Control of Horizontal Stripes on Cold-rolled Steel

Surface striated defects are important indicators affecting the surface quality of cold-rolled products. It is very difficult to solve the problem of surface ridges in many cold rolling mills in China, especially galvanizing units and continuous retreating units. The cause and solution of surface grain defects are an important part of the development of high-end cold-rolled product production technology.

How is the surface grain pattern formed? When the strip is subjected to a sudden transition from the elastic zone to the plastic zone by the stress exceeding the yield limit, due to the effect of the yielding platform, unevenness and local flow deformation will occur on the strip section, and the surface yielding transverse stripes will occur. The microscopic cause is due to the "nail anchor" effect of the Kodak air mass. When the solute atoms interact elastically with dislocations, they eccentrically gather around the dislocations to form a so-called "Kod's air mass." At this point, if the dislocations are to be moved, they must be freed from the air mass or dragged together with the air mass, which causes the strip to produce a yielding platform.

According to the above principle, to contorl the horizontal stripes on panel surface should capture the following influencing factors:

(1) Material composition. Generally speaking, the higher the content of carbon and nitrogen atoms in the steel plate, the more obvious the effect of the "nail anchor" formed by carbon-nitrogen atoms, the more pronounced the yielding platform, and the greater the probability of occurrence of horizontal grain defects.

(2) Annealing process. Experience has shown that the production of strips of the same specification and of the same material, the annealing furnace has a slow cooling section and the stripe produced without the slow cooling section has a small probability of horizontal grain defects. The annealing furnace has an overaged section with a lower probability of occurrence of transverse grain defects than the strip produced without the overage section. This is because the cooling rate is related to the degree of supersaturation of the elemental carbon and nitrogen in the solid solution in the yielding platform, and the overaging phase is related to the degree of precipitation of carbon and nitrogen atoms. The greater the degree of supersaturation, the longer the yielding platform; the longer the overaging time, the shorter the yielding platform.

(3) Cooling method after annealing. If the strip temperature is high, it will directly enter the leveling machine or the tension leveler. When the belt temperature is higher than 45 °C, it is easy to produce horizontal grain defects, and the higher the temperature, the more obvious the horizontal grain defects.

(4) Flat (light) finishing process. For different types of strip, horizontal grain defects will not occur as long as the proper leveling process is sufficient to eliminate the yielding platform. Experience has shown that the worse the deep drawability of the original plate or the higher the carbon content, the greater the amount of critical deformation required (the critical deformation amount refers to the amount of deformation when the yielding platform just disappears). In the case of carbon steel, the total elongation of the leveling machine plus the tension leveler is at least greater than 0.8% to completely eliminate the yielding platform. As a rule of thumb, the technique of eliminating or reducing surface graining at the leveling machine is: large rolling force and small tension to obtain proper elongation, which is beneficial to the elimination of the yielding platform. In addition, the wet flattening and wet-drawing process facilitates the elimination of the yielding platform because of the wetting of water or rolling fluid, which reduces the friction between the strip and the rolls or the bending rolls, and can be made larger under the same production conditions. The elongation rate. When the leveling machine completely eliminates the steel-based yielding platform, the post-tensioning leveling machine will not have any horizontal grain defects anyway.

(5) Pulling and finishing process. Experience has shown that the horizontal roller defect is not easy to occur when the roller diameter of the working roller of the tensioning machine is 40 mm or more, and the horizontal deviation is easy to occur when the roller diameter is less than or equal to 30 mm. As long as the maximum bending stress of the strip surface exceeds the yield point of the steel base, a large or small horizontal grain defect occurs, but the transverse grain defect can be obtained under the premise that the tension and the reduction amount are properly matched to obtain an appropriate elongation. The extent of it is minimized.