In Situ Investigation of the Iron Carbide Precipitation Process in a Fe-C-Mn-Si Q&amp;P Steel

Sébastien Allain,Frédéric Danoix,Mohamed Gouné,Michel Soler,Magali Bouzat,Samy Aoued,Jean-Christophe Hell,Angéline Quintin-Poulon,Guillaume Geandier

doi:10.3390/ma11071087

Abstract

Quenching and Partitioning (Q&P) steels are promising candidates for automotive applications because of their lightweight potential. Their properties depend on carbon enrichment in austenite which, in turn, is strongly influenced by carbide precipitation in martensite during quenching and partitioning treatment. In this paper, by coupling in situ High Energy X-Ray Diffraction (HEXRD) experiments and Transmission Electron Microscopy (TEM), we give some clarification regarding the precipitation process of iron carbides in martensite throughout the Q&P process. For the first time, precipitation kinetics was followed in real time. It was shown that precipitation starts during the reheating sequence for the steel studied. Surprisingly, the precipitated fraction remains stable all along the partitioning step at 400 °C. Furthermore, the analyses enable the conclusion that the iron carbides are most probably eta carbides. The presence of cementite was ruled out, while the presence of several epsilon carbides cannot be strictly excluded.

Highlights

Quenching and Partitioning (Q&P) is an annealing process proposed in 2003 to elaborate a new generation of advanced high strength steel (AHSS) for automotive construction [1,2]
We show that particles observed by microscopy in tempered martensite are definitely transition carbides, most probably eta carbides but without excluding the presence of epsilon carbides
The experiments presented in this paper uphold the carbon mass balance established by High Energy X-Ray Diffraction (HEXRD) in our previous papers to explain the carbon enrichment in austenite after established by HEXRD in our previous papers to explain the carbon enrichment in austenite after Q&P

Summary

Introduction

Quenching and Partitioning (Q&P) is an annealing process proposed in 2003 to elaborate a new generation of advanced high strength steel (AHSS) for automotive construction [1,2]. The steels manufactured according to this new route show high yield strengths thanks to a refined microstructure as well as good ductility provided by the large fraction of austenite retained after processing, which enables a transformation induced plasticity (TRIP) effect They are seen as possible solutions for automotive makers to lighten their body-in-white structures and to improve their crash resistance. This processing route comprises an incomplete quenching step after an austenitic soaking in order to partially transform austenite into martensite followed by an isothermal partitioning step during which carbon is supposed to diffuse out from martensite (α’) to austenite (γ).

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