ã€China Aluminum Network】 Aluminum melting, refining, horizontal continuous casting, sawing, heating, hot rolling, coarse, medium, finish rolling, and annealing are still widely used in many small and medium-sized aluminum plate production plants in China. This is because the horizontal continuous casting method has the advantages of continuous operation, unlimited spindle length, high production efficiency, easy operation, and a small investment in equipment and infrastructure. However, the in-depth investigation on the site found that: The surface “bubbles†of the hot-rolled billet and the “black pits†and “skinning†defects of the cost surface in the production process of these plants are related to the horizontal continuous casting method. To this end, the causes of the formation of “bubbles†and “black pits†were analyzed and discussed, and some opinions on the aspects of preventive measures were put forward.
1 Causes of "bubbles" on the surface of hot rolled blanks and their preventive measures 1.1 The "bubble" phenomenon is known on site. The general rule of "bubbles" appears as follows: Occasionally, batches appear on the surface of billets after hot rolling, and "bubbles" appear as "bubble packs". "Forms; not many in number, different sizes; often appear on one large surface of a billet.
1.2 Analysis of Causes The analysis was confirmed by observation and analysis: The basic reason was that the mold structure installed on the tundish was unreasonable. It was not conducive to the discharge of precipitated gas during the solidification of the aluminum melt, and it was retained on the upper side of the ingot. Caused by the surface. The horizontal continuous casting mold structure adopted at the scene is shown in Figure 1. As it can be seen, the crystallization process is not conducive to the evolution of the precipitated gas.
Aluminum melt easily absorbs hydrogen, and with the changes in temperature and state, the amount of equilibrium hydrogen absorption varies greatly. At high temperature, the primary electrolytic aluminum liquid has an equilibrium hydrogen absorption of 2.9ml/100gAL at 950°C and 1 at 750°C. 2ml/100g/AL. The equilibrium hydrogen absorption capacity 0.69ml/100gAL in the liquid aluminum at the melting point of 660°C, and the solid aluminum crystallized at this temperature was 0. 036mL/100gAL, which is a difference of nearly 20 times. This shows that, during continuous casting, the “melting†of hydrogen gas occurs in the aluminum melt near the crystallizing solidification interface, and the partial pressure thereof increases enough to nucleate and form “bubblesâ€. At this time, due to the restrictions of the asbestos baffle, the “bubbles†cannot escape through the middle body of the tundish and can only stay on the subsurface layer on the upper surface of the ingot, so that the “bubble†is formed under the action of gas expansion pressure after hot rolling. . In the on-site production, this method often uses the knife picking force method to try to eliminate its follow-up impact. I wonder if this may cause defects such as “skinning†and “black spots†on the surface of the product.
1.3 Prevention measures The first is to strengthen the refining gas effect, so that even if the oxygen "concentration" of the aluminum melt in the crystallizer does not reach the level of "bubbles", this is especially true for the direct use of high temperature electrolytic aluminum liquid as raw material. important. In addition to the use of effective refining agents and degassing methods, strict operating procedures are also of particular importance. There are sometimes no batch defects in production and this should be closely related to this; the crystallizer structure must be improved. It is necessary to improve the traditional tundish crystallizer with a very reasonable structure. For example, if the asbestos baffle aluminum liquid inlet seam is changed from the middle seam to the upper and lower two seams or the crystallizer length is appropriately increased, the asbestos baffle plate is simply removed and the melt in the crystallizer completely communicates with the melt in the small gap package. . This should be considered with reference to the method of horizontal continuous casting of copper and copper alloys with much higher melt static pressure and much higher temperatures.
2 The causes of “black burrs†on the surface and preventive measures 2.1 Phenomenon and regularity of “black burrs†On-site observations indicate that the so-called “black burrs†are present on the finished surface of the strip and are further processed and etched by the user. After polishing and other treatments, the surface is exposed with tiny, scattered black pockmarks; on the surface of the strip, it is in the form of zonal (block) line traces; in the case of severe alkali etching, black spots can be removed and removed; with hot rolling The use of milk is related to the use of time. After the “aging†of hot-rolled milk, the situation is more serious.
22 Causes of "Black Point" Defects Analysis According to the principle of "insertion on the surface of the processed product or near-surface is generally from the processing process", it can be determined that the "black spot" is the rolling after the horizontal ingot (mainly hot rolling) In the process, foreign matter pressures people. As the cause of this defect, the composition of factors is the leading factor and the contributing factor:
(1) Dominant factors - "aging" and "dirtification" of hot-rolling, lubricating, and cooling emulsions "aging" refers to changes in emulsion composition due to excessive force for a long time, lubrication Deterioration of performance, presence of oil and water layers, yellowing of emulsions, and increased surface contamination of products; “dirtification†refers to an increase in “dirty matter†in emulsions, including dust, sand particles, corroded piping systems, and exfoliated aluminum particles and their Emulsion formed creamy substances.
Due to the "aging" and "dirty" of the emulsion, conditions will be created 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 "aging" of the emulsion, the effects of the oil and moisture layers, and the carbon residue of the fuel oil. In this regard, the research carried out in Document 4 and the results of the analysis of the "black spots" were confirmed.
(2) Contributing factors - The bad surface roughness of the ingot.
In the horizontal continuous casting process, especially when the casting process is not properly controlled, a clear “cold strait line†is easily formed on the surface of the ingot, and “groove†is also generated when the mold is worn or damaged. The presence of "groove" is conducive to the retention of oil or dirt, and the coverage of the raised portion of the "stripe" during hot rolling causes the formation of 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 fluids (1) Use high-quality emulsions for hot-rolled aluminum to ensure that their working conditions are “aged†for a long period of time and are not likely to appear Oil moisture layer
(2) Under the circumstance that temperature, bacteria, and metal ions (especially Al + 3") make the emulsion "ageing" unavoidable, it can be "appropriately changed" under the condition of ensuring the amount of emulsion needed for hot rolling;
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