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

ISIJ International Vol. 56 (2016), No. 10

  • Evolution of Blast Furnace Process toward Reductant Flexibility and Carbon Dioxide Mitigation in Steel Works

    pp. 1681-1696

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    DOI:10.2355/isijinternational.ISIJINT-2016-210

    Blast furnace has been regarded as a highly optimized process as a result of various technological improvements over its long history. However, from the viewpoints of resources, energy and global warming, continuing evolution toward reductant flexibility and CO2 mitigation is desired. This review focuses on the progressive design of an ambitious blast furnace for the future.First, the history of techniques for reducing coke rate and reducing agent in the blast furnace are reviewed. Pulverized coal injection is currently common; however a more innovative process is desired in order to address the global warming issue. The low temperature blast furnace based on charging of high reactivity coke is a realistic process. The combination of the oxygen blast furnace with top gas recycling is also attractive. Although the top gas recycling process based on the oxygen blast furnace is very effective for reducing CO2 emissions, a total evaluation considering the role of the blast furnace to keep the energy self-sufficiency in the integrated steel works is necessary. The oxygen blast furnace enables injection of a large amount of natural gas, and optimized injection of natural gas and pulverized coal makes it possible to mitigate CO2 emissions while maintaining the energy supply to downstream processes. Moreover, owing to the high productivity of the oxygen blast furnace, the blast furnace profile can be downsized. The characteristics of several processes are quantitatively examined, and the concept of the advanced oxygen blast furnace as a next-generation process toward carbon dioxide mitigation is discussed.
  • Growth Kinetics of Metallic Iron Phase in Coal-based Reduction of Oolitic Iron Ore

    pp. 1697-1704

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    DOI:10.2355/isijinternational.ISIJINT-2016-253

    To understand the growth mechanism of metallic iron phase an oolitic iron ore was reduced isothermally by coal under various experimental conditions to examine the metallization process. The microstructural characteristics of metallic iron were investigated using scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). The grain size of metallic iron was measured by image analysis and the growth kinetics was analyzed based on the classical phenomenological kinetic theory. Experimental results showed that the metallization degree firstly increased and then gradually plateaued as reduction progressed. Metallic iron phase existed in the form of sphere-like grains inlaying in the slag phase. Reduction time and temperature had significant influence on the growth of metallic iron grain. The grain size of metallic iron increased with an increase in reduction time and temperature. The growth process of metallic iron grain was characterized by two stages with an inflection point at 30 min. The grain growth exponent and activation energy at the corresponding stage were determined, and the growth kinetics model was proposed to describe the metallic iron grain growth during coal-based reduction of oolitic iron ore. The metallic iron grain growth was controlled by chemical reaction of iron oxide minerals reduced to metallic iron at reduction time ≤30 min, and combined surface diffusion and metallic iron diffusion in the slag at reduction time >30 min.
  • Evaluating the Reduction-Softening Behaviour of Blast Furnace Burden with an Advanced Test

    pp. 1705-1714

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    DOI:10.2355/isijinternational.ISIJINT-2016-117

    A ferrous burden loses its permeability in the cohesive zone of a Blast Furnace (BF), where the iron burden materials soften and melt. A tailor-made, high-temperature furnace named ARUL (Advanced Reduction under Load) was used here to study the reduction-softening behaviour of acid and olivine pellets and basic sinter under simulated BF gas, temperature and pressure conditions.The ARUL test showed the best reduction-softening properties for the basic sinter. The sinter sample resisted up to 1329°C and achieved a reduction degree of 90.2% until a gas-impermeable structure was formed in a packed bed, whereas the acid pellet lost its permeability at 1160°C and only reduced to a reduction degree of 48.7%. The olivine pellet had intermediate reduction-softening properties with a final temperature of 1252°C and a final reduction degree of 68.7%. The differences between the test materials were assessed as being caused mainly by different chemistry, but it was also revealed that the sinter sample remained its macro-porosity markedly better in relation to the pellets, providing routes for reducing gases.The experimental results were compared to the phase diagrams calculated with the computational thermodynamic software FactSage. Phase diagrams for the 5-component FeO–SiO2–CaO–MgO–Al2O3 systems with constant CaO, MgO and Al2O3 contents were used to estimate the formation of liquid phases in the test materials. The computed phase diagrams gave an estimate of the liquid formation; however, some limitations were also found in the utilization of the computations because of the need to define the system in certain simplicity.
  • Fundamentals of Silico-Ferrite of Calcium and Aluminium (SFCA) and SFCA-I Iron Ore Sinter Bonding Phase Formation: Effects of Titanomagnetite-based Ironsand and Titanium Addition

    pp. 1715-1722

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    DOI:10.2355/isijinternational.ISIJINT-2016-162

    The thermal decomposition of titanomagnetite-based ironsand, and the effects of ironsand addition on the formation of SFCA and SFCA-I iron ore sinter bonding phases, was investigated using in situ X-ray diffraction. Titanium incorporation into the SFCA and SFCA-I structures was investigated via phase equilibria experiments and subsequent ex situ characterisation. Increasing ironsand addition from 1.3 to 6.7 and to 13.8 mass% in an otherwise synthetic sinter mixture composition designed to form SFCA-I did not significantly affect the thermal stability range of SFCA-I (~1373–1523 K), nor did it significantly affect the maximum concentration of SFCA-I attained (42–46 mass%). The main effect of ironsand addition was a small reduction in the thermal stability range of another complex calcium-rich ferrite, γ-CFF. In comparison, increasing ironsand addition from 2.4 to 3.9 and to 11.6 mass% in an otherwise synthetic sinter mixture composition designed to form SFCA resulted in a decrease in the maximum SFCA-I concentration, from 30, to 24 and 16 mass%, respectively, with a corresponding increase in the concentration of SFCA (16, to 23 and 33 mass%). The phase equilibria studies revealed that SFCA can incorporate more titanium in its structure (up to 1.2 mass% TiO2) than SFCA-I (0.6 mass% TiO2). The ‘total SFCA’ (i.e. SFCA-I + SFCA) content decreased as the ironsand content increased, as well as there being a general shift to higher temperature of the total SFCA concentration curves. Such effects are likely to exert an influence on the physical properties of iron ore sinter, as well as affecting sintering fuel requirements.
  • Control Technique of Coke Rate in Shaft Furnace by Controlling Coke Reactivity

    pp. 1723-1727

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    DOI:10.2355/isijinternational.ISIJINT-2016-244

    The shaft furnace, which is a scrap melting furnace, plays an important part as an energy supplier in a steel works because the shaft furnace produces high calorie gas. The shaft furnace is required either to reduce the coke rate or to increase the exhaust gas calorie, corresponding to the energy balance in the steel works. Because the coke rate and exhaust gas calorie are determined by coke gasification reactivity, the reactivity control technique is very important. In this study, the coke surface was coated with CaCO3, Fe2O3 and SiO2 to control coke reactivity, and the gasification rates were measured at 1573–1773 K. As the results, the gasification rate was accelerated by CaCO3 and Fe2O3 and decelerated by SiO2. The acquired gasification rates were applied to a one-dimensional mathematical model of the shaft furnace. The shaft furnace operation with controlled coke reactivity was simulated, and the effects of coke reactivity on the coke rate and exhaust gas calorie were estimated.
  • Phase Equilibria in the System CaO-SiO2-Al2O3-MgO-15 and 20 wt% “FeO” with CaO/SiO2 Ratio of 1.3

    pp. 1728-1737

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    DOI:10.2355/isijinternational.ISIJINT-2016-342

    The “FeO”-containing slags in the low part of blast furnace (BF) are of great importance to the operation of BF, particularly the primary and bosh slags. To optimise the slag components for the smooth operation of BF, the phase equilibria studies have been carried out in the system “FeO”-CaO-SiO2-Al2O3-MgO in equilibrium with metallic iron. High temperature equilibrations followed by quenching were conducted in experiments and electron probe X-ray microanalysis were employed to analyse the samples. For better interpretation and easy implementation of experimental results, the data obtained from measurements were symmetrically analysed and then plotted in the pseudo-ternary phase diagrams of (CaO+SiO2)-Al2O3-MgO with fixed CaO/SiO2 weight ratio of 1.3 and “FeO” of 15 and 20 wt%, respectively. The primary phases such as melilite, Ca2SiO4, merwinite, spinel and (Mg, Fe2+)O were observed with liquid phases and metallic iron in the composition range. The liquidus temperatures increase in melilite and spinel primary phase fields, but decrease in dicalcium silicate and merwinite primary phase fields with increasing Al2O3/(CaO+SiO2) ratio. In addition, the liquidus temperatures firstly increase then decrease with increasing MgO/(CaO+SiO2) ratio in dicalcium silicate and melilite primary phase fields, while they have an increasing trend in merwinite and monoxide primary phase fields. The data resulted from this study provide accurate experimental information that can be used for optimisation of the computed thermodynamic models.
  • The Conditions of Ettringite Formation by the Reaction of a Blast Furnace Slag with Aqueous Alkaline Solutions

    pp. 1738-1745

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    DOI:10.2355/isijinternational.ISIJINT-2016-347

    Blast furnace (BF) slags have been utilized in cement, concrete aggregate, roadbed materials, and earthwork materials. If an appropriate control of the elution and compound formation is developed under severer environmental conditions, their usage would be more diverse. Because the chemical composition of BF slag is similar to that of Portland cement, the possibility of ettringite (3CaO·Al2O3·3CaSO4·32H2O) formation from BF slag following a mechanism similar to that of cement hydration might be possible under a wet alkaline environment. Therefore, the effect of an alkaline solution on ettringite formation from BF slags was investigated by slag-leaching experiments and thermodynamic calculations using PHREEQC. The formation of ettringite was observed only for the high pH solutions in the experiments, whereas its thermochemical possibility from the air-cooled BF slags was always expected by the calculation. The kinetic analysis showed that the dissolution of alumina from the slag may control the whole reaction rate. The mixing of granulated BF slag with air-cooled ones tended to enhance the ettringite formation. Furthermore, a technique for removing the ettringite formed in the slag was also developed.
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  • Kinetics of Nitrogen Absorption and Desorption in High-Cr Molten Steel under Pressurized Atmosphere

    pp. 1746-1750

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    DOI:10.2355/isijinternational.ISIJINT-2016-202

    The rates of nitrogen absorption and desorption in high-Cr molten steel under a pressurized atmosphere were investigated using a pressurized directional solidification furnace. In this study, the melting experiments were performed under a maximum gas condition of 1.0 MPa. It was found that nitrogen absorption rate took the same value under the conditions of unified partial pressure on N2 gas in spite of the different total pressure. The mass transfer coefficient in the liquid phase of high-Cr molten steel at 1823 K was estimated as 0.0009 m·s−1. On the other hand, it was found that the rate constant at the gas/liquid interface for the nitrogen desorption reaction decreased with the increase of nitrogen partial pressure. This result indicates that the nitrogen activity in the molten steel has an influence on the chemical reaction at the gas/liquid interface. Furthermore, the mass transfer coefficient in the gas phase decreased with the increase of total pressure. As numerical kinetic analysis shows, this result suggests that mass transfer in the gas phase is very important for the nitrogen absorption and desorption reactions under a pressurized atmosphere.
  • Mechanisms Involved in the Roasting of Pellets Composed of Stainless Steel Slag and Sodium Hydroxide to Extract Chromium

    pp. 1751-1757

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    DOI:10.2355/isijinternational.ISIJINT-2016-320

    In order to extract the chromium from stainless steel (SSL) slag, the thermochemical processes involved in the roasting of pellets composed of SSL slag and sodium hydroxide were studied. It was found that Ca3Mg(SiO4)2, Ca2SiO4, MgCr2O4 and MgO are the main phases from the SSL slag. Most of chromium from the slag was present in spinel phase (MgCr2O4). In addition, the chromium was also found in metallic phase (Fe–Cr) and periclase phase (MgO). These three Cr-containing phases were embedded in the silicate phases. During the pellet roasting, the silicate phases were destroyed by liquid NaOH and the low valence of insoluble chromium species such as MgCr2O4 were converted to Na3CrO4 at 500–700°C and CaCrO3 at 800°C, respectively. Both were soluble in hot water. The residual chromium was only present in Cr–Fe–O phases. Many cavities were left on the pellet surface during the roasting, which facilitated the diffusion of both air and liquid NaOH. A high chromium extraction was achieved only when liquid NaOH had diffused into the cores of slag particles over 400°C.
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  • Generation Mechanism of Unsteady Bulging in Continuous Casting-1 -Development of Method for Measurement of Unsteady Bulging in Continuous Casting-

    pp. 1758-1763

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    DOI:10.2355/isijinternational.ISIJINT-2016-172

    In the continuous casting of steel, mold level fluctuation caused by unsteady bulging of the solidifying shell affects the surface quality of the product and stable operation of the continuous casting process. To clarify this problem, inter-roll bulging and unsteady bulging in experimental casting machines and commercial continuous casting machines have been measured by various methods in a number of studies. In this study, the fluctuation of inter-roll bulging with time in a commercial continuous casting machine was measured by an ultrasonic range finder using a water column. In these measurements, the fluctuation of the segment was also considered. The validity of the measured data was estimated by comparison with the mold level. The results showed that both inter-roll bulging and the mold level fluctuated with the cycle calculated from the roll pitch and casting speed, and the amplitude of the mold level fluctuation converted from the amount of fluctuation of inter-roll bulging corresponded to the actual mold level fluctuation. Therefore, the cycle and absolute amount of inter-roll bulging fluctuation measured in this study were considered reasonable. These results also revealed that the value measured in this study corresponded directly to the fluctuation of inter-roll bulging as such.
  • Generation Mechanism of Unsteady Bulging in Continuous Casting-2 -FEM Simulation for Generation Mechanism of Unsteady Bulging-

    pp. 1764-1769

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    DOI:10.2355/isijinternational.ISIJINT-2016-171

    In the continuous casting of steel, unsteady bulging contributes to degradation of slab quality. It has been reported that unsteady bulging is promoted by uneven solidification in the mold, but the effect of uneven solidification on unsteady bulging has not been clarified. In this study, a Finite Element Method (FEM) simulation was conducted. Shell deformation was calculated by an elasto-plastic analysis assuming that the slab moves between the rolls, considering time dependency. The bulging value and mold level fluctuation, which change corresponding to the solidified shell thickness, ferrostatic pressure and roll pitch, were obtained.In the simulation results, the shell is deformed by ferrostatic pressure. The bulging shell pushes out under the rolls in the thickness direction, and unsteady bulging occurs. While the shell is passing through rolls with the same pitch, unsteady bulging becomes larger. When the solidified shell is uneven, stress concentrates on the thinner portions, and this stress concentration accelerates unsteady bulging even at the same average shell thickness. Based on these results, an operational index for suppressing unsteady bulging by reducing unevenness of the solidified shell was proposed.
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  • Modeling of Inclusion Formation during the Solidification of Steel

    pp. 1770-1778

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    DOI:10.2355/isijinternational.ISIJINT-2016-243

    A thermodynamic model coupling microsegregation and inclusion formation using one ChemSage datafile is proposed. The thermodynamic equilibrium is calculated using ChemApp to determine the liquidus temperature, solute partition coefficients at the solidification interface and inclusion formation in the residual liquid. During the calculations, solute enrichment is predicted using Ohnaka’s model. The logarithm of the coupling microsegregation and inclusion formation is tested through an overall mass balance. With the proposed model, inclusion formation is predicted for the case of medium carbon steels alloyed with titanium and aluminum. The predicted types and compositions of inclusions agree well with the experimental results. Based on the predictions and measurements, the inclusion behavior during solidification is discussed.
  • PCA-LMNN-Based Fault Diagnosis Method for Ironmaking Processes with Insufficient Faulty Data

    pp. 1779-1788

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    DOI:10.2355/isijinternational.ISIJINT-2016-101

    Fault detection and fault classification are important in the modern ironmaking process. Some studies based on principal component analysis (PCA) techniques have been performed for fault detection in the ironmaking process. However, studies on fault classification in the ironmaking process remain limited. In this paper, problems that are related to the classification of abnormalities in blast furnaces are considered. We fuse historical abnormal data that were collected from three real blast furnaces to address the problem of insufficient historical faulty data. To extract common features for the same type of abnormalities, which are not affected by different operation points or different blast furnaces, we propose the use of a contribution vector as a fault feature, which is calculated by the PCA-based technique. The large marginal nearest neighbor (LMNN) technique is employed to train a classifier with contribution vectors as inputs. Twenty-one historical abnormalities in three different real blast furnaces are employed to validate the proposed method. The results indicate that this method achieves the desired performance.
  • Analyzing Nonlinear System Stability of a New Hydraulic Bilateral Rolling Shear

    pp. 1789-1795

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    DOI:10.2355/isijinternational.ISIJINT-2016-152

    New hydraulic bilateral rolling shear is a composite connecting rod shearing mechanism with the upper blade driven by servo-cylinders. This new creative system would be unstable because of interaction between the mechanical dynamics and hydraulic dynamics of the machine. What’s more, the positive feedback between hydro-cylinder & mechanic, which produces instability phenomenon. In this model, all instabilities result from the control of the fuel. In this study, by using the unsymmetrical valve to control unsymmetrical cylinder, this hydraulic servo model could get the nonlinear equation of the state between force and cylinder flow. Lyapunov stability theory is also used to analyze the stability of system to prove the consequence. According to the recorded data, actual trajectory agree well with given trajectory, which show that method in this paper can meet the practical demand in engineering. In addition, the experimental results show that the nonlinear system with multiple degrees of freedom is stable and its performance is outstanding.
  • In-situ TEM Observation of the Interaction between Dislocations and Particles of Cu-added Ferritic Stainless Steel under High-temperature Straining

    pp. 1796-1799

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    DOI:10.2355/isijinternational.ISIJINT-2015-711

    Cu is always present in the matrix when ferritic steels were prepared from ferrous scrap. When the ferritic steels are aged thermally, Cu particles start appearing and dispersing finely and homogeneously, which may result the steels strengthened by dispersion strengthening. In this study, the interactions between Cu particles and dislocations were examined via high-temperature in-situ TEM straining. Cu-added ferritic stainless steel (Fe-18.4%Cr-1.5%Cu) was used in the present study. Specimen was aged at 1073 K for 360 ks. Microstructure of specimen was analyzed by JEM-3200FSK and high-temperature in-situ TEM straining was conducted using JEM-1300NEF. Progressing dislocations in matrix contacted with the Cu particle at right angle. This result implies that there is an attractive interaction between dislocations and the Cu particle. Furthermore, dislocations pass through the particle after contacting it, so that the interaction with dislocations and particles should be explained by Srolovitz mechanism.
  • Effect of Hot Deformation and Controlled Cooling Process on Microstructures of Ti–Zr Deoxidized Low Carbon Steel

    pp. 1800-1807

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    DOI:10.2355/isijinternational.ISIJINT-2016-106

    It has been known that microstructures in steel welding heat affected zones can be improved by inclusion-induced intragranular ferrite nucleation. In this paper, the influence of inclusions on microstructure in steel base metal under thermo-mechanical conditions was further researched. Hot deformation and controlled cooling process were simulated with Ti–Zr deoxidized low carbon steels. Microstructural characteristics and transformation behaviors were analyzed. Results showed that inclusions were mainly (Ti, Zr)-rich oxide with MnS precipitation on the surface and they were effective for acicular ferrite nucleation in the coarse-grained heat affected zone. Acicular ferrite could still form in hot deformed and recrystallized austenite grains, but its volume fraction reduced with the decrease in deformation temperature. Accelerated cooling above 600°C and addition of boron significantly promoted acicular ferrite structure and the hardness value also increased. Hot rolling simulation indicated that for the experimental Ti–Zr–B steel, microstructure could not be further improved by applying conventional controlled rolling deformation. Instead, fine-grained acicular ferrite structure and high hardness value were obtained through high temperature deformation and controlled cooling process, during which (Ti, Zr)-oxide inclusions contributed to the intragranular acicular ferrite formation.
  • Thermo-mechanically Coupled Isotropic Analysis of Temperatures and Stresses in Cooling of Coils

    pp. 1808-1814

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    DOI:10.2355/isijinternational.ISIJINT-2016-073

    A 2D transient thermo-mechanically coupled axi-symmetric FE model has been implemented and used to predict the temperatures and stresses under cooling of coils. The temperature trajectory as a function of geometrical position in as-coiled steel strip products is affected by several parameters as the: initial temperature inherited from upstream cooling on the Run Out Table (ROT), coil dimensions, strip surface quality, contact conditions and the surrounding environmental conditions etc. The layered structure makes the thermal conductivity anisotropic where the interfacial contact condition depends on the transient stress state caused by thermal and initial effects. The coil cooling rates are for HSLA-steel grades of importance in achieving proper final mechanical properties where too fast temperature drops ceases the precipitation hardening solely diffusional driven. Furthermore is a parameter influence study made revealing process parameter significance. The model has been validated against two full-scale bell furnace trials. A main objective with this model development work was to keep the model fast and accurate applicable on real plant situation and for process controlling.
  • Strip Warpage Behavior and Mechanism in Single Roll Driven Rolling

    pp. 1815-1824

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    DOI:10.2355/isijinternational.ISIJINT-2016-188

    Model rolling experiment was conducted to investigate strip warpage behavior during single roll driven rolling. It became clear that direction of strip warpage changes with change in so-called rolling shape factor which is defined as ratio of contact arc length to strip thickness. In case of the shape factor being small, rolled strip tends to warp toward the idle roll side, which is the consequence of larger exit velocity of the rolled strip for the driven roll side due to higher peripheral velocity of the driven roll. On the other hand, in case of larger shape factor, rolled strip tends to warp toward the driven roll side owing to larger forward slip ratio caused by larger thickness reduction for the idle roll side. In addition, two dimensional steady-state rolling analyses by a rigid plastic finite element method were conducted to investigate mechanism of the strip warpage behavior. Utilizing fine FE mesh and precise boundary conditions, the results of analyses show good agreement both qualitatively and quantitatively in strip warpage behavior with the experimental results mentioned-above. Moreover elaborate mechanical investigation based on the FE analyses reveal the fact that rolling deformation is realized by a set of macroscopic shear bands which penetrate strip thickness and are inclined with respect to the strip thickness direction, and it is concluded that intensity and configuration of the shear bands determine strip warpage behavior.
  • Accelerative Effects of Diffusion Bonding of Recrystallization with Reversion of Deformation-induced Martensite in SUS304

    pp. 1825-1830

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    DOI:10.2355/isijinternational.ISIJINT-2016-283

    Diffusion bonding at low temperature is needed to improve the functional and mechanical properties of diffusion-bonded products. For example, metal micro pumps fabricated by diffusion bonding of stacked metal foils experience performance loss caused by a loss of yield strength due to grain growth during bonding. Accelerating the recrystallization by using pre-distorted base metals effectively decreases the diffusion-bonding temperature of metal materials. Pre-distorted SUS304 contains deformation-induced martensite before bonding, and then during diffusion bonding recrystallizes with reversion of this martensite. This study evaluates the effect of bonding time on bonding state and on recrystallization with the reversion of SUS304 that has deformation-induced martensite, and discusses the relationship between bonding behavior and microstructure change. Results revealed that the bonding area (evaluated based on line profile of pixel intensity on the bonding interface in a cross-sectional image) exceeded 70% in a specimen bonded at 973 K bonding temperature, 50 MPa bonding pressure, and 60 s bonding time. Results also showed that a bonding time longer than 900 s was necessary to obtain bonding strength comparable to that of the base material. On the other hand, analysis of the bonding area of pre-distorted SUS304 that had severely deformed austenite revealed recrystallized grains at the bonded area. Recrystallization of SUS304 that has deformation-induced martensite starts homogeneously due to reversion of this martensite. Therefore the most likely effect of reversion in diffusion bonding is that diffusion is promoted homogeneously in the bonding interface because recrystallization starts homogeneously during the bonding process.
  • Texture and Planar Anisotropy in Lean Duplex Stainless Steel Sheet

    pp. 1831-1839

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    DOI:10.2355/isijinternational.ISIJINT-2016-185

    The comparison of texture and planar anisotropy of the r-value between the lean Type 32101 (21.2%Cr-1.5%Ni-5.0%Mn-0.3%Mo-0.02%C-0.22%N) and standard Type 329J4L (25.2%Cr-6.9%Ni-0.7%Mn-3.0%Mo-0.02%C-0.11%N) duplex stainless steel cold-rolled and annealed sheets were examined. Type 32101 had a weaker texture, especially in the α phase, and a lower planar anisotropy of the r-value compared with Type 329J4L. These differences were caused by the cold rolling texture because the αbcc fiber (<011>//RD) texture in Type 32101 decreased during cold rolling. It was revealed that during cold rolling, inhomogeneous deformation easily occurred in Type 32101 because this steel contained a higher volume fraction of the γ phase harder than the α phase.
  • Effects of Dislocation Substructure on Creep Deformation Behavior in 0.2%C-9%Cr Steel

    pp. 1840-1846

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    DOI:10.2355/isijinternational.ISIJINT-2016-203

    In this paper, microstructural evolution under creep deformation was investigated by using a simple 0.2%C-9%Cr steel to reveal effects of dislocation substructure on creep strength in addition to creep deformation behavior in high Cr martensitic steels. Whereas strain versus creep rate curves are composed of simple primary and tertiary creep stages on the steel tested at higher applied stress condition, complicated variation in creep rate are found on the curves examined at lower stress levels under a temperature of 873 K. Formation of cellular dislocation network substructure is observed in the early periods of the primary creep stage on the steel tested at a lower stress. Such substructure is stable until onset of acceleration creep. The degradation of the substructure is due to coarsening and condensation of the carbides existing along the lath boundaries. An increase in yield stress at elevated temperature and internal back stress in the whole course of the transient creep region are provided by the cellular dislocation network substructure having highly dense dislocation walls where intralath dislocations move and integrate in the early periods after loading. The complicated variation in creep rate at the lower stress conditions is caused by accumulation and partial disappearance of dislocations at cell and lath boundaries. It can be concluded from above results that dislocations act a key role for strengthening against creep deformation.
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  • Proposal of Two-dimensional Local Curvature Multi-vertex Model for Grain Growth Simulation

    pp. 1847-1856

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    DOI:10.2355/isijinternational.ISIJINT-2016-107

    A local curvature multi-vertex model was developed. This model is a straightforward two-dimensional topological network model based on physical principles that consider the local curvatures of grain boundaries and the grain boundary tensions at triple junctions. Virtual vertices are set on the grain boundaries in order to calculate the driving forces of grain boundary and triple junction migration. Therefore, the accuracy of the developed model is higher than that of the conventional curvature model and the vertex model. In the proposed model, the generation and annihilation of virtual vertices maintained a proper configuration of virtual vertices, and high accuracy is expected with a suitable set of simulation parameters. The proposed model was verified by the grain growth simulation using adequately determined parameters for the artificially generated specimens with 5040 grains.
  • Proposal of Grain Growth Model Based on Two-dimensional Local Curvature Multi-vertex Model in the Presence of Pinning Particles

    pp. 1857-1865

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    DOI:10.2355/isijinternational.ISIJINT-2016-182

    A grain growth model based on a two-dimensional local curvature multi-vertex model in the presence of pinning particles was developed. This model is a physical model which pursues the minimum total grain boundary energy as the evaluation function, where the unpinning conditions are as follows. The first unpinning condition is that the total energy of the unpinned grain boundary is smaller than the total energy of the pinned grain boundary. The second unpinning condition is that the energy of the grain boundary necessary to surpass the energy barrier is assumed to be smaller than the jumping energy, which is presumably assisted by thermal lattice vibration. Using only the first condition, the Zener pinning effect caused by the finely dispersed particles during normal grain growth was reproduced. With the second condition, the selective abnormal grain growth was reproduced when the abnormally grown grain was surrounded by the grains with low-energy grain boundaries.
  • Internal Stress and Elastic Strain Energy in Pearlite and Their Accommodation by Misfit Dislocations

    pp. 1866-1873

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    DOI:10.2355/isijinternational.ISIJINT-2016-256

    Internal stress and elastic strain energy in pearlite caused by misfit between ferrite and lamellar cementite were theoretically analyzed based on micromechanics while taking into account an accommodation mechanism of the misfit strain between ferrite and cementite on pearlitic transformation. Two-dimensional large deformation analysis reveals that the Pitsch-Petch orientation relationship is most appropriate among already reported crystal orientation relationships under the condition that the interface contains a lattice invariant direction. However, the micromechanics analysis using a periodic function proves that the misfit strain generates very large elastic strain energy in pearlite even when the Pitsch-Petch orientation relationship is satisfied, which is almost comparable to the chemical driving force for pearlitic transformation. Assuming that an interval of misfit dislocations dynamically introduced at ferrite/cementite interface upon pearlitic transformation depends on the growth rate of pearlite, the total elastic strain energy reduces more effectively as the growth rate becomes lower. As a result, the elastic strain energy in pearlite changes widely depending on interlamellar spacing.
  • A Comparison between Ultra-high-strength and Conventional High-strength Fastener Steels: Mechanical Properties at Elevated Temperature and Microstructural Mechanisms

    pp. 1874-1883

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    DOI:10.2355/isijinternational.ISIJINT-2016-120

    A comparison is made between the mechanical properties of the ultra-high-strength steel KNDS4 of fastener grade 14.9 and of conventional, high-strength steels 34Cr4 of fastener grade 12.9 and 33B2 of grade 10.9. The results show that the ratio of the yield strength at elevated temperatures to the yield strength at room temperature is higher for the ultra-high-strength steel than for both conventional high-strength steels, especially at 500°C. Moreover, the results show a trend in which the nano-indentation creep rate is lower as the strength of the steels is higher. The improved mechanical properties of the KNDS4 steel compared to the conventional high-strength steels are related to the smaller size of the alloy carbides in the KNDS4 steel. Furthermore, the effect of an alternative (industrial) heat-treatment on the evolution of the microstructure and hardness of the KNDS4 steel was investigated. Changing the industrial heat treatment can increase the hardness of KNDS4 by about 8%, since more alloy carbides can nucleate and grow. However, the standard industrial heat treatment results in a refinement of the martensite microstructure (grain size), which might be more beneficial for the toughness of the steel. Independent of the heat treatment, the mechanical performance of KNDS4 fasteners at elevated temperature and the low nano-indentation creep rates are two strong indicators that fasteners made from KNDS4 steel might be used at higher service temperatures than traditional high strength fasteners.
  • Effects of Organic Matter on Acceleration of the Iron Elution Rate from Steelmaking Slag

    pp. 1884-1888

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    DOI:10.2355/isijinternational.ISIJINT-2016-098

    An iron (Fe) fertilization method using a mixture of steelmaking slag and compost for restoring seaweed beds in coastal areas of barren grounds has been developed, and mixing of organic matter other than compost with steelmaking slag has been shown to accelerate the Fe elution rate. In this study, Fe elution tests under conditions close to that of the actual sea environment were conducted in Tsushima, Nagasaki Prefecture, Japan, to elucidate the effects of the addition of powdered bamboo on the promotion of Fe elution. We examined the characteristics of Fe elution for three samples: 1) the mixture of steelmaking slag and compost; 2) the mixture of slag and powdered bamboo; and 3) the mixture of slag, compost, and bamboo. Our results showed that addition of bamboo to the mixture of slag and compost was the most effective for accelerating the Fe elution rate, suggesting that the amount of eluted organic matter was important, as were the structural characteristic of the material. The safety of this method was also evaluated by investigating the accumulation of heavy metals in sea creatures. We confirmed that six heavy metals, i.e., cadmium, arsenic, lead, zinc, selenium, and chromium, were not accumulated in the sea cucumber and sea snails.
  • Criticality of Sampler Locations in Inert-gas Shrouded Tundishes during Open Eye Events and Unbalanced Throughputs

    pp. 1889-1892

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    DOI:10.2355/isijinternational.ISIJINT-2016-239

    The standard sampling practice involves procuring lollipop samples from an opening on tundish floor close to the stopper rods. Analyzing these samples gives us an overall idea about the chemistry of steel and inclusions in the tundish. A mathematical model was built using computational fluid dynamics to determine the inclusion paths in cases of balanced and unbalanced throughput casting of steel. The calculated trajectories showed that the inclusions are mostly concentrated near the ladle shroud where the tundish open eye (TOE) forms. In order to investigate the effect of TOEs on the chemistry of steel, the ideal sampling location was determined from the mathematical model.

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