Effect of casting speed rising on the cleanliness of IF steel slab during the initial casting stage
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摘要: IF鋼連鑄開澆過程頭坯潔凈度水平較低，目前各企業一般將開澆坯作判廢或降級處理，這導致了產品質量不穩定，成材率低等問題。為研究IF鋼非穩態開澆階段連鑄坯的潔凈度水平及優化措施，采用現場取樣、檢測分析及數值模擬計算相結合的方法，分析了IF鋼開澆階段不同拉速變化曲線條件下連鑄坯沿拉坯方向的潔凈度變化規律。通過實驗檢測發現沿拉坯方向頭坯T[O]、[N]及顯微夾雜物含量呈現明顯下降趨勢，距離頭坯頭部6 m處 T[O]含量為13×10?6，接近正常坯水平，距離頭坯頭部7 m處 [N]含量約為19×10?6，接近正常坯水平，開澆工藝對于連鑄坯T[O]、[N]及顯微夾雜物含量影響不大。勻速、前快后慢和前慢后快三種拉速工藝條件下，大型夾雜物在距離頭坯頭部2 m處的質量分數約為0.2 mg?kg?1，但之后均存在不同程度的大型夾雜物數量波動現象，而前快后慢工藝影響范圍最小，在5.5 m后均達到正常水平。通過數值計算同樣發現，前快后慢的提拉速工藝條件下產生的頭坯潔凈度更快地接近正常坯潔凈度水平（約開澆430 s后，對應拉坯5 m）。基于本文研究，提出了一種優化的IF鋼連鑄開澆過程提拉速的工藝方法。Abstract: During the initial casting stage of the IF steel, the cleanliness of the first slab deteriorates. Steel plants treat the initial casting slabs as waste or demoting products. Moreover, the cleanliness deterioration of initial casting slabs leads to unstable quality and low product yield. In the present study, to investigate the cleanliness of continuous casting slabs and the optimization measures during the initial casting stage of the IF steel, the cleanliness of the initial casting slabs is evaluated by field sampling and experimental analysis. Moreover, the effect of the casting speed rising method on the cleanliness of the IF steel slab during the initial casting stage is investigated. Additionally, numerical simulations are conducted to reveal the level of fluctuations of the molten steel in the mold during the initial casting stage. The results show that the variations in T[O], [N], and the content of microinclusions in the slab decrease obviously along the casting direction. The contents of T[O] and [N] in the slab reach the normal slab level of 6 m (approximately 13×10?6) and 7 m (approximately 19×10?6) away from the dummy bar head, respectively. Nevertheless, the effects of the casting speed rising method on the contents of T[O], [N], and microinclusions in the initial casting slabs are inapparent during the open casting process. For the three conditions of open casting (different casting speed rising methods), the contents of the macroinclusions in the slab reach the normal slab level of around 2 m (approximately 2 mg/10 kg) away from the dummy bar head, respectively. Subsequently, the fluctuation of the macroinclusion content can be determined. Furthermore, under the condition of “quick-slow” casting speed rising, the fluctuation can be quickly alleviated, and the macroinclusion content normalizes. Numerical simulations reveal that the variations in the fluctuation level of the molten steel at different positions (near the nozzle, a quarter of the width, and near the narrow face) in the mold during the initial casting stage of the three casting speed rising modes are similar. First, the fluctuation level is extremely intense. Then, the fluctuation level gradually weakens for some time. Finally, the fluctuation level of the molten steel at different positions in the mold stabilizes. Additionally, the numerical simulation results reveal that the slab cleanliness can quickly normalize (430 s after initial casting and about 5 m away from the dummy bar head) under the “quick-slow” casting speed rising mode during the initial casting stage.
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Key words:
- IF steel /
- continuous casting slab /
- initial casting stage /
- casting speed /
- cleanliness
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圖 4 不同拉速工藝條件下頭坯T[O] (a)和[N] (b)含量變化
Figure 4. Variations in T[O] (a) and [N] (b) in slab along casting direction under different casting speed curves
圖 5 不同拉速工藝條件下顯微夾雜物數量變化
Figure 5. Variations in microinclusion number density in the slab along casting direction under different casting speed curves
圖 6 不同拉速工藝條件下大型夾雜物含量變化
Figure 6. Variations in macroinclusion mass fraction in the slab along casting direction under different casting speed curves
圖 7 不同拉速工藝條件下頭坯不同位置卷渣類大型夾雜物質量分數
Figure 7. Mass fraction of inclusions originated from slag entrainment in first slab samples
圖 8 開澆過程不同拉速工藝條件下結晶器液面波動的數值模擬計算結果. (a) 勻速工藝; (b) 前快后慢工藝; (c) 前慢后快工藝
Figure 8. Level fluctuation of steel–slag interface in mold during initial casting stage: (a) uniform; (b) fast and then slow; (c) slow and then fast
圖 9 開澆過程澆鑄時間與拉坯長度對應關系
Figure 9. Variation in casting length with pouring time during initial casting stage
表 1 IF鋼中間包鋼水樣化學成分（質量分數）
Table 1. Chemical components of IF steel samples obtained in the tundish（mass fraction）
% Heat Elevating speed method C Si Mn P S Alt Ti O N A Uniform 0.0008 0.0048 0.050 0.013 0.0063 0.0388 0.0588 0.0016 0.0016 B Fast and then slow 0.0010 0.0040 0.052 0.013 0.0066 0.0371 0.0579 0.0019 0.0015 C Slow and then fast 0.0008 0.0049 0.038 0.012 0.0061 0.0347 0.0597 0.0017 0.0017 
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