To suppress the occurrence of flaws at the time of tube making when Mannesmann piercing is mainly applied to diffucult-to-work materials which is inferior in hot workability and taken as being unsuitable to the material for seamless steel tubes or base stocks bad in quality in the center part of which porous cavity or the like is present because they remains as-cast.
In a method for manufacturing the seamless steel tubes by a skew piercing method using billets consisting of the difficult-to-work material or a material which is as-cast, the rolling is executed by keeping a piercing parameter PP positive, which is shown by the next equation, that is, PP =-0.00012BT+(1.6856×10-5×Dr)-(-0.000327×Vr)+0.0255 (Where, BT is the diameter of the billet of the base stock (mm), Dr is the diameter of the main roll for piercing (mm) and Vr is the number of revolution of the main roll for piercing (rpm).). Thus, by keeping the tip draft of a plug at ≥95%, the occurrence of the flaws on the inside surface of the tube is suppressed.
ARIIZUMI TAKASHI
AZUMA SHOZO
ONO KINUMASA
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