High strength, high toughness, seamless steel pipe for line pipe

A method for producing a high-strength, high-toughness seamless steel pipe used for a line pipe, a material for the steel pipe being a steel comprising 0.03 to 0.06% of C, 0.05 to 0.15% of Si, 1.6 to 2.0% of Mn, 0.010 to 0.10% of Al, 0.3 to 0.7% of Ni, 0.10 to 0.40% of Mo, 0.01 to 0.06% of V, 0.003 to 0.03% of Nb, 0.003 to 0.020% of Ti, and 0.0010 to 0.0100% of N, the relationships Mo+5V≧0.4% and 2Nb−V≦0% being satisfied, and the balance being Fe and incidental impurities, the method comprising the steps of:

• heating the material for the steel pipe to the Ac3 point or higher;
• making a pipe by hot rolling;
• performing direct quenching (DQ) of the pipe for cooling the pipe to the Ms point or lower immediately after the pipe-making step;
• and
• tempering the pipe at a temperature lower than the Ac1 point.

A method for producing a high-strength, high-toughness seamless steel pipe used for a line pipe, a material for the steel pipe being a steel comprising 0.03 to 0.06% of C, 0.05 to 0.15% of Si, 1.6 to 2.0% of Mn, 0.010 to 0.10% of Al, 0.3 to 0.7% of Ni, 0.10 to 0.40% of Mo, 0.01 to 0.06% of V, 0.003 to 0,03% of Nb, 0.003 to 0.020% of Ti, and 0.0010 to 0.0100% of N, the relationships Mo+5V≧0.4% and 2Nb−V≦0% being satisfied, and the balance being Fe and incidental impurities, the method comprising the steps of:

• heating the material for the steel pipe to the Ac3 point or higher;
• making a pipe by hot rolling;
• air-cooling the pipe to the vicinity of room temperature;
• performing reheating quenching (RQ) of the pipe for reheating the pipe to the Ac3 point or higher and cooling the pipe to the Ms point or lower;
• and
• tempering the pipe at a temperature range lower than the Ac1 point.