Small gauge high strength spring steel and method of production and use thereof

Abstract

The application provides a small-size high-strength spring steel and a production method and application thereof, the chemical components of the spring steel include C 0.64-0.69%, Si 1.38-1.58%, Mn 0.42-0.54%, Cr 0.52-0.67%, V 0.12-0.18% and satisfy the strength-plasticity index SE of 3.0-3.7%; the production method sequentially includes bloom continuous casting, high-speed wire controlled rolling, Stelmor wire controlled cooling and salt bath heat treatment; the wire rod prepared by the method is used to prepare automobile tail door spring steel wires, the tensile strength of the automobile tail door spring steel wires reaches 2275-2325 MPa, and the reduction of area is greater than or equal to 50%. The wire rod organization and performance of the spring steel are homogenized and regulated through the high-speed wire controlled cooling combined with the salt bath heat treatment and other processes, and the spring steel can be used to prepare high-strength automobile tail door springs.

Description

Technical Field

[0001] This invention relates to the field of spring steel technology, specifically to a small-size high-strength spring steel, its production method, and its application. Background Technology

[0002] The current new energy vehicle market is highly competitive, with significantly higher rates of application of functional and intelligent features compared to traditional fuel vehicles. Among these features, electric tailgates with one-button opening and closing have become standard equipment on mainstream models from various brands. The tailgate spring, a core component of the electric tailgate, is made from wire rod through processes such as drawing, quenching and tempering, and coiling. Strict requirements are placed on the material's strength, uniformity of microstructure, and fatigue performance.

[0003] For high-strength, small-diameter automotive tailgate spring steel, currently available technical solutions generally involve adding various alloying elements to the composition and employing different processes to obtain finished wire rods. These wire rods are then processed through drawing, quenching and tempering, and spring winding to produce high-strength electric tailgate springs. The existing spring steel and its production processes include the following:

[0004] Patent CN116770171A discloses a high-strength automotive tailgate spring steel and its manufacturing method. The main chemical composition (weight percentage) of the spring steel wire rod is: C 0.55%–0.80%, Si 1.30%–2.00%, Mn 0.30%–0.80%, Cr 0.30%–0.80%, and V 0.10%–0.30%. The main production process includes: pre-desulfurization of molten iron, converter smelting, LF refining, RH refining, continuous casting of large billets, billet preparation, grinding, and high-speed wire rod rolling. The resulting wire rod has a sorbite + ferrite microstructure with a grain size of 8–30 μm. The wire rod is made into spring steel wire after multiple drawing and quenching and tempering heat treatments (quenching temperature 850~950℃, tempering temperature 380-500℃). The tensile strength of the steel wire is ≥2200 MPa, the reduction of area is ≥35%, and the fatigue life is ≥100,000 cycles.

[0005] The existing technology's billet composition design allows for a wide range of settings for major alloying elements such as C, Si, Mn, Cr, and V, without additional limiting conditions. This makes it difficult to match the process with the composition, and thus hard to guarantee the stability of the finished wire rod's microstructure and properties. Furthermore, the wire rod is only cooled in a Stellmore line after rolling, and due to differences between overlapping and non-overlapping points, the performance of the same coil fluctuates significantly. In addition, the requirement for a semi-decarburized layer of ≤0.1 mm is not established in proportion to the wire rod diameter, posing a risk that the decarburized layer depth may not meet the usage requirements for small-diameter wire rods.

[0006] Patent CN111334708A discloses a high-strength spring steel with a tensile strength ≥2250 MPa and excellent fatigue performance, and its production method. The main chemical composition (weight percentage) of the spring steel wire rod is: C 0.70%~0.80%, Si 1.60%~2.00%, Mn 0.40%~0.60%, Cr 0.80%~1.00%, V 0.10%~0.20%, Nb 0.03%~0.05%, N 0.005%~0.008%. The main production process includes: electric furnace smelting, LF refining, vacuum refining, large billet continuous casting, billet preparation, controlled rolling and cooling, drawing, and heat treatment. After quenching and tempering, the spring steel wire has a tensile strength ≥2250 MPa, a reduction of area ≥40%, a fatigue strength ≥880 MPa, and a Bauschinger torsion test inverse curve area ≥300 mm². 2 .

[0007] This existing technology involves excessive addition of C, Si, and Cr elements in the steelmaking composition design, especially with a carbon content exceeding 0.70%. This leads to a large precipitation of carbides after quenching and tempering, making it difficult to consistently maintain the plasticity of the spring steel wire. The large square billet used in this method has a continuous casting size of 250 mm × 250 mm, and the intermediate billet size after initial casting is 150 mm × 150 mm. On the one hand, the dimensional change between the continuous casting billet and the intermediate billet is small, resulting in insufficient homogenization of the billet. On the other hand, the square cross-section of the continuous casting billet makes the light reduction process less effective, and controlling core segregation is difficult. Furthermore, this method does not clearly define key indicators such as the microstructure, properties, and decarburization of the base material wire rod, thus failing to consistently guarantee that the subsequently heat-treated steel wire meets the usage requirements.

[0008] Patent CN117344202A discloses a spring steel and its manufacturing method. The main components (by weight percentage) of the spring steel are: C 0.63%–0.68%, Si 1.30%–1.60%, Mn 0.40%–0.80%, Cr 0.30%–0.80%, V 0.03%–0.18%, and Al 0.001%–0.004%. The production method of the spring steel wire includes: converter / electric furnace smelting, LF refining, vacuum refining, continuous casting, hot rolling, drawing, and heat treatment. The microstructure of the quenched and tempered spring steel wire is tempered troostite and sorbite, with an average grain size ≤15μm. The wire strength is ≥2150 MPa, the reduction of area is ≥40%, and the fatigue life is ≥800,000 cycles.

[0009] The existing technology does not specify the specific process methods such as surface treatment, rolling and cooling control in the production of the base material wire rod, which makes the microstructure, mechanical properties, decarburization and segregation of the wire rod unpredictable. At the same time, the spring steel wire prepared from the wire rod by quenching and tempering can only reach a strength of 2150 MPa, which cannot meet the performance requirements of current high-strength electric tailgate springs.

[0010] In summary, the existing production methods for high-strength spring steel generally have the following problems: the composition design does not specify the main alloying elements; the specific process parameters for controlled rolling and controlled cooling are not clearly defined in the wire rod production process, thus failing to guarantee the uniformity of the microstructure and properties of the wire rod and the final quenched and tempered spring steel wire. Summary of the Invention

[0011] The purpose of this invention is to provide a small-size high-strength spring steel, its production method, and its application. By refining the alloying elements in the billet composition and using a salt bath heat treatment process after the hot-rolled wire rod is produced, the wire rod's structure and properties are uniformly controlled, thereby enabling the production of high-strength automotive tailgate springs with excellent performance.

[0012] To achieve the above objectives, the present invention proposes the following technical solution:

[0013] A small-size high-strength spring steel, wherein the chemical composition of the spring steel, by weight percentage, comprises the following components:

[0014] C: 0.64%~0.69%, Si: 1.38%~1.58%, Mn: 0.42%~0.54%, Cr: 0.52%~0.67%, V: 0.12%~0.18%, Mo: 0.03%~0.08%, Cu≤0.02%, Al≤0.005%, P≤70ppm, S≤80ppm, N≤50ppm, O≤20ppm, H≤0.8ppm, with the remainder being Fe and unavoidable impurities; and satisfying the strength-ductility index SE=[C]+0.3[Si]+1.2[Mn]+0.8[Cr]+5.0[V]+8.0[Mo], wherein the strength-ductility index SE is 3.0%~3.7%, and [C], [Si], [Mn], [Cr], [V], and [Mo] are the weight percentages of each element in the spring steel.

[0015] As a preferred embodiment of the present invention, the wire rod structure of the spring steel is ferrite + sorbite, wherein the sorbite is ≥95%, the grain size of the sorbite is 6.6~8.5μm, and the lamellar spacing of the sorbite is 140~170nm.

[0016] As a preferred embodiment of the present invention, the finished spring steel wire rod has a tensile strength of 1150-1190 MPa, a strength fluctuation of ≤25 MPa within the same coil, a reduction of area of ​​≥57%, and an elongation after fracture of ≥18%.

[0017] On the other hand, the present invention also provides an application of small-size high-strength spring steel, wherein the finished wire rod of the spring steel is sequentially drawn, quenched and tempered to produce automotive tailgate spring steel wire;

[0018] The tensile strength of the spring steel wire is 2275-2325 MPa, and the reduction of area is ≥50%.

[0019] On the other hand, the present invention also provides a method for producing small-size high-strength spring steel, the method comprising the following steps:

[0020] Large billet continuous casting: molten steel that meets the composition requirements is continuously cast, the superheat of the molten steel in the tundish and the fluctuation of the liquid level are controlled, electromagnetic stirring is configured in the secondary cooling zone and dynamic light pressure is adopted, and the continuous casting speed is controlled to obtain a large billet of the target size. Then the large billet is placed into the pit for slow cooling.

[0021] Diffusion billet preparation: The continuously cast large square billet is reheated and subjected to multiple passes of continuous rolling to obtain a small square billet of the target size;

[0022] High-speed wire rod rolling: The small square billet is heated to a higher temperature and continuously rolled to obtain the initial wire rod;

[0023] Stellmore Line Controlled Cooling: The initial wire rod is subjected to multi-stage controlled cooling to obtain intermediate wire rod;

[0024] Salt bath heat treatment: Under a protective atmosphere, the intermediate wire rod is heat-treated by selecting the corresponding induction heating temperature and salt bath temperature according to the strength-ductility index SE of the spring steel, and the corresponding wire speed is selected according to the diameter range of the wire rod to obtain the finished wire rod.

[0025] As a preferred embodiment of the present invention, the large billet continuous casting includes:

[0026] The molten steel that is continuously cast in the tundish is controlled by induction heating to maintain the superheat of the molten steel in the tundish at 20-25°C and the liquid level fluctuation at ±1mm.

[0027] Electromagnetic stirring is configured in the secondary cooling zone and dynamic light pressure is adopted, wherein the electromagnetic stirring frequency is 1.5 to 2.5 Hz and the total pressure reduction under dynamic light pressure is 17 to 20 mm.

[0028] By controlling the continuous casting billet pulling speed to 0.50–0.57 m / min, large continuously cast billets with cross-sectional dimensions of (300–330) mm × (390–450) mm and lengths of 5.5–5.8 m are obtained.

[0029] After the continuous casting billet is removed from the line, it is slowly cooled in a pit. The temperature in the pit is ≥650℃, the cooling time in the pit is ≥30 h, and the temperature when it comes out of the pit is ≤150℃.

[0030] The carbon segregation index of the continuously cast large square billet is ≤1.08, and the central segregation, central porosity, and corner cracks are all no greater than level 1.0.

[0031] As a preferred embodiment of the present invention, the diffusion blanking includes:

[0032] The continuously cast large square billet is heated, wherein the homogenization temperature is 1200~1230℃, the homogenization time is ≥270min, and the total heating time is 360~400min;

[0033] The small square billet with dimensions of (140~160)mm×(140~160)mm×(15000~15700)mm is obtained by multi-pass continuous rolling;

[0034] The initial rolling temperature is 920-950℃, the carbon segregation index of the small square billet is ≤1.05, and the central segregation, central porosity, and corner cracks are all no greater than level 0.5.

[0035] As a preferred embodiment of the present invention, the high-speed wire rod mill includes:

[0036] The heated small square billet is continuously rolled to obtain the initial wire rod with a wire rod specification of Φ5.0~10 mm;

[0037] The rolling temperature is 920–940℃, the finishing mill inlet temperature is 850–880℃, the finishing mill outlet temperature is 900–930℃, the sizing and reducing inlet temperature is 820–850℃, the wire drawing temperature is 800–830℃, and the rolling speed is 60–120 m / s.

[0038] As a preferred embodiment of the present invention, the Stellmore linear cooling system includes:

[0039] The initial coil is subjected to multi-stage controlled cooling to obtain intermediate coils:

[0040] (1) First rapid cooling section: control the roller speed to be 0.95~1.05 m / s, the cooling rate to be 6.5~8.5 ℃ / s, and the end temperature to be 635~655℃;

[0041] (2) Second slow cooling section: control the roller speed to be 1.05~1.15 m / s, the cooling rate to be 1.0~1.5 ℃ / s, and the end temperature to be 610~630℃;

[0042] (3) Third air cooling section: control the roller speed to be 1.15~1.25 m / s, the cooling speed to be 2.5~3.5 ℃ / s, and the winding temperature to be 480~510℃.

[0043] As a preferred embodiment of the present invention, the salt bath heat treatment includes:

[0044] The carbon potential of the atmosphere inside the heating furnace is controlled at 0.60% to 0.65%.

[0045] Based on the strength-ductility index (SE) of the spring steel, the induction heating temperature and salt bath temperature are controlled as follows:

[0046] When 3.0% ≤ SE < 3.25%, the induction heating temperature is 865~872℃, and the salt bath temperature is 620~628℃;

[0047] When 3.25% ≤ SE < 3.5%, the induction heating temperature is 875~882℃, and the salt bath temperature is 610~618℃;

[0048] When 3.5% ≤ SE ≤ 3.7%, the induction heating temperature is 885~892℃, and the salt bath temperature is 600~608℃;

[0049] The wire speed is controlled according to the wire diameter D:

[0050] When 5 mm ≤ D < 8 mm, the routing speed is 5.0~5.5 m / min;

[0051] When 8 mm ≤ D ≤ 10 mm, the routing speed is 4.3~4.8 m / min.

[0052] As can be seen from the above technical solutions, the technical solutions of the present invention provide small-size high-strength spring steel, its production method, and its applications, and compared with the prior art, they have the following beneficial effects:

[0053] (1) In the composition design of this invention, based on the differences in the influence of each alloying element on the strength and plasticity of spring steel, a strength-plasticity index SE=[C]+0.3[Si]+1.2[Mn]+0.8[Cr]+5.0[V]+8.0[Mo] is creatively proposed and its range is precisely defined as SE=3.0%~3.7%. Through the regulation of this strength-plasticity index, the fine compatibility of major alloying elements such as C, Si, Mn, Cr, V and Mo is achieved, which significantly reduces the performance fluctuation of the base material wire rod, has excellent microstructure uniformity, and can be stably used to prepare 2275~2325 MPa grade automotive tailgate spring steel wire.

[0054] (2) In the continuous casting process of large billets, this invention fully considers the operating conditions of the finished automotive tailgate spring, subsequent processing requirements, and the aforementioned composition design characteristics to rationally configure the continuous casting process parameters. Specifically, the continuous casting process is carried out within a suitable superheat range (20-25℃) and casting speed range (0.50-0.57 m / min). Excessive superheat or excessive casting speed will make it difficult to control core segregation, while insufficient superheat or excessively slow casting speed will easily induce surface and corner cracks. Based on this, the combination of electromagnetic stirring at the end of the secondary cooling zone and dynamic light reduction process (total reduction of 17-20 mm) effectively improves the carbon segregation in the core of the continuously cast billet. In addition, considering the high overall alloy element ratio and large internal stress of the high-strength spring steel of this invention, the large billet is immediately placed in the pit for slow cooling after being cast (pit temperature ≥650℃, pit cooling time ≥30 h, and pit exit temperature ≤150℃) after being cast from the line. This makes the cooling rate of the core and surface of the billet tend to be consistent, fully releasing the internal stress generated during solidification and phase transformation, thereby significantly reducing corner crack defects.

[0055] (3) In the controlled cooling process of the wire rod in the Stellmore line, the present invention adopts a segmented controlled cooling strategy and, in combination with the specific composition design and target performance requirements of the wire rod, rationally sets the cooling process of each segment. The first segment is the rapid cooling segment (cooling rate 6.5~8.5℃ / s, ending temperature 635~655℃), the purpose of which is to quickly pass through the ferrite phase region and reduce the precipitation ratio of proeutectoid ferrite; the second segment is the slow cooling segment (cooling rate 1.0~1.5℃ / s, ending temperature 610~630℃), the purpose of which is to allow the wire rod to undergo sufficient sorbite phase transformation under isothermal conditions and ensure that the sorbite ratio is ≥95%; the third segment is the air cooling segment (cooling rate 2.5~3.5℃ / s, coiling temperature 480~510℃), the purpose of which is to cool the wire rod to a suitable temperature for coiling, because if the temperature is too high, it will affect the subsequent sampling and packaging operations, and if the temperature is too low, the wire rod will be too elastic and prone to surface scratches. Considering that the wire rods involved in this invention are all small-sized (Φ5.0~10 mm) and rolled at a relatively fast speed, the roller speed is controlled at a relatively high level of 0.95~1.25 m / s to ensure the uniformity of the microstructure and properties within the same roll and to avoid the generation of abnormally hard and brittle microstructures such as martensite.

[0056] (4) In the wire rod salt bath heat treatment process of this invention, the austenitizing heating temperature, the sorbite isothermal phase transformation salt bath temperature, and the continuous production line speed are reasonably set according to the differences in the strength-plasticity index (SE) and the specification range of the wire rod. Specifically, when the SE is in different ranges (3.0%~3.25%, 3.25%~3.5%, 3.5%~3.7%), the corresponding induction heating temperature (865~892℃) and salt bath temperature (600~628℃) are matched respectively; at the same time, an appropriate line speed (4.3~5.5 m / min) is selected according to the wire rod diameter D (5.0~10 mm). Through the graded matching of the above parameters, the wire rod structure is made uniform, the sorbite grain size (6.6~8.5 μm) and lamellar spacing (140~170 nm) fluctuate within a small range, and the tensile strength of the finished wire rod fluctuates little and the plasticity is good. Meanwhile, to avoid surface decarburization during the austenitizing heating process, the diffusion tendency of C element is effectively suppressed by controlling the carbon potential of the furnace atmosphere (0.60%~0.65%), thus ensuring the compositional stability of the wire rod surface.

[0057] (5) Through the coordinated execution of the above-mentioned composition design and the entire process, the high-strength spring steel wire rod obtained by this invention has the following excellent microstructure and performance characteristics: the microstructure is ferrite + sorbite, wherein the proportion of sorbite is ≥95%, the sorbite grain size is 6.6~8.5 μm, and the sorbite lamellar spacing is 140~170 nm; the wire rod tensile strength is 1150~1190MPa, the strength fluctuation within the same coil is ≤25 MPa, the reduction of area is ≥57%, the reduction of area fluctuation within the same coil is ≤3%, and the elongation after fracture is ≥18%. Furthermore, the automotive tailgate spring steel wire made from this wire rod after drawing, quenching and tempering treatment can achieve a tensile strength of 2275~2325 MPa and a reduction of area of ​​≥50%, which fully meets the stringent requirements of automotive tailgate springs for ultra-high strength and good plasticity and toughness.

[0058] It should be understood that all combinations of the foregoing concepts and the additional concepts described in more detail below can be considered part of the inventive subject matter of this disclosure, provided that such concepts do not contradict each other. Detailed Implementation

[0059] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. Based on the described embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention. Unless otherwise defined, the technical or scientific terms used herein should have the ordinary meaning understood by those skilled in the art to which this invention pertains.

[0060] The terms "first," "second," and similar words used in the specification and claims of this patent application do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Similarly, unless the context clearly indicates otherwise, the singular forms of "an," "a," or "the," etc., do not indicate a quantity limitation, but rather indicate the presence of at least one. Terms such as "comprising" or "including" mean that the element or object preceding "comprising" encompasses the features, integrals, steps, operations, elements, and / or components listed following "comprising" or "including," and do not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components, and / or collections thereof.

[0061] This invention provides a small-size high-strength spring steel, wherein the chemical composition of the spring steel, by weight percentage, includes the following components:

[0062] C: 0.64%~0.69%, Si: 1.38%~1.58%, Mn: 0.42%~0.54%, Cr: 0.52%~0.67%, V: 0.12%~0.18%, Mo: 0.03%~0.08%, Cu≤0.02%, Al≤0.005%, P≤70ppm, S≤80ppm, N≤50ppm, O≤20ppm, H≤0.8ppm, with the remainder being Fe and unavoidable impurities; and satisfying the strength-ductility index SE=[C]+0.3[Si]+1.2[Mn]+0.8[Cr]+5.0[V]+8.0[Mo], wherein the strength-ductility index SE is 3.0%~3.7%, where [C], [Si], [Mn], [Cr], [V], and [Mo] are the weight percentages of each element in the spring steel.

[0063] The design principles of each chemical element involved in the embodiments of the present invention are explained as follows:

[0064] C (carbon): C is an inexpensive and effective strengthening element, which can synergistically improve the strength of spring steel through solid solution strengthening and carbide particle precipitation. However, excessive addition of C will cause the martensite morphology to change from lath to needle or plate, which will significantly reduce the plasticity of spring steel wire. Therefore, in this embodiment of the invention, the C content is set to 0.64% to 0.69%.

[0065] Si (silicon): Si effectively improves the elastic modulus of wire rod and the spring resistance of finished springs through solid solution strengthening. At the same time, it effectively reduces the number of inclusions through strong deoxidation, significantly improving the purity of molten steel. However, excessive addition deteriorates plasticity and induces complete decarburization, which will affect the stability of coil springs. Therefore, in this embodiment of the invention, the Si content is controlled at 1.38% to 1.58%.

[0066] Manganese (Mn): Mn is an effective solid solution strengthening and hardenability-enhancing element, but excessive Mn can lead to excessive hardenability, causing abnormal microstructure in the steel wire after quenching and tempering, thus affecting the coil spring forming performance. Therefore, in this embodiment of the invention, the Mn content is set to 0.42% to 0.54%.

[0067] Cr (chromium): Cr readily combines with C to form carbides that dissolve in cementite lamellars, enhancing the matrix strength. Similar to Mn, excessive addition can lead to excessive hardenability and abnormal martensitic structure, affecting plasticity. Therefore, in this embodiment of the invention, the Cr content is set to 0.52%–0.67%.

[0068] Vanadium (V): On one hand, V acts as a microalloying element, strengthening wire rod through solid solution and precipitation; on the other hand, V forms MC-type carbide particles with carbon (C), inhibiting C diffusion. During quenching and tempering, V effectively increases the proportion of retained austenite and delays martensite decomposition, simultaneously improving the strength and plasticity of the steel wire. Considering the role of V and its high price, the V content in this embodiment of the invention is set to 0.12%–0.18%.

[0069] Mo (Mo): As an important creep-resistant element, Mo can effectively improve the stability of martensite structure during tempering, largely retain lath characteristics, and ensure the strength of heat-treated steel wire. However, Mo is expensive. Considering all factors, the content of Mo in this embodiment of the invention is set to 0.03% to 0.08%.

[0070] Cu (copper): Cu is an impurity element that tends to segregate at grain boundaries and phase boundaries, increasing hot brittleness and affecting the hot deformation rolling effect. Therefore, in this embodiment of the invention, Cu is controlled to be ≤0.02%.

[0071] Al (aluminum): Al is a strong deoxidizing impurity element, which easily combines with O to form hard Al2O3 inclusions, becoming a concentration point under alternating stress, leading to premature fatigue failure of the finished spring. Therefore, in this embodiment of the invention, Al is controlled to be ≤0.005%.

[0072] Phosphorus (P): P is an easily segregated impurity element that tends to accumulate at grain boundaries, reducing grain boundary strength and decreasing the plastic deformation capacity of quenched and tempered structures. Higher strength grades of heat-treated steel wire require higher purity of P; therefore, P should be controlled to ≤70 ppm.

[0073] S (sulfur): S is an impurity element that easily causes central segregation. Furthermore, excessively high S content can lead to large-sized MnS inclusions, affecting fatigue life. Therefore, in this embodiment of the invention, S is controlled to be ≤80ppm.

[0074] Nitrogen (N): N is an impurity element that readily combines with V to form large-sized (C,N) composite particles, which can improve strength but deteriorate plasticity. Because the amount of V added in this embodiment of the invention is relatively small, N needs to be controlled to ≤50ppm.

[0075] O (Oxygen): O is an impurity element that combines with other impurity elements such as Al to form inclusions, affecting fatigue performance. Therefore, O should be controlled to ≤20ppm.

[0076] H (hydrogen): H is dissolved in the matrix as an impurity element, which makes the steel wire prone to delayed fracture during heat treatment. The higher the strength of the steel wire, the higher the requirement for the purity of H. Taking into account all factors, the embodiments of this invention control H ≤ 0.8ppm.

[0077] The strength-ductility index (SE) is calculated as: SE = [C] + 0.3[Si] + 1.2[Mn] + 0.8[Cr] + 5.0[V] + 8.0[Mo]. This index represents the strength-ductility index of the spring steel of this invention. It comprehensively considers the influence of major alloying elements (C, Si, Mn, Cr, V, Mo) on the strength and ductility of the spring steel, and further refines the limits on the content of each element. A higher index indicates higher strength but lower ductility; conversely, a lower index indicates lower strength but higher ductility. Therefore, according to the strength-ductility requirements of the high-strength heat-treated spring steel wire of this invention, the strength-ductility index is limited to 3.0%–3.7%.

[0078] Furthermore, the main preparation process of the small-size high-strength spring steel production method of the present invention is as follows: steelmaking → continuous casting of large billets → diffusion billet opening → grinding and flaw detection → heating of small billets → high-speed linear rolling → Steyrmo linear cooling → salt bath heat treatment.

[0079] In some embodiments of the present invention, the steelmaking steps mainly include: preparing pig iron and alloy materials in appropriate proportions according to the composition requirements of the target spring steel, smelting to obtain initial molten iron, and then pre-treating the molten iron, smelting in a converter, refining in an LF furnace, and vacuum degassing in an RH furnace to obtain molten steel that meets the composition requirements.

[0080] In some embodiments of the present invention, the continuous casting of large billets mainly includes: molten steel after smelting is continuously cast in a tundish, where the superheat of the molten steel is controlled at 20-25°C by induction heating, with a liquid level fluctuation of ±1 mm; the molten steel exiting the tundish enters the crystallizer through a stopper rod and an immersion nozzle, the secondary cooling zone of the crystallizer is equipped with electromagnetic stirring and dynamic light reduction, the electromagnetic stirring frequency is 1.5-2.5 Hz, and the total reduction is 17-20 mm; the continuous casting speed is controlled at 0.50-0.57 m / min, the cross-sectional dimensions of the continuously cast large billet are (300-330) mm × (390-450) mm, and the length is 5.5-5.8 m. After the fixed-length continuously cast billet is removed from the line, it is immediately placed in a pit and covered for slow cooling, the pit temperature is ≥650°C, the pit cooling time is ≥30 h, and the pit exit temperature is ≤150°C. The carbon segregation index of the continuously cast billet is ≤1.08, the center segregation is ≤1.0 grade, the center porosity is ≤1.0 grade, and the corner cracks are ≤1.0 grade.

[0081] In some embodiments of the present invention, diffusion billet opening mainly includes: heating the billet after the large continuously cast billet exits the pit, using a high-temperature diffusion heating method, with a homogenization temperature of 1200-1230℃, a homogenization time of ≥270 min, a total heating time of 360-400 min, and a heating air-fuel ratio of 0.50-0.55. The billet opening is carried out using 9-pass continuous rolling, with a descaling water pressure of ≥22 MPa, a rolling temperature of 920-950℃, and the size of the small billet after opening is (140-160) mm × (140-160) mm × (15000-15700) mm. The carbon segregation index of the small billet is ≤1.05, with center segregation ≤0.5 grade, center porosity ≤0.5 grade, and corner cracks ≤0.5 grade.

[0082] In some embodiments of the present invention, the grinding and flaw detection mainly includes: after blanking, the small square billet undergoes surface peeling through roughing and fine finishing, with a single-sided grinding amount of 1.0–1.2 mm and a corner grinding amount of 4–6 mm. After the billet is ground, the surface is shot blasted for flaw detection. S90 grade steel shot (diameter 1.0–1.3 mm) is used for shot blasting, and the surface roughness Ra after shot blasting is ≤20 μm. The flaw detection ensures that the crack depth at the edges and corners is ≤40 μm.

[0083] In some embodiments of the present invention, the heating of small square billets mainly includes: the ground small square billets are heated by a walking beam furnace, wherein the preheating section temperature is 850-950°C, the heating section temperature is 950-1030°C, the soaking section temperature is 1030-1080°C, the total heating time is 75-90 min, the soaking time is ≥45 min, the air-fuel ratio in the preheating and heating sections is 0.60-0.65, and the air-fuel ratio in the soaking section is 0.45-0.50.

[0084] In some embodiments of the present invention, high-speed wire rod controlled rolling mainly includes: continuous rolling of small square billets to obtain initial wire rods of the target size. The rolling process adopts continuous bar and wire rolling mill controlled rolling. The wire rod specifications are φ5.0~10 mm. After the billet exits the heating furnace, it is descaled by a high-pressure descaling machine with a descaling water pressure ≥20 MPa. The initial rolling temperature is 920~940℃, the finishing mill inlet temperature is 850~880℃, the finishing mill outlet temperature is 900~930℃, the sizing inlet temperature is 820~850℃, the wire drawing temperature is 800~830℃, and the rolling speed is 60~120m / s.

[0085] In some embodiments of the present invention, the initial wire rod obtained by high-speed wire rod rolling is cooled in a multi-stage controlled cooling manner using the Stellmore linear cooling process to obtain intermediate wire rod. The Stellmore linear cooling system mainly includes: (1) The first stage is the rapid cooling stage, during which fan #1 is turned on at 50%~100%, fan #2 at 20%~50%, fan #3 is turned off, insulation covers #1~4 are turned on, the roller speed is 0.95~1.05 m / s, the cooling rate is controlled at 6.5~8.5℃ / s, and the end temperature of the first stage is 635~655℃; (2) The second stage is the slow cooling stage, during which fans #4~7 are turned off, insulation covers #6~7 corresponding to fan #6 are turned on, the remaining insulation covers are turned off, the roller speed is 1.05~1.15 m / s, the cooling rate is controlled at 1.0~1.5℃ / s, and the end temperature of the second stage is 610~630℃; (3) The third stage is the air cooling stage, during which fans #8~12 are turned off and the corresponding insulation covers #9~13 are turned on, the roller speed is 1.15~1.25 m / s. The fan speed is 2.5–3.5℃ / s, the cooling rate is 2.5–3.5℃ / s, and the winding temperature is 480–510℃. The fan air volume of the Steilmo line is uniformly 260,000 m³ / s. 3 / h.

[0086] In some embodiments of the present invention, the salt bath heat treatment mainly includes: sequentially heating and isothermal salt bath treatment of intermediate wire rods obtained by Stellmore linear cooling to obtain finished wire rods. The combustion gas in the heating furnace is mainly methane, and methanol gas is added to control the carbon potential inside the furnace at 0.60%–0.65% as a protective atmosphere, fundamentally preventing or greatly reducing carbon loss (decarburization) on the surface of the wire rods. When 3.0% ≤ Polymer Index (SE) < 3.25%, the induction heating temperature is 865–872°C, and the salt bath temperature is 620–628°C; when 3.25% ≤ SE < 3.5%, the induction heating temperature is 875–882°C, and the salt bath temperature is 610–618°C; when 3.5% ≤ SE ≤ 3.7%, the induction heating temperature is controlled at 885–892°C, and the salt bath temperature is 600–608°C. When 5 mm ≤ wire rod diameter D < 8 mm, the wire speed is set to 5.0–5.5 m / min; when 8 mm ≤ D ≤ 10 mm, the wire speed is set to 4.3–4.8 m / min. Optionally, the salt used in the salt bath of this embodiment is a nitrate, such as a mixture of KNO3, NaNO2, and NaNO3.

[0087] Microstructure and properties of wire rod: The microstructure of finished spring steel wire rod is ferrite + sorbite, with sorbite content ≥95%, sorbite grain size 6.6~8.5μm, sorbite lamellar spacing 140~170 nm. The sorbite content was determined by “YB / T169-2014 Metallographic Test Method for Sorbite Content of High Carbon Steel Wire Rod”, and the sorbite grain size was determined by “GB / T 36165-2018 Determination of Average Grain Size of Metals by Electron Backscatter Diffraction (EBSD) Method”.

[0088] The finished wire rod has a tensile strength of 1150–1190 MPa, a strength fluctuation of ≤25 MPa within the same coil, a reduction of area ≥57%, and an elongation after fracture ≥18%. The finished wire rod, after drawing, quenching, and tempering, is used to prepare automotive tailgate spring wire, with a tensile strength of 2275–2325 MPa and a reduction of area ≥50%. Mechanical properties are tested according to "GB / T 228.1-2021 Metallic Materials—Tensive Testing—Part 1: Test at Room Temperature".

[0089] The following specific embodiments further illustrate the technical solution and beneficial effects of the present invention. Nine embodiments demonstrate the synergistic relationship between the composition range, process parameters, and product performance defined by the present invention. The production methods of each embodiment all follow the complete process flow described above, namely, steelmaking → continuous casting of large billets → diffusion billet opening → grinding and flaw detection → heating of small billets → high-speed wire rod rolling → Steyrmo wire rod cooling → salt bath heat treatment. The differences between the embodiments lie only in the specific chemical composition ratio (see Table 1) and the process parameters of some key processes (see Tables 2 to 5), in order to adapt to different specifications or performance targets. All embodiments prepare finished wire rods according to the aforementioned method and further process them into automotive tailgate spring steel wires. The corresponding microstructure and performance test results are shown in Table 6.

[0090] Table 1. Chemical composition of spring steel wire rods from Examples 1 to 9

[0091]

[0092] In Table 1, C, Si, Mn, Cr, V, Mo, Cu, and Al represent their respective element contents, all in wt%; P, S, N, O, and H represent their respective element contents, all in ppm; in addition to the elements mentioned in Table 1, the chemical composition of each embodiment also includes the balance of Fe and unavoidable impurities, in wt%.

[0093] Table 2. Large billet continuous casting process parameters in the spring steel production methods of Examples 1-9

[0094]

[0095] Table 3. High-speed wire rod controlled rolling process parameters in the spring steel production methods of Examples 1-9

[0096]

[0097] Table 4. Stellmordial controlled cooling process parameters in the spring steel production methods of Examples 1-9

[0098]

[0099] Table 5. Salt bath heat treatment process parameters in the spring steel production methods of Examples 1-9

[0100]

[0101] Table 6. Microstructure and properties of spring steel wire rods and spring steel wires from Examples 1-9

[0102]

[0103] As can be seen from the data in Tables 1 to 6, by strictly implementing the component design window and key process control of the present invention, Examples 1 to 9 all stably achieved the expected excellent performance.

[0104] From the perspective of matching composition and process, the core of this invention lies in the coordinated control of the "strength-ductility index (SE)" and the "salt bath heat treatment parameters." Taking Examples 2 (SE=3.57%), 3 (SE=3.70%), and 8 (SE=3.65%) as examples, their SE values ​​are in the higher range of this invention, indicating a higher total amount of alloying elements. To match their high hardenability, the production process correspondingly adopted a higher induction heating temperature (885-892℃) and a lower salt bath temperature (600-608℃) to ensure sufficient austenitization and suppress the precipitation of proeutectoid ferrite. Ultimately, the wire rods in these examples obtained fine sorbite grains (6.9-8.2μm) and ideal lamellar spacing (140-166nm). Based on this, after subsequent quenching and tempering treatment, the tensile strength of the spring steel wire all exceeded 2277 MPa, reaching a maximum of 2325 MPa.

[0105] Conversely, taking Examples 5 (SE=3.00%) and 6 (SE=3.11%) as examples, their SE values ​​are at the lower limit of the range. The process employed a relatively low induction heating temperature (869–872°C) and a relatively high salt bath temperature (620–628°C) to avoid grain coarsening and overheating. Despite the relatively low alloy content, thanks to precise process control, the finished wire rod exhibits particularly outstanding plasticity (up to 62.8% reduction of area) and spring steel wire (up to 54.9% reduction of area), while the tensile strength remains stable at a high level of 2288–2319 MPa. This fully demonstrates the scientific nature of the present invention's joint design of composition and process using the strength-plasticity index (SE).

[0106] Comparing examples of different specifications (Φ5.0 mm to Φ10.0 mm), such as Example 3 (Φ5.5 mm) and Example 7 (Φ10.0 mm), the smaller specification wire rod (Example 3) has a higher rolling speed (115 m / s), faster cooling, and is more likely to obtain a fine microstructure, but it also carries the risk of uneven performance. This invention effectively solves this problem through precise segmented controlled cooling on the Steyrmo line (the first segment rapidly passes through the ferrite region, and the second segment isothermally martensitizes). The same-ring strength fluctuation of Example 3 is controlled at 24 MPa, and the area shrinkage fluctuation is only 2.8%, demonstrating its excellent microstructure uniformity. Large-diameter wire rods (Example 7) are rolled at a slower speed (60 m / s), making it difficult for the core heat to dissipate. This invention uses salt bath heat treatment (induction heating 880℃, salt bath 615℃) combined with a slower wire speed (4.3 m / min) to ensure a uniform transformation of the entire cross-sectional structure. Ultimately, the wire rod tensile strength reaches 1190 MPa, and the steel wire tensile strength reaches 2325 MPa, demonstrating equally excellent performance.

[0107] Compared with the prior art, the technical solution of the present invention has achieved significant progress.

[0108] Simultaneous improvement of strength and plasticity: In existing technologies, the tensile strength of spring steel wire after quenching and tempering is generally in the range of 2150-2250 MPa, with a reduction of area of ​​about 35%-40%. However, in all embodiments of this invention, the tensile strength of the spring steel wire is consistently in the higher range of 2275-2325 MPa, while the reduction of area is ≥50%, achieving an excellent match between ultra-high strength and superior plasticity. This is crucial for improving the fatigue life and formability of automotive tailgate springs.

[0109] Uniformity control of microstructure properties: Existing technologies generally suffer from problems such as large fluctuations in the properties of the same coil of wire rod and unstable sorbitization. This invention, through a combined process of "high-speed wire rod controlled rolling + segmented controlled cooling on a Stellmo line + isothermal heat treatment in a salt bath," successfully controls the fluctuation of the strength of the same coil of the finished wire rod to ≤25 MPa, stabilizes the sorbitium ratio at over 95%, and maintains a narrow fluctuation range in grain size and interlamellar spacing (6.6–8.5 μm, 140–170 nm). This high uniformity provides an excellent base material foundation for subsequent drawing and quenching / tempering processes.

[0110] Refined Composition Design: This invention abandons a broad range of compositions and creatively proposes the Strength-Plasticity Index (SE). Based on this index, the content ranges of C, Si, Mn, Cr, V, and Mo are refined. This results in a very high degree of matching between the process and the composition, avoiding the risk of process failure due to composition fluctuations, and significantly improving the stability and reproducibility of product performance.

[0111] In summary, Examples 1-9 of this invention fully demonstrate that the technical solution for which protection is sought is clear, complete, and feasible. All embodiments successfully produced small-size high-strength spring steel wire rods with performance comprehensively superior to existing technologies, and the automotive tailgate spring steel wires made from them, achieving the beneficial effects described in the invention summary section.

[0112] While the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the invention. Those skilled in the art can make various modifications and refinements without departing from the spirit and scope of the invention. Therefore, the scope of protection of the present invention shall be determined by the claims.

Claims

1. A small-size high-strength spring steel, characterized in that, The chemical composition of the spring steel, by weight percentage, includes the following components: C: 0.64%~0.69%, Si: 1.38%~1.58%, Mn: 0.42%~0.54%, Cr: 0.52%~0.67%, V: 0.12%~0.18%, Mo: 0.03%~0.08%, Cu≤0.02%, Al≤0.005%, P≤70ppm, S≤80ppm, N≤50ppm, O≤20ppm, H≤0.8ppm, with the remainder being Fe and unavoidable impurities; and satisfying the strength-ductility index SE=[C]+0.3[Si]+1.2[Mn]+0.8[Cr]+5.0[V]+8.0[Mo], wherein the strength-ductility index SE is 3.0%~3.7%, where [C], [Si], [Mn], [Cr], [V], and [Mo] are the weight percentages of each element in the spring steel; The method for producing the spring steel includes the following steps: Large billet continuous casting: molten steel that meets the composition requirements is continuously cast, the superheat of the molten steel in the tundish and the fluctuation of the liquid level are controlled, electromagnetic stirring is configured in the secondary cooling zone and dynamic light pressure is adopted, and the continuous casting speed is controlled to obtain a large billet of the target size. Then the large billet is placed into the pit for slow cooling. Diffusion billet preparation: The continuously cast large square billet is reheated and subjected to multiple passes of continuous rolling to obtain a small square billet of the target size; High-speed wire rod rolling: The small square billet is heated to a higher temperature and continuously rolled to obtain the initial wire rod; Stellmore Line Controlled Cooling: The initial wire rod is subjected to multi-stage controlled cooling to obtain intermediate wire rod; Salt bath heat treatment: Under a protective atmosphere, the intermediate wire rod is heat-treated by selecting the corresponding induction heating temperature and salt bath temperature according to the strength-ductility index SE of the spring steel, and the corresponding wire speed is selected according to the diameter range of the wire rod to obtain the finished wire rod. The Stellmore linear cooling system includes: The initial coil is subjected to multi-stage controlled cooling to obtain intermediate coils: (1) First rapid cooling section: control the roller speed to be 0.95~1.05 m / s, the cooling rate to be 6.5~8.5 ℃ / s, and the end temperature to be 635~655℃; (2) Second slow cooling section: control the roller speed to be 1.05~1.15 m / s, the cooling rate to be 1.0~1.5 ℃ / s, and the end temperature to be 610~630℃; (3) Third air cooling section: control the roller speed to be 1.15~1.25 m / s, the cooling rate to be 2.5~3.5 ℃ / s, and the winding temperature to be 480~510℃; The salt bath heat treatment includes: The carbon potential of the atmosphere inside the heating furnace is controlled at 0.60% to 0.65%. Based on the strength-ductility index (SE) of the spring steel, the induction heating temperature and salt bath temperature are controlled as follows: When 3.0% ≤ SE < 3.25%, the induction heating temperature is 865~872℃, and the salt bath temperature is 620~628℃; When 3.25% ≤ SE < 3.5%, the induction heating temperature is 875~882℃, and the salt bath temperature is 610~618℃; When 3.5% ≤ SE ≤ 3.7%, the induction heating temperature is 885~892℃, and the salt bath temperature is 600~608℃; The wire speed is controlled according to the wire diameter D: When 5 mm ≤ D < 8 mm, the routing speed is 5.0~5.5 m / min; When 8 mm ≤ D ≤ 10 mm, the routing speed is 4.3~4.8 m / min.

2. The small-size high-strength spring steel according to claim 1, characterized in that, The wire rod microstructure of the spring steel is ferrite + sorbite, wherein the sorbite is ≥95%, the grain size of the sorbite is 6.6~8.5μm, and the interlamellar spacing of the sorbite is 140~170 nm.

3. The small-size high-strength spring steel according to claim 1, characterized in that, The finished spring steel wire rod has a tensile strength of 1150-1190 MPa, a strength fluctuation of ≤25 MPa within the same coil, a reduction of area ≥57%, and an elongation after fracture ≥18%.

4. An application of small-diameter high-strength spring steel as described in any one of claims 1 to 3, characterized in that, The finished wire rods of the spring steel are sequentially drawn, quenched, and tempered to produce automotive tailgate spring steel wires. The tensile strength of the spring steel wire is 2275-2325 MPa, and the reduction of area is ≥50%.

5. A method for producing small-diameter high-strength spring steel as described in any one of claims 1 to 3, characterized in that, The large billet continuous casting includes: The molten steel that is continuously cast in the tundish is controlled by induction heating to maintain the superheat of the molten steel in the tundish at 20-25°C and the liquid level fluctuation at ±1mm. Electromagnetic stirring and dynamic light pressure are applied in the secondary cooling zone, wherein the electromagnetic stirring frequency is 1.5 to 2.5 Hz and the total pressure reduction under dynamic light pressure is 17 to 20 mm. By controlling the continuous casting billet pulling speed to 0.50–0.57 m / min, large continuously cast billets with cross-sectional dimensions of (300–330) mm × (390–450) mm and lengths of 5.5–5.8 m are obtained. After the continuous casting billet is removed from the line, it is slowly cooled in a pit. The temperature in the pit is ≥650℃, the cooling time in the pit is ≥30 h, and the temperature when it comes out of the pit is ≤150℃. The carbon segregation index of the continuously cast large square billet is ≤1.08, and the central segregation, central porosity, and corner cracks are all no greater than level 1.

0.

6. The method for producing small-diameter high-strength spring steel according to claim 5, characterized in that, The diffusion blanking includes: The continuously cast large square billet is heated, wherein the homogenization temperature is 1200~1230℃, the homogenization time is ≥270 min, and the total heating time is 360~400 min; The small square billet with dimensions of (140~160)mm×(140~160)mm×(15000~15700)mm is obtained by multiple passes of continuous rolling; The initial rolling temperature is 920-950℃, the carbon segregation index of the small square billet is ≤1.05, and the central segregation, central porosity, and corner cracks are all no greater than level 0.

5.

7. The method for producing small-diameter high-strength spring steel according to claim 5, characterized in that, The high-speed wire rod controlled rolling includes: The heated small square billet is continuously rolled to obtain the initial wire rod with a wire rod specification of Φ5.0~10 mm; The rolling temperature is 920–940℃, the finishing mill inlet temperature is 850–880℃, the finishing mill outlet temperature is 900–930℃, the sizing and reducing inlet temperature is 820–850℃, the wire drawing temperature is 800–830℃, and the rolling speed is 60–120 m / s.

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