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

Tetsu-to-Hagané Vol. 89 (2003), No. 10

  • Formation of Iron Carbide in Fluidized Bed by the Reactions of Partially Pre-reduced Iron Ore with CO-H2-H2S Mixtures: Influence of the Reduction Degree

    pp. 1005-1010

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    In our previous studies of the iron carbide formation, the iron are particles were completely reduced to metallic iron and then the reduced iron was carburized with CO-H2-H2S mixtures. However, the reactivity of the reduced iron seems to definitely influence the rate of the iron carbide formation. Therefore, in this study, at the atmospheric pressure and 873K, iron are particles of hematite were first reduced to various reduction degrees with 50% N2-H2-H2S mixture and second exposed to CO-H2-H2S mixtures with aS=0.5. The remained iron oxides are reduced and then the fresh reduced iron ore of somewhat low sulfur contents is immediately carburized. It was experimentally confirmed that the lower the pre-reduction degree, the faster the carburization rate and the easier the formation of Fe5C2 and free carbon. The recommended condition for the fast and stable carbide formation is to be a low pre-reduction degree and the CO-H2-H2S mixtures containing less than 60% CO. From the microscopic observation of cross sections of samples, a new knowledge was obtained that the remained wustite and reduced iron are open to the reaction gas access through the pore of particles.
  • Kinetics of Iron Carbide Formation from Reduced Iron Ore in CO-H2-H2S Mixtures

    pp. 1011-1017

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    The rate of iron carbide formation by the reaction of completely reduced limonite with CO-H2-H2S mixtures of as=0.5 was gravimetrically measured. The weight gain by the reaction was evidenced to directly correspond to the carburization, i.e. taking in carbon from the gas phase, to form iron carbide by comparing the weight gain with the amount of carbon in iron carbides analyzed by Mossbauer spectroscopy. The rate was measured in the varied total flow rates and varied compositions of the reaction gas, and the varied weight and varied size of iron ore particles. It was concluded that the carburization rate on pore surface of completely reduced iron ore particles controls the iron carbide formation rate. By applying the integrated rate equation for the first order reaction to fθ versus t curves, the carburization rate was evaluated. The rate of the reduced limonite ore was much faster than the rate of a reduced hematite ore by the factor about 6. From the rate being nearly proportional to pCO to 80%CO, it was concluded that the main carburization reaction is CO+H2=[C]+H2O and the rate controlling step is the dissociative adsorption of CO molecules. The carburization rate decreased with increasing the reduction temperature but it hardly changed with the carburization temperature because the rate was strongly dependent on the specific surface are of reduced limonite. The dependencies of the rate on the reduction temperature for the carbide formation at 973K and on the carbide formation temperature from the reduced limonite at 1173K were quantitatively discussed.
  • Desulfurization of Steel Melt by Pulverized CaO Blasted through Top Lance of Vacuum Vessel in RH Degasser

    pp. 1018-1022

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    Experimental study of desulfurization reaction by pulverized CaO impinging upon steel melt in a vacuum vessel of a RH degasser was carried out to disclose the effects of powder and operation factors.
    Experiments were carried out by NO. 2 RH degasser at Mizushima works of Kawasaki Steel Corporation. Powder was blasted on steel melt in the vacuum vessel through KTB (Kawasaki Top oxygen Blowing) with a single hole. The powder was penetrated into steel melt and transferred to a ladle except powder directly drawn by suction through an exhauster. Powder size, powder feed rate and slag composition in a ladle were changed for the experiments. Initial sulfur content was 25-35 mass ppm.
    Sulfur content after the pulverized CaO blasting treatment reached below 10 mass ppm. Desulfurization ratio increased as the CaO unit consumption increased while the (MnO)+(T.Fe) and the oxygen activity in molten steel decreased. Because too small powder size caused the decrease in yield and too large size decreased the desulfurization rate, an optimal powder size existed for desulfurization efficiency.
  • Improvement of Hot Ductility in Continuously Cast Strand by Ferrite Precipitation Control

    pp. 1023-1030

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    In order to clarify the correlation between susceptibility to transverse cracking and microstructure of slab surface, an apparatus and a method of new hot tensile test were designed. Tensile specimens in a cold crucible type heater were in-situ remelted preceding the deformation. Two thermal history for simulating solidification process, mild cooling and SSC (Surface Structure Control) cooling, were examined. Mild cooling means gradual cooling after solidification corresponding to conventional secondary cooling. SSC cooling is a trial cooling pattern aimed at microstructure control which provides rapid cooling until the γ-α duplex phase region and reheating up to γ region.
    The results obtained are summarized as follows.
    (1) It was possible to evaluate hot ductility of slab surface with microstructures by means of this hot tensile tests simulating the solidification process.
    (2) By SSC cooling, the ductility is significantly improved and the fracture mode changes into transgranular ductile, because film-like ferrite is restrained.
    (3) Susceptibility to transverse cracking could be reduced with this microstructure.
    (4) Decrease of transverse cracking susceptibility and microstructure control result from fine precipitates dispersion, such as (Ti, Nb)(C, N), caused by SSC cooling.
    (5) By utilizing carbonitride precipitation, film-like ferrite along γ grain boundary could be restrained and idiomorphic ferrite could be formed throughout the matrix.
    (6) Remelting of specimen before deformation is indispensable to evaluate cracking susceptibility on the hot tensile test.
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  • Structural Analysis of Slag Using Multinuclear Solid State NMR

    pp. 1031-1037

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    A new approach to structural analysis of the slag has been proposed using multinuclear solid state NMR that is capable of obtaining the structural information of each element composing the slag such as Si and Al, and effective to both crystalline and non-crystalline solid materials. In this study, we applied 19F(I=1/2), 29Si(I=1/2) and 27Al(I=5/2) solid state NMR to two types of steel-making slags, that differ in Al contents; 2.94 mass% (slag A) and 0.10 mass% (slag B). Measurement of 19F magic-angle-spinning (MAS) NMR enables to determine which metal among Ca, Si and Al is directly bonded to F. In these slags, F was found to be mainly coordinated by Ca. The state of SiO4 network (Qn) was estimated by 29Si MAS spectra. As the result, it was found that highly condensed network of SiO4 tetrahedron such as Q2, Q3 and Q4 does not exist in these slag systems. In 27Al NMR measurement, two-dimensional 27Al multiple-quantum magic-angle-spinning (MQMAS) NMR was applied which can average the second-order quadrupolar interaction. Drastic improvement of spectral resolution at high field (16.4 T) 5QMAS experiment clarified the existence of at least 11 chemical sites of Al in slag A. From the above mentioned results, multinuclear solid state NMR is proved to be a very effective method in characterization of the slag.
  • Thermo-mechanical Treatment with Multi-direction Upsetting for Improvement of Superplasticity in SUS304

    pp. 1038-1043

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    Various thermo-mechanical treatments have been tried in order to improve the structural superplasticity in conventionally produced JIS SUS304.
    The attempts of thermo-mechanical treatment consisting of three or four cycles including cold working by roll-mill and subsequent annealing. By adopting the cold multi-direction upsetting method, which is suitable for the accumlation of enough volume of martensite phase, the grain size decreases into submicrons in diameter. Especially by thermo-mechanical treatment of four cycles, the grain size of 200 nm is achieved. The upper limit of strain rate to obtain significant superplastic behaviour is improved up to 1 × 10-2/s at 973K by the grain refinement.
  • Migration of Low Angle Grain Boundaries during Secondary Recrystallisation in Silicon Steel Sheet

    pp. 1044-1049

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    The sluggish migration of low angle grain boundaries was examined with regard to the progress of secondary recrystallisation in silicon iron processed by the two stage cold rolling method. The Goss secondary grains are frequently surrounded by low angle grain boundaries during their growth. In the present work, the retarded migration was demonstrated by measuring the frequency of grains at the perimeter of the secondary grains that could stunt growth through 'orientation pinning'. The poor migration of Goss secondary grains is concluded to be caused by their reduced mobility. The orientations of the stunting grains were observed to deviate less than 7° from those of the growing secondary recrystallisation grains. Such low angle grain boundaries are expected to be comprised of dislocation arrays and be much less mobile than general boundaries having misorientations of more than about 15°.
  • Effect of Cu Addition on Formation of Reversed Austenite and Hardness in 9% Ni Steels

    pp. 1050-1056

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    Copper (Cu) was added to a cryogenic 9% Ni steel as an alloying element for improving strength and controlling microstructure, and then the softening behavior and microstructural development during tempering were investigated in the 9% Ni steels with various amounts of Cu. Hardness of the specimens decreased with an increase in tempering time owing to recovery of martensite. However, the addition of Cu more than 1% significantly retarded the softening. In particular, the steels containing 2% Cu or more exhibited clear age hardening phenomena after tempering at 873K for 1.8 ks. The retardation of softening by Cu is found to be due to precipitation of fine Cu particles within martensite matrix. On the other hand, Cu addition promoted the formation of reversed austenite at tempering temperature, thus the volume fraction of the reversed austenite became much larger in a 3% Cu bearing steel than in a Cu free 9% Ni steel under a same tempering condition. Chemical analysis using XEDS revealed that not only Ni but also Cu concentrate into the reversed austenite and make the austenite more stable in the Cu bearing steel than in the Cu free steel. As a result, the hardness of Cu bearing 9% Ni steels was found to be estimated by the law of mixture of each hardness in martensite and reversed austenite.
  • Strengthening Mechanism in Hot-rolled High Strength Sheet Steel with Increase in Tensile Strength Induced by Bake Hardening

    pp. 1057-1064

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    A new type of bake-hardenable high strength hot-rolled sheet steel was developed, which shows remarkable increase in tensile strength (TS) as well as yield strength after strain age treatment such as paint baking to autobody. The new sheet steel possesses excellent crashworthiness and high fatigue strength, and also shows good formability equal to that of conventional high strength sheet steel. Furthermore, it shows good anti-aging property at room temperature. The mutually contradictory requirements of a high bake hardening capacity in TS and anti-aging property at room temperature are satisfied by optimizing the solute N content in the hot rolling process.
    The increased TS observed after paint baking is due to large work hardening relating with high dislocation density. In the case of developed steel, dislocations generated during prestrain are strongly locked by nitrogen after paint baking. Therefore, multiplication of dislocation encouraged at these locked dislocations to progress plastic deformation, resulting in high dislocation density. The external force required for movement of a dislocation in a dislocation group increases as the dislocation density becomes larger, leading to increase in TS. As the result of TEM investigation, nitrogen forms clusters and/or precipitates on locked dislocations after paint baking.
  • The Effects of Heat-treatment Conditions on Stretch-flangeability and Bendability of High-strength Low Alloy TRIP-aided Sheet Steels with Annealed Martensitic Matrix

    pp. 1065-1070

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    The effects of heat-treatment conditions on stretch-flangeability and bendability of 800 MPa grade 0.2C-1.5Si-1.5Mn low alloy TRIP-aided sheet steels with annealed martensitic matrix or "TRIP-aided annealed martensite steels" were investigated for automotive applications. Good stretch-flangeability and excellent bendability, as well as large ductility, were achieved in the steels intercritically annealed at 780-820°C, followed by austempering at 375-400°C for 100-1000 s. In this case, initial microstructure prior to intercritical annealing was martenite or fine bainite structure transformed at temperatures below 400°C. These good formabilities were considered to be caused by fine annealed lath matrix with low dislocation density, interlath stable retained austenites and relatively low difference in strength between second phase and matrix.
  • Hot Deformation Behavior in Mg-Al Alloy, AZ91D Grain Refined by Isothermal-rolling

    pp. 1071-1075

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    The isothermal-rolling mill has been developed for non-working materials, for example, like Mg-Al alloy. The recrystallization behavior of Mg-Al alloy, AZ91D has been investigated. The starting and finishing temperatures of recrystallization are 523K and 648K, respectively. AZ91D sheets tempered to fine grain size by recrystallization have been prepared and tensed at high temperatures. Both the total elongation and strain-rate sensitivity m have been obtained and discussed. The values of total elongation have exceeded over 100% at testing temperature over 623K with strain-rate below 1.0×10-2s-1 and 170% of maximum total elongation has been obtained. The m values have indicated over 0.5 in range of 1.0×10-4s-1 to 2.5.×10-4s-1 except testing temperature of 573K. The activation energy required for high temperature deformation has been calculated to be about 124 kJ·mol-1, which is almost same of the activation energy for self-diffusion coefficient in Mg. Therefore, the deformation of AZ91D has been accommodated by volume diffusion.
  • Evaluation of Mobile Dislocation Density of Modified 9Cr-1Mo Steel by Stress Change Test

    pp. 1076-1081

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    The quantitative evaluation of the relationship between creep deformation and microstructure is important in order to improve the accuracy of the residual creep life evaluation of power boilers and turbines. In this study to characterize the creep deformation mechanism of a Modified 9Cr-1Mo Steel used in newly constructed boilers, stress change tests were conducted during creep tests. It was confirmed that the dislocation behavior during the creep tests were in viscous manner because of no instant plastic strain observed at stress increments and transient backward creep behavior after stress reduction. Mobilities of dislocation evaluated by observed backward creep behaviors after stress reductions and internal stresses evaluated by the changes of creep strain rate in stress increments were stable during creep deformation. Mobile dislocation densities were evaluated with the estimated mobilities of dislocation and the changes of creep strain rate in stress increments. Variation of evaluated mobile dislocation densities with creep strain showed same tendency of variation of creep strain rate. Therefore mobile dislocation density is the dominant factor that influences the creep strain rate in creep deformation of this steel. The internal stress of mobile dislocation can be considered to be originated from the line tension of bowing mobile dislocation in this steel.
  • The Effect of Modified-ausforming on Giga-cycle Fatigue Properties in Si-Mn Steels

    pp. 1082-1089

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    This report reveals gigacycle fatigue properties for a modified-ausformed Si-Mn steel (SMn443) with the chemical composition of 0.42C-0.2Si-1.52Mn in mass%. The Si-Mn steel has an advantage in recycling since the steel does not contain Cr and Mo which are difficult to be removed in re-melting. On the other hand, disadvantages of the Si-Mn steel are low tempering resistance and low hardenablility.
    Modified-ausformed, oil-quenched and water-quenched steels were prepared for fatigue tests, followed by tempering at 473K and 673K to prepare two different strength specimens. The tensile strengths of steels tempered at 473K and 673K were nearly 1400 MPa and 2000 MPa, respectively. The problem of hardenablility was solved by modified-ausforming, while the tempering resistance was not improved. The low-strength modified-ausformed steel (AF1400) was free from fish-eye fracture with a fatigue limit of 770 MPa at 109 cycles, while the low strength oil-quenched steel (QT1400) caused fish-eye fracture. Although the high-strength modified-ausformed steel (AF2000) was suffered from fish-eye fracture, the fatigue limit at 1010 cycles was 830 MPa that was higher than 710 MPa of the water-quenched steel (QT2000W) tempered at 473K. The fatigue limit at 109 or 1010 cycles for both oil-quenched steels (QT1400 and QT2000) could not be determined because of temperature increase of specimens in 20 kHz tests. The fracture surfaces revealed optically dark areas (ODA) even in modified-ausformed steel since the modified-ausforming was not perfect in this research.
  • Evaluation by Statistics of Extreme of the Effect of Boron Addition on the Penetration Behaviour of Oxide Scale of Hot Forged Structural Steel

    pp. 1090-1092

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