The present invention relates to verticaltwo-roll disk shoe type seamless steel pipe rolling mills and more particularly to an apparatus for changing the rolling mill rolls and/or the disk shoes which are used in such rolling mills.

This type of rolling mill (a piercer, elongator or the like) is so designed that a heated material (hereinafter referred to as a billet) is passed between rolls which are arranged vertically and inclined relative to each other and a plug mountedon the forward end of a mandrel bar is forced into the billet from the opposite side thereby producing a thick-walled hollow crude pipe. Since the rolls used in this type of rolling mill are subject to wear or damage due to their use over a long periodof time, it is necessary to change the rolls with new rolls at times. The rolling mill also includes a pair of circular disk shoes which are arranged between the rolls so as to prevent the crude pipe from expanding between the rolls during the piercingof the material and the outer periphery of the disk shoes is formed into an arcuate shape corresponding to the external shape of billets thus making it necessary to change the disk shoes when the outer diameter of billets changes. Further, the outerperipheral surface of the disk shoes are always contacted with the hot billet and therefore it is necessary to change the disk shoes from time to time due to the wear and damage caused by a long period of use.

The seam-weld zone is a part which is heated, compressed, and rapidly cooled, so forms a fine uniform structure. Compared with a mainly ferrite pearlite base material and weld heat affected zone, the microstructure does not easily become a ferrite martensite dual-phase structure after heating to 760 to 830° C. If heating the vicinity of the seam, that is, the seam weld zone, once to the Ac3 point or more, the microstructure will approach a ferrite pearlite structure, the pipe body is heated to the austenite ferrite dual-phase region and quenched. The microstructure of the subsequent seam weld zone then becomes close to the structure of the base material and weld heat affected zone.

The steel pipe able to be used in the present invention includes seamless steel pipe, UOE steel pipe made by shaping steel plate into a tube and arc welding the end faces, etc., but seam-welded (ERW) pipe is preferable. The reason is that ERW pipe is produced from hot rolled steel plate as a material, so the thickness is uniform and, compared with seamless steel pipe, there are the features of excellent pipe expandability and crushing strength. If steel pipe is uniform in thickness, its expandability and crushing strength are improved. On the other hand, if it is not uniform in thickness, it will easily bend when expanded.