A high-stress spring steel and a process for producing the same

Abstract

The application relates to a high-stress spring steel and a preparation process thereof, and belongs to the technical field of spring steel. By controlling basic elements such as C, Si, Mn and Cr, optimizing the proportional relationship of V, Nb and N, adopting a segmented V, Nb adding process and a two-stage vacuum treatment process, nitrogen is coupled with micro-alloy elements to form dispersed carbonitride precipitation phases; meanwhile, calcium treatment is carried out in the late refining stage to realize inclusion spheroidization and refinement. After continuous casting, rolling and quenching and tempering treatment, a tempered martensite structure and a uniform precipitation strengthening structure are obtained. The method effectively improves the strength and impact toughness of the material, improves the inclusion distribution, and significantly improves the service performance of the spring steel under high-stress cyclic conditions.

Description

Technical Field

[0001] This invention relates to the field of spring steel technology, and in particular to a high-stress spring steel and its preparation process. Background Technology

[0002] Pneumatic actuators are widely used in petrochemical, energy equipment, and automated control systems, where they use compressed air to open, close, and regulate valves. In pneumatic actuators, the spring, as a key energy storage and reset element, is subjected to high-frequency reciprocating motion and high-stress cyclic loads over long periods, along with vibration, impact, and complex operating conditions. This places extremely high demands on the strength, elastic limit, and fatigue resistance of the spring material. Especially under high-cycle fatigue conditions, the microscopic defects within the material have a decisive impact on its service life.

[0003] In existing technologies, to improve the fatigue performance of spring steel, Si-Cr or Cr-V alloy systems are typically used, and the strength and toughness of the material are enhanced through optimized heat treatment processes. Simultaneously, during steel smelting, to reduce the adverse effects of non-metallic inclusions on fatigue performance, calcium treatment is commonly employed to modify inclusions. This involves adding calcium to the molten steel to transform high-melting-point, irregularly shaped inclusions into low-melting-point, relatively spherical composite inclusions, thereby reducing stress concentration to some extent and improving the material's fatigue performance.

[0004] However, existing calcium treatment techniques still have significant shortcomings: Firstly, calcium is highly reactive, with a narrow window of action in molten steel, making it prone to oxidation or burn-off, resulting in unstable inclusion modification effects. Secondly, in the presence of sulfur in the steel, calcium readily reacts with sulfur to form CaS inclusions, the size and distribution of which are difficult to control effectively and may still become sources of fatigue crack initiation. Furthermore, existing techniques struggle to precisely control the size and distribution of inclusions, resulting in the presence of large, harmful inclusions. Simultaneously, the calcium treatment process is typically independent of the precipitation strengthening process of microalloying elements (such as V and Nb), lacking a synergistic control mechanism, making it difficult to achieve synergistic optimization of inclusion control and precipitation strengthening.

[0005] Therefore, under high-stress cyclic service conditions, existing technologies still cannot effectively suppress the initiation and propagation of fatigue cracks, limiting further improvements in the reliability and long service life of spring steel for pneumatic actuators. Summary of the Invention

[0006] This application provides a process for preparing high-stress spring steel, comprising the following steps: Step S1. Mix molten iron and scrap steel and smelt them in a converter or electric furnace. At the same time, add ferrosilicon, ferromanganese and ferrochrome alloys to control the C, Si, Mn and Cr elements. During the tapping process, perform weak deoxidation and aluminum final deoxidation treatment to obtain low oxygen molten steel. Step S2. During the LF or RH refining stage, adjust the refining atmosphere and vacuum conditions, and perform the first stage of vacuum treatment and argon stirring to control the nitrogen content and promote the removal of oxide inclusions by flotation. Step S3. After the first stage of vacuum treatment, vanadium and niobium alloy are added in two stages, and a second stage of deep vacuum treatment is performed to allow vanadium, niobium and nitrogen to undergo a coupling reaction to form a precipitation precursor. Step S4. After the second stage of vacuum treatment, calcium is added and the reaction time and stirring intensity are controlled to transform the oxide inclusions into low-melting-point spherical composite inclusions and achieve stable distribution. Step S5. The molten steel is continuously cast into billets and then rolled and quenched and tempered to obtain a tempered martensite structure containing dispersed precipitates.

[0007] It should be noted that in steps S1 to S2, low oxygen control and the first-stage vacuum treatment preferentially remove oxide inclusions and reduce the O and S content in the molten steel, thereby reducing the formation of hard inclusions from the source. In step S3, V and Nb are added in stages and combined with the second-stage deep vacuum treatment, so that N preferentially couples with V and Nb to form stable carbonitride precipitation precursors, thereby inhibiting the enrichment of N at the inclusion interface and reducing the tendency of crack initiation. In step S4, calcium reacts with residual Al2O3 and sulfides to generate low-melting-point Ca-Al-OS composite inclusions, which spheroidize and soften the inclusions, significantly reducing the stress concentration effect. Finally, in step S5, heat treatment makes the dispersed precipitates uniformly distributed in the tempered martensite matrix, realizing the synergistic effect of precipitation strengthening and inclusion passivation, thereby significantly improving the fatigue resistance and service stability of the material under high stress cycling conditions.

[0008] As a preferred technical solution for the preparation process of high-stress spring steel, the spring steel comprises, by mass percentage: C: 0.52-0.60%, Si: 1.50-2.20%, Mn: 0.30-0.80%, Cr: 0.90-1.30%, V: 0.03-0.07%, Nb: 0.010-0.050%, N: 0.005-0.015%, Al: 0.005-0.020%, Ca: 0.0005-0.0050%, S≤0.003%, O≤0.0015%, with the balance being Fe and unavoidable impurities.

[0009] First, C, Si, Mn, and Cr constitute the basic strengthening system. C improves hardenability and forms a high-strength martensitic matrix, Si strengthens through solid solution and inhibits carbide coarsening, thereby improving tempering stability, Mn improves hardenability and promotes microstructure uniformity, and Cr further improves hardenability and resistance to tempering softening, enabling the material to obtain a stable high-strength tempered martensitic structure after heat treatment. Second, V and Nb, as microalloying elements, form dispersed (V,Nb)(C,N) nanoprecipitates with appropriate N content. These precipitates pin dislocations and grain boundaries. It significantly improves yield strength and inhibits crack propagation while refining grains. Furthermore, N, as a key regulating element, preferentially couples with V and Nb, causing nitrogen to transfer from the solid solution state to carbonitrides, thereby avoiding its enrichment at the inclusion interface and the formation of brittle sources. At the same time, Al is used for final deoxidation to control oxygen content, and combined with Ca to modify Al2O3 inclusions, generating low-melting-point Ca-Al-OS composite inclusions, which spheroidizes and refines the inclusions and reduces interfacial stress concentration. In addition, by strictly controlling the S and O content, the number and size of sulfide and oxide inclusions are reduced.

[0010] In a preferred technical solution for the preparation process of high-stress spring steel, the mass ratio of Ca to N in the steel is 0.1 to 0.33.

[0011] It should be noted that controlling the mass ratio of Ca to N in the steel to be between 0.1 and 0.33 allows calcium to preferentially modify oxide inclusions and inhibit nitrogen enrichment at the inclusion interface, while ensuring that nitrogen mainly forms carbonitride precipitates with vanadium and niobium, thereby reducing the tendency for crack initiation.

[0012] In a preferred technical solution for the preparation process of high-stress spring steel, the mass ratio of V and Nb to N is 6.7 to 8.

[0013] It should be noted that the mass ratio of V and Nb to N is 6.7 to 8, which ensures that nitrogen is fully consumed and preferentially forms dispersed carbonitride precipitates with vanadium and niobium, thereby reducing dissolved nitrogen and its enrichment at the inclusion interface and improving the material strength and fatigue resistance.

[0014] As a preferred technical solution for the preparation process of high-stress spring steel, the vacuum degree of the first stage vacuum treatment is ≤100 Pa and the time is 5 to 10 min, and the vacuum degree of the second stage vacuum treatment is ≤50 Pa and the time is 8 to 20 min.

[0015] It should be noted that the higher vacuum level in the first stage is used to promote gas escape and removal of inclusions by flotation, while the higher vacuum level and longer time in the second stage are used to enhance the coupling reaction of V, Nb and N and form a stable precipitation precursor, thereby achieving phased regulation of inclusion purification and precipitation control.

[0016] As a preferred technical solution for the preparation process of high-stress spring steel, vanadium and niobium are added in two stages, with 60-80% of the total amount added in the first stage and the remaining amount added after an interval of 3-8 minutes.

[0017] It should be noted that by adding V and Nb in two separate steps with an interval of 3 to 8 minutes, an initial dissolution and uniform diffusion base can be formed first. Then, the remaining alloy is added to suppress local segregation and promote uniform coupling reaction with nitrogen, thereby improving the uniformity of the precipitated phase distribution.

[0018] This invention, through synergistic optimization of the composition system and smelting process, controls the key ratios of (Ca / N) and (V+Nb) / N. It employs staged addition of V and Nb combined with two-stage vacuum treatment to preferentially couple nitrogen with microalloying elements, forming dispersed carbonitride precipitates. Simultaneously, a subsequent calcium treatment modifies inclusions, achieving spheroidization, refinement, and compositional optimization, thereby effectively reducing stress concentration and inhibiting crack initiation. Compared to traditional processes, this invention significantly improves the impact toughness of the material while maintaining high strength, achieving a good balance between strength and toughness. It exhibits superior microstructural stability and failure resistance, especially under high-stress cyclic service conditions, demonstrating significant engineering application value. Attached Figure Description

[0019] Figure 1 The images show the microstructure and energy dispersive spectroscopy (EDS) analysis results of the spring steel inclusions in Example 1 of this invention. (a) is a scanning electron microscope (SEM) image of the inclusions, and (b) is an EDS analysis result of the corresponding positions.

[0020] Figure 2 This is the X-ray diffraction (XRD) pattern of the spring steel in Embodiment 1 of the present invention. Detailed Implementation

[0021] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the examples in the specification.

[0022] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0023] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0024] Example 1

[0025] This embodiment provides a high-stress spring steel and its manufacturing process, wherein... The spring steel comprises, by mass percentage: C: 0.52%, Si: 1.50%, Mn: 0.30%, Cr: 0.90%, V: 0.03%, Nb: 0.01%, N: 0.005%, Al: 0.005%, Ca: 0.0005%, S: 0.0025%, O: 0.0015%, with the balance being Fe and unavoidable impurities; where Ca / N = 0.10, (V+Nb) / N = 8.

[0026] The manufacturing process of the spring steel includes the following steps: Step S1. Molten iron and scrap steel are added to the converter in a 7:3 ratio for smelting. The carbon content is adjusted by blowing oxygen and ferrosilicon, ferromanganese and ferrochrome alloys are added in batches to control the Si, Mn and Cr contents to reach the target range. At the end of smelting, the carbon content is controlled to 0.52%. During tapping, a weak deoxidation treatment is first performed to reduce the free oxygen content in the molten steel. Then aluminum is added for final deoxidation to stabilize the Al content in the molten steel at about 0.005% and control the oxygen content within 0.0015%, thereby obtaining a low-oxygen, uniformly composed molten steel with fewer inclusions.

[0027] Step S2. The molten steel is fed into the LF refining furnace and refined under a weakly reducing atmosphere, using bottom-blown argon gas (approximately 0.02 Nm³). 3 The refining process promotes the circulation of molten steel and the uniformity of temperature. During the refining process, the vacuum degree in the furnace is adjusted to about 100 Pa and maintained for 5 min as the first stage of vacuum treatment, so that the gaseous elements in the molten steel can escape and promote the removal of oxide inclusions by flotation. At the same time, the nitrogen content in the steel is stabilized at 0.005% by controlling the refining time and atmosphere conditions.

[0028] Step S3. After the first stage of vacuum treatment, add 60% of the total amount of ferrovanadium and ferroniobium alloy to the molten steel to allow it to dissolve preferentially and diffuse to form a preliminary uniform distribution. After an interval of about 3 minutes, add the remaining 40% of the alloying elements to replenish the alloy content and avoid local segregation. Then, perform the second stage of deep vacuum treatment, reduce the vacuum degree to about 50 Pa and maintain it for 8 minutes. Under this condition, promote the coupling reaction of vanadium, niobium and nitrogen and gradually form carbonitride precipitation precursors.

[0029] Step S4. After the second stage of vacuum treatment, the temperature of the molten steel is adjusted to about 1550℃. 0.5 kg / t of calcium wire is added by feeding the wire, and a weak stirring state is maintained for about 2 minutes after the addition, so that the calcium can diffuse evenly in the molten steel and react with Al2O3 inclusions and sulfides to generate low-melting-point Ca-Al-OS composite inclusions, thereby achieving spheroidization and softening of inclusions and reducing stress concentration.

[0030] Step S5. The treated molten steel is continuously cast to form a billet. Segregation is reduced by controlling the cooling intensity and casting speed of the secondary cooling zone. The billet is then heated and rolled, and after austenitizing quenching at about 850°C, it is rapidly cooled and then tempered at 420°C to give the steel a microstructure dominated by tempered martensite and to form dispersed precipitates of strengthening phases in the matrix.

[0031] Example 2

[0032] This embodiment provides a high-stress spring steel and its manufacturing process, wherein... The spring steel comprises, by mass percentage: C: 0.55%, Si: 1.70%, Mn: 0.45%, Cr: 1.00%, V: 0.04%, Nb: 0.02%, N: 0.008%, Al: 0.010%, Ca: 0.0015%, S: 0.0020%, O: 0.0012%; among them, Ca / N=0.19, (V+Nb) / N=7.5.

[0033] The manufacturing process of the spring steel includes the following steps: Step S1. Molten iron and scrap steel are added to an electric furnace in a 6:4 ratio for smelting. The carbon content is adjusted by blowing oxygen and ferrosilicon, ferromanganese and ferrochrome alloys are added in batches to control the Si, Mn and Cr contents to reach the target range. In the later stage of smelting, the carbon content is precisely controlled to 0.55%. During the tapping process, a weak deoxidation treatment is first performed to reduce the free oxygen content in the molten steel. Then aluminum is added for final deoxidation to stabilize the Al content in the molten steel at about 0.010% and control the oxygen content within 0.0012%.

[0034] Step S2. The molten steel is fed into the RH refining unit and refined under circulating vacuum conditions. The flow of molten steel and temperature homogenization are promoted by bottom blowing argon (about 0.03 Nm³ / min·t). The vacuum degree is adjusted to about 90 Pa and maintained for 7 min as the first stage of vacuum treatment, so that the gas can escape and promote the removal of inclusions by flotation. At the same time, the nitrogen content in the steel is stabilized at 0.008% by controlling the refining atmosphere.

[0035] Step S3. After the first stage of vacuum treatment, 70% of the vanadium and niobium alloy is added to the molten steel to allow it to fully dissolve and form a preliminary uniform distribution. After an interval of about 3 minutes, the remaining 30% of the alloying elements are added to avoid segregation. Then, the second stage of deep vacuum treatment is carried out, reducing the vacuum level to about 45 Pa and maintaining it for 12 minutes, so that vanadium, niobium and nitrogen can undergo a coupling reaction and form a uniform precipitation precursor.

[0036] Step S4. After the second stage of vacuum treatment, the temperature of the molten steel is adjusted to about 1580℃. Calcium of 1.0 kg / t steel is added by wire feeding and kept under weak stirring for about 3 minutes after addition, so that the calcium reacts with the oxides and sulfides in the steel to form low-melting-point spherical composite inclusions, thereby achieving the refinement and uniform distribution of inclusions.

[0037] Step S5. The molten steel is continuously cast into a billet and segregation is reduced by controlling the cooling regime. The billet is then rolled and quenched at about 900°C and then rapidly cooled. Finally, it is tempered at 450°C to obtain a refined tempered martensite structure and a dispersed precipitation strengthening structure.

[0038] Example 3

[0039] This embodiment provides a high-stress spring steel and its manufacturing process, wherein... The spring steel comprises, by mass percentage: C: 0.58%, Si: 2.00%, Mn: 0.65%, Cr: 1.20%, V: 0.06%, Nb: 0.02%, N: 0.012%, Al: 0.015%, Ca: 0.0030%, S: 0.0020%, O: 0.0010%; wherein, Ca / N = 0.25, (V+Nb) / N ≈ 6.7.

[0040] The manufacturing process of the spring steel includes the following steps: Step S1. Add molten iron and scrap steel to the converter in a 6:4 ratio for smelting. Adjust the carbon content by blowing oxygen and add ferrosilicon, ferromanganese and ferrochrome alloys in batches to control the Si, Mn and Cr contents to reach the target range. At the end of smelting, control the carbon content to 0.58%. During tapping, perform weak deoxidation treatment to reduce the free oxygen content in the molten steel. Then add aluminum for final deoxidation to stabilize the Al content at about 0.015% and control the oxygen content at 0.0010%.

[0041] Step S2. The molten steel is fed into the LF refining furnace and refined under a weakly reducing atmosphere, with bottom-blown argon gas (approximately 0.04 Nm³). 3 The vacuum level is controlled at approximately 80 Pa and maintained for 8 min as the first stage of vacuum treatment. This allows the gas to escape and promotes the removal of inclusions by flotation, while stabilizing the nitrogen content in the steel at 0.012%.

[0042] Step S3. After the first stage of vacuum treatment, 75% of vanadium and niobium alloy is added to the molten steel to allow it to dissolve and diffuse preferentially. After an interval of about 6 minutes, the remaining 25% of alloying elements is added to avoid local segregation. Then, the second stage of deep vacuum treatment is carried out, reducing the vacuum level to about 40 Pa and maintaining it for 15 minutes, so that vanadium, niobium and nitrogen can undergo a coupling reaction and form a stable precipitation precursor.

[0043] Step S4. After the second stage of vacuum treatment, the temperature of the molten steel is adjusted to about 1600℃. Calcium of 1.3 kg / t steel is added by wire feeding and the mixture is kept under weak stirring for about 4 minutes to allow the calcium to react with the inclusions to form low-melting-point spherical composite inclusions, thereby achieving the spheroidization, softening and uniform distribution of the inclusions.

[0044] Step S5 Molten steel is continuously cast to form a billet, and the cooling regime is controlled to reduce segregation. The billet is then rolled, quenched at about 920°C, rapidly cooled, and then tempered at 480°C to obtain a refined tempered martensite structure and uniformly dispersed precipitates.

[0045] Example 4

[0046] This embodiment provides a high-stress spring steel and its manufacturing process, wherein... The spring steel comprises, by mass percentage: C: 0.60%, Si: 2.20%, Mn: 0.80%, Cr: 1.30%, V: 0.07%, Nb: 0.05%, N: 0.015%, Al: 0.020%, Ca: 0.0050%, S: 0.0025%, O: 0.0012%; wherein, Ca / N≈0.33, (V+Nb) / N=8.

[0047] The manufacturing process of the spring steel includes the following steps: Step S1. Molten iron and scrap steel are added to an electric furnace in a 5:5 ratio for smelting. The carbon content is adjusted by blowing oxygen and ferrosilicon, ferromanganese and ferrochrome alloys are added in batches to control the Si, Mn and Cr contents to reach the target range. At the end of smelting, the carbon content is controlled to 0.60%. During tapping, a weak deoxidation treatment is first performed to reduce the free oxygen content in the molten steel. Then aluminum is added for final deoxidation to stabilize the Al content at about 0.020% and control the oxygen content at 0.0012%.

[0048] Step S2. The molten steel is fed into the RH refining system and refined under circulating vacuum conditions, using bottom-blown argon gas (approximately 0.05 Nm³). 3 The vacuum level is controlled at approximately 100 Pa and maintained for 10 minutes as the first stage of vacuum treatment. This allows the gas to escape and promotes the removal of inclusions by flotation, while stabilizing the nitrogen content in the steel at 0.015%.

[0049] Step S3. After the first stage of vacuum treatment, 80% of the vanadium and niobium alloy is added to the molten steel to allow it to dissolve and diffuse preferentially. After an interval of about 8 minutes, the remaining 20% ​​of the alloying elements are added to avoid segregation. Then, the second stage of deep vacuum treatment is carried out, reducing the vacuum level to about 35 Pa and maintaining it for 20 minutes, so that vanadium, niobium and nitrogen can undergo a coupling reaction and form a stable precipitation precursor.

[0050] Step S4. After the second stage of vacuum treatment, adjust the temperature of the molten steel to about 1600℃, add calcium at a rate of 1.5 kg / t of steel by wire feeding, and maintain a weak stirring state for about 5 minutes to allow the calcium to react with the inclusions to generate low-melting-point spherical composite inclusions, thereby achieving full spheroidization and uniform distribution of the inclusions.

[0051] Step S5. The molten steel is continuously cast into a billet and the cooling regime is controlled to reduce segregation. The billet is then rolled and quenched at about 950°C, followed by rapid cooling and tempering at 500°C to obtain a high-density dispersed precipitate phase and a stable tempered martensite structure.

[0052] Compare with Example 1 The only difference between Comparative Example 1 and Example 1 is that in step S3, vanadium and niobium alloy are added to the molten steel at once, while the other process conditions remain the same.

[0053] Compare with Example 2 The only difference between Comparative Example 2 and Example 1 is that the second stage deep vacuum treatment is cancelled in step S3, and only the first stage vacuum is retained.

[0054] Compare with Example 3 The only difference between Comparative Example 3 and Example 1 is that the timing of adding Ca in step S4 is brought forward to after the first stage vacuum (i.e., before S3).

[0055] Specific modifications: Ca is added immediately after the first stage of vacuum treatment, and V and Nb are added subsequently, followed by a second stage of vacuum treatment.

[0056] Compare with Example 4 The only difference between Comparative Example 4 and Example 1 is that the Ca content was adjusted so that Ca / N < 0.1.

[0057] Specific modifications: Ca changes from 0.0005% to 0.0002%, while N remains unchanged at 0.005%.

[0058] Compare with Example 5 The only difference between Comparative Example 5 and Example 1 is that the (V+Nb) / N ratio is reduced to <6.7.

[0059] Specific modifications: V changed from 0.03% to 0.020%, Nb changed from 0.01% to 0.005%, and the rest remained unchanged.

[0060] Performance testing methods

[0061] 1. Tensile strength and yield strength A universal testing machine was used to perform room temperature tensile tests on the specimens. The specimens were machined into round bars or plates according to standards (e.g., Φ5 mm standard specimens), and the tests were conducted according to GB / T 228.1. Constant-rate tension was applied, and stress-strain curves were recorded. The tensile strength (Rm) and yield strength (Rp0.2) were measured. At least three specimens were used in each group, and the average value was taken as the test result.

[0062] 2. Hardness The hardness of the samples was tested using a Rockwell hardness tester. After the samples were ground smooth, the tests were performed on their cross-section or surface using an HRC scale (load 150 kg). At least five different locations were selected for measurement on each sample, and the average value was taken after removing outliers as the hardness result for the material.

[0063] 3. Impact toughness Impact toughness was tested using a Charpy impact testing machine. The specimens were fabricated as standard V-notch specimens (10 mm × 10 mm × 55 mm). Impact tests were conducted at room temperature according to GB / T 229 standard, and the impact absorbed energy (kV value) was measured. At least three specimens were used in each group, and the average value was taken as the result.

[0064] Table 1. Mechanical property data of Examples 1 to 4 and Comparative Examples 1 to 5

[0065] In conjunction with Example 1 and Figure 1 The morphology and energy dispersive spectroscopy (EDS) results show that the inclusions in Figure (a) are generally nearly spherical or ellipsoidal with smooth boundaries and no obvious sharp edges, indicating that the original irregular oxide inclusions have undergone liquid-phase modification and tend to become spherical under the calcium treatment in step S4. The EDS in Figure (b) shows that the inclusions are mainly composed of Ca, Al, O, and S elements, and also contain small amounts of Mg, Si, and Fe elements. Al and O originate from the original Al2O3 inclusions. The presence of Ca indicates that calcium reacts with oxides to generate modified products, while the presence of S indicates the formation of a sulfur-containing composite phase. Overall, it is judged to be a Ca-Al-OS composite inclusion. Furthermore, the Fe peak mainly originates from the surrounding matrix signal. These results demonstrate that this embodiment achieves spheroidization, refinement, and composition optimization of inclusions through the preferential coupling of V, Nb, and N, combined with the synergistic regulation mechanism of subsequent calcium treatment. This effectively reduces stress concentration and inhibits fatigue crack initiation, thus improving the fatigue resistance of the material. In conjunction with Example 1 and Figure 2 The XRD pattern shows that the material exhibits obvious α-Fe diffraction peaks at approximately 44.7°, 65.0°, and 82.3° at 2θ, indicating that the matrix is ​​tempered martensite. Meanwhile, weak diffraction peaks appearing near 36°–43° and 60° correspond to (V,Nb)(C,N) carbonitride precipitates, indicating that under the conditions of segmented addition of V and Nb combined with a second-stage deep vacuum treatment, nitrogen preferentially couples with microalloying elements to form stable precipitates. Furthermore, characteristic peaks of Fe3C were detected in the range of 37°–49°, indicating the formation of dispersed carbides during tempering. These results demonstrate that this embodiment, through synergistic control of composition and process, achieves a composite microstructure of tempered martensite matrix and dispersed precipitates, thereby effectively improving the material's strength and suppressing crack initiation.

[0066] As can be seen from Examples 1 to 4 and Table 1, the spring steel obtained in the embodiments of the present invention has a tensile strength of 1680–1950 MPa, a yield strength of 1480–1720 MPa, a hardness of 47.5–53.0 HRC, and an impact toughness of 42–48 J. Overall, it maintains a good toughness match at a high strength level, indicating that under the defined composition range and process conditions, by controlling the (V+Nb) / N ratio, employing segmented addition and staged vacuum treatment, and combining calcium treatment to modify inclusions, a synergistic effect of precipitation strengthening and inclusion control can be achieved, thereby obtaining comprehensive mechanical properties with a good balance of strength and toughness.

[0067] As can be seen from Example 1, Comparative Example 1, and Table 1, in Comparative Example 1, due to the one-time addition of vanadium and niobium, the tensile strength decreased from 1680 MPa to 1600 MPa, the yield strength decreased from 1480 MPa to 1400 MPa, and the impact toughness also decreased from 42 J to 38 J. This is because the segmented addition method was not adopted, and V and Nb were prone to local segregation in the molten steel. This resulted in insufficient subsequent coupling reaction with N, uneven distribution of precipitates, and larger precipitates, thereby weakening the precipitation strengthening effect and increasing crack initiation sites, ultimately leading to a decrease in both the strength and toughness of the material.

[0068] As can be seen from Example 1, Comparative Example 2, and Table 1, the tensile strength and yield strength of Comparative Example 2 decreased to 1550 MPa and 1350 MPa, respectively, and the impact toughness decreased to 36 J, showing the most significant overall performance decline. This is because after canceling the second-stage deep vacuum treatment, gaseous elements in the steel were not sufficiently removed. Simultaneously, the coupling reaction conditions between V, Nb, and N were insufficient, leading to nitrogen existing in a solid solution state and accumulating at the inclusion interface. This not only weakens the carbonitride precipitation strengthening effect but also easily forms a brittle source, thus significantly reducing the material's strength and toughness.

[0069] As can be seen from Example 1, Comparative Example 3, and Table 1, the tensile strength and yield strength of Comparative Example 3 decreased to 1620 MPa and 1420 MPa, respectively, and the impact toughness decreased to 37 J. This is because calcium was added prematurely after the first-stage vacuum treatment, causing it to be consumed or rendered ineffective during subsequent refining. This prevented effective modification of the Al2O3 inclusions in the final stage, resulting in irregular inclusions that were difficult to spheroidize and soften, thus increasing stress concentration and reducing the material's resistance to crack initiation.

[0070] As can be seen from Example 1, Comparative Example 4, and Table 1, when the Ca / N ratio is below 0.1, the tensile strength and yield strength decrease to 1580 MPa and 1380 MPa, respectively, and the impact toughness further decreases to 35 J. This is because insufficient calcium content fails to adequately modify oxide inclusions in the steel, resulting in the retention of high-melting-point, sharp-shaped inclusions. These inclusions easily form stress concentration points under stress, significantly promoting crack initiation and thus adversely affecting the material's toughness.

[0071] As can be seen from Example 1, Comparative Example 5, and Table 1, when the (V+Nb) / N ratio decreases to approximately 5, the tensile strength decreases from 1680 MPa to 1630 MPa, the yield strength decreases from 1480 MPa to 1430 MPa, and the impact toughness also decreases. This is because the total amount of V and Nb is insufficient, preventing them from fully reacting with nitrogen in the steel to form dispersed carbonitride precipitates. Consequently, some nitrogen exists in solid solution form, weakening the precipitation strengthening effect and increasing the material's brittleness, ultimately leading to a decrease in both strength and toughness.

[0072] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A manufacturing process for high-stress spring steel, characterized in that, Includes the following steps: Step S1. Mix molten iron and scrap steel and smelt them in a converter or electric furnace. At the same time, add ferrosilicon, ferromanganese and ferrochrome alloys to control C, Si, Mn and Cr elements. During the tapping process, perform weak deoxidation and aluminum final deoxidation treatment to obtain low oxygen molten steel. Step S2. During the LF or RH refining stage, adjust the refining atmosphere and vacuum conditions, and perform the first stage of vacuum treatment and argon stirring to control the nitrogen content and promote the removal of oxide inclusions by flotation. Step S3. After the first stage of vacuum treatment, vanadium and niobium alloy are added in two stages, and a second stage of deep vacuum treatment is performed to allow vanadium, niobium and nitrogen to undergo a coupling reaction to form a precipitation precursor. Step S4. After the second stage of vacuum treatment, calcium is added and the reaction time and stirring intensity are controlled to transform the oxide inclusions into low-melting-point spherical composite inclusions and achieve stable distribution. Step S5. The molten steel is continuously cast into a billet and then rolled and quenched and tempered to obtain a tempered martensite structure containing dispersed precipitates; wherein, the spring steel comprises, by mass percentage: C: 0.52-0.60%, Si: 1.50-2.20%, Mn: 0.30-0.80%, Cr: 0.90-1.30%, V: 0.03-0.07%, Nb: 0.010-0.050%, N: 0.005-0.015%, Al: 0.005-0.020%, Ca: 0.0005-0.0050%, S≤0.003%, O≤0.0015%, with the balance being Fe and unavoidable impurities; The mass ratio of Ca to N in steel is 0.1 to 0.33; The mass ratio of total V and Nb to N is 6.7–8.

2. The preparation process according to claim 1, characterized in that, The vacuum degree of the first stage of vacuum treatment is ≤100 Pa and the time is 5 to 10 min. The vacuum degree of the second stage of vacuum treatment is ≤50 Pa and the time is 8 to 20 min.

3. The preparation process according to claim 2, characterized in that, Vanadium and niobium are added in two stages. The first stage adds 60-80% of the total amount, and the remaining amount is added after an interval of 3-8 minutes.

4. A method for preparing high-stress spring steel based on the preparation process described in any one of claims 1 to 3.

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