Microstructural study on retained austenite in advanced high-strength multiphase 3Mn-1.5Al and 5Mn-1.5Al steels

Purpose: The aim of the paper is to describe crystallographic and morphological features of retained austenite in thermomechanically processed bainite-martensite multiphase steels containing 3 and 5% Mn. Design/methodology/approach: Two groups of steels were designed and investigated: 3Mn-1.5Al and 5Mn-1.5Al were reference steels, whereas next two steels were microalloyed with niobium. The steels were thermomechanically processed using the Gleeble simulator. The isothermal holding temperature to enrich austenite in carbon was between 350 and 450°C. Metallographic investigations were carried out using light (LM) and scanning electron microscopy (SEM). The retained austenite amount and its carbon concentration was evaluated by X-ray analysis. Findings: Manganese addition results in the high hardenability of steels leading to bainitic-martensitic microstructures. A high-Al concept and isothermal holding of steel in a bainitic transformation range allow to obtain a high fraction of retained austenite as a result of an incomplete bainitic transformation phenomenon. New complex bainitic morphologies like degenerate upper and lower bainite were identified using SEM. The microstructure and retained austenite characteristics were correlated with the carbon content in y phase. Research limitations/implications: Further investigations (TEM, EBSD) to describe in detail the identified structural constituents and the effect of Nb microalloying on microstructure and mechanical properties are needed. Practical implications: The knowledge of the influence of the isothermal holding temperature on the microstructure and hardness of thermomechanically processed steels are of primary importance for hot rolling of these multiphase high-strength steels. Originality/value: A problem of the stabilization of retained austenite in advanced high-strength multiphase Nb-free and Nb-microalloyed steels with increased Mn content is discussed.