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Achieving successful stamping of AHSS

It’s all about the process

Advanced high-strength steels (AHSS) increasingly are specified in automotive applications, with some vehicles using these grades for more than 50 percent of the body structure. Nonautomotive applications also are growing; these grades are useful wherever a higher strength can result in weight savings.

The workhorse AHSS grade, DP350/600, will be as commonplace as HSLA 50XF (HSLA 350/450) in the coming years. The DP (dual-phase) steel has 50,000-PSI minimum yield strength and 87,000-PSI minimum tensile strength; the HSLA (high-strength, low-alloy) grade has the same yield strength and a minimum tensile strength of 65,000 PSI.

Unfortunately, it can be challenging to form products from AHSS grades successfully. Most of these challenges are related to the higher strength and thinner gauge associated with these grades. No individual issue is insurmountable, but understanding them is a necessary first step in designing countermeasures.

Press Feeding and Straightening

Material in a coil-fed line is straightened by bending the sheet metal around sets of rollers to alternately stretch and compress the upper and lower surfaces. Compared with processing thicker metal, straightening of thinner steels requires a number of smaller-diameter and closer-spaced rollers.

With closer-spaced rollers, more force is needed to bend and unbend the sheet steel, increasing the power requirements for processing these grades. Furthermore, since smaller-diameter work rolls are prone to deflect more when placed under higher forces, such as when processing higher-strength steels, the risk of coil shape problems and machine component wear increases. This risk is addressed with larger backup rolls and more robust bearings and gears.

Blanking

In cutting operations such as blanking, shearing, and piercing, snap-through shock or negative tonnage can occur. When sufficient cutting force is applied to the sheet metal, it fractures nearly instantaneously, leading to a sudden release of stored energy as vibrational waves. Components designed to be in tension are thrust into compression and vice versa. The shock force does not dissipate all at once, but instead oscillates down. At high press cycle rates, the oscillations may not fully dissipate before the next stroke, leading to further compounding of the vibration waves—which is more destructive to the press and die components.

One way to reduce the shock, vibration, and tonnage is to run slower. In addition to a productivity hit, another problem is the inability to achieve sufficient press energy when needed in the stroke. Also, remember that steel has positive strain rate sensitivity, meaning that it gets stronger as you run faster. If you want to increase the blanking speed, it will take more force to shear the steel, which increases the snap-through loads.

Cutting tools need to be sufficiently sharp. Compared to sharp tools, worn tools result in a 20 percent reduction in hole expansion (stretch flangeability) in mild steels, but a reduction of 50 percent or more in DP and TRIP (transformation-induced plasticity) grades.

Press Tonnage and Energy Capacity

Processing higher-strength sheet steels requires using a press with suitable tonnage and energy capacity. The metal’s strength determines the load necessary to plastically deform the sheet, which in turn influences the press tonnage requirements.

Remember that strength in the grade description (for example, DP-350/600) is the minimum value for that material. When you are determining press tonnage, the maximum strength value is far more useful. Unfortunately, several AHSS grade specifications don’t identify the maximum strength value for that material, so you will need to contact the steel supplier for that information.

Figure 1 – Shown here is the true stress/true strain curve through 13 percent elongation for three steel grades. (www.autosteel.org/research/ahss-data-utilization.aspx)

Also remember to focus on true strength rather than the more common engineering strength when comparing materials’ tensile strength. Strength is determined by dividing the tensile load by the cross-sectional area. When you are determining the engineering tensile strength, the cross-sectional area is assumed to be constant, such as the metal thickness multiplied by the ½-inch width of the tensile bar. This makes the calculations easier, but it is not realistic. As a tensile test progresses, the thickness and the width both decrease as the load increases.

Instead, dividing the load by the actual cross-sectional area determines the true strength. For more information about converting between engineering and true units, visit www.eqsgroup.com/all-about-steel/difference-between-true-stress-and-engineering-stress.asp.

Figure 1 shows true stress-strain curves up to a true strain value of 0.13 for three steel grades: HSLA 350/450 (also known as 50XF), DP-350/600, and DP-450/800. The maximum stresses for these grades are 73,000, 110,000, and 140,000 PSI, respectively. Therefore, if you want to stamp DP800 steel, you will need a press with twice the tonnage capacity as you would for stamping HSLA 50XF.

Determining the energy required to form the different grades is more complex and related to the area under the stress-strain curves. The area increases with both strength and elongation (draw depth). This is a concern if you slow the press down to form more challenging parts from these advanced grades or to avoid lubricant breakdown from excessive heat. When you reduce the cycle time in a mechanical press, the flywheel might not be able to generate the amount of energy required for these steels. The load application capacity of a mechanical press is lower than the rated tonnage at distances above bottom dead center, which is where deep drawing usually begins; hydraulic presses can exert maximum force throughout the stroke.

Blank Holding

One benefit of the AHSS grades is that you can reduce the sheet steel thickness. However, decreased thickness may result in increased wrinkling if die clearances are not adjusted to reflect the reduced gauge. Controlling wrinkling requires higher press forces, which may lead to the need for higher-tonnage presses.

The wrinkling, combined with the sheet steel's overall strength, increases the potential for die wear. Pulling the wrinkles out becomes increasingly more challenging as material strength goes up.

Not only are AHSS grades higher in strength, but they also work-harden faster than HSLA grades. This strength increase during the press stroke requires greater forming tonnage and blank holding. To control AHSS metal flow, you will need 20 percent higher tonnage than when working with conventional HSLA steel grades and twice the tonnage required for mild steels.

Lubrication

Increased blank holder and punch forces causes higher contact pressure between the sheet metal and tooling, which in turn results in higher interface temperatures. Without the use of lubricants designed to function at these higher temperatures and pressures, excessive die wear occurs, further increasing friction.

Studies by Dr. Taylan Altan and the researchers at The Ohio State University’s Center for Precision Forming indicate that the highest temperatures are found at the die corner radius. While 2-mm mild steel might reach a maximum of 120 degrees F, 1.6-mm DP600 reaches 175 degrees F, and 1.2-mm DP1000 increases to 210 degrees F (see References and Figure 2).

References
Taylan Altan Ph.D., “Advanced Methods for Forming AHSS and UHS Al alloy,” January 2015.
(ercnsm.osu.edu/sites/ercnsm.osu.edu/files/uploads/general.pdf)

Altan et al., “Evaluating lubricant using the cup drawing test,” STAMPING Journal, September/October 2016, p. 16.
(www.thefabricator.com/article/stamping/r-d-update-evaluating-lubricant-using-the-cup-drawing-test)

Figure 2 – The highest temperature during forming occurs around the die corner radius. (www.thefabricator.com/article/stamping/r-d-update-evaluating-lubricant-using-the-cup-drawing-test)

About the Author
Engineering Quality Solutions Inc.

Daniel J. Schaeffler

President

P.O. Box 187

Southfield, Michigan 48037

248-539-0162

Engineering Quality Solutions Inc. is a provider of practical solutions for sheet metal forming challenges.