The reduction of oxygen content and crack elimination of oxygen-free copper ingots in continuous casting were studied. Firstly, the heat preservation and melting processes are combined into one through the development of the integrated heat preservation melting furnace, and the traditional three-stage preparation method of copper alloy is upgraded to the two-stage preparation method of heat preservation and continuous casting, the Gate Valve Technology for controlling melt flow and stopping is put forward and realized. Finally, the crack defect of oxygen-free copper ingots with width-thickness ratio of 650 m/20 M is analyzed. By setting reasonable cooling parameters, the oxygen content of ingots is controlled to be less than 0.0004% , the casting density was increased to 8.94ー8.95 G/C M3, and the crack was completely eliminated.

Keywords Horizontal Continuous Casting; oxygen-free Copper Ingot; oxygen content;

Oxygen-free copper is used in electric vacuum and electric power industries because of its excellent conductivity, mainly used in the manufacture of vacuum tubes, computers, micro-radiators, flexible printed circuit board, connectors, coaxial RF cables, dry-type transformer windings and other high-end products [1] . There are two main ways to produce oxygen-free copper sheet and Strip in China. One is vertical continuous casting, hot rolling, cold rolling and air gap annealing

Second, horizontal continuous casting, cold rolling, hydrogen protection, horizontal annealing. The former has high energy consumption and long flow, while the latter has low energy consumption and short flow. Due to the market demand, the production of oxygen-free copper sheet and strip by domestic horizontal continuous casting has been greatly developed, but there are still some problems such as high oxygen content, cracks and low density of oxygen-free copper sheet, which can not meet the relevant requirements. The traditional process of copper alloy continuous casting is melting, heat preservation and casting in three stages, that is, melting and heat preservation are independent furnaces. At present, the domestic horizontal continuous casting method is more advanced than the international vacuum melting method, which has the following technical shortcomings: 1. The air is sucked in the process of melting and flowing; 2. The wide/thin ratio is easy to produce serious cracks on the surface and inside of the INGOT; 3. The Melt viscosity is high, graphite crystallization zone in crystallizer is easy to oxidize and shortens working life. In order to make the horizontal continuous casting approach or reach the level of low oxygen content and high density of the vacuum melting method, an integrated furnace was developed, the mould was improved, and the cooling condition of reducing the crack of the slab with width and thickness of 650 m/20 m was studied, the purpose is to provide reference for practical production.

1.piece furnace with bottom flow

1.1 furnace construction

In order to avoid the contact between air and transferred melt, a bottom-flow integrated furnace is designed, as shown in Fig. 1. The smelting furnace and the holding furnace are made into a whole body, and the Refining Chamber is added. The two ends are respectively connected with the smelting chamber and the Holding Chamber through an intermediate trough. The Refining Chamber can float oxides and impurities, and even the melt temperature to reduce the temperature shock of the holding chamber. The cross-section of the Middle Chute is shown in figure 2. The opening and closing of the chute are controlled by a gate valve, which is made of a refractory rod or solidified from a melt. When the cooling water channel flows through the cooling water channel, the melt in the middle chute solidifies to form a metal gate valve, which blocks the melt flow through the chute. When the cooling water supply is stopped, the gate valve melts, the Middle Chute opens, and the refractory Rod Gate Valve is driven by a mechanism.

1.2 nitrogen protection

Nitrogen is injected into the top and bottom of the three furnaces to protect the copper melt. The furnace body around the mold is lined with nitrogen pipes, which permeate into the melt. When the crystallization solidifies, nitrogen is released, further reducing the oxygen content of the billet. The Secondary Cooling zone of 300 m length was added at the exit of the original crystallizer, and the circulating cooling method was adopted. Nitrogen is also introduced into the mould to prevent high temperature oxidation of the INGOT. The experimental results show that the oxygen content of oxygen-free Copper Ingot is reduced and the service life of the mould is improved. The results are shown in Table 1.

2.Process parameters and cracks

2.1 crack morphology and its cause

Oxygen Free Copper Ingots with a width of 650 m and a thickness of 20 m often have macrocosm and microcosm cracks. Fig. 3 is a typical macro-crack on the surface of Ingot. It can be seen that the macro-cracks are located in the middle of the ingot width and extend along the direction of casting and crack along the grain boundary. The macro-cracks account for about 90% of the total cracks. The width of the Macro crack is 0.5ー1 m M M, the depth is 2ー5 m and the length is 20ー200 M. Fig. 4 is a typical micro-crack appearance in the INGOT, which is located at 200 ~ 300 m near the edge of the INGOT. The width of the crack is 0.2 ~ 0.4 m, the depth is 1 ~ 2 m, and the length is 5 ~ 20 M.

From the micro-crack Bending and macro-crack appearance, and the crack along grain boundary, it is inferred that the micro-crack of billet is formed in the mould, and the micro-crack of Ingot Center continues to develop into macro-crack after the mould is produced, and the reason of the crack formation is the non-uniform thermal stress. When the temperature distribution on the surface of the ingot is not uniform, the shell with higher temperature has lower strength, and the stress caused by shell shrinkage exceeds the critical stress and strain of the shell.

2.2 optimization of process parameters.

The higher local surface temperature of Ingot at the end of mould and out of mould is the main reason for the cracks. Therefore, the distribution of cooling water is an important factor for the cracks. In order to obtain a reasonable cooling water distribution, the temperature of the upper and lower surface of the ingot at the exit of mould was measured with a temperature sensor at the melt temperature of 1230ー1240 °C and casting speed of 1100ー120m/mi n for the slab with a width to thickness ratio of 650m/20m. The temperature distributions of the upper and lower surfaces along the width direction of the INGOT are shown in figures 5 and 6. It can be seen that the center temperature along the width direction is higher and the temperature on both sides gradually decreases. The temperature distribution curve of the upper surface is steeper than that of the lower surface, and the temperature gradient is larger, which is the reason why the upper surface cracks are more than the lower surface cracks.

According to the temperature distribution law of ingot surface, the flow rate of each cooling water pipe is designed and controlled to make the temperature of ingot surface nearly the same. In addition, the cooling intensity is enhanced, that is, the water flow rate is changed from 3 ~ 4 m 3/h to 5 ~ 7 m 3/h. Cooling water parameters are set according to reference [9] . After production inspection, the cracks in the Ingot were basically eliminated.

3.Conclusion

(1) the traditional three-stage production method of melting, heat preservation and continuous casting of copper alloy is changed into two-stage production method, which has positive effect on reducing oxygen content of oxygen-free Copper Ingot. 

(2) the gate valve technology for controlling the melt from the melting chamber to the Refining Chamber and then to the holding chamber in the integrated furnace is proposed and realized. 

(3) the reason of the crack in 650 mm/20 mm oxygen-free copper ingot was found out, and the crack was eliminated by optimizing the process parameters. The oxygen content ≤0.0004% and the density increased to 8.94ー8.95 G/CM 3. 

Source: Chinanews.com