ISIJ International
Online ISSN : 1347-5460
Print ISSN : 0915-1559
ISSN-L : 0915-1559
Advance online publication
Displaying 1-28 of 28 articles from this issue
  • Guoxuan Li, Jinshan Liang, Jun Long, Dong Guan, Zushu Li, Seetharaman ...
    Article ID: ISIJINT-2021-578
    Published: 2022
    Advance online publication: May 21, 2022

    Iron and phosphorus were successfully separated from CaO–SiO2–FeO–P2O5 slag through atmospheric control, B2O3 addition and a combination of magnetic separation and flotation. For the slag with basicity (CaO/SiO2) of 2.5 and B2O3 addition of 6% (weight percentage), iron and phosphorus in the slag were enriched in the form of magnetite (Fe3O4) and calcium phosphate (Ca10P6O25) phases respectively under Ar atmosphere. Using a combination of magnetic separation and flotation, the concentrates were obtained with Fe3O4 and P2O5 content of 92.84% and 37.66% respectively, corresponding to the recovery ratios of 85.8% for iron and 91.3% for phosphorus.

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  • Hiroki Hasegawa, Ade Kurniawan, Itsuki Iwamoto, Rochim Bakti Cahyono, ...
    Article ID: ISIJINT-2021-552
    Published: 2022
    Advance online publication: May 20, 2022

    The iron and steel industries currently face the depletion of high-grade ore and high CO2 emissions. Some initiatives that effectively utilize alternative carbon sources and abundant low-grade ores become the preferable solutions. This novel study aims to utilize municipal solid waste (MSW) as a reducing agent in ironmaking using low-grade (goethite) ores. As an initial fundamental approach, the comparison of decomposition behaviors between the model and actual MSW was investigated in thermogravimetric analysis. Both model and actual MSWs mainly decompose at 300–500°C. As for reduction tests, pellets containing MSWs and ores with different pretreatments were prepared. The pellets were reduced in an Ar atmosphere at different temperatures. The effect of different ores: high-grade and low-grade ones, on the decomposition of MSW and the iron reduction, were investigated. As a result, interestingly, the low-grade, goethite ore-containing pellet exhibits a more significant reduction degree than the high-grade ones. The reduction is completed in 5 minutes at 700°C and above, indicating a significant reduction by the decomposed carbon. The reduction degree extends at elevated temperature, which reaches more than 94% at 900°C.

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  • Sheng Chao Duan, Joo Hyun Park
    Article ID: ISIJINT-2022-015
    Published: 2022
    Advance online publication: May 18, 2022

    The oxidation behavior of various reactive elements, such as Al/Ti, B/Si, and rare earth metals (REM), by electroslag remelting (ESR) type slag has been investigated utilizing the systematic thermodynamic analysis based on the calculated activity in the fluoride-containing slags by the ion and molecule coexistence theory (IMCT). The results indicate that the IMCT model can be reliably applied to calculate the activity of each component in the ESR type slag. The oxidation behavior of the various reactive elements is completely different by changing the same component in the slag, such as CaO and Al2O3, during the ESR process. Therefore, it is indispensable to find a key parameter to control the homogeneity of the reactive elements in remelted ingots by comparing the effects of the activity of each component and temperature on the equilibrium content of the various reactive elements in alloy melts. The results demonstrate that the oxidation loss of Al/Ti, B/Si, and REM (Ce and La) can be effectively prevented by employing TiO2, B2O3/SiO2, and CaO during the ESR process. Oxidation behavior of B/Si and Ce(La)/Al is weakly susceptible to temperature fluctuation compared with that of Al/Ti in alloy melts, which can be controlled by adding TiO2.

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  • Hitomi Iwaizako, Masayuki Okugawa, Kenji Saito, Yuichiro Koizumi, Akih ...
    Article ID: ISIJINT-2021-441
    Published: 2022
    Advance online publication: May 16, 2022

    Fe–Cr–Co alloys are becoming important as half-hard magnet which can be subjected to plastic deformation process for their novel applications including non-contact electromagnetic brake because of its large hysteresis loss. Its magnetic hardness depends on the modulated structure formed by spinodal decomposition. It is important to clarify the effect of plastic deformation on the spinodal decomposition for optimizing the heat treatment after plastic deformation process. In the present study, we examined the spinodal-decomposed structures in Fe–Cr–Co sheets cold-rolled to 25% reduction and that without rolling to clarify the influences of cold rolling. Also, spinodal decomposition under the presence of dislocation structure have been simulated by phase field method for the case with the presence of dislocation cell boundary with a high in-plane solute diffusivity at various migrating speed. It has been found that the spinodal decomposition is accelerated around dislocation owing to the elastic field and higher diffusivity, which results in inhomogeneous microstructure with various wave length of modulation. The existence of dislocation enhances the initiation of phase decomposition and the growth particles. The decomposed structure greatly depends on the in-plane solute diffusivity and migrating speed of the dislocation cell boundary.

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  • Wenxin Huang, Sheng Chang, Zongshu Zou, Hao Song, Yingxia Qu, Lei Shao ...
    Article ID: ISIJINT-2021-600
    Published: 2022
    Advance online publication: May 09, 2022

    Swirling flow tundish was developed to enhance the coalescence of inclusions, so as to deeply clean the liquid steel. Inclusions would gather to the center of the swirling flow by centripetal force, due to the density difference between inclusions and liquid steel. Thus, small inclusions can coalesce into larger ones, and then float to the free surface by their self-buoyance. Physical experiments were carried out in a 1/2.5 scale single strand tundish to study the flow characteristics of tundish with swirling chamber. Numerical modeling was developed to simulate the movements of small inclusions in swirling flow. Discrete phase model was employed together with the O'Rourke algorithm to characterize the coalescence of the inclusions in the swirling flow. The removal of inclusions was investigated, considering the absorption by upper slag and trapping by outside wall of ladle shroud. Compared with a turbulence inhibitor, a swirling chamber shows a similar effect on flow improvement, while performs better in inclusion removal, owing to the inclusion coalescence caused by centripetal force. The results revealed that swirling chamber in diameter of 450 mm is an optimized scheme for deep cleaning of liquid steel, with only 1.66% of the inclusions flowing out of the tundish nozzle.

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  • Haoying Yang, Hongbing Wang, Haihua Li, Xiaoping Song
    Article ID: ISIJINT-2022-035
    Published: 2022
    Advance online publication: April 23, 2022

    Object detection algorithms like Faster R-CNN have been widely used in the field of industrial defect detection. For weld defect detection, its detection accuracy for some small targets and difficult-to-classify defects is not high. This paper proposes a Cascade R-CNN detection model for weld defects based on bidirectional multi-scale feature fusion and shape pre-classification. There are defects of different sizes in the weld. In order to improve the detection ability of the model for multi-size defects, the model adopts the bidirectional feature pyramid network, in which an extra bottom-up path after the top-down path aggregation network and an extra edge from the original input to output node are added. According to the statistics of the proportion distribution of long and short axes of weld defects, the defects can be divided into two categories: long strip defects with the proportion of about 2:1 and approximate circle defects with a much bigger proportion. Therefore, each cascade detector is connected in parallel with a two-categories classifier for long strip and approximate circle defects and a five-categories classifier for five specific defects, so as to realize the pre-classification of two morphological defects and mine the difference between the two shapes of defects. In order to avoid over fitting caused by small datasets. Firstly, noise is added to augment the data. Then the training samples are expanded by random flip and mirror in the training, and OHEM is introduced to balance the selection of positive and negative samples. The experimental results show that the detection accuracy of the model on small targets and difficult-to-classify defects is significantly improved. The mAP value is increased by about 9.3% compared with the traditional Faster R-CNN and about 3.3% compared with the traditional Cascade R-CNN.

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  • Dongdong Zhou, Yujie Zhou, Xuemin Zhang, Ke Xu
    Article ID: ISIJINT-2021-585
    Published: 2022
    Advance online publication: April 20, 2022

    Heavy and medium plate (HMP) is a valuable and irreplaceable material that is widely used in pipelines, bridges, ships, building construction, and power plants. The quality of a plate's surface is closely related to its strength, hardness, and corrosion resistance. HMP is still trimming to length after the continuous casting process. The goal of this research is to evaluate surface quality quickly by combining online detection information of normal and periodic defects with professional quality control skills. To begin, this study creates a set of assessment criteria for HMP surface quality based on length, total defect area, and total area of various types of surface defects on the plate. The analytic hierarchy process (AHP) is then developed to identify based on the operator's experience and expert knowledge, the weights of the classified defects are then determined using the analytic hierarchy process (AHP). Finally, the evaluation grades for each plate may be calculated. The evaluation findings may benefit not only in improving production efficiency, lessening labor intensity, and reducing waste caused by cutting to length, but also in boosting the intelligent control capability of the HMP manufacturing technique.

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  • Pan Gao, Guosen Zhu, Fei Wang, Yanhui Sun
    Article ID: ISIJINT-2021-445
    Published: 2022
    Advance online publication: April 02, 2022

    A 1/5 scale water model simulation was used to study the kinetic mechanism for 300 t mechanical stirring desulfurization station, factors including rotating speed, immersion depth and size of impellers were investigated. The results show the dispersion behavior of desulfurizer can be divided into three stages, (1) the entrainment depth of desulfurizer increases with the rotating speed increasing, (2) the entrainment depth of the desulfurizer remains unchanged, (3) the desulfurizer is thrown out into the water from impeller blades. For the reason that the desulfurizer is sticky to the top surface and the middle of impeller blades, which will cause the shortening of the impeller life, the optimum rotating speed is the minimum rotating rate that the desulfurizer reaches the upper surface of impellers, which is related with immersion depth of impellers compared with the bath level and the length of impellers compared with the diameter of the ladle. After the application of a larger impeller, with 14.0% impeller life longer than before and the sulfur element is removed to 0.0006% on average.

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  • Nozomu Adachi, Yoshikazu Todaka, Tashika Masaki, Yoshinori Shiihara, T ...
    Article ID: ISIJINT-2021-512
    Published: 2022
    Advance online publication: March 28, 2022

    This study developed a deformation process to form a uniform nano-crystalline layer with relatively high thermal stability that can retain even after an induction heating and quenching process on a surface of cylinder-shaped sample. The effect of the surface nano-crystalline layer on rolling contact fatigue life of carbon steels (JIS S45C and S55C) was investigated. The sample with the surface nano-crystalline layer showed lower friction coefficient under cylindrical rolling contact condition comparing to that without the layer. The rolling contact fatigue life was extended to 4 times higher cycles by forming the nano-crystalline layer. It is presumable that the improvement of a rolling contact fatigue is owing to not only the high hardness but also the reduction of friction coefficient during the test followed by a suppression of dynamic tempering softening and variation of stress distribution.

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  • Tim Marinus Johannes Nijssen, Johannes Alfonsius Maria Kuipers, Jan va ...
    Article ID: ISIJINT-2021-521
    Published: 2022
    Advance online publication: March 28, 2022

    In this work, large-scale simulations of the blast furnace hearth are presented, conducted using a model combining Computational Fluid Dynamics, the Volume of Fluid method, and the Discrete Element Method. Using a 5 m diameter, full-3D geometry, the influence of burden weight, bi-disperse packing, and blocked tuyeres on the liquid and solids flow within the hearth are investigated. Horizontal and vertical porosity profiles are presented, and the influence of the dynamic liquid level on the state of the deadman is evaluated. The liquid iron flow during tapping is visualised, and the influence of a coke-free space on the flow pattern is analysed. The magnitude of the circumferential flow through the corner of the hearth is analysed, and found to decrease with increasing burden weight pressure and coke diameter in the bed centre. A significant influence of the dynamic deadman on the liquid flow pattern is found, especially in case of a floating deadman. In addition to the liquid flow, the solid coke flow towards the raceways is analysed. Two pathways for coke particles towards the raceway are uncovered, one path through the actively flowing layer above the deadman, and a second path moving through the deadman and entering the raceways from below. The balance between these two mechanisms was found to change during the tapping cycle. Lastly, implementations for heat and dissolved carbon mass transfer are presented, and demonstrated using a full-scale 10 m hearth simulation. Additional closures for heat and mass transfer rates are required, but the current model is found in good shape for future work.

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  • Shengcun Lin, Zhaoyao Zhou
    Article ID: ISIJINT-2021-542
    Published: 2022
    Advance online publication: March 24, 2022

    The low mechanical properties of porous materials are a common problem. It is of great significance to produce new porous metal materials with higher impact properties by new preparation process. In this work, a new stainless steel metal powder and wire mesh composite porous plate was prepared by vertical rolling, folding, pressing and sintering. Most importantly, a device, with ultrasonic generator, could generate porous strip, was invented innovatively to feed powder and wire mesh automatically for applying in the first step of the preparation process. The porous strip that used to prepare porous plate had the unique characteristic of a powder-wire mesh-powder sandwich structure. The impact behavior and microstructure were comparatively investigated. The fracture morphologies were observed to determine the fracture mechanisms. The results manifested that sintering temperature had the outstanding influence on the impact properties and fracture morphologies, higher temperature got stronger impact properties. Wedge fracture, cone fracture and circular fracture were main type of fractured warp wire when porous plate was sintered at 1130°C. Quicker ultrasonic frequency, gave little change in fractographs, but brought lower porosity and excellent impact properties, which due to more powder participate in preparation. The gap between two rollers determined combined degree between powder and wire mesh then led to various impact strength, and diameter of the necking with roller gap of 0 mm was less than 0.1 mm.

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  • Nathan Barrett, Subhasish Mitra, Hamid Doostmohammadi, Damien O’dea, P ...
    Article ID: ISIJINT-2021-574
    Published: 2022
    Advance online publication: March 24, 2022

    Present day, the production of hot metal (HM) via the blast furnace route remains an integral part of the global steel industry. With global pressure to curb greenhouse gas emissions, injection of hydrogen is considered a promising solution while ironmaking transitions to alternate technologies. A comprehensive heat and mass balance model calibrated to an operating blast furnace was used to assess the operational limits of hydrogen injection through the tuyeres, replacing Pulverised Coal Injection (PCI). Constrained by a minimum top gas temperature and minimum Raceway Adiabatic Flame Temperature (RAFT), the maximum injection rate was determined to be 19.5 kg-H2/t-HM when replacing 37.4 kg-PCI/t-HM (i.e. a replacement ratio of 1.9 kg-PCI/kg-H2 or 1.54 kg-C/kg-H2). At the maximum hydrogen injection rate, the specific CO2,eq emissions were seen to decrease by 8% in the top gas. In the case where the increased level of hydrogen increases stack reduction efficiency, the maximum hydrogen injection rate is decreased, while the replacement ratio is increased significantly. A maximum hydrogen injection rate of 14.3 kg-H2/t-HM with a replacement ratio of 4.5 kg-PCI/kg-H2 was achieved when the stack reduction efficiency was 100%, with a CO2,eq emission decrease of 14%. The optimal scenario for injection of hydrogen was determined to be maintaining a constant production rate, allowing the RAFT to decrease, and replacing PCI.

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  • Yang Gu, Hui-bin Wu, Zhi-Hui Zhang, Peng-Cheng Zhang, Rui Yuan, Xin-ti ...
    Article ID: ISIJINT-2021-471
    Published: 2022
    Advance online publication: March 16, 2022

    Medium-Cr steel is considered as the most economical and effective pipeline material in a CO2 flooding environment; however, the effects of iron oxides on the corrosion resistance of medium-Cr steel are still not clear. Therefore, in the present study, a high-temperature, high-pressure reactor was used to accelerate the corrosion of a rolled oxidized sheet of 5Cr steel. The corrosion behavior of oxide scale leaching was studied by comparing the mass loss and gain rate with bare steel, through using XRD, Raman, XPS, SEM, EDS, EPMA and other methods. It was found that the 5Cr steel rolled scale was composed of three layers: the innermost layer contained Fe3O4, the middle layer consisted of Fe2O3, and the outer layer contained Fe3O4 and a small amount of Fe2O3. In the CO2 environment, the oxide scale mainly dissolved in the initial stage, and corrosion products were deposited in the later stage. The oxide scale delayed the occurrence of corrosion, and eventually, similar to the bare steel, a double-layer product film was formed. Also, the possible corrosion mechanism of the medium-Cr steel with the oxide scale was given.

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  • Reinhold Schneider, Valentin Wiesinger, Siegfried Gelder, Alec Mitchel ...
    Article ID: ISIJINT-2021-498
    Published: 2022
    Advance online publication: March 16, 2022

    Energy consumption is a vital aspect in the production of high quality materials by using electro slag remelting as a refining method. The electrical conductivity and the melting point of the slag, determined by the slag composition, as well as the fill ratio, the amount of slag and the melt rate have as significant influence. Several of these factors as well as a new method to measure the slag surface temperature with a two-color pyrometer were investigated in laboratory scale remelting trials. Two different slags with an average and a low electrical conductivity were used. While there is only a small impact of the slag cap height and the melt rate, the slag composition showed a strong effect, both on the slag surface temperature as well as on the specific energy consumption. Additional investigations confirmed that the effect of the slag composition on the amount and compositions of non-metallic inclusions, respectively the cleanliness level of the steel after remelting is rather minor. The results suggest possibilities for easily applicable, improved process parameters, which can combine high product quality with significantly reduced energy consumption.

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  • Xiaobo Yan, Xueyou Wang, Shisong Wang, Shaoda Zhang, Xubin Zhang, Qian ...
    Article ID: ISIJINT-2021-565
    Published: 2022
    Advance online publication: March 16, 2022

    In this study, the rheological and crystallization behavior of CaO–BaO–Al2O3-based mold slags was investigated through the measurement of the viscosity-temperature relationship and the record of crystallization behavior during the continuous cooling process from 1300 to 600°C applying a modified confocal scanning laser microscopy. Variations of the viscosity, break temperature, initial crystallization temperature and crystallization phases of mold slags with the substitution of SiO2 by Al2O3 and Al2O3 by BaO at a gradient interval of 4 mass% were discussed, and crystallization parameters of average crystallization rate, Ozawa index and effective crystallization activation energy were calculated to explain the crystallization performance. The slag viscosity at 1300°C, the melting and break temperature increased with the substitution of SiO2 (16 to 0 mass%) by Al2O3 (20 to 36 mass%), while those decreased with the substitution of Al2O3 (32 to 16 mass%) by BaO (4 to 20 mass%). With the gradual substitution of SiO2 by Al2O3, the initial crystallization temperature increased from 820 to 1273°C at the cooling rate of 1°C/s, major precipitated phases gradually changed from CaF2 to CaF2, LiAlO2 and BaAl2O4, the average crystallization rate and the Ozawa index fluctuated but had the same tendency. With Al2O3 gradually replaced by BaO, the initial crystallization temperature decreased from 970 to 775°C at the cooling rate of 1°C/s, major precipitated phases changed from CaF2 to CaF2 and BaAl2O4, the crystallization rate of slags was affected by the difference of the nucleation and growth rate of different crystals.

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  • Tomotaka Otsu, Hideya Nakamura, Shuji Ohsaki, Satoru Watano, Shohei Fu ...
    Article ID: ISIJINT-2022-009
    Published: 2022
    Advance online publication: March 12, 2022

    Wet granulation of iron ore powders is a key process in ironmaking. In wet granulation, it is important to determine the optimum content of water added to the original ore powders. To determine the optimum water content, it is important to understand the saturation state in wet ore powder, which can be done by measuring the agitation torque of the wet powder. This study proposes a methodology for determining the optimum water content of various iron ore powders using the agitation torque of wet ore powders. First, measurement of the agitation torque and wet granulation of various iron ore powders were conducted. By comparing the results, it was found that the optimum water content, which was defined as the minimum water content required to diminish fine particles in the original powder, corresponded to the water content exhibiting the maximum agitation torque, regardless of the original powder. Using the agitation torque at different water contents, the saturation degree S, which is the volume ratio of water to the interparticle voids, was calculated, resulting in a range of 0.999 ≤ S ≤ 1.173 at the optimum water content. This suggests that the state between the funicular and capillary states is a suitable saturation state for the wet granulation of ore powders. Consequently, it was demonstrated that it is possible to determine the optimum water content for wet granulation of various iron ore powders based on the water content exhibiting the maximum agitation torque of wet ore powders.

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  • Yulin Zhu, Tao Li, Guozhang Tang, Yingjian Gu, Henan Cui
    Article ID: ISIJINT-2021-551
    Published: 2022
    Advance online publication: March 10, 2022

    To improve the performance of steel and produce clean steel with higher quality, the inclusions in the molten steel should be removed and controlled strictly. The removal behaviors of the inclusions, such as flotation, are largely dependent on their size and morphology, which determines the cleanliness of molten steel. It is thus of great importance to clarify the floating behaviors of inclusion clusters with different morphologies. However, the previous work on the floating behaviors of the inclusions is mostly on basis of the spherical inclusions and the influence of the morphology of the inclusions has not been clarified systematically. In this study, the 3D structure of inclusion clusters was established by 3D modeling and fabricated by 3D Printing Technology. A series of water model experiments were performed to investigate the influence of diameter and fractal dimension of inclusion clusters on the terminal floating velocity. In this study, the formula of the terminal floating velocity of the inclusion clusters is derived from the experimental results based on the previous work, which considers the influence of the fractal dimension of the inclusion clusters. It would provide fundamental supports for the research on the removal behaviors of inclusion clusters in molten steel.

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  • Qian Meng, Xiaopei Guo, Tao Li, Bo Shang, Liying Ju, Fucheng Zhang
    Article ID: ISIJINT-2022-014
    Published: 2022
    Advance online publication: March 09, 2022

    The complex inclusions of Al2O3–SiO2 is a common inclusion in the steel, which leads to the stress concentration in the steel products. The formation of MnS on the surface of Al2O3–SiO2 could reduce the stress concentration due to the high plasticity of the MnS. In this study, the formation of MnS–Al2SiO5 complex inclusions is investigated at the atomic level by using first principles calculation based on the density functional theory (DFT). The adsorption energy of the atoms of Mn and S on the Al2SiO5 (110) surface was calculated with various initial positions and sequence. The interaction among the atoms was calculated to analyze the stable structures after the adsorption of the Mn and S on the Al2SiO5 (110) surface. The formation of the MnS on the surface of Al2SiO5 was proved by analyze the structure formed by the adsorbed atoms that shows the similar tetracyclic structure of the MnS crystal.

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  • Susumu Imashuku, Kazuaki Wagatsuma
    Article ID: ISIJINT-2021-393
    Published: 2022
    Advance online publication: March 02, 2022

    This paper reviews three emission spectrometric methods that can be applicable to the inclusion analysis in steel materials, which provides information on the number, the size and distribution, as well as the chemical composition of inclusion particles. Cathodoluminescence (CL), X-ray-excited optical luminescence (XEOL), and beam-scanning laser-induced breakdown spectrometry (LIBS) are employed for the imaging measurement of the inclusions. As a typical specimen, alumina inclusions are evaluated to compare their analytical performance. The easy handling and rapid response of them are significant features for an application to the on-site/in-line analysis in the production site of steel.

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  • Toshiyuki Takahashi
    Article ID: ISIJINT-2021-292
    Published: 2022
    Advance online publication: February 24, 2022

    To evaluate microbial potentials for the material development of iron and steel slag, this study particularly investigated the chemical effect of slag, which was artificially coated with a microbial biofilm, on buffer action. Prior to evaluating the slag, this study also developed a method to determine the amount of microbes adhering to slag. To encourage the growth of Bacillus bacteria on slag, the slag was mixed with the bacteria in LB medium for 24 hours. After extracting microbial DNA using the hot-alkaline DNA extraction method, the microbial quantity attaching tightly to slag was determined from the concentration of the microbial DNA using Pico Green-based fluorometry. The adsorption isotherm between the microbial quantity attached to the slag and the corresponding reacting microbial amount was analyzed using the Langmuir and Freundlich adsorption models. To examine the buffering action of slag coated with and without microbes, each slag was immersed in distilled water for seven days. Next, both pH levels of each slag-containing solution and each amount of microbes attached to slag were determined. The pH increased in both solutions containing slag coated without biofilm and with partially desquamated one; in contrast, the slag coated with well-preserved biofilm showed a buffering action, resulting in an inhibited increase in pH. These results show that slag coated with biofilm is distinctively different from an original slag coated without biofilm in terms of buffer action. This processing technique using microbes could contribute to the development of a novel application of slag as a recycled material.

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  • Seongtak Oh, Hirotoyo Nakashima, Ryosuke Yamagata, Satoru Kobayashi, M ...
    Article ID: ISIJINT-2021-309
    Published: 2022
    Advance online publication: February 23, 2022

    Effects of Zr addition on the microstructure formation of two intermetallic phases of Fe2Nb Laves (TCP) and Ni3Nb (GCP) in a carbon-free Fe-20Cr-35Ni-2.5Nb (at.%) novel austenitic heat-resistant steel at elevated temperatures were examined from the viewpoints of thermodynamics and kinetics. From a thermodynamic perspective, the maximum solubility of Zr in the matrix phase is about 0.1 at.% under homogenization treated states at around 1473 K. The dissolved Zr in the matrix eventually reduces the solubility limit of Nb, thereby playing an important role in promoting the formation of grain-boundary Fe2Nb phase as well as that of thermodynamically stable Ni3Nb-δ phase within grain interiors after long-term aging at 1073 K. The Zr dissolved in the matrix after the homogenization treatment is found to enrich in δ phase formed during the aging and raises the temperature limit of formation of δ phase by about 50 K. From a kinetic perspective, because of the increase in Nb supersaturation, the dissolved Zr not only enhances the precipitation of Laves phase at grain boundaries but also promotes the homogeneous nucleation of metastable Ni3Nb-γ″ phase within grain interiors from the beginning of aging and retards the phase transformation from γ″ to δ. These microstructure changes by Zr can be interpreted in terms of the phase equilibria and relative phase stability among different phases. The present results reveal that even a small amount of Zr in solution, not segregation to grain boundaries, has tremendous effects on the precipitation behavior and morphology of intermetallic phases.

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  • Hao FENG, Huabing LI, Xuze LI, Zhouhua JIANG, Shouxing YANG, Hongchun ...
    Article ID: ISIJINT-2021-447
    Published: 2022
    Advance online publication: February 18, 2022

    The precise control of nitrogen content in high nitrogen martensitic stainless steel is the guarantee of its excellent performance, and is a crucial issue for its manufacturing process using pressurized gas nitriding. In this paper, the effects of nitrogen pressure, temperature and alloy composition on nitrogen solubility and pressurized gas nitriding kinetics of Fe–Cr–Mo–C alloy melts were investigated. In order to correct the deviation of nitrogen solubility from Sieverts' law under high nitrogen pressure, the first- and second-order interaction parameters of nitrogen on itself were obtained; accordingly, the nitrogen solubility model under pressure was established and well verified. The nitrogen solubility decreased with increasing temperature and C content, and increased with increasing Cr and Mo contents. The nitrogen mass transfer in liquid phase was the nitriding rate-determining step under different nitrogen pressures. The apparent mass transfer coefficient of nitrogen was approximately 0.0218 to 0.0230 cm·s-1, and showed a weak dependence with pressure. The apparent mass transfer coefficient of nitrogen increased significantly with the increase of temperature and electromagnetic stirring, and was rarely affected by the content of Cr, Mo and C. Considering the influence of pressure change on nitriding rate in pressurization stage, a novel kinetic model for gas nitriding under high pressure was established and exhibited effectiveness in practice.

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  • Daisuke Itabashi, Kazumi Mizukami
    Article ID: ISIJINT-2021-416
    Published: 2022
    Advance online publication: February 12, 2022

    To determine the number density of fine precipitates in steels by asymmetric flow field-flow fractionation (AF4) with inductively coupled plasma–mass spectrometry (ICP–MS), an analysis method employing flow injection was investigated. For accurate calibration, matrix matching was performed by mixing the standard solution and AF4 carrier solution in front of a nebulizer. Two surfactants were used for AF4 separation; it was found that the appropriate selection of surfactants based on their acidity constant is essential to avoid salt precipitation. In addition, the effect of the AF4 retention time on recovery was investigated. A long retention time led to the adsorption and aggregation of the samples in the AF4 separation channel. Results showed that an AF4 retention time within 20 min facilitated superior recovery. Moreover, five types of AuNPs were analyzed via AF4–ICP–MS and quantified using flow injection analysis. Good analytical performance was achieved for all AuNPs and the recoveries exceeded 93%, and the coefficient of variation was within 5%. The effect of particle size on the recovery was not confirmed.

    Furthermore, the developed flow injection analysis for AF4–ICP–MS was applied to evaluate niobium carbide (NbC) precipitates in steels. The number density of nanometer-sized NbC was quantified to be within 1013 to 1014 particles per 1 g of Fe. It was quantitatively confirmed that the long-duration heat treatment led to an increase in the number density of nanometer-sized NbC. Hence, this method can be useful for quantitatively analyzing the size and number density of nanoprecipitates in steels.

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  • Takuya Mori, Takahiro Ozawa, Yuki Hiruta, Seigo Matsunaga, Kazunori Fu ...
    Article ID: ISIJINT-2021-419
    Published: 2022
    Advance online publication: February 05, 2022

    In this study, using two-dimensional X-ray absorption fine structure (2D-XAFS), the structures of atmospheric corrosion products on the steel surfaces were observed in the cross-sectional direction. For 2D-XAFS measurement, the sample was sliced from the surface of the rust toward the substrate using a microtome and thinned below 10 µm. In addition, the 2D-XAFS data incorporated an unsupervised learning denoising method based on the Noise2Noise algorithm for achieving high spatial resolutions. The results demonstrate that the structure of the amorphous rust exhibits an α-FeOOH-like structure, and the analysis of the composition ratio of Fe3O4, α-FeOOH, and γ-FeOOH revealed the distribution the Fe3O4 ratio. This distribution provided compelling evidence for proceedings of the reactions suggested by Evans et al. during the atmospheric exposure test.

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  • Motomichi Koyama, Takayuki Yamashita, Satoshi Morooka, Takahiro Sawagu ...
    Article ID: ISIJINT-2021-510
    Published: 2022
    Advance online publication: February 03, 2022

    The local plasticity and associated microstructure evolution in Fe-5Mn-0.1C medium-Mn steel (wt.%) were investigated in this study. Specifically, the micro-deformation mechanism during Lüders banding was characterized based on multi-scale electron backscatter diffraction measurements and electron channeling contrast imaging. Similar to other medium-Mn steels, the Fe-5Mn-0.1C steel showed discontinuous macroscopic deformation, preferential plastic deformation in austenite, and deformation-induced martensitic transformation during Lüders deformation. Hexagonal close-packed martensite was also observed as an intermediate phase. Furthermore, an in-situ neutron diffraction experiment revealed that the pre-existing body-centered cubic phase, which was mainly ferrite, was a minor deformation path, although ferrite was the major constituent phase.

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  • Hiroyuki Uchima, Masayoshi Kumagai, Junzo Shimbe, Akihiro Tanabe, Yuta ...
    Article ID: ISIJINT-2021-443
    Published: 2022
    Advance online publication: January 07, 2022

    Middle-carbon martensite steels are vital materials for mechanical components and their mechanical properties have attracted significant interest. However, the decrease in the elastic limit of the as-quenched materials is one of the remaining puzzles. Herein, we quantitatively characterized the dislocation density and its structure in the as-quenched and tempered martensite steel by neutron diffraction line profile analysis and discussed their impact on the yield stress. The dislocation density in the as-quenched specimen was the highest at 9.7 × 1015 m-2, while it decreased with an increase in the tempering temperature. In addition, the component ratios of edge and screw dislocations decreased and increased, respectively, depending on the increase in the tempering temperature. The dislocation arrangement parameter (M) varied between the tempering temperatures of 220 and 290°C. Although there was a large difference between the yield stress obtained from the tensile test and that estimated from the dislocation density, the experimental results could be explained by correcting them with the inverse of M value as an index showing the effective dislocation density ratio.

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  • Phatchada Santawaja, Shinji Kudo, Atsushi Tahara, Shusaku Asano, Jun-i ...
    Article ID: ISIJINT-2020-726
    Published: 2021
    Advance online publication: April 16, 2021

    Oxalic acid has been identified as a sustainable chemical enabling an efficient recovery of target metals from industrial minerals by dissolution. The dissolution process recently has attracted attention as a key reaction in a potential clean iron-making. In this application to efficiently produce a high-purity iron, the dissolution is required to occur in the absence of light, with no addition of other chemical reagents, and to produce high concentration iron oxalate aqueous solution as fast as possible. To reveal the chemistry of iron oxide dissolution for this application, in the present study, the dissolution experiments are carried out under various conditions with a particular focus on the iron oxide highly loaded in the oxalic acid aqueous solution. Highly acidic oxalic acid solution for dissolving the highly loaded iron oxide enabled the production of iron oxalates aqueous solution with the concentration of up to 0.56 mol-Fe/L. Different from conventional studies under diluted conditions with pH control, the dissolution followed a non-reductive mechanism, producing [Fe3+HC2O4]2+ as a dominant iron species, and highly correlated with a concentration of proton in the solution. The experimental results and proposed stoichiometries identified a minimum amount of oxalic acid required for the complete dissolution of iron oxide independently from the concentration and type of loaded iron oxide. Among iron oxides tested (α-Fe2O3, FeOOH and Fe3O4) as the feedstock, Fe3O4 had an advantage in the dissolution rate, but showed a relatively low iron recovery in the solution (80–90%) because of an unavoidable formation of FeC2O4·2H2O precipitates.

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  • Ryan ROBINSON, Liviu BRABIE, Magnus PETTERSSON, Marko AMOVIC, Rolf LJU ...
    Article ID: ISIJINT-2020-135
    Published: 2020
    Advance online publication: August 27, 2020

    Approximately 60–70% of the direct greenhouse gas emissions in electric arc furnace (EAF) steelmaking originate from the use of fossil carbon charge during melting of steel scrap. Regarding short-term solutions to mitigate the climate impact of steelmaking, there is greater potential to replace fossil carbon charge with renewable carbon in the EAF than in integrated blast furnace steelmaking where mechanical strength requirements on carbon charge are too demanding. Therefore, the present study aims to provide an experimental and practical foundation for using renewable biochar in the EAF as a relatively simple step to decrease the climate impact of steelmaking.

    In order to evaluate the inherent performance of biochar as a carburizing agent, lab-scale tests where completed using four different types of carbonaceous materials: synthetic graphite, anthracite coal and two types of biochar from woody biomass (BC1 and BC2). The first order dissolution rate constants from experiments ranged between 0.7 to 1.9 × 10-4 m/s, which agrees well with previously reported results. Furthermore, lab-scale results show that biochar properties commonly seen as detrimental, such as low carbon crystallinity and high porosity, do not necessarily constitute a disadvantage for biochar utilization as carburizer in steelmaking.

    In order to further assess the results from lab-scale tests, an industrial trial including six consecutive heats was performed in a 50 t EAF at the Höganäs Halmstad Plant. Results show that 33% substitution of standard Anthracite carbon charge with biochar BC2 gave no deviation from normal operating conditions in the EAF.

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