Generally, in eddy current testing (ECT), magnetic field variations generated by induced eddy currents are detected using a pickup coil. However, according to Faraday’s law of electromagnetic induction, the sensitivity of a pickup coil decreases significantly at low frequencies, making the detection of low-frequency magnetic fields difficult. As a result, conventional ECT often have limited capability for detecting deep flaws in conductive materials. In this study, we propose an eddy current testing system based on an optically pumped magnetometer (OPM) with low-frequency excitation. The proposed method aims to improve the detectability of not only surface flaws but also deeper flaws that are difficult to detect using conventional ECT. To enable measurements under geomagnetic field conditions, the magnetic signal detected by the probe was transferred to an OPM inside a magnetically shielded box via a flux transformer. Furthermore, to suppress the effects of external magnetic noise and environmental disturbances, the probe was covered with an aluminum alloy magnetic shield. Experiments were conducted using a 12-mm-thick aluminum alloy plate containing artificial flaws with a diameter of 3 mm and depths ranging from 2 to 6 mm. The experimental results demonstrate that the proposed OPM-ECT system successfully detected all backside flaws in the specimen, indicating the potential of this method for deep-flaw inspection in conductive materials.

When recycling scrap iron, impurities that are difficult to remove, called trump elements, become a problem. Among them, tin (Sn) has many opportunities for contamination and affects the processability of steel, so new removal methods are needed. Sn is highly reactive with iodine(I) and becomes a metal iodide gas at high temperatures. The purpose of this was to remove Sn from iron scrap using I. We prepared Carbon-saturated iron containing 2 mass% Sn as a sample, weighed 0.1 g, and put it into the furnace. I₂ gas was then injected into the furnace. The Sn concentration and Sn removal rate were calculated by dissolving the sample after the experiment and performing analysis by ICP-OES. As a result, the Sn concentration in the sample was reduced to 0.008%, and the removal rate was up to 99.5%.

Blast furnace slag is generated in large quantities as a by-product of ironmaking and is mainly utilized in bulk applications such as cement production. However, high-value utilization focusing on the functional properties of individual components included in the slag remains insufficient. Using a low-concentration citric acid aqueous solution, selective separation and recovery of silica from blast furnace slag could be performed but some impurities remained. Cation-exchange resin treatment reduced impurity metal ions such as Ca and Mg so effectively that the SiO₂ purity increased to approximately 95%. Water vapor adsorption measurements revealed type-IV isotherms with hysteresis that indicated the development of mesoporous structures. Practical desiccation performance was evaluated under closed vessels in comparison with a commercial silica gel. The recovered silica exhibited moisture adsorption behavior comparable to that of the commercial one and demonstrated its applicability as a desiccant material for humidity control applications. The proposed method enables efficient recovery of functional silica from blast furnace slag under relatively mild conditions and has potential as a sustainable resource-recycling utilization process.

Copper removal from ferrous scrap by a Metal-Immersion (MI) process using a molten Pb bath was investigated using a simulated scrap sample and Cu-clad steel. The specimens were immersed in molten Pb at 973-1273 K for 1 or 10 min under Ar atmosphere. In the simulated scrap sample, complete removal of Cu was achieved at 1173 and 1273 K for 10 min, whereas residual Cu remained after immersion at 1173 K for 1 min and at 1073 and 973 K. This behavior is attributed to a decrease in the dissolution rate of Cu with increasing Cu concentration in the Pb bath. In the Cu-clad steel sample, the Cu layer was completely removed by immersion at 1273 K for 10 min. Cross-sectional SEM-EDS observation showed negligible dissolution of Fe into the Pb bath, no intermetallic compound formation, and little diffusion of Pb into the steel phase. These results indicate that the Pb-bath MI process enables selective removal of Cu with minimal influence on the steel substrate, even for Cu-clad steel, which is difficult to separate by conventional physical methods.

Strain-induced lattice defects that form during the incubation stage of hydrogen embrittlement fracture in the plastic region were quantified and their relationship with mechanical properties and fracture morphologies was investigated. Pure iron was subjected to plastic strain by tensile testing at various strain rates and hydrogen charging conditions. After charging tracer hydrogen as a probe for detecting lattice defects under conditions that reached equilibrium, specimens were quickly cooled with liquid nitrogen to prevent hydrogen desorption, and total tracer hydrogen was detected using low-temperature thermal desorption spectroscopy (L-TDS), which is capable of continuously elevating the temperature and subsequently performing measurement from that temperature. Dislocation density was not affected by the strain rate or hydrogen content. However, the vacancy concentration increased in the presence of hydrogen and displayed strain rate dependence even at the same strain level. A comparison of the mechanical properties with/without hydrogen showed that the flow stress with hydrogen increased with a decreasing strain rate compared with that without hydrogen, i.e., dislocation mobility decreased. It was established that strain-induced vacancies, which were excessively generated in the presence of hydrogen and formed complexes with it, were responsible for reducing dislocation mobility. Furthermore, fractures, albeit predominantly quasi-cleavage ones, along the {001} plane, which is the cleavage plane in body centered cubic iron, were present on the fracture surfaces, and their proportion increased with decreasing dislocation mobility. This suggests that vacancy-hydrogen complexes contribute to cleavage fracture by inhibiting dislocation motion.

A flatness control in a cold rolling has a significant importance for product quality and productivity. An automatic feedback flatness control based on influence coefficient is therefore commonly used in 4Hi or 6Hi rolling mills. In this kind of conventional feedback control, to determine the influence coefficients, flatness actuators are independently operated in the actual mills. It is known that effects of flatness actuators on the strip flatness changes based on the thermal crown, wear of work rolls and so on. Building and managing a model which considers such effects of all parameters are unrealistic. On the other hand, due to the difficulty of determining the influence coefficients, such flatness control is not common in 12Hi or 20Hi multi-high rolling mills. In this study, a new flatness control based on deep learning, which doesn’t need influence coefficients was proposed. In the actual mill trials, same strip flatness was obtained with fewer manually operated flatness actuators. In the considerations, it was found that the correct influence coefficients can be learned in the training phase.

The present study was undertaken to investigate changes in the solidification microstructure consisting of Zn(hcp), α-Al(fcc), and Zn₂Mg phases in the hot-dip Zn–6Al–3Mg (mass%) alloy coating during isothermal holding at an elevated temperature of 200℃ (below the solidus temperature), in terms of the transformation from Zn₂Mg metastable phase to Zn₁₁Mg₂ stable phase. The macroscopic Vickers hardness increased approximately from 150 HV to 180 HV after holding at 200℃ for 36 ks. The hardness change corresponded well with the occurrence of the Zn₂Mg→Zn₁₁Mg₂ phase transformation identified by X-ray diffraction analyses. The Zn₁₁Mg₂ stable phase nucleated at the interface between Zn₂Mg and Zn(hcp) phases, and grew by consuming both phases formed in the initial solidification microstructure. Nanoindentation measurements revealed that the Zn₁₁Mg₂ phase had a lower hardness than the Zn₂Mg phase. Therefore, the significant growth of the Zn₁₁Mg₂ phase, which consumes a large part of the soft Zn(hcp) phase, could contribute to the observed hardening of the hot-dip Zn–Al–Mg alloy coating during isothermal holding.

Accurate determination of trace tungsten in steel is essential for quality control and process management in steel production. The solvent extraction spectrophotometric method based on the formation of tungsten(V)–thiocyanate complexes with tetraphenylarsonium chloride, specified in Annex 3 of JIS G 1220, has long been used in routine analysis. However, several operational steps in this procedure are based on empirical practices, and their chemical validity has not been sufficiently examined. In this study, critical analytical steps affecting the analytical results were systematically investigated, with particular attention to the sulfuric acid fuming step after sample digestion, reduction of tungsten(VI) by tin(II), temperature control during complex formation, and time management between solvent extraction and absorbance measurement. The procedure examined in this study was applied to certified reference materials of alloy tool steel and high-speed steel, showing good agreement between measured and certified tungsten contents. These results provide a chemical basis for the procedure specified in Annex 3 of JIS G 1220 and support its application in routine tungsten analysis of steel samples.

The addition of cobalt to steels alters their properties, such as decrease in temperature of martensitic dislocation and degradation of hardenability. Quantitative recoveries of the precipitated bis(1-nitoroso-2-naphtolato) cobalt complex are essential to ensure the accuracy of the gravimetric analysis of cobalt in steel using 1-nitroso-2-naphthol as a precipitant. Factors affecting quantitative recoveries of the bis(1-nitoroso-2-naphtolato) cobalt complex as precipitates were investigated, including a concentration of 1-nitroso-2-naphthol, an aging time, and coexisting ions. We found that optimal 1-nitroso-2-naphthol concentrations for quantitative precipitation of the complex were regulated by initial amounts of cobalt in steel samples to be determined. Above the optimal concentrations, the recoveries of the complex decreased with increasing the 1-nitroso-2-naphthol concentrations. This decrease in the recoveries is rationalized by the formation of higher-order complexes of divalent cobalt and the rate-determining step to release the ligands from the oxidized trivalent cobalt complexes. Under the optimal concentrations of 1-nitroso-2-naphthol, the achievement of the quantitative recoveries of the precipitates required a much longer aging time than that specified in the standard method. The coexisting zinc ions after the separation of the iron matrix did not interfere with the quantitative recoveries of the cobalt complex. Quantitative recoveries of cobalt in a certified standard steel sample were yielded during the precipitation and filtration steps under the optimum conditions specified here.

Tamahagane is compared to the lance metal with multicolor after extremely low temperature blowing in a test converter using a multifaceted process engineering approach. The following conclusions were reached:

(1) The golden portion of tamahagane consists of an oxide film approximately 20 nm thick, which is composed of three layers. This multilayered oxide film gives tamahagane its "gorgeous" color.

(2) Tamahagane, a high-purity iron, is produced through a metallurgical reaction network consisting of three reactions: a hydrogen production reaction due to the decomposition of moisture in the blast, a reverse water-gas shift reaction, and a carburization reaction due to aging (long-term refining).

(3) The temperature at which tamahagane is formed through this metallurgical reaction network is around the iron point (720°C) of the intersection point between Fe-FeO equilibrium and Boudouard equilibrium and the eutectoid temperature of the iron-carbon system (A1 point: 723°C).

(4) In the case of a weight absolute humidity of 0.0055 (kg-H₂O/kg-dry air) at a base temperature of 10°C, and 0.0085 after adiabatic humidification (compression ratio of 1.1), the carburization time for an average carbon content of iron (total weight of 3 ton) is approximately 10.5 to 16.0 hours.

(5) Humidity of environments where Tamahagane is made is another important factor as well as charcoal reactivity and TiO₂ content in iron sands.

Sulfur is known as one of the five ubiquitous elements of steel, and the addition of sulfur reduces the performance of steel. Therefore, the sulfur content in steel must be strictly controlled. In this study, we have been focusing on the iron separation–barium sulfate gravimetric method (JIS G 1215-1) specified in the Japanese Industrial Standards (JIS) as an absolute analytical method for the determination of sulfur in steel. In our previous paper, we demonstrated that the rinse process using hot water in JIS G 1215-1 causes dissolution of barium sulfate (BaSO₄), resulting in a significant decrease in recovery. Therefore, suppressing the dissolution of BaSO₄ during the rinse process specified in JIS G 1215-1 has been critical issue in this analytical method. In this paper, we aimed to establish a novel rinsing method to replace the conventional hot water rinsing. As a result, it was found that rinsing with an ethanol solution of acetic acid effectively removed excess Ba²⁺ and Cl⁻ without dissolving BaSO₄. With this novel rinsing method, the conventional problem of reduced recovery caused by the rinse process was resolved, and quantitative recovery of the BaSO₄ precipitate was achieved.

Control of trace elements in steel is essential for maintaining material quality, and reliable analytical methods are required to determine their concentrations accurately. The molybdenum blue spectrophotometric method, standardized in JIS G 1212 based on ISO 4829, is widely used for the determination of silicon in steel. Although this method has been employed for many years, several procedural steps still rely on empirical knowledge, and the chemical basis underlying its prescribed conditions has not been fully clarified. In this study, we reassessed the JIS procedure with a focus on the formation of molybdosilicic acid, its reduction to molybdenum blue, and the influence of reaction temperature on analytical performance. UV–Vis measurements showed that β-type heteropolyacid species, exhibiting a single absorption maximum near 810 nm, are preferentially formed around 20 °C, whereas higher temperatures promote conversion to α-type species with reduced absorbance. Calibration curves obtained at 20 °C demonstrated excellent linearity. The validity of the optimized conditions was confirmed by analyses of certified reference materials, in which measured silicon contents agreed well with certified values. These findings provide a scientific rationale for the long-established JIS method and highlight the importance of strict control of reaction temperature and reaction time to achieve reliable and reproducible measurements.

Silicon is one of the five ubiquitous elements of steel, and the addition of silicon improves the performance of steel. Therefore, the silicon content in steel must be strictly controlled. This paper focuses on the silicon dioxide gravimetric method (JIS G 1212) specified in the Japanese Industrial Standards (JIS) as an absolute analysis method for silicon content in steel. The contamination derived from glassware affected the analysis values. The silicic acid (Si(OH)₄) in the digested solution is in equilibrium with both the silicon dioxide (SiO₂) in the sample and the SiO₂ on the glass surface. Therefore, it was found that the use of glassware in JIS G 1212 plays a crucial role in suppressing the dissolution of silicon from the sample. The elements contained in the steel also influenced the analysis values. Especially, molybdate ion (MoO₄²⁻) in the digested solution, originating from steel samples, not only inhibited the formation of hydrated silica (SiO₂·nH₂O) but also significantly accelerated the dissolution of Si(OH)₄ during rinse. From these results, it was found that molybdenum contained in steel samples reduces the silicon recovery in the silicon dioxide gravimetric method. Silicon recovery in the entire JIS G 1212 was approximately 100% regardless of the steel sample used. This indicates contamination derived from glassware does not affect the recovery of silicon in the total process of JIS G 1212.

A simple photometric detector for titration was developed using a white LED and an RGB color sensor as the light source and detector, respectively. This detector could record the color change of the sample solution during titration. The color sensor has 12-bit resolution for each red, green, and blue channel. The detector included a magnetic stirrer, and the controller controlled the mixing of a sample solution. The RGB values were transferred to a PC via a USB port. They were automatically converted to absorbance and L*a*b* data, and the results were displayed as titration curves. However, the volume of titrant added to the sample must be manually entered into the PC. The detector is as compact as 125 mm × 90 mm × 90 mm. First, the analytical performance of the titrator was evaluated by standardizing the EDTA solution with calcium carbonate. Then it was applied to the determination of chromium ions in synthesized steel samples according to the method indicated in JIS G 1217:2017, Annex 1 (normative). This method uses ammonium peroxodisulfate oxidation followed by potassium permanganate titration. This detector's function is not only helpful for objectively identifying the endpoint of a titration but also valuable for transferring the veteran's skill in endpoint determination to a novice technician, since subtle color changes can be recorded as numerical data.

A microfluidic titration method using a cross-type microchannel device was developed for to the determination of total iron in steel samples. The redox titration reaction proceeded within the microchannel, and the titration endpoint was determined based on the differential value of grayscale intensity derived from blue-channel image analysis. The reaction behavior was visualized in real time, enabling quantitative evaluation of concentration-dependent color changes. Optimization of the flow rate and observation point revealed that a flow rate of 0.05 mL min⁻¹ and a detection position 1.5 cm downstream from the junction yielded the highest sensitivity. The obtained calibration plot exhibited a semi-logarithmic relationship with iron concentration. When applied to the certified reference material JSS 831-2 Taharoa iron sand, the total iron concentration determined by the microfluidic method (46.9 ± 1.9 mM) was statistically indistinguishable from that obtained by the conventional volumetric analysis based on JIS M 8212 (45.49 ± 0.18 mM), as confirmed by a t-test. These results demonstrate that the proposed system achieves analytical accuracy comparable to traditional wet chemical methods, while eliminating subjective endpoint determination, reducing sample and reagent consumption, and enabling automated and reproducible operation. The results demonstrate the feasibility of integrating redox titration within a microchannel as a new approach to miniaturized, automated wet analysis. The microfluidic titration platform offers a promising approach for miniaturized, automated wet analysis and has potential for extension to other volumetric techniques such as chelatometric and acid–base titrations.

In this study, we investigated the surface properties of iron subjected to corrosion in contact with frozen salt solutions, focusing on the effects of solution aeration and salt type on ferrous iron dissolution behavior. X-ray photoelectron spectroscopy (XPS) was used to examine the surface conditions after corrosion. Image analysis of the frozen media indicated that dissolved oxygen in freeze-concentrated solutions (FCS) plays a crucial role in the dissolution process. XPS analysis confirmed the formation of iron hydroxide and iron oxyhydroxide on the iron surface, suggesting a reaction mechanism similar to that observed under atmospheric conditions. Additionally, surface analysis revealed that specific salt ions—such as F⁻, Cl⁻, and Cs⁺—exhibit a tendency to adsorb onto the iron surface under conditions of pronounced dissolution. These hard anions form complexes with Fe (II) ions, thereby promoting their dissolution. Moreover, Cs⁺ ions readily adsorb onto FeOOH, creating a concentration gradient near and beyond the iron surface that further promotes iron dissolution.