The Formation Mechanism Of Surface Oxidation Defect Of Hot Rolled Picking Steel Strip

In hot-rolling process, slabs must be heated before hot rolling. It was inevitably that iron oxidewould be produced during the hot-rolling. The rates of finished products were decreased and thesurface quality of the slabs got worse. The surface oxidation defects were found in the hot rolledpicking steel strip, and these defects could not be removed by normal pickling.Stereomicroscope, SEM, EDS and X-ray Diffraction were used to study the oxide scales. Thestructural characteristics and the source of the defects were analyzed. The causes of the formation ofthe defects were analyzed, through researching the distribution of the defects in actual production.Hot rough rolling experiments were used to analysis the influence of hot rolling temperature on thesurface morphology of the steel plate. Finite element analysis software (MSC.Marc) was used tosimulate the stress and strain of the rolling surface in the hot-rolling process. On this basis,High-temperature oxidation experiments were used to study the microstructure and evolution of thehigh-temperature oxide layer in the hot-rolled heating process of low carbon steel.The results of optical electron microscopy, scanning electron microscopy and spectroscopyindicated：There were actually no complete oxide scale in the district of the defects. Iron matrix anddiscontinuous、irregular and incomplete oxidation layer were exposed after pickling. Oxide layerwas divided into a two-layer microstructure before pickling. The outer oxide layer which wasvertical and horizontal cracks was easy to be pickling removed; the intermediate oxide layer washard to be pickling removed, because the iron oxide and metallic iron were distributed unevenly andthe inner iron oxide was repressed and adhered by metallic iron.The results of high temperature roughing simulation indicated: the higher the rollingtemperature, the more serious the wrinkles of the surface of specimens, and the oxide scale wouldbe pressed into metal surface in the high-temperature hot-rolled. The wrinkles of the rolling surfacewould be reduced by lower rolling temperature. The results of stress and strain numerical simulationof the steel indicated: In high temperature roughing stage, slab surface along the width directionwas received by plastic compressive strain in the symmetrical center region, and compressive strainwas decreased from the center of the plate to outward, and the plastic strain was converted to tensilestrain in the width edge region of the slab. The distribution of the plastic strain could lead to themetal deformation behavior in high temperature. The simulation results were consistent with theactual results. The metal deformation behavior in high temperature could be affected by thedistribution of the plastic strainThe results of high-temperature heating oxidation experiments indicated: high-temperatureoxidation layer of low carbon steel could be divided into a five-layer structure. The inmostoxidation layer contained mainly FeO, while containing a certain amount of Fe3O4and Fe2O3; the inner oxidation layer contained mainly Fe3O4, FeO and Fe2O3; The middle oxidation layercontained Fe except Fe3O4, FeO and Fe2O3; the outer layer and the outmost layer contained Fe3O4and Fe2O3; the middle and outer oxidation layer was easy to be divided from the steel, because themiddle layer was incomplete and not dense and less adhesion of the role of matrix. However, theinnermost oxidation layer was hard to be removed, because it was dense and has the good adhesionwith steel substrate under the high temperature. In the high-temperature heating experiments, thecorrugated undulating phenomenon and more small-scale rough phenomenon was existed steelsurface and oxidation reaction boundary layer; The higher heating temperature and the longerholding time was, the larger the surface of the steel matrix roughness was, the severer the oxidescale with the steel matrix were meshed. When the heating temperature was1180℃~1200℃andthe heating time was less than120minutes, the corrugated undulating phenomenon and moresmall-scale rough phenomenon were lower; however, When the heating time was more than180minutes and the heating temperature was1180℃~1200℃, the corrugated and rough interface wouldbe more severe.The comprehensive research results indicated: the higher heating temperature as well as thelonger holding time would cause the corrugated undulating phenomenon and more small-scalerough phenomenon of steel surface and oxide scales were hard to be removed at the defect regionthen were pressed in subsequent rolling, which was the reason of the form of the oxidation defects.Two mechanism of the form of the oxide scales may be existed. One was that the residual oxidescales were pressed the bump metal iron steel in the subsequent hot rolling, the other was thatwrinkles behavior was suffered by plastic compressive strain in the width direction during thehigh-temperature roughing stage. Applied different techniques such as lower roughing temperatureand short the heating time appropriately, the oxidation defects of strip were controlled well.