ABSRTACT: This paper introduces the problems in the production of red copper strip by using up-drawing strip billet, and through adopting a series of quality control measures, the uniform crystallization on the surface of the strip billet is ensured, and the non-uniform cross-sectional tolerance of the strip billet is solved, the problem of uneven surface color of the Strip is eliminated.

Key words: Top Belt Billet; Red Copper Belt; quality control, classification number: TG339 document identification number: B Article Number: 1671-6795(2017)01-0030-03

At present, the copper strip is mainly produced by traditional three-stage process, I. E. Continuous Casting, hot rolling and cold rolling. The advantages of this process are large ingot size, high efficiency, compact strip structure, but large equipment investment, long process, low yield, high energy consumption. Therefore, starting from the 1990s, some domestic manufacturers began to try to use the up-drawing method to produce strips, followed by continuous extrusion + cold rolling or direct cold rolling method to produce copper strips, thus reducing equipment investment and shortening production flow. After nearly 30 years of exploration, the technology of producing copper strip by top drawing billet in China has become more and more mature. Some domestic manufacturers can produce wide and Takasumi oxygen-free copper strip by top drawing method. The overall yield is over 80% , high Purity oxygen-free Copper Strip [1-5] with a width of about 1M has been produced by some manufacturers.

Although the production of Red Copper Strip by top drawing billet has a series of advantages such as short flow, low investment and low energy consumption, however, it is very easy to cause the problems such as over-tolerance, rough crystal, micro-crack, hole, edge or middle crack and color inhomogeneity in the process of cold rolling. Therefore, it is particularly important to analyze the causes of the above-mentioned problems and take targeted measures to solve them.

1. Main problems and their causes in the production of top drawing strip

 1.1 disorder of rough crystal line on the surface of strip

In the long-term practice of producing Red Copper Strip from the upper drawing billet, it is found that the surface of the billet is rough and the crystallizing line is disordered, which leads to the difficulty of the following cold working of the billet, and the surface cracks and edge cracks often appear after cold rolling, even cold rolling a few passes straight after the break. Through analysis, the unreasonable structure design of mould and graphite mould results in uneven cooling of belt billet, which is the direct cause of the disorder of crystallizing line. The disorder of Crystal Line on the surface of the Strip indicates that the inner crystal structure is not uniform, which leads to the subsequent cooling, the difficulty of processing and even the crack on the surface of the strip.

1.2 Two side waves often appear in the process of opening up the upper guiding strip billet when the transverse tolerance of the upper guiding strip billet exceeds the standard, which is mainly caused by the uneven thickness of the cross section of the upper guiding strip billet and the large transverse tolerance (≥0.5 mm) . The main reasons for the large transverse tolerance of the upper guide strip blank are:

 (1) the manufacturing accuracy of the graphite die is not enough;

 (2) The graphite die is expanded and deformed and burned at high temperature.

1.3 after cold rolling, the surface cracks appear on the edge of the Strip, the surface cracks appear on the edge of the Strip and the edge of the cracks are sampled. The macroscopical analysis of the surface cracks and the edge of the cracks are carried out, and the macroscopically and microscopically high-power microstructure is analyzed, as shown in figures 1 and 2. As can be seen from Fig. 1, there are a large number of transverse cracks perpendicular to the rolling direction on the surface of the Strip, some of which are cracked. It can be seen from Fig. 2 that there are a lot of dense pores and voids in the sample, and the voids are distributed in the crystal and the grain boundary.

Therefore, it can be determined that there are a lot of dense pores and voids in the material, which lead to the deterioration of the material plasticity and the cracking in the process of deformation.

1.4 uneven surface color of copper strip during cold rolling is due to the oxidation reaction of copper. There are many factors that can cause oxidation reaction in the production of copper strip, there are mainly atmospheric environmental factors (temperature, humidity, gas, dust, etc.) and production process factors (annealing process, pickling process, cleaning process, etc.)[6,7] . In the process of cold rolling, with the increase of cold working rate, there are different light and dark sides along the Longitudinal Direction of the Strip, as shown in Fig. 3. In the case of large width-thickness ratio, the inner structure of the Strip is easy to be loose and the surface of the Strip is fine crack. In traditional three-stage processing, the total processing rate between two annealing can reach over 90% , however, when the total processing rate is more than 80% , after annealing, the coarse grain structure will extend along the Longitudinal Direction and the surface of the Strip will have different dark color, which can not be removed in the subsequent cold rolling.

2. Quality control measures 

2.1 strip quality control 

(1) high-quality electrolytic copper is used as far as possible, the surface of electrolytic copper is required to be free of copper beans and bronze, etc. 

(2) optimizing the process parameters of up-drawing continuous casting by controlling the temperature of melting furnace from 1080 °C to 1110 °C and the temperature of holding furnace from 1150 ± 10 °C, and keeping the coordination of temperature and traction speed In order to prevent oxidation during the solidification of copper liquid, a certain amount of graphite powder is added between the billet and the inner wall of the mould when the billet is drawn out of the mould (3) optimizing the design of the water channel and the graphite protective film inside the mould, adopting deionized water to reduce the scaling in the cooling water jacket, ensuring the cooling water quality, and paying attention to controlling the water pressure (1.5 ~ 2.5 Bar) and the temperature of the return water (35 °C ~ 42 °C) , (4) the addition of trace rare earth elements can refine the grains and purify the Matrix, which can effectively control the formation of structural defects such as bubbles and porosity.

2.2 transverse tolerance control for strip blanks 

(1) ensure the manufacturing accuracy of the graphite mould, the transverse deviation of the mould entrance is controlled within 0.2 mm, and the stoppers are used before use; 

(2) add a graphite clamp at the bottom of the graphite mould, to solve the problem of deformation of graphite mould at high temperature; (3) to improve the service life of graphite mould and reduce the burning loss by high temperature anti-oxidation treatment.

2.3 The measures to solve the color inhomogeneity of the Strip surface are analyzed from the causes of the color variation of the Strip surface. The key to solve the surface quality of the Strip is to improve the production process.

 (1) use brush and water to clean up the impurity on the surface of the copper strip, wrap the copper strip with paper backing before rolling to prevent the surface from being scratched. In addition, the whole oil rolling method should be adopted to reform the oil removing device of the rolling mill, and the rolling speed should be slowed down to remove the remaining pollutants on the surface of the red copper strip.

 (2) when the cold working rate of the blooming billet reaches about 80% , an intermediate annealing is added to make the material recrystallize, in order to eliminate the color disaccord of the Strip surface caused by the coarse grain during the cold rolling process after the subsequent annealing. In the process of Annealing, it is necessary to protect the surface of red copper strip by inert gas, and to enhance the cleaning of the surface of red copper strip to eliminate the oxide produced on the surface of Red Copper Strip in the rolling and annealing processes, so as to maintain high smoothness.

3. Conclusion

 (1) the main problems in the production of upper drawing continuous casting strip are loose organization, blowhole, disorder of surface crystallization and large lateral tolerance, etc. , among them, the loose structure, pores and crystal disorder on the surface are the basic causes of edge crack, surface crack and band break during cold rolling. 

(2) the charcoal is fully baked and the covering thickness is controlled at 100 ~ 120mm, the thickness of graphite powder covering in the holding furnace is controlled at 80 ~ 100 mm, 

(3)which can control the oxygen content in the upper lead-in strip billet at a lower leve

 (4) to ensure the manufacturing accuracy of the graphite die and to install a graphite clamp at the bottom of the graphite die can effectively solve the problems of large transverse tolerance of the upper drawing billet and billows on both sides during cold rolling; 

(5) when the cold working rate of the upper drawing billet reaches about 80% , adding one intermediate annealing can eliminate the color inhomogeneity of the Strip surface caused by the coarse grain after the subsequent annealing. 

Source: Chinanews.com, by Han Weiguang