In Japan, approximately 14 million tons of steelmaking slag were generated in FY2023. Owing to its excellent mechanical properties, steelmaking slag has been primarily used in construction materials, such as roadbeds. However, these applications are predicted to decrease, making it necessary to find new ways to utilize the slag. Steelmaking slag contains many components beneficial for plant growth, such as calcium, silicon, iron, and phosphorus. Technologies are being developed for marine and agricultural applications, including seabed restoration and yield improvement in rice cultivation. Laboratory-scale experiments have confirmed the elution of these components, the propagation of organisms, and the promotion of plant growth. Some of these findings have begun to be implemented in practical applications through on-site testing. At the same time, since steelmaking slag contains significant amounts of essential elements for society, such as manganese and phosphorus, research is being conducted to selectively recover these valuable elements. In particular, various separation methods have been reported for phosphorus recovery, taking advantage of the physical and chemical properties of the mineral phases in steelmaking slag. These methods include flotation separation, magnetic separation, capillary action, redox reactions, solution reactions, and combined processes. This report summarizes the current status of steelmaking slag and the progress of research and development for its utilization.

In hot rolling of steels, both microstructural evolution and work roll/steel interfacial state are critical for high quality products. Unfortunately, they are taking place in forms of black boxes because they cannot be readily detected during productions. Therefore, accurate modelling evolutionary behaviours of microstructures and surface scales during hot rolling and changes of as-rolled mechanical properties has become significantly important. In this paper, typical semi-empirical models developed since the 1970s and data-to-data models by artificial neural networks are briefly reviewed for their advantages and disadvantages. Then, physical metallurgy guided machine learning is discussed for its superiority in logicalities and accuracies. For the newest development, industrial foundation models (IFM) are proposed to integrate different processes in hot rolling and accelerated cooling, by which recrystallizations of austenite grains, strain induced precipitations, mechanical loading, and changes of interfacial friction coefficients during hot rolling can be simultaneously worked out, and dynamic continuous cooling transformation diagrams are instantly generated to account for variations of mechanical properties based on deep learning and heterogeneous data. Finally, typical applications to hot strip/plate lines for high efficiency production and stable control of mechanical properties are described.

Hydrogen-rich blast furnaces require the use of highly reactive coke due to their lower furnace temperature. In addition, a hydrogen-rich blast furnace contains a large amount of water vapor. We investigated the reaction behavior of highly reactive briquetting coke prepared from low-rank coal as the main raw material under a CO₂ + H₂O atmosphere. The reaction behavior and postreaction compressive strength of coke at different H₂O contents were examined. The changes in microcrystalline structure, functional groups, apparent porosity, micron pore structure, nitrogen and carbon dioxide adsorption properties of coke before reaction and after gasification reaction at different H₂O contents were investigated. Compared with traditional coke, briquetting coke not only has high reactivity at different H₂O contents but also high compressive strength after reaction. The reaction of briquetting coke under a CO₂ + H₂O atmosphere was significantly controlled by surface reactions.

The quality of interface bonding between inert maceral derived components (IMDC) and reactive maceral derived components (RMDC) in coke made from a variety of coal blends was quantified based on micro-CT images. A novel micro-CT image processing algorithm was developed to identify the boundary of IMDC and to determine the porosity profiles in the vicinity of IMDC surface. The bonding quality was assessed using excess porosity which is the difference between the peak and averaged porosity around IMDC surface. High excess porosity values indicate poor RMDC-IMDC bonding, while low or even negative excess porosity values indicate good quality bonding. Poor bonding with IMDC only occurs with one type of RMDC from a low- volatile matter high-rank parent coal. Further examination into this RMDC series indicates dependence of the excess porosity on the internal IMDC particle porosity at low IMDC phase density, combined with a lack of fusible material near the surface of the IMDC. The fusible material improves bonding, while low porosity near the IMDC inhibits bonding.

Cutting down CO₂ emissions from blast furnace (BF) ironmaking is a critical area of research. Utilization of hydrogen as a reducing agent has been regarded as one of the most promising solutions to address this challenge. This study examined the impact of using hydrogen injection on the isothermal reduction of commercial iron ore pellets under conditions simulating the center and wall areas of a BF. The effect of distinct reducing conditions, influenced by the radial position within a BF and the amount of hydrogen injected, on the reducibility, swelling, cracking, and porosity of pellets was investigated. A high-temperature furnace with a thermogravimetric analyzer was utilized to simulate the atmospheres of CO–CO₂–H₂–H₂O–N₂ at 700, 900 and 1100°C in 300-minute experiments. The changes in volume and porosity of the pellets were determined based on the results obtained using a gas pycnometer and manual measurement. The phase transformations were studied using microscopy and X-ray diffraction. The results show that the addition of hydrogen had an accelerating effect on the reduction rates excluding the experiment conducted at 700°C under center conditions of a BF, during which the high level of water vapor led to oxidation of the pellet on the surface rather than reduction at the beginning of the wüstite–metallic iron reduction stage. Furthermore, the pellets swelled less in hydrogen-enriched atmospheres. Also, it should be highlighted that the reduction was significantly faster near the center of a BF compared to the areas near the walls.

Accurate prediction of phosphorus content in molten steel at the end of the electric arc furnace (EAF) smelting process is crucial for optimizing smelting efficiency and ensuring product quality. Inaccurate predictions can result in increased resource consumption and compromised product quality. This study proposes a novel FA-MM-TabNet model, which integrates Firefly Algorithm (FA) optimization, metallurgical mechanisms (MM), and the deep learning-based TabNet algorithm to predict endpoint phosphorus content in EAF operations. The model was trained and tested on actual production datasets and demonstrated superior performance compared to competing models, achieving a mean absolute error of 0.0024, a root mean square error of 0.0034, and a hit rate of 90.82% within a ±0.005% error margin. Furthermore, an interpretability analysis using explainable artificial intelligence methods, including model-specific and SHAP analyses, confirmed the model’s alignment with metallurgical principles. This enhances the model’s reliability and applicability in industrial settings, contributing to more efficient steelmaking practices.

The present study proposes a synergistic reduction method for the efficient recovery of metals from BOF slag and copper slag. This process is accomplished while simultaneously stabilizing secondary slag, thereby achieving the valorization of metallurgical solid waste. The synergistic modification between the two slags has been shown to be conducive to the release of simple iron oxides, including FeO and Fe₂O₃, from complicated iron-containing mineral phases. This method is regarded as being advantageous for the deep extraction of iron resources. The impacts of mixed slag basicity, reduction temperature and C/O molar ratio on the metal recovery are explored, as well as the synergistic reduction mechanism. The results show that lower mixed slag basicity and higher reduction temperature can promote the metal particle aggregation, which improves the metal recovery rates. Optimum reduction parameters were utilized to yield a ferroalloy containing 95.85% Fe, 3.91% Cu, 0.12% and 0.12% Mn, with a recovery rate of 89.58% for Fe, 96.22% for Cu and 16.85% for Mn. Furthermore, the stable secondary slag contains 0.86% free CaO and 0.79% free MgO, and the hazardous elements from copper slag have been nearly eliminated, resulting in secondary slag that is the stable and non-toxic, and can be used as construction materials. This method enables the comprehensive utilization and valorization of metallurgical solid wastes.

The dynamic condition of the molten bath in a combined-blowing converter was decided by the operational parameters of the oxygen lance and bottom-blowing nozzle. Based on the movement directions of the kinetic energy sources generated by the top-blowing multi-jets and bottom-blowing bubbles, it could be inferred that these two sources created a kinetic energy cancellation phenomenon at the surface of the molten bath. Thus, the effect of three types of oxygen lance structures on the flow field of the molten bath in a 100 t K-OBM (Klockner Oxygen Blowing Maxhütte) converter has been analyzed by a serious of water experiments and numerical simulations. The results indicated that the neutralization of kinetic energy significantly improved as the top-blowing flow rate increased from 7200 to 9600 Nm³/h, leading to an extended mixing time. Meanwhile, in contrast to the traditional BOF converter, the K-OBM converter exhibited a noticeable reduction in top-blowing intensity and a significant enhancement in bottom-blowing intensity. Consequently, the variation in the average velocity of the molten bath generated by the three types of oxygen lances gradually decreased as the depth of the molten bath increased. Based on the results of industrial application, 2# lance is designated as the optimal oxygen lance for the K-OBM steelmaking process, which achieved the highest dephosphorization rate, the shortest melting time and the lowest T. Fe content, comparing with the original lance and #1 lance.

In rare-earth steel continuous casting, researchers examined the role of cerium oxide (CeO₂) in mold slag crystallization. Using single hot thermocouple technique (SHTT) and Raman spectroscopy, they investigated the CaO–SiO₂–Al₂O₃–Na₂O–CaF₂–(CeO₂) slag system. The CeO₂ was found to reduce structural polymerization, increase substance migration rates, and promote slag crystallization. Applying Johnson-Mehl-Avrami (JMA) and modified JMA models revealed distinct crystallization behaviors. In isothermal conditions, the crystal growth mechanism transitioned from one-dimensional to three-dimensional as temperature increased. Non-isothermal analysis consistently showed three-dimensional crystal growth. The models quantified how CeO₂ content influences crystallization kinetics, demonstrating altered growth patterns. As CeO₂ increased to 3%, the isothermal crystallization activation energy rose from 153.59 to 302.58 kJ/mol, indicating enhanced crystallization drive. Under non-isothermal conditions, with cooling rates of 1 to 20°C/s, the apparent activation energy ranged from −288.44 to −941.97 kJ/mol. The negative values suggest that accelerated cooling and increased CeO₂ concentration reduce crystallization process inhibition.

The formation of secondary inclusions during the solidification process of molten steel is a complex phenomenon triggered by microsegregation. Controlling the dispersion of secondary inclusions in solidified steel is an important issue that significantly affects the properties of steel; however, the distribution of inclusions after solidification does not always coincide with the locations of inclusion formation. Therefore, estimating when, where, and at what supersaturation level the inclusions crystallize in the liquid phase is difficult; therefore, clarifying their formation behavior is desired to control the dispersion of inclusions. In this study, we investigated the formation process of inclusions using a ternary model material of succinonitrile-water-Lumogen Yellow by in-situ observation, where the formation of oversaturated Lumogen Yellow can be regarded as inclusion formation. The frequency of inclusion formation was confirmed to increase markedly when the solution was held at lower temperatures, that is, when a large supersaturation ratio was applied. The results of the formation frequency indicated that the formation of inclusions occurred in the liquid phase according to classical nucleation theory.

To achieve both quality and productivity in continuous casting at a high dimension, a new electromagnetic flow control system was proposed, in which AC and DC magnetic fields are superimposed. Numerical simulations and experiments at an industrial continuous casting machine were carried out.

1) A model for predicting bubble defects and mold flux defects by focusing on the steel flow velocity on the wide face and the turbulent kinetic energy on the top surface of the molten steel was developed. The validity of the model was confirmed by comparing it with the actual defect tendency of a continuous casting machine.

2) Applying an AC magnetic field in the upper stage increases the molten steel flow velocity and suppresses entrapment of bubbles on the wide face, but flux engulfment and entrapment increase, presumably due to the larger random motion of particles caused by the increased turbulent kinetic energy.

3) When the AC and DC magnetic fields are superimposed in the upper stage, both bubble defects and mold flux defects can be reduced by optimizing the steel flow and turbulent kinetic energy. As a result, surface defects on steel sheets can be roughly halved compared to the conventional flow control method using only DC magnetic field.

4) The results of this study suggested that it may be possible to control the flow velocity and turbulent kinetic energy by optimizing the AC and DC magnetic field intensities according to the casting conditions, further reducing surface defects caused by bubbles and mold flux.

In a hot-dip galvanizing production line for high-aluminum zinc-aluminum-magnesium steel strip, the temperature of zinc pot may reach 600°C. The continuous zinc evaporation may cause serious pollution of zinc ash to the steel strip. In this paper, the confinement of zinc vapor by using a wind curtain is explored, in an attempt of preventing the zinc vapor flowing upward into the furnace. Numerical simulations are carried out to investigate the velocity, concentration, and temperature of zinc vapor in the snout of the hot-dip galvanizing production line. The impacts of the inflow velocity, position, and temperature of the curtain on the concentration of zinc vapor are also paid attention. The results show that the wind curtain can effectively prevent the upward flow of zinc vapor. Furthermore, the concentration of zinc vapor in above of the snout decreases with the increases of the inflow velocity and the height of the wind curtain, but it increases when the temperature of the wind curtain’s inflow is raised. The position of the wind curtain significantly influences the concentration of zinc vapor above the snout, which is the critical factor of confining the zinc vapor by using the wind curtain.

In order to improve both performance and safety of lithium-ion batteries, we investigated the use of steel sheets which have a higher melting point than aluminum currently used for cell cases of lithium-ion batteries, for cell cases. First, a coating metal that can suppress Fe dissolution was selected, because corrosion resistance to battery electrolyte is important for battery cell cases. We found that Ni has high corrosion resistance to battery electrolyte, and that Ni-coated steel sheets can reduce the risk of short circuits due to decrease in Fe dissolution and re-deposition compared to non-coated steel sheets.

Next, the performance of the battery using Ni-coated steel sheet as the cell case was shown, and the discharge capacity after 500 cycles was the same as that of the battery using aluminum as the cell case, confirming that there is no problem with the battery performance.

For battery cell cases, it is also important to have superior high-temperature strength to suppress burnout in the event of thermal runaway. Ni-coated steel sheets have superior high-temperature strength compared to aluminum.

These characteristics of Ni-coated steel sheets are expected to be applied battery cell cases to produce batteries with superior performance and safety.

During hot stamping, Zn oxidation occurs on the surfaces of Zn-coated steel sheets such as galvanized iron and galvannealed sheets. In order to elucidate the effect of Al in the Zn coating layer on the Zn oxidation, the present study investigated the amount of ZnO formed on the Zn-coated steel sheets with and without Al addition to the coating layer. The amount of ZnO was found to decrease upon Al addition. The microstructural analysis of the Zn-coated steel sheets with Al addition revealed that the added Al became the ZnAl₂O₄ layer at the interface between the ZnO layer and Zn coating layer after hot stamping. As a result, Zn oxidation is considered to be suppressed by the presence of the ZnAl₂O₄ layer.

The martensitic transformation, which influences the mechanical properties of metals, is essential for ensuring high strength in steel. Workability is also improved by the formation of ultrafine grains using this reverse transformation. This study evaluated the reverse transformation kinetics during cyclic ultrafast heating and cooling using femtosecond X-ray diffraction to measure dislocation densities in 50% rolled Fe–0.1 mass% C–2.0 mass% Mn martensitic steel. We also developed a unique cyclic ultrafast heating and cooling system to determine the reverse transformation mechanism from martensite (α′) to austenite (γ). The maximum heating and cooling rates achieved were 1.2 × 10⁴°C s⁻¹ and 4.0 × 10³°C s⁻¹, respectively, which were sufficient to avoid diffusive reversion and bainitic transformation. We then measured the dislocation density and reverse transformation during a two-cycle ultrafast heating and cooling process. The reverse transformation under ultrafast heating was massive in the first cycle, while a displacive transformation was exhibited in the second cycle. Additionally, we found that cyclic ultrafast heating and cooling results in a finer microstructure, irrespective of the martensitic transformation mode. These findings represent an advance in our understanding of functional steels.

Super-invar alloy, Fe–32%Ni–5%Co, is widely utilized in precision instruments due to its remarkably low thermal expansion coefficient. Additive manufacturing holds promise for fabricating complex-shaped components with this alloy. This study investigated the phase stability and thermal expansion properties of Super-invar alloy fabricated via Laser Powder Bed Fusion (AM sample), comparing them to those of conventionally cast material (Re-melt sample). Microstructural analysis indicates that the AM sample has a more stable austenitic structure, attributed to minimal micro-segregation. Furthermore, it was observed that the thermal expansion coefficient decreases consistently with higher cooling rates within the temperature range of 400–300 K. As a result, AM sample exhibits lower expansion coefficient and it maintains at lower temperatures.

As-quenched α’-martensitic structure and its mechanical properties of Fe-1.5%Mn-0.5%C-Al alloys containing various concentrations of Al were investigated. Microstructural observations of specimens heat-treated under various conditions confirmed that a austenite single-phase region can be obtained in the temperature range from 1373 K to 1473 K when Al is increased up to 3%. Dilatometry tests showed that the M_s temperature increases at a rate of 25 K/mass% with increasing Al content. The hardness of α’ tended to decrease due to the decrease in the concentration of carbon in solid solution by auto-tempering and the formation of coarse blocks near the prior austenite grain boundaries. The coarse blocks had a butterfly shape, and there were no lath structures or transformation twins inside them, but rather high-density dislocations and carbides. Tensile tests on the 2%Al steel showed that the above coarse blocks can produce a large strain, which suppresses early rupture, resulting in a significant increase in ductility without any loss of strength.

To expand the application of high-strength martensitic steel in automotive body components, improving the bendability and understanding the factors affecting it are essential. In this study, we investigated the formation of shear bands and surface cracks in Fe-0.24C-1.0Mn (mass%) lath martensite during three-point bending tests using scanning electron microscopy and electron backscatter diffraction, with a focus on the effects of tempering up to 400°C. During bending, fine striated steps and elongated notches formed on the surface, which were attributed to block boundary sliding along the {110}_α in-habit-plane slip system and the formation of shear bands, respectively. Boundary sliding dominated subsurface deformation in as-quenched martensite, and shear-band formation was enhanced by tempering at ≥200°C, with a more pronounced effect at higher tempering temperatures. In as-quenched martensite, most surface cracks initiated near less-deformed hard martensite packets, and tempering at ≥200°C retarded the initiation and growth of these cracks. The limiting strain for crack initiation, as evaluated through digital image correlation analysis, increased with an increase in tempering temperature at ≥200°C and was found to correlate with the critical fracture strain determined via tensile tests. These findings suggest that the inhibition of crack initiation and growth by tempering is primarily caused by the improvement in local deformability within shear bands and at crack tips, which is primarily due to the transition from boundary sliding to the enhanced activation of multiple slip systems. The reduction in surface local stress at the bend apex could also be a factor induced by tempering.

Microstructures of the two heats of Modified 9Cr-1Mo steel that exhibit comparable creep strength but largely different creep ductility were analyzed to identify the factors that can reduce their creep ductility. The critical difference between the microstructures of the two heats was the size and distribution of prior austenite grains (PAGs). For the heat with higher creep ductility, the size of the PAGs was ordinary, approximately 20 µm. In contrast, the microstructure of the heat with low creep ductility was characterized as a mixture of regions with ordinary-sized PAGs and extraordinary coarsened PAGs of several hundred micrometers. The creep deformation of the heat with low creep ductility was found to localize in the regions with ordinary-sized PAGs, which led to the development of creep cavities and cracks, eventually reducing the creep ductility.