Microstructural changes during intercritical annealing and cooling in relation to the formation of ferrite-martensite dual-phase steel have been studied to find ways to design optimum microstructure of the dual-phase steel. Austenite formation and shrinkage during the intercritical annealing have been observed and analized to investigate the mechanisms involved in the ferrite to austenite and the austenite to ferrite transformation. The effects of third microstructural constituents, such as retained austenite, bainite, and pearlite, on dual-phase properties have also been investigated. Four kinds of low-carbon steels of either Fe/Mn/Si or Fe/Mn/Si/Cr system have been used for present work.
Before ferrite/ferrite boundaries are site-saturated with austenite, austenite may grow fast along the boundaries with the rapid supply of carbon from cementite particles to growing fronts of austenite particles through newly formed austenite/austenite boundaries. After boundary planes are site-saturated with austenite particles, the growth rate of austenite may be controlled by volume diffusion of carbon in austenite. Then, the grain-boundary allotriomorphic austenite particles can also grow to Widmanstatten side plates, provided planar movement of ferrite/austenite interface may be insufficient for the rapid increase of austenite volume fraction due to a coarse-grained structure.
The presence of pearlite and/or bainite in a ferrite-martensite microstructure can not be directly linked to discontinuous yielding behavior. Mobile dislocation density for yielding ferrite matrix continuously may be obtained by about 2 percent austenite to martensite transformation.
If austenite volume fraction did not reach an equilibrium value during annealing before cooling, the austenite volume fraction increases to a peak value depending on cooling rate even on cooling but eventually decreases. Transformed ferrite does not grow epitaxially on retained ferrite but by a nucleation and growth mechanism. The sandwiched austenite shell between transformed and retained ferrite is retained on cooling from the intercritical annealing temperature since the prior ferrite/austenite interfaces, which formed before cooling, are much less mobile than the newly formed ferrite/austenite interfaces inside prior austenite pool during cooling. The strength of transformed ferrite is always higher than that of retained ferrite since the sandwiched austenite shell blocks atomic transport between two kinds of ferrites during cooling, resulting in decrease of the retained ferrite strength. Isolated retained austenite particles are the debris of the sandwiched austenite shell untransformed to martensite by a size stabilizing effect.
Retained austenite particles increase work hardening rate of dual-phase steel in the limited strain range of the first several percent. The increase in work hardening rate, which is caused by the strain induced transformation of retained austenite to martensite, can be ascribed to the transformation of the isolated retained austenite particles. Capsulated type of retained austenite, protected by martensite capsule, can survive much longer than the isolated type during straining.
On the basis of the above results, suitable ways for designing dual-phase microstructures of their optimum properties have been discussed and suggested.
