A Mechanical, Microstructural, and Damage Study of Various Tailor Hot Stamped Material Conditions Consisting of Martensi
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INTRODUCTION
HOT stamping (or press hardening) is now a mature and established technology which allows the forming of ultra high-strength steel (UHSS) automotive structural components that are both lightweight and meet the demands of passenger protection during crash. These properties are due to a solid-state phase transformation during the forming process, in which a fully austenitized blank is simultaneously formed and quenched (at cooling rates > ~ 27 C/s) within the tool, resulting in a fully martensitic as-formed microstructure which exhibits a tensile strength and total elongation of ALEXANDER BARDELCIK and CARYN J. VOWLES are with the University of Guelph, School of Engineering, 50 Stone Road East, Guelph, ON N1G 2W1, Canada. Contact e-mail: abardelc@ uoguelph.ca MICHAEL J. WORSWICK is with the University of Waterloo, Department of Mechanical and Mechatronics Engineering, 200 University Avenue West, Waterloo, ON N2L 3G1, Canada. Manuscript submitted August 9, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
~ 1500 MPa and ~ 0.10 strain, respectively.[1] Although conventionally hot stamped structural components possess excellent intrusion resistance characteristics, the overall crashworthiness of these parts stands to benefit from regions of lower strength and greater ductility for improved energy absorption.[2] This has led to the development of tailored hot stamping technologies, which are used to produce hot stamped parts with local regions consisting of lower strength and more ductile microstructures. Tailored hot stampings can be manufactured using a number of different processing routes, as reviewed in depth by Merklein et al.[3] Of these tailoring processes, the in-die heating (or heated tool) process relies on heating a section of the stamping die where tailored properties are desired. During the hot stamping process, the heated tooling section(s) within the die forces the cooling rate below 27 C/s as the part is quenched to the heated tool temperature. When the part is released from the die, the heated region further cools (slowly) due to convection.[4] Recently, this tailoring process has been applied to introduce soft zones
within the hot stamped rear frame member of the 2016 Honda Civic for optimized impact performance.[5] The tensile behavior of hot stamped and tailored microstructures has been investigated by multiple researchers, all of which studied Al-Si coated boron alloyed 22MnB5 hot stamping steel (Usibor 1500-AS). Bardelcik et al.[6,7] austenized and quenched boron steel specimens with compressed air to produce tailored microstructures (>268 HV) and develop a strain rate-sensitive constitutive model. Barcelona and Palmeri,[8] Min et al.[9] and Bardelcik et al.[10] all used a Gleeble thermal-mechanical simulator to simultaneously quench and deform boron steel at sub-critical cooling rates (< 27 C/s) and showed that deformation facilitated the decomposition of austenite into bainite and ferrite at cooling rates that would typically suppress these transformations in the absence of defor
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