The width of the fine stripes on the surface is generally less than 1mm and the depth is less than 0.2mm, but it can extend for hundreds of meters in the longitudinal direction of the board, even throughout the production process, until the shutdown, the fine stripes are still clearly visible after pickling. Some fine streaks are caused by leftover roller marks, and such fine streaks will generally disappear after pickling. A simple tensile test can show the defect and cause necking near the defect tissue, which proves that the cause of this type of defect is that there are some impurities that hinder the flow of molten metal.
Two Reasons for Blocking Flow
(1) There are impurity particles locally in the casting nozzle cavity. When the casting nozzle is inspected after the casting and rolling is stopped, it can be seen that some residues have accumulated on the lower surface of the casting nozzle. Through the electron microscope, it can be distinguished that there are titanium, boron particles and oxides in this part.
(2) The casting nozzle is in contact with the roller. At the beginning of the vertical plate, due to the low surface temperature of the roll and the graphite lubricant on the surface, when the aluminum liquid is under static pressure in the process of running the slag vertical plate, it flows from the front box through the cross runner into the casting nozzle to make the casting nozzle. The temperature rises rapidly, the casting nozzle begins to expand and deform when heated, and comes into contact with the roll. The graphite (carbon) on the roll surface accumulates on the lip. As the production continues, the oxide accumulates and thickens, and some accumulate into small pieces. Particles, contact with the roller, destroy the original oxide layer on the roller surface. After the aluminum liquid flows out from the casting nozzle and contacts the roller, the melt will solidify and adhere to the roller surface and move synchronously with the roller without relative sliding. The tension is used to separate the strip from the roll, and sometimes small particles of sticky aluminum are faintly visible at the scratches. The fine streaks on the surface pass through the casting nozzle to ensure a certain gap between the casting nozzle and the roll, and reduce the amount of graphite spraying. Most of the traces on the roll surface can be eliminated by using emery cloth.
Fine Stripes Treatment Measures
(1) Control the titanium wire addition speed (the titanium wire addition speed is about 10-12 m/ton per hour according to the aluminum flow rate), the adding position (at the outlet of the degassing furnace) and the melt temperature (melt temperature is higher than 710℃), Make the titanium content diffuse quickly and evenly in the melt to avoid precipitation.
(2) Adjust the gap between the casting nozzle and the roll when erecting the plate. It is found that the contact between the casting nozzle and the roller is adjusted in time to avoid the formation of thin strips on the roller surface.
(3) Make sure that the graphite layer on the roller surface is even and not too thick, and gently wipe the roller surface with a cotton cloth.