Spring steel and method for its production and heat treatment

Abstract

The application provides a spring steel and a production method and a heat treatment method thereof, and belongs to the technical field of spring steel.The application comprises the following components: Pr, La, C, Si, Mn, Cr, V, Al, P, S, O, Fe and other inevitable impurities, the plasticity index P of inclusions is controlled to be greater than or equal to 0.20, P=78.6* (%Pr)+29.5* (%La), the austenite grain size of the steel is finer than 9.5 levels, and the size of more than 95% of the inclusions is less than 8 microns; the austenitizing and isothermal partitioning heat treatment process is adopted; the structure is a nano-level structure; the nano-level bainite content is 60-80%; and a large number of dispersed fine carbide precipitated phases are accompanied; the product is higher than the general level spring steel in the market in the aspects of strength plasticity, fatigue performance and anti-bounce performance.

Description

Technical Field

[0001] This invention belongs to the field of spring steel technology, and particularly relates to a spring steel and its production method and heat treatment method. Background Technology

[0002] With the development of lightweight, heavy-duty, long-life and low-carbon automobiles and rail transit, higher requirements have been placed on their core components, such as springs and their materials. Spring steel is constantly required to improve its strength level while taking into account high plasticity and high fatigue performance.

[0003] Patent CN 105648332 A, published on June 8, 2016, discloses a high-performance spring steel. The chemical composition (by weight percentage) of the material is: C: 1.9-2.2 parts, Cu: 5-6 parts, Si: 1-1.2 parts, Cr: 0.8-1.2 parts, Nb: 0.2-0.5 parts, V: 0.2-0.3 parts, Al: 0.5-0.8 parts, with the remainder being Fe and non-removable impurities. The high-performance spring steel plate material is heated, formed, subjected to residual heat quenching, and tempered at medium temperature, finally forming the final product shape. Its beneficial effects are: good elasticity, suitable for normal operation under load, high yield strength for spring manufacturing, and long service life. However, this method adds 5% or more of copper, which easily causes copper embrittlement cracking in the steel. It cannot meet the performance requirements of high strength combined with high plasticity and high fatigue performance.

[0004] In addition, some spring companies at home and abroad are currently lowering the traditional heat treatment tempering temperature to improve the strength of steel and the spring design stress. This method increases the hydrogen embrittlement sensitivity of steel and deteriorates the fatigue performance of steel. There is an urgent need to develop new materials for spring steel and new heat treatment properties to improve the strength, toughness and fatigue performance of spring steel and meet the development needs of the spring industry. Summary of the Invention

[0005] The purpose of this invention is to provide a spring steel and its production method. By adding La and Pr to the steel composition and optimizing the production method, sufficient inclusion plasticization effect is obtained. The produced spring steel has an austenitic grain size of finer than 9.5 grade, and more than 95% of the inclusions are smaller than 8μm, exhibiting excellent comprehensive performance.

[0006] Another objective of this invention is to provide a heat treatment method for spring steel. This invention employs an austenitizing + isothermal partitioning heat treatment process, resulting in a nanoscale microstructure, wherein the nanoscale bainite area content is 60-80%, accompanied by a large number of dispersed fine carbide precipitates. After heat treatment, the yield strength reaches over 2000 MPa, the tensile strength reaches over 2200 MPa, the reduction of area is over 45%, the fatigue limit is over 1100 MPa, the fatigue ratio is over 0.50, and the area of ​​the spring resistance reduction Singer torsion inverse curve is ≥400 mm². 2 Its strength, plasticity, fatigue performance, and resistance to spring reduction are all higher than those of general-purpose spring steel on the market.

[0007] The specific technical solution of this invention is as follows:

[0008] A spring steel comprising the following components by weight percentage:

[0009] Pr 0.0010%-0.0030%, La 0.0025%-0.0045%, C 0.50%-0.60%, Si 1.70%-1.90%, Mn 0.40%-0.55%, Cr 0.70%-0.90%, V 0.30%-0.50%, Al trace -0.005%, P trace -0.015%, S trace -0.010%, O≤0.0010%, H≤0.0001%, with the remainder being Fe and other unavoidable impurities.

[0010] The composition of the spring steel also satisfies the following requirements: the inclusion plasticity index P value ≥ 0.20, P value = 78.6 × (%Pr) + 29.5 × (%La). The composition meets this requirement in order to obtain sufficient inclusion plasticity effect.

[0011] The spring steel has an austenitic grain size finer than grade 9.5, and more than 95% of the inclusions are smaller than 8 μm.

[0012] The present invention provides a method for producing spring steel, comprising the following process flow:

[0013] Smelting in an electric arc furnace or converter → refining in an LF furnace → RH vacuum degassing → continuous casting of large billets → heating of large billets → billet opening of small billets → heating of small billets → controlled rolling of high-speed wire rod → controlled cooling in a Steyrmo cooling line → finished wire rod product with diameters of Φ5.5-30mm.

[0014] The electric arc furnace or converter smelting process uses low-aluminum, low-carbon ferrosilicon as the deoxidizer. During tapping, the amount of steel remaining is controlled to be greater than 10 tons, and slag control is implemented to prevent slag from falling into the furnace. The argon blowing station uses bottom blowing argon, ensuring a blowing time of more than 5 minutes to promote the floating of inclusions.

[0015] The LF furnace refining process involves bottom blowing argon into the ladle throughout the process, with the argon flow rate determined so that the molten steel does not splash out of the ladle. Low-basicity slag is used and R is controlled at 1.3-1.5, thereby controlling the Al content in the steel to below 0.005%, and the inclusion composition is controlled in the low-melting-point plastic deformation zone through the low-basicity synthetic slag refining process.

[0016] The RH vacuum degassing process involves a soft blowing time of more than 15 minutes and a soft blowing flow rate of 60-100 L / min, ensuring that the molten steel is not exposed and the slag surface fluctuates slightly. Flow rates below this level will not be sufficient to remove slag, gas, or inclusions, while flow rates above this level will prevent the molten steel from contacting the air surface, thus increasing nitrogen content.

[0017] The large billet continuous casting: electromagnetic stirring is used during continuous casting. Pr and La lines are added to the crystallizer and their contents are adjusted. Protective casting is used throughout the process. Light reduction is applied, with a reduction of 10%-15%. If the reduction is lower than this amount, the center segregation cannot be effectively improved, thus deteriorating the fatigue performance. If the reduction is higher than this amount, internal cracks may occur in the billet, reducing product quality.

[0018] The heating of the large billet: control the homogenization temperature at 1250-1300℃. If the homogenization temperature is lower than 1250℃, the interior of the large billet cannot be fully heated, and the alloying elements cannot diffuse evenly, resulting in a heavy equipment load during billet opening and causing the steel to have uneven properties due to segregation. If the temperature is higher than 1300℃, the austenite grains begin to coarsen, and the tendency to decarburize increases significantly.

[0019] The heating of the rolled small square billet: control the homogenization temperature at 1000-1060℃. If the homogenization temperature is below 1000℃, the alloying elements cannot diffuse evenly, resulting in compositional segregation and brittleness in the steel; if it is above 1060℃, complete decarburization will occur.

[0020] The high-speed wire rod controlled rolling process involves controlling the sizing temperature at 820-850℃ and the wire drawing temperature at 800-840℃. Through low-temperature rolling with large deformation, deformation induces precipitation, and the interaction between recrystallization and precipitates, especially V precipitates, refines the grains. Above these temperatures, the grains coarsen, resulting in a lower surface shrinkage of the finished product. Below these temperatures, an extremely fine surface structure is produced, leading to breakage during drawing.

[0021] Through the above component ratios and production processes, the spring steel of this invention has an austenitic grain size finer than grade 9.5, and more than 95% of the inclusions are below 8μm in size.

[0022] The heat treatment method for the spring steel provided by this invention includes the following processes: First, austenitization is performed at an austenitizing temperature of 880-920℃, followed by placement in a salt bath isothermal furnace at an isothermal temperature of 330-370℃ for 3-4 hours, with a cooling rate of 200-240℃ / s, and cooling to room temperature. After heat treatment, a nanoscale microstructure is obtained, in which the bainite content is 60-80%, accompanied by a large number of dispersed fine carbide precipitates. This microstructure combines high strength, plasticity, fatigue performance, and resistance to spring reduction. This invention controls the isothermal temperature at 330-370℃; above this temperature, the tensile strength is insufficient, and below this temperature range, the plasticity and fatigue performance are insufficient. A suitable cooling oil is used to control the cooling rate at 200-240℃ / s; above this rate range, the steel will crack due to excessive cooling, and below this rate range, the microstructure is too coarse.

[0023] After heat treatment, the microstructure of the spring steel is nanoscale, wherein the content of nanoscale bainite is 60-80%, and the balance is nanoscale cementite and carbide precipitates, wherein the carbide precipitates are finely dispersed and below 20 nm.

[0024] After heat treatment, the yield strength of the spring steel reaches over 2000 MPa, the tensile strength reaches over 2200 MPa, the reduction of area is over 45%, the fatigue limit is over 1100 MPa, the fatigue ratio is over 0.50, and the area of ​​the spring steel under the Singer torsion curve is ≥400 mm². 2 Its strength, plasticity, fatigue performance, and resistance to spring reduction are all higher than those of general-purpose spring steel on the market.

[0025] The design concept of this invention is as follows:

[0026] Carbon (C): Carbon is the most basic and effective strengthening element in steel, essential for obtaining high-strength and high-hardness spring steel. While high carbon content is beneficial to the strength, hardness, elasticity, and spring-damping properties of steel, it is detrimental to the steel's plasticity and toughness, and reduces the yield strength ratio, increases decarburization sensitivity, and worsens the steel's fatigue resistance. The carbon content should be controlled between 0.50% and 0.60%.

[0027] Si: Si is an important strengthening element in steel, increasing its strength and hardness through solid solution treatment, while also improving the spring resistance and fatigue life of spring steel. Si mainly accumulates on the steel surface, further enhancing the corrosion resistance of the spring surface and thus increasing the fatigue life of the steel. However, increasing Si content can increase carbon diffusion in the steel, exacerbating decarburization. The Si content should be controlled between 1.70% and 1.90%.

[0028] Mn: Mn forms a solid solution with Fe, increasing the hardness and strength of ferrite and austenite in steel. Mn also improves the stability of the austenite structure, significantly enhancing the hardenability of the steel. However, excessive Mn will reduce the plasticity of the steel. The Mn content should be controlled between 0.40% and 0.55%.

[0029] Cr: Cr can form stable compounds with C, preventing the segregation of C or impurities, improving the stability of the matrix, and significantly improving the fatigue resistance of steel. In addition, some Cr dissolves into ferrite, producing solid solution strengthening, which can significantly increase the hardenability and tempering resistance of steel. However, excessive Cr increases the temper brittleness tendency of steel. The Cr content should be controlled between 0.70% and 0.90%.

[0030] V (Volume): V precipitates carbides in steel, promoting the precipitation of MC-type carbonitride second phases. During rolling, these precipitates pinnate grain boundaries, refining the microstructure and thus improving the steel's strength and plasticity. Furthermore, the combined use of V with Cr and Mn helps enhance the relaxation resistance of spring steel, maximizing its elastic limit. In addition, V carbide formation reduces the carbon content of the matrix, effectively mitigating the decarburization sensitivity of spring steel. However, excessive V content may cause precipitates to grow, losing their dispersed distribution effect. The V content should be controlled between 0.30% and 0.50%.

[0031] Pr: Pr is one of the main elements in the steel of this invention for achieving plasticization of inclusions. It can transform brittle inclusions such as Al2O3 into spherical rare earth composite oxysulfides, thereby achieving the harmlessness of inclusions. However, excessive Pr can easily cause molten steel to accumulate. The Pr content is controlled at 0.0010%-0.0030%.

[0032] La (La) and Pr form a composite modifier, which is more effective than the action of a single element. It purifies molten steel, promotes the dispersion of fine spherical inclusions, and significantly improves the fatigue performance of spring steel. However, La is easily oxidized, and excessive content can easily cause molten steel to accumulate. The La content should be controlled between 0.0025% and 0.0045%. In order to obtain sufficient inclusion plasticization effect, it is also necessary to ensure that the inclusion plasticization index P value is ≥0.20, P=78.6×(%Pr)+29.5×(%La). By using the La and Pr composite modifier, the inclusions are fully plasticized and dispersed, thereby effectively improving the fatigue resistance of the steel.

[0033] Al: Al easily forms brittle Al2O3 inclusions in steel, which deteriorates the fatigue performance of steel. This invention adopts an aluminum-free deoxidation process to control the Al content to within 0.005%.

[0034] S and P: Sulfur readily combines with manganese in steel to form MnS inclusions, which are detrimental to the steel's processing and fatigue properties. P is an element with a strong tendency to segregate, and it often causes the co-aggregation of sulfur and manganese, which is detrimental to the uniformity of the product's microstructure and properties. Control P ≤ 0.015% and S ≤ 0.010%.

[0035] O: O forms oxide inclusions in steel, which impairs the steel's processing and fatigue properties. O should be controlled to ≤ 0.0010%.

[0036] H: H is very harmful in high-strength steel, increasing the risk of delayed fracture of springs. H should be controlled to ≤ 0.0001%.

[0037] To ensure that non-metallic inclusions in steel undergo sufficient transformation and plasticization, thereby effectively improving the fatigue life of the steel, it is also necessary to ensure that the plasticization index P value of the inclusions is ≥0.20, P=78.6×(%Pr)+29.5×(%La), so that all inclusions achieve transformation and plasticization, and control the size of more than 95% of the inclusions to be below 8μm, so that the final processed springs have a high fatigue life.

[0038] Compared with existing technologies, by taking the composition range and ratio of the above-mentioned elements and using a specific production method, and employing an austenitizing + isothermal partitioning heat treatment process, which differs from the traditional spring steel oil quenching process, a nanoscale microstructure and finely dispersed carbide precipitates are obtained. The austenite grain size of the steel of this invention is finer than grade 9.5, with more than 95% of inclusions having a size below 8 μm. The heat-treated yield strength reaches over 2000 MPa, the tensile strength reaches over 2200 MPa, the reduction of area is over 45%, the fatigue limit is over 1100 MPa, the fatigue ratio is over 0.50, and the area of ​​the spring resistance reduction Singer torsion inverse curve is ≥400 mm². 2 Its strength, ductility, fatigue performance, and resistance to spring reduction are all superior to those of general-purpose spring steels on the market. This invention can provide ultra-high strength and toughness spring steel with excellent fatigue performance and resistance to spring reduction, which meets the requirements of automotive lightweighting development. Attached Figure Description

[0039] Figure 1 The microstructure of the heat-treated sample is shown in Example 1.

[0040] Figure 2 The microstructure is based on heat treatment ratio 1. Detailed Implementation

[0041] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0042] Examples 1-7

[0043] A spring steel comprising the following composition by mass percentage as shown in Tables 1 and 2, wherein the balance not shown in Tables 1 and 2 is Fe and unavoidable impurities.

[0044] Comparative Examples 1-6

[0045] A spring steel comprising the following composition by mass percentage as shown in Tables 1 and 2, wherein the balance not shown in Tables 1 and 2 is Fe and unavoidable impurities.

[0046] Table 1 Chemical composition (wt%) of the embodiments and comparative examples of the present invention

[0047]

[0048] Table 2 Chemical composition (wt%) of the embodiments and comparative examples of the present invention

[0049]

[0050] In Comparative Example 5, the composite rare earth RE contained 14% cerium, 15% praseodymium, 11% dysprosium, 5% holmium, 9% yttrium, and the balance was lanthanum.

[0051] Production methods of spring steel in various embodiments and comparative examples: Smelting in an electric arc furnace or converter → refining in an LF furnace → RH vacuum degassing → continuous casting of large billets → heating of large billets → billet roughing of small billets → heating of rolled small billets → controlled rolling of high-speed wire rod → controlled cooling in a Steyrmo cooling line → finished wire rod product with diameters of Φ5.5-30mm. Wherein:

[0052] Electric arc furnace or converter smelting: Low-aluminum, low-carbon ferrosilicon is used as the deoxidizer, and the amount of steel left after tapping is controlled to be greater than 10 tons. Slag control is implemented to prevent slag from falling onto the steel. Bottom blowing argon is used in the argon blowing station, and the blowing time is guaranteed to be greater than 5 minutes.

[0053] LF furnace refining: Argon is blown into the ladle throughout the process, and the argon flow rate is set so that the molten steel does not splash out of the ladle; low-basicity slag is used and R is controlled at 1.3-1.5.

[0054] RH vacuum degassing: soft blowing time is greater than 15 minutes, soft blowing flow rate is 60-100L / min, and the molten steel is not exposed and the slag surface fluctuates slightly.

[0055] Large billet continuous casting: Electromagnetic stirring is used during continuous casting. Pr and La lines are added to the crystallizer and their contents are adjusted. Protective casting is used throughout the process, and light reduction is applied with a reduction of 10%-15%.

[0056] Heating of large billets: control the homogenization temperature at 1250-1300℃.

[0057] Heating of rolled small square billets: control the homogenization temperature to 1000-1060℃.

[0058] High-speed wire rod controlled rolling: control the sizing temperature at 820-850℃ and the wire drawing temperature at 800-840℃.

[0059] The main production process parameters for each embodiment and comparative example are shown in Table 3.

[0060] Table 3. Control of key production parameters for each embodiment and comparative example.

[0061]

[0062] The performance testing methods for hot-rolled wire rods in each embodiment and comparative example are as follows:

[0063] Austenite grain size: The austenitizing heat treatment process is as follows: quenching at 890℃, oil cooling, quenching medium temperature 18-35℃, metallographic sample preparation after cooling, and inspection of austenite grain size level.

[0064] Inclusion size measurement: The size of inclusions on the longitudinal surface of the steel was measured and statistically calculated using an ASPEX scanning electron microscope.

[0065] The hot-rolled wire rods produced in Examples 1-7 and Comparative Examples 2-4 all adopted an austenitizing + isothermal partitioning heat treatment process. First, the steel was austenitized at a temperature of 880-920℃, and then placed in a salt bath isothermal furnace at an isothermal temperature of 330-370℃ for 3-4 hours. A suitable cooling oil was used, with a cooling rate of 200-240℃ / s.

[0066] Comparative Example 1 uses traditional quenching: quenching + tempering, quenching temperature 870-900℃, oil cooling, cooling rate 60-100℃ / s, followed by tempering, tempering temperature 400-440℃, water cooling.

[0067] Heat-treated microstructure: The cross-section was observed after heat treatment, and the content of nanoscale bainite was determined.

[0068] Tensile mechanical properties after heat treatment: After heat treatment, standard tensile specimens are precision machined and tensile tests are performed to measure yield strength, tensile strength, and reduction of area.

[0069] Fatigue strength: The material was subjected to austenitization + isothermal partitioning heat treatment as described above, and a rotational bending fatigue test was conducted according to GB / T 4337. The fatigue strength was tested using the lifting method, and the number of tests was 10. 7 Cycle number, and calculate fatigue ratio (fatigue ratio = fatigue strength / tensile strength);

[0070] Bauschinger Torsion Test Inverse Hysteresis Loop Determination: The material undergoes the aforementioned austenitization + isothermal partitioning heat treatment process, followed by the fabrication of a standard Bauschinger torsion test specimen. The test is then conducted on a standard torsion machine. The specimen is torsionally rotated to 90° at a torsion speed of 15° / min and then unloaded. Reloading is then performed to rotate the specimen back to 90° in the original direction before unloading again. A closed torsional hysteresis loop is obtained on the torque-torsion angle curve, and its area is calculated. The larger the area, the greater the spring resistance.

[0071] Table 4. Grain size, inclusion ratio, and heat treatment process of each embodiment and comparative example of the present invention.

[0072]

[0073] Table 5. Microstructure and properties of the embodiments and comparative examples of the present invention after heat treatment.

[0074]

[0075] Figure 1 The microstructure of the heat-treated sample in Example 1 is a nanoscale microstructure, with 61% nanoscale bainite content and a large number of dispersed fine carbide precipitates.

[0076] Figure 2 The microstructure of the heat-treated sample is as shown in ratio 1, with a nanoscale bainite content of 9%.

[0077] Examples 1-7 are cases where the range and proportion of chemical composition were appropriately controlled, and appropriate production and heat treatment methods were used. The resulting spring steel has fine grains, and all inclusions have been plasticized. After heat treatment, it has a nanoscale microstructure, exhibiting excellent comprehensive performance, with a strength level of over 2200 MPa, high plasticity, and excellent fatigue and springback reduction properties. Its strength, plasticity, fatigue performance, and springback reduction properties are all higher than those of commonly used spring steels on the market, meeting the development trend requirements of the automotive and other industries for longer service life and higher safety performance of spring steel.

[0078] The composition range and production method of Comparative Example 1 are within the appropriate range, but the traditional quenching and tempering heat treatment process was used, which failed to obtain sufficient nanoscale structure, resulting in strength, plasticity, fatigue performance and resistance to spring reduction not meeting the requirements of this invention.

[0079] The P value of Comparative Example 2 does not meet the requirements, the plasticization effect of inclusions is insufficient, and low-basicity slag is not used during smelting, resulting in large brittle inclusions in the steel, which leads to premature fracture during fatigue testing and insufficient fatigue performance.

[0080] Comparative Example 3 is a commonly used spring steel on the market. Its composition system is different from that of the steel of the present invention. Even with the heat treatment method of the present invention, its strength level has not been broken through to 2000MPa, and its fatigue performance and spring reduction resistance are also lower than those of the steel of the present invention.

[0081] The composition range and production method of Comparative Example 4 were within the appropriate range, and the austenitizing + isothermal partitioning heat treatment process was also adopted. However, its isothermal temperature was too low, which led to the entry into the martensitic phase region and the reduction of bainite content. Although the steel had high strength, its plasticity and fatigue performance deteriorated sharply.

[0082] The composition system of Comparative Example 5 differs from that of the present invention. Due to the addition of higher Al and Nb elements, more large-sized brittle inclusions are generated in the steel, resulting in poor fatigue performance. In addition, due to the addition of higher Ni, Mo, and V alloying elements, the hardenability is greatly improved. Therefore, although the isothermal partitioning method of the present invention is used, a large amount of martensite structure is still generated in the steel, and the content of nano-sized bainite is insufficient, resulting in the steel's plasticity, fatigue strength, and resistance to spring reduction being lower than the requirements of the present invention.

[0083] The above description of the embodiments is intended to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A spring steel, characterized in that, The spring steel comprises the following components by weight percentage: Pr 0.0010%-0.0030%, La 0.0025%-0.0045%, C 0.50%-0.60%, Si 1.70%-1.90%, Mn 0.40%-0.55%, Cr 0.70%-0.90%, V 0.30%-0.50%, Al trace -0.005%, P trace -0.015%, S trace -0.010%, O≤0.0010%, H≤0.0001%, the remainder being Fe and other unavoidable impurities; The composition of the spring steel also satisfies the following conditions: inclusion plasticity index P value ≥ 0.20, P = 78.6 × (%Pr) + 29.5 × (%La); The spring steel has an austenitic grain size finer than grade 9.5, and more than 95% of the inclusions are smaller than 8 μm. The heat treatment method for the spring steel includes the following process: first, austenitization is carried out at an austenitization temperature of 880-920℃, then it is placed in a salt bath isothermal furnace at an isothermal temperature of 330-370℃ for 3-4 hours, with a cooling rate of 200-240℃ / s, and cooled to room temperature. The microstructure of the spring steel after heat treatment is a nanoscale structure, wherein the volume content of nanoscale bainite is 60%~80%, and the balance is nanoscale cementite and carbide precipitates, and the size of the carbide precipitates is less than 20nm. After heat treatment, the yield strength of the spring steel reaches over 2000 MPa, the tensile strength reaches over 2200 MPa, the reduction of area is over 45%, the fatigue limit is over 1100 MPa, the fatigue ratio is over 0.50, and the area of ​​the spring steel under the Singer torsion curve is ≥400 mm². 2 .

2. A method for producing spring steel as described in claim 1, characterized in that, The production method includes the following process flow: Smelting in an electric arc furnace or converter → refining in an LF furnace → RH vacuum degassing → continuous casting of large billets → heating of large billets → billet opening of small billets → heating of small billets → controlled rolling of high-speed wire rod → controlled cooling in a Steyrmo cooling line → finished wire rod product with diameters of Φ5.5-30mm.

3. The production method according to claim 2, characterized in that, The LF furnace refining process involves bottom blowing argon into the ladle throughout the entire process, with the argon flow rate determined by preventing molten steel from splashing out of the ladle; low-basicity slag is used and R is controlled at 1.3-1.

5.

4. The production method according to claim 2, characterized in that, The large billet continuous casting: electromagnetic stirring is used during continuous casting. Pr and La lines are added to the crystallizer and their contents are adjusted. Protective casting is used throughout the process. Light reduction is applied, with a reduction of 10%-15%.

5. The production method according to claim 2, characterized in that, The heating of the large billet is controlled at a uniform heating temperature of 1250-1300℃; the heating of the rolled small billet is controlled at a uniform heating temperature of 1000-1060℃.

6. The production method according to claim 2, characterized in that, The high-speed wire rod controlled rolling process: the sizing temperature is controlled at 820-850℃, and the wire drawing temperature is controlled at 800-840℃.

7. A heat treatment method for spring steel as described in claim 1, characterized in that, The heat treatment method includes the following process: first, austenitization is performed at an austenitization temperature of 880-920℃, then the furnace is placed in a salt bath isothermal furnace at an isothermal temperature of 330-370℃ for 3-4 hours, with a cooling rate of 200-240℃ / s, and cooled to room temperature.

8. The heat treatment method according to claim 7, characterized in that, After heat treatment, the microstructure of the spring steel is nanoscale, wherein the content of nanoscale bainite is 60-80%, and the balance is nanoscale cementite and carbide precipitates, wherein the carbide precipitates are finely dispersed and below 20 nm.

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