Main Defects and Control Methods of Cold Rolling Leveling

The characteristics of cold-rolled strip are uniform thickness, beautiful surface, good shape, high dimensional accuracy and excellent formability.
Therefore, the product quality of cold-rolled strip directly affects the economic interests of users, as well as the economic effect and reputation of the enterprise. Here, we mainly analyze the causes of product defects in the flattening process, and strive to eliminate or reduce product defects and improve the yield and quality.
1 Two side waves and one side waves
Reason: The incoming side wave is serious, the roll curve is small during flattening or the inclination of the bending roll is not adjusted properly, which will cause waves on both sides or one side of the strip after flattening.

Elimination method: It can be eliminated or alleviated by adjusting the inclination, adjusting the bending roll or increasing the roll curve.

2 Intermediate waves
Reason: In the middle of the incoming material or the flattening process, the roll crown is too large, the positive bending force is too large, and the rolling leveling force is too small, which will cause the middle wave defect in the flat strip.

Solution: The intermediate wave of incoming materials can generally be eliminated by adjusting the roll shape and positive and negative bending rolls.

3 Warping (lateral bending)
Reason: Warping means that the middle of the strip steel is convex upward or downward. When cutting into a steel plate, the four corners are warped upwards, which is called scooping. The reason is that the plate shape is poor, the middle is thick and the two sides are thin, the work roll is too convex or the bending operation is wrong, which causes the middle extension of the strip to be larger than the two sides.

Solution: If the warpage is large, it can be eliminated by rationally preparing the roll shape, correct bending the roll operation or leveling again.

4 Slope
Reason: When the work roll curve is too large, the rolling pressure (leveling pressure) is too large, or the roll is expanded due to the high incoming temperature (leveling temperature requirement <=45 degrees Celsius), the strip surface will often be non-perpendicular and non-parallel. Sloping waves at the edge of the strip. Oblique wave defects are difficult to eliminate, often resulting in absolute waste. Solution: Only by controlling the temperature of the incoming material and using the roll shape reasonably, this kind of defect can be avoided. 5 Flatten scratches Reason: Scratch refers to the defect that scratches appear on the surface of the strip due to the mutual dislocation between the layers of the steel coil. On the one hand, the reason is that the incoming material is loose, and the coiling tension after rolling is too small; on the other hand, the flattening tension is too large. Solution: Reduce the leveling tension or leveling speed. 6 Roll and creasing Reason: The surface of the work roll is partially peeled off or adhered to the iron filings, and there are periodic surface marks (convex or concave) on the surface of the steel plate after leveling. In addition, the roll marks will also be caused by the deviation of the strip, the roll, and the broken strip. Elimination method: Check the surface of the strip frequently (every roll), and replace the rolls in time if any problems are found. 7 Flat flower (seat pattern) Reason: The flat flower refers to the stripes printed on the surface of the strip like leaves or mats. The reasons for this are: uneven thickness of the strip or wavy shape, improper adjustment of the amount of reduction when flat and thin specifications, and deviation of the strip. Wait. Solution: The tension roller device before and after the leveling machine can effectively prevent the mat line. Controlling the tension and roll shape is also the main preventive measure. 8 Flat Fold Flat-folding is a defect that is reflected in flatness due to annealing. It is a wrinkle-like bright stripe print, the length of the folded print varies, and it feels severe when touched by hand. During annealing, due to high local pressure and high temperature, lateral adhesion is caused. When the adhesion is torn off during flattening and unrolling, folding and printing will occur, which cannot be eliminated after flattening, leaving long wrinkled traces. Actually it is a bonding defect. The reasons for this are as follows: the temperature of the coil before flattening is greater than 45 °C, the metal molecules are more active when the temperature is high, and the strip is easy to jump from the elastic deformation zone to the plastic deformation zone when the strip is under tension, and slip occurs on the surface of the strip. belt, resulting in edge folding; the speed measuring roll, pressure roll and anti-wrinkle roll of the levelling machine are not properly positioned, and their axes are parallel to the rolls, resulting in excessive local tension acting on the strip to reach the yield strength, especially on both sides of the strip. 45° edge folding; the tension acting on the strip is unstable, resulting in instantaneous large or small tension, which is easy to produce edge folding. Control measures: The incoming temperature is controlled below 50℃. The equipment regularly checks whether the axis of each roll system is parallel to the rolls, and controls the quality of the flat raw materials (bonding, flat roll, inner diameter damage). 9 horizontal stripes The surface of the strip steel has periodic lateral chromatic aberration, which can be seen after surface grinding. In severe cases, it can be seen without grinding the side light. Generally, the width of the stripes is between 10-30mm. Reasons: The use time of the roll is too long, exceeding the roll changing period; vibration occurs during the rolling process; the grinding accuracy of the roll is not good. Solution: Pay attention to the strict implementation of the roller changing system during production; measure regularly to adjust the gap between the arches and the rollers; improve the quality of the grinding rollers. 10 yellow membrane Reason: The irregular color difference of light yellow appears on the surface of the strip after wet leveling, which generally occurs in the middle of the surface of the strip, and rarely occurs at the edge. The main reason for the occurrence is that the leveling liquid remains on the surface of the steel plate, and the impurities in the leveling liquid are formed by chemical reactions on the surface of the steel plate, which mostly occur in autumn and winter, and have a great relationship with factors such as temperature and humidity. Solution: It can be controlled by strengthening the purging of the leveling liquid and reducing the impurity content. 11 Rust The surface of the steel plate has yellow, brown and black dots and blocks, mainly due to the presence of water on the surface of the steel plate, resulting in a micro-battery effect and a redox reaction. The main factors affecting on-site production are: environmental factors, air humidity is high or there is water around the coil; the wet leveling fluid concentration is lower than the process requirements, or the leveling fluid is not clean; the storage time of intermediate products is too long. 12 scratches Continuous intermittent or continuous scratches caused by excessive wave shape of incoming material or protruding points of equipment. The main reason is that the speed of the roller system is not synchronized. During production, attention should be paid to controlling and handling the quality of flat raw materials (flat coils, loose coils, damaged inner diameter); for incoming loose coils, it is required to reduce tension, produce at low speed, and avoid uncoiling scratches; check whether there are protrusions on the roller surface that contacts the steel plate . Whether the equipment position on the upper and lower surfaces of the strip is correct. The device checks whether the rollers are synchronized. 13 Slip line During the stamping process, the strip produces twill like orange peel on the surface, which is essentially a Lüders strip. The causes are: insufficient leveling force; coarse grains caused by recrystallization in the critical region after hot rolling; coarse grains caused by excessive annealing temperature; grain boundary segregation affected by alloying elements.

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