| Brass tubes have extensive application in petrochemical industry, ocean engineering, architecture and other fields due to their high strength, good thermal conductivity and excellent corrosion resistance. Currently, the traditional process to produce brass tubes is "semi-continuous casting solid ingot?hot extrusion? cold rolling?intermediate annealing ? drawing (referred to as 'extrusion-rolling'method)", which has some problems, such as long process, high energy consumption, low product yield and high cost. In order to solve these problems, a new compact process technology for fabricating of copper alloy tubes is developed by our research group, which is based on the Heating-Cooling Combined Mold (HCCM) horizontal continuous casting technology. This compact process technology has been successfully applied to industrial production of BFe10-1-1 alloy tube, which can significantly shorten the production process, improve the yield and reduce the production cost. A feasible technological approach to solve the above-mentioned problems in brass tube production is provided by the new compact process technology.Based on the previous works, in this paper, the following research works were focused on. In order to avoid the serious surface spot defect and cracks formed on the brass tube during horizontal continuous casting, the formation mechanisms of the surface defects of the as-cast brass tubes were investigated and methods for eliminating the defects were proposed. The formation mechanism of the solidification microstructure of HA177-2 brass tube fabricated by HCCM horizontal continuous casting was studied and the reasonable process parameters were established. The influences of the grain size and crystallographic orientation on the mechanical properties and deformation mechanism of the HA177-2 brass alloy were studied. The idea that controls the grain size and crystallographic orientation of the alloy to improve its plasticity was proposed. The results can provide theoretical guidance for the solidification microstructure control of the tube. Based on the above results, the brass tube with high surface quality and excellent cold workability was fabricated by HCCM horizontal continuous casting and then subjected to large-deformation (>95%) cold rolling without surface milling and intermediate annealing.The innovative achievements of the thesis include:Using the H62 brass with high zinc content as the representative alloy, it is found that the spot defect on the tube outer surface was of mainly zinc-rich low-melting-point substance, whose zinc content reached 80 wt%. During casting, the volatilized zinc attached to the mold inner surface and formed zinc droplets which caused an erosion effect on the tube surface when contacting with the high temperature tube, leading to the formation of the zinc-rich spot defect on the tube surface. Rapidly lowering the temperature of the solidified tube in the cooling mold can reduce the zinc volatilization and eliminate the spot defects effectively. The periodic cracks formed on the upper part of the H62 brass tube fabricated by HCCM horizontal continuous casting were caused by the additional tensile thermal stress along the axial direction which was developed due to that the lower part of the tube solidified prior to the upper part of the tube. The cracks can be eliminated by improving the temperature of the melting alloy or heating section of the mold, or increase the continuous casting speed, attributing to the raising of temperature gradient at the solidification interface and decreasing the length of the solidification zone in the liquid-solid two phase region.Combining the HCCM continuous casting experiment and Cellular Automation-Finite Element (CAFE) solidification microstructure simulation method, the formation mechanism of the solidification structure of HA177-2 brass tube fabricated by HCCM horizontal continuous casting was ascertained. By reducing the number of the nucleation in the lower part of the tube and increasing the ratio of the temperature gradient and solidification rate at the liquidus by enhancing the temperature of the heating section of the mold from 1040? to 1080?, the tube with homogeneous axial-direction columnar grains along the circumferential direction was obtained. The appropriate process parameters for HCCM horizontal continuous casting of HA177-2 brass tube were established. Under the conditions of melting temperature 1100?, mold heating temperature 1080-1100?, mean drawing speed 90-100 mm/min, first cooling water flow rate 500-600 L/h and second cooling water flow rate 400 L/h, the tube with uniform mechanical properties and excellent plasticity, eg, the elongations to failure of all parts of the tube were larger than 80%, was produced.The columnar-grained HA177-2 brass with larger grain showed better ductility than the sample with small grain. With increasing grain size, the elongation to failure of the sample increased significantly from 70.4% for the grain size of 2.0 mm to 84.4% for the grain size of 6.0 mm. Better deformation uniformity mainly contributed to the ductility improvement of the larger-grained columnar-grained sample. The columnar-grained HA177-2 brass samples with the angles between the loading direction and grain growing direction of 0-30° deformed mainly by dislocation planar slip mechanism during tensile deformation, showing excellent plasticity, e.g. the elongation to failure was more than 80.0%. The 45-90° samples deformed with high strain concentration, resulted in the elongation to fracture of 62.9-65.2%. In order to obtain columnar-grained HA177-2 alloy with excellent cold workability, it is proposed to control the angle between columnar grain growing direction and main extension deformation direction in a range of 0-30° by directional solidification technology.Due to the random grain orientation of the equiaxed grain tube, intersecting planar dislocation structures, deformation twins and shear bands were produced during the cold rolling. In the columnar-grained tube with strong oriented grains and straight low-angle grain boundaries, parallel planar dislocation structures, deformation twins and shear bands formed, contributing to its lower residual stress. At 70% reduction, the circumferential and axial residual stresses of the columnar-grained tube were 201 MPa and 175 MPa, which were lower than those (281 MPa and 238 MPa) of the equiaxed grain tube, respectively. Visible cracks formed on the surface of the equiaxed grain tube at the accumulative reduction of 75%, while for the columnar-grained tube, the surface quality was good without any cracks when the accumulative reduction exceeded 95%. The strong oriented grains and straight low-angle grain boundaries gave rise to the uniform deformation microstructure and lower residual stress in the columnar-grained tube, which contributed to the better cold-rolled performance.Based on the above results, the columnar-grained HA177-2 brass tube with high surface quality and excellent cold workability was fabricated by the HCCM horizontal continuous casting. The as-cast tube was directly used for cold rolling and the accumulated cold-rolled reduction of the tube can exceeded 95%, which contributing to omitting the hot extrusion, surface milling, intermediate annealing and pickling process of the traditional process and finally realizing the compact process for fabricating the brass tube. |
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