New Mechanisms In A New Class of 3rd Generation AHSS Sheet
- April 9, 2014
D.J. Branagan, A.E. Frerichs, B.E. Meacham, L. Ma, I.A. Yakubtsov, S. Cheng, and A.V Sergueeva
Globally, more stringent vehicle emissions standards are driving automotive OEMs to improve vehicle efficiency at a demanding pace. In the United States, for example, a legislated fleet-wide average fuel economy standard of 54.5 miles per gallon (mpg) by 2025 is anticipated to result in passenger cars that must achieve over 61 mpg and light trucks over 33 mpg. . In order to reach these targets, the body structure of modern vehicle designs will carry a significant burden to shed weight while retaining functionality and maintaining safety standards. Due to its affordability, excellent stiffness-to-density ratio, and predictable forming characteristics, steel has historically dominated as the material selected for vehicle body structures. As light-weighting has gained momentum, steels have evolved to meet demand and the usage of Advanced High Strength Steels (AHSS) have proliferated during the past two decades, greatly increasing body structural efficiency. In the last decade, significant efforts have been made in developing the “third generation” of AHSS with strength-ductility combinations significantly better than in the first generation AHSS but at a lower cost as compared to second generation AHSS. A paradigm changing nanostructured 3rd Generation AHSS will be presented with a focus on the new mechanisms, enabling structures, and resulting mechanical properties. New mechanisms include the ability to create NanoScale structures through Static NanoPhase Refinement at high temperatures inherent to the industrial steel making processes and the ability to strain harden during cold deformation through the formation of high fractions of nanoscale precipitates through a Dynamic NanoPhase Strengthening mechanism. The steps in creating the enabling nanoscale structures will be detailed including the specific microstructural pathways to achieve the targeted High Strength NanoModal Structure. As a result of the new enabling mechanisms and formation of novel structure types, a wide range of tensile properties can be provided within and beyond the targeted 3rd Generation AHSS window which will be detailed. In addition to compelling combinations of strength and ductility, the ability to resist necking with high fraction of usable uniform ductility will be crucial for various cold forming strategies employed for forming structural parts in automotive applications. Thus, the new NanoSteel Class of AHSS is expected to extend the existing technological window for cold formable steels in advanced automobile design and manufacturing in body-in-white and other structural applications.
 2017 and Later Model Year Light-Duty Vehicle Greenhouse Gas Emissions and Corporate Average Fuel Economy Standards; Final Rule,” Environmental Protection Agency, October 15, 2012.