Ultra-high-strength spring steel with tensile strength larger than or equal to 2100 MPa for large-specification springs of engineering machinery and preparation method of ultra-high-strength spring steel

Abstract

The invention discloses ultra-high-strength spring steel with the tensile strength larger than or equal to 2100 MPa for a large-specification spring of engineering machinery and a preparation method of the ultra-high-strength spring steel, and relates to the technical field of steel production. 1.50% to 2.00% of Si; 0.80% to 1.50% of Mn; 0.90% to 1.50% of Cr; 0.10% to 0.40% of Mo; 0.40% to 0.80% of Ni; 0.10% to 0.40% of W; 0.08% to 0.15% of V; ti: 0.015% to 0.035%; b: 0.0008% to 0.0035% of the total weight of the P: less than or equal to 0.012%; s: < = 0.006%; o < = 0.0015%; and the balance of Fe and inevitable impurities. The prepared ultrahigh-strength spring steel has the advantages of being excellent in hardenability, high in strength, good in plasticity and toughness and the like.

Description

Technical Field

[0001] The present invention relates to the technical field of steel production, and in particular to an ultra-high strength spring steel with a tensile strength of ≥2100 MPa for large-size springs of engineering machinery and a preparation method thereof. Background Art

[0002] In recent years, with the rapid development of China's economy, the demand for various construction machinery has increased dramatically. Spring steel for construction machinery is mainly used to produce tension springs on construction machinery such as bulldozers and excavators. The working stress of large-tonnage construction machinery is relatively large, and the spring specifications are often larger. Spring steel is required to have a higher strength level and high hardenability to meet the urgent requirements of lightweight and high-performance spring materials for construction machinery. However, the performance of materials currently used to manufacture springs for construction machinery has reached its limit, and its strength level is basically below 2000 MPa. There is an urgent need to further develop ultra-high strength spring steel with high hardenability.

[0003] The invention patent "High-strength low-temperature toughness spring steel with guaranteed hardenability" with patent number ZL 201210005280.6 stipulates the content of elements such as C, Si, Mn, Cr, Ni, etc. in spring steel. The tensile strength after quenching + medium-temperature tempering is ≥1750 MPa, and the hardenability can reach J20 mm≥55HRC. It is suitable for making cylindrical helical springs with smaller diameters of ≤Φ50 mm. Not only can it not meet the high hardenability requirement of J40 mm≥55HRC, but the strength of the spring steel is also low, so it cannot meet the requirements of large-size springs of large-tonnage engineering machinery.

[0004] The invention patent with application number 202010061888.5, "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, Mo, etc., so that the hardenability J20 mm ≥ 55HRC, and the tensile strength after quenching + medium temperature tempering is ≥ 1700 MPa. However, it still cannot meet the J40 mm ≥ 55HRC requirement for large-size springs, and the strength level is relatively low.

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

[0006] In summary, the strength level and hardenability of the above materials cannot simultaneously meet the requirements of ultra-high strength spring steel for large-size springs of 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 of engineering machinery and a preparation method thereof.

[0008] In order to solve the above technical problems, the technical solution of the present invention is as follows: An ultra-high strength spring steel with a tensile strength of ≥2100 MPa for large-size springs of engineering machinery, whose 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 are Fe and unavoidable impurities.

[0009] As a preferred solution of the ultra-high strength spring steel with a tensile strength ≥2100 MPa for large-size springs of engineering machinery described in the present invention, the mass percentages of Mn, Cr, Mo, W, Ni, and B satisfy the relationship: 3.00≤Mn(%)+0.95Cr(%)+1.7Mo(%)+0.5W+0.6Ni+75B(%)≤4.00.

[0010] As a preferred solution for the ultra-high strength spring steel with a tensile strength ≥2100 MPa for large-size springs of engineering machinery described in the present invention, the mass percentages of V, Ti, and N satisfy the relationship: 0.07≤0.75V(%)+[Ti(%)-3.43N(%)]≤0.13.

[0011] The present invention also provides a method for preparing ultra-high strength spring steel with a tensile strength ≥2100 MPa for large-size springs of engineering machinery, which comprises the following steps in sequence: smelting in an electric arc furnace or a converter, refining outside the furnace, casting into steel ingots or continuous casting into billets, forging or rolling forming, and heat treatment.

[0012] As a preferred solution 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 heat treatment process comprises: Firstly carry out oil quenching at 900±30℃, and then carry out tempering at 350±20℃.

[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 of engineering machinery described in the present invention, the ultra-high strength spring steel has an end hardenability J40 mm ≥55HRC, a tensile strength ≥2100 MPa, a yield strength ≥1800MPa, an elongation after fracture ≥8%, a cross-sectional shrinkage ≥40%, and a room temperature V-type impact energy ≥15 J.

[0014] As a preferred solution of the method for preparing ultra-high strength spring steel with a tensile strength ≥ 2100 MPa for large-size springs of engineering machinery described in the present invention, the diameter of the prepared ultra-high strength spring steel is 80-100 mm.

[0015] The beneficial effects of the present invention are: The present invention ensures that the large-size steel with a diameter of more than Φ80 mm obtains a martensitic structure after quenching by reasonably combining Cr, Mn, Mo, B, Ni, and W elements (control parameter α); and compositely adding microalloying elements V and Ti ensures that there are appropriate amounts of undissolved M(C,N) particles rich in Ti and V to inhibit grain growth during austenitization heating, and at the same time, there is an appropriate amount of dissolved V (control parameter α). β ) plays a role in further hardenability; at the same time, the higher content of Si in the steel enables the steel to precipitate a large number of fine ε-carbides even when tempered at around 400°C, thereby achieving a good precipitation strengthening effect, thereby ensuring that it has good plasticity and toughness while achieving ultra-high strength. DETAILED DESCRIPTION

[0016] In order to make the content of the present invention more clearly understood, the present invention is further described in detail below according to specific implementation methods.

[0017] The present application provides an ultra-high strength spring steel for large-size springs of engineering machinery, whose 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: ≤0.015%; S: ≤0.010%; O≤0.0015%; the rest are Fe and unavoidable impurities.

[0018] The role and proportion of each element in this application are as follows: C: C is the most basic and effective strengthening element in steel and is necessary for spring steel to obtain high strength, hardness and elasticity. The C content in traditional spring steel is relatively high. For example, the carbon content of the 60Si2Mn and 60Si2CrV spring steels, which are currently the most widely used, is above 0.55%. However, too high a C content is not good for plasticity and toughness, increases the sensitivity to decarburization, and deteriorates the fatigue properties and processing properties of the steel. Therefore, the C content in the steel should be appropriately reduced to below 0.55%. However, in order to obtain the required ultra-high strength level after quenching and medium-temperature tempering, the C content must be above 0.46%, so the C content should be controlled to 0.46-0.55%. The strength loss caused by the reduction in C content can be compensated by solid solution strengthening of elements such as Si, Mo, and V, fine grain strengthening, and appropriately reducing the tempering temperature.

[0019] Si: Si is the main alloying element in traditional spring steel, with a strong solid solution strengthening effect. It is also the alloying element with the greatest influence on the spring reduction performance. Therefore, its content is appropriately increased and controlled above 1.50%. However, too high Si content will reduce plastic toughness, promote decarburization and graphitization tendency of steel during rolling and heat treatment, make smelting difficult and easy to form inclusions, and deteriorate the fatigue resistance of steel. Considering that the strong carbide forming element V can alleviate the above-mentioned adverse effects of Si, the Si content is controlled to 1.50-2.00%.

[0020] Mn: Mn can improve the hardenability of steel, and has a significant solid solution strengthening effect when dissolved in ferrite. It is also an effective element for deoxidation and desulfurization. When the content is less than 0.80%, it is difficult to play the above role. However, when quenched steel is tempered, Mn and P have a strong tendency to co-segregate at the grain boundaries, which promotes temper brittleness and deteriorates the toughness of the steel. Therefore, the Mn content is controlled below 1.50%.

[0021] 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 also reduce the activity of C, reduce the surface decarburization of steel during heating, rolling and heat treatment, and inhibit the graphitization tendency of high-Si steel, which is useful for obtaining high fatigue resistance. However, too high a content will deteriorate the elasticity and toughness of the steel, so the Cr content is controlled to 0.90-1.50%.

[0022] 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 elasticity reduction resistance of steel. However, excessive Mo affects the economy of steel, so the preferred Mo content range is 0.10-0.40%.

[0023] W: The effect of W element is similar to that of Mo element, which can improve the hardenability of steel, improve tempering resistance, reduce tempering brittleness, and improve the elastic loss resistance of steel. Therefore, the W content is controlled to be 0.10-0.40%.

[0024] Ni: Ni can significantly improve the hardenability of steel, and at the same time reduce the cold brittle transition temperature of steel, significantly improve the toughness of steel, especially low-temperature toughness, to ensure the toughness of steel under ultra-high strength; in addition, Ni can also inhibit the formation and depth of corrosion pits and improve corrosion fatigue resistance. However, too high Ni content not only affects the economy of steel, but also the iron oxide scale of Ni-containing steel is not easy to remove, affecting the surface quality and fatigue performance of steel. Therefore, the Ni content is controlled to 0.40-0.80%.

[0025] B: B can significantly improve the hardenability of steel by adding a small amount, and can also inhibit the grain boundary segregation of the impurity element P, play a role in purifying the grain boundary, improve the toughness of steel, and inhibit the oxidation and decarburization of steel during heat treatment, thus helping to improve the fatigue resistance of steel. In order to achieve the above effects, the B content must be above 0.0008%, but when the B content exceeds 0.0035%, the excess B will form coarse BN, which will deteriorate the toughness and fatigue resistance of the steel.

[0026] V: V is a strong carbide-forming element. The fine dispersed carbides formed by combining with C can prevent the grains from growing during heating, play a role in fine grain strengthening, and eliminate the coarse grain phenomenon easily caused by adding B to the steel, thereby improving the strength, toughness and fatigue performance of the steel at the same time. When the V content is lower than 0.08%, the above effects are not obvious; when the V content is higher than 0.15%, the above effects are saturated and the cost of the steel is increased. Therefore, the V content is controlled to be 0.08-0.15%.

[0027] Ti: Ti fixes N in steel, inhibits the formation of coarse BN, and ensures the above-mentioned good effects of B. In addition, Ti can also refine the grains, eliminate the coarse grain phenomenon easily caused by adding B to steel, and ensure the plasticity and toughness of steel. Ti content less than 0.015% does not play the above role, but the content exceeds 0.035% and the role is saturated, and coarse TiN is easily formed, which deteriorates the toughness and fatigue resistance of steel. Therefore, the Ti content is controlled to 0.015~0.035%.

[0028] P: P can form micro segregation when the molten steel solidifies, and then segregate at the grain boundaries when heated to the austenitizing temperature, which significantly increases the brittleness of the steel, so the P content is controlled below 0.015%.

[0029] S: An inevitable impurity in steel. The formation of MnS inclusions and segregation at grain boundaries will deteriorate the toughness and fatigue resistance of the steel, so its content is controlled below 0.010%.

[0030] N: N mainly forms Nb and V carbonitride M(C,N) with Nb and V in steel, which refines grains and improves delayed fracture resistance. Too high N content will significantly reduce the cold working performance of steel, so its content is controlled below 0.0060%.

[0031] O: O forms various oxide inclusions in steel. Under the action of stress, stress concentration is easily generated at these oxide inclusions, leading to the initiation of microcracks, thereby deteriorating the mechanical properties of steel, especially toughness and fatigue resistance. Therefore, measures must be taken to reduce its content as much as possible in metallurgical production. Considering economic efficiency, its content is controlled below 0.0015%.

[0032] In addition, in order to obtain good hardenability and meet the requirements of obtaining martensitic structure in the entire cross section after quenching for large-size springs, especially springs with a diameter of more than 80 mm, Cr+Mn+Mo+B+Ni+W multi-component addition is adopted, and 3.00≤Mn(%)+0.95Cr(%)+1.7Mo(%)+0.5W+0.6Ni+75B(%)≤4.00 is met, α= Mn(%)+0.95Cr(%)+1.7Mo(%)+0.5W+0.6Ni+75B(%). When the α value is less than 3.00, although the content of a single Cr, Mn, Mo, W, Ni or B element may be within the above most suitable range, it is difficult to obtain sufficiently high hardenability and full-section martensitic structure, which in turn affects the excellent strength, plasticity and toughness combination; when the α value is greater than 4.00, the hardenability is too high, affecting the processability and economy of the steel.

[0033] Secondly, in order to overcome the problem that adding B to steel will easily cause the structure to coarsen and obtain fine martensite structure, and thus ensure that ultra-high strength is obtained while having good plasticity and elasticity reduction resistance, V+Ti is used for composite microalloying. On the one hand, it ensures that there is an appropriate amount of undissolved Ti-rich V-rich M (C, N) to inhibit grain growth during austenitization heating, and at the same time, an appropriate amount of dissolved V plays a role in further improving hardenability. That is, its content must also meet the parameters β Relationship: 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 V and Ti elements may be within the above most suitable range, good plasticity and toughness cannot be obtained. β When the value is greater than 0.13, the effect is saturated and the cost of steel increases.

[0034] The present application also provides a method for preparing ultra-high strength spring steel with a tensile strength ≥ 2100 MPa for large-size springs of engineering machinery, the preparation method comprising the following steps in sequence: Electric arc furnace or converter smelting, refining outside the furnace, casting into ingots or continuous casting into billets, forging or rolling forming, heat treatment.

[0035] The heat treatment process includes: firstly performing oil quenching at 900±30°C, and then performing tempering at 350±20°C.

[0036] The above preparation method is used to produce ultra-high strength spring steel bar products for large engineering machinery springs with a diameter of Φ80 ~100 mm, and the 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 cross-sectional shrinkage ≥40%, and a room temperature V-type impact energy ≥15J.

[0037] The following is further described by means of specific examples.

[0038] According to the chemical composition range designed by the present invention, 4 heats of the present invention steel and 3 heats of comparative steel were smelted in a vacuum induction furnace. The specific chemical compositions are shown in Table 1, where heats 1 to 4 are the present invention steel and heats 5 to 7 are comparative steel. The steel ingots were forged into a diameter of F 80 mm, F 100 mm bars are slowly cooled after forging. They are processed into standard room temperature tensile specimens (gauge diameter) required for the experiment according to national standards. d 0=5 mm, length l 0=5 d0), Charpy V-type impact specimen (10 mm × 10 mm × 55 mm) and end hardenability specimen (25 mm in diameter, 100 mm in length, with convex edge). In the specific production process, the corresponding steel can be obtained by using the corresponding preparation process.

[0039]

[0040] Table 1 Chemical composition of the examples and comparative examples of the present invention (mass percentage, %) Performance Test: The samples were subjected to tensile and impact tests at room temperature and the results are listed in Table 2.

[0041]

[0042] Table 2 Mechanical properties and hardenability of the embodiments of the present invention and comparative examples It can be seen from Table 2 that the steel of the present invention is forged into two specifications (diameter F 80 mm, F 100 mm), hardenability J40 mm ≥ 55HRC; after heat treatment of steel bars (quenching: 900 ± 30 ℃, holding time is 30 ± 10 min, water cooling or oil cooling; tempering: 350 ± 20 ℃, holding time is 90~120 min, air cooling or water cooling), samples are taken from the 1 / 2 radius of the bar according to the national standard requirements, and mechanical properties of tensile strength ≥ 2100 MPa, yield strength ≥ 1800 MPa, elongation after fracture ≥ 8%, cross-sectional shrinkage ≥ 40%, room temperature impact energy (KV2) ≥ 15 J or more can be obtained, showing good strength, plasticity and toughness combination, and good hardenability. The mechanical properties and hardenability of the steel of the present invention are significantly improved compared with the comparative steel.

[0043] In addition to the above embodiments, the present invention may also have other implementation modes; any technical solutions formed by equivalent replacement or equivalent transformation shall fall within the protection scope required by the present invention.

Claims

1. An ultra-high strength spring steel with a tensile strength of ≥2100 MPa for large-size springs of engineering machinery, characterized in that: 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 are Fe and unavoidable impurities.

2. The ultra-high strength spring steel with a tensile strength of ≥2100 MPa for large-size springs of engineering machinery according to claim 1, characterized in that: The mass percentages of Mn, Cr, Mo, W, Ni and B satisfy the relationship: 3.00≤Mn(%)+0.95Cr(%)+1.7Mo(%)+0.5W+0.6Ni+75B(%)≤4.

00.

3. The ultra-high strength spring steel with a tensile strength of ≥2100 MPa for large-size springs of engineering machinery according to claim 1, characterized in that: The mass percentages of V, Ti and N satisfy the relationship: 0.07≤0.75V(%)+[Ti(%)-3.43N(%)]≤0.

13.

4. A method for preparing ultra-high strength spring steel with a tensile strength of ≥2100 MPa for large-size springs of engineering machinery according to any one of claims 1 to 3, characterized in that: The process includes the following steps: smelting in an electric arc furnace or a converter, refining outside the furnace, casting into steel ingots or continuous casting into billets, forging or rolling forming, and heat treatment.

5. The method for preparing ultra-high strength spring steel with a tensile strength of ≥2100 MPa for large-size springs of engineering machinery according to claim 4, characterized in that: The heat treatment process comprises: Firstly carry out oil quenching at 900±30℃, and then carry out tempering at 350±20℃.

6. The method for preparing ultra-high strength spring steel with a tensile strength of ≥2100 MPa for large-size springs of engineering machinery according to claim 4, characterized in that: The obtained ultra-high strength spring steel has an end hardenability J40 mm ≥ 55HRC, a tensile strength ≥ 2100 MPa, a yield strength ≥ 1800 MPa, an elongation after fracture ≥ 8%, a cross-sectional shrinkage ≥ 40%, and a room temperature V-type impact energy ≥ 15 J.

7. The method for preparing ultra-high strength spring steel with a tensile strength of ≥2100 MPa for large-size springs of engineering machinery according to claim 4, characterized in that: The diameter of the obtained ultra-high strength spring steel is 80-100 mm.

Images