Impact of Si on Microstructure and Mechanical Properties of 22MnB5 Hot Stamping Steel Treated by Quenching & Partiti
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EVER since its inception by Speer et al. in 2003, quenching and partitioning (Q&P) has become a major focus of steel research,[2–5] also attracting lots of attention from industry.[6] This is primarily based on two factors: On the one hand, many Q&P steels exhibit a superior combination of strength and ductility. On the other, very good properties can be achieved with reduced alloying contents, particularly, compared to second-generation Advanced High-Strength Steels (AHSS). With a focus on industrialization, such an aspect carries elevated importance as it impacts not only on costs but also on processability, most importantly welding. In contrast, hot stamping was overlooked for almost a decade after its first patent (SE435527, 1973) before being employed by the automotive industry.[7] What followed [1]
BERND M. LINKE is with the thyssenKrupp Steel Europe AG, Duisburg, Germany. Contact e-mail: [email protected] THOMAS GERBER is with the thyssenKrupp Steel Europe AG, Dortmund, Germany. ANSGAR HATSCHER is with Volkswagen AG, Wolfsburg, Germany. ILARIA SALVATORI is with Centro Sviluppo Materiali SPA, Rome, Italy. IN˜IGO ARANGUREN and MARIBEL ARRIBAS are with the Fundacio´n Tecnalia Research & Innovation, Derio, Basque Country, Spain. Manuscript submitted May 29, 2017. Article published online November 21, 2017 54—VOLUME 49A, JANUARY 2018
can only be described as a huge success with more hot stamped parts every year.[8] Amid rising demand for low-cost lightweight construction, it proved to be an outstanding and unique feature being able to reach an ultimate tensile strength (UTS) > 1500 MPa without forming difficulties like springback. However, the low level of ductility in martensitic 22MnB5, the backbone of hot stamping steels, limits potential applications. Besides increasing ductility, there are other good reasons to combine hot stamping with Q&P. To reach a two-phase mixture of martensite and austenite, both full austenitization and a high cooling rate to suppress ferrite and bainite formation are required. Being certainly achievable in general, this poses a challenge for many existing continuous annealing lines. In contrast, the hot stamping process includes these boundary conditions already. As high productivity and acceptable costs are paramount for a price sensitive industry, the Q&P cycle has to be tailored towards these demands to be incorporated into hot stamping. Commonly hot stamping lines produce around 4 parts per minute which should be reflected by the partitioning time. Also the current austenitization temperatures of 1173 K to 1223 K (900 C to 950 C) should be maintained to enable use of existing furnaces and limit energy consumption. A big challenge for Q&P alloy design for hot stamping poses its need for aluminum silicon (AS)
METALLURGICAL AND MATERIALS TRANSACTIONS A
coating which prevents scaling during austenitization. To preserve a martensite austenite phase balance, commonly 1.5 wt pct Si is added in order to inhibit cementite formation in this process.[9,10] Si, however, is
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