Porous iron oxide pellets prepared from the pulverized Brazilian ore, high quality hematite, were reduced with H2-H2O mixtures in the temperature range from 800°C to 1100°C and the rate of reduction was measured by a thermobalance. Cross sections of partially reduced pellets were examined under the microscope and poresize distributions were measured. The microscopic observation of the cross sections showed that the reduction from hematite to wustite had finished in the early stage of overall reduction period and the reduction from wustite to iron proceeded in the topochemical fashion. On the assumption that the region in more than 30% reduction in the reduction curve of pellet was correspond to the latter step in the reduction reaction, the process was analysed on the basis of the mixed control mechanism, in which the interface chemical reaction, gaseous diffusion in the particle and surface film, and the dilution effects of produced gas were taken into account. The chemical reaction constant kc and the intraparticle diffusivity De for the reduction in the temperature range from 900°C to 1100°C were obtained. The apperent activation energies of kc and De were 28.0 and 17.2kcal/mol, respectively. These large values are attributed to the high porosity of the pellets and to the large temperature dependence of physical properLies of reduced pellets, such as pore size and iron grain.
During solidification of rimmed steel, solutes are rejected at solid-liquid interface, causing deoxidation reactions to form CO, MnO, FeO and SiO2. Mechanism of the reactions is expressed by a mathematical formula, which could be applied to the C and Mn segregations in a rim zone and compositions of nonmetallic inclusions respectively. The amount of gas generated during solidification and that of oxygen absorbed from air are calculated by mass balance of solutes in several ingots. The amount of oxygen absorbtion reaches to 0.02kg/t·min, which gives a strong effect on CO generation. Therefore gas generation from a capped steel is far less than that of a rimmed steel. The gas generation is considered to be caused by two different sources (the one from the inside of diffusion boundary layer and the other from the outside of it), and the ratio of the former to the latter would be 1: 2-2. In a more accurate study on a mechanism of bubble growth during the solidification of rimmed steel, much more attention should be paid to the gas generation from the outside of the boundary layer rather than from the inside.
A study was carried out on the deformation of oxide inclusions in various 18-8 stainless steels containing 0.01-4.91% Si and 0.42-1.92% Mn. The changes of inclusions from amorphous to crystalline by hot working or heat treatment were discussed. The results are as follows: (1) When the content of manganese in steels is higher, the degree of deformation of inclusion increases with the silicon content, because most of the inclusions changes from MnO-Cr2O3 to mangano-silicate with increasing silicon content. In the case of lower manganese content in steel, however, the degree of deformation of inclusion is not so largely varied within the silicon content range of this experiment. (2) The inclusions in the as cast specimens containing about 1.0% Si and 0.5% Mn, is mainly amorphous, but are changed to spinel type MnO-Cr2O3 by heat treatment or hot working. (3) X-ray diffraction indicates that the inclusions in the as cast specimens containing about 1.0% Mn and 1.0% Si or about 1.8% Mn and 1.0% Si, are changed from an amorphous state to rhodonite (MnO-SiO2) by hot working or heat treatment. (4) As the chemical compositions of inclusions remain constant through their changes from amorphous states to crystalline one, these changes are considered to be due mainly to the crystallization of inclusions by heating.
The “step cooling method” was studied. In this method, the “water cooling” and the “natural cooling” were repeated alternately. The results obtained are as follows: 1. The method can prevent the bending of steel products caused by the forced water cooling. 2. With this method, one can control the cooling effect. 3. And the nozzel troubles by interruption are decreased. 4. Other conditions being the same, the total amount of the cooling water, necessary to cool the steel products for a definite temperature difference, is proportional to its flow rate. 5. With equal duration of “water cooling” and “natural cooling” the total time to cool the steel products for a definite temperature difference, depends on their repeating period.
The polarizing characteristics of synthesized carbides, Fe3C, Cr3C2, Cr23C6, M23C6, M6C, WC, V4C Nb4C3, Mo2C, ZrC, and TiC, were studied by means of potentiostat in order to determine the optimum condition for the electrolytic extraction of carbides from steels. From the results of polarization curves and natural electrode potentials of iron and carbides, some useful informations were obtained as to the electrolyte and the electrolytic potential for extraction. Moreover, the effects of various factors on polarizing characteristics were considered, that is, of pH and temperature of electrolyte, atmosphere, and kind, composition and form of carbides.
A new simple and reliable anodic dissolution technique using NaCl-EDTA (ethylenediaminetetraacetic acid) solution as an electrolyte has been developed for isolation of precipitates and inclusions from various steels.<BRThe steel sample covered with close-texture filter paper as a diaphragm and connected as an anode, is dissolved into 100 to 250ml of 1% NaCl-5% EDTA electrolyte (pH6-7) at a current density of 50mA/cm2 for 2 to 4hr. A residue of carbide, nitride, oxide and other constituent can be collected in the filter paper, and is separated and analyzed by a chemical procedure as directed in previous papers 1) 2). To the electrolyte, HNO3, HClO4 and H2SO4 are added and subsequently, it is evaporated to dense white fumes, and then the element as solid solution is determined by a chemical procedure similar to the above. NaCl-EDTA solution as an electrolyte gives good results than various electrolytes reported previously. Quantitative recovery of precipitates and inclusions from steel is obtained by using the NaCl-EDTA electrolyte. The method is applicable to the electrolytic isolation of precipitates and inclusions from low carbon steel, high carbon steel, low alloy steel and high alloy steel such as stainless steel with the same electrolytic condition.