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

Tetsu-to-Hagané Vol. 84 (1998), No. 12

  • Formation of Spinel Inclusions in Molten Stainless Steel under Al Deoxidation with Slags

    pp. 837-843

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    Morphology change of inclusions were experimentally investigated under Al deoxidation of molten stainless steel with CaO-SiO2-Al2O3-MgO slags in order to clarify the morphology control factor of inclusions. 15kg of molten 18mass%Cr-8mass%Ni stainless steels were deoxidized by aluminum at 1873K, and the samples were taken at intervals to observe the inclusions by SEM and EDS. As the results, MgO contents of alumina type inclusions were gradually increased with time and the maximum contents were affected by a CaO/SiO2 ratio in slags. The formation of MgO-Al2O3, spinel type inclusions were also observed in case of high CaO/Si02 ratio. The origin of Mg in inclusions was presumed to be deformation of MgO in slags.
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  • Iron Carbide Formation with CO Gas and CO-H2 Gas Mixture under Low Sulfur Potential Using the Two-step Process of Metallization and Carbidization

    pp. 844-849

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    Iron carbide is a good alternative of iron resource, but it tends to be decomposed into iron and carbon. In the previous works, iron oxide was reduced and converted to carbide in one step. The authors showed that carbon deposition was suppressed by adding slight sulfur to the reaction gas, and that the reduced iron was carbidized completely. At the temperature lower than 930K or with CO and a small amount of H2 gas mixture, the sulfur potential in gas had to be high enough to form sulfide. However, the sulfur content of product was relatively high.
    Thus, two-step process was introduced in this work; Iron are was reduced to metal and it adsorbed sulfur at 873K and 973K with hydrogen gas mixture containing smaller amount of H2S than sulfide is formed. Then, it was carbidized with CO gas or 10vol%H2-H2S-CO gas mixture at the same temperature. Sulfur was saturated on the pore surface of reduced iron in the reduction step. It stabilized carbide and prevented carbon deposition with carbidization gas mixtures containing little amount of sulfur. The conversion to carbide reached about 80% before 8ks. The sulfur content of product was less than 0.05mass%, which is low enough for steelmaking. The catastrophic carbon deposition and pulverization of iron ore, i.e. 'metal dusting', happened in H2-CO gas mixture after 16ks, although carbon deposition would be controlled up to the complete conversion to carbide.
    Using the two-step process, the authors will measure the rate of carbide formation by a thrmobalance without the disturbance of reduction and carbon deposition.
  • Dead-man Temperature Distribution Measurement in a Blast Furnace by Acoustic CT Method

    pp. 850-855

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    A new sensor measuring two dimensional temperature distribution of the dead-man in a blast furnace has developed by using an acoustic wave CT method based on the theory that acoustic velocity in gas depends on a temperature.
    The features of a new measurement apparatus transmitting and receiving the acoustic wave which decreased to a great extent in packed bed is as follows.
    Shock wave is used in transmitter to generate acousitc wave with large acousitc pressure. Reciever has the good sensitivity for low frequency. Then, the least square method is adopted for computed tomography since the number of acoustic velocity data is limited.
    Relationship between temperature and acoustic velocity in a packed bed was obtained in off-line experiment.
    The measurement in an actual blast furnace was carried out during the scheduled shutdown. The acoustic wave was transmitted from a tuyere and the wave propagated through the furnace were received at some tuyeres. The temperature calculated by this method showed the decreasing tendency from side wall to the center of the blast furnace. An isothermal line of the temperature distribution had the possibility to show the shape of the dead-man. The temperature measured by this method was about 100°C lower than that measured by thermocouple.
  • Formation Mechanism of Slab Surface Transverse Cracking in Continuously Cast Ni Bearing Steel

    pp. 856-860

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    The hot ductility behavior of low-alloy steels with Ni of 0-4.9mass% was investigated in the temperature range from 873K to 1273K. The mechanism of forming slab surface transverse cracking in continuously cast Ni bearing steel was discussed in terms of the behavior and heat transfer properties previously reported.
    For the hot ductility, significant influence of Ni content was observed below 1050K, while slight influence was observed over 1100K. The width of the ductility trough spread toward the lower temperature as the Ni content increase. The precipitates consist of Ni did not form even in Ni bearing steel. According to thermodynamic calculation, precipitation behavior such as carbide or nitride did not change depending on Ni content, therefore, embrittlement in Ni bearing steel was not caused by precipitates. The spread of ductility depending on Ni content could be attributed to the A3 transformation temperature change, and brought about the slab surface transverse cracking. In the temperature range over 1100K, corresponding to the low temperature austenite region, ductility loss was induced by precipitates, e.g. NbC and/or AIN. Then hot ductility was independent of Ni content in that temperature range.
    On the other hand, Ni content had influence not only on hot ductility but also on cooling property as previously reported. The breakdown temperature of film boiling became higher with Ni content because of adherent scale layer, thereby transition boiling begins at higher temperature. After all, uniformity of cooling intensity was impeded, and susceptibility to cracking was increased because of thermal stress and falling into ductility trough.
  • Effects of Roll Surface Deteriorations on Scale Defect in Hot Rolling

    pp. 861-867

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    Roll surface deteriorations and scale deformation behavior in hot rolling are investigated in order to clarify a mechanism of rolling, load increase and scale defect formation on using high speed steel roll. Several experiments are conducted; a rolling/sliding, friction test at elevated temperature and model rolling tests for the sheet coil and plate by using high Cr cast iron and high speed steel as roll material. The results obtained are as follows; (1) Worn surface of high Cr cast iron is flat and smooth in hot rolling. On the other hand, fine asperities are formed on worn surface of high speed steel. It is found that this peculiar topography of high speed steel consists of MC carbides which are left by prior wear of matrix in roll microstructure and black oxide film which prevent MC carbides from removing from worn surface, which leads higher friction coefficient and higher rolling load. (2) Fine asperities cause scale heterogeneous deformation of the steel products in roll bite. resulting in scale defect. (3) Heterogeneous scale deformation is easily occurs under conditions with the large oxide film on the steel before rolling and the large reduction in thickness and the high rolling temperature.
  • An Application of the System Free Energy Theory to the Ferrite/Pearlite Transformation in Hypo-eutectoid Steel

    pp. 868-873

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    The kinetics of pearlite transformation in Fe-0.5mass%C steel has theoretically been investigated based on the system free energy theory proposed by us, by which the kinetics of phase transformation for each phase can be estimated according to its energy priority. The interlamellar spacing of pearlite is precisely predicted by the energy analysis, and the calculated time -temperature- transformation diagrams consist with the experimental ones. It is obvious that the system free energy theory is also effective for analyzing the lamellar structure produced by, e.g., the pearlite transformation in steel.
  • Effect of Mn Content on r-value and Precipitates Morphology in Ti-added Ultra-low Carbon Sheet Steels

    pp. 874-880

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    The influences of precipitates morphology in Ti-added ultra-low carbon sheet steels with carbon content reduced less than 0.001%, in order to clarify the relationships between the r-value and Mn content, and slab reheating temperature, was closely investigated from the viewpoints of gain size of hot band and grain growth during annealing. The obtained results are as follows;
    (1) The mean r-value in 0.05% Mn steel improves with dropping the slab reheating temperature (SRT) and shows the highest value at 1050°C. Contrary, the mean r-value in 0.35% Mn steel remains almost a constant even if the slab reheating temperature changes.
    (2) Precipitates after hot rolling in 0.05% Mn steel is TiS at a SRT of 1050°C, while fine Ti4C2S2 at a SRT of 1200°C, which is assumed to precipitate during and/or after hot rolling. Contrary, coarse MnS is much observed in 0.35% Mn steel after slab reheating at 1050 and 1200°C, and consequently after hot rolling.
    (3) The change in mean r-value as functions of Mn content and SRT can be explained by the changes in hot band grain size and grain growth in relation to the precipitation behavior such as sulfides during slab reheating and hot rolling process.
  • Erosion Properties of the Stainless Steel and Cr-Mo Steel in Fluidized Bed Condition

    pp. 881-887

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    The erosion tests were carried out about the MITI SUS304J1HTB and MITI STBA28 used to be utility boiler in the actual boiler-simulated conditions. And the effect of tube materialsand impact velocity of bed material on the erosion properties were considered. The results obtained are as flows.
    (1) The higher the velocity of bed material became, the peak of erosion wastage shifted to the less angle side of the circumference of specimen.
    (2) In the range of lower temperature on specimen surface, MITI STBA28 had better erosion-resistant than MITI SUS304J1HTB, because of thick oxide formation on MITI STBA28. On the other hand, in the range of higher temperature, the above tendency became reverse, because of separation of oxide on MITI STBA28.
  • High Cycle Fatigue Properties of Austenitic Stainless Steel Welds at Cryogenic Temperatures

    pp. 888-895

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    High cycle fatigue properties of base and weld metals of SUS304L and SUS316L and the effects of welding structure and δ ferrite on the fatigue properties have been investigated at cryogenic temperatures in order to evaluate the long-life reliability of the structural materials used in liquid hydrogen huge tankers or storage tanks and to develop a welding process for those applications.
    The S-N curves of base and weld metals shifted to higher stress or longer life side with a decrease in test temperature. The ratio of fatigue strength at 106 cycles to tensile strength of the weld metals were 0.35 to 0.7 and getting lower than those of base metals with decreasing tem-perature. Fatigue crack initiation site of SUS304L weld metals initiated almost at blow holes of approximately 200700μm diameter, and that of SUS316L weld metals initiated at a virtual crack. A virtual crack of SUS316L weld metal was caused by carbides of M23C6, and AI/Mn oxides, which were formed at weld pass interface boundary during welding. The decrease in fatigue strength at 106cycles at cryogenic temperatures was due to very small welding defects and a virtual crack. Although δ ferrite reduces toughness at cryogenic temperatures, the effects of δ ferrite on high cycle fatigue properties are not clear or significant.
  • Relationship between Creep Behavior and Microstructure in Martensitic Heat Resistant Steel

    pp. 896-901

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    Relationship between creep deformation behavior and microstructural change has been investigated in a 12%Cr-0.12%C steel by means of optical and transmission electron microscopy. Applied stress assists the recovery of martensite at high temperature: Dislocation density is markedly decreased and Ostwald growth of M23C6 type carbide is promoted. Creep deformation behavior strongly depends on these microstructural changes in lath martensitic structure. In the early stage of creep test, M23C6 type carbide particles precipitate along lath boundary and form rod-like carbide. Such a carbide effectively suppresses the movement of dislocations and contributes to the holding of creep strength. But once the rod-like carbide is separated through Ostwald growth of carbide particles during creep deformation, dislocations become easy to move and the recovery of martensite proceeds rapidly. This leads to an acceleration of the creep deformation.
  • Effects of Shot-peening on Resistance to Softening during Tempering and Contact Fatigue Strength of Carburized and Carbo-nitrided JIS SCr420H Steels

    pp. 902-909

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    Effects of shot-peening were investigated on the resistance to softening during tempering at 573K and the contact fatigue strength of JIS SCr420H steels carburized conventionally and carbo-nitrided to a surface nitrogen content of about 0.70mass% N. Experimental methods involved in the investigation include hardness measurement, X-ray diffraction analysis and roller-pitting fatigue test.
    The resistance to softening during tempering in the carbo-nitrided layer is found to be higher than in the carburuized layer, which is due to the precipitation of γ'-Fe4N. It is further improved by shot-peening, which effect is larger after carbo-nitriding than after carburizing. The reason seems to be that recovery of dislocations introduced by shot-peening would be delayed by solute nitrogen and precipitation of γ'- Fe4N. Contact fatigue strength in terms of pitting durability and wear resistance is estimated from a roller-pitting test under a Hertz stress of 3.68 GPa added slip (-40%) under an assumption that the temperature of contact surfce rose to nearly 573K. Shot-peening is shown to improve the pitting life of the carbo-nitrided specimen by as much as 25 times, compared with only by 2times for the carburized specimen. Pitting life can be explained from the differences in the initial surface hardness and the resistance to softening. The pitting life was also found to correlate positively with the surface hardness of the specimen tempered at 573K.
  • Deviation from Matthiessen's Rule during Quench-aging of Mo Added Low Carbon Steels

    pp. 910-917

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    The formation and decomposition of Mo-C dipoles during quench-aging in 0.03 mass%C steels containing from 0 to 1.01mass% Mo is discussed. The changes in electrical resistivity and its deviation from Matthiessen's rule (DMR) during aging at 523K were examined for the steels just after quenching from 973 to 273K. The difference between the electrical resistivity measured at 77K and 273K was regarded as the value of the DMR. The electrical resistivity decreases for all steels during aging. Also the DMR decreases for steels except 1.01 mass% Mo steel. The decrease in electrical resistivity and DMR is largest in 0.25mass% Mo steel. Increment of lattice constant of ferrite with increase of Mo content in 0 to 0.25mass% Mo steels aged for 105s after quenching is less than that in 0.51 to 1.01mass% Mo steels. These results can be explained from the assumption that following two reactions occur during aging, 1) Mo-C dipoles existing in quenched ferrite matrix decompose in accordance with precipitation of carbides, 2) solid solute Mo atoms in quenched ferrite matrix decrease with precipitation of carbides containing Mo. The Mo-C dipoles may be formed at the solution treatment temperature by chemical interaction between Mo and C atoms, similarly Mn-C(N) dipoles in Fe-Mn-C(N) alloys. The amount of Mo-C dipoles increases with increase of Mo content. The binding energy of a Mo-C dipole is calculated to be approximately -5.7×10-20J.

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