Effect of cooling rate on mechanical properties and microstructure of controlled-rolled C–Mn–Nb steel

Abstract

The tensile properties, impact properties, and microstructures of a 0·09C–1·25Mn–0·02Nb steel have been measured in 13 mm plates cooled at rates of 1·8–26 Ks^−l after controlled rolling at 650°, 800°, and 1000°C with rolling reductions of 50, 67, and 83%. At finish-rolling temperatures of 800° and 1000°C, cooling rates of 1·8–6 Ks^–l raised the yield stress 20–60 N mm⁻² above the value for air-cooled plates, without impairing the impact properties. Further increases in yield stress at a cooling rate of 26 Ks^−l were accompanied by a decrease in impact toughness probably as a result of the presence of about 10% martensite in the microstructure, and a susceptibility to quench aging and strain aging developed. Lowering the finish-rolling temperature to 650°C produced yield stresses of about 550 N mm⁻² for cooling rates of 1·8–26 Ks^−l compared with 441Nmm⁻² for the air-cooled condition, and the impact properties were not impaired provided the cooling rate did not exceed 6 Ks⁻¹ and the rolling reduction was 67% or higher. The improvement in properties and the low dependence of properties on changes in processing parameters at rolling temperatures of 800° and 1000°C could make accelerated cooling commercially attractive for steels with suitable compositions.