Page 34 - MetalForming January 2011

The Science of Forming By Stuart Keeler
Don’t Believe Everything You Read
70 60 50 40 30 20 10
0
0 200 500 800 1100 1400 1700
Yield Strength (MPa)
HSL
TRIP
A
DP
Increased strength and increased stretchability
CP
MS
January is a time when many technical articles and columns review successes or failures of the past year. Others provide suggestions on how to improve (insert your own topic) during the coming year. This month’s column will present some metalforming statements that sound technical but are incorrect. Oversimplification can create a wrong statement. Others may originate from press-shop legends or war stories. Some were not proofread. The following state- ments need to be corrected regardless of the source.
• Stamping AHSS (advanced high-strength steel) can be challenging, as it has a higher
strength and lower formability
than conventional steel.
The first problem is the defini-
tion of conventional steel. One can
always set up an undefined apples
and oranges comparison and make
whatever argument they wish. If
you limit conventional steel to alu-
minum-killed draw quality (AKDQ)
or the current designations of
forming steel (FS) and drawing
steel (DS), high-strength low-alloy
(HSLA) steels will have higher
strength and lower formability.
Unfortunately the author is comparing a high-strength steel against a low-strength steel—apples versus oranges.
However, when discussing AHSS, HSLA becomes the con- ventional steel. When one compares AHSS against HSLA steels, the higher strength and lower formability statement is incorrect. The WorldAutoSteel (association of steel com- panies around the world) states that two targets are achieved with AHSS (Fig. 1). The first target is types of AHSS with bet- ter formability (stretching and bending) compared to equiv- alent yield strength HSLA steel. This is accomplished by the dual phase (DP) and transformation induced plasticity (TRIP) steels. The second target is different types of AHSS with
Stuart Keeler (Keeler Technologies LLC) is best known worldwide for his discovery of forming limit diagrams, development of circle grid analysis and implementa- tion of other press shop analysis tools. Stuart’s sheet- metal 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.
Keeler Technologies LLC
P.O. Box 283 | Grosse Ile, MI 48138 Fax: 734/671-2271 keeltech@comcast.net
Fig. 1—The arrow indicates the higher strength and higher total elongation of the AHSS compared to traditional HSLA steels.
Fig. 2—Engineering stress-strain curve shows the work hardening (strength- ening) between the yield strength and the ultimate tensile strength.
higher yield strength that have equivalent formability of HSLA steel. These steels are the complex phase (CP) and martensitic sheets (MS). The diagonal arrow in Fig.1 clearly illustrates that AHSS have higher strength and greater forma- bility than do HSLA steels when compared by yield strength. A sim- ilar curve for tensile strength shows
the same results. The vertical axis in Fig. 1 is total elongation, related to bendability. Similar graphs can be created for stretchability based on the n-value, dome stretch test, milled hole expansion or other stretch-based tests. For the future, simply remember that metallurgists created the new AHSS microstructures specifically to make them stronger and more formable, to allow more complex part designs
• Cold working sheet steel work-hardens the material.
Yes, cold working steel does increase its hardness. How- ever, forming modes such as stretchability and bendability do not correlate with hardness tests. This was stated back in 1940 by J. Dudley Jevons in his book The Metallurgy of Deep Drawing and Pressing. He said, “Hardness and cupping tests are best regarded by both suppliers and customers as an indi- cation of the probability that some particular sequence of mill operations has been carried out, not as an indication of true deep drawing and pressing properties.” The effect of cold working is shown by the stress-strain curve (Fig. 2). The slope of the curve from yield strength to tensile strength answers two questions. First, how much does the stress (strength) increase as the material is strained (cold worked) during form- ing in the press? Second, how much does the strain (defor- mation) increase as the applied stress (load) is increased in
UTS
Ultimate tensile strength
Stress @ 10% strain (“n” value calculation)
Yield strength
YS Fracture
e10 eTS Engineering Strain (Percent)
Uniform elongation
Total elongation
Instability region
32 MetalForming/January 2011
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Engineering Stress (ksi)
Total Elongation (%)