Gear steel isothermal normalizing treatment process

Abstract

This invention belongs to the field of steel smelting technology, specifically relating to an isothermal normalizing process for gear steel. Addressing issues such as uneven hardness, difficulty in achieving hardness standards for some steel grades, and low temperature control precision in batch processing of 20MnCr5 and 20CrMnTiH round steel during heat treatment, this invention achieves efficient batch processing of steel through a combination of precise furnace loading design, controlled heating time for each furnace cycle, coordinated temperature control via the cooling system, and precise and stable isothermal temperature control. After treatment, the steel's microstructure consists entirely of ferrite and pearlite (F+P), with no abnormal structures such as bainite or martensite, and a grain size reaching level 8. The Brinell hardness of the round steel remains stable above 150 HBW, fully meeting industry requirements. This process is compatible with existing conventional heat treatment equipment, requiring no additional equipment investment. Nitrogen cooling achieves oxidation-free decarburization of the steel, demonstrating feasibility and promotional value for large-scale industrial production.

Description

Technical Field

[0001] This invention relates to the field of heat treatment technology for metallic materials, specifically to a batch isothermal normalizing process for gear steel, which is particularly suitable for the industrial-scale batch isothermal normalizing of 20MnCr5 and 20CrMnTiH round steel for gears / shafts. Background Technology

[0002] Isothermal normalizing is a key heat treatment process for gear steel before subsequent processing. By controlling the heating, cooling, and isothermal processes, the metallographic structure of the steel can be optimized, ensuring core properties such as hardness and grain size, laying the foundation for the forging and machining of gears and shafts. As core steels for high-end gears / shafts, 20MnCr5 and 20CrMnTiH have specific industry requirements for their post-isothermal normalizing properties: 20MnCr5 round steel must have a Brinell hardness ≥160HBW, and 20CrMnTiH round steel must have a Brinell hardness ≥150HBW; simultaneously, the metallographic structure must be ferrite and pearlite (F+P), without abnormal structures such as bainite and martensite, and the grain size must be ≥8.

[0003] Currently, when using a 15-ton bogie hearth furnace in China to perform batch isothermal normalizing of the above-mentioned steel grades, the following technical challenges exist: (1) There is no standardized proportion when loading the furnace. Round steel of different specifications and grades is stacked randomly, resulting in uneven heating of steel during heating and cooling. After batch processing, the hardness fluctuates greatly and the performance of some areas does not meet the standards. (2) The heating time was fixed and not adjusted according to the furnace weight. When the furnace weight increased, the austenitization of the steel core was insufficient, which affected the subsequent microstructure transformation. (3) Poor coordination of the cooling system. The fog cannon, fan and nitrogen cooling operate independently. The steel area directly facing the cooling device cools too quickly and the temperature is too low, resulting in a large temperature difference with other areas, which leads to uneven structure and hardness. (4) The isothermal temperature control accuracy is low, and the furnace temperature is prone to a large surge after adjustment. The temperature fluctuation during the isothermal heat preservation stage is large, which cannot guarantee the full and uniform transformation of the steel structure. Therefore, the hardness requirement of 20CrMnTiH Φ52mm round steel after batch processing is far less than 150HBW, which has become a common technical problem in the industry, and the existing process has not proposed an effective control solution for this problem.

[0004] To address the aforementioned issues, there is an urgent need to develop a standardized batch isothermal normalizing process adapted to a 15-ton bogie hearth furnace. Through precise design of parameters for the entire process of furnace loading, heating, cooling, and isothermal treatment, this process can solve problems such as uneven hardness, difficulty in achieving the required hardness for some steel grades, and low batch processing efficiency. At the same time, it can ensure that the steel microstructure and grain size meet industry standards and be suitable for large-scale industrial production. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide an isothermal normalizing process for gear steel, which solves the problems of uneven hardness, difficulty in achieving the required hardness of 20CrMnTiH, and low temperature control accuracy in the batch heat treatment of 20MnCr5 and 20CrMnTiH round steel. It achieves efficient batch processing of 15 to 18 tons, and the properties and microstructure of the treated steel meet industry standards. At the same time, this process is compatible with existing conventional equipment, requires no additional investment, and has industrial promotion value.

[0006] To achieve the above objectives, this invention employs a technical solution of precise control of all process parameters and coordinated equipment operation, providing a batch isothermal normalizing process suitable for 20MnCr5 Φ60mm and 20CrMnTiH Φ52mm round steel. It utilizes a 15-ton bogie-type furnace with a maximum loading capacity of 20 tons, equipped with a mist cannon, a blower, and a nitrogen cooling system with controllable flow rate. Specifically, it includes four core steps: furnace loading, heating, isothermal cooling, and sampling and testing. The process parameters for each step are standardized, as detailed below: Step 1. Loading the Furnace The core of this step is standardized proportioning and uniform arrangement to solve the problem of uneven heating / cooling caused by random loading into the furnace, while also taking into account the efficiency of batch processing.

[0007] A 15-ton trolley-type furnace is used for loading, with the total loading weight controlled between 15.096 and 18.096 tons, and the total number of bundles between 123 and 144. The furnace is loaded in bundles according to steel type and specifications, with the specific proportions as follows: Preferably, when the steel in step (1) is 20MnCr5, the number of single bundles of Φ60mm and 6000mm specifications is 20 to 22, and the weight of a single bundle is 2.546 to 2.803 tons; the number of single bundles of Φ60mm and 4000mm specifications is 5 to 14, and the weight of a single bundle is 0.568 to 1.405 tons. Preferably, when the steel in step (1) is 20CrMnTiH, the number of single bundles of Φ52mm and 6000mm specifications is 9 to 10, and the weight of a single bundle is 0.976 to 1.084 tons; when loading the furnace, round steel of different specifications and steel types are evenly arranged to avoid local over-density stacking.

[0008] Furthermore, the main specifications for loading steel are 20MnCr5 Φ60mm and 6000mm (accounting for more than 80% of the total loading weight), with 22 pieces per bundle and a weight of 2.8 to 2.803 tons per bundle.

[0009] During the loading process, round bars of different specifications and steel grades are evenly arranged in the trolley-type furnace to avoid excessive local stacking and ensure uniform distribution of airflow and temperature in the furnace during heating and cooling.

[0010] Step 2. Heating Step The core of this step is to control the heating time for each furnace charge, adjusting the heating time according to the furnace charge weight to ensure that the steel core and edges are synchronized and fully austenitized.

[0011] After the closed bogie furnace, a stepped heating method is adopted (to avoid excessive temperature difference between the inside and outside of the steel caused by rapid heating). The heating time is adjusted according to the total furnace weight: that is, the total heating time corresponding to each ton of furnace weight is controlled to be 1.65~1.85h. Preferably, the stepped heating in step (2) is carried out in the following three stages: (1) Low temperature preheating stage: target temperature 200~250℃, heating rate 80~100℃ / h; heat preservation time is 30%~50% of the total heating time, and temperature fluctuation ≤±3℃; (2) Medium temperature heating stage: target temperature 550~600℃, heating rate 100~130℃ / h, heat preservation for 20%~30% of the total heating time, and temperature difference between steel surface and core ≤30℃; (3) High temperature heating stage: target temperature 880~930℃, heating rate 90~110℃ / h, heat preservation for 30%~50% of the total heating time, furnace temperature fluctuation ≤±5℃, after heat preservation, the temperature difference between the core and the surface ≤20℃, and then isothermal cooling can be started. When the furnace weight increases, the heating time is extended proportionally; when the furnace weight is 15.096 tons, the total heating time is controlled at 9 hours; when the furnace weight is 18.096 tons, the total heating time is controlled at 10 hours.

[0012] During the heating process, ensure uniform temperature inside the furnace and avoid localized overheating areas to ensure full austenitization of the steel, laying the foundation for the microstructure transformation in the subsequent isothermal stage.

[0013] Step 3. Isothermal Cooling Step This step is the core of the process. It solves the problems of uneven cooling and large fluctuations in isothermal temperature by coordinating the cooling system with temperature control and precise isothermal temperature stabilization. It is divided into two stages: preliminary cooling and isothermal holding. (1) Preliminary cooling Turn on the mist cannon, fan, and nitrogen cooling system in the pipeline. The three work together: the mist cannon achieves atomized cooling, the fan achieves forced convection cooling, and the nitrogen pipeline continuously supplies a stable flow of nitrogen to achieve an inert atmosphere protection inside the furnace, preventing steel oxidation and decarburization.

[0014] The initial cooling temperature control is as follows: the overall temperature of the steel is cooled to 620-650℃; for the steel area directly facing the fog cannon (which is prone to cooling too quickly and has a low temperature), the temperature of this area is precisely controlled at 560-600℃ by dynamically adjusting the fog output of the fog cannon and the fan speed, thereby reducing the temperature difference of steel in different areas of the furnace and ensuring uniform cooling.

[0015] (2) Isothermal insulation After initial cooling, the furnace temperature of the bogie hearth furnace is precisely adjusted to 610~620℃, and isothermal heat preservation treatment is carried out for a duration of not less than 2 hours, preferably 2-3 hours, to ensure that the steel structure is fully transformed into ferrite and pearlite.

[0016] During the isothermal process, if the furnace temperature surges due to the power supply for temperature compensation, it is necessary to quickly adjust the cooling system to strictly control the surge temperature within 630℃ and quickly bring it back to 610~620℃, ensuring that the overall furnace temperature fluctuation does not exceed ±10℃, thus achieving precise and stable control of the isothermal temperature.

[0017] Step 4. Sampling and Testing Procedures After the isothermal insulation is completed, all equipment is turned off, and the steel is allowed to cool naturally to room temperature before sampling and testing to ensure that the steel's properties and microstructure meet industry standards. The testing requirements are as follows: Sampling locations: Select samples from steel at different locations. Along the diameter of the sample, take samples at three locations: the core, 1 / 2R (midpoint of radius), and the edge to ensure the comparability of the sampling points. Testing items: Brinell hardness, metallographic structure, grain size; The standards for the prepared steel: ① The metallographic structure consists of ferrite and pearlite (F+P), without any abnormal structures such as bainite or martensite; ②Grain size ≥ grade 8; ③The average Brinell hardness of 20MnCr5 round steel is ≥160HBW; ④ The average Brinell hardness of 20CrMnTiH round steel is ≥150HBW, and the hardness difference between the core, 1 / 2R, and the edge is ≤13HBW. Beneficial effects

[0018] The process of this invention, through standardized design of all-process parameters and coordinated equipment operation, solves many technical pain points of existing batch isothermal normalizing processes, and has the following beneficial effects: Significantly improved performance uniformity: By standardizing the furnace loading ratio, heating in separate furnaces, and coordinating temperature control with the cooling system, the problem of uneven steel hardness has been solved. The hardness difference of the core, 1 / 2R and edge of 20CrMnTiH round steel is ≤8HBW, and the hardness of 20MnCr5 round steel has no obvious fluctuation, with the hardness difference of the core, 1 / 2R and edge being ≤13HBW. Core performance meets industry requirements: The Brinell hardness of 20MnCr5 round steel is stable at 160-190 HBW, which fully meets industry requirements; the average Brinell hardness of 20CrMnTiH round steel is close to 155 HBW, and its hardness is significantly improved, which can also meet production requirements and solve the hardness problem of batch processing of this steel. The microstructure and grain size meet the standards: the metallographic structure of the treated steel is all F+P, with no abnormal structures of bainite or martensite, and the grain size is stable at level 8, which meets the industry standard and provides a good microstructure basis for subsequent processing. High batch processing efficiency: The furnace loading capacity can reach 15.096 to 18.096 tons, which is close to the maximum loading capacity of a 15-ton bogie hearth furnace, realizing efficient batch heat treatment of alloy structural steel. The equipment is highly adaptable and requires no additional investment: This process is fully compatible with existing conventional 15-ton bogie furnaces and their supporting mist cannons, fans, and nitrogen pipelines, without the need for new equipment or modification of existing production lines, thus reducing industrial application costs. Excellent steel surface quality: Nitrogen gas is continuously introduced during the cooling process to achieve inert atmosphere protection, avoid steel oxidation and decarburization, ensure steel surface quality, and reduce subsequent grinding processes; The process is highly operable and easy to promote: the process parameters are standardized and quantitatively designed throughout the entire process, with no ambiguous operation requirements, making it easy for workers to learn and apply to the large-scale industrial production of steel heat treatment. Attached Figure Description

[0019] Figure 1 These are the furnace loading parameters for Embodiment 1 of the present invention.

[0020] Figure 2 These are the furnace loading parameters for Embodiment 2 of the present invention.

[0021] Figure 3 This is a 100x metallographic image of the edge of a Φ52mm round steel bar made of 20CrMnTiH in Example 1 of the present invention.

[0022] Figure 4 This is a 200x metallographic image of the edge of a Φ52mm round steel bar made of 20CrMnTiH in Example 1 of the present invention.

[0023] Figure 5 This is a 500x metallographic image of the edge of a Φ52mm round steel bar from 20CrMnTiH in Example 1 of the present invention.

[0024] Figure 6 This is a 100x metallographic image of the core of a Φ52mm round steel bar from 20CrMnTiH in Embodiment 1 of the present invention.

[0025] Figure 7 This is a 200x metallographic image of the core of a Φ52mm round steel bar from 20CrMnTiH in Embodiment 1 of the present invention.

[0026] Figure 8 This is a 500x metallographic image of the core of a 20CrMnTiH Φ52mm round steel bar in Embodiment 1 of the present invention.

[0027] Figure 9This is a 100x metallographic image of the edge of a 20MnCr5 Φ60mm round steel bar in Embodiment 1 of the present invention.

[0028] Figure 10 This is a 200x metallographic image of the edge of a 20MnCr5 Φ60mm round steel bar in Embodiment 1 of the present invention.

[0029] Figure 11 This is a 500x metallographic image of the edge of a 20MnCr5 Φ60mm round steel bar in Embodiment 1 of the present invention.

[0030] Figure 12 This is a 100x metallographic image of the core of a 20MnCr5 Φ60mm round steel bar in Embodiment 1 of the present invention.

[0031] Figure 13 This is a 200x metallographic image of the core of a 20MnCr5 Φ60mm round steel bar in Embodiment 1 of the present invention.

[0032] Figure 14 This is a 500x metallographic image of the core of a 20MnCr5 Φ60mm round steel bar in Embodiment 1 of the present invention. Detailed Implementation

[0033] The present invention will be further described in detail below with reference to several specific embodiments. The scope of protection of the present invention is not limited to the following embodiments. The described embodiments are only some embodiments of the present invention, not all embodiments. All equivalent transformations based on the technical solutions of the present invention are within the scope of protection of the present invention.

[0034] In this embodiment, the equipment used is: a 15-ton bogie-type furnace with a maximum loading capacity of 20 tons, one fog cannon, five fans, and a pipeline nitrogen cooling system with stable and controllable flow rate; the steel processed is 20MnCr5 Φ60mm (length: 6000mm, 4000mm) and 20CrMnTiH Φ52mm (length: 6000mm) round steel; the Brinell hardness test adopts the same industry standard, the metallographic structure test adopts metallographic microscope (100x, 200x, 500x), and the grain size test adopts the intercept method.

[0035] Example 1: 15.096 tons of mixed loading process. Figure 1 These are the furnace loading parameters for Embodiment 1 of the present invention.

[0036] Furnace loading: 20MnCr5 Φ60mm (6000mm and 4000mm) and 20CrMnTiH Φ52mm and 6000mm are mixed and loaded into the furnace. The total loading weight is 15.096 tons and the total number of bundles is 123. The specific proportions are as follows: 5 bundles of 20MnCr5 Φ60mm and 6000mm (11, 22, 22, 22, 22 pieces, corresponding to weights of 1.401, 2.803, 2.802, 2.801, and 2.8 tons, respectively, accounting for 83.5% of the total weight); 1 bundle of 20MnCr5 Φ60mm and 4000mm (14 pieces, 1.405 tons); 1 bundle of 20CrMnTiH Φ52mm and 6000mm (10 pieces, 1.084 tons); evenly distributed with no local stacking. Heating: Stepped heating, low-temperature preheating stage: target temperature 240℃, heating rate 80℃ / h; holding time is 40% of the total heating time, temperature fluctuation ≤±3℃; medium-temperature heating stage: target temperature 550℃, heating rate 100℃ / h, holding time is 20% of the total heating time, temperature difference between steel surface and core ≤30℃; high-temperature homogenization stage: target temperature 930℃, heating rate 100℃ / h, holding time is 40% of the total heating time, furnace temperature fluctuation ≤±5℃, after holding time the temperature difference between core and surface ≤20℃, then isothermal cooling begins; total heating time 9h, suitable for a low furnace charge of 15.096 tons; Isothermal cooling: ① Initial cooling: Turn on the cooling system to cool the steel as a whole to 620℃, and control the temperature of the area directly facing the fog cannon at 560℃; ②Isothermal heat preservation: The furnace temperature is adjusted to 620℃ and kept for 3 hours until the furnace temperature is stable without fluctuation; Sampling and testing: After isothermal cooling, 20CrMnTiH round steel, 20MnCr5 round steel (Φ60mm, 4000mm), and 20MnCr5 round steel (Φ60mm, 6000mm) were arranged in six rows from top to bottom, with the top row designated as the first row, and so on. The first row consisted of 20CrMnTiH round steel, the second row of 20MnCr5 (Φ60mm, 4000mm) round steel, and the third to sixth rows of 20MnCr5 (Φ60mm, 6000mm) round steel. A sample was taken from the right side of the first row and designated as C1; a sample was taken from the middle of the second row and designated as M2; samples were taken from the middle of the third and sixth rows and designated as M1 and M3, respectively. Thus, sample C1 was taken from 20CrMnTiH, and samples M1, M2, and M3 were taken from 20MnCr5. Samples were taken from the core, 1 / 2R, and edge of the samples for testing.

[0037] Test results:

[0038] Brinell hardness: 20CrMnTiH average 154.7 HBW (core 155.1 HBW, 1 / 2R 158.0 HBW, edge 151 HBW, hardness difference 7 HBW); 20MnCr5 M1: 167.3 HBW, M2: 164.6 HBW, M3: 168.5 HBW, all ≥160 HBW; Metallographic structure: all are F+P, with no abnormal bainitic or martensitic structures; Grain size: Grade 8, which meets the standard requirements.

[0039] Example 2: 18.096 tons of mixed loading process Figure 2 These are the furnace loading parameters for Embodiment 2 of the present invention.

[0040] Furnace loading: 20MnCr5 Φ60mm (6000mm / 4000mm) and 20CrMnTiH Φ52mm and 6000mm are mixed and loaded into the furnace. The total loading weight is 18.096 tons and the total number of bundles is 144. The specific proportions are as follows: 6 bundles of 20MnCr5 Φ60mm and 6000mm (with 22, 22, 22, 22, 22, 22, and 20 pieces respectively, corresponding to weights of 2.802, 2.802, 2.801, 2.8, 2.801, and 2.546 tons respectively, accounting for 91.5% of the total weight); 1 bundle of 20MnCr5 Φ60mm and 4000mm (5 pieces, 0.568 tons); and 1 bundle of 20CrMnTiH Φ52mm and 6000mm (9 pieces, 0.976 tons); evenly distributed with no local stacking. Heating: Stepped heating, low-temperature preheating stage: target temperature 240℃, heating rate 80℃ / h; holding time is 35% of the total heating time, temperature fluctuation ≤±3℃; medium-temperature heating stage: target temperature 550℃, heating rate 100℃ / h, holding time is 25% of the total heating time, temperature difference between steel surface and core ≤30℃; high-temperature homogenization stage: target temperature 930℃, heating rate 100℃ / h, holding time is 40% of the total heating time, furnace temperature fluctuation ≤±5℃, after holding time the temperature difference between core and surface ≤20℃, then isothermal cooling; total heating time 10h, suitable for 18.096-ton high-loading furnace, ensuring sufficient austenitization of the core; Isothermal cooling: ① Initial cooling: Turn on the cooling system to cool the steel as a whole to 650℃, some areas to 660℃, and control the temperature of the area directly facing the fog cannon at 600℃; ②Isothermal heat preservation: The furnace temperature is adjusted to 620℃ and kept for 3 hours. After the furnace temperature is powered on, it rises to 630℃ and then quickly falls back to 620℃. The temperature fluctuation throughout the process is ≤10℃. Sampling and Testing: After isothermal cooling, 20CrMnTiH and 20MnCr5 round steel bars were arranged in seven rows from top to bottom, with the top row designated as the first row, and so on. The first row consisted of 20CrMnTiH round steel bars, the second row consisted of 20MnCr5 round steel bars with diameters of 52mm and 4000mm, and the third to seventh rows consisted of 20MnCr5 round steel bars with diameters of 60mm and 6000mm. A sample was taken from the left side of the first row and designated as C2; a sample was taken from the middle of the second row and designated as M22; and samples were taken from the right side and the middle of the sixth row and designated as M21 and M23, respectively. Thus, sample C2 was taken from 20CrMnTiH, and samples M21, M22, and M23 were taken from 20MnCr5. Samples were taken from the core, 1 / 2R, and edge of the samples for testing.

[0041] Test results:

[0042] Brinell hardness: 20CrMnTiH average 153.6 HBW (core 153.5 HBW, 1 / 2R 155.3 HBW, edge 152.1 HBW, hardness difference 3.2 HBW); 20MnCr5 M21: 167.8 HBW, M22: 158.9 HBW, M23: 163.1 HBW, overall ≥160 HBW; Metallographic structure: all are F+P, with no abnormal bainitic or martensitic structures; Grain size: Grade 8, which meets the standard requirements.

[0043] Further through the Figure 3-14 A detailed analysis, including Figure 3-8 Metallographic diagram corresponding to 20CrMnTiH Φ52mm (6000mm) Figure 9-14 The metallographic diagram of 20MnCr5 Φ60mm (6000mm) is shown; the following conclusions can be drawn, which also demonstrate the advantages of the corresponding heat treatment process: (1) Good uniformity of metallographic structure: From Figure 3-14 As can be seen, whether it is 20CrMnTiH Φ52mm round steel or 20MnCr5 Φ60mm round steel, and whether it is the edge or the core, at 100x (scale bar in the figure is 100μm), 200x (scale bar in the figure is 50μm), and 500x (scale bar in the figure is 20μm), the metallographic structure is a mixed structure of ferrite and pearlite (F+P), and the distribution is relatively uniform. This indicates that the heat treatment process can enable the steel to obtain a consistent microstructure as a whole, avoiding performance differences caused by uneven microstructure.

[0044] (2) No abnormal structures were observed in the figure. Bainite, martensite and other abnormal structures were not observed. The presence of bainite and martensite often leads to an increase in the hardness and brittleness of steel, which may cause cracking and other problems during use. However, the steel treated by the heat treatment process of this invention does not have such abnormal structures, indicating that the process can effectively control the transformation of the structure and ensure the stability of the steel structure.

[0045] (3) Grain size meets the standard: The morphology and distribution of the metallographic structure can also be used to preliminarily judge that the grain size has reached the level 8 requirement; fine grain size helps to improve the strength and toughness of steel and improve the comprehensive mechanical properties of steel.

[0046] The above results demonstrate that the isothermal normalizing process for steel described in this invention has strong industrial applicability, specifically reflected in: Adaptable to existing production lines: The process is based entirely on the mainstream 15-ton bogie hearth furnace and supporting cooling equipment design in the domestic steel heat treatment industry. No new equipment or modification of existing production lines is required, and it can be directly applied. Batch processing scale adaptability: Furnace loading capacity of 15.096 to 18.096 tons, close to the maximum furnace loading capacity of the equipment, to meet the needs of large-scale industrial production and improve production efficiency; Standardized and easy-to-operate process: The entire process parameters are quantified and standardized, with no ambiguous operation requirements. Frontline workers can get started with simple training, reducing production management costs. Low overall production cost: Nitrogen cooling achieves decarburization of steel without oxidation, reducing subsequent grinding processes; hardness and microstructure meet standards in one go, eliminating rework and reducing raw material and energy consumption.

[0047] In summary, this invention, through precise furnace loading design, controlled heating time for each furnace cycle, and coordinated temperature control of the cooling system, ensures accurate temperature control of steel during heating and cooling, enabling the steel to undergo phase transformation according to the expected microstructure transformation path, thereby obtaining an ideal metallographic structure. By employing isothermal temperature control and other technological means, it achieves efficient batch processing of steel, and the processed steel exhibits stable metallographic structure and hardness. The Brinell hardness of the round steel remains stable above 150 HBW, meeting industry requirements, indicating that the process has good stability and reliability in large-scale production. 20MnCr5 fully meets industry requirements, and 20CrMnTiH also meets the requirements, enhancing the product's market competitiveness.

[0048] This process can be directly applied to the industrial batch isothermal normalizing treatment of 20MnCr5 and 20CrMnTiH round steel for gears and shafts. It can also provide technical reference for the batch isothermal normalizing treatment of other alloy structural steels in this field, and has broad industrial promotion value.

[0049] Note: The above embodiments are only used to illustrate the present invention and are not intended to limit the technical solutions described in the present invention. Therefore, although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the present invention. All technical solutions and improvements that do not depart from the spirit and scope of the present invention should be covered within the scope of the claims of the present invention.

Claims

1. A process for isothermal normalizing of a gear steel, characterized in that, The gear steel includes 20MnCr5 and 20CrMnTiH; The steps are as follows: (1) Loading the furnace: The steel is loaded into the furnace in bundles according to the steel type and specifications. The total weight of the load is controlled at 15.096 to 18.096 tons and the total number of bundles is 123 to 144. (2) Heating: The closed bogie furnace is heated in a stepped manner, and the heating parameters are adjusted according to the total weight of the furnace: that is, the total heating time corresponding to each ton of furnace weight is controlled to be 1.65~1.85h; (3) Isothermal cooling: ① Initial cooling: Turn on the fog cannon, fan and nitrogen pipeline to cool the steel to 620-650℃. By adjusting the fog output of the fog cannon and the fan speed, control the temperature of the steel area directly facing the fog cannon at 560-600℃. ②Isothermal heat preservation: The furnace temperature is precisely adjusted to 610~620℃, and the heat preservation time is not less than 2 hours; if the furnace temperature surge occurs during the isothermal process due to power compensation, the surge temperature is controlled within 630℃ and quickly returned to 610~620℃ to ensure that the overall furnace temperature fluctuation does not exceed ±10℃. (4) Sampling and testing: After the steel has cooled to room temperature, samples are taken from the core, 1 / 2R and edge of the steel at different locations to test the Brinell hardness, metallographic structure and grain size. Steel treated by isothermal normalizing process has a microstructure of ferrite and pearlite, without bainite or martensite, and a grain size ≥ 8.

2. The gear steel isothermal normalizing process of claim 1, wherein, In step (1), the steel is 20MnCr5, of which the single bundle of Φ60mm and 6000mm steel has 20 to 22 pieces and a single bundle weight of 2.546 to 2.803 tons, and the single bundle of Φ60mm and 4000mm steel has 5 to 14 pieces and a single bundle weight of 0.568 to 1.405 tons.

3. The gear steel isothermal normalizing process of claim 1, wherein, When the steel in step (1) is 20CrMnTiH, the number of single bundles of Φ52mm and 6000mm steel is 9 to 10, and the weight of a single bundle is 0.976 to 1.084 tons. When loading the furnace, round steel of different specifications and steel grades are evenly arranged to avoid local over-density stacking.

4. The gear steel isothermal normalizing process according to claim 2 or 3, characterized in that, The main specifications for loading steel 20MnCr5 in Φ60mm and 6000mm are 22 pieces per bundle, with a weight of 2.8 to 2.803 tons per bundle. Physical separation is used to distinguish them during loading.

5. The gear steel isothermal normalizing process of claim 1, wherein, Step (2) involves a stepped heating process, which is carried out in the following three stages: (1) Low temperature preheating stage: target temperature 200~250℃, heating rate 80~100℃ / h; heat preservation time is 30%~50% of the total heating time, and temperature fluctuation ≤±3℃; (2) Medium temperature heating stage: target temperature 550~600℃, heating rate 100~130℃ / h, heat preservation for 20%~30% of the total heating time, and temperature difference between steel surface and core ≤30℃; (3) High temperature heating stage: target temperature 880~930℃, heating rate 90~110℃ / h, heat preservation for 30%~50% of the total heating time, furnace temperature fluctuation ≤±5℃, after heat preservation, the temperature difference between the core and the surface ≤20℃, and then isothermal cooling can be started. When the furnace charge is 15.096 tons, the total heating time is controlled at 9 hours; when the furnace charge is 18.096 tons, the total heating time is controlled at 10 hours; to ensure that the steel core and edges are austenitized synchronously.

6. The isothermal normalizing process for gear steel according to claim 1, characterized in that, In step (3), the nitrogen flow rate in the nitrogen pipeline is stable, and nitrogen is continuously introduced into the furnace during the cooling process to achieve inert atmosphere protection for the steel and avoid oxidation and decarburization.

7. The isothermal normalizing process for gear steel according to claim 1, characterized in that, In step (4), the Brinell hardness test uses the same test standard. The sampling points for the core, 1 / 2R and edge are different positions in the same diameter direction to ensure the comparability of the test results.

8. The isothermal normalizing process for gear steel according to claim 1, characterized in that, The average Brinell hardness of 20MnCr5 round steel is ≥160HBW, and the average Brinell hardness of 20CrMnTiH round steel is ≥150HBW, with the hardness difference between the core, 1 / 2R, and edge ≤13HBW.

9. The application of the isothermal normalizing process for gear steel according to any one of claims 1-8 in the batch heat treatment of gear steel, characterized in that, The process is adapted to a 15-ton bogie hearth furnace for isothermal normalizing of 20MnCr5 and 20CrMnTiH gear steels, achieving a batch processing scale of 15 to 18 tons.

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