An increase in the melt (hot metal and slag) level in the lower part of a blast furnace can trigger serious troubles, including peripheral flow enhancement and, in extreme cases, melt backflow into tuyères. Conventional mass-balance management ignores time-varying voidage of the hearth coke packed bed, while sensor-based methods often suffer from installation constraints and noise. This study develops an online model that preserves continuity with the familiar mass-balance framework while explicitly incorporating the coke-bed voidage. The lower furnace is treated as a cylindrical packed bed; the melt volume is calculated from residual masses (production minus discharge) of metal and slag and converted to liquid level using cross-sectional area and voidage. Voidage at tap end is estimated from the balance between pressure loss of slag flow through the packed bed and hydrostatic pressure associated with the inclined slag surface near the taphole. Slag viscosity and density, effective coke particle size at the lower part, discharge flow at tap end, and inclination angle are derived from operational data and auxiliary models. Tap-wise discrete estimates are time-averaged and interpolated to yield continuous trajectories. Drift correction for cumulative balances and a moving base-tap window are introduced to improve robustness in practice.

Research on the machinability of steel materials, particularly ordinary carbon steels, has a long history, and numerous experimental data are available in textbooks and reference work. However, the majority of these studies focus on the machining characteristics of untreated base metals. In practical applications, many steel components undergo heat treatment to attain the desired mechanical properties, resulting in the formation of oxide layer (mill scale) on their surfaces. Consequently, mill scale removal becomes an essential consideration during the cutting process. Previous reports have indicated that the presence of mill scale significantly reduces tool life compared in case of only base material cutting under otherwise identical cutting conditions and leads to considerable variability in machinability. In this study, cutting experiments were conducted on high carbon steel with mill scale to experimentally evaluate the causes of instability during mill scale-affected cutting and the progression of tool wear. The results demonstrated that mill scale pieces become embedded between the tool flank and the workpiece material during cutting. Furthermore, the mill scale pieces entrapment induces an abrasive action that contributes to an increase in flank wear width.

Zn–11%Al–3%Mg–0.2%Si coated steel sheets exhibit high corrosion resistance. However, surface appearance issues, particularly a mottled pattern consisting of glossy and white areas, remain critical. In this study, the formation mechanism of this surface appearance was investigated by examining the relationship between surface roughness and coating microstructure. Microstructural characterization using SEM, EBSD, XRD, and GDS revealed that the contrast between the two areas originates from differences in smooth eutectic coverage over the rough preformed phases (primary Al and bulk MgZn₂). Insufficient eutectic coverage, which exposes the preformed phases, increases surface roughness in the white area, whereas sufficient eutectic coverage results in a smooth surface in the glossy area. Eutectic Zn phase exhibited distinct crystallographic orientations in the two areas, leading to Zn growing in different directions between the two areas: along the coating-thickness direction in the glossy area and along the coating-surface direction in the white area. No remarkable differences were detected in the composition of the two areas. Further crystallographic analyses indicated that bulk MgZn₂, eutectic MgZn₂, and eutectic Zn formed sequentially in this order during solidification. These findings indicate that the growth behavior of bulk MgZn₂ influences the eutectic coverage via the growth direction of eutectic Zn, and consequently the surface appearance. The results provide a fundamental basis for understanding surface appearance in Zn–Al–Mg–Si coatings.

The fatigue crack propagation behavior of newly developed SM490-grade steel plates under high stress ratios with a single overload was investigated in comparison with a conventional steel of the same grade. Under constant-amplitude loading, the developed steel exhibited approximately twice the crack propagation life at a stress ratio of R = 0.1, whereas the improvement was marginal at higher stress ratios of R = 0.3 and 0.5. In contrast, under single overload conditions, the developed steel showed a significant improvement in crack propagation life by up to 1.5 times even at R = 0.3. The crack growth rate after overload was lower in the developed steel, indicating a reduction in the effective crack driving force, as evaluated using the RPG (Re-tensile Plastic zone's Generating) load. Elasto-plastic FEM analysis was conducted to investigate the crack opening displacement behind the crack tip and the wedge effect associated with the overload-induced plastic zone. The results indicate that the wedge effect temporarily promotes crack opening immediately after overload, followed by enhanced crack closure during subsequent crack growth within the plastic zone. The mechanism of crack propagation life improvement considering combined effects of plasticity-induced crack closure and roughness-induced crack closure activated by the overload was discussed.

This study investigates the effects of copper (Cu) addition on the bending fatigue strength of lath martensite, focusing on dislocation structures, Cu precipitates, and crack initiation sites. Steel specimens with varying Cu contents of 0, 1, 2, and 4 mass% were prepared and tempered at 523 K or 723 K. Fatigue tests revealed that Cu addition significantly enhances fatigue strength, particularly at 723 K, where the 4Cu steel (4 mass% Cu) exhibited superior performance compared to the 0Cu steel (< 0.01 mass% Cu). Microstructural analysis by SEM and TEM showed that, after fatigue testing, the Cu precipitates at grain boundaries had undergone plastic deformation, indicating local stress relief at the grain boundaries. This stress relaxation effectively suppressed crack initiation at the grain boundaries and shifted the sites of crack initiation to the grain interior. Intragranular Cu precipitates were found to pin dislocations, delaying crack initiation within grains. It is also speculated that retardation of dislocation alignment during tempering, which is caused by Cu in solid solution and as fine precipitates, may further delay crack initiation, although direct confirmation of this effect is still required. At 523 K, Cu addition did not significantly improve fatigue performance, presumably due to the absence of Cu precipitates at grain boundaries. These findings suggest a dual mechanism of grain boundary strengthening by plastically deformable Cu precipitates and matrix pinning by intragranular Cu precipitates, which would explain the observed improvement in the crack initiation life and fatigue limit of Cu-added lath martensite.