Patent ID: 7661284

Claim:
An application method of a hot rolling Cr-plated mandrel bar repeatedly used in mandrel mill rolling by the Mannesmann tube making process, wherein when an opening width H of a surface defect generated on the mandrel bar by its use in the mandrel mill rolling is not less than 1.5 mm and a depth of the surface defect is in the range of 0.3 mm to less than 2.0 mm, a plated film is removed after said surface defect is conditioned, and finish-polishing or surface treatment is performed to the surface of the workpiece, followed by re-plating, thereby enabling the reclaimed mandrel bar to be used again as a tool of like size, and the surface of the mandrel bar is smoothly rounded off in conditioning said surface defect such that a conditioned length L1 (mm) in a longitudinal direction of the surface of the mandrel bar, a conditioned length L2 (mm) in a circumferential direction thereof, and a depth D (mm) of the surface defect thereof satisfy equations (1) and (2): 50 ≦L 1/ D (1), and 20 ≦L 2/ D (2).