Decarburization of Hot-Rolled Non-Oriented Electrical Steels
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Decarburization of Hot-Rolled Non-Oriented Electrical Steels Emmanuel J. Gutiérrez1 and Armando Salinas2 1 Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, Saltillo, Coahuila, PA 663, México ABSTRACT The high temperature decarburization-oxidation behavior of hot rolled, non-oriented electrical steel strips is investigated during air-annealing treatments. Annealing temperature and time are varied from 700 – 1050 °C and 10 to 150 min, respectively. The experimental results show that uniform external oxidation affects strongly the rate at which carbon can be removed from this material. The thickness of the oxide layer formed after 150 minutes of annealing increases linearly with increasing temperature in the range 828 and 920 ºC. The effect of temperature on the thickness of the oxide scale at temperatures outside this range is significantly smaller. These results indicate that the rate of oxidation in this material is strongly influenced by the microstructure of the steel during annealing. Decarburization rates are very slow during annealing at T 750 °C where the oxide layer is thin and porous. In contrast, fast and intense decarburization of the strips is observed as a result of annealing at temperatures between 800 and 850 °C. Finally, decarburization at T 875 °C becomes slower as the temperature is increased until at T 950 °C this process is practically inhibited. Measurements of C content as a function of time and temperature show that the observed decarburization kinetics follows Wagner’s model at 800 and 850 °C. However, at higher annealing temperatures decarburization is slower than that predicted by the model. This behavior is related to the increment of the oxide scale thickness and a transition from cracked to crack-free oxide structure which makes C diffusion through the oxide film very difficult. INTRODUCTION Electrical energy in terms of available resources, environmental and economic impact has become of great concern worldwide. Therefore, there is a driving force to produce more efficient electromagnetic components that allow minimizing the energy consumption. Electrical steels play an important role in the generation (generators), transmission and distribution (transformers) and consumption (motors) of electrical power and are the most important among the magnetic materials produced today [1]. Magnetic properties of these materials are affected by texture, grain size, second phase particles and residual stresses, which depend strongly on the processing variables [2]. Conventional manufacture of grain non-oriented (GNO) electrical steels involves several processing stages such as: hot rolling, cold rolling, intermediate annealing, temper-rolling and final decarburization annealing [2]. This last process is traditionally performed at temperatures below the Ac1 transformation temperature (-Fe-Fe+-Fe) during prolonged times (t 12 h), which implies high processing cost and consequently high cost of the final product. In a previous work [3], it has been demonstrat
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