Abstract: UOE steel pipe which has a tensile strength of at least 900 MPa, which does not develop delayed hydrogen cracking of a weld or fracture of the weld during pipe expansion, and which has a weld and a heat affected zone having excellent toughness is provided. The tensile strength of a weld formed on a base metal having a tensile strength of at least 900 MPa is adjusted to be in the range of 95-110% of the tensile strength of the base metal, the oxygen content of the weld metal in the weld is made at most 0.035 mass %, and the weld is preheated to the range of 75-250° C. before carrying out welding from the outer side.

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 method and device for manufacturing UOE steel pipes, wherein, when an opening width of the U-can obtained by U-ing press is larger than the die-diameter in an O-press of later step, while said opening is narrowed from the front end by the closing rolls disposed before said O-press and/or at least at the entrance portion of the O-press to be smaller than the die-diameter of said O-press, the U-can is simultaneously conveyed into the O-press, and then, after charging into the O-press, undergoes O-ing press to yield an O-can. The device is preferably configured to have the closing rolls disposed in a roll stand and consisted of multiple pairs, to have a mechanism to prevent the U-can from moving upward during narrowing the opening width, and to have the function of conveying the U-can. Thus, UOE steel pipes for high-strength application corresponding to X80 grade can be efficiently manufactured.

Abstract: A method and device for manufacturing UOE steel pipes, wherein, when an opening width of the U-can obtained by U-ing press is larger than the die-diameter in an O-press of later step, while said opening is narrowed from the front end by the closing rolls disposed before said O-press and/or at least at the entrance portion of the O-press to be smaller than the die-diameter of said O-press, the U-can is simultaneously conveyed into the O-press, and then, after charging into the O-press, undergoes O-ing press to yield an O-can. The device is preferably configured to have the closing rolls disposed in a roll stand and consisted of multiple pairs, to have a mechanism to prevent the U-can from moving upward during narrowing the opening width, and to have the function of conveying the U-can. Thus, UOE steel pipes for high-strength application corresponding to X80 grade can be efficiently manufactured.

Abstract: The high yield ratio problem intrinsic in high-strength steel pipes is solved and a method of producing a steel pipe, while securing the pipe roundness is provided. A steel pipe produced from a steel plate by forming and welding is subjected to 0.3 to 1.2% of expansion and then to 0.1 to 1.0% of reduction. The steel pipe produced by this method has a structure in which martensite and/or bainite accounts for more than 80% as expressed in terms of area percentage, and has a yield strength of not lower than 551 MPa and a yield ratio of not higher than 93%. The pipe reduction is desirably carried out at a lower work ratio as compared with the pipe expansion.

Abstract: The high yield ratio problem intrinsic in high-strength steel pipes is solved and a method of producing a steel pipe, while securing the pipe roundness is provided. A steel pipe produced from a steel plate by forming and welding is subjected to 0.3 to 1.2% of expansion and then to 0.1 to 1.0% of reduction. The steel pipe produced by this method has a structure in which martensite and/or bainite accounts for more than 80% as expressed in terms of area percentage, and has a yield strength of not lower than 551 MPa and a yield ratio of not higher than 93%. The pipe reduction is desirably carried out at a lower work ratio as compared with the pipe expansion.