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

ISIJ International Vol. 56 (2016), No. 7

  • Fluid Mixing in Ladle of RH Degasser Induced by Down Flow

    pp. 1119-1123

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    DOI:10.2355/isijinternational.ISIJINT-2015-714

    In order to understand the effect of down flow in RH ladle on mixing time, the relationship among flow patterns, circulation and mixing phenomena were examined by using three sizes of model RH. The temporal change in electric conductivity in ion-exchanged water was tracked after injecting KCl solution and the decoloration process of iodine color in ion-exchanged water was observed visually after injecting sodium thiosulfate solution. There were two mixing patterns in the ladle: One has a damping oscillation curve for a tracer response and the decoloration reaction occurred everywhere and uniformly, the other has a monotonical decreasing tracer curve after an overshoot and the decoloration delayed near the free surface of the ladle. The (tM/tR)/(Dleg/H) values decreased with the increase in Re below Re≒1.8×104 and it was kept to be constant above Re≒1.8×104 where tM: a mixing time (s), tR: a mean residence time (s) of fluid in the ladle, Dleg: inner diameter of down-leg (m) and H: length of jet axis between down-leg outlet and jet-impinged wall (m). In the region of Re>1.8×104, the mixing was controlled by the circulating flow, whereas in the region of Re<1.8×104 the mixing behavior was influenced by the stagnation zone near the free surface of the ladle.
  • Experimental Determination of the Phase Diagram for CaO-SiO2-MgO-10%Al2O3-5%TiO2 System

    pp. 1124-1131

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    DOI:10.2355/isijinternational.ISIJINT-2016-045

    The phase diagrams of Ti-bearing slag have fundamental guiding significance for the comprehensive utilization of Ti resources. In this article, the pseudo-melting temperatures were determined by the single hot thermocouple technique (SHTT) and high equilibrium experiments for the specified contents of 5% to 15% MgO in the CaO-SiO2-MgO-10%Al2O3-5%TiO2 phase diagram system. The 1250°C to 1350°C liquidus lines were firstly calculated based on the thermodynamic equations in the specific primary crystal field. The phase equilibrium relationship was determined experimentally at 1300°C using the high temperature experimental technique followed by X-Ray Fluorescence (XRF), X-Ray Diffraction (XRD), Scanning Electron Microscope (SEM) and Energy Dispersive X-ray spectroscope (EDX) analysis, and the liquid Phase, liquid coexist with solid solution phase of (2CaO·MgO·2SiO2, 2CaO·Al2O3·SiO2)ss were found. Therefore, the phase diagram was constructed for the specified region of the CaO-SiO2-MgO-10%Al2O3-5%TiO2 system, which was consistent with the existing phase diagram except for somewhat larger liquidus area at 1300°C. The comparison proved that the combination of liquidus temperature determination by SHTT and the high temperature experimental technique was enabled to construct the correct phase diagram for slag systems.
  • Catalytic Decomposition of Pyridine with Goethite-Rich Limonite in the Coexistence of Fuel Gas or Coke Oven Gas Components

    pp. 1132-1137

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    DOI:10.2355/isijinternational.ISIJINT-2016-052

    Catalytic performance of limonite in the decomposition of 100 ppmv pyridine (C5H5N) in the coexistence of fuel gas or coke oven gas (COG) components has been studied mainly with a cylindrical quartz reactor at 750–850°C under a high space velocity of 51000 h-1 to develop a novel hot gas cleanup method of removing the nitrogen in tar as N2. The limonite catalyst achieves the almost complete decomposition of C5H5N in He at 500–850°C and gives a high N2 yield of more than 85 N% at 500°C. When the decomposition run is performed in the presence of fuel gas or COG components, the coexistence of 20% CO/10% H2 at 750°C or 50% H2/30% CH4/5% CO at 850°C deactivates the limonite with remarkable formation of deposited carbon. On the other hand, the addition of a small amount of H2O or CO2 to these atmospheres can improve the catalytic activity without carbon deposition. When 3% H2O or 10% CO2 is added to 20% CO/10% H2, C5H5N conversion and N2 yield at 750°C become 80–95% and 65–80 N%, respectively, and the extent of the improvement is larger with the CO2 than with the H2O. The addition of 5% CO2 to 50% H2/30% CH4/5% CO also restores the conversion or the yield at 850°C to the high level of about 90% or 60–70 N%, respectively, and both values at 950°C are comparable to those at 500°C in inert gas.
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  • Effects of Sintering Materials and Gas Conditions on Formation of Silico-Ferrites of Calcium and Aluminium during Iron Ore Sintering

    pp. 1138-1147

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    DOI:10.2355/isijinternational.ISIJINT-2015-598

    Silico-ferrites of calcium and aluminium (SFCA) are desirable phases in a high quality iron ore sinter product. The effects of sintering temperature, CaO/SiO2 ratio, sintering gas atmosphere and cooling procedure on the phase composition of sintered specimens from an industrial sinter blend were examined with focus on the formation of SFCA phases. The proportions of mineral phases in specimens sintered at 1250–1325°C were quantitatively examined using image analysis. SFCA can be formed at low temperatures by solid state reactions, the formation of which was enhanced by increasing temperature. Further increasing sintering temperature promoted the reduction of Fe3+ in the SFCA crystal structure to Fe2+ and consequent decomposition of SFCA. At high temperatures, SFCA was produced during cooling via crystallisation from a silicate melt. Maintaining a high oxygen partial pressure favours the formation of SFCA, either via solid state reactions or from a melt. This is attributed to hematite being available as a reactant for SFCA formation. Similarly, increasing CaO/SiO2 ratio provides more CaO as a reactant and promotes SFCA formation.
  • Improving Energy Efficiency in Iron Ore Sintering through Segregation: a Theoretical Investigation

    pp. 1148-1156

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    DOI:10.2355/isijinternational.ISIJINT-2015-686

    In iron ore sintering, effective segregation of the particulate bed on the strand can result in productivity increases and decreased fuel rate. To find the optimum level of segregation for a given blend, intensive experimental work will be necessary to assess the impact of different segregation levels. The impact of segregation on sintering performance can also be quantified using mathematical modelling. In this work, a well-validated sintering mathematical model has been developed and the effects of mean granule size, bed voidage, bed bulk density, coke mass segregation as well as increased bed permeability on the sintering performance have been investigated. It was concluded that the variation in coke mass down the bed and increased bed permeability are the major factors giving increased sinter yield and productivity in segregated beds. Coke mass segregation has a large impact on maximum bed temperature, residence time above the critical temperature and, consequently, the total heat available to the bed at the critical melt formation period. The effect of variations in granule size and bulk density, caused by segregation, on coke combustion efficiency and bed temperature is small. Bed permeability has the largest impact on flame front speed and, therefore, sinter productivity. Based on differences in bottom and top bed temperatures, the optimal coke mass segregation level was identified. Results from this study provide useful guidelines on optimal segregation level to maintain sintering performance and reduce energy consumption and therefore ironmaking costs.
  • Crystallization Kinetics of 2CaO·Fe2O3 and CaO·Fe2O3 in the CaO–Fe2O3 System

    pp. 1157-1163

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    DOI:10.2355/isijinternational.ISIJINT-2015-710

    In this study, the non-isothermal crystallization kinetics of 2CaO·Fe2O3 and CaO·Fe2O3 were investigated by DSC measurement. Crystallization of CaO–Fe2O3 system includes three reactions and can be explained using Fe2O3–CaO phase diagram. The mechanisms of 2CaO·Fe2O3 and CaO·Fe2O3 crystallization were analyzed using Avrami and Mo models. Results of the Avrami model analysis indicated that growth of 2CaO·Fe2O3 and CaO·Fe2O3 include two stages, which are controlled first by a fibril-like mechanism followed by a spherulitic-type mechanism. The Mo model analysis yielded results similar to those of the Avrami model and further concluded that Avrami exponent/Ozawa exponent is constant despite the changes in cooling rate. Calculations using Kissinger method revealed that the activation energy of 2CaO·Fe2O3 and CaO·Fe2O3 crystallization were -464.16 kJ·mol-1 and -172.61 kJ·mol-1, respectively. Moreover, 2CaO·Fe2O3 crystallization occurs in a difficult manner but proceeds at a faster rate than CaO·Fe2O3 crystallization. Increasing the cooling rate promotes CaO·Fe2O3 crystallization but inhibits 2CaO·Fe2O3 crystallization, which is beneficial during sintering.
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  • Activities of FeO1.33 in the FeOx–CaO–SiO2 and FeOx–CaO–SiO2–Al2O3 Slags at 1573 K Under Oxygen Partial Pressures between 10−6 and 10−2 atm

    pp. 1164-1170

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    DOI:10.2355/isijinternational.ISIJINT-2016-065

    Qualities of sinters such as reducibility and strength are controlled by morphologies and mineral phases in iron ore sinters. Main mineral phases are hematite, magnetite and calcium ferrite. Slag phases are also included in sinters, which have been mainly liquid phases during the sintering process. Since iron ions in slags are hardly reduced to metallic irons in a blast furnace, iron oxides such as hematite and magnetite and calcium ferrite should be precipitated from molten slags during the sintering process. Precipitation of iron oxides and calcium ferrites from molten slags depends on the activity of FeOx in molten slags from the thermodynamic view point. Therefore, the activities of FeO1.33 have been measured over the wide ranges of chemical compositions in the single liquid phases of the FeOx–CaO–SiO2 and FeOx–CaO–SiO2–Al2O3 slags at 1573 K under oxygen partial pressures between 10-6 and 10-2 atm. It has been found that the aFeO1.33 values have maximum at the CaO/SiO2(mass%) ratios of 0.9–1.0 and 1.3–1.4 for the samples with 35 mass%FeOx equilibrated at PO2 = 1.0×10-2 atm and 1.0×10-4 atm, respectively. For the samples at PO2 = 2.5×10-6 atm, on the other hand, aFeO1.33 monotonically increases with increasing the CaO/SiO2 ratio within the compositional range of the single liquid region. As for the effect of the Al2O3 addition on the activities, the dependency of the activity on the CaO/SiO2 ratio becomes moderate with increasing the Al2O3 addition on the sample at PO2 = 2.5×10-6 atm.
  • Fundamental Forces Driving Analogue Sinter Mix Reshaping

    pp. 1171-1180

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    DOI:10.2355/isijinternational.ISIJINT-2016-069

    Major structural change occurs during sintering on melt formation. Melts activate surface forces that drive coalescence processes, as surface energy is reduced. The extent to which coalescence occurs depends on the relationship between surface and viscous forces, which in turn are determined by composition and temperature. In this study, a coal ash fusion furnace was utilised to investigate the impact of composition and temperature on analogue sinter mix tablet reshaping over alumina tiles. Sinter mix compositions were comparable to small size fractions of plant sinter mixes; as they are the first to form melt during sintering operations. A factorial experiment showed basicity to be the dominant driver for reshaping with increasing temperature. Alumina was found to retard reshaping, but only at low sintering temperatures. Material properties, calculated using FactSage and published correlations, were determined as a way to investigate forces acting in the system. Results showed the main determinant of reshaping was apparent viscosity, which was primarily dependent on the amount of melt formed in the sinter mix. The study also used a novel experimental technique, which demonstrated the ability of surface forces to drive reshaping and surface energy reduction when the tablet was suspended from a downward facing tile. This study found that while melt surface tension and wetting behaviour drive system reshaping to reduce surface energy, the extent of sinter mix reshaping was predominately determined by resistance from viscous forces.
  • Control of Inclusion Composition in Calcium Treated Aluminum Killed Steels

    pp. 1181-1187

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    DOI:10.2355/isijinternational.ISIJINT-2016-123

    Inclusions in slab samples with various total calcium, oxygen and sulfur content were investigated in low carbon aluminum killed steel (LCAK steel) with low sulfur content based on industrial experiments and the relationship between steel and inclusions was studied by analyzing inclusions characteristic being detected by SEM-EDS. It is found that T.Ca/T.O could better replace dissolved Ca to evaluate the extent of modification of alumina inclusions by Ca. Inclusions changed from Al2O3 based inclusions to Al2O3–CaS inclusions and finally to CaO–CaS inclusions with the increase of T.Ca/T.O in steel for slab samples and MgO and Al2O3 content in inclusions almost linearly decreased with T.Ca/T.O in steel. Increasing T.Ca/S in steel could improve the modification extent of alumina by Ca further increased CaS content of inclusions for slab samples with Al2O3–CaS inclusions. In addition, the formation mechanism of inclusions including Al2O3 based inclusions, Al2O3–CaS inclusions and CaO–CaS inclusions was discussed.
  • Removal of Inclusions Using Micro-bubble Swarms in a Four-strand, Full-scale, Water Model Tundish

    pp. 1188-1197

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    DOI:10.2355/isijinternational.ISIJINT-2016-077

    Water model experiments were performed in a full-scale, delta-shaped water model tundish, in order to study the removal of inclusions by micro-bubbles. Micro-bubbles were generated using a specially designed ladle shroud with twelve laser-drilled orifices. Gas flow rates, injection positions and multi-port injection were all taken into consideration to create different bubble conditions. Bubbles were recorded using a high speed camera and post-processed with commercial software, Image J. Hollow glass borosilicate microspheres, smaller than 100 µm, were used to simulate inclusions, and detected, in-situ, using a new generation of the Aqueous Particle Sensor, APS III. The results revealed that the effect of micro-bubbles on inclusion removal depends greatly on the gas injection protocols used. The optimum gas flow rate was an intermediate value, which indicates a minimum particle number density, np, of about 7.85/ml. This results from the counter-balancing effects of bubble sizes against the total number of bubbles. The highest inclusion removal rate was 80%, when gas was injected through the four ports located closest to the slide gate, at a gas flow rate of 0.2 L/min.
  • Thickness Measurement of Blast Furnace Staves with Curved Cooling Channel Using a Specially Designed Ultrasonic Probe

    pp. 1198-1203

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    DOI:10.2355/isijinternational.ISIJINT-2015-678

    Blast furnaces in steel industries employ staves, which provide cooling to the furnace shell for maintaining structural integrity. Motion of the raw material within the furnace during operation causes the cooling staves to wear. Thickness of the staves needs to be monitored on a regular basis to avoid catastrophic failure. The staves of the furnace in question have a curved cylindrical cooling channel as opposed to straight cooling channels; which makes the thickness measurement particularly challenging in this case due to mode conversion. Ultrasonic based thickness measurement is a widely used technique in industries for stave thickness measurement. This text describes a unique modification in a conventional ultrasonic probe to measure the thickness of the staves with curved cooling channels. The modified probe is designed to avoid mode conversion due to incidence of ultrasound at oblique angles. FEM simulations were done to validate the effectiveness of the shoe followed by trials in actual furnace.
  • Evaluation of Agglomeration Mechanisms of Non-metallic Inclusions and Cluster Characteristics Produced by Ti/Al Complex Deoxidation in Fe-10mass% Ni Alloy

    pp. 1204-1209

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    DOI:10.2355/isijinternational.ISIJINT-2016-030

    The characteristics of non-metallic single inclusions and clusters formed in a Fe-10mass% Ni alloy deoxidized with Ti, Al and Ti/Al were investigated. Laboratory experiments were performed, and samples were taken after deoxidation. The composition, number and size of the single inclusions and clusters in the samples were determined using SEM in combination with EDX. The agglomeration mechanism and collision rates of inclusions and clusters were considered in the Al and Ti/Al deoxidation experiments. The obtained results showed that the number and average size of clusters in all samples of the complex Ti/Al experiment are drastically smaller than those in the Al experiment. The Brownian, Stokes` and turbulent collisions between particle-particle in clusters, particle-cluster and cluster-cluster were evaluated to determine the cluster formation in the different deoxidation experiments depending on the holding time.
  • Experimental Investigation of High-temperature Steel Plate Cooled by Multiple Nozzle Arrays

    pp. 1210-1218

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    DOI:10.2355/isijinternational.ISIJINT-2015-691

    Thermo-mechanical controlled process (TMCP) technology has been widely used to improve controlled cooling technology during the hot rolled plate manufacturing process. Multiple impinging jets are the main cooling form used in plate cooling process of the steel mills. In this work, we mainly focused on the surface flow field and heat transfer of high-temperature plate (700°C) cooled by nine-nozzle arrays with different Reynolds numbers. The distribution of turbulent kinetic energy and water velocity vector were numerically studied. By analyzing the simulation results and measuring temperature data, the maximum heat flux, the corresponding surface temperature and time at each measurement point were found to be dependent on the distance from the impinging point and the jet velocity. No obvious maximum heat flux elevation for the locations between jets was observed, where the wetting speed was also slow with Reynolds numbers lower than 8278. These results are valuable in interpreting the heat transfer mechanism under jet arrays and predicting the cooling rate, and forecasting the microstructure of the steel product.
  • Influence of Heating Temperature on Edge Crack in Hot Rolling of 36%Ni-Fe Alloy

    pp. 1219-1225

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    DOI:10.2355/isijinternational.ISIJINT-2016-025

    For the purpose of edge crack control in hot rolling of 36%Ni-Fe alloys, a high temperature tensile test and laboratory-scale hot rolling experiment were carried out. Intergranular oxidation has been considered to be one of the major factors in edge cracking. Edge cracks which initiate from intergranular oxidation grow to the inner side along coarse grain boundaries as the thickness reduction ratio increases. The depth of intergranular oxidation increases at higher reheating temperatures. However, recrystallization occurs at the crack tip, and this has the effect of suppressing crack growth. This suggests that promoting recrystallization during hot rolling by increasing the reheating temperature, rather than inhibition of intergranular oxidation as such, is effective for suppressing edge cracking.
  • Friction Lap Joining of Thermoplastic Materials to Carbon Steel

    pp. 1226-1231

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    DOI:10.2355/isijinternational.ISIJINT-2016-042

    Dissimilar material joining of polyamide 6 and polyethylene plates to a plain carbon steel (SPCC) plate was performed using friction lap joining. The polyamide 6 and SPCC plates could be directly joined by friction lap joining, whereas the polyethylene and SPCC plates could not. Corona discharge treatment of the polyethylene surface enabled the joint formation with SPCC. The tensile shear fracture load of the SPCC/polyamide 6 and SPCC/corona-discharge-treated polyethylene joints increased with the joining speed up to 600 mm min-1, beyond which it decreased. These joints were fractured at the base material of the plastic plate at optimal joining speeds in the tensile shear test. Continuously joined interfaces of these materials were observed via cross-sectional microstructure analysis. Transmission electron microscopy and selected area diffraction patterns indicated that these materials were joined through the surface oxide layer of SPCC composed of Fe3O4. The relation between the tensile shear fracture loads and the results of XPS analysis indicated that polar groups such as amide, hydroxyl, and carboxyl groups on the plastic surfaces were highly effective for joint formation of the SPCC/plastic joints.
  • Effect of Arsenic and Copper+Arsenic on High Temperature Oxidation and Hot Shortness Behavior of C–Mn Steel

    pp. 1232-1240

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    DOI:10.2355/isijinternational.ISIJINT-2016-002

    To avoid surface hot shortness of C–Mn steels containing arsenic and copper+arsenic, high temperature oxidation behavior of the steels were systematically investigated. The results showed that the presence of arsenic and copper+arsenic accelerated oxidation degree of C–Mn steel. Meanwhile, arsenic alone contributed to grain boundaries oxidation, which resulted in the formation of noticeable oxide particles band along grain boundaries at 1000°C and 1050°C. Furthermore, arsenic enrichment degree and the penetration of copper-rich liquid phase containing arsenic into grain boundaries was most severe at 1050°C, while arsenic enrichment degree and the penetration of copper-rich liquid phase were reduced when oxidation temperatures exceeded 1100°C due to diffusion of arsenic and copper+arsenic back into the metal and the occlusion of arsenic-rich/copper-rich particles into oxide scale. The hot-compression testing results were confirmed to agree well with enrichment characteristics of arsenic and copper+arsenic in thermogravimetric (TG) investigations.
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  • Effect of Pre-heat Treatment on Hardening Behavior in Gas Nitrocarburized Carbon Steel

    pp. 1241-1247

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    DOI:10.2355/isijinternational.ISIJINT-2016-034

    To investigate the effect of the pre-heat treatments on hardening behavior in plane carbon steel during gas nitrocarburizing process, Fe-0.4mass%C-0.2mass%Si-0.8mass%Mn steel was nitrocarburized after pre-heat treatments, normalizing and low-temperature annealing. Hardness and nitrogen concentration in nitrocarburized layer of the normalized and annealed steel was lower than that of steel without pre-heat treatment. Nitrocarburized microstructure was observed by transmission electron microscopy. The observation showed that iron nitrides α”- Fe16N2 were precipitated. The α”- Fe16N2 grains formed in the normalized and annealed steel was larger than that in the steel without pre-heat treatment. These results suggest that both the nitrogen concentration and precipitation behavior of α”- Fe16N2 in the nitrocarburizing process contribute to the difference of the amount of hardening between the pre-heat treated steel and steel without pre-heat treatment.
  • Effect of Annealing Duration on the Structure and Hardness of Electrodeposited Ni–W Alloys

    pp. 1248-1255

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    DOI:10.2355/isijinternational.ISIJINT-2016-111

    Ni–W alloys were electrodeposited onto steel sheets with an initial Ni plating. The electrodeposition was conducted in unagitated sulfate solution containing citric acid at pH 5 and 60°C under coulostatic (9.0 × 105 C·m-2) and galvanostatic (100–3000 A·m-2) conditions. The effect of annealing duration on the structure and hardness of the deposited Ni–W alloys was investigated. Compared with the non-annealed case, annealing for several seconds significantly increased the crystallite size of the Ni in the deposits. Moreover, the crystallite size gradually increased with annealing duration. When the holding time at 700°C was increased to 100 s and 1000 s, the Ni4W and NiW precipitates increased and coarsened in the upper layers of the deposits. Annealing increased the hardness of all deposits, especially at high current densities (i.e., at high W content in the deposits). In deposits with 45 mass% W content, the hardness was maximized at a holding time of 10 s at 700°C, and gradually decreased as the holding time increased. Under the optimal precipitation conditions, the Ni4W and NiW precipitates were fine and uniformly distributed throughout the deposits.
  • Effect of Ce on the Evolution of Recrystallization Texture in a 1.2%Si-0.4%Al Non-oriented Electrical Steel

    pp. 1256-1261

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    DOI:10.2355/isijinternational.ISIJINT-2015-641

    The effect of Ce element on the evolution of recrystallization texture in a 1.2%Si-0.4% Al non-oriented electrical steel during annealing process was investigated by using OM, SEM/EBSD and OIM software. The results showed that the recrystallized grain size and recrystallization ratio of specimens containing Ce are coarser and higher than those of specimens without Ce respectively. The fraction of favourable textures in specimens with Ce during recrystallization is also higher than those in specimens without Ce. Moreover, the fraction of unfavorable {111} texture in specimens with Ce is very less. Appropriate Ce in non-oriented electrical steel purified steel, reduced fine MnS, coarsen and modified Al2O3 and spherified AlN inclusions. This result in more favorable textures appeared in non-oriented electrical steel containing Ce.
  • Effect of Nb Contents on Size of Ferrite Grains and Nb Precipitates in Ultra-low Carbon Steel for Cans

    pp. 1262-1267

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    DOI:10.2355/isijinternational.ISIJINT-2015-688

    Soft-tempered, fine-grained ferritic steel is preferred for film-laminated steel for drawn cans from the viewpoints of surface quality and high formability. Generally, ultra-low carbon steel (ULC) is soft but has coarse ferrite grains. On the other hand, low carbon steel (LC) has a fine ferrite grain size but is hard and has inferior formability, and thus is not suitable for drawing. In order to solve this problem, the contents of carbon and niobium in ULC steel were varied in order to control the size of niobium carbide precipitates. This study suggested that the newly-developed steel has the potential to have an excellent balance of ferrite grain size and formability.
  • Comparison of Constant Load, SSRT and CSRT Methods for Hydrogen Embrittlement Evaluation Using Round Bar Specimens of High Strength Steels

    pp. 1268-1275

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    DOI:10.2355/isijinternational.ISIJINT-2015-565

    Resistance to hydrogen embrittlement of low alloy steels was evaluated based on their critical hydrogen content and critical stress. Constant load test (CLT), Slow Strain Rate Test (SSRT) and Conventional Strain Rate Test (CSRT) were carried out using JIS-SCM435 and V-added steels in six laboratories. It was confirmed that the same test results were obtained in different laboratories under the same test conditions. Furthermore, the relationships between diffusible hydrogen content and nominal fracture stress obtained by means of CLT and SSRT were similar to each other. In CSRT, the nominal fracture stress was higher than that in CLT and SSRT under the same absorbed hydrogen content in the specimens. In SSRT and CSRT, fracture surfaces showed Quasi-cleavage mode under small hydrogen content, while they showed Inter-granular fracture under large hydrogen content. In order to compare the three methods considering the concentration of hydrogen in stress field, locally accumulated hydrogen content under the same fracture stress is calculated. The locally accumulated hydrogen under the same applied stress, in other words, critical hydrogen content to hydrogen embrittlement, is the following order; SSRT < CLT < CSRT in JIS-SCM435, and CSRT < CLT ≒ SSRT in V-added steels.
  • Improvement of Fatigue Properties of Resistance Spot Welded Joints in High Strength Steel Sheets by Shot Blast Processing

    pp. 1276-1284

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    DOI:10.2355/isijinternational.ISIJINT-2016-043

    This paper discusses the effect of shot blast processing on the fatigue strength of resistance spot welded joints of 980 MPa steel sheets and the mechanism by which the fatigue strength is improved.The fatigue limit load of shot blasted spot welded joints was extended to approximately twice that of the non-blasted types. While high compressive residual stress was conferred on the outer surface of the shot blasted steel sheets, there was little variation in the residual stress on the side of the overlapped surface. Shot blasting on the outer sheet surface increased the initiation life of the fatigue cracks that occurred on the overlapped surface and reduced the propagating speed of the cracks that grow from the overlapped surface toward the outer surface. The initiation and propagation of fatigue cracks were affected in the region where compressive residual stress was not conferred. An FE-analysis suggests that the compressive residual stress on the outer sheet surface reduces the opening of the sheet separation of the spot welded joints under the fatigue test load and reduces the maximum principal stress around the tip of the corona bond.
  • Absorbed Energy Distribution of Ductile Ni-resist Alloyed Iron Under Instrumented Impact Load at Low Temperatures

    pp. 1285-1288

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    DOI:10.2355/isijinternational.ISIJINT-2016-049

    In this study, in order to investigate the absorbed energy distribution on the low-temperature impact fracture process of ductile Ni-resist alloyed iron, the low-temperature impact tests of ductile Ni-resist austenitic alloyed iron (the main composition of 23.0 wt.% Ni and 4.0 wt.% Mn) were completed from room temperature to -193°C. Through the analysis of specific energy and load, the result shows that the impact crack initial energy Ei and unstable propagation energy Eup of ductile Ni-resist alloyed iron respectively present opposite trend with the decreasing of temperature, but their total energy Eadd is relatively stable. The metastable propagation strength energy Est under high load in the impact fracture process is the main decisive factor on the low-temperature impact property of ductile Ni-resist alloyed iron, and it is determined by the increasing of average energy valley load Fave and the decreasing of metastable propagation deflection dst under high load respectively at the front and back of the extreme point (-80°C).
  • On Strengthening of Austenitic Stainless Steel by Large Strain Cold Working

    pp. 1289-1296

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    DOI:10.2355/isijinternational.ISIJINT-2016-095

    The tensile properties of an S304H austenitic stainless steel processed by large strain cold rolling were studied. Cold rolling was accompanied by the deformation twinning and martensitic transformation. The fraction of strain-induced martensite comprised 0.65 after cold rolling to a total strain of 4. The transverse grain size progressively reduced to about 200 nm in both the austenite and martensite phases. The deformation microstructures were characterized by large internal distortions, which were attributed to high dislocation density well above 1015 m-2. Cold rolling resulted in significant strengthening. The yield strength increased from 290 MPa in the initial annealed state to 2050 MPa in the sample subjected to cold rolling to a total strain of 4. The strengthening could be adequately explained by increasing the dislocation density. In the framework of dislocation strengthening model the austenite and strain-induced martensite provided equal fractional contribution to the overall strength of the rolled steel.
  • Three-Dimensional Modeling of Interfacial Tension with Smoothed Particle Hydrodynamics by Using Pairwise Potential

    pp. 1297-1299

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    DOI:10.2355/isijinternational.ISIJINT-2016-031

    A new pairwise potential was introduced into the three-dimensional SPH simulation program and the dynamic behavior of small droplets on a solid substrate was calculated. The simulation could reproduce the experimental observations for an aluminum droplet on an alumina substrate as well as those of water on glass and Teflon substrates. The program was applied to an iron droplet on an aluminum substrate, and the droplet’s dynamic behavior was estimated.
  • Carbothermic Reduction of Alumina at 1823 K: On the Role of Molten Iron and Reaction Mechanisms

    pp. 1300-1302

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    DOI:10.2355/isijinternational.ISIJINT-2016-037

    A possible reaction mechanism is presented for the low temperature carbothermic reduction of alumina in the presence of molten iron under inert conditions. This reduction was found to take place in a number of stages that involved the carburization of molten iron, disintegration of alumina into sub-oxide gases, reduction of these gases by the solute carbon followed by the capture of reduced aluminum by molten iron due to its high affinity. With increasing levels of aluminum in the iron, the wettability of alumina with molten metal undergoes a transition from poor wetting (with Fe) to good wetting (with Fe–Al–C) leading to the dissolution and subsequent reduction of alumina in the Fe–Al–C melt. These reaction steps were observed to occur in Al2O3–Fe2O3–C system at 1823 K and atmospheric pressure.
  • A Simple Mathematical Model for Estimating Plume Hydrodynamics of Metallurgical Ladles

    pp. 1303-1305

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    DOI:10.2355/isijinternational.ISIJINT-2016-035

    In secondary steelmaking, the refining operation is typically carried out in a metallurgical ladle that is commonly furnished with gas injection facilities. Since gas injection plays an intrinsic role in determining the efficiency of secondary steelmaking and thus the quality of final products, a great volume of work has been conducted mostly utilizing air-water systems at room temperature. Despite a portion of the air-water correlations have been adopted in the literature, their applicability to the real argon-metal system is still questionable. The main motivation behind this paper is to present a simple mathematical model for plume hydrodynamics of metallurgical ladles based on the characteristic phenomena and underlying mechanisms of a buoyant plume, where bubble breakup and coalescence occur simultaneously. The main assumptions/simplifications and governing equations are firstly introduced. After that, the accuracy of the model is demonstrated by comparing predicted plume velocities with the ones measured in an industrial ladle.

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