| 摘要 |
PROBLEM TO BE SOLVED: To provide a square steel tube the outside dimension of formed thickened-part of which is made equal and whose outside surface is smoothed. SOLUTION: After heating the objective part of a tube body 1 with a heating means 20, the thickened part 2 is formed in the objective part by imparting compressive force in the longitudinal direction A to the tube body 1. After heating the whole tube body 1 on which the thickened part 2 is formed, the steel square tube is obtained by performing hot forming. By performing the hot forming, the outside dimension is equalized, also smoothed and the square steel tube which is in the fine state where there is no projecting member on the outside of the part where the thickened part is formed is made. By heating the whole, the residual stress of steel and deterioration of quality are removed and the tube is made into the square steel tube the residual strain by the heat of the steel material of which is sissolved. When the square steel tube is used for a strut, for example, and a beam is connected to the thickend part, because the outside surface of the thickened part is equalized in the outside dimension and smoothed, a clearance is not much remove between both or is uniform and small, so good welding (connection) is performed and the accuracy and the strength of welding are improved.
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