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

ISIJ International Vol. 54 (2014), No. 1

  • Effect of Slag Properties on Mixing Phenomena in Gas-stirred Ladles by Physical Modeling

    pp. 1-8

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    DOI:10.2355/isijinternational.54.1

    The effect of slag properties (thickness and viscosity), have been evaluated in terms of mixing time, exposed surface or ladle eye and energy dissipation. A nozzle configuration defined in terms of the number of nozzles, its radial position and gas flow rate has been employed to describe the influence of the top layer on mixing phenomena. It has been found a negative effect of both slag thickness and slag viscosity on mixing time, on the other hand, the same properties are useful to decrease the exposed surface or ladle eye.
    An empirical approach using water modeling is suggested to evaluate the average velocity of the bulk liquid. The method was used to define the fraction of stirring energy consumed by the top layer. The result is in agreement with a previous investigation.
  • Numerical Simulations of Inclusion Behavior and Mixing Phenomena in Gas-stirred Ladles with Different Arrangement of Tuyeres

    pp. 9-18

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    DOI:10.2355/isijinternational.54.9

    A CFD-PBM (Computational Fluid Dynamic-Population Balance Model) coupled model has been developed to investigate the effects of different number and position of bottom tuyeres and gas flow rate on the bubbly plume flow, inclusion removal and mixing phenomena in gas-stirred ladle. It is found that the dual blowing gives a shorter mixing time and higher inclusion removal ratio in comparison with the center blowing or eccentric blowing with one tuyere. With the increasing of separation angle of two tuyeres, the inclusion removal ratio increases, while mixing time decreases first and then increases. With the increasing of radial position of two tuyeres, the inclusion removal first increases and then decreases, and the mixing time decreases until the radial position exceeds 0.7R from the bottom center, where R is the bottom radius of ladle. It is recommended to use the two tuyeres placed at radial position of 0.6R and the angle of 135 deg in ladle to improve the joint efficiency both the inclusion removal and mixing. With the gas flow rate increasing, the efficiency both mixing and inclusion removal with the optimized tuyeres arrangement increases, however when the gas flow rate exceeds 300 NL/min in 150 ton ladle, the removal ratio and mixing time change little.
  • Changing Bed Bulk Density and other Process Conditions during Iron Ore Sintering

    pp. 19-28

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    DOI:10.2355/isijinternational.54.19

    Iron ore sintering blends in the Asia-Pacific contain significant levels of Australian ores which are lower in bulk density compared to Brazilian ores. This study explores the impact of further decreasing the bulk density of a fairly typical ore blend. This was done by introducing small amounts of a very porous ore into the blend. Measured decreases in bulk density were up to 3%. All the sinter quality parameters could be maintained or improved without the need to increase coke rate. Changes in sinter density results were not significant, indicating that the changes in bed bulk density did not have a significant effect on sinter porosity. The study was then extended to include a comparable blend and two hematite blends of higher bulk density. Decreasing green bed bulk density, bed shrinkage and sinter density did not have a detrimental effect on sinter tumble strength. The associated increase in porosity with the measured changes in sinter density is estimated to be up to 10%. Results showed that sinter density values obtained for the low bulk density mixes at increased coke addition were still lower than equivalent values for the hematite mixes. As expected flame front speed was dependent on post-ignition airflow rate and both these parameters influence sinter strength. Many blends studied have binders and results indicated that their inclusion into sinter mixes gave increased post-ignition airflow values.
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  • Decomposition Behavior of Fe3C under Ar Atmosphere

    pp. 29-31

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    DOI:10.2355/isijinternational.54.29

    Suppression of CO2 discharged from iron and steelmaking companies is an example of the biggest issues for the protection of global environment and sustainable growth of steelmaking industry. One of the efforts made to decrease the emission of CO2 in ironmaking process is blowing of hydrogen gas into blast furnace. Hydrogen gas can reduce iron oxide and form harmless H2O. Cementite (Fe3C) may be formed by introduction of hydrogen into blast furnace and play an important role on carburization and smelting behavior of reduced iron.
    In the present work, Fe3C sample was held at 800–1100 K under Ar atmosphere to clarify the stability and the behavior of Fe3C phase. It was confirmed that metastable Fe3C phase will decompose under Ar atmosphere at 800 K and rapidly decompose at temperature over 900 K. Also, it was found that composite of nano-size C and Fe will form when Fe3C decompose.
  • Nitrogen Solubility in Liquid Fe–C Alloys

    pp. 32-36

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    DOI:10.2355/isijinternational.54.32

    The nitrogen solubility in liquid Fe–C alloys containing carbon up to 5.2 mass% has been measured under reduced nitrogen partial pressures in the temperature range of 1773–1873 K. Previous studies on the C–N interaction in liquid iron have shown marked disagreement on its temperature dependency and the order of interaction. By the gas-liquid metal equilibration technique using a high frequency induction furnace with an accurate temperature measurement, precise nitrogen solubility data were obtained. The interaction between carbon and nitrogen in liquid iron has been expressed in terms of the first- and second-order interaction parameters. No temperature dependence of these values was observed in the temperature range from 1773 to 1873 K.

    [ This part of text is omitted ]

    (1.14 ≤ mass% C ≤ 4.95 at 1773 K, 0 ≤ mass% C ≤ 5.2 at 1823–1873 K)
  • Behavior of SO2 in the Process of Flue Gas Circulation Sintering (FGCS) for Iron Ores

    pp. 37-42

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    DOI:10.2355/isijinternational.54.37

    In order to reduce SO2 emission in the iron sintering process, the flue gas circulation sintering (FGCS) process has been recommended and put into practice. In this process, flue gas containing low SO2 concentration is recirculated to the sintering process, achieving the concentration of SO2 content in the off-gas, which is favorable for current flue gas desulphurization processes. In this study, SO2 behavior in different sintering zones was investigated under the simulated experimental conditions. In the moisture condensation zone and sinter mixture zone, SO2 of recirculated flue gas is absorbed by the moisture and slaked lime of sinter mixture; in the drying and preheating zone, part of SO2 in circulating flue gas is absorbed by dry slaked lime in sinter mixture under low temperature conditions, and SO2 is generated by oxidization of sulfides and re-released by decomposition of sulphurous acid; in the hot zone, SO2 in circulated gas is partially absorbed by the molten phases, newly crystallized minerals such as calcium ferrite and dissociative CaO; in sinter zone, part of SO2 is absorbed by residual CaO in sinter under the condition of humid circulated flue gas, which results in unfavorable increase of residual S in the up layer sinter.
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    3. Effect of Pre‑wetting Treatment on the Granulation Behavior of Iron Ore Fines ISIJ International Vol.54(2014), No.12
  • Influence of Operation Parameters on Dome Temperature of COREX Melter Gasifier

    pp. 43-48

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    DOI:10.2355/isijinternational.54.43

    Statistical analysis and theoretical calculation have been carried out to study the parameters affecting dome temperature of COREX melter gasifier by using actual plant data. The relationship between parameters and dome temperature has been realized qualitatively and quantitatively. It is found that the dome temperature is influenced significantly by metallization, fuel rate, coal rate, coke rate, and oxygen volume. High volatile matter and low moisture coal are beneficial to maintaining dome temperature. Regression analysis was carried out and equations have been developed to predict and regulate dome temperature.
  • Effect of Alumina and Silica on the Reaction Kinetics of Carbon Composite Pellets at 1473 K

    pp. 49-55

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    DOI:10.2355/isijinternational.54.49

    The influence of compositional changes in the CaO–FeOt–(Al2O3) or (SiO2) ternary oxide system on the reaction kinetics of carbon composite pellet was investigated by Thermo-Gravimetric Analyzer (TGA) at 1473 K (1200°C). Measured CO and CO2 gas by Infrared gas analyzer and surface area change from BET analysis assisted with the reaction kinetics of carbon composite pellet. As a result, alumina increased the reduction rate of iron oxide by increasing surface area, while silica decreased reduction rate by lowering surface area of pellet samples. A modified reaction kinetic model considering both the Boudouard reaction and surface area variation was developed for a better explanation of reduction mechanisms occurring in carbon composite pellets.
  • Effects of Particle Sizes of Iron Ore and Coal on the Strength and Reduction of High Phosphorus Oolitic Hematite-coal Composite Briquettes

    pp. 56-62

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    DOI:10.2355/isijinternational.54.56

    This study presents a process with coal-based direct reduction of high-phosphorus oolitic hematite before magnetic separation for producing direct reduction iron (DRI), where Ca(OH)2 and Na2CO3 were used as additives in order to inhibit the reduction of fluorapatite and promote the reduction of hematite. The effects of particle sizes of iron ore and coal on the strength and reduction of cold-bonded composite briquettes were investigated. It showed that the decrease of the particle sizes of iron ore and coal increased the strength of composite briquettes. At the temperature of 1200°C, a sizable increase of recovery and iron content of DRI was observed when the particle size of iron ore decreased from –4 mm to –0.1 mm, however, the phosphorus content of DRI obtained from –0.1 mm iron ore was much higher than that from coarser iron ore. The results of SEM-EDS analyses showed that the reduction of fluorapatite in the iron ore was activated when using –0.1 mm iron ore and the reduced phosphorus melting into metallic iron. Moreover, the particle size of coal has less effect on the direct reduction-magnetic separation than iron ore within the scope of the study.
  • Effect of Preheated Top Gas and Air on Blast Furnace Top Gas Combustion

    pp. 63-71

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    DOI:10.2355/isijinternational.54.63

    The coke consumption in blast furnaces (BF), directly related to the energy consumption and CO2 emission from BF, is reduced with the increase of hot blast temperature. To establish the flame temperature of more than the blast furnace gas (BFG) adiabatic flame temperature of 1587 K by using only BFG without any additional high calorific value gases such as coke oven gas, the effect of preheating of BFG and/or air on the BFG combustion temperature was investigated based on thermodynamic heat balance and the computational fluid dynamic (CFD) simulations. Thermodynamic evaluation and CFD simulated results showed that air preheating was not effective to raise the BFG combustion temperature compared with BFG preheating or simultaneous preheating of air and BFG at the same preheating temperature. The less efficiency of air preheating was explained based on the small heat content of preheated air. The blast temperature of 1700 K was obtained without adding high calorific value gases by preheating only BFG to 873 K or simultaneous preheating both BFG and air to 700 K. Compared with air or simultaneous preheating of air and BFG, BFG preheating will be a suitable approach to increase BFG combustion temperature. This is not only because the supply of heat content to the combustion zone, but also the enhanced mixing introduced by the large inlet gas velocity difference between air and BFG promotes rapid CO burn-up.
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    3. Formation and Growth of Iron Nuclei on Wüstite Surface at the Initial Stage of Reduction ISIJ International Vol.54(2014), No.8
  • Desulphurisation and Inclusion Behaviour of Stainless Steel Refining by Using CaO–Al2O3 Based Slag at Low Sulphur Levels

    pp. 72-81

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    DOI:10.2355/isijinternational.54.72

    The deep desulphurisation behaviour and steel cleanliness of stainless steel refining in the low sulphur content period (below 45 ppm [%S]) by using the current lime-fluorspar based slag and an optimized lime-alumina slag have been studied and compared. The desulphurisation effect was discussed based on the calculated sulphide capacity of slags and the equilibrated sulphur distribution ratio between the slag and steel. The sulphur evolution during the treatment was predicted with a previously developed kinetic model. The influence of slag chemistry on the compositional evolution of the steel and inclusions was discussed through thermodynamic considerations with respect to deoxidation and desulphurisation reactions. The sulphur content in the inclusions was predicted using the dissolved sulphur in the steel and their oxide chemistry. An equivalent steel cleanliness and desulphurisation effect was obtained with lime-alumina based slag as with lime-fluorspar based slag both in laboratory and industrial tests for low sulphur content stainless refining.
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    3. Effect of Additives on Viscosity of LATS Refining Ladle Slag ISIJ International Vol.46(2006), No.5
  • Liquid/liquid Mixing Pattern in a Mechanically-stirred Vessel

    pp. 82-86

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    DOI:10.2355/isijinternational.54.82

    In order to find out effect of operating factors on mixing pattern which affects liquid/liquid mass transfer rate drastically, cold model experiment was carried out with liquid paraffin or tetradecane as a dispersed phase and ion-exchanged water as a continuous phase in a mechanically stirred vessel. There exist three types of liquid/liquid mixing pattern in a mechanical agitation. I: region where each liquid phase separates and has no dispersion, II: region where vortex of dispersed phase (liquid/liquid interface) arrives at impeller position and its dispersion begins into continuous phase, III: region where gas/liquid interface as well as liquid/liquid one arrives at impeller position and dispersion occurs heavily. The transition of I–II accelerated along with the increases in rotation speed, ratio of dispersion phase volume to continuous one, density of dispersion phase, impeller diameter and vessel diameter, and the decrease in impeller depth. The transition of II–III accelerated along with the increases in rotation speed, density of dispersion phase and impeller diameter, and the decrease in impeller depth.
    The multi regression equation for the transition of I–II is expressed as,

    [ This part of text is omitted ]

    where H: distance between free surface of oil and upper part of impeller (mm), Hoil: bath depth of dispersed phase(mm), N: rotation speed(rpm), Voil/Vw: ratio of dispersion phase volume to continuous one(–), di: impeller diameter(mm). D: vessel diameter(mm), ρd: density of dispersion phase(kg/m3), whereas that on transition of II–III is H ∝ N2.18di1.96ρd1.33.
  • Mass Transfer between Different Phases in a Mechanically-stirred Vessel and its Comparison with that in a Gas-stirred One

    pp. 87-93

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    DOI:10.2355/isijinternational.54.87

    In order to understand effect of operating conditions on mass transfer between different phases in a mechanically stirred vessel, cold model study was carried out with liquid paraffin as a dispersion phase and ion-exchanged water as a continuous phase. Inner diameter of vessel, D, was varied in conjunction with both depth, H0, as D=H0=400 and 300 mm. Rotation speed, N, was changed between 50–240 rpm, volume ratio, Voil/Vw, of dispersed to continuous phase was 5.9×10–2 and 1.2×10–1.
    Liquid/liquid mass transfer rate showed characteristic trend depending on liquid/liquid mixing pattern. It was kept nearly constant at lower level in the region I, monotonically increased in the region II except near the region III and its increasing rate decreased in the region II near the region III. Liquid/liquid mixing pattern was grouped into three regimes. I: the region where liquid/liquid interface did not arrive at the impeller, II: the region where liquid/liquid interface attained at impeller position, III: the region where gas/liquid interface touched impeller.
    Under the same supply rate of mixing energy, liquid/liquid mass transfer rate of mechanical stirring corresponded to that of gas stirring at a point in the region II. In the region I and the first half of II, liquid/liquid mass transfer rate of gas stirring is larger than that of mechanical stirring, whereas that of gas stirring is smaller than that of mechanical stirring in the region III and the latter half of II.
    Gas/liquid mass transfer rate increased remarkably with an increase in N in the region III.
  • Numerical Simulation of Molten Steel Flow and Inclusions Motion Behavior in the Solidification Processes for Continuous Casting Slab

    pp. 94-102

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    DOI:10.2355/isijinternational.54.94

    A three-dimensional numerical simulation of transient fluid flow field during the solidification process of casting mold has been developed to predict the solidification front capturing the inclusions and the distribution of the inclusions at the different location in the inner surface layer has been given out. The motion behavior of non-metallic inclusions which are more than 50 μm has been simulated and visually displayed. The results show the large inclusions and the small inclusions cluster at different location in the inner surface layer through numerical simulation, and the inclusion cluster at the quarter of the broad face through numerical simulation and experiment.
  • Uneven Solidification during Wide-thick Slab Continuous Casting Process and its Influence on Soft Reduction Zone

    pp. 103-111

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    DOI:10.2355/isijinternational.54.103

    A two-dimensional heat transfer model was developed to analyze the uneven solidification, particularly the shape of solidification end in wide-thick slab continuous casting process. In order to ensure the accuracy of simulation, material properties of peritectic steel were calculated by weighted averaging of phase fractions, and the boundary conditions were obtained by realistic water flux distribution on the slab transverse surface. The model was verified by nail shooting results at different locations of strand, and the absolute value of relative error was found to be no more than 2.12%. According to the simulation results, the liquid core around 1/8 location of slab width increased after the water flux was decreased around the slab corner to reduce transverse corner cracks. The soft reduction (SR) zone was optimized to reduce macro-segregation both at the slab center and 1/8th location. The plant results showed that the inner quality of wide-thick slab was significantly improved in the whole transverse section with the optimized SR zone parameters.
  • Detection of Scarfing Faults on the Edges of Slabs

    pp. 112-118

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    DOI:10.2355/isijinternational.54.112

    At present, quality control is becoming a major issue in steel production. Thus we developed an algorithm that uses machine vision to detect scarfing faults on slabs, which impairs the steel quality of subsequent products such as steel plates. Scarfing faults typically occur in three locations: the top, middle, and edge of the slab. Our proposed algorithm is focused on detecting scarfing faults on the edge of slab, which is tiny and sometimes indistinct. A machine vision system with a line scan camera was designed, which facilitates the detection of brightness differences and texture differences between well-scarfed and poorly-scarfed slab surface. Scarfing faults are tiny on the edges, so we propose a new segmentation method that takes advantage of capabilities of the line scan camera. A segmented image is filtered using Gabor filters, which were designed to focus on the boundary with scarfing faults to identify specific regions with defect, referred to as defect candidates. Each defect candidate is classified using a Support Vector Machine (SVM) classifier based on its extracted features. Our proposed algorithm was effective according to the experimental trials using 2061 frame images acquired from real samples, where the true detection rate was 97.26% and the false detection rate was 1.66%. Our proposed system and algorithm based on machine vision technology facilitates scarfing faults detection, which can be detected before rolling process, resulting in improved steel quality.
  • Strip Steel Surface Defect Classification Method Based on Enhanced Twin Support Vector Machine

    pp. 119-124

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    DOI:10.2355/isijinternational.54.119

    The strip steel surface defect classification belongs to multi-class classification. It demands high classification accuracy and efficiency. However, traditional methods are not fit for abnormal datasets, such as the large-scale, sparse, unbalanced and corrupted dataset. So a novel classification method is proposed in this paper based on enhanced twin support vector machine (TWSVM) and binary tree. According to the density information, the large-scale dataset is pruned, the sparse dataset is added with unlabeled samples, and TWSVM is improved to multi-density TWSVM (MDTWSVM) which has efficient successive overrelaxation (SOR) algorithm. Finally, MDTWSVM and binary tree are combined together to realize multi-class classification. Some experiments are done on the strip steel surface defect datasets with the proposed algorithm. Experimental results show that MDTWSVM has higher accuracy and efficiency than the other methods of multi-class classification for the strip steel surface defect.
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  • In situ Measurements of X-ray Absorption Spectra during Transformation of Green Rust to Ferric Oxyhydroxide Via Aqueous Solution

    pp. 125-130

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    DOI:10.2355/isijinternational.54.125

    In situ measurements of X-ray absorption spectra at an Fe K absorption edge were carried out during the transformation of chloride-containing green rust (GR(Cl)) to oxyhydroxide (that is, lepidocrocite (γ-FeOOH)), by oxidation via aqueous solution. Results showed that the Fe K absorption edge in X-ray absorption near-edge structure (XANES) spectrum shifted toward the higher energy side with increasing oxidation time. The factor analysis of the XANES spectra, using reference data, implied that dissolution of GR(Cl) and precipitation of γ-FeOOH occur simultaneously. The fraction of γ-FeOOH in the suspension increased continuously with increasing oxidation time, in response to the continuous decrease of the fraction of GR(Cl). The results lead us to conclude that GR(Cl) transforms to γ-FeOOH during oxidation under the present experimental conditions.
  • Elemental Analysis of Iron and Steel by Solid-phase Extraction/ICP–MS Using an Anion Exchange Extraction Disk

    pp. 131-135

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    DOI:10.2355/isijinternational.54.131

    Various investigations were conducted to establish quantification of microelements in iron and steel using ICP–MS with solid-phase extractive separation capable of separating the matrix simply and rapidly as a pretreatment. A “zero-emission type analysis” free from EDTA as a masking agent and buffering agents used for pH adjustment was sought. To do so, an anion-exchange type solid-phase extraction disk was used as the solid-phase extraction agent, along with “skill-free analysis” requiring no cumbersome manipulations, and “quick analysis” for assessment in an extremely short period of time. Results show a target element in a solid-phase disk, whereas the sample solution at separation was maintained at higher than pH 1.8. The pH adjustment was conducted by dilution of the sample solution with about 400 cm3 of water. The target element held in the solid-phase disk was eluted with 10 cm3 of 3 kmol/m3 nitric acid. The detection limitations [3 σ; ng/g (ppb)] were the following: Ti 0.043, Ge 0.014, Zr 0.013, Nb 0.025, Mo 1.06, Sn 0.030, Hf 0.010, Ta 0.019, and W 0.12.
  • Comparison of the Analytical Performances of Laser-Induced Breakdown Spectroscopy and Spark-OES

    pp. 136-140

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    DOI:10.2355/isijinternational.54.136

    Most Czerny-Turner based LIBS systems are used widely. In the present work, an instrumental setup based on Paschen-Runge is introduced, and analytical parameters are optimized for LIBS. With optimum parameters, analytical performance measures, such as precision, accuracy and detection limit, are compared for LIBS and Spark-OES. The results show that the analytical accuracy obtained by LIBS is similar to that obtained with Spark-OES, while the precision and detection limit are notably close to those of Spark-OES. Thus, LIBS can be used satisfactorily for process analysis in the metallurgy industry.
  • W-Concentration 3D Mapping in SKH51 Steel by Dual-Energy K-Absorption Edge Subtraction Imaging

    pp. 141-147

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    DOI:10.2355/isijinternational.54.141

    Three-dimensional W-concentration mapping is attempted in a high speed tool steel, SKH51 by applying K-absorption edge subtraction imaging utilizing high energy X-ray in synchrotron radiation facility. Effect of a sample-to-detector distance on spatial resolution had already been reported in high-energy X-ray microtomography. Therefore, effect of the sample-to-detector distance on W-concentration obtained by the K-absorption edge subtraction imaging has been assessed in this study. A fine CT image was obtained in 65 mm sample-to-detector distance with the influence of both scattering and diffraction on spatial resolution. Although image quality depended on sample-to-detector distance, the distance did not affect W-concentration measured by the absorption edge subtraction imaging so much, because the effect is limited on object interface. The average W-concentration in whole specimen was consistent with the chemical composition in the SKH51 steel. The maximum W-concentration also agreed with the SEM-EDS result. It was not easy to assess W-concentration on a carbide particle by means of segmentation based on a linear absorption coefficient. The average W-concentration at an aggregated particle, which looks a large coarse particle in CT image, corresponded to the average W-concentration that was estimated based on SEM-EDS. Therefore, it was concluded that W-concentration obtained by K-absorption edge subtraction imaging was accurate. Three-dimensional W-concentration mapping was available in steels by dual-energy K-absorption edge subtraction imaging utilizing high energy X-ray.
  • Formation of Intergranular M23C6 in Sensitized Type-347 Stainless Steel

    pp. 148-152

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    DOI:10.2355/isijinternational.54.148

    Formation of intergranular M23C6 carbides and thereafter Cr-depletion zones in commercially available type-347 stainless steels were observed by optical microscopy, conventional transmission electron microscopy and analytical transmission electron microscopy. At the early stage of sensitization, only NbC carbides were observed both intragranularly and intergranularly. At the later stage, formation of intergranular M23C6 carbides and intergranular corrosion were found. A model is proposed to describe the formation kinetics of intergranular precipitation of NbC and of M23C6, as well as Cr-depletion zones, thermodynamically. Evolution mechanisms of intergranular precipitates were found in the order of 1) the formation of intergranular NbC, 2) the coarsening of NbC, 3) the formation of M23C6, then 4) the coarsening of M23C6 and formation of Cr-depletion zones.
  • Numerical Analysis of Influence of Hydrogen Charging Method on Thermal Desorption Spectra for Pre-strained High-Strength Steel

    pp. 153-159

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    DOI:10.2355/isijinternational.54.153

    A remarkable difference in thermal desorption spectra of hydrogen obtained from pre-strained high-strength steel specimens which were charged with hydrogen by two different methods was observed. One charging method is by immersion in NH4SCN solution and the other is by cyclic corrosion tests. In order to understand the difference, we simulated numerically thermal hydrogen spectra of the pre-strained high-strength steel. As a result, it was found that the difference of desorption spectra results from the difference of initial hydrogen states which is caused by the amount of charged hydrogen. It was also found that the desorption spectrum in the case of cyclic corrosion test is more sensitive to the initial hydrogen state than that of the immersion case because the amount of charged hydrogen in the former is not as enough as that in the latter.
  • Original Position Statistic Distribution Analysis for the Sulfides in Gear Steels

    pp. 160-164

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    DOI:10.2355/isijinternational.54.160

    The sulfides in some different gear steels were analyzed by original position statistic distribution analysis technique. The relationship between the proportion of the frequency of high intensity signals and the content of total sulfur and total oxygen in samples has been investigated. Multi-elemental correlation on high intensity signals has been performed in order to obtain the composition and proportion of different type of sulfides. It was found that the proportion of simple sulfide was more than that of oxide-sulfide duplex inclusion in six gear steel samples. The ratio of the oxide-sulfide duplex inclusion to the total sulfides were determined by the present method and the results had good coincidence with the value obtained by quantitative metallographic method and scanning electronic microscopy. The influence of the content of Ca on the composition of the sulfides was investigated. It was found that the relative proportion of the complex inclusions of Al–Ca–Mn–O–S increase with the rise of the content of Ca in gear steels. The ratio of the content of Ca to the content of S (Ca/S) also had great effect on the relative proportion of the oxide-sulfide duplex inclusion.
  • Development of a Rolling Chatter Model Considering Unsteady Lubrication

    pp. 165-170

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    DOI:10.2355/isijinternational.54.165

    Chatter, limiting the rolling speed for thin steel strips, has been identified as a serious limitation in increasing the efficiency of the cold rolling process. Many experimental investigations have led to the point that lubrication is one of the effective factors causing chatter. In this article, a new chatter model of the cold strip rolling with consideration of unsteady lubrication is proposed. The limiting shear stress concept and the variation of the lubricant viscosity with pressure are considered in this model. The results of the simulation are verified by comparing to experimental data. Also, a parametric study on the effect of some of the major parameters of rolling lubricant on the chatter critical speed is conducted. The results show that under the operating conditions of this paper, the limiting shear stress is an effective parameter.
  • Prediction of Mean Flow Stress during Hot Strip Rolling Using Genetic Algorithms

    pp. 171-178

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    DOI:10.2355/isijinternational.54.171

    In order to satisfy the demands for high accuracy and efficient rolling, it is necessary to establish favourable mathematical model of roll force calculation, which is one of the most important terms for process control. For this purpose it is necessary to predict the Mean Flow Stress (MFS) with good accuracy, since it is the predominant factor of the roll force model. In this paper this problem is dealt on data coming from a real industrial plant and hot compression tests. Various steels have been tested; these can be divided into 2 principal groups: Niobium/Titanium microalloyed, and plain Carbon-Manganese.
    Particularly, MFS has been found out by measurements taken in the industrial strip rolling mill converting log data in MFS using the Sims approach. Moreover, in order to evaluate the dispersion of MFS measurements, thermomechanical deformation tests have been conducted by a Gleeble 3800 thermomechanical simulator simulating all the seven passes of the studied finishing stand.
    The results have been analysed and compared to the predictions of some mathematical models developed in literature and it is shown how inadequate well known literature models are. Alternative models have been then proposed by improving existing formulae by means of genetic algorithms based optimization. The performance of the proposed methods have been compared. Moreover, their prediction abilities have been evaluated using the MFS dispersion data measured experimentally. The satisfactory results obtained by optimized based models put into evidence the advantages of the use of artificial intelligence techniques in the industrial framework.
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    1. Production and Technology of Iron and Steel in Japan during 1989 ISIJ International Vol.30(1990), No.4
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    3. Prediction of Bainite Start Temperature in Alloy Steels with Different Grain Sizes ISIJ International Vol.54(2014), No.4
  • Numerical Simulation of the New Post Tensioned Column Base with Bolted T-stubs

    pp. 179-187

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    DOI:10.2355/isijinternational.54.179

    A new post tensioned column base connection for earthquake resistant steel moment resisting frames (MRFs) is introduced. The proposed post tensioned column base consists of high strength PT bars, reinforcing plate and bolted T-stubs. In this connection, the PT bars provide restoring force for self-centering behavior while the T-stubs dissipate energy by yielding. By using this connection, the plastic hinge in the column bases is removed and plastic deformation is concentrated in energy dissipation device. An analytical model based on fibre elements was developed with OpenSees to model column base connection. Accordingly; the model accurately predicts the expected behavior of the new proposed column base connection under cyclic loading. The proposed connection provides similar characteristic behavior of the post tensioned column base. To investigate the cyclic behavior of the PT column base connections, a series of the PT column base connection were subjected to axial load and cyclic lateral displacements. Analysis variables included PT bar diameter, initial post tensioning force and dissipation device size. Results showed that under large earthquake rotations, this connection could reduce or eliminate the plastic rotation by its self-centering behavior as well as providing required strength and stiffness.
  • Toughness and Microstructure of Coarse Grain Heat Affected Zone with High Heat Input Welding in Zr-bearing Low Carbon Steel

    pp. 188-192

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    DOI:10.2355/isijinternational.54.188

    The microstructure and toughness of simulated coarse grained heat affected zone (CGHAZ) in low carbon steel have been investigated in this study. In order to simulate microstructure evolution in CGHAZ, specimens were subject to weld thermal cycle with heat input of 100 kJ/cm, 400 kJ/cm and 800 kJ/cm at 1400°C peak temperature using thermal simulator. As increase in heat input energy of weld thermal cycle, prior austenite grain size increases due to longer holding time at peak temperature. Excellent impact toughness of CGHAZ with heat input of 800 kJ/cm was obtained because of high volume fraction of fine acicular ferrite (AF) inside gain acting as an obstacle to cleavage propagation owe to its high angle grain boundary, forcing cleavage crack to change the route of propagation and effectively impedes the propagation of crack. The primary AF mainly originated from Zr-oxide particle with MnS and subsequently secondary AF nucleated sympathetically in form of side arms and grew from the primary AF in CGHAZ.
  • Improvement of the Tribological Properties of DLC/oxynitriding Duplex-treated AISI H13 Alloy Steel

    pp. 193-198

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    DOI:10.2355/isijinternational.54.193

    In this study, diamond-like carbon (DLC) films were prepared by DC-pulsed plasma CVD after the oxynitride treatment of AISI H13 tool steel. In order to investigate the tribological properties of DLC films, a Raman spectroscopy analysis, wear test, adhesion and roughness tests were performed. The main parameters of the DC-pulsed plasma CVD process includes various pretreatment times of argon plasma (15, 30, 45 and 60 min). Experimental results showed that an oxynitride layer and the DLC films could be completely obtained after DLC/oxynitriding duplex treatment. The duplex coating layers had optimal adhesion (critical load reached to 10.65 N) and wear properties after DC-pulsed plasma was CVD treated via a low pulse voltage (–1.5 kV), pretreatment times of the argon plasma were 15 min and the substrate temperature was kept at 40°C. Meanwhile, the optimal DLC/oxynitriding duplex treated specimens possessed the lowest wear volume (2.25 × 10–3 mm3) and a lower friction coefficient (0.06).
  • Improvement of Pitting Corrosion Resistance of Type 430 Stainless Steel by Electrochemical Treatments in a Concentrated Nitric Acid

    pp. 199-205

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    DOI:10.2355/isijinternational.54.199

    Electrochemical surface treatments have been attempted in 5 kmol/m3 HNO3 solution to improve pitting corrosion resistance of Type 430 stainless steel. Three different surface treatments were carried out for the same time period (2.9 h) under conditions of immersion, potentiostatic polarization at 0.9 V vs. Ag/AgCl/sat.KCl reference electrode (E=+0.222 V vs. SHE) and potential cycling between 0.6 and 0.9 V. The pitting corrosion resistance was evaluated by an ordinary pitting potential measurement and droplet tests which simulates atmospheric corrosion in marine environments. It was found that the potential cycling treatment is most effective among them to improve pitting corrosion resistance. The surface analysis by EPMA and XPS indicated that the potential cycling most successfully removes MnS inclusions and increases Cr content in the passive film.
  • Microstructural Features of Cold-Rolled Carbon Steel Evaluated by X-ray Diffraction Line Profile Analysis and Their Correlation with Mechanical Properties

    pp. 206-211

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    DOI:10.2355/isijinternational.54.206

    Line profile analyses of cold-rolled carbon steels were conducted to evaluate microstructural features such as dislocation density, crystallite size, and M values. After the samples were subjected to 40% cold-rolling, dislocation density increased from 7 × 1013 m−2 to 2 × 1015 m−2 and crystallite size decreased from 155 nm to 35 nm. The component ratio of screw and edge dislocations was approximately 1:1, as determined from the evaluation of the q values. The M value that indicated interaction of dislocations substantially decreased during the initial stage of cold-rolling, which means interaction of dislocations becomes strong. Proof stress, hardness, and tensile strength were increased by the cold-rolling process. Furthermore, the ratios between proof stress and hardness were initially 2 and increased to approximately 3. The correlation between the microstructures and the mechanical properties was demonstrated according to the Bailey-Hirsch relationship between flow shear stress and dislocation density. Variations in the proof stress and hardness as a function of the square root of dislocation density indicated that the work-hardening of the material is affected by not only the total amount of dislocations but also other factors, such as crystallite size and arrangement of dislocations.
  • Tensile Behavior of Ti,Mo-added Low Carbon Steels with Interphase Precipitation

    pp. 212-221

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    DOI:10.2355/isijinternational.54.212

    Tensile behavior and structure-property relationship of ferritic steels with nano-sized carbide dispersion were invesigated using Ti-added steel and Ti,Mo-added low carbon steels. By austenitizing followed by isothermal heat treatment at 700°C, polygonal ferrites containing very fine carbides of TiC and (Ti,Mo)C were obtained in the Ti-added and the Ti,Mo-added steels, respectively. The size of such carbides was finer in the Ti,Mo-added steel than in the Ti-added steel at the same isothermal holding. The results of tensile tests for these samples showed that the strength is higher as the carbide size is smaller. The structure-based strength calculation led to a good agreement with the experiments, when it was assumed that the Ashby-Orowan mechanism is dominant for precipitation strengthening of nano-sized alloy carbides. It was also suggested that a relatively large tensile ductility is related to enhanced recovery during the tensile deformation, accompanied with promotion of secondary slips or cross slips in a finer scale due to the nano-sized particles.
  • Refinement of Retained Austenite in Super-bainitic Steel by a Deep Cryogenic Treatment

    pp. 222-226

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    DOI:10.2355/isijinternational.54.222

    The effect of a deep cryogenic treatment on the microstructure of a super-bainitic steel was investigated. It was shown that quenching the super-bainitc steel in –196°C liquid nitrogen resulted in the transformation of retained austenite to two phases: ~20 nm thick martensite films and some nano carbides with a ~25 nm diameter. Some refinement of the retained austenite occurred, due to formation of fine martensite laths within the retained austenite. The evolution of these new phases resulted in an increase in the average hardness of the super-bainitic steel from 641 to ~670 HV1.
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    1. Isothermal Transformation Behaviour of Extremely Low Carbon 3%Mn-1%Cr Steels ISIJ International Vol.34(1994), No.1
    2. Time-Temperature-Transformation Diagram within the Bainitic Temperature Range in a Medium Carbon Steel MATERIALS TRANSACTIONS Vol.45(2004), No.12
    3. Some Microstructural Aspects of Bainite in High-carbon Si–Mn Steels ISIJ International Vol.47(2007), No.9
  • Determining the Conditions for Dynamic Recrystallization in Hot Deformation of C–Mn–V Steels and the Effects of Cr and Mo Additions

    pp. 227-234

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    DOI:10.2355/isijinternational.54.227

    The refinement of microstructure and its homogeneity during controlled hot strip rolling is primarily achieved by controlling the austenite recrystallization before its transformation during accelerated cooling. The paper describes the methodology to determine the deformation conditions favorable for dynamic recrystallization (DRX). Using this methodology it becomes possible to delineate the conditions for post-deformation static and metadynamic recrystallization as well. The work is based on viscoplastic power law formalism applied to steady state flow within wide range of deformation temperatures and strain rates. Two equations of the same form but with different coefficients can be used depending on whether the steady state flow is controlled by dynamic recovery (DRV) or DRX. The transition from DRV- to DRX at the corresponding value of Zener-Hollomon parameter Zt can be viewed as the demarcation between static and metadynamic recrystallization occurring after deformation. The approach is illustrated using low carbon Mn–V steel. Alloying with Cr and especially with Mo suppresses DRX and MDRX as manifested by increasing Zt.
  • An Original Way for Producing a 2.4 GPa Strength Ductile Steel by Rolling of Martensite

    pp. 235-239

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    DOI:10.2355/isijinternational.54.235

    A compositionally-graded steel composed of martensite with 0.4%C on the centre and bainite with 0.1%C on the surface was manufactured by partial decarburization. It is reported that the as quenched material can be cold rolled up to an equivalent strain of 1.5 without cracks. The mechanical properties of the cold-rolled material exhibits up to 2.5 GPa strength and ductility. A simple mechanical model is developed to predict the stress state after rolling of the graded structure explaining the good ductility of the present high strength materials.
  • Effects of Alloying Elements on the Hardenability, Toughness and the Resistance of Stress Corrosion Cracking in 1 to 3 mass% Cr Low Alloy Steel

    pp. 240-247

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    DOI:10.2355/isijinternational.54.240

    The effects of alloying elements on the hardenability, toughness and the resistance to stress corrosion cracking have been evaluated in the 1 to 3 mass%Cr low alloy steels for 1600 mmφ class large-sized turbine rotor forgings for geothermal power generation. The chemical composition suitable for the rotor forging which satisfied the required material properties and which reduced the C-segregation in the center portion of the ingot was chosen under the restrictive condition on the upper limit of Ni and Cr contents suitable for turbine rotors in a geothermal corrosive environment. An 8 ton sand mold ingot of the selected chemical composition was manufactured and the reduction of the C-segregation was verified. It is concluded that the selected 2.25Cr-0.9Ni-0.6Mo-0.25V-0.15Si-1.0Mn-0.25C steel (mass%) is hopeful as the chemical composition for large-sized turbine rotor forgings for geothermal power generation.
  • Effect of Solute Silicon on the Lattice Parameter of Ferrite in Ductile Irons

    pp. 248-250

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    DOI:10.2355/isijinternational.54.248

    The effect of solute silicon on the ferrite lattice parameter has been investigated using X-ray diffraction in cast ductile irons (DI) with nominal Si contents between 2.50 and 4.56 wt%. It was found that silicon changes the ferrite lattice parameter by –0.00185 Å per wt% Si. This contraction coefficient is three times larger than the most commonly used Si coefficient in the literature. Since substitutional solution by silicon contracts the ferrite lattice while the interstitial solution by carbon expands the lattice, the Si contraction coefficient found will have a significant effect on subsequent evaluation of the carbon content in austempered Si-alloyed ductile irons and steels.

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