Abstract: Provided is a hot-rolled steel sheet that has a chemical composition including, by mass %: C: 0.060% to 0.150%; Si: 0.15% to 0.70%; Mn: 1.00% to 1.90%; P: 0.10% or less; S: 0.010% or less; Al: 0.01% to 0.10%; N: 0.010% or less; Nb: 0.010% to 0.100%; and the balance including Fe and incidental impurities. The hot-rolled steel sheet has a microstructure containing ferrite of 18 ?m or less in average grain size by a volume fraction of at least 75% and pearlite of at least 2 ?m in average grain size by a volume fraction of at least 5%, the balance being low-temperature-induced phases, the pearlite having a mean free path of at least 5.0 ?m.

Abstract: A high-strength hot-dip galvanized steel sheet has excellent workability, namely, excellent ductility and hole expansion formability, and high yield ratio. The steel sheet has a chemical composition containing by mass %: C: 0.05-0.15%; Si: 0.10-0.90%; Mn: 1.0-1.9%; P: 0.005-0.10%; S: 0.0050% or less; Al: 0.01-0.10%; N: 0.0050% or less; Nb: 0.010-0.100%; and the balance being Fe and incidental impurities, in which: the steel sheet has a complex phase that includes: ferrite having an average crystal grain size of 15 ?m or less to at least 90% in volume fraction; martensite having an average crystal grain size of 3.0 ?m or less to 0.5% or more and less than 5.0% in volume fraction; pearlite to 5.0% or less in volume fraction; and the balance being a phase generated at low temperature.

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: Provided are a high-strength cold-rolled steel sheet having excellent formability, excellent ductility, excellent hole expansibility, and high yield ratio and a method for producing the same. The high-strength cold-rolled steel sheet contains 0.05% to 0.15% C, 0.10% to 0.90% Si, 1.0% to 2.0% Mn, 0.005% to 0.05% P, 0.0050% or less S, 0.01% to 0.10% Al, 0.0050% or less N, and 0.010% to 0.100% Nb, which are chemical components, on a mass basis, the balance being Fe and unavoidable impurities; has a microstructure which is a multi-phase structure containing 90% or more of a ferrite phase and 0.5% to less than 5.0% of a martensite phase on a volume fraction basis, the remainder being low-temperature transformation phases; and has a yield ratio of 70% or more.

Abstract: A method for manufacturing an ultra high strength member includes heating a steel sheet at a first heating temperature within a temperature range of 700 to 1000° C.; forming the steel sheet into a shape of a member at the first heating temperature and simultaneously cooling the steel sheet; after completion of the cooling, shear punching the steel sheet into a desired shape to obtain an ultra high strength member; and after the shear punching, subjecting the ultra high strength member to a first heat treatment including heating the ultra high strength member at second heating temperature within a temperature range of 100° C. or higher, but lower than 300° C. and retaining the member at the second heating temperature for 1 second to 60 minutes, wherein the resulting ultra high strength member has a tensile strength of 1180 MPa or more.

Abstract: An ultra.-high-strength cold-rolled steel sheet with excellent ductility and delayed fracture resistance includes 0.15% to 0.75 C. 1.0% to 3.0% Si, 1.5% to 2.5% Mn, 0.05% or less P, 0.02% or less 5, 0.01% to 0.05% Al, and less than 0.005% N on a mass ratio, the remainder being Fe and =avoidable impurities, the ultra-high-strength cold-rolled steel sheet having a metal microstructure including 40% to 85% of a tempered martensite phase and 15% to 60% of a ferrite phase on a volume fraction basis and a tensile strength of 1320 Mtn or more.

Abstract: A high strength galvanized steel sheet having excellent bendability and weldability, comprising by mass %: C: equal to or more than 0.05% and less than 0.12%; P: 0.001 to 0.040%; and S: equal to or less than 0.0050%, wherein a steel sheet surface layer, constituting a portion of the steel sheet up to a depth of 10 ?m measured from each surface of the steel sheet, has a structure containing more than 70% of ferrite phase by a volume fraction, a steel sheet inner layer portion, on an inner side than the depth of 10 ?m measured from each surface, has a structure containing 20 to 70% by a volume fraction of ferrite phase with an average crystal grain size equal to or smaller than 5 ?m, the steel sheet has a tensile strength equal to or larger than 980 MPa, and the steel sheet has a galvanized layer on a surface thereof.

Abstract: A high tensile-strength galvanized steel sheet includes C: at least 0.05% but less than 0.12%, Si: at least 0.01% but less than 0.35%, Mn: 2.0% to 3.5%, P: 0.001% to 0.020%, S: 0.0001% to 0.0030%, Al: 0.005% to 0.1%, N: 0.0001% to 0.0060%, Cr: more than 0.5% but not more than 2.0%, Mo: 0.01% to 0.50%, Ti: 0.010% to 0.080%, Nb: 0.010% to 0.080%, and B: 0.0001% to 0.0030%, the remainder being Fe and unavoidable impurities, wherein the high tensile-strength galvanized steel sheet has a microstructure that contains 20% to 70% by volume ferrite having an average grain size of 5 ?m or less. The high tensile-strength galvanized steel sheet has a tensile strength of at least 980 MPa, and excellent formability and weldability.

Abstract: A high-strength cold-rolled steel sheet having high chemical convertibility and a tensile strength of 590 MPa or more and a method for producing such a steel sheet are provided. The steel sheet contains, in terms of percent by mass, C: 0.05 to 0.3%, Si: 0.6 to 3.0%, Mn: 1.0 to 3.0%, P: 0.1% or less, S: 0.05% or less, Al: 0.01 to 1%, N: 0.01% or less, and the balance being Fe and unavoidable impurities. The coverage ratio of reduced iron on a steel sheet surface is 40% or more. In order to produce such a steel sheet, an oxidation treatment is performed after cold rolling. Subsequently, annealing is conducted in a furnace in a 1 to 10 vol % H2+balance N2 gas atmosphere with a dew point of ?25° C. or less.

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 high strength galvanized steel sheet including all in mass %, C: 0.05% to <0.12%, Si: 0.35% to <0.80%, Mn: 2.0 to 3.5%, P: 0.001 to 0.040%, S: 0.0001 to 0.0050%, Al: 0.005 to 0.1%, N: 0.0001 to 0.0060%, Cr: 0.01% to 0.5%, Ti: 0.010 to 0.080%, Nb: 0.010 to 0.080%, and B: 0.0001 to 0.0030%, optionally one or more of Mo: 0.01 to 0.15%, Ca: 0.0001 to 0.0050%, REM: 0.0001 to 0.1%, and Sb: 0.0001 to 0.1%, and Fe and unavoidable impurities as the balance, has a microstructure containing a ferrite phase with a volume fraction in a range of 20 to 70% and an average grain diameter equal to or smaller than 5 ?m, and has a galvanized layer on a surface thereof at a coating weight (per side) of 20 to 150 g/m2.

Abstract: A high strength steel sheet and a method for manufacturing the same has superior phosphatability properties and hot-dip galvannealed properties besides a tensile strength of 950 MPa or more and a high ductility, and also having a small variation in mechanical properties with the change in annealing conditions.

Abstract: A high tensile-strength galvanized steel sheet includes C: at least 0.05% but less than 0.12%, Si: at least 0.01% but less than 0.35%, Mn: 2.0% to 3.5%, P: 0.001% to 0.020%, S: 0.0001% to 0.0030%, Al: 0.005% to 0.1%, N: 0.0001% to 0.0060%, Cr: more than 0.5% but not more than 2.0%, Mo: 0.01% to 0.50%, Ti: 0.010% to 0.080%, Nb: 0.010% to 0.080%, and B: 0.0001% to 0.0030%, the remainder being Fe and unavoidable impurities, wherein the high tensile-strength galvanized steel sheet has a microstructure that contains 20% to 70% by volume ferrite having an average grain size of 5 ?m or less. The high tensile-strength galvanized steel sheet has a tensile strength of at least 980 MPa, and excellent formability and weldability.

Abstract: A high-strength cold-rolled steel sheet having a tensile strength of 340 MPa or more, which can prevent galling, can be manufactured even if a large number of the steel sheets are continuously press-formed. This is because a surface texture thereof is con-trolled so that the surface texture includes flat areas in which a roughness profile has a deviation of ±2 ?m or less from a filtered waviness curve and a dented portion having a maximum depth between 10 ?m and 50 ?m from the filtered waviness curve, wherein an average area of the dented portion is more than 0.01 mm2 and 0.2 mm2 or less, and an area fraction of the dented portion relative to the entire surface thereof is 5% or more and less than 20%.

Abstract: A method of manufacturing a high strength steel sheet. The method including casting a slab consisting essentially of 0.05 to 0.15% C, 0.5% or less Si, 1.00 to 2.00% Mn, 0.09% or less P, 0.01% or less S, 0.005% or less N, 0.01 to 0.1% Sol.Al, and a balance of Fe and inevitable impurities; hot-rolling the slab at a temperature of the Ar3 point or more; beginning cooling the hot-rolled steel sheet within 2 seconds after completing the hot-rolling at a temperature of from 750° C. to 600° C. at a cooling rate of 150° C./s or more; holding the cooled steel sheet at a temperature between 750° C. and 600° C. for 2 to 15 seconds; cooling the steel sheet at a cooling rate of 20° C./s or more; and coiling the cooled steel sheet at a temperature of 400° C. or less.

Abstract: A high strength steel sheet and a method for manufacturing the same has superior phosphatability properties and hot-dip galvannealed properties besides a tensile strength of 950 MPa or more and a high ductility, and also having a small variation in mechanical properties with the change in annealing conditions.

Abstract: The present invention provides a method for manufacturing an ultra high strength cold-rolled steel sheet, comprising the step of continuously annealing a cold-rolled steel sheet consisting essentially of, in terms of weight percentages, 0.07 to 0.15% C, 0.7 to 2% 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, 0.0003 to 0.003% B, and the balance being Fe, in which such continuous annealing comprises the steps of: heating the cold-rolled steel sheet at from 800° C. to 870° C. for 10 seconds or more; slowly cooling the heated steel sheet down to from 650° C. to 750° C.; rapidly cooling the slowly cooled steel sheet down to 100° C. or less at a cooling speed of over 500° C./sec; reheating the rapidly cooled steel sheet at from 325° C. to 425° C. for from 5 minutes to 20 minutes; cooling the reheated steel sheet down to room temperature; and coiling the cooled steel sheet.

Abstract: The high strength steel sheet consists essentially of 0.05 to 0.15% C, 0.5% or less Si, 1.00 to 2.00% Mn, 0.09% or less P, 0.01% or less S, 0.005% or less N, 0.01 to 0.1% Sol.Al, and balance of Fe and inevitable impurities; and contains 60% or more polygonal ferrite by volume, and 5 to 30% martensite by volume. The steel sheet is manufactured by the steps of: casting a slab having the specified composition; hot-rolling the slab at Ar3 point or more temperature; beginning cooling the hot-rolled steel sheet within 2 seconds after completing the hot-rolling to a temperature of from 750° C. to 600° C. at a cooling rate of 150° C./s or more; holding the cooled steel sheet at a temperature between 750° C. and 600° C. for 2 to 15 seconds; cooling the steel sheet at a cooling rate of 20° C./s or more; and coiling the cooled steel sheet at a temperature of 400° C. or less.

Abstract: The present invention provides a steel sheet for heat treatment consisting essentially of, by mass %, 0.05-0.09% C, below 1% Si, 1.6-2.4% Mn, 0.02% P or less, 0.02% S or less, 0.01-0.1% sol. Al, 0.005% N or less, 0.0003-0.003% B, Ti in a range satisfying (48/32)S+(48/14)N?Ti?2[(48/32)S+(48/14)N], and the rest of Fe, wherein the average diameter of iron carbides precipitating in steel is 2 ?m or smaller. The steel sheet for heat treatment according to the present invention has high strength and excellent hydrogen embrittlement resistance after being press formed and quenched, so that the steel sheet is well suited for automobile construction members.

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.