Abstract: A high-strength welded steel pipe is obtained by welding a seam weld portion of a steel plate that are formed in a pipe shape. In the high-strength welded steel pipe, a base metal of the steel plate includes, by mass %, C: 0.010% to 0.080%, Si: 0.01% to 0.50%, Mn: 0.50% to 2.00%, S: 0.0001% to 0.0050%, Ti: 0.003% to 0.030%, Mo: 0.05% to 1.00%, B: 0.0003% to 0.0100%, O: 0.0001% to 0.0080%, N: 0.006% to 0.0118%, P: limited to 0.050% or less, Al: limited to 0.008% or less, and the balance of Fe and inevitable impurities, Ceq is 0.30 to 0.53, Pcm is 0.10 to 0.20, [N]?[Ti]/3.4 is less than 0.003, the average grain size of the prior ? grains in heat affected zones in the steel plate is 250 ?m or less, and the prior ? grains include bainite and intragranular bainite.

Abstract: A high-strength welded steel pipe is obtained by welding a seam weld portion of a steel plate that are formed in a pipe shape. In the high-strength welded steel pipe, a base metal of the steel plate includes, by mass %, C: 0.010% to 0.080%, Si: 0.01% to 0.50%, Mn: 0.50% to 2.00%, S: 0.0001% to 0.0050%, Ti: 0.003% to 0.030%, Mo: 0.05% to 1.00%, B: 0.0003% to 0.0100%, O: 0.0001% to 0.0080%, N: 0.006% to 0.0118%, P: limited to 0.050% or less, Al: limited to 0.008% or less, and the balance of Fe and inevitable impurities, Ceq is 0.30 to 0.53, Pcm is 0.10 to 0.20, [N]—[Ti]/3.4 is less than 0.003, the average grain size of the prior ? grains in heat affected zones in the steel plate is 250 ?m or less, and the prior ? grains include bainite and intragranular bainite.

Abstract: The present invention provides high strength steel plate with excellent low temperature toughness, high strength steel pipe using this as a base metal, and methods of production of the same. The steel plate of the present invention contains Mo: 0.05 to 1.00% and B: 0.0003 to 0.0100%, has a Ceq of 0.30 to 0.53, has a Pcm of 0.10 to 0.20, and has a metal structure which has an area percentage of polygonal ferrite of 20 to 90% and has a balance of a hard phase comprised of one or both of bainite and martensite. To obtain this steel plate, strain-introducing rolling is performed with a start temperature of not more than Ar3+60° C., an end temperature of Ar3 or more, and a reduction ratio of 1.5 or more, then the plate is air-cooled and then acceleratedly cooled from Ar3?100° C. to Ar3?10° C. in temperature by 10° C./s or more.

Abstract: The present invention provides high strength steel plate with excellent low temperature toughness, high strength steel pipe using this as a base metal, and methods of production of the same. The steel plate of the present invention contains Mo: 0.05 to 1.00% and B: 0.0003 to 0.0100%, has a Ceq of 0.30 to 0.53, has a Pcm of 0.10 to 0.20, and has a metal structure which has an area percentage of polygonal ferrite of 20 to 90% and has a balance of a hard phase comprised of one or both of bainite and martensite. To obtain this steel plate, strain-introducing rolling is performed with a start temperature of not more than Ar3+60° C., an end temperature of Ar3 or more, and a reduction ratio of 1.5 or more, then the plate is air-cooled and then acceleratedly cooled from Ar3?100° C. to Ar3?10° C. in temperature by 10° C./s or more.