Main defects and control methods of cold-rolled steel plate flattening

The characteristics of cold-rolled strips are uniform thickness, beautiful surface, good plate shape, high dimensional accuracy, and excellent forming performance. Cold-rolled plates are widely used in automobiles, light industry, electrical, etc. Therefore, the product quality of cold-rolled strips directly affects the economic interests of users and also affects the economic effect and reputation of the enterprise. Here we mainly analyze the reasons for product defects in the leveling process and strive to eliminate or reduce product defects and improve the yield rate and quality.

1. Waves on both sides and waves on one side

Reason: The incoming material has serious side waves, the roll curve is small during leveling, or the bending roll inclination is improperly adjusted, which will cause the strip to have waves on both sides or one side after leveling.

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

2. Middle waves

Reason: The incoming material has large middle waves or the roll convexity is too large during leveling, the positive bending roll force is too large, and the rolling leveling force is too small, which will cause the strip to have middle waves after leveling.

Solution: The wave in the middle of the incoming material can generally be eliminated by reasonable roller type and positive and negative bending roller adjustment.

3. Warping (lateral bending)

Cause: Warping refers to the convex upward or downward bulging of the middle of the strip. When cut into steel plates, the four corners are warped upward, which is called scooping. The cause is poor plate shape, thick in the middle and thin on both sides, too large convexity of the working roll, or wrong bending roll operation, causing the middle of the strip to extend greater than the two sides.

Solution: If the warping is large, it can be eliminated by reasonable roll type, correct bending roll operation, or re-leveling.

4. Oblique waves

Cause: When the working roll curve is too large, the rolling pressure (leveling pressure) is too large, or the roll expands due to the high incoming material temperature (leveling temperature requirement <= 45 degrees Celsius), the strip surface often produces oblique waves that are neither vertical nor parallel to the edge of the strip. Oblique wave defects are difficult to eliminate and often result in absolute waste.

Solution: Only by controlling the temperature of the leveled incoming material and using the roller type reasonably can this defect be eliminated.

5. Scratches during leveling

Cause: Scratches refer to the defects of scratches on the surface of the strip due to the mutual displacement between the layers of the steel coil. The reasons are, on the one hand, the looseness of the incoming material and the too-small coiling tension after rolling; on the other hand, the leveling tension is too large.

Solution: Reduce the leveling tension or leveling speed.

6. Roller marks and indentations

Cause: The surface of the working roller is partially peeled off or adhered with iron filings, and periodic surface marks (convex or concave) are produced on the surface of the steel plate after leveling. In addition, roller marks can also be caused by strip deviation, roller strangulation, and strip breakage.

Elimination method: Check the surface of the strip frequently (every roll) and replace the roller in time if problems are found.

7. Leveling flowers (mat patterns)

Cause: Leveling flowers refer to stripes printed on the surface of the strip like leaves or mat flowers. The reasons for their occurrence are uneven thickness or wavy strips, improper adjustment of the pressure when leveling thin specifications, strip deviation, etc.

Solution: The front and rear tension roller devices of the leveling machine can effectively prevent mat patterns. Controlling the tension and roller shape is also the main preventive measure.

8. Flat folding

Flat folding is a defect caused by annealing and reflected in flatness. It is a wrinkle-like bright stripe print, the length of the folding is not uniform, and it is felt by hand when it is seriously touched. During annealing, due to the high local pressure and high temperature, horizontal adhesion is caused. When flattening and unrolling, the adhesion is torn off and folding is produced. After flattening, it cannot be eliminated, leaving long wrinkled marks. It is a bonding defect. The reasons for this are as follows: the temperature of the steel coil before flattening is greater than 45°C. When the temperature is high, the metal molecules are more active. When the strip is subjected to tension, it is easy to jump from the elastic deformation zone to the plastic deformation zone, resulting in a slip zone on the surface of the strip, causing edge folding; the speed measuring roller, pressure roller, and anti-wrinkle roller of the flattening machine are improperly positioned, and their axes are parallel to the rollers, resulting in excessive local tension on the strip reaching the yield strength, especially on both sides of the strip. It is easy to produce 45° edge folding; the tension acting on the strip is unstable, resulting in instantaneous tension that is large or small, which makes it easy to produce edge folding.

Control measures: The incoming material temperature is controlled below 50°C. The equipment regularly checks whether the axis of each roller system is parallel to the roller to control the quality of the flattened raw materials (adhesion, flat coil, inner diameter damage).

9. Horizontal stripes

The surface of the strip has periodic lateral color differences, which can be seen after surface polishing. In severe cases, it can be seen without polishing and side lights. The width of the general stripes is between 10-30mm.

Cause: The roller has been used for too long, exceeding the roller replacement cycle; vibration occurs during rolling; roller grinding accuracy is poor.

Solution: Pay attention to strictly implementing the roller replacement system during production; regularly measure and adjust the gap between the arch and the roller system; improve the quality of the grinding roller.

10. Yellow film

Cause: The surface of the strip steel has a light yellow irregular color difference after wet leveling. It usually occurs in the middle of the surface of the strip steel and rarely occurs on 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 react chemically on the surface of the steel plate. It often occurs in autumn and winter and has a lot to do 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

There are yellow or brown, black spots, and blocky spots on the surface of the steel plate. The main reason is that there is water on the surface of the steel plate, which causes a micro-battery effect and produces an oxidation-reduction reaction.

The main factors affecting production on site are: environmental factors, air humidity or water around the steel coil; wet leveling liquid concentration is lower than the process requirements, or the leveling liquid is not purged cleanly; the intermediate product is stored for too long.

12. Scratches

Continuous intermittent or continuous scratches caused by excessive incoming material waves or equipment protrusions. 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 the leveled raw materials (flat coils, loose coils, inner diameter damage); for incoming material loose coils, tension should be reduced, production should be slow, and unwinding scratches should be avoided; check whether there are protrusions on the roller surface in contact with the steel plate and whether the equipment position on the upper and lower surfaces of the strip is correct. The equipment checks whether the roller system is synchronized.

13. Slip line

During the stamping process, the surface of the strip produces a diagonal pattern similar to orange peel, which is a Ludes strip.

The causes are insufficient flattening force; coarse grains caused by recrystallization in the critical area after hot rolling; coarse grains caused by excessively high annealing temperature; and grain boundary segregation affected by alloying elements.