ISIJ International
Online ISSN : 1347-5460
Print ISSN : 0915-1559
ISSN-L : 0915-1559
Advance online publication
Displaying 1-20 of 20 articles from this issue
  • Yoshiya Matsukawa, Yusuke Nakamura, Daisuke Igawa, Takashi Matsui, Hid ...
    Article type: Regular Article
    Article ID: ISIJINT-2024-027
    Published: 2024
    Advance online publication: May 30, 2024
    JOURNAL OPEN ACCESS ADVANCE PUBLICATION

    Image analysis was performed on the analytical objects constructed by X-ray computed tomography images of coke with anthracite as low swelling and softening-melting coal to evaluate the pore structure three-dimensionally. The sphericity of the pores was calculated based on the volume and surface area of the pores. Tracers were inserted into all anthracite particles to distinguish coke matrices derived from anthracite from those derived from coking coal. This is the first success in identifying coke matrices derived from coking coal and those derived from other carbon source. The sphericity of pores in the surrounding area of the coke matrix derived from anthracite and the area without coke matrix derived from anthracite was extracted from the analytical object. The surrounding area of the coke matrix derived from anthracite contained more low-sphericity pores compared to the area without coke matrix derived from anthracite. This may be due to the free expansion of the coking coal in the surrounding area of the anthracite since the anthracite does not soften or expand during carbonization. The coking coal expanded excessively to fill the voids between the particles, resulting in the bubbles bursting and generating connected pores.

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  • Takayuki Yonezawa, Pengjun Luo, Seiichiro Tsutsumi
    Article type: Regular Article
    Article ID: ISIJINT-2024-124
    Published: 2024
    Advance online publication: May 29, 2024
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    In this paper, fatigue crack initiation and propagation properties of three structural steels with different static strength and cyclic softening behavior were evaluated and compared with the estimation results based on the local strain response. Steel C had the highest cyclic softening rate, which was 10 times that of steel A and twice that of steel B. The relationship between strain range and fatigue life in the fatigue life region shorter than 10^5 cycles was almost the same regardless of the test steels. The fatigue crack initiation life from the notch bottom of the SENT specimen was almost the same independent of static strength and cyclic softening rate. The crack initiation life estimated from the Strain range versus fatigue life equation using the local strain response measured by DIC was roughly in agreement with the experimental results. Steel C had the highest crack opening load and the slowest fatigue crack propagation rate compared to the other two steels. The local strain range at the fatigue crack tip showed a good correlation with the fatigue crack propagation rate irrespective of the steel grade. In addition, the estimates of fatigue crack propagation rate based on the local strain response were in almost agreement with the experimental results.

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  • Georg Gaiser, Peter Presoly, Christian Bernhard, Kerstin Baumgartner, ...
    Article type: Regular Article
    Article ID: ISIJINT-2024-072
    Published: 2024
    Advance online publication: May 27, 2024
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    Oxidation processes are unavoidable in continuous casting and further hot processing of steel. A deeper understanding of the occurring phenomena such as intergranular oxidation and liquid metal infiltration of grain boundaries is essential to continuously improve the quality of the products. In this study, oxidation experiments were performed with simultaneous thermal analysis for two thin slab casting and rolling applications under near-process conditions up to the point prior to the first reduction stage. The experiments were performed for two low-carbon steels contaminated with undesirable tramp elements (Cu, Sn, …). In addition, the two steels contain Silicon at different levels. The results show that for the "Endless Strip Production" process (ESP), intergranular oxidation is significantly less pronounced compared to a "Thin Slab Casting and Rolling process" with a gas-fired tunnel furnace (TSCR TF). Due to the short process time at high temperatures in the ESP process, hardly any liquid metal infiltration by copper appears. In low silicon steel, intergranular oxidation results from various oxides, and liquid metal infiltration appears simultaneously in the TSCR TF process. Furthermore, the yield loss from oxidation is significantly higher in the TSCR TF process. The change from a natural gas combustion atmosphere to a hydrogen combustion atmosphere further increases the oxidation rate and results in a higher mass loss.

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  • Miku Watanabe, Goro Miyamoto, Yongjie Zhang, Satoshi Morooka, Stefanus ...
    Article type: Regular Article
    Article ID: ISIJINT-2024-044
    Published: 2024
    Advance online publication: May 21, 2024
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    The mechanical properties of TRIP steels depend on heterogeneities of chemical composition and grain size in the retained γ structure, although these heterogeneities have not been characterized in detail. Therefore, in this study, we quantitatively investigate the inhomogeneous carbon concentration and grain size distribution, and its effects on the thermal stability of the retained γ in Fe-2Mn-1.5Si-0.4C (mass%) TRIP steel using FE-EPMA, EBSD, Mössbauer spectroscopy, and in-situ neutron diffraction during bainitic transformation at 673 K. In-situ neutron diffraction experiments detects high-carbon γ evolving during bainite transformation, in addition to the original γ, and the time variation of the volume fraction of high-carbon γ agrees well with the fraction of γ retained at room temperature. Williamson-Hall analysis based on peak width suggests that heterogeneity of carbon content exists even within the high-carbon γ. Compositional analysis using FE-EPMA and three-dimensional atom probe directly revealed that fine filmy γ was highly enriched with carbon compared to larger blocky γ, and the carbon content in blocky γ decreases with increasing blocky γ size. DICTRA simulation qualitatively reproduces the size dependency of carbon enrichment into γ. It was also found that γ tends to be retained at higher carbon content and smaller γ grain size since the smaller grain size directly improves thermal stability and the smaller γ size further contributes to the thermal stability via enhanced carbon enrichment.

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  • Shinya Miura, Teppei Tamura, Ken-ichiro Naito
    Article type: Regular Article
    Article ID: ISIJINT-2024-068
    Published: 2024
    Advance online publication: May 20, 2024
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    As for steelmaking process such as basic oxygen furnace (BOF) and electric arc furnace (EAF), slag foaming consists of introducing gas bubbles into molten metal and slag by chemical reaction. In the case of the BOF process, excessive foaming is over the converter capacity, a phenomenon called "slopping". Slopping reduces yield and equipment lifespan and increases production time. It is therefore important to control slag foaming properly. In previous studies by other investigators, the jet from top lance in BOF process effectively suppresses slag foaming. However, it is not obvious which mechanism of the jet from top lance is effective to suppress slag foaming, and its quantitative effect has not been reported. To clarify the relationship between slag foaming and the jet from top lance, the effects of the number of nozzle holes and lance height on the slag foaming were investigated by using a converter-shaped water-model device and test converter. The experimental results indicated that slag foaming height decreased as the number of nozzle holes increased. Also, slag foaming height changed instantly with the change in lance height, e.g., slag foaming height decreased as lance height increased, and vice versa. The foaming suppression mechanism of the jet from top lance is the entrainment of foaming slag into the jet. Consequently, slag foaming model that takes the effect of the jet from top lance into account is proposed. And it enables to predict the change in slag foaming height with time.

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  • Yuto Kadowaki, Yoko Yanagihara, Arinori Inagawa, Nobuo Uehara
    Article type: Regular Article
    Article ID: ISIJINT-2024-066
    Published: 2024
    Advance online publication: May 17, 2024
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    Total iron contents in iron ores have been accurately determined by JIS M 8212, in which iron ions in digested solutions of iron ores are reduced to divalent prior to redox titration. It is necessary for the iron reduction process that no reducing chemicals other than iron(II) in the decomposition solutions must not remain after the reduction with titanium(III). However, the redox reactions concerning the chemical species present in the decomposition solution has not been completely elucidated at the present time. In this paper, the redox reactions that occurred in the decomposition solution during the iron reduction in JIS M 8212 were studied by potentiometry and spectrophotometry under nitrogen atmosphere. The redox reaction of tin(II)/(IV) was very slow, causing significant effects on identifying the end point of the indicator for the iron reduction. The copper chloro-complexes were reduced with titanium(III) at a potential higher than that of indigo carmine used as a redox indicator, so that the reduced copper(I) gave a positive error to the potassium dichromate titration. The pentavalent vanadium was reduced with titanium (III) to form a complex with titanium, which also interfered with the potassium dichromate titration positively. To avoid these interferences, titanium(III) chloride was stoichiometrically added to the reaction mixture after addition of tin(II) chloride under nitrogen atmosphere so as to reduce only iron to divalent prior to the following redox titration. Combination of the proposed protocol with the potassium dichromate titration could successfully determine the iron content of certified reference materials of iron ores.

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  • Wei Mo, Yuxin Feng, Longlin Zhou, Jinlin Yang, Xiujuan Su, Jinpeng Fen ...
    Article type: Regular Article
    Article ID: ISIJINT-2024-006
    Published: 2024
    Advance online publication: May 14, 2024
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    Sodium modification is an effective approach for enhancing the properties of bentonite and reducing its usage in pellets. However, due to limited research, the relationship between the physicochemical properties of bentonite and its green ball properties remains unclear, and the optimal degree of modification for bentonite has rarely been discussed. Therefore, this paper proposes a novel research idea: to exploring the correlation between the five most commonly used indexes for evaluating the physicochemical properties of bentonite (water absorption, methylene blue index, swell capacity, colloid index, and cation exchange capacity) and the most frequently used evaluation indexes for assessing green ball performance (drop strength), in order to determine the optimal degree of sodium modification of bentonite for pellets. The response surface methodology was employed in this paper to investigate the quantitative relationship between the five indexes and the green ball drop strength. The results demonstrate that when the drop strength of the green ball reaches its optimal level, the five commonly used indicators of bentonite are as follows: water absorption is 545.27%, methylene blue index is 22.94g/100g, swell capacity is 72.36ml/g, colloid index is 35.95ml/g, and cation exchange capacity is 68.93mmol/100g. Under these conditions, it has been the predicted value for green ball drop strength is determined to be 12.88, which exceeds the maximum value in the experimental conditions by 48.05%. The study determined the optimal degree of sodium modification for bentonite in pelletizing, providing valuable guidance for optimizing the properties of bentonite.

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  • Ryota Higashi, Daisuke Maruoka, Yuji Iwami, Taichi Murakami
    Article type: Short Article
    Article ID: ISIJINT-2024-073
    Published: 2024
    Advance online publication: May 14, 2024
    JOURNAL OPEN ACCESS ADVANCE PUBLICATION

    The iron and steelmaking industry must focus on neutralizing CO2 emissions. One solution involves using hydrogen as a reducing agent for iron ore. However, carbon is an essential element as primary steel is produced by refining molten carbon-saturated iron (hot metal). Ironmaking processes applying CO2 capture and utilization have been suggested; however, they are limited to the reduction process. To satisfy the demand for primary steel production with net-zero CO2 emissions, a new carbon recycling ironmaking process capable of producing hot metal must be considered. This study proposes a carbon recycling ironmaking process using deposited carbon-iron ore composite (CRIP-D). In the CRIP-D process, hot metal is produced by using the solid carbon recovered by reforming exhaust gas as reducing and carburizing agents. Moreover, using the recovered solid carbon, iron oxides are reduced more rapidly, and reduced iron is melted at a lower temperature than that using fossil fuel-derived carbon. This means carbon-neutral steel can be produced more efficiently than conventional ironmaking processes. Using proven technologies, following hot metal production, primary steel can be produced while minimizing the burden on the steel mills for converting equipment. Thus, true carbon-neutral primary steel is feasible using the proposed CRIP-D process.

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  • Shuyi Zhou, Xiaoyan Liu
    Article type: Regular Article
    Article ID: ISIJINT-2023-459
    Published: 2024
    Advance online publication: May 10, 2024
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    3D vision technologies have been widely used in metallurgy industry to measure particle size distribution (PSD) of green pellets on conveyor. However, 3D camera only captures the point clouds of surface pellets, and algorithms measure the surface PSD. To what extent the measured surface PSD can reflect whole PSD is a question that hasn't been answered yet. In the present work, a simulation method is proposed to analyze the PSD measurement error of green pellets. First, the motion process of green pellets on conveyor is simulated by discrete element method to obtain PSD of whole pellets; then, a transformation method is proposed to generate point clouds of simulated surface pellets, and region growing-based method is adopted to measure the PSD of surface pellets; finally, the PSD measuring error can be obtained by comparing surface PSD and whole PSD of pellets. Error analysis of green pellet size distribution measurement on conveyors is conducted, in aspects of camera location, patch number of point clouds, thickness as well as size distribution of pellet bed. Results illustrate that although the PSD measuring error (up to 12.3%) cannot be neglected when camera is installed above conveyor, it can be effectively reduced by increasing the patch number of captured point clouds (reduced by more than 7.4%) or installing camera near discharge of conveyor (reduced to less than 3.1%).

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  • Song Liu, Weijian Feng, Jun Zhao, Zhiwei Zhao, Xiaojie Liu, Ran Liu, Q ...
    Article type: Regular Article
    Article ID: ISIJINT-2023-450
    Published: 2024
    Advance online publication: April 30, 2024
    JOURNAL OPEN ACCESS ADVANCE PUBLICATION

    Aiming at the problem of coadjustment of blast furnace raw materials and operation parameters, this paper proposes a cooptimization model of blast furnace batching that integrates Random Forest and NSGA-Ⅲ (Non-dominated Sorting Genetic Algorithm III) algorithm. First, blast furnace field data were collected for a two-year time span, and a predictive model for CO2 emissions and blast furnace permeability was constructed using the Random Forest algorithm; taking the goodness of fit (R2), mean square error (MSE) and mean absolute error (MAE) as the evaluation indexes, the R2 of the two prediction models obtained reached 0.93 and 0.96 respectively, and the MSE and MAE tended to be close to the zero value. Then, NSGA-Ⅲ was used to establish the blast furnace batching optimization model to optimally solve the batching scheme and the corresponding blast furnace operating parameters by taking the lowest batching cost, the lowest carbon dioxide emission and the maximum blast furnace permeability as the objective function, and the composition requirement of raw materials and the range limitation of operating parameters as the constraints; finally, the model was validated using the actual on-site data, and the application results showed that the output of the model conformed to the Finally, the results show that the model output meets the composition requirements and obtains a lower-cost dosage scheme than the original dosage ratio; moreover, this scheme corresponds to a blast furnace with less carbon dioxide emission, better blast furnace permeability and less slag. Therefore, the model can provide an effective reference for field operators to optimize blast furnace batching and operation.

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  • Dongfeng He, Chengwei Song, Yuanzheng Guo, Kai Feng
    Article type: Regular Article
    Article ID: ISIJINT-2024-058
    Published: 2024
    Advance online publication: April 30, 2024
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    Accurately predicting the end temperature of molten steel is significant for controlling ladle furnace (LF) refining. This paper proposes an error correction method called EC-CBR based on case-based reasoning (CBR) to reduce errors in the prediction models caused by discrepancies between actual production data and training data. The proposed method combines the incremental learning advantage of CBR with the ability of other models to fit nonlinear relations. First, a prediction model is established, and historical heats similar to the new heat are retrieved by CBR. Then, the model error of the new heat is calculated by employing the errors of similar heats. The prediction result is calculated by subtracting the error from the predicted value. Testing and comparison are conducted on the models (support vector regression, backpropagation neural network, extreme learning machine and mechanism model) and general CBR using actual production data. Results show that the EC-CBR is effective for both data-driven and mechanism models, with an increase of approximately 5% in hit rate within the range of ±5°C for data-driven models and an increase of 21.73% for mechanism model. Moreover, the corrected data-driven models show higher accuracy than the general CBR, further proving the effectiveness of the proposed method.

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  • Fuhai Liu, Chen Sun, Rong Zhu, Kai Dong, Guangsheng Wei, Ruiguo Bai
    Article type: Regular Article
    Article ID: ISIJINT-2023-169
    Published: 2024
    Advance online publication: April 25, 2024
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    The O2 and N2 mixing top-blowing method could effectively improve the mixing degree and suppress the temperature increase rate of the molten bath in vanadium extraction converter. In this paper, four kinds of top-blowing lances designed by an extra N2 flow rate and various Mach numbers have been investigated by a series of water experiments and numerical simulations. On the basis of result, the mixing time was first increased and then decreased with the increase of lance height, and the lance height of 1400mm obtained the longest mixing time. There were two high-velocity regions generated by impaction of top-blowing jets and stirring of bottom-blowing bubbles. Simultaneously, there were two low-velocity regions formed by the block of furnace wall, and one low-velocity region formed by the local eddy. Comparing with the current top-blowing lance, all three new kinds of top-blowing lances obviously improved the kinetic condition and impaction cavity area of molten bath, which would further be improved with a larger design Mach number. Therefore, an appropriate top-blowing lance had been selected in the industrial application research, which achieved a shorter melting time and a faster vanadium extraction rate, in contrast to the current lance.

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  • Vignesh Krishnan, Velmurugan Paramasivam
    Article type: Regular Article
    Article ID: ISIJINT-2024-011
    Published: 2024
    Advance online publication: April 25, 2024
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    Austenitic Stainless Steel (ASS) and Duplex Stainless Steel (DSS) are joined to optimize the Resistance Spot Welding (RSW) process parameters and to predict the parametric influence on the response of Tensile Shear Fracture Load (TSFL). The Response Surface Methodology (RSM) is an optimization technique is used in this research to develop the satisfactory quadratic mathematical model and to predict the response. The optimal parameters and their levels are found and reported as follows: welding current = 9 kA, welding time = 0.18 seconds and electrode tip radius = 3 mm. The actual and predicted values of TSFL for the optimized parameters are 17.6 kN and 17.9 kN respectively. The developed quadratic model is efficiently predicted the response with an average error percentage of 2.18. The significant and insignificant terms in the models has been identified by 95% confidence level using ‘p' test. The insignificant terms are removed from the model and the ANOVA table is formulated only with the significant terms. Significance or effect of each term in the ANOVA table is identified by calculating the percentage of contribution and noticed that welding current has the highest significance (46%) on TSFL. The macroscopic examination confirmed that the larger nugget is observed during the maximum welding current due to the high heat generation. Also, the variation in TSFL against the process parameters are observed as same as nugget length, because, TSFL and nugget length are perfectly correlated.

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  • Lili Meng, Jinxiang Liu, Ran Liu, Hongyang Li, Zhi Zheng, Yao Peng, Xi ...
    Article type: Regular Article
    Article ID: ISIJINT-2024-028
    Published: 2024
    Advance online publication: April 25, 2024
    JOURNAL OPEN ACCESS ADVANCE PUBLICATION

    The silicon content of hot metal is a key index for the determination of blast furnace status, and accurate prediction of the silicon content of hot metal is crucial for blast furnace ironmaking. First, 10992 sets of blast furnace data obtained from the site of an iron and steel enterprise were preprocessed. Then, 22 important feature parameters related to the silicon content of hot metal were screened by feature engineering. Finally, the hyperparameters of the Gradient Boosting Decision Tree (GBDT) algorithm model were optimized with the help of the Optuna framework, and the Optuna-GBDT model was established to predict the silicon content of hot metal. The experimental results show that compared with the Bayesian algorithm and the traditional stochastic search method, the Optuna framework can achieve better hyperparameter optimization with fewer iterations and smaller errors.The Optuna-GBDT model performs better in predicting the silicon content of hot metal compared with the optimized Random Forest (RF), Decision Tree and AdaBoost models, and the prediction results are basically in line with the actual values, with the mean absolute error (MAE) of 0.0094, the root mean square error (RMSE) of 0.0152, and the coefficient of determination (R2) of 0.975.The experimental results verified the validity and feasibility of establishing the Optuna-GBDT model to predict the silicon content of hot metal, which provides a reliable tool for iron and steel enterprises and helps to optimize the ironmaking process, improve production efficiency and product quality.

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  • Guojun Chen, Ying Ren, Minghui Wu, Weijian Wang, Lifeng Zhang
    Article type: Regular Article
    Article ID: ISIJINT-2024-049
    Published: 2024
    Advance online publication: April 25, 2024
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    In the current study, a novel laboratory experiment and a kinetic calculation were proposed to analyze the modification behavior of alumina inclusions in the molten steel. To obtain the shape and composition of a single Al2O3 inclusion at different times during the modification process, confocal scanning laser microscopy experiments were conducted to track the evolution of the Al2O3 inclusion particle on the surface of Ca-treated steel. Then, the composition evolution of the Al2O3 inclusion particle during the modification process was predicted using a kinetic model. It was assumed the product layer was homogeneous. The diffusion of dissolved [Ca], [Al], and [O] crosses through the inclusion-steel interface was considered. Experimental results agreed well with kinetic calculated results. Meanwhile, the kinetic model was used to analyze the modification behavior of Al2O3 inclusions in steel with various influence factors including the [Ca] content in steel, the [Al] content in steel, and the initial size of inclusions.

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  • Sunglock Lim, Ikuru Hatakeyama, Masashi Nakamoto, Takeshi Yoshikawa, T ...
    Article type: Note
    Article ID: ISIJINT-2024-017
    Published: 2024
    Advance online publication: April 22, 2024
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    The agglomeration of droplets dispersed in an immiscible liquid is often an issue in metallurgical processes. To determine the effect of wettability on droplet agglomeration, we conducted (1) wettability and (2) agglomeration experiments using immiscible liquid paraffin and glycerin aqueous solution. In the wettability experiment, a droplet of one liquid was settled on the other liquid and its shape was observed. The glycerin droplet was wrapped by liquid paraffin, while the paraffin droplet spread on the surface of the glycerin solution. Therefore, liquid paraffin wetted the glycerin droplet, while glycerin solution did not wet the paraffin droplet. In the agglomeration experiment, after the droplet settled or floated in the other liquid layer to arrive at the boundary between the two liquid layers, we measured the time required for droplet agglomeration in its liquid layer. The agglomeration of paraffin droplets from the glycerin solution was faster than that of glycerin droplets from liquid paraffin, indicating that non-wettability of droplets accelerated agglomeration.

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  • Yu Liu, Zhixin Ma, Xudong Wang, Yali Gao, Man Yao, Zhiqiang Xu, Miao Y ...
    Article type: Regular Article
    Article ID: ISIJINT-2023-449
    Published: 2024
    Advance online publication: April 17, 2024
    JOURNAL OPEN ACCESS ADVANCE PUBLICATION

    All 61 sticker breakouts and 183 false sticker breakouts were obtained based on the on-line mould monitoring system during the conventional slab continuous casting. The 16-dimensional temperature characteristics and temperature velocity characteristics of the sticker breakout were extracted. The sticker breakout recognition based on the XGBoost forward iterative model was developed and optimized by the mean square error algorithm. The results show that the prediction probability of the sticker breakout after optimization is in the range of 0.72∼1.00. The smallest output value 0.5 higher than that before optimization. When the threshold is set to 0.65, the optimized XGBoost model can correctly predict all sticker breakouts and has a 99.5% accuracy rate. The XGBoost model has a stronger generalization ability and higher prediction accuracy, which promotes the intelligent production of continuous casting.

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  • Hirokazu Kobayashi, Gentaro Takeda, Kenji Katoh, Tatsuro Wakimoto
    Article type: Regular Article
    Article ID: ISIJINT-2024-030
    Published: 2024
    Advance online publication: April 17, 2024
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    Gas jet cooling is widely used because the device is simple, oxidation can be prevented, and a uniform cooling capacity can be obtained with thin steel sheets. Because the gas jet cooling ability is affected by the physical properties of the gas such as the mixed gas ratio, a quantitative evaluation of the influence of these factors is very important. However, few studies concerning prediction of the cooling capacity of mixed gas jets in atmospheres with different concentrations have been published.

    In this research, the results of experiments and a fluid analysis with an air-helium gas jet in an air atmosphere were compared with the results obtained with Martin's non-dimensional empirical equations. As the nozzle condition, a single round nozzle with a tapered shape was examined. The helium concentrations with respect to air were 0, 20, 50, and 100 vol%, and the pressure conditions were 3 and 5 kPa.

    Compared with the experimental results, Martin's equations overestimated the improvement of cooling performance with increasing helium concentrations. In the analysis in the present study, it was found that mixing with ambient air increased as the helium concentration decreased.

    The trend of divergence between the experimental and predicted cooling capacity was clearly presented in this research. The results of this study will make it possible to improve the accuracy of predictions of the cooling capacity of impinging gas jets with different concentrations of the atmosphere and the gas jet.

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  • Yue Wang, Xianchun Dong, Yang Yu, Kaiguang Hu, Bing Yu, Shuhai Chen
    Article type: Regular Article
    Article ID: ISIJINT-2024-036
    Published: 2024
    Advance online publication: April 16, 2024
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    A thermomechanical simulator Gleeble 3800 was used to simulate the thermal cycles experienced by various heat-affected zones (HAZ) during the welding process. The influence of peak temperature (Tp, 500°C~1320°C) on the hardness, microstructure, precipitates, and properties of complex steel 780FB with microalloyed elements Ti, Nb, and V was systematically studied. The contributions of dislocation strengthening, precipitation strengthening, fine grain strengthening, and phase transformation strengthening increments to strength changes of samples after different thermal cycles were quantified, and the calculated results were found to be consistent with the experimental data. Compared with 780FB, there was little change in microstructure and properties when Tp was 500°C. When Tp was 650°C, the increase in VC density from 43/μm2 to 288/μm2 caused the enhancement of hardness and strength. The precipitation strengthening increment (49.84MPa) played a dominant role in strength improvement. As partial bainite in the microstructure of 780FB transformed into ferrite at Tp of 800°C, the weakening of phase transformation strengthening (-57.5MPa) became the main factor in strength change. The softening and strength reduction further increased when Tp was up to 980°C, as 780FB completely recrystallized and transformed into ferrite and MA islands. The phase transformation strengthening further reduced by 74.75MPa. When Tp was 1320°C, the VC density decreased from 43/μm2 to 13/μm2, and the (Ti,Nb)C density decreased from 34/μm2 to 14/μm2, leading to severe grain growth (2.24μm to 19.89μm) and bainite transformation. The decrease in precipitation strengthening (-26.86MPa) and fine grain strengthening (-87.91MPa) counteracted with the increase in phase transformation strengthening (51.62MPa), resulting a slight decrease in hardness and strength.

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  • Yuki Shimomura, Hyung-Won Park, Hyeon-Woo Park, Yuji Sato, Jun Yanagim ...
    Article type: Regular Article
    Article ID: ISIJINT-2024-014
    Published: 2024
    Advance online publication: February 29, 2024
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    A novel integrated constitutive equation of the flow curve for Ti–6Al–4V alloys is proposed by incorporating the effects of phase fraction in the hot-forging temperature range. The flow curve was obtained using hot-compression tests in the temperature range of 750–1050 °C and strain rate range of 1–25 s-1. The effects of friction and deformation heat generated during compression were corrected using the inverse analysis method to identify the ideal uniaxial flow curve. The obtained stress parameters were satisfactorily regressed using the rule of mixtures on the α and β phases considering changes in the phase fraction. The integrated flow curve equation incorporating the rule of mixtures of the two phases effectively expressed the flow curve throughout the investigated temperature range. The internal microstructural observation showed that the continuous dynamic recrystallization of the α phase is dominant in the α+β two-phase region, while the deformation of the β phase becomes dominant just below the β transus. The constitutive equation presented here is in good agreement with the temperature dependence of the microstructure.

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