Abstract: A steel plate for high-strength and high-toughness steel pipes has a chemical composition containing, by mass %, C: 0.03% or more and 0.08% or less, Si: more than 0.05% and 0.50% or less, Mn: 1.5% or more and 2.5% or less, P: 0.001% or more and 0.010% or less, S: 0.0030% or less, Al: 0.01% or more and 0.08% or less, Nb: 0.010% or more and 0.080% or less, Ti: 0.005% or more and 0.025% or less, and N: 0.001% or more and 0.006% or less, and further containing, by mass %, at least one selected from Cu: 0.01% or more and 1.00% or less, Ni: 0.01% or more and 1.00% or less, Cr: 0.01% or more and 1.00% or less, Mo: 0.01% or more and 1.00% or less, V: 0.01% or more and 0.10% or less, and B: 0.0005% or more and 0.0030% or less, with the balance being Fe and inevitable impurities. The steel plate has a microstructure in which an area fraction of ferrite at a ½ position of a thickness of the steel plate is 20% or more and 80% or less and deformed ferrite constitutes 50% or more and 100% or less of the ferrite.

Abstract: A high strength cold rolled steel sheet with a low yield ratio has a chemical composition containing C: 0.05% to 0.10%, Si: 0.6% to 1.3%, Mn: 1.4% to 2.2%, P: 0.08% or less, S: 0.010% or less, Al: 0.01% to 0.08%, N: 0.010% or less, and the remainder being Fe and incidental impurities, on a percent by mass basis, and a microstructure in which the average grain size of ferrite is 15 ?m or less, the volume fraction of ferrite is 70% or more, the volume fraction of bainite is 3% or more, the volume fraction of retained austenite is 4% to 7%, the average grain size of martensite is 5 ?m or less, and the volume fraction of martensite is 1% to 6%, wherein the average C concentration (percent by mass) in the retained austenite is 0.30% to 0.70%, yield ratio is 64% or less, and the tensile strength is 590 MPa or more.

Abstract: A high strength/highstrength, high toughness steel sheet having tensile strength, Charpy impact absorption energy, and ductile fracture rate are equal to or greater than specified value comprises, in mass %, 0.03-0.08% of C, 0.01-0.50% of Si, 1.5-2.5% of Mn, 0.001-0.010% of P, 0.0030% or less of S, 0.01-0.08% of Al, 0.010-0.080% of Nb, 0.005-0.025% of Ti, 0.001-0.006% of N, at least one substance selected from among 0.01-1.00% of Cu, 0.01-1.00% of Ni, 0.01-1.00% of Cr, 0.01-1.00% of Mo, 0.01-0.10% of V, and 0.0005-0.0030% of B, and a remainder of Fe and unavoidable impurities. At a position at ½ the sheet thickness, the area ratio of island-like martensite is less than 3%, the area ratio of bainite is 90% or more, and the average particle size of cementite within the bainite is 0.5 ?m or less.

Abstract: A high-strength, high-toughness steel plate having excellent surface properties and a high absorbed energy includes, by mass %, C: 0.03% to 0.08%, Si: 0.01% to 0.50%, Mn: 1.5% to 2.5%, P: 0.001% to 0.010%, S: 0.0030% or less, Al: 0.01% to 0.08%, Nb: 0.010% to 0.080%, Ti: 0.005% to 0.025%, and N: 0.001% to 0.006%, and further includes at least one selected from Cu: 0.01% to 1.00%, Ni: 0.01% to 1.00%, Cr: 0.01% to 1.00%, Mo: 0.01% to 1.00%, V: 0.01% to 0.10%, and B: 0.0005% to 0.0030%, with the balance being Fe and unavoidable impurities. In a surface portion and a central portion in the thickness direction, the area fraction of Martensite-Austenite constituent is less than 3% and the area fraction of bainite is 90% or more, and in the central portion in the thickness direction, the average particle size of cementite in bainite is 0.5 ?m or less.

Abstract: A high-strength galvanized steel sheet and method for producing the same. The high-strength galvanized steel sheet including a specified chemical composition and a microstructure comprising, in terms of area fraction, a tempered martensite phase: 30% or more and 73% or less, a ferrite phase: 25% or more and 68% or less, a retained austenite phase: 2% or more and 15% or less, and other phases: 10% or less (including 0%), the other phases comprising a martensite phase: 3% or less (including 0%) and a bainitic ferrite phase: less than 5% (including 0%), the tempered martensite phase having an average grain size of 8 ?m or less, the ferrite phase having an average grain size of 5 ?m or less, and the retained austenite phase having a C content less than 0.7% by mass.

Abstract: A high-strength galvanized steel sheet that includes a chemical composition containing, by mass %, C: 0.15% or more and 0.25% or less, Si: 0.50% or more and 2.5% or less, Mn: 2.3% or more and 4.0% or less, P: 0.100% or less, S: 0.02% or less, Al: 0.01% or more and 2.5% or less, and Fe and inevitable impurities. The steel sheet having a microstructure containing, by an area percentage basis, a tempered martensite phase: 30% or more and 73% or less, a ferrite phase: 25% or more and 68% or less, a retained austenite phase: 2% or more and 20% or less, and other phases: 10% or less (including 0%), the other phases containing a martensite phase: 3% or less (including 0%) and a bainitic ferrite phase: less than 5% (including 0%).

Abstract: A high strength galvanized steel sheet that has a tensile strength (TS) of 780 MPa or higher and excellent bendability, fatigue resistance, and surface appearance quality and a method for producing the high strength galvanized steel sheet. The steel sheet includes, at a position ½ of a thickness of the steel sheet, 5% or more and 80% or less of a ferrite phase in terms of area fraction, 20% or more and 70% or less of a martensite phase in terms of area fraction, and 0% or more and 25% or less of a bainite phase in terms of area fraction.

Abstract: A method for manufacturing a high-strength galvanized steel sheet. The method includes a first heating step of holding the steel sheet in a temperature range of 750° C. to 880° C. for 20 s to 600 s in an atmosphere having an H2 concentration of 0.05% to 25.0% by volume and a dew point of ?45° C. to ?10° C., a cooling step, a rolling step of rolling the steel sheet with a rolling reduction of 0.3% to 2.0%, a pickling step of pickling the steel sheet with a pickling weight loss of 0.02 gram/m2 to 5 gram/m2 in terms of Fe, a second heating step of holding the steel sheet in a temperature range of 720° C. to 860° C. for 20 sec. to 300 sec. in an atmosphere having an H2 concentration of 0.05% to 25.0% by volume and a dew point of ?10° C. or lower, and a galvanizing step.

Abstract: A method for manufacturing a high-strength galvanized steel sheet includes performing hot rolling, cold rolling, first annealing, pickling, and second annealing. The first annealing is performed to obtain a steel sheet having a steel microstructure including ferrite in an amount of 10% or more and 60% or less in terms of area ratio, and martensite, bainite, and retained austenite in a total amount of 40% or more and 90% or less in terms of area ratio. The second annealing includes heating to an annealing temperature of 750° C. or higher and 850° C. or lower, holding at the annealing temperature for 10 seconds or more and 500 seconds or less, cooling at an average cooling rate of 1° C./s or more and 15° C./s or less, performing a galvanizing treatment, and cooling to a temperature of 150° C. or lower at an average cooling rate of 5° C./s or more and 100° C./s or less.

Abstract: A high-strength galvanized steel sheet contains C: 0.010% or more and 0.06% or less, Si: more than 0.5% and 1.5% or less, Mn: 1.0% or more and 3.0% or less, P: 0.005% or more and 0.1% or less, S: 0.01% or less, sol.Al: 0.005% or more and 0.5% or less, N: 0.01% or less, Nb: 0.010% or more and 0.090% or less, and Ti: 0.015% or more and 0.15% or less, on a mass percent basis. The Nb and C contents of the steel satisfy the relation of (Nb/93)/(C/12)<0.20. C* satisfies 0.005?C*?0.025. Ferrite constitutes 70% by area ratio or more of the steel sheet. Martensite constitutes 3% by area ratio or more of the steel sheet. C*=C?(12/93)Nb?(12/48){Ti?(48/14)N}, wherein C, Nb, Ti, and N denote the C, Nb, Ti, and N contents of the steel.

Abstract: A method is provided for producing a high-strength galvanized steel sheet having a microstructure that contains martensite in an area proportion of 20% or more and 60% or less and ferrite in an area proportion of 40% or more and 80% or less includes, in sequence, hot-rolling a steel slab containing a specific component composition, performing cold rolling, performing primary annealing, performing pickling, performing secondary annealing, and performing galvanizing treatment, in which in the primary annealing, heating is performed at an average heating rate of 0.1° C./sec. or more and less than 3° C./sec. in the temperature range of 600° C. to 750° C., an annealing temperature of 750° C. to 850° C. is maintained for 10 to 500 seconds, and then cooling is performed from the annealing temperature range to a cooling stop temperature of 600° C. or lower at an average cooling rate of 1 to 15° C.,/sec, in which in the pickling, the pickling weight loss of the steel sheet is 0.

Abstract: There is provided a high-strength galvanized steel sheet including, on a mass percent basis, C: more than 0.060% and 0.13% or less, Si: 0.01% or more and 0.7% or less, Mn: 1.0% or more and 3.0% or less, P: 0.005% or more and 0.100% or less, S: 0.010% or less, sol.Al: 0.005% or more and 0.100% or less, N: 0.0100% or less, Nb: 0.005% or more and 0.10% or less, Ti: 0.03% or more and 0.15% or less, and the balance comprising Fe and incidental impurities, satisfying the relationship of (Nb/93+Ti*/48)/(C/12)>0.08 (wherein Ti*=Ti?(48/14)N?(48/32)S). The high-strength galvanized steel sheet has a structure including ferrite and martensite. The ferrite has an average grain diameter of 15 ?m or less and an area percentage of 80% or more. The martensite has an area percentage of 1% or more and 15% or less.

Abstract: A high strength cold rolled steel sheet includes a chemical composition containing, by mass %, C: 0.010% or more and 0.060% or less, Si: more than 0.5% and 1.5% or less, Mn: 1.0% or more and 3.0% or less, P: 0.005% or more and 0.100% or less, S: 0.010% or less, sol.Al: 0.005% or more and 0.500% or less, N: 0.0100% or less, Nb: 0.010% or more and 0.100% or less, Ti: 0.015% or more and 0.150% or less and the balance comprising Fe and inevitable impurities. The microstructure includes, in area fraction, 70% or more of a ferrite phase and 3% or more of a martensite phase. The tensile strength is 440 MPa or more and an average r value is 1.20 or more.

Abstract: A high strength cold rolled steel sheet with a low yield ratio has a chemical composition containing C: 0.05% to 0.10%, Si: 0.6% to 1.3%, Mn: 1.4% to 2.2%, P: 0.08% or less, S: 0.010% or less, Al: 0.01% to 0.08%, N: 0.010% or less, and the remainder being Fe and incidental impurities, on a percent by mass basis, and a microstructure in which the average grain size of ferrite is 15 ?m or less, the volume fraction of ferrite is 70% or more, the volume fraction of bainite is 3% or more, the volume fraction of retained austenite is 4% to 7%, the average grain size of martensite is 5 ?m or less, and the volume fraction of martensite is 1% to 6%, wherein the average C concentration (percent by mass) in the retained austenite is 0.30% to 0.70%, yield ratio is 64% or less, and the tensile strength is 590 MPa or more.

Abstract: A high strength hot-dip galvanized steel sheet has TS of 440 MPa or more and an average r value of 1.30 or more, where the absolute value of the planar anisotropy of the r value (?r) is 0.20 or less. A chemical composition contains C: 0.010% or more and 0.04% or less, Si: more than 1.0% and 1.5% or less, Mn: 1.0% or more and 3.0% or less, P: 0.005% or more and 0.1% or less, S: 0.01% or less, sol. Al: 0.005% or more and 0.5% or less, N: 0.01% or less, Nb: 0.010% or more and less than 0.05%, Ti: 0.015% or more and 0.120% or less, and the remainder composed of Fe and incidental impurities, wherein (Nb/93)/(C/12) 0.20 and 0.005<C*?0.020 are satisfied.