To prevent herringbone(rough surface), which is generated on an inner surface of a hot-extruded seamless steel tube when extruding the tube, and especially to prevent the generation of herringbone in hot-extruded seamless stainless steel tube having a high working degree.
In a manufacturing method for the hot-extruded seamless steel tube, a heated hollow billet, whose inner and outer surfaces are coated with glass powder as a lubricant, is inserted into a container, a mandrel 30, which has a diameter approximately equal to the inner diameter of the steel tube to be extruded, is mounted to a ram, and the hollow billet is extruded through a gap formed between the bore diameter of a die 20 and the outer diameter of the mandrel 30. A friction coefficient ratio, which is a ratio of a friction coefficient of the billet-mandrel to that of the billet-die, is 0.1-2.0.
SHO ATSUSHI
NISHIKAWA KIMISUKE
Mizuno Katsufumi