1 School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China 2 Functional Materials Branch, Advanced Technology and Materials Co. Ltd., Beijing 100081, China 3 Qian'an Steel Corp., Shougang Co. Ltd., Qian'an 064404, China
Cite this article:
Guimin ZENG,Haiwen LUO,Jun LI,Jian GONG,Xianhao LI,Xianhui WANG. Experimental Studies and Numerical Simulation on the Nitriding Process of Grain-Oriented Silicon Steel. Acta Metall Sin, 2017, 53(6): 743-750.
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.
Fig.1 Measured N contents of grain-oriented silicon steel sheet after nitriding for various periods at 750 ℃
Fig.2 EPMA line analyses of N concentration gradients across the thickness of steel sheet before and after nitriding
Fig.3 Microstructures (a, b) and EDS analyses (c, d) on the surface of cold rolled grain-oriented silicon steel sheet after both decarburization and nitriding(a) after decarburization at 850 ℃ in the atmosphere of 75%H2+25%N2 with the dew point of 70 ℃ for 3 min(b) after nitriding at 750 ℃ in the atmosphere of 15%NH3+85%(H2+N2) for 120 s(c) EDS analysis of inclusion in circle of Fig.3a(d) EDS analysis of inclusion in circle of Fig.3b
Fig.4 Schematic of surface reaction determined nitriding process (cNg, cNsurf and cNinterf are the N concentrations in gas, on the surface of steel sheet and at the interface of oxide layer and steel matrix, respectively)
Fig.5 Calculated nitriding kinetics by assuming the N-transfer coefficient f as a constant
Fig.6 Calculated nitriding kinetics by assuming f=a/t (a and n1—constants, t—nitriding time)
Fig.7 Possible dependences of f on time during nitriding, as described by f=constant, f=a/t and f=A(1-exp(-ktn)), respectively (k, A and n are constants)
Fig.8 Comparisons of the measured and calculated N contents during nitriding, the latter assumes the f=A(1-exp(-ktn))
Fig.9 Comparisons of calculated N concentration gradient and the measured one by EPMA across the thickness direction of silicon steel sheet after nitriding
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