Spring steel with high strength, high plasticity and excellent stress relaxation resistance as well as rapid induction heating preparation method and application thereof

Abstract

The invention relates to spring steel with high strength, high plasticity and excellent stress relaxation resistance as well as a rapid induction heating preparation method and application thereof. The steel comprises the following chemical components in percentage by weight: 0.53%-0.64% of C, 1.45%-1.49% of Si, 0.63%-0.68% of Mn, 0.62%-0.69% of Cr, 0.019%-0.03% of Cu, 0.02%-0.04% of Ni and 0%-0.12% of V. And through the rapid induction heating technology, the steel can be rapidly heated within a short time, and the heat preservation time is kept short. The production efficiency is remarkably improved, the tensile property and the stress relaxation resistance of the steel are improved by refining the grains, and the stress relaxation resistance of the steel at the high temperature is further improved by adding the V element. And the efficient and economical method is provided for production of the high-performance spring steel, and the remarkable industrial application value is achieved.

Description

Technical Field

[0001] The present invention belongs to the field of high-performance steel manufacturing. Specifically, it relates to a spring steel with high strength, high plasticity and excellent stress relaxation resistance, and its preparation method and application, especially to optimizing alloy components and rapid heating treatment to improve the stress relaxation performance of the material. Background Art

[0002] With the development of modern engineering technology, especially the increasing demand for high-performance materials in industries such as automotive, aerospace, and electronic information, lightweighting has become the key to improving system efficiency and reducing energy consumption. For this reason, the field of materials science is constantly promoting the development of high-strength steels to meet the increasingly stringent lightweighting requirements. High-strength steels can not only provide higher tensile strength but also reduce the amount of material used while ensuring strength, thus achieving the goal of overall lightweighting. In the automotive industry, the use of lightweight materials helps reduce the weight of the entire vehicle, lower fuel consumption, reduce carbon dioxide emissions, and improve vehicle safety and driving performance. High-strength steels, especially spring steels, as an important part of them, are widely used in key components such as suspension, steering, braking, and drive. With the rise of electric vehicles, intelligent vehicles, and high-performance vehicles, the demand for spring steel materials is also increasing continuously. Especially in environments that require long-term stable operation, high temperature, and high pressure, the performance of spring steel materials becomes particularly important.

[0003] As an important high-strength material, spring steel is widely used in various springs, supports, load-bearing, and buffer components. Spring steel not only requires good tensile strength and high elastic modulus but also needs to maintain stable performance under long-term load to avoid dimensional changes and performance degradation caused by stress relaxation. Therefore, how to improve the stress relaxation resistance of spring steel has become an urgent challenge in materials science.

[0004] However, although there have been many research results on the tensile and fatigue properties of spring steel, the research on the stress relaxation performance of spring steel under high temperature and long-term working conditions is relatively less. Stress relaxation refers to the phenomenon that the stress of a material gradually decreases over time under a long-term constant stress. This phenomenon is particularly significant in the application of spring steel. Especially in harsh environments such as high temperature and high pressure, the long-term load on the spring will cause stress relaxation of the material, resulting in the spring losing its original performance and affecting the stability and service life of the product.

[0005] Therefore, improving the stress relaxation resistance of spring steel, especially in high-temperature environments, is of great significance for enhancing the comprehensive properties of spring steel. Existing research has mainly focused on improving the tensile strength and elastic modulus of spring steel, etc., and there is little systematic research specifically targeting stress relaxation performance. In fact, the improvement of stress relaxation performance can not only extend the service life of springs but also ensure the mechanical property stability of springs during long-term use, avoiding material deformation and performance degradation caused by stress relaxation. Therefore, researching and developing spring steel materials with excellent stress relaxation performance has important application prospects and practical significance for meeting the requirements of modern industries for high-performance materials. Summary of the Invention

[0006] Based on this, it is necessary to provide a spring steel with high strength, high plasticity, and excellent stress relaxation resistance, as well as an ultra-fast induction heating production method for producing this steel.

[0007] The present invention proposes a technical solution for producing spring steel materials through a rapid heating process. In traditional heating processes, due to the long heating time, it may cause grain growth or excessive non-uniform structures in the materials, thus affecting their properties. However, through the rapid heating process, the material can reach the required heat treatment temperature within a short time, which can not only optimize the microstructure of the material but also shorten the production cycle while improving the material properties. Rapid heating can effectively improve the stress relaxation resistance of the material, especially in high-temperature environments, enabling the spring steel to exhibit more excellent properties under long-term load and high-temperature conditions.

[0008] In addition, the present invention discovers that adding vanadium element (V) can significantly improve the stress relaxation performance of the material, especially at high temperatures. The 0.12V alloy exhibits more excellent stress relaxation characteristics. Combining with the rapid heating process, the stress relaxation resistance of the 0.12V alloy at high temperatures is further enhanced, providing an efficient technical path to improve the comprehensive properties of spring steel.

[0009] The technical solution of the present invention is as follows:

[0010] A spring steel with high strength, high plasticity, and excellent stress relaxation resistance, its chemical composition and mass percentage are: C: 0.53% - 0.64%, Si: 1.45% - 1.49%, Mn: 0.63% - 0.68%, Cr: 0.62% - 0.69%, Cu: 0.019% - 0.03%, Ni: 0.02% - 0.04%, V: 0% - 0.12%, and the balance is Fe and inevitable impurities. Control the mass percentage of impurity chemical components: P ≤ 0.008%, S ≤ 0.008%, O2 ≤ 10 ppm, H2 ≤ 10 ppm.

[0011] In the composition design of a spring steel with high strength, high plasticity and excellent stress relaxation resistance described in the present invention, each element and its function are as follows:

[0012] C: Carbon is the most common strengthening element in steel. 1. It forms a solid solution structure, improving the strength of the steel. 2. It forms carbide structures, which can improve the hardness and wear resistance of the steel.

[0013] Mn: Manganese provides solid solution strengthening and enables the steel to obtain finer and stronger pearlite during the cooling process after hot rolling, and the content of pearlite increases with the increase of manganese content. Manganese is also a carbide-forming element, and manganese carbides can dissolve into cementite, thereby indirectly enhancing the strength of pearlite. Manganese can also enhance the hardenability of the steel, further improving the strength.

[0014] Si: Silicon forms a solid solution in ferrite or austenite, thereby enhancing the yield strength and tensile strength of the steel. Moreover, silicon can increase the cold working strain hardening rate of the steel and is a beneficial element in alloy steel. Silicon can improve the strength, hardness and wear resistance of the steel, and within a certain range, it will not significantly reduce the plasticity of the steel.

[0015] Cr: Chromium can improve the hardenability of the steel, making the steel have good comprehensive mechanical properties. Chromium forms various carbides with carbon, and has a greater affinity for carbon than iron and manganese elements. Chromium and iron can form intermetallic compounds.

[0016] Microalloying elements V and Cu: Adding microalloying elements V and Cu to the steel can refine the grains and greatly improve the strength and toughness of the steel. The microalloying elements in the present invention are beneficial, but considering other factors such as cost, it is not advisable to add too much. Compared with other high-alloy steels such as maraging steel, it can reduce the alloy cost while maintaining high mechanical properties.

[0017] The spring steel with high strength, high plasticity and excellent stress relaxation resistance of the present invention is produced by an ultra-fast induction heating method, which specifically includes the following steps:

[0018] (1) According to the chemical composition and ratio of the steel, iron ore, quicklime, vanadium ore, ferrosilicon alloy, ferromanganese alloy, ferrochromium alloy are placed in an oxidizing atmosphere furnace for melting and smelting to obtain molten steel, and the molten steel is continuously cast to obtain steel billets;

[0019] (2) The steel billets obtained in step (1) are placed in a walking beam reheating furnace for heating, and kept warm for 2.5 - 3 h under the condition of 1260 - 1300 °C to make all elements of C, Si, Mn, V, Cr, Cu, Ni dissolve;

[0020] (3) Hot-roll the steel billet after solution treatment in step (2), where the starting rolling temperature is 1100 - 1200 °C and the final rolling temperature is higher than 900 - 950 °C to obtain a hot-rolled steel bar, and then air-cool to room temperature;

[0021] (4) Cold-draw the hot-rolled steel bar obtained in step (3), and the total reduction of cross-sectional area during cold-drawing is 70% - 90%. The diameter of the cold-drawn steel wire after 70% - 90% cold-drawing is 3 - 10 mm;

[0022] (5) Perform ultra-rapid induction heating treatment on the cold-drawn steel wire obtained in step (4), including two stages of quenching and tempering. The specific operation is as follows: Heat the cold-drawn steel wire at a heating rate of 80 - 200 °C / s to 850 - 900 °C and hold for 2 - 4 seconds to complete austenitization, and then rapidly cool the temperature to 40 - 60 °C by water quenching, with a cooling rate of 50 - 250 °C / s; Heat the austenitized steel wire to 400 - 600 °C at a heating rate of 50 - 150 °C / s again and hold for 2 - 4 seconds, and then cool the steel wire temperature to 40 - 60 °C by water quenching, with a cooling rate of 50 - 250 °C / s.

[0023] Completing the above steps can obtain the spring steel with high strength, high plasticity and excellent stress relaxation resistance.

[0024] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0025] The method for preparing spring steel with high strength, high plasticity and excellent stress relaxation resistance provided by the present invention first processes the raw materials with appropriate proportions into a preliminary steel product, and then performs rapid heat treatment and cooling after cold-drawing. During the rapid heat treatment process, the preliminary steel product is heated to the target temperature through a large heating rate and held at this temperature for a short holding time. This process can ensure that the cementite and ferrite in the steel are completely transformed into austenite, while avoiding grain coarsening. Through rapid heat treatment and rapid cooling, the microstructure of the steel is transformed into a matrix of martensite, with a small amount of retained austenite and nano-carbides dispersed. This can not only significantly improve the strength and plasticity of the spring steel, but also improve its stability under high temperature and long-term load conditions. The rapid heating process rapidly heats the steel to the required temperature in a short time, promotes grain refinement and uniform dissolution of alloying elements, and effectively inhibits grain growth. Fine grains help prevent the movement of slip and dislocations, thereby improving the stress relaxation resistance of the steel under long-term load and reducing performance degradation. In addition, using a large heating rate and a short holding time can not only improve the performance of the steel, but also significantly reduce the processing time of the spring steel, and the production efficiency can be increased by 50% - 200% compared with traditional heat treatment.

[0026] Specifically, compared with traditional heat treatment processes, the high-carbon low-alloy spring steel wire obtained by the ultra-rapid induction heating treatment process has a reduced heating process and soaking process time and a shorter furnace length. Therefore, the surface quality of the product will be significantly improved; and due to the refinement of the product grains and the reduction of the material alloy content, the forming performance and service performance of the high-carbon low-alloy spring steel wire obtained by the technology of the present invention are also improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 is a flowchart of the rapid induction heating production of the spring steel with high strength, high plasticity and excellent stress relaxation resistance of the present invention.

[0028] Figure 2 are the results of the room temperature and quasi-static strain rate tensile tests of the spring steel products prepared by the rapid heating method in Examples 1 and 2 of the present invention.

[0029] Figure 3A are the stress relaxation curve results of the spring steel in Example 1 of the present invention at different temperatures when the initial stress is 1750 MPa and the holding time is 2000 seconds.

[0030] Figure 3B are the stress relaxation curve results of the spring steel in Example 2 of the present invention at different temperatures when the initial stress is 1750 MPa and the holding time is 2000 seconds.

[0031] Figure 3C are the stress relaxation curve results of the spring steel in Examples 1 and 2 of the present invention at 75 °C when the initial stress is 1750 MPa and the holding time is 2000 seconds.

[0032] Figure 4 is a transmission electron microscope tissue image of the spring steel obtained by rapid heat treatment in Example 2 of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0033] To make the objectives, technical solutions and advantages of the present invention clearer, the following will be described in detail in combination with each embodiment and the accompanying drawings. Many specific details are set forth in the following description to facilitate a thorough understanding of the present invention, but the embodiments of the present invention are not limited thereto. Those skilled in the art can make similar improvements without departing from the connotation of the present invention, so the present invention is not limited by the specific embodiments below.

[0034] Unless otherwise defined, the technical terms used hereinafter have the same meanings as commonly understood by those skilled in the art. All kinds of raw materials, reagents, instruments and equipment used in the present invention can be obtained through market purchases or can be prepared by existing methods.

[0035] The present invention provides a spring steel with high strength, high plasticity and excellent stress relaxation resistance, comprising, by mass percentage: C: 0.53% - 0.64%, Si: 1.45% - 1.49%, Mn: 0.63% - 0.68%, Cr: 0.62% - 0.69%, Cu: 0.019% - 0.03%, Ni: 0.02% - 0.04%, V: 0% - 0.12%; the balance being Fe and inevitable impurities.

[0036] In order to achieve excellent mechanical properties and outstanding stress relaxation resistance in this high-carbon spring steel, this study adopted a combination of rapid heating and optimized alloying elements. The main function of rapid heating is to significantly refine grains, maintain a high dislocation density, and increase the content of retained austenite by means of rapid temperature rise and short-time heat preservation. Grain refinement can effectively improve the mechanical properties of the material. The smaller the grain size, the higher the yield strength and tensile strength of the material usually are, and grain refinement can also significantly improve plasticity. And a high dislocation density can bring higher strength to the spring steel. Through this combined effect, rapid heating not only improves the strength and toughness of the steel, but also enhances its durability and stability under complex working conditions. On this basis, alloying elements such as silicon (Si) and trace amounts of vanadium (V) are precisely controlled to ensure the anti-elastic ability of the material in a high-temperature environment. The addition of silicon significantly enhances the anti-elastic and fatigue resistance of the steel, while the addition of trace vanadium effectively inhibits the stress relaxation phenomenon, enabling the spring steel to maintain stable performance under long-term load. The high carbon content not only provides good hardness and strength, but also provides sufficient hardening potential for the heat treatment process of the steel. By precisely controlling these factors, this study optimizes the plasticity and stress relaxation resistance of the spring steel while ensuring high strength, meeting the requirements of high-performance spring materials.

[0037] The preparation method of the spring steel with high strength, high plasticity and excellent stress relaxation resistance provided by the present invention includes Step 1, composition design; Step 2: solution treatment; Step 3: hot rolling; Step 4: cold drawing; Step 5: ultra-rapid induction heating heat treatment.

[0038] In Step 1, according to the chemical composition ratio of high-carbon low-alloy spring steel wire, iron ore, quicklime, vanadium ore, ferrosilicon alloy, ferromanganese alloy, and ferrochromium alloy are placed in an oxidizing atmosphere furnace for melting and smelting to obtain molten steel. Its chemical composition is: C: 0.53%-0.64%, Si: 1.45%-1.49%, Mn: 0.63%-0.68%, Cr: 0.62%-0.69%, Cu: 0.019%-0.03%, Ni: 0.02%-0.04%, V: 0%-0.12%; the rest are Fe and inevitable impurities. The Al content can be preferably adjusted within the range of 0 wt%-0.15 w% according to needs and then transferred to a reducing atmosphere furnace for refining to obtain refined molten steel, and the refined molten steel is continuously cast to obtain steel billets.

[0039] In Step 2, the steel billets obtained in Step 1 are placed in a walking beam reheating furnace for heating and held at 1260-1300°C for 2.5-3 h, so that all elements of C, Si, Mn, V, Cr, Cu, and Ni are solid-solved;

[0040] In Step 3, the steel billets after solid solution in Step 2 are hot-rolled, where the starting rolling temperature is 1200°C and the final rolling temperature is higher than 900°C to obtain hot-rolled steel bars, and then air-cooled to room temperature;

[0041] In Step 4, the hot-rolled steel bars obtained in Step 3 are cold drawn, and the total reduction of area of cold drawing is 70%-90%. The diameter of the cold-drawn steel wire after 70%-90% cold drawing is 3-10 mm;

[0042] In Step 5, the cold-drawn steel wire obtained in Step 4 is subjected to ultra-fast induction heating treatment, Figure 1 shows the rapid induction heating production process of the spring steel with high strength, high plasticity and excellent stress relaxation resistance manufactured by the present invention. As Figure 1As shown, the heat treatment process includes two stages: quenching and tempering. First, quenching is achieved through ultra-rapid induction heating. The specific operation is to heat the cold-drawn steel wire at a heating rate of 80 - 200 °C / s to 850 - 900 °C and hold for 2 - 4 seconds to complete the austenitization process. Then, the steel wire is rapidly cooled to 40 - 60 °C by water quenching, with a cooling rate of 50 - 250 °C / s, so as to obtain good hardness and microstructure. Next, tempering treatment is carried out. The austenitized steel wire is heated to 400 - 600 °C again at a heating rate of 50 - 150 °C / s and held at this temperature for 2 - 4 seconds. Finally, the temperature of the steel wire is cooled to 40 - 60 °C by water quenching, with the same cooling rate of 50 - 250 °C / s, to ensure the stability and excellent mechanical properties of the steel wire after tempering. This heat treatment process gives full play to the advantages of ultra-rapid induction heating technology, not only improving the strength and toughness of the steel wire, but also optimizing its stress relaxation resistance performance, meeting the requirements of high-performance spring materials.

[0043] The following describes in detail the spring steel with high strength, high plasticity and excellent stress relaxation resistance performance, its rapid induction heating preparation method and application according to the present invention in combination with specific embodiments.

[0044] Example 1 [[ID=??]]

[0045] A spring steel with high strength, high plasticity and excellent stress relaxation resistance performance is prepared according to the following steps:

[0046] (1) Molten steel containing the following chemical components is obtained through smelting: C: 0.56%, Si: 1.48%, Mn: 0.64%, Cr: 0.66%, Cu: 0.03%, Ni: 0.04%, and the balance is Fe and inevitable impurities. Then it is transferred to a refining furnace in a reducing atmosphere for refining to obtain refined molten steel, and continuous casting is carried out on the refined molten steel to obtain a steel billet.

[0047] (2) The steel billet is placed in a walking beam reheating furnace and heated at 1300 °C for 3 h to make all the elements of C, Si, Mn, Cr, Cu, and Ni dissolve;

[0048] (3) The solution-treated steel billet is hot-rolled, with the starting rolling temperature of 1200 °C and the finishing rolling temperature of 950 °C, to obtain a hot-rolled steel bar and then air-cooled to room temperature;

[0049] (4) Cold drawing is carried out on the obtained hot-rolled steel bar. The total reduction of cross-sectional area of cold drawing is 80%, and the diameter of the cold-drawn steel wire after 90% cold drawing is 3.9 mm;

[0050] It should be noted that there seems to be an error in the tag which is shown as in the original. It might be a mislabeling in the original text. I've translated it as as it is, but you may want to double-check the original for accuracy.(5) The obtained cold-drawn steel wire is subjected to ultra-rapid induction heating treatment. The specific operation is to heat the cold-drawn steel wire at a heating rate of 100 °C / s to 850 °C and hold for 2 - 4 seconds to complete the austenitization process. Then, the steel wire is rapidly cooled to 40 - 60 °C by water quenching, and the cooling rate is 150 °C / s. Next, tempering treatment is carried out. The austenitized steel wire is heated to 416 °C again at a heating rate of 80 °C / s and held at this temperature for 2 - 4 seconds. Finally, the temperature of the steel wire is cooled to 40 - 60 °C by water quenching, and the cooling rate is 100 °C / s.

[0051] Example 2

[0052] A spring steel with high strength, high plasticity and excellent stress relaxation resistance is prepared according to the following steps:

[0053] (1) Molten steel containing the following chemical components is obtained through smelting: C: 0.56%, Si: 1.48%, Mn: 0.64%, Cr: 0.66%, Cu: 0.03%, Ni: 0.04%, V: 0.12%, and the balance is Fe and inevitable impurities. Then it is transferred to a refining furnace in a reducing atmosphere for refining to obtain refined molten steel, and continuous casting is carried out on the refined molten steel to obtain steel billets.

[0054] (2) The steel billets are placed in a walking beam reheating furnace and heated at 1300 °C for 3 h to dissolve all the elements of C, Si, Mn, V, Cr, Cu, and Ni;

[0055] (3) The solution-treated steel billets are hot-rolled, with the starting rolling temperature of 1200 °C and the final rolling temperature of 950 °C, to obtain hot-rolled steel bars, and then air-cooled to room temperature;

[0056] (4) The obtained hot-rolled steel bars are cold-drawn, and the total reduction of cross-sectional area of cold drawing is 80%. The diameter of the cold-drawn steel wire after 90% cold drawing is 3.9 mm;

[0057] (5) The obtained cold-drawn steel wire is subjected to ultra-rapid induction heating treatment. The specific operation is to heat the cold-drawn steel wire at a heating rate of 100 °C / s to 890 °C and hold for 2 - 4 seconds to complete the austenitization process. Then, the steel wire is rapidly cooled to 40 - 60 °C by water quenching, and the cooling rate is 150 °C / s. Next, tempering treatment is carried out. The austenitized steel wire is heated to 424 °C again at a heating rate of 80 °C / s and held at this temperature for 2 - 4 seconds. Finally, the temperature of the steel wire is cooled to 40 - 60 °C by water quenching, and the cooling rate is 100 °C / s.

[0058] Example 3

[0059] A spring steel with high strength, high plasticity and excellent stress relaxation resistance is prepared according to the following steps:

[0060] (1) The molten steel containing the following chemical components is obtained through smelting: C: 0.56%, Si: 1.48%, Mn: 0.64%, Cr: 0.66%, Cu: 0.03%, Ni: 0.04%, V: 0.08%, and the balance is Fe and inevitable impurities. Then it is transferred to a reducing atmosphere furnace for refining to obtain refined molten steel, and the refined molten steel is continuously cast to obtain a steel billet.

[0061] (2) The steel billet is placed in a walking beam reheating furnace and heated, and kept at 1300 °C for 3 h to dissolve all the elements of C, Si, Mn, V, Cr, Cu, and Ni.

[0062] (3) The steel billet after solution treatment is hot-rolled, with the starting rolling temperature of 1200 °C and the final rolling temperature of 950 °C, to obtain hot-rolled steel bars, which are then air-cooled to room temperature.

[0063] (4) The obtained hot-rolled steel bars are cold-drawn, and the total reduction of cross-sectional area of cold-drawing is 80%. The diameter of the cold-drawn steel wire after 90% cold-drawing is 3.9 mm.

[0064] (5) The obtained cold-drawn steel wire is subjected to ultra-fast induction heating treatment. The specific operation is to heat the cold-drawn steel wire at a heating rate of 100 °C / s to 890 °C and keep it for 2 - 4 seconds to complete the austenitization process. Then, the steel wire is quenched in water to quickly reduce the temperature to 40 - 60 °C, and the cooling rate is 150 °C / s. Next, tempering treatment is carried out. The austenitized steel wire is heated to 442 °C again at a heating rate of 80 °C / s and kept at this temperature for 2 - 4 seconds. Finally, the temperature of the steel wire is cooled to 40 - 60 °C by water quenching, and the cooling rate is 100 °C / s.

[0065] Example 4

[0066] A spring steel with high strength, high plasticity and excellent stress relaxation resistance is prepared according to the following steps:

[0067] (1) The molten steel containing the following chemical components is obtained through smelting: C: 0.56%, Si: 1.48%, Mn: 0.64%, Cr: 0.66%, Cu: 0.03%, Ni: 0.02%, V: 0.12%, and the balance is Fe and inevitable impurities. Then it is transferred to a reducing atmosphere furnace for refining to obtain refined molten steel, and the refined molten steel is continuously cast to obtain a steel billet.

[0068] (2) The steel billet is placed in a walking beam reheating furnace and heated, and kept at 1300 °C for 3 h to dissolve all the elements of C, Si, Mn, V, Cr, Cu, and Ni.

[0069] (3) Hot-roll the solution-treated steel billet, with the starting rolling temperature being 1200 °C and the finishing rolling temperature being 950 °C, to obtain hot-rolled steel bars and then air-cool to room temperature;

[0070] (4) Cold-draw the obtained hot-rolled steel bars, with the total reduction of cross-sectional area in cold drawing being 80%, and the diameter of the cold-drawn steel wire after 90% cold drawing being 3.9 mm;

[0071] (5) Perform ultra-rapid induction heating treatment on the obtained cold-drawn steel wire. The specific operation is to heat the cold-drawn steel wire at a heating rate of 150 °C / s to 890 °C and hold for 2 - 4 seconds to complete the austenitization process. Then, the steel wire is rapidly cooled to 40 - 60 °C through water quenching, with a cooling rate of 150 °C / s. Next, perform tempering treatment, and heat the austenitized steel wire to 432 °C again at a heating rate of 80 °C / s and hold at this temperature for 2 - 4 seconds. Finally, cool the steel wire temperature to 40 - 60 °C through water quenching, with a cooling rate of 100 °C / s.

[0072] Example 5

[0073] A spring steel with high strength, high plasticity and excellent stress relaxation resistance is prepared according to the following steps:

[0074] (1) Obtain molten steel containing the following chemical components through smelting: C: 0.56%, Si: 1.48%, Mn: 0.64%, Cr: 0.66%, Cu: 0.03%, Ni: 0.04%, and the balance is Fe and unavoidable impurities. Then transfer it to a refining furnace under a reducing atmosphere for refining to obtain refined molten steel, and continuously cast the refined molten steel to obtain a steel billet.

[0075] (2) Place the steel billet in a walking beam heating furnace and heat it at 1300 °C for 3 h to make all elements of C, Si, Mn, Cr, Cu, and Ni dissolve;

[0076] (3) Hot-roll the solution-treated steel billet, with the starting rolling temperature being 1200 °C and the finishing rolling temperature being 950 °C, to obtain hot-rolled steel bars and then air-cool to room temperature;

[0077] (4) Cold-draw the obtained hot-rolled steel bars, with the total reduction of cross-sectional area in cold drawing being 80%, and the diameter of the cold-drawn steel wire after 90% cold drawing being 3.9 mm;

[0078] (5) The obtained cold-drawn steel wire is subjected to ultra-rapid induction heating treatment. The specific operation is to heat the cold-drawn steel wire at a heating rate of 150 °C / s to 850 °C and hold for 2 - 4 seconds to complete the austenitization process. Then, the steel wire is rapidly cooled to 40 - 60 °C by water quenching, and the cooling rate is 150 °C / s. Next, tempering treatment is carried out. The austenitized steel wire is heated to 415 °C again at a heating rate of 80 °C / s and held at this temperature for 2 - 4 seconds. Finally, the temperature of the steel wire is cooled to 40 - 60 °C by water quenching, and the cooling rate is 100 °C / s.

[0079] Example 6

[0080] A spring steel with high strength, high plasticity and excellent stress relaxation resistance is prepared according to the following steps:

[0081] (1) Molten steel containing the following chemical components is obtained through smelting: C: 0.64%, Si: 1.48%, Mn: 0.64%, Cr: 0.66%, Cu: 0.03%, Ni: 0.04%, V: 0.12%, and the balance is Fe and inevitable impurities. Then it is transferred to a refining furnace under a reducing atmosphere for refining to obtain refined molten steel, and the refined molten steel is continuously cast to obtain a steel billet.

[0082] (2) The steel billet is placed in a walking beam heating furnace and heated at 1300 °C for 3 h to make all the elements of C, Si, Mn, V, Cr, Cu, and Ni dissolve.

[0083] (3) The solution-treated steel billet is hot-rolled, with the starting rolling temperature of 1200 °C and the final rolling temperature of 950 °C, to obtain a hot-rolled steel bar, and then air-cooled to room temperature.

[0084] (4) The obtained hot-rolled steel bar is cold-drawn. The total reduction in cross-sectional area of the cold-drawing is 80%, and the diameter of the cold-drawn steel wire after 90% cold-drawing is 3.9 mm.

[0085] (5) The obtained cold-drawn steel wire is subjected to ultra-rapid induction heating treatment. The specific operation is to heat the cold-drawn steel wire at a heating rate of 150 °C / s to 890 °C and hold for 2 - 4 seconds to complete the austenitization process. Then, the steel wire is rapidly cooled to 40 - 60 °C by water quenching, and the cooling rate is 150 °C / s. Next, tempering treatment is carried out. The austenitized steel wire is heated to 440 °C again at a heating rate of 80 °C / s and held at this temperature for 2 - 4 seconds. Finally, the temperature of the steel wire is cooled to 40 - 60 °C by water quenching, and the cooling rate is 100 °C / s.

[0086] The spring steels with high strength, high plasticity and excellent stress relaxation resistance prepared in the above embodiments were respectively subjected to room temperature tensile property tests and stress relaxation property tests at an initial stress of 1750 MPa and different temperatures. The test results are shown in Table 1.

[0087] Table 1 Mechanical property test results of the spring steel wires prepared in each embodiment

[0088]

[0089] As can be seen from Table 1, the spring steels prepared by the scheme of the present invention are outstanding in terms of high strength, high plasticity and excellent stress relaxation resistance. Specifically, the yield strength and tensile strength of these materials are respectively between 2030 MPa and 2047 MPa and between 2200 MPa and 2263 MPa, showing excellent bearing capacity. At the same time, the total elongation of the materials all exceeds 15%, and the highest can reach 16.8%. Vanadium (V) has multiple strengthening mechanisms in steel. Vanadium can form fine carbides with good high-temperature stability in steel, which precipitate dispersedly during the heat treatment or hot working process of steel, hinder the movement of dislocations, and improve the strength; at the same time, V can inhibit the growth of austenite grains during heating, and its carbides pin the grain boundaries to prevent grain boundary migration, making the structures such as ferrite or pearlite after cooling fine, and the crack propagation path tortuous, not only improving the strength, but also improving the toughness and plasticity; in addition, part of V dissolves into the iron lattice to cause lattice distortion, increasing the resistance to dislocation movement, improving the strength and affecting the electronic structure to improve the mechanical properties. Moreover, the present invention uses a small amount of V, which not only reduces the cost but also can improve the strength and toughness of the steel. The above results prove that on the basis of ensuring strength, high plasticity can still be maintained, avoiding the risk of brittle fracture. Therefore, by adopting rapid heating treatment, not only the strength of the steel is improved, but good plasticity is effectively maintained. Compared with traditional materials such as maraging steels, the rapid heating process of the present invention has significant advantages in production efficiency and cost. Traditional high-performance steels usually require a long heat treatment process and have a high production cost, while the rapid heating technology of the present invention greatly shortens the heating time, simplifies the production process, reduces energy consumption and equipment complexity. In addition, the rapid heating process can ensure uniform heating, is particularly suitable for the production of small-sized wire rods, ensures the performance consistency of each wire rod, and thus improves the production stability and product quality.

[0090] However, due to the relatively high carbon content of these steel grades, their weldability may be affected. But for spring steels, welding is usually not necessary, so this shortcoming will not have a significant impact on their applications.

[0091] These steel grades also exhibit excellent stress relaxation resistance at different temperatures. At room temperature, their stress relaxation resistance rate is close to 99%; at 50°C, 75°C, and 125°C, the stress relaxation resistance rate still remains at a relatively high level. Specifically, the stress relaxation resistance rate exceeds 98% at 50°C, exceeds 96% at 75°C, and can still reach more than 85% at 125°C. This indicates that through rapid heating treatment, the stress relaxation behavior of the material has been effectively inhibited. Even in a high-temperature environment, it can maintain good mechanical properties for a long time, ensuring the long-term stability of the material.

[0092] Compared with conventionally treated steel, the steel processed by rapid heating not only shows a significant increase in yield strength and tensile strength but also has a substantial improvement in stress relaxation resistance. Conventionally treated steel may experience significant stress relaxation in a high-temperature environment, leading to performance degradation, while the steel of the present invention can maintain stable performance at high temperatures for a long time, making it particularly suitable for application scenarios that require long-term stable loading.

[0093] In summary, the rapid heating scheme of the present invention can not only significantly improve the strength and plasticity of steel but also effectively inhibit stress relaxation, ensuring the long-term stability of steel in a high-temperature environment. It has obvious advantages in terms of production efficiency, cost control, and application prospects, and is particularly suitable for large-scale industrial production, with broad application potential.

[0094] According to the room-temperature quasi-static strain rate tensile test results of the spring steel products prepared in Example 1 and Example 2 (as Figure 2 shown), it can be seen that the tensile properties of the two spring steels are similar, and both have good strength-plasticity coordination. Figure 3A and Figure 3B respectively show the stress relaxation curves of Example 1 and Example 2 at different temperatures when the initial stress is 1750 MPa and the holding time is 2000 seconds. It can be seen from the figure that as the temperature increases, the stress relaxation resistance of the spring steel gradually weakens. However, under high-temperature conditions, the stress relaxation resistance of Example 2 is significantly better than that of Example 1, as Figure 3C shown.

[0095] The key reason for this performance difference is that a small amount of vanadium element is added in Example 2. The vanadium (V) element reacts with carbon in the steel during tempering, precipitating fine vanadium carbide particles (as Figure 4 shown). These fine vanadium carbide particles play a key role in the stress relaxation process: they can effectively hinder the movement of dislocations, thereby inhibiting the plastic deformation of the material and significantly increasing the stress relaxation resistance rate. Especially at high temperatures, these vanadium carbide particles can stabilize the microstructure and prevent dislocation slip, thus enhancing the stress relaxation resistance of the material.

[0096] In summary, the addition of vanadium plays a significant synergistic role in optimizing the high-temperature stress relaxation resistance of spring steel. Vanadium can not only refine the microstructure of the steel but also effectively improve the stress relaxation resistance of the material at high temperatures, providing a longer service life and better performance stability for spring steel under high-stress and high-temperature working conditions.

[0097] The embodiments described above only represent some implementation modes of the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, for those skilled in the art, without departing from the concept of the present invention, they can still make several deformations and improvements to the technical solutions recorded in the foregoing embodiments, or make equivalent replacements for some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention. Therefore, the protection scope of this invention patent shall be subject to the appended claims, and the description and drawings can be used to explain the content of the claims.

Claims

1. A method for preparing spring steel with high strength, high plasticity and excellent stress relaxation resistance, characterized in that: The specific steps include: (1) placing iron ore, quicklime, vanadium stone, ferro-silicon alloy, ferro-manganese alloy, and ferro-chromium alloy in an oxidizing atmosphere furnace for melting and smelting according to the chemical composition and proportion of the steel to obtain molten steel, which is then continuously cast to obtain steel billets; (2) placing the steel billet obtained in step (1) in a walking beam heating furnace and heating it to dissolve all the elements C, Si, Mn, V, Cr, Cu, and Ni; (3) hot rolling the steel billet after solution treatment in step (2) to obtain a hot-rolled steel bar, and then air-cooling it to room temperature; (4) cold drawing the hot-rolled steel bar obtained in step (3); (5) subjecting the cold-drawn steel wire obtained in step (4) to ultra-fast induction heating treatment, including two stages of quenching and tempering; upon completion of the above steps, the spring steel having high strength, high plasticity and excellent stress relaxation resistance can be obtained.

2. The preparation method according to claim 1, characterized in that In step (1), the chemical composition and proportion of the steel are as follows: by mass percentage, C: 0.53%-0.64%, Si: 1.45%-1.49%, Mn: 0.63%-0.68%, Cr: 0.62%-0.69%, Cu: 0.019%-0.03%, Ni: 0.02%-0.04%, V: 0%-0.12%, and the balance is Fe and unavoidable impurities.

3. The preparation method according to claim 1, characterized in that In step (2), the heating is carried out at 1260-1300° C. for 2.5-3 h.

4. The preparation method according to claim 1, characterized in that In step (3), during the hot rolling process, the starting rolling temperature is 1100-1200°C, and the finishing rolling temperature is higher than 900-950°C.

5. The preparation method according to claim 1, characterized in that In step (4), the total area reduction rate of cold drawing is 70%-90%, and the diameter of the cold-drawn steel wire after 70%-90% cold drawing is 3-10 mm.

6. The preparation method according to claim 1, characterized in that In step (5), during the ultra-fast induction heating process, the specific operation of the quenching stage is: heating the cold-drawn steel wire to 850-900°C at a heating rate of 80-200°C / s and holding it for 2-4 seconds to complete austenitization, and then water quenching to quickly reduce the temperature to 40-60°C at a cooling rate of 50-250°C / s.

7. The preparation method according to claim 1, characterized in that In step (5), during the ultra-fast induction heating process, the specific operation of the tempering stage is as follows: heating the austenitized steel wire to 400-600°C again at a heating rate of 50-150°C / s, and keeping it warm for 2-4 seconds, and then cooling the steel wire temperature to 40-60°C by water quenching at a cooling rate of 50-250°C / s.

8. The preparation method according to claim 1, characterized in that The prepared spring steel has an anti-stress relaxation rate of more than 98% at 50°C, an anti-stress relaxation rate of more than 96% at 75°C, and an anti-stress relaxation rate of more than 85% at 125°C.

9. Spring steel with high strength, high plasticity and excellent stress relaxation resistance prepared by the preparation method according to any one of claims 1 to 8.

10. Use of the high-strength, high-plasticity and excellent stress relaxation resistance spring steel according to claim 9 in electric vehicles, smart cars and high-performance vehicles.

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