Real-Time Investigation of Recovery, Recrystallization and Austenite Transformation during Annealing of a Cold-Rolled Steel Using High Energy X-ray Diffraction (HEXRD)

Abstract

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.

A Physics-Based Mean-Field Model for Ferrite Recovery and Recrystallization

“…The density of dislocation in the deformed state has been estimated to be 1.7 × 10 15 m.m −3 . In the last part of this paper, the model will also be applied to the steel studied by Moreno et al (steel C) [7,23]. The purpose is to demonstrate that it is able to capture the effect of cold-reduction ratio on recrystallization kinetics.…”

“…Different equations are proposed in the literature to model the evolution of the dislocation density as a function of time t and temperature T [25]. The model developed by Verdier et al [26] for Al-Mg alloys was able to reproduce some observed trends in ferritic steels but not it did not lead to a perfect agreement with the experimental data (e.g., data of Moreno et al [23]). For this reason, we could not justify the use of the complex model of Verdier et al and reverted to the original relaxation model of Friedel et al [27]:…”

“…Those data were not used to calibrate the model, since the kinetics were measured by high energy x-ray diffraction (HEXRD) and a new peak counting procedure, which cannot be considered as a reference and an absolute method to measure the recrystallized fraction. All the details about this original and exploratory method have been published elsewhere [7,23]. Steel C is similar to the steel studied by Zhu et al except that the initial grain size is about 8 µm and the pearlite fraction was estimated to 16%.…”

Intercritical annealing of cold-rolled ferrite-pearlite steel: Microstructure evolutions and phase transformation kinetics

“…A new method has been introduced recently to track in situ by HEXRD the recrystallization kinetics, together with the phase transformation kinetics [55]. This method is related to the presence of individual spots of intensity inside the Debye-Scherrer diffraction rings, which are ascribed to the new recrystallizing ferrite grains.…”

“…A steel with composition Fe-0.1C-1.9Mn-0.2Cr-0.2Si wt.% with cold-rolled ferrite-pearlite initial microstructure is considered. A new experimental approach [55] is utilized to establish in situ by synchrotron high energy XRD the austenite transformation and the ferrite recovery/recrystallization global kinetics, in one single experiment. Three heating rates before isothermal annealing at 800°C are applied, fast (30 and 100°C.s -1 ) and slow (3°C.s -1 ) to induce strong or weak interactions.…”