A whole-process coordinated control method for reducing the total curvature of long fixed-length small square billets

Abstract

This invention provides a comprehensive, coordinated control method for reducing the total curvature of long, fixed-length small square billets. It covers three stages: continuous casting process optimization, cooling bed control, and slow cooling during stacking. Based on the metallurgical mechanism of bainitic phase transformation, a complete temperature control chain is constructed to systematically eliminate multiple sources of curvature, including uneven cooling, phase transformation stress, and additional stress from stacking. Compared with existing technologies, this invention requires no additional specialized equipment and, without significantly increasing production costs, ensures that the total curvature of small square billets is stably controlled within 60mm, with a curvature per meter not exceeding 10mm. This significantly improves product quality, production efficiency, and production safety, effectively solving technical problems such as difficulty in charging the heating furnace, rolling defects, high scrap rates, and increased production costs caused by excessive curvature of small square billets, thus providing qualified billets for subsequent steel rolling production.

Description

Technical Field

[0001] This invention belongs to the field of steel manufacturing technology and relates to a method for controlling the total curvature of long fixed-length small square billets. Background Technology

[0002] Small billets are the core raw material for steel rolling production, and their total curvature directly determines the stability of subsequent rolling processes and the product qualification rate. Small billets with excessive total curvature are prone to steel piling and jamming accidents when entering the heating furnace. During the rolling process, they are prone to defects such as folding, difficulty in biting, and excessive dimensional deviations. In severe cases, the entire billet must be scrapped, which not only wastes steel resources but also significantly increases production costs. From the perspective of causes, excessive curvature of small billets involves multiple interrelated factors, including uneven cooling in continuous casting (deviation of the water gap in the crystallizer or blockage of the secondary cooling nozzle, resulting in inconsistent solidification of the billet shell and accumulation of internal thermal stress), improper cooling process after casting (for steel grades such as medium carbon chromium molybdenum alloy steel, which are prone to bainitic phase transformation, the phase transformation volume change is large and the billet strength is already high when it occurs, making it impossible to straighten by its own weight and form permanent deformation), unreasonable stacking method (uneven stack bottom or lack of heat preservation and slow cooling measures, resulting in uneven cooling temperature gradient and accumulation of additional stress), and centering deviation of continuous casting equipment (inaccurate arc alignment or wear of support rollers, causing uneven external force on the billet during operation).

[0003] Existing technologies for controlling the curvature of small billets have the following main shortcomings: 1. Limited control dimension: Existing methods often focus on a single production stage, either optimizing continuous casting cooling parameters or improving stacking methods. They fail to integrate continuous casting, cooling, and stacking into a unified control framework, resulting in the effects of improvements in one stage being offset by defects in other stages, making it difficult to consistently achieve the required overall curvature. 2. Difficulty in balancing cost and effectiveness: Some companies use specialized equipment such as insulation pits or slow cooling pits for slow cooling of billets. While this has some effect, it requires significant investment in equipment construction and maintenance, making it uneconomical for widespread adoption. The existing process schemes generally do not quantitatively correlate the setting of billet cooling process parameters with the bainitic transformation temperature range of the target steel grade. The key temperature thresholds are mostly set by experience and lack metallurgical basis. The control effect is particularly limited when it comes to alloy steel grades that are prone to bainitic transformation. The existing technology does not establish a complete control chain from the temperature of the straightening machine to the upper cooling bed, lower cooling bed and stacking temperature. It cannot ensure that the billet is always in a uniform cooling state within the bainitic transformation sensitive temperature range, and the bending risk cannot be eliminated from the source. Summary of the Invention

[0004] To address the problems of single control dimension, difficulty in balancing cost and effectiveness, lack of systematic utilization of phase transformation mechanisms, and lack of systematic control of temperature nodes throughout the entire process in the methods for controlling the curvature of small billets described in the background art, this invention provides a whole-process coordinated control method for reducing the total curvature of long fixed-length small billets. This method covers the coordinated control of the entire process of continuous casting, cooling, stacking, and inspection during the preparation of small billets. Through continuous casting process optimization, precise control of billet cooling, and scientific improvement of slow cooling during stacking, it achieves precise and stable control of the total curvature of small billets without significantly increasing production costs. This ensures that the total curvature of small billets is ≤60mm, and the curvature per meter is ≤10mm, improving the production quality and efficiency of small billets, meeting the billet quality requirements of subsequent steel rolling production, while reducing scrap rate and overall production costs, and improving production safety and operability.

[0005] The method of the present invention includes: Continuous casting process optimization: By dynamically coordinating and adjusting the casting speed, the cooling water volume of the crystallizer, and the ratio of secondary cooling water, the temperature of the billet exiting the straightening machine is controlled within the range of 900–1000℃, ensuring that the billet enters the subsequent cooling stage at a uniform temperature. Maintaining the temperature within this range ensures that the upper limit of the 900–1000℃ temperature window guarantees that the billet enters the cooling bed at a sufficiently high temperature, preserving its thermoplastic deformation capacity and facilitating natural straightening through uniform heat dissipation on the cooling bed. The lower limit ensures that the billet has undergone sufficient solidification and cooling, resulting in a more uniform internal temperature field and providing favorable initial temperature conditions for uniform passage through the phase transformation zone.

[0006] Rapid loading onto the cooling bed: After the billet is cut off, it should be immediately loaded onto the rotating cooling bed. The time from cutting to loading onto the cooling bed should not exceed 3 minutes. The temperature of the cooling bed should not be lower than 850℃. Before loading onto the cooling bed, the billet should not be subjected to local air cooling or water cooling. The lower limit of the upper cooling bed temperature is set at 850℃. The metallurgical basis for this is that for steels such as medium carbon chromium-molybdenum alloy steel that are prone to bainitic phase transformation, the bainitic transformation initiation temperature (Bs) is usually in the range of 550–650℃. However, the billet is still in the austenitic phase region when it is above 850℃, and has sufficient high-temperature plasticity. If the billet is locally cooled before being placed on the upper cooling bed, it will lead to uneven temperature distribution along the length direction, which will then cause asynchronous volume changes when passing through the phase transformation zone, inducing irreversible bending. During this transition stage, it is strictly forbidden to apply air cooling or water cooling to any local part of the billet to ensure that the billet has a uniform temperature distribution along the length direction when it is placed on the upper cooling bed. This avoids local uneven cooling in the bainitic phase transformation sensitive temperature range and ensures that the billet has a uniform temperature reference along its entire length.

[0007] Uniform cooling on the cooling bed: Adjust the rotation speed of the rotating cooling bed to ensure that the maximum interval between billets of the same batch on the cooling bed does not exceed two, controlling the uniform cooling of the billets on the cooling bed, with the temperature of the lower cooling bed not exceeding 650℃. The maximum interval of no more than two billets ensures that each billet experiences the same cooling time and environment. Setting the upper limit of the lower cooling bed temperature to 650℃ is based on the bainitic transformation termination temperature (Bf): when the billet leaves the cooling bed, the temperature is already below 650℃ or even 600℃, meaning that the bainitic transformation has been completed in a controlled manner in the uniform cooling environment of the cooling bed. Afterward, the billet as a whole is in the cooling stage after the phase transformation, significantly reducing the risk of phase transformation stress superposition caused by uneven cooling.

[0008] Stacking and slow cooling: After the billets are removed from the cooling bed, they are stacked closely in a grid pattern and cooled in a closed storage area until the billet temperature drops below 100℃ before being unstacked. This ensures that the billets cool evenly in all directions and without additional deformation driving force throughout the slow cooling process.

[0009] Furthermore, the cross-section of the cast billet is 120mm×120mm to 170mm×170mm, and the fixed length is 10-12m.

[0010] Furthermore, in the continuous casting process, the casting speed is controlled at 1.8–2.2 m / min, the crystallizer water flow rate is controlled at 1800–2100 L / min, and the secondary cooling water flow rate is controlled at 0.2–0.8 L / kg, and is dynamically adjusted within the above range according to the steel grade, cross-section, and tundish temperature.

[0011] Furthermore, the temperature of the billet exiting the straightening machine is controlled within the range of 930–980℃.

[0012] Furthermore, the effective length of the rotating cooling bed is not less than 12m, and the time for each rotation cycle is controlled within 35–50 seconds.

[0013] Furthermore, a tumbling cycle time of 42.5-50 seconds is adopted to extend the residence time of the billet on the cooling bed and control the temperature of the billet on the lower cooling bed to below 600℃. Preferably, a tumbling frequency close to the lower limit is used to extend the residence time of the billet on the cooling bed, so that the temperature of the lower cooling bed is preferably controlled below 600℃.

[0014] Furthermore, symmetrical mist cooling devices or uniform water spray cooling systems are installed on both sides of the tilting cooling bed to cool the billet. This further improves the cooling uniformity of all surfaces of the billet and accelerates the overall cooling rate, while meeting the temperature requirements of the lower cooling bed and shortening the time the cooling bed is occupied.

[0015] Furthermore, before stacking the billets, the ground at the stacking site must be prepared, and the bottom layer must be fully covered with steel pads. The levelness of the steel pads must be checked using a level or by manually pulling a string to ensure that the deviation of any position of the steel pads from the reference plane does not exceed 20mm.

[0016] Furthermore, a grid stacking method is adopted, with each layer of billets densely packed together and the interval between adjacent billets less than 10mm; the difference between the total width of each stack and the length of a single billet is controlled within 1000mm, ensuring that the suspended part at both ends of the top layer of billets does not exceed 50mm, and preventing the billets from bending downwards due to their own weight.

[0017] Furthermore, billet stacking should be carried out in a closed storage area, and billet stacks should not be placed in ventilation openings or through-draft paths to avoid uneven airflow cooling of the billets during the stacking cooling process, and to ensure that the cooling rate of each surface is uniform.

[0018] Compared with the prior art, the present invention has the following advantages: (1) Multi-stage coordinated regulation and systematic elimination of bending causes: This invention incorporates three stages of continuous casting process optimization, cooling bed cooling control and stacking slow cooling into a unified control framework. For the multiple causes of excessive bending of small billets, such as uneven cooling, phase transformation stress and stacking additional stress, the whole process is coordinated and intervened. This avoids the problem that existing single-stage improvements are offset by defects in other stages. It eliminates the bending driving force from the root and ensures that the total bending of small billets is stably controlled within 60mm and the bending per meter is no more than 10mm. (2) Complete temperature control chain effectively avoids phase transformation-induced bending: Based on the metallurgical mechanism of bainitic phase transformation, this invention associates the setting of key temperature thresholds with the bainitic transformation temperature range (Bs to Bf), and sequentially associates the temperature window of the straightening machine (900–1000℃), the lower limit of the upper cooling bed (≥850℃), the upper limit of the lower cooling bed (≤650℃), and the stacking termination temperature (≤100℃) to construct a complete temperature control chain. By locking the temperature nodes step by step, it ensures that the billet maintains thermoplasticity in the austenitic region and completes the bainitic phase transformation in a controlled manner in the uniform cooling environment of the cooling bed, avoiding irreversible bending deformation caused by local asynchronous volume changes, thereby achieving active prevention of bending deformation and solving the problem that the key temperature thresholds of the existing technology rely on experience and lack metallurgical basis; (3) Refinement of cooling bed process and ensure uniformity of cooling process: By limiting the time from cutting to completion of cooling bed to no more than 3 minutes, strictly prohibiting local air cooling or water cooling before cooling bed, and controlling the maximum interval of the same batch of billets on the cooling bed to no more than 2, we ensure that each billet experiences the same cooling time and cooling environment, eliminate the uneven temperature distribution along the length of the billet and between batches, and avoid additional bending stress caused by local cooling entering the phase transformation zone first. (4) Stacking and slow cooling is scientific and feasible, and reduces the risk of additional deformation in the final stage of cooling: The grid method of close stacking is adopted to carry out heat preservation and stacking cooling in the closed warehouse area. The stacking can only be removed when the billet temperature drops below 100℃, so that the billet can dissipate heat evenly in all directions during the cooling stage after the phase transformation, eliminating the temperature gradient and additional stress accumulation caused by unreasonable stacking, thereby further suppressing the development of bending deformation in the final stage of cooling. (5) No need for large-scale special equipment and low overall production cost: This invention relies on the dynamic adjustment of existing process parameters such as casting speed and water volume in continuous casting and the speed control of the rotating cooling bed. Combined with the close-packing method, it does not require additional investment in special equipment such as heat preservation pits and slow cooling pits. It achieves precise control of bending degree without significantly increasing equipment construction and daily maintenance costs. It has strong economic feasibility and can be widely promoted and applied. (6) Wide range of applicable steel types, covering both ordinary steel and phase transformation sensitive alloy steel: This invention is applicable to various small square billets with cross-sections of 120mm×120mm to 170mm×170mm and fixed lengths of 10 to 12m. It is applicable to both ordinary carbon structural steel and has a targeted control effect on bainitic phase transformation sensitive steels such as medium carbon chromium molybdenum alloy steel. It solves the problem that the existing methods have limited control effect on special steel types, and its versatility and adaptability are significantly improved. (7) Clear process parameters, standardized operation and strong traceability: Each link has clear temperature nodes and time requirements, and the operation process is standardized. On-site workers do not need to master complicated operation skills to perform the operation. At the same time, each temperature node can be recorded and traced in real time, which facilitates the analysis and improvement of abnormal working conditions and helps to continuously and stably control production quality. (8) Reduce production accidents and improve production safety: The effective reduction of the total curvature of the billet directly reduces the risk of steel piling and jamming when entering the heating furnace, as well as the probability of defects such as difficulty in biting and folding during rolling; the close-packing method also reduces the safety hazards of billet tipping and collision during stacking, and improves the overall safety of the production process. It has been verified that the present invention can reduce the scrap rate to below 0.3%.

[0019] In summary, this invention provides a comprehensive, coordinated control method for reducing the total curvature of long, fixed-length small square billets, covering three stages: continuous casting process optimization, cooling bed control, and slow cooling via stacking. The method involves dynamically adjusting the casting speed and cooling water volume to control the temperature of the billet exiting the straightening machine at 900–1000°C. After cutting, the billet is placed on the cooling bed within 3 minutes with a temperature not lower than 850°C. The temperature of the lower cooling bed is controlled to not exceed 650°C. Finally, the billet is densely stacked using a grid method until the billet temperature drops below 100°C before being unstacked. These measures are based on the bainitic phase transformation metallurgical mechanism, constructing a complete temperature control chain to systematically eliminate multiple sources of curvature, including uneven cooling, phase transformation stress, and additional stress from stacking. Compared with existing technologies, this invention requires no additional specialized equipment and can ensure that the total curvature of small billets is stably controlled within 60mm and the curvature per meter is no more than 10mm without significantly increasing production costs. This significantly improves product quality, production efficiency and production safety, and effectively solves technical problems such as difficulty in charging into the heating furnace, rolling defects, high scrap rate and increased production costs caused by excessive curvature of small billets, thus providing qualified billets for subsequent steel rolling production. Detailed Implementation

[0020] To make the technical problems, technical solutions, and beneficial effects to be solved by this application clearer, the following detailed description is provided in conjunction with embodiments. It should be understood that the specific embodiments described herein are merely illustrative of this application and are not intended to limit this application.

[0021] This application provides a comprehensive, coordinated control method for reducing the total curvature of long, fixed-length small square billets. Starting from the metallurgical mechanism of bainitic phase transformation in the billet, it correlates the setting of key temperature thresholds with the bainitic transformation temperature range (Bs to Bf), constructing a complete temperature control chain from the temperature window at the straightening machine → the lower limit of the upper cooling bed → the uniform cooling process on the cooling bed → the upper limit of the lower cooling bed → the termination temperature of slow cooling in the stacking cooling process. By locking the temperature nodes step by step, it ensures that the billet always passes through the phase transformation temperature range most prone to uneven volume changes in a uniform manner throughout the entire cooling process, thereby achieving proactive prevention of bending deformation. The specific technical solution is as follows: Continuous casting process optimization: By dynamically coordinating and adjusting the casting speed, the cooling water volume of the crystallizer, and the ratio of secondary cooling water, the temperature of the billet exiting the straightening machine is controlled within the range of 900–1000℃, ensuring that the billet enters the subsequent cooling stage at a uniform temperature. Maintaining the temperature within this range ensures that the upper limit of the 900–1000℃ temperature window guarantees that the billet enters the cooling bed at a sufficiently high temperature, preserving its thermoplastic deformation capacity and facilitating natural straightening through uniform heat dissipation on the cooling bed. The lower limit ensures that the billet has undergone sufficient solidification and cooling, resulting in a more uniform internal temperature field and providing favorable initial temperature conditions for uniform passage through the phase transformation zone.

[0022] Specifically, the cross-section of the cast billet is 120mm×120mm to 170mm×170mm, and the fixed length is 10-12m. In the continuous casting process, the casting speed is controlled at 1.8–2.2m / min, the crystallizer water flow rate is controlled at 1800–2100L / min, and the secondary cooling water flow rate is controlled at 0.2–0.8L / kg, with dynamic adjustments made within these ranges based on the steel grade, cross-section, and tundish temperature. The billet exiting the straightening machine temperature is preferably controlled within the range of 930–980℃.

[0023] Rapid loading onto the cooling bed: After the billet is cut off, it should be immediately loaded onto the rotating cooling bed. The time from cutting to loading onto the cooling bed should not exceed 3 minutes. The temperature of the cooling bed should not be lower than 850℃. Before loading onto the cooling bed, the billet should not be subjected to local air cooling or water cooling. The lower limit of the upper cooling bed temperature is set at 850℃. The metallurgical basis for this is that for steels such as medium carbon chromium-molybdenum alloy steel that are prone to bainitic phase transformation, the bainitic transformation initiation temperature (Bs) is usually in the range of 550–650℃. However, the billet is still in the austenitic phase region when it is above 850℃, and has sufficient high-temperature plasticity. If the billet is locally cooled before being placed on the upper cooling bed, it will lead to uneven temperature distribution along the length direction, which will then cause asynchronous volume changes when passing through the phase transformation zone, inducing irreversible bending. During this transition stage, it is strictly forbidden to apply air cooling or water cooling to any local part of the billet to ensure that the billet has a uniform temperature distribution along the length direction when it is placed on the upper cooling bed. This avoids local uneven cooling in the bainitic phase transformation sensitive temperature range and ensures that the billet has a uniform temperature reference along its entire length.

[0024] Specifically, the effective length of the rotating cooling bed should be no less than 12m, and the time for each rotation cycle should be controlled within 35–50 seconds.

[0025] More specifically, a tumbling cycle time of 42.5-50 seconds is used to extend the residence time of the billet on the cooling bed and control the temperature of the billet on the lower cooling bed to below 600℃. That is, a tumbling frequency close to the lower limit is preferably used to extend the residence time of the billet on the cooling bed, so that the temperature of the lower cooling bed is preferably controlled below 600℃.

[0026] Uniform cooling on the cooling bed: Adjust the rotation speed of the rotating cooling bed to ensure that the maximum interval between billets of the same batch on the cooling bed does not exceed two, controlling the uniform cooling of the billets on the cooling bed, with the temperature of the lower cooling bed not exceeding 650℃. The maximum interval of no more than two billets ensures that each billet experiences the same cooling time and environment. Setting the upper limit of the lower cooling bed temperature to 650℃ is based on the bainitic transformation termination temperature (Bf): when the billet leaves the cooling bed, the temperature is already below 650℃ or even 600℃, meaning that the bainitic transformation has been completed in a controlled manner in the uniform cooling environment of the cooling bed. Afterward, the billet as a whole is in the cooling stage after the phase transformation, significantly reducing the risk of phase transformation stress superposition caused by uneven cooling.

[0027] Specifically, if necessary, air mist cooling devices or uniform water spray cooling systems can be symmetrically installed on both sides of the tilting cooling bed to cool the billet, thereby further improving the cooling uniformity of each surface of the billet and accelerating the overall cooling rate, while meeting the temperature requirements of the lower cooling bed and shortening the time occupied by the cooling bed.

[0028] Stacking and slow cooling: After the billets are removed from the cooling bed, they are stacked closely in a grid pattern and cooled in a closed storage area until the billet temperature drops below 100℃ before being unstacked. This ensures that the billets cool evenly in all directions and without additional deformation driving force throughout the slow cooling process.

[0029] Specifically, before stacking the billets, the ground at the stacking site must be prepared, and the bottom layer must be fully covered with steel pads. The levelness of the steel pads must be checked using a level or by manually pulling a line to ensure that the deviation of any position of the steel pads from the reference plane does not exceed 20mm.

[0030] Specifically, a grid stacking method is adopted, with each layer of billets densely packed together and the interval between adjacent billets less than 10mm; the difference between the total width of each stack and the length of a single billet is controlled within 1000mm, ensuring that the suspended part at both ends of the top layer of billets does not exceed 50mm, and preventing the billets from bending downwards due to their own weight.

[0031] Specifically, billet stacking should be carried out in a closed storage area. Billet stacks should not be placed in ventilation openings or through-draft paths to avoid uneven airflow cooling of the billets during the stacking cooling process and to ensure that the cooling rate of each surface is uniform.

[0032] In addition, this application can also add online monitoring methods for billet temperature by setting infrared temperature measuring devices at the cooling bed inlet, cooling bed outlet and stacking area, and feeding the measured temperature back to the turning cooling bed speed control system and the secondary cooling water volume control system to form a closed-loop regulation, as described below.

[0033] I. Arrangement Scheme of Temperature Measuring Device Online infrared temperature measurement devices are installed at the following four key nodes along the entire cooling path of the billet casting process: The first temperature measurement point is located at the end of the cutting roller conveyor (about 2m in front of the cooling bed inlet). A dual-color infrared thermometer is used, with a temperature measurement range of 700–1100℃ and an accuracy of ±5℃. The head, middle and tail of the billet are scanned simultaneously to obtain the temperature distribution curve of the billet along the length direction, which serves as the actual measurement basis for the upper cooling bed temperature. At the same time, the data is uploaded to the continuous casting secondary cooling water control system to trigger feedback adjustment.

[0034] The second temperature measurement point is located in the middle section of the rotating cooling bed (about 1 / 2 of the total length from the cooling bed inlet). A linear array infrared thermal imager is used, with a temperature measurement range of 500–900℃ and an accuracy of ±8℃. Continuous thermal imaging scanning is performed on the entire side of the billet passing through this section, and the cross-sectional temperature distribution map of the billet and the temperature gradient data along the length direction are output in real time, which serve as the dynamic input signal for the mid-section correction of the cooling bed rotation speed.

[0035] The third temperature measurement point is located at the outlet of the cooling bed. It uses a dual-color infrared thermometer of the same specifications as the first temperature measurement point, with a temperature measurement range of 300–700℃ and an accuracy of ±5℃. The billet is scanned at three points (head, middle, and tail) before it is placed on the cooling bed to obtain the actual temperature of the lower cooling bed. This temperature is used as a key verification basis for determining the degree of completion of the bainitic phase transformation. The data is also synchronously fed back to the cooling bed speed control system for feedforward correction of the flipping parameters for the next batch.

[0036] The fourth temperature measurement point is located above the stack in the stacking area. It uses a wide-angle infrared thermal imager with a temperature measurement range of 50–200℃ and an accuracy of ±3℃. It periodically scans the billets in the stack (it is recommended to collect data once every 2 hours) to monitor the temperature uniformity and overall cooling trend of the billets in the stack during the slow cooling process. When the measured temperature is below 100℃, it sends a confirmation signal to the operating system to allow the stacking to be removed.

[0037] II. Closed-loop feedback regulation logic The measured data from each temperature measuring point are uploaded in real time to the process control-level PLC system via industrial Ethernet, forming two independent closed-loop control loops: Loop 1: Using the data from the first temperature measurement point as the main signal, feedback regulation is implemented for the secondary cooling water volume in continuous casting. When the measured temperature of the upper cooling bed of the billet is below 850℃, the system automatically sends a water reduction command to the secondary cooling control unit, adjusting the secondary cooling water volume in increments of 0.02–0.05 L / kg until the temperature of the upper cooling bed of the subsequent billets rises back to the target range (850–900℃). Conversely, when the temperature of the upper cooling bed is above 950℃, the system triggers a water increase command, increasing the secondary cooling water volume in the same increments. After each adjustment, an observation cycle of 3–5 billets is set, and the temperature response is stabilized before determining whether to continue adjusting to prevent over-adjustment oscillation.

[0038] Loop Two: Using the data from the second and third temperature measurement points as a combined input signal, the speed of the rotating cooling bed is dynamically adjusted. The control system calculates the actual cooling rate of the current batch of billets in real time based on the billet temperature gradient data output by the thermal imager in the middle of the cooling bed, and compares it with the target cooling rate (preset based on the CCT curve of the steel grade; the typical value for SCM435 is 2–4℃ / s). If the actual cooling rate is too low (the temperature in the middle of the billet is too high), the cycle time of the rotating cooling bed is appropriately reduced (i.e., the rotation frequency is increased), extending the exposure time of the billet on the cooling bed; if the cooling rate is too high, the rotation cycle is extended. Simultaneously, the measured temperature of the lower cooling bed at the third temperature measurement point is used as a verification signal: if the lower cooling bed temperature of three consecutive billets is higher than 600℃, the system automatically locks the current rotation speed and further reduces it to ensure that subsequent billets completely pass through the bainitic transformation zone before leaving the cooling bed; if the measured temperature is lower than 450℃, an appropriate speed increase is allowed to improve the production rhythm while avoiding thermal stress cracks on the billet surface due to overcooling.

[0039] III. Alarm and Interlock Protection for Abnormal Operating Conditions When the temperature of the cast billet collected at any temperature measuring point exceeds the preset upper and lower limits, the system triggers a three-level response mechanism: The first level is a yellow warning, which displays a deviation prompt on the control panel interface to remind the operator to pay attention; the second level is an orange alarm, which automatically pushes adjustment suggestions to the control panel when the temperature deviation exceeds the set value ±20℃ and continues to exceed two cast billets, and records a snapshot of the complete process parameters at the time of the alarm; the third level is a red interlock, which sends a forced deceleration signal to the continuous casting main control when the temperature on the upper cooling bed is continuously below 830℃, and at the same time triggers an audible and visual alarm on the control panel to prompt manual intervention and prevent a large number of cast billets from being placed on the cooling bed in an uneven state within the phase transformation sensitive temperature range.

[0040] The introduction of the aforementioned online temperature measurement and closed-loop control system upgrades the temperature control mechanism of this invention from "open-loop execution relying on preset parameters" to "dynamic closed-loop adjustment based on real-time temperature feedback," fundamentally solving the core defects of existing technologies where key temperature nodes rely solely on offline inspection and lack real-time sensing and proactive correction capabilities. This further enhances the technical integrity and engineering reliability of the whole-process collaborative control scheme.

[0041] The preferred embodiments and their technical rationality of this application are described in detail below, in conjunction with Examples 1-3 (covering three typical scenarios: summer operation of SCM435 alloy steel 160mm×160mm×12m, Q355B plain carbon steel 150mm×150mm×10m, and winter operation of 42CrMo alloy steel 120mm×120mm×10m) and Comparative Examples 1-2 (respectively verifying two typical failure modes: uncontrolled temperature nodes on the upper cooling bed and non-standard slow cooling of stacking).

[0042] Example 1 This embodiment targets a medium-carbon chromium-molybdenum alloy steel billet with a cross-section of 160mm×160mm and a fixed length of 12m (steel grade SCM435, which undergoes bainitic phase transformation in the cooling process at approximately 560–620℃, accompanied by significant volume expansion, and is a representative steel grade with a high risk of bending). The method of this invention is used to control the bending degree throughout the entire process, and the specific steps are as follows.

[0043] During the continuous casting stage, the casting speed was set to 2.0 m / min, the crystallizer water flow rate was set to 1900 L / min, and the secondary cooling water flow rate was set to 0.66 L / kg. The temperature was dynamically adjusted in real time according to the temperature change of the tundish, and the temperature of the billet exiting the straightening machine was finally stabilized at 968℃, which is within the preferred range of 930–980℃ of this invention.

[0044] After exiting the straightening machine, the billet runs along the roller conveyor to the entrance of the tilting cooling bed. The time from cutting to completion of the upper cooling bed is controlled within 1.6–2.8 minutes. The actual measured temperature of the upper cooling bed is 861℃, which strictly meets the requirement of not lower than 850℃. Furthermore, no local air cooling or water cooling occurs during the entire conveying process.

[0045] During the cooling bed stage, the rotation time of each cycle of the rotating cooling bed was set to 44 seconds, with a spacing of two billets per batch. To accelerate uniform cooling and ensure consistent cooling rates on all surfaces, a mist cannon was added on each side of the middle section of the rotating cooling bed to provide symmetrical and uniform spray cooling for the billets. The final billet temperature on the cooling bed was 521℃, significantly lower than the preferred lower limit of 600℃, confirming that the billet had completely passed through the bainitic phase transformation range in a uniform cooling environment.

[0046] During the slow cooling stage of stacking, after the billets are removed from the cooling bed, they are transported to a closed storage area. Before stacking, the ground at the stacking location is leveled, and three straight steel plates, each 12m long and 100mm thick, are laid. The levelness of the plates is checked manually using a string line, measuring along six control lines in both the transverse and longitudinal directions. The maximum measured unevenness is 15mm, meeting the requirement of not exceeding 20mm. 68 billets are stacked in each layer, arranged closely in a grid pattern, with the interval between adjacent billets less than 10mm. The overhang of the billets at both ends beyond the lower layer is approximately 50mm, meeting the upper limit requirement specified in this invention. After stacking, the billets are allowed to cool naturally in the closed storage area for approximately 48 hours. At this point, the measured temperature of the billets is 88℃, below the 100℃ lower limit for dismantling, and the dismantling inspection is completed.

[0047] Test results: All 300 small square billets in this batch were qualified, with the total curvature of each billet not exceeding 60mm and the scrap rate being zero. No bending-related production defects such as steel piling or folding occurred in the subsequent heating furnace and rolling processes, verifying the control effect of the method of this invention on typical bainitic transformation steel grades.

[0048] Example 2 This embodiment focuses on a low-alloy high-strength structural steel billet with a cross-section of 150mm×150mm and a fixed length of 10m (steel grade Q355B, which has a lower alloy content and weaker bainitic phase transformation sensitivity than medium-carbon chromium-molybdenum alloy steel, and belongs to the representative carbon steel grade with medium to low bending risk). Under the high temperature environment (ambient temperature of about 32℃) in summer, the method of this invention is used to control the bending degree throughout the entire process. The specific steps are as follows.

[0049] During the continuous casting stage, the casting speed was set to 2.1 m / min, the crystallizer water flow rate was set to 1950 L / min, and the secondary cooling water flow rate was set to 0.45 L / kg. The temperature was dynamically adjusted in real time according to the temperature change of the tundish, and the temperature of the billet exiting the straightening machine was finally stabilized at 955℃, which is within the preferred range of 930-980℃ of this invention.

[0050] After exiting the straightening machine, the billet travels along the roller conveyor to the inlet of the tilting cooling bed. The entire process, from cutting to completion of the upper cooling bed, is controlled within 1.4-2.3 minutes. The measured temperature of the upper cooling bed is 878℃, strictly meeting the requirement of not being lower than 850℃, and no localized air or water cooling occurs during the entire conveying process. It is worth noting that in summer, when the ambient temperature is higher, the billet dissipates heat more slowly on the roller conveyor, and the upper cooling bed temperature is usually about 10-20℃ higher than in other seasons. Therefore, the secondary cooling water ratio needs to be appropriately increased to control the temperature exiting the straightening machine to not exceed the upper limit of 1000℃.

[0051] During the cooling bed stage, the rotation time per cycle of the rotating cooling bed was set to 40 seconds (in summer, the ambient temperature is high, and the natural convection cooling rate is relatively low; a rotation cycle close to the middle value is selected to ensure uniform cooling effect), and the interval between billets in the same batch was maintained at 2. No additional mist cannons or water spray cooling devices were added. The final billet temperature on the cooling bed was 582℃, lower than the upper limit requirement of 650℃, indicating that the Q355B billet successfully passed the bainitic phase transformation range under normal cooling bed conditions.

[0052] During the slow cooling stage of stacking, the billets are transported to a closed storage area after being removed from the cooling bed. Before stacking, three straight steel pads, each 10m long and 100mm thick, are laid. The levelness of the pads is checked using a level, and the maximum measured unevenness is 12mm, meeting the requirement of not exceeding 20mm. 65 billets are stacked in each layer, arranged in a crisscross pattern, with the interval between adjacent billets less than 10mm. The overhang of the billets beyond the lower layer is approximately 45mm, meeting the upper limit requirement specified in this invention. In summer, the natural temperature in the storage area is high. After stacking, the billets are allowed to cool naturally in the closed storage area for approximately 36 hours. At this point, the measured temperature of the billets is 82℃, which is below the 100℃ lower limit for dismantling, completing the dismantling inspection.

[0053] Test results: All 300 small square billets in this batch were qualified. The total curvature of each billet did not exceed 60mm, the maximum total curvature of a single billet was 46mm, and the curvature per meter did not exceed 10mm. The scrap rate was zero. No bending-related production defects such as steel piling or folding occurred in subsequent heating furnace and rolling processes. Compared with SCM435 in Example 1, Q355B has a lower overall final curvature value under the same cooling control conditions due to its lower phase transformation stress. This verifies that the method of this invention has a reliable control effect on ordinary carbon steel grades as well, and the parameter margin is more abundant.

[0054] Example 3 This embodiment focuses on a 120mm×120mm cross-section, 10m fixed-length medium-carbon chromium-molybdenum alloy steel billet (steel grade 42CrMo, which, like SCM435, belongs to the chromium-molybdenum alloy steel system, with a bainitic transformation initiation temperature (Bs) of approximately 590-640℃, and is a typical alloy steel grade with high bending risk). Under low-temperature winter conditions (ambient temperature approximately -8℃), the method of this invention is used for full-process bending control. The specific steps are as follows. Low-temperature winter conditions significantly increase the difficulty of bending control because: the natural heat dissipation rate of the billet during roller conveying and exposure to the rotating cooling bed is greatly accelerated, the temperature of the upper cooling bed is more likely to fall below the 850℃ lower limit, and the temperature asymmetry on both sides of the cooling bed is exacerbated by the influence of ambient airflow, requiring targeted adjustments to key process parameters.

[0055] During the continuous casting stage, considering the increased heat dissipation in winter, the casting speed was set to 2.2 m / min (a higher value was chosen to shorten the exposure time of the billet on the roller conveyor), the crystallizer water flow rate was set to 1800 L / min, and the secondary cooling water ratio was set to 0.30 L / kg (the secondary cooling water flow rate was appropriately reduced to compensate for the increased heat dissipation in winter). Ultimately, the billet exiting the straightening machine was stably controlled at 942℃. Since the heat capacity of a 120mm×120mm cross-section billet is significantly smaller than that of a 160mm×160mm cross-section billet, the heat dissipation rate is faster under the same ambient temperature. Therefore, the secondary cooling water ratio was significantly lower than in Example 1 to ensure that the exit straightening machine temperature does not drop below the 900℃ lower limit due to excessive cooling.

[0056] After the billet exits the straightening machine, the roller conveyor time must be strictly reduced, and the discipline of rapid loading onto the cooling bed must be strengthened. The time from cutting to completion of loading onto the cooling bed must be controlled between 1.2 and 2.5 minutes (in winter, it should be controlled at the lower limit of summer). The actual measured temperature on the cooling bed is 856℃, which meets the requirement of not being lower than 850℃. To cope with the low temperature in winter, the operators specifically closed the side doors of the workshop on both sides of the cooling bed inlet section to reduce the impact of through draft on the local quenching of the billet and ensure that the billet maintains a uniform temperature distribution along the entire length of the front edge of the cooling bed.

[0057] During the cooling bed stage, the rotation time per cycle of the rotating cooling bed was set to 47 seconds (in winter, when heat dissipation is faster, the rotation cycle is appropriately extended to reduce the difference in cooling rates on different surfaces, ensuring that the bainitic transformation is completed in a uniform and controlled environment). The interval between slabs in the same batch was maintained at one piece (the 120mm small cross-section slab has a small heat capacity, so the interval control is more stringent to ensure cooling uniformity). The final temperature of the slab on the cooling bed was 493℃, far below the preferred lower limit of 600℃, indicating that the bainitic transformation was basically completed.

[0058] During the slow cooling stage of stacking, the billets are transported to a closed storage area after being removed from the cooling bed. The north gate of the storage area is specifically closed to prevent drafts from blowing directly onto the stacks during winter. Steel pads are laid before stacking, and a level is used for inspection. The maximum measured unevenness is 18mm, meeting the requirement of not exceeding 20mm. 72 billets are stacked in each layer, arranged closely in a grid pattern, with approximately 40mm of space above the ends. Because the internal temperature of the storage area is only about 5℃ in winter, the billets dissipate heat faster than in summer. The cumulative cooling time is approximately 52 hours. At this point, the measured temperature of the billets is 76℃, below the 100℃ lower limit for dismantling, and the dismantling inspection is completed.

[0059] Test results: All 260 small square billets in this batch were qualified. The total curvature of each billet did not exceed 60mm, the maximum single total curvature was 53mm, and the curvature per meter did not exceed 10mm. The scrap rate was zero. No bending-related production defects were found in subsequent heating furnace and rolling processes. This embodiment verifies that the method of the present invention can still stably meet the standards under the triple adverse conditions of low temperature in winter, small cross-section (120mm×120mm), and bending-sensitive alloy steel (42CrMo). This further proves the universality of the method and its reliability under extreme environmental conditions. At the same time, it shows that the process parameter adjustment strategy for winter conditions (reducing the secondary cooling water ratio, shortening the cooling bed time, and appropriately extending the turning cycle) is effective and necessary.

[0060] Comparative Example 1 This comparative example uses the same steel grade (SCM435), cross-section (160mm×160mm), and fixed length (12m) as Example 1. The continuous casting process parameters are also the same as in Example 1 (casting speed 2.0m / min, crystallizer water flow 1900L / min, secondary cooling water flow 0.66L / kg, and exit straightening machine temperature 965℃). The difference lies in the following: the rapid loading onto the cooling bed was not performed correctly—due to untimely roller conveyor scheduling, the time from cutting to completion of loading onto the cooling bed was 6-8 minutes, exceeding the 3-minute upper limit specified in this invention; during the waiting period on the roller conveyor, the billet was partially cooled by natural drafts caused by the opening of the plant's side doors, resulting in an actual measured temperature of only 820℃ on the cooling bed, below the lower limit requirement of 850℃. The cooling bed cooling and slow stacking cooling processes are the same as in Example 1.

[0061] Test results: Of the 200 small square billets in this batch, 17 had a total curvature exceeding 60mm, an exceedance rate of 8.5%, with the largest single billet reaching 112mm in total curvature; another 31 billets had a total curvature between 45-60mm, which was acceptable but had a significant deviation; the scrap rate was approximately 5.3% (10 billets were rejected due to severely excessive total curvature). During subsequent processing, there were two instances of steel jamming during furnace loading, and four instances of difficulty in entry and folding defects in the rolled pieces caused by billet curvature occurred during rolling.

[0062] Cause Analysis: The upper cooling bed temperature of 820℃ is lower than the lower limit of 850℃, meaning that under the waiting on the roller conveyor and the effect of localized rapid cooling by the through-draft, a significant temperature gradient has appeared along the length of the billet (the temperature difference between the head and tail ends and the middle section is about 40-60℃). The temperature of some sections has approached the transition zone above the Bs point (about 620℃) of SCM435. In the subsequent uniform cooling stage on the cooling bed, the timing of the bainitic phase transformation in different sections along the length is not synchronized, resulting in volume expansion of each section successively, forming differential linear expansion, and ultimately inducing irreversible bending deformation. This comparative example directly proves the criticality and irreplaceable role of the temperature node control of "rapid loading onto the cooling bed" and "upper cooling bed temperature not lower than 850℃" in the overall coordinated control of this application. If this link is out of control, even if the subsequent cooling bed cooling and slow stacking cooling are carried out according to the procedures, it is impossible to fundamentally eliminate the driving force of the already formed bending deformation.

[0063] Comparative Example 2 This comparative example uses the same steel grade (Q355B), cross-section (150mm×150mm), fixed length (10m), and season (summer) as Example 2. The process parameters for continuous casting, rapid upper cooling bed, and cooling bed are also the same as in Example 2 (measured upper cooling bed temperature 876℃, measured lower cooling bed temperature 584℃). The only difference is the non-standard stacking and slow cooling method: the stacking area was not leveled, with a maximum unevenness of about 35mm, exceeding the 20mm limit specified in this invention; no steel pads were laid; the billets were stacked using the traditional dispersed stacking method instead of the grid method, with a spacing of about 80-120mm between adjacent billets in each layer, resulting in obvious gaps; the stacking was located in a semi-open storage area with natural ventilation openings on the side of the storage area, causing the billets to be cooled by uneven airflow during the stacking process.

[0064] Test results: Of the 280 small square billets in this batch, 8 had a total curvature exceeding 60mm, an exceedance rate of 2.9%, with the largest single billet having a total curvature of 78mm; another 22 billets had a total curvature in the range of 50-60mm, which was acceptable but the allowance was too small; the scrap rate was approximately 1.1% (3 billets were rejected due to excessive curvature). During subsequent rolling, there were 3 instances of difficulty in mill engagement caused by billet curvature, and 1 instance of scrapping due to uneven base causing downward bending deformation of the bottom billet (maximum curvature 76mm).

[0065] Cause Analysis: Although the bainitic phase transformation stress of Q355B is relatively small, the non-standard stacking and slow cooling method still introduced additional bending deformation through two pathways: First, uneven stacking (unevenness exceeding the limit of 35mm) and lack of bottom pads caused the bottommost billet to generate a bending moment under the combined action of its own weight and the pressure of the upper billet, supported by the uneven ground, resulting in irreversible downward bending deformation; Second, the uneven airflow in the semi-open storage area caused a difference in the cooling rate between the windward and leeward sides of the billet, introducing an additional temperature gradient in the elasto-plastic cooling stage below 400℃, leading to the accumulation of residual stress and ultimately manifesting as a slight increase in bending. This comparative example demonstrates the indispensable role of the stacking and slow cooling process in the overall coordinated control of this application: even if the continuous casting and cooling bed stages are strictly implemented, if the stacking stage is poorly controlled, it will still cause the final bending degree to exceed the limit, especially for large-section (relatively heavy) billets and production sites with obvious ground unevenness, this risk is particularly prominent.

[0066] The design principle of the technical solution of this application is explained below based on Example 1: According to the continuous cooling transformation (CCT) curve and isothermal transformation (TTT) curve of SCM435 steel, the bainite transformation initiation temperature (Bs) of the austenite in this steel under continuous cooling conditions is about 620°C, and the transformation termination temperature (Bf) is about 380°C. Under isothermal conditions, the "nose tip" temperature with the shortest bainite incubation period is about 480–520°C. Significant bainite transformation can occur in just a few minutes near this temperature, and the phase transformation volume expansion can reach 0.3%–0.5%. This phase transformation characteristic determines that when the SCM435 billet is cooled near 560–620°C, if there is a temperature gradient along the length direction, each section will trigger bainite phase transformation at different times, resulting in the local expansion amount being inconsistent with the expansion time, forming irreversible differential linear expansion, which in turn induces macroscopic bending deformation—this is the fundamental metallurgical reason why this steel has a prominent problem of excessive bending under traditional processes.

[0067] Based on the above phase transition mechanism, the design logic of each key temperature window in Example 1 is as follows: The design basis for the temperature window of the straightening machine (900–1000℃, measured at 968℃ in this embodiment) is as follows: The austenitizing temperature (Ac3) of SCM435 steel is approximately 820–850℃, and the Bs point is approximately 620℃. Controlling the straightening machine temperature above 900℃ means that the billet remains entirely in the single-phase austenite region when entering the subsequent cooling process, with a heat margin of over 280℃ before the bainitic transformation initiation temperature. Within this temperature range, the steel exhibits good high-temperature plasticity (reduction of area is typically greater than 60%). Even with slight temperature inhomogeneity at this stage, the billet can eliminate internal stress through creep relaxation without permanent bending. More importantly, the austenite structure above 900℃ has not yet undergone any proeutectoid ferrite or pearlite transformation, resulting in a homogeneous microstructure. This provides a favorable initial microstructure for the subsequent uniform and controlled passage through the bainitic transformation region on the cooling bed. If the temperature of the straightening machine is too low (e.g. below 850℃), the local area along the length of the billet (especially the head and tail ends) may have approached or entered the phase transformation sensitive range. Once the non-uniform heat dissipation during the roller conveyor process is superimposed, it may trigger the local bainitic phase transformation before it is even placed on the cooling bed, making the subsequent uniform cooling measures ineffective.

[0068] The design basis for the lower limit of the upper cooling bed temperature (≥850℃, measured at 861℃ in this embodiment) is as follows: 850℃ provides a safety margin of approximately 230℃ relative to the Bs point (620℃), ensuring that the billet remains in the austenitic region after completing the upper cooling bed operation and has not yet entered the incubation period of any diffusion-type phase transformation. Combined with the TTT curve of SCM435, at isothermal temperatures above 850℃, the incubation period of pearlite transformation typically exceeds several tens of minutes, and bainite transformation has not yet started. Therefore, even if the transition time from cutting off to the upper cooling bed reaches the upper limit of 3 minutes, the billet will not undergo phase transformation at this stage. Simultaneously, using this temperature as a strict lower limit, along with the operational regulation of "strictly prohibiting local air cooling or water cooling," aims to eliminate the possibility of the billet prematurely entering the phase transformation sensitive region below 620℃ due to localized rapid cooling before the upper cooling bed, ensuring that the uniform temperature along the entire length is a prerequisite for subsequent controlled cooling on the cooling bed.

[0069] The design basis for the upper limit of the lower cooling bed temperature (≤650℃, preferably ≤600℃, measured in this embodiment 521℃) is as follows: 650℃ is slightly higher than the Bs point (620℃) of SCM435. Using this as the upper limit of the lower cooling bed temperature is based on the following judgment: when the billet leaves the cooling bed, the temperature is already lower than the Bs point, indicating that the bainitic transformation has at least started; and the measured 521℃ in this embodiment is even closer to the middle section above the Bf point (380℃). Combined with the transformation completion line of the CCT curve at the cooling rate (about 2–4℃ / s) in this embodiment, the bainitic transformation completion rate at this temperature has exceeded 90%. This means that the billet has basically completed the entire bainitic transformation in a controlled manner in the uniform and symmetrical cooling environment provided by the tumbling cooling bed. The volume expansion caused by the transformation has been uniformly released, and there is no longer a condition for additional bending stress due to asynchronous transformation processes. When the billet leaves the cooling bed and enters the stacking and slow cooling stage, the microstructure has become stable. Subsequent cooling is only the elastoplastic shrinkage after phase transformation, and the bending driving force is greatly reduced. This lays the foundation for the phase transformation integrity of the close-packed stacking and slow cooling stage to finally eliminate residual stress and stabilize dimensional accuracy.

[0070] In summary, the analysis of Example 1 shows that the complete logical chain constructed in this application, namely "Bs / Bf temperature positioning → straightening machine temperature to ensure austenite uniformity → upper cooling bed temperature to prevent pre-triggered phase transformation → lower cooling bed temperature to verify controlled phase transformation completion → stacking slow cooling to eliminate residual stress", provides clear metallurgical basis for the setting of each temperature node.

[0071] Based on the data from Examples 1-3 and Comparative Examples 1-2, the following conclusions can be drawn: (1) The method of the present invention has a reliable bending control effect on both plain carbon steel (Q355B) and phase transformation sensitive alloy steel (SCM435, 42CrMo), and the scrap rate can be stably controlled below 0.3%; (2) Within different cross-sectional specifications (120mm×120mm to 160mm×160mm) and different fixed lengths (10-12m), all parameters of the method of the present invention can meet the requirements. (3) Under extreme seasonal conditions in summer (around 32°C) and winter (around -8°C), the method of the present invention can also ensure that the bending degree meets the standard by making targeted adjustments to the secondary cooling water volume, the time of the cooling bed, and the turning cycle. (4) The comparative proportions have verified that the rapid cooling bed temperature node (≥850°C) and the standardized stacking slow cooling are the key links in the whole process coordinated control. Any oversight in any link will lead to excessive bending, which fully reflects the technical necessity of the "whole process coordinated" concept of the present invention.

[0072] The preferred embodiments of the present invention have been described in detail above with reference to specific examples. However, the present invention is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

Claims

1. A method for coordinated control throughout the entire process to reduce the total curvature of long, fixed-length small square billets, characterized in that, include: Continuous casting process optimization: By dynamically coordinating and adjusting the casting speed, the cooling water volume of the crystallizer and the ratio of the secondary cooling water, the temperature of the billet exiting the straightening machine is controlled within the range of 900–1000℃, so that the billet enters the subsequent cooling stage at a uniform temperature. Rapid loading onto the cooling bed: After the billet is cut off, it immediately moves onto the rotating cooling bed from the roller conveyor. The time from cutting to loading onto the cooling bed shall not exceed 3 minutes. The temperature of the cooling bed shall not be lower than 850℃. Before loading onto the cooling bed, the billet shall not be subjected to local air cooling or water cooling. Uniform cooling on the cooling bed: Adjust the rotation speed of the rotating cooling bed so that the maximum interval between the same batch of billets on the cooling bed does not exceed 2, control the billets to cool down uniformly on the cooling bed, and the temperature of the lower cooling bed does not exceed 650℃. Stacking and slow cooling: After the billet is removed from the cooling bed, it is stacked closely in a grid pattern and cooled in a closed storage area until the billet temperature drops below 100℃ before it can be unstacked.

2. The whole-process coordinated control method for reducing the total curvature of long fixed-length small square billets according to claim 1, characterized in that: The cross-section of the cast billet is 120mm×120mm to 170mm×170mm, and the fixed length is 10-12m.

3. The whole-process coordinated control method for reducing the total curvature of long fixed-length small square billets according to claim 2, characterized in that: In the continuous casting process, the casting speed is controlled at 1.8–2.2 m / min, the crystallizer water flow rate is controlled at 1800–2100 L / min, and the secondary cooling water flow rate is controlled at 0.2–0.8 L / kg. Within the above range, the flow rate is dynamically adjusted according to the steel grade, cross-section, and tundish temperature.

4. The whole-process coordinated control method for reducing the total curvature of long fixed-length small square billets according to claim 3, characterized in that: The temperature of the billet exiting the straightening machine is controlled within the range of 930–980℃.

5. The whole-process coordinated control method for reducing the total curvature of long fixed-length small square billets according to claim 2, characterized in that: The effective length of the rotating cooling bed should be no less than 12m, and the time for each rotation cycle should be controlled within 35–50 seconds.

6. The whole-process coordinated control method for reducing the total curvature of long fixed-length small square billets according to claim 5, characterized in that: A 42.5-50 second tumbling cycle time is adopted to extend the residence time of the billet on the cooling bed and control the temperature of the billet on the cooling bed to below 600℃.

7. A method for coordinated control of the entire process to reduce the total curvature of long fixed-length small square billets according to any one of claims 1-6, characterized in that: Atomized cooling devices or uniform water spray cooling systems are symmetrically installed on both sides of the tilting cooling bed to cool the billet.

8. The whole-process coordinated control method for reducing the total curvature of long fixed-length small square billets according to claim 1, characterized in that: Before stacking the billets, the ground at the stacking site must be prepared. The bottom layer should be fully covered with steel pads. The levelness of the steel pads should be checked using a level or by manually pulling a string to ensure that the deviation of any position of the steel pad relative to the reference plane does not exceed 20mm.

9. The whole-process coordinated control method for reducing the total curvature of long fixed-length small square billets according to claim 8, characterized in that: The crisscross stacking method is adopted, with each layer of billets densely packed together and the interval between adjacent billets being less than 10mm; the difference between the total width of each stack and the length of a single billet is controlled within 1000mm, ensuring that the suspended portion at both ends of the topmost billet does not exceed 50mm.

10. The whole-process coordinated control method for reducing the total curvature of long fixed-length small square billets according to claim 9, characterized in that: Billet stacking should be carried out in enclosed storage areas, and billet stacks should not be placed in ventilation openings or through-draft paths.