Method for controlling looseness of control center of bearing steel produced by small-section casting blank and product thereof

Abstract

The invention provides a method for controlling looseness of a control center for producing bearing steel by a small-section casting blank and a product thereof, and relates to the technical field of steel smelting, and the method comprises the following steps: dividing the casting blank into a plurality of areas according to a water distribution area of a secondary cooling area; the actual surface temperature of each area of the casting blank is measured in real time in a secondary cooling area of the continuous casting process; calculating the theoretical surface temperature of each area of the casting blank in real time according to a casting blank heat transfer equation; if the actual surface temperature of a certain area in the casting blank is equal to the theoretical surface temperature of the area, the water distribution amount of the current secondary cooling area to the area is maintained; and if the actual surface temperature of a certain area in the casting blank is not equal to the theoretical surface temperature of the area, the water distribution amount of the current secondary cooling area to the area is adjusted till the actual surface temperature of the area is equal to the theoretical surface temperature. According to the combined strategy of guaranteeing cooling uniformity through dynamic water distribution, eliminating solute segregation through tail end electromagnetic stirring and making up solidification shrinkage gaps under dynamic soft reduction, the severity degree of internal defects of the casting blank is greatly reduced.

Description

Technical Field

[0001] This invention relates to the field of iron and steel smelting technology, specifically to a method and product for controlling the porosity of the control center in the production of bearing steel from small-section cast billets. Background Technology

[0002] In the continuous casting production of bearing steel, obtaining a billet with a uniform and dense internal structure is the primary prerequisite for ensuring the high fatigue life and reliability of the final product. Central porosity, a typical internal defect of continuously cast billets, manifests as tiny, dispersed pores in the central axis region of the billet caused by insufficient feeding of molten steel at the end of solidification. These defects are difficult to completely weld together during subsequent rolling, becoming stress concentration sources within bearing components and significantly reducing their contact fatigue life. This is one of the key bottlenecks affecting the quality stability of high-end bearing steel.

[0003] For the production of small-section billets, controlling central porosity presents a more severe challenge. Due to their rapid solidification rate and steep temperature gradient at the solidification front, the development of the central equiaxed crystal zone is relatively limited, resulting in a narrow and concentrated mushy region after final solidification. This makes it easy for solute elements (such as carbon and manganese) to accumulate in this region, and the voids generated by solidification shrinkage are difficult to be effectively replenished by external molten steel, thus exacerbating the tendency for central porosity and segregation. Therefore, developing efficient central porosity control technology suitable for the characteristics of small-section billets is an important issue for improving the continuous casting production level of high-quality special steels.

[0004] Currently, the industry focuses on three main technologies to improve the quality of the billet center: 1) Dynamic control technology for the secondary cooling zone: This technology aims to create ideal solidification conditions for the billet by optimizing the uniformity and precision of secondary cooling. Common methods include establishing heat transfer models and dynamically adjusting the water volume in each secondary cooling zone through online temperature feedback to reduce surface temperature fluctuations and the resulting uneven internal thermal stress. However, existing technologies mostly focus on overall surface temperature stability or following predetermined cooling curves. They lack specific design for how to accurately match dynamic secondary cooling with the rapid solidification process unique to small-section billets, especially for creating a stable and predictable solidification end position for subsequent metallurgical methods (such as stirring and reduction). 2) Front-end electromagnetic stirring technology (F-EMS): This technology applies electromagnetic force to the mushy zone at the end of billet solidification, driving the flow of solute-rich molten steel, thereby breaking up dendrites, homogenizing composition, and reducing segregation. Publicly available technologies (such as patent CN1021985C) have disclosed methods for applying electromagnetic stirring in continuous casting to improve quality. However, current practices are mostly based on large billets or conventional cross-sections, and the setting of stirring parameters (such as frequency and current) is more effective for cases where the paste-like zone at the end of solidification is long and the liquid cavity is deep. When directly applied to small-section billets, due to the short paste-like zone, the stirring range and intensity of conventional parameters often cannot be precisely matched, which may lead to insufficient stirring effect or energy waste, or even interfere with the already solidified fragile shell layer. 3) Light reduction technology: This technology applies a certain amount of mechanical reduction at the end of the billet solidification to compensate for the solidification shrinkage of the molten steel and physically press together the micro-pores between the dendrites. Dynamic light reduction technology can adjust the reduction position and value in real time according to the casting speed, superheat, etc., and has greater adaptability. Although existing research has confirmed that the combination of end electromagnetic stirring and light reduction technology has a synergistic effect on improving the center quality of large billets, for small-section billets, how to determine the optimal coupling point of stirring and reduction in time and space—that is, after the electromagnetic stirring has fully homogenized the composition of the paste-like zone, timely reduction to complete the feeding—has not yet formed a systematic parameter guide. Simply combining technologies may not achieve a synergistic effect of "1+1>2".

[0005] In summary, while existing technologies offer various methods to improve the center quality of cast billets, they have significant limitations when applied to the specific scenario of small-section bearing steel billets. The various techniques (dynamic secondary cooling, end-of-line electromagnetic stirring, and dynamic light pressure reduction) are often optimized independently or executed in a simple sequential manner, failing to form a deeply synergistic parameterized system tailored to the rapid solidification characteristics of small-section billets. Consequently, it is difficult to stably and efficiently suppress center porosity defects in small-section billets, thus limiting the efficiency and yield of high-quality bearing steel continuous casting production. Summary of the Invention

[0006] This invention aims to overcome the shortcomings of the prior art and provide a method specifically designed to control the central porosity of bearing steel produced in small-section cast billets. The core objective of this invention is to organically integrate and coordinate the parameters of dynamic secondary cooling control, end-stage electromagnetic stirring, and dynamic light reduction. Dynamic secondary cooling provides a stable and controllable solidification foundation for the entire process. Low-frequency, high-current electromagnetic stirring, specifically designed for small sections, precisely targets the short, pasty region. Finally, dynamic light reduction, matched to timing and spatial parameters, completes the final feeding, thus forming a complete, efficient, and suitable internal defect control system for small-section production.

[0007] The purpose of this invention is to address the shortcomings of existing technologies by providing a method for controlling the porosity of the control center in the production of bearing steel from small-section cast billets, comprising: The billet is divided into several areas according to the water distribution area of ​​the secondary cooling zone; The actual surface temperature of each region of the billet is measured in real time, and the theoretical surface temperature of each region of the billet is calculated in real time according to the billet heat transfer equation. If the actual surface temperature of a certain region of the billet is equal to the theoretical surface temperature of that region, the water supply of the current secondary cooling zone to that region is maintained. If the actual surface temperature of a certain region of the billet is not equal to the theoretical surface temperature of that region, the water supply of the current secondary cooling zone to that region is adjusted until the actual surface temperature of that region is equal to the theoretical surface temperature. Based on the real-time determination of the solidification end position of the billet according to the billet heat transfer equation, electromagnetic stirring is applied at the corresponding pasty zone position at the solidification end of the billet, and the direction of the generated Lorentz force is opposite to the direction of billet pulling. Then, dynamic light pressure is applied at the solidification end position of the billet.

[0008] Furthermore, the cross-sectional area of ​​the small-section casting is ≤25600mm². 2 .

[0009] Furthermore, the electromagnetic stirring device has a current intensity of 500~550A and a frequency of 1.5~2.5Hz.

[0010] Furthermore, the alternating magnetic field generated by the electromagnetic stirring device acts on the short, pasty region of the small-section casting.

[0011] Furthermore, the heat transfer equation for the cast billet is as follows: In the formula, The density of the billet, The isochoric heat capacity of the billet. The theoretical surface temperature, For time, Thermal conductivity, , , These represent the x-coordinate, y-coordinate, and height coordinates of a certain position on the casting billet. The strength of the internal heat source of the cast billet. This is the viscous dissipation term of molten steel.

[0012] Furthermore, In the formula, For the flow rate of molten steel at Components in direction, For the flow rate of molten steel at Components in direction, For the flow rate of molten steel at Components in direction.

[0013] Furthermore, the viscous dissipation term of the molten steel Calculated by the following formula: in, The dynamic viscosity of molten steel. , , The steel molten flow rate is respectively at , , Components in direction.

[0014] Furthermore, during continuous casting, the superheat is controlled between 20℃ and 35℃, and the casting speed is controlled between 1 and 3 m / min.

[0015] Furthermore, the total reduction under the dynamic light pressure is 3~8mm.

[0016] The bearing steel billet obtained by using the above-mentioned method of controlling the porosity of the bearing steel production center using small cross-section billets.

[0017] Compared with the prior art, the present invention has the following significant advantages: 1. A targeted collaborative control system has been formed: This invention organically integrates three process steps: dynamic secondary cooling control, end-stage electromagnetic stirring, and dynamic light pressing, with a high-precision billet heat transfer equation as the collaborative hub. Dynamic secondary cooling control ensures the stability and predictability of the solidification process, providing precise spatiotemporal coordinates for subsequent processes; low-frequency high-current electromagnetic stirring, specially designed for small cross-sections, achieves efficient homogenization of the composition in short, pasty regions; and dynamic light pressing implements precise feeding at the optimal time. The deep coupling of these three elements forms a complete defect suppression closed loop targeting the rapid solidification characteristics of small cross-sections.

[0018] 2. Significantly improved internal quality of the billet: Through a composite mechanism of "uniform solidification + composition homogenization + mechanical feeding," the central porosity and segregation level of small-section bearing steel billets can be significantly reduced. Examples show that billets produced using the method of this invention have significantly better central porosity ratings and carbon segregation indices than those produced using traditional processes and simple technology combinations.

[0019] 3. Clear parameters, strong operability and replicability: This invention provides specific process parameter ranges applicable to small cross-section working conditions (such as electromagnetic stirring current 500~550A, frequency 1.5~2.5Hz, and total light reduction 3~8mm), making the technical solution clear and easy to promote and apply on small cross-section continuous casting machines in different steel plants, so as to achieve stable production of high-quality bearing steel.

[0020] 4. Cost-effective and easy to implement: The core of this method lies in the upgrading of the control logic and the integrated application of key modules (such as adding infrared temperature measurement, adapting electromagnetic stirrers, and upgrading the pressure control system). It does not require large-scale modification of the main structure of the continuous casting machine, and has good economic efficiency and feasibility for industrial implementation.

[0021] Dynamic secondary cooling control: The billet is divided into several independent cooling control zones along its length in the secondary cooling zone. The actual surface temperature of the billet in each zone is acquired in real time using an online temperature measurement device (such as an infrared thermal imager). Simultaneously, based on a pre-established billet heat transfer equation embedded in the control system, the theoretical surface temperature of the corresponding zone is calculated in real time. By comparing the actual surface temperature with the theoretical surface temperature, the flow rate of the cooling medium in each zone is dynamically adjusted: if the actual temperature is higher than the theoretical temperature, the cooling intensity of that zone is increased; if the actual temperature is lower than the theoretical temperature, the cooling intensity is decreased. This closed-loop control aims to ensure that the billet surface temperature field strictly follows the ideal solidification path calculated by the model, thereby providing a uniform and stable temperature field foundation for the entire solidification process and accurately predicting the location of the solidification endpoint.

[0022] End-of-line electromagnetic stirring: An electromagnetic stirring device is installed in the secondary cooling zone of the continuous casting machine, corresponding to the position of the mushy zone at the end of the billet solidification. The device is activated to generate an alternating magnetic field. The electromagnetic stirring operates with a current intensity of 500-550A and a low frequency of 1.5-2.5Hz. Through specific magnetic pole design and arrangement, it is ensured that the alternating magnetic field generated under these parameters can effectively penetrate and act on the short mushy zone of the small-section billet, inducing eddy currents in the molten steel. These eddy currents interact with the magnetic field, generating a Lorentz force in the opposite direction to the billet pulling direction. This opposing force drives the solute-rich molten steel in the mushy zone to flow in a directional manner, effectively breaking up dendrites, homogenizing the composition, and reducing central segregation.

[0023] Dynamic light pressure: The solidification end position calculated in real time based on the billet heat transfer equation (usually expressed as solid fraction). The implementation range of the light reduction technology is dynamically set (using 0.8~0.9 as the criterion). Within this range, continuous mechanical reduction is applied to the billet by the reduction roller. The total reduction amount of the dynamic light reduction is controlled between 3 and 8 mm. This step follows or partially overlaps with the electromagnetic stirring period, aiming to mechanically compress the paste-like area that has been homogenized by electromagnetic stirring, directly compensating for the volume voids generated by the solidification shrinkage of the molten steel, and physically pressing together the micropores that have been formed. Attached Figure Description

[0024] Figure 1 This is a flowchart of the method of the present invention.

[0025] Figure 2 This is a low-magnification photograph of the billet sample from Example 1 of the present invention.

[0026] Figure 3 This is a low-magnification photograph of the billet sample from Example 2 of the present invention.

[0027] Figure 4 This is a low-magnification photograph of the billet sample of Comparative Example 1 of the present invention.

[0028] Figure 5 This is a low-magnification photograph of the billet sample of Comparative Example 2 of the present invention. Detailed Implementation

[0029] The present invention is further illustrated below by means of examples and comparative examples. Unless otherwise specified, all pharmaceutical products used in the following examples are commercially available products, and all methods used are conventional methods in the art.

[0030] A method for controlling porosity in the control center of bearing steel production using small-section cast billets includes: Real-time dynamic secondary cooling control. The billet is divided into several zones according to the water distribution area of ​​the secondary cooling zone. An online device (such as an infrared thermal imaging device) is installed in the secondary cooling zone of the continuous casting process to measure the surface temperature of the billet in real time. The actual surface temperature of each zone of the billet is measured in real time, and the theoretical surface temperature of each zone of the billet is calculated in real time according to the heat transfer equation of the billet. If the actual surface temperature of a certain zone of the billet is equal to the theoretical surface temperature of that zone, the water distribution of the current secondary cooling zone to that zone is maintained; if the actual surface temperature of a certain zone of the billet is not equal to the theoretical surface temperature of that zone, the water distribution of the current secondary cooling zone to that zone is adjusted until the actual surface temperature of that zone is equal to the theoretical surface temperature, so that the surface temperature field of the billet is uniform and conforms to the predetermined solidification path.

[0031] The secondary cooling zone is a critical stage for billet solidification. Uneven cooling leads to differences in solidification rates, resulting in uneven volume shrinkage and central porosity. Zoned measurement, adjusting the cooling intensity based on the temperature difference between the theoretical and actual surface temperatures, ensures that the cooling intensity of each zone matches the theoretical solidification requirements. This avoids excessively rapid or slow cooling in certain areas. Excessive cooling generates stress, while insufficient cooling exacerbates solute segregation, thus reducing central porosity caused by uneven solidification shrinkage at its source. Furthermore, parameters such as molten steel temperature and casting speed may fluctuate during continuous casting. Real-time measurement of the billet surface temperature and adjustment of the water distribution to respond to these fluctuations provides higher temperature control accuracy and stability compared to a fixed water distribution scheme, ensuring consistent solidification throughout the entire length of the billet. In addition, precise temperature control creates a stable temperature field environment for technologies such as electromagnetic stirring and dynamic light reduction, preventing the failure of subsequent technologies due to temperature fluctuations.

[0032] Electromagnetic stirring is used in the secondary cooling zone. During the solidification of bearing steel, solutes (such as carbon and chromium) tend to accumulate towards the center of the liquid core, forming a solute-rich zone. During solidification, the shrinkage rate in this zone differs significantly from the matrix, leading to central porosity. Electromagnetic stirring promotes convection in the molten steel, ensuring uniform solute diffusion and preventing solute accumulation in the center of the liquid core, thus suppressing central porosity from the perspective of compositional uniformity. The stirring action can eliminate localized hot or cold spots inside the billet, further improving the temperature uniformity of the secondary cooling zone. This synergistic effect with the billet surface temperature control provides dual protection for solidification consistency. The flow of molten steel can refine the solidification structure, reducing the proportion of columnar crystals and increasing the equiaxed crystal ratio, indirectly improving the density of the microstructure and reducing the risk of central porosity.

[0033] The electromagnetic stirring device is installed in the secondary cooling zone corresponding to the pasty area at the end of solidification. It is crucial to ensure that the main area of ​​the magnetic field coincides with the center of the liquid core, avoiding the magnetic field acting on the already solidified solid phase. This allows the reverse driving force to concentrate on the solute-enriched molten steel at the center of the liquid core. The pasty area at the end of solidification is a high-risk area for solute segregation and central porosity. In a liquid-solid coexistence state, solute easily accumulates in the liquid core, and solidification shrinkage voids easily form here. Installing the stirring device here allows the magnetic field to act directly on the solute-enriched molten steel at the center of the liquid core, avoiding the waste of magnetic field in the already solidified solid phase area, resulting in more targeted and efficient stirring. If the stirring device is installed at the beginning of solidification (the entire liquid phase zone), excessive stirring may impact the incompletely solidified billet shell. Installing it at the end of solidification can precisely improve the liquid core state without damaging the billet shell, balancing the surface quality and internal density of the billet.

[0034] The secondary cooling zone utilizes both electromagnetic stirring and dynamic light reduction. The reduction range of this dynamic light reduction is dynamically set based on the solidification end position calculated in real-time by the heat transfer model, with a total reduction of 3-8 mm. The core dimension for determining the solidification end position is the z-axis (casting direction). As the molten steel gradually solidifies from a completely liquid phase to a solid phase, the solidification end corresponds to the z-coordinate position where "the last point of liquid phase completely transforms into solid phase." The x and y axes are used to assist in calculating the solidification uniformity of the billet cross-section at this z-coordinate (avoiding quality defects caused by localized unsolidified areas). The solidification end position is typically determined by the solid fraction. The criterion is 0.8 to 0.9 for the solid fraction. The specific adjustment depends on the type of steel.

[0035] Dynamic light pressure applies slight pressure during solidification, eliminating tiny voids caused by solidification shrinkage and reducing shrinkage cavities and central cracks. Electromagnetic stirring focuses on homogenizing composition and temperature, suppressing segregation-type porosity, while dynamic light pressure focuses on compensating for solidification shrinkage voids, directly suppressing shrinkage-type porosity. The combination of the two provides dual control over homogenization and densification, which is more effective in reducing central porosity than using electromagnetic stirring alone.

[0036] In a preferred embodiment, the electromagnetic stirring device employs a specific magnetic pole arrangement. When the alternating magnetic field generated by the electromagnetic stirring device passes through the molten steel, eddy currents are induced in the solute-rich molten steel at the center of the liquid core. The interaction between the eddy currents and the magnetic field generates a Lorentz force. The specific magnetic pole arrangement directs the Lorentz force in the opposite direction to the billet pulling. During the billet pulling process, the solute-rich molten steel at the center of the liquid core tends to accumulate along the billet's movement direction, exacerbating center segregation. The opposite Lorentz force can generate a reverse driving force on the solute-rich molten steel, hindering its accumulation towards the center, promoting solute diffusion into the solid phase region, and reducing compositional segregation. The directional Lorentz force can form a stable circulating flow of molten steel, which, compared to non-directional stirring, can more efficiently break the solute-rich layer, further improving compositional uniformity and thus reducing center porosity caused by compositional segregation.

[0037] As a preferred embodiment, the electromagnetic stirring current intensity is 500~550A and the frequency is 1.5HZ. The small cross-section billet has a small liquid core volume and a fast solidification rate. A current intensity of 500~550A can generate sufficient Lorentz force to ensure that the stirring intensity can effectively act on the liquid core. The low-frequency stirring of 1.5HZ can avoid excessively violent flow of molten steel, preventing damage to the billet shell or the generation of new defects, while also matching the solidification rate.

[0038] As a preferred embodiment, the heat transfer equation of the cast billet is as follows: In the formula, The density of the billet, The isochoric heat capacity of the billet. The theoretical surface temperature, For time, Thermal conductivity, , , These represent the x-coordinate, y-coordinate, and height coordinates of a certain position on the casting billet. The strength of the internal heat source of the cast billet. This is the viscous dissipation term of molten steel.

[0039] in, The dynamic viscosity of molten steel. , , The steel molten flow rate is respectively at , , Components in direction.

[0040] in, The latent heat of solidification of bearing steel, denoted as solid fraction.

[0041] In the formula, For the flow rate of molten steel at Components in direction, For the flow rate of molten steel at Components in direction, For the flow rate of molten steel at Components in direction.

[0042] Existing heat transfer equations may neglect factors such as internal heat sources and molten steel convection, leading to large deviations between theoretical and actual temperatures. This equation comprehensively considers three-dimensional heat conduction, internal heat sources (such as latent heat of solidification), viscous dissipation of molten steel, and convection effects (molten steel flow velocities u, v, w), resulting in a theoretical surface temperature that more closely matches the actual solidification state of the cast billet.

[0043] As a preferred implementation method, during continuous casting, the superheat is controlled between 20℃ and 35℃, and the casting speed is controlled between 1 and 3 m / min. A superheat >35℃ will prolong solidification time, exacerbating solute segregation and central porosity; a superheat <20℃ will reduce the fluidity of the molten steel, leading to uneven solidification. A superheat of 20~35℃ balances fluidity and solidification speed, facilitating stirring and compositional homogenization while reducing segregation time. A casting speed of 1~3 m / min is suitable for small-section billets, ensuring production efficiency while allowing sufficient cooling and stirring time in the secondary cooling zone, avoiding insufficient cooling and stirring due to excessively fast casting speed, or low production efficiency due to excessively slow casting speed.

[0044] The three parameters of electromagnetic stirring in this invention can achieve precise matching of the flow field and solute homogenization in the short paste-like region of small cross-section bearing steel: (1) Low frequency (1.5~2.5Hz) achieves deep magnetic field penetration and stable flow field distribution, matching the fast solidification speed. The penetration depth of the alternating magnetic field is inversely proportional to the frequency. Low frequency can greatly improve the penetration depth of the magnetic field in the billet, and can completely cover the short paste-like liquid core of the small cross-section billet, avoiding the problem of "shallow magnetic field penetration and only acting on the surface of the liquid core" in conventional high frequency stirring, ensuring that the steel liquid in the entire short paste-like region can be driven to convect; at the same time, the steel liquid flow speed driven by low frequency stirring is mild and the flow field is stable, and will not produce local turbulence or eddy current impact on the thin solidified billet shell of the small cross-section billet, avoiding the deformation of the billet shell and the generation of surface cracks or internal intermediate cracks; in addition, the low frequency magnetic field action rhythm can match the fast solidification speed of the small cross-section, allowing the solute to have sufficient diffusion and homogenization time in the short paste-like region, solving the core contradiction of "the solute enrichment speed is faster than the diffusion speed" in the short paste-like region. (2) A higher current (500~550A) provides sufficient Lorentz force to drive effective convection of the small volume liquid core. The strength of the Lorentz force is positively correlated with the current intensity. The volume of the liquid core of the small cross-section billet is much smaller than that of the large billet. If the low current stirring of the conventional large billet is used, the generated Lorentz force is insufficient and cannot drive the small volume liquid core molten steel to form effective convection. Only the slight flow of the surface molten steel can be achieved, which cannot break the dendrite network and homogenize the central solute. A high current of 500~550A can generate a sufficiently strong Lorentz force to ensure a stable macroscopic convection flow field in a small volume liquid core, effectively breaking up the columnar dendritic network formed during solidification, allowing the solute elements (key elements of bearing steel such as carbon and chromium) enriched in the center of the liquid core to diffuse towards the solid phase region with the steel liquid convection, thus suppressing solute segregation in the short paste region from the root; at the same time, the current intensity is precisely controlled within the balance range of "effective stirring" and "bill shell protection", and the billet shell will not be deformed due to excessive Lorentz force, thus taking into account both internal solute homogenization and billet surface quality. (3) The opposite Lorentz force (opposite to the billet pulling direction) directionally hinders solute aggregation, realizing efficient solute homogenization in the short paste region. During the billet pulling process of small cross-section billets, the solute-rich steel liquid in the center of the liquid core will be "dragged" along the billet pulling direction, and the narrow space of the short paste region makes it impossible for the solute to diffuse naturally, and it is very easy to accumulate rapidly at the end of solidification, forming serious central segregation. The Lorentz force, opposite to the direction of billet pulling, forms a directional reverse driving force that directly hinders the dragging and accumulation of solute molten steel. At the same time, it forms a circulating convection flow field within the short mushy region: replenishing the fresh molten steel with uniform composition near the solid phase region to the center of the liquid core, while carrying the solute-enriched molten steel in the center towards the solid phase region for diffusion, greatly improving the efficiency of solute homogenization. Compared with non-directional stirring, the directional flow field of the reverse Lorentz force does not require additional energy to flow meaninglessly in molten steel, and can complete efficient composition homogenization within the limited space and time of the short mushy region, solving the core problem of "rapid solute enrichment" in small cross-sections.In summary, the combination of low frequency, high current, and reverse Lorentz force enables electromagnetic stirring to achieve "deep penetration, strong drive, and directional flow" in the short, pasty region of small-section bearing steel. This not only ensures the homogenization of the solute but also adapts to the structure and solidification characteristics of the small-section billet, laying a core foundation for uniform composition under subsequent dynamic light pressure.

[0045] This invention calculates the solidification end position in real time based on the billet heat transfer equation, and realizes precise coupling of electromagnetic stirring and dynamic light pressure in terms of solid fraction range, spatial position and action sequence, which is fully adapted to the rapid solidification process of small cross section billets. The specific matching relationship is as follows: (1) Spatial matching: Both act on the pasty area at the solidification end of the billet, and the electromagnetic stirring device is installed in the solid fraction range. The core position of the paste-like zone is 0.3~0.7. The pressing roller under dynamic light pressure is arranged close to the stirring device to ensure that the action area is highly overlapping, so that the homogenized molten steel after stirring can immediately enter the pressing stage, and avoid solute segregation in the molten steel during subsequent solidification. (2) Solid phase ratio timing matching: Electromagnetic stirring plays a dominant role in the early stage of solidification. At this stage, the molten steel has good fluidity and is the best window for solute homogenization; dynamic light pressure plays a dominant role in the later stage of solidification (solid phase ratio) (0.8~0.9), and the pressing interval partially overlaps with the stirring interval. During this stage, the fluidity of the molten steel gradually decreases and the rigidity of the billet shell gradually increases, which is the best window for mechanical feeding. This sequence of "stirring first, pressing later, and partial overlap" is just right to match the solidification law of small cross-section billets from "liquid and solid coexisting, easy convection" to "high solid phase ratio, easy feeding". The continuous process operation from "uniform composition" to "physical density" is completed in a very short solidification time. (3) Fitting the pressing amount: The billet shell of small cross-section billets is thin and the overall rigidity is poor. If the large pressing amount of conventional large square billets (such as 10mm in the comparative example) is used, it will cause the billet shell to sink and produce transverse or internal cracks. The small pressing amount of 3~8mm is "compensatory precise pressing" for small cross-section billets. Its pressing amount is just right to match the volume gap generated by the solidification shrinkage of small cross-section billets. While completing the feeding, it will not damage the billet structure. This real-time dynamic matching based on the heat transfer equation, unlike the traditional "fixed position, fixed parameter" technology superposition, allows the effects of electromagnetic stirring and dynamic light pressure to be seamlessly connected, providing a precise spatiotemporal process foundation for synergistic compensation.

[0046] The core function of electromagnetic stirring is to achieve component homogenization and suppress segregation-type central porosity; the core function of dynamic light pressure is to compensate for solidification shrinkage and eliminate shrinkage-type central porosity. The synergy between the two is not a simple process superposition, but forms a closed-loop defect suppression mechanism of "component homogenization improves the efficiency of mechanical shrinkage compensation, and mechanical shrinkage makes component homogenization land". Ultimately, it achieves a shrinkage compensation effect that is far superior to that of a single process or a simple superposition process. Specifically, it is manifested as follows: (1) Electromagnetic stirring provides a uniform component and structure basis for dynamic light pressure, making shrinkage compensation more efficient and more uniform. If electromagnetic stirring is not performed, the solute in the short paste-like area of ​​the small cross-section billet will be severely segregated, which will lead to uneven composition and large differences in solidification shrinkage rate in the paste-like area: the solidification shrinkage rate of the molten steel in the solute-rich area is much higher than that of the matrix, and the mechanical force of pressing cannot be uniformly transmitted in the heterogeneous paste-like area, and can only fill part of the voids, and is prone to new microcracks due to local shrinkage differences; however, after electromagnetic stirring, the composition of the paste-like area is highly uniform (carbon segregation index is close to 1), and solidification shrinkage occurs uniformly in the entire paste-like area. The mechanical force of pressing can be transmitted to all positions at the end of solidification without hindrance, so that the small pressing amount of 3~8mm can achieve precise and uniform feeding in the whole area, which greatly improves the feeding efficiency and effect of pressing. (2) Dynamic light pressing makes the homogenization effect of electromagnetic stirring come into play, and realizes billet densification from a physical level. Electromagnetic stirring can only achieve homogenization of the solute and solve the problem of "segregation porosity", but it cannot make up for the physical voids generated by volume shrinkage during the solidification of molten steel. If these voids are not filled in time, they will still form central porosity. Dynamic light pressing immediately applies mechanical pressing to the paste area after the homogenization of the composition is completed by stirring. Through the squeezing force of the pressing roller, the homogenized molten steel quickly fills the tiny voids generated by solidification shrinkage, physically pressing the gaps between dendrites, fundamentally eliminating shrinkage porosity. At the same time, the mechanical action of pressing can further refine the solidification structure and improve the internal density of the billet, so that the homogenization effect of electromagnetic stirring is transformed into the final high quality of the billet. (3) Dual suppression of defects avoids the limitations of a single process and is suitable for the rapid solidification characteristics of small cross sections. The core challenge of small-section cast billets is the rapid solidification rate and the extremely short time window for defect formation. A single process cannot simultaneously solve the two major problems of segregation and shrinkage in a short time: electromagnetic stirring alone cannot compensate for physical voids, and shrinkage-type porosity will still exist; dynamic light pressing alone cannot solve solute segregation, and segregation-type porosity will still exist, and the pressing effect will be greatly reduced. However, the synergistic process of this invention completes the continuous operation of "composition homogenization to mechanical densification" in a short time, and suppresses central porosity from two dimensions. This significantly reduces the central porosity rating of small-section bearing steel cast billets and brings the carbon segregation index close to 1. Its effect is far superior to the "1+1" of a single process or a simple superposition process, achieving a synergistic shrinkage compensation of "1+1>2".

[0047] The following provides Examples 1 and 2, as well as Comparative Examples 1 and 2, to better illustrate the present invention.

[0048] The processes using the method of the present invention in Examples 1 and 2 are as follows: Target billet: Example 1: Producing 160mm × 160mm (25600mm) billets 2 ) Cross-section casting billet; Example 2 also produces 160mm×160mm (25600mm) billets. 2 The cross-section of the billet is made of high-carbon chromium bearing steel (such as GCr15).

[0049] Basic process parameters: The ladle superheat is controlled at 25℃±5℃, and the continuous casting speed is 2.5m / min (Example 1) and 1.5m / min (Example 2).

[0050] Example 1 and Example 2 employ the same dynamic secondary cooling control: the secondary cooling zone is divided into 8 control zones along the billet pulling direction. Each zone is equipped with an infrared thermometer to collect surface temperature in real time. The control system has a built-in billet heat transfer equation, into which steel grade thermophysical parameters (including latent heat of solidification) are input, and the theoretical surface temperature of the billet in each control zone is calculated and output in real time. The controller compares the measured temperature with the theoretical temperature and uses a PID algorithm (i.e., proportional-integral-derivative control algorithm) to dynamically adjust the steam-water atomization cooling water volume in each zone, so that the actual surface temperature of the billet closely follows the theoretical surface temperature curve of the billet.

[0051] Example 1 and Example 2 use the same end electromagnetic stirring: the stirrer is installed on the solid phase ratio... Approximately 0.3-0.7 mm. Start the stirrer, setting the operating parameters to: current intensity 520A, frequency 2.0Hz. A special magnetic pole coil arrangement ensures the generation of an electromagnetic force field in the center of the slurry zone of the billet, with a dominant direction opposite to the casting direction.

[0052] Example 1 and Example 2 use the same dynamic light pressure: the heat transfer model calculates and outputs the solidification end (solid fraction) in real time. =0.9). The light pressing control system dynamically adjusts the opening of the pressing rollers accordingly. The pressing interval covers the solids content. In the region from 0.7 to 1.0, a continuous gradual reduction is applied within this range, with a total reduction of 5 mm.

[0053] Comparative Example 1 The same continuous casting machine and the same billet specifications as in Examples 1 and 2 were used. Secondary cooling employed static water metering; infrared temperature measurement and dynamic model control were not used. The end-effector electromagnetic stirring device and the light reduction device were not activated.

[0054] Comparative Example 2 Static secondary cooling was employed, but the same end electromagnetic stirring and light reduction as in Examples 1 and 2 were used. The stirring parameters were based on experience with large billets (current 450A, frequency 5Hz), the light reduction position was fixed, and the total reduction was 10mm.

[0055] Effect verification For all billets produced in the examples and comparative examples, samples were taken along the length for low-magnification microstructure pickling inspection. The central porosity was rated according to the "YBT153-2015 High-Quality Structural Steel Continuous Cast Billet Low-Magnification Microstructure Defect Rating Chart" (higher rating indicates more severe defects), and the carbon segregation index (central point carbon content / average carbon content) was determined using the electric probe microanalysis method. The results are compared below: Results Analysis: Reference Figures 2-5 Comparative Example 1 showed the most severe defects, indicating that traditional processes cannot meet the high-quality requirements of small-section bearing steel. Comparative Example 2 (simple superposition process) showed improvement compared to Comparative Example 1, but the effect was limited. The high-frequency electromagnetic stirring used in Comparative Example 2 may have failed to form an effective homogeneous flow field in the short paste-like region, and the large reduction rate and fixed position may not have been compatible with the solidification process of the small section.

[0056] Examples 1 and 2 (the method of this invention) achieved optimal and stable results. The center porosity rating was significantly reduced, the carbon segregation index approached 1, and the composition was extremely uniform. This fully demonstrates that the low-frequency high-current stirring parameters (520A, 2.0Hz) for small cross-sections proposed in this invention, as well as the precise spatiotemporal synergy of electromagnetic stirring and dynamic light compression (5mm) based on a dynamic secondary cooling model, have unexpectedly excellent effects on solving the center porosity problem of small cross-section bearing steel.

[0057] The above-described embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should all be included within the protection scope of the present invention.

Claims

1. A method of controlling the center porosity of a bearing steel produced in a small section billet, characterized by, include: The billet is divided into several areas according to the water distribution area of ​​the secondary cooling zone; The actual surface temperature of each region of the billet is measured in real time, and the theoretical surface temperature of each region of the billet is calculated in real time according to the heat transfer equation of the billet. If the actual surface temperature of a certain region of the billet is equal to the theoretical surface temperature of that region, the water supply of the current secondary cooling zone to that region is maintained. If the actual surface temperature of a certain area in the billet is not equal to the theoretical surface temperature of that area, then adjust the water distribution of the current secondary cooling zone to that area until the actual surface temperature of that area equals the theoretical surface temperature. Based on the real-time determination of the solidification end position of the billet according to the billet heat transfer equation, electromagnetic stirring is applied at the corresponding pasty zone position at the solidification end of the billet, and the direction of the generated Lorentz force is opposite to the direction of billet pulling. Then, dynamic light pressure is applied at the solidification end position of the billet.

2. The method for controlling the porosity of the bearing steel production center in small-section cast billets according to claim 1, characterized in that: The small section billet has a cross-sectional area ≤ 25600 mm 2 .

3. The method for controlling the porosity of the bearing steel production center in small-section cast billets according to claim 1, characterized in that: The electromagnetic stirring current intensity is 500~550A and the frequency is 1.5~2.5Hz.

4. The method for controlling the porosity of the bearing steel production center in small-section cast billets according to claim 1, characterized in that: The alternating magnetic field generated by the electromagnetic stirring acts on the short, pasty region of the small-section cast billet.

5. The method of claim 1, wherein the method is characterized by, The heat transfer equation for the cast billet is as follows: wherein is the density of the casting blank, is the specific heat capacity of the casting blank, is the theoretical surface temperature, is the time, is the thermal conductivity, , , are the horizontal, vertical and height coordinates of a certain position on the casting blank, respectively, is the internal heat source intensity of the casting blank, is the viscous dissipation term of the liquid steel.

6. The method for controlling the porosity of bearing steel production using small-section cast billets according to claim 5, characterized in that: wherein is the flow velocity of the steel liquid in the direction is the flow velocity of the steel liquid in the direction is the flow velocity of the steel liquid in the direction is the flow velocity of the steel liquid in the direction is the flow velocity of the steel liquid in the direction is the flow velocity of the steel liquid in the direction 7. The method of claim 5, wherein the method is characterized by, the viscous dissipation term of the steel liquid is calculated from the formula: wherein, is the dynamic viscosity of the liquid steel, , , are the components of the liquid steel flow rate in the directions x, y, z, respectively. , , are the components of the liquid steel flow rate in the directions x, y, z, respectively.

8. The method for controlling the porosity of the bearing steel production center in small-section cast billets according to claim 1, characterized in that: During continuous casting, the superheat is controlled between 20℃ and 35℃, and the casting speed is controlled between 1 and 3 m / min.

9. The method for controlling the porosity of the bearing steel production center in small-section cast billets according to claim 1, characterized in that: The total reduction under dynamic light pressure is 3~8mm.

10. Bearing steel billet obtained by the method for controlling the porosity of the bearing steel production center using any one of claims 1 to 9.

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