A novel combined process of Cold Stamping (CS) and Hot Stamping (HS) with Quenching and Partitioning (Q&P) treatment applied to advanced TRIP-assisted steel has been conducted by thermomechanical simulation to evaluate the influence of CS or HS in the Q&P processing. With this purpose, Q&P, CSQ&P, and HSQ&P cycles were designed to obtain multiphase microstructures containing ferrite, martensite, bainitic-ferrite, and the maximum retained austenite (RA) fraction after the processes. The objective was to investigate the effects of the variables involving the heat treatments, such as the intercritical austenitization temperature, the isothermal and non-isothermal deformation, the amount of deformation, and the temperature and partitioning times, and to analyze their influence on the microstructural and mechanical responses. Time-resolved X-ray diffraction using synchrotron radiation was undertaken in a thermomechanical simulator coupled to the synchrotron light source to understand the influence of time, temperature, and strain on the level of carbon enrichment in austenite. In addition, the in situ austenite transformation kinetics and lattice parameter evolution were tracked, making it possible to optimize the RA fraction at room temperature after Q&P processing. The newly developed combined process is promising as the transformation-induced plasticity phenomenon during deformation can contribute to the formability and energy absorption. The results also indicate that the deformation of austenite promotes the ferrite transformation while suppressing the bainite transformation. It was possible to plot the results in an elongation-mechanical strength diagram, coupled to material property charts, also known as, ‘banana curve’, allowing us to identify and correlate the thermal or thermomechanical treatment conditions that led to an increase in ductility or strength according to the volume fractions of the resulting phases. Comparing the results for the HSQ&P treatments, it was observed that isothermal strains at higher temperatures (≥800 °C) are more advantageous to increase mechanical strength, while non-isothermal strains (starting at 750 °C) are suggested if the objective is the increase in ductility, with mechanical strength being slightly sacrificed.

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A novel quenching and partitioning process (Q&P) including the hot stamping (HS) process was studied, using two stamping temperatures (750 °C and 800 °C) and two quenching temperatures (318 °C and 328 °C). This combination is here called Hot Stamping and Quenching and Partitioning process (HSQ&P). The partitioning step was performed at 400 °C for 100 s in all cycles. Microstructural features were comprehensively studied using electron backscattered diffraction and nanoindentation techniques. HSQ&P samples showed a good combination of ductility and high-strength due to the presence of: retained austenite, inter-critical ferrite with low stored internal strain energy, grain refinement via DIFT-effect (deformation induced ferrite transformation), martensite, and bainite. Significant internal stress relief was caused by carbon partitioning, which was induced by the DIFT-effect and the partitioning stage. This also led to a considerable stored energy, which was characterized by the Kernel average dislocation and geometrically necessary dislocation analysis. In addition, predominant {110}//strain direction crystallographic texture was identified, which promotes slip deformation and enhances the mechanical properties. Moreover, remarkable amounts of fine film-like retained austenite oriented along compact crystallographic directions (i.e., 〈111〉 and 〈112〉) were observed. Finally, subsize tensile test verified the optimum mechanical behavior of HSQ&P specimens.

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The modification of the matrix of ductile cast irons by heat treatments has been of interest of researchers for many years. Among these treatments, in the last years the Quenching & Partitioning (Q&P) process has emerged as a viable way to produce microstructures containing controlled amounts of martensite and retained austenite, providing a good combination of strength and ductility. In this work, the different mechanisms of phase transformations occurring during the Q&P heat treatment applied to a ductile cast iron alloy is investigated. Microsegregation, inherent to cast irons, was analyzed by means of Electron Probe Microanalysis (EPMA). Microstructural characterization was performed with Scanning Electron Microscopy (SEM) and Electron Backscattered Diffraction (EBSD), while kinetics of carbon redistribution and competitive reactions were studied using dilatometry and in situ synchrotron X-ray diffraction. It was found that either transition carbides or cementite precipitate in martensite depending on the partitioning temperature. Despite of carbides precipitation, evidence of carbon partitioning from martensite to austenite was obtained. Formation of bainitic ferrite occurs during the partitioning step, further contributing to carbon enrichment of austenite. The experimental results are compared with a local field model that computes the local kinetics of carbon redistribution by simultaneously considering carbides precipitation and growth of bainitic ferrite. Results showed that kinetics of carbon partitioning from martensite to austenite depends on the carbides free energy. More stable carbides do not dissolve and prevent the escape of carbon from martensite. Fast carbon partitioning occurs by dissolution of less stable carbides, but it is slowed down as growth of bainitic ferrite proceeds. This result is explained by the overlapping of the diffusion fields (soft impingement) of the carbon partitioned from martensite and the carbon rejected from growth of bainitic ferrite.@en