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

ISIJ International Advance Publication

  • Coke Microstructure and Graphitization Across the Hearth Deadman Regions in a Commercial Blast Furnace

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    DOI:10.2355/isijinternational.ISIJINT-2019-004

    Changes in porosity and graphitization degree of coke samples passing through different locations in the hearth deadman of a commercial blast furnace were investigated. SEM-EDS and Raman spectra were used to measure the coke microstructure and carbon structure. It indicates that the final slag migrate into the coke matrix and react with the minerals in the coke, resulting in the various conditions of the compositions of coke minerals. Al2O3 content in the coke matrix is higher compared with the final slag. With the deadman coke downward, the basicity of the slag increases firstly and then decreases. Si particles with diameter of about 30 um are deposited in the coke pores and different shaped Fe3Si are found in the deadman coke. The porosity of coke shows a liner relationship with coke descending. The porosity of coke above the taphole centerline is 27.74%. It is 33.72% near the taphole centerline and is with 42.47% below the taphole centerline. The structure as well as graphitization of deadman coke indicates higher ordering of coke below the taphole centerline regions than that of the other two samples. These results have an important practical significance for the dissolution reaction of coke in the blast furnace hearth.
  • Grain Size Effect on the Nitrogen Super-Saturation Process into AISI316 at 623 K

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    DOI:10.2355/isijinternational.ISIJINT-2019-069

    Coarse and fine grained AISI316 substrates were prepared to describe the grain size effect on the inner nitriding behavior at 623 K by using the high density plasma nitriding without precipitation of nitrides. In case of coarse grained AISI316, the nitriding process advanced homogeneously in one part of nitrided layer with high nitrogen content, and, heterogeneously in its other part. In the former, γα' two-phase, fine microstructure was uniformly formed by the phase transformation and plastic straining with the nitrogen supersaturation. In the latter, the nitrogen super-saturation localized to selectively modify the coarse grains to form the transformed α'-phase zones with the plastically strained γ-phase ones, even below the nitriding front end of 30 μm. In case of fine-grained AISI316, the nitriding took place homogeneously to form fine, two-phase microstructure down to the nitriding front end of 40 μm. This difference in the inner nitriding behavior came from the synergetic relationship between the nitrogen diffusion and super-saturation processes.
  • Optimization of Measuring Parameters for Two-dimensional Elemental Mapping in Laser-induced Breakdown Optical Emission Spectrometry Using 1-kHz Q-switched Nd:YAG Laser

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    DOI:10.2355/isijinternational.ISIJINT-2019-088

    This paper described how the lateral resolution of an elemental mapping was estimated in laser-induced breakdown optical emission spectrometry (LIBS), when the focus point of a high-frequency Q-switched Nd: YAG laser was moved on a sample surface, along with measuring the emission signal from the resultant plasma. Several measuring parameters were optimized to improve the lateral resolution; namely, they were an averaged laser power of 1 mJ/pulse, a laser repetition frequency of 1 kHz, a scanning rate of the laser beam of 0.5 mm/s, and an atmospheric gas pressure of He 1000 Pa. Using these optimal parameters, a lateral resolution was obtained to be ca. 20 μm in the one-dimensional direction of laser scan. Furthermore, two model samples, in which regularly-aligned copper circles were deposited on a nickel plate, were irradiated by a scanning laser beam to determine actual resolving abilities both in a line direction along travelling the laser and in a two-dimensional direction over a certain sample area. The sample having an interval of 85 μm between the copper circles could give an emission image which was appropriately resolved in the two-dimensional as well as the one-dimensional direction; however, in the other sample having the 25-μm interval, the two-dimensional resolution became degraded compared to the resolution of the line scan, probably because the ablation grooves, which were left on the sample surface, had a width of more than 100 μm and were overlapped with each other in the observed area.
  • Formation of Hematite Whiskers during Magnetite Concentrate Oxidation

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    DOI:10.2355/isijinternational.ISIJINT-2018-755

    Iron ore pellets prepared from magnetite concentrate are strengthened by oxidation of magnetite to hematite during pellet firing. In the present work, hematite whiskers were observed to grow on the oxidized surface of magnetite concentrate particles, over the entire temperature range studied (800°C to 950°C). The whisker thickness increased from approximately 30 nm for oxidation at 800°C to 200 nm at 950°C. The whiskers likely act as bridges between concentrate particles during pellet firing, contributing to pellet strength.
  • Effect of Substitutional Element Addition on Hall-Petch Relationship in Interstitial Free Ferritic Steels

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    DOI:10.2355/isijinternational.ISIJINT-2019-100

    The effects of individual substitutional element addition (Si, Al, Mn, Cu, Ni and Cr) on Hall-Petch relationship in interstitial free ferritic steels were systematically investigated by employing experimental examinations, the pie-up models and grain boundary segregation theory. Chemistry-dependent Hall-Petch coefficients were observed, which was principally interpreted based on the established correlations between the critical grain boundary shear stress and the grain boundary segregation depending on the kind of elements. Exceeded grain boundary segregation levels were found to be the predominant contributions of enhanced grain refinement strengthening abilities.
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    1. Formation of Hematite Whiskers during Magnetite Concentrate Oxidation ISIJ International Advance Publication
  • Addition Effect of Aromatic Amines on Coal Fluidity and Coke Strength

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    DOI:10.2355/isijinternational.ISIJINT-2018-815

    Coal fluidity is an important parameter in coal blending techniques for coke making because it strongly influences coke qualities. On the other hand, recently, the amount of high fluidity coal has been limited. To cope with this problem, caking additive method which improves fluidity of coal has been developed and commercialized. However, since tight supply of high fluidity coal is anticipated in the future, it is of great importance to develop more effective caking additive. Therefore, in this study, we investigated effect of 11 kinds of polyaromatic hydrocarbons which include oxygen, sulfur and nitrogen containing compounds on coal fluidity in order to search for more effective chemical substances. The additives were added to low fluidity coal, and fluidity analyses were carried out according to the Gieseler plastometer method. Addition of sulfur and oxygen containing compounds lowered fluidity of coal, whereas addition of aromatic amines enhanced fluidity of coal. Coal fluidity ameliorated with increasing the molecular weight of aromatic amine, and N,N'-di-2-naphthyl-1,4-phenylenediamine (DNPD) was the most effective aromatic amine in this study. Carbonization tests in an electric furnace were conducted to investigate an effect of DNPD on coke strength. As a result of adding only 1 wt% DNPD, fluidity of blended coal and coke strength (Drum Index) were highly improved.
  • Properties-to-microstructure-to-processing Inverse Analysis for Steels via Machine Learning

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    DOI:10.2355/isijinternational.ISIJINT-2019-089

    As traditional experimental science appears to be inefficient for designing novel materials with desired properties because of the complex combination of processing conditions and chemical compositions, data-driven materials science is becoming increasingly important for materials design. A properties-to-microstructure-to-processing inverse analysis for steels is attempted via a machine learning approach in this work, where a potential best balanced property of tensile strength (TS) and total elongation (tEL) TS × tEL and its corresponding microstructure and processing conditions are explored using a genetic algorithm, which is implemented by an independently developed machine learning tool called the Materials Genome Integration System Phase and Property Analysis (MIPHA). The results demonstrate that a property-to-microstructure/processing method is sufficient to identify a best model performance, potential TS × tEL, and a reasonable relationship description among the processing, microstructure and property. A microstructure with Widmanstatten ferrite, banite, and martensite is found to be beneficial to a good balanced property.
  • Quantitative Analyses of Chemical Structural Change and Gas Generation Profile of Coal upon Heating toward Gaining New Insights for Coal Pyrolysis Chemistry

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    DOI:10.2355/isijinternational.ISIJINT-2018-816

    Chemical structure of coal is evolutionary changed during pyrolysis that accompanies gas release. The chemical structural change and gas formation profiles play important roles in determining caking property and physical properties such as strength and size of the resultant coke. However, analyses of volatile components and structural analysis of solid char have been mostly performed individually, and it is difficult to combine both and to obtain quantitative understanding on the thermal decomposition of coal at mechanistic level. In this study, simultaneous analyses of solid chemical structures of the heat treated coals and gas formation profiles were conducted for two kinds of coals that were pyrolyzed at an identical condition. On-line gas analysis with a quadrupole mass spectrometer and spectroscopic methods (NMR and FT-IR) were employed for quantitative evaluation of gas formation characteristics and solid chemical structure, respectively. The information obtained were then integrated to acquire new insight for coal pyrolysis mechanism. Here an approach to quantify the transferable hydrogen that contributes to stabilize radicals formed in pyrolyzing coal was proposed. It includes the quantitative assessment of aromatic cluster growth, decomposition of hydroxyls, and releases of hydrogen and pyrolytic water into gas phase. The proposed approach suggested that a bituminous coal that exhibits plasticity during pyrolysis had 3.5 mol/kg-coal transferable hydrogen, whereas the amount of transferable hydrogen of the sub-bituminous coal, a non-caking coal, was 1.3 mol/kg-coal, during pyrolysis up to 500°C.
  • Insights into Accumulation Behavior of Harmful Elements in Cohesive Zone with Reference to Its Influence on Coke

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    DOI:10.2355/isijinternational.ISIJINT-2019-140

    The accumulation behavior of harmful elements (K, Na, Zn) in the upper area of the cohesive zone was reported for the first time. The alkalis-bearing aluminosilicate minerals and the kalsilite were found in the coke, while a number of zinc oxide crystals mainly existing as hexagonal wurtzite habit and zinc-bearing minerals were observed in the mixture-like phase of slag-iron. The findings further deepen the understanding of degradation behavior of coke in the cohesive zone.
  • Numerical Investigation of the Effects of Gas-release Rate and Viscosity under Various Heating Rates on Swelling Ratio of Coal in the Coke Production Process

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    DOI:10.2355/isijinternational.ISIJINT-2018-799

    The effect of heating rate on the swelling ratio of coke during heating of coal was investigated using numerical simulations. The mathematical model in our previous study was modified to observe the effect of the heating rate, and the equation for the reaction rate of coal pyrolysis and gas formation was changed to a first-order temperature-dependent equation. The original and viscosity-dependent classical bubble nucleation equations were compared, and the results showed that the swelling ratio in the numerical simulation results reduced with an increase in the mass transfer rate of gas from coal. At low mass transfer rate (1 × 10-13 m3/s), the dependence of the swelling ratio on the heating rate was not straightforward. The swelling ratio increased in the order of the following heating rates: 10, 30, and 3°C/min. At high mass transfer rate (1 × 10-12 m3/s), the swelling ratio increased with increasing heating rate. Experimental results of varying the heating rate agreed well with the results of the large mass transfer case. The numerical simulation results indicated two issues—the gas escaping from coal is a key factor affecting the swelling of coal during heating, and the swelling ratio increased with increasing heating rate because of the short gas release time.
  • Gigacycle Fatigue Fracture of Low Strength Carbon Steel, Tested using a Simulated Heat Affected Zone Microstructure

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    DOI:10.2355/isijinternational.ISIJINT-2019-086

    In this study, gigacycle fatigue properties were investigated for several microstructures prepared by heat treatment designed to simulate the heat-affected zone (HAZ) that results from welding. The results showed that internal matrix crack origin gigacycle fatigue becomes dominant in coarse-grained microstructures in spite of low tensile strength of only about 600 MPa. It was found that, high material strength is not always necessary and that microstructure plays an important role in the development of internal-origin gigacycle fatigue fractures.
  • Simultaneous Measurements of Corrosion Potential and Hydrogen Permeation Current in Atmospheric Corrosion of Steel

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    DOI:10.2355/isijinternational.ISIJINT-2019-113

    To investigate hydrogen absorption behavior into carbon steel during corrosion in an aqueous sodium chloride (NaCl) droplet, a simultaneous measurement system of the corrosion potential, Ecorr and hydrogen permeation current, iper was developed using the Kelvin probe (KP) technique and the Devanathan–Stachurski (DS) method, respectively. This system outputs the interrelation between corrosion and hydrogen absorption into steel throughout the drying process of an NaCl droplet. Our results showed that hydrogen absorption into the steel occurred when the Ecorr shifted in less noble direction under wet conditions, and ceased at a higher potential of Ecorr when the steel surface dried up. Based on the results of the transients of the iper, the amount of hydrogen absorbed during the drying of the NaCl droplet increased with NaCl concentration, which was attributed to the negative shift of the Ecorr. Furthermore, the amount of hydrogen absorbed within one wet-dry cycle changed with the number of cycles, due to the expansion of the corroded area and the formation of iron rust.
  • Behavior of Crystallization on a Continuous Solidification of Blast Furnace Slag

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    DOI:10.2355/isijinternational.ISIJINT-2019-147

    A continuous blast furnace slag solidification process was developed to promote the use of air-cooled slag coarse aggregate for concrete. In this process, molten slag can be solidified in only 120 seconds, and the thickness of the slag is about 25 mm. After crushing the slag, the water absorption ratio is much lower than that achieved in the past because gas generation is suppressed. With this apparatus, most of the slag is crystalline, but part of the slag has a glassy surface. Therefore, EPMA and XRD were used to study the glass transition phenomenon. It found that the thickness of the glass layer is about 2 mm. To discuss the glass transition and crystallization phenomena, the thermal history was simulated by heat transfer analysis. The results clarified the fact that all the slag on the mold has a glassy surface layer of about 2 mm, and good agreement between the calculation and experimental data concerning the layer was obtained. It was also shown that most of the slag crystallizes in the slag pit because the temperature inside the piled slags rises to more than 1173 K. The measured slag temperature and calculated temperature were also in good agreement.
  • Effects of Interfacial Oxygen Potential and Slag Phase Changing during Slag Formation Process on Dephosphorization Behavior

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    DOI:10.2355/isijinternational.ISIJINT-2019-052

    The effects of interfacial oxygen potential and slag phase changing during slag formation process on dephosphorization behavior when the decarburization slag is recycled are investigated by XRD and SEM. The research results show that the remove of phosphorus is mainly accomplished in the first 5 minutes for the condition 1(initial lump-sum addition of lime and Fe2O3), however the remove of phosphorus is mainly done from 5 to 10 minutes for the conditions 2(divided addition of lime and Fe2O3) and 3(initial lump-sum addition of lime and divided addition of Fe2O3). The dephosphorization rate of the condition 1 is about 78.9%, which is significantly higher than those of the conditions 2 and 3. The phase of decarburization slag is mainly made up of 2CaO·SiO2-3CaO·P2O5 and Ca3Fe2O5, the phosphorus mainly enriches in 2CaO·SiO2-3CaO·P2O5. After the recycling of decarburization slag, the slag phase with reactions under different conditions is basically the same, which is made up of FeO, 2CaO·SiO2, 2CaO·SiO2-3CaO·P2O5 and Ca3Fe2O5. However, the contents of 2CaO·SiO2 and 2CaO·SiO2-3CaO·P2O5 change seriously. The reason for the difference of dephosphorization behavior is that the interfacial oxygen potential and the slag phase changing during slag formation process are significantly different between different conditions.
  • Influence of Stress Re-distribution on Hydrogen-induced Fatigue Crack Propagation

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    DOI:10.2355/isijinternational.ISIJINT-2019-012

    In order to clarify influence of stress re-distribution effect on hydrogen-induced fatigue crack propagation, we investigated fatigue crack propagation rates and brittle-like fracture ratio. The experiments were conducted in nitrogen and hydrogen gas atmosphere with ferrite-pearlite steels having different pearlite ratio, respectively. The crack propagation rates and the brittle-like fracture ratio decreased as pearlite ratio increased. To explain the changes of crack propagation rates and fracture ratio, we proposed that the stress re-distribution effect causing stress and strain relaxation at a crack tip contributes to suppression of the hydrogen-induced fatigue crack propagation. As a verification, finite element methods were operated with models having different width of the hard phase and different distance between a crack tip and a hard phase in plane stress and strain conditions, respectively. The finite element method analysis showed that stress re-distribution effect was smaller in plane strain condition than that in plane stress condition, indicating that a large hardness difference is crucial in plane stress condition to suppress the hydrogen-induced fatigue crack propagation.
  • Microstructure and Phase of Carbon Brick and Protective Layer of a 2800 m3 Industrial Blast Furnace Hearth

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    DOI:10.2355/isijinternational.ISIJINT-2019-029

    In this paper, the residual thickness of carbon brick, residual carbon brick and skull of a Chinese 2800 m3 blast furnace hearth were studied in detail and the formation mechanism of skull and brittle layer were proposed. The results show that the remaining thickness of carbon brick is highly inhomogeneous in the height and circumferential direction. In the circumferential direction, the sidewall erosion in the range of 3.6 m under the taphole is more serious. In the height direction, the carbon brick at the distance of 1.0–2.0 m below the central line of the taphole is more obvious. The erosion of hearth bottom is "mumps face+ bowl" type erosion. The minerals of the hot surface of carbon brick used for more than nine years are mainly composed of KAlSiO4, KAlSi2O6, Zn2SiO4 and ZnO as well as a small amount of ZnS, KCl and ZnAl2O4. Micro cracks resulted from the KAlSiO4, KAlSi2O6, Zn2SiO4 and ZnAl2O4 are the inducement of formation of brittle layer. The main reason for the formation of macro cracks and brittle layer in carbon brick is the continuous accumulation of ZnO in micro cracks. The brittle layer mainly occurs in the region where the temperature of carbon brick is lower than 950°C. The skull above the central line of the taphole is mainly composed of Ca2Al2SiO7, Ca2MgSi2O7, CaTiO3 and KAlSiO4. The skull below the central line of the taphole is primarily comprised of Ca2Al2SiO7, Ca2MgSi2O7, CaS, Fe and Fe3Si. The blast furnace slag phase in the skull below the central line of taphole is derived from the blast furnace slag that penetrates into the deadman coke. The blast furnace slag can be present below the central line of the taphole and adhere to the hot surface of the carbon brick to isolate the direct contact between the molten iron and carbon brick.
  • Recycling of Blast Furnace Sludge to the Blast Furnace via Cold-Bonded Briquettes: Evaluation of Feasibility and Influence on Operation

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    DOI:10.2355/isijinternational.ISIJINT-2019-080

    Ore-based steelmaking generates various residues including dusts, sludges, scales and slags. Recycling of these residues within the process or via other applications is essential for sustainable production of steel. In blast furnace (BF) ironmaking, the gas-cleaning equipment generally recovers the particles in the off-gas as dust and sludge. Traditionally, the dry dust is recycled via the sinter or, in the case of pellet-based BF operation, via cold-bonded briquettes and injection. As the BF sludge mainly consists of iron and carbon, this residue is of interest to recycle together with the BF dust. However, depending on how the BF is operated, these two residues are more or less the major outlet of zinc from the furnace. Thus, to limit the recycled load of zinc, both materials cannot be recycled without dezincing the sludge prior to recycling. Dezincing and recycling of the low-zinc fraction of BF sludge via sinter have been reported whereas recycling via cold-bonded briquettes has not been performed. In the present study, cold-bonded briquettes containing the low-zinc fraction of dezinced BF sludge were charged as basket samples to the LKAB Experimental Blast Furnace (EBF). The excavated basket samples from the quenched EBF suggested that additions of up to 20 wt.% of upgraded BF sludge was feasible in terms of reducibility and strength. Based on these results, BF sludge were added to cold-bonded briquettes and charged in industrial-scale trials. The trials indicated that the annual generation of BF sludge, after dezincing, could be recycled to the BF.
  • Solubility of Sulfur in the Solid Oxide of the Calcium-Aluminate System

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    DOI:10.2355/isijinternational.ISIJINT-2019-127

    The formation of CaS around CaO–Al2O3 causes pitting corrosion in ferritic stainless steel. To prevent the precipitation of CaS, the solubility of CaS in both the liquid and solid CaO–Al2O3 system has to be clarified. In this study, the sulfur content in the CaS-saturated solid CaO–Al2O3 system was measured. The results showed that, sulfur was soluble only in 12CaO·7Al2O3 at approximately 1.2 mass% while the sulfur content in the other solid compounds was very low. In addition, the sulfur content of 12CaO·7Al2O3 was independent of the heating temperature and was higher than that in the liquid oxide of the same composition. Therefore, 12CaO·7Al2O3 can dissolve sulfur in the solid state, preventing CaS formation.
  • Hydrogen Embrittlement Susceptibility Evaluation of Tempered Martensitic Steels Showing Different Fracture Surface Morphologies

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    DOI:10.2355/isijinternational.ISIJINT-2019-130

    The effects of the crosshead speed, hydrogen content and temperature on fracture strength and fracture surface morphology were investigated using a tempered martensitic steel containing 1.67 mass% of Si (H-Si) and one containing 0.21 mass% of Si (L-Si). When L-Si specimens were charged with a small amount of hydrogen, fracture surfaces showed a transition from quasi-cleavage (QC) to intergranular-like (IG-like) to intergranular (IG) at room temperature. In contrast, when H-Si specimens were charged with a small amount of hydrogen, fracture surfaces showed a transition from QC to IG-like at room temperature. This transition in the fracture surface morphology can be explained by the magnitude relationship between intergranular and transgranular strengths under hydrogen charging. At a temperature of -196°C, hydrogen did not lower the fracture strength nor did it change the fracture surface morphology. Hence, hydrogen embrittlement at room temperature was presumably caused by hydrogen accumulation and lattice defect formation during stress application as well as by hydrogen trapped before stress was applied. Fracture strength decreased and converged to a constant value (lower critical stress) with decreasing crosshead speed. The crosshead speed for obtaining lower critical stress decreased as the fracture surface changed from IG to IG-like to QC. Therefore, the crosshead speed for obtaining lower critical stress should not be treated as a constant but should be determined experimentally for each type of fracture surface.
  • Formation of Non-Metallic Inclusion and Acicular Ferrite in Ti–Zr Deoxidized Steel

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    DOI:10.2355/isijinternational.ISIJINT-2019-008

    The formation of non-metallic inclusion and acicular ferrite (AF) during solidification is investigated by high temperature experiment, thermodynamic calculation and metallurgical analysis. The size distribution and composition of inclusions are carried out by optical microscope (OM), scanning electron microscope (SEM) equipped with energy-dispersive spectrometer (EDS) and image process software. The results indicate that adding Ti–Zr can modify the inclusions from Mn–Si–O–MnS to Ti–Zr–O–MnS, and the average size of inclusions can be refined obviously from 2.38 µm to 1.56 µm. The critical size of inclusion inducing AF is 0.21–0.37 µm, and the optimized size for AF nucleation induced by Ti–Zr–O–MnS inclusion is in range of 1.0–3.0 µm in this study. Moreover, for inducing AF nucleation, the inclusion larger than critical size is only necessary condition but not sufficient condition. A Mn-depletion zone resulted by MnS precipitating on Ti–Zr–O inclusion is observed adjacent to the Ti–Zr–O inclusion, which is believed to be one of the possible mechanisms to promote the nucleation of AF.
  • A Mill Set-up Model Using a Multi-output Regression Tree for a Tandem Cold Mill Producing Stainless Steel

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    DOI:10.2355/isijinternational.ISIJINT-2018-770

    In a tandem cold mill for stainless steel, an optimum reduction rate is necessary for each stand. A conventional mill set-up uses a lookup-table to optimize the rolling schedule. However, to reflecting all the input conditions and manual interventions on a model is difficult. In this paper, we propose a mill set-up model that can efficiently predict the reduction rate for each stand by considering various input conditions. The proposed prediction model has a multi-output tree structure with a smaller time complexity for easy interpretation. The key contribution to the proposed algorithm is variable selection. According to the results of an analysis of the time-complexity, the proposed algorithm is less time consuming and is capable of learning datasets with a large number of variables more efficiently than the single-output CART (classification and regression trees). To evaluate the performance of the proposed algorithm, we applied it to the rolling reduction rate of a tandem cold mill in POSCO. The proposed algorithm achieves a similar level of R-squared in only 18% of the computing time required for an existing single-output CART algorithm.
  • Evolution Mechanism of Inclusions in H13 Steel with Rare Earth Magnesium Alloy Addition

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    DOI:10.2355/isijinternational.ISIJINT-2019-094

    Rare earth metals have strong affinity with O and S in molten steel. However, due to high density of rare earth inclusions close to that of molten steel, rare earth inclusions are not easy to float up and it is not conducive to take away O and S. In this paper, the influence of rare earth magnesium alloy on the evolution mechanism of inclusions and deoxidization and desulfurization in H13 steel was analyzed, and the thermodynamic calculation was developed. The research results indicate that the composite inclusions of low-density MgO attaching or wrapping on the surface of high-density Ce inclusions are formed after adding Ce–Mg alloy to H13 steel. Compared to Ce inclusions alone, composite inclusions have lower average density and larger diameter, so that they have a faster floating rate according to Stokes law and are easier to float up. Therefore, rare earth magnesium treatment is beneficial to effectively remove the impurity elements such as O and S in the steel.
  • Evaluation of the Structure and Strength of Coke with HPC Binder under Various Preparation Conditions

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    DOI:10.2355/isijinternational.ISIJINT-2018-772

    Hypercoal (HPC) is examined as a caking additive to the mixture of strongly coking coal and non-slightly coking coal. Samples were coked, then their strength and crystallinity of the carbon structure by Raman spectra were measured. Hypercoal addition increased strength of coke, and a good correlation was observed between the mechanical strength of coke and the Raman parameter defined by the ratio of D3-band to G-band. Results suggest that mutual melting between blended coal and HPC brought strong carbonaceous structure with highly crystallized. The Raman parameter can predict the coke strength to some extent.
  • A New Data-driven Roll Force and Roll Torque Model Based on FEM and Hybrid PSO-ELM for Hot Strip Rolling

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    DOI:10.2355/isijinternational.ISIJINT-2018-846

    In this paper, a new Extreme Learning Machine (ELM) regression model of roll force and roll torque based on data-driven is proposed. The three-dimensional elastic-plastic finite element model (FEM) is established to solve the roll force and roll torque under different parameters (including rolling reduction rate, roll radius, rolling speed, average width of strip, entry temperature of strip). The regression model of ELM optimized by Particle Swarm Optimization (PSO) is established through using the datasets obtained by FEM. The PSO-ELM model prediction values of roll force and roll torque are compared with the single ELM and PSO-SVM model, and the error results of the prediction values are analyzed. The error results fully verify the feasibility and accuracy of the PSO-ELM model proposed. It is found that the new data-drive model of roll force and roll torque is simple in structure and it can make up for the deficiency of traditional mathematical mechanism model in dealing with nonlinear problems. The research result reveals that PSO-ELM method is suitable for parameters prediction and model optimization in strip rolling process.
  • Causes of Particle Trajectory Fluctuation on the Rotating Chute in Circumferential Direction at Bell-less Top with Parallel Type Hoppers

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    DOI:10.2355/isijinternational.ISIJINT-2018-862

    Uneven burden distribution at bell-less top has a negative influence on the smooth operation of blast furnace with parallel type hoppers. Although previous works agreed that the initially oval-shaped particle trajectory on the chute causes the above-mentioned segregation, the subsequent particle trajectory fluctuation against the circumferential direction was still not fully understood. As a result, this work employs both the discrete element method (DEM) simulation and the theoretical model calculation, to quantitatively elucidate the causes of particle fluctuating behaviors on the rotating chute. The consistent results show that, on the one hand, a sine-like particle velocity distribution causes the Coriolis force to have a maximum magnitude around 120 deg while a minimum around 300 deg in the circumferential direction. On the other hand, the alternately sparse and intensive granular flow on the chute causes the particle mass flow rate to present a sine-like result with a maximum rate around 220 deg while a minimum around 60 deg. The superposition of two results contributes to the particle trajectory fluctuation on the rotating chute in the circumferential direction at bell-less top with parallel type hoppers.
  • Characterization of Dislocation Evolution in Cyclically Loaded Austenitic and Ferritic Stainless Steels via XRD Line-profile Analysis

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    DOI:10.2355/isijinternational.ISIJINT-2019-077

    Dislocations in austenitic and ferritic stainless steels (SSs) under cyclic loading were quantitatively evaluated via X-ray diffraction line-profile analysis to determine the relationship between the dislocation density and low-cycle fatigue (LCF) life in both SSs. The dislocation density of the austenitic and ferritic SSs varied linearly with respect to the LCF life in a double-logarithmic graph, with different slopes of the line. The dislocation density normalized by the maximum work hardening for both SSs exhibited a log–log linear relationship with the LCF life. The fraction of screw dislocations in the ferritic SS decreased with decreasing LCF life owing to the easy cross-slip of dislocations. Because of the difficulty of the cross-slip of dislocations in the austenitic SS, the fraction of screw dislocations remained almost constant throughout the LCF life. Analysis of the crystallite size and the dislocation arrangement with respect to the dislocation density under tensile and cyclic loading revealed that the dislocation arrangement for cyclic loading was smaller than that for tensile loading. Thus, the dislocation arrangement was related to the cyclic loading. In the plot of the dislocation evolution versus the number of cycles, two stages were observed in the variation of the dislocation characteristics for both SSs. In the first stage, the dislocation density increased, and the crystallite size decreased. The dislocation arrangement parameter of the ferritic and austenitic SS decreased and remained the same, respectively, in the first stage. In the second stage, the dislocation density, dislocation arrangement parameter, and crystallite size remained constant.
  • Mechanism of Pore Formation in Novel Porous Permeable Ceramics Prepared from Steel Slag and Bauxite Tailings

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    DOI:10.2355/isijinternational.ISIJINT-2018-782

    Porous permeable ceramics (PPC) were prepared from composite ceramsites (CC) via a single firing process. CC were granulated with steel slag as a core and bauxite tailings in an outer-layer. XRD, SEM, EDS, mercury porosimetry and metallographic microscopy were used to study its properties and the pore formation mechanism. Results showed that during sintering process, gradual diffusion of cations from slag to tailings layers enhanced bonding among CC with formation of new crystals: anorthite and pyroxene. PPC had a wider distribution of pores from 0–300 µm sintered at 1160°C. With an increase in sintering temperature, ceramics were densified with disappearance of the small pores, which had an increasing threshold diameter values from 45 µm at 1180°C to 70 µm at 1190°C. Big pores larger than the threshold values would be remained and enlarged due to shrinkage of CC during the densification process. The decreasing amounts of pores and an increasing pore diameter had contrary effects on its permeable properties. PPC sintered at 1180°C with porosity of 27.5% and medium pore diameter of 92.7 µm had the optimum properties with bending strength of 10.92 MPa, water permeability of 0.039 cm/s and qualified leaching properties of harmful elements (Mn, Cr, V and Pb). This study would promote a more feasible and economic method for producing porous permeable ceramics and improving added value of steel slag and tailings.

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