取向硅鋼中滲氮過程的實驗研究與數值模擬取向硅鋼中滲氮過程的實驗研究與數值模擬Experimental studies and numerical simulation on the nitriding process of Grain-oriented Silicon steel 板坯低溫加熱工藝是國內外生產取向硅鋼的先進工藝，而滲氮是通過低溫工藝生產取向硅鋼的關鍵工序。本文通過在實驗室模擬取向硅鋼的滲氮生產工序，實測了不同滲氮時間下的滲氮量，并通過電子探針、場發射掃描電鏡等觀察了滲氮后氧化層的變化、氮在硅鋼帶厚度方向的濃度梯度。根據實測的組織形貌和滲氮動力學特點，首次建立了針對取向硅鋼的滲氮動力學模型，并成功地進行了數值模擬計算。研究結果表明： (1) 取向硅鋼帶的平均N含量在滲氮初期增加緩慢，之后逐漸加快，直至90 s時滲氮速率達到最大值之后保持恒定；在750 ℃滲氮2分鐘時間內，在近表面0.04 mm范圍內存在顯著的氮濃度梯度；(2) 脫碳退火后，取向硅鋼的氧化層主要為層狀氧化物，滲氮時氧化層被氫氣還原，層狀氧化物轉變為球狀，氧化層的變化對滲氮動力學影響恒定； (3) 分析了N由氣相穿過表面氧化層至鐵基體的傳質系數所遵循的可能模型，發現只有當傳質系數遵循氧化層還原動力學模型時，即Avrami函數模型f=1-exp?(-kt^n)，計算結果才能與實測的滲氮動力學特征高度吻合。 Grain-oriented silicon steel (GOSS) is an important functional material used as lamination cores in various transformers. Its magnetic properties are strongly dependent on the sharpness of Goss texture in product, which is developed after secondary recrystallization annealing. In order to save energy and reduce cut-down operation costs, Nippon Steel first lowered the slab-reheating temperature from 1350?1400 oC to 1150 oC and adopted the nitriding process to form nitride inhibitors before recrystallization annealing in 1970s. In this new process, nitriding is the critical process because it controls the size, distribution and volume fraction of nitride precipitates, which then determines the subsequent development of Goss texture. Although it is of great importance for good quality control of industrial GOSS product, unfortunately, a quantitative mathematic modeling on nitriding kinetics is still in lack. In this paper, nitriding kinetics were both measured experimentally and simulated by modeling. The nitrogen contents after various nitriding periods and N concentration gradient across thickness were both measured. It has been found that the N content increases slowly at the beginning of 60s and then much more rapidly during nitriding. there exists a sharp nitrogen concentration gradient within the depth of 0.03mm to the steel sheet surface, which diminishes after about 0.04mm depth. With the different assumptions on N-transfer coefficient from gas to the steel matrix, we were able to establish the first mathematic modeling on nitriding kinetics of GOSS. The simulation results suggest that only when the N-transfer coefficient, f, changes with time following the Avrami function, f=1-exp?(-kt^n), the calculated nitriding kinetics are consistent with the measurements. Such an Avrami-type dependence results from the reduction kinetics of oxide layer on the surface of silicon steel sheet during nitriding, in which both plate-like and spherical oxides were observed at the beginning but most of them became spherical after nitriding.
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