KurzfassungIm Zuge immer strikter werdender Regulierungen von einerseits CO2-Emissionen sowie andererseits der Bedeutung von Crash-Sicherheit und Insassenschutz ist die Warmumformung zu einer der bedeutendsten Leichtbautechnologien im Automobilbau geworden. Beschichtete Formplatinen der Güte 22MnB5 werden in langen gasbeheizten Rollenherdöfen auf eine Temperatur oberhalb von 900 °C erwärmt und anschließend in einem gekühlten Werkzeug simultan umgeformt und abgeschreckt. Die zeitintensive Erwärmung mittels Strahlung und Konvektion im Ofen bietet großes Potenzial für die Anwendung von Schnellerwärmungsmethoden wie der induktiven Erwärmung. An einer Versuchsfertigungslinie besteht die Möglichkeit, Formplatinen induktiv zu erwärmen und anschließend bei variablen Ofendauern das Austenitisieren durchzuführen. An somit hergestellten Bauteilen wird der Einfluss der Induktion auf die Material- sowie Schichtentwicklung im Hinblick auf essentielle Weiterverarbeitungskriterien wie Schweißbarkeit und Lackierbarkeit bewertet. Basierend auf den Ergebnissen dieser Untersuchungen kann ein Zeiteinsparpotenzial von 50 % bei vergleichbaren Eigenschaften ermöglicht werden.
In contrast to a cold‐forming process, a tempered forming process is able to deform high‐strength steel used for manufacturing automotive bodyworks in a more economic manner. Cold‐formed steel sheets are commonly coated with a Zn or ZnAlMg layer for cathodic corrosion protection. The tempering process would lead to diffusion processes at the steel/coating interface, which is accompanied by the formation of new phases in the coatings. This publication focuses on phase formation in Zn and ZnAlMg coatings on steel sheets, which are heat‐treated at 400 and 750 °C. the authors find that the pure Zn coating remains in the solid state and transforms into the intermetallic δ phase (FeZn10) during heat treatment at 400 °C. The coating melts during heating to 750 °C, but remains in the solid state after transformation into the Γ phase (Fe4Zn9) and α‐Fe. In the ZnAlMg coating, minor iron diffusion occurs at a temperature of 400 °C. Within a dwell time of 600 s, intermetallic Fe–Zn phases are not formed. During heat treatment at 750 °C, phase formation in the ZnAlMg coating is very similar to that in the pure Zn coating, during which Γ (Fe4Zn9) and α‐Fe are formed.
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 A C3 of the steel 22MnB5 (approx. 840 C) is decreased to enable a reduction in the heattreatment 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, e-martensite is formed. High Mn-alloyed steel 22MnB5 (9.5 mass% Mn, A C3 ¼ 744 C) possesses a high strength of R m ¼ 1618 MPa, similar to the initial material 22MnB5. Elongation -to-fracture decreases to A 5 ¼ 3.5% due to the formation of e-martensite. The material strength of the steel alloyed with 6 mass% manganese (A C3 ¼ 808 C) strongly increases to R m ¼ 1975 MPa as a result of α-martensite and solidsolution strengthening by the element manganese. This steel possesses a higher elongation-to-fracture of A 5 ¼ 7%.
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