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ONLINE ISSN: 1883-2954
PRINT ISSN: 0021-1575

Tetsu-to-Hagané Vol. 92 (2006), No. 7

  • Effect of Steel Compositions on Wettability between Steel Melt and Oxide Plates

    pp. 411-416

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    Effect of alloying elements of steel on the wettability between the steel melt and oxides was evaluated. Steels containing Ti, Nb, Mn and Al were melted on single-crystal and polycrystal plates of Al2O3 and MgO, and the contact angle between the melt and the oxide plate was measured. Titanium addition to steel was found to decrease the contact angle both on Al2O3 and MgO, but more titanium was needed to improve the wettability on MgO than on Al2O3. The addition of other elements, Nb, Mn and Al, did not improve the wettability between the steel melt and oxide plates. At the interface between Ti added well-wetted steel melt and oxide plate, reaction layer was formed. The layer was identified as Ti2O3 for 1% Ti added steel and TiO for 5% Ti added steel. Thermodynamic calculation supported the formation of these reaction products. It was concluded that the improvement of the wettability was achieved by the formation of the Ti-oxide reaction layers immediately after the melting of the steel and that the contact angles decreased depending mainly on the materials of reaction layers.
  • Effect of Inlet Gas Composition on Dioxins Emission in the Iron Ore Sintering Process

    pp. 417-426

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    The various factors of solid in raw materials have been pointed out for dioxin emissions from iron ore sintering process. It has been reported that copper and its components are catalysts and that ammonia and nitrogen-bearing organic compounds are inhibitors for dioxins formation. And it has been also reported that coke, which is the major fuel in the iron ore sintering process, gives the higher emissions than other carbonaceous materials, e.g., anthracite, coal, activated coke and coal.
    In addition, fundamental studies have indicated that the dioxin is considered to form with oxidation reaction of residual carbon particles formed by the solid fuels in the sintering process. So, inlet gas composition affects the oxidation reaction and dioxins formation.
    The main result is that increasing H2O concentration by injecting steam into inlet gas results in decreasing dioxin emission.This result suggests that H2O has a role of inhibiting residual carbon particles formation by promoting CO oxidation reaction to CO2. The phenomenon of promoting CO oxidation reaction was confirmed by exhaust gas analysis.
    The other main result is that decreasing O2 concentration to 17 vol% by injecting N2 gas into inlet gas also results in decreasing dioxin emission. This results suggests decreasing O2 concentration to 17 vol% is effective to inhibit the oxidation reaction of residual carbon particles without promoting residual carbon particles formation.
  • Apparent Growth Direction of Primary Dendrites in Chill Zone

    pp. 427-431

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    Apparent growth directions of dendrites in the initial solidification zone have been analyzed theoretically and experimentally. In a 2-D observation, the dendrite trunk can be observed as a projection to an observing cross section. Thus, the angle between heat flow direction and projected trunk may be lower than the real angle. In order to predict the distribution of apparent angle of dendrite trunks, a simple physical model has been developed and statistically analyzed. According to this model, dendrite trunks, the growth directions of which are close to the heat flow direction, become larger than the other. This prediction agrees well qualitatively with the experimental result which was carried out using SUS304.
  • Grain Selection during Initial Solidification for SUS304 and S45C

    pp. 432-438

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    In order to understand the initial solidification of the continuous casting process, the drop tests have been performed using SUS304 and S45C. The grain is defined as the group of dendrites that grow in the same direction. The grain size of SUS304 quickly increases from surface as solidification proceeds. On the other hand, the grain size of S45C remains constant in the range of experiment. This is due to the difference in the distribution of the growth direction of dendrites.The grain selection model has been modified by taking the distribution of growth direction into consideration. Then the change in number of grains with distance can be explained quantitatively. In order to clarify the difference in the growth direction of dendrites, the influence of the superheat has been evaluated. It has been found that the dendrites incline heavily and radially with increasing the superheat. This may be due to the melting of heterogeneous nuclei and enlarging the high-undercooling region near the chill plate.
  • The Improvement of Surface Cracks on Leaded Free-cutting Steel Bloom

    pp. 439-447

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    This study was carried out in order to prevent surface cracks on continuous cast of leaded free-cutting steel. As the leaded steel is likely to suffer from surface cracks, prevention of the cracks is necessary for higher reliability of products. The majority of surface cracks of billets of this steel were found out to be bloom cracks originated due to not uniform cooling at the initial solidification of bloom. Consequently, this improvement of bloom surface cracks is important for the decrease of surface cracks of billets of this steel.
    From the view point of the uniform initial solidification near meniscus, mold powder was optimized. Three types of mold powder with different viscosity and different carbon content were evaluated through commercial production processes. Uniform initial solidification was realized by use of the mold powder with higher viscosity and higher carbon content. Consequently, surface cracks of the blooms cast continuously and the rolled billets were decreased.
  • Effect of Nb(CN) Precipitation Conditions on Abnormal Grain Growth in Nb Added Steels

    pp. 448-456

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    In high temperature heat treatments like carburizing treatment, 4A or 5A elements that make nitride or carbide are doped to suppress abnormal grain growth. Nb, which is one of 4A elements, is known as one of the most effective elements to suppress abnormal grain growth during carburizing and added in commercial steels. Therefore the effects of Nb addition on abnormal grain growth in carburizing has been studied by many researchers, but there are few reports about the influence of Nb(CN) precipitation conditions before carburizing.
    In this study, Nb(CN) precipitation conditions were changed into solid solution, fine precipitations and coarse ones by pre-heat treatment, in order to investigate the effects of Nb(CN) precipitation conditions on abnormal grain growth behavior during a heat treatment at high temperature quasi-carburizing. The results are as follows:
    (1) Abnormal grain growth was suppressed in a sample containing solid solute Nb before heat treatment quasi-carburizing more than ones in which all Nb was precipitated as carbide.
    (2) If the size of Nb(CN) precipitates was too fine or too coarse, abnormal grain growth was stimulated. There is a suitable size of Nb(CN) precipitation to suppress abnormal grain growth. It is considered that factors to cause abnormal grain growth were not only amount and size of precipitation and austenite grain size, which were parameters in the theory of Hillert and Gladman on abnormal grain growth, but also Ostwald riping rate of precipitations.
  • Effects of the Grain Size and Volume Fraction of Second Hard Phase on Mechanical Properties of Dual Phase Steel

    pp. 457-463

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    The reduction in car weight has led to demand for higher strength steels. However, increasing strength generally has a negative effect on formability. The high strength steels being developed are divided broadly into two types: steels characterized by high uniform elongation such as DP and TRIP steels having a heterogeneous microstructure, and steels characterized by high local elongation such as bainite steel having a homogeneous microstructure. High strength steels with both high uniform elongation and local elongation are under investigation.
    In this study, various levels of carbon content and hot-rolling reduction in DP steel containing Si, Mn, Cr, and Mo were examined. The effects of grain size and volume fraction of martensite on the mechanical properties of DP steel were studied. It was found that increasing reduction, i.e., decreasing the diameter of the dispersed martensite, improved the balance between strength and total elongation. The authors investigated the total elongation in terms of both uniform elongation and local elongation. Reduction increased the balance of strength and uniform elongation as well as the balance of strength and local elongation. The balance of strength and uniform elongation increased markedly with increasing carbon content when the reduction was large. The reason for these results is explained in terms of work-hardening theory and SEM observation of deformed DP steels. An ideal microstructure for uniform and local elongation in high strength steels is concluded to be macroscopically homogeneous and microscopically heterogeneous.
  • Precise Estimation of Emission Intensity of Alloyed Elements in Steel by Using Internal Standard Line in Glow Discharge Optical Emission Spectrometry

    pp. 464-468

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    Precision in the emission intensity measurement of alloyed elements in steel is investigated when an emission line of iron is employed as an internal standard in d.c. glow discharge optical emission spectrometry. Long-term variations in their emission intensities can be well corrected using the internal standard of the iron emission line, probably because the fluctuation in the sputtering rate of samples is a major reason for their intensity variations. For instance, in an Fe-Cr alloy sample (Cr 0.50 mass%), the relative standard deviation (RSD) of the emission intensity of CrI 425.433 nm is 0.678% with the internal standard method whereas the RSD is 2.14% without the internal standard. Also, daily variations in calibration curves are much suppressed by using the internal standard line. For instance, in Fe-V alloy samples, the RSDs of the calibration factor for vanadium analytical lines are 1.10-1.84%, which means that a re-calibration procedure is hardly needed for the analytical application. The internal standard method contributes to the precise determination of minor alloyed elements in steel samples.

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