In recent years, it is necessary to reduce CO₂ emissions from the blast furnace, due to prevent global warning. Therefore, low coke ratio operation is required to reduce coke consumption in ironmaking process.

On the other hands, because coke works as a spacer in the blast furnace, low coke rate operation causes various problems, including deterioration of furnace permeability and delay of reduction reaction. Iron ore with low reduction ratio also expects to cause the deterioration of permeability, this is because FeO-containing slag promotes softening and melting of iron ore. Therefore, it is important to quantify the effect of ore melting behavior on permeability.

In this study, the effect of ore contraction behavior on permeability of cohesive zone was quantified by using the cohesive zone simulator, focusing on the difference in contraction behavior due to the difference in reduction ratio. In addition, the effect of ore contraction ratio on the permeability of blast furnace was estimated by the numerical simulation model.

In can industry, film-laminated materials are being commercialized as environmentally friendly alternatives to painted ones. Polyethylene terephthalate (PET) film is widely used as a laminating material for steel sheets in beverage and food cans because of its excellent formability, corrosion resistance, and adhesion properties. In food cans, steam retort treatment is used for sterilization. However, the discoloration phenomenon, known as retort blushing, resulting from the exposure of the can's outer surface to water vapor, has emerged as a significant issue.To mitigate retort blushing, it has been found effective to use a film that blends polybutylene terephthalate (PBT), which crystallizes rapidly, with PET. This method increases the crystalline content in the film during steam retort processing, thereby enhancing its barrier properties against water vapor. Nevertheless, the effect of the crystalline state produced by this method on retort blushing remains unclear. Therefore, we investigated the influence of the PET film's crystalline state on retort blushing by subjecting the PET film laminated steel sheets to heat treatment, which were manufactured by changing the lamination temperature, and analyzing the crystalline structure using Raman spectroscopy. Our study revealed two key findings: (1) Simply changing the lamination conditions to control the oriented crystalline content does not completely prevent retort blushing, but applying heat treatment after lamination can fully suppress it. (2) Heat treatment induces isotropic crystal growth within the PET film near the interface with the steel sheet, which is believed to prevent the vaporization and expansion of moisture during the retort process, thereby preventing void formation and effectively mitigating retort blushing.

Electrical resistivity measurement is a powerful method for evaluating the solute carbon concentration (C_sol) in BCC iron. In this study, the effect of carbide precipitation on the accuracy of C_sol quantification was examined by applying a model that predicts the electrical resistivity of composite microstructures containing dilute-dispersed spherical carbides. For Fe–C alloys with spheroidized cementite dispersed in ferrite, C_sol in ferrite could be estimated with an uncertainty of only several tens of ppm. In Fe–0.2Mn–C alloys, high accuracy comparable to that of Fe–C alloys was achieved when Mn enrichment in ferrite accompanying cementite precipitation was properly taken into account. For Fe–2Mn–0.5Si–C martensitic alloys, Mn partitioning, dislocation annihilation, and grain coarsening do not occur during tempering at low-temperature below 573 K. Therefore, changes in electrical resistivity during tempering directly reflected the decrease in C_sol, enabling accurate evaluation when the contribution of carbide precipitation was removed using the spherical dilute-dispersion model. In contrast, high-temperature tempering at 873 K induced pronounced Mn partitioning, and even slight uncertainties in the measured Mn concentration in ferrite led to errors of several hundred ppm in the estimated C_sol. Although errors associated with volume fraction and electrical resistivity of carbides also affected the evaluation, their impact was much smaller than that of Mn partitioning. Consequently, except for the high-temperature-tempered Fe–2Mn–0.5Si–C alloy, the final accuracy of C_sol measurement can be maintained within several tens of ppm when appropriate corrections are applied.

To clarify the mechanisms of plastic deformation and ductile fracture, it is crucial to elucidate the defect-formation processes occurring during plastic deformation. In this study, we performed in situ transmission electron microscopy (TEM) observations of the formation of small prismatic dislocation loops via the double cross-slip of screw dislocations in high-purity iron during tensile deformation at elevated temperatures. Conventionally, the type of a dislocation loop (vacancy or interstitial) has been determined by exploiting the dependence of its image contrast on diffraction conditions, as well as the diffuse scattering patterns generated by the loop in electron diffraction. However, such “static” identification methods are inapplicable to small dislocation loops (approx. <30 nm in diameter) whose image contrast becomes indistinct due to the comparatively large thickness of specimens required for dynamic in situ observations. To overcome these limitations, we propose a new “dynamic” identification method that utilizes the dynamic behavior of a dislocation forming a loop, rather than the intrinsic image contrast or diffuse scattering patterns of the loop itself. Applying this method, we reveal that the dislocation loops formed during tensile deformation at elevated temperatures are of the vacancy type. This finding suggests that loop formation via double cross-slip at elevated temperatures is one of the vacancy-formation mechanisms during plastic deformation, providing important insight into the ductile fracture processes of iron-based materials.

This study aims to clarify the influence of hardness distribution on the pitting fatigue strength of nitrided steels. Gas nitriding with controlled nitriding potential was applied to SCM440H and 31CrMoV9 steels, and roller pitting tests were conducted. The nitrided specimens exhibited superior pitting fatigue strength compared with carburized specimens at 10⁷ cycles, corresponding to the fatigue limit. In contrast, under high contact pressure, early pitting occurred within 10⁵ cycles, indicating reduced fatigue performance in the finite-life. The pitting fatigue strength of nitrided steels strongly depended on the case-hardened depth, and specimens with deeper hardened layers demonstrated higher durability. Under high contact pressure, most of the plastic deformation occurred in the early stage of testing, whereas work hardening developed progressively with increasing number of cycles. In nitrided steels, such plastic deformation induces tensile stress at the contact surface, and in combination with frictional stress, promotes the formation of vertical cracks originating from the compound layer. Therefore, increasing the case-hardened depth is effective for suppressing early pitting by reducing plastic deformation. For nitride steels, pre-running to induce work hardening also mitigated early pitting without reliance on alloying elements or prolonged nitriding durations. These findings indicate that, even when flaking initiates from the surface, the hardened layer depth plays a critical role in the rolling–sliding contact fatigue performance of nitrided steels. Consequently, for components such as gears that operate under rolling–sliding contact, it is essential to consider the influence of internal shear stress in strength design to achieve superior pitting fatigue strength.