High-performance spring flat steel for automobiles and method for manufacturing the same

Abstract

The application belongs to the technical field of steel material, and discloses a high-performance spring flat steel for automobiles and a preparation method thereof. p0.2 ≥1500MPa, the tensile strength R m ≥1700MPa, the elongation A is greater than or equal to 10%, and the reduction of area Z is greater than or equal to 38%, so that the steel has the characteristics of high strength, high plasticity and high corrosion resistance, and the production process is simple and the cost is low. p0.2 ≥1500MPa, the tensile strength R m ≥1700MPa, the elongation A is greater than or equal to 10%, and the reduction of area Z is greater than or equal to 38%; the inclusion (A, B, C, D) grade is not more than 1.0 grade, and the corrosion resistance coefficient I is greater than or equal to 6.8, so that the steel has the characteristics of high strength, high plasticity and high corrosion resistance, and the indexes are obviously better than those of the existing steel.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of steel materials, and particularly relates to a high-performance spring flat steel for automobiles and a preparation method thereof. BACKGROUND

[0002] Suspension springs are important elastic elements of automobile damping systems, and the quality and performance of suspension springs in automobile damping systems are directly related to the service safety and comfort of vehicles. The main material for manufacturing automobile suspension springs is spring flat steel, and different requirements for the strength, plasticity, corrosion resistance, fatigue performance, and hardenability of suspension springs are proposed due to different service environments of various automobiles.

[0003] With the rapid development of the automobile industry and the gradual implementation of the double-carbon policy, automobile safety and lightweight are the future development trend and an important way for automobile energy saving and emission reduction. Generally, the weight of automobile suspension springs accounts for about 9% of the total vehicle weight, and under the same working conditions, the higher the strength of the spring material, the thinner the required thickness. Therefore, it is imperative to improve the strength of spring flat steel and develop high-performance spring flat steel for automobiles.

[0004] Patent (CN 109161803 A) applies for a 1550MPa spring flat steel and a preparation method thereof, the tensile strength is less than 1600MPa, various inclusions are not more than 1.5 levels, the grain size is about 9 levels, and the addition of high Cr results in low plasticity.

[0005] Patent (CN 118241114 A) applies for a high-strength and plastic rare earth spring flat steel and a preparation method thereof, adds valuable elements Mo and Nb, and uses rare earth elements to improve inclusions, adopts 850-890℃ oil quenching + 410-440℃ tempering, the strength reaches 1600-1700MPa, and the elongation is only 10-11%.

[0006] Patent (CN 106978571 A) applies for a third-generation micro-alloyed spring flat steel for automobiles and a preparation method thereof, the tensile strength is only 1550MPa, the elongation is not less than 7%, the end quenching index is not less than 57HRC for 20mm thickness, and corrosion resistance is not involved.

[0007] Through the above analysis, the problems and defects of the prior art are:

[0008] At present, the alloy cost of the spring flat steel for automobiles is high, the inclusion level is high, and the process is complex, which is difficult to meet the technical requirements of high strength, high plasticity, and high corrosion resistance of high-performance spring flat steel for automobiles. SUMMARY

[0009] In view of the problems in the prior art, the present application provides a high-performance spring flat steel for automobiles and a preparation method thereof.

[0010] The present application is implemented in a high-performance spring flat steel for automobiles and a preparation method thereof, comprising:

[0011] A high-performance spring flat steel for automobiles, characterized in that the chemical composition of the high-performance spring flat steel for automobiles comprises, in terms of mass percentage, C: 0.55-0.60%, Si: 1.55-1.65%, Mn: 0.70-0.80%, Cr: 0.45-0.60%, V: 0.10-0.12%, Ni: 0.10-0.20%, Cu: 0.20-0.30%, B: 0.0010-0.0025%, Zr: 0.05-0.10%, P≤0.020%, S≤0.003%, H≤2ppm, and the balance of Fe and inevitable impurity elements, while satisfying a corrosion resistance index

[0012] I=26.01×[Cu]-33.39×[Cu]2-9.1×[Ni][P]-7.29×[Cu][Ni]+17.28×[P]+1.49×[Si]+1.2×[Cr]+3.88×[Ni]≥6.8.

[0013] Further, the thickness of the spring flat steel ranges from 6 to 20 mm.

[0014] Further, the preparation method of the high-performance spring flat steel for automobiles comprises the following steps:

[0015] Step 1, molten iron: KR desulfurization pretreatment is performed on the molten iron of a blast furnace to ensure that [S]≤0.002%.

[0016] Step 2, smelting and continuous casting process:

[0017] The chemical composition of the cast blank obtained by the smelting and continuous casting process comprises, in terms of mass percentage, C: 0.55-0.60%, Si: 1.55-1.65%, Mn: 0.70-0.80%, Cr: 0.45-0.60%, V: 0.10-0.12%, Ni: 0.10-0.20%, Cu: 0.20-0.30%, B: 0.0010-0.0025%, Zr: 0.05-0.10%, P≤0.020%, S≤0.003%, H≤2ppm, and the balance of Fe and inevitable impurity elements, while satisfying a corrosion resistance index

[0018] I=26.01×[Cu]-33.39×[Cu]2-9.1×[Ni][P]-7.29×[Cu][Ni]+17.28×[P]+1.49×[Si]+1.2×[Cr]+3.88×[Ni]≥6.8.

[0019] Low-S molten iron is smelted in a 150t top-and-bottom blowing converter with less than 15 tons of scrap steel added. The carbon content of the molten steel is controlled at 0.10-0.35% at tapping, and the tapping temperature is not lower than 1620℃. When 1 / 3 of the steel is tapped, ferrosilicon, high-carbon ferromanganese, ferrochrome, ferronickel, ferrovanadium alloy, and copper plate are added. The ladle is then transferred to an LF furnace for heating and refining. Ferroboron and ferrizirconium alloy are added according to the target composition, and the chemical composition is adjusted. At the same time, refining slag and quicklime are added for refining. White slag is maintained for more than 12 minutes, and soft blowing is performed for more than 12 minutes before being sent to the next process. Then, vacuum treatment is performed in an RH furnace with a vacuum degree ≤90Pa to significantly reduce harmful gases such as O, N, and H in the steel. Soft blowing for more than 15 minutes can further remove large particle inclusions, thereby smelting high-purity molten steel. Full-process protective casting is adopted and 160mm×160mm square billets are automatically fire-cut to length and then stacked for slow cooling.

[0020] Step 3, Rolling process:

[0021] The billet is cold-charged into the furnace and heated in sections. The preheating section temperature is no higher than 700℃, the soaking section temperature is 1200~1230℃, and the heating time is no less than 80 minutes. The roughing rolling temperature is 1060~1110℃; the finishing rolling temperature is 980~1050℃, and the finishing rolling temperature is 850~900℃. It is rolled into 6~20mm thick spring flat steel, which is then quickly removed from the cooling bed and stacked for slow cooling for no less than 48 hours. The temperature at which it leaves the cooling bed is no less than 500℃.

[0022] Step 4, Performance Testing:

[0023] Samples were taken for quenching and tempering heat treatment. The quenching and tempering heat treatment process was oil quenching at 840~870℃ + tempering at 500~560℃, and then performance testing was carried out.

[0024] Furthermore, the yield strength R of the spring flat steel p0.2 ≥1500MPa, tensile strength R m ≥1700MPa, elongation A≥10%, reduction of area Z≥38%.

[0025] Furthermore, the inclusion levels are as follows: Class A inclusions ≤ 1.0, Class B inclusions ≤ 1.0, Class C inclusions ≤ 1.0, and Class D inclusions ≤ 1.0.

[0026] The purpose of this invention is to provide a high-performance automotive spring flat steel manufacturing system comprising:

[0027] The hot metal pretreatment module is used to perform KR desulfurization pretreatment on blast furnace hot metal to ensure that [S] ≤ 0.002%;

[0028] The smelting and continuous casting module is used to determine the chemical composition (weight percentage) of the billet obtained from the smelting and continuous casting process as follows: C: 0.55–0.60%, Si: 1.55–1.65%, Mn: 0.70–0.80%, Cr: 0.45–0.60%, V: 0.10–0.12%, Ni: 0.10–0.20%, Cu: 0.20–0.30%, B: 0.0010–0.0025%. Zr: 0.05~0.10%, P≤0.020%, S≤0.003%, H≤2ppm, with the balance being Fe and unavoidable impurity elements, while also meeting the corrosion resistance index; I=26.01×[Cu]-33.39×[Cu]2-9.1×[Ni][P]-7.29×[Cu][Ni]+17.28×[P]+1.49×[Si]+1.2×[Cr]+3.88 ×[Ni]≥6.8; Low-S molten iron is smelted in a 150t top and bottom blowing converter, with scrap steel added at less than 15 tons. The C content of the molten steel is controlled at 0.10-0.35% at tapping, and the tapping temperature is not lower than 1620℃. When 1 / 3 of the steel is tapped, ferrosilicon, high-carbon ferromanganese, ferrochrome, ferronickel, ferrovanadium alloy and copper plate are added. The ladle is transferred to an LF furnace for heating and refining. Ferroboron and ferrizirconium alloy are added according to the target composition and the chemical composition is adjusted. At the same time, refining slag and active lime are added for refining. White slag is maintained for more than 12 minutes, and soft blowing is performed for more than 12 minutes before being sent to the next process. Then, vacuum treatment is performed in an RH furnace with a vacuum degree ≤90Pa to significantly reduce the harmful gases O, N and H in the steel. Soft blowing for more than 15 minutes can further remove large particle inclusions, thereby smelting into high-purity molten steel. Full-process protective casting is adopted and 160mm×160mm square billets are automatically fire-cut to length and then stacked for slow cooling.

[0029] The rolling module is used for cold charging of square billets into the furnace. It adopts segmented heating, with the preheating section temperature not exceeding 700℃, the soaking section temperature being 1200~1230℃, and the heating time not less than 80 minutes. The roughing rolling start temperature is 1060~1110℃; the finishing rolling start temperature is 980~1050℃, and the finishing rolling temperature is 850~900℃. It is rolled into 6~20mm thick spring flat steel, which is then rapidly removed from the cooling bed and slowly cooled by stacking for no less than 48 hours, with the removal temperature not less than 500℃.

[0030] The performance testing module is used to sample and perform quenching and tempering heat treatment. The quenching and tempering heat treatment process is oil quenching at 840~870℃ + tempering at 500~560℃, followed by performance testing.

[0031] Based on the above technical solutions and the technical problems solved, the advantages and positive effects of the technical solution to be protected by this invention are as follows:

[0032] This invention provides a high-performance automotive spring flat steel, employing a high-C, high-Si, Mn, and Cr alloy design, with the addition of microalloying elements such as V, Cu, Ni, B, and Zr to optimize the steel's overall mechanical properties. Through scientific component proportioning and refining control, this steel maintains high strength while exhibiting excellent plasticity and corrosion resistance. The final produced flat steel has a thickness ranging from 6 to 20 mm, meeting the stringent requirements of automotive springs for high stress and long fatigue life.

[0033] The spring flat steel of this invention achieves a yield strength R through a reasonable rolling and heat treatment process. p0.2 ≥1500MPa, tensile strength R m The steel exhibits a strength of ≥1700MPa, elongation A≥10%, and reduction of area Z≥38%. Furthermore, the inclusion grades (A, B, C, D) are all controlled to ≤1.0, ensuring material purity, reducing internal defects, and improving fatigue strength. Simultaneously, its corrosion resistance coefficient I≥6.8 is significantly superior to traditional spring steel, resulting in a longer service life in harsh environments such as automotive chassis.

[0034] The production process of this spring flat steel is simple and can be directly applied to existing metallurgical equipment without large-scale equipment modifications. High-performance production can be achieved simply by adjusting the relevant process parameters in the smelting, refining, rolling, and heat treatment stages. Specific optimization measures include: rationally controlling the white slag time during LF refining to improve the solubility and uniformity of microalloying elements; using RH vacuum treatment to reduce gas content and improve material purity; and employing controlled rolling and controlled cooling (TMCP) processes to improve grain refinement and achieve higher strength and toughness. These measures ensure the quality stability of the steel while reducing production costs, making it a promising candidate for industrial application.

[0035] The high-performance spring flat steel of this invention can be widely used in automotive suspension systems, heavy-duty vehicle springs, vibration damping components for engineering machinery, and the manufacture of high-strength springs. It is particularly suitable for automotive parts requiring high fatigue life and corrosion resistance. Compared to existing steel grades, this steel not only possesses higher strength and toughness but also significantly improves durability and corrosion resistance, adapting to the future development trend of lightweight and high-strength materials in automobiles. Therefore, the widespread application of this steel grade can improve the performance and reliability of automotive parts, reduce maintenance costs, and enhance industry competitiveness. Attached Figure Description

[0036] Figure 1 This is a flowchart of the method for preparing high-performance automotive spring flat steel provided in an embodiment of the present invention.

[0037] Figure 2 This is a structural block diagram of a high-performance automotive spring flat steel manufacturing system provided in an embodiment of the present invention.

[0038] Figure 3 The microstructure of the 20mm thick spring flat steel in the hot-rolled state provided in Embodiment 1 of this invention is mainly pearlite plus a small amount of ferrite. Detailed Implementation

[0039] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0040] The automotive spring flat steel provided by this invention has the following chemical composition by mass percentage: C: 0.55-0.60%, Si: 1.55-1.65%, Mn: 0.70-0.80%, Cr: 0.45-0.60%, V: 0.10-0.12%, Ni: 0.10-0.20%, Cu: 0.20-0.30%, B: 0.0010-0.0025%, Zr: 0.05-0.10%, P≤0.020%, S≤0.003%, H≤2ppm, with the balance being Fe and unavoidable impurities. It also meets the following corrosion resistance index:

[0041] I=26.01×[Cu]-33.39×[Cu]2-9.1×[Ni][P]-7.29×[Cu][Ni]+17.28×[P]+1.49×[Si]+1.2×[Cr]+3.88×[Ni]≥6.8.

[0042] like Figure 1 As shown, the method for preparing high-performance automotive spring flat steel according to an embodiment of the present invention includes the following steps:

[0043] S101, Molten Iron: The blast furnace molten iron is subjected to KR desulfurization pretreatment to ensure [S] ≤ 0.002%;

[0044] S102, smelting and continuous casting process:

[0045] The chemical composition (weight percentage) of the billet obtained by the smelting and continuous casting process is as follows: C: 0.55–0.60%, Si: 1.55–1.65%, Mn: 0.70–0.80%, Cr: 0.45–0.60%, V: 0.10–0.12%, Ni: 0.10–0.20%, Cu: 0.20–0.30%, B: 0.0010–0.0025%, Zr: 0.05–0.10%, P≤0.020%, S≤0.003%, H≤2ppm, with the balance being Fe and unavoidable impurities, while also meeting the corrosion resistance index.

[0046] I=26.01×[Cu]-33.39×[Cu]2-9.1×[Ni][P]-7.29×[Cu][Ni]+17.28×[P]+1.49×[Si]+1.2×[Cr]+3.88×[Ni]≥6.8;

[0047] Low-S molten iron is smelted in a 150t top-and-bottom blowing converter with less than 15 tons of scrap steel added. The carbon content of the molten steel is controlled at 0.10-0.35% at tapping, and the tapping temperature is not lower than 1620℃. When 1 / 3 of the steel is tapped, ferrosilicon, high-carbon ferromanganese, ferrochrome, ferronickel, ferrovanadium alloy, and copper plate are added. The ladle is then transferred to an LF furnace for heating and refining. Ferroboron and ferrizirconium alloy are added according to the target composition, and the chemical composition is adjusted. At the same time, refining slag and quicklime are added for refining. White slag is maintained for more than 12 minutes, and soft blowing is performed for more than 12 minutes before being sent to the next process. Then, vacuum treatment is performed in an RH furnace with a vacuum degree ≤90Pa to significantly reduce harmful gases such as O, N, and H in the steel. Soft blowing for more than 15 minutes can further remove large particle inclusions, thereby smelting high-purity molten steel. Full-process protective casting is adopted and 160mm×160mm square billets are automatically fire-cut to length and then stacked for slow cooling.

[0048] S103, rolling process:

[0049] The billet is cold-charged into the furnace and heated in sections. The preheating section temperature is no higher than 700℃, the soaking section temperature is 1200~1230℃, and the heating time is no less than 80 minutes. The roughing rolling temperature is 1060~1110℃; the finishing rolling temperature is 980~1050℃, and the finishing rolling temperature is 850~900℃. It is rolled into 6~20mm thick spring flat steel, which is then quickly removed from the cooling bed and stacked for slow cooling for no less than 48 hours. The temperature at which it leaves the cooling bed is no less than 500℃.

[0050] S104, Performance Verification:

[0051] Samples were taken for quenching and tempering heat treatment. The quenching and tempering heat treatment process was oil quenching at 840~870℃ + tempering at 500~560℃, and then performance testing was carried out.

[0052] The yield strength R of the spring flat steel provided in this embodiment of the invention p0.2 ≥1500MPa, tensile strength R m ≥1700MPa, elongation A≥10%, reduction of area Z≥38%.

[0053] The embodiments of the present invention provide inclusion levels of Class A (≤1.0), Class B (≤1.0), Class C (≤1.0), and Class D (≤1.0).

[0054] like Figure 2 As shown, an embodiment of the present invention provides a high-performance automotive spring flat steel manufacturing system comprising:

[0055] The hot metal pretreatment module is used to perform KR desulfurization pretreatment on blast furnace hot metal to ensure that [S] ≤ 0.002%;

[0056] The smelting and continuous casting module is used to determine the chemical composition (weight percentage) of the billet obtained from the smelting and continuous casting process as follows: C: 0.55–0.60%, Si: 1.55–1.65%, Mn: 0.70–0.80%, Cr: 0.45–0.60%, V: 0.10–0.12%, Ni: 0.10–0.20%, Cu: 0.20–0.30%, B: 0.0010–0.0025%. Zr: 0.05~0.10%, P≤0.020%, S≤0.003%, H≤2ppm, with the balance being Fe and unavoidable impurity elements, while also meeting the corrosion resistance index; I=26.01×[Cu]-33.39×[Cu]2-9.1×[Ni][P]-7.29×[Cu][Ni]+17.28×[P]+1.49×[Si]+1.2×[Cr]+3.88 ×[Ni]≥6.8; Low-S molten iron is smelted in a 150t top and bottom blowing converter, with scrap steel added at less than 15 tons. The C content of the molten steel is controlled at 0.10-0.35% at tapping, and the tapping temperature is not lower than 1620℃. When 1 / 3 of the steel is tapped, ferrosilicon, high-carbon ferromanganese, ferrochrome, ferronickel, ferrovanadium alloy and copper plate are added. The ladle is transferred to an LF furnace for heating and refining. Ferroboron and ferrizirconium alloy are added according to the target composition and the chemical composition is adjusted. At the same time, refining slag and active lime are added for refining. White slag is maintained for more than 12 minutes, and soft blowing is performed for more than 12 minutes before being sent to the next process. Then, vacuum treatment is performed in an RH furnace with a vacuum degree ≤90Pa to significantly reduce the harmful gases O, N and H in the steel. Soft blowing for more than 15 minutes can further remove large particle inclusions, thereby smelting into high-purity molten steel. Full-process protective casting is adopted and 160mm×160mm square billets are automatically fire-cut to length and then stacked for slow cooling.

[0057] The rolling module is used for cold charging of square billets into the furnace. It adopts segmented heating, with the preheating section temperature not exceeding 700℃, the soaking section temperature being 1200~1230℃, and the heating time not less than 80 minutes. The roughing rolling start temperature is 1060~1110℃; the finishing rolling start temperature is 980~1050℃, and the finishing rolling temperature is 850~900℃. It is rolled into 6~20mm thick spring flat steel, which is then rapidly removed from the cooling bed and slowly cooled by stacking for no less than 48 hours, with the removal temperature not less than 500℃.

[0058] The performance testing module is used to sample and perform quenching and tempering heat treatment. The quenching and tempering heat treatment process is oil quenching at 840~870℃ + tempering at 500~560℃, followed by performance testing.

[0059] The technical solution adopted by this invention to solve the above-mentioned technical problems is a high-performance automotive spring flat steel, the chemical composition (wt%) of which includes C: 0.55-0.60%, Si: 1.55-1.65%, Mn: 0.70-0.80%, Cr: 0.45-0.60%, V: 0.10-0.12%, Ni: 0.10-0.20%, Cu: 0.20-0.30%, and B: 0.0010-0.0025%. %, Zr: 0.05~0.10%, P≤0.020%, S≤0.003%, H≤2ppm, with the balance being Fe and unavoidable impurity elements, while satisfying the corrosion resistance index I=26.01×[Cu]-33.39×[Cu]2-9.1×[Ni][P]-7.29×[Cu][Ni]+17.28×[P]+1.49×[Si]+1.2×[Cr]+3.88×[Ni]≥6.8.

[0060] The manufacturing process of high-performance automotive spring flat steel of this invention is as follows: molten iron pretreatment → 150t converter smelting → LF furnace refining → RH furnace vacuum treatment → billet continuous casting → billet stacking cooling → billet heating and rolling → flat steel slow cooling → flat steel finishing → performance inspection.

[0061] This method includes smelting, rolling and other technological steps, as detailed below:

[0062] (1) Molten iron: The blast furnace molten iron is subjected to KR desulfurization pretreatment to ensure that [S] ≤ 0.002%.

[0063] (2) Smelting and continuous casting process: Low-S molten iron is smelted in a 150t top and bottom blowing converter. The amount of scrap steel added is less than 15 tons. The C content of the molten steel is controlled at 0.10-0.35% when tapping. The tapping temperature is not lower than 1620℃. When 1 / 3 of the steel is tapped, ferrosilicon, high-carbon ferromanganese, ferrochrome, ferronickel, ferrovanadium alloy and copper plate are added. The ladle is moved to the LF furnace for heating and refining treatment. Ferroboron and ferrizirconium alloy are added according to the target composition and the chemical composition is adjusted. At the same time, refining slag, active lime and other materials are added for refining treatment. White slag is maintained for more than 12 minutes. After soft blowing for more than 12 minutes, it is sent to the next process. Then, vacuum treatment is carried out in the RH furnace with a vacuum degree ≤90Pa to significantly reduce the harmful gases O, N and H in the steel. Soft blowing for more than 15 minutes can further remove large particle inclusions, thereby smelting into high-purity molten steel. Full-process protective casting is adopted and 160mm×160mm square billets are automatically fire-cut to length and then stacked for slow cooling.

[0064] (3) Rolling process: The billet is cold-charged into the furnace and heated in sections. The temperature of the preheating section is not higher than 700℃, the temperature of the soaking section is 1200~1230℃, and the heating time is not less than 80 minutes. The roughing rolling temperature is 1060~1110℃; the finishing rolling temperature is 980~1050℃, and the final rolling temperature is 850~900℃. It is rolled into 6~20mm thick spring flat steel. It is quickly removed from the cooling bed and stacked for slow cooling for not less than 48 hours. The temperature of the removed steel is not less than 500℃. At this time, the hydrogen content and stress in the steel can be removed, further reducing the risk of cracks in the steel.

[0065] (4) Performance test: Samples are taken for quenching and tempering heat treatment. The quenching and tempering heat treatment process is oil quenching at 840~870℃ + tempering at 500~560℃, and then performance test is carried out.

[0066] To ensure the objectives of this invention and to meet the requirements of high strength, high plasticity, and high corrosion resistance for high-performance automotive spring flat steel, the reasons for limiting the elements C, Si, Mn, Cr, V, Ni, Cu, B, Zr, P, S, and H in this invention are explained as follows:

[0067] Carbon (C) is a crucial element for ensuring the strength of spring flat steel. When the C content is too low, it's difficult to guarantee the high strength requirements of the spring flat steel; when the C content is too high, it deteriorates the quality of the billet, the steel's ductility and toughness, and its processing performance. Therefore, the C content of the steel in this invention is 0.55–0.60%.

[0068] Si is an important solid solution strengthening element, a major deoxidizing element during smelting, and a crucial element for ensuring the elasticity of spring flat steel. Therefore, the Si content of the steel in this invention is 1.55% to 1.65%.

[0069] Mn dissolved in ferrite can increase the strength of steel, as well as improve the hardenability and processing performance of spring flat steel. However, excessive Mn content will cause the steel grains to coarsen at high temperatures and reduce the toughness of the steel. Therefore, the Mn content of the steel in this invention is 0.70% to 0.80%.

[0070] Cr can improve the hardenability of spring flat steel, thereby increasing the strength and hardness of the steel. It can also improve the wear resistance and corrosion resistance of the steel. However, if the Cr content is too high, it will reduce the plasticity, toughness and processing performance of the steel. Therefore, the Cr content of the steel in this invention is 0.45% to 0.60%.

[0071] The carbonitrides formed by V can inhibit the recrystallization of austenite and prevent grain growth during rolling, thereby refining ferrite grains and improving the strength and toughness of the steel. Simultaneously, the precipitation of carbonitrides during tempering provides precipitation strengthening. Therefore, the V content in the steel of this invention is 0.10–0.12%.

[0072] Ni and Cu are beneficial for improving the corrosion resistance of steel, but excessive amounts will significantly increase the production cost of steel. Therefore, the Ni content of the steel in this invention is 0.10-0.20% and the Cu content is 0.20-0.30%.

[0073] B can significantly improve the hardenability of steel, but excessive B can affect the toughness of steel. Therefore, the B content of the steel in this invention is 0.0010 to 0.0025%.

[0074] The composite oxides of Zr modify inclusions such as MnS, improving the morphology, size and distribution of inclusions in steel, thereby reducing the inclusion level of steel and improving its ductility, toughness and corrosion resistance. However, Zr is expensive and adding too much will increase production costs. Therefore, the Zr content in the steel of this invention is 0.05 to 0.10%.

[0075] P and S are unavoidable impurity elements in steel, which are detrimental to the plasticity and toughness of steel. The lower their content, the better. Therefore, the steel of this invention has a P content of ≤0.020% and a S content of ≤0.003%.

[0076] Hydrogen (H) tends to accumulate in the gaps between inclusions and iron in the core of the billet, affecting its quality. Excessive H content can easily cause cracks. Therefore, the H content in the steel of this invention is ≤2ppm.

[0077] The chemical composition (wt%) of the embodiments and comparative examples of this invention is shown in Table 1:

[0078] Table 1

[0079]

[0080] The above embodiments use KR pretreated molten iron with an S content of 0.001-0.002%, all smelted in a 150t top-and-bottom combined blowing converter. The final carbon content at tapping is 0.20-0.30%, and the tapping temperature is 1628-1647℃. After deep desulfurization and refining in an LF furnace, the molten iron is soft-blown for 13-20 minutes, then transferred to an RH vacuum furnace for degassing, soft-blown for 18-25 minutes to fully remove large particle inclusions and ensure uniform composition. Then, the entire process is protected and cast into 160mm×160mm continuous casting billets, which are stacked and slowly cooled to below 200℃.

[0081] The billet is cold-charged into the heating furnace. The preheating temperature is set at 650℃, the homogenization temperature is set at 1220℃, the roughing rolling temperature is actually 1080~1105℃, the finishing rolling temperature is 1000~1030℃, the finishing rolling temperature is 865~890℃, and it is rolled into spring flat steel with a thickness of 6~20mm. It is then quickly removed from the line and stacked for slow cooling for no less than 48 hours.

[0082] The experimental samples were oil quenched at 840–870℃ and tempered at 500–560℃, and then the performance was tested.

[0083] Table 2 shows the main smelting process parameters for each embodiment.

[0084] Table 2

[0085]

[0086] Table 3 shows the main rolling and heat treatment process parameters for each embodiment.

[0087] Table 3

[0088]

[0089] After heat treatment, the spring flat steel was longitudinally processed into tensile specimens and metallographic specimens, and mechanical property tests and inclusion observations were performed. The test results are shown in Table 4.

[0090] Table 4

[0091]

[0092]

[0093] As shown in Table 4, the strength, elongation, inclusions, and grain size of the steel plates tested in the embodiments of the present invention are significantly better than those in the comparative example, indicating that the embodiments have better purity and physical properties. The corrosion resistance index I of the embodiments is greater than 6.8, which is significantly greater than that of the comparative example, indicating that they have better corrosion resistance.

[0094] Figure 3 The diagram shown is a microstructure of the hot-rolled spring flat steel with a thickness of 20mm in Example 1. The microstructure consists of pearlite and a small amount of ferrite.

[0095] This invention can be implemented in metallurgical enterprises. The process is simple, highly operable, and low in cost. It can be applied to the manufacture of various automotive suspension springs.

[0096] Example 1: Preparation method of high-performance flat steel

[0097] 1) Hot metal pretreatment:

[0098] The KR desulfurization process is used to desulfurize blast furnace hot metal to achieve [S] ≤ 0.002%, thereby removing sulfur and improving the purity of the steel.

[0099] 2) Smelting and refining:

[0100] The smelting process is carried out using a 150t top-and-bottom combined blowing converter. The amount of scrap steel added is controlled at 12 tons, the tapping temperature is 1625℃, and ferrosilicon (Si≥75%), high-carbon ferromanganese (C≥7.5%, Mn≥70%), ferrochrome (Cr≥65%), ferronickel (Ni≥15%), ferrovanadium (V≥50%), and copper plate (Cu≥99%) are added when 1 / 3 of the steel is tapped.

[0101] The material is fed into an LF furnace for refining, with the addition of ferroborone (B≥20%) and ferrizirconium (Zr≥30%). The white slag is maintained for 12 minutes, followed by soft blowing for 16 minutes.

[0102] The steel is fed into an RH furnace for vacuum treatment at a vacuum level of 81 Pa for 20 minutes to reduce the O, N, and H content and improve the purity of the steel.

[0103] 3) Continuous casting and slow cooling:

[0104] The billet is made of 160mm×160mm continuous casting. It is cast with full protection throughout the process. After casting, it is automatically cut to length and then slowly cooled in the stacking slow cooling zone for 48 hours to ensure the stability of the internal structure.

[0105] 4) Rolling process:

[0106] The billet is cold-charged into the furnace and heated in sections. The preheating section temperature is 650℃, the soaking section temperature is 1220℃, and the heating time is 85 minutes.

[0107] The roughing rolling temperature is 1090℃, the finishing rolling temperature is 1020℃, and the final rolling temperature is 880℃.

[0108] It is rolled into flat steel with a thickness of 12mm, quickly removed from the cooling bed, and slowly cooled in a stack for 48 hours, with a removal temperature of 510℃.

[0109] 5) Quenching and tempering heat treatment:

[0110] Oil quenching at 840℃ followed by tempering at 520℃ is used to improve the uniformity of the flat steel structure and ensure the strength and toughness of the steel.

[0111] 6) Chemical composition (mass percentage):

[0112] C: 0.58%, Si: 1.60%, Mn: 0.75%, Cr: 0.50%, V: 0.11%, Ni: 0.15%, Cu: 0.25%, B: 0.0015%, Zr: 0.08%, P: 0.018%, S: 0.002%, H: 1.8ppm, balance Fe.

[0113] 7) Control of non-metallic inclusions:

[0114] Category A ≤ Level 1.0, Category B ≤ Level 1.0, Category C ≤ Level 1.0, Category D ≤ Level 1.0.

[0115] Example 2: Preparation method of high-strength flat steel

[0116] 1) Hot metal pretreatment:

[0117] KR desulfurization treatment is used to ensure that [S] ≤ 0.002%, thus ensuring the purity of the molten steel.

[0118] 2) Smelting and refining:

[0119] Smelting in a 150t top and bottom blown converter, with 13 tons of scrap steel added and a tapping temperature of 1630℃, ferrosilicon (Si≥75%), high-carbon ferromanganese (Mn≥72%), ferrochrome (Cr≥68%), ferronickel (Ni≥18%), ferrovanadium (V≥52%), and copper plate (Cu≥99%) are added when 1 / 3 of the steel is tapped.

[0120] The slag is sent to the LF furnace for refining, and ferroborone (B≥22%) and ferrizirconium (Zr≥32%) are added. The slag is kept white for 15 minutes and then soft blown for 15 minutes.

[0121] The steel is subjected to vacuum treatment in an RH furnace with a vacuum level of 85 Pa and a soft blowing time of 23 minutes to reduce the O, N, and H content.

[0122] 3) Continuous casting and slow cooling:

[0123] A 160mm×160mm continuously cast square billet was used, and protective casting was adopted. After casting, the billet was automatically cut to length and then slowly cooled in the stacking slow cooling zone for 50 hours.

[0124] 4) Rolling process:

[0125] The billet is cold-charged into the furnace and heated in sections. The preheating section temperature is 680℃, the soaking section temperature is 1210℃, and the heating time is 90 minutes.

[0126] The roughing rolling temperature is 1085℃, the finishing rolling temperature is 1000℃, and the final rolling temperature is 870℃.

[0127] It is rolled into flat steel with a thickness of 16mm, quickly removed from the cooling bed, stacked and slowly cooled for 50 hours, and removed from the line at a temperature of 520℃.

[0128] 5) Quenching and tempering heat treatment:

[0129] The flat steel is quenched in oil at 860℃ and tempered at 540℃ to ensure its strength and toughness.

[0130] 6) Chemical composition (mass percentage):

[0131] C: 0.57%, Si: 1.62%, Mn: 0.76%, Cr: 0.55%, V: 0.12%, Ni: 0.18%, Cu: 0.28%, B: 0.0020%, Zr: 0.09%, P: 0.017%, S: 0.002%, H: 1.5ppm, balance Fe.

[0132] 7) Control of non-metallic inclusions:

[0133] Category A ≤ Level 1.0, Category B ≤ Level 1.0, Category C ≤ Level 1.0, Category D ≤ Level 1.0.

[0134] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any modifications, equivalent substitutions, and improvements made by those skilled in the art within the scope of the technology disclosed in the present invention, and within the spirit and principles of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A flat steel, characterized in that, The chemical composition of the flat spring steel by mass percentage comprises: C: 0.55~0.60%, Si: 1.55~1.65%, Mn: 0.70~0.80%, Cr: 0.45~0.60%, V: 0.10~0.12%, Ni: 0.10~0.20%, Cu: 0.20~0.30%, B: 0.0010~0.0025%, Zr: 0.05~0.10%, P≤0.020%, S≤0.003%, H≤2ppm, the balance being Fe and inevitable impurity elements; The flat spring steel is manufactured by the following method: KR desulfurization pretreatment of blast furnace molten iron is adopted to make [S]≤0.002%; Smelting is performed in a 150t top and bottom combined blowing converter, the amount of scrap steel is controlled to be less than 15 tons, the tapping temperature is not less than 1620℃, and silicon iron, high-carbon ferromanganese, ferrochrome, ferro-nickel, ferro-vanadium alloy and copper plate are added at 1 / 3 of the tapping time; An LF furnace is used for temperature rising and refining, boron iron and zirconium iron alloy are added, white slag is maintained for more than 12 minutes, and soft blowing is maintained for more than 12 minutes; Vacuum treatment is performed in an RH furnace, the vacuum degree is ≤90Pa, and soft blowing is maintained for more than 15 minutes; Full-process protection pouring is adopted and continuous casting is performed to form a 160mm×160mm square billet; The flat spring steel is subjected to a quenching and tempering heat treatment, comprising: 840~870℃ oil quenching; 500~560℃ tempering.

2. The flat steel according to claim 1, characterized in that The thickness of the flat spring steel ranges from 6mm to 20mm.

3. The flat steel according to claim 1, wherein The rolling method of the flat spring steel comprises: Square billet cold charging, segmented heating, preheating temperature not higher than 700℃, soaking temperature 1200~1230℃, and heating time not less than 80 minutes; Coarse rolling opening temperature 1060~1110℃, fine rolling opening temperature 980~1050℃, and finish rolling temperature 850~900℃; Rolled into 6~20mm thick spring flat steel, cold bed rapid offline, stacking slow cooling not less than 48 hours, offline temperature not less than 500℃.

4. The flat steel according to claim 1, wherein The non-metallic inclusion grade of the flat spring steel meets the following requirements: Class A inclusion grade ≤1.0 grade; Class B inclusion grade ≤1.0 grade; Class C inclusion grade ≤1.0 grade; Class D inclusion grade ≤1.0 grade.

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