ISIJ International
Online ISSN : 1347-5460
Print ISSN : 0915-1559
ISSN-L : 0915-1559
Volume 60 , Issue 5
Showing 1-31 articles out of 31 articles from the selected issue
Publication Data
Regular Articles
Fundamentals of High Temperature Processes
  • Guozhu Zhang, Longge Yan, Xinfang Zhang
    Type: Regular Article
    2020 Volume 60 Issue 5 Pages 815-822
    Published: May 15, 2020
    Released: May 16, 2020
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    The effect of pulsed electric current on the removal of MgO inclusions, which deteriorate mechanical and corrosion properties of magnesium alloys, was investigated. Through characterizing the number and size distribution of MgO inclusions, it’s found that the MgO inclusions were expelled to the surface (bottom and top) of the magnesium alloy melt and then absorbed by the coating agent, treated by the electropulsing. Besides, the electropulsing also promoted the aggregation of the inclusions, but the large size inclusion only exists on the surface of the sample. Based on the differences in electrical properties between the inclusions and the magnesium alloy melt, a force from electric current to electrically neutral MgO inclusion has been identified to drive the particles’ movement. Further, the results of numerical calculation show that under the action of pulse current, the migration of inclusions to the surface can reduce the system free energy, and the aggregation tendency between two inclusions with a relatively large radius is significantly stronger than the two inclusions with a smaller radius. The excellent agreement between numerical calculations and experimental data demonstrates that employing the electropulsing to separate the particles from the liquids is a fantastic new method in metal melt purification.

  • Jianjiang Xin, Nan Wang, Min Chen, Chen Chen
    Type: Regular Article
    2020 Volume 60 Issue 5 Pages 823-831
    Published: May 15, 2020
    Released: May 16, 2020
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    Vanadium oxides in metallized pellets can be reduced into metal phase by carbothermal reduction reaction in electric furnace smelting process. Smelting-separation parameters have an important influence on slag-metal separation and reduction behaviors of vanadium oxides. The effects of smelting temperature, slag basicity and smelting time on slag-metal separation, the mass fractions of V and C in metal phase and recovery degree of V for vanadium-bearing titanomagnetite metallized pellets were investigated in the present work. The results demonstrate that the increase of smelting temperature promotes the reduction of vanadium oxides, resulting in an increases of the mass fraction and recovery degree of V, whereas the mass fraction of C decreases continuously. Within proper slag basicity and smelting time ranges, increasing basicity and time is beneficial to the coalescence and settlement of metallic iron droplets and reduction of vanadium oxides in smelting separation process. The mass fraction and recovery degree of V increases firstly and then decreases with the increment of slag basicity and smelting time, whereas the mass fraction of C initially decreases and subsequently increases, showing an opposite tendency with that of V. Under the optimal conditions, smelting temperature of 1450°C, slag basicity of 0.7 and smelting time of 40 min, the recovery degree of V could reach the highest of 99.8%, and the mass fraction of V and C are 0.27% and 1.2%, respectively.

  • Xuefeng She, Zhenlong An, Zhuo Zhang, Tengfei Ma, Jingsong Wang
    Type: Regular Article
    2020 Volume 60 Issue 5 Pages 832-839
    Published: May 15, 2020
    Released: May 16, 2020
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    Crystallization behaviors of CaO–SiO2–CaF2–La2O3 synthesized slag with different basicity have been studied by differential scanning calorimetry (DSC), field emission scanning electron microscopy (SEM) and X-ray diffraction (XRD). It was found that there were mainly three kinds of crystalline phases (La7.58(Si1.048O4)6O2, CaSiO3 and Ca4Si2O7F2) precipitated in the slag during the cooling process. Crystalline phase Ca2SiO3 formed when the basicity was increased to 1.3. The observations confirmed that the rare earth phase (La7.58(Si1.048O4)6O2) precipitated firstly in each slag during the cooling process. With the increase of slag basicity, the crystallization temperature of the rare earth phase decreases while the precipitation peak temperature of CaSiO3 and Ca4Si2O7F2 increase. The morphology of rare earth phase is hollow hexagonal which is filled with substrate phase. The optimum condition for crystallization and separation of rare earth phase was obtained. The basicity of the slag should be controlled between 0.9 and 1.1. The morphology of rare earth phases can grow much better after isothermal heat treatment for 4 h at the crystallization temperature of rare earth phases. The mean size of rare earth phase could increase to more than 60 µm by isothermal heat treatment.

  • Dingli Zheng, Jing Li, Chengbin Shi
    Type: Regular Article
    2020 Volume 60 Issue 5 Pages 840-847
    Published: May 15, 2020
    Released: May 16, 2020
    [Advance publication] Released: January 24, 2020
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    To reduce environmental pollution caused by fluoride from conventional electroslag remelting (ESR)-type slag and meet the requirements of vacuum ESR, it is strongly needed to develop low-fluoride and fluoride-free slag. The crystallization behaviors and evaporation of CaF2–CaO–Al2O3–MgO–Li2O slag as a candidate for low-fluoride ESR-type slag were studied. The sequence of crystal precipitation in CaF2–CaO–Al2O3–MgO–Li2O slag during cooling process was 11CaO·7Al2O3·CaF2 to CaO, followed by CaF2. The dominant crystalline phase in the slag was 11CaO·7Al2O3·CaF2. The liquidus temperature and crystallization temperature of slag decreased significantly with increasing Li2O content from 1.59 mass% to 4.46 mass%. Increasing Li2O contents suppressed the crystallization behaviors of ESR-type CaF2–CaO–Al2O3–MgO–Li2O slag. The weight loss of the slag melts increased with increasing Li2O content in the slag from 1.59 mass% to 4.46 mass%. The gaseous species evaporated from the slag melts were mainly LiF and contained a few amounts of CaF2. A proper amount of Li2O could be considered as an effective component for the design of low-fluoride ESR-type slag. Serious fluoride evaporation of LiF from CaF2–CaO–Al2O3–MgO–Li2O slag melts takes place when Li2O content exceeds a critical value.

  • Yuewen Fan, Xiaojun Hu, Rong Zhu, Kuochih Chou
    Type: Regular Article
    2020 Volume 60 Issue 5 Pages 848-855
    Published: May 15, 2020
    Released: May 16, 2020
    [Advance publication] Released: December 26, 2019
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    In order to clarify the role of O2 and CO2 which are applied to steelmaking, the isotope tracing method has been used to study the decarburization reactions between CO2–O2 mixed gas and Fe–C melts containing different carbon contents at 1873 K. When the mixture of two isotope gases of 18O2 and 13CO2 reacts with melts, the isotopic composition of the reacted gas is monitored online by a mass spectrometer. According to the change of isotopic gas composition, the ratio of various reactions and the decarburization rate can be calculated, and the effects of gas flowrate and carbon content on the decarburization reaction can be discussed. The results showed that the utilization of CO2 was more than 97%, and approximately 52% of oxygen formed to CO, 17% to CO2, and 31% for post combustion under the conditions of 30 ml/min of flowrate, 1 cm of the gas injection height and the 4.00 to 1.00 wt% of original carbon content. And with the increase of the flowrate from 30 ml/min to 40 ml/min, the amount of CO2 generated by O2 increased, while the amount of CO decreased. In particular, the post combustion ratio improved significantly from 47% to 77%.

Ironmaking
  • Mingyin Kou, Heng Zhou, Zhibin Hong, Shun Yao, Shengli Wu, Haifa Xu, J ...
    Type: Regular Article
    2020 Volume 60 Issue 5 Pages 856-864
    Published: May 15, 2020
    Released: May 16, 2020
    [Advance publication] Released: December 28, 2019
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    Blast parameters are easy to be changed and are often applied to control the gas distribution in blast furnace. However, the effects of blast parameters on the gas and solid distribution characteristics were seldom investigated. Therefore, a 3D model considering gas and solid fluid phases is developed and used to analyze effects of blast parameters on the gas and solid distributions in the blast furnace B of Baiyi Steel. The results show that the gas temperature increases a little in the lower part while decreases in the middle and top parts of blast furnace when the oxygen enrichment ratio and blast temperature increase. The CO utilization ratio increases with the increase of oxygen enrichment ratio, humidified blast amount and blast temperature, and it increases most for the case of blast temperature. The metallization ratio at the bottom of the blast furnace increases with the increase of oxygen enrichment ratio and humidified blast amount while it decreases with the increase of gas temperature.

  • Stuart Nicol, Evgueni Jak, Peter Hayes
    Type: Regular Article
    2020 Volume 60 Issue 5 Pages 865-875
    Published: May 15, 2020
    Released: May 16, 2020
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    The cooling rate of the liquid oxide can be controlled in industrial sintering processes through the draft pressure and has the potential to influence microstructure formation. The solidification of a liquid within the hematite primary phase field in the ternary “Fe2O3”–CaO–SiO2 system in air was undertaken at different cooling rates to determine the impact of cooling rate on the formation of product microstructures. Samples with a bulk composition of 72.7 wt% Fe2O3 and a CaO/SiO2 ratio of 3.46, were cooled from 1623 K (1350°C) at 2 K/s, 0.5 K/s, 0.1 K/s and 0.01 K/s and quenched at 5 K temperature intervals from 1533 K (1260°C) to 1453 K (1180°C). During cooling, four stages of phase assemblage formation were consistently observed at all cooling rates; in order of formation these are, Liquid+hematite (I), Liquid+hematite+dicalcium silicate(C2S)(II), Liquid+C2S+calcium diferrite (CF2)(III) and C2S+CF2+calcium ferrite (CF)(IV). An intergrowth of silico-ferrite of calcium and aluminium-I (SFCA-I) and Ca7.2Fe2+0.8Fe3+30O53 was observed to form in some conditions in regions free of hematite, present in liquids solidifying at 0.5 K/s and 0.1 K/s. The sizes and shapes of microstructures were observed to systematically change with cooling rate, with a slower cooling rate typically resulting in coarser coupled microstructures and larger individual crystals. A larger proportion of coupled microstructures are observed at slower cooling rates, this appears to be related to the degree of undercooling prior to the nucleation of new phases. The equilibrium silico-ferrite of calcium (SFC) phase was not observed at any of the cooling rates investigated.

  • Stuart Nicol, Evgueni Jak, Peter Hayes
    Type: Regular Article
    2020 Volume 60 Issue 5 Pages 876-886
    Published: May 15, 2020
    Released: May 16, 2020
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    The solidification of “Fe2O3”–CaO–SiO2 liquids in air at a controlled cooling rate of 2.0 K/s for a range of Fe2O3 concentrations in the bulk has been investigated. The compositions investigated were selected such that the bulk compositions were within the hematite primary phase field and had a CaO/SiO2 ratio of 3.46 wt/wt.

    Non-equilibrium phase assemblages were formed for all bulk compositions investigated. Specifically, the silico ferrite of calcium (SFC) phase was not formed on cooling. The microstructures and proportions of the phase assemblages formed were found to vary with the Fe2O3 concentration in the bulk.

    Schematic diagram of the experimental apparatus for the controlled cooling of oxide liquids. Reproduced from Nicol. Fullsize Image
  • Yusuke Dohi, Kiyoshi Fukada, Tetsuya Yamamoto, Takashi Matsui, Hiroyuk ...
    Type: Regular Article
    2020 Volume 60 Issue 5 Pages 887-897
    Published: May 15, 2020
    Released: May 16, 2020
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    In our previous paper, we proposed a new measurement method for coal thermoplasticity called the “permeation distance method,” in which the permeation distance of thermally plastic coal into a glass bead layer adjacently placed on the coal sample is measured as a unique caking property. Although the maximum permeation distance measured by this method is roughly correlated with Gieseler fluidity, large deviation is observed, especially for high fluidity coals. Moreover, that study revealed that high MF coal having a longer maximum permeation distance forms thinner pore-wall structures in coke and that coke strength deteriorates when the coal blend includes longer maximum permeation distance coal. Therefore, a technique for reducing the adverse effects of long permeation distance coal on coke strength is necessary so as to utilize the coal more efficiently.

    In this paper, the influence of the grain size of mainly high MF coal on permeation distance and coke strength was investigated to clarify the possibility of controlling the permeation distance. As a result, it was found that the measured maximum permeation distance became shorter with decreasing coal size. Moreover, the coke strength deterioration caused by long permeation distance coal in a coal blend was suppressed as the size of the long permeation distance coal became smaller. Consequently, coal grain size design and control techniques for more effective utilization of long permeation distance coal were proposed.

  • Yong Deng, Qing Lyu
    Type: Regular Article
    2020 Volume 60 Issue 5 Pages 898-904
    Published: May 15, 2020
    Released: May 16, 2020
    [Advance publication] Released: January 17, 2020
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    The application of big data in industry can solve industrial problems through data analysis. In order to establish the evaluation and prediction system of comprehensive state in a commercial blast furnace (BF), problems of ironmaking data were observed, standardized processing technologies were employed to process ironmaking data, big data platform of ironmaking was deployed based on the collected data. Hot metal production, hot metal production quality [Si+Ti] and fuel rate were selected as the target parameters to reflect the comprehensive state of BF. 26 key parameters that were most closely related to BF production and target parameters were screened out through Python 3.7. The relationship between 26 key parameters and hot metal production, [Si+Ti], fuel rate was analyzed, respectively. The first five parameters with strong correlation with target parameters were selected as important data for analysis and prediction. The comprehensive state was scored according to scoring rules derived from data analysis, the total score could be obtained to evaluate the comprehensive state of the commercial BF. The prediction of comprehensive state was realized based on a large number of historical data. The optimization of model was completed and the model could be run online, the evaluation and prediction system helped operators optimize BF operation.

    Architecture of big data platform of ironmaking. (Online version in color.) Fullsize Image
  • Tomoyuki Kawashima, Akinori Murao, Naoki Yamamoto, Makoto Ando, Jun Ok ...
    Type: Regular Article
    2020 Volume 60 Issue 5 Pages 905-914
    Published: May 15, 2020
    Released: May 16, 2020
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    Influence of pulverized coal into the blast furnace is an invaluable technology for reducing ironmaking costs. In order to increase the rate of pulverized coal injection, it is necessary to investigate the combustion behavior of pulverized coal around the tuyere, and many numerical simulations as well as experimental studies have examined this issue. In this paper, the combustion behavior of individual pulverized coal particles was investigated by using the Extended CPD model, which is a new devolatilization model, and the Large Eddy Simulation (LES) model. The volume fraction of CO+CO2, which is the indicator of how much carbon included in pulverized coal is changed to the gas phase, was reasonably predicted by the new simulation model. The results of this study suggested that the devolatilization processes of pulverized coals with sizes under 40 µm varies widely between individual particles.

    N2 volume fraction distribution and particle distribution with particle diameter. Fullsize Image
Steelmaking
  • Jian-ming Su, Zhi-he Dou, Ting-an Zhang, Yan Liu
    Type: Regular Article
    2020 Volume 60 Issue 5 Pages 915-921
    Published: May 15, 2020
    Released: May 16, 2020
    [Advance publication] Released: December 28, 2019
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    To solve the technical problems such as serious splashing and low utilization rate of the desulfurizer in the traditional hot metal desulfurization by granular magnesium injection, a new idea of hot metal desulfurization with continuously controllable bottom blowing magnesium vapor is presented in this paper. An estimation model of the utilization rate of magnesium vapor was established based on the double-film theory, which shows that the utilization rate of magnesium vapor decreases with the increase of the flow rate of the carrier gas, and increases significantly with the decrease of the bubble radius of the magnesium vapor. The theoretical calculation results show that when the desulfurization temperature reaches 1573 K and the bubble radius of the magnesium vapor is refined to 0.175 mm, the residence time of magnesium vapor in the molten iron is equal to the reaction time, and the theoretical utilization rate of magnesium vapor can reach 100%. The model was validated by high temperature experiments using bottom blowing method. The experimental results indicate that the utilization rate of magnesium vapor in the desulfurization process is inversely proportional to the desulfurization temperature, the flow rate of the carrier gas and the mass of magnesium injection. Under the condition of 1573 K, the mass of hot metal 4.5 kg, the flow rate of 3 L/min and the injection mass of magnesium 1.55 g, the utilization rate of magnesium vapor can reach 83%. The calculated results of the model are well matched with the experimental data.

  • Naoki Kikuchi, Akitoshi Matsui, Yu-ichi Uchida
    Type: Regular Article
    2020 Volume 60 Issue 5 Pages 922-929
    Published: May 15, 2020
    Released: May 16, 2020
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    In order to clarify the effect of the lime (CaO) dissolution rate in slag on hot metal dephosphorization, a 150 kg scale hot metal dephosphorization experiment was carried out. The rate of decrease of free CaO and the dephosphorization rate were measured while varying the ratio of CaO and SiO2, which was defined as basicity.

    The dephosphorization rate showed its maximum value at basicity of around 1.0. At basicity higher than 1.0, the dephosphorization rate decreased due to poor dissolution of CaO in the flux.

    A thermodynamic calculation revealed that crystallization of 2CaO·SiO2(-3CaO·P2O5) in the liquid slag deteriorated CaO dissolution when basicity was higher than 1.5.

    The mass transfer coefficient of CaO in slag was calculated assuming that the interface between the CaO and liquid slag is saturated with CaO. High basicity showed a low mass transfer coefficient.

    The apparent slag viscosity was calculated in terms of the solid phase and showed a correlation with the CaO diffusion rate. The CaO diffusion rate in slag decreased with higher values of not only the liquid slag viscosity, but also the solid-liquid coexistent slag viscosity. These results suggest the existence of an optimum basicity for effective CaO dissolution.

Casting and Solidification
  • Tomohiro Nishimura, Kohei Morishita, Masato Yoshiya, Tomoya Nagira, Hi ...
    Type: Regular Article
    2020 Volume 60 Issue 5 Pages 930-938
    Published: May 15, 2020
    Released: May 16, 2020
    [Advance publication] Released: January 10, 2020
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    Time-resolved and in-situ observations using synchrotron radiation X-rays successfully proved that a massive-like transformation, in which the γ phase was produced through the solid–solid transformation and partitioning of substitute elements such as Mn and Si at the δγ interface was negligible, was selected in the unidirectional solidification of 0.3 mass% C steel at a pulling rate of 50 µm/s. The massive-like transformation produced fine γ grains near the front of the δγ interface. The coarse γ grains also grew behind the fine γ grains along the temperature gradient. Distance between the δγ front and the advancing front of coarse γ grains was as short as 200 µm. Namely, the fine γ grains disappeared within 10 s owing to growth of coarse γ grains. In addition, the observation of the δγ interface confirmed that a transition from the diffusion-controlled γ growth to the massive-like γ growth occurred at a growth velocity of 5 µm/s. Thus, the massive-like transformation is dominantly selected in the carbon steel during conventional solidification processes.

    Transmission images (left) and diffraction images (right) proved that diffusion-controlled growth of austenite phase was never observed and the massive-like δ-γ transformation was always selected during unidirectional solidification of Fe - 0.3 mass% C - 0.6 mass% Mn - 0.3 mass% Si at a pulling rate of 50 μm/s. The massive-like transformation is dominantly selected in conventional casting processes. Fullsize Image
Instrumentation, Control and System Engineering
  • Huaxin Yu, Dongcheng Wang, Hongmin Liu, Tongyuan Zhang, Lipo Yang
    Type: Regular Article
    2020 Volume 60 Issue 5 Pages 939-947
    Published: May 15, 2020
    Released: May 16, 2020
    [Advance publication] Released: January 18, 2020
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    For ultra-thin and super-wide cold rolled strips, the flatness problem are still very conspicuous. To ensure that the surface of a strip is not scratched, a whole-roller seamless flatness meter (abbreviated as WRS flatness meter) is developed. Due to the structure of the measuring roll, which has cylindrical holes under its surface, both the signal of the measuring channels and the signal of the adjacent channels will be significantly interfered when a load is applied to a WRS flatness meter, causing flatness measurement errors. To eliminate mutual interference between channels, flatness measuring principle and channel coupling mechanism are analyzed, and the concept and model of coupling coefficient are proposed. Then, some examples are given to illustrate significant errors caused by coupling, which demonstrates the necessity of decoupling the channels. Coupling coefficients between the channels are obtained by experimental calibration, and interference between the channels is eliminated with a decoupling matrix equation. Through simulation and industrial applications, it is shown that the theoretical model proposed in this paper realizes decoupling of the channels of the WRS flatness meter, which improves flatness detection and controlling accuracy.

  • Junichi Yotsuji, Kenji Ikushima, Hisato Yamada
    Type: Regular Article
    2020 Volume 60 Issue 5 Pages 948-953
    Published: May 15, 2020
    Released: May 16, 2020
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    Acoustically stimulated electromagnetic (ASEM) waves in thin steel sheets have been investigated for flaw detection. In the ASEM wave technique, magnetization is temporally modulated at the radio frequency (rf) of the irradiated ultrasonic waves through magnetomechanical coupling. The induced rf magnetic fields are detected by a resonant coil antenna and spatial images are obtained by scanning the ultrasound focal spot. In this work, we detected artificial defects (through holes) in thin steel sheets. Specific patterns of magnetic flux density caused by the hole were observed. By improving the sensitivity with a small coil antenna, we visualized a through hole 0.1 mm in diameter with a lift-off of 10 mm.

  • Fumito Ajioka, Zhi-Lei Wang, Toshio Ogawa, Yoshitaka Adachi
    Type: Regular Article
    2020 Volume 60 Issue 5 Pages 954-959
    Published: May 15, 2020
    Released: May 16, 2020
    [Advance publication] Released: January 18, 2020
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    We studied on automation of segmentation using deep learning, which has been remarkably developed in recent years.

    For the microstructural image of ferrite-martensite dual phase steel, we tried to segment the ferrite phase, martensite phase, and ferrite grain boundary in different colors individually. We created two models, SegNet and U-Net that can perform segmentation with high accuracy and compared the accuracy with an existing method.

    As a result, we demonstrated that models using deep leaning is more accurate than the existing method. In particular, U-Net model shows highly accuracy of segmentation for material microstructures.

  • Zhengmi Tang, Kohei Shimasaki, Mingjun Jiang, Takeshi Takaki, Idaku Is ...
    Type: Regular Article
    2020 Volume 60 Issue 5 Pages 960-970
    Published: May 15, 2020
    Released: May 16, 2020
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    In this study, the dynamic deflections and vibrations of a belt conveyor system operating in ironworks are observed using a high-speed telephoto mirror-drive active vision system that can simultaneously switch viewpoints and capture magnified images at the rate of hundreds of frames per second (fps). The active vision system captures images of a belt conveyor system at 160 fps using a pan-and-tilt scan mechanism. The images are captured as multiple high-frame-rate video images. The small deflections and vibrations, in multiple belts and pillars, which have peak frequencies of 10 Hz or more are estimated with a precision of dozens of micrometers using image analysis techniques such as digital image correlation when the camera system is placed at a distance of 5 m or more from the conveyor.

Chemical and Physical Analysis
  • Zhenzhen Wang, Yoshihiro Deguchi, Fangjung Shiou, Seiya Tanaka, Mincha ...
    Type: Regular Article
    2020 Volume 60 Issue 5 Pages 971-978
    Published: May 15, 2020
    Released: May 16, 2020
    [Advance publication] Released: December 28, 2019
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    The metallurgical industries are very important for social development. In order to improve the metallurgical techniques and quality of products, the real-time analysis and monitoring of iron and steel manufacturing processes are very significant. Laser-induced breakdown spectroscopy (LIBS) has been studied and applied for the contents measurement of iron and steel. In this paper, the remote open-path LIBS measurement was studied under different sample temperature, lens to target distance (LTD), sample angle conditions to clarify its online measurement features. The 3D profile measurement system of parallel laser beam fringes projection was also developed to measure the sample profile at different sample temperature. The measurement results demonstrated the robustness of remote open-path LIBS system and 3D profile measurement system. However, the correction is necessary to enhance the detection ability of LIBS online measurement. In order to improve the precision and accuracy of real-time elemental measurement, an innovative co-axial laser beam measurement system combining LIBS and 3D profile techniques is proposed to automatically adjust the focus unit and measure the sample components. The further study of this promising method will be developed for online application of iron and steel manufacturing processes.

    Proposed innovative co-axial laser beam measurement system of LIBS and 3D profile combination for iron and steel manufacturing processes. (Online version in color.) Fullsize Image
  • Daisuke Itabashi, Reiko Murao, Shunsuke Taniguchi, Kazumi Mizukami, Hi ...
    Type: Regular Article
    2020 Volume 60 Issue 5 Pages 979-987
    Published: May 15, 2020
    Released: May 16, 2020
    [Advance publication] Released: December 28, 2019
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    To investigate the performance of the size measurement by asymmetric flow field-flow fractionation (AF4), the measurement results of gold nanoparticles were compared among AF4, transmission electron microscopy (TEM), and small-angle X-ray scattering (SAXS) in terms of the average size and full width at half maximum (FWHM) of the size distribution. Although the average size was almost the same for the three methods, the FWHM measured using AF4 was larger than those measured using TEM and SAXS. This is attributed to the diffusion of the gold nanoparticles inside the AF4 instruments. The broadening coefficient of AF4 analysis was determined as 2.08 by the average of FWHM ratio of AF4 to TEM measured using the several sphere-like gold nanoparticles. In addition, the effect of particle shape on the above broadening coefficient was investigated using the sphere-like and plate-like silver nanoparticles. The broadening coefficient for plate-like particles apparently became smaller than that for sphere-like particles, possibly because the Brownian motion of plate-like particles was suppressed.

    Furthermore, the AF4 analysis with the FWHM correction method using the broadening coefficient was applied to niobium carbide (NbC) precipitates in steels. The average size measured by AF4 was mostly consistent with the results obtained in regions observed by TEM. Moreover, an increase in the number density of nanometer-sized NbC by heat treatment was successfully detected. The effect of particle shape on FWHM should be further investigated and improved; however, AF4 with the broadening coefficient can semi-quantitatively analyze the size distribution of nanoprecipitates in steels.

  • Francis Gyakwaa, Matti Aula, Tuomas Alatarvas, Tero Vuolio, Qifeng Shu ...
    Type: Regular Article
    2020 Volume 60 Issue 5 Pages 988-997
    Published: May 15, 2020
    Released: May 16, 2020
    [Advance publication] Released: January 09, 2020
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    The modification of MgO·Al2O3 spinel inclusions into less detrimental mixture phases of CaO–MgO–Al2O3 plays an essential role in refining calcium-treated aluminium killed steels. This study uses Raman spectroscopy for the characterisation of binary phase samples that contain MgO·Al2O3 spinel and calcium aluminate (CaO)x–(Al2O3)y phases. Samples were synthesised from MgO·Al2O3 spinel (MA), Al2O3 and calcium aluminate phases to achieve binary samples of CA–MA, C3A–MA, C12A7–MA and Al2O3–MA with varying phase fractions. The study also examined the possibility of a slight variation for non-stoichiometric spinel samples below the 1600°C region in an MgO–Al2O3 binary system. The relative intensities of the Raman band were used for the quantification of the phase fractions. For a quantitative prediction, linear regression calibration models were identified for each of the studied systems. This work demonstrates the use of Raman spectroscopy for the characterisation of calcium aluminate phases of CA, C3A, C12A7 and magnesium aluminate spinel phases along with Al2O3 and its potential application in inclusion characterisation.

Welding and Joining
  • Satoshi Yamane, Kouki Matsuo
    Type: Regular Article
    2020 Volume 60 Issue 5 Pages 998-1005
    Published: May 15, 2020
    Released: May 16, 2020
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    Plasma arc welding (PAW) was employed in joining thick materials with groove. Due to the high-density plasma arc, keyhole welding was used in the butt welding. The gap might be taken in places due to the heat distortion during the welding. To achieve a high-quality welding, the adaptive control is required according to the gap. The authors tried to apply CMOS camera to obtain information from the top surface and achieve synchronization between the camera shutter and welding current. Thus, a clear image of the weld pool with the keyhole and the gap was taken. The brightness distribution in the front of the weld pool becomes uneven due to the plasma arc. To classify the gap and select the welding conditions, such as the welding current and the plasma gas flow rate, the authors used one of the deep learning algorithms, convolutional neural network (CNN). In the training and testing data, the performance of the CNN was found to be satisfactory. However, if the plasma torch does not trace the welding line, the pool image will be different from the training image. Hence, the CNN will not be able to estimate the correct gap. To avoid this, the authors detected the welding line from the pool image and selected the image area near the welding line as the input of the CNN, i.e., the input area is selected based on the image obtained during the welding. The validity of the proposed method was verified by the welding experiments.

  • Tetsuro Hidaka, Nao-Aki Noda, Yoshikazu Sano, Nobuhiro Kai, Hiroyoshi ...
    Type: Regular Article
    2020 Volume 60 Issue 5 Pages 1006-1015
    Published: May 15, 2020
    Released: May 16, 2020
    [Advance publication] Released: January 18, 2020
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    In our earlier study, the authors revealed that the fatigue limit of ductile cast iron (DCI) specimens whose shapes are similar to the welded joint shapes is about three times larger than that of the welded joint specimens. However, since many defects are usually included in the DCI specimens, the fatigue limit of DCI joints decreases with increasing the maximum defect size. In this paper, therefore, the maximum defect size is estimated by using statistics of extremes. Then, the lowest fatigue limit corresponding to the maximum defect size is estimated from the 4 parameter model and compared with the lowest fatigue limit of the welded joint. As a result, it was confirmed that the lowest fatigue limit of the DCI specimens is about twice as large as the welded joint.

  • Kazuhiro Ogawa, Takahiro Osuki
    Type: Regular Article
    2020 Volume 60 Issue 5 Pages 1016-1021
    Published: May 15, 2020
    Released: May 16, 2020
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    In the weld metal the time-temperature-precipitation(TTP) diagram called as TTP curve of sigma phase in duplex stainless steel was investigated to clarify the difference from that in base metal or heat affected zone. Intermetallic phase such as sigma phase is harmful for mechanical properties of duplex stainless steels. The increasing of alloying elements of chromium and molybdenum effective for improving of corrosion resistance promotes the sigma phase precipitation in weldment. The weld metal consists of the different microstructure from the base metal. Because that consists of as solidified microstructure followed by transformation of a part of ferrite to austenite phase during cooling in welding process.

    Employing 25%Cr duplex stainless steel weld metal bearing 3% of Mo, 2% of W, 0.3% of N and various level of Ni, the time-temperature-precipitation(TTP) curve of each weld metal was obtained by the microstructure observation test with optical microscope after heated at various temperature. The TTP curve of weld metal is shifted to the direction of longer time comparing with that of the base metal with the almost same chemical composition. The mechanism of that shift was explained from a point of the difference of partitioning of alloying elements such as nickel and chromium between the phases by using the physical model proposed.

Surface Treatment and Corrosion
  • Xuehao Cheng, Xinfang Zhang
    Type: Regular Article
    2020 Volume 60 Issue 5 Pages 1022-1029
    Published: May 15, 2020
    Released: May 16, 2020
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    Precipitates that form at high temperatures detrimentally affect the mechanical properties and corrosion resistance of engineering structural materials. Different from the typical solution treatment, a method that incorporates electropulsing is proposed in this study. Electropulsing is applied to aged 316LN austenitic stainless steel, which undergoes the aging process at 650°C for 2000 h, microstructure characterization by SEM and TEM shows that electropulsing can aid in dissolving precipitates. Both the immersion and electrochemical tests showed a positive shift at corrosion potential and a decrease in the corrosion current density caused by electropulsing, and the corrosion resistance was improved. The change in the system’s free energy caused by the difference in the electrical conductivity between precipitates and matrix results in precipitate dissolution. This finding provides a technical reference for engineering applications, i.e., the on-line repair of properties in aged steels.

  • Le Quang Phan, Andrew Dennis Johnstone, Buyung Kosasih, Wayne Renshaw
    Type: Regular Article
    2020 Volume 60 Issue 5 Pages 1030-1039
    Published: May 15, 2020
    Released: May 16, 2020
    [Advance publication] Released: January 17, 2020
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    Smoothness of thin metallic coated strip produced in continuous galvanizing lines is influenced by fluctuations of the impinging wiping pressure. In this paper, vortex dynamics e.g. vortex production frequency and mixing of jet opposing shear layer vortices; and impinging pressure were numerically studied by Large Eddy Simulation (LES). The effects of jet nozzle width, d, and operational parameters (nozzle to strip distance, H, and mean jet velocity, Uo) were investigated. Vortex production rate is almost linearly correlated to Uo and mixing of shear layer vortices occurs when H/d ≥ 6. Dominant frequencies of impinging pressure fluctuation are significantly different between the two possible phenomena of i) Mixing of opposing shear layer vortices prior to jet impingement on the strip, or ii) No mixing of opposing shear layer vortices prior to jet impingement. The impinging pressure of a jet characterised by mixing of vortices is predominantly composed of frequencies lower than 10 kHz with the most significant components at less than 1 kHz. In contrast, for a jet with non-mixing of vortices, the impinging pressure fluctuations are comprised of frequencies greater than 10 kHz and the dominant frequency is approximately one half the vortex production frequency. Utilising existing model results for the coating thickness response to pressure and shear stress fluctuations12) the anticipated degree of coating thickness sensitivity to the mixing and non-mixing impinging jet cases of the present work has been elucidated. It is shown that a mixed vortices jet is most likely to cause surface ripples in the coating.

    Schematic diagram of the influence of mixing and non-mixing impinging jets on the time-varying wall pressure, and subsequently on the coating surface response. Not to scale. (Online version in color.) Fullsize Image
  • Ali Yahyaee soufiani, Joseph Robert Mcdermid, Andrew Nick Hrymak, Fran ...
    Type: Regular Article
    2020 Volume 60 Issue 5 Pages 1040-1051
    Published: May 15, 2020
    Released: May 16, 2020
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    This paper presents an investigation of the gas jet wiping process, which is used in the continuous galvanizing line to control the Zn-alloy coating thickness on steel substrates. In this study, a novel configuration of a multi-slot air knife was used as the wiping actuator in a parametric study of the gas jet wiping process. The main goal of the study was to identify the operating window in which lighter coating weights can be achieved with the multi-slot air knife at higher strip velocities. A laboratory scale wiping apparatus was designed and manufactured and the effects of various operating conditions, such as: main and auxiliary jet Reynolds numbers, strip velocity and jet-to-substrate distances on the final coating thickness were determined. Numerical simulations of multi-slot jet wiping were also performed under the same operating conditions using computational fluid dynamics modeling to estimate the pressure and shear stress profiles along with analytical models of the coating thickness to compare with the experimental measurements.

    It was observed that the experimental measurements, under different operating conditions for the multi-slot air knives, agreed with the coating weight predictions of analytical models available from the literature. The results showed that the coating weight produced by the multi-slot air knife, with a relatively low flow for the auxiliary jet (i.e. Rea/Rem ≤ 0.5), was lower than the final coating weight under similar main jet Reynolds from a single slot nozzle. Conversely, when Rea/Rem ~ 1, lower pressure gradient distribution found in the wiping region and consequently increasing the coating thickness.

Transformations and Microstructures
  • Eiji Tsuchiya, Yuta Matsumura, Yoshihiro Hosoya, Yuka Miyamoto, Takash ...
    Type: Regular Article
    2020 Volume 60 Issue 5 Pages 1052-1062
    Published: May 15, 2020
    Released: May 16, 2020
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    The effect of Nb addition of less than 0.05 mass% on the quenching and tempering behavior of spheroidized eutectoid steel, which has usually being applied to knitting needles, was investigated. The results obtained are as follows. 1) Hardenability upon brief heating was markedly improved by 0.01 mass% Nb addition. 2) Both quenching elongation and its standard deviation decreased with 0.01 mass% Nb addition compared with those of Nb-free steel. 3) While hardly any effect of Nb addition on the hardness was observed during low- temperature tempering, not only the impact toughness but also the fatigue durability was improved by 0.01 mass% Nb addition. 4) Atom probe tomography (APT) analyses revealed that the precipitation of carbon in solution directly resulted in the formation of ε- and/or θ-carbides with carbon contents of around 25 at% without the formation of clusters with 10–15 at% C upon the addition of a trace amount of Nb. 5) For the same P content, the average bulk concentration of P in the martensite phase markedly increased with the addition of up to 0.05 mass% Nb. 6) Regarding the optimum Nb content of 0.01 mass% for various mechanical properties upon the low-temperature tempering of martensite, it is considered that the mechanical properties are dominated by the balance between the positive effect of promoting carbide precipitation during low-temperature tempering by Nb addition and the negative effect of deteriorating the toughness with increasing bulk concentration of P in the martensitic phase upon the addition of more than 0.02 mass% Nb.

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