2000MPa class vanadium-containing 55SiCr spring steel hot-rolled wire rod and its production process

Abstract

This invention relates to a 2000 MPa class vanadium-containing 55SiCr hot-rolled wire rod and its production process. The wire rod employs a C-Si-Mn-Cr-V composition design. The hot-rolled extruded wire rod is first rapidly cooled to the bainite phase region in an online salt bath cooling tank at a wire rod cooling rate of ≥ 45°C / s to complete bainite transformation and increase retained austenite, thereby avoiding the formation of soft phases such as ferrite. Subsequently, heating and tempering are performed in an online salt bath high-temperature tempering tank to remove retained austenite. We have successfully manufactured hot-rolled wire rods with improved stability, containing tempered bainite by volume percentage ≥ 92% in the structure, with the remainder being retained austenite and nanoprecipitation phases. By effectively adjusting the strength and plasticity match, we have achieved a wire rod tensile strength ≥ 2010 MPa and a reduction ratio ≥ 18%, realizing the production of ultra-high-strength 55SiCrA spring steel wire rods with strong pull-out capabilities. This allows for the production of ultra-high-strength spring products by softening through tempering and then winding, without the need for a downstream oil quenching process.

Description

Technical Field

[0001] The present invention relates to the technical field of alloy materials, specifically to a 2000MPa grade vanadium-containing 55SiCr spring steel hot-rolled wire rod and its production process.

Background Art

[0002] With the evolution of China's industrialization process, the structure of the spring steel industry has been continuously upgraded. Although high-quality spring steel is in short supply to meet the demand, due to technical equipment reasons, there are few high-quality spring steel products in China, and the market competitiveness of the produced spring steel is insufficient. The 55SiCr spring steel grade is generally applied in manufacturing fields such as automobiles and railways with complex application environments that require high stress, high strength, anti-elasticity reduction performance, high fatigue resistance, etc., and is highly regarded. High-strength and high-plasticity spring steel hot-rolled wire rods are of great significance for improving the performance of spring products and saving process energy consumption.

[0003] 55SiCr spring steel wire rods produced in China are mainly air-cooled products, where billets are hot-rolled and then cooled using a Stermor air-cooling line. After the hot-rolled and extruded wire rod enters the air-cooling line, its cooling capacity is limited by the conveyor path length and blower airflow, resulting in a wire rod cooling rate of generally 5-12°C / s. Therefore, it takes a long time to control the microstructure and performance of the hot-rolled wire rod as it sequentially enters the ferrite and pearlite phases after extrusion. Consequently, only 1000MPa class spring steel wire rods (tensile strength 950-1150MPa, reduction ratio of 30% or more) with a microstructure consisting of soft phases of ferrite and pearlite can be produced, and the presence of harmful structures such as martensite or bainite that would affect use is not permitted. For example, in the controlled rolling and controlled cooling process applied to the 1960 MPa class spring steel 55SiCr disclosed in patent CN117488044A, after controlled rolling and controlled cooling, a pearlite spring steel hot-rolled wire rod with a tensile strength of only 1050-1100 MPa is obtained. Furthermore, because the cooling rate differs between the side exposed to air (sun side) and the side not exposed to air (shadow side) as the wire rod passes through the air-cooling line, it is more likely to cause performance variations in the hot-rolled wire rod and increase the risk of pull-out breakage.

[0004] In subsequent downstream wire-making processes, the low-strength level of hot-rolled wire limits the improvement of the strength level of the spring product. Therefore, it is generally necessary to further improve the strength through heat treatments such as oil quenching and tempering to obtain a high-strength spring product. For example, in the heat treatment process for high-strength Cr-V-Mo-based medium-carbon spring steel disclosed in patent CN117004888A, the hot-rolled wire, after cooling control, needs to undergo heat treatment processes such as austenitization by heating, oil quenching, and tempering in order to obtain a spring steel wire product with a tensile strength of 2000 MPa or more and a tempered troostite structure. Molten salt is a cooling / heat exchange medium with high specific heat capacity, high heat exchange capacity, and excellent stability. However, conventional salt baths are mostly offline salt bath heat treatments, meaning that high-strength spring products are obtained by further improving the strength of air-cooled wire through salt bath heat treatment. For example, in the heat treatment method for spring steel and the resulting spring steel disclosed in patent CN117625895A, the wire rod must undergo sequential heat treatment processes such as austenitization by heating, salt bath, quenching, and tempering in order to obtain spring steel with a tensile strength of 2000 MPa or more, containing martensite, retained austenite, lower bainite, and trace amounts of nanoscale precipitated phases. Consequently, the wire coils, which are hot-rolled wire rods removed from the Stermor air-cooling line and packaged, must undergo dismantling, single / multi-wire winding, scale removal, heat treatment, and wire drawing processes after being transported to downstream wire manufacturing users in order to obtain high-strength spring steel wire. This process increases energy consumption and costs downstream due to the heat treatment steps.

[0005] As can be seen from the above, at present there is no 55SiCr hot-rolled spring steel wire rod that possesses ultra-high strength and strong tensile strength, which can be used to manufacture ultra-high-strength spring products by softening through tempering and then winding, without requiring a downstream quenching process. [Overview of the project] [Problems that the invention aims to solve]

[0006] The present invention aims to solve at least to some extent one of the above technical problems, and enables the production of 55SiCr spring steel wire rods having ultra-high strength of 2000 MPa and strong drawability, and enables the production of ultra-high strength spring products by softening by tempering and then winding without requiring a downstream oil quenching process, and provides 2000 MPa class vanadium-containing 55SiCr spring steel hot-rolled wire rods and the production process thereof. [Means for solving the problem]

[0007] The technical means employed by the present invention to solve the technical problem are: This is a 2000 MPa class vanadium-containing 55SiCr hot-rolled spring steel wire rod with a chemical composition of C: 0.50%~0.70%, Si: 1.20%~1.60%, Mn: 0.50%~0.80%, Cr: 0.50%~0.80%, P≦0.020%, S≦0.015%, and V: 0.040%~0.080% by mass percentage, with the remainder being Fe and unavoidable impurities. Its microstructure contains tempered bainite ≥ 92% by volume, with the remainder being retained austenite and nanoprecipitation phases.

[0008] The design basis for the chemical composition and mass percentage of the hot-rolled wire rod described above includes the following: (1) Carbon, chromium, vanadium: Element C is used as an economical strengthening element for solid solution strengthening and the formation of a sufficient carbide precipitation strengthening phase, significantly increasing the material strength. Cr is a medium-strength carbide-forming element that significantly improves the hardenability of steel and strengthens the matrix. Microalloying element V is mainly used to isothermally form a dispersed precipitation strengthening phase VC in the medium temperature range, strengthening the matrix and reducing the hydrogen-induced cracking susceptibility of wire rods. However, if C is too high, the plasticity and toughness of the steel deteriorate; if Cr is too high, temper brittleness increases; and if V is too high, the wire rod cost increases significantly. Therefore, the mass percentage of C is limited to 0.50% to 0.70%, the mass percentage of Cr is limited to 0.50% to 0.80%, and the mass percentage of V is limited to 0.040% to 0.080%. (2) Silicon and Manganese: Si and Mn can be used as deoxidizing agents in the steelmaking process. Si performs a solid solution strengthening effect in steel, expanding the ferrite formation range and contributing to improved temper stability and improved plasticity of wire rods. Mn can expand the austenite phase region and contribute to the refinement of the phase transformation structure, the control of matrix structure content, and the improvement of steel strength. However, if the Si content is too high, the tendency for decarburization during the steel rolling process increases, and if the Mn content is too high, it promotes the growth of crystal grains and increases temper brittleness. Therefore, the mass percentage of Si is limited to 1.20% to 1.60%, and the mass percentage of Mn is limited to 0.50% to 0.80%. (3) Phosphorus and sulfur: P can cause grain coarsening, significantly reducing the plasticity and toughness of the steel. S can increase harmful impurities in the steel grade, causing red-hot brittleness. Therefore, the mass percentage of P is limited to ≤0.020%, and the mass percentage of S is limited to ≤0.0015%.

[0009] The above hot-rolled wire employs a C-Si-Mn-Cr-V composition design, with a microstructure primarily composed of tempered bainite, combined with small amounts of retained austenite and nanoprecipitation phases to form a mixed structure. Compared to ferrite and pearlite soft phase structures, the tempered bainite structure significantly increases wire strength, eliminates bainite brittleness, further toughens the matrix, improves the stability of retained austenite, and enables plasticization of the wire. This structure achieves strengthening and plasticization of high-V composition 55SiCr hot-rolled wire.

[0010] Preferably, the hot-rolled wire has a diameter of 6.5 to 14 mm, a tensile strength of ≥2010 MPa, a reduction ratio of ≥18%, a variation in mechanical properties at the same circumference of <30 MPa, and the nano-precipitation phase contains dispersed VC. In this way, the product is given excellent hydrogen embrittlement resistance and strength-plastic matching properties.

[0011] This is a production process for 2000MPa class vanadium-containing 55SiCr spring steel hot-rolled wire rods, in which hot rolling is performed based on the chemical composition of the 2000MPa class vanadium-containing 55SiCr spring steel hot-rolled wire rod described above, the wire rods are sequentially cooled in an online salt bath cooling tank at a wire rod cooling rate of ≥45℃ / s to the wire rod bainite phase region, and then heated and tempered in an online salt bath high-temperature tempering tank to complete the production of hot-rolled wire rods in which the microstructure contains tempered bainite by volume percentage ≥92%, with the remainder being retained austenite and nanoprecipitation phases.

[0012] Compared to the Stermore air-cooling line, which considers bainite as a brittle abnormal structure, hot-rolled wire rods enter an online salt bath cooling tank directly, allowing for extremely rapid cooling to the bainite phase region at a wire rod cooling rate of ≥45°C / s. Online molten salt quenching avoids the formation of ferrite and pearlite soft phases, enabling the transformation of most of the high-temperature austenite structure of the wire rod to a bainite structure. Furthermore, rapid temperature-raising tempering in an online salt bath high-temperature tempering tank achieves the transformation of the wire rod's bainite structure to a tempered bainite structure, stabilization of small amounts of distributed retained austenite, and large-scale dispersion and precipitation of alloy carbides (VC). This completes the control of the tempered bainite and retained austenite multiphase structure of ultra-high-strength vanadium-containing 55SiCr hot-rolled wire rods, and strengthens the wire rod by adjusting and controlling the dispersion and precipitation of alloy carbides (VC). Furthermore, by heat-treating the wire in an online salt bath, the wire surface can be sufficiently brought into contact with the molten salt in the salt bath. Compared to air cooling, there are no positive or negative surfaces, heat transfer is more uniform, and variations in mechanical properties along the same circumference can be further reduced.

[0013] Preferably, the process involves producing wire rods by hot rolling using a continuously cast billet, wherein the continuously cast billet is obtained from molten steel through a continuous casting process, and the carbon segregation index of the continuously cast billet is ≤1.05. By controlling the internal quality of the continuously cast billet, it is possible to avoid the problem of carbon segregation causing austenitization, which remains and cannot be removed, resulting in localized differences in mechanical properties after quenching and tempering, and further causing a significant decrease in strength and plasticity.

[0014] Preferably, the superheating of the continuous casting tundish in the continuous casting process is controlled to 12-33°C. By controlling the superheating, the proportion of equiaxed crystals is increased, improving carbon segregation. The crystallization apparatus molten metal level fluctuation is controlled to within ±5 mm. By controlling the crystallization apparatus molten metal level fluctuation, the uniformity of the billet shell is increased. The tip electromagnetic stirring current is 220-420 A with a frequency of 2-5 Hz, and the end electromagnetic stirring current is 380-560 A with a frequency of 8-12 Hz. By combining tip electromagnetic stirring and end electromagnetic stirring, impurities in the molten steel are sufficiently floated to reduce inclusions, the bridging phenomenon of columnar crystals is reduced, the degree of segregation is lowered, and the internal quality of the continuously cast billet is further improved.

[0015] Preferably, the process involves producing wire rods using a continuously cast billet through sequential heating furnace, hot rolling, and wire rod discharge processes, wherein the finish rolling temperature in the hot rolling process is 870-920°C, the finish rolling reduction is 5-10%, and the wire rod grain size is ≤20 μm. By effectively utilizing the deformation storage energy of the low-temperature finish rolling, the wire rod grain size can be refined, providing good structural preparation for subsequent wire rod structural transformation. In this way, the dispersion and precipitation of alloy carbides VC can be adjusted and controlled, further increasing the strength and plasticity of the high-V composition 55SiCr hot-rolled wire rod on the structure.

[0016] Preferably, the wire discharge temperature in the wire discharge process is 850 to 900°C. In controlled rolling and controlled cooling that combines hot rolling and a Stermore air cooling line, employing low-temperature rolling may cause the wire discharge or the initial stage of cooling control to be in a region sensitive to reticular carbide precipitation, potentially leading to the occurrence of reticular carbide defects. In contrast, the present invention employs a process design of low-temperature rolling and online molten salt quenching + tempering treatment. Online salt bath quenching rapidly reduces the temperature to below the reticular carbide precipitation temperature range, thus avoiding the precipitation of reticular carbides. In this way, most of the high-temperature austenite structure is transformed into a bainite structure, and furthermore, the advantages of grain refinement by low-temperature rolling and the online molten salt quenching + tempering treatment are achieved.

[0017] Preferably, the oxide scale on the surface of the continuous casting billet is removed with high-pressure scale removal water before rolling in the hot rolling process, with the pressure of the high-pressure scale removal water being 18-22 MPa. This prevents oxide scale from being pressed into the wire during the rolling process, which would degrade the surface quality of the wire.

[0018] Preferably, the molten salt temperature in the online salt bath cooling tank is 380-480°C, the wire cooling rate is ≤60°C / s, and the wire cooling time is 10-30 s. By exchanging heat with the molten salt, the wire can be rapidly cooled to the molten salt temperature. If the molten salt temperature is too high and the cooling time is too short, the amount of bainite structure transformation will be too low, resulting in a decrease in the bainite content after subsequent tempering and a decrease in strength. If the molten salt temperature is too low, it will cause the formation of martensite or troostite, and the plasticity of the wire will decrease after tempering.

[0019] Preferably, the molten salt circulation rate of the online salt bath cooling tank is 240 to 350 t / h, and the molten salt temperature rise is ≤ 15°C. The higher the molten salt circulation rate, the lower the molten salt temperature rise and the higher the energy consumption. By controlling the molten salt temperature rise, the control accuracy of online molten salt quenching can be further improved, and variations in the mechanical properties of the wire can be reduced.

[0020] Preferably, the molten salt temperature in the online salt bath high-temperature tempering tank is 450-500°C, and the wire isothermal time is 200-400 s. This temperature is in the medium tempering temperature range, which allows for further control of the transformation of the bainite structure to a tempered bainite structure and the stabilization of small amounts of distributed retained austenite. It is also in the VC precipitation temperature range, which allows for the stable dispersion and precipitation of a large amount of alloy carbide VC, forming a nanoscale precipitated phase. If the wire isothermal time is too short, the bainite structure will not transform sufficiently, and the wire material will become brittle. If the wire isothermal time is too long, the retained austenite content will increase, and the wire strength will decrease.

[0021] Preferably, the molten salt circulation rate of the online salt bath high-temperature tempering tank is 200 to 370 t / h, and the molten salt is circulated both in front of and behind the online salt bath high-temperature tempering tank along the wire production direction, with the front molten salt circulation rate being 130 to 160 t / h higher than the rear molten salt circulation rate, and the molten salt temperature rise being ≤ 10°C. Since the temperature when the wire enters the front of the online salt bath high-temperature tempering tank is lower than the temperature when it is in the rear of the online salt bath high-temperature tempering tank after heat exchange, increasing the molten salt circulation rate in the initial stages of the tempering process allows for rapid heat exchange and stabilizes the rise in molten salt temperature. This eliminates the need for excessively high molten salt circulation rates in the later stages of the tempering process, saving energy consumption by reducing the molten salt circulation rate, and thus controlling the rise in molten salt temperature further controls the accuracy of the wire performance.

[0022] This is an application of 2000MPa class vanadium-containing 55SiCr hot-rolled wire rod, used to manufacture ultra-high-strength springs through softening by tempering. In this way, downstream users of the hot-rolled wire rod can reduce energy consumption and costs by eliminating the need for an oil quenching process, and contribute to improving the strength level of the spring products. [Effects of the Invention]

[0023] Compared to conventional technologies, the beneficial effects of the present invention are as follows: (1) In the vanadium-containing 55SiCr spring steel hot-rolled wire rod with a tensile strength of 2000 MPa class described in the present invention, a C-Si-Mn-Cr-V component design is adopted, and the microstructure type is a mixed structure consisting mainly of tempered bainite with a small amount of retained austenite and nanoprecipitation phase. The tempered bainite toughens the matrix, improves the stability of retained austenite to plasticize the wire rod, and strengthens the wire rod by adjusting and controlling the dispersion and precipitation of alloy carbide VC. The strength and plasticity matching of the high V composition-containing 55SiCr hot-rolled wire rod is effectively adjusted on the microstructure, making it possible to achieve a wire rod tensile strength of ≥2010 MPa and a reduction ratio of ≥18%, and it can be used in application areas for manufacturing ultra-high strength springs, and ultra-high strength spring products can be manufactured by softening by tempering and then winding without requiring a downstream oil quenching process. (2) In the production process of the hot-rolled wire rod of vanadium-containing 55SiCr spring steel with a tensile strength of 2000 MPa grade described in the present invention, the wire rod after hot rolling is processed by online molten salt quenching + tempering treatment to avoid the formation of ferrite and pearlite soft phases. First, most of the high-temperature austenite structure is transformed into bainite structure, and then rapid heating tempering is carried out by an online salt bath high-temperature tempering tank to realize the transformation of the wire rod bainite structure into tempered bainite structure, the stabilization of a small amount of retained austenite, and the massive dispersion precipitation of alloy carbide VC, so as to realize the production of 55SiCr spring steel wire rod with ultra-high strength and strong drawing ability. (3) In the production process of the hot-rolled wire rod of vanadium-containing 55SiCr spring steel with a tensile strength of 2000 MPa grade described in the present invention, by further combining the advantages of low-temperature rolling deformation stored energy and online molten salt quenching + tempering treatment, the grain size can be further refined, the dispersion precipitation of alloy carbide VC can be regulated and controlled, and the strength and plasticity of the wire rod can be increased. By further controlling the internal quality of the continuous casting billet and combining the advantages of the online salt bath, the tissue uniformity and yield can be improved, the mechanical property variation of the same circumference of the hot-rolled wire rod can be made <30 MPa, and good industrial applicability can be provided.

[0024] The above and / or additional aspects and advantages of the present invention will become apparent and be easily understood from the description of the embodiments with reference to the following drawings.

Brief Description of the Drawings

[0025] [Figure 1] It is a macrostructure diagram of the continuous casting billet of Example 1 of the present invention. [Figure 2] It is a comparison diagram of the wire rod structures of Example 1 and Comparative Example 1 of the present invention. In FIG. 2, (a) shows the tempered bainite + retained austenite + nano-precipitation phase structure diagram of Example 1, and (b) in FIG. 2 shows the pearlite + ferrite structure diagram obtained in Comparative Example 1. [Figure 3] It is a comparison diagram of the wire rod structure of Example 2 of the present invention. [Figure 4] It is a comparison diagram of the wire rod structure of Example 3 of the present invention. [Figure 5] This is a comparative diagram of the wire structure of Embodiment 4 of the present invention. [Modes for carrying out the invention]

[0026] The embodiments described below with reference to the drawings are illustrative and intended to interpret the present invention, and should not be understood as limiting the present invention.

[0027] (Example 1) This is a production process for 2000 MPa class vanadium-containing 55SiCr hot-rolled wire rods. The hot-rolled wire rods have a chemical composition of C: 0.5%, Si: 1.32%, Mn: 0.75%, Cr: 0.68%, P: 0.018%, S: 0.015%, V: 0.67% by mass percentage, with the remainder being Fe and unavoidable impurities. The production process follows a process flow of continuous casting → heating furnace → hot rolling → wire rod discharge → online salt bath cooling tank → online salt bath high-temperature tempering tank. Specifically, The aforementioned continuous casting process is used to cast billets from high-temperature molten steel using a continuous casting machine. The superheating of the continuous casting tundish is controlled to 12-20°C, the crystallization device molten metal level fluctuation is controlled to within ±5 mm, and stirring is performed by combining front and end electromagnetic stirring. The front electromagnetic stirring current is 300 A at a frequency of 3 Hz, and the end electromagnetic stirring current is 400 A at a frequency of 9 Hz. The macrostructure diagram of the continuously cast billet is shown in Figure 1. The carbon segregation index of the continuously cast billet is 1.01, and the standard size was a 220 mm x 220 mm rectangular billet.

[0028] The heating furnace process is used to heat the continuous casting billet in a heating furnace until it becomes a rollable plastic high-temperature continuous casting billet. The hot rolling process is used to roll the high-temperature continuous casting billet into wire by hot rolling. The high-temperature continuous casting billet coming out of the heating furnace is treated with high-pressure scale removal water, which has a pressure of 19 MPa and is used to remove oxide scale from the surface of the continuous casting billet. Next, the high-temperature continuous casting billet after scale removal is rolled into wire on the rolling line. Low temperature is used for rolling, the finish rolling temperature is controlled to 870°C, and the finish rolling reduction is controlled to 6.5%. The wire grain is refined by the deformation storage energy of the low-temperature finish rolling, making the wire grain size ≤15 μm, providing good structural preparation for subsequent wire structure transformation. The wire discharge process is used to turn the wire material from the rolling line into wire using a wire discharge machine. The wire discharge temperature is 850°C, and the wire specification is a diameter of 12 mm.

[0029] The discharged wire is passed through an online salt bath cooling tank, the molten salt temperature in the online salt bath cooling tank is 380°C, the molten salt circulation rate in the online salt bath cooling tank is 260-270 t / h, the molten salt temperature rise is ≤15°C, the wire is rapidly cooled to the molten salt temperature at a wire cooling rate of 55°C / s, that is, online molten salt quenching is performed in the bainite phase region of the wire, the wire cooling time is 25 s, and the transformation of the high-temperature austenite structure of most of the wire to a bainite structure is achieved, avoiding the formation of ferrite, pearlite soft phase, and abnormal structures such as martensite or troostite.

[0030] The wire rod that has passed through the online salt bath cooling tank enters the online salt bath high-temperature tempering tank, where the molten salt temperature is 495°C. The molten salt is circulated both in front of and behind the online salt bath high-temperature tempering tank along the wire rod production direction. The molten salt circulation rate is 350 t / h in the front 1 / 3 of the length of the online salt bath high-temperature tempering tank, and 200 t / h in the rear 2 / 3 of the length of the online salt bath high-temperature tempering tank. The molten salt temperature rise is ≤9°C. The wire rod is heated to the molten salt temperature by heat exchange with the molten salt, i.e., tempering is performed in the medium-temperature tempering temperature range, with a wire rod isothermal time of 200 s. This further controls the transformation of the bainite structure into a tempered bainite structure, the stabilization of small amounts of distributed retained austenite, and the stable dispersion deposition of a large amount of alloy carbide VC. The microstructure of the wire removed from the online salt bath high-temperature tempering tank is shown in Figure 2(a). After sorting, baking, and winding, hot-rolled wire was obtained.

[0031] (Comparative Example 1) This is a production process for vanadium-containing 55SiCr spring steel hot-rolled wire rod, and differs from Example 1 in the following respects. The production process follows a process flow of continuous casting → heating furnace → hot rolling → wire rod discharge → Stermor air cooling line, specifically, The wire material discharged under the control of the aforementioned Stermore air cooling line was air-cooled at a cooling rate of 8.5°C / s, then placed in an insulating cover, where the cover entry temperature was 680°C. Phase transformation was completed inside the cover at a cooling rate of 0.8°C / s, and after being wound up and exiting the line, a hot-rolled wire material was obtained. The wire structure of the hot-rolled wire material is shown in Figure 2(b), and a hot-rolled wire material was obtained.

[0032] (Comparative Example 2) This is a production process for 55SiCr spring steel hot-rolled wire rod, and differs from Example 1 in the following respects: The steel grade does not contain vanadium, and hot-rolled wire rod was obtained.

[0033] (Example 2) This is a production process for 2000 MPa class vanadium-containing 55SiCr spring steel hot-rolled wire rod, wherein the hot-rolled wire rod has a chemical composition of C: 0.66%, Si: 1.20%, Mn: 0.80%, Cr: 0.50%, P: 0.018%, S: 0.007%, V: 0.80% by mass percentage, with the remainder being Fe and unavoidable impurities. The production process follows a process flow of continuous casting → heating furnace → hot rolling → wire rod discharge → online salt bath cooling tank → online salt bath high-temperature tempering tank, and specifically, The aforementioned continuous casting process was used to cast billets from high-temperature molten steel using a continuous casting machine. The superheating of the continuous casting tundish was controlled to 29-33°C, the crystallization device molten metal level fluctuations were controlled to within ±5 mm, and stirring was performed by combining front and end electromagnetic stirring. The front electromagnetic stirring current was 380 A at a frequency of 3 Hz, and the end electromagnetic stirring current was 520 A at a frequency of 11 Hz. The C segregation index of the continuously cast billet was 1.04, and the standard was a square billet of 220 mm x 220 mm.

[0034] The heating furnace process is used to heat the continuous casting billet in a heating furnace until it becomes a rollable plastic high-temperature continuous casting billet. The hot rolling process is used to roll the high-temperature continuous casting billet into wire by hot rolling. The high-temperature continuous casting billet coming out of the heating furnace is treated with high-pressure scale removal water, which has a pressure of 21 MPa and is used to remove oxide scale from the surface of the continuous casting billet. Next, the high-temperature continuous casting billet after scale removal is rolled into wire on the rolling line. Low temperature is used for rolling, the finish rolling temperature is controlled to 882°C, and the finish rolling reduction is controlled to 7%. The wire grain is refined by the deformation storage energy of the low-temperature finish rolling, the wire grain size is made to ≤16 μm, and good structural preparation is provided for the subsequent wire structure transformation. The wire discharge process is used to turn the wire material from the rolling line into wire using a wire discharge machine. The wire discharge temperature is 862°C, and the wire specification is a diameter of 9 mm.

[0035] The discharged wire is passed through an online salt bath cooling tank, the molten salt temperature in the online salt bath cooling tank is 420°C, the molten salt circulation rate in the online salt bath cooling tank is 265-275 t / h, the molten salt temperature rise is ≤14°C, the wire is rapidly cooled to the molten salt temperature at a wire cooling rate of 51°C / s, that is, online molten salt quenching is performed in the bainite phase region of the wire, the wire cooling time is 22 s, and the transformation of the high-temperature austenite structure of most of the wire to a bainite structure is achieved, avoiding the formation of ferrite, pearlite soft phase and abnormal structures such as martensite or troostite.

[0036] The wire rod that has passed through the online salt bath cooling tank enters the online salt bath high-temperature tempering tank, where the molten salt temperature is 483°C. The molten salt is circulated both in front of and behind the online salt bath high-temperature tempering tank along the wire rod production direction. The molten salt circulation rate in the front 1 / 3 of the length of the online salt bath high-temperature tempering tank is 340 t / h, and the molten salt circulation rate in the rear 2 / 3 of the length of the online salt bath high-temperature tempering tank is 210 t / h. The molten salt temperature rise is ≤ 9°C, and the wire rod is heated to the molten salt temperature through heat exchange with the molten salt, i.e., tempering is performed in the medium-temperature tempering temperature range, with a wire rod isothermal time of 276 s. This further controls the transformation of the bainite structure into a tempered bainite structure, the stabilization of small amounts of distributed retained austenite, and the stable dispersion deposition of a large amount of alloy carbide VC. Figure 3 shows the microstructure of the wire removed from the online salt bath high-temperature tempering tank. After sorting, baking, and winding, hot-rolled wire was obtained.

[0037] (Comparative Example 3) This is a production process for vanadium-containing 55SiCr spring steel hot-rolled wire rod, and differs from Example 2 in the following respects. The discharged wire was passed through an online salt bath cooling tank, where the molten salt temperature was 290°C. The wire was rapidly cooled to the molten salt temperature at a wire cooling rate of 59°C / s, and online molten salt quenching was performed, with a wire cooling time of 35 s. After passing through the online salt bath cooling tank, the wire entered an online salt bath high-temperature tempering tank, where the molten salt temperature was 460°C. The wire was heated to the molten salt temperature by heat exchange with the molten salt, with a wire isothermal time of 250 s.

[0038] (Comparative Example 4) This is a production process for vanadium-containing 55SiCr spring steel hot-rolled wire rod, and differs from Example 2 in the following respects. The extruded wire rod was passed through an online salt bath cooling tank, where the molten salt temperature was 470°C. The wire rod was rapidly cooled to the molten salt temperature at a wire cooling rate of 37°C / s, and online molten salt quenching was performed, with a wire cooling time of 30 s. After passing through the online salt bath cooling tank, the wire rod entered an online salt bath high-temperature tempering tank, where the molten salt temperature was 760°C. The wire rod was heated to the molten salt temperature by heat exchange with the molten salt, with a wire isothermal time of 200 s. After sorting, baking, and winding, hot-rolled wire rod was obtained.

[0039] (Example 3) This is a production process for 2000 MPa class vanadium-containing 55SiCr spring steel hot-rolled wire rod, wherein the hot-rolled wire rod has a chemical composition of C: 0.70%, Si: 1.53%, Mn: 0.50%, Cr: 0.66%, P: 0.016%, S: 0.012%, V: 0.40% by mass percentage, with the remainder being Fe and unavoidable impurities. The production process follows a process flow of continuous casting → heating furnace → hot rolling → wire rod discharge → online salt bath cooling tank → online salt bath high-temperature tempering tank, and specifically, The aforementioned continuous casting process was used to cast billets from high-temperature molten steel using a continuous casting machine. The superheating of the continuous casting tundish was controlled to 20-25°C, the crystallization device molten metal level fluctuations were controlled to within ±5 mm, and stirring was performed by combining front and end electromagnetic stirring. The front electromagnetic stirring current was 400 A at a frequency of 4 Hz, and the end electromagnetic stirring current was 520 A at a frequency of 8 Hz. The C segregation index of the continuously cast billet was ≤1.03, and the standard was a square billet of 220 mm × 220 mm.

[0040] The heating furnace process is used to heat the continuous casting billet in a heating furnace until it becomes a rollable plastic high-temperature continuous casting billet. The hot rolling process is used to roll the high-temperature continuous casting billet into wire by hot rolling. The high-temperature continuous casting billet coming out of the heating furnace is treated with high-pressure scale removal water, which has a pressure of 19 MPa and is used to remove oxide scale from the surface of the continuous casting billet. Next, the high-temperature continuous casting billet after scale removal is rolled into wire on the rolling line. Low temperature is used for rolling, the finish rolling temperature is controlled to 905°C, and the finish rolling reduction is controlled to 8%. The wire grain is refined by the deformation storage energy of the low-temperature finish rolling, the wire grain size is made to ≤17 μm, and good structural preparation is provided for subsequent wire structure transformation. The wire discharge process is used to turn the wire material from the rolling line into wire using a wire discharge machine. The wire discharge temperature is 885°C, and the wire specification is a diameter of 9 mm.

[0041] The discharged wire is passed through an online salt bath cooling tank, the molten salt temperature in the online salt bath cooling tank is 380°C, the molten salt circulation rate in the online salt bath cooling tank is 320-330 t / h, the molten salt temperature rise is ≤13°C, the wire is rapidly cooled to the molten salt temperature at a wire cooling rate of 55°C / s, that is, online molten salt quenching is performed in the bainite phase region of the wire, the wire cooling time is 25 s, and the transformation of the high-temperature austenite structure of most of the wire to a bainite structure is achieved, avoiding the formation of ferrite, pearlite soft phase, and abnormal structures such as martensite or troostite.

[0042] The wire rod that has passed through the online salt bath cooling tank enters the online salt bath high-temperature tempering tank, where the molten salt temperature is 490°C. The molten salt is circulated both in front of and behind the online salt bath high-temperature tempering tank along the wire rod production direction. The molten salt circulation rate is 340 t / h in the front 1 / 3 of the length of the online salt bath high-temperature tempering tank and 210 t / h in the rear 2 / 3 of the length of the online salt bath high-temperature tempering tank. The molten salt temperature rise is ≤9°C, further controlling the transformation of the bainite structure to a tempered bainite structure, the stabilization of small amounts of distributed retained austenite, and the stable dispersion and precipitation of a large amount of alloy carbide VC. The microstructure of the wire rod that has come out of the online salt bath high-temperature tempering tank is shown in Figure 4. After sorting, baking, and winding, hot-rolled wire rods were obtained.

[0043] (Comparative Example 5) This is a production process for vanadium-containing 55SiCr spring steel hot-rolled wire rod, and differs from Example 3 in the following respects. The hot rolling process was used to roll a high-temperature continuous casting billet into wire rod by hot rolling, with the finish rolling temperature controlled to 1050°C and the finish rolling reduction controlled to 4%, resulting in a wire rod grain size of 38 μm or more. The wire rod discharge process was used to turn the wire-making material from the rolling line into wire rod using a wire rod discharge machine, with a wire rod discharge temperature of 1029°C, resulting in hot-rolled wire rod.

[0044] (Example 4) This is a production process for 2000 MPa class vanadium-containing 55SiCr hot-rolled wire rod, wherein the hot-rolled wire rod has a chemical composition of C: 0.59%, Si: 1.60%, Mn: 0.76%, Cr: 0.80%, P: 0.16%, S: 0.006%, V: 0.71% by mass, with the remainder being Fe and unavoidable impurities. The production process follows a process flow of continuous casting → heating furnace → hot rolling → wire rod discharge → online salt bath cooling tank → online salt bath high-temperature tempering tank, and specifically, The continuous casting process described above was used to cast billets from high-temperature molten steel using a continuous casting machine. The superheating of the continuous casting tundish was controlled to 25-30°C, the crystallization device molten metal level fluctuation was controlled to within ±5 mm, and stirring was performed by combining front electromagnetic stirring and end electromagnetic stirring. The front electromagnetic stirring current was 250 A at a frequency of 3 Hz, and the end electromagnetic stirring current was 450 A at a frequency of 9 Hz. The C segregation index of the continuously cast billet was ≤1.05, and the standard was a square billet of 220 mm × 220 mm.

[0045] The heating furnace process is used to heat the continuous casting billet in a heating furnace until it becomes a rollable plastic high-temperature continuous casting billet. The hot rolling process is used to roll the high-temperature continuous casting billet into wire by hot rolling. The high-temperature continuous casting billet coming out of the heating furnace is treated with high-pressure scale removal water, which has a pressure of 20 MPa and is used to remove oxide scale from the surface of the continuous casting billet. Next, the high-temperature continuous casting billet after scale removal is rolled into wire on a rolling line. Low temperature is used for rolling, the finish rolling temperature is controlled to 875°C, and the finish rolling reduction is controlled to 7%. The wire grain is refined by the deformation storage energy of the low-temperature finish rolling, making the wire grain size ≤15 μm, providing good structural preparation for subsequent wire structure transformation. The wire discharge process is used to turn the wire material from the rolling line into wire using a wire discharge machine. The wire discharge temperature is 854°C, and the wire specification is a diameter of 12 mm.

[0046] The discharged wire was passed through an online salt bath cooling tank, the molten salt temperature in the online salt bath cooling tank was 468°C, the molten salt circulation rate in the online salt bath cooling tank was 300-325 t / h, the rise in molten salt temperature was ≤12°C, and the wire was rapidly cooled to the molten salt temperature at a wire cooling rate of 46°C / s. That is, online molten salt quenching was performed in the bainite phase region of the wire, achieving transformation of the high-temperature austenite structure of most of the wire to a bainite structure, avoiding the formation of ferrite, pearlite soft phase, and abnormal structures such as martensite or troostite, and the wire cooling time was 20 s.

[0047] The wire rod that has passed through the online salt bath cooling tank enters the online salt bath high-temperature tempering tank, where the molten salt temperature is 497°C. The molten salt is circulated both in front of and behind the online salt bath high-temperature tempering tank along the wire rod production direction. The molten salt circulation rate in the front 1 / 3 of the length of the online salt bath high-temperature tempering tank is 360 t / h, and the molten salt circulation rate in the rear 2 / 3 of the length of the online salt bath high-temperature tempering tank is 205 t / h. The molten salt temperature rise is ≤ 9°C, and the wire rod is heated to the molten salt temperature through heat exchange with the molten salt, i.e., tempering is performed in the medium-temperature tempering temperature range, with a wire rod isothermal time of 362 s. This further controls the transformation of the bainite structure into a tempered bainite structure, the stabilization of small amounts of distributed retained austenite, and the stable dispersion deposition of a large amount of alloy carbide VC. Figure 5 shows the microstructure of the wire removed from the online salt bath high-temperature tempering tank. After sorting, baking, and winding, hot-rolled wire was obtained.

[0048] (Comparative Example 6) This is a production process for vanadium-containing 55SiCr spring steel hot-rolled wire rod, and differs from Example 4 in the following respects. The aforementioned continuous casting process was used to continuously cast billets from high-temperature molten steel using a continuous casting machine. The superheating of the continuous casting tundish was controlled to 32-40°C, the crystallization apparatus molten metal level fluctuation was controlled to within ±8 mm, and stirring was performed using terminal electromagnetic stirring. The terminal electromagnetic stirring current was 220 A, the frequency was 6 Hz, the carbon segregation index of the continuously cast billet was 1.32, and the standard was a rectangular billet of 220 mm x 220 mm. Hot-rolled wire rods were obtained.

[0049] The microstructure and performance of the hot-rolled wire rods obtained in the above examples and comparative examples were investigated. The test method for the range of variation in mechanical properties within the same circumference is as follows: Two turns of wire rod were taken from a point 5 m away from the end of the wire coil, and the wire rod of each circumference was divided into eight equal parts on average, using the joint area position as the reference point. One tensile sample was taken from each part, and the range of strength after tensile testing of the taken tensile samples was the difference within the same circumference of the wire rod. The tensile test was performed using "GB-T228.1-2021 Metallic Materials Tensile Test Part 1: Room Temperature Test Method," and the tensile strength and reduction ratio were obtained. The obtained comparison results are shown in Table 1 below.

[0050] [Table 1]

[0051] As can be seen from the comparative results of Examples 1-4 and Comparative Example 1, the present invention adopts a C-Si-Mn-Cr-V component design and directly performs online rapid salt bath cooling and high-temperature salt bath tempering on the extruded 55SiCrA spring steel high-speed wire rod, thereby rapidly cooling the high-temperature extruded wire rod to the bainite transformation temperature and avoiding the formation of ferrite and pearlite soft phases. First, most of the high-temperature austenite structure is transformed into a bainite structure, completing the bainite transformation and improvement of retained austenite. Next, rapid temperature rise tempering is performed in an online high-temperature salt bath tempering tank, improving the stability of retained austenite through high-temperature salt bath tempering, achieving tempering and transformation of the wire rod bainite structure to a bainite structure, stabilization of small amounts of distributed retained austenite, and large-scale dispersion precipitation of alloy carbide VC, thereby effectively adjusting the strength and plasticity matching of the V-component-containing 55SiCr hot-rolled wire rod on the microstructure. Compared to 1000MPa class spring steel wire rod air-cooled products consisting of ferrite and pearlite soft phases obtained by hot-rolling billets and then controlling the cooling in a Stermore air-cooling line, the tensile strength can be increased to ≥2010MPa and the area reduction ratio to ≥18%. This achieves a level of strength comparable to that of conventional products that undergo austenitization, quenching, and tempering by downstream users after air-cooling and packaging. Furthermore, it can be applied to application fields for manufacturing ultra-high-strength springs, enabling the production of ultra-high-strength spring products by softening through tempering and then winding without the need for a downstream oil quenching process, further reducing energy consumption and costs, and contributing to an improvement in the strength level of spring products.

[0052] As can be seen from the comparison results between Example 1 and Comparative Example 2, the present invention employs a C-Si-Mn-Cr-V component design, where the V microalloy element is mainly used to isothermally form a dispersed precipitate strengthening phase VC in the medium temperature range to strengthen the matrix. By adjusting and controlling the dispersion precipitate of the alloy carbide VC, the strength and plasticity of the high-V composition 55SiCr hot-rolled wire can be further enhanced on the microstructure.

[0053] As can be seen from the comparison results of Example 2 and Comparative Example 3, the present invention rapidly cools the wire to the molten salt temperature by exchanging heat with the wire, transforming the high-temperature austenite structure of most of the wire into a bainite structure, thereby avoiding the generation of abnormal structures such as martensite or troostite and the decrease in wire plasticity after tempering. Furthermore, by performing rapid temperature rise tempering using an online salt bath high-temperature tempering tank, the transformation of the bainite structure of the wire into a tempered bainite structure can be achieved, significantly increasing the strength of the wire, eliminating the brittleness of bainite, further toughening the matrix, improving the stability of retained austenite, and plasticizing the wire.

[0054] As can be seen from the comparison results between Example 2 and Comparative Example 4, the present invention employs a C-Si-Mn-Cr-V component design, controls the wire rod to a medium tempering temperature range and a VC precipitation temperature range using an online salt bath high-temperature tempering tank, further transforms the bainite structure into a tempered bainite structure, stabilizes a small amount of distributed retained austenite, and controls the stable dispersion and precipitation of a large amount of alloy carbide VC to form a nanoscale precipitate phase. The product has excellent hydrogen embrittlement resistance and strength-plastic matching properties, as well as good industrial applicability.

[0055] As can be seen from the comparison results between Example 3 and Comparative Example 5, the present invention, by adopting a process design of low-temperature rolling and online molten salt quenching + tempering, effectively utilizes the deformation storage energy of low-temperature finish rolling to refine the wire grain, providing good microstructure preparation for subsequent wire structure transformation, and achieving the advantages of combining grain refinement by low-temperature rolling and online molten salt quenching + tempering.

[0056] As can be seen from the comparison results of Example 4 and Comparative Example 6, the present invention reduces the degree of segregation in continuously cast billets and controls the internal quality of continuously cast billets, thereby avoiding severe and irresolvable carbon segregation that reduces strength, severely reduces plasticity, and increases variations in mechanical properties around the same circumference.

[0057] Finally, the following should be explained: The above embodiments are for illustrating, and not limiting, the technical means of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that the technical means described in the above embodiments can be modified, or some or all of the technical features therein can be replaced with equivalent ones, and that such modifications or replacements will not cause the essence of the corresponding technical means to deviate from the scope of the technical means of each embodiment of the present invention.

Claims

1. The chemical composition, by mass percentage, is as follows: C: 0.50%–0.70%, Si: 1.20%–1.60%, Mn: 0.50%–0.80%, Cr: 0.50%–0.80%, P ≤ 0.020%, S ≤ 0.015%, V: 0.040%–0.080%, with the remainder being Fe and unavoidable impurities. The production process is characterized by comprising: producing wire rods by hot rolling based on the chemical composition; rapidly cooling the wire rods to the bainite phase region in an online salt bath cooling tank at a wire rod cooling rate of ≥ 45°C / s; performing heating and tempering in an online salt bath high-temperature tempering tank to complete the production of hot-rolled wire rods in which the microstructure contains tempered bainite by volume percentage ≥ 92%, with the remainder being retained austenite and nanoprecipitation phases; and the reduction ratio of the hot-rolled wire rods being ≥ 18%, wherein the hot-rolled wire rods are 2000 MPa class and vanadium-containing 55SiCr spring steel hot-rolled wire rods.

2. The 2000 MPa class vanadium-containing 55SiCr spring steel hot-rolled wire rod according to claim 1, characterized in that the diameter is 6.5 to 14 mm, the tensile strength is ≥ 2010 MPa, the variation in mechanical properties at the same circumference is < 30 MPa, and the nano-precipitation phase contains dispersed precipitated VC.

3. A production process for a 2000 MPa class vanadium-containing 55SiCr spring steel hot-rolled wire rod according to claim 1, comprising producing a wire rod by hot rolling using a continuously cast billet, wherein the continuously cast billet is obtained from molten steel through a continuous casting process, the C segregation index of the continuously cast billet is ≤ 1.05, the superheating degree of the continuous casting tundish in the continuous casting process is controlled to 12 to 33°C, the crystallization apparatus molten metal level fluctuation is controlled to within ±5 mm, the tip electromagnetic stirring current is 220 to 420 A with a frequency of 2 to 5 Hz, and the end electromagnetic stirring current is 380 to 560 A with a frequency of 8 to 12 Hz.

4. A production process for 2000 MPa class vanadium-containing 55SiCr spring steel hot-rolled wire rod according to claim 1, comprising producing wire rods sequentially through a heating furnace, hot rolling, and wire rod discharge process using a continuously cast billet, characterized in that the finish rolling temperature of the hot rolling process is 870 to 920°C, the finish rolling reduction is 5 to 10%, the wire rod grain size is ≤ 20 μm, and the wire rod discharge temperature of the wire rod discharge process is 850 to 900°C.

5. A production process for a 2000 MPa class vanadium-containing 55SiCr spring steel hot-rolled wire rod according to claim 4, characterized in that the oxide scale on the surface of the continuous casting billet is removed with high-pressure scale removal water before rolling in the hot-rolling process, and the pressure of the high-pressure scale removal water is 18 to 22 MPa.

6. A production process for a 2000 MPa class vanadium-containing 55SiCr spring steel hot-rolled wire rod according to any one of claims 1 to 5, characterized in that the molten salt temperature in the online salt bath cooling tank is 380 to 480°C, the wire rod cooling rate is ≤ 60°C / s, and the wire rod cooling time is 10 to 30 s.

7. The production process for a 2000 MPa class vanadium-containing 55SiCr spring steel hot-rolled wire rod according to claim 6, characterized in that the molten salt circulation rate of the online salt bath cooling tank is 240 to 350 t / h and the rise in molten salt temperature is ≤ 15°C.

8. A production process for a 2000 MPa class vanadium-containing 55SiCr spring steel hot-rolled wire rod according to any one of claims 1 to 5, characterized in that the molten salt temperature in the online salt bath high-temperature tempering tank is 450 to 500°C and the wire rod isothermal time is 200 to 400 s.

9. The production process for 2000 MPa class vanadium-containing 55SiCr spring steel hot-rolled wire rods according to claim 8, characterized in that the molten salt circulation rate of the online salt bath high-temperature tempering tank is 200 to 370 t / h, the molten salt is circulated in front of and behind the online salt bath high-temperature tempering tank along the wire rod production direction, the molten salt circulation rate in the front is 130 to 160 t / h higher than the molten salt circulation rate in the rear, and the rise in molten salt temperature is ≤ 10°C.

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