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

ISIJ International Vol. 55 (2015), No. 3

  • Wetting Behavior of Al2O3 Substrate by Calcium Ferrite Series Melts

    pp. 483-490

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    DOI:10.2355/isijinternational.55.483

    Calcium ferrite (CF) is the adhesive phase formed in high-basicity sinter. The wettability of CF with other solid phases plays an important role in assimilation process. In this study, an improved sessile drop technique was used to explore the wettability of a CF series slag, in which constant contents of Al2O3, MgO, SiO2, and TiO2 were added, with solid Al2O3 substrate at 1250°C. The interfacial structure and spreading mechanisms were also discussed. The dissolution of Al2O3 into the slag was found to be the driving force of the wetting process. The CF series slag could rapidly spread after melting until a low apparent contact angle of approximately 10°–20° was attained. The spreading time positively correlated with the dissolution amount. The addition of MgO, SiO2, and TiO2 improved the wettability of CF on Al2O3 and decreased the surface tension of CF melt. After cooling, a three-layer structure was formed in the slag phase.
  • Effect of Hot Metal on Decarburization in the EAF and Dissolved Sulfur, Phosphorous, and Nitrogen Content in the Steel

    pp. 491-499

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    DOI:10.2355/isijinternational.55.491

    The effect of hot metal additions on the decarburization and dissolved sulfur, phosphorous, and nitrogen content in the steel of a DC EAF was investigated. The addition of hot metal of maximum 36% provided heat into the EAF allowing faster melting and reduced power on times of the furnace. The increased melting rate lowered the FeO in the slag and seems to allow faster kinetics for CaO dissolution into the slag. Hot metal utilization into the slag resulted in higher phosphorous distribution ratios compared to hot metal free heats achieving comparable phosphorous levels at the ladle metallurgical furnace even though the initial input phosphorous was much higher than hot metal free heats. The effect of hot metal on the desulfurization was not pronounced and no apparent difference could be ascertained compared with the 100% scrap charge. With the addition of carbon saturated iron units in the EAF, the evolution of CO and foaming was promoted, which inhibited the infiltration of nitrogen and lowered the overall partial pressure of nitrogen resulting in lower nitrogen levels in the steel. In addition, the dilution effect of the tramp elements Cu and Sn with hot metal ensured the critical defect index to be less than 10% resulting in a 50% reduction of the quality defect index.
  • Viscosity of Molten Fe–B Alloy

    pp. 500-503

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    DOI:10.2355/isijinternational.55.500

    The viscosities of molten Fe–B alloys (Fe–12.5, 25, 37.5 mol% B alloy) were measured by using an oscillating viscometer. The viscosity measurements were carried out using 99.5% alumina crucible up to about 1873 K from the liquids temperatures. The viscosities of all alloys were higher than that of pure iron, and the obtained results showed very good Arrhenius type linearity, which indicates that no considerable change of liquid structure exists in each system at the temperatures studied. The viscosity and the activation energy for viscous flow of Fe–B alloy increased monotonously with increasing boron content, which also showed that there is no considerable change in the liquid structure and the mechanism of viscous flow. The viscosity of molten Fe–C alloy previously measured by the authors slightly decreased with increasing carbon content, which was totally different with the behavior of viscosity of Fe–B alloy. It was expected that the interatomic attracting force between the iron and boron was larger than that of iron and carbon because the free energies of formation of Fe2B and FeB in Fe–B system are larger than that of Fe3C in Fe–C system.
  • Production of (Mn,Fe)-Carbide Containing Low Phosphorus by Carbothermic Reduction of Mn Oxide and Fe Oxide

    pp. 504-512

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    DOI:10.2355/isijinternational.55.504

    The dephosphorization process for high phosphorus iron ores have recently been studied employing several methods. However, few researches of phosphorus removal from high phosphorus manganese ores have been carried out compared with those about the high phosphorus iron ores. As a potential approach for the removal of phosphorus contained in the composite, the formation of (Mn,Fe)-carbide by the carbothermic reduction of the oxides of manganese and iron was investigated at 1200°C in H2 atmosphere employing thermogravimetric analysis (TGA). It was confirmed that the phosphorus did not dissolve into the manganese iron carbide, (Mn,Fe)7C3 even though it has the high reactivity with Fe and Mn. Instead, the phosphorus generated by the decomposition of Ca3(PO4)2 reacted with Fe and Mn to form (Fe,Mn)3P. According to EPMA line mapping, (Mn,Fe)-carbide and (Fe,Mn)-phosphide coexist in the reduced particle. The current results could be applied for a promising treatment of dephosphorization of high phosphorous ores due to the low solubility of phosphorus in carbide.
  • Utilization of Magnetite Concentrate as an Additive in Adhering Fines of Quasi-particle and Its Effect on Assimilation Behavior

    pp. 513-520

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    DOI:10.2355/isijinternational.55.513

    In great efforts to utilize alternative iron ore resources in sintering process, it was attempted to use magnetite concentrate as an additive in adhering fines of quasi-particle and the effect of magnetite addition on assimilation behavior was investigated. A basic study was performed using the synthetic mixture of typical sinter composition with focus on the phase formation behavior of sinter. With small addition of magnetite, it was found that the formation of SFCA was increased with fixed CaO because Fe2+ probably replaced Ca2+ in the substitution mechanism of SFCA. Then the research was further extended by adopting quasi-particle concept using actual iron ores as a case study. Samples of coupled ore tablets simulating quasi-particle structure were prepared and experiments were carried out to investigate how magnetite addition affects the assimilation behavior of quasi-particle. Based on the experimental results and analysis, it was found that the small amount of magnetite addition, more specifically Fe2+ in magnetite, significantly influenced the physicochemical properties of melt as well as the structure of SFCA. The highest melt penetration depth was observed in N90-J10 sample containing 10 mass% magnetite ore J. As a result, it was concluded that the assimilation behavior was improved with proper amount of magnetite addition in adhering fines of quasi-particle.
  • Characteristics of Prepared Coke–biochar Composite and Its Influence on Reduction of NOx Emission in Iron Ore Sintering

    pp. 521-527

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    DOI:10.2355/isijinternational.55.521

    Substituting biofuel for coke breeze in the iron ore sintering process has become increasingly attractive. However, coke breeze and straw char separately distributed in the sinter bed lead to poor-quality sinter because of their different combustibilities. In this investigation, a coke–biochar composite (CBC) is produced from coking coal and raw straw, and its behaviour when used as a fuel for sintering is investigated. The results show that the coke and straw char in the CBC interpenetrate and adhere together. Compared with separately distributed coke breeze and straw char, this structure effectively restrains separate combustion of the CBC components. A CBC with 40–60% straw char is recommended as a replacement for coke breeze, with only small changes in sinter quality. The fuel nitrogen conversion rate is decreased by 8.3% during CBC combustion, possibly owing to reducing reactions between C (CO) and NOx, which further decrease NOx emission.
  • Flow Behavior of Plastic Particles in the Lower Part of Blast Furnace

    pp. 528-535

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    DOI:10.2355/isijinternational.55.528

    Injection of plastics into blast furnaces as an alternative reducing agent has been carried out for the purpose of mitigation of carbon dioxide emissions. Several studies on the pyrolysis of plastic particles have been reported, however flow behavior of plastic particles or unburnt char in actual blast furnace is not clear.
    In this study, hotmodel experiments and thermogravimetry analysis were conducted for the modeling of gasification and combustion behavior of plastic particles, then flow behavior of plastic particle and unburnt char in a blast furnace was calculated by numerical simulation. According to the observation results of combustion experiment, volatile matter of plastics seems to be evolved at the surface of the particles. Regarding reaction of char derived from plastics, thermogravimetry analysis showed that rate of gasification of unburnt char depended on the rate of heating. Gasification rate in the case of rapid heating condition tended to increase.
    As the results of numerical simulations, initial diameter of plastics and char diameter determined the flow behavior in the blast furnace. When relatively small plastic was injected or diameter of unburnt char was small, unburnt char went upward along with gas flow and most of it might be consumed at the cohesive zone. On the other hand, when relatively coarse plastics was injected and diameter of unburnt char was relatively coarse, it was suggested that unburnt char accumulated around the deadman.
    Therefore, it is considered that fine plastics injection is desirable for the stable operation of blast furnace.
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  • Effect of Coal Type on the Reduction and Magnetic Separation of a High-phosphorus Oolitic Hematite Ore

    pp. 536-543

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    DOI:10.2355/isijinternational.55.536

    Coal-based direct reduction followed by magnetic separation technique was employed to produce direct reduction iron powder (DRI powder) from a high-phosphorus oolitic hematite ore, and the effects of type and particle size of coal and C/O (Fixed carbon/Oxygen) mole ratio on this process were investigated. The results showed that when using coarse-sized and medium-sized coals, bitumite and lignite presented better iron recovery as compared with anthracite, while this advantage disappeared as the particle size of coals decreased. In addition, the iron content of the DRI powder increased with improving of the coal rank depended on fixed carbon contents, while decreased with the decrease of particle size of coals. Increasing the C/O ratio resulted in a sharp rise of P content of the DRI powder. X-Ray Diffraction (XRD) analysis revealed that more liquid phase was formed in the briquettes during reduction with anthracite as reductant. SEM (scanning electron microscope) observation confirmed that the size of the metallic iron grains formed in reduced briquettes decreased with decreasing of the rank of the coal and the particle size of coal.
  • Quantitative Estimation of Gas Permeability of Softening Sinter Layer with Liquid Phase

    pp. 544-551

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    DOI:10.2355/isijinternational.55.544

    Quantifying gas permeability of softening sinter layer was studied with the aim of increasing the precision of gas permeability in cohesive zone.
    In this study, the effect of liquid in packed bed on gas permeability was evaluated by a cold model experiment, which simulated sinter melting behavior by using sponge ball absorbed glycerin solution. As a result, gas permeability of packed bed with liquid was expressed by formula (a), based on Sugiyama’s formula.
    [ This part of text is omitted ]
    Formula (a) was adapted to actual softening sinter experiment result, and was confirmed to be good agreement.
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  • Effect of Na2O on Phosphorus Enrichment and Separation in P-Bearing Steelmaking Slag

    pp. 552-558

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    DOI:10.2355/isijinternational.55.552

    In order to recycle the phosphorus in P-bearing converter slag and make it used as slag phosphate fertilizer, the effect of Na2O in P-bearing steelmaking slag on phosphorus existence form, P2O5 solubility and magnetic separation behavior was researched systematically. The results show that the melting point of slag decreases obviously with the increasing of Na2O content in slag, and high phosphorus solid solution (Ca2SiO4–Ca3(PO4)2 and Na2Ca4(PO4)2SiO4) are generated by the continuous reaction of 6Ca2SiO4–Ca3(PO4)2 (for short 6C2S–C3P) solid solution precipitated in the slag with Na2O under the condition of 1623 K. The early precipitated 6C2S–C3P solid solution is reduced with the increasing content of Na2O even disappeared. If the addition of Na2O is increased or excess in the slag, the high phosphorus solid solution (Na2Ca4(PO4)2SiO4) that generated by the above mentioned reaction will continue to react with Na2O to produce Na3PO4, then the phosphorus content in the phosphorus-rich phase is heightened. Na2Ca4(PO4)2SiO4 and Na3PO4 have almost same good citric acid solubility with 6C2S–C3P solid solution. Therefore, adding appropriate Na2O content into slag can improve the slag P2O5 solubility, but the effect of different amounts of Na2O on the P2O5 solubility has little difference. Meanwhile, Added Na2O into slag can improve the metallization of slag and magnetism of iron-rich phase, make the magnetic substances content increased and separation of phosphorus and iron is incomplete, so it is adverse to phosphorus resources recovery from P-bearing slag by magnetic separation method. In order to recycle the phosphorus in P-bearing converter slag, adding appropriate Na2O content (<6%) into slag can be used as a flux to instead of fluorite for slag forming in steelmaking process.
  • Treatment of Hazardous Waste by Carbon Dioxide Capture from an Electric Arc Furnace

    pp. 559-563

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    DOI:10.2355/isijinternational.55.559

    The steel industry is important for the development of several economic activities in the world. Unfortunately, the electric arc furnace (EAF) used in production generates byproducts such as dust, which must be treated and confined to prevent pollution. In this study, the alkaline properties of solutions prepared with EAF dust were evaluated for use in CO2 capture. A process in which dust solution properties are regarded as an advantage is proposed. The results indicate that the developed process effectively minimizes operational problems. Specifically, the process neutralizes several hazardous pollutants contained in EAF dust, reduces CO2 emissions, and directly removes dioxin-forming dust from the exit gas stream.
  • Effect of Al2O3 on Enrichment of Phosphorus in Hot Metal Dephosphorization Slag

    pp. 564-569

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    DOI:10.2355/isijinternational.55.564

    The phosphorus enrichment behavior in dephosphorization slag was investigated by slag modification with Al2O3 in the present study. The mineral phase of the slag samples were measured by mineral phase microscope, SEM+EDS and XRD. The mechanism of Al2O3 modification was discussed according to FactSage calculation. The results show that phosphorus was mainly existed in nC2S–C3P solid solution and Al2O3 modification was beneficial to the enrichment of phosphorus. The content of phosphorus in phosphorus-rich phase was increased to 5.10% and 9.15% with 8% and 11% Al2O3 addition. With the slag temperature decreases, Ca3(PO4)2 and Ca2SiO4 firstly precipitated and formed the nC2P–C3P solid solution. The addition of Al2O3 has a positive influence on the reaction of Al2O3 and nC2S–C3P solid solution. Al2O3 could react with the initially precipitated low phosphorus nC2S–C3P solid solution to produce higher phosphorus n′C2S–C3P solid solution (n′<n) and Ca2Al2SiO7. When Al2O3 content was increased from 2% to 15%, the value of lnK decreased from 2.34 to 1.47.
  • Characterization of Metal Droplets in Slag after Desulfurization of Hot Metal

    pp. 570-577

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    DOI:10.2355/isijinternational.55.570

    Iron losses in slag during an intensive desulfurization of hot metal can reach 0.6–1.1% from the total amount of processed hot metal. The characteristics of different metal droplets (such as morphology, number, size, composition and solidification structure) in industrial slag samples after desulfurization were investigated using SEM. All metal droplets in the slag were classified into three groups according to morphology: Type A - spherical/oval; Type B - spherical/irregular; Type C - irregular. Thereafter, some mechanisms for involving of different metal droplets into the slag during desulfurization process were studied based on obtained characteristics of metal droplets. Moreover, a possibility to remove those metal droplets from the liquid slag was estimated based on Stokes law. In addition, the effect of some parameters (such as slag viscosity and size of different metal droplets in this slag) on the possibility to reduce the iron losses during desulfurization of hot metal was considered.
  • Effect of Nitrogen Content on Cold Working Properties of High-strength Mn–Cr–N Steels Made Using Pressurized ESR

    pp. 578-585

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    DOI:10.2355/isijinternational.55.578

    High-strength Mn–Cr–N steels with high nitrogen content were manufactured using a lab-scale pressurized electro-slag remelting furnace to study the deformability of the steels. Melting experiments were performed under 1.0 MPa pressure N2 gas in order to have various N contents. Gas porosity and severe macrosegregation were not observed in the remelted ingots. Microstructure observation revealed that nitrides and non-metallic inclusions were small enough not to affect the mechanical properties. After the ESR ingots were heat-treated and forged, the mechanical properties of the steels at a room temperature were measured. The grain sizes were measured in the range from 50 to 300 μm. The results of 0.2% proof stress showed that the steel became stronger with increasing N content according to solid solution hardening mechanism. In addition, with various strain rates, the tensile strain-hardening exponents were determined to be almost the constant values between 0.20 and 0.25. These results suggest that the methods of cold working for conventional 18Mn–18Cr–0.7N steel are applicable to the Mn–Cr–N steels containing over 1.0 mass% nitrogen.
  • Blowpipe Antenna in Blast Furnace Raceway Depth Measurement

    pp. 586-591

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    DOI:10.2355/isijinternational.55.586

    A high gain blowpipe antenna has been presented in this paper. The blowpipe is a pipe used to blow hot air into the Blast Furnace (BF). It has formed the antenna as the radiation unit. A multi-stages cylindrical waveguide has been designed and employed to be the feeding unit. The two units of antenna are separated outside and inside the BF. They are easy to assemble during the BF production process. The antenna operates from 24 to 26 GHz. The simulation result shows that the blowpipe antenna has 25 dBi gain, 20 dB return loss and 30° HPBW (Half Power Beam Width) at the frequency of 25 GHz. Prototype antenna has been fabricated and measured the raceway depth in a real BF. The experiment result shows that the blowpipe antenna can be used in raceway depth measurement. It has advantages of high gain and easy-to-assemble.
  • 3-Dimension Burden Surface Imaging System with T-shaped MIMO Radar in the Blast Furnace

    pp. 592-599

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    DOI:10.2355/isijinternational.55.592

    This paper presents a T-shaped MIMO radar imaging system for use within a Blast Furnace. A T-shaped MIMO antenna array consisting of 32 antenna elements has been developed. A dielectric loaded waveguide is employed as the antenna element. The antenna has good mechanical hardness and is resistant to dust and high temperature. A cooling system, employing a combination of Nitrogen and water, has been designed for this application. The function of cooling system is to maintain the temperature inside the radar between 30°C and 65°C whilst the outside temperature varies from 80°C to 200°C. A pilot study has been conducted using a prototype version of the radar system installed upon the coke surface. In this way it was possible to obtain measurement results concerned with a real burden surface distribution. These results have been carefully compared with results obtained via computer simulation.
  • Blast Furnace Gas Flow Strength Prediction Using FMCW Radar

    pp. 600-604

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    DOI:10.2355/isijinternational.55.600

    A method of predicting gas flow strength upon burden surface has been proposed in this paper. A frequency modulated continuous wave (FMCW) radar is employed to achieve the echo signal of burden surface. The radar, operating at 24–26 GHz, is mounted on top of blast furnace (BF). The burden surface scattering yields attenuation due to the burden surface dust. The propagation constant has been derived in terms of gas flow velocity, particle size, dust concentration and so on. The method has been validated in real blast furnace during production process and maintenance process, respectively. The experiment results show that the predicted value has close agreement with the measured value. The attenuation has the same periodic trend with the BF charging process.
  • Evaluation of Copper Stave Remnant Thickness in Blast Furnace Using Ultrasonic Method

    pp. 605-610

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    DOI:10.2355/isijinternational.55.605

    Blast furnace is one of the most important parts in an integrated iron and steel industry where smelting of iron is carried out. Staves are essential part of a blast furnace, which increase blast furnace campaign life by providing cooling in order to protect the external steel shell from heat as well as to maintain the inner profile of the blast furnace. Under high temperature environment, stave wear takes place due to downward and upward motion of materials and hot gases, respectively. Thickness of the stave should be monitored on a continuous basis to prevent any catastrophic failure. In this study an ultrasonic test method was adopted to carry out the thickness measurement of copper staves. Measuring copper stave thickness involves two challenges; direct accessibility from the outside and its inherent intricate geometry. In this paper, a complete methodology comprising ultrasonic sensor and fixture mechanism to overcome the above challenges is discussed. An innovative stave thickness measuring device was first developed and calibrated in the laboratory, which was then used to measure the remnant stave thickness of a running industrial blast furnace. The results were verified by hole drilling method that proved the developed methodology as a reliable one.
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  • Direct Quantification of Mn Concentration in Molten Steel Utilizing a Wavelength-Variable Laser

    pp. 611-615

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    DOI:10.2355/isijinternational.55.611

    A method for the on-line quantification of Mn concentration in molten steel was examined. Application of atomic absorption spectrometry using a wavelength-variable laser as the light source was attempted. When the laser emission wavelength was set to the absorption center wavelength in a laboratory melting furnace experiment, it was difficult to measure Mn concentrations of more than 1.0% in the molten steel. Investigation on the relationship between the laser emission wavelength and absorbance was performed using an atomic absorption burner and it was found that the absorption sensitivity could be adjusted by shifting the wavelength from the absorption center wavelength. We reduced the absorption sensitivity to about 1/10 by shifting the laser emission wavelength about 0.015 nm from the absorption center to the longer wavelength side, and performed a melting furnace experiment again. Possibility of directly quantifying Mn concentrations of up to 1.5% or higher was demonstrated.
  • Powder X-ray Diffraction Analysis of Lime-Phase Solid Solution in Converter Slag

    pp. 616-622

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    DOI:10.2355/isijinternational.55.616

    X-ray diffraction analysis of a converter slag was performed by focusing on the lime-phase solid solution. Solid solutions of Ca1-xFexO and Ca1-xMnxO (0<x<1) were prepared by mechanochemical processing or high-temperature solid-state reaction, and the relationships between solid solubility x and lattice parameters were determined. Crystallized lime could be distinguished from undissolved lime by the shift of the diffraction angle associated with the formation of the solid solution. The effect of slag aging treatment with water vapor was examined by comparing the solid solubility x and the amount of lime phase in the aged and non-aged slags. It was confirmed that x affects the hydration behavior of the lime phase and that crystallized lime with high x tends to remain unchanged even after aging. Moreover, about two-thirds of the lime phase was confirmed to have been converted by hydration as a result of the aging process.
  • An Experimental Protocol Development of Three-Dimensional Transmission Electron Microscopy Methods for Ferrous Alloys: Towards Quantitative Microstructural Characterization in Three Dimensions

    pp. 623-631

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    DOI:10.2355/isijinternational.55.623

    The majority of engineering steels are ferromagnetic and structurally inhomogeneous on scales ranging from nanometers to micrometers, and their physical properties depend on the three-dimensional (3D) features in their microstructures. Thus, obtaining a 3D image with a large field of view is desirable for transmission electron microscopy (TEM) based microstructure characterization in order to establish the relationship between the microstructure and the physical properties with a reasonable statistical relevancy. Here, we use a conventional sample preparation process, i.e., mechanical polishing followed by electropolishing, and optimizing experimental protocols for electron tomography (ET) of ferromagnetic materials, to carry out microstructural characterization of engineering steel. We determined that the sample thickness after the mechanical polishing step is a critical experimental parameter affecting the success rate of tilt-series image acquisitions. For example, for ferritic heat-resistant 9Cr steel, mechanical thinning down to 30 μm or less was necessary to acquire an adequate tilt-series image of the carbide precipitates in the annular dark-field scanning TEM (ADF-STEM) mode. However, acquiring tilt-series images of dislocation structures remains a challenge due to an unavoidable, significant electron beam deflection during specimen tilt, even with a thinned sample. To overcome the electron beam deflection problem, we evaluated several relatively accessible approaches including the “Low-Mag STEM and Lorentz TEM” modes. Although rarely used for ET, both modes reduce or even zero the objective lens current, likely weakening the magnetic interference between the ferromagnetic specimen and the objective lens magnetic field. The advantages and disadvantages of these experimental components are discussed.
  • Effect of Oil Film Thickness on Lubrication Property in Hot Rolling

    pp. 632-636

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    DOI:10.2355/isijinternational.55.632

    In hot rolling, lubrication between the work roll and hot strip plays an important role in reducing rolling force and protecting the work roll surface. However, the tribological behavior in hot rolling has not been clarified sufficiently. In this work, the effects of oil amount on the coefficient of friction in hot rolling were investigated in comparison with the case of cold rolling. The oil amount in hot rolling was measured by the amount of oil remaining on the work roll surface after rolling. The results of rolling tests clarified the following points: The coefficient of friction is reduced adequately with a small amount of oil. If the oil amount is increased, a few small oil-pits will form, but no further decrease in the coefficient of friction will be achieved. It is suggested that boundary lubrication is controlling in hot rolling, which is different from the case of cold rolling.
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  • Theoretical and Experimental Analysis of Chatter in Tandem Cold Rolling Mills Based on Wave Propagation Theory

    pp. 637-646

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    DOI:10.2355/isijinternational.55.637

    In this paper, a new theory is introduced to explain chatter mechanism based on wave propagation in elastic solids. It is believed that previous theories in chatter are based on a static analysis and normally are consistent just for low-frequency vibrations such as forced vibration. But chatter phenomenon is a dynamic high-frequency event and requires a consistent theory with a dynamic analysis. In this research, variations of inter-stand tensions are calculated based on wave propagation theory. Predicted results by wave propagation theory were evaluated by dynamic finite element method in a benchmark problem. Results show that wave propagation theory is consistent by dynamic finite element method, but previous theories are not sufficiently accurate in chatter conditions. Hereafter, a numerical model is constituted to simulate the vibrational behavior of a tandem rolling mill. Parameters for numerical simulations are adopted from an industrial two-stand tandem mill. Consequently, results of the analytical model are compared with the experimental measurements from that full scale industrial mill. Two main chattering characteristics, i.e., critical rolling speed and chatter frequency, obtained from the simulation program were found to be more consistent with the experimental results when considering the dynamic effects.
    x

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  • Effect of Interpass Time on the Dynamic Transformation of a Plain C–Mn and a Nb Microalloyed Steel

    pp. 647-654

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    DOI:10.2355/isijinternational.55.647

    Strip rolling simulations were carried out on a 0.06%C-0.3%Mn-0.01%Si and a 0.11%C-1.0%Mn-0.11%Si-0.03%Al-0.034%Nb steel over the temperature range 1000°C to 883°C. Pass strains of 0.4 were applied at a strain rate of 1 s–1 with interpass times of 0.5 s, 1 s, 1.5 s, 3 s and 5 s. Two different temperature schedules were employed, namely i) continuous cooling at 6°C/s and ii) isothermal holding. The mean flow stresses (MFS’s) applicable to strip rolling were determined by integration. The flow stress levels and MFS’s decrease when the interpass times are short. When they are long, the flow stress increases with decreasing temperature. These observations indicate that the austenite is transforming dynamically into ferrite and statically into austenite. The nucleation and growth of the ferrite reduce the rolling load and modify the microstructure. The addition of Nb in solid solution delays the occurrence of dynamic transformation (and the retransformation of ferrite back into austenite). The forward nucleation of ferrite occurs displacively while the retransformation back into austenite takes place by a diffusional mechanism.
  • Simulation on Changes in Microstructure and Texture during Normal Grain Growth of Steel Sheet by Two-dimensional Local Curvature Multi-vertex Model

    pp. 655-661

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    DOI:10.2355/isijinternational.55.655

    The two-dimensional local curvature multi-vertex model was applied to the normal grain growth of actual steel sheets for examination of the effect of the respective misorientation dependencies of grain boundary energy and mobility on grain growth and for comparison with experimental results. The simulation result revealed that the grain boundary energy had a major influence on the change in misorientation distribution with grain growth, whereas the grain boundary mobility did not have such a large influence. The simulation considering the misorientation dependency on grain boundary energy and mobility, in particular, accounting for Σ1 and high angle boundaries was constructed and was effective for reproducing the experimental results. Simulated microstructures were similar to the experimental ones; however, the detailed standard deviation of grain size distribution was smaller in the calculation than that in the experiment. The texture change with grain growth in the simulation was weaker than that in the actual steel sheets. As a whole, the developed model described the experimental grain growth well, and the difference in the results between the simulation and the experiment is probably attributable to the difference in dimension; i.e., two-dimension in simulation and three-dimension in experiment and the inaccuracy of the grain boundary conditions such as grain boundary energy and mobility in the model.
  • Effects of Aluminium on the Microstructure and Mechanical Properties in 0.2C–5Mn Steels under Different Heat Treatment Conditions

    pp. 662-669

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    DOI:10.2355/isijinternational.55.662

    The effects of aluminium on mcrostructure and mechanical properties in medium manganese steels were investigated in this study. It is found that addition of aluminium don’t significantly change the microstructure evolution during heat treatment process, aluminium can retard the precipitation of carbides in the following annealing, resulting in higher carbon content in several austenite. Though aluminium addition leads to a certain decline of tensile strength, the yield strength and elongation are neither decreasing significantly, especially the former is improved after heat treatment. Furthermore, the Al-bearing medium manganese steels can achieve optimal product of strength and ductility with austenization at 675°C and short time (5 min) annealing, the excellent mechanical properties are strength ~880 MPa, total elongation ~39.3% and product of tensile strength to total elongation (Rm*AT) ~34.6 GPa%.
  • Plastic Instability in ISO 5832-9 High-nitrogen Austenitic Stainless Steel

    pp. 670-678

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    DOI:10.2355/isijinternational.55.670

    ISO 5832-9 high-nitrogen austenitic stainless steel has shown promising results in the fabrication of temporary and permanent orthopedic prostheses, exhibiting better mechanical strength and corrosion resistance than the traditional ISO 5832-1 (ASTM F-138) steel. Recent studies have revealed that this alloy possesses unique properties, such as high mechanical strength and corrosion resistance and the presence of second phase particles (Z phase) in the matrix. However, it is not known how the microstructural and mechanical properties and the corrosion rate are correlated in regions of industrial processability of this alloy during hot forming. In this study, continuous and interrupted isothermal hot torsion tests were performed after solubilization heat treatment at 1473 K for 300 s, with temperature intervals varying from 1273 to 1473 K and strain rates ranging from 0.05 to 5 s–1. The purpose of these tests was to investigate the alloy’s workability and stress corrosion rate by means of electrochemical impedance spectroscopy (EIS) and to characterize its microstructure by optical microscopy. The results indicate that the yield stress is sensitive to the strain parameters. The peak stress decreases with increasing temperature and decreasing strain rate, with a high rate of dynamic recovery, with elongated grains generating areas of stress build-up. These regions of plastic instability present a higher degree of corrosion than as-received samples. The regions of high temperature and low strain rate exhibit good workability with a refined final microstructure of dynamically recrystallized grains.
  • In-situ Observations of Acicular Ferrite Growth Behavior in the Simulated Coarse-grained Heat-affected Zone of High-strength Low-alloy Steels

    pp. 679-685

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    DOI:10.2355/isijinternational.55.679

    The formation of acicular ferrite in the simulated heat-affected zone of high-strength low-alloy steels was investigated by means of in-situ observation. The acicular ferrite laths or plates were nucleated on intragranular Zr–Ti complex oxides in big austenite grains and then lengthened very fast. The growth of individual acicular ferrite lath or plate was completed in a very narrow temperature range, especially at low temperatures. The measured lengthening rates of ferrite laths or plates were ranged from 33.3 μm/s to 190.5 μm/s, which fell between limit calculated assuming para-equilibrium and negligible-partition local equilibrium modes from the semi-empirical equation proposed by Hillert.
  • Unusual Tempering Behavior of Fe–Cr–C Martensite

    pp. 686-690

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    DOI:10.2355/isijinternational.55.686

    The tempering behavior of an as-quenched high Cr-high C steel consisting of lath martensite, retained austenite and carbide was studied. The change in the total austenite volume fraction was monitored during tempering by in situ neutron diffraction. The results were compared with those obtained by X-ray diffraction, scanning electron microscopy/electron back scatter diffraction and transmission electron microscopy observations and ex situ neutron diffraction. The volume fraction of the blocky austenite grains increased by tempering at 573 K, whereas the thin film austenite decomposed to ferrite and cementite. The tempering behavior of the blocky austenite grains is an unusual phenomenon.
  • Hot Deformation Behavior of High Mn TWIP Steel Using the Processing Map

    pp. 691-696

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    DOI:10.2355/isijinternational.55.691

    Hot deformation behavior of 30Mn-0.5C-3.7Al-4Si TWIP steel was investigated in this study. Cylindrical specimens were used for hot compression tests at temperatures ranging from 750 to 1000°C and the strain rate range of 0.001–0.5 s−1. The effect of temperature and strain rate on the flow curves was addressed. Then, processing maps of hot deformation of the steel at the selected strains of 0.5 and 0.6 were developed. In addition, standard metallography procedures were conducted on the specimens after the hot compression tests. The generated processing maps indicated that there were only one unstable region and two stable regions in which dynamic recrystallization occurred. In addition, power dissipation was generally decreased with increasing the strain, while stability flow zone was gradually expanded. In the stable region, the dynamic recrystallization zone with a maximum efficiency 0.35 at a temperature of about 1000°C and the strain rate 0.1 s−1 was enlarged with increasing the strain up to 0.6. Finally, an optimum hot working condition for the TWIP steel was predicted from the processing maps.
  • Effect of Surface Oxidization on the Normal Spectral Emissivity of Straight Carbon Steel at 800–1100 K in Air

    pp. 697-705

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    DOI:10.2355/isijinternational.55.697

    This paper studies the effect of surface oxidization on the normal spectral emissivity of straight carbon steel Q235 in air at 1.5 μm over the temperature range from 800 to 1100 K. For this reason, the normal spectral emissivity of straight carbon steel Q235 is measured at sixteen definite temperatures over a 6-hour heating period. The normal radiance emitted from the specimen is received by an InGaAs photodiode detector. The temperature of specimen surface is measured by the two platinum-rhodium thermocouples. The variation of normal spectral emissivity with the heating time is studied at a given temperature. The variation of normal spectral emissivity with the temperature is evaluated at a definite heating time. The strong oscillation of normal spectral emissivity is discussed, which is affirmed to be connected with the thickness of oxide layer on the specimen surface, and originate from the interference effect between the radiation stemming from the oxide layer on the specimen surface and the radiation coming from the underlying metal substrate. The uncertainty of normal spectral emissivity contributed only by the surface oxidization is about 1.5–7.1%, and the uncertainty of temperature generated only by the surface oxidization is about 1.7–5.8 K. The models between the normal spectral emissivity and the heating time or temperature are evaluated. A simple functional form including the exponential and logarithmic functions can be used to reproduce well the variation of normal spectral emissivity with the heating time at a given temperature, including the reproduction of strong oscillations.
  • Reduction of Magnetite Concentrate Particles by H2+CO at 1673 K

    pp. 706-708

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    DOI:10.2355/isijinternational.55.706

    The purpose of this research was to determine the effect of CO addition to H2 on the reduction kinetics of magnetite concentrate particles at high temperatures. Experiments were carried out at 1673 K. The replacement of N2 by CO enhanced the reduction rate considerably. The kinetics of CO reduction is slower than that by hydrogen but significant, and the contributions by the two gases are additive.

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