Investigation of the Effect of Volume Fraction of Martensite and Different Tempering Conditions on the Microstructure and Mechanical Properties of St52 Dual-Phase Steel Used in the Automotive Industry

Dual-phase steels are a new class of low-strength alloy steels consisting mainly of ferrite and martensite phases. Dual-phase steels due to their special microstructure represent good mechanical properties and abrasion resistance. In this study, ST52 steel was selected as the prototype specimen because this type of steel after converting to the dual- phase steel will be used in automotive industry. After preparing some specimens of this steel, normalizing treatment was done on them. In order to achievement of dual-phase steel structure and evaluate the different tempering conditions, intercritical annealing was performed at 750 °C and 850 °C for 30 minutes on the specimens. Then tempering was carried out on specimens at 300 °C and 600 °C for one and two hours, respectively. The evaluation of microstructure and mechanical properties, revealed that with increment of the temperature during intercritical annealing, the volume fraction of martensite increases and with increment of the tempering temperature, the volume fraction of martensite decreases. Reducing the amount of martensite phase in most specimens caused to reduce the hardness and tensile strength, but the percentage of elongation increased. Also, increment of the tempering time same as tempering temperature had a similar effect on mechanical properties, while the rate of changes was more than the tempering time. Also, the obtained results showed that the highest hardness (450 Vikers) was related to specimen which intercritical treatment and tempering was performed at 850 °C and 300 °C for one hour, respectively. Additionally, this specimen revealed a highest amount of yield strength (1153 MPa) and a tensile strength (1199 MPa).