"Bubble" and "black pockmark" in the production of aluminum strip

In many small and medium-sized aluminum plate production plants in China, aluminum melting, refining→horizontal continuous casting, sawing→heating, hot rolling→crude, medium, finish rolling→annealing are still widely used. This is because the horizontal continuous casting method has the advantages of continuous operation, unlimited spindle length, high production efficiency, convenient operation, and small investment in equipment and infrastructure. However, the author conducted an in-depth investigation at the relevant site and found that the defects of the surface “bubble” of the hot-rolled billet and the “black hemp” and “peeling” of the cost surface in the production process are related to the horizontal continuous casting method. To this end, the reasons for the formation of "bubbles" and "black spots" were analyzed and discussed, and some views were put forward on the aspects of preventive measures.

1 The cause of "bubble" on the surface of hot rolled billet and its preventive measures

1.1 "Bubble" phenomenon

According to the on-site understanding, the general rule of “bubble” appears: occasionally in batches appearing on the surface of the billet after hot rolling, “bubble” is in the form of “bullet pack”; the quantity is not large, the size is different; often appears on a large surface of the billet .

1.2 Cause analysis

It is confirmed by observation and analysis that the root cause is that the crystallizer structure installed on the tundish is unreasonable, which is not conducive to the discharge of the gas precipitated during the solidification of the aluminum melt, so that it is retained at the near surface of the upper side of the ingot. The structure of the horizontal continuous casting mold used in the field is shown in Figure 1. As can be seen, the crystallization process is not conducive to the evolution of the evolved gas.

The aluminum melt is easy to absorb hydrogen, and the equilibrium hydrogen absorption varies greatly with temperature and state. The high-temperature electrolytic aluminum liquid has an equilibrium hydrogen absorption of 9.5 ° C / 550 ° C at 950 ° C. 2ml/100g/AL. The equilibrium hydrogen absorption force of liquid aluminum at a melting point of 660 ° C is 0.69 ml / 100 gAL, and the solid aluminum after crystallization at this temperature is 0. 036 mL / 100 gAL, that is, the difference between the two is nearly 20 times. This shows that in the aluminum melt near the crystal solidification interface during continuous casting, "concentration" of hydrogen occurs, and the partial pressure is increased enough to form a "bubble". At this time, due to the restriction of the asbestos baffle, the "bubble" can not escape through the middle body of the tundish, and can only stay on the subsurface of the upper surface of the ingot, so that after the hot rolling, the "bubble" is formed under the action of the gas expansion pressure. . In the field production, this method is often used to eliminate the subsequent influence of the knife. It is not known that this may form defects such as “skinning” and “black spots” on the surface of the product.

1.3 preventive measures

The first is to strengthen the effect of refining gas, so that the aluminum melt can not reach the degree of "bubble" even if the oxygen is "concentrated" in the crystallizer. This is especially important for the case where the high-temperature electrolytic aluminum liquid is directly used as a raw material. While using effective refining and degassing methods, strict operating procedures are also of particular importance. Sometimes such defects are not present in batches during production and should be closely related to this; further, the crystallizer structure is improved. The aforementioned conventional tundish crystallizer with a small and reasonable structure needs to be improved through practical exploration. If the asbestos baffle aluminum liquid inlet joint can be changed from the intermediate joint to the upper and lower joints, or the length of the crystallizer is appropriately increased, the asbestos baffle is simply removed, so that the melt in the crystallizer is completely in communication with the melt in the small compartment. . This point should be considered in consideration of the horizontal continuous casting method of copper and copper alloys with much larger hydrostatic pressure and much higher temperature.

2 The cause of the "black hemp point" on the surface and its preventive measures

2.1 "Black Hemp Point" Phenomenon and Law

On-site observations show that the so-called “black hemp spot” appears on the surface of the finished strip of sheet and strip, after the user has further processed, etched and polished, the surface is exposed with tiny, scattered black pockmarks; The area 忭 (blocking) stitches are dotted; the black spots can be removed after severe alkali corrosion; it is related to the hot rolling emulsion time, and the hot rolling emulsion is “aged”.

Analysis of the Causes of 22 "Black Hemp Point" Defects

According to the principle of "inlaid on the surface or near the surface of the processed product, generally from the processing", it can be determined that the "black hemp point" is caused by the foreign matter pressing during the rolling (main hot rolling) process after the horizontal ingot. As the causative component of this defect, it is the leading factor and the contributing factor:

(1) Dominant factor——The “aging” and “dirty” of the hot-rolled lubrication and cooling emulsion refers to the “aging” of the emulsion, which means that the composition of the emulsion changes due to the long-term use, and the lubrication changes. Deterioration of performance, oily water layer, yellowing of the emulsion and increased surface contamination of the product; "dirty" refers to an increase in "dirt" in the emulsion, including dust, sand, piping system rust, shed aluminum particles and their A creamy substance formed by an emulsion or the like.

Since the "aging" and "dirty" of the emulsion will create conditions for the formation of foreign matter on the bad surface of the plate during hot rolling. However, in terms of the formation of "black spots". More consideration should be given to the effects of emulsion "aging", oil moisture layer and fuel residue. In this regard, the research conducted by the literature 4 and the analysis results of the "small black spots" component were confirmed.

(2) Contributing factors - the poor flatness of the ingot is caused by poor flatness.

In the horizontal continuous casting process, especially when the casting process is not properly controlled, it is easy to form a clear “cold gap pattern” on the surface of the ingot; when the mold is worn or damaged, “groove” will appear. The presence of "grooves" facilitates the retention of oil or dirt, and the coverage of the "texture" raised portions during hot rolling causes subcutaneous inclusions to be formed so that the defects are not directly exposed on the finished person's surface.

2.3 Measures to reduce "black spots"

23.1 Reduce or eliminate the effects of hot-rolled pores

(1) Use high-quality emulsion for hot-rolled aluminum to ensure that its working conditions are long in the “aging” period, and it is not easy to appear oil-water layer;

(2) Under the condition that the temperature, bacteria, metal ions (especially Al+3) and other effects make the emulsion "aging" unavoidable, "appropriate amount can be changed" under the condition of ensuring the amount of emulsion required for hot rolling.

Concerned about surprises

Tag: Bubble Crystallizer Aluminum melt Hot rolled billet Aluminum strip

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