Welded steel pipe with excellent welded heat-affected zone toughness and process for producing same

A welded steel pipe with excellent welded heat-affected zone toughness, comprising a butt weld formed by prior welding of either an inner surface or an outer surface with a single layer for each of the inner and outer surfaces, wherein in the metallographic structure of a welded heat-affected zone, the martensite-austenite constituent (MA) area fraction is 4% or less, the average prior-austenite grain size is 400 μm or less, PLBZM calculated by the following equation (1) is 1,400 or less, CM1 calculated by the following equation (2) is 13 or less, and CM2 calculated by the following equation (3) is 13 or less:

• PLBZM=1.62D1t+0.84D2t−39t−89R1+81L+1510 Equation (1)
• CM1=0.0012(90−(K1+15))D1 Equation (2)
• CM2=0.0012(90−(K2+15))D2 Equation (3)
• where D1 (μm) is the average prior-austenite grain size of a welded heat-affected zone formed by prior welding, D2 (μm) is the average prior-austenite grain size of a welded heat-affected zone formed by subsequent welding, R1 (mm) is the bead width determined at a position 5 mm apart from the tip of a weld bead formed by subsequent welding, L (mm) is the lap of inside and outside welds, t (mm) is the wall thickness, K1 (°) is the fusion line tilt angle of a weld bead of prior welding, and K2 (°) is the fusion line tilt angle of a weld bead of subsequent welding.