A Fe-0.15C-5Mn-0.5Si-0.05Nb medium Mn steel annealed at 660℃ and 685℃ both exhibited inhomogeneous deformation with Lüders deformation and extremely high work hardening rates, but with different Lüders strain and work hardening behavior. In-situ neutron diffraction measurements during tensile test were performed to investigate changes in the phase stresses and in the contributed stresses to the strength of the constituent phases, and crystal orientation of austenite. The role of each constituent phase in the deformation and the effect of crystallographic orientation on austenite stability were discussed. Deformation induced martensite showed excellent phase stress and contributed to the strength approximately 1000 MPa, which is close to macroscopic tensile strength. Although austenite contributed less to the strength, but during Lüders deformation and work hardening stage, it continuously transformed to martensite as the deformation progressed, suggesting that it mainly contributed to the ductility of the steels through a transformation induced plasticity effect. Austenite transformed to martensite in all crystallographic orientations during Lüders deformation, but there was a tendency for more <311> austenite grains parallel to the tensile direction to remain.
Effects of material properties on the teeth accuracy of spline shafts processed by cold form rolling with rack dies have been experimentally investigated. The materials used were medium-carbon steel bars processed by four kinds of heat treatments. Three of them were quenched from 850℃ followed by tempering at 570℃, 630℃,700℃ respectively, and the fourth was annealed at 600℃. The remarks obtained by this study are as follows. (1) Teeth accuracy is improved with increasing tempering temperature, while the steel annealed at 600℃ has almost the same teeth accuracy as the steel tempered at 700℃. (2) Teeth accuracy shows high correlation with cumulative pitch deviation from the theoretical value. (3) The low teeth accuracy is due to that of the follower side teeth which shows lower formability than the driven side. (4) In order to find a material property which affects teeth accuracy, material flow inside the necking portion in the tensile test was focused from the fact that material flows of the follower side teeth are similar with that of the necking portion. (5) Cup and cone type fracture was observed in the tensile test. The value NPC defined as the width of tapered portion divided by outer diameter of tapered portion is proposed as an index of the formability in cold form rolling, and the reason why the teeth accuracy is improved with increasing this index is investigated on the basis of experimental results.
The local plasticity and associated microstructure evolution in Fe-5Mn-0.1C medium-Mn steel (wt.%) were investigated in this study. Specifically, the micro-deformation mechanism during Lüders banding was characterized based on multi-scale electron backscatter diffraction measurements and electron channeling contrast imaging. Similar to other medium-Mn steels, the Fe-5Mn-0.1C steel showed discontinuous macroscopic deformation, preferential plastic deformation in austenite, and deformation-induced martensitic transformation during Lüders deformation. Hexagonal close-packed martensite was also observed as an intermediate phase. Furthermore, an in-situ neutron diffraction experiment revealed that the pre-existing body-centered cubic phase, which was mainly ferrite, was a minor deformation path, although ferrite was the major constituent phase.
The color change of opal photonic crystal films (OPCFs) due to deformation was quantitatively evaluated using digital image correlation (DIC) analysis. OPCFs were pasted on specimens of three different gauge geometries, and random patterns were formed on the opposite side of each specimen for DIC analysis. To assess the applicability of using OPCFs-based strain characterization for analyzing steel structural components and associated metallurgical analyses, smooth, width-gradient, and holed specimens were prepared in this study. As deformation increased in the smooth specimen, the color of the OPCFs changed significantly. The color change in the OPCFs could be quantitatively converted into strain values through Hue value analysis. Heterogeneous strain distributions could also be quantitatively analyzed using OPCFs-based analysis at the submillimeter or millimeter scale. When the strain gradient is too high, for example, near a stress concentration site such as a hole, local peeling of the OPCFs away from the specimen surface can occur. Consequently, for quantitative characterization, we must take proper care when measuring this upper limit of the “strain gradient” as well as strain, which would depend on the adhesion and surface condition of the specimen.