Effects of Microstructure and Notches on the Mechanical Properties of Dual-Phase Steels

A low-carbon (0.1%) steel has been subjected to three different heat treatments to obtain dual-phase steels with different microstructures. The steel with the intercritically annealed microstructure of equiaxed ferrite–martensite exhibited the highest tensile strength, the lowest ductility, and intermediate fracture toughness properties. Step quenching also produced an equiaxed ferrite–martensite structure, but the material had the worst mechanical properties of the three different heat-treatment conditions. In contrast to the other two heat-treatment conditions, step annealing produced a fibrous (fine, needle-like) ferrite-plus-martensite structure. This gave rise to a material of intermediate tensile strength but with the highest ductility, notch strength, and fracture toughness. It is argued that optimum mechanical properties in a dual-phase steel can best be achieved by obtaining a microstructure containing fine, fibrous needle-like, martensite.