In hot rolling process of low carbon steel sheets, oxide scale formed on the sheets may result in surface defects on the rolled products. The major phase of the scale is wustite FeO, which shows sufficient plasticity to follow the sheet deformation only at elevated temperature. However, thick scale is cracked, fragmented and indented to the sheets by the rolling even at elevated temperature because scale surface is instantly cooled by cold rolls to brittle temperature. Therefore, thick scale should be removed by descalers just before the rolling. It is reported that manganese decreases the eutectoid temperature between ductile wustite and brittle magnetite. Therefore, manganese may have a positive effect to widen wustite window to lower temperature and to suppress surface defects. In this study, 0mass% and 2mass% manganese bearing steel sheets with controlled scale on surface were hot rolled in a laboratory. The sheets were reduced 30% in thickness by unlubricated rolling at temperature between 1173 K and 1373 K. Scanning electron microscopy on longitudinal section showed that manganese decreases crack depth and increases spacing between scales indented to the steel. It is concluded manganese makes the scale on steel more ductile and suppresses surface defects.
In this study, we performed scanning transmission X-ray microscopy with a spatial resolution of approximately 50 nm to investigate the two-dimensional mapping of the chemical states of carbon in Fe–C alloy. The lamellar texture (pearlite) consisting of ferrite (α-Fe) and θ-Fe3C with an interval of approximately 100 nm was identified by absorption from the carbon 1s→2p excitation in the X-ray absorption image. It was clearly observed that there exist more than two types of chemical states of carbon in θ-Fe3C depending on the microtextures. The differences in chemical states were found between grained θ-Fe3C and lamellar θ-Fe3C in pearlite, which might have originated from the texture and morphology of the θ-Fe3C. To consider the origins of the differences, we performed first-principles calculations by assuming the distortion and crystal anisotropy of the unit cell of the θ-Fe3C structure. The results suggest that the anisotropy of the crystal structure of θ-Fe3C and the lattice strain within lamellar θ-Fe3C fail to explain the differences, and therefore, other factors should be considered.
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.
The spray cooling of moving hot solids is widely performed in the steel industry. Understanding flow and heat transfer when droplets impinge on moving hot solids is important. By simultaneous visualization with flash photography and temperature measurement using thermography, the flow and heat transfer of a droplet train obliquely impinging on a moving solid at high temperatures was experimentally investigated. A rectangular test piece (SUS303) was heated to 500 °C at a moving velocity of 0.25–1.5 m/s. The test liquid was water at approximately 25 °C. The pre-impact droplet diameter, impact velocity, and inter-spacing between two successive droplets were 0.69 mm, 2.2 m/s, and 2.23 mm, respectively. The tilt and torsional angles were 50° and -30–60°, respectively. No coalescence of droplets was observed; the droplets deformed independently on the moving solid, even though the torsional angle generated a velocity component along the width of the solid. The surface temperature of solid after droplet impingements depended on the experimental conditions. Wavy temperature profile was obtained when the moving distance of solid was large during two successive collisions. The temperature changed continuously for the small distances. In this regard, a simple model considering droplet movement, collisional deformation behavior, and solid migration can explain this phenomenon by the overlap of the cooling regions of the droplets. Furthermore, experimental and numerical analyses show that the heat removal rate of individual droplets is constant at approximately 12.5 MW/m2 and depends on the total contact time when multiple droplets collide.