Microstructure-based behavior law for globular pearlitic steels

International audiencePearlitic steels are widely used in the industry for their good balance between strength, ductility and machinability. Sometimes, the classical lamellar structure can degenerate in a so-called globular structure which may significantly affect the behavior of the steel. To our knowledge, no size sensitive model is available in the literature to predict their mechanical behaviors and work-hardenings. Inspired by their analogy with ferrite–martensite dual-phase microstructures, we propose a new microstructure-based law for globular pearlite. It describes the evolution of the dislocation density into ferritic grains which is enhanced by the presence of hard particles. The model is as a consequence sensitive to ferritic grain but also to coarse carbide sizes after globularization. The model has been calibrated on a conventional steel grade and appears to be a practical tool for microstructure design

Dual-Phase Steels: The First Family of Advanced High Strength Steels

International audienceDual-Phase steels have been marketed since the early 1990s and are widely used in the sector of automotive and truck construction. They make up the majority of so-called modern or first-generation AHSS (Advanced High Strength Steels). The superior balance between strength, formability and ductility of DP steels comes from their particular composite microstructure, consisting mainly of ferrite and martensite. This article traces first the history of DP steels from their discovery in the 1960s to the most recent industrial developments. It then addresses current knowledge about the link between the microstructure and mechanical properties of these alloys. The mechanisms governing their microstructure morphogenesis are described in a third part. During those discussions, the contributions of metallurgical and micromechanical modeling in DP science shall be highlighted. Finally, the active fields of research and the anticipated developments of DP technologies will be evoked

Carbon heterogeneities in austenite during Quenching & Partitioning (Q&P) process revealed by in situ High Energy X-Ray Diffraction (HEXRD) experiments

International audienceBased on the evolution of the positions and intensities of the diffraction peaks, high energy X-ray diffraction (HEXRD) is recognized as the ultimate method to follow quantitatively in situ phase transformations in steels. However, the possible asymmetricity of diffraction peaks is seldom considered, and is known to bear information. A procedure for quantifying their skewness is proposed. In the case of a third generation high strength steel obtained by quench and partitioning

Carbon heterogeneities in austenite during Quenching & Partitioning (Q&P) process revealed by in situ High Energy X-Ray Diffraction (HEXRD) experiments

Based on the evolution of the positions and intensities of the diffraction peaks, high energy X-ray diffraction (HEXRD) is recognized as the ultimate method to follow quantitatively in situ phase transformations in steels. However, the possible asymmetricity of diffraction peaks is seldom considered, and is known to bear information. A procedure for quantifying their skewness is proposed. In the case of a third generation high strength steel obtained by quench and partitioning 1Partition du carbone dans les phases ferritiques nanostructurées: cinétiques et microstructuresDesign des Alliages Métalliques pour Allègement des Structure

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

An original mean field model for the nucleation and the growth of new recrystallized grains during annealing treatments of deformed, low-carbon ferritic steels is proposed in this paper. The model was calibrated on two steels extensively studied in the literature under both isothermal annealing and continuous heating schedules. It permits one to predict not only recrystallization kinetics but also advanced microstructural features (such as dislocation density, dislocation cell size and grain size) during complex heat treatments. Once calibrated, the model was applied to the case of a third ferrite/pearlite steel and was shown to accurately capture the effect of cold-rolling ratio on the recrystallization kinetics

Thermodynamic effetcs of substitutional elements on the CPE interface conditions in Q&P steels

International audienc

Thermodynamic effetcs of substitutional elements on the CPE interface conditions in Q&P steels

International audienc

Thermodynamic effetcs of substitutional elements on the CPE interface conditions in Q&P steels

International audienc

Numerical investigations of the effects of substitutional elements on the interface conditions during partitioning in quenching and partitioning steels

International audienceIn quenched and partitioned steels, carbon partitioning is considered to be driven by a constraint para-equilibrium at the martensite/austenite interface. Using Thermo-Calc calculations, we investigated the effect of non-partitioned elements on the resulting interface condition. Among all tested elements, only aluminum and chromium have significant effects. From this numerical study, a practical composition- and temperature-dependent relationship describing interface tie lines was derived and calibrated for Fe-C-2.5Mn-1.5Si-X wt pct alloys (X = Cr or Al)

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

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