The effect of microstructures on the mechanical properties of multi-phase steels has been studied. The amount and nature of low temperature transformation products of the steels are changed through heat treatment and controlled cooling after hot rolling. Ferrite plus bainite steel has superior stretch- flangeability and fatigue strength while it is inferior in strength-uniform elongation relationship to ferrite plus martensite (“dual-phase”) steel.
Introduction of a small amount of martensite to ferrite plus bainite steel gives rise to a decrease in yield ratio and increase in work hardening rate. The microscopic examination shows that pearlite and martensite particles nucleate cracks easily and replacement of pearlite and martensite particles by more ductile bainite particles results in improved ductility. Stretch-flangeability of triphase steel is also improved by decreasing martensite particle size. It is concluded that ferrite-bainite-martensite (triphase) steel may be preferred to ferrite-martensite steel in automobile applications.