Examples of the method of producing a seamless metal pipe include the Ugine Sejournet process based on a press method and the Mannesmann process based on a skew rolling method.
In the Ugine Sejournet process, a hollow round billet in which a through hole is formed at its axial center by machining or piercing press is prepared. Then, the hollow round billet is subjected to hot extrusion by use of an extrusion apparatus to produce a seamless metal pipe.
In the Mannesmann process, a round billet is piercing-rolled with a piercing machine to produce a hollow shell. The produced hollow shell is elongation-rolled with a rolling mill to reduce the diameter and/or thickness of the hollow shell, thus producing a seamless pipe. Examples of the rolling mill include a plug mill, a mandrel mill, a Pilger mill, a sizer, and the like.
The Ugine Sejournet process can process the round billet at a high reduction rate, and therefore is excellent in pipe workability. A high alloy generally has a high deformation resistance. Therefore, a seamless metal pipe made of a high alloy is usually produced by the Ugine Sejournet process.
However, the manufacturing efficiency of the Ugine Sejournet process is lower than that of the Mannesmann process. Further, it is difficult for the Ugine Sejournet process to produce a large diameter pipe and a long pipe. In contrast, the Mannesmann process has high manufacturing efficiency and is capable of producing large diameter pipes and long pipes. Therefore, to produce a seamless metal pipe made of a high alloy, it is preferable to employ the Mannesmann process than the Ugine Sejournet process.
However, inner surface flaws attributed to lamination defects may occur in the inner surface of a high-alloy seamless metal pipe produced by the Mannesmann process. The lamination defect is caused by the melting of a grain boundary within the wall (in a central part of the wall thickness) of the hollow shell. As described above, a high alloy has a high deformation resistance, and further when the Ni content of the high alloy is high, solidus temperatures in the phase diagram thereof are low. When such a high alloy is piercing-rolled with a piercing machine, due to high deformation resistance thereof, work-induced heat will increase accordingly. Such work-induced heat causes a portion in the billet being piercing-rolled where temperature becomes close to or exceeds the melting point of the billet. In such a portion, the grain boundary melts, and a crack occurs. Such a crack is referred to as a lamination defect. Therefore, inner surface flaws attributed to lamination defects are likely to occur in a seamless metal pipe made of a high alloy.
Techniques to suppress the occurrence of inner surface flaws are proposed in JP2002-239612A (Patent Document 1), JP5-277516A (Patent Document 2), and JP4-187310A (Patent Document 3).
Patent Documents 1 and 2 disclose the following matters. Patent Documents 1 and 2 have an object to produce a seamless steel pipe made of austenitic stainless steel such as SUS304 etc. In Patent Documents 1 and 2, the starting material is formed into a hollow shell by machining and charged into a heating furnace. Then, the heated hollow shell is elongation-rolled with a piercing machine. The amount of reduction when a hollow shell is piercing-rolled is smaller compared with the case of a solid round billet. Therefore, the amount of work-induced heat decreases, lamination defects are reduced, and therefore the occurrence of inner surface flaws is suppressed.
Patent Document 3 discloses the following matters. Patent Document 3 adopts a production method based on a so-called “double-piercing” method in which two piercing machines (first and second piercing machines) are utilized in the Mannesmann process. Patent Document 3 has its object to suppress the occurrence of inner surface flaws of the hollow shell in the second piercing machine (elongator). In Patent Document 3, the roll inclination angle and the elongation ratio of an elongator are adjusted to reduce the rolling load of the elongator. As a result, the occurrence of inner surface flows is suppressed. Other related literatures include JP64-27707A.