Patent ID: 8084144
Filing Date: 2011-12-27
Classification: B21C,B23K,C21D,C22C,Y10T

Abstract:
1. High strength thick line pipe use welded steel pipe, superior in low temperature toughness, the 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%, Mo: 0.10 to 1.50%, B: 0.0003 to 0.0030%, O: 0.0001 to 0.0080%, and a balance of iron and unavoidable impurities, wherein P and Al are present at most as impurities, wherein P is limited to 0.050% or less, and Al is limited to 0.020% or less, and wherein Ceq found by the formula is 0.30 to 0.53, Pcm found by the formula is 0.10 to 0.20, and C, Si, Mn, Ni, Cu, Cr, Mo, V, B, and Al are amounts of the 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 of a weld heat affected zone of no more than 150 μm.