WO2022209839A1

WO2022209839A1 - High-strength steel sheet and method for manufacturing same

Info

Publication number: WO2022209839A1
Application number: PCT/JP2022/011493
Authority: WO
Inventors: ティーフィンドアン; 寛長谷川; 英之木村
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2021-03-31
Filing date: 2022-03-15
Publication date: 2022-10-06
Also published as: US20240158881A1; KR20230148352A; EP4282993A4; CN117043381A; JP7168137B1; JPWO2022209839A1; EP4282993A1; MX2023011353A

Abstract

The purpose of the present invention is to provide a high-strength steel sheet having tensile strength, press formability, and bendability, and to provide a method for manufacturing the same.　The high-strength steel sheet has a predetermined component composition, and the mircostructure thereof has a specific structure in a surface layer region from the steel sheet surface to a position 1/10 of the sheet thickness and in an inner region from the position 1/10 of the sheet thickness to a position 3/10 of the sheet thickness. The average crystal grain size in the surface layer region from the steel sheet surface to a position 1/10 of the sheet thickness is at most 6 μm. The difference (HV2-HV1) between the hardness (HV1) of the surface layer region from the steel sheet surface to the position 1/10 of the sheet thickness and the hardness (HV2) of the inner region from the position 1/10 of the sheet thickness to the position 3/10 of the sheet thickness is 5-15% of [0.3 × tensile strength (Mpa)]. The high-strength steel sheet has a tensile strength of at least 980 Mpa, a uniform elongation of at least 6%, and the ratio R/t of the critical bending radius R and the sheet thickness t thereof is at most 1.5.

Description

High-strength steel plate and its manufacturing method

The present invention relates to a high-strength steel sheet and its manufacturing method. In particular, in addition to tensile strength of 980 MPa or more and uniform elongation of 6% or more, it has excellent bending workability, and is suitable as a material for trucks and passenger car frames, suspension parts, etc. It relates to a high strength steel sheet and a method for producing the same. .

Against the backdrop of automobile exhaust gas regulations aimed at curbing global warming, there is a demand for lighter automobiles. In order to reduce the weight of automobiles, it is effective to reduce the amount of materials used for the same automobile parts by increasing the strength of the materials used as the raw materials for automobile parts and making them thinner. Therefore, the application of high-strength steel sheets is increasing year by year. In particular, a high-strength steel sheet having a tensile strength of 980 MPa or more is expected as a material that can dramatically improve the fuel efficiency of automobiles by reducing weight.

On the other hand, if the tensile strength of the steel sheet is increased, the ductility of the steel sheet is lowered, so the press formability of the steel sheet is deteriorated. Automobile parts, especially underbody parts such as suspension parts, need to have complicated shapes to ensure rigidity. Therefore, high press formability, that is, ductility is required for materials for automobile parts.

Furthermore, if the tensile strength of the steel plate is increased, cracks are more likely to occur during bending. If a crack occurs in the bent portion, the crack may become a starting point for fatigue cracking, and the durability of the part assumed in the design may not be obtained. Therefore, excellent bending workability is required for materials used for automobile parts and the like.

Various techniques have been proposed to improve ductility and bendability while increasing the tensile strength of steel sheets.

Japanese Unexamined Patent Application Publication No. 2012-012701 WO2016/010004 JP 2013-117068 A JP 2017-115191 A WO2020/110855 WO2020/110843

However, the conventional techniques as described in Patent Documents 1 to 6 have the following problems.

With the techniques described in Patent Documents 1 and 2, a tensile strength of 980 MPa or more cannot be obtained. In both cases, hot-rolled steel sheets are considered to have excellent workability, and "elongation" is used as an index of workability. This "elongation" is also called total elongation (El), and represents the elongation at the time when the test piece breaks in the tensile test. In practice, however, necking occurs before breakage occurs. If necking occurs, the plate thickness becomes thin locally, resulting in product defects during press molding. Therefore, high total elongation alone is not sufficient to achieve excellent press formability. Further, Patent Documents 1 and 2 do not refer to bending workability.

The techniques described in Patent Documents 3 to 5 are said to yield high-strength steel sheets with excellent bending workability, but all of them focus only on cracks that occur on the outside of bending. If cracks occur during bending, regardless of whether they are on the outside or inside of the bend, the cracks will become fatigue crack initiation points, which may reduce the durability of the part. It cannot be said that ensuring sexuality is sufficient.

With the technology described in Patent Document 6, it is said that a high-strength steel sheet with excellent bending workability can be obtained, but attention is focused only on cracks that occur on the inner side of bending. If cracks occur during bending, regardless of whether they are on the outside or inside of the bend, those cracks can become fatigue crack initiation points, reducing the durability of the part. Otherwise, the performance of parts cannot be ensured.

As described above, the actual situation is that the technology for obtaining high-strength steel sheets with high levels of tensile strength, press formability, and bending workability has not yet been established.

The present invention has been made in view of the above-mentioned actual situation, and an object thereof is to provide a high-strength steel sheet having tensile strength, press formability, and bending workability, and a method for manufacturing the same.

In order to solve the above problems, the present inventors have found that a tensile strength of 980 MPa or more and a virtual stress-strain curve of steel sheets having various yield stresses and uniform elongations are created, and the stress-strain curve is used. We conducted press forming simulations for suspension parts. Then, based on the simulation results, the properties of the steel sheet required to obtain excellent press formability were examined.

As a result, it was found that in steel sheets with a tensile strength of 980 MPa or more, if a uniform elongation of 6% or more is secured, the thickness reduction during press forming can be minimized and press forming defects can be suppressed.

In addition, the inventors studied the optimal steel sheet structure in order to obtain a tensile strength of 980 MPa or more and a uniform elongation of 6% or more. As a result, the main phase is upper bainite, and a microstructure containing an appropriate amount of a hard secondary phase containing fresh martensite and/or retained austenite results in a high strength of 980 MPa or more and a uniform elongation of 6% or more. It has been shown that it is possible to combine

The upper bainite referred to here is an aggregate of lath-shaped ferrite with an orientation difference of less than 15°, and a structure having Fe-based carbides and/or retained austenite between lath-shaped ferrites (however, between lath-shaped ferrites (including the case of not having Fe-based carbides and/or retained austenite). Unlike lamellar (layered) ferrite in pearlite and polygonal ferrite, lath-like ferrite has a lath-like shape and a relatively high dislocation density inside. electron microscopy). When there is retained austenite between laths, only the lath-shaped ferrite portion is regarded as upper bainite and is distinguished from retained austenite. Fresh martensite is martensite that does not contain Fe-based carbides. Fresh martensite and retained austenite have similar contrast in SEM, but are distinguishable using electron backscatter diffraction (EBSD) methods.

Next, the inventors investigated the bending workability of high-strength steel sheets having a tensile strength of 980 MPa or more and a uniform elongation of 6% or more. Specifically, steel sheets with a tensile strength of 980 MPa or more and a uniform elongation of 6% or more, manufactured by different manufacturing methods, were subjected to a 90° V bending test to observe the fracture surface of bending cracks and the microstructure in the vicinity of the cracks. On the outside of the bending, the fracture surface of the crack was ductile fracture, and many voids were observed in the microstructure near the crack. On the other hand, on the inner side of the bend, the crack fracture surface is brittle fracture surface, and voids are not observed in the microstructure near the crack. Therefore, an improvement in ductility can suppress external bending cracks, and an improvement in compression embrittlement resistance can suppress internal bending cracks. Therefore, it was found that it is necessary to control the microstructure of the surface layer region and its neighboring region where bending cracks can occur.

The present invention has been made based on further studies based on the above findings, and the gist thereof is as follows.
[1] % by mass,
C: 0.05 to 0.20%,
Si: 0.5 to 1.2%,
Mn: 1.5-4.0%,
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% or less, with the balance being Fe and unavoidable impurities,
The microstructure includes upper bainite with an area ratio of 80% or more and fresh martensite and/or retained austenite with a total area ratio of 2% or more in the surface layer region from the steel plate surface to the position of 1/10 of the plate thickness,
The internal region from the plate thickness 1/10 position to the plate thickness 3/10 position contains upper bainite with an area ratio of 70% or more and fresh martensite and / or retained austenite with a total area ratio of 3% or more,
The average grain size in the surface layer region from the steel plate surface to the position of 1/10 of the plate thickness is 6 μm or less,
The difference (HV2-HV1) between the hardness (HV1) of the surface layer region from the steel plate surface to the 1/10 thickness position and the hardness (HV2) of the inner region from the 1/10 thickness position to the 3/10 thickness position 5% or more and 15% or less with respect to [0.3 × tensile strength (MPa)],
A high-strength steel sheet having a tensile strength of 980 MPa or more, a uniform elongation of 6% or more, and a ratio R/t of limit bending radius R to plate thickness t of 1.5 or less.
[2] The component composition further contains, in % by mass,
Cr: 1.0% or less, and Mo: 1.0% or less,
The high-strength steel sheet according to [1], containing at least one of
[3] The component composition further contains, in % 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], containing at least one of Nb: 0.3% or less and V: 0.3% or less.
[4] The component composition further contains, in % by mass,
Sb: 0.005-0.020%
The high-strength steel sheet according to any one of [1] to [3], containing
[5] The component composition further contains, in % by mass,
Ca: 0.01% or less,
The high-strength steel sheet according to any one of [1] to [4], containing at least one of Mg: 0.01% or less and REM: 0.01% or less.
[6] A method for manufacturing a high-strength steel sheet according to any one of [1] to [5],
Heating a steel material having the above composition to a heating temperature of 1150 ° C. or higher,
Then, after rough rolling,
A hot-rolled steel sheet is obtained by hot rolling under the conditions that the total rolling reduction in the temperature range of RC1 or less is 25% or more and 80% or less, and the finish rolling end temperature is (RC2-50°C) or more (RC2 + 120°C) or less,
Time from the end of hot rolling to the start of cooling of the hot-rolled steel sheet: within 2.0 s, average cooling rate at 3/10 thickness position: 15 ° C./s or more, cooling stop temperature: Trs or more, (Trs + 250 ° C. ) cooled under the following conditions,
The hot-rolled steel sheet after cooling is coiled at a coiling temperature of Trs or more and (Trs + 250 ° C.) or less,
A method for producing a high-strength steel sheet by cooling to 100°C or less at an average cooling rate of 20°C/s or less.
Note that RC1, RC2, and Trs are defined by the following formulas (1), (2), and (3), respectively.
RC1 (°C) = 900 + 100 x C + 100 x N + 10 x Mn + 700 x Ti + 5000 x B + 10 x Cr + 50 x Mo + 2000 x Nb + 150 x V (1)
RC2 (°C) = 750 + 100 x C + 100 x N + 10 x Mn + 350 x Ti + 5000 x B + 10 x Cr + 50 x Mo + 1000 x Nb + 150 x V (2)
Trs (° C.)=500-450×C-35×Mn-15×Cr-10×Ni-20×Mo (3)
Here, each element symbol in the above formulas (1), (2), and (3) represents the content (% by mass) of each element, and is set to 0 when the element is not contained.
[7] Manufacture of the high-strength steel sheet according to [6], wherein in the cooling step after the hot rolling is completed, the average cooling rate of the surface layer and the average cooling rate at the 3/10th thickness position satisfy the formula (4). Method.
Average cooling rate of the surface layer - Average cooling rate at the position of 3/10 of the plate thickness ≥ 10 ° C./s (4)

According to the present invention, a high-strength steel sheet having a tensile strength of 980 MPa or more, press formability, and bending workability can be obtained. Although the high-strength steel sheet of the present invention has high tensile strength, it is excellent in press formability and can be press-formed without forming defects such as necking and cracking. In addition, when the high-strength steel sheet of the present invention is applied to members of trucks and passenger cars, it is possible to reduce the weight of automobile bodies by reducing the amount of steel used while ensuring safety, thereby contributing to the reduction of environmental load.

In the present invention, excellent press formability means having a uniform elongation of 6% or more. In addition, excellent bending workability means that R/t, which is the ratio of the limit bending radius R and the plate thickness t at which cracks of 50 μm or more in depth do not occur on both the outer side and the inner side of the bend in the 90° V bending test, is 1.5. 5 or less.

The present invention will be specifically described below. In addition, the following description shows examples of preferred embodiments of the present invention, and the present invention is not limited thereto.

[Component composition]
First, the reasons for limiting the chemical composition of the high-strength steel sheet of the present invention will be described. In addition, "%" as a unit of content means "% by mass" unless otherwise specified.

C: 0.05-0.20%
C is an element that has the effect of improving the strength of steel. C promotes the formation of bainite by improving hardenability and contributes to high strength. In addition, C also contributes to high strength by increasing the strength of martensite. In order to obtain a tensile strength of 980 MPa or more, the C content must be 0.05% or more. Therefore, the C content should be 0.05% or more, preferably 0.06% or more. On the other hand, when the C content exceeds 0.20%, the strength of martensite increases excessively, and the difference in strength from upper bainite, fresh martensite and/or retained austenite as the main phase increases, resulting in Uniform elongation is reduced. Therefore, the C content should be 0.20% or less, preferably 0.18% or less.

Si: 0.5-1.2%
Si has the effect of suppressing the formation of Fe-based carbides and suppresses precipitation of cementite during upper bainite transformation. As a result, C is distributed in untransformed austenite, and by cooling after coiling in the hot rolling process, untransformed austenite becomes fresh martensite and/or retained austenite, and the desired fresh martensite and/or retained austenite are obtained. be able to. In order to obtain these effects, the Si content should be 0.5% or more. Preferably, the Si content is 0.6% or more. On the other hand, when the Si content exceeds 1.2%, fresh martensite and/or retained austenite are formed more than the desired area ratio, and as a result, the desired upper bainite area ratio cannot be obtained. may worsen sexuality. Therefore, the Si content should be 1.2% or less, preferably 1.1% or less.

Mn: 1.5-4.0%
Mn stabilizes austenite and contributes to the generation of fresh martensite and/or retained austenite. In order to obtain such effects, the Mn content must be 1.5% or more. Therefore, the Mn content is set to 1.5% or more, preferably 1.7% or more. On the other hand, when the Mn content exceeds 4.0%, fresh martensite and/or retained austenite are excessively generated, and as a result, the desired area ratio of upper bainite cannot be obtained, resulting in deterioration of bendability. Therefore, the Mn content should be 4.0% or less, preferably 3.8% or less.

P: 0.10% or less P is an element that forms a solid solution and contributes to an increase in the strength of steel. However, P is also an element that causes slab cracks during hot rolling by segregating at austenite grain boundaries during hot rolling. In addition, it segregates at grain boundaries to reduce uniform elongation. For this reason, it is preferable to keep the P content as low as possible, but the P content up to 0.10% is permissible. Therefore, the P content should be 0.10% or less. The lower limit is not particularly limited, but if the P content is less than 0.0002%, production efficiency is lowered, so 0.0002% or more is preferable.

S: 0.03% or less S combines with Ti and Mn to form coarse sulfides, which hasten the generation of voids, thereby lowering the uniform elongation. Therefore, it is preferable to keep the S content as low as possible, but an S content of up to 0.03% is permissible. Therefore, the S content is made 0.03% or less. The lower limit is not particularly limited, but if the S content is less than 0.0002%, production efficiency is lowered, so 0.0002% or more is preferable.

Al: 0.001-2.0%
Al is an element that acts as a deoxidizing agent 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 should be 0.001% or more, preferably 0.005% or more, and more preferably 0.010% or more. Al, like Si, has the effect of suppressing the formation of Fe-based carbides and suppresses precipitation of cementite during upper bainite transformation. This contributes to the generation of fresh martensite and/or retained austenite during cooling after winding. On the other hand, an excessive content of Al causes an increase in oxide-based inclusions and lowers the uniform elongation. Therefore, the Al content should be 2.0% or less, preferably 1.0% or less, and more preferably 0.1% or less.

N: 0.01% or less N precipitates as a nitride by combining with a nitride-forming element, and generally contributes to grain refinement. However, since N combines with Ti at high temperatures to form coarse nitrides, a content exceeding 0.01% causes a decrease in uniform elongation. Therefore, the N content is set to 0.01% or less. The lower limit is not particularly limited, but if the N content is less than 0.0002%, production efficiency is lowered, so 0.0002% or more is preferable.

O: 0.01% or less O forms oxides and deteriorates moldability, so the content must be suppressed. In particular, when O exceeds 0.01%, this tendency becomes remarkable. Therefore, the O content should be 0.01% or less, preferably 0.005%, more preferably 0.003%. The lower limit is not specified, but if it is less than 0.00005%, production efficiency may be remarkably lowered, so 0.00005% or more is preferable.

B: 0.0005 to 0.010%
B is an element that segregates at prior austenite grain boundaries, suppresses the formation of ferrite, promotes the formation of upper bainite, and contributes to the improvement of the strength of the steel sheet. In order to develop these effects, the B content must be 0.0005% or more. Therefore, the B content is set to 0.0005% or more, preferably 0.0006%, and more preferably 0.0007%. On the other hand, when the B content exceeds 0.010%, the above effects are saturated. Therefore, the B content is 0.010% or less, preferably 0.009% or less, more preferably 0.008% or less.

The balance consists of Fe and unavoidable impurities. Incidentally, examples of unavoidable impurities include Zr, Co, Sn, Zn, and W. When the component composition contains at least one of Zr, Co, Sn, Zn, and W as unavoidable impurities, the total content of these elements is preferably 0.5% or less.

The chemical composition of the high-strength steel sheet of the present invention can optionally contain at least one of the elements listed below.

Cr: 1.0% or less Cr is a carbide-forming element that segregates at the interface between the upper bainite and the untransformed austenite during the upper bainite transformation after winding, thereby reducing the driving force of the bainite transformation and causing the upper bainite to segregate. It has the effect of stopping metamorphosis. Untransformed austenite remaining after the transformation to upper bainite stops becomes fresh martensite and/or retained austenite by cooling after winding. Therefore, when Cr is added, Cr also contributes to the formation of a desired area ratio of fresh martensite and/or retained austenite. This effect is obtained when Cr is preferably 0.1% or more. However, when the Cr content exceeds 1.0%, fresh martensite and/or retained austenite are excessively generated, and as a result, the desired area ratio of upper bainite cannot be obtained, resulting in deterioration of bendability. Therefore, when Cr is added, the Cr content is 1.0% or less, preferably 0.9% or less, and more preferably 0.8% or less.

Mo: 1.0% or less Mo promotes formation of bainite through improvement of hardenability and contributes to strength improvement of the steel sheet. In addition, Mo, like Cr, is a carbide-forming element, and segregates at the interface between the upper bainite and the untransformed austenite during the upper bainite transformation after winding, thereby reducing the transformation driving force of the bainite and cooling the winding. It contributes to the later generation of fresh martensite and/or retained austenite. However, when the Mo content exceeds 1.0%, fresh martensite and/or retained austenite are excessively generated, and as a result, the desired area ratio of upper bainite cannot be obtained, which deteriorates uniform elongation. . This effect is obtained when Mo is preferably 0.1% or more. Therefore, when Mo is added, the Mo content is 1.0% or less, preferably 0.9% or less, and more preferably 0.8% or less.

In addition, the chemical composition of the high-strength steel sheet of the present invention can optionally contain at least one of the elements listed below.

Cu: 2.0% or less Cu is an element that forms a solid solution and contributes to increasing the strength of steel. Further, Cu promotes the formation of bainite through improvement of hardenability and contributes to strength improvement. This effect is obtained when Cu is preferably 0.01% or more. However, when the Cu content exceeds 2.0%, the surface properties of the high-strength steel sheet are deteriorated, and the bendability of the high-strength steel sheet is deteriorated. Therefore, when Cu is added, the Cu content is 2.0% or less, preferably 1.9% or less, and more preferably 1.8% or less.

Ni: 2.0% or less Ni is an element that forms a solid solution and contributes to increasing the strength of steel. In addition, Ni promotes the formation of bainite through improvement of hardenability and contributes to strength improvement. This effect is obtained when Ni is preferably 0.01% or more. However, when the Ni content exceeds 2.0%, fresh martensite and/or retained austenite excessively increase, and as a result, the desired area ratio of upper bainite cannot be obtained. deteriorate. Therefore, when Ni is added, the Ni content should be 2.0% or less, preferably 1.9% or less, and more preferably 1.8% or less.

Ti: 0.3% or less Ti is an element that acts to improve the strength of the steel sheet by precipitation strengthening or solid solution strengthening. Ti forms nitrides in the high temperature range of austenite. As a result, precipitation of BN is suppressed, and B becomes a solid solution. Therefore, when Ti is added, Ti also contributes to ensuring the hardenability necessary for forming upper bainite, and the strength is improved. This effect is obtained when Ti is preferably 0.01% or more. However, when the Ti content exceeds 0.3%, a large amount of Ti nitrides are formed, which reduces the uniform elongation. Therefore, when Ti is added, the Ti content should be 0.3% or less, preferably 0.28% or less, and more preferably 0.25% or less.

Nb: 0.3% or less Nb is an element that has the effect of improving the strength of the steel sheet by precipitation strengthening or solid solution strengthening. In addition, Nb, like Ti, raises the recrystallization temperature of austenite during hot rolling, enabling rolling in the austenite unrecrystallized region, refining the grain size of upper bainite, fresh martensite and / Or contribute to an increase in the area ratio of retained austenite. Nb, like Cr, is a carbide-forming element, and segregates at the interface between upper bainite and untransformed austenite during the upper bainite transformation after winding, thereby reducing the transformation driving force of bainite and untransformed austenite. It is an element that has the effect of stopping the upper bainite transformation while leaving the The untransformed austenite is then cooled to become fresh martensite and/or retained austenite. Therefore, when Nb is added, Nb also contributes to the formation of a desired area ratio of fresh martensite and/or retained austenite. This effect is obtained when Nb is preferably 0.01% or more. However, when the Nb content exceeds 0.3%, fresh martensite and/or retained austenite excessively increase, and as a result, the desired area ratio of upper bainite cannot be obtained, resulting in a decrease in uniform elongation. Therefore, when Nb is added, the Nb content should be 0.3% or less, preferably 0.28% or less, and more preferably 0.25% or less.

V: 0.3% or less V is an element that acts to improve the strength of the steel sheet by precipitation strengthening and solid solution strengthening. Further, similarly to Ti, V raises the recrystallization temperature of austenite during hot rolling, thereby enabling rolling in the austenite non-recrystallization region and contributing to refinement of the grain size of upper bainite. In addition, like Cr, V is a carbide-forming element, and segregates at the interface between upper bainite and untransformed austenite during upper bainite transformation after winding, thereby reducing the transformation driving force of bainite and untransformed austenite. It is an element that has the effect of stopping the upper bainite transformation while leaving the The untransformed austenite is then cooled to become fresh martensite and/or retained austenite. Therefore, when V is added, V also contributes to the formation of a desired area ratio of fresh martensite and/or retained austenite. This effect is obtained when V is preferably 0.01% or more. However, when the V content exceeds 0.3%, fresh martensite and/or retained austenite excessively increase, and as a result, the desired area ratio of upper bainite cannot be obtained, resulting in a decrease in uniform elongation. Therefore, when V is added, the V content is 0.3% or less, preferably 0.28% or less, and more preferably 0.25% or less.

In addition, the chemical composition of the high-strength steel sheet of the present invention can optionally contain the following elements.

Sb: 0.005-0.020%
Sb is an element that has the effect of suppressing nitridation of the surface of the steel material (slab) when the steel material (slab) is heated. By adding Sb, precipitation of BN in the surface layer of the steel material can be suppressed. As a result, the remaining solid solution B contributes to ensuring the hardenability necessary for the formation of bainite and thereby improving the strength of the steel sheet. When Sb is added, the Sb content is 0.005% or more, preferably 0.006% or more, more preferably 0.007% or more, in order to obtain the above effect. On the other hand, when the Sb content exceeds 0.020%, the toughness of the steel is lowered, and slab cracks and hot rolling cracks may occur. Therefore, when Sb is added, the Sb content is 0.020% or less, preferably 0.019% or less, and more preferably 0.018% or less.

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

Ca: 0.01% or less Ca controls the shape of oxide- and sulfide-based inclusions, and contributes to the suppression of cracking at sheared edge surfaces of steel sheets and the further improvement of bending workability. This effect is obtained when Ca is preferably 0.001% or more. However, if the Ca content exceeds 0.01%, the amount of Ca-based inclusions increases and the cleanliness of the steel deteriorates, which may rather cause shear edge cracks and bending cracks. Therefore, when Ca is added, the Ca content is set to 0.01% or less.

Mg: 0.01% or less Like Ca, Mg controls the shape of oxide- and sulfide-based inclusions, and contributes to the suppression of cracking at sheared edge surfaces of steel sheets and the further improvement of bending workability. This effect is obtained when Mg is preferably 0.001% or more. However, if the Mg content exceeds 0.01%, the cleanliness of the steel deteriorates, which may rather cause shear edge cracks and bending cracks. Therefore, when Mg is added, the Mg content is made 0.01% or less.

REM: 0.01% or less Like Ca, REM (rare earth metal) controls the shape of oxide- and sulfide-based inclusions, and contributes to the suppression of cracking at sheared edge surfaces of steel sheets and the further improvement of bending workability. do. This effect is obtained when REM is preferably 0.001% or more. However, if the REM content exceeds 0.01%, the cleanliness of the steel deteriorates, which may rather cause shear edge cracks and bending cracks. Therefore, when REM is added, the REM content is made 0.01% or less.

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

The high-strength steel sheet of the present invention includes upper bainite with an area ratio of 80% or more and fresh martensite with a total area ratio of 2% or more and / or residual Top bainite with an area ratio of 70% or more and fresh martensite with a total area ratio of 3% or more and/or residual It contains austenite and has an average crystal grain size of 6 μm or less in the surface layer region from the steel plate surface to the plate thickness 1/10 position, and the hardness (HV1) of the surface layer region from the steel plate surface to the plate thickness 1/10 position. The difference (HV2-HV1) in the hardness (HV2) of the inner region from the 1/10th thickness position to the 3/10th thickness position is 5% or more and 15% or less with respect to [0.3 × tensile strength (MPa)] It has a certain microstructure.

Upper bainite: 80% or more, fresh martensite and/or retained austenite: 2% or more in total area ratio in the surface layer region from the steel plate surface to the plate thickness 1/10 position In the high-strength steel plate of the present invention, the soft upper part By finely dispersing hard fresh martensite and/or retained austenite in bainite, ductility can be improved and external bending cracks can be suppressed. In order to obtain this effect, the surface layer should have an area fraction of upper bainite of 80% or more and an area fraction of fresh martensite and/or retained austenite of 2% or more. Preferably, the area ratio of upper bainite is 85% or more, and the area ratio of fresh martensite and/or retained austenite is 3% or more. On the other hand, if the total area ratio of fresh martensite and/or retained austenite is 20% or more, the bendability may decrease, so the total area ratio of fresh martensite and/or retained austenite is It is preferable to make it 20% or less. It is more preferably 18% or less, still more preferably 15% or less.

　Because the cooling rate is high in the surface layer region of the steel sheet, the bainite transformation progresses quickly, and the concentration of C for forming fresh martensite and/or retained austenite is less than in the interior. If the concentration of C is small, martensite transformation is suppressed. As a result, the area ratio of fresh martensite and/or retained austenite in the surface layer region of the steel sheet is smaller than that in the interior.

Upper bainite with an area ratio of 70% or more and fresh martensite and/or retained austenite with a total area ratio of 3% or more in the inner region from the plate thickness 1/10 position to the plate thickness 3/10 position. , upper bainite is included as a main phase in the inner region from the 1/10 thickness position to the 3/10 thickness position. If the area ratio of upper bainite is less than 70%, a tensile strength of 980 MPa or more and a uniform elongation of 6% or more cannot be achieved. Therefore, the area ratio of upper bainite is set to 70% or more, preferably 80% or more. In addition, in the present invention, fresh martensite and/or retained austenite are included in the internal region from the 1/10 thickness position to the 3/10 thickness position. Fresh martensite has the effect of improving uniform elongation by promoting work hardening and delaying the onset of plastic instability. Retained austenite can increase uniform elongation by TRIP (Transformation Induced Plasticity) effect. In order to obtain these effects, the total area ratio of fresh martensite and/or retained austenite is set to 3% or more, preferably 4% or more.
Further, in the present invention, the microstructure near the center of the plate thickness after the 3/10th position of the plate thickness has little effect on bendability, but from the viewpoint of ductility, the area ratio of upper bainite is preferably 60% or more. Fresh martensite/tempered martensite/retained austenite and the like may be contained up to 40% due to Mn segregation at the thickness center.

Average crystal grain size in the surface layer region from the steel plate surface to the position of 1/10 of the plate thickness: 6 μm or less Bending inner cracks are brittle fractures due to strong compression. That is, if the resistance to compression embrittlement is improved, internal bending cracks can be suppressed. Compression embrittlement is less likely to occur due to the refinement of crystal grains. In order to obtain this effect, the average crystal grain size in the surface layer region should be 6 μm or less, preferably 5 μm or less. As the average grain size becomes smaller, the effect of improving resistance to compression embrittlement can be obtained. Therefore, the average crystal grain size in the surface layer region is preferably 2 μm or more.

That is, in order to obtain a tensile strength of 980 MPa or more, a uniform elongation of 6% or more, and good bending workability, the effect of improving the uniform elongation of fresh martensite and/or retained austenite and the control of the surface layer microstructure. It can only be achieved by combining inhibitory effects.

The difference (HV2-HV1) between the hardness (HV1) of the surface layer region from the steel plate surface to the 1/10 thickness position and the hardness (HV2) of the inner region from the 1/10 thickness position to the 3/10 thickness position 5% or more and 15% or less with respect to [0.3 × tensile strength (MPa)] In the high-strength steel sheet of the present invention, the soft surface layer suppresses cracking on the outside of bending, and the hard inside adjacent to the surface layer prevents bending. Suppresses the growth of cracks in the plate thickness direction. In order to obtain the effect of suppressing the occurrence and growth of such bending cracks, the difference between the hardness of the surface layer region (HV1) and the hardness of the inner region (HV2) (HV2-HV1) is 0.3 × tensile strength (MPa ) to 5% or more. It is preferably 6% or more, more preferably 7% or more. On the other hand, if the hardness difference between the surface layer region and the inner region is large, a strain mismatch occurs between the surface layer and the inner region in the tensile test, making it impossible to obtain the target tensile properties. Therefore, the difference between the hardness of the surface layer region and the hardness of the inner region is set to 15% or less with respect to 0.3×tensile strength (MPa). It is preferably 14% or less, more preferably 13% or less. The above effect can be obtained by controlling the cooling rate on the surface and inside the thickness of the steel sheet.

The microstructure can further contain any structure (hereinafter referred to as "other structures") other than upper bainite, fresh martensite, and retained austenite. From the viewpoint of enhancing the effect of microstructure control, the total area ratio of other structures is preferably 3% or less. In other words, the total area ratio of upper bainite, fresh martensite, and retained austenite in the microstructure is preferably 97% or more. Other structures include, for example, cementite, polygonal ferrite, pearlite, tempered martensite, and lower bainite.

[Mechanical properties]
The high-strength steel sheet of the present invention has a tensile strength of 980 MPa or more, a uniform elongation of 6% or more, and R / t (limit bending radius R and plate thickness t at which cracks with a depth of 50 μm or more do not occur on both the outside and inside of the bend. ratio) is 1.5 or less. Therefore, the high-strength steel sheet of the present invention has excellent press formability despite its high tensile strength, and can be press-formed without causing forming defects such as necking and cracking. The durability of the part can be ensured without large cracks occurring on both the outside and inside of the bend. Therefore, safety can be ensured when applied to members of trucks and passenger cars.

The microstructure, hardness, and mechanical properties of the present invention can be determined by the measurement methods described in Examples below.

[Production method]
Next, a method for manufacturing a high-strength steel sheet according to one embodiment of the present invention will be described. In addition, the temperature in the following description represents the surface temperature of the object (steel material or steel plate) unless otherwise specified.

The high-strength steel sheet of the present invention can be produced by sequentially subjecting a steel material to the following treatments (1) to (5). Each step will be described below.
(1) heating (2) hot rolling (3) cooling (first cooling)
(4) Winding (5) Cooling (second cooling)
As the steel material, any material can be used as long as it has the chemical composition described above. The chemical composition of the finally obtained high-strength steel sheet is the same as the chemical composition of the steel material used. For example, a steel slab can be used as the steel material. Moreover, the manufacturing method of the steel material is not particularly limited. For example, molten steel having the above chemical 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 an ingot casting-blooming 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 may be subjected to the heating step after being cooled into a hot piece or a cold piece. good.

(1) Heating First, the steel material is heated to a heating temperature of 1150°C or higher. Usually, most carbonitride-forming elements such as Ti exist as coarse carbonitrides in steel materials. The presence of this coarse and non-uniform precipitates is generally required for high-strength steel sheets for truck and passenger car parts (e.g. shear edge crack resistance, bending workability, burring workability, etc.). aggravate. Therefore, it is necessary to heat the steel material prior to hot rolling to dissolve coarse precipitates. Specifically, the heating temperature of the steel material must be 1150° C. or higher in order to sufficiently dissolve the coarse precipitates. On the other hand, if the heating temperature of the steel material becomes too high, slab flaws will occur and the yield will decrease due to scale off. Therefore, from the viewpoint of improving the yield, it is preferable to set the heating temperature of the steel material to 1350° C. or lower. The lower limit of the heating temperature of the steel material is more preferably 1180°C or higher, and still more preferably 1200°C or higher. The upper limit of the heating temperature of the steel material is more preferably 1300° C. or lower, and still more preferably 1280° C. or lower.

In the heating, from the viewpoint of uniforming the temperature of the steel material, it is preferable to raise the temperature of the steel material to the heating temperature and then maintain it at the heating temperature. The time for which the heating temperature is maintained (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 longer. On the other hand, when the holding time exceeds 10000 seconds, the amount of scale generation increases. As a result, entrapment of scales and the like is likely to occur in subsequent hot rolling, leading to a decrease in yield due to defective surface defects. Therefore, the retention time is preferably 10000 seconds or less, more preferably 8000 seconds or less.

(2) Hot rolling Next, the heated steel material is hot rolled to form a hot rolled steel sheet. Hot rolling may consist of rough rolling and finish rolling. When rough rolling is performed, the conditions are not particularly limited. After rough rolling, descaling is preferably performed prior to finish rolling in order to remove surface scales. Descaling may be performed between stands in the finish rolling.

Next, in the present invention, in the finish rolling, when the temperature RC1 and the temperature RC2 are defined by the following formulas (1) and (2), the total rolling reduction in the temperature range of RC1 or less is 25% or more and 80% or less, In addition, the finishing temperature of finish rolling should be (RC2-50°C) or more and (RC2+120°C) or less.

RC1 is the austenite 50% recrystallization temperature estimated from the component composition, and RC2 is the austenite lower limit recrystallization temperature estimated from the component composition. If the total rolling reduction of RC1 or less is less than 25%, the average crystal grain size becomes large and good bending workability cannot be obtained. On the other hand, when the total rolling reduction in the temperature range of RC1 or less exceeds 80%, the dislocation density of austenite is high, the ductility of the bainite structure transformed from austenite in a state of high dislocation density is poor, and the uniform elongation is 6% or more. is not obtained. Therefore, the total rolling reduction in the temperature range of RC1 or less is set to 25% or more and 80% or less.

Further, hot rolling is performed under the conditions of finish rolling finish temperature: (RC2-50°C) or more and (RC2+120°C) or less. If the finish rolling finish temperature is lower than (RC2-50° C.), bainite transformation occurs from austenite in a state of high dislocation density. Since upper bainite transformed from austenite with a high dislocation density has a high dislocation density and poor ductility, the uniform elongation decreases. Also, when the rolling end temperature is low and the rolling is performed at the two-phase region temperature of ferrite + austenite, the uniform elongation decreases. Therefore, the finishing temperature of finish rolling should be (RC2-50° C.) or higher. On the other hand, if the finishing temperature of the finish rolling is higher than (RC2+120°C), the austenite grains become coarse and the average grain size of the upper bainite becomes large, resulting in a decrease in strength. Fresh martensite and/or retained austenite also become coarser, resulting in lower uniform elongation. Therefore, the finish rolling finish temperature is set to (RC2+120° C.) or less.
RC1 and RC2 are defined by the following formulas (1) and (2).
RC1 (°C) = 900 + 100 x C + 100 x N + 10 x Mn + 700 x Ti + 5000 x B + 10 x Cr + 50 x Mo + 2000 x Nb + 150 x V (1)
RC2 (°C) = 750 + 100 x C + 100 x N + 10 x Mn + 350 x Ti + 5000 x B + 10 x Cr + 50 x Mo + 1000 x Nb + 150 x V (2)
Here, each element symbol in the above formulas (1) and (2) represents the content (% by mass) of each element, and is set to 0 when the element is not contained.

(3) Cooling (first cooling)
Next, the obtained hot-rolled steel sheet is cooled (first cooling). At that time, the time from the end of hot rolling (end of finish rolling) to the start of cooling (cooling start time) is set within 2.0 seconds. If the cooling start time exceeds 2.0 seconds, grain growth of austenite grains occurs and a tensile strength of 980 MPa or more cannot be secured. The cooling start time is preferably within 1.5 seconds.

The average cooling rate at the plate thickness 3/10 position shall be 15°C/s or more. In the present invention, by rapidly cooling the surface layer from the inside, different microstructures are created between the surface layer and the inside. Due to the rapid cooling of the surface layer, the bainite transformation of the surface layer starts early, and the formation of martensite and retained austenite due to the enrichment of C is less than in the inside. If the average cooling rate in cooling is less than 15 ° C. / s, the surface layer is not cooled sufficiently rapidly, and the upper bainite with an area ratio of 80% or more and the total area ratio of 2% or more fresh martensite and / or residual An austenite surface layer structure cannot be obtained. Therefore, the average cooling rate is set to 15° C./s or higher, preferably 20° C./s or higher, 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 is too high, it becomes difficult to manage 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 present invention, by satisfying the average cooling rate of the surface layer - the average cooling rate at the plate thickness 3/10 position of 10 ° C./s or more, the formation of martensite and retained austenite due to the enrichment of C in the surface layer is reduced to the plate thickness. Less than the 3/10 position. As a result, a soft surface layer structure can be created. On the other hand, inside the steel sheet, the cooling rate is slower than the surface layer, and the progress of bainite transformation is slower than that in the surface layer. be able to. That is, a difference in hardness between the surface layer and the inside can be realized. If the average cooling rate of the plate thickness 3/10 position surface layer is less than 10 ° C / s, the above effect is not observed, so the average cooling rate of the surface layer - plate thickness The average cooling rate at the 3/10 position is 10° C./s or more. The average cooling rate is obtained by (temperature at the start of cooling - temperature at the end of cooling)/cooling time. The temperature of the surface layer is actually measured with a thermometer. The temperature at the 3/10 thickness position is obtained by calculating the temperature distribution in the cross section of the steel sheet by heat transfer analysis and correcting the result with the actual surface temperature of the steel sheet.

Also, in cooling, forced cooling may be performed so as to achieve the above average cooling rate. The cooling method is not particularly limited, but for example, water cooling is preferable.

The cooling stop temperature shall be Trs or higher and (Trs + 250°C) or lower. 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 their uniform elongation is remarkably low. 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, and a tensile strength of 980 MPa cannot be obtained. Therefore, the cooling stop temperature is set to (Trs+250° C.) or less.

Note that Trs is defined by the following equation (3).
Trs (° C.)=500-450×C-35×Mn-15×Cr-10×Ni-20×Mo (3)
Here, each element symbol in the above formula (3) represents the content (% by mass) of each element, and is set to 0 when the element is not contained.

(4) Winding Next, the cooled hot-rolled steel sheet is coiled under the conditions of a coiling temperature of Trs or more and (Trs+250° C.) or less. If the coiling temperature is lower than Trs, martensite transformation or lower bainite transformation proceeds after coiling, and desired fresh martensite and/or retained austenite cannot be obtained. Therefore, the winding temperature should be Trs or higher. On the other hand, if the coiling temperature is higher than (Trs+250° C.), ferrite is generated, and a tensile strength of 980 MPa cannot be obtained. Therefore, the winding temperature is set to (Trs+250° C.) or less.

(5) Cooling (second cooling)
After winding, it is further cooled to 100° C. or lower at an average cooling rate of 20° C./s or lower (second cooling). The average cooling rate affects the formation of fresh martensite and/or retained austenite. When the average cooling rate exceeds 20° C./s, most of the untransformed austenite transforms into martensite, the desired retained austenite cannot be obtained, and the uniform elongation decreases. Therefore, the average cooling rate is set to 20° C./s or less, preferably 2° C./s or less, more preferably 0.02° C./s or less. On the other hand, although the lower limit of the average cooling rate is not particularly limited, it is preferably 0.0001° C./s or more.

Cooling can be performed to any temperature below 100°C, but cooling to about 10 to 30°C (for example, room temperature) is preferable. It should be noted that the cooling can be performed in any form, for example, it may be performed in the state of a wound coil.

The high-strength steel sheet of the present invention can be manufactured by the above procedure. The winding and subsequent cooling may be carried out in accordance with a conventional method. For example, temper rolling may be applied, or pickling may be applied to remove scales formed on the surface.

Molten steel having the composition shown in Table 1 was melted in a converter, and a steel slab was produced as a steel material by continuous casting. The obtained steel material was heated to the heating temperature shown in Table 2, and then the heated steel material was subjected to hot rolling consisting of rough rolling and finish rolling to obtain a hot-rolled steel sheet. Table 2 shows the finishing temperature of hot rolling.

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

A test piece was taken from the obtained high-strength steel sheet, and the microstructure, surface roughness, and mechanical properties were evaluated according to the procedure described below.

(Microstructure)
A test piece for microstructure observation was taken from the obtained high-strength steel sheet so that the thickness cross-section parallel to the rolling direction was the observation surface. The surface of the obtained test piece was polished, and the surface was corroded using an etchant (3 vol.% nital solution) to expose the microstructure.

Then, the surface layer area from the surface of the test piece to the thickness 1/10 position, and the internal area from the thickness 1/10 position to the thickness 3/10 position, using a scanning electron microscope (SEM), 5000 times SEM images of the microstructure were obtained by photographing 10 fields of view at a magnification. The obtained SEM images were analyzed by image processing to quantify the area ratios of upper bainite (UB), polygonal ferrite (F), and tempered martensite (TM). In addition, fresh martensite (M) and retained austenite (γ) are difficult to distinguish by SEM, so they were identified using an electron backscatter diffraction (EBSD) method, and each area ratio and average crystal Particle size was determined. Table 3 shows the measured area ratio of each microstructure and the average crystal grain size of the surface layer structure. Table 3 also shows the total area ratio (M+γ) of fresh martensite and retained austenite.

(Hardness measurement)
From the obtained high-strength steel sheet, a sample for hardness measurement is taken so that the thickness cross section parallel to the rolling direction becomes the hardness measurement cross section, and the surface layer region from the steel plate surface to the thickness 1/10 position and the thickness 1/ The hardness of the internal region from the 10th position to the plate thickness 3/10th position was measured. The hardness of the surface layer region from the surface of the steel plate to the position of 1/10 of the plate thickness was measured at a position 50 µm away from the surface with an indentation interval of 250 µm. The hardness of the inner region from the 1/10 thickness position to the 3/10 thickness position was measured at the 1/5 thickness position with an indentation interval of 250 μm. All hardness measurement conditions were a load of 100 g, a holding time of 10 s, and an average of 5 measurement points.

(Tensile test)
A JIS No. 5 test piece (gauge length, GL: 50 mm) was taken from the obtained high-strength steel sheet so that the tensile direction was perpendicular to the rolling direction. Using the obtained test piece, a tensile test was performed in accordance with the provisions of JIS Z 2241, yield strength (yield point, YP), tensile strength (TS), yield ratio (YR), total elongation (El), Similar elongation (u-El) was determined. 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 was 980 MPa or more, it was evaluated as high strength. Moreover, when the uniform elongation was 6% or more, the press formability was evaluated as good.

(90° V bending test)
JIS Z 2248 (2014) using a test piece cut into a strip shape of 100 mm x 35 mm so that the longitudinal direction of the test piece is perpendicular to the rolling direction from the 1/2 position in the width direction of the obtained hot-rolled steel sheet. (V block 90° V bending test), a bending test was performed. The bending punch radius R is set from 0.5 mm to 2.0 times or more of the plate thickness t in increments of 0.5 mm. The presence or absence of bending cracks and their depth were measured at 1/4 position, 1/2 position and 3/4 position of the test piece width in a plane parallel to the longitudinal direction of the test piece and perpendicular to the plate surface after the bending test. After mirror-polishing the cross-sections cut at three locations, the cracks on the outside and inside of the bend of the test piece were observed with an optical microscope, and the maximum crack depths on the outside and inside of the bend that occurred in the three cross-sections were measured. A limit bending radius (minimum bending radius) was obtained in which the crack depth did not exceed 50 μm on both the outside and inside of the bend. An R/t of 1.5 or less was considered acceptable. Even if the critical bending radius is 2.0 times or more the sheet thickness t, if a crack of 50 μm or more occurs on the outside or inside of the bending, the bending workability is considered to be poor, and the critical bending radius R is not required. .

From the results in Table 3, all the invention examples have a tensile strength of 980 MPa or more, press formability, and bending workability.

Claims

in % by mass,
C: 0.05 to 0.20%,
Si: 0.5 to 1.2%,
Mn: 1.5-4.0%,
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%
and has a component composition consisting of the balance Fe and unavoidable impurities,
The microstructure includes upper bainite with an area ratio of 80% or more and fresh martensite and/or retained austenite with a total area ratio of 2% or more in the surface layer region from the steel plate surface to the position of 1/10 of the plate thickness,
The internal region from the plate thickness 1/10 position to the plate thickness 3/10 position contains upper bainite with an area ratio of 70% or more and fresh martensite and / or retained austenite with a total area ratio of 3% or more,
The average grain size in the surface layer region from the steel plate surface to the position of 1/10 of the plate thickness is 6 μm or less,
The difference (HV2-HV1) between the hardness (HV1) of the surface layer region from the steel plate surface to the 1/10 thickness position and the hardness (HV2) of the inner region from the 1/10 thickness position to the 3/10 thickness position 5% or more and 15% or less with respect to [0.3 × tensile strength (MPa)],
A high-strength steel sheet having a tensile strength of 980 MPa or more, a uniform elongation of 6% or more, and a ratio R/t of limit bending radius R to plate thickness t of 1.5 or less.
The component composition further, in mass %,
Cr: 1.0% or less, and Mo: 1.0% or less,
The high-strength steel sheet according to claim 1, containing at least one of
The component composition further, in mass %,
Cu: 2.0% or less,
Ni: 2.0% or less,
Ti: 0.3% or less,
The high-strength steel sheet according to claim 1 or 2, containing at least one of Nb: 0.3% or less and V: 0.3% or less.
The component composition further, in mass %,
Sb: 0.005-0.020%
The high-strength steel sheet according to any one of claims 1 to 3, containing
The component composition further, in mass %,
Ca: 0.01% or less,
The high-strength steel sheet according to any one of claims 1 to 4, containing at least one of Mg: 0.01% or less and REM: 0.01% or less.
A method for producing a high-strength steel sheet according to any one of claims 1 to 5,
Heating a steel material having the above composition to a heating temperature of 1150 ° C. or higher,
Then, after rough rolling,
A hot-rolled steel sheet is obtained by hot rolling under the conditions that the total rolling reduction in the temperature range of RC1 or less is 25% or more and 80% or less, and the finish rolling end temperature is (RC2-50°C) or more (RC2 + 120°C) or less,
Time from the end of hot rolling to the start of cooling of the hot-rolled steel sheet: within 2.0 s, average cooling rate at 3/10 thickness position: 15 ° C./s or more, cooling stop temperature: Trs or more, (Trs + 250 ° C. ) cooled under the following conditions,
The hot-rolled steel sheet after cooling is coiled at a coiling temperature of Trs or more and (Trs + 250 ° C.) or less,
cooling to 100° C. or less at an average cooling rate of 20° C./s or less;
A method for producing a high-strength steel plate.
Note that RC1, RC2, and Trs are defined by the following formulas (1), (2), and (3), respectively.
RC1 (°C) = 900 + 100 x C + 100 x N + 10 x Mn + 700 x Ti + 5000 x B + 10 x Cr + 50 x Mo + 2000 x Nb + 150 x V (1)
RC2 (°C) = 750 + 100 x C + 100 x N + 10 x Mn + 350 x Ti + 5000 x B + 10 x Cr + 50 x Mo + 1000 x Nb + 150 x V (2)
Trs (° C.)=500-450×C-35×Mn-15×Cr-10×Ni-20×Mo (3)
Here, each element symbol in the above formulas (1), (2), and (3) represents the content (% by mass) of each element, and is set to 0 when the element is not contained.
7. The method for producing a high-strength steel sheet according to claim 6, wherein in the cooling after hot rolling, the average cooling rate of the surface layer and the average cooling rate at the 3/10 thickness position satisfy the formula (4).
Average cooling rate of the surface layer - Average cooling rate at the position of 3/10 of the plate thickness ≥ 10 ° C./s (4)