Influence of the steel alloy composition on phase transitions and its applicability to hot forming process

Continuous Cooling Transformation (CCT) diagrams determined with a dilatometer were used to analyse the influence of the alloying elements on the phase transitions of high strength steel sheet alloys. Mo and Cr additions showed a significant influence on the extension of the bainitic region, whereas C and B additions showed noteworthy improvement on the quenchability of these steel alloys. These alloy properties are desirable requirements to be integrated in the hot formed components. Dilatometer samples of the 22MnB5 steel alloy were used to determine the resulting mechanical properties in function of the cooling rate. Hardness and micro-bending tests were carried out and they showed a qualitative trend with the cooling rate, which permits a preliminary estimation of the crash performance. A hot-forming process of a car B-pillar foot was simulated in commercial finite element code LS-DYNA considering experimentally determined input parameter for the 22MnB5 sheet alloy. These results were evaluated with an experimental trial showing a good approach. Results obtained in this work show an overview of essential research fields for the optimization of the hot forming process for the production of graded components, including the possibility of increasing the spectrum of alloys used in this process.