Abstract: A high-carbon steel sheet has a chemical composition represented by, in mass %, C: 0.60% to 0.90%, Mn: 0.30% to 1.50%, and Cr: 0.20% to 1.00%, and others, and has a structure represented by a concentration of Mn contained in cementite: 2% or more and 8% or less, a concentration of Cr contained in cementite: 2% or more and 8% or less, an average grain diameter of ferrite: 10 ?m or more and 50 ?m or less, an average particle diameter of cementite: 0.3 ?m or more and 1.5 ?m or less, and a spheroidized ratio of cementite: 85% or more.

Abstract: A hot stamping steel material, which secures good hydrogen embrittlement resistance even when the steel sheet after hot stamping is subjected to processing leading to remaining of stress, such as piercing and which is easily practicable, wherein the steel sheet has the chemical composition of: C: 0.18 to 0.26%; Si: more than 0.02% and not more than 0.05%; Mn: 1.0 to 1.5%; P: 0.03% or less; S: 0.02% or less; Al: 0.001 to 0.5%; N: 0.1% or less; O: 0.001 to 0.02%; Cr: 0 to 2.0%; Mo: 0 to 1.0%; V: 0 to 0.5%; W: 0 to 0.5%; Ni: 0 to 5.0%; B: 0 to 0.01%; Ti: 0 to 0.5%; Nb: 0 to 0.5%; Cu: 0 to 1.0%; and balance: Fe and impurities, in terms of % by mass, the concentration of a Mn-containing inclusion is not less than 0.010% by mass and less than 0.25% by mass, and the number ratio of a Mn oxide to the inclusion having a maximum length of 1.0 to 4.0 ?m is 10.0% or more.

Abstract: A hot stamping steel material, which secures good hydrogen embrittlement resistance even when the steel sheet after hot stamping is subjected to processing leading to remaining of stress, such as piercing and which is easily practicable, wherein the steel sheet has the chemical composition of: C: 0.18 to 0.26%; Si: more than 0.02% and not more than 0.05%; Mn: 1.0 to 1.5%; P: 0.03% or less; S: 0.02% or less; Al: 0.001 to 0.5%; N: 0.1% or less;O: 0.001 to 0.02%; Cr: 0 to 2.0%; Mo: 0 to 1.0%; V: 0 to 0.5%; W: 0 to 0.5%; Ni: 0 to 5.0%; B: 0 to 0.01%; Ti: 0 to 0.5%; Nb: 0 to 0.5%; Cu: 0 to 1.0%; and balance: Fe and impurities, in terms of % by mass, the concentration of a Mn-containing inclusion is not less than 0.010% by mass and less than 0.25% by mass, and the number ratio of a Mn oxide to the inclusion having a maximum length of 1.0 to 4.0 pm is 10.0% or more.

Abstract: A cold-rolled steel sheet has a predetermined chemical composition, in which a structure before and after a hot-stamping includes ferrite: 30 area % to 90 area %, martensite: 0 area % or more and less than 20 area %, pearlite: 0 area % to 10 area %, retained austenite: 5 volume % to 20 volume %, and rest structure: bainite, a hardness of the retained austenite measured with a nano indenter before and after the hot-stamping satisfies relations of H2/H1<1.1 and ?HM<20, and a relation of TS×El?20000 MPa·% is satisfied.

Abstract: Low carbon steel plate excellent impact resistance characteristics after carburizing and quenching and after tempering, characterized by having a predetermined chemical composition, an average grain size of carbides of 0.4 ?m to 2.0 ?m, an area ratio of pearlite of 6% or less, a ratio of a number of carbides at the ferrite grain boundaries to the number of carbides inside the ferrite grains of over 1, and a Vickers hardness of 100HV to 180HV.

Abstract: The present invention provides a steel sheet with chemical components including, by mass %, 0.18-0.35% of C, 1.0%-3.0% of Mn, 0.01%-1.0% of Si, 0.001%-0.02% of P, 0.0005%-0.01% of S, 0.001%-0.01% of N, 0.01%-1.0% of Al, 0.005%-0.2% of Ti, 0.0002%-0.005% of B, and 0.002%-2.0% of Cr, and the balance of Fe and inevitable impurities, wherein: by volume %, a fraction of the ferrite is 50% or more, and a fraction of a non-recrystallized ferrite is 30% or less; and Cr?/CrM is 2 or less, where Cr? is a concentration of Cr subjected to solid solution in iron carbide and CrM is a concentration of Cr subjected to solid solution in a base material, or Mn0/MnM is 10 or less, where Mn0 is a concentration of Mn subjected to solid solution in an iron carbide, and MnM is a concentration of Mn subjected to solid solution in a base material.

Abstract: A cold-rolled steel sheet for vitreous enameling has a predetermined chemical composition, in which a number density of Fe—Mn—Nb-based composite oxides having a diameter of 0.2 ?m to 10 ?m is 2×102 particle/mm2 to 1×104 particle/mm2; a fatigue limit ratio is higher than 0.42 after performing a heat treatment with an applied tensile strain of 10% at a heating temperature of 830° C. for a holding time of 5 minutes; voids are formed between the metallographic structure and the Fe—Mn—Nb-based composite oxides, and an equivalent circle diameter of the voids is 0.1 ?m to 0.6 ?m; and when each of the voids is approximated as a triangle and a long side of the triangle is set as a base, a value obtained by dividing a length of the base by a height of the triangle is 1.0 to 15.

Abstract: A hot stamped steel according to the present invention satisfies an expression of (5×[Si]+[Mn])/[C]>11 when [C] represents an amount of C by mass %, [Si] represents an amount of Si by mass %, and [Mn] represents an amount of Mn by mass %, a metallographic structure after hot stamping includes 40% to 90% of a ferrite and 10% to 60% of a martensite in an area fraction, a total of an area fraction of the ferrite and an area fraction of the martensite is 60% or more, a hardness of the martensite measured with a nanoindenter satisfies an H2/H1<1.10 and ?HM<20, and TS×?, which is a product of a tensile strength TS and a hole expansion ratio ? is 50000 MPa·% or more.

Abstract: A cold rolled steel sheet according to the present invention satisfies an expression of (5×[Si]+[Mn])/[C]>11 when [C] represents an amount of C by mass %, [Si] represents an amount of Si by mass %, and [Mn] represents an amount of Mn by mass %, a metallographic structure before hot stamping includes 40% to 90% of a ferrite and 10% to 60% of a martensite in an area fraction, a total of an area fraction of the ferrite and an area fraction of the martensite is 60% or more, a hardness of the martensite measured with a nanoindenter satisfies an H2/H1<1.10 and ?HM<20 before the hot stamping, and TS×? which is a product of a tensile strength TS and a hole expansion ratio ? is 50000 MPa·% or more.

Abstract: The present invention provides a method for manufacturing a hot stamped body having a vertical wall, the method including: a hot-rolling step; a coiling step; a cold-rolling step; a continuous annealing step; and a hot stamping step, in which the continuous annealing step includes a heating step of heating the cold-rolled steel sheet to a temperature range of equal to or higher than Ac1° C. and lower than Ac3° C.; a cooling step of cooling the heated cold-rolled steel sheet from the highest heating temperature to 660° C. at a cooling rate of equal to or less than 10° C./s; and a holding step of holding the cooled cold-rolled steel sheet in a temperature range of 550° C. to 660° C. for one minute to 10 minutes.

Abstract: Provided is a method for manufacturing a hot stamped body, the method including: a hot-rolling step; a coiling step; a cold-rolling step; a continuous annealing step; and a hot stamping step, in which the continuous annealing step includes a heating step of heating the cold-rolled steel sheet to a temperature range of equal to or higher than Ac1° C. and lower than Ac3° C.; a cooling step of cooling the heated cold-rolled steel sheet from the highest heating temperature to 660° C. at a cooling rate of equal to or less than 10° C./s; and a holding step of holding the cooled cold-rolled steel sheet in a temperature range of 550° C. to 660° C. for one minute to 10 minutes.

Abstract: A high-carbon steel sheet includes: a chemical composition represented by, in mass %, C: 0.30% to 0.70%, B: 0.0004% to 0.0035%, and others; and a structure represented by a spheroidized ratio of cementite: 80% or more; and an average diameter of cementite: 0.3 ?m to 2.2 ?m, wherein a coefficient of micro-friction of ferrite on a surface of the steel sheet is less than 0.5.

Abstract: In a hot stamped steel, when [C] represents an amount of C (mass %), [Si] represents an amount of Si (mass %), and [Mn] represents an amount of Mn (mass %), an expression of 5×[Si]+[Mn])/[C]>10 is satisfied, a metallographic structure includes 80% or more of a martensite in an area fraction, and optionally, further includes one or more of 10% or less of a pearlite in an area fraction, 5% or less of a retained austenite in a volume ratio, 20% or less of a ferrite in an area fraction, and less than 20% of a bainite in an area fraction, TS×?, which is a product of TS that is a tensile strength and ? that is a hole expansion ratio is 50000 MPa·% or more, and a hardness of the martensite measured with a nanoindenter satisfies H2/H1<1.10 and ?HM<20.

Abstract: Provided is a method for manufacturing a hot stamped body, the method including: a hot-rolling step; a coiling step; a cold-rolling step; a continuous annealing step; and a hot stamping step, in which the continuous annealing step includes a heating step of heating the cold-rolled steel sheet to a temperature range of equal to or higher than Ac1° C. and lower than Ac3° C.; a cooling step of cooling the heated cold-rolled steel sheet from the highest heating temperature to 660° C. at a cooling rate of equal to or less than 10° C./s; and a holding step of holding the cooled cold-rolled steel sheet in a temperature range of 550° C. to 660° C. for one minute to 10 minutes.

Abstract: When the amount of C, the amount of Si and the amount of Mn are respectively represented by [C], [Si] and [Mn] in unit mass %, the cold rolled steel sheet satisfies a relationship of (5×[Si]+[Mn])/[C]>10, the metallographic structure contains, by area ratio, 40% to 90% of a ferrite and 10% to 60% of a martensite, further contains one or more of 10% or less of a pearlite by area ratio, 5% or less of a retained austenite by volume ratio and 20% or less of a bainite by area ratio, the hardness of the martensite measured using a nanoindenter satisfies H20/H10<1.10 and ?HM0<20, and TS×? representing the product of TS that is a tensile strength and ? that is a hole expansion ratio is 50000 MPa·% or more.

Abstract: The present invention provides a method for manufacturing a hot stamped body, the method including: a hot-rolling step; a coiling step; a cold-rolling step; a continuous annealing step; and a hot stamping step, in which the continuous annealing step includes a heating step of heating the cold-rolled steel sheet to a temperature range of equal to or higher than Ac1° C. and lower than Ac3° C.; a cooling step of cooling the heated cold-rolled steel sheet from the highest heating temperature to 660° C. at a cooling rate of equal to or less than 10° C./s; and a holding step of holding the cooled cold-rolled steel sheet in a temperature range of 550° C. to 660° C. for one minute to 10 minutes.

Abstract: A middle/high carbon steel sheet according to an aspect of the present invention is a steel sheet, in which composition thereof contains, by mass %, C: 0.10% to 1.50%, Si: 0.01% to 1.00%, Mn: 0.01% to 3.00%, P: 0.0001% to 0.1000%, and S: 0.0001% to 0.1000%, and a remainder consisting of Fe and impurities, in which the steel sheet has a structure in which a total volume percentage of a martensite, a bainite, a pearlite, and a residual austenite is equal to or lower than 5.0%, and a remainder thereof is a ferrite and carbides, in which a spheroidizing ratio of carbide particles is 70% to 99%, and in which a proportion of a number of the carbide particles including a crystal interface at which an orientation difference is equal to or greater than 5° in the carbide particles is equal to or lower than 20% of a total number of the carbide particles.

Abstract: The present invention provides a method for manufacturing a hot stamped body having a vertical wall, the method including: a hot-rolling step; a coiling step; a cold-rolling step; a continuous annealing step; and a hot stamping step, in which the continuous annealing step includes a heating step of heating the cold-rolled steel sheet to a temperature range of equal to or higher than Ac1° C. and lower than Ac3° C.; a cooling step of cooling the heated cold-rolled steel sheet from the highest heating temperature to 660° C. at a cooling rate of equal to or less than 10° C./s; and a holding step of holding the cooled cold-rolled steel sheet in a temperature range of 550° C. to 660° C. for one minute to 10 minutes.

Abstract: This high strength hot-rolled steel sheet includes: in terms of percent by mass, C: 0.05 to 0.12%; Si: 0.8 to 1.2%; Mn: 1.6 to 2.2%; Al: 0.30 to 0.6%; P: 0.05% or less; S: 0.005% or less; and N: 0.01% or less, with the remainder being Fe and unavoidable impurities, wherein a microstructure includes specific ranges (in area %) of ferrite phases as well as martensite phases, and a maximum concentration of Al detected by a glow discharge emission spectroscopic analysis is in a range of 0.75 mass % or less in a region from a surface of the steel sheet to a thickness of 500 nm after being acid-pickled.

Abstract: The present invention provides a method for manufacturing a hot stamped body having a vertical wall, the method including: a hot-rolling step; a coiling step; a cold-rolling step; a continuous annealing step; and a hot stamping step, in which the continuous annealing step includes a heating step of heating the cold-rolled steel sheet to a temperature range of equal to or higher than Ac1° C. and lower than Ac3° C.; a cooling step of cooling the heated cold-rolled steel sheet from the highest heating temperature to 660° C. at a cooling rate of equal to or less than 10° C./s; and a holding step of holding the cooled cold-rolled steel sheet in a temperature range of 550° C. to 660° C. for one minute to 10 minutes.