Method for prolonging fatigue life of spring steel through rare earth treatment

Abstract

The invention discloses a method for prolonging the fatigue life of spring steel through rare earth treatment. The method specifically comprises the steps that the total adding amount of rare earth elements accounts for 0.02%-0.15% of the mass of molten steel; rare earth elements are added in the later stage of LF refining, after alloying is completed, small-gas-amount clean argon blowing is conducted for 10 min to 20 min, and the argon flow is 15 NL / min to 20 NL / min; after hot rolling, cooling to below 550 DEG C at the cooling speed of 10-20 DEG C / s; and heating the hot-rolled spring steel blank to 450-550 DEG C, keeping the temperature for 2-3 hours, and then performing air cooling. According to the method, molten steel is purified through rare earth elements, long-strip-shaped MnS and other inclusions are converted into spherical rare earth oxysulfide (such as Ce2OS), stress concentration sources are reduced, and the crack initiation probability is reduced. Austenite grain growth is inhibited through rare earth elements, so that the spring steel obtains a uniform and fine structure (the grain size is greater than or equal to 9 grades), and the crack propagation resistance of the material is enhanced. Through the synergistic effect of tempering treatment and rare earth treatment, the comprehensive mechanical property of the spring steel is improved, and the fatigue life is greatly prolonged.

Description

Technical Field

[0001] The invention belongs to the technical field of metal material processing, and in particular relates to a method for improving the fatigue life of spring steel through rare earth treatment. Background Art

[0002] Spring steel, a key mechanical manufacturing material, is widely used in the automotive, machinery, aerospace, and other fields, primarily in the manufacture of various spring components. Springs must withstand frequent alternating loads during operation, requiring spring steel to possess excellent fatigue properties. However, conventional spring steel has limitations in fatigue life, restricting its application in high-end applications.

[0003] During the production process of spring steel, numerous factors affect its fatigue life. Among them, the presence of inclusions is a key factor contributing to the decline in spring steel's fatigue performance. Inclusions, such as oxides and sulfides, disrupt the steel's continuity and easily trigger crack initiation and propagation under alternating loads. Grain size also significantly affects spring steel's fatigue life, with coarse grains reducing the material's fatigue resistance. Furthermore, traditional production processes lack control over steel purity and structural uniformity, making it difficult to fully realize the performance potential of spring steel.

[0004] Currently, there are several methods for improving spring steel performance, such as microalloying and optimizing heat treatment processes. However, relatively little research has focused on specifically improving spring steel fatigue life through rare earth element treatment. Due to their unique chemical properties, rare earth elements have potential advantages in improving inclusion morphology and grain refinement in steel, potentially offering a new solution for improving spring steel fatigue life.

[0005] Patent document CN114107841B, "A High-Strength, Corrosion-Resistant Spring Steel and Its Preparation Method," describes a preparation method that includes rare earth (RE) elements in its composition design. However, the invention does not clearly explain the role and importance of RE addition. Patent document CN117683970B, "A Rare Earth Treatment Method for High-Strength Wheel Steel," notes that the addition of RE elements can help improve the fatigue performance of wheel steel, but there is no specific description or detailed discussion of this specifically for spring steel. Summary of the Invention

[0006] This invention provides a method for improving the fatigue life of spring steel through rare earth treatment. This method uses rare earth treatment technology to optimize the internal structure and inclusion characteristics of spring steel, thereby significantly improving the fatigue life of the spring steel. By adding specific rare earth elements and precisely controlling the amount of rare earth elements added and related process parameters, the method improves the morphology of spring steel inclusions, refines the grain size, and enhances structural uniformity, thereby significantly improving the fatigue life of the spring steel.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] A method for improving the fatigue life of spring steel by rare earth treatment, specifically comprising:

[0009] 1) One or both of cerium (Ce) and lanthanum (La) are added to molten steel in the form of a pre-alloyed cored wire (rare earth-Fe alloy), with the total amount of rare earth elements added being 0.02% to 0.15% of the mass of the molten steel, in order to improve the yield of the rare earth elements and their uniform distribution in the molten steel.

[0010] 2) In the later stage of LF refining, the molten steel is refined in the LF furnace to ensure complete deoxidation and sufficient floating of inclusions. Then, rare earth elements are added to the molten steel in the form of cored wire. After alloying is completed, argon is blown in a small amount for 10 to 20 minutes with an argon flow rate of 15 to 20 NL / min to promote uniform diffusion of rare earth elements.

[0011] 3) Hot working process: The continuous casting billet is heated to 1100-1150°C and hot rolled. The hot rolling finishing temperature is controlled at 800-850°C. After rolling, the billet is cooled to below 550°C at a cooling rate of 10-20°C / s.

[0012] 4) Tempering treatment: Heat the hot-rolled spring steel billet to 450-550°C, keep it warm for 2-3 hours, and then air-cool it to eliminate residual stress and stabilize the structure.

[0013] The spring steel produced by the method of the present invention has a grain size of ≥9.0. The fatigue life of the spring steel reaches 5×10 6 More than one cycle.

[0014] Rare earth elements react chemically with elements like sulfur and oxygen in steel, transforming large, sharp, and brittle inclusions into small, spherical, or spindle-shaped ones. The diffuse distribution of these rare earth inclusions in the steel significantly reduces stress concentration points and the probability of crack initiation. Furthermore, the segregation of rare earth atoms at grain boundaries inhibits grain growth, refines the grain structure, and imparts to the spring steel enhanced resistance to crack growth under alternating loads.

[0015] Through extensive testing, the present invention has determined that the optimal mass fraction of rare earth elements in spring steel is 0.02-0.15%. Within this range, rare earth elements can fully exert their role in modifying inclusions and refining grains while avoiding the adverse effects of excessive addition. Under the same composition and processing conditions, the fatigue life of spring steel treated with the present invention can be increased by 30-50% compared to spring steel without rare earth addition.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] 1) The present invention utilizes rare earth elements to purify molten steel, transforming inclusions such as long strips of MnS into spherical rare earth oxysulfides (such as Ce2OS), thereby reducing stress concentration sources and lowering the probability of crack initiation.

[0018] 2) The present invention suppresses the growth of austenite grains by using rare earth elements, so that the spring steel obtains a uniform and fine structure (grain size ≥ 9.0 level), thereby enhancing the material's ability to resist crack growth.

[0019] 3) The tempering treatment and rare earth treatment in the present invention work synergistically to improve the comprehensive mechanical properties of the spring steel and significantly increase the fatigue life.

[0020] Conventional processes control the sulfur content in steel to 0.010-0.015%, the phosphorus content to 0.020-0.025%, and the oxygen content to 20-30 ppm. Mechanical properties include elongation of 14.5-19%, reduction of area of 33-46%, impact energy of 28-30 J, and fatigue life (number of cycles) of 3-4 million. The present invention, after using rare earth elements (RE treatment), controls the sulfur content in the steel to ≤0.005%, the phosphorus content to ≤0.020%, and the oxygen content to ≤15 ppm. Mechanical properties include elongation of 16%-20.5%, reduction of area of 41%-51%, impact energy of 35-38 J, and fatigue life (number of cycles) of more than 5 million. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a micrograph of inclusions in conventional technical steel.

[0022] Figure 2 This is a micrograph of inclusions in Example 1 of the present invention.

[0023] Figure 3 It is a metallographic diagram of the microstructure of conventional technical steel.

[0024] Figure 4 This is a metallographic microstructure diagram of Example 1 of the present invention. DETAILED DESCRIPTION

[0025] The specific embodiments of the present invention will be further described below in conjunction with examples. The following examples are used to specifically illustrate the content of the present invention. These examples are only general descriptions of the content of the present invention and do not limit the content of the present invention.

[0026] Example 1:

[0027] Raw material: 60Si2Mn spring steel

[0028]

[0029] Production process:

[0030] 1. After LF furnace refining, add La rare earth cored wire and stir with argon for 12 minutes at an argon flow rate of 19NL / min.

[0031] 2. The continuous casting billet is heated to 1120℃ and hot rolled. The hot rolling final temperature is controlled at 830℃ and cooled at a controlled rate (15℃ / s) to 500℃.

[0032] 3. Tempering: 500℃×2.5h.

[0033] Fatigue life test: After testing, the fatigue life of the spring steel reaches 5×10 6 cycles (the traditional process is 3×10 6 The oxygen content in the steel is controlled at 25 ppm, and the mechanical properties are: elongation 20.5%, reduction of area 51%, and impact energy (J) 38 J.

[0034] See Figure 1 and Figure 2 Comparison: The inclusions in spring steel produced by conventional technology are chain-like Al2O3, irregularly angular, 10-20 μm in size, and distributed along the rolling direction. The inclusions in spring steel produced by the present invention are spherical RE2O2S, 2-5 μm in size, and evenly dispersed. The inclusion level is reduced from 2.5 to 1.0. Figure 3 and Figure 4 Comparison: The microstructure of the spring steel of the present invention is compared with that of the spring steel of conventional technology: the grain size after rolling is reduced to 12-15μm, the refinement effect is significant, the grain size is increased from 8.0-8.5 level to 9.0-9.5 level, and the organization is more uniform.

[0035] Example 2:

[0036] Raw material: 55CrMnA spring steel

[0037]

[0038] Process:

[0039] 1. After LF furnace refining, add La-Ce (La:Ce=2:1 by weight) mixed rare earth cored wire and stir with argon for 15 minutes at an argon flow rate of 18NL / min.

[0040] 2. The continuous casting billet is heated to 1130℃ and hot rolled. The hot rolling final temperature is controlled at 820℃ and cooled at a controlled rate (18℃ / s) to 520℃.

[0041] 3. Tempering: 520℃×2.5h.

[0042] Fatigue life test: The fatigue life of this spring steel is 5.5×10 6cycles (the traditional process is 3.5×10 6 cycles). The oxygen content in the steel is 12 ppm; the mechanical properties are: elongation 18%, reduction of area 46%, and impact energy (J) 38 J.

[0043] Example 3:

[0044] Raw material: 60Si2CrVA spring steel

[0045]

[0046] Process:

[0047] 1. After LF furnace refining, add Ce rare earth cored wire and stir with argon for 18 minutes at an argon flow rate of 19NL / min.

[0048] 2. The continuous casting billet is heated to 1140℃ and hot rolled. The hot rolling finishing temperature is controlled at 840℃ and cooled at a controlled rate (20℃ / s) to 530℃.

[0049] 3. Tempering: 530℃×2.5h.

[0050] Fatigue life test: fatigue life reaches 6×10 6 cycles (the traditional process is 4×10 6 cycles). The oxygen content in the steel is 13 ppm; the mechanical properties are: elongation 16%, reduction of area 41%, and impact energy (J) 35 J.

Claims

1. A method for improving the fatigue life of spring steel by rare earth treatment, characterized in that: Specifically include: 1) The total addition amount of rare earth elements is 0.02% to 0.15% of the mass of the molten steel; 2) In the late stage of LF refining, rare earth elements are added to the molten steel. After alloying is completed, argon is blown in a small amount for 10 to 20 minutes, with an argon flow rate of 15 to 20 NL / min; 3) Controlled cooling after rolling: After hot rolling, cool to below 550°C at a cooling rate of 10-20°C / s; 4) Tempering treatment: Heat the hot-rolled spring steel billet to 450-550°C, keep it at this temperature for 2-3 hours, and then air-cool it.

2. The method for improving the fatigue life of spring steel by rare earth treatment according to claim 1, characterized in that: The rare earth element is one of cerium and lanthanum or a mixture of the two.

3. The method for improving the fatigue life of spring steel by rare earth treatment according to claim 1, characterized in that: In the step 2), rare earth elements are added to the molten steel in the form of cored wire.

4. The method for improving the fatigue life of spring steel by rare earth treatment according to claim 1, characterized in that: The continuous casting slab is heated to 1100-1150°C and hot rolled, and the hot rolling finishing temperature is controlled at 800-850°C.

5. The method for improving the fatigue life of spring steel by rare earth treatment according to claim 1, characterized in that: The spring steel has a grain size of ≥9.

0.

6. The method for improving the fatigue life of spring steel by rare earth treatment according to claim 1, characterized in that: The fatigue life of the spring steel reaches 5×10 6 More than one cycle.

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