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

ISIJ International Vol. 58 (2018), No. 5

  • Application of Phenomenological Theory to Chemical Metallurgy

    pp. 785-791

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    DOI:10.2355/isijinternational.ISIJINT-2018-120

    It is well known that, the metallurgy has been honored as “the father of chemistry”. However, the metallurgy as an independent discipline was only established around the Second World War. Why does it take so long for this period? That is because for any matured science a quantitative description is required instead of a qualitative description. Most metallurgical systems are with large number of components and higher melting points that are difficult to be dealt with theoretically. As a result, the empirical treatment will be adopted instead of a theoretical model. There are two kinds of theoretical treatments: physical model and phenomenological model. The former one has a very clear physical picture but it is difficult to be treated. The later one is easy to deal with due to less number of variables. As a result, many metallurgical problems will be solved by using the later theoretical models. In this presentation, it will be shown that how we apply the phenomenological models to solve the metallurgical problems both in theoretical topics and technical aspects.
  • Activity of CaO in CaO–SiO2–Al2O3–MgO Slags

    pp. 792-798

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    DOI:10.2355/isijinternational.ISIJINT-2017-735

    The activity of CaO in CaO–SiO2–Al2O3–MgO slags for blast furnace smelting was determined by a modified slag–metal equilibrium technique with Sn as the solvent metal and a known a(CaO) slag as the reference slag. Iso-activity contours of CaO in the CaO–SiO2–Al2O3–MgO pseudo-ternary system at w(Al2O3)=15% and w(MgO)/w(Al2O3)=0.35 were achieved from the measured values. Effects of R (basicity, R=w(CaO)/w(SiO2)), w(MgO)/w(Al2O3) and w(Al2O3) on a(CaO) were discussed. R was significant on a(CaO) and a(CaO) increased obviously with the increase of R at 1873 K. a(CaO) increased with the increase of w(MgO)/w(Al2O3) when R is 1.05 and w(Al2O3) is 15%. However, the effect of w(MgO)/w(Al2O3) on a(CaO) was not obvious for the slags with R=1.20. With the increase of w(Al2O3), a(CaO) decreased at fixed R=1.05 and w(MgO)/w(Al2O3)=0.35. When R is 1.20, a(CaO) increased with the increase of w(Al2O3).
  • Effect of Sodium Sulfate on Preparation of Ferronickel from Nickel Laterite by Carbothermal Reduction

    pp. 799-807

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    DOI:10.2355/isijinternational.ISIJINT-2017-611

    In this study, the effect of sodium sulfate on preparation of ferronickel from nickel laterite by carbothermal reduction was investigated. The thermodynamic analysis showed that the addition of sodium sulfate can effectively enlarge the region of liquid slag in the phase diagram. The experimental results of carbothermal reduction revealed that sodium sulfate was capable of enhancing the reduction of nickel laterite, as well as promoting the aggregation and growth of nickel-iron particles considerably. Both the size of ferronickel particles and the grade of Ni in the magnetic material increased with increase in the dosage of sodium sulfate. The Ni grade and its recovery ratio were only 2.4 mass% and 58.1% when the raw material was reduced in the absence of sodium sulfate. However, with addition of 8.0 mass% sodium sulfate, the grade of Ni in magnetic material and the recovery of Ni reached up to 9.7 mass% and 89.1%, respectively. The contents of S, P, C and Si in the magnetic materials was also discussed. In addition, the reaction mechanisms of reduction in the presence of sodium sulfate was revealed by investigating the phase transformation, growing character of ferronickel as well as TG and evolved gas analysis. The effective utilization coefficient of nickel (ENi) which is a new characterization method of normalization was proposed to compare different studies on preparation of ferronickel from various nickel laterite.
  • In Situ Temperature Measurement of Sinter Beds at High Spatial and Time Resolution

    pp. 808-814

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    DOI:10.2355/isijinternational.ISIJINT-2017-636

    In the sintering process during iron making, the sintering reaction proceeds in a packed bed along with the combustion of coke particles. Although detailed temperature information is necessary to improve the process, it is difficult to measure the temperature distribution inside the packed bed with high spatial and time resolution. We performed in situ temperature measurement inside the sinter bed at high spatial and time resolution, i.e., 2 mm and 10 s, respectively, during sintering. A sheathed thermocouple was scanned at optimized scan speed along the inside of the thin-wall alumina tube which was held perpendicular inside the sinter bed. The information on the temperature variation during sintering showed a clear correlation between the quality of the sinter and the sinter heat pattern for each layer. Further analysis also showed that the flame front speed is proportional to the O2 consumption in the sinter bed. The temperature measurement technique enabled an unprecedented detailed discussion with the temperature distribution inside the sinter bed during sintering. This technique will not only help to improve the sintering process but also provide beneficial information on the chemical reactions occurring inside packed beds.
  • Dissolution Mechanism of Carbon Brick into Molten Iron

    pp. 815-822

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    DOI:10.2355/isijinternational.ISIJINT-2017-659

    In order to investigate the dissolution mechanism of carbon brick into molten iron, cylindrical specimens were immersed into molten iron to carry out the experiments. The dissolution reaction of carbon was considered as the dominant reaction through thermodynamic analysis, the result of SEM revealed the hole diameter decrease from the reaction interface to the center position of carbon brick. The quantitative relationship between element content and erosion was obtained through the experimental results, the characteristic parameters were selected to compare the influence degree of element content on the erosion. The calculation model of mass transfer coefficient was established, the dissolution reaction of the sample is controlled by interfacial reaction and mass transfer of carbon when the phosphorus content up to 0.2% in molten iron. The adsorption of sulfur on the iron-carbon interface covers part of the effective surface, the degree of adsorption on the interface depends on the proportion of sites covered by sulfur.
  • Volumetric Shrinkage Characteristics and Kinetics Analysis of Vanadium Titanomagnetite Carbon Composite Hot Briquette during Isothermal Reduction

    pp. 823-832

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    DOI:10.2355/isijinternational.ISIJINT-2017-650

    The volumetric shrinking characteristics and kinetics of vanadium titanomagnetite carbon composite hot briquette (VTM-CCB) during isothermal reduction were investigated in this paper. It was found that the volumetric shrinking occurs as a combination of a loss of carbon and oxygen from VTM-CCB as well as sintering of iron oxide, suppression of growth of iron whiskers, formation of molten phases, and residual solid carbon content. In the temperature range from 900°C to 1100°C, the carbon gasification is viewed as the primary factor for the shrinking behavior. In the temperature range higher than 1100°C, the shrinking of VTM-CCB is mainly caused by the formation of molten slag, liquid iron, and residual solid carbon. It also should be pointed out that, in the higher temperature range, the volumetric shrinkage of VTM-CCB decreased with increasing FC/O ratio, which is mainly due to the fact that a higher FC/O ratio leads to a decreasing of molten slag generation proportion and a suppression of aggregation growth of liquid iron by the increasing of residual solid carbon. Based on the kinetics analysis, The formula for the shrinkage of VTM-CCB during reduction can be obtained, and the value of shrinking activation energy of VTM-CCB decreased obviously from 337.86 kJ/mol to 84.85 kJ/mol with increasing FC/O ratio from 0.8 to 1.4.
  • Recovery of Phosphorus from Modified Steelmaking Slag with High P2O5 Content via Leaching and Precipitation

    pp. 833-841

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    DOI:10.2355/isijinternational.ISIJINT-2017-454

    The P contained in steelmaking slag is regarded as a potential phosphate source, especially with regard to slag with high P2O5 content, which is generated from the utilization of high P iron ores. If P can be efficiently extracted from slag, the obtained P can be used as a phosphate fertilizer. Moreover, the remaining slag can be recycled inside the steelmaking process. Compared with other phases, the P-condensed C2S–C3P solid solution in slag is more easily dissolved in water; therefore, selective leaching was applied to recover P from slag with high P2O5 content. In this study, the effect of K2O modification on P dissolution in the citric acid solution was investigated, and subsequently, a process for extracting phosphate product from the leachate, via precipitation, was explored. It was determined that K2O modification promoted dissolution of the solid solution, resulting in a higher dissolution ratio of P. By modification, the majority of the solid solution was dissolved at pH 6, and other phases remained in residue, indicating that a better selective leaching of P occurred. As the pH decreased, the dissolution ratios of both P and Fe increased. Following leaching at pH 5, a residue with a higher Fe2O3 content and lower P2O5 content was obtained. When the pH of the leachate increased, the dissolved P in the aqueous solution was precipitated. Through separation and calcination, a phosphate product with a P2O5 content of 30% was obtained, which has the potential to be used as a phosphate fertilizer.
  • Effect of Main Gas Composition on Flow Field Characteristics of Supersonic Coherent Jets with CO2 and O2 Mixed Injection (COMI) at Steelmaking Temperature

    pp. 842-851

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    DOI:10.2355/isijinternational.ISIJINT-2017-580

    As an efficient oxygen supplying technology, coherent jets are widely applied in electric arc furnace (EAF) steelmaking processes to strengthen chemical energy input, speed up smelting rhythm and promote the uniformity of molten bath temperature and compositions. Recently, the supersonic coherent jets with CO2 and O2 mixed injection (COMI) was proposed and through industrial experiments, it can be found that the supersonic coherent jets with COMI showed remarkable advantages in reducing the dust production during EAF steelmaking. In this study, based on the eddy dissipation concept (EDC) model with the detailed chemical kinetic mechanisms (GRI-Mech 3.0), a computational fluid dynamics (CFD) model of supersonic coherent jets with COMI was built. Compared with one-step combustion reaction, GRI-Mech 3.0 consists of 325 elementary reactions with 53 components and can predict more accurate results. The numerical simulation results were validated by the combustion experiment data. The jet behavior and the fluid flow characteristics of supersonic coherent jets with COMI at steelmaking temperature 1700 K was studied and the results show that the chemical effect of CO2 significantly weakens the shrouding combustion reactions of CH4 and the relative importance of the chemical effect of CO2 increases with CO2 concentration increasing. The potential core length of supersonic coherent jets decreases with the volume fraction of CO2 increasing. Moreover, it also can be found that the potential core length of supersonic coherent jets can be prolonged with higher ambient temperature.
  • Effect of Side-blowing Arrangement on Flow Field and Vanadium Extraction Rate in Converter Steelmaking Process

    pp. 852-859

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    DOI:10.2355/isijinternational.ISIJINT-2017-463

    To improve the vanadium extraction rate, the iron ore powder was used to be injected into the molten bath by side-blowing. Both simulation numerical and water experiment model for analyzing the behaviors of molten bath flow field had been developed to investigate the mixing time, velocity profile and dead-zone volume of molten steel under various injection arrangements and gas flow rates. The result showed that the mixing time would reduce, with increasing the distance between side-blowing nozzle and molten bathe surface. Moreover, although the kinetic energy would be removed by the mutual influenced between bottom-blow bubbles and the side-blowing jet at a certain content, the stirring effect of molten bath is still improved by the side-blowing jet. Based on the result of simulation, an injection arrangement was used in the 150 t vanadium extraction converter. Based on the results, the vanadium content of semi-steel and T. Fe in the slag with side-blowing arrangement is 0.033% and 34.1%, respectively. And the vanadium content of semi-steel and T. Fe in the slag with side-blowing arrangement is 0.044% and 32.1%, respectively.
  • Optimum Conditions for Phosphorus Recovery from Steelmaking Slag with High P2O5 Content by Selective Leaching

    pp. 860-868

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    DOI:10.2355/isijinternational.ISIJINT-2017-669

    Selective leaching of P-concentrated solid solution is considered an effective method for recovering P from slag with high P2O5 content. This plays a significant role in stimulating the utilization of high-P iron ores. To determine the optimum conditions for selective leaching of P, we investigated the effects of the cooling rate of molten slag, Na2O content in slag, and pH on the dissolution behavior of the modified slag in the aqueous solution. Following leaching, a precipitation method was studied to recover P from the leachate. Compared to the quenched slag, the furnace-cooled slag exhibited a higher P dissolution ratio and lower Fe dissolution ratio, indicating that slow cooling was necessary to realize selective leaching. The addition of 2.5–4.0 mass% of Na2O to the slag was sufficient to cause most of the solid solution to dissolve at pH 6, Fe being difficult to dissolve. The dissolution ratio of P from the modified slag increased significantly when the pH decreased from 7 to 5. A further decrease in the pH promoted Fe dissolution. Therefore, the pH of the aqueous solution should be controlled between 5 and 6. After leaching, with an increase in the pH of the leachate, the precipitation ratio of P from the leachate increased, while the P2O5 content in the obtained phosphate product decreased. In this process, approximately 70% of P in the slag was recovered in the form of the phosphate product, which can be used as a phosphate fertilizer.
  • Change of Phosphorus-Concentrated Phase in Low Basicity Steelmaking Slag

    pp. 869-875

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    DOI:10.2355/isijinternational.ISIJINT-2017-706

    Recently, interest in the art of dephosphorization in steelmaking has turned toward relatively lower basicity slag saturated with a [2CaO·SiO2-3CaO·P2O5] solid solution rather than CaO-saturated slag. In this work, laboratory experiments were carried out in order to investigate the change of the phosphorus-concentrated phase in low basicity slag. Phosphorus-containing slag was added onto 10 kg of hot metal, and its basicity was lowered through the desiliconization reaction of the hot metal at 1573 K.EPMA observation of the slag after the experiment showed different mineral phases corresponding to the slag basicity (mass%CaO)/(mass%SiO2). When basicity was larger than 0.8, the phosphorus-concentrated phase (P-phase) was observed but was different from the solid solution phase in the initial slag. When basicity was lower than 0.8, the P-phase was not observed, and phosphorus was distributed through the slag at a low concentration. These results would reflect decomposition of the P-phase and formation of homogeneous liquid slag.Based on a thermochemical consideration with the phase diagram of the [CaO–SiO2–FeO] system, the dissolution of the P-phase observed in the present experiment would be due to the slag composition approaching the SiO2 saturated region and a resulting decrease in CaO activity. From the viewpoint of the phase diagram of the [CaO–SiO2–P2O5] system, lowering the slag basicity to CaO·SiO2 saturation would come to coexistence of a higher P-phase such as 5CaO·SiO2·P2O5 or even 3CaO·P2O5, which would lead to an increase in the driving force for phosphorus transfer from slag to metal (so-called rephosphorization) at lower slag basicity.
  • Behavior of Alloying Elements during Drawing-Ingot-Type Electroslag Remelting of Stainless Steel Containing Titanium

    pp. 876-885

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    DOI:10.2355/isijinternational.ISIJINT-2017-687

    Experimental and theoretical studies have been carried out to study the effects of slag and metal electrode compositions on alloying elements in ingot during Drawing-Ingot-Type electroslag remelting (ESR) with a focus of developing a numerical model to control titanium and aluminum. The mass transfer model based on the penetration and film theories is established to analyze the Fe+FeO, Ti+TiO2, Al+Al2O3 and Si+SiO2 concentration changes along the height of ESR ingot. The results show that the combination of slag containing high CaO content, extra TiO2 incessantly added into slag in the first slag-temperature-rising period, and extra SiO2 incessantly added into slag during the whole ESR process is suitable for Drawing-Ingot-Type ESR of AISI321 steel. The mass transfer model shows that the remelting rate has little effect on the change of silicon, aluminum and titanium content in ingot, while the titanium content in electrode has significant effect on silicon, aluminum and titanium content in ingot. Since the titanium in AISI321 stainless steel ingot ranges from 0.4% to 0.8%, the titanium in electrode needs to be larger than 0.9% because of the titanium’s reaction with silica. In addition, the alloying element content in ingot under different remelting rate, electrode and slag composition are estimated based on the mass transfer model.
  • Characteristic Transformation of Manganese-containing Inclusions during Al-killed Process in Ultra-low Carbon Interstitial-free Steel

    pp. 886-891

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    DOI:10.2355/isijinternational.ISIJINT-2017-682

    Ferromanganese alloys are usually used for the manganese adjustment to meet the requirement of IF steel grade; and the formation and transformation of manganese-containing inclusions were closely relevant with the recovery yield of manganese in the melt and the morphologies of final deoxidation products. Characteristic transformation of manganese-containing inclusions during Al-killed process in ultra-low carbon interstitial-free steel were studied through the factory trial and hot crucible experiment. Factory trial showed that the manganese recovery yield fluctuated between 14% and 92% mainly due to the formation of manganese-containing inclusions. Hot crucible experiment revealed that three different groups of inclusions, including FeO·xMnO (FM), FeO–Al2O3 (FA) and Al2O3 (AO) inclusions, appeared after semi Al-killed; and five typical Al2O3 inclusions, including spherical Al2O3, polygonal Al2O3, cluster Al2O3, dendritic Al2O3 and aggregated Al2O3, appeared successively based on different formation mechanisms. Under homogenous nucleation condition, the spherical Al2O3 appeared firstly, and changed to be cluster Al2O3 by collision growth or transformed to be large dendrite Al2O3 by single-direction diffusion growth. Under heterogenous nucleation condition, the manganese-containing inclusions were transformed to Al2O3 inclusions in the following order: spherical FeO·xMnO → spherical FeO·xMnO with coarse surface → polygonal FeO·Al2O3 → polygonal Al2O3 → aggregated Al2O3.
  • Investigation of Liquid/Solid Slag and Air Gap Behavior inside the Mold during Continuous Slab Casting

    pp. 892-898

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    DOI:10.2355/isijinternational.ISIJINT-2017-393

    The mold flux fundamentally determines solidification and friction by controlling heat transfer and lubrication within the mold, and it results in dominating the formation of various primary cracks. It is critical to reveal the distribution and evolution of the slag layers and the air gap, especially for control and optimization of the initial slab surface defects. Based on the measured temperatures in combination with the inverse heat transfer numerical model, this paper establishes a numerical model which describes the distribution of the slag layers and the air gap in the mold, aiming to calculate the non-uniform distribution of liquid slag, solid slag air gap and their evolution during actual casting process. The effect of slag solidification temperature and casting temperatures on the slag layers and air gap is investigated, and the liquid lubrication length, air gap formation position and evolution of the slag layers are also analyzed quantitatively. The results provide reference for deepening the understanding of complicated distribution and evolution of mold flux, and may lay the theoretical foundation for online visualization of non-uniform heat transfer and lubrication behavior inside the continuous casting mold.
  • Microsegregation Formation in Al–Cu Alloy under Action of Steady Magnetic Field

    pp. 899-904

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    DOI:10.2355/isijinternational.ISIJINT-2017-616

    The effect of a steady magnetic field (SMF) on microsegregation in the Al-4.5mass%Cu alloy during equiaxed solidification was investigated experimentally. It was found that the amount of microsegregation increased with increasing the SMF intensity at the same cooling rate. The variation of the microsegregation level with the SMF intensity at different cooling rates showed the consistent tendency. The effect of the change in various factors such as diffusivity in the solid phase, dendrite coarsening and undercooling on the microsegregation behavior was analyzed. The reduction of diffusivity in the solid phase in the SMF increased the microsegregation level. The increase in average nucleation undercooling of primary Al phase in the SMF, which was examined by the differential thermal analysis (DTA), led to decrease the amount of microsegregation. The enhancement of coarsening kinetics in the SMF increased the microsegregation level. Under the combined action of various factors, it was found that the increase in microsegregation in the range of the cooling rates under investigation was mainly attributed to the increase of the secondary dendrite arm spacing (SDAS) and the decrease in solid diffusion coefficient induced by the SMF.
  • Mechanism of Mild Cooling by Crystallisation of Mould Flux for Continuous Casting of Steel - A View from Apparent Thermal Conductivity under Steep Temperature Gradient -

    pp. 905-914

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    DOI:10.2355/isijinternational.ISIJINT-2017-661

    Effects of crystallisation on heat transfer across solid mould fluxes have been examined on the basis of apparent thermal conductivities including radiative contribution. The apparent thermal conductivities were measured on glassy and crystallised mould flux samples under steep temperature gradients using a parallel plate method improved in the present work. Both surfaces of the samples were coated with silver paste to reduce contact thermal resistance. Thermal resistance except the sample itself was experimentally determined to be 2.27 × 10−4 m2KW−1 based upon measurements on Inconel 600. To confirm the reasonableness of this value, the method was applied to fused silica. Apparent thermal conductivities were in good agreement with reported values. Apparent thermal conductivities of mould fluxes were measured up to 900°C at the high temperature side of the sample. The thermal conductivity of the glassy sample was 1.25 Wm−1K−1 below 300°C in the central temperature (Tc) of the sample, and was lower than those of the crystallised samples. With increasing degree of crystallinity, the thermal conductivities increased around room temperature. Samples with higher degrees of crystallinity showed negative temperature dependence more remarkably and resultantly were close to that of the glassy sample where Tc ~ 350–500°C. Where Tc > 500°C, the thermal conductivity of the glassy sample was 1.54 Wm−1K−1 and was greater than that of a crystallised sample, 1.32 Wm−1K−1, which would be due to the radiation. Apparent thermal conductivity at a practical temperature has also been estimated, which suggests that crystallisation enables radiative thermal conductivity to be reduced.
  • Optimization of Secondary Cooling Water Distribution for Improving the Billet Quality for a Small Caster

    pp. 915-920

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    DOI:10.2355/isijinternational.ISIJINT-2017-711

    In order to eliminate internal cracks and other defects of billet caused by improper secondary cooling water distribution for a small caster, a mathematical heat transfer model for billet continuous casting has been presented and solved by finite volume method. For calibrating model better, the surface temperature of billet has been measured by using high resolution CCD measurement system, which can effective eliminate the influence of randomly generated oxide scales for the surface temperature measurement. Then, the existing problems of the secondary cooling water distribution have been discussed and the secondary cooling water distribution of each zone has been optimized at key casting speeds. A new secondary cooling water distribution has been regressed and analyzed. This secondary cooling water distribution can increase the casting speed, minimize the reheating of billet, and minimize the wide fluctuations of secondary cooling water at higher casting speed region. This can improve the billet quality. Finally, the new secondary cooling water distribution has been applied on an actual small caster, it has shown that the billet defects have been decreased greatly and the quality of billet has been obviously improved.
  • Effect of Zr, Al Addition on Characteristics of MnS and Formation of Intragranular Ferrite in Non-Quenched and Tempered Steel

    pp. 921-928

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    DOI:10.2355/isijinternational.ISIJINT-2017-617

    In this paper, the characteristics of inclusions, microstructures and mechanical properties in non-quenched and tempered steel with Zr and Al addition were investigated to compare the effect of ZrO2 and Al2O3 particles on the precipitation behavior of MnS and the formation of intragranular ferrite. The results show that oxides, sulfides and oxy-sulfides in Zr addition steel are all finer than those in Al addition steel. As a result, the volume fraction of intragranular polygonal ferrite (IPF) induced by tiny MnS + V(C, N) particles in Zr addition steel has increased to 14.9%, resulting in higher transverse plasticity. Low lattice mismatch between ZrO2 and MnS inclusions plays an important role in refinement of MnS inclusions. Besides, the single ZrO2 particle, which is not wrapped by MnS inclusions, would induce formation of IPF further due to the small mismatch with α-Fe.
  • A MgO–SiO2–Al2O3–ZnO Ceramic-glass Coating to Improve the Anti-oxidation of Carbon Steel at High Temperature

    pp. 929-935

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    DOI:10.2355/isijinternational.ISIJINT-2017-602

    In this paper, a MgO–SiO2–Al2O3–ZnO ceramic-glass coating sprayed on carbon steel was studied at high temperature. The property of the MgO–SiO2–Al2O3–ZnO ceramic-glass coating was analyzed in a range of 900°C and 1150°C a share. The experimental results indicated that the MgO–SiO2–Al2O3–ZnO coating exhibited powerful anti-oxidation property for carbon steel. The MgO–SiO2–Al2O3–ZnO coating could improve the anti-oxidation performance of carbon steel by 84% at 1050°C for 60 min. The Ea of blank and coated samples were 108.65 and 202.55 kJ/mol, respectively. The kp of the blank sample (0.61 mg2·cm−4·s−1) was 7.6 times as much as that of coated sample (0.08 mg2·cm−4· s−1). It demonstrated the coating slowed down oxidation reaction rate and then improved the anti-oxidation performance of carbon steel. The possible protection mechanisms of the MgO–SiO2–Al2O3–ZnO ceramic-glass coating were also investigated using the SEM-EDS, XRD and TG-DTA characterization methods. The mixture formed between the coating and steel substrate (such as MgFe2O4, ZnFe2O4, MgSiO3, FeSiO4, ZnSiO4, SiO2, Al2O3, and (FexMg1−x)2SiO4 (x=0.4, 0.85, 0.94)) played a role in blocking the spread of ions and improved the oxidation resistance of carbon steel during heating treatment. The synergy of the formed Al, Mg and Zn compound layers in the coating could also block spread of oxygen and iron ions and exert an influence on enhancing anti-oxidation property.
  • Exposure Test Performance for Chromium Bearing Steel in Concrete under Coastal Environment

    pp. 936-942

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    DOI:10.2355/isijinternational.ISIJINT-2017-666

    The corrosion resistance of Cr bearing steel (CR) in concrete was estimated by the exposure test in a coastal environment. Carbon steel (SM) had a significant corrosion on the surface, and the concrete block had large cracks. On the other hand, CR had little corrosion, and there was no crack on the concrete. In the case of Electrochemical Impedance Spectroscopy (EIS) measurements, the impedance at low frequency region (ZL) of CR showed much higher values than that of SM. Cr and Si were enriched in inner rust of CR observed by Energy Dispersive Spectroscopy (EDS) measurement. Transmission Electron Microscopy (TEM) showed that nano complex iron oxides containing Cr and Si were formed in inner rust, which suppressed the corrosion of CR in the concrete.
  • Void Nucleation, Growth, and Coalescence Observed by Synchrotron Radiation X-ray Laminography during Tensile Deformation of Fe–0.02 mass% N Alloy

    pp. 943-951

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    DOI:10.2355/isijinternational.ISIJINT-2017-609

    In this study, the effect of the state of N atoms in a Fe–N alloy, namely, whether N exists as a dissolved atom or as a nitride on void nucleation, growth, and coalescence during tensile testing was examined and the factors dominating local elongation was discussed. Two types of Fe–0.02 mass% N alloy specimens were used; one was heated to the ferritic phase region before water-quenching, while the other was subjected to aging.The local elongation of the water-quenched specimen was lower than that of the aged specimen. Secondary-ion mass spectrometry and electron backscattering diffraction analyses revealed that N atoms were segregated around grain boundaries similarly in both the water-quenched and the aged specimens, while N atoms precipitated as Fe4N in the aged specimen. Void nucleation, growth, and coalescence were observed using synchrotron radiation X-ray laminography. In the water-quenched specimen, voids suddenly grew and coalesced before fracture, while in the aged specimen sudden growth and coalescence were suppressed. Nano-indentation hardness measurements showed that the difference in hardness between the regions around the grain boundaries and grain centers was smaller in the aged specimen compared to that in the water-quenched specimen. This result indicated a lower plastic strain gradient around the grain boundaries in the aged specimen. This lower plastic strain gradient in the aged specimen was caused by precipitation of N atoms as Fe4N. From these results, it was concluded that the main factor permitting the greater local elongation of the aged specimen was the lower plastic strain gradient around the grain boundaries.
  • Nanobainite Layer Prepared by Laser Hardening Combined with Isothermal Transformation

    pp. 952-957

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    DOI:10.2355/isijinternational.ISIJINT-2017-591

    Nanobainite layer was prepared by laser hardening combined with isothermal transformation (LHCIT) at 250°C, which between Ms and Md temperature. The microstructures of the nanobainite layer were analyzed by X-ray Diffraction (XRD) and transmission electron microscope (TEM). The nanohardness of the nanobainite layer were measured by the nano-mechanical tester. The results show that the residual stress value of the laser hardening layer is 319.15 ± 21.05 MPa, and the shear band such as stacking fault bundle, nano-twin and ε-martensite can be formed by the stress in austenite during LHCIT. The highest nano-indentation hardness of the nanobainite layer is 8.81 GPa and the average nanohardness of the nanobainite layer is evidently increased from 6.65 GPa to 7.76 GPa. The reduced hardness is bimodal in the laser hardening layer, and it is demonstrate that the high hardness of the nanobainite layer can be obtained by the formation of the shear band.
  • Effect of Dissolved Carbon on the Recrystallization Texture Formation in Electrical Steels

    pp. 958-964

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    DOI:10.2355/isijinternational.ISIJINT-2017-675

    Effect of dissolved carbon on the recrystallization texture formation was studied by comparing two electrical steels with the same composition. Both steels have very similar rolling texture. After annealing, the fully recrystallized steel A (0.002 wt% dissolved carbon) showed a stronger Goss intensity and a relatively weaker {111}<112> intensity, as compared to steel B (without dissolved carbon). Detailed analyses indicated that a high Goss intensity already appeared in steel A starting from the early stage of recrystallization. Nevertheless, the {111}<112> intensity of the recrystallized grains exhibited a relatively higher increasing rate in steel B than in steel A in the late stage of recrystallization. The favorable development of {111}<112> grains in steel B is attributed to the enhanced recovery rate.
  • Effect of Inclusion Size and Type on the Nucleation of Acicular Ferrite in High Strength Ship Plate Steel

    pp. 965-969

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    DOI:10.2355/isijinternational.ISIJINT-2017-696

    The mechanisms of acicular ferrite formation on non-metallic inclusion have been studied in high strength ship plate steel. The effect of inclusion size and type on the formation of acicular ferrite was studied by metallographic microscope and SEM-EDS. The inclusion composition was analyzed by Raman spectra. Experimental results showed that all the inclusions were nearly spherical in shape. Inclusion size was mainly from 1 µm to 4 µm in high strength ship plate steel. With increase of the inclusion size, the acicular ferrite nucleating rate increased. The probability of inducing nucleation increased gradually with the inclusion size increasing. The inclusions had an appropriate size to become nucleation center for acicular ferrite. This optimal size was about 3 µm. The inclusion was composed by Ti, Mn, S, Si, O, Al and Mg. The composition of Ti–O–Mn inclusion was Ti2O3 and MnS, but no MnTiO3. The surface of inclusion as an inert surface played an important role in nucleating of acicular ferrite. The lower low mismatch was also the important factors to promote the nucleation of acicular ferrite. Based on the two mechanisms, it would make it easier for acicular ferrite to nucleate.
  • Effect of Annealing Temperature and Coiling Temperature on r-value of Nb and B-added Extra Low-carbon Steel

    pp. 970-977

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    DOI:10.2355/isijinternational.ISIJINT-2017-645

    When a welded can body is expanded in the pail can manufacturing process, it shrinks in the direction of can height. The change of the can height is influenced by the r-value of the steel sheet.The effect of the annealing temperature and coiling temperature on the r-value of extra low-carbon steel with combined addition of niobium (Nb) and boron (B) was investigated by using commercial steels.The ferrite grain size of the annealed sheet decreased as the annealing temperature decreased. At the same time, the orientation density of {001} <110> increased and the r90°-value decreased.The behavior of the decrease of the r90°-value was dependent on the coiling temperature of the hot-rolled sheet. When a low coiling temperature was used, the r90°-value decreased rapidly as the annealing temperature decreased.
  • High-speed Tensile Deformation Behavior of 1 GPa-grade TRIP-aided Multi-phase Steels

    pp. 978-986

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    DOI:10.2355/isijinternational.ISIJINT-2017-635

    High-speed deformation behavior and strain rate dependency of mechanical properties of 1 GPa-grade TRIP-aided multi-phase (TRIP) steels were studied. The strain rate range in this study was between 3.3 × 10−6 and 103 s−1, and the effect of retained austenite (γR) shapes on TRIP effect in the 1 GPa-grade TRIP steel was also focused on. The effects of strain rate on tensile strength and flow stress in the TRIP steels were small whereas that on uniform elongation was large. The strain rate dependencies of tensile strength and uniform elongation in the TRIP steels were more closely to those of the metastable austenitic stainless steels than the conventional TRIP and dual-phase steels. The 1 GPa-grade TRIP steel with the γR shape of needle-like showed better tensile properties and absorbed energy in the present strain rate range. The volume fraction of γR more than 20% and the matrix microstructure of martensite seem to be important factors in the high-strength TRIP steels.

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