Sheet metal forming behaviour and mechanical properties of TRIP steels

摘要

Recently various kinds of high-strength sheet steels have beendeveloped to meet the requirements of the automotive industry such as passivesafety, weight reduction and saving energy. Usually the main problem of high-strength steels is their inferior ductility. Multiphase steels however show a verygood combination of strength and formability so that the applicable region ofhigh-strength steels has been widely enlarged. Multiphase steels have beendeveloped for various purposes because of their ability to tailor properties byadjusting the type, the amount, and the distribution of different phases. Especiallynew developed triple-phase steels which make use of the TRIP effect(transformation induced plasticity) can further improve formability as well asstrength due to the transformation of retained austenite to martensite during thedeformation. In this work the transformation behaviour and the mechanicalproperties of low alloyed TRIP steels were investigated. The influence of theannealing parameters on transformation behaviour and on the amount of retainedaustenite were determined. In addition the temperature dependence of themechanical properties and the effect of testing speed on the formability werestudied. The investigation was carried out on seven different TRIP steels withdifferent chemical compositions, especially the influence of the microalloyingelement niobium was considered. For reasons of comparison various mild andhigh-strength steels were tested parallel to the TRIP steels. It was found that theinvestigated TRIP steels offer very attractive combinations of elongation andstrength values. An interesting temperature dependence of the mechanicalproperties can be observed, in such a way that the elongation values of the TRIPsteels possess a maximum between +50 and +1000C. Due to its effect on grainsize and on precipitation behaviour the addition of niobium leads to higherstrength values without a strong decrease in ductility. In general, the mechanicalproperties are strongly affected by the type and the distribution of the differentphases. The most important parameters, however, to influence the mechanicalbehaviour are the amount and the stability of the retained austenite, which aremainly controlled by the heat treatment and the chemical composition.