발간논문

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As a fundamental research to establish an efficient process for the solid-state reduction of chromite, reduction behavior of natural chromites were investigated experimentally and interpreted in terms of thermodynamics. The results obtained were as follows; 1) The reduction of oxide components in natural chromite tended to process in the sequence of Fe₃O₄, FeCr₂O₄ and MgCr₂O₄. The tendency was pronounced at low temperatures. 2) Below 1200℃, reduction of chromite proceeded sequentially, i.e. (Mg, Fe)(Cr, Al, Fe)₂O₄→ Fe or Fe₃C+Fe-poor Non-stoichimetric chromite → (Fe, Cr)_7C₃+MgAl₂O₄ 3) Above 1200℃, reduction of chromite proceeded sequentially, i.e. (Mg, Fe)(Cr, Al, Fe)₂O₄ → (Fe, Cr)_7C₃+Non-stoichimetric chromite → (Fe, Cr)_7C₃+MgAl₂O₄ 4) Reduction degree of chromite was higher with increasing Fe/Cr ratio of chromite composition. 5) Reduction rate of various chromite ore by solid carbon is higher in order SA, RM, IH, PH and IM at the same condition.

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The effects of Nb and V addition on mechanical properties and microstructures were investigated in conventionally heat treated(quenched and tempered) Si-Cr spring steels. The specimens were austenitized at 850℃ for 15min and tempered at 470℃ for 15min. The addition of vanadium raised tensile strength by about 5kg/㎟, while that of niobium did not show any influence on tensile strength. The addition of V and Nb increased the hardness over the entire tempering temperatures, but did not have an effect on softening resistance which occurred by the Si addition. The impact values were not changed by the addition of vanadium and niobium. When tempered at 470℃, all steels showed the cementite precipitation in addition to asquenched microstructures. (Received July 4, 1994)

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Laser induced plasma(LIP) and its effect on weld penetration were discussed in this paper. High power carbon dioxide laser system was used to make bead-on-plate weld on structural steel. LIP formation sequence was recorded by a rotating prism type high speed camera 16㎜ tine film was loaded and the filming rate was 5,000 fps. Average elapse time for emerging, expanding and fading out of a LIP was about 1.4 msec. Effect of assist gas blowing on the reduction of LIP column height was remarkable especially when the rate was 101/min at the straight nozzle of 5mm orifice diameter. In accordance with increase of the assist gas blowing rate, the upper side of key hole was elongated and, at the same time, the shape of LIP showed small cone instead of round column. The calculated temperature of LIP column, based on the spectrum analysis results and black body radiation theory, was 11,290K or more.

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Effects of Cr content on the cylic oxidation behavior of W-Mo-(29-48w%)Cr alloys have been studid by measuring weight changes and observing oxide scales during an oxidation test at 1000, 1100, and 1200℃ in air. The W-Mo-Cr alloy ingots were prepared by a plasma are melting process. The oxidation resistance was found to improve as the Cr content increased but to decrease repidly as the temperature increased. The observed optimum Cr contents for oxidation resistance were more than 35% at 1000℃ and 48% at 1100℃. The W-26Mo-48Cr ally showed good oxidation resistance by forming an abherent and thin continuous layer of Cr₂O₃ at 1100℃ up to 100cycles. This is believed due to the formation of a new Cr₂O₃ healing layer laterally grown from the Cr₂O₃ layer was was damaged. However, an accelerated oxidation ocurred by first forming the porous multilayers of Cr₂O₃ and Cr₂WO_6 and then by spalling of these layers at 1200℃.

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Strain in InGaAs and AlAs epilayers grown on GaAs by atmospheric pressure metal organic chemical vapor deposition has been analysed using X-ray double crystal diffraction. It is seen that the epilayers grow tilted with respect to the substrate lattice. The measured tilt angles of the lattice matched epilayers agree well with Nagai`s model But the magnitude and the direction of the tilt of the partially relaxed epilayers were rather random and thought to be dependent on the details of the growth condition. In the partially relaxed epilayer, it is observed that the lattice relaxation is greater in [11￣0] than in [110]. So that, the epilayer lattice is thought to assume the orthorhombic distortion. The measured degree of the relaxation as a function of epilayer thickness agrees better with the half loop nucleation model than the mechanical equilibrium model or the energy balance model which have been used widely.

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With high power carbon dioxide laser system, C-Mn structural steel for automobile application was welded. Bead-on-plated welding process was applied with 6㎜ thick hot rolled steel plates, and the welding conditions such as travel speed, position of focus and laser output power were 1.2m/min, l.0㎜ under the sample surface and 2.1㎾, respectively. Microstructural examinations and calculation indicated that the microstructrue of the laser weld, which experienced very high heating and cooling cycle during welding process, may not be predicted from the founds in the conventional arc process. Test results also demonstrated tha the austenite nucleation takes place at the pearlite-ferrite boundaries instead of the interface between ferrite and cementite in the pearlite. In the laser welding, the initiation and completion temperature of austenite transformation are much higher than that in the arc welding. Hardness of martensite in the HAZ varies from 635 Hv to 350 Hv because the homogenization of austenite is not complete even at the temperatures far above the A₃.

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ZrN coatings were deposited on tool steel(SKH9) with the interlayer of Ni by a reactive D.C. magnetron sputtering. The microhardness, adhesion strength and wear-resistance of the films were investigated with microhardness tester, scratch tester, pin-on-disk method respectively. ZrN coatings were deposited with the condition of partial pressures of N2 gas(4∼10x10-4mbar) and the substrate bias voltage(0∼250V). The microhardness decreased from HV 1050 ㎏/㎜2 to HV 1000㎏/㎜2 as the thickness of Ni interlayer increased from 0.2 ㎛ to 1.3 ㎛. The adhesion strength of the Ni/ZrN double-layered coatings were 22N in the scratch test and it was four times higher than that of ZrN mono-layered coatings. The failure mode of the double-layered coatings was changed from the cohesive at the initial stage to the adhesive failure in the later stage of the failure. The wear-resistance of the Ni/ZrN double-layered coatings increased nearly twice as high as that of the ZrN mono-layered coatings.

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Various grades of refractories were tested for their corrosion characteristics when used as a lining material of ladle for Ti containing stainless steel melt. Three different commercial refractories were machined into crucibles to melt stainless steels containing various amounts of Ti. After melting in crucibles at 1550℃ under Ar atmosphere for 1 hour, analyses were conducted to measure the depth of attack by liquid steel. The magnesia material(90% MgO) showed the best corrosion resistance. The alumina refractory containing high silica(> 30%SiO₂) was found to corrode significantly by the reaction of sillimanite(Al₂O₃·SiO₂) with Ti in liquid steel melt to form complex oxides with low melting temperature. The high alumina refractory (84%Al₂O₃) showed a relatively good corrosion resistance. The plant trial tests using these refractory materials for ladle lining were also conducted to examine the inclusions in continuously cast slabs. The inclusions originated from refractory were significantly reduced when high alumina (84% Al₂O₃)refractory lining were used.

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The effect of plasma on the deposition rate of AlN film on a Si (111) substrate in PECVD process was investigated. AlCl₃ and NH₃ gases were used as precursors of aluminum and nitrogen, respectively. The process was carried out at relatively low temperatures under R.F plasma. Though some oxygen impurity was present in the film, the reasonably good stoichiometry of AlN was obtained. Employing the process variables such as temperature, plasma power, pressure and ammonia flow rate, the growth rate and refractivity index of coating layers were studied. The result of XRD analysis showed, the amorphous AlN film was formed at the low temperature of 500℃, while the crystalline structure was developed at higher temperatures.

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The corrosion property and formability of a steel treated with Zn powder containing paint were studied. Two kinds of substrates, hot rolled steel and Dacro treated steel, were painted with Zn powder containing paint in the various range of Zn powder contents, 60∼87 vol.%. In order to examine the corrosion property and formability, salt spray test, electrochemical polarization test and bend test were carried out. In the case of hot rolled steel plate, good corrosion resistance was obtained above 82 vol.% of Zn powder content. But the formability became severely worse above 82 vol.% of Zn powder content. Therefore, when Zn powder containing paint is used to treat steel surface, it can be applied to the parts that do not require subsequent forming process. In the case of Dacro treated steel, a Dacro layer was degraded by Zn powder containing paint layer. This phenomenon was caused by the increase in OH^- ion concentration produced by cathodic reaction at the steel surface. In fact, the corrosion potential of the Dacro treated layer increased in a 3% NaOH solution and the Dacro layer was completely dissolved by immersion in a 3% NaOH solution. When the Dacro treated plate was coated with a paint without Zn powder, the corrosion resistance was found to improve significantly and the formability was also very good.

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