Page 51 - MetalForming September 2013

ed in the low TS/YS ratio. This steel now can be compared to other higher- strength steels to evaluate relative stretchability.
For useful information about form- ing performance, consider the elastic- to-plastic transition portion of the stress-strain curve (Fig. 2). Steel A exhibits a gradual transition from the elastic modulus line to plastic (perma- nent) deformation. To determine YS, we construct a line parallel to the modulus line, offset by +0.2-percent strain. This smooth transition indicates the absence of stretcher strains (Lüder’s lines or bands).
In sharp contrast, steel B has two distinct changes in deformation. These well-defined changes in stress path, called the upper and lower yield stress- es, can easily change values depending on material processing. This yield- stress pattern indicates the presence of Lüder’s bands—thickness steps that divide areas of different amounts of deformation. Not only do they spoil the appearance of class A surfaces, but strain gradients can localize in these lines. The likelihood of kinking during bending operations and coil breaks when unwinding increases in the pres-
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measures the large deformation asso- ciated with the diffuse or width neck in the center of the tensile sample. The longer gauge length adds some of the lower deformation found in the uni- form elongation ends of the sample. The total elongation differences can be large—40 percent for A50 and 34 per- cent for A80, both measured simulta- neously in the same tensile sample.
The Science of Forming
Yield Strength
Steel
Upper
Yield Stress
Lower Yield Stress
Steel
AB
Engineering Strain Engineering Strain
Fig. 2—The smooth transition from elastic strain to plastic strain (steel A) is called yield strength. The upper and lower yield- stress states signify yield-point elongation and Lüder’s bands.
ence of Lüder’s bands.
On the other end of the forming spectrum, we use total elongation for a given test sample to compare steel bendability. Total elongation depends on the gauge length of the measuring exten- someter. Different parts of the world use different gauge lengths—two com-
mon gauge lengths are 2 in. (A50) and 3 in. (A80). The short- er gauge length
For a century or more, steel forma- bility has been determined by its hard- ness values. Hardness-testing equip- ment is simple and easy to operate, and the test results are easily under- stood. The hardness test proves very useful for determining wear resistance of surfaces. An indenter is driven into the steel at fixed load values. The com- pressive force creates a crater whose diameter indicates hardness. Various indenter shapes and sizes are used with different loads to conduct various types of hardness tests. While hardness values increase with YS, values for hardness and stretchability do not correlate well. Steel fails by application of a tensile stress and the resulting tensile strain within the sheet thickness. The result- ing thinning causes failure. This mode of deformation is completely opposite to the compressive cratering generated during a hardness test.
A few strange measurements still remain in the metalforming indus- try—the amount of lubricant placed on the workpiece, for example, measured in mg/ft.2. Who and why did someone create such a combination metric/Eng- lish unit? Let’s get this changed to g/m2. MF
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Engineering Stress
Engineering Stress