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

Abstract

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 30min had the lowest hardness and was characterized by best combination of mechanical properties with tensile strength of 950MPa, total elongation of 25.94%, and the product of tensile strength and total elongation of 24.62GPa%. 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.