Segregation control method and application of 8620H gear steel

Abstract

The invention relates to a segregation control method and application of 8620H gear steel, and the segregation control method of the 8620H gear steel comprises the following steps: (a) a continuous casting stage: carrying out atomization cooling when the solidification rate of a casting blank reaches 85-92%, and synchronously executing dynamic roll gap shrinkage; (b) a rolling stage: implementing single-pass pressing deformation which is greater than or equal to 35% in a temperature interval of 780-820 DEG C in a finish rolling area, wherein the accumulated pressing rate of last three passes is greater than or equal to 70%; and (c) finishing stage: carrying out quenching treatment within less than or equal to 5 seconds after the rolled piece is cut. According to the method, the segregation degree of the 8620H gear steel is effectively reduced and the fatigue life of a product is prolonged by accurately controlling the process parameters of the continuous casting stage, the rolling stage and the finishing stage.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of steel smelting, in particular to a segregation control method for 8620H gear steel and application. BACKGROUND

[0002] Gear steel is mainly used for transmission parts of automobiles, engineering machinery and mechanical manufacturing industry. In the process of transmission force and speed change, the tooth surfaces of mutual engagement have both rolling and sliding, and the tooth root part also bears the action of bending stress, so high-quality gear steel billet is needed to realize large-scale industrial processing.

[0003] Research has found that gear steel is prone to dendritic segregation during solidification. During the smelting process of gear steel, due to selective crystallization, the chemical composition in the crystal and between dendrites is uneven, and the dendrites and interdendritic are elongated when the gear steel continuous casting billet is forged and rolled.

[0004] At present, the method for improving or eliminating the segregation of gear steel at home and abroad is generally to raise the heating temperature and increase the holding time, so that the elements such as C, Cr, Mn and Si in the billet are fully diffused to weaken the element segregation and inhibit the generation of banded structure. However, this method has the disadvantage that the process parameters of the heating furnace cannot be accurately controlled, resulting in that the actual heating temperature and time of the billet in the heating furnace cannot reach or exceed the conditions required for eliminating banded structure, which increases the cost of actual production of enterprises and greatly reduces the production capacity.

[0005] 8620H is an alloy steel round steel that meets the ASTM A304 standard, mainly used in the fields of automobile gear manufacturing and mechanical manufacturing. Its chemical composition includes elements such as carbon C: 0.18-0.23, silicon Si: 0.15-0.35, manganese Mn: 0.70-0.90, chromium Cr: 0.40-0.60, nickel Ni: 0.40-0.70, and molybdenum Mo: 0.15-0.25, and the mechanical properties are as follows: tensile strength ≥ 980 MPa, yield strength ≥ 785 MPa.

[0006] At present, the segregation of 8620H gear steel is mainly caused by the following reasons: 1. Insufficient heating temperature will lead to insufficient element diffusion and aggravate segregation; 2. Insufficient reduction deformation (such as 30% or less) reduces the length and width of the element segregation band. SUMMARY

[0007] The present application is made in view of the above problems, and aims to provide a segregation control method for 8620H gear steel and application.

[0008] Specifically, the first aspect of the present application provides a segregation control method for 8620H gear steel, comprising the following steps:

[0009] (a) Continuous casting stage: Atomized cooling is performed in the range of 85-92% solidification rate of the billet, and dynamic roll gap shrinkage is performed simultaneously.

[0010] (b) Rolling stage: In the finishing rolling zone, a single pass of ≥35% reduction deformation is carried out in the temperature range of 780-820℃, and the cumulative reduction rate of the last three passes is ≥70%.

[0011] (c) Finishing stage: The rolled piece is rapidly cooled within ≤5s after cutting.

[0012] Furthermore, the solidification rate of the billet mentioned in step (a) is calculated in real time using the formula f = K√t, where K = 26-30 mm / min. 0.5 t is the solidification time.

[0013] Further, the water pressure for atomization cooling in step (a) is ≥1.5MPa, and / or the atomization cooling uses a 45° cross-jet nozzle group, the cooling medium is pure water, and / or the cooling rate is 50-70℃ / s.

[0014] Furthermore, the single-segment shrinkage amount of the dynamic roll gap shrinkage described in step (a) is 0.3-0.8 mm.

[0015] Furthermore, the billet heating temperature before rolling in step (b) is 1170-1190℃.

[0016] Furthermore, the single-pass reduction rate in step (b) is 35-40%, and the strain rate is ≥25s. -1 .

[0017] Furthermore, the cooling rate of the rapid cooling process described in step (c) is ≥30℃ / s.

[0018] Furthermore, the workpiece cutting in step (c) is performed using plasma cutting with a cutting current of 400-450A and a protective gas of Ar / H2.

[0019] Furthermore, the rapid cooling treatment described in step (c) uses a 9-12% NaCl aqueous solution.

[0020] Furthermore, the rapid cooling rate described in step (c) is ≥35℃ / s, and the final cooling temperature is 600-610℃.

[0021] A second aspect of this application provides the application of 8620H gear steel produced by the method described above in the manufacture of automotive transmission gears.

[0022] The present invention has the following beneficial effects:

[0023] (1) By precisely controlling the process parameters of continuous casting, rolling and finishing stages, the process of atomized strong cooling and roll gap shrinkage is adopted at the end of the solidification of the billet, so that the liquid phase between dendrites flows back, thereby eliminating macro segregation; in addition, a single pass of ≥35% reduction deformation is implemented in the finishing rolling zone, so that the segregation channel is mechanically broken, thereby dispersing the micro segregation; finally, the workpiece is rapidly cooled within 5s after cutting, so that the solute diffuses and freezes, thereby locking the uniformity of the structure, effectively reducing the degree of segregation of 8620H gear steel and improving the fatigue life of the product.

[0024] (2) The application of the 8620H gear steel of this application in the manufacture of automotive gearbox gears can significantly improve the wear resistance, fatigue resistance and service life of gears, reduce the failure rate, and improve the performance and reliability of the whole vehicle. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of this drawing or the prior art, 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 this drawing. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0026] Figure 1 The image shows the 100X-2.0 banded microstructure of the gear steel in Example 1.

[0027] Figure 2 The image shows the banded microstructure of 100X-2.0-1 gear steel from Example 1.

[0028] Figure 3 The image shows the 500X-1.5 banded microstructure of the gear steel in Example 1.

[0029] Figure 4 The image shows the banded microstructure of 500X-1.5-1 gear steel from Example 1.

[0030] Figure 5 The image shows the banded microstructure of the 500X-2.0 gear steel in Example 1.

[0031] Figure 6 This is a diagram showing the banded structure of the 500X-2.5 gear steel in Example 1.

[0032] The purpose, features, and advantages of this accompanying drawing will be further explained in conjunction with the embodiments and with reference to the accompanying drawing. Detailed Implementation

[0033] To make the objectives, technical solutions, and advantages of this application clearer, the following description and illustration are provided in conjunction with embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application. All other embodiments obtained by those skilled in the art based on the embodiments provided in this application without inventive effort are within the scope of protection of this application.

[0034] Obviously, the following description is merely some examples or embodiments of this application. Those skilled in the art can apply this application to other similar scenarios without any inventive effort. Furthermore, it is understood that although the effort involved in such development may be complex and lengthy, for those skilled in the art related to the content disclosed in this application, any changes to design, manufacturing, or production based on the technical content disclosed in this application are merely conventional technical means and should not be construed as insufficient disclosure of the content of this application.

[0035] The first aspect of this application provides a method for controlling segregation in 8620H gear steel, comprising the following steps:

[0036] (a) Continuous casting stage: Atomized cooling is performed in the range of 85-92% solidification rate of the billet, and dynamic roll gap shrinkage is performed simultaneously.

[0037] (b) Rolling stage: In the finishing rolling zone, a single pass of ≥35% reduction deformation is carried out in the temperature range of 780-820℃, and the cumulative reduction rate of the last three passes is ≥70%.

[0038] (c) Finishing stage: The rolled piece is rapidly cooled within ≤5s after cutting.

[0039] This invention employs a double-tapered crystallizer during the solidification process of the cast billet, with an upper taper of 1.5% / m and a lower taper of 0.8% / m. Simultaneously, it uses electromagnetic alternating rotation stirring with a current of 350A and a frequency of 2.5Hz to control the liquid level fluctuation of 2-4mm.

[0040] In step (a), the water pressure for atomizing cooling is ≥1.5MPa, and / or the atomizing cooling uses a 45° cross-jet nozzle group, the cooling medium is pure water, and / or the cooling rate is 50-70℃ / s. In step (a), the single-segment shrinkage of the dynamic roll gap shrinkage is 0.3-0.8mm, preferably 0.5mm.

[0041] Furthermore, basic cooling is initiated when the solidification rate reaches 70%, with a water pressure of 0.8 MPa and a specific water flow rate of 1.2 L / kg. When the solidification rate of the billet reaches 85%, high-pressure atomization cooling is activated. High-pressure atomization nozzles are arranged in sections 5-7 of a fan-shaped configuration (12-15 m from the meniscus), with a 45° cross-spray angle, and the water pressure is instantly increased to 2.0 MPa. When the solidification rate reaches 90%, the roll gap shrinks by 0.5 mm simultaneously. By dynamically adjusting the thermal stress distribution within the roll gap and the billet, the solidification structure of the billet is further optimized, reducing the formation of segregation bands. In addition, the use of atomization cooling not only accelerates heat exchange on the billet surface but also refines the grains on the billet surface through rapid cooling, further enhancing the uniformity of the steel's microstructure.

[0042] The solidification rate of the billet in step (a) is calculated in real time using the formula f = K√t, where K = 26-30 mm / min. 0.5 t is the solidification time. Preferably, K = 28 mm / min 0.5 .

[0043] In this embodiment, a walking beam furnace is used in the rolling process, with a heating temperature of 1170-1190℃, a holding time of 2.5h, and the oxygen content in the furnace is controlled to be ≤1.5%.

[0044] The initial rolling temperature in the roughing stage is ≥1050℃, the reduction rate of pass 1 is ≥30%, and the thickness is 200mm→140mm; the reduction rate of pass 2 is ≥25%, and the thickness is 140mm→105mm. The final rolling temperature is ≥980℃. This step avoids deformation in the two-phase region and provides a good deformation basis for the subsequent finishing rolling.

[0045] In the finishing rolling stage, the microstructure of the steel was further optimized by precisely controlling the temperature and reduction deformation. Specifically, single-pass reduction deformation of ≥35% and strain rate of ≥25s were achieved within the temperature range of 780-820℃. -1 Preferably, the single-pass reduction rate is 35-40%, and the temperature drop is ≤15℃ / pass. Ensuring that the cumulative reduction rate of the last three passes is ≥70% significantly enhances the density and uniformity of the steel and effectively reduces the formation of segregation bands.

[0046] In this embodiment, during the finishing stage, plasma cutting is used for the rolled piece, with a cutting current of 400-450A and an Ar / H2 protective gas. The single-piece cutting time is ≤3s. Then, a rapid quenching technique is employed. After cutting, the rolled piece undergoes rapid quenching within an extremely short time (≤5s), which not only avoids coarsening of the steel structure at high temperatures but also further fixes the deformed microstructure through rapid cooling. The 9-12% NaCl aqueous solution used as the cooling medium provides a stable cooling rate (≥35℃ / s), ensuring a rapid transition of the steel from high to low temperature, ultimately cooling the steel to 600-610℃.

[0047] Furthermore, the rapid cooling treatment lasts for 2-4 minutes to ensure a uniform decrease in the internal temperature of the steel and avoid excessive internal stress caused by uneven cooling in certain areas. After the rapid cooling treatment, the steel also needs to undergo tempering to eliminate residual stress and improve the toughness and plasticity of the steel.

[0048] In practice, the above parameters can be fine-tuned according to actual production needs. For example, when producing gear steel with high strength requirements, the water pressure and cooling rate of atomization cooling can be appropriately increased, as well as the cooling rate and duration of quenching treatment can be extended. Conversely, when producing gear steel with high toughness requirements, these parameters can be appropriately reduced to obtain better toughness performance.

[0049] A second aspect of this application provides the application of 8620H gear steel produced by the method described above in the manufacture of automotive transmission gears.

[0050] The segregation control method for 8620H gear steel presented in this application is not only applicable to the manufacture of automotive transmission gears, but can also be widely applied to other fields requiring high-quality gear steel, such as engineering machinery and shipbuilding. The 8620H gear steel produced using this method exhibits excellent mechanical properties and uniform microstructure, meeting the requirements of various complex working conditions and improving enterprise production efficiency and product quality.

[0051] Example

[0052] The following examples describe the disclosure of this invention in more detail. These examples are merely illustrative, as various modifications and variations will be apparent to those skilled in the art within the scope of this disclosure. Unless otherwise stated, all parts, percentages, and ratios reported in the following examples are based on weight. Unless otherwise stated, all reagents used in the examples are available commercially or synthesized using conventional methods and are ready for use without further processing. Unless otherwise stated, all instruments used in the examples are available commercially.

[0053] Example 1

[0054] A method for controlling segregation in 8620H gear steel includes the following steps:

[0055] (a) Continuous casting stage: Atomized cooling is carried out in the range of 85-92% solidification rate of the billet. The water pressure for atomized cooling is 2.5MPa, the cooling rate is 55℃ / s, and dynamic roll gap shrinkage of 0.5mm is performed simultaneously.

[0056] (b) Rolling stage: A walking beam furnace is used in the rolling process. The heating temperature is 1180℃ and the holding time is 2.5h. The initial rolling temperature in the roughing stage is 1100℃. The reduction rate of the first pass is 35% and the reduction rate of the second pass is 25%. In the finishing rolling zone, the single pass reduction deformation is ≥35% in the temperature range of 780-820℃, and the cumulative reduction rate of the last three passes is ≥70%. The final rolling temperature is ≥980℃.

[0057] (c) Finishing stage: The rolled piece is cut by plasma cutting with a cutting current of 400A and a protective gas of Ar / H2; the rolled piece is rapidly cooled within 4s after cutting, with 10% NaCl aqueous solution as the cooling medium, providing a cooling rate of 38℃ / s.

[0058] Example 2

[0059] This embodiment is basically the same as embodiment 1, except that the water pressure for atomization cooling in step (a) is 3MPa and the cooling rate is 65℃ / s.

[0060] Example 3

[0061] This embodiment is basically the same as embodiment 1, except that in step (b), a single pass of 38% reduction deformation is performed in the finishing rolling zone at a temperature range of 780-820°C, and the cumulative reduction rate of the last three passes is 78%.

[0062] Example 4

[0063] This embodiment is basically the same as embodiment 1, except that step (b) involves rapid cooling within 5 seconds after the rolled piece is cut.

[0064] Comparative Example 1

[0065] This comparative example is basically the same as Example 1, except that it adopts the traditional process, eliminates atomized cooling, reduces the single pass of finishing rolling by ≤25%, replaces rapid cooling with air cooling, and has a cooling rate of 3℃ / s.

[0066] Comparative Example 2

[0067] This comparative example is basically the same as Example 1, except that the water pressure in the atomization cooling process is 0.8 MPa and the cooling rate is 18°C / s.

[0068] Comparative Example 3

[0069] This comparative example is basically the same as Example 1, except that the reduction rate of the finishing rolling pass is reduced to 28%.

[0070] Comparative Example 4

[0071] This comparative example is basically the same as Example 1, except that the rolled piece was delayed in water cooling for 15 seconds after cutting, and the cooling rate was reduced to 22°C / s.

[0072] The performance of the 8620H gear steel produced in Examples 1-4 and Comparative Examples 1-4 was tested, and the results are shown in the table below:

[0073] Group Carbon segregation index Hardness difference (HV) Fatigue life / 10000 times Detection standard YB / T 4412-2014 ISO 6507 ISO 1143 Example 1 1.03 12 2.81 x 10 6 ]] Example 2 1.02 13 2.75 x 10 6 ]]> Example 3 1.04 15 2.69 x 10 6 ]] Example 4 1.05 14 2.78 x 10 6 ]] Comparative Example 1 1.12 37 1.05 x 10 6 ]]> Comparative Example 2 1.09 24 1.87 x 10 6 ]]> Comparative Example 3 1.07 19 2.23 x 10 6 ]]> Comparative Example 4 1.06 28 2.41 x 10 6 ]]

[0074] As can be seen from the table above, the 8620H gear steel in Examples 1-4 of this invention has excellent performance. This may be because the process of atomized strong cooling and roll gap shrinkage at the end of the solidification of the billet causes the liquid phase between dendrites to flow back, thereby eliminating macroscopic segregation. In addition, the single-pass reduction deformation of ≥35% in the finishing rolling zone causes the segregation channels to be mechanically broken, thereby dispersing the microscopic segregation. Finally, the rapid cooling treatment within 5 seconds after the rolled piece is cut causes the solute to diffuse and freeze, thereby locking the uniformity of the microstructure. The above treatments combined improve the fatigue life of the product.

[0075] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A method for controlling segregation in 8620H gear steel, characterized in that, Includes the following steps: (a) Continuous casting stage: Atomized cooling is performed in the range of 85-92% solidification rate of the billet, and dynamic roll gap shrinkage is performed simultaneously. (b) Rolling stage: In the finishing rolling zone, a single pass of ≥35% reduction deformation is carried out in the temperature range of 780-820℃, and the cumulative reduction rate of the last three passes is ≥70%. (c) Finishing stage: The rolled piece is rapidly cooled within ≤5s after cutting.

2. The segregation control method for 8620H gear steel according to claim 1, characterized in that, The water pressure for atomization cooling in step (a) is ≥1.5MPa, and / or the atomization cooling uses a 45° cross-jet nozzle group, the cooling medium is pure water, and / or the cooling rate is 50-70℃ / s.

3. The segregation control method for 8620H gear steel according to claim 1, characterized in that, The single-segment shrinkage amount of the dynamic roll gap shrinkage in step (a) is 0.3-0.8 mm.

4. The segregation control method for 8620H gear steel according to claim 1, characterized in that, In step (b), the billet heating temperature before rolling is 1170-1190℃.

5. The segregation control method for 8620H gear steel according to claim 1, characterized in that, The single-pass reduction rate in step (b) is 35-40%, and the strain rate is ≥25s. -1 .

6. The segregation control method for 8620H gear steel according to claim 1, characterized in that, The cooling rate of the rapid cooling process described in step (c) is ≥30℃ / s.

7. The segregation control method for 8620H gear steel according to claim 1, characterized in that, The workpiece cutting in step (c) is performed using plasma cutting with a cutting current of 400-450A and a protective gas of Ar / H2.

8. The segregation control method for 8620H gear steel according to claim 1, characterized in that, The rapid cooling process described in step (c) uses a 9-12% NaCl aqueous solution.

9. The segregation control method for 8620H gear steel according to claim 1, characterized in that, The rapid cooling rate described in step (c) is ≥35℃ / s, and the final cooling temperature is 600-610℃.

10. The application of 8620H gear steel produced by the method described in any one of claims 1-9 in the manufacture of automotive transmission gears.

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