| 摘要 |
Sheet steel for line pipe with a wall thickness of not less than 25 mm, and with excellent toughness and sour resistance is manufactured using steel billets that contain C, Si, Mn, Nb, and Ti, while Al, P, and N content is limited. The billets further contain Ca: 0.001-0.004%, there are limitations such that S: not more than 0.0008% and O: not more than 0.0030%, and the Ca, O, and S content meets the condition [Ca](1 - 124[O])/1.25[S] > 3.0. The billets are heated to a heating temperature (T1) in the range of 1000-1150°C that meets the condition T1 ? -7970/(log([Nb] × [C]) - 3.31) - 170, and are then rough milled, finish rolled, and subjected to accelerated cooling. The finish rolling therein is performed at a finishing temperature of not less than 800°C and at a 950°C or less reduction ratio of not less than 3 so that the plate thickness is not less than 25 mm, the cooling rate of the accelerated cooling is 10-30°C/s and the stop temperature is 200-500°C. Thus, it is possible to have both sour-resistance properties and drop weight tear properties. |
