External quality of cold rolled and galvanized products
One of the main reasons why cold-rolled sheet is a high-quality product is the strict requirements for surface quality of cold-rolled sheet. It can be said that in the entire metallurgical industry, cold-rolled sheet has the highest and most stringent requirements for surface quality, especially wide and thin cold-rolled steel strip products and products with strict requirements on stamping performance. This is also a requirement for downstream processes such as painting, coating, and stamping. For example, Baosteel has successfully developed O3 and O5 plates after several rounds of research and development, in order to supply cold-rolled products with high-level surface quality requirements to automobile manufacturers. Generally speaking, the surface quality of cold-rolled products is divided into three levels: ordinary surface quality, higher surface quality and advanced surface quality according to surface defects. The specific definitions are stipulated in relevant standards. The following table lists the types of surface defects that may occur in cold-rolled products and the possible processes and causes. Of course, the listed defects may not be complete, and the causes and processes may not be completely correct. This needs to be in actual production in the future. Gradually supplement and improve.
Surface Defects of Cold Rolled Steel Sheet and Strip
- Defects that are not allowed in steel plates and steel belts
1. Bubbles Steelmaking Bubbles are generated during steelmaking, and they are not welded during hot rolling. They are exposed to the outside after pickling and cold rolling.
2. Cracks Steelmaking, hot rolling and cold rolling and various processing procedures are caused by steelmaking thermal stress, rolling deformation or stress concentration caused by processing
3. Scars or nodules Pickling and cold rolling Pickling has not washed the oxide scale, and it is inlaid on the surface to form scars during rolling
4. Tensile cracking, cold rolling, galvanizing and leveling, caused by excessive tension, excessive tension fluctuations, and unstable tension
5. Inclusions Steelmaking Reasons for steelmaking
6. Folding Hot rolling, cold rolling Metal that is viscous and flowing during rolling is re-rolled and then embedded on the surface of the sheet
7. Delamination steelmaking, hot rolling and cold rolling
8. Black film or black belt Pickling after pickling, the drying effect is not good.
9. Emulsion spots Cold rolling and smoothing The emulsion remains on the surface of the steel strip.
10. Corrugation and folding, pickling, pickling, etc. (to be checked)
11. Barbs or burrs Cutting process The cutting blade is not sharp, the upper and lower cutting blades are staggered at a large angle, the cutting blade angle is not accurate, etc.
- Defects that are allowed to exist and are classified into different surface quality grades according to their different degrees
1. Pockmarks are caused by the adhesion of the plastic matrix metal to the surface of the high-speed rotating roll during cold rolling, smoothing and leveling rolling
2. Scratches, various processes, handling and hoisting processes, etc.
3. Scuffing, handling and hoisting process
4. Blue oxidized color Cold rolling and smoothing. The base metal heats up due to the rolling friction, which causes the base to turn blue, especially the strip edges are more likely to form this defect
5. Light yellow pickling color, pickling, caused by not drying after pickling
6. Roll indentation Cold rolling, smoothing and leveling Reasons for rolls
7. Scratching, handling and hoisting process
8. pits cold rolling pits are formed due to rolls and surface inclusions being rolled out of the surface of the substrate.
- Other surface quality defects
1. Bonding Cover annealing is caused by the reduction of residual iron powder on the surface of the steel coil to iron due to long-term heating in a full hydrogen atmosphere. In addition, bonding is also related to the coiling tension and cooling speed.
2. Surface carbon black cover annealing In a high-temperature atmosphere of full hydrogen, the residual rolling oil on the steel surface decomposes to form carbon black and deposits on the surface of the steel coil
3. Rust and corrosion Steel coil storage and transportation process. The quality of anti-rust oil is not good or the anti-rust oil is not coated or the amount of oil is insufficient, or it is caused by high humidity in the storage environment.
4. Under-pickling, pickling, showing that there is still iron oxide scale that has not been washed off
5. After pickling, pickling, it shows that clear rolling lines can be seen on the surface of the substrate