Growth rate and phase composition of oxide scales during hot rolling of low carbon steel

The rate of scale growth on low carbon steel in air over the temperature range 600-1200degreesC and the phase composition changes that occur between 750-1200degreesC were investigated. The low carbon steel was oxidized with the air velocity of 4.2 cm/s in order to approximate the formation of secondary and tertiary scales in hot rolling. In addition, some experiments were performed with a lower air velocity of 0.14 cm/s. Above 1000degreesC, with the air velocity of 4.2 cm/s, a transition from a parabolic rate of oxidation to a linear rate of oxidation was observed as the temperature increased. This transition in oxidizing mechanisms was related to the porosity of the oxide scale. The phase composition of the oxide scales changed with temperature and time. For the initial 30 s of oxidation, wustite was the predominant phase in the temperature range 800-1200degreesC and as oxidation proceeded, the percentages of magnetite and hematite increased. The homogeneity of the oxide decreased as the oxidation temperature increased. At 850degreesC, with the air velocity of 4.2 cm/s, the oxide was homogeneous, and for the first 120 s of oxidation, the oxide had a high percentage of wustite and a low percentage of hematite. This indicates that 850degreesC is the ideal temperature for the finishing strip mill in order to reduce work roll wear and surface defects.