Patent ID: 7879287

Claim:
A hot-rolled steel sheet for a high-strength electric-resistance welded pipe having sour-gas resistance and excellent weld toughness, comprising: 0.03% to about 0.06% C; about 0.05% to about 0.50% Si; about 0.5% to about 1.5% Mn; about 0.010% or less P; about 0.0010% or less S; about 0.01% to about 0.10% Al; about 0.01% to about 0.10% Nb; about 0.001% to about 0.025% Ti; about 0.001% to about 0.005% Ca; about 0.003% or less O; about 0.005% or less N, about 0.0001% or less B, and at least one element selected from the group consisting of about 0.01% to about 0.10% V; about 0.01% to about 0.50% Cu; about 0.01% to about 0.50% Ni; and about 0.01% to about 0.50% Mo on the basis of mass, wherein C, Si, Mn, Cu, Ni, Mo, and V satisfy Px given by Relationship 1: Px =(C)+(Si)/30+((Mn)+(Cu))/20+(Ni)/60+(Mo)/7+(V)/10≦0.17 (1) where (M) indicates the content by mass percent of an element M; Ca, O, and S satisfy Py given by Relationship 2: Py ={(Ca)−(130×(Ca)+0.18)×(O)}/(1.25×(S)) 1.2 ≦Py ≦3.6 (2) where (M) indicates the content by mass percent of an element M; the balance is Fe and incidental impurities; and the microstructure of the steel sheet is composed of about 95% by volume or more bainitic ferrite; the ratio of Nb precipitation in the steel sheet is from about 30% to about 70% by mass with respect to the total Nb content.