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

ISIJ International Vol. 60 (2020), No. 2

  • Molecular Dynamics Study of the Effect of Carbon Atoms on the Surface Tension of Silicon–carbon Alloy

    pp. 199-204

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    DOI:10.2355/isijinternational.ISIJINT-2019-308

    We conducted molecular dynamics simulations of Si–C alloy to understand the atomistic behavior of solute C atoms near the melt surface and to estimate the surface tension. The surface tensions of liquid Si and C were first evaluated and compared with experimental values and those for other metals. The composition dependence of the surface tension of Si–C alloy was then evaluated, and compared with estimates obtained using the modified Butler’s model. The behavior of C atoms at the surface of liquid Si–C alloys is also discussed.
  • Supergravity-Induced Separation of Oxide and Nitride Inclusions from Inconel 718 Superalloy Melt

    pp. 205-211

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    DOI:10.2355/isijinternational.ISIJINT-2019-321

    Herein, a method of supergravity-enhanced separation was used to remove oxide and nitride inclusions from Inconel 718 superalloy melt, with elucidating the inclusion removal behavior by varying the gravity coefficients (G) and separation times (t) used for melt treatment. Under supergravity conditions, inclusions concentrated at the sample top and are almost absent at the sample bottom. Moreover, the inclusion number density and average size showed a gradient distribution along the supergravity direction, and the steepness of this gradient rapidly increased with increasing G and t. The experimentally determined inclusion movement velocities agreed well with those calculated using Stokes’s law at G ≤ 210 and t ≤ 10 min. At G = 210 and t = 10 min, the total oxygen and nitrogen contents of the sample decreased from 34.4 to 8.7 ppm and 133.4 to 34.1 ppm, respectively, corresponding to oxide and nitride removal efficiencies of 74.7% and 74.4%, respectively.
  • Structural Evolution of Molten Slag during the Early Stage of Basic Oxygen Steelmaking

    pp. 212-219

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    DOI:10.2355/isijinternational.ISIJINT-2019-413

    The better understanding of structural effect of composition is of primary importance in the design of converter slag and for rationalizing the foaming performance of smelting process. In the present work, the CaO–SiO2–FexO samples with different compositions were prepared to simulate the converter slag of initial smelting stage. The compositions and structural units of slag samples were investigated by combining X-ray fluorescence spectroscopy and Raman spectroscopy. According to the results, the transformation behaviors of structural units and the degree of polymerization (DOP) of molten slag were further analyzed. The results of Raman spectra showed that when basicity increased from 0.38 to 0.97 and total iron content decreased from 32.77 to 13.26 mass%, increasing O2− led to the depolymerization of [SiO4]4− tetrahedrons from Q3 to Q0 units and the increasing [FeO4]5−/[FeO6]9− ratio. With further increasing basicity from 0.97 to 1.25, Q3 units disappeared and more O2− reacted with [FeO4]5− tetrahedrons to form [FeO6]9− octahedrons. Meanwhile, Fe3+ could probably form Si–O–Fe bond by replacing Si4+ cations in Q3 units. Overall, the depolymerization of [SiO4]4− tetrahedrons from Q3 to Q0 units was the main reason for the decreasing DOP of molten slag during the early stage of basic oxygen steelmaking.
  • Structural Transformation of Molten CaO–SiO2–Al2O3–FexO Slags during Secondary Refining of Steels

    pp. 220-225

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    DOI:10.2355/isijinternational.ISIJINT-2019-418

    According to the variation of compositions during steel secondary refining process, the ice-quenched samples of CaO–SiO2–FexO–Al2O3 system were prepared, the structure were detected via the method of Raman spectroscopy, and the evolution of structural units were further analyzed. The results showed that, for the Fe3+ cation, two types of units of tetrahedral fourfold coordination ([FeO4]) and octahedral sixfold coordination ([FeO6]) coexisted in the molten slag, and the ratio of [FeO6]/[FeO4] increases with the decreasing ratio of CaO/(SiO2+Al2O3). For the Al3+ cations, four types of aluminum units of Q2Al, Q3Al, Q4Al coexisted in the molten slag and the lower polymerized units Q2Al transform into the higher polymerized Q3Al and Q4Al along with the ratio of Al/(Al+Si) increasing from 0.41 to 0.85. For the Si4+ cations, Q0Si, Q1Si and Q2Si are the main types of [SiO4]-tetrahedral units, the Ca2+ cations of oxygen coordination in [SiO4]-tetrahedral units are gradually replaced by Al3+, which just act as network modifier, with the increase of Al/(Si+Al) ratio. Accordingly, the degree of polymerization of molten slag presented in NBO/T increases with the process of secondary steelmaking.
  • Erosion of Carbon Brick by Zinc in Hearth of Blast Furnace

    pp. 226-232

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    DOI:10.2355/isijinternational.ISIJINT-2019-142

    The service life of a blast furnace (BF) is affected by the accumulation of zinc. To clarify the erosion mechanism of the carbon bricks, due to the zinc action, a dissection investigation of a commercial BF was carried out. The results show that the zinc content reaches up to 10.59% in the tuyere coke. The carbon bricks were sampled in a region characterized by high erosion levels and molten iron was detected. More interestingly, zinc was detected between the molten iron and the carbon bricks: the high zinc content of the bosh gas of the BF induces the zinc vapor to penetrate into the molten iron surface. The zinc vapor and the molten iron mix together, and zinc migrates from the molten iron into the carbon bricks. The thermodynamic behavior of zinc was analyzed and the volume expansion rate (56.94%) was calculated when Zn oxidizes into ZnO. The oxidation process may be the main reason behind the carbon brick erosion. The results show that molten zinc flows to the brittle layer of the carbon bricks, and finally solidifies, the carbon bricks become easy to break at the brittle layer. Countermeasures to reduce the harm of zinc have been suggested based on the zinc balance calculation.
  • Effect of Magnetite on Mineral Phase Formation in Sintering Process

    pp. 233-237

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    DOI:10.2355/isijinternational.ISIJINT-2019-241

    Mineral phase formation behavior in the sintering process is one of the most important factors for quality and productivity of iron ore sinter. As resource of high-grade hematite ore is exhausting, it is expected that hematite ore can replaced by magnetite in iron-making. So that, the purpose of this study is to investigate the effect of magnetite on mineral phase formation. To clarify the effects of magnetite on mineral phases formation, sintering experiments using hematite and magnetite reagent were carried out. To research the effects of atmosphere and temperature, samples were sintered under oxidizing (air) and reducing (CO–CO2) atmosphere at 1250°C and 1350°C respectively. The results were analyzed by microscopic observation and image processing. Under both oxidizing and reducing atmosphere, the shapes of each phase after sintering of magnetite samples are likely to hematite samples. From the image processing results, the ratio of each phases formed after sintering of samples were at the same level. So, it is expected that magnetite can be used as raw material instead of hematite in sintering process. Under Air atmosphere, both hematite and magnetite samples formed more calcium ferrite phase when the sintering temperature was higher. Moreover, under Air atmosphere, the calcium ferrite formation ratio of both hematite and magnetite samples was larger than that of under CO–CO2 atmosphere. Therefore, it is very important to keep oxidation state in sintering process.
  • Removal of Fine SiO2 Composite Inclusions from 304 Stainless Steel Using Super-gravity

    pp. 238-246

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    DOI:10.2355/isijinternational.ISIJINT-2019-301

    The super-gravity technique was used to remove the SiO2 composite inclusions from 304 stainless steel. The effects of different super-gravity coefficients and super-gravity treatment time on the removal effect of inclusions were studied. It was found that the SiO2-based composite inclusions floated up to the top of the sample after the super-gravity treatment, and the inclusions in the lower part of the sample were largely removed. The volume fraction and number density of inclusions presented a gradient distribution along the direction of the super-gravity, which became steeper with increasing gravity coefficient and treatment time. The total oxygen content at the bottom of the sample was reduced from 150 ppm to 93 ppm within 15 min of super-gravity treatment under the gravity coefficient of G = 80.
  • Effect of Single Power Two Circuits Electroslag Remelting Process on the Cleanliness of the Remelted Ingot

    pp. 247-257

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    DOI:10.2355/isijinternational.ISIJINT-2019-450

    Single power two circuits electroslag remelting process with current carrying mould (ESR-STCCM) has been developed to remelt high alloy. In the present work, the laboratory experiments, physical simulations and numerical simulations were set up to systematically investigate the droplet size and cleanliness of the remelted ingot for ESR withdrawing process (ESRW) and ESR-STCCM. The results indicated that ESR-STCCM can change the distribution of electromagnetic force, thereby reducing the droplet size in the case of the same remelting power. ASPEX explorer was utilized to investigate the non-metallic inclusions of the remelted ingot for different remelting processes, and the result indicated that the types of the non-metallic inclusions for the different remelting processes were not changed, however, the number decreased by 42.3% for ESR-STCCM. Compared with the ESRW, the deoxidation ability of ESR-STCCM increased by 10.7% meanwhile, the desulfurization ability increased by 24.5%.
  • Kinetics of CO Gas Dissolution into Stirred Liquid Fe at 1823 K and Its Impact on Nozzle Clogging during Continuous Casting

    pp. 258-266

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    DOI:10.2355/isijinternational.ISIJINT-2019-355

    CO gas generated by a carbothermic reaction in Submerged Entry Nozzle (SEN) can reoxidize an Ultra Low C (ULC) steel during continuous casting. When Ti presents in the ULC steel, the CO gas oxidizes the liquid steel and FetO–Al2O3–TiOx liquid oxide mixed with solid alumina forms at the interface between the steel and the nozzle. The reoxidation is partly responsible for the nozzle clogging. In the present study, the kinetics of CO gas dissolution into liquid Fe at 1823 K was investigated in order to understand how fast the reoxidation occurs, which is responsible for the liquid oxide formation and the nozzle clogging. A series of gas-liquid reaction experiments were carried out under various conditions (gas flow rate, stirring speed, the partial pressure of CO). Dissolved C and O contents in the liquid Fe were analyzed in order to find possible rate controlling step. It was found that a gas phase mass transfer is a possible rate controlling step at low rate of CO gas supply if the flow rate (Q) is lower than 0.75 L min−1, which is thought to be higher than the actual CO gas supply rate in a typical SEN (~0.15 L min−1, volume corrected at room temperature). Therefore, the reoxidation is limited by the supply of CO gas to liquid steel. Decreasing CO gas generation from the nozzle is recommended to suppress the nozzle clogging.
  • Distribution Characteristics and Thermal Stability of Primary Carbide in Cast Ce-H13 Steel

    pp. 267-275

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    DOI:10.2355/isijinternational.ISIJINT-2019-364

    The primary carbide precipitated during the solidification process will act as the crack source to reduce the performance of H13 steel. It is necessary to obtain the nature of the primary carbide in H13 steel to reduce its detriment. Therefore, the distribution characteristics and thermal stability of the primary carbide in cast Ce-H13 steel were analyzed in this paper. There is a huge difference in the shape of the primary carbide between the 2D observation and the 3D observation. The shape of the primary carbide is a dendritic structure, and the branch is rich-V carbide and the trunk is rich-Ti-V carbide. The primary carbide size in the 3D observation increases gradually from the margin of the Ce-H13 ingot to the center. The rapid growth of the branch leads to an increase in size, and the decrease in the cooling rate is the main reason for the increase in size. When the heating temperature is 1150°C, the rich-V carbide starts to dissolve and dissolved completely at 1250°C. However, the rich-Ti-V carbide just starts to dissolve when the heating temperature is 1250°C. The number density and size of primary carbide decrease gradually with the increase of the heating temperature. Elemental Ce can effectively decrease the size of the primary carbide, but not for the number density. The calculated results are in keeping with the experimental observations. High-temperature heating can effectively reduce the primary carbide size, but cannot eliminate it.
  • In-situ Measurements of Solute Partition Coefficients between Solid and Liquid Phases in Fe–Cr–Ni–Mo–Cu Alloys during Solidification

    pp. 276-285

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    DOI:10.2355/isijinternational.ISIJINT-2019-444

    The in-situ measurements of the solute partition coefficients, k, between the solid and liquid phases in Fe–Cr–Ni–Mo–Cu alloys were conducted using X-ray transmission imaging and X-ray fluorescence spectroscopy (EDS) in a synchrotron radiation facility, SPring-8.A nearly planar solid/liquid interface was achieved in a furnace with a temperature gradient (5–10 K/mm) using X-ray transmission imaging. The measurement points in the solid and liquid phases and close to the solid/liquid interface were determined by X-ray imaging. The compositions of the solid and liquid phases were measured by EDS. The solute partition coefficients along the solidification path in the Fe - 19.89–25.82mass% Cr - 24.73–34.81mass% Ni - 4.46–10.28mass% Mo - 1.47–5.79mass% Cu alloy were determined at 56 different liquid compositions. At the beginning of solidification, the partition coefficients of Cr, Ni, Mo and Cu were 0.96, 0.97, 0.70 and 0.86, respectively. The partition coefficients of Cr and Ni were almost constant during the unidirectional solidification. The partition coefficient of Mo gradually changed from 0.7 to 0.6, leading to a severe microsegregation at the end of solidification. In contrast, the partition coefficient of Cu was dispersed in the range from 0.8 to 0.9. This study demonstrated that the in-situ measurement was effective for systematic measurement.
  • Decoupling Strategy and Dynamic Decoupling Model of Flatness Control in Cold Rolling Strip

    pp. 286-296

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    DOI:10.2355/isijinternational.ISIJINT-2019-328

    Taking a 1420 mm UCM six-high cold rolling mill as the research object, by calculating and analyzing the relative gain array of flatness adjustments, the flatness control strategy of independent control primary flatness, decoupling control quadratic and quartic flatness is proposed, which simplifies the complex three-loop decoupling to the two-loop decoupling, and facilitates the design of flatness control system. In order to overcome the shortcomings of the long response time and the process fluctuation of the static matrix decoupling control, based on the multi-input and multi-output decoupling control theory, a method and model for the whole process decoupling of quadratic and quartic flatness control loops is proposed by introducing dynamic decoupling matrix instead of static decoupling matrix. The simulation results show that the dynamic matrix decoupling control method can make the system adjust quickly and smoothly, and by controlling the primary, quadratic and quartic flatness, the cubic flatness can also be controlled effectively. This paper opens up a new way and method for developing a simple, practical and high performance flatness control system.
  • Tracking the Burden Surface Radial Profile of a Blast Furnace by a B-mode Mechanical Swing Radar System

    pp. 297-307

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    DOI:10.2355/isijinternational.ISIJINT-2019-362

    To continuously track the burden surface radial profile inside a blast furnace in every noncharging period, a new organizing structure for the scanned data of a mechanical swing radar system was proposed. The detection results of the radial shape of the burden surface in one scanning period can be organized into a matrix and represented by a composite image. Then, the extraction of the burden surface radial profile can be achieved by the segmentation of a featured region in the composite image. To address the incorrect segmentation results caused by the deterioration of the image quality in the later stage of each noncharging period, a priori curve-based image segmentation algorithm was proposed. The shape prior was constructed by a priori shape function and a current state function decomposed from the contours of the priori and current segmented regions, respectively. Compared with the classical region-scalable fitting segmentation algorithm, the proposed algorithm has the ability to provide more reasonable segmentation results during the entire noncharging period. The tracking of the burden surface radial profile can be accomplished by calculating the corresponding shape function from the contour of the segmented image region. Compared with the results produced by the existing A-mode radar data processing method, oscillations and local outliers can be avoided in the results of the proposed method. The goal of the continuous tracking of the burden surface radial profile was accomplished.
  • A Novel On-Line Model for the Prediction of Strip Profile in Cold Rolling

    pp. 308-317

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    DOI:10.2355/isijinternational.ISIJINT-2019-233

    This paper presents a new on-line model for the prediction of the roll force profile across the strip in cold rolling. Also presented is a new on-line model for the prediction of roll deformed profile in a six-high mill. It is shown that an integrated model may be formed for the prediction of the strip profile on the basis of them. The prediction accuracy of the proposed models is examined through comparison with the predictions from Finite Element simulation.
  • Effects of Temperature and Phase Transformation on Post-buckling Behavior of Non-oriented Electric Steel during Hot Finishing Rolling

    pp. 318-323

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    DOI:10.2355/isijinternational.ISIJINT-2019-383

    The traditional buckling model is based on the assumption of homogeneous material. However, for non-oriented electrical steel with high-temperature phase transformation, the transverse differences of temperature and phase transformation during the hot finishing rolling result in uneven distribution of material properties in the dual-phase region. In order to study the effect of inhomogeneous material on the post-buckling behavior of strip, the relationships between tangent modulus and temperature in the austenite region and ferrite region are firstly obtained by hot compression experiments. Secondly, the transverse distribution function of tangent modulus is calculated according to the distributions of temperature and phase structure in the dual-phase region. Finally, the large deflection theory of thin plate is modified, and the elastic modulus constant is replaced by the distribution function of tangent modulus. The post-buckling model considering inhomogeneous material is established to analyze the effect of temperature and phase transformation on the wave height. The results show that strip thickness and tension have great effect on the post-buckling deformation of global longitudinal wave, but little effect on local longitudinal wave. The temperature drop and phase transformation at the strip edge have no significant effect on the wave heights of global and local center waves, but they reduce the wave heights of global and local edge waves by 6% and 20%, respectively.
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  • Tailor Welded Partition Blanks: New Methods Improve the Ductility of Ultra-high-strength Welded Joint

    pp. 324-329

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    DOI:10.2355/isijinternational.ISIJINT-2019-114

    A novel quenching and partitioning (Q&P) processing was applied to the ultra-high-strength tailor welded blanks (TWBs) with an equiaxed martensite, retained austensit and carbides in the weld. The Q&P processing consisted of a cooling step and a partition step at 450°C for 10 s to 30 s. The fraction of martensite after the processing was nearly 64%. During partitioning, carbides precipitates with an average size of 30 nm formed inside the martensite. Currently, the interstitial content of austenite was increased to an average of almost 1.2 wt.%. After Q&P processing, the TWB of partition joints exhibited outstanding mechanical properties including a yield strength of 450 MPa, a tensile elongation of 15% at room temperature, and a formability ratio of 108.72% and 81.66%, with respect to the BMs DP1180 and DP590. Furthermore, the tempered martensite formation and austenite ductile-effect were attributed to the formability improvement of ultra-strength-steel TWBs.
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    DOI:10.2355/isijinternational.ISIJINT-2019-369

    In this study, Ni was bonded with steels having various microstructures to investigate the effect of various microstructures in steels on the bonding strength evolution by ultrasonic welding. It is found that at Ni/ferrite interface having similar hardness, the bonding can be produced by flattening of wear particles generated from the abrasion during ultrasonic welding to obtain a higher degree of plastic deformation, which is positive to bonding strength evolution. As for Ni/pearlite and Ni/martensite interfaces having dissimilar hardness, the bonding formation is difficult due to the presence of hard phases that limit the degree of plastic deformation near the interface, and Ni fragments are attached on the steel side. As a result, lower bonding strength evolution is correspondingly obtained due to slower increment of bonded area, whereas longer time is required for bonding formation between attached Ni fragments and the base metal Ni.
  • Passivation Mechanism of Galvanized Steel Rebar in Fresh Concrete

    pp. 337-345

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    DOI:10.2355/isijinternational.ISIJINT-2019-396

    We studied the passivation behavior of hot-dip galvanized steel (HDG) rebar and ordinary steel (black) rebar in fresh concrete using electrochemical techniques. Although the passivation behavior was considerably different, both types of rebar were fully passivated. The black rebar was immediately passivated after exposure to fresh concrete; however, the HDG rebar was passivated after being kept active for tens of hours. The corrosion rates of both types of rebars after passivation seemed comparable. To further investigate the difference, we monitored the passivation processes by electrochemical impedance spectroscopy using carbon steel and zinc electrodes in fresh mortar and saturated Ca(OH)2 solution, which simulated the water in the concrete pores; furthermore, we observed similar trends for fresh concrete. Initially, the Zn surface was partially covered with calcium hydroxy zincate (CHZ) whose coverage increased with exposure time. Finally, the surface was fully covered with a CHZ film after passivation.
  • Effect of Crystallographic Texture on Anisotropy of Mechanical Properties in High Strength Martensitic Steel

    pp. 346-351

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    DOI:10.2355/isijinternational.ISIJINT-2019-164

    The thermomechanical control process (TMCP) is widely applied as one of the effective processes for improving the strength and toughness of steel plates. In actual application, the anisotropies of mechanical properties arising from the crystallographic texture developed during the controlled rolling process are important issues. In this study, the effect of texture on the anisotropies of mechanical properties in an experimentally manufactured YP960 MPa class steel plate was investigated. Strength varied through the plate thickness from the surface to mid-thickness. At the surface, the strength in the longitudinal direction was higher than that in the transverse direction, and in contrast, at mid-thickness, the strength in the transverse direction was higher than that in the longitudinal direction. The major components of the texture at the plate surface were {110}<111> and {112}<111>, whereas those at mid-thickness were {332}<113> and {211}~{311}<011>. It is considered that the texture of the plate surface was formed by shear strain in the austenite region, whereas that at mid-thickness was formed by plane strain compressive. A crystal plasticity analysis based on the initial texture information obtained experimentally revealed that the anisotropies of mechanical properties were strongly affected by the crystallographic orientation.
  • Time Change in Scale Microstructure of Fe-5 mass%Ni Alloy at 1200°C

    pp. 352-358

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    DOI:10.2355/isijinternational.ISIJINT-2019-252

    Ni containing steel is known to form a complex oxide scale, which consists of an outer layer of Fe-oxides and an inner layer of FeO with complicated distribution of Ni(Fe) metal particles. Due to the complex microstructure, descaling of the oxide scale formed on Ni containing steel during a hot-rolling process is very difficult. In order to improve the descaling process, microstructural control of the inner oxide layer to eliminate its detrimental effect is necessary.In this study, the change in microstructure of the outer and inner layers formed on Fe-5 mass%Ni alloy during oxidation is investigated. In particular, the change in the microstructure of the metal particles in the inner layer with oxidation time is considered.The inner layer consisted of FeO, Ni(Fe), and voids. The concentration of Ni in the Ni(Fe) was found to increase across the inner layer from the scale/steel interface toward the outer/inner scale interface due to the equilibrium Ni concentration in the Ni(Fe) particles with FeO, which corresponded to the oxygen potential gradient in the inner layers. The number and area fraction of the Ni(Fe) metal particles decreased, whereas the size of the particles increased with oxidation time. This coarsening of the metal particles was proposed to be due to Ostwald ripening.
  • Effect of Cold Rolling on Stability of HCP and FCC Phases in Fe–Mn Alloys

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    DOI:10.2355/isijinternational.ISIJINT-2019-333

    The phase transformation behavior of an Fe–20%Mn alloy during a heating process after various cold-rolling reductions was investigated, and the phase stabilities of the γ and ε phases were discussed. The initial hot-rolled material was composed of an ε martensite matrix and a small amount of the γ austenite phase at room temperature. The deformation of the martensite alloy in the cold rolling was not homogeneous, and the microstructure of some regions was clearly adopted from that in the hot-rolled sample. Moreover, a residual γ phase was still detected even after 35% cold-rolling reduction. In the heating stage, a remarkable reverse transformation to the γ phase started at 200°C or higher, and its finishing temperature clearly increased with the rolling reduction ratio. However, the in situ X-ray diffraction and electron back scatter diffraction (EBSD) observations revealed that the reverse transformation had already started from the residual γ phase particles even at temperatures below 200°C. In addition, from the EBSD–image-quality map, the distribution of the dislocations was considered to remain in the γ phase even after the reverse transformation.
  • Effects of Alloy Elements on Carbon Partitioning in Early Stages of Proeutectoid Ferrite Transformation in Low Carbon Mn–Si Steels

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    DOI:10.2355/isijinternational.ISIJINT-2019-395

    Controlling the carbon concentration and distribution among constituent phases is one of the most important issues for achieving high strength and ductility in the design of steel. The carbon distribution near the α/γ interface in the early stage of isothermal holding at 750°C was measured and visualized in Fe–C–Mn–Si alloys containing 2 mass% Si and 1.5 or 2 mass% Mn using a high precision FE-EPMA developed recently by the authors, and the results were compared with the theory of ferrite growth in multi-component low alloy steel. The carbon concentrations at α/γ interfaces in austenite were generally between the NPLE/PLE and paraequilibrium α/(α + γ) boundary concentrations. In alloys with carbon contents smaller than the NPLE/PLE boundary, the α/γ interfaces appeared to migrate under a condition close to paraequilibrium or with partially developed spikes of alloy elements in the early stages. On the other hand, in alloys with a bulk composition on the boundary and its higher carbon concentration side, Mn enrichment was observed at the interfaces, and the carbon concentrations tended to be higher than those in alloys with lower carbon contents, albeit there were variations at individual interfaces.
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    DOI:10.2355/isijinternational.ISIJINT-2019-314

    Low–cycle fatigue tests of a polycrystalline Fe–3 mass%Si alloy were performed at room temperature under a constant total strain amplitude of 1 × 10−2. Dislocation structures were observed by high–voltage scanning transmission electron microscopy. The development of dislocation walls parallel to (110) started during the first few tens cycles of fatigue. The activation of a set of double slip systems, (211)[111] and (112)[111], contributed to the formation of (110) walls. The (110) walls lie in the directions bisecting the angles between the Burgers vectors of the two active dislocations of the double slip systems.
  • Effect of Initial Microstructure on Creep Strength of ASME Grade T91 Steel

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    DOI:10.2355/isijinternational.ISIJINT-2019-358

    To clarify the cause of heat-to-heat variation in the creep strength of Grade T91 steels, the influence of the initial microstructure on creep strength was investigated. The distribution of chromium concentration considered to be remaining segregation was observed as corresponding to lamellar contrasts parallel to the longitudinal direction of the boiler tube. Standard deviation (SD) of ΔCr was employed as an indicator of the degree of segregation, and a good correlation was found between the SD of ΔCr and the creep rupture life at 650°C. Remaining segregation was reduced by renormalizing heat treatment at 1200°C instead of 1250°C. The creep rupture life of steel subjected to renormalizing heat treatment at 1200°C and tempering at 760°C, followed by normalizing and tempering under standard heat treatment conditions for Grade T91 steel, was prolonged by a factor of 2.3–2.8. The strengthening effect of renormalizing at 1200°C to reduce the remaining segregation was confirmed by creep tests up to about 10000 h at 600°C and 650°C. Decreases in the number density of M23C6 carbide particles, length of high-angle boundaries and average KAM values during creep exposure are promoted by the presence of remaining segregation. Since diffusion is enhanced by the concentration gradient of elements, degradation due to microstructural change is promoted by the presence of remaining segregation. Segregation should be reduced to obtain high creep strength with homogenized concentration of chemical composition.
  • Relationship between Thermal Conductivity and Structure of the CaO–BO1.5–AlO1.5 System

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    DOI:10.2355/isijinternational.ISIJINT-2019-251

    During the continuous casting process in steel making, the mold flux plays an important role in establishing adequate heat flow. Therefore, it is important to optimize the thermal conductivity of the flux system to control this process. Boron oxide (B2O3) is one of the components of the mold flux system and its structural complexity is well known. With the aim of revealing the relationship between the thermal conductivity and flux structure, the authors previously studied BO1.5-containing mold flux systems. In this study, the thermal conductivity of the CaO–BO1.5–AlO1.5 flux system was measured above 1500 K for various compositions using the transient hot-wire method. The composition dependence of the flux thermal conductivity was investigated in terms of its local structure, as analyzed using Raman spectrometry and MAS-NMR. The non-additive change in the thermal conductivity of the CaO–BO1.5–AlO1.5 system, which is known as the borate anomaly, is attributed to the relative fraction of the BO1.5 structural unit or the three-dimensional (3D) structural network involving the [4]A–O–[3]B bond. The results obtained by Raman spectrometry revealed that the complexation of the flux structure by the 3D AlB3O7 structure can increase the thermal conductivity at a high BO1.5 content. The formation probability for this structure was calculated based on the MAS-NMR results. Thus, the increase in thermal conductivity can be adequately explained by the formation of the AlB3O7 structure. For practical purposes, the effect of substituting SiO2 for AlO1.5 on thermal conductivity was also investigated with fixed BO1.5 and CaO concentrations.
  • Extraction of Phosphorus and Recovery of Phosphate from Steelmaking Slag by Selective Leaching

    pp. 400-407

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    DOI:10.2355/isijinternational.ISIJINT-2019-298

    At the refining temperature of steelmaking slag, phosphorus is distributed between the liquid phase and solid solution phase of 2CaO·SiO2-3CaO·P2O5. By exploiting the differences in water solubilities between the solid solution and other phases, we are developing an acid leaching process to separate out phosphorus. In this paper, we determined the optimum conditions of leaching with nitric acid by investigating the control over Fe valency in the slag and the recovery of phosphorus from the leachate. pH of 3 was found to dissolve the solid solution, while a solid solution containing FeO showed a lower phosphorus dissolution ratio. To avoid the formation of a glassy phase, slow cooling was necessary which suppressed the dissolution of other phases at this pH. Leaching was further studied in artificially prepared steelmaking slags of compositions representing commercial slag fertilizers. The dissolution ratio of phosphorus reached about 91% while the phosphorus content in the residue was sufficiently low. After separating from the residue, the pH of the leachate was increased to precipitate the phosphate. At a pH of 7, over 80% of the phosphorus in the leachate was precipitated, and the phosphate content of the precipitate was approximately 25% after calcination.

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