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-assisted 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 bainite formation temperatures during continuous annealing resulting in cold-rolled 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 stability for the metastable retained austenite. In addition, due to high silicon 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. Regarding the thermomechanical path, the stabilization of the retained austenite in the final microstructure can be optimized by the application of the additional step of batch annealing between hot rolling and cold rolling. This additional thermomechanical step activates manganese diffusion in the ferrite matrix and manganese enrichment processes of the cementite. During the step of continuous annealing, the manganese enriched cementite is transformed into stabilization-optimized retained austenite. Regarding the final microstructure, a fine grained ferrite matrix of about 3 mu m grain size containing small islands of intragranular and intergranular stabilzation-optimized retained austenite can be obtained.