The concentration dependence of the activity coefficients of solutes in molten iron is expressed in terms of interaction parameters. The interaction parameters were determined by the experiments under thermodynamic equilibrium and the evaluation had been based only on the accuracy of the experiments. An evaluation of the interaction parameters is proposed due to the thermodynamic stability of homogeneous solution, and is applied to several interaction parameters of the solutes of O, S, Ca and Al in molten iron at 1600°C.

RH is one of the essential secondary refining processes, and the oxygen lance plays a significant part in its multi-functionalization. Therefore, it is necessary to dig deeper into the jet characteristics at low ambient pressure. This study examined the supersonic oxygen jet parameters, such as the jet Mach number, dynamic pressure, turbulence kinetic energy, and half-jet width with computational fluid dynamics, for RH vacuum furnaces operating at ambient pressures ranging from 4000 to 1000 Pa and 101325 Pa. Underexpanded jets were observed at the ambient pressures from 4000 to 1000 Pa, and single strong barrel shocks existed. It was shown that decreasing the back pressure significantly improved the axial and radial Mach number of the jet. However, this improvement was related to a sacrifice of the dynamic pressure. The length of the potential core region and supersonic region increased greatly with decreased ambient pressure. The decrease in ambient pressure also considerably increased the turbulence kinetic energy, indicating a heightened level of energy transfer. However, the correlation between the half-jet width and the ambient pressure was not a simple linear relationship. The half-jet width increased with the ambient pressure decreasing from 4000 Pa to 1000 Pa.

Recycling of Scrap will lead to the continuous accumulation of residual element Tin (Sn) in steel production. The influence of Sn on interstitial-free (IF) steel and the formation of Sn compounds were investigated by experiments. The types of inclusions in IF steel with Sn is TiN, Al₂O₃, MnS and Ti₂S₃, which are consistent with those in IF steel. After adding Sn from 0% to 0.5%, the starting solidification temperature of molten steel decreases, resulting in the increase of inclusion number density. The average size of inclusions decreases from 2.39 µm to 1.89 µm. After adding 0.065%La, the inclusions containing Sn, such as La–Sn, La–Sn–O and La–Sn–Ti, generate in IF steel with 0.1%Sn or 0.5%Sn. With the increase of Sn content in IF steel containing 0.065%La, the number density of inclusions decreases and the number of large size inclusions containing Sn (≥10 µm) increases from 121 inclusions to 284 inclusions. Furthermore, the addition of Sn and La refines the macrostructure of ingots. Thermodynamic calculation shows that La₂Sn and La₅Sn₄ inclusions can form during the solidification process in sample with 0.1%Sn and 0.065%La. And in sample with 0.5%Sn and 0.065%La, La₂Sn and La₅Sn₄ inclusions can form directly in the molten steel. The generation of inclusions containing Sn can change the existing state of Sn and is beneficial in inhibiting the grain boundary segregation of Sn. The experiment lays a theoretical foundation for the use of La to minimize the harmful effects of Sn in IF steel in the future.

In iron ore sintering, the green bed properties and sinter productivity are influenced by the ore blend mixture including the proportions of ultrafine iron ores in the blend. In this work, iron ore granules, green bed properties and sinter productivity were investigated for blends containing ultrafine iron ores. The granulation and sintering experiments were conducted in a granulation drum with an internal diameter of 500 mm and a length of 310 mm, and a steel pot with an internal diameter of 108 mm and a height of 500 mm, respectively. The results of this study show that the reference blend was representative of a commercial sinter plant mixture in terms of comparable granule size and green bed properties at a similar moisture content of approximately 7%. An increase in the proportion of ultrafine iron ores in the blend resulted in a deterioration of green bed properties. A simple correlation was derived to link green bed voidage (ε) to α, the ratio of granule SMD and dry sinter feed SMD, as . This equation provides a simple yet effective expression that relates green bed voidage to both sinter feed size and granule size, enabling the prediction of bed voidage with a level of accuracy that is comparable to Hinkley’s equation. It was concluded that an increase in the proportion of ultrafine iron ores has a negative impact on green bed properties and causes a decrease in the gas flow rate during sintering. Consequently, the productivity of the sinter pot decreased.

Steel Slag Hydrated Matrix was developed by mixing steelmaking slag as an aggregate with ground granulated blast furnace slag as a binder. In this study, a direct casting process from molten slag was investigated as an alternative to the complicated and expensive Matrix manufacturing process. However, cracks that occur during casting can reduce strength. To prevent crack formation, the mechanism of thermal stress crack initiation and the appropriate conditions for casting molten slag in rock form were investigated by casting experiments, sound measurements, and thermal stress analysis. Compared with slag cooled in the mold, the slag cast under the optimum cooling condition suppressed cracks and did not cause cracks inside the slag. The crack sound was measured by sound measurements. To suppress cracks, it was suggested that a uniform temperature in the slag should be achieved quickly within 10 min after slag injection. The required solidification shell thickness could be estimated based on the tensile stress that occurred on the slag surface. The casting experiments and thermal stress analysis revealed the cooling conditions for suppressing thermal stress cracks in slag casting. Specifically, it was found that thermal stress cracks can be avoided when the molten slag is poured into the mold and then is demolded at the stage where the solidification shell thickness sufficiently exceeds the strength of the slag surface against tensile stress applied to the solidification shell. The solidified slag is thermally insulated, and the temperature inside the slag is uniform and cools while remaining uniform.

Microsegregation of solute components during the solidification process leads to solute pile-up in the liquid phase, which considerably influences the formation behavior of inclusions. However, no quantitative evaluation of solute concentration distribution during dendritic growth has been conducted. In this study, we established an in-situ observation method for quantitatively evaluating solute concentration distribution using model materials with fluorescent reagents to clarify how solute pile-up progresses owing to microsegregation. In addition to evaluating the physical properties of the model materials required for this study, we successfully realized a quantitative evaluation of solute concentration distribution during dendritic growth. Numerical analysis, considering 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 through both actual measurements and numerical analysis, demonstrating that a relatively simple model can represent the progression of microsegregation.

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 solidification 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.

Multi-phase steels are often used to realize a combination of high strength and toughness and/or ductility. To optimize their mechanical properties, it is vital to accurately evaluate the grain size, hard phase size and distribution, and dislocation density. In this paper, we studied a new method for evaluating the morphology and phase fraction of the hard phase, i.e., the martensite-austenite constituent (M-A), which is an important component that governs the mechanical properties of high strength steels. Using a scanning electron microscope, martensite can be selectively visualized with a bright contrast by collecting high-angle backscattered electrons. This method identifies only martensite in isolation from other phases, whereas both martensite and austenite are highlighted with the conventional two-step etching method. In addition, machine learning image analysis allows accurate extraction of martensite even in the presence of inhomogeneous backscattered electron image contrast in the matrix. This method provides an accurate and simple evaluation of the morphology of martensite in multi-phase steels over a large area.

This study investigated the mechanism of cementite formation in JIS-SCM440 low-alloy steel subjected to high-temperature nitriding (NH) at 913 K, followed by quenching and aging. Specimens with and without prior quenching and tempering (QT) treatment were analyzed to understand the influence of the initial microstructures on cementite precipitation. Comprehensive characterization using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), X-ray diffraction (XRD), and electron probe microanalysis (EPMA) revealed that NH treatment promoted significant nitrogen diffusion into the steel matrix, resulting in the formation of a nitrogen compound layer and a hardened nitrogen martensite layer beneath the compound layer. In the NH specimens following QT treatment (QTNH), accelerated carbon diffusion to the surface led to cementite precipitation around the surface pores owing to the dissolution of fine carbides. Cementite enhanced the surface hardness of the porous regions compared to that of the NH specimens.

The paper focuses on DP980 dual-phase steel, aiming to deeply investigate the evolution and control strategies of residual stress during the run-out table cooling process. The temperature-phase transformation coupling model was solved using the finite difference method, and the viscoelastic-plastic constitutive relationship model considering stress relaxation was solved using LU decomposition. A rapid calculation model for shape during laminar cooling of DP980 was established and validated for accuracy using industrial actual data. Based on this model, the impact of different operational conditions on the residual stress in the strip was analyzed. The results indicate that smaller initial temperature difference and initial shapes such as middle-wave or quarter-wave lead to lower edge compressive stress in the strip. In addition, the sparse cooling and off ultra-fast cooling also help to alleviate the problem of excessive compressive stress at the edge of the strip. However, implementing rear section cooling may result in uneven stress distribution in the width direction of the strip, with a greater degree of stress reduction near the edges. Based on the analysis of stress impact laws, improvements were made to the original production line processes. These improvements reduce residual stress and effectively mitigate wave-shaped defects during the laminar cooling on the run-out table.

The simulation results for the E250 B0, S355J2, and X70M grades provide valuable insights into the effects of material thickness, deformation, and alloying elements on strain accumulation and microstructural evolution during the rolling process. For E250 B0, the low recrystallization temperature and minimal alloying led to moderate strain accumulation, with the highest reductions occurring above the RLT. In contrast, S355J2, with the addition of Nb, exhibited increased strain accumulation due to the precipitation of Nb(C,N), which enhanced the material’s resistance to deformation, especially in thinner specimens. The X70M grade, with a higher Nb content, demonstrated a more pronounced strain accumulation, particularly in the thicker specimens, due to the reduced recrystallization and the influence of strain-induced precipitation. Thinner specimens experienced more intense deformation and faster recrystallization, resulting in finer grains and higher strain accumulation. Meanwhile, thicker specimens underwent slower deformation and recrystallization, promoting grain growth and reducing strain accumulation. The study highlights the critical role of material composition, thickness, and processing parameters in controlling strain accumulation, microstructural homogeneity, and the final properties of the rolled products. The presence of Nb was particularly influential in improving strain resistance and refining the microstructure, which is essential for achieving optimal material performance.

Daily and seasonal changes in temperature (T) and relative humidity (RH) were monitored using a sensor network system installed in the Hokkaido Centennial Memorial Tower, built more than 50 years ago using weathering steel, to investigate its corrosion condition. Five T-RH sensors were set at the south side wall, inside the south tower, in the semi-open central area, inside the north tower, and on the north side wall on the 4th, 14th, and 24th floors. The T changed as a function of altitude, location in the floor, season, weather, solar radiation, diurnal cycle, distance from the wall, etc. The highest T of the south wall at daytime in the winter season could rise more than 30°C even if the outer temperature was below 0°C due to solar radiation causing the repetition of ice or snow melting in the daytime and freezing of water at night. The change in RH and T inside the tower followed a Tomashov-type RH-T curve (high RH at low T in the morning and evening). In winter, however, T and RH distribution, i.e., high-RH (> ca.60%) area below the freezing point and low-RH area with the high-T, caused air transportation inside the tower, condensation (and freezing) in the low-T area, and drying in the high-T area. In the visual inspections, severe corrosion, such as blistering and peeling, has been observed at the bottom of the tower, where snow has accumulated, and rainwater has stayed for a long time, especially at welds and joints.

The effects of high magnetic field (HMF) on microstructure and mechanical properties of Fe-0.3C-10Mn-3Al medium Mn steels tempered at low-temperature after deep cryogenic treatment (DCT) have been studied. The results show that the experimental steel has a high strength and a relative low elongation due to the presence of a large amount of martensite phases after the DCT. During the low temperature tempering process under the HMF, the martensite phases transform into ferrite gradually with the increase of magnetic flux density (MFD) as a result of a significant increase in elongation with a negligible decrease in tensile strength and maintaining a high work hardening capacity. With the MFD increases from 0 T to 10 T, the yield strength gradually decreases from about 1 GPa to 545 MPa with almost a constant strength more than 1.2 GPa. The uniform elongation has been improved for about 4.5 times and the product of strength and elongation has been improved for about 4 times. Besides, the volume fraction of austenite increases from 21.6% to 61.2% and the average grain size of austenite increases from 0.7 µm to 1.1 µm. This study provides new insights into the role of HMF in modulating the microstructure and mechanical properties of medium Mn steel.

When using steel scraps containing Cr and Mn as raw materials in electric arc furnace (EAF) steelmaking, EAF steelmaking slag containing Cr₂O₃ and MnO is inevitably produced as a by-product. Since Cr and Mn are valuable elements, it is economically and environmentally important to recover by reduction of EAF slag and reuse those elements into steelmaking process as alloying materials. In this study, to improve the recovery efficiency, flux addition to EAF slag was investigated both thermodynamically and experimentally. At first, thermodynamic calculations were performed using Al₂O₃, CaF₂ and SiO₂ as candidate fluxes, and it was found that SiO₂ has the greatest potential to increase the slag liquid fraction and to improve the recovery efficiency. Then, reduction experiments using an actual EAF slag containing 8 mass% Cr₂O₃ and 6 mass% MnO were carried out at 1573 K with SiO₂ flux addition. As a result, slag liquid fraction and the recovery efficiency of Cr and Mn increased with increasing the amount of SiO₂ addition. Up to 81% of Cr and 61% of Mn in an EAF slag could be recovered. Furthermore, reduction experiments using different reducing agents were conducted. Despite the slag liquid fraction being the same, differences in recovery efficiency were observed. It was suggested that not only slag liquid fraction but also liquid fraction of reduced metal is necessary to improve the recovery efficiency.