Increasing fracture strain in high-strength multiphase steels, without sacrificing tensile strength, can be achieved by increasing the content of retained austenite in the final microstructure, thus utilizing the well-known TRIP effect upon straining. Notably, in order to achieve a greater stability of retained austenite at lower temperatures, Si is often used as an alloying element as it suppresses the formation of carbides, thereby facilitating carbon enrichment in the retained austenite phase. However, some drawbacks of increased amounts of Si are its adverse effects on the coatability and wettability of molten zinc. Si can be replaced partially by Al, albeit at the expense of increased transformation temperatures during intercritical annealing, in particular, the Ac3 temperature, as well as complications during strip casting. Thus, the amount of both Si and Al should be kept to a minimum. Reduced amounts of Si and Al are also believed to be beneficial by virtue of a reduced susceptibility to liquid metal embrittlement.
Microalloying with Nb is known to be highly effective in order to increase strength through grain refinement and precipitation hardening by fine niobium carbides as well as retard recrystallization. However, the effect of Nb microalloying pertaining the stability of retained austenite in TRIP-assisted multiphase steels remains elusive. In the following, the impact of Nb on retained austenite and microstructure evolution and subsequently on the materials performance will be derived for a high-strength multiphase steel.
In particular, increasing amounts of Nb up to 400 ppm are shown to yield a greater fraction of ferrite in the final microstructure and impart changes in the bainite morphology from a lath-like bainite to a fine granular type of bainite, accompanied by a reduction of sigma 3 grain boundaries. Due to the changes in major microstructure constituents the amount of retained austenite increased linearly with the amount of Nb, thereby drastically improving ductility.