Copper rod is the main raw material of cable industry, and there are two main production methods - continuous casting and rolling method and up drawing continuous casting method. There are many production methods for continuous casting and rolling of low oxygen copper rod, which is characterized by that after the metal is melted in the shaft furnace, the copper liquid enters the closed mold cavity through the holding furnace, chute, and tundish, and is cooled with a larger cooling intensity to form a casting blank, and then rolled in multiple passes. The produced low oxygen copper rod is a hot processing structure, and the original casting structure has been broken, with the oxygen content generally between 200 and 400 ppm. The oxygen free copper rod is basically produced by up drawing continuous casting method in China. After the metal is melted in the induction furnace, it is continuously cast by up drawing through graphite mold, and then cold rolled or cold processed. The oxygen free copper rod produced is of casting structure, and the oxygen content is generally below 20ppm. Due to different manufacturing processes, there are great differences in many aspects, such as organizational structure, oxygen content distribution, impurity form and distribution.

1、 Drawability

The drawing property of copper rod is related to many factors, such as impurity content, oxygen content and distribution, process control, etc. The drawing performance of copper rod is analyzed from the above aspects.

1. Influence of Melting Mode on Impurities such as S

The production of copper rod by continuous casting and rolling is mainly to melt the copper rod through the combustion of gas. During the combustion process, some impurities can be reduced to some extent by oxidation and volatilization. Therefore, the continuous casting and rolling method has relatively low requirements for raw materials. The oxygen free copper rod is produced by upward continuous casting. Because it is melted by induction furnace, the "copper green" and "copper beans" on the surface of electrolytic copper are basically melted into the copper liquid. The molten S has a great influence on the plasticity of oxygen free copper rod, and will increase the wire breaking rate.

2. Ingress of impurities during casting

In the production process, the continuous casting and rolling process needs to transfer the copper liquid through the holding furnace, chute and tundish, which is relatively easy to cause the peeling of refractory materials. In the rolling process, it needs to pass through the roller to cause the fall of iron, which will cause external inclusions to the copper rod. However, the addition of oxides on and under the skin during hot rolling will adversely affect the drawing of low oxygen rod. The production process of the upward continuous casting method is relatively short. The copper liquid is completed through the submerged flow in the conjoined furnace, which has little impact on the refractory. The crystallization is carried out through the graphite mold, so there are fewer possible pollution sources and fewer opportunities for impurities to enter the process.

O. S and P are elements that can produce compounds with copper. In molten copper, oxygen can dissolve a part, but when copper condenses, oxygen can hardly dissolve in copper. The oxygen dissolved in the molten state is precipitated as copper=cuprous oxide eutectic and distributed at the grain boundary. The appearance of copper cuprous oxide eutectic significantly reduces the plasticity of copper.

Sulfur can be dissolved in molten copper, but its solubility is almost zero at room temperature. It appears at grain boundaries in the form of cuprous sulfide, which will significantly reduce the plasticity of copper.

3. Distribution of oxygen in low oxygen copper rod and oxygen free copper rod and its influence

Oxygen content has obvious influence on the performance of low oxygen copper rod. When the oxygen content increases to the optimum value, the breaking rate of copper rod is the lowest. This is because oxygen acts as a scavenger during the reaction with most impurities. Moderate oxygen is also conducive to removing hydrogen in copper liquid, generating steam overflow, and reducing the formation of pores. The optimum oxygen content provides the best conditions for the wire drawing process.

Distribution of low oxygen copper rod oxides: In the initial stage of solidification in continuous casting, heat dissipation rate and uniform cooling are the main factors determining the distribution of copper rod oxides. Inhomogeneous cooling will cause essential differences in the internal structure of copper rods, but the columnar crystals will usually be destroyed during subsequent thermal processing, making cuprous oxide particles fine and uniformly distributed. The typical case caused by the aggregation of oxide particles is center burst. In addition to the influence of oxide particle distribution, copper rods with smaller oxide particles show better pull wire characteristics, and larger Cu2O particles are easy to cause stress concentration points and fracture.

2、 Surface quality

In the process of producing electromagnetic wire and other products, the surface quality of copper rod also needs to be required. The surface of the copper wire to be drawn shall be free of burrs, copper powder and oil stain. The quality of copper powder on the surface is measured by torsion test and the recovery of copper rod is observed after torsion to determine whether it is good or not.

In the process of continuous casting and rolling, from casting to rolling, the temperature is high and the billet is completely exposed to air, so that a thick oxide layer is formed on the surface of the billet. During rolling, with the rotation of the roll, the oxide particles are rolled into the surface of the copper wire. As cuprous oxide is a brittle compound with high melting point, when it is rolled into deep cuprous oxide and stretched as a strip aggregate against the die, it will cause burrs on the outer surface of the copper rod, causing trouble for subsequent painting.

Low oxygen copper rod

Generally speaking, the sound cable prefers to use an oxygen free rod, which is related to that the oxygen free rod is single crystal copper and the oxygen free rod is polycrystalline copper.

Low oxygen copper rod and oxygen free copper rod have their own characteristics due to different manufacturing methods.

1、 About the inhalation and removal of oxygen and its existing state

The oxygen content of cathode copper for copper rod production is generally 10-50ppm, and the oxygen solid solubility in copper at room temperature is about 2ppm. The oxygen content of the low oxygen copper rod is generally 200 (175) - 400 (450) ppm, so the oxygen is inhaled in the liquid of copper. On the contrary, the oxygen of the oxygen free copper rod of the upward drawing method is reduced and removed after being kept under the liquid copper for a long time. Generally, the oxygen content of this rod is below 10-50 ppm, and the lowest can be 1-2 ppm. From the perspective of organization, the oxygen in the low oxygen copper exists near the grain boundary in the state of copper oxide, This can be said to be common for low oxygen copper rods but rare for oxygen free copper rods. The presence of copper oxide in the form of inclusions at the grain boundary has a negative impact on the toughness of the material. The oxygen in oxygen free copper is very low, so the copper structure is homogeneous single-phase structure, which is beneficial to toughness. Porosity is uncommon in oxygen free copper rods, while it is a common defect in low oxygen copper rods.

2、 Differences between hot-rolled microstructure and cast microstructure

As the low oxygen copper rod is hot-rolled, its structure belongs to the hot processing structure. The original casting structure has been broken, and the recrystallization form has appeared when the rod is 8mm, while the oxygen free copper rod belongs to the casting structure, and the grains are coarse. This is the inherent reason why the recrystallization temperature of oxygen free copper is higher, and higher annealing temperature is required. This is because recrystallization occurs near the grain boundary. The grain size of oxygen free copper rod is even several millimeters, so there are few grain boundaries. Even through drawing deformation, the grain boundaries are relatively small compared with that of low oxygen copper rod, so higher annealing power is required. The requirement for successful annealing of oxygen free copper is that the annealing power of the first annealing of the wire drawn from the rod but without casting structure should be 10-15% higher than that of low oxygen copper under the same conditions. After continuous drawing, enough allowance shall be reserved for the annealing power in the later stage and different annealing processes shall be implemented for low oxygen copper and oxygen free copper to ensure the flexibility of the wires in process and finished products.

3、 Inclusion, fluctuation of oxygen content, difference of surface oxide and possible hot rolling defects

The tensile property of oxygen free copper rod is superior to that of low oxygen copper rod in all wire diameters. In addition to the above organizational reasons, oxygen free copper rod has less inclusions, stable oxygen content, no defects that may occur during hot rolling, and the oxide thickness on the rod surface can be ≤ 15A. In the process of continuous casting and rolling, if the process is unstable and the oxygen monitoring is not strict, the unstable oxygen content will directly affect the performance of the rod. If the surface oxide of the rod can be made up in the continuous cleaning of the subsequent process, but the trouble is that there are quite a lot of oxides in the "skin", which has a more direct impact on the wire breakage. Therefore, in order to reduce the wire breakage during the drawing of micro fine wires and micro fine wires, sometimes the copper rod must be peeled, or even the reason for the secondary peeling, in order to remove the subcutaneous oxide.

4、 The toughness of low oxygen copper rod is different from that of oxygen free copper rod

Both of them can be pulled to 0.015mm, but the distance between the low temperature oxygen free copper filaments in the low temperature superconducting wire is only 0.001 mm

5、 There are differences in the economy from the raw materials of the rod to the thread.

High quality raw materials are required for manufacturing oxygen free copper rods. In general, when drawing copper wire with diameter>1mm, the advantage of low oxygen copper rod is obvious, while the advantage of oxygen free copper rod is to draw copper wire with diameter<0.5mm.

6、 The wire making process of low oxygen copper rod is different from that of oxygen free copper rod.

The wire making process of low oxygen copper rod cannot be transferred to the wire making process of oxygen free copper rod, at least the annealing process of the two is different. Because the flexibility of the wire is deeply affected by the material composition, rod making, wire making and annealing process, we can't simply say that the soft copper or oxygen free copper is the hard one.

Source: Security Group

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