Strain Rate and Temperature Effects on the Formability and Damage of Advanced High-Strength Steels
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INTRODUCTION
IN the interest of improving fuel efficiency, automotive manufacturers are driven to produce lighter weight vehicles without compromising passenger safety and environmental standards. Vehicle weight reduction can be achieved by substituting mild steel in automotive bodies with advanced high-strength steels, such as DP 600 and DP 780. These low-carbon microalloyed steels can attain very high strengths while retaining moderate ductility, making them ideal for energy absorption during automotive crash. Their inherent high strengths allow for thinner structural components, making a lighter and more fuel efficient vehicle. Commercial cold-rolled dual-phase grades are produced from steels of suitable chemical composition through cold reduction and intercritical annealing between the A1 and A3 temperatures of the Fe-C diagram. Soaking at the intercritical annealing temperature creates small pools of austenite in the ferrite matrix. Upon rapid cooling, austenite is transformed into martensite, producing a ferrite-martensite dualphase microstructure. The volumetric expansion of the S. WINKLER and C. SALISBURY, Research Associates, and M. WORSWICK, Professor and Associate Dean of Engineering, are with the Department of Mechanical Engineering, University of Waterloo, Waterloo, N2L 3G1, ON, Canada. Contact e-mail: swinkler@ lagavulin.uwaterloo.ca A. THOMPSON, formerly with the Department of Mechanical Engineering, University of Waterloo, is with Babcock and Wilcox, Cambridge, N1R 5V3, ON, Canada. I. VAN RIEMSDIJK, Senior Research Associate, is with Research and Development, Dofasco Inc., Hamilton, L8N 3J5, ON, Canada. R. MAYER, Staff Research Engineer, is with the GM R&D Center, MC 480-106-256, Warren, MI 48090-9055. Manuscript submitted September 20, 2007. Article published online April 15, 2008 1350—VOLUME 39A, JUNE 2008
martensite creates a high mobile dislocation density in the ferrite, which is responsible for the continuous yielding behavior and high initial work-hardening rate attributed to dual-phase grades.[1–3] The mechanical properties of dual-phase steels at low and intermediate strain rates have been studied by a number of researchers. Beynon et al.[3,4] conducted tensile tests on DP 500 and DP 600 sheets at strain rates of 0.001 and 100 s-1. The alloys studied displayed an increase in strength and a slight decrease in elongation to fracture with increasing strain rate. DP 600 and DP 800 alloys tested by Schael and Bleck[5] at quasi-static (QS) rates to 250 s-1 were found to exhibit a positive strain rate sensitivity. At rates above 100 s-1, a sharp increase in strain rate sensitivity and a sharp decrease in ductility were noted. Tarigopula et. al.[6] carried out tensile split Hopkinson bar (TSHB) tensile tests on DP 800 at strain rates up to 444 s-1. The results obtained are in agreement with Reference 5, but no change in ductility was noted. Recent work published by Huh et al.[7] on DP 600 and DP 800 type steel sheets showed that both flow stress and ductility increased with increasing strain rates when t
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