Patent ID: 7601231

Claim:
A high-strength steel pipe having a high low temperature toughness reliability and a high weld heat-affected zone toughness, comprising: a seam-welded portion having multiple layers obtained by applying multi-layer welding, wherein the portion is at least twice heated; and a base steel portion containing, in mass: C: 0.02 to 0.07%, Si: not more than 0.6%, Mn: 1.5 to 2.5%, P: not more than 0.015%, S: not more than 0.003%, Ni: 0.01 to 2.0%, Mo: 0.1 to 0.8%, Nb: 0.001% to 0.005% Ti: 0.005% to 0.030%, B: 0.0003 to 0.0030%, Al: not more than 0.070%, and N: not more than 0.0060%, so as to satisfy the expression Ti−3.4N≧0, wherein a balance of the base steel portion consists of Fe and unavoidable impurities, and the base steel portion does not contain Mg, wherein a P value of the steel is provided in a range of 2.5 and 4.0, wherein a microstructure of the steel being composed of martensite and bainite in the range from 90 to 100% in terms of a bainite and martensite fraction, and wherein the P value is defined by: P= 2.7C+0.4Si+Mn+0.8Cr+0.45(Ni+Cu)+2V+1.5Mo, so that the mother alloy of the pipe has a compositionally-derived high low temperature toughness reliability and the at least twice heated seam-welded portion has a compositionally-derived high heat-affected zone toughness after multiple welding-heatings, and wherein the high-strength steel pipe has tensile strength of not lower than 800 MPa and Charpy absorbed energy of the base steel is not lower than 200J at −40° C.