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

ISIJ International Vol. 58 (2018), No. 9

  • Preface to the Special Topics on “Cutting-edge Technologies and Scientific Researches of Zinc and Zinc Alloy Coated Steel Sheets”

    pp. 1549-1549

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    DOI:10.2355/isijinternational.58.1549

  • Crystal Structures and Mechanical Properties of Fe–Zn Intermetallic Compounds Formed in the Coating Layer of Galvannealed Steels

    pp. 1550-1561

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    DOI:10.2355/isijinternational.ISIJINT-2018-066

    The crystal structures of all intermetallics of the Fe–Zn system have been investigated by synchrotron X-ray diffraction combined with atomic-resolution scanning transmission electron microscopy, in order to elucidate a basic principle based on which these crystal structures are constructed. The plastic deformation behavior of all these Fe–Zn intermetallics have also been investigated by micropillar compression testing at room temperature, with the use of small-scale pillar specimens of micrometer size, in order to elucidate operative slip systems and their critical resolved shear stress values. The observed deformation behavior is discussed in the light of the deduced building principle of the crystal structures.
  • Automotive Corrosion and Accelerated Corrosion Tests for Zinc Coated Steels

    pp. 1562-1568

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    DOI:10.2355/isijinternational.ISIJINT-2018-159

    Numerous studies on zinc and zinc alloy coated steel sheets have appeared since corrosion problems were widely recognized in the automotive industry in the 1970s. In general, accelerated corrosion tests including the Salt Spray Test and Cyclic Corrosion Tests have been applied to evaluate the corrosion resistance of coated steels in the laboratory in order to shorten the product development period. However, the reproducibility of corrosion in actual use conditions by corrosion tests is always a controversial issue. In this paper, author’s previous studies regarding corrosion of coated steel sheets in automobiles and corrosion tests are reviewed. The perforation corrosion behaviors of coated steels at lapped parts in snowy and tropical regions were clarified on the basis of an analysis of corroded vehicles. Several types of accelerated corrosion tests used in the automotive industry were conducted for comparison with the corrosion observed in the actual vehicles. The Perforation Corrosion Index (PCI), which is the corrosion rate ratio of steel and zinc coating, was introduced as an index for comparing the correlation of corrosion behaviors. Coating weight is the major factor determining the perforation corrosion resistance of existing coated materials in automobile use environments. Corrosion tests that have a PCI value close to that in the use environment can simulate similar corrosion tendencies. The influence of the specimen configuration is also a key factor for adequate evaluation of corrosion resistance. Specimens without an intentional large clearance and bare metal surface in the lapped portion are recommended for perforation corrosion tests.
  • Fracture Toughness of the Fe–Zn Intermetallic Compounds Measured by Bend Testing of Chevron-Notched Single-Crystal Microbeams

    pp. 1569-1577

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    DOI:10.2355/isijinternational.ISIJINT-2018-068

    The fracture toughness values of the five intermetallic compounds in the Fe–Zn system have been investigated through bend testing of chevron-notched single-crystal microbeams. The intermetallic compounds of the Fe-lean phases, δ1p and ζ, exhibit fracture toughness values higher than the other compounds of the Fe-rich phases, Γ, Γ1, and δ1k. The δ1p-phase compound exhibits a strong anisotropy in fracture toughness while the other four compounds exhibit almost no anisotropy. The compositional dependence of fracture toughness is discussed in terms of the surface energies estimated by ab initio calculations as well as the capability of stress relaxation around a crack by dislocation emission. The Γ phase with a relatively wide solid solubility range exhibits no compositional dependence of the fracture toughness. The fracture toughness value of Γ-phase microbeams that include a grain boundary at the chevron-notch position is comparable to that of the Γ-phase single-crystal microbeams, indicating that the grain boundaries in the Γ phase may not be particularly weak.
  • Micro-Vickers Hardness of Intermetallic Compounds in the Zn-rich Portion of Zn–Fe Binary System

    pp. 1578-1583

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    DOI:10.2355/isijinternational.ISIJINT-2018-111

    Annealing temperature and composition dependences of hardness for the intermetallic compound phases, especially Γ-Fe4Zn9, δ1p-Fe13Zn126 and δ1k-FeZn7, which are obtained in Zn-rich portion of Fe–Zn alloys, were investigated by the micro-Vickers hardness test. Although the hardness of the Γ and δ1k phases only slightly decreases with increasing quenching temperature, it shows obvious composition dependences. However, the hardness of the disordered δ1p phase is basically lower than that of the ordered δ1k phase, which is hardly affected by composition change. The hardness of all the Zn-rich compound phases quenched from 500°C (or 550°C) are evaluated and compared.
  • Effect of Dew Point on Galvanizability in 4 Mass% Al Added Low Density Steel

    pp. 1584-1591

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    DOI:10.2355/isijinternational.ISIJINT-2018-010

    Hot dip galvanizing experiments were conducted on low density steel to investigate its galvanizability. The panels were annealed at 815°C for 150 s, and the dew point of the annealing atmosphere was varied from −40°C to +10°C. The surface morphologies of the coating and the substrate before dipping and after coating removal were demonstrated by SEM equipped with EDS. The cross sections prepared by common metallographic method and FIB were characterized by SEM and TEM respectively. It is found that the dew point of the annealing atmosphere has a great effect on the galvanizability of 4 mass% aluminium added low density steel. With the dew point increasing, the external Al oxidation converts into internal Al oxidation and an iron particle layer forms on the outmost surface of the matrix. The Al in the zinc bath mainly reacts with the iron particles to form Fe–Al–Zn inhibition layer at the interface of coating/matrix, resulting in both good wettability and strong coating adhesion of high aluminum low density steel sheet.
  • Improvement of Wettability between Steel and Liquid Zn–Al Alloy by Forced Wetting

    pp. 1592-1599

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    DOI:10.2355/isijinternational.ISIJINT-2018-147

    The main objective of the present work was to investigate the forced wetting of a partly oxidized steel by a liquid Zn–Al (0.2 wt.%) alloy. The wetting experiments are performed by means of the dispensed drop technique. The wetting is shown to be reactive with the formation of Fe2Al5Znx. The evolution of the contact angle and spreading diameter is determined as a function of spreading time.The final contact angle lies between the receding and the advancing contact angle and is a decreasing function of the initial kinetic energy of the droplet. The liquid zinc drop remains pinned in a metastable position, due to the contact angle hysteresis.During the first ms, the spreading diameter increases up to its maximal value Dmax. In good agreement with previous studies in the field of wetting at low temperatures, the maximal spreading diameter scales as D0 We1/4, We being the Weber number which compares inertial and capillary forces.The kinetic energy of the liquid metal droplet needed to reach the minimum receding contact angle was predicted from the model based on the Weber number to describe Dmax. This kinetic energy is in good agreement with the experimental results.As a final conclusion, an increase in the initial kinetic energy of the droplet leads to a decrease in the final contact angle. It is therefore expected that forced wetting could improve the galvanizability of steels by liquid Zn–Al alloys.
  • Effect of Si/Mn Ratio on Galvannealing Behavior of Si-added Steel

    pp. 1600-1607

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    DOI:10.2355/isijinternational.ISIJINT-2018-063

    The influence of the Si/Mn ratio on the galvannealing behavior of 1.5 wt% Si −1.5~2.5 wt% Mn-added steel in the Fe oxidation-reduction process was investigated. The Si/Mn ratio of the steel affected the formation of Si-containing oxides during the annealing process. The amount of SiO2 formed on the steel surface decreased with as the Si/Mn ratio decreased, while the amount of Mn2SiO4 increased. In addition, the internal oxide formed in a relatively narrow area near the surface in the lower Si/Mn ratio sample, which indicated that the content of solute Si near the surface was lower in the lower Si/Mn ratio sample. The galvannealing reaction was accelerated by decreasing the Si/Mn ratio of the steel. The species and morphology of the Si-containing oxides determined the galvannealing behavior of the Si-added steel. The Si-containing selective surface oxide affected the formation of the initial Fe–Zn intermetallic compounds (IMC) during hot-dipping in molten Zn. The formation of SiO2 was suppressed in the sample with the lower Si/Mn ratio, which resulted in accelerated Fe–Zn IMC formation. On the other hand, solute Si in the steel affected the growth of the Fe–Zn IMC during heating in the galvannealing process. The content of solute Si was assumed to be lower in the lower Si/Mn ratio sample, which resulted in acceleration of Fe–Zn IMC growth.
  • Enhanced Interfacial Reaction of Fe–Si Alloy Sheets Hot-Dipped in Zn Melt at 460°C

    pp. 1608-1615

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    DOI:10.2355/isijinternational.ISIJINT-2018-036

    In order to understand the effect of solute Si (in the steel sheet) on the interfacial reaction between liquid Zn and solid Fe (α-Fe phase) during the hot-dip galvanizing process, a change in the interfacial microstructure between Zn coating and Fe substrate in Fe–Si alloy sheets hot-dipped in Zn melt with dipping time at 460°C was examined. In pure Fe sheet, the Fe–Zn intermetallic layers form at the interface between solid Fe and liquid Zn at an early stage of dipping and subsequently grow to approximately 60 µm in thickness after 600 s. In Fe–1Si (wt%) alloy, the thickness of Zn coating substantially increases to beyond 500 µm after 600 s and the coarse ζ-FeZn13 phase with several facet planes was often observed in the Zn coating. The thickness of the Fe–1Si alloy sheets continuously decreases till 60 s and then is reduced significantly after 600 s. The thickness loss in the later stage of dipping is more significant in the Fe–Si alloy with higher Si content. These results indicate a significant Fe dissolution into liquid Zn could occur at the later stage of dipping the Fe–1Si alloy in Zn melt, which is distinguished from the interfacial reaction between pure Fe and liquid Zn. The enhanced interfacial reaction would be responsible for the decomposition of the initially formed ζ-FeZn13 phase layer to liquid and FeSi phases, which can be proposed based on thermodynamic calculations of the Fe–Zn–Si ternary system.
  • Role of Metal Cations on Corrosion of Coated Steel Substrate in Model Aqueous Layer

    pp. 1616-1622

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    DOI:10.2355/isijinternational.ISIJINT-2018-071

    To elucidate the role of Zn2+ on corrosion of coated steel, the effects of metal cations on the corrosion of carbon steel in the concentrated Cl aqueous solutions were studied by immersion tests, surface analysis and electrochemical tests. Among the examined metal cations, Zn2+ showed the significant effect on corrosion inhibition of carbon steel in the Cl aqueous solution at high concentration. XPS analysis results elucidated that Zn2+ can remain on the steel surface after immersed in the solutions with Zn2+. In addition, AFM results showed lower roughness of specimen immersed in the solution with Zn2+ than other solutions with metal cations. EIS measurements showed higher impedance in the solution with Zn2+ than other solutions, and the results suggested that Zn2+ reduced the defect points in the thin oxide film by forming a metal cation layer. Based on the experimental results, Zn2+ may form a layer on the oxide film that protects the Cl attack in the solution. The findings demonstrated that the formation of Zn layer on the oxide film is one of the main reason for showing high and longtime corrosion resistance of Zn coated steel substrate.
  • Influence of Cr Addition on Selective Oxidation Behavior of Mn-Added High-Strength Steel Sheet

    pp. 1623-1628

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    DOI:10.2355/isijinternational.ISIJINT-2018-075

    In the manufacture of high tensile strength sheet steels (HSS) containing Cr and Mn, selective surface oxidation of Cr and Mn affects the wettability of the HSS with molten Zn and leads to coating defects in the hot-dip galvanizing process. Therefore, it is important to clarify the selective surface oxidation behavior of Cr- and Mn-bearing steel from the viewpoint of active utilization of galvannealed HSS.The focus of the present study is an investigation of the influence of the amount of Cr addition on selective surface oxidation, which is thought to determine the wettability of cold-rolled HSS containing 0–0.6 mass% Cr-1.7 mass% Mn with molten Zn. The wettability was investigated by measuring the contact angle of molten Zn containing 0.14 mass% Al (Zn–Al) on the HSS. Surface and cross-sectional analyses were performed using secondary and transmission electron microscopes. Selective surface oxidation behavior was investigated by glow discharge spectroscopy.The main results obtained are as follows. The contact angle of Zn–Al on the HSS tended to decrease as the amount of Cr addition increased up to 0.6 mass% due to the increment of MnCr2O4, which is known as Cr–Mn spinel and reacts with molten Al more easily than MnO.
  • Influence of Annealing Temperature and Dew Point on Kinetics of Mn External Oxidation

    pp. 1629-1634

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    DOI:10.2355/isijinternational.ISIJINT-2018-213

    In the process of hot-dip galvanizing of steel, alloying elements such as Si and Mn are easily oxidized by H2O in the annealing atmosphere, causing coating defects. Because this selective oxidation depends on the annealing heat pattern, i.e., the soaking temperature and time, basic research on the kinetics of selective oxidation is important for clarifying the phenomenon of selective oxidation. In this study, the effects of the annealing temperature and dew point on the kinetics and compounds of Mn external oxidation were investigated experimentally, and the Mn external oxidation rate was estimated based on a diffusion equation and thermodynamic equilibrium, considering the diffusion coefficient and the activity coefficient of Mn in steel. The amount of Mn oxide increased in proportion to the square root of the soaking time. This result suggests that Mn oxidation is a diffusion limited process. The Mn oxidation rate increased with increasing temperature and reached a peak value, and at higher temperatures, the Mn oxidation rate became dramatically slower. The peak value also depended on the dew point. To clarify the reason for this slowdown of Mn oxidation, the Mn oxidation rate was estimated. Considering the activity coefficient and the diffusion coefficient of Mn in steel, the calculated Mn oxidation rate was consistent with the measured value. It is thought that the Mn oxidation rate slows at high temperature because the gradient of the Mn concentration around the steel surface becomes small at high temperatures near the equilibrium temperature of Mn/MnO.
  • Effect of Annealing Temperature on the Selective Oxidation and Reactive Wetting of a 0.1C-6Mn-2Si Advanced High Strength Steel During Continuous Galvanizing Heat Treatments

    pp. 1635-1643

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    DOI:10.2355/isijinternational.ISIJINT-2017-688

    The effects of peak annealing temperature and annealing time on the selective oxidation and reactive wetting of a prototype medium-Mn Fe-0.1C-6Mn-2Si third generation advanced high strength steel were investigated. Annealing heat treatments were carried out in a N2-5 vol% H2 243 K (−30°C) dew point process atmosphere at 963 K (690°C) and 1073 K (800°C) for 120 s and 600 s. TEM observations of the sample cross-sections revealed internal oxidation of the subsurface grains and grain boundaries. EELS results showed that the internal oxide network had a multi-layered structure with SiO2 at the oxide core and MnSiO3 as the surrounding shell; however, MnO was the only species detected at the surface of all samples. The effect of annealing temperature on the surface structure development and its impact on reactive wetting of the substrates annealed for 120 s at both peak annealing temperatures by a Zn-0.20 wt% Al (dissolved) galvanizing bath was also determined. In contrast to the 1073 K steel, the 963 K substrate showed superior reactive wetting, owing to the much thinner, finer and wider spacing of the MnO nodules on the pre-immersion surface. TEM+EELS analysis of the coated steels showed that infiltration of the bath alloy and partial reduction of MnO resulted in lift-off of the surface oxides and partial formation of Fe2Al5ZnX interfacial layer, indicating that reactive wetting had occurred for the 963 K × 120 s substrate.
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  • Roles of P2O5 Addition on the Viscosity and Structure of CaO–SiO2–Al2O3–Na2O–P2O5 Melts

    pp. 1644-1649

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    DOI:10.2355/isijinternational.ISIJINT-2018-138

    In this research, the effect of P2O5 additions on the viscosity of CaO–SiO2–Al2O3–Na2O–P2O5 melts was clarified using rotating cylinder method. In addition, the role of P2O5 on the microstructure of these melts was investigated by Raman, Fourier Transformation Infrared (FTIR) and Magic Angular Spinning Nuclear Magnetic Resonance (MAS NMR) spectroscopic techniques. We found that an increase of P2O5 content continuously increased the viscosity of melts and meanwhile increased the apparent activation energy for the viscous flow. The FTIR and Raman results show that the degree of polymerization (DOP) got increased with more P2O5 additions and this was consistent to the variation trend of melt viscosities. The MAS-NMR results show that there exist two kinds of PO4 tetrahedron, namely, Q0(P) and Q1(P), in the samples and a transformation appeared between them with increasing P2O5 content. The present study thus provided significant clues for stabilizing the pore structures in foam glass ceramics.
  • Effects of Temperature and Circulating Flue Gas Components on Combustion and NOx Emissions Characteristics of Four Types Quasi-particles in Iron Ore Sintering Process

    pp. 1650-1658

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    DOI:10.2355/isijinternational.ISIJINT-2018-185

    It’s significant to deeply understand the combustion and NOx yield characteristics of quasi-particles in flue gas recirculation sintering process. In present study, effects of temperature and different circulating flue gas components on the mass conversion rate and conversion of fuel-N to NOx of quasi-particles were investigated in a vertical quartz tube reactor. Four types quasi-particles were considered according to the existing state of coke in granules: C type, P type, S type and S’ type. The mass conversion rate of four types quasi-particles ranked in the order of C>P>S>S’ at different temperature. It was found that D1 model was most appropriate to describe the combustion characteristics of S, S’ and P type while D3 model was better than other models to describe the combustion characteristics of C type quasi-particles through the comparison of correlation coefficients calculated by different mechanism models. Effect of circulating flue gas components on the conversion of fuel-N to NOx of quasi-particles was studied. O2 has a positive impact on the conversion of fuel-N to NOx for S, S’ and C type but an inhibitory effect for P type quasi-particles. CO2 and CO could decrease the conversion rate of fuel-N to NOx for all types quasi-particles. Effect of H2O(g) on the conversion rate of fuel-N to NOx was different for four types quasi-particles. The conversion rate of fuel-N to NOx firstly increases then decreases for S’, S and C type quasi-particles while continuously increases for P type quasi-particles with increasing H2O(g) content.
  • Effect of Slag Composition on the Dissolution of Phosphorus from Steelmaking Slag by Selective Leaching

    pp. 1659-1668

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    DOI:10.2355/isijinternational.ISIJINT-2018-240

    Steelmaking slag is considered a potential source of P because of its huge production. In steelmaking slag, P is mainly distributed in the C2S–C3P solid solution which is more water-soluble than other phases. In order to recover P and recycle steelmaking slag, we have proposed the selective leaching of the P-concentrated solid solution. To clarify the applicability of this process to steelmaking slags with various compositions, the effects of P2O5 content, Fe2O3 content, and slag basicity on the dissolution behavior of the slag modified with the addition of Na2O were investigated in this study. It was determined that the dissolution ratio of P reached approximately 76% regardless of the P2O5 content in slag, while the dissolution of Fe and Mg was poor. In the case of high P2O5 content, a leachate with higher P concentration and lower Ca and Si concentrations was obtained, exhibiting excellent selective leaching of P. Increasing the Fe2O3 content in slag not only promoted the dissolution of P but also decreased the dissolution ratios of other elements. With the increase in slag basicity, the dissolution ratios of Ca, Si, and P all increased. Most of the solid solution was dissolved from each slag without a large dissolution of other phases at pH 6, indicating that selective leaching of the solid solution was achieved over a wide range of slag compositions. After leaching, a residue with low P2O5 content and high Fe2O3 content was obtained.
  • Computational Simulation Model for Metallurgical Effects during EAF Refining Stage: Waiting and Arcing Time

    pp. 1669-1678

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    DOI:10.2355/isijinternational.ISIJINT-2018-224

    3D computational fluid dynamics models using Fluent were developed to investigate the steel melt flow during waiting and arcing time. Both models were transient that analyzed 60 seconds to investigate the flow characteristics considering variation in steel melt thermo-physical parameters and operating conditions. The velocity of melt movement was high enough to make a turbulent flow (solved with realize k-ε turbulence model).It was found that the steel melt flow velocity increases by a combined effect of the steel melt temperature and composition, and slag pressure. The slag pressure increases by a double effect of slag density and height, and the steel melt fluid flow velocity changes with the slag pressure. The effect of the slag thickness is more significant than the effect of thermo-physical properties of steel melt. Although, the maximum steel melt velocity “during arcing time” may be as large as 0.67 m/s located at steel met outlet, the melt exhibits completely dead zones with minimum flow velocity distribution especially at the bottom and circumference areas. This indicates the importance of combined stirring and large reaction rates to achieve a complete homogeneous melt especially at bottom and circumference areas.
  • A Parametric Study on the Effects of Process Conditions on Dehydrogenation, Wall Shear and Slag Entrainment in the Vacuum Arc Degasser Using Mathematical Modelling

    pp. 1679-1686

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    DOI:10.2355/isijinternational.ISIJINT-2018-254

    The effect of vacuum pressure and argon flow rate on hydrogen degassing of molten steel in a triple plug, 100 tonne vacuum arc degasser has been examined using a three phase Eulerian CFD-mass transfer coupled model. The model takes into account the interaction between the slag, steel and argon phases over a 20-minute degassing period. Increasing the argon flowrate from 13–29 Nm3hr−1 produces a 10% increase in the hydrogen removal ratio, generating a faster melt velocity and larger slag eye. This also results in the maximum shear stress on the ladle walls increasing by a factor of 2.2 and the shear stress integrated across the wall increasing by a factor of 3.75, thus contributing to enhance refractory erosion. Within the same flowrate range the volume of entrained slag also increases by a factor of 1.4, which may result in increased nitrogen/oxygen pickup. Reducing the vacuum pressure maintains a low equilibrium hydrogen concentration and allows more efficient hydrogen removal, with a 38% reduction in the removal ratio between 102−104 Pa.
  • Influence of Large-Scale Vortex Movement in Lower Recirculation Zone on Instable Flow Field in the Mold

    pp. 1687-1694

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    DOI:10.2355/isijinternational.ISIJINT-2018-160

    During continuous casting of slab, the flow field in the mold is unstable, which will lead to defects in final products. To investigate the instability of molten steel in the mold, an unsteady Reynolds-average (URAN) method is used to study the unstable flow of molten steel in the mold. The results show that the large-scale vortex movement in the lower recirculation zone is a major factor causing the instability of flow field. These vortexes change the outflow of molten steel in the lower recirculation zone. Molten steel discharging from one side of the nozzle outlet reaches a certain depth, it refluxes to the bottom of nozzle, while it discharging from the other outlet flows deep into the liquid pool. This will cause an asymmetric distribution of internal defects in the casting productions. These vortexes also affect the meniscus level fluctuations of molten steel. They change the reverse flow direction and pressure field in the lower recirculation zone, which affect the position and the angle of stream impinging on the narrow face. The paper could provide a basis for stabilizing the flow field in the mold.
  • Mold Simulator Study of Effect of Mold Oscillation Frequency on Heat Transfer and Lubrication of Mold Flux

    pp. 1695-1704

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    DOI:10.2355/isijinternational.ISIJINT-2018-275

    The heat transfer and lubrication behavior of the slag film infiltrated into the mold/shell gap have a significant effect on the surface quality of continuous casting of steel slabs. With the mold simulator technique, the effect of mold oscillation frequency on the heat transfer and lubrication behavior of the infiltrated mold/shell slag film was studied in this article. The experiment results showed that the increase of the mold oscillation frequency from 1.00 to 2.00 Hz would cause the thickness of the infiltrated mold/shell slag film decreased by 8.08 pct. As a result, the thermal resistances between the mold and the shell decreased by 9.81 pct, and then the shell solidification factor increased by 3.98 pct due to the mold heat flux increased. Moreover, the thickness of liquid slag film decreased by 17.50 pct, and the liquid slag consumption decreased by 20.69 pct.
  • Optimization of Partial-least-square Regression for Determination of Manganese in Low-alloy Steel by Single-shot Laser-induced Breakdown Spectroscopy

    pp. 1705-1710

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    DOI:10.2355/isijinternational.ISIJINT-2018-134

    Laser-induced breakdown spectroscopy (LIBS) is employed for ultra-high-speed determination of the manganese content in various steel specimens. LIBS is widely known as a method for elemental analysis with a rapid response. It has several advantages such that it can work under ambient pressure, and that specimens can be tested without any pre-treatment such as acid digestion, cleaning, or polishing of surface of the specimens. We applied a laboratory-build LIBS system for the determination of Mn in a series of low-alloy steel certified reference materials by a multivariate analysis using partial-least-square regression. Considering enough intensities of Mn emission lines and spectral interferences from emission lines of the iron matrix in these alloys, two wavelength ranges for the spectrograph could be employed. By minimizing the predicted residual sum of squares and the root mean square error of prediction, the analytical result of the Mn concentrations could be obtained with reasonable accuracy and precision.
  • Hot Deformation Behavior of a Nickel-modified AISI 4330 Steel

    pp. 1711-1720

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    DOI:10.2355/isijinternational.ISIJINT-2018-034

    Hot compression of a nickel-modified AISI 4330 steel was studied over a wide range of temperatures and strain rates using Gleeble 3800 thermomechanical simulator. Samples were quenched at different stages of deformation to study microstructure evolution using optical and electron microscopies. Stress-strain curves showed that both dynamic recovery (DRV) and dynamic recrystallization (DRX) take place for certain deformation conditions. However, in comparison to low alloyed steels with lower nickel content, the nickel-modified AISI 4330 steel showed significantly lower workability particularly, resulting from the temperature and strain rate ranges over which DRX takes place. The kinetics of recrystallization were studied and the stress constants and apparent activation energies were determined for the investigated alloy and compared with similar alloy with lower nickel content so as to quantify the influence of nickel. The obtained results are interpreted in terms of the influence of nickel on dislocation dynamics and therefore the softening process at high temperature under various ranges of strain rate. The findings will help a better design of the forging process of these alloys which are prone to hot cracking during open die forging.
  • Effect of Post-weld Heat Treatment on the Dissimilar Cu/Al Joints Pro-duced by High Power Ultrasonic Spot Welding

    pp. 1721-1726

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    DOI:10.2355/isijinternational.ISIJINT-2017-694

    The effect of post-weld heat treatment (PWHT) on the microstructure and mechanical properties of ultrasonic spot welded Cu/Al dissimilar joints was studied in this paper. PWHT was performed for 1 h at four different temperatures of 200°C, 300°C, 400°C and 500°C, respectively. The microstructure and mechanical properties of joints were investigated by SEM, EDS, X-ray diffractometer, Microhardness tester and Universal testing machine, respectively. Results showed that no obvious intermetallic (IMC) layer was found at the as-welded joint and joint after PWHT at 200°C. But relatively higher shear strength of 2.458 KN was obtained when the temperature of PWHT was 200°C. As the temperature of PWHT exceeded 300°C, the shear strength of joints reduced obviously, due to the formation of brittle Al–Cu IMC layer. However, when the joint was subjected to PWHT at 500°C, maximum shear strength of 3.138 KN was acquired resulted from the increase of the bonded area, though large numbers of cracks were found at the Cu/Al interface. Besides, obvious grain boundary penetration occurred on the Al base metal adjacent to the Cu/Al interface when the joint after PWHT at 500°C. The micro-hardness at the Cu/Al interface was significantly higher than that of the Cu or Al base metal. With the temperature of PWHT increasing, the micro-hardness decreased both in Cu and Al base metals, but increased significantly at the Cu/Al interface. Fracture mode of joints, both as-welded and after PWHT, were the mixed-mode of ductile and brittle fractures.
  • Effect of Surface Oxidation on Decarburization of a Fe-3%Si Steel during Annealing

    pp. 1727-1734

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    DOI:10.2355/isijinternational.ISIJINT-2018-233

    The main purpose of decarburization annealing is to reduce the mass fraction of carbon in Fe-3%Si steels to avoid magnetic aging in silicon steels. Decarburization annealing experiments are carried out at different annealing temperatures, PH2O/PH2 and times. The morphological evolution of oxidized layer on steel surface, the changes of carbon and oxygen concentrations in a Fe-3% Si steel, and the influence of the oxidation of alloying elements on the decarburization are analyzed. Results show that decarburization rate of the tested silicon steel is the highest when annealing temperature is 1103 K and PH2O/PH2 is 0.374. When PH2O/PH2 and annealing temperature are lower than that of optimum annealing condition, increasing annealing temperature or PH2O/PH2 result in the increase of the convective mass transfer coefficient between steel and atmosphere, promoting the mass transfer and intensifying decarburization reaction. On the other hand, the amount of oxide particles also increases with further increasing PH2O/PH2 and annealing temperature, resulting in the hindrance of the diffusion of carbon atoms in the oxidized layer and slowdown of decarburization.
  • Influence of Induction Heating Conditions on Quenched Structure of SUJ2 Steel

    pp. 1735-1741

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    DOI:10.2355/isijinternational.ISIJINT-2018-106

    When JIS-SUJ2 steel is hardened by induction heating, high-temperature austenitization is applied in order to achieve high throughput. In this case, SUJ2 is quenched from the single-phase austenite zone. As a result, the properties of SUJ2 may be different from those obtained by a conventional furnace process involving SUJ2 quenched from the two-phase austenite/cementite zone. In this study, we experimentally investigated the relationship among the amount of undissolved carbide, austenitization temperature, and holding time. Additionally, we developed a formula for predicting the amount of undissolved carbide. The formula is constructed from the Arrhenius equation and the Kolmogorov-Johnson-Mehl-Avrami equation. Subsequently, the effect of the austenitization temperature and the amount of undissolved carbide on the hardness, retained austenite content, prior-austenite grain size, and martensite block size were investigated.
  • Holdup Characteristics of Melt in Coke Beds of Different Shapes

    pp. 1742-1744

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    DOI:10.2355/isijinternational.ISIJINT-2018-369

    Numerical analysis was carried out on the dripping and holdup behaviors in the lower part of a blast furnace for coke bed structures having different shapes; a discrete element method smoothed particle hydrodynamics scheme was used considering the size distribution immediately above the raceway. Even for coke beds with similar void fractions, the averaged-coke-shape factors such as ϕ and (ϕD)2 give little clear correlation for holdup sites. Instead of averaged-coke-shape information, only the direct evaluation of the void shape of the packed bed can explicitly trace the holdup site.
  • Split and Shift of ε-martensite Peak in an X-ray Diffraction Profile during Hydrogen Desorption: A Geometric Effect of Atomic Sequence

    pp. 1745-1747

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    DOI:10.2355/isijinternational.ISIJINT-2018-260

    Cryogenic X-ray diffraction measurements demonstrated a split of the ε-martensite peak at 193 K in a hydrogen-charged austenitic steel. Only the higher angle peak remained after aging at room temperature. This phenomenon can be interpreted by a change in the interstitial hydrogen position. Particularly, the motion of the leading partial involved in ε-martensitic transformation can move interstitial hydrogen from a tetrahedron to an octahedron site, expanding the lattice. Subsequently, the hydrogen can move back to the tetrahedron site, which relatively shrinks the lattice. The two different hydrogen positions cause the peak to split.
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