Stuart Keeler Stuart Keeler

Modern Forming Needs Upgraded Language

February 1, 2015

Sheetmetal forming is becoming more complex as new higher-strength steels find use for consolidating multiple stampings into one, with tighter dimensional tolerances and increased productivity. Too often these advances become hindered by the use of old terminology and misunderstanding of process problems. I describe four such problems here.

1) Misunderstanding of steel properties can occur due to use of different identification systems in different areas of the world. For example, does high-strength low-alloy (HSLA) 450 MPa mean a yield strength of 450 MPA, or a tensile strength of 450 MPa? Unfortunately, both can be correct. This steel has been listed by yield strength in North America and Europe but by tensile strength in Asia and the Pacific Rim. No problems existed when the steel was made and used in the same country. However, steel now made in one part of the world and then shipped to another area for production of stampings can create confusion. All advanced high-strength steels (AHSS) are specified by tensile strength throughout the world, but many steel users must order by yield strength.

Fig. 1—The n-values for some AHSS grades change with deformation and must be tracked as instantaneous values. WorldAutoSteel—AHSS Application Guidelines Version 5.0.
Fig. 1—The n-values for some AHSS grades change with deformation and must be tracked as instantaneous values. WorldAutoSteel—AHSS Application Guidelines Version 5.0.
One solution promoted by WorldAutoSteel (the technical division of Worldsteel) is use of a label printed with three pieces of information: steel type, plus yield strength and tensile strength values in MPa. One coil of steel could be labeled HSLA 350/450, and another coil (of AHSS) might be labeled TRIP 350/600. The user knows exactly the ordered properties of the steel. While both steels have equal yield strengths, their tensile strengths are different. By calculating the TS/YS ratio, some information about the workhardening exponent (n-value) can be obtained. The ratios are 1.29 for the HSLA and 1.71 for TRIP steels.

2) I often hear metalformers say that, “We convert uniform elongation into n-value because it is easier than curve-fitting to a line of data points.” The better option is to compute uniform elongation from the n-value. The uniform elongation depends only on one data point—ultimate tensile strength. This value could be an accidental blip anywhere in the stress-strain curve. The n-value is best computed from the slope of many points on the log true stress versus log true strain curve, and is not influenced by a single error point. The n-value then can be converted to uniform elongation. Modern tensile-test software automatically computes n-values.

Fig. 2—This hole-expansion study documents the tremendous reduction in edge stretchability for punched holes. R.R. HilsonMicroalloying 75.
Fig. 2—This hole-expansion study documents the tremendous reduction in edge stretchability for punched holes. R.R. HilsonMicroalloying 75.
Computing n-values from many datapoints becomes mandatory when determining n-values for AHSS DP, TRIP and TWIP steels. Unlike the single n-value associated with mild and HSLA steels, these three AHSS grades have n-values that change as the steel deforms. The single n-value is replaced by a graph of the instantaneous n-value plotted against deformation strain for each type of steel (Fig. 1). The important sector for DP steel lies between 3- and 8-percent strain, where the higher n-values help prevent the onset of localized stretch gradients. Note that the traditional 10- to 20-percent range of n-values for the HSLA and DP steels is identical and would not report the improved early n-values of the DP steel. The very high n-values above 8-percent strain for TRIP steel allow a greater degree of stretch throughout the stamping and a higher forming-limit curve (FLC).

3) Some measurements made many decades ago continue to be used for decisions today. The most common example: Taking hardness readings to determine formability capacity of a steel blank. Long ago the metalforming industry knew that cold working the steel increased its yield strength. Lacking the tensile-test machines of today, simple hardness tests were available to sort different coils and blanks of steel. Stronger steels with higher hardness values were diverted to less complex stampings needing increased load-carrying capacity. The process of cold working therefore was named workhardening; we know today the correct term should be work strengthening. Unfortunately, the term workhardening is so common in our culture that it will remain the phrase of choice.

Today’s knowledge of deformation teaches us that a pointed indenter under a standard load will compress the workpiece surface until a crater forms. We measure the diameter of the crater and give it a hardness number. A smaller crater with a higher hardness number signifies a stronger surface that resists indentations from rough dies or in-service applications. However, in most metalforming applications, we see stretch deformation through the entire thickness of the blank and not just a single surface crater. We measure the capacity for stretch forming using a tensile test, the FLC, stretch-bend and other formability tests.

4) Some metalforming professionals complain that the FLC is flawed, because the actual measured edge stretch before fracture is much less than predicted by the FLC. However, the real problem lies with improper application of the FLC, never intended as a tool for predicting the allowable stretch in sheared edges. The height of the FLC depends on the n-value and thickness of the as-received sheetmetal. Cold working the blank via cutting, shearing or punching greatly reduces the n-value of the edge, and the FLC. Fig. 2 shows a relatively undamaged milled edge with a hole-expansion ratio of 280 percent, and a traditional punched hole with a hole-expansion ratio of only 80 percent. The severe reduction in hole-expansion ratios can be even more severe for AHSS DP and TRIP steels, because the islands of martensite at the cut edge can initiate earlier fractures. MF
Industry-Related Terms: Blank, Cold Working, Compress, Edge, Forming, Martensite, Point, Shearing, Surface, Tensile Strength, Thickness
View Glossary of Metalforming Terms

Technologies: Materials, Quality Control


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