PURPOSE: To manufacture a seamless low alloy free-cutting steel tube without generation of defects by limiting a max. heating temp., a piercing ratio, and an outside surface temp. after piercing of a leaded free-cutting steel billet.

CONSTITUTION: A leaded free-cutting steel billet made by containing 0.04W0.3% lead in a low alloy steel is heated to a max. temp. in the range from 1180 to 1250°C. Then, by a diagonal rolling mill, piercing of the billet is finished in the outside surface temp. range of 1120W1240°C by setting a piercing ratio of ≤2.4. The reason to limit a heating temp. of ≥1180°C is considerations for a service life of a tool, prevention of the front and rear end cracks of a hollow stock after piercing and tear on the inside surface, and keeping of hot workability in a succeeding drawing stage. On the other hand, if a piercing temp. exceeds 1250°C, hole or front and rear end cracks are generated in a drawing stage, so that pulling out of a mandrel bar becomes difficult after rolling. If an outside surface temp. at finishing of piercing is <1120°C, hole or cracks are generated, so that a rolling load increases. If an outside surface temp. exceeds 1240°C, yield reduction or mill troubles occur.