High strength steel sheet and method for manufacturing the same

Abstract

A high strength steel sheet having 980 MPa or higher tensile strength and a method for manufacturing the same are provided. The high strength steel sheet has a specific chemical composition and is such that in a region at ¼ sheet thickness, the area fraction of tempered martensite is 38% or more and less than 90%, the volume fraction of retained austenite is less than 3%, the area fraction of the total of ferrite and bainitic ferrite is 10% or more and 60% or less, the average grain size of prior austenite is 20 μm or less, and the average of the proportions of packets having the largest area in prior austenite grains is 70% by area or less of the prior austenite grain.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This is the U.S. National Phase application of PCT / JP2023 / 002915, filed Jan. 30, 2023 which claims priority to Japanese Patent Application No. 2022-049757, filed Mar. 25, 2022, the disclosures of these applications being incorporated herein by reference in their entireties for all purposes.FIELD OF THE INVENTION

[0002] The present invention relates to a high strength steel sheet excellent in tensile strength, press formability, flatness in the width direction, and working embrittlement resistance, and to a method for manufacturing the same. The high strength steel sheet according to aspects of the present invention may be suitably used as structural members, such as automobile parts.BACKGROUND OF THE INVENTION

[0003] Steel sheets for automobiles are being increased in strength in order to reduce CO2 emissions by weight reduction of vehicles and to enhance crashworthiness by weight reduction of automobile bodies at the same time, with introduction of new laws and regulations one after another. To increase the strength of automobile bodies, high strength steel sheets having a tensile strength (TS) of 1180 MPa or higher grade are increasingly applied to principal structural parts of automobiles.

[0004] High strength steel sheets used in automobiles require excellent press formability. For example, high strength steel sheets with high El and excellent hole expansion ratio λ are suitably applied to automobile frame parts, such as bumpers. From the point of view of crash safety, excellent working embrittlement resistance is required.

[0005] Furthermore, high strength steel sheets used in automobiles require high flatness. Patent Literature 1 describes that warpage of a steel sheet causes operational troubles in forming lines and adversely affects the dimensional accuracy of products. The present inventors carried out extensive studies and have found that the dimensional accuracy of products is affected not only by the warpage of steel sheets but also by the flatness in the width direction that is evaluated as steepness. For example, the steepness in the width direction is suitably 0.02 or less in order to achieve excellent dimensional accuracy.

[0006] To meet the above demands, for example, Patent Literature 2 provides a hot-dip galvanized steel sheet with excellent press formability and low-temperature toughness that has a tensile strength of 980 MPa or more, and a method for manufacturing the same. While the steel sheet of Patent Literature 2 is improved in embrittlement at low temperatures, the technique does not take into consideration the working embrittlement of the steel sheet or the flatness in the width direction.Patent Literature

[0007] PTL 1: Japanese Patent No. 4947176

[0008] PTL 2: Japanese Patent No. 6777272Non Patent Literature

[0009] NPL 1: Journal of Smart Processing, 2013, Vol. 2, No. 3, pp. 110-118SUMMARY OF THE INVENTION

[0010] Aspects of the present invention have been developed in view of the circumstances discussed above. Objects of aspects of the present invention are therefore to provide a high strength steel sheet having 980 MPa or higher TS and being excellent in press formability, flatness in the width direction, and working embrittlement resistance; and to provide a method for manufacturing the same.

[0011] The present inventors carried out extensive studies directed to solving the problems described above and have consequently found the following facts.(1) 980 MPa or higher TS and excellent press formability can be realized by limiting the amount of tempered martensite to 38% or more and less than 90%, the amount of the total of ferrite and bainitic ferrite to 10% or more and 60% or less, and the amount of retained austenite to less than 3%.(2) The flatness in the width direction can be enhanced by limiting the proportion of a packet having the largest area in tempered martensite to 70% or less of a prior austenite grain.(3) Excellent working embrittlement resistance can be achieved by limiting the proportion of a packet having the largest area in tempered martensite to 70% or less of a prior austenite grain and by limiting the average prior austenite grain size in tempered martensite to 20 μm or less.

[0012] Aspects of the present invention have been made based on the above findings. Specifically, a summary of aspects of the present invention is as follows.[1] A high strength steel sheet having a chemical composition including, in mass %, C: 0.030% or more and 0.500% or less, Si: 0.01% or more and 2.50% or less, Mn: 0.10% or more and 5.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less, and O: 0.0100% or less, a balance being Fe and incidental impurities, the high strength steel sheet being such that in a region at ¼ sheet thickness, an area fraction of tempered martensite is 38% or more and less than 90%, a volume fraction of retained austenite is less than 3%, an area fraction of the total of ferrite and bainitic ferrite is 10% or more and 60% or less, an average grain size of prior austenite is 20 μm or less, and an average of the proportions of packets having the largest area in prior austenite grains is 70% by area or less of the prior austenite grain.[2] The high strength steel sheet according to [1], wherein the chemical composition further includes at least one element selected from, in mass %, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Co: 0.010% or less, Ni: 1.00% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less.[3] The high strength steel sheet according to [1] or [2], which has a coated layer on a surface of the steel sheet.[4] A method for manufacturing the high strength steel sheet according to [1] or [2], the method including providing a cold rolled steel sheet produced by subjecting a steel having the chemical composition described above to hot rolling, pickling, and cold rolling; heating the steel sheet at an annealing temperature T1 of 700° C. or above and 950° C. or below for a holding time t1 at the annealing temperature T1 of 10 seconds or more and 1000 seconds or less; cooling the steel sheet in such a manner that the average cooling rate from 750° C. to 600° C. is less than 20° C. / s, the average cooling rate from (Ms+50° C.) to a quench start temperature T2 is less than 5° C. / s wherein the quench start temperature T2 is (Ms−80° C.) or above and is below Ms where Ms is martensite start temperature (° C.) defined by formula (1), and the average cooling rate from the quench start temperature T2 to 80° C. is 300° C. / s or more; and heating the steel sheet at a tempering temperature T3 of 100° C. or above and 400° C. or below for a holding time t3 at the tempering temperature T3 of 10 seconds or more and 10000 seconds or less,Ms=519-474×[%⁢ C]-30.4×[%⁢ Mn]-12.1×[%⁢ Cr]-7.5×[%⁢ Mo]-17.7×[%⁢ Ni]-T⁢1 / 80(1)wherein [% C], [% Mn], [% Cr], [% Mo], and [% Ni] indicate the contents (mass %) of C, Mn, Cr, Mo, and Ni, respectively, and are zero when the element is absent.[5] The method for manufacturing the high strength steel sheet according to [4], further including performing a coating treatment.According to aspects of the present invention, a high strength steel sheet can be obtained that has 980 MPa or higher TS and excels in press formability, flatness in the width direction and working embrittlement resistance. Furthermore, for example, the high strength steel sheet according to aspects of the present invention may be applied to automobile structural members to reduce the weight of automobile bodies and thereby to enhance fuel efficiency. Thus, aspects of the present invention are highly valuable in industry.BRIEF DESCRIPTION OF THE DRAWINGSFIG. 1 is a set of views illustrating a structure of a packet having the largest area in a prior austenite grain according to an embodiment of the present invention, and how the calculation is made.

[0015] FIG. 2 is a set of views illustrating the concept of the steepness 0 of a steel sheet according to an embodiment of the present invention, and how the steepness is calculated.DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0016] Embodiments of the present invention will be described below.

[0017] First, appropriate ranges of the chemical composition of the high strength steel sheet and the reasons why the chemical composition is thus limited will be described. In the following description, “%” indicating the contents of constituent elements of steel means “mass %” unless otherwise specified.[C: 0.030% or more and 0.500% or less]

[0018] Carbon is one of the important basic components of steel. Particularly in accordance with aspects of the present invention, carbon is an important element that affects the fraction of martensite and the working embrittlement resistance. When the C content is less than 0.030%, the fraction of martensite is so small that realizing 980 MPa or higher TS is difficult. When, on the other hand, the C content is more than 0.500%, martensite becomes brittle to cause deterioration in working embrittlement resistance. Thus, the C content is limited to 0.030% or more and 0.500% or less. The C content is preferably 0.050% or more. The C content is preferably 0.400% or less. The C content is more preferably 0.100% or more. The C content is more preferably 0.350% or less.[Si: 0.01% or More and 2.50% or Less]

[0019] Silicon is one of the important basic components of steel. Silicon suppresses the occurrence of carbides during continuous annealing and promotes the formation of retained austenite. Thus, particularly in accordance with aspects of the present invention, silicon is an important element that affects TS and the amount of retained austenite. When the si content is less than 0.01%, realizing 980 MPa or higher TS is difficult. When, on the other hand, the Si content is more than 2.50%, the amount of retained austenite is increased excessively to make it difficult to achieve 85% or more YR. Thus, the Si content is limited to 0.01% or more and 2.50% or less. The Si content is preferably 0.05% or more. The Si content is preferably 2.00% or less. The Si content is more preferably 0.10% or more. The Si content is more preferably 1.20% or less.[Mn: 0.10% or More and 5.00% or Less]

[0020] Manganese is one of the important basic components of steel. Particularly in accordance with aspects of the present invention, manganese is an important element that affects the fraction of martensite and the working embrittlement resistance. When the Mn content is less than 0.10%, the fraction of martensite is so small that realizing 980 MPa or higher TS is difficult. When, on the other hand, the Mn content is more than 5.00%, martensite becomes brittle to cause deterioration in working embrittlement resistance. Thus, the Mn content is limited to 0.10% or more and 5.00% or less. The Mn content is preferably 0.50% or more. The Mn content is preferably 4.50% or less. The Mn content is more preferably 0.80% or more. The Mn content is more preferably 4.00% or less.[P: 0.100% or Less]

[0021] Phosphorus is segregated at prior austenite grain boundaries and makes the grain boundaries brittle, thereby lowering the ultimate deformability of steel sheets and causing deterioration in working embrittlement resistance. Thus, the P content needs to be 0.100% or less. The lower limit of the P content is not particularly specified. In view of the fact that phosphorus is a solid solution strengthening element and can increase the strength of steel sheets, the lower limit is preferably 0.001% or more. For the reasons above, the P content is limited to 0.100% or less. The P content is preferably 0.001% or more. The P content is preferably 0.070% or less.[S: 0.0200% or Less]

[0022] Sulfur forms sulfides and lowers the ultimate deformability of steel sheets to cause deterioration in working embrittlement resistance. Thus, the S content needs to be 0.0200% or less. The lower limit of the S content is not particularly specified but is preferably 0.0001% or more due to production technique limitations. For the reasons above, the S content is limited to 0.0200% or less. The S content is preferably 0.0001% or more. The S content is preferably 0.0050% or less.[Al: 1.000% or Less]

[0023] Aluminum raises the As transformation temperature to allow more ferrite to be contained in the microstructure. The fraction of martensite is correspondingly lowered to make it difficult to realize 980 MPa or higher TS. Thus, the Al content needs to be 1.000% or less. The lower limit of the Al content is not particularly specified. In view of the fact that aluminum suppresses the occurrence of carbides during continuous annealing and promotes the formation of retained austenite, the Al content is preferably 0.001% or more. For the reasons above, the Al content is limited to 1.000% or less. The Al content is preferably 0.001% or more. The Al content is preferably 0.500% or less.[N: 0.0100% or Less]

[0024] Nitrogen forms nitrides and lowers the ultimate deformability of steel sheets to cause deterioration in working embrittlement resistance. Thus, the N content needs to be 0.0100% or less. The lower limit of the N content is not particularly specified but the N content is preferably 0.0001% or more due to production technique limitations. For the reasons above, the N content is limited to 0.0100% or less. The N content is preferably 0.0001% or more. The N content is preferably 0.0050% or less.[O: 0.0100% or Less]

[0025] Oxygen forms oxides and lowers the ultimate deformability of steel sheets to cause deterioration in working embrittlement resistance. Thus, the O content needs to be 0.0100% or less. The lower limit of the O content is not particularly specified but the 0 content is preferably 0.0001% or more due to production technique limitations. For the reasons above, the O content is limited to 0.0100% or less. The O content is preferably 0.0001% or more. The O content is preferably 0.0050% or less.

[0026] The chemical composition of the high strength steel sheet according to an embodiment of the present invention includes the components described above, and the balance is Fe and incidental impurities. Here, the incidental impurities include Zn, Pb, As, Ge, Sr, and Cs. A total of 0.100% or less of these impurities is acceptable.

[0027] In addition to the components in the proportions described above, the high strength steel sheet according to aspects of the present invention may further include at least one element selected from, in mass %, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.010% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less. These elements may be contained singly or in combination.

[0028] When the contents of Ti, Nb, and V are each 0.200% or less, coarse precipitates and inclusions will not occur in large amounts and thus will not cause lowering of the ultimate deformability of steel sheets; hence there will be no deterioration in working embrittlement resistance. Thus, the contents of Ti, Nb, and V are each preferably 0.200% or less. The lower limits of the contents of Ti, Nb, and V are not particularly specified. These elements form fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing to increase the strength of steel sheets. In view of this fact, the contents of Ti, Nb, and V are each more preferably 0.001% or more. When titanium, niobium, and vanadium are added, the contents thereof are each limited to 0.200% or less for the reasons above. The contents are each more preferably 0.001% or more. The contents are each more preferably 0.100% or less.

[0029] When the contents of Ta and W are each 0.10% or less, coarse precipitates and inclusions will not occur in large amounts and thus will not cause lowering of the ultimate deformability of steel sheets; hence there will be no deterioration in working embrittlement resistance. Thus, the contents of Ta and W are each preferably 0.10% or less. The lower limits of the contents of Ta and W are not particularly specified. These elements form fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing to increase the strength of steel sheets in some cases. In view of this fact, the contents of Ta and W are each more preferably 0.01% or more. When tantalum and tungsten are added, the contents thereof are each limited to 0.10% or less for the reasons above. The contents are each more preferably 0.01% or more. The contents are each more preferably 0.08% or less.

[0030] When the B content is 0.0100% or less, inner cracks that lower the ultimate deformability of steel sheets will not form during casting or hot rolling and thus there will be no deterioration in working embrittlement resistance. Thus, the B content is preferably 0.0100% or less. The lower limit of the B content is not particularly specified. The B content is more preferably 0.0003% or more in view of the fact that this element is segregated at austenite grain boundaries during annealing and enhances hardenability. When boron is added, the content thereof is limited to 0.0100% or less for the reasons above. The content is more preferably 0.0003% or more. The content is more preferably 0.0080% or less.

[0031] When the contents of Cr, Mo, and Ni are each 1.00% or less, coarse precipitates and inclusions will not occur in increased amounts and thus will not cause lowering of the ultimate deformability of steel sheets; hence there will be no deterioration in working embrittlement resistance. Thus, the contents of Cr, Mo, and Ni are each preferably 1.00% or less. The lower limits of the contents of Cr, Mo, and Ni are not particularly specified. In view of the fact that these elements enhance hardenability, the contents of Cr, Mo, and Ni are each more preferably 0.01% or more. When chromium, molybdenum, and nickel are added, the contents thereof are each limited to 1.00% or less for the reasons above. The contents are each more preferably 0.01% or more. The contents are each more preferably 0.80% or less.

[0032] When the Co content is 0.010% or less, coarse precipitates and inclusions will not occur in increased amounts and thus will not cause lowering of the ultimate deformability of steel sheets; hence there will be no deterioration in working embrittlement resistance. Thus, the Co content is preferably 0.010% or less. The lower limit of the Co content is not particularly specified. In view of the fact that this element enhances hardenability, the Co content is more preferably 0.001% or more. When cobalt is added, the content thereof is limited to 0.010% or less for the reasons above. The content is more preferably 0.001% or more. The content is more preferably 0.008% or less.

[0033] When the Cu content is 1.00% or less, coarse precipitates and inclusions will not occur in increased amounts and thus will not cause lowering of the ultimate deformability of steel sheets; hence there will be no deterioration in working embrittlement resistance. Thus, the Cu content is preferably 1.00% or less. The lower limit of the Cu content is not particularly specified. In view of the fact that this element enhances hardenability, the Cu content is preferably 0.01% or more. When copper is added, the content thereof is limited to 1.00% or less for the reasons above. The content is more preferably 0.01% or more. The content is more preferably 0.80% or less.

[0034] When the Sn content is 0.200% or less, inner cracks that lower the ultimate deformability of steel sheets will not form during casting or hot rolling and thus there will be no deterioration in working embrittlement resistance. Thus, the Sn content is preferably 0.200% or less. The lower limit of the Sn content is not particularly specified. The Sn content is more preferably 0.001% or more in view of the fact that tin enhances hardenability (in general, is an element that enhances corrosion resistance). When tin is added, the content thereof is limited to 0.200% or less for the reasons above. The content is more preferably 0.001% or more. The content is more preferably 0.100% or less.

[0035] When the Sb content is 0.200% or less, coarse precipitates and inclusions will not occur in increased amounts and thus will not cause lowering of the ultimate deformability of steel sheets; hence there will be no deterioration in working embrittlement resistance. Thus, the Sb content is preferably 0.200% or less. The lower limit of the Sb content is not particularly specified. In view of the fact that this element enables control of the thickness of surface layer softening and the strength, the Sb content is more preferably 0.001% or more. When antimony is added, the content thereof is limited to 0.200% or less for the reasons above. The content is more preferably 0.001% or more. The content is more preferably 0.100% or less.

[0036] When the contents of Ca, Mg, and REM are each 0.0100% or less, coarse precipitates and inclusions will not occur in increased amounts and thus will not cause lowering of the ultimate deformability of steel sheets; hence there will be no deterioration in working embrittlement resistance. Thus, the contents of Ca, Mg, and REM are each preferably 0.0100% or less. The lower limits of the contents of Ca, Mg, and REM are not particularly specified. In view of the fact that these elements change the shapes of nitrides and sulfides into spheroidal and enhance the ultimate deformability of steel sheets, the contents of Ca, Mg, and REM are each more preferably 0.0005% or more. When calcium, magnesium, and rare earth metal(s) are added, the contents thereof are each limited to 0.0100% or less for the reasons above. The contents are each more preferably 0.0005% or more. The contents are each more preferably 0.0050% or less.

[0037] When the contents of Zr and Te are each 0.100% or less, coarse precipitates and inclusions will not occur in increased amounts and thus will not cause lowering of the ultimate deformability of steel sheets; hence there will be no deterioration in working embrittlement resistance. Thus, the contents of Zr and Te are each preferably 0.100% or less. The lower limits of the contents of Zr and Te are not particularly specified. In view of the fact that these elements change the shapes of nitrides and sulfides into spheroidal and enhance the ultimate deformability of steel sheets, the contents of Zr and Te are each more preferably 0.001% or more. When zirconium and tellurium are added, the contents thereof are each limited to 0.100% or less for the reasons above. The contents are each more preferably 0.001% or more. The contents are each more preferably 0.080% or less.

[0038] When the Hf content is 0.10% or less, coarse precipitates and inclusions will not occur in increased amounts and thus will not cause lowering of the ultimate deformability of steel sheets; hence there will be no deterioration in working embrittlement resistance. Thus, the Hf content is preferably 0.10% or less. The lower limit of the Hf content is not particularly specified. In view of the fact that this element changes the shapes of nitrides and sulfides into spheroidal and enhances the ultimate deformability of steel sheets, the Hf content is more preferably 0.01% or more. When hafnium is added, the content thereof is limited to 0.10% or less for the reasons above. The content is more preferably 0.01% or more. The content is more preferably 0.08% or less.

[0039] When the Bi content is 0.200% or less, coarse precipitates and inclusions will not occur in increased amounts and thus will not cause lowering of the ultimate deformability of steel sheets; hence there will be no deterioration in working embrittlement resistance. Thus, the Bi content is preferably 0.200% or less. The lower limit of the Bi content is not particularly specified. In view of the fact that this element reduces the occurrence of segregation, the Bi content is more preferably 0.001% or more. When bismuth is added, the content thereof is limited to 0.200% or less for the reasons above. The content is more preferably 0.001% or more. The content is more preferably 0.100% or less.

[0040] When the content of any of Ti, Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf, and Bi is below the preferred lower limit, the element does not impair the advantageous effects according to aspects of the present invention and is regarded as an incidental impurity.

[0041] Next, the steel microstructure of the high strength steel sheet according to aspects of the present invention will be described.[Area Fraction of Tempered Martensite: 38% or More and Less than 90%]

[0042] When the amount of tempered martensite is less than 38%, realizing 980 MPa or higher TS is difficult. When, on the other hand, the amount of tempered martensite is 90% or more, the amount of ferrite is lowered to cause a decrease in El and consequently press formability is lowered. Thus, the amount of tempered martensite is limited to 38% or more and less than 90%. The amount is preferably 40% or more. The amount is preferably 60% or less.

[0043] Here, tempered martensite is measured as follows. A longitudinal cross section of the steel sheet is polished and is etched with 3 vol % Nital. A portion at ¼ sheet thickness (a location corresponding to ¼ of the sheet thickness in the depth direction from the steel sheet surface) is observed using SEM in 10 fields of view at a magnification of ×2000. In the microstructure images, tempered martensite is structures that have fine irregularities inside the structures and contain inner carbides. The values thus obtained are averaged to determine the tempered martensite.[Amount of Retained Austenite: Less than 3%]

[0044] This configuration is a very important requirement that constitutes an aspect of the present invention. When the volume fraction of retained austenite is 3% or more, press formability is lowered. The reason for low press formability is that retained austenite with a high fraction gives rise to a lowering in λ by undergoing strain-induced transformation. Thus, the retained austenite is limited to less than 3%. The amount of retained austenite is preferably 1% or less. The lower limit of retained austenite is not particularly limited and may be 0%.

[0045] Here, retained austenite is measured as follows. The steel sheet is polished to expose a face 0.1 mm below ¼ sheet thickness and is thereafter further chemically polished to expose a face 0.1 mm below the face exposed above. The face is analyzed with an X-ray diffractometer using CoKα radiation to determine the integral intensity ratios of the diffraction peaks of {200}, {220}, and {311} planes of fcc iron and {200}, {211}, and {220} planes of bcc iron. Nine integral intensity ratios thus obtained are averaged to determine retained austenite.[Area Fraction of the Total of Ferrite and Bainitic Ferrite: 10% or More and 60% or Less]

[0046] This configuration is a very important requirement that constitutes an aspect of the present invention. When the total of ferrite and bainitic ferrite is less than 10%, El is lowered and consequently press formability is deteriorated. When, on the other hand, the total of ferrite and bainitic ferrite is more than 60%, realizing 980 MPa or higher TS is difficult. Thus, the total of ferrite and bainitic ferrite is limited to 10% or more and 60% or less. The total amount is preferably 35% or more. The total amount is preferably 55% or less.

[0047] Here, the total of ferrite and bainitic ferrite is measured as follows. A longitudinal cross section of the steel sheet is polished and is etched with 3 vol % Nital. A portion at ¼ sheet thickness (a location corresponding to ¼ of the sheet thickness in the depth direction from the steel sheet surface) is observed using SEM in 10 fields of view at a magnification of ×2000. In the microstructure images, ferrite is recessed structures having a flat interior and containing no inner carbides. In the microstructure images, bainitic ferrite is recessed structures having a flat interior and containing inner carbides. The values thus obtained are combined and are averaged to determine the total of ferrite and bainitic ferrite.

[0048] Possible microstructures other than those described above include pearlite, fresh martensite, and acicular ferrite. These microstructures do not affect characteristics as long as their fractions are 5% or less, and thus may be present within that range.[Average Grain Size of Prior Austenite: 20 μm or Less]

[0049] This configuration is a very important requirement that constitutes an aspect of the present invention. Reducing the average grain size of prior austenite can suppress crack propagation and thereby enhances the working embrittlement resistance of steel sheets. In order to obtain these effects, the average grain size of prior austenite needs to be 20 μm or less. The lower limit of the average grain size of prior austenite is not particularly specified. When, however, the average grain size of prior austenite is less than 2 μm, more retained austenite may form. Thus, the average grain size is preferably 2 μm or more. For the reasons above, the average grain size of prior austenite is limited to 20 μm or less. The average grain size is preferably 2 μm or more. The average grain size is preferably 15 μm or less. The average grain size is more preferably 3 μm or more. The average grain size is more preferably 10 μm or less.

[0050] Here, the average grain size of prior austenite is measured as follows. A longitudinal cross section of the steel sheet is polished and is etched with, for example, a mixed solution of picric acid and ferric chloride to expose prior austenite grain boundaries. Portions at ¼ sheet thickness (locations corresponding to ¼ of the sheet thickness in the depth direction from the steel sheet surface) are photographed with an optical microscope each in 3 to 10 fields of view at a magnification of ×400. Twenty straight lines including 10 vertical lines and 10 horizontal lines are drawn at regular intervals on the image data obtained, and the grain size is determined by a linear intercept method.[Average of the Proportions of Packets Having the Largest Area in Prior Austenite Grains: 70% by Area or Less]

[0051] This configuration is a very important requirement that constitutes an aspect of the present invention. The proportion of a packet having the largest area in a prior austenite grain affects the flatness in the width direction and the working embrittlement resistance. As illustrated in FIG. 1, a prior austenite grain contains up to four kinds of packets distinguished by crystal habit plane formed by transformation. The packet having the largest area in a prior austenite grain is the packet that occupies the largest area among such packets.

[0052] The proportion of one packet in a prior austenite grain is determined by dividing the area of the packet of interest by the area of the whole prior austenite grain. As a result of extensive studies, the present inventors have found that strain among the packets is reduced and the flatness in the width direction is improved by lowering the proportion of a packet having the largest area in a prior austenite grain. The present inventors have also found that lowering the proportion of a packet having the largest area in a prior austenite grain leads to a fine microstructure and suppresses crack propagation, thereby enhancing the working embrittlement resistance of the steel sheet. Thus, the average of the proportions of packets having the largest area in prior austenite grains is limited to 70% or less. The average proportion is preferably 60% or less. The lower limit of the average proportion of packets having the largest area in prior austenite grains is not particularly limited. The grains contain up to four kinds of packets. When four packets are evenly distributed, the proportion of a packet having the largest area in the prior austenite grain is 25%. Thus, the lower limit of the average proportion of packets having the largest area in prior austenite grains may be 25% or more but is not necessarily limited thereto.

[0053] Here, the average proportion of packets having the largest area in prior austenite grains is measured as follows. First, a test specimen for microstructure observation is sampled from the cold rolled steel sheet. Next, the sampled test specimen is polished by vibration polishing with colloidal silica to expose a cross section in the rolling direction (a longitudinal cross section) for use as observation surface. The observation surface is specular. Next, electron backscatter diffraction (EBSD) measurement is performed with respect to a portion at ¼ sheet thickness (a location corresponding to ¼ of the sheet thickness in the depth direction from the steel sheet surface) to obtain local crystal orientation data. Here, the SEM magnification is ×1000, the step size is 0.2 μm, the measured region is 80 μm square, and the WD is 15 mm. The local orientation data obtained is analyzed with OIM Analysis 7 (OIM), and a map (a CP map) that shows close-packed plane groups (CP groups) with different colors is created using the method described in Non Patent Literature 1. In accordance with aspects of the present invention, a packet is defined as a region or regions belonging to the same CP group. From the CP map obtained, the area of the packet having the largest area is determined and is divided by the area of the whole prior austenite grain to give the proportion of the packet having the largest area in the prior austenite grain. This analysis is performed with respect to 10 or more adjacent prior austenite grains, and the results are averaged to give the average proportion of packets having the largest area in prior austenite grains.

[0054] Next, a manufacturing method according to aspects of the present invention will be described.

[0055] In accordance with aspects of the present invention, a steel material (a steel slab) may be obtained by any known steelmaking method without limitation, such as a converter or an electric arc furnace. To prevent macro-segregation, the steel slab (the slab) is preferably produced by a continuous casting method.

[0056] In accordance with aspects of the present invention, the slab heating temperature, the slab soaking holding time, and the coiling temperature in hot rolling are not particularly limited. For example, the steel slab may be hot rolled in such a manner that the slab is heated and is then rolled, that the slab is subjected to hot direct rolling after continuous casting without being heated, or that the slab is subjected to a short heat treatment after continuous casting and is then rolled. The slab heating temperature, the slab soaking holding time, the finish rolling temperature, and the coiling temperature in hot rolling are not particularly limited. The lower limit of the slab heating temperature is preferably 1100° C. or above. The upper limit of the slab heating temperature is preferably 1300° C. or below. The lower limit of the slab soaking holding time is preferably 30 minutes or more. The upper limit of the slab soaking holding time is preferably 250 minutes or less. The lower limit of the finish rolling temperature is preferably Ar3 transformation temperature or above. Furthermore, the lower limit of the coiling temperature is preferably 350° C. or above. The upper limit of the coiling temperature is preferably 650° C. or below.

[0057] The hot rolled steel sheet thus produced is pickled. Pickling can remove oxides on the steel sheet surface and is thus important to ensure good chemical convertibility and a high quality of coating in the final high strength steel sheet. Pickling may be performed at a time or several. The hot rolled sheet that has been pickled may be cold rolled directly or may be subjected to heat treatment before cold rolling.

[0058] The rolling reduction in cold rolling and the sheet thickness after rolling are not particularly limited. The lower limit of the rolling reduction is preferably 30% or more. The upper limit of the rolling reduction is preferably 80% or less. The advantageous effects according to aspects of the present invention may be obtained without any limitations on the number of rolling passes and the rolling reduction in each pass.

[0059] The cold rolled steel sheet obtained as described above is annealed. Annealing conditions are as follows.[Annealing Temperature T1: 700° C. or Above and 950° C. or Below]

[0060] When the annealing temperature TI is below 700° C., the area fraction of the total of ferrite and bainitic ferrite is more than 60% to make it difficult to realize 980 MPa or higher TS. When, on the other hand, the annealing temperature T1 is above 950° C., prior austenite grains are excessively increased in size and the prior austenite grain size exceeds 20 μm to give rise to a decrease in working embrittlement resistance. Thus, the annealing temperature T1 is limited to 700° C. or above and 950° C. or below. The annealing temperature T1 is preferably 800° C. or above. The annealing temperature T1 is preferably 900° C. or below.[Holding Time t1 at the Annealing Temperature T1: 10 Seconds or More and 1000 Seconds or Less]

[0061] When the holding time t1 at the annealing temperature T1 is less than 10 seconds, austenitization is insufficient and the area fraction of the total of ferrite and bainitic ferrite is more than 60%. As a result, it is difficult to achieve 980 MPa or higher TS. When, on the other hand, the holding time at the annealing temperature T1 is more than 1000 seconds, the prior austenite grain size is excessively increased, and the working embrittlement resistance is lowered. For the reasons above, the holding time t1 at the annealing temperature T1 is limited to 10 seconds or more and 1000 seconds or less. The holding time t1 is preferably 50 seconds or more. The holding time t1 is preferably 500 seconds or less.[Average Cooling Rate from 750° C. to 600° C.: Less than 20° C. / s]

[0062] When the average cooling rate from 750° C. to 600° C. is 20° C. / s or more, the area fraction of the total of ferrite and bainitic ferrite is less than 10% to cause a decrease in El, thereby deteriorating press formability. For the reasons above, the average cooling rate from 750° C. to 600° C. is limited to less than 20° C. / s. The average cooling rate is preferably 15° C. / s or less.[Average Cooling Rate from (Ms+50° C.) to a Quench Start Temperature T2: Less than 5° C. / s]

[0063] This configuration is a very important requirement that constitutes an aspect of the present invention. When the average cooling rate from (Ms+50° C.) to a quench start temperature T2 is 5° C. / s or more, the area fraction of the total of ferrite and bainitic ferrite is less than 10% to cause a decrease in El, thereby deteriorating press formability. For the reasons above, the average cooling rate from (Ms+50° C.) to a quench start temperature T2 is limited to less than 5° C. / s. The average cooling rate is preferably 4° C. / s or less.[Quench Start Temperature T2: (Ms−80° C.) or Above and Below Ms]

[0064] This configuration is a very important requirement that constitutes an aspect of the present invention. The quench start temperature T2 is controlled to (Ms−80° C.) or above and below Ms to ensure that the martensite transformation rate before the start of quenching is 1% or more and 80% or less. In this manner, quenching can give microstructures in which the average proportion of packets having the largest area in prior austenite grains is 70% or less and the volume fraction of retained austenite is less than 3%. When the quench start temperature T2 is below (Ms−80° C.), the martensite transformation rate before the start of quenching exceeds 80% and consequently the volume fraction of retained austenite is 3% or more to cause a decrease in press formability. When, on the other hand, the quench start temperature T2 is above Ms, the martensite transformation rate before the start of quenching is less than 1% and the average proportion of packets having the largest area in prior austenite grains exceeds 70% to cause deterioration in flatness in the width direction and working embrittlement resistance. Thus, the quench start temperature T2 is limited to (Ms−80° C.) or above and below Ms. The quench start temperature T2 is preferably (Ms−50° C.) or above. The quench start temperature T2 is preferably (Ms−5° C.) or below. The martensite start temperature Ms (° C.) is defined by the following formula (1):Ms=519-474×[%⁢ C]-30.4×[%⁢ Mn]-12.1×[%⁢ Cr]-7.5×[%⁢ Mo]-17.7×[%⁢ Ni]-T⁢1 / 80(1)wherein [% C], [% Mn], [% Cr], [% Mo], and [% Ni] indicate the contents (mass %) of C, Mn, Cr, Mo, and Ni, respectively, and are zero when the element is absent.[Average Cooling Rate from the Quench Start Temperature T2 to 80° C.: 300° C. / s or More]When the average cooling rate from the quench start temperature T2 to 80° C. is less than 300° C. / s, the volume fraction of retained austenite is 3% or more to cause a decrease in press formability. Thus, the average cooling rate from the quench start temperature T2 to 80° C. is limited to 300° C. / s or more. The average cooling rate is preferably 800° C. / s or more. The upper limit is not necessarily specified but is preferably 2000° C. / s or less.[Tempering Temperature T3: 100° C. or Above and 400° C. or Below]

[0066] In accordance with aspects of the present invention, tempered martensite is a microstructure that is formed when martensite at 80° C. or below is heat-treated at a tempering temperature of 100° C. or above for a holding time of 10 seconds or more. Thus, martensite is not sufficiently tempered when the tempering temperature T3 is below 100° C. The resultant microstructures will be based on as-quenched martensite, which deteriorates the working embrittlement resistance. When, on the other hand, the tempering temperature T3 is above 400° C., martensite is excessively tempered to make it difficult to achieve 980 MPa or higher TS. For the reasons above, the tempering temperature T3 is limited to 100° C. or above and 400° C. or below. The tempering temperature T3 is preferably 150° C. or above. The tempering temperature T3 is preferably 350° C. or below.[Holding Time t3 at the Tempering Temperature T3: 10 Seconds or More and 10000 Seconds or Less]

[0067] In accordance with aspects of the present invention, tempered martensite is a microstructure that is formed when martensite at 80° C. or below is heat-treated at a tempering temperature of 100° C. or above for a holding time of 10 seconds or more. Thus, martensite is not sufficiently tempered when the holding time t3 at the tempering temperature T3 is less than 10 seconds. The resultant microstructures will be based on as-quenched martensite, which deteriorates the working embrittlement resistance. When, on the other hand, the tempering temperature T3 is more than 10000 seconds, martensite is excessively tempered to make it difficult to achieve 980 MPa or higher TS. For the reasons above, the holding time t3 at the tempering temperature T3 is limited to 10 seconds or more and 10000 seconds or less. The holding time t3 is preferably 50 seconds or more. The holding time t3 is preferably 5000 seconds or less.

[0068] Post-temper cooling is not particularly limited and the steel sheet may be cooled to a desired temperature in an appropriate manner. Incidentally, the desired temperature is preferably about room temperature.

[0069] Furthermore, the high strength steel sheet described above may be worked under conditions where the amount of equivalent plastic strain is 0.10% or more and 5.00% or less. The working may be followed by reheating at 100° C. or above and 400° C. or below.

[0070] When the high strength steel sheet is a product that is traded, the steel sheet is usually traded after being cooled to room temperature.

[0071] The high strength steel sheet may be subjected to coating treatment during annealing or after annealing.

[0072] For example, the coating treatment during annealing may be hot-dip galvanizing treatment performed when the steel sheet is being cooled or has been cooled from 750° C. to 600° C. at an average cooling rate of less than 20° C. / s. The hot-dip galvanizing treatment may be followed by alloying. For example, the coating treatment after annealing may be Zn—Ni electrical alloy coating treatment or pure Zn electroplated coating treatment performed after tempering. A coated layer may be formed by electroplated coating, or hot-dip zinc-aluminum-magnesium alloy coating may be applied. While the coating treatment has been described above focusing on zinc coating, the types of coating metals, such as Zn coating and Al coating, are not particularly limited. Other conditions in the manufacturing method are not particularly limited. From the point of view of productivity, the series of treatments including annealing, hot-dip galvanizing, and alloying treatment of the coated zinc layer is preferably performed on hot-dip galvanizing line CGL (continuous galvanizing line). To control the coating weight of the coated layer, the hot-dip galvanizing treatment may be followed by wiping. Conditions for operations, such as coating, other than those conditions described above may be determined in accordance with the usual hot-dip galvanizing technique.

[0073] After the coating treatment after annealing, the steel sheet may be worked again under conditions where the amount of equivalent plastic strain is 0.10% or more and 5.00 or less. The working may be followed by reheating at 100° C. or above and 400° C. or below.EXAMPLES

[0074] Steels having a chemical composition described in Table 1 and 2, with the balance being Fe and incidental impurities, were smelted in a converter and were continuously cast into slabs. Next, the slabs obtained were heated, hot rolled, pickled, cold rolled, and subjected to annealing treatment and tempering treatment described in Tables 3 to 5. High strength cold rolled steel sheets having a sheet thickness of 0.6 to 2.2 mm were thus obtained. Incidentally, some of the steel sheets were subjected to coating treatment during or after annealing.TABLE 1Chemical composition (mass %)SteelsCSiMnPSNOAlTiBNbCu OthersA0.2150.2802.240.0060.00110.0050.0060.047INV. EX.B0.2170.2981.980.0050.00090.0060.0050.053INV. EX.C0.1920.2512.040.0090.00080.0020.0030.015INV. EX.D0.1111.3322.040.0110.00070.0040.0050.024INV. EX.E0.1131.4642.220.0100.00140.0060.0020.018INV. EX.F0.0480.2622.250.008 0.00110.0040.0020.030INV. EX.G0.0210.1682.300.0070.00130.0030.0050.036COMP. EX.H0.4680.1662.090.0090.00120.0060.0020.059INV. EX.I0.5220.2482.000.0100.00150.0010.0070.015COMP. EX.J0.2120.0732.130.0150.00100.0050.0060.052INV. EX.K0.1910.0022.230.0130.00060.0020.0060.020COMP. EX.L0.2102.3392.220.0060.00100.0060.0070.054INV. EX.M0.2052.5321.980.0120.00070.0020.0060.025COMP. EX.N0.2030.2730.270.0060.00070.0010.0040.055INV. EX.O0.1870.3070.080.0120.00090.0070.0050.033COMP. EX.P0.1910.2714.980.0120.00110.0050.0020.052INV. EX.Q0.1970.1625.120.0070.00070.0050.0040.040COMP. EX.R0.2160.3142.120.0990.00050.0020.0030.012INV. EX.S0.2190.3332.060.1210.00100.0040.0030.049COMP. EX.T0.2190.1732.250.0060.01820.0060.0060.024INV. EX.U0.2050.1922.060.0100.02220.0040.0020.024COMP. EX.V0.2050.1652.180.0110.00100.0020.0030.976INV. EX.W0.1950.1652.100.0150.00090.0050.0021.135COMP. EX.X0.2140.2042.060.0110.00090.00890.0070.059INV. EX.Y0.1920.1861.930.0120.00070.01120.0020.032COMP. EX.Z0.1820.2292.220.0110.00070.0040.00900.037INV. EX.AA0.2060.2712.190.0090.00150.0060.01100.040COMP. EX.AB0.1860.3311.910.0070.00100.0050.0010.0380.002INV. EX.AC0.1900.3272.290.0130.00080.0060.0020.0270.187INV. EX.AD0.2060.3412.260.0110.00120.0060.0030.0380.223COMP. EX.AE0.1820.2582.060.0080.00090.0050.0050.0380.0002INV. EX.AF0.1890.3092.120.0060.00050.0070.0030.0480.0088INV. EX.AG0.2060.2392.160.0080.00090.0030.0050.0310.0121COMP. EX.AH0.1870.1642.270.0060.00130.0020.0040.0160.002INV. EX.AI0.2160.3082.200.0080.00110.0020.0030.0310.189INV. EX.AJ0.1900.1892.050.0120.00060.0030.0070.0510.203COMP. EX.AK0.1830.3452.030.0090.00130.0020.0050.0230.03INV. EX.Underlines indicate being outside the range of the present invention.TABLE 2Chemical composition (mass %)SteelsCSiMnPSNOAlTiBNbCuOthersAL0.2050.2931.980.0130.00120.0050.0020.0570.90INV. EX.AM0.1900.3172.290.0070.00100.0030.0070.0521.11COMP. EX.AN0.2140.2612.250.0070.00100.0050.0040.050V:0.070INV. EX.AO0.2050.3092.020.0070.00060.0020.0060.028Ta:0.05INV. EX.AP0.2130.2152.260.0140.00060.0060.0020.048W:0.03INV. EX.AQ0.1870.2632.100.0150.00070.0050.0030.036Cr:0.87INV. EX.AR0.1840.2282.060.0090.00110.0020.0040.029Mo:0.13INV. EX.AS0.1930.2882.290.0120.00050.0060.0040.028Co:0.008INV. EX.AT0.2190.2422.210.0070.00120.0020.0030.034Ni:0.33INV. EX.AU0.1850.2451.920.0120.00110.0070.0040.056Sn:0.012INV. EX.AV0.1850.3012.080.0090.00060.0050.0040.026Sb:0.005INV. EX.AW0.1960.2741.900.0100.00120.0040.0020.056Ca:0.0015INV. EX.AX0.2180.3391.950.0090.00090.0040.0040.022Mg:0.0086INV. EX.AY0.1860.3432.0100.0050.00080.0020.0020.051Zr:0.083INV. EX.AZ0.2170.2401.9900.0080.00130.0070.0030.024Te:0.092INV. EX.BA0.1021.3642.2700.0100.00140.0060.0050.016Hf:0.05INV. EX.BB0.1281.3901.9600.0120.00050.0050.0040.037REM:0.0092INV. EX.BC0.1371.4022.0200.0120.00070.0030.0050.030Bi:0.164INV. EX.BD0.1321.3281.9400.0070.00060.0040.0030.012Zn:0.03INV. EX.BE0.1131.4822.0900.0080.00070.0050.0070.041Pb:0.016INV. EX.BF0.1111.3742.0000.0070.00080.0050.0020.011As:0.040INV. EX.BG0.1161.3622.0800.0110.00110.0060.0060.012Ge:0.090INV. EX.BH0.1331.3872.2200.0090.00090.0010.0030.052Sr:0.065INV. EX.BI0.1081.3102.1500.0070.00120.0010.0040.037Cs:0.082INV. EX.BJ0.1980.8702.7000.0100.00030.0040.0010.0450.0070.00170.0140.18Ni:0.05INV. EX.BK0.2180.3262.0600.0090.00080.0070.0070.012INV. EX.BL0.1081.3521.9200.0130.00110.0030.0040.056INV. EX.BM0.1051.3312.0300.0070.00060.0040.0020.049INV. EX.BN0.2071.3741.9100.0070.00130.0030.0010.057INV. EX.BO0.1891.4142.0200.0090.00150.0030.0030.050INV. EX.Underlines indicate being outside the range of the present invention.TABLE 3Average Average cooling coolingrate inrate intemperature temperaturerange ofQuenchCoolingAnnealingHoldingrange of(Ms + 50° C.)-start rateTemperingHoldingtemp.time750-quench temp.from T2temp.timeT1t1600° C.start temp.Ms(Ms-80)T2to 80° C.T3t3Nos.Steels(° C.)(s)(° C. / s)T2 (° C. / s)(° C.)(° C.)(° C.)(° C. / s)(° C.)(s)Type* 1A796 322132330250321 905203854CRINV. EX. 2B783 348 73337257323 966189994CRINV. EX. 3B717 427122337257320 875172961CRINV. EX. 4B692 255102337257327 957215989CRCOMP. EX. 5B927 274 73336256322 862151606CRINV. EX. 6B965 399123335255316 882186555CRCOMP. EX. 7B758 63 93337257320 914188784CRINV. EX. 8B777  8 63336256322 863199706CRCOMP. EX. 9B785 896112336256324 888193931CRINV. EX.10B78310151143362563221000206590CRCOMP. EX.11B753 311174336256324 960198929CRINV. EX.12B924 404252335255321 872212563CRCOMP. EX.13B798 223124336256322 831162596CRINV. EX.14B792 243 92336256327 836198954CRINV. EX.15B759 207 73335255328 878215780CRINV. EX.16B785 335113335255318 832203862CRINV. EX.17B779 381 94337257327 842151693CRINV. EX.18B804 203126337257331 865177714CRINV. EX.19B790 241 73336256261 928156687CRINV. EX.20B803 260 74336256 20 847158671CRCOMP. EX.21B761 335 73336256409 894165571CRCOMP. EX.22B797 376144336256631 978168603CRCOMP. EX.23B771 369133336256316 312212510CRINV. EX.24B755 222102336256326 284201832CRCOMP. EX.25B764 414103336256323 34211741CRCOMP. EX.26B788 404 93337257331 915167879CRINV. EX.27B768 222113336256329 995111996CRINV. EX.28B768 437153337257325 846110639CRINV. EX.29B784 321 82337257322 910389763CRINV. EX.30B772 266134336256321 883398707CRINV. EX.31B755 465 83337257328 82116123CRINV. EX.32B790 311153336256326 89717312CRINV. EX.33B786 304 84336256317 8542109860CRINV. EX.34B779 485 73336256324 8111969878CRINV. EX.35B751 282 74336256330 951196726CRINV. EX.36B808 294143336256328 956178828CRINV. EX.37C799 211143336256 20 899152622CRCOMP. EX.38D760 208103294214544 875219917CRCOMP. EX.39D719 490 63295215281 818195563CRINV. EX.Underlines indicate being outside the range of the present invention.(*)CR: cold rolled steel sheet (no coating), GI: hot-dip galvanized steel sheet (no alloying of zinc coating), GA: galvannealed steel sheet, EG: electrogalvanized steel sheetTABLE 4Average cooling Average rate in cooling temperaturerate in range ofQuenchCoolingAnnealing Holding temperature(Ms + 50° C.)-start.rate TemperingHolding temp.timerange ofquenchtempfrom T2 temp.timeT1t1750-600° C.start temp. T2Ms(Ms-80)T2to 80° C.T3t3Nos.Steels(° C.)(s)(° C. / s)(° C. / s)(° C.)(° C.)(° C.)(° C. / s)(° C.)(s)Type*40D935452143294214276870159 565CRINV. EX.41D798 77103294214287938193 689CRINV. EX.42D756903153294214281827171 585CRINV. EX.43D805421193295215278837216 737CRINV. EX.44D794453114294214277876159 969CRINV. EX.45D786473 63294214279962151 846CRINV. EX.46D770485144295215285878215 607CRINV. EX.47D805453 92294214216877191 949CRINV. EX.48D793380123294214287972187 527CRINV. EX.49D751328114295215282324177 723CRINV. EX.50D796292 64294214282822215 652CRINV. EX.51D766311102295215288857114 508CRINV. EX.52D808328123295215281966391 882CRINV. EX.53D790421132295215286890216 12CRINV. EX.54D7642951132952152789801989910CRINV. EX.55D795344133294214283802177 855CRINV. EX.56D804332133295215284846160 813CRINV. EX.57D778361113295215276836208 558CFINV. EX.58D766334 84295215 21869169 742CRCOMP. EX.59D794330 72291214347962188 501GACOMP. EX.60D786472134295215289971168 859GAINV. EX.61D756347 74295215277857174 747GAINV. EX.62D778220102295215280845178 636EGINV. EX.63D799304 73295215287888199 977GAINV. EX.64D801233152294214288860177 518CRINV. EX.65E776328 93298218280905167 662CRINV. EX.66F768335102411331404986167 603GAINV. EX.67G766285 93420340413991204 546GACOMP. EX.68H775383123213133199818189 812GIINV. EX.69I806292103183103172915190 559GACOMP. EX.70J768465113334254320970164 534GAINV. EX.71K794423 83329249317952212 852GACOMP. EX.72L787397104342262331994174 783GAINV. EX.73M773409 84336256325943163 646GICOMP. EX.74N785205103393313383974176 537GAINV. EX.75O783203142408328390896209 789GACOMP. EX.76P779359133272192261813164 618GAINV. EX.77Q759297 63260180253957183 728GACOMP. EX.78R766394 83329249322879158 860GAINV. EX.Underlines indicate being outside the range of the present invention.(*)CR: cold rolled steel sheet (no coating), GI: hot-dip galvanized steel sheet (no alloying of zinc coating), GA: galvannealed steel sheet, EG: electrogalvanized steel sheetTABLE 5Average Average coolingcoolingrate in rate intemperaturetemperature.range of (Ms + CoolingAnnealingrange50° C.)-Quench ratetemp.Holding of 750-quench startstartfrom T2TemperingHolding T1time t1600° C.temp.Ms(Ms-80)temp. T2to 80° C.temp. T3time t3Nos.Steels(° C.)(s)(° C. / s)T2 (° C. / s)(° C.)(° C.)(° C.)(° C. / s)(° C.)(s)Type*79S784454133341261327930189933GICOMP. EX.80T801331122341261325901215887GAINV. EX.81U768416 63347267334919185770GACOMP. EX.82V771480113336256327824187944GAINV. EX.83W758214123352272345984164764GACOMP. EX.84X801499133352272335934171697CRINV. EX.85Y790401 63341261326971194829CRCOMP. EX.86Z791235 53339259333890203868GAINV. EX.87AA775413103329249312865174528GACOMP. EX.88AB787208 63350270333992193882GAINV. EX.89AC798455143345265340942170856GAINV. EX.90AD777477 74336256330977150508GACOMP. EX.91AE802364113348268335908156648GAINV. EX.92AF7992031223382583311000211653GAINV. EX.93AG752276 52326246313979168583CRCOMP. EX.94AH804473143331251320980185523CRINV. EX.95AI786370132345265336848182795CRINV. EX.96AJ803309103340260329977220587CRCOMP. EX.97AK807472142340260323900152738CRINV. EX.98AL798340133340260329834152641CRINV. EX.99AM755292103325245313843156644CRCOMP. EX.100AN705370 53351271340924194645CRINV. EX.101AO927268 94339259320858181678CRINV. EX.102AP773 51112346266338878214580CRINV. EX.103AQ767864 723182382981000211849CRINV. EX.104AR788208183326246313942168501CRINV. EX.105AS804235114340260333948170967CRINV. EX.106AT801287143321241314888164610CRINV. EX.107AU752236154349269332963171868CRINV. EX.108AV771260 72333253332807187716CRINV. EX.109AW785472133332252261921206904CRINV. EX.110AX807354143345265335325151504CRINV. EX.111AY758450133333253326833182679CRINV. EX.112AZ803289113338258321901108597CRINV. EX.113BA790344152283203274953394563CRINV. EX.114BB78237913329521527797715523CRINV. EX.115BC797237 942832032739182009851CRINV. EX.116BD799406114284204265940196667CRINV. EX.117BE799205 84306226292914168663CRINV. EX.118BF769466 82290210277844213732CRINV. EX.119BG768328122287207273812153757CRINV. EX.120BH769333 73292212287811150829CRINV. EX.121BI794352132293213281949207878CRINV. EX.122BJ8803101933312514201000180800CRCOMP. EX.123BK758416 83349269330994166744CRINV. EX.124BL800400144293213287843181895CRINV. EX.125BM800493 62284204267882207965CRINV. EX.126BN797330 63392312381931209677CRINV. EX.127BO810366 82403323392842181528CRINV. EX.Underlines indicate being outside the range of the present invention.(*)CR: cold rolled steel sheet (no coating), GI: hot-dip galvanized steel sheet (no alloying of zinc coating), GA: galvannealed steel sheet, EG: electrogalvanized steel sheetThe high strength cold rolled steel sheets obtained as described above were used as test steels. Tensile characteristics, flatness in the width direction, and working embrittlement resistance were evaluated in accordance with the following test methods.(Microstructure Observation)The amount of tempered martensite, the amount of retained austenite, the total amount of ferrite and bainitic ferrite, and the average grain size of prior austenite were determined by the methods described hereinabove.(Proportion of Packets Having the Largest Area in Prior Austenite Grains)The average proportion of packets having the largest area in prior austenite grains was determined by the method described hereinabove.(Tensile Test)A JIS No. 5 test specimen (gauge length: 50 mm, parallel section width: 25 mm) was sampled so that the longitudinal direction of the test specimen would be perpendicular to the rolling direction. A tensile test was performed in accordance with JIS Z 2241 under conditions where the crosshead speed was 1.67×10−1 mm / sec. TS and El were thus measured. In accordance with aspects of the present invention, 980 MPa or higher TS was determined to be acceptable.(Press Formability)

[0079] A hole expansion test was performed in accordance with JIS Z 2256 (2010). The steel sheets obtained were each cut to 100 mm×100 mm. A 10 mm diameter hole was punched with a clearance of 12%±1%. While holding the steel sheet on a die having an inner diameter of 75 mm with a blank holder force of 9 tons (88.26 kN), a conical punch with an apex angle of 60° was pushed into the hole to measure the critical hole diameter at the occurrence of cracking. The limiting hole expansion ratio λ (%) was determined from the formula below, and the flangeability was evaluated based on the value of limiting hole expansion ratio.Limiting⁢ hole⁢ expansion⁢ ratio: λ⁡(%)={(Df-D⁢0) / D⁢0}×100wherein Df is the hole diameter (mm) at the occurrence of cracking and DO is the initial hole diameter (mm).Based on the tensile strength (TS), the total elongation (El), and the hole expansion ratio (A) obtained as described above, TS×El×λ0.5 / 1000 was calculated. The steel sheet was evaluated as “excellent in press formability” when the calculated value was 80 or more.(Flatness in the Width Direction)

[0081] The cold rolled steel sheets obtained as described above were analyzed to measure the flatness in the width direction. The measurement is illustrated in FIG. 2. Specifically, a sheet with a length of 500 mm in the rolling direction (coil width x 500 mm L x sheet thickness) was cut out from the coil and was placed on a surface plate in such a manner that the warp at the ends would face upward. The height on the steel sheet was measured with a contact displacement meter by continuously moving the stylus over the width. Based on the results, the steepness 0 as an index of the flatness of the steel sheet shape was measured as illustrated in FIG. 2. The flatness was rated as “x” when the steepness was more than 0.02, as “o” when the steepness was more than 0.01 and 0.02 or less, and as “⊚” when the steepness was 0.01 or less. The steel sheet was evaluated as “excellent in the flatness in the width direction” when the steepness was 0.02 or less.(Working Embrittlement Resistance)

[0082] The working embrittlement resistance was evaluated by Charpy test. A Charpy test specimen was a 2 mm deep V-notched test piece that was a stack of steel sheets fastened together with bolts to eliminate any gaps between the steel sheets. The number of steel sheets that were stacked was controlled so that the thickness of the stack as the test piece would be closer to 10 mm. When, for example, the sheet thickness was 1.2 mm, eight sheets were stacked to give a 9.6 mm thick test piece. The sheets for stacking into the Charpy test specimen were sampled so that the width direction would be the longitudinal direction. As an index of the working embrittlement resistance, the ratio vE0% / vE10% of the absorbed impact energy at room temperature of the as-produced (unworked) steel sheet to that of the steel sheet after 10% rolling was measured. The working embrittlement resistance was rated as “x” when vE0% / vE10%: was less than 0.6, as “∘” when vE0% / vE10% was 0.6 or more and less than 0.7, and as “⊚” when vE0% / vE10% was 0.7 or more. The Charpy test specimen was evaluated as “excellent in working embrittlement resistance” when vE0% / vE10% was 0.6 or more. Conditions other than those described above conformed to JIS Z 2242: 2018.

[0083] The results are described in Tables 6 to 8. As shown in the tables, INVENTIVE EXAMPLES achieved 980 MPa or higher TS, excellent press formability, excellent flatness in the width direction, and excellent working embrittlement resistance. In contrast, COMPARATIVE EXAMPLES were unsatisfactory in one or more of TS, press formability, flatness in the width direction, and working embrittlement resistance.TABLE 6Proportion Total ofof largestferritepackets in andpriorPrior γTS ×FlatnessWorkingTempered Retained BainiticbainiticaustenitegrainEl ×inembrittle-martensiteausteniteFerriteferriteferritegrainssizeTSElλλ0.5 / width ment Nos.Steels(area %)(vol %)(area %)(area %)(area %)(area %)(μm)(MPa)(%)(%)1000directionresistance 1A551361046501516021059123⊚⊚INV. EX. 2B59133104353121791859110⊚⊚INV. EX. 3B41049756471010061542 98⊚⊚INV. EX. 4B340606665012 8271856111⊚⊚COMP. EX. 5B5714645060201758842 91⊚◯INV. EX. 6B52038644562414801058113⊚XCOMP. EX. 7B43050757501210121550107⊚⊚INV. EX. 8B350588665814 8961760118⊚⊚COMP. EX. 9B51139645551814251157118⊚◯INV. EX.10B54041849552115401044102⊚XCOMP. EX.11B8501131450 81947740 86⊚⊚INV. EX.12B941 16 752102031550 72⊚⊚COMP. EX.13B51138947551014711057111⊚⊚INV. EX.14B5714274949101687959117⊚⊚INV. EX.15B5904285049141752851100⊚⊚INV. EX.16B54136945551315451047106⊚⊚INV. EX.17B8411372050141973651 85⊚⊚INV. EX.18B920 22 455 92294457 69⊚⊚INV. EX.19B49240747571413901133 88⊚⊚INV. EX.20B526321042471015221025 76⊚⊚COMP. EX.21B5313984795101557957106XXCOMP. EX.22B5703394278111732949109XXCOMP. EX.23B5624264856121621934 85⊚⊚INV. EX.24B52640646591214581024 71⊚⊚COMP. EX.25B4573754246 811281421 72⊚⊚COMP. EX.26B52037744501215091056113⊚⊚INV. EX.27B5514354850111728945104⊚◯INV. EX.28B5413474148111684954111⊚◯INV. EX.29B53032941531412211243 96⊚⊚INV. EX.30B59035742521414771042 96⊚⊚INV. EX.31B5203794649 915181046103⊚◯INV. EX.32B52141748471015001049105⊚◯INV. EX.33B50137744461313541143 98⊚⊚INV. EX.34B52137845581414651054108⊚⊚INV. EX.35B5703694556101690954112⊚⊚INV. EX.36B51139443531314471156119⊚⊚INV. EX.37C52537946501314801023 71⊚⊚COMP. EX.38D5413664293 913841148105XXCOMP. EX.39D41054559581410351556116⊚⊚INV. EX.40D54044448481814741042 96⊚◯INV. EX.41D4205445856109831542 96⊚⊚INV. EX.42D5803744157161636958112⊚◯INV. EX.43D8401121350142138651 92⊚⊚INV. EX.44D56038104856121564950100⊚⊚INV. EX.45D54041344561314861052107⊚⊚INV. EX.46D8811431759122120649 89⊚⊚INV. EX.47D49242446591412011234 84⊚⊚INV. EX.Underlines indicate being outside the range of the present invention.TABLE 7TotalPropor- oftionferriteof largest andpackets bainiticin priorPrior γTS ×Flatness WorkingTemperedRetainedFerriteBainiticferriteaustenitegrainEl ×inembrittle-martensiteaustenite(areaferrite(areagrainssizeTSElλλ0.5 / width ment Nos.Steels(area %)(vol %)%)(area %)%)(area %)(μm)(MPa)(%)(%)1000directionresistance48D52142446561413421148102⊚⊚INV. EX.49D5724084849141582933 82⊚⊚INV. EX.50D55037845471114351048 99⊚⊚INV. EX.51D5514334646915861055118⊚◯INV. EX.52D50037542521310461448101⊚⊚INV. EX.53D5303584352813431153108⊚◯INV. EX.54D55139443551510601352 99⊚⊚INV. EX.55D581379464791627942 95⊚⊚INV. EX.56D52038442491413821155113⊚⊚INV. EX.57D5213954459913101152104⊚⊚INV. EX.58D4553674357810541426 75⊚⊚COMP. EX.59D4903964588912051250102XXCOMP. EX.60D520331043551413701152109⊚⊚INV. EX.61D57132941541315861060123⊚⊚INV. EX.62D51136743501513101242102⊚⊚INV. EX.63D51139645501312791245103⊚⊚INV. EX.64D52043346561513571154110⊚⊚INV. EX.65E5913374059101712956115⊚⊚INV. EX.66F42050555591010361451104⊚⊚INV. EX.67G331595645111 8191857111⊚⊚COMP. EX.68H5314084854112022756106⊚◯INV. EX.69I5104634955122035750101⊚XCOMP. EX.70J56035742481010781152 86⊚⊚INV. EX.71K551374415412 8201052 59⊚⊚COMP. EX.72L5723810485791869833 86⊚⊚INV. EX.73M44639948521012861226 79⊚⊚COMP. EX.74N421501060551210961357108⊚⊚INV. EX.75O350614655110 6872249106⊚⊚COMP. EX.76P5903844259151984854117⊚◯INV. EX.77Q551396455691789954118⊚XCOMP. EX.78R5213664252915291041 98⊚◯INV. EX.79S53038644481415331048106⊚XCOMP. EX.80T5803874547111719947106⊚◯INV. EX.81U5703294157121679945101⊚XCOMP. EX.82V4014875554910661448103⊚⊚INV. EX.83W300646704515 7921951107⊚⊚COMP. EX.84X5404084851131586953104⊚◯INV. EX.85Y51038644451413611143 98⊚XCOMP. EX.86Z5614354853915761048109⊚◯INV. EX.87AA5713710474691713949108⊚XCOMP. EX.88AB53041445511414491041 93⊚⊚INV. EX.89AC50044347551414961053109⊚◯INV. EX.90AD58139948571415331054113⊚XCOMP. EX.91AE591385435581772945107⊚⊚INV. EX.92AF5414334655131816854107⊚◯INV. EX.93AG5403894754101850859114⊚XCOMP. EX.Underlines indicate being outside the range of the present invention.TABLE 8Proportion Total ofof largestferrite packets inandprior Prior γTS ×Flatness WorkingTemperedRetainedBainiticbainitic austenitegrainEl ×inembrittle-martensiteausteniteFerriteferriteferritegrainssizeTSElλλ0.5 / widthmentNos.Steels(area %)(vol %)(area %)(area %)(area %)(area %)(μm)(MPa)(%)(%)1000directionresistance94AH5713754251131656951106⊚⊚INV. EX.95AI53033104347141696949107⊚◯INV. EX.96AJ541394434791742956117⊚XCOMP. EX.97AK5814444848111739855103⊚⊚INV. EX.98AL5304285055141771857107⊚◯INV. EX.99AM5404084857101810860112⊚XCOMP. EX.100AN40148755509103814 47100⊚⊚INV. EX.101AO52141445551614661058112⊚◯INV. EX.102AP4314795659810391548108⊚⊚INV. EX.103AQ571347415418161394395⊚◯INV. EX.104AR841116174910208165088⊚⊚INV. EX.105AS52141950521514761056110⊚⊚INV. EX.106AT5813744156151798947111⊚⊚INV. EX.107AU841145194811207165693⊚⊚INV. EX.108AV5023784549101331114598⊚⊚INV. EX.109AW550406465181536103186⊚⊚INV. EX.110AX502409494791444103484⊚⊚INV. EX.111AY5513694553131564953102⊚⊚INV. EX.112AZ5404244647131682957114⊚◯INV. EX.113BA5014074751910411549109⊚⊚INV. EX.114BB561397464781603956108⊚◯INV. EX.115BC52132941591513781154111⊚⊚INV. EX.116BD581338415314163394295⊚⊚INV. EX.117BE5203794652913891152110⊚⊚INV. EX.118BF51138644471212571253110⊚⊚INV. EX.119BG5713964550141632948102⊚⊚INV. EX.120BH5914044454111774940101⊚⊚INV. EX.121BI5403644060814031157117⊚⊚INV. EX.122BJ8909110889152094592XXCOMP. EX.123BK5713754245131743949110⊚⊚INV. EX.124BL5214244654813391147101⊚⊚INV. EX.125BM5114344757812551256113⊚⊚INV. EX.126BN50136642581414061152112⊚⊚INV. EX.127BO5903494346101828851104⊚⊚INV. EX.Underlines indicate being outside the range of the present invention.

Claims

1. A high strength steel sheet having a chemical composition comprising, in mass %,C: 0.030% or more and 0.500% or less,Si: 0.01% or more and 2.50% or less,Mn: 0.10% or more and 5.00% or less,P: 0.100% or less,S: 0.0200% or less,Al: 1.000% or less,N: 0.0100% or less, andO: 0.0100% or less,a balance being Fe and incidental impurities,the high strength steel sheet being such that in a region at ¼ sheet thickness,an area fraction of tempered martensite is 38% or more and less than 90%,a volume fraction of retained austenite is less than 3%,an area fraction of the total of ferrite and bainitic ferrite is 10% or more and 60% or less,an average grain size of prior austenite is 20 μm or less, andan average of proportions of packets having the largest area in prior austenite grains is 70% by area or less of the prior austenite grain.

2. The high strength steel sheet according to claim 1, wherein the chemical composition further comprises at least one element selected from, in mass %,Ti: 0.200% or less, Nb: 0.200% or less,V: 0.200% or less, Ta: 0.10% or less,W: 0.10% or less, B: 0.0100% or less,Cr: 1.00% or less, Mo: 1.00% or less,Co: 0.010% or less, Ni: 1.00% or less,Cu: 1.00% or less, Sn: 0.200% or less,Sb: 0.200% or less, Ca: 0.0100% or less,Mg: 0.0100% or less, REM: 0.0100% or less,Zr: 0.100% or less, Te: 0.100% or less,Hf: 0.10% or less, and Bi: 0.200% or less.

3. The high strength steel sheet according to claim 1, which has a coated layer on a surface of the steel sheet.

4. The high strength steel sheet according to claim 2, which has a coated layer on a surface of the steel sheet.

5. A method for manufacturing the high strength steel sheet according to claim 1, the method comprising:providing a cold rolled steel sheet produced by subjecting a steel having the chemical composition to hot rolling, pickling, and cold rolling;heating the steel sheet at an annealing temperature T1 of 700° C. or above and 950° C. or below for a holding time t1 at the annealing temperature T1 of 10 seconds or more and 1000 seconds or less;cooling the steel sheet in such a manner that:the average cooling rate from 750° C. to 600° C. is less than 20° C. / s,an average cooling rate from (Ms+50° C.) to a quench start temperature T2 is less than 5° C. / s wherein the quench start temperature T2 is (Ms−80° C.) or above and is below Ms where Ms is martensite start temperature (° C.) defined by formula (1), andan average cooling rate from the quench start temperature T2 to 80° C. is 300° C. / s or more; andheating the steel sheet at a tempering temperature T3 of 100° C. or above and 400° C. or below for a holding time t3 at the tempering temperature T3 of 10 seconds or more and 10000 seconds or less,Ms=519-474×[%⁢ C]-30.4×[%⁢ Mn]-12.1×[%⁢ Cr]-7.5×[%⁢ Mo]-17.7×[%⁢ Ni]-T⁢1 / 80(1)wherein [% C], [% Mn], [% Cr], [% Mo], and [% Ni] indicate contents (mass %) of C, Mn, Cr, Mo, and Ni, respectively, and are zero when the element is absent.

6. A method for manufacturing the high strength steel sheet according to claim 2, the method comprising:providing a cold rolled steel sheet produced by subjecting a steel having the chemical composition to hot rolling, pickling, and cold rolling;heating the steel sheet at an annealing temperature T1 of 700° C. or above and 950° C. or below for a holding time t1 at the annealing temperature T1 of 10 seconds or more and 1000 seconds or less;cooling the steel sheet in such a manner that:the average cooling rate from 750° C. to 600° C. is less than 20° C. / s,an average cooling rate from (Ms+50° C.) to a quench start temperature T2 is less than 5° C. / s wherein the quench start temperature T2 is (Ms−80° C.) or above and is below Ms where Ms is martensite start temperature (° C.) defined by formula (1), andan average cooling rate from the quench start temperature T2 to 80° C. is 300° C. / s or more; andheating the steel sheet at a tempering temperature T3 of 100° C. or above and 400° C. or below for a holding time t3 at the tempering temperature T3 of 10 seconds or more and 10000 seconds or less,Ms=519-474×[%⁢ C]-30.4×[%⁢ Mn]-12.1×[%⁢ Cr]-7.5×[%⁢ Mo]-17.7×[%⁢ Ni]-T⁢1 / 80(1)wherein [% C], [% Mn], [% Cr], [% Mo], and [% Ni] indicate contents (mass %) of C, Mn, Cr, Mo, and Ni, respectively, and are zero when the element is absent.

7. The method for manufacturing the high strength steel sheet according to claim 5, further comprising performing a coating treatment.

8. The method for manufacturing the high strength steel sheet according to claim 6, further comprising performing a coating treatment.

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