Large-specification alloy spring steel and production method thereof

Abstract

The invention relates to large-specification alloy spring steel and a production method thereof. The control process comprises the steps of steelmaking, bloom continuous casting, heating cogging, coping flaw detection, anti-decarburization segmented spraying, small square billet heating, high-speed wire controlled rolling, stelmor wire controlled cooling and coil collecting and packaging. And the surface quality and the structure property uniformity of the wire rod are effectively improved by combining segmented spraying of the anti-decarburization coating with a refined controlled rolling and controlled cooling process, and the wire rod can be used for preparing the large-size 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%. Process flow: 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 RH, argon blowing and silicon calcium core wire feeding are used for static treatment, and the static 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 soaking 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: 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); 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]; The hardenability index QE: 4.5-5.8%, QE=[C]+0.4[Si]+1.5[Mn]+3[Cr]+200[B]; The production method of the large-size alloy spring steel, comprising the following steps: Step S1, steelmaking, through molten iron pretreatment, converter smelting, LF combined RH double refining, to obtain molten steel meeting the chemical composition; Step S2, large billet continuous casting, the molten steel after smelting in step S1 is transported to the tundish, distributed from the tundish to the 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 the 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; Step S3, heating and breaking down, the large billet is heated in the heating furnace through the preheating section, the first heating section, the second heating section and the soaking section, the surface oxide is removed after the large billet exits the heating furnace by high-pressure water, and then enters the continuous rolling unit 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; 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; Step S5, after the flaw detection, the small square 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 square 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 square billet, and the tail part is a position 12 meters away from the end of the small square 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, and 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; Step S6, the small square billet is heated, the small square 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 DEG C, the heating section temperature is set to 850-1000 DEG C, and the soaking section temperature is set to 1000-1080 DEG C; the total heating time lasts for 70-90 min; Step S7, high-speed wire rolling control, after the small square billet is heated, the oxide layer is removed by using a high-pressure descaling machine, and the small square billet is rolled by using a continuous bar wire rolling mill; the rolling temperature is set to 910-940 DEG C, the entrance temperature of the finishing rolling is set to 860-880 DEG C, the entrance temperature of the reducing and sizing is set to 850-870 DEG C, and the wire drawing temperature is set to 830-850 DEG C; the circular coil is obtained, and the diameter is 18-26 mm; Step S8, Stelmor wire control cooling, the wire drawing machine is started, and the circular coil is staggered and laid on the roller way; 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, and the heat preservation cover is fully opened; the fast cooling section ends at 685-700 DEG C, and the temperature difference of the same circle is 10-15 DEG C; in the isothermal section, the No. 7 fan is operated at low load, and the heat preservation cover is opened; the isothermal section ends at 660-675 DEG C, and the temperature difference of the same circle is 5-8 DEG C; in the air cooling section, the No. 8-12 fans are turned off, and the heat preservation cover is fully opened; the air cooling section ends at 600-615 DEG C; Step S9, coiling and packaging, the coil is uniformly stacked layer by layer in the coiling barrel; the packaging is loaded in sections; the first section is set to 850-1000 MPa, the coil length is rapidly reduced to 2.0 m from the natural length after coiling, the second section is set to 1800-2000 MPa, the coil length is controlled to 1.55-1.60 m, and the third section keeps the coil pressure unchanged, the pressure is reduced to 1500-1600 MPa, and the coil is packaged after 3-5 s; 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; Further, in step S3, 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℃, the total heating time is set to 450-500min, the air-fuel ratio of the first and second heating sections is set to 0.60-0.65, and the air-fuel ratio of the soaking section is set to 0.55-0.60; After the bloom leaves the heating furnace, the surface oxide is removed by high-pressure water, the descaling water pressure is ≥20MPa, the open rolling temperature of the continuous rolling mill is 920-950℃, and the billet section size is 140×140mm 2 , and the length is 15.2-15.7m; 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; Further, in step S5, the chemical components of the sprayed anti-decarburization coating include, by weight percentage, Al2O3: 15-20%, MgO: 15-20%, ZrO2: 2-7%, CaO: 20-25%, SiO2: 15-18%, and Fe2O3: 20-25%; When spraying the head and 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; when spraying the middle part, 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; 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; Further, in step S7, when removing the oxide layer of the heated billet by using the high-pressure descaling machine, the descaling water pressure is set to ≥25MPa; Further, in step S8, the swing amplitude of the wire feeder is set to 4.5-6.5%, and the swing period is set to 10.5-13.5s; 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 volume is distributed horizontally at 12%, 23%, 15%, 15%, 23% and 12% respectively, the roller speed is 0.55-0.62 m / s, and the control cooling rate is 4.6-6.2 ℃ / s; In the isothermal section, the No. 7 fan runs at 10-30% power, the air volume is distributed horizontally at 5%, 35%, 10%, 10%, 35% and 5% respectively, the roller speed is 0.48-0.55 m / s, and the control cooling rate is 0.15-0.25 ℃ / s; In the air cooling section, the roller speed is 0.42-0.48 m / s, and the cooling rate is 1.5-2.0 ℃ / s; The air volume of the No. 1 fan to the No. 12 fan is 260,000 m 3 / h; 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° in the forward direction, and the baffle descending speed is 0.2-0.35 m / s; When packing, the first section is set to a pressure plate moving speed of 0.25-0.35 m / s, and the second section is set to a pressure plate moving speed of 0.10-0.15 m / s; Further, the obtained wire rod microstructure includes ferrite and sorbite, the sorbite proportion is ≥92%, the sorbite grain size is 8.8-10.5 μm, the sorbite lamella spacing is 120-150 nm, the original austenite grain size is 8.0-8.5 levels, the wire rod is free of full decarburization, and the total decarburization thickness is ≤120 μm; Under the precision of 50 μm requirement, the wire rod flaw defect number is ≤25 per coil; The wire rod tensile strength 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 fatigue cycle number of the finished suspension spring prepared by coiling is ≥1.5 million times.

[0009] Through the above technical scheme, compared with the prior art, the present application has the following beneficial effects: 1、The large-size alloy spring steel provided by the present application solves the problem that the core of the large-size spring steel is not easy to be quenched, and the composition is designed to ensure that the wire rod does not crack during quenching and the core of the wire rod is completely quenched; 2. The production method of the large-size alloy spring steel provided by the application, in the continuous casting process of the bloom, low superheat pouring, dynamic soft reduction and low pulling speed process are adopted to minimize the segregation of the core of the continuous casting billet; in the heating and cogging process, high-temperature diffusion heating is adopted to promote the diffusion of C in the core of the billet and further reduce segregation; 3. The production method of the large-size alloy spring steel provided by the application, in the anti-decarburization coating spraying process, according to the different decarburization tendencies of the billet in different areas of the heating furnace, the spraying process is reasonably optimized, the coating thickness of each area and each surface of the billet is controlled, the spraying effect and the protection effect are improved, and finally the full decarburization of the wire rod is realized; 4. The production method of the large-size alloy spring steel provided by the application, in the wire rod Stelmor line controlled cooling process, based on the requirement of the large-size wire rod for uniformity of the structure and performance, the wire rod laying machine swinging process and the segmented cooling process are reasonably set; through reasonable swinging of the wire rod laying machine, the lap density of the wire rod lap joint area can be effectively reduced, and the difficulty of uniform temperature control in the same circle is reduced; in the segmented cooling stage, based on the characteristics of the large-size wire rod, the horizontal air volume is reasonably distributed through the Jialing device, the temperature difference of the wire rod in the same circle is reduced, and the uniformity of the structure and performance of the wire rod is ensured; 5. The production method of the large-size alloy spring steel provided by the application, in the coiling and packaging process, through reasonable setting of the coiling temperature, the baffle angle and the descending speed, the elasticity and the layering error of the spring steel wire rod are effectively controlled, and at the same time, the reasonable setting of the descending speed can minimize the scratch of the wire rod and the coiling barrel; through segmented loading, the surface quality of the large-size wire rod is improved. BRIEF DESCRIPTION OF DRAWINGS

[0010] The application will be further described below in combination with the drawings and examples.

[0011] Figure 1 is a production process schematic diagram of the large-size alloy spring steel provided by the application. DETAILED DESCRIPTION

[0012] The application will be further described below in combination with the drawings and examples. The specific sizes used in the embodiment are only for illustrating the technical solutions and do not limit the protection scope of the application.

[0013] As set forth in the background, in the prior art of the large-size alloy spring steel, no corresponding and effective control means has been proposed for the surface decarburization, defects and uniformity of the structure and performance of the base material spring steel, so that the base material wire rod cannot meet the current requirements of the large-size spring steel wire rod.

[0014] To solve the above problems, the application provides a large-specification alloy spring steel and a production method thereof. The chemical composition is designed to make the subsequent obtained base material spring steel completely through-hardened without cracking. Then, the production method is designed to control the process, including steelmaking, bloom continuous casting, heating and blooming, flaw detection, anti-decarburization segmented spraying, small bloom heating, high-speed wire controlled rolling, Stelmor wire controlled cooling, and coiling and packaging. Especially, the anti-decarburization coating segmented spraying is combined with the refined controlled rolling and controlled cooling process, which effectively improves the wire rod surface quality and the uniformity of the microstructure and performance.

[0015] Firstly, the alloy spring steel provided by the application is for a diameter of 18 mm or more. The chemical composition includes, in terms of percentage by weight, 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).

[0016] The design of each of the above chemical elements follows the relevant principles, which are described as follows: C: C is a cheap and effective strengthening element. The cementite formed by combining C with Fe and the 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, affecting the uniformity of the performance. Therefore, C is set to 0.51-0.59%.

[0017] 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, improving the purity. However, excessive addition affects the plasticity and significantly increases the decarburization tendency. Therefore, Si is set to 1.50-1.70%.

[0018] Mn: Mn is a solid solution strengthening and hardenability improving element that can effectively improve the hardenability of large-specification wire rods and significantly improve the strength. However, excessive Mn can lead to the formation of abnormal bainite or martensite microstructure. Therefore, Mn is set to 0.65-0.80%.

[0019] Cr: Cr is a carbide-forming 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-specification wire rods and stabilize the performance of the spring. Excessive addition can cause abnormal microstructure during phase transformation. Therefore, Cr is set to 0.70-0.85%.

[0020] Cu: Cu is an impurity element for spring steel, which is easy to segregate at grain boundaries and phase boundaries and reduce the hot deformation strength, affecting the rolling and subsequent heat treatment process, so Cu≤0.03% needs to be controlled.

[0021] 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, so Al≤0.005% needs to be controlled.

[0022] V+Nb+Ti: V, Nb and Ti are effective micro-alloy strengthening elements, which form fine and dispersed particles by combining with C, significantly refine the grain and synergistically improve the strength and plasticity of the base material. However, excessive addition will strongly inhibit phase transformation and produce abnormal organization, so V+Nb+Ti is controlled at 0.12-0.18%; 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.

[0023] 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 diameter of large-size spring steel wire is large, and the core is not easy to harden. Appropriate addition of B is beneficial to obtain uniform and fine quenched structure. Excessive addition of B is prone to quenching cracking risk. Therefore, in order to ensure that the rod is completely hardened and does not crack, B is limited to 0.001-0.005%.

[0024] 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, so P≤100ppm needs to be controlled.

[0025] 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, so S≤80ppm needs to be controlled.

[0026] N: N combines with micro-alloy elements to form (C, N) composite particles, which effectively improve the strength, but high content will combine with B, affecting the improvement effect of B on hardenability, so N needs to be controlled at 50-80ppm.

[0027] O: O will combine with impurity elements to form various inclusions, reducing the purity of molten steel and affecting the fatigue performance, so O≤20ppm needs to be controlled.

[0028] H: H is an impurity element, which is easy to segregate in steel and cause hydrogen embrittlement problems for large-size high-strength alloy spring steel, so H≤1ppm needs to be controlled.

[0029] Another innovation in chemical composition design is to limit two indexes, namely the strength-plasticity index and the hardenability index, for large-size alloy spring steel. 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].

[0030] Regarding the strength-plasticity index, the higher the index, the higher the strength and the lower the plasticity, and vice versa. Therefore, according to the strength-plasticity requirements of large-size spring steel, the strength-plasticity index is limited to 2.1-2.55%. Regarding the hardenability index, the higher the index, the better the hardenability of large-size spring steel, but too good hardenability will cause cracking problems; the lower the index, the worse the hardenability, causing too large a difference in the performance of the wire core surface, which cannot meet the requirements of the suspension spring. Therefore, according to the hardenability requirements of large-size spring steel, the hardenability index is limited to 4.5-5.8%.

[0031] After the composition design, the present application further provides a production method for large-size alloy spring steel (as shown in the figure), step S1, steelmaking, through molten iron pretreatment, converter smelting, LF combined RH double refining, to obtain molten steel satisfying the chemical composition of the aforementioned large-size high-strength alloy spring steel. Figure 1

[0032] Considering that large-size wire rods are sensitive to core segregation, which will exacerbate the difference in the core-surface microstructure and performance of the wire rod, step S2, bloom continuous casting, the molten steel after step S1 smelting is delivered to a tundish, distributed from the tundish to a bloom crystallizer for continuous casting to form a bloom; through the low superheat pouring, dynamic soft reduction, and low pulling speed process in the continuous casting process, the core segregation of the continuous casting billet is minimized. Specifically, the superheat in the continuous casting process is set to 18-22°C, which is the optimal interval considering fluidity and solidification quality. Electromagnetic stirring is used in the secondary cooling area, and the electromagnetic stirring frequency is 1.8-2.3 Hz. Through the low-frequency electromagnetic field stirring of the un-solidified liquid steel in the billet, it is convenient to remove large inclusions in the molten steel and make the composition more uniform to inhibit the segregation of alloying elements; the solidification end of the billet is subjected to slight dynamic pressure by the withdrawal-straightening machine, and the total pressure reduction amount of the withdrawal-straightening machine is set to 19-22 mm, and the continuous casting speed is 0.45-0.50 m / min, to compensate for the volume shrinkage of the molten steel during solidification and make the billet inside more dense; the finally formed bloom has a cross-sectional size of (300-330) x (390-460) mm 2 , and a length of 5.2-5.7 m.

[0033] ​Step S3, heating the bloom, the bloom is heated in the heating furnace through a preheating section, a first heating section, a second heating section and a soaking section. The gradient temperature rising of preheating→sectional heating→soaking makes the temperature inside and outside the bloom uniform 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℃, and the soaking section temperature is set to 1180-1220℃. The total heating time lasts for 450-500min. The preheating section temperature is set lower when the bloom 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 bloom until most of the austenite transformation is completed, so that the plasticity of the bloom is significantly improved. Finally, the soaking section, as the last key process, completely eliminates the temperature gradient inside the bloom caused by sectional heating to avoid uneven deformation due to local temperature unevenness during rolling. At the same time, in order to achieve the optimal combustion efficiency, 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.

[0034] After the bloom leaves the heating furnace, high-pressure water is used to remove the surface scale. The descaling water pressure is ≥20MPa. Subsequently, the bloom enters the continuous rolling mill to be rolled into several billets. The open rolling temperature of the continuous rolling mill is 920-950℃ to ensure the balance between plasticity and deformation resistance. Finally, the billets with a size of 140×140mm, a length of 15.2-15.7m, a center segregation of ≤0.5 level, a center porosity of ≤0.5 level, and a center carbon segregation index of ≤1.08 are produced. 2

[0035] This step uses high-temperature diffusion heating to promote the diffusion of C in the core of the bloom, reduce segregation, and ensure the uniformity of the structure and performance of the subsequent rod.

[0036] Step S4, grinding and flaw detection, the small billets after blooming are ground, and the single-sided grinding amount is set to 1.0-1.2mm, and the corner grinding amount is set to 3-5mm. After grinding, the small billets are subjected to shot blasting operation by an automatic shot blasting machine. The shot is S90 grade steel shot with a diameter of 1.0-1.3mm. After shot blasting, the surface roughness Ra of the small billets is ≤20μm. Then the billets are subjected to magnetic particle flaw detection to ensure that the surface is free of defects.

[0037] Step S5, the small billets after grinding and flaw detection are rusted by steel wire brushes, and an automatic spraying machine is started to spray anti-decarburization coating. During the spraying process, sectional spraying is adopted. The small billets are divided into a head part, a middle part and a tail part. The head part is the position 4 meters from the end of the small billet, and the tail part is the position after 12 meters from the end of the small billet.

[0038] ​When spraying the head and tail, 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; after spraying is completed, the coating thickness on the upper surface of the billet is 1.2-1.5 mm, the coating thickness on the side surface is 1.5-1.8 mm, and the coating thickness on the lower surface is 1.8-2.1 mm.

[0039] When spraying the middle part (4-12 mm), 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; after spraying is completed, the coating thickness on the upper surface of the billet is 0.6-0.8 mm, the coating thickness on the side surface is 0.8-1.0 mm, and the coating thickness on the lower surface is 1.0-1.2 mm.

[0040] From the above results, it can be seen that the coating thickness after spraying the head and tail is greater than that of the middle part, because the billet heating adopts a regenerative heating furnace, and the burner is located at the head and tail of the billet, resulting in that the actual temperature of the head and tail during the heating process is higher than that of the middle part, and the decarburization tendency of the head and tail is greater than that of the middle part. Therefore, considering the higher decarburization requirement of large-size wire rod, a segmented control mode of the head, middle and tail is adopted, and the coating thickness of the head and tail is necessarily greater than that of the middle part, which ensures the decarburization requirement while reducing the coating cost and improving the subsequent high-pressure water descaling effect.

[0041] Of course, in the anti-decarburization coating spraying process, in order to ultimately achieve wire rod without full decarburization, the anti-decarburization coating is also crucial. The anti-decarburization coating provided by the present application preferably comprises, by weight percentage, Al2O3: 15-20%, MgO: 15-20%, ZrO2: 2-7%, CaO: 20-25%, SiO2: 15-18%, and Fe2O3: 20-25%.

[0042] Step S6, the billet is heated by a walking beam type heating furnace, the preheating section temperature is set to 750-850℃, the heating section temperature is set to 850-1000℃, the soaking section temperature is set to 1000-1080℃, 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.

[0043] Step S7, high line control rolling, after the small billet is heated, the oxide layer is removed by high pressure descaling machine, the descaling water pressure is set to be greater than or equal to 25 MPa, the iron scale on the surface of the billet 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 mill is used for controlled rolling of the small billet, the opening rolling temperature is set to be 910-940 DEG C, the precision rolling inlet temperature is set to be 860-880 DEG C, the reducing diameter inlet temperature is set to be 850-870 DEG C, the wire drawing temperature is set to be 830-850 DEG C, and the temperature gradient is set reasonably to ensure the stability of deformation in each rolling link, and finally the circular rod is obtained with a diameter of 18-26 mm.

[0044] Step S8, Stelmor line control cooling, the wire drawing machine swing function is started, the swing amplitude of the wire drawing machine is set to be 4.5-6.5%, and the swing period is set to be 10.5-13.5 s, and the circular rod is staggered and laid on the roller way. Through reasonable swing of the wire drawing machine, the lap joint density of the rod lap joint area can be effectively reduced, and the difficulty of uniform temperature control in the same circle is reduced.

[0045] In view of the problem that the temperature difference between the large specification rod lap joint point and the non-lap joint 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 organization, and make the strength of the finished rod fluctuate small.

[0046] Specifically, 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 closed, the air volume is distributed horizontally by the Jialing device, and the air volume is distributed horizontally by the Jialing device, respectively 12%, 23%, 15%, 15%, 23%, 12%, all the heat preservation covers are opened, the roller way speed is 0.55-0.62 m / s, the cooling speed is controlled to be 4.6-6.2 DEG C / s, the fast cooling section end temperature is 685-700 DEG C, and the temperature difference in the same circle is 10-15 DEG C. In the isothermal section, the No. 7 fan runs at 10-30% power, the heat preservation cover is opened, the air volume is distributed horizontally by the Jialing device, respectively 5%, 35%, 10%, 10%, 35%, 5%, the remaining fans and the heat preservation cover are closed, the roller way speed is 0.48-0.55 m / s, the cooling speed is controlled to be 0.15-0.25 DEG C / s, the isothermal section end temperature is 660-675 DEG C, and the temperature difference in the same circle is 5-8 DEG C. In the air cooling section, the No. 8-12 fans are closed, the heat preservation cover is opened, the roller way speed is 0.42-0.48 m / s, the cooling speed is 1.5-2.0 DEG C / s, and the air cooling section end temperature is 600-615 DEG C. The air volume of the No. 1 fan to the No. 12 fan is 260000 m 3 / h.

[0047] Step S9, coiling and packing, first the wire rod enters the coiling barrel for uniform layer-by-layer stacking, the coiling temperature and baffle angle are set to effectively control the spring steel wire rod elasticity and the problem of coil shape and layering. Regarding the problem of spring steel wire rod elasticity, if the temperature is too low, the elasticity is too large, and if the temperature is too high, it will affect the durability of the equipment, therefore the coiling temperature is set to 570-585°C; when the wire rod falls under the coiling barrel, it will have a certain inclination angle due to the forward and backward pulling action, therefore in order to keep the wire rod stacking angle consistent and avoid layering, a certain angle is added to the baffle for correction, preferably the baffle angle at the bottom of the coiling 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 wire rod and the coiling barrel, preferably the baffle falling speed is 0.2-0.35 m / s.

[0048] Next, the packing adopts segmented loading, the first segment sets the pressure plate pressure to 850-1000 MPa, the pressure plate moving speed to 0.25-0.35 m / s, quickly reduces the wire rod length from the natural length after coiling to 2.0 m, the second segment sets the pressure plate pressure to 1800-2000 MPa, the pressure plate moving speed to 0.10-0.15 m / s, controls the wire rod length to 1.55-1.60 m, the third segment keeps the pressure plate stationary, the pressure is reduced to 1500-1600 MPa, and after 3-5 s, the packing is carried out. The purpose of segmented loading is to increase the stacking density of the wire rod and reduce the extrusion damage between the wire rods before applying the final packing pressure, and to improve the surface quality of large-size wire rods.

[0049] The coiling and packing design of this step can reduce the problem of difficult control of the surface quality of large-size wire rods due to the large surface area of the wire rods, reduce the extrusion and scratch damage during the packing process of the wire rods, and reduce the number of flaw detection defects of large-size wire rods at the downstream customers.

[0050] Finally, through the above production method, the wire rod has a microstructure of ferrite + sorbite, the sorbite proportion 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, the wire rod has no full decarburization, and the total decarburization thickness is ≤120 μm. The sorbite proportion is determined by "YB / T 169-2014 Microstructure Determination 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".

[0051] The number of defects in the wire rod is ≤25 per coil under the precision requirement of 50 μm. 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 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”.

[0052] The finished suspension spring prepared by winding the spring has a fatigue cycle number of ≥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 Specification for Fatigue Test of Spiral Spring”.

[0053] Embodiments 1-9 are further given to verify the feasibility of the production method of the above large-size alloy spring steel.

[0054] The production processes of embodiments 1-9 are all: steelmaking→large billet continuous casting→heating and blooming→repairing and flaw detection→anti-decarburization section spraying→small billet heating→high wire controlled rolling→Stelmor wire controlled cooling→coiling and packaging.

[0055] 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.

[0056] Table 1

[0057] Table 2 is the large billet continuous casting process of the spring steels of embodiments 1-9.

[0058] Table 2

[0059] Table 3 is the continuous casting billet heating and blooming process of the spring steels of embodiments 1-9.

[0060] Table 3

[0061] Table 4 is the small billet anti-decarburization coating spraying process of the spring steels of embodiments 1-9.

[0062] Table 4

[0063] Table 5 is the controlled cooling process of the spring steels of embodiments 1-9.

[0064] Table 5

[0065] Table 6 is the spring steel wire rod coiling and packaging process of Example 1-Example 9.

[0066] Table 6

[0067] Table 7 is the spring steel wire rod organization performance and finished spring performance of Example 1-Example 9.

[0068] Table 7

[0069] From Table 7, it can be seen that by the large specification alloy spring steel and the production method thereof provided in the present application, the performance of the wire rod obtained meets the requirements of the large specification spring steel wire rod for the large self-renewable energy vehicle model.

[0070] Those skilled in the art can understand that, unless otherwise defined, all terms (including technical terms and scientific terms) used herein have the same meaning as commonly understood by those skilled in the art to which the present 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 as such.

[0071] The meaning of "and / or" described in the present application means that each single existence or both existences are included.

[0072] The meaning of "connection" described in the present application can be a direct connection between components or an indirect connection between components through other components.

[0073] Based on the above ideal embodiments according to the present application, through the above description, relevant personnel can certainly make various changes and modifications without deviating from the technical idea of the present application. The technical scope of the present application is not limited to the contents of the specification, and must be determined according to the scope of the claims.

Claims

1. 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].

2. The method of producing large size alloy spring steel of claim 1, wherein: The 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 of claim 1; 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 breaking 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 surface oxide skin is removed after the bloom leaves the heating furnace by using high-pressure water, and then the bloom is rolled into several small blooms in a continuous rolling unit; 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 breaking down is ground; the small bloom after grinding is subjected to shot blasting operation 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 part, a middle part and a tail part, the head part is a position 4 meters from the end of the small bloom, the tail part is a position 12 meters away from the end of the small bloom, 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 blank in 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 blank in 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. Step S6, billet heating, the billet coated with the anti-decarburization coating is heated by 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; Step S7, high line controlled rolling, after the oxidation layer of the heated billet is removed by a high-pressure descaling machine, the billet is controlled rolling by 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, and a circular rod with a diameter of 18-26 mm is obtained; Step S8, Stelmor line controlled cooling, the wire drawing machine is started to swing, and the circular rod 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 685-700 DEG C, and the temperature difference of the same circle is 10-15 DEG C; in the isothermal section, the No. 7 fan is operated at low load, the heat preservation cover is opened, the isothermal section ends at 660-675 DEG C, and the temperature difference of the same circle is 5-8 DEG C; 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 600-615 DEG C; Step S9, collecting and packaging, the rod is uniformly stacked layer by layer in the collecting barrel; the packaging is loaded in sections, the first section is set to 850-1000 MPa, the rod length is quickly reduced to 2.0 m from the natural length after collecting, the second section is set to 1800-2000 MPa, the rod length is controlled to 1.55-1.60 m, and the third section keeps the pressure plate still, the pressure is reduced to 1500-1600 MPa, and the rod is packaged after 3-5 s.

3. The method of producing large size alloy spring steel wire of claim 2, 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 ​ 4. The method of producing large size alloy spring steel of claim 2, 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.

5. The method of producing large size alloy spring steel of claim 2, 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 steel shot diameter is 1.0-1.3 mm.

6. The method of producing large size alloy spring steel of claim 2, wherein: In step S5, the anti-decarburization coating sprayed includes, 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%. When spraying the head and tail, 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 spraying the middle part, 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.

7. The method of producing large size alloy spring steel wire of claim 2, wherein: 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.

8. The method of producing large size alloy spring steel wire of claim 2, wherein: In step S7, when removing the oxide layer from the heated small square billet using a high-pressure descaling machine, the descaling water pressure is set to ≥25 MPa.

9. The method of producing large size alloy spring steel wire of claim 2, wherein: In step S8, the oscillation amplitude of the wire rod delivery machine is set to 4.5-6.5%, and the oscillation period is set to 10.5-13.5 s. 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 and No. 6 fans are turned off, the air flow is distributed horizontally at 12%, 23%, 15%, 15%, 23%, and 12%, respectively, and the roller speed is set to 0.55-0.62 m / s, with a controlled cooling rate of 4.6-6.2 ℃ / s. 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 set to 0.48-0.55 m / s, and the controlled cooling rate is 0.15-0.25 ℃ / s. In the air cooling section, the roller speed is 0.42-0.48 m / s, and the cooling rate is 1.5-2.0 ℃ / s. The air volume of No. 1 fan and No. 12 fan is 260000 m 3 / h.

10. The method of producing large size alloy spring steel of claim 2, wherein: In step S9, the coiling temperature is set to 570-585 ℃, and the bottom baffle angle of the coiling barrel is controlled to be 15-30° in the forward direction, with a baffle descent speed of 0.2-0.35 m / s. When packing, the first section is set to a pressure plate moving speed of 0.25-0.35 m / s, and the second section is set to a pressure plate moving speed of 0.10-0.15 m / s.

11. The method of producing large size alloy spring steel according to any one of claims 2 to 10, characterized in that: The obtained wire rod microstructure includes ferrite and sorbite, the sorbite proportion 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 has no full decarburization, and the total decarburization thickness is ≤120 μm. Under the precision requirement of 50 μm, the wire rod flaw number is ≤25 per coil. The wire rod tensile strength 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%. The finished suspension spring prepared by coiling has a fatigue cycle number of ≥150 million times under the conditions of a maximum stress of 1280 MPa and a stress amplitude of 850 MPa.

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