Decarburization of Hot-Rolled Non-Oriented Electrical Steels

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 °C. 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.