New developments and optimisation of high strength boron treated steels through the application of advanced boron monitoring techniques (OPTIBOS) : final report

This project was directed to high strength boron treated steels. Different processing routes have been addressed: Hot rolling followed by Q&T for plates and strips, direct quenching and subsequent tempering for plates and cold rolled and annealed sheet steels. The aim of this work is obtaining a better understanding of the behaviour of boron in Ti- protected and Al-protected steels. Materials were sampled from production or laboratory cast and pilot plant processed under different conditions. The material performance has been determined through the study of weldability, formability and hole expansion, wear, hardness, tensile and Charpy tests. The results have been linked to the composition, the microstructure and to the role of B at each stage of the production. Jominy and dilatometry tests have been carried out in order to investigate the effect of composition and processing conditions on B effectiveness. The study of the austenite recrystallization kinetics revealed the synergy between B and Nb. The capability of different characterisation techniques to monitor B allowed the selection of some of them as the most valuable and their application at different stages of the work. The sensitivity of current Thermodynamic predictions to BN and AlN precipitation has been investigated, indicating that work still remains to be done on the thermodynamic databases. Several available transformation models have also been assessed on the B-treated steels and the results are outlined in the text. Based on the developed knowledge, two new optimised compositions were proposed, cast and processed at laboratory and tested.