Thermodynamic analysis and isothermal bainitic transformation kinetics in lean medium-Mn steels
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Thermodynamic analysis and isothermal bainitic transformation kinetics in lean medium‑Mn steels M. Morawiec1 · V. Ruiz‑Jimenez2 · C. Garcia‑Mateo2 · A. Grajcar1 Received: 1 December 2019 / Accepted: 14 September 2020 © The Author(s) 2020
Abstract The work presents the results of thermodynamic analysis of two medium manganese steels with different Mn contents. The steels containing 3.1 and 3.6% of manganese were subjected to theoretical thermodynamic calculations using MUCG83 software and dilatometric experiments. The steels were heat-treated in two different isothermal holding temperatures of 400 and 350 °C for 15 min. The bainite transformation kinetics at different temperatures for different manganese contents was investigated. In the steel including 3.1% Mn, a complete transformation was obtained. The results indicated a strong influence of the holding temperature on the kinetics of bainitic transformation. It was related to the driving force of this process. When the manganese content was increased by 0.5%, an incomplete bainite transformation occurred. The microstructure investigations after heat treatment were performed using light and scanning electron microscopy. The XRD analysis to determine retained austenite amount and its carbon enrichment was performed. The microstructure of 3MnNb steel consisted of bainite and retained austenite with filmlike and blocky morphologies. The steel with the higher Mn content contained also fresh martensite for both isothermal holding temperatures. Keywords Isothermal bainitic transformation · Medium-Mn steel · Dilatometry · Thermodynamic calculations · Retained austenite
Introduction The development of new materials for automotive industry strongly affects various investigations conducted around the world. The design process needs to take into account safety and environmental aspects of car body production. One of the most important factors in car body manufacture is the material price. Medium-Mn steels are dedicated to the automotive sector due to their multiphase structure generating very beneficial properties. These steels are designed to compromise between cost and mechanical properties. Medium-Mn steels should have higher mechanical properties than first generation AHSS but be cheaper than second generation of AHSS (advanced high-strength steels) [1–3]. * M. Morawiec [email protected] 1
Department of Engineering Materials and Biomaterials, Silesian University of Technology, 18a Konarskiego Street, Gliwice, Poland
National Center for Metallurgical Research, Av. de Gregorio del Amo, 8, 28040 Madrid, Spain
2
The main structural constituent that gives these steels good formability is retained austenite. This phase undergoes strain-induced martensitic transformation. A local increase in strength accompanying this transformation allows prolonging deformation, which finally leads to higher formability and mechanical properties [4–6]. These aspects allow to implement such steels for cold-rolled steel coils for the automotive application. This approac
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