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

Tetsu-to-Hagané Vol. 87 (2001), No. 2

  • Segregation Behavior and Deformation Behavior during Soft-reduction of Unsolidified Steel Ingot

    pp. 71-76

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    Soft-reduction experiments on the unsolidified steel ingots were conducted to investigate segregation behavior and deformation behavior during reduction. V-type segregation was restrained when soft-reduction was applied under optimum conditions of a reduction value and reduction rate to compensate for the solidification shrinkage. Excess reduction caused inversed V-type segregation, negative segregation (white band) and internal cracks. The reduction rate should be controlled in accordance with the solidification rate of the unsolidified steel ingot. Apparent reduction stress was affected by the fluidity of the liquid core. Stress relaxation during repeated reduction at an interval of 15s was estimated to be about 30%.
  • Improvement of Semi-macro Segregation in Continuously Cast Slabs by Controlled Plane Reduction

    pp. 77-84

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    To improve center segregation in continuously cast slabs, a new technology, named 'controlled plane reduction', was developed. By this technology, the movement of molten steel in the final stage of solidification was restrained by both the support and the reduction of unsolidified slabs with two walking-bar blocks to prevent bulging and compensate for the solidification shrinkage. As a result, it was found that the center segregation in continuously cast slabs can be improved by the controlled plane reduction to a level comparable to that of interdendritic microsegregation. To thoroughly alleviate the center segregation by the controlled plane reduction, it is important to apply an optimum reduction taper to the unsolidified slab and to minimize the reduction taper difference between the two walking-bar blocks. The resulting optimum reduction taper by the controlled plane reduction experiments agreed with that estimated by a theoretical study based on the analysis of a molten steel flow.
  • Formation Mechanism of Sliver-type Surface Defect with Oxide Scale on Sheet and Coil

    pp. 85-92

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    Laboratory rolling test of continuously cast steel slab with artificial defects was conducted to determine the origin of sliver-type surface defects with oxide scale on sheet and coil. The results obtained are summarized as follows:
    (1) The surface crack and pinhole could be the origins of sliver-type surface defects with oxide scale.
    (2) The deformation behavior of crack-type artificial defects during rolling varied depending on the depth and width of artificial defects, and on the kinds of steel.
    (3) The crack-type artificial defects of Ti-SULC steel tended to remain after rolling than those of LC-Al-K steel and SUS430 steel, because of the difference in the oxidation behavior at the subsurface during heating.
    (4) The critical depth for the pinhole-type artificial defects to be exposed by rolling was estimated to be about 4 to 5 mm.
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  • Effect of Mn Depletion on Intra-granular Ferrite Transformation in Heat Affected Zone of Welding in Low Alloy Steel

    pp. 93-100

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    Role of Mn depleted zone formed around MnS precipitate in austenite (γ) in transformation of Intra-Granular Ferrite (IGF) in low alloy steel subjected to simulated thermal cycle of weld Heat Affected Zone (HAZ) has been studied. Mn distribution in γ adjacent to MnS were examined by Field-Emission type Transmission Electron Microscopy (FE-TEM) with Energy Dispersive X-ray Spectroscopy (EDS). Foil samples for TEM observation were prepared using Focused Ion Beam (FIB) in order to prevent ununiformity of thinning rate of MnS and steel, and to thin the nucleation site of IGF specified by optical or scanning electron microscopic observations. The authors demonstrated the direct measurement of Mn depleted zone at IGF nucleation site. Mn depletion varied from 0 to 0.4 mass% depending on annealing temperature and holding time at γ phase region. Calculated concentration profiles of Mn near the depleted zone for various thermal cycles showed good agreement with experimental results. Change in area fraction of IGF with thermal cycle conditions was well explained by the extent of Mn depletion. It was concluded that formation of IGF was controlled mainly by the Mn depletion effect in low alloy steels.
  • Tensile Property Changes Due to Phase Decomposition in Duplex Cast Stainless Steel

    pp. 101-106

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    Duplex stainless steels are subject to embrittlement due to the phase decomposition of ferrite to form Fe-rich phase and Cr-rich phases during thermal aging about 300°C. The decomposition fraction of ferrite and the volume fraction (fα') of Cr-rich phase were measured by Mösbauer spectroscopy as a function of aging time at 350°C. The result of fα', was consistent with a result obtained by TEM for the specimen aged for 10016 h. The phase decomposition was accompanied by the increase in hardness of ferrite and 0.2% proof stress, contrary to the decrease in fracture elongation. The origin of the strength change can be fully explained by a model proposed by Williams for fα'<12%, but partly for fα'>12%. Initiation of crack was observed near the ferrite boundary adjacent to arrested slip bands in the deformed austenite by the in-situ observation under tensile test. The crack nucleation is discussed on the basis of the Stroh's model.
  • Aged Microstructures of a Low Thermal Expansion 41Fe-29Ni-22Co-4Nb-2Cr-1Ti-0.5Al-0.5Si Superalloy

    pp. 107-112

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    41Fe-29Ni-22Co-4Nb-2Cr-1 Ti-0.5Al-0.5Si alloy (alloy 929C) is the latest low thermal expansion superalloy with a good resistance to SAGBO (Stress Accelerated Grain Boundary Oxidation) embrittlement at elevated temperatures. This alloy is strengthen by the precipitation of γ'(Ni3(Al, Ti); Ll2) phase in the alloy matrix. This study was carried out to elucidate the relation between the age-hardening and the growth of γ' precipitates in alloy 929C by micro Vickers hardness test and transmission electron microscopy.
    The results obtained in this study are as follows:
    ( 1 ) Comparing with the similar superalloys like alloys 909 and 929, the age-hardening rate in alloy 929C is slow, its peak hardness is high, and the overaging is suppressed.
    ( 2 ) Age-hardening proceeds with growing γ' precipitates and the hardness reaches to the maximum in the size of γ' precipitates 1113 nm. After reaching to the maximum value the hardness decreases gradually according to the coarsening of γ' phase. Though the γ' precipitates in the alloy 929C are relatively stable, they transform gradually to a stable η phase with increasing aging time but never transform to εphase unlike alloys 909 and 929.
    ( 3 ) The growth kinetics of the γ' precipitates are explained by Ostwald ripening theory of volume diffusion controlled growth. The apparent activation energy for the growth of γ' precipitates was estimated to be 259 kJ/mol which was nearly equal to those of diffusion of Ti or Fe atoms in γ-iron or nickel.
    ( 4 ) The γ' phase precipitates in sphere in the early stage of aging and shifts to in cubic with increasing aging time. The γ' precipitates with the size of 150200 nm change to a star-like form with eight horns according to the growth of <111>γ', direction.

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