Leveling principle:
After recrystallization annealing and zinc plating, strip steel often needs to be leveled to obtain the necessary properties and surface quality and meet the requirements of subsequent processing. In the temper mill of continuous hot dip galvanizing line, temper is essentially a rolling deformation with small reduction.
The galvanized strip steel has the following advantages after leveling:
1, which makes the coating surface smoother.
The surface of coated strip steel should be smooth and flat. There are zinc flowers on the surface of the coating. Due to the segregation of components in the crystallization process of zinc liquid, the zinc layer bulges at the first solidification and sinks at the subsequent solidification due to the shrinkage of zinc volume. If this strip steel is coated, these uneven parts will still be exposed on the coated surface. In order to completely eliminate these grooves, the strip steel on the surface of the small zinc flower can be further flattened to obtain a uniform surface.
2. Make the coating and steel structure stronger.
Using a leveling roller with roughness to level the strip steel can make the surface of the strip steel have a certain roughness. The rough surface of strip steel increases the surface area combined with the coating, which will make the coating and strip steel more firmly combined. In addition, the rough surface of strip steel can accumulate more oil on the plate surface, and the oil on the plate surface is more conducive to deep drawing.
3, peace, can eliminate the yield platform and prevent the strip from slipping during stretching or deep drawing. After heat treatment, although the plasticity of strip steel has been greatly improved, when it is elongated under the action of external force, it will yield and form a yield platform. When the strip is deep drawn or stretched, the yield platform will produce slip lines on the surface of the strip. The existence of slip line has no obvious influence on general use, but it is not allowed for products that require high surface quality and need to be painted after deep drawing, because its existence reduces the surface quality of products. In order to avoid slip lines during drawing or deep drawing, the strip can be cold rolled or flattened after annealing, so that the yield platform disappears. After the yield platform disappears, the steel plate can extend evenly during drawing or deep drawing.
The leveler on the galvanizing line can also eliminate the yield platform or make the yield platform inconspicuous by applying a certain rolling force to the strip during straightening. Practice has proved that the tendency to form slip lines can be eliminated when the flatness is 0.5%, and the slip lines can be completely eliminated when the flatness is 1%.
Eliminating slip lines by leveling is not eternal. If the steel plate is left for a long time, due to the effect of aging, the yield platform that disappeared after flattening will reappear, and slip lines will occur when the strip is stretched or deep drawn.
There are many reasons for aging. Chemical elements such as carbon, nitrogen and copper in steel have great influence on aging, especially the nitrogen content in steel has great influence on aging. Because at high temperature, nitrogen is dissolved in α-iron to form a solid solution, when the temperature is lowered, because the solubility of nitrogen in steel is not great, it becomes supersaturated and precipitates from grain boundaries and slip surfaces in the form of nitride. This phenomenon will happen quickly when the temperature is higher than 100℃, and steel can be artificially aged at 200℃ for 2 hours. If aluminum is added to steel, aluminum can form aluminum nitride with nitrogen because of its great affinity. After nitrogen forms aluminum nitride, it will not precipitate around the grain boundary, so the killed steel deoxidized by aluminum can prevent aging. Boiling steel should be used as soon as possible after leveling, otherwise slip lines will still appear when stretching or deep drawing after a period of time. If steel has no interstitial atoms, it will not age.
4. Flattening can improve flatness, and the surface of strip steel is smoother.