ABSRTACT: This paper analyses and discusses the key processes of Horizontal Continuous Casting and Planetary Rolling in cast-rolling process in detail, in order to provide reference for the determination of cast-rolling process parameters of white copper tubes. Key Words: Cupronickel Alloy Pipe; cast-rolling method; Horizontal Continuous Casting; Planetary Rolling Middle Diagram classification number: TG249.9 document identification number: A article number: 1671-6795(2016)05-0053-04

The shortage of freshwater resources in the world has become a problem of increasing concern. As an open source incremental technology of water resources, seawater desalination has become an important way to solve the global water resources crisis. Although the seawater desalination industry in China started late, it has developed rapidly and become a popular industry. It has been listed in the national "guide to the high tech key areas of current priority development" , it has been incorporated into such national medium-and long-term plans and documents as the special plan for the utilization of sea water, the outline of the national plan for the development of marine undertakings, and the opinions of the State Council on accelerating the development of the circular economy, with the technical and economic support of the relevant industrial policies of the state, in order to promote the leapfrog development of China's desalination industry and realize the sustainable utilization of water resources, to ensure the sustainable development of the national economy and society has played a positive role in promoting.

The white copper tube has good heat conductivity and corrosion resistance, especially the ability of resisting seawater impact corrosion, so it has been widely used in the fields of seawater desalination, ships, nuclear power station and so on as the condenser tube for heat exchanger, bFe10-1-1 is one of the most commonly used alloys for preparing cupronickel condenser tubes. At present, BFe10-1-1 Cupronickel tube is produced by traditional extrusion method at home and abroad, but it can not meet the market demand because of its low yield and small single weight. The cast-rolling process is an advanced technology for producing copper tubes, which has achieved great success in the production of red copper tubes for refrigeration and air-conditioning. At present, the process has been successfully applied to the production of BFe10-1-1 plain white copper pipe. The cast-rolling method can not only overcome the shortcomings of the traditional extrusion method, such as low efficiency, large investment, low yield and small coil weight, but also be easy to produce super-long tubes for tubular heat exchangers in high-power and large-capacity power plants and nuclear power plants, it is the development trend to produce cupronickel alloy pipe by casting-rolling method with advanced technology at present.

The process of casting and rolling is as follows: raw material，Horizontal Continuous Casting Tube Blank and 3-roll planetary rolling are two key processes in the casting-rolling process. The technical difficulties are mainly reflected in the following two aspects: (1) BFe10-1-1 alloy has high melting point and poor liquid metal fluidity, how to use horizontal continuous casting to prepare qualified BFe10-1-1 hollow tube blank (2) the 3-roll Planetary Rolling Process is a complicated deformation process with local cyclic loading, and the high strength of BFe10-1-1 Alloy requires planetary rolling process and equipment. The determination of reasonable process parameters is of great significance not only for obtaining BFe10-1-1 white copper tube with bright surface and qualified quality, but also for optimizing the design and investment of production equipment. In this paper, the production of BFe10-1-1 Cupronickel tube by casting-rolling method is analyzed and discussed in two processes of Horizontal Continuous Casting and Planetary Rolling.

1. Analysis and discussion of horizontal continuous casting process

Horizontal continuous casting is the first process to realize the casting-rolling process. The quality of horizontal continuous casting tube blank directly determines the quality of the finished alloy tube. In horizontal continuous casting, the alloy is melted in the melting furnace and kept warm in the holding furnace. When the pouring temperature is reached, the billet-drawing machine is started, the tube blank is cooled by water spray directly after exit from the mould. In the process of horizontal continuous casting, casting temperature and drawing schedule have important influence on the quality of tube blank.

1.1 the effect of casting temperature during the horizontal continuous casting of BFe10-1-1 Cupronickel tube billets, the molten metal enters the mold and starts to solidify to form the shell under the action of cooling water. With the decrease of temperature, the shell begins to contract. Because of the low temperature in the outer layer and the large amount of shrinkage, but the high temperature in the inner layer and the small amount of shrinkage, there is internal stress in the shell. In Formula F = e △ T [3] , F is internal stress of Slab, MPA, e is elastic modulus, MPA, linear shrinkage coefficient, 10-6/K, t is temperature difference between two points of Shell, K. It can be seen from the above formulas that the internal stress of casting is proportional to the temperature difference between inside and outside the shell when the material is certain, and the internal stress of the shell can be reduced by reducing the temperature difference between inside and outside the shell, thus the formation of cracks can be restrained. When the casting temperature is too high, the inner cooling of the Shell is slower but the outer cooling is faster, especially after entering the secondary cooling zone, the outer surface temperature of the shell decreases sharply under the effect of direct water cooling, resulting in a large temperature difference between the inner and outer of the Shell, it's prone to cold cracking. Therefore, low temperature casting should be used as far as possible in horizontal continuous casting. Of course, the lower the casting temperature is, the better, because the casting temperature is too low, the viscosity of the liquid metal increases, the mobility decreases, resulting in graphite mold filling is difficult to complete, the surface of the billet will appear cold insulation defects. Therefore, the reasonable choice of casting temperature has a vital influence on the quality of horizontal continuous casting tube blank. According to the relevant experience, the suitable casting temperature is 1230 °C ~ 1250 °C.

1.2 The influence of drawing system 

1.2.1 the selection of drawing mode in horizontal continuous casting the common drawing modes are as follows: 

(1) the drawingThe choice of drawing mode is related to the strength of the alloy. This casting mode has the following characteristics: (1) there is an intermediate stop time after drawing, so that the metal liquid in the mould has enough time to cool and solidify, so that the shell thickens and its strength is increased; (2) timely back pushing, the cold shrinkage of the billet can be compensated in time, the hot crack can be welded, the quality of the billet can be guaranteed, the shell can be loosened, the adhesion between the billet and the mould wall can be prevented, and the resistance of the billet can be reduced. 1.2.2 the influence of drawing speed on drawing speed is the key factor of influencing the quality of copper alloy tube blank. When other casting process parameters are determined, the higher the casting speed is, the deeper the liquid cavity depth is, and the greater the temperature difference between the bottom of liquid cavity and the surface layer of slab is, the higher the casting thermal stress is, which leads to the increase of hot cracking tendency. Therefore, the casting speed should be reduced properly in horizontal continuous casting, and the casting speed should be controlled at 330 ~ 350 MM/MIN. When the casting speed is fixed, the casting system with high frequency and small drawing distance can reduce the dummy ingot resistance and obtain the smooth and crack-free billet. 2. The core of the casting-rolling process is planetary rolling. By applying large deformation, the cast grain of the billet can be completely broken, so that the rolled billet has small and uniform grain, and the inner and outer surface is bright and non-oxidized, to provide high quality tube blanks for the subsequent drawing process. 2.1 The determination of rolling temperature Fig. 2 shows the change curve of mechanical properties of BFe10-1-1 Cupronickel Tube at high temperature. As can be seen from Fig. 2 curve, BFe10-1-1 Cupronickel tube blank has good plasticity at room temperature. With the increase of temperature, the tensile strength and elongation of the blank have a decreasing trend, especially at about 600 °C, the results show that the ductility of BFe10-1-1 billets is the worst at 600 °C, and the ductility of BFe10-1-1 billets is the worst at 600 °C, and the ductility of BFe10-1 billets is the worst at 600 °C. Therefore, BFe10-1-1 cupronickel billets should be rolled at low or high temperature without this temperature range.

If the tube is rolled at high temperature, the deformation resistance of the tube is small and the required rolling force is small, but the tube needs to be heated to a higher temperature and the energy consumption is large, at the same time, the significance of planetary rolling is lost (the advantages of planetary rolling are room temperature rolling and complete recrystallization) . In low temperature rolling, the friction heat and plastic deformation heat produced in rolling process make the temperature of the tube billet rise to about 850 °C, although the deformation resistance of the tube billet is high and the performance of the rolling mill and the die is required, the complete recrystallization of the billet is realized, the properties are improved obviously, and the advantages of planetary rolling can be fully exerted. Therefore, low temperature rolling should be adopted in BFe10-1-1 planetary rolling.

2.2 The selection of deflection angle is shown in Fig. 3. The Roll of the 3-high planetary rolling mill has a certain inclination angle and Deflection Angle, which provides the radial pressure for the workpiece, thus causing the workpiece to undergo radial compression deformation, and the deflection angle provides the axial forward force for the workpiece, which makes the workpiece move forward. In the production process, the reasonable reduction of the tube blank should be ensured, that is, the angle of inclination should be determined first, and then the rolling speed should be reasonable by adjusting the angle of inclination. The Inclination Angle of Planetary Rolling Mill is usually 50 ° ~ 55 ° , which can greatly increase the reduction of rolling section and carry out large deformation rolling and thin tube blank rolling. The Roll deflection angle is a very important parameter in the 3-roll planetary rolling process, which will affect the roll force and the exit speed of the workpiece. From the point of view of geometry, increasing the deflection angle will increase the forward velocity component of the rolled piece, thus the exit velocity of the rolled piece will be significantly increased and the production efficiency will be improved. But the bigger the deflection angle is, the bigger the contact area between the roller and the workpiece is, the bigger the friction force is, and the bigger the stress of the roller is. Therefore, by controlling the roll deflection angle in a reasonable range, the suitable exit speed of the workpiece can be ensured and the rolling force can not be too high. According to the relevant experience, the roll deflection angle should be set in the range of 7 ° ~ 8 ° . 3. Conclusion high quality BFe10-1-1 white copper tube blank with bright inner and outer surface and without oxidation can be produced by reasonable selection of main process parameters of horizontal continuous casting and planetary rolling on the premise of satisfying production demand. (1) the reasonable casting temperature for horizontal continuous casting of BFe10-1-1 white copper tube is from 1230 °C to 1250 °c; (2) the horizontal continuous casting of BFe10-1-1 white copper tube should adopt the mode of drawing stop and pushing to stop, and the casting speed should be controlled at 330 ~ 350 MM/MIN; After the casting speed is determined, the casting billet with smooth surface and no crack can be obtained by using the high frequency and small drawing distance, (3) the plasticity of BFe10-1-1 white copper tube billet is the worst at 600 °C, this temperature range should be avoided during planetary rolling. In order to make full use of the advantages of planetary rolling and save energy, low temperature rolling should be adopted in BFe10-1-1 planetary rolling. (4) the inclination angle of the rolling mill should be 50 ° ー55 ° and the roll deflection should be controlled at 7 ° ー8 ° in BFe10-1-1 planetary rolling. 

Source: Chinanews.com, by Gong Ran

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