THREE-ROLL STRETCH REDUCTION METHOD FO METALLIC TUBE

PURPOSE: To decrease angles in the peripheral direction of the inside surface of a product and to improve thickness deviation rate by arranging the rolls of roll stands imparting a specified angular displacement on a plane perpendicular to rolling pass line and executing rolling. CONSTITUTION: For example, after a steel strip 1 is heated to >=1200 deg.C in a heating furnace 2, both ends of the steel strip 1 are heated to 1380 deg.C with an edge heater 3. After that, by executing butt welding to both ends of the steel strip 1 with a tube making machine 4, the steel strip becomes a base tube 5. After stretch reducing this base tube 5 into a prescribed outside diameter and thickness with a stretch reducer 8 which is constituted of plural roll stands 7 making three rolls 6 which are arranged at the intervals of 120 deg. in the peripheral direction around the rolling pass line a set, the base tube is made into a product 10 by cutting it into a prescribed size with a cutting machine 9 and, after that, the product is transported to a cooling bed 11. Then, by executing rolling imparting the angular displacement that is a multiple of 9-15 deg. to the rolls 6 of the roll stand 7 on the plane perpendicular to the rolling pass line, the angles of the product 10 becomes >=24 angles and the out-of-roundness of the inside surface is improved.