High-strength spring steel with excellent delayed fracture resistance and heat treatment process thereof

Abstract

The application discloses a high-strength spring steel with excellent delayed fracture resistance and a heat treatment process thereof, and relates to the technical field of steel production. The chemical components and mass percentages of the high-strength spring steel are as follows: C: 0.50-0.60%; Si: 1.60-2.20%; Mn: 0.80-1.30%; Cr: 0.80-1.30%; W: 0.10-0.20%; V: 0.15-0.30%; Nb: 0.015-0.035%; P: ≤0.012%; S: ≤0.010%; O: ≤0.0015%; N: ≤0.0060%; H: ≤0.00012%; and the rest is iron and inevitable impurities. The mechanical properties and the delayed fracture resistance of the spring steel of the application are significantly improved compared with those of a comparative steel under the same treatment process.

Description

Technical Field

[0001] This invention relates to the field of steel production technology, and in particular to a high-strength spring steel with excellent resistance to delayed fracture and its heat treatment process. Background Technology

[0002] Springs are crucial load-bearing components for safety. With the rapid development of industries such as automobiles, railways, and construction machinery, increasingly higher strength requirements are being placed on spring steel to reduce weight and save energy. However, the increase in steel strength often leads to a more prominent problem of hydrogen-induced delayed fracture, which has become a major obstacle to further increasing the strength of high-strength steel. Delayed fracture is an environmental embrittlement phenomenon caused by the long-term interaction between material, environment (hydrogen), and stress; it is a form of hydrogen-induced material deterioration (hydrogen damage or hydrogen embrittlement). Even in a stationary state without operation, springs used in automobiles and railway trains must withstand the long-term load of their own vehicle weight. Spring steel is mostly quenched and tempered at medium temperature to obtain a medium-temperature tempered martensitic structure, thus having a high risk of delayed fracture. Even with protective treatments such as surface coating, springs inevitably suffer damage during long-term service due to factors such as flying stones, impacts, and reciprocating deformation. Hydrogen generated by corrosion at the damaged area still poses a significant risk of inducing delayed fracture. Therefore, in recent years, delayed fracture resistance design has received increasing attention in the research and development of spring steel.

[0003] The invention patent application No. 202010147746.0, entitled "An Ultra-High Strength Spring Steel with Excellent Resistance to Hydrogen-Induced Delayed Fracture and Its Production Method," mainly achieves excellent resistance to delayed fracture and fatigue life by controlling Al / N ≥ 3.5 to obtain fine austenite grains (≤ 10 μm) and cementite size (≤ 25 μm). Although grain size and cementite refinement can increase hydrogen trap density, both grain boundaries and cementite are reversible hydrogen traps. Under long-term stress, the adsorbed hydrogen can still diffuse and accumulate at stress concentration sites, making it difficult to guarantee the safety of long-term service.

[0004] The invention patent with application number 202311430070.6, entitled "A high-strength spring steel with hydrogen embrittlement resistance and delayed fracture performance, and its heat treatment method and production method", proposes to use an isothermal partitioning + medium-temperature tempering heat treatment process to obtain a multiphase structure of martensite + carbide-free bainite + retained austenite. However, the retained austenite in this structure often transforms into martensite under long-term stress, which not only seriously affects the elastic reduction performance of the spring steel, resulting in a reduction in spring height and affecting safe service, but also the newly transformed martensite is very brittle due to its high carbon content and lack of tempering treatment, and still has a great risk of delayed fracture.

[0005] The invention patent application No. 202210549788.6, entitled "A High-Strength Spring Steel Resistant to Hydrogen Embrittlement and Its Heat Treatment Method," proposes adding a relatively high content of Ni (2.8~3.5 wt.%), Cu (1.3~2.0 wt.%), and Al (0.9~1.30 wt.%), while controlling Ni / Cu ≤ 3 and Al / Cu ≤ 1, to maintain coherent NiAl and Cu precipitates with the matrix during tempering. However, adding a high content of Cu to the steel easily causes copper embrittlement cracking, and the high Al content also easily forms a large amount of coarse Al oxide inclusions (due to their precipitation during steel smelting and solidification), which seriously deteriorates the fatigue performance of the spring steel. In addition, the high content of precious elements such as Ni and Mo affects the economics of the steel and is not suitable for mass production applications. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a high-strength spring steel with excellent resistance to delayed fracture and its heat treatment process.

[0007] To solve the above technical problems, the technical solution of the present invention is as follows:

[0008] A high-strength spring steel with excellent resistance to delayed fracture has the following chemical composition and mass percentage: C: 0.50%~0.60%; Si: 1.60%~2.20%; Mn: 0.80%~1.30%; Cr: 0.80%~1.30%; W: 0.10%~0.20%; V: 0.15%~0.30%; Nb: 0.015%~0.035%; P: ≤0.012%; S: ≤0.010%; O ≤0.0015%; N ≤0.0060%; H ≤0.00012%; the remainder is iron and unavoidable impurities.

[0009] As a preferred embodiment of the high-strength spring steel with excellent resistance to delayed fracture described in this invention, the mass percentages of V, Nb, W, and N in the high-strength spring steel with excellent resistance to delayed fracture satisfy the following relationship: 0.25≤0.80V(%)+[Nb(%)-6.64N(%)]+W≤0.40.

[0010] The present invention also provides a heat treatment process for high-strength spring steel with excellent resistance to delayed fracture, comprising, in sequence: quenching heat treatment, first tempering heat treatment and second tempering heat treatment;

[0011] The quenching heat treatment is performed at a heating temperature of 900±30 ℃ and a holding time of 30±10 min.

[0012] As a preferred embodiment of the heat treatment process for the high-strength spring steel with excellent delayed fracture resistance described in this invention, the cooling method for the quenching heat treatment is oil cooling or water cooling.

[0013] As a preferred embodiment of the heat treatment process for the high-strength spring steel with excellent delayed fracture resistance described in this invention, the first tempering heat treatment includes:

[0014] Heated to 580±20 ℃ at a heating rate of 50±10 ℃ / s, held for 20±5 s, and then water cooled.

[0015] As a preferred embodiment of the heat treatment process for the high-strength spring steel with excellent resistance to delayed fracture described in this invention, the tempering temperature of the second tempering heat treatment is 350 ± 20 ℃, and the holding time is 90~120 min.

[0016] As a preferred embodiment of the heat treatment process for the high-strength spring steel with excellent resistance to delayed fracture described in this invention, the cooling method for the second tempering heat treatment is air cooling or water cooling.

[0017] The beneficial effects of this invention are:

[0018] The spring steel obtained by this invention is forged into a diameter Φ After controlled cooling, 16 mm thick bars can achieve mechanical properties including tensile strength ≥2050 MPa, yield strength ≥1800 MPa, elongation after fracture ≥10%, reduction of area ≥45%, and room temperature impact energy (KV2) ≥17 J, exhibiting a good balance of strength, plasticity, and toughness. The delayed fracture resistance (characterized by tensile strength after saturated hydrogen charging) is ≥1250 MPa, which is more than 30% higher than that of the conventional spring steel 60Si2CrV. The spring steel of this invention significantly improves both mechanical properties and delayed fracture resistance compared to the comparative steel under the same processing conditions. Detailed Implementation

[0019] To make the content of this invention easier to understand, the invention will be further described in detail below based on specific embodiments.

[0020] This application provides a high-strength spring steel with excellent resistance to delayed fracture, the chemical composition and mass percentage of which are as follows: C: 0.50%~0.60%; Si: 1.60%~2.20%; Mn: 0.80%~1.30%; Cr: 0.80%~1.30%; W: 0.10%~0.20%; V: 0.15%~0.30%; Nb: 0.015%~0.035%; P: ≤0.012%; S: ≤0.010%; O ≤0.0015%; N ≤0.0060%; H ≤0.00012%; the remainder is iron and unavoidable impurities.

[0021] The mass percentages of V, Nb, W, and N elements in the composition of the above-mentioned high-strength spring steel satisfy the following relationship: 0.25≤0.80V(%)+[Nb(%)-6.64N(%)]+W≤0.40; where V(%), Nb(%), W(%), and N(%) represent the mass percentages of V, Nb, W, and N, respectively.

[0022] The aforementioned spring steel is smelted in an electric arc furnace or converter with ladle refining, cast into ingots or continuously cast into billets, and then forged or rolled into bar and wire rod products, and rolled into diameters of... Φ The 15 mm steel bar, after heat treatment, has a tensile strength ≥2050 MPa, a yield strength ≥1800 MPa, an elongation after fracture ≥10%, a reduction of area ≥45%, and a room temperature V-shaped impact energy ≥17 J. Its resistance to delayed fracture is more than 30% higher than that of existing spring steel.

[0023] The roles and proportions of each element in this application are based on the following:

[0024] Carbon (C) is essential for spring steel to achieve high strength, hardness, and elasticity. To obtain the required high strength level after quenching and tempering, the C content must be above 0.50%. However, excessively high C content is detrimental to ductility and toughness, increases decarburization sensitivity, and worsens the fatigue and processing properties of the steel. Therefore, the C content should be controlled between 0.50% and 0.60%.

[0025] Si (Si): Si has a strong solid solution strengthening effect and also significantly improves the elastic reduction resistance of spring steel by refining carbides. However, excessive Si content will reduce plasticity and toughness, promote decarburization and graphitization of steel during rolling and heat treatment, make smelting difficult and prone to inclusion formation, and deteriorate the fatigue resistance of steel. Considering that strong carbide-forming elements V and Nb can mitigate the above-mentioned adverse effects of Si, the Si content is controlled at 1.60–2.20%.

[0026] Mn: Mn can improve the hardenability of steel, and when dissolved in ferrite, it has a significant solid solution strengthening effect. It is also an effective element for deoxidation and desulfurization. When the content is less than 0.80%, it is difficult to achieve the above effects. However, during the tempering of quenched steel, Mn and P have a strong tendency to co-segregate at grain boundaries, which deteriorates the toughness and resistance to delayed fracture of the steel. Therefore, the Mn content should be controlled below 1.30%.

[0027] Cr: Cr can significantly improve the hardenability and tempering resistance of steel; at the same time, Cr can reduce the activity of C, which can reduce the surface decarburization of steel during heating, rolling and heat treatment and inhibit the graphitization tendency of high-Si steel, thus helping to obtain high fatigue resistance. However, excessive content will deteriorate the elasticity and toughness of steel, so the Cr content is controlled at 0.80-1.30%.

[0028] W: W can improve the hardenability of steel, reduce temper brittleness, and improve the steel's resistance to elastic reduction. W, V, C, and N can form (W,V)(C,N) nanoscale carbides, which can act as effective hydrogen traps and improve the resistance to delayed fracture. However, excessive W affects the economics of steel, so the preferred W content range is 0.10-0.20%.

[0029] V: V is a strong carbide-forming element. The fine, dispersed carbides formed when V combines with C not only prevent grain growth during heating, thus strengthening the steel by refining its grains and improving its strength, toughness, and fatigue performance, but also act as effective hydrogen traps, inhibiting hydrogen diffusion and accumulation at stress concentration sites. When the V content is below 0.15%, these effects are not significant; when the V content is above 0.30%, these effects saturate, and the cost of the steel increases. Therefore, the V content is controlled between 0.15% and 0.30%.

[0030] Nb: Nb has a strong bonding force with C and N, and the resulting Nb(C,N) has a very effective grain refinement effect, which not only improves the strength and toughness of steel, but also acts as a very effective hydrogen trap, thereby improving the steel's resistance to delayed fracture. However, the strengthening effect of excessive Nb is no longer obvious, and it increases the crack susceptibility of steel, so it is controlled at 0.015~0.035%.

[0031] P: P can form micro-segregation when molten steel solidifies, and then agglomerates at the grain boundaries when heated to the austenitizing temperature, which significantly increases the brittleness of the steel. Therefore, the content of P should be controlled below 0.012%.

[0032] S: An unavoidable impurity in steel, which forms MnS inclusions and segregates at grain boundaries, deteriorating the toughness and fatigue resistance of steel. Therefore, its content should be controlled below 0.010%.

[0033] O₂ (O₂) forms various oxide inclusions in steel, thereby deteriorating the steel's mechanical properties, especially its toughness and fatigue resistance. Therefore, measures must be taken in metallurgical production to minimize its content. Considering economic efficiency, its content should be controlled below 0.0015%.

[0034] Nitrogen (N) mainly reacts with Nb and V in steel to form Nb and V carbonitrides M(C,N), which refine grains and improve resistance to delayed fracture. Excessive N content significantly reduces the cold workability of steel; therefore, its content is controlled below 0.0060%.

[0035] H: Excessive H in steel not only causes internal defects in the cast billet, but also severely deteriorates the steel's resistance to delayed fracture. Therefore, it should be controlled below 0.00012%.

[0036] Furthermore, to ensure both ultra-high strength and good ductility and delayed fracture resistance, a V+Nb+W composite alloy is employed. Firstly, a higher austenitizing temperature allows most of the V carbonitrides (V(C, N)) to dissolve, enabling them to recrystallize as nanoscale M(C, N) during subsequent high-temperature tempering, thus providing precipitation strengthening and effective hydrogen trapping. Secondly, a suitable amount of undissolved Nb(C, N) inhibits grain growth during austenitizing heating, resulting in a fine martensitic structure. Thirdly, the addition of a suitable amount of W accelerates the precipitation of M(C, N). Therefore, its content must also meet specific parameters. θ The relationship is: 0.25≤0.80V(%)+[Nb(%)-6.64N(%)]+W≤0.40,

[0037] θ = 0.80V(%) + [Nb(%) - 6.64N(%)] + W. When the value of θ is less than 0.25, even though the content of individual V and Nb elements may be within the above optimal range, good strength, toughness and resistance to delayed fracture properties cannot be obtained; when the value of θ is greater than 0.40, the effect is saturated and the cost of steel is increased.

[0038] To obtain excellent elasticity and resistance to spring reduction, spring steel is typically tempered at medium temperature after quenching to obtain tempered martensite (also known as tempered troostite). However, V (C, N) is difficult to precipitate in large quantities during medium-temperature tempering, meaning that traditional spring steel heat treatment processes cannot fully utilize the precipitation strengthening effect of V carbonitrides and the effective hydrogen trapping effect. To address this, this application also provides a heat treatment method for ultra-high strength spring steel with excellent resistance to delayed fracture, comprising the following steps:

[0039] 1) Quenching heat treatment to obtain a uniform quenched martensite structure, while dissolving most of the V(C,N) into the matrix structure. The quenching austenitizing heating temperature is 900±30 ℃, the holding time is 30 ±10 min, and the cooling method is oil cooling or water cooling.

[0040] 2) The first rapid induction tempering heat treatment was performed to obtain more nanoscale V(C,N) precipitates, wherein the temperature was raised to 580±20 ℃ at a heating rate of 50±10℃ / s, held for 20±5 s, and then water-cooled.

[0041] 3) A second tempering heat treatment is performed to obtain a medium-temperature tempered martensite (tempered troostite) structure. The tempering temperature is 350 ± 20 ℃, the holding time is 90~120 min, and the cooling method is air cooling or water cooling. Considering that the temperature of the first rapid tempering treatment is relatively high, the temperature of the second conventional tempering treatment is reduced to obtain the required strength level.

[0042] In specific production applications, the ultra-high strength steel of the present invention can be smelted in an electric arc furnace or converter + ladle refining, cast into steel ingots or continuously cast into billets, and then forged or rolled into products such as bars and wires. In the technical solution of this application: (1) Microalloying elements V+Nb are added to the steel to ensure that as many nano-sized M(C,N) precipitates as possible are obtained as effective hydrogen traps while obtaining fine grain size, and at the same time, precipitation strengthening effect is also achieved; (2) Two tempering treatments are adopted, namely, the first rapid high temperature tempering treatment to obtain nano-sized M(C,N) precipitates, and the second tempering treatment temperature is lower than the conventional tempering temperature to precipitate a large number of fine ε-carbides to obtain a good precipitation strengthening effect, so as to ensure that while obtaining ultra-high strength, it still has good plasticity and toughness.

[0043] The following specific examples will provide further details.

[0044] Sample preparation:

[0045] According to the chemical composition range designed in this invention, three heats of the steel of this application and three heats of comparative steel were smelted in a vacuum induction furnace. One heat was a commonly used 60Si2CrV spring steel, the specific chemical composition of which is shown in Table 1. Heat numbers 1# to 3# are the steel of this invention, and heat numbers 4# to 6# are the comparative steels, with 6# being a commonly used 60Si2CrV spring steel. The steel ingots were forged into shapes with a diameter of... Φ 15mm bars, forged and then slowly cooled. Processed according to national standards into standard room temperature tensile specimens (gauge length diameter) required for testing. d 0 = 5mm, length l 0=5 d0) Charpy V-type impact test specimen (10 mm × 10 mm × 55 mm). Heat treatment was performed according to the heat treatment process shown in Table 1. The delayed fracture resistance of the steel was evaluated according to GB / T39039-2020 "Evaluation Method for Hydrogen-Induced Delayed Fracture of High-Strength Steel". In specific production processes, the corresponding steel can be obtained by using the appropriate preparation process.

[0046]

[0047] Table 1

[0048] Performance testing:

[0049] The specimens were subjected to tensile and impact tests at room temperature, and the results are listed in Table 2.

[0050]

[0051] Table 2

[0052] As can be seen from Table 2, the diameter of the forged steel of this invention is... Φ After controlled cooling, 16 mm thick bars exhibit mechanical properties including tensile strength ≥2050 MPa, yield strength ≥1800 MPa, elongation after fracture ≥10%, reduction of area ≥45%, and room temperature impact energy (KV2) ≥17 J, demonstrating a good balance of strength, plasticity, and toughness. The delayed fracture resistance (characterized by tensile strength after saturated hydrogen charging) is ≥1250 MPa, which is more than 30% higher than that of the conventional spring steel 60Si2CrV. The steel of this invention shows significantly improved mechanical properties and delayed fracture resistance compared to the comparative steel under the same processing conditions.

[0053] In addition to the above embodiments, the present invention may have other implementation methods; all technical solutions formed by equivalent substitution or equivalent transformation fall within the protection scope claimed by the present invention.

Claims

1. A high-strength spring steel with excellent resistance to delayed fracture, characterized in that: Its chemical composition and mass percentage are as follows: C: 0.50%~0.60%; Si: 1.60%~2.20%; Mn: 0.80%~1.30%; Cr: 0.80%~1.30%; W: 0.10%~0.20%; V: 0.15%~0.30%; Nb: 0.015%~0.035%; P: ≤0.012%; S: ≤0.010%; O ≤0.0015%; N ≤0.0060%; H ≤0.00012%; the remainder is iron and unavoidable impurities; In the high-strength spring steel with excellent resistance to delayed fracture, the mass percentages of V, Nb, W, and N satisfy the following relationship: 0.25≤0.80V+Nb-6.64N+W≤0.

40.

2. A heat treatment process for high-strength spring steel with excellent resistance to delayed fracture as described in claim 1, characterized in that: In order, they include: Quenching heat treatment, first tempering heat treatment, and second tempering heat treatment; The quenching heat treatment is performed at a heating temperature of 900±30 ℃ and a holding time of 30±10 min.

3. The heat treatment process for high-strength spring steel with excellent resistance to delayed fracture as described in claim 2, characterized in that: The cooling method for the quenching heat treatment is oil cooling or water cooling.

4. The heat treatment process for high-strength spring steel with excellent resistance to delayed fracture as described in claim 2, characterized in that: The first tempering heat treatment includes: Heated to 580±20 ℃ at a heating rate of 50±10 ℃ / s, held for 20±5 s, and then water cooled.

5. The heat treatment process for high-strength spring steel with excellent resistance to delayed fracture as described in claim 2, characterized in that: The tempering temperature for the second tempering heat treatment is 350 ± 20 ℃, and the holding time is 90~120 min.

6. The heat treatment process for high-strength spring steel with excellent resistance to delayed fracture as described in claim 2, characterized in that: The cooling method for the second tempering heat treatment is air cooling or water cooling.

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