Stuart Keeler Stuart Keeler
President/owner

New Technology Means Back to School

May 1, 2015


Fig. 1—Many stampers only use yield strength, ultimate tensile strength and fracture information. However, the important stretchability data are contained in Zones A and B
Fig. 1—Many stampers only use yield strength, ultimate tensile strength and fracture information. However, the important stretchability data are contained in Zones A and B.

New metalforming technology developments surround us—growing perhaps at the fastest pace ever. Examples include completely new steels with significant increases in strength, servo presses and 3D printing. To remain competitive, manufacturers working with sheetmetal must strive to keep up. Here we focus on the most important mechanical property of steel that controls stretchability—workhardening exponent, or n-value.

In days gone by, a blacksmith would assess stretchability of a sheetmetal blank by using a strong pair of grips to grab the corner of the blank and bend it. The force required provided a measure of the material’s strength, and stronger meant less stretchability and formability. Unfortunately, this test was limited to thinner sheets, and the results were not numerical but mainly estimations and emotions.

Fig. 2—Plotting datapoints from Zone A in Fig. 1 allow computation of the n-value..
Fig. 2—Plotting datapoints from Zone A in Fig. 1 allow computation of the n-value.

A major breakthrough occurred with the development of the tensile-test machine and the use of prepared test samples. The output of this machine was a stress-strain curve that provided three major properties: yield strength, tensile strength and total elongation (Fig. 1). Today many properties of different types and grades of sheetmetal can be determined. Other research evaluated the requirements needed to form specific shapes in stampings. Matching the workpiece material to the die became numerical instead of theoretical.

As noted, an extremely important material property that affects stretchability is n-value, determined from plotting data from Zone A in Fig. 1 is according to a specific equation on log-log graph paper (Fig. 2). This graph shows the n-value as the slope of the curve. The 0.25 value indicates a vacuum-degassed interstitial-free (VD-IF) steel with excellent stretchability. Although n-values were available by 1940, mass utilization was delayed until the 1970s. Even today some press shops still do not utilize this valuable property.

High n-values reduce or prevent localized strain (stretch) gradients from growing to failure. The hemispherical-punch stretch test (Fig. 3) illustrates how high n-values reduce peak strains and redistribute deformation during stretching. Steels for the various n-values:

Fig. 3—Distribution of deformation depends strongly on the n-values shown in the center of the graphs. The dashed line indicates the direction for higher-strength steels with n-values less than 0.20.
Fig. 3—Distribution of deformation depends strongly on the n-values shown in the center of the graphs. The dashed line indicates the direction for higher-strength steels with n-values less than 0.20.
• 0.20, AKDQ

• 0.25, VD-IF

• 0.35/0.45, 300-series stainless steel

Higher-strength steels with n-values less than 0.20 would create sharper peaks with less stretchability, illustrated by the dashed line and arrow in Fig. 3.

The onset of material failure (Zone B in Fig. 1) occurs before total elongation and sheet fracture occur at the end of the stress-strain curve. The Zone B failure is called a local or through-thickness neck. The onset of this neck localizes the thinning while deformation in the remainder of the stamping is terminated. This local neck also is the forming-limit curve (FLC) that predicts forming severity and proximity to failure for most locations within the stamping. The FLC depends highly on n-value, as does the percent total elongation.

Stretchability and bendability of mild steels (VD-IF, AKDQ, etc.) and high-strength low-alloy steels (HSLA) depend on the n-value of the steel grade being formed. The n-value and FLC are major inputs to virtual forming—software that simulates forming before the first hard die is cut. This process also is used for simulated die tryout.

Fig. 4—Some AHSS grades have n-values that change when the steel deforms. This requires plotting of instantaneous n-values. Courtesy of AHSS Application Guidelines—Ver. 5.0
Fig. 4—Some AHSS grades have n-values that change when the steel deforms. This requires plotting of instantaneous n-values. Courtesy of AHSS Application Guidelines—Ver. 5.0
Advanced high-strength steels (AHSS), finding use for their increased stretchability compared to other steels with the same strength or higher strength, have microstructures that change properties as they deform. Unlike mild and HSLA steels that have a single n-value, the n-value of AHSS grades increases and decreases during forming. Therefore, the instantaneous n-value must be measured from the start of deformation to fracture. Fig. 4 shows these n-values for three steel grades with identical 50-ksi yield strengths—HSLA, dual-phase (DP) AHSS and transformation-induced plasticity (TRIP) AHSS.

The HSLA steel has a flat curve with an n-value around 0.16. The DP steel rises to a peak n-value of 0.20 at a very early strain of 3-percent engineering strain. Any area of the stamping that normally experiences greater deformation due to local high stresses from the die geometry will be restricted by the DP steel. The HSLA n-value of 0.12 is being replaced by an n-value of 0.20. Even an n-value increase of +0.02 can make a significant difference.

Metallurgists took the early increase in the DP n-value and caused the TRIP steel to keep repeating the n-value increase as deformation continued (Fig. 4). The resulting n-value of 0.25 has the same stretchability as the VD-IF steel but with a 30-ksi increase in yield strength. The DP steel has no FLC improvement compared to HSLA steels because the terminal n-values are identical. However, the higher terminal n-value of the TRIP steel creates a higher FLC that equals that of mild steel.

For additional information about AHSS and HSLA steels, visit the free download AHSS Application Guidelines—Ver. 5.0 at www.worldauto-steel.org. MF

Industry-Related Terms: Die, Download, Form, Forming, Tensile Strength, Blank, Center, Corner
View Glossary of Metalforming Terms

Technologies: Quality Control

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