Intercritical annealing with isothermal holding of TRIP CMnAlSi steel

Abstract

Purpose: The purpose was to obtain the TRIP microstructure in the modern CMnAlSi steel, after cooling from the temperature 850°C. Samples were continuously cooled with the rate 20°C/s to the R.T or isothermally annealed in the bainitic transformation range during 600s at 500, 450 and 400°C. The influence of cooling type on microstructure and amount of retained austenite were investigated. Design/methodology/approach: Dilatometric experiments of the CMnAlSi steel were done. Microstructures were investigated by light optical microscopy OM and scanning electron microscopy SEM. The amounts of retained austenite were investigated with X-ray diffraction technique and LePera color etching method. Quantitative analyses of phases were done using Image pro Plus program. Vickers hardness HV10 measurements were done. Findings: The TRIP type microstructure can be obtained for the investigated steel through continuous cooling from 850°C/60s or after cooling with the isothermal holding at the bainitic transformation temperatures range. The highest amount of austenite (~12%) was observed in samples isothermally annealed at the temperatures 450/600s and 400°C/600s. Practical implications: Steel CMnAlSi is well suited for production of TRIP grade in a large range of isothermal holding temperatures from 500°C to 400°C and also after continuous cooling with the rate 20°C/s to the R.T. The amount of 39 % austenite at temperature 850°C allows for production of TRIP microstructure with stable retained austenite. Originality/value: The additions of Al and/or Si to the CMn steels have the influence on Ac1 and Ac3 temperatures, morphology of bainite and on stability of retained austenite in the TRIP steels. Therefore it is important to determine the heat treatment parameters for each grade of TRIP steels to obtain optimal microstructure with good mechanical properties for such applications as car bodies and car parts.