A large gauge alloy spring steel and method of production thereof

Abstract

The present application relates to a kind of large-scale alloy spring steel and its production method, control flow is as follows: steelmaking, bloom continuous casting, heating breakdown, repair flaw detection, prevent decarburization section spraying, small bloom heating, high line controlled rolling, stelmor line controlled cooling and collection volume packing.Adopt the coating of prevent decarburization section spraying combined with fine controlled rolling and controlled cooling process, effectively promote the quality of rod surface and the uniformity of organization performance, can be used to prepare large-scale high-strength automobile suspension spring.

Description

TECHNICAL FIELD

[0001] The present application relates to a large-diameter alloy spring steel and a production method thereof, and belongs to the technical field of alloy spring steel, in particular to a large-diameter alloy spring steel with a diameter of greater than or equal to 18 mm. BACKGROUND

[0002] Suspension springs, as core components of suspension systems, are facing the development trend of higher strength, larger diameter and longer fatigue. Compared with conventional size (<φ18mm) suspension spring steel wire rods, the control of surface decarburization, surface defects and uniformity of microstructure and properties is more difficult for large-diameter spring steel, and a stable and reliable systematic solution is urgently needed.

[0003] Patent CN118595184A discloses a controlled rolling and controlled cooling process for large-diameter Si-Cr alloyed spring steel wire. The main components of the spring steel are: C 0.45-0.65%, Si 1.00-1.80%, Mn 0.40-1.00%, Cr 0.30-0.90%, Ni ≤0.20%, Cu ≤0.20%, Mo ≤0.20%, V ≤0.02%, Ti ≤0.01%. The wire rod size is φ18-24mm, the billet preheating temperature is 680-780℃, the heating temperature is 910-1000℃, the soaking temperature is 1030-1090℃, the opening rolling temperature is 940-1000℃, the final rolling temperature of reducing diameter is 760-780℃, and the wire drawing temperature is 780-880℃. The roller speed of the Stelmor line entrance section is 24±5m / min, the speed of the 8th section is 48±5m / min, and the speed increases by 3m / min from the entrance to the 8th section; the roller speeds of the 9th and 10th sections are 45±5 and 40±5m / min, respectively, and the roller speed of the outlet section is 35±5m / min. The Stelmor line heat preservation cover is opened at 1-6# and 19, 20#, among which the fan air volume of 1-6# is set to 40±5%. The microstructure of the finished wire rod is proeutectoid ferrite + sorbite + degenerated pearlite, and the sorbite interlamellar spacing is 95-200nm; the tensile strength of the wire rod is ≥1000MPa, and the reduction of area is ≥30%. However, the spring steel composition provided in the present application does not limit the impurity elements P, S, O and H, and there is no control means for surface decarburization and defects. The finished product also does not reflect the surface decarburization layer and surface defect result characterization, and the effect of the controlled rolling and controlled cooling process on the uniformity of the wire rod microstructure and properties is not clearly embodied. In addition, the heat-treated spring steel wire prepared from the wire rod and the finished spring performance are not clear.

[0004] Patent CN109735765A discloses a large size, ultra-fine grain, high strength and toughness spring steel and its production method. The main components of the spring steel are: C 0.47-0.52%, Si 0.15-0.35%, Mn 0.95-1.15%, Cr 0.90-1.10%, V 0.10-0.25%, Mo ≤0.30%, Cu ≤0.20%, Al ≤0.05%, P ≤0.020%, S ≤0.020%, Ti ≤0.035%, N ≤0.015%. The process flow is: converter smelting → ladle refining → vacuum degassing → round billet continuous casting → reversible breakdown → continuous rolling → controlled cooling. The proportion of converter molten iron is ≥90%, the tapping temperature is ≥1650℃, the end point C is ≤0.15%, and the end point P is ≤0.01%. In the RH vacuum degassing process, the vacuum degree is ≤67Pa, the holding time is ≤20min, and the end point H is ≤1.5ppm; after the RH is finished, the argon blowing is treated and the silicon calcium core wire is fed, and the settling time is not less than 20 minutes. The cross-sectional size of the continuous casting billet is ϕ500mm, and the pit cooling time of the continuous casting billet is ≥48h. The heating temperature of the continuous casting round billet breakdown is 1250-1280℃, and the heating time is ≥7h; the rolling heating temperature of the intermediate billet after breakdown is 1000-1050℃, and the final rolling temperature is 850-900℃. Although the application adopts high-purity smelting process, it does not reflect the wire rod rolling and cooling process, and does not explain the difficulties and control means of large size wire rod (such as surface decarburization, surface defects, wire rod structure performance uniformity, etc.), and there is no performance characterization of spring steel wire rod and finished spring.

[0005] Patent CN119685721A discloses a kind of engineering machinery big size spring with tensile strength ≥2100MPa ultra-high strength spring steel and its preparation method.Spring steel main component is: C 0.46-0.55%, Si 1.50-2.00%, Mn 0.80-1.50%, Cr 0.90-1.50%, Ni 0.40-0.80%, Mo 0.10-0.40%, W 0.10-0.40%, V 0.08-0.15%, Ti0.015-0.035%, B 0.0008-0.0035%, Cu ≤0.25%, Al ≤0.025%, P ≤0.012%, S ≤0.006%, O≤0.0015%.Mn, Cr, Mo, W, Ni, B satisfy: 3.00≤(Mn+0.95×Cr+1.7×Mo+0.5×W+0.6×Ni+75×B) / %≤4.00.V, Ti, N satisfy: 0.07≤(0.75×V+Ti-3.43×N) / %≤0.13.Process flow: electric furnace / converter smelting→refining→continuous casting→rolling breakdown→continuous rolling→heat treatment.Heat treatment process: 900±30 ℃ heating quenching, 350±20 ℃ tempering.The end of spring steel after heat treatment J40mm ≥55HRC, tensile strength ≥2100MPa, yield strength ≥1800MPa, elongation after fracture ≥8%, reduction of area ≥40%, room temperature V-type impact energy ≥15J.The application adds more alloy elements Ni, Mo, W, etc., and the alloy cost is relatively high.On the other hand, similar to the previous patent, the wire rod does not reflect the relevant rolling cooling process, and the difficulties and control means of large size wire rod are not described (such as surface decarburization, surface defects, wire rod structure performance uniformity, etc.).At the same time, although the patent characterizes the performance of the heat treated spring steel wire, the core index fatigue performance is not reflected, and it cannot be predicted whether the suspension spring can be prepared.

[0006] In summary, the existing large size alloy spring steel disclosed so far does not propose corresponding control means for the surface decarburization, surface defects and structure performance uniformity of the base material spring steel, and the performance of the base material wire rod cannot meet the requirements of the current large self-renewal energy vehicle model for large size spring steel wire rod.Therefore, it is urgent to propose a large size (φ18mm and above) high strength automobile suspension spring to meet the above requirements. SUMMARY

[0007] The present application provides a large size alloy spring steel and a production method thereof, which adopts a segmented spraying of anti-decarburization coating combined with a fine controlled rolling and controlled cooling process, effectively improves the surface quality and structure performance uniformity of the wire rod, and can be used to prepare a large size (φ18mm and above) high strength automobile suspension spring.

[0008] The technical solution adopted by the present application to solve its technical problems is:

[0009] A large-size alloy spring steel, for alloy spring steel with diameter ≥18mm, the chemical components include C: 0.51-0.59%, Si: 1.50-1.70%, Mn: 0.65-0.80%, Cr: 0.70-0.85%, Cu ≤0.03%, Al ≤0.005%, V+Nb+Ti: 0.12-0.18%, B: 0.001-0.005%, P ≤100ppm, S ≤80ppm, N: 50-80ppm, O ≤20ppm, H ≤1ppm, the rest is Fe and impurities, wherein V=(3-5)×(Nb+Ti) by weight percentage;

[0010] Meanwhile, set the strength-ductility index SE: 2.15-2.55%, SE=[C]+0.2[Si]+0.5[Mn]+0.7[Cr]+3.8[V+Nb+Ti];

[0011] Quenching index QE: 4.5-5.8%, QE=[C]+0.4[Si]+1.5[Mn]+3[Cr]+200[B];

[0012] The production method of the large-size alloy spring steel, comprising the following steps:

[0013] Step S1, steelmaking, through molten iron pretreatment, converter smelting, LF combined RH double refining, to obtain molten steel meeting the chemical composition;

[0014] Step S2, large billet continuous casting, the molten steel after smelting in step S1 is transported to a tundish, distributed from the tundish to a large billet crystallizer for continuous casting to form a large billet; the superheat degree in the continuous casting process is set, electromagnetic stirring is used in the secondary cooling area; the solidification end of the casting blank is subjected to slight dynamic pressure by a straightening mill, the total pressure reduction amount of the straightening mill is set to 19-22mm, and the continuous casting speed is 0.45-0.50m / min;

[0015] Step S3, heating and breaking down, the large billet is heated in a heating furnace through a preheating section, a first heating section, a second heating section and a soaking section, the surface oxide skin is removed after the large billet exits the heating furnace by high-pressure water, and then enters a continuous rolling unit to be rolled to form a plurality of small billets; the center segregation of the small billet is ≤0.5 level, the center porosity is ≤0.5 level, and the center carbon segregation index is ≤1.08;

[0016] Step S4, grinding and flaw detection, the small billet after breaking down is ground; the small billet after grinding is subjected to shot blasting operation by an automatic shot blasting machine, and the surface roughness Ra of the small billet after shot blasting is ≤20μm; the billet is further subjected to magnetic particle flaw detection;

[0017] Step S5, after the flaw detection, the small billet is rusted with a steel wire brush, and an automatic spraying machine is started to spray the anti-decarburization coating; during the spraying process, the small billets are divided into a head part, a middle part and a tail part, the head part is a position 4 meters away from the end of the small billet, the tail part is a position 12 meters away from the end of the small billet, the coating thickness of the head part and the tail part is greater than that of the middle part, the coating thickness of the upper surface of the head part and the tail part is 1.2-1.5 mm, the coating thickness of the side surface is 1.5-1.8 mm, the coating thickness of the lower surface is 1.8-2.1 mm, the coating thickness of the upper surface of the middle part is 0.6-0.8 mm, the coating thickness of the side surface is 0.8-1.0 mm, and the coating thickness of the lower surface is 1.0-1.2 mm;

[0018] Step S6, the small billet is heated, the small billet coated with the anti-decarburization coating is heated by using a walking beam type heating furnace, the preheating section temperature is set to 750-850℃, the heating section temperature is set to 850-1000℃, and the soaking section temperature is set to 1000-1080℃, and the total heating time lasts for 70-90 min;

[0019] Step S7, high-speed wire rolling control, after the small billet is heated, the oxide layer is removed by using a high-pressure descaling machine, and the small billet is rolled by using a continuous bar and wire rolling mill, the rolling temperature is set to 910-940℃, the entrance temperature of the finishing rolling is set to 860-880℃, the entrance temperature of the reducing and sizing is set to 850-870℃, and the wire drawing temperature is set to 830-850℃; and a circular coil is obtained, which has a diameter of 18-26 mm;

[0020] Step S8, Stelmor wire control cooling, the wire drawing machine is started to swing, and the circular coil is staggered and laid flat on the roller; the cooling section is divided into a fast cooling section, an isothermal section and an air cooling section, in the fast cooling section, the No.1 fan and the No.2 fan are operated at full power, all the heat preservation covers are opened, the fast cooling section ends at a temperature of 685-700℃, and the temperature difference of the same circle is 10-15℃; in the isothermal section, the No.7 fan is operated at low load, the heat preservation covers are opened, and the isothermal section ends at a temperature of 660-675℃, and the temperature difference of the same circle is 5-8℃; in the air cooling section, the No.8-12 fans are turned off, and all the heat preservation covers are opened, and the air cooling section ends at a temperature of 600-615℃;

[0021] Step S9, coiling and packaging, the coil enters the coiling barrel to be uniformly stacked layer by layer; the packaging is loaded in sections, the first section is set to a coil pressing pressure of 850-1000 MPa, the coil length is quickly reduced to 2.0 m from the natural length after coiling, the second section is set to a coil pressing pressure of 1800-2000 MPa, the coil length is controlled to be 1.55-1.60 m, and the third section keeps the coil pressing pressure unchanged, the pressure is reduced to 1500-1600 MPa, and the coil is packaged after 3-5 s;

[0022] Further, in step S2, the superheat of continuous casting is set to 18-22℃, the electromagnetic stirring vibration frequency is set to 1.8-2.3Hz, the total reduction of the straightening mill to the casting blank is set to 19-22mm, and the continuous casting speed is set to 0.45-0.50m / min; the formed bloom section size is (300-330)×(390-460)mm 2 , and the length is 5.2-5.7m;

[0023] Further, in step S3, the preheating section temperature is set to 700-850℃, the heating first section temperature is set to 850-1000℃, the heating second section temperature is set to 1000-1100℃, the soaking section temperature is set to 1180-1220℃, the total heating time is set to 450-500min, the air-fuel ratio of the heating first section and the heating second section is set to 0.60-0.65, and the air-fuel ratio of the soaking section is set to 0.55-0.60;

[0024] After the bloom leaves the heating furnace, the surface oxide skin is removed by high-pressure water, the descaling water pressure is ≥20MPa, the opening rolling temperature of the continuous rolling mill set is 920-950℃, and the billet section size is 140×140mm 2 , and the length is 15.2-15.7m;

[0025] Further, in step S4, the single-side grinding amount is set to 1.0-1.2mm, and the corner grinding amount is set to 3-5mm; the shot of the automatic shot blasting machine is S90 grade steel shot, and the steel shot diameter is 1.0-1.3mm;

[0026] Further, in step S5, the sprayed anti-decarburization coating includes, in terms of weight percentage, Al2O3: 15-20%, MgO: 15-20%, ZrO2: 2-7%, CaO: 20-25%, SiO2: 15-18%, and Fe2O3: 20-25%;

[0027] When spraying the head and the tail, the roller speed is set to 0.45-0.50m / s, the upper nozzle pressure is set to 0.15-0.18MPa, and the lower nozzle pressure is set to 0.18-0.22MPa; when spraying the middle part, the roller speed is set to 0.55-0.60m / s, the upper nozzle pressure is set to 0.08-0.11MPa, and the lower nozzle pressure is set to 0.11-0.14MPa;

[0028] Further, in step S6, the heating section air-fuel ratio of the walking beam furnace is set to 1.10-1.15, and the soaking section air-fuel ratio is set to 1.00-1.05;

[0029] Further, in step S7, when the high-pressure descaling machine is used to remove the oxide layer of the heated billet, the descaling water pressure is set to ≥25MPa;

[0030] Further, in step S8, the swing amplitude of the laying head is set to 4.5-6.5%, and the swing period is set to 10.5-13.5s;

[0031] In the fast cooling section, the No. 1 fan runs at full power, the No. 2 fan runs at full power, the No. 3 fan runs at 20-80% power, the No. 4 fan runs at 20-50% power, the No. 5 fan and the No. 6 fan are turned off, the air flow is distributed horizontally at 12%, 23%, 15%, 15%, 23% and 12% respectively, the roller speed is 0.55-0.62m / s, and the control cooling rate is 4.6-6.2℃ / s;

[0032] In the isothermal section, the No. 7 fan runs at 10-30% power, the air flow is distributed horizontally at 5%, 35%, 10%, 10%, 35% and 5% respectively, the roller speed is 0.48-0.55m / s, and the control cooling rate is 0.15-0.25℃ / s;

[0033] In the air cooling section, the roller speed is 0.42-0.48m / s, and the cooling rate is 1.5-2.0℃ / s;

[0034] The air flow of the No. 1 fan to the No. 12 fan is 260000m 3 / h;

[0035] Further, in step S9, the coiling temperature is set to 570-585℃, the bottom baffle angle of the coiling barrel is controlled to be 15-30°, and the baffle descending speed is 0.2-0.35m / s;

[0036] When packing, the first section is set to have a pressurized plate moving speed of 0.25-0.35m / s, and the second section is set to have a pressurized plate moving speed of 0.10-0.15m / s;

[0037] Further, the obtained wire rod microstructure includes ferrite and sorbite, the sorbite proportion is ≥92%, the sorbite grain size is 8.8-10.5μm, the sorbite lamella spacing is 120-150nm, the original austenite grain size is 8.0-8.5, the wire rod has no full decarburization, and the total decarburization thickness is ≤120μm;

[0038] Under the requirement of precision 50μm, the number of wire rod flaw defects is ≤25 per coil;

[0039] The tensile strength of the wire rod is 970-1020MPa, and the reduction of area is ≥50%;

[0040] Through quenching and tempering treatment, the tensile strength of the suspension spring steel wire is ≥2000MPa, and the surface reduction is ≥48%;

[0041] The finished suspension spring prepared by winding the spring has a fatigue cycle number of 1.5 million times under the condition of a maximum stress of 1280 MPa and a stress amplitude of 850 MPa.

[0042] Through the above technical scheme, compared with the prior art, the present application has the following beneficial effects:

[0043] 1. The large-specification alloy spring steel provided by the present application solves the problem that the core of a large-specification spring steel is not easy to be quenched by designing components, thereby ensuring that the rod is not cracked during quenching and ensuring that the core of the rod is completely quenched.

[0044] 2. The production method of the large-specification alloy spring steel provided by the present application adopts low superheat pouring, dynamic soft reduction and low pulling speed process in the bloom continuous casting process, thereby minimizing the segregation of the core of the continuous casting blank; and adopts high-temperature diffusion heating mode in the heating and cogging process, thereby promoting the diffusion of C in the core of the blank and further reducing segregation.

[0045] 3. In the anti-decarburization coating spraying process of the production method of the large-specification alloy spring steel provided by the present application, the spraying process is reasonably optimized according to the different decarburization tendencies of the blank in different regions of the heating furnace, the thickness of the coating on each region and each surface of the blank is controlled, and the spraying effect and protection effect are improved, so that the rod is finally free of complete decarburization.

[0046] 4. In the Stelmor wire controlled cooling process of the production method of the large-specification alloy spring steel provided by the present application, the wire drawing machine swinging process and the segmented cooling process are reasonably set based on the requirement of the large-specification rod for uniformity of structure and performance; the rod lap joint density is effectively reduced by reasonable swinging of the wire drawing machine, and the difficulty of uniform temperature control in the same circle is reduced; in the segmented cooling stage, the transverse air volume is reasonably distributed by the Jialing device based on the characteristics of the large-specification rod, the temperature difference in the same circle of the rod is reduced, and the uniformity of the structure and performance of the rod is ensured.

[0047] 5. In the collecting and packaging process of the production method of the large-specification alloy spring steel provided by the present application, the temperature, baffle angle and descending speed of the coil are reasonably set, so that the elasticity and layering of the spring steel rod are effectively controlled, and the descending speed is reasonably set to minimize the scratch of the rod and the coil barrel; the surface quality of the large-specification rod is improved by segmented loading. BRIEF DESCRIPTION OF DRAWINGS

[0048] The present application will be further described below in combination with the drawings and examples.

[0049] Figure 1 is a flowchart of the production method of the large-specification alloy spring steel provided by the present application. DETAILED DESCRIPTION

[0050] The application will be further described in detail in combination with the drawings. The specific dimensions used in the embodiments are only for illustrating the technical solutions and do not limit the protection scope of the application.

[0051] As set forth in the background, in the prior art of the large-diameter alloy spring steel, no corresponding and effective control means has been proposed for the surface decarburization, defects and uniformity of the microstructure and properties of the base material spring steel, so that the base material wire rod cannot meet the current requirements of the large-diameter spring steel wire rod.

[0052] To solve the above problems, the application provides a large-diameter alloy spring steel and a production method thereof. The chemical composition is designed to make the wire rod core completely through hardened under the premise of no cracking of the subsequently obtained base material spring steel. Then, the control process is designed from the production method, i.e., steelmaking→ bloom continuous casting→ heating and blooming→ flaw detection→ anti-decarburization segmented spraying→ small bloom heating→ high-speed wire rolling control→ Stelmor wire controlled cooling→ coiling and packaging. Especially, the anti-decarburization coating segmented spraying combined with the refined rolling and controlled cooling process effectively improves the surface quality of the wire rod and the uniformity of the microstructure and properties.

[0053] First, the chemical composition of the alloy spring steel with a diameter of ≥18 mm provided by the application, by weight percentage, includes C: 0.51-0.59%, Si: 1.50-1.70%, Mn: 0.65-0.80%, Cr: 0.70-0.85%, Cu ≤0.03%, Al ≤0.005%, V+Nb+Ti: 0.12-0.18%, B: 0.001-0.005%, P ≤100ppm, S ≤80ppm, N: 50-80ppm, O ≤20ppm, H ≤1ppm, and the rest is Fe and impurities; wherein, V=(3-5)×(Nb+Ti).

[0054] The design of each of the above chemical elements follows the relevant principles and is described as follows:

[0055] C: C is a cheap and effective strengthening element. The cementite formed by combining C with Fe and the dispersed fine carbide particles formed by combining C with alloying elements can significantly improve the strength of the suspension spring. However, excessive addition of C deteriorates the plasticity and causes segregation to affect the uniformity of the properties, so C is set to 0.51-0.59%.

[0056] Si: Si is an effective elastic improvement element that can improve the spring's resistance to elastic reduction. At the same time, as a strong deoxidizing element, it can greatly reduce the oxygen content and indirectly reduce the number of inclusions to improve the purity. However, excessive addition affects the plasticity and significantly increases the decarburization tendency, so Si is set to 1.50-1.70%.

[0057] Mn: Mn is a solid solution strengthening and hardenability enhancing element, which can effectively improve the hardenability of large diameter wire rod, and significantly improve the strength; but excessive Mn will lead to the formation of abnormal bainite or martensite structure, therefore Mn is set to 0.65-0.80%.

[0058] Cr: Cr is a carbide binding element, the carbide particles formed by Cr exist in the cementite lamella to improve the strength; at the same time, Cr can also improve the hardenability of large diameter wire rod and stabilize the spring performance; excessive addition will cause abnormal structure during phase transformation, therefore Cr is set to 0.70-0.85%.

[0059] Cu: Cu is an impurity element for spring steel, which is easy to segregate at grain boundaries and phase boundaries and reduce hot deformation strength, affecting rolling and subsequent heat treatment process, therefore Cu≤0.03% needs to be controlled.

[0060] Al: Al combines with O to form spinel-like hard Al2O3 inclusions, which is easy to form fatigue sources under alternating stress and greatly reduce the fatigue performance of the spring, therefore Al≤0.005% needs to be controlled.

[0061] V+Nb+Ti: V, Nb and Ti are effective micro-alloy strengthening elements, which form fine and dispersed particles by combining with C, significantly refining the grains and improving the strength and plasticity of the base material. However, excessive addition will strongly inhibit phase transformation and produce abnormal structure, therefore V+Nb+Ti is controlled to 0.12-0.18%; at the same time, considering the different effects of V, Nb and Ti on fine-grain strengthening, and preventing the proportion of large-size Nb-containing particles and hard Ti-containing particles from being too high, V=(3-5)×(Nb+Ti) is set.

[0062] Among the main chemical components of spring steel, the innovative part of the present application is the design of element B. B is a strong hardenability element. The core of large diameter spring steel wire is not easy to harden, and appropriate addition of B is beneficial to obtain uniform and fine hardened structure. Excessive addition of B is prone to quenching cracking risk. Therefore, in order to ensure that the wire rod is completely hardened without cracking, B is limited to 0.001-0.005%.

[0063] The remaining chemical components of spring steel are impurity elements, such as P: P is an impurity element, which is easy to enrich in grain boundaries and reduce grain boundary strength, reduce the plastic deformation ability of spring steel wire, and cause a series of problems such as spring breakage, therefore P≤100ppm needs to be controlled.

[0064] S: S is easy to cause center segregation, which affects the hot deformation performance together with Cu, and high content will produce large size MnS inclusions, affecting the fatigue life, therefore S≤80ppm needs to be controlled.

[0065] N: N combines with microalloying elements to form (C, N) composite particles, which effectively improves strength. However, if the content is too high, it will combine with B and affect the effect of B on hardenability. Therefore, N needs to be controlled at 50-80 ppm.

[0066] O: O can combine with impurity elements to form various inclusions, reducing the purity of molten steel and affecting fatigue performance. Therefore, O should be controlled to ≤20ppm.

[0067] H: H is an impurity element. For large-size high-strength alloy spring steel, it is easy to segregate in the steel and cause hydrogen embrittlement. Therefore, H needs to be controlled to ≤1ppm.

[0068] Another innovation in the chemical composition design is the limitation of two indices for large-size alloy spring steel: the strength-plasticity index and the hardenability index. The strength-plasticity index SE = [C] + 0.2[Si] + 0.5[Mn] + 0.7[Cr] + 3.8[V + Nb + Ti]; the hardenability index QE = [C] + 0.4[Si] + 1.5[Mn] + 3[Cr] + 200[B].

[0069] Regarding the strength-ductility index, considering the combined effects of added alloying elements on strength and ductility, a higher index results in higher strength but lower ductility; conversely, a lower index results in lower strength but higher ductility. Therefore, based on the strength-ductility requirements for large-size spring steel, the strength-ductility index is limited to 2.1-2.55%. Regarding the hardenability index, considering the combined effects of added alloying elements on hardenability, a higher index results in better hardenability for large-size spring steel, but excessive hardenability can lead to cracking. A lower index results in poorer hardenability, causing excessive differences in the properties of the steel wire core and surface, which cannot meet the requirements of suspension springs. Therefore, based on the hardenability requirements for large-size spring steel, the hardenability index is limited to 4.5-5.8%.

[0070] Following the composition design, this application further provides a method for producing large-size alloy spring steel. Figure 1 As shown), step S1, steelmaking, involves pretreatment of molten iron, converter smelting, and LF combined with RH double refining to obtain molten steel that meets the aforementioned chemical composition requirements for large-size high-strength alloy spring steel.

[0071] In view of the fact that large-size round billets are sensitive to core segregation, and the core segregation will exacerbate the difference in core-surface microstructure and performance of the round billets, in step S2, the molten steel obtained in step S1 is delivered to a tundish, distributed from the tundish to a bloom crystallizer, and then continuously cast to form a bloom; the low superheat pouring, dynamic soft reduction, and low casting speed process are adopted in the continuous casting process to minimize the core segregation of the continuously cast bloom. Specifically, the superheat in the continuous casting process is set to 18-22 ℃, which is the optimal interval for taking into account the fluidity and solidification quality. In the secondary cooling zone, electromagnetic stirring is adopted, and the electromagnetic stirring frequency is 1.8-2.3 Hz; the low-frequency electromagnetic field stirs the un-solidified liquid steel in the continuously cast bloom, which facilitates the subsequent removal of large inclusions in the molten steel and makes the composition more uniform to inhibit the segregation of alloying elements; the solidification end of the bloom is subjected to slight dynamic pressure by a withdrawal-straightening machine, the total soft reduction amount of the withdrawal-straightening machine is set to 19-22 mm, the continuous casting speed is 0.45-0.50 m / min, and the volume shrinkage of the molten steel during solidification is compensated for to make the continuously cast bloom more dense; and finally, the bloom has a cross-sectional size of (300-330) x (390-460) mm and a length of 5.2-5.7 m. 2

[0072] In step S3, the bloom is heated and broken down; the bloom is heated in a heating furnace through a preheating section, a first heating section, a second heating section, and a soaking section; the gradient temperature rising of preheating- segmented heating-soaking makes the temperature of the billet uniform inside and outside and reaches the plastic state required for rolling. The preheating section temperature is set to 700-850 ℃, the first heating section temperature is set to 850-1000 ℃, the second heating section temperature is set to 1000-1100 ℃, the soaking section temperature is set to 1180-1220 ℃, and the total heating time lasts for 450-500 min; the preheating section temperature is set to be relatively low when the billet enters the furnace to avoid thermal stress cracks caused by the rapid expansion of the surface; the first heating section and the second heating section rapidly increase the overall temperature of the billet until most of the austenite transformation is completed, so that the plasticity of the billet is significantly improved; finally, the soaking section, as the last key process, completely eliminates the temperature gradient in the billet caused by segmented heating to avoid uneven deformation due to local temperature unevenness during rolling. At the same time, in order to make the combustion efficiency optimal, the air-fuel ratio of the first heating section and the second heating section is 0.60-0.65, and the air-fuel ratio of the soaking section is 0.55-0.60.

[0073] After the bloom leaves the heating furnace, the surface scale is removed by high-pressure water, and the descaling water pressure is ≥20 MPa; then the bloom enters a continuous rolling mill to be rolled into a plurality of small billets, and the open rolling temperature of the continuous rolling mill is 920-950 ℃ to ensure the balance between plasticity and deformation resistance. Finally, a small billet with a size of 140 x 140 mm 2 , a length of 15.2-15.7 m, a center segregation of ≤0.5 level, a center porosity of ≤0.5 level, and a center carbon segregation index of ≤1.08 is obtained, which is accurate in size, clean in surface, and dense and uniform in internal structure.​

[0074] This step adopts high-temperature diffusion heating method to promote the diffusion of the billet core C, reduce segregation, and ensure the uniformity of the subsequent rod organization performance.

[0075] Step S4, grinding and flaw detection, the small billet after the grinding is set to 1.0-1.2mm, the corner grinding amount is 3-5mm. After the grinding, the small billet is subjected to the operation of the automatic shot blasting machine, the shot is S90 grade steel shot, the steel shot diameter is 1.0-1.3mm, and the surface roughness Ra of the small billet after the shot blasting is ≤20μm; continue to use the magnetic powder flaw detection to inspect the billet to ensure that the surface is free of defects.

[0076] Step S5, the small billet after the grinding and flaw detection is rusted by the steel wire brush, and the automatic spraying machine is started to spray the anti-decarburization coating. During the spraying process, the small billet is divided into the head, the middle and the tail, the head is the end of the small billet 4 meters away, and the tail is the position 12 meters away from the end of the small billet.

[0077] When spraying the head and the tail, the roller speed is set to 0.45-0.50m / s, the upper nozzle pressure is 0.15-0.18MPa, and the lower nozzle pressure is 0.18-0.22MPa; after the spraying is completed, the coating thickness on the upper surface of the billet is 1.2-1.5mm, the coating thickness on the side surface is 1.5-1.8mm, and the coating thickness on the lower surface is 1.8-2.1mm.

[0078] When spraying the middle part (4-12mm), the roller speed is set to 0.55-0.60m / s, the upper nozzle pressure is 0.08-0.11MPa, and the lower nozzle pressure is 0.11-0.14MPa; after the spraying is completed, the coating thickness on the upper surface of the billet is 0.6-0.8mm, the coating thickness on the side surface is 0.8-1.0mm, and the coating thickness on the lower surface is 1.0-1.2mm.

[0079] As can be seen from the above results, the coating thickness after the head and the tail spraying is greater than that of the middle part, which is because the small billet heating adopts the regenerative heating furnace, the burner is located at the head and the tail of the billet, which causes the actual temperature of the head and the tail to be higher than that of the middle part during the heating process, and the decarburization tendency of the head and the tail is greater than that of the middle part. Therefore, considering the higher decarburization requirement of large specification rod, the segmented control method of the head, the middle and the tail is adopted, and the coating thickness of the head and the tail is necessarily greater than that of the middle, which ensures the decarburization requirement while reducing the coating cost and improving the subsequent high-pressure water descaling effect.

[0080] Of course, in the anti-decarburization coating spraying process, in order to finally realize the wire rod without full decarburization, the anti-decarburization coating is also crucial, and the anti-decarburization coating provided by the application preferably comprises, in terms of percentage by weight, Al2O3: 15-20%, MgO: 15-20%, ZrO2: 2-7%, CaO: 20-25%, SiO2: 15-18%, and Fe2O3: 20-25%.

[0081] In step S6, the small billets are heated by using a walking beam furnace, the preheating section temperature is set to 750-850 DEG C, the heating section temperature is set to 850-1000 DEG C, the soaking section temperature is set to 1000-1080 DEG C, and the total heating time lasts for 70-90 min; the air-fuel ratio of the heating section is 1.10-1.15, and the air-fuel ratio of the soaking section is 1.00-1.05.

[0082] In step S7, high-speed wire rolling is performed on the heated small billets, the scale removal water pressure is set to be greater than or equal to 25 MPa, the scale on the surface of the billets is stripped by the impact of the high-pressure water flow, and a clean billet surface is provided for subsequent rolling, the continuous bar and wire rolling mill is used for controlled rolling of the small billets, the opening rolling temperature is set to 910-940 DEG C, the entry temperature of the finishing rolling is set to 860-880 DEG C, the entry temperature of the reducing sizing is set to 850-870 DEG C, and the wire drawing temperature is set to 830-850 DEG C, and through setting a reasonable temperature gradient, the deformation stability of each rolling link is ensured, and finally the circular wire rod with a diameter of 18-26 mm is obtained.

[0083] In step S8, the Stelmor wire controlled cooling is performed, the wire drawing machine swinging function is started, the swinging amplitude of the wire drawing machine is set to 4.5-6.5%, and the swinging period is set to 10.5-13.5 s, and the circular wire rod is staggered and laid on the roller.

[0084] In order to solve the problem that the temperature difference between the large-diameter wire rod overlapping point and the non-overlapping point is large and the cooling is not uniform, the cooling section is divided into a fast cooling section, an isothermal section and an air cooling section, the purpose is to reduce the temperature difference in the same circle, ensure the uniformity of the structure, and make the strength fluctuation of the finished wire rod small.

[0085] Specifically, in the rapid cooling section, the No. 1 fan runs at full power, the No. 2 fan runs at full power, the No. 3 fan runs at 20-80% power, the No. 4 fan runs at 20-50% power, the No. 5 fan and the No. 6 fan are turned off, the air volume is distributed horizontally by the Jialing device as 12%, 23%, 15%, 15%, 23%, and 12% respectively, all the heat preservation covers are opened, the roller speed is 0.55-0.62 m / s, the control cooling rate is 4.6-6.2 ℃ / s, the end temperature of the rapid cooling section is 685-700 ℃, and the temperature difference of the same circle is 10-15 ℃. In the isothermal section, the No. 7 fan runs at 10-30% power, the heat preservation covers are opened, the air volume is distributed horizontally as 5%, 35%, 10%, 10%, 35%, and 5% respectively, the remaining fans and heat preservation covers are turned off, the roller speed is 0.48-0.55 m / s, the control cooling rate is 0.15-0.25 ℃ / s, the end temperature of the isothermal section is 660-675 ℃, and the temperature difference of the same circle is 5-8 ℃. In the air cooling section, the No. 8-12 fans are turned off, all the heat preservation covers are opened, the roller speed is 0.42-0.48 m / s, the cooling rate is 1.5-2.0 ℃ / s, and the end temperature of the air cooling section is 600-615 ℃. The air volume of the No. 1 fan to the No. 12 fan is 260000 m 3 / h.

[0086] Step S9, the coil is packed, first the coil enters the coil barrel to stack uniformly layer by layer, the coil temperature and the baffle angle are set, the spring steel coil elasticity and the coil shape and layering problem are effectively controlled. Regarding the spring steel coil elasticity problem, if the temperature is too low, the elasticity is too large, and if the temperature is too high, the equipment durability will be affected, therefore the coil temperature is set to 570-585 ℃; when the coil falls in the bottom due to the front and rear pulling action, there is a certain inclination angle, therefore in order to keep the coil stacking angle consistent and avoid layering, the baffle is increased by a certain angle for correction, preferably the baffle angle at the bottom of the coil barrel is controlled to be 15-30° in the same direction. At the same time, a reasonable falling speed is set in this step to minimize the scratch of the coil and the coil barrel, preferably the baffle falling speed is 0.2-0.35 m / s.

[0087] Next the packing is loaded in sections, the first section is set to a pressurizing pressure of 850-1000 MPa and a pressurizing moving speed of 0.25-0.35 m / s, the coil length is quickly reduced from the natural length after the coil to 2.0 m, the second section is set to a pressurizing pressure of 1800-2000 MPa and a pressurizing moving speed of 0.10-0.15 m / s, the coil length is controlled to be 1.55-1.60 m, the third section keeps the pressurizing plate still, the pressure is reduced to 1500-1600 MPa, and the packing is performed after 3-5 s. The purpose of the section loading is to increase the coil stacking density and reduce the extrusion damage between the coils before the final packing pressure is applied, and to improve the surface quality of the large size coil.

[0088] The coiling and packaging design of this step can reduce the problem of difficult control of surface quality caused by the large surface area of the large-diameter wire rod, reduce the extrusion scratches during the wire rod packaging process, and reduce the number of flaw defects of the large-diameter wire rod in the downstream customer.

[0089] Finally, the wire rod obtained by the above production method has a microstructure of ferrite + sorbite, a sorbite proportion ≥ 92%, a sorbite grain size of 8.8-10.5 μm, a sorbite lamellar spacing of 120-150 nm, an original austenite grain size of 8.0-8.5, and no full decarburization of the wire rod, and the total decarburization thickness is ≤ 120 μm. The sorbite proportion is determined by the "YB / T 169-2014 Metallographic Detection Method for Sorbite Content of High Carbon Steel Wire Rod", and the original austenite grain size is determined by the oxidation method in "GB / T 6394-2017 Metal Average Grain Size Determination Method".

[0090] Under the precision of 50 μm, the number of flaw defects of the wire rod is ≤ 25 per coil. The tensile strength of the wire rod is 970-1020 MPa, and the reduction of area is ≥ 50%; the tensile strength of the suspension spring steel wire prepared by quenching and tempering treatment is ≥ 2000 MPa, and the surface reduction is ≥ 48%. The mechanical property detection is determined by "GB / T 228.1-2021 Metallic Materials Tensile Test Part 1: Room Temperature Test Method".

[0091] The finished product suspension spring prepared by winding the spring has a fatigue cycle number ≥ 1.5 million times under the conditions of maximum stress 1280 MPa and stress amplitude 850 MPa. The fatigue performance detection is determined by the fatigue test method in "GB / T 16947-2009 Spiral Spring Fatigue Test Specification".

[0092] Embodiments 1-9 are further given to verify the feasibility of the production method of the above large-diameter alloy spring steel.

[0093] The production processes of embodiments 1-9 are all: steelmaking → large billet continuous casting → heating and blooming → flaw detection → anti-decarburization section spraying → small billet heating → high wire controlled rolling → Stelmor wire controlled cooling → coiling and packaging.

[0094] Table 1 is the main chemical composition (wt%) of the spring steels of embodiments 1-9, and the impurity elements are controlled as follows: Cu ≤ 0.03%, Al ≤ 0.005%, P ≤ 100 ppm, S ≤ 80 ppm, N: 50-80 ppm, O ≤ 20 ppm, H ≤ 1 ppm, the rest is Fe and unavoidable impurities.

[0095] Table 1

[0096]

[0097] Table 2 is the spring steel bloom continuous casting process of Example 1-Example 9.

[0098] Table 2

[0099]

[0100] Table 3 is the spring steel continuous casting bloom heating and cogging process of Example 1-Example 9.

[0101] Table 3

[0102]

[0103] Table 4 is the spring steel billet decarburization prevention coating spraying process of Example 1-Example 9.

[0104] Table 4

[0105]

[0106] Table 5 is the spring steel wire rod controlled cooling process of Example 1-Example 9.

[0107] Table 5

[0108]

[0109] Table 6 is the spring steel wire rod coiling and packing process of Example 1-Example 9.

[0110] Table 6

[0111]

[0112] Table 7 is the spring steel wire rod structure and performance and finished spring performance of Example 1-Example 9.

[0113] Table 7

[0114]

[0115] From Table 7, it can be seen that through the large specification alloy spring steel and the production method thereof provided in the present application, the properties of the wire rod obtained all meet the requirements of the large specification spring steel wire rod for the large self-renewable energy vehicle model.

[0116] Those skilled in the art can understand that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. It should also be understood that terms such as those defined in a general dictionary should be understood to have meanings consistent with those in the context of the prior art, and should not be interpreted in an idealized or overly formal sense unless otherwise defined.

[0117] The meaning of "and / or" described in the present application means that each single existence or both existences are included.

[0118] The meaning of "connection" described in the present application can be direct connection between components or indirect connection between components through other components.

[0119] The above is the ideal embodiment according to the present application, and the above description can be changed and modified by the relevant staff without deviating from the technical idea of the present application. The technical scope of the present application is not limited to the content of the specification, and the technical scope must be determined according to the scope of claims.

Claims

1. A method of producing a large gauge alloy spring steel, characterized by: The alloy spring steel with a diameter of 18 mm or more comprises, in terms of weight percentage, C: 0.51-0.59%, Si: 1.50-1.70%, Mn: 0.65-0.80%, Cr: 0.70-0.85%, Cu ≤0.03%, Al ≤0.005%, V+Nb+Ti: 0.12-0.18%, B: 0.001-0.005%, P ≤100ppm, S ≤80ppm, N: 50-80ppm, O ≤20ppm, H ≤1ppm, and the rest is Fe and impurities; wherein, V=(3-5)×(Nb+Ti); Meanwhile, the strength-ductility index SE is set to 2.15-2.55%, SE=[C]+0.2[Si]+0.5[Mn]+0.7[Cr]+3.8[V+Nb+Ti]; The hardenability index QE is 4.5-5.8%, QE=[C]+0.4[Si]+1.5[Mn]+3[Cr]+200[B]; The production method comprises the following steps: Step S1, steelmaking, through molten iron pretreatment, converter smelting, LF combined RH duplex refining, to obtain molten steel meeting the chemical composition; Step S2, bloom continuous casting, the molten steel smelted in step S1 is transported to a tundish, distributed from the tundish to a bloom crystallizer for continuous casting to form a bloom; the superheat degree in the continuous casting process is set, electromagnetic stirring is used in the secondary cooling area; the solidification end of the casting blank is subjected to slight dynamic pressure by a straightening mill, the total pressure reduction amount of the straightening mill is set to 19-22 mm, and the continuous casting speed is 0.45-0.50 m / min; Step S3, heating and blooming, the bloom is heated in a heating furnace through a preheating section, a first heating section, a second heating section and a soaking section, the surface oxide skin of the bloom after leaving the heating furnace is removed by high-pressure water, and then the bloom enters a continuous rolling unit to be rolled into several small blooms; the center segregation of the small bloom is ≤0.5 level, the center porosity is ≤0.5 level, and the center carbon segregation index is ≤1.08; Step S4, grinding and flaw detection, the small bloom after blooming is ground; the small bloom after grinding is subjected to shot blasting by an automatic shot blasting machine, and the surface roughness Ra of the small bloom after shot blasting is ≤20 μm; the blank is further subjected to magnetic particle flaw detection; Step S5, the small bloom after grinding and flaw detection is rusted by a steel wire brush, and an automatic spraying machine is started to spray anti-decarburization coating; in the spraying process, the small bloom is divided into a head, a middle and a tail, the head is a position 4 meters from the end of the small bloom, the tail is a position 12 meters away from the end of the small bloom, the coating thickness of the head and the tail is greater than that of the middle, the coating thickness of the upper surface of the blank in the head and the tail is 1.2-1.5 mm, the coating thickness of the side surface is 1.5-1.8 mm, the coating thickness of the lower surface is 1.8-2.1 mm, the coating thickness of the upper surface of the blank in the middle is 0.6-0.8 mm, the coating thickness of the side surface is 0.8-1.0 mm, and the coating thickness of the lower surface is 1.0-1.2 mm. In step S6, the small billets coated with the anti-decarburization coating are heated by using a walking beam furnace, the preheating section temperature is set to 750-850 DEG C, the heating section temperature is set to 850-1000 DEG C, the soaking section temperature is set to 1000-1080 DEG C, and the total heating time lasts for 70-90 min; In step S7, after the small billets are heated, the oxide layer is removed by using a high-pressure descaling machine, and the small billets are controlled and rolled by using a continuous bar mill, the opening rolling temperature is set to 910-940 DEG C, the precision rolling inlet temperature is set to 860-880 DEG C, the reducing diameter inlet temperature is set to 850-870 DEG C, and the wire drawing temperature is set to 830-850 DEG C, so that the circular wire rod with a diameter of 18-26 mm is obtained; Step S8, Stelmor speed control, open the spinning machine swing function, the round cross-pile in the roller; set the spinning machine swing amplitude of 4.5-6.5%, swing period of 10.5-13.5 s; the cooling section is divided into fast cooling section, isothermal section and air cooling section, in fast cooling section, 1# fan and 2# fan full power operation, heat shield all open, 3# fan open 20-80% power operation, 4# fan open 20-50% power operation, 5# fan, 6# fan closed, set the air distribution respectively for 12%, 23%, 15%, 15%, 23%, 12% in horizontal, roller speed is 0.55-0.62 m / s, control cooling rate is 4.6-6.2 ℃ / s, fast cooling section end temperature is 685-700 ℃, the same circle temperature difference is 10-15 ℃; in isothermal section, 7# fan open 10-30% power operation, heat shield open, set the air distribution respectively for 5%, 35%, 10%, 10%, 35%, 5% in horizontal, roller speed is 0.48-0.55 m / s, control cooling rate is 0.15-0.25 ℃ / s, isothermal section end temperature is 660-675 ℃, the same circle temperature difference is 5-8 ℃; in air cooling section, 8#-12# fan closed, heat shield all open, roller speed is 0.42-0.48 m / s, cooling rate is 1.5-2.0 ℃ / s, air cooling section end temperature is 600-615 ℃; 1# fan-12# fan air volume is 260000 m 3 / h; In step S9, the wire rod enters the collecting barrel to be uniformly stacked layer by layer, and the wire rod is packed by using segmented loading, the first segment is set to have a wire rod pressing pressure of 850-1000 MPa, the length of the wire rod is rapidly reduced to 2.0 m from the natural length after the wire rod is collected, the second segment is set to have a wire rod pressing pressure of 1800-2000 MPa, the length of the wire rod is controlled to be 1.55-1.60 m, and the third segment keeps the wire rod pressing unchanged, the pressure is reduced to 1500-1600 MPa, and the wire rod is packed after 3-5 s.

2. The method of producing large size alloy spring steel of claim 1, wherein: In step S2, the superheat of continuous casting is set to 18-22℃, the electromagnetic stirring vibration frequency is 1.8-2.3Hz, the total reduction of the strand by the straightening mill is 19-22mm, the continuous casting speed is 0.45-0.50m / min; the formed bloom has a cross-sectional size of (300-330)×(390-460)mm and a length of 5.2-5.7m. 2 , the total reduction of the strand by the straightening mill is 19-22mm, the continuous casting speed is 0.45-0.50m / min; the formed bloom has a cross-sectional size of (300-330)×(390-460)mm and a length of 5.2-5.7m.

3. The method of producing large size alloy spring steel of claim 1, wherein: In step S3, the preheating section temperature is set to 700-850 DEG C, the first heating section temperature is set to 850-1000 DEG C, the second heating section temperature is set to 1000-1100 DEG C, the soaking section temperature is set to 1180-1220 DEG C, the total heating time lasts for 450-500 min, the air-fuel ratio of the first heating section and the second heating section is 0.60-0.65, and the air-fuel ratio of the soaking section is 0.55-0.60; After the bloom leaves the heating furnace, the surface scale is removed by high pressure water, the descaling water pressure is ≥ 20 MPa, the opening rolling temperature of the continuous rolling unit is 920-950℃, the section size of the billet is 140x140mm 2 , and the length is 15.2-15.7m.

4. The method of producing large size alloy spring steel of claim 1, wherein: In step S4, the single-side grinding amount is set to 1.0-1.2 mm, and the corner grinding amount is set to 3-5 mm; the steel shot of the automatic shot blasting machine is S90 grade steel shot, and the diameter of the steel shot is 1.0-1.3 mm.

5. The method of producing large size alloy spring steel of claim 1, wherein: In step S5, the anti-decarburization coating sprayed includes, in percentage by weight, Al2O3: 15-20%, MgO: 15-20%, ZrO2: 2-7%, CaO: 20-25%, SiO2: 15-18%, and Fe2O3: 20-25%; When the head and tail are sprayed, the roller speed is set to 0.45-0.50 m / s, the upper nozzle pressure is 0.15-0.18 MPa, and the lower nozzle pressure is 0.18-0.22 MPa; when the middle part is sprayed, the roller speed is set to 0.55-0.60 m / s, the upper nozzle pressure is 0.08-0.11 MPa, and the lower nozzle pressure is 0.11-0.14 MPa.

6. The method of producing large size alloy spring steel of claim 1, wherein: In step S6, the air-fuel ratio of the heating section of the walking beam furnace is set to 1.10-1.15, and the air-fuel ratio of the soaking section is set to 1.00-1.

05.

7. The method of producing large size alloy spring steel of claim 1, wherein: In step S7, when the high-pressure descaling machine is used to remove the oxide layer of the small billets after heating, the descaling water pressure is greater than or equal to 25 MPa.

8. The method of producing large size alloy spring steel of claim 1, wherein: In step S9, the collecting temperature is set to 570-585 DEG C, the angle of the bottom baffle of the collecting barrel is controlled to be 15-30 DEG C in the forward direction, and the descending speed of the baffle is 0.2-0.35 m / s; During packing, the first section is set to move the presser plate at a speed of 0.25-0.35 m / s, and the second section is set to move the presser plate at a speed of 0.10-0.15 m / s.

9. The method of producing large size alloy spring steel according to any one of claims 1 to 8, characterized in that: The obtained wire rod has a microstructure including ferrite and sorbite, the proportion of the sorbite is ≥92%, the sorbite grain size is 8.8-10.5 μm, the sorbite lamellar spacing is 120-150 nm, the original austenite grain size is 8.0-8.5 grade, the wire rod is free of full decarburization, and the total decarburization thickness is ≤120 μm; Under the precision of 50 μm, the number of defects of the wire rod is ≤25 per coil; The tensile strength of the wire rod is 970-1020 MPa, and the reduction of area is ≥50%; Through quenching and tempering treatment, the tensile strength of the suspension spring steel wire is ≥2000 MPa, and the surface reduction is ≥48%; Under the conditions of the maximum stress of 1280 MPa and the stress amplitude of 850 MPa, the finished suspension spring prepared by coiling has a fatigue cycle number of ≥15 million times.

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