INFLUENCE OF HEATING RATE ON GALVANIZED COATING EVOLUTION PRIOR TO HOT STAMPING

1. INFLUENCE OF HEATING RATE ON GALVANIZED COATING EVOLUTION PRIOR TO HOT STAMPING Amanda Barba1, Dr. J. G. Speer2 , X, Jin2 , Z. Ghanbari2 University of Texas at El Paso Colorado School of Mines, Golden Colorado Summary Introduction Results 22MnB5 galvanized sheet steel is a grade commonly used in the hot stamping process. It is desired that this steel would be zinc (Zn) coated to enhance corrosion resistance. During the hot stamping process, usually done above 850zinc melts and penetrates grain boundaries because of its low melting point (about 420). This can cause cracking of the substrate steel and a reduction in lifetime. If sufficient diffusion can occur between the coating and substrate by heating the samples to 775 at a slow enough rate, Zn liquefaction might be avoided. Qualitative phase identification in the 22MnB5 GI steel coating was obtained using the FESEM and compositions were analyzed by Energy Dispersive Spectroscopy (EDS). Based on the wt. % composition (obtained by EDS) various phases were concluded to be present. FESEM was used to quantify diffusion and the amount of alloying that was a result of the three heating rates. α+L 5 • 20 • 50 Methods • The heating rate experiments were all done using the Gleeble® because of its capability to run very controlled heating rates Galvanized as received 2 • The slower heating rate showed a decrease in coating thickness. • Larger amounts of alloying which allows for the coating to remain solid when it reaches 775/s. The reduced amount of zinc in the coating in the slower heating rate sample may be due to some zinc evaporation. • The faster heating rate had less alloying than did the slower heating rate, although there was still alloying due to diffusion. • Galvanized /s Galvanized 50/s • Galvanized 5/s Acknowledgments Temperature vs. Time indicating six heating rates employed to simulate heating prior to hot stamping. This work was supported by the National Science Foundation and the Air Force Office of Scientific Research under Grant No. DMR-1062797, and the Advanced Steel Processing and Products Research Center. • 22MnB5 GI sheet steel was heated to 775/s. The samples underwent these heating rates in an inert argon environment and were then helium quenched. • The Field Emission Scanning Electron Microscope (FESEM) was used to examine the etched GI samples to identify the composition and phases present on samples heated at 5, 20, or 50/s. • Phases obtained via EDS: • αFe, Γ1, δ References The heating rates were accurately controlled, as shown in the example thermal profile below. A.E. Tekkaya, H. Karbasian, “A Review on Hot Stamping,” Journal of Materials Processing Technology, no. 210, pp.2103-2118, July 2010. J. Kondratiuk, P. Kuhn, E. Labrenz, and C.Bischoff, “Zinc Coatings for Hot Sheet Metal Forming: Comparison of Phase Evolution and Microstructure During Heat Treatment, “ Surface & Coatings Technology, no. 205, pp. 4141-4153, March 2011. C. W. Lee, D. W. Fan, S. J. Lee, I. R. Sohn, and B. C. De Cooman, “Galvanized Coating Evolution During Hot Stamping,” Proceedings of GalvaTech 2011: 8th International Conference on Zinc and Zinc Alloy Coated Sheet Steel, Genova, Italy, 2011. J. G. Speer, Z. N. Ghanbari, “Zinc Coated Sheet Steel for Press Hardening,” Colorado School of Mines, ASPPRC, Research Report No. MT-SCR-012-002, March. 1, 2012 Phases obtained via EDS: αFe, Γ1, δ Phases obtained via EDS: αFe The thermal profiles show the Gleeble® program temperature and the thermocouple measured temperature. The labeled sites indicate the chemical compositions determined using EDS