A super strong spring steel for vehicle suspension and a method for manufacturing the same

Abstract

The application belongs to the technical field of metal materials, and particularly relates to a super strong spring steel for vehicle suspension and a preparation method thereof. The super strong spring steel has the following chemical components in percentage by mass: C: 0.48-0.56%, Si: 0.35-0.80%, Mn: 0.65-1.05%, Cr: 1.05-1.45%, Mo: 0.20-0.38%, V: 0.06-0.14%, Nb: 0.018-0.030%, B: 0.0015-0.0030%, Al: 0.020-0.035%, P: <=0.02%, S: <=0.01%, N: <=0.008%, and the balance of Fe and inevitable impurities. Through effective control of the component ratio, a heat treatment system and shot blasting parameters, the material has good fatigue cracking resistance and service stability while maintaining a high strength level.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials technology, specifically relating to a high-strength spring steel for vehicle suspension and its preparation method. Background Technology

[0002] The vehicle suspension system is a crucial component of the automobile chassis. Its primary function is to connect the frame and axles, buffering and absorbing energy generated by uneven road surfaces, load variations, and impact vibrations during vehicle operation, thereby improving the vehicle's ride comfort, handling stability, and structural safety. Within the suspension system, elastic components such as leaf springs and torsion bar springs are key parts for achieving load-bearing, buffering, and energy storage and release functions. The properties of the spring steel used directly affect the reliability and service life of the suspension system. Because this type of spring steel is subjected to alternating stress, impact loads, and complex environments during actual service, high requirements are placed on its strength, toughness, elastic limit, structural stability, and resistance to fatigue cracking.

[0003] Existing vehicle suspension spring steels mostly employ medium-to-high carbon alloy spring steel systems, and their overall performance is improved through processes such as quenching, tempering, and surface strengthening treatments. Shot peening, as a commonly used surface strengthening method for spring steel, primarily improves its resistance to fatigue cracking by introducing a favorable residual compressive stress state on the steel surface. This technical approach has been widely used in existing vehicle suspension spring steel applications, but its limitations are gradually becoming apparent as service conditions become increasingly stringent. While some spring steels can achieve higher tensile strength and hardness by increasing carbon content, alloy element content, or heat treatment strength, simply relying on increased strength or hardness does not necessarily lead to a simultaneous improvement in fatigue performance. Under high-strength conditions, materials are often more sensitive to surface defects, inclusions, residual stress changes, and microstructural fluctuations, making them prone to premature surface crack initiation, accelerated crack propagation, and increased service life dispersion under long-term cyclic loading.

[0004] Meanwhile, shot peening, a common method for improving the fatigue resistance of spring steel surfaces, primarily works by introducing a favorable residual compressive stress state through surface plastic deformation, thereby inhibiting fatigue crack initiation and delaying crack propagation. However, existing technologies often focus more on the formation of instantaneous residual compressive stress after shot peening, while neglecting the stability of the spring steel matrix structure, the adaptability of surface strengthening, and the ability to maintain the residual compressive stress state. This results in some materials achieving high surface residual compressive stress in the initial stage of shot peening, but this favorable compressive stress state is prone to relaxation and attenuation during subsequent thermal exposure, cyclic loading, or long-term service, thus weakening the strengthening effect. Furthermore, if the shot peening parameters are not properly matched with the material matrix state, it may also cause problems such as increased surface damage and roughness, thereby adversely affecting fatigue performance. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a high-strength spring steel for vehicle suspension and its preparation method. By synergistically designing the key chemical components of the spring steel and combining quenching, dual-temperature tempering, warm shot peening, and short-term stabilization treatment after shot peening, a material system is constructed that possesses high strength, good tempering stability, superior surface strengthening response, and residual compressive stress retention. Unlike the conventional approach where tempering is primarily used to improve toughness and shot peening is primarily used to introduce instantaneous surface compressive stress, the present invention, through synergistic matching of composition design and process steps, ensures that dual-temperature tempering, warm shot peening, and short-term stabilization treatment all serve the unified goal of constructing a high-strength matrix and maintaining a favorable surface stress state. Thus, the material maintains a high strength level while possessing good resistance to fatigue cracking and service stability. It is suitable for high-performance spring steel materials used in elastic components such as leaf springs and torsion bar springs in vehicle suspension systems, and is of great significance for improving the load-bearing capacity of suspension components, extending service life, and enhancing the overall reliability of vehicle operation.

[0006] The technical effects described in this invention are achieved through the following technical solutions: A type of high-strength spring steel for vehicle suspension has the following chemical composition by mass percentage: C: 0.48–0.56%, Si: 0.35–0.80%, Mn: 0.65–1.05%, Cr: 1.05–1.45%, Mo: 0.20–0.35%, V: 0.06–0.14%, Nb: 0.018–0.030%, B: 0.0015–0.0030%, Al: 0.020–0.035%, P: ≤0.02%, S: ≤0.01%, N: ≤0.008%, with the balance being Fe and unavoidable impurities; Among them, the chemical composition and process parameters of the material are preferably matched to the following relationship so that the material has good tempering stability and surface strengthening adaptability on the basis of high strength: (1) 0.15≤(V+Nb) / C≤0.25; (2) 1.5≤Mo / V≤3.2; (3) Under the conditions of controlling N≤0.008%, Al is 0.020~0.035% and low oxygen control in the refining stage, the effective utilization of B element is improved to reduce the risk of B being fixed by nitrogen and failing, and to enhance its effectiveness in improving hardenability at the austenite grain boundary; Among them, matching relationship (1) is used to characterize the degree of matching between the total addition level of microalloying elements and the carbon content of the matrix, so as to coordinate the balance between microstructure refinement, precipitation strengthening and toughness maintenance; matching relationship (2) is used to characterize the synergistic relationship between Mo and V in improving tempering stability, precipitation strengthening ability and microstructure stability. As for the element B, its effective utilization is mainly ensured by controlling the N and O content in the steel and combining it with the nitrogen fixation effect of Al, so as to reduce the risk of weakening the hardenability improvement effect due to B being fixed. Through the above synergistic control of components, it is beneficial to improve the stability of the matrix structure and the adaptability of subsequent surface strengthening treatment, thereby improving the comprehensive service performance of vehicle suspension spring steel.

[0007] The high-strength spring steel for vehicle suspension described in this invention does not employ a design approach that simply increases carbon content or hardness. Instead, it comprehensively regulates the material's strength base, tempering stability, microstructure refinement capability, and adaptability to subsequent surface strengthening through the synergistic combination of elements such as C, Cr, Mo, V, Nb, and B. Specifically, C provides a high strength and elasticity base; Cr improves the steel's hardenability and enhances its resistance to softening during tempering; Mo helps improve the tempering stability of the matrix and reduces the tendency for rapid microstructure recovery during service; V and Nb primarily improve the stability of the matrix structure by refining the microstructure, inhibiting austenite grain coarsening, and forming dispersed precipitates; and B, under low oxygen, low nitrogen conditions and with Al nitrogen fixation control, helps improve the steel's hardenability, thus avoiding excessive reliance on traditional high-silicon strengthening pathways. Through the synergistic design of these components, the steel, after quenching, tempering, and surface strengthening treatment, can maintain a high strength level while achieving good microstructure stability and surface strengthening response capability, thereby improving the fatigue crack resistance and service stability of vehicle suspension spring steel.

[0008] Further preferred, the chemical composition (by mass percentage) of the high-strength spring steel for vehicle suspension is as follows: C: 0.50–0.55%, Si: 0.45–0.75%, Mn: 0.70–1.00%, Cr: 1.10–1.40%, Mo: 0.22–0.30%, V: 0.08–0.10%, Nb: 0.020–0.025%, B: 0.0018–0.0025%, Al: 0.023–0.030%, P: ≤0.015%, S: ≤0.005%, N: ≤0.006%, with the balance being Fe and unavoidable impurities.

[0009] Based on the technical requirements for material composition and corresponding parameters mentioned above, another aspect of the present invention provides a method for preparing high-strength spring steel for vehicle suspension, specifically including the following steps: S1: Smelting and Refining; Pure iron, scrap steel, carbon raiser, ferrosilicon, ferromanganese, ferrochrome, and ferromolybdenum are weighed according to the formula as basic furnace charge and smelted in an electric furnace; the molten steel obtained from smelting is then subjected to LF refining; in the later stage of LF refining, aluminum wire is first added for deoxidation treatment, and then ferrovanadium, ferroniobium, and ferroboron are added for micro-alloying adjustment; subsequently, VD vacuum degassing treatment is performed; and finally, ingot casting is carried out to obtain steel billets; S2: Hot heating and hot rolling of steel billets; the steel billets obtained in step S1 are heated to 1130-1170℃ and held at that temperature for 1.5-2.5 hours for hot heating treatment; then hot rolling is carried out to produce flat steel billets for springs. After hot processing, the billets are air-cooled to room temperature to obtain the billets. S3: Spheroidizing annealing treatment; The billet obtained in step S2 is subjected to spheroidizing annealing treatment. The annealing temperature is controlled at 730-750℃ and held for 2-4 hours. Then, it is cooled in the furnace to below 520℃ and air-cooled. This step improves the uniformity of the billet's early structure, reduces the structural fluctuations during subsequent processing and heat treatment, and provides relatively stable initial structural conditions for subsequent austenitization and quenching treatment. S4: Steel pre-processing and austenitization; The billet after spheroidizing annealing in step S3 is surface cleaned, straightened and machined to obtain spring steel to be heat-treated; The spring steel to be heat-treated is then heated to 870-900℃ in a protective atmosphere for austenitization treatment and held for 20-40 minutes to fully austenitize the steel as a whole, which helps to suppress excessive growth of austenite grains and provides conditions for obtaining a more uniform strengthened structure in subsequent quenching; S5: Quenching treatment; The steel after the austenitization treatment in step S4 is immediately subjected to oil quenching treatment; The temperature of the quenching medium is controlled at 50-70℃; After quenching until the temperature of the steel drops to 90-120℃, it is taken out and then air-cooled to room temperature to obtain a high-strength quenched structure. S6: Dual-temperature zone tempering treatment; The steel after oil quenching in step S5 is subjected to dual-temperature zone tempering treatment to improve the stability of the matrix structure and improve the compatibility of subsequent surface strengthening. S7: Warm shot peening; Preheat the steel after the tempering treatment in step S6 to 210-240℃ and hold for 10-20 minutes; then perform warm shot peening, with steel shot as the peening medium; introduce a favorable residual compressive stress layer on the surface of the steel through warm shot peening, while controlling the increase in surface damage and roughness. S8: Short-term stabilization treatment after shot peening; The steel treated by shot peening in step S7 is kept at 180-210℃ for 25-35 minutes, and then air-cooled; Super-strong spring steel for vehicle suspension with surface strengthening treatment is obtained. In step S1, element B is preferably added in the later stage of LF refining. Before addition, aluminum wire is preferably added for deoxidation treatment to reduce the oxygen content in the molten steel, controlling the dissolved oxygen to ≤0.002%, and giving full play to the nitrogen-fixing effect of Al to reduce the free nitrogen content in the steel. After the addition of element B, inert gas stirring can be used to promote uniform composition, and the risk of oxidation and burn-off can be reduced by protective covering of the molten steel surface. Through the above control, it is beneficial to improve the actual yield of element B and the stability of composition control. Preferably, in step S2, during hot rolling, the final rolling temperature is controlled at 880–920°C; Preferably, in step S4, the cross-sectional dimensions of the spring steel to be heat-treated conform to the conventional specifications for spring steel used in vehicle suspension systems; the conventional specifications may include a thickness of 8–25 mm and a width of 40–120 mm. Preferably, in step S5, the oil quenching treatment uses rapid quenching oil.

[0010] Preferably, in step S6, the tempering process is specifically performed as follows: first tempering is performed, the temperature of the first tempering is controlled at 250-280℃, and the holding time is 60-80 minutes; then the furnace is removed and air-cooled to room temperature; then a second tempering is performed, the temperature of the second tempering is controlled at 450-480℃, and the holding time is 90-130 minutes. In step S6, the first tempering is mainly used to pre-stabilize the high-defect-density microstructure after quenching and reduce local instability factors within the microstructure. The second tempering further regulates the microstructure state, enabling the steel to maintain a high strength level while achieving better tempering stability and adaptability to subsequent surface strengthening. Thus, dual-temperature zone tempering is not simply a repetition of the tempering steps, but rather a phased regulation that makes the microstructure state more balanced and stable, which is more conducive to the introduction of beneficial residual compressive stress in the surface layer by subsequent shot peening and reduces its tendency to recover quickly. Preferably, in step S7, the steel shot particle size is 0.45–0.55 mm; the Alman strength of the warm shot peening treatment is 0.28–0.32 mmA, the coverage is controlled at 280–320%, and the warm shot peening time is 70–100 s. On the relatively stable matrix formed after dual-temperature tempering, warm shot peening helps improve the coordination of surface plastic deformation and introduces a beneficial residual compressive stress layer on the steel surface. Simultaneously, by matching and controlling the steel shot particle size, shot peening intensity, coverage, and shot peening time, both surface strengthening effect and surface damage control can be considered, thereby improving the effectiveness of the surface strengthening state. Step S8 is not aimed at significantly reducing residual compressive stress after shot peening, but rather at moderately adjusting the most unstable local plastic deformation state of the surface after shot peening under controlled temperature and time conditions, so as to reduce its tendency to recover rapidly in the early stage of subsequent service, thereby helping to maintain the stable state of favorable stress on the surface. By controlling the stabilization treatment temperature and holding time within the above range, the stability of the steel during service can be improved while taking into account the surface strengthening effect.

[0011] The beneficial effects of this invention are as follows: In its composition design, this invention employs a synergistic combination of C, Cr, Mo, V, Nb, and B. C provides a foundation for high strength and elastic limit; Cr enhances the hardenability and tempering resistance of the steel; Mo improves the tempering stability of the matrix; V and Nb refine the microstructure and enhance precipitation strengthening; and B, under low oxygen, low nitrogen conditions and with Al nitrogen fixation control, is beneficial for improving the hardenability of the steel. Through the synergistic effect of these elements, the material can achieve high strength and good microstructure stability without relying on traditional high-silicon strengthening pathways, and provides a relatively stable matrix foundation for subsequent heat treatment and surface strengthening processes.

[0012] In terms of process design, this invention adopts a continuous process route including smelting and refining, hot rolling, spheroidizing annealing, austenitizing quenching, dual-temperature tempering, warm shot peening, and short-term stabilization treatment after shot peening. Specifically, by combining LF refining with VD vacuum degassing to control the phosphorus, sulfur, nitrogen, and total oxygen content in the molten steel, it is beneficial to reduce harmful inclusions and gas content in the steel, thereby reducing fatigue crack initiation sites. The matched control of hot rolling temperature, austenitizing temperature, and quenching conditions helps to obtain a more uniform quench-strengthened microstructure. Spheroidizing annealing improves the uniformity of the early-stage microstructure, providing more stable initial microstructure conditions for subsequent heat treatment.

[0013] Furthermore, the dual-temperature zone tempering in this invention is not merely a conventional stress reduction and toughness improvement step, but rather a step that controls the microstructure after quenching in stages. This approach maintains high strength while improving the stability of the matrix structure and the compatibility with subsequent shot peening, making the material more suitable for bearing the surface plastic deformation and favorable residual compressive stress state introduced by subsequent warm shot peening.

[0014] Furthermore, this invention employs a combined process of warm shot peening and short-term stabilization treatment after shot peening. On a relatively stable matrix, warm shot peening introduces a favorable residual compressive stress layer on the surface, followed by short-term stabilization treatment to reduce the rapid recovery tendency of the most unstable plastic deformation structure on the surface. This helps maintain the relative stability of the surface-strengthened state during the early stages of service. Compared to spring steel strengthened only by conventional shot peening, this invention places greater emphasis on the formation quality and retention capability of the surface-strengthened state. Therefore, it helps suppress the preferential initiation of fatigue cracks on the surface and delay crack propagation, improving the fatigue crack resistance and service stability of spring steel used in vehicle suspensions under cyclic loading conditions.

[0015] This invention achieves a comprehensive improvement in material strength, microstructure stability, and fatigue service performance by synergistically matching component design with the process route, ensuring that each step serves the unified goal of constructing a high-strength matrix, enhancing shot peening response, and maintaining a stable surface stress state. Therefore, the ultra-strong spring steel for vehicle suspension produced by this invention possesses high strength, good microstructure stability, excellent shot peening compatibility, and superior resistance to fatigue cracking, making it suitable for vehicle suspension elastic components requiring high loads and long service lives. Attached Figure Description

[0016] Figure 1 The residual stress depth distribution diagrams are shown for the spring steels of Example 1 and Comparative Examples 4-6; Figure 2 This is a comparison chart of the residual stress retention rate of spring steel in Example 1 and Comparative Examples 5-6 under different heat exposure conditions; Figure 3 The XRD diffraction patterns of spring steel in Example 1 and Comparative Examples 4 and 6 are shown. Figure 4 The XRD diffraction patterns of spring steel in Example 1 and Comparative Examples 5-6 before and after heat exposure are shown. Detailed Implementation

[0017] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Unless otherwise specified, the raw materials involved in the present invention are all purchased through conventional commercial channels. Experimental methods without specific conditions are conventional methods and conditions well known in the art, or according to the conditions recommended by the instrument manufacturer. Example 1:

[0018] A method for preparing high-strength spring steel for vehicle suspension specifically includes the following steps: S1: Smelting and Refining; Weigh each raw material component according to the formula, with pure iron, scrap steel, carbon raiser, ferrosilicon, ferromanganese, ferrochrome, and ferromolybdenum serving as the basic furnace charge, and smelt using an electric furnace; The molten steel obtained from smelting is then subjected to LF refining; In the later stage of LF refining, aluminum wire is added for deoxidation treatment, controlling the dissolved oxygen in the molten steel to ≤0.002%, followed by the addition of ferrovanadium, ferroniobium, and ferroboron for microalloying adjustment; After the addition of ferroboron, argon gas is used for stirring to promote uniform composition, and a protective agent is applied to the surface of the molten steel to reduce the risk of oxidation and burn-off; Subsequently, VD vacuum degassing treatment is performed to further reduce the gas content and inclusion content in the steel; Finally, ingot casting is performed to obtain steel billets; S2: Steel billet homogenization and hot rolling; The steel billet obtained in step S1 is heated to 1150℃ and held at that temperature for 2 hours for homogenization treatment; then hot rolling is carried out, and the final rolling temperature is controlled at 900℃ to produce flat steel billet for springs. After hot processing, it is air-cooled to room temperature to obtain the billet. S3: Spheroidizing annealing treatment; The billet obtained in step S2 is subjected to spheroidizing annealing treatment. The annealing temperature is controlled at 740℃ and held for 3 hours. Then, it is cooled in the furnace to below 520℃ and then air-cooled. S4: Steel pre-processing and austenitization; The billet after spheroidizing annealing in step S3 is surface cleaned, straightened and machined to a thickness of 15mm and a width of 80mm to obtain spring steel to be heat-treated; The spring steel to be heat-treated is then heated to 885℃ in an argon atmosphere for austenitization treatment and held for 30min. S5: Quenching treatment; The steel after the austenitization treatment in step S4 is immediately subjected to oil quenching treatment with rapid quenching oil; The temperature of the quenching medium is controlled at 60℃; After quenching until the temperature of the steel drops to 105℃, it is taken out and then air-cooled to room temperature to obtain a high-strength quenched structure. S6: Dual-temperature zone tempering treatment; The steel after oil quenching in step S5 is subjected to dual-temperature zone tempering treatment. First, the first tempering is performed at a temperature of 265℃ and held for 70 minutes. Then, it is taken out of the furnace and air-cooled to room temperature. Then, the second tempering is performed at a temperature of 465℃ and held for 110 minutes. S7: Warm shot peening treatment; Preheat the steel after tempering in step S6 to 225℃ and hold for 15 minutes; then perform warm shot peening treatment, the shot peening medium is steel shot with a particle size of 0.5mm; the warm shot peening Alman strength is controlled at 0.3mmA, the coverage is controlled at 300%, and the warm shot peening time is controlled at 85s. S8: Short-term stabilization treatment after shot peening; The steel after shot peening in step S7 is placed at 195℃ for 30 minutes and then air-cooled to obtain ultra-strong spring steel for vehicle suspension with surface strengthening treatment. Example 2:

[0019] A method for preparing high-strength spring steel for vehicle suspension specifically includes the following steps: S1: Smelting and Refining; Weigh each raw material component according to the formula, with pure iron, scrap steel, carbon raiser, ferrosilicon, ferromanganese, ferrochrome, and ferromolybdenum serving as the basic furnace charge, and smelt using an electric furnace; The molten steel obtained from smelting is then subjected to LF refining; In the later stage of LF refining, aluminum wire is added for deoxidation treatment, controlling the dissolved oxygen in the molten steel to ≤0.002%, followed by the addition of ferrovanadium, ferroniobium, and ferroboron for microalloying adjustment; After the addition of ferroboron, argon gas is used for stirring to promote uniform composition, and a protective agent is applied to the surface of the molten steel to reduce the risk of oxidation and burn-off; Subsequently, VD vacuum degassing treatment is performed to further reduce the gas content and inclusion content in the steel; Finally, ingot casting is performed to obtain steel billets; S2: Hot heating and hot rolling of steel billet; The steel billet obtained in step S1 is heated to 1130℃ and held at that temperature for 2.5h for hot heating treatment; then hot rolling is carried out, and the final rolling temperature is controlled at 920℃ to produce flat steel billet for springs. After hot processing, it is air cooled to room temperature to obtain the billet. S3: Spheroidizing annealing treatment; The billet obtained in step S2 is subjected to spheroidizing annealing treatment. The annealing temperature is controlled at 730℃ and held for 4 hours. Then, it is cooled in the furnace to below 520℃ and then air-cooled. S4: Steel pre-processing and austenitization; The billet after spheroidizing annealing in step S3 is surface cleaned, straightened and machined to a thickness of 8mm and a width of 40mm to obtain spring steel to be heat-treated; The spring steel to be heat-treated is then heated to 870℃ in an argon atmosphere for austenitization treatment and held for 40min. S5: Quenching treatment; The steel after the austenitization treatment in step S4 is immediately subjected to oil quenching treatment with rapid quenching oil; The temperature of the quenching medium is controlled at 50℃; After quenching until the temperature of the steel drops to 120℃, it is taken out and then air-cooled to room temperature to obtain a high-strength quenched structure. S6: Dual-temperature zone tempering treatment; The steel after oil quenching in step S5 is subjected to dual-temperature zone tempering treatment. First, the first tempering is performed at a temperature of 250℃ and held for 80 minutes. Then, it is taken out of the furnace and air-cooled to room temperature. Then, the second tempering is performed at a temperature of 450℃ and held for 130 minutes. S7: Warm shot peening treatment; Preheat the steel after tempering in step S6 to 210℃ and hold for 20 minutes; then perform warm shot peening treatment, the shot peening medium is steel shot with a particle size of 0.45mm; the warm shot peening Alman strength is controlled at 0.28mmA, the coverage is controlled at 320%, and the warm shot peening time is controlled at 100s. S8: Short-term stabilization treatment after shot peening; The steel after shot peening in step S7 is placed at 180℃ for 35 minutes and then air-cooled to obtain ultra-strong spring steel for vehicle suspension with surface strengthening treatment. Example 3:

[0020] A method for preparing high-strength spring steel for vehicle suspension specifically includes the following steps: S1: Smelting and Refining; Weigh each raw material component according to the formula, with pure iron, scrap steel, carbon raiser, ferrosilicon, ferromanganese, ferrochrome, and ferromolybdenum serving as the basic furnace charge, and smelt using an electric furnace; The molten steel obtained from smelting is then subjected to LF refining; In the later stage of LF refining, aluminum wire is added for deoxidation treatment, controlling the dissolved oxygen in the molten steel to ≤0.002%, followed by the addition of ferrovanadium, ferroniobium, and ferroboron for microalloying adjustment; After the addition of ferroboron, argon gas is used for stirring to promote uniform composition, and a protective agent is applied to the surface of the molten steel to reduce the risk of oxidation and burn-off; Subsequently, VD vacuum degassing treatment is performed to further reduce the gas content and inclusion content in the steel; Finally, ingot casting is performed to obtain steel billets; S2: Hot heating and hot rolling of steel billet; The steel billet obtained in step S1 is heated to 1170℃ and held at that temperature for 1.5h for hot heating treatment; then hot rolling is carried out, and the final rolling temperature is controlled at 880℃ to produce flat steel billet for springs. After hot processing, it is air cooled to room temperature to obtain the billet. S3: Spheroidizing annealing treatment; The billet obtained in step S2 is subjected to spheroidizing annealing treatment. The annealing temperature is controlled at 750℃ and held for 2 hours. Then, it is cooled in the furnace to below 520℃ and then air-cooled. S4: Steel pre-processing and austenitization; The billet after spheroidizing annealing in step S3 is surface cleaned, straightened and machined to a thickness of 25mm and a width of 120mm to obtain spring steel to be heat-treated; The spring steel to be heat-treated is then heated to 900℃ in an argon atmosphere for austenitization treatment and held for 20min. S5: Quenching treatment; The steel after the austenitization treatment in step S4 is immediately subjected to oil quenching treatment with rapid quenching oil; The temperature of the quenching medium is controlled at 70℃; After quenching until the temperature of the steel drops to 90℃, it is taken out and then air-cooled to room temperature to obtain a high-strength quenched structure. S6: Dual-temperature zone tempering treatment; The steel after oil quenching in step S5 is subjected to dual-temperature zone tempering treatment. First, the first tempering is performed at a temperature of 280℃ and held for 60 minutes. Then, it is taken out of the furnace and air-cooled to room temperature. Then, the second tempering is performed at a temperature of 480℃ and held for 90 minutes. S7: Warm shot peening treatment; Preheat the steel after the tempering treatment in step S6 to 240℃ and hold for 10 minutes; then perform warm shot peening treatment, the shot peening medium is steel shot with a particle size of 0.55mm; the warm shot peening Alman strength is controlled at 0.32mmA, the coverage is controlled at 280%, and the warm shot peening time is controlled at 70s. S8: Short-term stabilization treatment after shot peening; The steel after shot peening in step S7 is placed at 210℃ for 25 minutes and then air-cooled; Super-strong spring steel for vehicle suspension with surface strengthening treatment is obtained. Example 4:

[0021] The process flow of Example 4 is consistent with that of Example 1, the main difference being the difference in raw material composition; in particular, Example 4 uses a combination of relatively low C, Si, Mn, Cr, Mo, V, Nb and B content to examine the effect of maintaining high strength and better comprehensive performance of spring steel under relatively low alloying conditions within the composition range of the present invention, in conjunction with the complete process route of the present invention. Example 5:

[0022] The process flow of Example 5 is consistent with that of Example 1, the main difference being the difference in the raw material composition; in particular, Example 5 uses a relatively high combination of C, Si, Cr, Mo, V and B content to investigate the variation law of spring steel strength base, tempering stability and surface strengthening response capability under the pre-strengthening design conditions within the composition range of the present invention. Example 6:

[0023] The process flow of Example 6 is consistent with that of Example 1, the main difference being the difference in raw material composition; in particular, Example 6 uses a composition design with higher levels of Mn, Cr and Si and moderate levels of Mo, V, Nb and B to investigate the changes in the structural stability, surface strengthening adaptability and comprehensive service performance of spring steel under different main alloy element matching methods within the scope of this invention.

[0024] Comparative Example 1: The raw material composition and process flow of Comparative Example 1 are basically the same as those of Example 1. The main difference is that the Nb content is increased in Comparative Example 1 so that (V+Nb) / C>0.25. This is used to investigate the effect of the total amount of microalloying elements added on the uniformity of material structure, precipitation state, compatibility of subsequent surface strengthening and comprehensive performance.

[0025] Comparative Example 2: The raw material composition and process flow of Comparative Example 2 are basically the same as those of Example 1. The main difference is that the Mo content is reduced in Comparative Example 2 so that Mo / V < 1.5. This is used to examine the effect of the deviation of the synergistic matching relationship between Mo and V on the tempering stability, surface strengthening stability and comprehensive performance of the material.

[0026] Comparative Example 3: The raw material composition and process flow of Comparative Example 3 are basically the same as those of Example 1. The main difference is that only the B content is reduced in Comparative Example 3, which weakens the effective utilization of B under the condition that the other conditions are basically the same. It is used to examine the effect of the effect of B on hardenability, matrix microstructure and comprehensive performance of the material when the effect of B on improving hardenability is weakened.

[0027] Comparative Example 4: The raw material composition and process flow of Comparative Example 4 are basically the same as those of Example 1. The main difference is that S6 in Comparative Example 4 is replaced by a single conventional tempering, the tempering temperature is 460℃, the holding time is 120min, and the product is air-cooled to room temperature after tempering. Subsequently, warm shot peening and short-term stabilization are still performed. This is used to investigate the effect of dual-temperature zone tempering on the tempering stability of the matrix, shot peening strengthening adaptability and fatigue performance.

[0028] Comparative Example 5: The raw material composition and process flow of Comparative Example 5 are basically the same as those of Example 1. The main difference is that the short-term stabilization treatment after shot peening was omitted in Comparative Example 5. This was used to investigate the effect of the short-term stabilization treatment after shot peening on the stability of the surface strengthening state and the overall fatigue performance.

[0029] Comparative Example 6: The raw material composition and process flow of Comparative Example 6 are basically the same as those of Example 1. The main difference is that in Comparative Example 6, step S7 adopts conventional room temperature shot peening treatment without preheating and heat preservation before shot peening. The other shot peening media, steel shot particle size, Alman strength, coverage and shot peening time are the same as those of Example 1. It is used to investigate the effect of warm shot peening treatment on the surface plastic deformation coordination, surface strengthening state and comprehensive fatigue performance compared with conventional room temperature shot peening treatment.

[0030] Performance testing: The chemical composition of the super-strong spring steel in Examples 1-6 and Comparative Examples 1-6 was tested according to GB / T 223, and the test results are shown in Tables 1 and 2 below; tensile properties were tested according to GB / T 228.1-2021; surface residual stress was tested according to GB / T 7704-2017; hardness was tested according to GB / T 230.1-2018; impact toughness was tested according to GB / T 229-2020; fatigue properties were tested according to GB / T 3075-2021; and other standards were applied. The total decarburized layer depth was tested in 224-2019. The test results are shown in Table 3 below (each group's performance test used 5 parallel samples, and the results were averaged; the impact absorption energy data used the average results obtained from the same batch of standard samples). Except for chemical composition, the tensile properties, hardness, impact toughness, fatigue properties, and total decarburized layer depth of each group of samples were tested based on the final state spring steel prepared according to their respective embodiments or comparative processes. Among them, the surface residual stress, residual stress depth distribution, residual stress retention rate after heat exposure, and XRD analysis were all tested on the surface of the final state spring steel of each group. The residual stress depth distribution is shown in Table 3 below. Figure 1 As shown; the residual stress retention rate after heat exposure is as follows: Figure 2 As shown. Considering that spring steel for vehicle suspension may be subjected to the combined effects of braking heat conduction, environmental heat accumulation, and long-term cyclic loads under complex working conditions, this invention selects 150℃×10h and 200℃×2h as comparative heat exposure conditions to characterize the stability difference of the material's surface reinforcement state under medium and relatively high temperature conditions.

[0031] As shown in Tables 1-3, the embodiments of the present invention, through synergistic matching and control of key elements such as V, Nb, Mo, and B, combined with dual-temperature tempering, warm shot peening, and short-term stabilization treatment after shot peening, enable the material to achieve higher surface residual compressive stress, smaller total decarburized layer depth, and better fatigue performance while maintaining high tensile strength, yield strength, and hardness levels. This indicates that the present invention does not simply rely on increasing hardness to achieve strengthening, but rather improves the stability of the matrix structure, the surface strengthening state, and its retention ability through synergistic matching of composition design and process route, thereby achieving comprehensive optimization of strength, toughness, and fatigue service performance.

[0032] As can be further seen from Table 3, the ultra-strong spring steels prepared in Examples 1 to 6 exhibit higher tensile strength, yield strength, residual compressive stress, hardness, and fatigue limit, while maintaining good impact toughness. Moreover, their overall performance is superior to that of the comparative examples. This indicates that by rationally controlling the component matching relationship and combining dual-temperature tempering, warm shot peening, and short-term stabilization treatment after shot peening, the present invention can further improve the matrix stability and surface strengthening state on the basis of high strength, thereby obtaining better comprehensive performance and fatigue resistance.

[0033] The tensile strength, yield strength, surface residual compressive stress, impact toughness, and fatigue performance of the material in Comparative Example 1 were all lower than those in Example 1, indicating that the total amount of microalloying elements added and the carbon content of the matrix are not necessarily better the higher they are. When (V+Nb) / C exceeds a reasonable range, it is easy to weaken the coordinated balance between precipitation strengthening, microstructure uniformity, and plasticity and toughness, which reduces the coordination ability of local deformation of the matrix and reduces the effectiveness and stability of subsequent surface strengthening to form a favorable stress state, thereby leading to a decline in overall performance.

[0034] The tensile strength, yield strength, surface residual compressive stress, hardness, and fatigue performance of the material in Comparative Example 2 all decreased significantly. This indicates that after the synergistic relationship between Mo and V was weakened, the matrix tempering stability was insufficient, the microstructure stability decreased after heat treatment, and the material's ability to support the favorable residual compressive stress state on the surface was weakened during subsequent surface strengthening treatment and service, thus adversely affecting the overall performance and fatigue resistance.

[0035] The strength, hardness, surface residual compressive stress, and fatigue performance of the material in Comparative Example 3 were all lower than those in Example 1. This indicates that the effective utilization of boron decreased after the B content was reduced, which is not conducive to fully exerting its role in improving hardenability and microstructure uniformity. As a result, the degree of matrix strengthening and microstructure integrity decreased after heat treatment, which in turn affected the overall performance and fatigue performance of the material.

[0036] Although the material in Comparative Example 4 still maintains a certain strength, its yield strength, surface residual compressive stress, impact toughness, and fatigue performance are all lower than those in Example 1. This indicates that dual-temperature tempering in this invention is not a simple conventional tempering step, but rather a combination of low-temperature pre-stabilization and further control at medium and high temperatures, enabling the matrix to achieve better microstructural stability and surface strengthening compatibility while maintaining high strength. When it is replaced by single tempering, the matrix's ability to bear and maintain the subsequent surface strengthening state decreases, resulting in a reduction in overall performance and fatigue resistance.

[0037] In Comparative Example 5, the tensile strength, yield strength, and hardness changed relatively little compared to Example 1, but the residual compressive stress and fatigue performance of the surface layer were still lower than those of Example 1. This indicates that the short-term stabilization treatment after shot peening has a limited direct impact on the static strength of the material. Its main function is to moderately adjust the relatively unstable plastic deformation state of the surface layer after shot peening, reduce its tendency to recover rapidly in the early stage of subsequent service, thereby improving the stability and retention of the favorable residual compressive stress state of the surface layer. After canceling this step, although the material can still maintain a certain level of static strength, the surface strengthening state is more likely to decay during subsequent service, which leads to a decrease in fatigue resistance.

[0038] The surface residual compressive stress, fatigue performance, and overall performance of the material in Comparative Example 6 were all lower than those in Example 1. This indicates that warm shot peening is more conducive to forming a surface plastic deformation layer with higher quality and more coordinated distribution and a favorable residual compressive stress layer under a more suitable material condition. Although room temperature shot peening can also introduce surface compressive stress, the surface strengthening state it forms is relatively weak in terms of coordination and stability. Therefore, both the overall performance and fatigue performance are reduced.

[0039] Depend on Figure 1 The results show that Example 1 formed a higher residual compressive stress peak and a deeper compressive stress influence layer in the surface and near-surface regions, while the residual compressive stress peak and compressive stress layer depth of Comparative Examples 4, 5 and 6 were all lower than those of Example 1. This indicates that the present invention, through the synergistic matching of composition design with dual-temperature tempering, warm shot peening and short-term stabilization treatment after shot peening, is more conducive to forming a higher quality and more coordinated favorable residual compressive stress state on the surface of spring steel. In Comparative Example 4, after single tempering replaced dual-temperature tempering, the stability of the matrix structure and the coordination of local deformation decreased. The formation quality and load-bearing capacity of the plastic deformation layer on the surface after shot peening were weakened, resulting in lower peak compressive stress and layer depth than in Example 1. In Comparative Example 6, the plastic flow capacity of the material surface was relatively weak under room temperature shot peening conditions, and the surface deformation was more likely to concentrate locally, which was not conducive to the formation of a more coordinated and deeper compressive stress layer, resulting in a lower residual compressive stress distribution than in Example 1. Although Comparative Example 5 could still form a certain level of initial surface compressive stress, due to the lack of short-term stabilization treatment after shot peening, some unstable dislocation entanglements and local high strain zones in the surface were not properly adjusted, making stress recovery more likely to occur later.

[0040] Depend on Figure 2The results show that under heat exposure conditions of 150℃×10h and 200℃×2h, the residual compressive stress retention rate of Example 1 was consistently higher than that of Comparative Examples 5 and 6, indicating that the favorable residual compressive stress state of the surface layer formed by the present invention not only has a higher initial level but also better thermal stability. This is because the short-term stabilization treatment after shot peening is not simply a release of compressive stress, but rather, under controlled temperature and time conditions, it causes limited adjustment to the relatively unstable plastic deformation structure of the surface layer, reducing the tendency of high-energy defect structures to rapidly recover during subsequent heat exposure or the early stages of service, thereby improving the stability retention capability of the compressive stress layer. Therefore, although the initial residual compressive stress of Comparative Example 5 remained at a certain level after this step was omitted, it decayed faster after heat exposure; although the retention rate of Comparative Example 6 was higher than that of Comparative Example 5, it was still lower than that of Example 1, indicating that the surface plastic deformation layer formed by warm shot peening is superior to that formed by room temperature shot peening in terms of coordination and stability, and is more conducive to maintaining a favorable residual compressive stress state.

[0041] XRD analysis was performed on the surface-strengthened spring steel surfaces of Examples 1 and Comparative Examples 4 and 6 using an XRD spectrometer. Simultaneously, the peak shape changes of the surface-strengthened spring steel surfaces of Examples 1 and Comparative Examples 5 and 6 before and after heat exposure were compared and tested. The XRD diffraction patterns are shown below. Figure 3 and Figure 4 As shown.

[0042] Depend on Figure 3 It can be seen that the XRD diffraction patterns of Example 1, Comparative Example 4, and Comparative Example 6 are dominated by α-Fe diffraction peaks, indicating that the overall phase composition of each group of samples has not changed substantially, but there are significant differences in peak shape and peak width. Among them, the diffraction peaks of Example 1 are more significantly broadened, and the peak positions show a slight low-angle shift, indicating that its surface layer has a higher level of micro-strain and lattice distortion, which is consistent with the higher surface residual compressive stress of Example 1. In contrast, the peak width of Comparative Example 4 is slightly reduced under single tempering conditions, indicating that its matrix structure stability and surface deformation coordination are lower than those of Example 1; Comparative Example 6, due to room temperature shot peening, has sharper diffraction peaks and weaker broadening, indicating that the degree of surface plastic deformation and micro-strain accumulation is lower than that of Example 1.

[0043] Depend on Figure 4It can be seen that after heat exposure at 150℃ for 10 hours and 200℃ for 2 hours, the diffraction peak shape of Example 1 changed little, and the peak width only narrowed slightly, indicating that the surface-strengthened state formed by the present invention has good stability under heat exposure conditions. This is because the present invention, through the matching control of composition design with dual-temperature tempering, warm shot peening, and short-term stabilization treatment after shot peening, is beneficial to stabilizing the high-energy defect structure on the surface and suppressing its rapid recovery under thermal activation conditions. In contrast, Comparative Examples 5 and 6 both showed more obvious peak narrowing and peak intensity recovery after heat exposure, with Comparative Example 5 showing the most significant change, indicating that it is more prone to dislocation rearrangement, micro-strain release, and recovery of the surface-strengthened state, thus resulting in faster decay of residual compressive stress.

[0044] In conclusion, Figure 3 and Figure 4 This invention not only facilitates the formation of a high level of beneficial residual compressive stress in the surface layer in the initial state, but also maintains a relatively stable surface layer strengthening state under thermal exposure conditions. Its essence lies in improving the stability of the matrix structure, the coordination of surface plastic deformation, and the ability to maintain the stability of high-energy defect structures.

[0045] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A type of high-strength spring steel for vehicle suspension, characterized in that, Its chemical composition by mass percentage is as follows: C: 0.48–0.56%, Si: 0.35–0.80%, Mn: 0.65–1.05%, Cr: 1.05–1.45%, Mo: 0.20–0.35%, V: 0.06–0.14%, Nb: 0.018–0.030%, B: 0.0015–0.0030%, Al: 0.020–0.035%, P: ≤0.02%, S: ≤0.01%, N: ≤0.008%, with the balance being Fe and unavoidable impurities.

2. The high-strength spring steel for vehicle suspension according to claim 1, characterized in that, Its chemical composition by mass percentage is as follows: C: 0.50–0.55%, Si: 0.45–0.75%, Mn: 0.70–1.00%, Cr: 1.10–1.40%, Mo: 0.22–0.30%, V: 0.08–0.10%, Nb: 0.020–0.025%, B: 0.0018–0.0025%, Al: 0.023–0.030%, P: ≤0.015%, S: ≤0.005%, N: ≤0.006%, with the balance being Fe and unavoidable impurities.

3. The high-strength spring steel for vehicle suspension according to claim 1, characterized in that, The following matching relationships are satisfied between V, Nb, C, and Mo: 0.15≤(V+Nb) / C≤0.25, 1.5≤Mo / V≤3.

2.

4. A method for preparing high-strength spring steel for vehicle suspension according to any one of claims 1 to 3, characterized in that, Specifically, the following steps are included: S1: Smelting and refining; according to the formula, pure iron, scrap steel, carbon raiser, ferrosilicon, ferromanganese, ferrochrome and ferromolybdenum are weighed as basic furnace charge and smelted in an electric furnace; the molten steel obtained from smelting is then refined by LF; In the later stage of LF refining, aluminum wire is first added for deoxidation treatment, followed by the addition of ferrovanadium, ferroniobium and ferroboron for microalloying adjustment; then VD vacuum degassing treatment is performed; and finally, ingot casting is carried out to obtain steel billets. S2: Steel billet homogenization and hot rolling; The steel billet obtained in step S1 is heated and kept at a constant temperature for homogenization treatment; then it is hot rolled to produce flat steel billet for springs. After hot processing, it is air-cooled to room temperature to obtain the billet. S3: Spheroidizing annealing treatment; The billet obtained in step S2 is subjected to spheroidizing annealing treatment; S4: Steel pre-processing and austenitization; The billet after spheroidizing annealing in step S3 is surface cleaned, straightened and machined to obtain spring steel to be heat-treated; The spring steel to be heat-treated is then heated in a protective atmosphere for austenitization treatment; S5: Quenching treatment; The steel after the austenitizing treatment in step S4 is subjected to oil quenching. S6: Dual-temperature zone tempering treatment; The steel after oil quenching in step S5 is subjected to dual-temperature zone tempering treatment. S7: Warm shot peening; Preheat and keep the steel after the tempering treatment in step S6 at a certain temperature; then perform warm shot peening, with steel shot as the peening medium. S8: Short-term stabilization treatment after shot peening; The steel treated by shot peening in step S7 is heated and kept at a constant temperature, and then air-cooled to obtain ultra-strong spring steel for vehicle suspension with surface strengthening treatment.

5. The method for preparing ultra-strong spring steel for vehicle suspension according to claim 4, characterized in that, In step S2, during hot rolling, the final rolling temperature is controlled at 880–920°C.

6. The method for preparing ultra-strong spring steel for vehicle suspension according to claim 4, characterized in that, In step S5, the oil quenching process uses rapid quenching oil.

7. The method for preparing high-strength spring steel for vehicle suspension according to claim 4, characterized in that, In step S6, the specific operation of the dual-temperature zone tempering process is as follows: First tempering is performed, with the temperature controlled at 250-280℃ and held for 60-80 minutes. Then, the furnace is removed and air-cooled to room temperature. Then, a second tempering is performed, with the temperature controlled at 450-480℃ and held for 90-130 minutes. Then, the furnace is removed and air-cooled to room temperature.

8. The method for preparing ultra-strong spring steel for vehicle suspension according to claim 4, characterized in that, In step S7, the steel preheating and heat preservation operation is to preheat to 210-240℃ and keep it at that temperature for 10-20 minutes.

9. The method for preparing ultra-strong spring steel for vehicle suspension according to claim 4, characterized in that, In step S7, the steel shot particle size is 0.45-0.55 mm; the Alman strength of the warm shot peening treatment is 0.28-0.32 mmA, the coverage is controlled at 280-320%, and the warm shot peening treatment time is 70-100 s.

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