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

ISIJ International Vol. 53 (2013), No. 8

  • Absorption/Desorption of Diffusible Hydrogen in Aluminized Boron Steel

    Cedric Georges, Thierry Sturel, Pascal Drillet, Jean-Michel Mataigne

    pp. 1295-1304

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    DOI:10.2355/isijinternational.53.1295

    Abstract

    Currently, boron steel with Al–Si coating experiences a rapid growth in the anti-intrusion applications in the car body due to its superior mechanical properties after hot stamping. However, the final microstructure can be sensitive to delayed fracture if the product is exposed to a critical combination of diffusible hydrogen content, stresses and other metallurgical factors. As the metallurgical parameters and stresses are usually defined, the proper control of the diffusible hydrogen content is the key parameter to improve the safety aspect of the product. However, this content is quite difficult to determine. In this paper, the parameters governing the absorption and desorption of diffusible hydrogen in aluminized boron steels is investigated. The present research shows that the dew point and the austenitizing holding time have a bigger influence on the diffusible hydrogen content than the austenitizing temperature. Simultaneously, four-point bending test, which is simple and representative of the stress field that may be encountered in car bodies, is used to determine the acceptable limit of the diffusible hydrogen amount. Using this test, a delayed fracture map is proposed, which can be used as a guideline to determine the safe process areas. The study reveals that fast cooling rates or the sheared edges lead to lower the critical diffusible hydrogen content. Nevertheless, under the standard industrial operating conditions, the materials remain safe. Finally, an e-coating process that is applied to the sample surface induces an efficient degassing that provides an additional safety margin.

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    3. Simulation of the Estimation of the Maximum Inclusion Size from 2-Dimensional Observation Data on the Basis of the Extreme Value of Statistics ISIJ International Vol.49(2009), No.7

  • Mechanical Anisotropy in Steels for Pipelines

    Min Sung Joo, Dong Woo Suh, Harshad Kumar Dharamshi Hansraj Bhadeshia

    pp. 1305-1314

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    DOI:10.2355/isijinternational.53.1305

    Abstract

    There has been a huge expansion in the laying of pipelines for the transmission of fossil fuels over large distances and in dire environments. Large diameter pipes can be manufactured by welding spirals of hot–rolled linepipe steels. This process has a cost advantage relative to one in which the steel is seam welded after bending into a tubular shape. However, one particular problem associated with the steels used to fabricate the pipes is that of the anisotropy of mechanical properties, especially the toughness. Even though properties such as the Charpy toughness and strength meet minimum specifications, the existence of orientation dependence can compromise, for example, the stability of the pipe to buckling. There is, therefore, a large international activity on understanding the anisotropy of pipeline steels. This review represents an attempt to critically assess the steels and the orientation dependence of their mechanical properties, with the aim of establishing a basis for further progress.

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  • Effects of Carbon and Oxygen on Fe–C–O Melt Surface Tension

    Keisuke Morohoshi, Masahito Uchikoshi, Minoru Isshiki, Hiroyuki Fukuyama

    pp. 1315-1319

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    DOI:10.2355/isijinternational.53.1315

    Abstract

    For this study, we measured surface tension of Fe–C and Fe–C–O melts accurately under controlled carbon and oxygen activities using an oscillating droplet method with an electromagnetic levitator (EML). The results are summarized as follows. The carbon activity has no influence on the surface tension of Fe–C melt at temperatures of 1823–2023 K under oxygen partial pressure of 10–10 Pa. For Fe–C–O melts, the carbon activity has no influence on the surface tension at a constant oxygen partial pressure of 10–7 Pa and temperatures of 1873–1973 K. It is noteworthy that oxygen activity is reduced by carbon in the melt because of the negative interaction between oxygen and carbon. Considering the interaction, the surface tension of the Fe–C–O melts was formulated as a function of carbon and oxygen concentrations and temperature.

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  • Effect of Fe Addition on the Activity Coefficient of Si in Cu–Fe–Si Melt at 1623 K

    Yuichi Kato, Takeshi Yoshikawa, Yutaka Kagawa, Kazuki Morita

    pp. 1320-1324

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    DOI:10.2355/isijinternational.53.1320

    Abstract

    The activity coefficient of Si in Cu–Si alloy and Cu–Si–Fe alloys was determined using three principles at 1623 K. Cu–Si(–Fe) alloys at a low Si concentration were equilibrated with SiO2 in a graphite crucible under a controlled CO partial pressure. Cu–Si(–Fe) alloys at a medium Si concentration were equilibrated with SiC in a graphite crucible. And finally, Cu–Si(–Fe) alloys at a high Si concentration were equilibrated in a Si3N4 crucible under a controlled partial pressure of N2. The activity coefficients of the alloys were calculated based on the equilibrium Si concentration. The results show that the activity coefficient of Si in Cu–Si–Fe alloy decreases with an increase in the concentration of Fe. The results were evaluated in terms of the interaction parameters in molten Cu for Wagner’s formalism, and the interaction coefficient in the concentrated solution.The evaluated thermodynamic properties of Cu–Si–Fe alloy indicated that the addition of Fe to Cu–Si alloy is effective in decreasing the activity of Si in the alloy.

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    3. Phase Relations in Ce–Al–Fe–S Based Grain Refiners for Steels ISIJ International Vol.49(2009), No.7

  • Thermodynamic Assessment of MnO and FeO Activities in FeO–MnO–MgO–P2O5–SiO2(–CaO) Molten Slag

    Sun-Joong Kim, Jun Takekawa, Hiroyuki Shibata, Shin-ya Kitamura, Katsunori Yamaguchi, Youn-Bae Kang

    pp. 1325-1333

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    DOI:10.2355/isijinternational.53.1325

    Abstract

    The activity coefficients of MnO and FeO in an FeO–MnO–MgO–P2O5–SiO2(–CaO) slag system were measured on the basis of the equilibrium between Ag and molten slag at 1673 K under a controlled atmosphere. On the basis of experimental results, the activity coefficients of MnO and FeO in this multicomponent slag were evaluated using a regular solution (RS) model, FactSage, and an empirical formula. In the case of the RS model, the interaction energies and conversion factors to fit the calculated values to experimental results were reassessed. By the used of the empirical formula, FactSage and the regular solution model of present work, the activity coefficient ratio of MnS and FeS in a Fe–Mn–O–S matte that was equilibrated with a FeO–MnO–MgO–P2O5–SiO2 slag system was evaluated. When the RS model was used to calculate the γMnO/γFeO ratio, the γMnS/γFeS ratio decreased slightly with an increase in the XMnS/XFeS ratio. In contrast, when the empirical formula and FactSage were used, the γMnS/γFeS ratio was almost constant when the XMnS/XFeS ratio was increased.

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    3. Phase Relations in Ce–Al–Fe–S Based Grain Refiners for Steels ISIJ International Vol.49(2009), No.7

  • In situ X-ray Diffraction Investigation of the Formation Mechanisms of Silico-Ferrite of Calcium and Aluminium-I-type (SFCA-I-type) Complex Calcium Ferrites

    Nathan A. S. Webster, Mark I. Pownceby, Ian C. Madsen

    pp. 1334-1340

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    DOI:10.2355/isijinternational.53.1334

    Abstract

    The formation mechanisms of the complex Ca-rich ferrite phase SFCA-I, an important bonding material in iron ore sinter, during heating of synthetic sinter mixtures in the temperature range 298–1623 K in air and at pO2 = 5 × 10–3 atm, were determined using in situ X-ray powder diffraction. In air, the initial formation of SFCA-I at ~1438 K (depending on composition) was associated with reaction of precursor phases Fe2O3, CaO·Fe2O3, SiO2, amorphous Al-oxide and a CFA phase of approximate composition 71.7 mass% Fe2O3, 12.9 mass% CaO, 0.3 mass% SiO2 and 15.1 mass% Al2O3. At temperatures above ~1453 K, the decomposition of another phase, γ-CFF, resulted in the formation of additional SFCA-I. At lower oxygen partial pressure the initial formation of SFCA-I occurred at similar temperatures and was associated with reaction between similar phases as its formation in air. However, the decomposition of γ-CFF did not result in the formation of additional SFCA-I, with the maximum SFCA-I concentration (25 mass%) lower than the values attained in air (54 and 34 mass%). Hence, more oxidising conditions appear to favour the formation of the desirable SFCA-I phase.

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  • Interaction between Iron Oxides and Olivine in Magnetite Pellets during Reduction to Femet at Temperatures of 1000–1300°C

    Pär Semberg, Charlotte Andersson, Bo Björkman

    pp. 1341-1349

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    DOI:10.2355/isijinternational.53.1341

    Abstract

    The addition of MgO to iron ore pellets is known to beneficially influences many high temperature reduction properties such as reducibility and swelling. When the pellet is metallized, MgO dissolved in the wustite concentrates in the unmetallized part, which is why MgO-levels much higher than the average concentration could be expected locally. In this work the impact of the elevated MgO-content on the reduction at 1000–1300°C was studied by SEM-EDS. The MgO content in the pellet was also varied by additions of a), highly reactive olivine b) unreactive olivine c) combined addition of reactive olivine and fine quartzite and d) combined addition of unreactive olivine and fine quartzite. Two cases of metallization were observed 1) a gradual reduction front with only moderate magnesium levels and 2) a sharp reduction front with strongly elevated magnesium levels before the metal front. The samples with added quartzite reduced a little better at 1100°C, compared to those with only olivine, but apart from that, reduction was not affected much by the additives in the range 1000–1200°C. The greatest difference in reduction degree appeared at 1300°C where a metal skin formed in most samples, hindering further reduction. At this temperature, the sample with addition of only reactive olivine had superior reducibility due to a porous morphology of the iron being mantained throughout the experiment.

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  • Kinetics of Reduction Step of Wustite to Iron of Hematite and Quaternary Calcium Ferrite Mixtures

    Daisuke Noguchi, Ko-ichiro Ohno, Takayuki Maeda, Kouki Nishioka, Masakata Shimizu

    pp. 1350-1357

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    DOI:10.2355/isijinternational.53.1350

    Abstract

    As a fundamental study for clarifying the reduction phenomena of sintered ore in a blast furnace, mixtures of iron oxide and quaternary calcium ferrite (Cf) were prepared and its kinetic behavior at the final stage of reduction with CO gas was studied. Reduction rate increased with increasing reduction temperature regardless of mineral ratio. Influence of mineral ratio were small and reduction rate were similar in every sample at 1000°C and above. While at 900°C and below, reduction rate increased with increaseng amount of Cf. The reason thought to be due to that the dense iron layer on surface of iron oxide particle inhibits the reduction at inner phase. Reduction reaction proceeded topochemically at higher temperature. On the other hand, reduction reaction did not proceed topochemically at lower temperature. Besides reduction reaction of rich Cf samples proceeded topochemically. Reduction data were analyzed based on the two interface unreacted core model, effective diffusion coefficient in outer layer and inner layer were determined. Reduction curves calculated by using the rate parameters obtained by the analysis agreed with observed data very well at 1000°C and above. Therefore reduction rate of two minerals mixture sample can be analyzed based on the two interface unreacted core model above 1000°C.

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  • Experimental Investigation on Direct Reduction Followed by Magnetic Separation for Nb2O5-Bearing Ore

    Xin Jiang, Fengman Shen, Ligang Liu, Xiaogang Li, Lin Wang

    pp. 1358-1364

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    DOI:10.2355/isijinternational.53.1358

    Abstract

    Recently a special attention is being paid on the ferroniobium production worldwide, especially in China. In present work, direct reduction followed by magnetic separation for Nb2O5-bearing ore is investigated. Reflected light microscope, scanning electron microscope with EDX, and high performance X-ray diffraction were used for fundamental analysis. The experimental results show that, (1) the optimum reduction parameters are 0.9 of C/O, 1200°C and 20 min. At these optimum parameters, the degree of metallization of ore-coal composite pellet is more than 90%. (2) In direct reduced pellets, metallic iron and slag have gathered respectively, and they can be magnetically separated. Nb and Ti are associated together and inserted in slag, and stay in slag phase after magnetic separation. (3) Based on the process of direct reduction followed by magnetic separation, 7.71% of Nb2O5-enriched slag is obtained, which is a good feed for producing ferroniobium or metallic niobium in electric furnace. The Nb2O5 content in Nb2O5-enriched slag is 1.8 times of the original ore, and the recovery of Nb is about 85%. These experimental results can give some theoretical references for industrial application in future.

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  • Validation and Calibration Approach for Discrete Element Simulation of Burden Charging in Pre-reduction Shaft Furnace of COREX Process

    Qiang Li, Mingxia Feng, Zongshu Zou

    pp. 1365-1371

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    DOI:10.2355/isijinternational.53.1365

    Abstract

    Discrete element method (DEM) has been an increasingly used tool to get better understanding of charging process and flow behaviors of granular materials in metallurgical reactors. However, validation and precision of DEM must be verified and calibrated. In this paper, a calibration approach is proposed for the sphere equivalence of irregular particles in DEM simulation of charging process. In this approach, the non-sphere behavior of irregular particles is characterized by a pair of apparent sliding and rolling resistance coefficients obtained by quantitative comparison of the angle of repose and discharging time of hopper based on laboratory measurement of physical benchmarking experiments. The calibration approach is applied in the DEM simulation of the charging process of a shaft furnace in COREX 3000. Validation of simulation results for flow trajectory and stream width after leaving chute and burden distribution and profile is investigated through comparison of DEM and experiments. The results show that, with such a calibration approach, DEM can be easily used to simulate solid flow of irregular particles.

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  • Enhancement of Iron Ore Sinter Reducibility through Coke Oven Gas Injection into the Modern Blast Furnace

    Elsayed Abdelhady Mousa, Alexander Babich, Dieter Senk

    pp. 1372-1380

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    DOI:10.2355/isijinternational.53.1372

    Abstract

    Energy network within the integrated steel works should be used more efficiency to reduce the energy consumptions and CO2 emissions. The injection of free resources of coke oven gas (COG), which is rich with hydrogen, into the modern blast furnace is one of such measures. In order to clarify the effect of COG injection on the reduction processes in the blast furnace; iron ore sinter was isothermally and non-isothermally reduced with different gas compositions at different temperature. The gas compositions were selected to simulate the conditions of middle (150 m3/tHM) and intensive (300 m3/tHM) injection of COG into the blast furnace. The results were compared to that obtained under typical blast furnace conditions without COG injection. The isothermal reduction at 900–1200°C indicated the enhancement of the reduction rate as COG injection increased. The non-isothermal reduction indicated the efficiency of intensive injection of COG in decreasing the direct reduction from 50% to only 5% at 1200°C. Reflected light microscopy, scanning electron microscopy and X-ray techniques were used to characterize the microstructure and the developed phases in the origin and reduced sinter. The rate controlling mechanism of sinter under different conditions was predicted from the correlation between apparent activation energy calculations and microstructure examination.

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  • Phase Equilibrium for the CaO–SiO2–FeO–5mass%P2O5–5mass%Al2O3 System for Dephosphorization of Hot Metal Pretreatment

    Xu Gao, Hiroyuki Matsuura, Masaki Miyata, Fumitaka Tsukihashi

    pp. 1381-1385

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    DOI:10.2355/isijinternational.53.1381

    Abstract

    Recently, after the restriction of the use of CaF2, dephosphorization process often generates large amount of slag, due to the neglect of refining functions of solid phases. Consequently, this brings environmental issues and influences refining. In order to improve the utilization efficiency of solid CaO and its compounds in the dephosphorization slag, multiphase flux refining has been proposed by considering the enrichment of phosphorus within the solid phases. As to provide theoretical fundamentals for both understanding on the reaction mechanism of phosphorus and practical slag control, phase relationship for the CaO–SiO2–FeO–5mass%P2O5–5mass%Al2O3 system has been studied based on chemical equilibration technique with oxygen partial pressure of 10−10 atm at 1673 K. In current work, the liquidus saturated with P2O5-rich solid solution has been firstly deduced on the CaO–SiO2–FeO ternary system, and the discussions on the relationship between solid solution and liquid phase has been proceeded. It has been found that the existence of Al2O3 enlarges the liquid phase area, but does not affect the composition of solid solution. On the other hand, the equilibrium solid phase has been confirmed as 2CaO·SiO2–3CaO·P2O5 solid solution, while the ratio between both varies along with liquidus. As expected, the large equilibrium partition ratio of phosphorus between solid solution and liquid slag has also been found and discussed.

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  • Characterization of Microstructure and Non-metallic Inclusions in Zr–Al Deoxidized Low Carbon Steel

    Min Jiang, Zhiyong Hu, Xinhua Wang, Jong-Jin Pak

    pp. 1386-1391

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    DOI:10.2355/isijinternational.53.1386

    Abstract

    Laboratory experiments were carried out to investigate non-metallic inclusions and microstructures in Zr–Al deoxidized low carbon steel. Contents of [Zr] in the steel samples were of different levels while [Al] varied in the range of 0.0012–0.0032%. It was found that inclusions and the developed microstructures varied greatly with the change of [Zr] in steel. In the high [Zr] (0.072%) steel sample with [S] about 0.012%, the produced ZrO2 inclusions in deoxidization favored the uniform precipitation of MnS and all inclusions were composed of dual phases of ZrO2+MnS. Microstructures consisted of pearlites and ferrites after water quenching directly from liquid to solid. Ferrites developed by ZrO2+MnS inclusions were not observed. With [Zr] at a medium level about of 0.0085% and [S] at about 0.012%, complex (ZrO2–TiOx)–(Al2O3–SiO2–MnO–(MnS)) inclusions were formed. The microstructures of steel were characterized by fine and interlocked intra-granular ferrites (IAF). SEM observation indicated that those complex inclusions were good nucleation sites for IAF. In the [Zr] steel sample with low [Zr] of about 0.0008% and [S] of about 0.0017%, very typical IAF was also induced by the complex (MgO–Al2O3–SiO2–MnO) inclusions of very high number density.

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  • Fracture Mechanics Investigation of MgO–C Bricks for Steelmaking by Bending and Fatigue Failure Tests Along with X-Ray CT Scan Observation

    Yuta Hino, Keisuke Yoshida, Yoshisato Kiyota, Michihiro Kuwayama

    pp. 1392-1400

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    DOI:10.2355/isijinternational.53.1392

    Abstract

    Fatigue failure tests were firstly carried out. The material constant derived from S-N curve was evaluated and compared with previous study. The compliance method was subsequently used to evaluate the fracture toughness of MgO–C brick and the relationship between the stress factor and crack growth rate. In order to use this method, the effects of brick carbon content on the crack growth rate and crack growth rate in high temperature were investigated.As a result, it was found that the crack growth rate increased when the carbon content in brick decreased. This result was confirmed by X-ray CT scans, which revealed large cracks in bricks with a lower carbon content, even in the middle of fatigue failure tests.Furthermore, the material constants obtained from fatigue failure test and K-V diagram were compared. A material constant was derived by evaluating the relationship between the crack growth rate and stress intensity factor, and the result was found to agree with the value derived from fatigue failure tests. This result confirms that that material constant derived from fatigue failure tests is the inherent property of the material and corresponds to the variation of crack growth rate with changes in stress factor. The effects of carbon content in MgO–C brick on the crack growth behaviour and fatigue failure mechanism were also investigated.

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  • Characteristics of Inclusions in Low Carbon Al-Killed Steel during Ladle Furnace Refining and Calcium Treatment

    Wen Yang, Lifeng Zhang, Xinhua Wang, Ying Ren, Xuefeng Liu, Qinglin Shan

    pp. 1401-1410

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    DOI:10.2355/isijinternational.53.1401

    Abstract

    A plant trial of the productions of LCAK steel was performed, and characteristics of inclusions during LF refining and calcium treatment were investigated. Besides, thermodynamic diagram among magnesium, aluminum, and oxygen as well as calcium, magnesium, aluminum, and oxygen in the steel melt were studied to understand the fundamentals of inclusion modification by calcium treatment in LCAK steel. Furthermore, the change mechanisms of oxide inclusions and the precipitation of calcium sulfide were discussed. The experimental results showed that oxide inclusions were partly changed along with the path of Al2O3→MgO·Al2O3→(MgO·)CaO·Al2O3, which was a little different from the thermodynamic calculation results due to the limited kinetic conditions. Calcium treatment somehow modified the inclusions, however, many large inclusions were generated, and the modification effect would be dramatically decreased by the formation of large amount of CaS inclusions, which appeared with three main distribution forms. The formation of inclusions after calcium treatment were discussed based on the thermodynamic analysis. In order to reach the target of modification, sulfur concertration in steel should be reduced to a small amount to decrease the formation of CaS.

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  • Aggregation Behavior of Desulfurization Flux in Hot Metal Desulfurization with Mechanical Stirring

    Yoshie Nakai, Ikuhiro Sumi, Naoki Kikuchi, Yasuo Kishimoto, Yuji Miki

    pp. 1411-1419

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    DOI:10.2355/isijinternational.53.1411

    Abstract

    The aggregation behavior of desulfurization flux in hot metal desulfurization with mechanical stirring was investigated by small-scale hot metal experiments. Desulfurization flux added from the top aggregated in the hot metal during stirring, reducing the interfacial area between the flux and hot metal, which had a large effect on the desulfurization rate. The size of aggregated slag can be estimated using an aggregation model based on granulation theory. The obtained aggregation rate constant, ka, is 4.5–40.9×10–11 (m3/s). The interfacial area between flux and hot metal can be estimated using the aggregation model, which can also estimate desulfurization behaviors under various conditions. The obtained slag size after desulfurization agreed with the calculated slag size in the equilibrium state, which is highly dependent on stirring conditions. The aggregation rate in commercial-scale (270 ton) desulfurization is larger than that in small-scale experiments due to the difference of their stirring energies.

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  • Finite-Element Analysis of Cylindrical-Void Closure by Flat-Die Forging

    Jong-Jin Park

    pp. 1420-1426

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    DOI:10.2355/isijinternational.53.1420

    Abstract

    Voids in a bloom produced by continuous casting are cylindrical in shape and located along the longitudinal direction. In the present study, the closure phenomenon of these voids by flat-die forging was investigated by the rigid-plastic finite-element analysis and experiments using plasticine specimens. The void closure was found to progress through contraction followed by collapse, and to be completed as the effective strain at the location of the void reached a certain value. The value was found to be dependent on the aspect ratio of the cross section of a void, but not on the size of the void. The relationship between the effective strain and the aspect ratio was established by which the closure of such a void with any aspect ratio can be predicted in terms of the effective strain in uniaxial forging. Compared to biaxial forging, uniaxial forging was found to be far more effective for the void closure in a bloom.

  • Experimental and Numerical Study of Twin Circular Water Jets Impinging on a Moving Thin Film

    Hitoshi Fujimoto, Yasuyuki Lee, Ryosuke Kato, Takayuki Hama, Hirohiko Takuda

    pp. 1427-1435

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    DOI:10.2355/isijinternational.53.1427

    Abstract

    In this study, we investigated the flow characteristics of two circular water jets impinging on a moving surface covered with a water film as fundamental research on strip cooling. Experiments and numerical simulations were conducted under non-heated surface conditions. The experiments were recorded on video. The jet velocity, nozzle-to-plate distance, nozzle-to-nozzle spacing, and flow rates of the water film were varied systematically. Depending on the flow conditions, three types of flows were found to exist between the two modes: stable, unstable, and transient. We propose a simple theoretical model for predicting the critical boundary at which the flow is in the “stable mode.” In the numerical simulation, the Navier–Stokes equation system for a three-dimensional incompressible unstable viscous fluid was solved using a finite difference method. The effects of viscosity, gravity, and presence of a free liquid surface with surface tension were considered. The flow characteristics of the “unstable mode” are discussed in detail to offer a better understanding of its physics.

  • A New Model for the Prediction of Evolution of the Residual Stresses in Tension Leveling

    Yongseok Cho, Hosung Ye, Sangmoo Hwang

    pp. 1436-1442

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    DOI:10.2355/isijinternational.53.1436

    Abstract

    The shape defects such as edge waves and center buckles may be formed in the rolled strip, since rolling can easily produce non-homogenous elongation across the strip width. The main purpose of tension leveling is to cure such defects by eliminating the differences in elongation and thus eliminating the residual stresses. In this paper, a new model is presented for the prediction of the evolution of the residual stress distribution in the strip undergoing tension leveling. The model consists of an analytic model for the prediction of the strip curvature at each roll, the residual stress distribution along the width of the strip, and the roll force at each roll. The prediction accuracy of the proposed model is examined through comparison with the predictions from a finite element model.

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  • Design and Strength Evaluation of Critical Gusset Plates in the Steel Bridge Using New Load and Resistance Factor Design Methods and Advanced FE Analyses

    Jong Wan Hu

    pp. 1443-1452

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    DOI:10.2355/isijinternational.53.1443

    Abstract

    The gusset plates typically employed for connecting steel members to each other in the lateral load resisting systems are always an important part of truss bridges and braced frames. We can recognize that when considering the tragic collapse of the I-35W Minnesota highway bridge on August 1, 2007, the failure of gusset plates in particular leaded to destroy the whole structure. Many researchers have been increasingly interested in the adequate design of such critical panel points as motivated by this accident. In order to follow up on this research trend, this study is intended to examine design strength models on the basis of feasible failure patterns for the gusset plates, thereby calculating their strength capacities. In addition, the inelastic behavior of existing gusset plates is investigated through a series of detailed finite element (FE) analyses, and then strength models specified in the current design specifications are evaluated based on the FE analysis results. According to individual failure modes, safety and load rating factors deliberately applied to gusset plate design are finally assured by observing the distribution of plastic stresses on the gusset plate.

  • Effect of Stress on Variant Selection in Lath Martensite in Low-carbon Steel

    Yamato Mishiro, Shoichi Nambu, Junya Inoue, Toshihiko Koseki

    pp. 1453-1461

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    DOI:10.2355/isijinternational.53.1453

    Abstract

    The effect of stress on the variant selection in lath martensite in a low-carbon steel (Fe–0.18%C–0.89%Mn–2.88%Ni–1.51%Cr–0.40%Mo) was investigated using electron backscatter diffraction pattern (EBSP) analysis. The steel was continuously cooled from a fully austenitic temperature to room temperature under uniaxial compressive stress applied during the martensitic transformation. It was demonstrated that certain variants maintaining the Kurdjumov-Sachs (K-S) orientation relationship with the prior austenite were preferentially selected under the applied stress only in blocks larger than the average block size. Otherwise, no clear variant selection was found. The applied stress and the external work done during the martensitic transformation, which was evaluated from the transformation strain, showed that the variants with greater external work values were more likely to be selected. However, both the shift in the martensite start temperature and the selected variants indicate that only the invariant line transformation strain was effective for variant selection in lath martensite in the low-carbon steel, unlike in nickel steels where the lattice-invariant shear has been additionally included in the literature.

  • Systematic Characterization of the Crashworthiness Properties of Low Alloys Silicon Bearing TRIP Steels

    Wael Dabboussi, James Alan Nemes

    pp. 1462-1470

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    DOI:10.2355/isijinternational.53.1462

    Abstract

    Metallurgical and mechanical properties of two low alloy, TRIP assisted, multi-phase steels are investigated. Tension and compression experiments are performed over a range of strain rates and temperatures to determine the kinetics of the austenite to martensite transformation. The volume fraction of retained austenite is measured using neutron diffraction which provides measurements of high accuracy. Results are used to assess the energy absorption characteristics of the steels for use in crashworthiness evaluations.

  • Abrasive Wear Behaviour of Heat Treated En31 Steel

    Abin Baburaj, K. B. Shishupal Singh Chaudhary, Rajesh K. Khatirkar, Sanjay G. Sapate

    pp. 1471-1478

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    DOI:10.2355/isijinternational.53.1471

    Abstract

    Ni–Cr–Mo steels are widely used in machine part members, gears and shafts. Steels with higher carbon content (~1%) are used for heavy machine parts and bearings. Abrasive wear resistance is often a very important requirement for these high carbon steels, apart from sliding wear properties. In the present study, En31 steel was subjected to varying heat treatments to generate different microstructures. An attempt has been made to correlate the two body abrasive wear resistance with the bulk hardness and microstructures. The microstructures were studied through a combination of scanning electron microscopy (SEM), energy dispersive spectrometer attached to SEM (i.e. SEM-EDS) and X-ray diffraction (XRD). The bulk hardness decreased with increase in tempering temperature from 423 K to 848 K. The precipitation of Cr7C3 after 598 K tempering did not cause an appreciable increase in the hardness. At higher tempering temperatures (848 K), the martensite decomposed to give ferrite and cementite. The abrasive wear tests were carried out on hardened and tempered specimens. The abrasive wear mass loss increased with increase in the tempering temperature. Hardness had a direct correlation with the two body abrasive wear behaviour in En31 steel – increase in hardness increased the abrasive wear resistance. The important material removal mechanism were micro cutting and micro ploughing, the relative contribution of each to total wear loss was influenced by abrasive wear test conditions.

  • Notch-fatigue Strength of Advanced TRIP-aided Martensitic Steels

    Junya Kobayashi, Nobuo Yoshikawa, Koh-ichi Sugimoto

    pp. 1479-1486

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    DOI:10.2355/isijinternational.53.1479

    Abstract

    The notch-fatigue limit and notch sensitivity of 0.1–0.6%C-1.5%Si-1.5%Mn transformation-induced plasticity (TRIP)-aided martensitic steels (TM steels) were investigated for use as common rails in next-generation automotive diesel engines. Also, these properties were related to the microstructural and retained austenite characteristics. When TM steels containing 0.2% to 0.4% C were subjected to heat treatment for isothermal transformation at 50°C and subsequent partitioning at 250°C, the steels achieved much higher notch-fatigue limits and lower notch sensitivities than those of conventional 0.2–0.4%C-1.0%Cr-0.2%Mo structural steels. This was principally associated with (i) plastic relaxation of localized stress concentration as a result of strain-induced transformation of 3–5 vol% metastable retained austenite and (ii) a large amount of finely dispersed martensite-austenite phase along prior austenitic, packet and block boundaries, as well as (iii) a small amount of carbide only in the wide lath-martensite structure, which may contribute to making fatigue crack initiation and/or propagation difficult.

  • Evaluation of Reduction Progress of Raw Titanomagnetite and Char Composite by Thermogravimetry

    Sung-Mo Jung

    pp. 1487-1489

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    DOI:10.2355/isijinternational.53.1487

  • © 2013 The Iron and Steel Institute of Japan

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