Activity coefficients of light elements in molten metals and alloys are important thermodynamic properties for refining and inclusion controls of metallic materials. Activity measurements of the activities of light elements in pure metals have been carried out by previous studies. Also activities in molten alloys have been previously investigated and summarized using interaction coefficients. However, it is still difficult to accurately explain the activities of light elements in molten alloys over a wide range of temperatures and concentrations. In this study, with the aim of unified understanding of the thermodynamic behavior and solubility of light elements in various molten alloys, we studied the activity coefficients of oxygen and nitrogen in molten alloys using solvation shell model, and examined the factors governing the activity coefficients of oxygen and nitrogen in molten alloys.

Microsegregation of solute components during the solidification process causes solute pile-up in the liquid phase, which strongly affects the formation behavior of inclusions. However, there is no quantitative evaluation of solute concentration distribution during dendritic growth. In this study, we established an in-situ observation method for quantitative evaluation of solute concentration distribution using model materials with fluorescent reagents to clarify how the solute pile-up progresses due to microsegregation. In addition to evaluating the physical properties of the model materials necessary for this study, a quantitative evaluation of solute concentration distribution during dendritic growth was successfully achieved. Numerical analysis, taking into account the equilibrium partition of solute components and solute diffusion in each phase, reproduced the measured solute concentration distribution in the liquid phase. Thus, the solute concentration distribution was evaluated by the actual measurement and numerical analysis, and it was clarified that a relatively simple model can represent the progress of microsegregation.

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 the solidified steel is an important issue that greatly affects the properties of the steel; however, the distribution of inclusions after solidification does not always coincide with the locations of inclusion formation. Therefore, it is still difficult to estimate when, where, and at what supersaturation level inclusions crystallize in the liquid phase, and it is desirable to clarify their formation behavior to control the dispersion of inclusions. In this study, we investigated the formation process of inclusion 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 the inclusion formation. It was confirmed that the frequency of inclusion formation increased significantly when the solution was held at lower temperatures, i.e., when a large supersaturation ratio was given. The results of the formation frequency indicated that the formation of inclusions occurred in the liquid phase according to the classical nucleation theory.

The formation of inclusions during solidification in steelmaking process is a critical issue for the optimal processing and the quality of steel products. Therefore, it is required to clarify the mechanism on the inclusion formation for its adequate control. In the present work, the evaluation method of inclusion distribution via the combination of inclusion positions analysis and image analysis of dendrite structure with machine learning is proposed. Image analysis using a conditional deep convolutional generative adversarial network enabled the detection of domain boundaries and the directions of secondary dendrite arms in the cross-sectional structure of unidirectionally solidified specimens. In addition, by combining this with the analysis of inclusion position, a correlation was confirmed between micro segregation behavior and the formation behavior of TiN inclusions.

In order to clarify the location of secondary inclusions formed during solidification, this study proposed a function to estimate the one-dimensional fraction solid at any position from the cross-section of unidirectionally solidified specimen. The function is based on an n-th order function expressed as a combination of the dimensionless distance (r/R) defined within the dendritic domain and the rotation angle θ measured from a specific secondary branch. The function has a simple form and uses only one parameter but the estimated values of fraction solid are ideal both for one-dimensional and two-dimensional ones. In addition, the predicted shape of the cross-sectional structure of the dendrite that can be derived from the proposed function is reasonable. It was also revealed that it is possible to express a variety of shapes from a cell shape without secondary branches to a dendrite shape with well-developed secondary branches, by changing the value of parameter n.

Forming conditions and compositional changes of primary inclusions in molten steel have been studied due to the demand for high cleanliness of steels. MnS, a common inclusion in steel, does not form in molten steel, although it is observed in steel with oxide inclusions such as MnO, Al₂O₃ and SiO₂. On the other hand, Mn and S are enriched in molten steel due to the segregation phenomenon during the solidification process which suggests that MnS form in molten steel during solidification. However, the precipitation behavior of MnS inclusions in molten steel due to the enrichment of Mn and S and the interaction between the primary inclusion and the molten steel is still unclear. In this work, a new experimental technique was developed and the precipitation behavior of MnS from molten steel onto solid Al₂O₃ was studied. Solid MnS precipitates were observed on the Al₂O₃ rod immersed in the sample with adding Al whereas precipitates containing MnO, A₂O₃ and MnS were observed on the Al₂O₃ rod in the sample without adding Al. Thermodynamic analysis revealed that Mn enriched in molten steel is oxidized to form MnAl₂O₄ when Al content is low and the MnAl₂O₄ reacts with S in molten iron to form molten MnO–Al₂O₃–MnS. MnS can precipitate from the molten MnO–Al₂O₃–MnS. On the other hand, Mn enriched in molten steel does not react with Al₂O₃ when Al content is high. Therefore, MnS can precipitate at the final period of solidification where Mn and S are significantly enriched in molten steel.

In order to reasonably control the precipitation of inclusions during solidification in TWIP steels, the precipitation behavior of typical MnS inclusions in high manganese steel was investigated by unidirectional solidification experiments. Through the combined analyses using ASEM-EDS, optical microscope, and thermodynamic calculation, it was found that Mn concentration in the liquid metal region were higher than those in the solid metal region. Furthermore, closer to the inclusion the liquid phase was, higher its Mn content was. In Fe–18mass%Mn–1Al–0.3C, MnS inclusions can precipitate at the positions located in the junction of dendrites at the end of the solidification (solid fraction f_S = 0.96), Mn content reaching 34.88 mass%. Already existing Al₂O₃ particles could become the core of MnS to form composite inclusions to promote the MnS precipitation during the solidification process. When f_S achieved 0.7 leading the Mn segregation in the liquid phase to 25 mass%, MnS starts to precipitate to attach the Al₂O₃ surface to form composite inclusion.

Mn-TRIP steels of which composition is mainly Fe–(0.5–3mass%)Al–(2–10mass%)Mn are expected to be new advanced high-strength sheet steels. During the solidification process of Fe–Al–Mn alloy, AlN inclusions precipitate at the grain boundary, which leads to the severe deterioration of hot ductility. However, the precipitation behavior of AlN inclusion is not known enough. In this work, a unidirectional solidification experiment of Fe–(0.5–2.0)mass%Al–2.0mass%Mn alloys and numerical analysis on the forming condition of AlN were carried out, and the precipitation behavior of AlN inclusions was studied. Al₂O₃ inclusions were observed in the alloy with 0.5 mass%Al. On the other hand, AlN inclusions were observed in alloys with 1.0, 1.5, and 2.0 mass%Al. The volume fraction of AlN inclusions increased with increasing Al content of the alloy. The thermodynamic analysis revealed that AlN is thermodynamically unstable at temperatures above the liquidus of the alloy. When Al content of molten steel is increased, AlN becomes thermodynamically stable. Accordingly, the forming amounts of AlN in the alloys during the solidification were analyzed considering the segregation. The results show that the precipitation of AlN inclusions increases significantly during solidification due to the enrichment of Al in the liquid phase. In the Fe–(1.0–2.0)mass%Al–2.0 mass%Mn alloy, Al₂O₃–AlN inclusions were also observed, where AlN is present around Al₂O₃. These inclusions are considered to be formed by the precipitation of AlN, which becomes stable as the Al concentration increases due to solidification segregation, on Al₂O₃, which is stable and precipitated in the early stage of solidification.

It is known that the size distribution of inclusions in steels has a significant effect on material properties. The solidification characteristics and TiC formation behaviour of alloy 800H were evaluated both by experiment and simulation in this work. The relationship between dendrite arm spacing and the cooling rate was estimated. TiC particles were observed at the interdendritic region. The size distribution of TiC particles was affected by the solidifacation cooling rate. A solidification analysis using the MPF (Multi-Phase Field) method revealed that TiC formation begins at a solid fraction of 0.79, and solidification accelerates due to TiC formation. It was thought that TiC particles generated in the latter part of solidification aggregate and coalesce without engulfment by the solidified shell. The size distribution of TiC particles was also affected by heat treatment after solidification.

Controlling the size, number, and composition of secondary inclusions is vital in the production of high-quality steels. In this study, experimental and computational investigation of the relationship between secondary inclusion formation in Fe-36mass%Ni alloy and cooling rate was carried out. Assuming the case of large ingots, solidification experiments using various cooling rates (0.17 to 128 K/min) were employed and the size, number, composition, and distribution of inclusions were analyzed by SEM-EDS automatic inclusion analysis. Like previous studies, inclusion number density increased with increasing cooling rate, while inclusion size decreased with increase of cooling rate. On the contrary, oxide inclusion area fraction was found to have little relationship with the cooling rate and was instead found related with oxygen content of the sample. As a new attempt to investigate the relationship between microsegregation and secondary inclusion formation, a combination of SEM-EDS analysis and EPMA mapping analysis was carried out. By superimposing information of microsegregation and inclusions, it was found that high-Al₂O₃ inclusions formed during the early stage of solidification, whereas low-Al₂O₃ inclusions formed during the later stage of solidification. These findings suggest that Al₂O₃ inclusions formed in the early stage of solidification reacted with the remaining Si-enriched liquid steel and changed into low-Al₂O₃ inclusions. Experimental results were also confirmed by thermodynamic calculations. Present work made it possible to understand deeper the relationship between microsegregation and secondary inclusion formation.

This study demonstrates the effect of Ti addition on phase selection and subsequent ferrite-austenite transformation in Fe–22mass%Mn–0.7mass%C alloy, where the austenite is the primary phase in equilibrium. X-ray radiography revealed that the metastable ferrite nucleated as equiaxed grains in the completely melted specimen. During subsequent cooling, the metastable ferrite massively transformed into the austenite in the solid state, forming multiple austenite grains in each metastable ferrite grain. The ferrite-austenite transformation was immediately followed by the coarsening of multiple austenite grains within each former metastable ferrite grain. Typical austenite grain size ranged from 100 to 500 μm. In the specimen after the observation, titanium carbonitride (Ti(C,N)), which acts as heterogeneous nucleation agent for the ferrite, was presented and overlaid manganese spinel (MnAl₂O₄) or Al-Ti oxide. Because disregistry between such oxides and Ti(C,N) can be relatively low, the oxides facilitated the formation of Ti(C,N) in the melt. Regarding the formation of the oxides, it can be postulated that titanium oxides, as a deoxidation product, first combined with soluble Al, Mn, and O to form liquid Al–Mn–Ti oxides. During cooling, MnAl₂O₄ or Al–Ti oxide was supersaturated in liquid Al-Mn-Ti oxides, which subsequently crystallized and dispersed in the melt. Thus, titanium oxide serves as a precursor to a multistep reaction leading to the formation of Ti(C,N), and its fine dispersion in the melt allows us to control the austenite grain size in the as-cast microstructure through promoting the metastable ferrite nucleation followed by the ferrite-austenite transformation.

In the production of high strength steel, it is necessary to improve the inner quality of steel slab. Among of the internal defects, the center segregation deteriorates the ductility and fatigue strength of high strength steel. Therefore, it is necessary to understand the generation mechanism of center segregation and develop the countermeasure technologies. In the center-segregated region of high phosphorous and hyper-peritectic steel slabs, iron-phosphide may be observed. However, the precipitation mechanism of iron-phosphide has not been fully clarified. Therefore, in this study, in order to clarify the formation mechanism of micro- and macro-segregation in the final solidification of CC slab, the melting behavior of the center segregation region of high phosphorous and hyper peritectic steel slab has been investigated by using high temperature observation equipment installed with the image furnace. Additionally, the enrichment of solute in the inter-dendritic region has been analyzed by using micro- segregation model with consideration of delta-gamma transition of steel. Under the assumption that the enriched solute in the inter dendritic region was transferred to the final solidification region because of fluid flow and the center-segregation was formed, the re-solidification of micro-segregated liquid was analyzed by using the modified micro-segregation model. At the same time, the possibility of precipitation of iron-phosphide was examined by using solubility limit of iron-phosphide. Moreover, under the basis of Fe-C-P ternary eutectic solidification system, the precipitation mechanism of iron-phosphide during the formation of center segregation was discussed.