발간논문

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The gas flow distribution behavior of the shaft type pre-reduction furnace of two stage-two step reduction method was investigated by a numerical method. Because the interaction between the ore and the reduction gas is the major factor of gas flow, to analyze the relation between gas and solid phases. Ergun`s equation was used. When the reduction gas is distributed uniformly in the shaft furnace, the condition of reducing ore is optimum. So, a three dimensional analysis of the height where flow is fully developed and the variation of pressure drop was accomplished. As the height of inlet becomes high, pressure drop decreases but the height where flow is fully developed and dead zone become higher. As the injected gas angle increases, pressure drop increases a little, but the height making the flow uniform doesn`t change. In case of the increase of the velocity and volume flow rate of gas, pressure drop increases but the height of uniform flow are identical. In case of the increase of the mean particle diameter and porosity of the packed bed, pressure drop decreases but the height of fully developed flow doesn`t change.

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In general practice, steels such as heat-treated hyper-eutectoid steel and alloy steel turn into a multi phasestructure containing retained austenite. There are many arguments on the role and influence of retained austenite on the mechanical properties of steels. If quantitative measurement of residual stress of retained austenite phase in steels becomes feasible, its influence on the mechanical properties may be evaluated by estimation of the stress state. This paper is a study of the derivation of the K value, X-ray stress constant, on the residual stress of the retained austenite using the V-ka X-ray. The K value is necessary to calculate the stress value. As a result, the X-ray stress constant of retained austenite was sensitive considering its volume fraction. But the stress value obtained by using -264MPa/deg as the average of X-ray stress constant on each volume fraction was found to be reliable.

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A new process for making Alnico type(Fe-Co-Ni-Al-Ti-Cu) permanent magnets was developed via the rapid solidifcation technology. The process includes extractive rapid solidifcation, pulverization and sintering. Magnetic properties of the Alnico type magnets were found to be subject to the shape and distribution(aspect ratio) of magnetically hard α₁ precipitates(Fe-Co rich). The higher the aspect ratio of the α₁ particles the better the magnetic properties were obtained. During heat treatment at a magnetic field(900∼600℃) an optimized aspect ratio of the α₁ particles was controllable for which parameters of the treatment are dependent upon the initial grain size of the rapidly solidified powders. The best magnetic properties of Alnico 8 type sintered magnets were obtained from the alloys containing 7.4 wt.%Ti heat treated isothermally at 820℃ for 15 minutes. The resulting properties were : H=2045 Oe, B_r = 7800 Gauss and (B.H)_(max) = 5.34 MGOe, respectively.

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The structures and mechanical properties of direct quenched martensitic 0.12C-0.30Si-1.00Mn-0.01Ti steels containing from 0 to 80 ppm of B were investigated. Tensile and impact tests were carried out and precipitation behavior of B on prior austenite grain boundary was studied using auger electron spectroscopy (AES) and scanning electron microscopy (SEM). The main results obtained are as follows. (1) Precipitation of B on prior austenite grain boundary was detected when more than 10 ppm B was added and the precipitates were consist of Fe, C and B. (2) Precipitation of Ti-rich phase was also observed on prior austenite grain boundary. (3) Ultimated tensile strength and impact energy increased as boron was added up to 10 ppm, however, the impact energy decreased rapidly after then. (4) The best combination of strength and toughness was obtained in 0.12C-0.30Si-1.00Mn-0.5Mo-0.01Ti-0.001B steel.

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An investigation was performed to examine the formation process and morphology of the precipitates on the prior-austenite grain boundary of boron bearing steel during continuous cooling. During the faster cooling rate of water quenching from 1350℃ or 20℃/sec from 1350℃ to 750℃ and 850℃, elements of boron and carbon were segregated to the prior-austenite grain boundary, and transformed to the unstable amorphous phase or a meta-stable and a partially crystallized one. The stable phases were Fe_(23)(B,C)_6 at step quenching temperature of 850℃ and Fe₃(C,B) at 750℃, respectively. But during the slower cooling rate of 1℃/sec from 1350℃ to 750℃ and 850℃, segregated boron and carbon were not transformed to a unstable amorphous phase but directly transformed to the stable Fe_(23)(B,C)_6 or Fe₃(C,B). Line fraction and volume fraction of the stable phase on the prior-austenite grain boundary were not depended upon the step quenching temperature and the holding time, but mainly upon the cooling rate from 1350℃ to 750℃ and 850℃. Morphology of precipitates on the prior-austenite grain boundary was changed from small and thin type at the faster cooling rate of water quenching to large and thick one at the slower cooling rate of 1℃ /sec.

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The present study is concerned with microstructural analyses of the surface layer of a ductile cast iron (DCI) roll modified by irradiation of high-energy electron beam. DCI roll specimens were irradiated using an electron accelerator, and the microstructures of the irradiated surface layers were examined. As a result of irradiation, the matrix phase was changed from banite to a mixture of retained austenite and martensite, and coarse spheroidal graphites were dissolved partially under the higher heat input. The microstructural modification by the irradiation improved greatly the hardness of surface layer because of the martensitic matrix, although retained austenite was presented around the spheroidal graphite. In order to investigate the transformation mechanism, the simulation test including thermal cycles of abrupt heating and quenching was carried out. Thermal analysis of the irradiated surface layer was also carried out for understanding of microstructural modification in terms of the irradiation parameters.

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(TiAl)N films were deposited on high speed steel(SKH9) by Cathodic Arc Plasma Deposition employing two separate targets of Al and Ti, and the formation behaviors of the films were investigated by AES, EDX, XRD and SEM. The composition of (TiAl)N film was varied with control of current ratios of Al and Ti targets. As the Al target current increased, the atomic ratio of Al in the film increased and the hardness of (TiAl)N film was obtained to be 2200∼2700 ㎏/㎟ depending on the stoichiometry of Al and Ti in the film. The deposition of (TiAl)N film significantly enhanced the wear resistance of substrate during ball-on-disc type wear test at elevated temperatures up to 600℃. The (TiAl)N film with high density and good adhesion caused a significant reduction of friction coefficient and weight loss of substrate between Al₂O₃ ball and (TiAl)N film sliding system and the most effective improvement of wear resistance was obtained by (TiAl)N film with Al/Ti atomic ratio of 1.10.

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Thermal resistant compound layer has been synthesized on hot working tool steel(SKD61) using plasma diffusion process a subsequent diffusion treatment after deposition. Ti and Al layers (Ti, Ti/Al and Ti/Al/Ti) were deposited on the steel substrate followed by diffusion-treatment with intense plasma heating through hollow cathode discharge effect at partial pressures of 1 torr to 2.5 torr of N₂ and H₂ mixture. Plasma diffusion at temperature range of 550℃ to 950℃ produced various intermetallic compounds of Fe₃Al and Fe₂Ti as well as nitride compounds of TiN and TiAlN at interface and surface. Compound formation enhanced surface hardness up to 3000Hk(0.1N). High temperature wear resistant also greatly increased at temperature range of 300℃ to 600℃ due to the thermal resistant compounds layer formation. Details of structure and high temperature wear behavior will be discussed with XRD, EDS and other analytical data.

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The effects of alloying elements(Ti, Nb and Ti+Nb) on the formability, mechanical and ridging properties of low C, N-18.5%Cr-1wt%Mo ferritic stainless steels have been investigated. The formability and ridging properties of cold rolled and annealed steels were effectively improved by addition of Ti and Nb together, 0.05∼0.1 wt% Ti and 0.2 ∼0.3% wt% Nb. The larger the amount of the Nb addition, the better results were achieved. The average r-value of the steel was the largest with the addition of (Ti+Nb) in the range of (Ti+Nb)/(C+N)=12∼20.

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Among various processes in fabricating metal matrix composites, melt-stirring technology is much more attractive route in industrial application because it is simpler and less expensive compared to squeeze casting or powder metallurgy. In the present study, effects of size and volume fraction of reinforcing particles on tensile strength and thermal expansion coefficients of α-Al₂O₃_(p)/AC8A composite, which was made by melt-stirring process, were investigated. And the effect of Mg addition on the above properties was also investigated. Composites with 3%Mg showed better wettability and distribution of particles than that of composites with no Mg. The addition of Al₂O₃ particles was not effective in increasing the tensile strength of the composite except the composite materials with 3%Mg, which showed an increased strength in the case of 5vol% addition of 16 ㎛ α-Al₂O₃ particles. On the other hand, thermal expansion coefficients of the composites decreased with increasing the volume fraction of reinforcing particles and Mg contents.

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