Abstract
It has been proven that through targeted quenching and partitioning (Q & P), medium manganese steels can exhibit excellent mechanical properties combining very high strength and ductility. In order to apply the potential of these steels in industrial press hardening and to avoid high scrap rates, it is of utmost importance to determine a robust process window for Q & P. Hence, an intensive study of dilatometry experiments was carried out to identify the influence of quenching temperature (TQ) and partitioning time (tp) on phase transformations, phase stabilities, and the mechanical properties of a lean medium manganese steel. For this purpose, additional scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and energy dispersive X-ray spectroscopy (EDX) examinations as well as tensile testing were performed. Based on the dilatometry data, an adjustment of the Koistinen–Marburger (K-M) equation for medium manganese steel was developed. The results show that a retained austenite content of 12–21% in combination with a low-phase fraction of untempered secondary martensite (max. 20%) leads to excellent mechanical properties with a tensile strength higher than 1500 MPa and a total elongation of 18%, whereas an exceeding secondary martensite phase fraction results in brittle failure. The optimum retained austenite content was adjusted for TQ between 130 °C and 150 °C by means of an adapted partitioning.
Highlights
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Manganese-boron steels such as 22MnB5 or 20MnB8 are used for this purpose. These steels have a ferritic-pearlitic (FP) microstructure, which is completely transformed into martensite with tensile strengths of 1500 MPa and higher by austenitizing and press hardening [4]
To the start (Ac1b ) and finish (Ac3 ) temperature of austenite formation, carbide solution was identified during the heating of dilatometric samples
Summary
The reduction in CO2 emissions is one of the most important issues in the automotive industry. One effective way of reducing emissions of passenger cars is to lower vehicle weight. Manganese-boron steels such as 22MnB5 or 20MnB8 are used for this purpose In their initial state, these steels have a ferritic-pearlitic (FP) microstructure, which is completely transformed into martensite with tensile strengths of 1500 MPa and higher by austenitizing and press hardening [4]. These steels have a ferritic-pearlitic (FP) microstructure, which is completely transformed into martensite with tensile strengths of 1500 MPa and higher by austenitizing and press hardening [4] Alloying elements such as carbon, manganese, chromium, and boron increase the strength attainable by this process and, above all, suppress the diffusion-controlled
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