The Change of Crystalline State of Coarse Grained Mild Steel during Static Tension

Abstract

The behaviour of the individual grain and that of the inside grain of the specimen deformed by tension were investigated by the back reflection divergent X-ray beam method, to confirm their correlation with the behaviour of the specimen as a whole.Coarse-grained polycrystalline plate specimens of low carbon steel were prepared by the strainannealing method. The grain sizes of the specimens were about 1.7, 7 and 5mm for sample A, B and C respectively.The horizontal type capillary X-ray tube apparatus (Rigaku Denki make) was used to obtain Fe-Kα radiation.First, to examine the structural change of crystals, pseudo-Kossel patterns were taken with the samples A and B at several stages of strain after the load were removed. Stresses and strains of specimens were also measured simultaneously to provide the nominal stress-strain diagram.Then, the relative lattice rotation was examined in an unstrained and strained state at two different positions in a grain with sample C.It was observed that the pseudo-Kossel patterns became irregular with deformation, showing local line broadening, kinking and shifting, when the specimens were stretched over the proportional limit, and that the diffraction lines broadened as a whole in the state of greater deformation.In the range between the proportional limit and the stress level at which linear hardening in the load-elongation diagram started, the kinking, shifting and broadening of the diffraction lines increased.When the specimen was deformed by tension, the relative lattice rotation was observed at two different positions in a grain, in the early stage of plastic deformation.In the stress range corresponding to the linear hardening, most diffraction lines of the obtained pseudo-Kossel patterns showed kinking, shifting and broadening, irrespective of the kinds of latticeplane.It was seen from these results that in the load-elongation diagram, the curved range between the proportional limit and the stress level at which linear hardening started was caused by the fact that the load elastically supported by individual grain or different part of a grain was different from each other.