발간논문

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In this study, CV graphite cast iron was made from adding small amount of A1 after making an incomplete spheroidal graphite by adding small amount of Fesi-Mg alloy. As a result, it was found that CV ratio and graphite length were increased as increasing amount of Al addition and holding time, but graphite width was decreased, Also, it was found that brinell hardness value 170-190 HB and tensile strength value 38-40 ㎏/㎟ of the CV graphite cast iron were closer to those values of the spheroidal than those values of the flake graphite cast iron, but thermal diffusivity value 7.8×10^(-6)㎡/sec of the CV graphite cast iron was closer to that value of the flake than that value of the spheroidal graphite cast iron.

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The stress tensor of the cold-rolled steel sheet was calculated for (211) reflection by Do¨lle`s theory which takes account of the texture in metals. But the result of (200) reflection was not that of quasi-isotropy case, because Do¨lle`s theory is based on the elastic anisotropy model which does not take other factors except the elastic anisotropy. So the classical equation, to know the residual stress, can not be applied to the graph of 2θ vs. sin²Ψobtained by (h00) or (hhh) reflection.

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The effect of different levels of cold prestrain on some creep characteristics at 620℃∼650℃ and 200MPa-230MPa of an 316 stainless steel was investigated. The prestrain influences the steady state creep rate and the structure. The steady state creep rate was minimum at 20%-30% prestrain condition. The stress exponent n of secondary creep rate expressed by ε_s＝Aσ^n, has been found to range from 7.68 to 7.96 at 620℃ creep and from 5.74 to 6.00 at 650℃ creep. Activation energy for creep in 30% prestrain samples was 97.3㎉/㏖ and in 10% prestrain samples was 87.2㎉/㏖.

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A theoretical model has been developed to calculate the aspect ratio distribution generated by random sectioning of an equilateral triangular prism. The calculations also show the extent to which this distribution depends upon the dimensional ratio of the prism. The results suggest that it would be feasible to compare the calculated and experimentally determined aspect ratio distributions and thereby establish the dimensional ratio of the real prisms.

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Nitrogen solubility in liquid Fe and Fe-Mn alloys has been measured by the levitation melting method under 1 atm N₂pressure to reduce vaporization of Mn from levitated alloys. Experiments were carried out at the temperature range of 1762-2271 K and Mn content of 0-15.8 wt.%. The nitrogen solubility measured in pure Fe was log [wt.%N]_Fe = -424/T－1.129 and in Fe-Mn alloys log [wt.% N ]_Fe-Mn = (-424/T－1.129)－(-94/T＋0.037) [wt.% Mn]

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A finite element model has been developed to analyze the bulging near the straightening zone, produced by elasto-plastic mechanism. The analysis leads to the following conclusions. 1) The bulging deformation can cause internal cracks in the withdrawal direction or in the width direction at the solidification front of the solid shell. 2) The tensile strain at the solidification front, resulting from the bulging deformation, increases with increasing roll spacing and with decreasing solid shell thickness. 3) The roll spacing is of primary concern, compared with solid shell thickness, in that irrespective of solid shell thickness, the bulging strain can hardly reach the critical strain when the roll spacing is small. 4) Even a small misalignment can lead to a remarkable increase of bulging strain by free bulging. 5) In spite of the ambiguity in the assumption of the temperature distribution in the solid shell and the temperature dependence of material properties, the results are in qualitative agreement with the experimental ones.

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