An ultra-high strength spring steel with a tensile strength of 2100 MPa or more for large-size springs of engineering machinery and a method for manufacturing the same

Abstract

The application discloses a kind of ultra-high strength spring steels with tensile strength ≥2100 MPa for engineering machinery large specification spring and preparation method thereof, and relates to the technical field of steel production, and its chemical composition and mass percentage are as follows: C:0.46%~0.55%;Si:1.50%~2.00%;Mn:0.80%~1.50%;Cr:0.90%~1.50%;Mo:0.10%~0.40%;Ni:0.40%~0.80%;W:0.10%~0.40%;V:0.08%~0.15%;Ti:0.015%~0.035%;B:0.0008~0.0035%;P:≤0.012%;S:≤0.006%;O≤0.0015%; the rest is Fe and unavoidable impurities.The ultra-high strength spring steel prepared by the application has excellent hardenability, high strength, good plasticity and toughness, etc.

Description

Technical Field

[0001] This invention relates to the field of steel production technology, and in particular to an ultra-high strength spring steel with a tensile strength ≥2100 MPa for large-size springs used in engineering machinery and its preparation method. Background Technology

[0002] In recent years, with the rapid development of China's economy, the demand for various types of construction machinery has increased dramatically. Spring steel for construction machinery is mainly used to produce tension springs for bulldozers, excavators, and other construction machinery. Large-tonnage construction machinery experiences greater working stress, and the spring specifications are often larger, requiring spring steel to have both higher strength levels and high hardenability to meet the urgent requirements of lightweight and high-performance spring materials for construction machinery. However, the properties of materials currently used to manufacture springs for construction machinery have reached their limits, with strength levels generally below 2000 MPa, necessitating the further development of ultra-high-strength spring steel with high hardenability.

[0003] The invention patent with patent number ZL 201210005280.6, "High-strength, Low-Temperature Toughness Spring Steel with Guaranteed Hardenability," specifies the content of elements such as C, Si, Mn, Cr, and Ni in the spring steel. After quenching and medium-temperature tempering, the tensile strength is ≥1750 MPa, and the hardenability can reach J20 mm≥55HRC. It is suitable for making cylindrical helical springs with a smaller diameter of ≤Φ50 mm. However, it not only fails to meet the high hardenability requirement of J40 mm≥55HRC, but also has low strength, thus failing to meet the requirements of large-scale springs for heavy-duty engineering machinery.

[0004] The invention patent with application number 202010061888.5, entitled "A high hardenability, high strength, low temperature toughness spring steel and its production method", optimizes the content of elements such as C, Si, Mn, Cr, Ni, and Mo to achieve a hardenability of J20 mm ≥ 55HRC and a tensile strength of ≥ 1700 MPa after quenching and medium temperature tempering. However, it still cannot meet the requirement of J40 mm ≥ 55HRC for large-size springs, and the strength level is relatively low.

[0005] The invention patent application number 202010147753.0, entitled "A high-strength spring steel with tensile strength ≥2250 MPa and excellent fatigue performance and its production method", obtained an ultra-high-strength spring steel with tensile strength ≥2250 MPa through the interaction of C, Si, Mn, Cr, V, Nb, Mg and N. However, its maximum steel specification is only Φ16 mm, and its hardenability cannot meet the requirements of large-size springs for engineering machinery. Moreover, the carbon content is too high (0.70~0.80 wt.%), which cannot guarantee the plasticity and toughness requirements of the spring steel. The invention patents with application number 202210754602.0, "2100 MPa grade spring steel wire and its production method", and application number 202311768900.6, "a 2200 MPa grade high strength and toughness spring steel wire and its preparation method", although they can obtain ultra-high strength (tensile strength ≥2100 MPa and ≥2200 MPa respectively), are also suitable for manufacturing small-sized spring steel wires.

[0006] In summary, neither the strength level nor the hardenability of the above materials can simultaneously meet the requirements for ultra-high strength spring steel for large-size springs in large-scale engineering machinery. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide an ultra-high strength spring steel with a tensile strength ≥2100 MPa for large-size springs in engineering machinery and its preparation method.

[0008] To solve the above technical problems, the technical solution of the present invention is as follows:

[0009] A type of ultra-high strength spring steel with a tensile strength ≥2100 MPa for large-size springs in engineering machinery has the following chemical composition and mass percentage: C: 0.46%~0.55%; Si: 1.50%~2.00%; Mn: 0.80%~1.50%; Cr: 0.90%~1.50%; Mo: 0.10%~0.40%; Ni: 0.40%~0.80%; W: 0.10%~0.40%; V: 0.08%~0.15%; Ti: 0.015%~0.035%; B: 0.0008~0.0035%; P: ≤0.012%; S: ≤0.006%; O ≤0.0015%; the remainder is Fe and unavoidable impurities.

[0010] As a preferred embodiment of the ultra-high strength spring steel with a tensile strength ≥2100 MPa for large-size springs in engineering machinery as described in this invention, the mass percentages of Mn, Cr, Mo, W, Ni, and B satisfy the following relationship: 3.00≤Mn(%)+0.95Cr(%)+1.7Mo(%)+0.5W+0.6Ni+75B(%)≤4.00.

[0011] As a preferred embodiment of the ultra-high strength spring steel with a tensile strength ≥2100 MPa for large-size springs of engineering machinery described in this invention, the mass percentages of V, Ti, and N satisfy the following relationship: 0.07≤0.75V(%)+[Ti(%)-3.43N(%)]≤0.13.

[0012] This invention also provides a method for preparing ultra-high strength spring steel with a tensile strength ≥2100 MPa for large-size springs in engineering machinery, which includes the following steps in sequence: smelting in an electric arc furnace or converter, refining outside the ladle, casting into steel ingots or continuous casting into billets, forging or rolling into shape, and heat treatment.

[0013] As a preferred embodiment of the method for preparing ultra-high strength spring steel with a tensile strength ≥2100 MPa for large-size springs in engineering machinery according to the present invention, the heat treatment process includes:

[0014] First, perform oil quenching at 900±30 ℃, then perform tempering at 350±20 ℃.

[0015] As a preferred embodiment of the method for preparing ultra-high strength spring steel with a tensile strength ≥2100 MPa for large-size springs of engineering machinery as described in this invention, the ultra-high strength spring steel obtained has the following characteristics: end hardenability J40 mm ≥55HRC, tensile strength ≥2100 MPa, yield strength ≥1800 MPa, elongation after fracture ≥8%, reduction of area ≥40%, and room temperature V-shaped impact energy ≥15 J.

[0016] As a preferred embodiment of the method for preparing ultra-high strength spring steel with a tensile strength ≥2100 MPa for large-size springs of engineering machinery according to the present invention, the diameter of the obtained ultra-high strength spring steel is 80~100 mm.

[0017] The beneficial effects of this invention are:

[0018] This invention achieves a martensitic structure in large-diameter steel (Φ80 mm and above) after quenching through a rational combination of Cr, Mn, Mo, B, Ni, and W elements (controlled parameter α). Furthermore, the addition of microalloying elements V and Ti ensures that an appropriate amount of undissolved Ti- and V-rich M(C,N) particles inhibit grain growth during austenitization heating, while simultaneously providing an appropriate amount of dissolved V (controlled parameter α). β It plays a role in further hardenability; at the same time, the high content of Si in the steel allows the steel to precipitate a large number of fine ε-carbides even when tempered at around 400℃, thus obtaining a good precipitation strengthening effect, so as to ensure that it still has good plasticity and toughness while obtaining ultra-high strength. 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 an ultra-high strength spring steel for large-size springs in engineering machinery, the chemical composition and mass percentage of which are as follows: C: 0.46%~0.55%; Si: 1.50%~2.00%; Mn: 0.80%~1.50%; Cr: 0.90%~1.50%; Mo: 0.10%~0.40%; Ni: 0.40%~0.80%; W: 0.10%~0.40%; V: 0.08%~0.15%; Ti: 0.015%~0.035%; B: 0.0008~0.0035%; P: ≤0.015%; S: ≤0.010%; O ≤0.0015%; the remainder is Fe and unavoidable impurities.

[0021] The roles and proportions of each element in this application are based on the following:

[0022] Carbon (C): Carbon is the most fundamental and effective strengthening element in steel, essential for spring steel to achieve high strength, hardness, and elasticity. Traditional spring steels have relatively high C content; for example, the most widely used 60Si2Mn and 60Si2CrV spring steels both have a carbon content above 0.55%. However, excessively high C content is detrimental to plasticity and toughness, increases decarburization sensitivity, and worsens the steel's fatigue and processing properties. Therefore, it is advisable to appropriately reduce the C content in the steel, controlling it below 0.55%. However, to obtain the required ultra-high strength level after quenching and medium-temperature tempering, the C content must be above 0.46%, thus the C content should ideally be controlled between 0.46% and 0.55%. The strength loss caused by reducing the C content can be compensated for by solid solution strengthening with elements such as Si, Mo, and V, grain refinement strengthening, and appropriately lowering the tempering temperature.

[0023] Si (Si): Si is a major alloying element in traditional spring steel, possessing strong solid solution strengthening properties and being the alloying element with the greatest impact on spring-load resistance. Therefore, its content is appropriately increased, controlled above 1.50%. However, excessive Si content will reduce ductility and toughness, promote decarburization and graphitization during rolling and heat treatment, and make smelting difficult and prone to inclusion formation, thus deteriorating the fatigue resistance of the steel. Considering that the strong carbide-forming element V can mitigate the above-mentioned adverse effects of Si, the Si content is controlled at 1.50–2.00%.

[0024] 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 these effects. However, during the tempering of quenched steel, Mn and P have a strong tendency to co-segregate at grain boundaries, promoting temper brittleness and deteriorating the toughness of the steel. Therefore, the Mn content should be controlled below 1.50%.

[0025] Cr: Cr can significantly improve the hardenability and tempering resistance of steel to obtain the required ultra-high strength; 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, which is beneficial to obtaining high fatigue resistance. However, excessive content will deteriorate the elasticity and toughness of steel, so the Cr content is controlled at 0.90-1.50%.

[0026] Mo: Mo can significantly improve the hardenability of steel, reduce temper brittleness, and lower the ductile-brittle transition temperature; adding Mo to spring steel can also improve the steel's resistance to elastic reduction. However, excessive Mo affects the economics of steel, so the preferred Mo content range is 0.10–0.40%.

[0027] W: The role of W is similar to that of Mo. It can improve the hardenability of steel, increase tempering resistance, reduce tempering brittleness, and improve the elasticity reduction resistance of steel. Therefore, the W content should be controlled at 0.10-0.40%.

[0028] Ni (Ni): Ni significantly improves the hardenability of steel, lowers its brittle-cold transition temperature, and significantly improves its toughness, especially its low-temperature toughness, ensuring toughness under ultra-high strength conditions. Furthermore, Ni can enhance corrosion resistance by inhibiting the formation and depth of corrosion pits. However, excessive Ni content not only affects the economics of steel but also makes it difficult to remove the iron oxide scale, impacting surface quality and fatigue performance. Therefore, the Ni content is controlled at 0.40–0.80%.

[0029] B: In addition to significantly improving the hardenability of steel even in trace amounts, B can also inhibit the grain boundary segregation of impurity element P, thus purifying the grain boundaries and improving the toughness of the steel. Simultaneously, B can inhibit oxidative decarburization during heat treatment, thereby improving the fatigue resistance of the steel. For these effects to be achieved, the B content needs to be above 0.0008%. However, when the B content exceeds 0.0035%, excess B will form coarse BN, which will worsen the toughness and fatigue resistance of the steel.

[0030] V (V): V is a strong carbide-forming element. The fine, dispersed carbides formed when V combines with C can prevent grain growth during heating, thus acting as a grain refiner and eliminating the grain coarsening phenomenon easily caused by adding B to steel. This simultaneously improves the strength, toughness, and fatigue performance of the steel. When the V content is below 0.08%, the above effects are not significant; when the V content is above 0.15%, the above effects are saturated, and the cost of the steel increases. Therefore, the V content is controlled between 0.08% and 0.15%.

[0031] Ti: Ti fixes nitrogen (N) in steel, inhibiting the formation of coarse boron (BN) and ensuring the aforementioned beneficial effects of boron. Furthermore, Ti refines the grain size, eliminating the coarse grains easily caused by adding boron to steel, thus guaranteeing the steel's ductility and toughness. A Ti content less than 0.015% does not achieve these effects, but a content exceeding 0.035% results in saturation and the formation of coarse TiN, which deteriorates the steel's toughness and fatigue resistance. Therefore, the Ti content is controlled between 0.015% and 0.035%.

[0032] 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 0.015%.

[0033] 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%.

[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] O₂ (O₂) forms various oxide inclusions in steel. Under stress, stress concentration easily occurs at these oxide inclusions, leading to the initiation of microcracks and thus deteriorating the mechanical properties of the steel, especially its toughness and fatigue resistance. Therefore, measures must be taken in metallurgical production to reduce its content as much as possible. Considering economic efficiency, its content should be controlled below 0.0015%.

[0036] Furthermore, to achieve good hardenability and meet the requirement that large-sized springs, especially those with a diameter of 80 mm or more, have a martensitic structure throughout the entire cross-section after quenching, a multi-element composite additive of Cr+Mn+Mo+B+Ni+W is used, satisfying the following conditions: 3.00≤Mn(%)+0.95Cr(%)+1.7Mo(%)+0.5W+0.6Ni+75B(%)≤4.00, and α= Mn(%)+0.95Cr(%)+1.7Mo(%)+0.5W+0.6Ni+75B(%). When the α value is less than 3.00, although the content of individual Cr, Mn, Mo, W, Ni, or B elements may all be within the above optimal range, it is difficult to obtain sufficiently high hardenability and a martensitic structure throughout the cross-section, thus affecting the achievement of excellent strength, ductility, and toughness balance. When the α value is greater than 4.00, the hardenability is too high, affecting the workability and economy of the steel.

[0037] Secondly, to overcome the tendency of adding boron to steel to cause coarse microstructure and instead obtain fine martensitic microstructure, and thus ensure good ductility, toughness, and resistance to elastic reduction while achieving ultra-high strength, a V+Ti composite microalloying method is used. This ensures that an appropriate amount of undissolved Ti- and V-rich M(C,N) inhibits grain growth during austenitization heating, while an appropriate amount of dissolved V further improves hardenability. In other words, the content must also meet certain parameters. β The relationship is: 0.07≤0.75V(%)+[Ti(%)-3.43N(%)]≤0.13. β =0.75V(%)+[Ti(%)-3.43N(%)]. when β When the value is less than 0.07, even though the contents of individual V and Ti elements may be within the optimal range mentioned above, good ductility and toughness cannot still be obtained; when β When the value is greater than 0.13, the effect is saturated and the cost of steel is increased.

[0038] This application also provides a method for preparing ultra-high strength spring steel with a tensile strength ≥2100 MPa for large-size springs in engineering machinery. The preparation method includes the following steps in sequence:

[0039] Smelting in an electric arc furnace or converter, refining outside the furnace, casting into steel ingots or continuously casting into billets, forging or rolling into shape, and heat treatment.

[0040] The heat treatment process includes: first, oil quenching at 900±30 ℃, and then tempering at 350±20 ℃.

[0041] Using the above preparation method, ultra-high strength spring steel bars for large engineering machinery with diameters of Φ80~100 mm were obtained, with end hardenability J40 mm ≥55HRC. After heat treatment, the ultra-high strength spring steel has a tensile strength ≥2100 MPa, a yield strength ≥1800 MPa, an elongation after fracture ≥8%, a reduction of area ≥40%, and a room temperature V-shaped impact energy ≥15J.

[0042] The following specific examples will provide further details.

[0043] Based on the chemical composition range designed according to the present invention, four heats of the steel of the present invention and three heats of the comparative steel were smelted in a vacuum induction furnace. Their specific chemical compositions are shown in Table 1, where heat numbers 1-4 are the steel of the present invention, and heat numbers 5-7 are the comparative steels. The steel ingots were forged into diameters of... F 80 mm F 100 mm 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 = 5 mm, length l 0=5 d 0) Charpy V-type impact test specimens (10 mm × 10 mm × 55 mm) and end-hardenability test specimens (25 mm in diameter, 100 mm in length, with raised edges). In specific production processes, the corresponding steel can be obtained by using appropriate preparation techniques.

[0044]

[0045] Table 1 Chemical composition (mass percentage, %) of the embodiments and comparative examples of the present invention

[0046] Performance testing:

[0047] The specimens were subjected to tensile and impact tests at room temperature, and the results are listed in Table 2.

[0048]

[0049] Table 2 Mechanical properties and hardenability of embodiments and comparative examples of the present invention

[0050] As can be seen from Table 2, the steel forging of this invention is made into two specifications (diameter). F 80 mm FThe steel bars (100 mm in diameter) exhibit a hardenability J40 mm ≥ 55 HRC. After heat treatment (quenching: 900 ± 30 ℃, holding time 30 ± 10 min, water cooling or oil cooling; tempering: 350 ± 20 ℃, holding time 90~120 min, air cooling or water cooling), samples were taken from half the radius of the bars according to national standards. All samples showed mechanical properties including tensile strength ≥ 2100 MPa, yield strength ≥ 1800 MPa, elongation after fracture ≥ 8%, reduction of area ≥ 40%, and room temperature impact energy (KV2) ≥ 15 J, demonstrating a good balance of strength, ductility, and toughness, as well as excellent hardenability. The mechanical properties and hardenability of this steel are significantly improved compared to the comparative steel.

[0051] 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. An ultra-high strength spring steel with a tensile strength > 2100 MPa for large size springs of construction machines, characterized in that: The chemical composition and mass percentage are as follows: C: 0.46%-0.55%; Si: 1.50%-2.00%; Mn: 0.80%-1.50%; Cr: 0.90%-1.50%; Mo: 0.10%-0.40%; Ni: 0.40%-0.80%; W: 0.10%-0.40%; V: 0.08%-0.15%; Ti: 0.015%-0.035%; B: 0.0008-0.0035%; P: ≤0.012%; S: ≤0.006%; O ≤0.0015%; the rest is Fe and inevitable impurities; The mass percentage of the Mn, Cr, Mo, W, Ni and B satisfies the relationship formula: 3.00 ≤ Mn+0.95Cr+1.7Mo+0.5W+0.6Ni+75B ≤4.00; The mass percentage of the V, Ti and N satisfies the relationship formula: 0.07 ≤ 0.75V+Ti-3.43N ≤0.

13.

2. A method of producing an ultra-high strength spring steel with a tensile strength > 2100 MPa for large-size springs of construction machines based on the steel according to claim 1, characterized in that: The process comprises the following steps in sequence: electric arc furnace or converter smelting, secondary refining, casting into ingot or continuous casting into billet, forging or rolling forming, heat treatment.

3. Method of manufacturing a spring steel with an ultimate tensile strength > 2100 MPa for large size springs of engineering machinery according to claim 2, characterized in that: The heat treatment process comprises: Firstly, 900±30 ℃ oil quenching treatment is carried out, and then 350±20 ℃ tempering treatment is carried out.

4. Method of manufacturing a spring steel with an ultimate tensile strength > 2100 MPa for heavy duty springs of construction machines according to claim 2, characterized in that: The prepared super high strength spring steel has the end hardenability J40 mm ≥55HRC, the tensile strength ≥2100 MPa, the yield strength ≥1800 MPa, the elongation after fracture ≥8%, the reduction of area ≥40%, and the room temperature V-type impact energy ≥15 J.

5. Method of manufacturing a spring steel with an ultimate tensile strength > 2100 MPa for heavy duty springs of construction machines according to claim 2, characterized in that: The prepared super high strength spring steel has a diameter of 80-100 mm.

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