Bubbles and black spots in the production of the h

2022-08-09
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In the production of aluminum sheet and strip, "bubbles" and "black spots"

in many small and medium-sized aluminum sheet production plants in China, the process of aluminum melting, refining → horizontal continuous casting, sawing → heating, hot rolling → rough, medium and finish rolling → annealing is still widely used. This is because the horizontal continuous casting method has the advantages of continuous operation, unlimited ingot length, high production efficiency, convenient operation and small equipment and infrastructure investment. In many small and medium-sized aluminum sheet production plants in China, the processes of aluminum melting, refining horizontal continuous casting, sawing heating, hot rolling rough, medium and finish rolling annealing are still widely used. This is because the horizontal continuous casting method has the advantages of continuous operation, unlimited ingot length, high production efficiency, convenient operation and small equipment and infrastructure investment. However, the author found through in-depth investigation on the relevant site that the surface bubbles of hot-rolled billets and the surface pockmarks, peeling and other defects of the cost surface in the production process of these plants are related to the horizontal continuous casting method. Therefore, the causes of bubbles and black spots are analyzed and discussed, and some views on preventive measures are put forward

1 causes and preventive measures of bubbles on the surface of hot rolled billets

1.1 bubble phenomenon

according to the field understanding, the general rule of bubbles is: occasionally appear on the surface of hot rolled billets in batches, and the bubbles are in the form of bulges; There are not many in quantity and different in size; It often appears on a large surface of the blank

1.2 cause analysis

through observation and analysis, it is confirmed that the fundamental reason is that the structure of the crystallizer installed on the tundish is unreasonable, which is not conducive to the discharge of the gas released during the crystallization and solidification of aluminum melt, leaving it stranded on the upper side of the ingot near the surface. The structure of horizontal continuous casting crystallizer used on site is shown in Figure 1. It can be seen that the crystallization process is not conducive to the discharge of precipitated gas

the aluminum melt is easy to absorb hydrogen, and the equilibrium hydrogen absorption changes greatly with the change of temperature and state. The equilibrium hydrogen absorption of high-temperature primary electrolytic aluminum liquid is 2 at 0 ℃ with the boom of global new energy vehicles. 9ml/100gal, 750 ℃ is 1. 2ml/100g/AL。 The equilibrium hydrogen absorption capacity of liquid aluminum with a melting point of 660 ℃ is 0.69ml/100gal, while that of solid aluminum after crystallization at this temperature is 0. 036ml/100gal, that is, the difference between the two is nearly 20 times. This shows that during continuous casting, hydrogen concentration will occur in the aluminum melt near the crystallization solidification interface, and its partial pressure will increase enough to nucleate and form bubbles. At this time, due to the limitation of asbestos baffle, bubbles cannot escape through the tundish middle body, and can only stay on the sub surface of the upper surface of the ingot, so that bubbles are formed under the action of gas expansion pressure after hot rolling. In on-site production, the knife pricking force method is often used to try to eliminate its subsequent impact. Do you know that this may cause defects such as peeling and black spots on the surface of products

1.3 preventive measures

the first is to strengthen the effect of refined gas, so that even if the aluminum melt is enriched with oxygen in the crystallizer, it cannot form a bubble for the future development direction of the high molecular material industry. This is particularly important for the direct use of high-temperature electrolytic aluminum liquid as raw material. While using effective refining agents and degassing methods, strict operating procedures are also particularly important. Product failure caused by improper material selection in production is often the most common fault of plastic product designers or engineers. Sometimes there is no batch of such defects, which should be closely related to this; Then improve the structure of crystallizer. The traditional tundish crystallizer with small and reasonable structure mentioned above needs to be improved through practical exploration. If it is possible to change the inlet seam of the molten aluminum of the asbestos baffle from the middle seam to the upper and lower side seams, or appropriately increase the length of the crystallizer, simply remove the asbestos baffle, so that the melt in the crystallizer is completely connected with the melt in the small ladle This should be considered with reference to the horizontal continuous casting method of copper and copper alloys with much higher melt static pressure and temperature, which should be feasible

2 causes and preventive measures of surface black spots

2.1 phenomenon and law of black spots

field observation shows that the so-called black spots are tiny and scattered black spots that appear on the surface of finished plate and strip products after further processing, etching and polishing by users; On the surface of the strip, it is distributed in the form of regional (block) trace dots; After severe alkali corrosion, the black spots can fall off and be removed; It is related to the use time of hot-rolled lotion, and the situation is more serious after the aging of hot-rolled emulsion

22 cause analysis of black spot defects

according to the principle that inclusions embedded on the surface or near the surface of processed products generally come from the processing process, it can be determined that black spots are caused by foreign bodies in the rolling (mainly hot rolling) process after horizontal ingots. As the contributing factor of this defect, there are two aspects: the leading factor and contributing factor:

(1) the leading factor is the aging and dirtiness of hot rolling lubricating and cooling emulsion. The aging of Wei lotion refers to the change of lotion composition, deterioration of lubricating performance, oil-water stratification, yellowing of lotion and aggravation of product surface pollution with the use of too long time due to multi force surface reasons; Dirtiness refers to the increase of dirt in lotion, including dust, sand particles, pipeline system rust, falling aluminum particles and the cream like substances formed with lotion

the aging and dirtiness of lotion will create conditions for the formation of foreign matters on the damaged surface of the plate during hot rolling. However, in terms of the formation of black spots What should be considered is the impact of lotion aging, oil-water stratification and fuel carbon residue. This is confirmed by the research carried out in document 4 and the analysis results of small black spots

(2) contributing factors (4) bad ingot and poor surface flatness of graphene based high-performance thermal interface materials used in the optical/electrical field

in the process of horizontal continuous casting, especially when the casting process is not controlled properly, it is easy to form obvious cold shut lines on the surface of ingots, and grooves will appear when the crystallizer is worn or damaged. The existence of grooves is conducive to the retention of oil or dirt, and the coverage of the convex part of the groove during hot rolling makes it form subcutaneous inclusions, so that the defects are not directly exposed on the surface of the finished product

2.3 measures to reduce black spots

23.1 reduce or eliminate the influence of hot-rolled pore liquid

(1) use high-quality emulsion for hot-rolled aluminum to ensure its working conditions, long aging period and not easy to have oil-water stratification

(2) when the aging of lotion is inevitable due to the effects of temperature, bacteria, metal ions (especially al+3), an appropriate amount of lotion can be changed under the condition of ensuring the amount of emulsion required for hot rolling

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