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 treatm ents to obtain dual-phase steels with different microstructures. The steel with the intercritically annealed microstructure of equiaxed ferrite-marten site exhibited the highest tensile strength, the lowest ductility, and inte rmediate fracture toughness properties. Step quenching also produced an equ iaxed ferrite-martensite structure, but the material had the worst mechanic al properties of the three different heat-treatment conditions. In contrast to the other two heat-treatment conditions, step annealing produced a fibr ous (fine, needle-like) ferrite-plus-martensite structure. This gave rise t o a material of intermediate tensile strength but with the highest ductilit y, notch strength, and fracture toughness. It is argued that optimum mechan ical properties in a dual-phase steel can best be achieved by obtaining a m icrostructure containing fine, fibrous needle-like, martensite. (C) Elsevie r Science Inc., 1999. All rights reserved.