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ONLINE ISSN: 1347-5460
PRINT ISSN: 0915-1559

ISIJ International Vol. 41 (2001), No. Suppl

  • Carburization and Metal Dusting on Iron

    Hans Jürgen Grabke, Else Marie Müller-Lorenz, André Schneider

    pp. S1-S8

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    DOI:10.2355/isijinternational.41.Suppl_S1

    Abstract

    Fundamental studies have been conducted on the kinetics and mechanisms of reactions of iron CH4-H2 and CO-H2-H2O mixtures, also in the presence of H2S, to understand carburization, iron carbide formation, metal dusting and carbon deposition. The rate equations and mechanisms of the surface reaction in carburization of iron are presented, the carburization in CH4-H2 is much slower than in CO-H2. Both reactions are retarded by adsorbed sulfur, the carburization rate becomes inversely proportional to the sulfur activity aSpH2S/pH2 with increasing aS. At carbon activities aC>1 cementite growth can be started in both gas mixtures on iron, however, the decomposition of this unstable carbide gives rise to a corrosion process 'metal dusting', i.e. the material disintegrates to a dust of metal particles and graphitic carbon. This disintegration can be prevented by adsorbed sulfur which hinders the nucleation of graphite. The stabilizing effect of sulfur on cementite and higher carbides such as Hägg carbide, allows fundamental studies about their thermodynamics, non-stoichiometry and diffusional growth mechanisms, on the other hand the presence of sulfur will allow iron carbide production in the reduction of iron ores in carburizing gas mixtures.

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  • Creep Deformation and the Corresponding Microstructural Evolution in High-Cr Ferritic Steels

    M. Igarashi, S. Muneki, H. Hasegawa, K. Yamada, F. Abe

    pp. S101-S105

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    DOI:10.2355/isijinternational.41.Suppl_S101

    Abstract

    Creep deformation and the corresponding microstructural evolution in high-Cr ferritic steels has been studied using model steels with the initial microstructures consisting of the various combination of α'', MX and M23C6 to clarify the role of each precipitate on creep deformation and the resultant creep strength of the steels. A carbon free steel strengthened by the α'' phase has exhibited high creep resistance at the initial stage of transient creep region but a quick transition to the acceleration creep at a small strain, due to less precipitation along boundaries such as lath, block, packet and the prior austenite grain boundary. Precipitation of M23C6 along these boundaries has been found to delay the transition to the acceleration creep, which gives a larger offset creep strain. Fine dispersion of MX has reduced much the creep rate in the transient creep region but enhances the heterogeneous creep deformation in the acceleration creep region. It is thus concluded that an optimum microstructure consists of fine dispersion of the α'' phase and MX carbonitride inside lath grain decorated with M23C6 along lath, block, packet and the prior austenite grain boundary.

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  • Strengthening Mechanism of Cu bearing Heat Resistant Martensitic Steels

    Yuichi Futamura, Toshihiro Tsuchiyama, Setsuo Takaki

    pp. S106-S110

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    DOI:10.2355/isijinternational.41.Suppl_S106

    Abstract

    Relation between microstructure and creep deformation behavior was investigated in 9%Cr base Cu bearing martensitic steels, and the strengthening mechanism by Cu particles was discussed in connection with the microstructure of lath martensite. All of as-quenched 9%Cr-(04)%Cu steels have typical lath-martensite single structure characterized by martensite-block and -packet structures, with high density of dislocation. As a result of creep testing at 873K-140MPa, it is found that minimum creep rate of these martensitic specimens is lowered and the rupture time is prolonged with increasing Cu content. The 9%Cr-4%Cu steel has about seven times as long creep-rupture time as the 9%Cr base steel. TEM observations in the crept 9%Cr-4%Cu steel revealed that dislocations tangled with fine Cu particles which precipitated within laths. This dislocation pinning effect by Cu particles contributes to retarding the recovery of martensite. Hence the dislocation density is kept higher in the 9%Cr-4%Cu steel than the 9%Cr base steel during creep deformation. The relation between the recovery and the creep deformation behaviors was successfully explained by the change in pinning force by Cu particles in all of 9%Cr-Cu steels. Discussed in detail, once the pinning force falls below the applied stress, the recovery rapidly proceeds and the creep strain rate is accelerated corresponding to the recovery behavior.

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  • Accelerated Coarsening of MX Carbonitrides in 12%Cr Steels during Creep Deformation

    Masaki Taneike, Masayuki Kondo, Tatsuo Morimoto

    pp. S111-S115

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    DOI:10.2355/isijinternational.41.Suppl_S111

    Abstract

    MX carbonitrides (M=Nb, V, Ta/X=C, N), which are finely dispersed in the matrix of 12%Cr ferritic heat resistance steels, increase the creep strength of the steels by the precipitation strengthening mechanism. However, MX particles usually coarsen during high temperature creep deformation, and this results in a decrease of creep strength. In order to clarify the coarsening process of MX particles in 12%Cr steels (10.5Cr-0.1C-(Nb, V, Ta)), TEM samples were prepared from the grip portion (creep stress free zone) and the gauge portion (stress zone) of ruptured creep test pieces. The changes of MX particle radii in the gauge portion were considerably larger than that in the grip portion, indicating that creep stress accelerated Ostwald ripening of the particles in the gauge portion. The Ostwald ripening equation was modified by introducing the effective diffusion coefficient as a sum of the lattice diffusion coefficient and dislocation diffusion coefficient, based on an assumption that Nb and V might diffuse not only by the lattice diffusion mechanism but also by the dislocation diffusion mechanism. The effect of dislocation diffusion in the gauge portion was larger than that in the grip portion because dislocations in the gauge portion were moved by creep stress, and a rate of solute atoms diffusing along the dislocations might be increased. The coarsening process of MX particles in the gauge portion could be simulated successfully by means of a modified ripening equation.

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  • Creep Properties affected by Morphology of MX in High-Cr Ferritic Steels

    Katsumi Yamada, Masaaki Igarashi, Seiichi Muneki, Fujio Abe

    pp. S116-S120

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    DOI:10.2355/isijinternational.41.Suppl_S116

    Abstract

    MX-type carbonitrides such as (Nb, V)(C, N) are well known to be effective to improve creep properties of the heat resisting steels at elevated temperatures. The control of precipitation behavior of MX is essential to obtain the stable microstructure that is suitable for heat resisting in the Ultra Super Critical plants. In this study, strong relationship between creep properties and precipitation behavior of MX is investigated by using the high-Cr steels with different C/N-balance.
    MX is categorized to three types from their characteristic morphology, type I, II and III. It is found that the C/N-balance of steel is one of factors to determine which MX is dominant in the tempered steels. Presence of type III-MX that is formed by secondary precipitation of VN adhering to Nb(C, N) seems to be unfavorable to long-term creep resistance by considering relatively decreasing in distribution density of fine VN, type II-MX. Precipitation sites such as Nb(C, N), type I-MX and excess solute nitrogen promote growth of type III-MX at the tempering stage. We conclude that the control of precipitation kinetics of MX is quite important to achieve good creep properties at high temperatures.

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  • Development of High Strength 15Cr Ferritic Creep Resistant Steel with Addition of Tungsten and Cobalt

    Kazuhiro Kimura, Kazuhiro Seki, Yoshiaki Toda, Fujio Abe

    pp. S121-S125

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    DOI:10.2355/isijinternational.41.Suppl_S121

    Abstract

    Effects of alloying elements of W and Co on a creep strength of full annealed 15Cr ferritic steel whose chemical composition is Fe-0.1C-15Cr-1Mo-3W-0.2V-0.05Nb-0.07N-0.003B has been investigated. Increases in creep strength with increase in W content and addition of Co have been obviously observed. About 10 times and 30 times longer time to rupture than that of the base steel have been obtained at 923K-100MPa by increase in W content from 3 mass% to 6 mass% and addition of 3 mass% of Co, respectively. Moreover, significant improvement of creep strength has been observed by the combination of increase in W content and addition of Co. Time to rupture of the steel strengthened by 6 mass% of W and 3 mass% of Co is 100 times longer than that of the base steel at 923K-100MPa. Creep rupture strength of the steel containing 6 mass% of W and 3 mass% of Co is almost the same as that of ASME T92 steel. Since Cr content of conventional ferritic creep resistant steels is less than 12 mass%, oxidation resistance of the 15Cr ferritic steel investigated in this study is better than those of conventional ferritic creep resistant steels. It has been concluded that a full annealed 15Cr ferritic creep resistant steel should be one of the candidate materials for new Ultra Supercritical (USC) power plant in the 21st century.

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  • Degradation of Creep Strength in Welded Joint of 9%Cr Steel

    Masakazu Matsui, Masaaki Tabuchi, Takashi Watanabe, Kiyoshi Kubo, Junichi Kinugawa, Fujio Abe

    pp. S126-S130

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    DOI:10.2355/isijinternational.41.Suppl_S126

    Abstract

    The degradation of creep strength in the heat affected zone (HAZ) of welded joint has been investigated for a tungsten-strengthened 9Cr steel, 9Cr-0.5Mo-1.8W-VNb. The creep test was carried out for the simulated HAZ specimens and the welded joint at 923K. The creep rupture strength of the welded joint is almost the same as that of the base metal at high stresses but it decreases rapidly and then it becomes almost the same as that of the Ac3 simulated HAZ specimen at low stresses. The creep fracture of the welded joint occurs at the fine-grained zone of HAZ, corresponding to the Ac3 heating, at low stresses. The fine-grained zone of HAZ contains higher density of dislocations than the base metal. The recovery of higher density of dislocations and the sparse distribution of large M23C6 carbides promote the formation of coarse subgrains near prior austenite grain boundaries. This results in the concentration of creep deformation in the coarse subgrains, which accelerates eventual creep fracture.

  • Production of direct reduced iron by a sheet material inserting metallization method

    Chikashi Kamijo, Masahiko Hoshi, Takazo Kawaguchi, Hideyuki Yamaoka, Yasuo Kamei

    pp. S13-S16

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    DOI:10.2355/isijinternational.41.Suppl_S13

    Abstract

    A new process for producing DRI called SMIMET (Sheet Material Inserting Metallization method), in which a sheet of material consisting of a coal and iron ore mixture is inserted in a rotary hearth furnace, is proposed here and basic experiments were operated. 4 types of iron ore and of coal were used in these experiments. One type of iron ore and one type of coal were mixed with water, and were formed into a sheet of material on a tray. The sheet was inserted into an electric furnace and reduced at 1373-1573K. Obtained results are summarized as follows:
    1) Over 90% metallization was achieved by increasing the coal ratio to over 20%.
    2) Temperature doesn't affect reduction very much at 1473K or more, but CO2 had great influence on it.
    3) Metallization of the sheet material is the same as that for green pellet made of the same material.
    4) A higher degree of metallization was achieved by using smaller particles of coal and iron ore.
    5) Metallization of the sheet was not affected by the various characteristics of the different type of ore. A higher degree of metallization was accomplished by using coal with less volatile matter.
    6) DRI (metallization degree=93%, fragment size=5-20mm) was produced from a sheet of material (1910dry-kg/m3, moisture=10.6%, thickness=10mm) reduced by 12.5min at 1273-1573K.

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  • New Coal-based Process to Produce High Quality DRI for the EAF

    Yoshitaka Sawa, Tetsuya Yamamoto, Kanji Takeda, Hiroshi Itaya

    pp. S17-S21

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    DOI:10.2355/isijinternational.41.Suppl_S17

    Abstract

    A new coal-based process to produce high quality DRI for EAF has been developed. This process is characterized by producing reduced iron directly from the mixture of fine ore and fine carbonaceous material, and by melting reduced iron to separate metal and slag in a rotary hearth furnace. The carbonaceous layer which has hollows on its surface is laid on the rotary hearth to make it possible to melt the reduced iron. A bench scale test was carried out to confirm the basic concept of the process. Quality of iron pebbles and productivity of the process were examined. The metal was obtained as iron pebbles without ash, gangue and micro-pores. These properties are desirable for EAF operation. The furnace temperature was one of the most important factors for the productivity. The productivity of a commercial plant with a capacity of 500, 000 ton per year was estimated using a one-dimensional, unsteady state mathematical model. The cost comparison of steelmaking using various iron sources was carried out. The steelmaking cost using iron source produced by this process was estimated to decrease by 18% compared with operation using 100% scrap. A bench scale test was also carried out for the treatment of BF dust after size separation.

  • Comparison of High Temperature Behavior of Self-Reducing Pellets Produced from Iron Ore with that of Dust from Sintering Plant

    Cyro Takano, Marcelo Breda Mourao

    pp. S22-S26

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    DOI:10.2355/isijinternational.41.Suppl_S22

    Abstract

    The main advantage of the self-reducing pellets is related to the reaction rate: due to the closeness of the reactants, their small size and nitrogen free environment, almost complete reduction can be achieved with reaction times ranging from 5 to less than 10 minutes for temperatures between 1000°C and 1200°C. These reaction times depend on the nature of the iron bearing material. This paper presents the comparison of experimental results when compact hematite iron ore and dust from sintering plant were used. Self-reducing pellets with 18 mm diameter were made with these materials composing with coal plus binder (4% Portland cement plus 4% Blast Furnace slag). The pellets were submitted to 8 different thermal paths for evaluating the high temperature behavior, that is: decrepitating, swelling, strength and reducing. The results showed that none of the pellets presented decrepitating and that the pellet with dust from sintering plant does not present any tendency for swelling. The abnormal swelling of the pellets containing hematite iron ore can be minimized or avoided by designing a self-reducing process with controlled heating rate and completing the reduction at temperature higher than 1100°C. The strength of 200400 N/pellet, at room temperature, decreased to a minimum of 2040 N/pellet after submitted to 900-950°C. The dust from sintering plant, to achieve high degree of reduction, is less dependent of the heat intensity supply and temperature than iron ore.

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  • Carbothermic Reduction by Microwave Heating

    Marcelo Breda Mouräo, Ivan Parreiras de Carvalho, Jr., Cyro Takano

    pp. S27-S30

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    DOI:10.2355/isijinternational.41.Suppl_S27

    Abstract

    The carbothermic reduction of iron ores is the most important reaction in ironmaking and has been performed mainly in the Blast Furnace. In the last years, several new processes have been proposed as alternatives, and many types of reactors have been tested. The most promising processes are those in which a mixture of iron ore and carbonaceous material is heated at high temperatures, promoting the reaction with formation of metallic iron. It became clear that one of the main obstacles to a fast reaction is heat transfer from the surroundings to the core of the mixture. On the other hand, several studies have shown that microwave heating is very effective in some industrial processes, like drying and sintering of ceramics. In the microwave heating, the material is heated from the inside, thus avoiding the constraints of heat transfer from the surroundings to the inner part of the material. In this work, microwave heating has been applied to the carbothermic reduction of hematite. The obtained results have shown that it is possible to heat iron ore-carbon mixtures above the reduction temperature, and the reaction rates have been compared to those obtained employing conventional heating with the same mixtures.

  • Reduction rate of cement bonded laterite briquette with CO-CO2 gas

    Hadi Purwanto, Taihei Shimada, Reijiro Takahashi, Jun-ichiro Yagi

    pp. S31-S35

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    DOI:10.2355/isijinternational.41.Suppl_S31

    Abstract

    In order to use the laterite as a raw material in ironmaking process, reduction rate of cement bonded laterite briquettes was investigated. The reduction experiments were carried out at temperature range of 973-1273K in a thermobalance reactor using two differences CO gas concentration. Reduction of laterite using CO-CO2 (30/70 vol.%) resulted in reducing of hematite to magnetite where nickel and cobalt completely reduced to metallic state. In the reduction using CO-CO2 (90/10 vol.%) indicated that the nickel, cobalt and iron oxides were reduced to metallic. A kinetic analysis of the reduction was conducted on the basis of the one interface reaction model considering intraparticle diffusion and chemical reaction of Fe2O3 in laterite. The results indicated that the reduction mechanism of laterite was mixed control of chemical reaction and CO-CO2 diffusion through the reacted laterite layer.

  • Prospect for Innovative Steel-making Process in the Forthcoming Century

    Katsukiyo Marukawa, Shigeta Hara, Masamichi Sano

    pp. S36-S41

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    DOI:10.2355/isijinternational.41.Suppl_S36

    Abstract

    Currently, there is a strong demand for development of innovative steel-making processes, because the energy saving is a key issue for global environment and sustainable development of human society. In order to match such a demand, we should grasp the problems and issues on the current processes without any prejudice. In this paper, we review process elements available for such innovative steel-making processes and discuss the process images.

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  • Ideas for Process Control of Inclusion Characteristics During Steelmaking

    Mia Göransson, Pär G. Jönsson

    pp. S42-S46

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    DOI:10.2355/isijinternational.41.Suppl_S42

    Abstract

    An optical emission spectroscopy (OES) method has been implemented for steel sample analysis in the steel plant at Ovako Steel. It makes it possible to receive feedback on oxygen content, inclusion size and inclusion composition within less than 10 minutes from sampling. A windows-based system has also been built to provide operators with easy access to the analysis data. A large number of production samples have been examined and evaluated using the new system. The information on inclusion size distribution and oxygen content in steel samples has been shown to correlate well with the oxygen content in rolled billet samples. It is therefore believed that actions taken during ladle refining to decrease the number of inclusions or the oxygen content can affect the steel cleanness in the finished material. Thus, it can be concluded that it is possible to obtain some control over inclusion characteristics during ladle refining.

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  • Swirling flow control in immersion nozzle for Continuous Casting process

    Shinichiro Yokoya, Shigeo Takagi, Manabu Iguchi, Katsukiyo Marukawa, Shigeta Hara

    pp. S47-S51

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    DOI:10.2355/isijinternational.41.Suppl_S47

    Abstract

    It has been acknowledged that swirling motion in the immersion nozzle is very effective to control the bulk mold flow. From the practical viewpoint, it is very reasonable for a swirling strength in the immersion nozzle to be as weak as possible while controlling the bulk mold flow. Here, a gradually diverging immersion nozzle is proposed and its characteristics and its effectiveness on controlling a bulk mold flow are investigated, using a water mold model.
    Using even considerably weak swirling velocity (0.24 m/s) in an immersion nozzle, maximum velocity at the outlet of the nozzle, surface flow on the meniscus and upward velocity near the narrow face were reduced. However a variation of velocity in those cases was observed.

  • Instability Phenomena at Bath Surface Induced by Top Lance Gas Injection

    Takehiko Kumagai, Manabu Iguchi

    pp. S52-S55

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    DOI:10.2355/isijinternational.41.Suppl_S52

    Abstract

    Top lance gas injection processes are extensively used in metallurgical engineering. When the exit of the top lance is placed near the bath surface, a downward bubbling jet is generated in the bath. The flow patterns of the downward bubbling jet are classified into seven categories mainly with gas flow rate. Instability phenomena of the bubbling jet appear when the gas flow rate is higher than a certain critical value. The phenomena are strongly dependent on the penetration depth of the bubbling jet into the bath, and hence, an empirical relation is proposed for the penetration depth as a function of the gas flow rate, nozzle diameter and vessel diameter.

  • Behavior of Bubbles Attaching to and Detaching from Solid Body of Poor Wettability

    Yoshiteru Mizuno, Manabu Iguchi

    pp. S56-S60

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    DOI:10.2355/isijinternational.41.Suppl_S56

    Abstract

    A circular cylinder, a rectangular cylinder or a sphere was placed in a water-air bubbling jet containing small bubbles. The behavior of bubbles attaching to and detaching from the solid body was observed with a still camera and a high-speed video camera. When the wettability of the solid body was poor, many bubbles attached to it and some of them were trapped near the upper surface. The trapped bubbles became large due to merging and they detached from the upper surface when their volumes attained a certain critical value. When the size of a trapped bubble was sufficiently smaller than the representative length of the solid body, the shape and size of the bubble just before detachment from the upper surface were approximately constant and predictable from an energy equation and a force balance equation for the bubble regardless of the shape and size of the solid body. A mercury-air system was also used. When the wettability of a circular cylinder was poor, the behavior of bubbles on the surface of the cylinder was similar to that in the water-air system.

  • Oxidation Behavior of Silicon and Carbon in Molten Iron-Carbon-Silicon Alloys with Carbon Dioxide

    Hideki Ono-Nakazato, Yukiyo Morita, Keishi Tamura, Tateo Usui, Katsukiyo Marukawa

    pp. S61-S65

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    DOI:10.2355/isijinternational.41.Suppl_S61

    Abstract

    Desiliconization of molten Fe-Si-C alloy is important to keep basicity of slag higher in the dephosphorization process and to minimize total slag generation rate in steelmaking process. However, there is a desiliconization limit because competitive decarburization reaction proceeds as silicon content of metal decreases. Therefore development of an effective method that promotes desiliconization without decarburization is desired. Oxidation behavior of silicon and carbon in molten Fe-C-Si alloys with CO2 is investigated mainly at 1573K in this study. Desiliconization limit with CO2 at 1573K is [mass%Si]=9.1×10-4 under the conditions of aC=1, aSiO2=1 and PCO2=1 atm and silicon in metal can be removed up to [mass%Si]=1.3×10-3 in the present experiment. However, in order to remove silicon to a desired level, it is necessary to remove formed SiO2 from the metal bath because SiO2 is reduced with carbon in the melt and Si is remelted into metal. Overall rate constant of silicon oxidation under the condition of aC=1 is about 3.1×10-5 m/s and is independent of CO2 flow rate. Overall rate constants of simultaneous carbon and silicon oxidation under the condition of PCO2=1 are 4.0×10-6 m/s and 5.0×10-6 m/s, respectively and the rate constant for silicon is smaller than that under the condition of aC=1. It is inferred simultaneous oxidation of carbon and silicon causes some retarding effects on the advance of each reaction.

  • Mathematical Model of Hot Metal Desulphurization by Powder Injection

    Zongshu Zou, Yousheng Zou, Libing Zhang, Nan Wang

    pp. S66-S69

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    DOI:10.2355/isijinternational.41.Suppl_S66

    Abstract

    A macro kinetic analysis of the hot metal desulphurization with powder injection has led to that there are three basic parameters which determine the desulphurization rate, as well as the power utilization efficiency. They are the penetration ratio (PR) of injected powder into the melt, the residence time distribution (RTD) of injected powder in the bath, as well as the homogeneous mixing time (HMT) of the bath. Both PR and HMT have been experimentally determined with water modeling, while the RTD has been determined from either flow field information or experimental measurements of water model with stimulus-response method, arriving at the correlation formulas between the parameter and the injection operation variables as well as powder characteristics. This has enabled the development of a mathematical model to predict the desulphurization process.
    Based on further analysis of the mechanism and macro kinetics of the process, a mathematical model has been developed, with the three basic parameters being taken into account, to simulate the variation of [S] in hot metal with time. Water-model experiment results, flow field information and industrial data are used to determine the model parameters and verify the model, leading to the result that the prediction from the model agrees well with practical results. In addition to CaO and CaC2 powders, the use of granular Mg has been particularly included and emphasized in the model for its more and more popular application. The coupling effect of hot metal desulphurization by injection of magnesium-CaC2 mixture was discussed.

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    2. The Effect of Calcium Carbide Particle Size Distribution on the Kinetics of Hot Metal Desulphurization ISIJ International Vol.34(1994), No.2
    3. Desulfurization of CaO–Al2O3–SiO2–TiO2 Slag System ISIJ International Vol.54(2014), No.10

  • Application of Capillarity of Solid CaO to Dephosphorization of Hot Metals

    Toshihiro Tanaka, Shigeta Hara, Ryosuke Oguni, Kazunori Ueda, Katsukiyo Marukawa

    pp. S70-S72

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    DOI:10.2355/isijinternational.41.Suppl_S70

    Abstract

    The penetration behavior of molten slag in solid CaO has been observed when liquid iron alloy containing phosphorus was oxidized by air or oxygen in CaO crucible. It was found from the present experiment that the phosphate was absorbed in the porosities of solid CaO accompanying the penetration of molten FeO slag when the carbon-saturated liquid iron alloy was oxidized at localized position. The phosphate is found to be fixed as 4CaOP2O5 from X-ray diffraction. Since the penetration can be evaluated theoretically to proceed much faster than the penetration rate obtained from the experimental results, the oxidation process of liquid iron alloy is considered to be the rate-determining step for the total reaction rate of the dephosphorization process using the capillarity of solid CaO. The application of the capillary phenomena to the dephosphorization of hot metal by solid CaO may be one of the useful engineering processes although the adequate oxidation procedures have to be considered.

  • Creep Deformation Behaviour and Kinetic Aspects of 9Cr-1Mo Ferritic Steel

    B. K. Choudhary, C. Phaniraj, K. Bhanu Sankara Rao, S. L. Mannan

    pp. S73-S80

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    DOI:10.2355/isijinternational.41.Suppl_S73

    Abstract

    This paper presents the detailed investigations on creep behaviour of 9Cr-1Mo ferritic steel with an emphasis to understand and unify the different stages of creep deformation in the framework of first order kinetic approach. The different values of stress exponent and apparent activation energy observed for the two stress regimes have been rationalized by invoking the concept of resisting stress. The detailed analysis of results revealed that both transient and tertiary creep obeyed first order kinetics with separate values of transient and tertiary creep parameters in the respective stress regimes. The two stress regimes with different values of stress exponent are manifested as separate master creep curves for transient and steady state creep. Similarly, the analysis of tertiary creep also revealed distinct master creep curves relating steady state and tertiary creep in the respective stress regimes. The paper also focuses attention on two important relationships, one obtained between transient and steady state creep, and the other between steady state and tertiary creep. The useful implications of these relationships in understanding the existing creep rate-rupture life relationships of Monkman-Grant type are also highlighted in this paper.

  • Development of Advanced Chromium Steels with Respect to Microstructure and Structural Stability

    Václav Foldyna, Jaroslav Purmenský, Zdenek Kubon

    pp. S81-S85

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    DOI:10.2355/isijinternational.41.Suppl_S81

    Abstract

    The really promising way how to improve the creep resistance of chromium steels was found in the early 80th and was based on increasing nitrogen content in the steel up to 0.07%. Vanadium nitride precipitating especially on dislocations inside subgrains during creep exposure plays the decisive role in the excellent creep strength of modified 9%Cr-1%Mo steel of the P91 type. The beneficial effect of tungsten on solid solution strengthening as well as on the creep resistance was appreciated especially in Japan and steel P92 has been developed. Nevertheless, high creep resistance of P92 steel is not only the result of increased tungsten content but boron addition to steel P92 has also the significant effect. It must be realized that the increasing of Mo and/or W content in the steel should be limited with respect to Laves phase precipitation (about 1%Mo or 2%W) with high coarsening rate and therefore limited effect on the precipitation strengthening. The precipitation strengthening of these steels is ruled by M23C6 and especially by vanadium nitride. Further increasing of CRS is possible by raising nitrogen content.

  • Creep Modeling for Life Evaluation of Heat-resistant Steel with a Martensitic Structure

    Kyu Seop Park, Fujimitsu Masuyama, Takao Endo

    pp. S86-S90

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    DOI:10.2355/isijinternational.41.Suppl_S86

    Abstract

    A tertiary creep model, the Ω method, previously proposed for creep life prediction was modified so as to be applicable to martensitic steels with a pronounced primary creep region. Primary creep behavior was described by a logarithmic creep equation with two parameters dependent on stress and temperature. Correlations between newly introduced primary creep parameters and tertiary ones were studied. The proposed creep model, which takes into account for primary and tertiary creeps, described the creep curves satisfactorily and predicted creep life accurately.

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    3. Estimation of Remaining Creep Life by Use of Tangential Strain on Outside Diameter of Boiler Tubes (2.25Cr-1Mo Steel) Tetsu-to-Hagané Vol.65(1979), No.7

  • Composite Pre-Reduced Pellet Quality as Affected by Reductant Reactivity

    R. C. Gupta, S. N. Misra

    pp. S9-S12

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    DOI:10.2355/isijinternational.41.Suppl_S9

    Abstract

    The composite pre-reduced (CPR) pellets have been developed to utilise steel plant waste fines (e.g. iron ore, flue dust, mill scale, coke, coal, lime etc.) into value added product suitable for blast furnace feed as metallised burden. This requires good reduction (%R), appreciable metallisation (%Femet), sufficient handling strength and low cost for acceptance as blast furnace feed. These properties could be achieved by optimising the process parameters. In this paper, the effect of reductant reactivity has been observed by studying the reduction behaviour of ore-char mixed composite pellet due to powdered coke, coal and woodchar while keeping the molar ratio of Fe2O3:C as 1:3 and identical heating schedule. It was observed that highly reactive form of carbon in woodchar rendered more reduction (46%R) compared to coke (22%R) and coal (39.5%R) but woodchar also seem to cause severe pellet cracking than coke or coal. The cracking is mainly due to whisker type of iron growth caused by rapid reduction rate.

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  • Continuously Cooled Microstructure and Creep Rupture Strength of Low C-2.25Cr-1.6W-V-Nb Steel

    Nobuyoshi Komai, Toshiyuki Imazato, Fujimitsu Masuyama

    pp. S91-96

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    DOI:10.2355/isijinternational.41.Suppl_S91

    Abstract

    The effect of the cooling rate after austenitizing on both the microstructure and the creep rupture strength of low C-2.25Cr-1.6W-V-Nb steel were investigated. As the cooling rate became lower, the as-normalized microstructure changed from bainitic ferrite (α°B) to granular bainitic ferrite (αB) with polygonal ferrite (αP). The hardness of materials after austenitizing followed by cooling at rates of 0.09 to 6.94Ks-1 and tempering was found to be about 190HV. Hardness was low at cooling rates of 0.03Ks-1 or under because of the increased volume of ferrite.
    The extrapolated 105h creep rupture strengths at 798, 823, and 848K with cooling at rates of 0.09 to 6.94Ks-1 were nearly the same, while the trend at 873K differed from others, with a peak observed at 0.09Ks-1. However the creep rupture strength of 0.03Ks-1 in the lower stress region was close to the creep rupture strength with cooling at rates of 0.09 to 6.94Ks-1. Three types of creep behavior tested at 873K and 175MPa were observed, since the microstructure changed due to the cooling rate when normalizing. It was found that, for cooling rates of approximately 0.09Ks-1 and greater, sufficient creep rupture strength could be secured. This rate can be attained through air-cooling for normal boiler application thickness.

  • EBSP analysis of Modified 9Cr-1Mo Martensitic steel

    Hideharu Nakashima, Daisuke Terada, Fuyuki Yoshida, Hiroyuki Hayakawa, Hiroshi Abe

    pp. S97-S100

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    DOI:10.2355/isijinternational.41.Suppl_S97

    Abstract

    In order to clarify the structure change of 9Cr-1Mo tempered martensitic steel during the high temperature creep, the structures deformed were observed by SEM-OIM and TEM method.
    It was found that initial block and packet structures were equi-axed. These structures were very stable during creep, but just before rupture changed to elongate grain. From an analysis of the grain boundary, Σ3 (rotation angle 70.5°), near Σ11 (rotation angle 50.5°) and small angle (rotation angle 10°) boundaries existed in the martensite. This result agrees with the result predicted with K-S orientation relationship. It was also found that the density of Σ3 boundary or near Σ11 boundary decreased during high temperature creep. But, it was found that the density of Σ3 boundary decreased, and that of near Σ11 and small angle boundaries increased during the high temperature deformation. In initial structure, relative frequency of <110> common axis (Block boundary) is largest to compare with other axes. But, just before the rupture, relative frequency of <110> (Block boundary) suddenly decreased and <123> and <223> (not Block or Packet boundaries) increased.

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  • © 2001 The Iron and Steel Institute of Japan

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