In-depth analysis of copper in cables What is good copper?

Copper rod is the main raw material of the cable industry, and there are two main production methods - continuous casting and rolling method and upward continuous casting method.

There are many production methods for continuous casting and rolling low-oxygen copper rods, which are characterized in that after the metal is melted in the shaft furnace, the copper liquid passes through the holding furnace, the chute, the tundish, and enters the closed mold cavity from the pouring pipe. The cooling strength is cooled to form a billet, and then multi-pass rolling is performed. The low-oxygen copper rod produced is a hot-worked structure. The original casting structure has been broken, and the oxygen content is generally between 200 and 400ppm. The oxygen-free copper rods are basically all produced by the upward continuous casting method in China. After the metal is melted in an induction furnace, the upward continuous casting is carried out through a graphite mold, and then cold-rolled or cold-worked. The produced oxygen-free copper rods have a casting structure with oxygen The amount is generally below 20ppm.

Due to different manufacturing processes, there are great differences in many aspects such as organizational structure, oxygen content distribution, form and distribution of impurities.

1. Drawing performance

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

1. The influence of melting method on impurities such as S (sulfur)

The copper rod produced by continuous casting and rolling mainly melts the copper rod through the combustion of gas. During the combustion process, through oxidation and volatilization, some impurities can be reduced to enter the copper liquid to a certain extent. Therefore, the continuous casting and rolling method has relatively high requirements on raw materials. lower. The above-mentioned continuous casting produces oxygen-free copper rods. Since it is melted in an induction furnace, the "patina" and "copper beans" on the surface of the electrolytic copper are basically melted into the copper liquid. Among them, the molten S has a great influence on the plasticity of the oxygen-free copper rod, which will increase the wire drawing breakage rate.

2. The entry of impurities in the casting process

In the production process, the continuous casting and rolling process needs to transfer the copper liquid through the holding furnace, the chute, and the tundish, which is relatively easy to cause the peeling of the refractory material. cause external inclusions. However, the rolling of oxides on and under the skin of the hot-rolled medium will adversely affect the wire drawing of the hypoxic rod. The production process of the upward continuous casting method is short. The copper liquid is completed by the submerged flow in the conjoined furnace, and the impact on the refractory material is not large. The crystallization is carried out in the graphite mold, so there may be fewer pollution sources and impurities in the process. There is less chance of entry.

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

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

3. Distribution of oxygen in hypoxic copper rods and oxygen-free copper rods and their effects

The oxygen content has a significant effect on the wire-pulling performance of the low-oxygen copper rod. When the oxygen content is increased to the optimum value, the wire breakage rate of the copper rod is the lowest. This is because oxygen acts as a scavenger in the process of reacting with most impurities. A moderate amount of oxygen is also beneficial to remove hydrogen in the copper solution, generate water vapor overflow, and reduce the formation of pores. The optimum oxygen content provides the best conditions for the wire drawing process.

Low-oxygen copper rod oxide distribution: During the initial stage of solidification in continuous casting, heat dissipation rate and uniform cooling are the main factors that determine the copper rod oxide distribution. Uneven cooling can cause intrinsic differences in the internal structure of the copper rod, but subsequent thermal processing, the columnar crystals are usually destroyed, resulting in a finer and uniform distribution of cuprous oxide particles. A typical condition resulting from agglomeration of oxide particles is center popping. In addition to the influence of oxide particle distribution, copper rods with smaller oxide particles show better wire-pulling properties, while larger Cu2O particles are prone to cause stress concentration points and fracture.

The oxygen content of oxygen-free copper exceeds the standard, the copper rod becomes brittle, the elongation decreases, the port of the tensile style is dark red, and the crystalline structure is loose. When the oxygen content exceeds 8ppm, the process performance deteriorates, which is manifested in the extremely high rod breakage and wire breakage rate during casting and drawing. This is because oxygen can form a cuprous oxide brittle phase with copper, forming a copper-cuprous oxide eutectic, which is distributed on the boundary in a network structure. This brittle phase has high hardness, and will be separated from the copper body during cold deformation, resulting in a decrease in the mechanical properties of the copper rod, and it is easy to cause fracture in the subsequent processing. High oxygen content can also lead to a decrease in the electrical conductivity of oxygen-free copper rods. Therefore, it is necessary to strictly control the upward continuous casting process and product quality.

4. The effect of hydrogen

In upward continuous casting, the control of oxygen content is low, and the side effects of oxides are greatly reduced, but the influence of hydrogen becomes a more significant problem. There is an equilibrium reaction in the melt after inhalation: H2O(g)=[O]+2[H];

Gas and looseness are formed by the precipitation and aggregation of hydrogen from supersaturated solutions during the crystallization process. Hydrogen precipitated before crystallization can reduce cuprous oxide to form water bubbles. Because the characteristic of upward casting is the crystallization of molten copper from top to bottom, the shape is similar to that of a cone. The gas precipitated before the crystallization of the copper liquid is blocked in the solidification structure during the floating process, and pores are formed in the casting rod during crystallization. When the gas content of the above guide is small, the precipitation hydrogen exists at the grain boundary to form looseness; when the gas content is large, it aggregates into pores. Therefore, the pores and looseness are formed by both hydrogen and water vapor.

In the continuous casting and rolling process, hydrogen is often controlled by moderately controlling the oxygen content. Cu2O+ H2= 2Cu+ H2O

Since the copper liquid crystallizes from the bottom to the top during the casting process, the water vapor generated by the oxygen and hydrogen in the copper liquid can easily float up and run out, and most of the hydrogen in the copper liquid can be effectively removed, thus affecting the copper rod. smaller.

2. Surface quality

In the process of producing magnet wire and other products, the surface quality of copper rods also needs to be required. The surface of the drawn copper wire needs to be free of burrs, less copper powder and no oil stains. The quality of the copper powder on the surface is measured by the torsion test and the recovery of the copper rod after torsion is observed to determine its quality.

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

The oxygen-free copper rod manufactured by the upward continuous casting process is completely isolated from oxygen due to casting and cooling, and there is no subsequent hot rolling process. , the above problems rarely exist.

Due to the different manufacturing methods, the low-oxygen copper rod and the oxygen-free copper rod are different and have their own characteristics.

1. About the inhalation and desorption of oxygen and its state of existence

The oxygen content of cathode copper used to produce copper rods is generally 10-50 ppm, and the solid solubility of oxygen in copper at room temperature is about 2 ppm. The oxygen content of the low-oxygen copper rod is generally 200 (175) - 400 (450) ppm, so the entry of oxygen is inhaled in the liquid state of copper, while the oxygen-free copper rod of the upward-drawing method is the opposite, and the oxygen is in the liquid copper. After holding for a considerable time, it is reduced and removed. Usually, the oxygen content of this rod is below 10-50 ppm, and the minimum can reach 1-2 ppm. From the organizational point of view, the oxygen in low-oxygen copper is in the state of copper oxide, It is present near the grain boundary, which is common for low-oxygen copper rods but rare for oxygen-free copper rods. The presence of copper oxide in the form of inclusions at grain boundaries negatively affects the toughness of the material. The oxygen in oxygen-free copper is very low, so the structure of this copper is a uniform single-phase structure, which is beneficial to toughness. Porosity is uncommon in oxygen-free copper rods, and a common defect in low-oxygen copper rods.

2. The difference between hot-rolled structure and cast structure

Because the low-oxygen copper rod has been hot-rolled, its structure is a hot-worked 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 is a casting structure with coarse grains. This is the inherent reason why oxygen-free copper has a higher recrystallization temperature and requires a higher annealing temperature. This is because recrystallization occurs near the grain boundary, and the grain size of the oxygen-free copper rod is coarse, and the grain size can even reach several millimeters, so the grain boundary is small. Even if it is deformed by drawing, the grain boundary is relatively low. There are still fewer oxygen copper rods, so a higher annealing power is required. The successful annealing requirements for oxygen-free copper are: the first annealing when the wire is drawn from the rod but has not yet been cast, the annealing power should be 10-15% higher than that of the low-oxygen copper in the same situation. After continuing to draw, the annealing power in the later stage should leave enough margin and perform different annealing processes for low-oxygen copper and oxygen-free copper to ensure the flexibility of the wire in process and the finished product.

3. Differences in inclusions, fluctuations in oxygen content, surface oxides and possible hot rolling defects

The drawability of oxygen-free copper rods is superior to that of low-oxygen copper rods in all wire diameters. In addition to the above organizational reasons, oxygen-free copper rods have less inclusions, stable oxygen content, and no defects that may be caused by hot rolling. , the thickness of the oxide on the rod surface can reach ≤15A. In the production 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 compensated in the continuous cleaning of the subsequent process, it is more troublesome that a considerable amount of oxide exists "under the skin", which has a more direct impact on the wire breakage. Therefore, when drawing fine wires, In the case of ultra-fine wires, in order to reduce the wire breakage, sometimes a last resort must be taken for the copper rod - peeling, or even the reason for secondary peeling, the purpose is to remove the subcutaneous oxide.

4. The toughness of the hypoxic copper rod and the oxygen-free copper rod is different

Both can be pulled to 0.015mm, but the low-temperature oxygen-free copper in the low-temperature superconducting wire has only a 0.001mm spacing between the filaments.

5. There is a difference from the raw material of the rod to the economy of the wire

Manufacturing oxygen-free copper rods requires high-quality raw materials. In general, when drawing copper wires with a diameter of >1mm, the advantages of low-oxygen copper rods are more obvious, and the oxygen-free copper rods are more superior when drawing copper wires with a diameter of <0.5mm.

6. The wire making process of the hypoxic copper rod is different from that of the oxygen-free copper rod

The wire-making process of the low-oxygen copper rod cannot be copied to the wire-making process of the oxygen-free copper rod, at least the annealing process of the two is different. Because the softness of the wire is deeply affected by the material composition and rod making, wire making and annealing process, it cannot be simply said that the low-oxygen copper or the oxygen-free copper is soft and hard.