Abstract: In a high-tensile steel plate according to the invention, the carbon equivalent Pcm represented in Expression (1) is from 0.180% to 0.220%, the surface hardness is a Vicker's hardness of 285 or less, the ratio of a Martensite Austenite constituent in the surface layer is not more than 10%, the ratio of a mixed structure of ferrite and bainite inside beyond the surface layer is not less than 90%, the ratio of the bainite in the mixed structure is not less than 10%, the thickness of the lath of bainite is not more than 1 ?m, the length of the lath is not more than 20 ?m, and the segregation ratio as the ratio of the Mn concentration in the center segregation part relative to the Mn concentration at a part in a depth equal to ¼ of the thickness of the plate from the surface is not more than 1.3. Pcm=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5B . . . (1) where the element symbols in Expression (1) represent the % by mass of the respective elements.

Abstract: In a high-tensile steel plate according to the invention, the carbon equivalent Pcm represented in Expression (1) is from 0.180% to 0.220%, the surface hardness is a Vicker's hardness of 285 or less, the ratio of a Martensite Austenite constituent in the surface layer is not more than 10%, the ratio of a mixed structure of ferrite and bainite inside beyond the surface layer is not less than 90%, the ratio of the bainite in the mixed structure is not less than 10%, the thickness of the lath of bainite is not more than 1 ?m, the length of the lath is not more than 20 ?m, and the segregation ratio as the ratio of the Mn concentration in the center segregation part relative to the Mn concentration at a part in a depth equal to ¼ of the thickness of the plate from the surface is not more than 1.3. Pcm=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5B . . . (1) where the element symbols in Expression (1) represent the % by mass of the respective elements.

Abstract: A steel plate for submerged arc welding having good low temperature toughness at a fusion line vicinity part and a boundary with the base material in HAZ as well as at the base material and the weld metal is provided. The steel plate for submerged arc welding according to the invention contains, by mass, 0.03% to 0.09% C, 1.5% to 2.5% Mn, 0.005% to 0.025% Nb, 0.005% to 0.02% Ti, 0.01% to 0.06% Al, at most 0.0005% B, 0.001% to 0.008% N, at most 0.015% P, at most 0.015% S, and at most 0.006% O, and the balance consists of Fe and impurities. Therefore, the steel plate according to the present invention has good low temperature toughness not only at the fusion line vicinity part and the boundary with a base material in the HAZ but also in the base material and weld metal.

Abstract: A high-strength welded steel pipe having a pipe base metal with a tensile strength of at least 760 MPa and which prevents the occurrence of transverse cracks in weld metal without carrying out preheating or postheating has a seam weld having weld metal with a tensile strength of at least 780 MPa and an average grain diameter of prior austenite in the weld metal of an inner seam weld of at least 90 ?m and at most 150 ?m. The pipe base metal contains C: 0.02-0.12%, Si: 0.01-0.50%, Mn: 0.4-2.5%, P: at most 0.015%, S: at most 0.003%, Nb: 0.005-0.10%, Al: 0.005-0.06%, and optionally at least one of Cu, Ni, Cr, Mo, V, and B, and the inner seam weld metal contains C: 0.02-0.12%, Si: 0.05-0.50%, Mn: 0.4-2.5%, Cr, Mo, and Ni: 0.1-3.0% each, Ti: 0.005-0.050%, and Al: 0.005-0.050%.

Abstract: A high-tensile-strength steel having excellent toughness throughout its thickness, excellent properties at welded joints, and a tensile strength (TS) of at least about 900 MPa (130 ksi), and a method for making such steel, are provided. Steels according to this invention preferably have the following composition based on % by weight: carbon (C): 0.02% to 0.1%; silicon (Si): not greater than 0.6%; manganese (Mn): 0.2% to 2.5%; nickel (Ni): 0.2% to 1.2%; niobium (Nb): 0.01% to 0.1%; titanium (Ti): 0.005% to 0.03%; aluminum (Al): not greater than 0.1%; nitrogen (N): 0.001% to 0.006%; copper (Cu): 0% to 0.6%; chromium (Cr): 0% to 0.8%; molybdenum (Mo): 0% to 0.6%; vanadium (V): 0% to 0.1%; boron (B): 0% to 0.0025%; and calcium (Ca): 0% to 0.006%. The value of Vs as defined by Vs=C+(Mn/5)+5P−(Ni/10)−(Mo/15)+(Cu/10) is 0.15 to 0.42. P and S among impurities are contained in an amount of not greater than 0.015% and not greater than 0.003%, respectively.

Abstract: High-tensile-strength steel having excellent arrestability and a TS of not less than 900 MPa, as well as a method of manufacturing the same. The steel of the invention has the following composition (% by weight): C: 0.02% to 0.1%; Si: not greater than 0.6%; Mn: 0.2% to 2.5%; Ni: greater than 1.2% but not greater than 2.5%; Nb: 0.01% to 0.1%; Ti: 0.005% to 0.03%; N: 0.001% to 0.006%; Al: not greater than 0.1%; and optional elements. Ceq of the B-free steel is 0.53-0.7%, and Ceq of the B-bearing steel is 0.4-0.58%. The microstructure of the steel may be a mixed structure of martensite (M) and lower bainite (LB) occupying at least 90 vol. % in the microstructure, LB occupying at least 2 vol. % in the mixed structure, and the aspect ratio of prior austenite grains is not less than 3.

Abstract: High-tensile-strength steel having excellent arrestability and a TS of not less than 900 MPa, as well as a method of manufacturing the same. The steel of the invention has the following composition (% by weight): C: 0.02% to 0.1%; Si: not greater than 0.6%; Mn: 0.2% to 2.5%; Ni: greater than 1.2% but not greater than 2.5%; Nb: 0.01% to 0.1%; Ti: 0.005% to 0.03%; N: 0.001% to 0.006%; Al: not greater than 0.1%; and optional elements. Ceq of the B-free steel is 0.53-0.7%, and Ceq of the B-bearing steel is 0.4-0.58%. The microstructure of the steel may be a mixed structure of martensite (M) and lower bainite (LB) occupying at least 90 vol. % in the microstructure, LB occupying at least 2 vol. % in the mixed structure, and the aspect ratio of prior austenite grains is not less than 3.