New thermomechanical strategies for the production of high strength low alloyed multiphase steel showing a transformation induced plasticity (TRIP) effect

In the last years a lot of research was done in the development of TRIP-ass isted multiphase steels. Two principal ways were proposed: controlled cooling during the hot-rolling process to obtain hot-rolled TRIP -assisted multiphase steels and the combination of intercritical annealing and isothermal holding at bainit e formation temperatures during continuous annealing resulting in cold-roll ed TRIP-assisted steel products. Unfortunately both proposed thermomechanical methods require a high silicon level to inhibit cementite precipitation in order to avoid a loss of stabi lity for the metastable retained austenite. In addition, due to high silico n levels, red scale surface defects and a moderate hot dip galvanizability appear. In this article, new thermomechanical strategies for the production of high strength low alloyed TRIP-assisted multiphase steels with good hot -dib galvanizability and without red scale defects will be presented. Regar ding the thermomechanical path, the stabilization of the retained austenite in the final microstructure can be optimized by the application of the add itional step of batch annealing between hot rolling and cold rolling. This additional thermomechanical step activates manganese diffusion in the ferri te matrix and manganese enrichment processes of the cementite. During the s tep of continuous annealing, the manganese enriched cementite is transforme d into stabilization-optimized retained austenite. Regarding the final micr ostructure, a fine grained ferrite matrix of about 3 mu m grain size contai ning small islands of intragranular and intergranular stabilzation-optimize d retained austenite can be obtained.