Stirring strands via traveling-wave magnetic fields to increase the equiaxed crystal ratio of stainless-steel slab castings
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摘要: 為揭示各種行波磁場鑄流攪拌的電磁冶金效果，基于計算域分段法建立了斷面1280 mm×200 mm板坯連鑄電磁、流動、傳熱和凝固的耦合模型，利用電氣參數和磁感應強度的實測值和預測值的對比驗證了模型的可靠性。研究表明：行波磁場攪拌器因電磁推力的方向性特點在板坯二冷區攪拌過程中均表現有不同程度與特征的端部效應，輥后箱式攪拌器(Box-typed electromagnetic stirrer, B-EMS)的單側安裝形式導致板坯內弧側磁感應強度遠大于外弧側，輥式攪拌器(Roller-typed electromagnetic stirrer, R-EMS)的對輥安裝形式則使磁感應強度呈現對稱分布。在400 kW和7 Hz的相同電氣參數下，R-EMS的電流強度比B-EMS高75 A；盡管箱式電磁攪拌的有效作用區域較輥式電磁攪拌大，鑄坯中心鋼液過熱耗散區域大，但輥式攪拌推動鋼液沖刷凝固前沿形核作用則明顯大于箱式攪拌。兩者均具有較好的抑制柱狀晶生長、促進凝固前沿等軸晶形核與發展的能力，將不銹鋼板坯等軸晶率提高至45%的門檻值以上，其中間隔型反向輥式攪拌器下的等軸晶率比箱式攪拌高約17%。綜合表明，基于行波磁場鑄流攪拌的間隔型反向輥式攪拌器有望更好地消除鐵素體不銹鋼板材表面皺折缺陷。Abstract: Electromagnetic stirring of strands by a traveling-wave magnetic field is a cutting-edge continuous casting technology for eliminating the columnar crystal structure that tends to develop in stainless- and/or silicon-steel slab castings. The common ridging defect on the surface of ferritic stainless strip products has been found to be closely related to the well-developed as-cast columnar crystal structure. To explore the various electromagnetic properties of the traveling-wave magnetic fields applied to the secondary cooling zone of a slab casting strand, we used the segmented computational domain method to develop a coupled math model to analyze the electromagnetic, fluid flow, heat transfer, and solidification behaviors, which had been previously determined in an electromagnetic measurement experiment to be a valid approach. The modeling analysis results regarding the traveling-wave magnetic fields show that molten steel stirring has some effect on the end of the slab strand. We also found that the intensity of the magnetic induction when using a box-type electromagnetic stirrer (B-EMS) is much greater on the inside of the strand than on the outside, as compared with its symmetric behavior when applying a roller-type electromagnetic stirrer (R-EMS). At an electrical power of 400 kW and frequency of 7 Hz, the current intensity of the R-EMS is higher than that of the B-EMS by 75 A, achieving a more efficient stirring effect for promoting equiaxed crystal nucleation in front of the solidified shell. In casting experiments in a stainless-steel slab caster, both the B-EMS and R-EMS are found to inhibit the growth of columnar crystals through nucleation of the heads of the dendrites, which realizes an equiaxed crystal ratio of the slab casting 45% higher than its threshold value. In addition, an R-MES with two pairs of rollers using inverse thrust EMS forces can produce an equiaxed crystal ratio 17% higher than that achieved by the B-EMS, and can thus be used in the casting production of ferritic stainless steels to obtain final strip products with no ridging defects.
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圖 1 攪拌器類型及其在板坯鑄流中安裝形式。（a）箱式攪拌器；（b）輥式攪拌器
Figure 1. Schematic of the strand stirrer type and installation: (a) B-EMS (b) R-EMS
圖 2 攪拌器布置與電磁推力特征。（a）箱式；（b）相鄰輥同向推力型；（c）間隔輥同向推力型；（d）間隔輥反向推力型
Figure 2. Schematic of the stirrer location and electromagnetic force: (a) B-EMS; (b) R-EMS-2; (c) R-EMS-T; (d) R-EMS-F
圖 4 磁感應強度測量值與計算值對比。（a）箱式攪拌器；（b）輥式相鄰同向型攪拌器
Figure 4. Comparison of measured and calculated magnetic flux densities: (a) B-EMS; (b) R-EMS-2
圖 5 電磁攪拌的電氣參數關系曲線。（a）電流與電壓；（b）電流與功率
Figure 5. Electrical parameters relationship for electromagnetic stirring: (a) current and voltage; (b) current and power
圖 6 攪拌器鑄坯橫截面(XY面)上磁感應強度分布特征。（a）箱式攪拌器；（b）輥式攪拌器
Figure 6. Distributions of magnetic flux density on the cross section (XY plane) of the stirrer: (a) B-EMS; (b) R-EMS
圖 7 板坯表面磁感應強度分布。（a）箱式；（b）相鄰輥同向推力型；（c）間隔輥同向推力型；（d）間隔輥反向推力型
Figure 7. Magnetic flux density distributions on the slab surface：（a）B-EMS；（b）R-EMS-2；（c）R-EMS-T；（d）R-EMS-F
圖 8 板坯內磁感應強度分布。（a）中心線上沿拉坯方向；（b）中心線上沿寬面方向
Figure 8. Magnetic flux density distribution in slab: (a) on center line along the casting direction; (b) on center line along the wide face direction
圖 9 不同攪拌方式下鑄坯窄面凝固前沿速度沿拉坯方向分布
Figure 9. Washing velocity distributions of the strand along the casting direction under different stirring modes
圖 10 鑄坯橫截面(Z=4.0 m)內液相分率分布與鋼液流線圖。（a）箱式；（b）相鄰輥同向推力型；（c）間隔輥同向推力型；（d）間隔輥反向推力型
Figure 10. Distributions of the liquid fraction in the cross-section (Z = 4.0 m) of the slab and the molten steel streamline: (a) B-EMS; (b) R-EMS-2; (c) R-EMS-T; (d) R-EMS-F
圖 11 板坯寬向中心面上鑄流方向溫度分布與鋼液流線圖。（a）箱式；（b）相鄰輥同向推力型；（c）間隔輥同向推力型；（d）間隔輥反向推力型
Figure 11. Temperature distributions and molten steel streamlines in the casting direction on the widthwise center plane of the slab: (a) B-EMS; (b) R-EMS-2; (c) R-EMS-T; (d) R-EMS-F
圖 12 鑄坯窄面中心坯殼厚度沿鑄流分布。（a）電磁推力起始側；（b）推向側
Figure 12. Distributions of the thickness on the narrow-face center of the strand along the casting direction: (a) start side of electromagnetic force; (b) end side of electromagnetic force
圖 13 鐵素體不銹鋼板坯鑄態組織形貌。（a）無攪拌；（b）箱式攪拌；（c）間隔輥反向推力型
Figure 13. As-cast structure and morphology of the ferritic stainless-steel slab: (a)without EMS (b) under B-EMS (c) under R-EMS-F
表 1 1Cr17鐵素體不銹鋼主要化學成分（質量分數）
Table 1. Chemical composition of 1Cr17 stainless steel
% C Cr Ni Mn P S Si ≤0.12 17 ≤0.6 ≤1.25 ≤0.035 ≤0.03 ≤0.75 
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表 2 計算用1Cr17不銹鋼熱物性參數和連鑄工藝參數
Table 2. Parameter values of 1Cr17 stainless-steel thermophysical properties and continuous casting practice
Parameters Value Parameters Value Slab cross section/mm2 1280×200 Liquidus temperature/K 1768 Distance to meniscus of B-EMS/m 4.0 Solidus temperature/K 1703 Distance to meniscus of R-EMS/m 3.8, 4.09, and 5.2 Specific heat/(J·kg?1·K?1) 720 Relative permeability of each material 1 Latent heat of solidification/(J·kg?1) 272000 Relative permeability of iron core 1000 Superheat degree/K 30 Conductivity of molten steel/(S·m?1) 7.14×105 Molten steel density/(kg·m?3) 7200 Specific water flow/(L·kg?1) 0.4 Molten steel viscosity/(kg·m?1·s?1) 0.0055 Casting speed/(m·min?1) 0.9 Thermal conductivity of molten steel/(W·m?1·K?1) 32
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