Viscosity measurements of a gas-liquid two-phase fluid (foam) with fine bubbles were conducted using a sphere pull-up method and the flow behavior in dam-break experiments was evaluated. The following results were obtained.

(1) Using known silicone oil, external forces were measured to determine the conversion constants under various sphere diameters and descent speeds. Subsequently, the viscosity of the foam was measured similarly. The results indicated that the foam exhibited shear-thinning behavior and could be classified as a pseudo-plastic fluid.

(2) The viscosity of the foam showed little variation between gas volume fractions of 0.4 and 0.65 but increased significantly near 0.8. This trend was consistent with the results obtained by Hatano et al. using a rotational viscometer.

(3) In the dam-break experiments, the traveling distance of the foam was proportional to time for gas volume fractions between 0.65 and 0.85, while at 0.95, the initial flow velocity was slow and increased gradually.

(4) Using the relationship between viscosity and shear rate of the foam measured by the sphere pull-up method, 3-D numerical fluid flow calculations were performed under dam-break conditions. Since the calculated time for traveling to the bottom was shorter than that of the experiment, an inverse analysis was performed to obtain a relationship between viscosity and shear rate that was compatible with the experiment. As a result, it was found that the viscosity at high shear rate was underestimated by the sphere pull-up method.

In this paper, the oxygen lance of a steel plant is taken as the research object. The jet behavior under the influence of two factors of oxygen nozzle deflection angle and oxygen flow is studied by combining numerical simulation with industrial experiment. The results show that the coalescence behavior of the oxygen jet is obvious, and the oxygen jet basically completes the coalescence at about 0.9 m at the outlet of the oxygen lance. When the nozzle deflection angle of oxygen lance is 12°, the surface of liquid steel fluctuates greatly. When the oxygen flow is increased, the impact crater area of scheme 3 is effectively expanded and the stability is increased. Under the three experimental schemes, the maximum exposed area of liquid steel are 7.18 m², 7.08 m² and 7.42 m² respectively. The reflection jet angle (θ) changes the most at low position, from 68.0° to 54.8°, 67.2° to 48.9°, 67.6° to 59.8°, respectively. When the oxygen lance position exceeds 1750 mm, the θ changes are less than 50°, the slag diffusion area is the largest, and the liquid steel reaction is dynamically stable. The industrial test shows that the composition of liquid steel and slag fluctuates greatly when 12° oxygen lance is used for blowing. When the nozzle deflection angle of oxygen lance increases to 13° and the oxygen flow increases to 42000 Nm³/h, the average oxygen content of liquid steel can be controlled to 573.28 ppm, the average total iron content in the slag can be reduced to 16.35%.

This paper presents a numerical investigation on the steel/slag interface movement behaviors in a two-strand slab tundish under varying gas curtain conditions of the flow, mass and heat transfer and its effect on the inclusion removal. The results show that the mathematical model accurately simulates turbulence and multiphase flow behaviors in the tundish, which were validated by the water model experiments. It has been found that the gas curtain can effectively enhance molten steel residence time, reduces dead volume, and facilitates removal of small particle inclusions. The metallurgical effect of the tundish initially improves with increasing blowing rate of the gas curtain, reaching its peak at 0.4 m³/h, beyond which it diminishes. The relationship between the slag eye area and blowing rate in the tundish is described by the equation y=−865.7x²+3002.8x−593.2 (0.25≤x≤0.55 m³/h). As the liquid level of tundish decreases, bubble rise paths become more vertical and narrower, driving reflux of liquid steel towards slag layer, thereby reducing steel/slag interface fluctuations and slag eye area. As the tundish liquid level decreases from 892 mm to 392 mm, the fluctuation height of the steel/slag interface reduces from 28.83 mm to 25 mm, and the molten steel exposure area from 394.5 cm² to 0.

Accurate prediction of oxygen supply in BOF steelmaking is essential for precise endpoint control, energy efficiency, and product quality. However, the high dimensionality and strong feature coupling of industrial data pose significant challenges for effective feature selection. This study proposes a comprehensive evaluation framework that integrates four filter-based methods: Pearson correlation coefficient (PCC), Spearman rank correlation coefficient (SCC), mutual information (MI), and maximal information coefficient (MIC), with five widely used regression models: elastic net (EN), support vector regression (SVR), extreme gradient boosting (XGBoost), deep neural networks (DNN), and k-nearest neighbors (KNN). The framework evaluates prediction accuracy, model sensitivity, and feature importance. Results show that MIC consistently outperformed the other methods, achieving the lowest average RMSE (197.4 m³) and highest R² (0.649), particularly improving the robustness of models sensitive to input features. In contrast, MI resulted in significantly higher errors across all models, with SVR reaching an RMSE of 231.1 m³. Furthermore, the study introduces a hybrid PI-SHAP interpretability approach to construct feature sets that are both predictive and mechanistically meaningful, further reducing DNN prediction error by 2%. The derived feature importance rankings align closely with metallurgical principles, highlighting the dual benefits of interpretable feature selection for accuracy enhancement and domain insight. This work offers a practical framework for feature selection in complex industrial modeling tasks within the steelmaking industry.

Titanium nitride (TiN) is detrimental to the mechanical properties of steel products and should be properly controlled. For this purpose, the precipitation and evolution of nitride inclusions during the unidirectional solidification of Fe-18Cr-2Mo alloy simulating stainless steel SUS444 were studied using thermodynamic calculations and experimental methods. The relationship between inclusion evolution and solid fraction (f_s), solidification rate, and especially dendritic microstructure was analyzed. (Ti,Nb)N was revealed to be the major inclusion type in all samples. Inclusion number density increased and peaked at intermediate f_s and then decreased towards unity. Cooling rate had limited influence on number density and size variation in the range between 1.25 and 20 K/min. Nitride average composition was found to evolve from Ti_0.75Nb_0.25N to TiN throughout the solidification process.

Following the global consensus on responding to climate change, reducing coal usage has become a critical task given that it is primary contributor to greenhouse gas (GHG) emissions in the steel industry. Given this context, the development of graphics processing units has led to an increasing research interest in applying various deep-learning technologies to enhance process efficiency in steel manufacturing operations. To this end, this study proposes an automatic control logic for supplying coal to rollers during briquette production. The proposed system employs convolutional neural network-long short-term memory for time-series prediction and you look only once for object detection to stabilize coal briquette quality in the FINEX process, replacing the existing manually controlled task in the hot metal production process. Actual process applications were verified by using these models to detect roller currents used as key control factors and briquettes produced below the roller and establish a correlation with compressive strength, which is the main quality indicator of briquettes. Based on these models, an automatic control logic was constructed and tested in actual processes, and the obtained results confirmed a quality improvement effect of ~30%. The results of this study are expected to contribute to improving coal usage efficiency and reducing GHG emissions in the FINEX process.

Load hold at peak stress can drastically reduce the fatigue life of Ti-6Al-4V alloys, a phenomenon known as dwell-fatigue life debit. Unlike normal fatigue, dwell-fatigue exhibits a characteristic three-stage strain accumulation curve: an initial rapid strain increase (primary), followed by a steady strain-increase (secondary), and finally an accelerated stage leading to failure (tertiary). In the current study, flat specimens made out of three distinct microstructures of Ti-6Al-4V alloys were subjected to uniaxial fatigue load with 120-second peak stress hold and 0.1 load ratio with real-time acoustic emission (AE) measurement. The microstructures, namely, equiaxed, bimodal, and lamellar, were tailored through different heat treatment procedures. Bayesian statistical analysis of AE data enabled both the detection of stage transition points using Markov chain Monte Carlo (MCMC) simulation and the selection of strain accumulation models through replica MCMC method. The results demonstrated that two change-points detected showed strong correlation with the transitions in the creep-like three-stage curve of strain accumulation. The strain accumulation behavior was successfully predicted using AE parameters. Furthermore, average frequency proved crucial in monitoring the beginning of dwell-fatigue process.

Recently, high-strength steels have been widely used in automobile bodies to improve collision safety and fuel efficiency. However, for high-strength steels with strengths of at least 780 and 1500 MPa, the cross-tension strength (CTS) and tensile-shear strength (TSS) of resistance spot-weld joints decrease, respectively, with increasing steel strength. This result may be attributed to hardening of the nugget or segregation of impurity elements such as P and S. Methods of improving CTS include post-heating, which tempers or diffuses solidification segregation elements in the nugget, improving its toughness, while TSS can be improved via nugget hardening. In this study, resistance spot welded joints fabricated from lab-melted 1.5-GPa-class steel were heat-treated to change the state of solidification segregation and prior γ grain size, and the effects of these on joint strength were investigated. TSS and CTS were expected to vary with solidification segregation and prior γ grain size. TSS improved upon increasing Vickers hardness owing to solidification segregation of P and S and under uniform Vickers hardness because of relaxation of solidification segregation. CTS improved upon increasing toughness owing to relaxation of the solidification segregation of P and S and refinement of the prior γ grain size.

In this study, SUS 304 stainless steel specimen was covered with an agar film containing Na₂S₂O₃ as a quasi-biofilm in a 0.1 kmol·m⁻³ NaCl solution, and the relationship between the Na₂S₂O₃ concentration in the film and the pitting potential of the specimen was determined by the potentiodynamic polarization test. In addition, the Na₂S₂O₃ concentration of the film surface in contact with the specimen was estimated from the pitting potential, and its change with the pre-immersion time was examined. As a result, the following findings were obtained. The Na₂S₂O₃ concentration in the film matched that in the Na₂S₂O₃ solution used in preparing the film from the powder. When the specimens with films containing 0.1 kmol·m⁻³ or more Na₂S₂O₃ were immersed in the NaCl solution, the pitting potential once decreased, then increased, and finally showed a steady value with increasing the immersion time. The Na₂S₂O₃ concentration in the film estimated from the pitting potential decreased rapidly, then slowly, and rapidly again with the immersion time. When a constant potential was applied for a specified period to the specimen covered with a film containing 1.0 kmol·m⁻³ Na₂S₂O₃, then pitting potential was obtained. The relationship between the Na₂S₂O₃ concentration in the film and the potential holding time was similar the relationship between the concentration and the immersion time.

Understanding the corrosion behavior of steel under realistic environmental conditions is essential for ensuring the durability of buried infrastructures. In this study, the corrosion behavior of carbon steel was investigated during a soil drying process under varying moisture content conditions. Electrochemical impedance spectroscopy combined with time-resolved analysis was used to monitor the temporal change in the corrosion rate, and the results were compared with the oxygen reduction behavior on Pt electrodes and the moisture dynamics evaluated using glassy carbon electrodes. The corrosion rate of carbon steel increased during drying, driven by enhanced oxygen transport through gas-phase pathways formed in the soil, which promoted the cathodic oxygen reduction reaction (ORR). However, the corrosion current remained substantially lower than the ORR-limited current on Pt electrodes. This was probably because of the suppression of anodic dissolution caused by the reduction in the liquid-phase volume near the steel surface, even as cathodic reactions became dominant. As drying further progressed, the NaCl concentration increased and the oxygen solubility decreased, leading to suppression of the ORR. In addition, the continued loss of the electrolyte phase further limited anodic processes, eventually resulting in the complete cessation of corrosion. These findings provide new insights into corrosion mechanisms under non-stationary soil moisture conditions, and they will contribute to more realistic assessment of the long-term performance of buried steel structures.

The third-generation advanced high-strength steels exhibit increased strength and formability due to their chemical composition and precisely controlled thermomechanical processing. The chemical composition also affects the properties of the semi-finished steel. Reliable production of these steels requires that the effects of chemical composition on microstructure development be understood during all steps of processing. This study focuses on the as-cast microstructure by characterizing laboratory-cast ingots containing 0.2 mass% C, with varying Mn, Si and Al concentrations. Isothermal heat treatments were also conducted to verify Thermo-Calc predictions of austenitization behavior. The microstructure constituents were compared against Thermo-Calc diffusion simulations. Results show that increasing the Al and Si concentrations increases the A₃ transformation temperature and the amount of ferrite in the as-cast state. Microstructures at room temperature are mainly bainite and ferrite. Steels containing C, Mn, and Si have proeutectoid ferrite and those with C, Mn, Si, and Al alloy have primary ferrite. A steel alloyed with only Al has a mixture of primary and proeutectoid ferrite where proeutectoid ferrite regrows from the primary dendrite remnants.

Creep deformation and precipitation behavior of 9Cr-1Mo-V-Nb steel with excess nitrogen introduced by solution nitriding were investigated. Precipitation of Cr₂N phase was confirmed in addition to M₂₃C₆ and MX phases in the tempered microstructure. The creep strength of the steel was significantly reduced by solution nitriding, while the creep rupture elongation was increased. To characterize the complex precipitation behavior of the nitrogen-added steel, a machine learning-based clustering method of the multidimensional scatter diagram of the X-ray intensity of the alloying elements in each pixel of a STEM-EDS map was developed. Reduced number density of precipitates and enhanced coarsening kinetics of both Cr₂N and MX were proposed as the mechanism of weakening caused by excess nitrogen.

New fracture process model of cleavage fracture initiated from a micro-crack, which is nucleated at hard phase in steels, was proposed. In addition, the equation to calculate the stress required for micro-crack propagation into the ferrite matrix was developed based on the Brechet-Louchet model. This equation can reproduce not only ferrite size dependence of cleavage fracture stress but both of test temperature dependence and strain rate dependence of fracture stress. Furthermore, in exchanging surface energy for grain boundary cohesive energy in the equation, grain boundary fracture stress can be also estimated.

The hydrogen embrittlement of steel can be mitigated by reducing the amount of solid-solution hydrogen that enters from the external environment. In this study, first-principles calculations were performed for various additive elements to identify which elements X effectively reduce hydrogen penetration from the material surface. We used substitutional (Si, Mn, P, S, Cu, Ni, Cr, Ti, Ge, Co, V, Zr, Nb, Mo, Sn, Sb, Ta, and W) and interstitial (C, B, N, and O) solid-solute atoms as additive elements to estimate the potential energy barrier for hydrogen penetration in Fe–X–H systems. Results showed that many additive elements are more effective in inhibiting hydrogen penetration than an Fe–H system and that the penetration pathways tend to be similar in each family of the periodic table. Moreover, it is suggested that the hydrogen penetration suppression effect of the additive elements is related to the first ionization energy in the case of the substitutional solid-solute atoms and to the change in the lattice constant in the case of the interstitial solid-solute atoms.

Changes in the initiation behavior of delayed fracture cracks have been investigated based on quantitative cross-sectional observation near the fracture surface in the previously reported sustained tensile-loading test during hydrogen charging of ultra-high strength steel sheets containing a small amount of retained austenite. The morphology of the delayed fracture surface depends on the applied stress and pre-deformation. With increasing applied stress, the number of delayed fracture cracks decreased in the martensite in the cross section immediately below the fracture surface, whereas the number at the prior austenite grain boundary tends to increase. Upon pre-deformation sufficient to transform from retained austenite to martensite, the number of cracks in the martensite increased at each applied stress in spite of the short time to delayed fracture. Even without fracture at low applied stress (200 MPa), short cracks exist in the martensite, but no long cracks are observed. The long cracks in the martensite increases with increasing applied stress or pre-deformation. The long cracks presumably result from connection of short cracks in the martensite. The long cracks probably enhance transgranular fracture. The results of the present study indicate that applied stress or pre-deformation affects delayed fracture behavior by changing the crack initiation and propagation behavior. With increasing applied stress in the elastic deformation region, the crack suppression effect of bainitic ferrite is likely to weaken, resulting in more transgranular fracture. In addition, the preferential crack initiation in deformation-induced martensite formed by plastic deformation probably shortens the time to delayed fracture.

DOI: https://doi.org/10.2355/isijinternational.ISIJINT-2024-368

The author has found that several values listed in Table 3 were inaccurate.

Only the values to be corrected are shown below.

Table 3. Evaluation of the interaction parameter of two solutes in molten iron by the Eq. (33) for nonlinear relation between log f_i and mass%.