Research and application of three-dimensional dynamic secondary cooling and accurate soft reduction for continuous casting slab
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摘要: 在現有工藝條件下，校驗和完善二冷區鑄坯凝固傳熱計算數學模型，開發三維二冷配水模型，解決目前設備狀況下冷卻水分布不均勻對鑄坯溫度的影響，從而控制鑄坯表面質量，特別是鑄坯的角部裂紋，同時對板坯連鑄二冷配水制度進行改進和優化，使之滿足高效連鑄生產條件和改善鑄坯質量的需要。提出壓下參數計算公式，結合所開發三維二冷配水模型，優化現有壓下工藝，提出并應用精準可控單段壓下、非穩態壓下控制，集中解決連鑄板坯中心偏析、中心疏松和縮孔等內部質量問題。同時優化模型數據庫，使之數據更加完備，模型計算更加準確，同時模型具備異鋼種混澆過程二冷及壓下控制功能，能夠進行凝固終點W形預測與控制，可進一步提高模型適用性和準確性。模型開發并成功在多家鋼廠現場應用，有效改善了鑄坯裂紋和偏析等鑄坯表面和內部的質量問題。Abstract: The surface and corner cracks of continuous casting billet form during the continuous casting process, especially in medium-carbon steel. Surface defects of such billets are directly related to the secondary cooling process of continuous casting, while the center deviation of the billet, center shrinkage, and center loosening in medium-carbon steel, medium-carbon alloy steel, and high-carbon steel are especially prominent. Such quality defects are related to the secondary cooling and press-down process of continuous casting. These two defects are the main factors restricting continuous casting production. The secondary cooling of the continuous casting process has an important influence on the surface and internal quality of the slab, especially the temperature of the slab corners, which directly affects the surface quality of the slab. Under the existing process conditions, the mathematical model for calculating the solidification heat transfer of the slab in the secondary cooling zone is calibrated and improved, and a three-dimensional secondary cooling model is developed to solve the influence of uneven cooling due to water distribution on the temperature of the slab. This controls the surface quality of the slab, especially the corner cracks of the slab, and improves and optimizes the slab continuous casting secondary cooling system to improve the slab quality. An equation for calculating the soft reduction parameters is proposed, and the existing soft reduction process is optimized by combining the developed three-dimensional secondary cooling model with the proposed and applied controllable single-stage soft reduction and unsteady soft reduction control to solve the internal quality problems, such as central segregation, central porosity, and shrinkage of the continuous casting slab. At the same time, the model database is optimized to make the data more complete and the model calculation more accurate. This model adds the technology of mixed casting of different grades of steel and the technology of predicting and controlling the W-shaped solidification to further improve the applicability and accuracy of the model. The model has been developed and successfully applied in several steel plants, and the result shows that the proposed model can improve the surface and internal quality of cast slabs effectively, such as cracks and segregation.
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Key words:
- continuous casting slab /
- secondary cooling /
- accurat soft reduction /
- double-target temperature /
- crack /
- segregation
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圖 4 連鑄坯表面冷卻速率實測與模型計算結果
Figure 4. Results of the measured and modeled cooling rates on the surface of the continuous casting slab
圖 11 穩態與非穩態過程典型扇形段壓下量的變化。（a）穩態時第8扇形段壓下量變化；（b）為非穩態時第7扇形段壓下量變化；（c）為非穩態時第8扇形段壓下量變化
Figure 11. Variation of the typical soft reduction at the steady state and nonsteady state: (a) variation of the reduction amount of seg.8 at the steady state；(b) variation of the reduction amount of segment.7 at the nonsteady state；(c) variation of the reduction amount of segment.8 at the nonsteady state
圖 14 Q345有效導熱系數分布及M值沿拉坯方向變化。（a）有效導熱系數分布；（b） M值沿拉坯方向變化
Figure 14. Effective thermal conductivity and M:
(a) distribution of effective thermal conductivity; (b) variation of M on the direction of casting 
圖 15 不同末端壓下量鑄坯中心偏析情況。（a）兩種末端壓下量對應C偏析度結果；（b）末端壓下2 mm低倍結果；（c）末端壓下10 mm低倍結果
Figure 15. Center segregation of different reduction amount at the end of solidification：(a) result of C segregation ratio；(b) macrostructure of C segregation of 2 mm reduction amount；(c) macrostructure of C segregation of 10 mm reduction amount
圖 16 非穩態壓下控制對比。（a）常規壓下方式非穩態過程控制；（b）本技術非穩態壓下控制
Figure 16. Comparison of different controls of reduction at the nonsteady state：(a)regular control of reduction at the nonsteady state;(b) this research control of reduction at the nonsteady state
圖 17 混合澆注數值計算結果。（a）碳元素質量分數分布；（b）碳含量和固?液相線溫度變化
Figure 17. Results of numerical calculation：(a) distribution of C mass fraction; (b) variation of carbon content and solid-liquid line
圖 18 W形凝固終點形狀預測與控制。（a）凝固終點W形；（b）W形模型預測；（c）W形消除；（d）W形模型消除
Figure 18. Prediction and control of W-shape at the end of solidification: (a) W-shape at the end of solidification; (b) model prediction of W-shape; (c) elimination of W-shape; (d) model elimination of W-shape
圖 19 投用前（1~4月）和投用后（5月）裂紋試槍合格率
Figure 19. Cracking rate before and after using the model
圖 21 投用前（a）和投用后（b）偏析缺陷對比
Figure 21. Comparison of segregation before and after using the model used for the Plant L
圖 22 模型計算溫度與實測溫度對比
Figure 22. Comparison of the model’s calculated temperature and measured temperature
表 1 實驗鋼種化學成分（質量分數）
Table 1. Chemical composition of experimental steel grades (mass fraction)
% Elements C Mn P S Si Nb V Ti Cu Alt N A32 0.16 1.11 0.0097 0.0067 0.242 0.0252 0.0039 0.0049 0.07 0.0319 0.0044 Q345EN 0.16 1.404 0.0076 0.0018 0.336 0.02296 0.0031 0.0123 0.05 0.0317 0.0033 
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表 2 混合澆注鋼種成分及其固?液相線溫度
Table 2. Composition of mixed cast steel and solid–liquid phase line temperature
Steel grades w/% Tl/℃ Ts/℃ C Si Mn P S Before mixed (AH36) 0.16 0.15 1.50 0.015 0.01 1514 1482 After mixed (Q345) 0.08 0.27 1.30 0.02 0.025 1520 1486
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表 3 模型上線前后軋后缺陷率統計
Table 3. Crack rate before and after using the model
Low-alloyed statistics of
the crack after rolling/ %Hot-rolled plate statistics of
the crack after rolling/ %Before use After use Before use After use 0.92 0.38 0.78 0.3 0.95 0.46 0.75 0.28 0.93 0.37 0.71 0.19
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