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

ISIJ International Advance Publication

  • Influence of Annealing Microstructure on the Low-cycle Fatigue Properties and Fatigue Microstructure of a Fe–15Mn–10Cr–8Ni–4Si Seismic Damping Alloy

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    DOI:10.2355/isijinternational.ISIJINT-2021-211

    We investigate the effects of the annealing microstructure on the low-cycle fatigue (LCF) life (Nf), cyclic stress behaviour and fatigue microstructure of a Fe–15Mn–10Cr–8Ni–4Si alloy that exhibits a deformation-induced transformation of austenite (γ-phase) into ε-martensite (ε-phase). The alloy rolled at 800°C was annealed at 600°C, 700°C, 800°C and 900°C to vary the grain size, the fraction of recrystallised grains and the texture intensity. Fully reversed axial strain-controlled LCF tests were conducted at total strain amplitudes, Δεt/2, ranging from 0.007 to 0.02. The alloy showed a higher Nf than common steels and ferrous high-Mn alloys in this strain range. This type of annealing microstructure was found to impact the Nf, fatigue behaviour and deformation-induced ε-martensitic transformation (ε-MT) in the studied alloy. The fully recrystallised and weakly textured austenite formed at T ≥ 800°C facilitated the uniform development of ε-martensite under cyclic deformation and led to an increased Nf. The partially recrystallised and textured austenite-containing substructure with high dislocation density formed at T ≤ 700°C suppressed the ε-MT, retarded reversible dislocation motions in the un-recrystallised regions and moderately decreased Nf. Moreover, Nf and the deformation-induced ε-MT were observed to be less sensitive to variations in grain size.
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    1. Premature Failure of Copper Staves and Applied Results for New Designed Staves ISIJ International Advance Publication
  • Premature Failure of Copper Staves and Applied Results for New Designed Staves

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    DOI:10.2355/isijinternational.ISIJINT-2021-158

    In a blast furnace employing a copper stave as a cooling system, premature stave wear is experienced in many steel mills. To confirm this failure mechanism, an online ultrasonic thickness measuring device was installed in the Pohang 4 blast furnace from blow-in to acquire data. Through this, the refractory damage in hot face and real-time wear data were obtained, and the correlation with the operational factors was analyzed. Through the CFD simulation, the effect of temperature due to the refractory was confirmed, and the possibility of acceleration of the copper body was confirmed in the absence of the refractory in hot face. To improve the life of the entire refractory as well as the stave body, a refractory stress test was conducted in the laboratory, and a stable groove structure was derived using the structure program DEFORM using the data obtained from cold tests. The new designed long-life copper stave was applied to the Pohang 4 blast furnace in November 2015, and it has been proved that the life of the stave has been significantly improved since wear has not been measured until now.
  • Effect of Heating Rate on Carbothermic Reduction and Melting Behavior of Iron Ore-Coal Composite Pellets

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    DOI:10.2355/isijinternational.ISIJINT-2021-175

    In the ironmaking processes via the carbothermic reduction with the tall-bed reactor, the composite pellets are rapidly reduced to highly metallized direct reduced iron (DRI) with high productivity under high temperature. Without coke making and ore sintering process, carbothermic reduction are not only help broaden the selection of raw materials but an environmentally friendly approach for the ironmaking process. In this study, experiments were conducted for iron ore-coal composite pellets under four heating rates. During the reduction, the reaction was quenched at different temperatures by quickly moving the specimens to room temperature. After the experiments, we performed phase analysis by X-ray diffraction, chemical composition analysis by wet method, and microstructure observation and element distribution analysis by scanning electron microscopy with energy dispersive spectrum. The pellet morphology during the experiments was also recorded. The pellets under fast heating rates (80°C/min and 40°C/min) collapsed over 1300°C. The pellets under the slow heating rate (20°C/min) maintained their spherical morphology until 1400°C. We found that the formation and melting of fayalite (2FeO–SiO2 or Fe2SiO4) played a key role in the collapsing behavior of pellets, which can be classified into three different stages, (a) between 1000°C and 1200°C, a large amount of fayalite compound (melting point at 1178°C) was formed. (b) above 1178°C, the fayalite compound started to melt and became a liquid phase. (c) between 1178°C and 1400°C, the pellets lost their strength, and collapsed due to the excess amount of liquid phase present.
  • Three-dimensional Quantitative Evaluation of the Lamellar Curvature in Pearlitic Steel Based on an Orientation Analysis of Cementite

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    DOI:10.2355/isijinternational.ISIJINT-2021-194

    To quantitatively evaluate the three-dimensional lamellar curvature in pearlitic steel, the crystallographic orientation rotation of ferrite and cementite was analyzed using scanning electron microscopy and electron backscattering diffraction. In the hypereutectoid steel containing 0.92% C, 0.30% Mn, 0.20% Si, and 0.19% Cr, the ferrite–cementite interfaces in some colonies were confirmed to match well with those having frequently observed habit planes with the Pitsch–Petch orientation relationship. It was also confirmed that the ferrite and cementite near the ferrite–cementite interface rotated toward the same orientation to maintain the Pitsch–Petch orientation relationship and the habit plane. Based on the confirmation that the ferrite–cementite interface is well approximated by (010) plane of the adjacent cementite, the lamellae were revealed to have three-dimensional anisotropic curvature.
  • Effects of Microstructural Anisotropy on the Dwell Fatigue Life of Ti-6Al-4V Bar

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    DOI:10.2355/isijinternational.ISIJINT-2021-245

    Cyclic fatigue, dwell fatigue and crack growth properties were evaluated in the axial direction (L) and transversal direction (T) of Ti-6Al-4V forged round bar. In the SN curve where the stress is normalized by 0.2% proof stress, the cyclic fatigue life in the L/T direction is almost the same, whereas the dwell fatigue life in the T direction is as short as 1/5. In dwell fatigue, ductile fracture occurred when the maximum stress was higher than 95% of 0.2% proof stress. At stresses below 870 MPa, the inelastic strain range and the strain increase rate in the T direction gradually decreased with decreasing stress, and the fracture mode transitioned to that with fatigue crack growth. The gradual change must have been caused by the mixture of anisotropic microtexture regions. At stresses below 825 MPa, the fracture mode transitioned rapidly in the L direction, where the soft oriented microtexture regions were dominant. In the low ΔK region (≤15 MPa√m), the crack growth rate in the axial direction was about twice that in the radial direction of the bar. The shorter dwell fatigue life in the T direction under stress conditions showing fatigue crack growth was explained by the significantly earlier crack initiation compared to that in cyclic fatigue and the faster crack growth along the microtexture in the axial direction of the bar.
  • An Improved CBR Model Using Time-series Data for Predicting the End-point of a Converter

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    DOI:10.2355/isijinternational.ISIJINT-2020-687

    The end-point temperature is one of parameters for the end-point control in the converter. Accurate prediction of the end-point temperature is helpful to improve the hit rate of the end-point. An improved CBR model using time-series data (CBR_TM) was proposed to predict the end-point carbon content and temperature in the converter according to the data types of process parameters. The attributes of the cases in the model not only include the influencing factors of single-value type such as composition and temperature of hot metal, but also include the influencing factors of time-series type such as lance position and oxygen flow, in the case retrieval process, the single-value data similarity and time-series data similarity between the cases were calculated based on the Euclidean distance and the dynamic time warping algorithm, and then weighted to obtain the comprehensive similarity. Then the influence of the weight of the time-series data similarity on the prediction accuracy was studied based on the production data. Finally, the prediction accuracy of the established model was also compared to models based on SVR and BPNN. The results show that: The prediction accuracy of the model increases at first and then decreases with the increase of similarity weight of time series data. The prediction accuracy of the model was the highest when the weight of time-series data similarity was 0.4 and was better than the SVR and BPNN models. The established can meet the requirements of field production.
  • TTP Diagrams of Graphitization of Creep Ruptured Carbon Steels and 0.5Mo Steel

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    DOI:10.2355/isijinternational.ISIJINT-2021-275

    Graphitization in carbon steels should be avoided because it results in the degradation of material performance. Safety management standards state that graphitization occurs at 698 K for carbon and carbon-Mo steels, although some standards state it to be above 738 K for carbon-Mo steels. However, recently, graphitization was found at 673 K in creep ruptured 0.3C steel. Herein, we investigated the graphitization behavior of creep ruptured 0.3C, 0.2C, and 0.5Mo steels. It was confirmed that the graphitization occurred below the specified temperatures of 673 K for the 0.3C and 0.2C steels and 723 K for the 0.5Mo steel. In addition, time-temperature-precipitation diagrams for graphite were obtained for all the steels. Elongated graphite and spherical graphite were confirmed in the 0.3C and 0.5Mo steels, while only spherical graphite was confirmed in the 0.2C steel. It was suggested that the elongated and spherical graphite were formed due to different mechanisms. The formation of elongated graphite was promoted by a higher carbon content, Mo addition, and higher applied stress, whereas that of spherical graphite was suppressed by Mo addition. Further, to accurately assess the risk of graphitization, time and temperature, as well as the stress level and different formation mechanisms, of the two types of graphite must be considered.
  • Residual Stress Control in Drawn Bar and Wire by Heating-Cooling-Drawing Process

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    DOI:10.2355/isijinternational.ISIJINT-2021-230

    We investigated the drawing process with a temperature gradient in the radial direction of the bar to achieve flexible control of the residual stress in the drawn bar with only the drawing process. The obtained results are as follows: (1) A heating-cooling-drawing process was developed to generate a temperature gradient in the radial direction of the bar. The optimum cooling time was determined by heat conduction analysis. A cooling time of 0.54 s is optimal for a steel bar with a diameter of 10 mm. (2) We experimentally confirmed that the proposed method is extremely effective for controlling the residual stress in a bar or wire. (3) The residual stress decreased by increasing the heating temperature up to 400°C. Above 400°C, the control of stress was small. (4) The combination of the proposed method and extremely small reduction drawing is effective for obtaining strong compressive residual stress in the surface layer. For a 0.4% or 0.6% reduction rate of the section area, residual stress reduction of 900 MPa was obtained. (5) It was confirmed that residual stress is reduced when the material is cooled down after drawing by finite element analysis considering thermal strain. The mechanism of residual stress reduction by the proposed method is the loss of thermal stress due to the drawing process and the thermal contraction at the center during cooling.
  • Crack Initiation and Propagation Behavior of Hydrogen-induced Quasi-cleavage Fracture in X80 Pipeline Steel with Stress Concentration

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    DOI:10.2355/isijinternational.ISIJINT-2021-228

    The processes leading to hydrogen-related fracture in X80 pipeline steel with a stress concentration have been investigated comprehensively through observations of fracture surfaces and subsidiary cracks, stress analyses, crack initiation and propagation analyses and crystallographic analyses of fracture surfaces. Fracture morphology showed quasi-cleavage (QC) fracture under various amounts of hydrogen. It was found that QC cracks initiated in hydrogen-charged specimens in an area ranging from the notch tip to 100 µm inside based on interrupted tensile tests until just before fracture strength. A fracture surface topography analysis (FRASTA) revealed that QC cracks initiated at the notch tip. A finite element analysis indicated that the equivalent plastic strain was maximum at the crack initiation site at the notch tip. A backscattered electron image showed that nanovoids of 50–250 nm in diameter were present near the initiation site. Regarding the crack propagation process, field emission scanning electron microscopy (FE-SEM), electron backscattered diffraction (EBSD) and FRASTA results indicated that some microcracks in ferrite grains coalesced in a stepwise manner and propagated. Trace analyses using EBSD revealed that the QC fracture surface consisted of {011} slip planes, {001} cleavage planes and non-specific index planes. These findings indicate that QC fracture initiated at the notch tip due to the interaction between dislocations and hydrogen associated with local plastic deformation, and propagated in a stepwise manner by coalescence through vacancies, nanovoids and microcracks on various planes associated with/without plastic deformation in ferrite grains.
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    2. Residual Stress Control in Drawn Bar and Wire by Heating-Cooling-Drawing Process ISIJ International Advance Publication
  • Influence of Slag Viscosity and Composition on the Inclusion Content in Steel

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    DOI:10.2355/isijinternational.ISIJINT-2021-213

    Influence of slag viscosity and composition on the inclusion content in the steel is studied using laboratory experiments and modeling simulations. The steel samples are taken during the experimental process to record the inclusion content change. Afterwards the prepared samples are analyzed using automated scanning electron microscope and energy dispersive spectroscopy (SEM/EDS) method. A simple steel/slag reaction model is constructed based on the effective equilibrium reaction zone (EERZ) method. The inclusion content evolution process is discussed by combining the experimental and calculated results. It is found that the inclusion content evolution in the steel is determined by the inclusion generation and removal.
  • Decoupling Strategy and Mechanism-intelligent Model of Non Square Flatness Control System

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    DOI:10.2355/isijinternational.ISIJINT-2020-662

    Taking 1420 mm UCM six-high cold rolling mill as the research object, a non square flatness control system with five input and four output is decoupled into a square subsystem with two input and two output which controls the primary and cubic flatness and a non square subsystem with three input and two output which controls the quadratic and quartic flatness by using the relative gain theory. By decomposing the unstable poles of the generalized inverse matrix of the non square system, the method of the generalized inverse matrix decoupling control the quadratic and quartic flatness is proposed, which solves the unstable problem of decoupling of non-square system. According to the characteristics of intermediate roll shifting, the variable model of roll shifting influence coefficient and the control strategy of minimum roll shifting adjustment and threshold are proposed. The dynamic characteristics of the system are improved and the adjustment of intermediate roll shifting is reduced. In order to overcome the shortcomings of low accuracy and poor generalization ability of shallow neural network, a mechanism-intelligent influence matrix model based on big data and deep neural network is proposed. Simulation calculation and industrial application show that the control system runs stably, the adjustment speed is fast, the control precision is high, the change of intermediate roll shifting is small, and it is suitable for online control.
  • Quick Recognition and Elimination of an Additional Signal Caused by Deflection of an Integral Roller Flatness Meter

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    DOI:10.2355/isijinternational.ISIJINT-2021-006

    Compared with the sectional roller flatness meter, the integral roller flatness meter is more suitable for products with higher requirements on surface quality, and thus becomes the trend of the contact-type flatness meter for cold rolling. Due to its structure design, some problems arise. Under the action of the dead weight or an external load, the deflection deformation of the roller changes the stress state of the sensor installed inside it and generates an additional signal. This paper studies the mechanism of the signal and the recognition and elimination method. First, according to the structure of the detection roller and the connection mode of the sensors, the ideal waveform characteristics of the flatness signal are analysed. Through an experiment on the 650 mm integral roller flatness meter platform, the ideal waveform and actual waveform characteristics are compared, and the influence of the additional signal on the detection signal is analysed. Then, according to the deflection deformation of the roller and its corresponding stress distribution characteristics, the generating mechanism of the additional signal is revealed. Finally, a minimum error method is proposed, which ensures the elimination of the deflection influence on all detection units in real time. During the process of flatness detection, the zigzag feature of the flatness distribution curve is eliminated; thus, during the process of flatness control, the bad influence of the roller deflection is removed. The industrial application shows that the work presented in this paper can obviously improve the products quality.
  • Effect of MC Type Carbides on Wear Resistance of High Wear Resistant Cast Iron Rolls Developed for Work Rolls of Hot Strip Mills

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    DOI:10.2355/isijinternational.ISIJINT-2021-127

    High-speed steel cast-iron rolls were developed around 1990 and have been widely used for the earlier stand of hot strip mills. However, for the later stand of hot strip mills, the use of high-speed steel cast-iron rolls has been limited due to the insufficient crack resistance. Therefore, in order to improve the wear resistance of the later stand, enhanced indefinite chilled rolls in which MC-type carbides of high hardness are crystallized in a conventional indefinite chilled roll has been developed. However, since the wear resistance of enhanced indefinite chilled rolls is significantly inferior to that of high-speed steel cast-iron roll, the development of a new cast iron roll with superior wear resistance applicable to the later stand of hot strip mills was studied. The present development roll has improved wear resistance by increased amount of the high hardness MC-type carbide-forming elements. In addition, the reduction of the carbon equivalent for less amount of eutectic carbide resulted in the reduction of the residual stress down to the same level as the indefinite chilled roll, which improved the crack resistance. As a result, it was confirmed that the wear resistance was improved about three times compared with the conventional indefinite chilled roll. In addition, the results suggest that the wear resistance of work rolls in hot strip mills is greatly controlled by the amounts of MC-type carbides, despite the roll hardness being the same.
  • Guide Vane Opening Prediction for Constant Speed Axial Blowers in Blast Furnace Ironmaking with Variation Information

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    DOI:10.2355/isijinternational.ISIJINT-2021-201

    Determining appropriate guide vane openings (GVOs) of axial blowers under varying industrial conditions is vital for smooth operations in blast furnace ironmaking. This work analyses the influence GVO variations have on outlet air flow rate and pressure by using data taken from operating, industrial blast furnaces, based on which a support vector machine (SVM)-based GVO prediction model is developed. Outcomes reveal that the change status of GVOs, i.e., whether the GVO angle increases or decreases, is critical in determining the relationship between air flow and pressure. By introducing the change status and removing the transition outliers, predictions for the optimal GVOs required to meet the desired air flow rate and pressure in real time can be more accurately determined. The measured values of GVOs range from 0% to 100%, and the SVM-based model developed in this work predicted the GVOs with an RMSE of 0.2480%, significantly improving upon the baseline model which had an RMSE of 0.6047%. The resulting method can provide insights into the operation of complex ironmaking processes, enabling a more efficient adjustment of GVOs.
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    1. Thermal Stability of Molten Slag in Blast Furnace Hearth ISIJ International Vol.61(2021), No.8
    2. Hydrogen Effects on the Migration of Nanoscale Cavities in Iron ISIJ International Vol.61(2021), No.8
  • Nonisothermal Investigation of Reaction Kinetics between Electric Arc Furnace Dust and Calcium Chloride under Carbon-Containing Conditions

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    DOI:10.2355/isijinternational.ISIJINT-2021-128

    The volatilization of zinc in the electric arc furnace dust–CaCl2 and ZnFe2O4–ZnO–CaCl2–C reaction systems was investigated. Experiments were conducted under an N2 atmosphere in an infrared lamp heating furnace, and the activation energy of the reaction rate of zinc volatilization was determined by the Ozawa method. The activation energy in the dust–CaCl2 reaction system was 123 ± 27 kJ/mol; when the dust was decarburized, the activation energy was reduced to 84 ± 4 kJ/mol. Further, the simultaneous carbothermic reduction by the carbon contained in the dust and chloride volatilization of zinc improved the reaction rate. In the temperature range where carbothermic reduction and chlorination occur simultaneously, carbothermic reduction is favored. The reduction of metal oxides in the dust inhibits the chlorination and carbothermic reduction of zinc, reducing the reaction rate and activation energy.
  • Influence of Annealing on Delamination Toughening of Mo-Bearing Medium-Carbon Steel with Ultrafine Elongated Grain Structure Processed by Warm Tempforming

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    DOI:10.2355/isijinternational.ISIJINT-2021-068

    Ultrafine elongated grain (UFEG) structure with a strong <110>//rolling direction fiber texture was created for a 0.4%C-2%Si-1%Cr-1%Mo steel (mass%) through deformation of tempered martensite using multi-pass caliber rolling at 773 K with a rolling reduction of 78% (i.e. warm tempforming). Annealing of the warm tempformed steel at 843 K enhanced delamination toughening at lower temperatures without a significant loss of tensile strength at 1.8 GPa. It was suggested that delamination toughening was controlled through the precipitation of nanoscale Mo-rich precipitates in the UFEG structure.
  • Viscosity Evaluation of Simulated Foaming Slag via Interfacial Reaction at Room Temperature

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    DOI:10.2355/isijinternational.ISIJINT-2020-640

    CaO-based slag used in hot metal pretreatment and converters in steelmaking processes typically contains dispersed gas phases. This is called foaming slag, which is known to degrade the quality of slag. The rheological behavior of this slag is dependent on the dispersed part of the gas phase. This gas is generated by the chemical reaction between the hot metal and the slag. In this study, simulated foaming slag was prepared by reacting sodium hydrogen carbonate and oxalic acid in glycerol, which disperses carbon dioxide. Next, we systematically investigated the effects of the volume fraction of the dispersed gas phase and the proportion of glycerol on the viscosity and bubble diameter. According to the model used in this study, the bubbles were smaller than those in the model in which the gas was directly dispersed. The bubble size increased as the gas phase ratio and liquid viscosity increased, likely because the bubble growth is promoted by increase in the gas phase ratio and liquid phase viscosity, and the frequency with which the bubbles contact one other. The increase of the gas phase ratio at low liquid-phase viscosity and low shear rate caused an increase in both apparent viscosity and relative viscosity, which was obtained by dividing the apparent viscosity by liquid-phase viscosity. However, these increases in viscosity were not observed at a high shear rate. This is likely because the mechanism of bubble diffusion and flow is affected by the liquid-phase viscosity and shear rate. We found that the model in this study exemplified a Herschel-Bulkley fluid. In addition, we proposed an equation for measuring viscosity from the gas phase ratio.
  • Dissolution of Iron Oxides Highly Loaded in Oxalic Acid Aqueous Solution for a Potential Application in Iron-Making

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    DOI:10.2355/isijinternational.ISIJINT-2020-726

    Oxalic acid has been identified as a sustainable chemical enabling an efficient recovery of target metals from industrial minerals by dissolution. The dissolution process recently has attracted attention as a key reaction in a potential clean iron-making. In this application to efficiently produce a high-purity iron, the dissolution is required to occur in the absence of light, with no addition of other chemical reagents, and to produce high concentration iron oxalate aqueous solution as fast as possible. To reveal the chemistry of iron oxide dissolution for this application, in the present study, the dissolution experiments are carried out under various conditions with a particular focus on the iron oxide highly loaded in the oxalic acid aqueous solution. Highly acidic oxalic acid solution for dissolving the highly loaded iron oxide enabled the production of iron oxalates aqueous solution with the concentration of up to 0.56 mol-Fe/L. Different from conventional studies under diluted conditions with pH control, the dissolution followed a non-reductive mechanism, producing [Fe3+HC2O4]2+ as a dominant iron species, and highly correlated with a concentration of proton in the solution. The experimental results and proposed stoichiometries identified a minimum amount of oxalic acid required for the complete dissolution of iron oxide independently from the concentration and type of loaded iron oxide. Among iron oxides tested (α-Fe2O3, FeOOH and Fe3O4) as the feedstock, Fe3O4 had an advantage in the dissolution rate, but showed a relatively low iron recovery in the solution (80–90%) because of an unavoidable formation of FeC2O4·2H2O precipitates.
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  • An Empirical Comparative Study of Renewable Biochar and Fossil Carbon as Carburizer in Steelmaking

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    DOI:10.2355/isijinternational.ISIJINT-2020-135

    Approximately 60–70% of the direct greenhouse gas emissions in electric arc furnace (EAF) steelmaking originate from the use of fossil carbon charge during melting of steel scrap. Regarding short-term solutions to mitigate the climate impact of steelmaking, there is greater potential to replace fossil carbon charge with renewable carbon in the EAF than in integrated blast furnace steelmaking where mechanical strength requirements on carbon charge are too demanding. Therefore, the present study aims to provide an experimental and practical foundation for using renewable biochar in the EAF as a relatively simple step to decrease the climate impact of steelmaking.In order to evaluate the inherent performance of biochar as a carburizing agent, lab-scale tests where completed using four different types of carbonaceous materials: synthetic graphite, anthracite coal and two types of biochar from woody biomass (BC1 and BC2). The first order dissolution rate constants from experiments ranged between 0.7 to 1.9 × 10-4 m/s, which agrees well with previously reported results. Furthermore, lab-scale results show that biochar properties commonly seen as detrimental, such as low carbon crystallinity and high porosity, do not necessarily constitute a disadvantage for biochar utilization as carburizer in steelmaking.In order to further assess the results from lab-scale tests, an industrial trial including six consecutive heats was performed in a 50 t EAF at the Höganäs Halmstad Plant. Results show that 33% substitution of standard Anthracite carbon charge with biochar BC2 gave no deviation from normal operating conditions in the EAF.
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