PRODUCTION METHOD FOR ROUND STEEL BAR FOR SEAMLESS PIPE COMPRISING HIGH Cr-Ni ALLOY, AND PRODUCTION METHOD FOR SEAMLESS PIPE USING ROUND STEEL BAR

摘要

The production method of a round bar for a seamless tube in which a continuously cast slab with a rectangular cross section, and made of high alloy containing Cr of 20 to 30 mass%, Ni of 30 to 50 mass%, and at least one of Mo and W as Mo + 0.5W of 1.5 mass% or more is subjected to a blooming and billet-making process to yield a round bar having a diameter of 150 to 400 mm as a starting material of the seamless tube; characterized in that the blooming and billet-making process is carried out under a condition satisfying a relation of 1.3‰¤H/D‰¤1.8 where a short side length of the cross section of the cast slab is defined as H (mm), and the diameter of the round bar is defined as D (mm). The round bar produced in this manner is subjected to piercing-rolling to make a hollow blank, and this hollow blank tube is subjected to elongation-rolling, further followed by diameter-adjusting-rolling to make a seamless tube made of high Cr-high Ni alloy, thereby preventing occurrence of the tube end cracking during piercing-rolling, and producing the seamless tubes in preferable yields.