Microstructures and properties of Transformation Induced Plasticity steels

摘要

The weight reduction of automobile, maintaining sufficient safety by using the high strength steel sheet, is of great concern to the carmakers who have to fulfil environmental standards. In particular for the structural and chassis parts, there has been the demand for replacing the conventional 390-440 Mpa grades steel sheets by the higher strength grades with sufficient formability. TRIP steel, where a small portion of austenite remains in the ferritic-bainitic basic matrix, is one of the candidate materials for 590-780 Mpa grades high strength steel sheet with superior formability. The excellent mechanical properties exhibited by the TRansformation Induced Plasticity steels are mainly due to the martensitic transformation of the metastable retained austenite induced by strain. Three TRIP steels grade have been tested: 590, 690 and 780. Mechanical properties are determined by uniaxial tensile tests and are correlated to the micro structures of the steels. The tensile tests up to fracture are been carried to analyse the relation between the different strain hardening behaviours and the transformation of retained austenite into martensite during deformation. The specimens for the metallo graphic investigations of micro structure are conventionally prepared, conventional etchings such as Le Per a and nital plus sodium metabisulphite, have been used. It has been possible by these etchings to identify the different phases present in TRIP steel and to distinguee the different morphologies of retained austenite. The relation between strain hardening behaviour and fraction of retained austenite is discussed.