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

Tetsu-to-Hagané Vol. 95 (2009), No. 2

  • 3D Analysis of Coke Microstructure Using μ-X-ray CT

    pp. 103-111

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    DOI:10.2355/tetsutohagane.95.103

    The performance of coke in blast furnace (BF) used to prefer a high strength and low reactivity, generally. One of the reasons is in the importance of the gas and liquid permeability in BF, which are related to the stable operation of BF. On the other hand, the energy saving on the BF will contribute the decrease of CO2 emission, because the proportion of CO2 emission in the iron making field is quite large. To decrease the reducing agent ratio the reaction efficiency in the BF should increase more and more. It is recognized that the reactivity and the strength of coke contradict each other. The high reactivity coke generally has a low strength. To overcome the contradiction, we are going to use the catalytic effect on the coke gasification. Therefore, it is important to clarify the mechanism of coke gasification concerning to the coke microstructure and porosity. In this study, the coke gasification were analyzed using μ-X-ray CT. The optimum conditions for the image processing of the data from the μ-X-ray CT were obtained through the comparison with the cross section of the coke embedded in the resin. Nondestructive observation became possible. It was found that the relatively larger structure (carbon and pore: ~0.5 mm) were almost the same between the μ-X-ray CT and the cross section of the sample embedded in resin, but smaller structure could not observed appropriately. The problems (ex.: existence of iron catalysts, difference of thickness for X-ray absorption, etc.) to get a proper image by μ-X-ray CT still remained.
  • Optimization of Steel Sheet Bar Gauge

    pp. 112-117

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    DOI:10.2355/tetsutohagane.95.112

    In order to manufacture products, various manufacturing specifications are necessary to be determined in advance. To manufacture cold-rolled steel sheets, as an example, specifications, such as additive elements, rolling conditions, heat treatment conditions, have freedom of choice. And they have to be properly determined, considering various conditions and restrictions. More concretely, manufacturing specifications are usually determined through laboratory experiments or trial manufacture at actual processes, considering their influences on quality, cost, and production throughput of products. As changing manufacturing specifications usually give influence on quality, cost, and production throughput of products, it is not easy to change them according to the conditions such as the amount of in-process inventory, change of production cost or production requirements, and so on. This paper presents a new approach to promptly determine manufacturing specifications, by analyzing and quantitatively modeling relations between manufacturing specifications and indices of estimation for consequent manufacture, and selecting manufacturing specifications for realizing optimum indices of estimation. This approach is applied for determining bar gauge, thickness of hot-rolled coils, for manufacturing cold-rolled steel sheets, and improvement of production is realized by adapting to the changes of requirements and circumstances of production.
  • A New Method for Size Estimation of Cu Nano-precipitates in Fe Based on Positron Quantum-dot Confinement

    pp. 118-123

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    DOI:10.2355/tetsutohagane.95.118

    A new method for size estimation of Cu (sub)nano-precipitates in Fe based on a positron quantum-dot-state is developed. Utilizing a unique characteristic of the positron quantum-dot confinement in positron affinitive embedded nano-precipitates, the smearing of the momentum distribution of positron-electron pair around the Fermi momentum in the Cu nano-precipitates is observed by two-dimensional angular correlation of positron annihilation radiation (2D-ACAR) measurements. Due to the uncertainly relation between position and momentum, the size of the Cu nano-precipitates is estimated from the width of the momentum smearing which is inversely proportional to the average size of the precipitates where positrons are confined. Fe–Cu alloys were aged at 475°C for 0.1 to 20 h to form the Cu nano-precipitates with various sizes. The sizes estimated from 2D-ACAR are compared with those by three-dimensional atom probe (3D-AP). Both the results agree with each other. Furthermore, the present method gives the sizes of the Cu subnano-precipitates which are too small to be detected even by the other technique, such as 3D-AP.
  • Heat-treatment Simulation by Use of a Convenient Database Creation Method for Structural Steel

    pp. 124-130

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    DOI:10.2355/tetsutohagane.95.124

    Heat treatment is one of the important manufacturing processes that determine the quality of metallic products. However, the heat treatment processes are usually based on artisan skills and experiences on the manufacturing floor, and therefore, it sometimes is costly and time-consuming to design the processes. There is room to refine the design of processes by simulation techniques. Here we describe a way to streamline the processes of heat treatment using a simulation technique. We developed a method for estimating the hardness of heat-treated steels. To predict the steel hardness after heat treatment, it is necessary to develop a heat transfer analysis procedure and to set up a database of required steels. First we built a heat treatment simulation program using the finite difference method. Then, to simulate the hardness after the heat treatment, we built a database using the Jominy test and the tempering parameter. Some examples of simulated results on heat-history and hardness of carbon steel cylinders during water-quenching and tempering are presented, and the validity of the program is evaluated by comparing the calculated values with the experimental values. The calculated hardness agreed well with the experimental values.
  • Analysis of Surface Imprinting in Sheet Coining by Elastic–Plastic Finite Element Method

    pp. 131-136

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    DOI:10.2355/tetsutohagane.95.131

    The surface roughness of cold rolled steel sheet is an important quality because it affects the press formability and image clarity after coating of steel sheet. The surface roughness is obtained by imprinting a roll surface onto a steel sheet surface in temper rolling process with or without lubricant. In the recent paper, we demonstrated that the surface imprinting is greatly affected by dull-finishing methods and lubricants using laboratory scale 4-high rolling mills. Various numerical analyses of surface imprinting have been performed, however imprinting mechanism has not been clarified. Because the roll surface textures depend on roll surface finishing methods and the deformation behaviors of rolled material are complicated.
    In this paper, sheet coining analysis by three dimensional elastic–plastic finite element method have been conducted to clarify the effects of roll surface texture and lubrication state on surface imprinting. Then, the results are compared with those obtained by temper rolling experiments, as follows: The tool of modeling the electric-discharged dull roll results in a better surface imprinting than the tool of modeling the shot dull roll. A small friction coefficient between the tool and the steel are results in better surface imprinting. This indicate that agree with the soluble lubricant with small friction coefficient is better surface imprinting than dry condition in the rolling experiment. Those results from the 3D FEM analysis agree with those obtain in the rolling experiment.
  • Superplastic Flow at Elevated Temperatures in As-rolled AZ91 Sheet

    pp. 137-143

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    DOI:10.2355/tetsutohagane.95.137

    n alternative processing method for superplastic magnesium alloys has been studied on the form of sheet of Mg–9Al–1Zn (AZ91) via large strain by ingot metallurgy with an isothermal-rolling. Constant initial strain rate tensile tests have been conducted at elevated temperatures for AZ91 sheets as rolled with reduction in area of 90%.
    The values of total elongation exceeded over 50% at all strain rates and at all testing temperatures except 673K, and the maximum total elongation exhibited 210%. The total elongation had a tendency to increase at low temperature side.
    The strain rate sensitivity, m-values, exceeded over 0.5 at strain rates from 1.0×10−4 to 2.5×10−4 s−1. The values of flow stress were lower than those of completely recrystallized AZ91 and did not depend so much on the tensile temperatures from 573 to 648K at high strain rates.
    The activation energy required for superplastic flow in the high-m-deformation was calculated to be 80–83 kJ·mol−1. This value was close to the grain boundary self-diffusion energy of Mg.
    Anisotropic textures were no seen in X-ray (0002) pole figure analysis (XPFA) by Schulz's reflection method. It was suggested that the stress concentrations through grain boundaries or in grains could be relaxed during superplastic deformation process and the superplastic flow was more easily associated with boundary diffusion.
  • Numerical Analysis Model of Galvanic Corrosion with Ion Movement and Reactions

    pp. 144-153

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    DOI:10.2355/tetsutohagane.95.144

    Numerical analysis models for galvanic corrosion has been developed in recent years, but most of the ordinary models only can calculate current density and potential distribution in an electrolyte solution. In order to make clear corrosion mechanism, more information such as pH, ion and corrosion product distributions are necessary. However, it is difficult to compute these phenomena by mathematical model because various kinds of ions and complex reactions exist in corrosion progress. Calculating ion movement with satisfying charge conservation is especially difficult.
    We have developed a new numerical analysis model for galvanic corrosion that can calculate ions movement and reactions. In this model, ion density distribution is corrected by solving Poisson's equation to satisfy charge conservation. Equilibrium reactions are computed after calculating ion transportation.
    Galvanic corrosion of a model Zn/steel couple in a NaCl solution was calculated with this model. The difference of ion and corrosion product distribution in 50 ppm and 50000 ppm NaCl solutions was discussed. Corrosion product distribution obtained by this numerical analysis model was well agreed with OH distribution on a galvanic corrosion specimen measured by FT-IR method qualitatively.
  • Change of Grain Orientation and Effect of Grain Size on It in Creep Deformed Hastelloy X

    pp. 154-160

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    DOI:10.2355/tetsutohagane.95.154

    In this study, EBSP (electron backscatter diffraction pattern) method was applied to evaluate changes in crystal orientation during creep deformation. The tested material was a solid solution strengthened Ni base superalloy, Hastelloy X, and creep tests were conducted at 1123K under the stress of 49 MPa. It was confirmed that a distribution of crystal orientation changed with increasing a creep deformation. In the ruptured specimen, grains having the crystal orientation of ‹101› in the direction of the stress axis had decreased, while grains having the crystal orientation of ‹001› or ‹111› in the direction of the stress axis had increased. A new parameter that indicates the distribution of the crystal orientation was proposed. The change of crystal orientation distribution with creep damage and the effect of grain size on it were described clearly and quantitatively by this parameter. This parameter was thought to be an effective parameter for the evaluation of local damages on high temperature components such as turbine blades and pressure tubes.
  • Mechanism of Austenite Grain Coarsening during Carburizing of Cold-forged Steel Parts

    pp. 161-168

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    DOI:10.2355/tetsutohagane.95.161

    It is known that austenite grain coarsening during carburizing is likely to occur in cold-forged steel parts. For efficient use of this production process, it is important to clarify the mechanism of grain coarsening. In this study, mechanism of austenite grain coarsening of the simulated cold-forged and carburized low-alloy steel was investigated by microstructural observation, measurement of crystal orientation by using EBSP (Electron backscatter diffraction pattern) and CAE analysis. Austenite grain coarsening occurred at highly shear-strained region of the cold-upset specimen. In the highly shear-strained region, recrystallized ferrite grains with relatively-randomized crystal orientation are formed during heating up to carburizing temperature. Because these recrystallized ferrite grains transform to fine austenite grains, the accelerated austenite grain coarsening occurs at the highly shear-strained region. It could be suggested that reduction of shear strain during cold forging prevent austenite grain coarsening.
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  • Clarification of Austenite Grain Formation Behavior during Carburizing of Cold-worked High-chromium Carburizing Steel

    pp. 169-175

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    DOI:10.2355/tetsutohagane.95.169

    It is known that austenite grain coarsening during carburizing is likely to occur in cold-forged steel parts. We estimate that carbide morphology of spheroidizing annealed microstructure prior to cold forging have an influence on the austenite grain coarsening behavior strongly through the ferrite recrystallization during heating to carburizing temperature. In this study, the formation of austenite grain during heating for carburizing of the cold-upset carburizing steel containing higher Cr than conventional JIS SCR420 was investigated. The carburizing steel with 2.1 mass% Cr showed relatively uniform distribution of spheroidized carbides after spheroidizing prior to cold working. It contained much less lamellar carbides than JIS SCR420 with almost 1 mass% Cr. This spheroidizing annealed microstructure is likely to deform more uniformly during cold working than lamellar carbide bearing microstructure. Consequently, both recrystallized ferrite grain and subsequent austenite grain were formed relatively coarse and uniform during heating for carburizing. Because of its higher austenite grain coarsening temperature, the carburinzing steel with 2.1 mass%Cr is suitable for the combined processes of cold forging and carburizing. In this steel, the formation of spherical carbide at ferrite and austenite interface occurs preferentially during spheroidizing and as a result, few lamellar carbides are observed.
  • Beneficial Effect of Re on the Long-term Creep Strength of High Cr Ferritic Heat Resistant Steels

    pp. 176-185

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    DOI:10.2355/tetsutohagane.95.176

    There has been a great demand for superior heat resistant steels in order to raise the thermal efficiency of fossil-fuel power plants and to reduce CO2 emission to the global environment. To this end, by using the d-electrons concept, 9–12% Cr ferritic steels were designed for use of a steam turbine rotor operated in the USC power plants at the steam temperature of 620 to 650°C. The crucial issue for the design is to suppress the deterioration of the long-term creep strength by alloying. First, the Re addition was found to give a beneficial effect on the creep strength of a 10%Cr–4%W steel. Then, the creep tests were performed with the six Re-free and 3.5% W ferritic steels to get an optimum Cr content in the range of 8.5 to 11.5%. As the result, it was found that an excess amount of Cr yielded a detrimental effect on the creep properties, and the 9% Cr steel was the best in view of the very long-term creep strength tested in the condition of 650°C, 98 MPa. Subsequently, a series of creep tests was conducted with the steels by fixing at 9% Cr but by varying the W content from 2 to 4% and the Re content from 0 to 0.5%. From the prolonged creep tests for more than 40,000 h, it was shown that the 9Cr–4 W–0.5Re steel had the longest creep rupture life among all the high Cr ferritic steels so far developed in the world.

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