Tetsu-to-Hagané
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ONLINE ISSN: 1883-2954
PRINT ISSN: 0021-1575

Tetsu-to-Hagané Vol. 83 (1997), No. 5

  • The Effect of Limonite Ore on the Sintering Mechanism

    pp. 287-292

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    The effects of pisolitic ore on the melting reaction rate and the fluidity of the melt were investigated to make clear the mechanism of the decrease of the sinter yield with the blending of pisolitic ore.
    Pisolitic ore, characterized by the high porosity, increases the melting depth by the promotion of melting reaction through the pore. As a result, CaO/(CaO + Fe2O3) in the melt decreases and the liquidus temperature of the melt rises. Thus, the time available for the pore rearrangement is decreased and the distribution of pore size changes with the blending of pisolitic ore.
  • Influence of Oxygen and Humidity of Inlet Gas on the Sintering Reaction

    pp. 293-298

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    The influence of oxygen and humidity of inlet gas on the sintering reaction was studied through mathematical model analysis. Then, the change of oxygen and humidity of inlet gas was investigated for the effects on the sintering time, product yield of sinter, heat wave in sintering bed and other parameters in all of the sintering pot tests.
    The flame front speed, the maximum temperature in the sintering bed and product yield of sinter decreased with decrease in oxygen content of the inlet gas.
    When the moisture content of the inlet gas was increased, the maximum temperature in the sintering bed decreased because it took time for moisture to evaporate. Moreover, the flame front speed decreased because of an increase in the gas flow resistance of the moisture condensation zone in the sintering bed.
    If the decrease of oxygen content and increase of moisture content of the inlet gas are combined, the adverse effect on the product yield of sinter becomes greater due to a drop in the heat level as a result of the decrease of maximum temperature in the sintering bed. In such case, the increase of the temperature of the inlet gas can be expected to improve this adverse effect.
  • Carburization Rate into Solid Iron at CO-CO2 Atmosphere

    pp. 299-304

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    In order to construct a mathematical model of a moving bed type ironmaking process using iron scrap as the burden materials, the carburization rate from CO-CO2 gas mixture into solid iron was studied experimentally and theoretically.
    The mass change of an iron sample during carburization was measured by a thermo-balance over a temperature range of 973 to 1423K and CO concentration of 70 to 100%. In CO atmosphere, carburization rate increased significantly with temperature over 973K, however it decreased over 1273K. In CO-CO2 atmosphere, carburization scarcely proceeded over 10% CO2 concentration at 1273K.
    The carburization rate equation was derived by considering CO dissociation and CO2 formation as two elemental processes. Rate constants were obtained from experimental data. The rate equation obtained agreed well with experimental results.
  • Effect of P on Hydraulic-descaling-ability in Si-added Hot-rolled Steel Sheets

    pp. 305-310

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    Scale defects, called "red-scale", frequently form in Si-added hot-rolled steel sheets because of poor hydraulic-descaling-ability in hot-rolling process. In this work, effect of P on the hydraulic-descaling-ability in Si-added hot-rolled steel sheets and the mechanism of descaling are investigated by red-scale reproduction test and the analysis of Fe-Si-P oxide.
    (1) Addition of P to Si-added steel improves the hydraulic-descaling-ability.
    (2) Because the addition of P lowers binary eutectic temperature of FeO/Fe2SiO4 oxides formed during slab soaking. Therefore the liquid eutectic compound in the scale/steel interface during the descaling improves the hydraulic-descaling-ability.
    (3) Furthermore high slab soaking temperature in addition to increase of P is more effective to improve the hydraulic-descaling-ability because of high descaling temperature.
    (4) The Fe-Si-P scales were synthesized from mixtures of the reagents of Fe, Fe3O4, SiO2 and P2O5. Decrease in the binary eutectic temperature of FeO/Fe2SiO4 with P2O5 is clarified by thermal analysis of them.
  • Identification of the Fe-Zn Intermetallic Phases by TEM

    pp. 311-316

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    A series map of selected area electron diffraction patterns (SADPs) and the microstructure criteria for the intermetallic phases in the Fe-Zn system, i.e., Γ (Fe3Zn10-bcc), Γ1 (FeZn4 or Fe5Zn21-fcc), δ1k (FeZn7-superlattice hexagonal), δ1p (FeZn10-hexagonal) and ζ (FeZn13-base centered monoclinic) were investigated by transmission electron microscopy (TEM). Following results are obtained:(1) A series of SADPs are very powerful handy-tools to identify each of the intermetallic phases in the Fe-Zn system. (2) Each of the intermetallic phases shows a characteristic microstructure such as dislocations, antiphase domain boundaries (APB) and stacking faults (SF).
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  • Effects of Carbon Content and Chromium Segregation on Creep Rupture Properties of Low Carbon and Medium Nitrogen Type 316 Stainless Steel

    pp. 317-322

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    The creep rupture properties of type 316 stainless steels containing 0.0050.022%C and 0.07%N have been investigated at 550°C and 600°C from the aspect of the grain boundary carbide precipitation which was changed with carbon content and chromium segregation. A small amount of carbide precipitated on grain boundaries during creep, because the solibility limit of the carbide is less than 0.005%. The creep rupture ductility of this steel increased with the reduction of carbon content from 0.010% to 0.005% while it decreased with increasing carbon content from 0.010% to 0.020%. Since the amount of grain boundary carbide decreased with reducing carbon content, the increase in ductility was due to the suppression of grain boundary embrittlement caused by the carbide. The creep rupture ductility of this steel was also improved by reducing chromium segregation. This behavior was attributed to the change in carbide morphology from concentrated type to dispersed one, which reduced the grain boundary embrittlement.
  • The Load Drop Phenomenon at the Fracture Toughness Test of the Heat Affected Zone of a Low Carbon Steel

    pp. 323-328

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    The load drop or pop-in phenomenon at the fracture toughness test of the heat affected zone (HAZ) of a low carbon steel has been investigated. Microstructural features such as the size of the bainitic ferrite were varied with the thermal cycles simulating a double pass welding. It is revealed that the initiation of the load drop is not dependent on test temperatures, whereas the arrest of the load drop decreases with the decrease of the test temperature, thus increasing the amount of the load drop. Fractographic examination has revealed that the amount of the load drop scales with the crack length formed by the local brittle fracture. Also, the size and density of cleavage facets increases with the load drop. It is discussed that the temperature dependent amount of a load drop is determined by the ductile linking of cleavage facets produced at bainitic ferrite grains.
  • Relation between Fracture Mode and Carbide of Hastelloy XR Creep-tested in Helium M23C6/M6C Environment

    pp. 329-334

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    Fracture modes and carbides of Hastelloy XR (a modified version of conventional Hastelloy X) have been examined metallographically. Creep tests were carried out at 800, 900 and 1000°C in a simulated environment of high-temperatute helium-cooled reactor (HTGR). At 800 and 900°C, wedge-type cracks were observed when specimens were tested under higher stresses, while cavity-type cracks were formed under lower stresses. At 1000°C, localized recrystallization was observed in fractured zone. Stress exponent for rupture life was 5.7 at 800°C and 3.4 at 1000°C, while the value changed from 5.9 to 3.3 in those with longer rupture life at 900°C. Nucleation and growth of crack were strongly associated with grain boundary carbides under tensile stress. Carbides were classified into two types at 900 and 1000°C; Cr-rich M23C6 and Mo-rich M6C, respectively. At 800°C, another type of carbide was found also. The two types of carbide had no different effect on nucleation and growth of crack. Comparison of the results in helium environment with that in air indicated that cracks and carbides were similar to each other except for the regions fractured under heavily oxidized condition. M23C6 was stable up to 1050°C and co-existed with M6C. In conventional Hastelloy X, precipitate of M6C predominated over M23C6 at 900°C.
  • Effects of Warm Forming on Stretch-flangeability of a TRIP-aided Dual-phase Sheet Steel

    pp. 335-340

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    To improve the stretch-flangeability of a high strength TRIP-aided dual-phase (TDP) sheet steel, the effects of forming temperature, forming rate and clearance on the stretch-flangeability were investigated. Warm hole-punching at 150-200°C and successive warm hole-expanding at about 150°C increased considerably the stretch-flangeability. The warm hole-punching minimized void initiation at matrix/second phase interface, work hardening and a depth of damage layer at hole surface by suppressing the strain-induced transformation in an early stage. Also, it increased the volume fraction of untransformed retained austenite, which contributes to large stretch-flangeability. The warm hole-expanding improved the localized ductility due to the transformation induced plasticity. When a clearance between punch and die was about 10%, the largest stretch-flangeability was obtained with the minimum surface damage on hole-punching. Punching rate influenced hardly upon the stretch-flangeability, although it did somewhat on surface damage.
  • Effect of Retained Austenite on Strength and Elongation in Metastable Austenitic Stainless Steels

    pp. 341-346

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    Effect of retained austenite on strength and ductility has been investigated in cold rolled metastable austenitic stainless steels having various austenite stabilities. Steels consist of from 50 to 60vol% martensite through 50% cold rolling at several temperatures. The steels achieved high strength by aging at 400°C for 3.6ks after 50% cold rolling. A stabilization of austenite generally enhanced the 0.2% proof stress and tensile strength. An increase in the strength results from dislocation morphology of cold rolled austenite and this is attributable mainly to planar dislocation of the austenite. In Erichness test, maximum ductility is obtained in steel with austenite stability Md(N) (= 580-520C-2Si-16Mn-16Cr-23Ni-300Ni-26Cu) of 40. Good ductility is a consequence of the formation of strain induced martensite during deformation. It is concluded that optimum austenite stability Md(N) is 40 to obtain high strength and ductility.

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