High-strength welded steel pipe for airbag inflator and method for manufacturing the same (as amended)

A high-strength welded steel pipe for airbag inflators, the welded steel pipe comprising:

• a tubular base material portion;
• and
• a welded portion formed in the base material portion and extending in an axial direction of the base material portion, wherein,
• the base material portion has a composition containing, in mass %,
• C:
• 0.02 to 0.08%, Si:
• 0.001 to 1.0%,
• Mn:
• 0.1 to 2.0%, P:
• 0.1% or less,
• S:
• 0.01% or less, Al:
• 0.01 to 0.1%,
• N:
• 0.01% or less, Ti:
• 0.01 to 0.20%,
• V:
• 0.01 to 0.50%, and Cr:
• 0.01 to 0.50%,
• with the balance including Fe and incidental impurities,
• the base material portion has a structure that includes a ferrite phase having an average grain size of 10 μm or less at an area fraction of 90% or more and a Ti, V-based carbide having an average grain size of 10 nm or less and dispersed in the ferrite phase,
• a high tensile strength TS of 780 MPa or more, a strength-elongation balance is TS×El of 15,000 MPa % or more, and the difference ΔHV in Vickers hardness between the base material portion and the welded portion is 60 points or less, and
• a softened portion has a softened width Ws in a circumferential direction of the welded steel pipe being 0.05 mm or less, the softened portion having a Vickers hardness different from the Vickers hardness of the base material portion by at least 30 points.