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Forming Higher-Strength Steels

By: Stuart Keeler and Pete Ulintz

Industry experts Stuart Keeler and Pete Ulintz author monthly columns in Metal-Forming magazine and conduct seminars for the Precision Metalforming Association. Keeler, an expert in steel-forming and material technology, is president of Keeler Technologies LLC, a consulting firm in Grosse Ile, MI; e-mail keeltech@comcast.net. Ulintz is advanced product engineering manager for Anchor Mfg. Group, Cleveland, OH, and has extensive experience in tool design and build. His e-mail: pete.ulintz@toolingbydesign.com.

Wednesday, April 01, 2009
 
 
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Forming higher-strength steel presents the same problems as forming
 Forming higher-strength steels
low-strength steel, except the problems are magnified and too often become disastrous. From the steel perspective, insufficient information about the higher-strength steels only makes these problems worse.

The following scenario occurs all too commonly in a die-build facility. A purchase order arrives with the part design, the allowed number of dies or progressive-die stations, and other die specifications. First, the die-build shop completes the process design, which allocates the type and amount of deformation for each die. Next comes die design and construction. Finally, the time arrives for die tryout. What about the steel? Oh, the trial coil has arrived from the customer and sits over in the corner. Tryout begins. The first piece splits in multiple locations with the stamping completely out of dimensional tolerance. What do you know about the coil of steel? Trade names, ASTM numbers, steel-mill process codes or internal customer identification systems provide little, if any, useful information.

The die can only respond to certain mechanical properties. For a complete evaluation, these properties include elastic modulus, yield strength, work-hardening exponent (n), anisotropy (r) and total elongation. One does not require all of these properties for every stamping. However, availability of the necessary properties throughout part design to production cycle provides critical information, creating up-front compensation before process and die designs even begin. Acquiring this information minimizes tryout and production problems. For dies already in production, knowing the mechanical properties of the current steel and the proposed higher-strength steel upgrade allows for identification of potential problems and corrective actions prior to the steel upgrade.

Formability

Too often we hear that steel formability decreases as strength increases. The term ‘formability’ is too broad because specific properties limit each different forming mode found in most stampings. Even simple stampings may encounter different amounts of stretching, bending, cylindrical cup drawing or deep drawing, with each mode strongly dependent on different properties.

Stretchability, or increase in length-of-line, depends on material strength. Laws of nature require payment for increasing the strength. For a given type of steel, allowable stretchability decreases as strength increases (Fig. 1).

Strength comparision of Various Steels 
Fig. 1—Graph shows decrease in strength over a range of steels from mild steel to 165-ksi martensitic steel.
WorldAuto-Steel
On a more technical level, stretchability relates to the workhardening capacity of the steel. One common measure of workhardening is the n-value from the equation,


which describes the steepness of the stress-strain curve. The n value is related to the tensile/yield-strength ratio, TS/YS. Thus, within a specific type of higher-strength steel, higher-yield-stress steel exhibits a flatter stress-strain curve, lower n-value, lower TS/YS ratio and reduced stretchability. Reduced stretchability is observed in two areas. First, the forming-limit curve (FLC) defining the different combinations of allowable stretch is directly proportional to the n-value and reduces the allowable stretch as steel strength increases. The lower n-value for higher-strength steels also provides less resistance to formation of localized strain gradients. Fig. 2 shows these two effects.

How n-Value Affects Stretchability
Fig. 2—Schematic shows reduced stretchability and increased strain-gradient severity as the n-value decreases with increased steel streng
Bendability and total elongation show a correlation. Imagine a tensile-test sample just under the outer surface of a bend sample. Since the higher-strength steel has a lower total elongation in tension, it has reduced bendability (outer-fiber stretch) as reflected in the bend radius/sheet thickness ratio, R/t. Therefore, higher-strength steels require a larger bend radius to avoid cracking and tearing at the outer surface of the bend.

Cup-drawing severity usually is measured by the limiting draw ratio (LDR) that determines the maximum blank diameter that can be successfully drawn into a cup with a given punch diameter. The LDR tends to be independent of steel strength for all hot-rolled and higher-strength-steel products. A simple analysis shows that the stronger steel moving in from the binder balances with the same stronger steel near the punch radius where failures usually occur.

Deep drawing generally refers to a square or roughly rectangular box. The corners, or the box, comprise one-quarter of the cup-drawing operation. However, most punch radii, embossments and other geometrical features combine bending and stretching, and become more likely to fail with increases in steel strength.

Summarizing formability of higher-strength steel, any area of the stamping subjected to a tensile stress and the resulting sheetmetal thinning becomes more critical as steel strength increases.

Springback

The atoms in steel bind via extremely strong elastic stresses. Applying a force to the steel generates a stress on the atomic structure. Tensile stress causes an increase in atomic spacing, while compressive stress causes a decrease. Remove the stress and the atoms return to their normal spacing, achieving a neutral stress state. This return to normal atomic spacing causes springback, the amount proportional to the yield stress of the material. Doubling the yield strength of steel doubles springback (Fig. 3).

Springback
Fig. 3—Springback is proportional to yield strength and increased flow stress (A).

As flow stress increases, resulting from cold working the material, elastic stress continues to increase proportionally with the flow stress. Therefore, the potential amount of springback in a stamping varies with location within the stamping. However, if the shape of the stamping now creates a geometrical restraint that does not allow all of the elastic stresses to return to their neutral stress state, those remaining elastic stresses remaining in the stamping become residual or trapped stresses. These trapped stresses cause additional springback during subsequent stamping operations.

Most experience with springback and springback compensation arises from work on mild steel with yield strengths in the range of 25 to 35 ksi. Different grades of high-strength low-alloy (HSLA) steels have yield strengths from 35 to 100 ksi. Some of the newer advanced high-strength steels (AHSS) have minimum yield strength of 200 ksi. (These AHSS are discussed further in the Science of Forming column on page 32 in this issue.)

Thermal Effects

Materials with higher yield strength obviously requires a greater force to create the same amount of strain. Less obvious: the increased energy required to deform each stamping. The energy required is proportional to the area under the true stress-strain curve, similar to that shown in Fig. 3. Both the greater force and energy required to form higher-strength steels have a major impact on die design and press selection. The increased energy imparted during forming creates internal heat that causes a temperature increase in the stamping. This temperature increase decreases steel strength. If the temperature of the whole sheet increases, the problem generally is limited to degradation of the lubricant or a change in die dimensions.

 

 
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