Abstract: A steel slab having a composition containing C: more than 0.07% and 0.2% or less, Si: 2.0% or less, Mn: 1.0% to 3.0%, Al: 0.1% or less, Ti: 0.05% to 0.3%, and V: 0.05% to 0.3% on a mass percent basis is heated to 1100° C. or more and is subjected to rough rolling and finish rolling. In the finish rolling, the total rolling reduction of two final passes is 30% or more, and the finish rolling temperature ranges from (Ar3 transformation temperature) to (Ar3 transformation temperature+120° C.). Cooling is started within 2 seconds after the finish rolling. Coiling is performed at an average cooling rate of 40° C./s or more at a coiling temperature in the range of 300° C. to 500° C. The resulting high-strength hot-rolled steel sheet has a microstructure in which a bainite phase constitutes more than 90% by volume, the average lath interval of bainite is 0.

Abstract: A high-strength hot-rolled steel sheet has a tensile strength TS of 980 MPa or more and is manufactured by controlling the steel sheet to have a chemical composition containing C: more than 0.1% and 0.2% or less, Si: 0.5% or more and 3.0% or less, Mn: 1.0% or more and 3.5% or less, P: 0.05% or less, S: 0.004% or less, Al: 0.10% or less, N: 0.008% or less, Ti: 0.05% or more and 0.15% or less, V: more than 0.10% and 0.30% or less, and the balance being Fe and inevitable impurities. Surface regions have a microstructure including mainly a ferrite phase, and an inner region has a microstructure including mainly a bainite phase. The proportions of the surface regions in the thickness direction of the steel sheet are 1.0% or more and 5.0% or less of the whole thickness respectively from the upper and lower surfaces of the steel sheet.

Abstract: There are provided a high-strength hot-rolled steel sheet having high burring formability and a method for manufacturing the high-strength hot-rolled steel sheet. A high-strength hot-rolled steel sheet having high burring formability contains, on a mass percent basis, C: 0.03% or more and 0.1% or less, Si: less than 0.5%, Mn: more than 0.7% and less than 1.2%, P: 0.05% or less, S: 0.005% or less, N: 0.01% or less, Al: 0.1% or less, and Ti: 0.1% or more and 0.25% or less such that C, S, N, and Ti satisfy Ti*?0.1 and C×(48/12)?0.14<Ti*<C×(48/12)+0.08 (wherein Ti*=Ti?N×(48/14)?S×(48/32), and C, S, N, and Ti denote the amounts (% by mass) of the corresponding elements), the remainder being Fe and incidental impurities, wherein the high-strength hot-rolled steel sheet has a microstructure in which a ferrite phase fraction is more than 90%, a carbide containing Ti is precipitated, and 70% or more of the carbide has a grain size of less than 9 nm.

Abstract: There are provided a high-strength hot-rolled steel sheet having high burring formability and a method for manufacturing the high-strength hot-rolled steel sheet. A high-strength hot-rolled steel sheet having high burring formability contains, on a mass percent basis, C: 0.06% or more and 0.13% or less, Si: less than 0.5%, Mn: more than 0.5% and 1.4% or less, P: 0.05% or less, S: 0.005% or less, N: 0.01% or less, Al: 0.1% or less, Ti: 0.05% or more and 0.25% or less, and V: more than 0.15% and 0.4% or less such that S, N, Ti, and V satisfy Ti.+V?0.35 (wherein Ti.?Ti—N×(48/14)?S×(48/32), and S, N, Ti, and V denote the amounts (% by mass) of the corresponding elements), the remainder being Fe and incidental impurities, wherein the high-strength hot-rolled steel sheet has a microstructure in which a ferrite phase fraction is more than 90%, a carbide containing Ti is precipitated, and 70% or more of the carbide has a grain size of less than 9 nm.

Abstract: There are provided a high-strength hot-rolled steel sheet having high burring formability and a method for manufacturing the high-strength hot-rolled steel sheet. The high-strength hot-rolled steel sheet having high burring formability contains, on a mass percent basis, C: 0.013% or more and less than 0.08%, Si: less than 0.5%, Mn: more than 0.8% and less than 1.2%, P: 0.05% or less, S: 0.005% or less, N: 0.01% or less, Al: 0.1% or less, and Ti: 0.03% or more and 0.15% or less such that C, S, N, and Ti satisfy 0.05?Ti*<0.1 and C×(48/12)?0.16<Ti* (wherein Ti*=Ti?N×(48/14)?S×(48/32), and C, S, N, and Ti denote the amounts (% by mass) of the corresponding elements), the remainder being Fe and incidental impurities, wherein the high-strength hot-rolled steel sheet has a microstructure in which a ferrite phase fraction is more than 90%, a carbide containing Ti is precipitated, and 70% or more of the carbide has a grain size of less than 9 nm.

Abstract: A high strength hot rolled steel sheet is provided with low yield ratio and that is excellent in low-temperature toughness. The hot rolled steel sheet can be used as a raw material of a steel pipe. The steel sheet has a chemical composition containing C: 0.03% or more and 0.10% or less, Si: 0.01% or more and 0.50% or less, Mn: 1.4% or more and 2.2% or less, P: 0.025% or less, S: 0.005% or less, Al: 0.005% or more and 0.10% or less, Nb: 0.02% or more and 0.10% or less, Ti: 0.001% or more and 0.030% or less, Mo: 0.01% or more and 0.50% or less, Cr: 0.01% or more and 0.50% or less, and Ni: 0.01% or more and 0.50% or less, in which the condition that Moeq is 1.4% or more and 2.2% or less.

Abstract: A hot rolled steel sheet for a square column for building structural members, the hot rolled steel sheet having a composition of, in terms of % by mass, C: 0.07 to 0.18%, Mn: 0.3to 1.5%, P: 0.03% or less, S: 0.015% or less, N: 0.006% or less, Al: 0.01 to 0.06%, and the balance being Fe and unavoidable impurities, and having a microstructure that includes a primary phase constituted of ferrite and a second phase constituted of pearlite or pearlite and bainite, wherein a second phase frequency defined by equation (1) below is 0.20 to 0.42 and a mean crystal grain diameter of the primary phase and the second phase together is 7 to 15 ?m Second phase frequency=(Number of second phase grains intersecting line segments of particular length)/(Number of primary phase grains and second phase grains intersecting line segments of particular length) ??(1).

Abstract: A high-strength hot-rolled steel sheet is provided having a high low-temperature toughness and a low yield ratio that is suitable for a steel pipe material. The hot-rolled steel sheet has a composition that contains C: 0.03% to 0.10%, Si: 0.01% to 0.50%, Mn: 1.4% to 2.2%, P: 0.025% or less, S: 0.005% or less, Al: 0.005% to 0.10%, Nb: 0.02% to 0.10%, Ti: 0.001% to 0.030%, Mo: 0.01% to 0.50%, Cr: 0.01% to 0.50%, and Ni: 0.01% to 0.50%. The composition of the hot-rolled steel sheet has an Moeq value in a range of 1.4% to 2.2%. The hot-rolled steel sheet includes an inner layer, which has a microstructure that contains a main phase and a second phase, and an outer layer, which has a microstructure that contains a tempered martensite phase or a tempered martensite phase and a tempered bainite phase.

Abstract: A thick-walled high-strength hot rolled steel sheet has a high tensile strength TS of 521 MPa or more and excellent low-temperature toughness. The steel material forming the sheet contains 0.02%-0.08% C, 0.01%-0.10% Nb, and 0.001%-0.05% Ti and is heated; C, Ti, and Nb satisfies (Ti+(Nb/2))/C<4.

Abstract: A steel strip for coiled tubing contains, in terms of percent by mass, C: 0.10% or more and 0.16% or less, Si: 0.1% or more and 0.5% or less, Mn: 0.5% or more and 1.5% or less, P: 0.02% or less, S: 0.005% or less, Sol. Al: 0.01% or more and 0.07% or less, Cr: 0.4% or more and 0.8% or less, Cu: 0.1% or more and 0.5% or less, Ni: 0.1% or more and 0.3% or less, Mo: 0.1% or more and 0.2% or less, Nb: 0.01% or more and 0.04% or less, Ti: 0.005% or more and 0.03% or less, N: 0.005% or less, and the balance of Fe and inevitable impurities.

Abstract: The steel sheet has a chemical composition containing, by mass %, C: 0.04-0.08%, Si: 0.50% or less, Mn: 0.8-2.2%, P: 0.02% or less, S: 0.006% or less, Al: 0.1% or less, N: 0.008% or less, and Cr: 0.05-0.8%, and further Nb: 0.01-0.08%, V: 0.001-0.12%, and Ti: 0.005-0.04% in adjusted amounts, with the balance including Fe and incidental impurities. The steel sheet has a surface layer having a microstructure containing bainite as a main phase, martensite as a second phase in a volume fraction of 0.5-4%, and at lease one of ferrite phase, pearlite, and cementite as a third phase in a total volume fraction of 10% or less.

Abstract: A low yield ratio and high-strength hot rolled steel sheet having a composition containing, on a mass percent basis, 0.03% to 0.10% C, 0.10% to 0.50% Si, 1.4% to 2.2% Mn, 0.005 % to 0.10% Al, 0.02% to 0.10% Nb, 0.001% to 0.030% Ti, 0.05% to 0.50% Mo, 0.05% to 0.50% Cr, and 0.01% to 0.50% Ni, in which Moeq preferably satisfies the range of 1.4% to 2.2%; and a microstructure including a main phase that contains bainitic ferrite having an average grain size of 10 ?m or less and a secondary phase that contains massive martensite having an aspect ratio of less than 5.0 in an area ratio of 1.4% to 15%.

Abstract: A high-strength hot rolled steel sheet with excellent bendability and low-temperature toughness includes a chemical composition including, in mass %, C: 0.08 to 0.25%, Si: 0.01 to 1.0%, Mn: 0.8 to 2.1%, P: not more than 0.025%, S: not more than 0.005% and Al: 0.005 to 0.10%, the balance including Fe and inevitable impurities, and a microstructure having a bainite phase and/or a tempered martensite phase as a main phase, the average grain diameter of prior austenite grains being not more than 20 ?m as measured with respect to a cross section parallel to a rolling direction and not more than 15 ?m as measured with respect to a cross section perpendicular to the rolling direction.

Abstract: A high-tensile-strength hot-rolled steel sheet is provided having a composition which contains 0.02 to 0.08% C, 0.01 to 0.10% Nb, 0.001 to 0.05% Ti and Fe and unavoidable impurities as a balance, wherein the steel sheet contains C, Ti and Nb in such a manner that (Ti+(Nb/2))/C<4 is satisfied, and the steel sheet has a structure where a primary phase of the structure at a position 1 mm away from a surface in a sheet thickness direction is one selected from a group consisting of a ferrite phase, tempered martensite and a mixture structure of a ferrite phase and tempered martensite, a primary phase of the structure at a sheet thickness center position is formed of a ferrite phase, and a difference ?V between a structural fraction (volume %) of a secondary phase at the position 1 mm away from the surface in the sheet thickness direction and a structural fraction (volume %) of a secondary phase at the sheet thickness center position is 2% or less.

Abstract: A high-tensile-strength hot-rolled steel sheet is provided having a composition which contains 0.02 to 0.08% C, 0.01 to 0.10% Nb, 0.001 to 0.05% Ti and Fe and unavoidable impurities as a balance, wherein the steel sheet contains C, Ti and Nb in such a manner that (Ti+(Nb/2))/C<4 is satisfied, and the steel sheet has a structure where a primary phase of the structure at a position 1 mm away from a surface in a sheet thickness direction is one selected from a group consisting of a ferrite phase, tempered martensite and a mixture structure of a ferrite phase and tempered martensite, a primary phase of the structure at a sheet thickness center position is formed of a ferrite phase, and a difference ?V between a structural fraction (volume %) of a secondary phase at the position 1 mm away from the surface in the sheet thickness direction and a structural fraction (volume %) of a secondary phase at the sheet thickness center position is 2% or less.

Abstract: A thick-walled high-strength hot rolled steel sheet having excellent hydrogen induced cracking resistance which is preferably used as a raw material for a high-strength welded steel pipe of X65 grade or more and a method of manufacturing the thick-walled high-strength hot rolled steel sheet are provided. The composition of the thick-walled high-strength hot rolled steel sheet contains by mass % 0.02 to 0.08% C, 0.50 to 1.85% Mn, 0.03 to 0.10% Nb, 0.001 to 0.05% Ti, 0.0005% or less B in such a manner that (Ti+Nb/2)/C<4 is satisfied or also contains one or two kinds or more of 0.010% or less Ca, 0.02% or less REM, and Fe and unavoidable impurities as a balance. The steel sheet has the structure formed of a bainitic ferrite phase or a bainite phase. Surface layer hardness is 230HV or less in terms of Vickers hardness.

Abstract: A high-tensile-strength hot-rolled steel sheet is provided having a composition which contains 0.02 to 0.08% C, 0.01 to 0.10% Nb, 0.001 to 0.05% Ti and Fe and unavoidable impurities as a balance, wherein the steel sheet contains C, Ti and Nb in such a manner that (Ti+(Nb/2))/C<4 is satisfied, and the steel sheet has a structure where a primary phase of the structure at a position 1 mm away from a surface in a sheet thickness direction is one selected from a group consisting of a ferrite phase, tempered martensite and a mixture structure of a ferrite phase and tempered martensite, a primary phase of the structure at a sheet thickness center position is formed of a ferrite phase, and a difference ?V between a structural fraction (volume %) of a secondary phase at the position 1 mm away from the surface in the sheet thickness direction and a structural fraction (volume %) of a secondary phase at the sheet thickness center position is 2% or less.

Abstract: A thick-walled high-strength hot rolled steel sheet has a high tensile strength TS of 521 MPa or more and excellent low-temperature toughness. The steel material forming the sheet contains 0.02%-0.08% C, 0.01%-0.10% Nb, and 0.001%-0.05% Ti and is heated; C, Ti, and Nb satisfies (Ti+(Nb/2))/C<4.

Abstract: A hot-rolled steel sheet for high-strength ERW pipes contains about 0.02% to about 0.06% C; about 0.05% to about 0.50% Si; about 0.5% to about 1.5% Mn; about 0.010% or less P; about 0.0010% or less S; about 0.01% to about 0.10% Al; about 0.01% to about 0.10% Nb; about 0.001% to about 0.025% Ti; about 0.001% to about 0.005% Ca; about 0.003% or less 0; and about 0.005% or less N, and at least one element selected from the group consisting of about 0.01% to about 0.10% V; about 0.01% to about 0.50% Cu; about 0.01% to about 0.50% Ni; and about 0.01% to about 0.50% Mo on the basis of mass. The group of C, Si, Mn, Cu, Ni, Mo, and V and the group of Ca, 0, and S satisfy specific relationships, and the microstructure of the steel sheet is composed of about 95% by volume or more bainitic ferrite.

Abstract: A hot-rolled steel strip having superior low temperature toughness and weldability, which is suitably used as a starting material for high strength electric resistance welding pipe, is provided at a low cost without constructing new production facilities and increasing cost. The hot-rolled steel strip is low carbon steel containing at least one of about 0.5.% or less of Cu, about 0.5% or less of Ni, and about 0.5% or less of Mo, wherein Pcm represented by the following equation (1) is 0.17 or less: Pcm=(% C)+(% Si)/30+((% Mn)+(% Cu))/20+(% Ni)/60+(% Mo)/7+(% V)/10??Equation (1), (where (% M) indicates the content of element M on a mass percent basis), and the balance includes Fe and incidental impurities. In addition, in the entire microstructure, the ratio of bainitic ferrite, which is a primary phase, is controlled to be about 95 percent by volume or more.