Development and application of intermixed length and composition variation model in continuous slab casting processes during a grade transition
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摘要: 基于建立的連鑄中間包及結晶器內鋼液混合過程的物理模型，開發了板坯連鑄異鋼種連澆過程混澆坯長度及成分變化模型。以某鋼廠單流板坯連鑄機220 mm×1560 mm斷面Q235與Q335Ti鋼的混澆過程為研究對象，采用水模型試驗結合數值模擬確定模型的關鍵參數，并通過開展現場試驗對混澆坯取樣驗證模型的準確性。結果證明：混澆坯成分取樣與模型預測的成分偏差小于5%，且模型預測的混澆坯長度與人工確定的一致。故采用該模型可跟蹤不同混澆工況下中間包內及鑄流上鋼液的混合行為，準確預測混澆坯的長度以及成分變化規律。采用該模型研究了拉速及中間包內剩余鋼液質量對混交坯長度及不同澆注長度鑄坯C元素質量分數變化的影響規律。發現當拉速保持不變時，中間包內剩余鋼液越多，混澆坯越長；當中間包內剩余鋼液質量保持不變時，拉速越大混澆坯越短。相比而言，中間包內剩余鋼液質量比拉速對混澆坯長度的影響更大。另外當拉速不變時，隨著中間包內剩余鋼液質量的增加，C元素質量分數由0.16%變化到0.18%的速率減慢；當中間包內剩余鋼液質量不變時，隨著拉速的增加，C元素質量分數由0.16%變化到0.18%的速率增加。因此異鋼種連澆過程，適當提高拉速以及減少中間包內剩余鋼液質量，可有效減少混澆坯長度，成分變化速率降低。Abstract: Based on a developed physical model during the mixing process in the tundish and the liquid pool of the strand, intermixed length and composition variation model have been established during steel grade changes in the continuous slab casting processes. The research object was the mixing process in the single-strand slab caster during steel grade changes of Q235 and Q335Ti steel with 220 mm × 1560 mm section. Key parameters of the model were determined using the water model test and numerical simulation, and the model was verified through plant tests, which were performed on the slab caster during the grade transition period of continuous casting. Real grade intermixed slabs were produced, and composition distributions were measured and compared. The carbon composition and the length of the intermixed slab predicted using the model were found to be in good agreement with the experimental data. Mixed behaviors in the tundish and strand were tracked using the model under various mixing conditions. In addition, the length and the composition change law of the intermixed slab can be precisely predicted. The effect of casting speed and the remaining molten steel in the tundish on length and the composition change law of the intermixed slab were studied by the model. As the casting speed remains unchanged, the intermixed-slab length increases with more remaining molten steel in the tundish. While the mass of remaining molten steel in the tundish keeps unchanged, the intermixed slab length decreases with more casting speed. In comparison, the remaining molten steel in the tundish has a larger effect on the intermixed slab length than the casting speed. As the mass of the remaining molten steel in the tundish increases with constant casting speed, the rate at which the mass fraction of C changes from 0.16% to 0.18% slows down; While the mass of remaining molten steel in the tundish keeps unchanged, the rate at which the mass fraction of C changes from 0.16% to 0.18% accelerates with an increase in the casting speed. Reducing the mass of the residual molten steel in the tundish and increasing the casting speed in the strand is beneficial for the reduction in the length of the intermixed slab and element composition variation rate. Moreover, the strategy of lowering the liquid level in the tundish and increasing the casting speed simultaneously can be adopted to decrease the intermixed slab length to the greatest extent.
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圖 1 在中間包內及鑄流上鋼液流動混合的物理模型示意圖
Figure 1. Schematic diagram of the fluid flow and mixing process in the tundish and strand
圖 2 異鋼種連澆過程模擬混澆的水模型試驗裝置
Figure 2. Water model test device of mixing process simulation during continuous casting grade transition
圖 3 異鋼種連澆過程中間包內鋼液混合過程流動的數值模擬。（a）中間包結構網格劃分；（b）模擬計算的鋼液混合過程流線圖
Figure 3. Numerical simulation of the mixing process in the tundish during continuous casting grade transition: (a) meshing of tundish structure; (b) streamline diagram during the mixing process in the tundish using numerical simulation
圖 4 異鋼種連澆過程結晶器內鋼液混合過程流動的數值模擬。（a）結晶器內鋼液速度云圖；（b）結晶器內鋼液流線圖
Figure 4. Numerical simulation of the mixing process in the mold during continuous casting grade transition: (a) velocity nephogram of flow in the mold; (b) streamline diagram during the mixing process in the mold using numerical simulation
圖 5 模型計算的混澆過程不同時刻中間包內鋼液的平均混合率以及鑄流混合率
Figure 5. Mixing rate at different times in the tundish and strand during continuous casting grade transition using a mixing prediction model
圖 6 模型計算的不同澆注長度鑄坯對應的鑄流混合率
Figure 6. Mixing rate at different casting lengths in the strand during continuous casting grade transition using a mixing prediction model
圖 7 混澆模型計算的混澆坯不同位置各元素質量分數變化
Figure 7. Mass fraction of elements in intermixed slab during continuous casting grade transition using a mixing prediction model
圖 8 模型計算的混澆坯不同位置C質量分數與實際鑄坯測量結果的對比
Figure 8. Comparison of C content between mixing prediction model calculation and actual measurement
圖 9 模型計算的拉速為1.2 m?min?1時不同中間包內剩余鋼液質量下不同澆注長度鑄坯對應的混合率。（a）15 t；（b）20 t；（c）25 t；（d）30 t
Figure 9. Mixing rate in the corresponding slab of different casting lengths with a casting speed of 1.2 m?min?1 under different masss of residual molten steel in the tundish: (a) 15 t; (b) 20 t; (c) 25 t; (d) 30 t
圖 10 模型計算的拉速為1.2 m?min?1時不同中間包內剩余鋼液質量下鑄流上不同澆注長度鑄坯C元素質量分數的變化。（a）15 t；（b）20 t；（c）25 t；（d）30 t
Figure 10. Chang of C content in the corresponding slab of different casting lengths with a casting speed of 1.2 m?min?1 under different masss of residual molten steel in the tundish: (a) 15 t; (b) 20 t; (c) 25 t; (d) 30 t
圖 11 模型計算的中間包內剩余鋼液質量為25 t時不同拉速下鑄流上不同澆注長度鑄坯對應的混合率。（a）1.1 m?min?1；（b）1.2 m?min?1；（c）1.3 m?min?1；（d）1.4 m?min?1
Figure 11. Mixing rate in the corresponding slab of different casting lengths with 25 t mass of residual molten steel in tundish under different casting speeds: (a) 1.1 m?min?1; (b) 1.2 m?min?1; (c) 1.3 m?min?1; (d) 1.4 m?min?1
圖 12 模型計算的中間包內剩余鋼液質量為25 t時不同拉速下鑄流上不同澆注長度鑄坯C元素質量分數的變化。（a）1.1 m?min?1；（b）1.2 m?min?1；（c）1.3 m?min?1；（d）1.4 m?min?1
Figure 12. Carbon element composition change in the corresponding slab of different casting lengths with 25 t mass of residual molten steel in the tundish under different casting speeds: (a) 1.1 m?min?1; (b) 1.2 m?min?1; (c) 1.3 m?min?1; (d) 1.4 m?min?1
表 1 連鑄機的主要技術參數
Table 1. Key technical parameters of a slab caster
Parameter Value Type of the caster Straight-arc type Strand of the caster 1 Metallurgical length of the caster/m 36 Section size of slab/(mm×mm) 220×1560 Effective length of mold copper plate/m 1 Type of submerged nozzle Two side outlet Type of tundish Rectangle flow control device Working capacity of tundish/t 32 
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表 2 混澆鋼種及其鋼包內的成分（質量分數）
Table 2. Element composition in ladle during continuous casting grade transitio
% Steel grade C Si Mn P S Ti Q235 0.18 0.10 0.30 0.019 0.0061 0 Q355Ti 0.16 0.10 0.35 0.018 0.0054 0.0514
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