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

Marc Moreno,Frédéric Bonnet,Sébastien Allain,Jean-Christophe Hell,Julien Teixeira,Guillaume Geandier,Mathieu Salib

doi:10.3390/met9010008

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.

Highlights

Dual-Phase (DP) Ferrite-Martensite steel presents a good formability, which permits the design of complex shapes for automotive structural elements [1]
This paper aims to demonstrate that high energy X-ray diffraction (HEXRD) on synchrotron beamline offers the possibility to follow concomitantly these mechanisms in situ and during a single experiment
Ac1, the recovery processes consist in the reorganization of the dislocation structures and in a decrease in the dislocation density in the deformed grains, which leads to a decrease in the stored strain energy

Summary

Introduction

Dual-Phase (DP) Ferrite-Martensite steel presents a good formability, which permits the design of complex shapes for automotive structural elements [1]. Their heterogeneous microstructures are made of hard martensite islands dispersed in a ductile ferrite/bainite matrix [2,3]. Most of low thickness industrial DP steel sheets are produced through a continuous annealing process of cold-rolled ferrite-pearlite semi-products at high temperature in the ferrite/austenite intercritical domain. The final microstructure is obtained after the last cooling of the annealing stage. The control of the austenite volume fraction at high temperature is, essential to achieve targeted mechanical properties [3].

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