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

Tetsu-to-Hagané Advance Publication

  • Determination of the Size Distribution of Nanoparticles Using Asymmetric Flow Field-flow Fractionation (AF4)

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    DOI:10.2355/tetsutohagane.TETSU-2020-088

    To investigate the performance of the size measurement by asymmetric flow field-flow fractionation (AF4), the measurement results of gold nanoparticles were compared among AF4, transmission electron microscopy (TEM), and small-angle X-ray scattering (SAXS) in terms of the average size and full width at half maximum (FWHM) of the size distribution. Although the average size was almost the same for the three methods, the FWHM measured using AF4 was larger than those measured using TEM and SAXS. This is attributed to the diffusion of the gold nanoparticles inside the AF4 instruments. The broadening factor of AF4 analysis was determined as 2.08 by the average of FWHM ratio of AF4 to TEM measured using the several sphere-like gold nanoparticles. In addition, the effect of particle shape on the above broadening factor was investigated using the sphere-like and plate-like silver nanoparticles. The broadening factor for plate-like particles apparently became smaller than that for sphere-like particles, possibly because the Brownian motion of plate-like particles was suppressed.Furthermore, the AF4 analysis with the FWHM correction method using the broadening factor was applied to niobium carbide (NbC) precipitates in steels. The average size measured by AF4 was mostly consistent with the results obtained in regions observed by TEM. Moreover, an increase in the number density of nanometer-sized NbC by heat treatment was successfully detected. The effect of particle shape on FWHM should be further investigated and improved; however, AF4 with the broadening factor can semi-quantitatively analyze the size distribution of nanoprecipitates in steels.
  • Influence of Organic Acid Complex Formation on the Elution Behavior of Steelmaking Slag Amorphous Phase into Freshwater

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    DOI:10.2355/tetsutohagane.TETSU-2020-067

    To recover the barren coast or degraded paddy field, the supply of nutrient elements such as Fe is effective. Since steelmaking slag contains various kinds of potent elements, it is expected to be used as an environmental restoration material. The dissolution mechanism of various elements from slag and the influence of surrounding organic matters and microorganisms must be clarified to utilize steelmaking slag in these methods effectively. In this research, the dissolution tests were conducted by using synthesized CaO-SiO2-FeO-Al2O3-P2O5 amorphous slag samples and aqueous solutions containing gluconic acid. Concentration of various elements in the solution, especially Fe, increased by the addition of gluconic acid. To evaluate the effect of gluconic acid on the dissolution behavior quantitatively, the existence forms of various elements in the solution were thermodynamically estimated based on the experimental results. The maximum ratio of chelated iron to total iron was 97%, indicating that the increase in iron concentration by adding gluconic acid was owing to the formation of iron complex ions. On the contrary, concentrations of Ca, showing complex formation ratio low, or Si and P, for which the complex formation has not been reported, also increased by adding the acid. This suggested the existence of elution mechanisms other than complex formation.
  • Production of High-Strength Coke from Low-Quality Coals Chemically Modified with Thermoplastic Components

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    DOI:10.2355/tetsutohagane.TETSU-2020-052

    In order to produce high-strength coke from low-quality coals, noncovalent bonds between O-functional groups in coal were cleaved by pyridine containing HPC pyridine soluble and HPC-derived thermoplastic components were introduced into the pores formed by swelling; thus, the synergistic effect during carbonization of the suppression of cross-linking reactions and the fluidity amplification due to close placement of coal and thermoplastic components was investigated. When HPC was extracted with pyridine, a decrease in O-functional groups was observed in the pyridine-soluble and pyridine-insoluble components. When HPC was extracted with MeOH, on the other hand, O-functional groups in HPC selectively moved into the soluble components. When non- or slightly-caking coal was chemically-modified with the prepared HPC pyridine-soluble components by utilizing the solvent-swelling effect of pyridine, the fluidity improved compared with the coals physically mixed with the soluble components or HPC. On the other hand, the fluidity of the chemically-modified sample with the MeOH-soluble components hardly changed from that of the original sample, and no effect of the modification with the thermoplastic component was observed. Furthermore, it was clarified that higher-strength coke can be produced from the chemically-modified sample with the HPC pyridine-soluble components than from the original coal or the physically mixed coal with the soluble components. The contraction behavior during carbonization of the chemically-modified sample with the soluble components and that of the original coal was investigated; as a result, a large difference was not observed between these two. Thus, it was found that high-strength coke can be produced from low-quality coals by the present method.
  • Influence of Heating Conditions on the Strength of Coke Produced from Slightly-Caking Coal Containing Chemically-Loaded Thermoplastic Components

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    DOI:10.2355/tetsutohagane.TETSU-2020-053

    In this work, we studies the production of higher-strength coke from chemically-loaded coal in which noncovalent-bonds between O-functional groups in coal are cleaved by pyridine and HPC-derived thermoplastic components are introduced into the pores produced by swelling. The effect of heating rate up to thermoplasticity temperatures of coal on coke strength is first investigated. To examine synergistic effects due to further fluidity enhancements caused by the increased proximity of coal to thermoplastic components during carbonization, the influence of heating rate on coke-strength prepared from pelleted-coal also examined, as described above, to clarify the optimal heating conditions for yielding high-strength coke from slightly-caking coal. An investigation of the use of a SUS-tube to produce high-strength coke from slightly-caking coal with chemically-loaded HPC pyridine-soluble components reveals that high-strength coke may be obtained by 20ºC/min to 400ºC and then continuing to heat at 3ºC/min to 1000ºC. On the other hand, when producing coke from formed specimens consisting of slightly-caking coal with chemically-loaded HPC pyridine-soluble components, we exhibit that, by heating first at 20ºC/min to 500-600ºC and then heating at 3ºC/min to 900ºC, it is possible to produce coke whose strength rivals that of coke produced by carbonization at 3ºC/min of strongly-caking coal. In addition, in producing high-strength coke from formed slightly-caking coal, an optimal amount of additive is present for all types of additive considered – HPC physical blend, chemically-loaded pyridine-soluble HPC and physical blend of pyridine-insoluble HPC components – and, with chemically-loaded pyridine-soluble HPC, it is possible to prepare particularly high-strength coke.
  • Influence of Additive Amount and Heating Conditions on the Strength of Coke Prepared from Non-Caking Coal

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    DOI:10.2355/tetsutohagane.TETSU-2020-054

    In the present study, we prepare several types of specimens from non-caking coal – including specimens in which noncovalent bonds between O-functional groups in coal are cleaved by pyridine and HPC-derived thermoplastic components are introduced into the pores produced by swelling, as well as specimens consisting of physical blends with HPC – and examine the influence of heating conditions and types of caking agents on the production of high-strength coke using a SUS tube. We also investigate the influence of heating conditions and types of caking agents on the strength of coke from pelleted specimens and determine the optimal conditions for producing high-strength coke from non-caking coal. HPC with a wide range of thermoplastic properties is more effective as caking agents than additives containing only low-molecular-weight or high-molecular-weight components. In addition, the strength of the produced coke depends on the amount of the additive, and optimal values of the additive amount are present. It was found that the following heating schedule is effective for producing high-strength coke from non-caking coal with added caking agents: First, high-speed heating (20ºC/min) to an intermediate temperature in the range 400-600ºC, recognized as the thermoplastic temperature range for typical caking coal; then, low-speed heating (3ºC/min) to the temperature range of 900-1000ºC. Moreover, we demonstrate that, by increasing the rate of heating in the thermoplastic temperature range, it is possible to reduce the amount of caking agent added.
  • Development of Mold Flux for Continuous Casting of High-Carbon Steel Bloom

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    DOI:10.2355/tetsutohagane.TETSU-2020-060

    Continuous casting of high-carbon steel containing 1% carbon tends to be operated with low mold flux consumption, resulting in insufficient lubricity. In this study, a mold flux was developed by increasing the viscosity to form a glassy film easily and improve the lubricity between the mold and the solidified shell. In the casting with the developed mold flux, a film with a thickness of 3 mm was stably formed inside the mold to cover the meniscus, bleeding was prevented, and the frequency of surface crack defects on the bloom was reduced by 80%. In the film of the developed mold flux, a 1.2 mm liquid layer lubricated the initial solidification shell.The increased thermal resistance at the film-mold interface reduced the heat flux in the mold, which contributed to the uniform initial solidification. The formation and the growth mechanisms of the crystalline layer in the film were as follows. Firstly, the mold flux flows into the gap between the mold and the initial solidification shell and forms a glassy film. Subsequently, crystallization of the glassy film starts from the mold plate side. Thereafter, the crystallization progresses in the thickness direction of the film until the position of the solidification temperature of the mold flux. The existence of two phases, a liquid layer and a solid layer, played an important role to achieve high lubricity and uniform initial solidification.
  • Sinter Pot for Temperature Measurement of the Top Layer during and After the Ignition

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    DOI:10.2355/tetsutohagane.TETSU-2020-068

    For achieving high sinter yield and quality, various technologies are being implemented and developed to control the heat pattern during the sintering reaction. Further improvements in these technologies necessitate detailed time-course profiles of temperature at all sinter-bed heights; however, no technique has yet been reported for determining the temperature distribution in the top layers of the sinter bed at high spatial and time resolutions. Herein, detailed heat patterns in these layers were visualized by a newly developed pot test apparatus having ~300-mm sinter-bed height. The developed apparatus demonstrated the effect of ignition time on heat patterns during combustion and immediately after ignition. Ignition times of 30, 60, and 90 s demonstrated that the high-temperature holding time increased with an increase in ignition time, and this effect is more evident in the top layer. All parameters, including high-temperature holding time, flue gas composition, and sinter yield, suggest that a longer ignition time intensified coke combustion in the top half layer. The developed technique to measure the temperature in the top layer will quantitatively clarify the effect of segregation or ignition condition on the heat pattern in the top layer.
  • Review on the High-Temperature Thermophysical Properties of Continuous Casting Mold Fluxes for Highly Alloyed Steels

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    DOI:10.2355/tetsutohagane.TETSU-2020-056

    Several recently developed highly alloyed steel grades have shown unsurpassed performance in terms of physical, chemical, and electromagnetic properties. However, broader commercialization of these steels has been hampered by limitations in mold flux performance. Newly developed steels containing considerable amounts of dissolved Al, Mn, and Ti actively react with typical CaO-SiO2-based mold fluxes, which severely changes the composition and subsequently the thermophysical properties of the mold flux that determine the external and internal quality of the as-cast steels. These dynamic changes result in nonuniform heat transfer, lubrication issues, surface defects, and caster breakouts. This work critically assesses the current status of the high-temperature thermophysical properties of CaO-SiO2-based and CaO-Al2O3-based mold fluxes intended for use in casting highly alloyed steel grades. Thermophysical properties, including viscosity, crystallization, thermal conductivity, and heat flux, have been evaluated. The effect of compositional variables including CaO/SiO2, CaO/Al2O3, and Al2O3/SiO2 mass ratios and the additions of CaF2, B2O3, Li2O, K2O, Na2O, TiO2, and BaO on these high-temperature thermophysical properties are discussed.
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  • Effect of MC Type Carbides on Wear Resistance of High Wear Resistant Cast Iron Rolls Developed for Work Rolls of Hot Strip Mills

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    DOI:10.2355/tetsutohagane.TETSU-2020-003

    High-speed steel type cast iron rolls were developed around 1990 and have been widely used in the former stands of hot strip mills. However, in the latter stands of the hot strip mills, the use of high-speed steel type rolls has been limited due to the insufficient crack resistance. Therefore, in order to improve the wear resistance of the latter stands, enhanced type high-nickel grain rolls in which MC type carbides of high hardness are crystallized in a conventional high-nickel grain roll has been developed. However, since the wear resistance of the enhanced type high-nickel grain roll is significantly inferior to that of the high-speed steel type roll. Therefore, The development of a new cast iron roll with superior wear resistance and is applicable to the latter stands of hot strip mills was studied. The present development roll has improved wear resistance by increasing the amount of addition of the high hardness MC type carbide forming elements. In addition, the reduction of the carbon equivalent for less amount of eutectic carbides resulted in the reduction of the residual stress down to the same level as the high-nickel grain roll, which improved the crack resistance. As a result, it was confirmed that the wear resistance was improved about three times compared with the conventional high-nickel grain rolls. In addition, the results suggest that the wear resistance of work rolls for the hot strip mill is largely controlled by the amount of MC carbides among the rolls with the same hardness.
  • In situ Observation of Abnormal Grain Growth of Austenite in Case Hardening Steel

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    DOI:10.2355/tetsutohagane.TETSU-2020-042

    In order to clarify the mechanism of abnormal grain growth of austenite in case hardening steel, in-situ observation by high temperature EBSD was performed using JIS SCM420 containing Nb (0.2C-0.2Si-0.8Mn-1.1Cr-0.2Mo-0.024Nb steel in mass%). After heating at 1143 K, two growing grains several times larger than the surrounding grains were observed. These growing grains grew abnormally by holding at 1193 K and were adjacent to each other. Since the boundary between the two abnormal grains is a twin boundary, the abnormal grains were observed as if they were one larger abnormal grain. The growth rate of abnormal grain is as high as the initial stage of growth and negligibly small at the latter stage of growth. That is, the grain size of abnormal grain growth of austenite is mainly decided by rapid grain growth for a short time after the start of grain growth. The generation of growing grain of abnormal grain growth of austenite is not affected by orientation and strain distribution. When distant grains which have a twin relationship are adjacent to each other in the grain growth process, grain connection occurs. It doubles the area surrounded by high angle grain boundaries without twin boundary. In addition to the encroachment of surrounding grains by larger grains, austenite grain connection through twin boundary also affects abnormal grain growth of austenite.
  • Development Technology for Prevention of Macro-segregation in Casting of Steel Ingot by Insert Casting in Vacuum Atmosphere

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    DOI:10.2355/tetsutohagane.TETSU-2020-034

    Important large components such as rotors for power generation steam turbines, pressure vessels and reaction vessels are manufactured by the ingot casting. However, it is difficult to manufacture the materials with sufficient properties due to macro-segregation of ingots.For the purpose of developing effective and versatile macro-segregation countermeasures in the casting of large steel ingots for manufacturing large parts for power plants, the insert casting in vacuum atmosphere, in which a core material with the same composition as the base steel is placed at the center of the steel ingot, was studied. The effectiveness of the proposed insert casting as a macro-segregation countermeasure was verified in laboratory experiments. In addition, it has been clarified that good bonding between the core material and the base material can be realized even under conditions where bonding by normal insert casting in air atmosphere is difficult.The smelting behavior of the core material and the solidification behavior of the molten steel in the experiments of macro-segregation reproduction casting and the insert casting were analyzed by the direct finite difference method. The mechanism by which this method suppresses the formation of macro-segregation and the solidification conditions for the suppression, the reasons and conditions for good bonding in this insert casting are clarified by the analyses.Furthermore, in the experiments of insert casting, the cause of internal cracks generated in the solidified shell on the core material surface was considered, and guidelines for preventing the internal cracks were presented.
  • Recovery of Zinc by Reaction between Electric Arc Furnace Dust and Calcium Chloride

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    DOI:10.2355/tetsutohagane.TETSU-2020-032

    The chlorination and volatilization behavior of zinc was investigated by the reaction between ZnO, ZnFe2O4 and CaCl2. Experiments were conducted in a nitrogen atmosphere by using horizontal electric furnace. As a result of changing the ratio of zinc oxide to zinc ferrite at 1000ºC, it was expected that ZnO and ZnFe2O4 were chloride volatilized at the same time. In the kinetic study at 950ºC-1030ºC, Jander’s equation showed the experimental result well in the reaction system of this study, and the activation energy was 237 ± 18 kJ / mol. It was also found that carbon influenced the reaction product determination. FeO and CaFe2O4 were formed in the presence of carbon, and Ca2Fe2O5 was formed in the absence of carbon.
  • Relationship between Creep Strength and Magnetic Properties of Cobalt-bearing High Chromium Ferritic Steel

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    DOI:10.2355/tetsutohagane.TETSU-2020-041

    In this study, the relation between the change of the magnetic properties and the creep strength with the addition of Co was investigated for ferritic steel containing 15 mass%Cr. Co addition up to 6 mass% hardly contributes to solid solution strengthening and precipitation strengthening at room temperature. However, in the range of 650ºC to 750ºC, it was confirmed that the addition of Co was effective for the creep strengthening because the steel with a larger amount of Co had higher creep strength. This creep strengthening is explained by a reduction in the diffusion rate associated with a change in magnetic properties by Co addition. The increase of the volume magnetization of the steel with increase of Co amount in the temperature range from room temperature to about 800ºC was confirmed. Comparing the difference in volume magnetization and the ratio of creep strain rate for steels with different amounts of Co, a clear correlation was found between the values. That is, at the temperature at which the difference in volume magnetization becomes maximum, the peak of the creep strain rate ratio was observed. This result is explained as follows. In a low temperature region where the magnetization is large or in a high temperature region above the Curie point of both steels, there is no significant difference in the creep strength between them. However, in the temperature where one steel loses the ferromagnetism but the other steel maintains ferromagnetism, a significant difference in the creep strength was observed.

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