Recently a new process has been developed to control the γ-α transformation of steels using MnS precipitates on the oxide inclusions as nuclei. In order to optimize the composition of inclusions, phase relations for the MnO-SiO2-MnS system have been investigated at temperatures ranging from 1473K to 1723K. The solubility of MnS increased with an increase in MnO/ (MnO+SiO2) ratio and showed a drastic increase at the ratio of 0.6, which was consistent with the maximum precipitation ratio of MnS in the oxide metallurgy process. An oxysulfide compound composed of 8MnO·3SiO2·MnS was newly found as a stable solid phase at lower temperature than 1573K. It has been considered that this phase is formed via the peritectic reaction between 2MnO·SiO2 and liquid phase. Deliberating the compositions of liquids saturated with various compounds at several temperatures, the phase diagram for the MnO-SiO2-MnS system has been estimated. Finally, the mechanism of the MnS precipitation in the oxide inclusions has been discussed.
Hot model experiments were carried out to clarify the combustion behavior in the raceway and the gas flow changes nearby the raceway at the massive coal injection into the blast furnace. From the experimental results, it was found that the location of maximum temperature in the raceway agrees with the peak position of the CO2 gas composition and it tends to move toward the tuyere when the amount of volatile matter carried by the injected coal increases. The combustion efficiency was influenced by the volatile matter content of the injected coal and the oxygen enrichement had little effect on the combustion efficiency. The heat load to the wall of the hot model increased by the approach of the maximum temperature position toward the tuyere and it decreased by the reduction of ash content in the coal. It was observed that the shell layer was formed at the depth of the raceway. Then, it was estimated that the peripheral gas flow was intensified and the heat load to the wall increased due to the formation of the shell layer. These tendencies were confirmed by the mathematical simulation on the gas flow.
A new method for separating inclusions from a molten metal by using a traveling magnetic field is proposed. A bundle of thin tubes for flowing molten metal is set between two linear induction motors which generates a transverse magnetic field traveling in the direction perpendicular to the molten melt flow. As the flow driven by the electromagnetic force which is induced to the traveling direction of magnetic field is constrained by the tube wall, inclusions move toward the tube wall opposite to the direction of the electromagnetic force. Two mathematical models to evaluate the electromagnetic force acting on inclusions were developed. The analytical formulations of the electromagnetic force in both the models were derived as a function of the design parameters of induction motors and the operating variables. The elimination principle of inclusions proposed here was confirmed in the experiment in a Al-Al2O3 system. A feasibility study on the application of the method to the elimination of inclusions from molten steel was performed.
Studies have been made on bubble formation at non-wetted nozzles and orifices and on the effect of bubble size on the rate of inclusion removal from molten metal. The bubble formation at a nozzle or an orifice in mercury has been investigated. The experimental values of size of the bubble formed at a nozzle agree with those calculated from correlating equations using outer diameter of the nozzle. In the case of lower gas flow rate, the bubble size increases with increasing outer diameter of the nozzle (dno<7mm). At nozzle diameter of dno>7mm, the bubble size is independent of dno, and the nozzle can be regarded as an orifice. The bubble size increases with increasing gas-chamber volume under the conditions where the gas flow rate is relatively low and the dimensionless gas-chamber volume, Nc', is larger than unity. The rate of SiO2 inclusion removal from molten Cu to slag with injecting Ar gas from multi-orifice and multi-nozzle has been examined. In the case of multi-orifice, the bubble size increases with increasing the number of orifices. This shows that the growing bubbles coalesce into one on the surface of the orifice plate. The rate of inclusion removal decreases with increasing bubble diameter. It is confirmed that the inclusion particles are removed from the melt mainly through adhesion to the bubble-metal interface.
Melting behavior of mold powder is very important for continuous casting operation and quality of steel. In this experimental study, the shrinking behavior of mold powder was measured by thermo-mechanical analysis in order to investigate the melting behavior of mold power. Thermodynamic calculation of the liquid fraction in the mold powder appears to show reasonable agreement with thermo-mechanical analysis data. Then, it may be concluded that thermo-mechanical analysis enables us to give useful information about the melting behavior of mold powder. The results of the thermo-mechanical analysis are summerized as follows; Fluorite is considered to be better fluorite source than sodium fluoride, sodium fluoride is better than soda ash as Na2O source and silica powder has a superiority over diatomaceous earth as SiO2 source.
As basic research to quantify the oxidation rate of molten steel in tundish, oxidation experiments were performed on nonkilled molten steel and aluminum-killed molten steel, and the effect of stirring on the oxidation rate was studied. The oxidation rate of the nonkilled molten steel by air is not affected by stirring and is controlled by diffusion of oxygen gas in the gas phase. Stirring changes the effect of the oxide film in the oxidation process of the aluminum-killed molten steel by air. The oxidation rate of the aluminum-killed molten steel in the still state is controlled by the diffusion of solute oxygen in the oxide film, while the oxidation rate of the aluminum-killed molten steel in the stirred state is controlled by the diffusion of oxygen gas in the gas phase.
To make clear the lubrication mechanism in the mold of continuous casting of steel, unsteady state friction analysis has been performed with the special attention to visco-elastic behaviour of the mold flux. The mold flux behaves as Maxwell type fluid. Elasticity of 10 Pa and 50 Pa are obtained at viscosity of 0.28 Pa·s and 2.3 Pa·s respectively. The difference between the obtained friction force and the predicted one as Newtonian fluid, and the phase shift of maximum friction peak become significant when the oscillation frequency exceeds 200 cpm. These transitions can be explained quantitatively by the introduction of elasticity to the mold powder characteristics.
The effect of rolls made of iron-chromium-carbon alloys and high speed steels on the lubrication characteristics are examined by rolling annealed low carbon steel sheets. Coefficients of friction for reduction in thickness and limit reductions of friction pick up are measured using the simulation testing machine of sliding-rolling type. The relation between the experimental results on the limit reductions on the one hand and carbide type, carbide area and carbide morphology of roll materials on the other is discussed. The conclusion obtained are as follows ; (1) The difference of coefficient of friction among roll materials is small in case that surface roughness of roll is constant. (2) The limit reduction of friction pick up has a strong correlation with the carbide mean spacing of roll materials. (3) Controlling the carbide spacing small is effective for prevention of friction pick up.
Effect of heat treatment in the temperature range from 200 to 550°C on sliding wear resistance of Ni-P electroplated Ti-6Al-4V has been investigated. The wear mechanism has also been discussed in terms of the film structure and the variations of hardness, toughness and internal stress of the plating films by the heat treatment. The wear resistance was influenced by the heat treatment. When the plating film was hardened from Hv700 (as plated) to approximately Hv1100 by the heat treatment at 350°C for 1h, the wear resistance slightly deteriorated. On the other hand, when the plating films were softened to below Hv800 by the heat treatment over 450°C, the wear resistance significantly improved. These behaviors were not in good agreement with the variations of the hardness nor internal stress of the films but with the number of the cracks which were microscopically observed on the wear tracks of the plating surfaces after the sliding wear test. As the number of the cracks decreased, the wear resistance increased. It is thought that the susceptibility to cracking relates to the toughness of the plating film, and that the film with good toughness obtained at temperature over 450°C is derived from the formation of soft Ni matrix due to the precipitation and growth of Ni3P phase by heat treatment. In conclusion, the sliding wear resistance of the Ni-P electroplated Ti-6Al-4V is considered to be largely governed by the mechanical property related to toughness of the plating films.
Quantitative analyses of crystalline compositions have been performed on the rusts which formed on the outer panel surface and in the crevice of the lapped portion on vehicles used in North America. X-ray diffraction using the internal standard technique was employed to analyze the rust composition quantitatively. Characteristic regions of the rust compositions were found in the α-FeOOH/(Fe3O4+γ-FeOOH)/(β-FeOOH+amorphous rust) ternary diagram that we proposed in the present study. The rust from the crevice in the lapped portion contained more (γ-FeOOH+Fe3O4) than that from the outer steel panels. The composition of rusts which formed in the accelerated corrosion tests indicated that the high content of (Fe3O4+γ-FeOOH) in the crevice rust of the lapped portion is caused by the high wetness. The rust on galvanized parts was mostly amorphous at the initial corrosion stage and then β-FeOOH in crystalline appeared characteristically. The composition of iron rust on galvanized parts moves towards the regions of steel parts at the following stage. A ternary diagram that we proposed in the present study is useful in correlating the results obtained in accelerated corrosion tests with those found on automobiles under service conditions.
The effect of the γ'-phase volume-fraction on the corrosion resistance was investigated of nickel-based superalloys in a Na2SO4-25wt%NaCL salt. Both Ni-Al-Cr-Ti-Ta-W system and Ni-Al-Cr-Ti-Ta-W-Mo system were used in this study and their compositions lay on a constant γ-γ' tie line. The corrosion resistance was evaluated by means of the coated test, the immersion test and the thermobalance test, and the corrosion products were identified by the EPMA analysis and the conventional X-ray diffraction method. In addition, morphological changes of the γ and the γ' phases at the interface between the salt and the base alloy were observed by scanning electron microscopy (SEM). It was found that the corrosion resistance did not change with the γ' volume-fractions in the alloys as far as they were positioned on the same γ-γ' tie-line. Furthermore, some kinds of oxides and sulfides were formed as the corrosion products in every alloy. The SEM observation of the corrosion interface indicated that a single phase region having a composition similar to that of the γ phase was produced underneath the corrosion products. This region is considered to be formed as the result of fading away the γ'-former elements such as Al and Ti from the surface.
For the development of a high Mn-Cr austenitic steel as one of the heat resisting materials, the present authors have investigated the high temperature strength, toughness and mirostructural phase instability of 12%Cr-15%Mn steels. High content of carbon (0.1 to 0.2 mass%C) formed the large amount of coarse precipitates of the type M23C6 carbide on grain boundaries and within grains after a long-term aging. These precipitates caused the intergranular and/or quasi-cleavage fracture and deteriorated the toughness. It was clarified that, for the improvement of the toughess, the lowering of carbon content to 0.02mass%C was necessary. The toughness and high temperature strength of the 12% Cr-15%Mn steels were compared and discussed with those of the ordinary SUS316 steel. Creep-rupture strength of the present materials was also compared with that of the TENELON which had been developed by Brady et al (US steel) as a high Mn austenitic heat resisting steel. The creep-rupture strength of the 12%Cr-15%Mn steel was superior to those of the type 316 steel and the TENELON.
Soft temper tinplate produced by continuous annealing from low carbon Alkilled steel would be sometimes faced with fluting trouble due to strain aging by solute carbon in product sheets. Temper rolling of high reduction could improve fluting resistance of tinplate, but would work-harden it too much for soft temper tinplate. In this work, the effect of temper rolling and overaging on the balance between hardness and fluting resistance of products was investigated. It was experimentally shown that the softness of temper T-3 in product and the acceptable fluting resistance after baking could be compatible in the continuous annealed sheets, the carbon content in solution in which sheets was decreased enough by long time (120s) overaging, by using high reduction temper rolling. This is because tinplate with long time (120s) overaging includes half amount of solute carbon, of that with short time (60s) overaging, after overaging to show short yield elongation after baking. It is considered that all amount of solute carbon in the annealed sheet contributes to the pinning of dislocations after baking, and that it has formed precipitates on dislocations, not atmosphere any longer.