CN119464925A - A kind of ultra-high strength spring steel for automobile hydraulic torque converter and its manufacturing method - Google Patents

Abstract

The invention relates to spring steel for an ultrahigh-strength automobile hydraulic torque converter, which comprises the following chemical elements in percentage by mass ：C:0.55-0.70%、Si:1.60-2.20%、Mn:0.30-0.80%、Cr:0.60-1.00%、V:0.10-0.30%、Mo：0.05-0.30%、P:≤0.020%、S:≤0.020%、Al:≤0.0010%、Ti:≤0.0010%、Cu:≤0.10%、Ni:≤0.30%、Nb:≤0.15%、H:≤0.00005%、N:≤0.008%、O:0.0007%-0.0025%; and the balance of Fe and unavoidable impurities. The spring steel for the ultrahigh-strength automobile hydraulic torque converter has the tensile strength of 2000-2200MPa, the steel grade has good tempering resistance and stability while meeting good fatigue performance and impact resistance, after being coiled into a spring, the heat-treated steel wire can meet the requirement that the hardness of the steel wire is reduced by less than or equal to 1.0HRC after being nitrided for 5-8 hours at 400-500 ℃, the ultrahigh-stress design requirement of the spring steel for the automobile hydraulic torque converter can be met, the shape of the steel wire for the spring is not limited to a circle, and the steel wire for the spring can be manufactured into oval, elliptic and other abnormal shapes.

Description

Spring steel for ultrahigh-strength automobile hydraulic torque converter and manufacturing method thereof

Technical Field

The invention belongs to the technical field of special steel smelting, and particularly relates to spring steel for an ultrahigh-strength automobile hydraulic torque converter and a manufacturing method thereof.

Background

The hydraulic torque converter is an important part in a power transmission system of a fuel automobile and a power transmission system of a new energy automobile, and is always a soft rib developed by the independent brand of the automobile in China, after the hydraulic torque converter of the automobile uses a spring, the impact caused by high rotating speed of the engine can be avoided or weakened, the power transmission of the engine is smooth, the starting stability, the acceleration speed or the softness of the automobile can be obviously promoted, the service life of the power transmission system of the automobile can be greatly prolonged, and the driving safety is improved.

In order to meet the requirements of high impact and high fatigue performance of an automobile hydraulic torque converter, the sectional area of a spring steel wire is equal to 7-20 square millimeters, the tensile strength is 2000-2200MPa, the area shrinkage is more than or equal to 40%, the fatigue life is more than or equal to 600 ten thousand times, the surface of the spring steel wire is subjected to surface nitriding treatment, and the hardness of the spring steel wire is reduced by less than or equal to 1.0HRC after the spring is nitrided for 5-8 hours at 400-500 ℃.

The spring steel material commonly used at present cannot meet the requirements of a spring for a hydraulic torque converter, so the invention aims to provide the spring steel for the hydraulic torque converter of the automobile, which has good tempering resistance stability, has the tensile strength of a steel wire reaching 2000-2200MPa and meets the high impact and high fatigue performance of the spring, and the manufacturing method thereof.

Disclosure of Invention

The invention aims to solve the technical problems of providing spring steel for an automobile hydraulic torque converter and a manufacturing method thereof aiming at the prior art, the steel has good tempering resistance and stability while meeting good fatigue performance, after being coiled into a spring, a heat-treated steel wire can meet the requirement that the hardness of the steel wire is reduced by less than or equal to 1.0HRC after nitriding for 5-8 hours at the temperature of 400-500 ℃, the ultrahigh stress design requirement of the spring steel for the automobile hydraulic torque converter can be met, the shape of the steel wire for the spring is not limited to a round shape, and the steel wire for the spring can be manufactured into oval shapes, elliptic shapes and the like.

The technical scheme adopted by the invention for solving the problems is that the spring steel for the ultrahigh-strength automobile hydraulic torque converter comprises the following chemical components in percentage by mass of ：C：0.55-0.70%,Si：1.60-2.20%,Mn：0.30-0.80%, Cr：0.60-1.00%, V:0.10-0.30%,Mo：0.05-0.30%,Ni：≤0.25%,Al：≤0.0008%,Nb:≤0.04%,P:≤0.020%,S:≤0.020%,Cu:≤0.10%, N:≤0.008%,Ti≤0.0012%,O：0.0007%-0.0020%,S:≤0.020%,H:≤0.00005%, and the balance of Fe and unavoidable impurities.

The design principle of each chemical element of the spring steel for the automobile hydraulic torque converter is as follows:

c is the most main strengthening element in spring steel for hydraulic torque converter, and has two functions, namely, on one hand, the action of dissolving in steel to form solid solution and form clearance solid solution strengthening, and on the other hand, the action of combining with strong carbide forming element to form carbide and form precipitation strengthening. When the C content is less than 0.55%, the strength requirement of the spring steel cannot be met, and the second phase carbon nitrogen compound formed by the micro alloy element cannot be met, but when the C content is more than 0.70%, massive carbide is formed in the spring steel, and the plasticity and toughness of the steel are reduced. Therefore, the content of C element in the spring steel for the automobile hydraulic torque converter is 0.55-0.70%.

Si has strong solid solution strengthening effect, can obviously improve the elastic limit, yield limit and yield ratio of steel, and ensures that the spring has higher strength and better elastic attenuation resistance, thereby ensuring that the spring has higher strength and better elastic attenuation resistance. Si is also a deoxidizer and a reducing agent in the steelmaking process, so that the oxygen content in steel can be reduced, and the fatigue life of the spring is prolonged. Therefore, the Si element content of the spring steel for the automobile torque converter is 1.30-2.00%.

Mn is dissolved in the matrix to have solid solution strengthening effect, so that the strength and the elastic reduction resistance of the matrix can be effectively improved. However, manganese has the effect of promoting grain growth, so that manganese is sensitive to overheating, and therefore, mn of the spring steel for the automotive torque converter cannot be excessively high, and therefore, the Mn element content of the spring steel for the automotive torque converter is 0.30-0.80%.

Cr can improve the tensile strength, hardness, yield strength and resistance to elastic decay of spring steel. Since Cr is a weak carbide-forming element, the addition of Cr appropriately can uniformly distribute carbide in the steel matrix, and large amounts of Cr form large blocks of carbide when added. Therefore, the Cr element content of the spring steel for the automobile torque converter is 0.30-0.80%.

The V element is a strong carbide forming element, can form a precipitation strengthening effect, remarkably improves the strength of steel without reducing plasticity, can effectively organize crystal grain growth, plays roles in refining the crystal grain and improving the toughness, and can also improve the elastic attenuation resistance and the hydrogen embrittlement resistance sensitivity of the spring. Therefore, the V element content of the spring steel for the automobile hydraulic torque converter is 0.10-0.30%.

Mo, molybdenum can raise tensile strength and elastic limit of spring steel, raise martensitic tempering stability, reduce hardness reducing range of spring when it is infiltrated with N, and in addition, it can greatly raise hardenability, refine crystal grain, raise toughness, reduce tempering brittleness, raise strength and make plasticity not reduce. Therefore, the Mo element content of the spring steel for the automobile hydraulic torque converter is 0.05-0.30%.

Al is a strong deoxidizing element and has a certain grain refining effect, but the formed brittle Al ₂O₃ inclusion has a fatal influence on the fatigue life of spring steel, so that the Al element content of the spring steel for the automobile hydraulic torque converter is less than or equal to 0.0010 percent.

Ti is also a strong deoxidizing element and has good grain refining effect, but forms TiN inclusions with higher hardness with N, and has larger influence on the fatigue life of spring steel, so the Ti element content of the spring steel for the automobile hydraulic torque converter is less than or equal to 0.0010 percent.

Cu element is easy to gather at the grain boundary, is extremely easy to generate thermal embrittlement and affects the toughness of the spring steel for the automobile hydraulic torque converter, so the Cu element content of the spring steel for the automobile hydraulic torque converter is less than or equal to 0.10 percent.

Nb is a carbon nitride forming element, can improve the nucleation rate of carbon nitride precipitation in the tempering process, prevents the growth of crystal grains to form the effect of refining the crystal grains, and can improve the strength of the steel wire. Therefore, the Nb element content of the spring steel for the automobile torque converter is less than or equal to 0.15 percent.

N is a nitrogen compound forming element, and carbon nitrogen compounds formed by combination with V, nb improve the matrix strength and refine the structure, but too high N content will produce coarse carbon nitrogen compounds, so the N element content of the spring steel for the automobile torque converter in the invention is less than or equal to 0.008 percent.

The too high H content can cause white spots, punctiform segregation and hydrogen-induced delayed fracture, so that the H element content of the spring steel for the automobile hydraulic torque converter is less than or equal to 0.00005 percent.

The O content represents the number of inclusions in steel, and the reduction of the oxygen content in the steel is beneficial to improving the fatigue life of spring steel, so that the O element content of the spring steel for the automobile hydraulic torque converter is 0.0007-0.0025%.

The spring steel for the automobile hydraulic torque converter has a matrix structure of tempered troostite and sorbite, wherein the matrix structure contains a second phase of vanadium, niobium and molybdenum carbon nitrogen compounds, and the size of the second phase is 8-80nm.

The austenitic grain size of the spring steel for the automobile hydraulic torque converter reaches 11-15 levels.

The strength of the spring steel for the automobile hydraulic torque converter is 2000-2200MPa, the area shrinkage is more than or equal to 40%, and the requirement of the spring steel for the automobile hydraulic torque converter on high strength and high toughness can be met.

The spring steel for the automobile hydraulic torque converter has the fatigue life of the spring of more than or equal to 600 ten thousand times, and can meet the requirement of the spring steel for the automobile hydraulic torque converter on the ultra-high fatigue life.

The application further aims to provide a manufacturing method of the spring steel for the ultrahigh-strength automobile hydraulic torque converter, which mainly comprises the following steps:

(1) S in molten iron is removed by adopting molten iron pretreatment, the S content after molten iron pretreatment is less than or equal to 0.005%, the S content in finished steel is less than or equal to 0.005%, the S content is unfavorable for the fatigue life of spring steel for an automobile hydraulic torque converter, and MnS inclusions are distributed along the rolling direction of a steel wire and are easy to become a fatigue source.

(2) The converter or the electric furnace is adopted for rough smelting, the S content of molten iron entering the rough smelting furnace is less than or equal to 0.005%, self-produced scrap steel in enterprises is selected, the introduction of harmful elements is reduced, particularly five-harmful elements As, sb, sn, pb, bi are reduced, the content of the five-harmful elements in finished steel is less than or equal to 0.0030%, the five-harmful elements influence the plasticity and tempering stability of the steel, the oxygen content in the steel is controlled by the rough smelting furnace, the content of end point C is improved, the P content in the steel during tapping is less than or equal to 0.012%, the S content is less than or equal to 0.005%, high-grade alloy is added during tapping, and the introduction of harmful elements such as P, S, ti, al is reduced.

(3) The coarse molten steel enters a ladle refining furnace, the alkalinity of refining slag of the ladle refining furnace is less than or equal to 1.4 (the mass percent of CaO content and SiO ₂ content in the refining slag), brittle inclusions in the steel can be removed by using weak alkaline slag, ds inclusions which are fatal to the fatigue life are reduced, the inclusions left in the steel have the capability of deforming during rolling, namely plastic inclusions are formed, because the plastic inclusions can be plastically deformed during rolling and drawing, the size of the inclusions is reduced, the purpose of innocuous inclusion is achieved, the smaller size of the inclusions is more beneficial to improving the fatigue life of a spring, the harmful gas N, H, O in the steel is removed by using a vacuum treatment RH or VD furnace, the N content in the steel is less than or equal to 0.0050%, the O content is less than or equal to 0.0017%, the H content is less than or equal to 0.0001%, the N content is high, the plasticity of the spring steel is reduced, the damage of the H to the ultrahigh-strength steel is fatal, the hydrogen-induced delay fracture is more easily generated, the inclusions in the steel are more, and the fatigue life of the spring is unfavorable.

(4) And the qualified molten steel is refined, the inclusion is ensured to float upwards sufficiently, the temperature of the molten steel is suitable for casting, the casting is protected during casting, the contact with air is reduced, and the molten steel is less in nitrogen absorption and oxygen absorption to influence the quality of the molten steel. The degree of superheat of molten steel entering the tundish is less than or equal to 25 ℃, and the lower the degree of superheat is, the better the lower the degree of superheat is, so that the control of the uniformity of the casting blank tissue is facilitated, and the harm caused by segregation is reduced. When the solid phase ratio of continuous casting is 0.63-0.75, the end soft reduction is used, the reduction is 2.1-3.6% of the thickness of the casting blank, the center segregation of the casting blank is less than or equal to 1.0 level, the center porosity is less than or equal to 1.0 level, the center carbon segregation index is less than or equal to 1.3, the deviation of the tensile strength of different steel wires is less than or equal to 50Mpa, and the cross section size of the steel blank cast by a continuous casting machine is 200-500mm in length and 200-550mm in width.

(5) The qualified continuous casting blank enters a continuous heating furnace, the temperature of the casting blank is controlled to be more than or equal to 400 ℃, the heating temperature 1120-1230 ℃ is determined according to the calculation summary of solid solution calculation of the alloy element of the steel grade and the second phase, the total heating time is controlled during heating, and the heating time is different according to different blank types, but the total heating time must ensure that the microalloy element carbon nitride compound is fully fused into austenite, namely the total heating time is 2.5min/mm-3.5min/mm (mm is the cross section size of the casting blank). After the casting blank is fully heated, the rolling deformation can be completely permeated into the core part, the defects of loosening, shrinkage cavity, microcrack and the like of the core part are welded, the center segregation and the tissue difference are reduced, the cross section of the rolling intermediate blank is 140mm-220mm long, 140mm-220mm wide, and the diameter of the round blank is 140mm-220mm.

(6) Defects such as pits, folds, cracks, scratches and the like which affect the surface quality of the steel wire exist on the surface of the intermediate billet after casting blank rolling, firstly, magnetic powder or infrared flaw detection is used for determining the positions of the defects, marking is carried out, and the defects at the marked positions are polished and cleaned. The intermediate blank without surface defects is used for completely removing the surface layer within 1-2mm near the surface of the intermediate blank by using equipment such as a grinding wheel and the like, and the decarburized layer on the surface is removed. Because process (7) further deepens the decarburization of the intermediate billet, the surface decarburization of the finished wire rod increases, and the surface hardness decreases to cause early fatigue failure of the spring.

(7) The intermediate blank with the surface decarburized layer and the defect removed is fed into a heating furnace, and the heating time is less than 2 hours and the heating temperature is 950-1150 ℃ in order to control and reach the aim of the austenite grain size of the finished steel wire of 11-15 grade.

(8) And immediately carrying out high-pressure water dephosphorization on the intermediate billet after the intermediate billet is discharged from the heating furnace, and removing the surface oxide layer and defects in the heating process. According to the thermoplastic and thermal simulation data of the steel grade, the initial rolling temperature is higher than 890 ℃, the process rolling temperature is 890-1050 ℃, and the spinning temperature is 830-900 ℃. Because the steel contains more alloy elements, and the wire rod has small size, abnormal martensite or bainite structure is easy to generate, and the cooling speed of the wire rod is 3.0-5.5 ℃ in order to obtain sorbite structure which is favorable for drawing.

(9) The wire rod is drawn into a steel wire in multiple passes, and in order to further improve the grain size of the steel wire and reduce surface defects, the wire rod is drawn to the size of a finished steel wire, and the total reduction ratio is more than or equal to 65%.

(10) The steel wire after drawing is subjected to heat treatment, the steel wire is subjected to heat treatment while being heated, namely, an online heat treatment mode is adopted, the heating temperature before quenching is 860-980 ℃ according to the austenitizing temperature of the steel, the quenching medium is water or oil, the temperature is controlled to be 20-50 ℃ when the quenching medium is water, and the temperature is less than or equal to 40 ℃ when the quenching medium is oil. The tempering temperature is selected to be 420-500 ℃ according to the requirement of the spring on the tensile strength of the steel wire, and the lower the tempering temperature is, the higher the tensile strength is. Before the steel wire is wound, flaw detection equipment is used for detecting surface defects, and color code marks to be sprayed are used for marking, wherein the depth of the surface defects is larger than 0.1 mm.

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

The spring steel for the automobile hydraulic torque converter has the tensile strength of 2000-2200MPa, the area shrinkage rate of more than or equal to 40%, the fatigue life of the spring of more than or equal to 600 ten thousand times, can meet the requirement that the hardness of the steel wire is reduced by less than or equal to 1.0HRC after nitriding for 5-8 hours at 400-500 ℃, can meet the ultrahigh stress design requirement of the spring steel for the ultrahigh strength automobile hydraulic torque converter, and can be manufactured into oval, elliptic and other abnormal shapes.

Drawings

FIG. 1 is a physical diagram of the product of example 1 of the present invention.

FIG. 2 is a physical diagram of the product of example 2 of the present invention.

Detailed Description

The technical scheme of the present invention will be described in more detail in connection with the preferred embodiments of the present invention. However, these examples are merely illustrative of preferred embodiments of the present invention and are not intended to limit the scope of the present invention in any way.

Example 1:

The production process of spring steel for ultrahigh strength automobile hydraulic torque converter includes hot metal with molten iron temperature 1320 deg.c, P content 0.095%, S content 0.043%, molten iron pre-treatment in KR process in ladle, S eliminating process at 1238 deg.c and P, S% content 0.093% and 0.005% content, and Fe and inevitable impurity.

The converter is used for smelting, the temperature of molten steel is 1626 ℃ during tapping, the C content is 0.18%, the P content is 0.008%, the S content is 0.004%, and high-quality alloy and carbon powder are used during tapping. The LF refining furnace is stirred by Ar gas and is accompanied with the whole smelting process, 945kg refining slag is added, the alkalinity of the refining slag is 1.25, namely the ratio of CaO percentage content to SiO ₂ percentage content is 1.25, and all the alloying elements to be added reach the target value after the refining smelting is finished. The vacuum treatment uses an RH furnace, the vacuum degree is 0.5 torr and kept for 21min, the H content is 0.00006%, the O content is 0.0012%, the N content is 0.0036% when the vacuum is finished, and the continuous casting is carried out after Ar gas is continuously blown for 11min after the vacuum is finished.

The whole process uses argon protection during continuous casting, reduces the contact with air, monitors the content of chemical component N in a continuous casting tundish to be 0.0042 percent, the superheat degree of the tundish to be 23 ℃, the size of the section of a crystallizer to be 390 mm, and the size of a casting blank after light reduction to be 382 mm, 509mm.

The surface temperature of the continuous casting blank is 493 ℃ when the continuous casting blank enters a heating furnace, the highest heating temperature of the continuous casting blank is 1172 ℃, the continuous casting blank is kept for 220 minutes, and the section size of a rolled intermediate blank is 160mm.

The intermediate blank is subjected to magnetic powder inspection, the found defects are manually polished, the total decarburization depth of the intermediate blank is checked to be 1.12mm, a grinding wheel is used for cleaning the surface decarburization layer, and the polishing depth is 1.3mm.

The intermediate blank with the surface polished clean enters a heating furnace, the highest heating temperature is 1120 ℃, the temperature is kept for 103min, the rolling temperature is 1012 ℃, the wire rod spinning temperature is 960 ℃, and the rolled wire rod specification phi is 7.5mm. All the Steyr fans are turned off, the cooling speed is 3.7 ℃ per second, and the test wire rod structure is 92% of the sorbitation rate.

The wire rod is subjected to pickling phosphorus saponification, phi 7.5mm is subjected to 7-pass drawing to obtain a steel wire with the diameter of phi 4.2mm, the drawn steel wire is subjected to heat treatment after 5 days of aging, the highest heating temperature is 923 ℃ before quenching, the heat is preserved for 43s, the steel wire rod is quenched by water at 35 ℃, and then is tempered at 432 ℃, eddy current flaw detection is used before winding, the flaw detection precision is 0.1mm, the total weight of the steel wire is 732kg, and 3 color marks with defects are formed. The mechanical properties of the steel wire are tested, the tensile strength is 2063MPa, and the surface shrinkage is 43%. The hardness of the steel wire at half radius was checked for 55.6HRC, and a spring for a torque converter was produced using the steel wire, nitrided at 435 ℃ for 5 hours, and the hardness of the steel wire at half radius was checked again for 54.6HRC. The fatigue verification of 600 ten thousand times shows that the springs of the batch have no fracture, crack and other phenomena, and the steel wire can meet the fatigue life requirement of the springs. Tensile strength 2060MPa and surface shrinkage 43%.

Example 2:

The production process of spring steel for ultrahigh strength automobile hydraulic torque converter includes the steps of pre-treatment of molten iron in ladle with KR process at molten iron temperature 1293 deg.c, P content 0.102% and S content 0.039%, S eliminating process at 1226 deg.c and P, S% content of 0.103% and 0.004% separately, and Fe and inevitable impurities as the rest.

The steel is smelted by using an electric furnace, the temperature of the molten steel is 1639 ℃ during tapping, the content of C is 0.09%, the content of P is 0.010%, the content of S is 0.005%, and high-quality alloy and carbon powder are used during tapping. The LF refining furnace is stirred by Ar gas and is accompanied with the whole smelting process, 923kg refining slag is added, the alkalinity of the refining slag is 1.25, namely, the ratio of CaO percentage content to SiO ₂ percentage content is 1.25, and all alloying elements to be added reach the target value after the refining smelting is finished. The vacuum treatment is carried out by using a VD furnace, the vacuum degree is 133Pa and kept for 21min, the H content is 0.0009%, the O content is 0.0014% and the N content is 0.0045% when the vacuum is finished, and the continuous casting is carried out after Ar gas is continuously blown for 18min after the vacuum is finished.

The whole process uses argon protection during continuous casting, reduces the contact with air, monitors the chemical component N content of 0.0049% in a continuous casting tundish, the superheat degree of the tundish is 21 ℃, the section size of a crystallizer is 300 x 340mm, and the size of a casting blank after light reduction is 291 x 3411 mm.

The surface temperature of the continuous casting blank is 521 ℃ when the continuous casting blank enters a heating furnace, the highest heating temperature of the continuous casting blank is 1172 ℃, the continuous casting blank is kept for 190 minutes, and the section size of the rolled intermediate blank is 150mm.

The intermediate blank is subjected to infrared flaw detection, the found defects are manually polished, the total decarburization depth of the intermediate blank is checked to be 1.02mm, a grinding wheel is used for cleaning a surface decarburization layer, and the polishing depth is 1.3mm.

The intermediate blank with the surface polished clean enters a heating furnace, the highest heating temperature is 1180 ℃, the heat is preserved for 100min, the rolling temperature is 1010 ℃, the wire rod spinning temperature is 958 ℃, and the rolled wire rod specification phi 9mm. All the Steyr fans are turned off, the cooling speed is 3.3 ℃ per second, and the test wire rod structure is 91% of the sorbitation rate.

The wire rod is subjected to pickling phosphorus saponification, phi 9mm is subjected to 7-pass drawing to obtain a steel wire with the diameter of phi 4.8mm, the drawn steel wire is subjected to heat treatment after 5 days of aging, the highest heating temperature is 923 ℃ and the heat is preserved for 51 seconds before quenching, oil is used as a quenching medium, 428 ℃ tempering is carried out, eddy current flaw detection is carried out before rolling, the flaw detection precision is 0.1mm, the steel wire has 652kg, and the number of defective color codes is 2. The mechanical property of the steel wire is checked, the tensile strength is 2043MPa, and the area shrinkage is 42%. The hardness of the steel wire at half radius was checked for 55.3HRC, and a spring for a torque converter was produced using the steel wire, nitrided at 435 ℃ for 5 hours, and the hardness of the steel wire at half radius was checked again for 54.4HRC. The fatigue verification of 600 ten thousand times shows that the springs of the batch have no fracture, crack and other phenomena, and the steel wire can meet the fatigue life requirement of the springs.

While the preferred embodiments of the present invention have been described in detail, it is to be clearly understood that the same may be varied in many ways by those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims (8)

1. A spring steel for ultra-high strength automobile torque converter, characterized in that the chemical composition and mass percentage of the spring steel are: C: 0.55-0.70%, Si: 1.60-2.20%, Mn: 0.30-0.80%, Cr: 0.60-1.00%, V: 0.10-0.30%, Mo: 0.05-0.30%, Ni: ≤0.25%, Al: ≤0.0008%, Nb: ≤0.04%, P: ≤0.020%, S: ≤0.020%, Cu: ≤0.10%, N: ≤0.008%, Ti≤0.0012%, O: 0.0007%-0.0020%, S: ≤0.020%, H: ≤0.00005%, and the balance is Fe and unavoidable impurities. 2. The ultra-high strength spring steel for automobile torque converter according to claim 1 is characterized in that the matrix structure of the spring steel is tempered troostite + troostite, the matrix structure contains a second phase of vanadium, niobium and molybdenum carbonitride, the size of which is 8-80nm, and the austenite grain size reaches 11-15. 3. The ultra-high strength spring steel for automobile torque converter according to claim 1, characterized in that the tensile strength of the spring steel is 2000-2200 MPa, the surface reduction rate is ≥40%, and the fatigue life of the spring is ≥6 million times. 4. A method for manufacturing ultra-high strength spring steel for automobile torque converter as claimed in claim 1, characterized in that the process flow of the method is: (1) pretreatment of molten iron; (2) rough refining using a converter or an electric furnace; (3) refining using a refining furnace + VD or a refining furnace + RH; (4) continuous casting; (5) heating and rolling the continuous casting billet into an intermediate billet; (6) flaw detection and fine trimming of the intermediate billet; (7) heating the intermediate billet; (8) controlled rolling and controlled cooling of the intermediate billet; (9) multi-pass drawing of the wire rod into steel wire; (10) heat treatment and flaw detection of the steel wire. 5. The method for manufacturing an ultra-high strength spring steel for a hydraulic torque converter for an automobile according to claim 4, characterized in that the method specifically comprises the following steps: (1) Use molten iron pretreatment to remove sulfur from the molten iron. The sulfur content after molten iron pretreatment is ≤0.005%; (2) Rough refining is carried out by using a converter or electric furnace. The S content of the molten iron entering the rough refining furnace is ≤0.005%, and the content of the five harmful elements in the finished steel is ≤0.0030%. The rough refining furnace controls the oxygen content in the steel and increases the final C content. The P content in the steel is ≤0.012% and the S content is ≤0.005% when the steel is tapped. Alloys are added during tapping to reduce the introduction of harmful elements. (3) After rough refining, the molten steel enters the ladle refining furnace. The basicity of the refining slag in the ladle refining furnace is less than or equal to 1.4. A vacuum treatment RH or VD furnace is used to remove harmful gases N, H, and O in the steel, so that the N content in the steel is ≤0.0050%, the O content is ≤0.0017%, and the H content is ≤0.0001%; (4) Refining qualified molten steel, ensuring that inclusions fully float, the temperature of the molten steel is suitable for pouring, using protective pouring during pouring, the superheat of the molten steel entering the tundish is ≤25°C, and the cross-sectional dimensions of the steel billet cast by the continuous casting machine are: length 200-500mm, width 200-550mm; (5) The qualified continuous casting billet enters the continuous heating furnace, and the temperature of the billet is controlled to be ≥400℃. According to the calculation summary of the alloy elements and second phase solid solution of this steel grade, the heating temperature is determined to be 1120-1230℃. The cross-sectional dimensions of the rolling intermediate billet are: the length of the square billet is 140mm-220mm, the width is 140mm-220mm, and the diameter of the round billet is 140mm-220mm; (6) After the casting is rolled, magnetic powder or infrared flaw detection is first used to determine the defect location and mark it. The defects at the marked location are ground and cleaned. For the intermediate billet without surface defects, the surface layer within 1-2 mm near the surface is completely removed to remove the decarburized layer on the surface; (7) The intermediate billet with the surface decarburized layer and defects removed enters the heating furnace. In order to control and achieve the target of 11-15 grade austenite grain size of finished steel wire, the heating time should be less than 2 hours and the heating temperature should be 950-1150℃; (8) After the intermediate billet comes out of the heating furnace, high-pressure water dephosphorization is immediately carried out to remove the surface oxide layer and defects during the heating process. According to the thermoplasticity and thermal simulation data of the steel grade, the starting rolling temperature is greater than 890°C, the process rolling temperature is 890-1050°C, the wire laying temperature is 830-900°C, and the cooling rate of the wire rod is 3.0-5.5°C/s; (9) The wire rod is drawn into steel wire in multiple passes, and the wire rod is drawn to the size of the finished steel wire, and the total reduction rate is ≥ 65%; (10) The steel wire needs to be heat treated after drawing. The steel wire is heat treated while being heated, that is, the steel wire is heat treated online. Before the steel wire is coiled, the surface defects are detected by flaw detection equipment. If the depth of the surface defects is greater than 0.1mm, the color code needs to be sprayed. 6. The method for manufacturing an ultra-high strength spring steel for a hydraulic torque converter of an automobile according to claim 5, characterized in that: in step (4), when the solid phase ratio of continuous casting is 0.63-0.75, a light pressure is applied at the end, and the pressure reduction amount is 2.1-3.6% of the thickness of the ingot, so that the center segregation of the ingot is ≤1.0 level, the center porosity is ≤1.0 level, and the center carbon segregation index is ≤1.3, and finally the deviation of the tensile strength of different steel wires is ≤50Mpa. 7. The method for manufacturing an ultra-high strength spring steel for a hydraulic torque converter of an automobile according to claim 5, characterized in that: during heating in step (5), the total heating time is controlled, and the heating time varies according to different billet shapes, but the total heating time must ensure that the carbonitride of the microalloying elements is fully integrated into the austenite, that is, the total heating time is 2.5 min/mm-3.5 min/mm, where mm is the cross-sectional size of the billet. 8. The method for manufacturing an ultra-high strength spring steel for a torque converter of an automobile according to claim 5, characterized in that: the online heat treatment in step (10) is based on the austenitizing temperature of the steel, the heating temperature before quenching is 860-980°C, the quenching medium is water or oil, when the quenching medium is water, the temperature needs to be controlled at 20-50°C, when oil is used, the temperature is ≤40°C, the tempering temperature is selected from 420-500°C according to the requirement of the spring for the tensile strength of the steel wire, the lower the tempering temperature, the higher the tensile strength.