Induction heating of a molten steel in a channel enhances inclusion removal though its purpose is heating. Thus, the optimum operating condition have not been clarified until now. In this study, theoretical analysis and numerical calculation of the inclusion behavior under the imposition of A.C. magnetic field using simple 2D model have been investigated. The rising velocity of the inclusion gradually increases as it approaches to the upper surface because the dominant driving force in the center area of the molten steel is buoyancy force and that in the upper region is a pinch force. Approximate mathematical expressions of the inclusion trajectory have been derived under the consideration of these dominant force. And removal time of the inclusion has been also theoretically derived. The shielding parameter in the range of 5 - 10 is the optimum condition to minimize the inclusion removal time, because the pinch force is weak in the case of small shielding parameter and the electromagnetic force dominant region is limited only in the vicinity of the metal surface in the case of large shielding parameter. The optimum shielding parameter relates channel size with frequency. Increase in magnitude of the magnetic flux density is another method to decrease the inclusion removal time. The pinch force in the vicinity of the channel wall overcome a force by turbulence and Saffman force under a certain condition which can be realized in industry, thus the inclusion reaches the channel wall.
The effect of atmospheric gas on gas absorption phenomena during tapping was studied by water model experiments. Atmospheric O2 gas composition around water stream was set as 20.9 (base), 11.0 and 4.0 vol% during tapping and changes of dissolved oxygen (DO) were observed. Experimental results were estimated with the kinetics formula considering dilution, volumetric coefficient (AkO) and saturated DO (CDO*). Experimental and calculated values were in good agreement with each other so that the utility of this method was confirmed.
Ar blow experiments were also estimated with the same method and atmospheric O2 gas consumption around plunge pool were discussed. Gas phase flow were visualized with volume fluid method (VOF) solver of OpenFOAM. In addition, O2 estimation model based on mass balance of oxygen was consisted. By using these methods, gas behavior around plunge pool were quantitatively discussed.
It is widely recognized that gas injecting into molten steel can be effective way to enhance inclusion removal. In this paper, bubble diameters from porous refractory nozzles, with different pore sizes, were measured, and the effect of gas flow, bubble size and inclusion size on inclusion removal ratio, was investigated by the water model experiments under high throughput conditions.
It was found that tracer removal ratio by bubbles was increased with smaller bubble diameter, because of large collision frequency function between bubbles and tracers. When bubble size was the same conditions, tracer removal ratio by bubbles was increased with larger flow rate, because of higher bubble density.
It can be concluded that the size and flow rate of bubbles were important for inclusion removal. Furthermore, it has been shown that Inclusion removal ratio by bubbles in high throughput conditions was expressed by the equation including bubble density, collision frequency function and Reynolds number.
Helium is a noble gas that has some important properties, such as low boiling point, low density, high thermal conductivity, and inertness. Thanks to its unique properties, helium is widely used in various areas of science and technology. But its price has risen steadily every year since its supply shortage has arisen in 2012. In the steel industry, helium is used as a carrier gas for thermal conductivity method which can analyze nitrogen in steel accurately and quickly. Therefore it is necessary to develop a nitrogen analysis method in steel without using helium as preparation for its shortage. We have developed new analytical method which enables highly precise and rapid quantitative analysis of nitrogen contained in steel by using argon instead of helium. The developed method is based on discharge molecular emission spectrometry, and its quantitation limit of nitrogen in steel was 8 mass ppm.
Cooling equipment of the pipe laminar type is utilized on run-out tables in hot strip mills. It is known that the responsiveness of starting and stopping cooling water injection affects cooling accuracy. In this study, experiments and calculations were performed in order to research the water drainage behavior of pipe laminar type cooling equipment. The experimental apparatus consisted of a single nozzle, orifice, header and air inlet tube. The calculation model considered Bernoulli’s theorem and pressure loss. The experimental results showed that the pressure loss of the air inlet tube and the nozzle affects water drainage time. The calculated results showed good agreement with the experimental results. Therefore, it is possible to estimate the water drainage time of pipe laminar equipment by the developed calculation model. Based on the calculation model, we suggested that the optimum diameter of the air inlet tube is 12 mm.
In sheet rolling, lubrication oil plays an important role in controlling the rolling condition and improving the sheet surface. However, the lubrication behavior in hot rolling has not been clarified sufficiently in comparison with that in cold rolling. In this work, the difference in oil pit formation and friction behavior at different rolling temperatures was investigated. Although the remaining oil amount was almost constant in the temperature range of 298 K to 973 K, the number of oil pits suddenly decreased in a range below 400 K. Furthermore, lubrication oil behavior was discussed based on the results of a numerical analysis of the oil film behavior at roll-bite. In hot rolling, it is thought that the lubricant oil behaves as follows: The oil film temperature varies greatly in the film thickness direction. Because the viscosity of the oil film on the work roll side remains high and flow is difficult, it does not contribute greatly to decreasing the friction coefficient. In contrast, the oil film on the sheet side has a low viscosity and flows easily, so only small oil pits are formed in the limited thickness with low oil viscosity.
In this study, Cu-added ferritic stainless steel sheets were investigated to understand the influences of Cu contents on the high-temperature strength during the high-temperature deformation. High-temperature proof stress at 700~900°C was improved greatly by addition of more than 1% of Cu. In the case of static aging at 900°C, the rod-shaped ε-Cu particles satisfied the K-S orientation relationship with the α matrix, but spherical-shaped ε-Cu particles without specific orientation relationship started appearing during the tensile deformation at 900°C. Similarly, the spherical-shaped and fine ε-Cu particles were observed during the thermal fatigue process in the temperature range from 200°C to 700°C with a 50% restriction ratio. During the observation using the high-temperature in-situ TEM straining, it was clarified that the ε-Cu particles were divided by dislocation shearing and parts of them were found dissolved. According to the LSW theory, it was suggested that the microstructure with fine and spherical-shaped ε-Cu particles was formed by the repetition of dissolution and precipitation during the high-temperature deformation.
In this study, the effect of Ni and Cr composition on mean thermal expansion coefficient (CTE) and γ’-Ni3 (Al, Ti) strengthening microstructure of austenitic heat resistant steel, Alloy 286, was investigated. The measurements revealed that the CTE of Alloy 286 decreases at wide range of temperature up to 800°C with addition of Ni and reduction of Cr concentration. The magnetization-temperature curve showed that the magnetic transition temperature, which the alloy transforms to ferromagnetic state to paramagnetic state, raise with addition of Ni and reduction of Cr. The CTE of alloys had good relations with magnetic transition temperature and mean valence electron numbers (e/a), suggesting that the magneto-volume (invar) effect and the suppression of lattice expansion strongly appear at high Ni and low Cr concentration. Ni and Cr optimized alloy exhibits γ phase microstructure with fine γ’ particles dispersed within the grain, and the hardness of grain interior is larger than Alloy 286. This alloy has low CTE comparable to conventional ferritic steels, together with higher strength than conventional austenitic steels.