and then the same temperature as _{T 1 was selected as the heating temperature T 2} , austenitization at a sufficiently high temperature could not be performed. Therefore, the amount of ferrite is large, the total fraction of tempered martensite and tempered bainite is low, and the value of the half-value range 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. In No. 6, the heating temperature T ₂ is low and the amount of ferride is large, and 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 T 1 is short, the value of the half-value range of the Mn concentration distribution in the carbon-enriched region is small, and as a result, the limit overhang height is low.

Sample No. In No. 9 _{, the holding time at the heating temperature T 2} is long, and the cooling shutdown temperature T ₃ is high. Therefore, 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 range 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. 16, rapid cooling start temperature T ₄ is fried low, many, tempered martensite and tempered bainite total fraction of low Feraido amount. As a result, the yield ratio is low.
Sample No. In No. 19, the reheating temperature T ₅ is high and the amount of retained austenite is small. As a result, the tensile strength, the value of TS × EL, and the limit overhang height are low.
Sample No. In No. 20, the reheating temperature T ₅ is low, and the amount of retained austenite is small. As a result, the value of TS × EL and the limit overhang height are low.

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

Sample No. In No. 24, the amount of Mn is small, the amount of ferrite is large, and the total fraction of tempered martensite and tempered bainite is low. As a result, the yield ratio and the value of TS × EL are low.
Sample No. In No. 25, the amount of Si + Al is low, the total fraction of tempered martensite and tempered bainite and the amount of retained austenite are low, and the average size of MA is large. 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. No. 27 has a large amount of Si + Al and a large average size of MA, and as a result, the value of TS × EL, the hole expansion rate, and the limit overhang height are low.

Claims (6)

C: 0.15% by mass to 0.35% by mass,
Total of Si and Al: 1.12 % by mass to 2.13 % by mass,
Mn: 1.59 % by mass to 2.51 % by mass,
P: 0.05% by mass or less,
S: 0.01% by mass or less,
Containing, the balance consisting of Fe and unavoidable impurities,
The steel structure consists of ferrite, MA, tempered martensite and tempered bainite, and the ferrite fraction is 4.2 % 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 plate having a half-value range of Mn concentration distribution of 0.3% by mass or more in a carbon-enriched region, which is equal to the amount of retained austenite. The high-strength steel plate according to claim 1, wherein the amount of C is 0.30% by mass or less. The high-strength steel sheet according to claim 1 or 2, wherein the Al content is less than 0.10% by mass. 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 plate according to any one of claims 1 to 3, which comprises one or more of the above. The method for producing a high-strength steel plate according to any one of claims 1 to 4.
To prepare a rolled material having the component composition according to any one of claims 1 to 4, and to prepare a rolled material.
After holding the rolled material at a temperature of between ₁ point and 0.2 × Ac ₁ point + 0.8 × Ac Ac ₃ point 5 seconds or more, then heated to a temperature of Ac ₃ point or more holding 5-600 seconds To make it austenite
After the austenitization, cooling is performed at a cooling rate of 10 ° C./sec or more from 650 ° C. to a cooling shutdown temperature between 100 ° C. and 300 ° C.
Heating from the cooling shutdown temperature to a reheating temperature in the range of 300-500 ° C.
A method for manufacturing a high-strength steel sheet, including. Cooling to the cooling stop temperature is performed at an average cooling rate of 0.1 ° C./sec or more and less than 10 ° C./sec to the quenching start temperature, which is a temperature of 650 ° C. or higher, and the cooling stop from the quenching start temperature. The method for producing a high-strength steel plate according to claim 5, which comprises cooling to a temperature at an average cooling rate of 10 ° C./sec or more.

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