Abstract
The process of press hardening is gaining importance in view of the increasing demand for weight reduction combined with higher crash safety in cars. An alternative to the established manganese-boron steel 22MnB5 is hot-formed martensitic chromium steels such as AISI 420C. Strengths of 1850 MPa and elongations of 12% are possible, exceeding those of 22MnB5. In industrial manufacturing, FE-simulation is commonly used in order to design car body parts cost-efficiently. Therefore, the characterization and the modeling of AISI 420C regarding flow stress, phase transformations as well as failure behavior are presented in this paper. Temperature-depended flow curves are determined, showing the low flow stress and hardening behavior at temperatures around 1000 °C. Cooling experiments are carried out, and a continuous cooling diagram is generated. Observed phases are martensite and retained austenite for industrial relevant cooling rates above 10 K/s. In addition, tests to investigate temperature-dependent forming limit curves are performed. As expected, the highest forming limit is reached at 1050 °C and decreases with falling temperature. Finally, a simulation model of a press-hardening process chain is set up based on the material behavior characterized earlier and compared to experimental values. The forming force, phase transformation and forming limit could be calculated with good agreement to the experiment.
Highlights
Major focus in the automotive industry has been on reducing energy consumption and emissions while improving crashworthiness as well as driving safety
The martensitic chromium steel AISI 420C offers the potential to improve these safety-relevant components currently made of 22MnB5 (Ref 3)
To investigate the transformation behavior of AISI 420C, CCT tests for cooling rates between 50 and 0.1 K/s were carried out on the dilatometer DIL 805A/D+T in quenching mode according to the standard ASTM A1033 (Ref 23)
Summary
Major focus in the automotive industry has been on reducing energy consumption and emissions while improving crashworthiness as well as driving safety. Apart from the higher mechanical properties, AISI 420C has a lower critical cooling rate and a lower martensite start temperature, which is advantageous for subsequent operations after the forming step such as hot trimming. Muller et al investigated the heat transfer coefficient during press hardening as a function of contact pressure and tool start temperature (Ref 9). Up to now no material models are available that can describe the temperature-dependent material behavior of AISI 420C with regard to flow stress, phase transformations and forming limit during press hardening. Material characterization and modeling methods are presented for the numerical simulation of hot forming and press hardening of martensitic chromium steel AISI 420C in 1.5 mm sheet thickness
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