High strength hot-rolled steel sheet for welded steel line pipe having excellent souring resistance, and method for producing same

A method for manufacturing a thick high-strength hot-rolled steel sheet for electric resistance welded line pipe having excellent sour resistance and a yield strength of 450 MPa or more, the method comprising heating a steel material, subjecting the steel material to hot rolling comprising rough rolling and finish rolling to manufacture a hot-rolled sheet, cooling the hot-rolled sheet after the hot rolling is finished, and then coiling the hot-rolled sheet, wherein the steel material has a composition containing 0.01% to 0.07% C, 0.40% or less Si, 0.5% to 1.4% Mn, 0.015% or less P, 0.003% or less S, 0.1% or less Al, 0.01% to 0.15% Nb, 0.1% or less V, 0.03% or less Ti, 0.3% or less Mo, 0.5% or less Cu 0.5% or less Ni, and 0.008% or less N on a mass basis, the remainder being Fe and inevitable impurities;

• Nb, V, and Ti satisfy the following inequality (1);
• Cm defined by the following equation (2) satisfies 0.12 or less;
• the heating temperature before the hot rolling ranges from 1,100° C. to 1,250° C.;
• the finish rolling is one in which the total rolling reduction at a temperature of 930° C. or lower is 40% to 85% and the finish rolling temperature is 760° C. to 870° C.;
• the cooling is performed to a cooling stop temperature of 500° C. or lower in terms of surface temperature at an average cooling rate of 30° C./s to 200° C./s in terms of thickness-wise center temperature, followed by natural cooling for more than 10 s after the cooling is stopped;
• and the coiling is performed at a coiling temperature of 400° C. to 620° C.;
• Nb+V+Ti<0.15 (1)
• Cm=C+Si/30+(Mn+Cu)/30+Ni/60+Mo/7+V/10 (2)
• where Nb, V, Ti, C, Si, Mn, Cu, Ni, Mo, and V are the content (mass percent) of each element.