Introduction
In steel production, round billets are a crucial intermediate product, and their quality has a crucial impact on subsequent processing and final product performance. As a key indicator of round billet quality, ovality has received widespread attention in recent years. Excessive round billet ovality can cause numerous problems during subsequent processing, such as piercing and rolling, seriously impacting production efficiency and product quality. Therefore, a thorough understanding of the importance of round billet ovality and its key control points are crucial for steel companies to improve product quality and reduce production costs.
The Concept of Round Billet Ovality
Round billet ovality, also known as out-of-roundness or loss of roundness, measures the difference between the maximum and minimum diameters of a circular rolled product (such as round steel and round steel pipe). Its calculation formula is typically: Ovality = (maximum diameter – minimum diameter) / nominal diameter × 100%. For example, for a round billet with a nominal diameter of 300mm, if the measured maximum diameter is 305mm and the minimum diameter is 295mm, the round billet’s ovality = (305 – 295) / 300 × 100% ≈ 3.33%. In actual production, different product standards have different allowable deviations for round billet ovality, generally around 0.5 times the sum of the positive and negative diameter deviations. For round tube billets, the tolerance is 0.75-0.8 times the diameter deviation.
The Importance of Round Billet Ovality
3.1 Impact on the Piercing and Rolling Process
In seamless steel pipe production, round billets undergo piercing. If the round billet ovality is excessive, the contact forces between the major and minor axes of the billet and the rolls will be inconsistent. When the ovality exceeds 8mm, the internal structure of the billet may be affected by uneven force during piercing, resulting in defects. During the horizontal continuous casting process, most round billets exhibit varying degrees of ovality due to factors such as gravity. Actual production measurements show that the ovality is generally within a certain range. If the billet ovality does not meet the required ovality, it is prone to rolling jams or head entrapment during piercing, leading to production interruptions and increased production costs. Furthermore, the internal structure of the billet may transform into a Widmanstätten structure, affecting the performance and quality of the steel pipe.
3.2 Impact on Product Quality
3.2.1 Impact on Steel Pipe Wall Thickness Uniformity
During the rolling process, round billets with excessive ovality will deform unevenly at different locations, resulting in uneven wall thickness in the produced steel pipe. For example, when rolling a round billet into a steel pipe, areas with greater ovality experience relatively greater deformation during rolling, while areas with less ovality experience relatively less deformation. This results in variations in the pipe’s wall thickness. Such uneven wall thickness reduces the pipe’s ability to withstand pressure during use, making it more susceptible to localized leakage or rupture, seriously impacting the product’s service life and safety, and reducing its market competitiveness.
3.2.2 Surface and Internal Defects
Excessive ovality in round billets is often accompanied by the development of intermediate and subsurface cracks. During the continuous casting process, the solidified shell of a round billet with large ovality shrinks unevenly, generating significant thermal stress within the billet, which can lead to cracks. These cracks may further expand during subsequent processing, affecting the internal quality of the product. Furthermore, the presence of surface cracks can reduce the product’s appearance, making it difficult to meet surface quality inspection standards and increasing scrap rates.
3.3 Impact on Production Efficiency
During production, if the round billet’s ovality does not meet the required specifications, the pipe threading machine will not be able to properly engage the billet, thus interrupting production. For example, during the production of round billets, one steel plant frequently encountered difficulties with the pipe threading machine due to excessive ovality. This problem led to an average of two to three production interruptions per day, each requiring one to two hours of equipment adjustments and billet replacements. This severely impacted production efficiency and increased production time and costs. Furthermore, to mitigate excessive ovality, some manufacturers were forced to reduce billet drawing speeds, significantly limiting production capacity.
Key Points for Controlling Round Billet Ovality
4.1 Control of Mold-Related Factors
4.1.1 Optimizing Mold Cooling Performance
Uneven mold cooling is a major cause of excessive round billet ovality. Factors such as mold powder properties, mold taper, and the type of steel being cast (such as peritectic steel) can affect the uniformity of heat dissipation in the mold, leading to uneven shrinkage of the billet shell and resulting in an elliptical shape. Therefore, the composition and properties of the mold powder must be strictly controlled to ensure good lubricity and uniform heat transfer. For example, for casting round billets of different steel grades, a matching mold powder type should be selected. Furthermore, the mold taper should be appropriately adjusted. The appropriate mold back taper should be determined based on the billet diameter and steel grade characteristics to ensure uniform growth of the billet shell within the mold.
4.1.2 Mold Structure and Maintenance
Deformation of the mold copper tube, uneven circumferential wear, and poor alignment of the submerged nozzle with the mold can all affect the shape of the round billet. Regularly inspect the wear of the mold copper tubes and replace any severely worn tubes promptly. At the same time, the alignment accuracy of the submerged nozzle and the mold must be ensured to prevent molten steel from flowing sideways within the mold, which could erode the solidified shell and cause uneven shell growth. For example, advanced alignment detection equipment should be used to regularly check and adjust the alignment of the submerged nozzle and mold to ensure that the alignment accuracy is within the specified range.
4.2 Straightening Equipment and Process Control
4.2.1 Optimizing Straightening Machine Pressure
Excessive straightening pressure in the straightening machine is one of the causes of increased ovality in round billets. During the straightening process, inappropriate total straightening pressure, especially excessive straightening pressure near the mold, can over-exert thinner shells with weaker deformation resistance, leading to excessive ovality. Therefore, the straightening machine pressure should be appropriately adjusted based on factors such as the round billet diameter, pouring temperature, straightening speed, and steel type. For example, for larger diameter round billets, due to the longer solidification time, the shell is thinner upon reaching the straightening machine and can withstand less pressure. Therefore, the straightening machine pressure should be appropriately reduced. At the same time, it is important to ensure that the pressure applied by the preceding straightener in each pair of adjacent straighteners along the billet’s travel direction is no greater than that applied by the succeeding straightener. This ensures billet stability during the straightening process and reduces ovality.
4.2.2 Straightening Unit Configuration and Coordination
Some advanced straightening equipment utilizes multiple straightening units working in coordination to control billet ovality. For example, a first straightening unit may comprise multiple first straighteners, and a second straightening unit may comprise one or two second straighteners. By properly configuring the operating pressures and coordination between the two straightening units, the indentation depth and ovality of the billet can be effectively reduced while ensuring stable and smooth operation of the continuous casting machine. In actual production, the number of second straighteners in the second straightening unit can be determined based on the specific ovality requirements and the billet’s process parameters. For applications with more stringent ovality control requirements, the number of second straighteners can be appropriately increased to further improve billet ovality.
4.3 Continuous Casting Process Parameter Control
4.3.1 Matching Pouring Temperature and Casting Speed
Pouring temperature and casting speed have a significant impact on the ovality of the cast billet. Generally speaking, high pouring temperatures and high casting speeds result in a thinner solidified billet shell, which has poor deformation resistance and is prone to increased ovality. Therefore, the pouring temperature and casting speed must be precisely controlled and properly matched based on the steel grade characteristics and billet specifications. For example, for a specific steel grade round billet, when the pouring temperature is controlled between 1520-1540°C, the casting speed should be controlled between 1.2-1.5 m/min. This parameter matching ensures a moderately thick solidified billet shell with good deformation resistance, thereby effectively controlling ovality. Furthermore, during the production process, fluctuations in pouring temperature and casting speed must be closely monitored and adjusted promptly to ensure the stability of process parameters.
4.3.2 Optimizing the Cooling System in the Secondary Cooling Zone
Uneven cooling in the secondary cooling zone or deformation of the support rollers that does not align with the curvature of the billet during continuous casting can cause uneven contraction of the billet during cooling, leading to excessive ovality. The cooling system in the secondary cooling zone should be optimized to achieve uniform cooling. Properly arranging the water spray system in the secondary cooling zone ensures uniform cooling of the billet circumference, promoting uniform growth of the billet shell. Furthermore, the condition of the support rollers should be regularly inspected and any deformed support rollers promptly replaced to ensure alignment with the curvature of the billet during continuous casting, thereby avoiding additional extrusion deformation of the billet. For example, advanced dynamic water distribution technology for the secondary cooling zone can be used to precisely adjust the water spray volume in each section of the secondary cooling zone based on the billet’s real-time temperature and position, achieving more precise cooling control.
4.4 Equipment Maintenance and Arc Precision Control
4.4.1 Regular Equipment Maintenance and Inspection
Roller roundness deviation is a significant factor affecting billet ovality. During equipment operation, rollers can experience roundness deviations due to wear and other factors. During rotation, the roller spacing changes, causing changes in the cross-sectional dimensions of the billet, resulting in indentations and, in turn, affecting ovality. Therefore, a comprehensive system for regular equipment maintenance and inspection is essential. Regular roller roundness testing is essential, and rollers with excessive roundness deviations should be promptly replaced. Additionally, other equipment components, such as the straightening machine and segments, should be inspected to ensure overall proper operation and minimize billet ovality issues caused by equipment failure.
4.4.2 Arc Alignment Accuracy Adjustment
Inaccurate alignment between the mold and the secondary cooling zone, as well as arc runout in the segments, can affect billet shape. During equipment installation and commissioning, strict control of the arc alignment accuracy between the mold and the secondary cooling zone is essential, requiring high-precision measuring instruments for inspection and adjustment. For example, a laser arc alignment instrument can be used to accurately measure the alignment between the mold and the secondary cooling zone to ensure that the arc error is within the allowable range. Segments should be designed with increased strength to prevent arc runout caused by thermal deformation during production. For example, support rods can be installed in the fan-shaped segments to enhance their structural stability and reduce arcing. At the same time, the arc alignment between the mold and the fan-shaped segments should be optimized in the hot state. Based on the thermal deformation of the equipment, the mold can be appropriately moved in the inner arc direction to offset arcing caused by thermal deformation and gaps at the joints, ensuring the shape accuracy of the round billet during the production process.
Conclusion
Round billet ovality is of great significance in steel production, directly and indirectly impacting the piercing and rolling process, product quality, and production efficiency. Strict control of mold-related factors, straightening equipment and processes, continuous casting process parameters, equipment maintenance, and arc accuracy can effectively reduce round billet ovality, improve billet quality, provide high-quality billets for subsequent processing, and enhance the market competitiveness of steel companies.
