Mn-Alloyed High-Strength Steels with a Reduced Austenitization Temperature: Thermodynamic Calculations and Experimental Investigations

Windmann, M. and Opitz, T. and Klein, S. and Röttger, A. and Theisen, W.

Volume: 89 Pages:
DOI: 10.1002/srin.201800166
Published: 2018

High-strength steels (e.g., 1.5528–22MnB5), processed by direct press-hardening, are widely used for security-relevant structures in automotive bodyworks. In this study, the austenitization temperature AC3 of the steel 22MnB5 (approx. 840 °C) is decreased to enable a reduction in the heat-treatment temperature. Thermodynamic calculations using the CALPHAD method are used to assess the effect of alloying elements on the α–γ transformation temperatures. On this account, 22MnB5 steel is alloyed with 6 to 9.5 mass% manganese, which decreases the α–γ transformation temperature to 744 °C. Simultaneously, the martensite finish temperature decreases below room temperature, which is accompanied by the presence of retained austenite after hardening. Furthermore, ϵ-martensite is formed. High Mn-alloyed steel 22MnB5 (9.5 mass% Mn, AC3 = 744 °C) possesses a high strength of Rm = 1618 MPa, similar to the initial material 22MnB5. Elongation-to-fracture decreases to A5 = 3.5% due to the formation of ϵ-martensite. The material strength of the steel alloyed with 6 mass% manganese (AC3 = 808 °C) strongly increases to Rm = 1975 MPa as a result of α-martensite and solid-solution strengthening by the element manganese. This steel possesses a higher elongation-to-fracture of A5 = 7%. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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