Page 26 - MetalForming April 2009

Forming Higher-Strength Steels
slipping when formed. Pad pressures for wiping dies generally equal the punch force required to form the part. Because higher-strength steels can have two to three times the initial strength of mild steels, the required pressure-pad force could be as much as 300 percent greater. This level of holding force may be difficult to achieve in small wiping dies or in a small die area.
Incorporate over-bending into wip- ing dies to compensate for springback. Expect as much as 10 deg. of springback depending on the steel grade. Higher forming stresses on die materials can create potentially large tool deflections. Key and properly heel forming steels to control tooling deflections and maintain part quality.
Where practical, consider rotary forming tools (Fig. 5) in place of flange- wipe dies. Rotary bending enables eas- ier adjustment of springback compen- sation while tensile loading, generated by the wiping tool, is absent.
General die design considerations
for bending and forming:
• Minimize punch radii, as they affect springback.
• Design small replaceable sec- tions in high wear areas—use inserts for radii and flange adjustments.
Rotary Forming Tool
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• Ensure that die sections have ade- quate aspect ratios (width to height) with heels and keys to resist thrust forces during forming.
• Consider higher grades of die mate- rial, at least one tool-steel-grade higher, and coat flange steels, preferably via a vanadium-carbide process.
• Use material gainers to increase length of line in low-strain areas to feed high-strain areas in flanges.
• Avoid placing trim bypass (mis- match) in stretch or compression areas
of the flange to help avoid splitting.
• Try to ‘fold’ the material around a radius, instead of drawing or stretching,
to reduce sidewall curl.
• Flatten the flange radius at the bot-
tom of the press stroke.
• Bottom the pad and all forming
steels at the bottom of the press stroke.
Draw Forming
Higher-strength steels require care- ful planning and consideration before choosing draw forming as a method of manufacture. Metalforming simulations allow exploration of different forming processes by predicting strains and residual stresses for different deforma- tion modes. Deep drawing over a binder and through draw beads may not be the best choice and simple die forming or rollforming may offer better options for some very-high-strength steels.
In draw development, maintain even draw depths and constant length-of- line to the greatest extent possible, and form as much of the part as possible to the full depth in the first forming oper- ation. Controlling material flow may require an increase in binder tonnage by as much as 20 percent when compared to working with conventional HSLA steel grades, and double that needed for mild steels. In the right press, these are not overwhelming issues. However, a single-acting press with a nitrogen cushion may be insufficient for draw forming some higher-strength grades.
In single-acting presses, high impact loads on the die and press occur high up from bottom dead center where draw- ing typically begins. The impact occurs at a point in the press stroke where slide velocities are higher and available
Sheet Die
Rocker
1. 2. 3.
Fig. 5—Consider rotary forming tools in place of wiping dies.