High strength welded steel pipe and manufacturing method thereof

The present invention provides a high strength welded steel pipe, wherein the forming a butt welding of the tubular portion of the steel sheet, the steel sheet base material of the mass percent basis, C: 0.010 ~ 0.080%, Si: 0.01 ~ 0.50% , Mn: 0.50 ~ 2.00%, S: 0.0001 ~ 0.0050%, Ti: 0.003 ~ 0.030%, Mo: 0.05 ~ 1.00%, B: 0.0003 ~ 0.0100%, O: 0.0001 ~ 0.0080%, N: 0.006 ~ 0.0118%, the remainder being Fe and unavoidable impurities, the following elements will be limited to P: 0.050% or less, Al: 0.008% or less, the Ceq is 0.30 ~ 0.53, Pcm of 0.10 ~ 0.20, [N] - [Ti] /3.4 below 0.003, an average crystal grain size of primary γ grains of the weld heat affected zone of the steel sheet is 250μm or less, comprising bainite and intragranular bainite within the primary γ grains.