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

ISIJ International Vol. 62 (2022), No. 3

  • Interaction Coefficients of Mo, B, Ni, Ti and Nb with Sn in Molten Fe–18mass%Cr Alloy

    pp. 405-412

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    DOI:10.2355/isijinternational.ISIJINT-2021-395

    Increasing the utilization of steel scrap is strongly required for reducing CO2 emission in iron- and steel-making processes. In steel scrap recycling, the content of tramp elements in steel (such as copper and tin) inevitably increases. Accordingly, it is important to understand the thermodynamic characteristics of relevance to the accumulation of tramp elements in molten steel. The values of the interaction coefficients of Mo, B, Ni, Ti, and Nb with Sn in molten iron were reported previously. However, little is known about the interaction coefficients of alloying elements with tramp elements in molten high-chromium steel. In this work, the interaction coefficients of Mo, B, Ni, Ti, and Nb with Sn in the molten Fe–18mass%Cr alloy were measured at 1873 K by a chemical equilibration technique that uses the liquid immiscibility of the Fe–18mass%Cr alloy and Ag, yielding the following results:[ This part of text is omitted ]The results show that the values of the interaction coefficients of M with Sn in the Fe-18mass%Cr alloy are smaller than those for molten iron, which were measured in the previous work, except for titanium. The interaction coefficients of M with Sn in Fe and Fe–18mass%Cr alloy were estimated based on a regular solution model. The estimated interaction coefficients of B, Ni, and Ti with Sn in molten iron and Ni and Ti with Sn in the molten Fe–18mass%Cr alloy reasonably agree with the measured values.
  • High Magnetic Field Effects on Cu-precipitation Behavior of Fe-1mass%Cu at 773 K

    pp. 413-417

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    DOI:10.2355/isijinternational.ISIJINT-2021-404

    Tramp elements in steel, such as Cu and Sn, cannot be removed by acid treatment. Since these elements condense by repeating recycling process, leading to deterioration of strength. Therefore, the methods for avoiding condensation or removing tramp elements are required. In this study, in-magnetic-field annealing process was focused on because magnetic field is effective for diffusion, phase transformation, phase diagram and precipitation. In-magnetic-field annealing of Fe-1mass%Cu at 773 K was performed in 5 and 10 T for investigating precipitation behavior of supersaturated Cu. From microstructural observation, precipitation of Cu-rich phase in Fe-matrix, and magnetic field effect on itself were not observed clearly. Increase of the hyperfine field was detected for the samples annealed at 5 T by Mössbauer spectroscopy, indicating the enhancement of the Cu-precipitation. On the contrary, hyperfine field for 10T-annealed sample was slightly smaller than that for 0 T. Therefore, in-field annealing effect on Cu-precipitation became unclear at 10 T. These magnetic field effects were discussed in the viewpoints of the change of Cu–Fe phase diagram and the atomic diffusion under magnetic field. Difference of the magnetic field effects on precipitation between 5 T and 10 T is explained by the competition between the enhancement of the driving force of the precipitation and suppression of the atomic diffusion. The obtained results indicated that there is optimized magnetic field intensity for controlling Cu-precipitation.
  • Investigation of CaO/Al2O3 Mass Ratio on the Properties and Structure of Ce2O3-Containing CaO–Al2O3-based Tundish Flux

    pp. 418-425

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    DOI:10.2355/isijinternational.ISIJINT-2021-438

    The properties and structure of Ce2O3-containing CaO–Al2O3-based tundish flux with different w(CaO)/w(Al2O3) have been investigated by hemispherical melting point method, the rotating cylinder method, and Raman spectroscopy, respectively. Results suggested that the melting temperature decreased from 1340°C to 1309°C, softening temperature from 1337°C to 1304°C, while the fluidity temperature changed not profound as the w(CaO)/w(Al2O3) from 0.9 to 1.3. The melting properties remained relatively steady when w(CaO)/w(Al2O3) was above 1.1. An increase in the w(CaO)/w(Al2O3) led to a decrease in viscosity, breaking temperature, and activation energy. A good property of the slag with w(CaO)/w(Al2O3) ranged from 1.1–1.3 in the current work. The main structural units of the AlO4-tetrahedral were Q2Al, Q3Al, and Q4Al, while the major structural units of SiO4-tetrahedral were Q0Si, Q1Si in the current system. With the increasing w(CaO)/w(Al2O3), the polymerized units Q3Al, and Q4Al were modified to Q2Al in aluminate structure. In silicate structure, the Q1Si unit was modified to Q0Si with the increasing w(CaO)/w(Al2O3). CaO could exert a stronger ability to stabilize the AlO4-tetrahedral unit in the current melts. Meanwhile, the increase of NBO/T and Λcorr indicated the decrease of DOP in the melt.
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    1. Influence Mechanism of Ce2O3 on Dephosphorization Process using CaO–Al2O3–SiO2–MnO Based Slag ISIJ International Vol.62(2022), No.3
  • Kinetics on Formation, Growth, and Removal of Alumina Inclusions in Molten Steel

    pp. 426-436

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    DOI:10.2355/isijinternational.ISIJINT-2021-475

    A series of Al deoxidation mechanisms from the nucleation and growth of Al2O3 nuclei immediately after the addition of Al to the growth, agglomeration, and removal of Al2O3 inclusions after the deoxidation equilibrium has been analyzed in light of the kinetics taking into consideration the influences of the interfacial properties on the basis of Al deoxidation experiments of molten steel. The nucleation number density of Al2O3 is (0.72 to 1.62) × 1014 m−3 and increases as the degree of supersaturation increases and the interfacial tension between the nuclei and molten steel decreases. These tendencies can be explained by the homogeneous nucleation theory, and the average interfacial tension, frequency factor, nucleation time, and average nucleation rate are respectively estimated to be 1.43 N·m−1, 4.27 × 1035 m−3·s−1, 0.01 s, and 1.96 × 1016 m−3·s−1 for the nucleation of Al2O3. Al2O3 nuclei rapidly grow to Al2O3 single inclusions having diameters of 2.0 to 2.6 µm through diffusion growth of supersaturated O in molten steel within 2.2 to 3.7 s after the addition of Al, and the molten steel reaches the deoxidation equilibrium. In the subsequent deoxidation equilibrium, the growth rate of Al2O3 single inclusions increases as the O concentration in molten steel increases, and their growth mechanism can be explained by Ostwald ripening. Meanwhile, Al2O3 cluster inclusions grow with the increase in the agglomeration force while agglomerating not only with single inclusions dispersed in molten steel but also with other cluster inclusions existing in the floating paths.
  • Numerical Simulation of Flash Reduction of Iron Ore Particles with Biomass Syngas

    pp. 437-448

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    DOI:10.2355/isijinternational.ISIJINT-2021-285

    Flash ironmaking technology has recently been developed to directly reduce in-flight hematite ore particles in a few seconds using gas reductant. Using biomass syngas as the reducer of this technology can decrease fossil fuel consumption and carbon dioxide emissions. This study investigated the flash reduction behavior of hematite particles using biomass syngas in a drop tube reactor based on a three-dimensional Eulerian-Lagrangian CFD model that includes heat and mass transfer, heterogeneous and homogeneous reactions, radiation, and interactions between gas and particles. The effects of the reduction temperature, biomass gas composition, and pressure on the reduction process were explored. Moreover, the particle characteristics and reaction rate during the flash reduction process were presented. The results showed that the water gas shift reaction proceeded in the reverse direction, and the methane steam reforming reaction proceeded in the forward direction during the flash reduction process. Under biomass steam gasification syngas, the reduction degree of hematite ore particles reached 95.90% within 1.4 s at 1573 K. Also, a higher temperature and operating pressure favored the reduction process by accelerating the reduction rate of hematite. These results provide a theoretical basis for using biomass syngas in flash ironmaking technology.
  • Powder Injection Effect on Hot Metal Desulfurization Behavior in the Kanbara Reactor: a Transient 3D Coupled Numerical Model

    pp. 449-456

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    DOI:10.2355/isijinternational.ISIJINT-2021-331

    The increasing demand and stringent requirements for high-quality steels necessitate their desulfurization efficiency enhancement and its robust substantiation. In this study, the Kanbara Reactor (KR) hot metal desulfurization process was simulated by a transient 3D coupled numerical model of two-phase flow, heat transfer, and desulfurizing agent (DA) particles. The multiple reference frame method was used to simulate the stirring blade rotation, and a desulfurization kinetic model was introduced to analyze the mass transfer rate of sulfur. The effect of addition of DA particles on the desulfurization efficiency was quantitatively evaluated under different conditions. The calculated average particle size was consistent with that reported by other researchers. The increased gas flow rate promoted the hot metal penetration of the DA, and the gas flow rate of 180 m/s corresponded to the desulfurization rate of 95.27%. When the lance was shifted further from the stirring shaft, higher desulfurization rates were obtained. However, an increased angle between the lance and the vortex liquid surface would induce the detrimental hot metal splashing phenomenon.
  • Promoting the Effective Utilization of Limonitic Nickel Laterite by the Optimization of (MgO + Al2O3)/SiO2 Mass Ratio During Sintering

    pp. 457-464

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    DOI:10.2355/isijinternational.ISIJINT-2021-362

    Limonitic nickel laterite always contains abundant high smelting components such as Al2O3 and MgO, which is extremely adverse to sintering. To weaken the negative effect of the abundant high smelting components, in this study, sinter pot tests of limonitic nickel laterite were conducted for better sintering performance by the optimization of (MgO + Al2O3)/SiO2 mass ratio at the same basicity. The relevant mechanism was revealed by the theoretical analysis and the measurement of chemistry and mineralogy of product sinter. At the optimum (MgO + Al2O3)/SiO2 mass ratio of 1.10, better sintering performance of limonitic nickel laterite can be achieved with tumble index and productivity increased by 8.22% and 7.22%, respectively, and solid fuel rate reduced by 7.41% compared with that of base case. Metallurgical performance of product sinter is excellent with reduction index (RI) and reduction degradation index (RDI+3.15 mm) of as high as 83.66% and 95.64%, respectively. In this case, liquid phase volume and fluidity are both significantly improved and nicely suitable for sintering. The optimization of (MgO + Al2O3)/SiO2 mass ratio contributes to the homogenization of liquid phase and the generation of tighter sinter microstructure with the obvious reduction of sinter porosity, the more formation of silico-ferrite of calcium and alumina (SFCA) and the improvement of bonding efficacy of solid phase by liquid phase. In the follow-up study, (MgO + Al2O3)/SiO2 mass ratio is considered to be adjusted by adding another type of nickel laterite with relatively lower MgO and Al2O3 contents for further improving limonitic nickel laterite sintering and eventually achieving its effective utilization.
  • Effects of High-Oxygen-Level Process Gas (40% O2) on the Temperature and Strength Development of a Magnetite Pellet Bed during Pot Furnace Induration

    pp. 465-476

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    DOI:10.2355/isijinternational.ISIJINT-2021-390

    As Sweden transitions to hydrogen-based steel production to enable fossil-free steelmaking, excess oxygen is likely to be generated through hydrogen production via water electrolysis based on green electricity. Further, during iron-ore pellet production, magnetite oxidises to hematite, releasing considerable heat. This excess oxygen and inherent heat can be used to promote exothermic oxidation, reducing the external fuel requirement, decreasing greenhouse gas emissions, and conforming to the Paris climate agreement. In this study, the effects of a high-oxygen-content (40% O2) inflow gas on pellet bed oxidation during induration were investigated, focusing on the resulting temperature profiles in the bed and the strength development of the produced pellets. An interrupted pot furnace experimental methodology was employed on the bed scale, with an approximate scale of 100 kg pellets per bed. The results indicate that the use of 40% O2 gas helps rapidly enhance the pellet properties and yields a more uniform pellet bed in terms of oxidation degree compared to the use of 13% O2 gas. In addition, improved cold compression strength (CCS) can be achieved when using 40% O2 inflow-gas. At temperatures above 1000°C, the oxidation degree and CCS are primarily enhanced by the high oxygen level of the inflow gas; this behaviour cannot be compensated for by increasing the temperature and residence time at a lower oxygen level. The positive effects on the bed-scale oxidation degree and strength are promising and may enable faster production rates in the future.
  • Evaluation and Improvement of Circumferential Uniformity for Blast Furnace Raceway

    pp. 477-486

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    DOI:10.2355/isijinternational.ISIJINT-2021-371

    The bustle pipe model of a running 2500 m3 blast furnace was established. The local and overall uniformity indexes of the raceway were defined and constructed. And then the effects of blast flow rate, the diameter and length of all tuyeres or No. 2 tuyere on the circumferential uniformity of raceway were quantitatively evaluated. It was shown that the circumferential uniformity of raceway decreased with the increasing blast flow rate, and when the blast flow rate increased from 4300 m3/min to 4700 m3/min, the circumferential uniformity index of raceway decreased from 0.1715 to 0.0760. The diameter of tuyere had a significant effect on the circumferential uniformity, while the length of tuyere showed little influence. The circumferential uniformity of raceway could be improved by increasing the diameter of all tuyeres or No. 2 tuyere with the minimum blast kinetic energy. When the diameter of all tuyeres increased to 140 mm, the overall uniformity index of raceway increased to 0.207, but the blast kinetic energy was only about 64 kJ/s which couldn’t meet the smelting. requirement. While the diameter of No. 2 tuyere near the hot blast main increased to 126 mm, the overall uniformity of the raceway increased to 0.124, and the blast kinetic energy was 120.11 kJ/s which was still within the reasonable range. it was feasible to improve the circumferential uniformity of raceway by adjusting the diameter of one or some off-average tuyeres but not all.
  • Effect of Briquette Thickness on Iron Nugget Formation in Fluxless Processing of Iron Sand Concentrate under Isothermal – Temperature Gradient Profiles

    pp. 487-495

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    DOI:10.2355/isijinternational.ISIJINT-2021-358

    Currently, electric furnaces and blast furnaces are used to process iron sand concentrate or titanomagnetite to produce pig iron and titanium slag. The titania content in the slag ranges from 10 to 25 mass% of blast furnaces and 30 to 35 mass% of electric furnaces. The low titania content in the slag is due to the addition of flux, some iron oxide must be retained in the slag to adjust the viscosity of the slag and the mixing of iron sand concentrate with ordinary iron ore. The low titania content in the slag makes titania extraction economically unattractive. One method to achieve high titania slag is the carbothermic reduction process without flux addition. A series of experiments have been carried out on the carbothermic reduction of cylindrical briquettes consisting of iron sand concentrate and coal under an isothermal – temperature gradient profile up to 1380°C to produce iron nuggets separated from slag. The briquettes have a diameter of 13 mm, and the thickness varied from 2 to 18 mm. The results showed that the thickness of the briquettes significantly affected the iron recovery in the nuggets. The briquettes with 2 mm thickness showed higher iron recovery in the nugget. The study also found that the sulfur content in coal may have contributed to the formation of iron nuggets.
  • Gasification Behavior of Phosphorus during Hydrogen-rich Sintering of High-phosphorus Iron Ore

    pp. 496-503

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    DOI:10.2355/isijinternational.ISIJINT-2021-468

    In order to realize the efficient utilization of high phosphorus iron ore resources, a new method of phosphorus gasification removal in hydrogen-rich sintering process was proposed. In this paper, the gasification behavior of phosphorus in hydrogen-rich sintering process of high-phosphorus iron ore was studied. Thermodynamic calculation of possible reactions is carried out by using FactSage6.1 software, and the phase transformation and distribution of phosphorus in the process of roasting reduction were analyzed by XRD, SEM-EDS and EPMA. The experimental results show that in hydrogen-rich atmosphere, the dephosphorization rate increased from 9.9% to 29.51% and then decreased to 8.62% in the temperature range of 900°C–1200°C, and the maximum value appeared at 1100°C. Compared to the carbon reduction, the dephosphorization rate in hydrogen-rich atmosphere increased significantly in the whole temperature range, and the maximal dephosphorization rate could be improved from 15.03% to 29.51%. The results of thermodynamic analysis showed that the initial temperature of direct reduction of apatite by hydrogen is higher, and adding SiO2 and Na2CO3 allowed to decrease the reduction temperature of apatite by hydrogen to about 946.50°C. With the increase of reduction temperature, the reduced phosphorus gas was absorbed by metallic iron to form stable iron phosphorus compounds, resulting in the decrease of dephosphorization rate. Therefore, in order to realize the gasification removal of phosphorus, the selective reduction of iron oxide and apatite should be realized, and the formation of liquid iron should be avoided as far as possible in the reduction process.
  • Kinetics Analysis of Steam Reforming of Methane on Sponge Iron

    pp. 504-514

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    DOI:10.2355/isijinternational.ISIJINT-2021-472

    The kinetics of steam reforming of methane catalyzed by sponge iron was studied at temperatures between 875°C and 1050°C. Results shows that sponge iron acts as a catalyst and methane conversion is increased in higher temperatures and with a higher H2/H2O ratio in the inlet gas. A kinetic model based on chemical reaction control fits well the experimental data up to methane conversion of 0.5 with an apparent activation energy of 258 kJ/mol. Pore diffusion limits the reaction rate more intensely at higher conversions of methane, higher temperatures, and larger particle size (from 9 to 17 mm). Two types of industrial pellets were compared showing that microstructural properties such as porosity, pore size, and grain size impact reduction rate with hydrogen and the catalytic property of the obtained sponge iron.
  • Influence Mechanism of Ce2O3 on Dephosphorization Process using CaO–Al2O3–SiO2–MnO Based Slag

    pp. 515-523

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    DOI:10.2355/isijinternational.ISIJINT-2021-491

    To improve the dephosphorization capacity of CaO–Al2O3–SiO2–MnO based ferromanganese slag, this work attempts to introduce Ce2O3 as a strong phosphorus fixative into the slag, and the influence mechanism of Ce2O3 on the dephosphorization process is investigated by thermodynamic analysis, structural analysis, melting temperature, and viscosity tests. The results show that after adding 20 mass% Ce2O3 into the slag, Ce2O3 can release the O2− and promote the P-O0 bond to P-O bond transformation, thus improving the phosphorus fixation capacity of melted slag. Also, the phosphorus enrichment phases are nCa2SiO3–Ca3P2O8 solid solution and CePO4 phase in Ce2O3-containing slow cooling slag. Furthermore, the melting temperature of slag with 0–20 mass% Ce2O3 addition is always about 1623 K, but increases obviously when the addition of Ce2O3 exceeds 20 mass%. Also, adding appropriate Ce2O3 content into the CaO–Al2O3–SiO2–MnO based slag can decrease the viscosity of slag, and the viscosity can remain relatively low with adding 15 mass%–25 mass% Ce2O3. In conclusion, adding appropriate Ce2O3 content into the CaO–Al2O3–SiO2–MnO based slag can improve the phosphorus fixation capacity of slag and ameliorate the fluidity of slag, and consequently, it is conducive to remove phosphorus from ferromanganese. Finally, the high-temperature ferromanganese dephosphorization experiments were carried out, and the results show that after adding 20 mass% Ce2O3 into the slag, the lowest [P] content decreases from 0.31% to 0.25%, and the dephosphorization rate increases from 31.1% to 45.6%. The above results not only verify that the feasibility of Ce2O3 as a strong phosphorus fixative used in the ferromanganese dephosphorization slag, but also provide the theoretical guidance for optimizing the composition of Ce2O3-containing dephosphorization slag.
  • Effect of Cooling Rate on Carbide Characteristics of the High Vanadium High-speed Steel

    pp. 524-531

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    DOI:10.2355/isijinternational.ISIJINT-2021-244

    Effect of cooling rate on carbide precipitation, morphology, size, distribution and composition of the high carbon and vanadium high-speed steel (HCVHSS) was investigated by field emission scanning electron microscope (FESEM), X-ray diffraction (XRD) and electron probe micro analyzer (EPMA). The results illustrated that the as-cast microstructure of HCVHSS mainly included the carbides of primary and eutectic MC, eutectic M2C, secondary M23C6 and the matrix of martensite, retained austenite. With the higher CR, the grain size is refined, the size and quantity of both primary MC and eutectic M2C decreased significantly, while an increase of eutectic clustered MC was obtained. Except for the more uniform distribution of MC and M2C, the poorer network continuity of M2C at grain boundary and fewer M23C6 carbides in the matrix were also found. For the composition of carbides, the weaker effect of selective crystallization and more uniform distribution of carbides composition were obtained in higher CR specimen. In addition, a higher content of V and Mo in MC and M2C respectively was found. Owing to the excellent microstructure obtained at higher CR, the hardness value of the specimen was higher too which was beneficial to the improvement of its wear resistance.
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  • A Hybrid Modeling Method Based on Expert Control and Deep Neural Network for Temperature Prediction of Molten Steel in LF

    pp. 532-541

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    DOI:10.2355/isijinternational.ISIJINT-2021-251

    The temperature control of molten steel in ladle furnace (LF) has a critical impact on steelmaking production. In this work, production data were collected from a steelmaking plant and a hybrid model based on expert control and deep neural network (DNN) was established to predict the molten steel temperature in LF. In order to obtain the optimal DNN model, the trial and error method was used to determine the hyperparameters. And the optimal architecture of DNN model corresponds to the hidden layers of 4, hidden layer neurons of 35, iterations of 3000, and learning rate of 0.2. Compared with the multiple linear regression model and the shallow neural network model, the DNN model exhibits stronger generalisation performance and higher accuracy. The coefficient of determination (R2), correlation coefficient (r), mean square error (MSE), and root-mean-square error (RMSE) of the optimal DNN model reached 0.897, 0.947, 2.924, 1.710, respectively. Meanwhile, in the error scope of temperature from −5 to 5°C, the hit ratio of the hybrid model acquired 99.4%. The results demonstrate that the proposed model is effective to predict temperature of molten steel in LF.
  • Development of Flow Visualization Measurement Method of Droplet Train Obliquely Impinging on Moving Hot Solid

    pp. 542-549

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    DOI:10.2355/isijinternational.ISIJINT-2021-427

    Spray cooling on moving hot solids is widely used in metal heat treatment processes. Understanding coolant droplet collision behavior with moving hot solids is of great importance toward improving heat treatment temperature control technology. Via flash photography, we experimentally investigated the hydrodynamics of droplet train obliquely impinging on a hot moving solid. The test piece was a rectangular steel piece (SUS303) heated to 500°C, 550°C, or 600°C with a moving velocity of 0.5 m/s, 1.0 m/s, or 1.5 m/s. The test liquid was water at approximately 20°C. The pre-impact diameter of droplets, droplet impact velocity, and inter-spacing between every successive two droplets were 0.64 mm, 2.2 m/s, and 1.91 mm, respectively. The tilt angle of the droplet train to the vertical was 50°. No coalescence of droplets was seen—the droplets deformed independently on the moving solid. The measured results of the maximum diameter and the residence time of the droplets agreed well with the empirical formulas that can be used for droplet impact on a stationary solid. It was found that the dynamics of a droplet train impinging on a hot moving solid are the same as the dynamics of a droplet train impinging on a hot stationary solid when the droplets deform independently on a moving solid. Taking advantage of said property such that it is equivalent to the dynamics of a droplet train impinging on a hot stationary solid, we proposed a critical condition for droplet coalescence and experimentally confirmed the validity of the critical condition.
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  • Erosion-Corrosion Behavior of Electroless Ni–P Coating on M2052 Alloy in Artificial Seawater

    pp. 550-560

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    DOI:10.2355/isijinternational.ISIJINT-2021-359

    M2052 damping alloy has good shock absorption and noise reduction ability, but the corrosion resistance and wear resistance are insufficient. In this study, a high phosphorus amorphous Ni–P coating with a thickness of about 22.1 µm was successfully deposited on the M2052 substrate by electroless plating. The wear experiments show that the main wear mechanism of Ni–P coating is adhesive wear, while the main wear mechanism of M2052 substrate is abrasive wear and corrosion wear. Compared with M2052 substrate, the electroless plating sample has a lower corrosion current and higher corrosion potential, and the corrosion resistance is greatly improved. In the erosion-corrosion environment, the corrosion rate of the uncoated sample is about 5 times that of the coated sample. However, the damage of Ni–P coating under the same environment corroded slightly, which effectively impedes the erosion of sediment flow and the corrosion of artificial seawater.
  • Coarsening Behavior of Ni(Fe) Particles in the Inner Oxide Layer Formed on Fe-5 mass%Ni Alloy at 1200°C in N2 Atmosphere

    pp. 561-567

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    DOI:10.2355/isijinternational.ISIJINT-2021-192

    In our previous study, we investigated the high-temperature oxidation behavior of Fe-5 mass%Ni alloy and discussed the changes in the microstructure of the inner layer, particularly the size and distribution of Ni(Fe) particles, with oxidation time. The number density and area fraction of Ni(Fe) particles in the inner layer decreased, whereas the size of the particles increased with oxidation time. This coarsening was proposed to arise from Ostwald ripening. However, this evaluation of the coarsening of Ni(Fe) particles includes the effect of oxygen potential in the oxide scale. In this study, for minimizing the effect of the oxygen potential gradient in the oxide scale on the coarsening of Ni(Fe) particles, the samples oxidized at 1200°C were subsequently held in pure N2 with low oxygen potential to decrease the oxygen potential gradient across the oxide scale, and the microstructural changes in the inner layer during heat treatment in N2 were investigated to confirm the coarsening of Ni(Fe) particles by Ostwald ripening, with a minimal effect of the change in the oxygen potential on the microstructural evolution.After the equilibrium condition was established in N2 with lower oxygen potential in the inner layer, the coarsening behavior of Ni(Fe) particles followed the relationship explained by Ostwald ripening. Ni(Fe) particles precipitated in the outer and inner layers because of a decrease in the oxygen potential. The precipitation of Ni(Fe) particles in the inner layer occurred only in the initial transient stage of heat treatment, until the equilibrium condition in N2 was established.
  • Identification of Chromium-Depleted Area around Chromium Nitride Precipitates in Heat-affected Zone of Lean-Duplex Stainless Steel and In-situ Observation of Preferential Dissolution by EC-AFM

    pp. 568-576

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    DOI:10.2355/isijinternational.ISIJINT-2021-462

    A Gleeble thermo-mechanical simulator was used to simulate the welds of duplex stainless steel S32101. A micro-precipitate of chromium-nitride and its surroundings in the simulated welds was analyzed in detail by SEM/AES and EC-AFM. The AES analysis revealed that a chromium-depleted area is present in the γ phase near the chromium-nitrides precipitated at the α/γ grain boundary. The EC-AFM observation revealed that the duplex stainless steel preferentially dissolves from the chromium-depleted area.
  • Phase-field Simulation of Abnormal Grain Growth due to the Existence of Second-phase Particles

    pp. 577-585

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    DOI:10.2355/isijinternational.ISIJINT-2021-299

    Grain growth processes are usually classified into two types. The first is a self-similar coarsening process, which is known as normal grain growth, while the second is characterized by the coarsening of a few grains at the expense of the surrounding matrix, and is known as abnormal grain growth. Although different mechanisms have been proposed for abnormal grain growth, the actual physical mechanism responsible for this phenomenon remains largely unknown. To inhibit normal grain growth in polycrystalline metals, dispersions of second-phase particles are often used. Using mean field analysis involving particle dispersions, Hillert and Humphreys predicted the condition where only abnormal grain growth occurs. In addition, Monte Carlo simulations on particle-assisted abnormal grain growth have been reported; however, the mechanism for this phenomenon has yet to be clarified. In this study, the abnormal grain growth caused by dispersed particles was reproduced using three-dimensional phase-field (PF) simulations. In particular, we investigated the influence of the particle dissolution rate on the intensity of abnormal grain growth. Furthermore, we evaluated the characteristics of individual grains exhibiting the maximum grain size at the end of the simulation. Our PF simulations revealed that size superiority under the initial condition is important for enhancing abnormal grain growth, as is the “growth environment,” i.e., the average grain size of adjacent grains that changes sequentially during the simulation. To deduce the effects of the pinning particles, the anisotropy in the interface properties was not considered in this study.
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    1. Effect of Cooling Rate on Carbide Characteristics of the High Vanadium High-speed Steel ISIJ International Vol.62(2022), No.3
  • Simultaneous Optimization of Rigidity and Strength of Super Invar Cast Steel using by Martensitic Reversion

    pp. 586-592

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    DOI:10.2355/isijinternational.ISIJINT-2021-478

    Super invar cast steel, Fe–32%Ni–5%Co by mass%, with excellent low coefficient of thermal expansion has disadvantages in the both of Young’s modulus and strength, because of coarse columnar solidification structure having <100> texture. To simultaneously overcome these disadvantages, the variations of microstructure and mechanical properties through the novel heat treatment referred to as cryo-annealing, which is consisting of subzero treatment and subsequent annealing, were investigated in a super invar cast steel. The cryo-annealing promoted fcc-bcc martensitic transformation and then bcc-fcc martensitic reversion. The bidirectional martensitic transformations led to the formation of duplex austenitic structure consisting of untransformed and reversed austenite with a coarse-grained structure similar to solidification structure. Furthermore, it is found that the austenitic structure was varied depending on the annealing temperature of the cryo-annealing; reversed austenite was remained at lower annealing temperature, while it recrystallized to fine-grained structure as increasing annealing temperature. The high-density dislocations in reversed austenite and the randomized orientation of recrystallized austenite contributed to the development of strength and Young’s modulus, respectively. Therefore, the simultaneous development of rigidity and strength is not achieved by single cryo-annealing, but can be achieved by two-cycle cryo-annealing. Increasing the first annealing temperature and lowering the second annealing temperature in the two-cycle cryo-annealing are appropriate to randomize crystal orientation through austenite recrystallization and to make volume fraction of reversed austenite higher, respectively. As a result, Young’s modulus and 0.2% strength were simultaneously optimized.
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    1. Effect of Internal Stress on Sequence of γεα′ Martensitic Transformations in Austenitic Stainless Steel ISIJ International Vol.62(2022), No.3
  • Effects of Crystallographic Texture on Subsurface Fatigue Crack Generation in Ti–Fe–O Alloy at Low Temperature

    pp. 593-601

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    DOI:10.2355/isijinternational.ISIJINT-2021-381

    Subsurface microcracks developed in a groove-rolled and cold-swaged Ti–Fe–O alloy were characterized to clarify the generation of subsurface fatigue crack. In addition, the effects of crystallographic texture on subsurface crack initiation and growth were discussed. A considerable number of microcracks were detected in the β grains, α grains, and at the α-β interface. The microcracks in the β grains grew negligibly into the neighboring α grains along the basal plane. This was because these grains were oriented with their c-axis almost perpendicular to the loading axis. The {1010}α fiber texture prevents the formation of basal facet and its growth on the basal plane. The stress concentration around the microcrack in the β grains could assist the growth of the microcrack into neighboring α grains along the prismatic plane (which is inclined to the loading axis at a suitable angle) or occasionally at a {1010}α twist boundary. The {1010}α fiber texture assisted microcrack growth, and thereby, formed aligned facets and yield longer microcrack length. The combination of the shear stress and opening stress on {1010}α results in a Mode II or III microcrack and causes microcrack growth on the prismatic plane in the neighboring grain.
  • Simulation of Thermal Decomposition of Partially Calcined Spherical Limestone Injected into a Molten Iron Bath

    pp. 602-605

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    DOI:10.2355/isijinternational.ISIJINT-2021-373

    To reduce the consumption of energy and materials, it is necessary to develop a more efficient method for refining iron. The use of partially calcined limestone as a refining flux is expected to increase the mass transfer and reaction area via thermal decomposition of CaCO3 and violent CO2 generation. A model was developed to simulate the decomposition of the limestone particles in molten iron using multi-physics analysis, in which the equations for multiphase flow, heat transfer, and chemical reactions were solved simultaneously. The particle penetration behavior and the temperature and mass distributions of CaCO3 were calculated as a function of time. A large amount of CO2 is generated in a short period, which is expected to generate a strong stirring effect and destroy the flux particles.
  • Hot Coke Strength in CO2 Reaction

    pp. 606-608

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    DOI:10.2355/isijinternational.ISIJINT-2021-305

    This study aims to investigate the strength of coke at high temperature in reaction atmosphere. A uniaxial compression test measured the fracture stress of coke at three conditions: (i) room temperature and air atmosphere; (ii) 1100°C and N2 atmosphere; (iii) 1100°C and CO2 atmosphere. The drum index (DI) and coke strength after reaction (CSR) were also measured. The DI and CSR of coke produced by a lump of caking coal were higher than those of coke produced by non- or slightly caking coal. The compression tests showed that the fractured stress at room temperature in the air atmosphere was lower than that at 1100°C in the N2 atmosphere. Also, the fractured stress at 1100°C in the N2 atmosphere was higher than that at 1100°C due to the CO2 reaction. The DI, CSR, and hot coke strength of coke produced by caking coal was higher than those of coke produced by non- or slightly caking coal.
  • Physical Modelling of Nitrogen Variation in Continuously Cast Blooms Resulting from Atmospheric Exposure of Steel in Tundish during Initial Stages of Teeming

    pp. 609-612

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    DOI:10.2355/isijinternational.ISIJINT-2021-473

    Exposure of liquid steel to air, consequent re-oxidation as well as nitrogen absorption are common in steelmaking and produce unacceptable variations in product chemistry. The phenomena, during continuous casting of the lead or first heat, have been investigated in a full-scale water model of a two-strand, bloom casting shroud-tundish system. Physical modelling results supported by industrial scale measurements have demonstrated that exposure of liquid steel to tundish atmosphere during initial period of ladle teeming and consequent nitrogen absorption, manifest over substantial period, influencing predominantly the first three blooms.
  • Effect of Internal Stress on Sequence of γεα′ Martensitic Transformations in Austenitic Stainless Steel

    pp. 613-616

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    DOI:10.2355/isijinternational.ISIJINT-2021-425

    It is well known that ε-martensite prefers to form before the nucleation of α′-martensite in austenitic steels with lower stacking fault energy, despite its lower phase stability. In the present study, the internal stress development during the formation of ε-martensite as an intermediate phase in the martensitic transformation from γ-austenite to α′-martensite was investigated in 18Cr–8Ni metastable austenitic stainless steel. The c/a value of hexagonal close-packed crystals of plate-shaped ε-martensite linearly increased as the plate thickness increased, proving that the internal stress was gradually stored by the growth of ε-martensite. The subsequent formation of α′-martensite decreased the c/a of ε-martensite, which suggests relaxation of the internal stress. Theoretical calculations based on micromechanics indicated that ε-martensite forms as the intermediate phase in austenitic steels because of (1) minimization of internal stress generation at the nucleation stage owing to its thin plate shape and (2) relaxation of internal stress by the coexistence of ε- and α′-martensite.
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    Readers Who Read This Article Also Read

    1. Simultaneous Optimization of Rigidity and Strength of Super Invar Cast Steel using by Martensitic Reversion ISIJ International Vol.62(2022), No.3
  • Contents and Publication Data

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  • Editorial Board, Copyright Page and Announcements

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