Metallurgical mechanisms and their interactions during the annealing of cold-rolled ferrite-pearlite steels : characterization and modeling

Abstract : Ferrite/Martensite Dual-Phase steels are largely used in the form of thin sheets in the automotive industry for their excellent balance between resistance and strength and thus for their lightening potential. They are elaborated by continuous casting, hot- and cold- rolling, followed by a continuous annealing. During the heating and the soaking stages of this latter process, the deformed ferrite/pearlite microstructure obtained after rolling evolves is transformed into a recrystallized ferrite-austenite microstructure. The experiments show that recrystallization and austenite transformation kinetics as well as the resulting spatial and morphological distribution of the phases are highly sensitive to the heating rate. This PhD thesis aims at understanding the different metallurgical mechanisms explaining this particular sensitivity as carbides ripening, recovery, recrystallization and austenite transformation and all their possible interactions. The mechanisms were characterized at different scales and by in situ technics on an industrial steel and model by physical based approaches in order to drive future production lines. After a first chapter dedicated to the experimental and modeling methods, the second chapter deals with the characterization of the morphogenesis of ferrite-austenite microstructures by Scanning Electron Microscopy (SEM). Chapter 3 is a study by Transmission Electron Microscopy (TEM) and by thermokinetic modeling (ThermoCalc, DICTRA) of the chemical composition of carbides along with manufacturing, from hot-rolling to annealing. Recovery and recrystallization are studied in chapter 4 by the means of in situ High Energy X-Ray Diffraction (HEXRD) experiments conducted on a synchrotron beamline and modeled by an original mean-field approach. Finally, chapter 5 proposes an analysis with DICTRA to understand austenite transformation kinetics as function of heating rates. The proposed approach is innovative as it accounts for intergranular carbides in the ferrite matrix, is conducted in non-isothermal conditions and propose a fine analysis of growth modes of austenite associated to manganese, a key alloying element of the studied steels.
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Theses
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https://hal.univ-lorraine.fr/tel-02349113
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Submitted on : Tuesday, November 5, 2019 - 3:22:41 PM
Last modification on : Wednesday, November 6, 2019 - 1:47:13 AM

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  • HAL Id : tel-02349113, version 1

Citation

Marc Moreno. Metallurgical mechanisms and their interactions during the annealing of cold-rolled ferrite-pearlite steels : characterization and modeling. Materials. Université de Lorraine, 2019. English. ⟨NNT : 2019LORR0068⟩. ⟨tel-02349113⟩

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