Austenite Decomposition and Precipitation Behavior of Plastically Deformed Low-Si Microalloyed Steel

Adam Grajcar,Wladyslaw Zalecki,Mateusz Morawiec

doi:10.3390/met8121028

Abstract

The aim of the present study is to assess the effects of hot deformation and cooling paths on the phase transformation kinetics in a precipitation-strengthened automotive 0.2C–1.5Mn–0.5Si steel with Nb and Ti microadditions. The analysis of the precipitation processes was performed while taking into account equilibrium calculations and phase transitions resulting from calculated time–temperature–transformation (TTT) and continuous cooling transformation (CCT) diagrams. The austenite decomposition was monitored based on thermodynamic calculations of the volume fraction evolution of individual phases as a function of temperature. The calculations were compared to real CCT and DCCT (deformation continuous cooling transformation) diagrams produced using dilatometric tests. The research included the identification of the microstructure of the nondeformed and thermomechanically processed supercooled austenite products formed at various cooling rates. The complex interactions between the precipitation process, hot deformation, and cooling schedules are linked.

Highlights

The strengthening of advanced ferrous alloys can be achieved in many ways, including solid solution strengthening, grain refinement, cold deformation, precipitation phenomena, phase transformations, high-pressure methods, etc. [1,2,3]
A special feature of transformation-induced plasticity (TRIP) steels is the presence of carbide-free bainite, which can be formed during a multistep heat treatment using increased Si and/or Al additions [14]
The results indicate that the plastic deformation significantly affects the technological windows required for producing multiphase microstructures consisting of ferrite, bainite, and retained austenite

Summary

Introduction

The strengthening of advanced ferrous alloys can be achieved in many ways, including solid solution strengthening, grain refinement, cold deformation, precipitation phenomena, phase transformations, high-pressure methods, etc. [1,2,3]. One of the most interesting and evolving groups of advanced steels for the automotive industry is the multiphase transformation-induced plasticity (TRIP)-aided steels consisting of ferrite, bainite, and retained austenite [4,5,6] These steels utilize different strengthening mechanisms to improve their strength level, but the most important is the strain-induced transformation of retained austenite into martensite [7,8,9]. This transformation takes place during the cold forming of steel sheets and results in improving both strength and plasticity. Such a high amount of this element deteriorates the sheet wettability by liquid zinc during hot-dip galvanizing [15]

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Conclusion