Heat treatment method of low-carbon steel vertical semi-continuous casting billet
By employing heat treatment methods for low-carbon steel vertical semi-continuous casting billets, including austenitization and staged controlled cooling, the problem of Widmanstätten structure in the billets was solved, the toughness and plasticity of the billets were improved, casting stress was released, and production efficiency and energy consumption were optimized.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-12-22
- Publication Date
- 2026-03-27
AI Technical Summary
Widmanstätten structure is easily formed during the solidification process of low-carbon steel vertical semi-continuous casting billets, which leads to reduced toughness, poor plasticity, and increased cracking sensitivity, affecting subsequent hot forging processing.
The heat treatment method for low-carbon steel vertical semi-continuous casting billets includes heating to Ac3+30℃ to Ac3+50℃ and holding for 0.5 to 2 hours for austenitization, followed by staged controlled cooling. The surface cooling rate is controlled by forced convection cooling or natural air cooling, and finally the temperature is slowly reduced to below 150℃. The internal uniformity of the billet is optimized by combining liquid core flow control and stirring technology.
It effectively eliminates Widmanstätten structure in the billet, improves the toughness and plasticity of steel, releases casting stress, enhances the overall performance of large-section low-carbon steel vertical semi-continuous casting billets, and reduces energy consumption and production cycle.
Smart Images
Figure CN121732741A_ABST
Abstract
Description
TECHNICAL FIELD
[0001] The present application relates to the technical field of metal material heat treatment, in particular to a heat treatment method of low-carbon steel vertical semi-continuous casting billet. BACKGROUND
[0002] In recent years, with the continuous growth of the demand for extra-large forgings, the industry has put forward new technical challenges to the preparation method of higher quality and larger section ingots. Under this background, the vertical semi-continuous casting technology of large diameter round billets gradually becomes the research and engineering breakthrough direction as a new large ingot production technology route. This technology is not a simple enlargement of the traditional continuous casting process, and there are essential differences in the solidification mode and heat flow distribution compared with the traditional mold casting. The solidification mode is vertical downward, low pulling speed and low stress, the self-weight effect of the billet is small, the solidification interface is stable, and the heat flow is more uniform, so that the grain growth mode, dendrite morphology, segregation distribution and densification process of the center organization are better than the gravity solidification mode of mold casting. However, in practical application, when the vertical semi-continuous casting technology is used to produce medium and low carbon steel round billets, the cast billet is prone to form widmanstatten structure during solidification, especially in the region close to the surface of the cast billet and close to the end of the dummy bar head. Widmanstatten structure has the characteristics of flaky and needle-like morphology, and its existence will cause the toughness of the steel to decrease significantly, the plasticity to be poor, the cracking sensitivity to be enhanced, and obvious adverse effects on the subsequent hot forging process. SUMMARY
[0003] The present application aims to solve the problem of forming widmanstatten structure in the solidification process of the cast billet.
[0004] To solve the above problems, the present application provides a heat treatment method of low-carbon steel vertical semi-continuous casting billet.
[0005] The present application provides a heat treatment method of low-carbon steel vertical semi-continuous casting billet, which is used for treating low-carbon steel vertical semi-continuous casting billets with a section diameter not less than φ1300mm, and includes the following steps: S1: taking out the low-carbon steel vertical semi-continuous casting billet with a surface temperature of 650-750℃ and an internal temperature not less than 800℃ from the crystallizer, heating to a temperature of Ac3+30-50℃, and keeping for 0.5-2 hours to make the full section of the low-carbon steel vertical semi-continuous casting billet reach austenitization; S2: cooling the low-carbon steel vertical semi-continuous casting billet as a whole to 575-625℃ and keeping for 36-72 hours, wherein the surface cooling speed of the low-carbon steel vertical semi-continuous casting billet is 10-30℃ / min; S3: cooling the low-carbon steel vertical semi-continuous casting billet as a whole to below 150℃.
[0006] Optionally, S1 further includes: liquid core flow control stirring of the low-carbon steel vertical semi-continuous casting billet.
[0007] Optionally, in S1, the low-carbon steel vertical semi-continuous casting billet is heated within 1 hour after being taken out from the crystallizer, so that the surface temperature drop of the low-carbon steel vertical semi-continuous casting billet during the process from being taken out to being heated is not more than 40 DEG C.
[0008] Optionally, in S2, the low-carbon steel vertical semi-continuous casting billet is cooled as a whole by forced convection cooling or air natural cooling.
[0009] Optionally, in S2, the surface cooling speed of the low-carbon steel vertical semi-continuous casting billet is 15-25 DEG C / min.
[0010] Optionally, the forced convection cooling is realized by a high-pressure axial flow fan, a wind deflector, short-time local air blowing or multi-point air blowing.
[0011] Optionally, the wind speed of the forced convection cooling is 5-15 m / s.
[0012] Optionally, in S3, the low-carbon steel vertical semi-continuous casting billet is cooled as a whole by natural furnace cooling, controlled convection or zoned open cover.
[0013] Optionally, in S3, the cooling speed of the low-carbon steel vertical semi-continuous casting billet cooled as a whole is 10-15 DEG C / h.
[0014] Optionally, in S2, the low-carbon steel vertical semi-continuous casting billet is cooled as a whole to 595-605 DEG C and is kept for 42-60 hours, wherein the surface cooling speed of the low-carbon steel vertical semi-continuous casting billet is 17-23 DEG C / min.
[0015] The low-carbon steel vertical semi-continuous casting billet heat treatment method has the advantages that the out-of-billet residual heat of the low-carbon steel vertical semi-continuous casting billet is utilized to directly connect the austenitizing and the multi-time cooling technology, so that the problems of high energy consumption and long cycle caused by traditional re-heating are effectively avoided. The method can eliminate the Widmanstatten structure easily formed in the casting billet, improve the toughness and plasticity of the steel material, release the casting stress, and thus improve the overall performance of the large-section low-carbon steel vertical semi-continuous casting billet. BRIEF DESCRIPTION OF DRAWINGS
[0016] Figure 1 Fig. 1 is a flowchart of the low-carbon steel vertical semi-continuous casting billet heat treatment method according to the embodiment of the present application; Figure 2 Fig. 2 is a cross-sectional structure diagram of the low-carbon steel vertical semi-continuous casting billet after heat treatment according to the embodiment 1; Figure 3 Fig. 3 is a cross-sectional structure diagram of the low-carbon steel vertical semi-continuous casting billet after heat treatment according to the comparative example 1. DETAILED DESCRIPTION
[0017] In order to make the above objectives, characteristics and advantages of the present application more apparent, comprehensible and easier to understand, specific embodiments of the present application will be described in detail below with reference to the accompanying drawings. Although some embodiments of the present application are shown in the drawings, it should be understood that the present application can be implemented in various forms, and should not be interpreted as being limited to the embodiments set forth herein, but rather, the embodiments are provided to make the present application more thorough and complete. It should be understood that the drawings and embodiments of the present application are for exemplary purposes only, and are not intended to limit the scope of protection of the present application.
[0018] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the description of the application herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application; As used herein, the term "comprising" and its variants are open-ended, meaning "including, but not limited to"; the term "based on" is "based, at least in part, on"; the term "one embodiment" means "at least one embodiment"; the term "another embodiment" means "at least one additional embodiment"; the term "some embodiments" means "at least some embodiments"; the term "optional" means "optional in at least some embodiments". Related definitions will be given in the description below.
[0019] As Figure 1 As shown in the drawings, an embodiment of the present application provides a heat treatment method for low-carbon steel vertical semi-continuous casting billets, which is used to treat low-carbon steel vertical semi-continuous casting billets with a cross-sectional diameter of no less than φ1300mm, and includes the following steps: S1: taking out a low-carbon steel vertical semi-continuous casting billet with a surface temperature of 650-750℃ and an internal temperature of no less than 800℃ from a crystallizer, heating to a temperature of Ac3+30-50℃, and keeping for 0.5-2 hours to make the full cross-section of the low-carbon steel vertical semi-continuous casting billet reach austenitization; S2: cooling the low-carbon steel vertical semi-continuous casting billet as a whole to 575-625℃ and keeping for 36-72 hours, wherein the surface cooling speed of the low-carbon steel vertical semi-continuous casting billet is 10-30℃ / min; S3: cooling the low-carbon steel vertical semi-continuous casting billet as a whole to below 150℃.
[0020] Specifically, low-carbon steel (mild steel) is carbon steel with a carbon content of less than 0.25%, and is also called soft steel due to its low strength and low hardness.
[0021] Low-carbon steel vertical semi-continuous casting billet: refers to a low-carbon steel casting billet with a cross-sectional size of no less than φ1300mm produced by a vertical semi-continuous casting process. The casting billet has specific organizational characteristics and thermal state during the solidification process.
[0022] Austenitization: This refers to the process of heating steel to the austenitic phase region and holding it for a period of time, causing the internal structure of the steel to completely transform into austenite. This process aims to eliminate the original undesirable structure and prepare for subsequent phase transformations.
[0023] Ac3 is the actual phase transformation temperature that occurs during the heating of carbon steel, corresponding to the offset value of the solid-state equilibrium critical temperature line A3. Because carbon steel exhibits phase transformation hysteresis during heating, the actual phase transformation temperature is higher than the theoretical equilibrium temperature; this offset temperature is defined as Ac3. The Ac3 temperature values for iron-carbon alloys typically range from 727℃ to 912℃.
[0024] In step S1, when the billet is removed from the crystallizer, its surface temperature is controlled within the range of 650°C to 750°C, while the internal temperature is not lower than 800°C. This state utilizes the residual heat of the billet, providing the initial conditions for subsequent heat treatment. The billet is drawn from the crystallizer and transported to the heat treatment section via roller conveyors or hanging devices in the shortest possible time, with a transport time required to be less than 1 hour (preferably within 30 minutes). This process maximizes the retention of residual heat, providing an "initial temperature platform" for rapid traversal of the Widmanstätten sensitive zone and reducing energy consumption. Subsequently, the billet is fed into heating equipment, such as a resistance furnace or induction furnace, and heated to a temperature of Ac3+30°C to Ac3+50°C. This temperature range ensures that the low-carbon steel vertical semi-continuous casting billet can achieve complete austenitization. The heating process can be carried out in various ways, such as by placing the billet in a preheated furnace or by radiant heating of the billet through an external heat source. After reaching the target temperature, the billet is held at that temperature for 0.5 to 2 hours. The heat preservation process ensures uniform temperature across the entire cross-section of the cast billet and allows the microstructure to fully transform into austenite. The heat preservation time can be adjusted according to the size and specific composition of the cast billet.
[0025] Further, steps S2 and S3 employ a staged air cooling method. In step S2, after austenitization, the billet is rapidly cooled as a whole to a temperature range of 575 to 625°C. The cooling process can be achieved in various ways, such as removing the billet from the furnace and allowing it to cool naturally in air, or transferring it to a temperature-controlled cooling zone. During this cooling process, the surface cooling rate of the billet is controlled at 10 to 30°C / min, reducing the billet temperature from Ac3+30°C to Ac3+50°C to 575 to 625°C in the shortest possible time to suppress acicular ferrite nucleation. The cooling rate can be controlled by adjusting the flow rate or temperature of the cooling medium, for example, by adjusting the power of the cooling fan or the flow rate of the cooling water. To improve uniformity, a guide sleeve or multi-point air supply can be installed on the billet surface to reduce radial temperature differences. Low-carbon steel is in a sensitive region for acicular ferrite / Widmanstätten nucleation within the 900–700℃ range. Increasing the crossing rate can prevent the austenite phase from fully diffusing and transforming, thereby inhibiting the nucleation of acicular ferrite and avoiding the formation of lamellar Widmanstätten that is difficult to eliminate through subsequent heat treatment. This necessity stems from the expansion sensitive region and thermal inertia characteristics of large-diameter billets in vertical semi-continuous casting.
[0026] The billet is held at a temperature range of 575 to 625°C for 36 to 72 hours. In the isothermal range around 600°C, a diffusion-dominated phase transformation mechanism promotes the steady-state transformation of austenite to pearlite / ferrite, thereby gradually transforming the non-equilibrium metastable phases remaining from the rapid cooling stage. Simultaneously, the long holding time allows for the attenuation of the internal thermal gradient, release of residual thermal stress and casting stress, and promotes grain fragmentation and equiaxing. This isothermal holding time range (36 to 72 hours) is not a traditional short-time isothermal homogenization, but rather an engineering parameter derived from the requirements of large-section deep reconstruction, enabling full-section austenite recrystallization and microstructure homogenization.
[0027] Therefore, in step S3, after the intermediate temperature holding is completed, the billet is cooled as a whole to below 150°C to achieve final microstructure stabilization. This final cooling process aims to bring the billet to room temperature or near room temperature to stabilize its final microstructure. Cooling methods may include removing the billet from the holding furnace and allowing it to cool slowly in ambient air, or transferring it to a dedicated cooling area for controlled cooling. During the cooling process, prolonged residence in the tempering brittleness-sensitive zone (150-350°C) must be avoided. Final cooling can be achieved through natural furnace cooling, controlled convection, or a partitioned open-top method to ensure controlled internal and external temperature differences and prevent hot cracking. By steadily reducing the temperature, the pearlite / ferrite microstructure completes its microphase transformation and stabilizes the residual stress field, preventing new stress concentrations and the formation of low-temperature brittle phases caused by temperature differences.
[0028] The low-carbon steel vertical semi-continuous casting billet, after heat treatment, enters the subsequent forging process.
[0029] In this embodiment, by utilizing the residual heat from the exit of the low-carbon steel vertical semi-continuous casting billet, austenitization and multiple cooling techniques are directly applied, effectively avoiding the high energy consumption and long cycle problems associated with traditional reheating. This method can eliminate the Widmanstätten structure that is easily formed in the billet, improve the toughness and plasticity of the steel, and release casting stress, thereby enhancing the overall performance of the large-section low-carbon steel vertical semi-continuous casting billet.
[0030] Optionally, S1 also includes: liquid core controlled flow stirring of low carbon steel vertical semi-continuous casting billet.
[0031] Specifically, liquid core controlled flow stirring refers to the forced agitation of the liquid metal inside the billet during the solidification process of the billet using external energy to control its flow pattern and heat and mass transfer processes. Its main function is to promote uniform mixing of the liquid metal inside the billet, eliminate temperature gradients and component segregation, thereby providing uniform initial conditions for subsequent solid-state phase transformation. One implementation method is to use electromagnetic stirring technology. By placing an electromagnetic coil outside the solidification region of the billet, a rotating magnetic field or traveling wave magnetic field is generated, applying a Lorentz force to the liquid metal and driving the metal flow within the liquid core. This method has the advantages of being non-contact and easy to control, and the stirring intensity and mode can be adjusted as needed. Another implementation method is to use mechanical stirring or gas stirring. For example, physical agitation can be achieved by inserting a stirring device into the liquid core region, or inert gas can be injected into the liquid core to generate bubbles, using the rising and bursting of the bubbles to drive the flow of liquid metal. These methods can also achieve the goal of promoting liquid core homogenization.
[0032] In this optional embodiment, before or during the heating of the ultra-large cross-section low-carbon steel vertical semi-continuous casting billet from the crystallizer to the austenitizing temperature, liquid core controlled flow stirring can effectively promote the uniform mixing of liquid metal inside the billet, significantly reduce the temperature gradient and compositional inhomogeneity inside the billet, and ensure that the equiaxed crystal ratio is above 50%, which is much higher than that of traditional ingot casting technology. Therefore, the holding time for secondary austenitizing can be greatly shortened, significantly improving production efficiency. This provides more uniform and ideal initial conditions for heating the billet to Ac3+30℃ to Ac3+50℃ and holding it in the subsequent S1 step to achieve full-section austenitizing. In view of this, this stirring operation can ensure that the austenitizing process is more complete and consistent, avoiding incomplete or uneven austenitizing caused by local temperature or compositional differences, thereby effectively suppressing the formation of undesirable structures such as Widmanstätten, and laying a solid foundation for subsequent microstructure control, ultimately obtaining a low-carbon steel vertical semi-continuous casting billet with uniform microstructure and excellent performance.
[0033] Optionally, in S1, the low-carbon steel vertical semi-continuous casting billet is heated within 1 hour after being taken out of the crystallizer, so that the surface temperature drop of the low-carbon steel vertical semi-continuous casting billet from taking out to heating does not exceed 40°C.
[0034] Specifically, low-carbon steel vertical semi-continuous casting billets are heated within one hour of being removed from the crystallizer. This aims to maximize the utilization of the residual heat from the billet's exit and minimize heat loss to the environment, thereby preventing a rapid drop in the billet's surface temperature. This can be achieved, but is not limited to: optimizing the production line layout by placing the heating furnace close to the crystallizer outlet to shorten the billet's transport distance and time; or employing automated conveyor systems, such as roller conveyors or robotic arms, to ensure the billet can be quickly and continuously transferred from the crystallizer to the heating equipment. Additionally, installing insulation covers or localized heating devices along the transport path can slow down the billet's temperature drop to some extent, allowing time for subsequent formal heating.
[0035] This method ensures that the surface temperature drop of low-carbon steel vertical semi-continuous casting billets does not exceed 40°C from removal from the mold to heating. It further quantifies the surface temperature control target during the transition from the mold to the furnace, ensuring the stability of the billet surface temperature and preventing uneven microstructure or premature Widmanstätten formation caused by excessive temperature drop. Achieving this characteristic can be achieved, but is not limited to: precisely controlling the billet transport speed and ambient temperature, such as by setting up localized insulation or preheating zones along the transport path; or by using non-contact temperature measuring equipment such as infrared thermometers to monitor the billet surface temperature in real time and dynamically adjusting the transport speed or insulation measures based on the monitoring results to ensure the temperature drop remains within the specified range. Additionally, applying a temporary insulation coating or covering the billet surface with insulation material can also mitigate heat loss.
[0036] In this optional embodiment, the heat treatment method for low-carbon steel vertical semi-continuous casting billets effectively connects the high-temperature state of the billet after exiting the mold with the subsequent austenitizing heating process by strictly limiting the time from when the billet is removed from the mold to the start of heating (within 1 hour) and the surface temperature drop during this period (not exceeding 40°C). This not only makes full use of the residual heat of the billet and reduces heating energy consumption, but more importantly, it ensures the uniformity and stability of the surface microstructure of the billet before entering the austenitizing stage, avoiding the formation of local Widmanstätten structures or uneven microstructure caused by excessive initial temperature drop. This provides good initial conditions for subsequent austenitizing and graded controlled cooling, ensuring the reliability and consistency of the final heat treatment effect, and ultimately obtaining a low-carbon steel vertical semi-continuous casting billet with excellent toughness and plasticity.
[0037] Optionally, in S2, the overall temperature of the low-carbon steel vertical semi-continuous casting billet is reduced by forced convection cooling or natural air cooling.
[0038] Specifically, forced convection cooling is a cooling method that accelerates heat transfer by driving fluid through the surface of an object using external mechanical force. Its core function lies in actively controlling the flow rate, volume, and direction of the cooling medium to precisely control the cooling speed of the object's surface and ensure uniform cooling. For example, multiple fans or nozzles can be used to uniformly spray cooling gas or liquid onto the surfaces of a low-carbon steel vertical semi-continuous casting billet to remove heat. Another method is to use guide plates or hoods within the cooling area to guide the cooling medium into a specific flow field, thereby optimizing the cooling effect and uniformity. Natural air cooling, on the other hand, utilizes the temperature difference between the object and the surrounding air to exchange heat through natural convection and radiation. This cooling method requires no additional mechanical energy input and is energy-saving and gentle. It is typically implemented by placing the low-carbon steel vertical semi-continuous casting billet in an open or semi-open space, allowing it to fully contact the ambient air, relying on natural airflow and thermal radiation to gradually reduce the temperature. For example, the speed and uniformity of natural cooling can be affected by adjusting the stacking spacing of the billets or controlling the ventilation conditions of the cooling area.
[0039] In this optional embodiment, forced convection cooling is a cooling method that accelerates heat transfer by driving fluid through the surface of an object using external mechanical force. Its core function lies in actively controlling the flow rate, volume, and direction of the cooling medium to achieve precise control over the cooling rate of the object's surface and ensure uniform cooling. For example, multiple fans or nozzles can be used to uniformly spray cooling gas or liquid onto the surfaces of the low-carbon steel vertical semi-continuous casting billet to remove heat. Another approach is to use guide plates or hoods within the cooling area to guide the cooling medium into a specific flow field, thereby optimizing the cooling effect and uniformity. Natural air cooling, on the other hand, utilizes the temperature difference between the object and the surrounding air to exchange heat through natural convection and radiation. This cooling method requires no additional mechanical energy input and is energy-saving and gentle. It is typically implemented by placing the low-carbon steel vertical semi-continuous casting billet in an open or semi-open space, allowing it to fully contact the ambient air, relying on natural airflow and thermal radiation to gradually reduce the temperature. For example, the speed and uniformity of natural cooling can be affected by adjusting the stacking spacing of the billets or controlling the ventilation conditions in the cooling zone.
[0040] Optionally, in S2, the surface cooling rate of the low-carbon steel vertical semi-continuous casting billet is 15 to 25 °C / min.
[0041] Specifically, surface cooling rate refers to the rate at which the surface temperature of a low-carbon steel vertical semi-continuous casting billet changes over time during the cooling process. This is a critical process parameter that directly affects the temperature gradient, phase transformation kinetics, and final microstructure and properties of the billet's interior and surface. Precise control of the surface cooling rate aims to avoid cooling too quickly or too slowly, thereby optimizing the transformation of austenite to the target microstructure, reducing thermal stress, and effectively suppressing the formation of Widmanstätten. One approach is to precisely adjust the flow rate, temperature, and spray pattern of the cooling medium (such as air or water mist). For example, in forced convection cooling, the surface cooling rate can be finely controlled by adjusting the fan speed and airflow, or by changing the length of the cooling zone and the distribution of the cooling medium. Another approach is to use temperature sensors to monitor the billet surface temperature in real time and combine this with a closed-loop control system to dynamically adjust cooling parameters according to a preset cooling curve. For example, an infrared thermometer can be used to continuously monitor the billet surface and feed the data back to the control system, which then automatically adjusts the cooling airflow or cooling time to ensure the surface cooling rate remains within the target range. In addition, the surface cooling rate can be indirectly controlled by adjusting the residence time or moving speed of the billet in the cooling zone.
[0042] In this optional embodiment, by precisely limiting the surface cooling rate of the low-carbon steel vertical semi-continuous casting billet to the range of 15 to 25 °C / min, this application effectively solves the problem of inaccurate cooling rate during the cooling process, thereby ensuring the uniformity of cooling and the stability of the microstructure. Specifically, controlling the surface cooling rate within this specific range can avoid the negative effects of excessively fast or slow cooling. If the cooling rate is too fast, the temperature difference between the surface and the interior of the billet may increase sharply, leading to thermal stress concentration and the generation of microcracks; if the cooling rate is too slow, it will prolong the entire process time, reduce production efficiency, and may not be able to completely suppress the formation of Widmanstätten structure. Through this precise control of the cooling rate, the cooling process is significantly optimized, which can maintain the stability of the overall temperature gradient of the billet and promote the transformation of austenite into a uniform microstructure, thereby effectively supporting the core objectives of eliminating Widmanstätten structure and releasing casting stress. At the same time, this precise control also makes full use of the residual heat after billet removal, further improving process efficiency and ensuring that the final product has excellent microstructure properties.
[0043] Alternatively, forced convection cooling can be achieved through high-pressure axial flow fans, surrounding air guide shrouds, short-term localized air jets, or multi-point air supply.
[0044] Specifically, a high-pressure axial flow fan is a ventilation device that utilizes axial airflow to generate a high-pressure differential. Its characteristics include large air volume and high air pressure, providing a powerful airflow to achieve rapid and efficient cooling of the billet surface. For example, multiple high-pressure axial flow fans can be used in parallel to increase the total air volume and coverage area; or the air volume and air pressure can be controlled by adjusting the fan speed or blade angle, thereby achieving precise adjustment of the cooling intensity. A surrounding airflow hood is a structure that surrounds or partially surrounds the object being cooled. Its internal design includes guide plates or specific geometries to guide the airflow, ensuring that the airflow generated by the high-pressure axial flow fan can uniformly and effectively cover the entire surface of the low-carbon steel vertical semi-continuous casting billet, reducing cooling dead zones and improving cooling uniformity. For example, it can be designed as an adjustable annular or semi-annular structure, adjustable according to the size and shape of the billet to optimize airflow distribution; or multiple guide vanes or holes can be set inside the hood to finely control the direction and speed of the airflow, ensuring uniform cooling. Short-duration localized air jetting refers to the concentrated airflow jetting of a localized area on a low-carbon steel vertical semi-continuous casting billet within a specific time period. This provides precise and rapid cooling intervention for areas with uneven temperature distribution or localized overheating on the billet surface, eliminating localized temperature differences and preventing uneven microstructure. For example, multiple independently controllable nozzles can be used in conjunction with an infrared temperature measurement system to monitor the billet surface temperature in real time and automatically open or close specific nozzles for localized air jetting based on temperature distribution. Alternatively, a mobile air jetting device can be used, moving along a specific path on the billet surface to provide short-duration, targeted air jetting to areas requiring cooling. Multi-point air supply refers to delivering cooling airflow to the surface of the low-carbon steel vertical semi-continuous casting billet from different locations through multiple independent air outlets or air supply channels. This ensures that the cooling airflow acts on the billet surface from multiple directions and angles, achieving a comprehensive and balanced cooling effect and avoiding uneven cooling caused by unidirectional or limited-directional air supply. For example, multiple air outlets can be evenly arranged around the cooling area, with each outlet equipped with an independent airflow regulating valve to achieve precise control of the cooling intensity of each area; or a layered or zoned air supply system can be adopted to provide differentiated air supply strategies according to the cooling needs of different heights or areas of the billet.
[0045] In this optional embodiment, the high-pressure axial flow fan provides a strong and stable cooling airflow, ensuring the efficiency of the cooling process. The surrounding airflow guide precisely directs this airflow to the surface of the billet, avoiding cooling dead zones and ensuring cooling uniformity. The combination of short-duration localized air jets and multi-point air supply makes the cooling process highly flexible and precise, allowing for dynamic adjustments to address temperature differences in different parts of the billet, effectively preventing localized overcooling or overheating. These measures work together to achieve more precise cooling rate control and superior temperature uniformity during the overall cooling process. This precise and uniform cooling helps suppress the formation of Widmanstätten structure, promotes the uniform transformation of austenite to pearlite or bainite, thereby effectively eliminating Widmanstätten structure inside the billet and releasing casting stress, ultimately improving the toughness and plasticity of the billet and providing high-quality billets for subsequent processing.
[0046] Optionally, the forced convection cooling air velocity is 5 to 15 m / s.
[0047] Specifically, the air velocity in forced convection cooling refers to the flow rate of the cooling medium (e.g., air) acting on the surface of the low-carbon steel vertical semi-continuous casting slab during forced convection cooling. Precise control of this air velocity is crucial for achieving the desired heat exchange efficiency and temperature gradient management. For example, the air velocity can be precisely controlled by adjusting the fan speed or by adjusting the baffle opening in the duct to ensure that the airflow acts uniformly on the surface of the low-carbon steel vertical semi-continuous casting slab at a stable speed. Furthermore, optimizing the design of the nozzles or guide vanes can ensure that the airflow is evenly distributed within the range of 5 to 15 m / s, thereby avoiding localized overcooling or undercooling.
[0048] In this optional embodiment, when performing forced convection cooling on the low-carbon steel vertical semi-continuous casting billet, the wind speed is precisely limited to the range of 5 to 15 m / s, effectively solving the problem of unstable cooling caused by insufficient wind speed control. This wind speed range avoids both excessively high wind speeds leading to rapid surface cooling of the low-carbon steel vertical semi-continuous casting billet, thus preventing excessive thermal stress concentration and microstructure inhomogeneity, and excessively low wind speeds resulting in low cooling efficiency and prolonged heat treatment cycles. Therefore, by precisely controlling the wind speed of forced convection cooling, it is possible to ensure that the low-carbon steel vertical semi-continuous casting billet cools at a more stable and uniform rate in step S2, thereby promoting the homogenization of its internal structure, effectively releasing casting stress, and ultimately improving the overall performance and production efficiency of the low-carbon steel vertical semi-continuous casting billet.
[0049] Optionally, in S3, the overall temperature of the low-carbon steel vertical semi-continuous casting billet is reduced by means of natural furnace cooling, controlled convection, or partitioned opening.
[0050] Specifically, natural furnace cooling refers to allowing the low-carbon steel vertical semi-continuous casting billet to cool naturally within the furnace after high-temperature holding, without applying any external forced cooling medium. One method is to directly shut off the furnace heating power after the S2 holding step and keep the furnace door or lid closed, allowing the furnace temperature to drop naturally over time, thus slowly cooling the low-carbon steel vertical semi-continuous casting billet. Another method is to adjust the furnace's sealing after shutting off the heating power, for example, by slightly opening the exhaust vents, to slightly adjust the cooling rate without introducing forced airflow, while still maintaining the overall natural cooling characteristics.
[0051] Controlled convection refers to the cooling of low-carbon steel vertical semi-continuous casting billets by actively controlling the flow of the cooling medium. Specifically, one method involves installing multiple fans or blowers inside the furnace. By adjusting the fan speed or airflow, cooling air flows over the surface of the low-carbon steel vertical semi-continuous casting billet at a controlled speed and direction, thus achieving convective cooling. Another method involves installing cooling ducts outside the furnace. External cooling air is introduced and evenly guided into the furnace using guide plates or nozzles to form a controlled convection field, thereby achieving precise temperature control.
[0052] Zonal cooling refers to a method for controlling the localized cooling of large, low-carbon steel vertical semi-continuous casting slabs. This involves designing the furnace lid in sections and selectively opening or closing specific sections as needed. For example, one approach is to design the furnace lid as multiple independent, individually operable modules, each capable of opening or closing independently. During cooling, based on the temperature distribution or cooling requirements of the low-carbon steel vertical semi-continuous casting slab, parts of the furnace lid are selectively opened, exposing the corresponding areas of the slab to ambient air for cooling. Another approach involves using a sliding or flip-up zonal structure for the furnace lid, with a mechanical transmission device controlling the opening and closing degree of different sections, thereby finely adjusting the heat dissipation in localized areas and achieving zonal cooling.
[0053] In this optional embodiment, a variety of flexible and efficient cooling methods are introduced, making the cooling process in step S3 more controllable and efficient. These methods work synergistically to effectively solve the problems of uneven cooling, stress concentration, and high energy consumption. Specifically, natural furnace cooling provides a slow and uniform cooling environment, effectively reducing thermal shock and internal stress accumulation, and avoiding the risk of uneven microstructure and cracking caused by rapid cooling; controlled convection enables precise control of the cooling rate, ensuring that the temperature of the entire cross-section of the low-carbon steel vertical semi-continuous casting billet drops uniformly, thereby optimizing microstructure and effectively releasing casting stress; and partitioned opening can specifically address the cooling needs of different parts of the billet, optimizing heat distribution, improving cooling efficiency, and reducing overall energy consumption. Therefore, while ensuring the microstructure stability of the low-carbon steel vertical semi-continuous casting billet and the full release of casting stress, the energy efficiency of the overall heat treatment process is significantly improved, and potential defects caused by improper cooling are avoided.
[0054] Optionally, in S3, the overall cooling rate of the low-carbon steel vertical semi-continuous casting billet is 10 to 15℃ / h.
[0055] Specifically, the overall cooling rate of the low-carbon steel vertical semi-continuous casting billet is 10 to 15 °C / h. This means that in step S3, during the process of cooling the low-carbon steel vertical semi-continuous casting billet from a higher temperature to below 150 °C, the average temperature drop rate is precisely controlled within the range of 10 to 15 °C per hour. This specific cooling rate aims to ensure that the billet can release internal thermal stress uniformly and slowly during the cooling process, and to provide sufficient time for the complete transformation of austenite into stable structures such as ferrite and pearlite. This cooling rate can be achieved in various ways, such as by precisely controlling the furnace temperature drop rate of the cooling furnace, adjusting the flow rate and temperature of the cooling medium (such as air), or optimizing the thermal insulation performance and heat dissipation conditions of the cooling environment. For example, a programmed furnace cooling method or slow natural convection cooling in a controlled atmosphere can be used to ensure that the cooling curve conforms to the preset range.
[0056] In this optional embodiment, the cooling rate of the low-carbon steel vertical semi-continuous casting slab is precisely controlled at 10 to 15 °C / h. This effectively avoids the drastic temperature gradient caused by excessively rapid cooling, thereby significantly reducing the concentration of thermal stress inside the slab. This slow and uniform cooling rate provides sufficient time for the complete transformation of austenite into stable structures such as ferrite and pearlite, promoting microstructure homogenization and preventing the re-formation of inhomogeneous or Widmanstätten structures. Simultaneously, the controlled cooling process helps to smoothly release internal stresses in the slab, minimizing the risk of cracking due to excessive stress. Ultimately, this technical solution ensures that the slab possesses excellent plasticity and toughness, providing a reliable material basis for subsequent processing and use.
[0057] Optionally, in S2, the low-carbon steel vertical semi-continuous casting billet is cooled to 595 to 605°C and held for 42 to 60 hours, wherein the surface cooling rate of the low-carbon steel vertical semi-continuous casting billet is 17 to 23°C / min.
[0058] In this optional embodiment, the specific process window of "surface cooling rate of 17 to 23°C / min + isothermal temperature of 595 to 605°C + isothermal time of 42 to 60h" is simultaneously satisfied, which can completely eliminate Widmanstätten in the surface layer and the double high-incidence area at the ingot head end of the vertical semi-continuous casting ultra-large cross-section round billet to below 0.5%, and synergistically achieve extremely low residual stress and excellent forging plasticity.
[0059] The present invention will be further described below with reference to specific embodiments.
[0060] Example 1: A heat treatment method for low-carbon steel vertical semi-continuous casting billets, comprising the following steps: 1. The low-carbon steel vertical semi-continuous casting billet uses 25 steel (chemical composition wt%: C 0.24, Si 0.28, Mn 0.55, P 0.012, S 0.008, balance Fe and unavoidable impurities), billet diameter φ1600mm, produced by vertical semi-continuous casting process, with Widmanstätten proportions of 28% in the original cast surface layer and 36% in the ingot head region.
[0061] 2. Billet unloading and conveying: After the vertical semi-continuous casting round billet exits the crystallizer, the surface temperature is controlled at 710℃ and the core temperature at 885℃. Within 15 minutes after exiting the billet, it is directly sent into the heat treatment furnace through a hanging device, and the surface temperature drop does not exceed 40℃.
[0062] 3. Secondary austenitizing treatment: The furnace temperature is set to 900℃ (the Ac3 value of 25 steel is about 860℃), and the temperature is held for 1.2 hours to make the entire cross section completely austenitized.
[0063] 4. Staged cooling system: (1) Rapid cooling stage: Open the furnace cover and start 4 sets of high-pressure axial flow fans (wind speed 12m / s) + surrounding wind guide hood to make the average cooling rate of the surface of the billet 20℃ / min, and cool the billet from 900℃ to 600℃ in 14.8 minutes. (2) Isothermal transition stage: Close the furnace lid and precisely control the furnace temperature at 600℃±3℃ for 54 hours; (3) Final cooling stage: After the heat preservation is completed, the furnace is naturally cooled to 120°C before being taken out of the furnace. The total final cooling time is about 38 hours.
[0064] Test results of heat-treated low-carbon steel vertical semi-continuous casting billets: Widmanstätten content in the surface layer (depth 0 to 50 mm) was 0.3%; Widmanstätten content in the ingot head region (longitudinal section 0 to 500 mm from the bottom) was 0.4%; (The text abruptly ends here, so the translation stops as well.) Figure 2As shown, the microstructure of the entire cross section is uniform pearlite + network ferrite + a small amount of blocky ferrite; the longitudinal Charpy impact energy at -40℃ is 96J (average value, 3 samples); the maximum residual stress (blind hole method) is 38MPa; the subsequent forging into Φ800mm bars with a compression ratio of 4:1, without any macro or micro cracks; compared with the traditional die casting + overall reheating process, this embodiment reduces 1.5 heating cycles and reduces the overall energy consumption by 42%.
[0065] Comparative Example 1, a heat treatment method for low-carbon steel vertical semi-continuous casting billets, including the following steps: 1. The low-carbon steel vertical semi-continuous casting billet uses 25 steel (chemical composition wt%: C 0.24, Si 0.28, Mn 0.55, P 0.012, S 0.008, balance Fe and unavoidable impurities), billet diameter φ1600mm, produced by vertical semi-continuous casting process, with Widmanstätten proportions of 28% in the original cast surface layer and 36% in the ingot head region.
[0066] 2. Billet unloading and conveying: After the vertical semi-continuous casting round billet exits the crystallizer, the surface temperature is controlled at 710℃ and the core temperature at 885℃. Within 15 minutes after exiting the billet, it is directly sent into the heat treatment furnace through a hanging device, and the surface temperature drop does not exceed 40℃.
[0067] 3. Secondary austenitizing treatment: The furnace temperature is set to 900℃ (the Ac3 value of 25 steel is about 860℃), and the temperature is held for 1.2 hours to make the entire cross section completely austenitized.
[0068] 4. Staged cooling system: (1) Rapid cooling stage: Open the furnace cover and start 4 sets of high-pressure axial flow fans + surrounding air guide hoods to make the average cooling rate of the surface of the billet 12℃ / min, and cool the billet from 900℃ to 600℃; (2) Isothermal transition stage: Close the furnace lid and precisely control the furnace temperature at 600℃±3℃ for 54 hours; (3) Final cooling stage: After the heat preservation is completed, the furnace is naturally cooled to 120°C before being taken out of the furnace. The total final cooling time is about 38 hours.
[0069] Test results of heat-treated low-carbon steel vertical semi-continuous casting billets: Widmanstätten content in the surface layer (depth 0 to 50 mm) was 11.8%; Widmanstätten content in the ingot head region (longitudinal section 0 to 500 mm from the bottom) was 27.3%; (The text abruptly ends here, so the translation stops as well.) Figure 3 As shown, the cross-sectional microstructure is Widmanstätten; the maximum residual stress (blind hole method) is 162 MPa.
[0070] Comparative Example 2, a heat treatment method for low-carbon steel vertical semi-continuous casting billets, including the following steps: 1. The low-carbon steel vertical semi-continuous casting billet uses 25 steel (chemical composition wt%: C 0.24, Si 0.28, Mn 0.55, P 0.012, S 0.008, balance Fe and unavoidable impurities), billet diameter φ1600mm, produced by vertical semi-continuous casting process, with Widmanstätten proportions of 28% in the original cast surface layer and 36% in the ingot head region.
[0071] 2. Billet unloading and conveying: After the vertical semi-continuous casting round billet exits the crystallizer, the surface temperature is controlled at 710℃ and the core temperature at 885℃. Within 15 minutes after exiting the billet, it is directly sent into the heat treatment furnace through a hanging device, and the surface temperature drop does not exceed 40℃.
[0072] 3. Secondary austenitizing treatment: The furnace temperature is set to 900℃ (the Ac3 value of 25 steel is about 860℃), and the temperature is held for 1.2 hours to make the entire cross section completely austenitized.
[0073] 4. Staged cooling system: (1) Rapid cooling stage: Open the furnace cover and start 4 sets of high-pressure axial flow fans + surrounding air guide hoods to make the average cooling rate of the surface of the billet 20℃ / min, and cool the billet from 900℃ to 580℃; (2) Isothermal transition stage: Close the furnace lid and precisely control the furnace temperature at 580℃±3℃ for 54 hours; (3) Final cooling stage: After the heat preservation is completed, the furnace is naturally cooled to 120°C before being taken out of the furnace. The total final cooling time is about 36 hours.
[0074] Test results of the treated low-carbon steel vertical semi-continuous casting billet: Widmanstätten content in the surface layer (depth 0 to 50 mm) is 6.2%; Widmanstätten content in the ingot head area (longitudinal section 0 to 500 mm from the bottom) is 13.5%; maximum residual stress (blind hole method) is 118 MPa.
[0075] Comparative Example 3, a heat treatment method for low-carbon steel vertical semi-continuous casting billets, including the following steps: 1. The low-carbon steel vertical semi-continuous casting billet uses 25 steel (chemical composition wt%: C 0.24, Si 0.28, Mn 0.55, P 0.012, S 0.008, balance Fe and unavoidable impurities), billet diameter φ1600mm, produced by vertical semi-continuous casting process, with Widmanstätten proportions of 28% in the original cast surface layer and 36% in the ingot head region.
[0076] 2. Billet unloading and conveying: After the vertical semi-continuous casting round billet exits the crystallizer, the surface temperature is controlled at 710℃ and the core temperature at 885℃. Within 15 minutes after exiting the billet, it is directly sent into the heat treatment furnace through a hanging device, and the surface temperature drop does not exceed 40℃.
[0077] 3. Secondary austenitizing treatment: The furnace temperature is set to 900℃ (the Ac3 value of 25 steel is about 860℃), and the temperature is held for 1.2 hours to make the entire cross section completely austenitized.
[0078] 4. Staged cooling system: (1) Rapid cooling stage: Open the furnace cover and start 4 sets of high-pressure axial flow fans + surrounding air guide hoods to make the average cooling rate of the surface of the billet 20℃ / min, and cool the billet from 900℃ to 620℃; (2) Isothermal transition stage: Close the furnace lid and precisely control the furnace temperature at 620℃±3℃ for 54 hours; (3) Final cooling stage: After the heat preservation is completed, the furnace is naturally cooled to 120°C before being taken out of the furnace. The total final cooling time is about 40 hours.
[0079] Test results of the treated low-carbon steel vertical semi-continuous casting billet: Widmanstätten content in the surface layer (depth 0 to 50 mm) is 9.1%; Widmanstätten content in the ingot head area (longitudinal section 0 to 500 mm from the bottom) is 21.7%; maximum residual stress (blind hole method) is 139 MPa.
[0080] Comparative Example 4, a heat treatment method for low-carbon steel vertical semi-continuous casting billets, including the following steps: 1. The low-carbon steel vertical semi-continuous casting billet uses 25 steel (chemical composition wt%: C 0.24, Si 0.28, Mn 0.55, P 0.012, S 0.008, balance Fe and unavoidable impurities), billet diameter φ1600mm, produced by vertical semi-continuous casting process, with Widmanstätten proportions of 28% in the original cast surface layer and 36% in the ingot head region.
[0081] 2. Billet unloading and conveying: After the vertical semi-continuous casting round billet exits the crystallizer, the surface temperature is controlled at 710℃ and the core temperature at 885℃. Within 15 minutes after exiting the billet, it is directly sent into the heat treatment furnace through a hanging device, and the surface temperature drop does not exceed 40℃.
[0082] 3. Secondary austenitizing treatment: The furnace temperature is set to 900℃ (the Ac3 value of 25 steel is about 860℃), and the temperature is held for 1.2 hours to make the entire cross section completely austenitized.
[0083] 4. Cold treatment: Open the furnace cover and allow the furnace to cool naturally, cooling the billet from 900℃ to 120℃ before unloading.
[0084] Test results of the treated low-carbon steel vertical semi-continuous casting billet: surface layer (depth 0 to 50 mm) Widmanstätten content 23.4%; ingot head region (longitudinal section 0 to 500 mm from bottom) Widmanstätten content 38.6%; residual stress (blind hole method) maximum 204 MPa.
[0085] It is evident that the heat treatment method with a rapid cooling stage reduces the surface layer and the high-incidence area of Widmanstätten at the ingot head end, which are unique to vertical semi-continuous casting of ultra-large cross-section round billets. Furthermore, by simultaneously satisfying the specific process window of "surface cooling rate of 17 to 23℃ / min + isothermal temperature of 595 to 605℃ + isothermal time of 42 to 60h", the surface layer and the high-incidence area of Widmanstätten at the ingot head end, which are unique to vertical semi-continuous casting of ultra-large cross-section round billets, can be completely eliminated to below 0.5%, and extremely low residual stress and excellent forging plasticity are achieved in synergy, resulting in significant and unexpected technical effects.
[0086] While the present invention has been disclosed above, its scope of protection is not limited thereto. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention, and all such changes and modifications will fall within the scope of protection of the present invention.
Claims
1. A heat treatment method for a low-carbon steel vertical semi-continuous casting billet, characterized in that, The process for processing low-carbon steel vertical semi-continuous casting billets with a cross-sectional diameter of not less than φ1300mm includes the following steps: S1: Take out a low-carbon steel vertical semi-continuous casting billet with a surface temperature of 650℃ to 750℃ and an internal temperature of not less than 800℃ from the crystallizer, heat it to a temperature of Ac3+30℃ to Ac3+50℃, and hold it for 0.5 to 2 hours to make the entire cross section of the low-carbon steel vertical semi-continuous casting billet austenitized. S2: Cool the entire low-carbon steel vertical semi-continuous casting billet to 575 to 625°C and hold it at that temperature for 36 to 72 hours, wherein the surface cooling rate of the low-carbon steel vertical semi-continuous casting billet is 10 to 30°C / min. S3: Cool the entire low-carbon steel vertical semi-continuous casting billet to below 150°C.
2. The heat treatment method for low-carbon steel vertical semi-continuous casting billets according to claim 1, characterized in that, S1 further includes: performing liquid core controlled flow stirring on the low carbon steel vertical semi-continuous casting billet.
3. The heat treatment method for low-carbon steel vertical semi-continuous casting billets according to claim 1, characterized in that, In step S1, after the low-carbon steel vertical semi-continuous casting billet is taken out of the crystallizer, it is heated within 1 hour so that the surface temperature drop of the low-carbon steel vertical semi-continuous casting billet does not exceed 40°C from the time it is taken out to the time it is heated.
4. The heat treatment method for low-carbon steel vertical semi-continuous casting billets according to claim 1, characterized in that, In step S2, the overall temperature of the low-carbon steel vertical semi-continuous casting billet is reduced by forced convection cooling or natural air cooling.
5. The heat treatment method for low-carbon steel vertical semi-continuous casting billets according to claim 4, characterized in that, In S2, the surface cooling rate of the low-carbon steel vertical semi-continuous casting billet is 15 to 25°C / min.
6. The heat treatment method for low-carbon steel vertical semi-continuous casting billets according to claim 4, characterized in that, The forced convection cooling is achieved by using a high-pressure axial flow fan, an enclosed airflow guide shroud, short-term localized air jets, or multi-point air supply.
7. The heat treatment method for low-carbon steel vertical semi-continuous casting billets according to claim 4, characterized in that, The forced convection cooling airflow velocity is 5 to 15 m / s.
8. The heat treatment method for low-carbon steel vertical semi-continuous casting billets according to claim 1, characterized in that, In step S3, the overall temperature of the low-carbon steel vertical semi-continuous casting billet is reduced by means of natural furnace cooling, controlled convection, or partitioned opening.
9. The heat treatment method for low-carbon steel vertical semi-continuous casting billets according to claim 1, characterized in that, In S3, the overall cooling rate of the low-carbon steel vertical semi-continuous casting billet is 10 to 15℃ / h.
10. The heat treatment method for low-carbon steel vertical semi-continuous casting billets according to claim 1, characterized in that, In step S2, the low-carbon steel vertical semi-continuous casting billet is cooled to 595 to 605°C and held at that temperature for 42 to 60 hours. The surface cooling rate of the low-carbon steel vertical semi-continuous casting billet is 17 to 23°C / min.