Oxygen exchange reaction rates between CO2-CO gas and molten oxides containing iron oxide have been measured by isotope exchange technique with 18O at 1773K with PCO2/PCO=1. Measured results represent that the oxygen exchange reaction rates show the maximum values at the beginning of the reaction and sharply decrease within approximately 20 s, and then gradually decrease with the reaction time proceeds. These changes indicate that the rate-limiting step would change from the dissociative adsorption of oxygen on the surface of molten oxide in the initial stage of reaction to the mass transfer of oxide ion from the surface to the inside of molten oxide in the following stage. Based on the kinetic analysis of the oxygen exchange process, CO2 dissociation rate constant, CO2 re-formation rate constant and the mass transfer coefficient of oxide ion in molten oxides were calculated for the FeOx-CaO and FeOx-SiO2 systems. Addition of CaO and SiO2 to molten oxide increases and decreases the oxygen exchange reaction rate, respectively.
In the continuous casting process, it is important to reduce the off-quality portion due to ladle exchange or variation in process parameters. In the present study, experimental investigation and thermodynamic calculation has been carried out for analysis origin of non-metallic inclusions in ladle transition part of continuously cast slabs. The following results are obtained. (1) In the present study, entrapped nonmetallic inclusions in the transition part of ladle exchange slabs are mainly spherical inclusions (CaO-Al2O3). (2) According to thermodynamic calculation, origin of spherical inclusions are mainly ladle slag and tundish slag. (3) Applying thermodynamic calculation for the equilibrium between slag and molten steel is useful for analysis origin of nonmetallic inclusions in the continuously cast slabs.
Experimental and theoretical studies were carried out to clarify shape control of alumina inclusions during calcium double treatment in a line pipe steel for sour service. Calcium (Ca) double addition method were experimented by 280 t-scale commercial plant. In this method, Ca powder was firstly injected for shape control of alumina inclusions by flux injection process (FI), and was secondly added for shape control of MnS to CaS in tundish of continuous casting machine after eliminating the first step Ca by RH degasser. The degree of shape control of alumina inclusions of this method was increased compared with that of the conventional Ca treatment by FI or tundish addition, and approaches to the equilibrium condition among Ca-CaO-Al2O3 system. To explain the shape control rate of alumina inclusions, the non-reacted core model fitted by the laboratory scale experimental results was applied for this experimental data. The calculated values from this model in the Ca double addition method agreed well with the observed ones.
Mold flux has a large influence to the surface quality of ultra-low-carbon steel slabs. Especially, its involvement in the continuous casting mold must be prevented in order to reduce inclusions in the surface layer of the slabs. In this work, experimental continuous cast with the pilot caster was conduct and the influence of mold flux basicity to the surface quality of the slabs was researched. Spherical inclusions, which existed in the surface layer of the slabs and were considered to be caused by the involvement of molten flux in the mold, decreased in number or size, with the increase of mold flux basicity. The effect by the basicity could be explained in terms of interfacial tension or wettability between mold flux and molten steel. Thus, the number and the size of spherical inclusions decreased with the increase of interfacial tension and contact angle of molten flux on the surface of molten steel. This effect was considered to be larger than that by the viscosity or density of mold flux.
Recently, hot rolling with a severely high redution was developed to make the ultra fine grained steels. The high reduction in the hot rolling caused some troubles such as increase of the rolling load, occurrence of friction pick up and so on. To solve these problems, new lubricants with high coefficient of friction at biting and low coefficient of friction during hot rolling must be developed. The lubricants cannot be estimated using the two-disks testing machine or the pin-on-disk testing machine. So, a testing machine to simulate hot rolling in the laboratory has been developed by the author et al. The performance of the simulator is evaluated at a hot rolling temperature of 800°C using SPHC strip with a width of 22 mm and a thickness of 9 mm, SKD61 roll with a diameter of 100 mm and a width of 60 mm and new lubricants. Results from experiment to measure the coefficient of friction decided the lubricity of the lubriconts in hot rolling with a severely high reduction can be evaluated.
A three-dimensional high frequency tube welding heat model has been developed by using electro magnetic and heat conduction finite element methods. Eddy current density distribution at a static state is calculated by means of a frequency response analytic law. Heat transfer in the tube circumferential and longitudinal direction is calculated with heating and cooling. This model is verified by experiments with laboratory-scale electric resistance welding mill. The analysis has good agreement with the actual measurement. This model can satisfactorily simulate the eddy current density distribution and the temperature distribution in the high frequency tube welding. The influences of welding frequency, welding speed, and magnetic permeability of impeder on a temperature distribution of a tube are quantitatively clarified.
The oxidation behavior of Si-containing steels at high temperature has been the subject of numerous investigations. In many cases, the addition of Si to the steel in sufficient amounts to form an external film of SiO2 or other Si-rich oxide on the steel surface has resulted in extremely slow oxidation rates. However, the steel in which Si content is insufficient forms an external scale and an internal scale on the steel surface and a subscale in the steel interior which are less protective. In order to suppress the growth of the scales on the hot slab which causes surface bruise of hot rolled Si-containing steel sheets, many researchers paid attention to the de-scaling characteristics of Si-containing steels. However, the effects of Si content on the oxidation kinetics, scale composition and scale structure, especially three-dimensional scale morphology formed on the Si-containing steels at high temperatures are not necessarily understood adequately. In this study, the high-temperature oxidation behavior and scale morphology of (0, 0.5, 1.5, 3.0 mass%) Si-containing steels exposed to the flowing air and the LNG combustion gas atmosphere at 1373K or 1473K were investigated. The external scale and the internal scale formed on Si-containing steels were composed of the laminated α-Fe2O3 and Fe3O4 and the mixture of FeO and Fe2SiO4, respectively. The subscale may be composed of vitreous SiO2. The kinetics of the external and internal scale formation were followed by parabolic rate laws. The SEM images of the three-dimensional subscale extracted from the oxidized steels by means of an organic solvent system dissolution technique showed that an intergranular oxide formed thin walls along not only grain boundaries but also twin boundaries of the steel matrix and an intragranular oxide formed isolated particles in the steel grains near the internal scale.
Formation of a surface layer containing a high concentration of aluminum on SUS304 stainless steel was tried by electrodeposition of Al on the steel using potentiostatic cathodic polarization method in NaCl-KCl melt containing AlF3. In addition, improvement in spallation resistance of the steel was accomplished by electrodeposition of La in the same molten salt containing LaF3. The cyclic oxidation resistance of the electrodeposited sample was investigated at 1273K in air containing water vapor. The deposited layer formed after Al electrodeposition uniformly coated the substrate steel, and was adhesive to the substrate steel. The deposited layer consisted mainly of Fe aluminides. The particles of La were observed on the surface of the deposited layer when the electrodeposition of La was carried out after the Al electrodeposition. The cyclic oxidation test in atmosphere containing water vapor showed that for the untreated steel, a large mass loss was observed, while for the steel with Al deposition, this behavior was largely improved. For the SUS304 steel with Al and La depositions, the mass loss became small as compared with the steel with only Al deposit, showing that the cyclic oxidation resistance of the steel with Al and La depositions was extremely high. For this case, it was found that a protective scale consisting of α-Al2O3 was formed on the deposited layer surface.
To clarify the behavior of the formation of boron nitrides in P122 heat resistant steel containing 0.003 mass% B and 0.06 mass% N, the influence of heat treatments, remelting and hot working were investigated by SEM observations on boron nitrides at the fractured surfaces of the steel samples and by the EDS analysis. Boron nitrides start to precipitate at temperatures between 1150 and 1200°C during the cooling process after hot forging or rolling. They agglomerate to a very large size of about 20 to 30 μm at a very slow cooling rate of 100°C/h. However, they only grow to 1 to 3 μm at a medium slow cooling rate and never precipitate at a very fast cooling rate such as in water quenching. The precipitation behavior of boron nitrides has also been found to be affected by the cooling rate after normalizing but not by the microstructure of the steel resulting from casting or forging.
Several kinds of Zr and/or Nb added Fe3Al based intermetallic alloys (i.e., Fe3Al-Zr, Fe3Al-Nb and Fe3Al-Zr-Nb) were arc-melted, homogenized, hot-rolled and then annealed to evaluate microstructure and tensile property at room temperature as well as at a high temperature (873K). After annealing above 1073K, the rolled alloys exhibited a recrystallized microstructure containing coarse Laves phase particles, except for the Nb-added alloy with a minor content of Nb, which showed a single phase microstructure. For the as hot-rolled and recrystallized specimens, relatively high tensile elongation as well as high tensile strength was observed at room temperature in the Zr-added alloys in comparison with those in the Nb-added alloys. Additionally, fracture mode of the Zr-added alloys exhibited intergranular fracture whereas that of the Nb-added alloys showed transgranular cleavage fracture. The recrystallized specimens except for the Nb-added alloys with a minor amount of Nb also showed relatively high tensile strength and elongation at high temperature (873K). These results suggest that tensile ductility as well as strength of Fe3Al-based alloys can be improved by introduction of the Laves phase dispersions. The mechanism responsible for the observed improved ductility (and strength) was discussed in association with the existence of coarse second phase particles.