The Effect of Ultrafast Heating on Cold-Rolled Low Carbon Steel: Formation and Decomposition of Austenite

Felipe Castro Cerda,Bernd Schulz,Spyros Papaefthymiou,Roumen Petrov,Alfredo Artigas,Alberto Monsalve

doi:10.3390/met6120321

Felipe Castro Cerda, Bernd Schulz + Show 4 more

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https://doi.org/10.3390/met6120321

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Abstract

The effect of heating rate on the formation and decomposition of austenite was investigated on cold-rolled low carbon steel. Experiments were performed at two heating rates, 150 °C/s and 1500 °C/s, respectively. The microstructures were characterized by means of scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). Experimental evidence of nucleation of austenite in α/θ, as well as in α/α boundaries is analyzed from the thermodynamic point of view. The increase in the heating rates from 150 °C/s to 1500 °C/s has an impact on the morphology of austenite in the intercritical range. The effect of heating rate on the austenite formation mechanism is analyzed combining thermodynamic calculations and experimental data. The results provide indirect evidence of a transition in the mechanism of austenite formation, from carbon diffusion control to interface control mode. The resulting microstructure after the application of ultrafast heating rates is complex and consists of a mixture of ferrite with different morphologies, undissolved cementite, martensite, and retained austenite.

Highlights

Very fast heating rates have been applied in the case hardening of the structural steels, ultrafast heating (UFH) of flat steel products is considered to be among the new processing routes [1,2,3,4,5]
The methodology of the present study considers the formation of austenite in cold-rolled low carbon steel as a process independent from recrystallization, which is still sensitive to the changes in cementite morphology
The features of austenite formation will be described in conjunction with the martensite size and morphology found in the final microstructure

Summary

Introduction

Very fast heating rates have been applied in the case hardening of the structural steels, ultrafast heating (UFH) of flat steel products is considered to be among the new processing routes [1,2,3,4,5]. A recent study on cold-rolled low carbon steel [5] shows that both strength and ductility can be enhanced at the same time after the application of ultrafast heating rates. The improvement in mechanical properties stems from the variety of microstructures resulting from the application of UFH. Several simultaneous processes are taking place during the continuous heating of steel, impacting the final microstructure. Austenite formation during UFH experiments is summarized elsewhere [6]. Kaluba et al [8] proposed a so-called “bainitic” mechanism active

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