An Approach for the Design of Multiphase Advanced High-Strength Steels Based on the Behavior of CCT Diagrams Simulated from the Intercritical Temperature Range

Most of information regarding the construction of continuous cooling transformation (CCT) diagrams considers the decomposition of austenite above the critical transformation temperature Ac3. The development of new grades of multiphase advanced high-strength steels (AHSS) usually involves an intercritical annealing stage, making it necessary the construction of CCT diagrams from the intercritical range. The availability of CCT diagrams constructed from the intercritical austenite is practically non-existent in the open literature. The present research reports an approach for the design of AHSS based on the behavior of CCT diagrams calculated from the intercritical temperature range as a function of chemical composition. Changes in chemical composition were conducted to promote the fabrication of multiphase AHSS steels under conditions that simulate continuous annealing and galvanizing lines (CAGL). To validate the results obtained from computer simulations, the steel was then fabricated, processed at laboratory scale and subjected to thermal cycles that simulate CAGL. Although there is a good approximation between theoretical and experimental results, it was observed that the software presents some limitations regarding the effects of plastic deformation and carbon partitioning during the isothermal bainitic treatment (IBT) on the final microstructures.