HIGH-STRENGTH STEEL SHEET AND METHOD FOR MANUFACTURING THE SAME

Abstract

The high-strength steel sheet has a predetermined chemical composition and has a microstructure containing a specific microstructure in a surface layer region and in an inner region. The surface layer region has an average grain size of 6 μm or less. A difference between a hardness of the surface layer region extending from the surface of the steel sheet to the position of one-tenth of the thickness of the steel sheet and a hardness of the inner region is 5% or more and 15% or less of [0.3×tensile strength (MPa)], and the steel sheet has a tensile strength of 980 MPa or more, a uniform elongation of 6% or more, and a ratio of a critical bending radius to a thickness of 1.5 or less.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT/JP2022/011493, filed Mar. 15, 2022, which claims priority to Japanese Patent Application No. 2021-062132, filed Mar. 31, 2021, the disclosures of these applications being incorporated herein by reference in their entireties for all purposes.

FIELD OF THE INVENTION

The present invention relates to a high-strength steel sheet and a method for manufacturing the high-strength steel sheet. In particular, aspects of the present invention relate to a high-strength steel sheet that has a tensile strength of 980 MPa or more, a uniform elongation of 6% or more, and high bendability, and that is suitable as a material for a frame, a suspension component, or the like of a truck or a passenger car, and a method for manufacturing the high-strength steel sheet.

BACKGROUND OF THE INVENTION

Against the background of automobile exhaust emission control aimed at global warming mitigation, there is a need for lighter vehicles. To reduce the vehicle weight, it is effective to reduce the amount of material used for an automotive part by increasing the strength and reducing the thickness of the material used for the automotive part. Thus, high-strength steel sheets have been increasingly used year by year. In particular, high-strength steel sheets with a tensile strength of 980 MPa or more are expected as materials that can dramatically improve the mileage of automobiles through weight reduction.

However, steel sheets with higher tensile strength have lower ductility and press formability. Automotive parts, particularly chassis parts, such as suspension components, need to have complex shapes to ensure rigidity. Thus, materials for automotive parts require high press formability or ductility.

Furthermore, steel sheets with increased tensile strength tend to have a crack during bending. A crack in a bent portion may become a fatigue crack initiation point and may result in parts with lower durability than assumed in the design. Thus, materials for automotive parts and the like require high bendability.

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

PATENT LITERATURE

• PTL 1: Japanese Unexamined Patent Application Publication No. 2012-012701
• PTL 2: International Publication No. WO 2016/010004
• PTL 3: Japanese Unexamined Patent Application Publication No. 2013-117068
• PTL 4: Japanese Unexamined Patent Application Publication No. 2017-115191
• PTL 5: International Publication No. WO 2020/110855
• PTL 6: International Publication No. WO 2020/110843

SUMMARY OF THE INVENTION

However, the related art as described in Patent Literature 1 to Patent Literature 6 has problems as described below.

The techniques described in Patent Literature 1 and Patent Literature 2 cannot achieve a tensile strength of 980 MPa or more. Although it is argued in Patent Literature 1 and Patent Literature 2 that hot-rolled steel sheets have high workability, “elongation” is used as a measure of workability. This “elongation”, which is also referred to as the total elongation (El), represents the elongation at the point in time when a test specimen breaks in a tensile test. In practice, however, necking (constriction) occurs before breakage. Necking locally reduces the thickness of the sheet and results in a defective product during press forming. Thus, high total elongation is not sufficient for high press formability. Furthermore, Patent Literature 1 and Patent Literature 2 do not describe bendability.

Although it is argued that a high-strength steel sheet with high bendability can be manufactured by the techniques described in Patent Literature 3 to Patent Literature 5, attention is given only to a crack formed outside the bend. A crack formed during bending, whether outside or inside the bend, may become a fatigue crack initiation point and decrease the durability of the component. Thus, it cannot be said that the bendability is sufficiently ensured unless cracks inside the bend are reduced.

Although it is argued that a high-strength steel sheet with high bendability can be manufactured by the technique described in Patent Literature 6, attention is given only to a crack formed inside the bend. A crack formed during bending, whether outside or inside the bend, may become a fatigue crack initiation point and decrease the durability of the component. Thus, the performance of the component cannot be ensured unless cracks are reduced both inside and outside the bend.

Thus, a technique for manufacturing a high-strength steel sheet with a high level of tensile strength, press formability, and bendability has not yet been established.

Aspects of the present invention have been made in view of such situations and aim to provide a high-strength steel sheet with high tensile strength, press formability, and bendability, and a method for manufacturing the high-strength steel sheet.

To solve the above problems, the present inventors made hypothetical stress-strain curves of steel sheets with a tensile strength of 980 MPa or more and with various yield stresses and uniform elongations, and performed a press forming simulation of a suspension component using the stress-strain curves. The characteristics of a steel sheet required for high press formability were examined on the basis of the results of the simulation.

As a result, it was found that, having a uniform elongation of 6% or more, a steel sheet with a tensile strength of 980 MPa or more can have the minimum thickness reduction during press forming and is less likely to have press forming defects.

The present inventors have also studied an optimum steel sheet microstructure to achieve a tensile strength of 980 MPa or more and a uniform elongation of 6% or more. As a result, it was found that a microstructure that contains upper bainite as a main phase and contains an appropriate amount of hard second phase containing fresh martensite and/or retained austenite can achieve both a high strength of 980 MPa or more and a uniform elongation of 6% or more.

The term “upper bainite”, as used herein, refers to an aggregate of lath ferrite with an orientation difference of less than 15 degrees and refers to a microstructure with Fe-based carbide and/or retained austenite between lath ferrites (including a microstructure without Fe-based carbide and/or retained austenite between lath ferrites). Unlike lamellar (layered) ferrite or polygonal ferrite in pearlite, lath ferrite has a lath shape and has a relatively high dislocation density inside, so that they can be distinguished with a scanning electron microscope (SEM) or a transmission electron microscope (TEM). In the presence of retained austenite between laths, only a lath ferrite portion is regarded as upper bainite and is distinguished from the retained austenite. The fresh martensite refers to martensite without Fe-based carbide. Fresh martensite and retained austenite have a similar contrast in SEM but can be distinguished by an electron backscatter diffraction patterns (EBSD) method.

Next, the present inventors examined the bendability of a high-strength steel sheet with a tensile strength of 980 MPa or more and a uniform elongation of 6% or more. 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-degree V bending test, and a fracture surface of a flex crack and the microstructure near the crack were observed. On the outside of the bend, a crack fracture surface was a ductile fracture surface, and many voids were observed in the microstructure near the crack, indicating that cracking outside the bend was ductile fracture. On the other hand, on the inside of the bend, a crack fracture surface was a brittle fracture surface, and no voids were observed in the microstructure near the crack. Thus, it was found that cracking inside the bend was brittle fracture due to strong compression. Thus, the improvement of ductility can reduce cracking outside the bend, and the improvement of compression embrittlement resistance characteristics can reduce cracking inside the bend. Thus, it was also found that the microstructure should be controlled in a surface layer region where a flex crack can be formed and in the vicinity thereof.

Aspects of the present invention have been made by further examination based on these findings and are as follows.

[1] A high-strength steel sheet which comprises:

• • a chemical composition containing, in mass %:
  • C: 0.05% to 0.20%,
  • Si: 0.5% to 1.2%,
  • Mn: 1.5% 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,
  • B: 0.0005% to 0.010%, and the remainder being Fe and incidental impurities;
  • a microstructure in a surface layer region extending from a surface of the steel sheet to a position of one-tenth of a thickness of the steel sheet containing 80% by area or more of upper bainite and 2% by area or more of fresh martensite and/or retained austenite in total; and
  • a microstructure in an inner region extending from the position of one-tenth to a position of three-tenths of the thickness of the steel sheet containing 70% by area or more of upper bainite and 3% by area or more of fresh martensite and/or retained austenite in total,
  • wherein:
  • the surface layer region extending from the surface of the steel sheet to the position of one-tenth of the thickness of the steel sheet has an average grain size of 6 μm or less;
  • the steel sheet has a tensile strength of 980 MPa or more, a uniform elongation of 6% or more, and a ratio R/t of a critical bending radius R to a thickness t of 1.5 or less; and
  • a difference (HV2−HV1) between a hardness (HV1) of the surface layer region extending from the surface of the steel sheet to the position of one-tenth of the thickness of the steel sheet and a hardness (HV2) of the inner region extending from the position of one-tenth to the position of three-tenths of the thickness of the steel sheet is 5% or more and 15% or less of [0.3×tensile strength (MPa)].

[2] The high-strength steel sheet according to [1], wherein the chemical composition further contains, in mass %, at least one of

• • Cr: 1.0% or less and
  • Mo: 1.0% or less.

[3] The high-strength steel sheet according to [1] or [2], wherein the chemical composition further contains, in mass %, at least one of

• • Cu: 2.0% or less,
  • Ni: 2.0% or less,
  • Ti: 0.3% or less,
  • Nb: 0.3% or less, and
  • V: 0.3% or less.

[4] The high-strength steel sheet according to any one of [1] to [3], wherein the chemical composition further contains, in mass %,

Sb: 0.005% to 0.020%.

[5] The high-strength steel sheet according to any one of [1] to [4], wherein the chemical composition further contains, in mass %, at least one of

• • Ca: 0.01% or less,
  • Mg: 0.01% or less, and
  • REM: 0.01% or less.

[6] A method for manufacturing the high-strength steel sheet according to any one of [1] to [5], which comprises:

• • heating a steel material having the chemical composition to a heating temperature of 1150° C. or more;
  • then hot-rolling including rough rolling and finish rolling the steel material into a hot-rolled steel sheet, the finish rolling being performed under conditions of a finishing temperature: (RC2−50° C.) or more and (RC2+120° C.) or less and a total rolling reduction of 25% or more and 80% or less at a temperature of RC1 or less;
  • cooling the hot-rolled steel sheet under conditions of a time from completion of the hot rolling to start of cooling: 2.0 seconds or less, an average cooling rate at the position of three-tenths of the thickness of the steel sheet: 15° C./s or more, and a cooling stop temperature: Trs or more and (Trs+250° C.) or less;
  • coiling the hot-rolled steel sheet after the cooling at a coiling temperature: Trs or more and (Trs+250° C.) or less; and
  • cooling the hot-rolled steel sheet to 100° C. or less at an average cooling rate of 20° C./s or less,
  • wherein RC1, RC2, and Trs are represented by the following formulae (1), (2), and (3), respectively,

RC1(° C.)=900+100×C+100×N+10×Mn+700×Ti+5000×B+10×Cr+50×Mo+2000×Nb+150×V  (1)

RC2(° C.)=750+100×C+100×N+10×Mn+350×Ti+5000×B+10×Cr+50×Mo+1000×Nb+150×V  (2)

Trs(° C.)=500−450×C—35×Mn−15×Cr—10×Ni−20×Mo  (3)

• • where each element symbol in the formulae (1), (2), and (3) denotes a corresponding element content (% by mass) and is 0 in the absence of the element.

[7] The method for manufacturing the high-strength steel sheet according to [6], wherein in the cooling after completion of the hot rolling, an average cooling rate of the surface layer and the average cooling rate at the position of three-tenths of the thickness of the steel sheet satisfy the formula (4):

Average cooling rate of surface layer−average cooling rate at position of three-tenths of thickness of steel sheet≥10° C./s  (4).

Aspects of the present invention can provide a high-strength steel sheet with a tensile strength of 980 MPa or more, high press formability, and high bendability. A high-strength steel sheet according to aspects of the present invention has high press formability in spite of high tensile strength and can be press-formed without a forming defect, such as necking or cracking. The application of a high-strength steel sheet according to aspects of the present invention to a member of a truck or a passenger car can reduce the amount of steel material used while ensuring safety, and reduce the weight of the automobile body, thus contributing to reducing the effects on the environment.

The phrase “high press formability”, as used herein, refers to a uniform elongation of 6% or more. The phrase “high bendability” means that the ratio R/t of the critical bending radius R at which no crack with a depth of 50 μm or more is formed both outside and inside the bend in a 90-degree V bending test to the thickness t of a steel sheet is 1.5 or less.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Embodiments of the present invention are specifically described below. The following description shows an example of a preferred embodiment of the present invention, and the present invention is not limited to this.

[Chemical Composition]

First, the reasons for limiting the chemical composition of a high-strength steel sheet according to aspects of the present invention are described below. Unless otherwise specified, “%” as a unit of the content refers to “% by mass”.

• • C: 0.05% to 0.20%

C is an element with the effect of improving the strength of steel. C improves hardenability, thereby promotes the formation of bainite, and contributes to increase the strength. C increases the strength of martensite and also contributes to increase the strength. To achieve a tensile strength of 980 MPa or more, the C content should be 0.05% or more. Thus, the C content is 0.05% or more, preferably 0.06% or more. On the other hand, a C content of more than 0.20% results in martensite with excessively increased strength, a larger difference in strength between upper bainite serving as a main phase and fresh martensite and/or retained austenite, and consequently lower uniform elongation. Thus, the C content is 0.20% or less, preferably 0.18% or less.

• • Si: 0.5% to 1.2%

Si has the effect of reducing the formation of Fe-based carbide and reduces the precipitation of cementite during upper bainite transformation. This distributes C to non-transformed austenite, and cooling after coiling in a hot-rolling process transforms the non-transformed austenite into fresh martensite and/or retained austenite, thereby forming the desired fresh martensite and/or retained austenite. These effects require a Si content of 0.5% or more, preferably 0.6% or more. On the other hand, a Si content of more than 1.2% may result in the formation of fresh martensite and/or retained austenite in an amount larger than the desired area ratio, consequently an undesired area ratio of upper bainite, and lower bendability. Thus, the Si content is 1.2% or less, preferably 1.1% or less.

• • Mn: 1.5% to 4.0%

Mn stabilizes austenite and contributes to the formation of fresh martensite and/or retained austenite. Such effects require a Mn content of 1.5% or more. Thus, the Mn content is 1.5% or more, preferably 1.7% or more. On the other hand, a Mn content of more than 4.0% results in excessive formation of fresh martensite and/or retained austenite, consequently an undesired area ratio of upper bainite, and lower bendability. Thus, the Mn content is 4.0% or less, preferably 3.8% or less.

• • P: 0.10% or less

P is an element that contributes to an increase in the strength of steel through solid solution. However, P is also an element that segregates at an austenite grain boundary during hot rolling and thereby causes slab cracking during the hot rolling. P also segregates at a grain boundary and reduces the uniform elongation. Thus, the P content is preferably minimized and may be 0.10% or less. Thus, the P content is 0.10% or less. Although the lower limit is not particularly limited, the P content is preferably 0.0002% or more because a P content of less than 0.0002% causes a decrease in production efficiency.

• • S: 0.03% or less

S binds to Ti or Mn and forms a coarse sulfide, which accelerates the formation of a void and reduces the uniform elongation. Thus, the S content is preferably minimized and may be 0.03% or less. Thus, the S content is 0.03% or less. Although the lower limit is not particularly limited, the S content is preferably 0.0002% or more because a S content of less than 0.0002% causes a decrease in production efficiency.

• • Al: 0.001% to 2.0%

Al is an element that acts as a deoxidizing agent and is effective in improving the cleanliness of steel. This effect is insufficient at an Al content of less than 0.001%. Thus, the Al content is 0.001% or more, preferably 0.005% or more, more preferably 0.010% or more. Like Si, Al is effective in reducing the formation of Fe-based carbide and reduces the precipitation of cementite during upper bainite transformation. This contributes to the formation of fresh martensite and/or retained austenite during cooling after coiling. On the other hand, an excessively high Al content results in an increased number of oxide inclusions and lower uniform elongation. Thus, the Al content is 2.0% or less, preferably 1.0% or less, more preferably 0.1% or less.

• • N: 0.01% or less

N binds to a nitride-forming element, thereby precipitates as a nitride, and generally contributes to grain refinement. However, N binds to Ti at high temperatures and forms a coarse nitride. Thus, a N content of more than 0.01% results in lower uniform elongation. Thus, the N content is 0.01% or less. Although the lower limit is not particularly limited, the N content is preferably 0.0002% or more because a N content of less than 0.0002% causes a decrease in production efficiency.

• • O: 0.01% or less

O generates an oxide and reduces formability. Thus, the O content should be reduced. In particular, this tendency is remarkable at an O content of more than 0.01%. Thus, the O content is 0.01% or less, preferably 0.005%, more preferably 0.003%. Although the lower limit is not particularly specified, the O content is preferably 0.00005% or more because the production efficiency may be significantly lowered at an O content of less than 0.00005%.

• • B: 0.0005% to 0.010%

B is an element that segregates in a prior-austenite grain boundary, reduces the formation of ferrite, thereby promotes the formation of upper bainite, and contributes to improving the strength of a steel sheet. These effects require a B content of 0.0005% or more. Thus, the B content is 0.0005% or more, preferably 0.0006% or more, more preferably 0.0007% or more. On the other hand, these effects are saturated at a B content of more than 0.010%. Thus, the B content is 0.010% or less, preferably 0.009% or less, more preferably 0.008% or less.

The remainder is composed of Fe and incidental impurities. Examples of the incidental impurities include Zr, Co, Sn, Zn, and W. When the chemical composition contains at least one of Zr, Co, Sn, Zn, and W as incidental impurities, the total amount of the element(s) is preferably 0.5% or less.

The chemical composition of a high-strength steel sheet according to aspects of the present invention may optionally contain at least one of the following elements.

• • Cr: 1.0% or less

Cr is a carbide-forming element and has the effect of reducing driving force for bainite transformation by segregating at an interface between upper bainite and non-transformed austenite during upper bainite transformation before coiling, thereby stopping the upper bainite transformation. Non-transformed austenite remained due to the stopped upper bainite transformation becomes fresh martensite and/or retained austenite by cooling after coiling. Thus, Cr, if added, also contributes to the formation of a desired area ratio of fresh martensite and/or retained austenite. This effect is preferably produced at a Cr content of 0.1% or more. However, a Cr content of more than 1.0% results in excessive formation of fresh martensite and/or retained austenite, consequently an undesired area ratio of upper bainite, and lower bendability. When Cr is added, the Cr content is 1.0% or less, preferably 0.9% or less, more preferably 0.8% or less.

• • Mo: 1.0% or less

Mo promotes the formation of bainite through the improvement of hardenability and contributes to improving the strength of a steel sheet. Like Cr, Mo is a carbide-forming element, segregates at an interface between upper bainite and non-transformed austenite during upper bainite transformation before coiling, thereby reduces transformation driving force for bainite, and contributes to the formation of fresh martensite and/or retained austenite after coiling and cooling. However, a Mo content of more than 1.0% results in excessive formation of fresh martensite and/or retained austenite, consequently an undesired area ratio of upper bainite, and lower uniform elongation. This effect is preferably produced at a Mo content of 0.1% or more. Thus, when Mo is added, the Mo content is 1.0% or less, preferably 0.9% or less, more preferably 0.8% or less.

The chemical composition of a high-strength steel sheet according to aspects of the present invention may optionally contain at least one of the following elements.

• • Cu: 2.0% or less

Cu is an element that contributes to an increase in the strength of steel through solid solution. Cu promotes the formation of bainite through the improvement of hardenability and contributes to improving the strength. This effect is preferably produced at a Cu content of 0.01% or more. However, a Cu content of more than 2.0% results in a high-strength steel sheet with lower surface quality and bendability. Thus, when Cu is added, the Cu content is 2.0% or less, preferably 1.9% or less, more preferably 1.8% or less.

• • Ni: 2.0% or less

Ni is an element that contributes to an increase in the strength of steel through solid solution. Ni promotes the formation of bainite through the improvement of hardenability and contributes to improving the strength. This effect is preferably produced at a Ni content of 0.01% or more. However, a Ni content of more than 2.0% results in an excessive increase of fresh martensite and/or retained austenite, consequently an undesired area ratio of upper bainite, and a high-strength steel sheet with lower ductility. Thus, when Ni is added, the Ni content is 2.0% or less, preferably 1.9% or less, more preferably 1.8% or less.

• • Ti: 0.3% or less

Ti is an element with the effect of improving the strength of a steel sheet by precipitation strengthening or solid-solution strengthening. Ti forms a nitride in a high-temperature region of austenite. This reduces the precipitation of BN and results in B in a solid solution state. Thus, Ti, if added, also contributes to ensuring hardenability necessary for the formation of upper bainite and improves the strength. This effect is preferably produced at a Ti content of 0.01% or more. However, a Ti content of more than 0.3% results in a large amount of Ti nitride and lower uniform elongation. Thus, when Ti is added, the Ti content is 0.3% or less, preferably 0.28% or less, more preferably 0.25% or less.

• • Nb: 0.3% or less

Nb is an element with the effect of improving the strength of a steel sheet by precipitation strengthening or solid-solution strengthening. Like Ti, Nb increases the recrystallization temperature of austenite during hot rolling, thereby enables rolling in an austenite non-recrystallization temperature range, and contributes to decreasing the grain size of upper bainite and increasing the area ratio of fresh martensite and/or retained austenite. Like Cr, Nb is a carbide-forming element and is an element that segregates at an interface between upper bainite and non-transformed austenite during upper bainite transformation before coiling and thereby has the effect of reducing transformation driving force for bainite and stopping the upper bainite transformation while leaving the non-transformed austenite. The non-transformed austenite becomes fresh martensite and/or retained austenite by cooling thereafter. Thus, Nb, if added, also contributes to the formation of a desired area ratio of fresh martensite and/or retained austenite. This effect is preferably produced at a Nb content of 0.01% or more. However, a Nb content of more than 0.3% results in an excessive increase of fresh martensite and/or retained austenite, consequently an undesired area ratio of upper bainite, and lower uniform elongation. Thus, when Nb is added, the Nb content is 0.3% or less, preferably 0.28% or less, more preferably 0.25% or less.

• • V: 0.3% or less

V is an element with the effect of improving the strength of a steel sheet by precipitation strengthening and solid-solution strengthening. Like Ti, V increases the recrystallization temperature of austenite during hot rolling, thereby enables rolling in an austenite non-recrystallization temperature range, and contributes to decreasing the grain size of upper bainite. Like Cr, V is a carbide-forming element and is an element that segregates at an interface between upper bainite and non-transformed austenite during upper bainite transformation before coiling and thereby has the effect of reducing transformation driving force for bainite and stopping the upper bainite transformation while leaving the non-transformed austenite. The non-transformed austenite becomes fresh martensite and/or retained austenite by cooling thereafter. Thus, V, if added, also contributes to the formation of a desired area ratio of fresh martensite and/or retained austenite. This effect is preferably produced at a V content of 0.01% or more. However, a V content of more than 0.3% results in an excessive increase of fresh martensite and/or retained austenite, consequently an undesired area ratio of upper bainite, and lower uniform elongation. Thus, when V is added, the V content is 0.3% or less, preferably 0.28% or less, more preferably 0.25% or less.

The chemical composition of a high-strength steel sheet according to aspects of the present invention may further optionally contain the following elements.

• • Sb: 0.005% to 0.020%

Sb is an element with the effect of reducing nitriding of the surface of a steel material (slab) when the steel material is heated. The addition of Sb can reduce the precipitation of BN in a surface layer portion of a steel material. As a result, remaining solid solution B contributes to ensuring hardenability necessary for the formation of bainite and thereby improving the strength of a 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, to produce the effects. On the other hand, an Sb content of more than 0.020% results in steel with lower toughness and may cause slab cracking and hot-rolling cracking. Thus, when Sb is added, the Sb content is 0.020% or less, preferably 0.019% or less, more preferably 0.018% or less.

The chemical composition of a high-strength steel sheet according to aspects of the present invention may optionally contain at least one of the following elements. The following elements contribute to further improvement of press formability or other characteristics.

• • Ca: 0.01% or less

Ca controls the shape of an oxide or sulfide inclusion and contributes to reducing cracking of a sheared end face of a steel sheet and further improving the bendability of a steel sheet. This effect is preferably produced at a Ca content of 0.001% or more. However, a Ca content of more than 0.01% may result in an increase of a Ca-based inclusion and steel with lower cleanliness and rather result in sheared end face cracking or bending cracking. Thus, when Ca is added, the Ca content is 0.01% or less.

• • Mg: 0.01% or less

Like Ca, Mg controls the shape of an oxide or sulfide inclusion and contributes to reducing cracking of a sheared end face of a steel sheet and further improving the bendability of a steel sheet. This effect is preferably produced at a Mg content of 0.001% or more. However, a Mg content of more than 0.01% may result in steel with lower cleanliness and rather result in sheared end face cracking or bending cracking. Thus, when Mg is added, the Mg content is 0.01% or less.

• • REM: 0.01% or less

Like Ca, a rare-earth metal (REM) controls the shape of an oxide or sulfide inclusion and contributes to reducing cracking of a sheared end face of a steel sheet and further improving the bendability of a steel sheet. This effect is preferably produced at a REM content of 0.001% or more. However, a REM content of more than 0.01% may result in steel with lower cleanliness and rather result in sheared end face cracking or bending cracking. Thus, when REM is added, the REM content is 0.01% or less.

[Microstructure]

Next, the reasons for limiting the microstructure of a high-strength steel sheet according to aspects of the present invention are described below.

A high-strength steel sheet according to aspects of the present invention has a microstructure containing 80% by area or more of upper bainite and 2% by area or more of fresh martensite and/or retained austenite in total in a surface layer region extending from a surface of the steel sheet to a position of one-tenth of the thickness of the steel sheet and 70% by area or more of upper bainite and 3% by area or more of fresh martensite and/or retained austenite in total in an inner region extending from a position of one-tenth to a position of three-tenths of the thickness of the steel sheet. The surface layer region extending from the surface of the steel sheet to the position of one-tenth of the thickness of the steel sheet has an average grain size of 6 μm or less. A difference (HV2−HV1) between a hardness (HV1) of the surface layer region extending from the surface of the steel sheet to the position of one-tenth of the thickness of the steel sheet and a hardness (HV2) of the inner region extending from the position of one-tenth to the position of three-tenths of the thickness of the steel sheet is 5% or more and 15% or less of [0.3× tensile strength (MPa)].

Upper bainite: 80% or more and fresh martensite and/or retained austenite: 2% by area or more in total in surface layer region extending from surface of steel sheet to position of one-tenth of thickness of steel sheet

In a high-strength steel sheet according to aspects of the present invention, hard fresh martensite and/or retained austenite can be finely dispersed in soft upper bainite to improve ductility and reduce cracking outside the bend. To produce this effect, in a surface layer, the area fraction of upper bainite is 80% or more, and the area ratio of fresh martensite and/or retained austenite is 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, the bendability may be lowered when the total area ratio of fresh martensite and/or retained austenite is 20% or more. The total area ratio of fresh martensite and/or retained austenite is therefore preferably 20% or less, more preferably 18% or less, still more preferably 15% or less.

In a surface layer region of the steel sheet, due to a high cooling rate, bainite transformation proceeds rapidly, and carbon concentrating for forming fresh martensite and/or retained austenite is lower than that in the inside of the steel sheet. Lower carbon concentrating reduces martensite transformation. This results in a smaller area ratio of fresh martensite and/or retained austenite in the surface layer region than in the inside of the steel sheet.

70% by area or more of upper bainite and 3% by area or more of fresh martensite and/or retained austenite in total in inner region extending from position of one-tenth to position of three-tenths of thickness of steel sheet

In accordance with aspects of the present invention, an inner region extending from a position of one-tenth to a position of three-tenths of the thickness of a steel sheet contains upper bainite as a main phase. When the upper bainite has an area ratio of less than 70%, a tensile strength of 980 MPa or more and a uniform elongation of 6% or more cannot be achieved. Thus, the upper bainite has an area ratio of 70% or more, preferably 80% or more. In accordance with aspects of the present invention, an inner region extending from a position of one-tenth to a position of three-tenths of the thickness of a steel sheet contains fresh martensite and/or retained austenite. Fresh martensite has the effect of promoting work hardening, delaying the onset of plastic instability, and thereby improving the uniform elongation. Retained austenite can improve the uniform elongation by the transformation induced plasticity (TRIP) effect. To produce these effects, the total area ratio of fresh martensite and/or retained austenite is 3% or more, preferably 4% or more. In accordance with aspects of the present invention, in the microstructure extending from a position of three-tenths of the thickness to the vicinity of the center of the thickness of a steel sheet, the area ratio of upper bainite is preferably 60% or more from the perspective of ductility, although the effect on bendability is small. 40% or less of fresh martensite/tempered martensite/retained austenite and the like may be contained due to Mn segregation at half the thickness of a steel sheet.

Average grain size in surface layer region extending from surface of steel sheet to position of one-tenth of thickness of steel sheet: 6 μm or less

Cracking inside the bend is brittle fracture due to strong compression. Thus, compression embrittlement resistance characteristics can be improved to reduce cracking inside the bend. Grain refinement can reduce compression embrittlement. To produce this effect, the average grain size in a surface layer region is 6 μm or less, preferably 5 μm or less. Although compression embrittlement resistance is improved as the average grain size decreases, an excessively small average grain size may result in higher strength and lower elongation and cracking in outer bending. Thus, the average grain size in the surface layer region is preferably 2 μm or more.

Thus, a tensile strength of 980 MPa or more, a uniform elongation of 6% or more, and high bendability can be achieved only by combining the uniform elongation improving effect of fresh martensite and/or retained austenite with the flex cracking suppressive effect by controlling a surface layer microstructure.

Difference (HV2−HV1) between hardness (HV1) of surface layer region extending from surface of steel sheet to a position of one-tenth of thickness of steel sheet and hardness (HV2) of inner region extending from a position of one-tenth to a position of three-tenths of thickness of steel sheet is 5% or more and 15% or less of [0.3×tensile strength (MPa)]

In a high-strength steel sheet according to aspects of the present invention, a soft surface layer reduces cracking outside the bend, and a hard inner portion adjacent to the surface layer reduces the growth of a flex crack in the thickness direction of the steel sheet. To produce the effects of reducing the occurrence and growth of a flex crack, the difference (HV2−HV1) between the hardness (HV1) of the surface layer region and the hardness (HV2) of the inner region is 5% or more, preferably 6% or more, more preferably 7% or more, with respect to 0.3×tensile strength (MPa). On the other hand, a large difference between the hardness of the surface layer region and the hardness of the inner region results in strain mismatch between the surface layer and the inner portion in a tensile test, thus failing to achieve target tensile properties. Thus, the difference between the hardness of the surface layer region and the hardness of the inner region is 15% or less, preferably 14% or less, more preferably 13% or less, with respect to 0.3×tensile strength (MPa). The above effects can be produced by controlling the cooling rate on the surface of the steel sheet and in the inside of the steel sheet.

The microstructure may further contain a microstructure other than upper bainite, fresh martensite, and retained austenite (hereinafter referred to as “another microstructure”). From the perspective of enhancing the effect of microstructure control, another microstructure preferably has a total area ratio of 3% or less. In other words, the upper bainite, fresh martensite, and retained austenite in the microstructure preferably have a total area ratio of 97% or more. Examples of the other microstructure include cementite, polygonal ferrite, pearlite, tempered martensite, and lower bainite.

[Mechanical Properties]

A high-strength steel sheet according to aspects of the present invention has a tensile strength of 980 MPa or more, a uniform elongation of 6% or more, and an R/t (the ratio of the critical bending radius R at which no crack with a depth of 50 μm or more is formed both outside and inside the bend to the thickness t of the steel sheet) of 1.5 or less. Thus, a high-strength steel sheet according to aspects of the present invention has high press formability in spite of its high tensile strength, can be press-formed without a forming defect, such as necking or cracking, and can ensure the durability of a component without forming a large crack outside and inside a bent portion. Thus, a high-strength steel sheet according to aspects of the present invention applied to a member of a truck or a passenger car can ensure safety.

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

[Manufacturing Method]

Next, a method for manufacturing a high-strength steel sheet according to an embodiment of the present invention is described. Unless otherwise specified, the temperature in the following description represents the surface temperature of an object (a steel material or a steel sheet).

A high-strength steel sheet according to aspects of the present invention can be manufactured by sequentially subjecting a steel material to the following treatments (1) to (5). Each of the steps is described below.

• • (1) Heating
  • (2) Hot rolling
  • (3) Cooling (first cooling)
  • (4) Coiling
  • (5) Cooling (second cooling)

The steel material may be any material with the chemical composition described above. The chemical composition of the final high-strength steel sheet is the same as the chemical composition of the steel material used. The steel material can be a steel slab, for example. The steel material may be manufactured by any method. For example, a molten steel with the chemical composition described above can be obtained by steelmaking by a known method using a converter or the like, and a steel material can be manufactured by a casting method, such as continuous casting. It is also possible to use a method other than the continuous casting method, such as an ingot casting and blooming method. Scrap may also be used as a raw material. The steel material manufactured by the continuous casting method or the like may be directly subjected to the next heating step or may be cooled and subjected as a hot piece or a cold piece to the heating step.

(1) Heating

First, the steel material is heated to a heating temperature of 1150° C. or more. In general, carbonitride-forming elements, such as Ti, are mostly present as coarse carbonitrides in steel materials. The presence of the coarse and nonuniform precipitates causes degradation of various characteristics generally required for high-strength steel sheets for truck and passenger car components (for example, sheared edge cracking resistance, bendability, burring formability, and the like). Thus, the steel material should be heated before hot rolling to dissolve coarse precipitates. More specifically, the heating temperature of the steel material should be 1150° C. or more to sufficiently dissolve coarse precipitates. On the other hand, an excessively high heating temperature of the steel material results in the occurrence of a slab defect and a decrease in yield due to scaling off. Thus, from the perspective of improving the yield, the heating temperature of the steel material is preferably 1350° C. or less. The lower limit of the heating temperature of the steel material is more preferably 1180° C. or more, still more preferably 1200° C. or more. The upper limit of the heating temperature of the steel material is more preferably 1300° C. or less, still more preferably 1280° C. or less.

In the heating, from the perspective of uniformizing the temperature of the steel material, the temperature of the steel material is preferably increased to the heating temperature and is then maintained at the heating temperature. The holding time at the heating temperature is preferably, but not limited to, 1800 seconds or more from the perspective of improving the temperature uniformity of the steel material. On the other hand, a holding time of more than 10,000 seconds results in an increased amount of scales. This often causes scale biting or the like in the subsequent hot rolling and decreases the yield due to defective surface flaws. Thus, the holding time is preferably 10,000 seconds or less, more preferably 8000 seconds or less.

(2) Hot Rolling

The heated steel material is then hot-rolled to form a hot-rolled steel sheet. The hot rolling may include rough rolling and finish rolling. The rough rolling may be performed under any conditions. After the rough rolling, descaling is preferably performed to remove surface scales before the finish rolling. In the finish rolling, descaling may be performed between stands.

Next, in accordance with aspects of the present invention, in the finish rolling, when the temperature RC1 and the temperature RC2 are defined by the following formulae (1) and (2), the total rolling reduction at a temperature of RC1 or less is 25% or more and 80% or less, and the finishing temperature is (RC2−50° C.) or more and (RC2+120° C.) or less.

RC1 denotes the austenite 50% recrystallization temperature estimated from the chemical composition, and RC2 denotes the austenite recrystallization lower limit temperature estimated from the chemical composition. When the total rolling reduction at RC1 or less is less than 25%, the average grain size increases, and high bendability cannot be achieved. On the other hand, when the total rolling reduction at a temperature of RC1 or less is more than 80%, austenite has a high dislocation density, a bainite microstructure transformed from the austenite with a high dislocation density has low ductility, and the uniform elongation of 6% or more cannot be achieved. Thus, the total rolling reduction at a temperature of RC1 or less is 25% or more and 80% or less.

The hot rolling is performed under the condition of finishing temperature: (RC2−50° C.) or more and (RC2+120° C.) or less. At a finishing temperature below (RC2−50° C.), bainite transformation occurs from austenite with a high dislocation density. Upper bainite transformed from austenite with a high dislocation density has a high dislocation density and low ductility, resulting in a decrease in the uniform elongation. The uniform elongation also decreases when the finish rolling temperature is low and the rolling is performed at a two-phase temperature of ferrite+austenite. Thus, the finishing temperature is (RC2−50° C.) or more. On the other hand, at a finishing temperature above (RC2+120° C.), austenite grains coarsen, and the average grain size of upper bainite increases, resulting in a decrease in the strength. Furthermore, fresh martensite and/or retained austenite also coarsens, and the uniform elongation decreases. Thus, the finishing temperature is (RC2+120° C.) or less. RC1 and RC2 are defined by the following formulae (1) and (2):

RC1(° C.)=900+100×C+100×N+10×Mn+700×Ti+5000×B+10×Cr+50×Mo+2000×Nb+150×V  (1)

RC2(° C.)=750+100×C+100×N+10×Mn+350×Ti+5000×B+10×Cr+50×Mo+1000×Nb+150×V  (2)

• • wherein each element symbol in the formulae (1) and (2) denotes a corresponding element content (% by mass) and is 0 in the absence of the element.

(3) Cooling (First Cooling)

Next, the hot-rolled steel sheet thus manufactured is cooled (first cooling). The time from completion of the hot rolling (completion of the finish rolling) to the start of the cooling (cooling start time) is 2.0 seconds or less. A cooling start time of more than 2.0 seconds results in the growth of austenite grains, thus failing to achieve a tensile strength of 980 MPa or more. The cooling start time is preferably 1.5 seconds or less.

The average cooling rate at a position of three-tenths of the thickness of the steel sheet is 15° C./s or more. In accordance with aspects of the present invention, the surface layer is cooled more rapidly than the inside to form a microstructure that differs between the surface layer and the inside. The rapid cooling of the surface layer advances the bainite transformation start in the surface layer and forms a smaller amount of martensite and retained austenite due to carbon concentrating in the surface layer than in the inside. When the average cooling rate of the cooling is less than 15° C./s, the surface layer is not sufficiently rapidly cooled, and the surface layer microstructure containing 80% by area or more of upper bainite and 2% by area or more of fresh martensite and/or retained austenite in total cannot be formed. Thus, the average cooling rate is 15° C./s or more, preferably 20° C./s or more, more preferably 50° C./s or more. On the other hand, although the average cooling rate may have any upper limit, an excessively high average cooling rate makes it difficult to control the cooling stop temperature. Thus, the average cooling rate is preferably 200° C./s or less. The average cooling rate is specified on the basis of the average cooling rate at the surface of the steel sheet.

In accordance with aspects of the present invention, when the average cooling rate of the surface layer−the average cooling rate at a position of three-tenths of the thickness of the steel sheet is 10° C./s or more, the formation of martensite or retained austenite due to carbon concentrating in the surface layer is less than that at the position of three-tenths of the thickness of the steel sheet. Consequently, a soft surface layer microstructure can be formed. On the other hand, in the inside of the steel sheet, the cooling rate is lower than that in the surface layer, and the progress of the bainite transformation is slower than that in the surface layer, so that the formation of martensite or retained austenite due to carbon concentrating is more than in the surface layer, and an internal microstructure with high hardness can be formed. Thus, a difference in hardness between the surface layer and the inside can be realized. When the average cooling rate of the surface layer−the average cooling rate at a position of three-tenths of the thickness of the steel sheet is less than 10° C./s, the above effects are not recognized. Thus, the average cooling rate of the surface layer−the average cooling rate at the position of three-tenths of the thickness of the steel sheet is 10° C./s or more. The average cooling rate is determined 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 position of three-tenths of the thickness of the steel sheet is determined by calculating the temperature distribution in a cross section of the steel sheet by heat transfer analysis and correcting the result by the actual temperature of the surface of the steel sheet.

In the cooling, forced cooling may be performed at the average cooling rate. The cooling method is preferably, but not limited to, water cooling, for example.

The cooling stop temperature is Trs or more and (Trs+250° C.) or less. At a cooling stop temperature below Trs, the microstructure becomes tempered martensite or lower bainite. Both tempered martensite and lower bainite have a high-strength microstructure but have significantly low uniform elongation. Thus, the cooling stop temperature is Trs or more. On the other hand, at a cooling stop temperature above (Trs+250° C.), ferrite is formed, and the tensile strength of 980 MPa cannot be achieved. Thus, the cooling stop temperature is (Trs+250° C.) or less.

Trs is defined by the formula (3):

Trs(° C.)=500−450×C—35×Mn−15×Cr—10×Ni−20×Mo  (3)

• • wherein each element symbol in the formula (3) denotes a corresponding element content (% by mass) and is 0 in the absence of the element.

(4) Coiling

Next, the hot-rolled steel sheet after the cooling is coiled under the condition of a coiling temperature: Trs or more and (Trs+250° C.) or less. When the coiling temperature is below Trs, martensite transformation or lower bainite transformation proceeds after the coiling, and the desired fresh martensite and/or retained austenite cannot be formed. Thus, the coiling temperature is Trs or more. On the other hand, at a coiling temperature above (Trs+250° C.), ferrite is formed, and the tensile strength of 980 MPa cannot be achieved. Thus, the coiling temperature is (Trs+250° C.) or less.

(5) Cooling (Second Cooling)

After the coiling, cooling to 100° C. or less is further performed at an average cooling rate of 20° C./s or less (second cooling). The average cooling rate has an influence on the formation of fresh martensite and/or retained austenite. At an average cooling rate of more than 20° C./s, most of the non-transformed austenite undergoes martensite transformation, the desired retained austenite cannot be formed, and the uniform elongation decreases. Thus, the average cooling rate is 20° C./s or less, preferably 2° C./s or less, more preferably 0.02° C./s or less. On the other hand, the lower limit of the average cooling rate is preferably, but not limited to, 0.0001° C./s or more.

The cooling can be performed to any temperature of 100° C. or less, preferably in the range of approximately 10° C. to 30° C. (for example, room temperature). The cooling may be performed in any form, for example, in the state of a coil.

A high-strength steel sheet according to aspects of the present invention can be manufactured in accordance with the procedure described above. The coiling and the subsequent cooling may be followed by routine procedures. For example, temper rolling may be performed, or pickling may be performed to remove scales formed on the surface.

EXAMPLES

A molten steel with the composition shown in Table 1 was obtained by steelmaking in a converter, and a steel slab as a steel material was manufactured by a continuous casting method. The steel material was heated to the heating temperature shown in Table 2, and the steel material after the heating was then subjected to hot rolling including rough rolling and finish rolling to manufacture a hot-rolled steel sheet. The finishing temperature in the hot rolling was shown in Table 2.

Next, the 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 hot-rolled steel sheet after the cooling 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 manufacture a high-strength steel sheet. The cooling was followed by skin pass rolling and pickling as post-treatment. The pickling was performed at a temperature of 85° C. using aqueous hydrochloric acid with a concentration of 10% by mass.

A test specimen was taken from the high-strength steel sheet, and the microstructure, surface roughness, and mechanical properties were evaluated in accordance with the following procedures.

(Microstructure)

A test specimen for microstructure observation was taken from the high-strength steel sheet such that a thickness cross section parallel to the rolling direction was an observation surface. The surface of the test specimen was polished and was corroded with a corrosive liquid (3% by volume nital solution) to expose the microstructure.

A surface layer region extending from the surface of the test specimen to a position of one-tenth of the thickness of the steel sheet and an inner region extending from the position of one-tenth to a position of three-tenths of the thickness of the steel sheet were then photographed with a scanning electron microscope (SEM) at a magnification of 5000 times in 10 visual fields to acquire a SEM image of the microstructure. The SEM image was analyzed by image processing to quantify the area ratios of upper bainite (UB), polygonal ferrite (F), and tempered martensite (TM). Fresh martensite (M) and retained austenite (γ) were difficult to distinguish with the SEM and were therefore identified by an electron backscatter diffraction patterns (EBSD) method, and the area ratio and the average grain size of each were determined. Table 3 shows the area ratio of each microstructure and the average grain size of the surface layer microstructure thus measured. Table 3 also shows the total area ratio (M+γ) of fresh martensite and retained austenite.

(Measurement of Hardness)

A sample for hardness measurement was taken from the high-strength steel sheet such that a thickness cross section parallel to the rolling direction was a hardness measurement cross section. The hardness was measured in a surface layer region extending from the surface of the steel sheet to a position of one-tenth of the thickness of the steel sheet and in an inner region extending from the position of one-tenth to a position of three-tenths of the thickness of the steel sheet. The hardness of the surface layer region extending from the surface of the steel sheet to the position of one-tenth of the thickness of the steel sheet was measured at an indentation distance of 250 μm at a position away from the surface by 50 μm. The hardness of the inner region extending from the position of one-tenth to the position of three-tenths of the thickness of the steel sheet was measured at an indentation distance of 250 μm at a position of one-fifth of the thickness of the steel sheet. The hardness measurement conditions included a load of 100 g, a holding time of 10 seconds, and averaging at five points of measurement.

(Tensile Test)

A JIS No. 5 test specimen (gauge length (GL): 50 mm) was taken from the high-strength steel sheet such that the tensile direction was perpendicular to the rolling direction. The test specimen was subjected to a tensile test in accordance with JIS Z 2241 to determine the yield strength (yield point, YP), tensile strength (TS), yield ratio (YR), total elongation (El), and uniform elongation (u-E1). The tensile test of each high-strength steel sheet was performed twice, and the average of the measured values is shown in Table 3 as the mechanical characteristics of the high-strength steel sheet. In accordance with aspects of the present invention, a TS of 980 MPa or more was rated as high strength. A uniform elongation of 6% or more was rated as high press formability.

(90-Degree V Bending Test)

A bending test was performed in accordance with JIS Z 2248 (2014) (a V block 90-degree V bending test) on a test specimen with a rectangular shape of 100 mm×35 mm, which was taken from the hot-rolled steel sheet at a ½ position in the width direction such that the longitudinal direction of the test specimen was perpendicular to the rolling direction. The bending punch radius R ranged in 0.5 mm increments from 0.5 mm to 2.0 times or more of the thickness of the steel sheet t. To determine the presence and depth of a flex crack, the test specimen after the bending test was cut at three positions of a quarter, half, and three quarters of the width of the test specimen in a plane parallel to the longitudinal direction of the test specimen and perpendicular to the surface of the test specimen. The cross-sections were mirror-polished, and a crack outside and inside the bend of the test specimen was observed with an optical microscope to measure the maximum crack depth outside and inside the bend in the three cross-sections, thereby determining the critical bending radius (minimum bending radius) at which the crack depth outside and inside the bend was less than 50 μm. R/t of 1.5 or less was judged to be acceptable. A crack of 50 μm or more outside or inside the bend at a critical bending radius of 2.0 times or more of the thickness t is considered to indicate low bendability, and the critical bending radius R is not determined.

TABLE 1
Type of	Chemical composition (% by mass) *
steel	C	Si	Mn	P	S	Al	N	O	B	Others
A	0.114	1.20	2.90	0.015	0.0010	0.042	0.0050	0.0013	0.0020	—
B	0.183	0.90	3.31	0.010	0.0008	0.040	0.0041	0.0025	0.0015	—
C	0.142	0.75	3.11	0.011	0.0007	0.061	0.0056	0.0009	0.0015	—
D	0.161	1.15	3.21	0.010	0.0009	0.050	0.0045	0.0020	0.0013	Sb: 0.007
E	0.172	1.03	2.41	0.015	0.0013	0.038	0.0035	0.0011	0.0015	Cr: 0.20,
										Ca: 0.0035
F	0.122	0.73	3.48	0.015	0.0010	0.045	0.0045	0.0020	0.0018	—
G	0.134	1.08	2.77	0.005	0.0006	0.040	0.0035	0.0016	0.0025	Cr: 0.30
H	0.144	0.89	2.47	0.009	0.0017	1.910	0.0055	0.0012	0.0006	Cr: 0.60,
										Sb: 0.019
I	0.139	0.96	2.96	0.011	0.0009	0.050	0.0040	0.0009	0.0017	Mo: 0.23
J	0.142	0.88	2.55	0.021	0.0006	0.037	0.0029	0.0007	0.0020	Mo: 0.41
K	0.118	0.72	2.43	0.012	0.0013	0.048	0.0036	0.0011	0.0032	Mo: 0.34
L	0.163	1.06	3.56	0.008	0.0006	0.040	0.0028	0.0023	0.0095	Cu: 1.02
M	0.145	0.99	2.96	0.014	0.0023	0.047	0.0038	0.0010	0.0010	Ni: 0.48,
										Mg: 0.0022
N	0.122	1.15	2.88	0.013	0.0009	0.043	0.0040	0.0010	0.0015	Ti: 0.021,
										REM: 0.0016
O	0.108	1.11	2.77	0.019	0.0018	0.041	0.0098	0.0013	0.0007	Ti: 0.082,
										Nb: 0.037
P	0.125	0.99	2.82	0.034	0.0008	0.041	0.0041	0.0012	0.0009	Ti: 0.125,
										V: 0.013
Q	0.070	0.69	1.80	0.0018	0.0025	0.043	0.0052	0.0031	0.0016	Cr: 0.45,
										Ti: 0.12
a	0.043	1.05	2.50	0.029	0.0012	0.031	0.0044	0.0021	0.0017	—
b	0.212	0.99	3.08	0.012	0.0039	0.028	0.0035	0.0024	0.0016	—
c	0.145	0.35	3.42	0.025	0.0031	0.045	0.0055	0.0011	0.0019	—
d	0.134	1.35	2.80	0.015	0.0005	0.058	0.0034	0.0015	0.0012	—
e	0.138	0.87	2.81	0.013	0.0012	0.052	0.0037	0.0012	0.0003	—
f	0.142	0.96	1.24	0.020	0.0015	0.065	0.0042	0.0025	0.0019	Cr: 0.80
g	0.133	0.71	4.15	0.010	0.0015	0.047	0.0062	0.0011	0.0014	—
h	0.118	1.17	3.05	0.009	0.0009	0.051	0.0036	0.0017	0.0020	Cr: 1.10
k	0.140	0.92	2.95	0.014	0.0014	0.042	0.0041	0.0021	0.0008	Ti: 0.350
	Type of	RC1	RC2	Trs	RC2-50	RC2 + 120	Trs + 250
	steel	(° C.)	(° C.)	(° C.)	(° C.)	(° C.)	(° C.)	Notes
	A	951	801	347	751	921	597	Conforming
								steel
	B	959	809	302	759	929	552	Conforming
								steel
	C	953	803	327	753	923	577	Conforming
								steel
	D	955	805	315	755	925	565	Conforming
								steel
	E	951	801	335	751	921	585	Conforming
								steel
	F	956	806	323	756	926	573	Conforming
								steel
	G	957	807	338	757	927	588	Conforming
								steel
	H	949	799	340	749	919	590	Conforming
								steel
	I	964	814	329	764	934	579	Conforming
								steel
	J	970	820	339	770	940	589	Conforming
								steel
	K	969	819	355	769	939	605	Conforming
								steel
	L	1000	850	302	800	970	552	Conforming
								steel
	M	949	799	326	749	919	576	Conforming
								steel
	N	964	806	344	756	926	594	Conforming
								steel
	O	1074	859	354	809	979	604	Conforming
								steel
	P	1035	841	345	791	961	595	Conforming
								steel
	Q	1022	830	399	780	950	649	Conforming
								steel
	a	938	788	393	738	908	643	Comparative
								steel
	b	960	810	297	760	930	547	Comparative
								steel
	c	959	809	315	759	929	565	Comparative
								steel
	d	948	798	342	748	918	592	Comparative
								example
	e	944	794	340	744	914	590	Comparative
								steel
	f	945	795	381	745	915	631	Comparative
								steel
	g	962	812	295	762	932	545	Comparative
								steel
	h	964	814	324	764	934	574	Comparative
								steel
	k	1193	920	334	870	1040	584	Comparative
								steel
	The underlines are outside the scope of the present invention.
	* The remainder is composed of Fe and incidental impurities.

	TABLE 2
	Manufacturing conditions
	First cooling
	Time from
	Hot rolling	completion	Average cooling
		Heating	Total rolling		of hot	rate at position
		Heating	reduction at	Finishing	rolling to	of three-tenths
	Type	temper-	temperature	temper-	start of	of thickness
	of	ature	of RC1 or less	ature	cooling	of steel sheet
No.	steel	(° C.)	(%)	(° C.)	(s)	(° C./s)
1	A	1250	35	840	1.0	60
2	A	1210	45	920	1.4	45
3	A	1200	30	870	0.5	80
4	A	1240	35	880	0.8	55
5	B	1230	45	875	1.0	75
6	C	1220	35	855	1.6	37
7	D	1220	60	865	1.0	92
8	E	1270	30	845	0.6	75
9	F	1200	45	875	0.8	62
10	G	1250	50	820	1.0	55
11	G	1230	35	910	1.4	38
12	G	1260	45	880	0.5	32
13	G	1240	30	885	0.8	95
14	H	1230	60	865	1.0	47
15	I	1250	40	850	1.6	38
16	J	1240	30	880	1.0	46
17	K	1235	35	860	0.6	58
18	L	1210	50	920	0.8	92
19	M	1235	45	870	1.4	62
20	N	1220	50	860	0.5	38
21	O	1300	60	940	0.8	77
22	P	1270	75	890	1.0	64
23	Q	1240	40	900	1.6	70
24	a	1250	35	900	1.0	88
25	b	1220	40	900	0.6	29
26	c	1210	55	890	0.8	16
27	d	1225	45	910	0.9	60
28	e	1215	40	905	1.0	46
29	f	1230	35	895	1.4	32
30	g	1260	45	905	0.5	67
31	h	1220	35	920	0.8	78
32	k	1290	75	930	1.0	57
33	A	1245	15	900	1.6	60
34	A	1240	90	870	1.0	55
35	A	1250	45	735	1.4	50
36	A	1230	30	1020	0.5	70
37	A	1270	50	915	0.8	4
38	A	1260	40	900	1.0	45
39	A	1240	30	920	1.6	50
40	A	1240	40	895	1.0	37
41	B	1220	40	750	0.6	49
42	C	1190	35	1040	0.8	56
43	D	1215	30	920	1.0	3
44	E	1230	40	880	1.4	34
45	G	1250	60	915	0.5	28
46	H	1230	50	905	0.8	67
	Manufacturing conditions
	First cooling
		Average cooling
		rate of surface
		layer - average
	cooling rate		Second cooling
		at position of	Cooling	Coiling		Cooping
		three-tenths	stop	Coiling	Average	stop
		of thickness	temper-	temper-	cooling	temper-
		of steel sheet	ature	ature	rate	ature
	No.	(° C./s)	(° C.)	(° C.)	(° C./s)	(° C.)	Notes
	1	15	435	445	5	50	Example
	2	17	470	460	8	55	Example
	3	18	360	375	4	35	Example
	4	16	570	550	6	80	Example
	5	22	455	470	11	45	Example
	6	16	530	520	17	75	Example
	7	21	425	445	5	40	Example
	8	15	500	490	7	80	Example
	9	18	360	390	3	40	Example
	10	19	410	420	3	50	Example
	11	18	405	430	7	70	Example
	12	22	355	345	4	85	Example
	13	21	580	570	8	75	Example
	14	17	420	440	6	70	Example
	15	16	475	490	4	55	Example
	16	15	520	510	9	70	Example
	17	15	405	430	5	50	Example
	18	20	385	410	6	55	Example
	19	15	500	495	7	80	Example
	20	16	410	430	7	55	Example
	21	18	525	510	4	35	Example
	22	20	540	530	9	75	Example
	23	16	460	440	6	70	Example
	24	14	400	420	10	35	Comparative
							example
	25	15	450	470	8	45	Comparative
							example
	26	17	400	435	6	50	Comparative
							example
	27	19	460	445	7	65	Comparative
							example
	28	15	390	425	4	25	Comparative
							example
	29	16	520	510	5	50	Comparative
							example
	30	15	500	480	3	85	Comparative
							example
	31	16	400	430	3	80	Comparative
							example
	32	21	520	510	7	50	Comparative
							example
	33	15	480	465	8	60	Comparative
							example
	34	16	470	435	5	45	Comparative
							example
	35	17	430	450	5	35	Comparative
							example
	36	15	525	510	3	45	Comparative
							example
	37	8	420	440	3	85	Comparative
							example
	38	15	310	330	4	55	Comparative
							example
	39	9	650	630	3	75	Comparative
							example
	40	16	450	470	25	35	Comparative
							example
	41	15	430	440	6	55	Comparative
							example
	42	15	390	410	3	50	Comparative
							example
	43	20	430	460	2	35	Comparative
							example
	44	15	280	300	6	55	Comparative
							example
	45	17	670	630	3	65	Comparative
							example
	46	20	525	510	35	30	Comparative
							example
	The underlines are outside the scope of the present invention.

	TABLE 3
	Average	Hardness (HV)
		Microstructure	grain size	Surface
	Type	Area ratio(%)	of surface	layer
	of	Surface layer region*)	inner region**)	layer*)	region*)
No.	steel	UB	M	γ	M + γ	F	TM	UB	M	γ	M + γ	F	TM	(μm)	HV1
1	A	90	8	2	10	0	0	88	9	3	12	0	0	4.9	416
2	A	88	11	1	12	0	0	86	12	2	14	0	0	4.8	403
3	A	93	5	2	7	0	0	91	6	3	9	0	0	4.6	429
4	A	83	13	4	17	0	0	81	14	5	19	0	0	5.0	412
5	B	82	13	5	18	0	0	80	14	6	20	0	0	4.8	447
6	C	89	6	5	11	0	0	87	7	6	13	0	0	4.2	414
7	D	85	12	3	15	0	0	83	13	4	17	0	0	3.5	428
8	E	90	6	4	10	0	0	88	7	5	12	0	0	4.9	422
9	F	82	16	2	18	0	0	80	17	3	20	0	0	4.7	438
10	G	85	12	3	15	0	0	83	13	4	17	0	0	4.5	423
11	G	85	13	2	15	0	0	83	14	3	17	0	0	4.3	426
12	G	92	5	3	8	0	0	90	6	4	10	0	0	4.6	455
13	G	82	14	4	18	0	0	80	15	5	20	0	0	4.0	410
14	H	82	15	3	18	0	0	81	16	3	19	0	0	3.8	404
15	I	86	11	3	14	0	0	84	12	4	16	0	0	5.0	418
16	J	85	12	3	15	0	0	83	13	4	17	0	0	4.8	418
17	K	82	13	5	18	0	0	81	14	5	19	0	0	4.5	420
18	L	80	16	4	20	0	0	78	17	5	22	0	0	4.1	433
19	M	86	13	1	14	0	0	84	14	2	16	0	0	4.6	406
20	N	88	9	3	12	0	0	86	10	4	14	0	0	4.8	408
21	O	86	10	4	14	0	0	84	11	5	16	0	0	3.8	407
22	P	90	8	2	10	0	0	88	9	3	12	0	0	3.4	431
23	Q	98	2	0	2	0	0	94	2	1	3	0	0	4.0	355
24	a	94	5	1	6	0	0	92	6	2	8	0	0	4.9	328
25	b	91	6	3	9	0	0	89	7	4	11	0	0	4.9	468
26	c	99	1	0	1	0	0	98	2	0	2	0	0	4.5	428
27	d	65	32	3	35	0	0	63	33	4	37	0	0	4.3	420
28	e	69	27	4	31	0	0	68	26	6	32	0	0	5.0	425
29	f	100	0	0	0	0	0	99	1	0	1	0	0	4.7	395
30	g	69	26	5	31	0	0	69	26	5	31	0	0	4.9	452
31	h	69	24	7	31	0	0	69	24	7	31	0	0	4.4	475
32	k	69	29	2	31	0	0	67	30	3	33	0	0	2.9	468
33	A	91	8	1	9	0	0	89	9	2	11	0	0	7.6	398
34	A	94	5	1	6	0	0	92	6	2	8	0	0	1.7	425
35	A	90	7	3	10	0	0	88	8	4	12	0	0	2.4	465
36	A	89	9	2	11	0	0	87	10	3	13	0	0	6.8	388
37	A	79	7	4	11	10	0	77	10	3	13	10	0	4.0	328
38	A	7	1	0	1	0	92	6	2	0	2	0	92	3.2	478
39	A	0	0	0	0	100	0	0	0	0	0	100	0	5.4	318
40	A	67	31	2	33	0	0	65	32	3	35	0	0	4.6	388
41	B	92	6	2	8	0	0	90	7	3	10	0	0	2.5	452
42	C	90	7	3	10	0	0	88	8	4	12	0	0	8.2	397
43	D	0	0	0	0	100	0	0	0	0	0	100	0	5.0	325
44	E	2	1	1	2	0	96	2	2	0	2	0	96	3.2	492
45	G	0	0	0	0	100	0	0	0	0	0	100	0	5.7	328
46	H	2	95	3	98	0	0	0	96	4	100	0	0	3.8	508
				Hardness
				difference
	with respect	Mechanical properties
	to [0.3 ×		Critical
	Hardness (HV)	tensile		bending
		Inner	Hardness	strength						radius
		region**)	difference,	(MPa)]	YP	TS	YR	El	u-El	R/thick-
	No.	HV2	HV2-HV1	(%)	(MPa)	(MPa)	(%)	(%)	(%)	ness t	Notes
	1	453	37	10.0	1000	1232	81	15.9	7.1	1.0	Example
	2	436	32	9.0	976	1195	82	17.8	9.2	0.8	Example
	3	475	46	12.0	1025	1273	80	14.6	8.4	0.9	Example
	4	463	51	14.0	951	1221	78	16.7	7.3	1.1	Example
	5	490	44	11.0	1068	1325	81	12.7	6.7	0.8	Example
	6	447	33	9.0	1028	1228	84	13.8	7.2	0.9	Example
	7	459	30	8.0	1080	1270	85	16.9	9.1	1.0	Example
	8	460	38	10.0	1102	1252	88	16.2	8.8	0.9	Example
	9	481	43	11.0	1116	1298	86	13.9	6.1	0.9	Example
	10	469	45	12.0	1004	1255	80	14.8	7.2	1.0	Example
	11	464	38	10.0	1023	1263	81	14.7	7.3	1.1	Example
	12	491	36	9.0	1121	1350	83	13.8	6.5	0.8	Example
	13	454	44	12.0	1057	1215	87	17.2	7.8	1.2	Example
	14	454	50	14.0	993	1196	83	17.2	8.8	1.1	Example
	15	459	41	11.0	954	1239	77	15.8	8.2	0.9	Example
	16	451	33	9.0	980	1238	79	15.9	8.7	0.8	Example
	17	450	30	8.0	1010	1246	81	16.2	9.0	0.7	Example
	18	472	39	10.0	1118	1285	87	13.9	7.1	0.9	Example
	19	446	40	11.0	950	1203	79	15.9	8.1	0.6	Example
	20	452	44	12.0	1004	1210	83	14.9	8.1	0.6	Example
	21	443	36	10.0	1036	1205	86	15.7	9.3	0.7	Example
	22	466	35	9.0	1125	1278	88	13.5	7.5	1.3	Example
	23	393	38	12.0	980	105C	93	18.1	8.9	0.8	Example
	24	369	41	14.0	870	970	90	18.3	9.6	0.7	Comparative
											example
	25	514	46	11.0	1265	1390	91	13.2	3.8	2.1	Comparative
											example
	26	463	34	9.0	1080	1270	85	13.1	4.9	1.8	Comparative
											example
	27	455	35	9.5	870	1234	71	14.1	6.0	2.0	Comparative
											example
	28	455	30	8.0	1096	1260	87	15.2	4.8	1.9	Comparative
											example
	29	430	35	10.0	1020	1170	87	16.4	5.4	1.6	Comparative
											example
	30	496	44	11.0	871	1340	65	11.8	4.2	2.0	Comparative
											example
	31	526	51	12.0	931	1410	66	10.8	3.2	2.0	Comparative
											example
	32	510	42	10.0	1100	1390	79	11.7	5.2	2.0	Comparative
											example
	33	433	35	10.0	1002	1178	85	13.8	6.4	1.7	Comparative
											example
	34	459	34	9.0	1167	1260	93	12.2	5.5	1.6	Comparative
											example
	35	514	50	12.0	1158	1379	84	12.9	5.1	2.0	Comparative
											example
	36	437	48	16.7	905	965	94	13.8	7.2	1.7	Comparative
											example
	37	340	12	4.1	860	970	89	13.8	7.2	1.3	Comparative
											example
	38	521	43	10.0	1136	1420	80	9.8	3.2	2.0	Comparative
											example
	39	329	11	3.9	840	940	89	14.8	7.2	1.3	Comparative
											example
	40	423	35	10.0	897	1150	78	13.7	6.3	1.6	Comparative
											example
	41	496	44	11.0	1126	1340	84	12.6	5.4	1.7	Comparative
											example
	42	439	42	12.0	905	1175	77	15.9	7.1	1.6	Comparative
											example
	43	354	29	10.0	797	960	83	17.8	9.2	1.3	Comparative
											example
	44	531	39	9.0	1358	1460	93	8.8	3.2	2.0	Comparative
											example
	45	357	29	10.0	930	970	96	14.7	8.3	0.9	Comparative
											example
	46	551	43	9.5	1163	1510	77	7.9	3.1	2.0	Comparative
											example
	The underlines are outside the scope of the present invention.,
	UB: upper bainite
	M: fresh martensite
	y: retained austenite
	F: polygonal ferrite martensite
	*)A surface layer region extending from the surface to a position of one-tenth of the thickness of the steel sheet.
	**)An inner region extending from one-tenth of the thickness of the steel sheet to a position of three-tenths of the thickness.

The results in Table 3 show that all the examples have a tensile strength of 980 MPa or more, high press formability, and high bendability.

Claims

1. A high-strength steel sheet comprising:

a chemical composition containing, in mass %:

C: 0.05% to 0.20%,

Si: 0.5% to 1.2%,

Mn: 1.5% 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,

B: 0.0005% to 0.010%, and the remainder being Fe and incidental impurities;

a microstructure in a surface layer region extending from a surface of the steel sheet to a position of one-tenth of a thickness of the steel sheet containing 80% by area or more of upper bainite and 2% by area or more of fresh martensite and/or retained austenite in total; and

a microstructure in an inner region extending from the position of one-tenth to a position of three-tenths of the thickness of the steel sheet containing 70% by area or more of upper bainite and 3% by area or more of fresh martensite and/or retained austenite in total,

wherein:

the surface layer region extending from the surface of the steel sheet to the position of one-tenth of the thickness of the steel sheet has an average grain size of 6 μm or less;

the steel sheet has a tensile strength of 980 MPa or more, a uniform elongation of 6% or more, and a ratio R/t of a critical bending radius R to a thickness t of 1.5 or less; and

a difference (HV2−HV1) between a hardness (HV1) of the surface layer region extending from the surface of the steel sheet to the position of one-tenth of the thickness of the steel sheet and a hardness (HV2) of the inner region extending from the position of one-tenth to the position of three-tenths of the thickness of the steel sheet is 5% or more and 15% or less of [0.3×tensile strength (MPa)].

2. The high-strength steel sheet according to claim 1, wherein

the area of fresh martensite and/or retained austenite in total in the surface layer region is smaller than the area of fresh martensite and/or retained austenite in total in the inner region.

3. The high-strength steel sheet according to claim 1, wherein the chemical composition further contains at least one selected from following groups A to D consisting of:

Group A: in mass %, at least one of Cr: 1:0% or less and Mo: 1.0% or less;

Group B: in mass %, at least one of

Cu: 2.0% or less,

Ni: 2.0% or less,

Ti: 0.3% or less,

Nb: 0.3% or less, and

V: 0.3% or less;

Group C: in mass %, Sb: 0.005% to 0.020%; and

Group D: in mass %, at least one of Ca: 0.01% or less, Mg: 0.01% or less, and REM: 0.01% or less.

4. The high-strength steel sheet according to claim 2, wherein the chemical composition further contains at least one selected from following groups A to D consisting of:

Group A: in mass %, at least one of Cr: 1.0% or less and Mo: 1.0% or less;

Group B: in mass %, at least one of Cu: 2.0% or less, Ni: 2.0% or less, Ti: 0.3% or less, Nb: 0.3% or less, and V: 0.3% or less;

Group C: in mass %, Sb: 0.005% to 0.020%; and

Group D: in mass %, at least one of Ca: 0.01% or less, Mg: 0.01% or less, and REM: 0.01% or less.

5. (canceled)

6. A method for manufacturing the high-strength steel sheet according to claim 1, comprising:

heating a steel material having the chemical composition to a heating temperature of 1150° C. or more;

then hot-rolling including rough rolling and finish rolling the steel material into a hot-rolled steel sheet, the finish rolling being performed under conditions of a finishing temperature: (RC2−50° C.) or more and (RC2+120° C.) or less and a total rolling reduction of 25% or more and 80% or less at a temperature of RC1 or less;

cooling the hot-rolled steel sheet under conditions of a time from completion of the hot rolling to start of cooling: 2.0 seconds or less, an average cooling rate at the position of three-tenths of the thickness of the steel sheet: 15° C./s or more, and a cooling stop temperature: Trs or more and (Trs+250° C.) or less;

coiling the hot-rolled steel sheet after the cooling at a coiling temperature: Trs or more and (Trs+250° C.) or less; and

cooling the hot-rolled steel sheet to 100° C. or less at an average cooling rate of 20° C./s or less,

wherein RC1, RC2, and Trs are represented by the following formulae (1), (2), and (3), respectively, RC1(° C.)=900+100×C+100×N+10×Mn+700×Ti+5000×B+10×Cr+50×Mo+2000×Nb+150×V  (1) RC2(° C.)=750+100×C+100×N+10×Mn+350×Ti+5000×B+10×Cr+50×Mo+1000×Nb+150×V  (2) Trs(° C.)=500−450×C—35×Mn−15×Cr—10×Ni−20×Mo  (3)

where each element symbol in the formulae (1), (2), and (3) denotes a corresponding element content (% by mass) and is 0 in the absence of the element.

7. The method for manufacturing the high-strength steel sheet according to claim 6, wherein in the cooling after the hot rolling, an average cooling rate of the surface layer and the average cooling rate at the position of three-tenths of the thickness of the steel sheet satisfy the formula (4):

Average cooling rate of surface layer−average cooling rate at position of three-tenths of thickness of steel sheet≥10° C./s  (4).

8. A method for manufacturing the high-strength steel sheet according to claim 2, comprising:

heating a steel material having the chemical composition to a heating temperature of 1150° C. or more;

then hot rolling including rough rolling and finish rolling the steel material into a hot-rolled steel sheet, the finish rolling being performed under conditions of a finishing temperature: (RC2−50° C.) or more and (RC2+120° C.) or less and a total rolling reduction of 25% or more and 80% or less at a temperature of RC1 or less;

cooling the hot-rolled steel sheet under conditions of a time from completion of the hot rolling to start of cooling: 2.0 seconds or less, an average cooling rate at the position of three-tenths of the thickness of the steel sheet: 15° C./s or more, and a cooling stop temperature: Trs or more and (Trs+250° C.) or less;

coiling the hot-rolled steel sheet after the cooling at a coiling temperature: Trs or more and (Trs+250° C.) or less; and

cooling the hot-rolled steel sheet to 100° C. or less at an average cooling rate of 20° C./s or less,

wherein RC1, RC2, and Trs are represented by the following formulae (1), (2), and (3), respectively, RC1(° C.)=900+100×C+100×N+10×Mn+700×Ti+5000×B+10×Cr+50×Mo+2000×Nb+150×V  (1) RC2(° C.)=750+100×C+100×N+10×Mn+350×Ti+5000×B+10×Cr+50×Mo+1000×Nb+150×V  (2) Trs(° C.)=500−450×C—35×Mn−15×Cr—10×Ni−20×Mo  (3)

where each element symbol in the formulae (1), (2), and (3) denotes a corresponding element content (% by mass) and is 0 in the absence of the element.

9. The method for manufacturing the high-strength steel sheet according to claim 8, wherein in the cooling after the hot rolling, an average cooling rate of the surface layer and the average cooling rate at the position of three-tenths of the thickness of the steel sheet satisfy the formula (4):

Average cooling rate of surface layer−average cooling rate at position of three-tenths of thickness of steel sheet≥10° C./s  (4).

10. A method for manufacturing the high-strength steel sheet according to claim 3, comprising:

heating a steel material having the chemical composition to a heating temperature of 1150° C. or more;

then hot rolling including rough rolling and finish rolling the steel material into a hot-rolled steel sheet, the finish rolling being performed under conditions of a finishing temperature: (RC2−50° C.) or more and (RC2+120° C.) or less and a total rolling reduction of 25% or more and 80% or less at a temperature of RC1 or less;

cooling the hot-rolled steel sheet under conditions of a time from completion of the hot rolling to start of cooling: 2.0 seconds or less, an average cooling rate at the position of three-tenths of the thickness of the steel sheet: 15° C./s or more, and a cooling stop temperature: Trs or more and (Trs+250° C.) or less;

coiling the hot-rolled steel sheet after the cooling at a coiling temperature: Trs or more and (Trs+250° C.) or less; and

cooling the hot-rolled steel sheet to 100° C. or less at an average cooling rate of 20° C./s or less,

wherein RC1, RC2, and Trs are represented by the following formulae (1), (2), and (3), respectively, RC1(° C.)=900+100×C+100×N+10×Mn+700×Ti+5000×B+10×Cr+50×Mo+2000×Nb+150×V  (1) RC2(° C.)=750+100×C+100×N+10×Mn+350×Ti+5000×B+10×Cr+50×Mo+1000×Nb+150×V  (2) Trs(° C.)=500−450×C—35×Mn−15×Cr—10×Ni−20×Mo  (3)

where each element symbol in the formulae (1), (2), and (3) denotes a corresponding element content (% by mass) and is 0 in the absence of the element.

11. The method for manufacturing the high-strength steel sheet according to claim 10, wherein in the cooling after the hot rolling, an average cooling rate of the surface layer and the average cooling rate at the position of three-tenths of the thickness of the steel sheet satisfy the formula (4):

Average cooling rate of surface layer−average cooling rate at position of three-tenths of thickness of steel sheet≥10° C./s  (4).

12. A method for manufacturing the high-strength steel sheet according to claim 4, comprising:

heating a steel material having the chemical composition to a heating temperature of 1150° C. or more;

then hot rolling including rough rolling and finish rolling the steel material into a hot-rolled steel sheet, the finish rolling being performed under conditions of a finishing temperature: (RC2−50° C.) or more and (RC2+120° C.) or less and a total rolling reduction of 25% or more and 80% or less at a temperature of RC1 or less;

cooling the hot-rolled steel sheet under conditions of a time from completion of the hot rolling to start of cooling: 2.0 seconds or less, an average cooling rate at the position of three-tenths of the thickness of the steel sheet: 15° C./s or more, and a cooling stop temperature: Trs or more and (Trs+250° C.) or less;

coiling the hot-rolled steel sheet after the cooling at a coiling temperature: Trs or more and (Trs+250° C.) or less; and

cooling the hot-rolled steel sheet to 100° C. or less at an average cooling rate of 20° C./s or less,

wherein RC1, RC2, and Trs are represented by the following formulae (1), (2), and (3), respectively, RC1(° C.)=900+100×C+100×N+10×Mn+700×Ti+5000×B+10×Cr+50×Mo+2000×Nb+150×V  (1) RC2(° C.)=750+100×C+100×N+10×Mn+350×Ti+5000×B+10×Cr+50×Mo+1000×Nb+150×V  (2) Trs(° C.)=500−450×C—35×Mn−15×Cr—10×Ni−20×Mo  (3)

where each element symbol in the formulae (1), (2), and (3) denotes a corresponding element content (% by mass) and is 0 in the absence of the element.

13. The method for manufacturing the high-strength steel sheet according to claim 12, wherein in the cooling after the hot rolling, an average cooling rate of the surface layer and the average cooling rate at the position of three-tenths of the thickness of the steel sheet satisfy the formula (4):

Average cooling rate of surface layer−average cooling rate at position of three-tenths of thickness of steel sheet≥10° C./s  (4).