The main quality indicators of hot-rolled strip products include surface quality, shape and size accuracy, and mechanical properties.
With the widespread application of automation control technology in hot rolling production lines, the production line has been very accurate in controlling the quality of strip product shape, and the quality indicators of the products have been greatly guaranteed.
The annual design production capacity of a 2,250mm hot rolling production line in a certain plant is 4.6 million tons, and the entire line adopts automation control. During the production and use of the equipment, due to problems in the process and control system, some quality problems have occurred in the product shape and size, which has caused some quality complaints and objection compensation from customers. This article discusses the hot rolling production process and analyzes some quality problems and solutions in hot rolling production.
Hot rolling production
Introduction to the main equipment of hot rolling line
The main equipment of the hot rolling production line includes: walking beam heating furnace, high-pressure water descaling, fixed width press, roughing mill, flying shear, finishing mill, coiler, flattening and coiling unit, etc.
Heating furnace
The walking beam heating furnace is mainly used for production buffering and slab heating before slab rolling.
In order to meet the needs of billet heating and realize flexible heating of billets at different stages, the heating furnace is divided into eight heating sections.
At the same time, the heating furnace adopts an automatic temperature control system to achieve accurate adjustment of the temperature of each heating section of the heating furnace, and continuously heats slabs of different steel grades and specifications along the length direction.
High pressure water descaling machine
During the rolling process, scaly substances such as iron oxide will be produced, which need to be removed by high-pressure water spraying.
Width setting machine
The width-fixing press is used for the first width reduction before rough rolling. It is arranged between the dephosphorization machine and the 2-roll reversible rough rolling mill. The main function of the width-fixing press is to change the appearance of the material through the extrusion action of the extrusion hammer to obtain uniform material deformation.
Roughing mill
The rough rolling mill is a mechanical equipment mainly used for side rolling to obtain intermediate billets with uniform width and neat edges, so as to prevent cracks and bulging on the edges of the rolled pieces and reduce the metal consumption coefficient.
Flying Shears
The flying shear is mainly used to cut the steel billet into a certain length to facilitate the subsequent rolling process. The flying shear is powered by two DC motors and driven by a geared coupling, a crank shaft, a spur gear reducer, etc. Driven by the motor and the crank, the flying shear cuts up and down quickly, thereby completing the shearing of the steel billet according to a certain length specification.
Finishing mill
The finishing mill uses automatic control and automatic compensation systems such as hydraulic AGC to perform high-precision rolling on slabs through hydraulic systems and rollers. It is mainly driven and transmitted by motors, couplings, reducers, etc. to obtain rolled products with high-precision appearance dimensions.
Coiler
The coiler is mainly used to coil the strip steel to make it into steel coils for bundling, transportation and storage. The coiler is also hydraulically driven and automatically controlled by roller gap hydraulic fine adjustment, auxiliary coiling roller step control SC, coiling tension control system, etc. The equipment has the advantages of long service life, less inner ring scratches, and less wear and maintenance.
Leveling unit
The leveling unit is mainly responsible for leveling various types of steel coils such as carbon steel. The ideal plate shape is obtained through the extrusion of the leveling roller, thereby improving the quality of the plate and facilitating the subsequent processing of the steel coil.
Hot rolled strip technology
The billet for hot-rolled strip is generally a slab or a continuous casting slab. Its appearance, curvature, dimensional tolerance and chemical composition are all technical requirements that need to be focused on. The quality of the slab is the basis for normal hot rolling processing. It should be required that there should be no looseness, pores, inclusions, segregation and other defects inside and outside the slab. The hot-rolled strip mainly has the following technical requirements:
High dimensional accuracy requirements
Dimensional accuracy mainly includes length, width and thickness accuracy. Dimensional accuracy is the most difficult technical indicator to control in the production of hot-rolled plates and strips, and has a great impact on the subsequent reprocessing and performance of the plates and strips. Among them, the thickness accuracy is more prominent. If the error is too large, it will have an adverse effect on the subsequent stamping process; and if the dimensional accuracy is not well controlled, it is also easy to cause metal waste in the subsequent processing links. Some minor thickness size changes will affect the performance and metal consumption of the plate.
When the heating furnace heats the billet, the heating temperature, heating time and steel passing speed are all process indicators that need to be controlled.
According to the process requirements of each type of steel, the heating system of the preheating section, heating section, soaking section and other links should be reasonably formulated to prevent uneven heating of the billet, as well as overburning, oxidation, steel sticking, decarburization and other phenomena. Due to its processing characteristics, hot-rolled products have relatively great difficulty in controlling thickness dimensions.
Therefore, hot rolling is often called the precursor of cold rolling, and its products are mostly used as billets for cold rolling. In order to maximize the rolling accuracy, it is generally produced according to the negative tolerance rolling process.
The board shape should be good
Hot-rolled products have relatively high requirements for plate shape. Not only does it require the plate shape to have no obvious appearance defects, but the thickness and curvature must also strictly meet the requirements.
The curvature of ordinary thin plates should be controlled within 20mm; the curvature of ordinary medium and thick plates per meter length is not more than 15mm. For wide and thin plates and strips, they are very sensitive to uneven deformation and are prone to uneven deformation.
The uneven deformation of the plate shape during plate and strip rolling is the main cause of poor plate shape and uneven thickness, which is closely related to the accuracy of plate and strip thickness control.
Good surface quality
Hot-rolled strip is often used as structural steel, so its surface is not allowed to have defects such as cracks, scabs, folds, scratches, bubbles, rust, and iron oxide scale intrusion.
Since these defective parts can withstand less stress than other normal parts, these defective parts are also prone to become weak links where stress is concentrated in the strip, and are the source of rust and cracking of the strip.
For example, when producing silicon steel sheets, the iron oxide scale on the surface will also be quickly stamped onto the strip surface during the deep drawing process, which will lead to cracking, roughness, and rapid wear of the stamping tools on the strip surface. Therefore, the surface quality of hot-rolled strips must be guaranteed, and the heating, rolling, high-pressure water descaling and other processes must be strictly controlled to eliminate quality risks.
Requirements for mechanical properties
The requirements for the mechanical properties of plate and strip steel mainly include: process performance, mechanical properties, chemical properties and physical properties of special plates and strips, etc. The mechanical characteristics of hot-rolled products are mainly manifested in the uneven organization and performance, and their strength index and plasticity index are between cold-worked hardened products and fully annealed products.
General structural steel plates have high requirements for process performance such as welding and cold bending, but are not strict on mechanical properties. For steel plate products with special uses, such as silicon steel sheets, stainless steel plates, high-temperature alloy plates, etc., there are relatively high requirements for their high-temperature performance and acid resistance, alkali resistance, corrosion resistance, etc. For structural steel plates with important uses, their chemical composition and process performance such as strength and shaping must be strictly controlled.
Analysis of hot rolling production process
The process flow of hot rolling production mainly includes: heating, high-pressure water descaling, rough rolling, uncoiling, shearing, finishing rolling, cooling, and curling.
The specific process flow is as follows: First, the slab to be rolled is taken out from the slab warehouse, placed on the furnace side roller, and loaded into the furnace by the steel loader.
In the heating furnace, the slab will be heated according to the established heating system. After the slab reaches a certain temperature, it will be carried out by the steel tapping machine and placed on the furnace side roller. Then the slab is sent to the rough rolling process under the roller conveyor, and the high-pressure water descaling, width setting, rough rolling, shearing, finishing rolling, descaling and other processing processes are carried out in sequence.
Finally, the slab is processed and rolled into a certain specification of plate and strip products and sent to the output roller. After laminar cooling, it is sent to the curling section by the pinch roller. The coiler, coiling roller, etc. complete the curling of the plate and strip, and the baler is packaged and sprayed with numbers and transported to the warehouse.
Rough rolling
After the slab comes out of the heating furnace, a layer of iron oxide scale with a thickness of about 2~5mm will form on the surface. It must be cleaned up, otherwise it will be pressed into the slab in the subsequent rolling process, becoming a quality defect that affects the surface quality and mechanical properties of the slab.
The removal of iron oxide scale on the slab surface generally uses high-pressure water descaling equipment, and the iron scale attached to the slab surface is cleaned by the flushing force of water pressure. Hot-rolled strip steel uses continuous casting billets. In order to meet the rolling requirements of slabs of different varieties and specifications, the width of the slab needs to be adjusted by the width setting process.
Finishing
Finishing rolling is mainly for high-precision rolling of strip thickness, plate shape and other appearance dimensions. Its precision control is achieved by the currently more advanced AGC roll gap adjustment and compensation system, which eliminates various interference factors in rolling through roll gap adjustment, thereby meeting the high-precision appearance dimension requirements of high-quality strip products.
Cooling, curling
After the strip is finished rolled, the final rolling temperature is about 800℃~900℃, and it needs to be cooled to reach the temperature of 550℃~650℃ required for the coiling process. The commonly used strip cooling processes in hot rolling production mainly include laminar cooling, high-pressure water spray cooling, water curtain cooling, etc. After the strip is cooled, it immediately enters the coiling process. The coiler uses the remaining temperature after the strip is cooled to quickly coil the strip under the action of the auxiliary coiling roller. The main function of the coiler is to coil the finished rolled product into a fixed coil shape, which becomes a steel coil, for easy bundling and transportation.
Finishing process
The hot-rolled strip finishing equipment unit includes leveling, coiling, shearing and other processes. The shearing of the strip is mainly completed by the flying shear equipment. The flying shear equipment uses the rapid cutting action of the shear blade to cut the strip into plates of certain specifications for subsequent plate processing.
Common quality problems and causes in hot-rolled strip production
Crack problem
Cracks are a common surface quality problem of strip steel.
Hot rolled strip surface cracks mostly appear in the form of longitudinal cracks with different depths and lengths. Various crack defects existing in the slab itself are the main cause of cracks in strip processing, and the cracks carried by the raw material billet cannot be eliminated in subsequent processes. In some cases, due to unscientific crystallizer and heat flow control, defects on the surface of the crystallizer copper plate, etc., the slab shell grows unevenly, and the stress imbalance causes cracks in the slab.
Scar problem
The scar problem mainly affects the flatness of the surface of the strip steel product, and it is easy to become a weak stress link, causing the strip steel to break in the subsequent rolling process.
The scar will be distributed on the surface of the strip steel in the form of irregular strips, spots, etc., and severe cases will cause rough pitting on the surface of the strip steel. The scar problem is mainly caused by the pressure of iron oxide scale.
Find the reasons from the production process:
First, the slab heating time is too long, and the iron oxide scale formed on its surface is more and thicker. It is difficult to remove these iron oxide scales in the descaling process, causing them to be pressed into the surface of the strip steel in the subsequent rolling process;
Second, the descaling water pressure is not enough or the descaling process is unscientific, resulting in poor descaling effect;
Third, the roller tonnage is too high or the rolling force setting is unreasonable, resulting in the wear of the strip steel surface and pitting.
Indentation problem
The indentation defects on the strip surface are generally distributed periodically, with relatively fixed length and width. The indentation defects are mainly caused by roller surface damage, roller surface slag or rolled piece falling off. The indentation problem seriously affects the flatness of the hot rolled strip surface.
Surface bulge
The problem of strip surface bulge generally occurs at the edge of the strip, and is distributed in the shape of strips along the rolling direction of the strip. The main reason for the bulge defect on the edge of the hot-rolled strip is the local wear of the working roll.
Common quality problems of hot-rolled strip steel and their solutions
Solutions to crack problems
First of all, we must pay attention to the maintenance of continuous casting process equipment to avoid coating peeling during the use of the equipment; we must also pay attention to improving the protective slag system, and through the summary of production experience of different steel grades, formulate crystallizer flow field areas and taper tables suitable for different steel grades and specifications, so as to improve the production quality of hot-rolled strip.
Solutions to the streak problem
The main thing to do is to prevent the iron oxide scale from being pressed in. First of all, it is necessary to formulate reasonable heating measures to avoid excessive iron oxide scale generation due to too long heating time and too high heating temperature; then, it is necessary to ensure the descaling effect by adjusting the descaling water pressure, cleaning the nozzle blockage and other measures to ensure the descaling water pressure; at the same time, it is also necessary to reasonably adjust the rolling force and tonnage of the rolls to avoid wear on the strip surface.
Solutions to the indentation problem
To solve the indentation problem of hot rolled strip, first check whether the roll surface is worn, sticky, etc. If the wear is light, the roll surface can be repaired and ground. If the indentation is serious, the roll must be replaced immediately.
Solutions to the bulge problem
Optimize the roller crown and improve the roller grinding accuracy. Also, avoid problems such as poor water cooling effect and uneven roller wear caused by nozzle blockage. Rollers with severe wear should be repaired and replaced in time.
