Alloy steel strip material for coiled spring leaf and method of manufacturing the same

Abstract

This application relates to the technical field of steel, and in particular to an alloy steel strip material for coiled springs, its preparation method, and coiled springs. The alloy steel strip material for coiled springs comprises the following chemical composition by mass percentage: C 0.71-0.84%, Si 0.24-0.3%, Mn 0.6-0.72%, P≤0.016%, S≤0.006%, Cr 0.3-0.43%, Mo 0.001-0.006%, Ni 0.005-0.02%, Cu 0.005-0.02%, with the balance being Fe. It has the advantage of improving the comprehensive properties of steel strip materials such as strength, plasticity, service life, and hardness.

Description

Technical Field

[0001] This application relates to the technical field of steel, and in particular to an alloy steel strip material for coiled springs, a method for preparing the same, and a coiled spring. Background Technology

[0002] The main steel strip materials used for coil springs currently include high-carbon tool steel SK5 (containing 0.75-0.85wt% carbon, tensile strength ≥1050MPa, hardness 520-560HV, typical life 6000-7000 cycles) and manganese steel 65Mn (containing 0.62-0.70wt% carbon, tensile strength ≥980MPa, hardness 450-490HV, typical life 5000–6500 cycles). Both are prepared by hot rolling → pickling → cold rolling → continuous annealing → quenching + medium-temperature tempering process, and have formed standard systems such as GB / T 1222–2016 and JIS G 4801, and are widely used in low- and mid-range mechanical springs.

[0003] The main problems faced by existing technologies are: SK5 has problems such as high cost, poor hardenability, sensitivity to temper brittleness, and a high tendency to crack during winding; 65Mn has low strength and hardness, insufficient fatigue life, and poor high-temperature dimensional stability, making it difficult to meet the dual requirements of high-end equipment for finished product life greater than 8000 cycles and tensile strength greater than 1100MPa.

[0004] Therefore, there are still many challenges in solving the problem of synergistic optimization of strength, plasticity, lifespan, and hardness. Summary of the Invention

[0005] In order to provide a steel strip material with optimized strength, plasticity, service life and hardness, this application provides an alloy steel strip material for coiled springs, a method for preparing the same, and a coiled spring.

[0006] In a first aspect, this application provides an alloy steel strip material for coiled springs, employing the following technical solution: An alloy steel strip material for coiled springs comprises the following chemical composition by mass percentage: C 0.71-0.84%, Si 0.24-0.3%, Mn 0.6-0.72%, P≤0.016%, S≤0.006%, Cr 0.3-0.43%, Mo 0.001-0.006%, Ni 0.005-0.02%, Cu 0.005-0.02%, with the balance being Fe.

[0007] By adopting the above technical solution, this application controls the carbon content in the high carbon range of 0.71-0.84%, and combines it with medium and low carbon equivalents of Mn, Si and trace amounts of Cr and Mo alloying elements. Utilizing the martensitic phase transformation strengthening mechanism of hypereutectoid steel, while ensuring the hardness of the matrix, it reduces impurity segregation and inclusion levels by strictly controlling P≤0.016% and S≤0.006%. The aim is to provide an alloy steel strip material for coiled spring sheets with a high tensile strength matrix, high toughness and surface quality, enabling it to meet the basic mechanical properties required for high-end equipment with tensile strength greater than 1100MPa and high fatigue life.

[0008] Preferably, the alloy steel strip material further includes K at a mass percentage of 0-0.05% and Nb at a mass percentage of 0-0.05%, wherein the amounts of K and Nb added are not both 0.

[0009] By employing the above technical solution, potassium (K) is a strong deoxidizer and desulfurizer. In molten steel, potassium reacts with oxygen and sulfur to form high-melting-point fine sulfides or oxysulfides (K₂O, K₂S). These compounds, acting as heterogeneous nucleation sites, can "spheroidize" strip-shaped and sheet-shaped harmful MnS inclusions, transforming them into fine, dispersed spherical composite inclusions (such as K-containing Mn-SO₄ composite phases); they can also hinder the growth of inclusions during solidification, refining the inclusion size from the micrometer level to the submicrometer level. Strip-shaped MnS inclusions are the most dangerous fatigue crack initiation sites in coiled spring steel. When they are spheroidized and refined, the crack propagation path along the inclusion becomes tortuous, and the stress concentration factor at the tip of the spherical inclusion is much lower than that of the sharp-angled inclusion. This makes it less likely for cracks to initiate from surface inclusions when the finished steel strip undergoes >8000 repeated bending cycles, significantly improving fatigue life. Potassium has a large atomic radius and is surface-active, tending to segregate towards grain boundaries during solidification. It can interact with brittle elements in steel (such as P, S, Sn, and As), or by occupying grain boundary sites, it blocks the segregation channels of impurity atoms at the grain boundaries, thus purifying the grain boundaries and improving their bonding strength. Purifying grain boundaries reduces brittle phases at these sites, and spheroidized inclusions no longer disrupt the continuity of the matrix metal. This results in better plasticity and toughness in the steel strip during cold rolling and subsequent winding into coiled sheets. The micro-boiling effect of potassium during solidification also helps break up micro-shrinkage cavities between dendrites, promoting compositional homogenization and reducing micro-segregation.

[0010] As a strong carbonitride forming element, Nb precipitates nano-sized Nb(C,N) particles during hot rolling and annealing to pin grain boundaries and refine austenite grains. Without significantly increasing alloy costs, it breaks through the traditional bottleneck of matching strength and plasticity in high-carbon steel and further improves the toughness and fatigue resistance of the material through microalloying.

[0011] Preferably, the mass percentage of K is 0.01-0.05%, and the mass percentage of Nb is 0.01-0.05%.

[0012] By adopting the above technical solution, the mass percentage content of K and Nb is further optimized and limited to a narrow window range of 0.01-0.05%. This ensures that the K element can effectively modify non-metallic inclusions (making them spheroidized and refined), and also ensures that the Nb element is sufficient to form a dispersed precipitate phase without enrichment and agglomeration at the grain boundaries. This avoids the risk of insufficient addition of trace alloying elements resulting in no significant grain refinement effect, or excessive addition leading to billet cracking and reduced toughness, thus achieving the best synergy between strength and plasticity.

[0013] Secondly, this application provides a method for preparing alloy steel strip material for coiled springs, using the following technical solution: A method for preparing an alloy steel strip material for coiled spring sheets includes the following steps: S1. Prepare a continuous casting slab that conforms to the chemical composition and content of each chemical component of the alloy steel strip material. The end face specifications of the continuous casting slab are (150-250) mm × (1250-2000) mm. S2. Hot rolling: The continuously cast slab is heated to 1100-1350℃, held for 100-200 minutes, and the final rolling temperature is controlled at 840-950℃. The thickness of the steel plate after hot rolling is 1.5-3.0 mm. Pickling: Pickling hot-rolled steel plates to obtain pickled plates; Cold rolling: The pickled sheet is cold rolled five times to a thickness of 0.18-0.26 mm to obtain cold-rolled strip steel; Continuous annealing: The cold-rolled strip steel is continuously annealed at a temperature of 750-850℃ and a holding time of 140-300s. The protective atmosphere is nitrogen doped with hydrogen, with a hydrogen volume fraction of (2-4.5)% and a dew point of (-42)-(-30)℃. The belt speed is 10-15m / min. Quenching: Transfer the annealed strip to the quenching heating section, heat to 800-900℃, hold for 70-120s, and oil cool to below 15℃; Tempering: The quenched strip is tempered under nitrogen protection at a temperature of 350-500℃ for 100-150s with a dew point of ≤-40℃. After tempering, it is naturally cooled to room temperature to obtain alloy steel strip material for coiled springs.

[0014] By adopting the above technical solutions, this application constructs a complete short-process preparation process including hot rolling temperature control, pickling and descaling, multi-pass cold rolling, continuous annealing, quenching and medium-temperature tempering. In particular, recrystallization and carbide spheroidization are achieved through continuous annealing (750-850℃), combined with quenching at 800-900℃ to obtain high-saturation martensite, and nitrogen-protected tempering at 350-500℃ to control the degree of martensite decomposition. The aim is to obtain a microstructure composed of fine needle-like tempered martensite or tempered troostite, so that the steel strip has moderate yield strength and excellent elastic limit while maintaining high hardness, meeting the service requirements of repeated bending of coiled spring sheets.

[0015] As a preferred embodiment: In S1, when the alloy steel strip material contains K and Nb, K and Nb are added before tapping the continuously cast slab during preparation. The K is fed in as a potassium salt cored wire, and the Nb is added as a niobium-iron master alloy.

[0016] By adopting the above technical solutions, potassium salt cored wire feeding method is used for potassium (K) element, which is volatile and difficult to dissolve. The coating layer delays the reaction time to achieve uniform retention of K in molten steel. For niobium (Nb) element, which has a high melting point and is prone to segregation, niobium-iron master alloy is added to ensure full melting and diffusion at steelmaking temperature. This solves the problems of unstable yield and component segregation of trace active elements in the smelting process, while ensuring the high uniformity of chemical composition inside the continuously cast slab and the consistency of the final product's performance.

[0017] Preferably, in step S2, the pickling step is as follows: The hot-rolled steel plates are transferred to a pickling unit for pickling. A hydrochloric acid solution with a concentration of 150-250 g / L is used at a temperature of 60-80℃ for 100-200 seconds. After rinsing with water and drying, the residual iron salts are ≤0.72 g / m³. 2 The pickled plate was obtained.

[0018] By adopting the above technical solution, a high-concentration (150-250 g / L), medium-temperature (60-80℃) hydrochloric acid pickling process is used, and the residual iron salt is strictly controlled to be ≤0.72 g / m³. 2 By utilizing the rapid dissolution ability of hydrochloric acid on iron oxide to remove oxide scale, and simultaneously blocking the residual chloride ions through high-pressure water rinsing and drying, hot-rolled iron oxide scale can be thoroughly removed, preventing surface pitting and potential pitting corrosion sources. This provides a clean and defect-free surface substrate for subsequent cold rolling, thereby eliminating the early initiation source of fatigue cracks.

[0019] Preferably, in step S2, the reduction rates for the five cold rolling passes are 12-15%, 15-16.5%, 16.5-17.5%, 15-16%, and 13-15% respectively, and the tension for the final pass is 85-100 N / mm. 2Plate shape waviness value ≤ 8I, surface roughness Ra ≤ 0.45μm.

[0020] By adopting the above technical solution, a five-pass decreasing reduction rate distribution and 85-100 N / mm were set. 2 The high final pass tension, combined with ≤8I strip shape waviness control and ≤0.45μm surface roughness control, utilizes accumulated deformation energy storage to promote annealing recrystallization, and eliminates edge and center waviness through tension straightening, thereby obtaining cold-rolled strip steel with high thickness accuracy, flat strip shape and uniform surface texture, ensuring that the final finished steel strip has good tightness, dimensional accuracy and low residual stress when wound into a spring.

[0021] Secondly, this application provides a coiled spring sheet, which adopts the following technical solution: A coiled spring, the coiled spring being made of alloy steel strip material.

[0022] By adopting the above technical solution, the coiled spring product prepared in this application has a long service life (greater than 8000 cycles), good anti-relaxation performance, and is not prone to plastic deformation or fracture under extreme working conditions, thus solving the problem of early failure of existing coiled springs due to insufficient material performance.

[0023] In summary, this application includes at least one of the following beneficial technical effects: 1. This application controls the carbon content in the high carbon range of 0.71-0.84%, and combines it with medium and low carbon equivalents of Mn, Si and trace amounts of Cr and Mo alloying elements. Utilizing the martensitic phase transformation strengthening mechanism of hypereutectoid steel, while ensuring the hardness of the matrix, it reduces impurity segregation and inclusion levels by strictly controlling P≤0.016% and S≤0.006%. The aim is to provide an alloy steel strip material for coiled spring sheets with a high tensile strength, high toughness and surface quality, enabling it to meet the basic mechanical properties required for high-end equipment with tensile strength greater than 1100MPa and high fatigue life.

[0024] 2. Further optimization limits the mass percentage of K and Nb to a narrow window range of 0.01-0.05%. This ensures that K can effectively modify non-metallic inclusions (making them spheroidized and refined) and that Nb is sufficient to form a dispersed precipitate without enrichment and agglomeration at grain boundaries. This avoids the risk of insufficient addition of trace alloying elements resulting in no significant grain refinement effect, or excessive addition leading to billet cracking and reduced toughness, thus achieving the best synergy between strength and plasticity.

[0025] 3. This application constructs a complete short-process preparation process including hot rolling temperature control, pickling and descaling, multi-pass cold rolling, continuous annealing, quenching and medium-temperature tempering. In particular, recrystallization and carbide spheroidization are achieved through continuous annealing (750-850℃), combined with quenching at 800-900℃ to obtain high-saturation martensite, and nitrogen-protected tempering at 350-500℃ to control the degree of martensite decomposition. The aim is to obtain a microstructure composed of fine needle-like tempered martensite or tempered troostite, so that the steel strip has moderate yield strength and excellent elastic limit while maintaining high hardness, which meets the service requirements of repeated bending of coiled spring sheets. Detailed Implementation

[0026] The following provides a more detailed description of this application in conjunction with specific details.

[0027] raw material The raw materials used in the embodiments of this application are all commercially available products. Example

[0028] Examples 1-3 An alloy steel strip material for coiled springs, the elemental composition and content of which are shown in Table 1, is prepared as follows: S1. Preparation of continuously cast slabs According to the mass percentage of each element in Table 1, scrap steel, pig iron, and molten iron are added to the electric arc furnace for smelting. The tapping temperature is controlled at 1650℃. During the smelting process, the P content of the raw materials entering the furnace is controlled to be ≤0.020%, and the S content is controlled to be ≤0.015%. The molten steel is transferred to a ladle refining furnace (LF furnace) for fine-tuning of its composition. Intermediate alloys of ferrosilicon, ferromanganese, ferrochrome, ferromolybdenum, ferronickel, and ferrocopper are added. Aluminum ingots are used for deoxidation to control AlS (acid-soluble aluminum) between 0.015% and 0.030%. White slag is then used for reduction refining to further desulfurize the steel, controlling S ≤ 0.010%. After refining, it is transferred to a vacuum degassing unit (VD) and processed for 15 minutes under a vacuum of ≤67Pa to remove hydrogen and nitrogen, controlling H≤1.5ppm and N≤40ppm; Before tapping, potassium is fed in the form of potassium salt cored wire (KCl cored wire), and its uniform distribution in the molten steel is controlled by vapor pressure; at the same time, niobium is added in the form of ferroniobium. Molten steel is cast using a fully protective casting method to prevent secondary oxidation. The overheating temperature of the tundish is controlled at 20-35℃, and the billet pulling speed is 1.0m / min. The end face specifications of the continuous casting billet are 200mm×1500mm. After the continuous casting slab is cut off, it is immediately sent to a slow cooling pit for slow cooling. The slow cooling time is ≥48 hours, so that the core temperature of the billet is uniformly reduced to below 300℃ to eliminate casting thermal stress, prevent the generation of microcracks, and obtain a continuous casting slab for subsequent hot rolling. S2. Hot rolling: Take the continuously cast slab, heat it to 1220℃ and hold it for 150 minutes to make the austenitization uniform. The final rolling temperature is controlled at 895℃ and the thickness of the rolled steel plate is 2.0mm. Pickling: The hot-rolled steel plate is transferred to the pickling unit for pickling using a 200g / L hydrochloric acid solution at 70℃ for 150s. After high-pressure water rinsing and hot air drying, the surface oxide scale is completely removed, leaving 0.72g / m³ of residual iron salts. 2 The pickled plate was obtained; Cold rolling: Pickled sheet is cold rolled in five passes using a single-stand six-roll reversible cold rolling mill to a finished thickness of 0.22 mm, yielding cold-rolled strip steel; the reduction rates for the five passes are 14.5%, 16.3%, 17.1%, 15.8%, and 14.0% respectively, with a final tension of 92 N / mm. 2 Plate shape waviness value 7I, surface roughness Ra=0.35μm; Continuous annealing: The cold-rolled strip is transferred to a vertical continuous annealing furnace, the annealing temperature is 820℃, the holding time is 180s, the protective atmosphere is nitrogen doped with hydrogen with a hydrogen volume fraction of 3.5%, the dew point is -35℃, and the belt speed is 12.5m / min, to achieve recrystallization and carbide homogenization. Quenching: The annealed strip steel is directly transferred to the quenching heating section, heated to 860℃, heating rate 17℃ / s, held for 90s, and then quickly immersed in 120℃ graded quenching oil for oil cooling to about 15℃ below Ms point to obtain fine acicular martensite structure. Tempering: The quenched strip is transferred to a N2 protective tempering furnace, tempered at 440℃ for 120s, with a dew point of -42℃. After tempering, it is air-cooled to room temperature to obtain alloy steel strip material for coiled springs.

[0029] Table 1. Elemental composition and content (%) of Examples 1-3

[0030] Example 4 An alloy steel strip material for coiled springs differs from that of Example 2 in that its elemental composition does not include K and Nb, while the remaining steps are the same as those of Example 2.

[0031] Example 5 An alloy steel strip material for coiled springs differs from that of Example 2 in that the added K in its elemental composition is replaced with an equal mass of Nb, while the remaining steps are the same as those in Example 2.

[0032] Example 6 An alloy steel strip material for coiled springs differs from that of Example 2 in that the added Nb in its elemental composition is replaced with an equal mass of K, while the remaining steps are the same as in Example 2.

[0033] Example 7 An alloy steel strip material for coiled springs differs from that in Example 2 in that the tempering temperature in step S2 is 350°C, while the remaining steps are the same as in Example 2.

[0034] Example 8 An alloy steel strip material for coiled springs differs from that in Example 2 in that the tempering temperature in step S2 is 500°C, while the remaining steps are the same as in Example 2.

[0035] Comparative Example Comparative Example 1 An alloy steel strip material for coiled springs differs from that in Example 4 in that the tempering temperature in step S2 is 300°C, while the remaining steps are the same as in Example 4.

[0036] Comparative Example 2 An alloy steel strip material for coiled springs differs from that in Example 4 in that the tempering temperature in step S2 is 550°C, while the remaining steps are the same as in Example 4.

[0037] Performance testing Detection methods / test methods Alloy steel strip materials were prepared according to the preparation methods of Examples 1-8 and Comparative Examples 1-2, and then tested according to the following testing methods. The test results are shown in Table 2.

[0038] Tensile properties, including tensile strength and yield strength, were tested according to GB / T 228.1-2021; Vickers hardness was converted according to GB / T 231.1-2018; and axial fatigue tests (R=0.1, σ) were conducted according to GB / T 16825.1-2008. a =620MPa).

[0039] Tensile strength between 1128-1220 MPa, yield strength between 426-500 MPa, hardness between 502-530, and lifespan between 8320-9000 cycles. Table 2. Detection results of Examples 1-8 and Comparative Examples 1-2

[0040] As can be seen from the test data in Table 2, the tensile strength of the alloy steel strip materials prepared in this application is all above 1128 MPa, the yield strength is all above 426 MPa, the hardness is all above 502 HV, and the service life is all above 8320 cycles; indicating that the alloy steel strip materials prepared in this application have excellent strength, plasticity and hardness, as well as excellent fatigue resistance and service life.

[0041] Combining Examples 2 and 4-6, adding K and Nb simultaneously ensures that K can effectively modify non-metallic inclusions (making them spheroidized and refined), while also ensuring that Nb is sufficient to form a dispersed precipitate phase without enrichment and agglomeration at grain boundaries. This avoids the risk of insufficient addition of trace alloying elements resulting in no significant grain refinement effect, or excessive addition leading to billet cracking and reduced toughness, thus achieving the best synergy between strength and plasticity.

[0042] In conjunction with Examples 2 and 7-8, and Comparative Examples 1-2, nitrogen-protected tempering was performed at 350-500℃ to control the degree of martensite decomposition. The aim was to obtain a microstructure composed of fine acicular tempered martensite or tempered troostite, enabling the steel strip to maintain high hardness while possessing moderate yield strength and excellent elastic limit, meeting the service requirements of repeated bending of coiled spring sheets. The tempering temperature has a significant impact on various material properties and should not exceed this range.

[0043] This specific embodiment is merely an explanation of the present invention and is not intended to limit the invention. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they are within the scope of the claims of the present invention.

Claims

1. An alloy steel strip material for coiled spring sheets, characterized in that: It comprises the following chemical composition by mass percentage: C 0.71-0.84%, Si 0.24-0.3%, Mn 0.6-0.72%, P≤0.016%, S≤0.006%, Cr 0.3-0.43%, Mo 0.001-0.006%, Ni 0.005-0.02%, Cu 0.005-0.02%, with the balance being Fe.

2. The alloy steel strip material for coiled springs according to claim 1, characterized in that: The alloy steel strip material also includes K at a mass percentage of 0-0.05% and Nb at a mass percentage of 0-0.05%, wherein the amounts of K and Nb added are not both 0.

3. The alloy steel strip material for coiled springs according to claim 2, characterized in that: The mass percentage of K is 0.01-0.05%, and the mass percentage of Nb is 0.01-0.05%.

4. A method for preparing an alloy steel strip material for coiled springs according to any one of claims 1-3, characterized in that: It includes the following steps: S1. Prepare a continuous casting slab that conforms to the chemical composition and content of each chemical component of the alloy steel strip material. The end face specifications of the continuous casting slab are (150-250) mm × (1250-2000) mm. S2. Hot rolling: The continuously cast slab is heated to 1100-1350℃, held for 100-200 minutes, and the final rolling temperature is controlled at 840-950℃. The thickness of the steel plate after hot rolling is 1.5-3.0 mm. Pickling: Pickling hot-rolled steel plates to obtain pickled plates; Cold rolling: The pickled sheet is cold rolled five times to a thickness of 0.18-0.26 mm to obtain cold-rolled strip steel; Continuous annealing: The cold-rolled strip steel is continuously annealed at a temperature of 750-850℃ and a holding time of 140-300s. The protective atmosphere is nitrogen doped with hydrogen, with a hydrogen volume fraction of 2-4.5%, a dew point of (-42)-(-30)℃, and a belt speed of 10-15m / min. Quenching: Transfer the annealed strip to the quenching heating section, heat to 800-900℃, hold for 70-120s, and oil cool to below 15℃; Tempering: The quenched strip is tempered under nitrogen protection at a temperature of 350-500℃ for 100-150s with a dew point of ≤-40℃. After tempering, it is naturally cooled to room temperature to obtain alloy steel strip material for coiled springs.

5. The method for preparing an alloy steel strip material for coiled springs according to claim 4, characterized in that: In step S1, when the alloy steel strip material contains K and Nb, K and Nb are added before tapping the continuously cast slab during preparation. The K is fed in as a potassium salt cored wire, and the Nb is added as a niobium-iron master alloy.

6. The method for preparing an alloy steel strip material for coiled springs according to claim 4, characterized in that: In step S2, the pickling process is as follows: The hot-rolled steel plates are transferred to a pickling unit for pickling. A hydrochloric acid solution with a concentration of 150-250 g / L is used at a temperature of 60-80℃ for 100-200 seconds. After rinsing with water and drying, the residual iron salts are ≤0.72 g / m³. 2 The pickled plate was obtained.

7. The method for preparing an alloy steel strip material for coiled springs according to claim 4, characterized in that: In step S2, the reduction rates for the five cold rolling passes are 12-15%, 15-16.5%, 16.5-17.5%, 15-16%, and 13-15% respectively, with a final pass tension of 85-100 N / mm. 2 Plate shape waviness value ≤ 8I, surface roughness Ra ≤ 0.45μm.

8. A coiled spring, characterized in that: The coil spring is made of alloy steel strip material as described in any one of claims 1-3.

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