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行波磁場鑄流攪拌提升不銹鋼板坯等軸晶率

Stirring strands via traveling-wave magnetic fields to increase the equiaxed crystal ratio of stainless-steel slab castings

  • 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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