Common defects and control measures of pickled boards

1. Overview of pickling products
Pickled strip is an intermediate product that uses hot-rolled steel coils as raw materials, and after pickling, the surface quality and usage requirements are between hot-rolled and cold-rolled sheets. Compared with hot-rolled plates, the advantages of pickled plates mainly lie in: good surface quality, high dimensional accuracy, improved surface smoothness, enhanced appearance effect, and reduced environmental pollution caused by dispersed pickling by users. In addition, compared to hot-rolled products, pickled products are easier to weld due to the removal of surface oxide scales, which is also conducive to surface treatment such as oiling and painting.
The surface quality level of hot-rolled products is generally FA, pickled products are FB, and cold rolled products are FB/FC/FD. Pickled products can replace cold rolled products to make some structural parts, that is, using heat instead of cold.
2. Common defects of pickled boards
The common defects in the production process of pickled boards include: iron oxide skin pressing, oxygen spots (surface landscape painting), waist folding (horizontal folding printing), scratches, yellow spots, under pickling, over pickling, etc.
(Note: Defects are linked to standard or protocol requirements, and those that do not comply are called defects. For the convenience of expression, defects are used to replace a certain type of morphology.)
2.1 Iron oxide sheet pressing
Iron oxide sheet pressing is a surface defect formed during the hot rolling process. After acid washing, it often appears as black dots or long strips pressed in, with a rough surface, usually with a touch, and sporadic or dense appearance.
The reasons for the formation of iron oxide scale are related to various factors, mainly including heating in the heating furnace, descaling process, rolling process, roller material and status, roller track status, and rolling plan.
Control measures: Optimize the heating process, increase the number of descaling passes, regularly inspect and maintain the roller table and rollers, and keep the rolling line in good condition.
2.2 Oxygen spots (surface landscape painting defects)
Oxygen spot defect refers to the point, line, or pit shaped morphology left after the surface of the hot rolled iron oxide is washed off, which visually shows irregular color difference spots. Due to its shape similar to landscape painting, it is also called a landscape painting defect. Visually, it appears as a dark pattern of undulating mountain peaks, with overall or local distribution on the steel plate surface. Essentially, it is an oxide scale stain, which is a layer of material floating on the surface without a touch and may have a darker or lighter color. The dark areas are relatively rough, and after electrophoresis, they have a certain impact on the appearance, but do not affect the usability.
Reason for oxygen spot (landscape painting defect): The essence of this defect is that the oxide scale on the surface of the hot rolled strip steel is not removed completely, and is then pressed into the substrate after subsequent rolling, which highlights after acid washing.
Control measures for oxygen spots: reduce the tapping temperature of the heating furnace, increase the number of rough rolling descaling passes, and optimize the cooling water process for precision rolling.
2.3 Waist Fold
Waist folding is a transverse wrinkling, bending, or rheological zone perpendicular to the rolling direction. When opening the roll, it can be identified with the naked eye, and in severe cases, it can be felt by touching with the hand.
The reason for waist bending: Low carbon aluminum killed steel has an inherent yield platform. When the steel coil is uncoiled, under the action of bending stress, yield deformation effect occurs, causing the originally uniform bending to become uneven bending, resulting in the appearance of waist bending morphology.
2.4 Macules
Yellow spots appear locally or on the entire surface of the strip steel, which cannot be covered after applying oil, affecting the quality and appearance of the product.
The reason for the occurrence of yellow spots: the surface activity of the strip steel that has just emerged from the pickling tank is high, and the rinsing water has not played a normal role in washing the strip steel, resulting in oxidation and yellowing of the strip steel surface; The spray beam and nozzle of the rinsing tank are blocked, with unequal angles.
The control measures for macula include: regularly checking the condition of the spray beam and nozzle, and regularly cleaning the nozzle; Ensure the pressure of rinsing water, etc.
2.5 Scratches
The surface has a certain depth of scratches and irregular shapes, which affect the surface quality of the product.
Reason for scratches: Improper tension of the loop; Wear of nylon lining plate; Poor shape of incoming plate; Loose inner ring of hot coil, etc.
Control measures for scratches: 1) Increase the tension of the loop appropriately; 2) Regularly inspect the surface condition of the lining plate and replace the lining plate with abnormal surface condition in a timely manner. 3) Repair the steel coils with poor incoming plate shape and loose inner ring.
2.6 Underpickling
The so-called underpickling refers to the incomplete and incomplete removal of local oxide scales on the surface of the strip steel, resulting in a grayish black surface with fish scales or horizontal water ripple like stains.
The cause of underpickling is related to the process of acid solution and the surface condition of the steel plate. In terms of production process, there are mainly factors such as insufficient acid concentration, low temperature, too fast strip steel operation speed, inability of strip steel to immerse in acid solution, uneven thickness of hot rolled iron oxide scale, and wave shaped steel coils. In general, the location of under pickling is often prone to occur at the head, tail, and edges of the strip steel.
Control measures for insufficient pickling: adjust the pickling process, optimize the hot rolling process, control the shape of the strip steel plate, and establish a reasonable process system.
2.7 Overpickling
Excessive pickling refers to excessive pickling, and the surface with steel plates often turns dark black or brownish black, presenting blocky, flaky black or yellow spots, and the surface is generally rough.
Reason for over pickling: Contrary to under pickling, if the acid concentration is relatively high, the temperature is relatively high, and the conveyor speed is relatively slow, it is easy to cause over pickling. The parts that have been pickled should be more likely to appear in the middle and width of the strip steel.
Control measures for excessive pickling: adjust and optimize the pickling process, establish appropriate process systems, and carry out quality training to improve quality management level.
3. Understanding of Quality Management for Pickled Plate and Strip
Compared to hot-rolled plates and strips, pickling plates and strips only have an additional pickling process, and it is generally believed that producing high-quality pickling plates and strips should be easier. However, practice has shown that to ensure the quality of pickling products, not only is the pickling line in good condition, but also the production and operation status of the previous processes (steelmaking and hot rolling) should be kept stable, so as to ensure the quality of hot rolled incoming materials. Therefore, it is necessary to adhere to a consistent quality management method and ensure that the quality of each process is in a normal state in order to ensure the quality of the final product.