The P contained in steelmaking slag is regarded as a potential phosphate source, especially with regard to slag with high P₂O₅ content, which is generated from the utilization of high P iron ores. If P can be efficiently extracted from slag, the obtained P can be used as a phosphate fertilizer. Moreover, the remaining slag can be recycled inside the steelmaking process. Compared with other phases, the P-condensed C₂S–C₃P solid solution in slag is more easily dissolved in water; therefore, selective leaching was applied to recover P from slag with high P₂O₅ content. In this study, the effect of K₂O modification on P dissolution in the citric acid solution was investigated, and subsequently, a process for extracting phosphate product from the leachate, via precipitation, was explored. It was determined that K₂O modification promoted dissolution of the solid solution, resulting in a higher dissolution ratio of P. By modification, the majority of the solid solution was dissolved at pH 6, and other phases remained in residue, indicating that a better selective leaching of P occurred. As the pH decreased, the dissolution ratios of both P and Fe increased. Following leaching at pH 5, a residue with a higher Fe₂O₃ content and lower P₂O₅ content was obtained. When the pH of the leachate increased, the dissolved P in the aqueous solution was precipitated. Through separation and calcination, a phosphate product with a P₂O₅ content of 30% was obtained, which has the potential to be used as a phosphate fertilizer.

Solidification shell deformations within the mold during continuous casting have been calculated in order to clarify the influence of mold flux infiltration variability on the cooling rate, the width of the low heat flux region, the height of air gap, the unevenness of solidified shell, and the resulting strain in the solidified shell. A sequentially coupled thermal-mechanical finite element model has been developed to perform the calculations. The simulation includes heat transfer and shell deformation in a growing solidified shell, along with the delta-to-gamma transformation. Further, it takes into account the effects of variability in mold flux infiltration and air gap formation on heat transfer into the mold, as well as the effect of cooling rate on the thermal expansion resulting from delta-to-gamma transformation. The results showed that mild cooling and small width of low heat flux region (i.e. low variability in mold flux infiltration) strongly decrease the height of the air gap, the unevenness in the solidified shell and the strain in the solidified shell. It is confirmed that it is important to optimize the cooling rate and prevent the variation in mold flux infiltration, especially at near the meniscus region of δ to γ transformation in order to minimize longitudinal crack formation.

Heat transfer properties of lamination stacks of the non-oriented electrical steels which important for heat management of high-performance motor core, were investigated. The thermal conductivity of in-plane direction of lamination stack almost coincided with the value which was estimated from the electrical resistance of the steel. The thermal conductivity of through-stack direction was depend on fixing methods of lamination stack and decreased with increase of the inter-lamellar air gap. Removal of the inter-lamellar air gap using glue is effective to improve heat transfer of the high performance motor core which adopted electrical steels with higher alloy and/or thinner gauge.

Spray cooling on moving hot solids is widely used in metal heat treatment processes. Understanding coolant droplet collision behavior with moving hot solids is of great importance toward improving heat treatment temperature control technology. Via flash photography, we experimentally investigated the hydrodynamics of droplet train obliquely impinging on a hot moving solid. The test piece was a rectangular steel piece (SUS303) heated to 500°C, 550°C, or 600°C with a moving velocity of 0.5 m/s, 1.0 m/s, or 1.5 m/s. The test liquid was water at approximately 20°C. The pre-impact diameter of droplets, droplet impact velocity, and inter-spacing between every successive two droplets were 0.64 mm, 2.2 m/s, and 1.91 mm, respectively. The tilt angle of the droplet train to the vertical was 50°. No coalescence of droplets was seen—the droplets deformed independently on the moving solid. The measured results of the maximum diameter and the residence time of the droplets agreed well with the empirical formulas that can be used for droplet impact on a stationary solid. It was found that the dynamics of a droplet train impinging on a hot moving solid are the same as the dynamics of a droplet train impinging on a hot stationary solid when the droplets deform independently on a moving solid. Taking advantage of said property such that it is equivalent to the dynamics of a droplet train impinging on a hot stationary solid, we proposed a critical condition for droplet coalescence and experimentally confirmed the validity of the critical condition.

To clarify the relaxation behavior of compressive residual stress during the first push and pull loading cycle, an in situ X-ray stress measurement method was formulated, in which a fine particle peening-treated hourglass-shaped specimen was fixed on an axial-loading fatigue testing machine, and the surface stress of the specimen—which is the sum of applied stress and residual stress—was directly measured via X-ray diffraction without removing the specimen from the testing machine. A noticeable relaxation in compressive residual stress occurred under the first compressive loading process, and slight relaxation was observed then onward. During the first compressive loading, the surface stress decreased almost linearly as the applied compressive stress increased; however, when the stress exceeded a certain threshold value, the relation between the applied stress and the surface stress deviated from the linear relation. This threshold value is important with regard to compressive residual stress relaxation. Furthermore, the relaxation behavior during the first compressive loading process can be explained by a master diagram that shows the relationship between the applied stress and the stress measured via X-ray diffraction. The diagram consistently shows that with an increase in the applied compressive stress, there is an increase in the amount of relaxed residual stress.

Fe-Cr-Co alloys are becoming important as half-hard magnet which can be subjected to plastic deformation process for their novel applications including non-contact electromagnetic brake because of its large hysteresis loss. Its magnetic hardness depends on the modulated structure formed by spinodal decomposition. It is important to clarify the effect of plastic deformation on the spinodal decomposition for optimizing the heat treatment after plastic deformation process. In the present study, we examined the spinodal-decomposed structures in Fe-Cr-Co sheets cold-rolled to 25% reduction and that without rolling to clarify the influences of cold rolling. Also, spinodal decomposition under the presence of dislocation structure have been simulated by phase field method for the case with the presence of dislocation cell boundary with a high in-plane solute diffusivity at various migrating speed. It has been found that the spinodal decomposition is accelerated around dislocation owing to the elastic field and higher diffusivity, which results in inhomogeneous microstructure with various wave length of modulation. The existence of dislocation enhances the initiation of phase decomposition and the growth particles. The decomposed structure greatly depends on the in-plane solute diffusivity and migrating speed of the dislocation cell boundary.

Effects of Si and Mn contents on V-bending in high strength TRIP-aided dual-phase (TDP) steel sheets with polygonal ferrite matrix were investigated for automotive applications. V-bending test was performed on a hydraulic testing machine using a rectangular specimen (50 mm in length, 5 mm in width, 1.2 mm in thickness) and 88-degree punch (2.0 mm in punch radius) and 88-degree die (12 mm in die width, 0.8 mm in die radius) at a processing speed of 1 mm/min. The main results are as follows.

(1) The 0.2C-(1.0-2.5)Si-(1.0-2.0)Mn, mass% TDP steel sheets were able to perform V-bending by strain-induced martensitic transformation of TRIP effect.

On the other hand, ferrite-martensite dual-phase (MDP0) steel sheet of 900 MPa grade was not able to perform 90-degree V-bending because of initiation of crack in tension area.

(2) The 0.2C-2.5Si-1.5Mn, mass% TDP-G steel sheet of 980 MPa grade was able to enable the 90-degree V-bending that considered an amount of springback (Δθ=θ₂−θ₁), in which the θ₁ and the θ₂ are a bending angle on loading and a bending angle after unloading respectively, of more than 2-degree by controlling a displacement of punch bottom dead center.

Effect of retained austenite characteristics on V-bending in ultrahigh strength TRIP-aided steel sheets with bainitic ferrite matrix (TBF steel) was investigated for automotive applications. V-bending test was performed on a hydraulic testing machine using a rectangular specimen (50 mm in length, 5 mm in width, 1.2 mm in thickness) and 88-degree top angle punch (2.0 mm in punch top radius) and 88-degree groove angle die (12 mm in die groove width, 0.8 mm in die shoulder radius) in a processing speed of 1 mm/min. The main results are as follows.

(1) The 0.2C-1.5Si-1.5Mn (mass%) TBF steel sheets were able to perform V-bending by strain-induced martensitic transformation of TRIP effect. On the other hand, ferrite-martensite dual-phase (MDP0) steel sheet of 900 MPa grade was not able to perform 90-degree V-bending because of initiation of crack in tension area.

(2) The TBF375 steel sheet of 1100 MPa grade was able to enable the 90-degree V-bending that considered an amount of springback (Δθ= θ₂−θ₁), in which the θ₁ and the θ₂ are a bending angle on loading and a bending angle after unloading respectively, of more than 2-degree by controlling a displacement of punch bottom dead center.

Effect of hydrogen on spot welded tensile properties in ultrahigh strength TRIP-aided martensitic (TM) steel sheet was investigated for automotive applications. Tensile test was performed on a tensile testing machine at a crosshead speed of 1 mm/min (strain rate of 2.8×10⁻⁴/s), using base metal and spot welded specimens with or without hydrogen charging.

The results are as follows.

(1) The difference between the tensile strength (TS) of 1532 MPa for base metal specimen without hydrogen charging and the maximum stress (TS-H) of 1126 MPa for the base metal specimen with hydrogen charging (ΔTS-H=TS−TS-H) in TM steel was smaller than that of hot stamped steel (HS1 steel) and superior to that of HS1 steel. On the other hand, the TS-H of 725 MPa for the base metal specimen with hydrogen charging was halved in comparison with the TS of 1438 MPa for base metal specimen without hydrogen charging in the HS1 steel. It is considered that this was because retained austenite suppressed the strength reduction due to hydrogen embrittlement of TM steel.

(2) The amount of hydrogen decreased in the order of HS1 steel, TM steel, and tempered martensitic steel (HS7 steel), and HS1 steel was the highest. This is thought to be due to the high dislocation density of HS1 steel.

(3) The difference between the maximum stress (TS-W) of spot welded specimen without hydrogen charging and the maximum stress (TS-WH) of spot welded specimen with hydrogen charging (ΔTS-WH=TS-W−TS-WH) in TM steel and that of HS1 steel were similar. It was considered that this is partly due to effect of stress concentration on heat affected zone (HAZ) softening of hardness distribution of spot weld.