| Hot rolled product quality indicators mainly include dimensional accuracy, surface quality and mechanical properties. With the increase of hot-rolled steel grades, the application fields are enlarging. The customers do not pay more attention only to properties of steel, but also the surface quality. Oxide scale of hot rolled steel is one of most important factors about surface quality. But it had not been paid attention closely, so the problem was not solved. The thickness, structure and corrosion resistance of hot rolled steel oxide scale were studied by metallographic analysis, SEM, EDS and XRD, and formation mechanisms and influence factors of several plate surface defects were also studied. The chief original works of this paper are as follows:(1) The structure and thickness of oxide scale evolution rule were investigated. It was found that wustite crystal grew in the form of triangular pyramidal, and magnetite grew in the form of columnar crystal in the initial oxidation, while into the complex structures of columnar crystal and clusters in later oxidation. Hematite developed in three ways:whiskers, platelets and polygonal grains. Hematite whiskers did not influence the weight gain kinetic. However, the relative hematite thickness increased when whiskers were observed. Oxidation weight gain curves of510L and610L were measured by isothermal oxidation kinetics tests. Simulation programme of oxide scale thickness evolution was developed basing on non-isothermal oxidation kinetics. Numerical simulation results were in good agreement with the measured values, indicating that the simulation method was useful for adjusting rolling parameters to control scale formation during the finishing rolling process.(2) The effects of initial rolling temperature, finish rolling temperature, coiling temperature and cooling rates on oxide scale structure were investigated. With the increase of the initial rolling temperature, the ratio of magnetite gradually decreased. And with the increase of the finish rolling temperature, the ratio of magnetite increased. The isothermal transformation curves for low carbon steel of SPHC and micro-alloy steel of510L and610L were measured. The results showed that isothermal time-temperature-transformation curves for the wustite layer of them were found to be in a "C" shape. The range of the eutectoid and pro-eutectoid reaction "nose" temperature of SPHC was450℃～550℃. The range of the pro-eutectoid reaction "nose" temperature of510L was450℃～550℃, and the eutectoid reaction "nose" temperature was350℃～-450℃. The range of the pro-eutectoid reaction "nose" temperature of610L was350℃～400℃, and the eutectoid reaction "nose" temperature was350℃～400℃, indicating that the eutectoid and pro-eutectoid reactions for610L were more difficult than that for SPHC and510L. When the cooling rate was25℃/min, the wustite layer was not found to eutectoid transform in the range of350℃to650℃. If the simulated coiling temperature was in the range of400℃to500℃, and the cooling rate was15℃/min, then the wustite layer was found to transform into mixture of mostly magnetite-iron eutectoid. Strip-cooling rates of5℃/min and1℃/min always produced eutectoid transform in the coiling-temperature range of400℃to550℃. The "nose" temperature for wustite in the continuous cooling curve was400℃～500℃, in the range of which the eutectoid structure was obtained at slow cooling rate.(3) The corrosion weight gain for the samples with eutectoid structure in oxide scale being more than70%was biggest, while that for those without eutectoid and pro-eutectoid structure in oxide scale was smallest during80cycles at alternate dry-wetting accelerating test. Corrosion kinetics curves for the samples with four different kinds of oxide scale followed linear law in initial corrosion, and parabolic law in middle and late corrosion. Corrosion potential and current were measured by polarization curve. The current for the samples with eutectoid structure in oxide scale being more than70%was biggest and potential was more negative. The current for those without eutectoid and pro-eutectoid structure in oxide scale was smallest. The porosity of oxide scale with above70%eutectoid structure of oxide scale was biggest by electrochemical test.(4) Chemical compositions of HY490were optimized by tests, reducing the Si content to less than0.1%. And the process was also optimized to control the structure of oxide scale.(The initial and finish rolling temperature was1000℃～1030℃, and870℃～-890℃. Respectively, and the coiling temperature was550℃～590℃.) According to different production processes of downstream firms, the "flexible controlling strategy" of oxide scale structure was proposed. It was proposed that the ratio of magnetite should be more than70% for oiling steel coils, on the contrary, the ratio of magnetite was40%～70%and wustite should be distributed island in magnetite, by which oxide scale breaking phenomenon was eliminated successfully for high strength steel (thickness^8mm). The cleaner production was implemented.(5) The surface defects were one of key problems about restricting plate quality. But the formation of them was not clear. Point defects consisted of multi-layered oxide scales. The EDS analysis showed that there were not only Fe and O but also Si and Cr in the sticky point defects. In the spontaneous oxide scale formed on hot rolled steels, no Cr contamination can be observed, indicating that the multi-layered scales containing Cr could have been spalled from the working rolls. Pressed in oxide scale on hot rolled plates could be classified into shallow pressed-in and embedded oxide scales. The results showed that the mould powder adhered to the oxide scales formed on the slab surfaces in the reheating furnace. After descaling, the remained mould powder and oxide scale were pressed deeply into plate surfaces during rough rolling, resulting in the formation of the embedded oxide scale. Pockmark was made from oxide scale which was broken into small pieces during hot rolling. The effective methods to eliminate the defect for A32and X46were proposed on the base of formation mechanism of surface defect, obtaining good surface quality in industrial test. The methods consisted of controlling Si content, reheating temperature and improving roller surface quality by shortening time of grinding and replacing roller. |
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