Over many years, mesoscale analysis such as the phase-field method has been the mainstream for numerical simulation of solidification. In contrast, our group has taken the initiative in applying molecular dynamics (MD) simulation to various problems in solidification. In this review, recent advances and contributions of MD simulations for solidification are presented. The primary contribution of MD simulation is the derivation of solid-liquid interfacial properties since it is not easy to measure these properties experimentally with high precision. In addition, recent significant progress in computational environments has dramatically expanded the possibilities of MD simulations for solidification. Now, MD simulations with a scale of billion atoms at the micrometer-scale have become a reality, enabling the exploration of analyses previously dominated by mesoscale methods, such as grain growth and dendrite growth. In particular, the dendrite growth at the micrometer-scale presented here represents the first achievement of directly simulating a typical four-fold symmetrical dendrite structure solely through atomic-scale simulations, to the best of the author’s knowledge. Moreover, new attempts at the fusion of data-driven methods and MD simulations are presented in this review, aiming to contribute to the rapid development of the field of solidification in the future.

Mixed powder of SiO₂ and SiC was heated to produce Si in air by irradiating multi-mode microwave at 2.45 GHz using a porous alumina crucible of sintered cement. SiC generated heat inside the mixture. Mullite layer was produced inside of crucible wall. Molten Si was produced at the apparent temperature between 1550°C and 1600°C during 5400s and 6000s. The apparent temperature was much lower than 1778°C determined thermodynamically. This is the characteristics of microwave that heat generates at the contact points of particles and the pointed parts of the surface in powder. A furnace for producing high-quality Si by microwave heating is proposed.

Recycled scrap is used as a raw material in an electric arc furnace (EAF). Certain EAF systems preheat the scrap using its exhaust gas to save energy. However, the actual operations cannot recover sufficient thermal energy of gas owing to non-uniform flow distribution such as blow-out and stagnation to cause the portion that is not melted. This study investigated the relationship between packed structure and gas permeability in a tank filled with random- and multiple-shaped solids. Visualization and flow measurement experiments were conducted. The packing structure was measured using laser-induced fluorescence by scanning a laser sheet through the tank to measure the three-dimensional distribution of the packed structure. The flow velocity distribution was measured using particle image velocimetry by preparing multiple directions of the laser sheet with respect to the water tank and reconstructing a three-dimensional three-component velocity distribution. Under high packing ratio, the flow structure was obstructed by the packing material, resulting in stagnation areas with low flow velocity. In contrast, at low packing ratio, the stagnation area was smaller, and the global flow field was stable. Furthermore, histograms of the flow velocity distributions suggested that stagnation occurred under high packing ratio conditions, while a global flow field occurred at low packing ratios. These results are applicable in the design of preheating equipment, such as exhaust gas recycling, preheating furnace, or clamshell. Thus, this study provides valuable insights into flow nonuniformity and the design of preheating equipment to improve operational efficiency and safety.

The role of impurities introduced to the steel melt through ferrosilicon addition is of considerable importance in determining the steel cleanness of Si-killed, Si-alloyed steel grades. The silicon requirement is often met with ferrosilicon (FeSi) addition at the industrial scale. In the present study, a detailed investigation of commercial purity ferrosilicon (FeSi) alloy has been conducted to assess its impurity content. Aluminium and titanium were found to be the main impurities among others. Si–O equilibria in liquid steel has been established at 1873 K using FeSi alloy for a wide range of silicon concentration to examine the influence of its impurities. The actual Si–O equilibria established through FeSi addition was compared with true Si–O equilibria which was established using High-purity silicon (HPS) addition. It has been observed that impurities (mainly aluminium and titanium) from FeSi perturbed the true Si–O equilibria. Thermodynamic and kinetic considerations pertaining to this deviation have been elaborated in the present study.

The heats of fusion of Fe, Ni, and Co were measured using a closed-type aerodynamic levitation method to prevent chemical interactions with the container and oxidation of the samples. The hypercooling limits of these metals were experimentally determined using the correlation between the undercooling temperature and thermal plateau time. The heats of fusion of the metals were obtained as the product of the hypercooling limit and the isobaric heat capacity. The experimentally determined hypercooling limits for Fe, Ni, and Co were 280, 414, and 360 K, respectively. Using these hypercooling limits, the heats of fusion of pure Fe, Ni, and Co were determined as 12.7 ± 2.2 kJ mol⁻¹, 16.9 ± 5.6 kJ mol⁻¹, 14.8 ± 2.8 kJ mol⁻¹, respectively. Notably, these experimentally determined heats of fusion using the closed-type aerodynamic levitation method closely align with the literature values within the range of experimental uncertainty, affirming the reliability of this measurement technique.

Controlling the shape, size, and arrangement of residual defects (pores) in additive-manufactured materials is essential for improving their strength and reliability. However, quantifying the shape and arrangement of individual pores in such materials remains a challenge. This study aimed to clarify the effect of pore configurations that determine the tensile properties of laser powder-based fusion (L-PBF) materials. First, the 3D pore-configurations of pure titanium L-PBF materials fabricated under different beam energy densities were visualized using high-intensity X-ray computed tomography (CT). Subsequently, the porosity, volume equivalent diameter, and sphericity of each pore were quantified by 3D analysis of each CT image, and their correlations with the tensile properties were analyzed. The results showed that, unlike conventional sintered materials, the 0.2% yield stress did not correlate with the porosity of the specimen, suggesting heterogeneity in the hydrostatic component of stress acting on pores. This was connected to periodic fluctuation in the local porosity of the layers sliced perpendicular to the building direction. Furthermore, for specimens fabricated under relatively low beam energy densities, the porosity of the lowest density sliced layer was negatively correlated with tensile strength and total elongation, whereby the local low-density layer dominated the tensile properties. For specimens fabricated under the high energy densities where keyholes were generated, the maximum pore diameter rather than the local layer porosity was more predominate. Thus, it is evident that local structures such as local low-density regions or larger pores dominate the ductile properties of Ti L-PBF materials in terms of their tensile properties.

A new type of online accelerated cooling equipment for seamless steel pipe and its application experiments were introduced in the present work. The equipment can simultaneously cool the inner and outer surface of seamless steel pipes. By using online accelerated cooling process, the mechanical properties of the test steel pipe made of Q345B composition meet the requirements of Q460E steel grade index, with the yield strength of 460–475 MPa, the tensile strength of 601–603 MPa, the elongation of 21.5–26.5%, and the −40°C impact energy of 136J.

This study investigates the impact of paint baking on the macro and micro-mechanical properties of resistance spot welds in quenched and partitioned 980 steels. It is observed that paint baking enhances both peak load and energy absorption during cross-tension tests, as indicated by load-displacement curves. Four different regions were identified from the load-displacement curves after paint baking. An intriguing observation was a quick increase in the loading rate following a prior decrease, attributed to change in crack propagation behavior rather than improved work hardening. The study further simulated the upper-critical heat-affected zone using a Gleeble thermo-mechanical simulator to evaluate flow strength and work hardening. The Kocks-Mecking strain-hardening model was employed to analyze work hardening behavior in the studied conditions.

In this study, linear friction welding is applied to join JIS-S45C medium carbon steel with ferrite and pearlite structures at temperatures above and below the A₁ point. Additionally, low-strain-rate tensile tests are conducted both in air and with a cathodic hydrogen charge to evaluate the hydrogen-embrittlement susceptibility of the linear friction-welded joints under both joining conditions. Results of hydrogen thermal-desorption analysis show that the hydrogen-charging conditions in this study simulated atmospheric corrosion conditions. The joining zone of the above-A₁ joint comprises fine martensite and ferrite, whereas that for the below-A₁ joint comprises ultrafine ferrite and cementite. In air tensile tests, both joints fractured in the base-metal region, thus suggesting the high reliability of the joints. In the hydrogen-charged tensile test, the above-A₁ joints exhibit premature fracture at the joining zone. By contrast, the below-A₁ joints exhibit base-metal fractures, thus suggesting that the joints are highly reliable in a hydrogen environment. Fracture-surface observations show that the above-A₁ joints exhibit cleavage fractures in the martensite-dominated region. Tensile tests on heat-treated martensite S45C specimens show that their fracture strength decreased significantly in a hydrogen environment. Therefore, the joint fracture is due to the significant decrease in the fracture strength of martensite formed in the above-A₁ joints in the hydrogen environment. The linear friction-welded medium carbon steel joints below the A₁ temperature can ensure reliability even in a hydrogen environment.

Previous research has shown that Fe–Mn–Cr–Ni–Si alloys offer excellent low-cycle fatigue resistance via reversible bidirectional transformation between face-centered cubic (FCC) γ-austenite and hexagonal closed-packed (HCP) ε-martensite. The alloy shows superior low-cycle fatigue life and is used for seismic damping applications, but there have been concerns over their resistance to highly corrosive environments. In this study, Fe–15Mn–aCr–bNi–4Si alloys were prepared with different Cr and Ni concentrations to evaluate the effects on the fatigue and corrosion resistances: Z1 with (a, b) = (14, 10.1), Z2 with (a, b) = (12.5, 8.8), Z3 with (a, b) = (11, 7.5), Z4 with (a, b) = (9.5, 6.1), and Z5 with (a, b) = (8, 4.8). Z2 had the longest fatigue life. The alloy showed Gibbs free energy difference between γ-austenite and ε-martensite phases close to the ideal of zero and the α′-martensitic transformation was suppressed well, which agreed with the design criteria for achieving bidirectional transformation-induced plasticity. The developed alloys showed superior corrosion resistance in seawater. Local pitting corrosion was observed that was attributed to the high Mn concentration of the alloys, although this was greatly mitigated by adjusting the Cr and Ni concentrations, especially with Z1 and Z2.

The carburizing and nitriding, essential surface modification methods for steels, enhance wear, fatigue, and corrosion resistance by forming fine carbides, nitrides, and nanoclusters involving alloy elements. Understanding the interactions between interstitial X (C or N) and substitutional elements M is critical for optimizing these processes and tailoring the material properties to specific applications. This study investigates the interaction energies in diatomic and triatomic clusters involving C/N atoms and substitutional elements of Al, Si, Ti, V, Cr, Mn, Co, Ni, Cu, Zr, Nb, and Mo. Using the first-principles calculations, this study reveals the intricate balance of interactions within these clusters, highlighting how atomic arrangements and specific element combinations can lead to either repulsion or attraction. We found that the interaction energies for triatomic clusters can be represented using a linear combination of interaction energies for diatomic clusters. Stable triatomic clusters comprise the second nearest neighbor M–X interactions for Fe–Ti–N, Fe–V–N, and Fe–Nb–N alloys. This finding was consistent with experimental observations of the monolayer clusters. Our analysis using the multiple linear regression and stratified analysis reveals that the metallic radius of element M influences interaction in M–X clusters: a larger metallic radius causes repulsion in the first nearest neighbor clusters and attraction in the second and third nearest neighbor clusters due to strain relief.

This paper reports new research progress on characterizing the burden surface particles of the blast furnace. An algorithm is proposed to extract the contour of the particles on the burden surfaces from their digital elevation models. The statistical distributions of particle size corresponding to the coke and sintered ore burden surfaces are counted from the extraction results of the particle contours. The statistical results obtained in the former research are compared with those of here. The particle surface height distributions can be approximated based on the burden particle size distributions. The peak positions of the estimated particle surface height distribution are consistent with that of the burden surface height distribution.

In ferritic steels, solute carbon (C) causes two types of discontinuous stress fluctuations that are accompanied by local deformation bands in the stress–strain curves. One is the yield drop with the Lüders band at yielding, and the other is the serrated flow stress with Portevin–Le Chatelier (PLC) bands during the strain hardening stage, that is, the PLC effect. Lüders band and PLC bands can be explained by static strain aging and dynamic strain aging, SSA and DSA, respectively. These difference in strain aging mechanism distinguish the Lüders band and PLC bands and qualitatively explain when they appear in the stress–strain curve at the yielding and strain-hardening stages. Nevertheless, Lüders band and PLC effect occur in carbon steels at room temperature and 373–473 K, respectively. Therefore, fundamental difference between these bands remains unclear because it is difficult to compare them under the same tensile conditions. In this study, low-strain-rate tensile tests were performed on ultralow-carbon ferritic steel at ambient temperature to compare the bands under the same deformation conditions. In addition to the Lüders band, the formation and propagation of PLC bands were observed at strain rates lower than 1.0 × 10⁻⁴ s⁻¹, and the PLC effect became more pronounced as the strain rate decreased and the carbon content increased. Furthermore, local strain analysis using digital image correlation revealed that the dislocation movement was much faster than C diffusion only in the Lüders band, which is attributed to the difference in the strain-aging mechanism.