Page 32 - MetalForming May 2009

Tooling Technology
Stuart Keeler (Keeler Technologies LLC) is best known worldwide for his discovery of forming limit diagrams, development of circle grid analysis and implementation of other press shop analysis tools. Stuart’s sheetmetal forming experience includes 24 years at National Steel Corporation and
12 years at The Budd Company Technical Center, enabling him to bring a very diverse background to this column and the many seminars he teaches for PMA. His most recent project is technical editor of the AHSS Application Guidelines—Version 4, which now is available for downloading free from www.worldautosteel.org. Keeler Technologies LLC
P.O. Box 283
Grosse Ile, MI 48138
Fax: 734/671-2271
E-mail: keeltech@comcast.net
Stuart Keeler’s next seminar is “Troubleshooting Formability Problems” scheduled for May 7 in Indianapolis, IN. Check www.metalforming.com for this and other seminars.
You have previously formed parts with 50-ksi-yield-strength HSLA steel and feel confident about your capabilities to form most any high- strength steel. That is until a new RFQ lands on your desk. The part has a very complex shape with areas of large increases in length-of-line. Tight speci- fications limit dimensional variations. So far you are okay—until you look at the mechanical-property specifications for the part. The final part must have a martensitic microstructure with mini- mum yield strength of 150 ksi.
What is known about the specified martensitic steel? The as-received prop- erties are 150 ksi min. yield strength, 220 ksi min. tensile strength, and a typ- ical total elongation (2-in. gauge length) of 5 percent. The workhardening expo- nent (n-value) is so low that it is immeas- urable by standard test procedures. The forming-limit diagram has an FLC0 of 15 percent. Springback is a major source of dimensional variation. Since the elastic stresses that cause springback are pro- portional to yield strength, the spring- back is about three times that of 50-ksi- yield steel. A usual mode of deformation for martensitic steel is rollforming because die forming is not possible or limited to very shallow, simple shapes.
To quote your part, you may need to break the part down into four separate pieces and then weld them together. Considering the extreme amount of springback compensation required and the introduction of dimensional varia- tion in welding these four pieces, what is the probability of keeping dimen- sional tolerances?
Some industries, especially the auto- motive industry, have created many final parts with 150 ksi and higher yield strengths by taking a radically different
approach. Nothing in the RFQ says the as-received steel must start with those high-strength properties; only the final part needs to meet those specifications. These companies successfully make the parts by hot forming the steel. The prem- ise is simple. Process low-strength steel through the following steps in the schematic: 1) External to the die, cut and preheat the blank to high temperatures to lower the yield strength for maxi- mum stretchability, 2) Quickly transfer the hot blank to the die and form the part without elastic springback, and 3) In-die quench the part to generate the required very high-strength final martensitic microstructure. Process information is amplified below.
• While several steels can be used in the hot-forming process, the most com- mon is a boron-manganese (22MnB5) steel. The as-received properties are 50 ksi min. yield strength, 70 ksi min. ten- sile strength, and a typical total elonga- tion of 23 to 27 percent. The blank- cutting tools must be designed to withstand these properties.
• The blank is heated to a target tem- perature of 1650 F. In less than 10 min., the microstructure transforms to austen- ite. At this temperature, austenite has a relatively constant yield strength of 6 ksi and allowable elongations greater than 50 percent. Because of the high temperature, the steel needs a protective coating to prevent formation of a surface oxide.
• The blank is transferred to a water- cooled die for immediate forming. At the end of the forming stroke, the punch is held at bottom dead center to allow the punch and die to quickly quench the formed part to a martensitic micro- structure. This locks in the geometrical shape of the part. The part properties now are 150 ksi min. yield strength,
30 METALFORMING / MAY 2009
www.metalformingmagazine.com
THE SCIENCE OF FORMING
Alternate Forming of Very High-Strength Steel
STUART KEELER