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INTRODUCTION

A good understanding of the main aspects of phase transformations is very important for an optimal control of the steel properties. The kinetics of austenite formation and decomposition is of great interest, as it is the source phase from which many distinct microstructures are generated. With the development of steels with complex mechanical properties, there is less tolerance in the process variations. Consequently, a good understanding of the mechanisms of austenite formation and decomposition occurring during critical steps in the processing route is important. Austenite decomposition is perhaps the most important process in the fabrication of steels as a wide variety of constituents can be generated by the decomposition of austenite. Pearlite forms as a result of a cooperative process of ferrite and cementite nucleation and growth from the austenite.[1,2] Its morphology consists of parallel plates of cementite surrounded by ferritic phase. The mechanical properties of the steels are directly related to the thickness of the plates.[3] In hypoeutectoid

HENRIQUE DUARTE ALVARENGA, Researcher, is with the Research Group Electrochemical and Surface Engineering (SURF), Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium, and also with the Department of Metallurgy and Materials Science, Universiteit Gent University, Technologiepark 903, 9052 Ghent, Belgium. Contact e-mail: [email protected] NELE VAN STEENBERGE, Researcher, is with ArcelorMittal Global R&D Gent OCAS, Pres. J.F. Kennedylaan 3, 9060 Zelzate, Belgium. JILT SIETSMA, Professor, is with the Department of Materials Science and Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands, and also with the Department of Metallurgy and Materials Science, Universiteit Gent University. HERMAN TERRYN, Professor, is with the Research Group Electrochemical and Surface Engineering (SURF), Vrije Universiteit Brussel, and also with the Department of Materials Science and Engineering, Delft University of Technology. Manuscript submitted July 8, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A

steels, pearlite forms at the interface of the newly formed pro-eutectoid ferrite and the dissolving austenite. Pearlite structures grow at a rate determined by the carbon diffusion in austenite, just ahead of the growth front. This is caused by the carbon partitioning between cementite and ferrite during pearlite formation.[1] The evolution of austenite decomposition predominantly determines the morphology of the carbides. The coiling temperature during the production process has a strong influence on the kinetics of austenite decomposition, consequently on the morphology of carbides.[4,5] Austenite formation has been extensively studied in the past years, although to a less extent as its decomposition. These studies conclude that austenite forms by a preferential nucleation at the grain boundaries, followed by a diffusional growth mechanism.[6–13] The kinetics of austenitization is very complex. Due to the high carbon solubili