Study On Surface State And Development Of The Control Process Of The Hot-Rolled Oxide Scale

EPMA and TGA were used to explore the high temperature oxidation behaviour of the low carbon steel under oxidant gas condition with different oxygen content. The oxidation activation energy and growth morphology of the scale were obtained. Combined with Fe-O phase diagrams, the law of structure transformation of the scale was studied when the plate is under continuous cooling conditions with different experimental coiling temperature. The formation mechanism of the color difference defect and red scale defect were studied, and pertinent improvements of the process were proposed to eliminate the defects. Main research contents are as following:1. The oxidation mass gain curves of DC04oxidized under oxidant gas condition with different oxygen content were determined through the high temperature oxidation behaviour experiment. Parabolic law was observed from the oxidation mass gain curves, and oxidation activation energy of DC04steel was calculated to be around230kJ/mol. By observing surface morphology of the scale, it was found that wustite crystal was alveolate; magnetite grew in form of columnar crystal and hematite developed in form of whiskers.2. By studying the law of microstructure evolution of the scale under continuous cooling conditions, the schematic diagram of relationship of scale type, starting coiling temperature and cooling rate was obtained. Under continuous cooling conditions, the curve of the scale evolution followed a C-curve trend, and the nose temperature of the C-curve appeared at about450℃. The eutectoid reaction can be restrained under high cooling rate condition, but pre-eutectoid reaction can never be hindered. When the simulative coiling temperature was under350℃/min, because of the decreasing of the atom diffusibility, the eutectoid reaction was restrained completely.3. The results of the detection and analysis of the scale on hot-rolled plate showed that when the coiling temperature was too high, the structure and thickness of the scale on hot-rolling plate surface can be lateral inhomogeneity,then the color different defect will emerge, and it will be effectively eliminated when the coiling temperature is lowered to proper value. The scale on plate surface will be fragmental when the finishing temperature is too low. The fragmental scale would be oxidized into Fe2O3finally, and then the red scale defect would be found on the plate surface.4. Combined with industrial production, after the process adjusted, the color difference defect and red scale defect could be eliminated. By means of high rolling temperature and rolling speed, the color difference defect was eliminated when the coiling temperature was adjusted from640℃to550℃. After adjusting the finishing temperature from850℃to890℃, the red scale defect was almost eliminated.