This study systematically examined the abnormal grain growth (AGG) behavior of ferrite in pure iron subjected to cold torsion followed by annealing. Continuous shear strain in the radial direction of the cylindrical test specimen was introduced by cold torsion. Two torsion angles 1300 ° and 3700 ° were applied, and the resulting microstructural change was characterized through hardness distribution, optical microscopy, electron backscatter diffraction (EBSD), and inclusion particle analysis. Annealing at 1123 K revealed that AGG consistently occurred at regions with a shear strain of approximately 1.5, independent of the torsion angle or radial position. The abnormal grains formed in a circumferential chain, reaching diameters of 6 mm and 2 mm for 1300 ° and 3700 ° torsion, respectively. The recovery and recrystallization behavior were influenced by the shear strain. The emergence of AGG was successfully interpreted using the defect-model proposed by Hillert, which defines AGG conditions based on the balance between the driving force of grain growth and the pinning force exerted by dispersed particles.

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.

We attempted to improve the strength–ductility balance in tempered martensite steel using multi-objective Bayesian optimization. The martensite steels were tempered at two stages, and fine and coarse cementite particles were mixed. Water-quenching rather than furnace-cooling between the first and second temper stages provided a better balance of strength and ductility. Moreover, the strength–ductility balance was also improved by tempering at a low temperature in the first stage and a high temperature in the second stage, rather than tempering at a high temperature in the first stage and a low temperature in the second stage. Based on these experimental results, multi-objective Bayesian optimization was used to further improve both tensile strength and total elongation. The strength–ductility balance that is better than the experimental results was achieved with a minimal number of optimization times. Additionally, machine learning suggested that it is crucial to control the average aspect ratio of cementite particles to less than 1.8 and the size of coarse cementite particles to less than 2.0 μm in order to improve the strength–ductility balance.

In the point of view of reducing coke production cost and future resource depletion, it is necessary to produce high-strength coke from low-rank coal.

The declining quality of iron ore fines and increasing re-use requirement of steelmaking slag have led to higher Al2O3 content in sinter products. High grade concentrated hematite ore (concentrate) has been emerged as a potential solution for reducing Al2O3 levels of sinter. However, research regarding on the melting and assimilation behavior of concentrates in sintering process is limited. On the other hand, our previous work demonstrated that Parallel Granulation with Inclined Mixing of Limestone, based on the creation and mixing of pseudo-particles with high and low CaO/Fe2O3 ratios, can improve the sinter strength and productivity. To investigate the melting and assimilation characteristics of concentrates, we conducted melt dripping tests various kind of iron ore, revealing that adhesive layers with high concentrate content have low melt retention capacity. Furthermore, sinter pot tests under single granulation condition showed that when a large amount of concentrate is blended, fragile sinter cake is formed due to insufficient melting and assimilation. Sinter pot tests using Inclined Mixing of Limestone demonstrated that melting and assimilation are promoted by positioning concentrate near abundant melt sources.

In the JIS standard case-hardening steel, spheroidizing annealing can improve cold forgeability but inevitably results in uneven distribution of carbides (a mixture of lamellar carbides and spherical carbides, and uneven carbides distribution). This degrades cold forgeability and promotes grain coarsening during carburizing. To address this problem, new SA methods combined with suitable chemical compositions are required to achieve a uniform distribution of carbides in a hypoeutectoid steel.

The reducibility and mechanical properties of iron ore sinter in blast furnace is critical to effective plant operation. The reduction reaction of sinters progresses heterogeneously owing to microstructures with various mineral phases and pore networks. The reduction process was investigated by semi-microbeam synchrotron X-ray multimodal analysis. Heterogeneous chemical state evolution of Fe and trigger sites of crack formation were visualized using two-dimensional Fe K-edge X-ray absorption near-edge structure analysis and were discussed based on reduction gas transfer. The elemental composition map and X-ray diffraction microanalysis were also combined to reveal the microprocesses during the reduction, such as calcium ferrite decomposition and crystal grain growth.