The difference between the performance of low-oxygen copper rod and oxygen-free copper rod

Due to the different processes for producing copper rods, the oxygen content and appearance of the produced copper rods are different. The copper rod produced by Shangyin has an oxygen content below 10ppm, which is called oxygen-free copper rod; the copper rod produced by continuous casting and continuous casting is hot-rolled under protected conditions, and the oxygen content is in the range of 200-500ppm, but sometimes it is as high as 700ppm or more. Generally, the copper produced by this method has a bright appearance and low-oxygen copper rod. 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. The characteristic is that after the metal is melted in the shaft furnace, the copper liquid passes through the holding furnace, chute, and tundish, and enters the closed mold cavity from the pouring tube. It is cooled with a large cooling intensity to form a billet, and then rolled for multiple passes. The low-oxygen copper rod produced is a thermally 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 by upward continuous casting in China. After the metal is melted in an induction furnace, it is continuously cast upward through a graphite mold, and then cold-rolled or cold-processed. The oxygen-free copper rod produced is a cast structure, and the oxygen content is generally below 20ppm. Due to the 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 rod 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 respectively.

1. Effect of melting method on impurities such as S

The production of copper rods by continuous casting and rolling is mainly through the combustion of gas to melt the copper rods. During the combustion process, through oxidation and volatilization, some impurities can be reduced to a certain extent from entering the copper liquid. Therefore, the continuous casting and rolling method has relatively low requirements for raw materials. Upward continuous casting produces oxygen-free copper rods. Because induction furnaces are used for melting, the "patinous green" and "copper beans" on the surface of electrolytic copper are basically melted into the copper liquid. The fused S has a great influence on the plasticity of the oxygen-free copper rod, which will increase the wire breaking rate.

2. The entry of impurities in the casting process

In the production process, the continuous casting and rolling process needs to transfer molten copper through holding furnaces, chutes, and tundishes, which is relatively easy to cause spalling of refractory materials. During the rolling process, it needs to pass through rolls, causing iron to fall off and causing external inclusions to copper rods. However, the rolling in of oxides on and under the skin during hot rolling will have an adverse effect on the wire drawing of the hypoxic 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 material. The crystallization is carried out through the graphite mold, so there are fewer possible pollution sources in the process and fewer opportunities for impurities to enter.

O, S, and P are elements that form compounds with copper. In molten copper, oxygen can dissolve partly, 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 copper in the melt, but at room temperature, its solubility is almost reduced to zero, and it appears in the form of cuprous sulfide at the grain boundary, which will significantly reduce the plasticity of copper.

3. Oxygen distribution in low-oxygen copper rods and oxygen-free copper rods and its influence

Oxygen content has a significant effect on the drawing properties of low-oxygen copper rods. 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 during the reaction with most impurities. Moderate oxygen is also beneficial to remove hydrogen in copper liquid, generate water vapor overflow, and reduce the formation of pores. The optimum oxygen content provides the best conditions for the wire drawing process.

Distribution of oxides in low-oxygen copper rods: In the initial stage of solidification in continuous casting, the rate of heat dissipation and uniform cooling are the main factors that determine the distribution of oxides in copper rods. Uneven cooling will cause essential differences in the internal structure of the copper rod, but the subsequent thermal processing will usually destroy the columnar crystals, making the cuprous oxide particles finer and evenly distributed. A typical situation arising from agglomeration of oxide particles is center 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 are prone to cause stress concentration points and break.

The oxygen content of oxygen-free copper exceeds the standard, the copper rod becomes brittle, the elongation rate decreases, the end of the stretched style is dark red, and the crystal structure is loose. When the oxygen content exceeds 8ppm, the process performance deteriorates, which is manifested as a very high rate of broken rods and broken wires during casting and stretching. This is because oxygen can form a brittle phase of cuprous oxide 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, which is likely to cause fracture in subsequent processing. High oxygen content can also lead to a decrease in the conductivity of oxygen-free copper rods. Therefore, the upward continuous casting process and product quality must be strictly controlled.

4. The effect of hydrogen

In upward continuous casting, the control of oxygen content is lower, 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 suction: H2O(g)=[O]+2[H];

Gas and porosity are formed by the precipitation and accumulation of hydrogen from the supersaturated solution during the crystallization process. The hydrogen precipitated before crystallization can reduce cuprous oxide to generate water bubbles. Since the upward casting is characterized by the crystallization of molten copper from top to bottom, the shape of the liquid crystal formed is similar to that of a cone. The gas precipitated before the copper liquid crystallization is blocked in the solidification structure during the floating process, and pores are formed in the casting rod during crystallization. When the gas content is low, the precipitated hydrogen exists at the grain boundary, forming loose; when the gas content is high, it gathers into pores. Therefore, the pores and pores are formed by both hydrogen and water vapor.

Hydrogen comes from various process links in the production process of Shangyin, such as the "patinous green" of raw material electrolytic copper, auxiliary charcoal **, climate environment **, graphite crystallizer is not dry, etc. Therefore, the surface of the copper liquid in the melting furnace should be covered with baked charcoal, and the electrolytic copper should try to remove "patina", "copper beans" and "ears", which is very important for improving the quality of oxygen-free copper rods.

In the continuous casting and rolling process, moderate control of oxygen content is often used to control hydrogen. Cu2O+ H2= 2Cu+ H2O

Since the copper liquid crystallizes from bottom to top during the casting process, the water vapor produced by the oxygen and hydrogen in the copper liquid can easily float up and escape, and most of the hydrogen in the copper liquid can be effectively removed, so the impact on the copper rod is small.

2. Surface quality

In the process of producing electromagnetic wires and other products, the surface quality of copper rods also needs to be required. The surface of the drawn copper wire needs to have no burrs, less copper powder, and no oil. And through the torsion test to measure the quality of the copper powder on the surface and observe the recovery of the copper rod after torsion to judge whether it is good or bad.

In the continuous casting and rolling process, from casting to before rolling, the temperature is high and it is completely exposed to the air, so that a thicker oxide layer is formed on the surface of the slab. During the rolling process, as the roll rotates, 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 cuprous oxide rolled into a deep place, when the strip-shaped aggregates meet the mold stretching, burrs will be generated on the outer surface of the copper rod, which will cause trouble for subsequent painting.

However, the oxygen-free copper rod manufactured by the continuous casting process is completely isolated from oxygen during casting and cooling, and there is no subsequent hot rolling process. The surface of the copper rod has no oxides rolled into the surface, and the quality is better. There is less copper powder after drawing, and the above problems rarely exist.

Oxygen-free copper rods are also made by imported equipment and domestic equipment, but currently imported products have no obvious advantages. After the copper rod products come out, the difference is not very big. As long as the copper plate is selected well and the production control is relatively stable, domestic equipment can also produce copper rods that can stretch 0.05. Imported equipment is generally Finland’s Outokumpu equipment.

There are mainly two kinds of low-oxygen copper rod imported equipment in the world, one is the American South Wire equipment, the English is SOUTHWIRE, the domestic manufacturer is Nanjing Huaxin, Jiangxi Copper, and the other is the German CONTIROD equipment, the domestic manufacturer is Changzhou Jinyuan and Tianjin Seamless.

Oxygen-free and hypoxic rods are easy to distinguish from the oxygen content. The oxygen content of oxygen-free copper is below 10-20 PPM, but currently some manufacturers can only achieve below 50 PPM. The low-oxygen copper rod is 200-400 PPM, and the oxygen content of good rods is generally controlled at about 250 PPM. Springback angle and winding performance. However, hypoxic rods are relatively harsh on the wire drawing conditions. Similarly, if the wire drawing conditions are not good, ordinary anaerobic rods can be pulled, while good hypoxic rods will break. The way the skin handles the hypoxic rod to stretch the 0.03 line. But I'm not very clear about the content of this aspect.

Audio cables generally prefer to use anaerobic rods, which is related to the fact that the anaerobic rods are monocrystalline 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 state of existence

The oxygen content of cathode copper for the production of copper rods is generally 10-50ppm, and the solid solubility of oxygen in copper is about 2ppm at room temperature. The oxygen content of low-oxygen copper rods is generally 200 (175)-400 (450) ppm, so the entry of oxygen is inhaled in the liquid state of copper, while the up-drawing oxygen-free copper rod is the opposite. After a certain period of time in the liquid copper, the oxygen is reduced and removed. Usually, the oxygen content of this rod is below 10-50 ppm, and the lowest can reach 1-2 ppm. From the perspective of structure, the oxygen in low-oxygen copper exists near the grain boundary in the state of copper oxide. , which can be said to be common for low-oxygen copper rods but rare for oxygen-free copper rods. The presence of copper oxide at grain boundaries in the form of inclusions 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, while it is a common defect in low-oxygen copper rods.

2. The difference between hot rolling structure and casting structure

Because the low-oxygen copper rod has been hot-rolled, its structure is a hot-worked structure. The original cast structure has been broken, and recrystallization has appeared in the form of 8mm rods, while the oxygen-free copper rod is cast. This is because recrystallization occurs near the grain boundaries, and the oxygen-free copper rods have coarse grains, and the grain size can even reach several millimeters, so there are few grain boundaries. Even through drawing deformation, the grain boundaries are still relatively few compared with low-oxygen copper rods, so higher annealing power is required. The requirements for successful annealing of oxygen-free copper are:

The annealing power for the first annealing of wires that have not yet been cast, should be 10-15% higher than that of low-oxygen copper in the same situation. After continuous drawing, sufficient margin should be left in the annealing power in the later stage and different annealing processes should be implemented for the difference between low-oxygen copper and oxygen-free copper, so as to ensure the softness of in-process and finished wires.

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

The pullability of oxygen-free copper rods is superior to low-oxygen copper rods in all wire diameters. In addition to the above structural reasons, oxygen-free copper rods have less inclusions, stable oxygen content, and no defects that may be caused by hot rolling. The oxide thickness on the rod surface can reach ≤15A. In the continuous casting and rolling production process, if the process is unstable and the monitoring of oxygen 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 post-process, but the more troublesome thing is that there are quite a lot of oxides existing in the "under the skin", which has a more direct impact on the wire breakage. Therefore, when drawing micro-fine wires and ultra-fine wires, in order to reduce wire breakage, sometimes a last resort must be taken on the copper rod-the reason for peeling, or even secondary peeling, is to remove the subcutaneous oxides.

4. The toughness of low-oxygen copper rods and oxygen-free copper rods is different

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

Fifth, there is a difference in the economy from the raw materials for making rods to making wires.

The manufacture of oxygen-free copper rods requires higher quality raw materials. Generally, when drawing copper wires with a diameter > 1 mm, the advantages of low-oxygen copper rods are more obvious, while oxygen-free copper rods are more superior for drawing copper wires with a diameter of < 0.5 mm.

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 the 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 which is hard.