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15 June 2025, Volume 60 Issue 6
    

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    Technical Reviews
  • Progress in preparation of inorganic fiber materials from typical bulk solid wastes in metallurgical processes
    CHEN Wei, YU Jianyu, ZHANG Yuzhu, XIAO Yongli, WANG Baoxiang, ZHAO Kai, ZHEN Changliang
    Iron and Steel. 2025, 60(6): 1-15. https://doi.org/10.13228/j.boyuan.issn0449-749x.20240666
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    Under the current "dual-carbon" framework, the high-value utilization of typical large-scale solid wastes from metallurgical processes and other major industrial sources has emerged as a critical issue demanding urgent resolution within the industry. Large-scale solid wastes from metallurgical processes, including blast furnace slag and dust ash, as well as other major industrial solid wastes such as fly ash and coal gangue, are rich in valuable resources like silicon and aluminum. Through appropriate conditioning and proportioning treatments, these solid wastes can be transformed into inorganic fiber materials exhibiting excellent thermal insulation and refractory properties, thereby achieving high-value utilization of both metallurgical and other major industrial solid wastes. The research progress in preparing mineral wool fibers is reviewed, using conditioned blast furnace slag as the raw material and silicon-aluminum-based ceramic fiber materials using dust ash in combination with fly ash or coal gangue. It analyzes the advancements in the production of inorganic fibers via the spray-blowing method and the centrifugal method from three perspectives, fundamental principles, experimental studies, and industrial practices. Pilot-scale experimental studies are conducted to produce mineral wool fibers from conditioned blast furnace slag using both spray-blowing and centrifugal methods. The results indicate that spray-blowing pressure significantly affects the content of fiber slag balls, while having minimal impact on the average fiber diameter. Specifically, increasing the spray-blowing pressure from 0.20 MPa to 0.38 MPa reduces the fiber slag ball mass fraction from 25% to 16%, with negligible changes in fiber diameter. Conversely, roller speed has little effect on fiber slag ball content but significantly influences fiber diameter; as roller speed increases, the average fiber diameter decreases from 3.17 μm to 2.73 μm. The study further compares the differences between mineral wool fibers produced by the two methods in terms of fiber diameter, fiber slag ball content, and fiber compressive strength. Additionally, it outlines the application prospects of silicon-aluminum-based inorganic fiber materials, derived from metallurgical large-scale solid wastes in conjunction with other major industrial solid wastes, in sectors such as construction, industry, aerogels, and photocatalytic materials. Building upon the existing research foundation, future research directions for the production of inorganic fiber materials from metallurgical large-scale solid wastes is proposed, aiming to further advance waste valorization, energy conservation, and carbon reduction.
  • Research progress on application of acid retardation method for recovering stainless steel pickling waste liquid
    ZHAO Linxiu, YAO Shuhui, HAN Mingfeng, MIAO Yuxin, LI Wenjuan, GAO Jianfeng
    Iron and Steel. 2025, 60(6): 16-25. https://doi.org/10.13228/j.boyuan.issn0449-749x.20250032
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    Stainless steel has the advantages of exquisite appearance, high strength, strong corrosion resistance, and good heat resistance, and is widely used in petrochemical, electronic, medical, aerospace and other fields. China is the largest producer and consumer of stainless steel, and pickling is an essential process in stainless steel production. Each ton of stainless steel acid washed will produce 1-3 m3 of pickling waste liquid. The acid pickling waste liquid contains a large amount of free acids and metal ions, which have the characteristics of high corrosiveness, high metal toxicity, large production, and difficult treatment. It poses a serious threat to the environment and ecology, and has become the main bottleneck restricting the upgrading, development, and sustained growth of China's stainless steel industry. The current treatment methods for stainless steel pickling waste liquid are first compared. The acid retardation method has the advantages of short process flow, flexible equipment scale, low energy consumption, low investment, significant environmental and economic benefits, and has broad application prospects in the field of stainless steel pickling waste liquid recovery. Secondly, a systematic review is conducted on the overall application research progress of acid retardation method from four aspects,the discovery and mechanism research of acid retardation, the research progress of acid retardation process and equipment, the research of acid retardation materials, and the application status of acid retardation method for recovering stainless steel pickling waste liquid. The aim is to provide new ideas for the treatment and recovery of stainless steel pickling waste liquid in Chinese stainless steel enterprises. Finally, based on the current research status, prospects are made for future research directions on acid retardation method. Future research should focus on the study of acid retardation mechanism, the development of domestically produced acid retardation equipment, the diversification of acid retardation materials, and the joint application of acid retardation method and other metal recovery methods to achieve efficient recovery of acid pickling waste liquid resources, ultimately promoting the widespread application of acid retardation method in the field of stainless steel acid pickling waste liquid recovery in China.
    • Raw Material and Ironmaking
  • Preparation characteristics of laterite nickel ore pellets and feasibility analysis of its use for enhancement of RKEF process
    ZHU Deqing, CAO Wen, YANG Congcong, PAN Jian, GUO Zhengqi, LI Siwei, YANG Qing
    Iron and Steel. 2025, 60(6): 26-35. https://doi.org/10.13228/j.boyuan.issn0449-749x.20250024
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    In view of the current problems of the rotary kiln reduction-electric furnace smelting （RKEF） process, such as low reduction efficiency and easy ring formation, study on the preparation characteristics of laterite nickel ore pellets were carried out using one type of saprolitic laterite nickel ore as raw material, and the effect of internal carbon on the roasting performance of laterite nickel ore pellets was investigated. On this basis, pilot-scale reduction roasting tests in a rotary kiln was further carried out, and the performance of pre-reduced pellets from pilot-scale tests was compared with the rotary kiln calcine from industry. The results show that the roasted laterite nickel ore pellets with compressive strength of more than 200 N/pellet and AC drum strength of 98% in hot and cold conditions can be prepared under the conditions of roasting at 1 000 ℃ for 60 min with carbon mass fraction of 5%. In the pilot-scale rotary kiln tests, the dry pellets of nickel laterite are directly charged into the kiln and give good process strength and reduction results under conditions of total carbon mass fraction of 5%, the mass ratio of inside to outside reductant in the pellets of 60∶40, reduction temperature of 1 000 ℃, and reduction time of 60 min. Specifically, the resultant pre-reduced pellets possess compressive strength of larger than 100 N/pellet, AC drum strength of greater than 95%, and the nickel and iron metallization rate of 68.55% and 1.84%, respectively. Compared with the rotary kiln calcine from industry, the pre-reduced pellets made from laterite nickel ore possess higher nickel metallization rate, lower loss on ignition （LOI）, and significantly less fines generation of less than 3 mm, which helps reduce the risk of ring formation in the rotary kiln, improve the gas permeability of charging burdens, increase smelting efficiency and reduce power consumption of the submerged arc furnace smelting process.
  • Feature detection model of green pellets based on image processing
    LIU Yimin, WEN Shanshan, LIANG Ye, HE Zhijun, GAO Lihua
    Iron and Steel. 2025, 60(6): 36-43. https://doi.org/10.13228/j.boyuan.issn0449-749x.20250010
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    As one of the core components of the national basic industries, the steel industry currently has an enormous total carbon emission, and the task of carbon reduction is extremely arduous. Pelletized ore, with lower energy consumption and emissions compared to sintered ore, makes high-proportion pellet smelting an important way to save energy and reduce emissions in the iron-making process. During the production of pelletized ore, accurately detecting its features is crucial for ensuring product quality. Image recognition technology was used to compare and select the most suitable light intensity for detection and two operators with high recognition rate of key pellet features under different lighting conditions in the simulated field, integrate and compare with three convolutional neural networks suitable for detection of green pellet features, and conduct a comprehensive evaluation of these algorithms from various indicators such as accuracy. For the feature monitoring task, the MobileNetv3 algorithm introduced the convolutional block attention module （CBAM） in the module splicing part to improve the expressiveness of defect features and the anti-interference ability of the model, enabling it to stably detect features such as cracks in complex environments. When the generative adversarial network （GAN） model was used for detection, the pelletized ore data set was input into the generator to generate defect prediction images, which were then compared with real images to judge the defect situation. It had a strong ability to recognize tiny and blurred defects. The internal feature extraction and classification process of faster region-based convolutional neural network （Faster R-CNN） algorithm was relatively intuitive and interpretable. The extraction of pellet features and areas of concern by the network could be observed through visualization techniques. The experimental results show that the Kirsch operator-integrated convolutional neural network Faster R-CNN algorithm proposed performs excellently in green pellet feature detection, with the model accuracy reaching 96.2% and the recall rate reaching 91.0%. The pellet feature detection model proposed has broad application prospects in the fields of industrial automation and quality control, bringing new ideas and new methods for the innovation of pellet detection technology.
  • Disintegration mechanism of typical blast furnace lump ore
    SONG Yunfei, ZHAN Wenlong, ZHANG Meng, TIAN Zengchao, GAO Lihua, ZHANG Junhong, HE Zhijun
    Iron and Steel. 2025, 60(6): 44-54. https://doi.org/10.13228/j.boyuan.issn0449-749x.20240686
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    To grasp the differences in metallurgical properties of various lump ores at high temperatures, optimize the burden structure charging into the furnace, and ensure the stable operation of the blast furnace, the low-temperature reduction disintegration performance and decrepitation behavior of typical imported and domestic lump ores from a domestic enterprise were studied. The phase evolution, interface morphology, and disintegration mechanism were also analyzed. Results show that with the increase of reduction temperature, the low-temperature reduction disintegration index of PB lump ore becomes better. In contrast, low-temperature reduction disintegration index of the Baishan lump ore becomes worse, and the reduction and anti-pulverization properties of Baishan is better than that of PB. In the process of lump ore reduction, there are crystal water decomposition, phase transitions among the iron-bearing spinel phase, iron-bearing olivine phase, and Wüstite phase, and large thermal stresses between each phase, resulting in cracks. Long, narrow, tearing large cracks dominate the PB lump ore, while short and fine cracks dominate the Baishan lump ore. As the temperature increases, the decrepitation index of both lump ores increases, with the PB lump ore exhibiting a significantly higher degree of decrepitation compared to the Baishan lump ore. The PB lump ore is prone to forming a porous structure due to the decomposition of crystalline water, and the increased internal vapor pressure further leads to decrepitation and the formation of through-cracks in the lump ore, resulting in a neat fracture surface and severe disintegration. In contrast, the Baishan lump ore develops dendritic cracks due to the decomposition of carbonates, and the anisotropy of these dendritic cracks prevents excessive disintegration.
  • Performance of semi-coke for blast furnace injection and its influencing factors
    NING Xiaojun, REN Zheng, WU Junyi, ZHANG Xueting, WANG Guangwei, YANG Yuzhuo
    Iron and Steel. 2025, 60(6): 55-65. https://doi.org/10.13228/j.boyuan.issn0449-749x.20250009
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    The efficient utilization of semi-coke is of great significance in realizing the hierarchical conversion and stepwise utilization of coal resources. The blast furnace injection performance of five semi-cokes was analyzed in depth, including proximate analysis, elemental analysis, ash composition and other key indexes, as well as the combustion characteristic kinetics, XRD and industrial application of the coke. The results show that, relative to the remaining 4 semi-cokes, semi-coke E has the lowest ash and volatile matter content and the highest fixed carbon content. The order of high and low content of alkaline metal oxides in semi-coke ash is consistent with the order of high and low content of CaO. The excessive alkaline metal oxides in the ash may affect the integrated combustion characteristic index S value. The abradability of semi-coke is negatively correlated with its ash content. The ash content of semi-coke E is the lowest and the abradability is the best with 80.21. The ignition point of semi-coke is in the temperature range of 573-673 K. It is non-explosive, which meets the requirements of safe production in the blast furnace of the iron and steel enterprises. The combustion performance of semi-coke is between that of bituminous coal and anthracite. Semi-coke B has the highest S value of and the best combustion performance. The kinetic curves of different samples has good linear relationship, in which the lowest combustion activation energy of bituminous coal is 22.86 kJ/mol, and the highest combustion activation energy of semi-coke E is 69.65 kJ/mol. The combustion performance is affected by the activation energy and the pre-finger factor together. A small amount of semi-coke B is introduced into the blast furnace injection process for the test. The results show that the fuel ratio of the blast furnace is reduced and the utilization efficiency of the blast furnace is improved after adding semi-coke B. However, after further elevating the proportion of semi-coke injection, it is found that the fuel ratio of the blast furnace gradually increases and the gas utilization efficiency gradually decreases.
  • Analysis of burden distribution in pipe-distributor of hydrogen-rich gas-based shaft furnace during charging process
    PEI Xiaoyu, XIA Heng, BANG Jiawen, CHENG Shusen
    Iron and Steel. 2025, 60(6): 66-76. https://doi.org/10.13228/j.boyuan.issn0449-749x.20250015
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    In order to obtain effect of the changing parameters on the burden, a three-dimensional model of the pipe distributor was established based on the discrete element method （DEM）. Particle movement and shape feature of the burden in the shaft furnace are investigated. The effects of particle mass ratio, the distance between the pipe and the center of the furnace, the number of pipes and the inclination angle of the bin on the circumferential distribution of the particle in the shaft furnace, particle segregation and porosity along the radial direction are calculated. The airflow distribution is also evaluated. The results show that particles fall into the furnace from pipe in a divergent way and burden apex is formed under the pipe. The small particles are concentrated under pipe, while the large particles are dispersed on the burden corners. The burden apex under the pipe is approximately 350 mm thicker than the center and edge of the burden. θ3 at the burden apex is greater than θ1 at the edge when the distance between the pipe and the center of the furnace is 860 mm. The particles are segregated in the circumferential direction. There is a maximum value of the average particle size in the area between burden apexes. The number of pipes, the distance between the pipes and the center have the greatest influence on the particle distribution. The circumferential distribution of the burden is the most uniform when the number of pipe is 6 and the distance between pipe and the center of the furnace is 860 mm. The more pipes, the smaller distance of pipes from the center, the more uniform circumferential distribution of the burden. Positive segregation occurs for small particles in the burden apex along the radial direction, while positive segregation occurs for large particles in the center and edge areas. The particle distribution does not directly reflect the porosity distribution. There is a minimum value of porosity in the 4-8 area of the burden apex. The area with the smallest porosity at the burden apex is closer to the center of the furnace as the pipe approaches the furnace center. As the distance between pipe and the center decreases, the area with smaller porosity is closer to the center. Strong airflow intensity is showed at the center and edge but weak gas flow intensity in the burden apex along the radial direction. The unit pressure drop in the area 1 is the largest when the distance between pipe and the center of the furnace. Increasing the distance between the pipe and the center is conducive to the development of the central gas flow.
    • Steelmaking
  • Effect of EAF co-injection on flow and mixing behavior of molten pool
    LI Zihao, XIE Qinghua, LI Ying, NI Peiyuan
    Iron and Steel. 2025, 60(6): 77-86. https://doi.org/10.13228/j.boyuan.issn0449-749x.20250053
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    Under the "dual carbon" strategy, the short-process steelmaking has became the key development route of green steel production due to its advantages of low carbon emission and resource recycling. Electric arc furnace （EAF） is one of the key equipment in short-process steel production. The improvement of EAF-refining efficiency is crucial for high efficiency and green steelmaking. In order to improve the flow uniformity and mixing efficiency of the molten pool, the oxygen lance jet combined with bottom blowing stirring method was studied. The effects of side blowing, bottom blowing, side blowing+bottom blowing and swirling flow side blowing+bottom blowing on multiphase flow and mixing behavior in EAF were investigated.Firstly, a mathematical model was established with a ratio of 4∶1 to a 75 t industrial EAF. The reliability of mathematical model was verified by comparing the measurement data of mixing time in water model with the predicted values. Then, parameter studies were carried out to investigate the flow field, mixing time and bubble distribution behavior under different injection modes based on the validated numerical model and physical framework. The results show that the mixing time is reduced by 63.4% and 11.2% by using the side blowing combined with bottom blowing stirring method, compared to the bottom blowing and side blowing. Also, the averaged velocity of EAF is increased by 25.0% and 133.3%. When the swirling flow side blowing + bottom blowing is applied, the mixing time is further reduced by 29.8% while the averaged velocity of EAF is increased by 86.7% and the dead zone in EBT （eccentric bottom tapping） region is significantly reduced, compared to that obtained by the conventional side blowing + bottom blowing method. By applying the swirling flow side blowing + bottom blowing method, the jet is sprayed into the EAF by the side-blowing oxygen gun and have momentum exchange with the fluid, and a swirling fluid flow in molten pool was produced. This flow pattern generates a uniform velocity distribution. Due to the influence of swirling flow on bubbles, the range of gas/liquid interaction is increased, which is beneficial for bubbles to drive molten steel and for improving mass transfer as well as mixing efficiency. It has guiding significance to the optimization of EAF steelmaking process in actual production.
  • 18Ni250 vacuum arc remelting simulation and TiN formation mechanism
    YUAN Wenjie, NING Jing, WANG Ao, WANG Jiaming, LI Yuan, SU Jie, CHONG Xiaoyu
    Iron and Steel. 2025, 60(6): 87-102. https://doi.org/10.13228/j.boyuan.issn0449-749x.20250042
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    In addressing the challenge of controlling TiN inclusions in aerospace-grade ultra-pure 18Ni250 martensitic aging steel during the ϕ660 mm ingot vacuum consumable remelting process, the precipitation mechanisms of TiN and optimization pathways were systematically investigated through a combined approach of numerical simulation and experimental validation. Building upon the current steady melting rate of 5.1 kg/min, the Meltflow-VAR multi-physical field simulation model for the vacuum consumable remelting process of 18Ni250 martensitic aging steel was established. Through multiple iterations and validation against actual samples, the model's accuracy met engineering application requirements, providing a reliable numerical experimentation platform for process optimization. Utilizing high-temperature confocal microscopy observations and in-situ characterization techniques for inclusions, the precipitation patterns of TiN during solidification were analyzed. Laboratory isothermal and variable cooling rate simulation tests determined the solidification characteristic temperatures of the material （liquidus at 1 450 ℃, solidus at 1 370 ℃, and the sensitive temperature for TiN precipitation at 1 390 ℃）. It was observed that when the residual liquid phase ratio reached 30.6%, the local concentration gradient of Ti between dendrites exceeded the critical threshold of 0.543 55%, triggering TiN nucleation. Further quantitative analysis revealed the cascading relationship between cooling rate, dendritic spacing, and elemental segregation. It was proposed that by controlling the instantaneous cooling rate at the solidification front to exceed 0.021 42 ℃/s, the concentration of Ti positive segregation could be maintained within a safe range, concurrently leading to the refinement of TiN inclusion sizes. Ultimately, based on the validated simulation model and experimental data, an optimized steady-state melting rate of 4.5 kg/min is determined, which ensures the cleansing capacity of the melt pool while achieving coordinated control over Ti segregation levels and TiN sizes. The result elucidates the formation mechanisms and control pathways of TiN inclusions within the vacuum consumable remelting process, and the established simulation model and process optimization strategies can provide theoretical support and industrial application guidance for the high-purity smelting of large-sized 18Ni250 steel ingots.
  • Mold breakout prediction models based on computer vision and machine learning
    WANG Yanyu, CHENG Yonghui, ZHU Guoqiang, CHEN Baiyu, ZHANG Lihui, WANG Qican, YAO Man, WANG Xudong
    Iron and Steel. 2025, 60(6): 103-112. https://doi.org/10.13228/j.boyuan.issn0449-749x.20240651
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    Many abnormal conditions may occur in the process of continuous casting, among which breakout accident is one of the most serious accidents. One of the typical signs before the occurrence of breakout accidents is the formation of a "V" shaped expansion region on the copper plate, which needs to be detected and predicted by a stable and reliable method. Using computer vision technology, the temperature signal of thermocouple collected from the surface of the mold copper plate and the calculated temperature rate were mapped to the color space, and the abnormal sticking region was characterized by two-dimensional plane thermographic image. By extracting the dynamic and static features of the sticking region in the graph, the ten-dimensional feature vector representing the sticking region was constructed. Based on the statistical report of breakout accident of a steel plant, sample dataset of sticking region feature vector was established. At the same time, two machine learning models, support vector machine （SVM） and random forest （RF）, were used to learn and identify the true and false sticking region features. The test results show that, compared with the RF model, SVM model can more effectively identify the abnormal temperature pattern of sticking breakout regions. The RF model has two cases of missing alarms in the prediction results, while the SVM model can reach 100% of the reported rate of sticking breakout and control the false alarm rate below 10% （9.93%）. In terms of Gmean score （0.95） and model AUC （0.98）, SVM model is also significantly better than RF model, which indicates that the model can meet the requirements of breakout prediction. Based on the above results, a mold breakout prediction model based on computer vision and machine learning is established, which provids reference for data-driven process abnormality detection and prediction technology in continuous casting production.
  • Formation mechanism of longitudinal cracks at wide edge of stainless steel continuous casting slabs
    ZHANG Hejun, WANG Yadong, FU Zhixiang, WANG Fachao, ZHAO Deli, YIN Qing, WU Xiaolin, ZHANG Lifeng
    Iron and Steel. 2025, 60(6): 113-120. https://doi.org/10.13228/j.boyuan.issn0449-749x.20250039
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    Longitudinal off-corner cracks on the wide face of stainless steel slabs were found in a domestic steel plant. The formation mechanism of longitudinal off-corner cracks on the wide edge of continuous casting slabs was revealed through laboratory testing and analysis, and stress simulation. The grains near the longitudinal off-corner cracks were coarse, and there was a layer of pre-precipitated ferrite with a thickness ranging from tens to hundreds of microns at the original austenite grain boundary, and the cracks mainly occurred along the filmy pre-precipitated ferrite. The characteristic element Na of the mold powder was detected inside the longitudinal off-corner cracks on the wide face of slabs, therefore it was determined that the longitudinal off-corner cracks were generated in the mold. When the distance from meniscus was greater than 0.2 m, the stress on wide surface of continuous casting slab increased with the distance from corner. The maximum value was reached near the corner, and then the stress value gradually decreased and tended to be stable. The corners of continuous casting slab were subjected to two-dimensional cooling, resulting in a large solidification shrinkage and led to the appearance of gaps, which weakened the cooling intensity of the corners and caused the thinnest point of the solidified shell to appear near corner on the wide face of slab. Due to the solidification shrinkage, there was maximum stress near the edge of continuous casting slab. When the taper of mold could not completely match the solidification shrinkage, depressions or cracks would appear at the thinnest point of solidified shell at the wide face edge of continuous casting slab. The formation mechanism of longitudinal off-corner cracks on the wide face of stainless steel slabs is revealed, and a theoretical basis for controlling surface defects in continuous casting slabs is provided.
    • Metal Forming
  • Prediction model for influence of phosphorus content in steel with phosphorus on outlet thickness difference of cold rolling
    WANG Zhuo, ZHANG Ji, JI Haodong, SUN Rongsheng, BAI Zhenhua
    Iron and Steel. 2025, 60(6): 121-129. https://doi.org/10.13228/j.boyuan.issn0449-749x.20240664
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    Considering the equipment and process characteristics of acid rolling mill, elastoplastic curve and performance characteristics of steel with phosphorus, it is determined that the outlet thickness difference of steel with phosphorus in cold rolling is mainly affected by incoming material properties, that is, phosphorus content. On this basis, steel grade DQ1461H5 was selected as an example to study the effect of phosphorus content on strip properties. The carbon equivalent calculation model of steel with phosphorus was constructed by data regression method, and the optimization function was established to make the element coefficient significantly improved and more consistent with the production practice. Then, according to the quantitative relationship between phosphorus content and strip deformation resistance, the deformation resistance model under the influence of multiple factors was derived, and the corresponding rolling force calculation model was further given. Finally, the longitudinal outlet thickness prediction model of steel with phosphorus was established considering the influence factors such as the fluctuation of incoming material properties. In order to verify the reliability of theoretical model, the outlet thickness difference prediction model of steel with phosphorus was applied to the off-line prediction calculation during the production of DQ1461H5 by the 1420 acid rolling unit of a domestic steel plant. The results show that the proposed model can be used to predict the outlet thickness difference of steel with phosphorus during cold rolling. The measured value of outlet thickness difference is in good agreement with the predicted value of the model, and the error between the measured value and the predicted value decreases from less than 10 μm before application to less than 5 μm. After verifying the accuracy of the model, the relation curve between phosphorus content fluctuation and outlet thickness difference of steel with phosphorus is obtained by substituting relevant parameters, which indicates that the phosphorus content fluctuation of steel with phosphorus is positively correlated with the outlet thickness difference of cold rolling. At the same time, the proposed model can be applied to the precision prediction process of cold rolled plate thickness AGC combined with the actual production field as an auxiliary mathematical model, and adjust the process parameters such as the reduction amount and rolling speed according to the predicted results to optimize the control strategy, which provides a theoretical basis for the comprehensive control of the production quality of phosphorus-containing steel products.
    • Materials
  • Effect of ausforming temperature on microstructure and properties of low-carbon bainitic steel
    LI Jianhua, YANG Dapeng, ZHAO Minghui, WANG Ruiting, YI Hongliang, DI Hongshuang
    Iron and Steel. 2025, 60(6): 130-139. https://doi.org/10.13228/j.boyuan.issn0449-749x.20250050
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    Ausforming, as a classical thermomechanical treatment process, can effectively refine the bainitic microstructure, providing a promising solution for the production of low carbon CFB steels with a combination of high strength and excellent ductility. However, systematic studies of the relationships among ausforming temperature, bainitic transformation kinetics, microstructure characteristics and their effects on mechanical properties in low carbon CFB steels are still limited. Therefore, a low-carbon CFB steel was subjected to ausforming treatment at different temperatures （600, 500, 400 ℃） prior to isothermal bainitic transformation at 350 ℃ and compared with conventional isothermal heat treatment. The effects of ausforming temperature on bainitic transformation kinetics, microstructural evolution, and mechanical properties were studied using a combination of DIL805A/D dilatometry, SEM, XRD, EBSD, TEM, and hot rolling experiments. The results show that, compared to the conventional isothermal heat treatment, high-temperature （600 ℃） ausforming has little effect on the bainitic transformation kinetics, while ausforming at medium-temperature （500 ℃） and low-temperature （400 ℃） significantly accelerates the bainitic transformation kinetics and the maximum bainite volume fraction increases. The ausforming temperature also influences the morphology of the CFB steel microstructure. When ausforming at 600 ℃, the volume fraction of granular features of bainitic ferrite （BF） increased, whereas the morphology of BF was almost lath-like when ausforming at 400 ℃. Additionally, the content of martensite/austenite （M/A） islands produced during secondary cooling in the ausformed samples was reduced and its size was smaller. While maintaining the same strength level as the conventional process, the uniform elongation increased from 6.8% to 17.6% when ausforming at 500 ℃. This is primarily attributed to the large amount of RA in the microstructure, which undergoes a progressive transformation-induced plasticity （TRIP） effect over a larger strain range during tensile deformation, thus enhancing the work hardening capability. The result provides a new method for the development of low-carbon CFB steels with high strength and plasticity, which has great potential for industrial applications.
  • Effect of microstructure evolution of pearlitic heavy rail steel on fatigue crack propagation rate
    WANG Kaidi, CEN Yaodong, YU Bo, LU Yuting, BAO Xirong, WANG Dongmei
    Iron and Steel. 2025, 60(6): 140-149. https://doi.org/10.13228/j.boyuan.issn0449-749x.20250041
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    To investigate the influence of the microstructure in pearlitic heavy rail steel on the fatigue crack growth rate and to clarify the relationship between strain at the inflection points of the fatigue crack growth rate curve in Regions I, II, and III and the fracture behavior, fatigue tests on three types of pearlitic rail steel using fatigue testing machine were conducted. Optical microscopy （OM） and scanning electron microscopy （SEM） were employed to observe and analyze the microstructure, pearlite lamellar characteristics, fatigue cracks, and fracture morphology. The strain values at the inflection points of fatigue crack growth rate curve between Regions I/II and II/III were measured using strain gauge.The results indicate that the pearlite lamellar spacings of the three rail steels are 186.2, 129.5,116.8 nm, respectively. Under the same loading conditions, the smaller the pearlite lamellar spacing, the longer the fatigue life of the rail steel, with the number of cycles to fracture failure records as 6.7×10⁵, 7.05×10⁵,7.7×10⁵, respectively. The fracture surfaces in the near-threshold and crack steady-state propagation regions exhibites characteristics such as fatigue striations, cleavage steps, and river patterns. Secondary cracks are observed in the steady-state propagation region. The fatigue striation spacings of the three rail steels are measured as 325.8, 236.6, 162.4 nm, respectively. Both the pearlite lamellar spacing and fatigue striation spacing exhibites a negative correlation with fatigue life, where larger lamellar spacing leads to faster fatigue crack growth rate.Strain test results show that the strain values at the I/II inflection point for the three rail steels are 0.115, 0.163, 0.484, while at the II/III inflection point, they are 0.254, 0.400, 0.616, respectively. Higher strain values indicate greater strain energy and stronger deformation resistance, which can impede and delay crack propagation, whereas lower strain values facilitate crack growth. Refining pearlite lamellae is beneficial for improving the fatigue resistance of pearlitic rail steel.
  • In situ observation on bainite transformation and crystallography of low-carbon carbide-free bainite steel
    LIANG Yalin, ZHOU Mingxing, CHEN Zhenye, YIN Weifan, TIAN Junyu, ZHAO Linlin, ZHAO Yanqing, GAO Yunzhe
    Iron and Steel. 2025, 60(6): 150-159. https://doi.org/10.13228/j.boyuan.issn0449-749x.20250048
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    Low-carbon carbide-free bainite has a wide range of applications in engineering machinery, rail transit and other fields due to its excellent strength, toughness and weldability. It is very important to understand the bainite transformation law for further improving its mechanical properties. A low-carbon carbide-free bainitic steel was taken as the object, and the bainite transformation kinetics and crystallography were studied using high-temperature confocal microscopy and backscattered electron diffraction simultaneously. The results show that the lengthening rate of bainite decreases first and then changes little when the transformation temperature decreases from 450 ℃ to 390 ℃. Two classical models of bainite growth （Zener-Hillert model and Trivedi model） are compared and it is found that the Trivedi model can better describe the effect of temperature on lengthening rate. But the calculated values are much faster than the experimental ones for both models （the calculated values are 2.5-3.0 times those of experimental ones）. The main reason is that the effect of alloying elements such as Mn, Cr and Mo on the lengthening rate of bainite is not considered in the theoretical calculation. The theoretical model was modified by increasing the activation energy of C diffusion. It is found that the calculated results match well with the measured values when the diffusion activation energy of C increases by about 6 700 J/mol compared to the theoretical value for the experimental steel. However, the calculated values in low-temperature region are lower than the measured ones after modification, which may be due to that C supersaturation in bainitic ferrite increases with decreasing temperature. In addition, the Nishiyama-Wassermann （N-W） orientation relationship was used to calibrate the bainite variants of the experimental steel. It is found that the bainite sheaves composed of V1,V2,V3 variants always preferentially nucleate and grow at the original austenite grain boundaries, which may be related to the crystallographic compatibility, interfacial energy, etc. These studies provide a reference for further understanding the phase transformation characteristics of low-carbon carbide-free bainitic steel.
  • Influence of cryogenic treatment on microstructure and mechanical properties of nitrogen-containing martensitic stainless steel
    AN Xin, KANG Yan, WANG Rui, FANG Jing, YU Zhiqiang, YAN Zhijie
    Iron and Steel. 2025, 60(6): 160-169. https://doi.org/10.13228/j.boyuan.issn0449-749x.20250029
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    7Cr14Mo nitrogen-containing martensitic stainless steel with high hardness, high strength, high wear resistance and other excellent properties, has been widely used in industrial production. The steel has high contents of carbon and other alloying elements, and the big eutectic carbides M7C3 easily forms. The method of quenching+cryogenic treatment+tempering was adopted to achieve the purpose of eliminating large carbides M23C6 in martensitic stainless steel. The effects of cryogenic treatment on size and distribution of the second-phase particles in the steel were studied by transmission electron microscope （TEM）, and the martensite and retained austenite were also identified. The fracture morphologies of the samples were observed by the scanning electron microscope （SEM）, and the crack initiation and propagation mechanism were analyzed. The tensile strength and elongation of steels were obtained by tensile testing, and the influencing relationship between microstructure and mechanical performances was illustrated. The results show that while tempering temperature is increased in the quenching+tempering samples, the volume fraction of retained austenite, quantity and size of M23C6 are increased, however, quantity and size of Cr2N are decreased. The same evolvement process is also observed in the quenching+cryogenic treatment+tempering samples. With increasing of tempering temperature in the two types of samples, the impact energy and elongation are gradually increased, while the hardness and tensile strength are gradually decreased. The quantity and the size of Cr2N in the quenching+tempering steel are lower than that of the quenching+cryogenic treatment+tempering steel at different tempering temperatures. When tempering temperature is at 160 ℃ and 360 ℃, the volume fraction of residual austenite, the quantity and size of M23C6, the impact energy in the quenching+cryogenic treatment+tempering steel are lower than that of in the quenching+tempering steel. The hardness and tensile strength of the quenching+cryogenic treatment+tempering steel are higher than that of the quenching+tempering steel. Compared with tempering temperature at 160 ℃ and 360 ℃, when tempering temperature is at 560 ℃, the evolvement process of precipitates and mechanical properties in the quenching+cryogenic treatment+tempering steel are opposite. The evolution laws of microstructures during the cryogenic treatment are revealled in this research, providing a theoretical basis for establishing the relationship between the microstructure and mechanical properties. It has certain guiding significance for practical engineering application of the steels.
  • Effect and mechanism of rare earth on high-temperature oxidation behavior of GCr15 bearing steel
    ZHANG Di, LI Hui, WU Bingbing, ZHOU Shuhao, LIU Longxin, SHANG Dianzuo, XU Rongchang, XING Yunxiang
    Iron and Steel. 2025, 60(6): 170-178. https://doi.org/10.13228/j.boyuan.issn0449-749x.20250035
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    As one of the key components in the field of high-end intelligent equipment, precision bearings have higher requirements for the mechanical properties, high-temperature oxidation performance, and corrosion resistance of bearing steel. The high-temperature oxidation behavior of the heating process for bearing steel is the key factor affecting the dimensional accuracy and performance of bearing parts, and controlling and optimizing it is key. The influence of rare earth elements on the oxidation properties of GCr15 bearing steel was tested and characterized through oxidation experiments in the process of high-temperature heating and heat preservation, and the related mechanisms were discussed and analyzed. The structure and phase composition of the oxide layer on the surface of bearing steel with different rare earth element contents were characterized by XRD, SEM, EDS, and other methods. The thermodynamic phase diagram of the composition and evolution of the oxide coating layer is calculated and analyzed, and an oxidation kinetic model is constructed. The experimental results show that the oxidation rate constants of bearing steel with rare earth addition （mass fraction） of 0, 0.003% and 0.015% were 3.44, 3.28 and 1.55, respectively, during the oxidation process at 900 ℃. Compared with the bearing steel without the addition of rare earth elements, the thickness of the oxide layer is reduced from 99.00 μm to 76.67 μm after the addition of 0.015% rare earth elements, which is 22.56% thinner. The oxide film is composed of inner oxide layer, middle oxide layer, and outer oxide layer, which is mainly composed of dense spinel oxide, and the formation of the middle Cr-rich layer and the inner layer of Si-rich oxide affects the high-temperature oxidation resistance of rare earth steel. The oxide layer of bearing steel without rare earth elements is thick, and there are numerous cracks and voids. The oxide layer of bearing steel with the addition of rare earth elements is relatively dense and continuous. In the initial oxidation stage at high temperatures, rare earth elements increases the diffusion rate of Si and Cr in the bearing steel, promotes the formation of dense oxide layer with high Fe2SiO4 and FeCr2O4 content on the surface, and effectively slow down the oxidation rate. During the growth process of the oxide film, rare earth elements hinder the outward diffusion of Cr3+, leading to a shift in the dominant growth of the oxide film towards inward diffusion of O2-, and significantly improves the high temperature oxidation resistance of GCr15 bearing steel.
  • Effect of heat treatment on microstructure and properties of 2 100 MPa grade stainless steel
    QIU Yu, LIU Zhenbao, TIAN Shuai, MENG Qihan, YIN Jiancheng
    Iron and Steel. 2025, 60(6): 179-187. https://doi.org/10.13228/j.boyuan.issn0449-749x.20250056
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    To improve the fatigue performance of 2 100 MPa grade ultra-high strength stainless steel, the effects of different solution treatment temperatures and aging durations on the microstructure, mechanical properties, and fatigue behavior of the experimental steel were investigated using characterization techniques including scanning electron microscopy （SEM）, transmission electron microscopy （TEM）, X-ray diffraction （XRD） and other means. The intrinsic relationships among heat treatment process, microstructure and properties were established. The results show that reducing the solution treatment temperature （from 1 100 ℃ to 1 070 ℃） can decrease the average grain size, effectively prevent the initiation and propagation of fatigue cracks, thereby improving the fatigue performance. However, after solution treatment at 1 070 ℃ for 70 min, a large amount of coarse M6C phase in the steel remains undissolved, which reduces the content of alloying elements with strengthening effects dissolved in the matrix, thus affecting the precipitation of precipitation-hardening phases during the subsequent aging process and having an adverse effect on the strength. At the same time, a large amount of M6C phase will disrupt the continuity of the matrix, hindering the slip of dislocations, and also have an adverse effect on plasticity and toughness. After aging, fine and dispersed M2C and Laves phases precipitate in the steel. When the solution treatment temperature is 1 080 ℃, during the process of increasing the secondary aging time from 2 h to 7 h, the volume fraction of precipitated phases increases, the pinning effect on dislocations is enhanced, resulting in increasing strength and fatigue performance, but decreasing the plasticity and fracture toughness. Taking into account the fatigue performance, strength and plasticity and toughness, 1 080 ℃ is selected as the optimal solution treatment temperature, and 4 h as the optimal secondary aging holding time. With this treatment, the experimental steel has tensile strength greater than 2 100 MPa, yield strength greater than 1 700 MPa, elongation greater than 10%, reduction of area greater than 50%, fracture toughness greater than 60 MPa·m1/2, and good fatigue performance. The result offers some guidance on improving mechanical and fatigue properties through microstructural evolution.
  • Hydrogen embrittlement susceptibility of new 2 000 MPa grade Ti microalloyed ultra-high strength steel
    Lü Wanqing, CAI Hong, YU Wenchao, WU Zhifang, YAN Yongming, SHI Jie, WANG Maoqiu
    Iron and Steel. 2025, 60(6): 188-196. https://doi.org/10.13228/j.boyuan.issn0449-749x.20240687
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    Owing to the superior mechanical properties, 2 000 MPa grade ultra-high strength steels have been extensively used in various industries, such as automobile manufacturing, engineering machinery and mining equipment. However, the exceptional strength also introduces the challenge of hydrogen embrittlement. The microstructure and mechanical properties of a novel Ti microalloyed low alloy ultra-high strength steel by SEM, TEM and chemical phase analysis were investigated. The hydrogen embrittlement susceptibility of the experimental steel was examined by electrochemical hydrogen charging, thermal desorption spectrometry （TDS） and slow strain rate tensile （SSRT） testing. The results indicate that the Ti element incorporated in steel predominantly precipitates as MC-type precipitates after heat treatment. The microstructure in steel consists of lath martensite, exhibiting tensile strength of 2 060 MPa and high yield ratio of 0.79, while maintaining excellent plasticity. The combination of Ti precipitation and controlled rolling technology resulted in an average grain size of approximately 4.3 μm in the experimental steel. According to TDS experimental results, the diffusible hydrogen content within the specimen has reached saturation when the charging time exceeded 48 h. Furthermore, the TDS curves reveal diffusible hydrogen desorption peak at around 142 ℃, corresponding to a hydrogen desorption activation energy Ea of 15.0 kJ/mol, which is consistent with the hydrogen trapping site of grain boundary. Additionally, the SSRT experimental demonstrate that the experimental steel maintains excellent strength and plasticity when the hydrogen content is below 0.000 027%. However, the fracture mode changes from ductile fracture to brittle fracture when the hydrogen content in the experimental steel exceeds 0.000 027%, the post-fracture elongation and the reduction of area significantly decrease with inclusions identified as the fracture initiation source. Therefore, for the experimental steel, there is a risk of brittle fracture when the hydrogen content exceeds 0.000 027%.
  • Thermodynamic analysis for occurrence state of rare earth in low alloy wear-resistant steel
    ZHOU Jianbo, WANG Xinhua
    Iron and Steel. 2025, 60(6): 197-204. https://doi.org/10.13228/j.boyuan.issn0449-749x.20250066
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    Low alloy wear-resistant steels are widely utilized in mining machinery and engineering equipment due to their excellent wear resistance, strength, and toughness. With increasing industrial demands for material performance under extreme conditions, cerium （Ce） introduction has become crucial for optimizing these steels. The influence of Ce on inclusion evolution and steel cleanliness under varying total oxygen contents was systematically investigated through FactSage thermodynamic calculations. By establishing a thermodynamic database incorporating rare earth oxysulfides and silicates, combined with actual NM450 steel composition, inclusion dynamics was simulated during refining （1 600 °C） and solidification processes. Key findings reveal that untreated steel contains brittle Al2O3 and CaO-Al2O3 inclusions prone to crack initiation. Adding 0.000 5% （mass fraction） Ce modifies inclusions into softer CeAlO3 phases, with complete transformation occurring at 0.010 0% Ce mass fraction, potentially enhancing machinability and wear resistance. Ce dissolution efficiency shows strong w（T[O]） dependence. Under low w（T[O]） （0.001 0%）, Ce solubility increases from 0.000 3% to 0.008 6% with treatment intensity （mass fraction of Ce addition, 0.000 5%-0.010 0%）, achieving full inclusion modification.However, at high w（T[O]） （0.004 0%）, Ce solubility remains limited to 0.000 4%, leaving residual Al2O3 and CaO-Al2O3 phases, which are difficult to metamorphize completely, resulting in weak inclusion control effect. During solidification, Ce treatment regulates precipitation behavior. With 0.001 0% Ce（mass fraction） treatment, sequential precipitation of CeAlO3, CaS-MnS, and TiN occurs. Increasing Ce to 0.005 0% suppresses TiN formation by reducing Ti activity, decreasing both precipitation temperature and quantity. This suppression is particularly beneficial. The reduction of TiN effectively improves the toughness and wear resistance of steel because ocoarse TiN particles typically induce stress concentration. The result identifies that maintaining w（T[O]）≤0.002 0% and Ce treatment intensity between 0.005 0%-0.010 0% can ensure effective inclusion modification and Ce dissolution. The occurrence mechanism of rare earth Ce and the control law of inclusions are revealed by thermodynamic calculation, and the process strategy of w（T[O]） and Ce treatment intensity optimization is proposed. These findings provide theoretical guidance for enhancing wear-resistant steel performance through Ce microalloying, suggesting future work should focus on experimental validation and exploring synergistic effects between Ce and other microalloying elements to expand industrial applications.
    • Metallurgical Process Engineering
  • System dynamic laws and regulation methods of "one ladle from BF to BOF" mode at iron and steel interface
    ZHENG Zhong, YANG Zhipeng, YANG Yongjie, WANG Yongzhou, GAO Xiaoqiang
    Iron and Steel. 2025, 60(6): 205-216. https://doi.org/10.13228/j.boyuan.issn0449-749x.20240682
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    The "one ladle from BF to BOF mode", as an innovative interface technology adopted by long-process steel enterprises, demonstrates significant advantages in optimizing operations, particularly in enhancing ladle turnover rates and reducing temperature drops during iron transport processes. To fully leverage the technical advantages of this mode and facilitate the transformation of steel enterprises towards high-efficiency and low-carbon operation control, the system dynamics operation law of the "one ladle from BF to BOF" mode was investigated based on metallurgical process engineering and system dynamics theory. Through systematic analysis, dynamic control methods for timely adjustment of iron ladle operations, both online and offline, were elucidated. By analyzing thrust and tension sources at the iron-steel interface using metallurgical process engineering, the fundamental operating characteristics of ferrous material flow were identified. System dynamics methodology was employed to establish causal relationships between main state variables and decision variables at the interface, constructing a comprehensive system dynamics model for ferrous material flow operation. This model clarified dynamic level equations of the interface system and formulates calculation formulas for input and output rate equations. Under the "one ladle from BF to BOF" paradigm, a model for the iron and steel packet turnover process was established. Quantitative analysis of dynamic characteristics was conducted using actual production data from an interface comprising 3 large blast furnaces and 2 steel plants. The results demonstrate that the system dynamics model effectively captures the influence of production rhythm changes in blast furnaces and steel plants on system operation status through input and output rate variations of ferrous material flow. Furthermore, the number of iron ladles accurately reflects the system's operational patterns in practical production scenarios. Utilizing Vensim dynamic system simulation software, a dynamic ladle adjustment strategy based on material flow rate matching was proposed, enabling real-time regulation of ladle quantities according to rate differences between iron and steel sides. Theoretical support for optimizing the operation mechanism of iron-steel interfaces is provided by this research, and decision-making guidance for production operation optimization in steel enterprises is offered.
    • Technology Exchange
  • Bidirectional multi-scale deep prediction method for vibration characteristics of turbine generator units
    XUE Zenghong, WEI Chunhu, CHENG Feng, ZHANG Xing
    Iron and Steel. 2025, 60(6): 217-228. https://doi.org/10.13228/j.boyuan.issn0449-749x.20240685
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    To address the difficulty of proactively assessing equipment condition when key vibration features of a turbine generator units cross predefined thresholds,thereby preventing unplanned shutdowns,a vibration trend prediction method was proposed based on a bidirectional multi-scale deep prediction network （MSBi-LSTM）. This approach analyzed the evolving trend of vibration signals to forecast when vibration amplitudes would reach maintenance thresholds, enabling maintenance planning to be scheduled in advance and optimized alongside production schedules, to enhance the operational reliability and economic benefits of the equipment. Specifically, MSBi-LSTM integrated a multi-scale CNN to extract hierarchical features from operational data, employed Bi-LSTM to capture both forward and backward temporal dependencies in the vibration signals, and incorporated an attention mechanism to emphasize critical features, thereby improving both prediction accuracy and robustness. Experimental results on both simulated and industrial datasets demonstrate that, compared with conventional time-series forecasting methods, MSBi-LSTM reduces RMSE by at least 36.13% and RMAE by 36.43%. Furthermore, ablation studies confirm that the number of channels and the kernel sizes in multi-scale CNN module critically influence model performance. Appropriately tuning these hyperparameters can further enhance predictive accuracy. In summary, the proposed MSBi-LSTM method offers an efficient and precise solution for vibration trend forecasting in turbo-generator sets, with strong potential for industrial application. Its adoption can markedly improve equipment reliability and production efficiency in the steel industry, thereby promoting high-efficiency, stable, and sustainable operation.
Started in 1954 (Monthly)
ISSN 0449-749X　CN 11-2118/TF
Manager: China Association for Science and Technology
Sponsor: The Chinese Society for Metals
Central Iron & Steel Research Institute Co., Ltd.
CISRI Boyuan Publishing Co., Ltd.
Editor-in-Chief: TIAN Zhiling
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