PURPOSE: To obtain rolling conditions capable of preventing the occurrence of cracks and flaws by specifying strain at the time of forming a high Cr ferritic steel of specific composition into a stock for seamless steel tube under specific conditions and applying piercing and skew rolling to the stock.
CONSTITUTION: A high Cr ferritic steel, having a composition consisting of, by weight, 0.01-0.15% C, 8-11% Cr, 0.05-1.5% Mo, 0.05-4% W, small amounts of Mn, P, S, Si, N, Nb, V, B, and Ni, and the balance Fe with inevitable impurities, is used. A cast billet of this steel is cogged at ≥60% reduction of area and then soaked at 1000-1150°C for ≥24hr and formed into a stock for seamless steel tube. At the time of subjecting this stock to heating to ≤1280°C, to piercing, and to elongation by means of skew rolling, the strain () represented by equation is regulated to ≤0.5. In the equation, t=ln(t1/t0) and L=ln(L1/L0) are satisfied, where t0 and t1 mean the wall thicknesses (mm) of a hollow tube stock before and after elongation rolling, respectively, and also L0 means the length (mm) before rolling and L1 means the length (mm) after rolling. By this method, cracking due to delta ferrite can be prevented.
OGAMI MASAHIRO
HASEGAWA HIROSHI
