Evolution of Oxide Structures of Low-Alloy Steel Surface During Short-Time Oxidation at High Temperature

Abstract

During hot rolling, high-temperature oxidation has a direct effect on the thickness and structures of oxide scales of hot-rolled steel, even influences the descaling and the final surface quality of product. The GLEEBLE thermal simulator was used to investigate the short-time (30–900 s) oxidation of low-alloy steel Q345B in the temperature ranging from 850 to 1050 °C. The structures, thickness and proportion of each layer were characterized by SEM and EDS. The research showed that the initial surface of the scale was in mesh pattern. The scale blistered with increasing time and the grown crystals surrounded by mesh-like gullies could be observed. The scale thickness increased with the elevating temperature, ranged from 5.19 to 207.59 μm depending on the experiments process. It was less than 10 μm within 60 s at 850 °C, increased slowly with the oxidation time below 950 °C, but increased sharply above 950 °C before 120 s. The oxide scales had triple layers after oxidized at 850 °C, and at 950 °C for 30 s. The scale was a double-layer structure at high temperature, consisted of FeO and Fe₃O₄. With increasing temperature and time of oxidation, the proportion of loose layer (FeO) increased, but compact layer (Fe₃O₄) decreased from 70.13 to 7.30% in the two layers. The bottom oxide layer with main component of Fe₂SiO₄ was found between the scale and substrate. It can improve the scale adhesion and is bad for scale separation from substrate.