Evolutions of Microstructural and Mechanical Properties of Tempered Dual-Phase Steels Influenced by Silicon Content and the Intercritical Annealing Temperature

This study evaluated the effect of tempering along with intercritical annealing temperature change on the microstructure and mechanical behavior of dual-phase steels with the various silicon content. To get different ratios of ferrite and martensite, steels were intercritically annealed at three temperatures of 750, 775, and 800 °C. The steels were then tempered in the temperature range of 200-400 °C for 1 hour. In the non-tempered samples, strength increased by increasing intercritical annealing temperature, whereas ductility decreased continuously up to 800 °C. The effect of silicon content in the range of 0.34 to 2.26 wt.% on strength and ductility of non-tempered steel was different from that of the intercritical annealing temperature so that by increasing Si content, the strength decreased and ductility improved. It is found that the martensite volume fraction (MVF) decreased with increasing Si content. The microstructural analysis revealed that tempering treatment resulted in the carbide precipitation and martensite decomposition. The increase in the tempering temperature from 200 to 400 °C causes an intensification in the decomposition of martensite. It was also observed that increasing the tempering temperature decreases strength while increasing the ductility. Fractography also illustrated that the tempering has a so extreme influence on the fracture type. Tempering led to more ductile fracture. The results were discussed based on the microstructural evolutions during tempering.