KOKOSZA Adam AGH University of Science and Technology

The results of studies of the influence of intercritical annealing parameters on the austenite formation and its stability in TRIP steel (0.41 % C, 1.52 % Mn and 1.22 % Si) were presented in the paper. Based on the results of a dilatometric analysis and metallographic investigation it was noted that the pearlite to austenite transformation does not occur at a constant temperature, which is referred to as Ac1, but rather within some, possible to determine, temperature range which is bounded by the values Ac1s and Ac1f. It was shown that to achieve austenite with high carbon content, it is required to perform such annealing at a temperature slightly higher than the temperature at which the pearlite to austenite transformation begins (Ac1s). Obtained on this way austenite (highly enriched in carbon ) in further stages of the heat treatment such TRIP steels should favor remaining a increased volume fraction of retained austenite in their microstructure. Dilatometric analysis of cooling curves confirmed these observations. It was noted that the austenite formed at a temperature of Ac1s+10 °C transformed into martensite at a much lower temperature (Ms) than austenite formed during annealing in two-phase temperature range, ie between Ac1f and Ac3 temperatures. Moreover, it was found that formed austenite was the more stable the shorter annealing time was applied. Based on the obtained results the isothermal time-temperature-austenitizing diagram (IHT) was developed. It may to become a useful tool supporting the proper selection of intercritical annealing parameters for the TRIP steel heat treatment.