Abstract: A high-strength steel sheet is useful for applications to automobile steel sheets and the like and has excellent deep drawability, a tensile strength (TS) of as high as 440 MPa or more, and a high r value (average r value ?1.2), and a process for producing the steel sheet. The steel sheet has a composition containing, by % by mass, 0.010 to 0.050% of C, 1.0% or less of Si, 1.0 to 3.0% of Mn, 0.005 to 0.1% of P, 0.01% or less of S, 0.005 to 0.5% of Al, 0.01% or less of N, and 0.01 to 0.3% of Nb, the Nb and C contents in steel satisfying the relation, (Nb/93)/(C/12)=0.2 to 0.7, and the balance substantially including Fe and inevitable impurities. The steel microstructure contains a ferrite phase and a martensite phase at area ratios of 50% or more and 1% or more, respectively, and the average r value is 1.2 or more.

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: There is provided a high-stiffness high-strength thin steel sheet having a tensile strength of not less than 590 MPa and a Young's modulus of not less than 225 GPa, which comprises C: 0.02-0.15%, Si: not more than 1.5%, Mn: 1.5-4.0%, P: not more than 0.05%, S: not more than 0.01%, Al: not more than 1.5%, N: not more than 0.01% and Nb: 0.02-0.40% as mass %, provided that C, N and Nb contents satisfy 0.01?C+(12/14)×N?(12/92.9)×Nb?0.06 and N?(14/92.9)×(Nb?0.01) and the remainder being substantially iron and inevitable impurities, and has a texture comprising a ferrite phase as a main phase and having a martensite phase at an area ratio of not less than 1%.

Abstract: A steel sheet containing 0.004 to 0.02% C, 1.0% or less Si, 0.7 to 3.0% Mn, 0.02 to 0.15% P, 0.02% or less S, 0.01 to 0.1% Al, 0.004% or less N, 0.2% or less Nb, by mass %, optionally Ti, Bi or at least one element selected from the group consisting of Cr Mo, Ni and Cu, and the balance being Fe; the Nb content satisfying a formula of (12/93)×Nb*/C?1.0, wherein Nb*=Nb?(93/14)×N, and wherein C, N and Nb designate the content in mass % of carbon, nitrogen and niobium, respectively; and a yield strength and an average grain size of the ferritic grains which satisfy a formula of YP??120×d+1280, wherein YP designates yield strength in MPa, and d designates an average size of ferritic grains in ?m.

Abstract: There is provided a high-stiffness high-strength thin steel sheet having a tensile strength of not less than 590 MPa and a Young's modulus of not less than 230 GPa, which comprises C: 0.02-0.15%, Si: not more than 1.5%, Mn: 1.0-3.5%, P: not more than 0.05%, S: not more than 0.01%, Al: not more than 1.5%, N: not more than 0.01% and Ti: 0.02-0.50% as mass %, provided that C, N, S and Ti contents satisfy Ti*=Ti?(47.9/14)×N?(47.9/32.1)×S?0.01 and 0.01?C?(12/47.9)×Ti*?0.05 and the remainder being substantially iron and inevitable impurities, and has a texture comprising a ferrite phase as a main phase and having a martensite phase at an area ratio of not less than 1%.

Abstract: A galvannealed steel sheet is manufactured by preparing a hot-dip galvanized steel sheet, alloying the steel sheet, and controlling time and temperature for alloying thereof depending on Si and Al contents. The hot-dip galvanized steel sheet contains 0.05 to 0.30% C, 0.01 to 2.0% Si, 0.08 to 3.0% Mn, 0.003 to 0.1% P, 0 to 0.07% S, 0.01 to 2.5% Al, 0 to 0.007% N, by mass, and the balance being Fe and inevitable impurities. Alloying time and temperature are controlled by the formula [Si+Al?1.5×10?7×t0.75×(T?465)3+0.117], where t is the total time (sec) of holding the sheet at 465° C. or more on alloying the coating layer thereon, and T is the average temperature (° C.) of the sheet during the total time t (sec) of holding the steel sheet at 465° C. or more on alloying the coating layer thereon.

Abstract: The present invention provides a high-strength steel sheet useful for applications to automobile steel sheets and the like and having excellent deep drawability, a tensile strength (TS) of as high as 440 MPa or more, and a high r value (average r value?1.2), and a process for producing the steel sheet. The steel sheet has a composition containing, by % by mass, 0.010 to 0.050% of C, 1.0% or less of Si, 1.0 to 3.0% of Mn, 0.005 to 0.1% of P, 0.01% or less of S, 0.005 to 0.5% of Al, 0.01% or less of N, and 0.01 to 0.3% of Nb, the Nb and C contents in steel satisfying the relation, (Nb/93)/(C/12)=0.2 to 0.7, and the balance substantially including Fe and inevitable impurities. The steel microstructure contains a ferrite phase and a martensite phase at area ratios of 50% or more and 1% or more, respectively, and the average r value is 1.2 or more.

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 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 minutes; cooling the reheated steel sheet down to room temperature; and coiling the cooled steel sheet.

Abstract: A steel sheet containing 0.004 to 0.02% C, 1.0% or less Si, 0.7 to 3.0% Mn, 0.02 to 0.15% P, 0.02% or less S, 0.01 to 0.1% Al, 0.004% or less N, 0.2% or less Nb, by mass %, optionally Ti, Bi or at least one element selected from the group consisting of Cr Mo, Ni and Cu, and the balance being Fe; the Nb content satisfying a formula of (12/93)×Nb*/C≧1.0, wherein Nb*=Nb−(93/14)×N, and wherein C, N and Nb designate the content in mass % of carbon, nitrogen and niobium, respectively; and a yield strength and an average grain size of the ferritic grains which satisfy a formula of YP≦−120×d+1280, wherein YP designates yield strength in MPa, and d designates an average size of ferritic grains in &mgr;m.

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 steel sheet comprises: a ferritic phase having ferritic grains of 10 or more grain size number and ferritic grain boundaries; and at least one kind of Nb precipitates and Ti precipitates. The ferritic grain has a low density region with a low precipitate density in the vicinity of grain boundary. The low density region has a precipitate density of 60% or less to the precipitate density at center part of the ferritic grain. The steel sheet consists essentially of 0.002 to 0.02% C, 1% or less Si, 3% or less Mn, 0.1% or less P, 0.02% or less S, 0.01 to 0.1% sol.Al, 0.007% or less N, at least one element of 0.01 to 0.4% Nb and 0.005 to 0.3% Ti, by mass %, and the balance being Fe.

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.

Abstract: The steel sheet comprises: a ferritic phase having ferritic grains of 10 or more grain size number and ferritic grain boundaries; and at least one kind of Nb precipitates and Ti precipitates. The ferritic grain has a low density region with a low precipitate density in the vicinity of grain boundary. The low density region has a precipitate density of 60% or less to the precipitate density at center part of the ferritic grain. The steel sheet consists essentially of 0.002 to 0.02% C, 1% or less Si, 3% or less Mn, 0.1% or less P, 0.02% or less S, 0.01 to o.1% sol.Al, 0.007% or less N, at least one element of 0.01 mass %, and the balance being Fe.

Abstract: A hot-dip galvanized steel sheet is produced by rough rolling a steel, finish rolling the rough rolled steel at a temperature of Ar3 point or more, coiling the finish rolled steel at a temperature of 700° C. or less, and hot-dip galvanizing the coiled steel at a pre-plating heating temperature of Ac1 to Ac3. A continuous hot-dip galvanizing operation is performed by soaking a pickled strip at a temperature of 750 to 850° C., cooling the soaked strip to a temperature range of 600° C. or less at a cooling rate of 1 to 50° C. per second, hot-dip galvanizing the cooled strip, and cooling the galvanized strip so that the residence time at 400 to 600° C. is within 200 seconds. The steel sheet has a structure consisting essentially of ferrite and martensite.

Abstract: A hot-dip galvanized steel sheet is produced by rough rolling a steel, finish rolling the rough rolled steel at a temperature of Ar3 point or more, coiling the finish rolled steel at a temperature of 700° C. or less, and hot-dip galvanizing the coiled steel at a pre-plating heating temperature of Ac1 to Ac3. A continuous hot-dip galvanizing operation is performed by soaking a pickled strip at a temperature of 750 to 850° C., cooling the soaked strip to a temperature range of 600° C. or less at a cooling rate of 1 to 50° C. per second, hot-dip galvanizing the cooled strip, and cooling the galvanized strip so that the residence time at 400 to 600° C. is within 200 seconds. The steel sheet has a structure consisting essentially of ferrite and martensite.

Abstract: A steel slab containing not more than 0.005 wt % carbon, 1.0 to 4.0 wt % silicon, 0.1 to 1.0 wt % manganese, not more than 0.1 wt % phosphorus, not more than 0.005 wt % sulfur, and 0.1 to 2.0 wt % aluminum is directly sent to a hot rolling step without brief soaking, whereby Al and N are made soluble with respect to species other than AlN inevitably precipitated during hot rolling, and uniform and coarse AlN precipitation is formed by a subsequent annealing treatment in which the hot rolled plate is soaked at a temperature of 800.degree. to 1000.degree. C. for a period of time t defined as exp (-0.018T+19.4) .ltoreq.t.ltoreq.exp (-0.022T+25.4) wherein T is the soaking temperature (.degree.C.) and t is the soaking time (minutes). Uniform and satisfactory ferrite grain growth at the recrystallization annealing is provided thereby.

Abstract: A method of producing non-oriented electrical steel sheets having excellent magnetic properties which comprehends heating a steel at the low temperature for hot rolling, thereby to minimize re-solution of AlN particles when a slab is cooled, so that coarsening of AlN particles is facilitated during annealing the hot rolled sheet; and coiling at low temperatures for controlling the amount of scale generation. De-scaling is perfectly accomplished after the hot rolling, and the hot rolled sheet is annealed in a non-annealing atmosphere, thereby to control the oxidation and the nitriding to a minimum during annealing the hot rolled sheet, and the annealing conditions of the hot rolled sheet are specified for proper coarsening of the cohesion, taking into consideration the magnetic properties and the economics.