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

ISIJ International Vol. 52 (2012), No. 10

  • Recent Progress on Long Service Life Design of Chinese Blast Furnace Hearth

    pp. 1713-1723

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    DOI:10.2355/isijinternational.52.1713

    In recent years, Chinese steel companies suffered a spate of blast furnace hearth incidents, such as hearth sidewall breakout or temperature abnormal increase, causing Chinese blast furnace ironmakers to pay a great deal attention on long hearth service life technology. In this paper, the progress of Chinese blast furnace long hearth service life design concept was analyzed deeply from design of hearth structure, selection of hearth refractory, arrangement of hearth cooling system and establishment of online hearth monitoring system, and the future design proposal for long hearth service life of Chinese blast furnace was put forward.
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    2. Blast Furnace Dynamics Using Multiple Autoregressive Models with Exogenous Inputs ISIJ International Vol.52(2012), No.10
    3. Diffusion Behaviors of He and CH4 in Air Flow through Packed Bed ISIJ International Vol.52(2012), No.8
  • Ring Formation in the Smelting of Saprolite Ni-ore in a Rotary Kiln for Production of Ferro-nickel Alloy: Mechanism

    pp. 1724-1729

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    DOI:10.2355/isijinternational.52.1724

    The clarification of mechanism of SR (Slag-Ring) formation in the smelting of saprolite Ni-ore by the rotary kiln has been successfully attempted. The walls in the kiln have a higher temperature than the raw materials, in particular, the point where the raw materials enter the bed have the highest temperature. In that point, the released low-MgO and high-SiO2·FeO silicate accompanied by the recrystallization of non-stoichiometric serpentine are transformed to the primary melt, the fine parts of ores with much FeO and Al2O3 and limestone being melted by Ostwald ripening, which allows the formation of the secondary melt. Subsequently, its entrance into the bed leads to the cooling and precipitation on the wall and accretions, which allows the increase in the sticking force. The slight transportation of raw materials from the SR sticking zone toward the discharge end leads to the rapid decrease in the sticking. This is explained from the fact that the large growth of the particles by Ostwald ripening causes the reduction of melting amount into the melt, allowing the small precipitation on the wall and accretions. The resulting SR is locally formed. The fine parts of low-MgO and high-FeO type ore have higher concentration of point defects than that of the high-MgO and low-FeO type ore. This provides the lower melting point and higher diffusion rate to the former than the latter, which allows the former to have higher possibility of SR formation than the latter.
  • Numerical Analysis of Surface Tension Gradient Effect on the Behavior of Gas Bubbles at the Solid/Liquid Interface of Steel

    pp. 1730-1739

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    DOI:10.2355/isijinternational.52.1730

    The mechanism which governs the behavior of bubbles in the vicinity of the solidification front where both the concentration and temperature gradients exist was investigated and the effect of the solutal and thermal marangoni forces to the entrapment of bubble were compared and discussed.
    When a bubble approaches the solid/liquid interface, it first encounters the thermal boundary layer, which is much thicker than the concentration boundary layer, and experiences the thermal marangoni force. This force, which occurs due to the temperature gradient in the thermal boundary layer, pushes the bubble away from the solidification front if the sulfur concentration is lower than the critical value which was found to be around 47–60 ppm, but pulls it toward the solidification front if the sulfur concentration is higher than the critical value. Only if the bubble passes through the thermal boundary layer successfully, it then arrives at the concentration boundary layer where a strong solutal marangoni force comes into action on the bubble to pull it to the solid/liquid interface.
    The above analysis predicts that the bubble-induced surface defect of a cast be strongly affected by the sulfur concentration and there exists a critical sulfur concentration above which the surface defect becomes increasing. The above prediction was validated by the CFD (Computational Fluid Dynamics) simulation and the trial at the commercial casting plant, and they were in good agreement with each other.
  • Relaxation Phenomena of Ultrasonic Waves on Molten Alkali Silicates

    pp. 1740-1749

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    DOI:10.2355/isijinternational.52.1740

    The temperature and the frequency dependencies of velocities and absorption coefficients of ultrasonic waves have been measured on molten alkali silicates (33(mol%)M2O–67SiO2 (M = Na and K)) using the pulse transmission technique. The velocities and the absorption coefficients have been determined by the Fourier transform of ultrasonic pulses. It has been found that the velocities, c, increases with decreasing temperature, while the absorption per wavelength, αλ, increases and then decreases with decreasing temperature. It has also been found that the c and αλ values are identical irrespective of frequencies above ca. 1400 K for 33Na2O·67SiO2 and ca. 1500 K for 33K2O·67SiO2, respectively, while dispersion, i.e., frequency dependency, appears below these temperatures. The dispersions of acoustic properties have been successfully interpreted using a structural relaxation model.
  • Hot Airflow Ignition with Microwave Heating for Iron Ore Sintering

    pp. 1750-1756

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    DOI:10.2355/isijinternational.52.1750

    At present coal gas with high calorific value is used in ignition (CGI) of iron ore sintering, which brings about problems of high energy consumption and environmental pollution. In this paper, the ignition model using microwave heating (MHI) was developed. Took raw materials of a domestic sintering plant as sample, model calculations showed that the minimum ignition temperature was 638.54°C and the energy consumption was 42.03 MJ/m2 for MHI process in ignition time of 1.5 min. According to the sintering experiments, with ignition at 660°C for 1.5 min the main indexes of sinter qualities and production in MHI were better than those of CGI (1050°C) process. And the ignition energy consumption (IEC) in MHI was 43.77 MJ/m2, which was far less than 185.13 MJ/m2 of CGI. Results also show other advantages of sufficient combustion of coke, better mineralized sinter and less polluted flue gas in MHI process.
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    3. Reducing Net CO2 Emissions Using Charcoal as a Blast Furnace Tuyere Injectant ISIJ International Vol.52(2012), No.8
  • Beneficiation of High Phosphorus Limonite Ore by Sodium-carbonate-added Carbothermic Reduction

    pp. 1757-1763

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    DOI:10.2355/isijinternational.52.1757

    The characteristics of Huimin high phosphorus limonite ore and the beneficiation of this iron ore by sodium-carbonate-added carbothermic reduction, ultrafine grinding and magnetic separation were investigated. Iron particle size in reduced ore without Na2CO3 additive is tiny and the fayalite is abundant. It is indicated that the formation of fayalite is the main hindrance to accelerate the reduction of limonite. With a mass ratio of Na2O3 to ore of 10% additive, the reduction of limonite can be reinforced. The reinforcing affect may be caused by the increase of the reducing reaction activity of FeO and the acceleration of the carbon gasification reaction rate. Fluorapatite were not reduced in the low temperature reduction process and entered to gangue phases, after ultrafine grinding-magnetic separation process, a qualified iron concentrate with 76.47% Fe, a recovery of 73.20% is obtained with simultaneous decrease in the phosphorus content down to 0.25%.
  • Blast Furnace Dynamics Using Multiple Autoregressive Models with Exogenous Inputs

    pp. 1764-1771

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    DOI:10.2355/isijinternational.52.1764

    Autoregressive models with exogenous inputs are useful tools for analyzing systems with unknown dynamics, but are limited by the assumption that the relations between inputs and output(s) are linear. For complex systems with nonlinear or abruptly changing dynamics it is possible to modify the technique by allowing for multiple local models and designing a strategy for switching between them. A method by which this can be realized is developed in the paper. The technique is applied on a complex problem in the metallurgical industry, i.e., the prediction of hot metal silicon content in the blast furnace. A set of local models is developed for different parts of a training set, using a statistical criterion for model selection. The resulting local models are then applied to predict future values of the silicon content. It is demonstrated that the method is capable to develop models, among which a proper choice can be made for prediction. The potential of multi-step predictions is also studied. Finally, some conclusions concerning the method and the results are drawn.
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    2. Hot Airflow Ignition with Microwave Heating for Iron Ore Sintering ISIJ International Vol.52(2012), No.10
    3. Reducing Net CO2 Emissions Using Charcoal as a Blast Furnace Tuyere Injectant ISIJ International Vol.52(2012), No.8
  • Innovative Methodology for Separating of Rare Earth and Iron from Bayan Obo Complex Iron Ore

    pp. 1772-1777

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    DOI:10.2355/isijinternational.52.1772

    A new process was proposed in this research, in order to address the problems of difficult treatment, low efficiency and heavy pollution of Bayan Obo complex iron ore. The isothermal reduction experiments, using carbon-bearing pellets which were mainly made of Bayan Obo complex iron ore and pulverized coal, were investigated in the temperature range of 1623–1723 K with different heating time. The results indicate that the pellets could not melt well at 1623 K and 1723 K, and the iron nugget and slag can separate in a clear manner at 1673 K for 12 min. The contents of C and S in iron nugget are 2.09% and 1.62% respectively. The iron nugget can be used partly to substitute the steel scrap for EAF steelmaking. The RE2O3 content is 14.19% in the rare-earth-rich slag. Nearly all rare earth is concentrated into one phase during solidification, which is identified as cefluosil ([7(Ca, Ce, La, Nd)2·SiO4] (F, O)10). The slag was leached by hydrochloric acid and the leaching efficiency of rare earth is 98.70%. After being filtered, the solution can be used to extract rare earth and the leached residue will be treated to recover CaF2 and ThO2.
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  • Gasification and Reduction Behavior of Iron Ore-Carbon Composite under High Pressure

    pp. 1778-1784

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    DOI:10.2355/isijinternational.52.1778

    Iron oxide-carbon composites have a potential to play a significant role in mitigating CO2 emission from the ironmaking process. However, efficiency of gasification and reduction of iron ore-carbon composite seem to be limited under the condition of blast furnace. In this study, effect of pressure on the gasification and reduction of the composite was evaluated. Hematite reagent and ore powders were mixed with graphite powder to make composite samples. They were heated up to different target temperatures at a heating rate of 0.167°C/s under different pressures. Weight loss fraction was obtained by the difference between sample masses before and after reduction. Gasification temperature decreased and weight loss fraction at the target temperature increased with an increase in pressure. Further, molar ratio of CO to (CO+CO2) in the outlet gas decreased with increasing pressure, after it reached the coexisting condition of wustite and metallic iron phases. These results show that an increase in pressure leads to increase in the reaction rate of indirect reduction. Further, this can be attributed to an increase in the partial pressure of CO gas and the prevention of gasification reaction.
  • Simulation of Sintering Process -Effects of Air Flow, Liquid Film Cohesion Force and Fixation Process on Large Scale Crack-

    pp. 1785-1793

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    DOI:10.2355/isijinternational.52.1785

    The formation and the growth of void (crack) in the sintering bed have not been explored. In this study the motions of particles and the air in the nearly actual scale sintering bed were simulated to elucidate the void formation and the growth mechanisms to large scale crack by the simultaneous calculation of Navier-Stokes equations and the Lagrangian DEM equations based on the simple sintering model in which the phase change of particles, the cohesion force due to the liquid film between particles and the fixation process above the melting zone were considered. The air flow among particles facilitated to grow the crack and finally to produce the large scale crack. The cohesion force by the liquid film caused the agglomeration among particles and grew the voids in the melting zone. In the fixation zone the large cohesive force which was 10 or 30 times larger than that in the melting zone in this study advanced the agglomeration and grew the void. Therefore the cohesion force between particles mainly affects the occurrence of the large scale crack. The decrease of mobility of particle motion by the fixation process in the fixation zone generated the locally large contact force which was about 250 times larger than the usual cohesion force between particles in the agglomerate and the large velocity difference between agglomerates. They broke down the agglomerate particle. Through the fixation zone the cracks (voids) further grew and merged to a large scale crack.
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    3. Diffusion Behaviors of He and CH4 in Air Flow through Packed Bed ISIJ International Vol.52(2012), No.8
  • Heat and Acid Leach Treatments to Lower Phosphorus Levels in Goethitic Iron Ores

    pp. 1794-1800

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    DOI:10.2355/isijinternational.52.1794

    Phosphorus associated with goethite in high-phosphorus (>0.10 mass% P) iron ores was lowered to below 0.075 mass% P with a heat treatment at 300 or 350°C for 1 h followed by a sulphuric acid (H2SO4) leach. This phosphorus removal was associated with a sample weight loss of 10–20 mass% due to dissolution of iron oxides. After heating at 900°C for 1 h, a sulphuric acid leach resulted in similar phosphorus removal but with dissolution of less than 3 mass% of the sample.
    The weight losses in the leach are associated with phase changes of the phosphorus-containing goethite phase during heating. Heating at 300 or 350°C resulted in conversion of the goethite into an intermediate hematite phase (protohematite), while heating at 900°C gave a dense hematite phase. Compared with goethite in the ore, the more porous protohematite phase was more soluble in the sulphuric acid resulting in dissolution of iron with the phosphorus, while the dense hematite phase was much less soluble and little iron was dissolved in the leach.
    Leaching at 25 mass% solids for 3 h at 60°C, at a pH of 0.5 or lower, gave significant lowering of phosphorus levels. Leaches were with 0.1–1M H2SO4; the concentration of acid required depended on the amount of phosphorus to be removed. Recycling of the acid leach liquor four times did not show evidence for precipitation of phosphorus and resulted in leach solutions with up to 1 g/L P and 134 g/L Fe.
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  • Chemical Interactions in Al2O3–C/Fe System at 1823 K: Implications for Refractory Recycling

    pp. 1801-1808

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    DOI:10.2355/isijinternational.52.1801

    An investigation was carried out on recycling spent refractories comprising of Al2O3–12.9%C refractory substrate after 30 minutes of interactions with molten iron at 1823 K. XRF results showed that the spent refractory was contaminated with iron (0.798 wt%). Spent refractories in 10, 20 and 30 wt% proportions were mixed with virgin refractory composition of Al2O3–10%C. High temperature interactions of recycled refractory substrates with liquid iron were investigated at 1823 K to understand chemical reactions occurring in the metal/refractory interface as well as in the bulk of the refractory. The sessile drop technique was used to determine the interfacial wetting behaviour and the phase transformations during interactions with molten iron were determined using SEM and EDS. These investigations determined the role of spent refractory constituents which contains degraded alumina, carbon and iron. The presence of iron and reactive alumina in the refractory substrates was seen to affect both physical and chemical interactions, contact angles and carbon pick-up. The drop in contact angles was in direct correspondence with an increase in carbon pickup. The structural integrity and bonding of the recycled refractory was also compromised, and interfacial cracks were observed along with increases in the interfacial area of contact of metal droplet with refractory. Video images also showed whisker formation on metal droplets during 30 minutes of reaction time. The results from this study have shown that metal contaminated spent refractories tend to enhance refractory degradation during recycling and therefore may be unsuitable for steelmaking applications.
  • Effect of Propane Gas on Hot Metal Desulfurization by CaO Based Flux

    pp. 1809-1816

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    DOI:10.2355/isijinternational.52.1809

    The effect of propane gas blowing on hot metal desulfurization was investigated in laboratory-scale experiments and commercial plant tests. Desulfurization tests were carried out by two methods, flux injection and mechanical stirring.
    Flux injection:
    In the 4 ton-scale and commercial-scale tests, the flux efficiency for desulfurization increased by 1.2–1.7 times by mixing of propane gas with the nitrogen carrier gas at propane gas ratio of 10–58%. The effect of propane gas mixing on improving desulfurization efficiency is presumed to be caused by a decrease in the local oxygen potential, together with an increase in the interfacial area between hot metal and flux.
    Mechanical stirring:
    A top-blowing propane gas technique was examined in both 4 ton-scale tests and commercial hot metal desulfurization operation by mechanical stirring with an impeller. The flux efficiency for desulfurization increased by 1.25 times with top-blowing propane gas. The presumed effects of the top-blowing propane gas are enhancement of stirring energy in addition to reduction of the oxygen potential in hot metal and increase in the interfacial area between hot metal and flux. The sulfur partition ratio between slag and hot metal with mechanical stirring was higher than that with flux injection in a torpedo car at the same oxygen potential level with utilizing propane gas.
  • A Novel Approach to Determine the Mixing Time in a Water Model of an AOD Converter

    pp. 1817-1823

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    DOI:10.2355/isijinternational.52.1817

    During the argon-oxygen-decarburization (AOD) process high-chromium steel melts are decarburized by oxygen and inert gas injection through sidewall tuyeres and a top-lance. The tap-to-tap time of the AOD process depends mainly on the time which is necessary to produce a homogeneous distribution of all required components in the melt. This mixing time is correlated to the process time. Shorter tapping times lead to a higher productivity, lower energy consumption and lower operating costs. Prior to the reduction stage, the mixing behavior influences the melting of the solid slag layer after the addition of ferro-silicon. Fast and efficient melting of the solid slag compounds is essential to attain sufficient reduction rates. Conventional approaches to experimentally investigate the mixing efficiency in aqueous models (e.g. the 95%-mixing time criterion), yield results which show a large variance concerning the mixing time for a single operating point. In the present study a novel approach for the determination of the mixing time in a water model of an AOD converter is presented and verified. The results show a lower variance and an increased reproducibility as compared to the prior measurement technique. Using these experimental results, the vessel shape and the required volume flow rate of the AOD process gas can be optimized. Furthermore, numerical simulations can be validated using the presented results. The measurement technique can be utilized in water models representing other metallurgical processes.
  • Numerical Computation for Metallurgical Behavior of Primary Inclusion in Compact Strip Production Mold

    pp. 1824-1831

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    DOI:10.2355/isijinternational.52.1824

    Steady molten steel flow in CSP (Compact Strip Production) thin slab mold has been calculated via numerical simulation with heat transfer and solidification considered, and then the trajectories of inclusions have been calculated based on the flow field simulated. The solidifying shell has great influence on fluid flow and heat transfer in thin slab CSP mold, thus it should not be neglected while investigating the inclusion metallurgical behavior. The influence of solidifying shell on inclusion behavior has been particularly evaluated. Because the complex and irregular geometry of solidification front, grid of solidification front in mold has been used to judge whether inclusion is absorbed. Through the trajectories of inclusions, a statistics method to study the motion and distribution of inclusions by the “collision event” between inclusions and sampling surfaces has been used. The influence of inclusion diameter, density and casting speed on inclusion removal, also inclusion distribution within the solidifying shell in mold have been evaluated using this method. It is concluded that the distribution of inclusions in solidifying shell is not well-proportioned mainly due to fluid flow pattern in mold, diameter of inclusion, density of inclusion and casting speed. The ratio of inclusion floating to free surface decreases with casting speed rising, and the time for inclusions to float to free surface gets shorter. Inclusions with larger diameter are more possible to be frozen in solidifying shell with shallow skin depth. Removal of inclusions with smaller diameter is not sensitive to the above factors.
  • Effects of Ti Addition on Austenite Grain Growth during Reheating of As-Cast 0.2 mass% Carbon Steel

    pp. 1832-1840

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    DOI:10.2355/isijinternational.52.1832

    Effects of Ti addition on grain growth in reversely-transformed austenite structure during reheating of the as-cast 0.2 mass% C steel have been investigated for a Ti concentration range between 0 and 0.2 mass% and heating rates from 0.014 to 2.5°C/s. The austenite grain growth during reheating is retarded by the Ti addition and such an effect becomes stronger with the addition of higher amount of Ti. This retarding effect is ascribable to the pinning effect of fine Ti(C,N) particles which should precipitate from the as-cast structure during the reheating process. The experimental results on the grain growth behavior are well explained by the grain growth model including the Zener force and counting reduction of the pinning effect due to the existence of the coarse Ti(C,N) particles crystallized during the solidification.
  • Effects of Second Phase Particles on Migration of α/γ Interface during Isothermal α to γ Transformation

    pp. 1841-1847

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    DOI:10.2355/isijinternational.52.1841

    The effects of insoluble particles on migration of ferrite (α)/austenite (γ) interface during isothermal α to γ transformation at 1133 K have been studied by means of a model experiment using diffusion couple method and a multi-phase field simulation. It was found that the insoluble particles, ZrO2 or TiO2 particles, could retard the migration of α/γ interface. And the tendency that the retarding effect becomes stronger with higher volume fraction of TiO2 particle was observed. However, the retarding effect of ZrO2 on α/γ interface is not as strong as that on δ–ferrite (δ)/austenite (γ) interface during peritectic transformation. The phase field simulation indicates that this reduction of retarding effect originates from the appearance of carbon pile-up in γ phase, which is also caused by the existence of particles. In addition, it is shown that the difference between σα/p and σγ/p, the interfacial energy between matrix (α or γ) and particles, strongly affects the migration velocity of α/γ interface.
  • Influence of Magnesium Content and Coating Type on Graphite Degeneration in Surface Layer of Iron Castings in Resin Sand – P-Toluol Sulphonic Acid (PTSA) Moulds

    pp. 1848-1855

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    DOI:10.2355/isijinternational.52.1848

    The performance of coatings on furan resin sand moulds [P-Toluol Sulphonic Acid (PTSA) as hardener] [FRS-PTSA], was compared to ‘no sulphur’ Novolak resin coated sand [NRS] moulds by analysing the graphite characteristics in the surface layer of Mg-treated irons (0.020–0.054%Mg) and over the entire section. The extent of an abnormal surface layer is influenced by the different Mg contents for compacted or nodular graphite irons, the sulphur content of the moulding system and the type of mould coating (containing sulphur or desulphurization materials). With lower Mg content, more graphite degeneration was apparent in the cast surface layer, especially at less than 0.03%Mgres [typical Mg content for vermicular/compacted graphite cast irons] with increasing differences between FRS-PTSA and NRS moulds, and uncoated and coated moulds results. Un-coated FRS-PTSA moulds prompted five times more degenerate graphite in the surface layer than NRS moulds. Sulphur bearing coatings increased the layer up to five times the thickness in NRS moulds and up to 50% in FRS-PTSA moulds, especially for less than 0.030%Mg residual. Desulphurization type coatings based on MgO appear to be efficient for high Mg-content iron (0.05%Mg, ductile iron) but not active enough for less than 0.03%Mg (compacted graphite iron type). The change in graphite characteristics in the centre of samples evolved in a clear relationship with degenerate graphite in the surface layer, for the experimental solidification conditions.
  • Effect of Elements on Peritectic Reaction in Molten Steel Based on Thermodynamic Analysis

    pp. 1856-1861

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    DOI:10.2355/isijinternational.52.1856

    The characteristics of the steel solidification in continuous casting, especially the peritectic reaction, have a considerable effect on the surface quality of the cast blanks. This study employed the FactSage software and thermodynamic calculation to determine the effects of various elements on the carbon content at the peritectic point (Cp) at equilibrium conditions for regular carbon and low alloy steels. For typical elements, we found that Cp=0.1763+0.0616[%Al]–2.5275[%S]+0.2652[%P]–0.0023[%Si]–0.0344[%Mn]+1.5250[%S][%Mn]–0.0210[%Si][%Mn]. Furthermore, the accuracy for the carbon content at the peritectic reaction was indirectly validated by comparing the liquidus, solidus and austenite formation temperatures from the calculated phase diagrams with literature data. Moreover, statistical analysis was performed on the longitudinal crack ratio for slabs produced in a commercial plant to determine the tendency for the peritectic reaction. The results show that when the cooling rate is taken into consideration, the difference between actual carbon content and predicted Cp is negatively correlated with the longitudinal crack rate.
  • An On-Line Hierarchical Decomposition Based Bayesian Model for Quality Prediction during Hot Strip Rolling

    pp. 1862-1871

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    DOI:10.2355/isijinternational.52.1862

    Mathematical models have been widely used for the prediction of the microstructure and mechanical properties in hot rolling of strip. However, their accuracy is insufficient for quality control purposes. To accurately predict these characteristics, it is necessary to create models which can replicate the thermomechanical state of the material and its evolution during processing. In addition, these models should be able to capture the uncertainties introduced in the processing by the dynamics of the casting process and the subsequent rolling. These uncertainties lead to considerable variations in the material and mechanical state of the rolled strip. This paper presents the development of a hybrid model (MICROL) which uses the mills setting and the real time plant data such as chemical composition; forces and temperatures; and integrates them a Bayesian format to predict the desired quality attributes as well as microstructural features. This information is combined into Bayesian Hierarchical models to create an on-line tool that predicts the properties of each individual rolled coil, as well as provide information on the batch-to-batch and heat-to-heat variations. Case study from a steel Plant is presented which illustrates the implementation, calibration and validation of this model across different materials grades. Model results are found to be within the 5% tolerance of the measured values for many steel grades and rolling conditions.
  • Mechanical Properties of Strength-Gradient Steel Sheets Produced by Solution Nitriding

    pp. 1872-1878

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    DOI:10.2355/isijinternational.52.1872

    Solution nitriding and vacuum annealing treatments were applied to type 316L stainless steel plates in order to fabricate steel sheets that have a continuous strength change throughout their thickness direction. We produced three types of strength-gradient steel sheets through these treatments, with the hardness profile of each type of steel sheet reflective of the nitrogen concentration. The mechanical properties of the obtained steel sheets were investigated to demonstrate the effects of a strength gradient. As a result of tensile and bending tests, the uniaxial tensile deformation behavior of strength-gradient steel sheets is never affected by its strength gradient; however, it is governed by the average nitrogen content, while the bending deformation behavior of the sheets is significantly dependent upon the strength gradient through the thickness direction.
  • A Comparative Study of Intergranular Corrosion of AISI 304 Stainless Steel and Chrome-Manganese Austenitic Stainless Steel

    pp. 1879-1887

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    DOI:10.2355/isijinternational.52.1879

    The use of Chrome-Manganese austenitic stainless steel (Cr–Mn ASS) has tremendously increased in past few years in applications like home accessories, office appliances, light poles, etc and it is serving as an alternative to 300 series. But, the study related to sensitization behavior of these steels is scanty. Therefore, this paper aims a systematic comparison on intergranular corrosion of AISI 304 stainless steel and Cr–Mn ASS. The qualitative and quantitative comparison of degree of sensitization of these steels was carried out on the basis of optical microscopy (ASTM standard A-262 practice A test) and electrochemical test (double loop electrochemical potentiokinetic reactivation (DLEPR)) respectively. The isothermal time-temperature-sensitization diagrams were used to compare the effect of heat treatment on sensitization zone of both the steels. The DLEPR results showed that the AISI 304 SS starts to recover from sensitization when ageing time is prolonged beyond 2880 min. (at 700°C), whereas for Cr–Mn ASS, the degree of sensitization keeps on increasing to a very high value of 60.70% (11520 min.). Chromium depleted zone was identified and compared using Electron Probe Micro-analyzer (EPMA) line scan of both the steels. The depleted region of sensitized Cr–Mn ASS is found to be much wider than that of AISI 304 SS.
  • Microstructural Change and Hardness Profile of Roll Materials after Laser Irradiation and Tempering Process

    pp. 1888-1893

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    DOI:10.2355/isijinternational.52.1888

    Laser irradiation, using a continuous wave CO2 crosscurrent laser with generated beam power of 1 kW, was performed on an adamite steel and an indefinite chilled cast iron roll materials which were used in the industries. Optical microscopy, Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) were applied to reveal the microstructural details. In addition, microvickers hardness test was conducted on the laser beam irradiated samples to measure hardness profiles of the laser treated area. The results indicate that, laser irradiated areas of two roll samples sequentially consist of melting zone, phase transformation zone and non-transformed heat affected zone on sections. The hardnesses at the melting zone on both samples are relatively low when the samples were irradiated by the laser, but increase dramatically after tempering at 540°C for 1 hr. On the other hand, the hardnesses at the phase transformation zones sharply decrease from high values when they were subjected to the tempering process for the irradiated samples. There are many small and well distributed FeS and MnS inclusions in the melting zone at the both roll samples. It also was observed that, there are occasional hot tears occurred in the indefinite chilled cast iron sample, but not in the adamite steel.
  • Effects of the Addition of Cr, Mo and Ni on the Microstructure and Retained Austenite Characteristics of 0.2% C–Si–Mn–Nb Ultrahigh-strength TRIP-aided Bainitic Ferrite Steels

    pp. 1894-1901

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    DOI:10.2355/isijinternational.52.1894

    Cr, Mo and/or Ni were added to TRIP-aided bainitic ferrite (TBF) steel (0.2% C, 1.5% Si, 1.5% Mn and 0.05% Nb ultrahigh-strength TBF steel) in order to increase its hardenability. In addition, the effects of the alloying elements on the Vickers hardness, microstructure and retained austenite characteristics of the TBF steels were investigated. When the TBF steels were austempered at temperatures between MS and Mf, the Vickers hardness increased from HV300 to HV430 with increasing hardenability. The microstructure consisted of martensite and bainitic ferrite lath structures and retained austenite phases and the volume fraction of retained austenite increased with increasing hardenability. Conversely, the carbon concentration of the retained austenite decreased with increasing hardenability. Simultaneously, the quantity of the hard blocky martensite phase (M-A constituent) with refined interlath retained austenite films increased with increasing hardenability. These characteristics are mainly caused by the delayed bainite transformation during austempering through the addition of Cr, Mo and/or Ni. The addition of Ni lowered the T0 line further. The retained austenite phases of Cr- and/or Mo-bearing TBF steels were relatively stable against straining, despite their low carbon concentrations.
  • Composition Dependence of Viscosity in Binary Silicate Melts Using Double Exponential Function

    pp. 1902-1908

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    DOI:10.2355/isijinternational.52.1902

    Development of a model for estimating the viscosity of a molten silicate is important because viscosity plays an important role in high-temperature processes. However, there is still much controversy with regard to the type of equation that is appropriate for expressing viscosity changes with changes in composition and temperature. In the present work, the composition dependence of viscosity for molten silicates in binary systems was investigated based on a double exponential function. The relationship between the double logarithm of viscosity, which is an inverse of a double exponential function, and the composition was clarified using the available experimental data. We found that the double logarithm of viscosity is linearly related to the composition in certain composition ranges. The double exponential function derived empirically by fitting it to the experimental data shows a reasonable viscosity dependence on composition and temperature. The linear relationship between the double logarithm of viscosity and the composition can be derived by assuming that the probability function of the occurrence of flow in a binary silicate melt has a Gumbel distribution.
  • The Effect of Humic Substances on Iron Elution in the Method of Restoration of Seaweed Beds Using Steelmaking Slag

    pp. 1909-1913

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    DOI:10.2355/isijinternational.52.1909

    The transformation of formally productive coastal areas into barren ground is a serious problem in Japan and worldwide. Although several factors have been proposed for this phenomenon, this study especially focused on the effect of lack of dissolved iron in limiting the growth of seaweed beds. A method has been developed where a mixture of steelmaking slag and compost including humic substances, is supplied to seawater in order to stimulate seaweed bed restoration. This method increases dissolved iron concentration, since complexes called iron-humates are formed from the iron in steelmaking slag and the humic substances in compost. In this study, an evaluation was made whether humic substances increases dissolved iron concentration. A laboratory based iron elution test using seawater was attempted. Three kinds of samples were evaluated for iron elution namely, steelmaking slag in isolation, compost in isolation, and a mixture of steelmaking slag and compost. The change in dissolved iron concentration using each method was monitored over time. We found that the iron elution rate was more rapid within the mixture of steelmaking slag and compost than that in the steelmaking slag. Results of the study indicated that the structural characteristics of the humic substances were related to the increased iron elution from steelmaking slag. The method of using a mixture of steelmaking slag and compost was more effective not only for increasing dissolved iron concentration within seawater, but also for extending the life time of Fe elution.
  • Decomposition Process for Polychlorobiphenyls by Means of Basic Molten Salts

    pp. 1914-1920

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    DOI:10.2355/isijinternational.52.1914

    The decomposition of polychlorobiphenyls (PCBs) by means of basic molten salts was investigated in order to construct a safe, simple and highly efficient waste treatment system for PCBs. PCBs (mono, di, tri, tetra, penta, hexa, hepta-chlorobiphenyls) solution was injected into basic molten salts (KOH–K2CO3 or NaOH–Na2CO3) at 773–973 K with oxygen or imitation air. The mass of residual PCBs in the exhaust after the decomposition and the mass of PCBs condensed inside the reaction vessel were measured by using gas chromatograph mass spectrometry, and the decomposition efficiency of PCBs was determined. The decomposition efficiency was also determined from the residual concentration of PCBs in the exhaust in some experiments. The decomposition efficiency was high regardless of the number of chlorine contained in PCBs and reached to very high of 99.999% in an optimum condition. Chlorine of PCBs was captured in basic molten salts, and organic compounds containing chlorine was not emitted from the molten salts. Some by-products comprising benzene rings were found at 773–873 K, but the by-products disappeared at 973 K.
  • Comparison of Mixing Time Values between Gas Injections from Centered and Off-centered L-shaped Top Lance in the Presence of Swirl Motion

    pp. 1921-1923

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    DOI:10.2355/isijinternational.52.1921

  • Development of an Electric Arc Furnace Simulator Considering Thermal, Chemical and Electrical Aspects

    pp. 1924-1926

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    DOI:10.2355/isijinternational.52.1924

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