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

ISIJ International Vol. 54 (2014), No. 8

  • Novel Process of Ferronickel Nugget Production from Nickel Laterite by Semi-molten State Reduction

    pp. 1749-1754

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    DOI:10.2355/isijinternational.54.1749

    Rotary kiln-electric furnace (RKEF) process is the main technology to deal with nickel laterite for the ferronickel alloy production in the world. However, this process needs huge amount of electric power due to the large ratio of slag to metal. Therefore, a novel process was proposed to directly produce ferronickel alloy nugget at a related low temperature from nickel laterite by the semi-molten reduction in the reactor like the rotary hearth furnace (RHF). The effects of temperature and basicity of the slag on the separation between the slag and metal were investigated, the results revealed that it is reasonable to achieve the ferronickel alloy nugget directly at 1400°C when the quaternary basicity ((mCaO+mMgO)/(mSiO2+mAl2O3)) was fixed at 0.60. Bad wettability of the refractory by the slag is good for the discharge of product from RHF and avoiding the corrosion of the slag. The high-temp wettability of refractory materials by the molten slag was also carried out with the sessile drop method, the results shows that the wettability order of the refractory by the slag from good to bad is Al2O3, MgO and graphite. It seems that the graphite is the suitable refractor material for bottom of RHF. However, the anti-oxidation of the graphite in the charging and discharging area is another potential problem which needs further study.
  • Effect of Al Content on the Characteristics of Inclusions in Al–Ti Complex Deoxidized Steel with Calcium Treatment

    pp. 1755-1764

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    DOI:10.2355/isijinternational.54.1755

    To improve the effect of calcium treatment and the cleanliness of steel and to make use of fine TiOx to refine the microstructure of steel, the effect of aluminum content on inclusion characteristics of aluminum-titanium complex deoxidized and calcium treated steel is investigated based on the experiment with SEM/EDS, Image Pro-Plus 6.0 and FactSage 6.1 softwares and thermodynamic calculation in the present work. The results show that the inclusions in two steels with different aluminum content are obviously spherical composite inclusions with a two-layer complex structure, consisting of an Al2O3–CaO–TiOX core surrounded by MnS. In low aluminum steel, the oxide core of inclusions contains much TiOx and CaO, and their composite structure is mosaic compared with bundle in high aluminum steel, the number of inclusions is 2.5 times more than that in high aluminum steel, the thickness of MnS on oxides surface is also thinner. In addition, melting point of the inclusions in low aluminum steel is lower, the cleanliness of the steel is relatively improved because of the inclusions floating up, and the deformation aspect ratio of calcium aluminate inclusions with a certain amount of Ti2O3 is effectively improved, which is about 1–2 while the composition of oxide core is xAl2O3 = 35–55%, xTi2O3 = 15–35%, and xCaO = 10–25%. As a result, less calcium is needed to modify the alumina inclusions to liquid calcium aluminate in the case of lower aluminum deoxidized steel, thus the calcium treatment effect can be improved. The low aluminum in steel is more effective to control the inclusion characteristics to reduce the harm of MnS and to improve the cleanliness of steel.
  • Formation and Growth of Iron Nuclei on Wüstite Surface at the Initial Stage of Reduction

    pp. 1765-1771

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    DOI:10.2355/isijinternational.54.1765

    Formation and growth of iron nuclei on the (001) surface of Fe1–xO by hydrogen ion bombardment were observed using scanning tunneling microscope (STM) and low energy electron diffraction (LEED) in order to understand the change in the nano-level structure of the Fe1–xO surface at the initial stage of reduction. It has been found from the STM images that prior to the formation of iron nuclei, hydrogen ion bombardment increases the number of large dents called “depression” on the terrace of the Fe1–xO surface, which may imply that O atoms are deficient on the surface. Simultaneously, the number of steps on the Fe1–xO surface increases probably owing to the reconstraction of the surface structure. According to the LEED pattern, the (2×2) long range periodic structures have appeared which may also demonstrate that the O atom-deficient surface is produced by hydrogen ion bombardment. As the reduction proceeds, the iron nuclei with the shape of a slightly rounded truncated four-sided pyramid are preferentially formed on the surface area where the steps are densified. The side length and the height of the pyramids are 4–8 nm and 1.2–1.8 nm, respectively. Subsequently, the iron nuclei are also formed on the terraces.
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    Readers Who Read This Article Also Read

    1. Fault Diagnosis for Blast Furnace Ironmaking Process Based on Two-stage Principal Component Analysis ISIJ International Vol.54(2014), No.10
    2. Viscosities of Iron Blast Furnace Slags ISIJ International Vol.54(2014), No.9
    3. Characterisation of Coke Packed Beds after Liquid Slag Flow at 1500°C by Image Analysis ISIJ International Vol.54(2014), No.8
  • Modeling the Gas-solid Flow in the Reduction Shaft of COREX

    pp. 1772-1780

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    DOI:10.2355/isijinternational.54.1772

    COREX is a promising alternative to blast furnace ironmaking. It includes two main reactors: a reduction shaft (RS) for the direct reduction of iron ore and a melter gasifier for the melting reduction of directly reduced iron. This work uses a two-dimensional slot model to investigate the gas-solid flow in the RS by a combined computational fluid dynamics and discrete element method approach. The three-dimensional flow of cohesive solids is then examined for three RS designs by the discrete element method. The effects of gas flow, the stickiness between particles, the rotational speed of screws, and different designs are depicted in terms of gas-solid flow pattern, overall bed pressure drop and solid flowrate. The results show that the effect of gas flow is insignificant on gas-solid flow pattern due to the small gas-solid interaction forces under the considered conditions. Solid flow varies in a complex manner with the rotational speed of screws and the sticking force, and a correlation is formulated for predicting the solid flowrate based on the simulated results. It is also shown that the effect of geometrical design on solid flow is complicated and significant. Caution should be taken for any changes in the design. The findings should be useful for the design, control and optimization of the RS operation.
  • Binders and Bonding Mechanism for RHF Briquette Made from Blast Furnace Dust

    pp. 1781-1789

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    DOI:10.2355/isijinternational.54.1781

    Blast furnace dust is a kind of solid waste that produced in the process of iron smelting and it contains large amount of Fe and non-ferrous metal elements. It’s not only a very good Fe-contained resource, but also a very important non-ferrous metal resource. Application of blast furnace dust to RHF (Rotary Hearth Furnace) briquette is an effective and comprehensive utilization method. However the strength of the briquettes is low, therefore the application of binder to the briquette should be developed. In this study, physical and chemical characteristics of blast furnace dust were investigated firstly. And the experiments of different binder used in the briquettes were studied. Then the bonding mechanisms of binders were discussed. The experiment results showed that different binder has different bonding mechanism of the briquettes. According to the application effect of binders, it should be priority to use the composite binder, and the excellent collocation pattern is starch binder together with silicon-containing binder, such as sodium silicate.
  • Characterisation of Coke Packed Beds after Liquid Slag Flow at 1500°C by Image Analysis

    pp. 1790-1796

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    DOI:10.2355/isijinternational.54.1790

    The flow of slag in the lower zone of the ironmaking blast furnace was experimentally simulated by adding slag to a high temperature laboratory scale coke packed bed. The flow of slag through packed beds of coke with packing densities varying from 50% to 65% was examined at 1500°C. Since the liquid flow through a packed bed depends on packing properties such as particle size, particle shape, pore size and pore neck size, it was necessary to characterise these properties of the beds.
    In this work, image analysis of successive sections of the tested beds was utilised to characterise the bed packing properties. In addition, the slag distribution and holdup was also measured. The procedure was that at the end of each experiment, the beds were cooled down, cold mounted in resin, then sectioned into approximately 4 mm thick slices. Each section was then analysed to measure particle and pore geometric properties, and slag distribution. It was found that the slag holdup was well distributed within the bed and was varied in position between bed cross sections, indicating no significant wall effect. The average pore size and average pore neck size were found to decrease as the packing density increased. Slag flow caused the particle sphericity and the average pore neck size to increase in comparison to an unreacted bed due to the slag-coke interaction.
    x

    Readers Who Read This Article Also Read

    1. Flow of Molten Slag through a Coke Packed Bed ISIJ International Vol.54(2014), No.4
    2. Formation and Growth of Iron Nuclei on Wüstite Surface at the Initial Stage of Reduction ISIJ International Vol.54(2014), No.8
    3. Effect of Powder–Liquid Interaction on Their Accumulation Behavior in Packed Bed ISIJ International Vol.54(2014), No.6
  • Effects of Immersion Depth on Flow Turbulence of Liquid Steel in a Slab Mold Using a Nozzle with Upward Angle Rectangular Ports

    pp. 1797-1806

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    DOI:10.2355/isijinternational.54.1797

    Fluid flow of liquid steel in a slab mold influenced by a submerged entry nozzle (SEN) with ports of high aspect ratio and upward angle of 100 was studied using a water modeling approach and experimental techniques including tracer injection, particle image velocimetry and ultrasound velocimetry. Fluid dynamics near the ports indicate that the discharging jet is subdivided into upper and lower jets when the SEN is at its deep position (185 mm) forming a double roll flow (DRF). At the shallow position (115 mm) the tendency to form two jets decrease with a general trend to form a single jet and a single roll flow (SRF) pattern. These flows are attributed to the difference of the hydrostatic pressures between both submergences. Both, the upper flow in the DRF and the upper flow in the SRF induce free shear flows near the free bath surface that give origin to vortexes and unstable meniscus dynamics. Therefore, nozzle ports with upward angles create small pressure gradients which, in spite of their small magnitudes, have profound effects on fluid flow patterns of the fluid in the whole working mold volume. The results indicate that this nozzle works with less turbulence in the shallow position.
  • Fundamental Study of Sn Removal from Hot Metal by NH3 Gas Blowing

    pp. 1807-1812

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    DOI:10.2355/isijinternational.54.1807

    Development of a practical method of Sn removal in the steelmaking process is necessary from the viewpoints of promoting use of scrap procured in the market and reducing energy consumption. It is well known that Sn promotes surface cracks of billets in hot rolling by coexisting with Cu. Although various methods of Sn removal have been investigated in laboratory experiments, enough Sn removal efficiency for commercially use has not been obtained. In the present study, Sn removal from high-S hot metal by NH3 gas blowing was investigated in laboratory experiments as a new method of Sn removal. The laboratory experiment on Sn removal from hot metal was carried out using up to a 10 kg-scale vacuum induction melting furnace. Sn removal was accelerated while blowing NH3 gas, and the evolution of gas bubbles were observed at the hot metal surface. Within the ranges of these experiments, higher temperature and higher concentrations of S and N were advantageous for Sn removal. The mechanism of the acceleration of Sn removal by NH3 gas blowing could be estimated that oversaturated N or H in hot metal made small bubbles to increase the hot metal surface for SnS evaporation. In the estimation of Sn removal ratio in plant-scale operation, it could reach 40%. For further rapid Sn removal, it was necessary to maximize [N] of hot metal by optimizing the lance height or flow rate of NH3 gas.
  • Effective Desiliconization Method with Strong Mixing of Agent in Swirling Flow of Hot Metal

    pp. 1813-1822

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    DOI:10.2355/isijinternational.54.1813

    Control of the silicon content in hot metal is one important strategy for better hot metal dephosphorization and subsequent BOF operation. For this purpose, hot metal desiliconization is carried out by the addition of iron oxide at the tilting runner in the blast furnace casthouse. Since hot metal desiliconization is determined by mass transfer of FexO in the slag as well as mass transfer of silicon in the metal in the low FexO concentration range, attention should be focused on mixing of the desiliconizing agent and hot metal at the tilting runner. The authors therefore applied a swirling flow to the hot metal, aiming at effective silicon removal. A basic investigation of the swirling flow was carried out in a water model experiment. The swirling flow was generated in a funnel-shaped container with various flow rates, outlet diameters, etc. The experiment showed that mass transfer of a tracer in model slag was enhanced under the swirling flow condition. A 5-ton hot metal experiment was carried out with a specially designed vessel to generate a swirling flow of hot metal. A desiliconization experiment with addition of iron ore showed that silicon removal was enhanced by the swirling flow of hot metal, especially with slag of low FexO content.
  • Sintering Behavior of Silica Filler Sands for Sliding Nozzle in a Ladle

    pp. 1823-1829

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    DOI:10.2355/isijinternational.54.1823

    A study has been carried out to clarify the sintering behavior of silica filler sand. Sintering experiments were firstly conducted and the experimental data were compared with the thermodynamic calculations by FactSage and MELTS software.
    It was found that sintering of the silica sand proceeded through the evolution of liquid phases, which were originated from alkali feldspars. The liquid proportion was found to increase with an increase in temperature. It wan considered that the viscous liquid phases connecting silica particles contributed to the strength of the sintered sand. The thermodynamic calculations revealed that MELTS software could sufficiently simulate the phases in equilibrium with each other. The comparison of the experimental data with the thermodynamic simulations demonstrated that the whole system of the silica sand did not attain equilibrium. It was hence inferred that filler sands should be designed to prevent the state fully sintered by controlling the particle size.
  • Numerical Modeling of Fluid Flow and Solidification Characteristics of Ultrasonically Processed A356 Alloys

    pp. 1830-1835

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    DOI:10.2355/isijinternational.54.1830

    The main objectives of this study is to determine the fluid flow characteristics and cavitation phenomena during molten A356 alloys processed via high power ultrasonic treatment (UST) as well as the resulting solidification microstructure of the ultrasonically processed A356 alloys. A previously developed UST modeling approach is applied to obtain the simulation results in this study. The modeling approach consists of two main models: (1) A CFD UST cavitation model is applied to simulate acoustic streaming, cavitation and bubble dynamics during molten alloy processing and (2) A stochastic mesoscopic modeling to predict microstructure evolution during alloy solidification. The first model can predict the amount of the cavitated phase that is used to determine the number of potential nuclei formed due to the UST processing. The second model uses the predicted number of nuclei from the first model as an input to predict the microstructure evolution during the solidification of cast alloys. In addition, the UST model predicts the fluid flow characteristics that include flow profile, velocity vectors and magnitude, pressure contours, and streamlines under two different gravity conditions. The fluid flow characteristics are analyzed to ensure that proper ultrasonic stirring and cavitation of the melt are achieved.
  • Two-stage Particle Swarm Optimization-based Nonlinear Model Predictive Control Method for Reheating Furnace Process

    pp. 1836-1842

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    DOI:10.2355/isijinternational.54.1836

    The steel slab temperature control of reheating furnace process plays an important role in the production of high quality reheated slab. Because of the characteristics of nonlinearity, long time-delay and uncertainty, high-accuracy slab temperature control is a challenging problem. This paper proposes a two-stage particle swarm optimization (PSO)-based nonlinear model predictive control (NMPC) method to solve the problem. In this method support vector machine (SVM) is utilized to construct the nonlinear predictive model based on the real production data. To obtain better predictive model dynamically, PSO optimizes the parameters of SVM for different problems. Then PSO solves the rolling optimization problem in NMPC to obtain the proper control variables. Finally, the production data collected from a real reheating furnace process are utilized to test the proposed method. Numerical experiments are done by computer simulation based on the real production data. The experiment results illustrate that the PSO-based SVM can obtain accurate predictive model. Moreover, the proposed nonlinear model predictive control method can obtain outstanding control accuracy in steel slab temperature control.
  • Correlation of Optical Microstructure and Macroscopic Intensity of Defects on Steel Strip Surface

    pp. 1843-1848

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    DOI:10.2355/isijinternational.54.1843

    Low contrast defects which originate from inclusions under the surface layer are often observed as patterns of shallow bright streaks with the exception of exposed inclusions on the surface of steel strips. These defects also appear brighter than the normal part under most optical conditions. In this research, the relationship between the macroscopic intensity of the defective parts and the defect microstructure was studied in detail with galvannealed (iron–zinc alloy) steel strips. The following points were observed: 1) Defective parts contain a number of microscopic flat portions which have mirror reflection parallel to the surface, and the reflected intensity of these micro-flat portions is dominant in specular reflection. 2) A clear correlation exists between the specular reflected intensity in the macroscopic images of the defective part and the unit area ratio of the microscopic flat portions. 3) When the intensity from macroscopic observations can be estimated while considering the polarized reflection characteristics of the defect and treating the flat portion as a mirror facet, it agrees with the measured intensity.
    The results of this study led to a better understanding of the reason why defects originating from inclusions under the surface layer appear slightly brighter than the normal area on steel strips.
  • A Novel Structure of Blackbody Cavity Sensor for Continuous Measurement of Molten Steel Temperature

    pp. 1849-1855

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    DOI:10.2355/isijinternational.54.1849

    For continuous measurement of molten steel temperature, this work proposed a novel structure of blackbody cavity sensor to overcome the shortcomings of the traditional sensor. The traditional sensor had slow response speed and poor thermal shock resistance, which was due to its double layer tubes and thermal stress concentration in the closed bottom of an inner corundum tube respectively. The double layers included an outer protective tube of Al2O3–C refractory and the inner tube which formed the blackbody cavity and isolated the fume generated by the outer tube. The novel structure was composed of a measurement cavity of an Al2O3–C tube and a fume exhaust system. The fume exhaust system, including an inlet, an exhaust pipe and two outlets, eliminated the fume influence of obstructing the optical path. Furthermore, the fume exhaust system was based on the relationship between the fume and temperature distribution. Results showed that the fume formed below approximately 1320°C, meaning the fume existed in and above the slag layer in the internal cavity of the sensor. The open bottom of the exhaust pipe should locate at the interface between the slag layer and molten steel to remove the fume. Additionally, the sensor should dip into molten steel up to 220 mm for high accuracy. It improved the response time from 340 to 240 seconds and enhanced the thermal cycling times from about 4 to beyond 10 times.
  • Finite Element Analysis of Three-Dimensional Hot Bending and Direct Quench Process Considering Phase Transformation and Temperature Distribution by Induction Heating

    pp. 1856-1865

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    DOI:10.2355/isijinternational.54.1856

    In this study, a coupled thermo-mechanical-metallurgical finite element analysis (FEA) method was developed to investigate the deformation behavior in the three-dimensional hot bending and direct quench processes. In the developed FEA procedure, the temperature distribution was calculated by two methods. First was a three dimensional electromagnetic and heat conduction analysis that considered a non-linearity of permeability and magnetic transformation. Second was a simplified method that used an original heat source model for induction heating. In the deformation analysis, temperature, micro structure and strain rate dependencies of flow stress were taken into consideration. As for the microstructure evolution, an experimental formula was used to track the ferrite-austenite transformation, and Koistinen-Marburger relationship was employed to describe the austenite-martensite change. To confirm the effectiveness of the developed FEA method, the thickness change upon bending and the camber by inhomogeneous cooling were simulated. The results were in good agreement with the experimental measurements.
  • Metallurgical Characterization and Diffusion Studies of Successively Buttered Deposit of Ni–Fe Alloy and Inconel on SA508 Ferritic Steel

    pp. 1866-1875

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    DOI:10.2355/isijinternational.54.1866

    The present work aims to investigate the major problem of carbon diffusion in dissimilar metal weld (DMW) between ferritic and austenitic stainless steel used in Nuclear Power Plants. For such DMW joints, Inconel 82 is often deposited on ferritic steel with Gas Tungsten Arc Welding (GTAW) process, but the problems associated with carbon diffusion persist. In the present study, Ni–Fe alloy (ERNiFe-CI) and Inconel 82 (ERNiCr-3) have been successively deposited with Gas Metal Arc Welding (GMAW) process. The substrates of SA508Gr.3Cl.1 ferritic steel were used for deposition. First buffer layer of Ni–Fe alloy was deposited with GTAW in order to have low heat input initially and the subsequent deposition was carried out using GMAW over the buffer layer. The effect of operating temperature of DMW joints on carbon migration from the ferritic steel to buttering zone was studied by carrying out the thermal ageing 450°C for 240 h. Diffusivity of carbon at different ageing temperatures was quantified. The effectiveness of buttering deposit was addressed with respect to metallurgical properties and carbon diffusion by carrying out the heat flow analysis, Electron Probe Micro Analysis (EPMA), Optical Emission Spectroscopy (OES), martensite formation analysis at fusion interfaces, micro-hardness variation across the fusion interfaces and the microstructure evolution. Significant amount of martensite was observed to be formed at the fusion interface and the subsequent effect of stress relieving was addressed. Momentous reduction in carbon diffusion and favourable metallurgical properties owing to successive buttering deposit could increase the life of DMW joints with cost effective GMAW over the GTAW process at Nuclear Plants.
  • Ultrasonic C-scan Detection for Stainless Steel Spot Welds Based on Signal Analysis in Frequency Domain

    pp. 1876-1882

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    DOI:10.2355/isijinternational.54.1876

    In this paper, ultrasonic C-scan detection is conducted on stainless steel spot welds, and C-scan images are obtained respectively through characteristic signal analysis in time domain and frequency domain. C-scan image of frequency domain characteristics signal, which is less affected by detecting conditions, can reflect the weld nugget morphology more truly. After C-scan image enhancement and edge detection processing, the dimension of spot weld nugget can be automatically obtained through equivalent diameter algorithm procedure. The dimension of nugget contains corona bond, and it is in good agreement with corona bond external diameter through metallographic measured value. So it can be taken as the reference to evaluate the quality of spot welds. Without the influence of corona bond, detection accuracy is very high. The error is less than 0.066 mm.
  • Welding Defects Occurrence and Their Effects on Weld Quality in Resistance Spot Welding of AHSS Steel

    pp. 1883-1889

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    DOI:10.2355/isijinternational.54.1883

    An investigation was made on the weldability of DP600 steel. Influences of electrode force, welding current and welding time on welding defects occurrence were discussed. Metallographic analysis and tensile-shear tests were implemented for weld quality detections. Weld quality dependency on welding defects was studied. It was found that welding defects were easy to occur during spot welding process of DP600 steel, which could be explained by its rich chemistry. Lower electrode force, larger welding current and longer welding time all contributed to the easy occurrence of welding defects. HAZ softening caused by martensite tempering was observed under strong welding parameters. For interfacial failure, welding defects effects on weld quality should not be underestimated. As to pullout failure, the effect of expulsion on weld quality was not that significant as suggested by other researches. In the end, a method was introduced to predict peak load and maximum tensile-shear displacement for pullout failure.
  • Influence of Substrate Molybdenum on Galvannealing Kinetics of an Advanced High Strength Steel

    pp. 1890-1897

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    DOI:10.2355/isijinternational.54.1890

    Advanced high strength steels (AHSS) are used for car body applications in the automotive industry to improve passenger safety, improve fuel efficiency and to lower harmful carbon emissions. Galvannealed AHSS are used to provide corrosion protection. AHSS are alloyed with several elements such as Mn, Si, Al, P, Mo, etc. to achieve better mechanical properties. However, the knowledge of the effects of Mo on galvannealing behavior is limited. This investigation was conducted to study the influence of molybdenum in steel on galvannealing kinetics. Five dual-phase steels with Mo varying from 0 wt% to 0.4 wt% were used for this study. The results showed a delay in the galvannealing kinetics with an increase in the steel molybdenum content. No obvious improvement in the coating appearance was observed with increasing molybdenum.
  • Assessment of Inter-granular Corrosion Susceptibility of 304L Stainless Steel Using Non-destructive Electrochemical Techniques

    pp. 1898-1905

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    DOI:10.2355/isijinternational.54.1898

    The effect of heat treatment on the corrosion resistance of AISI 304L stainless steel is studied in the present investigation. As-received samples of 304L stainless steel were solution annealed at 1050°C for 90 minutes followed by thermal ageing at 750°C for various time durations (upto 24 days). ASTM standard A-262 practice A test was used to detect susceptibility of inter-granular attack. Electron dispersive spectroscopy attached to scanning electron microscopy was performed for identification of carbides. It was observed that no significant amount of attack took place upto 1 day. The extent of sensitization was quantitatively evaluated using double loop electrochemical potentiodynamic reactivation test. The results obtained showed that corrosion resistance decreased with an increase in thermal ageing duration. This behavior was attributed to the precipitation of chromium carbides causing depletion of chromium in the areas adjacent to the grain boundaries (as determined by electron probe micro analyzer). The degree of sensitization of highly aged sample was found to be 24.6%. Further, the degree of sensitization in terms of polarization resistance was computed in the middle of the transpassive potential region (at 1.1 V) using electrochemical impedance spectroscopy.
  • Electrodeposition of Zn–V Oxide Composites from a Strongly Agitated Solution without Dispersed Particles

    pp. 1906-1912

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    DOI:10.2355/isijinternational.54.1906

    The electrodeposition of a Zn–V oxide composite under galvanostatic conditions from an agitated sulfate solution without dispersed particles and containing Zn2+ and VO2+ at pH 2 and 313 K was investigated. Although the V content in the deposits initially decreased with increasing current density, irrespective of the flow rate of electrolyte, a further increase in the current density resulted in an increase in the V content of the deposits. The curves, which show the relationship between the V content in the deposits and the current density, shifted to the higher-current-density region with increasing flow rate of the electrolyte. Agitation of the electrolyte decreased the V content of the deposits but reduced the segregation of V oxide. EDX point analysis of the cross-section of the deposits revealed that the V oxide concentrated at the surface of the deposits. The polarization curves in 3% NaCl solution revealed that the corrosion potential of the deposited Zn–V oxide films depended on the V content in the deposits, irrespective of the flow rate of electrolyte, and that the corrosion potential shifted toward the more noble direction with the codeposition of V oxide when the V content in the deposits was less than 2 mass%. At V contents of <4 mass%, the corrosion current density of the deposits decreased with increasing V content. The corrosion current densities of the deposits obtained from agitated solutions were smaller than those of the deposits obtained from unagitated solutions.
  • Nano Structure of the Rust Formed on an Iron-based Shape Memory Alloy (Fe–Mn–Si–Cr) in a High Chloride Environment

    pp. 1913-1919

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    DOI:10.2355/isijinternational.54.1913

    The corrosion resistance of an Iron-based shape memory alloy (28 Mn: Fe–Mn–Si–Cr) was estimated by using wet and dry corrosion test. The structure of the rust formed on the alloy was examined via EELS (Electron Energy Loss Spectroscopy) using TEM (Transmission Electron Microscopy) analysis. The electrochemical behavior of the 28 Mn alloy was investigated via EIS (Electrochemical Impedance Spectroscopy). The 28 Mn alloy showed a much higher corrosion resistance than SM (carbon steel) in wet and dry corrosion test. In EIS measurement following the corrosion tests, the 28 Mn alloy exhibited a much larger value of corrosion resistance (Z1mHz) as compared to SM, which can be attributed to rust formation. TEM analysis indicated that the inner rust was composed of Mn-rich, Cr-Si rich and interface layer. The corrosion resistance was primarily dependent on the Cr-Si rich layer, and further supported by the Mn rich layer. It was found that the 28 Mn alloy could maintain a protective rust layer composed of effective elements in the wet and dry environment.
  • Effects of Temperature and Oxygen Concentration on the Characteristics of Decarburization of 55SiCr Spring Steel

    pp. 1920-1926

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    DOI:10.2355/isijinternational.54.1920

    Effects of temperature and oxygen concentration on decarburization of 55SiCr spring steel were investigated at the temperature range of 500°C–1200°C by using a Muffle furnace (an atmosphere of ambient air) and a Simultaneous Thermal Analyzer (low oxygen concentration with 2% O2 and 98% N2). In ambient air, the decarburization behavior can be divided roughly into five temperature ranges: T≥1200°C, no decarburization; TG (A3 for 55SiCr when its carbon concentration equals to zero)<T<1200°C, partial decarburization; A3<T<TG, complete and partial decarburization; T<A3, complete decarburization; T<A1, no decarburization. However, it is observed that complete decarburization did not occur in all the testing temperature ranges under low oxygen concentration. For 55SiCr spring steel, decarburization behavior under low oxygen concentration can be divided into two regions: T<700°C, no decarburization; T>700°C, only partial decarburization. Lower oxygen concentration results in decrease in a growth rate of the oxide scale and leads to different characteristics of decarburization.
  • Preventing Abnormal Grain Growth of Austenite in Low Alloy Steels

    pp. 1927-1934

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    DOI:10.2355/isijinternational.54.1927

    Abnormal grain growth (AGG) in austenitic state is studied in low alloy steel in relation with precipitation state. It is observed that initial austenite grain size and precipitation state plays more important role in defining the normal or abnormal grain growth condition than the final one obtained after a heat treatment. Precipitate volume fraction evolution with time-temperature having similar quantity at the end but different initial grain sizes, showed different grain growth phenomenon. Arguments are presented to rationalize the presented experience. A simplified AGG model is applied to understand the effects of initial mean austenite grain size and precipitate size distribution on the subsequent AGG occurrence.
  • Effect of Surface Mechanical Attrition Treatment on Microstructure and Oxidation Behavior in T91 Steel at High Temperature

    pp. 1935-1942

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    DOI:10.2355/isijinternational.54.1935

    Traditional techniques and methods can not resolve the contradictions of the microstructure requirement between creep resistance and oxidation resistance in T91 steel at high temperature. In the present work, oxidation resistance of T91 steel were successfully improved by surface mechanical attrition treatment (SMAT). The variations of ferrite grains and precipitation behavior in the surface layer at different tempering temperature were also observed to investigate thermal stability. A FeCr2O4 spinel inner layer with higher Cr content could rapidly form and protect the material from further oxidation in T91 steel due to the nanocrystalline surface layer.
  • Effects of Microalloying on Stretch-flangeability of Ultrahigh-strength TRIP-aided Martensitic Steel Sheets

    pp. 1943-1951

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    DOI:10.2355/isijinternational.54.1943

    The effects of Cr, Mo, and Ni addition on the microstructure and stretch-flangeability of a 0.2%C–1.5%Si–1.5%Mn–0.05%Nb (mass%) transformation-induced plasticity (TRIP)-aided martensitic steel sheet produced by an isothermal transformation process at a temperature below martensite transformation-finish temperatures were investigated in order to develop third-generation steel sheet for automobiles requiring high hardenability. When 0.5% or 1.0% Cr was added to the base steel, a tensile strength of 1.5 GPa and a hole-expanding ratio of 40% was attained. On the other hand, the addition of Cr–Mo or Cr–Mo–Ni had a minimal influence on stretch-flangeability and stretch-formability, although it increased the yield and tensile strengths as compared to the base steel. The good balance of the Cr-bearing steel was mainly caused by a suitable combination of (1) volume fraction and (2) interparticle path of a finely dispersed martensite–austenite complex phase, which suppressed void initiation at the matrix/complex-phase interface on hole-punching and void coalescence or crack extension on hole-expanding.
  • Austenite Stability and Its Effect on the Ductility of the Cold-rolled Medium-Mn Steel

    pp. 1952-1957

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    DOI:10.2355/isijinternational.54.1952

    The mechanical and thermal stability of retained austenite in the cold-rolled medium-Mn (Fe-0.1C-5Mn) steel were studied by the tensile deformation at different temperature. The volume fractions of retianed austenite and microstructure of the starting and deformed materials were analyzed by X-ray diffraction, the scanning electron microscopy (SEM) and electron back scattered diffraction (EBSD). It was found that the volume fraction of retained austenite gradually decreased with strain increasing during tensile deformation. It showed high mechanical stability of retained austenite, which resulted in good comprehensive mechanical properties with ultrahigh strength and plasticity. Meanwhile, the tensile results at different temperature showed pretty high plasticity at low temperature (–40°C and –80°C) tensile deformation of the cold-rolled steel, which indicated the retained austenite maintained certain stability at low temperature. The results of this study indicate that the ductility in the medium-Mn steel with high contents of retained austenite can be altered by control of the austenite stability.
  • Influence of Grain Size on the Ductile Fracture Toughness of Ferritic Steel

    pp. 1958-1964

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    DOI:10.2355/isijinternational.54.1958

    The ductile fracture toughness of ferritic steel was assessed in terms of crack tip opening displacement (CTOD). The CTOD is composed of two parts: elastic and plastic. In the ductile fracture region, as compared to the elastic part, the fraction of the plastic part is dominant. The CTOD linearly increases with an increase in ferrite grain size, but grain coarsening simultaneously increases the possibility of cleavage fracture. As yield strength increases, the CTOD decreases due to the decreased plastic deformation ability.
  • Effect of Hydrogen on Deformation Behavior of Carbon Steel S25C —Measurement of Time Evolution of Strain Distribution until Crack Initiation Using Digital Image Correlation Method—

    pp. 1965-1970

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    DOI:10.2355/isijinternational.54.1965

    To explain hydrogen embrittlement, it is important to understand the effect of hydrogen on the plastic deformation of materials. In this study, we measured the plastic deformation process until crack initiation in hydrogen-charged and hydrogen-uncharged carbon steel S25C using the digital image correlation method. As a result, we found that the equivalent strain at crack initiation decreased at the stress-concentrated regions owing to the presence of hydrogen, whereas the size of regions with a high equivalent strain rate increased at an earlier stage. Comparing the equivalent strain rate in regions with roughly the same equivalent strain, we found that there is no significant difference between hydrogen-charged and hydrogen-uncharged specimens for small equivalent strain; however, the equivalent strain rate increased rapidly for large equivalent strain in hydrogen-charged specimens.
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    1. Influence of Grain Size on the Ductile Fracture Toughness of Ferritic Steel ISIJ International Vol.54(2014), No.8
    2. Effects of Temperature and Strain Rate on Tensile Properties of a Lean Duplex Stainless Steel ISIJ International Vol.54(2014), No.8
  • Effects of Temperature and Strain Rate on Tensile Properties of a Lean Duplex Stainless Steel

    pp. 1971-1977

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    DOI:10.2355/isijinternational.54.1971

    Effects of temperature and strain rate on tensile properties in a lean duplex stainless steel S32101 were investigated. The 0.2% proof stress and tensile strength increase with a decrease in the temperature and an increase in the strain rate. The uniform elongation decreased with an increase in the strain rate. In the temperature dependence on uniform elongation, it increased from 273 K to 283 K and indicated the maximum uniform elongation at 258 K. This is closely associated with TRIP effect because austenite is transformed to stress-induced martensite at temperatures below 283 K from the x-ray diffraction experiments. The stress-induced transformation behavior at 258 K, at which the maximum uniform elongation was obtained, had things in common with the case of metastable austenitic stainless steels. When the tensile properties were compared between the S32101 and the metastable austenitic stainless steels, the increase in the uniform elongation due to TRIP effect was almost the same. At low temperatures below about 250 K, the uniform elongations of the metastable austenitic steels were smaller than that of the S32101 because of the large amount of stress-induced martensite at small strains.
  • Utilisation of Residues of Nickel Production in EAF Steelmaking Route

    pp. 1978-1982

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    DOI:10.2355/isijinternational.54.1978

    A stock of over 100 Mt of residues (colas or tailings) from metallurgical production of nickel exists presently in Cuba with concentrations of around 0.4 wt% Ni, 42 wt% Fe and 2.0 wt% Cr. The annual production of colas in year 2005 will be about 10 Mt/year.
    The aim of this study is to develop a sustainable recycling concept in EAF steelmaking route for utilising a part of those residues, thus reducing impact on the environment and recycling valuable materials.
    Experiments have been carried out in suitably equipped 50 kg lab-scale electric induction furnace by adding, inserting or injecting of tailings at 1600°C with or without reducing agents (e.g. coal) to study recovery of the metals contained to the steel melt. The experiments were carried out under equilibrium, quasi-equilibrium and non-equilibrium conditions.
    Thermodynamical calculations at RWTH lab-scale conditions had shown nearly complete reducibility of the compounds NiO, Fe3O4 and Cr2O3 from the colas. The experimental conditions have confirmed this, resulting in about 90% recovery of Ni, 80% of Cr and 99% of Fe.
    Industrial tests at Antillana de Acero in Cuba in a 1.5 ton EAF with the injection of tailings in combination with coal have presented around 75% of recovery of chromium, 100% of nickel and 85% of iron.
  • Fundamental Experiment to Extract Phosphorous Selectively from Steelmaking Slag by Leaching

    pp. 1983-1990

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    DOI:10.2355/isijinternational.54.1983

    Steelmaking slag, including hot metal dephosphorization slag, is usually in the dicalcium silicate (C2S) saturated composition range. C2S is known to form a pseudo-binary solid solution with tricalcium phosphate (C3P) over a wide composition range, and most of the phosphorus in the slag forms a solid solution. The authors investigated the dissolution behavior of the solid solution and matrix phases in aqueous solutions, and the dissolution ratio of each element in matrix phase was much lower than that in the solid solution at every pH. To clarify the possibility of selective extraction of the solid solution from slag, leaching experiments were conducted on the steelmaking slag. The CaO–SiO2–Fe2O3 steelmaking slag system was made by a mixture of reagents and ground into particles smaller than 53 μm. After immersion, holes were observed on the surface of the slag particles. The area that selectively dissolved is considered as the solid solution phase prior to leaching. At pH 3, most of the Ca and Si in the solid solution dissolved after 120 min; however, the dissolution ratio of P was approximately 65% smaller than that of Ca. Compared to the matrix composition, the CaO/SiO2 ratio in the residue was close to the matrix and P2O5/Fe2O3 ratio was slightly larger than that of the matrix. XRD analysis revealed that the peak corresponding to the solid solution disappeared in the residue. The mass ratio of the residue to the dissolved slag was close to the ratio of the matrix to the solid solution before leaching.
  • Effect of CO and CO2 Content in Suction Gas on Sintering Process for Iron Ores

    pp. 1991-1993

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    DOI:10.2355/isijinternational.54.1991

    To serve fundamental information for the operation of flue gas circulation sintering (FGCS) process, this study mainly focuses on the influence of the CO and CO2 content in the suction gas on the iron ore sintering process. The results indicated that the presence of a small amount of CO in suction gas was beneficial to improve the combustion efficiency of coke and the sintering productivity, yield and sinter strength. However, excessive of CO2 in suction gas worsen the combustion efficiency of coke and sintering indexes. The CO2 content in the suction gas should be less than 5 vol%.
  • Re-Occurrence Time of Swirl Motion Caused by Transient Increase in Water Bath Depth

    pp. 1994-1996

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    DOI:10.2355/isijinternational.54.1994

    The swirl motion of a bubbling jet generated in a bottom blown bath ceases temporarily under a certain gas injection condition due to a sudden increase in the bath depth. Re-occurrence time of the swirl motion, Ts,sa, is defined as the period from the sudden supply of water into the bath to the moment at which re-occurrence of the swirl motion is observed in the bath. Measurements of Ts,sa, are carried out to propose its estimation method.

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