Growth Rate and Phase Composition of Oxide Scales during Hot Rolling of Low Carbon Steel

Abstract

The rate of scale growth on low carbon steel in air over the temperature range 600-1200°C and the phase composition changes that occur between 750-1 200°C 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 1 000°C, 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 800-1 200°C and as oxidation proceeded, the percentages of magnetite and hematite increased. The homogeneity of the oxide decreased as the oxidation temperature increased. At 850°C, 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 850°C is the ideal temperature for the finishing strip mill in order to reduce work roll wear and surface defects.

Effect of a grain-refined microalloyed steel substrate on the formation mechanism of a tight oxide scale

“…Compared the parabolic rate constants of previous studies [41,[45][46][47][48] with the present work, the solid line signified the current study lies along the upper limit of the spread of k p values for the dot line. The activation energy of 179.2 kJ/mol corresponds closely to that of iron reported 194.9 kJ/mol (46.6 kcal/mol) between 640 and 805 C [45].…”

In situ observations of early oxide formation in steel under hot‐rolling conditions

“…Traditionally, the kinetics of oxide formation has been studied by the use of thermo‐gravimetric analytical techniques, which allow for the calculation of the oxidation rate based on mass changes during oxidation at predetermined temperatures (Basabe & Szpunar, 2004; Lee & Choi, 2005). When thermo‐gravimetric techniques are applied to the study of oxidation, they are inherently limited to long‐term experiments and it is not possible to directly observe the progress of the oxidation process.…”

Effect of processing parameters on scale formation during hot steel strip rolling

“…These studies were usually limited to specific materials at fixed temperatures and constrained oxidation times in experimental simulations [1][2][3]6 . However, only a few works investigated the scales formed on steels in factory, and showed great difference compared to the laboratory results, not only in the thickness but also in the phases compositions and strip surface quality 2,5,7 . It is essential to study the influence of processing parameters on oxide scales formation during the hot strip rolling, because the secondary and tertiary scales should be removed before subsequent processing, i.e., cold rolling the steel.…”

“…The frictional phenomena and thermal conductivity at the interface between the roll and the steel will affect the rolling process, by changing rolling forces, torques, power consumptions, temperature gradient near the steel surface, roll wear and surface quality and even the mechanical and other properties of the bulk materials [1][2][3][4] . On carbon steel, the tertiary scale formed at high temperatures usually consists of three iron oxide phases, the inner wüstite (FeO), magnetite (Fe 3 O 4 ) and the outer layer hematite (a-Fe 2 O 3 ) [1][2][3][4][5] . The thickness of these three phases will be different with the changes of oxidation conditions, steel compositions and surface finish 3,6 .…”