Page 27 - MetalForming May 2010

Graph of instantaneous values of n for three 50-ksi (350-MPA) higher strength steels. Courtesy of WorldAutoSteel AHSS Application Guidelines
error. However, the solution is quite simple. The computer analysis program must be capable of accepting the true stress-true strain curve, which captures any change in n-value over the entire range of deformation
The forming limit curve or maxi- mum allowable stretch (strain) also depends on the n-value. Here an n- value at a large deformation or plateau strain is required. The two curves shown in the graph have the same high n- value. This means the forming limit curves for these HSLA and DP steel coils will be identical. This was con- firmed by experimental determination of the actual forming limit curves.
The second AHSS is the transfor- mation induced plasticity (TRIP) steel, beginning to gain applications in the automotive industry. If the initial high- er n-value of DP steel is so beneficial, why not create steel that keeps regener- ating the higher n-value over a wide range of deformation? The TRIP steel (dotted line in the graph) is designed to accomplish this major improvement. Now the n-value not only changes with strain, but has an increased value as traditionally measured between 10 per- cent strain and uniform elongation. Even here, the measured value is an average n-value between the two meas- urement points and still provides no information about the changing n-val- ues outside that range. Again, the true stress-true strain curve would capture the variable nature of the n-value for use by the computational program.
Unfortunately, the TRIP steel has another problem. The properties of the as-received TRIP steel are not the prop- erties experienced by the stamping dur-
ing the forming operation. The increase in the instantaneous n-value is a func- tion not only of initial steel composition and processing, but also the strain path, strain history, forming speed, tempera- ture and other production factors. Therefore, the instantaneous n-value and the forming limit diagram depend on the actual deformation, which changes not only from part design to part design but also with location with- in the part. Thus, capturing the actual property levels active during deforma- tion is a problem still unsolved, which makes output of TRIP steel analyses questionable.
Most aluminum alloys have a com- parable problem. Their n-values are not constant but decrease with increased deformation. Therefore, n-value data often are presented as 0.35 (0.22), where the first value is the initial n-value and the second number in parentheses is the average value over a wide range of strain. Other metal alloys have their own set of property idiosyncrasies. One must understand the input to the com- puter program to assess the validity of the output.
The correct properties for virtual forming and other analyses depend on the accuracy desired for the final stamp- ing. Springback prediction requires much more accuracy of input properties than does designing a stamping to hold water. The need for accurate properties extends to troubleshooting and press- shop areas other than virtual forming. Therefore, discussion of accurate prop- erties continues next month. Topics include blanked-edge damage, local elongation, strain-rate hardening and friction. MF
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0.30 0.25 0.20 0.15 0.10 0.05
0
0 5 10 15 20 25 30
Engineering Strain (%)
n-value
TRIP 350/600 DP 350/600 HSLA 350/450
•NoAir Consumption
The Pax EGD conveyor is an electrically driven, oscillating type conveyor that utilizes a motorized, elliptical gear drive (EGD) to convey parts and scrap out of the press area.
• Elliptical Gear Drive
PATENT PENDING
• Snap-On Tray
Instantaneous n-Value