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

ISIJ International Vol. 55 (2015), No. 1

  • Recent Advances in the Fluid Dynamics of Ladle Metallurgy

    pp. 1-6

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    DOI:10.2355/isijinternational.55.1

    Ladle or secondary metallurgy in steelmaking is an essential process step for the production of high-quality steel. There has been a vast amount of research conducted in this area, particularly regarding refining reactions and inclusion chemistry. More recently, the fluid dynamic aspects of the process have been examined in more detail to provide better control of the process. In this Honorary Lecture two recent aspects related to fluid dynamics are reviewed; gas-liquid plumes and entrainment of slag droplets in metal.
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  • Ironmaking Technology for the Last 100 Years: Deployment to Advanced Technologies from Introduction of Technological Know-how, and Evolution to Next-generation Process

    pp. 7-35

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    DOI:10.2355/isijinternational.55.7

    The 150 year history of the Japanese steel industry dates from the first western blast furnace, which was built by T. Ohashi in 1857. Modern blast furnace operation at integrated steel works in Japan started in 1901 with the first blow-in of Higashida No. 1 blast furnace at Yawata Steel Works. Throughout the prewar and postwar periods, the steel industry has supported the Japanese economy as a key industry which supplies basic materials for social infrastructure and development.
    After the period of recovery following the destruction caused by World War II, Chiba Works of Kawasaki Steel Corporation (now JFE Steel Corporation) was built and began operation in 1953 as the first integrated steel works in the Keiyo Industrial Region after the war. During Japan’s period of high economic growth, many coastal steel works with large blast furnaces having inner volumes of more than 3000 m3 and even 5000 m3 were built to enable efficient marine transportation of raw materials and steel products. Japanese steel makers introduced and improved the most advanced technologies of the day, which included high pressure equipment, stave cooler systems, bell-less charging systems, etc. As a result, Japanese steel works now lead the world in low reducing agent rate (RAR) operation, energy saving, and long service life of blast furnaces and coke ovens.
    Following the Oil Crises of the 1970s, the Japanese steel industry changed energy sources from oil to coal and implemented cost-oriented operation design and technology. In 2012, the Japanese steel industry produced approximately 80 million tons of hot metal from 27 blast furnaces, including large-scale furnaces with inner volumes over 5000 m3. During this period, the industry has faced many economic and social challenges, such as the high exchange rate of the yen, oligopoly in the mining industry, global warming, and the surge in iron ore and coal prices driven by the rapid growth of the BRICs. The industry has successfully responded to these challenges and maintained its international competitiveness by developing advanced technologies for pulverized coal injection, expanded use of low cost iron resources, recycling for environmental preservation, and CO2 mitigation.
    In this paper, the prospects for ironmaking technologies in the coming decades are described by reviewing published papers and looking back on the history of developments in ironmaking during the last 100 years.
  • Steelmaking Technology for the Last 100 Years: Toward Highly Efficient Mass Production Systems for High Quality Steels

    pp. 36-66

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    DOI:10.2355/isijinternational.55.36

    Progress of steelmaking technology in Japan over the last 100 years is overviewed covering hot metal pretreatment, primary steelmaking with open hearth furnaces, converters and electric arc furnaces, secondary refining of steel with degassers and ladle furnaces, and ingot- and continuous-casting.
    Key issues that contributed considerably to the progress of the unit processes are highlighted with scientific, technological and engineering breakthroughs involved. Also, systematization of the unit processes is depicted for optimizing full cost, productivity and quality of steel products to meet the constraints on the resources and socioeconomic demands of the steel market at times.
    Possible future development of steel technology is briefly commented on the basis of the above observation.
  • Thick Plate Technology for the Last 100 Years: A World Leader in Thermo Mechanical Control Process

    pp. 67-78

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    DOI:10.2355/isijinternational.55.67

    The history of construction of thick plate mills in Japan and trends in the development of rolling technology (gauge control, plan view control, and crown control) during the 100 year history of plate technology in this country are discussed in outline, and the Thermo Mechanical Control Process (TMCP) is reviewed.
    In 1901, the blast furnace at the state-owned Yawata Steel Works was blown-in and the medium gauge plate mill was started up, followed by startup of a 3-high rolling mill in 1905. Today, Japan has an annual thick plate production capacity of more than 10 million tons. Various important technical advances have also been achieved over the years, including gauge control, plan view pattern control, and crown control. The Thermo Mechanical Control Process was applied to controlled rolling for the first time in the 1960s, and accelerated cooling was applied in the 1980s. These technologies first reached full maturity in Japan and are now global technologies. The heat-treatment on-line process was also developed and continues to be a world-leading technology.
  • Sheet Steel Technology for the Last 100 Years: Progress in Sheet Steels in Hand with the Automotive Industry

    pp. 79-88

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    DOI:10.2355/isijinternational.55.79

    Development in sheet steels has progressed in strong relation with automotive industry in Japan. To meet the requirements from automotive industry, various types of sheet steels including high and ultra high strength steel sheets have been developed. Progresses in three types of steel series will be discussed by checking the historical facts and technologies and their contributions. Introductions of interstitial free (IF) steel and continuous annealing system are the important events in mild steel developments for panels. Extensive work on finding the optimum mixtures of hard and soft phases to improve elongation of steels contributed to improve the crashworthiness of autobodies. Continuous annealing system also played an important role in producing these advanced high strength steels. Precipitation is used in a particular way which is to scavenge solute carbon and nitrogen and to prevent coarse cementite particle precipitation. It is also worth to point out that the collaborative activities particularly strong in Japan between steel manufacturers and auto companies have affected the progress in advanced sheet steels.
  • Rolling Technology and Theory for the Last 100 Years: The Contribution of Theory to Innovation in Strip Rolling Technology

    pp. 89-102

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    DOI:10.2355/isijinternational.55.89

    Rolling theory has made remarkable progress for the last 100 years. The history of rolling theory is described in this report, comparing rolling theory with innovation of strip rolling technology. In Japan, computer control system started to be introduced to iron and steel company in 1960s. Therefore, research of rolling theory became in dispensable. Before 1960, it was published mainly in the Western countries. It is said that two-dimensional rolling theory had been completed. First of all, in Japan flow stress was investigated to calculate rolling load precisely, and approximate three-dimensional analysis for deformation of rolled strip was researched to predict the distribution of rolling pressure in width direction, which made up crown and shape of rolled strip. After that, FEM (finite element analysis method) replaced the approximate three-dimensional analysis, by which three-dimensional deformation of rolled strip could be calculated precisely. The results of these researches supported the innovation of crown and flatness control technology and the invention of many rolling mills with high functional ability for crown and flatness control. Next, the continuous strip rolling theory was completed chiefly in Japan, by which the static and dynamic characteristics of tandem strip mills could be understood. It is said that the continuous rolling technology for cold and hot strip tandem mills would not be realized without the continuous strip rolling theory.
  • Growth Rate of Copper Sulfide Precipitates in Solid Low Carbon Steel

    pp. 103-108

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    DOI:10.2355/isijinternational.55.103

    The kinetics of copper sulfide growth has been investigated using low carbon steel samples such as Fe-0.3mass%Cu-0.03mass%S-0.1mass%C and Fe-0.1mass%Cu-0.01 mass%S-0.1mass%C. The samples were heat-treated at 1273, 1423 and 1573 K for 100 s – 14.4 ks for precipitation of copper sulfides and then subjected to observation by a scanning electron microscope (SEM) and a transmission electron microscope (TEM) to measure the size of copper sulfides precipitated in the samples. The growth kinetics of copper sulfides has been found to be well described by the Ostwald ripening model, as follows:

    [ This part of text is omitted ]

    where Rt and R0 are the radii of copper sulfide precipitates, respectively, at t = t and t = 0 where t is time, kO is the rate constant in this model and T is thermodynamic temperature. The diffusion coefficients and activation energy derived from values of kO are close to those of copper in austenitic iron. On the basis of the growth kinetics, it has been proposed that the heat-treatment for as-cast strip steel should be conducted at around 1273 K, at which the size of copper sulfide precipitates can be controlled to be as small as 20–30 nm in several tens of minutes.
  • Numerical Study of Self-Induced Rotary Sloshing Caused by an Upward Periodic Jet in a Cylindrical Container

    pp. 109-116

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    DOI:10.2355/isijinternational.55.109

    Jet-induced rotary sloshing caused by an upward periodic jet in a partially filled cylindrical container is computationally simulated for investigating a stable sloshing condition and its basic characteristics under various values of AQ, Tjet and QL0 in a given function of the inlet jet: QL(t) = AQ sin[(2π /Tjet)t] + QL0. Then, the stable rotary sloshing is found not to be observed when either the minimum or maximum flow rate of QL(t) reaches a threshold value of the sloshing-occurrence boundary. The free surface motion like a beat phenomenon caused by interference between the sloshing wave and the periodicity of the inlet jet is also observed in the signal of the free surface elevation. Furthermore, the period of the envelope seems to match with Tjet, and this phenomenon is successfully formulated by the superposition between the sloshing wave and the free surface wave due to the periodic inlet jet.
  • Mathematical Modeling of the Melting Rate of Metallic Particles in the EAF under Multiphase Flow

    pp. 117-125

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    DOI:10.2355/isijinternational.55.117

    The sponge iron or Direct Reduced Iron (DRI) is an important feedstock in the Electric Arc Furnace (EAF). The main sources of iron units for the EAF can be steel scrap, DRI, hot metal and combinations of these materials. The EAF has become a melting reactor and its melting rate plays a key role in furnace productivity. In this work, the melting rate of porous metallic particles is analyzed employing CFD tools, having the computational domain of an industrial size EAF. The molten pool is comprised of two liquid phases, steel and slag. In order to compute the melting rate as a function of particle size and arc length, three sub-models were developed, one computes the instantaneous power delivery as a function of arc voltage and arc length, the second one computes the velocity and temperature fields and finally the third sub-model computes the melting rate. Comparisons of melting rates when the particles are immersed in its own melt and the case where immersion is carried out in the steel/slag system is included in the analysis. A contribution from this work is a more realistic approach to compute the convective heat transfer coefficient using the estimated values of the velocity fields.
  • Inclusion Evolution after Calcium Addition in Low Carbon Al-Killed Steel with Ultra Low Sulfur Content

    pp. 126-133

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    DOI:10.2355/isijinternational.55.126

    Inclusions in steel samples collected 0.5, 2, 5, 10 and 20 min after calcium addition were investigated in Low Carbon Aluminum Killed steel (LCAK steel) with ultra low sulfur content (0.04–0.065% Al, 9 ppm S). It’s found that inclusions change from CaO to CaO–CaS and finally to CaO–Al2O3–CaS with time. Al2O3 in inclusions linearly decreases by increasing T.Ca/T.O of the steel and disappears when T.Ca/T.O exceeds 3. CaS/CaO of the inclusions linearly increases by increasing S/T.O of the steel. Contents of CaO, Al2O3 and CaS can be estimated through T.Ca, S and T.O. CaO–CaS clusters are found at 5 min. The mechanism of inclusion evolution and formation of CaO–CaS clusters are discussed.
  • Selective Phase Transformation Behavior of Titanium-bearing Electric Furnace Molten Slag during the Molten NaOH Treatment Process

    pp. 134-141

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    DOI:10.2355/isijinternational.55.134

    In this paper, selective phase transformation for titanium-bearing phase (Ti-bearing phase) and impurity phase in titanium-bearing electric arc furnace molten slag (Ti-bearing EAF slag) was effectively realized during the molten NaOH treatment process. The phase transformation mechanism based on thermodynamic calculation was investigated in detail by X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectrum. It is indicated that the Ti-bearing phases in Ti-bearing EAF slag such as anosovite solid solution and Mg2TiO4 can be easily converted to Na2TiO3 with NaCl-type crystal structure, whilst the main impurity phase such as MgAl2O4 was apt to be decomposed by molten NaOH to form NaAlO2 and MgO. In addition, with increasing of roasting temperature and time, as well as decreasing of Ti-bearing EAF slag/NaOH mass ratio (Rslag/NaOH), the formed Na2TiO3 could be partly changed to NaMO2 (M=Mg, Ti, Fe) with α-NaFeO2-type crystal structure, due to the coexisting metal ions like Mg2+ and Fe3+ in the slag be doped into or even substituted the Ti atoms of Na2TiO3 to form NaMO2 (M=Ti, Fe, Mg).
  • Separation of FeO and P2O5 from Steelmaking Slag Utilizing Capillary Action

    pp. 142-148

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    DOI:10.2355/isijinternational.55.142

    The recoverability of FeO and P resources from steelmaking slag was investigated using a novel mechanical approach involving capillary action. After determining the phase relationship of the CaO–FeO–SiO2–P2O5 system with the coexisting 2CaO·SiO2 and MgO phases, it was found that P2O5 was concentrated in the 2CaO·SiO2 phase and was also distributed in the liquid phase. Due to the density difference of the FeO rich liquid phase and P2O5 containing 2CaO·SiO2 phase, the P2O5 concentration in the upper part of the sample was 2.2–3.6 times higher than that of the lower part. Also, the FeO concentration in the upper part of the sample was nearly half of that in the lower part. To improve the efficiency of the separation of the solid 2CaO·SiO2 phase and FeO rich liquid phase in steelmaking, capillary action was used to facilitate penetration into sintered CaO. Once the liquid phase had penetrated the CaO sinter, it was found that that the solid 2CaO·SiO2 phase and FeO rich phase could be effectively separated. When a sintered CaO sphere was added to a mixture of the solid 2CaO·SiO2 phase and FeO rich liquid phase, it was possible to recover 87% of the P2O5 and 90% of the FeO from steelmaking slag. The recovered CaO sinter, which includes a FeO rich liquid phase, may be used as Fe source or a dephosphorization agent, and the 2CaO·SiO2 phase, which includes P2O5, may be used as P source.
  • Melting and Reduction Behaviour of Individual Fine Hematite Ore Particles

    pp. 149-157

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    DOI:10.2355/isijinternational.55.149

    HIsarna is a new alternative ironmaking process of ULCOS program, which is under intensive development at EU. It uses coal and fine iron ore directly as raw materials instead of coke and pellet. In this context, the melting and pre-reduction behaviour of hematite ore in the smelting cyclone of HIsarna process was studied in the laboratory scale. The experimental study which used the typical reaction conditions of smelting cyclone in the pilot plant supplies a good reference to the HIsarna process and the results are helpful on determining the temperature range, gas composition and particle size in the smelting cyclone. All the experiments were performed in a High-temperature Drop Tube Furnace. The experimental temperature was varied from 1550 K to1750 K. A series of experiments have been conducted with different reaction time which was varied from 210 ms to 2020 ms, thus enabling the characterization of partially reduced samples. It was found that the reduction degree increases with the increase of temperature and residence time, while decreases with the increase of particle size. The maximum reduction degree at the studied conditions is approximately in the range of 23–30%. Both solid sample and molten sample were obtained at the current experimental conditions. The completely gas-solid particle reduction took place at 1550 K, 1600 K for the residence time up to 2020 ms. Fully molten particles can only be obtained at the temperature higher than 1700 K.
  • Co-modification and Crystalline-control of Ti-bearing Blast Furnace Slags

    pp. 158-165

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    DOI:10.2355/isijinternational.55.158

    The present paper investigated how the SiO2 and B2O3 additions influenced the crystallization behaviors of Ti-bearing blast furnace (Ti-BF) slags with a purpose of recycling the titanium. Ti-BF slags were co-modified by different SiO2 and B2O3 contents. Isothermal experiments were carried out using a Single Hot Thermocouple Technique (SHTT) to in-situ determine the crystal formation, and accordingly Time Temperature Transformation (TTT) curves were established. The results showed that the small amount of B2O3 addition can efficiently promote the transformation of primary crystalline phase from perovskite to rutile, whereas the precipitation of perovskite was suppressed. The kinetics of precipitation of the crystalline phase was explored and the results indicated that the growth of rod shape rutile was 1-D with the rate-controlling step of interfacial reaction; whereas the precipitated perovskite presented a 3-D growth style. With the increase of holding time, the nucleation rate of the formed crystals including rutile and perovskite became smaller.
  • Effective Use of CH4 Gas as a Reducing Agent in Suspension Reduction Process

    pp. 166-174

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    DOI:10.2355/isijinternational.55.166

    The current study investigated the reduction and carburization in the suspension reduction of the fine Chinese Benxi magnetite ore with CH4-containing gas. A very high fractional reduction was achieved by CH4-containing gas at 1300°C compared with that by H2 or CO gas. The reduction rate of the ore by CH4 was higher than that by H2 above 1150°C. The activation energies of the ore reduction by 11 vol%CH4 and 20 vol%H2 were calculated to be 113.6 and 76.9 kJ/mol, respectively. The reduction mechanism analysis showed that the formation of thin Fe layer reduced from wustite and the carbon dissolved into metallic shell by carburization had great influences on the further reduction progress. It is believed that the subsequent progress is related to the formation of less dense shell of metallic Fe with some cracks by the building up of gas pressure at Fe/wustite interface. Furthermore, the synergistic effect of H2 addition to CH4 gas on the reduction of the magnetite ore was also clarified.
  • Blast Furnace Operation with 100% Extruded Briquettes Charge

    pp. 175-182

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    DOI:10.2355/isijinternational.55.175

    Industrial stiff vacuum extrusion briquettes (BREX) producing line can efficiently provide for the small-scale blast furnace (BF) operation with 100% briquetted charge. Physical and metallurgical properties of the BREX are being investigated. A new effect of the non-linear strengthening of the BREX bonded with the combined Portland cement and Bentonite binder is described. Application of this combined binder results in the local maximum of the cold compressive strength of the BREX on third day of strengthening. Hot strength mechanism of the BREX is explained and was found to be related with creation of the metallized shell on their surface, sintering of the iron-containing particles and further creation of the metallicc phase surrounded by the silicates and ferrites. The addition of the iron ore fines to the BREX composition can improve their sintering during reduction thus helping to keep the integrity of the agglomerates. Results of the industrial operation of the small-scale BF with BREX are analyzed. The consumption of coke in the BF at 100% of BREX does not exceed 500 kg/t compared with 680 kg/t for the smelting without BREX.
  • Fractionation of Phosphorus in Steelmaking Slags and Aquatic Particulate Materials Using a Sequential Extraction Technique

    pp. 183-189

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    DOI:10.2355/isijinternational.55.183

    We measured the concentrations and chemical forms of phosphorus (P), an essential nutrient in marine ecosystems, in steelmaking slag (Slag) to determine whether use of Slags as a basal medium for artificial tidal flats and seagrass beds was an environmentally acceptable option for the reuse and recycling of Slags. We tested six Slag samples, including dephosphorization slags and converter slags. The total P concentrations were one order of magnitude higher in the Slags than in the aquatic particulate materials. We used a sequential extraction technique in which the strength of the extractants was increased stepwise in order to separate the P pools in the Slags into four fractions; exchangeable P, Fe (III)-bound P, authigenic P-like component, and detrital P-like component. We considered P contained in the first two fractions to be easily mobilized under the conditions found in some marine environments, such as the highly reduced zones of subsurface sediments. The mobile P accounted for 55–660 μmol P g–1 and for 56–81% of the total P in the Slags. The results strongly suggested that Slags have substantial potential to function as a P source that could sustain biological productivity in artificial tidal flat and seagrass bed ecosystems. On the other hand, Slags also have the potential to mitigate intense and rapid release of P from basal media, because mechanisms exist for adsorption and co-precipitation of phosphate with major components of Slags, including iron, calcium, and magnesium.
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  • Inclusion Modification in Si–Mn Killed Steels using Titanium Addition

    pp. 190-199

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    DOI:10.2355/isijinternational.55.190

    Modification of inclusions by titanium (Ti) additions in low-alloyed Si–Mn killed steels was studied. Formation of the modified inclusions was studied by measuring the inclusion count and chemistry. Mn–Si–Al–S–O based inclusions were observed in the steel, after Si–Mn de-oxidation, to which ferro-titanium (FeTi) additions were made. Thermochemical software (FactSage) was used to study the equilibrium between steel and inclusions and subsequent modification due to Ti addition. Inclusion count and chemistry in the steel were measured at different time intervals after the FeTi additions. MnO–SiO2 inclusions transformed to TiO2–MnO–SiO2 based inclusions with Ti replacing both Mn and Si in the inclusions. Si removal was more prominent from the inclusions compared to Mn. When more FeTi was added to the steel the inclusions further decreased in their Mn and Si content. With further FeTi addition Ti based inclusions with Al and Mn content less than 10% and Si content less than 5% were formed. MnS inclusions were also observed in the steel and formed as a result of segregation during solidification.
  • Influence of CaO on Existence form of Vanadium-containing Phase in Vanadium Slag

    pp. 200-206

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    DOI:10.2355/isijinternational.55.200

    A novel de-Phosphorisation & V-extraction process (de-P & V-extraction) is proposed in which lime is added during the production of vanadium slag. This is expected to promote both phosphorus removal and environment-friendly vanadium extraction from the vanadium-containing hot metal simultaneously. The influence of CaO addition on the chemical components and mineralogical morphology of vanadium-containing phase in vanadium slags has been investigated by XRD and SEM/EDS. Results show that the increase in CaO contents changes both species of vanadium-containing phase and the morphologies of vanadium-containing spinels in vanadium slags. Vanadium concentrates as (Mn,Fe,Mg)V2O4 in the spinel phase in vanadium slag with CaO content less than 10 mass%. When CaO content is higher than 12.89 mass%, vanadium exists both as (Mn,Fe,Mg)V2O4 in dendritic-like spinel and as Ca3V2(SiO4)3 in goldmanite in the matrix. Precipitation of perovskite results in the dendritic-like shape of spinels while V3+ diffusing away from spinel lattice leads to the formation of goldmanite Ca3V2(SiO4)3. Thermodynamic calculation of phase equilibrium has been conducted to understand the phase evolution. This work demonstrates the influence of CaO addition on the existence form of vanadium-containing phase in vanadium slag and thus provides the theoretical foundation for the proposed novel de-P & V-extraction process.
  • A Fast Detection Method for Region of Defect on Strip Steel Surface

    pp. 207-212

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    DOI:10.2355/isijinternational.55.207

    In order to meet the increasing demands of high efficiency and accuracy for strip steel production line, a fast detection method for region of defect (ROD) on strip steel surface is proposed in this paper. Firstly, the efficiency requirement of ROD detection algorithm is described. Secondly, mean filter improved in speed is used to filter noise. Then, five statistical projection features are extracted from detection region on surface image. Finally, based on distinct feature vector dataset, extreme learning machine (ELM) classifier, region of background (ROB) pre-detection and classifiers selection are combined together to realize two-class classification of ROD and ROB. Experimental results show that the novel method proposed in this paper not only is of high detection accuracy and efficiency but also can satisfy on-line ROD detection.
  • Measurement of Excited Argon Atom in Glow Discharge Plasma by Diode Laser Coherent Forward Scattering Spectrometry (DL-CFS)

    pp. 213-217

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    DOI:10.2355/isijinternational.55.213

    When non-metallic atoms are excited in a low-pressure glow discharge plasma, absorption transitions can be observed in the 640–930 nm wavelength range where laser diodes are commercially available. These excited atoms can be sensitively probed by diode laser atomic absorption spectrometry (DL-AAS). Because an atomic absorption transition can also be detected by coherent forward scattering (CFS) spectrometry and CFS spectrometry with a diode laser has more attractive features than DL-AAS, diode laser coherent forward scattering spectrometry (DL-CFS) was employed to investigated the absorption transition of excited argon atom at 842.46 nm. Ar (I) 842.46 nm line was adopted due to the tunable wavelength range of available diode laser in this experiment. Excited argon atoms were produced in a glow discharge plasma. CFS signal intensity at 842.46 nm attained maximum at discharge current of 20 mA in 3 Torr (399 Pa) of argon and at magnetic field of 160 mT. Calibration curve of argon was prepared to mix a small amount of argon into 6 Torr (798 Pa) of helium. The signal intensity depended on the 4.4th power of the number density of argon. The 4.4th power dependence is too large to be explained by the theoretically predicted quadratic dependence. When a small amount of molecular nitrogen was mixed into argon plasma, strong suppression of CFS intensity was observed. Molecular gases such as air were found to be unsuitable for the plasma gas to excite the target atoms.
  • Strengthening Mechanisms of Ultrafine Grained Dual Phase Steels Developed by New Thermomechanical Processing

    pp. 218-226

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    DOI:10.2355/isijinternational.55.218

    Dual phase (DP) steels have been investigated using a new approach utilizing simple cold-rolling and subsequent intercritical annealing of a martensite-ferrite duplex starting structure. The ferrite grain size and volume fraction of martensite were varied by changing the rolling reduction and intercritical annealing time. Ultrafine grained DP (UFG-DP) steel with an average grain size of about 2 μm was achieved by short intercritical annealing of the 80% cold–rolled duplex microstructure. Tensile testing revealed superior mechanical properties (the ultimate tensile strength of 1100 MPa and elongation of 13%) for the new DP steel in comparison with the commercially used high strength DP980 steel. The variations in hardness, strength and elongation of the specimens with rolling reduction and intercritical holding time were correlated to microstructural characterizations. The inherent mechanism for strengthening of the DP steel was discussed and the contribution of each strengthening factor was quantitatively calculated. The results showed that the calculated yield strength (430.6 MPa) is very close to the measured value (422 MPa).
  • Mechanism of Abnormal Surface Carbon Content Reduction in Nb-bearing Case Hardening Steel Gas Carburized after Machining

    pp. 227-234

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    DOI:10.2355/isijinternational.55.227

    In our previous paper, we reported that the surface carbon concentration (Cs) in carburized Nb-bearing steel decreases substantially with increasing the machining speed before carburizing. In the present study, a systematic study was made to clarify the effects of Cr,Nb addition and machining speed on the Cs. Uniform Cr oxide film was observed on the surface of specimens exhibited a large decrease in Cs. The carbon concentration profile was calculated assuming that carbon absorption was ceased after the complete coverage of specimen surface by Cr oxide film. It is confirmed that the present calculation fits well with the measured carbon concentration profile of low Cs. Moreover, it is shown that the value of Cs becomes smaller as the time span between the start of carburization and the time of Cr oxide film formation becomes shorter. The role of Nb is considered to be the retardation of recovery, recrystallization and grain growth through the pinning effect of Nb(CN) during carburizing. The high density of lattice defects introduced by machining is maintained by Nb(CN) and promotes faster Cr diffusion. In the present study, it has been clarified that in machined specimens of Nb-bearing steel, entire coverage of specimen surface by Cr oxide film during carburizing occurs in steels containing even a small amount of Cr like 1%.
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  • Quantitative Evaluation of Stability of Water Flow Injected from Pipe Laminar Nozzle

    pp. 235-240

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    DOI:10.2355/isijinternational.55.235

    Cooling equipment of the pipe laminar type is utilized on run-out tables in hot strip mills. It is known that the stability of the water flow injected from the cooling equipment nozzles affects cooling performance. In this study, new criteria and an experimental method for quantifying water flow stability are proposed. The experimental apparatus consists of the water injection equipment, an electric circuit and a logger. The time variation of voltage represents the electric resistance of the injected water and is measured under various conditions of flow rate and nozzle diameter. A new laminar stability index, Rσ/RAVE was proposed, in which Rσ and RAVE mean the standard deviation and the average value of electric resistance of water flow injected from the nozzle, respectively. The proposed index qualitatively shows good agreement with the appearance of the laminar flow in the experiment. Based on the experiment, an equation of continuous laminar length is suggested.
  • Effects of Metal Types on Residual Stress in Electron-Beam Welding Joints with Sheet Metals

    pp. 241-249

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    DOI:10.2355/isijinternational.55.241

    The effect of metal types on the residual stresses has been researched through X-ray stress measurement for the electron-beam welding joints made of sheet metals with a thickness of approximately 10 mm. The finite-element method (FEM) has also been used to analyze the experimental results and verify the residual stress characteristics. Consequently, it has been revealed that the residual stress near the weld toe in the longitudinal direction along the weld axis becomes tensile to a larger extent as the yield strength of the testing material is higher. By contrast, in the transverse direction, the residual stress close to the weld toe has been found to be compressive to a higher degree as the yield strength of the testing material is bigger. These correlations can be observed not only for ferritic and martensitic steel materials but also for other types of metals such as austenitic stainless steel of SUS304 and aluminum alloy of A5052.
  • Acicular Ferrite Formation on Ti-Rare Earth Metal-Zr Complex Oxides

    pp. 250-256

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    DOI:10.2355/isijinternational.55.250

    Acicular ferrite (AF) formation potency of Ti-Rare earth metal (REM)-Zr (TRZ) complex oxide has been investigated in the simulated heat affected zone of low carbon steel. The TRZ complex oxide shows higher AF formation potency than Ti and Al oxides. A TRZ oxide particle is composed of REM-rich and Zr-rich phases. AF crystals nucleate on the interface between austenite and the Zr-rich oxide phase, having a crystal structure that is face-centered cubic with lattice parameter of 0.44 nm. The Zr-rich phase and AF have an orientation relationship described by (011)AF//(011)Oxide, [100]AF//[011]Oxide (the AF-TRZ orientation relationship), which allows good lattice coherency. It is suggested that the formation of this orientation relationship promotes AF nucleation on TRZ complex oxides. The AF also satisfies the Kurdjumov-Sachs (K-S) orientation relationship with the austenite matrix. It is considered that the coexistence of the AF-TRZ and K-S “three phase” orientation relationships is caused by variant selection of AF in addition to the formation of a rational orientation relationship between the Zr-rich oxide phase and the austenite matrix during the HAZ thermal cycle.
  • Influence of Cr on Weld Solidification Cracking in Fe-15Mn-0.5C-3.5Al-xCr Alloys

    pp. 257-263

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    DOI:10.2355/isijinternational.55.257

    The solidification cracking sensitivity of Fe-15Mn-0.5C-3.5Al-xCr (x= 5, 12 mass%) alloys was evaluated using a longitudinal Varestraint test and compared with that of austenitic Fe-18Mn-0.6C alloy. Weld microstructures of Low Cr (5 mass%) and High Cr (12 mass%) alloys revealed duplex structures with a mixture of austenite and δ ferrite. The amounts of residual δ ferrite and (Cr, Fe, Mn)23C6 carbide remarkably increased with increasing Cr content. A small amount of Cr addition (5 mass%) provided negligible influence on the solidification cracking susceptibility. However, the welds of high Cr alloys demonstrated excellent resistance to solidification cracking due to healing by the eutectic liquid. The addition of Cr enhanced the formation of a low melting point (γ + (Cr, Fe,Mn)7C3) eutectic during solidification by increasing the eutectic formation temperature and simultaneously decreasing the eutectic C content.
  • Liquid-Metal-Induced Embrittlement Related Microcrack Propagation on Zn-coated Press Hardening Steel

    pp. 264-271

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    DOI:10.2355/isijinternational.55.264

    The coatings on press hardening steel are needed to suppress high temperature oxidation and decarburization. Additional corrosion protection is provided by Zn coatings which provide cathodic protection to press hardened parts. Due to the low melting temperature of Zn and Zn–Fe intermetallic compounds, the Zn-coated PHSs are susceptible to LMIE during the die-quenching process. In the present work, the mechanical properties of Zn coated PHS were evaluated in terms of tensile properties and bending properties. A deterioration of the room temperature bendability, due to microcrack formation and propagation, was observed. The presence of Γ-Fe3Zn10 observed in the microcracks at room temperature correspond to liquid Zn at the die-quenching temperature, making it possible to trace the progress of the liquid Zn phase during microcrack formation. The results suggest that Zn-grain boundary diffusion causes the phase transformation to ferrite of the austenite grain boundary region. This results in intergranular cracking due to the lower strength of ferrite. The LMIE microcrack formation is most severe in areas where the applied stress and the friction are highest during the forming process, making the occurrence of LMIE-mitigated microcrack propagation dependent on the local deformation conditions.
  • Aluminizing of High-carbon Steel by Explosive Welding and Subsequent Heat Treatment

    pp. 272-277

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    DOI:10.2355/isijinternational.55.272

    A carbon steel plate containing 0.87 mass% C was coated with an aluminum foil with a thickness of 100 μm by explosive welding. This aluminum-coated steel was heat-treated in the temperature range of 973–1273 K for up to 7.2 ks in the air to investigate reactions between molten aluminum and high-carbon steel from the viewpoint of aluminide coating. An aluminized layer was basically composed of Fe2Al5, FeAl, Fe3Al containing carbon (Fe3Al(C)), and ferrite stabilized by aluminum diffusion (α-Fe(Al)). FeAl2 was detected with Fe2Al5 at heating temperatures of more than 1223 K, whereas FeAl had two layers due to its chemical composition. In addition, defects like a crack and a void were observed in the vicinity of the FeAl/α-Fe(Al) interface after heating at 1273 K for 3.6 ks. To reduce brittle Fe2Al5 and FeAl2 in the aluminized layer, the steel coated with aluminum with a thickness of 50 μm was prepared and then heat-treated under the same conditions. At 1173 K, the region consisting of FeAl, Fe3Al(C), and α-Fe(Al) accounted for a large part of the aluminized layer. This aluminized steel was subjected to quenching and tempering on the basis of features of the used bare steel. As a result, the hardness near the surface of the aluminized layer was approximately equal to that of the steel substrate.
  • Crystallographic Texture Based Analysis of Fe3O4/α-Fe2O3 Scale Formed on a Hot-rolled Microalloyed Steel

    pp. 278-284

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    DOI:10.2355/isijinternational.55.278

    Oxide scale formed on the strip surface during hot rolling has posed a serious obstacle to ensure a defect-free surface of steel products in an ecologically friendly way. Recently, an influential idea is that the tertiary oxide scale with a tailored texture can be expected to enhance surface quality and tribolgoical properties during particular lubrication. In this study, texture evolutions of magnetite (Fe3O4) and hematite (α-Fe2O3) in deformed oxide layers formed on a hot-rolled microalloyed steel were investigated by electron back-scattering diffraction (EBSD). Fe3O4 develops a strong θ fibre parallel to the oxide growth, and α-Fe2O3 has a dominant {0001}<1010> texture component. This could be explained by surface energy minimisation during oxides growth and transformation between two oxides, which can also be affected by propagation of cracks along high angle grain boundaries in Fe3O4. Our data further demonstrate that a high thickness reduction (>28%) can reduce α-Fe2O3 wedging through Fe3O4 cracks, and tailoring Fe3O4 texture to {111} components can prevent the α-Fe2O3 growth titling 54.76° from the <001> crystal direction of Fe3O4. As such, these means can dramatically alleviate disturbance from ‘red scale’ (α-Fe2O3) during high-temperature steel processing.
  • Sensitivity Analysis of the Finite Difference 2-D Cellular Automata Model for Phase Transformation during Heating

    pp. 285-292

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    DOI:10.2355/isijinternational.55.285

    Sensitivity analysis of the Finite Difference Cellular Automata model for Dual Phase steel phase transformation during heating was performed in the present work. The main goal of the work was to determine the process parameters that are most important throughout transformation and should be particularly considered during the identification of model parameters: deformation, coefficients related with grains nucleation, activation energy, pre-exponential factor, and curvature parameters. The Morris OAT is a screening method capable of recognising the important factors of model and global sensitivity analysis was computed using this method. Different responses of the model outcomes were obtained by changing subsequent model input parameters. Results from Morris OAT design showed that deformation and activation energy have the most significant impact on the kinetics of phase transformation whereas average grain size strongly depends on all of the model parameters. Next, local sensitivity analysis was considered to check the behavior of each parameter locally. Finally both global and local sensitivities were compared and it was found that local sensitivity analysis in the case of such complex models can lead to inaccurate results.
  • Formation of the Fe2Hf Laves Phase Along the Eutectoid-type Reaction Path of δ-Fe→γ-Fe+Fe2Hf in an Fe-9Cr Based Alloy

    pp. 293-299

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    DOI:10.2355/isijinternational.55.293

    This paper reports the precipitation kinetics along the eutectoid-type reaction path of δ-Fe → γ-Fe+Fe2Hf in an Fe-9Cr-0.3Hf (wt.%) alloy. Three types of precipitation modes were observed in the reaction path: (1) interphase precipitation forming periodically arrayed rows of fine Fe2Hf particles, (2) the precipitation of Fe2Hf phase in the δ phase followed by a phase transformation from the δ phase to the γ phase and (3) the precipitation of Fe2Hf phase in the γ phase matrix preceded by a δγ phase transformation. The nose for the interphase precipitation in time-temperature- transformation diagram is located at a few second and at 1100°C. The interphase precipitation is expected to be used as a tool for strengthening heat resistant ferritic steels, but the heat treatment window for the precipitation mode is narrow in the ternary alloy.
  • Flow Softening-based Formation of Widmanstätten Ferrite in a 0.06%C Steel Deformed Above the Ae3

    pp. 300-307

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    DOI:10.2355/isijinternational.55.300

    Compression tests were carried out at a strain rate of 1 s–1 on a 0.06%C-0.3%Mn-0.01%Si steel over two temperature ranges: i) 920°C to 980°C, and ii) 500 to 750°C. Optical and scanning electron microscopy images indicated that significant volume fractions of Widmanstätten ferrite were formed dynamically above the Ae3 temperature. The ferrite plates coalesced into polygonal grains during straining. The double differentiation method was applied to the stress-strain curves, providing average values for the dynamic transformation (DT) and dynamic recrystallization (DRX) critical strains of 0.12 and 0.20, respectively. These results are interpreted in terms of the flow softening-based transformation model by calculating both the driving forces promoting the transformation as well as the energy barriers that oppose it. The model predicts the temperature range over which DT can occur as well as the observed critical strains.
  • Effect of Texture on the Cold Cracking in Weld Zone of T23 Steel

    pp. 308-311

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    DOI:10.2355/isijinternational.55.308

    Y-slit welded joints without preheating were used to prepare the cold cracks in T23 steel. Then the effect of texture on the cold cracking was studied from the views of macrotexture and microtexture. The results showed that the propagation path of the cold cracks was strongly affected by the macrotexture of the columnar grain structure after solidification. The fracture direction was changed at almost light-etched columnar grain but within each columnar grain an average fracture direction was maintained. The crack finally took the same initial direction before meeting another light-etched columnar grain. The strain distribution obtained by electron backscattered diffraction (EBSD) showed that different crystallography of the columnar grain induced large strain concentration, which led to the big angle transition of the main crack. In addition, transform texture, also named as microtexture in this study, had a large effect on the exact propagation paths of the cold cracks. The cold cracks propagated along lath boundaries which were almost parallel to {011}α plane.
  • Design of Alloy Composition in 5%Mn-Cr-C Austenitic Steels

    pp. 312-318

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    DOI:10.2355/isijinternational.55.312

    The range of chemical compositions that can obtain an austenitic single structure was defined for medium-manganese (Mn) carbon (C) steels. Among the potential compositions, Fe-5%Mn-4%Cr-(0.8–1.4)%C (mass%) was selected as the optimized composition range to form a stable austenitic structure. The tensile properties and deformation substructure were investigated in the austenitic steels having this composition. The work hardening behavior of the steels varied depending on the carbon content, which was closely related to the deformation microstructure. In the 0.8%C steel, both a deformation-induced martensitic phase as well as the formation of deformation twins generated a high work hardening until fracture. With an increasing carbon content, which increased the stacking fault energy (SFE), the deformation tended to shift towards dislocation slipping, resulting in a lower work hardening rate. This trend appears similar to conventional twinning-induced plasticity steel where the work hardening behavior is tied to the SFE.
  • Microstructures and Wear Behavior of the TiC Ceramic Particulate Locally Reinforced Steel Matrix Composites from a Cu–Ti–C System

    pp. 319-325

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    DOI:10.2355/isijinternational.55.319

    The steel-matrix composites locally reinforced by in situ TiC particulates were successfully fabricated via the SHS-casting route using a Cu–Ti–C system. Effects of Cu content in the Cu–Ti–C system on the microstructure, hardness and wear behavior of the composites were investigated. The results showed that the locally reinforced regions in the steel matrix composites consisted mainly of TiC, Cu and austenite. The hardness and wear resistance of the locally reinforced regions were significantly higher than those of the matrix. With the increase in the Cu content, the hardness value decreased monotonically, while the wear resistance of the reinforced region first increased and then decreased. The composites fabricated using the 20–30 wt.% Cu–Ti–C system had the best wear resistance. The wear characteristics were micro-cutting and abrasive wear under the condition of low and/or moderate load (50–90 N). Moreover, with the increase in Cu content and the applied load (90–110 N), the adhesive wear could also be clearly observed on the wear surface of the test samples besides the micro-cutting and abrasive wear.
  • A Water Model Study of Simultaneously Releasing Multi-Particles onto a Molten Metal Bath

    pp. 326-328

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    DOI:10.2355/isijinternational.55.326

    Uniform dispersion of refining agents in a molten iron bath is of practical importance for efficient desulfurization. As a fundamental study of enabling uniform dispersion, a simple method was proposed to simultaneously release multi-particles in the atmosphere. As a first step, two solid spheres of different diameters were released onto a water bath. The dynamic behavior of the spheres entering the bath and associated air cavity formation were observed with a high-speed camera.
  • Desulfuration Behavior of Low-grade Iron Ore-coal Briquette during the Process of Direct Reduction Followed by Magnetic Separation

    pp. 329-331

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    DOI:10.2355/isijinternational.55.329

    The process of coal-based direct reduction followed by magnetic separation was employed to produce direct reduction iron powder (DRIP) from a refractory low-grade iron ore. The desulfuration behavior of the composite briquettes during this process was studied. Experimental results showed that this process presents advanced sulfur removal ability. The sulfur contained in the composite briquette was almost reserved in the form of troilite at the reduction stage; however, after grinding and magnetic separation the sulfur was removed efficiently into the tailing, and the DRIP with low content of sulfur was obtained.
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  • Determination of Trace Amounts of Bismuth in Steel by ICP-MS Through a Cascade-preconcentration and Separation Method

    pp. 332-334

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    DOI:10.2355/isijinternational.55.332

    Determination of trace amounts (~0.05 μg) of bismuth in steel by inductively coupled plasma mass spectrometry (ICP-MS) has been carried out by a cascade-preconcentration and separation method that combines the one-drop solvent concentration method and solid-phase extraction method. The bismuth(III) ion from acid decomposition of the steel sample formed a complex with iodine, and this subsequently formed an ion pair with tetrabutylammonium (TBA). This ion pair was almost completely adsorbed by passage through a solid-phase packed column. The ion pair was eluted with ethyl acetate, and a small volume of dimethyl sulfoxide (DMSO) was added to the resulting solution; successful preconcentration of the iodobismuthate complex/TBA ion pair into the DMSO phase was accomplished by removing the ethyl acetate using a compact evaporator in a water bath (40°C). The sample was diluted with an internal standard (thallium) solution and an acidic solution and was then tested by ICP-MS. The calibration curve for the proposed method showed good linearity (r2 = 0.9997) in the 0.05–10 μg range. A recovery test was performed, in which 0.05 μg and 2.5 μg of bismuth was spiked into an acidic decomposition solution of pure iron (JSS 003-6), and the recovery was 98% for both solutions, and the relative standard deviations (n = 5) were 8.8% and 1.2%, respectively. Good results were obtained applying the method to analysis of CRMs.
  • Detection of Charged Hydrogen in Ferritic Steel through Cryogenic Secondary Ion Mass Spectrometry

    pp. 335-337

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    DOI:10.2355/isijinternational.55.335

    Hydrogen uptake in a ferritic steel was investigated through secondary ion mass spectrometry (SIMS) at 83 K, where hydrogen diffusion is sufficiently suppressed. Additionally, the SIMS was operated with cold trap and Si sputtering to reduce the back ground effect. Thanks to the suppression of hydrogen diffusion during the measurements, the cryogenic SIMS could demonstrate reproducible results which showed a significant difference in hydrogen content between hydrogen-charged and uncharged specimens. Namely, hydrogen in the ferritic steel was successfully detected similarly to austenitic steels.

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