Processing and properties of C-Si-Mn trip steels

In the processing of steels, various types of alloy compositions and heat treatments have been used to improve strength and ductility. Unfortunately, these methods are usually accompanied by a decrease in formability. It has been well recognized that the presence of retained austenite, in microstructures which include ferrite and bainite, can significantly strengthen the steel and improve ductility by transforming to martensite under an applied stress or strain. This is the basic principal behind the TRIP (transformation-induced plasticity) effect.In this work, the effects of thermomechanical processing (TMP) and chemical composition on the structure and properties of C-Si-Mn TRIP steels were investigated. It was found that the addition of Nb to these steels not only increases the volume fraction of retained austenite, but also improves the total elongation and tensile properties. It was also revealed that the combination of strength and ductility is further optimised by varying the C content in the steel. For C-Si-Mn Nb-bearing TRIP steels, the bainite transformation conditions are the critical parameters in dictating the final mechanical properties.The effects of retained austenite characteristics on the mechanical properties were examined. Although the retained austenite volume fraction is indeed a key factor in controlling the final properties, there are other parameters that must be considered, such as the morphology, particle size, solute enrichment and the mechanical stability of the retained austentite. In this work, the approach involved a systematic investigation of the effects of TMP parameters on the state of the retained austenite characteristics and finally, an evaluation of the final mechanical properties. It was revealed that, for a given ferrite/bainite structure, the tensile strength and total elongation are controlled by the volume fraction and the mechanical stability of the retained austenite. Increasing the mechanical stability of retained austenite leads to transformation at higher strains. (Abstract shortened by UMI.