連鑄特厚板坯二冷強冷及表層組織控制研究
[Abstract]:The effect of cooling rate on the law of microstructure transformation was studied by thermal expansion test. The effect of temperature record on the thermal plasticity of steel was studied by thermal expansion test. The mechanism of high temperature brittleness in third brittle zone of S355 steel was obtained, and the cooling rate was clarified. The effect of rate and cooling temperature on the microstructure and plasticity of high temperature was studied. Secondly, the improvement effect of temperature on the thermal plasticity of steel was studied by the high temperature tensile test of the back temperature and temperature drop after cooling, and the best cooling rate and cooling temperature of the heavy plate slab continuous casting were put forward. On the basis of the study of the phase transition and high temperature plastic characteristics, based on the calculation and analysis of the solidification heat transfer of the cast billet, a new technology for the strong cold of the vertical section and the bending section of the thick slab caster is put forward. Based on the structure analysis of the two cold spray system of the casting machine, the selection and optimization of the nozzle of the strong cold zone are put forward, and the two cold strength of the thick slab continuous casting is formulated. The cold process test scheme and the production verification test were organized. The effect of refining the surface structure and reducing the surface crack rate was obtained. Microalloy steel S355 was used as the research object. The microalloy steel S355 was studied by the thermal expansion analyzer, metallographic microscope, scanning electron microscope, transmission electron microscope and high temperature laser confocal microscope. However, the transformation law of microstructures in the process shows that the critical cooling rate of bainite transformation in microalloy steel S355 is 2 /s, and the critical cooling rate of martensitic transformation is 15 C /s, while the phase transformation of pearlite does not occur when the cooling rate is above 7 C /s, and the unhomogeneous grain size of the original austenite is easy to cause the mixed crystal group. The Gleeble thermal simulation equipment system was used to study the high temperature plastic behavior of microalloy steel S355 under different temperature records. The distribution of fracture tissue and precipitates was observed by scanning electron microscope and transmission electron microscope. The study showed that the thin film ferrite formed at the grain boundary of the original austenite resulted in the low plasticity of the third brittle zone in the S355 steel. The main factor of the valley is that the thermal plasticity of the material is obviously improved after the strong cold return temperature recooling process, and the ferrite in the net film near the fracture surface is obviously reduced. The ferrite is formed inside the austenite grain; most of the precipitates are evenly distributed in the crystal and the dislocations are larger in the intersection position. The strong cold work is simulated and analyzed by the mathematical model. The effect of art on the surface temperature of continuous casting billet is checked. The existing fan section spray system is checked, the nozzle selection of the strong cold zone is optimized, the best position and cooling speed of the strong cooling in the continuous casting machine are determined. The two cold strong cooling process scheme is put forward and the production test is organized. The test results show that the surface temperature of the slab is reached after the strong cold. Under 720 centigrade, the surface structure of the slab is refined, the grain degree uniformity is improved, the coarse dendrite structure is basically eliminated, the surface grain size of the strong cold cast billet reaches 11, the depth of the surface fine crystal area reaches 5mm, and the surface crack rate of the slab is reduced from 7.53% to 3.29%, and the surface structure of the slab is realized and the surface structure of the cast billet is realized. The purpose of control of surface crack. Contrast of strong cold cast billet, the result of hot rolling test shows that the grain size of the thick slab and steel plate can be refined, and the quality of the heart of the special thick steel plate has been greatly improved. It shows that the development of the strong cold process and the renovation of the spray system in the strong cold zone have been successful. The innovation points are as follows: 1) put forward and verified. Two cold local strong cooling technology is used to refine the surface structure of the slab and reduce the rate of crack occurrence. 2) it is revealed that the preeutectoid ferrite film and its thickness of S355 steel are the key factors affecting the ductility of the steel third brittle zone, and the effective control of the ferrite film and precipitates on the surface of the billet is realized through the control of the heat record of the billet. 3) 55 steel thick slab continuous casting is taken as an example. An industrial scale two cold local cold test is carried out, which proves the feasibility of the ferrite control technology on the surface of the heavy plate.
【學位授予單位】:鋼鐵研究總院
【學位級別】:博士
【學位授予年份】:2017
【分類號】:TF777.1
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