High Temperature Oxidation and Its Effects on Microstructural Changes of Hot-Rolled Low Carbon Non-oriented Electrical Steels During Air Annealing

This paper reports the influence of temperature on external oxidation and its effect on microstructural changes of hot-rolled non-oriented electrical steels during air-annealing treatments. Annealing during 150 min at temperatures above 700 A degrees C, promotes the formation of two oxide layers: an inner iron-silicon-aluminum oxide and an outer three-layered wustite-magnetite-hematite oxide. Thickness and oxide characteristics depend on temperature and influence other microstructural changes. Significant decarburization occurs at 800 and 850 A degrees C when thin and cracked oxide structures are formed. At higher temperatures, decarburization becomes slower due to the increase of oxide thickness and a transition from cracked to crack-free structures, until at 950 and 1,050 A degrees C, decarburization is practically inhibited. Absence of decarburization is confirmed by the increment of carbides volume fraction resulting from gamma-Fe -> alpha-Fe + Fe3C phase transformation. Finally, slow decarburization leads to normal grain growth, while intense decarburization favors abnormal growth with significant reduction in the amount of secondary particles.