To provide a manufacturing method of a seamless tube that is excellent in the durability of piercing plug.
In this manufacturing method, a billet is arranged on the pass line of roll which are arranged obliquely to the advancing direction of the billet and pierced with piercing plug. The billet is composed of alloy steel containing ≥1wt.% at least one among Cr, Mo and Ni, or an alloy consisting essentially of either of Cr, Mo and Ni. A steel sheet is joined at least on the end face to be pierced of the billet. The same plug is used for piercing the billet, piercing is executed in a stage where piercing and cooling with the plug 1 are alternately repeated and, from start of piercing till forced cooling is executed after completing the piercing, the surface temp. of the plug 1 in the rear part from a position A where is rearer than a distance L1 of ≥10% of the entire length L0 from the tip of the plug 1 is maintained at ≤800°C. Cooling of the plug 1 is executed by making a cooling liquid having the jetting pressure of ≥0.2 kgf/cm2 G collide against the surface of the plug 1 at ≤20° to the vertical direction of the surface of the plug 1.
ARIIZUMI TAKASHI
NAKASEKO MAKOTO
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