A high-strength Cr-V-Mo medium-carbon spring steel and its heat treatment process and applications

Abstract

The application relates to the technical field of medium-carbon spring steel, and specifically discloses a high-strength Cr-V-Mo medium-carbon spring steel, a heat treatment process thereof and application. The high-strength Cr-V-Mo medium-carbon spring steel is characterized by comprising the following elements in percentage by mass: C: 0.55-0.57%, Si: 1.47-1.53%, Mn: 0.65-0.69%, P: <=0.010%, S: <=0.010%, Cr: 0.71-0.75%, Al: <=0.010%, Ti: 0.060-0.090%, V: 0.02-0.04%, Mo: 0.40-0.60%, Ca: <=0.002%, Ni: <=0.05%, Cu: <=0.05%, Sn: <=0.01%, As: <=0.01%, N: <=0.0040%, O: <=0.0010%, H: <=0.0001%, and the rest is Fe and other uncontrollable trace residual elements. The medium-carbon spring steel disclosed by the application can be used for automobile suspensions and has the advantages of high strength and high toughness.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of medium-carbon spring steel, and more particularly to a high-strength Cr-V-Mo medium-carbon spring steel and a heat treatment process and application thereof. BACKGROUND

[0002] The development concept of automobile lightweight has become one of the mainstream development directions of the automobile industry. From the perspective of saving resources, energy and environmental protection, the demand for lightweight and high-strength of automobile parts will be increasingly large in the future. The research and development of high-performance suspension spring steel is an important proposition in the development of automobile materials. The Ministry of Industry and Information Technology has included automobile suspension spring steel with a tensile strength of more than 2000 MPa and a fatigue life of more than 100 million times in the “First Batch of Application Demonstration Guidance Catalogue of Key New Materials (2021 Edition)”. The design stress of the suspension spring currently reaches 1300 MPa at the highest, which requires the tensile strength of the spring steel to be more than 2100 MPa, the reduction of area to be more than 35%, and the total elongation to be more than 8%. The tensile strength of high-strength spring steel in South Korea and Japan can reach 2100-2200 MPa at the highest, and has good plasticity and toughness. The currently widely used high-strength spring steel such as SAE9254 of the American standard, 54SiCrV6 of the European standard and UHS2000 of Japan has a strength of about 2000 MPa.

[0003] Suitable alloy design and heat treatment process are the main methods for the spring steel to obtain excellent microstructure and properties, and through the mutual cooperation of the two, the strength and toughness of the steel can be optimized. At present, 55SiCr is widely used as the main grade of passenger car suspension spring, and the composition design is mainly based on the composition of 55SiCr by adding alloying elements such as Nb, V, Ti and Mo to obtain better microstructure and properties. Quenching-tempering process is the mainstream heat treatment process for spring steel 55SiCr, and the quenching and tempering parameters are important process parameters for determining the microstructure and properties of the spring steel. Lower tempering temperature can improve the strength of the spring steel wire, but the toughness will be lost, resulting in reduced fatigue life. However, higher tempering temperature will result in lower strength.

[0004] Therefore, the current research focus is to achieve better strength and toughness through reasonable composition design and heat treatment process. SUMMARY

[0005] In order to improve the strength and toughness of the medium-carbon spring steel wire, the application provides a high-strength Cr-V-Mo medium-carbon spring steel and a heat treatment process and application thereof.

[0006] The first aspect, the application provides a kind of high-strength Cr-V-Mo medium carbon spring steel, using the following technical solutions: a kind of high-strength Cr-V-Mo medium carbon spring steel, comprising the following mass percentage of element composition:

[0007] C:0.55-0.57%, Si:1.47-1.53%, Mn:0.65-0.69%, P:≤0.010%, S:≤0.010%, Cr:0.71-0.75%, Al:≤0.010%, Ti:0.060-0.090%, V:0.02-0.04%, Mo:0.40-0.60%, Ca:≤0.002%, Ni:≤0.05%, Cu:≤0.05%, Sn:≤0.01%, As:≤0.01%, N:≤0.0040%, O:≤0.0010%, H:≤0.0001%, the rest is Fe and other uncontrollable trace residual elements.

[0008] By using the above technical scheme, C is the most basic element in steel, and also the most economical strengthening element, which improves the strength of steel through solid solution strengthening and precipitation strengthening of carbide forming elements, but the higher the C content, the higher the hardness of the steel, which will adversely affect the plasticity and toughness of the steel, and the high C content will reduce the welding performance of the steel, reduce the atmospheric corrosion resistance of the steel, and increase the cold brittleness and aging sensitivity of the steel. The C content range of the application is determined to be 0.55-0.57%, which belongs to the category of 55SiCr medium carbon spring steel.

[0009] Si can be used as a deoxidizing element, and is also a basic solid solution strengthening element and improves the transparency. Silicon can significantly improve the elastic limit, yield limit and yield ratio of steel. Many spring steels use silicon as the main alloying element. Silicon has solid solution strengthening effect and does not form carbide, and basically exists in solid solution state in steel. Among commonly used alloying elements, the solid solution strengthening effect of silicon is the strongest. Silicon can change the amount, size and morphology of precipitated carbide during tempering, and improve the tempering stability of steel. When the alloying element and carbon content are within a certain range, the contribution of silicon to elastic reduction resistance is the first among various alloying elements. In the range of 0.30-2.30% of silicon content, the effect of silicon on elastic reduction resistance increases with the increase of silicon content. However, if the Si content is too high, it will promote the decarburization and graphitization tendency of steel during rolling and heat treatment. In the application, the Si content range is determined to be 1.47-1.53%, which belongs to the category of 55SiCr medium carbon spring steel, and is also the main deoxidizing element.

[0010] Manganese (Mn) is a solid solution strengthening element for steel, significantly improving its solubility and heat treatment properties. It strengthens the steel matrix and refines pearlite, thereby increasing the steel's strength and hardness. Furthermore, manganese alloys are relatively inexpensive and readily available, making them a preferred element in the design of high-strength steels. However, excessively high Mn content in steel can lead to significant temper brittleness, and Mn promotes grain growth, resulting in increased overheating sensitivity and cracking tendency. In this application, the Mn content is determined to be in the range of 0.65-0.69%, falling within the category of medium-carbon spring steel 55SiCr.

[0011] Cr is a medium-grade carbide-forming element. Among all carbides, chromium carbides are the finest, distributing evenly throughout the steel volume, thus contributing to high strength, hardness, yield point, and wear resistance. Cr slows down the decomposition of austenite, lowering the critical cooling rate during melting, thereby promoting martensite formation and stability. Therefore, Cr steels exhibit excellent permeability and minimal quenching deformation. However, excessive Cr content raises the brittle-brittle transition temperature, impairing hot strength and promoting temper brittleness. This application defines the Cr content range as 0.71-0.75%, falling within the category of medium-carbon spring steel 55SiCr.

[0012] Mo is a carbide-forming element. When combined with Cr, it can improve solubility and refine grain size. A certain amount of Mo can form fine, dispersed carbides in steel, inhibiting dislocation movement and improving the steel's resistance to elasticity and brittleness. When the Mo content reaches 0.40-0.60%, it can slow down the dissolution rate of carbide particles at high temperatures. Even after heating to 1100℃ and holding for 10 minutes, some carbide particles will not dissolve into austenite, thus preventing the pinning of austenite grain boundaries and inhibiting grain growth. For quenched and tempered alloy steels, it can significantly reduce temper brittleness and improve the steel's toughness. Mo can slow down the aggregation and growth of carbide particles during tempering, thereby improving strength and plasticity. When Mo segregates on grain boundaries, it increases the binding energy of the grain boundaries, inhibiting grain boundary embrittlement caused by P.

[0013] V is a strong carbide-forming element. It readily forms fine V particles in steel, resulting in dispersed precipitation strengthening. When V dissolves in austenite, it significantly improves the stability of austenite and the hardenability of the steel, ensuring the formation of a fine lamellar sorbite structure in the wire rod. In this application, the V composition is primarily designed to enhance strength.

[0014] Ti reacts with carbon and nitrogen in steel to form TiCN compounds, which inhibit austenite grain growth during heating and disperse and precipitate during wire rod cooling, improving the wire rod's strength and plasticity. When the Ti content is sufficient to form TiC, TiC acts as a hydrogen trap, improving the spring's resistance to delayed fracture and pitting corrosion.

[0015] This application achieves sufficient refinement of austenite grains by adding V and Ti components, and at the same time, through the reasonable addition of Mo and V components, it achieves the best match between high strength and high toughness of spring steel wire during low-temperature tempering.

[0016] Secondly, this application provides a heat treatment process for high-strength Cr-V-Mo medium-carbon spring steel, employing the following technical solution:

[0017] A heat treatment process for high-strength Cr-V-Mo medium-carbon spring steel includes the following steps:

[0018] S1: First heat treatment: austenitization and heat preservation: heat the steel wire to 1000-1020℃ and hold for 8-10 minutes;

[0019] Quenching: Quenching is completed by immersing austenitized steel wire in quenching oil;

[0020] Tempering: Reheat the quenched steel wire to 350-400℃, hold for 15-30 minutes, and then cool it with water to room temperature;

[0021] S2: Secondary heat treatment: Repeat the above steps. First, heat the steel wire to 940-960℃ and hold for 5-10 minutes to complete austenitization. Then, immerse the austenitized steel wire in quenching oil to complete quenching. After quenching, reheat the steel wire to 400-440℃ and hold for 15-30 minutes. Finally, water cool the steel wire to room temperature.

[0022] By adopting the above technical solution, compared with the traditional single heat treatment process, this application uses a two-stage quenching-tempering heat treatment technology to refine the original austenite structure and carbide size, achieving an austenite grain size of approximately 20-30 μm, thereby improving the strength and toughness of the steel. The produced finished steel wire has a tensile strength >2100 MPa, a reduction of area >43%, and a fracture toughness greater than 140 J / cm². 2 It has a corrosion resistance rate greater than 90% and features high toughness and resistance to corrosion. After coiling, its fatigue life exceeds 500,000 cycles under a working stress of 1500 MPa.

[0023] Preferably, in the primary heat treatment, the heating temperature of the steel wire is 1010-1015℃, and the holding time is 9 minutes.

[0024] By adopting the above technical solution, when the steel wire is heated to 1010±10℃, large-sized carbides in the steel wire will partially dissolve into smaller-sized carbides. Some of the smaller carbide particles will not completely dissolve in the austenite, and these fine carbide particles will remain on the steel matrix. These carbide particles can then play two roles: first, pinning austenite grain boundaries to prevent austenite grain growth; second, acting as nucleation sites for austenite, increasing the number of austenite nuclei, thereby refining the austenite grains. Furthermore, controlling the holding time to 8-10 minutes prevents the complete dissolution and disappearance of fine carbide particles, while simultaneously dissolving and refining large-sized carbide particles. This increases the transformation temperature of quenched martensite, reduces internal stress in the steel part, and decreases the content of retained austenite.

[0025] Preferably, in the primary heat treatment, the reheating temperature of the quenched steel wire is 370-385℃, and the holding time is 20-25 minutes.

[0026] By adopting the above technical solution, during the tempering process, due to the fine austenite grains, the martensite grains formed after quenching are also fine, increasing the amount of cryptocrystalline martensite. Cryptocrystalline martensite possesses high strength, high plasticity, and high toughness, thus improving the properties of tempered martensite. In this process, Mo can slow down the aggregation and growth of carbide particles during tempering, therefore, even at high tempering temperatures, dispersed carbide particles can still be obtained in the steel matrix.

[0027] Preferably, in the secondary heat treatment, the steel wire is first heated to 950-955°C for austenitization, and after quenching, the steel wire is heated to 430-435°C during the second tempering.

[0028] By adopting the above technical solution, the tempering temperature in the secondary heat treatment is significantly lower than that in the primary heat treatment. Generally, the lower the tempering temperature, the better it is for improving the strength of the steel wire, but at the same time it will reduce the toughness. The secondary heat treatment process of this application can ensure the microstructure, strength, toughness and other properties of the steel wire at a lower tempering temperature, ensure the uniform distribution of carbides, make the grains more refined, and help improve the strength and toughness of medium carbon spring steel.

[0029] Preferably, the quenching oil temperature during quenching in S1 is -20℃ to -30℃, and the immersion time in the quenching oil is 10S-20S.

[0030] By adopting the above technical solution, oil cooling is used during quenching, resulting in a smoother overall cooling process. Cooling at the quenching oil temperature in this application is more conducive to the transformation of austenite into martensite, ensuring the completeness of the transformation, and thus obtaining medium carbon spring steel with a more uniform microstructure and good strength and toughness.

[0031] Preferably, the water cooling temperature during tempering in S1 is 10-30℃.

[0032] By adopting the above technical solution, water cooling has the characteristic of rapid cooling compared to oil cooling. Rapid cooling during tempering is beneficial to improving the strength of medium carbon spring steel.

[0033] Thirdly, this application provides an application of medium carbon spring steel prepared by a heat treatment process, using the following technical solution:

[0034] An application of a medium-carbon spring steel prepared by the above heat treatment process is used in the manufacture of automobile suspensions.

[0035] By adopting the above technical solution and heat treatment process, the medium carbon spring steel obtained has the advantages of high strength and good toughness when used in automotive suspension spring steel.

[0036] In summary, this application has the following beneficial effects:

[0037] 1. This application achieves sufficient refinement of austenite grains by adding V and Ti components, and at the same time, achieves the best match between high strength and high toughness of steel wire during low-temperature tempering by reasonably adding Mo and V components.

[0038] 2. This application adopts a two-stage quenching-tempering heat treatment process, which enables the steel wire to achieve high strength and high toughness that cannot be achieved by a typical one-stage quenching-tempering heat treatment process. Moreover, the heat treatment process is simple and easy to implement, and does not require complex equipment and operations. Attached Figure Description

[0039] Figure 1 The CCT curve of the spring steel wire in this application is shown, with an austenitizing temperature of 1010℃.

[0040] Figure 2 The data for the end quenching test of the spring steel wire in this application are as follows: austenitizing temperature 950℃.

[0041] Figure 3 This is the tempered troostite structure of the spring steel wire in this application. Detailed Implementation

[0042] Raw material source:

[0043] All raw materials used in the embodiments of this application are commercially available. The following disclosure of raw material sources is merely for the purpose of full disclosure and should not be construed as limiting the scope of protection.

[0044] The quenching oil is U8132 fast bright quenching oil, with an operating temperature of 40-80℃.

[0045] Example

[0046] Examples 1.1-1.4

[0047] A high-strength Cr-V-Mo medium-carbon spring steel comprises the following elements by mass percentage:

[0048] C: 0.55-0.57%, Si: 1.47-1.53%, Mn: 0.65-0.69%, P: ≤0.010%, S: ≤0.010%, Cr: 0.71-0.75%, Al: ≤0.010%, Ti: 0.060-0.090%, V: 0.02-0.04%, Mo: 0.40-0.60%, Ca: ≤0.002%, Ni: ≤0.05%, Cu: ≤0.05%, Sn: ≤0.01%, As: ≤0.01%, N: ≤0.0040%, O: ≤0.0010%, H: ≤0.0001%, with the remainder being Fe and other uncontrollable trace residual elements.

[0049] The elemental composition of the spring steel in Examples 1.1-1.4 is shown in Table 1.

[0050] Table 1. Elemental composition of spring steel in Examples 1.1-1.4

[0051]

[0052]

[0053] The remainder consists of Fe and other uncontrollable trace residual elements.

[0054] A steelmaking and rolling process for spring steel includes the following steps: pretreatment of raw materials in hot metal; converter smelting; LF refining; RH refining; continuous casting; (bill preparation); billet cleaning, flaw detection, and grinding; billet heating; high-pressure water descaling; high-speed wire rod rolling; controlled rolling and cooling; wire rod inspection; packaging; and warehousing.

[0055] Hot metal pretreatment: The sulfur content in the hot metal should be ≤0.005%. The hot metal temperature should be >1300℃, and the silicon content should be >0.30%. If the hot metal temperature and composition do not meet the requirements, adjust the amount of scrap steel added in advance.

[0056] Converter smelting: A top-and-bottom combined blowing process is adopted. The converter final temperature is >1620℃, and the converter final composition is C: 0.05-0.15%, P≤0.010%, S≤0.010%, and O content 350-450 PPM. No more than one supplementary blowing is required. Slagging is performed throughout the converter blowing process, and iron-containing materials such as briquettes are used appropriately to prevent re-drying. During the converter tapping process, recarburizer, ferrosilicon, other alloys (Ti ferro, Mn ferro, Mo ferro, V ferro, etc.), and pre-melted slag are added sequentially. The tapping time is 3-5 minutes, ensuring a smooth steel stream without any slurry.

[0057] LF Refining: Power supply frequency should be ≤3 times / furnace; ferrosilicon should be added during refining heating for deoxidation; deoxidation is considered complete when the top slag is white, light green, or transparent; lime and quartz should be added according to the converter slag amount for basicity adjustment, with the top slag basicity at 1.0-1.4. The total amount of quicklime added during refining should be ≥700kg / furnace, added in small batches multiple times, with an appropriate amount of fluorite added to ensure good melting; molten steel must not be exposed during soft blowing; slight positive pressure operation at the ladle mouth should be ensured while maintaining environmental protection; soft blowing time should be ≥40 minutes, with the slag surface fluctuation ensuring no exposed molten steel.

[0058] RH refining: Vacuum degree ≥100Pa, processing time not less than 20 minutes, pure degassing time not less than 5 minutes, soft blowing time not less than 15 minutes, RH oxygen content at the station ≤12ppm.

[0059] Continuous casting: The continuous casting process employs a crystallizer, end-of-line electromagnetic stirring, and low-superheat casting, with superheat controlled at 15-25℃ (liquidity line 1495℃), casting speed of 1.25-1.45 m / min, crystallizer water flow rate of 1100-1400 m³ / h, and secondary cooling water flow rate of 0.30-0.60 L / kg. The carbon segregation index of the continuously cast billet is controlled to not exceed 1.12. Superheat control is achieved using a plasma heating device in the tundish, with superheat controlled at 15-25℃. The electromagnetic stirring process requires the use of a high-intensity mode (electromagnetic stirring current: 450 A, electromagnetic stirring frequency: 1.5 Hz).

[0060] It adopts a ladle-protected casting method with an integral immersion nozzle in the tundish, and uses automatic control based on "stopper position + molten steel level" and automatic detection of slag discharge from the ladle.

[0061] Blanking: If two types of firewood are used, blanking is required. The heating temperature is 1180-1210℃ and the heating time is 180-240 minutes.

[0062] Steel billet finishing: cleaning, flaw detection, and full grinding to 1.5mm.

[0063] Billet heating: Heating temperature 1040-1010℃, heating time 90-110 minutes, controlled by reducing atmosphere. Initial rolling temperature 960-1000℃. Descaling water pressure >20 bar.

[0064] Controlled rolling and cooling: The temperature entering the finishing mill is 830–860℃, the temperature entering the sizing mill is 860–900℃, and the wire drawing temperature is controlled at 840–880℃. Upon entering the Stellmore cooling line, the number of openings of the insulation covers, the fan opening degree, and the roller speed are adjusted to control the cooling rate of the wire rod before phase transformation at 15–20℃ / s, the phase transformation temperature at 580–620℃, the post-phase transformation reddening temperature at 630–650℃, and the cooling rate after phase transformation reddening at 0.5–1℃. To reduce decarburization, an atomizing cooling device is installed after the wire drawing machine to reduce the residence time in the 600–900℃ temperature range.

[0065] Cable inspection - packaging - warehousing: Offline protection devices are used to prevent scratches on the cable surface.

[0066] A heat treatment process for high-strength Cr-V-Mo medium-carbon spring steel includes the following steps:

[0067] S1: First heat treatment: austenitization and heat preservation: heat the spring steel wire to 1010℃ and hold for 9 minutes;

[0068] Quenching: The austenitized steel wire is immersed in quenching oil to complete the quenching process. The temperature of the quenching oil during quenching is -20℃, and the immersion time in the quenching oil is 15 seconds.

[0069] Tempering: Reheat the quenched steel wire to 380℃, hold for 20 minutes, and then water cool it to room temperature at a water cooling temperature of 20℃.

[0070] S2: Secondary heat treatment: Repeat the above steps. First, heat the steel wire to 950℃ and hold for 8 minutes to complete austenitization. Then, immerse the austenitized steel wire in quenching oil to complete quenching. After quenching, reheat the steel wire to 425℃ and hold for 25 minutes. Then, water cool the steel wire to room temperature.

[0071] Examples 2.1-2.9

[0072] A heat treatment process for high-strength Cr-V-Mo medium-carbon spring steel differs from that in Example 1.1 in that the heat treatment process parameters are different.

[0073] The heat treatment process parameters for Examples 2.1-2.9 are shown in Table 2.

[0074] Table 2. Heat treatment process parameters for Examples 2.1-2.9

[0075]

[0076]

[0077] Comparative Example

[0078] Comparative Example 1

[0079] A high-strength Cr-V-Mo medium-carbon spring steel differs from Example 1.1 in that the amount of V is 0.01%.

[0080] Comparative Example 2

[0081] A high-strength Cr-V-Mo medium-carbon spring steel differs from Example 1.1 in that the amount of V is 0.06%.

[0082] Comparative Example 3

[0083] A high-strength Cr-V-Mo medium-carbon spring steel differs from Example 1.1 in that the amount of Mo used is 0.2%.

[0084] Comparative Example 4

[0085] A high-strength Cr-V-Mo medium-carbon spring steel differs from Example 1.1 in that the amount of Mo is 0.8%.

[0086] Comparative Example 5

[0087] A high-strength Cr-V-Mo medium-carbon spring steel differs from Example 1.1 in that the amount of Ti is 0.04%.

[0088] Comparative Example 6

[0089] A high-strength Cr-V-Mo medium-carbon spring steel differs from Example 1.1 in that the amount of Ti is 0.11%.

[0090] Performance testing

[0091] The spring steel wire from Example 1.1 of this application was tested, and its CCT curve, end hardenability test, and tempered troostite structure were observed. Figures 1-3 .

[0092] Figure 1 The CCT curve is the basis for formulating the quenching-tempering process. The CCT curve measured in this embodiment is related to the composition design and provides support for the design of the heat treatment process. By analyzing the CCT curve, the most suitable cooling rate and transformation temperature are determined, thereby optimizing the heat treatment process and obtaining the required microstructure and properties.

[0093] Figure 2 The data from the end-quenching experiment show that the spring steel wire with the best composition and heat treatment exhibits excellent hardenability and fully meets the requirements for use.

[0094] Figure 3It is a tempered troostite structure, which is the normal structure of spring steel wire. The structure observed under a 100X light microscope shows that the structure of the spring steel in this embodiment is uniform, and the tempered troostite structure is relatively fine and uniform, achieving the purpose of heat treatment.

[0095] The performance of the medium carbon spring steel wires of Examples 1.1-1.4, Examples 2.1-2.9, and Comparative Examples 1-6 was tested, and the test results are shown in Table 3.

[0096] The testing items and standards shall be conducted in accordance with GB / T1222-2016 for spring steel;

[0097] The tensile strength of medium carbon spring steel wire shall be tested according to the method specified in GB / T 228.1-2021;

[0098] The hardness of medium carbon spring steel wire shall be tested in accordance with the method specified in GB / T 231.1-2018;

[0099] The fracture toughness of medium carbon spring steel wire was tested according to the method specified in GB / T 38769-2020;

[0100] Table 3 Performance test results of spring steel wire

[0101]

[0102]

[0103] The fatigue life of the medium carbon spring steel wires of Examples 1.1-1.4, Examples 2.1-2.9 and Comparative Examples 1-6 was tested under the same test stress. The fatigue life test was conducted in accordance with the method specified in GB / T6398-2017. The test results are shown in Table 4.

[0104] Table 4 shows the life test results of carbon spring steel wire.

[0105] Serial number Test stress, MPa Fatigue life, (thousands of cycles) Example 1.1 1500 50 Example 1.2 1500 50 Example 1.3 1500 50 Example 1.4 1500 50 Example 2.1 1500 50 Example 2.2 1500 50 Example 2.3 1500 50 Example 2.4 1500 50 Example 2.5 1500 48.5 Example 2.6 1500 49 Example 2.7 1500 48.3 Example 2.8 1500 48.6 Example 2.9 1500 48.5 Comparative Example 1 1500 47.6 Comparative Example 2 1500 47.8 Comparative Example 3 1500 47.5 Comparative Example 4 1500 47.7 Comparative Example 5 1500 47.6 Comparative Example 6 1500 47.7

[0106] Note: A fatigue test of 500,000 cycles is considered to have met the fatigue life standard, and further testing will cease.

[0107] Combining Examples 1.1-1.4 and Comparative Examples 1-6 with Tables 3 and 4, it can be seen that the steel wires of Examples 1.1-1.4 have better strength, toughness, and service life than those of Comparative Examples 1-6. This indicates that, under the lower tempering temperature environment of this application, the addition of the three alloying elements vanadium, molybdenum, and titanium in the amounts specified in this application improves the strength of the steel while avoiding the loss of toughness after the tempering temperature is lowered. The reason for this is that, generally, the lower the tempering temperature, the higher the strength of the steel wire, but the lower the toughness. In order to ensure the performance of spring steel, a higher tempering temperature is usually selected to ensure its toughness. However, this application can ensure its toughness and strength under low-temperature tempering, especially the secondary tempering temperature of only 400-440 degrees Celsius, which is an important temperature range for improving the strength and toughness of the steel wire. The double quenching-tempering process further ensures the uniform distribution of carbides and finer grains, which in turn helps to improve toughness and strength.

[0108] As can be seen from Examples 2.1-2.4 and 2.5-2.6, and in conjunction with Tables 3 and 4, the austenitizing temperature of this application enables the steel wire to possess better strength and toughness. This is because at this temperature, large-sized carbides in the steel wire partially dissolve into smaller-sized carbides. Some of these smaller carbide particles do not completely dissolve in the austenite, remaining on the steel matrix. These carbide particles then play two roles: first, they pin austenite grain boundaries, preventing austenite grain growth; second, they act as nucleation sites for austenite, increasing the number of austenite nuclei and thus refining the austenite grains, thereby improving the strength, toughness, and fatigue performance of the steel wire.

[0109] As can be seen from Examples 2.2 and 2.7, and Tables 3 and 4, this application controls the holding time for austenitization to no more than 10 minutes after the steel part reaches a uniform temperature, to prevent the complete dissolution and disappearance of fine carbide particles, while simultaneously dissolving and refining large carbide particles. This increases the transformation temperature of quenched martensite, reduces internal stress in the steel part, decreases the content of retained austenite, and thus ensures improved strength and toughness of the steel wire.

[0110] As can be seen from Examples 2.2 and 2.8-2.9, and Tables 3 and 4, this application employs a two-stage heat treatment process of low-temperature tempering to heat-treat the steel wire, which can ensure both the strength and toughness of the steel wire. The first heat treatment process prepares the alloy structure for the second heat treatment process, so the tempering temperature in the first heat treatment process is slightly higher than that in the second heat treatment process. However, when the tempering temperature is too high (as in Example 2.8), the strength of the steel wire will decrease because carbides aggregate and grow during tempering, making it difficult to ensure the uniformity of carbide particle size, resulting in a decrease in strength. When the tempering temperature is low (as in Example 2.9), although the strength of the steel wire is high, the toughness decreases significantly, making it difficult to simultaneously ensure both strength and toughness of the steel wire.

[0111] As can be seen from Examples 2.2 and Comparative Examples 1-2, and Tables 3-4, this application can improve the strength of steel wire by controlling the amount of vanadium added to be 0.02-0.04%.

[0112] As can be seen from Examples 2.2 and Comparative Examples 3-4 and Table 3-4, this application can improve the stability of tempering and avoid the agglomeration and growth of carbides by controlling the amount of molybdenum added to 0.4-0.6%, thereby ensuring the uniformity of carbide particles.

[0113] As can be seen from Examples 2.2 and Comparative Examples 5-6 and Tables 3-4, since delayed fracture is prone to occur under low-temperature tempering conditions, titanium is added in this application, and the amount of titanium added is controlled at 0.06-0.09% to ensure that the steel wire still has good strength during low-temperature tempering.

[0114] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A heat treatment process of high-strength Cr-V-Mo medium-carbon spring steel, characterized in that: The method comprises the following steps: S1: primary heat treatment: austenitizing and holding: heating the steel wire to 1010-1015 DEG C, holding for 9 minutes; quenching: quenching the austenitized steel wire by immersing in quenching oil, the quenching oil temperature during quenching is-20 DEG C to-30 DEG C, and the time of immersing in quenching oil is 10-20 seconds; tempering: re-heating the quenched steel wire to 350-400 DEG C, holding for 15-30 minutes, then water cooling, and cooling the steel wire to room temperature; S2: secondary heat treatment: repeating the above steps, first heating the steel wire to 940-960 DEG C, holding for 5-10 minutes to complete austenitizing, immersing the austenitized steel wire in quenching oil to complete quenching, re-heating the quenched steel wire to 400-440 DEG C, holding for 15-30 minutes, then water cooling, and cooling the steel wire to room temperature; in the S1, the steel wire comprises the following mass percentage of element composition: C: 0.55-0.57%, Si: 1.47-1.53%, Mn: 0.65-0.69%, P: ≤0.010%, S: ≤0.010%, Cr: 0.71-0.75%, Al: ≤0.010%, Ti: 0.060-0.090%, V: 0.02-0.04%, Mo: 0.40-0.60%, Ca: ≤0.002%, Ni: ≤0.05%, Cu: ≤0.05%, Sn: ≤0.01%, As: ≤0.01%, N: ≤0.0040%, O: ≤0.0010%, H: ≤0.0001%, and the rest is Fe and other uncontrollable trace residual elements.

2. The heat treatment process of a high-strength Cr-V-Mo medium-carbon spring steel according to claim 1, characterized in that, in the primary heat treatment, the re-heating temperature of the quenched steel wire is 370-385 DEG C, and the holding time is 20-25 minutes.

3. The heat treatment process of a high-strength Cr-V-Mo medium-carbon spring steel according to claim 1, characterized in that, in the secondary heat treatment, the austenitizing of the steel wire is first heated to 950-955 DEG C, after quenching, the steel wire is heated to 430-435 DEG C for the second time.

4. The heat treatment process of high-strength Cr-V-Mo medium-carbon spring steel according to claim 1, characterized in that, the water cooling temperature during tempering in the S1 is 10-30 DEG C.

5. Use of a medium carbon spring steel prepared by the heat treatment process according to any one of claims 1 to 4, characterized in that, application to prepare automobile suspension.

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