Effect of temperature on the lower yield strength and static strain ageing in low-carbon steels

Three different low-carbon steels, namely ASTM 516 Gr70 PVQ, CSA G40.21 350 WT and CSA G40.21 350 AT plates, were tensile tested at temperatures from ambient to 623 K over a range of strain rates (1.48×10^–5–1.48×10^–3 s^–1). The lower yield strength generally decreases with increasing temperature, but there is a strength plateau, or a small peak, at temperatures between 423 and 573 K. The results of computer modelling show that concentration of nitrogen in the atmospheres at dislocations decreases with increasing temperature, approaches a minimum value at a temperature around 423 K, then increases with increasing temperature for temperatures above 423 K. This effect of temperature on the concentration of nitrogen in atmospheres at dislocations results in the strength plateau or small peak.