Abstract: A method for the manufacturing of high strength cold rolled steel sheets includes continuously annealing a cold rolled steel sheet that has a composition containing C: 0.05-0.3% mass, Si: 0.6-3.0% mass, Mn: 1.0-3.0% mass, P: ?0.1% mass, S: ?0.02% mass, Al: 0.01-1% mass, N: ?0.01% mass, and Fe and inevitable impurities: balance, in a manner such that the cold rolled steel sheet is heated in a furnace using an oxidizing burner to a steel sheet temperature of ?700° C., then soak-annealed in a reducing atmosphere furnace at 750-900° C., then cooled so the average cooling rate between 500° C. and 100° C. is ?50° C./s. High-Si cold rolled steel sheets with high strength and good phosphatability while containing Si?0.6% are obtained without controlling conditions so as to increase the dew point in the reducing atmosphere in the soaking furnace or to increase the vapor hydrogen partial pressure ratio.

Abstract: A method for the manufacturing of high strength cold rolled steel sheets includes continuously annealing a cold rolled steel sheet that has a composition containing C: 0.05-0.3% mass, Si: 0.6-3.0% mass, Mn: 1.0-3.0% mass, P: 0.1% mass, S: ?0.02% mass, Al: 0.01-1% mass, N: ?0.01% mass, and Fe and inevitable impurities: balance, in a manner such that the cold rolled steel sheet is heated in a furnace using an oxidizing burner to a steel sheet temperature of ?700° C., then soak-annealed in a reducing atmosphere furnace at 750-900° C., then cooled so the average cooling rate between 500° C. and 100° C. is ?50° C./s. High-Si cold rolled steel sheets with high strength and good phosphatability while containing Si?0.6% are obtained without controlling conditions so as to increase the dew point in the reducing atmosphere in the soaking furnace or to increase the vapor hydrogen partial pressure ratio.

Abstract: The present invention provides a high strength cold-rolled steel sheet, consisting essentially of, by mass %, 0.04 to 0.10% C, 0.5 to 1.5% Si, 1.8 to 3% Mn, 0.02% or less P, 0.01% or less S, 0.01 to 0.1% Sol. Al, 0.005% or less N, and the balance being iron and inevitable impurities, and having a structure which substantially comprises ferrite phase and martensite phase. Since the high strength cold-rolled steel sheet of the invention has ductility in which an elongation is 18% or more, excellent stretch-flangeability in which a hole-expanding ratio is 60% or more and a tensile strength of 780 MPa or more, it is favorable for use in a structural member of automobile.

Abstract: The present invention relates to a steel sheet that consists essentially of 0.10 to 0.37% C, 1% or less Si, 2.5% or less Mn, 0.1% or less P, 0.03% or less S, 0.01 to 0.1% sol.Al, 0.0005 to 0.0050% N, 0.0003 to 0.0050% B, by mass, and balance of Fe, [14B/(10.8N)] being 0.5 or more, average particle size of precipitate BN being 0.1 &mgr;m or more, and grain size of prior austenite after the hardening treatment being 2 to 25 &mgr;m. The steel sheet according to the present invention allows an inexpensive hardening treatment method to be applied to improve the strength to a level required for the structural components and the functional components of automobiles, while providing excellent toughness after the hardening treatment.

Abstract: An ultra-high strength cold rolled steel sheet having a tensile strength of 880 to 1170 MPa, that consists essentially of 0.01 to 0.07% C, 0.3% or less Si, 0.1% or less P, 0.01% or less S, 0.01 to 0.1% sol.Al, 0.0050% or less N, 1.6 to 2.5% of the sum of at least one element selected from the group consisting of Mn, Cr, and Mo, by mass, and a balance of Fe, and that has an inner zone deeper than 10 &mgr;m from the surface of the steel sheet being substantially a martensitic single phase structure. Since the steel sheet has a hole expansion ratio of 75% or more, specified by the Standard of Japan Iron and Steel Federation, JFST1001-1996, the steel sheet has a tensile strength in a range of from 880 to 1170 MPa, and an excellent mechanically joining property, and is suitable for fabricating automobile seat frames.

Abstract: The present invention relates to a steel sheet that consists essentially of 0.10 to 0.37% C, 1% or less Si, 2.5% or less Mn, 0.1% or less P, 0.03% or less S, 0.01 to 0.1% sol.Al, 0.0005 to 0.0050% N, 0.0003 to 0.0050% B, by mass, and balance of Fe, [14B/(10.8N)] being 0.5 or more, average particle size of precipitate BN being 0.1 &mgr;m or more, and grain size of prior austenite after the hardening treatment being 2 to 25 &mgr;m. The steel sheet according to the present invention allows an inexpensive hardening treatment method to be applied to improve the strength to a level required for the structural components and the functional components of automobiles, while providing excellent toughness after the hardening treatment.

Abstract: The present invention relates to an ultra-high strength cold rolled steel sheet having 880 to 1170 MPa of tensile strength, that consists essentially of 0.01 to 0.07% C, 0.3% or less Si, 0.1% or less P, 0.01% or less S, 0.01 to 0.1% sol.Al, 0.0050% or less N. 1.6 to 2.5% of sum of at least one element selected from the group consisting of Mn, Cr, and Mo, by mass, and balance of Fe, and that has an inner zone deeper than 10 &mgr;m from the surface of the steel sheet being substantially martensitic single phase structure. Since the steel sheet has hole expansion ratio of 75% or more, specified by the Standard of Japan Iron and Steel Federation, JFST1001-1996, tensile strength in a range of from 880 to 1170 MPa, and excellent mechanically joining property, it is suitable for automobile seat frames.