Upper-side formed glass for hot boring and process for producing billet for tube formation by hot extrusion

A hollow billet is subjected to hot boring in which the hollow billet is inserted into a container and bored by hot-expanding the hollow with a plug inserted from above.  A formed glass is used in the hot boring as an upper-side lubricant for the hollow billet, the formed glass comprising: a formed-glass ring part which has a disk shape having, at the center, a circular opening for inserting the plug thereinto; and a projecting ring part which has been fixed to the periphery of the opening of the formed-glass ring part and has a ring shape protruding perpendicularly to the formed-glass part.  Hot boring with the formed glass is safe and can prevent the inner surface of the billet from developing defects.