Common defects of pickled coils and their solutions
I. Defects of Pickled Plates
1.1 Raw Material Defects
1.1.1 The roll diameter of the raw material is inappropriate and too small.
Especially for thin-sized hot-rolled coils. Mainly due to the thinness of the steel strip, during the cooling process of the hot-rolled coil off the production line, the inner ring shrinks, becomes loose and the head droops. This leads to a reduction in the inner diameter. This issue has affected the first volume to a certain extent. The treatment method is to remove the inner ring manually by gas cutting.
1.1.2 The edges of the raw materials are uneven, heavy, cracked and scratched. The problem is more serious within a 10-meter range of the head. The main cause is that during the winding process, the edges are uneven due to the equipment and side guide plates. During the hoisting and transportation process, if the hoisting tools are used improperly, the edges will be scratched. Defects such as edge cracks and heavy piles caused by severe scratches will also occur accordingly. Treatment methods:
(1) Adjust the equipment. Keep its working state within the design range;
(2) Modify the hoisting tools.
1.1.3 Sickle Bend
The zinc knife of the raw material is severely bent, the plate shape is poor, the production deviation is serious, the production speed cannot be improved, and it has a great impact on production. The main cause of its occurrence:
(1) The hot-rolled slabs are wedge-shaped and the thickness of the slabs is uneven. Deviation from the rolling centerline leads to uneven deformation.
(2) The actual roller gap values are different. The steel strip on the side with a smaller roll gap value deforms more. It has a large extension.
Treatment method:
(1) The side guide plates of the rolls are stably set.
(2) Adjust the tension.
1.1.4 There are still some other defects in the raw materials, such as bubbles, longitudinal bends, scabs, inclusions, etc. This kind of defect has existed since the slab and is relatively rare.
1.1.5 Severe edge waves at the tail of raw materials. It is about 80 meters long. The main reason is:
(1) During hot rolling, if the rolling force is too large, the rolls will be overly bent, and the gap value at the middle part will be smaller than that in the middle. As a result, the edge of the steel strip will deform significantly, causing edge waves. Moreover, the head of the hot-rolled coil is in an unstable rolling stage, and there is a cooling section of approximately 70 meters from the last rolling mill to the coiling. The steel strip has no tension, making this defect even more obvious.
(2) If one sector block of the coiler is worn more severely than the others, resulting in a smaller tension in that section of the steel strip, edge waves will occur. When this situation occurs, the forming block should be replaced in time to eliminate the edge waves it generates.
1.1.6 Width issue: There is a situation where hot-rolled coils are too wide at both ends and too narrow in the middle. The main reason for this is that the tension at the head and tail is too small during rolling. As the tension becomes too high during acceleration, setting an appropriate tension value can improve this situation.
1.2 Defects of the rubber-washed board
After pickling, water rinsing and dry bath, the surface of the steel strip should be grayish-white or silver-white. However, if a problem occurs in a certain link of production, it will lead to certain defects in the pickled plate, thereby reducing the yield and qualification rate and affecting production. The common defects of pickled plates are summarized as follows.
1.2.1 Pickling defects
(1) Insufficient pickling
After pickling of the steel strip, there are some unwashed iron oxide scales remaining on the surface. After rolling, the surface appears dark or mottled. In severe cases, the iron oxide scale pressing on a person can cause black spots. After rolling, defects such as wavy shapes or spring curves occur due to uneven elongation. Sometimes, the iron oxide scale can firmly adhere to the surface of the rolls, resulting in roll marks and the like.
The main causes are:
a) The pickling time is short, and the reaction between the iron oxide and the acid has not yet been completed.
b) When the concentration of the acid solution is low, the reaction rate between iron oxides and the acid decreases, resulting in incomplete reactions and the presence of residual iron oxide scale.
c) The temperature of the acid solution is low, and the reaction rate is slow;
d) The acid solution injection is minimal and the acid solution volume is insufficient;
e) The turbulent effect is not good.
Treatment method: Ensure that the concentration, temperature and spray of the acid solution are within the set range, and enhance the fluidity of the acid solution. Controlling the process speed means ensuring the acid washing time to make the reaction complete. It can eliminate under-acid washing.
Picture
Figure 1 Picture of under-pickling
(2) Over-acid washing
If the steel strip stays in the pickling solution for too long, its surface gradually turns rough and pitted. The ductility of the steel strip after pickling is greatly reduced, making it prone to breakage and fragmentation during rolling, and it is also likely to cause deviation and dry rolling.
There are multiple reasons for its occurrence:
a) The pickling time is long. The reaction between the iron hydride and the acid is completed, and then the body is corroded.
b) When the concentration of the acid solution is high, the reaction rate between iron oxides and the acid increases. After the reaction is fully completed, the steel strip has not yet left the acid tank and thus corrodes the substrate.
c) The temperature of the acid solution is high, and the reaction rate is fast;
d) The acid injection volume is large and the acid supply is sufficient;
e) High turbulence, etc.
Treatment method:
a) Reduce the pickling concentration;
b) Appropriately reduce the temperature of the acid solution;
c) The injection volume of the small acid solution;
d) Add an appropriate amount of corrosion inhibitor. The corrosion inhibitor forms a protective film on the substrate to protect it from corrosion.
Picture
Figure 2 pickling picture
(3) Water rust
After pickling of steel strips, if there is residual acid on the surface that has not been thoroughly rinsed off or if it is not dried well after rinsing, especially at the edges, water rust is prone to occur. After rolling, the rusted areas may darken, or the finished products may rust again as a result in storage. The main causes are as follows: insufficient rinsing, inadequate injection pressure and flow, insufficient water supply, resulting in the concentrations of Fe 2+ and CI- in the water after rinsing exceeding the set range.
In this situation, several aspects usually need to be checked
a) Inspect the squeezing roller and determine whether it needs to be replaced based on the degree of wear.
b) Whether the jet pressure in the water rinsing section is normal;
c) Check whether the temperature of the dryer is at the set value;
d) Check the conductivity of the rinsing water and add supplementary water.
(4) Parking spots
The vehicle stopped for some reason. Cause a section of steel strip to be immersed in acid-neutralizing water. This causes the board surface to undergo acid washing and rust again. This defect is caused by shutdown. Before starting up, we must ensure that all equipment is in good operating condition and try to avoid or minimize shutdown as much as possible.
1.2.2 Surface defects
(1) Double skin
The local delamination that appears on the surface of the steel strip mostly occurs at the head or edge of the steel strip and forms a continuous area along the rolling direction. Most of them show delamination after cold rolling. In severe cases, it can cause leaks, broken belts and dry rollers.
(2) Scarring
Heavy parts of surfaces with different shapes and degrees. Inclusions or iron oxide scale often present in scars.
(3) Scratches
New scratches that occur during the operation of the steel strip pickling unit: The depth of the scratch shall not exceed 50% of the thickness deviation, and there shall be no obvious hand feel.
There are mainly two types of scratches: a) Regular scratches; b) Random scratches.
For regular scratches, the first step is to determine the distance between the scratches to identify the section where the scratch occurred. Confirm again and eliminate. For irregular scratches. The scratches are mainly caused by the unstable operation of the steel strip, unstable tension or poor plate shape.
The methods to eliminate this defect include: a) ensuring the normal operation of the equipment; b) Improve the shape of the board.
1.2.3 Edge defects
During the production process, due to the edge cutting shears and other equipment, many defects occur at the edges, such as uneven cutting, notches, uncut waste edges, burrs, etc. The main reasons for its occurrence are:
If the set value of the tool clearance is too large, the edges are prone to unevenness and burrs.
(2) The knife is heavy and its edges are curved and uneven.
(3) If the blade breaks, there will be periodic "flesh" on the edge with the key.
(4) Excessive wear of the cutting tool. Burrs are prone to appear on the edges.
This requires frequent inspection of the knife gap and weight. Whether the set values are appropriate and whether there is excessive wear or chipping of the cutting tool. When problems are found, the tool should be replaced in a timely manner or the tool clearance and weight should be adjusted according to the situation of the defects that have occurred.
1.2.4 Shape defects
Due to the reasons of the coiler and other equipment, tower shape defects sometimes occur. When the single-side tower shape exceeds 30mm, it is regarded as scrap. The main causes are as follows:
(1) The strokes of the upper and lower cylinders of the belt coiler are different, resulting in different forces on the belt coiler to wrap the core shaft. When the steel belt enters the core shaft for coiling, the tightness of the steel belt coiling is also different. On the side with a larger stroke of the cylinder, the steel belt is coiled tighter, while on the side, it is relatively looser. The loose side has a larger roll diameter and is prone to form a tower shape when rolled up.
(2) There is a problem with the threading program. When threading automatically. When the original design allows the belt to be wound up for two and a half turns, the low-level proximity switch sends a signal to open the belt helper. At this time, the tension has not yet been established. When the belt helper opens, the two and a half turns that have already been wound will become loose.
Treatment method:
(a) Level the belt coiler to ensure uniform wrapping force between the belt and the core shaft;
(b) When using automatic threading, change the original design of the steel strip coil from two and a half turns to four turns (plate thickness 3.0-4.0mm) to five turns (plate thickness 2.0-3.0mm).
(c) The belt of the original designed belt coiler was a bit narrow. It was originally designed to be 400mm, but now it has been widened to 500mm, making the coiling function more distinct.
(d) Strengthen the inspection of equipment to ensure its stable operation.
Ii. Quality Management
2.1 Strengthen quality management
A good management system and effective management methods are of vital importance to an enterprise. It can be said that they are related to the survival and development of an enterprise. In light of the specific circumstances of the workshop, we have established detailed rules and regulations, which are implemented layer by layer. As soon as a problem is identified, it will be solved without exception, and no problem will be overlooked.
2.2 Enhance the skill levels of employees
Highly automated equipment. High-quality products can only be produced when there are highly skilled employees to operate. The factory and workshops attach great importance to the improvement of employees' skills. They often organize learning sessions, covering both theory and practice, and invite experienced and knowledgeable senior masters to explain the common problems that occur in production and how to handle them.
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