In the manufacture of seamless tube by the Mannesmann mandrel mill method, first, a round billet or a square billet is heated to 1200-1260° C. in a heating furnace, and it is then subjected to piercing by a piercer to produce a hollow shell. Subsequently, a mandrel bar is inserted into the bore of the hollow shell, and the hollow shell is subjected to rolling for elongation in a mandrel mill so as to reduce the wall thickness to a predetermined thickness, thereby forming a tube. The mandrel bar is then withdrawn from the tube having the reduced wall thickness, and the tube is subjected to rolling for sizing in a sizer so as to obtain a desired outer diameter, resulting in the manufacture of a seamless tube as a product.
During elongation rolling, galling (seizure) between a mandrel bar and a hollow shell easily occurs. Therefore, a lubricant is applied to the surface of the mandrel bar. A lubricant which has been most commonly used is a graphite-based lubricant containing graphite which has excellent wear resistance and anti-galling properties. Ideally, after a lubricant applied to the surface of a mandrel bar has dried, the mandrel bar is transported to a mandrel mill while it is in contact with a conveyor such as transport rolls, and it is used for elongation rolling. However, in an actual manufacturing operation, it is often not possible to keep enough time for the applied lubricant to completely dry. Therefore, the lubricant which has not dried drips off during transport of the mandrel bar and adheres to a part of the conveyor beneath the mandrel bar. Even when transport is carried out after the lubricant has completely dried, the film of the applied lubricant drops off or peels off due to vibrations or the like during transport and it adheres to the conveyor. Therefore, a conveyor for a mandrel bar is always contaminated by graphite which is contained in the lubricant deposited thereon. Since a conveyor for a mandrel bar is contaminated by graphite in this manner, a mandrel bar which is transported by the conveyer in contact therewith is also contaminated by graphite.
When a mandrel bar which has been contaminated by graphite in this manner is used for elongation rolling of a hollow shell made of a low carbon steel such as SUS 304L having a carbon content of at most 0.04% (in this description, unless otherwise specified, % means mass %), the inner surface of the tube produced by elongation rolling is unavoidably carburized.
Conceivable countermeasures for preventing this carburization include not using a graphite-based lubricant for elongation rolling of a hollow shell with any type of steel, providing additional processing equipment for elongation rolling in which only a non-graphite-based lubricant is used, or thoroughly washing a conveyor for a mandrel bar when manufacturing a seamless tube of a low carbon stainless steel by elongation rolling in processing equipment in which a graphite-based lubricant has been used.
However, a non-graphite-based lubricant is generally more expensive than a graphite-based lubricant, and provision of new processing equipment requires additional capital investment, so these measures are difficult to carry out from the standpoint of economy. Therefore, the primary countermeasure has been to wash a conveyor for a mandrel bar.
For example, Patent Document 1 discloses an invention using a non-water resistant graphite-based lubricant for improving the washability of a conveyor. Patent Document 2 discloses an invention in which a mandrel bar and a mandrel bar conveyor are washed by spraying with high pressure steam or water such that the amount of adhesion of graphite to the surface of a mandrel bar is controlled to at most 100 mg/m2.
Patent Document 1: JP 2002-28705 A1
Patent Document 2: JP 2000-24706 A1
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