The activities of MnO and SiO2 in the liquid phase region of the MnO-SiO2-CrO, x ternary system saturated with MnO·Cr2O3 solid solution were determined by conducting the equilibrium experiments between the above-mentioned ternary liquid phase and molten iron under the condition of the oxygen partial pressure, PO2=2×10-6Pa at 1873K. By using the thermodynamic data of the MnO-SiO2-CrOx system determined in the previous and the present studies by the authors, the effect of the amount of Si addition on the equilibrium oxygen concentration in molten stainless steel and the composition of non-metallic inclusions under the various conditions of Si-deoxidation process were calculated with the aid of the thermodynamic data related to molten steel. The behavior of inclusions formed during the process is discussed based on the results obtained.
A method for preheating steel scraps using waste heat in a continuous casting process was proposed. Experiment and numerical analysis were carried out to examine the ability of scrap preheating and to determine the factors affecting the rise in scrap temperature. The commercial FEM software ANSYS was used for simulation of temperature distributions in the scrap. The simulated results agreed well to the experimental ones with the emissivities of heat source and samples of 0.85 and 0.65, respectively. The thickness (Hs) of the scrap sample and the distance (D) between the heat source and sample were found to affect the rise in sample temperature largely. The overall heat transfer can be given by the following dimensionless relation: Nu0=[(5.505D*H*0.75-0.403)Ra0.18t*-1-26.21(D*H*)1/3+9.96]×(1+0.268D*-2.19H*-0.73) A higher preheating efficiency is anticipatable to combine the proposed method with the conventional ways of scrap preheat using waste gas.
Improvement in coke strength against abrasion is needed for a stable operation of blast furnace. Pore structure of coke (e.g. pore size distribution and tortuosity) is important for an improvement of coke abrasion strength, because its effect influences CO2 diffusion coefficient and coke strength after CO2 reaction. In this study, production of coke samples having different pore structure was attempted by blending different size of slightly caking coal to coal blend. The effect of pore structure on CO2 reactivity and abrasion after CO2 reaction was investigated. In case that larger size (2-3 mm) slightly caking coal was added to coal blend and carbonized (Coke A), the pore size distribution of coke became wider and contained larger pore which had higher value of tortuosity than the coke produced by adding smaller size (1-2 mm) slightly caking coal particles (Coke B). Difference in pore structure is considered to be originated by bubble size generated during softening of coal particles. Relationship between pore size and tortuosity is caused by the quantity of bottle-neck type pores in coke. The reactivity of Coke A was higher and the extent of the reaction at inner part of coke lump analyzed by X-ray computerized tomography was larger, compared with the Coke B. These differences are considered to be based on the fact that large pore enhances diffusion of CO2 into inner part of the coke, and that concentration of shear stress easily occurs around large pores.
The reaction behavior of low porosity carbon composite pellet produced by hot briquetting process was examined under various gas atmospheres such as N2, CO and 50vol%CO-50vol%CO2. The reduction reaction was affected largely by atmospheric gas at low temperature, and affected small at high temperature. On the other hand the gasification of coal was affected by not atmospheric gas but temperature. The effect of atmospheric gas on the reaction behavior was explained by variation of pressure in sample during reaction. The reactions in nitrogen gas atmosphere above 1000°C were controlled by the gasification of coal. Therefore, kinetic analysis in nitrogen gas atmosphere was carried out by using the first order reaction model. The temperature dependencies of reaction rate constants for the gasification of coal obtained by kinetic analysis were as follows. kCC=exp(5.51-130×103/RT) kC2=exp(5.67-160×103/RT)
Carbon composite iron ore hot briquet was new material for ironmaking and was made by binder-less briquetting in use of thermal plasticity of coal. Physical and chemical properties of the briquet were low slag rate, low porosity and high crushing strength without curing in comparison with carbon composite iron ore cold bonded pellet. In this paper, application of the briquet to shaft furnace was discussed. In order to clarify reduction and carburization of the briquet on condition of increasing in temperature, reduction experiments were carried out. The following results were obtained. (1) During reduction, crushing strength of the briquet did not decrease in widely difference from cold bonded pellet. (2) In heat-up reduction tests under load, reduction and carburization of the briquet occur in lower temperature in comparison with sinter and iron ore pellet. (3) Using the briquet in blast furnace simulator, temperature of thermal reserve zone decrease and CO gas utilization increase in comparison with sinter. It was expected that reaction efficiency of lumpy zone in blast furnace was improved by using the briquet.
This paper describes an automatic scheduling system for steelmaking process from refining converter to continuous caster. Conventionaldaily scheduling system does not get an optimal solution automatically, but gets a feasible solution with no machine conflicts between processes. It is very difficult to get the optimal solution for the large-scale and complex problem of steelmaking process scheduling, because this scheduling involves particular constraints and varying production requirements of each process. The proposed scheduling procedure consists of optimal method by using modified genetic algorithm (GA). This method applies multi-constraints to the reduction of the search space obtaining feasible solutions. And it can suppress the calculation time. The system has two benefits. One is an extension of the weekly scheduling system from the daily one. The other is an improvement in the quality of the schedule. The cost effectiveness is low waiting time of secondary refining process and low changing of alternative secondary refining process. This algorithm was installed in the scheduling system of NKK Keihin Works and has contributed to improving its efficiency in the steelmaking process.
To obtain the ductile and corrosive coating treatment for steel, the aluminizing treatments using aluminum foil on mild steel surface were performed at diffusion temperature from 700 to 1000°C. The foil aluminized steels had less voids in diffusion layers, than the hot-dip aluminized steels. The foil aluminized steels at diffusion temperature not higher than 800°C had a thinner brittle layer of aluminum rich intermetallic Fe2Al5, and that at diffusion temperature not lower than 900°C had thicker layers of iron rich intermetallics such as FeAl and Fe3Al. It was found that the three body abrasive wear rates of aluminized steels with FeAl and Fe2Al that diffused at not lower than 900°C against free SiC grains were smaller wear rate than those of the steels diffused at higher than 800°C.
The effects of thermomechanical processing in an α+γ region on microstructure and retained austenite characteristics were investigated to enhance the workability limit of a newly developed high strength low alloy TRIP-aided ferrous steel. Ultra fine polygonal ferrite grains of 1 to 2 μm in diameter were formed inside prior martensite lath structure with interlath retained austenite plates or needles when the steels with martensite single phase were subjected to 30-50% reduction at intercritical annealing temperatures between 760 and 800°C, followed by austempering at 375-425°C for 100 s. It was confirmed by EBSP analysis that some of the ultra fine polygonal ferrite grains possessed a large difference in orientation from neighboring grains. Under the same conditions, volume fraction of retained austenite was significantly increased with a small decrease in its carbon concentration, compared to the steel subjected to only heat treatment. The refining of polygonal ferrite grain was principally caused by interlath austenite plates or needles suppressing the grain growth of polygonal ferrite recrystallized just after intercritical reduction.
To elucidate the origin of the onset of accelerating creep in a single crystal nickel-based superalloy, CMSX-4, the correlation between the creep rate during the accelerating creep stage and the thickness of γ channel was investigated by using the single crystals crept at 1273K in a wide stress range of 100-400 MPa. The shape of γ' phase and the thickness of γ channel at the time of the onset of accelerating creep turn to different ones with decreasing the stress. At the stress of 160 MPa, the cuboidal γ' phase turns its shape to a rafted one. While at the stresses less than 160 MPa the rafted γ' phase appears before the onset of accelerating creep. At the stresses higher than 250 MPa, the cuboidal γ' phase still remains. The correlation between the thickness of γ channel, λγ and the creep rate during the accelerating creep stage, ε, was evaluated, and the following equation, ε∝λ4γ is proposed and confirmed in a wide stress range of 100-400 MPa, independent of the shape of γ' phase. In addition, it was also confirmed that the radius of dislocation curvature within γ channel of the crept specimens was proportional to the thickness of γ channel defined by TEM. Consequently, the origin of the onset of accelerating creep in a single crystal nickel-based superalloy was interpreted by the loss of creep resistance due to an increase in the thickness of γ channel, λγ.
A seam welded Cr-Mo steel pipe elbow that had been used as main steam piping at a thermal power plant was investigated. Creep damages at fine grained heat affected zone of the seam weld, called as type IV damages, were observed. Hook-shaped microcracks surrounded by many polygonal hollows were observed by optical microscopy and scanning electron microscopy. Those polygonal hollows were seemed to form the same hook-shaped lines as microcracks. Fine bainite grains surrounded by coarse carbides and creep cavities were also lined in the same manner as the hollows. From the appearances of the hook, it was inferred that these damages were originated locally at the former austenite grain boundaries. Dense precipitations of carbides (DPCs) were found on extraction replicas by transmission electron microscopy. At the beginning of the usage, there might be high-carbon-content fine bainite grains and DPCs were created in these grains by aging. Because of the similarities in shapes, sizes and distributions, DPCs were regarded as root causes of the hollows and the cracks. Many creep cavities would appear at DPCs and fine bainite grains would easily be isolated. As DPCs line on the former austenite grain boundaries, exfoliated fine bainite grain boundaries would link each other to form a microcrack. In order to suppress type IV damage, it would be effective to avoid the origination of fine bainite with high-carbon-content grains on the former austenite grain boundaries. Adding normalizing twice or more to the heat treatment process for the plate materials before welding is proposed.