Titanium metal has the advantages of low specific gravity, high strength, strong corrosion resistance, and good biocompatibility, and is widely used in aerospace, ships, metallurgy, petrochemical, medical and other fields. Cold-rolled titanium strip is made of high-quality sponge titanium, produced and supplied in rolls, and the commonly used thickness specification is 0.5mm~3.0mm. It has the advantages of large output, good surface quality, and uniform structure.
During the processing of cold-rolled titanium strips, in order to eliminate work hardening, improve the plasticity and toughness of titanium strips, and obtain better mechanical properties, it is necessary to anneal titanium strips. Since titanium is a metal with relatively active chemical properties, it is easy to react with O2, N2, H2 and water vapor when heated. In order to ensure the production of high-quality titanium coils, the titanium coils are annealed under the protection of vacuum or inert gas. At present, there are mainly three kinds of annealing equipment at home and abroad: vacuum annealing furnace, argon continuous annealing line, and argon-protected bell annealing furnace. Among them, bell furnace annealing has the advantages of low cost, short cycle, and good uniformity. However, when titanium coils are annealed in bell furnaces, due to various reasons, oxidation after annealing often occurs, and the oxidation of titanium coils seriously affects the surface of titanium strips. The quality and yield will even cause the titanium coil to be scrapped. This paper analyzes the causes and control methods of titanium coil annealing and oxidation.
1 Titanium coil annealing process
The bell annealing furnace is mainly composed of a hearth, a heating mantle, a cooling mantle, an inner mantle, a vacuum system, a heating system, a cooling system, an electrical system, etc. During annealing, one furnace with two rolls is used, and the two rolls are placed vertically.
The operation process of the titanium coil bell annealing furnace is as follows: Titanium coil loading furnace → install the inner cover, heating cover → vacuumize → fill with argon gas → heat → keep warm → remove the heating cover → install the cooling cover → cool → remove the cooling cover, the inner cover → Titanium rolls to cooling table.
The raw material for bell annealing is pure titanium coil (TA1 or TA2), the specification is (0.5~3.0) mm×1250mm×L, and the weight of a single coil is 6t. The titanium coil is hoisted onto the hearth by the material rack, and the inner cover is hoisted in place. A mechanical locking device is used to press the water-cooled rubber ring between the inner cover and the hearth to form a sealed cavity. When the protective atmosphere and working pressure required in the furnace are reached, the heating system and circulation fan start to work to make the protective gas in the furnace circulate strongly, and the temperature is uniform, and all processes such as heating, heat preservation, air cooling, and water cooling are completed under the control of the automatic electrical control system process.
2 Annealing oxidation defects and cause analysis
In order to obtain a bright surface quality, titanium coil annealing is carried out in a vacuum and a protective atmosphere, and titanium coil annealing generally uses argon as a protective gas. However, due to equipment failure, improper operation, surface pollution and other problems, when the titanium coil is annealed, the content of oxidizing media such as oxygen, carbon dioxide and water vapor in the atmosphere is relatively large, and the surface of the titanium coil will be oxidized to form a dense oxide film. . The oxidation reaction equation of titanium is Ti+O=TiO2, and the reaction condition is high temperature heating.
When the heating temperature is low, the oxide film on the surface of titanium is almost transparent, which is difficult for people to detect with the naked eye, but when the temperature rises, the oxide film will gradually thicken and interfere with light. The eyes will also show different colors. Generally, titanium coil annealing oxidation presents a light yellow, tan or blue imprint gradually changing from the edge to the middle, and the contour line is arc-shaped and irregular. Generally, the edge is more serious, as shown in Figure 2. After analysis, the following reasons usually cause the increase of the oxidizing medium in the furnace atmosphere, which leads to oxidation on the surface of the titanium coil after annealing.
2.1 Equipment problems
2.1.1 Equipment leaks
Before annealing, the hood-type annealing furnace needs to be evacuated to a certain degree of vacuum, and the equipment is in a sealed state, but the long-term use and aging of the equipment will cause air leakage in the inner cover, pipeline or seal of the equipment, oxygen will enter the furnace, and the vacuum degree in the furnace will drop. Titanium coils are oxidized during high temperature annealing.
2.1.2 Equipment pollution
When the titanium coil is annealed in the furnace, the material tray, inner cover, bracket and other parts are heated together in the furnace, and the diffusion pump is connected with the atmosphere in the furnace through a pipeline. If the furnace inner cover, material tray and other parts (as shown in Figure 3) There are pollutants such as oil, or there is oil leakage in the diffusion pump. When the temperature rises during annealing, the oil is carbonized to produce oxidizing media such as carbon monoxide, carbon dioxide, and nitrogen oxides. The oxidizing media reacts with the titanium coil at high temperature, causing the titanium coil to oxidize.

2.2 Argon pollution
Argon is an inert gas that does not react chemically with other substances at room temperature and is insoluble in liquid metals at high temperatures. It is very inactive in nature and can neither burn nor support combustion.
During the annealing process of the titanium coil, the furnace should be filled with argon for protection. If the purity of the argon is not enough, or the argon pressure is not enough during the argon filling operation, the gas in the furnace is mixed with oxygen, and the titanium coil will be oxidized during high temperature annealing.
2.3 Titanium Coil Contamination
Since the bell annealing is vacuum argon-filled annealing, the titanium coil needs to be degreased and cleaned before annealing to remove the surface oil. The degreasing process is to remove the surface oil by spraying the degreasing liquid and brushing the roller brush. After washing with rinsing water and drying with hot air, it meets the requirements of bell annealing. The degreasing of titanium coils is not clean (as shown in Figure 4), which will lead to oxidation defects of titanium coils, mainly including:
(1) The cleaning ability of the degreasing liquid is insufficient, and the oil and degreasing on the surface of the titanium coil are not clean;
(2) The roller brush of the degreasing equipment is aging, and the scrubbing ability is insufficient, so the surface oil cannot be effectively removed;
(3) The rinse water was not replaced in time, and the degreasing solution on the surface of the titanium coil was not rinsed;
(4) The hot air blowing ability is insufficient, and the titanium coil is not completely dried after cleaning, and there will be water vapor on the surface of the titanium coil. When the polluted titanium coil is loaded into the furnace, the oil and water vapor will volatilize in the furnace at high temperature, increasing the oxidizing medium and causing oxidation on the surface of the titanium coil.
2.4 Poor roll shape
A large number of production data show that when the shape of the coil is poor after rolling, the oxidation will increase. After analysis, when the coil has side waves, the gap between the layers of the titanium coil increases after coiling (as shown in Figure 5), and the gas circulation in the furnace is not smooth during annealing, and it is easy to generate eddy currents and other phenomena, thereby creating a negative pressure zone , It is easy to cause the gas outside the furnace to infiltrate into the furnace, and the gas is easy to infiltrate into the interior of the coil along the edge wave area to cause oxidation.
2.5 Operational issues
Improper operation during the bell annealing process will also cause oxidation of titanium coils. Improper operation has the following aspects:
(1) During the process from degreasing and cleaning the titanium coils to loading into the furnace, it is necessary to ensure that there is no pollution. If the operator does not wear clean gloves during the furnace loading process, the tooling sling has oil, which will cause secondary pollution of the titanium coils. oxidation.
(2) During the annealing process, the operator does not operate according to the process requirements, the vacuum degree in the furnace is not up to standard or the argon filling pressure is not enough during annealing, which will cause annealing and oxidation of titanium coils.
(3) The implementation of equipment maintenance and spot inspection is not in place, and the furnace table, inner cover, material tray and other parts are not cleaned cleanly, resulting in annealing and oxidation of titanium coils.

3 Oxidation control of titanium coils
According to the analysis of the above reasons, in order to avoid or reduce the occurrence of titanium coil annealing and oxidation, it can be controlled from the following aspects:
3.1 Equipment Maintenance
Do a good job in regular maintenance and daily spot inspection of equipment, regularly check the working conditions of each component of the equipment, and pass the pressure rise rate test and equipment leak detection to ensure that the equipment does not leak air or oil; Clean important parts such as discs, inner covers, vacuum chambers, etc., and confirm before the titanium coil is loaded into the furnace to avoid oxidation defects caused by oil contamination of the titanium coil.
3.2 Argon gas control
Argon with a purity of 99.99% or above is used as the protective gas, and argon is always kept in the pipeline to avoid air entering the pipeline. The pipeline pressure is guaranteed during the argon filling operation, and the argon pressure in the furnace meets the process requirements.
3.3 Titanium Volume Control
Strengthen the degreasing and drying effect of the degreasing equipment, do a good job in spot inspection and maintenance of the roller brush of the degreasing equipment, avoid the titanium coil degreasing due to the aging of the roller brush, and formulate the degreasing inspection standard of the titanium coil to ensure that the titanium coil is degreased clean and pollution-free , meeting the requirements of bell annealing.
Control the rolling plate shape of titanium coils, and eliminate edge waves by leveling or straightening after rolling to ensure that there are no obvious gaps between layers and no overflow at the ends of titanium coils after degreasing, which is conducive to the circulation of gas in the furnace and the discharge of exhaust gas during annealing.
3.4 Personnel training
Formulate bell-type annealing operation standards, strengthen personnel training, strictly implement operating standards and process document requirements for furnace loading and annealing processes, standardize equipment maintenance, and check and confirm maintenance results. During the whole annealing process of titanium coils, the changes of vacuum degree and temperature in the furnace are constantly observed, and abnormal situations are reported and resolved in time.
Through the above control measures, the titanium coil can appear bright silvery white after annealing, which can greatly improve the surface quality of the titanium coil and increase the product yield.