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

ISIJ International Vol. 62 (2022), No. 7

  • Mechanical Properties of Cementite

    pp. 1313-1333

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    DOI:10.2355/isijinternational.ISIJINT-2022-048

    This review focuses on the mechanical properties of single-phase cementite. The mechanical properties of interest are 1) sound velocity, 2) elastic constants, 3) hardness, 4) plastic deformation mechanism, 5) wear, 6) fracture toughness, and 7) crystal orientation anisotropy. The effects of temperature, magnetic transition, and alloying elements on the sound velocity, elastic constants, and hardness were reviewed. Furthermore, experimental values of the above mechanical properties as well as other parameters, such as the specimen shape, the quantity of alloying elements, and the measurement method, were collected. A large variation was observed in the reported experimental values. The main reason for this is that cementite is metastable, and it is difficult to prepare large single-phase samples. Other factors, such as sample shape, measurement method, alloying element, magnetic transformation, and crystal orientation anisotropy, also influenced the measured values. Studies using first-principles calculations of cementite were also reviewed. The crystal orientation anisotropy of each elastic constant of single-crystal cementite based on the first-principles calculations are summarized, and its comparison with the experimental results is discussed. By comparing the elastic constants obtained by the first-principles calculations with the measured values, the former values of Young’s modulus and shear modulus are several % and bulk modulus and Poisson’s ratio are several tens of % larger than those of the latter. This is thought to be because of the difference in temperature between 0 K (first-principles calculations) and room temperature (measured value), and theoretical and experimental studies in which the temperature is changed are expected.
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    1. Recrystallization Behavior of IF Steel at the Interface of Aluminum Junction ISIJ International Vol.62(2022), No.7
    2. Strengthening via Grain Refinement in Lath Martensite on Low Carbon Fe–18Ni Alloys ISIJ International Vol.62(2022), No.7
    3. Crack Propagation Behavior of SNCM439 Steels in High-pressure Hydrogen Gas ISIJ International Vol.62(2022), No.7
  • Effect of B2O3 Addition to Slag on the Dynamic Change Behavior of Interfacial Tension between Liquid Iron and Molten Slag

    pp. 1334-1340

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    DOI:10.2355/isijinternational.ISIJINT-2021-605

    The interfacial tension between liquid steel and molten slag is an important consideration for the continuous casting process because it significantly affects the inclusion of mold flux in liquid steel when the metal/slag interface is disturbed. Specifically, dynamic changes in interfacial tension are observed when liquid iron and molten silicate slag are contacted. This behavior is explained well by a mechanism in which oxygen generated by the decomposition of SiO2 adsorbs at the interface, temporally decreasing interfacial tension, followed by the desorption of this oxygen from the interface to the bulk metal, recovering the interfacial tension. B2O3 has emerged as an alternative to fluorides as a component of mold flux that controls its physicochemical properties, such as liquid viscosity and crystallization behavior. However, its effect on interfacial-tension dynamics is not currently understood. Accordingly, in this study, the dynamic interfacial tension between liquid iron and molten B2O3-bearing slag was investigated by the floating lens method. When B2O3-bearing slag was used, the interfacial tension significantly decreased to a minimum in the initial stage then slowly increased and finally recovered to the value observed when the B2O3-free slag was contacted with liquid iron. The decomposition of both B2O3 and SiO2, providing oxygen to the metal/slag interface, was proposed as a reason for the significant decrease in interfacial tension. Furthermore, it was proposed that the B2O3 decomposition occurs continuously, which causes the slow increase of interfacial tension after the initial stage.
  • Thermodynamic Activity of SiO2 in CaO–SiO2–Al2O3–MnO–MgO System Molten Slags

    pp. 1341-1351

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    DOI:10.2355/isijinternational.ISIJINT-2022-018

    To understand the thermodynamic characteristics of the reaction between Al and SiO2 during the continuous casting process of the high-aluminum steel, the activity coefficients of SiO2 and MnO in molten slags of (18–43%) CaO-(33–64%) SiO2-(9–21%) Al2O3-(2–3%) MgO-(<2.4%) MnO were measured at 1400°C, 1450°C and 1500°C by the experiments of the Si and Mn equilibrium between liquid copper and molten slag in the graphite crucible under the mixed gas atmosphere of CO and Ar. The effects of SiO2, Al2O3, the basicity, the radio of CaO/Al2O3 and the temperature on the activity coefficients of SiO2 and MnO in molten slag were discussed. The quadratic regression relationships among the activity coefficient of SiO2 or MnO, the concentration of component, the basicity and the temperature were investigated by the regression analysis method.
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  • Wettability of Molten Fe–Al Alloys against Oxide Substrates with Various SiO2 Activity

    pp. 1352-1362

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    DOI:10.2355/isijinternational.ISIJINT-2022-093

    The contact angle between molten Fe–Al alloy with 0.03, 0.3, and 3 mass% Al composition, and Y2O3 matrix oxide substrate with 0.002, 0.32, and 1 SiO2 activity was measured using sessile drop method in Ar atmosphere at 1873 K, and the interfacial tension was evaluated. The contact angle and interfacial tension between the molten Fe-0.3 Al alloy and the Y2Si2O7 + SiO2 (aSiO2 = 1) substrate decreased over time during 60 s after the molten alloy was dropped onto the substrate. The decrease of the contact angle was 20°, and that of the interfacial tension was 628 mN·m−1 Conversely, the other contact angles and the other interfacial energies were almost stable during the same period. The decrease of the contact angles ranged between 0° and 7°, and that of the interfacial tensions ranged 4 and 195 mN·m−1. By observing the wetting behavior for 60 min, it was recognized that the interfacial reaction between the Fe–Al alloy and the oxide substrate was the redox reaction between Al composition in the alloy and SiO2 composition in the substrate, composed of SiO2 decomposition reaction and Al2O3 formation reaction between oxygen absorbed at the interface and Al composition in the alloy. In addition, it was indicated from the interfacial tension dependence on SiO2 activity that the medium SiO2 volume slag for the molten low-Al steel and the low SiO2 volume slag for the molten high-Al steel were effective in preventing the small droplets of molten slag into the molten steel.
  • Effect of Granule’s Moisture on Granulation Rate of Iron Ore

    pp. 1363-1370

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    DOI:10.2355/isijinternational.ISIJINT-2021-568

    For countermeasures of deterioration in iron ore, development of pretreatment technology for high quality iron ore fine is critical issue in the sintering process. However with increasing small particles, sinter productivity is decreased by lower permeability due to increment of un-granulated particles or considerably large granules. Therefore understanding of granulation phenomena becomes more important issue. Since moisture of iron ore largely relates to the granulation behavior, it is necessary to reveal the effect of moisture on granulation rate for controlling the process. In this study, granulation tests were performed and the size distribution of granule was analyzed with probability model and new model considering granulation rate. Based on the calculation results, the effect of moisture on granulation rate was discussed.
  • Prediction of Pulverized Coal Combustibility by Measuring Chemi-luminescence of Radical Species around Tuyere

    pp. 1371-1380

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    DOI:10.2355/isijinternational.ISIJINT-2021-558

    To develop novel technologies for reducing CO2 emission from ironmaking process, steelmaking companies in Japan are engaging in the national project called COURSE50. In this project, a large amount of H2 is co-injected into blast furnace (BF) with pulverized coal (PC). In order to clarify how the co-injection affects PC combustibility, PC combustibility should be measured in COURSE50 BF. However, it was difficult to measure PC combustibility in BF without bothering operation. Therefore, we developed the non-contact measurement method for PC combustibility (ηPC) by utilizing chemi-luminescence spectra of PC combustion field in the lab-scale combustion furnace. We also applied the method to the COURSE50 experimental BF to ensure the accuracy of the method. And then we found that;1) Several chemi-luminescence peaks of hydrocarbon radicals and coal ash were detected in the combustion field of PC. Among these peaks, the peak intensity ratio of OH radicals to CH radicals was most appropriate for deriving the formula for the ηPC estimation.2) In the result of the measurement in COURSE50 experimental BF, ηPC in the experimental BF that estimated by the formula agreed well with the results in lab-scale combustion furnace. The validity of the method for estimating ηPC was confirmed.
  • Determining Optimum Water Content for Iron Ore Granulation using Agitation Torque of Wet Ore Powder

    pp. 1381-1388

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    DOI:10.2355/isijinternational.ISIJINT-2022-009

    Wet granulation of iron ore powders is a key process in ironmaking. In wet granulation, it is important to determine the optimum content of water added to the original ore powders. To determine the optimum water content, it is important to understand the saturation state in wet ore powder, which can be done by measuring the agitation torque of the wet powder. This study proposes a methodology for determining the optimum water content of various iron ore powders using the agitation torque of wet ore powders. First, measurement of the agitation torque and wet granulation of various iron ore powders were conducted. By comparing the results, it was found that the optimum water content, which was defined as the minimum water content required to diminish fine particles in the original powder, corresponded to the water content exhibiting the maximum agitation torque, regardless of the original powder. Using the agitation torque at different water contents, the saturation degree S, which is the volume ratio of water to the interparticle voids, was calculated, resulting in a range of 0.999 ≤ S ≤ 1.173 at the optimum water content. This suggests that the state between the funicular and capillary states is a suitable saturation state for the wet granulation of ore powders. Consequently, it was demonstrated that it is possible to determine the optimum water content for wet granulation of various iron ore powders based on the water content exhibiting the maximum agitation torque of wet ore powders.
  • Solute Concentration Distribution in the Vicinity of Solid-Liquid Interface under the Imposition of a Time-Varying Force

    pp. 1389-1395

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    DOI:10.2355/isijinternational.ISIJINT-2021-177

    Mass transfer is often the rate determining step for solid-liquid reaction, such as an electroplating process in automotive industry and a refining process in metallurgical industry. The decrease of concentration boundary layer thickness through the excitation of convection is adapted to enhance the solid-liquid chemical reaction rate. Therefore, traditional methods excite a macro-scale flow in the bulk liquid. Because the concentration boundary layer exists in the velocity boundary layer, the traditional methods have the limitation in enhancing mass transfer rate. Therefore, a new method was proposed, which imposes force directly near the solid-liquid interface. In the past research, force, with or without an oscillating component was imposed near the solid-liquid interface during the dissolution of a Cu anode into a Cu2+ aqueous solution. The increase of Cu2+ concentration under the force imposition with oscillating component was suppressed compared to that by imposing the force without oscillating component just above the center of the anode. This research evaluated the dissolved Cu2+ concentration distribution and the liquid flow pattern in the whole vicinity of the solid-liquid interface under the force imposition with or without oscillation component. The results indicated that by imposing the force with oscillating component, the increase of the Cu2+ concentration was suppressed in the whole vicinity of the solid-liquid interface, and the Cu2+ concentration distributed more uniformly near the solid-liquid interface. This might be because of the excitation of circulating micro-scale flows near the side parts of the anode surface.
  • Analytical Evaluation of Heat Flow in Oxygen Steelmaking

    pp. 1396-1407

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    DOI:10.2355/isijinternational.ISIJINT-2021-378

    Dynamic models for a BOF (Basic oxygen furnace) process have been developed to quantitatively capture the variation in refining kinetics and heat flow during the blowing period. Previous studies have developed extensive models to understand the kinetics and reaction mechanisms for a BOF process. The understanding of heat flow during the refining period helps in optimizing the process interms of energy consumption and minimizing the overall excess heat (heat losses). The present work aims to develop an approach that will capture the heat flow during the blowing period once the refining profiles are known. In the present calculation the instantaneous heats were calculated based on the refining profiles from Cicutti et al.1) industrial trial and Imphos pilot plant data.2) The developed model reveals different possibilities for process optimization by identifying the periods of overall excess energy formation. It was observed by compiling Cicutti’s data that an average of 7.5 MJ/min/t of hot metal excess heat is generated during the blowing period spanning from 6 to 14 minutes. Moreover, the present study demonstrates that the parameters such as scrap quantity, flux added, and oxygen flowrate can play a vital role in optimizing the process in terms of the utilization of the overall excess energy during the refining period.
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    1. Numerical Simulation of Powder Spraying at the Bottom of Converter Based on Gas-liquid-solid Coupling Model ISIJ International Vol.62(2022), No.7
  • Water Model Study on the Flotation Behaviors of Inclusion Clusters in Molten Steel

    pp. 1408-1417

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    DOI:10.2355/isijinternational.ISIJINT-2021-551

    To improve the performance of steel and produce clean steel with higher quality, the inclusions in the molten steel should be removed and controlled strictly. The removal behaviors of the inclusions, such as flotation, are largely dependent on their size and morphology, which determines the cleanliness of molten steel. It is thus of great importance to clarify the floating behaviors of inclusion clusters with different morphologies. However, the previous work on the floating behaviors of the inclusions is mostly on basis of the spherical inclusions and the influence of the morphology of the inclusions has not been clarified systematically. In this study, the 3D structure of inclusion clusters was established by 3D modeling and fabricated by 3D Printing Technology. A series of water model experiments were performed to investigate the influence of diameter and fractal dimension of inclusion clusters on the terminal floating velocity. In this study, the formula of the terminal floating velocity of the inclusion clusters is derived from the experimental results based on the previous work, which considers the influence of the fractal dimension of the inclusion clusters. It would provide fundamental supports for the research on the removal behaviors of inclusion clusters in molten steel.
  • Numerical Simulation of Powder Spraying at the Bottom of Converter Based on Gas-liquid-solid Coupling Model

    pp. 1418-1429

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    DOI:10.2355/isijinternational.ISIJINT-2022-020

    In this work, the powder injection process at the bottom of the converter was numerically simulated by establishing a coupled gas-liquid-solid mathematical model. The effects of powder injection speed, solid-gas ratio, particle size, and injection position on the trajectory and residence time of particles in the molten pool are studied. The discrete phase and continuous phase coupling solution method is used to analyze the change of the molten pool flow field after powder injection. It is found that increasing the spraying rate can reduce the particle concentration near the liquid surface from 2.3 kg/m3 to 1.18 kg/m3. Increasing the solid-gas ratio from 10 kg/m3 to 30 kg/m3 can increase the powder distribution ratio from 70.9% to 93.1%. The larger the size of the particles, the easier it is to stay near the liquid level, and the maximum can reach 2.13 kg/m3. Finally, it was also found that spraying powder at 0.7 R can make the powder more uniformly distributed in the molten pool.
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  • Carbide Characteristics of High Vanadium High-speed Steel Manufactured by Electroslag Remelting

    pp. 1430-1438

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    DOI:10.2355/isijinternational.ISIJINT-2021-490

    The carbide characteristics of high vanadium high-speed steel (W3Mo4Cr5V6) manufactured by electroslag remelting (ESR) are analyzed by field emission scanning electron microscopy (FESEM), electron probe microanalysis (EPMA) and micro-Vickers. The results illustrate that three types of carbides are mainly formed in W3Mo4Cr5V6 which includes the lumpy and strip-like MC in the intracrystalline, lamellar M2C at grain boundary with discontinuous network and globular secondary carbides dispersed in matrix, among these, the eutectic strip-like MC occupies the main part. The more uniform distribution and finer size for both MC and M2C are obtained with high solidification rate (SR). Both 2D and 3D morphologies of MC carbides show obvious symmetry with clustered distribution and various shapes. The MC and M2C are rich in V and Mo, W respectively, the compositional characteristics are not only related to the carbide type and morphology, but also the precipitation order and distribution density. The Vickers hardness of primary MC, eutectic MC and M2C are measured and the average values are 2205 HV, 850 HV and 896 HV respectively which includes the adverse effects of soft matrix on the test results of hard carbides. A new and exact test method for pure carbides should be developed in the future.
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  • Removal of Inclusions using Swirling Flow in a Single-Strand Tundish

    pp. 1439-1449

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    DOI:10.2355/isijinternational.ISIJINT-2021-600

    Swirling flow tundish was developed to enhance the coalescence of inclusions, so as to deeply clean the liquid steel. Inclusions would gather to the center of the swirling flow by centripetal force, due to the density difference between inclusions and liquid steel. Thus, small inclusions can coalesce into larger ones, and then float to the free surface by their self-buoyance. Physical experiments were carried out in a 1/2.5 scale single strand tundish to study the flow characteristics of tundish with swirling chamber. Numerical modeling was developed to simulate the movements of small inclusions in swirling flow. Discrete phase model was employed together with the O’Rourke algorithm to characterize the coalescence of the inclusions in the swirling flow. The removal of inclusions was investigated, considering the absorption by upper slag and trapping by outside wall of ladle shroud. Compared with a turbulence inhibitor, a swirling chamber shows a similar effect on flow improvement, while performs better in inclusion removal, owing to the inclusion coalescence caused by centripetal force. The results revealed that swirling chamber in diameter of 450 mm is an optimized scheme for deep cleaning of liquid steel, with only 1.66% of the inclusions flowing out of the tundish nozzle.
  • Prediction Study on Vortex Center Position and Surface Velocity in a Steel Continuous Casting Slab Strand Using Mathematical Modeling

    pp. 1450-1460

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    DOI:10.2355/isijinternational.ISIJINT-2022-050

    In the current study, the movement of the vortex center position and the prediction of the maximum velocity at the top surface with different casting parameters were studied in a steel continuous casting slab strand using the Eulerian–Eulerian approach. One, two, and three vortexes were generated under the flow pattern of single roll flow, double roll flow, and complex roll flow, respectively. The vortex center position migrated from the meniscus to the submerged entry nozzle in the upper recirculation zone and moved downward along the mold height in the lower recirculation zone with the increasing of the casting speed, respectively. With the increasing of the argon flow rate, the movement trajectory of vortex center was opposite to the increasing of the casting speed. The vortex center position moved from the meniscus to the submerged entry nozzle with the outport angle of submerged entry nozzle increased and migrated from the submerged entry nozzle to the meniscus with mold width increased. In addition, nonlinear fitting for the maximum velocity of the molten steel at the top surface under different cast parameters was performed, and the regression equation was verified by nail board measurements The on-line prediction of the maximum velocity at the top surface was realized.
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  • Surface Quality Evaluation of Heavy and Medium Plate Using an Analytic Hierarchy Process Based on Defects Online Detection

    pp. 1461-1468

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    DOI:10.2355/isijinternational.ISIJINT-2021-585

    Heavy and medium plate (HMP) is a valuable and irreplaceable material that is widely used in pipelines, bridges, ships, building construction, and power plants. The quality of a plate’s surface is closely related to its strength, hardness, and corrosion resistance. HMP is still trimming to length after the continuous casting process. The goal of this research is to evaluate surface quality quickly by combining online detection information of normal and periodic defects with professional quality control skills. To begin, this study creates a set of assessment criteria for HMP surface quality based on length, total defect area, and total area of various types of surface defects on the plate. The analytic hierarchy process (AHP) is then developed to identify based on the operator’s experience and expert knowledge, the weights of the classified defects are then determined using the analytic hierarchy process (AHP). Finally, the evaluation grades for each plate may be calculated. The evaluation findings may benefit not only in improving production efficiency, lessening labor intensity, and reducing waste caused by cutting to length, but also in boosting the intelligent control capability of the HMP manufacturing technique.
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  • Recrystallization Behavior of IF Steel at the Interface of Aluminum Junction

    pp. 1469-1477

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    DOI:10.2355/isijinternational.ISIJINT-2021-505

    Two cold-rolled IF steel sheets sandwiching an aluminum sheet were heat-treated at 650°C, and the recrystallization behavior in the interfacial region between the IF steel and pure aluminum was observed and analyzed. The tongue-like structure of the η-Fe2Al5 phase appeared in the reaction zone and the recrystallized structure of the IF steel was characterized by an equiaxed structure including subgrains in the region surrounded by the η phase. In contrast, a bcc-ferrite phase elongated with a pancake shape was mainly observed in the other regions. The η phase grew preferentially along the c-axis, and the growing η phase caused distortion of the surrounding iron due to the difference in the molar volume. This suggests that compressive strain is generated along both a-axis and b-axis, and tensile strain is generated along c-axis of the η phase. The growth of the η phase obviously changed the recrystallization behavior of the IF steel. The development of the γ-fiber crystalline texture (<111>/ND), a typical recrystallization texture of IF steel, was not observed in the region surrounded by the tongue-like η phase. The local misorientation in the vicinity of the two-phase interface could not explain the texture change. It is suggested that the growth of the tongue-like η phase caused constraint of the surrounding region and affected nucleation in the recrystallization of the ferrite.
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    3. Mechanical Properties of Cementite ISIJ International Vol.62(2022), No.7
  • Modelling of Sigma Phase Dissolution by Solution Treatment in Super Duplex Stainless Steel

    pp. 1478-1484

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    DOI:10.2355/isijinternational.ISIJINT-2021-587

    The modelling of the dissolution behavior of the sigma phase by solution treatment in duplex stainless steel was investigated to clarify the condition of heating temperature and time necessary to finish dissolving the harmful phase into the matrix. To determine that dissolution condition of the sigma phase experimentally, the heat treatment test at various temperatures and durations was conducted after pre-aging to form an amount of sigma phase using the super duplex stainless steels with 25%Cr-6/7%Ni-3/4%Mo-0/2%W-0.3%N. Using the experimental results, the model which considering the effect of initial situation of sigma phase on the condition of solution treatment to achieve the dissolution condition, was suggested. The solution treatment at higher temperature and for longer heating time was necessary to dissolve the retained sigma phase in the initial microstructure consisting of two phases of austenite and sigma without ferrite compared to that with ferrite phase. The critical condition by the experimental results was described as a linear function of the inverse of solution temperature and the logarithm of holding time. The effect of the chemical composition of the steel on that critical condition was reflected in a parameter in that function proposed.
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    3. Mechanical Properties of Cementite ISIJ International Vol.62(2022), No.7
  • Weld Defect Cascaded Detection Model Based on Bidirectional Multi-scale Feature Fusion and Shape Pre-classification

    pp. 1485-1492

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    DOI:10.2355/isijinternational.ISIJINT-2022-035

    Object detection algorithms like Faster R-CNN have been widely used in the field of industrial defect detection. For weld defect detection, its detection accuracy for some small targets and difficult-to-classify defects is not high. This paper proposes a Cascade R-CNN detection model for weld defects based on bidirectional multi-scale feature fusion and shape pre-classification. There are defects of different sizes in the weld. In order to improve the detection ability of the model for multi-size defects, the model adopts the bidirectional feature pyramid network, in which an extra bottom-up path after the top-down path aggregation network and an extra edge from the original input to output node are added. According to the statistics of the proportion distribution of long and short axes of weld defects, the defects can be divided into two categories: long strip defects with the proportion of about 2:1 and approximate circle defects with a much bigger proportion. Therefore, each cascade detector is connected in parallel with a two-categories classifier for long strip and approximate circle defects and a five-categories classifier for five specific defects, so as to realize the pre-classification of two morphological defects and mine the difference between the two shapes of defects. In order to avoid over fitting caused by small datasets. Firstly, noise is added to augment the data. Then the training samples are expanded by random flip and mirror in the training, and OHEM is introduced to balance the selection of positive and negative samples. The experimental results show that the detection accuracy of the model on small targets and difficult-to-classify defects is significantly improved. The mAP value is increased by about 9.3% compared with the traditional Faster R-CNN and about 3.3% compared with the traditional Cascade R-CNN.
  • Preferential Site for Scaling on Carbon Steel with Corrosion Products

    pp. 1493-1501

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    DOI:10.2355/isijinternational.ISIJINT-2022-021

    Although it has been pointed out that corrosion products are the preferred scaling site, the detail research has not been conducted. In this study, the initial scaling sites on carbon steel with corrosion product were investigated and scaling mechanisms were discussed. Carbon steel sheets were immersed in a solution supersaturated condition for magnesium silicate under normal standard state. Scaling at a corroded part on carbon steel was easier to occur than that at non-corroded part on carbon steel. The corrosion product was comprised of Fe2O3 (Hematite), Fe3O4 (Magnetite), and β-FeOOH (Akaganeite). When the particles of Fe2O3, Fe3O4, and β-FeOOH were individually immersed in the solution, the formation of magnesium silicate occurs only on β-FeOOH. One of the preferred scaling sites for magnesium silicate was β-FeOOH. The physical and chemical interactions were investigated. The physical interactions were evaluated by zeta potential, and the results suggested that the repulsion occurs between them. On the other hand, the chemical interaction was evaluated by IR and Raman analyses. Only IR spectrum of β-FeOOH changed. The change was derived from absorption range of Fe–OH in β-FeOOH. The OH group in β-FeOOH may react with silanol group by the dehydration-condensation reaction.
  • Strengthening via Grain Refinement in Lath Martensite on Low Carbon Fe–18Ni Alloys

    pp. 1502-1511

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    DOI:10.2355/isijinternational.ISIJINT-2022-036

    The lath martensite structure in steel offers high strength with a complex substructure, and its strength increases with carbon content. However, the mechanism of carbon strengthening is yet to be elucidated. In this study, we evaluate the tensile properties of as-quenched lath martensite without retained austenite in Fe–18Ni alloys containing 4–570 ppm carbon. In the 4 mass ppm carbon alloy, whose carbon is almost trapped by Ti(CN) particles, the work hardening behavior during uniform elongation is constant regardless of the size of the effective grain surrounded by a high angle boundary. In contrast, the yield point (YP), 0.2% proof stress (σ0.2%), 0.6% proof stress (σ0.6%), and maximum tensile strength (TS) in 7, 110, and 570 mass ppm carbon alloys increase with the refinement of their effective grain, consistent with the Hall–Petch relationship. The Hall–Petch intercepts for the YP, σ0.2%, σ0.6%, and TS are constant and unaffected by the carbon content. This suggests that the non-occurrence of solution hardening by solute carbon atoms in the lath martensite. The Hall–Petch coefficients for the YP, σ0.2%, σ0.6%, and TS increase with carbon content and are proportional to the square root of the carbon content. This indicates that the increase in carbon content increases the strength of the lath martensite via the refinement of effective grains and the increase in the effectiveness of grain refinement.
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  • Analysis of the Oxidation and Nitridation of Ti-17 (Ti-5Al-2Sn-2Zr-4Mo-4Cr) Alloys with Added Si under Atmospheric Heating

    pp. 1512-1521

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    DOI:10.2355/isijinternational.ISIJINT-2022-053

    We investigated the oxidation and nitridation of a near β-type Ti-17 (Ti-5Al-2Sn-2Zr-4Mo-4Cr) alloy with up to 1 mass% of added Si. The alloys were heated in air at 923–1223 K for up to 518.4 ks. The mass gain was determined, and the scales and oxygen-rich α-case layer that were formed were analyzed. At temperatures below 1123 K, the primary product was rutile-type TiO2, along with traces of Al2O3. The presence of Si4+ decreased the oxygen vacancy concentration in TiO2, whereas the presence of SiO2 caused the relaxation of the internal stress in the scale. These factors were instrumental in lowering the parabolic rate constant and expanding the region conforming to the parabolic rate law. A TiN layer was detected beneath the oxide layer at 1223 K. At the interface between the TiN layer and the Ti alloy, a Sn-rich layer was observed. Both the presence of the Sn-rich layer and the incubation period following the formation of the Sn-rich layer play a crucial role in the formation of the TiN layer. Meanwhile, Si addition had no discernible effect on the formation of the α-case. The parabolic rate constants that were predicted via machine learning concurred well with the measured values. The current study is the first report that describes the scale and α-case formation in Ti-17 alloys, and it may serve as a guideline for determining the optimal application conditions, designing a practical manufacturing process, and improving the oxidation resistance of Ti-17 alloys with Si.
  • Effect of Solution Temperature on Electrodeposition Behavior of Zn–Ni Alloy from Alkaline Zincate Solution

    pp. 1522-1531

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    DOI:10.2355/isijinternational.ISIJINT-2022-076

    Zn–Ni alloys were electrodeposited on a Cu electrode at 10–500 A·m−2 and 5 × 104 C·m−2 in an unagitated zincate solution at 293, 313, and 333 K. The effect of solution temperature on the electrodeposition behavior of Zn–Ni alloys from alkaline zincate solutions was investigated. The transition current density at which the deposition behavior shifted from a normal to an anomalous co-deposition was almost identical at 293 and 313 K but increased at 333 K, due to enhanced H2 evolution and Ni deposition at 333 K. The current efficiency for alloy deposition increased with solution temperature in both the normal (10–50 A·m−2) and anomalous (500 A·m−2) co-deposition regions. In the normal co-deposition region, Ni deposition and H2 evolution mainly occurred, and the current efficiency increased with solution temperature due to the stronger promotional effect of increase in solution temperature on Ni deposition. In the anomalous co-deposition region at 500 A·m−2, Zn deposition and H2 evolution mainly occurred, and Zn deposition appeared to proceed based on a mixed rate-determining process comprising the charge transfer and diffusion of Zn ions. The current efficiency increased with solution temperature due to the acceleration of the Zn ions diffusion. The Ni content in the deposited films increased with the solution temperature at all the current densities, since Ni deposition was accelerated to a greater degree than Zn deposition by increasing the solution temperature in the region where the charge transfer process was rate-limiting. The γ-phase of the deposited films also increased with increasing solution temperature.
  • Strengthening of Super Invar Cast Steel by Precipitation of Intermetallic Compounds

    pp. 1532-1539

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    DOI:10.2355/isijinternational.ISIJINT-2021-602

    In previous studies, we reported a cryo-annealing process consisting of subzero treatment and subsequent annealing as a novel heat treatment to overcome the low rigidity and strength caused by the coarse solidified <100> texture in super invar cast steel. In the present study, aging was added to the cryo-annealing process of Ti and Si-bearing super invar cast steels for further strengthening. By aging at 723 K after the subzero treatment, the intermetallic compounds L12–Ni3Ti and Ni3Si were finely precipitated within lenticular martensite, but not within untransformed austenite. This is because of the higher driving force for precipitation and faster atomic diffusion in the body-centered cubic matrix of martensite, as well as the high-density lattice defects that acted as preferential nucleation sites. The precipitated intermetallic compounds remained even after the matrix reverted from martensite to austenite, which led to significant strengthening of the reversed austenite. The high-strength reversed austenite contributed to the enhanced strain hardenability in the duplex structure composed of untransformed and reversed austenite, and consequently, the tensile strength was enhanced to over 800 MPa while maintaining an excellent low thermal expansion coefficient. Furthermore, an analysis of the strength mechanism based on the Orowan and Ashby-Orowan models reveals that the intermetallic compounds precipitated in reversed austenite acted as weak obstacles against dislocation motion.
  • Crack Propagation Behavior of SNCM439 Steels in High-pressure Hydrogen Gas

    pp. 1540-1547

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    DOI:10.2355/isijinternational.ISIJINT-2022-017

    To investigate the effect of high-pressure hydrogen gas on the fracture of high-strength low-alloy steels, rising load tests were conducted on JIS SNCM439 steel in high-pressure (20 MPa) hydrogen gas at room temperature (20–25°C), and its hydrogen-induced crack initiation behavior was investigated. The load-crack opening displacement curve obtained for rising load tests in hydrogen began to deviate from that obtained in air at very low loads, indicating that the stress intensity factor at crack initiation was significantly smaller in hydrogen. Scanning electron microscopy observations of the fractured surfaces of the specimens unloaded during the middle of the rising load test confirmed that hydrogen-induced cracks had already occurred at a load lower than the deviation point. The stretch zone that appeared in the rising load test in air was not observed for the test in hydrogen, and the hydrogen-induced cracks were found to directly initiate from the tip of the fatigue pre-crack. The hydrogen-induced cracks were initiated at almost the same stress intensity factor value as that at which the stretch zone was observed in air, indicating that plastic slip and the resulting hydrogen ingress from the new surface were the causes of the hydrogen embrittlement. In addition, it was shown that the stress intensity factor for crack initiation in hydrogen increased and the effect of hydrogen decreased with the increase in loading rate, inferring that dislocation migration and hydrogen penetration into the steel are key factors for hydrogen embrittlement.
  • Effect of Neodymium Content on Creep Properties of 9Cr-3Co-3W-Nd-B Steel

    pp. 1548-1555

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    DOI:10.2355/isijinternational.ISIJINT-2022-057

    The effects of the neodymium content on the creep properties and microstructures of 9Cr-3Co-3W-Nd-B steel were investigated. Neodymium had a mild effect on the creep rupture strength at contents up to 0.056 mass%. This suggested that the effects of neodymium compounds and solid-dissolved neodymium were minimal in the microstructures and uniformly creep-deformed parts after normalizing and tempering heat treatment. On the other hand, the reduction of area after creep rupture was improved by the addition of neodymium. Creep rupture occurred at the prior austenite grain boundaries on steel without neodymium. Therefore, neodymium conclusively adhered to the segregated sulfur at the prior austenite grain boundaries to suppress the formation of creep cracks.
  • A Novel Process for Separation of Magnetite and Phosphorous Phases from a CaO–SiO2–FeO–P2O5 Slag

    pp. 1556-1559

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    DOI:10.2355/isijinternational.ISIJINT-2021-578

    Iron and phosphorus were successfully separated from CaO–SiO2–FeO–P2O5 slag through atmospheric control, B2O3 addition and a combination of magnetic separation and flotation. For the slag with basicity (CaO/SiO2) of 2.5 and B2O3 addition of 6% (weight percentage), iron and phosphorus in the slag were enriched in the form of magnetite (Fe3O4) and calcium phosphate (Ca10P6O25) phases respectively under Ar atmosphere. Using a combination of magnetic separation and flotation, the concentrates were obtained with Fe3O4 and P2O5 content of 92.84% and 37.66% respectively, corresponding to the recovery ratios of 85.8% for iron and 91.3% for phosphorus.
  • Correction of the parameters reported in the paper “Hot Coke Strength in CO2 Reaction” [ISIJ International, Vol. 62 (2022), No. 3, pp. 606-608]

    pp. 1560-1560

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    DOI:10.2355/isijinternational.62.1560

    https://doi.org/10.2355/isijinternational.ISIJINT-2021-305 The authors have found that Figure 3 reported in the above paper is incorrect. The authors would like to correct the figures: Original figures (incorrect)[ This part of text is omitted ]Corrected figures[ This part of text is omitted ]Fig. 3. Stress-strain curve of coke samples: (a) Coke A; (b) Coke C. 01 and 02 are sample numbers. (Online version in color.)
    x

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  • Editorial Board, Copyright Page and Announcements

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