Reducing the weight of the automobile while maintaining sufficient safety levels through the use of high strength steel sheet is of great concern to carmakers in their aim to fulfil environmental standards. In particular, there is strong interest in replacing conventional 390 - 440 MPa grade steel sheets used for structural and chassis parts with higher strength grades offering sufficient formability. TRIP steel, in which a small portion of austenite remains in the ferritic-bainitic basic matrix, is one of the candidate materials for 590 - 780 MPa grade high strength steel sheet with superior formability. The excellent mechanical properties exhibited by transformation induced plasticity (TRIP) steels are mainly due to the martensitic transformation of the metastable retained austenite induced by strain. Three TRIP steel grades with tensile strengths around 590, 690 and 780 MPa were tested. Mechanical properties were determined by uniaxial tensile tests and were correlated to the steel microstructures. Tensile tests up to fracture were performed to analyse the relation between the different strain hardening behaviours and the transformation of retained austenite into martensite during deformation. Specimens for the metallographic microstructure investigations were conventionally prepared and conventional etchants such as Le Pera were used as well as a two-step etching starting with nital and then sodium metabisulfite. These etchants made it possible to identify the different phases present in TRIP steel and to distinguish the different retained austenite morphologies. The relationship between strain hardening behaviour and fraction of retained austenite is discussed.
Correlations between microstructures and properties of TRansformation Induced Plasticity steels
LOVICU, GIAN FRANCO;DE SANCTIS, MASSIMO;VALENTINI, RENZO;SOLINA, ADRIANO
2006-01-01
Abstract
Reducing the weight of the automobile while maintaining sufficient safety levels through the use of high strength steel sheet is of great concern to carmakers in their aim to fulfil environmental standards. In particular, there is strong interest in replacing conventional 390 - 440 MPa grade steel sheets used for structural and chassis parts with higher strength grades offering sufficient formability. TRIP steel, in which a small portion of austenite remains in the ferritic-bainitic basic matrix, is one of the candidate materials for 590 - 780 MPa grade high strength steel sheet with superior formability. The excellent mechanical properties exhibited by transformation induced plasticity (TRIP) steels are mainly due to the martensitic transformation of the metastable retained austenite induced by strain. Three TRIP steel grades with tensile strengths around 590, 690 and 780 MPa were tested. Mechanical properties were determined by uniaxial tensile tests and were correlated to the steel microstructures. Tensile tests up to fracture were performed to analyse the relation between the different strain hardening behaviours and the transformation of retained austenite into martensite during deformation. Specimens for the metallographic microstructure investigations were conventionally prepared and conventional etchants such as Le Pera were used as well as a two-step etching starting with nital and then sodium metabisulfite. These etchants made it possible to identify the different phases present in TRIP steel and to distinguish the different retained austenite morphologies. The relationship between strain hardening behaviour and fraction of retained austenite is discussed.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
