Tailoring by Direct Contact Heating During Hot Forming/Die Quenching
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INTRODUCTION
THE proliferation of ultrahigh strength steel (UHSS) body-in-white components is driven by increasingly stringent environmental regulations that promote lighter, more fuel-efficient vehicles. These steels, chief among them 22MnB5 alloy, are transformed into automotive parts through hot forming/die quenching (HFDQ), otherwise known as hot stamping or press hardening. In this technique, as-received blanks consisting of a ferrite/pearlite structure are first austenitized through heating, most often with a roller-hearth furnace. The austenitized, ductile blanks are then transferred at high temperature to a water-chilled die mounted within a hydraulic press, which simultaneously quenches and forms the blanks into martensitic parts. The blanks are often coated with a protective Al-Si layer that, upon heating, reacts with iron below the coating to form a permanent Al-Si-Fe intermetallic layer that prevents oxidation and decarburization within the furnace, and provides some long-term corrosion
NATALIE N. FIELD, MASSIMO DI CIANO, ADRIAN P. GERLICH, and KYLE J. DAUN are with the Department of Mechanical and Mechatronics Engineering, University of Waterloo, 200 University Ave W, Waterloo, ON N2L 3G1, Canada. Contact e-mail: [email protected] Manuscript submitted July 5, 2018. Article published online May 23, 2019 METALLURGICAL AND MATERIALS TRANSACTIONS A
protection. Today’s auto frames consist of up to 40 pct UHSS components, and over 600 million parts were produced by over 400 manufacturing lines in 2015.[1] Increased international competition among OEMs and parts suppliers motivates continuous improvements in process efficiency. Recent advances are summarized by Mori et al.[2] The heating phase of hot stamping is a particularly potent area for improvements, given that it is far more time-, energy-, and space-intensive compared to the forming phase. Most hot-stamping lines use indirectly fired roller hearth furnaces[3] to austenitize the blanks, typically in ~ 5 minutes, while the blanks are subsequently formed and quenched in under 30 seconds. Consequently, most roller hearth furnaces must be very long (typically 30 to 40 m) in order for the heating cycle time (i.e., the time between successive blanks leaving the furnace) to match that required for forming/quenching. The furnaces are also costly, and consume considerable amounts of floor space and energy. Furnaces used to heat uncoated blanks must have atmosphere control to prevent decarburization and oxidation, while, in the case of coated blanks, liquefaction of the Al-Si coating allows the blank to slide upon the ceramic rollers and relocate to new positions, complicating blank transfer from the furnace to the press. The molten coating also impregnates the ceramic rollers, causing them to fail, and the consequential roller replacement costs and furnace downtime represent a major expense in roller furnace operation.
VOLUME 50A, AUGUST 2019—3705
The shortcomings of furnace-based austenitization strategies motivate the development of alternative heating/austeni
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