Is the copper in all cables the same? What kind of copper is good?
Copper rod is the main raw material in the cable industry. There are two main production methods - continuous casting and rolling and upward continuous casting. There are many production methods for continuous casting and rolling of low-oxygen copper rods. The characteristic is that after the metal is melted in the shaft furnace, the copper liquid passes through the holding furnace, chute, tundish, and enters the closed mold cavity from the pouring pipe. The cooling intensity is used to cool down to form a cast slab, which is then rolled in multiple passes. The produced low-oxygen copper rod has a hot-processed structure. The original casting structure has been broken, and the oxygen content is generally between 200 and 400 ppm. Oxygen-free copper rods are basically produced in China using the upward continuous casting method. After the metal is melted in an induction furnace, it is continuously cast through graphite molds, and then cold rolled or cold worked. The oxygen-free copper rods produced have a cast structure and contain 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, impurity form and distribution, etc.
1. Drawing performance
The drawing performance of copper rods is related to many factors, such as the content of impurities, oxygen content and distribution, process control, etc. The following is an analysis of the drawing performance of copper rods from the above aspects.
1. The influence of melting method on impurities such as S
Continuous casting and rolling to produce copper rods mainly melts the copper rods through the combustion of gas. During the combustion process, through oxidation and volatilization, some impurities can be reduced from entering the copper liquid to a certain extent. Therefore, the continuous casting and rolling method has relatively high raw material requirements. Lower. The upper continuous casting produces oxygen-free copper rods. Since the induction furnace is used for melting, the "patina" and "copper beans" on the surface of the electrolytic copper are basically melted into the liquid copper. The melted S has a great influence on the plasticity of the oxygen-free copper rod and will increase the wire drawing breakage rate.
2. The entry of impurities during the casting process
During the production process, the continuous casting and rolling process requires the transfer of molten copper through holding furnaces, chutes, and tundishes, which is relatively easy to cause the refractory material to peel off. During the rolling process, it needs to pass through the rollers, causing the iron to fall off and causing damage to the copper rods. Cause external inclusions. The rolling in of oxides on and under the skin during hot rolling will have an adverse effect on the drawing of hypoxic rods. The production process of the upward continuous casting method is short. The copper liquid is completed through the submersible flow in the combined furnace, which has little impact on the refractory materials. The crystallization is carried out in the graphite mold, so there are fewer pollution sources and impurities that may be generated in the process. There are fewer opportunities to get in.
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 precipitates as copper = cuprous oxide eutectic and is distributed at the grain boundaries. The emergence 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. It appears at grain boundaries in the form of cuprous sulfide, which will significantly reduce the plasticity of copper.
3. Oxygen distribution patterns and effects in low-oxygen copper rods and oxygen-free copper rods
The oxygen content has a significant impact on the wire drawing performance of low-oxygen copper rods. When the oxygen content increases to the optimal value, the copper rod has the lowest breakage rate. This is because oxygen acts as a scavenger in its reaction with most impurities. Moderate oxygen is also conducive to removing hydrogen from the copper liquid, generating water vapor to overflow, and reducing the formation of pores. The optimal 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 that determine the distribution of copper rod oxides. Uneven cooling will cause essential differences in the internal structure of the copper rod, but in subsequent thermal processing, the columnar crystals will usually be destroyed, resulting in the refinement and uniform distribution of cuprous oxide particles. A typical situation resulting from the aggregation of oxide particles is central bursting. In addition to the influence of oxide particle distribution, copper rods with smaller oxide particles show better wire drawing characteristics, and larger Cu2O particles easily cause stress concentration points and break.
2. Surface quality
In the process of producing products such as electromagnetic wires, requirements are also required for the surface quality of copper rods. The surface of the drawn copper wire needs to be free of burrs, less copper powder, and free of oil stains. The quality of the copper powder on the surface is measured through a torsion test and the recovery of the copper rod after torsion is observed to determine its quality.
During the continuous casting and rolling process, from casting to rolling, the temperature is high and completely exposed to the air, causing a thick oxide layer to form on the surface of the cast slab. During the rolling process, as the rollers rotate, the oxide particles Rolled into the surface of the copper wire. Since cuprous oxide is a brittle compound with a high melting point, for cuprous oxide that is rolled deeper, when the strip-shaped aggregates are stretched by the mold, burrs will be generated on the outer surface of the copper rod, causing trouble for subsequent painting.
Low oxygen copper rod
Audio cables generally prefer to use oxygen-free rods. This is related to the fact that the oxygen-free rods are single crystal copper and the hypoxic rods are polycrystalline copper.
Low-oxygen copper rods and oxygen-free copper rods are different due to different manufacturing methods and have their own characteristics.
1. About the inhalation and removal of oxygen and its existence state
The oxygen content of cathode copper used in the production of copper rods is generally 10-50ppm, and the solid solubility of oxygen in copper at room temperature is about 2ppm. The oxygen content of low-oxygen copper rods is generally 200 (175) - 400 (450) ppm, so the oxygen is inhaled under the liquid copper state, while the upward-drawing oxygen-free copper rod is on the contrary, the oxygen is inhaled under the liquid copper After being kept for a considerable period of time, it is reduced and removed. Usually, the oxygen content of this kind of rod is below 10-50ppm, and the lowest can be 1-2ppm. From a tissue point of view, the oxygen in low-oxygen copper is oxidized. The copper state exists near the grain boundaries, 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 has a negative impact on 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 is a common defect in low-oxygen copper rods.
2. The difference between hot rolled structure and cast structure
Since the low-oxygen copper rod has been hot-rolled, its structure is a hot-processed structure. The original casting structure has been broken, and recrystallization has appeared in the 8mm rod. The oxygen-free copper rod has a cast 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 boundaries. The oxygen-free copper rod structure has coarse grains and the grain size can even reach several millimeters. Therefore, there are few grain boundaries. Even if it is deformed by drawing, the grain boundaries are relatively low. There are still fewer oxygen copper rods, so higher annealing power is required. The requirements for successful annealing of 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 low-oxygen copper in the same situation. After continuous drawing, sufficient margin should be left for the annealing power in subsequent stages and different annealing processes should be performed on low-oxygen copper and oxygen-free copper to ensure the softness of the in-process and finished wires.
3. Differences in inclusions, oxygen content fluctuations, 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-mentioned structural reasons, oxygen-free copper rods have fewer inclusions, stable oxygen content, and no defects that may arise from hot rolling. , the oxide thickness on the rod surface can reach ≤15A. During the continuous casting and rolling production process, 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 for in the continuous cleaning in the post-process, the more troublesome thing is 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, When working with ultra-fine wires, in order to reduce wire breakage, sometimes the copper rod has to be peeled or even peeled twice as a last resort to remove the subcutaneous oxide.
4. There is a difference in toughness between low-oxygen copper rods and oxygen-free copper rods
Both can be stretched to 0.015mm, but in the low-temperature grade oxygen-free copper in the low-temperature superconducting wire, the spacing between the filaments is only 0.001mm.
5. There are differences in the economy from the raw materials for rod making to the thread making.
Manufacturing oxygen-free copper rods requires higher quality raw materials. Generally, when drawing copper wires with a diameter >1mm, the advantages of low-oxygen copper rods are more obvious, while oxygen-free copper rods are even more superior when drawing copper wires with a diameter <0.5mm.
6. The wire-making process of low-oxygen copper rods is different from that of oxygen-free copper rods.
The wire-making process of low-oxygen copper rods cannot be copied to the wire-making process of oxygen-free copper rods. At least the annealing processes of the two are different. Because the softness of the wire is deeply affected by the material composition and rod making, wire making and annealing processes, we cannot simply say who is softer or harder, low oxygen copper or oxygen-free copper.
