HIGH-STRENGTH STEEL SHEET AND METHOD FOR PRODUCING THE SAME

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT/JP2023/011909, filed Mar. 24, 2023 which claims priority to Japanese Patent Application No. 2022-055658, filed Mar. 30, 2022, 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 (specifically, hot-rolled steel sheet) that is suitable as an automotive member and improved particularly in terms of strength and fatigue resistance and a method for producing the high-strength steel sheet.

BACKGROUND OF THE INVENTION

Reductions in CO₂ emissions have been anticipated in a worldwide framework from the viewpoints of global environment conservation. In particular, there has been a strong demand for increases in the mileage of automobiles. Reductions in the weights of automotive bodies have been anticipated. It is effective to increase the strengths of steel sheets used as materials for automotive members and reduce the thicknesses of the steel sheets for reducing the weights of automotive bodies without reducing the strengths of the automotive bodies. In particular, a steel sheet having a tensile strength of 980 MPa or more has been considered as a promising material that enables the reductions in the thicknesses of the steel sheets and thereby markedly increases the mileage of automobiles.

In order to maintain durability, which is likely to degrade with reductions in the thicknesses of automotive parts, it is necessary to enhance the fatigue resistance of steel sheets. Automotive parts, that is, specifically, undercarriage parts, such as suspension parts, are subjected to cyclic loading through tires. Therefore, if the fatigue strengths of the above parts are low, the durability of the parts may fall below the designed durability with an increase in the distance travelled. Commonly, an increase in the strength of a steel sheet does not always result in an increase in the fatigue strength of the steel sheet.

For enhancing the fatigue strength of a steel sheet while increasing the tensile strength of the steel sheet, various studies (Patent Literatures 1 to 3) have been conducted in the related art.

• • Patent Literature 1 discloses a technique related to a high-strength hot-rolled steel sheet having excellent formability and excellent fatigue resistance, which is produced by controlling the manufacturing conditions under which hot rolling is performed, forming ferrite as a primary phase, and controlling the shape of inclusions and the mode in which the inclusions are dispersed.
  • Patent Literature 2 discloses a technique related to a high-strength hot-rolled steel sheet having excellent stretch flangeability and excellent fatigue resistance, which is produced by controlling the manufacturing conditions under which hot rolling is performed, forming bainite as a primary phase, dispersing a fine hard secondary phase therein, and controlling the amount of solute Ti.
  • Patent Literature 3 discloses a technique related to a high-strength hot-rolled steel sheet having excellent formability, excellent fracture properties, and excellent fatigue resistance, which is produced by forming ferrite as a primary phase and controlling the number density of cementite in ferrite grains, the size of a hard secondary phase, and the number density of inclusions.

PATENT LITERATURE

• • PTL 1: Japanese Unexamined Patent Application Publication No. 2014-31560
  • PTL 2: Japanese Unexamined Patent Application Publication No. 2012-012701
  • PTL 3: Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2021-505759

SUMMARY OF THE INVENTION

The techniques known in the related art, such as those described in Patent Literatures 1 to 3, have the following issues.

In the technique described in Patent Literature 1, a tensile strength of 980 MPa or more cannot be achieved.

In the technique described in Patent Literature 2, a fatigue strength at which the steel sheet is practically used as an automotive part is not studied sufficiently.

In the technique described in Patent Literature 3, a high-strength steel sheet having excellent fatigue resistance is reportedly produced. However, fatigue resistance is not described specifically.

Thus, a technique related to a high-strength hot-rolled steel sheet having a tensile strength of 980 MPa or more and excellent fatigue resistance has not been established in the related art.

Accordingly, aspects of the present invention were developed in light of the above-described circumstances. An object of aspects of the present invention is to provide a high-strength hot-rolled steel sheet having a tensile strength of 980 MPa or more and a markedly high fatigue strength and a method for producing the high-strength hot-rolled steel sheet.

In order to achieve the above object, the inventors of the present invention conducted extensive studies of a technique for enhancing the fatigue resistance of a hot-rolled steel sheet while maintaining a tensile strength of 980 MPa or more and consequently found the following facts. Specifically, a microstructure that includes upper bainite as a primary phase and an adequate amount of a martensite and/or retained austenite phase, which serves as a hard secondary phase, is formed. Furthermore, the dislocation density in all the phases included in the surface layer region which extends from the surface of the steel sheet to the position 1/10 of the thickness of the steel sheet increases. Moreover, the grain sizes of all the phases are controlled. This enables a steel sheet having a high strength of 980 MPa or more and a markedly high fatigue strength to be formed subsequent to a heat treatment that corresponds to baking coating.

Note that the term “upper bainite phase” refers to a microstructure phase that is an assembly of lath-shaped ferrite grains having a misorientation of less than 15° and includes a Fe-based carbide and/or a retained austenite phase interposed between the lath-shaped ferrite grains. Note that the above microstructure phase does not always include Fe-based carbide and/or retained austenite interposed between the lath-shaped ferrite grains.

Since lath-shaped ferrite grains have a lath-like shape and the inside of the grains has a relatively high dislocation density unlike lamellar (laminar) ferrite or polygonal ferrite in pearlite, they can be distinguished from each other using a SEM (scanning electron microscope) or a TEM (transmission electron microscope).

In the case where retained austenite is present between lath grains, only the lath-shaped ferrite parts are considered as upper bainite and distinguished from retained austenite.

A martensite and/or retained austenite phase appears brighter in a SEM image than an upper bainite phase, a lower bainite phase, or a polygonal ferrite phase. Thus, a martensite phase and/or retained austenite phase can be distinguished from the above microstructure phase using a SEM.

Although martensite and retained austenite phases appear to have the same degrees of brightness with a SEM, they can be distinguished from each other using an electron backscatter diffraction patterns (EBSD) method.

The dislocation density can be determined by irradiating a steel material with an X-ray and analyzing the resulting strength curve (line profile) with respect to the angle or energy of the diffracted X-ray. The analysis of line profile is conducted in accordance with “Method for Evaluating Dislocation Density Using X-ray Diffraction”, Material and Process, vol. 17 (2004) No. 3, p. 396-399. In accordance with aspects of the present invention, the dislocation density is calculated from the half-widths of (110), (211), and (220).

The inventors of the present invention conducted further studies on the basis of the above findings and consequently devised aspects of the present invention. The summary of aspects of the present invention is as follows.

• • [1] A high-strength steel sheet having a chemical composition containing, by mass, C: 0.03% to 0.15%, Si: 0.1% to 3.0%, Mn: 0.8% to 3.0%, P: 0.001% to 0.1%, S: 0.0001% to 0.03%, Al: 0.001% to 2.0%, N: 0.001% to 0.01%, and B: 0.0002% to 0.010%, the chemical composition further containing at least one selected from Ti: 0.01% to 0.30%, and Nb: 0.001% to 0.10%, with a balance being Fe and incidental impurities, a microstructure consisting of, in a surface layer region of the high-strength steel sheet extending from a surface of the steel sheet to a position 1/10 of a thickness of the steel sheet, 75% or more and less than 98.5% by area of an upper bainite phase as a primary phase, 1.5% or more and less than 25% by area of a martensite phase and/or a retained austenite phase as a secondary phase, and 2.0% or less by area of a remaining microstructure phase other than the upper bainite phase, the martensite phase and/or the retained austenite phase, wherein an average grain size of all phases included in the surface layer region extending from the surface of the steel sheet to the position 1/10 of the thickness of the steel sheet is 6.0 μm or less; and wherein a dislocation density in all the phases included in the surface layer region extending from the surface of the steel sheet to the position 1/10 of the thickness of the steel sheet is 8.0×10¹⁴/m² or more.
  • [2] The high-strength steel sheet according to [1], wherein the chemical composition further contains, by mass, at least one group selected from Groups a to c below: Group a: at least one element selected from Cu: 0.005% to 2.0%, Ni: 0.005% to 2.0%, Cr: 0.005% to 2.5%, V: 0.001% to 0.5%, and Mo: 0.005% to 1.0%, Group b: at least one element selected from Sb: 0.005% to 0.2%, and Sn: 0.001% to 0.05%, and Group c: at least one element selected from Ca: 0.0005% to 0.01%, Mg: 0.0005% to 0.01%, and REM: 0.0005% to 0.01%.
  • [3] A method for producing the high-strength steel sheet according to [1] or [2], the method including heating a steel material having the chemical composition to a heating temperature of 1150° C. or more; rough-rolling the heated steel material into a steel sheet; finish-rolling the steel sheet such that a total rolling reduction achieved in a temperature range of (RC1−150)° C. or more and RC1° C. or less is 35% or more and a finish rolling delivery temperature is (RC2−100)° C. or more and (RC2+50)° C. or less; cooling the finish-rolled steel sheet such that a time interval between an end of finish rolling to a start of cooling is 2.0 s or less, an average cooling rate at a surface of the steel sheet is 20° C./s or more, and a cooling stop temperature is Trs°C or more and (Trs+180)° C. or less; coiling the cooled steel sheet such that a coiling temperature is Trs°C or more and (Trs+180)° C. or less; performing cooling to a temperature of (Trs−250)° C. or less at an average cooling rate of 1° C./s or less; and performing temper rolling at a rolling reduction of 0.1% or more and 5.0% or less,
  • wherein RC1, RC2, and Trs are defined by Formulae (1), (2), and (3) below, respectively,

RC ⁢ 1 ⁢ ( °C ) = 900 + 120 × C + 100 × N + 10 × Mn + 500 × Ti + 5000 × B + 10 × Cr + 50 × Mo + 1500 × Nb + 150 × V ( 1 ) RC ⁢ 2 ⁢ ( °C ) = 750 + 120 × C + 100 × N + 10 × Mn + 250 × Ti + 5000 × B + 10 × Cr + 50 × Mo + 750 × Nb + 150 × V ( 2 ) Trs ⁢ ( °C ) = 500 - 450 × C - 35 × Mn - 15 × Cr - 10 × Ni - 20 × Mo ( 3 )

• • where each of element symbols used in Formulae (1), (2), and (3) above represents the content (% by mass) of the element and is zero when the element is absent.

According to aspects of the present invention, a high-strength steel sheet having a tensile strength of 980 MPa or more and excellent fatigue resistance and a method for producing the high-strength steel sheet can be provided.

Applying the high-strength steel sheet according to aspects of the present invention to automotive undercarriage parts, such as a suspension, structural parts, framework parts, or truck frame parts enhances safety, allows reductions in the weights of the automotive bodies, and therefore produces markedly advantageous effects from the viewpoint of industry.

In accordance with aspects of the present invention, the expression “excellent fatigue resistance” means that the ratio (fatigue limit ratio) of the fatigue strength at 2×10⁶ cycles of plane bending performed in an alternating plane bending fatigue test relative to the tensile strength is 0.50 or more.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE is a schematic diagram illustrating the shape of a test specimen used for a plane bending fatigue test in accordance with aspects of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Embodiments of the present invention are described below. Note that the following description is intended to be illustrative of preferable embodiments of the present invention, and the present invention is not limited by the following embodiments.

The steel sheet has the chemical composition described below. In the following description, the symbol “%” used as the unit of the content of an element in the chemical composition means “% by mass” unless otherwise specified.

<C: 0.03% to 0.15%>

C is an element that effectively facilitates the formation of bainite and increases strength by enhancing hardenability. If the C content is less than 0.03%, the above advantageous effects are not produced to a sufficient degree, and a tensile strength of 980 MPa or more cannot be achieved. Accordingly, the C content is 0.03% or more, is preferably 0.04% or more, and is more preferably 0.05% or more. On the other hand, if the C content is more than 0.15%, the amount of martensite and retained austenite increases and, consequently, sufficiently high fatigue resistance cannot be achieved. Accordingly, the C content is 0.15% or less, is preferably 0.14% or less, and is more preferably 0.13% or less.

<Si: 0.1% to 3.0%>

Si contributes to an increase in steel strength by solid solution strengthening of steel. Accordingly, the Si content is 0.1% or more, is preferably 0.3% or more, and is more preferably 0.5% or more. However, since Si is an element that facilitates the formation of ferrite, a Si content exceeding 3.0% causes the formation of ferrite and degrades fatigue resistance. Accordingly, the Si content is 3.0% or less, is preferably 2.5% or less, and is more preferably 2.0% or less.

<Mn: 0.8% to 3.0%>

Mn is an element that stabilizes austenite. Mn is also an element effective for suppressing the formation of ferrite and increasing strength. If the Mn content is less than 0.8%, the above advantageous effects are not produced to a sufficient degree and ferrite, etc. are formed. Consequently, a tensile strength of 980 MPa or more cannot be achieved. Accordingly, the Mn content is 0.8% or more, is preferably 1.0% or more, and is more preferably 1.2% or more. On the other hand, if the Mn content is more than 3.0%, the amount of martensite and retained austenite increases and, consequently, sufficiently high fatigue resistance cannot be achieved. Accordingly, the Mn content is 3.0% or less, is preferably 2.8% or less, and is more preferably 2.5% or less.

<P: 0.001% to 0.1%>

Since P degrades weldability, it is desirable to minimize the P content. The maximum P content allowable in accordance with aspects of the present invention is 0.1%. Thus, the P content is 0.1% or less. Since a P content of less than 0.001% reduces production efficiency, the lower limit is set to 0.001% or more.

<S: 0.0001% to 0.03%>

Since S degrades weldability, it is desirable to minimize the S content. The maximum S content allowable in accordance with aspects of the present invention is 0.03%. Thus, the S content is 0.03% or less. Since a S content of less than 0.0001% reduces production efficiency, the lower limit is set to 0.0001% or more.

<Al: 0.001% to 2.0%>

Al is an element that serves as a deoxidizing agent and effectively enhances the cleanliness of steel. If the Al content is excessively low, the advantageous effects cannot always be produced to a sufficient degree. Accordingly, the Al content is 0.001% or more, is preferably 0.01% or more, and is more preferably 0.02% or more. However, since Al is an element that facilitates the formation of ferrite, an Al content exceeding 2.0% causes the formation of ferrite and reduces a fatigue strength. Accordingly, the Al content is 2.0% or less, is preferably 1.8% or less, and is more preferably 1.6% or less.

<N: 0.001% to 0.01%>

N binds to an element that forms a nitride to precipitate in the form of a nitride and thereby contributes to a reduction in grain size. For producing the above advantageous effects, the N content needs to be 0.001% or more. However, since N is likely to bind to Ti at high temperatures to form coarse nitride particles, an excessively high N content degrades fatigue resistance. Accordingly, the N content is 0.01% or less, is preferably 0.008% or less, and is more preferably 0.006% or less.

<B: 0.0002% to 0.010%>

B is an element that effectively facilitates the formation of upper bainite and increases the strength of the steel sheet by segregating at prior-austenite grain boundaries to suppress the formation of ferrite. For producing the above advantageous effects, the B content needs to be 0.0002% or more. Therefore, the B content is 0.0002% or more, is preferably 0.0005% or more, and is more preferably 0.0007% or more. However, if the B content is more than 0.010%, the above advantageous effects become saturated. Thus, the B content is 0.010% or less, is preferably 0.009% or less, and is more preferably 0.008% or less.

<One or More Elements Selected From Ti: 0.01% to 0.30% and Nb: 0.001% to 0.10%>

Ti and Nb are elements that form a carbide and effectively increase strength by precipitation strengthening. Therefore, one or more elements selected from Ti and Nb need to be included in the chemical composition. The lower limits for the Ti and Nb contents are set to Ti: 0.01% or more and Nb: 0.001% or more. The Ti and Nb contents are preferably Ti: 0.02% or more and Nb: 0.002% or more and are more preferably Ti: 0.03% or more and Nb: 0.003% or more. However, if the Ti and Nb contents are more than Ti: 0.30% and Nb: 0.10%, carbide particles become coarsened to degrade hardenability and, consequently, it may become impossible to form the steel microstructure intended in accordance with aspects of the present invention. Accordingly, the upper limits for the Ti and Nb contents are set to Ti: 0.30% or less and Nb: 0.10% or less. The Ti and Nb contents are preferably Ti: 0.25% or less and Nb: 0.08% or less and are more preferably Ti: 0.20% or less and Nb: 0.05% or less.

The balance includes Fe and incidental impurities.

The above are constituents of the fundamental chemical composition of the high-strength steel sheet according to aspects of the present invention. The chemical composition of the high-strength steel sheet may optionally contain the following elements as needed.

Cr, Ni, Cu, V, and Mo are elements that stabilize austenite and are also elements effective for suppressing the formation of ferrite and increasing strength. In order to produce the above advantageous effects, one or more elements selected from the above elements are preferably included in the chemical composition. In the case where one or more elements selected from Cr, Ni, Cu, V, and Mo are included in the chemical composition, the contents of the above elements are preferably Cu: 0.005% to 2.0%, Ni: 0.005% to 2.0%, Cr: 0.005% to 2.5%, V: 0.001% to 0.5%, and Mo: 0.005% to 1.0%. If the Cr, Ni, Cu, V, and Mo contents are more than the respective upper limits described above, martensite and retained austenite are likely to remain and, consequently, it may become impossible to form the steel microstructure intended in accordance with aspects of the present invention. The lower limit for the Cr content is more preferably 0.1% or more. The upper limit for the Cu content is more preferably 0.6% or less. The lower limit for the Ni content is more preferably 0.1% or more. The upper limit for the Ni content is more preferably 0.6% or less. The lower limit for the Cu content is more preferably 0.1% or more. The upper limit for the Cu content is more preferably 0.6% or less. The lower limit for the V content is more preferably 0.005% or more. The upper limit for the V content is more preferably 0.3% or less. The lower limit for the Mo content is more preferably 0.1% or more. The upper limit for the Mo content is more preferably 0.5% or less.

Sb is an element that reduces the likelihood of elements being removed from the surface of a steel material when the steel material is heated and thereby effectively limits a reduction in steel strength. Accordingly, in the case where the chemical composition contains Sb, the Sb content is preferably 0.005% to 0.2%. If the Sb content is more than the above upper limit, embrittlement of the steel sheet may occur. The lower limit for the Sb content is more preferably 0.01% or more. The upper limit for the Sb content is more preferably 0.050% or less.

Sn is an element that suppresses the formation of pearlite and thereby effectively limits a reduction in steel strength. In order to produce the above advantageous effects, in the case where the chemical composition contains Sn, the Sn content is preferably 0.001% to 0.05%. If the Sn content is more than the above upper limit, embrittlement of the steel sheet may occur. The lower limit for the Sn content is more preferably 0.005% or more. The upper limit for the Sn content is more preferably 0.03% or less.

Ca, Mg, and REMs are elements that effectively enhance workability by shape control of inclusions. In order to produce the above advantageous effects, one or more elements selected from the above elements are preferably included in the chemical composition. In the case where the chemical composition includes one or more elements selected from Ca, Mg, and REMs, the contents of the above elements are preferably Ca: 0.0005% to 0.01%, Mg: 0.0005% to 0.01%, and REM: 0.0005% to 0.01%. However, if the Ca, Mg, and REM contents are more than the respective upper limits, the amount of inclusion increases and workability may become degraded consequently. The lower limit for the Ca content is more preferably 0.001% or more. The upper limit for the Ca content is more preferably 0.005% or less. The lower limit for the Mg content is more preferably 0.001% or more. The upper limit for the Mg content is more preferably 0.005% or less. The lower limit for the REM content is more preferably 0.001% or more. The upper limit for the REM content is more preferably 0.005% or less. Note that the term “REM (rare-earth element)” used herein refers collectively to Sc, Y, and the 15 elements from lanthanum (La) with an atomic number of 57 to lutetium (Lu) with an atomic number of 71. The term “REM content” used herein refers to the total content of these elements.

The advantageous effects according to aspects of the present invention are not impaired even when the Mo, V, Cr, Ni, Cu, Sb, Sn, Ca, Mg, and REM contents are less than the respective lower limits described above. Thus, when the contents of the above constituents are less than the respective lower limits described above, it is considered that the chemical composition contains the above elements as incidental impurities.

The microstructure of the high-strength steel sheet according to aspects of the present invention is described below.

A surface layer region of the high-strength steel sheet according to aspects of the present invention which extends from the surface of the steel sheet to the position 1/10 of the thickness of the steel sheet has the microstructure described below. Specifically, an upper bainite phase, the area fraction of which is 75% or more and less than 98.5%, is the primary phase, and a microstructure phase consisting of a martensite phase and/or a retained austenite phase, the area fraction of which is 1.5% or more and less than 25%, is the secondary phase. Furthermore, the average grain size of all the phases included in the surface layer region is 6.0 μm or less, and the dislocation density in all the phases is 8.0×10¹⁴/m² or more.

<Upper Bainite Phase: 75% or More and Less than 98.5% by Area>

The microstructure of the high-strength steel sheet according to aspects of the present invention includes upper bainite as a primary phase. If the area fraction of upper bainite is less than 75%, a markedly high fatigue strength cannot be achieved. Accordingly, the lower limit for the area fraction of upper bainite is set to 75% or more and is preferably 85% or more. However, if the upper bainite phase is 98.5% or more, the intended dislocation density cannot be achieved. Accordingly, the upper limit for the area fraction of upper bainite is set to be less than 98.5% and is preferably 97% or less.

<Martensite Phase and/or Retained Austenite Phase: 1.5% or More and Less Than 25% by Area>

The microstructure of the high-strength steel sheet according to aspects of the present invention includes a martensite phase and/or a retained austenite phase. If the martensite phase and/or retained austenite phase is less than 1.5%, it becomes impossible to achieve a tensile strength of 980 MPa or more and excellent fatigue resistance. On the other hand, if the area fraction of martensite and/or retained austenite is 25% or more, the amount of interfaces between martensite and/or retained austenite and upper bainite, which may act as the origins of fatigue cracks, increases and, consequently, fatigue resistance may become degraded. For the above reasons, it is necessary to limit the total area fraction of martensite and/or retained austenite to be less than 25%. The above total area fraction is preferably 20% or less and is more preferably 15% or less. Note that, in accordance with aspects of the present invention, the term “martensite” refers to as-quenched martensite.

The advantageous effects according to aspects of the present invention are not impaired when the area fraction of the remaining microstructure phase, which is a phase other than any of upper bainite and martensite and/or retained austenite, is 2.0% or less at the maximum. The remaining microstructure phase includes known microstructure phases, such as ferrite and pearlite.

<Average Grain Size: 6.0 μm or Less>

It is considered that fatigue cracks are formed as a result of slip deformation occurring in the crystal grains included in the surface layer. Grain boundaries reduce the likelihood of the slip deformation being propagated to adjacent crystal grains and consequently delay the occurrence of cracking. That is, fatigue strength can be increased by reducing the size of crystal grains. Reducing the grain size also contributes to an increase in strength. Accordingly, the average grain size is 6.0 μm or less and is preferably 5.0 μm or less. However, if the average grain size is excessively small, elongation may be reduced with an increase in strength. Accordingly, the average grain size is preferably 2.0 μm or more. Note that the term “average grain size” used herein refers to the average grain size of all the phases included in the surface layer region of the steel sheet which extends from the surface of the steel sheet to the position 1/10 of the thickness of the steel sheet. In the case where the surface layer region, which extends to the position 1/10 of the thickness, includes the remaining microstructure phase, the remaining microstructure phase is also included in the “all the phases”.

<Dislocation Density: 8.0×10¹⁴/m² or More>

Most of the fatigue cracks occur at the surface of the steel sheet. After the fatigue cracks have grown to several tens of micrometers in length, a fatigue crack propagation stage starts. In high-cycle fatigue, fatigue life is primarily affected by the number of the cycles performed until the occurrence of cracking. Therefore, for increasing a fatigue strength at 2×10⁶ cycles, it is necessary to reduce the formation of cracks. It is important to control the dislocation behavior of the surface layer region, which extends from the surface of the steel sheet to the position 1/10 of the thickness. In the high-strength steel sheet according to aspects of the present invention, the dislocations introduced to the microstructure are pinned as a result of the heat treatment being performed in the subsequent step to obstruct the movement of the dislocations. This prevents the movement and rearrangement of the dislocations, delays cyclic softening, and consequently enhances fatigue resistance. In order to produce the above advantageous effects, the dislocation density is limited to 8.0×10¹⁴/m² or more. The dislocation density is preferably 1.0×10¹⁵/m² or more and is more preferably 1.2×10¹⁵/m² or more. The upper limit for the dislocation density is not set but preferably 4.0×10¹⁵/m² or less. Although it is most important to control the dislocation density in the primary phase included in the surface layer region, which extends from the surface of the steel sheet to the position 1/10 of the thickness, it is difficult to measure the dislocation density in the primary phase only. Therefore, the dislocation density measured in accordance with aspects of the present invention is the dislocation density in all the phases included in the surface layer region, which extends to the position 1/10 of the thickness. In the case where the surface layer region, which extends to the position 1/10 of the thickness, includes the remaining microstructure phase, the remaining microstructure phase is also included in the “all the phases”.

The high-strength steel sheet according to aspects of the present invention has a tensile strength of 980 MPa or more and a fatigue limit ratio of 0.50 or more. The term “fatigue limit ratio” used herein refers to the ratio of the fatigue strength at 2×10⁶ cycles of plane bending to the tensile strength. Thus, the high-strength steel sheet according to aspects of the present invention has a high tensile strength and is capable of maintaining safety even when the thickness of the steel sheet is reduced. The high-strength steel sheet according to aspects of the present invention can be applied to members for trucks or automobiles.

Note that, in accordance with aspects of the present invention, the area fractions and mechanical properties of the above microstructure phases are determined by the methods described in Examples below.

A method for producing the high-strength steel sheet according to an embodiment of the present invention is described below. In the following description, the symbol “° C.” used for describing temperature refers to the surface temperature of the object (steel material or steel sheet) unless otherwise specified.

The high-strength steel sheet according to aspects of the present invention can be produced by subjecting a steel material to the treatments (1) to (6) below in order. Each of the steps is described below.

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

Note that the steel material may be any steel material having the above-described chemical composition. The high-strength steel sheet that is to be produced finally has the same chemical composition as the steel material used. The steel material may be, for example, a steel slab. The method for producing the steel material is not limited. For example, a molten steel having the above-described chemical composition is prepared using a known method, such as a converter, and the molten steel is formed into a steel material by a casting method, such as continuous casting. A method other than continuous casting, such as ingot casting-blooming rolling, may also be used. Alternatively, steel scrap may also be used as a raw material. After the steel material has been produced by continuous casting or the like, it may be directly subjected to the subsequent heating step. In another case, the steel material may be cooled to prepare warm or cold steel pieces, which are subjected to the heating step.

(1) Heating

First, the steel material is heated to a heating temperature of 1150° C. or more. In the steel material cooled to low temperatures, most of the carbonitride-forming elements, such as Ti, are present in the form of coarse carbonitride particles in a nonuniform manner. The presence of the coarse and nonuniform precipitates degrades various properties (e.g., strength and fatigue resistance) commonly required for high-strength steel sheets used for producing parts for trucks or automobiles. Therefore, it is necessary to heat the steel material prior to hot rolling to dissolve the coarse precipitates. Accordingly, the temperature to which the steel material is heated is 1150° C. or more. The above heating temperature is preferably 1180° C. or more and is more preferably 1200° C. or more. However, if the temperature to which the steel material is heated is excessively high, slab flaws may occur. Furthermore, yields may be reduced due to descaling. Accordingly, the temperature to which the steel material is heated is preferably 1350° C. or less, is more preferably 1300° C. or less, and is further preferably 1280° C. or less.

In the heating step, it is preferable to hold the temperature of the steel material at the above heating temperature after the temperature of the steel material has been increased to the heating temperature in order to make the temperature of the steel material uniform. Although the amount of time (holding time) during which the temperature of the steel material is held at the heating temperature is not limited, the holding time is preferably 1800 seconds or more in order to increase the uniformity in the temperature of the steel material. However, if the holding time is more than 10000 seconds, the amount of the scale generated may be increased. This increases, for example, the possibility of entanglement of the scale in the subsequent hot rolling step and consequently may reduce yields due to surface flaw defects. Accordingly, the holding time is preferably 10000 seconds or less and is more preferably 8000 seconds or less. Alternatively, subsequent to casting, the steel material that has not been hot-rolled may be directly subjected to hot rolling (hot direct rolling) while the temperature of the steel material is high (i.e., while the temperature of the steel material is held to fall within the above heating temperature range).

(2) Hot Rolling

The heated steel material (or, the as-cast steel material having a high temperature) is subjected to a hot rolling step in which rough rolling and finish rolling are performed. The conditions under which the rough rolling is performed are not limited and may be any conditions with which the resulting sheet bar has intended dimensions. The steel material is rough-rolled to form a rough-rolled steel sheet bar. Prior to finish rolling, the rough-rolled steel sheet bar may be subjected to descaling (high-pressure water descaling) in which high-pressure water is sprayed at the entry side of the finish rolling mill.

In accordance with aspects of the present invention, when temperatures RC1 and RC2 are defined by Formulae (1) and (2) below, finish rolling is performed such that the total of the rolling reductions achieved in the temperature range of equal to or more than (RC1−150)° C. and equal to or less than RC1° C. is 35% or more. The amount of residence time during which the temperature is held in the temperature range is not limited and may be 3 seconds or more and 20 seconds or less. Moreover, the finish rolling delivery temperature is set to a temperature equal to or more than (RC2−100)° C. and equal to or less than (RC2+50)° C. RC1 is an austenite 50%-recrystallization temperature estimated from the chemical composition and RC2 is the lower limit for the austenite recrystallization temperature which is estimated from the chemical composition. If the above total rolling reduction achieved in the temperature range of equal to or more than (RC1−150)° C. and equal to or less than RC1° C. is less than 35%, the average grain size increases and, consequently, it becomes impossible to enhance fatigue resistance. Accordingly, the total rolling reduction achieved in the temperature range of equal to or more than (RC1−150)° C. and equal to or less than RC1° C. is 35% or more. The above total rolling reduction is preferably 45% or more and is more preferably 60% or more.

Hot rolling is performed such that the finish rolling delivery temperature is equal to or more than (RC2−100)° C. and equal to or less than (RC2+50)° C. If the finish rolling delivery temperature is less than (RC2−100)° C., ferrite is formed and, consequently, a tensile strength of 980 MPa or more cannot be achieved. Accordingly, the finish rolling delivery temperature is equal to or more than (RC2−100)° C., is preferably equal to or more than (RC2−90)° C., and is more preferably equal to or more than (RC2−70)° C. On the other hand, if the finish rolling delivery temperature is more than (RC2+50)° C., austenite grains become coarsened and the average grain size of upper bainite increases consequently. This reduces strength. Accordingly, the finish rolling delivery temperature is equal to or less than (RC2+50)° C., is preferably equal to or less than (RC2+40)° C., and is more preferably equal to or less than (RC2+30)° C. RC1 and RC2 are defined by Formulae (1) and (2) below.

RC ⁢ 1 ⁢ ( °C ) = 900 + 120 × C + 100 × N + 10 × Mn + 500 × Ti + 5000 × B + 10 × Cr + 50 × Mo + 1500 × Nb + 150 × V ( 1 ) RC ⁢ 2 ⁢ ( °C ) = 750 + 120 × C + 100 × N + 10 × Mn + 250 × Ti + 5000 × B + 10 × Cr + 50 × Mo + 750 × Nb + 150 × V ( 2 )

• • where each of element symbols used in Formulae (1) and (2) represents the content (% by mass) of the element and is zero when the element is absent.

(3) Cooling (First Cooling)

The hot-rolled steel sheet is cooled (first cooling). In the cooling step, the time interval between the end of hot rolling and the start of cooling (cooling start time) is limited to 2.0 s or less after the end of finish rolling. If the above cooling start time is less than 2.0 s, austenite grains grow disadvantageously and a tensile strength of 980 MPa or more cannot be achieved consequently. Accordingly, the cooling start time is 2.0 s or less, is preferably 1.5 s or less, and is more preferably 1.0 s or less.

In the cooling step, if the average cooling rate at which the temperature is reduced from the finish rolling delivery temperature to the cooling stop temperature is excessively low, ferrite transformation may disadvantageously occur prior to upper bainite transformation and the intended area fraction of upper bainite phase cannot be formed consequently. Accordingly, the average cooling rate is 20° C./s or more, is preferably 30° C./s or more, and is more preferably 50° C./s or more. Although the upper limit is not set, if the average cooling rate is excessively high, it becomes difficult to control the cooling stop temperature and, consequently, it may become difficult to form the intended microstructure. Therefore, the average cooling rate is preferably 500° C./s or less, is more preferably 300° C./s or less, and is further preferably 150° C./s or less. In the cooling step, forced cooling may be performed such that the above average cooling rate can be achieved. The cooling method is not limited. It is preferable to perform water cooling or the like.

The cooling stop temperature is set to a temperature equal to or more than Trs°C and equal to or less than (Trs+180)° C. If the cooling stop temperature is less than Trs°C, the microstructure includes lower bainite. Although lower bainite is a microstructure phase having a high strength, it may have low fatigue resistance after subjected to a heat treatment. Accordingly, the cooling stop temperature is set to a temperature equal to or more than Trs°C. On the other hand, if the cooling stop temperature is more than (Trs+180)° C., ferrite may be generated disadvantageously. This makes it impossible to achieve a tensile strength of 980 MPa or more. Accordingly, the cooling stop temperature is set to a temperature equal to or less than (Trs+180)° C. Trs is defined using Formula (3) below.

Trs ⁢ ( °C ) = 500 - 450 × C - 35 × Mn - 15 × Cr - 10 × Ni - 20 × Mo ( 3 )

• • where each of element symbols used in Formula (3) represents the content (% by mass) of the element and is zero when the element is absent.

(4) Coiling

The cooled hot-rolled steel sheet is coiled at a coiling temperature equal to or more than Trs°C and equal to or less than (Trs+180)° C. If the coiling temperature is less than Trs°C, lower bainite transformation may occur subsequent to coiling and, consequently, intended martensite and/or retained austenite cannot be formed. Accordingly, the coiling temperature is limited to a temperature equal to or more than Trs°C, is preferably equal to or more than (Trs+10)° C., and is more preferably equal to or more than (Trs+30)° C. On the other hand, if the coiling temperature is more than (Trs+180)° C., ferrite may be generated disadvantageously. This makes it impossible to achieve a tensile strength of 980 MPa or more. Accordingly, the coiling temperature is limited to a temperature equal to or less than (Trs+180)° C., is preferably equal to or less than (Trs+150)° C., and is more preferably equal to or less than (Trs+120)° C.

(5) Cooling (Second Cooling)

Subsequently, the temperature is reduced to a temperature equal to or less than (Trs−250)° C. at an average cooling rate of 1° C./s or less (second cooling). If the average cooling rate at which the temperature is reduced from the coiling temperature to a temperature equal to or less than (Trs−250)° C. is more than 1° C./s, bainite transformation does not occur to a sufficient degree and the amount of martensite and retained austenite increases consequently. This makes it impossible to form the microstructure intended in accordance with aspects of the present invention. Accordingly, the average cooling rate at which the temperature is reduced from the coiling temperature to a temperature equal to or less than (Trs−250)° C. is limited to 1° C./s or less, is preferably 0.8° C./s or less, and is more preferably 0.5° C./s or less. Although cooling may be performed to any temperature equal to or less than (Trs−250)° C., it is preferable to reduce the temperature to about 10° C. to 30° C. The steel sheet may be cooled in any form. For example, the steel sheet may be cooled after it has been wound into a coil.

(6) Temper Rolling

The cooled steel sheet is temper-rolled at a rolling reduction of 0.1% or more and 5.0% or less. If the rolling reduction is less than 0.1%, the dislocation density becomes insufficient and a markedly high fatigue strength cannot be achieved. Accordingly, the rolling reduction is limited to 0.1% or more, is preferably 0.3% or more, and is more preferably 0.5% or more. However, if temper rolling is performed at a rolling reduction of more than 5.0%, the amount of load applied to the rolls increases. This disadvantageously increases the number of times the rollers need to be replaced and the manufacturing costs. Accordingly, the rolling reduction is limited to 5.0% or less, is preferably 4.0% or less, and is more preferably 3.0% or less.

The high-strength steel sheet according to aspects of the present invention can be produced by the above-described steps. Optionally, for example, pickling may be performed after temper rolling in accordance with the conventional method in order to remove scales formed on the surface of the steel sheet.

Examples

Molten steels having the compositions described in Table 1 were prepared using a converter and formed into steel slabs by continuous casting, which were used as steel materials.

TABLE 1
Steel	Chemical composition (mass %) *

type	C	Si	Mn	P	S	Al	N	B	Ti	Nb	Others
A	0.065	0.68	1.75	0.015	0.0010	0.042	0.0050	0.0020	0.13	0.000	—
B	0.100	0.87	2.48	0.010	0.0008	0.040	0.0041	0.0015	0.06	0.000	—
C	0.108	1.44	2.26	0.011	0.0007	0.061	0.0056	0.0015	0.13	0.000	—
D	0.137	1.09	1.44	0.010	0.0009	0.050	0.0045	0.0013	0.14	0.000	Sb: 0.007
E	0.033	0.19	1.98	0.015	0.0013	0.038	0.0035	0.0015	0.00	0.050	Cr: 0.20,
											Ca: 0.0035
F	0.076	1.20	1.26	0.015	0.0010	0.045	0.0045	0.0018	0.00	0.042	—
G	0.068	0.87	1.41	0.005	0.0006	0.040	0.0035	0.0025	0.00	0.065	Cr: 0.30
H	0.098	0.79	1.89	0.009	0.0017	0.039	0.0055	0.0002	0.00	0.023	Cr: 0.60,
											Sb: 0.019
I	0.106	0.82	1.69	0.011	0.0009	0.050	0.0040	0.0017	0.13	0.021	Mo: 0.23
J	0.092	1.49	1.55	0.021	0.0006	0.867	0.0029	0.0020	0.24	0.028	Mo: 0.41
K	0.049	1.39	1.47	0.012	0.0013	0.048	0.0036	0.0032	0.23	0.051	Mo: 0.34
L	0.082	0.71	1.54	0.008	0.0006	0.039	0.0028	0.0095	0.09	0.062	Cu: 1.02
M	0.070	1.28	1.79	0.014	0.0023	1.821	0.0038	0.0010	0.16	0.022	Ni: 0.48,
											Mg: 0.0022
N	0.066	1.15	2.18	0.013	0.0009	0.043	0.0040	0.0015	0.17	0.031	REM: 0.0016
O	0.144	0.97	0.92	0.019	0.0018	0.041	0.0098	0.0005	0.12	0.006	—
P	0.100	0.24	1.79	0.034	0.0008	0.041	0.0041	0.0007	0.06	0.007	V: 0.013,
											Sn: 0.009
Q	0.070	0.52	2.16	0.0018	0.0025	0.043	0.0052	0.0016	0.06	0.039	Ca: 0.002
a	0.022	0.87	1.03	0.029	0.0012	0.031	0.0044	0.0017	0.24	0.000	—
b	0.178	1.14	2.13	0.012	0.0039	0.028	0.0035	0.0016	0.20	0.000	—
c	0.094	3.21	1.64	0.025	0.0031	0.045	0.0055	0.0019	0.12	0.000	—
d	0.083	0.99	2.33	0.013	0.0012	0.052	0.0037	0.0001	0.00	0.072	—
e	0.062	1.41	0.68	0.020	0.0015	0.065	0.0042	0.0019	0.00	0.023	Cr: 0.80
f	0.071	0.67	3.21	0.010	0.0015	0.047	0.0062	0.0014	0.00	0.018	—
g	0.090	1.11	1.95	0.016	0.0017	0.055	0.0038	0.0019	0.00	0.123	Cr: 0.18
h	0.096	1.41	1.96	0.016	0.0015	0.045	0.0042	0.0026	0.05	0.000	Cr: 2.70
i	0.080	1.23	1.90	0.009	0.0009	0.051	0.0036	0.0020	0.34	0.000	Cr: 1.10
j	0.048	1.42	1.96	0.078	0.0014	0.042	0.0041	0.0008	0.00	0.000	—
k	0.077	1.98	0.92	0.002	0.0003	0.012	0.0028	0.0140	0.08	0.012	V: 0.021,
											Mo: 0.002

					RC1 −	RC2 −	RC2 +	Trs +	Trs −
	Steel	RC1	RC2	Trs	150	100	50	180	250
	type	(° C.)	(° C.)	(° C.)	(° C.)	(° C.)	(° C.)	(° C.)	(° C.)	Remarks
	A	1001	818	410	901	718	868	590	160	Conforming
										steel
	B	975	810	368	875	710	860	548	118	Conforming
										steel
	C	1009	826	372	909	726	876	552	122	Conforming
										steel
	D	1008	823	388	908	723	873	568	138	Conforming
										steel
	E	1009	821	413	909	721	871	593	163	Conforming
										steel
	F	994	813	422	894	713	863	602	172	Conforming
										steel
	G	1036	837	416	936	737	887	596	166	Conforming
										steel
	H	975	807	381	875	707	857	561	131	Conforming
										steel
	I	1047	848	389	947	748	898	569	139	Conforming
										steel
	J	1119	888	396	1019	788	938	576	146	Conforming
										steel
	K	1145	900	420	1045	800	950	600	170	Conforming
										steel
	L	1111	892	409	1011	792	942	589	159	Conforming
										steel
	M	1045	838	401	945	738	888	581	151	Conforming
										steel
	N	1069	853	394	969	753	903	574	144	Conforming
										steel
	O	1000	815	403	900	715	865	583	153	Conforming
										steel
	P	977	807	392	877	707	857	572	142	Conforming
										steel
	Q	1032	837	386	932	737	887	566	136	Conforming
										steel
	a	1042	832	454	942	732	882	634	204	Comparative
										steel
	b	1051	851	345	951	751	901	525	95	Comparative
										steel
	c	998	818	400	898	718	868	580	150	Comparative
										steel
	d	1043	839	381	943	739	889	561	131	Comparative
										steel
	e	967	799	436	867	699	849	616	186	Comparative
										steel
	f	975	812	356	875	712	862	536	106	Comparative
										steel
	g	1126	884	389	1026	784	934	569	139	Comparative
										steel
	h	974	812	381	874	712	862	561	131	Comparative
										steel
	i	950	800	381	850	700	850	561	131	Comparative
										steel
	j	931	780	410	831	680	830	590	160	Comparative
										steel
	k	931	780	410	831	680	830	590	160	Comparative
										steel
	The underlined parts are outside the scope of the invention

The steel materials were heated to the respective heating temperatures described in Table 2. The heated steel materials were subjected to a hot-rolling process consisting of rough rolling and finish rolling to form hot-rolled steel sheets. Table 2 lists the finish rolling delivery temperatures in the hot rolling process.

The hot-rolled steel sheets were each cooled (first cooling) with the average cooling rate and the cooling stop temperature listed in Table 2. The cooled hot-rolled steel sheets were coiled at the respective coiling temperatures listed in Table 2. The coiled steel sheets were cooled (second cooling) with the respective average cooling rates listed in Table 2 to form high-strength steel sheets. Subsequent to cooling, temper rolling was performed at the rolling reductions listed in Table 2. Then, pickling was performed. Pickling was performed at a temperature of 85° C. using a 10-mass % aqueous solution of hydrochloric acid. Subsequently, the steel sheets were subjected to a heat treatment (170° C. and 20 minutes) that corresponded to baking coating. Hereby, high-strength hot-rolled steel sheets were prepared.

	Manufacturing conditions

Hot rolling

Total rolling reduction

Finish

First cooling

		Heating	in temperature range of	rolling	Time interval between	Average	Cooling
		Heating	(RC1 − 150° C.) or	delivery	end of hot rolling	cooling	stop
	Steel	temperature	more and RC1 or less	temperature	and start of cooling	rate	temperature
No.	type	(° C.)	(%)	(° C.)	(s)	(° C./s)	(° C.)
1	A	1250	40	790	1.0	60	455
2	A	1180	45	820	1.4	45	470
3	A	1210	45	850	0.5	45	465
4	A	1200	40	750	0.8	80	515
5	A	1240	50	800	1.0	55	570
6	A	1245	25	820	0.8	60	480
7	A	1250	45	690	2.2	50	430
8	A	1230	80	890	1.0	70	525
9	A	1270	50	825	0.6	4	440
10	A	1260	40	780	0.8	45	310
11	A	1240	45	760	1.0	50	650
12	A	1240	40	810	1.4	37	450
13	A	1230	50	800	1.2	40	475
14	B	1230	45	850	1.6	75	455
15	B	1220	40	700	0.5	49	430
16	C	1220	65	855	1.0	37	530
17	C	1190	75	950	0.8	56	515
18	D	1220	60	740	0.6	92	450
19	D	1215	85	785	1.0	3	430
20	E	1270	40	765	0.8	75	500
21	E	1230	40	775	1.2	34	280
22	F	1200	45	780	1.0	62	520
23	G	1170	70	840	1.0	95	580
24	G	1250	50	810	1.4	55	485
25	G	1230	55	825	0.5	38	470
26	G	1260	45	785	0.8	32	445
27	G	1250	60	800	0.6	28	670
28	H	1230	60	850	1.6	47	445
29	H	1230	50	820	1.8	67	525
30	I	1250	40	795	1.0	38	490
31	J	1240	65	930	0.6	46	520
32	K	1235	45	910	0.8	58	510
33	L	1210	50	875	1.4	92	555
34	M	1235	45	790	0.5	62	500
35	N	1220	50	860	0.8	38	460
36	O	1300	60	800	1.0	77	525
37	P	1270	75	780	1.6	64	540
38	Q	1240	40	845	1.0	70	460
39	a	1250	50	865	0.6	88	515
40	b	1220	40	780	0.8	29	470
41	c	1210	55	760	1.0	26	435
42	d	1215	40	780	1.4	46	440
43	e	1230	35	815	0.5	32	520
44	f	1260	45	805	0.8	67	500
45	g	1240	50	890	1.0	70	510
46	h	1245	65	780	1.6	82	520
47	i	1220	45	810	1.0	78	490
48	j	1300	75	785	1.4	57	520
49	k	1210	30	790	0.8	48	500

Manufacturing conditions

Second cooling

Temper

			Coiling	Average	Cooling	rolling
			Coiling	cooling	stop	Rolling
		Steel	temperature	rate	temperature	reduction
	No.	type	(° C.)	(° C./s)	(° C.)	%)	Remarks
	1	A	445	<0.5	50	1.5	Invention
							example
	2	A	460	<0.5	55	1.6	Invention
							example
	3	A	455	<0.5	55	2.4	Invention
							example
	4	A	420	<0.5	35	1.2	Invention
							example
	5	A	550	<0.5	80	0.6	Invention
							example
	6	A	465	<0.5	60	1.5	Comparative
							example
	7	A	450	<0.5	35	2.4	Comparative
							example
	8	A	510	<0.5	45	1.1	Comparative
							example
	9	A	440	<0.5	85	0.7	Comparative
							example
	10	A	380	<0.5	55	2.0	Comparative
							example
	11	A	600	<0.5	75	2.7	Comparative
							example
	12	A	470	2	35	0.8	Comparative
							example
	13	A	465	<0.5	30	0.0	Comparative
							example
	14	B	470	<0.5	45	2.6	Invention
							example
	15	B	440	<0.5	55	0.9	Comparative
							example
	16	C	520	<0.5	75	0.7	Invention
							example
	17	C	410	<0.5	50	1.3	Comparative
							example
	18	D	445	<0.5	40	2.1	Invention
							example
	19	D	460	<0.5	35	0.6	Comparative
							example
	20	E	490	<0.5	80	2.5	Invention
							example
	21	E	370	<0.5	55	2.0	Comparative
							example
	22	F	450	<0.5	40	1.9	Invention
							example
	23	G	570	<0.5	75	2.9	Invention
							example
	24	G	420	<0.5	50	1.8	Invention
							example
	25	G	430	<0.5	70	2.5	Invention
							example
	26	G	450	<0.5	85	0.5	Invention
							example
	27	G	630	<0.5	65	2.4	Comparative
							example
	28	H	440	<0.5	70	1.0	Invention
							example
	29	H	510	3	30	1.0	Comparative
							example
	30	I	490	<0.5	55	1.7	Invention
							example
	31	J	510	<0.5	70	1.5	Invention
							example
	32	K	430	<0.5	50	2.2	Invention
							example
	33	L	410	<0.5	55	0.5	Invention
							example
	34	M	495	<0.5	80	1.2	Invention
							example
	35	N	430	<0.5	55	0.8	Invention
							example
	36	O	510	<0.5	35	0.6	Invention
							example
	37	P	530	<0.5	75	2.5	Invention
							example
	38	Q	440	<0.5	70	1.0	Invention
							example
	39	a	420	<0.5	35	2.5	Comparative
							example
	40	b	450	<0.5	45	1.4	Comparative
							example
	41	c	435	<0.5	50	2.2	Comparative
							example
	42	d	425	<0.5	25	2.4	Comparative
							example
	43	e	510	<0.5	50	2.1	Comparative
							example
	44	f	480	<0.5	85	3.0	Comparative
							example
	45	g	430	<0.5	60	1.6	Comparative
							example
	46	h	440	<0.5	60	1.7	Comparative
							example
	47	i	430	<0.5	80	1.6	Comparative
							example
	48	j	510	<0.5	50	4.3	Comparative
							example
	49	k	490	<0.5	40	1.2	Comparative
							example
	The underlined parts are outside the scope of the invention

A test specimen was taken from each of the high-strength steel sheets, and the microstructure and mechanical properties of the specimen were determined by the following steps.

<Microstructure>

A test specimen for microstructure observation was taken from each of the high-strength steel sheets such that a cross section of the steel sheet which was taken in the thickness direction so as to be parallel to the rolling direction was exposed as an observation plane. The surface of the test specimen was ground and then corroded with an etchant (3% nital solution) in order to cause the microstructure to appear. Subsequently, an image of the surface layer that extended from the surface to the position 1/10 of the thickness of the steel sheet was taken using a scanning electron microscope (SEM) at a 5000-fold magnification in 10 fields of view in order to obtain SEM images of microstructure. The SEM images were analyzed by image processing in order to determine the area fractions of upper bainite (UB), polygonal ferrite (F), and lower bainite (LB). Since it is difficult to distinguish martensite (M) and retained austenite (γ) from each other with a SEM, the area fractions and average grain sizes thereof were determined by making identifications using an electron back scatter diffraction patterns (EBSD) method. Table 3 lists the area fractions and average grain sizes of the above microstructure phases. Table 3 also lists the total area fractions (M+γ) of martensite and retained austenite.

<Tensile Test>

A JIS No. 5 test piece for tensile test (JIS Z 2201) was taken from each of the hot-rolled steel sheets such that the tensile direction of the test piece was perpendicular to the rolling direction. A tensile test was conducted in conformity with JIS Z 2241 at a strain rate of 10-3/s in order to determine tensile strength. In accordance with aspects of the present invention, an evaluation of “Passed” was given when the tensile strength was 980 MPa or more. Table 3 lists the results.

<Plane Bending Fatigue Test>

A test specimen having the dimensions and shape illustrated in the FIGURE was taken from each of the hot-rolled steel sheets such that the longitudinal direction of the test specimen was perpendicular to the rolling direction. A plane bending fatigue test was conducted in conformity with JIS Z 2275. The stress loading mode was such that the stress ratio R was −1 and the frequency f was 25 Hz. The amplitude of loading stress was changed in six stages, and the number of stress cycles applied until rupture occurred was measured. An S-N curve was determined, and a fatigue strength (fatigue limit) at 2×10⁶ cycles was calculated. In accordance with aspects of the present invention, an evaluation of “excellent fatigue resistance” was given when a fatigue limit ratio calculated by dividing the fatigue limit by the tensile strength determined in the tensile test was 0.50 or more. Table 3 lists the results.

	TABLE 3
	Plane bending

Microstructure of surface layer

Mechanical

fatigue

Fatigue

	Average	Dis-	property	strength	limit
	grain	location	Tensile	at 2 × 106	ratio,

Steel

Area fraction (%)

size

density

strength

cycles, σw

σw/TS

No.

type

UB

M

γ

M + γ

F

LB

(μm)

(1014/m2)

(MPa)

(MPa)

(—)

Remarks

1	A	90	8	2	10	0	0	5.4	10.0	1137	603	0.53	Invention
													example
2	A	88	11	1	12	0	0	5.1	15.0	1080	562	0.52	Invention
													example
3	A	89	10	1	11	0	0	5.1	8.0	1190	690	0.58	Invention
													example
4	A	93	5	2	7	0	0	4.9	10.0	1188	653	0.55	Invention
													example
5	A	83	13	4	17	0	0	5.5	15.0	1002	601	0.60	Invention
													example
6	A	91	8	1	9	0	0	8.3	9.0	1068	374	0.35	Comparative
													example
7	A	76	5	1	6	18	0	1.8	3.5	937	525	0.56	Comparative
													example
8	A	89	9	2	11	0	0	7.5	12.0	1182	449	0.38	Comparative
													example
9	A	69	16	5	21	10	0	4.8	5.5	975	371	0.52	Comparative
													example
10	A	7	1	0	1	0	92	5.3	13.5	1006	507	0.34	Comparative
													example
11	A	0	0	0	0	100	0	5.6	4.5	940	342	0.54	Comparative
													example
12	A	53	45	2	47	0	0	5.2	15.0	1256	508	0.41	Comparative
													example
13	A	92	3	5	8	0	0	5.9	7.0	991	515	0.47	Comparative
													example
14	B	82	13	15	18	0	0	5.2	12.0	1097	669	0.61	Invention
													example
15	B	65	6	2	8	27	0	5.1	15.0	876	515	0.43	Comparative
													example
16	C	89	6	5	11	0	0	4.6	10.0	1068	555	0.52	Invention
													example
17	C	90	7	3	10	0	0	9.0	8.0	970	377	0.33	Comparative
													example
18	D	85	12	3	15	0	0	3.8	12.0	1163	593	0.51	Invention
													example
19	D	15	0	0	0	85	0	5.4	2.0	894	320	0.52	Comparative
													example
20	E	90	6	4	10	0	0	5.9	13.0	1171	644	0.55	Invention
													example
21	E	4	0	0	0	0	96	5.6	9.5	1087	465	0.36	Comparative
													example
22	F	82	16	2	18	0	0	5.3	8.0	1075	570	0.53	Invention
													example
23	G	85	12	3	15	0	0	5.1	8.0	1036	559	0.54	Invention
													example
24	G	85	13	2	15	0	0	4.9	14.0	1022	593	0.58	Invention
													example
25	G	92	5	3	8	0	0	5.8	14.0	1113	579	0.52	Invention
													example
26	G	79	16	5	21	0	0	4.7	10.0	1132	623	0.55	Invention
													example
27	G	0	0	0	0	100	0	7.6	2.5	870	391	0.38	Comparative
													example
28	H	83	11	6	17	0	0	4.2	10.0	1066	629	0.59	Invention
													example
29	H	69	28	3	31	0	0	5.9	12.5	1346	331	0.38	Comparative
													example
30	I	86	11	3	14	0	0	5.7	8.0	1105	575	0.52	Invention
													example
31	J	85	12	3	15	0	0	5.8	16.5	1079	669	0.62	Invention
													example
32	K	77	18	5	23	0	0	5.2	9.0	1070	578	0.54	Invention
													example
33	L	80	16	4	20	0	0	4.9	10.0	1034	579	0.56	Invention
													example
34	M	86	13	1	14	0	0	5.4	13.0	1106	586	0.53	Invention
													example
35	N	88	9	3	12	0	0	5.2	9.0	1057	581	0.55	Invention
													example
36	O	86	10	4	14	0	0	4.1	14.0	1112	578	0.52	Invention
													example
37	P	90	8	2	10	0	0	3.8	13.0	1131	679	0.60	Invention
													example
38	Q	96	2	2	4	0	0	4.2	8.0	1119	615	0.55	Invention
													example
39	a	94	5	1	6	0	0	5.5	12.0	970	534	0.55	Comparative
													example
40	b	91	6	3	9	0	0	5.7	15.0	1017	427	0.42	Comparative
													example
41	c	93	3	4	7	0	0	4.8	14.0	1160	452	0.39	Comparative
													example
42	d	73	1	2	3	24	0	5.8	4.5	887	426	0.48	Comparative
													example
43	e	100	0	0	0	0	0	5.5	3.0	1045	376	0.36	Comparative
													example
44	f	88	7	5	12	0	0	5.7	14.0	1200	564	0.47	Comparative
													example
45	g	48	45	7	52	0	0	4.9	12.0	1062	350	0.33	Comparative
													example
46	h	70	2	1	3	27	0	4.3	6.5	959	451	0.47	Comparative
													example
47	i	77	21	2	23	0	0	3.2	11.0	922	479	0.52	Comparative
													example
48	j	84	9	7	16	0	0	5.2	10.0	1214	364	0.30	Comparative
													example
49	k	66	32	2	34	0	0	4.3	14.0	1312	604	0.46	Comparative
													example
The underlined parts are outside the scope of the invention
UB: upper bainite,
M: martensite,
γ: retained austenite,
F: polygonal ferrite,
LB: lower bainite

Invention Examples were all high-strength steel sheets having a tensile strength of 980 MPa or more and excellent fatigue resistance. In contrast, Comparative Examples, which were outside the scope of the present invention, did not have a tensile strength of 980 MPa or more or excellent fatigue resistance.