Sensitivity of the final properties of tailored hot stamping components to the process and material parameters
Introduction
Hot stamping has been used for many years to produce high strength structural automotive components. The high tensile strength achievable by hot stamping is beneficial where the intrusion during a vehicle crash is not desirable – e.g. for the vehicle occupant compartment. On the other hand, there is a need for high ductility in the crumple zones to absorb crash energy via plastic work. Fig. 1 shows an example of a B-Pillar component, where both requirements should be satisfied. To achieve increased ductility in the local zones of the final part, the cooling rate during hot stamping should be slower than the critical value of 20 °C/s–30 °C/s (depending on the steel grade) to produce softer steel phases such as bainite, ferrite and pearlite in these zones. The local adjustment of the final properties within different areas in a single part using hot stamping is commonly referred to as tailored hot stamping. In tailored hot stamping, the accurate prediction of the temperature history is critical to determine the final phase fractions and the hardness values. Unlike the conventional hot stamping process, which has been well established in production for a number of years, the tailored hot stamping process is new and still needs more attention in terms of specification of the final mechanical properties as functions of the input (process and material) parameters.
The main challenge in the accurate prediction of the final mechanical properties in the hot stamped components is the accurate prediction of the temperature history and the phase transformation kinetics. As shown in Fig. 2, there are a large number of interacting thermal–mechanical–metallurgical parameters that must be considered. When designing the tailored hot stamping process, each of these parameters must be carefully considered so that the desired cooling rates (and resulting microstructure/properties) are achieved precisely during production. Therefore, a sound understanding of the influence of each of the parameters on the temperature history and final phase fractions is necessary. Some researchers have investigated the effects of some of the input parameters on the phase fraction and/or the final mechanical properties, with tool temperature being the most common parameter examined. Feuser et al. (2011) showed that increasing the tool temperature from 200 °C to 400 °C decreases the tensile strength by about 32%. The same decrease in the Vickers hardness has been reported by George et al. (2011) for the same change in the tool temperature. The effects of the tool temperature and the heat contact conductance between the blank and tool material on the temperature history have been shown by Oldenburg and Lindkvist (2011). It was shown that the most important parameter is the tool temperature, while the process sensitivity to the heat contact conductance is low. The effects of tool temperature, contact pressure, transfer time, die closure time and the blank thickness on the tensile strength and bending angle have been investigated by Feuser et al. (2011), which seems to be the most comprehensive parametric study in this field to the authors’ knowledge. The fully martensitic microstructure in the final component was achieved at tool temperatures below 200 °C and the tool temperatures higher than 400 °C was necessary to obtain other phases. At tool temperature of 500 °C, other parameters had less significant effects due to the cooling down and keeping the specimens at a temperature higher than martensitic start temperature. No clear dependency of the material properties to the process parameters was observed in this case. This implies the necessity of more investigations to obtain a better understanding of the process sensitivity with respect to even more input parameters at different process conditions.
In this study, a thermal–mechanical–metallurgical model of a simple hot stamping process is created. The model includes internal dependencies of many parameters and a modified phase transformation model – based on JMAK-type equations and Scheil's additive rule with the inclusion of incubation time for the austenite decomposition into ferrite, pearlite and bainite – which increases the accuracy of the results. The paper provides detailed explanation of the mechanical–thermal–metallurgical model used, and validation of selected results with experimentation and the literature, which will be of benefit to future studies in this area.
Based on the developed and validated model, a series of parametric studies have been performed under four different process conditions. The significance of this study is the consideration of a large number of input parameters which is the first comprehensive parametric study in this field. In this study, the sensitivity of the phase fractions and the final mechanical properties to an arbitrary variation of ±10% in the input parameters have been investigated for four process scenarios. These scenarios were chosen to simulate a conventional hot-stamping process, and three possible methods in which tailored hot stamping can be achieved. The purpose is to understand the sensitivity of the hot stamping process to small variations of the input parameters under a number of operating condition scenarios.
Section snippets
Thermal–mechanical model
The finite element software ABAQUS Standard V6.12 was used to simulate the hot stamping of a simple hat-shaped component (Fig. 3), including transfer from furnace to the tool, forming, quenching and air cooling processes. The model does not include a blank holder, representing the crash-forming-type process often used when hot stamping long structural members. The flange region of the part, which is typically formed by the blank holder in traditional stamping processes, is formed by the action
Jominy end-quenching tests
The temperature histories results from the finite element model and experimental set up which were explained in Section 2.3 have been compared. As shown in Fig. 7, the predicted temperature history compares well with the experimental measurements. As expected, the material near the water-quenched end (A) experiences the fastest cooling rates, with progressively slower cooling rates experienced with increasing distance from the position A.
The predicted phase fraction distribution at the end of
Conclusions
In this paper, a thermal–mechanical–metallurgical model of hot stamping was created. The internal dependencies of most parameters were considered and a modified phase transformation model was used to increase the accuracy of the numerical results of phase fractions and the hardness values. The model was validated against experimental results from Jominy end-quench tests and instrumented hot stamping tests.
This validated numerical model was used to investigate the sensitivity of the final phase
References (20)
- et al.
Austenite decomposition during press hardening of a boron steel – computer simulation and test
J. Mater. Process. Technol.
(2006) - et al.
A review on hot stamping
J. Mater. Process. Technol.
(2010) - et al.
Tailor die quenching in hot stamping for producing ultra-high strength steel formed parts having strength distribution
Cirp. Ann.-Manuf. Technol.
(2010) - et al.
Numerical modelling of the tailored tempering process applied to 22MnB5 sheets
Finite Elem. Anal. Des.
(2014) Hot Work Tool Steel-W300
(2014)- et al.
Modeling of the temperature field, transformation behavior, hardness and mechanical response of low alloy steels during cooling from the austenite region
J. Heat Treat.
(1990) - et al.
The coupled thermo-mechanical-microstructural finite element modeling of hot stamping process in 22MnB5 steel, NUMISHEET, 2014
- et al.
The finite element analysis of austenite decomposition during continuous cooling in 22MnB5 steel
Modell. Simul. Mater. Sci. Eng.
(2014) - et al.
A Thermo-mechanical-metallurgical FE approach for simulation of tailored tempering
- et al.
Partially hot-formed parts from 22MnB5-process window material characteristics and component test results
Proc. ICTP
(2011)
Cited by (30)
Effect of retained austenite on the fracture behavior of a novel press-hardened steel
2023, Journal of Materials Science and TechnologyCitation Excerpt :The bending curve of the 1 MPa sample begins to deviate from the initial linearity earlier than the 7 MPa and 4 MPa samples, indicating that it has the lowest YS among all three samples which are consistent with the tensile test results from Table 2. The magnitude of the contact pressure will change the interfacial heat transfer coefficient, resulting in a change in the cooling rate and consequently the degree of auto-tempering during the die quenching process, and eventually, it affects the final microstructure of PHS [1, 29, 30]. Firstly, the newly developed PHS with the combined addition of Cr and Si in this study contains RA after the conventional hot forming process [15].
Optimized welding process of residual stress control of P91 steel considering martensitic transformation
2021, International Journal of Pressure Vessels and PipingExperimental and numerical investigation on temperature field and tailored mechanical properties distribution of 22MnB5 steel in spray quenching process
2020, Journal of Manufacturing ProcessesInfluence of microstructure on the fracture toughness of hot stamped boron steel
2019, Materials Science and Engineering: A