Using a thermobalance, an industrial hematite pellet was reacted with pressurized H2-CO gas mixtures at temperatures of 9731073K to produce more effectively an iron carbide pellet. H2S of low pressures unable to form FeS was added to the mixtures not to precipitate soot. Gas pressurization to 5 atm enhanced iron carbidization rates a few times more than those under 1 atm. This carbidization occurred as soon as metallic iron was formed on the outer surface of pellet even when unreduced iron oxides remained inside, finally leading iron percarbide χ(FeχC, Monoclinic), not θ(Fe3C, Orthorhombic) as final products. During a moderate period, 4 phases of θ, Fe, FeO, and Fe3O4 existed macro-topochemically in turn from the outer layer. The lower temperature and the more gas pressure, the higher the iron carbidization rates. The minimum sulfur content in carbides was as low as 0.012 mass% S. Carbidization rates of a reduced iron pellet were analysed by an unreacted core model to investigate this process.
A new concept of burden distribution control and charging system have been developed for a stable operation under the high productivity, high pulverized coal rate and high small sized ratio operation condition. A safety factor analysis based on the soil mechanics was carried out. Reduction of charging weight at a unit area and inclination angle are effective to avoid the layer collapse. The fluidization index indicates that the optimization of particle diameter at the wall is necessary for the stability of burden at the lumpy zone and the dropping zone. According to these results, multi batch charging, flat burden surface profile and vertical falling trajectory were proposed for a new charging system. Stabilizer, multi-paralleled bunker and high speed rotation chute were applied to No.6 blast furnace at Chiba Works to realize the new charging system. To improve the controllability of the burden distribution, new methods to calculate surface profile and falling trajectory were incorporated to No.6 blast furnace burden distribution simulator.
Chiba No. 6 blast furnace was blown out after the world record of service life, twenty years and nine months. During the blowing out operation, tracer was injected from the No. 23 tuyere. Further, metal, coke and slag in the lower part of the furnace were gathered and analysed to investigate the metal flow at the hearth bottom, and the protection mechanism of the hearth brick. As a result, a stable solidified layer of hot metal formed during most of the operation period has played a key role to prevent the hearth brick wear. The formation of the solidified layer is explained by the stagnant metal flow which is caused by the "low permeability zone".
To improve the sintering productivity and yield, the effects of the magnetic force produced by magnets equipped under the plate feeder on the structure of the sintering bed and sintering performance were investigated by the laboratory charging experiments and sintering tests. Introducing the magnetic force on the sinter mixture during charging, void fraction in the sintering bed was increased with the reduction of falling velocity caused by the increase of the friction of particles against the plate. Moreover return fine and mill scale, which improved the melt fluidity, were segregated toward the upper layer by changing those trajectory. As these results, the sintering productivity and the sinter yield were increased. From the above findings, a new type feeder was developed and applied to Mizushima No. 3 sinter plant, confirming the principle of the method.
In order to get high productivity on continuous casting process, slab must be cast on higher speed. But high speed cast slab has caused defects on cold rolled sheet owing to the powder used in the mold. In case of producing tin plate sheet by slabs cast under the condition of high speed, the defect which contain composition of the mold powder has been found and named as white spot defect. In this paper, formation mechanism of white spot defect is discussed based on an analysis of this defect and investigation of the operational conditions. White spot defects reappear in the laboratory experiment and formation mechanism presumed in this paper proves to be right. The results are summarized as follows; (1) White spot defects on tin plate sheet are consisted of inclusions which include the mold powder of continuous casting process. (2) The formation mechanism is corrosion of tin plate sheet caused by NaOH. This NaOH is formed by chemical reaction between Na of mold powder and water which invaded into the inclusion during tin plating process. (3) White spot defect can be decreased by using mold powder containing less amount of Na on continuous casting process.
In the production of cast slab for high grade sheet steel, the powder in continuous casting mold is entrained into the molten steel and caused the surface defects in the products. The typical example is the white spot defect for tin plate steel. For preventing the white spot defect, new continuous casting powders are developed. Those powders are consist of CaO-SiO2-Al2O3-MgO-CaF2-AlF3 system and less Na content. The results of study in laboratory and casting in plant are summarized as follows. (1) By replacing Na materials with the fluorides of alkali-earth metal, the improved powder can be produced. (2) When Na in powder is less than 1 mass percent, the occurrence of white spot defect of the tin plate steel produced by high-speed casting is remarkably decreased. (3) Characteristics of the improved powder are as follows. (1) The powder is designed of higher viscosity and surface tension. (2) The crystallization temperature is lower. (3) The variation in viscosity due to Al2O3 pick up during casting is less.
A new shape control technology with the variable roll cooling of a work roll in plate mill has been proposed by authors. In this study, to realize this technology in the actual plate mill with the roll-cooling header divided into roll barrel direction, an integral approach has employed combing the following aspects: -development of accurate plate crown and gauge meter models, -construction of these models to the pass schedule and mill setting method, -installing the fuzzy control of roll cooling on rolling campaign. Results of actual operation of this technology have shown remarkable improvement in plate crown and shape.
The effects of material properties of steel sheet, such as mechanical properties, microstructure and chemical composition, on the fatigue limit of gas shielded metal arc welded (GMAW) joints were investigated. Axial tension fatigue tests were conducted to lap-fillet welded joints of seven different hot-rolled sheet steels. Followings are main results. (1) The fatigue limit of welded joints varies to large extent, depending on the steels tested. (2) The fatigue limit depends mainly on toe radius of weld bead, which determines the stress concentration factor of welds. And the fatigue limit of welded joint is slightly affected by the fatigue limit of base metal. (3) There is a close correlation between chemical compositions of base metal and the fatigue limit of welded joint. The increase of Si content improves the fatigue limit remarkably, because it enlarges the toe radius of weld.
The precipitation behavior of 17-4PH stainless steel, aged at 400-900°C for 0.1-4 h, was studied to understand the strengthening effect of mainly Cu and Nb. MC, M6C, and Cu-rich phases were identified through the x-ray diffraction of the extracted residue, the measurement of electric resistivity, and TEM observation. Among these, the Cu-rich phase was most finely (around 20 nm) dispersed in the matrix, and was supposed to have a dominating effect to strengthen the steel. The precipitation of Cu-rich phase was considerably rapid to start, showing the nose of a c-curve with a shorter aging condition than 700°C ×0.1 h, and the high temperature limit of precipitation being about 700°C. The Cu-rich phase was supposed to have the largest effect for strengthening at the earlier stage of precipitation, which was clarified using the tempering parameter separating the age hardening and the matrix softening by martensite recovery. A quantitative discussion, based on a morphological study of microstructure, was carried out through the mean free path calculation for dislocation. Also, several combinations of high-temperature and low-temperature aging (2-step aging) was tried for effective age hardening, and conditions for a better-balance of tensile strength, hardness and ductility than the H900 heat treatment in JIS was obtained. The difference between single and double-step aging was discussed.
Polycrystals of 18% Cr ferritic stainless steel were hot-compressed at various strain rates and various temperatures, and immediately cooled by water. Equiaxed grains surrounded by clearly etched boundaries were observed in the specimens deformed at higher temperatures or lower strain rates. The detailed microstructural observations by optical microscopy and TEM and the crystallographic determinations by TEM/Kikuchi-line analysis and SEM/EBSP analysis showed that these equiaxed grains are the dynamically recrystallized grains with both large angle grain boundaries and dislocation substructures. In spite of the occurrence of dynamic recrystallization, a large drop in stress was not observed in stress-strain curves. At a constant strain, the occurrence of dynamic recrystallization was generally determined by Zener-Hollomon parameter (Z). For example, it appeared under the condition of Z<1014s-1 at a true strain of 0.8. The dynamically recrystallized grains were very much larger than that of the subgrains formed under the same deformation condition, which suggests that the dynamic recrystallization occurred by nucleation and growth mechanism. Such characteristics of dynamic recrystallization of ferrite is quite similar to that in the IF steel previously reported. On the other hand, the Z<1014s-1 rule did not stand at 700°C, at which a number of carbides precipitated at initial grain boundaries and pinned the boundaries. It was also clarified that the size of the dynamically recrystallized grains is considerablly smaller than that of the grains statically or meta-dynamically recrystallized during cooling after the same hot-deformation condition.
For clarification of the grain boundary energy and its structure of α-<110> symmetric tilt boundaries, the atomic structure was obtained by molecular dynamic method. The obtained results are summarized as follows. The grain boundary energy of <110> symmetric tilt boundary strongly depends on misorientation angle. There are two large energy cusps at the angles which correspond to (112) Σ3 and (332)Σ11 symmetric tilt boundaries and are small cusps at (334)Σ17 and (114)Σ9 boundaries. It was found that (112)Σ3 and (111)Σ3 boundaries are consist with the single structure unit, and (332)Σ11, (334)Σ17 and (114)Σ9 boundaries are consist with the structure units composed from two different structure units, one by one.