High-performance automobile spring flat steel and preparation method thereof

Abstract

The invention belongs to the technical field of steel and iron materials, and discloses high-performance spring flat steel for an automobile and a preparation method thereof.The high-performance spring flat steel for the automobile with the thickness being 6-20 mm is produced by adopting a high-C-high-Si-Mn-Cr system as the basis and being supplemented with a proper amount of V, Cu, Ni, B, Zr and other elements through reasonable smelting, rolling and heat treatment processes, and the yield strength Rp0.2 is larger than or equal to 1500 MPa, the tensile strength Rp0.2 is larger than or equal to 10 MPa, and the tensile strength Rp0.2 is larger than or equal to 10 MPa. The tensile strength Rm is larger than or equal to 1700 MPa, the ductility A is larger than or equal to 10%, and the percentage reduction of area Z is larger than or equal to 38%, so that the steel has the characteristics of high strength, high plasticity, high corrosion resistance and the like, the production process is simple, and the cost is low. According to the spring flat steel, the yield strength Rp0.2 is larger than or equal to 1500 MPa, the tensile strength Rm is larger than or equal to 1700 MPa, the ductility A is larger than or equal to 10%, and the percentage reduction of area Z is larger than or equal to 38%; the grade of inclusions (A, B, C and D) does not exceed 1.0 grade, the corrosion resistance coefficient I is larger than or equal to 6.8, the steel has the characteristics of high strength, high plasticity, high corrosion resistance and the like, and various indexes are obviously superior to those of existing steel types.

Description

Technical Field

[0001] The invention belongs to the technical field of steel materials, and in particular relates to a high-performance spring flat steel for automobiles and a preparation method thereof. Background Art

[0002] Suspension springs are important elastic components of automobile shock absorption systems. The quality and performance of suspension springs in automobile shock absorption systems are directly related to the service safety and comfort of vehicles. The main material for manufacturing automobile suspension springs is spring flat steel. At the same time, due to the different service environments of various types of automobiles, different requirements are put forward for the strength, plastic toughness, corrosion resistance, fatigue performance, hardenability and other properties of suspension springs.

[0003] With the rapid development of the automobile industry and the gradual implementation of the dual-carbon policy, automobile safety and lightweight are the future development trends and important ways to save energy and reduce emissions. Generally speaking, the weight of automobile suspension springs accounts for about 9% of the weight of the entire vehicle. 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 automobile spring flat steel.

[0004] The patent (CN 109161803 A) applied for a 1550MPa spring flat steel and its preparation method, with a tensile strength of less than 1600MPa, various inclusions not exceeding level 1.5, a grain size of about level 9, and low plasticity due to the addition of a relatively high Cr.

[0005] The patent (CN 118241114 A) applied for a high-strength plastic rare earth spring flat steel and its preparation method, adding precious elements Mo and Nb, and using rare earth elements to improve its inclusions. It adopts 850-890℃ oil quenching + 410-440℃ tempering, and the strength reaches 1600-1700MPa, and the elongation is only 10-11%.

[0006] The patent (CN 106978571 A) applies for a micro-alloyed third-generation automotive spring flat steel and its preparation method. The tensile strength is only 1550MPa, the elongation is not less than 7%, and the end quenching index ensures that the 20mm thickness is not less than 57HRC. It does not involve corrosion resistance.

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

[0008] At present, the alloy cost of automotive spring flat steel is high, the inclusion level is high, and the process is complicated, which makes it difficult to meet the technical requirements of high strength, high plasticity and high corrosion resistance of high-performance automotive spring flat steel. Summary of the invention

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

[0010] The present invention is achieved by: a high-performance automobile spring flat steel and a preparation method thereof comprising:

[0011] A high-performance automobile spring flat steel, characterized in that the chemical composition of the high-performance automobile spring flat steel is calculated by mass percentage 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, the balance is Fe and unavoidable impurity elements, and the corrosion resistance index is satisfied at the same time.

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

[0014] Furthermore, the method for preparing the high-performance automotive spring flat steel comprises the following steps:

[0015] Step 1, molten iron: perform KR desulfurization pretreatment on blast furnace molten iron to ensure [S] ≤ 0.002%;

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

[0017] The chemical composition content (weight percentage) of the ingot 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, the balance is Fe and unavoidable impurity elements, and the corrosion resistance index is met at the same time.

[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] The low-S molten iron is smelted in a 150t top and bottom combined blown converter, the amount of scrap steel added is less than 15 tons, the C content of the molten steel is controlled to be 0.10-0.35% when tapping, the tapping temperature is not less than 1620℃, and ferrosilicon, high carbon ferromanganese, ferrochrome, ferronickel, ferrovanadium alloy and copper plate are added when 1 / 3 of the steel is tapped; the ladle is moved to the LF furnace for heating and refining treatment, ferroboron and ferozirconium alloy are added according to the target composition and the chemical composition is adjusted, and refining slag, active lime, etc. are added for refining treatment, the white slag is kept for more than 12 minutes, and soft blowing is carried out for more than 12 minutes before being sent to the next process; then vacuum treatment is carried out in an RH furnace with a vacuum degree of ≤90Pa to significantly reduce the harmful gases of O, N, and H in the steel, and soft blowing for more than 15 minutes can further remove large particle inclusions, thereby smelting high-purity molten steel; full-process protection casting is adopted to form 160mm×160mm square billets, and the square billets are automatically fire-cut to size and then stacked and slowly cooled;

[0020] Step 3, rolling process:

[0021] The billet is cold-charged into the furnace and heated in sections. The preheating section temperature is not higher than 700°C, the soaking section temperature is 1200-1230°C, the heating time is not less than 80 minutes, the rough rolling start temperature is 1060-1110°C; the finishing rolling start temperature is 980-1050°C, the final rolling temperature is 850-900°C, and the billet is rolled into 6-20mm thick spring flat steel. The cooling bed is quickly unloaded and stacked for slow cooling for not less than 48 hours, and the unload temperature is not less than 500°C.

[0022] Step 4, performance test:

[0023] The samples are taken for tempering heat treatment, the tempering heat treatment process is 840-870℃ oil quenching + 500-560℃ tempering, and then the performance is tested.

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

[0025] Furthermore, the inclusion grade of Class A is ≤1.0, that of Class B is ≤1.0, that of Class C is ≤1.0, and that of Class D is ≤1.0.

[0026] The object of the present invention is to provide a high-performance automobile spring flat steel manufacturing system comprising:

[0027] Hot metal pretreatment module, used for KR desulfurization pretreatment of blast furnace hot metal, ensuring [S] ≤ 0.002%;

[0028] The smelting and continuous casting module is used for the chemical composition content (weight percentage) of the ingot obtained by the smelting and continuous casting process: 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 is Fe and unavoidable impurity elements, and at the same time meets 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; smelting low-S molten iron in a 150t top and bottom combined blown converter, adding less than 15 tons of scrap steel, controlling the C content of molten steel to 0.10-0.35% when tapping, and the tapping temperature to not less than 1620℃. When 1 / 3 of the steel is tapped, adding ferrosilicon, high carbon ferromanganese, ferrochrome, ferronickel, ferrovanadium alloy and copper plate; moving the ladle to the LF furnace for heating and refining treatment, adding ferroboron and ferozirconium alloy according to the target composition and adjusting the chemical composition, while adding refining slag, active lime, etc. for refining treatment, keeping the white slag for more than 12 minutes, and soft blowing for more than 12 minutes before sending it to the next process; then vacuum treatment is carried out in an RH furnace with a vacuum degree of ≤90Pa to significantly reduce the harmful gases of 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; adopting full-process protection casting and 160mm×160mm square billets, the square billets are automatically fire-cut to size and then stacked and slowly cooled;

[0029] The rolling module is used for cold charging of square billets into the furnace. It adopts segmented heating. The temperature of the preheating section is not higher than 700℃, the temperature of the soaking section is 1200-1230℃, the heating time is not less than 80 minutes, the rough rolling start temperature is 1060-1110℃; the finishing rolling start temperature is 980-1050℃, the final rolling temperature is 850-900℃, and the billets are rolled into 6-20mm thick spring flat steel. The cooling bed is quickly unloaded and stacked for slow cooling for not less than 48 hours, and the unload temperature is not less than 500℃.

[0030] The performance inspection module is used to take samples for tempering heat treatment. The tempering heat treatment process is 840-870℃ oil quenching + 500-560℃ tempering, and then the performance inspection is carried out.

[0031] In combination with the above technical solutions and the technical problems solved, the advantages and positive effects of the technical solutions to be protected by the present invention are as follows:

[0032] The present invention provides a high-performance flat steel for automobile springs, which is designed with a high C-high Si-Mn-Cr alloy, and on this basis, micro-alloy elements such as V, Cu, Ni, B, and Zr are added to optimize the comprehensive mechanical properties of the steel. Through scientific composition ratio and refining control, the steel has excellent plasticity and corrosion resistance while maintaining high strength. The thickness of the flat steel finally produced ranges from 6 to 20 mm, meeting the stringent requirements of automobile springs for high stress and high fatigue life.

[0033] The spring flat steel of the present invention has achieved a yield strength R through reasonable rolling and heat treatment processes. p0.2 ≥1500MPa, tensile strength R m ≥1700MPa, elongation A≥10%, and section shrinkage Z≥38%. In addition, the inclusion level (A, B, C, D) of the steel is controlled at ≤1.0 level to ensure the purity of the material, reduce internal defects, and improve fatigue strength. At the same time, its corrosion resistance coefficient I≥6.8 is significantly better than traditional spring steel, and has a longer service life in harsh environments such as automobile 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 modification. High-performance production can be achieved by adjusting the corresponding process parameters in the smelting, refining, rolling and heat treatment links. Specific optimization measures include: reasonable control of white slag time during LF refining to improve the solubility and uniformity of micro-alloy elements; RH vacuum treatment to reduce gas content and improve material purity; controlled rolling and controlled cooling (TMCP) process to improve grain refinement and achieve higher strength and plastic toughness. These measures ensure the quality stability of steel while reducing production costs, giving it a good prospect for industrial promotion.

[0035] The high-performance spring flat steel of the present invention can be widely used in automobile suspension systems, heavy-load vehicle springs, engineering machinery shock-absorbing components and high-strength spring manufacturing, and is particularly suitable for automobile parts that require high fatigue life and corrosion resistance. Compared with existing steel grades, this steel not only has higher strength and plastic toughness, but also greatly improves durability and corrosion resistance, and adapts to the development trend of lightweight and high-strength materials for future automobiles. Therefore, the promotion and application of this steel grade can improve the performance and reliability of automobile parts, reduce maintenance costs, and enhance industry competitiveness. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 The present invention provides a flowchart of a method for preparing a high-performance automotive spring flat steel according to an embodiment of the present invention.

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

[0038] Figure 3 The hot-rolled metallographic structure of the 20 mm thick spring flat steel of Example 1 provided in the embodiments of the present invention is mainly pearlite + a small amount of ferrite. DETAILED DESCRIPTION

[0039] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0040] The spring flat steel for automobiles provided by the present invention has the following chemical compositions 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, the remainder is Fe and unavoidable impurity elements, and the corrosion resistance index is satisfied at the same time:

[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, a method for preparing a high-performance automotive spring flat steel provided by an embodiment of the present invention comprises the following steps:

[0043] S101, molten iron: KR desulfurization pretreatment is performed on blast furnace molten iron to ensure [S] ≤ 0.002%;

[0044] S102, smelting and continuous casting process:

[0045] The chemical composition content (weight percentage) of the ingot 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, the balance is Fe and unavoidable impurity elements, and the corrosion resistance index is met at the same time.

[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] The low-S molten iron is smelted in a 150t top and bottom combined blown converter, the amount of scrap steel added is less than 15 tons, the C content of the molten steel is controlled to be 0.10-0.35% when tapping, the tapping temperature is not less than 1620℃, and ferrosilicon, high carbon ferromanganese, ferrochrome, ferronickel, ferrovanadium alloy and copper plate are added when 1 / 3 of the steel is tapped; the ladle is moved to the LF furnace for heating and refining treatment, ferroboron and ferozirconium alloy are added according to the target composition and the chemical composition is adjusted, and refining slag, active lime, etc. are added for refining treatment, the white slag is kept for more than 12 minutes, and soft blowing is carried out for more than 12 minutes before being sent to the next process; then vacuum treatment is carried out in an RH furnace with a vacuum degree of ≤90Pa to significantly reduce the harmful gases of O, N, and H in the steel, and soft blowing for more than 15 minutes can further remove large particle inclusions, thereby smelting high-purity molten steel; full-process protection casting is adopted to form 160mm×160mm square billets, and the square billets are automatically fire-cut to size and then stacked and slowly cooled;

[0048] S103, rolling process:

[0049] The billet is cold-charged into the furnace and heated in sections. The preheating section temperature is not higher than 700°C, the soaking section temperature is 1200-1230°C, the heating time is not less than 80 minutes, the rough rolling start temperature is 1060-1110°C; the finishing rolling start temperature is 980-1050°C, the final rolling temperature is 850-900°C, and the billet is rolled into 6-20mm thick spring flat steel. The cooling bed is quickly unloaded and stacked for slow cooling for not less than 48 hours, and the unload temperature is not less than 500°C.

[0050] S104, performance test:

[0051] The samples are taken for tempering heat treatment, the tempering heat treatment process is 840-870℃ oil quenching + 500-560℃ tempering, and then the performance is tested.

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

[0053] The embodiment of the present invention provides that the inclusion level of Class A is ≤1.0, Class B is ≤1.0, Class C is ≤1.0, and Class D is ≤1.0.

[0054] like Figure 2 As shown, a high-performance automobile spring flat steel manufacturing system provided by an embodiment of the present invention includes:

[0055] Hot metal pretreatment module, used for KR desulfurization pretreatment of blast furnace hot metal, ensuring [S] ≤ 0.002%;

[0056] The smelting and continuous casting module is used for the chemical composition content (weight percentage) of the ingot obtained by the smelting and continuous casting process: 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 is Fe and unavoidable impurity elements, and at the same time meets 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; smelting low-S molten iron in a 150t top and bottom combined blown converter, adding less than 15 tons of scrap steel, controlling the C content of molten steel to 0.10-0.35% when tapping, and the tapping temperature to not less than 1620℃. When 1 / 3 of the steel is tapped, adding ferrosilicon, high carbon ferromanganese, ferrochrome, ferronickel, ferrovanadium alloy and copper plate; moving the ladle to the LF furnace for heating and refining treatment, adding ferroboron and ferozirconium alloy according to the target composition and adjusting the chemical composition, while adding refining slag, active lime, etc. for refining treatment, keeping the white slag for more than 12 minutes, and soft blowing for more than 12 minutes before sending it to the next process; then vacuum treatment is carried out in an RH furnace with a vacuum degree of ≤90Pa to significantly reduce the harmful gases of 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; adopting full-process protection casting and 160mm×160mm square billets, the square billets are automatically fire-cut to size and then stacked and slowly cooled;

[0057] The rolling module is used for cold charging of square billets into the furnace. It adopts segmented heating. The temperature of the preheating section is not higher than 700℃, the temperature of the soaking section is 1200-1230℃, the heating time is not less than 80 minutes, the rough rolling start temperature is 1060-1110℃; the finishing rolling start temperature is 980-1050℃, the final rolling temperature is 850-900℃, and the billets are rolled into 6-20mm thick spring flat steel. The cooling bed is quickly unloaded and stacked for slow cooling for not less than 48 hours, and the unload temperature is not less than 500℃.

[0058] The performance inspection module is used to take samples for tempering heat treatment. The tempering heat treatment process is 840-870℃ oil quenching + 500-560℃ tempering, and then the performance inspection is carried out.

[0059] The technical solution adopted by the present invention to solve the above technical problems is a high-performance spring flat steel for automobiles, wherein the chemical composition content (wt%) 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%, B: 0.0010-0.0025 %, Zr: 0.05~0.10%, P≤0.020%, S≤0.003%, H≤2ppm, the balance is Fe and unavoidable impurity elements, and at the same time satisfies 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 the high-performance automobile spring flat steel of the present invention includes: molten iron pretreatment→150t converter smelting→LF furnace refining→RH furnace vacuum treatment→square billet continuous casting→square billet stack cooling→square billet heating and rolling→flat steel slow cooling→flat steel finishing→performance inspection.

[0061] The method includes smelting, rolling and other process steps, which are as follows:

[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 double-blown converter, the amount of scrap steel added is less than 15 tons, the C content of the molten steel is controlled to be 0.10-0.35% when tapping, the tapping temperature is not less than 1620℃, and ferrosilicon, high-carbon ferromanganese, ferrochrome, ferronickel, ferrovanadium alloy and copper plate are added when 1 / 3 of the steel is tapped; the ladle is moved to the LF furnace for heating and refining treatment, ferroboron and ferozirconium alloy are added according to the target composition and the chemical composition is adjusted, and refining slag, active lime, etc. are added for refining treatment, the white slag is kept 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 of ≤90Pa to significantly reduce the harmful gases of 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 protection casting is adopted to produce 160mm×160mm square billets, and the square billets are automatically fire-cut to size and then stacked for slow cooling.

[0064] (3) Rolling process: The billet is cold loaded into the furnace and heated in sections. The preheating section temperature is not higher than 700°C, the soaking section temperature is 1200-1230°C, the heating time is not less than 80 minutes, the rough rolling start temperature is 1060-1110°C; the finishing rolling start temperature is 980-1050°C, the final rolling temperature is 850-900°C, and the billet is rolled into a 6-20 mm thick spring flat steel. The cooling bed is quickly stacked and slowly cooled for not less than 48 hours, and the offline temperature is not less than 500°C. 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: Take samples for tempering heat treatment. The tempering heat treatment process is 840-870℃ oil quenching + 500-560℃ tempering, and then perform performance test.

[0066] In order to ensure the purpose of the present invention and meet the characteristics of high strength, high plasticity, high corrosion resistance and the like of high-performance automobile spring flat steel, the reasons for limiting the elements such as C, Si, Mn, Cr, V, Ni, Cu, B, Zr, P, S, and H in the present invention are as follows:

[0067] C is an important element to ensure the strength of spring flat steel. When the C content is too low, it is difficult to ensure the high strength requirements of spring flat steel. When the C content is too high, the quality of the square billet, the plasticity and toughness of the steel and the processing performance are deteriorated. Therefore, the C content of the steel of the present invention is 0.55-0.60%.

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

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

[0070] Cr can improve the hardenability of spring flat steel, thereby improving the strength and hardness of the steel, and can also improve the wear resistance and corrosion resistance of the steel. However, too high a Cr content will reduce the plasticity and processing properties of the steel. Therefore, the Cr content of the steel of the present invention is 0.45% to 0.60%.

[0071] The carbonitride formed by V can inhibit the recrystallization of austenite and prevent the growth of grains during the rolling process, thereby refining the ferrite grains and improving the strength and toughness of the steel. At the same time, the carbonitride precipitated during tempering plays a precipitation strengthening role. Therefore, the V content of the steel of the present invention is 0.10-0.12%.

[0072] Ni and Cu are beneficial to improving the corrosion resistance of steel, but too much of them will significantly increase the production cost of steel. Therefore, the Ni content of the steel of the present invention is 0.10-0.20%, and the Cu content is 0.20-0.30%.

[0073] B can obviously improve the hardenability of steel, but excessive B will affect the toughness of steel. Therefore, the B content of the steel of the present invention is 0.0010-0.0025%.

[0074] The composite oxide of Zr modifies inclusions such as MnS, improves the shape, size and distribution of inclusions in steel, thereby reducing the inclusion level of steel and improving the plasticity, toughness and corrosion resistance of steel. However, Zr is expensive and adding too much will increase production costs. Therefore, the Zr content of the steel of the present invention is 0.05-0.10%.

[0075] P and S are inevitable impurity elements in steel and are harmful to the plasticity and toughness of steel. The lower their content, the better. Therefore, the P content of the steel of the present invention is ≤0.020%, and the S content is ≤0.003%.

[0076] H easily gathers in the gap between inclusions and iron in the center of the billet, affecting the quality of the billet. When the content is too high, it is easy to cause cracks. Therefore, the H content of the steel of the present invention is ≤2ppm.

[0077] The chemical compositions (weight percentage by weight) of the embodiments of the present invention and the comparative examples are shown in Table 1:

[0078] Table 1

[0079]

[0080] The above embodiment adopts KR pretreatment molten iron, the molten iron S content is 0.001-0.002%, all of which are smelted in a 150t top and bottom double-blown converter, the C content at the end of the furnace is 0.20-0.30%, the tapping temperature is 1628-1647°C, and then subjected to LF furnace deep desulfurization and refining treatment, soft blowing time is 13-20 minutes, transferred to the RH vacuum furnace for degassing, soft blowing for 18-25 minutes, fully removing large particle inclusions, and ensuring uniform composition, and then the whole process is protected and cast into 160mm×160mm continuous casting billets, and the billet stacks are slowly cooled to below 200°C.

[0081] The square billet is cold charged into the heating furnace, the preheating section temperature is set at 650℃, the soaking temperature is set at 1220℃, the actual rough rolling start temperature is 1080~1105℃; the finishing rolling start temperature is 1000~1030℃, the final rolling temperature is 865~890℃, and it is rolled into 6~20mm thick spring flat steel, which is quickly stacked off the line and slowly cooled for no less than 48 hours.

[0082] Experimental samples were taken for oil quenching at 840-870℃ + tempering at 500-560℃, and then the performance was tested.

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

[0084] Table 2

[0085]

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

[0087] Table 3

[0088]

[0089] The spring flat steel after heat treatment was longitudinally processed into tensile specimens and metallographic specimens, and mechanical properties tests and inclusion observations were carried out. The test results are shown in Table 4.

[0090] Table 4

[0091]

[0092]

[0093] As can be seen from Table 4, the strength, elongation, inclusions and grain size indicators of the test steel plates of the embodiments of the present invention are significantly better than those of the comparative examples, indicating that the embodiments have good 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 examples, indicating that they have good corrosion resistance.

[0094] Figure 3 The figure shows the hot-rolled organizational structure of the 20 mm thick spring flat steel in Example 1, which is pearlite + a small amount of ferrite.

[0095] The present invention can be implemented in metallurgical enterprises, has simple process flow, strong operability and low cost, and can be applied to the manufacture of various automobile 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 molten iron to make [S] ≤ 0.002%, removing sulfur elements to improve the purity of steel.

[0099] 2) Smelting and refining:

[0100] A 150t top and bottom combined blowing converter is used for smelting, 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] Send it to LF furnace for refining, add ferroboron (B≥20%) and ferrozirconium (Zr≥30%), keep white slag for 12 minutes, and soft blow for 16 minutes;

[0102] The steel was sent to the RH furnace for vacuum treatment with a vacuum degree of 81Pa and a soft blowing time of 20 minutes to reduce the O, N, and H contents in the steel and improve the purity of the steel.

[0103] 3) Continuous casting and slow cooling:

[0104] 160mm×160mm continuous casting square billets are used, and the whole process is protected for pouring. After pouring, they are automatically cut to size by fire, and then slow-cooled in the stacking slow cooling area for 48 hours to ensure the stability of the internal structure.

[0105] 4) Rolling process:

[0106] The billet is cold loaded into the furnace and heated in sections, with the preheating section temperature at 650°C, the soaking section temperature at 1220°C, and the heating time at 85 minutes;

[0107] The starting temperature of rough rolling is 1090℃, the starting temperature of finishing rolling is 1020℃, and the final rolling temperature is 880℃;

[0108] The flat steel is rolled into a thickness of 12mm, and the cooling bed is quickly taken off the line. The stacking slow cooling time is 48 hours, and the off-line temperature is 510℃.

[0109] 5) Quenching and tempering heat treatment:

[0110] 840℃ oil quenching + 520℃ tempering is used to improve the uniformity of flat steel structure and ensure the strength and plasticity 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, and the balance is Fe.

[0113] 7) Non-metallic inclusion control:

[0114] Class A ≤1.0, Class B ≤1.0, Class C ≤1.0, Class D ≤1.0.

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

[0116] 1) Hot metal pretreatment:

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

[0118] 2) Smelting and refining:

[0119] In a 150t top and bottom combined blown converter, 13 tons of scrap steel is added, the tapping temperature is 1630℃, and 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] Send it to LF furnace for refining, add ferroboron (B≥22%) and ferrozirconium (Zr≥32%), keep white slag for 15 minutes, and soft blow for 15 minutes;

[0121] RH furnace vacuum treatment, vacuum degree is 85Pa, soft blowing time is 23 minutes, to reduce the O, N and H contents in the steel.

[0122] 3) Continuous casting and slow cooling:

[0123] 160mm×160mm continuous casting square billet is used, and protected pouring is adopted. After pouring, it is automatically cut to size by fire and slow-cooled in the stacking slow cooling area for 50 hours.

[0124] 4) Rolling process:

[0125] The billet is cold loaded into the furnace and heated in sections, with the preheating section temperature at 680°C, the soaking section temperature at 1210°C, and the heating time at 90 minutes.

[0126] The starting temperature of rough rolling is 1085℃, the starting temperature of finishing rolling is 1000℃, and the final rolling temperature is 870℃;

[0127] The flat steel is rolled into a thickness of 16mm, and the cooling bed is quickly taken off the line. The stacking slow cooling time is 50 hours, and the off-line temperature is 520℃.

[0128] 5) Quenching and tempering heat treatment:

[0129] 860℃ oil quenching + 540℃ tempering is used to ensure the strength and plasticity of the flat steel.

[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, and the balance is Fe.

[0132] 7) Non-metallic inclusion control:

[0133] Class A ≤1.0, Class B ≤1.0, Class C ≤1.0, Class D ≤1.0.

[0134] The above description is only a specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Any modifications, equivalent substitutions and improvements made by any technician familiar with the technical field within the technical scope disclosed by the present invention and within the spirit and principle of the present invention should be covered by the protection scope of the present invention.

Claims

1. A flat steel, characterized in that: Measured in percentage by mass, the chemical composition of the flat steel 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%, B: 0.0010-0.0025%, Zr: 0.05-0.10%, P≤0.020%, S≤0.003%, H≤2ppm, and the remainder is Fe and unavoidable impurity elements.

2. The flat steel according to claim 1, characterized in that The thickness of the flat steel is in the range of 6 to 20 mm.

3. The flat steel according to claim 1, characterized in that The flat steel is manufactured by the following method: Use KR desulfurization to pre-treat blast furnace hot metal to make [S] ≤ 0.002%; Smelting in a 150t top and bottom double-blown converter, controlling the amount of scrap steel added to less than 15 tons, the tapping temperature to not less than 1620℃, and adding ferrosilicon, high carbon ferromanganese, ferrochrome, ferronickel, ferrovanadium alloy and copper plate when 1 / 3 of the steel is tapped; Use LF furnace for heating and refining, add ferroboron and ferrozirconium alloy, keep white slag for more than 12 minutes, and soft blow for more than 12 minutes; Carry out vacuum treatment in RH furnace, vacuum degree ≤90Pa, soft blowing for more than 15 minutes; The whole process is protected by pouring and continuous casting to form a 160mm×160mm square billet.

4. The flat steel according to claim 3, characterized in that The rolling method of the flat steel comprises: The billet is cold loaded into the furnace and heated in sections. The temperature of the preheating section is not higher than 700°C, the temperature of the soaking section is 1200-1230°C, and the heating time is not less than 80 minutes. The starting temperature of rough rolling is 1060~1110℃, the starting temperature of finishing rolling is 980~1050℃, and the final rolling temperature is 850~900℃; It is rolled into 6-20mm thick spring flat steel, quickly taken off the line on the cooling bed, slowly cooled on the stack for no less than 48 hours, and the off-line temperature is no less than 500℃.

5. The flat steel according to claim 3, characterized in that: The flat steel is subjected to a quenching and tempering heat treatment, and the quenching and tempering heat treatment comprises: 840~870℃ oil quenching; Tempering at 500-560℃.

6. The flat steel according to claim 3, characterized in that The non-metallic inclusion grade of the flat steel meets the following requirements: Class A inclusion level ≤1.0; Class B inclusions: ≤1.0; Class C inclusions: ≤1.0; Class D inclusion level ≤1.

0.

7. A method for preparing flat steel, characterized in that: The following steps are involved: Step 1: Perform KR desulfurization pretreatment on blast furnace molten iron to make [S] ≤ 0.002%; Step 2: Use a 150t top and bottom combined blowing converter for smelting, control the amount of scrap steel added to less than 15 tons, the tapping temperature is not less than 1620℃, and add ferrosilicon, high carbon ferromanganese, ferrochrome, ferronickel, ferrovanadium alloy and copper plate when 1 / 3 of the steel is tapped; Step 3: LF furnace refining treatment, adding ferroboron and ferrozirconium alloy, keeping white slag for more than 12 minutes, and soft blowing for more than 12 minutes; Step 4: RH furnace vacuum treatment, vacuum degree ≤ 90Pa, soft blowing for more than 15 minutes; Step 5: Use full-process protection pouring and continuous casting to form 160mm×160mm square billets. After the square billets are automatically fire-cut to size, they are stacked and slowly cooled.

8. The method for preparing flat steel according to claim 7, characterized in that: The rolling method of the flat steel comprises the following steps: The billet is cold loaded into the furnace and heated in sections. The temperature of the preheating section is not higher than 700°C, the temperature of the soaking section is 1200-1230°C, and the heating time is not less than 80 minutes. The starting temperature of rough rolling is 1060~1110℃, the starting temperature of finishing rolling is 980~1050℃, and the final rolling temperature is 850~900℃; The spring flat steel is rolled into a thickness of 6 to 20 mm, and is quickly taken off the cooling bed. The stacking slow cooling time is not less than 48 hours, and the off-line temperature is not lower than 500°C.

9. The method for preparing flat steel according to claim 7, characterized in that: The flat steel is subjected to quenching and tempering heat treatment, comprising the following steps: 840~870℃ oil quenching; Tempering at 500-560℃.

10. The method for preparing flat steel according to claim 7, characterized in that: The non-metallic inclusion grade of the flat steel is controlled as follows: Class A inclusion level ≤1.0; Class B inclusions: ≤1.0; Class C inclusions: ≤1.0; Class D inclusion level ≤1.0.

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