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

Tetsu-to-Hagané Vol. 104 (2018), No. 6

  • Control of Macrosegregation by Size and Shape of Equiaxed Grains

    pp. 293-302

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    DOI:10.2355/tetsutohagane.TETSU-2017-096

    In order to investigate the possibility for control of macrosegregation by size and shape of equiaxed grains, laboratory scale casting experiments have been performed, using Al-10mass%Cu alloy. The mold was newly developed in order to produce macrosegregation, intentionally bringing about a bridging in the middle of a cast. Equiaxed grain size (d) was controlled by the amount of the modifier. The complexity of the equiaxed grain morphology was controlled by the mechanical vibration and was characterized using fractal dimension (Df). Macrosegregation was quantitatively characterized by the areal fraction of eutectic structure (fE). Multiple regression analysis for the maximum value of fE with respect to d and Df have been performed. It was found that the maximum of fE decreased with decreasing d and with increasing Df. Based on the multiple regression analysis, the concept of a size and shape map for macrosegregation free zone has been developed.
  • Suppression Effect of Fretting Wear in Railway Axle Journal Bearings by Means of Hard-film Coatings

    pp. 303-311

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    DOI:10.2355/tetsutohagane.TETSU-2017-088

    Axle journal bearings composed of an inner ring, a backing ring, etc. are used in railway applications. The bearing often show fretting wear due to slight relative slip between the inner ring and the backing ring. In this work, the authors have investigated the effects of hard films coated on the backing ring on the fretting wear. Rotational tests using full-scale railway axle bearings with backing rings of side faces which are coated with either of three hard films (titanium nitride (TiN), chromium nitride (CrN) and diamond-like carbon (DLC)) were conducted. It was found that the fretting wear with the TiN film is the worst as a result of the decohesion of whole TiN film, and the fretting wear of the backing ring coated with the CrN film is slight, although the film was not able to follow the deformation of the base material. It was also found that the fretting wear of the backing ring coated with the DLC film which is the thinnest and the hardest among the three films is as slight as that of the backing ring coated with the CrN film, though some of the DLC film left. Accordingly, it is supposed that the fretting wear of the backing ring can be suppressed, if the film coating has a lower friction coefficient, higher hardness and higher bonding strength with the base material.
  • Flying Gauge Control for a Specified Section Including a Welding Point in a Cold Strip Mill

    pp. 312-321

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    DOI:10.2355/tetsutohagane.TETSU-2017-094

    In a continuous cold strip mill, the succeeding strip and the preceding strip are welded together and then rolled continuously. The section near the welding point is often processed as scrap due to the constraints of the downstream manufacturing processes. Therefore, thickness reduction is desired as a means of reducing the scrap weight. To this end, this paper presents a new flying gauge change (FGC) controller design which achieves thickness change in the section including the welding point. Because of the short length of the target section, two thickness change points can exist simultaneously in the mill. In the conventional FGC, the set values of the roll gap and the roll speed are calculated without considering the positions of the two thickness change points in the mill. However, this can possibly result in a poor roll speed set value and cause large strip tension deviations. The proposed scheme utilizes the transition pattern of the positions of the two thickness change points in the mill during FGC to calculate the set values of the roll gap and the roll speed. Thickness change with small strip tension deviations is achieved by applying these set values to the control systems of the rolling mill according to the positions of the two thickness change points. The validity of this approach was verified by computer simulations and experiments with a real process. Application of this technology has improved cold strip yield.
  • Effect of Surface Textures of Iron substrate on the Crystal Orientation Relationship between Electrodeposited Zinc and Iron

    pp. 322-330

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    DOI:10.2355/tetsutohagane.TETSU-2017-097

    Zn deposition was performed galvanostatically at 1500 A/m2 and a charge of 1.48×104 C/m2 onto both the high purity electrolytic iron and the cold rolled steel sheets in an agitated sulfate solution at 40 °C to investigate the effect of surface textures of Fe on the crystal orientation relationship between Fe and Zn. Zn deposited on the high purity electrolytic iron with large grain size showed the orientation relationship of {110}Fe//{0001}Zn. However, with increasing the angle of inclination of {110} Fe plane from the surface of substrate, the deviation of orientation relationship of {110}Fe//{0001}Zn increased. This result suggests that the orientation relationship of {110}Fe//{0001}Zn is difficult to be completed in the middle of deposition with increasing the angle of inclination of {110} Fe plane from the surface of substrate, as a result, the epitaxial growth of Zn easily changes to random growth. On the other hand, Zn deposited on the cold rolled steel sheets with small grain size showed preferred orientation of {0001} regardless of orientation of Fe, which indicates that the orientation of deposited Zn is more affected by deposition overpotential than by the orientation of Fe substrate. Although the strain was introduced to the high purity electrolytic iron with sandblasting, the orientation relationship of {110}Fe//{0001}Zn hardly changed with sandblasting, showing that the strain of Fe substrate has scarcely effect on the orientation relationship between Fe and deposited Zn.
  • Structural Changes in Galvannealed Coating during Hot-stamping Heating

    pp. 331-337

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    DOI:10.2355/tetsutohagane.TETSU-2017-100

    Hot-stamping process is widely applied to coated steel sheets including galvannealed steel sheets (GA) to produce high-strength automobile components. In this study, we investigated the structural changes in galvannealed coating during hot-stamping heating. It was revealed that the original δ1 phase decomposed to the Γ phase and Zn liquid above the peritectic temperature of the δ1 phase, then the Γ phase further decomposed to the Fe-Zn solid-solution and Zn liquid above the peritectic temperature of the Γ phase, and finally the coating changed to a single phase of the Fe-Zn solid-solution. The series of structural changes in GA coating can be explained with the Fe-Zn binary phase diagram.
  • Slow Strain Rate Tensile Test Properties of Iron-Based Superalloy SUH660 in Hydrogen Gas

    pp. 338-345

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    DOI:10.2355/tetsutohagane.TETSU-2017-084

    To investigate dependence of strain rate of tensile test for iron-based superalloy SUH660 (A286), tensile tests were conducted for the specimens in 70 MPa hydrogen gas and air at 150°C. Nominal stress-nominal strain curve of each strain rate in 70 MPa hydrogen gas showed same behavior to maximum load via yield point in comparison with that in air, however, each elongation at breaking point in 70 MPa hydrogen was a little shorter than that in air. The values of tensile strength didn’t depend on the strain rate in 70 MPa hydrogen as well as those in air. In addition, the difference in tensile strength wasn’t observed between that in 70 MPa hydrogen gas and that in air for the strain rate. However, it’s proved that relative reduction of area in 70 MPa hydrogen to that in air was significantly affected by strain rate of tensile test. Those values were 80%, 51%, and 32% in the case of strain rate 5.0×10–5, 7.5×10–6, and 1.25×10–6 s–1, respectively. The morphology of fracture surface also changed from dimple to quasi-cleavage (QC), with a decrease in strain rate. Simulation of hydrogen gas diffusion form surface to inside during experiment showed that the hydrogen diffusion layer of specimen with QC fracture surface (RRA 51%, strain rate 7.5×10–6 s–1) was only 0.25 mm in depth. That implies that hydrogen content at crack tips is much higher than that of simulation due to hydrogen concentration by a couple of defects. That tendency seems to become stronger with a decrease in strain rate.

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