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

Tetsu-to-Hagané Vol. 93 (2007), No. 4

  • Decarburization Behavior of Molten Steel Using Top-blown Oxygen Jet under Reduced Pressure

    pp. 265-270

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    DOI:10.2355/tetsutohagane.93.265

    In steelmaking, the top-blown oxygen jet is used in refining processes and its behavior is therefore an important factor for decarburization and spitting behavior of molten metal in steel refining. Numerous studies have been conducted on decarburization and spitting behavior with top-blown oxygen under atmospheric pressure conditions; however, the behavior under reduced pressure for vacuum refining processes is still not fully understood.
    In this study, decarburization and spitting behavior with top-blown oxygen under reduced pressure was investigated by small-scale experiment. Decarburization rate in oxygen supply-controlling region increased as the ambient pressure decreased. Also, total amount of spitting in decarburization increased as the ambient pressure decreased. These results can be explained by the attenuation rate parameter, AR, calculated from the top-blown oxygen and ambient pressure conditions.
  • Prevention of Surface Cracks on Billets by Refining Austenite Grain of Bloom Surface before Rolling

    pp. 271-280

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    DOI:10.2355/tetsutohagane.93.271

    This study was carried out in order to make clear the crack generation mechanism of case-hardening steels.
    It was suggested from the average oxide particle radius around surface cracks of the rolled billets that most of the surface cracks should be generated at the bloom rolling process.
    From the point of view of surface crack generation at the rolling process, austenite grain sizes just before rolling were pointed out. In general, in order to prevent surface cracks at the rolling process, bainite or ferite/bainite structure before reheating is suitable for fine grain structure just before rolling. In order to get these structures, the cooling rate of the bloom surface should be higher than 2.0K/s under the cooling start temperature of 1023-1173K. It was proved that refining austenite grain in the bloom surface just before rolling was effective to decrease the crack generation.
  • Improvement of Transition Part of Slab Surface Cleanliness with In-mold Electromagnetic Stirring

    pp. 281-288

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    DOI:10.2355/tetsutohagane.93.281

    To react to user's quality demands and the needs for saving the production cost to realize the high quality slab, the transition part of slab surface cleanliness has been improved with electromagnetic stirring (EMS). In the present study, experimental investigation and 3-D fluid dynamic calculation have been carried out for analysis of the influence of velocity on the slab surface cleanliness. The following results are obtained.
    (1) A steady level of surface cleanliness is possible to be maintained by providing an average velocity of about 0.2 to 0.4 m/s along the solidification front, irrespective of the cleanliness of the molten steel.
    (2) According to 3-D magnetohydrodynamics calculation, electromagnetic stirring technique is useful for improvement of transition part of slab surface cleanliness, especially bottom end part of continuously cast slab.
  • Finite Elemental Analyses of Forming Ultra Heavy Gauge Square Tube Using Hot Sizing Mill

    pp. 289-295

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    DOI:10.2355/tetsutohagane.93.289

    Ultra heavy gauge steel square tubes have been required as columns to promote the beautiful design of such as glass-faced buildings and shopping arcades. There have been some methods of manufacturing square columns, but manufacturing heavy gauge tubes was difficult in either method. Then hot forming was suggested as a method of manufacturing heavy gauge tubes by reshaping round cross-sectional mother pipe into square shape. Sharpness of the corners and flatness of the sides are both essentially required. Rigid plastic finite elemental method was applied to simulate the deformation of the material during the hot forming, and influence of forming condition on the both requirements was discussed. 2-roll-type hot sizing mill was applied as the instrument of the hot forming. The simulations were carried out in 4 cases with difference of total reduction or distribution of reduction at each stand. The square tube was experimentally formed under one of the above 4 cases.
    The calculated result was in a good accordance with the experimental result. The shapes of corners become sharper as growth of the total reduction. The sides become flatter as the reduction at the final stand is smaller. Large total reduction with small reduction at the final stand is recommendable to satisfy the both of sharp corners and flat sides.
  • Evaluation of Interfacial Fracture Toughness between Paint and Steel Sheet in Paint-coated Steel Sheet System Using DCB Specimen

    pp. 296-302

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    DOI:10.2355/tetsutohagane.93.296

    A method for evaluating the interfacial adhesion strength between paint layer and chromate-coated steel sheet in the paint-coated steel sheet system was proposed. Three kinds of the steel sheets modified with chromic acid which contained various content of colloidal silica and a paint composed of a mixture of alkyd resin and silica particles were used as specimens for the experiment. The adhesion strength was evaluated by the critical strain energy release rate at interfacial crack initiation. To measure the critical strain energy release rate, an asymmetric double cantilever beam specimen was used, where an interfacial pre-crack was induced between the paint layer and steel sheet. When the crack propagates along the interface between paint layer and chromate-coated steel sheet, it was observed that the critical strain release rate increase with increasing the content of colloidal silica in the chromate-coated steel sheet. Similar dependency of the silica content on the adhesion strength had been also confirmed in the cross cut tests. This signified that the interfacial adhesion strength could be evaluated quantitatively by the critical energy release rate.
  • Studies on Adhesive Properties of Steel Coating—Examinations of Influenced Factor between Steel Treatment Methods and Coating Thickness—

    pp. 303-308

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    DOI:10.2355/tetsutohagane.93.303

    This paper examined adhesive properties of steel coating alkid-melamine film (silica content 30 wt%) that has the relations between surface treatments of steel coating and thickness of steel coats. Especially we experimentally studied the influenced factors of adhesive properties between steel surface treatments and strength of coating film by using the interfacial cutting system (Saicas method).
  • Effect of Stress Ratio on Fatigue Properties for Ti-6Al-4V Alloy

    pp. 309-316

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    DOI:10.2355/tetsutohagane.93.309

    Effect of stress ratio on fatigue properties was investigated for 900 and 1100 MPa-class Ti-6Al-4V alloys. Numbers of heats were 3 for each tensile strength class. Fatigue tests were conducted under stress ratios of R=-1, 0 and 0.3, and under the condition fixing the maximum stress at the yield stress (σmaxy test). Although heat C for 900 MPa-class and heat F for 1100 MPa-class did not show internal fracture under R=-1, the internal fracture occurred in all heats under stress ratios above R=0. The fracture sites of the internal fracture revealed no inclusion but matrix including several facets under stress ratios above R=0. The size of a facet was almost equal to the α grain size and the areas of the internal fracture origin containing the facets were 100-300 μm. The fatigue limits determined at 108 cycles were below a modified Goodman line under R=0 and 0.3, although they were above under around R=0.7. The drops of fatigue limits under around R=0 suggested that the internal fracture properties were more sensitive to the stress ratios in case of the Ti-6Al-4V alloys. Moreover, in comparing the fatigue limits with ΔKth, the dependency of the fatigue limits on the stress ratios was similar to that of ΔKth.
  • Forming Limit Prediction and Work-hardening Behavior under Strain-path Changes

    pp. 317-323

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    DOI:10.2355/tetsutohagane.93.317

    The work-hardening behavior under strain path changes for dual phase (DP) steel has been investigated by using simple shear sequences, in comparison with that for interstitial free (IF) steel. The results show that a strong hardening followed by softening in an orthogonal path change, which is normally seen in IF steel, is not significant in DP steel. Based on these results, analytical approaches to predict forming limit strain and stress in monotonic and orthogonal paths by using Hill criterion have been conducted. It is known that stress based forming limit curves are path independent, while strain based forming limit curves are strongly path dependent. For IF steel, the localized necking is started just after the second loading in orthogonal sequence, which thus shows that even stress based forming limits are path dependent in IF steel. On the other hand, DP steel exhibits localized necking at the same amount of total strain both for monotonic and orthogonal paths and thus the stress based forming limits are path independent, which reflects differences in work-hardening under strain path changes in DP and IF steels. It might be pointed out that these differences are originated in the microstructural evolution of both steels.
  • Temper Embrittlement Characteristic of Cr-Mo-V Steel Steam Turbine Rotors in Long Term Service

    pp. 324-329

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    DOI:10.2355/tetsutohagane.93.324

    Temper embrittlement is one of the typical material degradations of Cr-Mo-V steel high and intermediate pressure steam turbine rotors used at high temperature in thermal power plants, and so it is important to know the embrittlement characteristics of the turbine rotors serviced for a long term. For this purpose, FATTs (50% fracture appearance transition temperatures) at around the center hole of retired turbine rotors serviced for 150,000-240,000 h were investigated.
    It was found that Cr-Mo-V turbine rotors were embrittled at the estimated service temperature of 350-460°C, and the maximum shift in FATT (ΔFATT) occurred at around 400°C. This embrittlement has been well correlated with an embrittling factor K2 (K2=(2Si+Mn+Cu+Ni) · X, where X=(10P+5Sb+4Sn+As)). It is thus concluded that the reduction of impurities is effective to reduce the susceptibility to the emberittlement during the service.
    With this findings, the equation, which could estimate the amount of emberittlement ΔFATT of Cr-Mo-V steel steam turbine rotor, was obtained as a function of the embrittling factor K2, the service time and the service temperature. According to the equation, it was estimated that the embrittlement increased rapidly until about 100,000 h, but slowly after about 150,000 h because of approaching the saturation of temper embrittlement.
  • Measurement of Concentrations of Tramp Elements in Shredder Scrap and Automobile Shredder Residue (ASR) and Their Mass Balance Analyses

    pp. 330-335

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    DOI:10.2355/tetsutohagane.93.330

    Contamination of undesirable tramp metals in steel scraps from end-of-life vehicles has been of interest for many years. In this work, typical 5 mini-vehicles and 5 small-size passenger cars were dismantled and put into shredder process, in which most of the parts end-of-life vehicles were removed thoroughly before shredding process. The weights of removed parts, shredder scrap, separated non-ferrous metals and automobile shredder residue (ASR) were measured to figure out the mass balances of the end-of-life vehicles. Then, the concentration of Cu, Cr, Sn in shredder scraps and ASRs were measured. It was found that the concentrations of Cu, Cr, Sn in shredder scrap were 0.078%, 0.004%, 0% for mini-vehicles, and 0.08%, 0.015%, 0.001% for small-size passenger cars. On the other hand, relatively high concentrations of Cu, Cr, Sn in ASR were observed, which was 0.42-0.85%, 0.03-0.04%, 0.05-0.06% for mini-vehicles, and 0.59-0.74%, 0.02%, 0.04-0.15% for small-size passenger cars, respectively. It was found that most of Cu contained in the end-of-life vehicles were removed before shredding process.
  • Relative Sensitivity Factor of Nitrogen in Steel by Helium/Glow Discharge-Mass Spectrometry

    pp. 336-340

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    DOI:10.2355/tetsutohagane.93.336

    We have already reported that, in argon glow discharge-mass spectrometry, relative sensitivity factors (RSF) for nitrogen are different among various steel samples. In this paper, the RSF was measured in detail when helium was employed as the plasma gas, and further RSFs for other elements such as carbon were also checked. 30 disk-shaped standard reference samples used in this experiment comprised four different groups depending on their chemical compositions, including stainless steel of Japanese Iron and Steel Certified Reference Materials (JSS CRMs). The sample surfaces were prepared up to a mirror-polishing with alumina (1 μm). The preliminary discharge time was set to be 120 min, as considering the results of preliminary experiments. The average RSF-value obtained for different alloy groups were 0.990-1.314 in the helium ion source, whereas the values were 11.03-28.69 in the argon ion source, indicating that their variations were much smaller in the case of helium. Variations of RSF-value for other elements were expressed as the RSDs of less than 6%. When using the helium ion source, the analytical result of nitrogen for actual samples, BS CRM 12B, well agree with the corresponding certified values. Reproducibility of analytical values for minor and trace elements was within 3%, except for the 4.5% of boron.

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