Forged steel cold rolling process

The difference between the production process of forged steel cold-rolled strip and the general cold-rolled strip production process is that the forged steel cold-rolled strip must be annealed before cold rolling and the surface of the strip must be kept clean at all times during the production process to improve the finished product Rate and corrosion resistance.

The production process of forged steel cold-rolled strip is roughly as follows: hot-rolled strip (blank)-annealing, alkali, pickling-inspection and grinding-cold rolling-annealing, alkali, pickling-smoothing-polishing-shearing-inspection classification One package, one warehousing.

　　 Ferritic steel and martensitic steel have a longer annealing time, in order to facilitate recrystallization and dissolution of carbides. Usually annealing in a bell furnace, the annealing temperature is about 800 ℃, and the holding time is 2 to 6 hours. Ferritic steel should be rapidly cooled in the air to prevent embrittlement. Martensitic steel is not allowed to cool quickly, so as not to cause excessive internal stress and hardening cracks. The austenitic steel is heated in a continuous furnace at a temperature of 1000 to 1100°C, and is rapidly cooled in water or air.

　　The annealed strip steel is first shot blasted to break the surface oxide scale, and then enter the pickling tank after brushing to completely remove the oxide scale and passivate the surface. Generally, nitric acid or sulfuric acid is used for pickling. The temperature of the nitric acid solution is 20 to 55°C, and the temperature of the sulfuric acid solution is 50 to 70°C.

　　 Annealing and pickling of cold rolled forged steel strips are generally carried out in an annealing and pickling line.

　　After the pickled strip steel is inspected, the surface defects are repaired on the belt grinding machine, and then sent to the cold rolling mill for rolling. Forged steel cold rolling is a difficult-to-deform steel, and it is easy to produce work hardening during cold rolling, especially during multi-pass low reduction rolling. Forged steel cold-rolled strips are generally rolled on four-high rolling mills and multi-high rolling mills, such as offset eight-high rolling mills (MKW rolling mills) and 20-high rolling mills. For austenitic steels that are easier to roll, the compression ratio per pass does not exceed 25%, and the total compression ratio per rolling pass does not exceed 75%. For martensitic steels with higher carbon content, the compression ratio per pass is 15%, and the total compression ratio per rolling pass is not more than 50%.

　　After one rolling pass is completed, intermediate annealing and pickling are required to eliminate cold-rolling work hardening (recrystallization annealing). For austenitic steel, the heating temperature is 1050～1080°C. For ferritic and martensitic steels, the heating temperature is about 800°C. After heating, it is quenched in water, air or steam. The iron sheet produced during the heat treatment is oxidized and loosened in the salt bath furnace to facilitate pickling.

　　After pickling, the surface of the strip steel should be inspected, and the defects should be removed on the grinding machine. The finished cold-rolled forged steel strip is bright annealed, that is, the final recrystallization annealing is performed in a non-oxidizing atmosphere. Usually decomposed ammonia is used as a protective gas.

　　 After bright annealing, grinding and polishing are generally not carried out to maintain the surface roughness of the strip steel obtained. The purpose of leveling is to improve the surface quality of the strip, while also improving the shape of the strip and eliminating the yield platform. The compression rate during flattening is generally not more than 2%. In order to obtain an extremely smooth surface of the strip, the rolls should be polished frequently during the leveling process.

　　In order to improve the corrosion resistance of forged steel cold-rolled strips, grinding and polishing are required for some special types of forged steel cold-rolled strips. Generally, wet grinding (oil and emulsion) is used, and the burn spots or cracks caused by the poor thermal conductivity of forged steel cold rolling should be prevented during grinding. The polishing process consists of two parts: polishing and wiping. Generally, emulsion is used as polishing agent. In order to prevent the surface of the strip from being scratched, the surface of all rollers in contact with the strip must be very smooth during the entire process, or rubber-coated rollers must be used. When reeling, paper tape must be placed between each layer. The collection of finished products requires paper between each layer to protect the surface from scratching each other.