발간논문

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As mentioned above, "How to obtain the sound ingots" is the first, most important technical problem than any others in high quality special steel manufacturing. And the hydrogen-boiling in the ingot of the low C-special steel melting is one of the most troublesome problems, especially in the humid season. We suffered formerly about this problem for a long time too. So that here I introduced the considerations on our bitter experiences, for reference. I showed at first, many statistics about [H]_Tap which we measured and then the test results in our practical melting operations. Finally I summarize the instructions, how to suppress the increase of [H] and to avoid the hydrogen-boiling in the low C-special steel. (1) About the increase of [H] from the oxidizing period to the reduction stage. In order to keep [H] (in the connection with [O]) in the lower level at the earlier stage of reduction, it is necessary to keep [H]²×[O] value at the end of oxidizing period as smaller as possible, and to keep Δ[H](=k₁[H]²×Δ[C]＋k₂) as higher as possible. For these purposes, the most important point is, after all, how to choose the optimum temperature to begin the oxygen-blowing and at the same time its finishing temperature. The effective conditions for decarburization are also effective for dehydrogenation. Especially in this case, we must pay attention to the fact that, in the earlier stage of reduction, [H] is apt to increase to higher amount in low C steels than in medium, high C steels, even when the values of [H]²×[O] at the end of the oxidizing period are same. (2) About the degree of slag-off at the end of oxidizing period: as completely(by another reason) and as rapid as possible. About 1m exposure of bath surface may be sufficient. (3) After slag-off of the oxidizing period, we can say there is no difference in the charging sequence, that is, at first deoxidizing agent, or at first reducing slag. But we can say that [H]_(Tap) in the case when we use 50% Ca CO₃is lower than that of 100% CaO in the humid season. (4) The limes must be perfectly dried. It must be aimed at least H₂O<0.5%. About the ferro-alloys (especially the ferro-Cr) and other charging materials (including the scraps) too. (5) Reducing period must be operated as quick as possible. Basicity of reducing slag must be at least>2.0 and is better to be kept somewhat higher, as nearer to the end of reducing period. Of course, never sudden change of basicity. Slag conditions must be kept in the state of A or B, for the hydrogen problem in the humid season. And for this purpose the important point is the temperature control, that is, keeping the temperature in only gradually progressive drop without specially strong heating in the way from the end of the oxygen-blowing to the tapping. (6) Even though with the full cautions mentioned above, if H_(Tap) increases over the limited value, we can suppress such increasing amount of [H] by the argon-blowing max. 1.5㎥/Ton of steel, and avoid surely the hydrogen-boiling. Many practical data in this report are those which were measured formerly by our "Hydrogen Problem Committee" of Japan Special Steel Co. This time, I could introduce here these data, under permission of Japan Special Steel Co. I thank for that permission. At last for your eager listening, I want to express the thankfulness.

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The quantitative effects of iron in the electro-boronizing treatment, the mechanism of deterioration by iron, and the process of recovery by adding aluminium into the deteriorated molten borax, have been studied. Using iron anode, to promote the deterioration of the molten borax electrolyte, the approximate diffusivity of boron into the mild steel specimen has been decreased rapidly as the iron content in the molten salt increased up to about 3.3% iron and above this content of iron the boronized depth into the specimen was not detected. And when iron oxide (Fe₂O₃) were added into the molten salt, similar effects were founded. This deterioration by iron were thought to be the formation of a porous metallic iron (which was detected by X-ray technique) surrounding the specimen, and the formation of this porous iron film were thought to be effects to reduce the reduction of a active boron in the molten salt. Adding 2% aluminium into the deteriorated molten borax, the extensive recovery of the diffusivity of boron into the specimen were detected. This phenomena were thought to be the reduction of the amount of iron content in the molten salt by the formation of Fe₃Al (which was detected by X-ray technique) which was combined with iron in the deteriorated molten salt and was floated up onto the surface slag layer. The distribution of the micro-vickers-hardness of the boronized layer, the weight gain and the length change of the specimen, the structure of the boronized layer by X-ray Debye-Scherrer method, and also the approximate activation energy of the boron to diffuse into the mild steel, which were 1.3×10⁴ ㎈/㏖ and 3.5×10⁴ ㎈/㏖ in the range of temperature above and below 860℃, respectively, were detected.

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The formation of iron oxide in a synthetic picklic sulphuric acid studied at elevated temperature and pressures. Waste picklic sulphuric acid was neutralized by adding sodium hydroxide, resulting hydrous ferrous oxide. The hydrous ferrous oxide thus formed was digested under a pressure of 3 to 5 atmospher at 100℃ for two hours. The results obtained were as follows; 1) When Fe(OH)₂ and picklic acid of pH 7.9 were reacted, 74% of iron was recovered in the form of α-goethite. 2) When Fe(OH)₂ and ferric sulphate solution (spent solution) were reacted, the yield in magnetite was better than 90%. 3) When Fe(OH)₂ of pH 12 reacted alone, δ-goethite was formed and yield was higher than 90%. When magnetite and α-goethite obtained were calcined at 600℃ it had been shown that they both could be turned into α-ferric oxide of the pigment grade, a mixture of α-ferric oxide and magnetite was also formed, when σ-goethite was calcinated at 600℃.

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Copper was diffused into high purity CdS single crystal specimens with various electrical resistivities. For C-axis, the diffusion constant D have been measured as 9.2×10^(-3) exp(-1.23 eV/kT) ㎠/sec. In this measurement, the rate of diffusion was larger in the specimen with low resistivity. It was also conformed from the present experiment that the method of capacitance and the metallurgical microscope technique could be applied to the study of diffusion of cadmium sulfide with low resistivity (of the order of ohm-cm to kilo ohm-cm).

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The aging effect of Fe-Cr-Ni alloys at around 475℃ embrittlement temperature has been studied by means of electron microscopy. The modurated structure was found to be very similar to the theoretical pattern of the spinodal decomposition. The embrittlement phenomena of this ternary alloy seams to the spinodal decomposition separated into two phase of iron-rich and chromium-rich solid solution with consideration of the spectrum of Mo¨ssbauer effect, the microstructure changes by potentiostatic etching, microhardness observation and the electron microscopic study.

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In order to study the effect of pressure applied up to 1000 ㎏/㎠ during solidification on the stabilizing in Zn-22%Al alloy, dimensional changes of the alloy have been meassured at various temperature, and microstructural observation and hardness test were also performed. The calculated activation energies with various applied pressure were lower with higher applied pressure as Q_0=60,200 ㎈/㏖, Q_(250)=46,600 ㎈/㏖, Q_(500)=32,900 ㎈/㏖ and, Q_(750) and Q_(1000) were almost equal to Q_(500). The rate of eutectoid decomposition was accelerated with applied pressure up to 500 ㎏/㎠, thus stabilization was greatly affected.

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The effects of mischmetal addition from 0.02% to 0.08% on the mechanical properties of 18 Ni 300 grade maraging steel were investigated. DTA and electrical resistivity measurements at a constant heating rate showed singular points and aging treatments were performed at these singular temperatures. Mechanical properties of the maraging steels were examined according to the aging treatment. Also the precipitates were investigated by the thin film electron microscope technique. It was found that 18 Ni 300 grade managing steel maintains its strength by an addition of mischmetal even after a prolonged aging treatment at temperatures below 482℃. Over-aging is not particularly detrimental to its mechanical properties. The coherency of the thin plate-like precipitates is maintained during the aging treatment for 160 hr at 482℃. Their orientations with respect to matrix are (011)α//(001)pp, [100]α//[100]pp, [01￣1]α//[010]pp or occasionally {112}α//{001}pp. The reason for such an unsual behavior is believed due to the fact that mischmetal inhibits the coarsening of precipitates by decreasing the diffusion rate of Ni and Mo forming orthorhombic Ni₃Mo.

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The effects of magnesium addition, pouring temperatures, holding time and the double treatment together with sulfur on the refinement of primary silicon crystals in high silicon-aluminum alloys were examined by metallography, X-Ray diffraction and EPMA line analysis and compared with those of copper-treatment. The results obtained are as follows: 1) The refining could be achieved by addition of less than 1% magnesium. 2) The refining effect was observed to increase with increasing pouring temperature. 3) The longer holding time turned out to lower the refining effect. 4) The refining effect by the double treatment together with sulfur was better than that of the single treatment by magnesium. 5) The addition of magnesium and the addition of copper promoted the growth of silicon with feathering and plate-like silicon respectively.

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