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| Method for manufacturing seamless steel pipe or tube |
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Even if the techniques proposed by the Patent Documents 1 to 6 described above are applied, the internal surface defects in the piercing and rolling stage would not necessarily be suppressed in high-Cr seamless steel pipe or tubes containing 10.50 to 14.0% of Cr. That is to say, the technique proposed by the Patent Document 1 does not regulate the condition of the piercing and rolling by a piercing mill. Therefore, the internal fracture flaws would not necessarily be suppressed even if a steel stock with low contents of P and s is used. Although the technique proposed by the Patent Document 2 regulates the condition of the piercing and rolling by a piercing mill, the purpose is only to suppress occurrence of -ferrite by reducing an average strain rate. Therefore, like the technique of the above-mentioned Patent Document 1, it would not necessarily suppress internal fracture flaws. The technique proposed by the Patent Document 3 does not regulate the condition of the piercing and rolling by a piercing mill as it is described that "a general means can be used in the rolling process of a seamless steel pipe or tube. Therefore, the occurrence of internal fracture flaws is often unavoidable". Since the technique proposed by the Patent Document 4 only regulates the above-mentioned content as the condition of the piercing and rolling by a piercing mill, the internal fracture flaws would not necessarily be suppressed. The technique proposed by the Patent Document 5, also, describes that "the rolling process of a seamless steel pipe or tube can be a general manufacturing process of a seamless steel pipe or tube" similar to the above-mentioned Patent Document 3, and does not regulate the condition of the piercing and rolling by a piercing mill. Therefore, the internal fracture flaws are often unavoidable. Since the technique proposed by the Patent Document 6 simply regulates the cross angle and the feed angle as the condition of the piercing and rolling by a piercing mill, the internal fracture flaws would not necessarily be suppressed. For solving the problem as described above, the present inventors observed actual internal fracture flaws on the high-Cr steel pipes or tubes in detail in order to examine the causal relationship between occurring the flaws and the conditions, such as the round billet heating condition or various conditions of the piercing and rolling. This resulted in finding that all the internal fracture flaws on steel pipes or tubes could not be necessarily distinguished clearly between the material-originated ones and the machine- originated ones. That is to say, it was found that the internal fracture flaws on steel pipes or tubes have strong correlation with not only material-originated flaws but also "piercing efficiency" that is one of factors related to Mannesmann effect of a piercing mill. From the viewpoint of the above-mentioned content, it is an objective of the present application technology to provide a method for manufacturing a high-Cr seamless steel pipe or tube, particularly a high-Cr seamless steel pipe or tube containing, by mass %, 10.50 to 14.00% of Cr, capable of suppressing, internal fracture flaws among the internal surface defects caused at the time of piercing and rolling by a piercing mill. |