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

Tetsu-to-Hagané Vol. 96 (2010), No. 1

  • Sensitivity Analysis of Physical Parameters in Discrete Element Method Compared with Blast Furnace Cold Model Experiments

    pp. 1-10

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    DOI:10.2355/tetsutohagane.96.1

    Application of discrete element method (DEM) to blast furnace attracts special interest, because the accurate solid motion analysis in the blast furnace is feasible by DEM. In the previous studies in this area, although the descending velocity and physical parameters of particle in DEM were important factors for accurate solid motion analysis, a priori assumptions were frequently introduced to the calculations. In this study, the sensitivities of these parameters were cautiously examined on the basis of cold model experiments. The favorable combination of contact and rolling frictions was introduced to represent the shape of particles. A scale-down three-dimensional cold model of blast furnace was used for analyzing motion of polyethylene and coke particle to confirm the influence of these coefficients. Accordingly, the relationship between the descending behavior of particle and the rolling friction coefficient became clear by comparison of model experiments and DEM simulation. Basically, the rolling friction coefficient is dependent on diameter of particle. In these experimental results, the reduced descending velocity in the packed bed had little influence on the burden descending behavior. It was estimated that the calculated results by DEM in the reduced time scale could quantitatively show the burden behavior in the actual blast furnace. DEM calculation with the appropriate parameters is profitable for the macro flow and local particle behavior analyses in the blast furnace.
  • Thermodynamic Analysis of Selective Oxidation Behavior of Si and Mn-added Steel during Recrystallization Annealing

    pp. 11-20

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    DOI:10.2355/tetsutohagane.96.11

    In order to establish the necessary conditions for producing high strength hot-dip Zn galvanized steel sheets for automotive use on a Continuous Galvanizing Line (CGL), a thermodynamic calculation of the selective oxidation behavior of Si, Mn-added high strength steel sheets was introduced, assuming a model in which an equilibrium is reached locally at the outermost surface layer of the steel sheet. The applicability of that model is confirmed by comparison with experimental results. Both the calculated chemical potential diagram for an Fe–Si–Mn–O system and an isothermal pseudo ternary phase diagram for an FeO–SiO2–MnO system, explain the reaction path of the selective oxidation behavior in 1 mass% Si and 0.01–3.01 mass% Mn-added steel. As this simulation model demonstrates good agreement with experimental results, this thermodynamic calculation is extremely appropriate for prediction of the surface oxidation behavior of Si, Mn-added high strength steel sheets. The transition from selective surface oxidation to internal oxidation can be explained by considering the oxygen flux in the oxidation film and the effect of the selective surface oxides on the inward diffusion behavior of oxygen. By thermodynamic calculation of the suppression condition of SiO2 film formation, which deteriorates the molten Zn wettability of the surface of annealed sheets, a process control model for industrially stable production of high strength hot-dip Zn galvanized steel sheets with arbitrary chemical compositions is proposed. Based on this research, a comprehensive surface control technology for high strength steel sheets is established.
  • Formation of Ultrafine-grained Structure at Drill-hole Surface of Martensitic Steels by High-speed Drilling and Their Mechanical Properties

    pp. 21-28

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    DOI:10.2355/tetsutohagane.96.21

    Microstructure and mechanical properties of martensitic steels after ultrafine-crystallized drilling were investigated. Nano- (at top surface) and submicron-crystalline structures (UFG structure) were formed near drill-hole surface at more than Ac1 by ultrafine-crystallized drilling. The nanocrystalline structure has extremely high hardness and high thermal stability. These characteristics of nanocrystalline structure were similar to those formed by ball milling and shot peening. The tensile property of specimen with UFG structure near the specimen surface showed the similar deformation behavior to that without UFG structure. The fatigue life of specimen with surface UFG structure increased independently of the existence of residual compression stress as compared to that without UFG structure.
  • The Effects of Cold-rolling Strain on Microstructure and Formability of Al Bearing TRIP-aided Cold-rolled Steel Sheets with Annealed Bainitic Lath Structure Matrix

    pp. 29-35

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    DOI:10.2355/tetsutohagane.96.29

    The effects of cold-rolling strain on microstructure and formability of 0.2%C–0.5–1.5%Si–1.5%Mn–0.04–1.0%Al, in mass%, TRIP-aided cold-rolled sheet steels with annealed bainitic lath structure matrix (TAB steels) were investigated. Cold rolling straining changed the matrix structure after intercritical annealing from annealed bainitic lath structure to granular structure with a change of retained austenite morphology, if the structure before cold-rolling was changed to bainitic lath structure. Both the largest elongation and the best stretch-flangeability were achieved in 1.0% Al bearing TAB steel subjected to cold-rolling strain of 40%, followed by intercritical annealing and austempering. This was caused by the increased retained austenite stability and the remained bainitic lath structure due to Al addition.
  • Effect of Stress Ratio on Giga-cycle Fatigue Properties for Ti–6Al–4V Alloy

    pp. 36-41

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    DOI:10.2355/tetsutohagane.96.36

    The effect of stress ratio on giga-cycle fatigue properties was investigated for three heats of 900-MPa-class Ti–6Al–4V alloy. Fatigue tests were carried out using ultrasonic fatigue testing at 20 kHz and conventional fatigue testing at 120 Hz. The ultrasonic and conventional fatigue testing used 3 and 6 mm specimens, respectively. These fatigue tests were conducted under stress ratio of R=0 and 0.3 and under the condition fixing the maximum stress at the yield stress. As the result, internal fracture occurred in all heats, and in that case, frequency effects were negligible. This result meant that the specimen size effect was also negligible. In these tests, no specimen failed at over 109 cycles, which meant that the Ti–6Al–4V alloy probably revealed fatigue limits in a giga-cycle region in spite of occurrence of internal fracture. The fracture sites of the internal fracture revealed no inclusion but a cluster of several facets. The size of a facet was almost equal to the α grain size and the size of the cluster was 100~300 μm. In comparison with steels, the Ti–6Al–4V alloy revealed lower fatigue strength under R=0 in spite of almost equal under R=−1. Hence, the fatigue limits determined at 1010 cycles were below a modified Goodman line under R=0 and 0.3. In comparing the fatigue limits with ΔKth, the dependency of the fatigue limits on the stress ratios was similar to that of ΔKth.
  • Effect of Cementite Volume Fraction on Static Tensile Properties in Ultrafine-grained Ferrite–Cementite Steels

    pp. 42-50

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    DOI:10.2355/tetsutohagane.96.42

    Effect of cementite volume fraction on stress–strain relations in ultrafine-grained ferrite-cementite (FC) steels was studied. The ultrafine-grained FC steels with various cementite volume fractions between 0.3 and 13.7% and average ferrite grain size of about 0.25 μm were prepared by carbon steels. Lower yield stress, tensile strength and flow stress increased with increasing of the volume fraction of cementite. Effect of dispersed cementite on the work-hardening rate in the present ultrafine-grained FC steels was discussed by using Ashby's theory. It is very important for the work-hardening rate of the ultrafine-grained FC steels to control or maintain the size of finely dispersed cementite. By using the experimental results, true stress (σpdl) and true strain (εpdl) at plastic deformation limit were calculated. Effects of the volume fraction of cementite and the grain size on the σpdl−εpdl balance were summarized from the viewpoints of yield strength and the work-hardening rate at σpdl.
  • Development of Procurement Management System at Re-roll Maker

    pp. 51-55

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    DOI:10.2355/tetsutohagane.96.51

    The global increase in demand for steel has caused the depletion of resources, and the demand for material procurement is growing among steel manufacturers. Furthermore, measures to improve the transparency of processes related to material procurement are required in terms of compliance and internal control. Under such circumstances, Toyo Kohan Co., Ltd. has introduced a procurement management system to ensure stable and inexpensive material procurement and to improve efficiency and transparency by systemization of the procurement processes. This system comprises: (1) a procurement system with functions including procurement arrangement, receiving inspection, inventory control, and material requirements planning (MRP) calculations based on production volume, (2) a work flow system from the initial request from a division to the receiving verification, and (3) an EDI trade system with suppliers. This paper reports on the outline, developments and effects of introducing this system.

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