Technological advances in the field of structural materials have seen the development of advanced high steel with excellent properties for various applications such a lightweight vehicle. Alternatively, with the increasing global concern about environmental control and strict regulations, more efforts have been devoted to enhancing the properties of these materials. To this end, researchers have been looking for alternatives of forming steel sheets onto automotive parts and have identified stretch-flangeability as a promising solution. Recent research has shown that understanding the relationship between the microstructure of steels and macroscale characteristics will be a key contribution towards developing advanced steels with desired properties.
In a recent research work published in the research journal, Materials Science and Engineering, Pohang University of Science and Technology scientists led by Professor Hyoung Seop Kim investigated the effect of grain size of stretch-flangeability of twinning induced plasticity steels. In particular, among the factors affecting microstructural features such as morphology and distribution, the grain size is generally easy to control and much effective. Altogether, their aim was to clarify the Hall-Petch relationship between the stretch-frangibility and grain size.
In brief, the research team commenced their experimental works by a detailed cross-examination of the effect of grain size on the stretch-flangeability of the steels. First, the experiment comprised of plastic steel materials based on the single-phase twinning which was chosen to prevent the effects of the microstructure. Secondly, two samples of twinning induced plasticity steels were used. Their grain sizes were either increased by annealing at 1100°C or reduced by subjecting to high-pressure torsion followed by annealing at 650°C. Furthermore, an electron backscatter diffraction was utilized to analyze the microstructural features.
The authors observed that the stretch-flangeability of a steel material was improved by controlling the microstructural features. Specifically, it was possible to obtain coarse grain twinning induced plasticity through heat treatment of the materials at 1100°C. In addition, hole expansion ratio analysis revealed that the stretch-flangeability was majorly dominated by fracture toughness. This was attributed to the fact that the fracture toughness increased by grain refinement.
In summary, the study by Pohang University of Science and Technology scientists successfully presented an approach to analyzing the effects of grain size on stretch-flangeability. Generally, the Hall-Petch relationship between the stretch-frangibility and grain size were properly understood. For instance, both of the hole-expansion ratio and the fracture toughness followed the Hall-Petch correlation. Altogether, the proposed framework offers a promising solution in developing advanced high strength steels. Furthermore, it provides a reference for future studies with the main interest being improving on the stretch-flangeability of high strength steels. This includes redesigning the microstructural features to enhance their fracture toughness.
Yoon, J., Lee, H., Jung, J., & Kim, H.S. (2018). Effect of grain size on stretch-flangeability of twinning-induced plasticity steels. Materials Science and Engineering: A, 735, 295-301.Go To Materials Science and Engineering