To produce a long-size hollow bar steel increased in the ratio between wall thickness and outside diameter by respectively specifying, at a cross helical rolling mill, the temp. before rolling, rolling temp., and outside diameter reduction rate of a steel having a specific composition where Cr, V, N, etc., are added and further specifying the structure and crystalline grain size of the steel, respectively.
A round billet A1, having a composition consisting of, by weight, 0.20-0.60% C, 0.05-1.50% Si, 0.30-2.00% Mn, 0.02-1.00% Cr, 0.01-0.30% V, 0.005-0.020% N, ≤0.20% Cu, ≥1.0% Ni, ≤0.20% Mo, ≤0.40% Nb, ≤0.05% Al, ≤0.35% Pb, ≤0.20% Bi, ≤0.10% S, ≤0.20% Te, ≤0.01% Ca, and the balance Fe, is heated to 950-1250°C in a heating furnace 11 and pierced into a hollow stock satisfying (wall thickness)/(outside diameter) ≥0.1. A mandrel 13 is inserted into the hollow stock, and then, diameter reduction working and wall thickness working, satisfying ≤0.2 outside diameter reduction rate and (wall thickness draft)/(outside diameter reduction rate) <0.55, are applied at 700-950°C by means of a cross helical rolling mill. By the above procedure, the ratio of wall thickness to outside diameter is regulated to 0.25-0.40 and a structure composed of ferrite and pearlite is formed, and further, the crystalline grain size of ferrite is regulated to JIS grain size number 6-12.
ASAKAWA MOTOO
FURUKATA MUNEKATSU
