Abstract: In a hot-rolled steel sheet, a microstructure contains, in area ratio, at a location at ¼ a sheet thickness, a total amount of 90% or more of one or more of martensite and tempered martensite and a total amount of 10% or less of one or more of ferrite, bainite, and pearlite, at a location of 200 ?m from the surface, a total amount of 70% or more and less than 90% of one or more of martensite and tempered martensite and a total amount of more than 10% and 30% or less of one or more of ferrite, bainite, and pearlite.

Abstract: This hot-rolled steel sheet has a predetermined chemical composition, the number % of crystal grains of bainite that are in contact with both tempered martensite and residual austenite is 80% or more of all crystal grains of the bainite, a C concentration in the residual austenite is 0.80 mass % or more, an average crystal grain size of the residual austenite is 0.70 ?m or less, and a standard deviation of Vickers hardness is 25 HV0.01 or less.

Abstract: This hot-rolled steel sheet has a predetermined chemical composition, in a microstructure, in terms of area %, residual austenite is less than 3.0%, ferrite is 15.0% or more and less than 60.0%, and pearlite is less than 5.0%, an E value that indicates periodicity of the microstructure is 10.7 or more, and an I value that indicates uniformity of the microstructure is 1.020 or more, a standard deviation of a Mn concentration is 0.60 mass % or less, and a tensile strength is 980 MPa or more.

Abstract: This hot-rolled steel sheet has a predetermined chemical composition, in a microstructure, in terms of area %, residual austenite is less than 3.0%, ferrite is 15.0% or more and less than 60.0%, and pearlite is less than 5.0%, an E value that indicates periodicity of the microstructure is 10.7 or more, and an I value that indicates uniformity of the microstructure is less than 1.020, a standard deviation of a Mn concentration is 0.60 mass % or less, and a tensile strength is 980 MPa or more.

Abstract: This hot-rolled steel sheet has a predetermined chemical composition, in which a metallographic structure contains, by area %, less than 3.0% of residual austenite, 15.0% or more and less than 60.0% of ferrite, and less than 5.0% of pearlite, has a ratio L60/L7 of a length L60 of a grain boundary having a crystal misorientation of 60° to a length L7 of a grain boundary having a crystal misorientation of 7° about a <110> direction of 0.60 or more, has a standard deviation of a Mn concentration of 0.60 mass % or less, and has a tensile strength of 980 MPa or more.

Abstract: A hot-rolled steel sheet has, as a chemical composition, by mass %: C: 0.01% to 0.30%; Si: 0.01% to 3.00%; Mn: 0.20% to 3.00%; P: 0.030% or less; S: 0.030% or less; Al: 0.001% to 2.000%; N: 0.0100% or less; and Ni: 0.02% to 0.50%, in which among measurement points at which elemental analysis is performed at a measurement pitch of 1 ?m using an EPMA in a region of 250 ?m×250 ?m on a surface, the percentage of measurement points having a Ni content of 0.5 mass % or more is 10% to 70%.

Abstract: This hot-rolled steel sheet has a predetermined chemical composition, in which a microstructure contains, by area %, less than 3.0% of residual austenite, 15.0% or more and less than 60.0% of ferrite, and less than 5.0% of pearlite, has a ratio L60/L7 of a length L60 of a grain boundary having a crystal misorientation of 60° to a length L7 of a grain boundary having a crystal misorientation of 7° about a<110> direction of less than 0.60, has a standard deviation of a Mn concentration of 0.60 mass % or less, and has a tensile strength of 980 MPa or more.

Abstract: This hot rolled steel sheet is a hot rolled steel sheet including a base steel sheet and a scale formed on a surface of the base steel sheet, in which the base steel sheet has a predetermined chemical composition, the scale has a layer structure composed of wustite, magnetite, and hematite in order from the base steel sheet side or a layer structure composed of the wustite and the magnetite in order from the base steel sheet side, and, when a thickness of the scale is represented by s, a thickness of the hematite is represented by h, and a thickness of the magnetite is represented by m, the s, the h, and the m satisfy formula (1) and formula (2). (h+m)/s<0.

Abstract: This hot-rolled steel sheet has a predetermined chemical composition, in which a metal microstructure contains, by area %, 3.0% or more of residual austenite, has a ratio L52/L7 of a length L52 of a grain boundary having a crystal orientation difference of 52° to a length L7 of a grain boundary having a crystal orientation difference of 7° about a <110> direction of 0.10 or more and 0.18 or less, has a standard deviation of a Mn concentration of 0.60 mass % or less, and has a tensile strength of 980 MPa or more.

Abstract: A hot-rolled steel sheet includes, by mass %, C: 0.030% or more and less than 0.075%, Si+Al: 0.08% to 0.40%, Mn: 0.5% to 2.0%, and Ti: 0.020% to 0.150%, and includes a microstructure having a ferrite and a martensite. The hot-rolled steel sheet includes a microstructure having, by area %, 90% to 98% of the ferrite, 2% to 10% of the martensite, 0% to 3% of a bainite, and 0% to 3% of a pearlite. In the martensite, the number proportion of martensite grains having a hardness of 10.0 GPa or more is 10% or less, and a ratio N1/N2 of the number N1 of martensite grains having a hardness of 8.0 GPa or more and less than 10.0 GPa to the number N2 of martensite grains having a hardness of less than 8.0 GPa is 0.8 to 1.2.

Abstract: A hot-rolled steel sheet according to the present invention has a predetermined chemical composition, in which, when a height profile of a surface of the hot-rolled steel sheet is measured in each of five measurement ranges in a rolling direction and a direction perpendicular to the rolling direction, a distance in a height direction from an average height position which is an average of a height position of a point having a highest height position and a height position of a recessed part which is a point having a lowest height position to the recessed part is indicated as R1 (?m) in each of the height profiles, and an average of heights of two measurement points away from the recessed part in the rolling direction or the direction perpendicular to the rolling direction by 5 ?m is indicated as R2 (?m), an average value of radii of curvature r represented by Expression (1) is 10 ?m or more, and a tensile strength of the hot-rolled steel sheet is 500 MPa or more. r=(25+|R2?R1|2)/2|R2?R1| . . .

Abstract: There is provided a hot-rolled steel sheet in which a composition contains: in mass %, C: 0.01% to 0.2%; Si: 2.5% or less; Mn: 4.0% or less; 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; Ti: 0.01 to 0.30%; and the balance being composed of iron and impurities and a structure is composed of by volume fraction, 90% or more of tempered martensite with an average aspect ratio of 2 or less, or 90% or more in total of both tempered martensite and lower bainite.

Abstract: This hot-rolled steel sheet has a predetermined chemical composition. The metallographic structure at a sheet thickness ¼ depth from a surface and at a center position in a sheet width direction in a sheet width cross section parallel to a rolling direction contains, by area %, 77.0% to 97.0% of bainite and tempered martensite in total, 0% to 5.0% of ferrite, 0% to 5.0% of pearlite, 3.0% or more of residual austenite, and 0% to 10.0% of martensite. The average grain size of the metallographic structure excluding the residual austenite is 7.0 ?m or less. The C concentration in the residual austenite is 0.5 mass % or more. The number density of iron-based carbides having a diameter of 20 nm or more is 1.0×106 carbides/mm2 or more.

Abstract: The present invention provides a hot-rolled steel sheet capable of preventing softening of the strength of a sheet-thickness central portion of the steel sheet in thermal treatment, even in the case where an amount of working performed on the steel sheet is small and a work hardening rate is low. A hot-rolled steel sheet of the present invention consists of chemical components of, in mass %, C: 0.040 to 0.150%, Si: 0 to 0.500%, Mn: 0.10 to 1.50%, P: 0 to 0.050%, S: 0 to 0.020%, Al: 0.010 to 0.050%, N: 0.0010 to 0.0060%, Nb: 0.008 to 0.035%, Cu: 0 to 0.10%, Ni: 0 to 0.10%, Cr: 0 to 0.02%, Mo: 0 to 0.020%, V: 0 to 0.020%, Ca: 0 to 0.0100%, B: 0 to 0.0050%, and the balance: Fe and impurities. The hot-rolled steel sheet contains 0.005 to 0.030% dissolved Nb. An area fraction of ferrite structure is 85% or more, the balance is cementite and/or pearlite structure, and an average crystal grain size of ferrite is equal to or more than 5 ?m and equal to or less than 20 ?m.

Abstract: A hot-rolled steel sheet including predetermined components, in which the amounts of Cr and Al added satisfy Expression (1) below, a metallographic structure has, by % by volume, a ferrite fraction of more than 90% and 98% or less, a martensite fraction of 2% to less than 10%, and, a fraction of a residual structure made of one or more of pearlite, bainite, and residual austenite being less than 1%, the ferrite has an average circle-equivalent diameter of 4 ?m or more and a maximum circle-equivalent diameter of 30 ?m or less, and the martensite has an average circle-equivalent diameter of 10 ?m or less and a maximum circle-equivalent diameter of 20 ?m or less. [Cr]×5+[Al]?0.50??Expression (1) Here, in Expression (1), [Cr] represents an amount of Cr (mass %), and [Al] represents an amount of Al (mass %).

Abstract: A hot-rolled steel sheet includes, by mass %, C: 0.030% or more and less than 0.075%, Si+Al: 0.08% to 0.40%, Mn: 0.5% to 2.0%, and Ti: 0.020% to 0.150%, and includes a microstructure having a ferrite and a martensite. The hot-rolled steel sheet includes a microstructure having, by area %, 90% to 98% of the ferrite, 2% to 10% of the martensite. 0% to 3% of a bainite, and 0% to 3% of a pearlite. In the martensite, the number proportion of martensite grains having a hardness of 10.0 GPa or more is 10% or less, and a ratio N1/N2 of the number N1 of martensite grains having a hardness of 8.0 GPa or more and less than 10.0 GPa to the number N2 of martensite grains having a hardness of less than 8.0 GPa is 0.8 to 1.2.

Abstract: There is provided a hot-rolled steel sheet in which a composition contains: in mass %, C: 0.01% to 0.2%; Si: 2.5% or less; Mn: 4.0% or less; P: 0.10% or less; S: 0.03% or less; Al: 0.001% to 2.0%; N: 0.01% or less; 0: 0.01% or less; Ti: 0.01 to 0.30%; and the balance being composed of iron and impurities and a structure is composed of by volume fraction, 90% or more of tempered martensite with an average aspect ratio of 2 or less, or 90% or more in total of both tempered martensite and lower bainite.

Abstract: There is provided a hot-rolled steel sheet in which a composition contains: in mass %, C: 0.01% to 0.2%; Si: 2.5% or less; Mn: 4.0% or less; 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; Ti: 0.01 to 0.30%; and the balance being composed of iron and impurities and a structure is composed of by volume fraction, 90% or more of tempered martensite with an average aspect ratio of 2 or less, or 90% or more in total of both tempered martensite and lower bainite.

Abstract: A hot-rolled steel sheet has a composition containing: in mass %, C: 0.01 to 0.2%; Si: 2.5% or less; Mn: 4.0% or less; P: 0.10% or less; S: 0.03% or less; Al: 0.001 to 2.0%; N: 0.01% or less; and O: 0.01% or less, and one kind or a total of two kinds of Ti and Nb for 0.01 to 0.30%. An average effective crystal grain diameter at a sheet thickness ¼ part is 10 ?m or less, and an average effective crystal grain diameter at a part of a range of 50 ?m from a surface is 6 ?m or less. A structure of the steel sheet is tempered martensite or lower bainite, and a volume fraction thereof is 90% or more as a total.

Abstract: The present invention provides a hot-rolled steel sheet capable of preventing softening of the strength of a sheet-thickness central portion of the steel sheet in thermal treatment, even in the case where an amount of working performed on the steel sheet is small and a work hardening rate is low. A hot-rolled steel sheet of the present invention consists of chemical components of, in mass %, C: 0.040 to 0.150%, Si: 0 to 0.500%, Mn: 0.10 to 1.50%, P: 0 to 0.050%, S: 0 to 0.020%, Al: 0.010 to 0.050%, N: 0.0010 to 0.0060%, Nb: 0.008 to 0.035%, Cu: 0 to 0.10%, Ni: 0 to 0.10%, Cr: 0 to 0.02%, Mo: 0 to 0.020%, V: 0 to 0.020%, Ca: 0 to 0.0100%, B: 0 to 0.0050%, and the balance: Fe and impurities. The hot-rolled steel sheet contains 0.005 to 0.030% dissolved Nb. An area fraction of ferrite structure is 85% or more, the balance is cementite and/or pearlite structure, and an average crystal grain size of ferrite is equal to or more than 5 ?m and equal to or less than 20 ?m.