ABSTRACT: According to the characteristics of each working procedure and the quality standard of oxygen-free copper in the production of oxygen-free Copper Pipe Rod, the variation law of oxygen content in the process of melting, casting, extruding, drawing and annealing was analyzed, the key measures to control oxygen content in the production of oxygen-free Copper Tube Bar are put forward. 

Keywords: oxygen-free Copper Pipe; smelting; casting; extrusion; annealing; determination of oxygen content

Foreword

Oxygen-free copper (generally copper products with oxygen content below 20 ~ 10 ~ 6) is widely used in the production and manufacture of electronic, electrical and electronic devices due to its excellent conductivity and thermal conductivity. Oxygen-free copper materials are used in high-frequency transmitting tubes, transfer tubes, magnetron tubes and vacuum switch tubes. The oxygen content in copper is strictly controlled because the vacuum device is sealed in hydrogen. The production of oxygen-free Copper Tube and bar has a history of more than 20 years. The products are widely used in electric vacuum industry. The following is a summary of the production process of oxygen-free copper pipe rod and the control measures of oxygen content in the production process.

1. Quality requirements for oxygen-free copper tubes and bars and their production processes 

1.1 quality requirements for oxygen-free copper tubes and bars

Chemical composition and oxygen content of oxygen-free copper in GB/t 5231-2001[2](table 1). 

(2) the technical requirements for oxygen-free copper for electric vacuum materials are shown in table 2. The specifications are as follows. First, electric vacuum devices require a high degree of purity. Because in the electric vacuum device, oxygen-free copper is often used as conductive, thermal conductive materials. Oxygen-free copper.

The higher the purity, the better the conductivity, the longer the service life of the tube. Therefore, in the actual production, the chemical composition of the Ingot should meet the requirements of GB. Second, oxygen content requirements. Copper for electric vacuum devices requires very low oxygen content. This is because in the production of electronic tubes, parts are produced, assembled, sealed and processed in a reducing atmosphere. If oxygen is normally present in copper as Cu2O, it is heated in a reducing atmosphere, hydrogen and carbon monoxide react with Cuprous oxide at high temperatures.

Cu2O+H2=2Cu+H

2OCu2O+CO=2Cu+CO2

When the pressure is higher than the strength of the metal, it will cause the material to crack along the grain boundary. In serious case, the surface bubble can be seen by naked eye. Therefore, the oxygen content of oxygen-free copper should be controlled strictly in order to avoid micro-cracks. Third, the content of phosphorus requirements. For oxygen-free copper directly sealed with glass, a lower phosphorus content is required. When oxygen-free copper high phosphorus content, sealing, glass and metal combination is not strong, and easy to cause tube leakage.

1.2 production technology of oxygen-free copper tube and bar in a factory

1) the production process of pipe material. Melting-semi-continuous casting (determination of oxygen content in ingots)-extrusion (determination of oxygen content in extruded products)-rolling-drawing (determination of oxygen content in finished products). 

(2) production technology of bar. Melting-semi-continuous casting (determination of oxygen content in ingots)-extrusion (determination of oxygen content in extruded products)-drawing (determination of oxygen content in finished products).

2. Oxygen content control in smelting process

(1) procedure for smelting oxygen-free copper. 1 The electrolytic copper is cut, selected and dried, sent to the hearth and immediately added into the furnace; 2 the number of times of adding material during smelting shall not exceed 3 times; 3 after adding material, it shall be immediately covered with calcined charcoal, covering thickness of 150-200 mm, and the charcoal shall be added once, cover the liquid metal surface and the stove head evenly, do not add charcoal in the middle; 4 after charcoal is added, cover the Stove Lid, do a good job of sealing, tight with heat-resistant material where the LID is not tight; 5 times the Stove Lid can not be opened more than 3 times. Matters needing attention in melting process. Because oxygen content of oxygen-free copper must be controlled below 0.003% , the raw material of oxygen-free copper must be selected strictly. Generally, manufacturers choose standard cathode copper as the raw material of oxygen-free copper. The electrolytic copper plate should be made: 1 The electrolytic copper with copper bean and Verdigris on the surface can not be used; 

2 the periphery of the electrolytic plate must be cut off; 3 the electrolytic plate of the standard quality, selected and cut, must be put into the box oven to bake. The electrolytic cathode copper used in the production of oxygen-free copper mainly includes: Guiye brand cathode copper, Yunye brand cathode copper and so on; 5 degassing mainly includes degassing and degassing. Oxygen-free copper smelting, not only to control the hydrogen content, but also must strictly deoxidize. In general, Copper Deoxidation, the addition of copper phosphorus master alloy, the effect is better. However, the deoxidization of oxygen-free copper is usually carried out by covering agent, which is calcined charcoal. 6 charcoal is a common covering agent for smelting copper and its alloys. However, if the charcoal does not burn through, often contains gas, once heated, it will precipitate a large number of gas (see figure 1) , so in order to avoid the introduction of harmful gas and water charcoal affect the quality of casting ingot, to cover with calcined charcoal.

3. Control of oxygen content in semi-continuous casting ingot

The production of oxygen-free Copper Ingot has two main processes: Melting and casting. The control of oxygen content in the two processes has great influence on the oxygen content of Ingot. (1) operation specification of casting process. Operating rules for the casting process of oxygen-free copper INGOT: 1. Before discharging, the furnace head should be tilted, and the casting bracket should be dried thoroughly; 2. The pouring pipe should be aligned with the center of the Mould; 3. The liquid level of the mould should be kept stable during casting; 4. A round ingot of 180mm diameter should be cast, the gate diameter is 8 mm, 5 the liquid surface is covered with carbon black, 6 no more raw materials are added to the furnace after casting. (2) matters needing attention in casting process. In the process of semi-continuous casting, special attention should be paid to prevent secondary oxygen absorption. Casting temperature is too high, easy to cause ingots structure grain size, easy to cause secondary oxygen absorption. If the temperature is too low, the INGOT is easy to cause the inclusion defect. On the premise of ensuring other aspects of the quality of the INGOT, in principle, it is necessary to increase the intensity of primary and Secondary Water Cooling and increase the speed of drawing and casting in order to avoid the secondary oxygen absorption which may occur in the casting process. The purpose of the liquid cover is to avoid secondary oxygen inhalation and lubricate the mold wall. In the process of oxygen-free copper casting, carbon black is usually used to protect the mould surface. Carbon black must be a powder, not agglomerate, low sulfur, small ash. The carbon black must be dried before use, and then evenly covered to the metal surface. If the carbon black is not dried, it is likely to bring in moisture, causing the metal to absorb oxygen. (3) measurement of oxygen content in Ingot. First, the oxygen content of the test method. The following methods are commonly used: 1 repeated bending method, 2 metallographic crack comparison method (hydrogen crack rating method) , 3 density test method, 4 metallographic dot method, 5 oxygen content analyzer infrared determination method. Oxygen content was measured by oxygen determinator when oxygen-free copper was produced in a factory. Secondly, the sampling of INGOT. After checking the internal and external quality of the INGOT, get the qualified product and start checking the oxygen content. The sampling procedure of oxygen content in ingot is as follows: 1300 MM OF INGOT head and tail can not be used as sampling position; 2300 mm of Ingot Head, middle and tail are removed, 3 samples are taken at Ingot head, middle and tail, and their numbers and labels are numbered; 3 they are sent to metallographic examination room for testing; 4 The test results are judged. Third, the qualification of ingot casting. If the results of head, middle and tail are all larger than Grade 2, the Ingot is qualified and can be sawn and sent to the next step. If one of the results of head, Middle and tail is grade 3 or above, the INGOT can not be used for oxygen-free copper, should be used for general copper ingot.

4. Control of oxygen content during extrusion

(1) extrusion process operation. The qualified billet can be sent to extrusion process after numbering. Extrusion process operation rules: Billet number-medium Frequency Induction Furnace heating-peeling extrusion-water seal-discharge. The whole extrusion process is carried out at high temperature. Therefore, during the extrusion process, the oxygen content should be paid close attention to, and the oxygen infiltration should be strictly controlled to ensure that the oxygen content of the extrusion products does not exceed the standard. Problems needing attention in extrusion heating process. Squeeze Casting Ingot is heated in a medium frequency induction furnace. According to the hot working plastic diagram of Red Copper (Fig. 2) , the hot working temperature range of red copper is 775-850 °C. because induction heating is not vacuum heating, the surface and both ends of the INGOT will oxidize and permeate oxygen at high temperature. According to the oxygen permeation of red copper at high temperature (Fig. 3) , the heating of oxygen-free copper should follow the following principles: 1 in the plastic zone of red copper at high temperature, the heating temperature should be as low as possible; Increase the heating rate as much as possible. Problems needing attention in extrusion process. The INGOT is heated and enters

Extrusion process. Although the heating temperature and speed are controlled, the surface and both ends of the INGOT are still oxidized and surface oxygen permeated. Therefore, the extrusion process should be controlled as much as possible. Actual production, the use of peeling extrusion, in addition to increase the length of pressure surplus. Strictly control water seal, ensure water cooling strength, products should be reduced to 50 °C below, can be moved to step-out rack. Oxygen content of the extruded product is measured and the finished product is drawn into the rolling or drawing process. (4) determination of oxygen content of extruded products. The method for determining oxygen content of extruded products is the same as that for ingots. Extrusion products to adopt a complete inspection method. Take 3 test pieces at the head, middle and end of each extruded product and send them to metallographic laboratory for testing. Test results are reported in numbered reports. (5) judgment of qualified extruded products. According to the inspection report, the finished product is judged to be qualified if 3 samples taken from a extruded product are all of grade 3 or above. If one of 3 samples oxygen content is reported to be grade 4 or above, it is judged to be unqualified. 5. Control of oxygen content during rolling and drawing 

(1) operating procedures for oxygen-free Copper Rolling and drawing processes. Oxygen-free copper pipes are usually cold-rolled. The main purpose is to increase the density of the microstructure and to adjust the ECCENTRICITY. Oxygen-free copper bars generally do not need to go through the rolling process. In addition, some specifications of oxygen-free copper tubes also need to carry out vacuum annealing (soft delivery) . Drawing process of oxygen-free Copper Tube: pickling-head-rolling-drawing-peeling-drawing-vacuum annealing; drawing process of oxygen-free Copper Rod; pickling-head-drawing. 

(2) control of oxygen content in annealing process. Some specifications of oxygen-free copper tubes, because the production process is relatively long (generally some small products) , in which the annealing will be used through-type annealing, because through-type annealing is exposed annealing, therefore, to control the annealing temperature and through time, minimize Oxidation and penetration, annealing, to strictly pickling, must increase peeling process. Of course, if the production costs allow, the best intermediate annealing vacuum annealing.

 (3) sampling of drawn articles. All drawn products, whether pipe or bar, are fully inspected. Take one sample of each product and number, send it to metallographic laboratory to determine oxygen content. The determination adopts the metallographic examination method. (4) judgment of drawing qualified products. Test results, oxygen content of three or above is qualified, other grades are unqualified. 6. Conclusion oxygen-free copper is produced by induction furnace melting, semi-continuous casting, induction heating, peeling and water-seal extrusion, and then by rolling and drawing. The key is to control oxygen content in each link. 

(1) smelting process, to select materials strictly, strictly dry, strictly cover the charcoal baking, reduce the number of feeding, do a good job of sealing furnace cover; 

(2) casting, to create a dry environment. 

(3) the heating of extrusion casting ingot should follow the principle of low temperature and fast speed, and the extruding process should be carried out strictly according to the process regulations;

 (4) the sampling and judging system of extrusion products should be strictly enforced; Do the pickling and peeling well in drawing and rolling process to ensure the quality of drawn products. 

Source: Chinanews.com, by Hu Yanping