A fundamental study on some aspects of nonisothermal reduction of iron ore by both solid and gaseous reductions has been carried out. The aim is two fold: (a) to develop appropriate mathematical procedure for the analyses of nonisothermal kinetic data and evaluation of kinetic parameters and (b) to develop a theory for kinetic studies of reduction under fluctuating but measured temperature. These theories have been tested against actual experimental results on reduction of iron ore by solid and gaseous reductants. A mathematical procedure conveniently amenable to computation is described which can be used to predict the course of reaction under fluctuating temperature conditions.
High N-20Cr-10Ni stainless steels in the range of 0.25-0.7%N were melted by a high pressure induction furnace. These molten metals were deoxidized and desulfurized by calcium after pre-deoxidation by aluminum. It is concluded that the calcium addition is one of the most promising methods for desulfurization and deoxidation in the production of high nitrogen stainless steel by high pressure melting method. The interaction parameters eOCa and eSCa are obtained according to the Gustafsson's method, of which values are nearly equal to those proposed by Gustafsson. The equilibrium contents among Ca, S, O and Al components are computed using the apparent equilibrium constants and the interaction parameters obtained in the present study. The influence of aluminum on the calcium deoxidation and desulfurization is large, and therefore, pre-deoxidation by aluminum is important for the deoxidizing and desulfurizing process of calcium.
The reduction, carburization, and melt-down properties at high temperatures of pellets and sinter in packed beds were studied using a quenching method. The results obtained are summarized as follows: (1) Reduction retardation of pellets is attributed to the disappearance of the micropores caused by sintering of iron under the presence of a liquid exuded from the wustite core. (2) Carburization proceeds mainly through the diffusion of carbon from the iron surface, brought about by melting reduction, directly contacting solid carbon. (3) Improved pellet reduction retardation was brought about by adjusting the chemical composition of the core part to have a higher solidus temperature. The melt-down property was ameliorated by increasing the basicity of the entire pellet.
Formation behavior of coke non-packed regions (free space) in a blast furnace hearth was analyzed using a two-dimensional cold model of a blast furnace. Based on the obtained results, the influence of the free space shape, the packed structure of dead-man and the depth of a taphole on the hot metal flow and the heat transfer in the blast furnace hearth was examined by numerical calculation. The results obtained are summarized as follows; (1) An upward motion of hearth coke towards the raceway occurs due to the rising of stored molten iron level. This motion results in formation of free space at the corner of a hearth. (2) The shape of free space and the packed structure of dead-man significantly affect the hot metal flow and the heat transfer in a hearth. (3) Circumferential flow in a hearth is caused by the presence of free space. As a result, refractories opposite to a taphole in the corner of the hearth bottom are subjected to significant heat loads. (4) If the dimensionless depth of a taphole (I/R (I; depth of a taphole from inner wall of a hearth, R; hearth radius)) is 0.33 which is almost equal to that in usual blast furnaces, refractories at an angle of 30° with a taphole horizontally are subjected to significant heat loads.
In the absence of static recrystallization, dynamic recrystallization is the principal softening mechanism operating during strip rolling. This process takes place when the interpass time is short, there is little strain-induced precipitation, and the presence of alloying elements in solution retards static recrystallization. It can lead to austenite grain sizes below 5 μm and ferrite grain sizes of about 3 μm when cooling is carried out at 10°C/s. Increasing the roughing-to-finishing delay time or the delay time between successive passes leads to an increase in the density of Nb(CN) precipitates, which in turn promotes the formation of pancaked austenite. When the latter structure is cooled, 7 μm ferrite grain sizes are produced, which are coarser than the ferrite structures obtained from dynamically recrystallized austenite rolled over the same temperature range.
The character and amount of elastic strains created by the f.c.c. to b.c.c. martensitic transformation of small Fe-Co precipitate particles in a Cu matrix were investigated by analyzing moiré fringes in a transmission electron microscope. Taking account of the internally-twinned microstructure of the transformed particles, theoretical evaluation of the elastic strains was made by applying the equivalent inclusion method in micromechanics. From reasonable agreement between the experimentally observed elastic strains and theoretically evaluated ones, it has been concluded that although the internal twinning can partly accommodate the transformation strains, considerable amounts of elastic strains still remain in the as-transformed spherical martensite. This is in contrast to the internally-twinned thin-plate martensite where the accommodation of the transformation strains is considered to be nearly perfect.
Welding of comparatively thick (4 mm) C-Mn-Cr-Mo dual phase steel has been carried out by resistance spot welding process. Weldability of this steel has been studied by varying the electrode force and the primary welding parameters affecting the heat input such as the effective current and weld time. The influence of these welding parameters on the morphology, microhardness and the tensile shear strength of the weldment are investigated. Optimum welding parameters producing maximum joint strength are established as electrode force of 615 kg, effective current of 6 kA and weld time of 80 cycle. Weakening of weldment caused by excess tempering of martensite at the outer region of HAZ was not observed in the range of optimum welding conditions.
A large number of steel plates is currently produced by HCR (Hot Charge Rolling) process. In this study, the effect of HCR condition on the mechanical properties of Nb bearing steel is investigated. If the charging temperature is above Ar1, the toughness of Nb bearing steel through HCR process is inferior to conventionally rolled one. This phenomenon comes from the larger amount of Nb in solid solution in the HCR process than in the CCR (Cold Charged Rolling) process. This Nb in solid solution markedly retards the recrystallization of austenite and then it precipitates finely as Nb(CN) after rolling in the HCR process. To improve the balance between the tensile strength and ductile-brittle transition temperature in Nb bearing steel at a charging temperature above Ar1, 1100°C is the most favorable for the rolling start temperature, because the finest austenite grain is obtained at this temperature range due to the homogeneous recrystallization.
With ternary Ni-40at%Co-X alloys the solid-solution hardening (ssh) due to solutes X of transition-metal group (TM) elements like Ta and Nb and of B-subgroup (B) ones like Sn and Al was investigated. The results were compared with the previous study on the ssh of pure Ni. The value of ssh, Δσ, for a solute X in the Ni-Co alloy is greater than that for X in pure Ni. This increase in Δσ, however, is not explicable in terms of the changes in size-misfit and shear modulus with alloying Co in Ni solvent because these changes are not so great. Instead, for the ssh normalized by the size-misfit, there found a trend of it being greater in the order of the alloy group (A) of Ni-X making a continuous solid solution, the group (B) of Ni-X having a solid solubility, and the group (C) of Ni-Co-X having a smaller solid solubility than that for the group B. This suggests that the increase in the normalized ssh for the Ni-Co-X alloy might be deeply related with the reduction in solid solubility due to the addition of Co. A probable reason for this has been discussed.
This study was carried out to clarify the properties and antioxidation mechanisms of newly developed oxidation inhibitor consisting of refractory powder-SiO2-Si-SiC-synthetic mica-colloidal silica-surface active agent and caking bond. By applying this type of oxidation inhibitor to grain-oriented silicon steel, it was possible to achieve significant reductions in scaling and production of fayalite-based slag, thereby substantially increasing yield. During heating, Al2O3 is formed as a result of the decomposition by Si (metallic silicon) of mullite (3Al2O3·2SiO2) contained in refractory powder. Furthermore, the fine SiC powder is oxidized and changes gradually to protective cristobalite-SiO2(C-SiO2) layer which acts as an excellent barrier to oxygen diffusion from atmosphere. The protective C-SiO2 is not formed from the C-SiO2 which is added initially in the oxidation inhibitor but is newly formed through the oxidation process of the SiC. On the other hand, Al2O3 which is formed by the decomposition of mullite becomes Al2O3·SiO2 in combination with SiO2. On the steel surface, however, it becomes highly protective FeO·Al2O3 or 3FeO·Al2O3·3SiO2 layer and, at the same time, it prevents the formation of low melting point material such as fayalite (2FeO·SiO2). It has been clarified that this oxidation inhibitor exhibits the excellent antioxidability due to superposed effect of above-mentioned reactions.