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The Science of Forming
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DP 350/600 HSLA 350/450
True Strain
Fig. 2—The area under the true stress-strain curves more accurately describe deforma- tion of HSLA and AHSS alloys. The blue area represents the added energy required to form the DP steel to the same strain as the HSLA steel.
sample narrows, the force to continue elongation decreases. For almost zero crosssection (near the end of elonga- tion) almost no force is required to continue deformation—certainly not the maximum force required to deform the full amount of crosssection. The full analysis also provides for an increase in the flow stress of the steel as it workhardens. We use the true stress- strain curves to compute the rate of workhardening, or n-value, of the steel.
Why the strong interest in energy to deform? Fig. 2 shows the energy curves for two 350-MPa yield-strength steels—a traditional high-strength low-alloy steel and a dual-phase (DP) AHSS. The area under the true stress- strain line represents the amount of energy required to achieve the given amount of strain—0.125 true strain or 13-percent engineering strain. Deforming a DP steel to the same strain would require an extra amount of energy (shown in blue). The ten- sile test only allows the curve to reach the uniform elongation or ultimate tensile strength. At that point the ten- sile sample begins a diffuse or width neck and the pulling stress is decreas- ing (Fig. 1). The curve can be extend- ed by creating an equation to define the existing curve and then extending
the curve to higher values of strain. Another research process mechani- cally extends the deformation to high- er true strain values.
The amount of energy consumed is critical in metalforming.
1) Energy is a cost item supplied by the electric company to the electric motor to maintain flywheel speed in traditional presses, or the charging of the capacitor in servo-driven presses.
2) Forming stamped parts requires energy. Increased force or length of press stroke can increase energy use, as can an increase in strokes per minute— this creates additional heat in the die. The increase in die temperature lowers the viscosity of most lubricants. Exces- sive heat can cause vaporization of lubricants, leading to galling, scoring or even a die freeze up.
AHSS and virtual forming are a nat- ural partnership in this new press-shop technology. Soft-tool tryouts are no match for many of the very high- strength steels. Some of these steels change properties that assist some dif- ficult forming modes. Virtual forming allows troubleshooting of the stamping and die design before starting to cut the first die. These forming codes require true stress-strain curves to understand how each AHSS performs. MF
www.metalformingmagazine.com
MetalForming/May 2014 51
True Stress (MPa)
