Patent ID: 8039118

Claim:
High strength welded steel pipe for line pipe superior in low temperature toughness, the high strength welded steel pipe comprising steel pipe obtained by seam welding a base material steel plate formed into a pipe shape, wherein said base material steel plate comprises, by mass %, C: 0.010 to 0.050%, Si: 0.01 to 0.50%, Mn: 0.50 to 2.00%, S: 0.0001 to 0.0050%, Ti: 0.003 to 0.030%, O: 0.0001 to 0.0080%, B: 0.0003 to 0.0030%, and a balance of iron and unavoidable impurities, wherein P, Al, and Mo are present at most as impurities, and P is limited to 0.050% or less, Al is limited to 0.020% or less, and Mo is limited to less than 0.10%, and wherein Ceq found from the formula, C eq =C+Mn/6+(Ni+Cu)/15+(Cr+Mo+V)/5, is 0.30 to 0.53, Pcm found by the formula, P cm =C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5B, is 0.10 to 0.20, wherein C, Si, Mn, Ni, Cu, Cr, Mo, V, and B are the amounts of the individual elements in mass %, and wherein a metal structure of said base material steel plate has a polygonal ferrite area ratio of no more than 20%, a bainite area ratio of at least 80%, an effective crystal grain size of no more than 20 μm, and an effective crystal grain size in a weld heat affected zone of no more than 150 μm.