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

ISIJ International Vol. 55 (2015), No. 11

  • Effect of Applied Voltage on Electroreduction with Controlled Oxygen Flow of Molten Slag Containing FeO at 1723 K

    pp. 2273-2282

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    DOI:10.2355/isijinternational.ISIJINT-2014-826

    This paper reports on electroreduction with controlled oxygen flow (COF) for extraction of iron along with oxygen by-product from molten slag containing FeO at 1723 K. An electrolytic cell with COF was constructed by a cathode and a porous platinum anode sintered on a one-end-closed magnesia partially stabilized zirconia based solid electrolyte tube. The process for electroreduction with COF was analyzed. Effect of applied voltage on electroreduction behavior of FeO in SiO2–CaO–Al2O3–MgO molten slag with iron rod serving as cathode was investigated by means of the linear sweep and the potentiostatic electrolysis. The possibility of zirconia membrane as conductive noncorrosive anode material was also discussed. The results show that decreased oxygen partial pressure in anode atmosphere can contribute to lower decomposition voltage of FeO in molten slag. Higher applied voltage results in better effect of FeO electroreduction. Electrolytic product can be pure iron with dendrite or ferrosilicon alloy based on applied voltage. Besides, the phase transformation of zirconia grain and the slag penetration can lead to an increase of electronic conductivity of the zirconia membrane. The corrosion of the molten slag to the zirconia membrane during electrolysis is evidently aggravated by large the applied voltage. In order to obtain pure iron and minimize the corrosion of the molten slag to the zirconia membrane under this condition of experimentation, the applied voltage of 1.5 V is preferred.
  • Activity of P2O5 in Solid Solution between Di-calcium Silicate and Tri-calcium Phosphate at 1823 and 1873 K

    pp. 2283-2288

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    DOI:10.2355/isijinternational.ISIJINT-2015-019

    By applying the chemical equilibrium method, the equilibrium between liquid iron and 2CaO∙SiO2-3CaO∙P2O5 solid solution has been measured with oxygen partial pressure of 5.22×10−12 atm at 1823 K and 1.41×10−11 atm or 4.25×10−10 atm at 1873 K. The phosphorus partition ratio between 2CaO∙SiO2-3CaO∙P2O5 solid solution and liquid iron was observed. The activity of P2O5 relative to hypothetical pure liquid P2O5 was determined from the phosphorus content in liquid iron and the reported thermodynamic data. The activity of P2O5 in 2CaO∙SiO2-3CaO∙P2O5 solid solution increased with the increase of 3CaO∙P2O5 content in solid solution. The activity coefficient of P2O5 in 2CaO∙SiO2-3CaO∙P2O5 solid solution also increased with the increase of 3CaO∙P2O5 content in solid solution.
  • In-situ Investigation on the Reduction of Magnesiochromite with Ferrosilicon between 1373–1573 K

    pp. 2289-2296

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    DOI:10.2355/isijinternational.ISIJINT-2015-126

    Certain amount of the chromium in slags exists in the form of magnesiochromite (MgCr2O4) during the decarbonisation of stainless refining. It is mainly recovered through high temperature reduction. In this study, an interface of ferrosilicon and synthetic MgCr2O4 is made and the reduction behaviour is in-situ observed by confocal scanning laser microscope (CSLM) between 1373–1573 K. The samples were analysed afterwards through electron probe micro analyser (EPMA). Reduction of MgCr2O4 can already initiate at 1373 K within around 30 min. The heat generated from the reaction between Si and Cr2O3 can increase the local temperature of the area surrounding the reaction zone. This makes the liquefaction of Fe–Si alloy possible even at measured bulk temperatures lower than its melting point. Although Fe does not participate in the reduction of Cr, it can diffuse into the reaction zone to form a Fe–Cr–Si alloy phase, together with the reduced Cr metal and residual Si. The liquid Fe–Cr–Si alloy can act as a transfer medium for Si to promote the reduction.
  • Effect of Na2O on Dissolution Rate of Alumina in CaO–Al2O3–MgO–SiO2 Slag

    pp. 2297-2303

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    DOI:10.2355/isijinternational.ISIJINT-2015-211

    As the last step of inclusion removal in tundish, the dissolution of Al2O3 in tundish slag is very important and has received many interests by previous researchers. In the present work, the effect of Na2O addition on dissolution of Al2O3 in CaO–Al2O3–MgO–SiO2 slag at 1823 K was investigated by using rotating finger method. The interface between rod and molten slag was investigated using SEM-EDS technique. The dissolution of Al2O3 in CaO–Al2O3–MgO–SiO2–Na2O system increases with increase of rotating speed and immersing time. A linear relationship between logarithm of dissolution rate and logarithm of periphery velocity was found with a slope of 0.53, which indicates that rate determining step for dissolution of Al2O3 is the mass transfer of solute in boundary layer of molten slag. The dissolution rate of alumina in CaO–Al2O3–MgO–SiO2–Na2O slag increases with increase of Na2O content, which could be attributed to the kinetic and thermodynamic factors. The decrease of viscosity of molten slag with increasing Na2O content could lead to increased mass transfer in boundary layer of molten slag. On the other hand, the increase of Na2O would lead to increase of thermodynamic driving force of Al2O3 dissolution. Some intermediate compounds such as MgAl2O4 and CaAl4O7 were found at interface between alumina rod and slag, indicating indirect dissolution of Al2O3 in slags.
  • Distribution Characteristics of Phosphorus in the Metallic Iron during Solid-State Reductive Roasting of Oolitic Hematite Ore

    pp. 2304-2309

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    DOI:10.2355/isijinternational.ISIJINT-2015-212

    The distribution characteristics of phosphorus in the metallic iron, and the reduction behaviors of hematite and fluorapatite during solid-state reductive roasting of a phosphorus-rich (1.3 mass%) oolitic hematite ore were investigated in the temperature range of 900°C–1200°C. Experimental results show that phosphorus in the iron is present in the forms of αFe and Fe3P, resulted from the simultaneous reduction of fluorapatite and hematite at temperatures up to 1100°C. The phosphorus content in the metallic iron particles decreases from the edge to the center in the pellet cross-section. For the aim of phosphorus removal, the selective reduction of hematite and fluorapatite should be achieved; while for the enrichment of phosphorus as ferrophosphorus, the reduction of fluorapatite, and affinity of P2 and Fe should be guaranteed. These findings could provide guidance for technique development for the utilization of phosphorus-rich oolitic iron ores via pyro-methods.
  • Phase Equilibria in the CaO–SiO2–Al2O3–MgO System with CaO/SiO2 Ratio of 1.3 Relevant to Iron Blast Furnace Slags

    pp. 2310-2317

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    DOI:10.2355/isijinternational.ISIJINT-2015-263

    Phase equilibria and liquidus temperatures in the CaO–SiO2–Al2O3–MgO system with CaO/SiO2 weight ratio of 1.30 have been experimentally determined by means of high temperature equilibration and quenching technique followed by electron probe X-ray microanalysis. Isotherms between 1723 and 1773 K were determined in the primary phase fields of melilite, dicalcium silicate, spinel, merwinite and periclase that are relevant to ironmaking slags. Effects of Al2O3, MgO and CaO/SiO2 ratio on the liquidus temperatures have been discussed. Compositions of the solid phases corresponding to the liquidus have been accurately measured that will be used for development of the thermodynamic database.
  • Reassessment of TiN(s)=Ti+N Equilibration in Liquid Iron

    pp. 2318-2324

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    DOI:10.2355/isijinternational.ISIJINT-2015-265

    Thermodynamics of nitrogen solubility and TiN formation in liquid Fe–Ti alloys was reassessed over the temperature range from 1823 to 1973 K by combining new experimental data for the Ti–N–TiN relations in liquid iron at 1823–1873 K together with the authors’ previous data. Using the Wagner’s Interaction Parameter Formalism(WIPF), the first-order interaction parameters of [ This part of text is omitted ], [ This part of text is omitted ] and [ This part of text is omitted ], and the equilibrium constant for the TiN formation reaction in liquid iron, logKTiN were newly determined. The interaction parameters showed no temperature dependence in this temperature range. The calculated solubility product of titanium and nitrogen for TiN formation in liquid iron was in excellent agreement with the available experimental results measured at various temperatures.
  • Electronic/Ionic Properties of FexO–SiO2–CaO–Al2O3 Slags at Various Oxygen Potentials and Temperatures

    pp. 2325-2331

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    DOI:10.2355/isijinternational.ISIJINT-2015-325

    As a fundamental study on properties of the FexO-bearing slags, the total electrical conductivity and electronic/ionic properties of FexO–SiO2–CaO–Al2O3 slags were measured at different oxygen potentials (controlled by CO–CO2 mixture gas) and temperatures by using four-electrode method. From experiments results, it can be seen that the total conductivity changes little as increasing the ratio of CO to CO2 (decreasing the oxygen potential), while the electronic and ionic conductivities of all slags decreases and increases monotonously, respectively. The temperature dependences of the total electrical conductivity, electronic, and ionic conductivities follow the Arrhenius law. It was also found that with increasing CaO/Al2O3 ratio, the total electrical conductivity and ionic conductivity firstly decrease and then increase, while electronic conductivity firstly almost keeps constant but then increases from CaO/Al2O3=1. The minimum values of the total electrical conductivity and ionic conductivity occurs near CaO/Al2O3 = 1, which is mainly resulted from the charge compensation effect of Al3+ ions.
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  • Sensitive Influence of Floating State of Blast Furnace Deadman on Molten Iron Flow and Hearth Erosion

    pp. 2332-2341

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    DOI:10.2355/isijinternational.ISIJINT-2014-665

    Based on the survey of two Chinese large blast furnaces (BFs), it was found there were complex relationships between hearth sidewall and bottom temperature. The highest temperature positions for these two BFs were distinctly different, which was indicated that the positions of the most serious erosion were different. Therefore, the mathematical model of molten iron flow in BF hearth bottom was established and the influence of floating state of deadman on molten iron flow and wall shear stress was analyzed. The results showed that the status of molten iron flow was determined by the floating state of deadman so that the erosion position was influenced. The floating height of deadman of BF A was higher than that of BF B obviously, which led to the difference between hearth sidewall and bottom temperature and also the erosion migration. When deadman was sinking at bottom, the molten iron flow was faster and wall shear stress was larger near the bottom corners than that at other bottom positions so that the elephant-foot-type erosion occurred easily. When deadman was floating, the erosion position migrated from the hearth bottom corners to the corners between bottom of deadman and hearth sidewall. The permeability of deadman would also affect the erosion. Finally, the erosion profiles and their forming reasons were discussed at different floating heights of deadman.
  • Influence of Preformation Process on Combustibility of Biochar and its Application in Iron Ore Sintering

    pp. 2342-2349

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    DOI:10.2355/isijinternational.ISIJINT-2015-332

    A study was carried out into the use of preformation process to prepare biochar for replacing coke breeze in iron ore sintering. Two types of preformation process, thermo-compression process (TCP) and normal temperature process (NTP), were adopted. Results show that preformation contributed to improving the density of both raw biomass and biochar. Recommended preformation parameters for TCP were 200°C, 1 min and 120 MPa respectively, while the recommended preformation pressure for NTP was 180 MPa. Compared with non-preformed biochar, the specific surface area and porosity of preformed biochar were considerably reduced. The inner structure of biochar was therefore compacted to close to that of coke breeze. These changes bridged the differences in terms of combustion properties between biochar and coke breeze. Laboratory-scale sintering tests showed that the proper replacement percentage of biochar to coke breeze was increased from 20% to 40% after preformation process, and the emission reduction of SOx and NOx were correspondingly increased from 18.37% and 15.88% to about 38.00% and 27.00% respectively.
  • Effect of Oxygen Enrichment on Mineral Texture in Sintered Ore with Gaseous Fuel Injection

    pp. 2350-2357

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    DOI:10.2355/isijinternational.ISIJINT-2015-322

    The effects of oxygen enrichment on the mineral texture in sintered ore with the gaseous fuel injection and its operational results were evaluated in the actual plant.
    The mineral composition is an important factor affect to sinter strength. The mineral composition of sintered ore consists of Fe2O3, Fe3O4, and calcium ferrite and so on. Then, the strength of calcium ferrite is highest of all textures. As the results of sintering test with electrical furnace, calcium ferrite ratio in sintered ore increased with an increase of oxygen concentration in the atmosphere. The increase of oxygen concentration makes calcium ferrite to become stabilized.
    Gaseous fuel and oxygen injection technology have been installed at Chiba sinter plant since 2014. It was confirmed the expansion of the temperature zone between 1473 K and 1673 K which is proper for sintering reaction to form the calcium ferrite texture by the gaseous fuel and oxygen injection through the actual plant test. Oxygen enrichment shifted ignition position of coke and gaseous fuel to lower temperature side and the proper temperature zone was expanded. These results denoted the same tendency of the laboratory test results. Moreover, the calcium ferrite ratio of sintered ore increased in the actual plant. As these results, the effects of gaseous fuel and oxygen injection technology were confirmed in the actual machine similarly to the laboratory test.
  • Formation and Growth Mechanism of Clusters in Liquid REM-alloyed Stainless Steels

    pp. 2358-2364

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    DOI:10.2355/isijinternational.ISIJINT-2015-293

    REM-oxide clusters extracted from 253MA stainless steel grade samples from a pilot trial were investigated using a 2%TEA electrolyte. The samples were taken from liquid steel at different holding times after an addition of an appropriate amount of mischmetal. Thereafter, SEM in combination with EDS was deployed for three dimensional (3D) investigations of the characteristics of the extracted REM-oxide clusters. A reliable cluster size distribution (CSD) was obtained by improving the observation method and it was used to explicate the formation and growth mechanism of REM-oxide clusters. A correlation between morphology of clusters and their growth rate was found. This was used to divide the clusters into two different groups, which form and grow in accordance to different mechanisms. The results also show that the growth of clusters is governed by different types of collisions dependent up on size of the clusters. It has been concluded that for REM-oxide clusters turbulent collisions are the main controlling mode for the growth rate.
  • Simulation and Application of Bottom-Blowing in Electrical Arc Furnace Steelmaking Process

    pp. 2365-2373

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    DOI:10.2355/isijinternational.ISIJINT-2015-352

    Characteristics of flow field and stirring effects of six kinds of bottom-blowing arrangements on the molten bath were researched in a 75 t electrical arc furnace. The mixing time was measured by water experiment under different flow rates. Flow field characteristics of three-phase flow were simulated by Fluent software. At the tested conditions for the 75 t EAF, it was found that increasing flow rate, weakening impeding force of sidewall and improving stirring effect on the molten bath in EBT (Eccentric Bottom Tapping) region would decrease mixing time and improve stirring ability. Moreover, the flow velocity of molten steel was higher which was near surface of molten bath or near bottom of furnace. Compared with no bottom-blowing conditions at industrial application research, it showed that the bottom-blowing could stir molten better in steelmaking process for the 75 t EAF, which agreed well with the experimental results of the water experiments and the numerical simulations. Finally, an optimum bottom-blowing arrangement was determined.
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  • Microstructural Features and Formation Processes of As-cast Austenite Grain Structures in Hypoperitectic Carbon Steels

    pp. 2374-2382

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    DOI:10.2355/isijinternational.ISIJINT-2015-240

    Formation processes of as-cast austenite grain structures in hypoperitectic carbon steels have been investigated by means of a rapid directional solidification method, the cooling conditions of which are similar to those in the vicinity of slab surfaces in continuous casting processes. Coarse Columnar austenite Grain (CCG) structure was observed in all the hypoperitectic carbon steels employed in this study. It was demonstrated that its formation mechanism is ascribable to the discontinuous grain growth from Fine Columnar austenite Grain (FCG) formed in the delta phase, in which both the delta phase and residual liquid phase act as the pinning phase in the grain growth process. Then, a summary of the findings was provided regarding the microstructural features and the formation mechanisms of as-cast austenite grain structures formed in the rapid directional solidification in carbon steels with the carbon composition ranging from 0.05 to 0.45 mass%.
  • Effects of Manufacturing Parameters in Planar Flow Casting Process on Ribbon Formation and Puddle Evolution of Fe–Si–B Alloy

    pp. 2383-2390

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    DOI:10.2355/isijinternational.ISIJINT-2015-349

    The effects of manufacturing parameters in the planar flow casting process on the ribbon formation and the puddle stability of Fe78Si9B13 alloy are investigated experimentally. The ribbon morphology, surface quality, and puddle geometry are examined at different conditions and the transient evolution processes of puddle for molten metal passing through a rectangular nozzle are observed. The successful operability window for the production of Fe78Si9B13 ribbon is established and it is found the scope is different from that of Al-based alloy. The trend lines of the alloys on the plane of Reynolds number versus Weber number corresponding to obtain a successful ribbon are established. The ribbon thickness is found to vary with the applied pressure across the crucible and the wheel speed to the power of 0.45 and −0.9, respectively. The formation of small air pockets could be enhanced by increasing the applied pressure difference and wheel speed, or by decreasing the nozzle-wheel gap and the jetting temperature.
  • Rapid Sorting of Stainless Steels by Open-air Laser-induced Breakdown Spectroscopy with Detecting Chromium, Nickel, and Molybdenum

    pp. 2391-2396

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    DOI:10.2355/isijinternational.ISIJINT-2015-316

    Laser-induced breakdown spectroscopy (LIBS) is a promising method for rapid determination of compositions of stainless steels in steel scrap. LIBS is widely known as a method for elemental analysis that enables a rapid determination. It has several advantages such that it can work under ambient pressure, and specimens can be tested without any pre-treatment such as acid digestion, cleaning, or polishing of surface of specimens. We applied a laboratory-build LIBS system for sorting of six types of stainless steels. The standard reference materials of JISF FXS 324–334, 335–343, and 344–349, which are respectively Fe–Ni, Fe–Cr, and Fe–Mo binary alloys, were employed for making calibration lines of them. Considering spectral interferences from emission lines of the iron matrix in these alloys, seven emission lines could be chosen. Longer gate width, shorter delay time, high stability of pulse laser energy, and more number of laser shots can decrease the fluctuation of emission intensity. Utilizing these parameters mentioned above, the sorting of stainless specimens by detecting chromium, nickel, and molybdenum could be achieved.
  • Effect of Groove Shape on Closure of Center Defects in Symmetric Rolling of Round Billets

    pp. 2397-2405

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    DOI:10.2355/isijinternational.ISIJINT-2015-192

    Although the closure of defects at the center of round billets on rolling such billets is an important subject, a method of quantity evaluation for such closure has not yet been clarified. Therefore, to explain the effect of rolling conditions including grooved shape, we carried out the experiments with round billet which has an artificial defect and finite element analysis, especially with respect to the integration of the hydrostatic stress Gm. The results showed that grooved shape affected the closure of center defect, and Gm could express the relationship between closure and rolling conditions without the consideration of the difference in the ratio of defect size and roll size. However we found that original Gm could not express the influence of the grooved shape, at the same time.
  • Correlation of Strain Accommodation Factor with the State of Microstructural Components in a Multiphase Steel

    pp. 2406-2415

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    DOI:10.2355/isijinternational.ISIJINT-2015-213

    A mathematical approach to strain accommodation was employed to address the deformation behavior and microstructural evolutions of a two-phase steel during intercritical deformation. For this purpose, uniaxial compression tests were conducted at 640–960°C at strain rates of 10−3 to 10−1 s−1 up to the strain of 0.6. Constitutive equations were used to assign a strain accommodation factor for austenite and ferrite at each deformation condition. Then, a correlation between the strain accommodation factor, volume fractions of ferrite and austenite and dynamic microstructural evolutions was established. The electron backscatter diffraction and optical observations indicated that ferrite is completely responsible for strain accommodation at 760°C when the microstructure is occupied by 70% ferrite. The strain accommodation factor of ferrite decreased sharply with increasing deformation temperature and increased slightly with increasing strain rate. At 800°C, when the phase fractions of austenite and ferrite were close to equal, ferrite seemed to have bore 72% of the total strain in average. Under such conditions, the strain accommodation factor of ferrite showed considerable strain rate sensitivity due to proper load transferring from austenite toward ferrite and effective strain accommodation in ferrite. However, when the deformation temperature was increased only by 40°C to 840°C and the austenite fraction exceeded 60%, the strain bearing phase changed from ferrite to austenite, which was seen as coarse subgrains in ferrite. The strain accommodation transition point is believed to lie between 800 and 840°C, where the volume fraction of ferrite was less than 40%.
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  • Proposal for New Hearth Roller Consisting of Ceramic Sleeve and Steel Shafts Used in the Heat Treat Furnace

    pp. 2416-2425

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    DOI:10.2355/isijinternational.ISIJINT-2015-246

    Conventional rollers used in a heat furnace are called hearth roll. They must be changed very frequently since high temperature of the furnace induces wear on the roll surface in short period. This paper therefore discusses a new roller structure consisting of ceramic sleeve and steel shaft connected by shrink fitting. Although all ceramic sleeve has high temperature resistance and high corrosion resistance, attention should be paid for the risk of fracture due to the thermal expansion of the steel shaft that is much larger than the one of ceramic. Simple double cylinder simulation suggests that thinner structure is useful for reducing thermal stress. The finite element analysis shows that tapered shaft thickness is desirable for the ceramics hearth roller. Finally, an application of ceramic roller to steel manufacturing machinery is considered by changing geometry and material. Since only low shrink fitting ratio can be applied to the new roller, failure analysis is also considered for preventing the coming out of the shaft from the ceramic sleeve.
  • Ferrite Formation above the Ae3 Temperature during the Torsion Simulation of Strip Rolling

    pp. 2426-2434

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    DOI:10.2355/isijinternational.ISIJINT-2015-320

    Torsion simulations of 7-pass strip rolling were carried out on a 0.06%C-0.3%Mn-0.01%Si and a 0.11%C-1.0%Mn-0.11%Si-0.03%Al-0.034%Nb steel using pass strains of 0.4 applied at 1 s−1. The deformations were imposed isothermally at 910°C and 930°C for the C–Mn and the Nb microalloyed steel, respectively. The flow curve levels decreased from pass to pass as a result of softening by both dynamic transformation (DT) and dynamic recrystallization (DRX). The application of double differentiation to the stress-strain curves led to average critical strains for the initiation of DT and DRX of about 0.06 and 0.11, respectively. Optical microscopy revealed that the volume fraction of DT ferrite increased continuously right up to the last pass. The fraction of DT ferrite formed and retained was significantly higher when short interpass times were used. Comparison of the behaviors of the C–Mn and Nb steels indicates that Nb addition retards both the forward as well as the reverse transformation.
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  • Hydrogen Diffusivity in the Welding Zone of Two High Strength Experimental Microalloyed Steels

    pp. 2435-2442

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    DOI:10.2355/isijinternational.ISIJINT-2015-259

    Using permeation tests, determination of hydrogen trapping and microhardness measurements, the effect of the microstructure on the hydrogen diffusivity was analyzed in the welding zone of two high-strength experimental microalloyed steels called M1 steel, with a martensite and bainite microstructure and M2 steel with a quasi-polygonal ferrite (QPF), acicular ferrite (AF) and martensite-austenite (M/A) microstructure. Determination of the diffusivity was performed using electrochemical permeation tests, and hydrogen traps were determined with the silver decoration technique.
    One-pass welds without filler material were simulated with the Gas Tungsteng Arc Welding (GTAW) process, and samples of the welding zone were collected: base material (BM), heat affected zone (HAZ) and fusion zone (FZ). A Devanathan and Stachurski type electrochemical cell was constructed to conduct permeation tests. From the microstructural analysis, the permeation testing and silver decoration, it was observed that the hydrogen diffusivity decreases with the increase in traps, promotion of the formation of the M/A microconstituent and AF and the reduction of martensite and bainite microconstituents. The lower diffusivity of all zones of both steels is presented by the BM of M2 steel, which is associated with a QPF, AF and M/A microstructure and low microhardness. The highest relative amount of traps are in the coarse grained heat affected zone (CGHAZ) of both steels, however because these are reversible traps, these subzones could be the most susceptible to hydrogen induced cracking (HIC).
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  • Electrochemical Roles of Anti-corrosive Pigments in Sacrificial Corrosion Protection of Painted Galvanized Steel and their Relation to Organic Coating Delamination

    pp. 2443-2449

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    DOI:10.2355/isijinternational.ISIJINT-2015-249

    The effect of anti-corrosive pigments on the delamination of an organic film on a Zn coated steel was investigated using a cyclic wet-dry corrosion test (according to ISO 16539 Method A). The organic film was made from a mixture of polyvinyl butyral-co-vinyl acetate (PVB) ethanol, and a pigment. SrCrO4 or Na2HPO4 was added as a pigment. The delaminated area around a deep scratch on the specimen with the SrCrO4-containing film was smaller than those of the Na2HPO4-containing and pigment-free films. The corrosion rate and potential of the Zn layer coupled with the steel were estimated on the basis of the iR-drop in the films and the polarization curves of the Zn and steel electrodes. Also, the electrode potentials of the Zn under the PVB films were measured by scanning Kelvin probe force microscopy. From these results, it was confirmed that the delamination of the PVB films on galvanized steels was mainly caused by the anodic reaction of the Zn layer. The galvanic coupling of the delamination fronts at the interface of the Zn layer and the PVB film to the steel substrate at the scratches, inhibits the oxygen reduction reaction on the steel, in effect preventing Zn corrosion.
  • Delta Phase Characterization in Inconel 718 Alloys Through X-ray Diffraction

    pp. 2450-2454

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    DOI:10.2355/isijinternational.ISIJINT-2015-111

    One of the main issues of concern regarding the quality control of Inconel alloys is the precipitation of δ-phase, which in certain quantities and morphologies can lead to a loss in fracture toughness; it is therefore important to be able to properly detect and quantify it. X-ray diffraction (XRD) is a well-established and widely used technique for qualitative and quantitative analysis; however, the quality of its results depends on prior knowledge of the exact structure of each phase. The information on δ-phase available in the literature is incomplete and several discrepancies exist among different publications; furthermore, doubts persist regarding the interaction of δ and γ’’ phases. The aim of this work is to evaluate the structure of δ-phase in Inconel 718 alloys. This was achieved through a combination of experimental results, obtained for samples of the alloy that were subjected to three different heat treatments in order to produce different concentrations of δ-phase, and comparison of these with simulated results. This methodology allowed a global view of the mechanics of phase transformations in the alloy and to the correct identification of diffraction lines for δ-phase.
  • Phase-field Simulation of Habit Plane Formation during Martensitic Transformation in Low-carbon Steels

    pp. 2455-2462

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    DOI:10.2355/isijinternational.ISIJINT-2015-039

    The origin of the habit plane of the martensite phase (α′) in low-carbon steels is elucidated by three-dimensional phase-field simulations. The cubic → tetragonal martensitic transformation and the evolution of dislocations with Burgers vector aα/2〈111〉α in the evolving α′ phase are modeled simultaneously. By assuming a static defect in the undercooled parent phase (γ), we simulate the heterogeneous nucleation in the martensitic transformation. The transformation progresses with the formation of the stress-accommodating cluster composed of the three tetragonal domains of the α′ phase. With the growth of the α′ phase, the habit plane of the martensitic cluster emerges near the (111)γ plane, whereas it is not observed in the simulation in which the slip in the α′ phase is not considered. We observed that the formation of the (111)γ habit plane, which is characteristic of the lath martensite that contains a high dislocation density, is attributable to the slip in the α′ phase during the martensitic transformation.
  • Intergranular Fatigue Crack Initiation and Its Associated Small Fatigue Crack Propagation in Water-quenched Fe–C Fully Ferritic Steel

    pp. 2463-2468

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    DOI:10.2355/isijinternational.ISIJINT-2015-305

    The fatigue crack initiation and propagation behavior of a water-quenched binary Fe–C fully ferritic steel was investigated though rotating-bending fatigue testing. Intergranular and transgranular crack initiation and propagation were observed. The intergranular crack propagation did not stop, while the transgranular crack propagation was retarded by crack closure and strain aging. As a result, intergranular cracking was the dominant cause of fatigue damage in the steel. A considerable number of cracks were initiated; these propagated through coalescence, which occurred mainly at the grain boundaries. Dominancy of the intergranular fatigue crack propagation increased with increasing stress amplitude. In addition, the steel showed coaxing effect significantly. The coaxing effect suppresses crack initiation as well as crack propagation.
  • Low Cycle Fatigue Behavior of a 10% Cr Martensitic Steel at 600°C

    pp. 2469-2476

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    DOI:10.2355/isijinternational.ISIJINT-2015-336

    In this work, the low cycle fatigue (LCF) behavior of a 10% Cr martensitic steel was studied under fully reversed tension-compression loading at 600°C for constant strain amplitudes between ±0.20 to ±1.0%. The cyclic stress response exhibited a gradual softening regime until fracture after an initial short period of a nearly stable stress. The fatigue lifetime curve at 600°C was determined to be described by the Basquin-Manson-Coffin relationship. At constant strain amplitudes of ≥±0.35%, the cyclic strain resistance of the steel mostly depends on its plastic properties. The cyclic softening effect was attributed to the coarsening of the laths and subgrains, accompanied by a decrease in the dislocation density.
  • Effect of Hydrogen Partial Pressure on the Hydrogen Embrittlement Susceptibility of Type304 Stainless Steel in High-pressure H2/Ar Mixed Gas

    pp. 2477-2482

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    DOI:10.2355/isijinternational.ISIJINT-2015-232

    Slow strain rate testings (SSRTs) were carried out on Type304 stainless steel (Type304SS) in room temperature mixed gasses controlled to various pressure levels of up to 75 MPa, and the variation of embrittlement susceptibility of the tested steel under various hydrogen partial pressure was measured. Regardless of gas composition and at a hydrogen partial pressure level of 0.3 MPa or more in the gasses, elongation as well as relative reduction of area lowered, and brittle fracture surfaces appeared with accompanying quasi-cleavage. The critical PH2 at which Type304SS embrittles in a room temperature inert gas environment with mixed hydrogen is inferred to be between 0.05 and 0.1 MPa. From a conservative judgment it is concluded that hydrogen embrittlement fractures would be unlikely to occur to members made of Type304SS when PH2 is around one tenth of the atmospheric pressure or lower (i.e. 0.05 MPa).
  • First-principles Calculation of Effects of Carbon on Tetragonality and Magnetic Moment in Fe–C System

    pp. 2483-2491

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    DOI:10.2355/isijinternational.ISIJINT-2015-276

    The effects of carbon content on the tetragonality and magnetic moment of the Fe–C system have been evaluated using the first-principles calculation. Three types of supercell, Fe54C1, Fe54C2, and Fe128C1 (which correspond to Fe-0.40C, Fe-0.79C, and Fe-0.17C mass%, respectively) are used for the calculation. The main results are as follows: (1) The total and mechanical energies of the Fe–C system with carbon atoms at the octahedral sites are smaller than those of the system with carbon atoms at the tetragonal sites. The carbon atom at the octahedral site produces a relatively large expansion in one direction; (2) The tetragonality of the Fe–C system obtained using the first-principles calculation increases linearly with increasing carbon content and agrees well with experimental results. The average magnetic moment of the Fe atom increases with increasing carbon content; (3) The magnetic moment of the Fe atom at the carbon atom nearest neighbor site is lower than that of pure iron and increases with increased distance between the Fe and carbon atoms. The projected density of states exhibits a hybridization with the main contributions being from Fe d and C p states, which leads to the abovementioned decrease in the magnetic moment of the Fe atom. (4) In Fe54C2, the tetragonality and magnetic moment of the Fe atom change with the distance between two carbon atoms, with the tetragonality being 0.981, 1.036, or 1.090. When the Fe–C–Fe pair, which consists of the first carbon atom and its two nearest neighbor Fe atoms, is perpendicular to the second pair, which consists of the second carbon atom and its two nearest neighbor Fe atoms, the tetragonality is 0.981 and does not agree with the experimental value. The mechanical energy is relatively large. On the other hand, when the first pair is parallel to the second pair, the tetragonality is 1.036, which agrees well with experimental data. In this case, the mechanical energy is relatively small. When a straight C–Fe–C pair is formed, the tetragonality is 1.090; (5) In an Fe54C2 supercell, the formation enthalpy is relatively low when the calculated tetragonality is 1.036, and the existence probability under the assumption of a Boltzmann distribution is high. In other cases, the formation enthalpy is relatively high and the existence probability is almost zero; (6) The average magnetic moment of an Fe atom is proportional to volume, but it is not clearly related to the tetragonality. It is believed that the increase in the magnetic moment of the Fe atom by the addition of a carbon atom is primarily due to the magnetovolume effect, and is not due to the tetragonality effect.
  • Influence of Specimen Thickness on Thermal Desorption Spectrum of Hydrogen in High Strength SCM435 Steel

    pp. 2492-2498

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    DOI:10.2355/isijinternational.ISIJINT-2015-345

    Thermal desorption analysis (TDA) of hydrogen was carried out in high strength SCM435 steel in which the strain field of dislocation presumably is a major hydrogen trap site. Cylindrical specimens of radius 0.5–5 mm were cathodically charged until saturated and were heated at a rate ranging from 25 to 200°C/hr, aiming to vary the condition of hydrogen desorption from detrapping- to diffusion-control. For specimens of radius 0.5 mm the trap energy of hydrogen evaluated from the Choo-Lee (C-L) plot was as high as 33.8 kJ/mol, while for thicker specimens it was significantly smaller, i.e. 25.6–27.5 kJ/mol. The possible causes for the dependence of the trap energy on specimen thickness are discussed in terms of the influence of initial hydrogen distribution on the peak temperature and the deviation from local equilibrium of hydrogen during desorption. If pre-exposure is carried out for a sufficiently long time prior to TDA, the C-L plot seems to give a correct detrap energy even in the mixed-control desorption.
  • Effect of Water Granulation Conditions on Density and Grain Size of Granulated Blast Furnace Slag

    pp. 2499-2508

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    DOI:10.2355/isijinternational.ISIJINT-2015-254

    The water granulation conditions for producing high density, coarse granulated blast furnace slag were investigated in a laboratory-scale experiment. The influence of slag temperature, water temperature and nozzle shape on the density and grain size of granulated slag was clarified. The influence of these factors on the density and grain size of the slag was confirmed by using various nozzles in a slag water granulation system.
    Neural network computation was applied to estimation of the density and grain size of granulated blast furnace slag. The influence of water granulation conditions on the density and grain size predicted by neural network computation.
    Based on the results of this research, we proposed a new slag granulation system and manufactured a high density, coarse blast furnace slag fine aggregate.
    x

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  • Aqueous Carbonation of Steel Slag: A Kinetics Study

    pp. 2509-2514

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    DOI:10.2355/isijinternational.ISIJINT-2015-142

    In this study, the kinetics of aqueous carbonation of steel slag in an atmospheric three-phase system containing steel slag, water, and CO2 gas was studied. Also, some factors likely affecting this process were investigated, such as reaction time and temperature, steel slag particle size (d0.5), CO2 flow rate, and the mass ratio of liquid to solid (L/S). The particle size of steel slag and the reaction temperature were found to be the major factors affecting the carbonation degree. The carbonation degree was determined to be 26.4% under the following conditions: reaction time of 3 h, temperature of 60°C, CO2 flow rate of 600 ml/min, d0.5=12.8 µm, L/S=10, corresponding to a capacity of 0.264 kg CO2/kg steel slag. The experimental results were employed to study the reaction mechanism using the shrinking core model. The aqueous carbonation process of steel slag was found to be limited by the diffusion of calcium carbonate through the product layer. The apparent activation energy of the aqueous carbonation of steel slag was found to be 4.8 kJ/mol. It was confirmed that aqueous carbonation of steel slag is an effective approach for enhancing the CO2-sequestration capacity and reducing the environmental impacts of steel slag.
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  • Effect of Pearlite Structure on Lattice Strain in Ferrite Estimated by the Williamson-Hall Method

    pp. 2515-2517

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    DOI:10.2355/isijinternational.ISIJINT-2015-407

    The effect of pearlite on the X-ray diffraction peak reflected from ferrite phase in ferrite-pearlite steel was investigated using normalized carbon steels with different volume fraction of pearlite and a hypereutectoid steel with different interlamellar spacing. The lattice strain in ferrite phase, which causes the broadening of X-ray diffraction peak, was increased in proportion to both of the volume fraction of pearlite and the inverse of interlamellar spacing. As a result, the lattice strain in ferrite-pearlite steel can be simply formulated as a function of them. On the other hand, TEM observation reveals that pearlite has low-density dislocation in ferrite phase. This result suggests that the misfit between ferrite and cementite in pearlite generates the significant amount of elastic strain, which leads to the increase in lattice strain. Therefore, the dislocation density must be overestimated in carbon steels with pearlite, if it is estimated from the experimental lattice strain directly.

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