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

ISIJ International Vol. 50 (2010), No. 10

  • Control of MgO·Al2O3 Spinel Inclusions in Stainless Steels

    Joo Hyun Park, Hidekazu Todoroki

    pp. 1333-1346

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    DOI:10.2355/isijinternational.50.1333

    Abstract

    The latest publications regarding the development of technology to control inclusion compositions focusing on MgO·Al2O3 spinel inclusions were summarized in this review article. The problems caused by spinel inclusions, which affect practice as well as products were shown. The formation mechanism of MgO·Al2O3 spinel inclusions is secondly explained thermodynamically from the view points of chemistries of molten steels and slag compositions. Furthermore, crystallization behaviour of spinel was introduced. Countermeasures conducted in practices and laboratories were shown along with some problems still left that should be solved in the future.

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    3. Effect of Mg Addition on Equiaxed Grain Formation in Ferritic Stainless Steel ISIJ International Vol.53(2013), No.12

  • Physical Modeling Study on Combined Side and Top Blowing AOD Refining Process of Stainless Steel: Back-attack Phenomenon of Gas Streams with Horizontal Side Blowing and Its Influence on Erosion and Wear of Refractory Lining

    Ji-He Wei, Hong-Li Zhu, Qing-Yuan Jiang, Guo-Min Shi, He-Bing Chi, Hai-Jiang Wang

    pp. 1347-1356

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    DOI:10.2355/isijinternational.50.1347

    Abstract

    The back-attack phenomenon of the gas side blowing streams and its influence on the erosion and wear of the refractory lining during the combined side and top blowing AOD refining process of stainless steel were investigated on a water model unit of a 120 t AOD converter. Sufficiently full kinetic similarity between the model and its prototype was maintained. The influences of the gas flow rates for side and top blowing, the side tuyere position and number were examined. The preliminary industrial experiments were conducted. The results indicated that the presence of a gas top blowing jet and using of multiple tuyeres did not change the basic features of the back-attack, but could give it some distinctive behaviors. The back-attack could indeed bring about the evident and uneven erosion and wear of the lining. On the back-attack, the gas streams of the main tuyeres had a decisive role, and the subtuyere streams showed a certain suppression and alleviation effect. At a given tuyere number and position, its frequency and pressure and the total average pulse number in per unit time in the processes of this work increased with an increase in the gas side blowing rate. At a given tuyere position and gas side blowing rate, the back-attack and its influence on the erosion and wear of the lining enhanced with decreasing the tuyere number (increasing the gas flow rate for single tuyere). The gas top blowing jet could make the back-attack become more uniform and its frequency decrease, and its intensity and the total average pulse number increase; and it could reduce the eroded and worn rate of the lining at a given tuyere number and position and gas side blowing rate. The increased amplitude of the back-attack intensity and the extension of the damaged area of the lining caused by the buoyancy in a combined blowing were smaller and lower than those in a simple side blowing. As another important reason resulted in the back-attack and the erosion and wear of the lining, the effectiveness of the circulatory motion of the liquid in a combined blowing is different from that in a simple side blowing. At a given tuyere number, properly increasing the angle between each tuyere could be beneficial to alleviating the back-attack and to slowing down the erosion and wear of the lining. Under the conditions of this work, the back-attack actions and the lining eroded and worn extents and rates with 7 tuyeres and 22.5° or 6 tuyeres and 27° were all gentler and lower than those with the other tuyere equipments and arrangements. The results obtained from the physical modeling studies on the refining process were reliable, believable and valid. Suitably increasing the angle between each tuyere of the 120 t AOD converter could raise the life of its lining by a big margin, and remarkably improve the technical and economic indications of the process.

  • Structure of S45C Steel Solidified under the Simultaneous Imposition of a Static Magnetic Field and a Direct Current

    Kazuhiko Iwai, Takenori Kohama

    pp. 1357-1361

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    DOI:10.2355/isijinternational.50.1357

    Abstract

    S45C carbon steel has been solidified under the simultaneous imposition of a static magnetic field in vertical direction and a direct current having a horizontal component. Thus, an electromagnetic force was excited in the steel sample and it affected structure formation. Samples solidified under the different electromagnetic conditions were cut and chemically etched for the observation of the macro- and micro-structures. Solidified structure without the electromagnetic field was dendritic-structure. On the contrary, globular structure was obtained under the simultaneous imposition of the 1 T static magnetic field and the direct current of 20 A in this experimental condition. The solidified structure was also globular structure, when the magnetic field intensity decreased to 0.3 T. In the case only the 20 A direct current was applied to the sample, dendritic structure and globular structure coexisted. For the modification of the solidified structure from dendritic structure to globular structure, the simultaneous imposition of the static magnetic field and the direct current is preferable to the simultaneous imposition of the static magnetic field and the alternating current from the viewpoint of the current intensity.

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  • A Sulphide Capacity Prediction Model of CaO–SiO2–MgO–Al2O3 Ironmaking Slags Based on the Ion and Molecule Coexistence Theory

    Cheng-bin Shi, Xue-min Yang, Jin-sha Jiao, Chuang Li, Han-jie Guo

    pp. 1362-1372

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    DOI:10.2355/isijinternational.50.1362

    Abstract

    A sulphide capacity prediction model of CaO–SiO2–MgO–Al2O3 ironmaking slags has been developed based on the ion and molecule coexistence theory (IMCT) and verified by two groups of sulphide capacity data of CaO–SiO2–MgO–Al2O3 ironmaking slags by different researchers. A hot metal pretreatment slags of CaO–SiO2–MgO–Al2O3 with high binary basicity is also applied to verify the feasibility of the developed IMCT model. The predicted sulphide capacity of CaO–SiO2–MgO–Al2O3 ironmaking slags at 1773 K as well as high alumina CaO–SiO2–MgO–Al2O3 ironmaking slags in a temperature range of 1773–1873 K by the developed IMCT model has higher accuracy than the measured as well as the predicted by other sulphide capacity prediction models. The calculated equilibrium mole numbers, mass action concentrations of structural units or ion couples and optical basicity are recommended to represent slag composition for correlating with sulphide capacity of the slags compared with mass percentage of components or binary slag basicity. The developed IMCT model can calculate not only the total sulphide capacity of the slags but also the respective sulphide capacity of free CaO and MgO in the slags. Largely increasing Al2O3 content from 15 to 25% and decreasing CaO content from 40 to 34%, MgO content from 9 to 4% can improve contribution of free CaO from 97 to 99% while decreasing contribution of free MgO from 3 to about 1% to the total sulphide capacity of CaO–SiO2–MgO–Al2O3 ironmaking slags.

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    3. Sulphide Capacities of CaO–Al2O3–SiO2–MgO–MnO Slags in the Temperature Range 1673–1773K ISIJ International Vol.49(2009), No.2

  • Thermodynamic Interaction between Chromium and Titanium in Liquid Fe–Cr Alloys Containing 30 mass% Cr

    Jong-Oh Jo, Wan-Yi Kim, Chang-Oh Lee, Jong-Jin Pak

    pp. 1373-1379

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    DOI:10.2355/isijinternational.50.1373

    Abstract

    Thermodynamic interaction of chromium on titanium in liquid Fe–Cr alloys was studied by measuring the effect of chromium on the solubility product of TiN in liquid Fe–Cr–Ti–N alloys by the metal–nitride–gas equilibration technique in the temperature range from 1873 to 1973 K. Titanium nitride formed in the melt was identified as a pure solid stoichiometric TiN. As the chromium content increases in the melt saturated with TiN, the critical nitrogen solubility increases significantly, while the titanium solubility decreases. Using Wagner's formalism, the present results were thermodynamically analyzed to determine the first- and the second-order interaction parameters between chromium and titanium given as follows.
    eTiCr = 406.7/T−0.1933  rTiCr = −20.6/T−0.011  (0 < mass% Cr < 30)

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  • Multi-objective Optimization of Ironmaking in the Blast Furnace with Top Gas Recycling

    Hannu Helle, Mikko Helle, Frank Pettersson, Henrik Saxén

    pp. 1380-1387

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    DOI:10.2355/isijinternational.50.1380

    Abstract

    Concern about the growing carbon dioxide content in the atmosphere has induced increasing research activities in the search for means to suppress the emissions of CO2 in primary steelmaking. Blast furnace top gas recycling, combined with CO2 stripping, has been proposed as a promising concept. The paper presents a numerical analysis of top gas recycling under massive oxygen enrichment of the blast based on a simulation of the process chain from coal and ore to liquid steel. Because of the conflicting goals of reducing both production costs and emissions, the task is formulated as a multi-objective optimization problem. The optimal states of the system studied were found to vary significantly on the Pareto frontier, which demonstrates that fundamentally different states of operation may be selected to strongly reduce the emissions, still keeping the steelmaking economically feasible. The findings stress the importance of selecting a proper state of operation for achieving a cost-efficient production of steel with reduced environmental impact. The results also show how emissions can be “artificially” reduced by minimizing the arising emissions within the system boundary.

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  • Reaction behavior of Formed Iron Coke and Its Effect on Decreasing Thermal Reserve Zone Temperature in Blast Furnace

    Seiji Nomura, Kenichi Higuchi, Kazuya Kunitomo, Masaaki Naito

    pp. 1388-1395

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    DOI:10.2355/isijinternational.50.1388

    Abstract

    Usage of highly reactive coke in order to decrease the thermal reserve zone temperature in blast furnaces is promising to increase reaction efficiency in blast furnaces and to decrease the reducing agent rate. We focused on the catalytic effect of iron and succeeded in producing highly reactive formed iron coke with high iron content. In this paper the reaction behavior of formed iron coke when mixed with conventional coke and in the presence of alkali was investigated and the following results were obtained. When the mixture of iron coke and conventional coke is heated in a reaction gas, iron coke selectively and preferentially reacts near the thermal reserve zone temperature (900°C), which causes a decrease in the thermal reserve zone temperature, while conventional coke barely reacts and is protected from degradation. It was also confirmed that the catalytic activity of Fe and that of K is independent of each other and that in the presence of alkali, the reaction beginning temperature of iron coke is lower than that of conventional coke. These results show that the use of formed iron coke could decrease the thermal reserve zone temperature in an actual blast furnace where coke reactivity is promoted by condensed alkali vapor.

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  • Blast Furnace Pellet Textures during Reduction and Correlation to Strength

    Ulrika Leimalm, Seija Forsmo, Anna Dahlstedt, Lena Sundqvist Ökvist, Bo Björkman

    pp. 1396-1405

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    DOI:10.2355/isijinternational.50.1396

    Abstract

    The blast furnace is the most common means of producing hot metal. As the amounts of reduction agents increases, which influence in-furnace conditions such as ascending gas properties, temperature profiles and the ore-to-coke ratio, new demands are put on the iron-bearing material in terms of both reducibility and mechanical strength. To investigate the possibilities to use the Pellet Multi Press (PMP) equipment for compression strength measurements of reduced pellets and to gain a deeper understanding of the correlation between pellet texture and strength, an initial study of pellets taken from the LKAB Experimental Blast Furnace (EBF) was conducted. Furthermore, the pellet pieces generated after compression tests were characterized using light optical microscopy. In order to correlate the texture of pellet pieces to the pellet texture prior to breakage, a characterization of the chronological pellet texture development during reduction in the EBF was performed. The original pellet texture remained in the beginning of reduction and differences receded through the EBF shaft as wustite and Femet was formed. Occurrence of Femet in the pellet texture increased the compression strength, while less reduced and less sintered textures showed the reverse effect. So far, the results from compression strength tests indicate that disintegration of pellets takes place at a reaction front, at the transition between different texture types of iron oxide or at the location of a visible surface crack.

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  • Influence of Blast Furnace Inner Volume on Solid Flow and Stress Distribution by Three Dimensional Discrete Element Method

    Zhengyun Fan, Satoru Igarashi, Shungo Natsui, Shigeru Ueda, Tianjun Yang, Ryo Inoue, Tatsuro Ariyama

    pp. 1406-1412

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    DOI:10.2355/isijinternational.50.1406

    Abstract

    Since the enlargement of blast furnace inner volume is generally carried out two dimensionally, the diameter of the throat, bosh and hearth increase with the inner volume, while the height is kept approximately constant. Due to these changes, blast furnace enlargement influences burden descending behavior and stress distribution in the furnace. The present investigation intends to elucidate the influence of the inner volume of the blast furnace on the solid flow and stress distribution through a three-dimensional analysis by the discrete element method (DEM). Firstly, the enlargement of blast furnace volume increases the deadman volume in the lower part of the blast furnace in accordance with the previous study. As a result of this change in the deadman volume, the solid motion in the lower furnace is influenced by the enlargement of blast furnace inner volume. In large blast furnaces, the stress between particles near the wall at bosh level is suppressed due to the active particle movement between the wall and the deadman, and at the same time, the particles in these regions tend to cause slipping. The profile of the blast furnace, such as the bosh and shaft angle, has little influence on these phenomena. Totally, the blast furnace inner volume has an effect on the stability of solid movement through the enlarged deadman.

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  • A Unified Approach to the Fluid Dynamics of Gas–Liquid Plumes in Ladle Metallurgy

    Krishnakumar Krishnapisharody, Gordon A. Irons

    pp. 1413-1421

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    DOI:10.2355/isijinternational.50.1413

    Abstract

    The paper focuses on the fundamental aspects of the fluid mechanics of unconstrained gas–liquid plumes relevant to Ladle Metallurgy Practice. A mathematical model previously proposed by the authors is justified by comparison with Eulerian–Lagrangian models. Further, a unified analytical framework is proposed to describe the fluid dynamic and similarity characteristics of two-phase plumes. Despite the apparent complexity of the system, the analysis demonstrates that the plume cross-sectional area, void fraction and gas and liquid velocities can be quantified in terms of two parameters: a dimensionless gas flow rate and the normalized axial height. The analysis clarifies the proper form of the Froude number similarity which is important for process design and scale-up.

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  • Chemical Reaction of Glazed Refractory with Al-deoxidized and Ca-treated Molten Steel

    Jung-Hwan Son, In-Ho Jung, Sung-Mo Jung, Henri Gaye, Hae-Geon Lee

    pp. 1422-1430

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    DOI:10.2355/isijinternational.50.1422

    Abstract

    The influence of a high alumina refractory, glazed by the typical CaO–MgO–Al2O3–SiO2 ladle slag, on the generation of non-metallic inclusions in Al-deoxidized and Ca-treated molten steel was investigated at 1600°C. The variations of microstructure and chemistry of the glazed refractory were investigated with reaction time. Significant chemical reaction of the glazed refractory was observed due to the infiltration of liquid glaze into porous spinel matrix region in the refractory. Various non-metallic inclusions of the CaO–MgO–Al2O3–CaS system were generated in molten steel. The major inclusions were liquid Ca–Al–Mg–O–S inclusion and its mixtures with solid phase such as CaS, MgO and MgAl2O4. Thermodynamic analysis using the FactSage software was also carried out to understand the chemical reactions of the glazed refractory with molten steel more clearly.

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  • Thermal Deformation Analysis of the Rotating Roller in Planar Flow Casting Process

    Heping Liu, Wenzhi Chen, Yuanqing Chen, Guodong Liu

    pp. 1431-1440

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    DOI:10.2355/isijinternational.50.1431

    Abstract

    In planar flow casting (PFC) process for the manufacture of amorphous ribbon, the thermal deformation of the rotating roller is an important phenomenon that is not only for accuracy controlling the proper nozzle/substrate gap size to obtain the desired ribbon with uniform thickness and excellent quality, but also for stabilizing the manufacture and minimizing its cost. In this paper, by mapping and interpolating 2D temperature results (which are obtained by a conjugated solution of the thermal-fluid dynamics of the melt puddle zone and the heat transfer of rotating roller zone) into a 3D temperature distribution of the roller, the thermal behavior and the roller deformation features of rotating roller at a quasi-steady state and over a initial transient evolution in PFC process are analyzed. The effect of operating parameters and PFC configurations on the thermal and deformation behavior is theoretically examined. Results show that in PCF process, the plastic deformation of the roller occurs in a very thin surface layer under melt puddle, sharply decaying a lower value along the circumferential revolution. The thermal expansion is non-homogeneous on the roller surface and the location of the maximum roller expansion is beyond the coverage of the melt puddle due to the roller revolution. There is a rapid increase of the thermal expansion from the melt contact to ribbon separation. However, for the small melt puddle length, the fluctuation of the expansion along circumferential direction is very small (<10 μm) while the variation of the expansion along ribbon width direction is larger. With the evolution of the time, the thermal expansion rapidly increases at the beginning of the casting process and gradually increases, and the fluctuation of the roller surface expansion increases until a quasi-steady state when the heat balance is reached. The operating parameters and PFC configurations affect the magnitude and profile of roller thermal expansion. It is necessary to pay great attention to the effect of large roller deformation on the small nozzle/roller gap and thin ribbon thickness to avoid some qualities and operating problems in PFC process for the formation of amorphous ribbon.

  • Effect of YAG Laser Cutting on Stretch-flangeability of Ultra High Strength TRIP Sheet Steel with Bainitic Ferrite Matrix

    Akihiko Nagasaka, Yuichi Kubota, Koh-ichi Sugimoto, Atsushi Mio, Tomohiko Hojo, Koichi Makii, Masahiro Kawajiri, Mitsunari Kitayama

    pp. 1441-1446

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    DOI:10.2355/isijinternational.50.1441

    Abstract

    The effect of YAG laser cutting on stretch-flangeability of ultra high strength TRIP-aided steel sheets with bainitic ferrite matrix (TBF steel sheet) austempered at 375 or 450°C, was investigated for automotive applications. Holes of 5 mm diameter for hole-expanding test were produced by YAG laser cutting and mechanical punching and the stretch-flangeability was evaluated by measurement of the hole-expanding ratio (λ).
    In TBF steel sheet, laser cut specimens show higher stretch-flangeability than mechanically punched specimens. The hole-expanding ratio (λ) of TBF steel sheet austempered at 375°C in the case of laser cutting at powers between 50 and 100 W was higher than those austempered at 450°C. Furthermore, the strength–stretch-flangeability balance (TS×λ) of TBF steel sheet austempered at 375°C showed the highest value after laser cutting at 100 W. Compared to mechanical punching, YAG laser cutting contributed to the improvement of the TS×λ to 1100 MPa with TBF steel sheet possessing fine bainitic ferrite matrix.

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  • Effect of Scale on Coefficient of Friction in Hot Sheet Rolling of Steel

    Akira Azushima, Yoshifumi Nakata

    pp. 1447-1452

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    DOI:10.2355/isijinternational.50.1447

    Abstract

    The effect of scale on coefficient of friction in hot rolling was investigated by changing the scale thickness of Si–Mn steel, using the simulation testing machine developed by the authors. The simulation testing machine for the evaluation of the lubrication behavior consisted of a main stand, a substand, a furnace and a tension device. The rolling force and the rolling torque were measured and the coefficients of friction were calculated by changing the scale thickness, the reduction and the emulsion concentration. The scale thicknesses were obtained by the control of the flow rate of Ar gas and the heating time at 800°C in the image furnace. The experiments were carried out at constant rolling conditions of a velocity ratio of 20, a rolling speed of 50 m/min and a furnace temperature of 800°C at the rolling reductions of 0.3, 0.5 and 1.0 mm. The colza oil was used as the base oil. The emulsion concentrations were 0.1 and 3.0 mass%. At an emulsion concentration of 3 mass%, the coefficient of friction at rolling reductions of 0.3 and 0.5 mm remains constant above 60 μm, whereas below 60 μm it increases with decreasing scale thickness. The coefficient of friction at a rolling reduction of 1.0 mm remains constant above 110 μm, whereas below 110 μm it increases with decreasing scale thickness. At an emulsion concentration of 0.1 mass%, the coefficient of friction at a rolling reduction of 0.3 mm increases with increasing scale thickness above 60 μm, whereas below 60 μm it increases with decreasing with scale thickness. The coefficient of friction at a rolling reduction of 0.5 mm increases with increasing scale thickness above 110 μm, whereas below 110 μm it increases with decreasing with scale thickness. The coefficient of friction at a rolling reduction of 1.0 mm remains constant above 110 μm, whereas below 110 μm it increases with decreasing scale thickness. The increase in the coefficient of friction for the specimens with a thinner layer of scale is due to the formation of white zones in which the scale is removed from the interface between scale and steel. For these specimens, the ratio of the Fe3O4 layer of the scale surface is higher and the black layer is observed on the roll surface after rolling.

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  • Effects of Sintering Processes on Composite Tool Steels Containing Self-synthesized TiC

    J. K. Chen, H. J. Chiu, C. Liang, B. S. Yu, S. H. Chang

    pp. 1453-1459

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    DOI:10.2355/isijinternational.50.1453

    Abstract

    In present study, synthesized TiC was formed via mechanical alloying by planetary ball milling Ti and carbon black. Nano-size TiC powders in spherical shape were synthesized by milling for longer than 8 h at 400 rpm. Raman spectroscopy and TEM analyses both confirmed TiC formation. 0–44 vol% of these TiC and tool steel powders were then sintered by combining two powder metallurgy routes, vacuum sintering and hot isostatic pressing (HIP) without encapsulation. It was found that the nano-size TiC particles distributed uniformly among the tool steel powders. HIP had positive effects in both reducing internal voids and increasing flexural strengths of vacuum sintered composites. Nearly full sintering density was achieved without encapsulation. The amount of TiC added was up to 44 vol%, and the hardness of composite such formed reached as high as 71.6 HRc.

  • Evolution of Interface Microstructure and Mechanical Properties of Titanium/304 Stainless Steel Diffusion Bonded Joint Using Nb Interlayer

    Sukumar Kundu, Subrata Chatterjee

    pp. 1460-1465

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    DOI:10.2355/isijinternational.50.1460

    Abstract

    In the present investigation, the evolution of interface microstructure and mechanical properties of diffusion bonded joints of titanium to Type 304 stainless steel with a niobium interlayer were studied. The joints were processed in the temperature range of 800 to 1000°C for 0.5 h in vacuum. The stainless steel/niobium interface was free from intermetallic phase up to 900°C; however, Fe2Nb+Fe7Nb6 phase mixture was observed at and above a processing temperature of 950°C. The niobium/titanium interface was free from intermetallic compounds at all processing temperatures. A maximum tensile strength of 297 MPa (~93% of Ti) and shear strength of 217 MPa (~75% of Ti) along with a 6.9% ductility were achieved, when processed at 900°C processing temperature. The failure of bonded samples took place through the stainless steel–niobium interface at all processing temperatures during loading.

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  • Development of a Low Solution Resistance Type Solution Flow Droplet Cell and Investigation of Its Electrochemical Performance

    Masatoshi Sakairi, Takuya Murata, Tatsuya Kikuchi, Koji Fushimi

    pp. 1466-1470

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    DOI:10.2355/isijinternational.50.1466

    Abstract

    A solution resistance reduced solution flow type micro-droplet cell with co-axial dual capillary tubes was developed. A Pt counter electrode wire, 50 μm diameter, inserted in the inner capillary tube, successfully reduced solution resistance between the working and counter electrodes. The potentio-dynamic polarization measurements showed the electrochemical performance of developed droplet cell to be very similar to that of traditional macro sized electrochemical cells. The developed droplet cell was applied to obtain line profiles of a current at a constant potential across the surface of for cross-sections of model metal interfaces. The current changed at the model interface between deposited nickel and substrate, and the spatial resolution of the developed droplet cell was reduced with increasing scanning speed of the cell. The results reported in this paper suggest that this technique would be able to measure differences in corrosion resistance in welded metal components and base metals.

  • Ultra Grain Refinement in a Plain Carbon Steel by Means of Warm Torsion Testing

    Welbert Ribeiro Calado, Ronaldo Antônio Neves Marques Barbosa

    pp. 1471-1475

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    DOI:10.2355/isijinternational.50.1471

    Abstract

    Ultra grain refinement of a plain carbon SAE-AISI 1010 steel has been obtained by means of warm torsion testing. Tests were conducted at 500°C, constant strain rate of 0.1 s−1 and total equivalent strain of 0.8, 1.6, 4.0 and 5.6. The average ferritic grain size obtained was smaller than 0.5 μm. The steel microstructure consisted of elongated ferrite grains and fragmented and spheroidized cementite particles, similar to the microstructures found in carbon steel severely plastic deformed, like in the ARB and ECAP processes. The average ferritic grain size is reasonably predicted by the Hall–Petch equation. The mechanisms by which ultra grain refinement has been achieved here has been attributed to dynamic recovery and continuous dynamic recrystallization of the ferrite during warm plastic deformation. Warm torsion testing proved to be a suitable tool to study grain refining mechanism through severe plastic deformation.

  • Effects of Si and Cr on Bainite Microstructure of Medium Carbon Steels

    Takeshi Suzuki, Yoshiki Ono, Goro Miyamoto, Tadashi Furuhara

    pp. 1476-1482

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    DOI:10.2355/isijinternational.50.1476

    Abstract

    Effects of Si and Cr contents on bainite microstructure of medium carbon steels have been examined in order to obtain important information for good strength–ductility balanced spring steel. Four practical medium carbon steels, JIS-S55C, SUP9, SUP7 and SUP12 were isothermally transformed at temperatures between 300°C and 500°C after austenitized at 1000°C. The microstructure was observed by means of scanning electron microscopy and transmission electron microscopy. While carbide was formed from the early stage of bainite transformation in S55C without Si and Cr, carbide precipitation was suppressed by the increase of Si particularly in SUP7 and SUP12. The fraction of retained austenite was increased with increasing of Si and Cr at the intermediate stage of bainite transformation because of the increase of C concentration in retained austenite. Particularly, Si addition promoted carbide-free bainitic ferrite and leading to larger amounts of retained austenite by carbon enrichment.

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  • Effect of Melting Process and Aluminium Content on the Microstructure and Mechanical Properties of Fe–Al Alloys

    Shivkumar Khaple, R. G. Baligidad, M. Sankar, V. V. Satya Prasad

    pp. 1483-1487

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    DOI:10.2355/isijinternational.50.1483

    Abstract

    This paper presents the effect of air induction melting with flux cover (AIMFC) versus vacuum induction melting (VIM) on the recovery of alloying element, reduction of impurities, workability and mechanical properties of Fe–(7–16mass%)Al alloys. Three Fe–Al alloy ingots containing 7, 9 and 16 mass% Al were prepared by both AIMFC and VIM. All these ingots were hot-forged and hot-rolled at 1373 K and were further characterized with respect to chemical composition, microstructure and mechanical properties. The recovery of aluminium as well as reduction of oxygen during both AIMFC and VIM is excellent. AIMFC ingots exhibit low level of sulphur and high concentration of hydrogen as compared to VIM ingots. VIM ingots of all the three alloys were successfully hot worked. However, AIMFC ingots of only those Fe–Al alloys containing lower concentration of aluminium could be hot worked. The tensile properties of hot-rolled Fe–7mass%Al alloy produced by AIMFC and VIM are comparable. The present study clearly demonstrates that it is feasible to produce sound ingots of low carbon Fe–7mass%Al alloy by AIMFC process with properties comparable to the alloy produced by VIM.

  • Tensile Properties of Chromium-bearing Extra Low Carbon Steel Sheets

    Yoshimasa Funakawa, Takumi Ujiro

    pp. 1488-1495

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    DOI:10.2355/isijinternational.50.1488

    Abstract

    The influence of chromium on tensile properties of cold-rolled and annealed extra low carbon steel sheets was investigated. Tensile strength increased with an increase in chromium content whereas elongation decreased. Yield strength showed the minimum at chromium content of 5%. The yield strength in each steel changed with ferrite grain diameter according to Hall–Petch relationship. The coefficient in Hall–Petch equation linearly decreased with an increase in chromium content. Since ferrite region hot-rolling and generation of α′ phase did not affect the coefficient, the change can be attributed to reduction of solute carbon at grain boundaries by chromium carbide precipitation. The slope of the decrease in the coefficient per 1 mass% chromium was 0.02 MPa·m1/2. The friction term in Hall–Petch equation showed the minimum at chromium content of 5% like the yield strength. The change in the friction term is the reason why 5% Cr steel inhibited the minimum yield strength. Aging at 150°C made the friction terms of low chromium steels up and friction term became to show linear relationship for chromium content. This result indicated that the lowest yield strength which 5% Cr steel showed is attributed to the retardation of aging by chromium during cooling after the annealing. It is concluded that solute chromium exhibited solid-solution strengthening. The increase in solid-solution strengthening of yield strength is about 5.6 MPa per 1 mass% chromium.

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  • Absorption of Hydrogen in High-strength Low-alloy Steel during Tensile Deformation in Gaseous Hydrogen

    Koichi Takasawa, Ryoji Ishigaki, Yoru Wada, Rinzo Kayano

    pp. 1496-1502

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    DOI:10.2355/isijinternational.50.1496

    Abstract

    Absorption of hydrogen in a high-strength nickel–chromium–molybdenum steel during tensile deformation in 0.5 MPa gaseous hydrogen was examined using a thermal desorption analysis method. The tensile strength of the specimen was varied in the range from 1214 to 947 MPa by heat treatment. The dislocation density of the specimens was measured by X-ray diffractometry after tensile testing in a hydrogen atmosphere. The hydrogen content absorbed during tensile deformation increased with increasing tensile strain in proportional elastic range until just before yielding. The yield stress was defined as 0.2% proof stress in this work. At the same tensile strains, the hydrogen content of lower-strength specimens was larger than that of higher-strength specimens. The dislocation density gradually decreased until just before yielding, corresponding to the proportional increase of hydrogen content to the tensile strain. This implies that the hydrogen absorption behavior during tensile deformation in gaseous hydrogen is related to the motion of mobile dislocations initially contained in the specimens. The activation energy for desorption of hydrogen absorbed during tensile deformation did not depend on the strength of the steel. This indicates that the trap sites of hydrogen atoms created through the tensile deformation were the same regardless of the strength levels.

  • Prediction Model of Global Demand for Iron Source by Utility of Stock Hypothesis—Verification of Predictive Power and Outlook for Demand to 2050—

    Sumio Kozawa, Fumitaka Tsukihashi

    pp. 1503-1510

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    DOI:10.2355/isijinternational.50.1503

    Abstract

    The Utility of Stock hypothesis, which assumes that an in-use stock of constructional material is a function of GDP, was formulated and a clear correlation between the world steel stock and the world GDP led to the estimation that the world demand for iron ore (primary iron) depends not on the volume of GDP but on the variation of GDP, as already reported. It also became clear that the flow of primary iron has the controlling effect on the world production of crude steel. In this study, the predictive power of the Utility of Stock hypothesis is verified. Based on the verification, the global demand for iron source until 2050 is projected by deciding a world 1-region model.

  • Effect of Test Condition on Decrepitation Index and Test Repeatability for Lump Iron Ore

    Keiichi Aotsuka, Takazo Kawaguchi, Yoshikazu Nimiya

    pp. 1511-1513

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    DOI:10.2355/isijinternational.50.1511

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  • Complex Permittivity and Permeability of α-Fe2O3 and Fe1−xO Powders in the Microwave Frequency Range between 0.2 and 13.5 GHz

    Masahiro Hotta, Miyuki Hayashi, Kazuhiro Nagata

    pp. 1514-1516

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    DOI:10.2355/isijinternational.50.1514

  • © 2010 The Iron and Steel Institute of Japan

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