PURPOSE: To produce an ultrahigh strength resistance welded tube by subjecting a slab of a steel with a specific composition for tube making to hot rolling under specific temp. conditions and to coiling, producing a resistance welded tube by the use of the resulting coiled hot rolled steel plate, and then performing heat treatment at specific temp.
CONSTITUTION: A slab of a steel which has a composition containing, by weight, 0.005-0.30% C, 0.05-1.5% Si, 1.0-3.0% Mn, <0.02% P, <0.006% S, 0.01-0.08% Al, 0.01-0.15% Ti, 0.0003-0.003% B, and <0.005% N or further containing one or ≥2 kinds among <1.5% Ni, <2.0% Cr, <1.5% Mo, <0.05% Nb, and <0.10% V is hot-rolled and finish rolling is completed at a temp. between the Ar3 transformation point and 950°C, and the resulting plate is coiled at ≥600°C. After a resistance welded tube is produced by using this hot rolled steel plate as a stock, this resistance welded tube is annealed at a temp. between (Ac1 transformation point + 50°C) and 950°C and air-cooled, or, the above resistance welded tube after annealing is cold-drawn into square tube or tube of special form and then annealed. By subjecting the resulting tube to untempering heat treatment after tube making, without hardening treatment, and then to cold drawing which is added if necessary, the ultrahigh strength resistance welded tube can be produced.
MATSUDA HIROO
KONDO TETSUMI
KAWADA YASUYUKI
