The manufacturing of Dual-Phase steels includes as a crucial step the annealing of a cold-rolled ferrite-pearlite (F/P) microstructure, which involves numerous and interacting metallurgical mechanisms, namely recovery/recrystallization of ferrite, globularization, manganese enrichment, coarsening of cementite and finally austenite transformation. Present study focuses on the austenite transformation considering its interaction with the ferrite recrystallization and the influence of the chemical composition of the cementite. The behavior of a cold-rolled F/P microstructure is studied at three heating rates to induce weak and strong interactions between the mechanisms, in particular using post mortem microstructure observations but also in situ High Energy X-Ray Diffraction experiments on a synchrotron beamline. Slow heating leads to a necklace austenite distribution whereas fast heating conducts to a banded topology. This particular microstructure morphogenesis is explained by the presence of numerous intergranular (or isolated) carbides inside the ferrite matrix, inherited from the hot-rolling. Thermokinetic analysis accounting for the cementite composition shows that the pearlite islands transformation necessarily involves the partition of substitutional elements. Conversely, the dissolving isolated carbides undergo a partition/partitionless transition on heating. After the dissolution of the cementite, a final ferrite/austenite transformation takes place. The phase transformation kinetics increases with increasing heating rates, despite the thermal-activated nature of the austenite growth process. This is interpreted thanks to kinetic simulations with DICTRA software, which allow to analyze the austenite growth regimes involving or not the partition of the alloying elements.

The annealing process of cold-rolled ferrite/pearlite steel involves numerous metallurgical mechanisms as recovery/recrystallization of deformed phases, ripening of carbide microstructure, and austenite transformation in the intercritical domain. The interactions between these mechanisms govern the morphogenesis of the transformed austenite microstructure and, thus, the final properties of the steel. This paper demonstrates that high energy X-ray diffraction (HEXRD) on synchrotron beamline offers the unique possibility to follow concomitantly these mechanisms in situ during a single experiment. A cold-rolled ferrite-pearlite steel dedicated to the industrial production of Dual-Phase steel serves as case-study. Synchrotron experiments have been conducted in transmission at 100 keV with a 2D detector. Diffraction patterns acquired all along an annealing treatment are first analyzed after circular integration. A Rietveld refinement procedure coupled with a Williamson-Hall approach is used to determine phase transformation and recovery kinetics. In this paper, a new method inspired by the 3D X-ray diffraction tomography is proposed to follow recrystallization kinetics at the same time. It is based on a systematic detection of individual diffraction spots related to newly recrystallized grains appearing on Debye-Scherrer rings. The deduced recrystallization kinetics is compared and validated by more conventional ex situ methods.

No abstract