Investigation of the Microstructural Evolution during Hot Stamping of a Carburized Complex Phase Steel by Laser-Ultrasonics

Abstract

Prior carburization of semi-finished steel sheets is a new process variant in hot stamping to manufacture parts with tailored properties. Compared to conventional hot stamping processes, a complex phase typed steel alloy is used instead of 22MnB5. Yet recent investigations focused on final mechanical properties rather than microstructural mechanisms cause an increase in strength. Thus, the influence of additional carburization on the microstructural evolution during hot stamping of a complex phase steel CP-W®800 is investigated within this work. The phase transformation behavior, as well as the grain growth during austenitization, is evaluated by in-situ measurements employing a laser-ultrasound sensor. The results are correlated with additional hardness measurements in as-quenched condition and supplementary micrographs. The experiments reveal that the carburization process significantly improves the hardenability of the CP-W®800. However, even at quenching rates of 70 K/s no fully martensitic microstructure was achievable. Still, the resulting hardness of the carburized samples might exceed the fully martensitic hardness of 22MnB5 derived from literature. Furthermore, the carburization process has no adverse effect on the fine grain stability of the complex phase steel. This makes it more robust in terms of grain size than the conventional hot stamping steel 22MnB5.

<i>In-situ</i> Laser Ultrasonics Measurements of Ferrite Formation during Stepped Cooling of A 0.1C-2Mn Dual Phase Steel

“…Owing to its non-contact nature, laser ultrasonics (LUS) has been gaining attention as an in-situ monitoring technique in steel manufacturing lines [4][5][6][7] . Laser ultrasonics has been reported to successfully measure phase transformations in steels during continuous cooling, as benchmarked with conventional techniques including dilatometry [8][9][10][11] .…”

“…above the magnetic transition temperature, the velocity change has not provided any quantitative measure of austenite formation whereas dilatometry has shown an increase of the austenite fraction. Further, Horn et al 11) applied Krüger at al. 's 13) ultrasonic velocity technique to investigate phase transformation kinetics at different quenching rates on state-of-art hot stamping steels containing complex phases in different carburized conditions.…”