通过在实验室模拟取向硅钢的渗氮生产工序,实测了不同渗氮时间下的渗氮量,并通过EPMA、FESEM等观察了渗氮后氧化层的变化、N在硅钢带厚度方向的浓度梯度。根据实测的组织形貌和渗氮动力学特点,首次建立了针对取向硅钢的渗氮动力学模型,并成功地进行了数值模拟计算。研究结果表明：(1) 取向硅钢带的平均N含量在渗氮初期增加缓慢,之后逐渐加快,直至90 s时渗氮速率达到最大值之后保持恒定;在750 ℃渗氮2 min时间内,在近表面0.04 mm范围内存在显著的N浓度梯度;(2) 脱碳退火后,取向硅钢的氧化层主要为层状氧化物,渗氮时氧化层被H2还原,层状氧化物转变为球状,氧化层的变化对渗氮动力学影响显著;(3) 分析了N由气相穿过表面氧化层至Fe基体的传质系数所遵循的可能模型,发现只有当传质系数遵循氧化层还原动力学模型时,即Avrami函数模型f=A(1-exp(-ktn)),计算结果才能与实测的渗氮动力学特征高度吻合。
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, which is developed during the secondary recrystallization annealing of product. In order to save energy and reduce cut-down operation costs, Nippon steel first lowered the slab-reheating temperature from 1350~1400 ℃ to 1150 ℃ and adopted the nitriding process to form nitride inhibitors before recrysta-llization 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 deve-lopment 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 work, nitriding kinetics were both measured experimentally and simulated by modeling. The N 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 60 s and then much more rapidly during nitriding. There exists a sharp N concentration gradient within the depth of 0.03 mm to the steel sheet surface, which diminishes after about 0.04 mm depth. With the different assumptions on N-transfer coefficient from gas to the steel matrix, the first mathematic modeling on nitriding kinetics of GOSS has been successfully established and solved numerically. The simulation results suggest that only when the N-transfer coefficient, f, changes with time following the Avrami function, f=A(1-exp(-ktn)), 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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