A spring steel hot-rolled steel strip and its preparation process

Abstract

This invention discloses a hot-rolled spring steel strip and its preparation process, belonging to the field of spring steel production technology. The hot-rolled steel strip has a specific chemical composition, a thickness of 3.0-8.0 mm, and is grade 51CrV4, possessing excellent properties such as a pearlite spheroidization grade ≥3. Its heat treatment process encompasses smelting, continuous casting, heating, rolling, cooling, coiling, pickling, and spheroidizing annealing. By controlling parameters at each stage, such as tundish temperature, rolling cooling temperature, pickling conditions, and spheroidizing annealing process, synergy between the upstream and downstream processes of hot rolling and spheroidizing annealing is achieved. This process can significantly shorten the spheroidizing annealing time by ≥25% compared to existing technologies, improving production efficiency, reducing costs, and increasing the single-pass cold rolling reduction rate, which is beneficial for the production of ultra-thin cold-rolled steel strips and enhances product added value.

Description

Technical Field

[0001] This invention relates to the field of spring steel production technology, specifically to a hot-rolled spring steel strip and its preparation process. Background Technology

[0002] Spring steel 51CrV4 is widely used in the manufacture of disc springs and diaphragm springs due to its high elastic limit, high strength, high wear resistance, and good fatigue resistance. To meet these performance requirements, disc springs and diaphragm springs both domestically and internationally primarily utilize cold-rolled heat-treated spring steel strips. These cold-rolled strips undergo processes such as stamping, quenching, tempering, and shot peening. Considering the relatively high strength and deformation resistance of 51CrV4 spring steel in its hot-rolled state, an annealing process is required before and between cold-rolling passes to reduce its strength and meet the requirements of cold rolling. The annealing process is an indispensable step in the production of cold-rolled 51CrV4 spring steel strips, directly affecting the spheroidization grade, annealing production efficiency, cold-rolled finished product thickness, and production costs.

[0003] Regarding the annealing heat treatment process of 51CrV4 spring steel strip, currently published patents mainly employ spheroidizing annealing. Research focuses on the annealing process itself. For example, patents with publication numbers CN105401073B and CN108385019A use a four-stage isothermal spheroidizing annealing process. Although the spheroidization rate after annealing is ≥90%, the spheroidizing annealing process is complex, with a single annealing time of at least 40 hours, resulting in low production efficiency. Patent CN106311789B employs a three-stage multi-step spheroidizing annealing process, which is also complex and requires a single annealing time of at least 40 hours. The spheroidizing annealing process has several drawbacks. First, the spheroidizing annealing time is long. Second, the material is kept in the range of 670~720℃ for a long time, which is not conducive to the control of the decarburization layer depth and grain boundary oxidation of spring steel. Third, the patent with publication number CN114985453B adopts a three-stage isothermal spheroidizing annealing process. Although the annealing spheroidization rate is stable at ≥95%, according to the heating rate and cooling process of the annealing furnace, there are problems such as complex annealing process, single annealing time of at least 37 hours and long spheroidizing annealing time. Fourth, the patent with publication number CN112077141B adopts a one-stage isothermal spheroidizing annealing process. The annealing holding time is 30-60 hours, which also has the problem of long spheroidizing annealing time.

[0004] Spheroidal pearlite or granular pearlite is mainly obtained through spheroidizing annealing. The spheroidization grade is not only related to the spheroidizing annealing process, but also closely related to the hot-rolled microstructure. The hot-rolled microstructure has heritability and is directly related to the efficiency of spheroidizing annealing and the spheroidization grade. However, there are currently no published patents on the synergistic control of hot-rolled microstructure and spheroidizing annealing process to improve the spheroidization grade of the material and improve the production efficiency of spheroidizing annealing. For example, publication numbers CN111876664B and CN115537634A only focus on the microstructure and performance control of hot-rolled 51CrV4.

[0005] In summary, regarding the annealing process of 51CrV4 spring steel strip, on the one hand, the currently published patents mainly adopt the spheroidizing annealing process, and research focuses on the annealing process itself, which suffers from problems such as complex spheroidizing annealing process, long spheroidizing annealing time, and low production efficiency; on the other hand, there is basically no information on the synergistic control of hot-rolled microstructure and spheroidizing annealing process to improve the spheroidization grade of the material and improve the spheroidizing annealing production efficiency. Summary of the Invention

[0006] To address the problems in the prior art, the present invention aims to provide a hot-rolled spring steel strip and its preparation process. Through reasonable smelting, continuous casting, heating, rolling, cooling, coiling, pickling, and spheroidizing annealing processes, the spheroidization grade of 51CrV4 hot-rolled steel strip is ≥3, significantly shortening the spheroidizing annealing time, improving production efficiency, and reducing production costs.

[0007] This invention can also increase the single-pass cold rolling reduction rate, which is beneficial to the production of ultra-thin 51CrV4 cold-rolled steel strip and increases the added value of the product.

[0008] This invention can also control the depth of the decarburized layer and intergranular oxidation in hot-rolled steel strips.

[0009] To achieve the above objectives, the present invention is implemented through the following technical solution:

[0010] This invention provides a process for preparing hot-rolled spring steel strip, including smelting, continuous casting, heating, rolling, cooling, coiling, pickling, and spheroidizing annealing steps. In the rolling process, the finishing rolling temperature is 820–880℃, and the strip is cooled to 520–580℃ before coiling. The spheroidizing annealing process involves heating the steel strip from room temperature at a heating rate of r = 60–100℃ / h to a holding temperature T = 750–800℃, with a holding time t = 0.5–1h, followed by a heating rate of V1 = 30–50℃. The spring steel hot-rolled strip is cooled at a rate of ℃ / h for the first stage, then held at a second stage temperature of T1=670~720℃ for t1=4~6h. It is then cooled again at a rate of V2=30~50℃ / h to a second stage cooling temperature of T2=470~520℃. Finally, it is cooled at a third stage cooling rate of V3=40~60℃ / h to a third stage cooling temperature of ≤85℃ before being removed from the furnace. The thickness of the spring steel hot-rolled strip is 3.0~8.0mm, and the grade is 51CrV4.

[0011] As a further optimization of the present invention, the continuous casting process adopts dynamic light reduction and electromagnetic stirring, the billet is lightly reduced by 0~5mm, the electromagnetic stirring frequency is 2.40~2.80Hz, the current is 290~300A, and the center segregation C of the continuous casting billet is controlled to be ≤1.0.

[0012] As a further optimization of the present invention, in the smelting process, the steel strip is smelted in a converter and refined with LF+RH according to its composition, and the target temperature of the tundish is controlled at 15~30°C above the liquidus temperature.

[0013] As a further optimization of the present invention, in the heating process, the slab exit temperature is 1200~1250℃ and the excess air coefficient in the last two sections of the heating furnace is <1.0.

[0014] As a further optimization of the present invention, in the pickling process, hot-rolled steel strip cooled to ≤45℃ is pickled, the acid solution temperature is 70~80℃, the acid solution concentration is 120 / L~180g / L, and the pickling rate is 30~60m / min.

[0015] As a further optimization of the present invention, the amount of oil applied to the surface of the steel plate after pickling is 0 g / m². 2 .

[0016] As a further optimization of the present invention, in the rolling, cooling and coiling process, a 2-stand roughing mill and a 7-stand finishing mill hot continuous rolling mill are used for rolling, and the final rolling temperature of the roughing mill is 1050~1100℃; the laminar flow cooling adopts a segmented cooling mode, the first stage cooling is cooled to 620~680℃ at a cooling rate of ≥50℃ / s, then the second stage cooling is air-cooled with 620~680℃ as the starting temperature, the air-cooling time is 5~10s, the air-cooling rate is 4~8℃ / s, then the third stage cooling is cooled at a cooling rate of ≥30℃ / s and then coiled, the hot coil is placed in a slow cooling pit for slow cooling, and the cooling rate is ≤5℃ / h.

[0017] As a further optimization of the present invention, the chemical composition and weight percentage content of the hot-rolled spring steel strip are as follows: C: 0.47%~0.55%, Si: 0.17%~0.30%, Mn: 0.80%~1.10%, Cr: 0.90%~1.20%, V: 0.10%~0.25%, Als: 0.015%~0.040%, P: ≤0.015%, S: ≤0.003%, with the remainder being Fe and unavoidable inclusions.

[0018] As a further optimization of the present invention, the chemical composition and weight percentage content of the hot-rolled spring steel strip are as follows: C: 0.50%, Si: 0.25%, Mn: 0.98%, Cr: 1.05%, V: 0.16%, Als: 0.025%, P: 0.010%, S: 0.002%, with the remainder being Fe and unavoidable inclusions.

[0019] The present invention also provides a hot-rolled spring steel strip prepared by the aforementioned preparation process, wherein the hot-rolled spring steel strip has a pearlite spheroidization grade ≥3 and a yield strength R p0.2 =300~400MPa, tensile strength R m =500~600MPa, elongation A 80 ≥20%, hardness ≤190HV10, decarburized layer depth is 0μm, and intergranular oxide thickness is ≤8μm.

[0020] In the design of the smelting and continuous casting process, the chemical composition can be implemented according to the 51CrV4 standard. The target temperature of the tundish is controlled at 15~30℃ above the liquidus temperature. The use of dynamic light reduction and electromagnetic stirring during continuous casting is to control the C-type segregation of the continuous casting billet center to ≤1.0, reduce the banded structure of 51CrV4 hot-rolled steel strip, and facilitate the improvement of the subsequent pearlite spheroidizing annealing rate and the pearlite spheroidization grade.

[0021] In the heating process design of this invention, the slab exit temperature is 1200~1250℃ and the excess air coefficient in the last two sections of the heating furnace is <1.0, mainly based on the consideration of controlling the depth of the decarburized layer and the thickness of the intergranular oxidation of the hot-rolled steel strip.

[0022] In the rolling and cooling process design, this invention adopts a control approach of "low-temperature rolling + segmented cooling + low-temperature coiling". The combination of low-temperature rolling and low-temperature coiling is beneficial for controlling the decarburized layer depth, especially the intergranular oxide thickness. Furthermore, it facilitates the acquisition of bainitic microstructure. Compared to pearlite and ferrite+pearlite microstructures, bainitic microstructure is more conducive to improving the pearlite spheroidization rate and grade, and shortening the spheroidization annealing time. Pearlite transformation is a diffusion-type transformation, primarily influenced by carbon atom diffusion in the spheroidization rate. Since carbon is supersaturated in bainite, long-distance carbon atom diffusion is not required during spheroidization, resulting in a faster spheroidization rate and better microstructure uniformity. The segmented cooling process is mainly for controlling the shape of the hot-rolled strip. 51CrV4 steel contains a relatively high amount of hardenable alloying elements; centralized cooling and excessively high cooling rates are detrimental to shape control.

[0023] In the pickling process design of this invention, the main purpose is to remove iron oxide scale from the surface of hot-rolled steel strip. After pickling, the oil coating amount on the steel plate surface is 0 g / m. 2 This is mainly based on reducing the pretreatment time required for subsequent spheroidizing annealing processes due to the removal of oil and moisture from the steel strip surface, thereby improving spheroidizing annealing efficiency.

[0024] In the design of the spheroidizing annealing process, this invention adopts an isothermal spheroidizing annealing process, such as... Figure 1As shown, the process specifically includes heating, primary cooling, primary isothermal cooling, secondary cooling, and tertiary cooling. The heating rate is controlled at 60~100℃ / h primarily to improve the temperature uniformity along the length and width of the spring steel strip, laying the foundation for the uniformity of the microstructure after isothermal spheroidizing annealing. The first-stage holding temperature is controlled at 750~800℃. Excessive holding temperature will, on the one hand, increase the depth of the decarburized layer in the spring steel strip, hindering decarburization layer control; on the other hand, it will reduce the number of undissolved carbide points, leading to a more uniform carbon concentration distribution in austenite, which is more likely to transform into lamellar pearlite during subsequent cooling transformation, thus hindering the improvement of the pearlite spheroidization grade. The first-stage holding time is controlled at 0.5~1h. Too short a holding time will prevent the lamellar pearlite in the microstructure from fully dissolving and breaking down; too long a holding time will cause most of the pearlite to dissolve into austenite, resulting in a smaller amount of undissolved carbides, which is detrimental to the formation of spheroidized carbides. The cooling rates for the first and second stages are controlled at 30-50℃ / h. A faster cooling rate will result in the formation of directional, dot-like carbides in localized areas, which is detrimental to the uniformity of the spheroidized structure. The first-stage cooling process involves cooling to a second-stage holding temperature T1 = 670-720℃. If the isothermal temperature is too low, the diffusion ability of iron and carbon atoms weakens, hindering the carbide spheroidization process and making it easier to form lamellar pearlite. If the isothermal temperature is too high, on the one hand, the undercooling is relatively small, resulting in slower carbide nucleation in austenite and a reduced number of unmelted carbides, leading to fewer non-spontaneous nucleation points, although some lamellar pearlite will still be produced. On the other hand, a higher isothermal temperature will cause the diameter of the formed carbide particles to increase, both of which are detrimental to improving the spheroidization grade of pearlite. The second-stage holding time should be controlled at 4-6 hours. If the second-stage holding time is too short, the carbide spheroidization will be incomplete; if the second-stage holding time is too long, the amount of unmelted carbides remaining will decrease, the austenite composition will tend to be uniform, and the probability of lamellar pearlite formation will increase. The second-stage cooling, cooling to the second-stage termination temperature T2 = 470-520℃, is mainly to avoid the formation of bainite structure during the subsequent air cooling process due to excessively high final cooling temperature, which is not conducive to improving the pearlite spheroidization grade.

[0025] This invention emphasizes controlling the spheroidizing annealing grade and rate of spring steel strip from the perspective of coordinated control of upstream and downstream processes of hot rolling-spheroidizing annealing, rather than controlling it solely through a single process like hot rolling or spheroidizing annealing. The key to good spheroidizing effect lies in the diffusion distance of carbon atoms and the number of undissolved carbide spots. Therefore, the original microstructure type in the hot-rolled state and the isothermal spheroidizing annealing process parameters (such as heating temperature, holding time, and cooling rate) affect the diffusion distance of carbon atoms and the number of undissolved carbide spots, thus influencing the spheroidizing effect of the material. Therefore, spheroidizing annealing needs to be controlled from the perspective of the entire process flow, exploring reasonable process parameters that match the upstream and downstream processes.

[0026] The beneficial effects of this invention are as follows:

[0027] 1. The 51CrV4 spring steel strip with a thickness of 3.0~8.0mm produced using this invention has a pearlite spheroidization grade ≥3, R p0.2 =300~400MPa, R m =500~600MPa, A80≥20%, hardness≤190HV10, decarburized layer depth is 0μm, intergranular oxide thickness≤8um. While exhibiting a high pearlite spheroidization grade, the material has low strength and hardness, which can increase the single-pass cold rolling reduction rate, facilitating the production of ultra-thin 51CrV4 cold-rolled steel strip and increasing product added value.

[0028] 2. By adopting the angle of coordinated control of the upstream and downstream processes of hot rolling and spheroidizing annealing, the spheroidizing annealing grade of spring steel strip is improved. The hot rolling adopts a low temperature rolling + low temperature coiling process to obtain bainitic structure, matched with a two-stage isothermal annealing process. The process is simple and can obtain a high spheroidized pearlite structure of 51CrV4 while significantly shortening the spheroidizing annealing time. The single spheroidizing annealing time is shortened by ≥25% compared with the existing technology, which significantly improves production efficiency and reduces production costs. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the isothermal annealing process corresponding to the present invention.

[0030] Figure 2 This is a typical microstructure of hot-rolled steel strip corresponding to Example 1 of the present invention.

[0031] Figure 3 This is a typical microstructure of hot-rolled steel strip after spheroidizing annealing, corresponding to Example 1 of the present invention.

[0032] Figure 4 This is a typical microstructure of hot-rolled steel strip after spheroidizing annealing, corresponding to Example 2 of the present invention.

[0033] Figure 5 This is a typical microstructure of hot-rolled steel strip after spheroidizing annealing, corresponding to Example 3 of the present invention.

[0034] Figure 6 This is a typical microstructure of hot-rolled steel strip corresponding to Comparative Example 1 of the present invention.

[0035] Figure 7 This is a typical microstructure of hot-rolled steel strip after spheroidizing annealing, corresponding to Comparative Example 1 of this invention.

[0036] Figure 8 This is a typical microstructure of hot-rolled steel strip after spheroidizing annealing, corresponding to Comparative Example 2 of this invention.

[0037] Figure 9 This is a typical microstructure of hot-rolled steel strip after spheroidizing annealing, corresponding to Comparative Example 3 of this invention. Detailed Implementation

[0038] To enable those skilled in the art to better understand the technical solutions of the present invention, the preferred embodiments of the present invention are described below in conjunction with specific examples. However, it should be understood that the accompanying drawings are for illustrative purposes only and should not be construed as limiting the present patent.

[0039] The chemical composition and weight percentage of the cold-rolled spring steel strip provided by the present invention are as follows: C: 0.47%~0.55%; Si: 0.17%~0.30%; Mn: 0.80%~1.10%; Cr: 0.90%~1.20%; V: 0.10%~0.25%; Als: 0.015%~0.040%; P: ≤0.015%; S: ≤0.003%; the remainder is Fe and unavoidable inclusions.

[0040] The chemical compositions of the examples and comparative examples are shown in Table 1. The hot rolling process parameters corresponding to the examples and comparative examples are shown in Table 2. The spheroidizing annealing process parameters corresponding to the examples and comparative examples are shown in Table 3. The spheroidizing annealing process parameters corresponding to the examples and comparative examples are shown in Table 4. The mechanical properties, decarburized layer depth, intergranular oxide thickness, and pearlite spheroidization grade corresponding to the examples and comparative examples are shown in Table 5.

[0041] Table 1 Chemical composition (wt%) of the examples and comparative examples

[0042]

[0043] Table 2 Hot rolling process parameters for the examples and comparative examples

[0044]

[0045] Table 3. Pickling process parameters for the examples and comparative examples

[0046]

[0047] Table 4 Spheroidizing annealing process parameters for examples and comparative examples

[0048]

[0049] The formula for calculating the spheroidizing annealing time is: (T - room temperature) / r + t + (T - T1) / V1 + t1 + (T1 - T2) / V2 + (T2 - three-stage cooling termination temperature) / V3, where the room temperature is 25℃.

[0050] Table 5 Mechanical properties, decarburized layer depth, intergranular oxidation depth, and pearlite spheroidization grade of the examples and comparative examples.

[0051]

[0052] This invention focuses on the research of hot-rolled spring steel strip and its heat treatment process. By setting up examples and comparative examples, the performance indicators under different process parameters are compared, comprehensively verifying the advantages of the invention. The specific contents are as follows:

[0053] 1. As can be seen from Tables 2-5, the coiling temperature of 685℃ in Comparative Example 1 is outside the range of this invention (520~580℃), resulting in the hot-rolled steel strip having a lamellar pearlite structure. The spheroidization rate of the pearlite structure is slower than that of the bainite structure. Even if the spheroidizing annealing process parameters are within the range of this invention, the final pearlite spheroidization grade is only level 1. Furthermore, high-temperature coiling is not conducive to the control of intergranular oxide thickness, and the intergranular oxide thickness is 15μm, which exceeds the requirements of this invention. In Comparative Example 2, the first-stage holding temperature T=820℃, the first-stage holding time t=0.4h, and the second-stage holding temperature T1=732℃ in the spheroidizing annealing process are all outside the scope of this invention. The first-stage holding temperature is too high, which is not conducive to controlling the decarburized layer depth and will reduce the number of undissolved carbide points, which is not conducive to improving the spheroidization grade of pearlite. The holding time is too short, and the lamellar pearlite in the microstructure cannot be fully dissolved and broken, which is not conducive to obtaining spheroidized pearlite. In addition, the second-stage holding temperature is too high, the supercooling is relatively small, the nucleation of carbides in austenite is slower, the number of unmelted carbides remaining is reduced, resulting in a reduction of non-spontaneous nucleation points, which is also not conducive to obtaining spheroidized pearlite. In Comparative Example 3, the second-stage heat preservation temperature T1=660℃ and the second-stage heat preservation time t1=3.5h are both outside the scope of this invention. The low second-stage heat preservation temperature and short second-stage heat preservation time weaken the diffusion ability of iron atoms and carbon atoms, hinder the spheroidization process of carbides, and the incomplete spheroidization of carbides easily forms lamellar pearlite, which is not conducive to obtaining spheroidized pearlite.

[0054] 2. Excellent comprehensive performance: As shown in Table 5, the spring steel strip in the embodiment has a pearlite spheroidization grade ≥3 and a yield strength Rp 0.2 At 300-400MPa, tensile strength R m At 500-600 MPa, elongation A 80 With a content of ≥20%, hardness ≤190HV10, decarburized layer depth of 0μm, and intergranular oxide thickness ≤8μm, it has good overall performance and can meet the needs of various applications.

[0055] 3. Improve cold rolling production efficiency: The material has low strength and hardness, which can increase the single-pass cold rolling reduction rate, which is beneficial for the production of ultra-thin 51CrV4 cold-rolled steel strip and increases the added value of the product.

[0056] 4. Shorten spheroidizing annealing time: As shown in Table 4, the spheroidizing annealing time data and the comparison with the comparative example show that by adopting the coordinated control of the upstream and downstream processes of hot rolling-spheroidizing annealing and matching the two-stage isothermal annealing process, the single spheroidizing annealing time is shortened by ≥25% compared with the published patent, which significantly improves production efficiency and reduces production costs.

[0057] 5. Optimize organizational performance: Figure 1 The spheroidizing annealing process shown (heating → segmented cooling → isothermal holding) is key to obtaining spherical pearlite. Figure 2 Typical microstructure of hot-rolled steel strip in Example 1 shown. Annealed microstructure of Examples 1-3 ( Figures 3-5 The structure showed high spheroidization grade spheroid pearlite, with a spheroidization grade ≥ 3 (Table 5), while the annealed structures of Comparative Examples 1-3 ( Figures 7-9 The presence of a large amount of lamellar pearlite or insufficient spheroidization, with a spheroidization grade of only 1.0-2.0, proves that the process parameters of this invention (such as heating temperature 750-800℃, isothermal temperature 670-720℃, cooling rate control, etc.) can effectively promote carbide spheroidization.

[0058] The present invention has been described in detail above with reference to the embodiments. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention. Furthermore, the embodiments of the present invention and the features thereof can be combined with each other unless otherwise specified.

Claims

1. A process for preparing hot-rolled spring steel strip, comprising the steps of smelting, continuous casting, heating, rolling, cooling, coiling, pickling, and spheroidizing annealing, characterized in that, In the heating process, the slab exit temperature is 1200-1250℃; In the rolling process, the finishing rolling temperature is 820-880℃, and the coil is cooled to 520-580℃ before being coiled. The laminar flow cooling adopts a segmented cooling mode. The first stage of cooling cools to 620-680℃ at a cooling rate of ≥50℃ / s. Then, the second stage of cooling starts at 620-680℃ and is air-cooled for 5-10s at a cooling rate of 4-8℃ / s. Then, the third stage of cooling cools to ≥30℃ / s before coiling. After the hot coil is coiled, it is placed in a slow cooling pit for slow cooling at a cooling rate of ≤5℃ / h. The spheroidizing annealing process is as follows: the steel strip is first heated from room temperature to a first-stage holding temperature of 750-800℃ at a heating rate of 60-100℃ / h, and the first-stage holding time is 0.5-1h. Then, it is cooled to a second-stage holding temperature of 670-720℃ at a cooling rate of 30-50℃ / h, and the second-stage holding time is 4-6h. Then, it is cooled to a second-stage cooling temperature of 470-520℃ at a cooling rate of 30-50℃ / h, and the second-stage cooling is terminated at a cooling rate of 40-60℃ / h. Finally, it is cooled to a third-stage cooling temperature of ≤85℃ and then removed from the furnace. The thickness of the hot-rolled spring steel strip is 3.0 to 8.0 mm, its pearlite spheroidization grade is ≥3, its decarburization layer depth is 0 μm, and its intergranular oxide thickness is ≤8 μm; the grade of the hot-rolled spring steel strip is 51CrV4.

2. The preparation process of hot-rolled spring steel strip according to claim 1, characterized in that, The continuous casting process employs dynamic light reduction and electromagnetic stirring. The billet is lightly reduced by 0–5 mm, the electromagnetic stirring frequency is 2.40–2.80 Hz, and the current is 290–300 A. The center segregation C of the continuously cast billet is controlled to be ≤1.

0.

3. The preparation process of hot-rolled spring steel strip according to claim 1, characterized in that, In the smelting process, the steel strip is smelted in a converter and refined with LF+RH according to its composition, and the target temperature of the tundish is controlled at 15-30°C above the liquidus temperature.

4. The preparation process of hot-rolled spring steel strip according to claim 1, characterized in that, In the heating process, the excess air coefficient in the last two stages of the heating furnace is <1.

0.

5. The preparation process of hot-rolled spring steel strip according to claim 1, characterized in that, In the pickling process, hot-rolled steel strip cooled to ≤45℃ is pickled, the acid temperature is 70~80℃, the acid concentration is 120 / L~180g / L, and the pickling rate is 30~60m / min.

6. The preparation process of hot-rolled spring steel strip according to claim 1 or 5, characterized in that, The amount of oil applied to the steel plate surface after pickling is 0g / m. 2 .

7. The preparation process of hot-rolled spring steel strip according to claim 1, characterized in that, In the rolling, cooling and coiling process, a 2-stand roughing mill and a 7-stand finishing mill hot continuous rolling mill are used for rolling, and the final rolling temperature of the roughing mill is 1050-1100℃.

8. The preparation process of hot-rolled spring steel strip according to claim 1, characterized in that, The chemical composition and weight percentage of the hot-rolled spring steel strip are as follows: C: 0.47%–0.55%, Si: 0.17%–0.30%, Mn: 0.80%–1.10%, Cr: 0.90%–1.20%, V: 0.10%–0.25%, Als: 0.015%–0.040%, P: ≤0.015%, S: ≤0.003%, with the remainder being Fe and unavoidable inclusions.

9. The preparation process of hot-rolled spring steel strip according to claim 8, characterized in that, The chemical composition and weight percentage of the hot-rolled spring steel strip are as follows: C: 0.50%, Si: 0.25%, Mn: 0.98%, Cr: 1.05%, V: 0.16%, Als: 0.025%, P: 0.010%, S: 0.002%, with the remainder being Fe and unavoidable inclusions.

10. A hot-rolled spring steel strip prepared by the preparation process described in any one of claims 1-9, characterized in that, The yield strength R of the hot-rolled spring steel strip p0.2 =300~400MPa, tensile strength R m =500~600MPa, elongation A 80 ≥20%, hardness ≤190HV10.

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