| 摘要 |
PROBLEM TO BE SOLVED: To provide a welded steel pipe preventing brittle cracks of fused metal by reducing stress concentration to an easily cracking weld heat-affected zone, and utilizing high corrosion resistance of a Zn-Al-Mg alloy plated layer. SOLUTION: The Zn-Al-Mg alloy plated steel pipe is manufactured by molding hot dipped steel plate, in which a Zn-Al-Mg alloy plated layer is formed, the alloy containing 0.05-10 mass % Mg and 4-22 mass % Al, into an open pipe shape, and then welding both cross-direction ends through upset welding. The metal flow angleαof a weld part projected outwardly from the circumference of the steel pipe is controlled by the expressions (1) or (2) shown below depending on the Mg content. The Zn-Al-Mg alloy plated layer contains 0.002-0.1 mass % Ti 0.001-0.045 mass % B and 2.0 mass % Si, as needed. When % Mg <=3 mass %,α<=-8.6×%Mg+75.9...(1) When % Mg >3 mass %,α<=2.9×%Mg+41.2...(2).
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