Precipitation and grain growth modelling in Ti-Nb microalloyed steels

Peer ReviewedPostprint (author's final draft

Design and development of complex phase steels with improved combination of strength and stretch-flangeability

This study presents the design and development of a hot-rolled bainitic steel, presenting a good combination of strength and stretch-flangeability, for automotive applications. Ti, Nb, and Mo were added in the steel composition in order to control austenite grain sizes, enhance precipitation hardening, and promote the formation of bainite. This study focuses on the effect of process parameters on final microstructures and mechanical properties. These parameters are the finishing rolling temperature, which conditions the austenite microstructure before its decomposition, and the coiling temperature, which conditions the nature and morphology of the ferritic phases transformed. A preliminary study allowed to determine the austenite grain growth behavior during reheating, the recrystallization kinetics, and the continuous cooling transformation curves of the studied steel. Then, a first set of parameters was tested at a semi-industrial scale, which confirmed that the best elongation properties were obtained for homogeneous bainitic lath/granular microstructures, that can be produced by choosing a coiling temperature of 500 °C . When choosing those parameters for the final industrial trial, the microstructure obtained consisted of a homogeneous lath/granular bainite mixture that presented a Ultimate Tensile Strength of 830 MPa and a Hole Expansion Ratio exceeding 70%.Peer ReviewedPostprint (published version

Microstructure evolutions during hot rolling of low carbon microalloyed steels

Il existe actuellement une dynamique de développement d’aciers à très haute résistance, destinés à l’industrie automobile, et présentant des propriétés de formabilité améliorées. Ces travaux font partie d'un projet de recherche qui vise à produire des aciers à phase complexe (CP) par laminage à chaud, en recherchant une combinaison améliorée de résistance et d’aptitude à l’étirement des bordures. Ainsi, ces travaux ont porté sur la description et la compréhension des évolutions de la microstructure au cours des différentes étapes du laminage à chaud d’aciers microalliés à faible teneur en carbone. Dans un premier temps, l’évolution de l’état de précipitation et de la taille des grains austénitiques au cours du réchauffage des brames ont été étudiés. Un modèle de précipitation a été développé et couplé à un modèle de croissance de grain simple basé sur l'épinglage Zener pour décrire les évolutions de la microstructure qui se produisent pendant le réchauffage. Ensuite, les évolutions microstructurales prenant places après la déformation à chaud lors du laminage ont été analysées. Les cinétiques de recristallisation de l'austénite et de la précipitation induite par déformation ont été étudiées par des essais de relaxation de contraintes couplées à observations directes de la microstructure. Des modèles ont également été développés pour décrire ces évolutions microstructurales. Un dernier chapitre a été consacré à l'établissement des relations entre les paramètres de laminage à chaud, les microstructures et les propriétés mécaniques obtenues. Les transformations de phase au cours de refroidissements continus ont été établis, en prenant en compte l’effet de la déformation de l’austénite. Ensuite, six procédés différents de laminage à chaud différents ont été appliqués et les microstructures et propriétés mécaniques résultantes ont été caractérisées de manière détaillée. Ce travail de thèse a permis de mieux comprendre les évolutions de la microstructure se produisant lors du laminage à chaud, ainsi que les microstructures et les propriétés mécaniques qui en résultent. Les travaux de modélisation présentés pourraient s’étendre à l’étude d’autres alliages. Plusieurs stratégies visant à améliorer la combinaison de résistance et d’aptitude à l’étirement des bordures ont été proposées.There are nowadays major driving forces for the development of Advanced High Strength steels presenting enhanced formability properties for automotive applications. This PhD-work is part of a research project that aims at producing complex phase (CP) steels by hot rolling, seeking for an enhanced combination of strength and stretch flangeability. Thus, this PhD-work focused on the description and the understanding of the microstructural evolutions during the various steps of the hot rolling process of low carbon microalloyed steels. First, the evolution of the precipitation state and austenite grain sizes during the reheating stage of hot rolling were studied. A precipitation model was developed and coupled to a simple grain growth model based on Zener pinning to describe microstructural evolutions that occurs during reheating. Then, the microstructural evolutions occurring after the hot rolling deformation passes were analyzed. The kinetics of austenite recrystallization and strain-induced precipitation were determined by stress relaxation and direct microstructural observations. Models were also developed for describing these microstructural evolutions. A final chapter focused on establishing the relationships between the hot rolling parameters, the microstructures, and the mechanical properties obtained. The phase transformation during continuous cooling was established, including the effect of austenite deformation. Then, six different hot rolling processes were applied, and the resulting microstructures and mechanical properties were extensively characterized. This PhD-work provided a better understanding of the microstructural evolutions taking place during hot rolling and of the resulting microstructures and mechanical properties. The modeling work presented could extended to the study of other alloys. Several strategies for improving the combination of strength and stretch flangeability were proposed

Evolutions microstructurales au cours du laminage à chaud d’aciers bas carbone microalliés

There are nowadays major driving forces for the development of Advanced High Strength steels presenting enhanced formability properties for automotive applications. This PhD-work is part of a research project that aims at producing complex phase (CP) steels by hot rolling, seeking for an enhanced combination of strength and stretch flangeability. Thus, this PhD-work focused on the description and the understanding of the microstructural evolutions during the various steps of the hot rolling process of low carbon microalloyed steels. First, the evolution of the precipitation state and austenite grain sizes during the reheating stage of hot rolling were studied. A precipitation model was developed and coupled to a simple grain growth model based on Zener pinning to describe microstructural evolutions that occurs during reheating. Then, the microstructural evolutions occurring after the hot rolling deformation passes were analyzed. The kinetics of austenite recrystallization and strain-induced precipitation were determined by stress relaxation and direct microstructural observations. Models were also developed for describing these microstructural evolutions. A final chapter focused on establishing the relationships between the hot rolling parameters, the microstructures, and the mechanical properties obtained. The phase transformation during continuous cooling was established, including the effect of austenite deformation. Then, six different hot rolling processes were applied, and the resulting microstructures and mechanical properties were extensively characterized. This PhD-work provided a better understanding of the microstructural evolutions taking place during hot rolling and of the resulting microstructures and mechanical properties. The modeling work presented could extended to the study of other alloys. Several strategies for improving the combination of strength and stretch flangeability were proposed.Il existe actuellement une dynamique de développement d’aciers à très haute résistance, destinés à l’industrie automobile, et présentant des propriétés de formabilité améliorées. Ces travaux font partie d'un projet de recherche qui vise à produire des aciers à phase complexe (CP) par laminage à chaud, en recherchant une combinaison améliorée de résistance et d’aptitude à l’étirement des bordures. Ainsi, ces travaux ont porté sur la description et la compréhension des évolutions de la microstructure au cours des différentes étapes du laminage à chaud d’aciers microalliés à faible teneur en carbone. Dans un premier temps, l’évolution de l’état de précipitation et de la taille des grains austénitiques au cours du réchauffage des brames ont été étudiés. Un modèle de précipitation a été développé et couplé à un modèle de croissance de grain simple basé sur l'épinglage Zener pour décrire les évolutions de la microstructure qui se produisent pendant le réchauffage. Ensuite, les évolutions microstructurales prenant places après la déformation à chaud lors du laminage ont été analysées. Les cinétiques de recristallisation de l'austénite et de la précipitation induite par déformation ont été étudiées par des essais de relaxation de contraintes couplées à observations directes de la microstructure. Des modèles ont également été développés pour décrire ces évolutions microstructurales. Un dernier chapitre a été consacré à l'établissement des relations entre les paramètres de laminage à chaud, les microstructures et les propriétés mécaniques obtenues. Les transformations de phase au cours de refroidissements continus ont été établis, en prenant en compte l’effet de la déformation de l’austénite. Ensuite, six procédés différents de laminage à chaud différents ont été appliqués et les microstructures et propriétés mécaniques résultantes ont été caractérisées de manière détaillée. Ce travail de thèse a permis de mieux comprendre les évolutions de la microstructure se produisant lors du laminage à chaud, ainsi que les microstructures et les propriétés mécaniques qui en résultent. Les travaux de modélisation présentés pourraient s’étendre à l’étude d’autres alliages. Plusieurs stratégies visant à améliorer la combinaison de résistance et d’aptitude à l’étirement des bordures ont été proposées

An alternative method for the measurement of precipitate volume fractions in microalloyed steels by the means of atom probe tomography

International audienceKnowledge of the volume fraction of precipitates is crucial for estimating the impact of precipitation on microstructure evolution or mechanical properties. However, its experimental determination is often a difficult task. In this work, atom probe tomography was applied to an industrial TieNb microalloyed steel, to follow the evolution of austenite solute composition in titanium and niobium as a function of temperature in the austenitic domain. These composition measurements were used to calculate the volume fraction of (Ti x , Nb 1-x)C carbides in austenite, using mass balance. This type of measurement is made possible by the considerable evolutions experienced by tomographic atom probes over the past 20 years in terms of volume analyzed and mass resolution. Atom probe tomography is nowadays not only able to help determining volume fractions below 0.1%, but also provides unique information related to solubility limits as low as a few tens of ppm, most useful for developing phase diagrams, or assessing existing ones

An alternative method for the measurement of precipitate volume fractions in microalloyed steels by the means of atom probe tomography

International audienceKnowledge of the volume fraction of precipitates is crucial for estimating the impact of precipitation on microstructure evolution or mechanical properties. However, its experimental determination is often a difficult task. In this work, atom probe tomography was applied to an industrial TieNb microalloyed steel, to follow the evolution of austenite solute composition in titanium and niobium as a function of temperature in the austenitic domain. These composition measurements were used to calculate the volume fraction of (Ti x , Nb 1-x)C carbides in austenite, using mass balance. This type of measurement is made possible by the considerable evolutions experienced by tomographic atom probes over the past 20 years in terms of volume analyzed and mass resolution. Atom probe tomography is nowadays not only able to help determining volume fractions below 0.1%, but also provides unique information related to solubility limits as low as a few tens of ppm, most useful for developing phase diagrams, or assessing existing ones

Design and Development of Complex Phase Steels with Improved Combination of Strength and Stretch-Flangeability

© 2020 by the authors.This study presents the design and development of a hot-rolled bainitic steel, presenting a good combination of strength and stretch-flangeability, for automotive applications. Ti, Nb, and Mo were added in the steel composition in order to control austenite grain sizes, enhance precipitation hardening, and promote the formation of bainite. This study focuses on the effect of process parameters on final microstructures and mechanical properties. These parameters are the finishing rolling temperature, which conditions the austenite microstructure before its decomposition, and the coiling temperature, which conditions the nature and morphology of the ferritic phases transformed. A preliminary study allowed to determine the austenite grain growth behavior during reheating, the recrystallization kinetics, and the continuous cooling transformation curves of the studied steel. Then, a first set of parameters was tested at a semi-industrial scale, which confirmed that the best elongation properties were obtained for homogeneous bainitic lath/granular microstructures, that can be produced by choosing a coiling temperature of 500 ∘C . When choosing those parameters for the final industrial trial, the microstructure obtained consisted of a homogeneous lath/granular bainite mixture that presented a Ultimate Tensile Strength of 830 MPa and a Hole Expansion Ratio exceeding 70%.The authors gratefully acknowledge the funding received from the European Commission, Research Fund for Coal and Steel, under grant agreement 709803 (NANOFORM).Publisher’s versio

Design and Development of Complex Phase Steels with Improved Combination of Strength and Stretch-Flangeability

International audienceThis study presents the design and development of a hot-rolled bainitic steel, presenting a good combination of strength and stretch-flangeability, for automotive applications. Ti, Nb, and Mo were added in the steel composition in order to control austenite grain sizes, enhance precipitation hardening, and promote the formation of bainite. This study focuses on the effect of process parameters on final microstructures and mechanical properties. These parameters are the finishing rolling temperature, which conditions the austenite microstructure before its decomposition, and the coiling temperature, which conditions the nature and morphology of the ferritic phases transformed. A preliminary study allowed to determine the austenite grain growth behavior during reheating, the recrystallization kinetics, and the continuous cooling transformation curves of the studied steel. Then, a first set of parameters was tested at a semi-industrial scale, which confirmed that the best elongation properties were obtained for homogeneous bainitic lath/granular microstructures, that can be produced by choosing a coiling temperature of 500 ∘ C . When choosing those parameters for the final industrial trial, the microstructure obtained consisted of a homogeneous lath/granular bainite mixture that presented a Ultimate Tensile Strength of 830 MPa and a Hole Expansion Ratio exceeding 70%

Precipitation and grain growth modelling in Ti-Nb microalloyed steels

International audienceMechanical properties of microalloyed steels are enhanced by fine precipitates, that ensure grain growth control during subsequent heat treatment. This study aims at predicting austenite grain growth kinetics coupling a precipitation model and a grain growth model. These models were applied to a titanium and niobium microalloyed steel. The precipitate size distributions were first characterized by TEM and SEM and prior austenite grain boundaries were revealed by thermal etching after various isothermal treatments. From CALPHAD database, a solubility product was determined for (Ti,Nb)C precipitates. A numerical model based on the classical nucleation and growth theories was used to predict the time evolution of (Ti,Nb)C size distributions during various isothermal heat treatments. The precipitation model was validated from TEM/SEM analysis. The resulting precipitate size distributions served as entry parameters to a simple grain growth model based on Zener pinning. The pinning pressure was calculated using the whole size distribution. The resulting austenite grain growth kinetics were in good agreement with the experimental data obtained for all investigated heat treatments

Precipitation and grain growth modelling in Ti-Nb microalloyed steels

International audienceMechanical properties of microalloyed steels are enhanced by fine precipitates, that ensure grain growth control during subsequent heat treatment. This study aims at predicting austenite grain growth kinetics coupling a precipitation model and a grain growth model. These models were applied to a titanium and niobium microalloyed steel. The precipitate size distributions were first characterized by TEM and SEM and prior austenite grain boundaries were revealed by thermal etching after various isothermal treatments. From CALPHAD database, a solubility product was determined for (Ti,Nb)C precipitates. A numerical model based on the classical nucleation and growth theories was used to predict the time evolution of (Ti,Nb)C size distributions during various isothermal heat treatments. The precipitation model was validated from TEM/SEM analysis. The resulting precipitate size distributions served as entry parameters to a simple grain growth model based on Zener pinning. The pinning pressure was calculated using the whole size distribution. The resulting austenite grain growth kinetics were in good agreement with the experimental data obtained for all investigated heat treatments