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

ISIJ International Vol. 60 (2020), No. 3

  • Agglomeration and Removal of Alumina Inclusions in Molten Steel with Controlled Concentrations of Interfacial Active Elements

    pp. 409-418

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    DOI:10.2355/isijinternational.ISIJINT-2019-107

    In this study, Al deoxidation experiments have been performed in a mildly stirred steel bath with controlled O and S concentrations, to investigate the effects of interfacial active elements on the agglomeration and removal of Al2O3 inclusions in molten steel. The decrease rate constants of total Al2O3 inclusions, Al2O3 cluster inclusions, and Al2O3 single inclusions as well as the maximum average diameter of Al2O3 cluster inclusions decrease with increasing O and S concentrations in molten steel. However, the effect of O is much greater than that of S. These experimental results have been analyzed based on the kinetics of Al2O3 inclusion removal and the interfacial chemical interaction between Al2O3 inclusions in molten steel. The following findings have been obtained on the agglomeration and removal mechanisms of Al2O3 inclusions in molten steel. The Al2O3 inclusions in molten steel are removed by a mechanism whereby large Al2O3 cluster inclusions, formed by Al deoxidation, float and separate while repeatedly agglomerating and coalescing with fine Al2O3 single inclusions suspended in molten steel. The agglomeration of Al2O3 inclusions during floating and separation can also be explained by a mechanism whereby the agglomeration force due to the cavity bridge force is exerted between the Al2O3 inclusions and the Al2O3 inclusions come in complete contact when the Al2O3 inclusions with thermodynamically agglomerating tendency are approaching each other. The effects of O and S interfacial active elements are considered in both these mechanisms.
  • Effect of the Silicate Structure on Calcium Elution Behaviors of Calcium-silicate Based Mineral Phases in Aqueous Solution

    pp. 419-425

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    DOI:10.2355/isijinternational.ISIJINT-2019-263

    In this study, the effect of the silicate structure of calcium-silicate based mineral phases on their Ca elution behaviors into water was investigated. The Ca elution behaviors of Ca-silicate based mineral phases with different skeleton silicate structures in ion-exchanged water were analyzed using the powder leaching test. The elution amount of Ca was in the order alite (Ca3SiO5) > belite (γ-Ca2SiO4) > rankinite (Ca3Si2O7) > pseudowollastonite (α-CaSiO3) > wollastonite (β-CaSiO3) ≈ cuspidine (Ca4Si2O7F2) > diopside (CaMgSi2O6) > hedenbergite (CaFeSi2O6) > tremolite (Ca2Mg5Si8O22(OH)2) > anorthite (CaAl2Si2O8). This suggests that the elution amount of Ca decreased with the skeleton silicate structure became more complicated.
  • Growth of Initial Clog Deposits during Continuous Casting of Ti-ULC Steel – Formation and Reduction of the Initial Deposits at Nozzle/Steel Interface

    pp. 426-435

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    DOI:10.2355/isijinternational.ISIJINT-2019-384

    In order to elucidate a nozzle clogging mechanism during continuous casting of Ti-ULC (Ti containing Ultra Low C) steel, a series of experiments employing a rotating finger method was employed. In the context of the carbothermic generation of CO – oxidation of Ti-ULC steel at the interface between the nozzle and the liquid steel – reduction of FetO in the oxidized product composed of FetO–Al2O3–TiOx (“FAT”) by the liquid steel, the FAT was intentionally coated on usual nozzle refractories (“CZ”). It was found that the refractory components (CaO, ZrO2, SiO2) rapidly dissolved into the FAT. The FetO in the FAT was rapidly reduced by Al and Ti in the Ti-ULC steel. As a result, reduced Fe metallic droplets mixed with CaO–Al2O3–TiOx–ZrO2–SiO2 oxide were found as the clog deposit. This was in good agreement with the previous reports. In case of pure liquid Fe, the FetO was not reduced but remained in the deposit. A likely reaction mechanism for the growth of the initial clog deposit was proposed.
  • Experimental Evaluation of Interfacial Free Energy of Solid Iron

    pp. 436-441

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    DOI:10.2355/isijinternational.ISIJINT-2019-388

    Interfacial free energy of solid iron was experimentally evaluated for solid/liquid interface and grain boundary of austenite phase in solid Fe/liquid FeO–SiO2 system. Multi-phase equilibrium method was adopted and two kinds of angles such as dihedral angle between solid/liquid interface and grain boundary, and contact angle of sessile drop of molten oxide on solid iron were measured. On the basis of experimental results and literature data of activity of FeO, surface tension of liquid FeO–SiO2 oxide and solid Fe, interfacial tension (interfacial free energy) of solid/liquid interface and grain boundary of γ–Fe were evaluated. Oxygen partial pressure in the experimental atmosphere was evaluated as 1 × 10−12~10−11atm. Under this condition, solid/liquid interfacial free energy was evaluated as 1440–1500 mJ/m2 and grain boundary free energy of γ–Fe as 860–940 mJ/m2 at 1350°C. The evaluated value for the grain boundary was agreed well with the data of previous works.
  • Phosphorus Migration Behavior of Medium-Phosphorus Magnetite Ore during Carbothermic Reduction

    pp. 442-450

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    DOI:10.2355/isijinternational.ISIJINT-2019-214

    Time-dependent phosphorus migration characteristics during the carbothermic reduction process have been investigated based on the iron and phosphorus status in medium-phosphorus magnetite ore. The results show that the iron metallization enhanced from 84.38% to 96.49% with the reduction time varying from 10 min to 60 min. However, gasificating dephosphorization first increased from 21.03% to 33.07% and then decreased to 31.61% as a result of the large absorption of reduced phosphorus gas into metallic iron. Optimal phosphorous gasification was achieved at a reduction time of 50 min. Moreover, SEM-EDS and EPMA analyses indicated that the ratio of phosphorus content distributed between the iron phase and gangue minerals evidently increased from 0.12 to 1.01 and 1.18, as the reduction continued from 10 min to 30 min and 50 min. Meanwhile, the phosphorus content in the iron phase increased along with an increase in the carbon and Fe3C contents. Furthermore, the aggravated phosphorus migration into the iron phase is inherently attributed to the smaller lattice discrepancy between FexP and Fe3C in contrast with that between FexP and γ-Fe, which accelerated the entrance of phosphorus into the lattice of Fe3C and the formation of FexP.
  • Melting Behaviour of Iron Ore Pellet Bed under Nut Coke Mixed Charge Conditions

    pp. 451-462

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    DOI:10.2355/isijinternational.ISIJINT-2019-246

    The melting and dripping behaviour of an iron ore pellet bed mixed with nut coke are investigated through a series of quenching, melting and dripping experiments. In the melting bed of iron ore pellets, nut coke acts as a frame to maintain the passage for the gas flow. The iron carburisation level of the pellet shell is found to control the melting temperature of the pellet bed. Simultaneous and layer-wise melting is observed for the pellet bed with and without mixed nut coke, respectively.In the case of pellet bed mixed with nut coke, the liquid dripping starts at a lower temperature (1500°C) compared to the case when nut coke is absent (1518°C). Subsequently, a steady rate of liquid dripping is observed for the pellet bed mixed with nut coke. However, in the case of the pellet bed without nut coke, most of the liquid drips (~50 wt%) at high temperature (1550°C). The difference in carbon content of the quenched pellets and the dripped metal reveals that a substantial iron carburisation occurs when liquid iron flows over the regular coke particles.The nut coke is noticed to consumed preferentially in place of the regular coke. Additionally, the total coke consumption decreases with an increase in nut coke addition in the pellet bed. These results give support for more extensive use of nut coke as a replacement of the regular coke in the ironmaking blast furnace.
  • Estimation of Particle Segregation Behavior in Ore-coke Mixed Layer Using Screening Layer Model

    pp. 463-469

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    DOI:10.2355/isijinternational.ISIJINT-2019-416

    To achieve low RAR operation by coke mixed charging, it is important to control coke segregation behavior in mixed layer at blast furnace top. In this study, a numerical simulator based on screening layer model was developed to estimate the distribution of mixed coke ratio in mixed layer. The results are summarized as follows:(1) The parameters required for the screening layer model to estimate the segregation behavior of the burden materials were determined by PIV test and numerical fitting.(2) The screening layer model containing parameters obtained by experiments and fittings was taken into the blast furnace burden distribution simulator. The simulation results showed that the distribution of mixed coke ratio of the small coke in the ore can be accurately estimated under the charging conditions of the actual furnace.(3) The influence of the difference in tilting direction of the rotating chute on the distribution of mixed coke ratio was evaluated. In the reverse tilting, the radial distribution of the mixed coke ratio became more uniform as compared with the forward tilting charging. Therefore, it is considered that reverse tilting is more effective for carrying out coke mixed charging.
  • Bubble Growth and Floating Behavior during Degassing Process of Molten Steel/(N2, H2) System

    pp. 470-480

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    DOI:10.2355/isijinternational.ISIJINT-2018-875

    Dissolved gas flotation method has been developed to remove inclusions in molten steel. The principle is that bubbles formed on inclusions by vacuum treatment of nitrogen or hydrogen supersaturated molten steel can carry the inclusions to slag. A kinetic model was constructed to analyze the bubble growth and floating behavior during the degassing process of the method, and its accuracy was verified by related experiments. The results show that pretreatment pressure, bubble nucleation depth and gas type have significant effects on bubble growth and floating, while vacuum treatment pressure and inclusion radius have little effects on it. The growth rate and floating velocity of bubbles increase with the increase of pretreatment pressure or the decrease of bubble nucleation depth. The growth rate and floating velocity of hydrogen bubbles are much larger than those of nitrogen bubbles. Calculation results indicate that the diameters of the bubbles are mostly 0.2–10 mm during floating process via this method. Moreover, the distribution of the bubbles nucleating on the inclusions is dispersive. In addition to the bubbles carrying inclusions to slag directly, these dispersive fine bubbles have a high probability of inclusion adhesion resulting in an improvement of the inclusion removal.
  • Fluid Dynamics Analysis of O2–CaO Jet with a Shrouding Flame for EAF Steelmaking

    pp. 481-491

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    DOI:10.2355/isijinternational.ISIJINT-2019-207

    Compared with the traditional addition methods of lumpy lime into the electric arc furnace (EAF) for slag making, the technology of O2–CaO jet can deliver lime powder directly into the EAF molten bath with high speed carrier gas, which demonstrates much advantages in quick melting and effective phosphorus removal. Recently, the shrouding combustion flame was proposed and applied to strengthen the CaO import capability of the O2–CaO jet. In this study, combining the discrete particle model (DPM) and the Eddy Dissipation Concept (EDC) model with the detailed chemical kinetic mechanisms (GRI-Mech 3.0), computational fluid dynamics (CFD) models of the O2–CaO jet with shrouding flame, with shrouding O2 and without shrouding gas were developed. The numerical results of CFD models were firstly validated by the experimental data. The interaction between the particles and the gas jet of the O2–CaO jet was analyzed and how the shrouding combustion flame affected the fluid flow characteristics of the O2–CaO jet were clarified. The shrouding high-temperature combustion flame could delay the attenuation of the axial velocity of the O2–CaO jet, heat the CaO particles effectively and make the CaO particles cluster together in a much longer distance, which is helpful to strengthen the jet impact, accelerate the meltdown of CaO particles and improve the utilization efficiency of CaO.
  • Crystallization Kinetics and Structure of CaF2–CaO–Al2O3–MgO–TiO2 Slag for Electroslag Remelting

    pp. 492-498

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    DOI:10.2355/isijinternational.ISIJINT-2019-461

    The crystallization kinetics and structure of CaF2–CaO–Al2O3–MgO–TiO2 slag for electroslag remelting (ESR) were investigated by differential scanning calorimetry and Raman spectroscopy, respectively. The results show that increasing TiO2 content from 4.2 mass% to 16.8 mass% in the slag lowers the crystallization rate of the slag. The crystallization of the primary crystalline phase (11CaO·7Al2O3·CaF2) in the slags with 4.2–12.6 mass% TiO2, and primary crystalline phase (11CaO·7Al2O3·CaF2 and CaTiO3) in slag with 16.8 mass% TiO2 originates from constant nucleation rate, interface reaction controlled and one-dimensional growth, irrespective of the TiO2 contents of the slag. Raman spectroscopy study indicates that TiO2 plays a network-modifier role in relatively more complex Al–O–Al band and Q4 units by forming Q2 units and less complex Ti2O64- chain unit, resulting the decreasing of the polymerization degree of the slag. The variation in slag structure is in agreement with the analysis of crystallization kinetics.
  • Multiphase Flow Behavior in a Single-Strand Continuous Casting Tundish during Ladle Change

    pp. 499-508

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    DOI:10.2355/isijinternational.ISIJINT-2019-506

    The three-phase flow behavior in a single-strand continuous casting tundish during ladle change was investigated using physical modeling. These phenomena observed from physical modeling were explained by employing the multiphase model volume of fluid, which can track the interface behavior between the liquid steel, slag, and air during this operation. The effects of the refilling time and lowest operating level on the slag entrainment and the steel exposure during ladle change were analyzed and discussed, respectively. Increasing the refilling time significantly decreased the amount of entrained oil and the exposed area in the impact zone during ladle change. However, the increase in the lowest level had little influence on reducing the slag entrainment. To reduce the slag entrainment and the steel exposure during ladle change, the refilling time in the prototype should be larger than 3 minutes. Furthermore, the use of the turbulence inhibitor has also been evaluated. By diminishing the turbulence intensity in the impact zone and the velocity magnitude at the steel-slag interface, the turbulence inhibitor reduced considerably the amount of entrained slag and the steel reoxidation. The results indicated that the emulsification phenomenon during ladle change could be eliminated using TI-2, and the maximum exposed area fractions in the impact zone for different refilling times and lowest levels were less than 13% and 23%, respectively. Therefore, the TI-2 was recommended to improve the steel cleanliness during ladle change.
  • Non-equilibrium Precipitation Behavior of TiC during Rapid Solidification of TiC-reinforced Wear-resistant Steel

    pp. 509-518

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    DOI:10.2355/isijinternational.ISIJINT-2019-535

    Precipitation behavior of TiC in TiC-reinforced wear-resistant steel was investigated using both thermodynamics and experiments. The carbide in TiC-reinforced wear-resistant steel is principally Ti-rich MC-type, which starts to precipitate at 1460°C in the solid–liquid zone. As the temperature decreased, there is no major change in the concentration of Ti and C in TiC. The results calculated by using equilibrium assumptions show that TiC can form when the solid fraction exceeds 0.24. However, the isolated primary TiC was observed in the sample cooled at rate of 162 K/s and 267 K/s, indicating that TiC can precipitate from the melts at the initial stage of solidification process. As an extension of Brody and Fleming’s model, the interface response functions relating the cooling rate with the interface composition and interface temperature, which consider interface non-equilibrium effect on the basis of incomplete mixing of solute in the liquid during solidification, were used to evaluate TiC precipitation behavior during rapid solidification for different cooling rates. In comparison with lower cooling rate, the effect of non-equilibrium behavior is strengthened under the condition of higher cooling rate, so that primary TiC can precipitate from the melts at the initial stage of solidification when a high cooling rate is imposed.
  • A Visual PCI Blockage Detection in Blast Furnace Raceway

    pp. 519-527

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    DOI:10.2355/isijinternational.ISIJINT-2019-367

    The pulverized coal injection (PCI) blockage detection is critical to the stable operation of blast furnace. In recent years, tuyere cameras have been widely applied, which provides a channel to detect the PCI blockage. However, the visual impression of images strongly varies between different raceways, it requires detection method should be robust and convenient to fine-tune for different blast furnace images. This paper presents an intelligent image-based method to detect the PCI blockage. An adaptive image preprocessing technique combining de-noising algorithm and image enhancement algorithm is applied to remove image noise and improve image quality, laying the foundation for subsequent work. The fitting ellipse based on Hough transform is used to locate the tuyere region, which can separate the tuyere region from the background. The adaptive threshold segmentation algorithm combining Otsu and Bernsen is used to obtain binarized image. However, it is difficult to obtain the pulverized coal cloud only by binarization due to the similarity between pulverized coal cloud and lance in gray-level. The multi-scale fully convolutional network (FCN) based on deep learning is investigated to detect the lance region, and pulverized coal cloud can be extracted by removing lance in binarized image. The flow rate of PCI can be characterized by the extracted area information to some extent, which can be used to detect PCI blockage. Extensive videos captured from real production lines are used to evaluate the detection method. The experiment results show that the method can accurately detect the PCI blockage.
  • 3D Crystal Orientation Mapping of Recrystallization in Severely Cold-rolled Pure Iron Using Laboratory Diffraction Contrast Tomography

    pp. 528-533

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    DOI:10.2355/isijinternational.ISIJINT-2019-405

    The mechanism of recrystallization texture development of cold-rolled metal and steel largely depends on the material chemical composition, cold-rolling reduction, and annealing treatment conditions. To clarify the mechanism, it is important to identify the locations where recrystallization starts and progresses within cold-rolled materials.Using laboratory diffraction contrast tomography (LabDCT), three-dimensional (3D) crystal orientation mapping corresponding to different stages of recrystallization has been successfully performed for pure iron sheets that were severely cold-rolled and heated at different temperatures.In cold-rolled iron with 99.2% reduction, the deformation texture was a strong α-fiber (RD//<110>). During annealing in the temperature range of 773–973 K, recrystallized grains were formed with textural components of {100}, {211}, {111} and {411}, and the α-fiber changed to the {100}<012> component. Recrystallized grains were generated at rather random locations within the sample. The size of recrystallized grains in the center region was 20–30% larger than that in the surface region. These results suggest that the nucleation is driven by the large strain caused by severe rolling. The number of recrystallization sites was larger in the surface region than in the center region and the competition of selective growth among recrystallized grains was more severe in the surface region, resulting in a smaller grain size.The volume data of the 3D crystal orientation mapping obtained by LabDCT provided crucial information for understanding the recrystallization mechanism including the nucleation and/or selective growth.
  • A Statistical Analysis on the Complex Inclusions in Rare Earth Element Treated Steel

    pp. 534-538

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    DOI:10.2355/isijinternational.ISIJINT-2019-428

    In present work the complex inclusions were investigated in the rare earth element treated steels. Based on the results of automated inclusion analysis in the La and Ce added wheel steel a post-processing method was proposed to correct the errors in size distribution, amount and chemical composition introduced by the double-threshold scan. According to the automatic result, the peaks for light-element and heavy-element inclusion were both in range of 0.14–0.53 µm, with the value of 66.5/mm2 and 25.9/mm2 respectively, and the results were corrected in post-processing to be 27.0/mm2 and 15.9/mm2 respectively. Meanwhile, the peak of number density for complex inclusion was in range of 0.9–1.3 µm with the value of 10.3/mm2. The amount of complex inclusions accounted for about one third of the total. La and Ce dominated in the complex inclusions due to their large size.
  • Enhanced Faster Region Convolutional Neural Networks for Steel Surface Defect Detection

    pp. 539-545

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    DOI:10.2355/isijinternational.ISIJINT-2019-335

    Bar steel surface defects detection is very important to steel production and quality control. Many traditional computer vision methods have been applied to industrial defects detection, but they are usually environmentally sensitive and not robust enough. In this paper, a deep learning defects detection method based on Faster Region Convolutional Neural Networks (Faster R-CNN) is proposed. Firstly, to solve the problem of missed detection of a large number of small defects, we introduce Weighted Region of Interest (RoI) Pooling instead of RoI pooling, which eliminates the area misalignment caused by the two quantization processes in the latter, and the small defects detection rate is significantly improved. Secondly, considering that most of the defects are irregular in shape, we use deformable convolution in upper layers to adapt to various shapes by learning the positional offset in convolution. Thirdly, owing to the diversity of bar steel defects, multi-scale feature extraction network with Feature Pyramid Networks (FPN) is proposed to build feature pyramids. Finally, we propose Strict-Non-Maximum Suppression (Strict-NMS) algorithm to reduce overlapping bounding boxes as much as possible. Experiments on defect datasets in real industrial environments show that the detection rate of this method can reach 97%, which is much higher than state-of-the-art methods.
  • Microstructure Development with Thermomechanical Processing in Alloy MA956

    pp. 546-555

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    DOI:10.2355/isijinternational.ISIJINT-2019-131

    Microstructural and texture development with thermomechanical processing was performed through a combination of cold-rolling and annealing, in MA956 plate consisting of a layered and inhomogeneous microstructure. The alloy contained in mass percent, 20 Cr, 0.02C, 4.8 Al, 0.4 Ti, 0.4 Y2O3, and the balance iron. The starting material was as-hot-rolled plate, 9.7 mm thick. The as-hot-rolled plate was subjected to 40%, 60%, and 80% cold-rolling reduction and subsequently annealed at 1000°C, 1200°C, and 1380°C. Assessment of microstructural and texture developments before and after cold-rolling and annealing was performed using light optical microscopy (LOM), Vickers hardness testing, and electron backscatter diffraction (EBSD). Locally introduced misorientations by cold-rolling in each region were evaluated by Kernel Average Misorientation (KAM) maps. The as-hot-rolled condition contained a layered and inhomogeneous microstructure consisting of thin and coarse elongated grains, and aggregated regions which consisted of fine grains and sub-grains with {100} <011> texture. The microstructure of the 40% cold-rolled condition contained deformation bands, and the 60% and 80% cold-rolled conditions also contained highly deformed regions with intersecting deformation bands. The magnitude of KAM angles varied through the thickness depending on the initial microstructures. Recrystallization occurred in regions where high KAM angles were dense after annealing, and nucleation sites were fine elongated grain regions, deformation bands, and highly deformed regions. The shape and size of the recrystallized grains varied depending on the nucleation sites.
  • Behavior of V-Ti Elements in Warm-Rolled Transformation-Induced Plasticity Steel with Medium Manganese Content

    pp. 556-563

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    DOI:10.2355/isijinternational.ISIJINT-2019-329

    To reveal the law governing V-Ti precipitation behavior in warm-rolled transformation-induced plasticity (TRIP) steel, two steel samples including 0.072V-0.051Ti steel (Bear-V-Ti steel) and 0.001V-0.001Ti steel (Free-V-Ti steel) are designed. Based on a comparative analysis, the former has an excellent combination of mechanical properties including a total elongation (TE) of 37%, ultimate tensile strength (UTS) of 935 MPa, and UTS×TE of 34 GPa·% after annealing at a temperature of 650°C. The superior mechanical properties are attributable to particles that precipitate in a composite form of (V-Ti)C/N at 650°C. These particles can refine the grains and improve the tensile strength of Bear-V-Ti steel. However, it should be noted that the two roles of precipitates in steel are size dependent. For sizes between 10–20 nm, the role mainly entails pinning the dislocation. This accounts for the precipitation strength. As the size increases to 20–60 nm, the role mainly involves pinning of the grain boundary, which leads to fine crystal reinforcement. These results provide useful data for the production of medium manganese steel using the warm-rolled process.
  • Effect of High Nitrogen Addition on Microstructure and Mechanical Properties of As-cast M42 High Speed Steel

    pp. 564-572

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    DOI:10.2355/isijinternational.ISIJINT-2019-417

    This study systematically investigated the influence of high nitrogen (N) addition (0.205 wt.%) on microstructure and mechanical properties of as-cast M42 high speed steel. The results demonstrate that the conventional and high-nitrogen M42 cast ingots are mainly composed of martensite, retained austenite and various precipitates (M2C, M6C as well as MC in M42 cast ingot or M(C, N) in M42N cast ingot). The addition of N could increase the retained austenite content, trigger the transformation of MC to M(C, N), favor the formation of M2C at the expense of M6C, and improve the distribution uniformity of M6C at the macroscopic scale. Moreover, the addition of N could lead to the reduction of the secondary dendrite arm spacing as well as the decrease of the thickness and area fraction of eutectic carbides, and improve the distribution uniformity of eutectic carbides at the microscopic scale. The M(C, N) particles form directly from the liquid phase prior to the formation of primary austenite, which could act as the heterogeneous nuclei of primary austenite and thus promote the refinement of the as-cast microstructure. The addition of N slightly decreases the macro-hardness and ultimate compression strength of the cast ingot but increases its ductility, which could be ascribed to the increase of retained austenite content and the reduction in the amount of eutectic carbides. Therefore, high N addition can significantly improve the as-cast microstructure of M42 high speed steel, which is promising for the further enhancement of the mechanical property and service life of the final product.
  • Flow Stress Measurement and Dynamic Response Analysis of Hot Compression Test Machine at High Strain Rates

    pp. 573-581

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    DOI:10.2355/isijinternational.ISIJINT-2019-426

    Flow stress is the most important information for hot strip rolling as it affects the rolling force and the thickness of the rolled product. A high-speed compression test up to a strain rate of 300 s−1, which is the compression speed of 3600 mm∙s−1 for a 12-mm-high cylindrical specimen, is necessary as a strain rate of 100–300 s−1 is the normal rate in the production of hot-strip-rolled steel sheets. An experiment is conducted using a servo-hydraulic compression test machine, which enables a high compression speed and a high temperature, but the oscillation is observed in stress-strain curve at high strain rate over 50 s−1. To determine the natural frequency of the compression test machine, the Savitzky–Golay filtering method is used for regression and the fast Fourier transformation (FFT) is adopted. To explain the mechanism of this phenomenon, a spring–mass–damper model is used and the results are compared with the FFT analysis result. After eliminating oscillation on the time versus load curve, a flow curve is obtained by inverse analysis, which compensates for the nonuniform strain rate, the inhomogeneous distribution of deformation, and the temperature increase during deformation.
  • Heterogeneous Nano-structure and its Evolution in Heavily Cold-rolled SUS316LN Stainless Steels

    pp. 582-589

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    DOI:10.2355/isijinternational.ISIJINT-2019-445

    Evolution of heterogeneous nano-structure in heavily cold-rolled SUS316LN stainless steels was investigated in detail. Transmission electron microscopic observations from the transverse direction (TD) of the 92% rolled specimen revealed the formation of a typical hetero-nano structure composed of ultra-fine lamellar grains embedded with deformation twin domains. The twin domains had prolate ellipsoidal shape elongated parallel to TD. Two types of twin domains with different crystallographical orientations to matrices could be identified, i.e., i) <211> // rolling direction (RD) and <110> // TD or ii) <110> // RD and <211> // TD, although all the {111} twining planes of both twin domains were oriented nearly parallel to the rolling planes. The ultra-fine lamellar grains were elongated along <100> direction and nearly parallel to RD. Deformation twins with a few nano-meter spacing were also frequently observed to develop in the lamellar grains. Evolution sequence of the hetero-nano structure during cold rolling was also investigated. At an early stage of rolling, deformation twins were gradually formed in the whole grains. Then, the regions fragmented grains by twins were further subdivided by a numerous number of shear bands inclined at about 20–45° from the RD, resulting in the formation of “eye-shaped” twin domains surrounded by shear bands and their crystallographical rotation. Cold rolling up to 50% caused a considerable increase in strength and decrease in ductility. While the strength was raised more with increasing reduction up to 92%, both the strength and ductility eventually slightly decreased by further rolling.
  • Development of Wide-range Viscometer and the Viscosity Measurement for SiO2–Na2O–NaF System

    pp. 590-596

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    DOI:10.2355/isijinternational.ISIJINT-2019-412

    A rotating crucible viscometer previously developed by the authors was improved in order to measure wide-range of viscosity values with high reliability. Two sets of inner cylinder and crucible with different geometries were employed as an attempt to accomplish this objective. The centers of these components were matched with accuracy of ± 0.1 mm by adopting X-Y stage for positioning. Relative error in torque measurement was less than 1.5%, and calibration curves had a good consistency for all experiments. By using the improved viscometer, a typical molten salt, LiF–10 mol% NaF–45 mol% KF (FLiNaK), a high-temperature low viscosity liquid, was measured and the results agreed with the theoretical values estimated from pure substances relations. Viscosity of the SiO2–Na2O–NaF system (CNa2O/CSiO2 = 0.67 in mol%) was obtained by varying concentration range from 0 to 80 mol% NaF. All melts’ composition tested followed Arrhenius-type temperature dependence, and a viscosity relation at 1473 K based on composition was also obtained. Drastic decreases in activation energy for viscous flow were observed at the concentrations of 5 and 80 mol% NaF.
  • Magnetic, Electrical, Thermal and Elastic Properties of High-Mn Electrical Steel

    pp. 597-601

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    DOI:10.2355/isijinternational.ISIJINT-2019-512

    We have studied influence of Mn doping on electrical resistivity, magnetic properties, specific heat and elastic constants of electrical steels. The electrical resistivity of the steel with Mn content of 1.5 mass% (HM steel) is by 13% higher than that of the steel with Mn content of 0.3 mass% (LM steel) at 5 K, and the difference decreases as temperature increases. In addition, magnetocrystalline anisotropy constant K1 of the HM steel is smaller than that of the LM steel by 15% or more in a wide temperature range. Despite of these benefits as electrical steel, the spontaneous magnetization of the HM steel is by 1.9% lower and specific heat is by 0.9% higher than that of the LM steel. Elastic constants of the two steels are almost the same. Moreover, Curie temperature of the HM steel is by 14 K lower than that of the LM steel.
  • Recycling Nickel Slag by Aluminum Dross: Iron-extraction and Secondary Slag Stabilization

    pp. 602-609

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    DOI:10.2355/isijinternational.ISIJINT-2019-173

    Nickel slag is a metallurgical solid waste from nickel refineries, which can be recycled as one of excellent secondary sources due to valuable iron contents. In this work, the approach of recycling nickel slag by aluminum dross was proposed, and the processes of network modification of slags and reduction were successively investigated at 1773 K. Upon the thermodynamic calculations, CaO was chosen as the modifier in order to obtain a higher activity of ‘FeO’, and basicity of the modified slag was determined as 1.0. Element mapping analysis of the modified slag showed that ‘FeO’ had been separated from the structure of nickel slag. After aluminothermic reduction for 120 min, the recovery degree of iron and copper was 94.35% and 97.89%, respectively. In addition, the secondary slag stabilization was discussed, and the utilization of the produced Fe–Cu alloy and the secondary slag was analyzed.
  • Optimization of Discharge Parameters for a Glow Discharge Emission Spectrograph with Two-dimensional Spatial Resolution

    pp. 610-612

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    DOI:10.2355/isijinternational.ISIJINT-2019-205

    This paper describes a specified measuring system for glow discharge emission spectrograph, which can provide a spatial/radial distribution of analyte atoms on the sample surface, while the conventional system provides only the information on the elemental distribution in depth direction. For this purpose, a spectrometer system consisting of an image spectrograph and an intensified charge coupled device (ICCD) detector was employed. The delay time and gate width of the ICCD detector was principally selected to improve the spatial resolution of the emitting zone. The objective of this paper was to determine an optimized set of the experimental parameter for better spatial resolution. The best spatial resolution was obtained when the gate width was 1 μs and the delay time was 60 μs. Better spatial resolution was obtained at narrower gate width, because the re-emission from the analyte atoms could be observed to a less extent when the observation was conducted more instantly just after start of the pulsed discharge.
  • Viscosity Measurements of CrO-bearing CaO–SiO2–5%Al2O3–CrO Slag Equilibrating with Metallic Cr

    pp. 613-615

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    DOI:10.2355/isijinternational.ISIJINT-2019-377

    This paper presents a fundamental investigation of the effects of CrO on the viscosity and degree of polymerization (DOP) of CaO–SiO2–5%Al2O3–CrO (R=0.5 and 0.8) slags for the purpose of efficiently recycling the valuable elements from the steelmaking slags. The results show that both Cr2O3 and CrO have a basic characteristic when R=0.5 and 0.8. The slag viscosity decreases with increasing CrO content and the CrO acts as a network modifier based on calculated DOP variations of quenched slags. The activation energy of the CrO content further validates these observations. The DOP is also found to increase with the addition of Al2O3.

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