PRODUCTION OF NON-HEATTREATED STEEL EXCELLENT IN DURABILITY RATIO

PURPOSE:To obtain the ferrite-pearlite type non-heattreated steel by subjecting a steel having a specific composition containing specific amounts of V to rolling, cooling, rolling, cooling, and precipitation stabilizing treatment under respectively specified conditions. CONSTITUTION:A steel having a composition consisting of, by weight, 0.15-0.60% C, 0.05-1.00% Si, 0.60-1.80% Mn, 0.10-0.60% Cr, 0.005-0.070% Al, 0.05-0.40% V, and the balance Fe is used. Rolling is applied to this steel at a temp. in the range between 800 and 1100 deg.C and not exceeding (recrystallization starting temp. +100) deg.C at >=30% total draft, by which austenite crystalline grains are refined. This steel is cooled down to a temp. in the unrecrystallization temp. region between 650 and 900 deg.C and rolled in this temp. range at >=30% total draft, and a fine ferrite-pearlite structure is obtained at the time of cooling. This steel is cooled down to 600 deg.C at a rate of (1 to 200) deg.C/min, by which the contamination with bainite structure is prevented. Then the steel is cooled temporarily down to ordinary temp. and subjected to precipitation stabilizing treatment at 400-700 deg.C for 3-6hr, by which supersaturated V is precipitated in the form of carbonitride.