This invention relates to a plug and a mandrel

In the specification of the present invention, the plug used in each of the aforesaid pipe rolling machines is properly called as a piercing plug, an elongating plug or the like by applying a name of the pipe rolling machine. In addition, in the case of the mandrel mill, the aforesaid rod-like member corresponds to a plug for another rolling machine and this is called as a mandrel bar. In order to manufacture a seamless steel pipe by applying a Mannesman Pipe Manufacturing Method, for example, at first a round steel piece heated up to a predetermined temperature (hereinafter called as a billet) is punched with the aforesaid plug by applying a slant type drilling machine called as a piercing machine and rolled to make a hollow raw pipe (hereinafter called as a hollow member). Then, the hollow member is rolled under application of a plug or a mandrel bar (hereinafter merely called as a bar) by the extending and rolling machine such as an elongating machine, a plug mill, a reel machine or a mandrel mill or the like in the same manner as that described above so as to reduce its wall thickness. Further, as required, after the rolled pipe is heated again, its outer diameter is reduced without using any plug in a squeezing rolling machine or a non-proportional rolling machine such as a reducing machine and a sizing machine. In this case, the hollow raw pipe rolled by a plug mill, a reel machine and a mandrel mill or the like is called as a shell. However, the plug is always exposed under a high temperature and a high load through a continuous contact with the heated billet or hollow member at the stage of punching and rolling operation performed by the aforesaid piercing machine, resulting in that the plug is quite easily worn out and melt lost. In the prior art, since the plug was made of steel, the plug was heat treated at about 900 to 1000 C. as a countermeasure against a worn-out loss and a scale film of several 10 to several 100 m was formed. However, when such a plug as above is used in making a pipe of high alloy steel containing Cr of 5 wt. % of which demand is increased in particular in recent years, the plug is merely endurable against several number of billets and its life is remarkably short. In turn, as a countermeasure for extending the life of the plug, there is provided a usage of plug in addition to a modification of the aforesaid material quality of the plug. Either the official gazette of Japanese Patent Laid-Open No. Sho 51-133167 or the official gazette of Japanese Patent Laid-Open No. Hei 1-180712, for example, has a proposal about a technology that the punching and rolling can be carried out while lubricant is being injected from the extremity end of the plug and in turn the official gazette of Japanese Patent Laid-Open No. Hei 5-138213 has a proposal about a technology that the punching and rolling can be performed after lubricant is coated in advance at the surface of the plug, respectively. However, the technology for injecting the lubricant from the extremity end of the plug shows a problem that the injection hole at the extremity end is clogged and in turn the technology for coating lubricant before punching and rolling operation shows a problem that only a required amount of lubricant can be uniformly coated at the surface of the plug. In addition, these both technologies have a common economical problem of increasing cost in manufacturing a pipe under the application of lubricant and so they may not employed in view of their industrial application. In view of the circumstances described above, the present inventors have vigorously studied about an extension of life of the plugs and bars for use in manufacturing a seamless steel pipe. The study was carried out such that as the rolled material, the billet of high alloy steel was selected in the model mill of which size was reduced to about 1/3 of that of the actual machine, various kinds of experiments of punching and rolling as well as extending and rolling were performed and then the attained results were confirmed afterwards by the actual machine. As a result, the countermeasure based on the method of using the plug together with lubricant shows a problem of which resolution is quite difficult such as a clogging at the injection hole caused by lubricant or its increased cost. To the contrary, it has been concluded that the improvement caused by modification of material quality of the plug needs a certain amount of improvement under utilization of a recent development or an improved raw material. In other words, it is an object of the present invention to provide a new plug and a new bar for use in rolling a seamless steel pipe capable of more remarkably improving a life than that of the prior art even under a severe condition of application always exposed to a high temperature and a high load. Then, the present invention provides a method for attaining a seamless steel pipe having a more superior quality than that of the prior art under application of these members. Accordingly, the present invention is constructed such that as raw material for the plug and the bar for a seamless steel pipe, ceramics is employed, resulting in that a life of these members is substantially improved. In addition, as a result of the arrangement in which either the aforesaid plug or the aforesaid bar is applied for manufacturing the seamless steel pipe, time required for performing a replacing work for these members is shortened and its productivity is remarkably improved more than that of the prior art.