Method of making high strength steel tube

A method of converting pre-rolled precipitation-hardenable fine-grain medium-carbon low alloy steel plates or strips into steel profiles having a yield strength of .gtoreq.500N/mm.sup.2, a tensile strength of .gtoreq.600N/mm.sup.2 and high toughness, comprising the first step of cooling the rolled steel plates or strips from a final rolling temperature above the A.sub.1 -temperature level, down to a temperature of about 400.degree. C. in an atmosphere which in part supresses radiation and convection heat losses and thus reduces the cooling rate;

• the second step of thereafter cooling the steel plates or strips to room temperature;
• the third step of cold-processing the cooled steel plates or strips to form an open profile therefrom;
• the fourth step of subjecting the profile to annealing so as to dissolve coarse precipitants thereon;
• and the fifth step of cooling the profile to form finely dispersed precipitants therein;
• winding up the steel to form a coil before said first step, and thereafter cooling the coil by overaging at high temperature corresponding to said temperature in said first step;
• said third step comprising further the step of converting the open profile into a tube by welding adjacent longitudinal edges of the profile together;
• said fourth step comprising further solution-annealing the profile above the A.sub.C3 -temperature;
• the steel having a composition of:
• ______________________________________ 0.13 to 0.35% carbon 0.10 to 0.80% silicone 0.90 to 1.70% manganese max. 0.035% phosphorous max. 0.035% sulfur max. 0.70% copper max. 0.80% nickel max. 0.020% nitrogen max. 0.08% aluminum 0.02 to 0.20% niobium and/or titanium and/or vanadium, ______________________________________
• the balance being iron and impurities.