| 摘要 |
PURPOSE:To obtain a high-toughness and high-strength bent steel pipe by submerged arc welding by specifying in the gross heat treatment conditions, rolling conditions, weld metal components, bending conditions and heat treatment conditions after working from the compsn. of stock. CONSTITUTION:Hot rolling is started by heating the steel consisting of 0.02-0.10% (wt%, hereinafter the same) C, <=0.20% S, <=1.60% Mn, <=0.10% Mo, 0.008-0.020% Ti, 0.0008-0.004% B, the balance Fe and impurities and having 0.25-0.35% carbon equiv. Ceq determined from the formula to the AC3 point - 1,280 deg.C. The plate is allowed to cool after the rolling is ended at 690-800 deg.C or the plate is forcibly cooled at 5-50 deg.C/sec rate. The straight welded steel pipe of which the weld metal in the seam part contains <=0.07% Mo, <=300ppm O and <=50ppm N and the carbon equiv. determined by the formula is 0.28-0.38% is formed from the steel plate obtd. in the above-mentioned manner. The steel pipe is heated within 5min at the AC3 point - (AC3 point + 80 deg.C) and is bent. The bent pipe is tempered at the temp. of the AC1 or below after hardening.
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