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. (C) Elsevier Science Inc., 1999. All rights reserved.