60Si2CrVAT spring steel and low-temperature homogenizing annealing process thereof

Abstract

The invention discloses 60Si2CrVAT spring steel and a low-temperature homogenizing annealing process thereof, and belongs to the technical field of 60Si2CrVAT spring steel polishing material preparation, and the low-temperature homogenizing annealing process comprises the following steps: carrying out gradient heating, isothermal diffusion and slow cooling on hot-rolled 60Si2CrVAT spring steel, and then carrying out post-treatment of straightening and surface treatment. According to the 60Si2CrVAT spring steel prepared through the low-temperature homogenizing annealing technology, the tensile strength is larger than or equal to 1900 MPa, and the thickness of a decarburized layer is smaller than or equal to 50 micrometers. The low-temperature annealing process is adopted, the structure is refined while the strength of the 60Si2CrVAT spring steel is kept, and the plasticity is improved; and the obtained decarburized layer is reduced in thickness and uniform in internal structure, so that the fatigue performance is remarkably improved.

Description

Technical Field

[0001] This invention belongs to the technical field of 60Si2CrVAT spring steel polishing material preparation, specifically 60Si2CrVAT spring steel and its low-temperature homogenization annealing process. Background Technology

[0002] 60Si2CrVAT is a typical high-strength and high-toughness spring steel, mainly used in automotive suspensions, railway vibration damping devices, and construction machinery, where high-stress cyclic loads and long service life are required. Compared with the widely used 60Si2Mn, this steel exhibits superior fatigue resistance, stress relaxation resistance, and corrosion resistance under extreme service conditions, but several key bottlenecks still restrict its further engineering application.

[0003] First, high-temperature annealing is commonly used in industry to achieve higher yield strength. However, when the spring diameter or effective thickness is large, the radial temperature gradient during cooling is significant, easily leading to residual tensile stress between the core and the surface layer. This stress can rapidly evolve into fatigue crack initiation under subsequent alternating loads. Second, while adding composite microalloying materials such as Cr and V can effectively refine the grains, it can result in uneven carbide distribution during subsequent heat treatment, affecting the balance between strength and toughness.

[0004] Furthermore, during hot rolling and subsequent annealing, the high Si content in the steel preferentially oxidizes at high temperatures. Simultaneously, complete decarburization occurs on the surface, reaching a depth of 80-100 µm. This internal oxide layer and decarburization zone significantly reduce surface hardness and fatigue strength, becoming sensitive areas for the preferential initiation of fatigue cracks. While the currently widely used hydrogen-nitrogen mixed protective atmosphere can suppress surface oxidation to some extent, it cannot effectively suppress both decarburization and Si internal oxidation simultaneously. This results in a narrow process window, making it difficult to meet the dual requirements of large-size springs for uniform microstructure and surface integrity. Summary of the Invention

[0005] To overcome the aforementioned technical problems, this invention provides 60Si2CrVAT spring steel and its low-temperature homogenization annealing process. This invention employs a low-temperature annealing process, which refines the microstructure and improves the plasticity of 60Si2CrVAT spring steel while maintaining its strength; the resulting decarburized layer thickness is reduced, and the internal microstructure is more uniform, thus significantly improving fatigue performance.

[0006] The present invention solves the above-mentioned technical problems through the following technical solutions.

[0007] This invention discloses a low-temperature homogenization annealing process for 60Si2CrVAT spring steel, which includes the following steps: subjecting the hot-rolled 60Si2CrVAT spring steel to gradient heating, isothermal diffusion and slow cooling, followed by straightening and surface treatment.

[0008] The 60Si2CrVAT spring steel comprises, by mass percentage: 0.56~0.64% C, 1.40~1.80% Si, 0.40~0.70% Mn, S≤0.030%, P≤0.030%, 0.90~1.20% Cr, Ni≤0.35%, Cu≤0.25%, and 0.10~0.20% V.

[0009] According to some embodiments of the present invention, the diameter of the hot-rolled 60Si2CrVAT spring steel is 10~80mm, preferably 20~60mm.

[0010] According to some embodiments of the present invention, the first stage of the gradient heating is to heat to 400~450℃ at a heating rate of ≤80℃ / h, and the second stage is to heat to the isothermal diffusion temperature at a heating rate of 30~100℃ / h. Preferably, the first stage of the gradient heating is to heat to 400°C at a heating rate of 50~80°C / h, and the second stage is to heat to the isothermal diffusion temperature at a heating rate of 40~60°C / h.

[0011] According to some embodiments of the present invention, the isothermal diffusion temperature is 550~600℃; preferably, the isothermal diffusion temperature is 580±5℃.

[0012] According to some embodiments of the present invention, the isothermal diffusion time is 12-16 hours.

[0013] According to some embodiments of the present invention, the slow cooling is performed at a cooling rate of 15~25℃ / h to 450~500℃; and then at a cooling rate of 5~15℃ / h to ≤300℃.

[0014] For example, the slow cooling is achieved by cooling to 450~500℃ at a cooling rate of 20℃ / h, and then cooling to ≤300℃ at a rate of 10℃ / h.

[0015] According to some embodiments of the present invention, the gradient heating and the isothermal diffusion are both carried out in a protective atmosphere containing 1-10% hydrogen, 0.5-2% carbon dioxide and the balance nitrogen. Preferably, the protective atmosphere contains 3-5% hydrogen, 0.5-2% carbon dioxide, and the balance nitrogen.

[0016] According to some embodiments of the present invention, the total deformation of the straightening is 0.1-1%, preferably 0.1-0.5%.

[0017] According to some embodiments of the present invention, the straightening amplitude is 0.05~0.2mm, preferably 0.05~0.1mm.

[0018] According to some embodiments of the present invention, the surface treatment includes surface shot peening, alkaline washing, acid washing, and passivation.

[0019] According to some embodiments of the present invention, after shot peening, stress-relief annealing is required, which is performed at 150~250℃ for 0.5~2h.

[0020] 60Si2CrVAT spring steel is obtained by the aforementioned low-temperature homogenization annealing process of 60Si2CrVAT spring steel.

[0021] According to some embodiments of the present invention, the tensile strength of the 60Si2CrVAT spring steel is ≥1900MPa, preferably 1920~1980MPa.

[0022] According to some embodiments of the present invention, the thickness of the decarburized layer of the 60Si2CrVAT spring steel is ≤50μm, preferably 20~43μm.

[0023] Based on common knowledge in the field, the above-mentioned preferred conditions can be combined arbitrarily to obtain various preferred embodiments of the present invention.

[0024] Compared with the prior art, the beneficial effects of the present invention are: By employing a low-temperature, long-time diffusion annealing process, grain coarsening can be suppressed while promoting uniform diffusion of alloying elements, thereby prioritizing the improvement of tensile strength without sacrificing toughness.

[0025] During the annealing stage, a H2-N2 composite protective atmosphere containing trace amounts of CO2 is introduced. By utilizing the regulating effect of CO2 on carbon activity and the reducing properties of H2, the surface decarburized layer thickness is ≤50μm, and the formation of the oxide layer inside Si is significantly suppressed.

[0026] Post-treatment can effectively seal potential microcracks and block the intrusion path of corrosive media, thereby improving fatigue life. Detailed Implementation

[0027] To facilitate understanding of the present invention, the present invention will be described more fully and in detail below with reference to preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0028] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0029] The "range" disclosed in this invention is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be combined arbitrarily; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for specific parameters, it is understood that ranges of 60-110 and 80-120 are also expected. Furthermore, if minimum range values ​​1 and 2 are listed, and if maximum range values ​​3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this invention, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" have been listed herein; "0-5" is merely a shortened representation of these numerical combinations. Furthermore, when a parameter is described as an integer greater than or equal to 2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0030] Unless otherwise specified, all embodiments and optional embodiments of the present invention can be combined with each other to form new technical solutions.

[0031] Unless otherwise specified, all technical features and optional technical features of this invention can be combined to form new technical solutions.

[0032] Unless otherwise specified, all steps of the present invention may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order; for example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0033] Unless otherwise specified, the terms "comprising" and "including" as used in this invention can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0034] Unless otherwise specified, the term "or" is inclusive in this invention. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0035] Example 1 By mass fraction, 60Si2CrVAT spring steel comprises: 0.61%C, 1.52%Si, 0.55%Mn, 0.018%S, 0.013%P, 1.13%Cr, 0.22%Ni, 0.193%Cu, and 0.17%V; the diameter of hot-rolled 60Si2CrVAT spring steel is 60mm. The gradient heating and isothermal diffusion of the aforementioned 60Si2CrVAT spring steel were both conducted in a protective atmosphere containing 3% hydrogen, 1.0% carbon dioxide, and the balance nitrogen, with a flow rate of 20 m³ / s. 3 / h; The first stage of the gradient heating is to heat to 400℃ at a heating rate of 60℃ / h, and the second stage is to heat to the isothermal diffusion temperature at a heating rate of 50℃ / h; the isothermal diffusion temperature is 580℃, and the isothermal diffusion time is 14h. Slow cooling involves cooling to 480℃ at a rate of 20℃ / h, followed by cooling to 250℃ at a rate of 10℃ / h before air cooling.

[0036] At room temperature, a roller straightener was used for straightening, with a straightening amplitude of 0.08 mm and a total straightening deformation of 0.3%. The steel surface is then shot blasted with cast steel shot at a pressure of 0.5 MPa and a blasting angle of 90°. After shot blasting, the steel is held at 200°C for 1 hour for stress-relief annealing. The steel is prepared by alkaline washing with sodium hydroxide solution at 70°C for 5 minutes, acid washing with 10% citric acid solution at 60°C for 10 minutes, and finally passivation treatment with 5% sodium nitrate and 1% sodium molybdate solution at pH=9 and 60°C for 10 minutes. After drying, 60Si2CrVAT spring steel can be obtained.

[0037] Example 2 The difference between this embodiment and Embodiment 1 is as follows: The temperature for isothermal diffusion is 550℃; All other steps and parameters are the same as in Example 1.

[0038] Example 3 The difference between this embodiment and Embodiment 1 is as follows: The temperature for isothermal diffusion is 620℃; All other steps and parameters are the same as in Example 1.

[0039] Example 4 The difference between this embodiment and Embodiment 1 is as follows: The protective atmosphere contains 4.5% hydrogen, 0.5% carbon dioxide, and the balance nitrogen. All other steps and parameters are the same as in Example 1.

[0040] Example 5 The difference between this embodiment and Embodiment 1 is as follows: The protective atmosphere contains 5% hydrogen and the balance nitrogen. All other steps and parameters are the same as in Example 1.

[0041] Example 6 The difference between this embodiment and Embodiment 1 is as follows: This embodiment does not involve shot peening. All other steps and parameters are the same as in Example 1.

[0042] Comparative Example 1 The difference between this comparative example and Example 1 is as follows: In this comparative example, hot-rolled 60Si2CrVAT spring steel was heated to 650℃ at a heating rate of 60℃ / min, held at that temperature for 10 hours, and then cooled in the furnace. All other steps and parameters are the same as in Example 1.

[0043] Comparative Example 2 The difference between this comparative example and Example 1 is as follows: The isothermal diffusion time is 8 hours; All other steps and parameters are the same as in Example 1.

[0044] Test case The 60Si2CrVAT spring steels prepared in the above embodiments and comparative examples were subjected to strength tests and decarburization layer tests. The decarburization test standard was in accordance with GB / T224. The test results are shown in Table 1.

[0045]

[0046] Unless otherwise specified, all raw materials, reagents, instruments, and equipment used in this invention can be purchased commercially or prepared using existing methods. The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of this invention. It should be understood that the above descriptions are merely specific embodiments of this invention and are not intended to limit the invention. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of this invention should be included within the scope of protection of this invention.

Claims

The low-temperature homogenization annealing process for 1.60Si2CrVAT spring steel is characterized by, Includes the following steps: The hot-rolled 60Si2CrVAT spring steel is subjected to gradient heating, isothermal diffusion and slow cooling, followed by straightening and surface treatment.

2. The low-temperature homogenization annealing process for 60Si2CrVAT spring steel as described in claim 1, characterized in that, The 60Si2CrVAT spring steel comprises, by mass percentage: 0.56~0.64% C, 1.40~1.80% Si, 0.40~0.70% Mn, S≤0.030%, P≤0.030%, 0.90~1.20% Cr, Ni≤0.35%, Cu≤0.25%, and 0.10~0.20% V.

3. The low-temperature homogenization annealing process for 60Si2CrVAT spring steel as described in claim 1, characterized in that, The diameter of the hot-rolled 60Si2CrVAT spring steel is 10~80 mm, preferably 20~60 mm.

4. The low-temperature homogenization annealing process for 60Si2CrVAT spring steel as described in claim 1, characterized in that, The isothermal diffusion temperature is 550~600℃; preferably, the isothermal diffusion temperature is 580±5℃. And / or, the isothermal diffusion time is 12~16h.

5. The low-temperature homogenization annealing process for 60Si2CrVAT spring steel as described in claim 4, characterized in that, The first stage of the gradient heating is to heat to 400~450℃ at a heating rate of ≤80℃ / h, and the second stage is to heat to the temperature of isothermal diffusion at a heating rate of 30~100℃ / h. Preferably, the first stage of the gradient heating is to heat to 400°C at a heating rate of 50~80°C / h, and the second stage is to heat to the isothermal diffusion temperature at a heating rate of 40~60°C / h.

6. The low-temperature homogenization annealing process for 60Si2CrVAT spring steel as described in claim 5, characterized in that, The slow cooling is achieved by cooling to 450-500℃ at a rate of 15-25℃ / h, followed by cooling to ≤300℃ at a rate of 5-15℃ / h.

7. The low-temperature homogenization annealing process for 60Si2CrVAT spring steel as described in any one of claims 1 to 6, characterized in that, The gradient heating and the isothermal diffusion are both carried out in a protective atmosphere containing 1-10% hydrogen, 0.5-2% carbon dioxide and the balance nitrogen. Preferably, the protective atmosphere contains 3-5% hydrogen, 0.5-2% carbon dioxide, and the balance nitrogen.

8. The low-temperature homogenization annealing process for 60Si2CrVAT spring steel as described in claim 1, characterized in that, The total deformation during straightening is 0.1% to 1%, preferably 0.1% to 0.5%. And / or, the straightening amplitude is 0.05~0.2mm, preferably 0.05~0.1mm.

9. The low-temperature homogenization annealing process for 60Si2CrVAT spring steel as described in claim 1, characterized in that, The surface treatment includes surface shot peening, alkaline washing, acid washing, and passivation; After shot peening, stress-relief annealing is required, which is performed at 150~250℃ for 0.5~2h. 10.60Si2CrVAT spring steel, characterized in that, It is prepared by the low-temperature homogenization annealing process of 60Si2CrVAT spring steel as described in any one of claims 1 to 9.

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