Manufacturing method of 2.0 GPa grade gradient strength steel plate spring

Abstract

The invention discloses a manufacturing method of a 2.0 GPa grade gradient strength steel plate spring, and belongs to the technical field of automobile parts. The manufacturing method comprises rolling, straightening, punching, lug rolling, edge cutting, lug wrapping, aerosol quenching, tempering, shot blasting, paint spraying and assembling, and is characterized in that aerosol quenching comprises the following steps: (1) after the lug wrapping procedure of the steel plate spring is finished, the steel plate spring is conveyed into a heating furnace to be heated and subjected to heat preservation, and the whole steel plate spring is uniformly austenitized; (2) the steel plate spring is rapidly transferred into an aerial fog quenching device, a quenching medium is adopted for conducting local aerial fog quenching on different parts of the steel plate spring obtained after uniform austenitizing, and the cooling rate of local aerial fog quenching ranges from 0.5 DEG C / s to 60 DEG C / s; the steel plate spring has gradient strength in the length direction and the thickness direction and has a gradient structure. Non-uniform mechanical property distribution can be achieved, the strength gradient gradually changing in the length direction and the thickness direction is obtained, and meanwhile the combined performance of high strength and high toughness is obtained.

Description

Technical Field

[0001] This invention belongs to the field of automotive parts technology, and relates to a method for manufacturing a leaf spring, particularly a method for manufacturing a 2.0GPa grade gradient strength leaf spring. Background Technology

[0002] With the increasing popularity of automobiles and the rapid development of the industry, the demands for vehicle safety and lightweighting have also increased. Using higher-strength leaf springs for lightweight design can reduce energy consumption and improve vehicle power and braking performance while meeting design performance requirements. However, as the strength of leaf springs increases, their toughness will decrease accordingly.

[0003] In actual production applications, the stress on different parts of a leaf spring is not exactly the same. This requires leaf springs to have different strength and toughness properties in different parts. For example, the strength requirement is higher in the middle, while the toughness is considered to be higher at the two ends. The strength and toughness of the parabolic segment are in between.

[0004] In current leaf spring manufacturing processes, the quenching process involves heating and holding the leaf spring at a specific temperature before immersing it entirely in a quenching medium. This results in a uniform distribution of properties across the leaf spring in all directions. For example, Chinese Patent No. CN202310409366.3, published on April 14, 2023, entitled "A Spring Steel Material, a Single-Layer Leaf Spring and its Manufacturing Method," describes a method for manufacturing spring steel material and a single-layer leaf spring. This patent involves oscillating the leaf spring sheet with the quenching fixture during the quenching process, ensuring full contact between the leaf spring sheet and the quenching oil. This solves the problem of thick-section leaf springs being difficult to quench through, resulting in leaf springs with high stress and long service life. However, the quenching process uses quenching oil with a uniform temperature, leading to consistent overall mechanical properties in the resulting leaf spring. This results in insufficient strength in stress concentration areas, while other parts have redundant strength. The patent addresses the contradictory demand for performance optimization that cannot be specifically tailored. Chinese patent number CN201810642545.0, published on November 23, 2018, is titled "A High-Stress, Long-Life Variable Cross-Section Single-Plate Steel Leaf Spring and Its Manufacturing Method." Inventors Le Wenchao et al. provide a method for manufacturing steel leaf springs with sufficient strength and long service life. This patent utilizes high-strength spring flat steel with tensile strength ≥1870MPa and yield strength ≥1680MPa. After processes including material cutting, center hole machining, heating, variable cross-section forming, ear rolling, forming heat treatment, shot peening, electrophoretic painting, and warehousing, its lifespan is significantly improved compared to the previous method. However, the uniform rapid cooling process used in the forming heat treatment process does not consider the different requirements for high strength and high toughness in different parts of the steel leaf spring. Therefore, the overall mechanical properties of the steel leaf spring are uniformly distributed, failing to achieve higher strength in stress concentration areas and higher toughness in areas subjected to alternating loads.

[0005] Based on this, it is desirable to obtain a steel leaf spring manufacturing method that differs from existing technologies, which has a strength gradient that gradually changes along the length and thickness directions. Summary of the Invention

[0006] To address the difficulties in local strength adjustment and strength distribution optimization encountered in the current manufacturing of high-strength steel leaf springs, this invention proposes a method for preparing high-strength steel leaf springs with real-time controllable process and variable strength. By performing local heat treatment on different regions, a gradually varying strength gradient is obtained along the length and thickness of the leaf spring, thereby simultaneously achieving a combination of high strength and high toughness.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] A method for manufacturing a 2.0 GPa grade gradient strength steel leaf spring includes the following steps: rolling, straightening, punching, earing, trimming, ear wrapping, air mist quenching, tempering, shot peening, painting, and assembly. The improvement of this invention lies in the air mist quenching process, specifically including the following steps:

[0009] S1. After the leaf spring is finished with the lug process, the leaf spring is transported to a heating furnace for heating and held at a certain temperature for a period of time to make the leaf spring uniformly austenitized.

[0010] S2. The uniformly austenitized steel leaf spring is quickly transferred to the air mist quenching device. Different parts of the uniformly austenitized steel leaf spring are subjected to local air mist quenching. The quenching temperature range of the air mist quenching is 700-900℃, and the cooling rate range of the local air mist quenching is 0.5-60℃ / s. Finally, a steel leaf spring with gradient structure and properties is obtained.

[0011] Furthermore, in step S1: the heating temperature range is 800-920℃, so that the entire leaf spring reaches the critical temperature A. c3 The heating and holding time is 90-120 minutes to ensure that the steel leaf spring steel can form a uniform austenitic structure.

[0012] Furthermore, in S2: the aerosol quenching device consists of nozzles, transport pipes, a temperature monitor, and quenching medium. The central nozzle 5 is connected to the transport pipe 4 and is located directly above the center of the leaf spring 1. The quenching medium passes through the transport pipe 4, and the nozzle 5 sprays it onto the upper surface of the center of the leaf spring 1. The end nozzles 7 are connected to the transport pipe 4 and are located directly above the two ends of the leaf spring 1. The quenching medium passes through the transport pipe 4, and the nozzle 7 sprays it onto the upper surface of the two ends of the leaf spring 1. The parabolic nozzle 6 is connected to the transport pipe 4 and is located directly above the parabolic segment between the center and the two ends of the leaf spring 1. The quenching medium passes through the transport pipe 4, and the nozzle 6 sprays it onto the upper surface of the parabolic segment between the center and the two ends of the leaf spring 1. The temperature monitor 2 is located directly above the leaf spring 1 and is used to monitor the real-time temperature of the upper surface of the leaf spring 1.

[0013] Furthermore, in S2: the distance between the bottom of the nozzle of the air mist quenching device and the quenching surface of the steel leaf spring is 300-1000mm;

[0014] Furthermore, in S2: the quenching medium includes compressed gas and water, wherein the CO2+N2 content in the compressed gas is not less than 70%, and the O2 content is less than 5%.

[0015] High-strength steel leaf springs can be divided into three regions based on different performance requirements. The central region has the highest strength requirement and the lowest toughness requirement; the two ends of the leaf spring have the lowest strength requirement and the highest toughness requirement; the parabolic segment between the central and two ends of the leaf spring has strength and toughness requirements that fall between the two. During the air-mist quenching process, since the upper surface of each region always contacts the quenching medium first, the cooling rate of the leaf spring in the thickness direction also exhibits a certain gradient variation, forming a gradient metallographic structure. This results in a leaf spring with gradient strength along both the length and thickness directions.

[0016] To achieve the above requirements, the gradient strength of the leaf spring is realized by controlling the surface temperature of the leaf spring at the start of air-mist quenching and the cooling rate of air-mist quenching at different locations of the leaf spring. This includes the following steps:

[0017] The first step is to ensure that the nozzles at each position on the upper surface of the leaf spring are perpendicular to the leaf spring surface. Adjust the distance between each nozzle and the upper surface of the leaf spring. The middle nozzle 5 is closest to the leaf spring, the two end nozzles 7 are farthest from the leaf spring, and the distance between the parabolic nozzle and the leaf spring is in between.

[0018] The second step is to start the nozzle 5 to perform localized aerosol quenching on the middle area of ​​the leaf spring when the surface temperature of the middle area of ​​the leaf spring cools down to 850-900℃. The spray cone angle of the nozzle 5 is above 15°, the pressure range of the compressed gas in the quenching medium is 0.1-0.9MPa, and the pressure range of the water is 0.1-0.9MPa.

[0019] The third step is to start the nozzle 6 to perform localized aerosol quenching on the parabolic segment between the middle and both ends of the leaf spring when the surface temperature of the parabolic segment of the leaf spring cools to 780-850℃. The spray cone angle of the nozzle 6 is 10-20°, the pressure of the compressed gas in the quenching medium is 0.1-0.6MPa, and the pressure of the water is 0.1-0.6MPa.

[0020] The fourth step is to start the nozzle 7 to perform localized aerosol quenching on the ends of the leaf spring when the surface temperature of the end areas on both sides of the leaf spring cools down to 700-780℃. The spray cone angle of the nozzle 7 is less than 15°, the pressure range of the compressed gas in the quenching medium is 0.1-0.5MPa, and the pressure range of the water is 0.1-0.5MPa.

[0021] The beneficial effects of this invention are as follows:

[0022] (1) The present invention uses nozzles arranged above the steel leaf spring to quench different parts and controls the amount of spray from the nozzles to achieve different cooling rates in different parts. During the air mist quenching process, the temperature distribution and cooling rate of the quenched surface are monitored in real time by the temperature monitoring system to correct the amount of air mist sprayed from the air mist quenching device, thereby achieving a non-uniform mechanical property distribution and thus obtaining a strength gradient that gradually changes along the length direction. Since the air mist quenching is performed above the steel leaf spring, the upper surface of the steel leaf spring cools down faster than the lower surface, forming a certain temperature gradient, thereby achieving a gradient difference in microstructure and a strength gradient along the thickness direction, so as to obtain a combination of high strength and high toughness at the same time.

[0023] (2) This invention allows for convenient and easy control of air-jet quenching parameters, and enables the acquisition of more diverse strength distribution characteristics through different combinations of air-jet quenching parameters, thereby achieving greater strength and toughness requirements. Furthermore, this invention allows for easier real-time measurement and monitoring of the temperature distribution and cooling rate on the surface of the uniformly austenitized steel leaf spring during the air-jet quenching process, providing feedback to correct the air-jet quenching parameters and more accurately achieving the specified strength distribution characteristics. This results in a manufacturing process that is highly efficient, precise, versatile, and low-cost. It also enables the fulfillment of multiple performance requirements for a single part, improving the overall mechanical properties of the part. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the air-mist quenching process in a specific embodiment of the present invention;

[0025] Figure 2 This refers to the nozzle position corresponding to the upper surface of the leaf spring;

[0026] Figure 3 This refers to the spray range of the nozzle corresponding to the upper surface of the leaf spring;

[0027] Figure 4 This is a microstructure diagram of region A on the upper surface of a steel leaf spring;

[0028] Figure 5 This is a microstructure diagram of region A' on the lower surface of a steel leaf spring.

[0029] Figure 6 This is a microstructure diagram of region B on the upper surface of a steel leaf spring.

[0030] Figure 7 Microstructure diagram of region B' on the lower surface of a steel leaf spring;

[0031] Figure 8 This is a microstructure diagram of region C on the upper surface of a steel leaf spring;

[0032] Figure 9 Microstructure diagram of region C' on the lower surface of a steel leaf spring;

[0033] Figure 10 A schematic diagram showing the location of the sampling points for the leaf spring;

[0034] Figure 11 This is a hardness distribution diagram of the upper and lower surfaces of a leaf spring at different distances from the center.

[0035] In the diagram: 1. Leaf spring; 2. Temperature monitor; 3. Leaf spring forming machine; 4. Transport pipeline; 5. Central nozzle; 6. Parabolic segment nozzle; 7. End nozzles on both sides. Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0037] Figure 1 This is a schematic diagram of the air-mist quenching process in a specific embodiment of the present invention, as shown below. Figure 1 As shown, the aerosol quenching device in this embodiment consists of nozzles, transport pipes, a temperature monitor, and quenching medium. The central nozzle 5 is connected to the transport pipe 4 and is located directly above the center of the leaf spring 1. The quenching medium passes through the transport pipe 4 and is sprayed from the nozzle 5 onto the upper surface of the center of the leaf spring 1. The end nozzles 7 are connected to the transport pipe 4 and are located directly above the two ends of the leaf spring 1. The quenching medium passes through the transport pipe 4 and is sprayed from the nozzle 7 onto the upper surface of the two ends of the leaf spring 1. The parabolic nozzle 6 is connected to the transport pipe 4 and is located directly above the parabolic segment between the center and the two ends of the leaf spring 1. The quenching medium passes through the transport pipe 4 and is sprayed from the nozzle 6 onto the upper surface of the parabolic segment between the center and the two ends of the leaf spring 1. The temperature monitor 2 is located directly above the leaf spring 1 and is used to monitor the real-time temperature of the upper surface of the leaf spring 1. The quenching medium includes compressed gas and water, wherein the compressed gas contains 85% CO2 + N2 and 4% O2.

[0038] High-strength steel leaf springs can be divided into three regions based on different performance requirements. The central region has the highest strength requirement and the lowest toughness requirement, with a tensile strength greater than 2000 MPa and a hardness above 55 HRC. Because the upper surface of the central region (region A) is the first to contact the quenching medium, it has a higher cooling rate, so the hardness of the upper surface (region A') is higher than that of the lower surface (region A'). Region B, located on the parabolic segment between the central region and the two ends of the leaf spring, has strength and toughness requirements between the two, with a tensile strength between 1800-2000 MPa. The hardness is between 50-55 HRC. Because the upper surface of the leaf spring 1 is the first to contact the quenching medium, its cooling rate is higher, so the hardness of region B on the upper surface of the leaf spring is higher than that of region B' on the lower surface. The end regions on both sides of the leaf spring have the lowest strength requirements and the highest toughness requirements. Their tensile strength is between 1600-1800 MPa, and their hardness is between 45-50 HRC. Because the upper surface of the leaf spring 1 is the first to contact the quenching medium, its cooling rate is higher, so the hardness of region C on the upper surface of the leaf spring is higher than that of region C' on the lower surface.

[0039] To achieve the above requirements, the gradient strength of the leaf spring is realized by controlling the surface temperature of the leaf spring at the start of air-mist quenching and the cooling rate of air-mist quenching at different locations of the leaf spring. This includes the following steps:

[0040] First, ensure that the nozzle angles at each position on the upper surface of the leaf spring are perpendicular to the leaf spring surface. Adjust the distance between each nozzle and the upper surface of the leaf spring, as shown in the image. Figure 1 , 2 As shown, the angles are all perpendicular to the surface of the leaf spring. The distance between nozzle 5 and area A is 650mm, nozzle 6 is 900mm from area B, and nozzle 7 is 1000mm from area C.

[0041] The second step involves activating nozzle 5 to perform localized aerosol quenching on region A of the leaf spring when the surface temperature naturally cools to 850℃. The spray effect area of ​​nozzle a on the surface of region A is region α. Figure 3 As shown. The spray cone angle of nozzle 5 is 20°, the pressure of the compressed gas is 0.3 MPa, and the pressure of the water is 0.3 MPa;

[0042] The third step is to start the nozzle 6 to perform localized aerosol quenching on the B area of ​​the leaf spring when the surface temperature of the B area of ​​the leaf spring naturally cools down to 800℃. The spray effect area of ​​the nozzle 6 on the surface of the B area is the β area. The spray cone angle of the nozzle 6 is 15°, the pressure of the compressed gas is 0.3MPa, and the pressure of the water is 0.2MPa.

[0043] Fourth step: When the surface temperature of region C cools naturally to 750℃, start nozzle 7 to perform localized aerosol quenching on region C. The spray influence area of ​​nozzle 7 on the surface where region C is located is region γ. The spray cone angle of nozzle 7 is less than 15°, the pressure of compressed gas is 0.2MPa, and the pressure of water is 0.1MPa.

[0044] After the air-mist quenching of the leaf spring 1 is completed, samples are taken from various parts of the leaf spring 1. Samples are taken every 200 mm starting from the center of the leaf spring. The specific locations are as follows: Figure 10 As shown, the hardness values ​​of the upper and lower surfaces of the sample were measured, and the metallographic structure of the sample was observed using a metallographic microscope. The microstructure of regions A and A' after phase transformation is martensite, as shown... Figure 4 , 5 As shown; the microstructure of regions B and B' after phase transformation is martensite and bainite, as shown in the figure. Figure 6 , 7 As shown; the microstructure of regions C and C' consists of ferrite and pearlite, as... Figure 8 , 9 As shown, the specific hardness value distribution is as follows: Figure 11 As shown, the results indicate that all planned values ​​were achieved.

[0045] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for manufacturing a 2.0 GPa grade gradient strength steel leaf spring, the manufacturing method comprising the steps of: rolling, straightening, punching, earing, trimming, ear wrapping, air mist quenching, tempering, shot peening, painting, and assembly, the improvement being the air mist quenching, specifically: the air mist quenching process includes the following steps: S1. After the leaf spring is finished with the lug process, the leaf spring is transported to a heating furnace for heating and heat preservation, so that the leaf spring is uniformly austenitized. S2. The uniformly austenitized steel leaf spring is quickly transferred to an air mist quenching device. Different parts of the uniformly austenitized steel leaf spring are locally air mist quenched using a quenching medium. The quenching temperature range of the air mist quenching is 700-900℃, and the cooling rate range of the local air mist quenching is 0.5-60℃ / s, forming a steel leaf spring with gradient strength and gradient structure along both the length and thickness directions.

2. The manufacturing method of a 2.0 GPa grade gradient strength steel leaf spring according to claim 1, characterized in that, In S1: the heating temperature range is 800-920℃, so that the entire leaf spring reaches the critical temperature A. c3 The heating and holding time is 90-120 minutes to ensure that the steel plate spring steel can form a uniform austenitic structure.

3. The method for manufacturing a 2.0 GPa grade gradient strength steel leaf spring according to claim 1, characterized in that, In S2: the aerosol quenching device includes a nozzle, a transport pipe (4), a temperature monitor (2), and a quenching medium, wherein the nozzle includes a central nozzle (5), parabolic segment nozzles (6) on both sides of the central nozzle (5), and an outermost end nozzle (7); each nozzle is connected to the transport pipe (4); specifically: The central nozzle (5) is located directly above the center of the leaf spring (1), and the quenching medium is sprayed onto the upper surface of the center of the leaf spring (1) through the central nozzle (5). The end nozzle (7) is located directly above the two ends of the leaf spring (1), and the quenching medium is sprayed onto the upper surface of the two ends of the leaf spring (1) through the end nozzle (7). The parabolic segment nozzle (6) is located directly above the parabolic segment between the middle and both sides of the leaf spring (1). The quenching medium is sprayed through the parabolic segment nozzle (6) onto the upper surface of the parabolic segment between the middle and both sides of the leaf spring (1). The temperature monitoring instrument (2) is located directly above the leaf spring (1) and is used to monitor the real-time temperature of the upper surface of the leaf spring (1).

4. The method for manufacturing a 2.0 GPa grade gradient strength steel leaf spring according to claim 1, characterized in that, In S2: The distance between the bottom of the nozzle of the air mist quenching device and the quenching surface of the steel leaf spring (1) is 300-1000mm.

5. The method for manufacturing a 2.0 GPa grade gradient strength steel leaf spring according to claim 1, characterized in that, In S2: The quenching medium includes compressed gas and water, wherein the CO2+N2 content in the compressed gas is not less than 70% and the O2 content is less than 5%.

6. The method for manufacturing a 2.0 GPa grade gradient strength steel leaf spring according to claim 1, characterized in that, In S2 of the air-mist quenching process, the gradient strength of the leaf spring is achieved by controlling the surface temperature of the leaf spring at the start of air-mist quenching and the cooling rate of air-mist quenching at different positions of the leaf spring.

7. The method for manufacturing a 2.0 GPa grade gradient strength steel leaf spring according to claim 6, characterized in that, Includes the following steps: S2.1, the nozzle angles at each position on the upper surface of the leaf spring (1) are perpendicular to the surface of the leaf spring (1). Adjust the distance between each nozzle and the upper surface of the leaf spring (1). The middle nozzle (5) is closest to the leaf spring (1), the end nozzles (7) on both sides are farthest from the leaf spring (1), and the parabolic nozzle (6) is between the two. S2.2 When the surface temperature of the middle area of ​​the leaf spring (1) cools down to 850-900℃, the middle nozzle (5) is started to perform localized air mist quenching on the middle area of ​​the leaf spring (1); S2.3 When the surface temperature of the parabolic segment of the leaf spring cools to 780-850℃, the parabolic segment nozzle (6) is started to perform localized air mist quenching on the parabolic segment between the middle and both ends of the leaf spring (1). S2.4 When the surface temperature of the end areas on both sides of the leaf spring cools to 700-780℃, start the end nozzle (7) to perform localized air mist quenching on both sides of the leaf spring.

8. The method for manufacturing a 2.0 GPa grade gradient strength steel leaf spring according to claim 6, characterized in that, In S2.2, the spray cone angle of the central nozzle (5) is 15° or more, the pressure range of the compressed gas in the quenching medium is 0.1-0.9MPa, and the pressure range of the water is 0.1-0.9MPa.

9. The method for manufacturing a 2.0 GPa grade gradient strength steel leaf spring according to claim 6, characterized in that, In S2.3, the spray cone angle of the parabolic segment nozzle (6) is 10-20°, the pressure of the compressed gas in the quenching medium is 0.1-0.6MPa, and the pressure of the water is 0.1-0.6MPa.

10. A method for manufacturing a 2.0 GPa grade gradient strength steel leaf spring according to claim 6, characterized in that, In S2.4, the spray cone angle of the end nozzle (7) is less than 15°, the pressure range of the compressed gas in the quenching medium is 0.1-0.5MPa, and the pressure range of the water is 0.1-0.5MPa.

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