Influence of Ni and Process Parameters in Medium Mn Steels Heat Treated by High Partitioning Temperature Q&P Cycles

In this work, two medium Mn steels (5.8 and 5.7 wt pct Mn) were subjected to a quenching and partitioning (Q&P) treatment employing a partitioning temperature which corresponded to the start of austenite reverse transformation (ART). The influence of a 1.6 wt pct Ni addition in one of the steels and cycle parameters on austenite stability and mechanical properties was also studied. High contents of retained austenite were obtained in the lower quenching temperature (QT) condition, which at the same time resulted in a finer microstructure. The addition of Ni was effective in stabilizing higher contents of austenite. The partitioning of Mn and Ni from martensite into austenite was observed by TEM–EDS. The partitioning behaviour of Mn depended on the QT condition. The lower QT condition facilitated Mn enrichment of austenite laths during partitioning and stabilization of a higher content of austenite. The medium Mn steel containing Ni showed outstanding values of the product of tensile strength (TS) and total elongation (TEL) in the lower QT condition and a higher mechanical stability of the austenite.This research is partially funded by the European Commission in the HIGHQP Project, which has received funding from the Research Fund for Coal and Steel under Grant Agreement No 709855. This study reflects only the author's views and the European Commission is not responsible for any use that may be made of the information contained therein. The authors thank for technical and human support provided by SGIker (UPV/EHU/ERDF, EU). The authors also acknowledge the support of the sponsors of the Advanced Steel Processing and Products Research Center

Characterization of a Medium Mn-Ni Steel Q&P Treated by a High Partitioning Temperature Cycle

In this work, a medium Mn-Ni steel was treated through Quenching and Partitioning (Q&P) with a partitioning temperature (PT) of 650 °C, which corresponded to the start of the austenite reverse transformation (ART) phenomenon. The influence of the quenching temperature (QT) and partitioning time (Pt) on austenite stabilization and mechanical properties was investigated. A strong influence of the quenching temperature was observed. Results were compared with those obtained after a Q&P treatment with 400 °C partitioning temperature. The Q&P cycle with quenching to room temperature and a high partitioning temperature produced a steel with a high retained austenite (RA) volume fraction and exceptional strength–ductility balance. The analysis of the mechanical stability of the retained austenite revealed a significant stress-induced transformation. Nevertheless, the austenite, which was stable at stresses above the yield stress, provided significant TRIP-assisted ductility. Bending, hole expansion and post-stamping properties were also evaluated for the most promising conditions.This research was funded by the Research Fund for Coal and Steel, grant number 70985

Evolution of microstructure and mechanical properties of medium Mn steels and their relationship

Lors d’un recuit inter-critique d’un acier dit « Medium Manganèse », dont la teneur en Mn est située entre 4 et 12 %, avec une microstructure initiale complètement martensitique, la formation de l’austénite obéit à un mécanisme spécifique qui porte le nom d'ART - « Austenite Reverted Transformation » (transformation inverse de l’austénite). L’objectif de ce travail de thèse était d’étudier et de modéliser les évolutions microstructurales en lien avec les propriétés mécaniques lors d’un recuit ART. Il a été déterminé que la microstructure finale se compose de phases de nature (ferrite, austénite résiduelle et martensite de trempe) et morphologie (en forme d’aiguille et polygonale) différentes. Une attention particulière a été accordée aux cinétiques de dissolution des carbures et de formation de l’austénite. Une vision complète de ces processus a été construite. En outre, le mécanisme de stabilisation de l’austénite résiduelle à la température ambiante a été étudié et discuté. Enfin, des essais de traction ont été réalisés afin d’évaluer le comportement mécanique de l’acier après différents recuits ART et établir le lien avec la microstructure. Une analyse plus détaillée du comportement de chaque constituant de la microstructure a été effectuée. A l'issue de cette thèse, un modèle complet est disponible pour calculer les courbes de contrainte vraie - déformation vraie d’un acier Medium MnDuring the intercritical annealing of fully martensitic Medium Mn steel, containing from 4 to 12 wt.% Mn, the formation of austenite happens through the so-called “Austenite Reverted Transformation” (ART) mechanism. In this PhD work, the evolution of both microstructure and tensile properties was studied as a function of holding time in the intercritical domain. The microstructure evolution was studied using a double experimental and modeling approach. The final microstructure contained phases of different natures (ferrite (annealed martensite), retained austenite and fresh martensite) and of different morphologies (lath-like and polygonal). A particular attention was paid to the kinetics of austenite formation in connection with cementite dissolution and to the morphology of the phases. A mechanism was proposed to describe the formation of such microstructure. The critical factors controlling thermal austenite stability, including both chemical and size effects, were determined and discussed, based on the analysis of the retained austenite time-evolution. At last, tensile properties of the steel were measured as a function of holding time and the relation between microstructure and mechanical behavior was analyzed. Advanced analysis of the individual behavior of the three major constituents was performed. As a final output of this work, a complete model for predicting the true-stress versus true-strain curves of medium Mn steels was propose

Critical factors governing the thermal stability of austenite in an ultra-fined grained medium-Mn steel

The time-evolution of retained austenite at 650 °C is shown to be a fair indicator of the factors governing the austenite stability. The grain size contribution to austenite stability is evidenced and a martensite start temperature (Ms) law applicable to medium-Mn steels and including both the chemical and size effects is determined

Characterization and modeling of mechanical behavior of quenching and partitioning steels

International audienceQ&P annealing was applied to cold rolled carbon-manganese steel with Si. Q&P cycles with different partitioning temperature and time were simulated and the evolution of microstructure and mechanical properties was investigated. All the microstructures were composed of three constituents: partitioned martensite, laths of retained austenite and MA islands. Fine microstructure characterization confirmed that C diffusion plays an important role for the stabilization of retained austenite at room temperature and further TRIP effect during mechanical loading. Good compromise between yield strength (similar to 1200 MPa) and uniform elongation (similar to 11%) was found in the case of 400 degrees C partitioning for 300 s due to the enhanced mechanical stability of retained austenite. Evolution of microstructure and mechanical properties was discussed and some mechanisms were proposed to explain the observations. Mechanical model for the prediction of stress-strain curves of Q&P steels was proposed, based on the obtained experimental data. Accurate prediction of stress-strain curves using model was achieved

Link between microstructure and mechanical behavior of double annealed medium Mn steel

Double annealing of low carbon medium Mn steel was studied. The second intercritical annealing was done at 650°C within a range of holding time: 3min to 30h. Tensile properties of the steel were measured as a function of holding time and the relation between microstructure and mechanical behavior was analyzed. Furthermore, a model, based on the mixture law combined with the considerations of equivalent increment of work in each microstructural constituent during mechanical loading, was proposed. The individual mechanical behavior of each considered microstructural constituent was described with the approaches existing in the literature. The complete model shows a very good agreement with the experimental stress-strain curves and predicts well the optimum strength-ductility balance after 2h holding

Mechanism of ultra-fine microstructure formation during ART-annealing of a Medium-Mn steel

International audienceThe medium Mn steels are a topic of interest from both practical and scientific point of view. Many studies were focused on the microstructure characterization, but only some of them addressed the mechanisms of austenite formation and stabilization. Hence, there are still remaining questions regarding the link between the optimum retained austenite fraction and stability and the austenite formation including both the morphological and kinetics aspects. In this work, different ART annealing treatments were performed on a cold rolled 0.1C – 4.7Mn (wt.%) steel. SEM and TEM observations as well as XRD and magnetic measurements were done to characterize the resulting microstructures. Microstructure evolution was analyzed as a function of soaking time: precipitation and dissolution of cementite; austenite nucleation, growth and stabilization. The experimental observations were compared with the predictions from thermodynamic calculations. Based on the obtained results, a mechanism of austenite formation and stabilization during ART annealing is proposed

Characterization and modeling of manganese effect on strength and strain hardening of martensitic carbon steels

Carbon and manganese combined effect on the mechanical behavior of martensite was characterized and analyzed using literature and new experimental data of various carbon-manganese steels. A synergy effect of carbon and manganese on the martenstite strength and strain hardening was detected and was then taken into account in a specific way in the simplified model, based on a Continuous Composite Approach. Model was adjusted with only one fitting parameter and the obtained results are in good agreement with experimental stress-strain curves