Abstract: A screw coupling joint having a sealing portion between an outer circumference formed in convex in an axial direction of the coupling joint at the end of the male screw portion and an inner circumference formed in taper at an inner part of the female screw portion. The female screw portion has a stopper end positioned to be contacted, when the coupling joint is assembled, by the inner end of the male screw portion. The taper is between 1.degree. and 3.degree. and the inner diameter D.sub.1 of the female screw and the outer diameter D.sub.2 of the male screw have the relationship D.sub.2 >D.sub.1 at a tangent point therebetween. The end of the male screw portion and the end of the stopper are vertical or perpendicular to the axis of the coupling joint.

Abstract: In a plug mill, the plug is contained in a container supported by a carriage which is moved from the plug recovering position to the inlet guide provided in front of the mill along a straight transfer rail positioned at an angle with respect to the pass line of the mill and at a speed sufficient to impart a predetermined moment of inertia to the plug. When the carriage is advanced to a position near the inlet guide, the carriage is rapidly stopped to eject and supply the plug to the front end of the mandrel bar by utilizing the moment of inertia.