Abstract
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.
Highlights
There is a continuous industrial demand for new steels that present better combinations of ductility and mechanical properties without increasing production costs
The distinction between ferrite and martensite based on comparing the shape of the grains and the shades of gray is validated by nanohardness and Kernel misorientation data (Figure 27b) in these regions
Various heat and thermomechanical treatments were applied to an advanced high-strength TRIP-assisted steel consisting of quenching (Q), quenching and partitioning (Q&P), hot stamping (HS), and the combined process of HS or cold stamping (CS) with subsequent quenching and partitioning (HSQ&P or cold stamping followed by quenching and partition (CSQ&P))
Summary
There is a continuous industrial demand for new steels that present better combinations of ductility and mechanical properties without increasing production costs. This demand meets the needs of vehicle weight reduction, CO2 emission reduction, and increasing passengers and pedestrians safety. Improving mechanical resistance without loss of ductility leads to reductions in weight, CO2 emission, and fuel consumption by decreasing the thickness of the sheets. The challenge to be faced to optimize the vehicle structure weight is the development of steels with high mechanical strength without compromising their formability.
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