| 摘要 |
PURPOSE: To provide a weld heat-affected zone with seawater corrosion resistance and to properly secure the toughness, weldability, and wear resistance of a base material by forming a welded steel tube by using a steel sheet prepared by applying controlled rolling and controlled cooling to an extra low C-Cr-Mn type low alloy steel of specific composition. CONSTITUTION: A steel, having a composition consisting of, by weight ratio, 0.03-0.07% C, 0.05-0.5% Si, 0.5-2.0% Mn, <=0.03% P,η0.005% S, 0.1-0.3% Ni, 0.8-1.5% Cr, 0.0005-0.005% Ca, and the balance essentially Fe. Moreover, if necessary, small amounts of Mo, Nb, V, Ti, and B are further added selectively, is used. A slab of steel with this composition is heated to >=900 deg.C and hot-rolled under the conditions of a draft in an unrecrystallized temp. region of >=30% and a finishing temp. not lower than the Ar3 transformation point. Subsequently, the resulting hot rolled steel plate is subjected to accelerated cooling from a temp. of >=(Ar3 transformation point -50 deg.C) down to <=600 deg.C at a rate of 1 deg.C/sec. Then, the steel plate is cold-formed and formed into welded steel tube.
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