발간논문

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The hardened layer of the high-C (1%) steel with a thickness of 25㎛ was obtained by the irradiation of CO₂laser with a power density of 2×10^5 W/㎠ for about 0.23 second (spot size of 0.23㎜, scanning speed of 6㎝/min). When the thickness of hardened layer was compared with the temperature calculated by the conduction of heat in solid, the hardening mechanism was due to the solid-state transformation in steel.

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A computer program has been developed which is capable of calculating phase boundaries in a ternary system with multiple phases when binary thermodynamic data are known. The important features of the program are as follows: (1) The binary data are expressed in the form of Redlich - Kister equation or Legendre polynomial. (2) The ternary free energy of mixing is calculated using three different model equations which are equivalent to Kohler, Toop and the shortest distance composition path equations. (3) The phase boundary is determined by minimizing the ternary free energy of mixing using simplex and hill-climbing methods. The calculated phase diagrams of the Co-Ni-Fe system at 873˚K and 1073˚K and of the Cr-Ni-Fe system at 1373˚K are discussed in comparison with the measured ones.

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Hardness, tensile properties and impact toughness of 0.07C-1.4Mn-0.08Nb and 0.07C-1.4Mn-0.09V steels, which had been cold rolled by different degrees, were studied after aging treatment at 600℃ for varying period of times. By increasing aging times, there appeared two peaks of hardness values or strength. Aging time, which caused the hardness peaks, were independent of cold rolling degree and were constant for the same steel. When compared at the same hardness level, ductility and impact Goughness of the samples of secondary hardness peak were much better than that of the samples of primary hardness peak. Test results of mechanical properties are discussed in relation with the precipitation particle size and its distribution, coherency of the particles, and recrystallization.

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A stdy has been made on tempered embrittlement of AISI 4340 steel having martensite-bainite duplex structure, as compared with that in tempered martensite, metallographically. For making martensite-bainite duplex structure, lower bainite formation was achieved in a salt bath held at 315℃, upper bainite at 400℃, and subsequently quenched into iced brine. After the isothermal transformation, they were tempered for lhr in the range between 200℃ and 500℃, and then the mechanical properties were examined. Impact toughness was measured by Charpy Impact Tester, microstructure was examined by means of optical and transmission electron microscope, and fracture surface scanning electron microscope. The lower bainite was precipitated acicularly so as to partition prior austenite grain and the upper bainite was precipitated massively so as to fill prior austenite grain. These shapes of bainite had an effect on the impact toughness, temper embrittlement, and brittle fracture resistance of martensite-bainite duplex structure. The 300℃-tempered martensite embrittlement was greatly improved by martensite-bainte duplex structure and the ductile-brittle transition temperature of martensite-bainite duplex structure was lower than that of martensite. The former, the improvement of tempered martensite embrittlement by martensite-bainite duplex structure, may have been attributed to the shape and distribution of carbide which precipitated differently compared with those of martensite structure during tempering. They also affected on the fracture behavior. And, it seemed that the latter, the increment of brittle fracture resistance, have been due to the partitioning of prior austenite grain, with the result of fining cleavage crack path by lower bainite.

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The specimens have compositions of 70wt% Mn, 28-29wt% Al and three different carbon contents (0.48wt%, 0.92wt%, 1.24wt%). After solutionized at 1100℃ for 3 hr, the specimens were quenched into solicon oil maintained at zoom temperature and then annealed in a salt bath for the various period of time at the predetermined temperatures ranging from 400℃ to 700℃. Their phase transformations and magnetic properties were investigated. The results obtained from the present experiments can be summarized as follows; 1) The specimen with 0.48wt% C showed the best magnetic properties when annealed for 60 to 120 min. at 550℃. It was believed that the transformation of εto τphase was overshadowed by the decomposition of τinto β-Mn and γphase. 2) The specimen with 1.24wt% C which is more than two times of carbon solubility of τphase (0.48wt% C), showed the highest values of (BH)_(max) when compared with the others. The reason for it is believed to be resulted from a particular morphology of carbide precipitates, i.e. a repeated lamellar structures of Mn₃AlC in the τphase. This type of structure seems to inhibits the decomposition process of τphase to β-Mn phase in addition to the negative effect on the domain wall motion in that phase by pinning it.

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Together with the deoxidation and desulphurizaton behaviors of RE (rare earth element) treated molten steel, the changes in shape, size, and constitution of the RE inclusions after RE treatment were studied in 0.3%C-1.3%Mn-0.02%S steel using high frequency induction furnace with magnesia crucible. It was found that little additional deoxidation occurred when RE addition was made to the molten steel pre-deoxidized with 0.05% Al, whilst considerable desulphurization was observed. The effectiveness of desulphurization increased with the amount of RE added. It is believed that the desulphurization occurred mainly by the formation of RE-sulphide rather than RE-oxysulphide and the growth of RE-sulphide was accelerated by the induction stirring. During holding after RE treatment, resulphurization and fading of globular RE-inclusions, accompanied by the formation of MnS, occurred possibly due to the reoxidation of the molten steel.

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