1. Description of the actual defects of the off-square
Traditional de-squareness defects generally refer to rhombus change, that is, the billet changes from a square to a rhombus, and the billet has two obtuse angles and two acute angles. The degree of de-squareness is usually expressed by the ratio of the difference between the two diagonals to the average value of the two diagonals, that is:
R=[(a-b)÷(a+b)／2]×100％
When the R value is greater than the standard value, it is judged as a diamond-shaped scrap. When R>3% of the standard value, cracks often appear at the blunt angle of the ingot; if R>6% of the standard value, it will cause inconvenience in pushing the steel in the heating furnace and failure to bite during rolling.
In reality, there are many shapes of billets with diagonal deviations on the billet end faces, which can be roughly divided into three categories according to actual statistics:
Class I is diamond-shaped and deformed;
Class II is bulging and deformed;
Class III is irregular single-angle protrusion or defective deformed, with irregular shape changes.
These different types of de-square defects have different effects on the quality of the ingot and subsequent processing, but they all seriously restrict the product quality and the smooth progress of production.
2. Analysis of the reasons for the loss of prescription:
2.1 The impact of equipment factors on the square
2.1.1 Crystallizer
In the actual production of small billet continuous casting machines, the material of the copper tube of the crystallizer has a significant impact on the quality of the billet. At present, most copper tubes are made of Cu-Ag material, and some are made of Cu-Cr-Zr copper tubes. The inner surface of the copper tube is usually plated with Cr, Ni-Fe or Ni-Co, etc. Many domestic manufacturers use copper tubes with Cr plated on the inner surface, but the simple Cr plated layer is easy to peel off, affecting the life of the crystallizer. For example, a crystallizer copper tube that was used not long ago in a steel plant had local coating peeling off on the corners and hot surface, and serious de-squareness and steel leakage occurred frequently during production. The peeling of the coating will cause the air gap between the copper tube and the billet to change too much, resulting in uneven cooling and uneven thickness of the primary billet shell, and then de-squareness deformation.
The water-jacketed crystallizer is composed of multiple parts, and the uniformity of the water seam is crucial to the cooling effect. Whether the water seam is uniform depends on whether the water jacket is deformed and whether the outer dimensions of the copper tube are uniform and consistent. On-site measurements found that the difference in the diagonal of the inner water jacket of the de-squared crystallizer is large, and the control accuracy of the outer dimensions of the copper tube is low, resulting in the water seam not meeting the requirements and large changes in cooling intensity, which causes the billet to de-square. The original crystallizer water jacket is integrally drawn and formed, with limited manufacturing accuracy. It is difficult to adjust the copper tube water seam and assemble the crystallizer during installation. It is easy to deform after a period of use, resulting in large water seam deviation and uneven cooling at the corners, affecting the quality of the billet.
The wear of the copper tube of the crystallizer is directly related to the molding quality of the ingot. The wear usually starts from the corners and the lower mouth of the crystallizer, and gradually expands upward. The wear of the corners is more serious. When the copper tube is severely worn, the chrome plating layer peels off irregularly, and the deformation reaches a certain degree, the thickness of the primary shell in the crystallizer will be uneven. During the billet drawing process, the air gap increases and the cooling is uneven, which aggravates the square-off phenomenon. In addition, the asynchronous intermittent boiling of the four sides of the crystallizer will also cause uneven cooling of the four sides of the shell, resulting in inconsistent shell thickness. The directionality of square-off depends on the weakness of the corner shell.
2.1.2 Equipment arc alignment and billet deviation
Small billet continuous casting machines are mostly arc-shaped. The outer arc surface of the crystallizer copper tube should coincide with the outer arc surface of the continuous casting machine, and the two sides need to be centered with the center line of the casting stream as the center.
Once the outer arc of the crystallizer and the casting machine are not on the same plane, the entire cooling system will be misaligned. In the crystallizer, the molten steel cools and solidifies to form a primary billet shell. At this time, the billet shell thickness is only 10-15mm, but it bears the important responsibility of supporting the static pressure of the molten steel and maintaining the shape of the billet.
Uniform billet shell thickness is the key to maintaining the initial shape of the billet, and the heat transfer conditions in the crystallizer play a decisive role in the uniform development of the billet shell. The uniform distribution of cooling water is a key factor affecting the heat transfer conditions. If the crystallizer is not cooled evenly, especially the corners are overcooled, causing the billet shell to shrink violently, it will not only cause the initial billet shell to deform rhombus-shaped, but also hinder the thickening of the billet shell at the corners due to the air gap thermal resistance.
After the billet leaves the crystallizer, due to the weakness of the corners and the initial rhombus deformation, the rhombus deformation will develop rapidly under the strong cooling of the secondary cooling water, resulting in a square defect. If the secondary cooling is not uniform, the degree of square deviation will be further increased, and lateral thermal stress will be generated, causing the billet to bend to one side and deviate. After the billet deviates, it loses alignment with the nozzle, and the secondary cooling becomes more uneven, forming a vicious cycle. Setting a billet side guide device in the secondary cooling guide section is an effective way to control the billet deviation, which can reduce the influence of thermal stress and lateral mechanical stress on the billet and prevent the billet deviation from aggravating square deviation.
2.2 Influence of process factors on de-squareness
2.2.1 Chemical composition
The chemical composition of the steel grade has a significant effect on de-squareness. The shrinkage and thermal conductivity of the steel will change with different carbon contents, thus affecting the degree of de-squareness. For example, when the carbon content in the steel is within a certain range, the heat extraction of the crystallizer and the solidification range of the steel will change, causing uneven growth of the shell and increasing the possibility of de-squareness. Sulfur is an element that easily segregates in steel and will lower the zero plasticity temperature of the steel. Its segregation at the grain boundaries and the precipitation of (Mn, Fe)S at the austenite grain boundaries will weaken the grain boundary bonding force, promote grain boundary sliding, and increase the probability of de-squareness. Properly increasing the Mn/S ratio is conducive to the granular distribution of manganese sulfide on the grain boundaries and the body, improving the high temperature performance of the steel, enhancing the ability of the shell to resist stress deformation, and reducing de-squareness.
2.2.2 Control of superheat during pouring process
During the pouring process, the superheat of molten steel has a great influence on square stripping. When the temperature of molten steel is too high, the heat taken away by the cooling water in the crystallizer is reduced, the primary shell becomes thinner, the strength and rigidity are reduced, and it is easy to deform under the action of unbalanced forces, which promotes square stripping. At the same time, high superheat may also lead to the intermittent boiling of cooling water in the crystallizer, further affecting the uniform growth of the shell. On the contrary, if the temperature of molten steel is too low, the drawing speed often needs to be increased, which may make the copper mould tube unable to adapt and is also prone to square stripping. Therefore, strictly controlling the superheat of molten steel within an appropriate range is crucial to reducing square stripping.
2.2.3 Alignment of nozzle
The centering of the tundish nozzle in the crystallizer directly affects the uniformity of the molten steel flow field. When the nozzle is deflected, the thickness of the solidified shell on each surface of the crystallizer will be inconsistent. The shell near the injection area becomes thinner due to the erosion of molten steel, while the shell in the opposite area becomes thicker accordingly, and the unevenness of the shell is aggravated, resulting in irregular shape of the billet and excessive de-squareness. Ensuring the centering of the nozzle can make the molten steel evenly distributed in the crystallizer, which is conducive to the uniform growth of the shell and reduces the occurrence of de-squareness.
2.2.4 Crystallizer cooling water quality
The quality of cooling water in the crystallizer has an important impact on the quality of the ingot. The scale deposited on the outer wall of the copper tube in the cooling water will increase the local thermal resistance of the copper wall, aggravate intermittent boiling and deformation of the copper tube. This not only affects the heat transfer effect of the crystallizer, but also causes the ingot to de-square. An inspection of the crystallizer water supply system of a certain factory during a period of severe de-squareness found that the protective net of the water supply tower was damaged, there was a large amount of mud and debris in the filter, a layer of scale adhered to the outer wall of the crystallizer copper tube and local overburning was serious. Testing found that the scale substances are mainly Fe ions and fungi. To improve the water quality, measures such as adjusting the reagents, adding sterilizing agents, and regularly cleaning the filters can be taken.
2.2.5 Secondary cooling water quality
Poor secondary cooling water quality can cause frequent nozzle blockage, resulting in uneven cooling of the ingot after it leaves the crystallizer, exacerbating the de-squareness of the ingot. During a period of severe de-squareness in a certain factory, even the turbid water turned yellow and the turbidity increased, reaching more than 200 at the highest. At the same time, the secondary cooling chamber nozzles were frequently blocked, and the pressure difference before and after the filter changed greatly. After checking the water system, replacing the existing turbid water, clearing the side pipes, repairing the damaged cement culverts, and using the downtime to clean and discharge the turbid water pool, vortex well, etc., the turbidity of the turbid water was reduced to below 30, and the blockage of the nozzles was significantly improved.
2.2.6 Reasonable secondary cooling water distribution
The rationality of water distribution in each zone is crucial to the uniformity of billet cooling. In actual production, different steel grades and different drawing speeds require different cooling intensities. If the secondary cooling water distribution is unreasonable, it will lead to uneven cooling of the billet, thereby aggravating the square deviation. The traditional water meter method of water distribution has disadvantages, and it is difficult to accurately adjust the water volume according to the actual situation. The use of dynamic water distribution and real-time adjustment of the secondary cooling water volume according to the steel grade, drawing speed, etc. can effectively improve the uniformity of billet cooling and reduce the occurrence of square deviation.
2.2.7 Blockage of water tap
The blockage of the water nozzle will cause insufficient or excessive cooling of the local part of the billet, resulting in uneven cooling, which will cause the billet to fall off. The cause of the blockage of the water nozzle may be impurities, scale, etc. in the water. Therefore, regular cleaning of the water nozzle and strengthening of water quality management are important measures to prevent the blockage of the water nozzle.
3. Control measures
3.1 Equipment
Regularly check the wear of the copper mould tube of the crystallizer, establish a copper tube usage file, and record the number of furnaces used, degree of wear and other information of the copper tube. When the wear of the copper tube reaches a certain degree, such as severe peeling and deformation of the chrome plating layer affecting the quality of the ingot, replace the copper tube in time. Optimize the design of the crystallizer water jacket and adopt a high-precision flow-guiding water jacket. For example, change the original ordinary water jacket to a combined assembly structure, and use high-precision CNC machine tools to process the arc of the water jacket to ensure the accuracy of the inner and outer arcs of the water jacket and the overall assembly size. Use fixed-line positioning to adjust the inner water seam so that the cooling water remains uniform and at a constant speed in the inner water jacket, thereby improving the cooling effect and cooling intensity of the crystallizer copper tube and stabilizing the quality of the ingot.
Strengthen the detection and adjustment of the arc accuracy of the equipment, regularly measure the arc of the continuous casting machine, ensure that the outer arc surface of the crystallizer copper tube coincides with the outer arc surface of the continuous casting machine, and the two sides are well centered.
Set up a reliable side guide device for the billet in the secondary cooling guide section to reduce the possibility of the billet running off, ensure that the billet is evenly stressed during the cooling process, and avoid uneven cooling and aggravated de-squareness caused by running off.
3.2 Process
⑴ Strictly control the chemical composition of molten steel, and accurately adjust the content of carbon, sulfur, manganese and other elements according to the requirements of different steel grades. For steel grades that are easy to de-square, appropriately increase the Mn/S ratio to enhance the ability of the shell to resist stress deformation. At the same time, stabilize the superheat of molten steel, avoid excessively high or low temperatures, and reduce the impact of superheat fluctuations on shell growth.
⑵Optimize the pouring process to ensure that the tundish nozzle is accurately centered in the crystallizer. Use appropriate nozzle structure and installation method, use special tools to install the nozzle, and ensure the relative position accuracy between the nozzle and the crystallizer. Control the pouring temperature and pulling speed to match them to avoid intermittent boiling of cooling water in the crystallizer and deformation of the billet shell due to improper temperature and pulling speed.
⑶ Strengthen the water quality management of the crystallizer and the secondary cooling zone. Use soft water for the crystallizer cooling water, test the water quality regularly, and clean the filter in time to prevent scale and debris from depositing on the outer wall of the copper tube. Treat the turbid water in the secondary cooling zone, clean the turbid water pool and vortex well regularly to ensure that the nozzle is unobstructed and the cooling is uniform. According to the changes in steel type and casting speed, reasonably adjust the secondary cooling water distribution system. Use a dynamic water distribution model to accurately control the water volume of each cooling section according to real-time data to ensure uniform cooling of the ingot in the secondary cooling zone and reduce the occurrence of squareness.
