Medium-carbon non-quenched and tempered steel for high-strength control arm of high-end automobile and manufacturing method of medium-carbon non-quenched and tempered steel

Abstract

The invention relates to medium-carbon non-quenched and tempered steel for a high-strength control arm of a high-end automobile and a manufacturing method of the medium-carbon non-quenched and tempered steel, and belongs to the field of alloys. The steel comprises the following chemical components in percentage by mass: 0.38 to 0.55 percent of C, 0.10 to 0.80 percent of Si, 1.10 to 1.80 percent of Mn, less than or equal to 0.010 percent of P, 0.020 to 0.070 percent of S, 0.10 to 0.35 percent of Cr, less than or equal to 0.25 percent of Ni, less than or equal to 0.050 percent of Al, less than or equal to 0.035 percent of Ti, 0.08 to 0.35 percent of V, 0.008 to 0.025 percent of N and the balance of Fe and inevitable impurity elements, and a microscopic structure mainly comprises ferrite and pearlite. According to the control arm obtained by forging the steel, the tensile strength Rm is larger than or equal to 1200 MPa, the yield strength Rp0.2 is larger than or equal to 745 MPa, the ductility A% is larger than or equal to 13%, and the surface hardness is larger than or equal to 330 HBW.

Description

Technical Field

[0001] This invention belongs to the field of iron-based alloy technology, specifically relating to a non-quenched and tempered steel for automobiles and its manufacturing method. Background Technology

[0002] Automotive lightweighting refers to reducing the weight of a vehicle body as much as possible without compromising its strength and safety performance. Driven by both the pressure to conserve energy and reduce emissions and the demand for improved performance in new energy vehicles, automotive lightweighting is accelerating and is a key area for future development.

[0003] According to statistics, for every 10% reduction in the weight of a traditional vehicle, fuel consumption can be reduced by 6%-8%. The need for lightweighting is even more urgent for new energy vehicles. Lightweighting is one of the important means to improve the driving range of electric vehicles. For every 10kg reduction in the weight of a pure electric vehicle, the driving range can be increased by 2.5km. The lightweighting of materials is the most important component of automotive lightweighting.

[0004] As a guiding and force-transmitting element of the automotive suspension system, the control arm elastically connects the wheels and the vehicle body via ball joints or bushings. It transmits various forces acting on the wheels to the vehicle body while ensuring the wheels move along a specific trajectory, which is crucial for comfortable and smooth driving. The control arm should possess sufficient strength, rigidity, and service life. Currently, the main materials used in control arm products both domestically and internationally are ductile iron, 850MPa strength grade quenched and tempered steel and non-quenched and tempered steel, and aluminum alloys. These materials suffer from insufficient strength and high cost, respectively. A search revealed: Chinese patent document CN 114941111 A, published on September 5, 2023, entitled "A Low-Carbon Non-Quenched and Tempered Steel for Automobile Control Arms and Its Preparation Method," discloses a method for preparing low-carbon non-quenched and tempered steel for automobile control arms. The steel is composed of the following elements by mass percentage: C 0.25~0.29%, Si 0.45~0.55%, Mn 1.60~1.70%, Cr 0.20~0.25%, Cu≤0.08%, Ni≤0.08%, Mo≤0.0050%, V 0.11~0.15%, Nb≤0.006%, As≤0.0070%, Als 0.015~0.040%, P≤0.020%, S0.025~0.040%, B≤0.0005%, Ti0.015~0.030%. N 0.015~0.020%, balance being Fe and unavoidable impurity elements. The low-carbon non-quenched and tempered steel for automotive control arms produced using this invention exhibits the following properties in hot-rolled state: room temperature U-shaped impact energy ≥ 65J, lower yield strength ≥ 600MPa, tensile strength ≥ 840MPa, elongation ≥ 21%, and reduction of area ≥ 57%. Summary of the Invention

[0005] The purpose of this invention is to provide a non-quenched and tempered high-strength steel and its manufacturing method for manufacturing high-end automotive control arms, thereby solving the problems of high cost of alloy raw materials and high energy consumption of heat treatment.

[0006] The technical solution adopted by this invention to solve the above problems is as follows: a medium-carbon non-quenched and tempered steel for high-strength control arms of high-end automobiles. The chemical composition of this steel by mass percentage is: C: 0.38-0.55%, Si: 0.10-0.80%, Mn: 1.10-1.80%, P: ≤0.010%, S: 0.020-0.070%, Cr: 0.10-0.35%, Ni: ≤0.25%, Al: ≤0.050%, Ti: ≤0.035%, V: 0.08-0.35%, N: 0.008-0.025%, with the balance being Fe and unavoidable impurity elements.

[0007] The mechanical properties of round steel forged into automotive control arm products meet the following requirements: Rm≥1200MPa, Rp0.2≥745MPa, A%≥13%, and surface hardness≥330HBW.

[0008] The preferred chemical composition of the round steel, by mass percentage, is: C: 0.45–0.50%, Si: 0.62–0.70%, Mn: 1.54–1.60%, P: ≤0.008%, S: 0.055–0.065%, Cr: 0.25–0.35%, Ni: 0.15–0.20%, Al: 0.012–0.018%, Ti: 0.012–0.018%, V: 0.22–0.25%, N: 0.016–0.022%, with the balance being Fe and unavoidable impurity elements.

[0009] The rationale for the chemical composition design of non-quenched and tempered steel is as follows: C is the most basic and effective strengthening element in steel, and it is also the most economical to use. Considering that the crankshaft surface needs to be subjected to medium-frequency induction hardening, in order to ensure that the material has sufficient strength / hardness and hardenability while taking into account a certain degree of plasticity, the C content is determined to be 0.38-0.55%, with C being preferred at 0.45-0.50%.

[0010] Si can be used as a deoxidizer and reducing agent in steelmaking. At the same time, Si has a strong solid solution strengthening effect in steel, which can significantly improve the elastic limit, yield strength and tensile strength of steel, especially the yield strength. The Si content is determined to be 0.10-0.80%, and the preferred Si content is 0.62-0.70%.

[0011] Mn is an effective element for deoxidation and desulfurization in steel, improving its strength and hardness, significantly enhancing its hardenability, and improving its hot working properties. Simultaneously, Mn is the element with the strongest bonding force with sulfur in steel, and the resulting MnS compound helps improve the machinability of the steel. However, Mn ≥ 1.65% easily produces bainitic structure. The recommended Mn content is 1.10–1.80%, with a preferred Mn content of 1.54–1.60%.

[0012] P is a harmful impurity in steel. It is a low-melting-point element that is prone to segregation. Its content in steel needs to be controlled. The determined P content is ≤0.010%, and the preferred P content is ≤0.008%.

[0013] S is usually a harmful impurity in steel, but when it reaches a certain content, it can significantly improve the cutting performance of steel. While the steel of the present invention has increased strength, it will inevitably cause the deterioration of cutting performance during processing. Therefore, S element is artificially added, and the S content is determined to be 0.020-0.070%, preferably 0.055-0.065%.

[0014] Cr can effectively improve the hardenability of steel to obtain the required strength and reduce the tendency of surface decarburization during steel heating. However, since it is a strong carbide precipitation element, it will affect the performance of the induction hardened layer. Therefore, the content cannot be too high. The determined content is: Cr: 0.10~0.35%, preferably Cr: 0.25~0.35%.

[0015] Ni can improve the strength of steel while ensuring good plasticity and toughness, but it is scarce and expensive. Therefore, the optimal Ni content is determined to be ≤0.25%, with 0.15-0.20% being preferred.

[0016] Al is an effective deoxidizer, and the formed AlN can refine grains. However, its effect is not significant at low concentrations, while high concentrations easily lead to the formation of coarse inclusions, thus deteriorating the steel's properties. Therefore, the recommended Al content is ≤0.050%, preferably 0.012–0.018%. Ti and N have strong bonding, and the resulting TiN particles can pin grain boundaries and inhibit grain growth during forging. However, high content can easily lead to the formation of irregularly shaped, high-hardness TiN inclusions. Therefore, Ti content is determined to be ≤0.035%, with a preferred Ti content of 0.012–0.018%.

[0017] V has high solubility in steel and is one of the most commonly used and effective strengthening elements in non-quenched and tempered steel. It affects the microstructure and properties of steel by forming V(C,N) compounds that precipitate at grain boundaries, thereby refining ferrite grains and improving the strength and plasticity of steel. V, like Ni, is a precious alloy. The determined V content is 0.08-0.35%, with a preferred V content of 0.22-0.25%.

[0018] Nitrogen (N) is a common element in non-quenched and tempered steel. Its main function is to enhance precipitation strengthening and refine grains. Increasing the N content in steel expands the precipitation range of carbonitrides and enhances the effective role of microalloying elements. Equivalent mechanical properties can be obtained with a smaller content of microalloying elements. The determined N content is 0.008–0.025%, with a preferred N content of 0.016–0.022%.

[0019] This invention provides a method for manufacturing the above-mentioned medium-carbon non-quenched and tempered steel, characterized in that it includes: Step 1, Smelting: Using scrap steel and molten iron as raw materials, and adding pyrite, the chemical element mass percentage is adjusted to the design range through primary smelting, refining, and vacuum degassing. Step 2, Continuous casting: Using the continuous casting process, molten steel is poured into continuously cast billets of 300mm × 340mm or larger; Step 3: Heating: The continuously cast billet is heated in the furnace to fully austenitize the microstructure and fully dissolve the alloying elements; Step 4: Rolling: After the billet exits the heating furnace, it is descaled using high-pressure water. The initial rolling temperature is set at 1000-1060℃. It is rolled into round steel using 6 primary rolling mills + 6 intermediate rolling mills + 6 finishing rolling mills. The final rolling temperature is controlled to be ≤930℃. Low-temperature final rolling yields a fine grain structure of ≥7.5 grade. Step 5, Cooling: After rolling, the steel is cooled on a cooling bed. The stepping speed is controlled so that the temperature of the round steel on the cooling bed is ≤450℃ and the cooling rate is ≤1.5℃ / s. The bainite content in the steel is controlled to be no more than 3%.

[0020] As one of the preferred options, in step one, the proportion of molten iron accounts for more than 50% of the raw materials.

[0021] As one of the preferred options, in step two, the casting superheat is controlled at 20-35℃, the casting speed is 0.70m / min, the secondary cooling intensity is 0.18l / kg, the crystallizer electromagnetic stirring intensity is 2.5Hz / 120A, and the end electromagnetic stirring intensity is 15Hz / 200A.

[0022] As one of the preferred options, in step three, a walking beam furnace is used to heat the billet. The temperature of the preheating section is controlled at 650-800℃ and the preheating time is ≥90min to ensure that the billet heats up slowly and is heated evenly. The temperature of the heating section is controlled at 1000-1180℃ and the temperature of the soaking section is controlled at 1160-1220℃. The total heating time is 400min or more.

[0023] As one of the preferred options, step four involves rolling to a specification of φ50mm or larger. Detailed Implementation

[0024] The following detailed description, in conjunction with embodiments, illustrates the present invention of a medium-carbon non-quenched and tempered steel for high-strength control arms in high-end automobiles and its manufacturing method. These embodiments are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0025] The present invention uses the chemical composition shown in Table 1 to manufacture three furnaces of the medium carbon non-quenched and tempered steel material described in the present invention (Examples 1-3).

[0026] Table 1 Elemental composition of the examples

[0027] The medium-carbon non-quenched and tempered steel for the high-strength control arm of high-end automobiles in this embodiment of the invention is manufactured according to the following production process: smelting Using scrap steel and molten iron as raw materials, with an iron content of about 50%, and adding pyrite, the amount of pyrite alloy and pyrite wire added later is minimized to reduce production costs. The molten steel after the initial smelting in the electric furnace is then refined outside the furnace and degassed. The mass percentage of each chemical element is strictly controlled in accordance with the standard requirements.

[0028] Continuous casting Casting was performed using a 300mm×340mm full-arc continuous casting machine. The casting superheat was controlled at 20-35℃, the casting speed was 0.70m / min, and the secondary cooling intensity was 0.18l / kg. Electromagnetic stirring in the crystallizer (2.5Hz / 120A) and electromagnetic stirring at the end (15Hz / 200A) were used to improve the surface and internal quality of the billet. heating When heating the billet using a walking beam furnace, the preheating zone temperature should be controlled at 650–800℃ and the preheating time should be ≥90min to ensure that the billet heats up slowly and is heated evenly. The heating zone temperature should be controlled at 1000–1180℃ and the soaking zone temperature should be controlled at 1160–1220℃. The total heating time should be 400min or more to ensure that the V microalloying element in the steel can be fully dissolved during the heating process and play a strengthening role. Rolling After exiting the heating furnace, the billet is descaled using high-pressure water. The initial rolling temperature is 1000–1060℃, followed by rolling using a combination of 6 primary rolling mills, 6 intermediate rolling mills, and 6 finishing rolling mills. The final rolling temperature is controlled to be ≤930℃, and a microstructure of ≥7.5 is obtained through low-temperature final rolling. The rolled specification is φ50mm. cool down After rolling, the steel is placed on a cooling bed, and the stepping speed is controlled to ensure that the temperature of the lower cooling bed is ≤450℃ and the cooling rate is ≤1.5℃ / s, so as to ensure that the bainite content in the obtained steel is not greater than 3%. The φ50mm material obtained from the chemical composition of Examples 1-3 was used to manufacture (forge) control arm products, and the mechanical properties of the products are shown in Table 2.

[0029] Table 2 Mechanical properties of control arm products

[0030] As shown in Table 2, the mechanical properties of the control arm products manufactured in Examples 1-3 all meet the requirements of the invention design, and a medium carbon non-quenched and tempered steel for high-strength control arms of high-end automobiles is obtained. Moreover, the manufacturing process is simple, and it has high application value and advantages in the field of non-quenched and tempered steel for high-end automobile chassis components.

[0031] In addition to the above embodiments, the present invention also includes other embodiments. All technical solutions formed by equivalent transformation or equivalent substitution should fall within the protection scope of the claims of the present invention.

Claims

1. A medium-carbon non-quenched and tempered steel for high-strength control arms in high-end automobiles, characterized in that: The chemical composition by mass percentage is as follows: C: 0.38–0.55%, Si: 0.10–0.80%, Mn: 1.10–1.80%, P: ≤0.010%, S: 0.020–0.070%, Cr: 0.10–0.35%, Ni: ≤0.25%, Al: ≤0.050%, Ti: ≤0.035%, V: 0.08–0.35%, N: 0.008–0.025%, with the balance being Fe and unavoidable impurity elements. The microstructure is mainly ferrite + pearlite.

2. The steel according to claim 1, characterized in that: The chemical composition by mass percentage is as follows: C: 0.45–0.50%, Si: 0.62–0.70%, Mn: 1.54–1.60%, P: ≤0.008%, S: 0.055–0.065%, Cr: 0.25–0.35%, Ni: 0.15–0.20%, Al: 0.012–0.018%, Ti: 0.012–0.018%, V: 0.22–0.25%, N: 0.016–0.022%, with the balance being Fe and unavoidable impurity elements.

3. The steel according to claim 1, characterized in that: The control arm obtained by forging steel has the following properties: tensile strength Rm≥1200MPa, yield strength Rp0.2≥745MPa, elongation A%≥13%, and surface hardness≥330HBW.

4. A method for manufacturing the steel of claim 1, characterized in that: include, Step 1, Smelting: Using scrap steel and molten iron as raw materials, and adding pyrite, the chemical element mass percentage is adjusted to the design range through primary smelting, refining, and vacuum degassing. Step 2, Continuous casting: Using the continuous casting process, molten steel is poured into continuously cast billets of 300mm × 340mm or larger; Step 3: Heating: The continuously cast billet is heated in the furnace to fully austenitize the microstructure and fully dissolve the alloying elements; Step 4: Rolling: After the billet exits the heating furnace, it is descaled using high-pressure water. The initial rolling temperature is set at 1000-1060℃. It is rolled into round steel using 6 primary rolling mills + 6 intermediate rolling mills + 6 finishing rolling mills. The final rolling temperature is controlled to be ≤930℃. Low-temperature final rolling yields a fine grain structure of ≥7.5 grade. Step 5, Cooling: After rolling, the steel is cooled on a cooling bed. The stepping speed is controlled so that the temperature of the round steel on the cooling bed is ≤450℃ and the cooling rate is ≤1.5℃ / s. The bainite content in the steel is controlled to be no more than 3%.

5. The method according to claim 4, characterized in that: Step 1: The proportion of molten iron in the raw materials should be more than 50%.

6. The method according to claim 4, characterized in that: Step 2: Control the casting overheating at 20-35℃, the casting speed at 0.70m / min, the secondary cooling intensity at 0.18l / kg, the electromagnetic stirring intensity of the crystallizer at 2.5Hz / 120A, and the end electromagnetic stirring intensity at 15Hz / 200A.

7. The method according to claim 4, characterized in that: Step 3: Use a walking beam furnace to heat the billet. The preheating temperature should be controlled at 650-800℃ and the preheating time should be ≥90min to ensure that the billet heats up slowly and is heated evenly. The heating temperature should be controlled at 1000-1180℃ and the soaking temperature should be controlled at 1160-1220℃. The total heating time should be 400min or more.

8. The method according to claim 4, characterized in that: Step 4: Rolling specifications of φ50mm and above.

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