Page 24 - MetalForming May 2014

Advanced High Strength Steels
particles trigger surface fractures. Researchers hope to develop forming- limit curves for these last two forming modes.
Cold Forming–Stretchability: For traditional low- and medium-strength steels, n-value decreases as yield strength increases. As n-value decreas- es so does overall stretchability. A lower n-value encourages the formation of highly localized strain gradients, which can cause early failure.
Increasing n-value is one goal of AHSS. For DP steels, an increase in n- value in the engineering strain range of 4 to 8 percent helps to prevent the for- mation of severe gradients. However, n- value decreases back to that of a high- strength low-alloy steel of the same yield strength. Therefore, no increase occurs in the terminal n-value or FLC. In con- trast, TRIP steels have an initial n-value climb that is maintained throughout deformation. These steels prevent the initiation of gradients and increase the allowable stretchability defined by the
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FLC–a double benefit.
Hot Forming: The use of hot form-
ing (HF) has increased greatly during the last 5 yr. Direct HF can form severe and complex shapes. Stampings exhibit the super-high strength of martensite after quenching. And, the process results in nearly no springback.
The HF process (Fig. 2) begins with heating a coated manganese-boron blank to above 900 C and then rapidly transferring the blank into a press. The press quickly forms the stamping, which has a tensile strength of only 100 MPa. The stamping then dwells in a water-cooled die until transformation to martensite is complete—tensile strength of 1900 M, total elongation of about 5 percent.
Press Requirements: Because AHSS products have significantly different forming characteristics than conven- tional steels, they challenge conven- tional mechanical and hydraulic press- es and related equipment. Some of these differences include lower forma-
bility/ductility, variations in through- coil properties, higher forming loads and increased springback
The stamping industry continues to develop more-advanced die designs and advanced manufacturing tech- niques to help reduce fractures and scrap created when stamping AHSS using traditional equipment. Servo-dri- ven presses are a great example—these presses adjust slide speed throughout the stroke to ensure shape and dimen- sional accuracy of the formed product. A pressure sensor within the servo die cushion (similar to that of a hydraulic cushion) controls the position of the die cushion, allowing for the optimiza- tion of material flow in the flange between the die and the blankholder. These characteristics have proven valu- able when forming complex geometries from AHSS products.
Lubrication: Deforming higher strength steels (especially AHSS) requires more energy. Higher forming energy causes the stamping and the
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