High hardenability microalloyed spring steel plate and method for manufacturing the same

Abstract

The application relates to the technical field of steel materials, and discloses a high hardenability micro-alloyed plate spring steel and a preparation method thereof. The high hardenability micro-alloyed plate spring steel comprises the following chemical components in mass ratio: C: 0.45-0.69%; Si: 1.30-2.20%; Mn: 0.60-1.30%; Cr: 0.10-0.35%; Al: 0.075-0.15%; N: 0.008-0.025%; the balance being Fe; and further comprising the following chemical components for controlling impurities: Pb, Sn, Zn, Sb and Bi <=0.03%; (O2+H2) <=30ppm; S and P <=0.025%; Cu <=0.25%. The performance of the high hardenability micro-alloyed plate spring steel prepared by the application is as follows: the hardness of raw materials is <=HB330, the tensile strength can reach more than 1600MPa after heat treatment, the yield strength can reach about 1350MPa, the elongation is >=8%, the reduction of area is >=25%, and the fatigue cycle is greater than 250,000 cycles.

Description

Technical Field

[0001] This invention relates to the field of steel technology, and more specifically, to a high hardenability microalloyed leaf spring steel and its preparation method. Background Technology

[0002] As a fundamental component and part of equipment manufacturing, the expansion of spring production scale and variety, as well as the improvement of its quality level, are prerequisites for ensuring the performance improvement of mechanical equipment.

[0003] Spring steel refers to steel specifically used for manufacturing springs and elastic components due to its elasticity in the quenched and tempered state. Based on chemical composition, it is divided into non-alloy spring steel (carbon spring steel) and alloy spring steel. Currently, the main spring steels used in domestic trailers are 60Si2Mn (thickness <16mm) and SUP9 (thickness <25mm). These not only require custom manufacturing, resulting in high costs, but also easily lead to material mixing during production, which is detrimental to production management. Furthermore, its strength is only around 1350 MPa, severely restricting the range of new energy vehicles and the weight reduction and emission reduction of various types of vehicles, failing to meet the development needs of high-end trailers. Developing spring steel with ultra-high strength, high elongation, high reduction of area, and fatigue resistance will be an inevitable trend to improve my country's independent supporting capabilities for high-end equipment components and effectively replace imports. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the purpose of this invention is to provide a high-hardenability microalloyed leaf spring steel, which has the advantages of high mechanical strength, large elongation, high reduction of area, and good fatigue resistance, as well as ultra-high hardenability (ideal quenching diameter exceeding 170mm). This allows for the use of a single steel grade, eliminates material mixing, simplifies the production process, and significantly reduces the cost of trailer leaf springs. This invention also provides its preparation method, which is scientific, reasonable, simple, and easy to implement.

[0005] A high hardenability microalloyed leaf spring steel, comprising the following chemical composition by mass ratio:

[0006] C: 0.45-0.69%; Si: 1.30-2.20%; Mn: 0.60-1.30%; Al: 0.075-0.15%; Cr: 0.10-0.35%; N: 0.008-0.025%; balance Fe;

[0007] Preferably, the chemical composition of impurities is controlled as follows: Pb, Sn, Zn, Sb and Bi ≤ 0.03%; O2 and H2 ≤ 30ppm; S and P ≤ 0.025%; Cu ≤ 0.2%.

[0008] The dosage standards and functions of each chemical component are as follows:

[0009] C: 0.45-0.69 wt.%;

[0010] Carbon strengthens spring steel through solid solution treatment, improving its elastic strength, hardness, and wear resistance. However, it reduces the plasticity, toughness, and fatigue strength of spring steel. When carbon is controlled within 0.45-0.69 wt.% and is present with other alloying elements, the optimal combination of strength, fatigue life, and economic benefits can be obtained.

[0011] Si: 1.30-2.20 wt.%;

[0012] Silicon strengthens ferrite through solid solution, significantly improving the elasticity of spring steel. However, excessive silicon increases the tendency for decarburization and graphitization, generating inclusions and deteriorating the fatigue performance of the spring. This invention uses a Si content controlled at 1.30-2.20 wt.%, slightly higher than conventional spring steel, aiming to utilize its contribution to the elastic strength of spring steel while minimizing its impact on decarburization, grain coarsening, and fatigue strength.

[0013] Mn: 0.60-1.30%;

[0014] Mn can improve the strength of steel through solid solution treatment. At the same time, it can improve the hardenability of steel and reduce the rejection of carbon, thus reducing decarburization. However, excessive Mn will promote temper brittleness. Therefore, the Mn content needs to be controlled between 0.60-1.30%.

[0015] Al: 0.075-0.15%;

[0016] Al can refine the grain structure of steel and inhibit the aging of low-carbon steel. It reduces notch sensitivity, especially lowering the ductile-brittle transition temperature of steel and improving the toughness of steel at low temperatures. This invention introduces Al into spring steel to utilize the affinity between Al and nitrogen, thereby achieving deoxidation, nitrogen fixation, and grain refinement. It can also significantly improve strength, low-temperature toughness, and fatigue strength. However, excessive aluminum affects the hot working properties of steel. Through experiments, the optimal Al content should be controlled between 0.075% and 0.15%.

[0017] Cr: 0.10-0.35 wt.%;

[0018] Chromium can improve the strength of steel through solid solution, as well as its hardenability and tempering stability, which is beneficial to improving the performance and dispersion precipitation of spring steel. However, the Cr content used in this invention is much lower than that of conventional spring steel. The purpose is to prevent excessive chromium from forming chromium carbide, which would reduce the plasticity and toughness of the steel. Therefore, the Cr content is controlled at 0.10-0.35 wt.%.

[0019] N: 0.008-0.025 wt.%;

[0020] Nitrogen (N) plays a similar role to carbon in steel, enhancing its elasticity, strength, and hardness through stronger solid solution strengthening. However, it has a smaller weakening effect on the plasticity, toughness, and fatigue strength of spring steel than carbon. In particular, the martensite formed has an Fe-CN structure, resulting in higher fatigue strength. Nitrogen-added microalloyed spring steel can have higher strength, toughness, and fatigue life. An N content of 80-250 ppm is the optimal N content determined in this invention.

[0021] S and P ≤ 0.025%;

[0022] Inclusions such as sulfur (S) and phosphorus (P) are unavoidable in steel. S and P form inclusions with alloying elements, such as MnS, which not only counteract the beneficial effects of the alloying elements, but also cause S and P to agglomerate, weakening the toughness of the steel and becoming fatigue crack initiation sites, severely reducing the fatigue strength of the spring. Therefore, the content of S and P in this steel should be strictly controlled to within 0.025 wt.%.

[0023] Cu≤0.2wt.%

[0024] Since the spring will undergo subsequent hot working, Cu will severely reduce the thermoplasticity of the material and easily generate microcracks during forging, which will seriously affect the strength of the spring. Therefore, it should be strictly controlled. Since the scrap contains copper wire, the amount of Cu used in the scrap should be strictly controlled to ≤0.25wt.

[0025] The spring steel described in this invention has a thickness of 6-32 mm or a diameter of 6-35 mm.

[0026] Metallographic analysis revealed that the microstructure of the spring steel consisted of more than 95% sorbite and a small amount of ferrite, and contained only sorbite and ferrite without any other structures.

[0027] A method for preparing high hardenability microalloyed leaf spring steel involves sequentially melting, secondary refining, RH or VD vacuum degassing, continuous casting and cooling into steel ingots, peeling off the ingots, reheating and continuous rolling, controlled cooling, quenching and tempering to obtain the spring steel product.

[0028] The spring steel raw material can be made from some scrap steel, but the scrap steel contains copper wire, etc. Therefore, the amount of scrap steel used needs to be controlled within 25% of the total mass of the spring steel raw material.

[0029] The melting temperature is 1580-1700℃, and the time is 20-60 minutes; the refining temperature is 1500-1580℃, and the time is 20-60 minutes. Electromagnetic stirring is used during the refining process. Electromagnetic stirring ensures a uniform microstructure and prevents the segregation of alloying elements.

[0030] The RH or VD vacuum degassing is performed with a vacuum degree ≤180Pa and a vacuum time of 20-60 minutes.

[0031] The continuous casting and cooling process for steel ingots ensures that the ingot dimensions and the final hot-rolled spring dimensions have a compression ratio greater than 15:1. The ingots are first cooled to below 1100℃ at a rate greater than 35℃ / min, and then allowed to cool naturally to room temperature. This process confines inclusions as much as possible to the centerline of the ingot, minimizing their impact on the performance of the rolled steel product.

[0032] The depth of the peeling of the steel ingot is greater than 1.0 mm.

[0033] The reheating continuous rolling process has an initial rolling temperature of 930-1050℃ and a final rolling temperature of 780-920℃. This allows rolling to be performed in the austenitic region, maximizing the material's deformation properties and providing favorable conditions for subsequent cooling.

[0034] The controlled cooling process involves first rapidly cooling to 600℃, then holding at that temperature and slowly cooling to room temperature; the rapid cooling rate is ≥35℃ / min, and the holding-at-temperature slow cooling rate is ≤10℃ / min. This prevents surface decarburization and maintains a low hardness, which is beneficial for subsequent shearing processes.

[0035] The quenching method is oil quenching, the quenching temperature is 820-880℃, the holding time is 0.7-1.3 minutes / mm, and the tempering temperature is 370-500℃.

[0036] The high hardenability microalloyed leaf spring steel preparation process described in this invention further involves feeding raw materials into a converter, with the scrap steel content controlled to within 20%. To control impurity content, electromagnetic stirring and RH or VD vacuum degassing are used throughout the secondary refining process, resulting in a uniform fiber structure with fewer bubbles and pores, and a dense structure. After vacuum degassing, continuous casting is performed to form a stable macrostructure. Heated continuous rolling ensures uniform dimensional structure. Controlling the cooling temperature reduces the decarburized layer and ensures shear hardness. After cooling to room temperature, quenching and tempering are performed to obtain the finished product.

[0037] The beneficial effects of this invention are as follows:

[0038] (1) The properties of the high hardenability microalloyed leaf spring steel prepared by the present invention are as follows:

[0039] The raw material has a hardness ≤ HB330, and after heat treatment, its tensile strength can reach over 1600 MPa, its yield strength can reach around 1350 MPa, its elongation is ≥ 8%, its reduction of area is ≥ 25%, and its fatigue cycles are greater than 250,000.

[0040] (2) The semi-decarburized layer of spring steel is less than or equal to 0.20 mm, and there is no fully decarburized layer.

[0041] (3) After heat treatment, the grain size is greater than ASTM 9.0 grade.

[0042] (4) The preparation method described in this invention is scientific, reasonable, simple and easy to implement. The use of electromagnetic stirring and RH vacuum degassing can reduce bubbles and pores, making the microstructure more uniform and dense. Attached Figure Description

[0043] Figure 1 These are the hardness gradient curves of leaf spring steel processed by different techniques according to the present invention;

[0044] Figure 2 This is a comparison of the surface hardness distribution of leaf spring steel treated with different processes according to the present invention. Detailed Implementation

[0045] The subject matter described herein will now be discussed with reference to exemplary embodiments. It should be understood that these embodiments are discussed only to enable those skilled in the art to better understand and implement the subject matter described herein, and changes may be made to the function and arrangement of the elements discussed without departing from the scope of this specification. Various processes or components may be omitted, substituted, or added as needed in the examples. Furthermore, some features described in the examples may be combined in other examples.

[0046] Example 1

[0047] This embodiment presents a high hardenability microalloyed leaf spring steel, comprising the following chemical composition by mass ratio:

[0048] C: 0.69%; Si: 1.30%; Mn: 0.60%; Cr: 0.10%; Al: 0.075%; N: 0.008%; balance Fe.

[0049] It also includes the following chemical composition for controlling impurities: Pb: 0.01, Sn: 0.002, Zn: 0.005, Sb: 0.006 and Bi: 0.001; (O2+H2): 20ppm; S and P: 0.02:0; Cu: 0.2:1%.

[0050] Example 2

[0051] This embodiment presents a high hardenability microalloyed leaf spring steel, comprising the following chemical composition by mass ratio:

[0052] C: 0.69%; Si: 2.20%; Mn: 1.30%; Cr: 0.35%; Al: 0.15%; N: 0.025%; balance Fe.

[0053] It also includes the following chemical composition for controlling impurities: Pb, Sn, Zn, Sb and Bi: 0.03%; (O2+H2): 30ppm; S and P: 0.025%; Cu: 0.25%.

[0054] Example 3

[0055] This embodiment presents a high hardenability microalloyed leaf spring steel, comprising the following chemical composition by mass ratio:

[0056] C: 0.50%; Si: 1.80%; Mn: 0.90%; Cr: 0.25%; Al: 0.10%; N: 0.015%; balance Fe.

[0057] It also includes the following chemical composition for controlling impurities: Pb: 0.01, Sn: 0.002, Zn: 0.005, Sb: 0.006 and Bi: 0.001; (O2+H2): 20ppm; S and P: 0.02:0; Cu: 0.2:1%.

[0058] Example 4

[0059] This embodiment proposes a method for preparing high hardenability microalloyed leaf spring steel: the steel raw material is successively smelted, refined twice, continuously poured and cooled into steel ingots, the steel ingots are peeled off, and then heated and continuously rolled and cooled in a controlled manner to obtain the hot-rolled trailer leaf spring flat steel bar product.

[0060] in:

[0061] The melting temperature is 1580℃ and the time is 20 minutes; the refining temperature is 1500℃ and the time is 20 minutes. The refining process uses electromagnetic stirring.

[0062] The steel ingots are continuously cast and cooled. The dimensions of the continuously cast ingots and the final hot-rolled trailer leaf spring flat steel bars meet a compression ratio greater than 15:1. The ingots are first cooled to below 1100℃ at a rapid cooling rate of 35℃ / min, and then allowed to cool naturally to room temperature.

[0063] The depth of the steel ingot peeling is greater than 1.0 mm.

[0064] The reheating continuous rolling process starts at 930℃ and ends at 780℃.

[0065] The cooling control is as follows: first, rapidly cool to 600℃ at a rate of 35℃ / min; then, maintain the temperature and slowly cool to room temperature at a rate of 10℃ / min.

[0066] The quenching method is oil quenching, the quenching temperature is 830℃, the holding time is 0.7 minutes / mm, and the tempering temperature is 370℃.

[0067] Example 5

[0068] The difference between this embodiment and Embodiment 4 is that the melting temperature is 1640℃ and the time is 40 minutes; the refining temperature is 1540℃ and the time is 40 minutes.

[0069] The steel ingot is continuously poured and cooled, first cooled to below 1100℃ at a rapid cooling rate of 40℃ / min, and then naturally cooled to room temperature.

[0070] The reheating continuous rolling process starts at 1000℃ and ends at 810℃.

[0071] The cooling control is as follows: first, rapidly cool to 600℃ at a rate of 40℃ / min; then, hold at the temperature and slowly cool to room temperature at a rate of 8℃ / min.

[0072] The quenching method is oil quenching, the quenching temperature is 850℃, the holding time is 1.0 minute / mm, and the tempering temperature is 420℃.

[0073] Example 6

[0074] The difference between this embodiment and Embodiment 4 is that the melting temperature is 1700℃ and the time is 60 minutes; the refining temperature is 1580℃ and the time is 60 minutes.

[0075] The steel ingot is continuously poured and cooled, first cooled to below 1100℃ at a rapid cooling rate of more than 45℃ / min, and then naturally cooled to room temperature.

[0076] The reheating continuous rolling process starts at 1080℃ and ends at 850℃.

[0077] The cooling control is as follows: first, rapidly cool to 600℃ at a rate ≥ 45℃ / min; then, maintain the temperature and slowly cool to room temperature at a rate ≤ 5℃ / min.

[0078] The quenching method is oil quenching, the quenching temperature is 880℃, the holding time is 1.3 minutes / mm, and the tempering temperature is 500℃.

[0079] Example 7 (SS1500-1)

[0080] The high hardenability and low cost microalloyed leaf spring steel for trailers in this embodiment is prepared by the following method:

[0081] Molten iron is added to a 12-ton induction heating furnace and smelted at 1670℃. After 35 minutes, the steel is tapped and 22% scrap steel is added to adjust the temperature to 1600℃. The steel is then transferred to a refining furnace, where ferrosilicon, ferromanganese, ferrochrome, manganese nitride, and pure aluminum wire are added under electromagnetic stirring. The chemical composition is adjusted for 40 minutes, followed by VD vacuum degassing for 35 minutes (vacuum degree ≤180Pa). The steel is then continuously cast into 150×200mm billets, cooled to 1100℃ at a rate of 28℃ / min, and then air-cooled to room temperature. The billets are then peeled to a depth of 2mm and reheated to 1150℃. The billets are then continuously rolled into 24*89mm spring strips at an initial rolling temperature of 1070℃ and a final rolling temperature of 850℃. After rolling, the strips are rapidly cooled to 600℃ at a rate of 36℃ / min and then slowly cooled to room temperature at a rate of 8℃ / min.

[0082] The 24*89mm strip steel was prepared according to the above method. Its chemical composition is shown in Table 1. It was further processed into single leaf springs. After quenching at 870℃ and tempering at 450℃, tensile specimens were processed and tensile tests were conducted according to GB / T228-2002. The yield strength, elongation and reduction of area were tested. The assembled leaf springs were subjected to more than six effective fatigue tests according to GB / T228-2002. The results are shown in Table 2.

[0083] Example 8 (SS1500-2)

[0084] The high hardenability and low cost microalloyed leaf spring steel for trailers in this embodiment is prepared by the following method:

[0085] Molten iron is added to a 60-ton converter and smelted at 1650℃. After 45 minutes, the steel is tapped and 19% scrap steel is added to adjust the temperature to 1650℃. The steel is then transferred to a refining furnace, where ferrosilicon, ferromanganese, ferrochrome, and manganese nitride are added under electromagnetic stirring. Pure aluminum wire is slowly fed in at 1515±15℃, and the chemical composition is adjusted for 60 minutes. The steel is then degassed under RH vacuum (vacuum degree ≤130Pa) and continuously cast into 200×200mm billets. The billets are cooled to 1150℃ at a rate of 30℃ / min and then air-cooled to room temperature. The billets are then peeled to a depth of 2.1mm and reheated to 1200℃. The billets are then continuously rolled into 24*89mm spring strips at an initial rolling temperature of 1010℃ and a final rolling temperature of 850℃. After rolling, the strips are rapidly cooled to 600℃ at a rate of 35℃ / min and then slowly cooled to room temperature at a rate of 10℃ / min.

[0086] The 24*89mm strip steel was prepared according to the above method. Its chemical composition is shown in Table 1. It was further processed into single leaf springs. After quenching at 870℃ and tempering at 450℃, tensile specimens were processed and tensile tests were conducted according to GB / T228-2002. The yield strength, elongation and reduction of area were tested. The assembled leaf springs were subjected to more than six effective fatigue tests according to GB / T228-2002. The results are shown in Table 2.

[0087] Example 9 (SS1500-3)

[0088] The high hardenability and low cost microalloyed leaf spring steel for trailers in this embodiment is prepared by the following method:

[0089] Molten iron is added to a 120-ton converter and smelted at 1700℃. After 40 minutes, the steel is tapped and 16% scrap steel is added to adjust the temperature to 1650℃. The steel is then transferred to a refining furnace, where ferrosilicon, ferromanganese, ferrochromium-molybdenum, ferrovanadium, ferroniobium, and manganese nitride are added under electromagnetic stirring. Pure aluminum wire is slowly fed in at 1535±15℃, and the chemical composition is adjusted for 30 minutes. The steel is then degassed under RH vacuum (vacuum degree ≤130Pa) and continuously cast into 200×200mm billets. The billets are cooled to 1130℃ at a rate of 35℃ / min, then air-cooled to room temperature. The billets are then peeled to a depth of 2.0mm and reheated to 1200℃. The billets are then continuously rolled into 24*89mm spring strips at an initial rolling temperature of 990℃ and a final rolling temperature of 870℃. After rolling, the strips are rapidly cooled to 600℃ at a rate of 40℃ / min, and then slowly cooled to room temperature at a rate of 9℃ / min.

[0090] The 24*89mm strip steel was prepared according to the above method. Its chemical composition is shown in Table 1. It was further processed into single leaf springs. After quenching at 870℃ and tempering at 480℃, tensile specimens were processed and tensile tests were conducted according to GB / T228-2002. The yield strength, elongation and reduction of area were tested. The assembled leaf springs were subjected to more than six effective fatigue tests according to GB / T228-2002. The results are shown in Table 2.

[0091] Comparative Example 1—SUP9A

[0092] The standard steel SUP9, after testing, has the chemical composition shown in Table 1. It is further processed into 24x89mm single-leaf springs, quenched at 870℃ and tempered at 480℃, and then subjected to tensile testing and processing according to GB / T228-2002. The assembled leaf springs undergo at least six effective fatigue tests according to GB / T228-2002. In addition, its yield strength, elongation, and reduction of area are tested, and the results are shown in Table 2.

[0093] Comparative Example 2---60Si2MnA

[0094] The standard steel 60Si2MnA, after testing, has the chemical composition shown in Table 1. It is further processed into 24x89mm single-leaf springs, quenched at 8700℃ and tempered at 480℃, and then subjected to tensile testing and processing according to GB / T228-2002. The assembled leaf springs undergo at least six effective fatigue tests according to GB / T228-2002. In addition, its yield strength, elongation, and reduction of area are tested, and the results are shown in Table 2.

[0095] Table 1. Comparison of chemical composition between Examples 1-3 and Comparative Examples 1-3

[0096]

[0097] Table 2 Test Results

[0098]

[0099] The results show that, under similar conditions of plasticity, toughness, reduction of area Z, and elongation A, the strength of the spring steel of the present invention, including yield strength (Rp0.2) and tensile strength (Rm), is significantly improved, especially the fatigue strength is improved by more than 4 times, making it particularly suitable for manufacturing weight-reducing springs with fewer leaf springs.

[0100] Example 10

[0101] This embodiment proposes a method for preparing high-hardenability microalloyed leaf spring steel, specifically a reheating continuous rolling method, comprising the following steps:

[0102] Combined descaling pretreatment: High-pressure water jet combined with mechanical brush rollers is used to remove the oxide scale on the surface of the leaf spring steel, followed by alkaline soaking and neutralization treatment;

[0103] Pulsed plasma surface activation treatment: The steel surface is treated with pulsed plasma at a frequency of 0.5-5kHz to form active sites and rough structure on the surface;

[0104] Staged oxidation pretreatment: The activated steel surface undergoes a three-stage oxidation treatment to form a surface structure suitable for the bonding of rare earth elements;

[0105] Rare earth element micro-coating: Select a combination of rare earth elements according to the silicon content of the leaf spring steel, prepare a coating slurry and coat it on the surface of the steel.

[0106] Deep diffusion enhancement treatment: Pulsed thermal cycling treatment is used to promote the diffusion of rare earth elements to the surface of steel, forming a gradient transition structure layer;

[0107] Rolling process control: Rolling is carried out in an oxygen-deficient atmosphere, and the rolling temperature and speed are controlled. After rolling, a segmented cooling method is used for treatment.

[0108] in:

[0109] In the combined descaling pretreatment, the pressure of the high-pressure water jet is 15-25MPa, the action angle is 60°-80°, and the speed of the mechanical brush roller is 800-1200r / min.

[0110] In pulsed plasma surface activation treatment, argon with a purity of ≥99.99% is used as the main working gas, and hydrogen with a volume ratio of 5%-15% is added as the reducing gas. The vacuum degree of the treatment environment is 50-100Pa, and the treatment temperature is 150-250℃.

[0111] The staged oxidation pretreatment includes:

[0112] First stage of oxidation: In a gaseous environment with an oxygen content of 0.5%-1%, heat the steel to 350-400℃ and hold for 30-60 seconds;

[0113] The second stage of oxidation involves adjusting the oxygen content in the gaseous environment to 1%-2%, raising the temperature to 450-500℃, and holding it for 60-120 seconds.

[0114] The third stage of stabilization: Under argon protection, the temperature is maintained at 500-550℃ for 120-180 seconds.

[0115] In rare earth element micro-coating, for leaf spring steel with silicon content <1.5%, a Ce-La-Y combination is used with a mass ratio of 60:30:10; for leaf spring steel with silicon content ≥1.5%, a Ce-La-Nd combination is used with a mass ratio of 50:30:20.

[0116] In rare earth element micro-coating, the coating slurry is composed of rare earth oxide fine powder with a purity of ≥99.9%, organic solvent, and binder with a mass fraction of 2%-5%, and the particle size of the rare earth oxide fine powder is ≤5μm.

[0117] The deep diffusion-enhancing treatment employs a pulsed thermal cycling method, including:

[0118] First cycle: Heat to 600-650℃ at a rate of 50-80℃ / min, hold for 30-60s, and then cool to 400-450℃ at a rate of 30-50℃ / min.

[0119] Second cycle: Heat to 650-700℃ at the same rate, hold for 40-80 seconds, and cool to 450-500℃;

[0120] Third cycle: Heat to 700-750℃ at the same rate, hold for 50-100 seconds, and cool to room temperature.

[0121] Deep diffusion-enhanced treatment was carried out in a protective atmosphere with an oxygen content of ≤0.05%.

[0122] In the rolling process control, pre-rolling heating is carried out in an atmosphere with an oxygen content of 0.2%-0.5% and a heating temperature of 850-950℃.

[0123] In the rolling process control, the initial rolling temperature is 900-950℃, the final rolling temperature is 780-850℃, the single-pass reduction rate is 10%-20%, and the oxygen content in the rolling zone is controlled at 0.1%-0.3%.

[0124] Example 11

[0125] This embodiment proposes a method for preparing high-hardenability microalloyed leaf spring steel, specifically the reheating continuous rolling method, which mainly includes the following steps:

[0126] 1. Combined descaling pretreatment

[0127] The leaf spring steel billet to be rolled undergoes a combined descaling pretreatment, specifically including:

[0128] Mechanical descaling: High-pressure water jet combined with mechanical brush rollers is used to remove coarse oxide scale from the surface of the leaf spring steel. The pressure of the high-pressure water jet is controlled within the range of 20MPa, the action angle is 70°, and the rotation speed of the mechanical brush roller is 1000r / min, so that the oxide scale on the surface of the leaf spring steel is removed while avoiding damage to the base material.

[0129] Alkaline soaking: The steel billet after mechanical descaling is soaked in an alkaline solution with a pH of 11 at a temperature of 70°C for 8 minutes. This weakens the bond between the residual oxide scale and the substrate, making it easier to remove later.

[0130] Neutralization treatment: The steel billet after alkaline soaking is neutralized with a weak acidic solution with a pH of 5.5 for 2 minutes to neutralize and remove the alkaline residue on the surface of the steel billet, forming a clean surface to prepare for subsequent treatment.

[0131] The mechanical descaling in this step uses a combination of non-contact high-pressure water jet as the main method and mechanical brush roller as the auxiliary method. Compared with the traditional single mechanical sand roller descaling, this method avoids damage to the substrate material and improves the descaling efficiency.

[0132] 2. Pulsed plasma surface activation treatment

[0133] Pulsed plasma surface activation treatment is one of the core steps of this invention, specifically including:

[0134] Plasma generation: Plasma is generated using a frequency-adjustable pulsed DC power supply, with the frequency controlled within the range of 3.5kHz, a duty cycle of 30%, and an output power density of 1.2W / cm².

[0135] Gas selection: Argon with a purity of ≥99.99% is used as the main working gas, and hydrogen with a volume ratio of 10% is added as the reducing gas. The total gas flow rate is controlled within the range of 22L / min.

[0136] Processing parameters: The vacuum level of the processing environment is controlled within 80 Pa, the processing time is 5 min, the processing temperature is controlled within 200 ℃, and the distance between the steel billet and the plasma source is maintained within 75 mm.

[0137] Processing trajectory: A combination of moving multi-point discharge and stepping trajectory overlapping processing is adopted to ensure uniform distribution of plasma energy on the steel plate surface. Specifically, the plate surface is divided into multiple processing areas with a 40% overlap between adjacent processing areas. The processing sequence is advanced in a "Z" shape to avoid uneven processing areas.

[0138] The following effects were achieved through pulsed plasma treatment:

[0139] Surface activation: High-energy particles from pulsed discharge bombard the surface of steel, causing the chemical bonds of surface atoms to break and forming a large number of dangling bonds and active sites;

[0140] Surface roughening: Forming a rough surface structure at the nanoscale increases the surface area by 250%;

[0141] Oxide removal: Hydrogen in the plasma reacts with residual oxides on the surface to generate water vapor, which is then removed.

[0142] Temperature control: The pulsed discharge method controls the surface temperature of the steel, avoiding microcracks caused by thermal stress.

[0143] 3. Staged oxidation pretreatment

[0144] A staged oxidation pretreatment is an important step in this invention, the purpose of which is to form a surface structure suitable for the bonding of rare earth elements, specifically including:

[0145] First stage of oxidation: In a gaseous environment with an oxygen content of 0.8%, the steel is heated to 380°C and held for 45 seconds to form an initial oxide layer with a thickness of about 80nm on the surface of the steel.

[0146] The second stage of oxidation involves adjusting the oxygen content in the gaseous environment to 1.5%, raising the temperature to 480℃, and holding it for 90 seconds to allow the oxide to grow along the dominant crystal orientation, forming a directional needle-like or network-like microstructure.

[0147] The third stage of stabilization: Under the protection of argon gas (purity ≥99.99%), the temperature is maintained at 525℃ for 150 seconds to stabilize the formed oxide structure.

[0148] Through these three stages of oxidation treatment, an oxide layer with an oriented structure is formed on the surface of the leaf spring steel. This oxide layer provides a good foundation for the subsequent bonding of rare earth elements and enhances the bonding strength between rare earth elements and the steel surface.

[0149] 4. Rare Earth Element Micro-coating Process

[0150] The rare earth element micro-coating process is the core step of the decarburization prevention and protection in this invention, and specifically includes:

[0151] Coating formulation selection: Choose a suitable combination of rare earth elements based on the silicon content of the leaf spring steel.

[0152] For leaf spring steel with a silicon content of <1.5%: a Ce-La-Y combination is used with a mass ratio of 60:30:10;

[0153] For leaf spring steel with a silicon content ≥1.5%: a Ce-La-Nd combination is used with a mass ratio of 50:30:20.

[0154] Coating slurry preparation: Fine powder of rare earth oxides with a purity of ≥99.9% (particle size ≤5μm) is dispersed in an organic solvent (ethanol or isopropanol), and a binder with a mass fraction of 3% is added. The mixture is ultrasonically dispersed for 45 minutes to form a uniform and stable coating slurry.

[0155] Coating method selection: Choose a suitable coating method based on the dimensions of the leaf spring steel.

[0156] For narrow strip steel with a width ≤ 200mm: use dip coating method, with the dip speed controlled at 8mm / s and the pull-out speed at 4mm / s;

[0157] For wide strip steel with a width > 200 mm: use the spraying method, with the distance between the nozzle and the steel surface controlled at 180 mm, the spraying angle at 80°, the travel speed at 400 mm / s, and the overlap ratio of adjacent sprayed strips at 450%.

[0158] Coating drying: Use infrared or hot air drying at 100℃ for 4 minutes to ensure a uniform coating free of bubbles and cracks.

[0159] Coating thickness control: By adjusting the slurry concentration and coating parameters, the thickness of the dried coating is kept within the range of 8-15 μm to ensure the effectiveness of subsequent diffusion treatment.

[0160] 5. Deep diffusion promotion treatment

[0161] Deep diffusion-enhanced treatment is a key step in ensuring a stable bond between rare earth elements and the steel surface, and specifically includes:

[0162] Pulse-type thermal cycling treatment: The coated steel is treated using a pulse-type heating-cooling cycle method, specifically as follows:

[0163] First cycle: Heat to 625°C at a rate of 65°C / min, hold for 45 seconds, and then cool to 425°C at a rate of 40°C / min.

[0164] Second cycle: Heat to 680℃ at the same rate, hold for 60 seconds, and cool to 480℃;

[0165] Third cycle: Heat to 725°C at the same rate, hold for 75 seconds, and cool to room temperature.

[0166] Thermal cycling control: The heating rate is controlled within the range of 65℃ / min, and the cooling rate is controlled within the range of 70℃ / min. Through this pulsed thermal cycling, thermal stress is used to promote the diffusion of rare earth elements into the matrix.

[0167] Protective atmosphere: The entire diffusion process is carried out in a protective atmosphere (argon or nitrogen) with an oxygen content of ≤0.05% to prevent oxidation of the steel surface during diffusion.

[0168] Through the above-mentioned pulsed thermal cycling treatment, rare earth elements diffuse to the surface of steel to a depth of 3-5 μm, forming a composite structural layer with gradient transition characteristics. This enables the rare earth elements to form a metallurgical bond with the steel substrate, improving the adhesion and high-temperature stability of the coating.

[0169] 6 Rolling process control

[0170] Rolling process control is a crucial step in ensuring the integrity and functionality of the anti-decarburization coating during rolling, and specifically includes:

[0171] Pre-rolling heating: Heating is carried out in an oxygen-deficient atmosphere with an oxygen content controlled at 0.3%, the heating temperature is 900℃, and the heating time is determined according to the thickness of the steel, with 1.0 min of heating time required per millimeter of thickness.

[0172] Rolling parameter control:

[0173] The rolling temperature is controlled within the range of 925℃.

[0174] The final rolling temperature is controlled within the range of 820℃.

[0175] The single-pass reduction rate is controlled within 15%.

[0176] Rolling speed: 2.5 m / s for the initial pass and 5 m / s for the final pass.

[0177] Rolling atmosphere control: By injecting a small amount of nitrogen or argon into the rolling area, a rolling environment with an oxygen content of 0.2% is formed, reducing the decarburization reaction during the rolling process.

[0178] Post-rolling cooling control: The rolled steel is treated by segmented cooling. First, it is rapidly cooled to below 650℃ at a cooling rate of ≥30℃ / min, and then slowly cooled to room temperature at a cooling rate of ≤10℃ / min.

[0179] By controlling the rolling process as described above, the stability and effectiveness of the anti-decarburization protective layer formed by rare earth elements are ensured during the high-temperature rolling process, achieving the anti-decarburization protection effect throughout the entire process.

[0180] Through the above methods, the present invention achieves the following technical effects:

[0181] Using the rolling method of this invention, the decarburized layer depth of high-silicon leaf spring steel (Si content ≥ 1.5%) is reduced from 0.15-0.25 mm in traditional processes to 0.01-0.03 mm, a reduction of 85%-95%. For standard leaf spring steel (Si content < 1.5%), the decarburized layer depth is controlled within the range of 0.005-0.015 mm. This effect is mainly achieved by rare earth elements forming a stable oxide protective layer at high temperatures, effectively preventing the outward diffusion of carbon elements.

[0182] Because decarburization is controlled, the surface hardness distribution uniformity of the leaf spring steel rolled by the method of this invention is improved. The difference between surface hardness and core hardness is reduced from 15%-25% in the traditional process to 3%-8%, and the surface hardness variation coefficient (Cv) is reduced from 5%-8% to 1.5%-3%. This effect has a significant impact on improving the fatigue performance of leaf spring steel, as the uniformity of surface hardness directly affects the initiation and propagation of fatigue cracks.

[0183] The surface quality of leaf spring steel treated by the method of this invention is improved, mainly in the following ways: Surface roughness is reduced from Ra 1.6-2.5μm in traditional processes to Ra 0.8-1.2μm; The number of surface defects (such as oxide scale indentation, scratches, cracks, etc.) is reduced by more than 80%; The distribution of residual stress on the surface is more uniform, and the residual compressive stress value increases by 15%-25%. These improvements in surface quality stem from the surface regulation effect of the combined descaling pretreatment and pulsed plasma activation treatment.

[0184] By combining staged oxidation pretreatment with deep diffusion-promoting treatment, the high-temperature stability of the rare earth element anti-decarburization coating is improved. In a high-temperature environment of 900-950℃, traditional coatings fail within 5-10 minutes, while the coating of this invention can maintain effective protection for 30-60 minutes, improving stability by 5-6 times. This is mainly due to the formation of a gradient transition layer structure, which creates a metallurgical bond between the coating and the substrate, rather than a simple physical adhesion.

[0185] The leaf spring steel rolled using the method of this invention exhibits improved fatigue performance and service life after standard heat treatment: high-cycle fatigue limit is increased by 20%-30%; fatigue life is extended by over 300% under the same stress level; and the service life of the spring in actual operating environments is extended by 250%-350%. This effect is mainly attributed to controlled surface decarburization and improved surface quality, which reduces the sources of fatigue cracks and slows down crack propagation.

[0186] Experimental verification

[0187] To verify the technical effectiveness of this embodiment, the following experimental results were conducted:

[0188] Overview of Experimental Verification

[0189] The experiment employed a comparative analysis method, comparing leaf spring steel samples treated with the method of this invention with those treated with traditional methods to comprehensively evaluate the degree of improvement in technical effectiveness. The experimental samples used two materials: standard leaf spring steel (silicon content 1.0%) and high-silicon leaf spring steel (silicon content 2.0%), to verify the adaptability of the method of this invention to leaf spring steels with different silicon contents.

[0190] 1. Experiment to verify the effect of reducing the decarburized layer depth

[0191] 1.1 Experimental Objective

[0192] The effectiveness of the method of the present invention in reducing the depth of the decarburized layer on the surface of leaf spring steel compared with traditional methods was verified, especially the decarburization protection effect on high silicon leaf spring steel.

[0193] 1.2 Experimental Materials and Equipment

[0194] Materials: Standard leaf spring steel sample (Si content 1.0%, size: 100mm×50mm×10mm); high silicon leaf spring steel sample (Si content 2.0%, size: 100mm×50mm×10mm); rare earth element coating material (prepared according to the formula of this invention); traditional anti-decarburization coating (commercially available product).

[0195] Equipment: High-pressure water jet descaling device; pulsed plasma treatment system (frequency adjustable: 0.5-5kHz); controlled atmosphere furnace (temperature range: room temperature - 1000℃); metallurgical microscope (magnification: 50-1000x); electron probe microanalyzer (EPMA); scanning electron microscope (SEM); rolling simulation device

[0196] 1.3 Experimental Methods

[0197] Sample grouping:

[0198] Group A: Standard leaf spring steel processed using conventional methods (Si 1.0%).

[0199] Group B: Standard leaf spring steel (Si 1.0%) processed by the process of this invention;

[0200] Group C: High-silicon leaf spring steel (Si 2.0%) processed using conventional methods;

[0201] Group D: High-silicon leaf spring steel (Si 2.0%) processed by the process of this invention.

[0202] Sample processing:

[0203] Groups A and C use traditional processes (mechanical descaling + traditional anti-decarburization coating + ordinary heating + conventional rolling).

[0204] Groups B and D are processed using the process of this invention (all processes are carried out according to the process flow of Embodiment 1).

[0205] Rolling parameters:

[0206] Heating temperature: 900℃;

[0207] Rolling temperature: initial rolling temperature 920℃, final rolling temperature 820℃;

[0208] Reduction rate: 15% for a single pass, 50% for the total reduction rate;

[0209] Rolling speed: 3m / s.

[0210] Methods for measuring the depth of decarburized layer:

[0211] The samples are rolled and then subjected to standard heat treatment (quenching + tempering).

[0212] Metallographic specimens are prepared by cutting along a direction perpendicular to the rolling direction;

[0213] Corrosion was performed using a 4% nitric acid-alcohol solution;

[0214] The depth of the decarburized layer was observed and measured under a metallographic microscope;

[0215] Ten measurements were taken at different locations for each sample, and the average value was calculated.

[0216] 1.4 Experimental Results and Analysis

[0217] 1. Measurement results of decarburized layer depth

[0218] Table 1. Measurement results of decarburized layer depth of leaf spring steel treated with different processes (unit: mm)

[0219]

[0220] 2. Distribution characteristics of the decarburized layer

[0221] The microstructure of the decarburized layer was observed by scanning electron microscopy, and the following results were obtained: the interface between the decarburized layer and the matrix of the samples treated by the traditional process (group A and group C) was blurred and showed a gradual change, especially the decarburization phenomenon of high silicon leaf spring steel (group C) was more serious; the decarburized layer of the samples treated by the process of the present invention (group B and group D) was extremely thin and the interface with the matrix was clear.

[0222] 1.5 Conclusion Analysis

[0223] Comparison of decarburized layer depth: Compared with the traditional process, the decarburized layer depth of the leaf spring steel samples treated by the process of this invention is significantly reduced.

[0224] Standard leaf spring steel (Si 1.0%): reduced from 0.100mm to 0.011mm, a reduction rate of approximately 89%;

[0225] High-silicon leaf spring steel (Si 2.0%): reduced from 0.217mm to 0.0225mm, a reduction rate of approximately 90%;

[0226] The effect on high-silicon leaf spring steel is obvious: The process of this invention has a particularly obvious effect on decarburization protection of high-silicon leaf spring steel, and solves the technical problem of easy decarburization of high-silicon leaf spring steel in traditional processes.

[0227] 2. Verification Experiment on the Improvement of Surface Hardness Distribution Uniformity

[0228] 2.1 Experimental Objective

[0229] To verify whether the method of the present invention can significantly improve the uniformity of surface hardness distribution of leaf spring steel compared with the traditional method, and to evaluate the degree of reduction in the hardness difference between the surface and the core, as well as the reduction in the surface hardness variation coefficient.

[0230] 2.2 Experimental Materials and Equipment

[0231] Materials: Four groups of leaf spring steel samples (A, B, C, and D) processed in Section 1; standard hardness blocks (Rockwell and Vickers hardness).

[0232] Equipment: Rockwell hardness tester (HRC); Vickers microhardness tester (load range: 10g-1kg); data acquisition and statistical analysis system; automatic hardness distribution mapping instrument.

[0233] 2.3 Experimental Methods

[0234] Sample heat treatment:

[0235] All samples underwent standard quenching and tempering heat treatment:

[0236] Quenching: Heat to 860℃, hold for 30 minutes, then oil quench;

[0237] Tempering: Hold at 450℃ for 2 hours, then air cool.

[0238] Cross-sectional hardness testing method:

[0239] The sample was cut along a direction perpendicular to the rolling direction to prepare a metallographic specimen;

[0240] Vickers microhardness tester, 500g load;

[0241] Measure every 0.1 mm from the surface to the core, up to a depth of 1.5 mm;

[0242] Take one test line every 45° around the sample, for a total of 8 test lines;

[0243] Record the hardness values ​​at different depths and their changing trends.

[0244] Surface hardness distribution test method:

[0245] 9×9=81 test points are evenly arranged on the sample surface (with a spacing of 10mm).

[0246] The hardness values ​​at various points were measured using a Rockwell hardness tester.

[0247] Calculate the average surface hardness, standard deviation, and coefficient of variation (Cv);

[0248] Draw a heat map of surface hardness distribution.

[0249] 2.4 Experimental Results and Analysis

[0250] 1. Results of surface and core hardness difference test

[0251] Table 2. Differences in surface and core hardness of leaf spring steel treated with different processes (Vickers hardness HV)

[0252]

[0253] 2. Hardness gradient curve

[0254] Figure 1Hardness gradient curves of leaf spring steel processed by different techniques.

[0255] 3. Analysis of Surface Hardness Distribution Uniformity

[0256] Table 3. Statistical results of surface hardness distribution of leaf spring steel treated with different processes (Rockwell hardness HRC)

[0257]

[0258] 4. Surface hardness distribution heatmap

[0259] A thermal map was created using the hardness values ​​at 81 test points on the sample surface to visually demonstrate the hardness distribution.

[0260] Figure 2 Comparison of surface hardness distribution heat maps of leaf spring steel treated with different processes.

[0261] 2.5 Conclusion Analysis

[0262] Surface and core hardness difference: In the leaf spring steel samples treated by the process of this invention, the hardness difference between the surface and core is significantly reduced.

[0263] Standard leaf spring steel (Si 1.0%): The percentage of hardness difference decreased from 15.4% to 2.7%;

[0264] High silicon leaf spring steel (Si 2.0%): The percentage of hardness difference decreased from 21.5% to 4.9%.

[0265] Hardness gradient characteristics:

[0266] Samples treated with traditional processes show a significant decrease in hardness near the surface, with a large gradient change.

[0267] The sample processed by the present invention exhibits a gentle hardness gradient from the surface to the core, with the surface hardness approaching the core hardness.

[0268] Surface hardness uniformity: The surface hardness distribution of samples treated by the process of this invention is more uniform.

[0269] Standard leaf spring steel: coefficient of variation decreased from 5.7% to 1.4%;

[0270] High-silicon leaf spring steel: The coefficient of variation decreased from 7.8% to 1.7%.

[0271] Technical effect evaluation:

[0272] The process of this invention effectively controls decarburization, thus maintaining the surface hardness of the leaf spring steel at a high level.

[0273] The uniformity of surface hardness distribution is significantly improved, and there are no obvious hardness fluctuation areas on the surface.

[0274] The improvement effect of high-silicon leaf spring steel is particularly significant, solving the technical problem of uneven surface hardness of high-silicon leaf spring steel in traditional processes.

[0275] The embodiments of the present invention have been described above. However, the embodiments are not limited to the specific implementation methods described above. The specific implementation methods described above are merely illustrative and not restrictive. Those skilled in the art can make more equivalent embodiments under the guidance of the present embodiments, and all of them are within the protection scope of the present embodiments.

Claims

1. A high-hardenability microalloyed leaf spring steel, characterized in that, The chemical composition includes the following mass ratios: C: 0.45-0.69%; Si: 1.30-2.20%; Mn: 0.60-1.30%; Cr: 0.10-0.35%; Al: 0.075-0.15%; N: 0.008-0.025%; with the balance being Fe. It also includes the following controlled impurity chemical composition: Pb, Sn, Zn, Sb, and Bi ≤ 0.03%; (O2+H2) ≤ 30 ppm; S and P ≤ 0.025%; Cu ≤ 0.25%. The steel used for leaf springs has a thickness of 6-32 mm or a diameter of 6-35 mm; the microstructure of the steel used for leaf springs is greater than 95% sorbite and a small amount of ferrite, and contains only sorbite and ferrite, without other structures; the semi-decarburized layer of the steel used for leaf springs is less than or equal to 0.20 mm, and there is no fully decarburized layer; after heat treatment, the grain size is greater than ASTM 9.0 grade.

2. A method for preparing the high hardenability microalloyed leaf spring steel according to claim 1, characterized in that: The steel raw material is successively smelted, refined twice, continuously poured and cooled into steel ingots, the steel ingots are peeled off, and then heated and continuously rolled and cooled in a controlled manner to obtain steel for leaf springs.

3. The method for preparing a high-hardenability microalloyed leaf spring steel according to claim 2, characterized in that: The melting temperature is 1580-1700℃ and the time is 20-60 minutes; the refining temperature is 1500-1580℃ and the time is 20-60 minutes, and the refining process uses electromagnetic stirring.

4. The method for preparing a high-hardenability microalloyed leaf spring steel according to claim 2, characterized in that: The continuous casting and cooling process produces steel ingots with a compression ratio greater than 15:1 between the ingot size and the final steel size for the leaf springs. The ingots are first cooled to below 1100°C at a rapid cooling rate of greater than 35°C / min, and then allowed to cool naturally to room temperature.

5. The method for preparing a high-hardenability microalloyed leaf spring steel according to claim 2, characterized in that: The depth of the peeling of the steel ingot is greater than 1.0 mm.

6. The method for preparing a high-hardenability microalloyed leaf spring steel according to claim 2, characterized in that: The reheating continuous rolling process has an initial rolling temperature of 930-1080℃ and a final rolling temperature of 780-850℃.

7. The method for preparing a high-hardenability microalloyed leaf spring steel according to claim 2, characterized in that: The controlled cooling process specifically involves: first, rapid cooling to 600℃ at a rate ≥35℃ / min; then, holding at the temperature and slowly cooling to room temperature at a rate ≤10℃ / min.

8. The method for preparing a high-hardenability microalloyed leaf spring steel according to claim 2, characterized in that, The reheated continuous rolling process includes the following steps: Combined descaling pretreatment: High-pressure water jet combined with mechanical brush rollers is used to remove the oxide scale from the surface of the leaf spring steel, followed by alkaline soaking and neutralization treatment; Pulsed plasma surface activation treatment: Pulsed plasma with a frequency of 0.5-5kHz is used to treat the steel surface, forming active sites and a rough structure; Staged oxidation pretreatment: The activated steel surface undergoes a three-stage oxidation treatment to form a surface structure suitable for rare earth element bonding; Rare earth element micro-coating: A combination of rare earth elements is selected according to the silicon content of the leaf spring steel, a coating slurry is prepared and coated onto the steel surface; Deep diffusion promotion treatment: Pulsed thermal cycling treatment is used to promote the diffusion of rare earth elements to the steel surface, forming a gradient transition structure layer; Rolling process control: Rolling is carried out in an oxygen-deficient atmosphere, and the rolling temperature and speed are controlled. After rolling, a segmented cooling method is used for treatment.

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