EFFECT OF HEAT TREATMENT AND STRAIN RATES ON STRUCTURE AND MECHANICAL PROPERTIES OF TRIP STEEL

Abstract

Automotive applications demand high strength and high energy absorbing materials to ensure passenger safety. TRIP (Trabsformation Induced Plasticity) steels are one such class of materials that satisfy these automotive demands and hence are under extensive research. Over time, a lot many routes have been designed to process multiphase steels like Dual Phase, bainitic and Tri Phase steels. These are still under consistent development and have successfully replaced the conventional steel grades in various walks of life. But obtaining a good combination of strength and ductility was still an issue till the TRIP effect was discovered. TRIP stands for Transformation Induced Plasticity. Steels exploiting this effect are called TRIP assisted steels. This work aims at investigating the microstructure, crystallography and mechanical properties of a medium carbon 0.33C - 1.67 Si – 0.89Mn – 0.2 Cr (wt %) TRIP steel sample using optical and electron microscopy along with X – ray diffraction and tensile tests. The carbon content of this sample is just right to because a carbon content too high can lead to weldability issues. While a low carbon content can be ineffective in retaining sufficient amounts of austenite because carbon is by far the most important element in improving the stability of retained austenite. Manganese is another important austenite stabilizer while silicon is essential in avoiding carbide formation so that the carbon is instead available for austenite stabilization. The sample received as a leaf suspension was cut down to smaller samples in order to ensure uniform heat treatment. Also round tensile samples of 25mm gauge length and 5 mm diameter were prepared out of the as received sample. The cuboid pieces and tensile samples were subjected to complete austenitization followed by continuous cooling for varied durations and then isothermal bainitic transformation around the bainitic bay temperatures. Here they were isothermally held to ensure transformation of austenite to bainite. The parent austenite is already enriched with carbon due to the nucleation of intercritical ferrite during the continuous cooling. This austenite is further enriched by carbon in the bainitic region making it highly stable even upto the room temperature. After holding at this temperature, the samples were water quenched. With different combinations of the IBT temperature and CC duration, five different heat treatments were carried out. v The heat treated samples were then polished for optical microscopy and etched with Nital and Sodium Metabisulphite to reveal austenite grains in slightly different colour. EDS and XRD analysis was done to further confirm the presence of retained austenite. A carbon content above 1.2% in the austenite islands is indicative of highly stable austenite that can contribute to the TRIP effect. Also FCC peaks in the XRD data confirm the presence of retained austenite and this also allows the calculation of exact percentage of RA. FE-SEM micrographs reveal the variation in the phases with the heat treatment incorporated. tensile tests were performed (along with varying strain rates) on the fabricated samples to get the trend in mechanical properties with the heat treatment parameters. Fractographic analysis after tensile tests helps with explaining the trend in mechanical properties with varying strain rates.