Research On The Oxidation Color On The Surface Of The Cold Rolled Strip Steel

With the good flatness, smoothness, high press performance, anddimensional accuracy, cold rolled strip steel is widely used in various fields, suchas construction, machinery, household appliances, and plating industry. Itsvarious advantages make it be still widely used in various industries long after therapid rise. In the actual production process, cold rolled strip steel is oftenannealed, in order to reduce its hardness or to obtain a good performance.The cold rolling strip steel was chosen as experimental samples to carry outthe simulation experiment on the influence factors which effected the steel surfaceoxidation color. The influence factors included temperature when the sample began toleave the heating furnace, hydrogen flow rate, annealing system, heating rate androlling oil. The measurement methods, which consisted of SEM, EDS, XPS andelectrochemical test methods, were used to character the morphology, elementalcomposition, film composition, organization structure and corrosion resistance. Thereasons of influence factors on surface oxidation color of cold-rolled strip steel werealso analyzed in this paper.The results indicate that the elements of Mn and C will be enriched after annealingby comparing the normal samples before and after annealing. The surface roughnessreduces and the planeness improves after annealing. Before annealing, the normalsample mainly composes of Fe phase. While, C8phase appears after the annealing.For the cold rolled band in cleaned surface, over-high tapping temperatures canlead to form oxidation color on the surface. Different tapping temperatures can result invarious oxidation colors. The surface has no obvious change when the tappingtemperature is under160℃. When the tapping temperature is beyond165℃, the bandbegins to be oxidized. The yellow color appears on the surface when the tappingtemperature is beyond200℃.The yellow color continues to deepen when the tappingtemperature changes from200℃to250℃. When the temperature is280℃, red-browncolor appears. When the temperature is320℃, blue oxide film can be found on the steelsurface. For cold rolled band in a cleaned surface, the main reason of formation on oxidation color is because of the formation of iron oxide film whose thickness is innanoscale of380nm. The film compositions consist of FeO, Fe3O4and Fe2O3. Besides,the three compositions exist in the film on a certain scale. The formation process ofoxide film can be described as follows. From inside to outside, the FeO layer wasgenerated on the Fe substrate, and then the Fe3O4and Fe2O3layers are formed. Thereason of color difference is mainly due to the oxidation behavior of steel plate. Theoxide film only has an effect on the material appearance and corrosion resistance andhas no obvious effect on the structures and properties of the substrate. On account of thechange of different surface states, the antioxidant properties of materials will bechanged. No matter what kinds of oxidation colors can make the corrosion resistance ismuch weaker than normal cold rolled strip steel. Guarantee the tapping temperaturelower than160℃can avoid oxidation color for cold rolled strip steel in cleanedsurface.For the cold rolled strip steel where exists rolling oil, the rolling oil can lead toblack spots and white oxidation color. The too high rolling oil concentration is the maincause of the black spot, and it cannot be easy to remove. In addition to oil concentration,hydrogen flow rate, annealing system and heating rate are the factors to form whiteoxidation film. Among all these factors, annealing system and heating rate haveconsiderable influence on forming the white film process. Improving the annealingconditions reasonably can avoid the formation of a white oxide color. Experiments showthat there is no any oxide color appears when heating rate is at3℃/min and under theannealing system of350℃for an hour and700℃for an hour. Even though minorhydrogen rate of50mL/min and higher concentration of3%, there is still no any oxidecolor appears. The formation of white oxidation film maybe relates to the enrichment ofO and Mn, in addition to the associated with the concentration of Ca, S and Al element.