Physical simulation and dilatometric study of double-step heat treatment of medium-Mn steel
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(2020) 20:136
ORIGINAL ARTICLE
Physical simulation and dilatometric study of double‑step heat treatment of medium‑Mn steel A. Skowronek1 · M. Morawiec1 · V. Ruiz‑Jimenez2 · C. Garcia‑Mateo2 · A. Grajcar1 Received: 21 August 2020 / Revised: 2 October 2020 / Accepted: 16 October 2020 © The Author(s) 2020
Abstract The work addresses physical simulation and dilatometric study of one-step and double-step heat treatments of medium-Mn steel designed for automotive sheets. The conventional one-step isothermal bainitic transformation was applied as the reference heat treatment. The newly implemented heat treatment consisted of isothermal holding in a bainitic region followed by additional holding of the material at reduced temperature also in the bainitic range. This step was added to refine the microstructure, which led to the stabilization of the retained austenite. Calculations of equilibrium state and non-equilibrium cooling and simulations of the developed thermal cycles were performed using the thermodynamic JMatPro software. The physical simulations of the heat treatment were performed in the dilatometer. The obtained samples were subjected to microscopic observations using light and SEM microscopy. One- and two-step heat treatments allowed to obtain bainitic structures with high contents of retained austenite. Lowering the temperature of one-step isothermal holding resulted in the bainite refinement and adjacent retained austenite. The increased Mn content in steel increased its susceptibility to form coalesced bainite resulting in the partial formation of thicker plates despite a decrease in a process temperature. Keywords Dilatometric study · Medium-Mn sheet steel · Complex microstructure · Bainitic transformation · Retained austenite
1 Introduction Medium-manganese sheet steels containing 3–12 wt% Mn and belonging to the third generation of AHSS (Advanced High Strength Steels) are currently very popular among researchers and industry due to the combination of high strength properties and relatively good plasticity. There are several variants of heat treatment for these steels: intercritical annealing after cold rolling [1] or hot rolling [2], and isothermal holding in a bainitic region [3].The goal of each is to obtain a large fraction of retained austenite in the structure characterized by optimal thermal and mechanical stabilities. To do this, it is necessary to enrich the austenite with stabilizing elements—carbon and manganese—thanks * A. Skowronek [email protected] 1
Department of Engineering Materials and Biomaterials, Silesian University of Technology, Konarkiego 18a St., 44‑100 Gliwice, Poland
National Center for Metallurgical Research, Av. de Gregorio del Amo 8, 28040 Madrid, Spain
2
to which the M sγ (martensite start) temperature is reduced and thus there is no martensitic transformation during the steel cooling to room temperature. Retained austenite as a metastable phase [4] increases the work-hardening rate and plasticity at the same time due to the TRIP (Transformati
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