Patent ID: 8764918
Filing Date: 2014-07-01
Classification: B23K,C21D,C22C,F16L

Abstract:
1. A high strength steel pipe for line pipe superior in low temperature toughness characterized by comprising a steel plate shaped into a tube with seam portions of the steel plate welded by one layer each at the inner side and outer side, wherein a base metal of the steel pipe has a chemical composition containing, by mass %, C: 0.020 to 0.080%, Si: 0.01 to 0.50%, Mo: 0.01 to 0.09%, Al: 0.0005 to 0.030%, and Nb: 0.013 to 0.030% in a range of C+0.25Si+0.1Mo+Al+Nb: 0.100% or less further containing, Mn: 1.50 to 2.50%, Ti: 0.003 to 0.030%, and B: 0.0001 to 0.0030%, and limiting P: 0.020% or less and S: 0.0030% or less, with the balance of Fe and unavoidable impurities, a mixture of austenite and martensite present along prior austenite grain boundaries of reheated parts of a heat affected zone having a width of 10 μm or less and a length of 50 μm or less and wherein the weld metal has a chemical composition containing, by mass %, C: 0.010 to 0.100%, Si: 0.01 to 0.50%, Mn: 1.00 to 2.00%, Ni: 1.30 to 3.20%, Al: 0.0005 to 0.100%, Ti: 0.003 to 0.050%, and O: 0.0001 to 0.0500% further containing a total of one or more of Cr, Mo, and V: 1.00 to 2.50%, limiting P: 0.020% or less and S: 0.0100% or less with the balance of Fe and unavoidable impurities.