The possibility of direct reduction of TiO2 by molten magnesium has been investigated using TiO2 single crystals and powders. First, reduction experiments were carried out using TiO2 single crystals at 1173 K for up to 8 h, resulting in formation of two layers consisting of oxide phases with different Mg and Ti concentrations between magnesium and TiO2, in which layers TiO2 was reduced only to TiO0.5 as a maximum and there was no metallic titanium obtained. The thicknesses of both the oxide layers increased according to the parabolic rate law, and Mg is reported to have diffusivity greater than the others, suggesting that the reduction rate of TiO2 is controlled by the diffusion of Mg across the oxide layers. Second, reduction experiments were carried out using TiO2 powers having 45 and 100 μm diameters at 1173 K for up to 48 h, resulting in reduction of TiO2 to form metallic titanium, TiO0.5 and TiO, indicating the effect of change in figuration of TiO2 from single crystal to powder. However, difference in powder size did not appear in the degree of reduction presumably due to sintering effect during reduction. Finally, reduction experiments were carried out using TiO2 powers having 45 μm diameters with additions of MgCl2 at 1173 K for 30 h because MgCl2 is miscible with MgO. TiO2 was reduced to form only metallic titanium and TiO0.5, without TiO remaining as titanium oxide, suggesting the removal effect of MgO by MgCl2 which may allow continuous reduction on the TiO2 surface. Consequently, it has been found that additions of MgCl2 are effective to promote reduction of TiO2 by molten magnesium.
The viscosity of slag is a very important factor for blast furnace operation. For instance, the viscosity of slag exerts a big influence on the liquidity of slag in the hearth, the tapping of pig iron and slag, softening, shrinkage, melting down, permeability in the cohesive zone, and permeability in the deadman. In addition, the viscosity of slag itself is affected by temperature and composition changes when the slag is formed. Furthermore, the fact that slag viscosity is greatly increased by the deposition of coke powder and the solid phase is well known. However models that consider such effects are few. Therefore, we have developed an estimation model that takes into account the increase in the viscosity of slag due to the deposition of the solid phase. The calculation result of the model was in good agreement with the measured results. As an example application of the estimation formula, we have examined the melting point and viscosity of the slag of quaternary Al2O3-SiO2-CaO-MgO along with slag containing FeO.
In the recent operation of blast furnace, it is supposed that high gas permeability of burden is important for low RAR and high PCR operation. In this work, sinter quality for improvement in gas permeability of blast furnace was investigated with reduction degradation and under-load-reduction tests. As the results, the reduction degradation of sinter is deteriorated by increasing H2 concentration in the reduction gas under the condition of below 3.8vol% H2. However, over 3.8vol% H2, increase of H2 has no effect on the reduction degradation because the diffusion of reduction gas in the sinter is limited. On the other hand, from the under-load-reduction test, there is possibility that increase in H2 concentration of reduction gas and decrease in slag ratio in sinter are effective to improve gas permeability of lower part of blast furnace rather than reducibility of sinter. Due to adoption of these experimental results to a 2-dimentional mathematical simulation model, the precision of pressure drop calculation of blast furnace was improved. It is considered from the evaluation by this model calculation that the RDI, a slag ratio and the slag viscosity as the sinter properties are greatly influence on the permeability of blast furnace.
In order to improve the utilization of CaO based flux for hot metal desulfurization with mechanical stirring, the effect of a bottom inclination vessel on the hot metal desulfurization reaction was investigated in experiments with a water model and 70kg-scale induction furnace. In the water model experiments, it was observed that the vortex generated by impeller rotation was inclined when using the bottom inclination vessel. The number of particles entrained into the water during stirring in the bottom inclination vessel increased by twice compared with that in the normal vessel. In the hot model experiments, the desulfurization rate during 0-120 seconds after flux top addition increased by 3 times when using the bottom inclination vessel compared with that with the normal vessel. In the bottom inclination vessel, the estimated diameter of desulfurization flux entrained into the hot metal was half of that with the normal vessel. The change in the water and hot metal flows was investigated by numerical simulation. In the bottom inclination vessel, the vertical downward flow velocity increased locally at the fluid-air interface. It is considered that entrainment of desulfurization flux into hot metal can be improved by this higher vertical downward velocity. The bottom inclination vessel was tested in a commercial scale (350t) hot metal desulfurization vessel. Utilization of CaO based flux for desulfurization with the bottom inclination vessel increased by 1.25 times compared with the normal vessel.
New welding method (F-MAG) based on CO2 gas-shielded arc welding method (MAG) was developed to increase deposition rate of weld metal. In F-MAG, filler wire (hot-wire) heated using direct-current was put into rear part of weld-pool formed with main electrode used in MAG. Multi-layered weld metals were prepared using both F-MAG and MAG. Microstructures of both as-weld and reheated zones in uppermost layer of the multi-layered weld metals were examined and the effects of microstructures on mechanical properties were analyzed. Both the as-weld and reheated zones of weld metal formed with F-MAG consisted of acicular ferrite (AF) and/or equiaxed ferrite and so on. Both strength and elongation in as-weld and reheated zones formed with F-MAG were superior compared with those formed with MAG. It could be suggested that strength increased by refinement of AF due to increase in the concentration of alloy elements being contained in hot-wire. Larger elongation of weld metal in F-MAG compared with that in MAG could be analyzed in terms of the aggregate of AFs with almost parallel slip systems between neighboring AFs, hereafter referred as to ALPS. The number of AFs contained in an ALPS formed in weld metal formed with F-MAG is larger than that with MAG, in spite of the sizes of ALPSs in weld metals formed with MAG and F-MAG being almost same between them. Deformation occurs over a lot of AFs in the case of finer AF through the rotation of tensile test piece during deformation.
The toughness and the resistance to delayed fracture are deteriorated with increasing strength of martenstic steels. It has been reported that these properties could be improved by grain refinement of martensite. In this study, the influence of both hot stamping conditions such as heating rate, heating temperature, cooling rate, and material conditions such as hot band microstructure, cold rolling reduction, addition of micro-alloys on the grain refinement of hot stamped ultra-high strength martenstic steel components are investigated. The investigation revealed that low temperature and short time heating, high heating and cooling rates, the decrease in the coiling temperature of hot bands, the increase in cold rolling reduction and addition of micro-alloys are preferable for grain refinement of martensite. The mechanism of the grain refinement was discussed from the viewpoints of the increase in the nucleation rate of austenite during the α→γ transformation and the suppression of the austenite grain growth.
In order to clarify the dominant factors controlling stretch flange ability of hot-rolled high-strength steel sheets, a model to predict crack growth under plastic strain gradient beneath pierced edge was constructed based on fracture mechanics. The model predicts crack growth by equating crack growth driving force Japp which is numerically calculated, and crack growth resistance Jmat which is obtained by estimating strain distribution under pierced edge introduced by piercing and has taken into consideration the influence of pre-strain on crack growth resistance. The model could predict actual crack growth on pierced edge at acceptable accuracy. The calculation using the model suggests that stretch flange ability of high-strength steel sheets can be improved by raising crack growth resistance of the material, or reducing strain due to piercing.
Gigacycle fatigue tests were conducted for hydrogen charged SCM440 low-alloy and SUJ2 bearing steels prepared with a double-melting method to improve their cleanliness. In case of the SCM440 steel, 200 and 550ºC tempered samples were prepared. The fatigue tests were conducted mostly using an ultrasonic fatigue testing machine. In case of the SUJ2 steel and the 200ºC tempered version of the SCM440 steel, fish-eye fracture occurred both in base and hydrogen charged steels, and the hydrogen charged steels showed large drops of fatigue strength. The 550ºC tempered version of the SCM440 steel revealed only surface fracture below 107 cycles and the fatigue strength was almost equal between the base and hydrogen charged steels, while the hydrogen charged steel showed fish-eye fracture above 108 cycles, decreasing the fatigue strength. These results meant that the effects of hydrogen were large on the fish-eye fracture properties, while they were small on the surface fracture properties. The degradation of fatigue strength was caused by diffusible hydrogen absorbed in the matrix. Although the SUJ2 steel revealed non-diffusible hydrogen trapped by insoluble carbides, the fatigue strength of the SUJ2 steel was almost equal to that of the 200ºC tempered version of the SCM440 steel. This meant that the non-diffusible hydrogen trapped by insoluble carbides had negligible effects on the fatigue strength.