JP6875916B2 - 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 various applications such as 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 hole expansion ratio. ing.
Specifically, the following are required for each of the tensile strength, the strength ductility balance, the yield ratio, the deep drawing characteristics, and the hole expansion ratio.
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.
Further, 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 0.75 or more is required.

Regarding the strength ductility balance, the product (TS × EL) of TS and total elongation (EL) is required to be 20000 MPa% or more. Further, in order to ensure the moldability at the time of forming a part, it is also required that the LDR indicating the deep drawing property is 2.05 or more and the hole expanding rate λ indicating the hole expanding property is 20% or more. .. 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), LDR, and hole expansion ratio (λ), and a method for producing 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,
The average size of MA is 1.0 μm or less,
The average size of retained austenite is 1.0 μm or less,
A high-strength steel plate having a size of 1.5 μm or more and a retained austenite of 2% or more of the total 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.
By heating the rolled material to _{a temperature of 3} points or more to make it austenite,
After the austenitization, the mixture is cooled between 650 ° C. and 500 ° C. at an average cooling rate of 15 ° C./sec or more and less than 200 ° C./sec, and at a cooling rate of 10 ° C./sec or less within the range of 300 to 500 ° C. for 10 seconds. As mentioned above, staying for less than 300 seconds and
After the retention, cooling is performed at an average cooling rate of 10 ° C./sec or higher from a temperature of 300 ° C. or higher 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 to 500 ° C.
It is a manufacturing method of a high-strength steel sheet including.

Aspect 6 of the present invention is the production method according to Aspect 5, wherein the retention is maintained at a constant temperature within the range of 300 to 500 ° C.

According to the present invention, tensile strength (TS), cross tensile strength of weld (SW cross tension), yield ratio (YR), product of (TS) and total elongation (EL) (TS × EL), LDR and It is possible to provide a high-strength steel plate having a high hole expansion ratio (λ) 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. By setting the amount of γ: 10% or more, the average size of MA: 1.0 μm or less, the average size of retained austenite: 1.0 μm or less, and the residual austenite of size 1.5 μm or more: 2% or more of the total amount of retained austenite. , Tensile strength (TS), yield ratio (YR), product of (TS) and total elongation (EL) (TS × EL), LDR and hole expansion ratio (λ) are all at high levels. I found what I could get.

1. 1. 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, more preferably 0%.
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 TS × EL of 20000 MPa% or more 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 the diffraction intensity ratio of ferrite (including bainite, tempered bainite, tempered martensite and untempered martensite in X-ray diffraction) and austenite by X-ray diffraction. .. Co-Kα rays can be used as the X-ray source.

(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) Average size of retained austenite: 1.0 μm or less and size 1.5 μm or more Retained austenite: 2% or more of the total amount of retained austenite The average size of retained austenite is 1.0 μm and the size is 1.5 μm or more. It was found that excellent deep drawing property can be obtained by setting the ratio (volume ratio) of retained austenite to total retained austenite to 2% or more.

If the inflow stress of the flange portion is smaller than the tensile stress of the vertical wall portion formed during the deep drawing, the drawing will proceed easily, and good deep drawing can be obtained. As for the deformation behavior of the flange portion, since compressive stress is strongly applied from the plate surface direction and the circumference, the flange portion is deformed in a state where isotropic compressive stress is applied. On the other hand, since the martensitic transformation is accompanied by volume expansion, the martensitic transformation is less likely to occur under isotropic compressive stress. Therefore, work-induced martensitic transformation of retained austenite at the flange portion is suppressed and work hardening is reduced.
As a result, the deep drawing property is improved. The larger the size of retained austenite, the greater the effect of suppressing martensitic transformation.

Further, in order to increase the tensile stress of the vertical wall portion formed by deep drawing, it is necessary to maintain a high work hardening rate during deformation. Unstable retained austenite that easily undergoes work-induced transformation under relatively low stress and stable retained austenite that does not cause work-induced transformation unless under high stress are mixed to cause work-induced transformation over a wide stress range. By doing so, a high work hardening rate can be maintained during deformation. Therefore, it was examined to obtain a steel structure containing a predetermined amount of coarse and unstable retained austenite and fine and stable retained austenite. Then, the present inventors set the average size of retained austenite to 1.0 μm and the ratio (volume ratio) of the amount of retained austenite having a size of 1.5 μm or more to the total amount of retained austenite to 2% or more. It has been found that a high work hardening rate can be maintained during deformation and excellent deep drawing property (LDR) can be obtained.

Further, as described above, when the retained austenite undergoes processing-induced transformation, a TRIP phenomenon occurs and a large elongation can be obtained. On the other hand, the martensite structure formed by the work-induced transformation is hard and acts as a starting point of fracture. Larger martensitic tissues are more likely to be the starting point of destruction. By setting the average size of retained austenite to 1.0 μm or less and reducing the size of martensite formed by process-induced transformation, the effect of suppressing fracture can also be obtained.

A Phase map is created using the EBSD (Electron Back Scatter Diffraction Patterns) method, which is a crystal analysis method using SEM, for the average size of retained austenite and the ratio of the amount of retained austenite having a size of 1.5 μm or more to the total amount of austenite. It can be obtained by From the obtained Phase map, the area of each austenite phase (retained austenite) was obtained, the circle-equivalent diameter (diameter) of each austenite phase was obtained from the area, and the average value of the obtained diameters was taken as the average size of retained austenite. To do. Further, by integrating the area of the austenite phase having a circle equivalent diameter of 1.5 μm or more and obtaining the ratio to the total area of the austenite phase, the ratio of the retained austenite having a size of 1.5 μm or more to the total austenite can be obtained. .. The ratio of retained austenite having a size of 1.5 μm or more to all austenites thus obtained is an area ratio, which is equivalent to a volume ratio.

(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.

2. 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: 0.10 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 0.10% 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 0.13% or more, more preferably 0.15% 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 2.5%
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, if the total of Si and aluminum exceeds 2.5%, the deformability of steel decreases and TS × EL decreases. 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. In order to effectively exert such an action, it is necessary to add 1.0% or more. However, if it exceeds 4.0%, bainite transformation is suppressed and relatively coarse residual γ cannot be formed. Therefore, the deep drawing property cannot be improved. It is preferably 1.5% or more, more preferably 2.0% or more. Also, 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.

3. 3. Characteristics As described above, the high-strength steel sheet of the present invention has high levels of TS, YR, TS × EL, LDR and λ. 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 0.75 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 0.80 or more.

(3) Product of TS and total elongation (EL) (TS x EL)
TS × EL is 20000 MPa% or more. By having TS × EL of 20000 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) Deep drawing (LDR)
LDR is an index used for evaluation of deep drawing property. In cylindrical drawing, the diameter of the obtained cylinder is d, and the maximum diameter of a disk-shaped steel plate (blank) that can obtain a cylinder without breaking in one deep drawing is D, and d / D is defined as d / D. It is called LDR (Limiting Drawing Ratio). More specifically, a disk-shaped sample having a plate thickness of 1.4 mm and various diameters is deeply drawn into a cylinder with a die having a punch diameter of 50 mm, a punch angle radius of 6 mm, a die diameter of 55.2 mm, and a die angle radius of 8 mm. The LDR can be obtained by obtaining the sample diameter (maximum diameter D) that has been squeezed out without breaking.

The high-strength steel plate of the present invention has an LDR of 2.05 or more, preferably 2.10 or more, and has excellent deep drawing properties.

(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 a heat treatment (multi-step austempering treatment) described in detail later, and as a result, the above-mentioned desired properties. It was found to obtain a high-strength steel sheet with.
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) Austenitizing treatment As shown in [1] and [2] of FIG. 1, the austenitizing treatment is performed by heating to a temperature of _{3 or more points of Ac.} It may be held at this heating temperature for 1 to 1800 seconds. The upper limit of the heating temperature is preferably Ac ₃ points or more and Ac ₃ points + 100 ° C. or less. This is because coarsening of crystal grains can be suppressed by setting the temperature at ₃ points of Ac to + 100 ° C. or lower. The heating temperature is more preferably Ac ₃ points + 10 ° C. or higher, Ac ₃ points + 90 ° C. or lower, and even more preferably Ac ₃ points + 20 ° C. or higher, Ac ₃ points + 80 ° C. or lower. This is because it can be more completely austenitized and the formation of ferrite can be suppressed, and the coarsening of crystal grains can be suppressed more reliably.
The heating at the time of austenitization, which is shown in [1] of FIG. 1, may be performed at an arbitrary heating rate, and preferred average heating rates include 1 ° C./sec or more and 20 ° C./sec.

(2) Cooling and retention in the temperature range of 300 ° C to 500 ° C After the above austenitization, the mixture is cooled and as shown in [5] of FIG. It is allowed to stay for 10 seconds or more and less than 300 seconds at a cooling rate of 10 ° C./sec or less in a temperature range of 300 to 500 ° C.
Cooling is performed at an average cooling rate of 15 ° C./sec or more and less than 200 ° C./sec at least between 650 ° C. and 500 ° C. This is because the formation of ferrite during cooling is suppressed by setting the average cooling rate to 15 ° C./sec or more. Further, by setting the cooling rate to less than 200 ° C./sec, it is possible to prevent the occurrence of excessive thermal strain due to rapid cooling. As a preferable example of such cooling, as shown in [3] of FIG. 1, relatively low average cooling of 0.1 ° C./sec or more and 10 ° C./sec or less up to the quenching start temperature of 650 ° C. or higher. Cool at a rate, and as shown in [4] of FIG. 1, the average cooling rate is 20 ° C./sec or more and less than 200 ° C./sec from the quenching start temperature to the retention start temperature of 500 ° C. or lower. be able to.

It is allowed to stay for 10 seconds or more at a cooling rate of 10 ° C./sec or less in a temperature range of 300 to 500 ° C. That is, in the temperature range of 300 to 500 ° C., the cooling rate is set to 10 ° C./sec or less for 10 seconds or more. The state where the cooling rate is 10 ° C./sec or less includes the case where the cooling rate is maintained at a substantially constant temperature (that is, the cooling rate is 0 ° C./sec) as shown in [5] of FIG.
This retention partially forms bainite. And since bainite has a lower solid solution limit of carbon than austenite, it expels carbon exceeding the solid solution limit. As a result, a carbon-enriched austenite region is formed around bainite.
This region undergoes cooling and reheating, which will be described later, and becomes slightly coarse retained austenite. By forming this slightly coarse retained austenite, the deep drawing property can be enhanced as described above.

If the retention temperature is higher than 500 ° C., the carbon-enriched region becomes too large, and not only retained austenite but also MA becomes coarse, so that the hole expansion rate decreases. On the other hand, when the retention temperature is lower than 300 ° C., the carbon-enriched region is small, the amount of coarse retained austenite is insufficient, and the deep drawing property is lowered.
On the other hand, if the residence time is shorter than 10 seconds, the area of the carbon-enriched region becomes small, the amount of coarse retained austenite becomes insufficient, and the deep drawing property deteriorates. On the other hand, when the residence time is 300 seconds or more, the carbon-enriched region becomes too large, and not only the retained austenite but also the MA becomes coarse, so that the hole expansion rate decreases.
Further, if the cooling rate during retention is higher than 10 ° C./sec, sufficient bainite transformation does not occur, so that a sufficient carbon-enriched region is not formed, and the amount of coarse retained austenite is insufficient.

Therefore, it is allowed to stay for 10 seconds or more at a cooling rate of 10 ° C./sec or less in a temperature range of 300 to 500 ° C. It is preferable to keep the temperature within the temperature range of 320 to 480 ° C. at a cooling rate of 8 ° C./sec or less for 10 seconds or more, and to hold the temperature at a constant temperature for 3 to 80 seconds during that period.
More preferably, it is allowed to stay in a temperature range of 340 to 460 ° C. at a cooling rate of 3 ° C./sec or less for 10 seconds or more, and during that time, it is held at a constant temperature for 5 to 60 seconds.

(3) Cooling to a cooling shutdown temperature between 100 ° C. and lower than 300 ° C. After the above-mentioned retention, between 100 ° C. and 300 ° C. from the second cooling start temperature of 300 ° C. or higher as shown in [6] of FIG. Cool at an average cooling rate of 10 ° C./sec or higher to the cooling shutdown temperature of. In one of the preferred embodiments, as shown in FIG. 1 [6], the above-mentioned retention end temperature (for example, the holding temperature shown in FIG. 1 [5]) is set as the second cooling start temperature.
This cooling causes martensitic transformation while leaving the above-mentioned carbon-enriched region as austenite. By controlling the cooling shutdown temperature within the 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.

If the cooling rate is slower than 10 ° C./sec, the carbon-enriched region expands more than necessary during cooling, and MA becomes coarse, so that the hole expansion rate decreases. If the cooling shutdown temperature is lower than 100 ° C., the amount of retained austenite is insufficient. As a result, although the TS becomes high, the EL decreases and the TS × EL balance becomes insufficient.
When the cooling shutdown temperature is 300 ° C. or higher, coarse untransformed austenite increases and remains even after subsequent cooling, so that the MA size finally becomes coarse and the hole expansion rate λ decreases.
The preferable cooling rate is 15 ° C./° C. or higher, and the preferable cooling shutdown temperature is 120 ° C. or higher and 280 ° C. or lower. A more preferable cooling rate is 20 ° C./s or more, and a more preferable cooling stop temperature is 140 ° C. or more and 260 ° C. or less.

As shown in [7] of FIG. 1, the temperature may be maintained at the cooling shutdown temperature. 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.

(4) Reheating to a temperature range of 300 to 500 ° C. As shown in [8] of FIG. 1, heating is performed from the above-mentioned cooling shutdown temperature to a reheating temperature in the range of 300 to 500 ° C. The heating rate is not particularly limited. After reaching the reheating temperature, 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.

By this reheating, carbon in martensite can be expelled, carbon concentration to surrounding austenite is promoted, and austenite can be stabilized. Thereby, the amount of retained austenite finally obtained can be increased.
If the reheating temperature is lower than 300 ° C., carbon diffusion is insufficient and a sufficient amount of retained austenite cannot be obtained, resulting in a decrease in TS × EL. 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.
If the reheating temperature 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 precipitated as cementite in the same industry. Therefore, the holding time is preferably 1200 seconds or less.
The preferred reheating temperature is 320 to 480 ° C., in which 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. 10 ° C./sec can be mentioned as a preferable average cooling rate up to 200 ° C. or lower.
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 A cast material having the chemical composition shown in Table 1 was produced by vacuum melting, and then this cast material was hot-forged into a steel sheet having a thickness of 30 mm and then hot-rolled. _{It should be noted that the hot rolling conditions, in which 3} points of Ac calculated from the composition are also described in Table 1, do not have an essential effect on the final structure and characteristics of the present patent, but after heating to 1200 ° C., the plate is subjected to multi-step rolling. The thickness was 2.5 mm. 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. Reference numerals 1, 4 and 7 are samples in which the sample was not retained for 10 seconds or more at a cooling rate of 10 ° C./sec or less in a temperature range of 300 to 500 ° C. in the step corresponding to [5] of FIG. In particular, sample No. Reference numerals 1 and 26 are samples in which quenching was started at 700 ° C. and then cooled to 200 ° C. at once (samples in which the steps corresponding to [5] and [6] were skipped in FIG. 1). Reference numeral 7 denotes a sample that has not been cooled to a cooling shutdown temperature between 100 ° C. and lower than 300 ° C. (a sample in which the steps corresponding to [6] to [8] in FIG. 1 are skipped).

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, the ferrite fraction, the total fraction of tempered martensite and tempered bainite (listed in "Tempering M / B" in Table 3), the amount of retained austenite (the amount of residual γ), and the amount of MA Obtain the average size, the average size of retained austenite (residual γ average size), and the ratio of retained austenite having a size of 1.5 μm or more to the total austenite (Table 3 shows “residual γ ratio of 1.5 μm or more”). It was. 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 LDR, 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 Samples No. 9, 11-13, 17-21 and 36-46, which are example samples satisfying the conditions of the present invention, have a tensile strength of 980 MPa or more, a yield ratio of 0.75 or more, and a TS of 20000 MPa% or more. × EL, LDR of 2.05 or more, hole expansion ratio of 20% or more, and SW cross tension of 6 kN or more are achieved.
On the other hand, sample No. In No. 1, since the austenite was not retained in the temperature range of 300 to 500 ° C., the amount of retained austenite having a size of 1.5 μm or more was not sufficient, and as a result, sufficient deep drawing property could not be obtained. ..
Sample No. No. 2 has a long residence time in the temperature range of 300 to 500 ° C. after austenitization. In No. 3, since the average cooling rate from the second cooling start temperature (“[5] holding temperature” shown in Table 2) to the cooling stop temperature is slow, the MA average size becomes excessive, and as a result, sufficient hole expansion is performed. No rate was obtained.

Sample No. In No. 4, since the holding time in the temperature range of 300 to 500 ° C. was short after austenite formation, the amount of retained austenite having a size of 1.5 μm or more was not sufficient, and sufficient deep drawing property could not be obtained.
Sample No. After austenitization, No. 5 was retained at a temperature higher than the temperature range of 300 to 500 ° C., so that the MA average size became excessive, and as a result, a sufficient hole expansion ratio could not be obtained.
Sample No. Since No. 6 was retained at a temperature lower than the temperature range of 300 to 500 ° C. after austenite formation, the amount of retained austenite having a size of 1.5 μm or more was not sufficient, and as a result, sufficient deep drawing property could not be obtained.

Sample No. Since No. 7 has not been cooled to a cooling shutdown temperature between 100 ° C. and lower than 300 ° C., the total amount of tempered martensite and tempered bainite is insufficient, the MA average size is excessive, and the average size of retained austenite is also large. It became excessive. As a result, sufficient hole expansion ratio and deep drawing property could not be obtained.

Sample No. In No. 8, since the heating temperature for austenitization was low, the amount of ferrite was excessive, and the total amount of tempered martensite and tempered bainite was insufficient, and as a result, a sufficient yield ratio could not be obtained.
Sample No. In No. 10, since the cooling shutdown temperature is lower than the temperature range of 100 ° C. to 300 ° C., the amount of retained austenite is small, and the amount of retained austenite having a size of 1.5 μm or more is not sufficient. As a result, a sufficient TS × EL value is obtained. And sufficient deep drawing property was not obtained.

Sample No. In No. 14, since the cooling rate from the quenching start temperature to the retention start temperature (“[5] holding temperature” in Table 2) is slow, the amount of ferrite becomes excessive, and the total amount of tempered martensite and tempered bainite becomes insufficient. And the MA average size became excessive. As a result, sufficient tensile strength, yield ratio and hole expansion ratio could not be obtained.
Sample No. In No. 15, since the reheating temperature is higher than the temperature range of 300 ° C. to 500 ° C., the amount of retained austenite is small, the average MA size is excessive, and the amount of retained austenite having a size of 1.5 μm or more is not sufficient. No good tensile strength, TS × EL and deep drawing property were obtained.
Sample No. In No. 16, since the reheating temperature is lower than the temperature range of 300 ° C. to 500 ° C., the amount of ferrite is excessive, the amount of retained austenite is small, the average MA size is excessive, and the amount of retained austenite having a size of 1.5 μm or more is sufficient. There wasn't. As a result, sufficient yield ratio, TS × EL and deep drawing property were not obtained.

Sample No. In No. 22, the amount of C was small, the amount of retained austenite was insufficient, and the amount of retained austenite having a size of 1.5 μm or more was not sufficient. As a result, sufficient TS × EL and deep drawing property could not be obtained.
Sample No. In No. 23, the amount of Mn was large, the amount of retained austenite was insufficient, and the amount of retained austenite having a size of 1.5 μm or more was not sufficient. As a result, sufficient TS × EL and deep drawing property could not be obtained.

Sample No. In No. 24, the amount of Mn was small, the amount of ferrite was excessive, and the total amount of tempered martensite and tempered bainite was insufficient, and as a result, a sufficient yield ratio and TS × EL could not be obtained.
Sample No. In No. 25, the amount of Si + Al was small, the total amount of tempered martensite and tempered bainite was insufficient, and the amount of retained austenite was small. As a result, sufficient TS × EL, hole expansion ratio and deep drawing property could not be obtained.

Sample No. In No. 26, the amount of C was excessive, and after austenitization, the mixture was retained at a temperature lower than the temperature range of 300 to 500 ° C., so that sufficient SW cross tensile strength could not be obtained.
Sample No. In No. 27, the amount of Si + Al was excessive, and sufficient TS × EL could not be obtained.

Claims (6)

C: 0.15% by mass to 0.35% by mass,
Total of Si and Al: 1.06 % by mass to 2.13 % by mass,
Mn: 1.60 % 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, with a ferrite fraction of 0%.
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,
The average size of retained austenite is 1.0 μm or less,
A high-strength steel plate in which retained austenite having a size of 1.5 μm or more is 2% or more of the total 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. further,
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.
By heating the rolled material to _{a temperature of 3} points or more to make it austenite,
After the austenitization, the mixture is cooled between 650 ° C. and 500 ° C. at an average cooling rate of 15 ° C./sec or more and less than 200 ° C./sec, and at a cooling rate of 10 ° C./sec or less within the range of 300 to 500 ° C. for 10 seconds. As mentioned above, staying for less than 300 seconds and
After the retention, cooling is performed at an average cooling rate of 10 ° C./sec or higher from a temperature of 300 ° C. or higher 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 ° C. to 500 ° C.
A method for manufacturing a high-strength steel sheet, including. The production method according to claim 5, wherein the retention is maintained at a constant temperature in the range of 300 to 500 ° C.

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