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

ISIJ International Vol. 55 (2015), No. 4

  • Crystallization Kinetics of CaO-SiO2(CaO/SiO2=1)-TiO2-10 mass%B2O3 Glassy Slag by Differential Thermal Analysis

    pp. 709-716

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    DOI:10.2355/isijinternational.55.709

    Crystallization characteristics of the CaO-SiO2-TiO2-10%B2O3 glassy slag at w(CaO)/w(SiO2)=1 have been studied by Differential Thermal Analysis (DTA) and Matusita-Sakka method. Crystallization products have been distinguished by employing X-ray diffraction (XRD) and Scanning Electron Microscopy equipped with Energy Dispersive Spectroscopy (SEM-EDS). As the TiO2 content is within 10–18%, the crystal phase precipitated is mainly CaSiO3, and the effective activation energies for crystal growth increase with the increase of TiO2 content. Crystallization mechanism for CaSiO3 shifted from surface crystallization to one-dimensional growth with increase of TiO2. As the TiO2 content in slag further increases to 22% and 26%, CaTiSiO5 becomes the predominant crystal phase precipitated, and the effective activation energies for crystal growth decrease with the increase of TiO2 content. Crystallization mechanism for CaTiSiO5 is mainly surface crystallization. Therefore, with the increase of TiO2 content, the crystallization ability of the CaO-SiO2-TiO2-10%B2O3 glass system decreases initially and then increases.
  • Thermodynamic and Kinetic Aspects of Sulfur Evaporation from Fe–C Alloy Droplets

    pp. 717-722

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    DOI:10.2355/isijinternational.55.717

    Evaporative loss of sulfur from Fe-C-0.1 mass% S alloys was investigated using electromagnetic levitation. An inert atmosphere was maintained with a constant flow of purified argon. The influence of carbon concentration, up to carbon saturation, on the reaction kinetics was determined at 1873 K. The effect of carbon on the activity coefficient of sulfur in liquid iron was also evaluated. While the value for the first order interaction parameter was found to be in good agreement with data in the literature, the value for the second order parameter was higher than those measured using a conventional crucible approach involving gas dissolution into liquid iron. The results are consistent with the premise that sulfur loss is primarily due to the evolution of monatomic sulfur vapor.
  • Evaluation of the Chemical Vacuum Effect in Decarburization Treatment by Argon-injected Steel under Normal Atmosphere

    pp. 723-726

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    DOI:10.2355/isijinternational.55.723

    For the mass production of ultra-low-carbon steel, application of the chemical vacuum method was examined in depth. In the present study, based on the results of 600-kg-scale experiments and theoretical analysis of them, the possibility of innovative creation producing ultra-low-carbon products without using a vacuum degasser was obtained.
    To confirm the effect, a 60-t-scale AOD plant test was carried out. As a result, by using the chemical vacuum effect, the carbon content is reduced to 9 ppm in normal steel because of its much higher CO partial pressure than that of stainless steel.
    The results of the examination show that, theoretically, the reaction rate can be further enhanced. However, it is necessary to construct a dispersion condition in a large-scale metal vessel with small argon bubbles because applying the current technique of small bubble production cannot produce a sufficiently high argon injection flow rate.
    For the construction of an innovative secondary refining process in the future steelmaking process, the gas dispersion technique is strongly preferred.
  • Solid State Reduction of Preoxidized Chromite-iron Ore Pellets by Coal

    pp. 727-735

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    DOI:10.2355/isijinternational.55.727

    A comparative study on coal-based solid state reduction of preoxidized chromite pellets and chromite-iron ore pellets was conducted to investigate the reduction behavior of these two types of pellets. And the enhancement mechanism of chromite reduction reactions due to the pre-oxidation process of chromite with magnetite was revealed by means of optical microscopy, SEM with EDS. The results show the presence of iron ore and pre-oxidation process both are advantageous to the reduction of chromite. Pre-oxidation of chromite and iron ore mixture at 1323 K for 15 min contributes to more formation of sesquioxide solid solution (Fe, Cr, Al)2O3 than pure chromite oxidation. Massive cations exchange and diffusion occur between chromite and iron ore, which normally cause the decrease of magnesium content and aluminum content and the increase of iron content in spinel solid solution. Accordingly, the chromite reduction is enhanced with the metallization rate of chromite raised from 20.83% in prereduced chromite pellets to 47.98% in prereduced composite pellets at temperature of 1473 K for 90 min and C/O ratio=1.06, but no further improvement is observed above 1473 K. The mechanism study based on carbothemic reduction thermodynamics and the microstructure of prereduced pellets reveals the reduction of sesquioxide solid solution (Fe, Cr, Al)2O3 is more thermodynamically supported and the high iron, low magnesium and aluminum spinel solid solution formed in pre-oxidation process is also favorable for chromite reduction because of lower outward diffusion resistance of Fe2+ to the interface with CO gas.
  • Reduction and Nitriding Behavior of Hematite with Ammonia

    pp. 736-741

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    DOI:10.2355/isijinternational.55.736

    This paper describes the reduction and nitriding behavior of hematite with ammonia in the context of ironmaking. The effects of temperature and ammonia concentration on the products were investigated. In the temperature range from 793 to 863 K, hematite was directly reduced to magnetite by ammonia (1/2Fe2O3 + 1/9NH3 → 1/3Fe3O4 + 1/6H2O + 1/18N2). The ammonia started to decompose at 873 K, triggered by the generation of α-Fe. Magnetite was reduced mainly to iron by hydrogen generated from the decomposition of ammonia (1/3Fe3O4 + 4/3H2 → Fe + 4/3H2O). According to in-situ X-ray diffraction (XRD) measurements, α-Fe was immediately nitrided to an ε-Fe3-xN (0 ≤ x ≤ 1) phase, and the N/Fe atomic ratio decreased gradually with increasing temperatures. The rate of hematite reduction increased with the ammonia concentration for 5% to 20% NH3, but plateaus for NH3 concentrations greater than 20%. This was attributed to the mechanism of ammonia decomposition; the amount of hydrogen generated also plateaus at ammonia concentrations above 20%. The reduction rate was therefore limited by the rate at which hydrogen was generated during ammonia decomposition. The N content of the product was affected not only by the temperature but also by the nitriding potential (KN = PNH3/PH23/2). The nitriding potential increased with increasing ammonia concentrations and decreasing temperatures. The addition of nitrogen gas to the reactive gas inhibited ammonia decomposition and increased the nitrogen potential and N content of the product.
  • High Temperature Mineralization Behavior of Mixtures during Iron Ore Sintering and Optimizing Methods

    pp. 742-750

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    DOI:10.2355/isijinternational.55.742

    The structure of sinter composes of a melt zone and unfused ores. Sinter strength is mainly subjected to the properties of melt zone since unfused ores are wrapped by melt zone is proposed. It facilitates obtaining sinter of high strength with the increase of liquid phase generated in melt zone and the formation of columnar & acicular SFCA (calcium ferrite containing silicate and alumina) during melt condensation. The mineralization behaviors show that, when Ca/Fe (molar ratio) in melt zone is 0.3–0.4, the content of SiO2 (mass ratio) is about 5%, Al2O3 is less than 1.8% and MgO is as low as possible, it would benefit the generation of liquid phase and columnar & acicular SFCA. Research of optimizing ore blending indicates that, as the chemical components in melt zone satisfies the conditions of mineralization, the yield and tumbler strength of sinter can be improved, and the solid fuel consumption can be reduced.
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  • Carbothermal Reduction of Boron-bearing Iron Concentrate and Melting Separation of the Reduced Pellet

    pp. 751-757

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    DOI:10.2355/isijinternational.55.751

    Comprehensive utilization of low grade ludwigite is critical important for the safe supply of boron resource of China. Boron-bearing iron concentrate is the intermediate product of the ore dressing process of crude low grade ludwigite and extracting boron from it has great effect on the total boron yield. Pre-reduction of the boron-bearing iron concentrate is one of the important steps of the pyrometallurgical boron and iron separation process. The carbothermal reduction study of boron-bearing iron concentrate was carried out in the present work. The reduction rate was improved with the increasing of heating temperature and carbon content. The appropriate temperature and C/O (mole ratio of fixed carbon to reducible oxygen) were 1200 to 1300°C and 0.8 to 1.2, respectively. In addition, the microstructure and phase evolution of the pellet during the reduction process were characterized by means of SEM and XRD. The apparent activation energy of the iron oxide reduction in the composite pellet was 114.32 kJ/mol based on the first-order reaction model. The iron and slag separate well when the reduced pellets were heated at 1550°C. The B2O3 content of the slag and the boron element content of the pig iron were 10.8 wt% and 0.74 wt%, respectively. The main minerals in the slag were olivine, kotoite, periclase and spinel after slow cooling, and the efficiency of extraction of boron (EEB) of the slag was 68.4%. The boron-rich slag and boron-bearing pig iron can be used as the raw materials for boron extraction and steel making.
  • Recycling of Blast Furnace Dust in the Iron Ore Sintering Process: Investigation of Coke Breeze Substitution and the Influence on Off-gas Emissions

    pp. 758-764

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    DOI:10.2355/isijinternational.55.758

    In iron ore sinter plants blast furnace dust and other fine-grained residues which are rich in iron or carbon content are recycled. The carbon contained in the blast furnace dust can be used to replace coke breeze which is added to the charge as fuel for the sintering process. However, there is no information in the literature on the carbon replacement factor of fine-grained blast furnace dust. Nor is detailed information on the effect of blast furnace dust recycling on sinter plant off-gas emissions available. Recycling of blast furnace dust increases the zinc content of the sinter feed mixture. The degree of volatilisation of zinc in the sintering process and its discharge with the off-gas is of interest because zinc has to be limited in the burden of the blast furnace. The sinter pot tests performed in this study revealed that the carbon replacement factor of the carbon content in fine-grained blast furnace dust is very close to 1.0. The removal of sulphur contained in the sinter charge depends on the basicity of the sinter and on the total sulphur input. About 25% of the zinc contained in the blast furnace dust was volatilised in the sintering process and emitted with the off-gas.
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  • Modification of Reactivity and Strength of Formed Coke from Victorian Lignite by Leaching of Metallic Species

    pp. 765-774

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    DOI:10.2355/isijinternational.55.765

    Binderless briquetting of lignite at 100–200°C and subsequent carbonization produces formed coke with tensile strength (ST) of 5–40 MPa, while the briquetting often requires mechanical pressure over 100 MPa. High reactivity is another feature of the lignite-derived coke, and this arises from highly dispersed metallic species such as alkali/alkaline-earth metallic species and ferrous/ferric ones that catalyze CO2 gasification. This work investigated effects of leaching of those metallic species in aqueous solution of hydrochloric acid, acetic acid or oxalic acid on the reactivity and ST of resulting coke from a lignite. The leaching at pH ≤ 1 removed catalytic metallic species near completely, reducing the coke reactivity by a factor of 8–15. The reduced reactivity was similar to the reactivity of coke from a typical coking coal. The leaching at pH ≤ 2.2 increased ST from 6 to 13 MPa for briquetting at 200°C and 32 MPa. The performance of leaching with oxalic acid, of that solution had pH of 0.75 at 1 mol/L, was much better than that with acetic acid. This work also examined another type of leaching, oxidation of the lignite in aqueous solution of hydrogen peroxide, which produced organic mono-/di-acids in-situ from the oxidation of aromatic carbons of the lignite. The degree of reduction of the coke reactivity was between that for leaching at pH of 1 and 2. The degradation of macromolecules enhanced plasticizability of the lignite under briquetting and increased ST of the resulting coke to 22 MPa.
  • Interaction Steel/Slag/Submerged Entry Nozzle and its Impact on Refractory Wear – Thermochemical Process Simulation

    pp. 775-780

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    DOI:10.2355/isijinternational.55.775

    This work presents the generation and application of a process model which aims to increase the understanding of submerged entry nozzle (SEN) wear during continuous casting and its dependence on steel and slag compositions. An effective equilibrium reaction zone model was built and applied to calculate the equilibrium compositions of steel and slag at their common interface and in contact with the SEN. The results allow the estimation of the interface tension gradient responsible for Marangoni convection. Further, they include redox reactions at the slag/steel/refractory interface. Calculations were performed for a LC and a TRIP steel grade together with their respective mold fluxes. In both cases, the same SEN was used and the wear was assessed after service. In the case of the TRIP steel, the SEN showed by far lower wear and a less pronounced corrosion groove. The simulation showed that the interface tension difference responsible for Marangoni convection will be rather negligible for the case of TRIP steel, but it amounts to 150 mN/m for LC steel. Moreover, removal of carbon from the SEN zirconia-graphite sleeve will be caused by the reduction of silica in the slag. This is only possible at the three-phase boundary where the activity of the silicon formed is lowered by dissolution in the steel. Also, this effect is by far more distinct for LC steel and is in agreement with the different wear rate and corrosion groove. As a spin-off, the calculated interface tension might be interesting with respect to possible slag intrusion.
  • Effects of Alloying Elements on the Ferrite Potential of Peritectic and Ultra-Low Carbon Steels

    pp. 781-790

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    DOI:10.2355/isijinternational.55.781

    Solidification of steels involving the peritectic reaction are inherently prone to longitudinal facial cracking during casting, owing primarily to the large volume shrinkage associated with delta-ferrite to austenite transformation occurring at high temperatures. Therefore casting of those steels requires a lot of caution. Ferrite Potential (FP) is widely accepted as a measure of the tendency of steels to form delta-ferrite during solidification. However, the most commonly used definition of Ferrite Potential is strictly applicable only to plain carbon steels while commercial steels may contain several other alloying elements. Therefore, the existing definition may not express their actual Ferrite Potential. In view of this, a new correlation has been developed in the present work for evaluating the true Ferrite Potential for multi-component steels. Influences of several common alloying/solute elements have been considered while developing the present model. Present model development involved rigorous thermodynamic calculations aimed at determining the interactions among various solute elements and their overall influence on the peritectic compositions and the peritectic transformation temperature. In addition, assumed quadratic dependence of peritectic compositions and temperature on the concentrations of alloying elements has been employed besides the application of non-linear multi-variate regression analysis. Finally, present correlations have been extensively validated against literature data as well as actual plant observations. In comparison to other models, reasonably better agreement between the current model predictions and plant data has been obtained. The present model has helped the plant operators in selecting the right parameters for casting operations.
  • Transport Phenomena in a Beam-Blank Continuous Casting Mold with Two Types of Submerged Entry Nozzle

    pp. 791-798

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    DOI:10.2355/isijinternational.55.791

    A three-dimensional full-coupled mathematical model is established to study the fluid flow, heat transfer and solidification in a 450 mm × 350 mm × 90 mm beam-blank mold with two different types of submerged entry nozzle (SEN), namely single-port straight SEN and three-port radial flow SEN. Water modeling experiments, industrial trials and public results available in literature are performed to validate the numerical results. The results show that, with the straight SEN which has been widely applied in beam-blank continuous casting, there is a very inactive top free surface in the mold which level fluctuation magnitude is less than 1 mm and velocity magnitude is far from a reasonable interval, and the shell thickness distribution at the mold exit is very uneven, thick at the web but thin at the fillet. Moreover, there exists a “wavy contour” at the flange due to the washing effect of the off-center molten steel jet. While with the new designed radial flow SEN, a suitable meniscus status and a more uniform shell thickness at the mold exit can be obtained, which is helpful to avoid the breakouts caused by the rupture of thin fillet and the flange depression. The “self-braking” effect caused by two radial flow SENs provides good flow stability at the web center.
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  • Distribution and Detriment of Bubbles in Continuous Casting Interstitial Free Steel Slab

    pp. 799-804

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    DOI:10.2355/isijinternational.55.799

    The distribution and detriment of bubbles in ultra low carbon interstitial free steel were studied by X-ray radiographic and cold-rolled experiment; a total of 24 pieces samples with size 230 mm × 65 mm × 2 mm were detection and 150 bubbles were counted; the results showed that: (1) Two bubbles bands formed in continuous casing slab; bubbles distributed asymmetrically in thickness direction of the slab; small bubbles mainly concentrated in inner art side 1/8 to 1/4 in thickness direction; large bubbles were main in center of the slab in thickness direction; (2) Hundreds of inclusions with sizes 2 μm to 10 μm accumulated in cone-shape region after bubbles. Inclusions or Fe and FeO particles often attached or mixed with the bubbles. (3) Small bubbles entered into down-flow easily and rushed out the mould, bubbles and inclusions accumulative band formed in inner art side of the slab. (4) The bubbles, inclusions around the bubbles or large Fe and FeO particles mixing with bubbles became source of the sliver defect.
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  • Instability and Periodicity of Asymmetrical Flow in a Funnel Thin Slab Continuous Casting Mold

    pp. 805-813

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    DOI:10.2355/isijinternational.55.805

    Characteristics of the transient turbulent flow in a funnel thin slab continuous casting mold are studied using a large eddy simulation (LES) computational approach. Validations were done through comparison with previous experimental data of the mean velocities and the instantaneous velocities, and good qualitative and quantitative agreements are obtained. The turbulence flow inside the mold consists of various scales vortices; many pronounced large scale vortex structures were clear found inside the mold, containing various small scale vortices between them. The boundary shear flow can separate from the narrow wall and transport vorticity into the interior flow, and then possibly develop into a vortex. The intermittent chaotic vortex formation on both sides of the SEN is found at the meniscus. Three types of vortex phenomena are identified: one big vortex, two vortices and three small vortices. The positions and sizes are different, and the vortexes are located at the low velocity sides adjacent to the SEN. Significant asymmetry is seen in the instantaneous flow in the two halves of the thin slab mold cavity, especially in the lower recirculation zone. The periodical behavior of asymmetric flow inside the liquid pool was identified and characterized. The spectrum of velocities shows that the mean time interval for periodical changeover is 14.28 seconds.
  • Simulation Research of Flow Field in Continuous Casting Mold with Vertical Electromagnetic Brake

    pp. 814-820

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    DOI:10.2355/isijinternational.55.814

    The narrow face of continuous casting mold is the first impact region of the jet from the submerged entry nozzle (SEN), and against the characteristic a new pattern of electromagnetic brake device is proposed, which is called Vertical Electromagnetic Brake (V-EMBr) and its magnetic poles can cover the free surface and the frontier region of solidified shell. The effect laws of magnetic flux density, casting speed and the submergence depth of the SEN on flow field in mold with V-EMBr are investigated by numerical simulation method. The results show that with the magnetic flux density increasing, inhibitory effect of electromagnetic force on molten steel from SEN is increased gradually, impact strength of molten steel stream on narrow face is weakened gradually, the vortex center of lower recirculation zone is moved up gradually and the flow velocity of free surface is decreased gradually. These can reduce the fluctuations of free surface and chance of slag effectively, suppress impinging depth of molten steel in lower recirculation zone and float air bubbles and inclusions beneficially. In addition, the V-EMBr technology can also be applied to different casting speed and different submergence depth of the SEN in continuous casting process. The significantly metallurgical effects and flexibility of application range which are generated by V-EMBr is in line with the original intention of the V-EMBr design.
  • Melting and Heat Transfer Behavior of Fluorine-Free Mold Fluxes for Casting Medium Carbon Steels

    pp. 821-829

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    DOI:10.2355/isijinternational.55.821

    As fluxing agent, fluorine is important to the properties of mold flux; however, it tends to cause serious environment and health problems. In this paper, the melting and heat transfer behaviors have been studied by using Single Hot Thermocouple Technique (SHTT) and Infrared Emitter Technique (IET). The results show that the melting temperature range of F-free mold flux decreases with the addition of Na2O/Li2O and B2O3 contents; and the heat flux decreases with the increase of basicity and Na2O/Li2O, as well as the decrease of B2O3 contents. The analysis of EDS and XRD show that Ca11Si4B2O22 and Ca14Mg2(SiO4)8 are the two main precipitated crystalline phases in F-free mold fluxes, and the Ca11Si4B2O22 is common and stable crystalline phase in the designed F-free mold fluxes system that shows the potential to replace Ca4Si2O7F2 in the conventional F-containing mold fluxes. Besides, through the comprehensive comparison, Sample E (Basicity 1.15, Na2O 7.92, Li2O 1.97 and B2O3 5.98) shows the closest performances with the benchmark conventional commercial mold flux and has the potential to be used for casting medium carbon steels.
  • Effect of TiO2 Addition on Crystallization Characteristics of CaO-Al2O3-based Mould Fluxes for High Al Steel Casting

    pp. 830-836

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    DOI:10.2355/isijinternational.55.830

    Crystallization behaviors of new developed CaO–Al2O3 based mould fluxes with TiO2 addition for casting of high-Al steels were investigated by using DTA techniques combined with SEM-EDS and XRD analysis. XRD and SEM analyzed on the crystallized samples showed that the sequence of crystal precipitation for TiO2-free mould flux during cooling was MgO, and followed by Ca12Al14O33 during cooling. The sequence of crystal formation for TiO2-bearing mould fluxes during cooling is CaTiO3 to MgO, and then Ca12Al14O33. Continuous cooling transformation diagrams (CCT) were constructed for analysis of the crystallization behaviors. The Undercooling values for onset crystallization of various crystals were calculated by using liquidus temperature obtained by heating DTA and crystallization temperature of various crystals. The crystallization temperatures of CaO–Al2O3 based mould fluxes increased with increasing TiO2 content. The undercooling values for onset crystallization of CaTiO3 decreased with increasing TiO2 content, which indicating that the crystallization of CaTiO3 was enhanced with increasing TiO2 content. The undercooling values for onset crystallizations of Ca12Al14O33 and MgO only changed slightly with increasing TiO2. This indicated that crystallizations of Ca12Al14O33 and MgO crystals were only slightly influenced by TiO2 addition. The overall crystallization of mould fluxes was enhanced with increasing TiO2 content.
  • Simulation of Flow and Heat Fields in a Seven-strand Tundish with Gas Curtain for Molten Steel Continuous-Casting

    pp. 837-844

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    DOI:10.2355/isijinternational.55.837

    The present study involves physical and mathematical simulations to study the effect of gas bubbling curtains on the flow and temperature fields in a seven-strand tundish. In 1/3 scale tundish model experiments, RTD (Residence Time Distribution) curves were used to study the flow characteristics in the tundish with different configurations. The flow uniformity for all strands is evaluated through variance analysis of the RTD curves in different tundish cases. Besides, the tracer dispersion experiment was performed to visualize the fluid flow under the effect of the gas curtain. In order to investigate the effect of a gas curtain on the flow in the tundish, mathematical simulations were carried out mainly for two cases with or without gas bubbling curtains. The RTD curves were obtained from the species transport model. The results obtained show that gas bubbling curtains can help improve fluid flow characteristics in multi-strand tundish, and that the location of the curtains plays a key role in optimizing multi-strand tundishes configuration. Gas bubbling curtains can decrease the differences in residence times and molten steel temperatures at different outlets in multi-strand tundishes effectively. Heat transfer between different areas is promoted and the temperature field is uniformed significantly in such tundishes.
  • Adaptive Least Squares Support Vector Machine Predictor for Blast Furnace Ironmaking Process

    pp. 845-850

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    DOI:10.2355/isijinternational.55.845

    Blast furnace system is one of the most complex industrial systems and, as such, there are still many unsolved theoretical and experimental difficulties, such as silicon prediction. For this reason, based on recursive updating algorithm, an adaptive least squares support vector machine (LS-SVM) predictor is presented for prediction task of silicon content in blast furnace (BF) hot metal. The predicator employs recursive updating algorithm to get the precise solution of the latest LS-SVM model and avoid the long process of running through the whole model. Theoretically, the computational complexity is reduced significantly from O(n3m + m4) to O(n3 + m3). Experiments on two different BF data sets demonstrate that the proposed adaptive LS-SVM predicator is suitable for the task of predicting BF ironmaking process for its high hitting percentage and time saving.
  • Camber Regulation in Rough Rolling Process Using Wedge Estimation of Incoming Bar

    pp. 851-857

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    DOI:10.2355/isijinternational.55.851

    In this paper, a new algorithm to reduce camber based on an estimated entry-side wedge of the bar during rough rolling process is proposed. The entry-side wedge is estimated based on a simple mathematical model with the aids of the tangent vector variation and curvature, which can both be obtained using the measured bar centerline. The differential roll gap between drive-side and work-side was then adjusted to minimize the delivery-side curvature of the bar using this estimated entry-side wedge information. A three-dimensional simulator that describes a rough rolling process was constructed using FEM (Finite Element Method). The effectiveness of the proposed scheme was validated using FEM simulation combined with the proposed camber reducing algorithm.
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  • Decorated Dislocations with Fine Precipitates Observed by FIB-SEM Slice-sectioning Tomography

    pp. 858-862

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    DOI:10.2355/isijinternational.55.858

    Dispersion behavior of intragranular NbC precipitates in Nb added austenitic stainless steel were investigated via nanoscopic characterization in detail, FIB-SEM slice-sectioning tomography, orientation image microscopy, energy-dispersive X-ray spectrometry (EDS), selected area electron diffraction pattern (SAEDP) and transmission electron microscopy (TEM). The heterogeneous dispersion of fine intragranular NbC precipitates were visualized, and in particular, it was found that they were on the {111} slip plane and associated with dislocations.
  • Visualization Study of Flow Stability in Reverse Roll Coating

    pp. 863-869

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    DOI:10.2355/isijinternational.55.863

    Reverse roll coating is widely used to coat a thin liquid layer onto a moving substrate. A metered liquid layer is created within the gap between a pair of co-rotating rolls. To avoid roll damage due to roll run-out, substrate caliper change or splice passage, when the gap between the rolls is small, one of the rolls will have a compliant polymer cover. The existence of a deformable cover in the gap between a metal roll and a rubber roll creates an elastrohydrodynamic flow field in that region. As the liquid passes through the coverging-diverging section within the gap, it generates pressure, and this pressure can deform the elastic roll surface, which in turn alters the geometry of the gap and the flow. Therefore, the uniformity of coating is affected differently than what is observed when only rigid rolls are used.
    In this study, visualizations of the flow between a reverse deformable roll and solid stainless steel roll are done to determine how the uniformity of coating in the high roll speed region is affected by operating parameters, namely, the speed ratio between the rolls, roll cover properties and liquid properties. The wavelength of ribbing is investigated to verify the effect of speed ratio, wet coating thickness and viscosity.
    For these experiments, the roll coating apparatus is used with 4 inch diameter rolls installed one above the other. A high-resolution camera is used for visualizations of the flow, and the ribbing wavelength is measured from photographed images. The uniformity of coating and its dependence on capillary number is evaluated.
    A numerical simulation of the flow between metal and rubber rolls enabling prediction of the ribbing wavelength is also performed.
  • In situ Observation of the Influence of Al on Deformation-induced Twinning in TWIP Steel

    pp. 870-876

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    DOI:10.2355/isijinternational.55.870

    The deformation twinning behavior of Fe18Mn0.6C and Fe18Mn0.6C-2.5Al twinning-induced plasticity steels was compared by in-situ electron backscattering diffraction. Al suppressed deformation-induced twinning. A constitutive model considering the effect of Al on the twin formation kinetics was used to show that the work hardening and the ultimate tensile strength were lowered by the suppression of deformation twinning despite the pronounced solution hardening contribution of Al.
  • Recrystallization of Isolated Deformed Grains in High Purity Iron

    pp. 877-883

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    DOI:10.2355/isijinternational.55.877

    Early stage of recrystallization behavior was investigated in high purity deformed iron with isolated crystal grains to elucidate the mechanism of recrystallization. Recrystallization of the isolated deformed grains was greater than that of the bi-crystal matrix. The recrystallized grains were observed in the RD×ND section at the tip portion of the deformed isolated grains extending in the rolling direction. Recrystallized grains were also observed inside the deformed isolated grains. The crystal orientations of the recrystallized nucleus were positioned where they were in the perimeter portion of the orientation spread of the deformed matrix, or there may have been the intermediate orientation between the matrix and the isolated grains.
  • Modelling the Evolution of Multiple Hardening Mechanisms during Tempering of Fe–C–Mn–Ti Martensite

    pp. 884-893

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    DOI:10.2355/isijinternational.55.884

    We model the hardness evolution of martensite during tempering as a linear addition of multiple hardening mechanisms that is combined with a microstructural Kampmann-Wagner-Numerical (KWN) model to simulate the nucleation and growth of TiC-precipitates during tempering. The combined model is fitted to the measured hardness evolution during tempering at 300°C and 550°C of martensitic steels with and without the addition of titanium. The model predicts TiC-precipitate sizes in agreement with experimental observations and generates fitting parameters in good agreement with literature. The microstructural components that give the highest contribution to the overall hardness are Fe3C precipitates (88 HV) and dislocations (54 HV). Both Fe3C- and dislocation-strengthening decreases rapidly during the initial stage and stabilise after 10 minutes of tempering. The model shows that the decrease in dislocation density due to recovery is slowed down due to the presence of TiC-precipitates. Titanium atoms in solid solution give a stable hardness contribution (25 HV) throughout the tempering process. TiC-precipitate strengthening generates a minor contribution (3.5 HV). The model shows that less than 1% of the equilibrium volume fraction of TiC-precipitates forms during isothermal tempering at 550°C due to the large misfit strain (1.34 GJ/m3) and a limited density of potential nucleation sites in the martensite. The model shows that the hardness of tempered martensitic steels could potentially be increased by increasing the TiC-precipitate density by reducing the misfit strain.
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    1. A Bainite-Ferrite Multi-Phase Steel Strengthened by Ti-Microalloying MATERIALS TRANSACTIONS Vol.52(2011), No.11
    2. Development of Continuous Steelmaking Slag Solidification Process Suitable for Sensible Heat Recovery ISIJ International Vol.55(2015), No.4
  • Development of Continuous Steelmaking Slag Solidification Process Suitable for Sensible Heat Recovery

    pp. 894-903

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    DOI:10.2355/isijinternational.55.894

    The COURSE50 project aims at developing technologies to reduce CO2 emissions from steel works by approximately 30% in Japan. In order to supply the energy required to separate CO2, a technology for recovering sensible heat from steelmaking slag is being developed as one theme of COURSE50. A twin roll type continuous slag solidification process to obtain a shape suitable for sensible heat recovery was investigated.
    Sheet-like slag was shaped to a thickness of about 7 mm in a twin roll pilot-scale experiment. The slag thickness depended on the adhesion of the molten slag rather than the thickness of the solidified slag on the roll. The slag condition suitable for the twin roll method was identified as a liquid phase ratio of 60% or more. Based on a laboratory-scale experiment and heat transfer calculations, a combination process using the twin roll method and a countercurrent flow packed bed is expected to achieve a heat recovery ratio of 30% or more from sheet-like slag.
  • Effective Thermal Conductivity of Slag Crust for ESR Slag

    pp. 904-906

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    DOI:10.2355/isijinternational.55.904

    x

    Readers Who Read This Article Also Read

    1. Physical Model of Slag Foaming ISIJ International Vol.33(1993), No.1
    2. Thermal Conductivity of the CaO–Al2O3–SiO2 System ISIJ International Vol.46(2006), No.3
    3. Prediction of Slag Pool Thickness in Continuous Casting Mould ISIJ International Vol.39(1999), No.8
  • Effect of Basicity on Titanomagnetite Concentrate Sintering

    pp. 907-909

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    DOI:10.2355/isijinternational.55.907

    The effect of binary basicity (mass ratio of CaO to SiO2) on the sintering of titanomagnetite concentrate was investigated in an experimental-scale sintering pot. The results indicated that increasing basicity can improve yield, productivity, and reduction index (RI) of the finished sinter. However, sinter strength and reduction disintegration index (RDI+3.15) have a significant decrease at the basicity varying from 1.0 to 2.0. It also proved that titanomagnetite and titanohematite are the main minerals in sinters, while kirschsteinite and calcium silicate (CS) are the main melted phases. The amount of perovskite increases then slightly decreases with the increasing basicity as silicoferrite of calcium and aluminum (SFCA) began to generate and increase with basicity from 1.5 to 3.0. It is concluded that adjusting sinter basicity is capable to restrain the adverse effect of TiO2 on sintering of titanomagnetite concentrate.
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    Readers Who Read This Article Also Read

    1. Reduction of Titania-Ferrous Ore by Hydrogen ISIJ International Vol.44(2004), No.6
    2. Sintering Characteristics of Titanium Containing Iron Ores ISIJ International Vol.54(2014), No.10

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