High-strength spring steel and method for producing the same

Abstract

The application discloses a high-strength spring steel and a preparation method thereof. The high-strength spring steel comprises the following components in percentage by mass: C: 0.50-0.65%, P: less than or equal to 0.007%, and S: less than or equal to 0.0025%. The method comprises the following process: KR hot metal pretreatment, converter smelting, LF refining, RH vacuum, large square billet continuous casting, hot charging and hot delivery, cogging and grinding, high-speed wire rolling, Stelmor air cooling, finished wire rod, quenching and tempering. The application adopts whole-process sulfur control, converter high-carbon low-phosphorus smelting, control of the S and P contents of the molten steel, converter tapping carbon powder and weak deoxidization alloying, refining process slag system change control of inclusion types, RH process degassing through vacuum degree change, inclusion removal, and reduction of erosion on refractory materials, improvement of cleanliness, continuous casting segregation control, high-temperature rolling segregation reduction, and improvement of wire rod quality. The wire rod is further quenched and tempered, and the spring steel with high strength and high fatigue life is obtained.

Description

Technical Field

[0001] The invention relates to high-strength spring steel and a preparation method thereof, belonging to the technical field of spring steel manufacturing. Background Art

[0002] Springs are essential components widely used in automobiles, machinery, railways, and other fields. As safety-bearing components, springs are subject to high-cycle alternating loads during service, and their primary mode of failure is fatigue fracture. Numerous studies have shown that impurity elements, large brittle inclusions, segregation, and surface quality are the primary causes of fatigue failure in high-strength, high-cycle fatigue springs.

[0003] Phosphorus is one of the main impurity elements that degrades the performance of spring steel. It is a harmful element in spring steel, increasing the steel's cold brittleness, reducing the material's plasticity, and worsening its cold bending properties, making it prone to brittle fracture under high-load operating conditions. Inclusions have a relatively weak bond with the matrix, especially brittle inclusions such as alumina, magnesia spinel, titanium nitride, and large-sized silicate inclusions. Under service conditions, stress concentration occurs at the interface between the inclusion and the matrix. When the local stress exceeds a threshold, a crack source forms. Subsequently, under alternating loads, the crack gradually expands, ultimately causing the spring to break. Inhomogeneous wire rod composition, segregation, or microstructure can lead to unstable spring performance and make fatigue fracture more likely during service. Surface defects such as cracks, pits, and scratches can directly lead to fracture during the spring's service life.

[0004] High-strength automotive suspension springs have very high technical requirements for impurity element content, inclusions, segregation, microstructure, decarburization, and surface quality. These factors directly impact the spring's fatigue life, and thus the overall lifespan and safety of the vehicle. To produce high-strength, high-performance spring steel, it is necessary to improve the control of impurity elements and inclusions during the production process; address uneven composition and internal and surface quality issues during the continuous casting process; and control decarburization, microstructure uniformity, and surface quality during the rolling process. This will comprehensively improve the quality and fatigue life of high-strength automotive suspension springs.

[0005] Patent 2022109583109 provides a spring steel wire rod and its production method. This process controls the content of impurity elements such as P, S, Al, and Ti, as well as gases, in the molten steel through the smelting process and certain auxiliary materials. Segregation of the ingot is controlled through continuous casting and high reduction. Combined with heating, the overall quality of the spring steel is controlled by shortening the holding time, heating at low temperatures, controlling the furnace atmosphere, and controlling surface decarburization through full grinding. Although this process controls resulfurization through KR and converters, and uses a high-basicity slag system for desulfurization during the refining and vacuum processes, the high-basicity slag system significantly increases the Al2O3 and CaO components in the molten steel, hindering the control of inclusion types. In addition, since it is silicon-manganese deoxidized steel, acidic inclusions have a certain corrosive effect on refractory materials. At the same time, the reduction corrosion of refractory oxides by C in high carbon steel under vacuum is also relatively serious. The method provided by this patent does not provide a control technology method for refractory materials such as vacuum and continuous casting, which is not conducive to the control of foreign large-sized inclusions; at the same time, the continuous casting reduction is simply divided into three sections according to the solidification center. If the reduction amount of a single section is too large, it may cause problems such as cracks inside the ingot; and the use of low holding time and low heating temperature to heat the ingot in the heating furnace is not conducive to the weakening and elimination of element diffusion and segregation; in addition, the use of wire as a strapping tape can easily cause marks and scratches on the surface of the spring coil.

[0006] Patent 2013106819801 provides a spring steel wire production process for controlling inclusions, comprising the following steps: 1) smelting using an aluminum-free deoxidation process; 2) combined top and bottom blowing in a converter; 3) argon blowing in an Ar station; 4) argon refining in a LF ladle, with a basicity target of 0.7-2.0; 5) vacuum treatment; 6) pouring into billets, which are then rolled into wire through a high-speed wire rolling process. Although this process utilizes aluminum-free deoxidation, the low-basicity slag used in this process does not effectively absorb silicate inclusions produced by Si / Mn deoxidation. Furthermore, low-basicity slags can exacerbate refractory corrosion, increasing the number of foreign inclusions introduced into the refractory, such as alumina and magnesia-alumina spinel. These inclusions pose the greatest threat to fatigue life.

[0007] Patent 2012102042739 discloses a method for controlling spring steel wire rod and its inclusions. The process involves: 1) high-carbon tapping from a converter (BOF); 2) post-BOF silicon-manganese deoxidation; 3) LF refining, with the oxygen content of the final steel controlled at 20-40 ppm; 4) continuous casting with low-superheat pouring; and 5) continuous rolling and wire rod processing. Inclusions in the steel are limited to a width of no more than 10 μm, and the aspect ratio is greater than 3. However, the process involves a high oxygen content (20-40 ppm) and an acid-soluble aluminum content in the molten steel between 0.01% and 0.02%, which can lead to an increase in non-metallic brittle inclusions in the spring steel, significantly impacting the fatigue life of the spring during service.

[0008] Patent 2016102603055 provides a smelting process for controlling inclusions in spring steel, comprising 1) primary refining in an electric furnace; 2) argon blowing; 3) refining in a LF furnace; 4) refining in a VD furnace; and 5) continuous casting. During the tapping process in the electric furnace, a Si / Mn aluminum-free deoxidation process is used for pre-deoxidation, along with the use of activated lime, a composite agent, and refined slag. While this process effectively controls brittle inclusions in the steel, the slag basicity in the later stages of refining is controlled at 0.7-1.4. Acidic slag severely corrodes the refractory materials in the ladle and continuous casting tundish, easily causing excessive foreign inclusions.

[0009] High-grade, critical-use springs have stringent requirements for inclusions, composition and microstructure uniformity, and surface quality. Therefore, controlling inclusions, composition and microstructure uniformity, and surface quality has always been a key focus in spring steel R&D. Analysis shows that inclusions need to be controlled from two perspectives: plasticization and small size. Large inclusions are primarily categorized as intrinsic and extrinsic. Intrinsic inclusions are primarily deoxidation products, while extrinsic inclusions originate from sources such as steel slag, refractory materials, and mold slag. Control measures can be categorized as reducing the generation of extrinsic inclusions and eliminating large inclusions. Composition and microstructure uniformity, as well as surface quality, are primarily controlled through the continuous casting and rolling processes. The continuous casting process achieves highly uniform ingots through process parameter settings. The ingot opening and grinding process controls the surface and subsurface quality of the ingot. High-temperature coatings, combined with high-temperature heating in the furnace and extended holding times, control surface decarburization while promoting uniform element diffusion and improving segregation and carbon lattice in the ingot. Summary of the Invention

[0010] In order to solve the above problems, the present invention discloses a high-strength spring steel and a preparation method thereof, and the specific technical solutions are as follows:

[0011] A high-strength spring steel, wherein the chemical composition of the high-strength spring steel comprises, by mass percentage, C: 0.50-0.65%, Si: 1.35-1.65%, Mn: 0.60-0.80%, Cr: 0.55-0.80%, V: 0.10-0.30%, Ni≤0.02%, Cu≤0.015%, Mo≤0.005%, P≤0.007%, S≤0.0025%, TO≤0.0010%, N≤0.0025%, H≤0.0002%, Al≤0.0015%, and Ti≤0.0008%.

[0012] Furthermore, the metallographic structure of the high-strength spring steel wire comprises, by volume percentage, troostite and pearlite, ≥95% of the total content of martensite and ferrite ≤5%. The spring prepared using the high-strength spring steel wire has a strength of ≥2000 MPa and a fatigue life of ≥1 million times.

[0013] Furthermore, the carbon content ratio of the most severely segregated region and other matrix regions on the cross section of the high-strength spring steel wire is ≤1.05, Cr content ratio is ≤1.15, and Mn content ratio is ≤1.15, in terms of mass percentage, and the hardness difference is ≤20HV.

[0014] Furthermore, the surface of the high-strength spring steel wire has no folds, the maximum surface depth is ≤25 μm, the diameter of the high-strength spring steel wire is defined as D mm, the maximum depth of its surface decarburization layer is ≤D*0.2% mm, and there is no complete decarburization layer on its surface.

[0015] Furthermore, the inclusions in the high-strength spring steel include SiO2-MnO-(CaO-Al2O3-MgO) inclusions, wherein the mass proportion of (CaO-Al2O3-MgO) is ≤25%, and the number density of inclusions larger than 5 μm is ≤0.15 / mm 2 Under the GB / T10561 standard, the ratings of Class A, B, C, and D inclusions are all ≤1.0, and the rating of Class Ds inclusions is ≤0.5.

[0016] The method for preparing high-strength spring steel is carried out according to the process flow of KR molten iron pretreatment - converter smelting - LF refining - RH vacuum - large square bloom continuous casting - hot charging and hot delivery - billet opening and grinding - high-strength wire rolling - Stelmor air cooling - finished wire rod - quenching - tempering; the production specifically includes the following steps:

[0017] Step 1: KR hot metal pretreatment: The hot metal is subjected to three slag removal processes, two deep desulfurization processes with the addition of a desulfurizer, and the final slag removal process by blowing out the slag from the bottom of the ladle to remove all slag. This results in hot metal with an S content of ≤0.001% and a slag block area ratio of ≤5% on the entire hot metal surface.

[0018] Step 2: Converter smelting: The converter is loaded with 140-150t of steel and the iron-metal ratio is 80-90%. Clean scrap steel is used for smelting. The converter adopts a three-step slag making and two-step slag pouring process. After the last slag making, the slag is not poured and the slag is retained for smelting to obtain molten steel with a phosphorus content of ≤0.006%.

[0019] Step 3: At the beginning of tapping, 10-20% of low nitrogen recarburizer and metallic manganese are added to the ladle for deoxidation and alloying. When 85% of the steel is tapped, silicon carbide, low titanium and low aluminum ferrosilicon, ferrochrome, ferrovanadium and the remaining low nitrogen recarburizer, metallic manganese are added. After all the silicon carbide, low titanium and low aluminum ferrosilicon, metallic manganese, ferrochrome, ferrovanadium and low nitrogen recarburizer are melted in the molten steel, synthetic slag is added for slagging, stirred for 3-5 minutes and then transported to the LF furnace for treatment;

[0020] Step 4: LF furnace refining: open the bottom blowing argon throughout the process, the double-permeable brick flow control ratio is 1:3-1:2, the slag basicity is controlled at 0.5-0.8 during the refining process, after the molten steel composition and temperature are all adjusted to meet the standards, add high basicity synthetic slag, adjust the slag basicity to 1.5-2.0, stir for 3-5 minutes and then tap;

[0021] Step 5: RH furnace refining: RH furnace enters the station for rapid vacuum treatment, the vacuum chamber working pressure is ≤1mbar, and the gas flow rate is increased to 150-200Nm 3 / h, processing time ≥15min, then the ladle is lowered 10-30cm, the E4 and E5 two-stage vacuum pumps are turned off, the vacuum chamber pressure is increased to above 10mbar, and the lifting gas flow rate is reduced to 100-150N m 3 / min, after circulating for ≥5min, soft stirring and calming treatment are carried out after breaking the air, the soft stirring time is 10-15min, the flow control ratio of double permeable bricks during soft stirring is 1:3-1:2, the argon flow rate of the maximum permeable brick bottom blowing is 60-90NL / min, after the soft stirring is completed, the molten steel is allowed to stand for ≥10min and then transported to the continuous casting;

[0022] Step 6: Continuously cast a large square bloom with a cross-section of 300 mm × 390 mm, with full protection during continuous casting. A low-alkalinity, low-alumina tundish covering agent is used. The tundish electromagnetic induction heating equipment is used to control the superheat of the molten steel to fluctuate by ≤5°C, and the crystallizer liquid level fluctuation is ≤2mm. The crystallizer electromagnetic stirring current is 450-650A, the frequency is 6-8Hz, the continuous casting speed is controlled at 0.5-0.7m / min, and the reduction is controlled and distributed as follows: 4th stage: 3.5-4.5mm, 5th stage: 3-4mm, 6th stage: 2.5-3.5mm, 7th stage: 2-3mm, 8th stage: 1.5-2.5mm, 9th stage: 1-2mm, and 0.5-1mm after 10th stage, to obtain a C segregation index of the slab of 0.95-1.05.

[0023] Step 7: Opening and grinding, high temperature heating, improving the segregation and surface quality of the blank;

[0024] Step 8: Rolling and air cooling, controlling cooling intensity, improving structure and performance;

[0025] Step 9: Quenching;

[0026] Step 10: Tempering.

[0027] Furthermore, the specific process of step 1 is as follows: the molten iron is transported to the KR processing station, the slag is removed for the first time, the molten iron slag brought by the blast furnace iron-making is removed, and the surface slag is removed; the second slag removal: after the first slag removal, a desulfurizer is added to the molten iron for desulfurization, and after S is removed to below 0.005%, the desulfurization slag is removed, and the slag block occupies an area ratio of ≤10% of the entire molten iron surface; the third slag removal is: after the second slag removal, the desulfurizer is continued to be added for further deep desulfurization to below 0.0010%, and then the slag is removed; during the third slag removal, the bottom blowing of the ladle is opened to drive out the slag, and the bottom blowing argon flow rate is 200-300NL / min.

[0028] Furthermore, the step 2 is specifically as follows: the converter adopts the three-slag slag retention method for smelting, and the converter adopts the large bottom blowing stirring in the early stage of blowing, and the flow rate of the large bottom blowing is 5-10Nm 3 / min, the top gun oxygen flow rate is 24000-26000Nm 3 / h, adding pellets and lime to make slag, the slag basicity is 1.8-2.2, the T.Fe content in the slag is 20-30%, the molten steel temperature is 1400-1450℃ at the end of the first stage, then the slag is poured out, and the slag is ≥70%; in the second stage of blowing, the bottom blowing flow rate is 3-6Nm 3 / min, top gun oxygen flow rate is 32000-36000Nm 3 / h, add lime, light burn, pelletize to make slag, slag basicity 3.0-4.0, T.Fe content in slag 15-25%, blow to C content ≤ 0.5%, start slag pouring, pour out slag ≥ 60%, then carry out the third slag blowing, bottom blowing flow rate is 5-10Nm 3 / min, top gun oxygen flow rate is 30000-34000Nm 3 / h, slag basicity ≥5.0, T.Fe content in slag 15-25%, after blowing, slide plate is used to block slag, steel is tapped and slag is retained for next furnace smelting.

[0029] Furthermore, in step 3: the low nitrogen recarburizer and metallic manganese are added twice, the first time is when the steel is tapped, 10-20% of the low nitrogen recarburizer and metallic manganese are added to the ladle, and the second time is when 85% of the steel is tapped, the remaining low nitrogen recarburizer and metallic manganese are added;

[0030] The order of adding silicon carbide, low titanium and low aluminum ferrosilicon, manganese metal, ferrochrome, ferrovanadium and low nitrogen recarburizer is: first add silicon carbide, then add low titanium and low aluminum ferrosilicon, manganese metal, ferrochrome, ferrovanadium and finally add carbon powder, all of which are added in sequence;

[0031] At the beginning of tapping, the bottom blowing flow rate is 800-1200NL / min. When 85% of the steel is tapped, the bottom blowing flow rate is 400-800NL / min. After the alloy and carbon powder are all melted in the molten steel, synthetic slag is added for slagging, and the bottom blowing flow rate of the ladle is reduced to 150-250NL / min.

[0032] Furthermore, in the step 4: during LF furnace refining, the maximum argon flow rate of the air-permeable brick is 400-500NL / min during charging and alloying, the maximum argon flow rate of the air-permeable brick is 300-400NL / min during the heating period, and the maximum argon flow rate of the air-permeable brick is 100-150NL / min during the rest of the time;

[0033] After the molten steel composition and temperature are all adjusted to meet the standards, the flow rate of the double bottom blowing air bricks is controlled at 50-80NL / min;

[0034] LF tapping temperature is 1560-1580℃.

[0035] Furthermore, the step 7 specifically includes: opening and grinding the continuous casting billet, heating and soaking the continuous casting billet in a billet opening heating furnace under controlled furnace atmosphere, controlling the soaking period to 250-350 minutes, and controlling the temperature of the continuous casting billet in the billet opening heating furnace to 1150-1200° C. and the rolling start temperature to 1120-1150° C., wherein the total content of H2O and O2 in the furnace atmosphere is ≤1.2% by volume;

[0036] After the billet is opened, the rolled billet is obtained, and the rolled billet is subjected to magnetic particle inspection, and then the surface is fully ground, with a grinding depth of ≥0.5mm. The locations with obvious flaws on the surface are further repaired, with an average grinding depth of ≥1.2mm.

[0037] Furthermore, the step 8 specifically includes: rolling and air cooling, spraying the rolled billet with a coating, and spraying the surface of the ground rolled billet with a high-temperature resistant coating with a coating thickness of 1.3-2.3 mm;

[0038] Under the condition of controlling the atmosphere in the furnace, the sprayed rolled billet is heated and heat-insulated in a steel rolling heating furnace, the heating rate in the adding stage is 30-50°C / min, and the temperature is kept after reaching the target temperature. The furnace time is controlled to be 140-180 minutes, and the rolled billet temperature in the steel rolling heating furnace is controlled to be 1080-1150°C, the start rolling temperature is controlled to be 940-990°C, the finishing rolling entrance temperature is 900-930°C, and the finishing rolling exit temperature is 1020-1040°C, wherein the total content of H2O and O2 in the furnace atmosphere is ≤1.2% by volume;

[0039] The rolled wire obtained in the high-speed wire rolling process is subjected to temperature-controlled air cooling, and the air-cooled wire spinning temperature is controlled at 865-885°C;

[0040] The wire rods are packaged with wide steel strapping tape, and the packaging pressure is controlled at 350-400 MPa to control the surface quality.

[0041] Furthermore, step 9 is specifically as follows: the quenching temperature is controlled at 880-920° C., the holding time is 45-65 minutes, and oil cooling is adopted.

[0042] Furthermore, step 10 is specifically as follows: controlling the tempering temperature at 380-420° C., holding time at 1.5-2.5 hours, and air cooling.

[0043] Furthermore, in the step 1, the KR arrival temperature is ≥1380°C, the molten iron S content is ≤0.035%, the C content is 4.0-4.5%, the Si content is 0.25-0.65%, the Ti content is ≤0.03%, the P content is ≤0.10%, and the rest are Fe and other unavoidable impurity elements; the exit temperature is ≥1350°C, the exit S content is ≤0.001%; ​​the desulfurizer components used for the first desulfurization of KR are: CaO content 80-90%, CaF2 content 5-10%, and the desulfurizer particle size of 1-5mm accounts for ≥90%; the desulfurizer components used for the second desulfurization are CaO content 70-80%, CaF2 content 10-20%, metal aluminum powder 5-10%, and the desulfurizer particle size of 1-5mm accounts for ≥90%.

[0044] Furthermore, the clean scrap steel used in the converter includes P≤0.015%, S≤0.0050%, Ti≤0.01%, and the rest are conventional C, Si, Al, Mn and Fe elements; after the converter blowing, the C content is ≥0.06%, the O content is ≤0.055%, the P content is ≤0.006%, the temperature is ≥1610℃, and the converter tapping capacity is 130~140t.

[0045] Furthermore, the N content of the low-carbon recarburizer added during the converter steelmaking process is ≤0.035%, and the rest is C and unavoidable impurity elements; the Mn content in metallic manganese is ≥99%, and the rest is iron and unavoidable impurity elements; the SiC content in SiC is ≥98%, and other unavoidable impurity elements; the Si content in low-titanium and low-aluminum ferrosilicon is 75-80%, the Al content is ≤0.005%, the Ti content is ≤0.002%, the P content is ≤0.01%, and the S content is ≤0.005%, and the rest is iron and unavoidable impurity elements; the Cr content in ferrochrome is 55-60%, the C content is ≤1.5%, and the P content is ≤0.015%, and the rest is iron and unavoidable impurity elements; the V content in ferrovanadium is 45-50%, the P content is ≤0.012%, and the rest is iron and unavoidable impurity elements; the synthetic slag added during the converter steelmaking process mainly comprises: CaO: 35-40%, SiO2 45-50%, MnO 3-5%, MgO ≤3%, Al2O3≤1.5%, and other inevitable impurity components.

[0046] Furthermore, the ladle used in LF refining is made of magnesia-carbon bricks with an Al2O3 content of ≤2.5%, a C content of ≤10%, a density of 2.8-3.5 g / cm3, a flexural strength of ≥38 MPa, and a porosity of ≤10%. The high-basicity synthetic slag primarily comprises: CaO 55-65%, SiO2 15-25%, CaF2 5-10%, Al2O3 ≤1.5%, MgO ≤3%, and other unavoidable impurities.

[0047] Furthermore, the refractory materials used for the immersion tube and bottom tank of the RH vacuum furnace are mainly magnesia-chromium ultra-low carbon bricks, wherein the C content is ≤1.5%, MgO: 85-95%, Cr2O3: 5-12%, Al2O3 ≤3%, and other inevitable impurity components.

[0048] Furthermore, the induction heating current of the tundish is 1500-1800A, the voltage is 1600-2000V, the superheat of the continuous casting molten steel is 20-25°C, the tonnage of the tundish is maintained at 48±1 tons, and the tonnage during the tundish change is 40-45 tons; the continuous casting is cast at a constant pulling speed, the water volume of the crystallizer is 2800±50NL / min, and the water volume of the secondary cooling section is 600±30NL / min.

[0049] Furthermore, the main components of the continuous casting low-basicity, low-alumina tundish coating include: CaO 25-35%, SiO2 45-55%, Al2O3 ≤2%, MgO 3-6%, and other inevitable components. The main components of the tundish inner wall magnesium spray coating include MgO ≥80%, CaO 5-10%, SiO2 1-3%, and other inevitable components. Particles ≤2mm account for 70-80%, 2-3mm account for more than 20%, and 3mm and above account for ≤5%. Casting is carried out using an integrated submerged nozzle with an insertion depth of 10-15mm, a mold slag layer thickness of 10-20mm, and a consumption of 0.15-0.25kg / ton. The main components of the mold slag are CaO 20-25%, SiO2 40-45%, Na2O 10-15%, MgO 3-5%, CaF2 20-25%, and other inevitable impurities.

[0050] Furthermore, the submerged nozzle and the stopper rod supporting the submerged nozzle are made of magnesium carbon, the stopper rod head has an MgO content of 80-85%, a C content of 8-12%, an Al2O3 content ≤1.5%, a SiC content of 1-4%, a SiO2 content of 2-3%, and other unavoidable impurity components, and a density of 2.4-2.7 g / cm 3, porosity ≤ 14%, flexural strength ≥ 40MPa; the inner wall thickness of the submerged nozzle is 5-7mm, the composition includes MgO content 75-80%, C content 6-10%, SiC 3-5%, SiO2 5-10%, and other unavoidable impurities, and the density is 2.4-2.6g / cm 3 , porosity ≤13%; the inner wall surface coating thickness is 0.5-1.5mm, the main components are SiO2≥95%, CaSiO3 1-3%, and other unavoidable components.

[0051] Furthermore, the rolling mill is sprayed with a high-temperature resistant coating for protection before entering the heating furnace. The main components of the coating are, by mass, CaO•SiO2: 10-20%, CaO•Al2O3: 10-20%, MgO•SiO2: 35-45%, SiO2: 15-25%, ZrO2: 3-5%, C: 1-5%, and a small amount of alkali metal oxides, inorganic binders, surfactants and other substances. The particle size of the mixture is below 180 meshes, and the coating slurry density is adjusted to 1.3-1.6 g / cm with water. 3 .

[0052] The present invention also applies for protection of: high-strength spring steel prepared by the method for preparing high-strength spring steel of the present invention.

[0053] The present invention also applies for protection of: a high-strength spring prepared from the high-strength spring steel prepared by the present invention.

[0054] Spring steel requires higher fatigue resistance, so it has very high requirements for cleanliness, segregation, decarburization, surface quality, etc. In order to obtain high-purity molten steel, highly homogenized ingots and high-quality wire rods, the present invention, based on conventional smelting technology, first, removes the S in the molten steel to an extremely low level during the molten iron smelting stage. At the same time, the converter uses high-quality scrap steel to reduce the amount of back-sulfurization, thereby comprehensively reducing desulfurization in the refining process. LF refining desulfurization mainly relies on large bottom blowing and strong stirring to promote slag-metal reaction desulfurization. This method is very likely to cause slag rolling or slag-metal reaction to change the type of inclusions. Therefore, the present invention reduces the S content to a lower level in the KR and converter, avoiding strong slag-metal reaction, so that large-sized inclusions are generated in the molten steel for easy removal. Low-Ti molten iron and clean scrap steel are selected for smelting, and at the same time, the converter uses a slide plate to block the slag, which can effectively avoid the problem of slag causing the return of Al, Ti, P, and other problems, and further avoid the precipitation of brittle inclusions such as aluminum oxide, titanium oxide, and titanium nitride during the continuous casting process.

[0055] Secondly, during the converter blowing stage, a three-stage slagging and two-stage slagging method is employed. The first stage produces a medium-low basicity slag system. Low molten iron temperatures during the early stages of converter blowing facilitate dephosphorization but hinder lime melting. Therefore, the slag basicity is reduced during the first stage, and pellets are used to increase the oxidizing properties of the slag, promoting slag melting. Extensive bottom blowing and agitation are also used to enhance dephosphorization. Simultaneously, the oxygen flow rate is reduced to slow the oxidation rate of Si and C in the molten steel, thereby slowing the temperature rise rate and facilitating dephosphorization. Maintaining a low temperature, medium basicity, and highly oxidizing slag, along with extensive bottom blowing and agitation during the first stage of converter smelting, ensures sufficient dephosphorization and effective slagging. This facilitates the discharge of large quantities of dephosphorization slag and reduces phosphorus reversion during the converter decarburization stage. The second stage of converter blowing primarily involves decarburization, creating a high-basicity slag system to prevent rephosphorization upon heating. A moderate amount of pellets is added to maintain the oxidizing properties of the slag, further minimizing rephosphorization. High-basicity, highly oxidizing slag is used to avoid rephosphorization and further remove phosphorus from the molten steel. The dephosphorized slag is then discharged. In the third stage, the phosphorus content in the molten steel is already low, so high-basicity, highly oxidizing slag is continued for further deep dephosphorization. Due to the relatively low amount of dephosphorization, slag can be retained for the next smelting furnace, reducing slag consumption for the next furnace and enhancing early slag removal.

[0056] Secondly, during high-carbon steel tapping from the converter, the oxygen content in the molten steel is kept low. Pre-deoxidation is also performed by adding a small amount of carbon powder and metallic manganese in the early stages of tapping to further reduce the oxidizability of the steel and significantly reduce the total amount of silicon oxide, aluminum oxide, and other substances formed during the deoxidation process. Incomplete deoxidation using carbon powder and metallic manganese, combined with small to medium-sized bottom blowing operations in the ladle, can prevent significant aspiration of the molten steel, which can lead to a significant increase in nitrogen content. Silicon carbide, low-titanium, low-aluminum ferrosilicon, and low-carbon recarburizers are added during the middle and later stages of deoxidation and alloying. Silicon carbide increases carbon and silicon content in the molten steel, reducing the amount of carbon powder added and shortening the aspiration issues caused by high-intensity stirring with the addition of carbon powder. This also reduces the total amount of oxide inclusions formed. The LF refining process utilizes an asymmetric agitation mode throughout the bottom blowing of the molten steel, minimizing the problem of convection in the molten steel and the resulting expansion of the stirring dead zone caused by uniform agitation. This promotes molten steel circulation, promotes temperature and composition uniformity, and facilitates the floating of inclusions. No slag is added during the converter tapping process. After the slag is added until the end of refining, small and medium bottom blowing is carried out on the molten steel throughout the process to weaken the slag-metal reaction. A low-alkalinity acidic slag system is produced during the refining process to ensure that the inclusions are SiO2-MnO low-melting-point inclusions, reducing the sources of CaO and Al2O3.

[0057] Finally, the use of ladle refractory materials, vacuum furnace refractory materials, and continuous casting components with low alumina content further reduces the source of alumina inclusions. At the same time, the quality requirements for refractory materials are improved, refractory erosion is reduced, and magnesium foreign inclusions are avoided. According to relevant research results, high-melting-point foreign inclusions such as alumina, magnesia, and magnesia-aluminum spinel are the main inclusions that cause spring fatigue fracture. In the early stage of the RH vacuum treatment process, deep vacuum and high-alkalinity slag systems are used to quickly remove acidic, low-melting-point SiO2-MnO inclusions. In the middle and late stages, a weak circulation stirring mode is used to prevent the molten steel from continuously and strongly eroding the refractory materials, allowing a small amount of complex inclusions formed by refractory erosion to float up and be removed. After breaking the air, further soft stirring and calming are performed to promote the floating of inclusions and obtain high-purity molten steel.

[0058] During the deoxidation and alloying process of the converter, high bottom blowing is used to quickly complete the deoxidation of the molten steel and achieve uniform composition. The bottom blowing of the molten steel is then quickly reduced to promote the floating of the deoxidation products while avoiding slag curling. Small and medium bottom blowing is used throughout the LF refining process. Medium bottom blowing is used during charging and alloying to promote slag melting and uniform alloy composition. All other operations adopt a small bottom blowing mode to avoid slag curling and further promote the floating and removal of inclusions. At the same time, a high-basicity slag system is produced in the later stages of LF refining. The deoxidation products of spring steel are mainly high-SiO2 oxides. Combined with the small and medium bottom blowing and stirring in the later stages of converter tapping, the residual inclusions in the molten steel still maintain a high SiO2 component when entering the refining process. The high-basicity slag system in the later stages of LF refining is conducive to the large-scale adsorption of acidic inclusions. In addition, the slag-metal reaction is relatively mild during the later stages of refining, soft stirring, and RH vacuum treatment, avoiding the increase of Al content in the molten steel due to excessive basicity and maintaining the inclusions as low-melting silicate inclusions. The continuous casting process uses high-alkalinity magnesia gunning material to absorb inclusions, and at the same time uses technologies such as electromagnetic induction heating of the tundish and electromagnetic stirring of the crystallizer to further promote the floating and removal of inclusions.

[0059] The continuous casting process uses a narrow range of stable superheat casting, with a moderate superheat temperature. This avoids low superheat causing excessive hardness at the corners of the billet, which can easily cause cracks due to reduction, and also prevents excessive segregation due to excessive superheat. The appropriate reduction process and cooling water volume are matched to ensure the proportion of liquid phase in each section. A gradually decreasing reduction mode is used to ensure that the total reduction remains unchanged, reducing center segregation while avoiding problems such as internal cracks caused by excessive reduction. Stable control of mold liquid level fluctuations can eliminate the occurrence of mold slag rolls. The weak deoxidation process for converter steelmaking and RH vacuum degassing technologies, combined with continuous casting cooling intensity control and constant drawing speed casting technology, reduce the generation of precipitates.

[0060] The heating furnace utilizes high-temperature heating and extended holding time to further promote uniform diffusion of alloying elements and reduce segregation and carbon retardation. Furthermore, full grinding technology is employed after the initial development to eliminate surface quality issues with the original ingot. High-temperature delamination is employed in the rolled ingot to mitigate surface decarburization and improve microstructure and performance uniformity. Controlled rolling and cooling techniques are utilized to achieve sorbite and pearlite structures, while reducing martensite and ferrite structures, resulting in high-quality spring steel wire rod products.

[0061] The beneficial effects of the present invention are:

[0062] (1) In the conventional process of smelting spring steel, the desulfurization task is generally mainly carried out at the LF refining station. The present invention controls the S content in the molten steel at an extremely low level in the KR and converter, effectively avoiding the change of slag rolls and inclusion types caused by large-scale desulfurization during refining, effectively reducing the size of inclusions, and accurately controlling the type of inclusions.

[0063] (2) The converter adopts the three-time slag making and two-time slag pouring method for smelting, and the slag is retained at the end. This not only ensures the acquisition of low-phosphorus molten steel, but also enables high-carbon steelmaking. It is very beneficial to control the P content of molten steel, alloy consumption, and cleanliness.

[0064] (3) The ladle, RH vacuum furnace, continuous casting refractory and auxiliary materials are all made of high-quality standard materials to reduce the erosion problems caused by high carbon, acidic slag, silicate inclusions, etc., effectively control foreign inclusions, and provide an effective control method for foreign harmful inclusions in high-quality wire.

[0065] (4) An original asymmetric flow control method for the entire process of the double bottom blowing ladle was proposed, which effectively reduced the dead zone of molten steel agitation caused by convection, greatly promoted the floating removal of inclusions, and reduced gas consumption, which has positive significance for the quality of molten steel and cost reduction.

[0066] (5) The whole process system design takes into account the control of various impurity elements and inclusions in the steel, reduces the impurity element content and gas content in the steel, and at the same time greatly reduces the size and number of endogenous and exogenous inclusions, significantly improving the cleanliness of the molten steel.

[0067] (6) Accurately design the deoxidation and alloying process of converter steelmaking, breaking the traditional process method. The converter steelmaking is designed to first use carbon powder and metallic manganese for weak deoxidation, combined with silicon carbide for strong deoxidation and carbonization, to avoid air absorption during the deoxidation and alloying process and to accurately control SiO2-MnO type low-melting point inclusions; LF refining and RH vacuum use a variable alkalinity operation process to strengthen deoxidation and promote inclusion adsorption, which not only reduces the number and size of inclusions, but also controls the type of low-melting point plastic inclusions.

[0068] (7) A precise segmented pressing process is adopted, combined with the high temperature and long-term heat preservation technology of the heating furnace to further promote the uniform diffusion of elements, and the surface full grinding and high-temperature resistant coating technology are adopted to effectively control the decarburization problem of the surface of the ingot and comprehensively improve the quality of the ingot.

[0069] The present invention adopts deep desulfurization and high carbon and low phosphorus steelmaking in converter to control the S and P content of molten steel; carbon powder and metallic manganese are added to the converter for deoxidation and alloying, and the slag system is changed during the refining process to control the type of inclusions; the RH process reduces the circulation flow by reducing the insertion depth of the immersion tube and increasing the gas flow, thereby reducing the erosion of the refractory material, strengthening the removal of inclusions, etc., and improving the cleanliness; the physical and chemical indexes of the refractory material, alloy and auxiliary materials are combined to control the foreign brittle inclusions; continuous casting adopts the technology of precise pressing according to the position of the casting machine to reduce segregation; high temperature heating, billet protection, increased heating time, etc. reduce segregation and improve the quality of the wire rod. BRIEF DESCRIPTION OF THE DRAWINGS

[0070] Figure 1 It is a flow chart of the method of the present invention. DETAILED DESCRIPTION

[0071] The present invention will be further described below in conjunction with specific embodiments. It should be understood that the following specific embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention.

[0072] The chemical composition of high-strength spring steel includes, by mass percentage: C: 0.50-0.65%, Si: 1.35-1.65%, Mn: 0.60-0.80%, Cr: 0.55-0.80%, V: 0.10-0.30%, Ni≤0.02%, Cu≤0.015%, Mo≤0.005%, P≤0.007%, S≤0.0025%, TO≤0.0010%, N≤0.0025%, H≤0.0002%, Al≤0.0015%, Ti≤0.0008%.

[0073] The metallographic structure of high-strength spring steel wire includes sorbite and pearlite, which are ≥95% by volume, and the total content of martensite and ferrite is ≤5%.

[0074] The carbon content ratio between the most severely segregated area and other matrix areas on the cross section of the high-strength spring steel wire is ≤1.05, Cr content ratio is ≤1.15, Mn content ratio is ≤1.15, and the hardness difference is ≤20HV.

[0075] The surface of the high-strength spring steel wire has no folds, the maximum surface depth is ≤25 μm, the diameter of the wire is D, the maximum depth of the surface decarburization layer is ≤D*0.2%mm, and there is no complete decarburization layer on the surface.

[0076] The number density of inclusions larger than 5μm in high-strength spring steel wire is ≤0.15 / mm 2 Under the GB / T10561 standard, the ratings of A, B, C, and D inclusions are all ≤1.0, and the rating of Ds inclusions is ≤0.5; the inclusions in the steel are mainly SiO2-MnO-(CaO-Al2O3-MgO) inclusions, among which (CaO-Al2O3-MgO) is ≤25%.

[0077] The spring steel products made from high-strength spring steel wire have a strength of ≥2000MPa and a fatigue life of ≥1 million times.

[0078] The preparation process of high-strength spring steel is as follows: the production is carried out according to the process flow of molten iron pretreatment - converter smelting - LF refining - RH vacuum - large square bloom continuous casting - hot charging and hot delivery - cogging and grinding - high-speed wire rolling - Stelmor air cooling - finished wire rod - quenching - tempering. The high-strength spring steel produced according to the inventive method has a TO content of ≤0.0010%, and the inclusions in the steel are mainly SiO2-MnO-(CaO-Al2O3-MgO) inclusions, of which (CaO-Al2O3-MgO) is ≤25%, and the number density of inclusions larger than 5μm is ≤0.15 pieces / mm 2 , and the ratings of A, B, C, and D inclusions are all ≤1.0, the rating of Ds inclusions is ≤0.5, the wire rod includes troostite and pearlite, which are ≥95% by volume, and the sum of martensite and ferrite content is ≤5%. The spring prepared using the wire rod has a strength of ≥2000MPa and a fatigue life of ≥1 million times.

[0079] The specific preparation method is:

[0080] Step 1: KR molten iron pretreatment. After the molten iron arrives at the station, it is first subjected to slag skimming treatment to remove the surface slag, and then desulfurizer is added for desulfurization. The S content is removed to below 0.005%. Slag skimming treatment is carried out, and the area ratio of slag blocks to the entire molten iron surface is ≤10%. Then desulfurizer is added for a second desulfurization, and the S content is removed to below 0.001%. Slag skimming treatment is carried out again. At the same time, the bottom blowing of the ladle is turned on to remove the slag cleanly, and the area ratio of slag blocks to the entire molten iron surface is ≤5%.

[0081] The arrival temperature of KR is ≥1380℃, the S content of molten iron is ≤0.035%, the C content is 4.0~4.5%, the Si content is 0.25~0.65%, the Ti content is ≤0.03%, the P content is ≤0.10%, and the rest are Fe and other unavoidable impurity elements; the exit temperature is ≥1350℃, the exit S content is ≤0.001%; ​​the desulfurizer components used for the first desulfurization of KR are: CaO content 80-90%, CaF2 content 5-10%, and the desulfurizer particle size of 1-5mm accounts for ≥90%; the desulfurizer components used for the second desulfurization are CaO content 70-80%, CaF2 content 10-20%, metal aluminum powder 5-10%, and the desulfurizer particle size of 1-5mm accounts for ≥90%.

[0082] Step 2: Converter smelting, the converter loading capacity is 140-150t, the molten iron ratio is 80-90%, clean scrap steel is used for smelting, the converter adopts the three-slag slag retention method, and the converter adopts large bottom blowing and stirring (5-10Nm 3 / min), top gun oxygen flow rate is 24000-26000Nm 3 / h, adding pellets and lime to make slag, the slag basicity is 1.8-2.2, the T.Fe content in the slag is 20-30%, the molten steel temperature is 1400-1450℃ at the end of the first stage, then the slag is poured out, and the slag is ≥70%; in the second stage of blowing, the bottom blowing flow rate is 3-6Nm 3 / min, top gun oxygen flow rate is 32000-36000Nm 3 / h, add lime, light burn, pelletize to make slag, slag basicity 3.0-4.0, T.Fe content in slag 15-25%, blow to C content ≤ 0.5%, start slag pouring, pour out slag ≥ 60%, then carry out the third slag blowing, bottom blowing flow rate is 5-10Nm 3 / min, top gun oxygen flow rate is 30000-34000Nm 3 / h, slag basicity ≥5.0, T.Fe content in slag 15-25%, after blowing, slide plate is used to block slag, steel is tapped and slag is retained for next furnace smelting.

[0083] The composition of clean scrap steel used in the converter includes P≤0.015%, S≤0.0050%, Ti≤0.01%, and the rest are conventional C, Si, Al, Mn and Fe elements; at the end of converter blowing, the C content is ≥0.06%, the O content is ≤0.055%, the P content is ≤0.006%, the temperature is ≥1610℃, and the converter tapping capacity is 130~140t.

[0084] Step 3: At the beginning of tapping, 10-20% of low-nitrogen recarburizer and metallic manganese are first added to the ladle for deoxidation and alloying. The bottom blowing flow rate is 800-1200NL / min. When 85% of the steel is tapped, silicon carbide, low-titanium and low-aluminum ferrosilicon, metallic manganese, ferrochrome, ferrovanadium and the remaining low-nitrogen recarburizer are added. The bottom blowing flow rate is 400-800NL / min. After the alloy and carbon powder are completely melted in the molten steel, synthetic slag is added for slagging. The bottom blowing flow rate of the ladle is reduced to 150-250NL / min. After stirring for 3-5 minutes, it is transported to LF for treatment.

[0085] The low-carbon recarburizer added during converter tapping has a nitrogen content of ≤0.035%, with the remainder being carbon and unavoidable impurities. The manganese metal has a Mn content of ≥99%, with the remainder being iron and unavoidable impurities. The SiC content in SiC is ≥98%, along with other unavoidable impurities. The low-titanium, low-aluminum ferrosilicon has a Si content of 75-80%, an Al content of ≤0.005%, a Ti content of ≤0.002%, a P content of ≤0.01%, and a S content of ≤0.005%, with the remainder being iron and unavoidable impurities. The chromium content in ferrochrome is 55-60%, a C content of ≤1.5%, and a P content of ≤0.015%, with the remainder being iron and unavoidable impurities. The vanadium content in ferrovanadium is 45-50%, a P content of ≤0.012%, with the remainder being iron and unavoidable impurities. The synthetic slag added during converter tapping mainly comprises: CaO: 35-40%, SiO2 45-50%, MnO 3-5%, MgO ≤3%, Al2O3≤1.5%, and other inevitable impurity components.

[0086] Step 4: LF furnace refining, open bottom blowing argon throughout the whole process, double air brick flow control ratio is 1:3-1:2, the maximum air brick argon flow during charging and alloying is 400-500NL / min, the maximum air brick argon flow during heating is 300-400NL / min, and the rest of the time the maximum air brick argon flow is 100~150NL / min; the slag basicity is controlled at 0.5-0.8 during the refining process. After the molten steel composition and temperature are all adjusted to meet the standards, high basicity synthetic slag is added to adjust the slag basicity to 1.5-2.0, and the double bottom blowing air brick flow is controlled at 50-80NL / min. Stir for 3-5 minutes before tapping.

[0087] The ladle used in LF refining is magnesia carbon brick, in which Al2O3 content is ≤2.5%, C content is ≤10%, and density is 2.8-3.5g / cm 3 , flexural strength ≥38MPa, porosity ≤10%. The main components of high basicity synthetic slag include: CaO 55-65%, SiO2 15-25%, CaF2 5-10%, Al2O3 ≤1.5%, MgO ≤3%, and other inevitable impurities.

[0088] Step 5: RH furnace refining, RH enters the station for rapid vacuum treatment, the working pressure of the vacuum chamber is ≤1mbar, and the gas flow rate is increased to 150-200Nm 3 / h, processing time ≥15min, then the ladle is lowered 10-30cm, the E4 and E5 two-stage vacuum pumps are turned off, the vacuum chamber pressure is increased to above 10mbar, and the gas flow rate is increased to 100-150N m 3 / min, after circulating for ≥5min, break the air for soft stirring and calming treatment, the soft stirring time is 10-15min, the flow control ratio of double permeable bricks during soft stirring is 1:3-1:2, the maximum permeable brick bottom blowing argon flow rate is 60-90NL / min, after the soft stirring is completed, let the molten steel stand for ≥10min and then transport it to continuous casting.

[0089] The refractory materials used for the immersion tube and bottom tank of the RH vacuum furnace are mainly magnesia-chrome ultra-low carbon bricks, with C content ≤1.5%, MgO: 85-95%, Cr2O3: 5-12%, Al2O3 ≤3%, and other inevitable impurity components.

[0090] Step 6: Use large square blooms for continuous casting, with a cross-section of 300mm×390mm, and protect the casting throughout the continuous casting process. Use a low-alkalinity, low-alumina tundish covering agent, and use the tundish electromagnetic induction heating equipment to control the superheat of the molten steel in the tundish to fluctuate ≤5°C. The tundish induction heating current is 1500-1800A, and the voltage is 1600-2000V. The superheat of the continuous casting molten steel is 20-25°C. The tundish tonnage is maintained at 48±1 tons, and the tonnage during the tundish change is 40-45 tons.

[0091] The water volume of the crystallizer is 2800±50NL / min, the water volume of the secondary cooling section is 600±30NL / min, the liquid level fluctuation of the crystallizer is ≤2mm, the electromagnetic stirring current of the crystallizer is 450-650A, the frequency is 6-8Hz, the continuous casting is cast at a constant pulling speed, the continuous casting pulling speed is controlled at 0.5-0.7m / min, the reduction amount is controlled and distributed, 4th section: 3.5-4.5mm, 5th section: 3~4mm, 6th section: 2.5-3.5mm, 7th section: 2-3, 8th section: 1.5-2.5mm, 9th section: 1-2mm, and 0.5-1mm after 10th section, and the C segregation index of the ingot is 0.95-1.05.

[0092] The main components of the low-basicity, low-alumina tundish coating for continuous casting include: CaO 25-35%, SiO2 45-55%, Al2O3 ≤2%, MgO 3-6%, and other unavoidable components. The main components of the magnesium spray coating for the tundish inner wall include MgO ≥80%, CaO 5-10%, SiO2 1-3%, and other unavoidable components. Particles ≤2mm account for 70-80%, 2-3mm account for more than 20%, and 3mm and above account for ≤5%. Casting is performed using an integrated submerged nozzle with an insertion depth of 10-15mm. The mold slag layer thickness is 10-20mm, and the mold slag consumption is 0.15-0.25kg / ton. The main components of the mold slag are CaO 20-25%, SiO2 40-45%, Na2O 10-15%, MgO 3-5%, CaF2 20-25%, and other unavoidable impurities.

[0093] The materials of the stopper rod and nozzle are mainly magnesia carbon. The MgO content of the stopper rod head is 80-85%, the C content is 8-12%, the Al2O3 content is ≤1.5%, the SiC content is 1-4%, the SiO2 content is 2-3%, and other inevitable impurities. The density is 2.4-2.7g / cm 3 , porosity ≤ 14%, flexural strength ≥ 40MPa; the inner wall thickness of the submerged nozzle is 5-7mm, the composition includes MgO content 75-80%, C content 6-10%, SiC 3-5%, SiO2 5-10%, and other unavoidable impurities, and the density is 2.4-2.6g / cm 3 , porosity ≤13%; inner wall surface coating thickness 0.5-1.5mm, main components are SiO2 ≥95%, CaSiO3 1-3%, and other unavoidable components.

[0094] Step 7: Opening and grinding the continuous casting billet. Under the condition of controlling the atmosphere in the furnace, the continuous casting billet is heated and soaked in the billet heating furnace. The soaking period is controlled to be 250-350 minutes. The temperature of the continuous casting billet in the billet heating furnace is controlled to be 1150-1200°C, and the rolling temperature is controlled to be 1120-1150°C. The total content of H2O and O2 in the atmosphere in the furnace is ≤1.2% by volume.

[0095] After the billet is opened, the rolled billet is obtained, and the rolled billet is subjected to magnetic particle inspection, and then the surface is fully ground, with a grinding depth of ≥0.5mm. The locations with obvious flaws on the surface are further repaired, with an average grinding depth of ≥1.2mm.

[0096] Step 8: rolling and air cooling, spray coating on the billet, spray high temperature resistant coating on the surface of the ground billet, with a coating thickness of 1.3-2.3mm;

[0097] Under the condition of controlling the atmosphere in the furnace, the sprayed rolled billet is heated and heat-insulated in a steel rolling heating furnace, with a heating rate of 30-50°C / min in the adding section. After reaching the target temperature, the billet is kept warm, and the furnace time is controlled at 140-180 minutes. The rolled billet temperature in the steel rolling heating furnace is controlled at 1080-1150°C, the start rolling temperature is controlled at 940-990°C, the finishing rolling inlet temperature is 900-930°C, and the finishing rolling outlet temperature is 1020-1040°C. Among them, in the atmosphere components in the furnace, the total content of H2O and O2 is ≤1.2% by volume.

[0098] Before the rolling mill enters the heating furnace, the high-temperature resistant coating is sprayed for protection. The main components of the coating are CaO•SiO2: 10-20%, CaO•Al2O3: 10-20%, MgO•SiO2: 35-45%, SiO2: 15-25%, ZrO2: 3-5%, C: 1-5%, and a small amount of alkali metal oxides, inorganic binders, surfactants and other substances. The particle size of the mixture is below 180 meshes, and the coating slurry density is adjusted to 1.3-1.6g / cm with water. 3 .

[0099] The rolled wire obtained in the high-speed wire rolling process is subjected to temperature-controlled air cooling, and the air-cooled wire spinning temperature is controlled at 865-885°C.

[0100] The wire rods are packaged with wide steel strapping tape, and the packaging pressure is controlled at 350-400 MPa to control the surface quality.

[0101] Step 9: Quenching temperature is controlled at 880-920℃, holding time is 45-65 minutes, and oil cooling is used.

[0102] Step 10: Control the tempering temperature at 380-420℃, keep warm for 1.5-2.5 hours, and air cool. Example

[0103] The preparation process of high-strength spring steel: production is carried out according to the process flow of molten iron pretreatment - converter smelting - LF refining - RH vacuum - large square bloom continuous casting - hot charging and hot delivery - billet opening and grinding - high-strength wire rolling - Stelmor air cooling - finished wire rod - quenching - tempering.

[0104] Step 1: KR hot metal pretreatment

[0105] After the molten iron arrives at the KR processing station, the initial slag from the blast furnace is removed. A desulfurizer is then added for desulfurization. The desulfurizer used in the first KR desulfurization step consists of 80-90% CaO, 5-10% CaF2, and ≥90% 1-5mm particle size. The sulfur content of the molten iron is reduced to below 0.005%, and the desulfurization slag is then removed. The slag remains at ≤10% of the molten iron surface area. A second desulfurization step is then added. The desulfurizer used in the second desulfurization step consists of 70-80% CaO, 10-20% CaF2, 5-10% aluminum powder, and ≥90% 1-5mm particle size. Deep desulfurization is then performed to reduce the sulfur content to below 0.0010%, followed by deslagging. During this process, ladle bottom blowing is used to remove the slag. KR desulfurization technology transfers the molten iron to the converter for blowing.

[0106] Table 1 Main parameters of molten iron arriving at KR station

[0107]

[0108] Table 2 Main parameters of KR desulfurization treatment

[0109]

[0110] Step 2: Converter smelting

[0111] After the converter is charged with desulfurized molten iron and clean scrap, blowing begins. The converter utilizes a three-stage, two-stage slag-retention method. During the initial blowing phase, high bottom blowing and stirring are employed, while the top lance oxygen flow rate is appropriately reduced. Pellets and lime are added to create slag. The basicity, oxidizability, and molten steel temperature of the slag are controlled at the end of the first stage of blowing. Slag is then drained, with the discharged slag reaching ≥70%. The second stage of blowing then begins, with the bottom blowing flow rate appropriately reduced and the top lance oxygen flow rate increased. Lime, light calcination, and pellets are added to create slag, controlling the basicity and oxidizability of the second-stage slag. When the carbon content is ≤0.5%, slag draining begins, with the discharged slag reaching ≥60%. The third stage of slag-making and blowing is then conducted, with the bottom blowing flow rate increased again and the top lance oxygen flow rate appropriately reduced to control the basicity and oxidizability of the third-stage slag. At the end of the converter blowing phase, a slide plate is used to block the slag. The tapping rate is 130-140 tons, with the slag retained for the next batch of smelting.

[0112] The components of clean scrap steel used in the converter include P≤0.015%, S≤0.0050%, Ti≤0.01%, and the rest are conventional C, Si, Al, Mn and Fe elements.

[0113] Table 3 Main parameters of converter smelting

[0114]

[0115] Table 4 Key indicators of converter smelting process

[0116]

[0117] Step 3: Tapping Steel from Converter

[0118] At the start of tapping from the converter, high bottom blowing flow is used to agitate the ladle, and 10-20% of low-nitrogen recarburizer and metallic manganese are added. When 85% of the steel has been tapped, moderate bottom blowing flow is used to agitate the ladle, and the remaining alloy and low-nitrogen recarburizer are added. Silicon carbide, low-titanium, low-aluminum ferrosilicon, metallic manganese, ferrochrome, ferrovanadium, and low-nitrogen recarburizer are added in the following order: silicon carbide first, followed by low-titanium, low-aluminum ferrosilicon, metallic manganese, ferrochrome, ferrovanadium, and finally carbon powder, all added sequentially. After the alloy and carbon powder have been completely dissolved in the molten steel, synthetic slag is added for slagging, and the bottom blowing flow and agitation are further reduced. The molten steel is then transported to LF refining for further processing.

[0119] The low-carbon recarburizer added during converter tapping has a nitrogen content of ≤0.035%, with the remainder being carbon and unavoidable impurities. The manganese metal has a Mn content of ≥99%, with the remainder being iron and unavoidable impurities. The SiC content in SiC is ≥98%, along with other unavoidable impurities. The low-titanium, low-aluminum ferrosilicon has a Si content of 75-80%, an Al content of ≤0.005%, a Ti content of ≤0.002%, a P content of ≤0.01%, and a S content of ≤0.005%, with the remainder being iron and unavoidable impurities. The chromium content in ferrochrome is 55-60%, a C content of ≤1.5%, and a P content of ≤0.015%, with the remainder being iron and unavoidable impurities. The vanadium content in ferrovanadium is 45-50%, a P content of ≤0.012%, with the remainder being iron and unavoidable impurities. The synthetic slag added during converter tapping mainly comprises: CaO: 35-40%, SiO2 45-50%, MnO 3-5%, MgO ≤3%, Al2O3≤1.5%, and other inevitable impurity components.

[0120] Table 5 Bottom blowing flow rate during converter tapping

[0121]

[0122] Step 4: LF Refining

[0123] Bottom argon blowing is used throughout the LF refining process, with the flow rate of the double-permeable bricks controlled at a ratio of 1:3-1:2. The slag basicity is controlled at 0.5-0.8 during the refining process. After the molten steel composition and temperature are adjusted to meet the standards, high-basicity synthetic slag is added to adjust the slag basicity to 1.5-2.0. After stirring for 3-5 minutes, the steel is tapped and transported to the RH for processing. The bottom blowing flow rate during the LF refining process varies depending on the operating conditions, such as alloying, heating, slagging, and waiting.

[0124] The ladle used in LF refining is magnesia carbon brick, in which Al2O3 content is ≤2.5%, C content is ≤10%, and density is 2.8-3.5g / cm 3 , flexural strength ≥38MPa, porosity ≤10%. The main components of high basicity synthetic slag include: CaO 55-65%, SiO2 15-25%, CaF2 5-10%, Al2O3 ≤1.5%, MgO ≤3%, and other inevitable impurities.

[0125] Table 6 Main parameters of LF refining process

[0126]

[0127] Table 7 Bottom blowing control during LF refining process

[0128]

[0129] Step 5: RH Vacuum Treatment

[0130] After the molten steel is transported to the processing station, the RH is quickly vacuumed to reduce the working pressure of the vacuum chamber to the minimum, and the lifting gas flow is increased for circulating degassing. After a certain period of degassing, the ladle is lowered by 10-30cm, and the E4 and E5 two-stage vacuum pumps are closed, the vacuum chamber pressure is increased, and the lifting gas flow is reduced. After the circulation treatment, the air is broken for soft stirring and calming treatment, and then it is transported to the continuous casting and pouring.

[0131] The refractory materials used for the immersion tube and bottom tank of the RH vacuum furnace are mainly magnesia-chrome ultra-low carbon bricks, with C content ≤1.5%, MgO: 85-95%, Cr2O3: 5-12%, Al2O3 ≤3%, and other inevitable impurity components.

[0132] Table 8 Main process parameters of RH treatment process

[0133]

[0134] Table 9 Bottom blowing control during RH treatment

[0135]

[0136] Step 6: Bloom Continuous Casting

[0137] After RH treatment, the molten steel is transported to the continuous casting process, where it is cast in large square blooms with a cross-section of 300mm x 390mm. Protective casting is applied throughout the continuous casting process. A low-alkalinity, low-alumina tundish covering agent is used, and electromagnetic induction heating equipment is used to control the superheat fluctuation of the tundish steel to ≤5°C, maintaining a stable weight during ladle changes and continuous casting. The crystallizer utilizes electromagnetic stirring to control cooling intensity. Casting is performed at a constant casting speed. The reduction during continuous casting is controlled and distributed according to the following parameters: 4th stage: 3.5-4.5mm, 5th stage: 3-4mm, 6th stage: 2.5-3.5mm, 7th stage: 2-3mm, 8th stage: 1.5-2.5mm, 9th stage: 1-2mm, and 10th stage and beyond: 0.5-1mm. The resulting C segregation index for the cast bloom is 0.95-1.05.

[0138] Table 10 Main process parameters of continuous casting tundish

[0139]

[0140] Table 11 Main process parameters of continuous casting mold

[0141]

[0142] The main components of the low-basicity, low-alumina tundish coating for continuous casting include: CaO 25-35%, SiO2 45-55%, Al2O3 ≤2%, MgO 3-6%, and other unavoidable components. The main components of the magnesium spray coating for the tundish inner wall include MgO ≥80%, CaO 5-10%, SiO2 1-3%, and other unavoidable components. Particles ≤2mm account for 70-80%, 2-3mm account for more than 20%, and 3mm and above account for ≤5%. Casting is performed using an integrated submerged nozzle with an insertion depth of 10-15mm. The mold slag layer thickness is 10-20mm, and the mold slag consumption is 0.15-0.25kg / ton. The main components of the mold slag are CaO 20-25%, SiO2 40-45%, Na2O 10-15%, MgO 3-5%, CaF2 20-25%, and other unavoidable impurities.

[0143] The stopper rod and nozzle used in continuous casting are mainly made of magnesia carbon. The MgO content of the stopper rod head is 80-85%, the C content is 8-12%, the Al2O3 content is ≤1.5%, the SiC content is 1-4%, the SiO2 content is 2-3%, and other inevitable impurities. The density is 2.4-2.7g / cm 3 , porosity ≤ 14%, flexural strength ≥ 40MPa; the inner wall thickness of the submerged nozzle is 5-7mm, the composition includes MgO content 75-80%, C content 6-10%, SiC 3-5%, SiO2 5-10%, and other unavoidable impurities, and the density is 2.4-2.6g / cm3 , porosity ≤13%; inner wall surface coating thickness 0.5-1.5mm, main components are SiO2 ≥95%, CaSiO3 1-3%, and other unavoidable components.

[0144] Step 7: Opening and grinding

[0145] The large square billet produced in the above production process is then subjected to blooming and rolling. The continuously cast billet is heated and soaked in a blooming furnace under controlled furnace atmosphere, with key process parameters such as billet temperature, holding time, start rolling temperature, and furnace atmosphere controlled. After blooming, the rolled billet is subjected to magnetic particle inspection and then fully surface-grinded. Any obvious surface defects detected are then spot-repaired to ensure a defect-free surface.

[0146] Table 12 Main process parameters of blank opening and grinding

[0147]

[0148] Step 8: Rolling and air cooling

[0149] The rolled billet obtained after blanking and grinding is sprayed with a high-temperature resistant coating and then transferred to a heating furnace for heating. The sprayed billet is heated and soaked in a steel rolling furnace under controlled furnace atmosphere, with key process parameters such as heating rate, furnace time, billet temperature, start rolling temperature, finish rolling temperature, and furnace atmosphere controlled. The rolled wire obtained in the high-speed wire rolling process is cooled with controlled air cooling. The cooled wire rod is then bundled with wide steel strapping tape, with a pressure controlled at 350-400 MPa to control surface quality.

[0150] Before the rolling mill enters the heating furnace, the high-temperature resistant coating is sprayed for protection. The main components of the coating are CaO•SiO2: 10-20%, CaO•Al2O3: 10-20%, MgO•SiO2: 35-45%, SiO2: 15-25%, ZrO2: 3-5%, C: 1-5%, and a small amount of alkali metal oxides, inorganic binders, surfactants and other substances. The particle size of the mixture is below 180 meshes, and the coating slurry density is adjusted to 1.3-1.6g / cm with water. 3 .

[0151] Table 13 Main process parameters of rolling and air cooling

[0152]

[0153] Step 9: Heat Treatment Process

[0154] The obtained high-quality spring steel wire rod is heat-treated to produce high-strength, high-quality spring steel products.

[0155] Table 14 Main process parameters of heat treatment

[0156]

[0157] The composition of molten steel obtained by KR hot metal pretreatment, converter smelting, LF refining, and RH vacuum treatment smelting process is as follows:

[0158] Table 15 Main chemical composition of high-strength spring steel wire rod

[0159]

[0160] Table 16 Impurity element composition in high-strength spring steel wire rod

[0161]

[0162] The metallographic structure of the wire rod obtained by the continuous casting, blanking and rolling process of the present invention is calculated by volume percentage; the area with the most serious segregation and other matrix areas on the cross section of the high-strength spring steel wire are calculated by mass percentage; the area with the most serious segregation and other matrix areas on the cross section of the high-strength spring steel wire are calculated by mass percentage;

[0163] Table 17 Measurement results of microstructure and element indexes of high-strength spring steel wire rod

[0164]

[0165] The high-strength spring steel wire rod has no folds on its surface. The wire rod has a diameter of D, a maximum depth of a surface decarburization layer ≤ D*0.2%mm, and no complete decarburization layer. The high-strength spring steel wire rod contains few large-sized inclusions and brittle, high-melting-point inclusions. Under the GB / T10561 standard, the A, B, C, and D inclusion ratings are all ≤1.0, and the Ds inclusion rating is ≤0.5. The steel primarily contains SiO2-MnO-(CaO-Al2O3-MgO) inclusions, with the (CaO-Al2O3-MgO) content ≤25%.

[0166] The springs produced by heat treating the high-purity and high-quality wire rods have high tensile strength and long fatigue life.

[0167] Table 18 Wire rod surface defects and inclusions inspection results and prepared spring properties

[0168]

[0169] The technical means disclosed in the solution of the present invention are not limited to the technical means disclosed in the above technical means, but also include technical solutions composed of any combination of the above technical features.

[0170] With the above-described preferred embodiments of the present invention as a guide, and with reference to the above description, relevant personnel are fully capable of making various changes and modifications without departing from the technical scope of this invention. The technical scope of this invention is not limited to the contents of the specification and must be determined according to the scope of the claims.

Claims

1. A method for preparing high-strength spring steel, characterized in that: The chemical composition of the high-strength spring steel is as follows by mass percentage: C: 0.50-0.65%, Si: 1.35-1.65%, Mn: 0.60-0.80%, Cr: 0.55-0.80%, V: 0.10-0.30%, Ni≤0.02%, Cu≤0.015%, Mo≤0.005%, P≤0.007%, S≤0.0025%, TO≤0.0010%, N≤0.0025%, H≤0.0002%, Al≤0.0015%, Ti≤0.0008%, and the balance is iron and unavoidable impurities; the production is carried out according to the process flow of KR molten iron pretreatment - converter smelting - LF refining - RH vacuum - large square bloom continuous casting - hot charging and hot delivery - cogging and grinding - high-speed wire rolling - Stelmor air cooling - finished wire rod - quenching - tempering; and the production specifically comprises the following steps: Step 1: KR hot metal pretreatment: The hot metal is subjected to three slag removal processes, two deep desulfurization processes with the addition of a desulfurizer, and the final slag removal process by blowing out the slag from the bottom of the ladle to remove all slag. This results in hot metal with an S content of ≤0.001% and a slag block area ratio of ≤5% on the entire hot metal surface. The desulfurizer components used in the first desulfurization of KR are as follows: CaO content 80-90%, CaF2 content 5-10%, and desulfurizer particle size 1-5mm accounts for ≥90%; The desulfurizer components used in the second desulfurization are CaO content 70-80%, CaF2 content 10-20%, metal aluminum powder 5-10%, and the desulfurizer particle size 1-5mm accounts for ≥90%; The main components of synthetic slag added to converter tapping include: CaO: 35-40%, SiO2 45-50%, MnO 3-5%, MgO ≤3%, Al2O3 ≤1.5%, and other inevitable impurities; Step 2: Converter smelting: The converter is loaded with 140-150t of steel and the iron-metal ratio is 80-90%. Clean scrap steel is used for smelting. The converter adopts a three-step slag making and two-step slag pouring process. After the last slag making, the slag is not poured and the slag is retained for smelting to obtain molten steel with a phosphorus content of ≤0.006%. Specifically: the converter adopts the three-slag slag retention method for smelting, and the converter adopts large bottom blowing and stirring in the early stage of blowing, and the flow rate of large bottom blowing is 5-10Nm 3 / min, the top gun oxygen flow rate is 24000-26000Nm 3 / h, adding pellets and lime to make slag, the slag basicity is 1.8-2.2, the T.Fe content in the slag is 20-30%, the temperature of the molten steel is 1400-1450℃ at the end of the first stage, and then the slag is poured out, and the slag discharged is ≥70%; In the second stage of blowing, the bottom blowing flow rate is 3-6Nm 3 / min, top gun oxygen flow rate is 32000-36000Nm 3 / h, add lime, light burn, pelletize to make slag, slag basicity is 3.0-4.0, T.Fe content in slag is 15-25%, when blowing to C content ≤ 0.5%, start slag pouring, pour out slag ≥ 60%; Then the third slag blowing is carried out, and the bottom blowing flow rate is 5-10Nm 3 / min, top gun oxygen flow rate is 30000-34000Nm 3 / h, slag basicity ≥5.0, T.Fe content in slag 15-25%, after blowing, slide plate is used to block slag, steel is tapped and slag is retained for next smelting; Step 3: At the beginning of tapping, 10-20% of low nitrogen recarburizer and metallic manganese are added to the ladle for deoxidation and alloying. When 85% of the steel is tapped, silicon carbide, low titanium and low aluminum ferrosilicon, ferrochrome, ferrovanadium and the remaining low nitrogen recarburizer, metallic manganese are added. After all the silicon carbide, low titanium and low aluminum ferrosilicon, metallic manganese, ferrochrome, ferrovanadium and low nitrogen recarburizer are melted in the molten steel, synthetic slag is added for slagging, stirred for 3-5 minutes and then transported to the LF furnace for treatment; Step 4: LF furnace refining: open the bottom blowing argon throughout the process, the double-permeable brick flow control ratio is 1:3-1:2, the slag basicity is controlled at 0.5-0.8 during the refining process, after the molten steel composition and temperature are all adjusted to meet the standards, add high basicity synthetic slag, adjust the slag basicity to 1.5-2.0, stir for 3-5 minutes and then tap; During LF furnace refining, the maximum argon flow rate of the gas brick is 400-500NL / min during charging and alloying, 300-400NL / min during the heating period, and 100-150NL / min at other times. After the molten steel composition and temperature are all adjusted to meet the standards, the flow rate of the double bottom blowing air bricks is controlled at 50-80NL / min; LF tapping temperature 1560-1580℃; The composition of high basicity synthetic slag includes: CaO 55-65%, SiO2 15-25%, CaF2 5-10%, Al2O3≤1.5%, MgO≤3%, and other inevitable impurities; Step 5: RH furnace refining: RH furnace enters the station for rapid vacuum treatment, the vacuum chamber working pressure is ≤1mbar, and the gas flow rate is increased to 150-200Nm 3 / h, processing time ≥15min, then the ladle is lowered 10-30cm, the E4 and E5 two-stage vacuum pumps are turned off, the vacuum chamber pressure is increased to above 10mbar, and the lifting gas flow rate is reduced to 100-150N m 3 / min, after circulating for ≥5min, break the air; After RH breaks through the air, soft stirring and calming treatment are carried out. The soft stirring time is 10-15 minutes. During soft stirring, the flow rate control ratio of double-permeable bricks is 1:3-1:2, and the argon flow rate of the bottom of the maximum permeable brick is 60-90NL / min. After the soft stirring is completed, the molten steel is allowed to stand for ≥10 minutes before being transported to continuous casting; Step 6: Continuous casting of large square blooms, with a cross-section of 300 mm × 390 mm, with full protection during continuous casting. A low-alkalinity, low-alumina tundish covering agent was used. The tundish electromagnetic induction heating equipment was used to control the superheat of the molten steel to fluctuate by ≤5°C, and the crystallizer liquid level fluctuation was ≤2mm. The crystallizer electromagnetic stirring current was 450-650A, the frequency was 6-8Hz, the continuous casting speed was controlled at 0.5-0.7m / min, and the reduction was controlled and distributed as follows: 4th stage: 3.5-4.5mm, 5th stage: 3-4mm, 6th stage: 2.5-3.5mm, 7th stage: 2-3mm, 8th stage: 1.5-2.5mm, 9th stage: 1-2mm, and 0.5-1mm after 10th stage. The resulting C segregation index of the cast bloom was 0.95-1.

05. Step 7: Opening and grinding, high temperature heating, improving the segregation and surface quality of the blank; Step 8: rolling and air cooling, controlling the cooling intensity, improving the wire rod structure and performance; Rolling billet spray coating: spray the surface of the ground rolling billet with a high-temperature resistant coating with a coating thickness of 1.3-2.3mm; Under the condition of controlling the atmosphere in the furnace, the sprayed billet is heated and heat-insulated in a steel rolling heating furnace, the heating rate in the adding stage is 30-50°C / min, and the temperature is kept after reaching the target temperature. The furnace time is controlled to be 140-180 minutes, and the billet temperature in the steel rolling heating furnace is controlled to be 1080-1150°C, the start rolling temperature is controlled to be 940-990°C, the finishing rolling entrance temperature is 900-930°C, and the finishing rolling exit temperature is 1020-1040°C. In the furnace atmosphere composition, the total content of H2O and O2 is ≤1.2% by volume; The rolled wire obtained in the high-speed wire rolling process is cooled by controlled air cooling, and the spinning temperature of the air-cooled wire is controlled at 865-885℃; The wire rods are packaged with wide steel strapping tape, and the packaging pressure is controlled at 350-400 MPa to control the surface quality; Step 9: Quenching; Step 10: Tempering.

2. The method for preparing high-strength spring steel according to claim 1, characterized in that: The specific process of step 1 is as follows: the molten iron is transported to the KR processing station, and the slag is removed for the first time, that is, the molten iron slag brought by the blast furnace is removed, and the surface slag is removed; The second slag skimming is as follows: after the first slag skimming, desulfurizer is added to the molten iron for desulfurization. After the sulfur content is reduced to below 0.005%, the desulfurization slag is skimmed off. The area ratio of the slag block to the entire molten iron surface is ≤10%; The third slag removal is: after the second slag removal, continue to add desulfurizer to further desulfurize to below 0.0010%, and then remove the slag again; during the third slag removal, open the iron ladle bottom blowing to remove the slag, and the bottom blowing argon flow rate is 200-300NL / min.

3. The method for preparing high-strength spring steel according to claim 1, characterized in that: In step 3, the low nitrogen recarburizer and metallic manganese are added twice, the first time is when the steel is tapped, 10-20% of the low nitrogen recarburizer and metallic manganese are added to the ladle, and the second time is when 85% of the steel is tapped, the remaining low nitrogen recarburizer and metallic manganese are added; The order of adding silicon carbide, low titanium and low aluminum ferrosilicon, manganese metal, ferrochrome, ferrovanadium and low nitrogen recarburizer is: first add silicon carbide, then add low titanium and low aluminum ferrosilicon, manganese metal, ferrochrome, ferrovanadium and finally add carbon powder, all of which are added in sequence; At the beginning of tapping, the bottom blowing flow rate is 800-1200NL / min. When 85% of the steel is tapped, the bottom blowing flow rate is 400-800NL / min. After the alloy and carbon powder are all melted in the molten steel, synthetic slag is added for slagging, and the bottom blowing flow rate of the ladle is reduced to 150-250NL / min.

4. The method for preparing high-strength spring steel according to claim 1, characterized in that: The step 7 specifically comprises: opening and grinding the continuous casting billet, heating and soaking the continuous casting billet in a billet opening heating furnace under controlled furnace atmosphere, controlling the soaking period to be 250-350 minutes, and controlling the temperature of the continuous casting billet in the billet opening heating furnace to be 1150-1200° C. and the rolling start temperature to be 1120-1150° C., wherein the total content of H2O and O2 in the furnace atmosphere is ≤1.2% by volume; After the billet is opened, the rolled billet is obtained, and the rolled billet is subjected to magnetic particle inspection, and then the surface is fully ground, with a grinding depth of ≥0.5mm. The locations with obvious flaws on the surface are further repaired, with an average grinding depth of ≥1.2mm.

5. The method for preparing high-strength spring steel according to claim 1, characterized in that: The step 9 specifically includes: controlling the quenching temperature to 880-920° C., holding time to 45-65 minutes, and using oil cooling.

6. The method for preparing high-strength spring steel according to claim 1, characterized in that: The step 10 specifically includes: controlling the tempering temperature at 380-420° C., holding time at temperature for 1.5-2.5 hours, and air cooling.

7. The method for preparing high-strength spring steel according to claim 1, characterized in that: In the step 1, the KR arrival temperature is ≥1380°C, the molten iron S content is ≤0.035%, the C content is 4.0-4.5%, the Si content is 0.25-0.65%, the Ti content is ≤0.03%, the P content is ≤0.10%, and the rest are Fe and other unavoidable impurity elements; the exit temperature is ≥1350°C, and the exit S content is ≤0.001%.

8. The method for preparing high-strength spring steel according to claim 1, characterized in that: The components of the clean scrap steel used in the converter in step 2 are P≤0.015%, S≤0.0050%, Ti≤0.01% by mass, and the rest are conventional C, Si, Al, Mn and Fe elements and unavoidable impurities; at the end of converter blowing, the C mass content is ≥0.06%, the O mass content is ≤0.055%, the P mass content is ≤0.006%, the temperature is ≥1610°C, and the converter tapping capacity is 130-140t.

9. The method for preparing high-strength spring steel according to claim 1, characterized in that: The mass content of the low-carbon recarburizer N added during the converter tapping is ≤0.035%, and the rest is C and unavoidable impurity elements; The mass content of Mn element in metallic manganese is ≥99%, and the rest is iron and unavoidable impurity elements; SiC mass content in SiC ≥ 98%, and other unavoidable impurity elements; Low titanium and low aluminum ferrosilicon has a Si mass content of 75-80%, an Al mass content of ≤0.005%, a Ti mass content of ≤0.002%, a P mass content of ≤0.01%, and an S mass content of ≤0.005%, with the remainder being iron and unavoidable impurity elements; The Cr mass content in ferrochrome is 55-60%, the C mass content is ≤1.5%, the P mass content is ≤0.015%, and the rest are iron and inevitable impurity elements; the V mass content in ferrovanadium is 45-50%, the P mass content is ≤0.012%, and the rest are iron and inevitable impurity elements.

10. The method for preparing high-strength spring steel according to claim 1, characterized in that: The ladle used in the LF refining is a magnesia carbon brick, wherein the Al2O3 mass content is ≤2.5%, the C mass content is ≤10%, and the density is 2.8-3.5g / cm 3 , flexural strength ≥38MPa, porosity ≤10%.

11. The method for preparing high-strength spring steel according to claim 1, characterized in that: The refractory materials used for the immersion tube and bottom tank of the RH vacuum furnace are magnesia-chromium ultra-low carbon bricks, of which C≤1.5%, MgO: 85-95%, Cr2O3: 5-12%, Al2O3≤3%, and other inevitable impurity components are calculated by mass.

12. The method for preparing high-strength spring steel according to claim 1, characterized in that: The induction heating current of the tundish is 1500-1800A, the voltage is 1600-2000V, the superheat of the continuous casting molten steel is 20-25°C, the tonnage of the tundish is 48±1 tons, and the tonnage during tundish change is 40-45 tons; the continuous casting is cast at a constant pulling speed, the water volume of the crystallizer is 2800±50NL / min, and the water volume of the secondary cooling section is 600±30NL / min.

13. The method for preparing high-strength spring steel according to claim 1, characterized in that: The components of the continuous casting low basicity low alumina tundish covering agent are as follows by mass: CaO 25-35%, SiO2 45-55%, Al2O3≤2%, MgO 3-6%, and other inevitable components; The composition of the magnesium spray coating on the inner wall of the tundish is calculated by mass content: MgO ≥ 80%, CaO 5-10%, SiO2 1-3%, and other inevitable components. Particles with a size of ≤ 2mm account for 70-80%, particles with a size of 2mm < < 3mm account for more than 20%, and particles with a size of 3mm ≤ account for ≤ 5%; Adopting integral submerged nozzle casting, the submerged nozzle insertion depth is 10-15mm, the thickness of the mold protection slag layer is 10-20mm, and the consumption is 0.15-0.25kg / ton; The components of the protective slag are calculated by mass: CaO 20-25%, SiO2 40-45%, Na2O 10-15%, MgO 3-5%, CaF220-25%, and other inevitable impurities.

14. The method for preparing high-strength spring steel according to claim 1, characterized in that: The submerged nozzle and the stopper rod matched with the submerged nozzle are made of magnesium carbon. The MgO mass content of the stopper rod head is 80-85%, the C mass content is 8-12%, the Al2O3 mass content is ≤1.5%, the SiC mass content is 1-4%, the SiO2 mass content is 2-3%, and other inevitable impurity components, with a density of 2.4-2.7g / cm 3 , porosity ≤ 14%, flexural strength ≥ 40MPa; The inner wall thickness of the submerged nozzle is 5-7mm, and the composition by mass content is: MgO content 75-80%, C content 6-10%, SiC3-5%, SiO2 5-10%, and other unavoidable impurities. The density is 2.4-2.6g / cm 3 , porosity ≤13%; inner wall surface coating thickness is 0.5-1.5mm, SiO2 ≥95%, CaSiO3 1-3%, and other unavoidable components.

15. High-strength spring steel produced by the method for producing high-strength spring steel according to any one of claims 1 to 14.

16. The high-strength spring steel according to claim 15, characterized in that: In the metallographic structure of the high-strength spring steel wire, the volume percentage of sorbite and pearlite is ≥ 95%, and the strength of the spring prepared by using the high-strength spring steel wire is ≥ 2000 MPa; The inclusions in the high-strength spring steel wire include SiO2-MnO-(CaO-Al2O3-MgO) type inclusions.

17. The high-strength spring steel according to claim 15, characterized in that: In the metallographic structure of the high-strength spring steel wire, the total content of martensite and ferrite is ≤5%, and the fatigue life is ≥1 million times.

18. The high-strength spring steel according to claim 15, characterized in that: The most severely segregated region on the cross section of the high-strength spring steel wire and other matrix regions have, in terms of mass percentage, a carbon content ratio of ≤1.05, a Cr content ratio of ≤1.15, and a Mn content ratio of ≤1.15, and a hardness difference of ≤20 HV.

19. The high-strength spring steel according to claim 15, characterized in that The surface of the high-strength spring steel wire has no folds, and the maximum surface depth is ≤25 μm. The diameter of the high-strength spring steel wire is defined as D mm, then the maximum depth of the surface decarburization layer is ≤D×0.2% mm, and there is no complete decarburization layer on the surface.

20. The high-strength spring steel according to claim 15, characterized in that: The mass proportion of (CaO-Al2O3-MgO) in the SiO2-MnO-(CaO-Al2O3-MgO) inclusions is ≤25%, and the number density of inclusions larger than 5μm is ≤0.15 pieces / mm 2 Under the GB / T10561 standard, the ratings of Class A, B, C, and D inclusions are all ≤1.0, and the rating of Class Ds inclusions is ≤0.

5.

21. A high-strength spring made from the high-strength spring steel according to claim 15.

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