Spring steel with high strength and hydrogen embrittlement and delay fracture resistance, and heat treatment method and production method thereof

Abstract

The application discloses a spring steel with high strength and hydrogen embrittlement resistance and delay fracture resistance and a heat treatment method and 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%, Mo 0.12%-0.25%, 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 produced at low cost has the following properties: tensile strength≥2350MPa, yield strength≥1850MPa, elongation≥20%, reduction of area≥43%, hydrogen-induced delay fracture resistance life≥1200sec, and simultaneously has high strength and hydrogen embrittlement resistance and delay fracture resistance.

Description

Technical Field

[0001] This invention belongs to the field of spring steel technology, specifically relating to a high-strength spring steel with resistance to hydrogen embrittlement and delayed fracture, as well as its heat treatment method and production method. Background Technology

[0002] In recent years, with the introduction of lightweight automotive goals, the research and development of related component materials has also shifted towards this objective, including springs, a crucial safety component in automobiles. The main approach to automotive lightweighting is to improve material composition and heat treatment processes to increase material strength while reducing weight, ultimately achieving the desired lightweighting. However, this brings a new challenge: increased hydrogen embrittlement sensitivity. Currently, the prevailing view among scholars is that hydrogen embrittlement sensitivity increases with material strength. Hydrogen embrittlement is primarily caused by atomic hydrogen, leading to reduced toughness or cracking. Hydrogen embrittlement poses a significant threat to component safety, and avoiding this phenomenon is a critical technical consideration that must be carefully addressed during the development of high-strength, high-toughness spring steel.

[0003] Chinese Patent CN 111321346 A discloses an ultra-high strength spring steel with excellent resistance to hydrogen-induced delayed fracture and its production method, specifically disclosing the following chemical composition by weight percentage: C 0.75%–0.85%, Si 1.60%–2.00%, Mn 0.40%–0.60%, Cr 0.80%–1.00%, V 0.20%–0.30%, Nb 0.03%–0.05%, Mo 0.10%–0.30%, Re 0.01%–0.03%, Al 0.015%–0.040%, N 0.005%–0.008%, P ≤ 0.015%, S ≤ 0.015%, O ≤ 0.0015%, with the remainder being Fe and other unavoidable impurities; and Al / N ≥ 3.5%.

[0004] 29.1C+5.2Si+1.8Mn+3.5Cr+2.6V+0.8Nb+4.9Mo≥36%; produced by electric furnace smelting-LF furnace-vacuum degassing-continuous casting-billing-wire rolling. This invention avoids the use of large amounts of precious metal element Ni and Cu element, which easily causes copper brittle fracture in steel. Under the interaction of C, Si, Mn, Cr, V, Nb, Mo, Re, and N, ultra-high strength spring steel with a tensile strength ≥2300MPa is produced at low cost under specific processes, and it also has excellent resistance to delayed fracture and fatigue performance. Although the spring steel disclosed in this patent has excellent resistance to delayed fracture, it requires the addition of more alloying elements, resulting in higher production costs. Summary of the Invention

[0005] To solve the above-mentioned technical problems, the present invention provides a high-strength spring steel with hydrogen embrittlement resistance and delayed fracture resistance, as well as its heat treatment method and production method. The low-cost production yields spring steel with tensile strength ≥2350MPa, yield strength ≥1850MPa, elongation ≥20%, reduction of area ≥43%, and hydrogen-induced delayed fracture life ≥1200sec, which simultaneously possesses high strength and hydrogen embrittlement resistance and delayed fracture resistance.

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

[0007] A high-strength spring steel with resistance to hydrogen embrittlement and delayed fracture 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%, Mo 0.12%–0.25%, 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 metallographic structure of the high-strength, hydrogen-embrittlement-resistant, delayed fracture-resistant spring steel is martensite + carbon-free bainite + retained austenite.

[0009] The high-strength, hydrogen-embrittled, delayed fracture-resistant spring steel has a tensile strength ≥2350MPa, a yield strength ≥1850MPa, an elongation ≥20%, a reduction of area ≥43%, and a hydrogen-induced delayed fracture life ≥1200sec.

[0010] The present invention also provides a heat treatment method for the high-strength, hydrogen-embrittled, delayed fracture-resistant spring steel, the heat treatment method comprising the following steps: rapidly heating the spring steel to 860-900℃ and holding it thereafter, then cooling it to 70-90℃ and holding it thereafter, then holding it at 240-280℃, 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 70-90℃ and holding it at that temperature for 30-60 seconds, and then holding it at 240-280℃ for 70-120 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, preferably 25℃ / s, and held for 20-40 minutes. Then, it is cooled to the T2 quenching medium water (70-90℃) and held for 30-60 seconds. The T2 temperature is Ms (the starting temperature of martensite transformation) - M fThe material is held at a temperature between the martensitic transformation end temperature and the quenching termination temperature for a period of time. Preferably, this invention uses a T2 temperature of 80℃ for 30 seconds. This step yields a mixed microstructure of martensite and retained austenite. Then, it is held at a T3 temperature (240-280℃) for 70-120 minutes before water cooling to room temperature. The T3 temperature is a specific temperature higher than the Ms temperature. At this temperature, the fractionation and tempering processes are completed. The fractionation process mainly involves carbon fractionation. 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. The tempering process involves the precipitation of carbides from microalloying elements. In this invention, the fractionation temperature is 250℃ and the fractionation time is 100 minutes. Ultimately, the material achieves a strength of 2350 MPa and an elongation of 20%. Simultaneously, its resistance to hydrogen embrittlement is enhanced. 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 high-strength, hydrogen-embrittled, delayed fracture-resistant spring steel, 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 process, the heating temperature is 1235-1285℃, and the heating time is 240-270 minutes. This ensures that the internal inclusions are fully dissolved and that the alloying elements diffuse and the composition is homogeneous. After rolling, the billet undergoes peeling and finishing.

[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 → Φ5.5~10mm wire rod finished product.

[0021] To ensure no decarburized layer on the surface, the peeling depth should be at least 1.2 mm; the heating temperature should be controlled at 1060–1110℃, preferably 1065–1085℃, and the soaking time should be 110–130 min; the final rolling temperature should be 780–825℃, preferably 800–820℃. 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 is to suppress the coarsening of recrystallized austenite grains and ultimately obtain a fine-grained structure; the wire drawing temperature should be 780–820℃, preferably 790–810℃.

[0022] The functions and controls of each component in the high-strength, hydrogen-embrittlement-resistant, delayed-fracture 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 Mo, which can improve resistance to hydrogen-induced delayed fracture and fatigue performance. However, excessive dispersed precipitation of VC within the grains will lead to a decrease in steel toughness. The V content should be controlled between 0.10% and 0.20%.

[0028] Mo: Mo can improve the hardenability and strength of materials. Simultaneously, Mo can interact with V and C to form (Mo,V)C nanoprecipitates, capturing free hydrogen and increasing resistance to hydrogen embrittlement. However, excessive Mo content leads to higher material costs. Therefore, the Mo content in materials is controlled between 0.12% and 0.25%.

[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 grains. They can also form tiny hydrogen traps to capture hydrogen atoms. However, increasing the Al content leads to coarser nitrides, deteriorating the steel's machinability. Therefore, the Al content is controlled between 0.020% and 0.040%.

[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-strength spring steel with hydrogen embrittlement resistance and delayed fracture performance provided by this invention employs a specific quenching-partitioning + tempering heat treatment process instead of the traditional quenching-tempering process. After quenching, the spring steel needs to be briefly held at a certain temperature T2 before being heated to a certain temperature T3 for partitioning + tempering treatment. After quenching at T2, the microstructure is a mixture of martensite and retained austenite. During the partitioning + tempering stage at T3, the partitioning process mainly involves the diffusion of carbon from martensite to austenite. Face-centered cubic austenite has a higher carbon solubility than body-centered cubic martensite, and carbon atoms in the supersaturated martensite phase diffuse into the austenite phase, enriching carbon atoms in the austenite to obtain stable retained austenite. Figure 3 The dark field arrow in the mid-transmission image shows an increased content of retained austenite, which absorbs more hydrogen atoms and passively dulls cracks, significantly reducing the hydrogen embrittlement sensitivity of high-strength spring steel. Simultaneously, the increased retained austenite enhances the material's plasticity, further improving its elongation.

[0035] On the other hand, during the tempering process, the addition of alloying elements such as Mo and V forms complex carbides with carbon in the steel, increasing the material's resistance to hydrogen embrittlement. Furthermore, the addition of these trace alloying elements during tempering can play a precipitation strengthening role, further enhancing the material's strength. Ultimately, this results in a steel with high strength, toughness, and resistance to hydrogen embrittlement. Simultaneously, corresponding restrictions were placed on production conditions, strictly controlling the nitrogen content through the RH process to reduce the harm caused by TiN inclusions. Through composition design, heat treatment processes, and corresponding adjustments to the production process, the invented steel achieves a tensile strength of 2350 MPa, a yield strength ≥1850 MPa, an elongation ≥20%, and a reduction of area ≥43% after heat treatment, while also improving its resistance to hydrogen embrittlement. Attached Figure Description

[0036] Figure 1 A schematic diagram of the heat treatment process for high-strength spring steel with resistance to hydrogen embrittlement and delayed fracture.

[0037] Figure 2 The image shows the metallographic structure of the spring steel in Example 1.

[0038] Figure 3 This is a diagram showing the austenitic grain size of the spring steel in Example 1. Detailed Implementation

[0039] This invention provides a high-strength spring steel with resistance to hydrogen embrittlement and delayed fracture, comprising, 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%, Mo 0.12%–0.25%, 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.

[0040] The heat treatment method for the high-strength, hydrogen-embrittled, delayed fracture-resistant spring steel 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 70-90℃ and holding it at that temperature for 30-60 seconds, then holding it at 240-280℃ for 70-120 minutes, and finally water cooling it to room temperature.

[0041] The production method of the high-strength, hydrogen-embrittled, delayed fracture-resistant spring steel 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] In the continuous casting step, a 250mm square billet is continuously cast.

[0046] In the billet rolling process, the heating temperature is 1235-1285℃ and the heating time is 240-270min.

[0047] In the billet rolling step, the billet opening process rolls a 250mm×250mm large billet into a 150mm×150mm billet.

[0048] The wire rod rolling process includes the following steps: billet peeling → heating → high-speed wire rod controlled rolling → Steyrmo cooling line controlled cooling → Φ5.5~10mm wire rod finished product.

[0049] To ensure that there is no decarburized layer on the surface, the peeling depth is more than 1.2 mm; the heating temperature is controlled at 1060-1110℃, preferably 1065-1085℃, and the heating time is 110-130 min; the final rolling temperature is 780-825℃, preferably 800-820℃; and the wire drawing temperature is 780-820℃, preferably 790-810℃.

[0050] The present invention will now be described in detail with reference to the embodiments.

[0051] This invention uses spring steel with specific compositions. 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 diameters of 5.5-10 mm for comparison. Two comparative examples were used. The comparative examples used conventional 60Si2CrVA steel. Comparative Example 1 used the same heat treatment method as the embodiments, but without the addition of Mo / V elements. Comparative Example 2 used a conventional quenching-tempering process. The steel was first rapidly heated at a rate of 25℃ / s to the austenitizing temperature of 880±20℃, held for 30±10℃ min, then rapidly oil-quenched, and then heated to 420℃, held for 120±10℃ min, and finally water-cooled to room temperature. The heat treatment process is shown in Table 3. The microstructure and properties after heat treatment are shown in Table 4.

[0052] Table 1. Chemical composition (wt%) of embodiments and comparative examples of the present invention.

[0053] Example 1 0.60 0.55 1.65 1.13 0.15 0.18 0.025 0.010 0.005 0.0011 0.0040 Example 2 0.59 0.54 1.65 1.15 0.14 0.17 0.024 0.010 0.004 0.0010 0.0039 Example 3 0.61 0.55 1.66 1.14 0.16 0.16 0.022 0.008 0.003 0.0008 0.0030 Example 4 0.60 0.54 1.65 1.14 0.15 0.17 0.023 0.010 0.004 0.0009 0.0041 Comparative Example 1 0.60 0.53 1.66 1.15 / / 0.025 0.009 0.003 0.0010 0.0042 Comparative Example 2 0.59 0.54 1.65 1.13 0.16 0.17 0.026 0.009 0.005 0.0009 0.0038

[0054] Table 2 Production conditions of embodiments and comparative examples of the present invention

[0055]

[0056]

[0057] Table 3

[0058]

[0059] In Table 3, the steel composition of Comparative Example 3 is the same as that of Example 1.

[0060] Hydrogen-induced delayed fracture test: A flat experimental piece (65mm*10mm*1.5mm) was machined from a billet during rolling and subjected to a cathode charging-4-point bending test. The applied bending stress was 1400MPa, and the solution was a mixed solution of 0.5mol / L H2SO4 + 0.01mol / L KSCN. A voltage of -700mV compared to the SCE reference electrode was applied using a voltage regulator. The time from the start of charging to fracture was measured as the fracture life, which was used as the evaluation index for hydrogen-induced delayed fracture resistance.

[0061] Table 4 Mechanical properties and resistance to hydrogen embrittlement of embodiments and comparative examples of the present invention.

[0062]

[0063] The above detailed description of a high-strength spring steel with hydrogen embrittlement resistance and delayed fracture performance, as well as its heat treatment method and production method, 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 strength and resistance to hydrogen embrittlement and delayed fracture, 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%, Mo 0.12%~0.25%, 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-strength, hydrogen-embrittled, delayed fracture-resistant spring steel has a tensile strength ≥2350MPa, a yield strength ≥1850MPa, an elongation ≥20%, a reduction of area ≥43%, and a hydrogen-induced delayed fracture life ≥1200 sec. The heat treatment method for the high-strength, hydrogen-embrittled, delayed fracture-resistant spring steel includes the following steps: rapidly heating the spring steel to 860-900℃ and holding it thereafter, then cooling it to 70-90℃ and holding it thereafter, then holding it at 240-280℃, and finally water-cooling it to room temperature.

2. The high-strength, hydrogen-embrittlement-resistant, delayed-fracture spring steel according to claim 1, characterized in that, The metallographic structure of the high-strength, hydrogen-embrittlement-resistant, delayed fracture-resistant spring steel is martensite + carbon-free bainite + retained austenite.

3. The heat treatment method for high-strength spring steel with hydrogen embrittlement resistance and delayed fracture 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℃ and holding it at that temperature, then cooling it down to 70-90℃ and holding it at that temperature, then holding it at 240-280℃, 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 70-90℃ and holding it at that temperature for 30-60 seconds, then holding it at 240-280℃ for 70-120 minutes, and finally water cooling it to room temperature.

5. The method for producing high-strength spring steel with hydrogen embrittlement resistance and delayed fracture properties 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 continuous casting step, the target temperature of the molten steel is controlled at 20~45℃ above the liquidus temperature for single casting and 15~40℃ above the liquidus temperature for continuous casting. The cooling rate and casting speed are adjusted to be relatively low, and electromagnetic stirring is performed. Finally, cooling is carried out, and the cooling time is ≥48 hours.

7. The production method as described in claim 5, characterized in that, In the billet rolling process, the heating temperature is 1235-1285℃ and the heating time is 240-270min.

8. 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 → Φ5.5~10mm wire rod finished product.

9. The production method as described in claim 5, characterized in that, Peeling depth is 1.2mm or more; control heating temperature at 1060~1110℃, final rolling temperature at 780~825℃, and wire drawing temperature at 780~820℃.

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