Rolling Mill Production Line: Cold Rolling Process and Characteristics of Forged Steel

Tuesday, June 7, 2022

Cold Rolling Process and Characteristics of Forged Steel

The so-called cold rolling of forged steel is not simply "cold rolling". Annealing, pickling, grinding, etc. are required before cold rolling, and leveling, straightening, shearing, stacking, etc. are required after cold rolling, so it is actually the whole process from the hot rolling coil to cold rolling finished products.

Due to the characteristics of cold rolling of forged steel and special requirements for product quality, the cold rolling production process has the following characteristics:

1. Cold rolling of forged steel is a kind of high alloy steel, which has great rolling deformation resistance. In order to carry out high-efficiency and high-precision rolling, the mill with high rigidity should be used, and the multi-roll cold rolling mill is generally used.

2. When the strip steel is cold rolled on the reversing mill, the head and tail parts wound on the coiler cannot be pressed down and are cut off as scrap. In order to change this situation and improve the yield, both ends of the strip steel should be welded before rolling; In addition, if the weight of the hot-rolled coil is too small, in order to improve the rolling efficiency and yield, the steel coil needs to be welded in advance; On the continuous annealing and pickling unit, the head and tail connections of the steel strip also need to be welded due to the continuous operation. Therefore, welding is an indispensable link in the cold rolling of forged steel. However, the cold rolling welding of forged steel is different from that of ordinary steel, which is much more difficult to weld; In particular, some welds need to be under pressure, and the requirements for welding quality are much stricter. Therefore, the special welding process is also a feature of the cold-rolled strip production of forged steel.

3. During the cold rolling production of forged steel, the raw materials (hot-rolled coil) shall be annealed, the intermediate annealing shall be conducted during the cold rolling process, and the final products shall be annealed. Therefore, annealing is an important link in production. However, there are many kinds of cold-rolled forged steel. The properties of various steels are different. The purpose, methods and requirements of heat treatment are different from those of general steel. There is a unique process system.

4. Cold rolling of forged steel is a kind of high-grade steel product. The requirements for surface quality are very strict. Not only metallurgical defects caused by the previous process, but also obvious defects caused by the cold rolling process are not allowed. Therefore, a series of elimination and protection measures have been taken in the production process. For example, in order to eliminate the hot-rolled scale, the hot-rolled strip steel should be shot-peened and pickled; In order to eliminate the defects caused by billets and after cold rolling and heat treatment, the strip steel often needs to be polished on the grinding unit; In order to ensure the surface quality after cold rolling, there are very strict requirements for roll grinding; In order to prevent scratches in the production process, the steel coils of each unit are required to be rolled tightly and neatly, and many units before and after cold rolling must pad process paper between the layers of the steel coils。

In addition, some special protective measures have been taken for operations and equipment that are prone to scratch; In order to obtain a good and uniform surface gloss, the finished product should be pickled after annealing, and the bright plate with special requirements should be annealed in the protective atmosphere; In order to protect the surface of finished products, some products have to be coated on the surface. In a word, the production of cold rolling of forged steel is an exquisite process, which is incomparable with other steel grades.

5. The finishing of cold rolling of forged steel is also different from that of ordinary steel. There are some special requirements. For example, the flatness process should not only improve the shape of the plate, but also use rollers with a high finish (except for hemp panels) to produce a smooth 2B steel plate. For ferritic steel, the formability of the steel plate should be improved by controlling the flattening reduction (or elongation), which is actually a kind of quenching and tempering rolling. Cold rolling of forged steel must be straightened with a strong straightener because the steel is hard and has strict requirements on flatness, and products of different thicknesses often need different straightening methods. Cold-rolled products of forged steel are usually delivered in two ways: coil forming and plate cutting, and some products need to be delivered in strips. Therefore, the modern cold rolling mill of forged steel should be equipped with longitudinal and transverse cutting units respectively.

5. The production of cold rolling stainless steel is a joint operation of multiple units. Process balance and process management are very important. A strong production scheduling system is necessary to ensure continuous high-quality production. In addition, the equipment of a cold-rolled stainless steel plant is relatively complex. There are many units and many high-quality, precision and advanced equipment, involving the technology of machinery, electricity, instrument automation, thermal engineering and other aspects. Doing well in equipment management is the fundamental guarantee to improving the production level and product quality.

Cold rolling process of forged steel

The difference between the production process of forged steel cold rolled strip and that of the general cold-rolled strip 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 any time in the production process, so as to improve the yield and corrosion resistance.

The production process of forged steel cold rolled strip is as follows: hot rolled strip (blank) - annealing, alkali and pickling - Inspection and grinding - cold rolling - annealing, alkali and pickling - leveling - polishing - shearing - Inspection and classification - Packaging - warehousing.

Ferritic and martensitic steels have a long annealing time to facilitate recrystallization and dissolution of carbides. It is usually annealed in a bell-type furnace at an annealing temperature of about 800 ℃ for 2 ~ 6h. Ferritic steel shall be rapidly cooled in air to prevent embrittlement. Rapid cooling of martensitic steel is not allowed to avoid excessive internal stress and hardening crack. Austenitic steel is heated at 1000 ~ 1100 ℃ in a continuous furnace and cooled rapidly in water or air.

The annealed strip steel shall be shot blasted to break the iron oxide scale on the surface, and then enter the pickling tank after brushing to completely remove the iron 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 ~ 55 ℃, and that of the sulfuric acid solution is 50 ~ 70 ℃.

The annealing and pickling of cold-rolled strips of forged steel are generally carried out in the annealing and pickling line.

After the pickling strip steel is inspected, the surface defects are repaired on the belt grinder, and then sent to the cold rolling mill for rolling. Cold rolling of forged steel is a kind of hard to deform steel, which is easy to produce work hardening during cold rolling, especially during multi-pass low compressibility rolling. Forged steel cold-rolled strip is generally rolled on four high mill and multi high mills, such as partial eight high mills (MKW mill) and twenty high mills. For austenitic steel that is easy to roll, the compression ratio of each pass shall not exceed 25%, and the total compression ratio of each rolling pass shall not exceed 75%. For martensitic steel with high carbon content, the compression ratio of each pass is 15%, and the total compression ratio of each rolling pass is not more than 50%.

After the completion of one rolling pass, it needs to be subject to intermediate annealing and pickling to eliminate cold rolling work hardening (recrystallization annealing). For austenitic steel, the heating temperature is 1050 ~ 1080 ℃. For Ferritic and martensitic steels, the heating temperature is about 800 ℃. Quenching in water, air or steam after heating. The iron scale produced during heat treatment is oxidized and loosened in the salt bath furnace to facilitate acid pickling.

After pickling, the surface of the strip steel shall be inspected and the defects shall be removed from the grinding machine. The finished cold rolled strip of forged steel is subject to bright annealing, that is, the last recrystallization annealing in a non-oxidizing atmosphere. Decomposed ammonia is usually used as protective gas.

After bright annealing, grinding and polishing are generally not carried out to maintain the obtained surface roughness of the strip steel. The purpose of flatness is to improve the surface quality of strip steel, improve the shape of strip steel and eliminate the yield platform. The compression ratio during leveling is generally not more than 2%. In order to obtain a very smooth strip surface, the roll should be polished frequently during the leveling process.

In order to improve the corrosion resistance of forged steel cold rolled strip, some special kinds of forged steel cold rolled strips must be ground and polished. Generally, wet grinding (oil and emulsion) is used to prevent burning spots or cracks caused by the poor thermal conductivity of cold-rolled forged steel. The polishing process consists of polishing and wiping. Generally, the emulsion is used as a polishing agent. In order to prevent the surface of the strip steel from being scratched, the surface of all rollers in contact with the strip steel in the whole process must be very smooth, or the rubber-coated roller shall be used. During coiling, the paper tape must be padded between each layer. For the collection of finished products, the paper shall be padded between each layer to protect the surface from mutual scratch.

If you are interested in cold rolling mill machine or relevant machine, please mail at marketing2@hanrm.com or kerrirollingmill@gmail.com.

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