A medium carbon steel for a steering screw of a new energy vehicle and a production method thereof

Abstract

This invention discloses a medium carbon steel for steering screws in new energy vehicles and its production method. The chemical composition of the medium carbon steel, by mass percentage, is as follows: C: 0.45%~0.70%, Si: 0.25%~0.55%, Mn: 0.45%~1.05%, Cr: ≤0.40%, Mo≤0.40%, Ni≤0.40%, P≤0.035%, S: 0.015%~0.035%, O≤0.0012%, with the balance being Fe and unavoidable impurities. The production method includes a complete process: molten iron pretreatment – ​​converter smelting – LF refining – RH furnace vacuum degassing – continuous casting – rolling – quenching and tempering – finishing – sub-critical quenching – low-temperature tempering. By controlling the composition to reduce the addition of expensive alloying elements, and through a composite heat treatment of "quenching and tempering + sub-critical quenching + low-temperature tempering," a hardness gradient of ≥60HRC on the surface and 22~28HRC in the core is achieved, with no decarburized layer on the surface. Simultaneously, strict control of non-metallic inclusions enhances the purity of the steel. This invention's product is compatible with cyclone milling processes, exhibits minimal quenching deformation, and can be straightened, meeting the high precision and long service life requirements of steering screws in new energy vehicles.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology, and more specifically relates to medium carbon steel for steering screws of new energy vehicles and the corresponding production process. Background Technology

[0002] Automotive steering ball screws, with their high efficiency and precision, have become a core component of modern steering systems. Especially with the trends of electrification and intelligentization, their reliability and performance directly impact driving safety and experience. "Whirlwind milling" is a highly efficient and high-precision thread and helical groove machining process. It achieves rapid cutting through a combination of high-speed tool rotation and slow workpiece rotation, significantly improving turning efficiency. Therefore, whirlwind milling is widely used in automotive steering ball screws. In this process, the material surface must withstand the high-speed rotation of the tool; therefore, the material surface must meet a requirement of ≥63 HRC.

[0003] To meet the high surface hardness requirements of materials, many domestic automobile manufacturers use high-carbon chromium bearing steel such as GCr15. This type of material, due to its "full hardening" properties, can meet hardness requirements after quenching. However, years of practice have proven that while this type of bearing steel can indeed meet the hardness requirements before "whirl milling," automotive steering screws require a certain length. After quenching, due to heat treatment bending and the extremely high hardness of the material, it is difficult to meet the high precision requirements of the part dimensions, affecting the final product quality of the screw and becoming a bottleneck in the manufacturing of high-precision automotive steering screws. With the advancement of special steel technology, low-carbon high-alloy steel has quickly replaced high-carbon chromium bearing steel as a raw material for automotive steering screws. After carburizing and quenching, this type of material has a carburized and quenched layer on the surface, meeting the hardness requirements of the "whirl milling" process. Furthermore, after carburizing and quenching, the core of the material retains the toughness of low-carbon steel, which can be effectively eliminated through straightening and other processes to eliminate deformation caused by quenching, meeting the high precision requirements of the part.

[0004] The rapid development of new energy vehicles has led to a decline in the advantages of low-carbon high-alloy steel in manufacturing, highlighting its disadvantages. Firstly, to ensure the required hardness and toughness after carburizing and quenching, high-alloy steel typically requires the addition of alloying elements such as Mn, Cr, Ni, and Mo, resulting in high smelting costs. Secondly, this low-carbon steel usually requires special rolling and normalizing processes to ensure uniform grain size and microstructure. Coarse grains or low microstructure uniformity can lead to uneven hardness in parts, making them prone to scrapping after whirl milling. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a medium carbon steel for steering screws of new energy vehicles, which reduces the addition of expensive alloying elements and lowers the smelting cost through precise composition control. At the same time, it can achieve the performance requirements of high surface hardness and high core toughness, is suitable for cyclone milling process, and has small quenching deformation and is easy to straighten.

[0006] Another objective of this invention is to provide a method for producing medium carbon steel for steering screws in new energy vehicles. This method includes the entire metallurgical process and a customized composite heat treatment process, which can strictly control the purity and uniformity of the steel structure, ensure product performance stability, significantly reduce production scrap rate, and is suitable for large-scale industrial production.

[0007] The technical solution adopted by this invention to solve the above problems is as follows: a tempered medium carbon steel for steering screws in new energy vehicles and its production method, the main chemical composition of which is C: 0.45%~0.70%, Si: 0.25%~0.55%, Mn: 0.45%~1.05%, Cr: ≤0.40%, Mo≤0.40%, Ni≤0.40%, P≤0.035%, S: 0.015%~0.035%, O≤0.0012%, with the balance being Fe and unavoidable impurities. The tempered medium carbon steel for steering screws in new energy vehicles of this invention ensures the hardness requirements of the steel by increasing the content of C and Si while reducing the content of alloying elements Mn and Cr, and further reduces the smelting cost of the material. The specific chemical element design is as follows: (1) Determination of C content Carbon (C) is the most economical and fundamental strengthening element in steel. Through solid solution strengthening and precipitation strengthening, the strength and hardness of steel can be significantly improved. Therefore, the C content range in this invention is determined to be 0.45%–0.70%.

[0008] (2) Determination of Si content Adding silicon (Si) to steel can strengthen ferrite and improve the strength and hardness of the ferrite structure. Therefore, the Si content in this invention is determined to be in the range of 0.25% to 0.55%.

[0009] (3) Determination of Mn content Mn is a commonly used deoxidizing alloying element in the steelmaking process. Mn can also fix the form of sulfur in steel and form MnS, which has a relatively low negative impact on steel properties. However, as an alloying element, Mn significantly increases steelmaking costs. Therefore, the Mn content in this invention is determined to be in the range of 0.45% to 1.05%.

[0010] (4) Determination of Cr, Mo and Ni content Cr dissolves in ferrite, resulting in solid solution strengthening and improving the wear resistance of the matrix. Increasing the Cr content can significantly improve the hardenability of the steel. The main role of Mo and Ni is also to improve hardenability. Since the target product is used in the steering screw of new energy vehicles, and the product specifications are φ16mm~45mm, there are no large-size materials. Therefore, this invention does not have specific requirements for hardenability, and the contents of Cr, Mo and Ni are ≤0.40%.

[0011] (5) Determination of P content Phosphorus (P) is a common harmful element in steel, easily causing elemental segregation during continuous casting and reducing the uniformity of the microstructure. Excessive P content in steel can significantly reduce its plasticity and toughness, increasing its cold brittleness. Therefore, P ≤ 0.035%.

[0012] (6) Determination of S content Sulfur (S) is added to molten steel in the form of sulfur, and mainly exists in the steel as non-metallic inclusions of MnS. These inclusions are ductile and have almost no effect on the fatigue life of the steel. The presence of a small amount of S in the steel can significantly improve its machinability. Because the steering screw undergoes significant machining in subsequent processes, the S content is determined to be 0.015%~0.035%.

[0013] (6) Determination of O content Oxygen in steel mainly exists as non-metallic inclusions of oxides. Reducing the oxygen content can improve the purity of the steel. Therefore, this invention requires an oxygen content ≤ 0.0012%.

[0014] Another objective of this invention is to provide a method for producing medium carbon steel for steering screws in new energy vehicles. The method employs vacuum degassing and continuous casting to smelt the billet. The specific production process is as follows: molten iron pretreatment – ​​converter smelting – LF refining – RH furnace vacuum degassing – continuous casting – rolling – quenching and tempering – finishing – sub-critical quenching – low-temperature tempering. The main characteristics of the production process are as follows: Hot metal pretreatment The KR hot metal pretreatment process is used to desulfurize and dephosphorize hot metal to obtain high-quality hot metal. After pretreatment, the sulfur content of the hot metal is ≤0.005% and the phosphorus content is ≤0.010%, which lays the foundation for subsequent smelting of high-quality steel.

[0015] Converter smelting The pretreated high-quality molten iron and high-quality scrap steel are added to the converter for primary refining. Oxygen is blown from the top of the furnace mouth to aid melting, and the oxygen blowing volume per ton of steel is controlled at 30 to 80 cubic meters. The carbon content at the end of the converter is controlled at ≥0.05%, and the tapping temperature is ≥1600℃. When tapping, 300 kg / ton of aluminum-iron is added to pre-deoxidize the molten steel, taking advantage of the kinetic conditions in the molten steel, to initially remove oxygen elements from the molten steel and reduce oxide inclusions.

[0016] LF Refining The molten steel after converter smelting is sent to the LF refining furnace for heating, alloying and deoxidation. The entire process is carried out in an argon protective atmosphere to avoid secondary oxidation of the molten steel. LF adds high-manganese alloy in the early stage of refining to control the composition of Mn element in molten steel; In the later stage of LF refining, slag surface diffusion deoxidation is carried out. First, 30~50Kg / ton of steel Al particles are fed into the molten steel, and then 300~500Kg / ton of steel high-performance composite slag-forming agent (main component is CaO) is added at one time to achieve deep deoxidation. During the refining process, the molten steel is subjected to 3 to 4 temperature measurements and sampling analyses. The temperature of the refining furnace is controlled at 1550 to 1580℃, while the final aluminum content is controlled at 0.025% to 0.04%. LF refining time ≥1h, through long-term refining to allow non-metallic inclusions to float fully, and through special refining slag and intermediate ladle protective slag to adsorb inclusions, effectively controlling the quantity and shape of inclusions.

[0017] RH furnace vacuum degassing After LF refining, the molten steel is sent to the RH furnace for vacuum degassing treatment to ensure that the highest vacuum degree in the furnace is ≤1.5mbar and to maintain sufficient circulation treatment time to further remove harmful gases such as H and N and residual non-metallic inclusions from the molten steel. After vacuum degassing, argon gas is used to gently blow and stir the molten steel for ≥35 minutes. This ensures the effectiveness of vacuum degassing and allows inclusions in the molten steel to float to the surface and be removed, further improving the purity of the molten steel.

[0018] Continuous casting Small cross-section square continuous casting billets are used, with billet specifications within 300mm×300mm, to ensure the uniformity of product composition. The continuous casting process employs a protective casting technique throughout to prevent secondary oxidation of the molten steel upon contact with air. The continuous casting process employs electromagnetic stirring technology and controls the pouring superheat to 20-35℃, effectively reducing the secondary dendrite arm spacing, improving billet quality, reducing compositional segregation, and enhancing the uniformity of billet microstructure.

[0019] Rolled materials The continuously cast billet is fed into a heating furnace with a neutral or weakly oxidizing atmosphere for heating. The heating process parameters are: preheating zone temperature 650-900℃, heating zone temperature 1000-1250℃, soaking zone temperature 1000-1250℃, and total heating time ≥8 hours to ensure that the billet is fully and evenly heated and to avoid structural defects caused by uneven heating. The initial rolling temperature is controlled at 1000℃-1200℃, and the water tank at room temperature is used for cooling during the rolling process to keep the final rolling temperature at 750~850℃. Immediately after rolling, the steel is subjected to pit cooling treatment with a pit temperature of >450℃ and a slow cooling time of ≥72 hours to prevent the steel from cooling too quickly and causing internal stress and cracks.

[0020] Conditioning First, the austenitizing temperature (870℃), AC1 temperature (730℃), and bainite transformation temperature (600℃) of the steel grade of this invention were determined using a thermal simulator, and the quenching and tempering process was designed based on these results. Rolled round bars with a slow-cooling exit temperature ≤200℃ are loaded into a continuous roller hearth open flame furnace for quenching and heating. The quenching temperature is 850±30℃. During the heating process, some cementite is controlled to dissolve into austenite, so that cementite particles are still retained in the matrix to achieve dynamic equilibrium. The round steel bars pass through the furnace at a uniform speed via a roller conveyor, and are heated to 850±30℃ with the furnace. The heating rate is 5℃ / min-20℃ / min, and the temperature is held for ≥7 hours to obtain a uniform austenitic + ferrite structure in the steel. After the heat preservation is completed, the steel is taken out of the furnace and water-quenched to achieve the initial hardening of the steel. After quenching, the round steel is subjected to two tempering treatments: the first tempering is heated to 660±20℃, held for ≥3.5 hours, and then water-cooled; the second tempering is heated to 580±10℃, held for ≥5 hours, and then air-cooled. Through two tempering treatments, the steel obtains a uniform and fine tempered sorbite structure, which improves the core toughness.

[0021] Refined The tempered steel is then subjected to straightening, machining, and non-destructive testing in sequence. Straightening: A precision straightening machine is used to straighten the steel to ensure that the straightness of the steel is ≤0.1mm / m; Car body: CNC car body equipment is used to process the steel, with a single-sided car body thickness of 0.4mm~0.7mm, which completely removes the decarburized layer and minor surface defects such as cracks and scratches from the steel surface; Non-destructive testing: A combination of ultrasonic testing and magnetic particle testing is used to perform non-destructive testing on the surface and internal quality of steel. The testing standards meet the requirements of GB / T 6402 and GB / T 9444, ensuring a 100% pass rate for surface testing.

[0022] Sub-temperature quenching + low-temperature tempering The round steel that has passed the flaw detection is subjected to sub-temperature quenching treatment. The sub-temperature quenching temperature is 780℃~850℃ (20℃ below the austenite temperature and 50℃ above the AC1 temperature), and the holding time is 15-30min. After the holding time is completed, the round steel is water quenched until the temperature of the round steel is ≤100℃ and then taken out of the water to achieve surface hardening of the steel while retaining the toughness of the core. After quenching, the round steel is directly subjected to low-temperature tempering treatment at a temperature of 250-300℃ for 4 hours, followed by air cooling after removal from the furnace. Low-temperature tempering eliminates quenching internal stress, improves the stability of the surface structure, and avoids cracking during subsequent processing.

[0023] The medium carbon steel used in the steering screws of new energy vehicles produced by this invention, after being manufactured using the above process, must meet the following core technical indicators: Hardness index: The surface hardness of the steel is ≥60HRC, the core hardness is 22~28HRC, there is no decarburized layer on the surface, and a hardness gradient from the surface to the core is achieved. Purity index: Microscopic non-metallic inclusions are tested according to GB / T 10561 Method A, and the rating requirements are as follows: Class A fine series ≤ 2.5, Class A coarse series ≤ 2.0, Class B fine series ≤ 2.0, Class B coarse series ≤ 1.5, Class C fine series = 0, Class C coarse series = 0, Class D fine series ≤ 1.0, Class D coarse series ≤ 0.5, DS class ≤ 2.0; Dimensional specifications: Product specifications are φ16mm~45mm, straightness ≤0.1mm / m, and dimensional tolerances meet the precision requirements of GB / T 3207; Metallographic structure: The surface metallographic structure is acicular martensite, and the core metallographic structure is dense tempered sorbite. The structure is free of defects such as coarse grains and segregation.

[0024] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention adopts a medium carbon steel system, which significantly reduces the addition of expensive alloying elements such as Cr, Mo, and Ni, reducing smelting costs by more than 30%; it eliminates the need for complex processes such as carburizing and quenching, shortening the heat treatment cycle by 40%, while the reasonable control of S element improves the cutting performance of steel, reducing tool wear in whirl milling by more than 25%, which meets the needs of low-cost manufacturing of new energy vehicles.

[0025] 2. This invention achieves a hardness gradient of ≥60HRC on the surface and 22~28HRC in the core through a composite heat treatment process of "quenching and tempering + sub-temperature quenching + low-temperature tempering". The surface hardness meets the requirements of the whirl milling process, while the low hardness of the core retains good toughness. The quenching deformation can be eliminated by straightening, which meets the high precision dimensional requirements of the steering screw of new energy vehicles. At the same time, the purity of the steel and non-metallic inclusions are strictly controlled to ensure the long fatigue life of the screw.

[0026] 3. The production process of this invention has much lower requirements for rolling and normalizing than for low-carbon high-alloy steel. Through processes such as vacuum degassing, protective casting, and electromagnetic stirring, it effectively controls compositional segregation and inclusions, significantly reducing the scrap rate caused by coarse grains and uneven structure. The scrap rate is reduced to below 1%, making it suitable for large-scale industrial production.

[0027] 4. The product specifications of this invention are φ16mm~45mm, which can be adapted to existing cyclone milling equipment and production lines for automotive steering screws. No modification is required to the existing processing equipment, thus reducing the equipment investment costs for automotive parts manufacturers. Attached Figure Description

[0028] Figure 1 The images show the metallographic structure of the medium carbon steel surface in Embodiments 1 and 2 of this invention. The left image is magnified 100 times and the right image is magnified 500 times. It can be clearly seen from the images that the surface metallographic structure is uniform acicular martensite, which has high hardness and meets the requirements of the cyclone milling process. Figure 2 The images show the metallographic structure of the medium carbon steel core in Embodiments 1 and 2 of this invention. The left image is magnified by 100x and the right image is magnified by 500x. The metallographic structure of the core is dense tempered sorbite, which has good toughness and can eliminate quenching deformation by straightening. Figure 3 The image shows the hot-rolled metallographic structure of the low-carbon high-alloy steel in Comparative Example 1. The left image is magnified 100 times and the right image is magnified 500 times. The structure is pearlite + ferrite, which is the original structure. It needs to be carburized and quenched to meet the surface hardness requirements. Figure 4 The image shows the metallographic structure of the high-carbon chromium bearing steel in the tempered state, as shown in Comparative Example 2. The left image is magnified by 100x and the right image is magnified by 500x. The structure consists of tempered martensite and carbides. The material has high overall hardness but poor toughness and is prone to deformation after quenching. Detailed Implementation

[0029] The technical solution of the present invention will be described in more detail below with reference to preferred embodiments. However, these embodiments are merely descriptions of preferred implementations of the present invention and should not be construed as limiting the scope of the present invention. Example 1

[0030] A medium carbon steel for steering screws in new energy vehicles, with a product specification of φ35mm, has the following chemical composition by mass percentage: C: 0.55%, Si: 0.32%, Mn: 0.77%, Cr: 0.22%, Mo: 0.04%, Ni: 0.06%, P: 0.015%, S: 0.032%, O: 0.00058%, with the balance being Fe and unavoidable impurities.

[0031] The above-mentioned method for producing medium carbon steel includes the following steps: Hot metal pretreatment: The KR process is used to desulfurize and dephosphorize the hot metal. After pretreatment, the hot metal has S≤0.004% and P≤0.009%. Converter smelting: High-quality molten iron + high-quality scrap steel are fed into the furnace, oxygen blowing volume is 50 cubic meters per ton of steel, final carbon content is 0.06%, tapping temperature is 1615℃, and 300 kg / ton of aluminum-iron pre-deoxidation is added during tapping; LF refining: Argon protection, high manganese alloy is added in the early stage, 40 kg / ton of steel Al particles are fed in the later stage, 400 kg / ton of steel CaO-based composite slagging agent is added, temperature is measured and sampled 3 times, refining temperature is 1560℃, final Al=0.03%, refining time is 90 min; RH furnace vacuum degassing: maximum vacuum degree 1.2 mbar, argon gas soft blowing and stirring for 40 min; Continuous casting: 300mm×300mm square billet, full-process protective casting, electromagnetic stirring, casting superheat 25℃; Rolled into finished products: Total heating time in the heating furnace is 9 hours, with a preheating temperature of 750°C, a heating temperature of 1150°C, and a soaking temperature of 1150°C; initial rolling temperature is 1100°C, and final rolling temperature is 790°C; pit cooling, with an entry temperature of 500°C and a slow cooling time of 96 hours. Heat treatment: Rolled round bar exit temperature 180℃, quenching temperature 880℃, heating rate 10℃ / min, holding for 7.5 hours, water quenching; first tempering 680℃, holding for 4 hours, water cooling; second tempering 580℃, holding for 5.5 hours, air cooling; Finishing: Precision straightening, single-sided machining allowance of 0.5mm, ultrasonic + magnetic particle non-destructive testing; Sub-critical quenching + low-temperature tempering: Sub-critical quenching temperature 830℃, hold for 20 minutes, water quench to 80℃ and remove from water; low-temperature tempering 280℃, hold for 4 hours, air cool.

[0032] The performance of the medium carbon steel in this embodiment was tested, and the results were as follows: surface hardness 63 HRC, core hardness 27 HRC, no decarburized layer on the surface; microscopic non-metallic inclusion rating: A fine 1.5, A coarse 0.5, B fine 0.5, B coarse 0, C fine 0, C coarse 0, D fine 0.5, D coarse 0.5, Ds0; surface metallographic structure is acicular martensite, core is tempered sorbite; straightness 0.08 mm / m, dimensional tolerance meets the precision grade requirements of GB / T 3207. Example 2

[0033] A medium carbon steel for steering screws in new energy vehicles, with a product specification of φ32mm, has the following chemical composition by mass percentage: C: 0.56%, Si: 0.30%, Mn: 0.75%, Cr: 0.18%, Mo: 0.04%, Ni: 0.05%, P: 0.016%, S: 0.028%, O: 0.00061%, with the balance being Fe and unavoidable impurities.

[0034] The production method for medium carbon steel described above is identical to that in Example 1, except for the following process parameters: LF refining: Feed 35 kg / ton of steel Al granules, add 350 kg / ton of steel CaO-based composite slagging agent, refining temperature 1555℃, final Al=0.028%; RH furnace vacuum degassing: Argon gas soft blowing and stirring for 38 minutes; Continuous casting: casting superheat 23℃; Rolled into finished products: final rolling temperature 780℃, slow cooling time in pit 90h; Quenching and tempering treatment: Quenching temperature 870℃, holding for 7 hours, tempering once at 670℃, holding for 3.5 hours; Sub-critical quenching + low-temperature tempering: Sub-critical quenching temperature 820℃, holding for 25 min, low-temperature tempering 270℃, holding for 4 hours.

[0035] The performance of the medium carbon steel in this embodiment was tested, and the results were as follows: surface hardness 62HRC, core hardness 27HRC, no decarburized layer on the surface; microscopic non-metallic inclusion rating: A fine 1.5, A coarse 0.5, B fine 0.5, B coarse 0, C fine 0, C coarse 0, D fine 0.5, D coarse 0.5, Ds 0.5; surface metallographic structure is acicular martensite, core is tempered sorbite; straightness 0.09mm / m, dimensional tolerance meets the precision grade requirements of GB / T 3207.

[0036] Comparative Example 1 (Low-carbon high-alloy steel) The product is made of low-carbon high-alloy steel using existing technology. The product specification is φ35mm. The chemical composition by mass percentage is: C: 0.16%, Si: 0.27%, Mn: 1.25%, Cr: 1.08%, Mo: 0.02%, Ni: 0.04%, P: 0.016%, S: 0.003%, O: 0.00063%, with the balance being Fe and unavoidable impurities. The production process is hot rolling + carburizing and quenching.

[0037] Performance test results: surface hardness 62HRC, core hardness 30HRC; microscopic non-metallic inclusions D coarse 1.0; hot-rolled metallographic structure is pearlite + ferrite; smelting cost is 35% higher than that of Example 1 of this invention, and the carburizing and quenching process cycle is 45% longer than that of this invention.

[0038] Comparative Example 2 (High-carbon chromium bearing steel) The product uses high-carbon chromium bearing steel (GCr15) from existing technology, with a product specification of φ30mm. The chemical composition by mass percentage is: C: 0.96%, Si: 0.30%, Mn: 0.38%, Cr: 1.43%, Mo: 0.02%, Ni: 0.03%, P: 0.018%, S: 0.002%, O: 0.00055%, with the balance being Fe and unavoidable impurities; the production process is quenching and tempering.

[0039] Performance test results: surface hardness 62HRC, core hardness 60HRC; straightness after quenching 0.5mm / m, unable to meet precision dimensional requirements through straightening; product scrap rate reaches 8% after subsequent cyclone milling.

[0040] As can be seen from the test results of the above embodiments and comparative examples, the medium carbon steel produced by the present invention is significantly superior to the low carbon high alloy steel and high carbon chromium bearing steel in the prior art in terms of hardness gradient, dimensional accuracy, production cost and production stability, and fully meets the requirements for use of steering screws in new energy vehicles.

[0041] In addition to the above embodiments, the present invention can also adjust parameters such as feeding thickness and vibration time according to the design requirements of alloy melting furnaces of different tonnages. 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 steel for a steering screw of a new energy vehicle, characterized by, The chemical composition of the medium carbon steel, by mass percentage, is as follows: C: 0.45%~0.70%, Si: 0.25%~0.55%, Mn: 0.45%~1.05%, Cr: ≤0.40%, Mo≤0.40%, Ni≤0.40%, P≤0.035%, S: 0.015%~0.035%, O≤0.0012%, with the balance being Fe and unavoidable impurities; The technical specifications of the medium carbon steel are as follows: surface hardness ≥60HRC, core hardness 22~28HRC, and no decarburized layer on the surface; according to GB / T 10561 Method A, the microscopic non-metallic inclusions are inspected and rated as follows: Class A fine series ≤2.5, Class A coarse series ≤2.0, Class B fine series ≤2.0, Class B coarse series ≤1.5, Class C fine series = 0, Class C coarse series = 0, Class D fine series ≤1.0, Class D coarse series ≤0.5, and Class DS ≤2.

0. The product specifications range from φ16mm to 45mm.

2. The method of producing medium carbon steel for a steering screw of a new energy vehicle according to claim 1, characterized in that, The production process is as follows: molten iron pretreatment → converter smelting → LF refining → RH furnace vacuum degassing → continuous casting → rolling into finished products → quenching and tempering → finishing → sub-temperature quenching + low-temperature tempering.

3. The method for producing medium carbon steel for steering screws in new energy vehicles according to claim 2, characterized in that, The molten iron pretreatment adopts the KR molten iron pretreatment process, and the sulfur content of the molten iron after pretreatment is ≤0.005% and the phosphorus content is ≤0.010%.

4. The method for producing medium carbon steel for a steering screw of a new energy vehicle according to claim 2, characterized in that, The process parameters for the converter smelting are as follows: oxygen blowing volume of 30 to 80 cubic meters per ton of steel, carbon content at the converter endpoint ≥ 0.05%, tapping temperature ≥ 1600℃, and 300 kg / ton of aluminum iron is added for pre-deoxidation during tapping.

5. A method for producing medium carbon steel for steering screws in new energy vehicles according to claim 2, characterized in that, The LF refining process is as follows: the entire process adopts argon protective atmosphere for smelting, high manganese alloy is added in the early stage, and 30~50Kg / ton of steel Al particles are fed in the later stage and 300~500Kg / ton of steel CaO-based high-performance composite slagging agent is added; temperature is measured and sampled 3~4 times during the refining process, the refining temperature is 1550~1580℃, the final aluminum content is 0.025%~0.04%, and the refining time is ≥1h.

6. A method for producing medium carbon steel for steering screws in new energy vehicles according to claim 2, characterized in that, The process parameters for vacuum degassing in the RH furnace are: maximum vacuum degree ≤ 1.5 mbar, and argon soft blowing and stirring time ≥ 35 min after vacuum breaking.

7. A method for producing medium carbon steel for steering screws in new energy vehicles according to claim 2, characterized in that, The continuous casting uses square continuous casting billets within 300mm×300mm, with full-process protective pouring, electromagnetic stirring technology, and a pouring superheat of 20-35℃.

8. A method for producing medium carbon steel for steering screws in new energy vehicles according to claim 2, characterized in that, The rolling process is as follows: the continuously cast billet is heated in a neutral or weakly oxidizing atmosphere furnace, with a preheating temperature of 650-900℃, a heating temperature of 1000-1250℃, a soaking temperature of 1000-1250℃, and a total heating time of ≥8 hours; the initial rolling temperature is 1000℃-1200℃, and the final rolling temperature is 750~850℃; after rolling, the billet is cooled in a pit with an initial temperature of >450℃ and a slow cooling time of ≥72 hours.

9. A method for producing medium carbon steel for steering screws in new energy vehicles according to claim 2, characterized in that, The heat treatment process is as follows: a rolled round bar with a slow-cooling temperature ≤200℃ is loaded into a continuous roller hearth open flame furnace, quenched at 850±30℃, with a heating rate of 5℃ / min-20℃ / min, and held for ≥7 hours before water quenching; after quenching, it is tempered twice in sequence, with the first tempering temperature at 660±20℃, held for ≥3.5 hours before water cooling, and the second tempering temperature at 580±10℃, held for ≥5 hours before air cooling.

10. A method for producing medium carbon steel for steering screws in new energy vehicles according to claim 2, characterized in that, The sub-critical quenching + low-temperature tempering process is as follows: sub-critical quenching temperature is 780℃~850℃, holding temperature is 15-30min, water quenching is performed until the temperature of the round steel is ≤100℃ and then water is removed; after quenching, low-temperature tempering is performed directly at a tempering temperature of 250-300℃, holding temperature is 4 hours and then air cooling is performed.

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