Patent ID: 9181609
Filing Date: 2015-11-10
Classification: B21C,C21D,C22C,F17D

Abstract:
1. A welded steel pipe for a linepipe having the composition which contains by mass % 0.02 to 0.06% C, 0.01 to 0.5% Si, 0.8 to 1.6% Mn, 0.012% or less P, 0.0008% or less S, 0.01 to 0.08% Al, 0.005 to 0.050% Nb, 0.005 to 0.025% Ti, 0.0005 to 0.0035% Ca, 0.0020 to 0.0060% N, and Fe and unavoidable impurities as a balance, wherein C(%)−0.065Nb(%) is 0.025 or more, a CP value is 0.95 or less, a Ceq value is 0.28 or more, Ti/N is a value which falls within a range of 1.5 to 4.0, and the steel pipe has metal microstructure where a fraction of bainite is 80% or more, a fraction of martensite-austenite constituent (MA) is 2% or less, and an average grain size of bainite is 5 μm or less, wherein the CP value is expressed by the formula: wherein the Ceq value is expressed by the formula: