Basic knowledge of hot rolled strip

Generally, their specifications are written as (Thickness*Width*C, C means length. Since it is usually rolled up, it is expressed by C. If it is a rectangular steel, it will have a length). For example, we have seen Ruifeng strip steel 2.5*315*C , WISCO hot coil 11.5*1500*C, where 315 and 1500 represent the widths of 315mm and 1500mm. From this, we believe that the difference between strip steel and hot coil is that the strip width is below 1000mm, so Ruifeng’s strips can be distinguished at a glance through the specifications. As for hot coils of steel and Wuhan Iron and Steel Co., as to why some people use strip steel and others use hot coils, it depends on what width their rolling mills can roll, and the requirements are also different. As for the Q235 mentioned downstairs, it means that the steel can withstand a pressure of 235 MPa without deformation. It is used to indicate the material of the steel. Similarly, Q195 naturally does not deform under a pressure of 195 MPa. hot rolled strip
2.1 Compact layout of hot strip rolling mill

Hot strip hot strip rolling mills mainly have three layout types: fully continuous, semi-continuous, and 3/4 continuous. Their differences are concentrated in the rough rolling area. The main feature of the fully continuous hot strip rolling mill is that the rolling mills are all irreversible rolling mills. When the strip is rolled in the rough rolling zone, each rolling mill only rolls one line in the forward direction of the slab and does not form continuous rolling; semi-continuous The main point of the hot strip hot continuous rolling mill is that there is at least one reversible rolling machine. The strip steel is rolled in the rough rolling area by reversible rolling and is pressed in multiple passes. There is no continuous rolling in the rough rolling unit; 3/ 4 The main feature of the continuous hot strip rolling mill is that the strip adopts reversible rolling in some rolling mills in the rough rolling area, and continuous rolling is formed in the last two rough rolling mills. The fully continuous hot strip rolling mill has a high degree of automation and high output, but it has a lot of equipment, a large investment, and a long rolling process. Therefore, the heat loss of the rolled piece is too much, which is not conducive to heat preservation and heat-rush rolling. Especially during the production process, since each rolling mill only rolls once, the rough rolling mill is idle most of the time, so the equipment utilization rate is too low. For this reason, semi-continuous and 3/4 continuous hot strip rolling mills are widely used to save equipment investment, improve the utilization rate of the rough rolling mill, shorten the length of the rolling line, and reduce the heat loss of the rolled products. In recent years, due to the improvement of the rough rolling mill control level and the improvement of the rolling mill structure, the strength of the rolling mill arch has increased, the rolling speed has also increased relatively, the single-stand production capacity of the rough rolling mill has increased, and the output of the rolling mill is no longer restricted by the output of the rough rolling mill. Therefore, , the semi-continuous roughing mill developed rapidly. However, due to the complex operation and maintenance of reversing rolling mills and high energy consumption, for strip mills with an annual output of more than 3 million tons, 3/4 continuous hot strip rolling mills have become the mainstream. 2.2 AWC vertical roller short stroke control

The slab is alternately rolled by vertical rollers and horizontal rollers in the roughing mill, and the width is controlled by the side pressure of the vertical rollers. However, compared with the middle part of the rolled piece, the head and tail have no outer end restriction, so in the vertical rolling pass There will be craniocaudal width. If the width loss phenomenon at the head and tail is not effectively controlled, the width accuracy and yield will be adversely affected. The heavy-duty vertical roller mill with AWC (automatic width control) function is a modern rolling mill produced to adapt to the development of continuous casting and to facilitate the hot charging of hot-rolled strip steel slabs. This type of vertical roller mill has an advanced structure, high power of the main drive motor, large side pressure capacity, and the ability to adjust, control, and control the head and tail shape of the strip during the rolling process, which can not only reduce the width specifications of the continuous casting slab , and is conducive to realizing hot charging of hot-rolled steel strip slabs, improving strip width accuracy and reducing cutting losses. ​

According to different control methods, AWC is divided into: rolling force feedback control (RF-AWC), feed-forward control (FF-AWC) and short-stroke control (SS-AWC). ​

Rolling force feedback control (RF-AWC) is based on the principle that the different hardness of the material along the length of the slab during side pressure will cause different bounce amounts in the vertical roll mill, resulting in changes in the rolling force. The measured rolling force is Change, the hydraulic AWC device quickly changes the roll gap, thereby changing the rolling pressure, keeping the slab width constant, so that the width of the horizontally rolled slab is uniform in the length direction. ​

Feedforward control (FF-AWC) is aimed at the watermark where the temperature is lower than other parts due to the influence of the water-cooled slide when the slab is heated in the heating furnace. When horizontal rolling is carried out after the vertical roller side pressure, the material width at the watermark is larger than other parts. Part of the material is widened, resulting in a width difference in the length direction. The watermark is tracked during lateral pressure. The hydraulic AWC is preset to increase the lateral pressure at the watermark to eliminate the excess width generated at the watermark and make the level The rolled slab reaches the set width value. ​

Short stroke control (SS-AWC) solves the problem that when the lateral pressure of the slab is large, the metal tends to flow to the middle or two corners, causing the head and tail of the slab to become wider. In addition, the side pressing edges of the slab are convex, small at both ends and large in the middle. After horizontal rolling, the head and tail widths are increased. The hydraulic AWC device is used to control the head and tail of the slab in a short stroke to adjust the amount of side pressure. , so that the head and tail of the slab tend to be rectangular after horizontal rolling, so that the width of the entire slab in the length direction is uniform, there is less head and tail cutting loss, and the product yield is improved. 2.3 Intermediate billet insulation technology and edge induction heating technology

The strip length at the outlet of the rough rolling mill can reach 80~90m. During the rolling process of the finishing mill, in order to reduce the temperature drop on the conveyor roller table and save energy consumption, in recent years many factories have also installed thermal insulation covers or compensation on the conveyor roller table. Heating furnace (device). The inner surface of the insulation cover is attached with a special alloy layer that absorbs heat quickly, has a high temperature rise and has high heat reflectivity, which effectively increases the temperature of the intermediate billet entering the finishing rolling, thus reducing the billet exit temperature from the heating furnace, increasing the yield rate and saving fuel consumption. . It can also increase the temperature at the end of the strip, reduce the temperature difference between the head and the tail of the strip, make the temperature of the strip more uniform, and roll out wider, thinner, heavier, and higher-precision, performance-quality coils. ​

During the rolling process of the strip, the temperature drop at the edges is greater than the temperature drop at the middle due to faster heat dissipation, and the temperature difference is about 100°C. The temperature difference between the edges is large, the grain structure is uneven on the cross section of the strip, and the properties vary greatly. At the same time, it will also cause edge cracks and serious uneven wear on the rolls during rolling. Therefore, the edges of the strip are heated in front of the finishing rolling mill to compensate the temperature to be consistent with the temperature in the middle. Generally, electromagnetic induction heaters are used to increase the temperature of the edge of the strip by 30 to 50°C, making the transverse temperature of the strip more uniform, thus reducing cracks on the edge of the strip, so as to adapt to rolling thin products and silicon steel, stainless steel, high carbon Steel and other special types of steel.