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

Tetsu-to-Hagané Vol. 83 (1997), No. 8

  • Analysis of Flow Behavior of Fluid during Sintering Process with Large Amounts of Pisolitic Ore by Hot Stage X-ray CT Scanner

    pp. 473-478

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    The relationship between a flow behavior of fluid during sintering process, and the pore structure and agglomeration degree of the sinter cake with the change of pisolitic are ratio were investigated. A new experimental method employing hot stage X-ray computerized tomography has successfully made it possible to observe the cross section of sinter cake during sintering. The pore structure and the agglomeration of the sinter cake were evaluated quantitatively by the pore network analysis, cluster analysis and fluid displacement analysis developed in this work. The index of fluid displacement was defined as the area of the images which change from solid regions to pore ones or change reversely within a unit time. The growth rate of the branch width increased with the increase in the index of fluid displacement. The mean size of the cluster was strongly correlative with the branch width. Increase of pisolitic ore ratio reduced the fluid displacement, hence the development of pore and agglomeration was suppressed. The addition of the mill-scale improved the fluidity and accelerated the agglomeration of sinter cake.
  • A Precise Quantitative Analysis of Precipitates in Ti-bearing Interstitial-free Steel

    pp. 479-484

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    A more precise quantitative analysis of precipitates in Ti-bearing Interstitial-Free (IF) steel is made through the combination of both a Field-Emission Transmission Electron Microscope (FE-TEM) analysis and an analysis based upon chemical extraction and isolation. The non-stoichiometry of precipitates is precisely determined using either Energy Dispersive X-ray spectroscopy (EDX) or a new method by plasmon energy in Electron Energy-Loss Spectroscopy (EELS) attached to FE-TEM. The non-stoichiometric compositions of each precipitate are taken into consideration in the quantitative analysis.
    The precise quantitative method is applied to both the hot-rolled and the annealed steels in the process condition: the reheating temperature: 1220 deg. C, the finishing hot-rolling temperature: 890 deg. C, the hot-rolling coiling temperature: 680 deg. C and the annealing temperature: 770 deg. C. The precipitates are quantitatively analyzed from the viewpoint of the interstitial elements; nitrogen, sulfur and carbon. All the nitrogen is precipitated as TiN when hot-rolling is completed. Most of the sulfur-related precipitate is TiS1.25 in the hot-rolled steel. The rest are Ti4C2S2 and (Mn, Fe) S1.25 After annealing, TiS1.25 decreases, while Ti4C2S2 increases significantly and (Mn, Fe) S1.25 slightly increases. Not all the carbon is combined with Ti in the hot-rolled steel and a lot of "free carbon" still remains even after the hot-rolling process is finished. After the annealing process, both TiC0.85 and Ti4C2S2 increase in quantity dramatically.
  • Flattening of Grooves Formed of Intergranular Corrosion for Austenic Stainless Steel Strip in Cold Rolling

    pp. 485-489

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    High-speed cold rolling of stainless steel strip are attempted in a cluster type mill with small-diameter work rolls, or in a tandem cold-rolling mill with large-diameter work rolls. Surface brightness of the strip deteriorates with high-speed rolling, or with large-diameter work rolls. Authors found that the surface brightness was strongly affected by microdefects on the surface, and the brightness was improved with decreasing surface area ratio of micro-defects. A groove formed of intergranular corrosion is one of the micro-defects on SUS304 steel strip.
    In this study, effect of conditions in annealing, pickling and cold rolling on occurrence of the grooves is investigated, and the mechanism for the occurrence of the grooves is discussed. The grooves by intergranular corrosion occurred in cooling of strip immediately after annealing, and the depth and width of the grooves were enlarged in pickling. Occurrence of the grooves was related to layer of insufficient chromium. The grooves flattening on the surface of cold rolled strip were affected by the diameter of work rolls, surface roughness of the roll, and the reduction in thickness. The effects of rolling speed and of viscosity of the rolling oil were little under the experimental conditions investigated.
  • Analysis on Warping of Steel Plate under Controlled Cooling

    pp. 490-495

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    A numerical analysis on the warping of steel plates during the controlled cooling, has been conducted by beam model with one-dimensional stress and thermo-elastic-plastic material. In order to prove the validity of this analysis, experiments were conducted using a newly designed experimental device. The continuous changes in temperature and warping were measured in water cooling.
    (1) The validity of this analysis was quantatively proved by comparison with the experiment results. It is expected that this caluulation technique will be applied to control of warping of steel plates and shape steels in the controlled cooling process.
    (2) From the results of this analysis, it was found that the main factors of warping were phase transformation, yield stress and thermal conductivity.
    (1)In the case of low-temperature-transformation, both warping behavior during cooling and residual warp are large, because of large expansion in phase transformation and high yield stress at low temperature.
    (2)A higher yield stress leads to a larger warping.
    (3)A higher thermal conductivity leads to a high, steep temperature slope in thickness, which causes a higher thermal stress and larger warping.
  • Development and Evaluation of Model for Mechanism of Formation of Surface Properties of Cold-Rolled Stainless Steel Sheet

    pp. 496-501

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    The purpose of this study is to build a model for the transudation of rolling oil in cold rolling of stainless steel by referring to a micropool lubrication mechanism. The rolling oil transudation function or F value calculated by the model is proposed as a rolling pass characteristic parameter to describe the relationship between the kinematic behavior of the rolling oil mechanically trapped in micropits on the strip surface before cold rolling and the surface smoothness of the cold-rolled strip.
    1) It is shown that the kinematic behavior of the rolling oil in the roll bite in rolling passes can be represented by the parameter of F value.
    2) Increasing the F value toward 1 in early passes promotes the transudation of the rolling oil from micropits on the strip surface, and reduces the micropits of the mother strip.
    3) In later passes the F value necessarily becomes smaller due to using smooth-surfaced rolls for getting the surface gloss, but even in this case increasing the F value makes the transudation of the rolling oil from micropits, and eliminates the oilpit-like micropits.
    4) It is made clear that the very smooth surface is obtained by using rough-surfaced rolls in early passes and smooth -surfaced rolls in two or more later passes as an effective pass schedule based on the F value.
  • Influence of Additives in the Bath and in Iron Specimens on the Composition and Structure of Siliconized Layer Formed on the Specimens by Molten Magnesium Bath with SiC Powder

    pp. 502-507

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    When a molten magnesium bath is used to siliconize iron and steel specimens embeded in SiC powder by keeping them in the bath for 3.6ks at 1123K, the siliconized layer obtained consists of two sublayers; composite and noncomposite layers. The former is composed of Fe3Si as the matrix and SiC as a dispersed particulate phase, while the latter is only of Fe3Si.
    The effect of addition of about 10 mass% of Ag, Al, Cd, Cu, La, Li, Ni, Pb, Sn and Zn, respectively, to the bath on the composition and structure of siliconized layer was examined in this experiment. Behavior of alloy elements in the steel specimens was also investigated, using pure iron, carbon steels, cast iron and alloy steels.
    The addition of Ag and Li, respectively, to the bath, increased the thickness of the layer, while the addition of Al and La decreased the thickness and detered the formation of the composite layer. The addition of the other elements to the bath did not change the thickness, significantly.
    The change of the amount of carbon in carbon steels did not affect the siliconized layer, while free carbon in cast iron segregated at the boundary between the siliconized layer and the base iron. Molybdenum and Mn in steels also segregated at the boundary. Chromium and Co could distribute throughout the layer, while W and V concentrated only in the noncomposite layer. Nickel did not dissolve in the layer, but segregated at the surface of specimens. Micro-Vickers hardness was also measured on the siliconized layer of the steels.
  • Mineralizing of Siliconized Layer of Iron and Steel Treated in the Molten Magnesium Bath with Silicon Carbide Powder

    pp. 508-513

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    A molten magnesium bath can be used for siliconizing of iron specimens embedded in SiC powder by keeping for 3.6ks at 1123K, to obtain an Fe3Si-SiC composite layer on the surface of the specimens, as reported previously1). Then, another composite layer made of Fe3Si and other materials can be also obtained by setting an extra layer of the materials powder between SiC powder layer and the base iron, and then treating in the bath. In the present paper, many materials powders including ceramics, such as nitrides, carbides, oxides and two refractory metals, Mo and W, were used to mineralize the siliconized layer of iron, most of which, except some, have been composed successfuly.
  • Influence of Nano Substructure of Cementite Lamellae on the Mechanical Properties of Galvanized Pearlitic Steel Wires

    pp. 514-519

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    It is well known that Si improves the mechanical properties of hot dip Zn-galvanized pearlitic steel wires. However, the improving mechanism is still not clear. Thus, hot dip galvanized pearlitic steel wires were investigated by TEM observation from the viewpoint of the sub-structure of cementite lamellae. The presence of nano sized substructure was observed in electrolytically extracted cementite lamellae after hot dip Zn-galvanization process. The mean diameter of cementite sub-structure in the high tensile strength Si-containing pearlitic steel is much finer than that in a conventional low Si steel. Hence, it is concluded that Si improves the mechanical properties of galvanized pearlitic steel wires by refining the substructure of cementite lamellae.
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  • Estimation of Creep Strength based on Interparticle-Spacing in 2.25Cr-1Mo Steels Used in Actual Boilers

    pp. 520-525

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    In order to ascertain that a method to evaluate a creep strength based on the interparticle spacing between carbides is applicable, creep strength measurements were made on about 14 kinds of 2.25Cr-1Mo steel used in actual boilers operated at up to 870K and compared with those estimated by calculation. The results of this study are as follows:
    (1) The creep rupture curve obtained by the calculation, which is based on the change in interparticle spacing added by connections considering decrease in the cross section area of the test piece due to oxidation and creep deformation, well fit the result of creep rupture test.
    (2) The creep rupture time obtained by this calculation is in the range of 2/3 to 3/2 times of that obtained by creep rupture test.
  • Analysis of Crack Initiation and Propagation process in Delayed Fracture by FRASTA (Fracture-surface Topography Analysis) Method

    pp. 526-531

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    Crack initiation and propagation process in delayed fracture of high tensile strength steel bolt have been studied by Fracture-surface topography analysis (FRASTA) method. The delayed fracture process thus involves a series of crack initiations rather than the continuous growth of a single crack. Crack initiates at the region of maximum triaxial-stress state. After first crack initiation, new cracks initiate at new regions of sever triaxial-stress state, and these cracks join in propagation process.
  • Delayed Fracture Dynamics of Maraging Steel by Quantitative Analysis of Acoustic Emission

    pp. 532-537

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    Microfracture dynamics in the delayed fracture of a maraging steel was investigated using multi-channel AE monitoring and advanced signal processing system. Source location and moment tensor analaysis of 7 channel monitoring system were utilized to determine both the fracture mode and theoretical transfer function of the specimen. Source wave was estimated by iteration of the out-of-plane displacement so as to best represent the corresponding measured displacement. Delayed fracture of maraging steel was found to be dominated by Mode-1 cracks of 10 to 30 μm in size. These cracks generated within 1 to 3 μ sec.

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