Page 46 - MetalForming January 2020

The Science of Forming By Daniel J. Schaeffler, Ph.D.
Material Selection: The Rest of the Story
Automotive bodies include increasing amounts of higher- strength steels and aluminum alloys to address the challenges of improving fuel economy and safety while reducing tailpipe emissions. Stamping and assembly hurdles are tackled regularly, with the impact of these materials choices reaching beyond the metal forming community and affecting the entire supply chain.
Producing higher-strength steels requires upgrades at the steel mill. The molten steel requires tighter control of alloying chemistry. Hot and cold rolling demand greater rolling mill forces, placing greater strain on mill stands. Achieving improved thickness tolerances requires upgrades in roll profiling and maintenance schedules. Producing advanced high-strength steels (AHSS) often necessitates sig- nificant capital expenditures due to the thermal cycle needed to achieve the relevant microstructures.
These grades, once produced, often must undergo additional processing. Shape issues such as flatness, waves
Danny Schaeffler, with 30 years of materials and applications experi- ence, is co-founder of 4M Partners, LLC and founder and president of Engineering Quality Solu- tions (EQS). EQS provides product-applications assistance to materials and manufacturing com-
panies; 4M teaches fundamentals and practical details of material properties, forming technolo- gies, processes and troubleshooting needed to form high-quality components. Schaeffler, who also spent 10 years at LTV Steel Co., received his Bach- elor of Science degree in Materials Science and Engineering from the Johns Hopkins University in Baltimore, MD, and Master of Science and Doctor of Philosophy degrees in Materials Engineering from Drexel University in Philadelphia, PA.
Danny Schaeffler
248/66-STEEL • www.EQSgroup.com
E-mail ds@eqsgroup.com or Danny@learning4m.com
Fig. 1—The Jaws of Life evolved to tackle advanced automotive body structures in extraction of vehicle-crash victims.
and coil buckles require upgraded lev- eling equipment, especially when working the highest-strength materials. Slitting or blanking may be particularly challenging. In addition to the issues associated with increased material strength, cut edges of advanced grades, with their engineered microstructures, behave differently than those of other grades. Optimal clearances and cutting steels likely will need upgrades or else premature failure will occur.
Cutting-knife sharpness, alignment, maintenance and capabilities demand careful consideration even outside of coil processing shops. After the first form operation, metal stamping com- panies trim away the binder and addendum, with the scrap collected, bundled and shipped to a scrap proces- sor. These processors further cut, shred and bale these steels. By mid-decade, automotive usage of steel grades with tensile strength of at least 100,000 psi is expected to grow five-fold.
Crash Access
The high strength of alloys used for passenger-cage parts promotes greater occupant safety but doesn’t eliminate the risk of a crash. When crashes do
occur with passengers trapped inside, rescue personnel use power tools, including the Jaws of Life (Fig. 1), to cut through sections of the passenger cage such as the B-pillar or roof rail.
When the first Jaws of Life patent was filed, the highest-strength steel available had a tensile strength of about 50,000 psi. Today, passenger-cage parts typically feature steel with 200,000-psi tensile strength, with 300,000-psi ten- sile grades becoming commercialized. Over the past decade, rescue teams had to purchase new tools capable of attacking these ultra-high-strength sec- tions, while learning vehicle-specific best practices of how and where they should target their efforts.
Early challenges faced by rescue teams encountering ultra-high- strength steels: teeth burning off of reciprocating-saw blades; axes bounc- ing off of some components; and sparks produced when cutting. Regarding the last, sparks can start a fire, which now brings higher risk given the use of high- voltage batteries in electric vehicles.
In addition to producing B-pillars from ultra-high-strength steels, some automakers save even more weight by using tailor rolled blanks. These vari-
44 MetalForming/January 2020
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