Introduction to steel rolling production process, bar and wire technology (71)
1. Process flow of bar production lineBillet acceptance → heating → rolling → double-length shearing → cooling → shearing → inspection → packaging → measurement → storage
(1), billet acceptance
The quality of the billet is the key to the quality of the finished product and must be checked and accepted.
①. The billet acceptance procedures include: material card check, external dimension measurement, surface quality inspection, recording, etc.
②. The billet acceptance shall be carried out according to the billet technical standards and internal control technical conditions, and unqualified billets shall not be put into the furnace.
(2), billet heating
Billet heating is an important process in the hot rolling production process.
①The purpose of billet heating The purpose of billet heating is to improve the plasticity of the steel and reduce the deformation resistance to facilitate rolling; the correct heating process can also eliminate or reduce the internal structural defects of the billet. The heating process of steel is directly related to various technical and economic indicators such as steel quality, rolling mill output, energy consumption, and rolling mill life.
②, three-stage continuous heating furnace so-called three-stage namely: preheating section, heating section and soaking section. The function of the preheating section: use the waste heat of the heating flue gas to preheat the billet to save fuel. (Generally preheated to 300~450℃) The function of the heating section: reheat the preheated billet to 1150~1250℃, which is the main heating section of the heating furnace and determines the heating production capacity of the furnace. The function of the soaking section: reduce the temperature difference between the inside and outside of the billet, eliminate the black mark of the water-cooled slide, and stabilize and evenly heat the quality.
③, several common defects of billet heating
a. Overheating
When the billet is heated at high temperature for a long time, it is prone to overheating. The phenomenon of overheating of the billet is mainly manifested in the excessive growth of the grains of the steel into a coarse-grained structure, thereby reducing the bonding force between the grains and reducing the plasticity of the steel. Overheated steel is prone to cracking during rolling, especially at the corners. When slightly overheated, cracks occur on the surface of the steel, affecting the surface quality and mechanical properties of the steel. In order to avoid overheating defects, the heating temperature and heating time must be strictly controlled.
b. Overheating
When the billet is heated at high temperature for a long time, it will become a coarse crystalline structure, and at the same time, the low-melting point non-metallic compound on the grain boundary is oxidized and the crystalline structure is destroyed, so that the steel loses its due strength and plasticity. This phenomenon is called overheating. burn. Overfired steel can suffer severe cracking during rolling. Therefore, overburning is a heating defect that is more serious than overheating. Overburned steel cannot be salvaged except re-smelting. Ways to avoid over-burning: reasonably control the heating temperature and the oxidizing atmosphere in the furnace, strictly implement the correct heating system and the system to be rolled, and avoid excessive temperature.
c. Uneven temperature
This phenomenon is easy to occur when the billet heating speed is too fast or the output of the rolling mill is greater than the heating capacity. For billets with uneven temperature, the dimensional accuracy of the rolling stock is difficult to control stably during rolling, and it is easy to cause rolling accidents or equipment accidents. Avoidance methods: reasonably control the furnace temperature and heating speed; do a good job in the connection between rolling and heating.
d. Oxidative burn loss
The steel billet is oxidized at room temperature, but the oxidation speed is slow. With the increase of the heating temperature, the oxidation speed is accelerated. When the steel billet is heated to 1100-1200 °C, it is strongly oxidized under the action of the furnace gas to form iron oxide scale. . The production of iron oxide scale increases the heating and burning loss, resulting in a decrease in the yield index. Measures to reduce oxidative burning loss: reasonable heating system and correct operation, and control the atmosphere in the furnace.
e. Decarbonization
When the billet is heated, the phenomenon that the carbon content on the surface decreases is called decarburization. The steels that are easy to decarburize are generally carbon structural steels and alloy steels with high carbon content. These steels have their own special purposes. After decarburization, the carbon content on the surface and the interior of the steel is inconsistent, which reduces the strength of the steel and affects the performance. Especially for steels that require high wear resistance, high elasticity and high toughness, the surface hardness and performance are greatly reduced due to decarburization, and even waste products are caused. Control method: strict heating system, reasonable control of furnace temperature and oxidizing atmosphere in furnace.
(3), rolling
The rolling process is the core of the entire steel rolling production process. The deformation process completed by billet rolling becomes the product required by the user. The rolling process plays a decisive role in product quality.
The quality of rolled products includes: product geometry, dimensional accuracy, internal organization, process mechanical properties and surface finish. Therefore, the rolling process must formulate corresponding rolling process technical regulations and process management systems according to product technical standards or technical requirements, production product characteristics and production technology and equipment capabilities, as well as production costs and labor conditions of workers. In order to ensure the optimization of rolled product quality and technical and economic indicators.
The rolling process generally consists of one row (group) or two rows (group) of rough rolling mills, intermediate rolling mills and finishing rolling mills. It undertakes the functions of billet section compression, semi-finished product rolling deformation and finished product rolling respectively. The finished rolling mill pass or finishing rolling pass is K1 hole, the pre-finished rolling mill pass is K2 hole, the pre-finished front rolling mill pass is K3 hole, and so on.
Continuous rolling mill and continuous rolling constant continuous rolling mill means that several rolling mill stands are arranged in a row according to the rolling direction, the rolling stock is rolled and deformed in several rolling mills at the same time, and the rolling speed of each stand increases with the length of the rolling stock The rolling mode that increases and maintains the second flow of metal in each rolling mill is equal or has a slight pile-pulling relationship is called a continuous rolling mill. The continuous rolling mill has high mechanization, automation, rolling speed and rolling precision, so the labor productivity is high and the product quality is good. Keeping the rolling stock through each rolling mill with the same volume in unit time is called continuous rolling.
Continuous rolling constant=F1D1N1= F2D2N2= F3D3N3= …… = FnDnNn
In the formula: F—the cross-sectional area of ??the rolled piece mm2
D—roller working diameter mm
N—roll revolutions (1/min)
(4), finishing
①, steel cooling
The cooling bed is the main cooling equipment for rolled bar products. The cooling roller table is the main cooling equipment for wire products.
The role of the cooling bed and the cooling roller table is to cool the high-temperature rolling stock above 800 °C to below 150-100 °C, so as to restore the inherent physical properties of the steel and facilitate the guarantee of shear quality and subsequent process operations.
Cooling method According to the chemical composition of the steel, the state of the structure, the application, and the defects that may occur after cooling, the following cooling methods are determined:
Natural air cooling Natural air cooling is a commonly used cooling method for carbon structural steel, low alloy structural steel, carbon structural steel, and austenitic stainless steel. physical properties.
Forced rapid cooling Forced rapid cooling generally adopts air blowing, spraying, water spraying, etc. The process feature is that the steel is quickly cooled to a certain temperature within a certain period of time and then naturally cooled. This cooling method can generally change the internal structure of the steel, which will affect the physical properties of the steel. Such as improving the mechanical properties of ordinary wire; eliminating steel mesh carbide and so on.
Controlled slow cooling Steels with strong sensitivity to stress after cooling are generally cooled by controlled slow cooling, such as high-speed tool steel, martensitic stainless steel, high-alloy tool steel, and high-alloy structural steel. This cooling method can prevent the steel structure from changing and stress concentration from causing crack defects.
②. Steel shearing Purpose of steel shearing: Cut off the parts that affect the use (defects) of steel, such as the head and tail of the steel; cut into the length required by the user.
Steel shearing equipment is divided into cold shearing and hot shearing. Hot shearing is often used for semi-finished product cutting head, tail cutting or double ruler cutting; cold shearing is often used for finished steel cutting head, tail cutting or fixed (through) ruler cutting.
③. Steel inspection Steel inspection is a key process to ensure that product quality meets product technical standards and technical conditions. Product inspection usually includes: steel dimensions, surface quality, length to length, weight deviation, process performance, mechanical properties, etc. Steel that does not meet product quality standards must be picked out and sorted for disposal.
Common defects and preventive measures on the surface of products: rod and wire products usually stipulate that the surface should not have defects such as cracks, folds, ears, scars, delaminations and inclusions.
a. The occurrence and prevention characteristics of crack defects: There are different shades and scattered hair-like fine lines on the surface of the billet or steel, which are generally unevenly arranged along the rolling direction. Causes: subcutaneous bubbles, surface pores, non-metallic inclusions, uneven heating temperature, too low steel temperature or improper cooling after rolling (such as carbon structural steel) in the continuous casting billet.
Preventive measures: steel smelting and degassing of molten steel should be done well, the tapping temperature should be reduced, and protection casting should be used to avoid secondary oxidation; steel rolling should reasonably control the furnace temperature and cooling rate.
b. Folding features: locally long or continuous defects similar to cracks on the surface of the steel along the rolling direction. Generally linear. Cause: The semi-finished product of the rolling piece has ears, serious scratches, or the edge and corner of the rolling piece after rolling with wrong pass roll cannot be extended.
Preventive measures: Reasonably control the size of the semi-finished rolling stock. During the production process, a wooden bar should be often used to check whether there are ears and wrong rolls on both sides of the rolling stock roll gap; pay attention to observe the rolling stock running condition.
c. Ear characteristics: Overfilling of both sides or one side of the steel roll gap along the rolling direction causes partial or continuous bulge. Causes: The incoming material of the front hole of the finished product is large; the imported guide is biased and loose, and the rolling piece is not straightened; the roller moves axially; the heating is uneven or the temperature is too low; the wear of the finished pass produces a bump with steps.
Preventive measures: reasonably control the temperature of the heating furnace and the size of semi-finished products; strictly adjust the guide and guard devices; improve the pre-installation accuracy of the rolling mill; timed and quantitative inverted hole patterns.
d. Scarring characteristics: "tongue-shaped" or "nail-shaped" scars on the surface of the steel that are lumpy or scaly, with uneven size, uneven thickness and irregular shape. Closed or not closed; rooted or unrooted. There is usually a scale of iron oxide below the scar. Raised scars are also called warped skins. Causes: The billet has defects such as scarring, heavy skin, inclusions, etc.; the semi-finished product has local bumps; the pass is dropped or trachoma; The surface of the rolled piece; the semi-finished rolled piece is scratched by external objects, etc. Preventive measures: unqualified billets shall not be put into the furnace; when the pass adopts nicks or welding scars, the shape and height of the engraving and welding marks should be smooth and smooth; strengthen the quality inspection of rolls; reasonable pass design; strictly prohibit low-temperature, black-headed steel rolling; often Check the wear of the pass and reverse the pass in time; the rolling stock conveying equipment and operating place should be neat and smooth.
e. Scratch (scratch, abrasion) characteristics: There are partial or intermittent groove marks on the surface of the steel, generally in a straight line or arc. Causes: Improper processing and installation of import and export guides or scratches on rolling stock transportation equipment; unfavorable rolling stock removal from grooves. Preventive measures: Correct processing, installation, and use of import and export guide facilities; rolling stock transportation equipment and operating places should be neat and smooth.
f. Features of pits: There are local periodic or irregular pit defects on the surface of the steel. Causes: There are bumps in the rolling pass or iron oxide scale is adhered to it; there is no root scarring on the surface of the steel; foreign metal objects are substituted into the pass and formed after rolling and falling off. Preventive measures: The cooling water of the hole type should be clean and the water quantity should be sufficient; the quality of the billet is qualified; the production environment is free of debris.
2. Wire production process
Billet acceptance→heating→rolling→wire laying→uncoil cooling→coil collection→coiling→inspection→packaging→measurement→storage wire rod production. Billet acceptance, heating and rolling process are basically the same as bar production process, but The cooling of the wire is carried out by cooling rollers. The cooled bulk wire needs to be collected into a coil by a concentrator, and then compacted by a press coiler and then packaged in small coils. Then, after inspection, classification and packaging into large bundles, they are measured and put into storage.
2. Analysis and treatment of faults in the production process of rods and wires
(1) Reasons for the failure of the rolling piece: the rolling piece is rotten, split, black, etc.; the imported guide is installed incorrectly or too small; there is sticky iron or foreign matter in the imported guide; the rolling angle is too large or too small ; Rolling guide wheel does not rotate or bearing burns out; Roll does not rotate due to breakage or connection; Improper adjustment of roll gap or unsatisfactory size of incoming material; Roll pass friction coefficient is too small (new pass), etc.
Analytical processing and preventive measures
1. Analyze the reasons for the failure of the rolled piece, strengthen the inspection of the head defect of the rolled piece and deal with it in time;
②. After the inlet guide is installed, use the inner caliper to measure the width of the inlet cavity; illuminate with light and check from the exit direction whether the inlet guide is in the middle hole type and fasten the guide bolts;
③. Use calipers to check and measure the actual size of the rolled piece to ensure the correct size of the incoming material;
④. The roll gap size is strictly adjusted according to the rolling requirements;
⑤. Frequently inspect the rolling guide guide wheel and the twist outlet and replace it in time; frequently check the import guide plate for iron and iron oxide deposits or debris;
⑥. The new hole type should be tested with small material first.
(2) Failure of rolling stock stacking
Rolling stock stacking failure refers to the phenomenon of stagnation after stacking of rolling stock between rolling mills or rolling mill units, which is called stacking failure.
①, the cause
a. The incoming size and cross-sectional area of ??the rolled piece are too large; the rolled piece itself has surface or internal quality defects;
b. The friction coefficient of the roll pass is not enough, causing the rolling piece to slip;
c. The volume flow per second of the incoming rolling mill is greater than the volume flow per second of the receiving rolling mill;
d. The electrical control system is wrong or out of control. e. The mechanical running parts of the automatic looper system or the photoelectric control instruments are installed incorrectly, out of control or out of order.
②, analysis processing and preventive measures
a. Adjust the size and area of ??incoming materials to meet a reasonable continuous rolling constant relationship;
b. Dynamically and closely monitor the running status of the rolling stock and adjust it in time;
c. Ensure the quality of the billet entering the furnace; the semi-finished rolling stock with quality defects shall be dealt with in time;
d. Regularly check the revolutions, looper system and electrical control system of each rolling mill to ensure that the operating equipment and electrical control system are stable and normal.
(3) Failure of rolling steel drawing
The steel drawing failure of the rolling stock refers to the phenomenon that the rolling stock is too stretched or broken between the rolling mills or the rolling mill units, which is called the steel drawing failure. Due to the failure of drawing steel, the size of the rolled piece is often changed, and in severe cases, it may even lead to the out-of-tolerance dimensions or out-of-roundness of the finished product, resulting in scrap.
①, the cause
a. The incoming size and cross-sectional area of ??the rolled piece are too small;
b. The volume flow per second of the incoming rolling mill is less than the volume flow per second of the receiving mill;
c. The electrical control system is wrong or out of control.
②, analysis processing and preventive measures
a. Adjust the size and area of ??incoming materials to meet a reasonable continuous rolling constant relationship.
b. Ensure that electrical appliances and electronic control equipment systems are operating normally.
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