The formation of manganese sulfides, (Mn,Fe)S, during solidification of steel and subsequent cooling is a critical factor influencing the quality of continuously cast products, generally impacting hot tearing sensitivity and surface cracking on the strand. Understanding and predicting the formation of sulfides in the process is, therefore, essential for steelmakers. In this study, phase equilibria in two isopleth sections in the systems Fe-0.02C-0.50Mn-S and Fe-0.02C-2Mn-S (in wt.-%, S = 0.002–0.30 wt.-%) were experimentally characterized between 700°C and 1550°C. Alloys were produced in a high-frequency remelting (HFR) furnace. The morphology and distribution of (Mn,Fe)S inclusions in selected HFR samples were then examined via Scanning Electron Microscopy (SEM) coupled with Energy Dispersive X-Ray Analysis (EDX). High-temperature phase equilibria were analyzed by Differential Scanning Calorimetry (DSC) over the temperature range of 700 to 1550°C. To further elucidate the dissolution behavior of (Mn,Fe)S during the melting, phase transformations in a selected sample were observed in situ using High-Temperature Laser Scanning Confocal Microscopy (HT-LSCM). A CALPHAD-type thermodynamic database for the Fe–C–Mn–S system was created based on literature assessments and used to perform thermodynamic calculations of (Mn,Fe)S stability, showing excellent agreement with the DSC data. The critically evaluated database was used to derive the analytical equilibrium constant for the stability of stoichiometric manganese sulfide, (MnS) → [Mn] + [S], in equilibrium with the liquid, δ-ferrite (BCC), and austenite (FCC) phases.

This research investigates the formation mechanisms of inclusions in ultra-low carbon steel with Al–Ti complex deoxidation. Al2O3–TiOx–(FeOx) complex inclusions can induce nozzle clogging and surface defects. Many studies have examined the formation of Al2O3–TiOx–(FeOx) complex inclusions in molten steel, and considerable effort has been made due to the difficulty in distinguishing between thermodynamic equilibrium and transient states. The primary focus is on confirming the effects of thermodynamically stable inclusions in the RH process with Al–Ti complex deoxidation using sampling techniques and high-temperature confocal microscopy. The samples were analyzed using SEM-EDS and SEM-FIB techniques for precise analysis, and the inclusions were extracted for SEM-EDS analysis. The inclusions analyzed in the RH samples with low oxygen potential, melted under deoxidized Ar gas conditions in a high-temperature microscope, were Al₂O₃, which is consistent with the results of the equilibrium phase diagrams reported by many researchers. Utilizing in-situ high-temperature confocal microscopy, this study confirms the formation of Al2O3–TiOx–(FeOx) complex inclusions during the reoxidation process. The results indicate that these Al2O3–TiOx–(FeOx) complex inclusions are not formed under thermodynamic equilibrium in the RH process but are instead a product of reoxidation. This results provides grounds for preventing reoxidation to reduce the formation of Al2O3–TiOx–(FeOx) complex inclusions, which cause nozzle clogging and surface defects in the steelmaking process.

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.

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.

The effect of the cooling rate on the morphology, number density, and size distribution of MnS inclusions in a high sulfur steel was investigated. With the increase of the cooling rate from 15 K/min to 300 K/min, the number density of MnS inclusions ranged from 91 #/mm² to 192 #/mm², and the average size of MnS inclusions decreased from 7.4 µm to 5.3 µm. A coupled model was developed to predict the precipitation and growth of MnS inclusions. The precipitation temperature of MnS at different cooling rates was approximately 1823 K, which agreed well with experimental results. Through the high-temperature confocal scanning laser microscope, the precipitation process of MnS in a high sulfur steel during the solidification was observed. Inclusions of MnS primarily precipitated at grain boundaries, while the morphology of MnS before and after solidification was influenced by the cooling rate. A higher cooling rate promoted elongated MnS inclusions precipitated, and smaller MnS inclusions were precipitated after solidification. Conversely, a slower cooling rate led to the massive MnS inclusions precipitated during solidification, leading to the formation of elongated MnS particles.

In the current study, a novel laboratory experiment and a kinetic calculation were proposed to analyze the modification behavior of alumina inclusions in the molten steel. To obtain the shape and composition of a single Al₂O₃ inclusion at different times during the modification process, confocal scanning laser microscopy experiments were conducted to track the evolution of the Al₂O₃ inclusion particle on the surface of Ca-treated steel. Then, the composition evolution of the Al₂O₃ inclusion particle during the modification process was predicted using a kinetic model. It was assumed the product layer was homogeneous. The diffusion of dissolved [Ca], [Al], and [O] crosses through the inclusion-steel interface was considered. Experimental results agreed well with kinetic calculated results. Meanwhile, the kinetic model was used to analyze the modification behavior of Al₂O₃ inclusions in steel with various influence factors including the [Ca] content in steel, the [Al] content in steel, and the initial size of inclusions.

The strand surface and subsurface cracks could be prevented through the control of the strand surface microstructure, which correlates with the precipitation behavior of carbonitrides in the microalloyed steel. In this study, the carbonitride precipitation behavior was characterized in-situ with a high-temperature confocal laser scanning microscope, and the effects of cooling rate on the morphology and distribution of precipitates was investigated. The results show that the carbonitride precipitation process is usually accompanied by the formation of dark particles due to the volume expansion of the solute depletion region. The evolution of dark particles suggests that carbonitrides mainly precipitate between 910°C and 1085°C, and the “fast-growing region” ranges from 910°C to 960°C. As the cooling rate increases, the size and volume fraction of carbonitrides decrease. Meanwhile, the nucleation location changes from grain boundary to grain interior. To quantify the pinning capacity of the carbonitrides, the pinning force factor σ is defined according to the classical Gladman equation, which is a power function of the cooling rate, namely σ=0.43−0.1(1+2.76Vc^3.48)⁻¹. Combining the above research, a new secondary cooling method is proposed and has been applied on an actual caster, which improves the crack resistance of the bloom surface microstructures.

Time-resolved and in-situ observation of solidification by X-ray imaging is a powerful method to empirically understand the solidification process of metallic materials. Observations began with low-melting-point alloys such as Al, and were later applied to the observation of Fe-based alloys in the temperature range above 1750 K. In Ti–Al binary system, there is a γ phase (TiAl intermetallic compound) in addition to an α phase and a β phase, which results in a relatively complex phase diagram with multiple peritectic solidification. Therefore, although there are various possibilities for phase selection and microstructure formation during the solidification process, the solidification process has not been empirically clarified. Therefore, we used single crystal MgO as the sample holding container to suppress the reaction with the reactive molten Ti as much as possible, and for the first time, we successfully observed the solidification process of TiAl alloys. Time-resolved and in-situ observations showed that a solid state phase transformation with repeated nucleation and growth was selected instead of the peritectic solidification from α to γ phases shown in the Ti–Al binary phase diagram. The α/γ transformation should also be taken into account in TiAl alloys, since the α/γ transformation formed a fine γ phase and affected the microstructure formation. This understanding of the α/γ transformation was expected to provide new guidelines for controlling the solidification structure of TiAl alloys.

Understanding the mechanism of particle aggregation and dispersion at a liquid surface is important for the design and fabrication of novel materials. The behaviors of particles with various compositions at the interface of high-temperature melts, including the rapid growth of particles, the linear aggregation or curvilinear motion, and the wetting and separation processes, were observed in situ by high-temperature confocal laser scanning microscopy (HT-CLSM). We experimentally investigated the interaction force on particle pairs as a function of the interparticle distance and the size, composition and shape of particles. The experimental results indicate the attractive force between particle pairs decreases with increasing distance, and it increases as the size of the guest particle increases. The acting length of particles increases in the order of the alumina-magnesia particle (30 µm)< calcium aluminate particle with low CaO contents (80 µm)<alumina particle (110 µm). The estimated attractive interaction between particle pairs based on in situ observation increases in the order of Al₂O₃·35%CaO< Al₂O₃·19%MgO < Al₂O₃·38%MgO < Al₂O₃·15%CaO < Al₂O₃. The complex selective attraction has been observed in the in situ experiments which is attributed to the polydirectional attractive force and anisotropy in the particle morphology. The relationship between the roundness with acceleration rate of particles indicates that the attraction between spherical particles tends to be less than the attraction between irregular particles with edges.