A study of microstructure and phase transformations of CMnAlSi TRIP steel

Purpose: Purpose was to obtain the TRIP-type microstructure in the CMnAlSi steel. Heat treatment consisted of the partial austenitization at 900*C/60s and continuous cooling with rates: 0.5-40*C/s, was examined. Also the effect of Al and Si on Ac1 and Ac3 temperatures, and the volume fractions of austenite in CMnSi, CmnAl and CMnAlSi steels was investigated. Design/methodology/approach: The effect of alloying elements on Ac1 and Ac3 temperatures was investigated using Thermo-calc program. The influence of cooling rates on phase transformations and microstructures of samples austenitized at 900*C/60s was examined using dilatometer, light optical microscopy and scanning electron microscopy. X-ray diffraction technique was used to calculate the amount of retained austenite. Quantitative analyses of phases were done using Image pro Plus 3.0 program. The mechanical properties and Vickers hardness (HV10) measurements were also investigated. Findings: The TRIP-aided microstructure consisted of ferrite matrix, bainitic ferrite and metastable retained austenite can be obtained for the CMnAlSi steel through intercritical annealing at 900*C/60s and continuous cooling with the rate 20*C/s to the R.T. Isothermal holding at bainitic temperature range (600-400*C) during cooling is not necessary, because of the Al and Si additions to the steel. Practical implications: The CMn steel with addition of 1% Al and Si is well-suited for production of TRIP steel sheets in a large range of temperatures: 800-900*C. The advisable cooling rates are in the range from 10 to 40*C/s. Originality/value: In the TRIP steels the amount of residual austenite in structure at the R. T. strongly depends on the heat treatment parameters such as annealing temperature, cooling rates and amounts of added alloying elements. It is very important to determine the optimal annealing parameters for each TRIP steel grade to obtain the steel with the best mechanical properties and microstructure.