Formability of Ultrafine-Grained Interstitial-Free Steels

The stretch formability of ultrafine-grained (UFG) interstitial-free steel (IF-steel) produced by equal-channel angular extrusion/pressing (ECAE/P) via various strain paths was investigated with a miniaturized Erichsen test. A coarse-grained (CG) sample demonstrated high formability with an Erichsen index (EI) of 4.5 mm. Grain refinement by ECAE decreased the formability, but increased the required punch load (F _EI) depending on the applied strain paths. The EI values were 0.35, 2.90, and 3.91 mm for 8A-, 8Bc-, and 8C-processed samples, respectively. Decrease in the biaxial stretch formability was attributed to the limited strain-hardening capacity of the UFG microstructure. Also, the grain morphology of the UFG microstructure was found to be very influential on stretch formability. Heavily elongated grain morphology in the 8A-processed microstructure resulted in the lowest formability due to the increased cracking tendency through elongated grain boundaries. However, the UFG microstructures with equiaxed grains obtained after 8C and 8Bc ECAE resulted in better formability compared to 8A. The UFG microstructure reduced the roughness (orange peel effect) of the free surface of the biaxial stretched samples by decreasing the non-uniform grain flow leading to the so-called orange peel effect. It should be noted that the strength and ductility values gained from uniaxial tensile tests are not comparable directly to the EI and F _EI values determined from the Erichsen tests. Finally, it is important to emphasize that the UFG microstructure produced by a suitable strain path leading to equiaxed grains below 1 μm could be highly deformed even under multiaxial stress conditions.