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The hot strip production has four ways according to the hot strip width and production process: wide strip steel hot continuous rolling, wide strip steel reversible hot rolling, narrow strip hot continuous rolling, hot rolling with planetary rolling mill. Strip steel materials include carbon steel, low-alloy steel, stainless steel and silicon steel. The final product mainly uses as a raw material for cold-rolled strip, welded pipe, cold-formed and welded section steel or to make various components.
The hot rolled strip production process mainly includes: billet preparation, heating, descaling, rough rolling, cutting end, finishing rolling, cooling, winding, finishing.
BILLET PREPARATION: The billet of hot-rolled strip steel is generally continuous casting billet. Its chemical composition, dimensional tolerance, bending and end shape should meet the requirements. The cold-loaded billet should be checked. The hot-charged billet should be free of defects, that is, no visible defects on the surface, and no shrinkage, porosity, segregation inside.
BILLET HEATING: Billet heating mainly controls heating temperature, time, speed and temperature schedule (including preheating, heating and heat-balance section temperature), preventing overheating, over-burning, oxidation, de-carbonization or sticking steel.
DESCALING: The descaling devices includes flat roll descaling machine, vertical roll descaling machine and high pressure water descaling box. It is widely used to remove the oxidized iron sheet with high pressure water (10 ~ 15MPa) after vertical roll edging.
ROUGH ROLLING: The rough rolling is to compress and extend the blank, provide the blank with the required size and shape for the finishing rolling. In the roughing process, the amount and speed of each pass should be controlled according to the setting. The workpiece temperature from the roughing mill should be increased as much as possible. The workpiece thickness and width should be guaranteed. In order to shorten the distance between the mill stand, the last two roughing mill units are arranged in a continuous manner.
CUTTING ENDS: The cutting head is to cut off the workpiece head and end, suitable for the finishing mill biting and the coiler pick-up.
FISHING ROLLING: Finishing rolling is carried out according to the reduction amount of each mill, rolling temperature and rolling speed in the rolling schedule. It is generally controlled by equal flow per second or constant tension mode. The thickness is controlled by hydraulic or electric AGC. The temperature control of the rolling process includes the final rolling temperature and the temperature difference between the head and the end. In order to control plate shape, roll shape and pre-bending roll device are used to ensure the transverse thickness difference of the steel strip.
REELING UP: The steel strip temperature after finishing rolling is 900 ~ 950℃. It must be cooled to 600 ~ 650℃ in a few seconds for reeling up. Generally, laminar cooling and water curtain cooling are used to cooling the steel strip. Regarding laminar cooling with low water pressure and large water, the water quantity can be automatically adjusted according to the strip thickness and the final rolling temperature. The strip steel cooled by water curtain can get uniform transverse cooling at the fast cooling speed and high cooling capacity.
In order to ensure the microstructure and properties of hot rolled strip steel to meet the requirements, it must be coiled at a lower temperature and a higher speed after rolling, generally at 500~650℃.The grain will grow if the coiling temperature is high.
FINISHING: Finishing is to unroll the hot rolled steel strip, after cutting the head and edge, cutting to the length and leveling, then packed in rolls or packed in a single sheet for delivery.
2. Technical requirements for the hot rolled strip mainly includes dimensional accuracy, plate shape, surface quality and performance.
(1)High dimensional accuracy: Dimensional accuracy is one of the important technical indexes of hot rolled strip steel, which directly affects the service performance and processing performance of strip steel. Dimensional accuracy mainly includes length, width and thickness accuracy. The thickness accuracy control is especially important.
(2)The plate shape should be good. The plate should be flat, without wave-shaped buckling, to ensure the strip steel use. However, because the strip is wide and thin, it is particularly sensitive to uneven deformation. So it is not easy to maintain a good plate shape. The thinner the band, the greater the susceptibility to uneven deformation and the more difficult it is to maintain a good shape. Obviously, the defect of the plate shape comes from the uneven deformation, and the uneven deformation often leads to the uneven thickness. Therefore, the plate shape is often directly related to the thickness accuracy.
(3)Good surface quality: The surface quality requires no defects, no rust, etc., to ensure the durability and beauty of the strip. Strip steel is a kind of steel product with the largest surface area per unit volume, and it is often used as a structural part. So its surface quality must be ensured. Surface defects not only damage the appearance of plate parts, but also often corrupt the performance or become the source of cracking and rust. At the end, it becomes the weak point of stress concentration.
(4) Hot rolled strip steel also needs to have good mechanical properties, process properties and special physical or chemical properties.
3. Hot-rolled steel strip production characteristics are as follows.
(1) It is simple and easy to change the product specifications because of flat roll rolling out. The adjustment operation is convenient, easy to achieve comprehensive computer control and automation production.
(2) Strip steel can be produced in rolls due to simple shape. The national economy needs large quantity. Therefore, it needs high speed continuous rolling production.
(3) Due to the large wide thickness ratio and the surface area, the rolling pressure in the production is very large, up to tens of millions of Newtons. The rolling mill equipment is complex and huge. It is very difficult and complex to control the size accuracy, plate shape and surface quality.
The hot strip production has four ways according to the hot strip width and production process: wide strip steel hot continuous rolling, wide strip steel reversible hot rolling, narrow strip hot continuous rolling, hot rolling with planetary rolling mill. Strip steel materials include carbon steel, low-alloy steel, stainless steel and silicon steel. The final product mainly uses as a raw material for cold-rolled strip, welded pipe, cold-formed and welded section steel or to make various components.
The hot rolled strip production process mainly includes: billet preparation, heating, descaling, rough rolling, cutting end, finishing rolling, cooling, winding, finishing.
BILLET PREPARATION: The billet of hot-rolled strip steel is generally continuous casting billet. Its chemical composition, dimensional tolerance, bending and end shape should meet the requirements. The cold-loaded billet should be checked. The hot-charged billet should be free of defects, that is, no visible defects on the surface, and no shrinkage, porosity, segregation inside.
BILLET HEATING: Billet heating mainly controls heating temperature, time, speed and temperature schedule (including preheating, heating and heat-balance section temperature), preventing overheating, over-burning, oxidation, de-carbonization or sticking steel.
DESCALING: The descaling devices includes flat roll descaling machine, vertical roll descaling machine and high pressure water descaling box. It is widely used to remove the oxidized iron sheet with high pressure water (10 ~ 15MPa) after vertical roll edging.
ROUGH ROLLING: The rough rolling is to compress and extend the blank, provide the blank with the required size and shape for the finishing rolling. In the roughing process, the amount and speed of each pass should be controlled according to the setting. The workpiece temperature from the roughing mill should be increased as much as possible. The workpiece thickness and width should be guaranteed. In order to shorten the distance between the mill stand, the last two roughing mill units are arranged in a continuous manner.
CUTTING ENDS: The cutting head is to cut off the workpiece head and end, suitable for the finishing mill biting and the coiler pick-up.
FISHING ROLLING: Finishing rolling is carried out according to the reduction amount of each mill, rolling temperature and rolling speed in the rolling schedule. It is generally controlled by equal flow per second or constant tension mode. The thickness is controlled by hydraulic or electric AGC. The temperature control of the rolling process includes the final rolling temperature and the temperature difference between the head and the end. In order to control plate shape, roll shape and pre-bending roll device are used to ensure the transverse thickness difference of the steel strip.
REELING UP: The steel strip temperature after finishing rolling is 900 ~ 950℃. It must be cooled to 600 ~ 650℃ in a few seconds for reeling up. Generally, laminar cooling and water curtain cooling are used to cooling the steel strip. Regarding laminar cooling with low water pressure and large water, the water quantity can be automatically adjusted according to the strip thickness and the final rolling temperature. The strip steel cooled by water curtain can get uniform transverse cooling at the fast cooling speed and high cooling capacity.
In order to ensure the microstructure and properties of hot rolled strip steel to meet the requirements, it must be coiled at a lower temperature and a higher speed after rolling, generally at 500~650℃.The grain will grow if the coiling temperature is high.
FINISHING: Finishing is to unroll the hot rolled steel strip, after cutting the head and edge, cutting to the length and leveling, then packed in rolls or packed in a single sheet for delivery.
2. Technical requirements for the hot rolled strip mainly includes dimensional accuracy, plate shape, surface quality and performance.
(1)High dimensional accuracy: Dimensional accuracy is one of the important technical indexes of hot rolled strip steel, which directly affects the service performance and processing performance of strip steel. Dimensional accuracy mainly includes length, width and thickness accuracy. The thickness accuracy control is especially important.
(2)The plate shape should be good. The plate should be flat, without wave-shaped buckling, to ensure the strip steel use. However, because the strip is wide and thin, it is particularly sensitive to uneven deformation. So it is not easy to maintain a good plate shape. The thinner the band, the greater the susceptibility to uneven deformation and the more difficult it is to maintain a good shape. Obviously, the defect of the plate shape comes from the uneven deformation, and the uneven deformation often leads to the uneven thickness. Therefore, the plate shape is often directly related to the thickness accuracy.
(3)Good surface quality: The surface quality requires no defects, no rust, etc., to ensure the durability and beauty of the strip. Strip steel is a kind of steel product with the largest surface area per unit volume, and it is often used as a structural part. So its surface quality must be ensured. Surface defects not only damage the appearance of plate parts, but also often corrupt the performance or become the source of cracking and rust. At the end, it becomes the weak point of stress concentration.
(4) Hot rolled strip steel also needs to have good mechanical properties, process properties and special physical or chemical properties.
3. Hot-rolled steel strip production characteristics are as follows.
(1) It is simple and easy to change the product specifications because of flat roll rolling out. The adjustment operation is convenient, easy to achieve comprehensive computer control and automation production.
(2) Strip steel can be produced in rolls due to simple shape. The national economy needs large quantity. Therefore, it needs high speed continuous rolling production.
(3) Due to the large wide thickness ratio and the surface area, the rolling pressure in the production is very large, up to tens of millions of Newtons. The rolling mill equipment is complex and huge. It is very difficult and complex to control the size accuracy, plate shape and surface quality.
