JP2021147646A - High-strength steel sheet and method for producing the same - Google Patents

Abstract

To provide a high-strength steel sheet that has both tensile strength of 1180 MPa or more and uniform elongation of 6% or more, and impact fracture resistance.SOLUTION: Provided is a high-strength steel sheet which contains, in mass%, C: 0.10 to 0.20%, Si: 0.7 to 1.4%, Mn: 1.8 to 4.0%, P: 0.10% or less, S: 0.03% or less, Al: 0.001 to 2.0%, N: 0.01% or less, O: 0.01% or less, and B: 0.0005 to 0.010%, has the MSC defined by Equation (1) below of 3.0 to 4.2 mass%, and the balance of Fe and unavoidable impurities, and has a microstructure in which upper bainite is 70% or more in area fraction, and fresh martensite and retained austenite is 7 to 30% in total area fraction, and the area fraction of the retained austenite is 2% or more. MSC(mass%)=Mn+0.2×Si+1.7×Cr+2.5×Mo...(1)SELECTED DRAWING: None

Description

The present invention relates to a high-strength steel plate, and is particularly suitable as a material for trucks, passenger car frames, suspension parts, etc., because it has a tensile strength of 1180 MPa or more, a uniform elongation of 6% or more, and excellent impact fracture resistance. Regarding a high-strength steel plate. The present invention also relates to a method for producing the high-strength steel sheet. Here, excellent impact fracture resistance means that the separation index SI described later is 0.1 mm ^-1 or more.

Against the background of automobile emission regulations aimed at controlling global warming, weight reduction of automobiles is required. Since it is effective to increase the strength of materials used as materials for automobile parts in order to reduce the weight of automobiles, the application of high-strength hot-rolled steel sheets is increasing year by year. In particular, a high-strength hot-rolled steel sheet having a tensile strength of 1180 MPa or more is expected as a material that can dramatically improve the fuel efficiency of automobiles through weight reduction.

On the other hand, if the tensile strength of the steel sheet is increased, the ductility is lowered, so that the press formability of the steel sheet is deteriorated. Since automobile parts, especially suspension parts such as suspension parts, need to have a complicated shape in order to secure rigidity, the material of the automobile parts needs to have high press formability, that is, ductility.

Further, when the tensile strength of the steel sheet is increased, the toughness is lowered, so that the impact fracture resistance is deteriorated. The impact fracture resistance is a performance capable of stopping the propagation of brittle cracks when an impact load is applied to a suspension component or the like to generate brittle cracks. If this impact fracture resistance is small, the parts may be brittlely broken and separated in the event of a collision or the like, and the strength of the parts expected in the design may not be obtained. Therefore, impact fracture resistance is required for materials used for automobile parts and the like. It should be noted that this impact fracture resistance is a characteristic that is difficult to evaluate in a normal Charpy test in which a notch is introduced by machining.

So far, various techniques for improving toughness and press formability while increasing the tensile strength of the steel sheet have been proposed.

For example, in Patent Document 1, a microstructure having a predetermined component composition contains (1) tempered martensite or (2) tempered martensite and lower bainite as the main phase at a volume fraction of 90% or more. A hot-rolled steel sheet containing and having an average aspect ratio of 2 or less of the main phase has been proposed.

Further, in Patent Document 2, the {211} <011> orientation which has a two-phase structure consisting of ferrite having an area division of 40 to 90% and the remaining bainite and is parallel to the rolling surface of the steel sheet and parallel to the rolling direction. A high-strength hot-rolled steel sheet has been proposed in which the X-ray random intensity ratio of Bainite is 2.5 or less, the average particle size of ferrite grains is 5.0 μm or less, and the standard deviation of ferrite particle size is 2.0 μm or less. ..

In Patent Document 3, the micro has a predetermined component composition, the area fraction of pearlite is 5% or less, the total area fraction of martensite and retained austenite is 0.5% or less, and the balance is ferrite and / or bainite. It has a structure, the average grain size of ferrite and bainite is 10 μm or less, the average particle size of unmatched precipitated alloy martensite containing Ti and Nb is 20 nm or less, and the yield ratio is 0.85 or more. High yield specific hot-rolled steel sheets with a tensile strength of 600 MPa or more have been proposed.

International Publication No. 2014/185405 JP-A-2015-199987 International Publication No. 2013/022043

However, the prior art as described in Patent Documents 1 to 3 has the following problems.

According to the technique proposed in Patent Document 1, a hot-rolled steel sheet having high strength of 1180 MPa class and low temperature toughness is obtained. However, in Patent Document 1, no consideration is given to uniform elongation and impact fracture resistance. In the examples of Patent Document 1, "total elongation (El)" is measured. The "total elongation" represents the elongation at the time when the test piece is broken in the tensile test. However, in reality, necking occurs before the rupture occurs. When necking occurs, the plate thickness becomes locally thin, resulting in a product defect. Therefore, it cannot be said that high total elongation is sufficient to realize excellent press formability.

Although uniform elongation (uniform elongation) is taken into consideration in Patent Document 2, the tensile strength is at most 847 MPa, and the technique of Patent Document 2 achieves both excellent press moldability and a tensile strength of 1180 MPa or more. I can't let you. In addition, Patent Document 2 does not consider impact fracture resistance.

In the technique proposed in Patent Document 3, the impact energy absorption characteristic is improved in order to suppress the occurrence of brittle cracks, but the brittle cracks are stopped after the brittle cracks are once generated from the embrittled portion. Performance has not been investigated. In addition, the tensile strength of the steel sheet obtained in Patent Document 3 is 700 MPa at the highest, and a high strength of 1180 MPa class has not been obtained. Further, Patent Document 3 only evaluates the total elongation, and does not consider the uniform elongation.

As described above, the technique for obtaining a high-strength steel plate having high levels of strength, press formability, and impact fracture resistance has been established.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a high-strength steel plate having a tensile strength of 1180 MPa or more, a uniform elongation of 6% or more, and impact fracture resistance.

In order to solve the above problems, the present inventors have created a virtual stress-strain curve of a steel plate having a tensile strength of 1180 MPa or more and various yield stresses and uniform elongation, and use the stress-strain curve. A press molding simulation of suspension parts was performed. Then, based on the result of the simulation, the characteristics of the steel sheet required to obtain excellent press formability were examined.

As a result, it was found that in a steel sheet having a tensile strength of 1180 MPa or more, if a uniform elongation of 6% or more is secured, wall thinning during press forming can be minimized and press forming defects can be suppressed.

Generally, in order to obtain a high strength of 1180 MPa or more, at least one of lower bainite and tempered martensite, which have a high hardness structure, is used as the main phase of the microstructure of the steel sheet. However, these tissues are inferior in uniform elongation. Therefore, the present inventors have investigated the optimum steel sheet structure in order to increase the uniform elongation of the steel sheet.

As a result, the main phase is upper bainite, and the second phase has a microstructure containing both fresh martensite and retained austenite in appropriate amounts, so that high strength of 1180 MPa or more and uniform elongation of 6% or more can be achieved. It was clarified that they are compatible.

Furthermore, it was clarified that it is necessary to add Si, Mn, and Cr in a well-balanced manner in order to obtain a microstructure containing both fresh martensite and retained austenite in appropriate amounts.

The upper bainite referred to here is an aggregate of lath-like ferrites having an orientation difference of less than 15 °, and has a structure having Fe-based carbides and / or retained austenite between the lath-like ferrites (however, between the lath-like ferrites). It means (including the case where it does not have Fe-based carbide and / or retained austenite). Unlike lamellar (layered) ferrite and polygonal ferrite in pearlite, lath-like ferrite has a lath-like shape and has a relatively high dislocation density inside, so both are SEM (scanning electron microscope) and TEM (scanning electron microscope) and TEM (scanning electron microscope). It can be distinguished by using a transmission electron microscope). When there is retained austenite between laths, only the lath-shaped ferrite portion is regarded as upper bainite to distinguish it from retained austenite. Further, the fresh martensite is martensite having no Fe-based carbide. Fresh martensite and retained austenite have similar contrasts in SEM, but can be distinguished using the Electron Backscatter Diffraction Patterns (EBSD) method.

Next, the impact fracture resistance of a steel sheet having a tensile strength of 1180 MPa or more and a uniform elongation of 6% or more was examined. Specifically, a sample in which an arc welded portion is arranged is prepared using a steel sheet having a tensile strength of 1180 MPa or more and a uniform elongation of 6% or more, which are manufactured by different manufacturing methods, and the strain rate of 1 / s at −40 ° C. is prepared for the sample. The impact fracture characteristics were investigated by applying the impact load of.

As a result, a sample in which brittle cracks were generated from the embrittled part of arc welding and separated through the entire steel sheet, and a sample in which the brittle cracks stopped after a little growth and did not separate. Existed. A detailed examination of the fracture morphology of the samples that did not separate revealed that the transition of brittle cracks to ductile cracks stopped the brittle cracks and suppressed the propagation of the brittle cracks. ..

Subsequently, a V-notch Charpy test was performed at −40 ° C. on the base material of the separated sample and the non-separated sample in accordance with JIS Z 2242. As a result, in the sample in which the separation occurred, almost no separation was observed on the fracture surface of the Charpy test piece. On the other hand, many separations were observed in the samples in which the brittle cracks stopped, that is, the impact fracture resistance was exhibited. It is considered that this is because the plastic restraint at the tip of the brittle crack was relaxed by the separation and the driving force for the propagation of the brittle crack decreased. Therefore, when the separation was quantified using the separation index SI, it was found that the material showing impact fracture resistance had an SI of 0.1 or more.

As a result of further studies based on the above results, the present inventors have produced a high-strength steel sheet having a tensile strength of 1180 MPa or more, a uniform elongation of 6% or more, and an SI of 0.1 or more. I found the conditions that can be done.

The present invention has been further studied based on the above findings, and the gist of the present invention is as follows.

1. 1. By mass%
C: 0.10 to 0.20%,
Si: 0.7-1.4%,
Mn: 1.8-4.0%,
P: 0.10% or less,
S: 0.03% or less,
Al: 0.001 to 2.0%,
N: 0.01% or less,
O: 0.01% or less, and B: 0.0005 to 0.010%
Containing, the balance consists of Fe and unavoidable impurities,
The MSC defined by the following formula (1) has a component composition of 3.0 to 4.2% by mass, and has a component composition.
As the prime minister, the upper bainite with a surface integral of 70% or more,
Contains 7-30% fresh martensite and retained austenite in total surface integral, and
It has a microstructure in which the surface integral of the retained austenite is 2% or more, and has a microstructure.
A high-strength steel sheet having mechanical properties such as a uniform elongation of 6% or more, a tensile strength of 1180 MPa or more, and a separation index SI defined by the following equation (2) of 0.1 mm ^{-1 or more.}
MSC (mass%) = Mn + 0.2 × Si + 1.7 × Cr + 2.5 × Mo… (1)
Here, each element symbol in the above formula (1) represents the content (mass%) of each element, and is set to 0 in the case of an element that is not contained.
SI (mm ^-1 ) = ΣL / A ... (2)
here,
ΣL: Sum of separation lengths of 0.1 mm or more in the Charpy test at -40 ° C (mm)
A: Area of fracture surface (mm ² ).

2. The component composition is further increased by mass%.
Cr: 1.0% or less, and Mo: 1.0% or less,
The high-strength steel sheet according to 1 above, which contains one or both of them.

3. 3. The component composition is further increased by mass%.
Cu: 2.0% or less,
Ni: 2.0% or less,
Ti: 0.3% or less,
The high-strength steel sheet according to 1 or 2 above, which contains at least one selected from the group consisting of Nb: 0.3% or less and V: 0.3% or less.

4. The component composition is further increased by mass%.
Sb: 0.005 to 0.020%
The high-strength steel sheet according to any one of 1 to 3 above, which contains the above.

5. The component composition is further increased by mass%.
Ca: 0.01% or less,
The high-strength steel sheet according to any one of 1 to 4 above, which contains at least one selected from the group consisting of Mg: 0.01% or less and REM: 0.01% or less.

6. The method for manufacturing a high-strength steel sheet according to any one of 1 to 5 above.
A steel material having the above composition is heated to a heating temperature of 1150 ° C. or higher.
The heated steel material is hot-rolled under the condition of a reduction rate of 5% or more at a temperature lower than the IC to obtain a hot-rolled steel sheet.
The hot-rolled steel sheet is cooled under the conditions of the time from the end of hot rolling to the start of cooling: 2.0 s or less, the average cooling rate: 5 ° C./s or more, the cooling stop temperature: Trs or more, and (Trs + 250 ° C.) or less. ,
The cooled hot-rolled steel sheet is wound under the conditions of winding temperature: Trs or more and (Trs + 250 ° C.) or less.
Cool to 100 ° C or lower with an average cooling rate of 20 ° C / s or less.
The IC is defined by the following formula (3), and the Trs is defined by the following formula (4), a method for manufacturing a high-strength steel sheet.
IC (℃) = 1000 --250 x C --80 x Mn --15 x Cr ... (3)
Trs (℃) = 500 --450 x C --35 x Mn --15 x Cr --10 x Ni --20 x Mo ... (4)
Here, each element symbol in the above equations (3) and (4) represents the content (mass%) of each element, and 0 is set in the case of an element not contained.

7. The method for manufacturing a high-strength steel sheet according to any one of 1 to 5 above.
A steel material having the above composition is heated to a heating temperature of 1150 ° C. or higher.
The heated steel material is hot-rolled under the conditions of finish rolling end temperature: IC or higher to obtain a hot-rolled steel sheet.
The hot-rolled steel sheet is cooled under the conditions of the time from the end of hot rolling to the start of cooling: 2.0 s or less, the average cooling rate: 5 ° C./s or more, the cooling stop temperature: Trs or more, and (Trs + 250 ° C.) or less. ,
The cooled hot-rolled steel sheet is wound under the conditions of winding temperature: Trs or more and (Trs + 250 ° C.) or less.
Cool to 100 ° C or lower with an average cooling rate of 20 ° C / s or less.
The cooled hot-rolled steel sheet is rolled in a temperature range lower than the temperature IC under a condition of a reduction rate of 5% or more.
The IC is defined by the following formula (3), and the Trs is defined by the following formula (4), a method for manufacturing a high-strength steel sheet.
IC (℃) = 1000 --250 x C --80 x Mn --15 x Cr ... (3)
Trs (℃) = 500 --450 x C --35 x Mn --15 x Cr --10 x Ni --20 x Mo ... (4)
Here, each element symbol in the above equations (3) and (4) represents the content (mass%) of each element, and 0 is set in the case of an element not contained.

According to the present invention, it is possible to obtain a high-strength steel plate having excellent impact fracture resistance in addition to a tensile strength of 1180 MPa or more and a uniform elongation of 6% or more. Although the high-strength steel plate of the present invention has high tensile strength, it is excellent in press formability and can be press-formed without causing molding defects such as necking and cracking. Further, when the high-strength steel plate of the present invention is applied to a member of a truck or a passenger car, the weight of the automobile body can be reduced while ensuring safety, which can contribute to the reduction of environmental load.

Hereinafter, the present invention will be specifically described. The following description shows an example of a preferred embodiment of the present invention, and the present invention is not limited thereto.

[Ingredient composition]
First, the reason for limiting the component composition of the high-strength steel sheet of the present invention will be described. In the present specification, "%" as a unit of content means "mass%" unless otherwise specified.

C: 0.10 to 0.20%
C is an element having an action of improving the strength of steel. C promotes the formation of bainite by improving hardenability and contributes to high strength. C also contributes to increasing the strength by increasing the strength of martensite. In order to obtain a tensile strength of 1180 MPa or more, the C content needs to be 0.10% or more. Therefore, the C content is 0.10% or more, preferably 0.12% or more, and more preferably 0.13% or more. On the other hand, when the C content exceeds 0.20%, the intensity of martensite increases excessively, and the intensity difference between upper bainite as the main phase and fresh martensite and retained austenite becomes large, and as a result, it is uniform. Growth decreases. Therefore, the C content is 0.20% or less, preferably 0.18% or less, and more preferably 0.17% or less.

Si: 0.7-1.4%
Si has an action of suppressing the formation of Fe-based carbides and suppresses the precipitation of cementite during the transformation of upper bainite. As a result, C is distributed to the untransformed austenite, and upon cooling after winding, the untransformed austenite becomes fresh martensite and / or retained austenite, and the desired fresh martensite and retained austenite can be obtained. In order to obtain these effects, the Si content needs to be 0.7% or more. Therefore, the Si content is set to 0.7% or more, preferably 0.8% or more. On the other hand, Si is an element that forms a subscale on the surface of the steel sheet during hot rolling. If the Si content exceeds 1.4%, the subscale becomes too thick, the surface roughness of the surface of the steel sheet after descaling becomes excessive, and the pretreatment property of the hot-rolled steel sheet deteriorates. Therefore, the Si content is 1.4% or less, preferably 1.3% or less, and more preferably 1.2% or less.

Mn: 1.8-4.0%
Mn stabilizes austenite and contributes to the production of fresh martensite and / or retained austenite. In order to obtain such an effect, the Mn content needs to be 1.8% or more. Therefore, the Mn content is set to 1.8% or more, preferably 2.3% or more. On the other hand, when the Mn content exceeds 4.0%, fresh martensite and retained austenite are excessively produced, and the uniform elongation is lowered. Therefore, the Mn content is 4.0% or less. It is preferably 3.6% or less, more preferably 3.2% or less.

P: 0.10% or less P is an element that dissolves in a solid solution and contributes to an increase in the strength of steel. However, P is also an element that causes slab cracking during hot rolling by segregating at the austenite grain boundaries during hot rolling. In addition, segregation occurs at the grain boundaries to reduce uniform elongation. Therefore, it is preferable to reduce the P content as much as possible, but the content of P up to 0.10% is acceptable. Therefore, the P content is set to 0.10% or less.

S: 0.03% or less S combines with Ti and Mn to form coarse sulfide, which accelerates the generation of voids and reduces uniform elongation. Therefore, it is preferable that the S content is as low as possible, but the content of S up to 0.03% is acceptable. Therefore, the S content is set to 0.03% or less.

Al: 0.001 to 2.0%
Al is an element that acts as a deoxidizer and is effective in improving the cleanliness of steel. If the Al content is less than 0.001%, the effect is not sufficient, so the Al content is set to 0.001% or more. Further, Al has an effect of suppressing the formation of Fe-based carbides like Si, and suppresses the precipitation of cementite during the transformation of upper bainite. This contributes to the formation of fresh martensite and / or retained austenite in cooling after winding. On the other hand, excessive addition of Al causes an increase in oxide-based inclusions and reduces uniform elongation. Therefore, the Al content is set to 2.0% or less.

N: 0.01% or less N is precipitated as a nitride by combining with a nitride-forming element, and generally contributes to grain refinement. However, since N combines with Ti at a high temperature to form a coarse nitride, a content of more than 0.01% causes a decrease in uniform elongation. Therefore, the N content is set to 0.01% or less.

B: 0.0005 to 0.010%
B is an element that segregates into the old austenite grain boundaries and suppresses the formation of ferrite, thereby promoting the formation of upper bainite and contributing to the improvement of the strength of the steel sheet. In order to exhibit these effects, the B content needs to be 0.0005% or more. Therefore, the B content is set to 0.0005% or more. On the other hand, when the B content exceeds 0.010%, the above-mentioned effect is saturated. Therefore, the B content is set to 0.010% or less.

The high-strength steel sheet according to the embodiment of the present invention can contain the above elements and have a component composition consisting of the balance Fe and unavoidable impurities.

Examples of the unavoidable impurities include Zr, Co, Sn, Zn, and W. When the component composition contains at least one selected from the group consisting of Zr, Co, Sn, Zn, and W as an unavoidable impurity, the total content of these elements may be 0.5% or less. preferable.

In addition, the component composition of the high-strength steel sheet in another embodiment of the present invention can optionally further contain at least one of the following elements.

Cr: 1.0% or less Cr is a carbide-forming element and segregates at the interface between the upper bainite and untransformed austenite during the upper bainite transformation after winding the hot-rolled steel plate, reducing the driving force of the bainite transformation. , Has the effect of retaining the upper bainite transformation. The untransformed austenite remaining after the transformation to the upper bainite is retained becomes fresh martensite and / or retained austenite by cooling after winding. Therefore, when Cr is added, Cr also contributes to the formation of fresh martensite and retained austenite in the desired surface integral. However, since Cr is an element that deteriorates corrosion resistance and coating pretreatment property, when Cr is added, the Cr content is set to 1.0% or less.

Mo: 1.0% or less Mo promotes the formation of bainite through the improvement of hardenability and contributes to the improvement of the strength of the steel sheet. Further, Mo is a carbide-forming element like Cr, and segregates at the interface between the upper bainite and the untransformed austenite during the transformation of the upper bainite after winding the hot-rolled steel sheet, thereby reducing the transformation driving force of the bainite. Contributes to the formation of fresh martensite and bainite after winding and cooling. However, when the Mo content exceeds 1.0%, fresh martensite and retained austenite are excessively produced, which worsens uniform elongation. Therefore, when Mo is added, the Mo content is set to 1.0% or less.

In addition, the component composition of the high-strength steel sheet in another embodiment of the present invention can optionally further contain at least one of the following elements.

Cu: 2.0% or less Cu is an element that dissolves in solid solution and contributes to increasing the strength of steel. Further, Cu promotes the formation of bainite through the improvement of hardenability and contributes to the improvement of strength. However, if the Cu content exceeds 2.0%, the surface texture of the hot-rolled steel sheet is deteriorated, and the fatigue characteristics of the hot-rolled steel sheet are deteriorated. Therefore, when Cu is added, the Cu content is set to 2.0% or less.

Ni: 2.0% or less Ni is an element that dissolves in solid solution and contributes to increasing the strength of steel. In addition, Ni promotes the formation of bainite through the improvement of hardenability and contributes to the improvement of strength. However, when the Ni content exceeds 2.0%, fresh martensite and retained austenite are excessively increased, which deteriorates the ductility of the hot-rolled steel sheet. Therefore, when Ni is added, the Ni content is set to 2.0% or less.

Ti: 0.3% or less Ti is an element that has the effect of improving the strength of the steel sheet by precipitation strengthening or solid solution strengthening. Ti forms a nitride in the high temperature range of austenite. As a result, the precipitation of BN is suppressed and B becomes a solid solution state. Therefore, when Ti is added, Ti also contributes to ensuring the hardenability required for the formation of upper bainite, and the strength is improved. However, when the Ti content exceeds 0.3%, a large amount of Ti nitride is produced, which lowers the uniform elongation. Therefore, when Ti is added, the Ti content is set to 0.3% or less.

Nb: 0.3% or less Nb is an element having an action of improving the strength of the steel sheet by precipitation strengthening or solid solution strengthening. Further, like Ti, Nb enables rolling in the unrecrystallized region of austenite by raising the recrystallization temperature of austenite during hot rolling, and makes the particle size of upper bainite finer and fresh martensite and fresh martensite. Contributes to an increase in the volume ratio of retained austenite. Further, Nb is a carbide-forming element like Cr, and segregates at the interface between the upper bainite and the untransformed austenite during the transformation of the upper bainite after winding the hot-rolled steel plate, thereby reducing the transformation driving force of the bainite. It is an element that has the effect of stopping the upper bainite transformation while leaving untransformed austenite. Untransformed austenite is then cooled to fresh martensite and / or retained austenite. Therefore, when Nb is added, Nb also contributes to the formation of fresh martensite and retained austenite in the desired surface integral. However, when the Nb content exceeds 0.3%, fresh martensite and retained austenite increase excessively, and uniform elongation decreases. Therefore, when Nb is added, the Nb content is set to 0.3% or less.

V: 0.3% or less V is an element having an action of improving the strength of the steel sheet by precipitation strengthening and solid solution strengthening. Further, like Ti, V enables rolling in the austenite unrecrystallized region by raising the recrystallization temperature of austenite during hot rolling, and contributes to the miniaturization of the particle size of the upper bainite. Further, V is a carbide-forming element like Cr, and segregates at the interface between the upper bainite and the untransformed austenite during the transformation of the upper bainite after winding the hot-rolled steel plate, thereby reducing the transformation driving force of the bainite. It is an element that has the effect of stopping the upper bainite transformation while leaving untransformed austenite. Untransformed austenite is then cooled to fresh martensite and / or retained austenite. Therefore, when V is added, V also contributes to the formation of fresh martensite and retained austenite in the desired surface integral. However, when the V content exceeds 0.3%, the second phase increases excessively and the uniform elongation decreases. Therefore, when V is added, the V content is set to 0.3% or less.

In addition, the component composition of the high-strength steel sheet in another embodiment of the present invention may further optionally contain the following elements.

Sb: 0.005 to 0.020%
Sb is an element having an effect of suppressing nitriding of the surface of the steel material when the steel material (slab) is heated. By adding Sb, precipitation of BN in the surface layer portion of the steel material can be suppressed. As a result, the remaining solid solution B contributes to ensuring the hardenability required for the formation of bainite and thereby improving the strength of the steel sheet. When Sb is added, the Sb content is set to 0.005% or more in order to obtain the above effect. On the other hand, if the Sb content exceeds 0.020%, the toughness of the steel may decrease, causing slab cracking and hot rolling cracking. Therefore, when Sb is added, the Sb content is set to 0.020% or less.

In addition, the component composition of the high-strength steel sheet in another embodiment of the present invention can optionally further contain at least one of the following elements. The elements listed below contribute to further improvement of properties such as press moldability.

Ca: 0.01% or less Ca controls the shape of oxide and sulfide-based inclusions, and contributes to the suppression of cracking of the sheared end face of the steel sheet and the further improvement of bending workability. However, if the Ca content exceeds 0.01%, Ca-based inclusions increase and the cleanliness of the steel deteriorates, which may cause shear end face cracks and bending cracks. Therefore, when Ca is added, the Ca content is set to 0.01% or less.

Mg: 0.01% or less Mg controls the shape of oxide and sulfide-based inclusions, and contributes to the suppression of cracking of the sheared end face of the steel sheet and the further improvement of bending workability. However, if the Mg content exceeds 0.01%, the cleanliness of the steel deteriorates, which may cause shear end face cracks and bending cracks. Therefore, when Mg is added, the Mg content is set to 0.01% or less.

REM: 0.01% or less REM (rare earth metal), like Ca, controls the shape of oxide and sulfide-based inclusions, and contributes to the suppression of cracking of the sheared end face of steel sheets and further improvement of bending workability. do. However, if the REM content exceeds 0.01%, the cleanliness of the steel deteriorates, which may cause shear end face cracks and bending cracks. Therefore, when REM is added, the REM content is set to 0.01% or less.

MSC: 3.0-4.2% by mass
In order to obtain high uniform elongation while maintaining high strength of 1180 MPa or more, it is necessary to control the surface integral of fresh martensite and retained austenite within an appropriate range, as will be described later. In order to control the surface integral of fresh martensite and retained austenite, the addition balance of Mn, Si, Cr (when added), and Mo (when added) is important. Specifically, the following (1) The MSC value defined by the formula should be 3.0 to 4.2% by mass. In a high-strength steel plate having a tensile strength of 1180 MPa or more, if the MSC value deviates from the above range, a uniform elongation of 6% or more cannot be obtained. The MSC is preferably 3.1% by mass or more. The MSC is preferably 3.7% by mass or less, and more preferably 3.5% by mass or less.
MSC (mass%) = Mn + 0.2 × Si + 1.7 × Cr + 2.5 × Mo… (1)
Here, each element symbol in the above formula (1) represents the content (mass%) of each element, and is set to 0 in the case of an element that is not contained.

[Micro tissue]
Next, the reason for limiting the microstructure of the high-strength steel sheet of the present invention will be described.

The high-strength steel plate of the present invention contains (1) upper bainite as a main phase having a surface integral of 70% or more, and (2 total area fractions of 7 to 30% fresh martensite and retained austenite). It has a structure, and the surface integral of the retained austenite is 2% or more. In the present specification, "%" representing the proportion of microstructure means the surface integral unless otherwise specified. do.

Upper bainite: 70% or more The microstructure of the high-strength steel plate of the present invention contains upper bainite as the main phase. If the surface integral of the upper bainite is less than 70%, it is not possible to realize a tensile strength of 1180 MPa or more and a uniform elongation of 6% or more. Therefore, the surface integral of the upper bainite is set to 70% or more, preferably 80% or more.

Fresh martensite and retained austenite: 7-30%
The microstructure of the high-strength steel plate of the present invention contains fresh martensite and retained austenite. If the total surface integral of fresh martensite and retained austenite is less than 7%, it is not possible to achieve a tensile strength of 1180 MPa or more and a uniform elongation of 6% or more. Therefore, the total surface integral of fresh martensite and retained austenite shall be 7% or more. On the other hand, when the total surface integral exceeds 30%, the combined growth of voids formed at the interface between fresh martensite and retained austenite and the main phase is promoted, and the uniform elongation is lowered. Therefore, the total surface integral is 30% or less, preferably 20% or less, and more preferably 16% or less.

Residual austenite: 2% or more Fresh martensite has the effect of improving uniform elongation by promoting work hardening and delaying the onset of plastic instability. However, in order to obtain a uniform elongation of 6% or more in a high-strength steel plate having a tensile strength of 1180 MPa or more, fresh martensite alone is not sufficient, and it is necessary to contain 2% or more of retained austenite. Therefore, the surface integral of retained austenite is set to 2% or more.

That is, a tensile strength of 1180 MPa or more and a uniform elongation of 6% or more can be achieved only by combining the work hardening improving ability of fresh martensite and the strain dispersion ability due to the work-induced transformation (TRIP) effect of retained austenite. can.

The microstructure can further contain any tissue other than upper bainite, fresh martensite, and retained austenite (hereinafter referred to as "other tissue"). However, from the viewpoint of enhancing the effect of microstructure control, it is preferable that the total surface integral of the other tissues is 3% or less. In other words, the total surface integral of upper bainite, fresh martensite, and retained austenite in the microstructure is preferably 97% or more. Other tissues include, for example, cementite, pearlite, tempered martensite, and lower bainite.

[Mechanical characteristics]
Uniform elongation: 6% or more Tensile strength: 1180 MPa or more As described above, the high-strength steel plate of the present invention has a tensile strength of 1180 MPa or more and a uniform elongation of 6% or more. Therefore, the high-strength steel plate of the present invention is excellent in press formability despite its high tensile strength, and can be press-formed without causing molding defects such as necking and cracking.

SI: 0.1 mm ^-1 or more Further, the high-strength steel plate of the present invention has a separation index SI defined by the following equation (2) of 0.1 mm ^-1 or more. Therefore, the high-strength steel plate of the present invention has excellent impact fracture resistance and can be extremely suitably used as a material for automobile parts and the like.
SI (mm ^-1 ) = ΣL / A ... (2)
here,
ΣL: Sum of separation lengths of 0.1 mm or more in the Charpy test at -40 ° C (mm)
A: Area of fracture surface (mm ² ).

[Production method]
Next, the method for producing the high-strength steel sheet of the present invention will be described. Unless otherwise specified, the temperature in the following description represents the surface temperature of the object (steel material or steel plate).

(First Embodiment)
In the first embodiment of the present invention, a high-strength steel sheet satisfying the above-mentioned conditions can be produced by sequentially applying the following treatments (1) to (5) to the steel material. Hereinafter, each step will be described.
(1) Heating (2) Hot rolling (3) Cooling (first cooling)
(4) Winding (5) Cooling (second cooling)

(Steel material)
As the steel material, any material having the above-mentioned component composition can be used. The composition of the finally obtained thick steel sheet is the same as the composition of the steel material used. As the steel material, for example, a steel slab can be used.

The method for producing the steel material is not particularly limited. For example, molten steel having the above composition can be melted by a known method such as a converter, and a steel material can be obtained by a casting method such as continuous casting. A method other than the continuous casting method, such as a ingot-lump rolling method, can also be used. Moreover, scrap may be used as a raw material. The steel material may be directly subjected to the next heating step after being manufactured by a method such as a continuous casting method, or the steel material which has been cooled to become hot or cold pieces is subjected to the heating step. May be good.

(heating)
Heating temperature: 1150 ° C. or higher First, the steel material is heated to a heating temperature of 1150 ° C. or higher. Usually, most of the carbonitride-forming elements such as Ti are present as coarse carbonitrides in the steel material. The presence of this coarse and non-uniform deposit has various characteristics (for example, shear end face cracking resistance, bending workability, burring workability, etc.) that are generally required for high-strength steel sheets for truck and passenger car parts. It causes deterioration. Therefore, it is necessary to heat the steel material prior to hot rolling to dissolve the coarse precipitates as a solid solution. Specifically, in order to sufficiently dissolve the coarse precipitate, it is necessary to set the heating temperature of the steel material to 1150 ° C. or higher. On the other hand, if the heating temperature of the steel material becomes too high, slab defects will occur and the yield will decrease due to scale-off. Therefore, from the viewpoint of improving the yield, it is preferable that the heating temperature of the steel material is 1350 ° C. or lower. The heating temperature of the steel material is more preferably 1180 ° C. or higher and 1300 ° C. or lower, and further preferably 1200 ° C. or higher and 1280 ° C. or lower.

In the heating, from the viewpoint of making the temperature of the steel material uniform, it is preferable to raise the temperature of the steel material to the heating temperature and then maintain the temperature at the heating temperature. The time for holding at the heating temperature (holding time) is not particularly limited, but from the viewpoint of improving the temperature uniformity of the steel material, it is preferably 1800 seconds or more. On the other hand, when the holding time exceeds 10,000 seconds, the amount of scale generated increases. As a result, scale biting or the like is likely to occur in the subsequent hot rolling, which causes a decrease in yield due to poor surface defects. Therefore, the holding time is preferably 10,000 seconds or less, and preferably 8,000 seconds or less.

(Hot rolling)
Next, the heated steel material is hot-rolled to obtain a hot-rolled steel sheet. The hot rolling may consist of rough rolling and finish rolling. When rough rolling is performed, the conditions are not particularly limited. Further, after rough rolling, it is preferable to perform high-pressure water descaling prior to finish rolling in order to remove surface scale. In the finish rolling, descaling may be performed between the stands.

Reduction rate below temperature IC: 5% or more In the first embodiment of the present invention, the reduction rate below temperature IC is 5% or more in the hot rolling. When a high-strength steel sheet is manufactured under the condition that the reduction rate below the temperature IC is less than 5%, separation hardly occurs in the Charpy test of the obtained high-strength steel sheet, and the SI is less than 0.1. The reduction rate below the temperature IC is preferably 8% or more, and more preferably 10% or more.

In the first embodiment of the present invention, the other conditions of the hot rolling are not particularly limited. However, if the finish rolling end temperature is less than 400 ° C., the uniform elongation may decrease. Therefore, it is preferable that the finish rolling end temperature is 400 ° C. or higher.

The IC is the transformation temperature from austenite to ferrite estimated from the component composition, and is defined by the following equation (2).
IC (℃) = 1000 --250 x C --80 x Mn --15 x Cr ... (2)
Further, RC is the lower limit temperature for austenite recrystallization estimated from the component composition, and is defined by the following formula (3).
RC (℃) = 800 + 100 × C + 100 × N + 10 × Mn + 700 × Ti + 5000 × B + 10 × Cr + 50 × Mo + 2000 × Nb + 150 × V… (3)
Here, each element symbol in the above equations (2) and (3) represents the content (mass%) of each element, and is set to 0 in the case of an element not contained.

(cooling)
Cooling start time: Within 2.0 s after the finish rolling is completed Next, the hot-rolled steel sheet is cooled (first cooling). At that time, the time from the end of hot rolling to the start of cooling (cooling start time) is set to 2.0 s or less. If the cooling start time exceeds 2.0 s, grain growth of austenite grains occurs, and a tensile strength of 1180 MPa or more cannot be secured. The cooling start time is preferably 1.5 s or less.

Average cooling rate: 5 ° C./s or more If the average cooling rate in the cooling is less than 5 ° C./s, ferrite transformation occurs before upper bainite transformation, and the desired area fraction of upper bainite cannot be obtained. Therefore, the average cooling rate is 5 ° C./s or higher, preferably 20 ° C./s or higher, and more preferably 50 ° C./s or higher. On the other hand, the upper limit of the average cooling rate is not particularly limited, but if the average cooling rate becomes too large, it becomes difficult to control the cooling stop temperature. Therefore, the average cooling rate is preferably 200 ° C./s or less. The average cooling rate is defined based on the average cooling rate on the surface of the steel sheet.

In the cooling, forced cooling may be performed so as to have the above average cooling rate. The cooling method is not particularly limited, but it is preferably performed by, for example, water cooling.

Cooling stop temperature: Trs or more, (Trs + 250 ° C.) or less When the cooling stop temperature is less than Trs, the microstructure becomes tempered martensite or lower bainite. Tempered martensite and lower bainite are both high-strength structures, but have significantly lower uniform elongation. Therefore, the cooling stop temperature is set to Trs or higher. On the other hand, if the cooling stop temperature is higher than (Trs + 250 ° C.), ferrite is generated, so that a tensile strength of 1180 MPa cannot be obtained. Therefore, the cooling stop temperature is set to (Trs + 250 ° C.) or less.

The Trs are defined by the following equation (4).
Trs (℃) = 500 --450 x C --35 x Mn --15 x Cr --10 x Ni --20 x Mo ... (4)
Here, each element symbol in the above equation (4) represents the content (mass%) of each element, and is set to 0 in the case of an element that is not contained.

(Volume)
Winding temperature: Trs or more and (Trs + 250 ° C.) or less Next, the cooled hot-rolled steel sheet is wound under the conditions of winding temperature: Trs or more and (Trs + 250 ° C.) or less. If the take-up temperature is less than Trs, martensite transformation or lower bainite transformation proceeds after take-up, and the desired fresh martensite and retained austenite cannot be obtained. Therefore, the winding temperature is set to Trs or higher. On the other hand, if the winding temperature is higher than (Trs + 250 ° C.), ferrite is generated, so that a tensile strength of 1180 MPa cannot be obtained. Therefore, the winding temperature is set to (Trs + 250 ° C.) or less.

(cooling)
Average cooling rate: 20 ° C./s or less After the winding, the product is further cooled to 100 ° C. or less at an average cooling rate of 20 ° C./s or less (second cooling). The average cooling rate affects the production of fresh martensite and retained austenite. When the average cooling rate exceeds 20 ° C./s, the untransformed austenite is almost martensitic transformed, the desired retained austenite cannot be obtained, and the uniform elongation is lowered. Therefore, the average cooling rate is set to 20 ° C./s or less, preferably 2 ° C./s or less, and more preferably 0.02 ° C./s or less. On the other hand, the lower limit of the average cooling rate is not particularly limited, but is preferably 0.0001 ° C./s or more.

The cooling can be performed to any temperature of 100 ° C. or lower, but it is preferable to cool to about 10 to 30 ° C. (for example, room temperature). The cooling can be performed in any form, and may be performed, for example, in the state of a wound coil.

By the above procedure, the high-strength steel plate of the present invention can be produced. After winding and subsequent cooling, temper rolling may be performed according to a conventional method, or pickling may be performed to remove scale formed on the surface.

(Second Embodiment)
In order to set the separation index SI to 0.1 mm ^-1 or more, it is necessary to perform rolling with a rolling reduction ratio of 5% or more below the temperature IC. The rolling may be performed in hot rolling as in the first embodiment, but may be performed after the second cooling. Therefore, in the second embodiment of the present invention, the steel material is sequentially subjected to the following treatments (1) to (7) to produce a high-strength steel plate satisfying the above-mentioned conditions. Of the steps (1) to (7), (1), (3), (4), and (5) can be performed under the same conditions as in the first embodiment. Hereinafter, the steps (2) and (6) will be described. The points not particularly mentioned are the same as those in the first embodiment.
(1) Heating (2) Hot rolling (3) Cooling (first cooling)
(4) Winding (5) Cooling (second cooling)
(6) Rolling

In the second embodiment, the finish rolling end temperature in hot rolling is set to IC or higher, while rolling is performed in a temperature range lower than the temperature IC after the second cooling. Then, the rolling reduction is set to 5% or more. As a result, the separation index can be set ^{to 0.1 mm -1 or more.} The reasons for limiting the rolling conditions will be described below.

(rolling)
In the second cooling, the hot-rolled steel sheet is cooled to a temperature of 100 ° C. or lower, and then the cooled hot-rolled steel sheet is rolled to obtain a high-strength steel sheet. At that time, if the temperature of the hot-rolled steel sheet becomes IC or higher, the microstructure formed in the process up to the second cooling is impaired, so that a desired microstructure cannot be finally obtained. Therefore, the rolling is performed in a temperature range lower than the temperature IC. On the other hand, the lower limit of the temperature at which the rolling is performed is not particularly limited, but from the viewpoint of preventing the steel sheet from breaking during rolling, it is preferable to perform the rolling at a temperature of 0 ° C. or higher. That is, it is preferable to perform rolling in a temperature range of 0 ° C. or higher and lower than IC.

After the second cooling, reheating can be performed prior to the rolling. When reheating is performed, the hot-rolled steel sheet cooled in the second cooling may be heated to a reheating temperature lower than the IC, and then rolled.

In addition, when the rolling reduction is less than 5%, the separation index SI in the Charpy test of the obtained high-strength steel sheet is less than 0.1. Therefore, the rolling reduction is set to 5% or more. The reduction rate is preferably 8% or more, and more preferably 10% or more.

After the rolling is performed, the temperature may be cooled to room temperature, for example, although it is not particularly limited. The cooling is not particularly limited, but can be performed by, for example, at least one selected from the group consisting of air cooling, air cooling, gas cooling, and water cooling.

The molten steel having the composition shown in Table 1 was melted in a converter to produce a steel slab as a steel material by a continuous casting method. The obtained steel slab was heated to the heating temperature shown in Table 2, and then the heated steel slab was subjected to hot rolling consisting of rough rolling and finish rolling to obtain a hot-rolled steel sheet. Table 2 also shows the rolling reduction rate below the temperature IC in the hot rolling and the finish rolling end temperature. In addition, "-(hyphen)" in the column of the reduction rate below the temperature IC indicates that the reduction was not performed below the temperature IC.

Next, the obtained hot-rolled steel sheet was cooled under the conditions of the cooling start time, the average cooling rate, and the cooling stop temperature shown in Table 2 (first cooling). The cooled hot-rolled steel sheet was wound at the winding temperature shown in Table 2, and the wound steel sheet was cooled at the average cooling rate shown in Table 2 (second cooling) to obtain a high-strength steel sheet. .. After the cooling, skin pass rolling and pickling were performed as post-treatment. The pickling was carried out at a temperature of 85 ° C. using an aqueous hydrochloric acid solution having a concentration of 10% by mass.

Further, in the example in which the rolling was not performed below the temperature IC in the hot rolling step, after the second cooling, rolling was performed at the temperature shown in Table 2 to obtain a final high-strength steel sheet. .. The rolling reduction ratios in the rolling are also shown in Table 2.

A test piece was taken from the obtained high-strength steel plate, and the microstructure and mechanical properties were evaluated by the procedure described below.

(Micro tissue)
From the obtained high-strength steel plate, test pieces for microstructure observation were taken so that the sheet thickness cross section parallel to the rolling direction became the observation surface. The surface of the obtained test piece was polished, and the surface was further corroded with a corrosive solution (3% by mass nital solution) to reveal a microstructure.

Next, the surface of the test piece at the plate thickness 1/4 position was photographed in 10 fields at a magnification of 5000 times using a scanning electron microscope (SEM) to obtain an SEM image of a microstructure. The SEM image was analyzed by image processing to quantify the surface integrals of upper bainite (UB), polygonal ferrite (F), and tempered martensite (TM). In addition, since it is difficult to distinguish between fresh martensite (M) and retained austenite (γ) by SEM, they were identified using the Electron Backscatter Diffraction Patterns (EBSD) method, and the area fractions of each were determined. .. The surface integral of each microstructure measured is shown in Table 3. Table 3 also shows the total surface integral (M + γ) of fresh martensite and retained austenite.

(Tensile test)
From the obtained high-strength steel plate, JIS No. 5 test pieces (distance between marked lines (gage length, GL): 50 mm) were collected so that the tensile direction was perpendicular to the rolling direction. Using the test piece, a tensile test is performed in accordance with JIS Z 2241 to determine the yield strength (yield point, YP), tensile strength (TS), total elongation (El), and uniform elongation (u-El). I asked. The tensile test was performed twice for each high-strength steel sheet, and the average of the obtained measured values is shown in Table 3 as the mechanical properties of the high-strength steel sheet. In the present invention, when TS is 1180 MPa or more, it is evaluated as high strength, and when uniform elongation is 6% or more, press moldability is evaluated as good.

(Charpy test)
From the obtained high-strength steel plate, a Charpy test piece was collected so that the longitudinal direction of the test piece was perpendicular to the rolling direction, and a 2 mm V notch was formed by machining. Using the test piece, a Charpy test was carried out at a test temperature of −40 ° C. in accordance with JIS Z 2242. The fracture surface of the test piece after the Charpy test was photographed with a digital microscope, and the obtained image was analyzed by image processing software (WinROOF). In the analysis, the length L of the separation existing on the fracture surface was measured, and the total length ΣL of all the separations having a length of 0.1 mm or more was obtained. The separation index SI was calculated by dividing the ΣL by the area A of the fracture surface. The length of the fracture surface was 8 mm, and the width of the fracture surface was the same as the plate thickness t (mm) of the high-strength steel plate. Therefore, the area A of the fracture surface is 8 t (mm ² ).

Claims (7)

By mass%
C: 0.10 to 0.20%,
Si: 0.7-1.4%,
Mn: 1.8-4.0%,
P: 0.10% or less,
S: 0.03% or less,
Al: 0.001 to 2.0%,
N: 0.01% or less,
O: 0.01% or less, and B: 0.0005 to 0.010%
Containing, the balance consists of Fe and unavoidable impurities,
The MSC defined by the following formula (1) has a component composition of 3.0 to 4.2% by mass, and has a component composition.
As the prime minister, the upper bainite with a surface integral of 70% or more,
Contains 7-30% fresh martensite and retained austenite in total surface integral, and
It has a microstructure in which the surface integral of the retained austenite is 2% or more, and has a microstructure.
A high-strength steel sheet having mechanical properties such as a uniform elongation of 6% or more, a tensile strength of 1180 MPa or more, and a separation index SI defined by the following equation (2) of 0.1 mm ^{-1 or more.}
MSC (mass%) = Mn + 0.2 × Si + 1.7 × Cr + 2.5 × Mo… (1)
Here, each element symbol in the above formula (1) represents the content (mass%) of each element, and is set to 0 in the case of an element that is not contained.
SI (mm ^-1 ) = ΣL / A ... (2)
here,
ΣL: Sum of separation lengths of 0.1 mm or more in the Charpy test at -40 ° C (mm)
A: Area of fracture surface (mm ² ). The component composition is further increased by mass%.
Cr: 1.0% or less, and Mo: 1.0% or less,
The high-strength steel plate according to claim 1, which contains one or both of them. The component composition is further increased by mass%.
Cu: 2.0% or less,
Ni: 2.0% or less,
Ti: 0.3% or less,
The high-strength steel plate according to claim 1 or 2, which contains at least one selected from the group consisting of Nb: 0.3% or less and V: 0.3% or less. The component composition is further increased by mass%.
Sb: 0.005 to 0.020%
The high-strength steel plate according to any one of claims 1 to 3. The component composition is further increased by mass%.
Ca: 0.01% or less,
The high-strength steel sheet according to any one of claims 1 to 4, which contains at least one selected from the group consisting of Mg: 0.01% or less and REM: 0.01% or less. The method for producing a high-strength steel sheet according to any one of claims 1 to 5.
A steel material having the above composition is heated to a heating temperature of 1150 ° C. or higher.
The heated steel material is hot-rolled under the condition of a reduction rate of 5% or more at a temperature lower than the IC to obtain a hot-rolled steel sheet.
The hot-rolled steel sheet is cooled under the conditions of the time from the end of hot rolling to the start of cooling: 2.0 s or less, the average cooling rate: 5 ° C./s or more, the cooling stop temperature: Trs or more, and (Trs + 250 ° C.) or less. ,
The hot-rolled steel sheet after cooling is wound under the conditions of winding temperature: Trs or more and (Trs + 250 ° C.) or less.
Cool to 100 ° C or lower with an average cooling rate of 20 ° C / s or less.
The IC is defined by the following formula (3), and the Trs is defined by the following formula (4), a method for manufacturing a high-strength steel sheet.
IC (℃) = 1000 --250 x C --80 x Mn --15 x Cr ... (3)
Trs (℃) = 500 --450 x C --35 x Mn --15 x Cr --10 x Ni --20 x Mo ... (4)
Here, each element symbol in the above equations (3) and (4) represents the content (mass%) of each element, and is set to 0 in the case of an element not contained. The method for producing a high-strength steel sheet according to any one of claims 1 to 5.
A steel material having the above composition is heated to a heating temperature of 1150 ° C. or higher.
The heated steel material is hot-rolled under the conditions of finish rolling end temperature: IC or higher to obtain a hot-rolled steel sheet.
The hot-rolled steel sheet is cooled under the conditions of the time from the end of hot rolling to the start of cooling: 2.0 s or less, the average cooling rate: 5 ° C./s or more, the cooling stop temperature: Trs or more, and (Trs + 250 ° C.) or less. ,
The cooled hot-rolled steel sheet is wound under the conditions of winding temperature: Trs or more and (Trs + 250 ° C.) or less.
Cool to 100 ° C or lower with an average cooling rate of 20 ° C / s or less.
The cooled hot-rolled steel sheet is rolled in a temperature range lower than the temperature IC under a condition of a reduction rate of 5% or more.
The IC is defined by the following formula (3), and the Trs is defined by the following formula (4), a method for manufacturing a high-strength steel sheet.
IC (℃) = 1000 --250 x C --80 x Mn --15 x Cr ... (3)
Trs (℃) = 500 --450 x C --35 x Mn --15 x Cr --10 x Ni --20 x Mo ... (4)
Here, each element symbol in the above equations (3) and (4) represents the content (mass%) of each element, and 0 is set in the case of an element not contained.