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Opt for Qualitative, Rather Than Quantitative, Analysis of Steel Data

By: Stuart Keeler

Sunday, September 01, 2013
 

Computers, electronic measuring devices, video recorders and other technological advancements entering our pressrooms often hinder our understanding of the metalforming processes. While mathematically correct, the massive amount of data spewing from these machines often is confusing, misinterpreted, overly time consuming and not useful. When it comes to comparing different types and grades of steel, often qualitative analyses of simple data will provide sufficient information.

Fig. 1—For a given increment of strain, the steel with the largest increase in stress from YS to TS (steel B) has the larger n-value and greater stretching capacity.
Assume a tensile-test machine reports the n-values (workhardening exponent) of steel A as 0.213849 and steel B as 0.213122. The larger n-value (steel A) generally means more stretchability. For decades, n-value only was valid to two decimal places, but within the last 20 years the third decimal place has gained creditability—when measured under strict test procedures. Just because the computer is programmed to calculate and print three additional decimal places does not mean they contain more useful information. In our example, steels A and B have the same n-value (0.213), typical of an aluminum-killed deep-draw steel.

This type of simple qualitative information often will prove more useful in the press shop than will extremely detailed quantitative data. Two candidate steels are evaluated for a part requiring extra stretchability. Both have yield strengths (YS) of 350 MPa; tensile strength (TS) of steel A is 525 MPa, and for steel B is 600 MPa. Immediately one can conclude that steel B should have the most stretchability.

The n-value of the true stress-strain curve dictates the amount of stretchability, by reducing the growth of strain gradients and increasing the maximum allowable strain combinations predicted by the forming-limit curve. The n-value is proportional to the TS/YS ratio—1.5 and 1.7, respectively, for steels A and B. Fig. 1 displays the basis for this qualitative relationship. Note the stress-strain curve of steel B climbs more steeply than for steel A. This is caused by the higher n-value workhardening the steel more per unit of strain. One can make a good qualitative decision without requiring full stress-strain curves from which the actual n-values are computed.

This simple stretchability evaluation becomes more difficult when using the inverse ratio YS/TS (0.67 for steel A, 0.59 for steel B). As n-value (and stretchability) increases, YS/TS decreases. Having all four values (n-value, TS/YS, amount of stretchability and the stress-strain increase in Fig. 1) going in the same direction is much easier to comprehend. Confusion occurs when a TS/YS value of 0.59 represents more stretchability than a steel with a higher TS/YS ratio value.

Some specifications have a special code that indicates the minimum TS-YS gap for a given steel. Steel with a code showing a minimum 70-MPa gap has more stretchability than a code showing only a 35-MPa gap.

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.

The global interchange of sheetmetal, dies and stampings has created property identification problems. A coil of high-strength low-alloy (HSLA) steel is received with an HSLA 550-MPa label. Is 550 MPa the YS or TS? It is YS for coils made in North America or Europe, but TS for coils made in the Pacific Rim. To rectify this problem, the worldsteel organization, representing major steel companies around the world, uses unique steel identifiers. It labels a 550-MPa steel as HSLA 550/650 MPa in the United States. Immediately one can compute a TS/YS ratio of 1.18. The reduced stretchability of this high-strength steel is reflected 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 forming 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 (permanent) 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 stresses, 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 presence 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 extensometer. Different parts of the world use different gauge lengths—two common gauge lengths are 2 in. (A50) and 3 in. (A80). The shorter gauge length measures the large deformation associated 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 uniform elongation ends of the sample. The total elongation differences can be large—40 percent for A50 and 34 percent for A80, both measured simultaneously in the same tensile sample.

For a century or more, steel formability has been determined by its hardness values. Hardness-testing equipment is simple and easy to operate, and the test results are easily understood. The hardness test proves very useful for determining wear resistance of surfaces. An indenter is driven into the steel at fixed load values. The compressive 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 resulting 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 industry—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/English unit? Let’s get this changed to g/m2. MF

 

Related Enterprise Zones: Materials/Coatings

 


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