To provide a high strength steel sheet for a line pipe which has high strength of an API 5LX65 to 80 class, and further has excellent carbonate stress corrosion cracking resistance.
The stock for rolling having a composition containing 0.01 to 0.04% C, <0.5% Si, 0.2 to 2.00% Mn, ≤0.005% S, 0.01 to 0.10% Al, 0.1 to 0.6% Cu and 0.0003 to 0.0050% B, and the balance Fe with inevitable impurities is heated to the temperature range of the Ac3 transformation point to 1,350°C, and is hot-rolled. After the completion of the hot rolling, the steel sheet is isothermally held in the temperature range of 500 to 800°C for 30 to 7,200 s, or is cooled in the above temperature range at a cooling rate of ≤1°C/s for 30 to 7,200 s, and is thereafter subjected to air cooling or accelerated cooling. Further, the steel sheet can contain one or more kinds selected from Nb, Ti and V, further, one or more kinds selected from Ni, Cr, Mo and W, and further, one or two kinds selected from Ca and rare earth metals.
OKATSU MITSUHIRO
AMANO KENICHI
