High-strength spring steel wire rod and preparation method and application thereof

Abstract

The present application relates to spring steel preparation technical field, specifically relates to a kind of high-strength spring steel wire rod and its preparation method and application.The wire rod is by controlling the content of C, Si, Mn, Cr, Mo, V, Nb and the like, and limit the strength plastic index SE is 1.25~1.75%, while the wire rod after hot rolling of continuous casting large square billet is using Stelmor line three-section controlled cooling: first with 4.2~5.5 ℃ / s fast cooling to 670~710 ℃, then with 0.3~0.5 ℃ / s slow cooling to 620~630 ℃, finally with 1.8~2.2 ℃ / s air cooling to 530~550 ℃.The present application is by the synergistic design of component and process, effectively solve the problem that the high-strength spring steel wire rod for existing electric tailgate spring is difficult to simultaneously ensure excellent organization uniformity, good plasticity and excellent fatigue performance when realizing ultrahigh strength.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of spring steel preparation, in particular to a high-strength spring steel wire rod and a preparation method and application thereof. BACKGROUND

[0002] With the development of new energy automobile electric tailgate towards lightweight and high reliability, the market requirements for spring steel wire performance are increasingly stringent, which need to meet tensile strength > 2200MPa, ton steel flaw defect point number ≤ 10 (detection accuracy 0.1mm), reduction of area ≥ 46% and fatigue life ≥ 100000 times. Although the market demand is clear, domestic high-end spring steel wire rod still mainly depends on import. The fundamental reason for this situation is that the wire rod produced by the existing domestic technology is difficult to realize the coordinated improvement of uniformity, good plasticity and excellent fatigue performance while ensuring ultra-high strength.

[0003] Through the analysis of the existing technical solutions, it can be seen that the current technical route has some obvious defects. Some technical solutions add expensive alloy elements such as nickel and tungsten, which can improve the material strength to 2280MPa, but the material plasticity is significantly reduced, and the reduction of area is less than 40%. Some other technical solutions use conventional continuous casting process, and the center segregation of the casting blank reaches 1.5 level, which seriously affects the uniformity of the wire rod organization. In addition, some technical solutions try to improve the performance by adjusting the composition range, but due to the lack of corresponding process control means, the obtained wire rod sorbite transformation rate is not ideal, and the fluctuation range of lamellar spacing is as high as 35%, which leads to the significant decline of the material fatigue performance and plasticity stability. These technical defects together lead to the difficulty of the finished electric tailgate spring wire rod to meet the comprehensive requirements of ultra-high strength, excellent uniformity, good plasticity and excellent fatigue performance. SUMMARY

[0004] The present application provides a high-strength spring steel wire rod and a preparation method and application thereof, to solve the problem that the existing high-strength spring steel wire rod for electric tailgate spring is difficult to simultaneously ensure excellent uniformity, good plasticity and excellent fatigue performance while achieving ultra-high strength.

[0005] In a first aspect, the present application provides a preparation method of a high-strength spring steel wire rod, comprising the following steps:

[0006] The following chemical components are used for smelting and continuous casting, in mass percentage: C: 0.56-0.63%, Si: 1.22-1.38%, Mn: 0.55-0.72%, Cr: 0.81-0.94%, Mo: 0.02-0.10%, V: 0.11-0.19%, Nb: 0.02-0.06%, P≤0.008%, S≤0.005%, Al≤0.002%, Cu≤0.02%, Ti≤0.002%, H≤1ppm, N≤50ppm, O≤30ppm, and the rest is Fe and inevitable impurities; and the strength-plasticity index SE=0.2[Si]+0.5[Mn]+0.3[Cr+10×Mo]+2[V+3×Nb]=1.25-1.75% is satisfied.

[0007] After the large bloom obtained by continuous casting is processed into a wire rod through hot rolling, the wire rod enters a Stelmor line controlled cooling system for three-stage controlled cooling: first, fast cooling at a cooling rate of 4.2-5.5°C / s to 670-710°C, then slow cooling at a cooling rate of 0.3-0.5°C / s to 620-630°C, and finally air cooling at a cooling rate of 1.8-2.2°C / s to 530-550°C, to obtain a high-strength spring steel wire rod.

[0008] The following describes the component design of the spring steel:

[0009] C: C is an effective strength-enhancing element, mainly existing in the form of cementite. However, excessive addition will cause core segregation, affecting the strength-plasticity uniformity of the wire rod. Therefore, C is limited to 0.56-0.63% in consideration of the overall situation.

[0010] Si: Si has a solid solution strengthening effect, which can significantly improve the spring resistance of the finished product; at the same time, Si is a strong deoxidizing element, which can effectively reduce the number of inclusions and improve the purity. However, excessive addition of Si will affect the plasticity and worsen the decarburization. Therefore, Si is limited to 1.22-1.38% in consideration of the overall situation.

[0011] Mn: Mn is also an effective solid solution strengthening element, which can enhance the hardenability of the steel and significantly improve the strength. However, excessive Mn will affect the plasticity. Therefore, Mn is limited to 0.55-0.72% in consideration of the overall situation.

[0012] Cr: Cr is a carbide-forming element, and the carbide particles formed by the combination of Cr and C can significantly improve the strength by solid solution in the matrix. However, excessive addition of Cr will cause abnormal microstructure due to excessive hardenability. Therefore, Cr is limited to 0.81-0.94% in consideration of the overall situation.

[0013] Mo: Mo is an important strengthening and anti-creep element, which can improve the strength while maintaining the mechanical properties of the wire rod at high temperature, and is helpful for preparing ultra-high strength quenched and tempered steel wire. However, Mo is relatively expensive. Therefore, Mo is limited to 0.02-0.10% in consideration of the overall situation.

[0014] V: V is an effective micro-alloying element, which forms fine and dispersed particles with C, and synchronously improves the strength and plasticity of the wire rod by fine-grain strengthening. Excessive addition will strongly inhibit the pearlite transformation and produce abnormal microstructure. Therefore, V is limited to 0.11-0.19%.

[0015] Nb: Nb is also a commonly used micro-alloying element. Since the solid solution temperature of Nb is higher than that of V, low-temperature rolling process is needed for the dispersion precipitation of Nb. Therefore, Nb is limited to 0.02-0.06%.

[0016] Cu: Cu is an impurity element for spring steel, which is easy to enrich at the grain boundary and phase boundary and reduce the strength, thereby affecting the strength and plasticity of the steel wire after quenching and tempering. Therefore, Cu is limited to ≤0.02%.

[0017] Al: Al is an impurity element for spring steel, which is easy to combine with O to form spinel-like hard Al2O3 inclusions, thereby significantly deteriorating the fatigue performance. Therefore, Al is limited to ≤0.002%.

[0018] Ti and N: Ti and N are impurity elements for spring steel, which are easy to form large-sized TiN hard particles in the steelmaking process, thereby affecting the fluidity of the molten steel and deteriorating the fatigue performance of the finished spring. Therefore, Ti is limited to ≤0.002% and N is limited to ≤50 ppm.

[0019] P and S: S is easy to form long strip-shaped MnS inclusions with Mn, thereby affecting the plasticity and toughness; P is easy to segregate at the grain boundary, thereby reducing the grain boundary strength and deteriorating the plasticity of the quenched and tempered steel wire. Therefore, P is limited to ≤0.008% and S is limited to ≤0.005%.

[0020] O: Most of O combines with impurity elements to form inclusions, thereby reducing the purity of the molten steel and affecting the fatigue performance. Considering the smelting cost and the fatigue performance requirement, O is limited to ≤30 ppm.

[0021] H: High content of H may cause delayed fracture risk for the ultra-high strength heat-treated steel. Considering the smelting cost and the strength grade of the heat-treated steel wire, H is limited to ≤1 ppm.

[0022] 0.2[Si]+0.5[Mn]+0.3[Cr+10×Mo]+2[V+3×Nb]: This equation is a formula for calculating the strength and plasticity index (SE) of the high-strength spring steel wire rod according to the present application. In the formula, each symbol represents the mass percentage (wt%) of the corresponding alloying element in the steel. Considering the differences in phase transformation strengthening, solid solution strengthening and precipitation strengthening of each element, the lower limit of the index is limited to 1.25% and the upper limit is limited to 1.75% in order to ensure that the steel has certain strength and plasticity and good processability.

[0023] In an alternative embodiment, the three-stage controlled cooling is continuously carried out on a Stelmor line comprising 12 air blowers and 14 heat covers, wherein the air blowers and the heat covers are staggered along the running direction of the wire rod, the nth air blower is responsible for the cooling of the area 50% behind the nth heat cover and the area 50% in front of the (n+1)th heat cover; the wire rod sequentially passes through the following three cooling stages:

[0024] (1) The first fast cooling stage: this stage corresponds to the areas of the 1st to 4th heat covers, the roller speed is controlled at 0.72-0.80 m / s, the 1st to 4th heat covers are opened, the 1st air blower (corresponding to the rear part of the 1st heat cover and the front part of the 2nd heat cover) is opened and the opening degree is controlled at 30%-80%, the air volume is controlled at 250000-270000 m 3 / h, so that the wire rod is rapidly cooled from the spinning temperature of 790-810℃ to 670-710℃ at a cooling rate of 4.2-5.5℃ / s;

[0025] (2) The second slow cooling stage: this stage corresponds to the areas of the 5th to 10th heat covers, the roller speed is controlled at 0.68-0.72 m / s, the 6th to 7th heat covers are opened and the remaining heat covers in this stage are closed, the 6th air blower (corresponding to the rear part of the 6th heat cover and the front part of the 7th heat cover) is opened and the opening degree is controlled at 10%-20%, the air volume is controlled at 250000-270000 m 3 / h, and the air flow is distributed to the wire rod overlap area by using the air direction baffle, so that the wire rod is slowly cooled to 620-630℃ at a cooling rate of 0.3-0.5℃ / s, and the temperature difference between the wire rod overlap point and the non-overlap point is controlled to be ≤10℃;

[0026] (3) The third air cooling stage: this stage corresponds to the areas of the 11th to 14th heat covers, the roller speed is controlled at 0.60-0.65 m / s, the 11th to 14th heat covers are opened, all air blowers are closed, and the wire rod is naturally cooled to the coiling temperature of 530-550℃ at a cooling rate of 1.8-2.2℃ / s.

[0027] In an alternative embodiment, the smelting and continuous casting comprises the following processes:

[0028] (1) Hot metal pre-desulfurization: the initial S of the blast furnace hot metal is controlled to be ≤0.08%, the P is controlled to be ≤0.10%, the S content of the hot metal after desulfurization pretreatment is ≤0.003%, the slag skimming rate is ≥99%, and the treatment temperature is ≥1400℃;

[0029] (2) Converter smelting: desulfurized molten iron is smelted in a converter to an endpoint of C≤0.15%, P≤0.008%, Si≤0.10%, a tapping temperature of 1600-1615°C, and a tapping amount of one-third, and then low-titanium low-aluminum ferrosilicon with Ti≤0.05%, metallic manganese, preheated 700-780°C chromium molybdenum alloy blocks, and low-nitrogen carbon additive with N≤0.1wt% are sequentially added;

[0030] (3) LF refining: 5.2-6.8kg of lime and 1.8-2.5kg of fluorite are added per ton of molten steel to prepare refining slag, the refining slag basicity is controlled to be 1.10-1.25, the ladle is bottom-blown with argon gas at 250-350L / min for soft stirring for 10-15min; after slagging, 3.2-4.5kg / ton of steel of vanadium-niobium alloy blocks are added, the argon gas is simultaneously increased to 400-500L / min for stirring for 5-8min, and finally the composition target is adjusted, the LF refining tapping temperature is 1585-1600°C; wherein the vanadium-niobium alloy block comprises 55-65wt% V and 12-17wt% Nb;

[0031] (4) RH refining: the vacuum degree is controlled to be ≤2mbar, the molten steel vacuum degassing time is ≥18min (net circulation ≥10min), after treatment, N≤30ppm, O≤15ppm, H≤1ppm, and the molten steel temperature when breaking the vacuum is 1555-1570°C;

[0032] (5) Continuous casting: long shroud (argon flow rate 80-150L / min) sealed by argon gas, alkaline tundish covering agent, and submerged nozzle are used for full protection pouring; the crystallizer cooling water flow rate is 2600-2680L / min and the water temperature difference is 5-10°C; the molten steel temperature in the tundish is controlled to be 1485-1500°C, the molten steel liquid level fluctuation in the crystallizer is-1.5-1.5mm, and the casting speed is 0.50-0.65m / min, to produce 300×390mm large square billets (length 5.2-5.7m); preferably, the secondary cooling zone is configured with electromagnetic stirring (frequency 1.5-2.5Hz) and end dynamic soft reduction (reduction amount 12-15mm).

[0033] In an alternative embodiment, the hot rolling process comprises a blooming process, a grinding treatment process, and a controlled rolling process in sequence.

[0034] In an alternative embodiment, the cogging process comprises a hot charging cogging process, which comprises: charging the continuous casting billet into a heating furnace at 680-750℃, sequentially passing through a 780-850℃ preheating section, an 880-950℃ first heating section (air-fuel ratio 0.55-0.65), a 950-1050℃ second heating section (air-fuel ratio 0.55-0.65) and a 1100-1180℃ soaking section (air-fuel ratio 0.55-0.65), and controlling the soaking time to be ≥300min and the total heating time to be 420-480min; after discharging, using ≥20MPa high-pressure water to remove scale, and rolling into 140×140mm small square billets (length 15.2-15.7m) at a rolling temperature of 960-1000℃, and controlling the center segregation of the obtained small square billets to be ≤0.5 level, the center porosity to be ≤0.5 level and the center carbon segregation index to be ≤1.1;

[0035] And / or, the grinding treatment process comprises: the small square billets after cogging are subjected to two-pass grinding treatment, the single-side grinding amount is controlled to be 0.8-1.0mm, the corner grinding amount is controlled to be 3-5mm, then the surface roughness Ra is controlled to be ≤25μm by using steel shots (such as S90 grade) with a diameter of 1.2-1.5mm for shot blasting treatment, and the surface crack depth is ensured to be ≤50μm by magnetic powder detection.

[0036] And / or, the controlled rolling process comprises: the billets after grinding are sequentially subjected to preheating at 720-820℃, first heating at 850-920℃, second heating at 940-1020℃ and soaking at 1060-1120℃ (air-fuel ratio 0.45-0.55) in a heating furnace, and the total heating time is controlled to be 80-100min; after discharging, ≥22MPa high-pressure water is used to remove scale, and the open rolling temperature is controlled to be 910-940℃, the finish rolling inlet temperature is controlled to be 830-850℃, the finish rolling outlet temperature is controlled to be 820-840℃, the reducing diameter inlet temperature is controlled to be 810-830℃ and the wire drawing temperature is controlled to be 790-810℃ by the controlled rolling process, and finally the wire rod with a diameter of 5.5-10mm is prepared.

[0037] In a second aspect, the application further provides a high-strength spring steel wire rod prepared by the above-mentioned method for preparing a high-strength spring steel wire rod.

[0038] In an alternative embodiment, the microstructure of the high-strength spring steel wire rod is ferrite and sorbite, the proportion of sorbite is ≥92%, the sorbite grain size is 7.2-8.5μm, the interlamellar spacing is 140-160nm, there is no full decarburization and the total decarburization thickness is ≤40μm, the ton steel flaw detection defect point number is ≤10, the tensile strength is 1140-1180MPa, the reduction of area is ≥50%, the tensile strength fluctuation of the same coil is ≤45MPa and the surface reduction fluctuation is ≤4.2%.

[0039] In a third aspect, the application further provides a high-strength spring steel wire, which is made of the high-strength spring steel wire rod as described above after drawing and quenching and tempering treatment.

[0040] In an optional embodiment, the drawing process comprises: the wire rod is subjected to shot blasting, flaw detection and single-pass drawing to obtain a standard round steel wire, and the wire rod is reduced in diameter by 8-12%;

[0041] And / or, the quenching and tempering treatment process comprises: the drawn steel wire is heated to 880-920 DEG C for austenitizing and then oil quenching, and then tempered at 420-450 DEG C for 60-90 min;

[0042] And / or, the high-strength spring steel wire has a tensile strength of 2230-2300 MPa, a reduction of area of greater than or equal to 46%, and a fatigue cycle number of greater than or equal to 200,000 times under the conditions of a maximum stress of 1250 MPa and a stress amplitude of 750 MPa for a finished spring prepared by winding the spring.

[0043] In a fourth aspect, the application further provides an application of the high-strength spring steel wire rod as described above or the high-strength spring steel wire as described above in the preparation of an automobile electric tail gate spring.

[0044] In the application, unless otherwise specified, the percentage of components refers to mass percentage.

[0045] The technical scheme of the application has the following advantages:

[0046] 1. A method for preparing high-strength spring steel wire rod, comprising the following steps: smelting and continuous casting according to the following chemical composition, in mass percent: C: 0.56-0.63%, Si: 1.22-1.38%, Mn: 0.55-0.72%, Cr: 0.81-0.94%, Mo: 0.02-0.10%, V: 0.11-0.19%, Nb: 0.02-0.06%, P≤0.008%, S≤0.005%, Al≤0.002%, Cu≤0.02%, Ti≤0.002%, H≤1 ppm, N≤50 ppm, O≤30 ppm, and the balance being Fe and unavoidable impurities; and satisfying the strength-plasticity index SE = 0.2[Si] + 0.5[Mn] + 0.3[Cr + 10 × Mo] + 2[V + 3 × Nb] = 1.25-1.75%; after hot rolling the large bloom obtained by continuous casting into a wire rod, the wire rod is subjected to three-stage controlled cooling in a Stelmor line control cooling system: first, fast cooling at a cooling rate of 4.2-5.5°C / s to 670-710°C, then slow cooling at a cooling rate of 0.3-0.5°C / s to 620-630°C, and finally air cooling at a cooling rate of 1.8-2.2°C / s to 530-550°C, to obtain a high-strength spring steel wire rod. Through the design of alloy composition and process optimization, the present application successfully overcomes the problem that the existing high-strength spring steel wire rod for electric tailgate springs is difficult to simultaneously ensure excellent microstructure uniformity, good plasticity, and excellent fatigue performance while achieving ultra-high strength. In the composition design, the content ranges of key elements such as C, Si, Mn, Cr, Mo, V, and Nb are precisely controlled, while the steel purity is ensured (impurity contents such as P≤0.008%, S≤0.005%, Al≤0.002%, and O≤30 ppm are strictly controlled), and a scientific proportioning relationship of the strength-plasticity index SE = 1.25-1.75% is established, laying a foundation for material performance. Combined with the three-stage controlled cooling process of the Stelmor line (4.2-5.5°C / s fast cooling to 670-710°C; 0.3-0.5°C / s slow cooling to 620-630°C; 1.8-2.2°C / s air cooling to 530-550°C), a spring steel wire rod with excellent comprehensive performance is successfully prepared. Through the synergistic effect of alloy elements and precise process control, the present technical solution achieves the synergistic improvement of four key performances of the electric tailgate spring steel wire rod: in terms of ultra-high strength, the tensile strength of the steel wire after quenching and tempering treatment reaches 2230-2300 MPa; in terms of high plasticity, the wire rod reduction of area is ≥50%, and the reduction of area of the steel wire after treatment remains ≥46%; in terms of fatigue performance, the fatigue cycle number of the finished spring is ≥200,000 times under the conditions of a maximum stress of 1250 MPa and a stress amplitude of 750 MPa; and in terms of microstructure uniformity, the proportion of sorbite in the wire rod is ≥92%, the grain size is 7.2-8.5 μm, there is no complete decarburization, and the total decarburization thickness is ≤40 μm.

[0047] 2. The present application adopts a cogging process combining hot charging and hot feeding at 680-750℃ and high-temperature diffusion at 1100-1180℃ for a long time, and effectively promotes the full diffusion of alloying elements in the core of the blank through ≥300min soaking treatment. This process design effectively eliminates the problems of large temperature gradient and insufficient element diffusion caused by repeated heating in conventional cold charging process, and significantly improves the segregation of carbide-forming elements such as V, Nb and Cr.

[0048] 3. The present application effectively improves the uniformity of the solidification structure of the casting blank by the combined application of electromagnetic stirring and end dynamic light pressing technology during the bloom continuous casting process. Electromagnetic stirring effectively breaks the dendritic structure of the solidification front through the action of electromagnetic force of 1.5-2.5Hz, increases the proportion of equiaxed crystals, and end dynamic light pressing compensates for the shrinkage gap at the end of the solidification of the casting blank through a precise pressing amount of 12-15mm, and inhibits the formation of center porosity. DETAILED DESCRIPTION

[0049] In order to better further understand the present application, the following examples are provided, but the following examples do not constitute limitations on the content and scope of protection of the present application, and any product identical or similar to the present application obtained by the inspiration of the present application or the combination of the present application with other existing technical features falls within the protection scope of the present application.

[0050] If the specific experimental steps or conditions are not specified in the examples, they are operated according to the conventional experimental steps or conditions in the art. If the reagents or instruments used are not specified by the manufacturer, they are conventional reagent products or instruments that can be obtained by market purchase.

[0051] Example 1 (denoted as S1)

[0052] The present embodiment provides a preparation method of high-strength spring steel wire rod, and the specific steps are as follows:

[0053] 1. Component design:

[0054] The chemical composition of the high-strength spring steel wire rod is as follows in mass percentage (wt%): C 0.59%, Si 1.27%, Mn 0.62%, Cr 0.87%, Mo 0.05%, V 0.13%, Nb 0.05%, P 0.007%, S 0.004%, Al 0.001%, Cu 0.015%, Ti 0.001%, and the gas content is controlled as [H]=0.6ppm, [N]=45ppm, [O]=22ppm, the rest is Fe and inevitable impurities. According to the formula of the strength-plasticity index SE=0.2[Si]+0.5[Mn]+0.3[Cr+10×Mo]+2[V+3×Nb]=1.2×1.27+0.5×0.62+0.3[0.87+10×0.05]+2[0.13+3×0.05], the SE value of the embodiment is 1.54.

[0055] 2. Smelting and continuous casting process:

[0056] (1) Hot metal pre-desulfurization: control the initial S of the hot metal from the blast furnace at 0.06%, P at 0.08%, after pre-treatment by desulfurization, the S content of the hot metal is reduced to 0.002%, the slagging rate is 99.2%, and the treatment temperature is 1410℃;

[0057] (2) Converter smelting: the desulfurized hot metal is smelted in the converter by blowing oxygen until the end point C is 0.12%, P is 0.006%, and Si is 0.08%, the tapping temperature is 1610℃, and when the tapping amount reaches one-third, low-titanium low-aluminum ferrosilicon with Ti 0.0wt4%, Al 0.04wt%, metal manganese, preheated 750℃ chromium molybdenum alloy block (composition: Cr 72wt%, Mo 10wt%) and low-nitrogen carbon additive with N 0.08wt% are sequentially added, wherein the addition amount of the chromium molybdenum alloy block is 38kg / ton of steel;

[0058] (3) LF refining: 6.0kg of lime and 2.2kg of fluorite are added per ton of molten steel to adjust the refining slag, the slag basicity is controlled at 1.18, the ladle is subjected to soft stirring by argon gas blowing at 300L / min for 12min; after slagging, 3.8kg / ton of steel of vanadium-niobium alloy block (composition: V 60%, Nb 15%) is added, the argon gas flow is simultaneously increased to 450L / min for stirring for 6min, and finally the composition target is adjusted, the LF refining tapping temperature is 1595℃;

[0059] (4) RH refining: the vacuum degree is controlled at 1.5mbar, the vacuum degassing time of the molten steel is 20min (net circulation time 12min), after treatment, the gas content in the steel is N 25ppm, O 12ppm, H 0.8ppm, and the molten steel temperature is 1565℃ when the vacuum is broken;

[0060] (5) Continuous casting: long shroud (argon flow rate 120 L / min) with argon gas sealing protection, basic tundish covering agent and submerged nozzle for full protection pouring; control the crystallizer cooling water flow rate 2640 L / min and the temperature difference between inlet and outlet water 8℃; configure electromagnetic stirring (frequency 2.0 Hz) and end dynamic soft reduction (reduction amount 13 mm) in the secondary cooling zone; control the molten steel temperature in the tundish 1495℃, the molten steel liquid level fluctuation in the crystallizer ±1.2 mm, and the casting speed 0.58 m / min to produce 300×390 mm bloom (length 5.5 m).

[0061] 3. Hot rolling process

[0062] (1) Hot charging and blooming: the 300×390 mm bloom obtained by continuous casting is hot charged into the heating furnace at 693℃, and is sequentially treated in the 797℃ preheating section, 889℃ first heating section (air-fuel ratio 0.58), 992℃ second heating section (air-fuel ratio 0.60) and 1133℃ soaking section (air-fuel ratio 0.55), with the soaking time controlled to 321 min and the total heating time controlled to 446 min; after discharging, the scale is removed by high-pressure water at 22 MPa, and the bloom is rolled into 140×140 mm billet (length 15.5 m) at a rolling temperature of 975℃, with the center segregation of the obtained billet being 0.4 level, the center porosity being 0.3 level and the center carbon segregation index being 1.05;

[0063] (2) Grinding treatment: the billet after blooming is treated by two passes of grinding, with the single-side grinding amount controlled to 0.9 mm and the corner grinding amount controlled to 4 mm, and then the surface roughness Ra value is reduced to 22 μm by using S90 grade steel shot with a diameter of 1.3 mm for shot blasting treatment, and the surface crack depth is ensured to be ≤40 μm by magnetic particle inspection;

[0064] (3) Controlled rolling process: the billet after grinding is sequentially treated in the heating furnace at 750℃ preheating, 880℃ first heating section, 980℃ second heating section and 1090℃ soaking section (air-fuel ratio 0.50), with the total heating time controlled to 90 min; after discharging, the scale is removed by high-pressure water at 23 MPa, and the hot rolling process is controlled to have a rolling temperature of 927℃, a finish rolling inlet temperature of 830℃, a finish rolling outlet temperature of 827℃, a reducing inlet temperature of 811℃ and a wire rod temperature of 799℃, so as to finally produce a wire rod with a diameter of 6.5 mm.

[0065] 4. Controlled cooling process

[0066] Three-stage controlled cooling is implemented on a Stelmor line comprising 12 fans and 14 heat preservation covers:

[0067] (1) First fast cooling section: 1# fan (corresponding to the rear part of 1# heat preservation cover and the front part of 2# heat preservation cover) is opened, with the fan opening degree being 40% and the fan air volume being 260000 m 3 / h, the 1#-4# heat preservation covers are synchronously opened, the roller speed is controlled at 0.75 m / s, the wire rod is rapidly cooled from the spinning temperature 799℃ to 688℃ at a cooling rate of 4.8℃ / s;

[0068] (2) the second slow cooling section: the 6# fan (corresponding to the rear of the 6# heat preservation cover and the front of the 7# heat preservation cover) is opened, the fan opening is 10%, the fan air volume is 260000 m 3 / h, the air flow is distributed to the wire rod lap joint area through the air direction baffle, the 6#-7# heat preservation covers are synchronously opened and the remaining heat preservation covers in this section are closed, the roller speed is controlled at 0.69 m / s, the wire rod is slowly cooled to 624℃ at a cooling rate of 0.3℃ / s, and the temperature difference between the wire rod lap joint point and the non-lap joint point is controlled at 8℃;

[0069] (3) the third air cooling section: all the fans are closed, the 11#-14# heat preservation covers are opened, the roller speed is controlled at 0.63 m / s, the wire rod is naturally cooled to the coiling temperature 550℃ at a cooling rate of 1.8℃ / s, and the high-strength spring steel wire rod is obtained.

[0070] Examples 2-11 (referred to as S2-S11 in turn)

[0071] Examples 2-11 provide a high-strength spring steel wire rod and a preparation method thereof, and the detailed parameters are shown in Tables 1-6, and the remaining unlisted process parameters and operating conditions are the same as those of Example 1.

[0072] Example 12 (referred to as S12)

[0073] This example provides a spring steel wire rod and a preparation method thereof, and the process route and parameters are basically the same as those of Example 1, and the difference lies in that the hot charging and blooming process is changed to cold charging and blooming, the large bloom obtained by continuous casting is first cooled to room temperature, and then reheated to the blooming temperature in the heating furnace, the charging temperature is about 25℃, the hot charging link of 693℃ is cancelled, and the total heating time is correspondingly prolonged to 550 min, and the detailed parameters are shown in Tables 1-6, and the remaining unlisted process parameters and operating conditions are the same as those of Example 1.

[0074] Example 13 (referred to as S13)

[0075] This example provides a spring steel wire rod and a preparation method thereof, and the process route and parameters are basically the same as those of Example 1, and the difference lies in that in the large bloom continuous casting process, the electromagnetic stirring and the terminal dynamic soft reduction process are cancelled. The remaining unlisted process parameters and operating conditions are the same as those of Example 1.

[0076] Comparative Example 1 (referred to as D1)

[0077] The comparative example provides a spring steel wire rod and a preparation method thereof, a process route and parameters of which are basically same as those of Example 1, except that the composition is controlled as follows: C 0.58%, Si 1.15%, Mn 0.50%, Cr 0.78%, Mo 0.18%, V 0.08%, Nb 0.01%, and SE is calculated as 0.2 x 1.15 + 0.5 x 0.50 + 0.3 x (0.78 + 10 x 0.18) + 2 x (0.08 + 3 x 0.01) = 1.18%, and the rest of the impurity elements, smelting, continuous casting, hot rolling, and controlled cooling process parameters are consistent with those of Example 1.

[0078] Comparative Example 2 (denoted as D2)

[0079] The comparative example provides a spring steel wire rod and a preparation method thereof, a process route and parameters of which are basically same as those of Example 1, except that a conventional single-stage cooling process is used in the Stelmor controlled cooling process instead of the three-stage controlled cooling. Specifically, after wire drawing, all the fans are turned off and all the heat preservation covers are opened, so that the wire rod is naturally cooled at an average cooling rate of about 1.2 ℃ / s to the coiling temperature, and the rest of the process parameters are consistent with those of Example 1.

[0080] Comparative Example 3 (denoted as D3)

[0081] The comparative example provides a spring steel wire rod and a preparation method thereof, a process route and parameters of which are basically same as those of Example 1, except that the composition is controlled as follows: C 0.52%, and the strength-plasticity index SE is calculated as 1.54%, and the rest of the conditions are same as those of Example 1.

[0082] Comparative Example 4 (denoted as D4)

[0083] The comparative example provides a spring steel wire rod and a preparation method thereof, a process route and parameters of which are basically same as those of Example 1, except that the composition is controlled as follows: Si 1.45%, and the strength-plasticity index SE is calculated as 1.57%, and the rest of the conditions are same as those of Example 1.

[0084] Comparative Example 5 (denoted as D5)

[0085] The comparative example provides a spring steel wire rod and a preparation method thereof, a process route and parameters of which are basically same as those of Example 1, except that a “two-stage controlled cooling” process is used in the Stelmor controlled cooling process. Specifically, after the fast cooling stage (cooled to 688 ℃ at 4.8 ℃ / s), the 6# fan is turned off, and the 5#, 8#, 9# and 10# heat preservation covers are turned on, so as to cancel the controllable cooling in the slow cooling stage, and the wire rod is directly naturally cooled at an average cooling rate of 1.0 ℃ / s from 688 ℃ to 650 ℃, and then enters the air cooling stage to be cooled to the coiling temperature, and the rest of the conditions are same as those of Example 1.

[0086] Comparative Example 6 (referred to as D6)

[0087] This comparative example provides a spring steel wire rod and a preparation method thereof, the process route and parameters of which are basically the same as those of Example 1, except that in the fast cooling section of the Stelmor controlled cooling process, the cooling rate is controlled to be 3.5 ℃ / s by adjusting the fan opening, and the end temperature of this section is about 730 ℃, and the other conditions are the same as those of Example 1.

[0088] Table 1 Chemical composition of spring steel (wt%)

[0089]

[0090] Table 2 Chemical composition of spring steel (wt%)

[0091]

[0092] Table 3 Hot charging and blooming process parameters

[0093]

[0094] Table 4 Controlled rolling process parameters

[0095]

[0096] Table 5 Controlled cooling process parameters (first fast cooling section)

[0097]

[0098] Table 6 Controlled cooling process parameters (second holding section, third air cooling section)

[0099]

[0100] Test Example 1

[0101] The high-strength spring steel wire rods prepared in the above examples and comparative examples were tested for microstructure and mechanical properties, and the test results are shown in Tables 7 and 8.

[0102] Among them, the proportion of sorbite is determined by "YB / T 169-2014 High Carbon Steel Wire Rod Sorbite Content Metallographic Detection Method";

[0103] The sorbite grain size and sorbite lamella spacing are determined by the area method in "GB / T 6394-2017 Metal Average Grain Size Determination Method";

[0104] The total decarburization layer thickness of the wire rod is determined by "GB / T 224-2019 Steel Decarburization Layer Depth Determination Method";

[0105] The ton steel flaw detection points are detected by an ultrasonic flaw detector (precision 0.1 mm) according to "GB / T 7736-2015 Steel Ultrasonic Inspection Method for Macrostructure and Defects";

[0106] The tensile strength and the area reduction are determined by "GB / T 228.1-2021 Metallic Materials Tensile Testing Part 1: Room Temperature Test Method";

[0107] The determination method of the tensile strength fluctuation and the area reduction fluctuation is as follows: 10 tensile samples are taken at equal intervals on the same coil rod, and the difference between the maximum value and the minimum value of the tensile strength (or the area reduction) is the fluctuation value;

[0108] The fatigue performance detection is determined by the fatigue test method in "GB / T 16947-2009 Spiral Spring Fatigue Test Specification", and is determined under the conditions of a maximum stress of 1250 MPa and a stress amplitude of 750 MPa.

[0109] Table 7 Spring steel coil rod structure and performance

[0110]

[0111] Table 8 Coil rod / product spring performance

[0112]

[0113] According to the test results of the above examples and comparative examples, the spring steel coil rods prepared in examples S1-S11 have a tensile strength of the finished steel wire of 2250 MPa or more, an area reduction of 48% or more, a fatigue life of 300,000 times or more, and a ton steel flaw detection point number of 6 or less; the coil rod structure uniformity indexes (tensile fluctuation ≥ 37 MPa, area reduction fluctuation ≥ 3.5%) and the fatigue life (≤ 31 million times) of examples S12 (canceling the hot charging process) and S13 (canceling the electromagnetic stirring and light pressing process) have decreased to a certain extent, but still meet the basic performance requirements and are significantly better than all the comparative examples. The comparative examples D1-D6 have one or more performance not meeting the standards in terms of tensile strength (> 2200 MPa), structure uniformity (ton steel flaw detection point number ≤ 10), plasticity (area reduction ≥ 46%), or fatigue life (≥ 10 million times) due to composition exceeding the standard (D1, D3, D4) and poor process control (D2, D5, D6), and cannot meet the comprehensive requirements of ultrahigh strength, structure uniformity, high plasticity, and excellent fatigue performance of the electric tail door spring.

[0114] Obviously, the above embodiments are merely example for clearly illustrating but not limitation to the embodiments. Based on the above description, other different forms of changes or variations can be made by those skilled in the art. Here, all the embodiments need not and can not be enumerated. The obvious changes or variations derived from this still fall within the protection scope of the present application.

Claims

1. A method for producing a high-strength spring steel wire rod, characterized by, The method comprises the following steps: The smelting and continuous casting is carried out according to the following chemical components in percentage by mass: C: 0.56-0.63%, Si: 1.22-1.38%, Mn: 0.55-0.72%, Cr: 0.81-0.94%, Mo: 0.02-0.10%, V: 0.11-0.19%, Nb: 0.02-0.06%, P≤0.008%, S≤0.005%, Al≤0.002%, Cu≤0.02%, Ti≤0.002%, H≤1ppm, N≤50ppm, O≤30ppm, and the rest is Fe and inevitable impurities; and the following condition is met: SE=0.2[Si]+0.5[Mn]+0.3[Cr+10×Mo]+2[V+3×Nb]=1.25-1.75%; After the large bloom obtained through continuous casting is processed into a wire rod through hot rolling, the wire rod enters a Stelmor line controlled cooling system to be subjected to three-stage controlled cooling: first, fast cooling at a cooling rate of 4.2-5.5 ℃ / s to 670-710 ℃, then slow cooling at a cooling rate of 0.3-0.5 ℃ / s to 620-630 ℃, and finally, air cooling at a cooling rate of 1.8-2.2 ℃ / s to 530-550 ℃, to obtain a high-strength spring steel wire rod; The smelting and continuous casting comprises the following procedures: (1) Pre-desulfurization of molten iron: the initial S of the molten iron from the blast furnace is controlled to be ≤0.08%, the P is controlled to be ≤0.10%, the S content of the molten iron after pre-treatment through desulfurization is ≤0.003%, the slag tapping rate is ≥99%, and the treatment temperature is ≥1400 ℃; (2) Converter smelting: the desulfurized molten iron is subjected to oxygen blowing smelting in a converter to reach the end point of C≤0.15%, P≤0.008%, Si≤0.10%, the tapping temperature is 1600-1615 ℃, and when the tapping amount reaches one-third, low-titanium low-aluminum ferrosilicon with Ti≤0.05% and Al≤0.05%, metal manganese, preheated 700-780 ℃ chromium molybdenum alloy blocks, and low-nitrogen carburizer with N≤0.1wt% are sequentially added; wherein, the composition of the chromium molybdenum alloy blocks comprises 70-75wt% Cr and 8-12wt% Mo, and the chromium molybdenum alloy block addition amount per ton of molten steel is 35-40 kg; (3) LF refining: 5.2-6.8 kg of lime and 1.8-2.5 kg of fluorite are added per ton of molten steel to prepare refining slag, the refining slag basicity is controlled to be 1.10-1.25, and the ladle is subjected to soft stirring for 10-15 min through argon gas blowing at 250-350 L / min; after slagging, 3.2-4.5 kg / ton of steel of vanadium niobium alloy blocks are added, the argon gas is simultaneously increased to 400-500 L / min for stirring for 5-8 min, and finally, the composition target is adjusted, and the LF refining tapping temperature is 1585-1600 ℃; wherein, the composition of the vanadium niobium alloy blocks comprises 55-65wt% V and 12-17wt% Nb; (4) RH refining: the vacuum degree is controlled to be ≤2 mbar, the molten steel vacuum degassing time is ≥18 min, and after treatment, N≤30 ppm, O≤15 ppm, and H≤1 ppm, the molten steel temperature when the vacuum is broken is 1555-1570 ℃; (5) continuous casting: full protection casting is carried out by using argon sealing protection long nozzle, alkaline tundish covering agent and submerged nozzle; the crystallizer cooling water flow is 2600-2680 L / min and the water temperature difference is 5-10 ℃; the molten steel temperature in the tundish is controlled to be 1485-1500 ℃, the molten steel liquid level fluctuation in the crystallizer is-1.5-1.5 mm, the casting speed is 0.50-0.65 m / min, and a 300*390 mm bloom is prepared; the secondary cooling zone is provided with electromagnetic stirring and end dynamic soft reduction.

2. The production method according to claim 1, characterized by, The three-stage controlled cooling is continuously carried out on a Stelmor line comprising 12 fans and 14 heat preservation covers, wherein the fans and the heat preservation covers are staggered arranged along the running direction of the rod, the nth fan is responsible for the cooling of the area after the nth heat preservation cover and the area before the (n+1)th heat preservation cover; the rod passes through the following three cooling stages in turn: (1) The first fast cooling section: this section corresponds to the 1# to 4# holding cover area, the roller way speed is controlled at 0.72~0.80m / s, the 1# to 4# holding cover is opened, the 1# fan is opened and the opening degree is controlled at 30%~80%, the air volume is controlled at 250000~270000m 3 / h, so that the wire rod is rapidly cooled from the spinning temperature of 790~810℃ to 670~710℃ at the cooling rate of 4.2~5.5℃ / s; (2) The second slow cooling section: this section corresponds to the 5# to 10# holding cover area, the roller speed is controlled at 0.68~0.72m / s, the 6# to 7# holding cover is opened and the rest of the holding cover in this section is closed, the 6# fan is opened and the opening degree is controlled at 10%~20%, the air volume is controlled at 250000~270000m 3 / h, and the air flow is distributed to the wire lap area by using the air direction baffle, so that the wire is slowly cooled to 620~630℃ at a cooling rate of 0.3~0.5℃ / s, and the temperature difference between the wire lap point and the non-lap point is controlled at ≤10℃. (3) the third air cooling stage: this stage corresponds to the area of the 11th to 14th heat preservation covers, the roller speed is controlled to be 0.60-0.65 m / s, the 11th to 14th heat preservation covers are opened, all the fans are closed, and the rod is naturally cooled to the collecting temperature of 530-550 ℃ at the cooling rate of 1.8-2.2 ℃ / s.

3. The production method according to claim 1 or 2, characterized by, The hot rolling process comprises a blooming process, a grinding treatment process and a controlled rolling process which are carried out in turn.

4. The production method according to claim 3, characterized by, The blooming process comprises hot charging blooming process, which comprises: hot charging the continuous casting billet into the heating furnace at 680-750 ℃, sequentially passing through the 780-850 ℃ preheating section, the 880-950 ℃ first heating section, the 950-1050 ℃ second heating section and the 1100-1180 ℃ soaking section, controlling the soaking time to be greater than or equal to 300 min and the total heating time to be 420-480 min; after discharging, the scale is removed by using high-pressure water with a pressure greater than or equal to 20 MPa, and the rod is rolled into a 140*140 mm small bloom at a rolling temperature of 960-1000 ℃, and the center segregation of the obtained small bloom is controlled to be less than or equal to 0.5 level, the center porosity is controlled to be less than or equal to 0.5 level, and the center carbon segregation index is controlled to be less than or equal to 1.1; And / or, the grinding treatment process comprises: the small bloom after blooming is treated by two passes of grinding, the single-side grinding amount is controlled to be 0.8-1.0 mm, the corner grinding amount is controlled to be 3-5 mm, then the surface roughness Ra is controlled to be less than or equal to 25 μm by adopting steel shot with a diameter of 1.2-1.5 mm for shot blasting treatment, and the surface crack depth is ensured to be less than or equal to 50 μm by magnetic particle inspection; And / or, the controlled rolling process comprises: the billet after grinding is sequentially treated by preheating at 720-820 ℃, first heating at 850-920 ℃, second heating at 940-1020 ℃ and soaking at 1060-1120 ℃ in the heating furnace, and the total heating time is 80-100 min; after discharging, the scale is removed by using high-pressure water with a pressure greater than or equal to 22 MPa, the rolling temperature is controlled to be 910-940 ℃ by the controlled rolling process, the entry temperature of the finishing rolling is controlled to be 830-850 ℃, the exit temperature of the finishing rolling is controlled to be 820-840 ℃, the entry temperature of the reducing diameter is controlled to be 810-830 ℃, and the wire rod temperature is controlled to be 790-810 ℃, and finally the wire rod with a diameter of 5.5-10 mm is prepared.

5. A high-strength spring steel wire rod, characterized in that It is prepared by the preparation method of the high-strength spring steel wire rod according to any one of claims 1-4.

6. High-strength spring steel wire rod according to claim 5, characterized in that The high-strength spring steel wire rod has a microstructure of ferrite and sorbite, a sorbite proportion of ≥92%, a sorbite grain size of 7.2-8.5 μm, a lamellar spacing of 140-160 nm, no full decarburization and a total decarburization thickness of ≤40 μm, a ton steel flaw point number of ≤10, a tensile strength of 1140-1180 MPa, a reduction of area of ≥50%, a same-circle tensile strength fluctuation of ≤45 MPa and a surface reduction fluctuation of ≤4.2%.

7. A high-strength spring wire, characterized in that The high-strength spring steel wire rod is drawn, quenched and tempered to obtain the high-strength spring steel wire.

8. The high-strength spring wire according to claim 7, characterized in that The drawing process comprises: the wire rod is subjected to shot blasting, flaw detection and single-pass drawing to obtain a standard round steel wire, and the wire rod reduction is 8-12%; The quenching and tempering process comprises: the drawn steel wire is heated to 880-920 ℃ for austenitization, then oil quenched, and then tempered at 420-450 ℃ for 60-90 min. The high-strength spring steel wire has a tensile strength of 2230-2300 MPa, a reduction of area of ≥46%, and a finished spring prepared by winding the spring has a fatigue cycle number of ≥200,000 times under the conditions of a maximum stress of 1250 MPa and a stress amplitude of 750 MPa.

9. Use of the high-strength spring steel wire rod of claim 5 or 6 or the high-strength spring steel wire of claim 7 or 8 in preparing a spring for an automobile electric tail gate.

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