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

ISIJ International Vol. 49 (2009), No. 11

  • Thermodynamic Properties of the MgO–BO1.5, CaO–BO1.5, SiO2–BO1.5, MgO–BO1.5–SiO2 and CaO–BO1.5–SiO2 Slag Systems at 1 873 K

    A. Semih Sunkar, Kazuki Morita

    pp. 1649-1655

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    DOI:10.2355/isijinternational.49.1649

    Abstract

    Ferroboron alloys have been used in the production of various materials from conventional steels to metallic glasses. Recently, the demand for these alloys has been increasing. Commercial ferroboron processes strongly depend on the use of pure B2O3 or H3BO3 as starting materials in reduction processes. However, the use of pure B2O3 instead of natural B minerals leads to the loss of B through evaporation and a decrease in process efficiency. In order to overcome this loss, this study evaluated raw materials other than pure B2O3 from a thermodynamic point of view.
    Thermodynamic properties of the components in the binary MgO–BO1.5, CaO–BO1.5, and SiO2–BO1.5 systems, as well as in the ternary MgO–BO1.5–SiO2 and CaO–BO1.5–SiO2 systems, were investigated at 1873 K by means of chemical equilibration, with Cu alloy as a reference melt. During experiments, considerable amounts of B and Si were dissolved in Cu. Prior to the analysis of the thermodynamic properties of the slags, such as activities and activity coefficients of BO1.5, those of the Cu–B and Cu–B–Si melts were clarified at 1873 K.
    The investigated oxide melts were compared with each other for their possible use as raw materials in the production of ferroboron. Among the binary systems investigated, MgO–BO1.5 slags were found to be the most suitable candidates for the production of carbothermic ferroboron.
    Finally, it was found that in both ternary systems, the activities and activity coefficients of BO1.5 decrease significantly with the addition of SiO2. It was found that the effect of SiO2 in the MgO–BO1.5–SiO2 system was more pronounced than that in the CaO–BO1.5–SiO2 system. According to the results, the addition of SiO2 to binary slags is likely to cause a decrease in the smelting efficiency of ferroboron.

  • Influence of Neodymium on the Deoxidation and Desulfurization Equilibria of Liquid Iron in the Fe–Nd–O–S(–Al) System at 1 873 K

    Hideki Ono, Yasuhisa Tachiiri, Katsuhiro Yamaguchi, Tateo Usui

    pp. 1656-1660

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    DOI:10.2355/isijinternational.49.1656

    Abstract

    Deoxidation and desulfurization equilibria of liquid iron, as influenced by neodymium, were investigated at a temperature of 1 873 K. The following thermodynamic values for neodymium deoxidation and desulfurization were derived:
    [Equation]

    Using the thermodynamic data derived in this study, deoxidation and desulfurization equilibria of liquid iron by neodymium are presented for the Fe–Nd–O–S system. Moreover, the interaction parameter, Al for Nd (eNdAL) was determined to be 4.9(±4.1) at 1 873 K.

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  • CFD Model for Emulsification of Slag into the Steel

    Petri Sulasalmi, Aki Kärnä, Timo Fabritius, Jari Savolainen

    pp. 1661-1667

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    DOI:10.2355/isijinternational.49.1661

    Abstract

    A CFD model based on water model experiments has been created to simulate slag entrainment and droplet formation. Multiphase Volume of Fluid (VOF) method is used to track the interface between slag and steel and User Defined File (UDF) code is applied to track separate droplets. Four oil–water systems and three slag–metal systems were simulated. The main objective of this research was to obtain droplet diameter distributions and the average droplet diameter in several different cases. With slag–metal cases the main interest was the effect of interfacial tension to droplet formation. With oil–water systems we studied the effect oil layer width and oil viscosity. The obtained droplet diameter distributions show that the dominant droplet size is 2–3 mm in every oil–water systems. In slag–metal systems the dominant size varies between 1–2 mm and 2–3 mm. The simulations show also that the average droplet size in all cases is 2.78–3.63 mm. The results were compared to the studies available in the literature.

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  • Effect of Chromium on Nitrogen Solubility in Liquid Fe–Cr Alloys Containing 30 mass% Cr

    Wan-Yi Kim, Chang-Oh Lee, Chul-Wook Yun, Jong-Jin Pak

    pp. 1668-1672

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    DOI:10.2355/isijinternational.49.1668

    Abstract

    The nitrogen solubility in liquid Fe–Cr alloys has been measured by the gas–liquid metal equilibration technique in the temperature range from 1873 to 1973 K. The nitrogen dissolution follows the Siverts' law for liquid Fe–Cr alloys containing chromium up to 30 mass%. Chromium increased the nitrogen solubility linearly at Cr content less than 3 mass%, and the second-order effect of Cr on nitrogen solubility was apparent at higher Cr content. Using Wagner's formalism, the first- and second-order interaction parameters between chromium and nitrogen were determined from the equilibrium relation between chromium and nitrogen contents in liquid Fe–Cr–N alloys as a function of temperature:
    [Equation]

  • Production of Mn–Fe Alloy from Slag Generated in Mn-removal Treatment of Molten Cast Iron

    Takahito Takagawa, Shigeru Ueda, Hiroyuki Ike, Kouji Iwashimizu, Ryo Inoue

    pp. 1673-1677

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    DOI:10.2355/isijinternational.49.1673

    Abstract

    Production of Fe–Mn alloy from the slag generated in the Mn-removal treatment of cast iron was investigated by using the method of carbon reduction of FeO–SiO2–MnO and FeO–SiO2–MnO–CaO slags at 1723 K.
    In the first stage, almost all the Fe was reduced from the slags, followed by the reduction of Si and Mn. Finally, Fe–Mn–Si metal was obtained as a result of the reduction treatment. The addition of CaO to the FeO–SiO2–MnO slag had two effects: (1) the slag viscosity decreased, thereby improving slag reactivity, and (2) the SiO2 activity in the slag melt decreased, thereby lowering the Si content in the metal. It was confirmed that the reaction between carbon and the slag is rate limiting reaction. The contact condition between the slag and carbon has a significant influence on the slag reactivity; improvement of the contact condition leads to an increase in the slag reactivity.

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  • Conditions for CeS Formation During Manufacturing of Ce–S–Al Based Grain Refiners for Steels

    Erlend F. Nordstrand, Øystein Grong, Casper van der Eijk, Sean Gaal

    pp. 1678-1685

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    DOI:10.2355/isijinternational.49.1678

    Abstract

    In the present investigation the phase relations within the Ce–S–Al system have been clarified, using a combination of optical microscopy and WDS microprobe analyses. As a starting point high-purity charge materials of cerium, aluminium and Ce2S3 were melted and superheated to about 2000°C within small tantalum crucibles inside a dedicated laboratory furnace filled with cleaned argon. The main constituent phases detected in the as-solidified samples are CeS, Ce3Al, CeAl and γ-Ce, where the CeS phase constitutes a discontinuous dendritic network within the grain refiners. The melting experiments show that pure cerium can dissolve about 6 wt% of sulphur at 2000°C, which drops to approximately 1.8 wt% at 1500°C. The measured sulphur solubility is considerable lower than that inferred from the existing binary Ce–S phase diagram, which therefore should be revised to comply with these new measurements. Because alloying with aluminium reduces the sulphur solubility in liquid cerium, the addition of this element should be restricted if a high volume fraction of CeS is desired in the grain refiners. At the same time the use of Ce2S3 as a sulphur source in replacement of pyrite (FeS2) means that aluminium is not actually needed to prevent the grain refiners from disintegrating in contact with air due to internal oxidation.

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  • Lowering Reduction Temperature of Iron Ore and Carbon Composite by Using Ores with High Combined Water Content

    Taichi Murakami, Takeshi Nishimura, Eiki Kasai

    pp. 1686-1693

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    DOI:10.2355/isijinternational.49.1686

    Abstract

    The steel industry is facing two difficult and urgent tasks to reduce CO2 emissions and to use low-grade iron resources effectively. The utilization of an iron ore–carbon composite is one of the promising methods to solve the former. The latter is concerning to goethite ores such as Australian Marra Mamba and pisolite ores, which contain high concentration of combined water. In this study, the effect of combined water on the reduction behavior of the iron ore–coal composites at elevating temperature was examined under inert gas flow.
    Below 1200 K, the reduction of iron oxide in Marra Mamba ore–coal and pisolite ore–coal composites proceeded faster than that in hematite ore–coal composite. It can be attributed to the larger specific surface area of the ores after decomposition of the combined water. Metallic iron also formed at lower temperature in the composites containing Marra Mamba and pisolite ores. The generation rate of CO gas from these composites showed the maximum value at approximately 1170 K; however, that from the hematite-coal composite gave no peak, because formed metallic iron could act as a catalyst for the gasification of carbon. These results indicate that the reduction of ores with high combined water concentration can proceed at lower temperature. Above 1373 K, however, the reduction rate of these ores in the composite significantly decreased due to a drastic decrease in the specific surface area of the ores and the formation of slag.

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  • Experimental Research on Reducing the Dust of BOF in CO2 and O2 Mixed Blowing Steelmaking Process

    Cao Yi, Rong Zhu, Bo-yu Chen, Can-rong Wang, Jian-Xiang Ke

    pp. 1694-1699

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    DOI:10.2355/isijinternational.49.1694

    Abstract

    The thermodynamic theory and experimental research on reducing the dust generation in CO2 and O2 Mixed Blowing (COMB) steelmaking process have been undertaken. In labo-scale experiment, data show that the amounts of dust and iron loss keep on decreasing as CO2 blowing rate increases. Further, dust sampled from 30 t BOF conventional process in Fujian Sanming Iron and Steel Plant has been analyzed, and the dust generation rate in the conventional process reduced from 666 g/s at the 1.5 min point to 215 g/s at the end of blowing. Introducing the technology of the stepped CO2 blow, the average generation rate of dust is reduced by 12.5% in this experiment compared with the conventional one, while the average generation rate of T-Fe in dust is reduced by 12.7%. This improvement has been made keeping the same decarburization rates. Therefore the stepped CO2 blowing rate has remarkable effect on dust generation. This research has proved that COMB steelmaking process is effective on a decrease in dust generation along with improved iron yield.

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  • Development of the Rheocast Diesel Engine Block with ADC10 Alloy via ART (Advanced Rheocasting Technology) System

    J. G. Sim, Y. S. Jang, J. Y. Moon, J. M. Kim, K. H. Min, C. P. Hong

    pp. 1700-1709

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    DOI:10.2355/isijinternational.49.1700

    Abstract

    A new rheocasting process was applied to develop a diesel engine block using ADC10 alloy. This type of diesel engine block requires much higher mechanical properties for a high engine power, and this is why heat treatment for improving mechanical properties should accompany processing. Therefore, the manufacturing process for a diesel engine block must prevent inner defects such as blow-holes, gas porosities and shrinkages. Rheocasting is considered as a manufacturing technique for this purpose. In order to apply the rheocasting technique in manufacturing a diesel engine block, it is important to produce large-volume slurry. Establishing the injection condition has been particularly focused on for the integral inner parts. The Advanced Rheocasting Technology (ART) system, in this study, was used for large-volume slurry with ADC10 alloy. Through computer simulation, the injection condition was determined to stabilize feeding patterns which were verified from experimental results. Various heat treatments were tested to determine the optimal condition for the rheocast diesel engine block. The rheocast engine block showed fine and uniform microstructures, where the average diameter of primary α-globules was 75 μm and its form-factor indicating the degree of globularity was 0.83 including gas contents below 4cc/100 g Al. Mechanical properties indicated 317 MPa Ultimate Tensile Strength (UTS) and 1.2% elongation good enough for the objective usage after modified T6 heat treatment.

  • Radiative Heat Transfer in Transition Metal Oxides contained in Mold Fluxes

    Jiang Diao, Bing Xie, Jianping Xiao, Chengqing Ji

    pp. 1710-1714

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    DOI:10.2355/isijinternational.49.1710

    Abstract

    Transition metal oxides FeO, MnO and TiO2 contained in mold fluxes were prepared and measured by a FTIR spectrometer. Characteristics of absorption/extinction coefficient were obtained through infrared spectrum analysis. Relation between radiative heat transfer and the transition oxides was calculated by a heat exchange model. The result indicates that the transition oxides have the great negative effect on radiative heat transfer during the wavelength of 1–6 μm. Radiation heat flux, q12 decreases from 5.5–6.3×104 W m−2 to 4.3–5.1×104 W m−2, 3.4–4.2×104 W m−2 and 3.9–5.4×104 W m−2 with 2–8% MnO, 1–3% FeO and 2–8% TiO2 added, respectively. Due to the great refraction and scattering at surface and grain boundaries, the negative effect in crystalline samples was much larger than that happened in the glassy ones. MnO and TiO2 have great influence on viscosity and melting temperature of mold fluxes, but FeO has little influence. XRD results show that Mn2SiO4, Fe2SiO4, CaTiO3 and other minor phases were precipitated after transition oxides added. Grain size of crystals enlarges from 12.5 to 100 μm with increasing of holding temperature. At 800°C, the radiation heat flux is 1.25×104 W m−2, and decreases to 0.84×104 W m−2 at 900°C. Above 900°C, the radiation heat flux increased on the contrary. The radiation heat flux increased from to 0.84×104 W m−2 to 1.0×104 W m−2. Industrial trial shows that the transition oxides contained in mold fluxes are good at coordination of heat transfer controlling and strand lubricating, and the occurrence of longitudinal cracks is greatly decreased. Further studies in the transition metal oxides contained in mold fluxes will be valuable for improving strand surface quality of crack sensitive peritectic steels.

  • Heat Transfer Analysis for Solidifying Ferrous Multi-component Alloys Using Computational Thermodynamics

    Kenichi Ohsasa, Hiroyuki Shirosawa

    pp. 1715-1721

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    DOI:10.2355/isijinternational.49.1715

    Abstract

    Temperature–enthalpy curves of ferrous multi-component commercial alloys of S45C steel, SUS304 stainless steel and FC200 cast iron were calculated based on the thermodynamic calculation using the Thermo-Calc for both an equilibrium state and non-equilibrium state with no diffusion in solid. The heat transfer simulations during the solidification of the alloys based on the enthalpy method were carried out using the calculated temperature–enthalpy curves. Thermal analysis experiment of the alloys under a furnace cooling condition was carried out and measured cooling curves were compared with simulated ones. The simulated cooling curves of S45C carbon steel and SUS304 stainless steel obtained with the non-equilibrium condition showed similar shapes to the experimentally measured ones. Phase transformation kinetics during the eutectic solidification of FC200 cast iron was taken into account for calculating the temperature–enthalpy curve of FC200 cast iron and simulated cooling curve showed similar shape to measured cooling curve.

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  • Effects of Both Crystallisation and Iron Oxides on the Radiative Heat Transfer in Mould Fluxes

    Masahiro Susa, Atsushi Kushimoto, Hiroaki Toyota, Miyuki Hayashi, Rie Endo, Yoshinao Kobayashi

    pp. 1722-1729

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    DOI:10.2355/isijinternational.49.1722

    Abstract

    To suggest a guideline for designing a mould flux for mild cooling, the radiative heat transfer characteristic of mould fluxes has been investigated using apparent reflectivity and transmissivity data on the basis of an optical-process model. The apparent reflectivities and transmissivities of mould fluxes were measured systematically as functions of the degree of crystallinity and the concentration of iron oxides, where the degree of crystallinity ranged between 0% and about 60%, and the concentration of iron oxides ranged between 0% and 2.0% on a mass basis. The apparent reflectivities and transmissivities were measured over the wavelength range 300–2600 nm using a spectrophotometer with an integrating sphere. It has been found that crystallisation for mould fluxes increases the apparent reflectivity and decreases the apparent transmissivity, and that additions of iron oxides increase the apparent absorptivity of crystallised mould fluxes and instead decrease the apparent reflectivity. The radiative heat transfer characteristic has been evaluated using apparent reflectivity and transmissivity data on the basis of an optical-process model, which has suggested that crystallisation of mould fluxes leads to mild cooling due to increased apparent reflectivities and decreased apparent transmissivities. Furthermore, it has also been suggested that crystallised mould fluxes with no iron oxides can reduce radiative heat transfer even more by decreasing the apparent absorptivity and increasing the apparent reflectivity.

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  • Characterization of Formation and Oxidation of Green Rust (Cl−) Suspension

    Futoshi Nagata, Katsuya Inoue, Kozo Shinoda, Shigeru Suzuki

    pp. 1730-1735

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    DOI:10.2355/isijinternational.49.1730

    Abstract

    Green rust (GR) containing both ferrous (Fe(II)) and ferric (Fe(III)) ions is formed in an aqueous solution with a relatively low electrochemical potential. In this study, in order to understand the formation and oxidation of GR, a suspension of GR containing chloride ions (GR(Cl−)) was synthesized and subsequently oxidized by injecting nitrogen gas containing oxygen under different conditions. X-ray diffraction (XRD) measurements were performed for identifying the solid particles formed under different conditions. The results show that the GR(Cl−) suspension was formed from specific concentrations of hydroxyl, ferrous, and ferric ions. The pH values and oxidation-reduction potential (ORP) of the aqueous solution were measured during oxidation of the GR(Cl−) suspension. It was observed that because of oxygen injection, GR(Cl−) was transformed into iron oxides such as α-FeOOH, γ-FeOOH, and Fe3O4. These iron oxide and oxyhydroxide species formed by oxidation were dependent on the oxidation conditions such as temperature. This suggested that conditions of aqueous solution are crucial for the transformation of GR(Cl−) into different iron oxides and oxyhydroxides.

  • Correlation between Sheet Deformation and Hydraulic Pressure Variation during Sheet Hydroforming

    Takayuki Hama, Keita Matsushima, Toru Kitajima, Hitoshi Fujimoto, Hirohiko Takuda

    pp. 1736-1743

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    DOI:10.2355/isijinternational.49.1736

    Abstract

    In our series of studies, the outflow characteristics of a pressure medium used for a sheet hydroforming process have been investigated. The variation of hydraulic pressure during a square-cup sheet hydroforming process was examined, focusing on correlation with sheet deformation. In the initial stage, the outflow of the pressure medium remained stable because a greater extent of pre-bulging arose around the die shoulder just after the process commenced. When the pre-bulging ceased, the pressure medium was sealed in the chamber. Thus, the hydraulic pressure began to increase rapidly within the chamber, and fell in the flange area. The punch stroke at which the hydraulic pressure in the chamber began to rise was highly dependent on the blank holding force because the height of pre-bulging was determined by this force. When the sheet conformed completely to the die shoulder, the pressure medium was also sealed at the shoulder. Therefore, the hydraulic pressure on the die shoulder as well as in the flange area started to fall. The correlation between the critical punch stroke for fracture and the hydraulic pressure in the flange area was also examined.

  • Electrode Degradation in Resistance Spot Welding of Magnesium Alloy

    B. Lang, D. Q. Sun, G. Z. Li, B. Q. Zhu

    pp. 1744-1748

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    DOI:10.2355/isijinternational.49.1744

    Abstract

    Mechanism of electrode degradation in resistance spot welding (RSW) of magnesium alloy has been investigated using scanning electron microscopy (SEM) and energy-dispersive spectrum (EDS). The results show that electrode degradation experienced mainly four basic steps (i.e. magnesium alloy pickup, electrode alloying with magnesium, electrode tip face pitting, and cavitation). Magnesium alloy pickup began from the first weld as tiny drops of molten magnesium alloy were transferred from the sheet surface to the electrode tip face. A complex alloy layer (Cu2Mg and CuMg2) was formed by the molten magnesium alloy reacting with the electrode. Electrode pitting occurred by brittle fracture of alloyed region formed on the electrode tip face resulting in material removal from the electrode tip face. Once the electrode pitting started, large cavities formed by combining smaller pits. Therefore, it is favorable to periodic cleaning of the electrode tip face for extending electrode life and obtaining well appearance of joint surface.

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  • Influence of Bonding Temperature on Structure and Strength Properties of Titanium and Micro-duplex Stainless Steel Diffusion Bonded Joints

    Sukumar Kundu, Subrata Chatterjee

    pp. 1749-1754

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    DOI:10.2355/isijinternational.49.1749

    Abstract

    In the present study, diffusion bonding of titanium and micro-duplex stainless steel was investigated in vacuum. Diffusion interfaces were characterized using light microscopy, scanning electron microscopy and X-ray diffraction technique. The inter-diffusion of the chemical species across the diffusion interfaces were evaluated by electron probe microanalysis (EPMA). The maximum tensile strength of ~97% and shear strength of ~80% of those of Ti along with 6.9% elongation were obtained for the diffusion couple processed at 900°C joining temperature. Fracture surface observation in SEM using EDS demonstrates that, failure takes place through β-Ti phase when bonding was processed up to 850°C, however, failure takes place through λ+FeTi+β-Ti for the diffusion couples processed at 900°C and above bonding temperature.

  • Aerodynamic Investigation about the Cause of Check-mark Stain on the Galvanized Steel Surface

    Hyun Gi Yoon, Gi Jang Ahn, Sang Jun Kim, Myung Kyoon Chung

    pp. 1755-1761

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    DOI:10.2355/isijinternational.49.1755

    Abstract

    When galvanized steel strip is produced through a continuous hot-dip galvanizing process, the thickness of the adhered zinc film is controlled by impinging a thin plane nitrogen gas jet. In such a gas wiping process frequently there appears stain of check-mark shape. The check-mark stain is caused by non-uniform zinc coating over the surface. In order to find the main reason of the appearance of the check-mark stain, the unsteady compressible flow field is numerically simulated by using LES turbulence model for two cases of L/d=6.67 in the Case 1, 11.54 in the Case 2 where d is the nozzle width and L is the lip-to-plate distance. It was found that there are alternating plane-wise vortices near the impinging stagnation region, and that such alternating vortices move almost periodically to the right and to the left sides on the stagnation line due to the jet flow instability and alternating pressure peaks along the stagnation line. Since higher stagnation pressure removes more molten zinc adhered on the surface, the zinc coating thickness is thinner there. In addition, since the strip moves upward at a constant speed, the non-uniform coating surface is formed with a variety of patterns like “W”, “V” and “X”. The angle of the check-mark was calculated by using both the moving speeds of the steel strip and the sidewise movement of the vortices along the stagnation line. It was favorably compared with the experimental measurement.

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  • Effect of Nitrocarburizing Time on the Microstructures and Erosion Behavior of Cold-work Tool Steel

    Dong Cherng Wen

    pp. 1762-1768

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    DOI:10.2355/isijinternational.49.1762

    Abstract

    In this study we examined the influence of nitrocarburizing holding time on the surface microstructures and erosion behavior of JIS SKD11 modified cold-work tool steel (DC11 tool steel). The steel was nitrocarburized at 570°C for varying durations of 1, 3, and 5 h. The microstructures and hardness of the nitrocarburized coatings were then analyzed. Particle erosion was examined at different impinging angles (15–90°) and impact speeds (20.2–45.6 m/s).
    The results show that a single diffusion zone is formed on the specimens at 1 h nitrocarburizing while a compound layer together with diffusion zone are formed on the specimens for the nitrocarburizing time beyond 3 h. In addition, the compound layer formed on the specimens exhibits a higher erosion resistance. The nitrocarburizing treatment not only increases the surface hardness but also improves the erosion resistance of the experimental steel. This improvement in erosion rates is more obvious at higher impact speeds and lower impinging angles. The maximum erosion rate appears at an impinging angle of 30° for all specimens. In this condition, plough grooves and cutting lips appear in the eroded surface; however, the erosion tracks are more superficial for nitrocarburized specimens than untreated specimen. The exponent in the power law Ė=kVn varied between 1.9–2.3 for impinging angles between 15° and 90°.

  • Effect of Preadsorption of Polyethylene Glycol on the Appearance and Morphology of Electrogalvanized Steel Sheets

    Hiroaki Nakano, Satoshi Oue, Yusuke Hamaguchi, Shigeo Kobayashi, Hisaaki Fukushima

    pp. 1769-1775

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    DOI:10.2355/isijinternational.49.1769

    Abstract

    Zn electrodeposition was performed galvanostatically on a steel sheet at 1500 A/m2 in an unagitated sulfate solution at 40°C to investigate the effect of preadsorption of polyethylene glycol (PEG) on the lightness and morphology of Zn. The lightness of deposited Zn was increased by the preadsorption at coating masses of 10–80 g/m2, except in the initial stage of the deposition. The overpotential for Zn deposition was increased by the preadsorption, and its polarization effect was maintained even when Zn deposition proceeded. The platelet crystals of Zn were smaller and more random in growth direction when Zn deposited on the cathode preadsorbed by PEG. The decrease in size of platelet crystals of Zn with the preadsorption is attributed to both the increase in overpotential for Zn deposition and the decrease in epitaxial growth of Zn. The orientation of {0001} Zn basal plane decreased because of an increase in overpotential for Zn deposition by the preadsorption of PEG. The lightness of deposited Zn increased in spite of a decrease in orientation of the {0001} plane with the preadsorption. This indicates that the lightness of deposited Zn was more easily affected by the decease in size of Zn platelet crystals than by the crystal orientation of Zn.

  • Investigation on Boronizing of N80 Tube Steel

    Z. G. Su, X. Tian, J. An, Y. Lu, Y. L. Yang, S. J. Sun

    pp. 1776-1783

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    DOI:10.2355/isijinternational.49.1776

    Abstract

    In present paper, boronizing was applied to N80 steel tube by pack boronizing, and its effect on mechanical properties, wear and corrosion of N80 tube steel was investigated. A dual-phase boride layer composed of FeB and F2B phases was formed on the surface of the steel substrate in a hardness range of 1220–1340 HV. A set-up was designed to reduce usage of the boriding agent and accelerate the pipe's cooling process after boronizing treatment. In order to meet the tensile properties of N80 steel required by API SPEC 5L, different cooling methods were employed including annealing, normalizing, fan-cooling and graphite-bar assisted rapid cooling. Among these methods, graphite-bar assisted rapid cooling resulted in the highest amount of pearlite in the steel substrate and the highest mechanical properties, satisfying the mechanical properties of API SPEC 5L. The boronized N80 steel exhibited an abrasive wear mechanism and showed a higher wear resistance under a given sliding condition due to the great hardness and integrity of the boride layer, and it also displayed an excellent corrosion resistance in both H2SO4 and HCl acid environments.

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  • In-situ Observation of Butterfly-type Martensite in Fe–30mass%Ni Alloy during Tensile Test Using High-resolution EBSD

    Hisashi Sato, Stefan Zaefferer, Yoshimi Watanabe

    pp. 1784-1791

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    DOI:10.2355/isijinternational.49.1784

    Abstract

    The microstructual change of the thermal butterfly-type martensite (α′) in Fe–30mass%Ni alloy during a tensile test was investigated by in-situ observation using high-resolution electron backscatter diffraction (EBSD). When the specimen is plastically deformed, the α′ plate in a thermal butterfly-type α′ starts to grow toward its width direction from the α′–γ interface with {2 2 5}γ habit plane. As the α′ plate grows, the crystal orientation relationship (OR) between α′ and γ changes from Greninger–Troiano to Kurdjumov–Sachs. Moreover, an orientation gradient is formed in the α′ plate as its growth proceeds. The latter is due to the inheritance of the piled-up transformation dislocations into the α′ plate. On the other hand, the orientation gradient in the γ matrix around the α′–γ interface decreases. These results indicate that the thermal butterfly-type α′ changes its character to lath-type during growth. This is caused by the change of the accommodation process from single slip for butterfly-type martensite to multiple dislocation slip due to the tensile deformation at R.T. It is concluded that the growth behaviour of the butterfly-type α′ during tensile deformation depends on the change of the accommodation process.

  • Abnormal α to γ Transformation Behavior of Steels with a Martensite and Bainite Microstructure at a Slow Reheating Rate

    Takuya Hara, Naoki Maruyama, Yasuhiro Shinohara, Hitoshi Asahi, Genichi Shigesato, Masaaki Sugiyama, Toshihiko Koseki

    pp. 1792-1800

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    DOI:10.2355/isijinternational.49.1792

    Abstract

    The same coarse austenite (γ) grains as those before austenitizing emerge when a martensite or bainite steel with coarse grains is reheated to an austenite region at a slow reheating rate. This is called abnormal ferrite (α) to austenite (γ) transformation or γ grain memory. In this paper, α to γ transformation behavior is investigated in order to clarify the mechanism of abnormal α to γ transformation from the viewpoint of the roles of cementite and retained γ. (1) Coarse γ grains and fine globular γ grains that nucleate along the coarse γ grain boundaries are formed when bainite or martensite steel is reheated above the AC3 temperature. The size distribution of γ grain is the same as that before reheating. (2) Coarse γ grains are formed by the growth, impingement, and coalescence of acicular γ grains that corresponds to retained γ between laths. (3) Abnormal α to γ transformation is suppressed by decreasing the amount of retained γ and by increasing the amount of cementite before reheating. These results suggest that α to γ transformation behavior is governed by competition between the nucleation and growth of newly formed γ from the dissolution of cementite and the growth and coalescence of retained γ. Abnormal α to γ transformation occurs when the growth and coalescence of retained γ dominates rather than the nucleation and growth of globular γ grains.

  • Effects of Pre-exsisting Boundaries on Microstructure Obtained by Plasma-nitriding of Fe–18%Cr Alloy

    Goro Miyamoto, Atsushi Yonemoto, Yusuke Tanaka, Tadashi Maki, Tadashi Furuhara

    pp. 1801-1805

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    DOI:10.2355/isijinternational.49.1801

    Abstract

    The present authors have studied the plasma-nitriding behavior of Fe–high Cr alloys and reported that ferrite grains are newly formed by holding the strict Σ9 coincide site lattice (CSL) relation with respect to the original ferrite grain [G. Miyamoto et al., Acta Mater. 54 (2006), 4771]. In this study, effects of pre-exsiting grain boundaries on the growth behavior of the newly formed ferrite grains in the plasma-nitriding of an Fe–18mass%Cr alloy is investigated by means of electron backscatter diffraction analysis. The growth of newly formed ferrite grains is mostly stopped at pre-existing boundaries misoriented larger than 5 degrees because of losing the Σ9 CSL relation at the boundaries. Instead, ferrite grains nucleate at such boundaries by holding the Σ9 CSL relationships with respect to the ferrite grain in front. By two-step nitriding, it is confirmed that ferrite grains can nucleate even inside of grains without a presence of grain boundaries or the free specimen surface.

  • Estimating the Effects of Microscopic Stress Concentrations on the Fatigue Endurance of Thin-walled High Strength Steel

    Shunsuke Toyoda, Yoshikazu Kawabata, Kei Sakata, Akio Sato, Naotake Yoshihara, Jun'ichi Sakai

    pp. 1806-1813

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    DOI:10.2355/isijinternational.49.1806

    Abstract

    The effects of microscopic surface stress concentrations on the fatigue endurance of thin-walled high strength steel were systematically estimated by numerical analysis and plane-bending fatigue tests with Schenck-type specimens, using the commercially available static implicit method FEA software I-DEAS ver. 11 for the stress distribution calculations. The microscopic stress concentration factor αi, from notch depth t=50 μm and notch root radius ρ=6 μm microscopic surface ridges, monotonously increased with increases in the roughness ridge direction, θ, from 1 to 7 in the bending mode. A fitted curve was developed for deriving the calculated stress concentration, αθ, from the superposition of the principal stresses. In the twisting mode, αi varied from about 4 to 7. The θ dependency of αi was smaller than that in bending mode. The empirical rule that the specimen collection direction has a lesser effect in twisting mode fatigue was supported by the αi value. It is reported that the fatigue notch factor β increased linearly with increases in the macroscopic stress concentration factor αa of up to 3. On the other hand, β slowly increased with increases in αi until it exceeded about 2. This marked difference might be due to differences between their respective stress gradients, which was well described by Nisitani and Endo by using a parameter ρ. A plane-bending fatigue test was performed with an artificial surface micro-groove of θ=0, 90° using 590 MPa class strength circumferentially flattened electric resistance welded tube. The θ=0° micro-groove had little effect on the fatigue endurance in bending. On the other hand, the fatigue cracks of all the θ=90° specimens initiated at the basilar part of the micro-groove without any nonpropagating cracks. The fatigue notch factor β seems to be determined by only αi independent of ρ in the microscopic stress concentration field.

  • Repulsive Capillary Force between two Plates Linked by a Concave Molten Slag Bridge

    Yasushi Sasaki, Hyun-Soo Kim, Manabu Iguchi

    pp. 1814-1815

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    DOI:10.2355/isijinternational.49.1814

  • Particle Size Segregation in the Falling Stream of Burden Materials

    Mriganshu Guha, Samik Nag, Subhashis Kundu, Mantu Patra, Subhash Sinha, Sanjay Kumar

    pp. 1816-1818

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    DOI:10.2355/isijinternational.49.1816

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  • Erratum: A Model for Activity Coefficient of P2O5 in BOF Slag and Phosphorus Distribution between Liquid Steel and Slag
    [ISIJ Int. 47(8): 1236-1238 (2007)]

    Somnath Basu, Ashok Kumar Lahiri, Seshadri Seetharaman

    pp. E1-E1

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    DOI:10.2355/isijinternational.49.E1

    Abstract

    Equation (10) of the article published in ISIJ International, Vol. 47, No. 8, pp. 1236.1238, (2007) was not correct.
    Error:
    [Equation1]

    Correct:
    [Equation2]

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  • © 2009 The Iron and Steel Institute of Japan

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