Physical simulation and dilatometric study of double-step heat treatment of medium-Mn steel

A Skowronek,C Garcia-Mateo,M Morawiec,A Grajcar,V Ruiz-Jimenez

doi:10.1007/s43452-020-00144-9

A Skowronek, C Garcia-Mateo + Show 3 more

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https://doi.org/10.1007/s43452-020-00144-9

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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.

Highlights

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
The results have shown that along with the decrease in the isothermal temperature from 400 °C to 350 °C, the bainitic transformation is characterized by the lower dynamics generating less signal of the processed sample
The work concerned a detailed study of the physical simulation and dilatometric study of double-step heat treatment of the 3.3 wt.% Mn, 0.17 wt.% C, 1.6 wt.% Al, 0.23 wt.% Mo and 0.22 wt.% Si medium-Mn steel

Summary

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. Depending on the strength requirements of medium-Mn steels, different complex structures can be obtained using appropriate heat treatments. The intercritical annealing is carried out in the α + γ area following the cold rolling of steel sheets [9]. During annealing at 650–720 °C (depending on a chemical composition) the γ phase is formed upon heating as a result of the austenite reverse transformation (ART) from roomtemperature martensite.

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