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

ISIJ International Vol. 54 (2014), No. 10

  • Interdiffusion in Co-Based Co–Al–W Ternary Alloys at Elevated Temperatures

    pp. 2129-2133

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    DOI:10.2355/isijinternational.54.2129

    L12-type Co3(Al,W) compound is a very important phase in Co-based high temperature materials. This study investigated the interdiffusions of Co-based Co–Al–W ternary alloys at 1423, 1473 and 1523 K. Two types of diffusion couples were employed to estimate four diffusivity matrix coefficients ([ This part of text is omitted ], [ This part of text is omitted ], [ This part of text is omitted ] and [ This part of text is omitted ]). A modified ternary Boltzmann-Matano method was employed for the evaluation of interdiffusion coefficients. The estimated cross-interdiffusion coefficients ([ This part of text is omitted ] and [ This part of text is omitted ]) had opposite signs at 1523 K which is not contradictory to but rather consistent with the Onsager’s reciprocity theorem. The second derivative of the Gibbs free energy with respect to the Al concentration, ([ This part of text is omitted ]), may have a sign opposite to that of W concentration, ([ This part of text is omitted ]). Experimental results revealed that the effect of W on the Al diffusion flux is greater than that of Al on the diffusion flux of W by about one order of magnitude.
  • Development of a Quasi-chemical Viscosity Model for Fully Liquid Slags in the Al2O3–CaO–‘FeO’–MgO–SiO2 System: The Revised Model to Incorporate Ferric Oxide

    pp. 2134-2143

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    DOI:10.2355/isijinternational.54.2134

    A model has been developed that enables the viscosities of the fully liquid slag in the multi-component Al2O3–CaO–FeO–Fe2O3–MgO–SiO2 system close to and at metallic iron saturation to be predicted within experimental uncertainties over a wide range of compositions and temperatures based on the Eyring equation to express viscosity. The model links both the activation and pre-exponential energy terms to the slag internal structure through the concentrations of various Si0.5O, Men+2/nO and Men+1/nSi0.25O viscous flow structural units, of which the concentrations are derived from a quasi-chemical thermodynamic model of the liquid slag. The model describes a number of slag viscosity features including the charge compensation effect specific for the Al2O3-containing systems. The present paper describes application of recent significant improvements in the model formalism to the multi-component system Al2O3–CaO–FeO–Fe2O3–MgO–SiO2, where both Fe2+ and Fe3+ effects on viscosity are individually evaluated. The present model reproduces viscosities of slags equilibrated with metallic iron, which mainly reflects Fe2+ effects on viscosity including the charge compensation effect of the Fe2+ as well as Ca2+ and Mg2+ cations on the formation of tetrahedrally-coordinated Al3+. The model can also reproduce the compositional tendency of viscosity of the SiO2-free CaO–FeO–Fe2O3 slag in air by incorporating the charge compensation effect of Fe3+ to form tetrahedral coordination by basic cations such as Ca2+ and Fe2+ and to indicate viscosity maximum at an intermediate composition. Further analysis of the behaviour of the Fe3+ cation in the silicate structure to describe corresponding effect on viscosities and to improve viscosity predictions is essential.
  • Reaction between MnO–SiO2–FeO Oxide and Fe–Mn–Si Solid Alloy during Heat Treatment

    pp. 2144-2153

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    DOI:10.2355/isijinternational.54.2144

    The diffusion behavior of each element at the interface has to be clarified to comprehensively understand the reaction between oxide inclusions and solid steel at heat treatment temperatures. To this end, we performed diffusion couple experiments to investigate the interaction between Fe–Mn–Si alloy and MnO–SiO2–FeO oxide. As the first step, we investigated the composition of the oxide equilibrated with the molten steel deoxidized by Mn–Si. Results indicate that the equilibrium oxide was not a binary compound of MnO and SiO2 but a ternary compound of MnO, SiO2, and FeO. In diffusion couple experiments, fine oxide particles and fine metal particles were observed near the interface in the alloy and in the bulk oxide, respectively, after heat treatment at 1673 K and 1473 K. This phenomenon indicated that the alloy and bulk oxide in the diffusion couple were not equilibrated at these temperatures, despite the equilibration of both phases at 1823 K. The precipitation behavior of the particles can be related to the diffusion of oxygen from the oxide to the alloy. The oxide particles were precipitated near the interface, and the Mn content in the alloy decreased in this region owing to the consumption of Mn for particle formation. The formation of metal particles and the decrease of FeO content in oxide indicate the reduction of unstable FeO in oxide at these temperatures.
  • Surface Tension of the Molten Blast Furnace Slag Bearing TiO2: Measurement and Evaluation

    pp. 2154-2161

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    DOI:10.2355/isijinternational.54.2154

    The influence of TiO2 content and basicity on the surface tension of the molten blast furnace slag has been investigated using the dispensed drop method at 1673 K under the Argon atmosphere (Pressure ≈ 1.2 atm). The measurement shows that TiO2 plays a role as a surfactant in the CaO–SiO2–MgO–Al2O3–TiO2 system, and the decrease of the surface tension of the melt results from both the compositional changes and the structural change along with the composition variation. The surface tension of the slag rises from 413 mN/m up to 456 mN/m along with CaO/SiO2 mass ratio increasing from 0.90 to 1.20, while the structural variation only attenuates the rise of the surface tension of the slag. The thermodynamic model for determining the surface tension of molten slag by considering the surface tension and molar volume of the pure oxide components, and their ionic radii was applied to this molten slag bearing TiO2. The temperature dependence of the surface tension for pure TiO2 has been determined by comparing the measurement and the model calculation. The calculated surface tensions based on the present model are in good agreement with the experimental values. The maximum error and the average error are –7.52% and 3.27%, respectively. The influence of MgO and Al2O3 in the 5-component slag system has also been studied, and the addition of MgO or Al2O3 can increase the surface tension of the slag.
  • Preparation of Prereduced Pellets by Pyrite Cinder Containing Nonferrous Metals with High Temperature Chloridizing-reduction Roasting Technology

    pp. 2162-2168

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    DOI:10.2355/isijinternational.54.2162

    Pyrite cinder contains considerable amount of nonferrous metals and the iron grade of pyrite cinder is low for blast furnace burden, so processing the pyrite cinder has become a significant issue for the utilization of valuable resources and environmental protection. In this study, a technique with high temperature chloridizing-reduction roasting is presented in aim to remove nonferrous metals from iron and produce blast furnace burden with high quality. The effects of chloridizing temperature and time, chlorinating agent dosage, reduction temperature and time are investigated. The optimum process parameters are determined as follows: chloridizing at 1125°C for 10 min, CaCl2 dosage of 2% and reduction at 1100°C and the coal/cinder ratio of 2.5 for 40 min. Under these conditions, the compressive strength of prereduced pellets is 1147N per pellet, the metallization degree is 84.53% and the removal rates of Cu, Pb and Zn are 37.87%, 99.31% and 80.20%, respectively.
  • Development on Iron Ore Pelletization Using Calcined Lime and MgO Combined Flux Replacing Limestone and Bentonite

    pp. 2169-2178

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    DOI:10.2355/isijinternational.54.2169

    CaO and MgO bearing fluxes are widely used in sinters and pellet to improve their basicity and other properties viz. strength and physico-chemical properties especially, for lowering reduction degradation index of high alumina ore agglomerate. Limestone (CaCO3 mineral) is presently used as CaO input in agglomeration. Endothermic calcination of CaCO3 at elevated temperature is energy consuming and kinetic driven process. Grinding to powder also consumes energy. In contrary, calcined lime is relatively softer and disintegrates on hydration. Calcined and hydrated lime both has very good green bonding property also. Though, calcined/hydrated lime is used in sintering replacing limestone, it is hardly used in pelletization. This is because of facing problems viz. crack formation in pellet, loss in bonding efficiency of bentonite, handling of hydrated lime etc. However, suitable adjustment of flux composition and process parameters can make the calcined lime usable in pelletization as advantage. This study is concentrated on optimization of MgO (olivine) flux addition and basicity and finally used calcined lime in place of limestone in developing good quality iron ore pellets with combined flux. It is found that calcined lime fluxed pellet without bentonite and limestone shows very good dry compressive strength (3.5 kg/pellet) and improved green compressive strength (1.5 kg/pellet), drop numbers (12 Nos ), cold crushing strength (310 kg/pellet) and reduction properties. The complete replacement of limestone and bentonite is found with better pellet properties. The bentonite elimination will help minimizing alumina and silica input in pellet and reduce the cost in pelletization.
  • Understanding Coalescence in Iron Ore Sintering Using Two Bench-scale Techniques

    pp. 2179-2188

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    DOI:10.2355/isijinternational.54.2179

    With the formation of melt, the structure of a sintering bed transforms because of material coalescence. The drivers of coalescence were studied using two bench-scale techniques. Analogue sinter mixes of varying the basicity and gangue levels were taken to high temperatures using thermomechanical analysis (TMA) and in an ash fusion test (AFT). In TMA, the penetration of a piston into the sinter mix as melt was generated provided information on the deformation, shrinkage, densification and flow of the sample as a function of temperature. Projected sample shapes in the AFT were used to determine sample density and densification level. A computer model FactSage and reported equations were used to provide estimates of melt volume and viscosity. Trends indicated by the TMA and AFT results were similar and large changes in results were only obtained with significant melt generation. Differences in results between the samples could not always be explained because varying the composition of the sinter mix altered the porosity of the sample. Increasing sample porosity meant that the generated melts were not as connected and more work is required to achieve the same level of densification. On a sinter strand, coalescence occurs under a normal load and this effect is simulated in the TMA. However, the excessive flow of melt from the crucible and chemical reactions means that TMA results are unreliable at temperatures greater than 1300°C. For this reason, the AFT is the preferred technique to understand the factors that cause material coalescence on a sinter strand.
  • Sintering Characteristics of Titanium Containing Iron Ores

    pp. 2189-2195

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    DOI:10.2355/isijinternational.54.2189

    The effect of Ti-oxide on the sintering behaviour of iron ore has been studied by doping pure TiO2 to a sinter blend in laboratory tests. The results showed a considerable effect of TiO2 on sinter strength, where the tumble index increased with increasing TiO2% (up to 2.0%). However, doping more than 2.0% TiO2 to the sinter blend decreased its strength. The melting point of the sinter was also affected by increasing the TiO2 content, indicating a possible change in the coke consumption during industrial-scale sintering of Ti-bearing ores. Optical and electron microscopy studies of sinter structures confirmed a general improvement in the overall pore structure and sinter melt by increasing TiO2 content of the sinter blend up to 2.0%. An important feature of the sinter structure was the observation of perovskite phase formation and an increase in the volume fraction of this phase obtained by increasing TiO2. The formation of this perovskite phase was considered as an important reason for the reduction of the sinter melting point. Also, doping TiO2 to the sinter mix resulted in stabilizing and increasing the volume fraction of the larnite phase in the sinter structure.
  • Thermal Decomposition Behaviour of Fine Iron Ore Particles

    pp. 2196-2205

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    DOI:10.2355/isijinternational.54.2196

    In the smelting cyclone of HIsarna process, both thermal decomposition and gaseous reduction of iron ore contribute to the expected pre-reduction degree about 20%. However, the fine ore reduction and melting process in the smelting cyclone is extremely fast and it is very difficult to differentiate between the thermal decomposition and gaseous reduction. This study focused on the thermal decomposition mechanism of the fine iron ore under different conditions. Firstly, the theoretical evaluation has been conducted based on the thermodynamics, and then the laboratory investigation was conducted in three stages with three reactors: the TGA-DSC, the electrically heated horizontal tube furnace and the High-temperature Drop Tube Furnace (HDTF). According to the experimental results of the first two stages and the theoretical evaluation, it was found that the temperature of intensive thermal decomposition of Fe2O3 in the inert gas environment is in the range of 1473–1573 K, while the thermal decomposition of Fe3O4 could be sped up when the temperature is above 1773 K in the inert gas. Temperature plays an important role in the thermal decomposition degree and reaction rate. Finally, it was found that the thermal decomposition of the individual iron ore particles took place very rapidly in the HDTF and no significant influence of the particle size and residence time (t ≤ 2020 ms) on the equivalent reduction degree could be observed, when the particle diameter was smaller than 250 μm in the CO2 gas.
  • Extruded Briquettes – New Charge Component for the Manganese Ferroalloys Production

    pp. 2206-2214

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    DOI:10.2355/isijinternational.54.2206

    The results of the laboratory study and the full-scale industrial testing of the extruded briquettes (BREX) application for the production of the Ferroalloys are discussed. Stiff vacuum extrusion agglomeration provides for the high metallurgical properties of the agglomerates leading to the economic attractiveness of this technology. BREX can be efficiently used as one of the essential charge component (up to 30% of the ore part of the charge) for the Silicomanganese smelting thus improving technical and economical parameters of the furnace and decreasing the self-cost of the Silicomanganese production.
  • Precipitation and Growth Behaviors of BN Inclusions in Molten Steel

    pp. 2215-2220

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    DOI:10.2355/isijinternational.54.2215

    The existing form of BN particles, a new environmentally friendly free cutting particle, is very important in steel for the purpose of optimizing cutting performance. However, precipitation and growth behaviors of BN inclusions in steel are not very clear until now. In this research, the morphologies of BN inclusions in steel were discussed and the formation mechanisms of BN inclusions were proposed. First, the precipitation behaviors of BN inclusions in steel have been investigated by theoretical calculation using Factsage software. Then, the actual three-dimensional morphology of BN inclusions and composite inclusions were observed in laboratory. Moreover, Growth model of BN inclusions via diffusion and collision are established employing Matlab, respectively. The results revealed that BN precipitate greatly in the solidification end and BN can nucleate heterogeneously on the surface of nucleation site (such as Al2O3, spherical MnS) besides homogeneous nucleation. The size of the BN ranged from 1 to 2 um after the diffusion growth, and then BN particles and other particles in steel, relying on its own Brownian motion, collided and agglomerated with each other, thereby formed a large cluster-like inclusion. The results agreed well with the simulated results based on BN growth model.
  • Effects of Carbon Content and Grain Orientation on the Crack Growth Behaviour in Magnesia-carbon Refractory Bricks

    pp. 2221-2229

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    DOI:10.2355/isijinternational.54.2221

    Effects of carbon content and loading direction/grain orientation on the crack growth resistance of MgO–C refractories were examined via evaluating R-curve, critical stress intensity factor and bridging stress derived from the results of three point bending testing on single edged notched beam samples. The results indicated that crack growth resistance, critical stress intensity factor and bridging stress all increased with increasing the carbon content in the refractories, and were greater in the loading direction horizontal to than perpendicular to the sample pressing direction. Nevertheless, the stress intensity factor at crack initiation was similar in all cases of samples containing different levels of carbon. Based on these, it can be concluded that an MgO–C refractory with a higher carbon content would have better resistance against crack propagation after initiation, but would not show obvious improvement in its resistance against crack initiation.
  • Dissolution Behavior of Mg from MgO into Molten Steel Deoxidized by Al

    pp. 2230-2238

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    DOI:10.2355/isijinternational.54.2230

    The formation of MgO·Al2O3 spinel-type inclusions has often been reported even when Mg is not added during treatment. Many researchers have investigated the dissolution behavior of Mg from slag; however, studies on the reaction between molten steel and MgO-type refractory are limited. In this paper, the inclusion composition of Al-deoxidized steel melted in a MgO crucible, and mass transfer rates of Mg from a MgO rod and MgO in slag to Al-deoxidized molten steel were investigated. These studies clarified that Mg dissolved in the molten steel and the spinel formed not only with the reaction of molten steel and slag but also with the reaction of molten steel and MgO crucible. The dissolution rate of Mg from MgO rod increased as the rotation rate and Al content in steel increased. The MgO·Al2O3 spinel layer formed at the interface between the metal and MgO rod. The Mg content was higher for the reaction between molten steel and MgO in slag compared to the reaction between molten steel and MgO rods, as it equilibrated with MgO activity in slag.
  • Cold Model Study of Submerged Peripheral Gas Bubbling from a Cylindrical Dispenser

    pp. 2239-2247

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    DOI:10.2355/isijinternational.54.2239

    Modeling of the emerging gas flow through the cylindrical dispenser was important and critical where magnesium vapors were generated through chemical reaction between MgO and Al at 1673 K inside the dispenser and forced out through its perforations along with a carrier gas (Ar) into the liquid iron bath for desulphurization of hot metal. At lower part of the dispenser and 5 mm above from bottom, six equidistant perforations each of 1 mm diameter were drilled along the axis of the dispenser. Cold model study was carried out to assess the flow rate of carrier gas, dispenser clearance and bath height on the rate of ascent of gas bubble through the bath influence and mixing time, when gas emerges from the periphery of a cylindrical dispenser immersed into a liquid bath. Water was used for representing the liquid bath. The diameter of the gas bubbles which were released through the perforations of immerged dispenser increased with increase in gas flow rate and was not significantly influenced by dispenser clearance. The minimum gas flow rate required to escape the gas bubble from dispenser decreased with clearance. The mixing time in the bath due to the agitation caused by the bubbles was found to be a function of the gas flow rate, clearance and the bath height. The rate of ascent and the mixing time could be modeled successfully with mathematical formulations. The data were used for predicting the behavior of gas bubbles in 5 ton liquid iron bath.
  • Desulfurization of CaO–Al2O3–SiO2–TiO2 Slag System

    pp. 2248-2254

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    DOI:10.2355/isijinternational.54.2248

    The desulfurization ability of CaO–Al2O3–SiO2–TiO2 slag at 1873 K was studied through the slag-metal desulfurization experiment by using a 10 kg inductive furnace and through theoretical calculation by using thermodynamic software FactSage. The experimental index considered were the sulfur distribution coefficient of the slag (LS) and the desulphurization ratio (ηS). The effect factors were binary basicity, the mass percentage of Al2O3 and TiO2. Research results are as follows. The effect of binary basicity is significant, compared with the mass percentage factors of Al2O3 and TiO2. Under the optimal experimental conditions, with the binary basicity is 7, the mass percentage of Al2O3 is 30% and TiO2 is 3%, the corresponding highest sulfur distribution coefficient for the slag are 58.14. While the content of TiO2 in the CaO–Al2O3–SiO2–TiO2 slag is in a range lower than 3–4%, the desulfurization ability of the CaO–Al2O3–SiO2–TiO2 slag is equal to that of the CaO–Al2O3–SiO2 slag. The slag system with TiO2 can meet the desulfurization requirements of steelmaking, which is a good way for resource utilization of solid waster containing titanium.
  • Optimization of Combined Blown Converter Process

    pp. 2255-2262

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    DOI:10.2355/isijinternational.54.2255

    A 1/6th scaled down physical model was used to study and optimize the stirring condition of a 30 t converter. A number of parameters were studied and their effects on the mixing time were recorded. A new bottom tuyere scheme with an asymmetrical configuration was found to be one of the best cases with respect to a decreased mixing time in the bath. Mathematical modeling was employed to study the flow field characteristics caused by the new tuyere scheme. In the mathematical model, a comparison between the existing and the new tuyere setups was made with regards to the mixing time and turbulence in the bath. In addition, a new volumetric method for calculating the mixing time was applied. The results showed that, on average, a 23.1% longer mixing time resulted from the volumetric method compared to the standard method where discrete point are used to track the mixing time. Furthermore, an industrial investigation was performed to check the effects of the new tuyere scheme in a converter by analyzing the [O], [C] and [P] contents in the bath. The results showed that the application effects of the new tuyere scheme yield a better stirring condition in the bath compared to the original case.
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  • A Novel Approach for Numerical Modeling of the CAS-OB Process: Process Model for the Heat-Up Stage

    pp. 2263-2272

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    DOI:10.2355/isijinternational.54.2263

    The objective of this work was to develop a process model for the CAS-OB (Composition Adjustment by Sealed argon bubbling-Oxygen Blowing). The CAS-OB is designed to homogenize and control the steel composition and temperature before the casting. In the heating mode (OB) studied here, a refractory bell is lowered and submerged 30 cm below the liquid steel surface of the ladle and under this well-defined sealed volume, oxygen gas is injected to oxidize solid aluminum particles that are fed and molten at the surface. Under consideration were the melting of the solid aluminum particles, the oxidation of pure molten aluminum, and the oxidation of dissolved species, in this case Al, Mn, C and Si, and the solvent Fe. We also considered the formation and oxidation of steel droplets formed in the blowing when they pass through and react with the surface slag and also the reaction of pure aluminum on the top of the slag layer. Based on our simulations, only 30–40% of the chemical energy can be used to heat up the steel. A fraction of 0.8–0.85 of the O2 can be utilized in the process; these values correspond to those obtained in previous work. The main part of the heating energy comes from the oxidation of the fed Al. FeO is primarily an intermediate product of the reactions. The model was tested against industrial trials for steel temperature and compositions of slag and steel, and it succeeded in capturing correct trends and absolute accuracy within the analyzing accuracy.
  • Magnetohydrodynamic Phenomena, Fluid Control and Computational Modeling in the Continuous Casting of Billet and Bloom

    pp. 2273-2282

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    DOI:10.2355/isijinternational.54.2273

    Magnetohydrodynamic flow phenomena with electromagnetic stirring were studied in billet casting. Solidification microstructure and oscillation marks were also investigated. EMS with 310 A current was proven to increase central equiaxed zone from 10–15% in unstirred billet to over 45% in stirred billet for steel grade 40CrMo4. Dendritic microstructures at the billet corners were found to deflect downwards due to the sustained upward flow along the mould corners. Rhomboidity was observed to relate to the unsteady flow in the mould during changes of casting speed and EMS intensities. Computational modeling was performed and validated with the results obtained from the casting experiments. Through modeling, a declining phenomenon of the injected stream from the SEN was observed towards the back wall under certain stirring intensities. This would influence thermal distribution of the strand shell and casting qualities, thus requiring correct matching of the vertical shear flow field with the horizontal rotation flow in order to obtain a favorable flow field. In addition, vortexes were found on the free surface around the SEN tube, which were influential to the cleanliness of steel melt. Modeling was also performed for a bloom caster with various SEN designs in the context of cleanliness. It was found that ported SENs showed obvious advantage on inclusion removal over the straight bore SEN. Reducing casting speed is actually an effective way to improve cleanliness. Casting trials with 65 bloom casting heats verified the modeling results and remarkable improvement in cleanliness was observed by using a 2-port SEN.
  • Application of a Corner Chamfer to Steel Billets to Reduce Risk of Internal Cracking during Casting with Soft Reduction

    pp. 2283-2287

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    DOI:10.2355/isijinternational.54.2283

    In order to get the brittle temperature range for internal crack, the solidification phase transformation model is established and LIT and ZDT are calculated. Then the fully coupled thermo-mechanical finite element models are developed using the commercial software ABAQUS to investigate stress and strain states in the brittle temperature range of the chamfer billets during soft reduction. The relation between chamfer angle and maximal principal stress, as well as equivalent plastic strain is analyzed, and the influence of chamfer angle of billet on the internal crack is studied. The results indicate that the maximal principal stress and the tensile stress decrease with the increase of chamfer angle. Reducing the soft reduction can lower the stress and strain in the brittle temperature range. It can be obtained that the larger reduction effect and lesser internal crack risk by adjusting the chamfer angle of the billet.
  • Effect of Cooling Rate and Titanium Additions on Microstructure of Thin-Walled Compacted Graphite Iron Castings

    pp. 2288-2293

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    DOI:10.2355/isijinternational.54.2288

    This article addresses the effect of cooling rate and of titanium additions on the exhibited microstructure of thin-walled compacted graphite iron (TWCI) castings as determined by changing molding media, section size and Ferro Titanium. The research work was carried out on TWCI castings and reference castings of 2–5 mm and 13 mm wall thickness, respectively. Various molding materials were employed (silica sand and insulating sand ‘‘LDASC’’) to achieve different cooling rates. Thermal analysis was implemented for determinations of the actual cooling rates at the onset of solidification. This study shows that the cooling rates exhibited in the TWCI castings varies widely (70–14°C/s) when the wall thickness is changed from 2 to 5 mm. In turn, this is accompanied by a significant variation in the compacted graphite fraction. The resultant cooling rates were effectively reduced by applying an insulating sand in order to obtain the desired graphite compactness. In addition, good agreement was found between the theoretical predictions of the solidification process and the experimental outcome. Ti additions in combination with LDASC sand molds were highly effective in promoting the development of over 80% compacted graphite in castings with wall thicknesses of 2 and 3 mm as evidenced by quantitative metallographic analyses.
  • Effect of Sub-critical Tempering and Deep Cryogenic Treatment on Slurry Erosion of Cr–Mn–Cu White Cast Irons

    pp. 2294-2301

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    DOI:10.2355/isijinternational.54.2294

    The effect of deep cryogenic treatment and air cooling subsequent to sub-critical tempering treatment on the structural evolution and the resulting wear performance of Cr–Mn–Cu white cast iron in sand-water slurry media were studied. The as-cast matrix of Cr–Mn–Cu white cast iron is predominantly austenitic which can be transformed to a martensitic matrix embedded with fine M7C3 carbides, by a sub-critical tempering treatment followed by air cooling. But the retained austenite is not totally converted. Quenching of alloy in liquid nitrogen bath after subcritical tempering enables more amount of transformation of austenite with simultaneous homogeneous precipitation of fine carbides. The phase transformation and corrosion-erosion behavior were characterized by optical and scanning electron microscopy, bulk and micro-hardness measurement and x-ray diffraction analysis, measurement of corrosion rate and slurry erosion test. The slurry erosive wear performance has been found to be much better in the cryotreated alloys compared to those with conventional sub-critically tempered alloys.
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  • Grain Refinement and Delta Ferrite Reduction of High Cr Steel Ingots by Thermal Control

    pp. 2302-2308

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    DOI:10.2355/isijinternational.54.2302

    High Cr steel (X12CrMoWVNbN10-1-1) ingots were prepared by thermal control by varying three parameters: the melt pouring temperature, the mold temperature and the melt superheating temperature. The effects of these parameters on the macro-/micro-structure of the ingots were investigated through a group of orthogonal experiments. The results showed that superheating temperature was the dominating parameter that affects the grain size and fraction of delta ferrite. The optimized parameters for the sound ingot were 1893 K for superheating temperature, 1073 K for mold temperature and 1873 K for pouring temperature. Under that condition, the ingot was composed of fine equiaxed grain with a radius of 1.1 mm, and the fraction of the delta ferrite was less than 2%. Mechanisms that explain the increment in the proportion of the equiaxed fraction and the grain refinement were proposed, and the mechanisms for the delta ferrite reduction were explained.
  • Fracture Mechanism of Ferritic Ductile Iron under Instrumented Impact Load at Low Temperatures

    pp. 2309-2313

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    DOI:10.2355/isijinternational.54.2309

    The impact toughness of 400-18L ductile iron (FCD400-18L) was measured by V-notch Charpy impact test at room and low temperatures. SEM and TEM were used to analyze the fracture mechanism. Experimental results show that the energy absorbed by impact test decreases significantly with the decrease of temperature. The fracture properties were measured using instrumented impact testing at temperatures between –80 and 20°C. The total impact fracture energy, the crack initiation and propagation energy, the dynamic loads and the ductile to brittle temperature were measured. The 400-18L ductile iron exhibited much higher resistance to ductile fracture at high test temperatures, while its resistance to brittle fracture at low test temperatures was reduced. The fracture morphology analysis was discussed in terms of the two fracture types: ductile fracture mechanism upon the BDTT (brittle to ductile transition temperature) and brittle fracture mechanism below the BDTT.
  • Measurements of Molten Steel Surface Velocity and Effect of Stopper-rod Movement on Transient Multiphase Fluid Flow in Continuous Casting

    pp. 2314-2323

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    DOI:10.2355/isijinternational.54.2314

    Surface velocity of the molten steel in the mold is critical to final product quality during continuous casting of steel, and is one of the few flow parameters that can be measured in the plant to validate fluid flow models. Surface velocity was measured using two different sensors: Sub-meniscus Velocity Control (SVC) devices and nail dipping, to evaluate their performance, and to quantify surface velocities in a commercial steel caster under different casting speeds, argon gas fractions, and mold widths. A correlation between the height difference of the solidified lump on the nail and surface velocity is confirmed and extended. Reasonable agreement between the two sensing methods was obtained, both in trends and magnitudes for both time-averaged velocity and transient flows. Transient CFD models are applied to simulate multiphase flow of steel and gas bubbles in the Submerged Entry Nozzle (SEN) and mold and are validated with nail dipping measurements. To obtain the transient inlet boundary conditions for the simulation, two semi-empirical models, a stopper-position-based model and a metal-level-based model, predict the liquid steel flow rate through the SEN based on recorded plant data. The model system was applied to study the effects of stopper rod movements on transient flow in the SEN and mold. Mold level fluctuations were calculated using a simple pressure method and compared with plant measurements. The results show that severe stopper rod movements cause significant disturbances of the meniscus level, which may cause slag entrapment, leading to sliver defects in the final product.
  • Analysis of Transient Transport and Entrapment of Particle in Continuous Casting Mold

    pp. 2324-2333

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    DOI:10.2355/isijinternational.54.2324

    A mathematical model has been developed to simulate the transient turbulent flow, solidification, transport and entrapment of particles in a continuous casting mold. The turbulence of molten steel inside the liquid pool is calculated using the large eddy simulation (LES). The enthalpy-porosity approach is used to simulate the solidification of steel. A Laplacian equation is used to approximate the pull velocity in the solid region. The transport of particle (bubble and non-metallic inclusion) inside the liquid pool is considered according to the Lagrangian approach. A criterion for particle entrapment in the solidified front is developed using the user-defined functions (UDF) of FLUENT software, considering force balance involving eight different forces acting on a particle at the mushy-zone. The predicted asymmetrical flow pattern is compared with the water model experiment results. The entrapped positions of particles in the solidified shell are predicted and compared with the ultrasonic flaw results of rolled steel plates and the previous study of inclusions distribution in slabs. The model has successfully reproduced many known phenomena and other new predictions, including the transient asymmetrical flow pattern, transient asymmetrical distribution of particles inside the liquid pool, transient entrapment ratios and asymmetrical positions of different particles in the solidified shell.
  • Fault Diagnosis for Blast Furnace Ironmaking Process Based on Two-stage Principal Component Analysis

    pp. 2334-2341

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    DOI:10.2355/isijinternational.54.2334

    Monitoring an ironmaking process is a very challenging task as it often fluctuates frequently and lacks of direct measurements. Principal component analysis (PCA) technique has been widely used in various industrial fields, mainly due to its advantage of not requiring the information about the principle knowledge of the process and faults. However, the PCA based application results in ironmaking process are still limited. In this paper, based on the dataset collected from a real blast furnace with a volume of 2000 m3, a fault diagnosis method by incorporating the PCA technique in two stages will be presented. To overcome the adverse effects of the peak-like disturbances caused by switching between two distinct hot-blast stoves, they are identified and removed from the dataset through the first-stage PCA. Experimental results show that our method outperforms the existing algorithm and the operators’ monitoring in detecting the getting cold accident of the blast furnace.
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    3. Effect of Preheated Top Gas and Air on Blast Furnace Top Gas Combustion ISIJ International Vol.54(2014), No.1
  • Effect of Specimen Shape on Fatigue Behavior in Thin Pure Copper Sheet for Smart Stress-memory Patch

    pp. 2342-2348

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    DOI:10.2355/isijinternational.54.2342

    Smart stress-memory patch is a promising method for monitoring long-term fatigue damage in structure. The patch can estimate the number of cycles and the stress amplitude using fatigue crack growth properties of thin metal sheets. In this study, fatigue crack growth behavior of thin pure copper sheet was investigated by changing the specimen shape under strain-controlled testing in order to improve measuring range of smart stress-memory patch. To characterize the initiation as well as the stable growth of fatigue cracks, the relationship between stress intensity factor range and crack growth rate was successfully fitted to one equation regardless of strain amplitude, strain ratio and specimen shape. Based on the experimental results, an equation for estimating fatigue cycles from fatigue crack length was derived. In addition, the detectable range of stress amplitude was evaluated and its dependence on sensor shape and stress ratio was shown. Since this patch needs neither power supply nor wiring, it provides a great potential for long-term structural health monitoring with easy maintenance.
  • Effect of Friction on Tandem Cold Rolling Mills Chattering

    pp. 2349-2356

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    DOI:10.2355/isijinternational.54.2349

    Chatter phenomenon in the rolling mills is the undesired mechanical vibrations which reduces the productivity and quality of the products. Many studies have led to the point that friction is one of the most effective factors incurring chatter. In this paper, a chatter simulation model for the tandem cold rolling mills based on the well-known Coulomb and the Tresca friction models is proposed. By considering the effects of the material strain-hardening and the elastic deformation of the work roll, the model relaxes certain assumptions. The simulation results are verified by comparing to experimental data obtained from Mobarakeh Steel Company. In addition, a numerical method is developed to introduce a relationship between the Tresca friction factor and the Coulomb friction coefficient. A parametric study on the effects of friction coefficients of the two stand rolling mill on the chatter critical speed is conducted. The results show that an optimal setting of friction coefficients improves the process efficiency. Under the operating conditions stated in this paper, the friction coefficient of the second stand should be increased as much as possible in order to get closer to its optimum point. The optimum friction coefficient of the first stand depends on the friction coefficient of the second stand.
  • Universal Rolling Deformation of Asymmetric Unequal-leg Angles

    pp. 2357-2363

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    DOI:10.2355/isijinternational.54.2357

    Universal rolling of asymmetric unequal-leg angles having an L-section was studied by a model rolling experiment and finite element (FE) analysis. In the experiment, flange spread displayed a linear relationship against the reduction balance, which was defined as the difference of the flange and web thickness strains. Similar linear behaviors of web height, flange depth and bulge height against the reduction balance were also observed. Rolling deformation was also investigated in detail by FE simulation. Although the trends of deformation in the simulation were similar to the experimental results, two significant differences were noted: The inclination of the regression lines of deformation (flange spread parameter) was slightly different, and the change of flange depth was not linear in the high reduction balance region. Furthermore, unexpected web spread was observed in the experiment and FE analysis.
    FE simulation of universal rolling of channels (U-section), in which the stock had the same half-section as the section of the unequal-leg angle (L-section), was carried out to investigate the similarities in the deformation behaviors of these two products. From a comparison of the upstream section in the two rolling simulations, unstable stock rotation at the entry of the rolls was discovered in unequal-leg angle rolling. The rotation angle was influenced by the reduction balance, and this rotation was considered to be the cause of the distinctive deformation observed in unequal-leg angle rolling. Finally, the importance of restricting rotation was examined with the aim of improving the stability of rolling deformation.
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  • Surface Oxide Formation during Rapid Heating of Zn-coated Press Hardening Steel

    pp. 2364-2368

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    DOI:10.2355/isijinternational.54.2364

    During the conventional die-quenching processing of a galvanized PHS steel, a thick ZnO layer is formed at the surface. When the heating rate is increased, the oxide at the surface is a thin Al2O3 layer. This remarkable change in surface oxide during rapid heating is due to the partial melting of the coating instead of the solidification of the coating and the formation of Fe–Zn intermetallics. In the present study, the characterization of the surface oxide formation at different heating rates is presented.
  • Resistance of Hydrogen Embrittlement on Hot-sheared Surface during Die-quench Process

    pp. 2369-2374

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    DOI:10.2355/isijinternational.54.2369

    This study investigated the resistance of hydrogen embrittlement on a hot-sheared and quenched surface of 22MnB5 steel sheets. The specimens were sheared at 750°C and 650°C after austenitization, and then quenched by water cooling. Additionally, these specimens were cathodically hydrogenized for 48 h to accelerate cracking by hydrogen embrittlement. This sequence resulted in a residual tensile stress of over 1 GPa on the hot-sheared surface and a diffusible hydrogen density of about 1.5 ppm. Despite these severe conditions, cracking by hydrogen embrittlement did not arise. The state of the microstructure in the vicinity of the sheared surface might cause this high resistance against cracking. Indeed, sub-micron grained ferrite or deformed uncertain soft and hard phases, which might be more ductile than martensite, were observed around the sheared surface.
  • Effects of Welding Schedules on Resistance Spot Welding of DP600 Steel

    pp. 2375-2379

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    DOI:10.2355/isijinternational.54.2375

    In this study, experiments were made to investigate effects of electrode force, welding current and welding time on weld quality of resistance spot welded DP600 steel. Microstructure, microhardness and mechanical behavior of welded joints were analyzed. Martensite formation was observed in the fusion zone. Heat affected zone softening caused by martensite tempering could be detected. Increasing welding parameters would promote pullout failure occurrence and heat affected zone softening. However, the associated welding defects would have negative effects on weld quality. Welding current influence on weld quality was the most significant among the three selected welding parameters. In addition, penetration rate could probably be used to judge the weld quality.
  • Effect of Silicon in Dual Phase Steel on the Alloy Reaction in Continuous Hot-dip Galvanizing and Galvannealing

    pp. 2380-2384

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    DOI:10.2355/isijinternational.54.2380

    The mechanical properties of steels can be significantly improved by alloying with Si. However, Si in dual phase steel retarded the alloy reaction in hot-dip galvanizing and galvannealing; specifically, a continuous Fe–Zn alloy layer was absent on the steel containing 0.44 wt% Si after 25 s of galvannealing at 500°C. This retardation may be due to the Si-rich molten Zn layer, in which the Si resulted from the aluminothermic reduction of Si oxide and the dissolution from the steel substrate.
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  • Onset of Secondary Recrystallization in High Purity 3.3%Si Steel

    pp. 2385-2393

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    DOI:10.2355/isijinternational.54.2385

    Possibility of the occurrence of secondary recrystallization in high purity material without containing inhibitor elements was investigated experimentally. Only in the sample with primary recrystallization annealing performed at 900°C, onset of secondary recrystallization was observed. The orientation of secondary recrystallized grains proved to be (110)[001] (Goss) orientation. It was also shown that normal grain growth was suppressed in the sample in which secondary recrystallization occurred. Narrow grain size distribution and strong texture accumulation were considered to help the stabilization of the matrix grains.
    Texture development during secondary recrystallization in the absence of inhibitors was discussed based on proposed growth models. It was shown from Grain Boundary Character Distribution (GBCD) analysis that the High Energy (HE) boundary model was applicable to secondary recrystallization and the Solid State Wetting (SSW) model was applicable to normal grain growth. The HE boundary was related theoretically to high mobility through high grain boundary diffusion coefficient. It was considered that reducing impurity elements exerted inherent high mobility of the HE boundary and hence led to the onset of the secondary recrystallization.
  • Martensite Nucleation at Grain Boundaries Containing Intrinsic Grain Boundary Dislocations

    pp. 2394-2403

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    DOI:10.2355/isijinternational.54.2394

    It is shown that grain boundaries containing intrinsic grain boundary dislocations act as preferential nucleation site for the martensitic transformation. The microstructure and crystallography of BCC α’-martensite formed in a sensitized AISI 304 stainless steel was studied in detail by means of convergent beam Kikuchi pattern analysis. The orientation relationship between martensite and austenite was determined to be Kurdjumov-Sachs type. The presence of intrinsic grain boundary dislocations was observed at a grain boundary 10.83° from the exact Σ11 CSL boundary orientation, where a martensite nucleus formed by the faulting and extension of intrinsic grain boundary dislocations. The selection of the crystallographic variant for the martensite nucleus was not related to a reduction in interfacial energy at nucleation, nor was it related to the accommodation of transformation strain by easy slip in the austenite.
  • Impact of Vanadium Addition on API X100 Steel

    pp. 2404-2410

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    DOI:10.2355/isijinternational.54.2404

    Laboratory heats of an API X100 steel (Mn–Mo–Nb) with two levels of vanadium (0.004% and 0.063%) were melted and control rolled simulating a coiled plate production process. Extensive microstructural analyses were performed, including optical, EBSD and TEM. Texture analyses of the two alloys were also compared. Mechanical properties were determined for various rolling and pipe axis orientations. Only minor differences in microstructure and texture properties were observed between the two alloys. Yet the 0.063% V alloy had from 8 to 14% higher yield and tensile strengths in all directions, while the toughness and ductility measurements were similar for both alloys. Higher strength of the V added pipeline steel was partially due to its smaller grain size and larger fraction of subgrain boundaries. Some, but perhaps not all, of the strength differences could be related to the observed smaller precipitate size of the higher vanadium steel. The vanadium addition was necessary in this alloy to ensure meeting the required strength properties of X100 steel.
  • Digital Image Correlation Measurement of Localized Deformation in Carbon Steel in the Presence of Hydrogen

    pp. 2411-2415

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    DOI:10.2355/isijinternational.54.2411

    Although there have been many reports related to localized plastic deformation in the presence of hydrogen, this process has not been clearly elucidated to date. In our previous report, we conducted tensile tests on hydrogen-charged carbon steel S25C. Using the digital image correlation method, we were able to measure the time evolution of the equivalent strain distribution and the equivalent strain rate distribution up to the crack initiation around the notch bottom. We then revealed that the hydrogen induced an increase in the equivalent strain rate during the deformation process and a decrease in the equivalent strain when the time the crack started to grow. In this study, we performed similar measurements on carbon steels S15C and S55C and evaluated how the deformation progressed until the crack started to grow depending on the carbon content. The results showed a decrease in the local equivalent strain when the time the crack started to grow in all of the investigated materials, but we could not identify any clear trend with respect to a carbon-content dependence. However, we observed that smaller carbon contents (i.e., higher ferrite volume fractions) resulted in greater significance of the localization of the deformation due to the hydrogen and in a greater rate of increase in the local strain rate. In addition, on the basis of the equivalent strain distribution, we revealed that the deformation tended to localize in the ferrite phase.
  • Flow Curve Modelling of an Mg-PSZ Reinforced TRIP-matrix-composite

    pp. 2416-2420

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    DOI:10.2355/isijinternational.54.2416

    A prediction model for cold flow curves was introduced for a new class of composite materials known as TRIP-matrix-composites consisting of three phases, austenite, strain-induced martensite and ZrO2. The content of the ceramic phase was varied between 0 and 30%, whereas the particle size of the ceramic was selected to be 10 to 30 μm. For the manufacturing of the composite material the powder metallurgical route including hot press procedure was chosen and very dense material could be produced.
    Included in the model is the hydrostatic stress σm close to the circumferential surface of the compression test sample. The hydrostatic stress was varied using different material compositions and true strain values. To calculate the cold flow curves the ISO-E-method was applied. The calculated results show a consistent congruency with the experimental data.
  • One-Dimensional Glide Motion of “Naked” Nanoscale 1/2<111> Prismatic Dislocation Loops in Iron

    pp. 2421-2424

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    DOI:10.2355/isijinternational.54.2421

    One-dimensional (1D) glide diffusion of interstitial-type prismatic dislocation loops is believed to play key roles in microstructural evolutions within nuclear-fusion and fission materials upon energetic particle irradiation. In the present study, using in-situ transmission electron microscopy, we have examined behaviors of nanoscale 1/2<111> loops in high-purity α-iron at “low temperatures” ranging from 15 to 290 K, under zero external stress. We observed 1D intermittent and fast motion of loops above approximately 100 K, which was significantly different from the 1D continuous and viscous motion observed at “high temperatures” above approximately 450 K. The estimated lower-limit values of diffusivities for “low temperatures” were considerably higher than the values obtained through the extrapolation of the temperature dependence of the diffusivities at “high temperatures.” These results support the idea that strong obstacles for loop motion are formed by heating of the specimens to “high temperatures” after the introduction of loops.
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