Page 26 - MetalForming May 2014

Advanced High Strength Steels
die to increase in temperature. This decreases lubricant viscosity, and usu- ally increases the lubricant’s coefficient of friction (COF). Lubricant breakdown can occur, causing galling and scoring of the sheetmetal and dies.
In the end, blankholder restraining force increases, defeating the goal of more material flowing into the die. Draw beads in the binder area further complicate the problem. Likewise, increasing the number of strokes/min. increases the amount of heat generated with a corresponding increase in work- piece and die temperature.
To battle heat buildup when forming AHSS, stampers opt for better lubri- cants less prone to viscosity changes and lubricant breakdown. Water-based lubricants disperse more heat than do oil-based lubricants. Some AHSS stampings may require tunnels drilled inside the tooling for circulating cooling liquids. These tunnels target hot spots (thermal gradients) that tend to localize deformation and lead to failures.
Some lubricant manufacturers have developed stable, low-COF dry (barrier) lubricants. These lubricants (mainly polymer based) completely separate the workpiece material from the die. A dry lube’s COF for the same combi- nation of steel and die can be 0.03, compared to 0.12 to 0.15 for a good wet lubricant. That means doubling the blankholder force to maintain very flat binders for joining purposes will still reduce the binder restraining force by one-half or more. That reduction in binder restraining force allows much more material to flow into the die. The amount of punch stretching can drop from the FLC-red failure zone to well into the green safe zone.
The complete separation of sheet- metal and die by the barrier lubricant also will isolate any differences in coat- ing characteristics. Most important, the COF tends to be temperature- insensitive, resulting in a more robust forming system. Finally, a known and constant COF over the entire stamping
greatly improves the accuracy of com- puterized die tryout (virtual forming). Springback: Springback has been cited as the leading hurdle to meeting part dimensional specifications when stamping AHSS. The elastic stresses causing springback are on the atomic level and cannot be seen or even easily measured during forming. These stress- es are proportional to workpiece-mate- rial yield strength and can be very high for AHSS. While bending 175-MPa mild steel might result in 2 to 4 deg. of springback, bending 1400-MPa AHSS could result in 16 to 32 deg. of spring-
back.
The AHSS Application Guidelines
includes extensive explanations of the types of springback and correction techniques. Three main correction cat- egories are:
1) Change the elastic stresses from unfavorable to favorable: A commonly used technique is post-stretching.
2) Reduce or minimize the magni- tude of the stresses: The most effective
24 MetalForming/May 2014
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