CN114134431A

CN114134431A - 2000 Mpa-grade high-strength high-toughness high-hardenability spring steel prepared by continuous casting and rolling of square billet and manufacturing method thereof

Info

Publication number: CN114134431A
Application number: CN202110505106.7A
Authority: CN
Inventors: 谭亮; 罗元东; 白云; 吴小林; 葛龙桂; 李正中; 李伟
Original assignee: Jiangyin Xingcheng Special Steel Works Co Ltd
Current assignee: Jiangyin Xingcheng Special Steel Works Co Ltd
Priority date: 2021-05-10
Filing date: 2021-05-10
Publication date: 2022-03-04
Anticipated expiration: 2041-05-10
Also published as: CN114134431B

Abstract

The invention relates to a 2000 Mpa-grade high-strength high-toughness high-hardenability spring steel prepared by continuous casting and rolling of a billet, which takes Fe as a basic element and also comprises the following chemical components in percentage by mass: c: 0.38-0.45%, Si: 1.60-2.20%, Mn: 0.10-0.75%, P: less than or equal to 0.010 percent, S: less than or equal to 0.002%, Cr: 1.80-2.30%, Ni: 1.60-2.40%, Mo: 0.4-0.7%, Cu: 0.2-0.5%, Al: 0.01-0.04%, V: 0.10 to 0.30%, Nb: 0.010-0.030%, N: less than or equal to 0.006 percent, Ca: 0.0005 to 0.005, and Ca/S is not less than 1, and inevitable impurity elements. The diameter of the obtained steel bar is 40-70 mm, the steel bar is quenched and tempered after complete austenitizing and heat preservation, the Brinell hardness of the whole section of the obtained steel bar is HBW 540-HBW 570, the static tensile strength is larger than or equal to 2000Mpa, the yield strength is larger than or equal to 1750Mpa, the elongation A is larger than or equal to 8%, the reduction of area Z is larger than or equal to 35%, and the normal-temperature-to-summer specific impact energy is larger than or equal to 40J.

Description

2000 Mpa-grade high-strength high-toughness high-hardenability spring steel prepared by continuous casting and rolling of square billet and manufacturing method thereof

Technical Field

The invention belongs to the technical field of special steel, and particularly relates to high-strength, high-toughness and high-hardenability spring steel with the tensile strength of 2000Mpa and a corresponding manufacturing method. The method is suitable for manufacturing the elastic element with large section and high stress requirement.

Background

The development of spring steel to high strength is a trend shown in the production and application of spring steel at present, the expansion of parts to the high stress application field can be promoted, the weight reduction of the parts can be realized from the aspects of energy conservation and economy, and the use cost is reduced. Research on high strength of spring steel shows that the strength level of the traditional spring steel is difficult to meet the requirement of rapid development of modern industry, and particularly, the spring material for large and heavy vehicles requires high strength, high toughness and good hardenability so as to meet the application of large-section springs. At present, the strength of widely applied high-strength spring steel such as American standard SAE9254, European standard 54SiCrV6, Japanese UHS2000 and the like can reach about 2000Mpa, but the high-strength spring steel can only be applied to spring parts with smaller specifications due to the limit of hardenability, and the high-strength spring steel with the hardenability of more than 30mm has difficult core hardening and cannot be applied to spring elements with large specifications. At present, 45CrNiMoVA is used as a material with a large section and a high stress, the Russia is introduced into the material, the material is applied to military heavy vehicles in China, but the strength of the steel is difficult to exceed 1800Mpa on the premise that the toughness of the steel is ensured by medium-temperature tempering, the strength is improved in recent years, the processing technology is changed into low-temperature tempering, the strength of the steel reaches about 2000Mpa, but the toughness loss is obvious, and the stable and high fatigue life is difficult to obtain.

The invention with Chinese patent publication No. CN103409698A discloses a steel material for a torsion shaft, which is 40Si2Ni2CrMoV, and comprises the following alloy elements in percentage by weight: c: 0.35-0.5, Si: 1.5-2.3, Mn: less than or equal to 0.05, Cr: 0.5-1.5, Ni: 0.8-3.5, Mo: 0.4-0.8, Ca: 0.0005-0.008, P: less than or equal to 0.010, S: less than or equal to 0.005, 0.05-0.2V or 0.02-0.1 Nb or 0.05-0.15V and 0.02-0.06 Nb, and the balance of Fe and impurities. The yield strength of the steel manufactured by the method can reach 1600-2000Mpa, but the smelting manufacturing mode needs to adopt an electroslag remelting process, and the electroslag remelting process has high manufacturing cost and poor stability of different batches and cannot meet the requirement of mass production, so the use of the steel is limited.

Disclosure of Invention

The technical problem to be solved by the invention is to provide the spring steel with high strength, high toughness and high hardenability aiming at the prior art, the use of an elastic element with a section of which the diameter is more than or equal to 30mm can be realized, and the low-cost manufacturing mode of continuous casting and rolling is met.

The technical scheme adopted by the invention for solving the problems is as follows: the square billet continuous casting and rolling 2000 Mpa-grade high-strength high-toughness high-hardenability spring steel takes Fe as a basic element and also comprises the following chemical components in percentage by mass: c: 0.38-0.45%, Si: 1.60-2.20%, Mn: 0.10-0.75%, P: less than or equal to 0.010 percent, S: less than or equal to 0.002%, Cr: 1.80-2.30%, Ni: 1.60-2.40%, Mo: 0.4-0.7%, Cu: 0.2-0.5%, Al: 0.01-0.04%, V: 0.10 to 0.30%, Nb: 0.010-0.030%, N: less than or equal to 0.006 percent, Ca: 0.0005 to 0.005, and Ca/S is not less than 1, and unavoidable impurity elements; the carbon equivalent CEV (C + Mn/6+ (Cr + Mo + V)/5+ (Cu + Ni)/15) should be 1.15-1.35%.

The function and amount of the components contained in the present invention are specifically described below:

1) determination of C content

C is the most basic element in steel and the most economic strengthening element, the strength of the steel is improved by solid solution strengthening and precipitation strengthening of carbide forming elements, but the higher the carbon content of C is, the higher the hardness of the steel is, the adverse effect is brought to the plasticity and the toughness of the steel, and meanwhile, the higher the C content is, the welding performance of the steel is reduced, the atmospheric corrosion resistance of the steel is reduced, and the cold brittleness and the aging sensitivity of the steel are increased. The content range of the C in the item is determined to be 0.35-0.50%, and the C belongs to the category of medium carbon steel.

2) Determination of the Si content

Si is a basic solid-solution strengthening element as a deoxidizing element and improves hardenability. Silicon can obviously improve the elastic limit, yield limit and yield ratio of steel, and many spring steels take silicon as a main alloy element, the silicon has a solid solution strengthening effect, does not form carbide, basically exists in the steel in a solid solution state, and the solid solution strengthening effect of the silicon is strongest in common alloy elements. Silicon can change the quantity, size and form of precipitated carbide during tempering and improve the tempering stability of the steel. When the alloying elements and carbon content are within certain ranges, the contribution of silicon to sag resistance is the first of the various alloying elements that carbon acts on. In the range of 0.30 to 2.30% silicon, the effect of silicon on the increase in the anti-ballistic properties increases with increasing silicon content. However, if the Si content is too high, the tendency of decarburization and graphitization of the steel during rolling and heat treatment is promoted. The Si content of this item is determined to be in the range of 1.50 to 2.50%.

3) Determination of Mn content

Mn is a solid solution strengthening element of steel, can obviously improve the hardenability of the steel, improves the heat treatment performance of the steel, strengthens the matrix of the steel and refines pearlite, thereby improving the strength and the hardness of the steel, and the manganese alloy is cheap and easy to obtain, and is a preferred element in the design of high-strength grade steel. However, the steel contains excessive Mn, so that the phenomenon of relatively obvious temper brittleness can be generated, and Mn promotes the growth of crystal grains, so that the overheating sensitivity and the crack tendency of the steel are enhanced. The Mn content range of the project is determined to be 0.20-0.80%.

4) Determination of the Cr content

Cr is a medium carbide former, and among all the carbides, chromium carbide is the finest one, which is uniformly distributed in the steel volume, and therefore has high strength, hardness, yield point and high wear resistance. Cr slows the decomposition rate of austenite, reduces the critical cooling rate during quenching, and thus contributes to the formation of M-bodies and the improvement of the stability of the M-bodies, so that Cr steel has excellent hardenability and less quenching deformation. The influence of Cr on the strength and toughness of steel is that the strengthening effect is most obvious when Cr is gradually enhanced to 2% when the Cr is less than 2%. But exceeds this limit. However, if the Cr content is too high, the brittle transition temperature of the steel is increased, and the hot strength of the steel is deteriorated, thereby promoting the temper brittleness of the steel. The Cr content range of the project is determined to be 1.20-2.50%.

5) Determination of Ni content

Ni can improve the strength of steel, and maintain good plasticity and toughness, and the steel containing Ni is not easy to overheat, so that it can prevent the growth of crystal grains at high temperature, and can still maintain fine grain structure. Since the lattice constant of Ni is close to that of iron, it can be a continuous solid solution. This is beneficial to improving the hardenability of steel, and Ni can reduce the critical point and increase the stability of austenite, so the quenching temperature can be reduced, and the hardenability is good. Meanwhile, the welding performance and the low-temperature performance of the steel added with Cr and Ni are good, and the other important function is to inhibit the initiation and the expansion of corrosion pits in a corrosion environment, so that the steel has higher fatigue limit. Therefore, the Cr content of the steel grade of the project is determined to be 0.80-2.50%.

6) Determination of Mo content

Mo is a carbide forming element, can improve hardenability and refine crystal grains by combining with Cr, and can greatly reduce the temper brittleness and improve the toughness of the heat-treated alloy structural steel. Certain content of Mo can form fine dispersed carbide in steel to prevent dislocation movement and improve the elasticity-reducing resistance of the steel. The Mo content of the steel of the item is 0.30-0.60%.

7) Determination of the V content

V and O, N have great affinity and are strong carbide elements, fine and dispersed MC type carbide separated out in a solid state has strong precipitation strengthening effect, and the strength and the hardness of the steel are improved, and the anti-elasticity performance of the steel is also improved. The common VC has high dispersivity and high stability, so that it can utilize deoxidation and deaeration to obtain compact fine crystal structure, raise plasticity, toughness and high strength, and its impact property and fatigue strength are higher than those of V-free steel, and its strength and toughness are high at high temp. and low temp. (less than 0 deg.C). Meanwhile, the high dispersion of vanadium carbide prevents coarse grains of a welding seam, so that the weldability of the steel can be improved. The range of the V content in the steel grade of the project is 0.05-0.15%.

8) S, P determination of content

S, P are inevitable impurity elements in steel, and are prone to form defects such as segregation and inclusion. P dissolves in ferrite to coarsen grains and increase cold brittleness. S causes hot shortness of the steel, reducing ductility and toughness of the steel. The project is to ensure the high strength and toughness performance of the steel, and the P is controlled to be less than or equal to 0.015 percent and the S is controlled to be less than or equal to 0.008 percent.

9) Determination of Al content

Al is added as a deoxidizing element in steel, except for reducing dissolved oxygen in molten steel, Al and N form dispersed and fine aluminum nitride particles to refine grains, but the content of Al is high, brittle inclusions such as Al2O3 and the like are easily formed in the molten steel smelting process, and the purity of the molten steel is reduced, so that the content of Al in the project is determined to be less than or equal to 0.03%.

10) Determination of the Nb content

Is an element which plays a significant role in grain refinement in the rolling process. In the recrystallization rolling stage, Nb is precipitated through strain induction to hinder the recovery and recrystallization of deformed austenite so as to refine grains, thus providing a foundation for the fine structure of the steel after quenching and tempering treatment and being beneficial to improving the toughness of the steel. However, too high Nb cannot be dissolved by the limitation of the C content and the influence of the heating temperature, and does not work and increases the cost. In addition, too high a content of Nb has an adverse effect on the welding performance. The content of the invention is controlled to be less than or equal to 0.03 percent.

11) Determination of the Cu content

Cu can improve the hardenability and atmospheric corrosion resistance of steel and reduce the hydrogen induced crack sensitivity of steel. However, too high Cu content is disadvantageous in weldability of steel, and also tends to cause copper embrittlement, which deteriorates surface properties of steel. The Cu content is controlled to be 0.05-0.30%.

12) Determination of the Ti content

The low-carbon Ti steel is usually mixed with non-metal due to high viscosity of molten steel, is not easy to separate and float out, and meanwhile, the titanium has high affinity with N, O, so that TiN and TiO2 are easy to form, and the defects of more non-metal impurities, subcutaneous porosity and the like are formed at a lower temperature of a steel billet. The invention controls the Ti content to be less than or equal to 0.06 percent.

13) Determination of As, Sn, Sb, Pb content

As, Sn, Sb, Pb and other trace elements belong to low-melting-point nonferrous metals, and the existence of the elements in steel can cause soft spots on the surface of parts and uneven hardness, so the elements are regarded As harmful elements in the steel, and the content ranges of the elements are determined to be less than or equal to 0.04 percent of As, less than or equal to 0.03 percent of Sn, less than or equal to 0.005 percent of Sb and less than or equal to 0.002 percent of Pb.

14) Determination of Ca content and Ca-S ratio

Ca has obvious effect on the deterioration of inclusions in steel, so that the inclusions are spheroidized and uniformly distributed, the adverse effect on toughness is reduced, and meanwhile, the fluidity of molten steel is improved, and the problem of nozzle blockage is solved. In the invention, the Ca content is controlled to be 0.0005-0.005, and the Ca/S is more than or equal to 1.

15) Determination of carbon equivalent

Carbon equivalent is an important index for evaluating hardenability of steel. The low CEV is beneficial to the welding performance of steel but not beneficial to forming a high-strength martensite structure during quenching, and the high CEV is beneficial to forming the martensite structure, so that the hardenability of the steel bar is improved. In order to ensure the excellent hardenability of the steel and coordinate the addition of each alloy element on the premise of designing the medium carbon of the steel, the CEV is controlled to be 1.15-1.35%. The carbon equivalent calculation formula is CEV (═ C + Mn/6+ (Cr + Mo + V)/5+ (Cu + Ni)/15).

The manufacture of the high-strength high-toughness high-hardenability steel bar of the 2000Mpa grade by the continuous casting billet is realized by the following steps:

preparing smelting raw materials according to the chemical composition of the steel, and sequentially carrying out KR molten iron pretreatment, converter smelting, LF refining, RH refining and continuous casting to produce high-purity molten steel and molten steel with a section area of more than or equal to 10 multiplied by 10⁴mm²The continuous casting billet has low center segregation and porosity, no cracks and high purity (the center segregation is equal to or better than 1.0 grade, the center porosity is equal to or better than 1.0 grade, no center cracks, corner cracks and triangular region cracks, the inclusions are A, B, D coarse and fine systems which are less than or equal to 1.0, and the C coarse and fine systems are less than or equal to 0.5). The content of H in molten steel after RH treatment is less than or equal to 0.0001%.

Annealing the continuous casting billet after the continuous casting is finished so as to reduce the structural stress and prevent stress cracking; and simultaneously reducing the H content in the steel billet so as to avoid hydrogen-induced cracking of the steel bar, wherein the annealing temperature is more than or equal to 650 ℃, the heat preservation time is more than or equal to 36 hours, and then the steel bar is cooled to below 300 ℃ along with the furnace and taken out of the furnace for air cooling.

And heating the treated continuous casting slab to 1150-1300 ℃, and keeping the temperature for more than or equal to 3 hours to fully dissolve alloy elements in the steel, so as to play a strengthening and toughening role and ensure the components and the performance of a final product. After the heat preservation is finished, high-pressure water descaling treatment is carried out, then rolling is carried out, the initial rolling temperature is 1050 ℃ and 1150 ℃, the final rolling temperature is 930 +/-40 ℃, and the rolling is carried out until the target size is reached. The rough rolling in the rolling process adopts large rolling reduction to ensure that the deformation permeates into the core of a rolled piece, the rolling reduction of the first two rough rolling is more than or equal to 50 percent, and the rolling reduction of the second two rough rolling is more than or equal to 30 percent. The total compression ratio of the rolled blank is more than or equal to 35 so as to ensure the comprehensive mechanical property of the rolled blank. Avoiding rapid cooling after rolling, cutting to length and then annealing again, wherein annealing needs to be carried out within 4 hours after rolling, the annealing temperature is 650-750 ℃, and the time is as follows: the steel bar is cooled to below 300 ℃ in a steel bar furnace and then taken out of the furnace for air cooling to room temperature, and the structure of the steel bar is converted into ferrite and uniformly distributed fine carbide particles in the state, so that the hardness of the rolled steel bar can be reduced to below 280HBW, and the steel bar is beneficial to subsequent use and processing.

In order to obtain good mechanical properties of steel and meet the requirements of large-size spring components for equipment such as large vehicles, rail transit, engineering machinery and the like, the material needs to be quenched and tempered, the final metallographic structure is tempered troostite (quenched at 920-940 ℃ and tempered at 380-450 ℃) or tempered martensite (quenched at 920-940 ℃ and tempered at 200-270 ℃) and the static mechanical properties of the material can reach the tensile strength of more than or equal to 2000MPa, the yield strength of more than or equal to 1750MPa, the elongation A of more than or equal to 8 percent and the reduction of area Z of more than or equal to 35 percent under the conditions of the two heat treatment processes and the corresponding metallographic structure. In actual use, a medium-temperature tempering process can be selectively adopted according to the design and performance requirements of parts so that the workpiece has higher toughness, or a low-temperature tempering process is selected so that the workpiece has higher strength.

Compared with the prior art, the invention has the advantages that:

the spring steel bar manufactured by the invention has the characteristics of high strength, high toughness and high hardenability, and meets the use requirements of large-section and high-strength parts made of steel. The steel bar is manufactured by hot rolling of a continuous casting billet, and the application diameter of the steel bar can reach 40-70 mm.

The spring steel bar manufactured by the invention is manufactured by using the continuous casting billet as the blank, simplifies the production process compared with the manufacturing method by using a die-cast steel ingot or an electroslag ingot as the blank, simultaneously improves the yield of the steel bar, reduces the overall manufacturing cost of the steel bar, and has obvious production advantages in industrial production.

Drawings

FIG. 1 is a CCT curve relating to a steel rod in an embodiment of the present invention; (ii) a

FIG. 2 is a graph relating to the quenching hardness of a steel bar as a function of the quenching temperature in an example of the present invention;

FIG. 3 is a photograph of the annealed microstructure of a steel bar according to an embodiment of the present invention;

FIG. 4 is a photograph (1000 times enlarged) of a microstructure of a steel bar in a 930 ℃ quenched state and a 400 ℃ tempered state according to example 1 of the present invention;

FIG. 5 is a photograph (magnified 1000 times) of a microstructure of a steel bar in a 930 ℃ quenched state and a 260 ℃ tempered state according to example 2 of the present invention.

Detailed Description

The present invention will be described in further detail below with reference to the accompanying examples and comparative examples.

Example 1

The embodiment relates to a high-strength high-toughness high-hardenability alloy bar with the diameter of phi 50mm, which comprises the following components in percentage by mass: c: 0.40%, Si: 1.98%, Mn: 0.25%, P: 0.007%, S: 0.002%, Cr: 1.96%, Mo: 0.50%, Ni: 1.95%, Cu: 0.15%, V: 0.30%, Nb: 0.02%, N: 0.0054%, Al: 0.020%, Ca: 0.0016 percent, the balance of iron and inevitable impurity elements, and the H content in the molten steel after RH treatment is 0.00007 percent.

The production process of the high-strength high-toughness high-hardenability alloy bar comprises the following steps:

preparing smelting raw materials according to the chemical composition, and sequentially carrying out KR molten iron pretreatment, electric furnace smelting, LF refining, VD refining, continuous casting (continuous casting billet section: 300 x 340mm), continuous casting billet annealing, continuous casting billet cleaning, heating (heat preservation treatment), high-pressure water descaling, controlled rolling, cutting, rolled material annealing and straightening.

Further, the specific process of the heating, controlled rolling and cooling stages comprises the following steps: annealing the produced continuous casting billet (center segregation: C class 0.5 grade, center porosity: 0.5 grade, no center crack, corner crack and triangular region crack, inclusion: A, B, C class coarse system is 0, D class coarse system is 0.5, A, B class fine system is 0.5, D class fine system is 1.0, C class fine system and Ds class 0), keeping the annealing temperature at 750 ℃, keeping the temperature for 36 hours, furnace-cooling to below 300 ℃, discharging and air-cooling.

Heating the continuously cast bloom processed by the steps to 1270 ℃, preserving heat for 4.5 hours, discharging from a furnace for rolling after heat preservation is finished, and carrying out high-pressure water descaling treatment before rolling, wherein the start rolling temperature is 1120 ℃, the finish rolling temperature is 945 ℃, and the rolling is carried out to phi 50 mm. The rough rolling in the rolling process adopts large rolling reduction, the rolling reduction of the first two rough rolling is 50-60%, and the rolling reduction of the second two rough rolling is 30-40%. Rolling the blank until the total material compression ratio is 51, carrying out annealing treatment within 3 hours after rolling, sizing and cutting, keeping the annealing temperature at 750 ℃, keeping the temperature for 36 hours, cooling to below 300 ℃ along with the furnace, discharging and air cooling.

The bar obtained by the process is sampled to prepare a standard sample, and the mechanical property is detected: oil quenching +400 ℃ according to a quenching temperature of 930 ℃ (the temperature reaching and holding time is 35min), and the measured mechanical properties are as follows:

TABLE 1

Mechanical property sampling in the example mechanical property samples were taken from rolled round steel products according to the requirements of the ISO 377 standard, and used according to the requirements of the ISO 6892 standard

Heat treating the sample made of the blank, and measuring the longitudinal mechanical property of the steel;

charpy impact performance test in this example used the impact strength values of standard U-notch impact test specimens of 10mm x 55 mm.

Example 2

This example relates to a high strength, high toughness and high hardenability alloy bar having a diameter of phi 62mm, comprising the same components and mass percentages and carbon equivalent CEV as in example 1. The manufacturing process of this example is essentially the same as example 1, with the main difference being the rolling and subsequent heat treatment, as follows:

preparing smelting raw materials according to the chemical composition, and sequentially performing KR molten iron pretreatment, converter smelting, LF refining, RH refining, continuous casting (continuous casting billet section: 390 x 510mm), continuous casting billet annealing, continuous casting billet cleaning, heating (heat preservation treatment), high-pressure water descaling, cogging and rolling (intermediate billet section: 200 x 200mm), intermediate billet annealing, intermediate billet cleaning, heating (heat preservation treatment), high-pressure water descaling, controlled rolling, cutting, rolled material annealing and straightening.

Heating the continuously cast slab subjected to the treatment to 1270 ℃, preserving heat for 6 hours, discharging the continuously cast slab out of a furnace after heat preservation is finished, performing high-pressure water descaling treatment before rolling, wherein the rolling temperature is 1135 ℃, the final rolling temperature is 955 ℃, and the continuously cast slab is rolled to the middle specification of 200 x 200 mm. And (3) adopting a large reduction process in the cogging rolling process, penetrating deformation into the core of a casting blank, carrying out annealing treatment within 3 hours after rolling, sizing and cutting, keeping the annealing temperature at 750 ℃, keeping the temperature for 36 hours, cooling to below 300 ℃ along with a furnace, discharging and air cooling.

Heating the annealed 200-square intermediate billet to 1220 ℃, preserving heat for 2 hours, discharging the intermediate billet out of a furnace after heat preservation is finished, performing cogging rolling, and performing high-pressure water descaling treatment before rolling, wherein the cogging temperature is 1085 ℃, the finish rolling temperature is 910 ℃, and the intermediate billet is rolled to phi 62 mm. The rough rolling in the rolling process adopts large rolling reduction, the rolling reduction of the first two rough rolling is 50-60%, and the rolling reduction of the second two rough rolling is 30-40%. The total compression ratio of the rolled blank is 65, annealing treatment is carried out within 3 hours after rolling, sizing and cutting, the annealing temperature is 750 ℃, the temperature is kept for 36 hours, and the blank is taken out of the furnace and cooled to below 300 ℃ along with the furnace.

The bar obtained by the process is sampled to prepare a standard sample, and the mechanical property is detected: according to the quenching 930 ℃ (the temperature reaching and holding time is 35min), oil quenching +260 ℃ (the temperature reaching and holding time is 150min), and the measured mechanical properties are as follows:

TABLE 3

In addition to the above embodiments, the present invention also includes other embodiments, and any technical solutions formed by equivalent transformation or equivalent replacement should fall within the scope of the claims of the present invention.

Claims

1. The high-strength high-toughness high-hardenability spring steel of 2000Mpa grade is characterized in that the spring steel takes Fe as a basic element and comprises the following chemical components in percentage by mass: c: 0.38-0.45%, Si: 1.60-2.20%, Mn: 0.10-0.75%, P: less than or equal to 0.010 percent, S: less than or equal to 0.002%, Cr: 1.80-2.30%, Ni: 1.60-2.40%, Mo: 0.4-0.7%, Cu: 0.2-0.5%, Al: 0.01-0.04%, V: 0.10 to 0.30%, Nb: 0.010-0.030%, N: less than or equal to 0.006 percent, Ca: 0.0005 to 0.005, and Ca/S is not less than 1, and unavoidable impurity elements; the carbon equivalent CEV (C + Mn/6+ (Cr + Mo + V)/5+ (Cu + Ni)/15) value is 1.15-1.35%.

2. The square billet continuous casting and rolling 2000 Mpa-grade high-strength high-toughness high-hardenability spring steel according to claim 1, which is characterized in that the diameter of the spring steel is 40-70 mm, and the internal quality meets the AAA-grade flaw detection requirement of GB/T4162; and quenching and tempering the bar after complete austenitizing and heat preservation, wherein the metallographic structure is tempered lath martensite or tempered troostite structure with retained martensite orientation from the surface to the center, and the Brinell hardness of the obtained steel bar on the whole section is HBW 540-HBW 570.

3. The square billet continuous casting and rolling 2000 Mpa-grade high-strength high-toughness high-hardenability spring steel as claimed in claim 2, wherein the static tensile strength of the spring steel is greater than or equal to 2000Mpa, the yield strength is greater than or equal to 1750Mpa, the elongation A is greater than or equal to 8%, and the reduction of area Z is greater than or equal to 35% under the metallographic structure condition.

4. The method for producing a high-strength, high-toughness, high-hardenability spring steel of 2000Mpa grade according to claim 1, comprising the steps of: the method comprises the steps of preparing smelting raw materials according to chemical compositions, sequentially carrying out KR molten iron pretreatment, electric furnace smelting, LF refining, VD refining, continuous casting billet annealing, continuous casting billet cleaning, heating, high-pressure water descaling, controlled rolling, cutting, rolled material annealing, heat treatment and straightening.

5. The method for manufacturing the high-strength, high-toughness and high-hardenability spring steel of the square billet continuous casting and rolling 2000Mpa level according to claim 4, wherein the method comprises the following steps: the raw materials are smelted and continuously cast to obtain high-purity molten steel and the section area of the molten steel is more than or equal to 10 multiplied by 10⁴mm²The continuous casting billet with low center segregation and porosity, no crack and high purity, the center segregation of the continuous casting billet: equal to or better than grade 1.0, central porosity: equal to or better than grade 1.0, no central cracks, no corner cracks and no triangular zone cracks; inclusion: A. the B, D class coarse and fine system is less than or equal to 1.0, the C class coarse and fine system is less than or equal to 0.5, and the H content in the molten steel after RH treatment is less than or equal to 0.0001%.

6. The method for manufacturing the high-strength, high-toughness and high-hardenability spring steel of the square billet continuous casting and rolling 2000Mpa level according to claim 4, wherein the method comprises the following steps: and the annealing temperature of the continuous casting billet is more than or equal to 650 ℃, the time is more than or equal to 36 hours, and the continuous casting billet is cooled to below 300 ℃ and discharged for air cooling.

7. The method for manufacturing the high-strength, high-toughness and high-hardenability spring steel of the square billet continuous casting and rolling 2000Mpa level according to claim 4, wherein the method comprises the following steps: the heating temperature of the continuous casting billet is 1150-1300 ℃, and the heat preservation time is more than or equal to 3 hours.

8. The method for manufacturing the high-strength, high-toughness and high-hardenability spring steel of the square billet continuous casting and rolling 2000Mpa level according to claim 4, wherein the method comprises the following steps: the initial rolling temperature of the continuous casting billet during rolling is 1050-: cooling to below 300 deg.c for more than 36 hr, and air cooling to room temperature.

9. The method for manufacturing the high-strength, high-toughness and high-hardenability spring steel of the square billet continuous casting and rolling 2000Mpa level according to claim 4, wherein the method comprises the following steps: the heat treatment comprises quenching and tempering treatment, the metallographic structure is finally obtained through quenching at 920-940 ℃ and tempering at 380-450 ℃, or the metallographic structure is finally obtained through quenching at 920-940 ℃ and low-temperature tempering at 200-270 ℃, and is tempered martensite.