What Is a Strip Mill?

Strip cold rolling is a production process when processing steel, using cold extrusion (not heating, otherwise called hot rolling). Cold-rolled strip and sheet generally have a thickness of 0.1 to 3 mm and a width of 100 to 2000 mm; both are made of hot-rolled strip or steel plate and rolled into cold rolled mills at room temperature.

Cold-rolled strip and sheet generally have a thickness of 0.1 to 3 mm and a width of 100 to 2000 mm; both are made of hot-rolled strip or steel plate and rolled into cold rolled mills at room temperature. Cold-rolled strip and sheet have the advantages of smooth and flat surface, high dimensional accuracy and good mechanical properties. Most of the products are coiled, and a large part is processed into

The process of cold-rolled strip is slightly different according to the variety and specifications of raw materials and products.

Pickling

A continuous pickling unit was used to remove the iron oxide scales on the surface of the hot rolled coil. The acid solution used sulfuric acid and hydrochloric acid. Before pickling, they are welded and rolled, and some are continuously black annealed. After pickling, it is washed, dried, trimmed and rolled.

Cold rolled

The strip after pickling is rolled to the thickness of the finished product on a cold rolling mill, and generally does not undergo intermediate annealing. The five-stand continuous rolling mill has a total cold rolling reduction rate of 60 to 80%, and mainly rolls automobile sheets, enamel sheets and galvanized sheets with a thickness of 0.3 to 3 mm. The six-stand continuous rolling mill mainly rolls tin-plated sheets with a thickness of 0.15 to 0.5 mm. The total cold rolling reduction is generally 70 to 90%. The rolling amount distribution of each stand (or pass) during rolling is comprehensively formulated according to the pressure, power and speed allowed by the rolling mill, taking into account factors such as output and quality.
3. Process lubrication

For lubrication and cooling. Generally, emulsions with good cooling properties are used, and pure oil lubricants with good lubrication properties are used when rolling thin or hard metals, such as rolling tin plate or stainless steel palm oil.
4. Annealing

The purpose is to eliminate the cold rolling work hardening, soften the recrystallization of the steel sheet, and have good plasticity. Annealing methods include coil furnace annealing and continuous furnace annealing. Coil annealing is divided into tight coil annealing and loose coil annealing; continuous furnace annealing is divided into vertical continuous furnace annealing and horizontal continuous furnace annealing. Protective gas is generally introduced into the furnace. Most of the furnaces are annealed. Although the processing cycle is long, due to the large number of furnaces, they are flexible to use and save investment. Continuous furnace annealing has a large output, of which horizontal continuous furnace annealing is only used to deal with special steels with low output, such as decarburization annealing of silicon steel.
5. Leveling

The purpose is to avoid the occurrence of annealed steel sheet during stamping.

Cold rolling is usually performed by longitudinal rolling. The cold rolling production process generally includes raw material preparation, pickling, rolling, degreasing, annealing (heat treatment), finishing, and the like. Cold rolling uses hot-rolled products as raw materials. The raw materials must be descaled before cold rolling to ensure that the surface of the cold-rolled products is clean. Rolling is the main process of deforming the material. The purpose of degreasing is to remove the lubricating grease attached to the rolling material during rolling, so as not to contaminate the surface of the steel during annealing, and to prevent carbon addition to stainless steel. Annealing includes intermediate annealing and heat treatment of the finished product. The intermediate annealing is to eliminate the work hardening caused by cold deformation through recrystallization to restore the plasticity of the material and reduce the deformation resistance of the metal. The purpose of the heat treatment of the finished product is to eliminate hardening by recrystallization. It is also based on the technical requirements of the product to obtain the required organization (such as various textures, etc.) and product properties (such as deep drawing, electromagnetic properties, etc.). Finishing includes inspection, cutting, straightening (leveling), printing, sorting and packaging. Cold rolled products have high packaging requirements to prevent the surface of the product from being scratched during transportation. In addition to the above processes, there are also special processes for the production of some special products. For example, when rolling a silicon steel sheet, decarburization annealing should be performed before cold rolling, and coating film, high temperature annealing, tensile straightening (see tension straightening), and tempering should be performed after rolling.
Rolling mills for cold-rolled strip include two-roll mills, four-roll mills and multi-roll mills. The most widely used are four-high rolling mills. Rolling thinner products requires a multi-roll mill. There are many types of multi-roll rolling mills, such as six-high rolling mills, partial eight-high rolling mills, twelve-high rolling mills, twenty-high rolling mills, etc. (see rolling mills). With the increase of the requirements for the shape, many technologies have been developed to improve the shape, such as bending roll technology, shifting roll technology and cross roll technology. Cold rolling strip rolling mills can be divided into two types: single-machine reversible or irreversible and multi-machine continuous according to the arrangement of stands. The former is suitable for the case of multiple varieties, small batches or a large proportion of alloy steel products. It has low investment and fast plant construction, but low output and large metal consumption. Multi-stand continuous rolling is suitable for the case where the product variety is relatively single or has little change. It has the advantages of high production efficiency and large output, but the investment is large ^[3]

Compared with hot-rolled strip (see production process of hot-rolled wide strip steel and hot-rolled narrow strip production), the rolling process of cold-rolled strip has the following characteristics:

(1) Use process lubrication and cooling to reduce deformation resistance during rolling and cool the rolls;

(2) High tension rolling is used to reduce the deformation resistance and maintain the stability of the rolling process. The average unit tension value used is 10% to 60% of the yield strength of the material, and generally does not exceed 50%;

(3) Multi-pass rolling is adopted. Cold rolling causes work hardening of the material. When the total deformation reaches 60% to 80%, it becomes difficult to continue to deform. To this end, intermediate annealing is performed, so that the rolling can continue after the material is softened. In order to obtain the required thin strip, such intermediate annealing may be performed multiple times. Rolling between two intermediate anneals is called a rolling pass. The annealing of the cold-rolled strip is performed in a continuous annealing furnace or a bell annealing furnace with a protective gas. The minimum thickness of cold-rolled strip can reach 0.05mm, and the cold-rolled foil can reach 0.001mm ^[4]

The main progress of cold strip rolling technology is as follows:
Increasing productivity is mainly to increase the speed of continuous rolling mills and the weight of single coils. The rolling speed has been increasing over the years. The weight of single coils has increased from 20 tons in the 1950s to 40-60 tons in the 1970s. Due to the adoption of high-efficiency circulating oil supply and some improvements in rolling mill equipment, in 1971, a group of six-stand cold continuous rolling mills in Japan achieved a rolling speed of 2500 m / min. In order to facilitate the control and guarantee the quality, the actual speed of the general rolling mill is still 1500 1800m / min. In the late 1960s, computer-controlled reduction procedures were set and rolling mill parameters were set and adjusted. Belt automation and tail removal, winding and unwinding, and roll changing were automated, which improved the mill's efficiency.

Improving product quality In the late 1960s, an automatic thickness control system (AGC) was generally installed. Most of the new rolling mills in the 1970s adopted hydraulic AGC with fast response speed and high accuracy, which reduced the tolerance of cold-rolled strip to ± 0.005mm. In plate shape control, hydraulic bending roll and roller shape adjustment devices are generally used, and various plate shape detectors and various roller shape adjustment devices have been developed to form an automatic plate shape control system.
The key to shortening the production cycle is to make each process continuous. In 1971 Japan established the first fully continuous rolling mill. In the early 1980s, a continuous pickling and full continuous rolling mill connection was established, and a continuous annealing line and a continuous rolling mill combined operation lines. In order to shorten the storage time of products between various processes, speed up the production cycle and achieve continuous process, systems such as computer planning, scheduling, equipment diagnosis and repair are being established. ^[4]