Double-furnace heating method for realizing high-temperature diffusion of bearing steel

Abstract

The invention provides a double-furnace heating method for realizing high-temperature diffusion of bearing steel, which belongs to the technical field of heat treatment, and comprises the following steps: transforming a first heating furnace and a second heating furnace which are arranged in parallel into a series heating system, and carrying out the following steps: feeding a steel billet into the second heating furnace for preheating treatment; the preheated steel billets are transferred to a first heating furnace through a heat preservation sealing channel and a transverse moving device, and heat preservation and heat compensation are conducted in the transferring process; performing three-stage high-temperature diffusion treatment on the steel billet in the first heating furnace, wherein the three-stage high-temperature diffusion treatment comprises a carbide dissolution stage, a transition cooling stage and a carbide re-precipitation stage which are performed in sequence; and after high-temperature diffusion treatment of the steel billet is completed, gradient cooling treatment is conducted on the steel billet in the first heating furnace, so that the tapping temperature of the steel billet meets the rolling starting temperature requirement of the follow-up rolling procedure. According to the double-furnace heating method for achieving high-temperature diffusion of the bearing steel, the billet heating capacity is improved, and the problem that alloy elements in a billet core area are not fully diffused is solved.

Description

Technical Field

[0001] This invention belongs to the field of heat treatment technology, and more specifically, relates to a dual-furnace heating method for achieving high-temperature diffusion of bearing steel. Background Technology

[0002] As a key basic material in the machinery manufacturing field, bearing steel's performance directly determines the precision, lifespan, and reliability of bearings, and it is widely used in high-end equipment manufacturing industries such as automobiles, machine tools, and aerospace. The core performance requirements of bearing steel include high hardness, high wear resistance, high elastic limit, and good toughness. The realization of these properties highly depends on the high-temperature diffusion treatment process during production. The essence of high-temperature diffusion is to promote the diffusion of alloying elements such as carbon and chromium formed during the solidification of the steel billet in a prolonged high-temperature environment, eliminating compositional segregation and refining the grain structure. This lays a uniform microstructure foundation for subsequent rolling and heat treatment processes, ultimately ensuring the consistency of the mechanical properties and service life of the bearing steel.

[0003] In the production of bearing steel bars and coils, the industry commonly adopts a parallel arrangement of multiple heating furnaces to accommodate the different heating requirements of ordinary steel and bearing steel. However, in existing technologies, parallel-configured heating furnaces often exhibit inconsistent design parameters: some furnaces, due to their shorter design length (typically ≤25m), fail to meet the minimum residence time required for the high-temperature diffusion process of bearing steel (industry standard ≥4h). These furnaces can only heat ordinary steel and remain idle for extended periods during the bearing steel production cycle, resulting in insufficient utilization of fixed assets (commonly below 30% in the industry) and a widespread waste of equipment resources.

[0004] Meanwhile, heating furnaces capable of meeting the high-temperature diffusion requirements of bearing steel (typically ≥30m in length and 80-100 tons / h in heating capacity) have inherent upper limits on their single-furnace operating capacity. Given the continuously increasing market demand for bearing steel, the single-furnace heating mode is insufficient to meet the demands of large-scale production. More significantly, existing heating furnaces meeting high-temperature diffusion requirements must simultaneously perform the dual functions of billet preheating and high-temperature diffusion. The furnace is only divided into three sections: a first heating section, a second heating section, and a soaking section. The first heating section is used for preheating, while only the second heating section and the soaking section are used for high-temperature diffusion, resulting in a limited effective diffusion length (approximately 25 meters). For billets with a cross-sectional dimension greater than 150mm, the long heat conduction path leads to insufficient diffusion of alloying elements in the core area, resulting in poor internal microstructure uniformity and subsequent quality problems such as hardness deviations and reduced fatigue life after processing. Summary of the Invention

[0005] The purpose of this invention is to provide a dual-furnace heating method for achieving high-temperature diffusion of bearing steel, aiming to solve the problems of limited production capacity and uneven diffusion of steel billets.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is: to provide a dual-furnace heating method for achieving high-temperature diffusion of bearing steel, comprising the following steps: S0. The first and second heating furnaces arranged in parallel are transformed into a series heating system. The second heating furnace is set as a gradient preheating unit and the first heating furnace is set as a high-temperature diffusion unit. The length of the second heating furnace is shorter than that of the first heating furnace. The second heating furnace is limited by the length of the furnace body and cannot independently achieve high-temperature diffusion of the steel billet. S1. The steel billet is sent into the second heating furnace for preheating treatment; S2. The preheated steel billet is transferred to the first heating furnace through an insulated and sealed channel and a transverse transfer device. Insulation and heat replenishment are carried out during the transfer process. S3. The steel billet is subjected to a three-stage high-temperature diffusion treatment in the first heating furnace. The three stages include a carbide dissolution stage, a transition cooling stage, and a carbide re-precipitation stage performed sequentially. S4. After the steel billet completes the high-temperature diffusion treatment, it undergoes a gradient cooling process in the first heating furnace to ensure that the billet exiting the furnace is compatible with the initial rolling temperature requirements of the subsequent rolling process.

[0007] In one possible implementation, in step S1, the preheating process involves sequentially setting up a first preheating section, a second preheating section, and a third preheating section in the second heating furnace along the billet's travel direction. The temperature of the first preheating section is 600-750℃, the temperature of the second preheating section is 750-900℃, and the temperature of the third preheating section is 900-1050℃. The temperature difference between adjacent sections is controlled at 100-150℃.

[0008] In one possible implementation, in step S2, the heat-insulating sealing channel connects the steel tapping end of the second heating furnace with the steel loading end of the first heating furnace. The inner wall of the heat-insulating sealing channel is provided with a high-temperature resistant heat-insulating layer, and multiple independent infrared radiation heat-replenishing sections are arranged along the length of the heat-insulating sealing channel. During the transfer process, the power of each heating section is dynamically adjusted according to the billet's transfer temperature to control the billet surface temperature drop rate to ≤5℃ / min.

[0009] In one possible implementation, in step S3, the carbide dissolution stage involves heating the billet to a target diffusion temperature of 1200-1220°C at a heating rate of 5-8°C / min; the transition cooling stage involves reducing the billet temperature by 5-10°C at a cooling rate of 1-2°C / min; and the carbide reprecipitation stage involves restoring the billet to the target diffusion temperature and holding it at that temperature for 4-7 hours.

[0010] In one possible implementation, both the second and first heating furnaces are purged with a protective atmosphere of mixed nitrogen and hydrogen, with hydrogen accounting for 5-15% of the volume and nitrogen accounting for 85-95% of the volume, and the furnace pressure is maintained at 0.01-0.03 MPa.

[0011] In one possible implementation, For steel billets with a carbon content of 0.9-1.0%, the holding time for the carbide re-precipitation stage is 4-5 hours, and the final temperature of the gradient cooling treatment is 1020-1050℃. For steel billets with a carbon content of 1.0-1.1%, the holding time for the carbide re-precipitation stage is 5-6 hours, and the final temperature of the gradient cooling treatment is 1000-1030℃. For steel billets with a carbon content of 1.1-1.2%, the holding time for the carbide re-precipitation stage is 6-7 hours, and the final temperature of the gradient cooling treatment is 950-1000℃.

[0012] In one possible implementation, step S4, the gradient cooling includes: In the first cooling stage, the steel billet is cooled from its high-temperature diffusion temperature to 1150℃ at a rate of 7.5-10℃ / min; and In the second cooling stage, the billet temperature is reduced from 1150℃ to 1050-1100℃ at a rate of ≤10℃ / min.

[0013] In one possible implementation, the cooling rate of the second cooling stage is adapted to the billet cross-sectional dimensions. When the billet cross-sectional dimensions are ≤150mm, the cooling rate is 8-10℃ / min; when the billet cross-sectional dimensions are >150mm and ≤200mm, the cooling rate is 6-8℃ / min; and when the billet cross-sectional dimensions are >200mm, the cooling rate is 4-6℃ / min.

[0014] In one possible implementation, the effective length of the furnace body occupied by the billet undergoing the high-temperature diffusion treatment in the first heating furnace is not less than 85% of the total length of the first heating furnace.

[0015] In one possible implementation, the exhaust pipe of the first heating furnace is connected to the air preheating device of the second heating furnace, and the combustion air of the second heating furnace is preheated to 200-300°C by the high-temperature flue gas discharged from the first heating furnace before being introduced into the second heating furnace.

[0016] The beneficial effects of the dual-furnace heating method for achieving high-temperature diffusion of bearing steel provided by the present invention are as follows: Compared with the prior art, the dual-furnace heating method for achieving high-temperature diffusion of bearing steel of the present invention effectively revitalizes underutilized fixed assets and avoids investment waste by transforming the idle second heating furnace into a gradient preheating unit.

[0017] At the same time, the first heating furnace was freed from the dual load of preheating and high-temperature diffusion, and the effective diffusion path was fully extended through the three-stage high-temperature diffusion treatment. This solved the problem of insufficient diffusion of alloying elements in the core area of ​​steel billets with a cross-sectional size greater than 150mm, significantly improved the uniformity of the bearing steel structure, and thus improved its hardness consistency and fatigue life.

[0018] The dual-furnace operation significantly increases the heating capacity of bearing steel, better meeting the incremental market demand. The heat preservation and heat replenishment design during the transfer process avoids the impact of billet temperature drop on the diffusion effect. The gradient cooling treatment in the first heating furnace achieves precise matching between the furnace exit temperature and the subsequent rolling start temperature, ensuring production continuity. Ultimately, while improving equipment utilization and solving the problems of limited capacity and uneven diffusion, it fully guarantees the core performance of bearing steel and meets the high-quality requirements of the high-end equipment manufacturing industry for bearing steel. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 A flowchart of a dual-furnace heating method for achieving high-temperature diffusion of bearing steel is provided as an embodiment of the present invention; Figure 2 This is a schematic diagram of the layout structure of the first heating furnace and the second heating furnace provided in an embodiment of the present invention.

[0021] Explanation of reference numerals in the attached figures: 1. First heating furnace; 2. Second heating furnace; 3. Insulated and sealed channel; 4. Transverse movement device; 5. Roughing mill. Detailed Implementation

[0022] To make the technical problems to be solved, the technical solutions, and the beneficial effects of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the present invention and are not intended to limit the present invention.

[0023] Please see Figures 1 to 2This invention provides a dual-furnace heating method for achieving high-temperature diffusion in bearing steel. This method first modifies existing equipment, transforming the originally parallel-connected first heating furnace 1 and second heating furnace 2 into a series heating system. Simultaneously, the second heating furnace 2 is designated as a gradient preheating unit, specifically responsible for the preheating process of the steel billet; the first heating furnace 1 is designated as a high-temperature diffusion unit, focusing on the high-temperature diffusion and gradient cooling processes of the steel billet. In application, the length of the second heating furnace 2 is shorter than that of the first heating furnace 1. Before the modification, the second heating furnace 2 was limited by its furnace length and could not independently achieve high-temperature diffusion of the steel billet.

[0024] In the traditional parallel operation mode, the second heating furnace 2 is too short, resulting in insufficient billet dwell time to meet diffusion requirements and long-term idleness. Meanwhile, the first heating furnace 1 must simultaneously perform preheating and diffusion functions, compressing the effective diffusion length. By converting it into a series connection, on the one hand, the heating capacity of the second heating furnace 2 can be fully utilized, separating the preheating process from the first heating furnace 1, allowing the first heating furnace 1 to be used only for high-temperature diffusion, significantly extending the billet diffusion time and increasing the effective diffusion length. On the other hand, the series connection of the two furnaces forms a continuous production line of "preheating-diffusion-cooling," eliminating the need for billets to wait outside the furnace for extended periods, thus greatly improving production efficiency.

[0025] To achieve stable operation of the two furnaces connected in series, a heat-insulating and sealed channel 3 and a transverse transfer device 4 are added between the steel tapping end of the second heating furnace 2 and the steel loading end of the first heating furnace 1. This enables continuous and heat-insulating transfer of steel billets from the second heating furnace 2 to the first heating furnace 1, preventing temperature loss and oxidation / decarburization during the transfer process. The roughing mill 5 in the subsequent rolling process is located at the steel tapping end of the first heating furnace 1.

[0026] By upgrading the dual-furnace system, the utilization rate of workshop equipment has been improved from the original operation of only the first heating furnace 1 to the coordinated operation of both furnaces. The method of preheating the billet with the second heating furnace 2 and performing high-temperature diffusion on the billet with the first heating furnace 1 has greatly increased the effective diffusion length of the billet, providing space and time guarantees for the full diffusion of large-sized billets. At the same time, the continuous production mode has significantly shortened the production cycle and increased production capacity.

[0027] In application, the specific steps of the dual-furnace heating method are as follows: S1. The steel billet is sent into the second heating furnace for preheating treatment.

[0028] In this step, the bearing steel billet to be processed is sent into the second heating furnace for a three-stage gradient preheating treatment. The specific process parameters are as follows: along the billet's travel direction (from the loading end to the unloading end), preheating stages one, two, and three are set sequentially. The temperature of preheating stage one is 600-750℃, the temperature of preheating stage two is 750-900℃, and the temperature of preheating stage three is 900-1050℃. The temperature difference between adjacent stages is strictly controlled within 100-150℃.

[0029] In applications, the initial temperature of bearing steel billets is typically room temperature (20-30℃) or ambient temperature (≤100℃). If directly fed into a high-temperature furnace at 1200-1220℃, the large temperature difference will generate severe thermal stress, leading to cracks in the billet. Furthermore, residual stress within the billet (generated during casting or cold storage) will further exacerbate microstructural inhomogeneity if not released beforehand during high-temperature diffusion. Therefore, preheating treatment of the billet is necessary.

[0030] The reason for using a three-stage gradient preheating method instead of a one-stage or two-stage preheating method in this step is that a one-stage preheating method results in an excessively wide temperature range, a large temperature difference between the billet surface and core, and still significant thermal stress; a two-stage preheating method has a steep temperature gradient, resulting in insufficient release of residual stress. The three-stage preheating method, by setting a reasonable temperature gradient (100-150℃), gradually increases the billet temperature from room temperature to approximately 1000℃. Each temperature increase lays the foundation for the next stage of preheating, ensuring the gradual release of thermal stress while avoiding accelerated surface oxidation and decarburization caused by excessively rapid preheating.

[0031] To ensure the effectiveness of the three-stage gradient preheating, the specific operation in application is as follows: First, implement zoned temperature control for the second heating furnace. The furnace chamber is divided into three heating sections, each independently equipped with a burner group (8-10 gas burners per section) and temperature detection devices (3-4 thermocouples per section, located on the furnace top and sides), enabling precise temperature control of each preheating section. For example, the first preheating section uses low heat load combustion (burner power 30-40kW), with the temperature controlled at around 650℃; the second preheating section increases the burner power to 50-60kW, with the temperature controlled at around 800℃; and the third preheating section further increases the burner power to 70-80kW, with the temperature controlled at around 980℃, ensuring a stable temperature difference of 120-130℃ between adjacent sections, forming a smooth temperature gradient.

[0032] Second, control the preheating rate of each section. Considering the combustion efficiency of the second heating furnace, the thermal conductivity of the steel billet, and the continuous production rhythm, the heating rate of each preheating section is set as follows: Preheating (from room temperature 25℃ to 600-750℃): The heating rate is controlled at 16-18℃ / min, and the billet residence time is 30-40 minutes. This allows the temperature to rise steadily from room temperature to the target temperature range, avoiding thermal stress concentration in the initial stage. Preheating stage 2 (from 600-750℃ to 750-900℃): The heating rate is controlled at 4-5℃ / min, and the billet residence time is 35-45 minutes. This balances the release of thermal stress with preheating efficiency and ensures uniform temperature penetration inside and outside the billet. Preheating in three stages (from 750-900℃ to 900-1050℃): The heating rate is controlled at 3-4℃ / min, and the billet residence time is 40-50 minutes. As the high-temperature diffusion stage approaches, the rate is reduced to avoid surface overheating and to ensure a smooth connection with the first heating furnace process.

[0033] Meanwhile, the heating rate adjustment for each preheating stage needs to be adapted to the actual operating environment variables: When the billet cross-sectional size is less than or equal to 150mm, the upper limit value is taken for each section heating rate; when the billet cross-sectional size is 150-200mm, the middle value is taken; when the billet cross-sectional size is greater than 200mm, the lower limit value is taken.

[0034] When the furnace loading temperature is room temperature (25℃), the heating rate described above shall be followed; when the furnace loading temperature is greater than or equal to 300℃, the heating rate of each preheating section may be increased by 20%, but shall not exceed 20℃ / min, 6℃ / min, and 4.5℃ / min, respectively, to avoid overheating that could lead to abnormal tissue structure.

[0035] Third, the billet traveling speed is controlled. Based on the length of the second heating furnace (22 meters) and the total preheating time (1.5-2 hours), the billet traveling speed is set to 0.2-0.3 m / min to ensure that the billet stays in each preheating zone for a uniform time, ensuring that the temperature penetrates evenly to the core area of ​​the billet, so that the temperature difference between the surface and the core of the billet when it exits the furnace is ≤30℃, providing a uniform temperature basis for subsequent high-temperature diffusion.

[0036] Through three-stage gradient preheating, a gentle temperature gradient is formed inside the steel billet, resulting in a stable heat conduction rate. This facilitates the uniform transfer of heat from the surface to the core, avoiding uneven microstructure caused by excessively high or low local temperatures. Simultaneously, the uniform preheating temperature allows for a faster temperature response of the steel billet during subsequent high-temperature diffusion, improving diffusion efficiency and providing a solid guarantee for the smooth operation of the entire high-temperature diffusion process.

[0037] S2. The preheated steel billet is transferred to the first heating furnace through an insulated and sealed channel and a transverse transfer device, and insulated and reheated during the transfer process.

[0038] In applications, the preheated steel billet temperature reaches as high as 950-1050℃. If traditional open-air transport methods are used, the billet surface is in direct contact with the air, causing a rapid temperature drop due to convective and radiative heat transfer (the temperature drop rate can reach 15-20℃ / min). This not only further increases the temperature difference between the billet surface and the core (potentially reaching 50-80℃), but also exacerbates surface oxidation and decarburization, and may even form a hardened layer on the billet surface. These problems severely affect the subsequent high-temperature diffusion effect: on the one hand, excessive temperature difference requires the billet to be reheated after entering the first heating furnace, prolonging the diffusion time and reducing production efficiency; on the other hand, the hardened surface layer hinders the diffusion of alloying elements at high temperatures, further widening the microstructure difference between the surface and the core, affecting product quality.

[0039] In this step, a high-temperature resistant sealed channel is used to rigidly connect the outlet of the second heating furnace to the inlet of the first heating furnace. The length of the high-temperature sealed channel is determined based on the existing equipment layout in the workshop, and its cross-sectional dimensions are determined based on the diameter of the largest steel billet to ensure that the billet can pass through smoothly. The inner wall of the high-temperature sealed channel is lined with two layers of high-temperature resistant insulation: the inner layer is ceramic fiber cotton (material Al2O3-SiO2, high temperature resistance ≥1300℃, thickness 50mm), and the outer layer is rock wool insulation board (thickness 80mm); high-temperature resistant sealed doors (material heat-resistant steel, lined with ceramic fiber) are installed at both ends of the channel to ensure the sealing performance of the channel and reduce heat loss.

[0040] During implementation, 3-5 independent infrared radiation heating sections (each 1.5-2 meters long) are evenly arranged along the length of the high-temperature sealed channel. Each heating section uses an infrared heating tube as the heating element. The arrangement principle of the heating sections is: the heating section near the steel tapping end of the second heating furnace has a lower power (5-6kW), and the heating section near the steel loading end of the first heating furnace has a higher power (8-10kW), forming a gradient heating to compensate for the natural temperature drop during the billet transfer process and ensure the stability of the billet temperature.

[0041] During implementation, two thermocouples (arranged at the top of the channel, close to the billet surface) are installed in each reheating section to monitor the billet surface temperature in real time. Simultaneously, a temperature sensor is installed on the billet transfer trolley (transverse transfer device) to monitor the billet core temperature. The temperature signals are transmitted to the central control system. Based on the initial temperature (T0) of the billet as it exits the second heating furnace and the target transfer temperature (T1, ≥950℃), the system dynamically adjusts the power of each reheating section: if the billet surface temperature drops at a rate exceeding 3℃ / min, the power of the corresponding reheating section is automatically increased; if the temperature drop rate is ≤2℃ / min, the power of the reheating section is appropriately reduced. This ensures that the billet surface temperature drop rate is strictly controlled to ≤5℃ / min, the core temperature drop rate is ≤3℃ / min, and after transfer, the billet surface temperature is ≥950℃, the core temperature is ≥930℃, and the temperature difference between the surface and core is ≤20℃.

[0042] During implementation, the transverse transfer device employs a hydraulically driven translation trolley. The trolley platform is covered with high-temperature resistant insulation panels to prevent heat loss from the steel billet through conduction. The trolley's operating speed is set at 0.5-1 m / min, and the transfer time is controlled within 5-10 minutes to ensure the shortest possible residence time of the steel billet in the channel, minimizing heat loss. Simultaneously, a labyrinthine sealing structure is used between the trolley and the insulated, sealed channel to further reduce air intrusion and lower the risk of oxidation and decarburization.

[0043] In this step, by employing an insulated and sealed channel and dynamic heat-replenishing transfer, the surface temperature drop rate of the billet can be controlled at 2-3℃ / min, and the core temperature drop rate can be controlled at 1-2℃ / min. After transfer, the surface temperature of the billet is ≥960℃, the core temperature is ≥940℃, and the temperature difference between the surface and the core is ≤15℃. Compared with traditional open-air transfer, the temperature drop rate is reduced by 70-80%, and the heating time of the billet after entering the first heating furnace is shortened by 40-50%, effectively improving production efficiency. At the same time, it avoids the formation of a surface hardened layer, ensuring the uniformity of subsequent high-temperature diffusion and providing an important guarantee for stable product quality.

[0044] S3. The steel billet is subjected to a three-stage high-temperature diffusion treatment in the first heating furnace. The three stages include a carbide dissolution stage, a transition cooling stage, and a carbide re-precipitation stage performed sequentially.

[0045] In the as-cast microstructure of bearing steel billets, carbides (mainly Fe3C and Cr7C3) are unevenly distributed. In some areas, carbides aggregate to form coarse particles (5-10 μm in size), while in other areas, carbides are sparse, leading to compositional segregation. The core objective of high-temperature diffusion is to eliminate the uneven distribution of carbides and achieve uniform diffusion of carbon and chromium. Traditional high-temperature diffusion adopts a "single temperature, long holding time" mode (e.g., holding at 1200℃ for 4-7 hours). Although this can promote carbide dissolution to some extent, it has two key problems: first, the coarse carbides are not fully dissolved, and the remaining carbide particles become the core of subsequent microstructure inhomogeneity; second, prolonged high temperature can easily lead to excessive grain growth (grain size can reach 100-150 μm), reducing the plasticity and toughness of the steel billet.

[0046] This step employs a three-stage high-temperature diffusion treatment process, specifically including: Carbide dissolution stage: By rapidly heating to a higher target diffusion temperature, sufficient activation energy is provided to promote the rapid dissolution of coarse carbides, allowing carbon and chromium elements to enter the austenitic matrix; Transitional cooling stage: Appropriately reduce the temperature to decrease the concentration gradient of carbon and chromium in the austenitic matrix, laying the foundation for subsequent uniform precipitation; Carbide redeposition stage: The temperature is restored to the target diffusion temperature and held to allow carbon and chromium to diffuse fully in the austenitic matrix. Then, carbides are redepositioned in a fine and uniform form (size ≤1μm), while the grains are refined through dynamic recrystallization.

[0047] This three-stage process of "dissolving-homogenizing-reprecipitating" can completely eliminate carbide aggregation and compositional segregation, and avoid excessive grain growth, thus achieving the dual goals of homogenization and grain refinement.

[0048] During implementation, in the carbide dissolution stage, the billet is heated from its post-transfer temperature (950-960℃) to the target diffusion temperature of 1200-1220℃ at a heating rate of 5-8℃ / min. If the heating rate is too fast (>8℃ / min), the temperature difference between the billet surface and core will increase again, causing the surface carbides to dissolve too quickly while the core carbides dissolve lag behind, creating new compositional differences. If the heating rate is too slow (<5℃ / min), it will prolong the heating time, reduce production efficiency, and easily lead to premature grain growth. When the billet core temperature reaches 1200-1220℃, it enters a holding state for 1-1.5 hours to ensure that coarse carbides are fully dissolved (dissolution rate ≥95%), laying the foundation for subsequent element diffusion.

[0049] During the transition cooling stage, the billet temperature is reduced by 5-10℃ from the target diffusion temperature (1200-1220℃) to 1190-1210℃ at a slow cooling rate of 1-2℃ / min. The core objective of this stage is to reduce the supersaturation of carbon and chromium in the austenite matrix, decrease the concentration gradient, and create conditions for subsequent uniform re-precipitation. A cooling rate that is too slow (<1℃ / min) will result in insufficient element diffusion and difficulty in eliminating the concentration gradient; a cooling rate that is too fast (>2℃ / min) may lead to non-uniform carbide precipitation in localized areas. The cooling time should be controlled within 3-8 minutes (adjusted according to the cooling range) to ensure a smooth temperature decrease and prepare for carbide re-precipitation.

[0050] During the carbide re-precipitation stage, the billet is restored to the target diffusion temperature (1200-1220℃), and the holding time is adjusted according to the carbon content of the billet, specifically as follows: For steel billets with a carbon content of 0.9-1.0% (such as GCr15SiMn, with a carbon content of 0.95%), the holding time is 4-5 hours. These billets have relatively low carbon content and fewer carbides, making diffusion easier; a shorter holding time is sufficient to achieve uniform elemental distribution. For steel billets with a carbon content of 1.0-1.1% (such as GCr15, with a carbon content of 1.05%), the holding time is 5-6 hours. These billets have a moderate carbon content and relatively uniform carbide distribution, but sufficient time is needed to ensure adequate diffusion of carbon and chromium in the core area. For steel billets with a carbon content of 1.1-1.2% (such as GCr18Mo with a carbon content of 1.15%), the holding time is 6-7 hours. These billets have a high carbon content, resulting in a large number of carbides that are prone to aggregation and diffusion, requiring a longer holding time to promote carbide dissolution and element diffusion.

[0051] During the heat preservation process, the temperature fluctuation within the first heating furnace is controlled within ±5℃ using the temperature control system, ensuring uniform overall temperature of the steel billet. Simultaneously, the billet's travel speed within the first heating furnace remains stable (0.1-0.15 m / min), ensuring consistent residence time for each billet in the high-temperature diffusion zone and preventing quality fluctuations caused by differences in residence time.

[0052] After a three-stage high-temperature diffusion treatment, the compositional segregation of the billet is significantly reduced, and the carbides exhibit a uniform and fine dispersed distribution with uniform grain size and no coarse or uneven grains. For large-sized billets (cross-sectional dimensions ≥150mm), the difference in carbon and chromium concentration between the core region and the surface region is ≤0.03%, fully meeting the compositional uniformity requirements for high-end bearing steel. Compared with traditional single-furnace diffusion, the carbide solubility is increased by 20-30%, the compositional segregation is reduced by 70-80%, and the grain size is refined by 40-50%, laying an excellent microstructure foundation for subsequent rolling and heat treatment processes.

[0053] In practice, a mixed protective atmosphere of nitrogen and hydrogen is introduced into both the second heating furnace (preheating unit) and the first heating furnace (high temperature diffusion unit), with hydrogen accounting for 5-15% of the volume and nitrogen accounting for 85-95% of the volume, and the furnace pressure is maintained at 0.01-0.03 MPa.

[0054] The reasons for choosing a mixed protective atmosphere of nitrogen and hydrogen in the method of this invention are as follows: First, nitrogen is an inert gas (chemically stable and does not react with steel billets at high temperatures), widely available and inexpensive, and can be used as the main protective gas to fill the furnace space and prevent air intrusion; Second, hydrogen has reducing properties and can reduce the low-valence iron oxide formed on the surface of the steel billet, reducing the formation of oxide scale. At the same time, hydrogen has a high thermal conductivity (about 7 times that of nitrogen), which can improve the temperature uniformity in the furnace; Third, the synergistic effect of the mixed atmosphere: although nitrogen protection alone can isolate air, it cannot reduce the oxide scale that has already been formed; hydrogen protection alone is costly, and there is a risk of explosion when the hydrogen concentration is too high. Therefore, a mixed gas with a hydrogen volume ratio of 5-15% and a nitrogen volume ratio of 85-95% is chosen, which can give full play to the reducing effect of hydrogen, ensure production safety, and control the cost of the protective atmosphere.

[0055] S4. After the steel billet completes the high-temperature diffusion treatment, the steel billet is subjected to gradient cooling treatment in the first heating furnace so that the billet exiting the furnace temperature is adapted to the starting rolling temperature requirements of the subsequent rolling process.

[0056] In applications, after the billet undergoes high-temperature diffusion at 1200-1220℃, its temperature is relatively high. If it is directly removed from the furnace and air-cooled or rapidly cooled, the following problems will occur: the temperature difference between the billet surface and the core will increase sharply, generating huge thermal stress, which may lead to cracking of the billet; the austenitic structure at high temperature will transform into hard and brittle structures such as martensite or bainite during rapid cooling, reducing the plasticity of the billet and making it prone to fracture during subsequent rolling; excessively high billet exit temperature (>1100℃) will lead to accelerated oxidation and burning loss during rolling, and the excessive plasticity of the steel will easily lead to uneven rolling deformation; excessively low exit temperature (<950℃) will lead to a decrease in the plasticity of the steel, an increase in rolling force, an increase in energy consumption, and even the inability to achieve normal rolling.

[0057] In this step, after the billet has undergone high-temperature diffusion treatment, the purpose of performing gradient cooling treatment on the billet in the first heating furnace is as follows: First, to gradually reduce the billet temperature, reduce the temperature difference between the surface and the core, release thermal stress, and avoid cracking; Second, to control the transformation process of the austenite structure, so that the billet maintains a certain degree of plasticity when it exits the furnace, and is suitable for the initial rolling temperature of the subsequent rolling process (usually 950-1050℃); Third, to adjust the cooling rate according to the cross-sectional size of the billet, because the larger the cross-sectional size, the longer the heat conduction path and the slower the heat dissipation. If the cooling rate is too fast, the accumulation of thermal stress is prone to cracking, so the cooling rate needs to be reduced accordingly.

[0058] To achieve the purpose of gradient cooling, the gradient cooling process includes: The first cooling stage involves reducing the steel billet from its high-temperature diffusion temperature (1200-1220℃) to 1150℃ at a rate of 7.5-10℃ / min. During this stage, the billet temperature is relatively high (1150-1220℃), the austenitic structure is stable, the billet has good plasticity, and strong thermal stress release ability. Therefore, a relatively fast cooling rate can be used to shorten the cooling time and improve production efficiency.

[0059] The cooling process is achieved by adjusting the burner power of the first heating furnace and increasing the flue gas emission flow rate, while maintaining a protective atmosphere to prevent billet oxidation during the cooling process. The duration of the first cooling stage is calculated based on the cooling rate: if the temperature drops from 1210℃ to 1150℃, a decrease of 60℃, and a cooling rate of 8℃ / min, the cooling time is 7.5 minutes, maximizing production efficiency while ensuring effective cooling.

[0060] The second cooling stage: The billet is cooled from 1150℃ to the target tapping temperature at a rate of ≤10℃ / min (adjusted according to the billet's carbon content: 1020-1050℃ for 0.9-1.0% carbon content; 1000-1030℃ for 1.0-1.1% carbon content; 950-1000℃ for 1.1-1.2% carbon content). During this stage, the billet temperature gradually decreases, the austenitic structure becomes unstable, and the risk of thermal stress accumulation increases. Therefore, the cooling rate needs to be precisely adjusted according to the billet's cross-sectional dimensions. Specifically: When the cross-sectional size of the steel billet is ≤150mm (such as round billets with diameters of 120mm or 150mm), the heat conduction path is short, heat dissipation is fast, and the cooling rate can be controlled at 8-10℃ / min. For example, for a steel billet with a diameter of 150mm, cooling from 1150℃ to 1030℃ (suitable for carbon content of 0.95%) requires a temperature drop of 120℃, a cooling rate of 9℃ / min, and a cooling time of approximately 13.3 minutes. When the cross-sectional dimensions of the steel billet are >150mm and ≤200mm (such as a round billet with a diameter of 180mm), the heat conduction path is moderate, and the cooling rate is controlled at 6-8℃ / min. For example, for a steel billet with a diameter of 180mm, cooling from 1150℃ to 1010℃ (suitable for a carbon content of 1.05%) requires a cooling range of 140℃, a cooling rate of 7℃ / min, and a cooling time of approximately 20 minutes. When the cross-sectional size of the steel billet is greater than 200mm (such as a round billet with a diameter of 220mm), the heat conduction path is long, the heat dissipation is slow, and the cooling rate is controlled at 4-6℃ / min. For example, for a steel billet with a diameter of 220mm, cooling from 1150℃ to 980℃ (suitable for a carbon content of 1.15%) requires a temperature drop of 170℃, a cooling rate of 5℃ / min, and a cooling time of approximately 34 minutes.

[0061] The second cooling stage involves further reducing burner power, partially shutting down burners, and adjusting the flue gas damper opening to control the heat dissipation rate. Throughout the process, the surface and core temperatures of the steel billet are continuously monitored to ensure a temperature difference ≤40℃ and that thermal stress is controlled below the billet's yield strength to prevent cracking.

[0062] In this step, the gradient cooling treatment of the steel billet is based on thermal stress theory and rolling process requirements: According to thermal stress calculation theory, thermal stress is generated during the cooling process of the steel billet, and the yield strength of the steel billet decreases with increasing temperature. At 1150℃, the yield strength is relatively low (approximately 100-150 MPa), and it can withstand greater thermal stress. Therefore, a faster cooling rate can be used in the first cooling stage. As the temperature decreases below 1000℃, the yield strength gradually increases (approximately 200-300 MPa), but thermal stress also accumulates due to the increased temperature difference. Therefore, the cooling rate needs to be reduced in the second cooling stage. For large-section steel billets, according to Fourier's law, the heat conduction time is proportional to the square of the cross-sectional size. Doubling the cross-sectional size quadruples the heat conduction time. Therefore, the cooling rate needs to be significantly reduced to ensure that the heat in the core area has sufficient time to transfer to the surface and reduce the temperature difference.

[0063] Meanwhile, according to the rolling process requirements, the initial rolling temperature of bearing steel needs to be controlled within the austenite recrystallization temperature range (950-1050℃). Within this temperature range, the steel exhibits good plasticity and low deformation resistance, enabling uniform rolling. The furnace exit temperature after gradient cooling precisely matches this range, and the billet microstructure remains uniform austenite, creating conditions for dynamic recrystallization during subsequent rolling processes and further refining the grains.

[0064] In practical applications, the effective length of the furnace body occupied by the steel billet undergoing high-temperature diffusion treatment in the first heating furnace shall not be less than 85% of the total length of the first heating furnace.

[0065] During implementation, the total length of the first heating furnace is 35 meters. If the effective diffusion length is insufficient, the residence time of the steel billet in the high-temperature diffusion zone will be shortened, failing to meet the requirements for sufficient diffusion of carbon and chromium elements. This problem is particularly pronounced for large-sized steel billets. Setting the effective diffusion length to ≥85% (i.e., ≥30 meters) is based on diffusion kinetic calculations and actual production verification: the billet's travel speed in the first heating furnace is 0.1-0.15 m / min, and the residence time corresponding to an effective diffusion length of 30 meters is 30 ÷ 0.1 = 300 minutes (5 hours) to 30 ÷ 0.15 = 200 minutes (approximately 3.3 hours). Combining the process requirements of three-stage diffusion (total diffusion time ≥6 hours), plus the holding time before gradient cooling, it can be ensured that the total residence time of the steel billet in the high-temperature zone meets the requirements, providing sufficient time for sufficient element diffusion.

[0066] Meanwhile, the explicit requirements for the effective diffusion length ratio can avoid the decline in diffusion effect caused by temperature fluctuations at both ends of the furnace (the charging end and the tapping end are affected by the external environment and the temperature is prone to be lower), ensuring that the billet is in a stable high-temperature diffusion environment in most areas of the furnace, thus ensuring diffusion uniformity.

[0067] In this method, the exhaust pipe of the first heating furnace is connected to the air preheating device of the second heating furnace. The high-temperature flue gas discharged from the first heating furnace is used to preheat the combustion air of the second heating furnace to 200-300°C before it is introduced into the second heating furnace.

[0068] During the high-temperature diffusion process in the first heating furnace, the exhaust gas temperature reaches as high as 350-400℃. This high-temperature flue gas contains a large amount of heat energy (approximately 1200-1500 kJ per cubic meter), and direct emission would result in significant energy waste. If room temperature air (20-30℃) is used directly for combustion in the second heating furnace, the combustion efficiency is low, and more fuel is required to maintain the preheating temperature. By recovering waste heat from the flue gas and preheating the combustion air to 200-300℃, combustion efficiency can be significantly improved, fuel consumption reduced, and energy-saving goals achieved.

[0069] The present invention provides a dual-furnace heating method for achieving high-temperature diffusion of bearing steel, which has the following advantages compared with the prior art: (a) Capacity increase With the two furnaces connected in series, the second furnace focuses on preheating, while the first furnace focuses on high-temperature diffusion and gradient cooling, forming a continuous production mode. Under stable operation, the overall heating capacity can reach 130-160 tons / h (of which the minimum capacity is 130-140 tons / h, and the optimal capacity can reach 150-160 tons / h). Compared with the traditional single furnace (which takes into account both preheating and diffusion) of 95 tons / h, the minimum capacity is increased by 36.8%, and the optimal capacity is increased by 68.4%.

[0070] (ii) Product quality has improved significantly, and the supply capacity in the high-end market has been enhanced. Through three-stage high-temperature diffusion, sufficient effective diffusion length (≥30 meters) and temperature control, the elemental segregation of the billet is significantly reduced compared with traditional single-furnace diffusion, eliminating the performance differences caused by component segregation from the root. The carbides are uniformly and finely dispersed, with uniform grain size and no coarse carbides or grain inhomogeneity, providing an excellent microstructure for subsequent processing.

[0071] (iii) Equipment utilization rate is significantly improved, and the benefits of fixed assets are maximized. The second heating furnace, which had been idle for a long time, has been upgraded to achieve 100% utilization.

[0072] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A dual-furnace heating method for achieving high-temperature diffusion in bearing steel, characterized in that, Includes the following steps: S0. The first and second heating furnaces arranged in parallel are transformed into a series heating system. The second heating furnace is set as a gradient preheating unit and the first heating furnace is set as a high-temperature diffusion unit. The length of the second heating furnace is shorter than that of the first heating furnace. The second heating furnace is limited by the length of the furnace body and cannot independently achieve high-temperature diffusion of the steel billet. S1. The steel billet is sent into the second heating furnace for preheating treatment; S2. The preheated steel billet is transferred to the first heating furnace through an insulated and sealed channel and a transverse transfer device. Insulation and heat replenishment are carried out during the transfer process. S3. The steel billet is subjected to a three-stage high-temperature diffusion treatment in the first heating furnace. The three stages include a carbide dissolution stage, a transition cooling stage, and a carbide re-precipitation stage performed sequentially. S4. After the steel billet completes the high-temperature diffusion treatment, it undergoes a gradient cooling process in the first heating furnace to ensure that the billet exiting the furnace is compatible with the initial rolling temperature requirements of the subsequent rolling process.

2. The dual-furnace heating method for achieving high-temperature diffusion of bearing steel as described in claim 1, characterized in that, In step S1, the preheating treatment involves sequentially setting up a first preheating section, a second preheating section, and a third preheating section in the second heating furnace along the billet's travel direction. The temperature of the first preheating section is 600-750℃, the temperature of the second preheating section is 750-900℃, and the temperature of the third preheating section is 900-1050℃. The temperature difference between adjacent sections is controlled at 100-150℃.

3. The dual-furnace heating method for achieving high-temperature diffusion of bearing steel as described in claim 2, characterized in that, In step S2, the heat-insulating and sealing channel connects the steel tapping end of the second heating furnace with the steel loading end of the first heating furnace. The inner wall of the heat-insulating and sealing channel is provided with a high-temperature resistant heat-insulating layer, and multiple independent infrared radiation heat-replenishing sections are arranged along the length of the heat-insulating and sealing channel. During the transfer process, the power of each heating section is dynamically adjusted according to the billet's transfer temperature to control the billet surface temperature drop rate to ≤5℃ / min.

4. The dual-furnace heating method for achieving high-temperature diffusion of bearing steel as described in claim 1, characterized in that, In step S3, the carbide dissolution stage involves heating the billet to the target diffusion temperature of 1200-1220℃ at a heating rate of 5-8℃ / min; the transition cooling stage involves reducing the billet temperature by 5-10℃ at a cooling rate of 1-2℃ / min; and the carbide reprecipitation stage involves restoring the billet to the target diffusion temperature and holding it at that temperature for 4-7 hours.

5. The dual-furnace heating method for achieving high-temperature diffusion of bearing steel as described in claim 1, characterized in that, Both the second and first heating furnaces are purged with a protective atmosphere of nitrogen and hydrogen, with hydrogen accounting for 5-15% of the volume and nitrogen accounting for 85-95% of the volume, and the furnace pressure is maintained at 0.01-0.03 MPa.

6. The dual-furnace heating method for achieving high-temperature diffusion of bearing steel as described in claim 4, characterized in that: For steel billets with a carbon content of 0.9-1.0%, the holding time for the carbide re-precipitation stage is 4-5 hours, and the final temperature of the gradient cooling treatment is 1020-1050℃. For steel billets with a carbon content of 1.0-1.1%, the holding time for the carbide re-precipitation stage is 5-6 hours, and the final temperature of the gradient cooling treatment is 1000-1030℃. For steel billets with a carbon content of 1.1-1.2%, the holding time for the carbide re-precipitation stage is 6-7 hours, and the final temperature of the gradient cooling treatment is 950-1000℃.

7. The dual-furnace heating method for achieving high-temperature diffusion of bearing steel as described in claim 1, characterized in that, In step S4, the gradient cooling includes: In the first cooling stage, the steel billet is cooled from its high-temperature diffusion temperature to 1150℃ at a rate of 7.5-10℃ / min; and In the second cooling stage, the billet temperature is reduced from 1150℃ to 1050-1100℃ at a rate of ≤10℃ / min.

8. The dual-furnace heating method for achieving high-temperature diffusion of bearing steel as described in claim 7, characterized in that, The cooling rate of the second cooling stage is adjusted according to the cross-sectional size of the billet. When the cross-sectional size of the billet is ≤150mm, the cooling rate is 8-10℃ / min; when the cross-sectional size of the billet is >150mm and ≤200mm, the cooling rate is 6-8℃ / min; when the cross-sectional size of the billet is >200mm, the cooling rate is 4-6℃ / min.

9. A dual-furnace heating method for achieving high-temperature diffusion of bearing steel as described in claim 1, characterized in that, The effective length of the furnace body occupied by the steel billet undergoing the high-temperature diffusion treatment in the first heating furnace shall not be less than 85% of the total length of the first heating furnace.

10. A dual-furnace heating method for achieving high-temperature diffusion of bearing steel as described in claim 1, characterized in that, The exhaust pipe of the first heating furnace is connected to the air preheating device of the second heating furnace. The high-temperature flue gas discharged from the first heating furnace is used to preheat the combustion air of the second heating furnace to 200-300°C before it is introduced into the second heating furnace.

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