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

ISIJ International Vol. 59 (2019), No. 5

  • A Kinetic Model of Mass Transfer and Chemical Reactions at a Steel/Slag Interface under Effect of Interfacial Tensions

    pp. 737-748

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    DOI:10.2355/isijinternational.ISIJINT-2018-496

    A new kinetic model was developed to predict the dynamic change of the interfacial oxygen content and the steel/slag interfacial tension. This model mainly describes the following interfacial physicochemical phenomena: i) Silica decomposition and oxygen adsorption at the interface, ii) Oxygen and aluminum reactions at the interface, iii) Oxygen desorption from the interface, iv) Silica mass transfer from the slag to the interface, v) Dissolution of the formed alumina into the slag and its transfer in slag and vi) Blockage on the silica mass transfer, to come in contact with the steel, by the accumulation of the formed alumina at the interface. With this model, the dynamic changes of the interfacial oxygen contents under different aluminum contents in steel and different slag viscosities were predicted. Overall, the interfacial oxygen content was found to increase with a decreased aluminum content and a decreased slag viscosity. Furthermore, the aluminum reaction rate can significantly influence the interfacial oxygen content as well as the interfacial tension. In addition, the model captured the fast increase of the interfacial tension after passing the minimum value point for the system of a high-Al content steel and a low viscous slag, which is in agreement with the experimental observations. Furthermore, a parameter study was carried out to show the influence of various parameters on the dynamic interfacial phenomena.
  • Influence of Al/Ti Ratio in Ti-ULC Steel and Refractory Components of Submerged Entry Nozzle on Formation of Clogging Deposits

    pp. 749-758

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    DOI:10.2355/isijinternational.ISIJINT-2018-672

    Nozzle clogging during continuous casting of Ti-ULC (Ultra Low C) steel was investigated by inspections of plant-used SENs and laboratory scale experiments using a rotating finger method. Various Al/Ti ratios in the steel and different kinds of nozzles (with or without CaO) were employed. Clog deposits found in the used SENs were composed of complicated oxide (CaO–Al2O3–TiOx-…), Fe drops and chunks. Increasing Ti concentration increased amount of the deposit the metallic deposits. In the laboratory scale experiments, increasing Al/Ti ratios was effective to suppress the formation of the deposit and the oxidation loss of Ti in the steel. When a refractory without CaO was used, increasing Al/Ti ratio decreased the portion of Ti oxide in the deposit. Almost no Fe drops were observed, except for a thin layer of Fe on surface of the oxide deposit. When CaO-lined refractory was used, many numbers of Fe drops were found inside the deposit. Increasing Al/Ti ratio lowered Ti peak in the oxide deposit. The present results lend a strong support that Ti-ULC steel is oxidized by CO gas from nozzle refractory, forming FetO-containing liquid oxide and solid alumina. The liquid oxide is reduced to leave Fe drops and Al2O3–TiOx layer. If CaO presents in the nozzle, then a complicated oxide of CaO–Al2O3–TiOx forms, as was found in the SEN from the practical operation.
  • Surface Tension Calculation of Molten Slag in SiO2–Al2O3–CaO–MgO Systems Based on a Statistical Modelling Approach

    pp. 759-767

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    DOI:10.2355/isijinternational.ISIJINT-2018-746

    A calculation model of surface tension for molten slags in SiO2–Al2O3–CaO–MgO systems has been developed in the present study, based on a statistical modelling approach. A total of 832 experimental data of slags have been critically selected and used for the optimization of model parameters, and the present model has a fairly lower average deviation with 4.60% within the applicable composition and temperature ranges. And also, the composition and temperature dependences of surface tension have been discussed using the model. The results indicate that the surface tensions always decrease with increasing SiO2 content and increase with increasing MgO content. The surface tension will decrease with increasing temperature.
  • Improvement of Sinter Strength and Reducibility through Promotion of Magnetite Ore Oxidation by Use of Separate Granulating Method

    pp. 768-777

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    DOI:10.2355/isijinternational.ISIJINT-2018-518

    In general, Fe content in iron ore is gradually decreasing. This fact affects worse performance of BF operation, for example, increase of RAR and Slag ratio. Depletion of high grade iron ore deposits is moving us to use concentrates in sintering process.Through magnetite concentration deteriorates reducibility because of high FeO content in sinter product. Such situation makes it to promote oxidation of magnetite iron ore during sintering process for improving sinter reducibility. In addition, promoting oxidation of magnetite possibly increases sinter strength with using oxidation heat.ISIJ sinter research group for utilization of magnetite concentration suggests that restricting melt formation is critical for promoting oxidation of magnetite concentration.In this paper, It is confirmed that “Separate Granulation” has been examined to apply their suggestion by sinter pot test.The main results obtained are described as follows:(1) “Separate Granulation” in case that magnetite is pre-granulated with high Al2O3 iron ore without limestone and coke breeze resulted in decrease of FeO in sinter and improvement of both sinter reducibility and sinter strength.(2) Sinter micro structure featured restriction of pore, low circle factor and small mineral texture, which supported that melting restriction worked during sintering.(3) Magnetite decreased and hematite increased as sinter mineral, which corresponded with decrease of FeO content.(4) These facts shown (1) to (3) concluded that “Separate Granulation” is effective to improve both sinter reducibility and sinter strength due to restriction of melting during sinter reaction.
  • Effect of Nut Coke Addition on Physicochemical Behaviour of Pellet Bed in Ironmaking Blast Furnace

    pp. 778-786

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    DOI:10.2355/isijinternational.ISIJINT-2018-580

    One of the primary causes that limit the blast furnace productivity is the resistance exerted to the gas flow in the cohesive zone by the ferrous burden. Use of nut coke (10–40 mm) together with ferrous burden proves beneficial for decreasing this resistance. In present study, effect of nut coke addition on the olivine fluxed iron ore pellet bed is investigated under simulated blast furnace conditions. Nut coke mixing degree (replacement ratio of regular coke) was varied from 0 to 40 wt% to investigate the physicochemical characteristics of the pellet bed. Three distinct stages of bed contraction are observed and the principal phenomena governing these stages are indirect reduction, softening and melting. It is observed that nut coke mixing enhances the reduction kinetics, lowers softening, limits sintering and promotes iron carburisation to affects all three stages. In the second stage, the temperature and displacement range is reduced by 60°C and 24%, respectively upon 40 wt% nut coke mixing. Addition of nut coke exponentially increases the gas permeability (represented by pressure drop and S-value). A higher degree of carburisation achieved on the pellet shell (iron) is suggested to be the principal reason for decrease in the pellet melting temperature. The pellets softening temperature increases by approximately 4°C, melting and dripping temperature drops by 11°C and 12°C, respectively, for every 10 wt% nut coke addition. Consequently, the nut coke addition shortens the softening, melting and dripping temperature ranges, which shows improved properties of the cohesive zone.
  • Direct Reduction Recycling of Mill Scale Through Iron Powder Synthesis

    pp. 787-794

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    DOI:10.2355/isijinternational.ISIJINT-2018-628

    Mill scale, a potential raw material for recycling from hot rolling mill operations is chosen and one step thermo-chemical reduction technique is employed to beneficiate the iron content in the form of powdered iron. Experiments are conducted at various temperature (600–1300°C) and time (1–4 h) combinations using hydrogen as reducing atmosphere. Physical and chemical properties of mill scale iron powders (MIP) are analysed using particle size analyser, gas pycnometer and wet chemical testing. MIP are also characterized for phase and morphology using X-ray diffractometer and scanning electron microscopy respectively. Effect of parameters like temperature of reduction, time of reduction, particle size of raw material, sintering and grinding on the iron powder synthesis is well studied. Mill scale iron powder with > 99% degree of metallization, 97% Fe (T), > 96% Fe (met) and 2.63 g/cc apparent density is obtained at 1200°C, 4 h and 1300°C, 4 h parameters and this material would stand promising for recycling through nutrition supplements, body warmers, water purification, sound insulators, etc applications.
  • A Phase Equilibrium of the Iron-rich Corner of the CaO–FeO–Fe2O3–SiO2 System in Air and the Determination of the SFC Primary Phase Field

    pp. 795-804

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    DOI:10.2355/isijinternational.ISIJINT-2018-575

    Experimental studies of the CaO–FeO–Fe2O3–SiO2 system in air from 1200°C to 1260°C have been carried out in the Fe-rich region with particular focus on the phase equilibria associated with silico-ferrite of calcium (SFC) phase. The measurements have been made possible through the use of a modified experimental technique involving high temperature equilibration and rapid quenching followed by electron probe X-ray microanalysis (EPMA). Isothermal sections for 1220°C, 1240°C, 1255°C and 1260°C are reported providing information on the limits of stability of the SFC solid solution and the liquidus in the SFC primary phase field for the first time.
  • Effect of Gas Atmosphere on the Phase Chemistry in the CaO–FeO–Fe2O3–SiO2 System Related to Iron Ore Sinter-making

    pp. 805-809

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    DOI:10.2355/isijinternational.ISIJINT-2018-631

    Experimental phase equilibria studies have been carried out to investigate the phase chemistry in the CaO–“Fe2O3”–SiO2 system related to the iron ore sinter making in 1 atm CO2 at 1200–1230°C. The measurements have been performed using a modified experimental technique involving high temperature equilibration and rapid quenching followed by electron probe X-ray microanalysis (EPMA). In contrast to the equilibria in air, the experimental results show that no silico-ferrite of calcium (SFC solid solution) is formed in 1 atm CO2 within the temperature range investigated; that is, the SFC phase is not stable at liquidus temperatures. The liquidus for the iron-rich corner of the CaO–“Fe2O3”–SiO2 system in CO2 atmosphere for the spinel (magnetite Fe3O4) and dicalcium silicate (C2S = Ca2SiO4) primary phase fields has been proposed.
  • Numerical Investigation of Applying High-carbon Metallic Briquette in Blast Furnace Ironmaking

    pp. 810-819

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    DOI:10.2355/isijinternational.ISIJINT-2018-673

    Application of high-carbon metallic briquette (HCMB) in the blast furnace (BF) ironmaking for coke saving was previously proposed. This paper is focused on clarifying the in-furnace phenomena and demonstrating the advantages of applying the HCMB in BF ironmaking. A mathematical model has been formulated based on the gas-solid counterflow and its validity was confirmed by the comparison of the simulation results with the averaged industrial data from a BF with a productivity of 6250 tHM/day. Afterward, BF operations under two HCMB mixing ratios (5% and 10%) were simulated. Simulation results indicate that charging HCMB in BF can suppress the coke gasification and improve the ore reduction above the CZ. Coke of approximately 12 kg could be saved for producing one-ton hot metal from the ore under an HCMB mixing ratio of 5%, and approximately 17 kg under an HCMB mixing ratio of 10%. Simulation results still indicate that the reasonable mixing ratio of HCMB is less than 5%, under which, significant changes of the BF operation conditions for HCMB charging are not required.
  • ADEM Simulation for Analysis of Particle Breakage of Irregular Shaped Particles

    pp. 820-827

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    DOI:10.2355/isijinternational.ISIJINT-2018-688

    Breakage behavior of irregular shaped particles was analyzed by ADEM (Advanced Distinct Element Method). This paper attempted to determine ADEM parameters to represent breakage behavior of irregular shaped particles. Compression test have been carried out experimentally to obtain information about particle breakage. In the simulation, joint spring coefficient and maximum strain were parameters for reproducing the experimental results. Pumice was chosen as a sample, and the shape was measured by 3D scanner. Obtained 3D data was converted to cluster particle which is agglomerate of primary particles. Simulation results were compared to experimental results from the view point of load-displacement curve and the shape of samples after breakage. In the simulation, the gradient of load in response to displacement obviously increases with an increase in the joint spring coefficient. Compressive strength increases with an increase in the maximum strain. Impact test was carried out to confirm whether the parameters determined by compression test can also be applied to other breakage behavior. In the impact test, a stainless steel ball was free-falling on the cluster particle from 200 or 400 mm in drop height. In experiment, the number of dropping times needed for breakage decrease with increase in falling height. Same tendency could be observed in the simulation. Breakage behavior in impact test could express by ADEM simulation using the simulation parameters obtained from compression test. According to these results, it is suggested that ADEM has a possibility to become a useful tool to analyze particle breakage behavior.
  • Formation and Evolution of Non-Metallic Inclusions in Calcium Treatment H13 Steel during Electroslag Remelting Process

    pp. 828-838

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    DOI:10.2355/isijinternational.ISIJINT-2018-537

    In the current study, the evolution of inclusions in H13 steel electrode with different Ca content refined through ESR process were analyzed, the formation and evolution of sulfide and oxide inclusions during ESR process was determined based on experiments and thermodynamic calculation. The results showed: The inclusions of remelted ingot were Al2O3, CaO–Al2O3–CaS and CaO–Al2O3–MgO–CaS, more than half of the total inclusions were smaller than 3 µm. The cleanliness and inclusions of remelted ingot had no difference with different Ca content in electrode. MnS inclusions in electrode precipitated while the solid fraction reached 0.98. MnS inclusions cannot precipitate due to the low level of S content in remelted ingot and the decrease of Mn and S segregation on the front edge of solidified shell. The low-melting-temperature inclusions were dissolved and disappeared completely before liquid metal droplets collected in the liquid metal pool. The different of local cooling intensity during ESR process and high Al2O3 content of liquid inclusion in molten pool promoted the formation of large amount of tiny Al2O3 inclusions in remelted ingot.
  • Phosphorus Partition and Phosphate Capacity of TiO2 Bearing Basic Oxygen Steelmaking Slags

    pp. 839-847

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    DOI:10.2355/isijinternational.ISIJINT-2018-683

    The phosphorus partition (LP) and phosphate capacity ([ This part of text is omitted ]) were measured for TiO2 bearing basic oxygen steelmaking slags in the CaO–SiO2–MgO–FetO–(TiO2–MnO–Al2O3–P2O5) system at 1650°C. The effect of slag and metal additions were tested by varying the TiO2 content from 0.0 to 18.0 mass% and the [Ti] content from 0.009 to 0.301 mass%. A recently published LP model was used to assess the experimental LP data using the measured slag composition and temperature. Experimental LP data from this study and literature data were used to modify the published model to include titania.Increasing the TiO2 concentration of the slag was found to decrease the LP and [ This part of text is omitted ] of basic oxygen steelmaking (BOS) slags. Capacity values in the range of 2.2×1016 to 1.5×1018 at 1650°C were obtained. An empirical model for determining [ This part of text is omitted ] was developed for BOS slags using a large dataset of published slag and pO2 data for relevant slag systems, including published data for titania bearing slags. The predicted [ This part of text is omitted ] from the empirical model was found to agree with the experimentally determined [ This part of text is omitted ] data from this study.
  • Effect of Cooling Rate on the Solidification Microstructure and Characteristics of Primary Carbides in H13 Steel

    pp. 848-857

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    DOI:10.2355/isijinternational.ISIJINT-2018-524

    The microstructure, alloying elements segregation and characteristics of primary carbides in AISI H13 steel that solidified at different cooling rate were investigated by optical microscope (OM), field emission scanning electron microscope (FE-SEM), electro-probe microanalyzer (EPMA) and automated inclusion analyzer ASPEX. The microstructure of H13 steel samples become more refined with increased cooling rate. The equation of relationship between cooling rate (RC) and secondary dendrite arm spacing (λ) for H13 steel could be expressed as [ This part of text is omitted ]. Primary carbides are located in interdendritic region, where existed obvious Cr, Mo, V and C segregation. Higher cooling rate promoted higher alloying elements segregation and facilitated earlier precipitation of primary carbides during solidification process. The number, size, amount and mean area of primary carbides decreased significantly with the increased cooling rate, however the shape of the primary carbides were insensitive to cooling rate. Thermodynamic calculation indicated that V-rich primary carbides precipitated at solid fraction larger than 0.94, Mo-rich primary carbides precipitated at solid fraction larger than 0.99 in the cooling rate range investigated. Lower cooling rate suppressed alloying elements segregation, but the precipitation of primary carbides could not be avoided in the cooling rate range.
  • Effect of Silicon on AHSS As-Cast Microstructure Development and Properties

    pp. 858-864

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    DOI:10.2355/isijinternational.ISIJINT-2018-598

    This work linked properties and performance in as-cast condition for 3rd generation advanced high strength steel (AHSS) by examining the effects of chemical composition and microstructure on mechanical properties. Elevated levels of carbon, manganese, and silicon in new AHSS grades lead to a complex evolution of microstructure during solidification that can lead to castability problems. Three lab cast ingots with 0.2 wt% C, 3 wt% Mn, and 0.5, 1.5, and 3 wt% Si were characterized by their microstructure and mechanical properties. Light optical microscopy (LOM) and Scanning Electron Microscopy (SEM) confirmed that the microstructure of steels was mostly granular bainite, with some proeutectoid ferrite allotriomorphs at 3 wt% Si. Tensile testing showed Si increased strength and that ductility of all samples was low. Higher silicon levels were found to promote formation of proeutectoid ferrite allotriomorphs and changed the cracking propagation behavior. Some comparisons between the observed microstructures and those expected in continuously cast slabs were also discussed.
  • Simulation of Crack Initiation on the Slab in Continuous Casting Machine by FEM

    pp. 865-871

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    DOI:10.2355/isijinternational.ISIJINT-2018-679

    In continuous casting of steel, prevention of surface cracks on the slab is an important issue. For quantitative evaluation of cracks that occur in the continuous casting machine, the critical strain for crack generation was analyzed by a high-temperature tensile test and FEM simulation. Based on obtained material property values, a model for crack generation by tensile strain was constructed. The local strain at the notch relative to the strain in the whole specimen was determined by a simulation of the tensile test, and the critical strain for crack generation εc was calculated. The results of a crack simulation by FEM using εc showed that the average strain until crack initiation was small under deep notch conditions. The average strain for crack generation calculated by the simulation model was in good agreement with the value measured in the tensile test. As a result of the simulation applying temperature distribution to the slab, the depth change of the oscillation mark was more influential to crack formation than the change of the width. The effect of the shape of the oscillation mark on the crack cannot be organized only by the stress concentration factor. Simulation analysis that includes the shape of the oscillation mark is considered to be effective. Using this simulation model, it is possible to predict the generation of cracking when the temperature distribution or the oscillation mark shape in actual operation changes.
  • Solidification Structure, Non-metallic Inclusions and Hot Ductility of Continuously Cast High Manganese TWIP Steel Slab

    pp. 872-879

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    DOI:10.2355/isijinternational.ISIJINT-2018-694

    In the current study, the solidification structure, non-metallic inclusions and hot ductility of continuously cast high manganese TWIP steel slab have been investigated and the inclusion formation behavior have been revealed by FactSage (CRCT-ThermFact Inc., Montréal, Canada). The area ratio of equiaxed grain zone of the TWIP steel slab is 0.18. Two main types of inclusions in the TWIP steel slab are single AlN particle and AlN+MnS aggregates. It is found that MgAl2O4 and AlN particles can precipitate in the initial solidification stage, which can act as heterogeneous nuclei of other inclusions. In the high temperature tension test, the reduction of area (RA) of the TWIP steel slab samples are higher than 40 pct in the temperature range from 873 K to 1473 K (600°C to 1200°C). Brittle fractures are observed in the fracture surface of the TWIP steel slab samples with dimples. Contents of manganese, carbon, sulfur and phosphorus, strain rate, and dynamic recrystallization (DRX) are factors influencing the hot ductility of TWIP steel slab.
  • Interactive Relationship between the Superheat, Interfacial Heat Transfer, Deposited Film and Microstructure in Strip Casting of Duplex Stainless Steel

    pp. 880-888

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    DOI:10.2355/isijinternational.ISIJINT-2018-747

    In this study, a new droplet solidification test has been developed to simulate the process of sub-rapid solidification of 2205 duplex stainless steel (DSS) strip casting process. The results suggest that the maximum heat flux and amount of heat removed increase with the repeat of dropping tests. Furthermore, the final wetting angle between the solidified droplet and substrate decreases, and the bottom area of the droplet increases with the successive addition of solidifying material on prior film. The major composition of the films is detected mainly as oxides containing O, Fe, Si, Mn, Cr and Cu, which precipitate from DSS samples and substrate. The microstructure of the sub-rapid solidified DSS samples consists of δ-ferrite (60–65%) and Widmanstätten-like austenite (35–40%) phases. The element N mainly segregates in austenite phase and elements Si, Mo tend to stabilize ferrite phase. The finer microstructure in HS4 (the 12th droplet experiment with high superheat) sample implies that a higher interfacial heat flux (10.4360 MW m−2) brings a higher cooling rate, and the nucleation rate can be increased markedly, causing a less space for each grain to grow and further refine the as-cast grain size.
  • Numerical Simulation on Interfacial Creep Generation for Shrink-fitted Bimetallic Roll

    pp. 889-894

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    DOI:10.2355/isijinternational.ISIJINT-2018-749

    The bimetallic work rolls are widely used in the roughing stands of hot rolling stand mills. The rolls are classified into two types; one is a single-solid type, and the other is a shrink-fitted assembled type consisting of a sleeve and a shaft. Regarding the assembled rolls, the interfacial creep sometimes appears between the shaft and the shrink-fitted sleeve. This interfacial creep means the relative displacement on the interface between the sleeve and the shaft. This creep phenomenon often causes damage to the roll such as shaft breakage due to fretting cracks. Although to clarify this creep mechanism is an important issue, experimental simulation is very difficult to be conducted. Since few studies are available, in this paper, the interfacial creep phenomenon is simulated by using the elastic finite element method (FEM) analysis. Here, the roll rotation is replaced by the road shift on the fixed roll surface. It is found that the interface creep can be explained as the accumulation of the relative circumferential displacement along the interface.
  • Analysis of Heat Crystallization Behavior of Polyester Film on Laminated Steel Sheet

    pp. 895-899

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    DOI:10.2355/isijinternational.ISIJINT-2018-733

    Polyester films have been used widely as laminate films of beverage and food cans due to their excellent properties of formability, corrosion resistance, and adhesion to steel sheets. Recently, excellent formability under a higher processing degree has been required in laminated steel sheets for drawn and ironing (DI) cans, which are used as beverage and food cans. Polyester films which are almost amorphous are used in this application since high formability is needed in the laminate film. In this study, we investigated the thermal crystallization behaviors of a near-amorphous oriented polyester film and a non-oriented film by thermal analysis and Raman spectroscopy. We found that the two types of films display different thermal crystallization behaviors.
  • Heat Transfer Characteristic of Slit Nozzle Impingement on High-temperature Plate Surface

    pp. 900-907

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    DOI:10.2355/isijinternational.ISIJINT-2018-576

    Heat transfer mechanism of a slit jet impingement was thoroughly studied to improve capacity and uniformity of a hot steel strip/plate during its ultrafast cooling or quenching. The impact angle has a significant influence on the heat transfer characteristics of the stationary slit jet impinging process. Heat transfer capability and rewetting front propagation, which include such parameters as qmax, tMHF, and TMHF, differ significantly between the upstream and downstream regions. Parallel flow and intense sputtering in the downstream region are apparent for the forward-moving inclined slit jet impingement cooling process. The antiparallel flow in the upstream region is thinner, and the sputtering is reduced and is relatively stable. As the plate moves forward, the wetting front expands and forms almost a straight line with synchronized and uniform heat transfer. The inclined angle increases from 0 to 45°, which significantly increases the heat transfer intensity and shortens the time to nucleate boiling stage as well as the width of the transitional boiling region. A higher moving velocity reduces and promotes qmax moving to the downstream region.
  • Effect of Low-temperature Nitridation on Sulfide Stress Corrosion of 321 Austenitic Stainless Steel in H2S-Containing Environments

    pp. 908-917

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    DOI:10.2355/isijinternational.ISIJINT-2018-515

    The effect of low-temperature nitridation on corrosion resistance and sulfide stress corrosion (SSC) behavior of 321 austenitic stainless steel in H2S-containing environments was investigated. Varying four-point bend loading of 80 pct yield stress, 100 pct yield stress, 120 pct yield stress were respectively applied to nitrided 321 austenitic stainless steels. Although no macroscopic corrosion cracking is observed, pits are formed on the alloy surfaces. As the bending stress increases, the numbers and depths of pitting corrosion on the surface of the nitrided samples are getting more and more serious. Under 100 pct YS, the surface of unnitrided sample is completely destroyed and a series of jagged corrosion pits appear. The corrosion degree of the tension side is more serious than that of the compression side for a same sample. The result demonstrates that the low-temperature liquid nitridation can effectively improve the stress corrosion resistance and SSC behavior. The low-temperature nitrided sample has better stress corrosion resistance than the unnitrided sample in H2S-containing environments and the SSC resistance of 321 austenitic stainless steel can be apparently improved by low-temperature nitridation.
  • Effects of Surface Microstructure on Selective Oxidation Morphology and Kinetics in N2 + 5%H2 Atmosphere with Variable Dew Point Temperature

    pp. 918-926

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    DOI:10.2355/isijinternational.ISIJINT-2018-727

    Starting surface microstructure has been shown to influence selective oxidation morphology and kinetics on cold-rolled grades of CMnSi advanced high strength steels (AHSSs). The research presented below examined this phenomenon after 120 and 1800 s and under N2 + 5%H2 with 0°C and −30°C dewpoint temperatures (DPTs). Starting microstructures were altered by pre-annealing, which led to decarburization and grain growth. All samples were oxidized in a high temperature confocal scanning laser microscope (CSLM) at 850°C. External oxides were subsequently examined using secondary electron scanning electron microscopy (SE SEM) and energy-dispersive spectroscopy (EDS). Cross-sections were produced and examined in a focused ion beam scanning electron microscope (FIB-SEM). Oxidizing atmosphere, surface microstructure, and time all influenced selective oxidation behavior. High and low DPT atmospheres had expected effects. Variation between microstructures was only apparent in 1800 s samples oxidized in a high DPT atmosphere. Decarburized samples oxidized at higher DPTs exhibited surfaces covered with discrete iron nodules which may facilitate reactive zinc wetting.
  • Crystallographic Characterisation of Hydrogen-induced Twin Boundary Separation in Type 304 Stainless Steel Using Micro-tensile Testing

    pp. 927-934

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    DOI:10.2355/isijinternational.ISIJINT-2018-608

    Micro-tensile behaviour and the corresponding microstructural evolution under hydrogen pre-charging conditions were examined on single-crystalline and twinned bi-crystalline specimens with the same [111] loading axis to elucidate the hydrogen-induced twin boundary separation in type 304 stainless steel. A hydrogen pre-charge increased the flow stress during tensile testing but decreased the elongation-to-failure in both single-crystalline and twinned specimens. Although the hydrogen-charged single-crystalline specimen exhibited a quasi-cleavage, the presence of a twin boundary induced a premature failure at the twin boundary interface. Flat-facetted features due to the twin boundary separation had linear steps in the three <110> directions, which corresponded to the intersections between the twin plane and the other {111} close-packed planes of austenite. Matching halves of the fracture surface along the three directions perpendicular to the linear steps, i.e. <112> on the (111) twin plane, revealed two sets of concavity–flat surface and a peak-and-valley correspondence. In addition, electron backscatter diffraction analysis of the substructures below the fracture surfaces revealed that martensite variants developed mainly with their habit planes parallel to the most favourably shear-stressed plane in each crystal, and they grew towards the concavities on the fracture surfaces. These findings suggest that the hydrogen-induced twin boundary separation is triggered by cracks generated by the high hydrogen concentration at the twin boundary due to deformation-induced martensitic transformation, and this is followed by coalescence of cracks through hydrogen-enhanced alternating shear on the slip planes situated symmetrically with respect to the twin boundary.
  • Effect of Nitrogen on Planar Anisotropy of the r-value and Texture in Lean Duplex Stainless Steel Sheets

    pp. 935-938

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    DOI:10.2355/isijinternational.ISIJINT-2018-633

    The planar anisotropy of the r-value of duplex stainless steel sheets showed reverse V-type. To ascertain the possibility of control, the anisotropies and textures of cold-rolled and annealed sheets with different amounts of nitrogen, based on 21.2%Cr-1.5%Ni-5.0%Mn-0.02%C steel, and for different annealing temperatures after hot rolling were investigated. With a decrease in the amount of nitrogen and an increase in the annealing temperature before cold rolling, the anisotropy changed to V-type from reverse V-type. In addition, the singular texture with weak αbcc-fiber and strong Goss orientation in the α phase of the low-nitrogen added sheet was observed.

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