High-strength steel sheet and method for manufacturing the same

Abstract

The present invention aims to provide a high-strength steel sheet having a tensile strength of 980 MPa or more, press formability, and fatigue resistance, and a method for manufacturing the same. The high-strength steel sheet has a composition defined by a certain formula with an MSC of 2.7 to 3.8 mass%, a microstructure containing a certain structure in a surface layer region from the surface of the steel sheet to a depth of 100 μm and in an internal region other than the surface layer region, a maximum height of surface roughness of the steel sheet of 30 μm or less, a tensile strength of 980 MPa or more, an uniform elongation of 6% or more, and a ratio of the 10 7 second plane complete fatigue strength to the tensile strength (fatigue limit ratio) of 0.45 or more.

Description

TECHNICAL FIELD

[0001] The present application relates to a high-strength steel sheet and a method for manufacturing the same. In particular, it relates to a high-strength steel sheet having a tensile strength of 980 MPa or more and an uniform elongation of 6% or more, and further having excellent fatigue resistance, and a method for manufacturing the same, which is suitable as a material for a frame, a suspension member, or the like of a truck or a passenger car. BACKGROUND

[0002] Under the background of emission regulations for automobiles aimed at controlling global warming, the light-weighting of automobiles is required. For the light-weighting of automobiles, it is effective to reduce the amount of material used in the same automobile component by making the material used as a base material for an automobile component high-strength and thin-walled. Therefore, the use of high-strength steel sheets has been increasing year by year. In particular, a high-strength steel sheet having a tensile strength of 980 MPa or more is expected as a base material that can significantly improve the fuel efficiency of an automobile through light-weighting.

[0003] On the other hand, if the tensile strength of a steel sheet is increased, the ductility decreases, and thus the press formability of the steel sheet deteriorates. Automobile components, particularly chassis components such as suspension members, require complex shapes to ensure rigidity. Therefore, the base material for an automobile component needs to have high press formability, i.e., ductility.

[0004] Further, in order to ensure the durability of a component, it is necessary to increase the fatigue strength of a steel sheet. However, increasing the tensile strength of a steel sheet does not necessarily increase the fatigue strength. If the fatigue strength is low, it can be impossible to obtain the component durability assumed in design. Therefore, the material used for an automobile component or the like needs to have excellent fatigue resistance.

[0005] So far, in order to increase the fatigue resistance while increasing the tensile strength of a steel sheet, for example, the techniques of Patent Documents 1 to 3 have been proposed.

[0006] PRIOR ART DOCUMENTS

[0007] PATENT DOCUMENTS

[0008] Patent Document 1: International Publication No. 2016 / 010004

[0009] Patent Document 2: Japanese Patent Application Publication No. 2012-012701

[0010] Patent Document 3: International Publication No. 2014 / 188966 SUMMARY

[0011] However, the prior art described in Patent Documents 1 to 3 has the following problems.

[0012] In the technologies described in Patent Documents 1 and 2, a tensile strength of 980 MPa or more is not obtained. In addition, although both are described as having excellent workability, only "elongation" is used as an index of workability. This "elongation" is also referred to as total elongation (El), and indicates the elongation at the time of fracture of a test piece in a tensile test. However, in reality, necking occurs at a stage before fracture. When necking occurs, the thickness of the steel sheet locally becomes thin, and this leads to product defects at the time of press forming. Therefore, for achieving excellent press formability, it is not enough that only the total elongation is high.

[0013] In the technology described in Patent Document 3, a high-strength steel sheet having excellent fatigue properties can be obtained, but since the main phase is a tempering martensite or lower bainite phase that lacks ductility, the ductility of the steel sheet is insufficient, and in the case of application to a member such as an automobile chassis member that requires high ductility, there is a possibility that forming defects will occur.

[0014] As described above, the actual situation is that a technology for obtaining a high-strength steel sheet that has a high level of tensile strength, press formability, and fatigue resistance has not been established.

[0015] The present application was made in view of the above-described actual situation, and aims to provide a high-strength steel sheet that has a tensile strength of 980 MPa or more, press formability, and fatigue resistance, and a method for manufacturing the same.

[0016] The present inventors and others produced a virtual stress-strain curve of a steel sheet having a tensile strength of 980 MPa or more, various yield stresses, and a uniform elongation, and using the stress-strain curve, performed press forming simulation of a suspension member. Then, based on the results of the simulation, the characteristics of the steel sheet required to obtain excellent press formability were studied.

[0017] As a result, it was found that in a steel sheet having a tensile strength of 980 MPa or more, if the uniform elongation is ensured to be 6% or more, the thinning at the time of press forming can be controlled to a minimum, and press forming defects can be suppressed.

[0018] In addition, the present inventors and others studied the optimum steel sheet structure for obtaining a tensile strength of 980 MPa or more and a uniform elongation of 6% or more. As a result, it was shown that by forming a microstructure in which the main phase is upper bainite and an appropriate amount of a hard second phase containing fresh martensite and / or residual austenite is contained, a high strength of 980 MPa or more and a uniform elongation of 6% or more can be obtained.

[0019] Furthermore, it was also clarified that in order to obtain a microstructure containing an appropriate amount of a hard second phase containing fresh martensite and / or residual austenite, it is necessary to add Si, Mn, Cr, and Mo in balance.

[0020] Note that the upper bainite here refers to a collection of lath-shaped ferrite having an orientation difference of less than 15°, and a structure having Fe-based carbide and / or residual austenite between the lath-shaped ferrite (including the case where there is no Fe-based carbide and / or residual austenite between the lath-shaped ferrite). The lath-shaped ferrite is distinguished from thin-plate (lamellar) ferrite in pearlite and polygonal ferrite because of the shape of the lath and the high dislocation density inside, and can be distinguished using SEM (scanning electron microscope) and TEM (transmission electron microscope). Note that in the case where there is residual austenite between the laths, only the lath-shaped ferrite portion is regarded as the upper bainite, and distinguished from the residual austenite. In addition, fresh martensite refers to martensite having no Fe-based carbide. Fresh martensite and residual austenite have the same contrast under SEM, but can be distinguished using Electron Backscatter Diffraction Patterns (EBSD).

[0021] In addition, in general, the fatigue life of a steel sheet is determined by the time required for the generation and propagation of a fatigue crack, and by delaying these times, a steel sheet having excellent fatigue properties can be obtained. The present inventors have newly found that by controlling the maximum height (Ry) of the surface roughness of a high-strength steel sheet, the generation of an initial crack can be delayed, and the fatigue resistance can be improved. Furthermore, it has also been found that by controlling the microstructure of the surface layer of a steel sheet, the propagation of an initial fatigue crack can also be delayed, and the fatigue resistance can be further improved.

[0022] The present application was further researched based on the above insight, and the gist is as follows.

[0023] [1] A high-strength steel sheet having the following composition consisting of, in mass%: C: 0.05 to 0.20%, Si: 0.6 to 1.2%, Mn: 1.3 to 3.7%, P: 0.10% or less, S: 0.03% or less, Al: 0.001 to 2.0%, N: 0.01% or less, O: 0.01% or less, and B: 0.0005 to 0.010%, with the remainder consisting of Fe and unavoidable impurities, and having an MSC defined by the following (1) formula of 2.7 to 3.8 mass%:

[0024] The microstructure is such that in a surface layer region from the surface of the steel sheet to a depth of 100 μm, 70% or more by area of upper bainite and 2% or more by total area of fresh martensite and / or residual austenite are contained, the average grain diameter of the upper bainite is 7 μm or less, the average grain diameter of the fresh martensite and / or residual austenite is 4 μm or less, and the number density of the fresh martensite and / or residual austenite is 100 pieces / mm 2 or more.

[0025] In the internal region other than the above-described surface layer region, 70% or more by area ratio of upper bainite and 3% or more by total area ratio of fresh martensite and / or residual austenite are contained;

[0026] The maximum height of the surface roughness of the steel sheet is 30 μm or less;

[0027] The tensile strength is 980 MPa or more, the uniform elongation is 6% or more, and the 10 7 The ratio of the second plane bending fatigue strength to the tensile strength (fatigue limit ratio) is 0.45 or more;

[0028] MSC (mass%) = Mn + 0.2 x Si + 1.7 x Cr + 2.5 x Mo... (1)

[0029] In the above-described formula (1), each of the element symbols represents the content (mass%) of each element, and the elements other than the above are 0.

[0030] [2] The high-strength steel sheet according to [1], wherein the component composition further contains at least one of Cr: 1.0% or less and Mo: 1.0% or less in mass%.

[0031] [3] The high-strength steel sheet according to [1] or [2], wherein the component composition further contains 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 in mass%.

[0032] [4] The high-strength steel sheet according to any one of [1] to [3], wherein the component composition further contains Sb: 0.005 to 0.020% in mass%.

[0033] [5] The high-strength steel sheet according to any one of [1] to [4], wherein the component composition further contains at least one of Ca: 0.01% or less, Mg: 0.01% or less, and REM: 0.01% or less in mass%.

[0034] [6] A manufacturing method of a high-strength steel sheet, which is the manufacturing method of the high-strength steel sheet according to any one of claims 1 to 5,

[0035] heating a steel material having the above-described component composition to a heating temperature of 1150°C or more,

[0036] subjecting the heated steel material to hot rolling composed of rough rolling and finish rolling,

[0037] at least twice between the start of rough rolling and the start of finish rolling, and water pressure of 15 MPa or more once within 5 s before the start of finish rolling,

[0038] the finish rolling is performed at a total reduction ratio of 25 to 80% in a temperature range of RC1 or less and at a finish rolling end temperature of (RC2-50°C) to (RC2+120°C), thereby producing a hot-rolled steel sheet,

[0039] the hot-rolled steel sheet is cooled at an average cooling rate of 5°C / s or more within 2.0 s from the end of hot rolling to the start of cooling, and at a cooling stop temperature of Trs to (Trs+250°C),

[0040] the hot-rolled steel sheet after the cooling is coiled at a coiling temperature of Trs to (Trs+250°C),

[0041] cooled to 100°C or less at an average cooling rate of 20°C / s or less,

[0042] Note that RC1, RC2, and Trs are defined by the following (2), (3), and (4), respectively,

[0043] RC1 (°C) = 900 + 100 x C + 100 x N + 10 x Mn + 700 x Ti + 5000 x B + 10 x Cr + 50 x Mo + 2000 x Nb + 150 x V... (2)

[0044] RC2 (°C) = 750 + 100 x C + 100 x N + 10 x Mn + 350 x Ti + 5000 x B + 10 x Cr + 50 x Mo + 1000 x Nb + 150 x V... (3)

[0045] Trs (°C) = 500 - 450 x C - 35 x Mn - 15 x Cr - 10 x Ni - 20 x Mo... (4)

[0046] wherein each of the element symbols in the above (2), (3), and (4) represents the content (mass%) of each element, and the content of an element not listed is 0.

[0047] According to the present application, a high-strength steel sheet having a tensile strength of 980 MPa or more, press formability, and fatigue resistance can be obtained. The high-strength steel sheet of the present application has a high tensile strength, but has excellent press formability, and can be press formed without forming defects such as necking and cracking. In addition, when the high-strength steel sheet of the present application is used for parts of a truck or a passenger car, the amount of steel used can be reduced while ensuring safety, and thus the weight of the automobile body can be reduced, which contributes to reduction of environmental load.

[0048] Note that in the present application, excellent press formability means having a uniform elongation of 6% or more. In addition, excellent fatigue resistance means that the ratio of the 10 7 times plane bending fatigue strength to the tensile strength (fatigue limit ratio) is 0.45 or more. BRIEF DESCRIPTION OF DRAWINGS

[0049] Figure 1 is a schematic view showing the shape of a test piece for a plane bending fatigue test in the examples. DETAILED DESCRIPTION

[0050] The present application will be specifically described below. Note that the following description is an example showing a preferred embodiment of the present application, and the present application is not limited thereto.

[0051] [Component Composition]

[0052] First, the reason for the limitation of the component composition of the high-strength steel sheet of the present application will be described. Note that "%" as a unit of content means "mass %" unless otherwise specified.

[0053] C: 0.05 to 0.20%

[0054] C is an element that has an effect of increasing the strength of steel. C promotes the generation of bainite by increasing hardenability, and contributes to high-strength. In addition, C also contributes to high-strength by increasing the strength of martensite. In order to obtain a tensile strength of 980 MPa or more, it is necessary to have a C content of 0.05% or more. Therefore, the C content is 0.05% or more, and preferably 0.06% or more. On the other hand, if the C content exceeds 0.20%, the strength of martensite excessively increases, and the difference in strength between upper bainite as the main phase and fresh martensite and / or residual austenite becomes large, as a result of which the uniform elongation decreases. Therefore, the C content is set to 0.20% or less, and preferably 0.18% or less.

[0055] Si: 0.6 to 1.2%

[0056] Si has an effect of inhibiting the formation of Fe-based carbides, and inhibiting the precipitation of cementite at the time of upper bainite transformation. Thereby, C is distributed to untransformed austenite, and at the time of cooling after coiling in the hot rolling step, the untransformed austenite becomes fresh martensite and / or residual austenite, and the desired fresh martensite and / or residual austenite can be obtained. In order to obtain these effects, it is necessary to set the Si content to 0.6% or more. The Si content is preferably 0.7% or more. On the other hand, Si is an element that forms a subscale on the surface of the steel sheet in hot rolling. If the Si content exceeds 1.2%, the subscale becomes too thick, and the surface roughness of the steel sheet surface after rust removal becomes too large, and the pre-painting workability, fatigue characteristics of the high-strength steel sheet deteriorate. Therefore, the Si content is 1.2% or less, and is preferably 1.1% or less.

[0057] Mn: 1.3 to 3.7%

[0058] Mn stabilizes austenite, and contributes to the generation of fresh martensite and / or residual austenite. In order to obtain this effect, it is necessary to set the Mn content to 1.3% or more. Therefore, the Mn content is 1.3% or more, and is preferably 1.4% or more. On the other hand, if the Mn content exceeds 3.7%, fresh martensite and / or residual austenite are generated in excess, and the uniform elongation decreases. Therefore, the Mn content is 3.7% or less, and is preferably 3.6% or less, and more preferably 3.5% or less.

[0059] P: 0.10% or less

[0060] P is an element that contributes to an increase in the strength of the steel by solid solution. However, P is also an element that causes cracking of the slab at the time of hot rolling by segregation at the grain boundaries of austenite at the time of hot rolling. In addition, segregation at the grain boundaries decreases the uniform elongation. Therefore, it is preferable to reduce the P content as much as possible, but P in an amount of 0.10% or less can be allowed. Therefore, the P content is 0.10% or less. The lower limit is not particularly limited, but since a decrease in production efficiency occurs when the P content is less than 0.0002%, it is preferable to be 0.0002% or more.

[0061] S: 0.03% or less

[0062] S forms coarse sulfides in combination with Ti and Mn, which decreases the uniform elongation by accelerating the generation of voids. Therefore, it is preferable to reduce the S content as much as possible, but S in an amount of 0.03% or less can be allowed. Therefore, the S content is 0.03% or less. The lower limit is not particularly limited, but since a decrease in production efficiency occurs when the S content is less than 0.0002%, it is preferable to be 0.0002% or more.

[0063] Al: 0.001 to 2.0%

[0064] Al acts as a deoxidizer and is an element effective for improving cleanliness of the steel. Since the effect is insufficient when the Al content is less than 0.001%, the Al content is set to 0.001% or more, preferably 0.005% or more, and more preferably 0.010% or more. In addition, Al has an effect of suppressing formation of Fe-based carbides, like Si, and suppressing precipitation of cementite at the time of upper bainite transformation. Thereby, generation of fresh martensite and / or residual austenite is promoted in cooling after coiling. On the other hand, an excessive amount of Al causes an increase in oxide inclusions, resulting in a decrease in uniform elongation. Therefore, the Al content is set to 2.0% or less, preferably 1.0% or less, and more preferably 0.1% or less.

[0065] N: 0.01% or less

[0066] N is precipitated as a nitride by combining with a nitride-forming element and generally contributes to grain refinement. However, since N forms coarse nitrides by combining with Ti at high temperatures, an excessive amount of N causes a decrease in uniform elongation. Therefore, the N content is set to 0.01% or less. The lower limit is not particularly specified, but since an N content of less than 0.0002% causes a decrease in production efficiency, it is preferably 0.0002% or more.

[0067] O: 0.01% or less

[0068] Since O deteriorates formability by forming oxides, the content needs to be controlled. In particular, if O exceeds 0.01%, the tendency becomes significant. Therefore, the O content is set to 0.01% or less, preferably 0.005%, and more preferably 0.003%. The lower limit is not particularly specified, but since less than 0.00005% sometimes causes a significant decrease in production efficiency, it is preferably 0.00005% or more.

[0069] B: 0.0005 to 0.010%

[0070] B segregates at prior austenite grain boundaries, suppresses generation of ferrite, thereby promoting generation of upper bainite, and is an element effective for improving strength of the steel sheet. In order to exhibit these effects, the B content needs to be 0.0005% or more. Therefore, the B content is set to 0.0005% or more, preferably 0.0006% or more, and more preferably 0.0007% or more. On the other hand, if the B content exceeds 0.010%, the above effects are saturated. Therefore, the B content is set to 0.010% or less, preferably 0.009% or less, and more preferably 0.008% or less.

[0071] The remainder is composed of Fe and inevitable impurities. It should be noted that, as inevitable impurities, for example, Zr, Co, Sn, Zn, and W can be cited. When the composition contains at least one of Zr, Co, Sn, Zn, and W as inevitable impurities, the total content of these elements is preferably 0.5% or less.

[0072] The composition of the high-strength steel sheet of the present application can further arbitrarily contain at least one of the elements cited below.

[0073] Cr: 1.0% or less

[0074] Cr is a carbide-forming element, and has the effect of reducing the driving force of the bainite transformation at the interface between the upper bainite and the untransformed austenite at the time of the upper bainite transformation before coiling, and stopping the upper bainite transformation. By stopping the transformation of the upper bainite, the remaining untransformed austenite becomes fresh martensite and / or retained austenite through cooling after coiling. Therefore, Cr also contributes to the formation of fresh martensite and / or retained austenite at the desired area ratio in the case where Cr is added. This effect is obtained when Cr is preferably 0.1% or more. However, if the Cr content exceeds 1.0%, fresh martensite and / or retained austenite are excessively generated, and the uniform elongation decreases, so the Cr content is 1.0% or less, preferably 0.9% or less, and more preferably 0.8% or less in the case where Cr is added.

[0075] Mo: 1.0% or less

[0076] Mo promotes the formation of bainite through the improvement of the hardenability, and contributes to the increase in the strength of the steel sheet. In addition, Mo, like Cr, is a carbide-forming element, and reduces the driving force of the bainite transformation at the interface between the upper bainite and the untransformed austenite at the time of the upper bainite transformation before coiling, and contributes to the generation of fresh martensite and / or retained austenite after coiling. This effect is obtained when Mo is preferably 0.1% or more. However, if the Mo content exceeds 1.0%, fresh martensite and / or retained austenite are excessively generated, and the uniform elongation deteriorates. Therefore, the Mo content is 1.0% or less, preferably 0.9% or less, and more preferably 0.8% or less in the case where Mo is added.

[0077] In addition, the composition of the high-strength steel sheet of the present application can further arbitrarily contain at least one of the elements cited below.

[0078] Cu: 2.0% or less

[0079] Cu is an element which contributes to an increase in strength of the steel by solid solution. In addition, Cu promotes the formation of bainite by an increase in hardenability, and contributes to an increase in strength. This effect can be obtained when Cu is preferably 0.01% or more. However, if the Cu content exceeds 2.0%, the surface properties of the high-strength steel sheet are deteriorated, and the fatigue characteristics of the high-strength steel sheet are deteriorated. Therefore, in the case where Cu is added, the Cu content is set to 2.0% or less, preferably 1.9% or less, and more preferably 1.8% or less.

[0080] Ni: 2.0% or less

[0081] Ni is an element which contributes to an increase in strength of the steel by solid solution. In addition, Ni promotes the formation of bainite by an increase in hardenability, and contributes to an increase in strength. This effect can be obtained when Ni is preferably 0.01% or more. However, if the Ni content exceeds 2.0%, fresh martensite and / or residual austenite are excessively increased, and the ductility of the high-strength steel sheet is deteriorated. Therefore, in the case where Ni is added, the Ni content is set to 2.0% or less, preferably 1.9% or less, and more preferably 1.8% or less.

[0082] Ti: 0.3% or less

[0083] Ti is an element which has an effect of increasing the strength of the steel sheet by precipitation strengthening or solid solution strengthening. Ti forms a nitride in a high-temperature range of austenite. Thereby, the precipitation of BN is suppressed, and B becomes in a solid solution state. Therefore, in the case where Ti is added, Ti also contributes to securing the hardenability required for the generation of upper bainite, and an increase in strength. This effect can be obtained when Ti is preferably 0.01% or more. However, if the Ti content exceeds 0.3%, Ti nitrides are generated in a large amount, and the uniform elongation is reduced. Therefore, in the case where Ti is added, the Ti content is set to 0.3% or less, preferably 0.28% or less, and more preferably 0.25% or less.

[0084] Nb: 0.3% or less

[0085] Nb is an element having an effect of improving the strength of the steel sheet by precipitation strengthening or solid solution strengthening. In addition, Nb, like Ti, increases the recrystallization temperature of austenite at the time of hot rolling, thereby enabling rolling in the range where austenite is not recrystallized, contributing to the refinement of the grain size of upper bainite and the increase in the area ratio of fresh martensite and / or retained austenite. In addition, Nb, like Cr, is a carbide-forming element, and at the time of upper bainite transformation before coiling, Nb is segregated at the interface between upper bainite and untransformed austenite, thereby having an effect of reducing the transformation driving force of bainite and stopping the transformation of upper bainite in the state where untransformed austenite remains. The untransformed austenite becomes fresh martensite and / or retained austenite by subsequent cooling. Therefore, in the case where Nb is added, Nb also contributes to the formation of fresh martensite and / or retained austenite having a desired area ratio. This effect can be obtained when Nb is preferably 0.01% or more. However, if the Nb content exceeds 0.3%, fresh martensite and / or retained austenite excessively increases, and the uniform elongation decreases. Therefore, in the case where Nb is added, the Nb content is made 0.3% or less, preferably 0.28% or less, and more preferably 0.25% or less.

[0086] V: 0.3% or less

[0087] V is an element having an effect of improving the strength of the steel sheet by precipitation strengthening and solid solution strengthening. In addition, V, like Ti, increases the recrystallization temperature of austenite at the time of hot rolling, thereby enabling rolling in the range where austenite is not recrystallized, contributing to the refinement of the grain size of upper bainite. In addition, V, like Cr, is a carbide-forming element, and at the time of upper bainite transformation before coiling, V is segregated at the interface between upper bainite and untransformed austenite, thereby having an effect of reducing the transformation driving force of bainite and stopping the transformation of upper bainite in the state where untransformed austenite remains. The untransformed austenite becomes fresh martensite and / or retained austenite by subsequent cooling. Therefore, in the case where V is added, V also contributes to the formation of fresh martensite and / or retained austenite having a desired area ratio. This effect can be obtained when V is preferably 0.01% or more. However, if the V content exceeds 0.3%, fresh martensite and / or retained austenite excessively increases, and the uniform elongation decreases. Therefore, in the case where V is added, the V content is made 0.3% or less, preferably 0.28% or less, and more preferably 0.25% or less.

[0088] In addition, the high-strength steel sheet of the present application can further arbitrarily contain the elements listed below in the composition.

[0089] Sb: 0.005 to 0.020%

[0090] Sb is an element having an effect of suppressing nitriding of the surface of the steel blank when the steel blank is heated. By adding Sb, precipitation of BN in the surface layer portion of the steel blank can be suppressed. As a result, the solid-solution B remaining contributes to securing hardenability required for generation of bainite and improvement of strength of the steel sheet resulting therefrom. In the case where Sb is added, in order to obtain the above effect, the Sb content is made 0.005% or more, preferably 0.006% or more, and more preferably 0.007% or more. On the other hand, if the Sb content exceeds 0.020%, the toughness of the steel decreases, and sometimes causes cracking of the blank and hot rolling. Therefore, in the case where Sb is added, the Sb content is made 0.020% or less, preferably 0.019% or less, and more preferably 0.018% or less.

[0091] Further, the high-strength steel sheet of the present application can further arbitrarily contain at least one of the elements listed below. The elements listed below contribute to further improvement of properties such as press formability.

[0092] Ca: 0.01% or less

[0093] Ca controls the shape of oxide, sulfide-based inclusions, and contributes to suppression of cracking of the sheared end surface of the steel sheet and further improvement of bend workability. This effect can be obtained when Ca is preferably 0.001% or more. However, if the Ca content exceeds 0.01%, Ca-based inclusions increase and the cleanliness of the steel deteriorates, and sometimes becomes a cause of cracking of the sheared end surface, bend work cracking. Therefore, in the case where Ca is added, the Ca content is made 0.01% or less.

[0094] Mg: 0.01% or less

[0095] Mg, like Ca, controls the shape of oxide, sulfide-based inclusions, and contributes to suppression of cracking of the sheared end surface of the steel sheet and further improvement of bend workability. This effect can be obtained when Mg is preferably 0.001% or more. However, if the Mg content exceeds 0.01%, the cleanliness of the steel deteriorates, and sometimes becomes a cause of cracking of the sheared end surface, bend work cracking. Therefore, in the case where Mg is added, the Mg content is made 0.01%.

[0096] REM: 0.01% or less

[0097] REM (rare earth metal) is also effective in controlling the shape of inclusions of oxide and sulfide systems to suppress cracks in the sheared end surface of the steel sheet and further improve the bend workability. The effect is obtained when the REM content is preferably 0.001% or more. However, if the REM content exceeds 0.01%, the cleanliness of the steel deteriorates, and sometimes becomes a cause of cracks in the sheared end surface and cracks in the bend work. Therefore, in the case where REM is added, the REM content is set to 0.01% or less.

[0098] In the present application, it is characterized that MSC defined in the following (1) formula is 2.7 to 3.8 mass%. In order to maintain the tensile strength of 980 MPa or more while obtaining a high uniform elongation, it is necessary to control the area fraction of fresh martensite and / or retained austenite within an appropriate range, as described later. For the control of the area fraction of fresh martensite and / or retained austenite, it is important to add Mn, Si, Cr (at the time of addition) and Mo (at the time of addition) in balance, and specifically, it is necessary to set the MSC value defined in the following (1) formula to 2.7 to 3.8 mass%. In a high-strength steel sheet having a tensile strength of 980 MPa or more, if the MSC value is outside the above range, a uniform elongation of 6% or more cannot be obtained. It is preferable that the MSC value be 2.75 mass% or more, and more preferably 2.80 mass% or more. It is preferable that the MSC value be 3.75 mass% or less, and more preferably 3.70 mass% or less.

[0099] MSC (mass%) = Mn + 0.2 x Si + 1.7 x Cr + 2.5 x Mo... (1)

[0100] In the above (1) formula, each element symbol represents the content (mass%) of each element, and the element not included is 0.

[0101] [Microstructure]

[0102] Next, the reason for the limitation of the microstructure of the high-strength steel sheet of the present application will be described.

[0103] The high-strength steel sheet of the present application has a microstructure in which the surface layer region from the surface of the steel sheet to a depth of 100 μm contains 70% or more of upper bainite in terms of area fraction, and 2% or more of fresh martensite and / or retained austenite in terms of total area fraction, the average grain diameter of the upper bainite is 7 μm or less, the average grain diameter of the fresh martensite and / or retained austenite is 4 μm or less, and the number density of the fresh martensite and / or retained austenite is 100 pieces / mm 2 and has a microstructure in which the internal region other than the surface layer region contains 70% or more of upper bainite in terms of area fraction, and 3% or more of fresh martensite and / or retained austenite in terms of total area fraction.

[0104] First, the microstructure of the surface layer region from the surface of the steel sheet to a depth of 100 μm is described.

[0105] upper bainite: 70% or more

[0106] The microstructure of the high-strength steel sheet of the present application contains upper bainite as a main phase. If the area ratio of the upper bainite is less than 70%, it is not possible to achieve a tensile strength of 980 MPa or more and a uniform elongation of 6% or more. Therefore, the area ratio of the upper bainite is made 70% or more, preferably 80% or more.

[0107] fresh martensite and / or retained austenite: 2% or more in total area ratio

[0108] In order to improve the fatigue property, the fresh martensite and / or retained austenite is provided at 2% or more, preferably 3% or more in total area ratio. On the other hand, if the total area ratio of the fresh martensite and / or retained austenite reaches 30% or more, the interface of the fresh martensite and / or retained austenite which can be a starting point of fatigue crack generation increases, and the fatigue property can be lowered, and therefore, the total area ratio of the fresh martensite and / or retained austenite is preferably 30% or less. More preferably, it is 25% or less, and further preferably, it is 20% or less.

[0109] In the surface layer region from the surface of the steel sheet to a depth of 100 μm, the progress of the bainite transformation is fast due to the fast cooling rate, and therefore, the C enrichment for forming the fresh martensite and / or retained austenite is less than that in the interior. As a result, the area ratio of the fresh martensite and / or retained austenite in the surface layer region from the surface of the steel sheet to a depth of 100 μm is less than that in the interior, and the difference is about 1%.

[0110] the average grain diameter of the upper bainite is 7 μm or less, and the average grain diameter of the fresh martensite and / or retained austenite is 4 μm or less

[0111] Fatigue crack generation is considered to be caused by slip deformation within the crystal grains of the surface layer. This slip deformation is difficult to propagate to the adjoining crystal grains due to the grain boundaries, and as a result, it is possible to delay crack generation. That is, it is possible to improve the fatigue strength by grain refinement. In order to obtain this effect, the average grain diameter of the upper bainite is made 7 μm or less. Preferably, it is 6 μm or less. The average grain diameter of the fresh martensite and / or retained austenite is made 4 μm or less, preferably 3 μm or less. The smaller the average grain diameter becomes, the more it is possible to obtain the effect of delaying fatigue crack generation. However, if the average grain diameter becomes too small, the elongation can possibly be lowered while the strength is increased. Therefore, it is preferable to make the average grain diameter of the upper bainite 2 μm or more. It is preferable to make the average grain diameter of the fresh martensite and / or retained austenite 0.5 μm or more.

[0112] The number density of the fresh martensite and / or the retained austenite is 100 / mm 2 The above

[0113] Fatigue cracks mostly occur on the surface of the steel sheet, and after the length is extended to several tens of μm, the fatigue crack propagation phase is entered. In high-cycle fatigue, the number of cycles until the crack propagation phase is entered accounts for a large part of the fatigue life. Therefore, in order to improve the fatigue strength of 10 7 times, it is important to control the microstructure of the surface layer up to a depth of 100 μm. In the high-strength steel sheet of the present application, the hard fresh martensite and / or the retained austenite are finely dispersed in the soft upper bainite, thereby preventing rearrangement of dislocations that proliferate upon repeated loading, and delaying repeated softening, and in order to improve the fatigue properties, the number density of the fresh martensite and / or the retained austenite is made to be 100 / mm 2 The above, preferably 200 / mm 2 The above.

[0114] Next, the microstructure of the internal region other than the surface layer region will be described.

[0115] Upper bainite: 70% or more

[0116] The microstructure of the high-strength steel sheet of the present application contains the upper bainite as the main phase in the internal region as well as in the surface layer region. If the area ratio of the upper bainite is less than 70%, the tensile strength of 980 MPa or more and the uniform elongation of 6% or more cannot be achieved. Therefore, the area ratio of the upper bainite is made to be 70% or more, preferably 80% or more.

[0117] Fresh martensite and / or retained austenite: 3% or more in total area ratio

[0118] The microstructure of the high-strength steel sheet of the present application contains the fresh martensite and / or the retained austenite. The fresh martensite has an effect of promoting work hardening to delay the occurrence of plastic instability and thereby improving the uniform elongation. The uniform elongation can be improved due to the TRIP (Transformation Induced Plasticity) effect. In order to obtain these effects, the area ratio of the fresh martensite and / or the retained austenite is made to be 3% or more, preferably 4% or more. On the other hand, if the total area ratio of the fresh martensite and / or the retained austenite reaches 30% or more, the interface between the fresh martensite and / or the retained austenite, which can be the starting point of fatigue cracks, and the bainite increases, and the fatigue properties can possibly decrease, and therefore the area ratio of the fresh martensite and / or the retained austenite is preferably 30% or less. More preferably, 25% or less, and further preferably, 20% or less.

[0119] The above microstructure can further contain any structure other than upper bainite, fresh martensite and residual austenite (hereinafter, referred to as "other structure"). From the viewpoint of improving the effect of microstructure control, the total area ratio of the other structure is preferably 3% or less. In other words, the total area ratio of the upper bainite, fresh martensite and residual austenite in the above microstructure is preferably 97% or more. As the other structure, for example, cementite, polygonal ferrite, pearlite, tempered martensite and lower bainite, etc. can be given.

[0120] Maximum height of surface roughness (Ry) of the steel sheet: 30 μm or less

[0121] If the maximum height of surface roughness (Ry) of the steel sheet is large, local stress concentration occurs in the concave portion of the surface layer at the time of the flat bending fatigue test, and a fatigue crack is generated at an early stage, and excellent fatigue characteristics cannot be obtained. Therefore, in order to ensure good fatigue characteristics of the high-strength steel sheet, the maximum height of surface roughness (Ry) of the steel sheet is made to be 30 μm or less. The fatigue characteristics are improved as the maximum height of surface roughness (Ry) of the steel sheet is smaller, and therefore, the maximum height of surface roughness (Ry) of the steel sheet is preferably 25 μm or less, and more preferably 20 μm or less.

[0122] [Mechanical properties]

[0123] The high-strength steel sheet of the present application has a tensile strength of 980 MPa or more, an uniform elongation of 6% or more and a fatigue limit ratio (ratio of the secondary flat bending fatigue strength to the tensile strength) of 0.45 or more. Therefore, the high-strength steel sheet of the present application, although having a high tensile strength, has excellent press formability, and can be subjected to press forming without forming defects such as necking and cracking, and can ensure safety when used in parts of a truck or a passenger car. 7

[0124] Note that the microstructure, surface roughness and mechanical properties of the present application can be obtained according to the measurement methods described in the Examples described later.

[0125] [Manufacturing method]

[0126] Next, the manufacturing method of the high-strength steel sheet in one embodiment of the present application will be described. Note that the temperature in the following description indicates the surface temperature of the object (steel material or steel sheet) unless otherwise specified.

[0127] The high-strength steel sheet of the present application can be manufactured by sequentially applying the following (1) to (5) to a steel material. Hereinafter, each step will be described.

[0128] (1) Heating

[0129] (2) Hot rolling​

[0130] (3) cooling (first cooling)

[0131] (4) coiling

[0132] (5) cooling (second cooling)

[0133] Note that, as the steel billet, any steel billet having the above-described composition can be used. The composition of the high-strength steel sheet finally obtained is the same as the composition of the steel billet used. As the steel billet, for example, a slab can be used. Further, the method of manufacturing the steel billet is not particularly limited. For example, molten steel having the above-described composition can be smelted by a known method such as a converter, and a steel billet can be obtained by a casting method such as continuous casting. A method other than the continuous casting method such as an ingot-cogging method can also be used. Further, as the raw material, scrap iron can be used. The steel billet manufactured by the continuous casting method or the like can be directly supplied to the subsequent heating step, and the steel billet of a hot sheet or a cold sheet manufactured by cooling can be supplied to the heating step.

[0134] (1) heating

[0135] First, the steel billet is heated to a heating temperature of 1150°C or higher. Generally, carbonitride forming elements such as Ti in the steel billet exist almost as coarse carbonitrides. The presence of such coarse and uneven precipitates generally causes deterioration of various characteristics (for example, shear end face crack resistance, bend workability, burr workability, and the like) required for a high-strength steel sheet for parts for trucks and passenger cars. Therefore, it is necessary to heat the steel billet before hot rolling and to solid-solute the coarse precipitates. Specifically, in order to sufficiently solid-solute the coarse precipitates, it is necessary to set the heating temperature of the steel billet to 1150°C or higher. On the other hand, if the heating temperature of the steel billet becomes too high, the yield is reduced due to the generation of slab defects and the peeling of the scale. Therefore, from the viewpoint of improving the yield, the heating temperature of the steel billet is preferably 1350°C or lower. The lower limit of the heating temperature of the steel billet is more preferably 1180°C or higher, and further preferably 1200°C or higher. The upper limit of the heating temperature of the steel billet is more preferably 1300°C or lower, and further preferably 1280°C or lower.

[0136] During the heating, from the viewpoint of making the temperature of the steel material uniform, after the steel material is heated to the above-mentioned heating temperature, it is preferable to be held at the heating temperature. The time of the heating temperature holding (holding time) is not particularly limited, but from the viewpoint of improving the uniformity of the temperature of the steel material, it is preferable to be 1800 seconds or more. On the other hand, if the holding time exceeds 10000 seconds, the amount of scale generation increases. As a result, scale biting and the like easily occur in the subsequent hot rolling, leading to a reduction in yield due to surface defects. Therefore, the holding time is preferably 10000 seconds or less, and more preferably 8000 seconds or less.

[0137] (2) Hot rolling

[0138] Next, the heated steel material is subjected to hot rolling, and a hot-rolled steel sheet is produced. The hot rolling can be composed of rough rolling and finish rolling. When rough rolling is performed, the conditions are not particularly limited, but in order to reduce the surface roughness of the steel sheet, it is necessary to remove the surface scale from the start of the rough rolling to the start of the finish rolling.

[0139] In the present application, between the start of the rough rolling and the start of the finish rolling, derusting is performed at least twice or more, and derusting with a water pressure of 15 MPa or more is performed once or more within 5 seconds before the start of the finish rolling. The temperature of the steel sheet is high in the rough rolling or before the finish rolling, and a thick surface scale is easily formed. In order to remove such a surface scale, derusting is performed at least twice or more, and preferably three times or more. Also, the removal of the surface scale within 5 seconds before the start of the finish rolling is very effective in reducing the surface roughness. Therefore, in order to control the maximum height (Ry) of the surface roughness of the steel sheet to be 30 μm or less, in addition to the derusting at least twice or more, it is necessary to set the water pressure of the derusting to be 15 MPa or more within 5 seconds before the start of the finish rolling. If the water pressure of the derusting is less than 15 MPa, the scale remains on the surface of the steel sheet before the finish rolling, the unevenness of the surface of the steel sheet after the finish rolling becomes large, and the maximum height of the surface roughness of the steel sheet exceeds 30 μm. Therefore, the water pressure of the derusting within 5 seconds before the start of the finish rolling is set to be 15 MPa or more. It is preferable to be 30 MPa or more, and more preferably 60 MPa or more.

[0140] Note that the water pressure of the derusting other than the derusting performed within 5 seconds before the start of the finish rolling can be 10 MPa or more.

[0141] Next, in the present application, when the temperature RC1, the temperature RC2 are defined by the following equations (2), (3) at the time of the finish rolling, the total reduction in the temperature range below RC1 is 25% to 80%, and the finish rolling end temperature is (RC2 - 50°C) to (RC2 + 120°C).

[0142] RC1 is an austenite 50% recrystallization temperature estimated from the composition of the ingredients, and RC2 is an austenite recrystallization lower limit temperature estimated from the composition of the ingredients. When the total reduction ratio at or below RC1 is less than 25%, the average grain diameter becomes large, and the fatigue characteristic improvement effect is not obtained. On the other hand, if the total reduction ratio in the temperature range at or below RC1 exceeds 80%, the dislocation density of the austenite becomes high, and the bainite structure obtained from the austenite phase change in the state of high dislocation density lacks ductility, and a uniform elongation of 6% or more is not obtained. Therefore, the total reduction ratio in the temperature range at or below RC1 is 25% to 80%.

[0143] In addition, the hot rolling is performed under the condition that the finish rolling temperature is (RC2 - 50°C) to (RC2 + 120°C). If the finish rolling temperature is less than (RC2 - 50°C), the bainite phase change occurs from the austenite in the state of high dislocation density. The upper bainite obtained from the austenite phase change in the state of high dislocation density is high in dislocation density and lacks ductility, and thus the uniform elongation is reduced. In addition, when the finish rolling temperature is low and the rolling is performed at the temperature in the two-phase region of ferrite + austenite, the uniform elongation is also reduced. Therefore, the finish rolling temperature is set to (RC2 - 50°C) or more. On the other hand, if the finish rolling temperature is higher than (RC2 + 120°C), the austenite grains become coarse, and the average grain diameter of the upper bainite becomes large, and thus the strength is reduced. In addition, the fresh martensite and / or the residual austenite also become coarse, and as a result, the uniform elongation is reduced. Therefore, the finish rolling temperature is set to (RC2 + 120°C) or less.

[0144] It should be noted that RC1 and RC2 are defined by the following (2) and (3) formulas.

[0145] RC1 (°C) = 900 + 100 x C + 100 x N + 10 x Mn + 700 x Ti + 5000 x B + 10 x Cr + 50 x Mo + 2000 x Nb + 150 x V... (2)

[0146] RC2 (°C) = 750 + 100 x C + 100 x N + 10 x Mn + 350 x Ti + 5000 x B + 10 x Cr + 50 x Mo + 1000 x Nb + 150 x V... (3)

[0147] Here, each of the element symbols in the above (2) and (3) formulas represents the content (mass %) of each element, and the elements not included are 0.

[0148] (3) Cooling (first cooling)

[0149] Next, the obtained hot-rolled steel sheet is cooled (first cooling). At this time, the time from the end of hot rolling (end of finish rolling) to the start of cooling (cooling start time) is made to be within 2.0 s. If the cooling start time exceeds 2.0 s, grain growth of austenite grains occurs, and it is not possible to ensure the tensile strength of 980 MPa or more. The cooling start time is preferably within 1.5 s.

[0150] The average cooling rate is set to 5°C / s or more. In the present application, the microstructure of the surface layer is made different from the inside by cooling the surface layer more rapidly than the inside. Due to the rapid cooling of the surface layer, the bainite transformation of the surface layer starts early, and the martensite and residual austenite formed due to the enrichment of C are less than the inside. If the average cooling rate during cooling is less than 5°C / s, the surface layer is not sufficiently rapidly cooled, and the surface layer structure of 70% or more in area ratio of upper bainite and 2% or more in total area ratio of fresh martensite and / or residual austenite is not obtained. Therefore, the average cooling rate is made to be 5°C / s or more, preferably 20°C / s or more, and more preferably 50°C / s or more. On the other hand, the upper limit of the average cooling rate is not particularly limited, and if the average cooling rate becomes too large, the management of the cooling stop temperature becomes difficult. Therefore, the average cooling rate is preferably 200°C / s or less. It should be noted that the average cooling rate is defined based on the average cooling rate of the surface of the steel sheet.

[0151] In addition, during cooling, forced cooling can be performed at the above average cooling rate. The method of cooling is not particularly limited, and for example, it is preferable to be performed by water cooling.

[0152] The cooling stop temperature is Trs to (Trs + 250°C). If the cooling stop temperature is less than Trs, the microstructure is tempered martensite or lower bainite. Both the tempered martensite and the lower bainite are high-strength structures, but the uniform elongation is significantly lower. Therefore, the cooling stop temperature is made to be Trs or more. On the other hand, if the cooling stop temperature is higher than (Trs + 250°C), ferrite is generated, and therefore the tensile strength of 980 MPa is not obtained. Therefore, the cooling stop temperature is made to be (Trs + 250°C) or less.

[0153] It should be noted that Trs is defined by the following (4).

[0154] Trs (°C) = 500 - 450 x C - 35 x Mn - 15 x Cr - 10 x Ni - 20 x Mo... (4)

[0155] Here, each of the element symbols in the above (4) represents the content (mass %) of each element, and the elements not included are 0.

[0156] (4) Winding

[0157] Next, the cooled hot-rolled steel sheet is coiled at a coiling temperature: Trs~(Trs+250°C). If the coiling temperature is less than Trs, a martensite transformation or a lower bainite transformation occurs after coiling, and the desired fresh martensite and / or residual austenite is not obtained. Therefore, the coiling temperature is made to be Trs or more. On the other hand, if the coiling temperature is higher than (Trs+250°C), ferrite is generated, and thus a tensile strength of 980 MPa is not obtained. Therefore, the coiling temperature is set to be (Trs+250°C) or less.

[0158] (5) Cooling (second cooling)

[0159] After coiling, further cooling to 100°C or less is performed at an average cooling rate of 20°C / s or less (second cooling). The average cooling rate affects the generation of fresh martensite and / or residual austenite. If the average cooling rate exceeds 20°C / s, the untransformed austenite mostly undergoes a martensite transformation, and the desired residual austenite is not obtained, and the uniform elongation decreases. Therefore, the average cooling rate is made to be 20°C / s or less, preferably 10°C / s or less, and more preferably 1°C / s or less. On the other hand, the lower limit of the above average cooling rate is not particularly limited, and is preferably 0.0001°C / s or more.

[0160] The cooling can be performed to an arbitrary temperature of 100°C or less, and is preferably cooled to around 10 to 30°C (for example, room temperature). Note that the cooling can be performed in an arbitrary form, and for example, can be performed in a state of a coiled coil.

[0161] By the above steps, the high-strength steel sheet of the present application can be manufactured. Note that after coiling and the subsequent cooling, a conventional method can be used. For example, a thermo-mechanical rolling can be performed, and in addition, pickling to remove an oxide scale formed on the surface can be performed.

[0162] Example

[0163] A molten steel having the composition shown in Table 1 was melted in a converter, and a steel slab was manufactured as a steel slab material by a continuous casting method. The obtained steel slab material was heated to the heating temperature shown in Table 2, and next, the heated steel slab material was subjected to hot-rolling composed of rough rolling and finish rolling, and a hot-rolled steel sheet was manufactured. The finish rolling end temperature in the hot-rolling is shown in Table 2. Note that the water pressure at the time of rust removal was performed twice or more, and the water pressure at the time of rust removal other than the one shown in Table 2 was 10 MPa.

[0164] Next, the obtained hot-rolled steel sheet was subjected to cooling (first cooling) under the conditions of the average cooling rate and the cooling stop temperature shown in Table 2. The cooled hot-rolled steel sheet was coiled at the coiling temperature shown in Table 2, and the steel sheet obtained by coiling was subjected to cooling (second cooling) at the average cooling rate shown in Table 2, to obtain a high-strength steel sheet. Note that, after the cooling, skin pass rolling and pickling were performed as post-treatments. The pickling was performed using a 10 mass% hydrochloric acid aqueous solution at a temperature of 85°C.

[0165] From the obtained high-strength steel sheet, test pieces were taken, and microstructure, surface roughness, and mechanical properties were evaluated according to the following procedures.

[0166] (Microstructure)

[0167] From the obtained high-strength steel sheet, a test piece for microstructure observation was taken, and a cross section of the sheet thickness parallel to the rolling direction was used as the observation surface. The surface of the obtained test piece was polished, and then the surface was etched using an etching solution (3 vol.% nitric acid ethanol solution) to expose the microstructure.

[0168] Next, using a scanning electron microscope (SEM), 10 fields of view of the surface layer and the internal region other than the surface layer from the surface to a depth of 100 μm were photographed at a magnification of 5000 times, to obtain SEM images of the microstructure. The area ratios of upper bainite (UB), polygonal ferrite (F), and tempered martensite (TM) were quantified by image processing analysis of the obtained SEM images. In addition, since fresh martensite (M) and residual austenite (γ) are difficult to distinguish under SEM, identification was performed using an Electron Back scatter Diffraction Patterns (EBSD) method, and the area ratios and average grain diameters of each were calculated. The area ratios of the obtained microstructures and the average grain diameters of the surface layer structure are shown in Table 3. Note that, in Table 3, the total area ratio of fresh martensite and residual austenite (M + γ) is also indicated.

[0169] (Surface roughness)

[0170] From the obtained high-strength steel sheet, test pieces for surface roughness measurement (size: t (sheet thickness) x 50 mm (width) x 50 mm (length)) were taken at 5 positions of the surface of the steel sheet at different sheet width positions, and measurement of the maximum height (Ry) of the surface roughness was performed in accordance with JIS B 0601. In addition, for each of the test pieces taken at 5 positions of different sheet width positions, measurement of the maximum height Ry was performed three times in a direction orthogonal to the rolling direction, and the average value was calculated as the maximum height Ry of the test piece. The maximum height Ry of the high-strength steel sheet was evaluated using the average value of the 5 test pieces taken at 5 positions of different sheet width positions.

[0171] (Tensile test)

[0172] JIS 5 specimens (gauge length (GL): 50 mm) were taken from the obtained high-strength steel sheets, with the tensile direction orthogonal to the rolling direction. Tensile tests were performed on the obtained specimens according to JIS Z 2241 to determine the yield strength (yield point, YP), tensile strength (TS), yield ratio (YR), total elongation (E1), and uniform elongation (u-E1). Two tensile tests were performed on each high-strength steel sheet, and the average of the measured values ​​is shown in Table 3 as the mechanical properties of that high-strength steel sheet. In this invention, a TS of 980 MPa or higher is evaluated as high strength. Furthermore, a uniform elongation of 6% or higher is evaluated as good press formability.

[0173] (Plane bending fatigue test)

[0174] Take from the obtained high-strength steel plate Figure 1 The specimens, with the dimensions and shape shown, were subjected to planar bending fatigue tests according to JIS Z 2275, with the long side of the specimen orthogonal to the rolling direction. The stress loading mode was: stress ratio R = -1, frequency f = 25 Hz. The load stress amplitude was varied in 6 stages, and the stress period until fracture was measured to obtain the S-N curve and the 10-degree stress ratio. 7 The fatigue strength (fatigue limit) of the second test. In this invention, when the value obtained by dividing the fatigue limit by the tensile strength (TS) obtained in the tensile test is 0.45 or higher, it is evaluated as having good fatigue characteristics.

[0175]

[0176] [Table 2]

[0177]

[0178] Underlined text indicates content outside the scope of this invention.

[0179] According to the results in Table 3, all examples of the present invention possess tensile strength, compression molding properties, and fatigue resistance of over 980 MPa.

Claims

1. A high-strength steel plate, It has the following components: It contains, by mass%, C: 0.05-0.20%, Si: 0.6-1.2%, Mn: 1.3-3.7%, P: less than 0.10%, S: less than 0.03%, Al: 0.001-2.0%, N: less than 0.01%, O: less than 0.01%, and B: 0.0005-0.010%, with the remainder consisting of Fe and unavoidable impurities, and the MSC as defined by the following formula (1) is 2.7-3.8% by mass; The microstructure is as follows: in the surface region from the surface of the steel plate to a depth of 100 μm, it contains more than 70% upper bainite by area ratio and more than 2% fresh martensite and / or retained austenite by combined area ratio; the average grain diameter of upper bainite is less than 7 μm; the average grain diameter of fresh martensite and / or retained austenite is less than 4 μm; and the number density of fresh martensite and / or retained austenite is 100 grains / mm. 2 above; In the internal regions other than the surface region, there are more than 70% upper bainite by area ratio and more than 3% fresh martensite and / or retained austenite by total area ratio. The maximum height of the surface roughness of the steel plate is below 30 μm; Tensile strength is above 980 MPa, uniform elongation is above 6%, and 10 7 The ratio of the planar bending fatigue strength to the tensile strength, i.e., the fatigue limit ratio, is greater than 0.

45. MSC (mass%) = Mn + 0.2 × Si + 1.7 × Cr + 2.5 × Mo…(1) In equation (1), each element symbol represents the mass percentage of each element, and the element that is not present is 0.

2. The high-strength steel plate according to claim 1, characterized in that, The combined area ratio of fresh martensite and / or retained austenite in the surface region is less than the combined area ratio of fresh martensite and / or retained austenite in the inner region.

3. The high-strength steel plate according to claim 1 or 2, wherein, The composition further contains, by mass%, at least one of the following groups A to D: Group A: At least one of Cr: less than 1.0% and Mo: less than 1.0%; Group B: At least one of Cu: less than 2.0%, Ni: less than 2.0%, Ti: less than 0.3%, Nb: less than 0.3%, and V: less than 0.3%; Group C: Sb: 0.005–0.020%; Group D: At least one of Ca: less than 0.01%, Mg: less than 0.01%, and REM: less than 0.01%.

4. A method for manufacturing a high-strength steel plate, as described in any one of claims 1 to 3. The steel billet having the aforementioned composition is heated to a temperature of 1150°C or higher. The heated steel billet is then subjected to hot rolling consisting of roughing and finishing rolling. From the start of rough rolling to the start of finish rolling, at least two rust removal processes shall be performed, and within 5 seconds of the start of finish rolling, at least one rust removal process with a water pressure of 15 MPa or higher shall be performed. The finishing rolling is carried out under the conditions of a total reduction rate of 25% to 80% in the temperature range below RC1 and a finishing rolling end temperature of (RC2-50℃) to (RC2+120℃), thereby producing hot-rolled steel sheets. The hot-rolled steel sheet is cooled under the following conditions: the time from the end of hot rolling to the start of cooling is within 2.0 seconds, the average cooling rate is above 5℃ / s, and the cooling stop temperature is Trs~(Trs+250℃). The cooled hot-rolled steel sheet is wound at a winding temperature of Trs~(Trs+250℃). Cool to below 100°C at an average cooling rate of less than 20°C / s. in, RC1, RC2, and Trs are defined by equations (2), (3), and (4) respectively, with units of ℃. RC1=900+100×C+100×N+10×Mn+700×Ti+5000×B+10×Cr+50× Mo + 2000 × Nb + 150 × V…(2) RC2=750+100×C+100×N+10×Mn+350×Ti+5000×B+10×Cr+50× Mo + 1000 × Nb + 150 × V…(3) Trs=500-450×C-35×Mn-15×Cr-10×Ni-20×Mo…(4) In equations (2), (3), and (4), the symbols of each element represent the mass percentage of each element, and the element that is not present is represented by 0.

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