The effect of pre-straining at intermediate temperatures on the mechanical behavior of high-bainite dual phase (HBDP) steels

High-bainite dual phase (HBDP) steels with 34 vol.% ferrite were subjected to tensile strains of 1%, 3%, and 5% at intermediate temperatures (150-450 °C). Mechanical behavior of pre-deformed steels was then investigated at room temperature. A maximum value for both yield stress and ultimate tensile strength was observed for pre-deformed steels. A minimum elongation was also found in the same pre-deformed specimens at the range of 250-350 °C. It was found that dynamic strain aging (DSA) plays the major role in mechanical properties of pre-strained HBDP steels. The DSA indicator parameter (DSAP) was introduced to evaluate DSA mechanism. Maxima of DSAP were likewise observed at the range of 250-350 °C. At higher pre-deformation temperatures the role of DSA was retarded and as a result, DSAP decreased. In order to shed more light on this matter, response surface methodology (RSM) was employed for the evaluation of important factors (temperature and pre-strain) on DSA. The results showed the statistically significant effects of first order term of both factors as well as second order term of temperature. However, there was no statistically significant interaction between these two factors. The optimization of these two factors for achieving maximum DSAP revealed that the maximum DSAP is 47.5 under the following conditions: temperature of 312 °C; pre-strain of 5%. Moreover, fatigue tests were conducted on pre-deformed specimens at 312 °C. It was demonstrated that higher pre-deformation would lead to higher fatigue limit.