Abstract: A weld joint with an excellent CTOD property is produced with a weld metal, using a steel plate as a base metal. The steel plate has a chemical composition including C: 0.03% to 0.09%, Si: 0.01% to 0.35%, Mn: 1.3% to 2.0%, P: 0.012% or less, S: 0.0035% or less, Al: 0.01% to 0.06%, Ni: less than 0.3%, Mo: less than 0.10%, Nb: 0.005% to 0.023%, Ti: 0.005% to 0.025%, B: less than 0.0003%, N: 0.002% to 0.005%, Ca: 0.0005% to 0.0050%, and O: 0.0030% or less, with the components additionally satisfying a predetermined relationship. The weld metal has a chemical composition including C: 0.040% to 0.090%, Si: 0.1% to 0.8%, Mn: 1.0% to 2.5%, Al: 0.020% or less, Ni: 0.1% to 1.0%, Mo: 0.05% to 0.50%, Ti: 0.005% to 0.050%, and B: 0.0015% or less, the balance being Fe and incidental impurities.

Abstract: A low alloy high strength thick-walled seamless steel pipe for oil country tubular goods is provided having a wall thickness of 40 mm or more and a yield strength of 758 MPa or more, the steel pipe including a composition containing, in terms of mass %, C: 0.25 to 0.31%, Si: 0.01 to 0.35%, Mn: 0.55 to 0.70%, P: 0.010% or less, S: 0.001% or less, O: 0.0015% or less, Al: 0.015 to 0.040%, Cu: 0.02 to 0.09%, Cr: 0.8 to 1.5%, Mo: 0.9 to 1.6%, V: 0.04 to 0.10%, Nb: 0.005 to 0.05%, B: 0.0015 to 0.0030%, Ti: 0.005 to 0.020%, and N: 0.005% or less, and having Ti/N of 3.0 to 4.0, with the balance being Fe and inevitable impurities, wherein a cumulative frequency rate at a measurement point at which a Mo segregation degree by a predetermined expression is 1.5 or more is 1% or less.

Abstract: The steel pipe of the present invention is a low alloy high strength seamless steel pipe for oil country tubular goods including a composition containing, in terms of mass %, C: 0.23 to 0.27%, Si: 0.01 to 0.35%, Mn: 0.45 to 0.70%, P: 0.010% or less, S: 0.001% or less, O: 0.0015% or less, Al: 0.015 to 0.080%, Cu: 0.02 to 0.09%, Cr: 0.8 to 1.5%, Mo: 0.5 to 1.0%, Nb: 0.02 to 0.05%, B: 0.0015 to 0.0030%, Ti: 0.005 to 0.020%, and N: 0.005% or less, and having a value of a ratio of the Ti content to the N content (Ti/N) of 3.0 to 4.0, with the balance being Fe and inevitable impurities, the steel pipe having a ratio of a stress at a strain of 0.7% to a stress at a strain of 0.4% in a stress-strain curve; of 1.02 or less and a yield strength of 655 MPa or more.

Abstract: A low alloy high strength seamless steel pipe for oil country tubular goods is provided including a composition containing, in terms of mass %, C: 0.25 to 0.31%, Si: 0.01 to 0.35%, Mn: 0.45 to 0.70%, P: 0.010% or less, S: 0.001% or less, 0: 0.0015% or less, Al: 0.015 to 0.080%, Cu: 0.02 to 0.09%, Cr: 0.8 to 1.5%, Mo: 1.1 to 1.6%, V: 0.01 to 0.06%, Nb: 0.005 to 0.015%, B: 0.0015 to 0.0030%, Ti: 0.005 to 0.020%, and N: 0.005% or less, and having a ratio of the Ti content to the N content of 3.0 to 4.0, with the balance being Fe and inevitable impurities, the steel pipe having a ratio of a stress at a strain of 0.7% to a stress at a strain of 0.4% in a stress-strain curve of 1.02 or less and a yield strength of 861 MPa or more.

Abstract: The high-strength seamless steel pipe has a volume fraction of tempered martensite of 95% or more, and a prior austenite size number of 8.5 or more, and contains nitride inclusions having a size of 4 ?m or more and whose number is 100 or less per 100 mm2, nitride inclusions having a size of less than 4 ?m and whose number is 700 or less per 100 mm2, oxide inclusions having a size of 4 ?m or more and whose number is 60 or less per 100 mm2, and oxide inclusions having a size of less than 4 ?m and whose number is 500 or less per 100 mm2, in a cross section perpendicular to a rolling direction.

Abstract: A seamless steel pipe having a specific chemical composition. A ratio of Ti to N is in a range of 2.0 to 5.0. The steel pipe has a microstructure including tempered martensite having a volume ratio of 95% or more, and prior austenite grains having a grain size number of 8.5 or more. In a cross-section perpendicular to a rolling direction of the steel pipe a number of nitride-based inclusions having a particle size of 4 um or more is 100 or less per 100 mm2, a number of nitride-based inclusions having a particle size of less than 4 ?m is 1000 or less per 100 mm2, a number of oxide-based inclusions having a particle size of 4 ?m or more is 40 or less per 100 mm2, and a number of oxide-based inclusions having a particle size of less than 4 ?m is 400 or less per 100 mm2.

Abstract: A round bar tensile test specimen for a sulfide stress corrosion cracking test of a steel, the specimen including a parallel section, a shoulder section, and a grip section. A sectional shape of the shoulder section being formed by a curve having two or more radii of curvature. A radius of curvature R1 (mm) of a portion of the curve adjacent to the parallel section being 15 mm or more, and the radius of curvature R1 (mm) satisfying (0.22??119)?R1?100 in terms of load stress ? (MPa) of the sulfide stress corrosion cracking test. A length X1 (mm) of the portion of the curve having the radius of curvature R1 in a longitudinal direction of the test specimen satisfying X1??{(r/8)×(R1?r2/4)}. Additionally, other radii of curvature of the curve being smaller than the radius of curvature R1.

Abstract: A high-tensile steel plate has a chemical composition containing, by mass, specific amounts of C, Mn, Si, P, S, Al, Ni, B, N, one or more elements selected from Cr, Mo, V, Cu, Ti, and Ca as needed, Ceq?0.80, and a center-segregation zone hardness index HCS satisfying 5.5[C]4/3+15[P]+0.90[Mn]+0.12[Ni]+0.53[Mo]?2.5. The hardness of a center-segregation zone satisfies HVmax/HVave?1.35+0.006/C?t/750. A steel having the above-described chemical composition is subjected to hot rolling at a specific slab-heating temperature at a specific rolling reduction ratio, subsequently reheated, cooled at a cooling rate of 0.3° C./s or more until the temperature of a central portion in a plate-thickness direction reaches 350° C. or less, and tempered to a specific temperature range.

Abstract: A high-strength seamless steel pipe for oil country tubular goods comprising, by mass %, C: 0.20% to 0.50%, Si: 0.05% to 0.40%, Mn: 0.3% to 0.9%, P: 0.015% or less, S: 0.005% or less, Al: 0.005% to 0.1%, N: 0.006% or less, Mo: more than 1.0% to 3.0% or less, V: 0.01% to less than 0.05%, Nb: 0.001% to less than 0.01%, B: 0.0003% to 0.0030%, O: 0.0030% or less, and Ti: 0.003% to 0.025%, and wherein Ti/N: 2.0 to 5.0 is satisfied, a volume fraction of a tempered martensitic is 95% or more, prior austenite grains have a grain size number of 8.

Abstract: A high-strength seamless steel pipe for oil country tubular goods comprising, by mass %, C: 0.20% to 0.50%, Si: 0.05% to 0.40%, Mn: more than 0.6% to 1.5% or less, P: 0.015% or less, S: 0.005% or less, Al: 0.005% to 0.1%, N: 0.006% or less, Mo: more than 1.0% to 3.0% or less, V: 0.05% to 0.3%, Nb: 0.001% to 0.020%, B: 0.0003% to 0.0030%, O: 0.0030% or less, and Ti: 0.003% to 0.025%, and wherein Ti/N: 2.0 to 5.0 is satisfied, a volume fraction of a tempered martensitic is 95% or more, prior austenite grains have a grain size number of 8.

Abstract: The present invention provides a high tensile strength steel plate having a chemical composition containing, in percent by mass, 0.03% to 0.12% of C, 0.01% to 0.30% of Si, 0.5% to 1.95% of Mn, 0.008% or less of P, 0.005% or less of S, 0.015% to 0.06% of Al, 0.011% to 0.05% of Nb, 0.005% to 0.02% of Ti, 0.001% to 0.006% of N, 0.0005% to 0.003% of Ca, optionally, one or two or more of Cr, Mo, V, Cu, and Ni, in which Ceq is 0.44 or less, Ti/N is 1.5 to 3.5, and parameter formulas composed of specific elements for controlling the sulfide morphology and the degree of center segregation in the steel are satisfied, and the balance being Fe and incidental impurities, in which the hardness of the center segregation area of the steel sheet is further specified.

Abstract: Provided is a high-strength seamless steel pipe having the composition which contains, by mass %, 0.20 to 0.50% C, 0.05 to 0.40% Si, 0.3 to 0.9% Mn, 0.015% or less P, 0.005% or less S, 0.005 to 0.1% Al, 0.008% or less N, more than 0.6% and 1.7% or less Cr, more than 1.0% and 3.0% or less Mo, 0.01 to 0.30% V, 0.001% or more and less than 0.01% Nb, 0.0003 to 0.0030% B, and 0.0030% or less O (oxygen). The high-strength seamless steel pipe has the microstructure where a volume fraction of a tempered martensitic phase is 95% or more, and prior austenitic grains have a grain size number of 8.5 or more, and a segregation degree index Ps which is defined by a formula Ps=8.1 (XSi+XMn+XMo)+1.2XP is set to less than 65.

Abstract: A thick-walled high-strength steel plate with excellent low-temperature toughness (Charpy impact and CTOD properties of a weld bond) in a multilayer weld zone, and a method for manufacturing the steel plate.

Abstract: Provided is a high-strength seamless steel pipe having the composition which contains, by mass %, 0.20 to 0.50% C, 0.05 to 0.40% Si, 0.3 to 0.9% Mn, 0.015% or less P, 0.005% or less S, 0.005 to 0.1% Al, 0.008% or less N, 0.6 to 1.7% Cr, 0.4 to 1.0% Mo, 0.01 to 0.30% V, 0.01 to 0.06% Nb, 0.0003 to 0.0030% B, and 0.0030% or less O (oxygen). The high-strength seamless steel pipe has the microstructure where a volume fraction of a tempered martensitic phase is 95% or more, and prior austenitic grains have a grain size number of 8.5 or more, and a segregation degree index Ps which is defined by a formula Ps=8.1 (XSi+XMn+XMo)+1.2XP relating to XM which is a ratio between a segregated portion content and an average content is set to less than 65.

Abstract: A thick steel plate having a chemical composition containing, by mass %, C: 0.200% or more and 0.350% or less, Si: 0.05% or more and 0.45% or less, Mn: 0.50% or more and 2.00% or less, P: 0.020% or less, S: 0.005% or less, Al: 0.005% or more and 0.100% or less, at least one of Cu, Ni, Cr, Mo, V, Nb, Ti, B, REM, Ca, and Mg, and Fe and inevitable impurities. The thick steel plate being such that CI, which is defined by a particular equation, is 40 or more. Additionally, the thick steel plate having a microstructure in which the area fraction of a bainite phase is 60% or more, the area fraction of Martensite-Austenite constituent is 5% or more and less than 20%, and the remaining constituent phases are at least one of a ferrite phase, a pearlite phase, and a martensite phase.

Abstract: A round bar tensile test specimen for a sulfide stress corrosion cracking test of a steel, the specimen including a parallel section, a shoulder section, and a grip section. A sectional shape of the shoulder section being formed by a curve having two or more radii of curvature. A radius of curvature R1 (mm) of a portion of the curve adjacent to the parallel section being 15 mm or more, and the radius of curvature R1 (mm) satisfying (0.22??119)?R1?100 in terms of load stress ? (MPa) of the sulfide stress corrosion cracking test. A length X1 (mm) of the portion of the curve having the radius of curvature R1 in a longitudinal direction of the test specimen satisfying X1??{(r/8)×(R1?r2/4)}. Additionally, other radii of curvature of the curve being smaller than the radius of curvature R1.

Abstract: A high-tensile-strength steel plate is provided with a new chemical composition design that guarantees the same properties as a 50 mm thick steel plate even in a steel plate with a thickness of 100 mm or greater, without the yield stress being affected by the plate thickness. By mass %, the chemical composition includes C: 0.02% to 0.08%, Si: 0.01% to 0.35%, Mn: 1.4% to 2.0%, P: 0.007% or less, S: 0.0035% or less, Al: 0.010% to 0.060%, Ni: 0.5% to 2.0%, Mo: 0.10% to 0.50%, Nb: 0.005% to 0.040%, Ti: 0.005% to 0.025%, B: less than 0.0003%, N: 0.002% to 0.005%, Ca: 0.0005% to 0.0050%, and O: 0.003% or less, with the components additionally satisfying a predetermined relationship.

Abstract: An abrasion resistant steel plate having excellent low-temperature toughness and excellent cracking resistance in a portion which has been heated to a temperature range in which low-temperature temper embrittlement occurs and a method for manufacturing the steel plate. The steel plate includes a microstructure at positions located at ¼ of the thickness and at ¾ of the thickness including a martensite single phase microstructure having an average prior austenaite grain diameter in the range of 20 ?m to 60 ?m, or a mixed microstructure of martensite and bainite having a proportion of martensite-austenite constituent of less than 5% in terms of area ratio with respect to the whole microstructure.

Abstract: A thick-walled high-strength steel plate with excellent low-temperature toughness (Charpy impact and CTOD properties of a weld bond) in a multilayer weld zone, and a method for manufacturing the steel plate.

Abstract: The present invention provides a high tensile strength steel plate having a chemical composition containing, in percent by mass, 0.03% to 0.12% of C, 0.01% to 0.30% of Si, 0.5% to 1.95% of Mn, 0.008% or less of P, 0.005% or less of S, 0.015% to 0.06% of Al, 0.011% to 0.05% of Nb, 0.005% to 0.02% of Ti, 0.001% to 0.006% of N, 0.0005% to 0.003% of Ca, optionally, one or two or more of Cr, Mo, V, Cu, and Ni, in which Ceq is 0.44 or less, Ti/N is 1.5 to 3.5, and parameter formulas composed of specific elements for controlling the sulfide morphology and the degree of center segregation in the steel are satisfied, and the balance being Fe and incidental impurities, in which the hardness of the center segregation area of the steel sheet is further specified.