Secondary cooling control based on solidification characteristics of non-quenched and tempered steel
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摘要: 提出了基于非調質鋼凝固特性的二次冷卻控制方法。在凝固特性研究方面，運用高溫共聚焦顯微鏡、場發射掃描電鏡研究了冷速對第二相粒子析出規律的影響，并闡釋了先共析鐵素體的相變機制。結果表明，第二相粒子在1086 ℃開始析出，并在912 ℃達到峰值。當冷速在0.1～5 ℃·s?1時，隨著冷速增大，第二相粒子尺寸和數量均減小，且第二相粒子由晶界處的鏈狀分布向晶體內的彌散分布過渡，提高冷速有助于削弱第二相粒子的釘扎作用，強化鑄坯表層微觀組織；在二冷配水優化方面，建立了考慮鑄坯橫向水量分布的凝固傳熱數學模型，提出了基于非調質鋼凝固特性的二冷配水優化方案，即對出結晶器后的鑄坯實施強冷，以滿足控制第二相粒子析出的合理冷速和溫度區間的要求。工業試驗證實了技術方案的可行性。此外，研究表明，降低噴淋距離有助于改善連鑄坯橫向冷卻不均勻性。本研究統籌考慮二冷水量與噴淋距離對非調質鋼裂紋敏感性的影響，通過開展“縱?橫”凝固冷卻控制研究對連鑄二次冷卻進行系統優化，提出的二冷優化方案有助于改善連鑄坯的表面及皮下裂紋。Abstract: Due to the high crack sensitivity of non-quenched and tempered steel and the difficulty of accurate control of secondary cooling, surface cracks of the continuous casting strand occur frequently. A secondary cooling control method based on the solidification characteristics of non-quenched and tempered steel was proposed. For the solidification characteristics, the effect of the cooling rate on the secondary phase precipitation was studied using a confocal microscope and field emission scanning electron microscopy (FESEM), and the phase transformation mechanism of proeutectoid ferrite was clarified. Results show that the second-phase particles start to precipitate at 1086 °C and reach a peak at ~912 °C. When the cooling rate ranges from 0.1 to 5 °C·s?1, the size and volume fraction of the second-phase particles decrease with the increase of the cooling rate, and the second-phase particles transition from a chain-like distribution at the grain boundaries to a uniform distribution in the matrix. Increasing the cooling rate is helpful to weaken the pinning effect of the precipitates and strengthen the microstructure of the bloom surface. As for the secondary cooling optimization, a heat transfer and solidification model considering a transverse water distribution was established, and a secondary cooling optimization method based on the solidification characteristics of non-quenched and tempered steel was proposed. Strong cooling is performed after the strand leaves the mold to meet the requirements of a reasonable cooling rate and temperature range for controlling the precipitation of particles. Industrial trials confirm the feasibility of the technical solution. In addition, the study shows that reducing the spray distance can improve the transverse non-uniformity of secondary cooling water. In this study, the influence of the secondary cooling water amount and spray distance on the crack sensitivity of non-quenched and tempered steel was comprehensively considered, and the secondary cooling process was optimized by studying the “longitudinal?transverse” solidification cooling. The proposed optimization scheme contributes to the improvement of surface and subsurface cracks of continuous casting bloom.
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圖 2 SG02鋼凝固進程的原位觀察。（a）1086 ℃；（b）912 ℃；（c）723 ℃；（d）665 ℃
Figure 2. In-situ observation of the solidification process of the SG02 steel: (a) 1086 ℃; (b) 912 ℃; (c) 723 ℃; (d) 665 ℃
圖 3 碳氮化物析出的熱力學計算。（a）碳氮化物的質量分數；（b）碳氮化物的主要成分
Figure 3. Thermodynamic calculation of carbonitride precipitation: (a) mass fraction of carbonitrides; (b) main component of a carbonitride
圖 4 不同冷速下碳氮化物粒子的析出形貌。（a）0.1 ℃·s?1；（b）0.5 ℃·s?1；（c）1 ℃·s?1；（d）3 ℃·s?1；（e）5 ℃·s?1；（f）能量衍射分析
Figure 4. Precipitation morphology of carbonitride particles at different cooling rates: (a) 0.1 ℃·s?1; (b) 0.5 ℃·s?1; (c) 1 ℃·s?1; (d) 3 ℃·s?1; (e) 5 ℃·s?1; (f) energy-dispersive spectrum analysis
圖 6 SG02鋼熱物性參數。（a）固相率；（b）密度和熱焓；（c）熱導率
Figure 6. Thermal-physical properties of SG02 steel: (a) solid fraction; (b) density and enthalpy; (c) thermal conductivity
圖 7 模型修正及計算結果。（a）鑄坯表面中心測量值與計算值對比；（b）表面中心和角部溫度變化
Figure 7. Model modification and calculation results: (a) comparisons of the measured and simulated temperatures of the bloom surface center; (b) surface center and corner temperature changes
圖 8 連鑄坯表面中心瞬時冷速
Figure 8. Instantaneous cooling rate at the surface center of the continuous casting billet
圖 10 二冷配水優化后連鑄坯特征位置溫度變化
Figure 10. Temperature change of characteristic position after secondary cooling water distribution optimization
圖 12 噴淋高度優化前、后水量分布。（a）二冷一段，噴嘴A；（b）二冷二段，噴嘴B；（c）二冷三段，噴嘴C；（d）二冷四段，噴嘴D
Figure 12. Water distribution before and after spray height optimization: (a) SegmentⅠ, Nozzle A; (b) Segment Ⅱ, Nozzle B; (c) Segment Ⅲ, Nozzle C; (d) Segment Ⅳ, Nozzle D
圖 13 噴嘴配置優化后連鑄坯特征位置溫度變化
Figure 13. Temperature change of characteristic position after nozzle configuration optimization
表 1 SG02鋼主要化學成分(質量分數)
Table 1. Main chemical composition of the SG02 steel
% C Si Mn P S N V Nb Ti 0.43 0.45 1.41 0.01 0.0125 0.0115 0.07 0.017 0.015 
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表 2 SG02鋼主要連鑄工藝參數
Table 2. Main casting parameters of the SG02 steel
Item Value Sectional dimension/(mm×mm) 220×220 Casting speed/(m·min?1) 1.05 Pouring temperature/°C 1524 Superheat/℃ 35 Water flux of mold cooling/(m3·h?1) 150 Temperature difference of mold water/°C 7.0 Water flux of secondary cooling / (L·kg?1) 0.32 Water temperature/℃ 35 Ambient temperature/℃ 25
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表 3 噴淋高度優化方案
Table 3. Spray height optimization scheme
mm Schemes Nozzle A Nozzle B Nozzle C Nozzle D Before optimization 165
(ModeⅠ)178
(ModeⅠ)170
(ModeⅡ)175
(ModeⅠ)After optimization 140
(ModeⅡ)145
(Mode Ⅲ)140
(Mode Ⅲ)135
(Mode Ⅲ)
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