Spring steel with high bending fatigue properties and method of heat treatment and production thereof

Abstract

The application discloses a spring steel with high bending fatigue performance and a heat treatment method and a production method thereof, the spring steel contains the following components in percentage by weight: C 0.56-0.64%, Si 1.40-2.00%, Mn 0.35-0.75%, Cr 0.90-1.30%, V 0.10-0.20%, Nb 0.03-0.06%, Al 0.020-0.045%, P≤0.015%, S≤0.010%, O≤15ppm, [H]≤1.5ppm, [N]≤50ppm, and the rest is Fe and other inevitable impurities; the spring steel with the tensile strength≥2300MPa, the surface shrinkage rate≥50%, the bending fatigue strength≥940MPa and the austenitic grain size≥8.0 grade is produced at low cost, and the spring steel has high strength and high bending fatigue performance.

Description

Technical Field

[0001] This invention belongs to the field of spring steel technology, specifically relating to a spring steel with high bending fatigue performance and its heat treatment and production methods. Background Technology

[0002] In recent years, with the introduction of the goal of lightweighting in automobiles, the research and development of automotive-related component materials has also shifted towards this goal, including springs, which are important safety components in automobiles. The main idea of ​​automotive lightweighting is to improve the composition and heat treatment process to reduce the weight of materials while increasing their strength, thus achieving the requirements for lightweighting. In addition to meeting strength requirements, springs are also subjected to high bending stress during service, requiring materials with high bending fatigue strength.

[0003] Chinese patent CN 109735765 A discloses a large-size, ultra-fine-grained, high-strength and high-toughness spring steel and its production method. Its main chemical composition (by weight percentage) is: C: 0.47-0.52, Si: 0.15-0.35, Mn: 0.95-1.15, P: ≤0.020, S: ≤0.020, Cu: ≤0.20, Cr: 0.90-1.10, Mo: ≤0.30, Al: ≤0.050, V: 0.10-0.25, Nb: ≤0.040, Ti: ≤0.035, N: ≤0.015, with the remainder being Fe. Under specific processes, the produced spring steel product achieves an austenitic grain size of 8.0, an impact absorption energy (KU2) of not less than 20J, and a tensile strength of not less than 1800MPa. However, this method involves the addition of excessively high levels of titanium, which is difficult to control during production.

[0004] Chinese patent CN 115125450 A discloses a high-performance rail fastener spring clip, its manufacturing method, and its application. Its main chemical composition (by weight percentage) is: 0.2%≤C≤0.8%, 0.5%≤Mn≤3.0%, 1.0%≤Si≤2.5%, Cr≤2.0%, Ti≤0.15%, Nb≤0.15%, V≤0.15%, Mo≤0.50%, B≤0.004%, P≤0.02%, S≤0.03%, N≤0.02%, with the remainder being Fe and unavoidable impurities. A unique heat treatment process achieves a spring clip with a hardness ≥44HRC, a reduction of area ≥25%, a room temperature impact absorption energy KV2 ≥10J, and a total decarburized layer depth ≤0.2mm. Furthermore, the spring clip exhibits excellent high-cycle fatigue resistance. However, this method is limited to laboratory processes, requires a relatively high amount of alloy additives, and necessitates strict control of elements such as P, S, and N, which is detrimental to the fatigue performance of the spring. Summary of the Invention

[0005] To solve the above-mentioned technical problems, the present invention provides a spring steel with high bending fatigue performance, its heat treatment method and production method, and the spring steel obtained by low-cost production has a tensile strength ≥2300MPa, a reduction of area ≥50%, a bending fatigue strength ≥940MPa, and an austenite grain size ≥8.0 grade, which simultaneously possesses high strength and high bending fatigue performance.

[0006] The technical solution adopted in this invention is as follows:

[0007] A spring steel with high bending fatigue performance contains, by weight percentage: C 0.56%–0.64%, Si 1.40%–2.00%, Mn 0.35%–0.75%, Cr 0.90%–1.30%, V 0.10%–0.20%, Nb 0.03%–0.06%, Al 0.020%–0.045%, P≤0.015%, S≤0.010%, O≤15ppm, [H]≤1.5ppm, [N]≤50ppm, with the remainder being Fe and other unavoidable impurities.

[0008] The microstructure of the spring steel with high bending fatigue performance is martensite + carbon-free bainite + retained austenite.

[0009] The high bending fatigue performance spring steel has a tensile strength ≥2300MPa, a reduction of area ≥50%, a bending fatigue strength ≥940MPa, and an austenite grain size ≥8.0 grade.

[0010] The present invention also provides a heat treatment method for the high bending fatigue performance of spring steel, the heat treatment method comprising the following steps: rapidly heating the spring steel to 860-900℃ and holding it at that temperature, then cooling it to 75-100℃ and holding it at that temperature, then holding it at 230-280℃ and then heating it to 300-400℃ for tempering, and finally water cooling it to room temperature.

[0011] The heat treatment method includes the following steps: rapidly heating the spring steel to 860-900℃ at a heating rate of 20-30℃ / s and holding it at that temperature for 20-40 minutes, then cooling it to 75-100℃ and holding it at that temperature for 20-60 seconds, then holding it at 230-280℃ for 30-70 minutes, and then heating it to 300-400℃ for tempering for 30-80 minutes. During this process, the steel is divided and tempered, and finally water-cooled to room temperature.

[0012] In the above heat treatment process, the steel is first rapidly heated to the austenitizing temperature of 860-900℃ at a rate of 20-30℃ / s and held for 20-40 minutes. Then, it is cooled to the T2 quenching medium oil (75-100℃) and held for 20-60 seconds. The T2 temperature is Ms (the starting temperature of martensite transformation) - M fThe quenching process involves holding the material at a temperature between the martensitic transformation end temperature and the quenching termination temperature for a period of time. In this invention, the preferred T2 temperature is 80℃ for 30 seconds. This step yields a mixed structure of martensite and retained austenite. The material is then held at T3 temperature (230-280℃) for 30-70 minutes and then heated to T4 temperature (300-400℃) for tempering for 30-80 minutes. Finally, the material is water-cooled to room temperature. T3 temperature is a specific temperature higher than Ms temperature. At this temperature, the material is partitioned. The partitioning process mainly involves the partitioning of carbon. Face-centered cubic austenite has a higher carbon solubility than body-centered cubic martensite. Carbon atoms in the supersaturated martensite phase diffuse into the austenite phase, enriching carbon atoms in the austenite to obtain stable retained austenite. In this invention, the preferred partitioning temperature is 240℃, and the preferred partitioning time is 40 minutes. The material is then heated to T4 for tempering. The tempering process involves the precipitation of carbides from microalloying elements. In this invention, the preferred tempering temperature is 320℃, and the preferred tempering time is 40 minutes. Ultimately, the tensile strength of the heat-treated material is ≥2300 MPa, and the reduction of area is ≥50%. The microstructure consists of martensite (initial quenched martensite + secondary quenched martensite), carbon-free bainite, and retained austenite, such as... Figure 2 As shown.

[0013] The present invention also provides a method for producing the spring steel with high bending fatigue performance, the method comprising the following steps: converter smelting—LF refining—RH treatment—continuous casting—square billet rolling—wire rod rolling—heat treatment; the heat treatment is performed using the heat treatment method described in the present invention.

[0014] In the LF refining step, the LF furnace refining process uses aluminum deoxidation to adjust C, Si, Mn, Cr, V, and Mo to the target values.

[0015] In the RH treatment step, the vacuum level and vacuum time are 70 Pa and 20 min, respectively, and [H] ≤ 1.5 ppm, strictly controlling the source of hydrogen atoms. Simultaneously, to ensure that the nitrogen content meets the standards, silicon-calcium wire is used for inclusion modification treatment.

[0016] In the continuous casting step, the target temperature of the molten steel is controlled at 20-45°C above the liquidus temperature for single casting and 15-40°C above the liquidus temperature for continuous casting. Electromagnetic stirring is also performed, and finally, cooling is carried out for ≥48 hours to eliminate stress.

[0017] In the continuous casting step, a 250mm square billet is continuously cast.

[0018] In the billet rolling step, the heating temperature is 1240-1280℃, and the heating time is 240-270 min, preferably 250-280 min. This ensures that the internal inclusions are fully dissolved and that the alloying elements diffuse and the composition is uniform.

[0019] In the billet rolling step, the billet opening process rolls a 250mm×250mm large billet into a 150mm×150mm billet. The whole process can increase the compression ratio of steel and improve the internal quality of finished wire rod. At the same time, billet rolling also avoids the center segregation that is prone to occur in round billet rolling.

[0020] The wire rod rolling process includes the following steps: billet peeling → heating → high-speed wire rod controlled rolling → Steyrmo cooling line controlled cooling → Φ18mm wire rod finished product.

[0021] To ensure no decarburized layer on the surface, the peeling depth should be greater than 1.2 mm; the heating temperature should be controlled at 1080–1120℃, preferably 1080–1110℃, with a soaking time of 120–140 min; the final rolling temperature should be 790–830℃, preferably 810–830℃. Temperatures above this range will cause network carbides to form in the subsequent cooling process, while temperatures below this range will cause the phase transformation temperature to be too low, resulting in abnormal bainite structure. This process is designed to suppress the coarsening of recrystallized austenite grains and ultimately obtain a fine-grained structure; the wire drawing temperature should be 770–810℃, preferably 770–800℃.

[0022] The functions and controls of each component in the high bending fatigue performance spring steel provided by this invention are as follows:

[0023] Carbon (C): Carbon is the most fundamental and effective strengthening element in steel. It is crucial for hardness and wear resistance in spring steel, and is essential for obtaining high-strength and high-hardness spring steel. While high carbon content is beneficial to the strength, hardness, elasticity, and resilience of steel, it is detrimental to its plasticity and toughness, reduces the yield strength ratio, increases decarburization sensitivity, and worsens the fatigue resistance and machinability of the steel. The carbon content should be controlled between 0.56% and 0.64%.

[0024] Si: Si is an important strengthening element in steel, increasing its strength and hardness through solid solution treatment, while also improving the resilience of spring steel. Silicon is mainly concentrated on the steel surface, also improving the stability of the rust layer and enhancing the steel's resistance to pitting corrosion. Furthermore, during the heat treatment process of this invention, Si can inhibit the diffusion of carbon from austenite during the partitioning process, ensuring its stability. However, increasing the Si content can increase carbon diffusion in the steel, exacerbating decarburization. The Si content is controlled between 1.40% and 2.00%.

[0025] Mn: Mn forms a solid solution with Fe, increasing the hardness and strength of ferrite and austenite in steel. Simultaneously, Mn improves the stability of the austenite structure, significantly enhancing the hardenability of the steel. However, excessive Mn will reduce the plasticity of the steel. The Mn content should be controlled between 0.35% and 0.75%.

[0026] Cr: Cr can form stable compounds with C, preventing the segregation of C or impurities, improving the stability of the matrix, and significantly improving the oxidation resistance of steel. Cr dissolves in ferrite, producing solid solution strengthening, which can significantly increase the hardenability and tempering resistance of steel. Cr can form a dense oxide film on the steel surface, improving the passivation ability of steel. However, excessive Cr increases the temper brittleness tendency of steel. The Cr content should be controlled between 0.90% and 1.30%.

[0027] Vanadium (V) is an excellent deoxidizer for steel. Adding vanadium to steel can refine the grain structure and improve strength and toughness. V forms fine carbonitrides with Nb, which can improve the fatigue performance of the material. However, excessive VC dispersion within the grains will lead to a decrease in steel toughness. The V content should be controlled between 0.10% and 0.20%.

[0028] Nitrogen (Nb): Nitrogen (Nb) is a highly effective microalloying element for refining grain size. In steel, Nb primarily increases the recrystallization temperature of austenite, thereby refining the austenite grains. Combined with vitamin V, it forms complex carbides during the tempering stage, thus increasing the steel's strength. However, excessive Nb diminishes its strengthening effect and increases the steel's susceptibility to cracking. Therefore, the optimal Nb addition level is between 0.03% and 0.06%.

[0029] Al: In steel, Al combines with N to form AlN. These particles tend to aggregate at grain boundaries during rolling and heat treatment, refining the grain size. Simultaneously, they can form tiny hydrogen traps to capture hydrogen atoms. In this invention, similar to silicon, they can suppress the precipitation of carbides in the residual austenite during the partitioning stage. However, on the other hand, increasing the Al content leads to coarser nitrides, deteriorating the steel's processing properties. Therefore, the Al content is controlled between 0.020% and 0.045%.

[0030] S and P: Sulfur readily combines with manganese in steel to form MnS inclusions, which are detrimental to the steel's processing and fatigue properties. P is an element with a strong tendency to segregate, and it often causes the co-aggregation of sulfur and manganese, which is detrimental to the uniformity of the product's microstructure and properties. Control P ≤ 0.015% and S ≤ 0.010%.

[0031] [O]: O forms oxide inclusions in steel, which impair the steel's processing and fatigue properties. O should be controlled to ≤15ppm.

[0032] [N]: N mainly forms fine precipitates with Al in steel, thereby improving the steel's resistance to hydrogen-induced delayed fracture. However, excessive N precipitates Fe4N in steel, which diffuses slowly, leading to aging of the steel. At the same time, N also reduces the cold workability of steel. Therefore, the N content needs to be controlled at ≤50ppm.

[0033] [H]: Excessive H in steel can cause internal defects in the cast billet; therefore, H ≤ 1.5 ppm.

[0034] The heat treatment method for high-flexural-fatigue-performance spring steel provided by this invention employs a specific quenching-partitioning + tempering heat treatment process, with the tempering temperature higher than the partitioning temperature, replacing the quenching-tempering heat treatment process. After partitioning, the microstructure is martensite + retained austenite, while the partitioning stage completes the diffusion of carbon from martensite to austenite, enriching carbon atoms in the austenite to obtain stable retained austenite. Figure 3 As shown, the increased content of retained austenite enhances the material's plasticity, thereby improving its elongation. During bending stress, the carbon-rich retained austenite undergoes a phase transformation, which strengthens the material's fatigue resistance.

[0035] On the other hand, the added Nb and V elements are highly effective microalloying elements, which can increase the recrystallization temperature of austenite, thereby refining the austenite grains and resulting in a more refined microstructure. During tempering, due to the addition of alloying elements such as Nb and V, they form composite carbides with carbon elements in the steel, which can play a precipitation strengthening role during the tempering stage, further improving the strength of the material. At the same time, corresponding restrictions are also imposed on production conditions, strictly controlling the content of N and H elements through the RH process to reduce the harm caused by inclusions.

[0036] This invention, through the design of the composition, the heat treatment process, and the corresponding adjustment of the production process, enables the invented steel to achieve a tensile strength of 2300MPa, a reduction of area ≥50%, a bending fatigue strength ≥940MPa, an austenite grain size ≥8.0 grade, and an increased bending fatigue strength after heat treatment. Attached Figure Description

[0037] Figure 1 A schematic diagram of the heat treatment process for spring steel with high bending fatigue performance;

[0038] Figure 2 The image shows the metallographic structure of the spring steel in Example 1.

[0039] Figure 3 This is a diagram showing the austenite grain size of the spring steel in Example 1;

[0040] Figure 4 This is a dimensional diagram of a rotating bending fatigue specimen. Detailed Implementation

[0041] This invention provides a spring steel with high bending fatigue performance, containing, by weight percentage: C 0.56%–0.64%, Si 1.40%–2.00%, Mn 0.35%–0.75%, Cr 0.90%–1.30%, V 0.10%–0.20%, Nb 0.03%–0.06%, Al 0.020%–0.045%, P ≤ 0.015%, S ≤ 0.010%, O ≤ 15 ppm, [H] ≤ 1.5 ppm, [N] ≤ 50 ppm, with the remainder being Fe and other unavoidable impurities.

[0042] The heat treatment method for the spring steel with high bending fatigue performance includes the following steps: rapidly heating the spring steel to 860-900℃ at a heating rate of 20-30℃ / s and holding it at that temperature for 20-40 minutes, then cooling it to 75-100℃ and holding it at that temperature for 20-60 seconds, then holding it at 230-280℃ for 30-70 minutes, then heating it to 300-400℃ for tempering for 30-80 minutes, and finally water cooling it to room temperature.

[0043] The production method of the spring steel with high bending fatigue performance includes the following steps: converter smelting—LF refining—RH treatment—continuous casting—square billet rolling—wire rod rolling—heat treatment; the heat treatment is carried out using the heat treatment method described in this invention.

[0044] In the LF refining step, the LF furnace refining process uses aluminum deoxidation to adjust C, Si, Mn, Cr, V, and Mo to the target values.

[0045] In the RH treatment step, the vacuum level and vacuum time are 70 Pa and 20 min, respectively, and [H] ≤ 1.5 ppm, strictly controlling the source of hydrogen atoms. Simultaneously, to ensure that the nitrogen content meets the standards, silicon-calcium wire is used for inclusion modification treatment.

[0046] In the continuous casting step, the target temperature of the molten steel is controlled at 20-45°C above the liquidus temperature for single casting and 15-40°C above the liquidus temperature for continuous casting. Electromagnetic stirring is also performed, and finally, cooling is carried out for ≥48 hours to eliminate stress.

[0047] In the continuous casting step, a 250mm square billet is continuously cast.

[0048] In the billet rolling process, the heating temperature is 1240-1280℃ and the heating time is 230-280min.

[0049] In the billet rolling step, the billet opening process rolls a 250mm×250mm large billet into a 150mm×150mm billet.

[0050] The wire rod rolling process includes the following steps: billet peeling → heating → high-speed wire rod controlled rolling → Steyrmo cooling line controlled cooling → Φ18mm wire rod finished product.

[0051] To ensure there is no decarburized layer on the surface, the peeling depth should be more than 1.2 mm; control the heating temperature to 1080~1120℃, the soaking time to 120~140min; the final rolling temperature to 790~830℃, and the wire drawing temperature to 770~810℃.

[0052] The present invention will now be described in detail with reference to the embodiments.

[0053] This invention uses spring steel with a specific composition. The compositions of the embodiments and comparative examples are shown in Table 1. All compositions in Table 1 were produced by converter smelting and rolled into wire rods with a diameter of 18 mm for comparison. Three comparative examples were used. Comparative example 1 used conventional 60Si2CrVA steel. Comparative example 1 employed the same heat treatment method as the embodiments, but without the addition of Nb and V elements. Comparative example 2 used a conventional quenching-tempering process, first rapidly heating the steel to the austenitizing temperature of 880±20℃ at a rate of 25℃ / s, holding it at that temperature for 30±10℃ min, then rapidly oil quenching it, followed by heating it to 420℃, holding it at that temperature for 120±10℃ min, and then water cooling it to room temperature. Comparative example 3 used the same chemical composition and heat treatment method as embodiment 1, except that some heat treatment parameters were not properly controlled. The heat treatment processes of each embodiment and comparative example are shown in Table 3. The microstructure and properties after heat treatment are shown in Table 4.

[0054] Table 1. Chemical composition (wt%) of embodiments and comparative examples of the present invention.

[0055] Example 1 0.59 0.56 1.66 1.14 0.14 0.04 0.024 0.009 0.004 0.0010 0.0039 Example 2 0.60 0.55 1.66 1.14 0.15 0.05 0.023 0.009 0.005 0.0009 0.0040 Example 3 0.60 0.54 1.65 1.15 0.15 0.04 0.023 0.009 0.005 0.0009 0.0038 Example 4 0.59 0.55 1.66 1.13 0.16 0.04 0.024 0.010 0.005 0.0010 0.0036 Comparative Example 1 0.59 0.54 1.65 1.14 / / 0.023 0.009 0.004 0.0009 0.0038 Comparative Example 2 0.60 0.54 1.66 1.14 0.15 0.05 0.024 0.010 0.004 0.0010 0.0039

[0056] Table 2 Production conditions for embodiments and comparative examples of the present invention

[0057]

[0058]

[0059] Table 3

[0060]

[0061] In Table 3, the steel composition of Comparative Example 3 is the same as that of Example 1.

[0062] The rotational bending fatigue test was conducted according to the national standard GB / T 4337-1984 "Metallic materials - Rotational bending fatigue test method", and the specimen dimensions were as follows. Figure 4As shown. All specimens were heat-treated, with the oxide and decarburized layers removed from the specimen surface through finishing. The testing machine used a four-point force application, a testing speed of 5000 r / min, a strain cycle ratio R = -1, a frequency of 83 Hz, and a sine wave loading waveform for the fatigue test. Specimen failure or reaching 10... 7 The test was then stopped. Based on the fatigue curve of the test steel, the conditional fatigue limit of the test steel was calculated.

[0063] Table 3 Mechanical properties, grain size, and flexural fatigue strength of embodiments and comparative examples of the present invention.

[0064]

[0065] The above detailed description of a spring steel with high bending fatigue performance, its heat treatment method, and its production method, with reference to the embodiments, is illustrative rather than limiting. Several embodiments can be listed within the defined scope. Therefore, variations and modifications without departing from the overall concept of the present invention should be within the protection scope of the present invention.

Claims

1. A spring steel with high bending fatigue performance, characterized in that, It contains, by weight percentage: C 0.56%~0.64%, Si 1.40%~2.00%, Mn 0.35%~0.75%, Cr 0.90%~1.30%, V 0.10%~0.20%, Nb 0.03%~0.06%, Al 0.020%~0.045%, P ≤0.015%, S ≤0.010%, O ≤15ppm, [H] ≤1.5ppm, [N] ≤50ppm, with the remainder being Fe and other unavoidable impurities; The high-bending-fatigue-performance spring steel has a tensile strength ≥2300MPa, a reduction of area ≥50%, a bending fatigue strength ≥940 MPa, and an austenite grain size ≥8.0 grade. The heat treatment method for the spring steel with high bending fatigue performance includes the following steps: rapidly heating the spring steel to 860~900℃ at a heating rate of 20~30℃ / s and holding it at that temperature, then cooling it to 75~100℃ and holding it at that temperature, then holding it at 230~280℃ and then heating it to 300~400℃ for tempering, and finally water cooling it to room temperature.

2. The spring steel with high bending fatigue performance according to claim 1, characterized in that, The microstructure of the spring steel with high bending fatigue performance is martensite + carbon-free bainite + retained austenite.

3. The heat treatment method for spring steel with high bending fatigue performance as described in claim 1 or 2, characterized in that, The heat treatment method includes the following steps: rapidly heating the spring steel to 860-900℃ at a heating rate of 20-30℃ / s and holding it at that temperature, then cooling it to 75-100℃ and holding it at that temperature, then holding it at 230-280℃ and then heating it to 300-400℃ for tempering, and finally water cooling it to room temperature.

4. The heat treatment method as described in claim 3, characterized in that, The heat treatment method includes the following steps: rapidly heating the spring steel to 860-900℃ at a heating rate of 20-30℃ / s and holding it at that temperature for 20-40 minutes, then cooling it to 75-100℃ and holding it at that temperature for 20-60 seconds, then holding it at 230-280℃ for 30-70 minutes, then heating it to 300-400℃ for tempering for 30-80 minutes, and finally water cooling it to room temperature.

5. The method for producing spring steel with high bending fatigue performance as described in claim 1 or 2, characterized in that, The production method includes the following steps: converter smelting—LF refining—RH treatment—continuous casting—square billet rolling—wire rolling—heat treatment; the heat treatment is carried out using the heat treatment method in claim 4.

6. The production method as described in claim 5, characterized in that, In the billet rolling step, the heating temperature is 1240~1280℃ and the heating time is 230~280min.

7. The production method as described in claim 5, characterized in that, The wire rod rolling process includes the following steps: billet peeling → heating → high-speed wire rod controlled rolling → Steyrmo cooling line controlled cooling → Φ18mm wire rod finished product.

8. The production method as described in claim 7, characterized in that, Peeling depth is 1.2mm or more; control heating temperature at 1080~1120℃, final rolling temperature at 790~830℃, and wire drawing temperature at 770~810℃.

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