Characterization and methodology for calculating the mechanical properties of a TRIP-steel submitted to hot stamping and quenching and partitioning (Q&P)
Introduction
Increased requirements for low fuel consumption and improved safety in the automotive industry have stimulated the development of new generations of high strength steels. To reduce fuel consumption, the decrease of the car's body weight is achieved by using thinner and therefore necessarily stronger and ductile steel sheets. A promising way of obtaining a favorable compromise between strength and ductility is the Quenching and Partitioning (Q&P) heat treatment [1]. Q&P heat treatment can produced a martensitic microstructure with a considerable amount of fine-dispersed retained austenite (RA).
In this process, the steel is austenitized (either fully or partially) and then quenched to a temperature between the martensite start (Ms) and martensite finish (Mf) temperatures, to create a controlled mixture of martensite and austenite (and any intercritical ferrite present during annealing). The steel is then isothermally held at the quenching temperature (QT) (one-step Q&P treatment) or heated to a higher temperature (PT) (two-step Q&P treatment) to allow carbon partition from martensite towards austenite. At this step, carbon diffusion occurs from the supersaturated martensite to the untransformed austenite (carbon partitioning), stabilizing this phase during the final quench to room temperature. Carbon-enriched austenite, which is stable at room temperature, is capable of contributing to mechanical properties in the same manner as it does in transformation induced plasticity (TRIP) steels [2]. The carbon distribution, specifically the local concentration of carbon in the individual phases, grain boundaries or interface boundaries, is assumed to be the key parameter that influences the phase transformation and mechanical properties.
Additionally, hot stamping (HS), also known as hot press forming or press hardening, is one of the most effective methods to produce ultra-high strength steels for automotive bodies. HS is a non-isothermal process designed for sheet metals, in which forming and quenching take place at the same forming stage [3]. Its main advantages are the excellent shape accuracy of the components, allowing the use of thinner gauge sheet metal. Thus, weight reduction can be achieved while maintains structural integrity, by enabling the production of ultra-high strength parts without any springback [4]. Recently, Liu et al. [5] pointed out that although the Q&P process is one of the most promising methods for producing advanced high-strength steels (AHSS) with both high strength and ductility, the heat treatment methods do not consider the strain induced phase transformations. Few studies investigated the potential of combining the hot stamping process with the quenching and partitioning [5], [6], [7], [8], [9]. However, these studies were limited to isothermal deformation with fully austenitized samples and one-step Q&P treatments. An exception is the work of Chang et al. [7], who performed two-step Q&P treatment with quenching time of one minute before the partitioning treatment. De Knijf et al. [10] pointed out that the isothermal holding time at the quenching temperature in Q&P steels need to be kept short (usually five to ten seconds), in order to avoid the precipitation of isothermal transformation products at the quenching temperature. In this work, a novel process (herein referred to as HSQ&P) applied to TRIP-assisted steels is proposed, which combines intercritical annealing with non-isothermal hot stamping, followed by two-step Q&P treatment. Experiments were conducted using thermo-mechanical simulation equipment, in order to compare the microstructure and the mechanical properties of a TRIP-assisted steel submitted to quenching (Q), HS, Q&P, and HSQ&P. Several characterization and modeling techniques were used: nanoindentation, object-oriented finite element method, and micro-tensile tests. The influence of time, temperature, and strain on the retained-austenite (RA) stability, as well as the role of transformation induced plasticity on the formability and energy absorption of AHSS sheets were studied in situ, using time resolved X-ray diffraction on the XTMS synchrotron radiation beamline of the Brazilian synchrotron facility. The combination of thermomechanical and thermal treatments suggest a new generation of high strength, formable sheet steel that answers the demands of the automotive industry.
Section snippets
Material and thermomechanical treatments
TRIP-assisted steel (Fe-0.23C-1.23Si-1.50Mn, wt%) sheets were subjected to Q, HS, Q&P, and HSQ&P heat and thermomechanical treatments. Q&P and HSQ&P cycles were designed in order to obtain ferrite, martensite, and retained austenite at the end of both processes. A Gleeble®3S50 thermomechanical simulator was used to reproduce the thermo-mechanical treatments conditions (heating and cooling rates, and deformations) as schematically depicted in Fig. 1. The as-received multiphase specimens
Intercritical temperature
The volume fractions of the constituent phases as predicted by Thermo-Calc® based on an equilibrium thermodynamic analysis are shown in Fig. 5. In this figure it is possible to see that when the temperature is below the Ae1 temperature (≈700 °C), the material consists of approximately 98% of ferrite () and 2% of cementite (). Both cementite and ferrite start to transform to austenite () above the Ae1 temperature, until the temperature (≈715 °C) is reached, at which the cementite is
Summary and conclusions
A novel process (HSQ&P) applied to TRIP-assisted steels was proposed, which combines intercritical annealing with non-isothermal hot stamping (HS), followed by a two-step quenching and partitioning (Q&P) treatment. Experiments were conducted using a Gleeble®3S50 thermo-mechanical simulator equipment, in order to compare the microstructures and the mechanical properties of a TRIP-assisted steel submitted to quenching (Q), HS, Q&P, and HSQ&P. From the obtained results, the following conclusions
Acknowledgments
The authors gratefully acknowledge financial support from CAPES (Process n° 23038.006737/2012-56), CNPq (Grant 235297/2014-3 PVE), and FAPESP (through Grant 2014/11793-4). The Brazilian Nanotechnology National Laboratory (LNNano) and the Brazilian Synchrotron Light Laboratory (LNLS) are also acknowledged for the use of the XTMS facility at the XRD1 beamline. To Dr. Arun Sreeranganathan, M.Sc. Vanessa Seriacopi, and Dr. Kyoo Sil Choi for their helping during the construction of the OOF-Abaqus
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