PURPOSE: To improve the durability of a piercing plug in manufacture of a seamless steel pipe and to prolong its life.
CONSTITUTION: A hollow base stock is made by hot piercing a billet consisting of alloy steel with the piercing plug and the seamless steel tube is manufactured by rolling the obtained hollow base stock. Before executing hot piercing with the plug, a steel plate is preliminarily joined to at least the tip face on the side to be pierced of the billet so as not to be projected to the outside of the outer peripheral face of the billet in the longitudinal and orthogonal directions of the billet and the cross-sectional area C of the steel plate satisfies the next formula. S×0.2≤C≤S×1.2... Formula. Where, S: (the radius of the bottom part of the plug)2×number π, C: the cross-sectional area of the steel plate.
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