Microstructure and mechanical property relationship in an ultrahigh strength 980Â MPa grade high-Al low-Si dual phase steel

A high-Al low-Si dual phase (DP) steel processed through thermo-mechanical processing practice involving combination of hot rolling and water quenching was studied in terms of structure-mechanical property relationship. The structure of the steel was strongly influenced by the start rolling temperature. At start rolling temperature of 1150 °C, blocky-type martensite was obtained, but on decreasing the start rolling temperature to 1000 °C, long strip-type martensite formed along the rolling direction. The steel with long strip-type martensite was subsequently tempered at different temperatures. Steel tempered at 200 °C for 30 min had the lowest hardness and was characterized by best combination of mechanical properties with tensile strength of 950 MPa, total elongation of 25.94%, and the product of tensile strength and total elongation of 24.62 GPa%. The decrease in the hardness of martensite enhanced the strain hardening ability, while ferrite influenced the fracture characteristics. There was a high tendency to cleavage, when large number of ferrite grains with {100} orientation and hardness was high in the adjacent area between martensite and ferrite. However, the refinement of ferrite grain size and increase in ferrite-ferrite grain boundary misorientation led to increase in ductile behavior.