Tensile mechanical properties of 1.6Si-1.58Mn-0.195C TRIP (transformation-induced plasticity) steels under high strain rate and effects of DP (dual-phase) treatments were studied and compared to the quasi-static tensile behavior. The results show that the increasing of strain rate leads to increasing in their strengths and decreasing in the uniform elongation remarkably. Because the stable retained austenite in TRIP steel can transform to martensite during tensile testing and the material exhibits excellent characteristic of transformation induced plasticity, the plastic deformation behavior is evidently improved and the combination of strength and elongation is superior to that of dual-phase steel, although its strength is smaller than that of DP steel. However, DP treated steel shown lower elongation under dynamic tension in spite of higher strength. A model was proposed to explain the excellent elongation rate of TRIP steel compared with DP steel on the basis of SEM analysis and the strength of the components in microstructure.
X.C. Wei, L. Li, R.Y. Fu and W. ShiSchool of Materials Science and Engineering, Shanghai University, Shanghai 200072, China Manuscript received 18 June 2001
Research status of high strength low alloy TRIP (transformation induced plasticity) steels for automobile structural parts is briefly described. Composition and microstruc-ture factors especially the morphology, size and volume fraction of retained austenite, which largely influence the strength and ductility of the steel, are reviewed and discussed one after another. Modelling of the inter-critical annealing and martempering processes as well as the designing of the TRIP steel aided by commercial software are introduced. Some special aspects of the dynamic mechanical properties of TRIP steel are firstly reported.
With the sublattice model, equilibrium compositions of ferrite (a) and austenite (7 ) phases, as well as the volume percent of austenite (7) at 780℃ in different TRIP steels were calculated. Concentration profiles of carbon, Mn, Al and Si in the steels were also estimated under the lattice fixed frame of reference so as to understand the complex mechanical behavior of TRIP steels after different isothermal bainitic transformation treatments. The effect of Si and Mn on transformation induced plasticity (TRIP) was discussed according to thermodynamic and kinetic analyses. It is recognized that Al also induces phase transformation in the steels but its TRIP effect is not as strong as that of Si.
The relationships between microstructure of 0.195C-1.6Si-1.58 Mn TRIP steel and its dynamic mechanical properties at high strain rate were investigated.The effect of microstructures on dynamic properties was discussed and the comparison with its static mechanical properties was also presented.The specimens of TRIP steel via three heat treatment techniques exhibit different morphological structures,responsible for their dynamic mechanical performances.The dynamic tensile testing was performed on self-made pneumatic tensile impact tester.The results showed that the size,volume fraction,morphology and distribution of retained austenite all affect the final mechanical properties at high strain rate.Among them,the second phase(retained austenite + bainite) with net structure severely decreases the elongation of TRIP steel in spite of the fact that it enhances strength because it restrains ferrite deformation.In order to obtain the excellent combination of strength and elongation,rational matching of morphology,size and volume fraction of several phases in TRIP steel can be obtained via proper heat treatment techniques.
