Development of Formable Ultra High-strength TRIP-aided Steel Sheets and Hydrogen Embrittlement Performance

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In order to develop formable ultra high-strength TRIP-aided sheet steels with bainitic ferrite matrix (TBF steels), the effects of heat treatment conditions and alloying element on microstructure, retained austenite characteristics, tensile properties and formability of the TBF steels have been examined. In addition, hydrogen embrittlement performance of the steels was investigated. The main results are summarized as follows.(1)Complex additions of 0.02-0.05%Nb and 0.1-0.2%Mo to 0.2%C-1.5%Si-1.5%Mn TBF steel improved the balance of ductility and stretch-flangeability, accompanied with increase in tensile strength by 1100-200MPa due to fine NbMoC precipitates. This was caused by that prior austenitic grain and lath structure were refined and retained austenite stability was highly maintained. The complex additions also raised optimum austempering temperature to 450-500℃ which corresponds to hot-dip galvanizing temperature.(2)Total elongation of the TBF steel was considerably improved by warm forming of 50-100℃ and 200-300℃. This was associated with strain-induced martensite or bainite transformation of retained austenite and/or dynamic aging.(3)Complex additions of 0.02%Nb and 0.1%Mo to 0.2%C-1.5%Si-1.5%Mn TBF steel achieved far high delayed fracture strength. This was caused by that retained austenite films and fine NbMoC precipitates trapped most of solute hydrogen and resultantly suppressed grain boundary fracture or cleavage fracture.(4)Commercial 980 and 1470MPa grade 0.2-0.25%C-1.5Si-Mn-NbMMo TBF steel sheets were produced on continuous annealing line of a certain company. It was certified that the sheets have a good stretch-flangeability and high hydrogen embrittlement resistance.