High-cleanliness free-cutting non-quenched and tempered steel and method for manufacturing same

Abstract

The present application relates to a kind of high clean free-cutting non-quenched and tempered steel and its manufacturing method, and its chemical element mass percentage includes: C:0.38%~0.41%, Si:0.20%~0.30%, Mn:0.60%~0.70%, P≤0.018%, S:0.020%~0.028%, Cr:0.15%~0.25%, V:0.04%~0.05%, N:0.007%~0.009%, Al:0.02%~0.04%, Cu≤0.10%, Ca:0.0003~0.0015%, Ce:0.001~0.002%, the balance is Fe and other inevitable impurities;Can be in the premise of not sacrificing steel strength and toughness and cleanliness, with low cost, realize the accurate control of sulfide spindling, ensure process smooth, obtain the high clean aluminum calming sulfur free-cutting non-quenched and tempered steel with excellent comprehensive performance.

Description

Technical Field

[0001] This invention belongs to the field of non-quenched and tempered steel manufacturing, specifically relating to a high-purity free-machining non-quenched and tempered steel and its manufacturing method, which is particularly suitable for hot-forged parts such as automobile crankshafts and rods that have high requirements for strength, toughness and machinability. Background Technology

[0002] Non-quenched and tempered steels, by adding microalloying elements such as V, Ti, and Nb, can achieve or approach the mechanical properties of quenched and tempered steels after rolling or forging without further quenching and tempering. They offer significant advantages in energy saving and consumption reduction, and are widely used in shafts and other components in the automotive and construction machinery industries. For example, crankshafts are key components of automotive engines, requiring them to withstand complex bending and torsional stresses and demanding high strength and toughness. As their manufacturing increasingly relies on CNC machine tools, the requirements for the steel's machinability are becoming increasingly stringent. To improve the machinability of non-quenched and tempered steels, sulfur is typically added. However, excessive sulfur content can lead to hot brittleness, and the resulting MnS inclusions are prone to deformation and elongation during hot working, worsening the steel's transverse mechanical properties and machinability anisotropy.

[0003] Although some methods for controlling sulfide morphology have been proposed in the existing technology, the following technical bottlenecks still exist in manufacturing free-machining non-quenched and tempered steel for hot forging parts such as crankshafts that require high strength and high cleanliness:

[0004] I. To improve the size and distribution of MnS inclusions, Al, Ti, Ca, and rare earth elements are added to free-machining steels. For example, patent CN113913676B discloses a metallurgical method for improving the morphology of cast sulfides in medium-carbon, high-sulfur free-machining steel. After LF refining, Ca alloy is added to control the Ca and Al content and transform the sulfides into spherical sulfides. However, the steel has a high Mn and S content, and after Ca treatment, the proportion of large-sized MnS is still relatively large, and the control of sulfide morphology is still not ideal. The toughness and machinability of the steel will be poor. Further increasing the Ca content will easily produce large-particle D. S inclusions cause tool breakage during the cutting process; Patent CN114015937B discloses a free-cutting non-quenched and tempered steel and its production method. The steel contains Ti, and after VD is broken up, MnN lines, La / Ce rare earth alloys and S lines are added to form spherical or spindle-shaped high-melting-point compounds. However, the steel has a high Mn and S content, and the addition of high rare earth elements leads to high cost. Further reducing the Mn and S content in the steel results in insufficient total sulfide and difficulty in nucleation. The anisotropy of sulfide morphology will increase, and the steel cleanliness will be insufficient, making it difficult to simultaneously achieve high strength, high toughness and excellent cutting performance.

[0005] II. To improve the machinability of non-quenched and tempered steel, methods such as increasing the amount of MnS, reducing the length of individual MnS strands, or controlling the size and morphology of inclusions are employed. For example, patent CN115852239B discloses a high-strength and tough free-machining non-quenched and tempered steel and its manufacturing method. Rare earth element Ce is added to the steel, and sulfur wire is fed before RH or VD vacuum treatment, followed by calcium wire feeding after vacuum breaking, resulting in long, strip-shaped MnS inclusions. However, the high Mn and S content in the steel leads to low Ce yield, making cost control difficult. The poor morphology of the long, strip-shaped sulfides causes stress concentration during cutting, leading to decreased machinability and mechanical properties. Furthermore, improper timing of calcium treatment can easily result in high melting points. CaS formation at the nozzle causes nodules, affecting smooth production. Patent CN120719198A discloses a high-quality non-quenched and tempered crankshaft steel rectangular billet and its preparation method. It uses strong deoxidation, uniform stirring, refining slag system, and soft blowing time as the core to control the size and morphology of inclusions and obtain spindle-shaped MnS morphology. However, the steel has high Mn and S content, resulting in insufficient toughness. Insufficient aluminum treatment deoxidation will affect the cleanliness of the steel. Further reducing the Mn and S content in the steel results in insufficient total sulfide and difficulty in nucleation. The increase in hard inclusions will cause wear on the cutting tools and other tools. Moreover, its process window is narrow, and the control requirements of the continuous casting process are extremely high, making it difficult to achieve large-scale stable production.

[0006] Based on the above existing technologies, although they each have their own focus on sulfide morphology control, they generally have one or more of the following problems: they may not be able to simultaneously achieve high strength, high toughness and excellent machinability; or they may have a narrow process window and be difficult to control in production. In particular, for steel used in hot forging parts such as crankshafts with high cleanliness requirements, improper treatment of aluminum or calcium can easily lead to blockage of the continuous casting nozzle, affecting smooth production. Summary of the Invention

[0007] The present invention aims to solve at least one of the above-mentioned technical problems. The present invention provides a high-purity free-cutting non-quenched and tempered steel and its manufacturing method, which can achieve precise control of sulfide spindle formation at low cost without sacrificing the strength, toughness and cleanliness of the steel, while ensuring smooth process operation, thereby obtaining a high-purity aluminum-killed sulfur-containing free-cutting non-quenched and tempered steel with excellent comprehensive performance.

[0008] The technical solution adopted by this invention to solve its technical problem is:

[0009] The first aspect of this invention is to provide a high-purity, free-machining, non-quenched and tempered steel, wherein the chemical element mass percentages comprise: C: 0.38%~0.41%, Si: 0.20%~0.30%, Mn: 0.60%~0.70%, P≤0.018%, S: 0.020%~0.028%, Cr: 0.15%~0.25%, V: 0.04%~0.05%, N: 0.007%~0.0 0.9%, Al: 0.02%~0.04%, Cu≤0.10%, Ca: 0.0003~0.0015%, Ce: 0.001~0.002%, with the balance being Fe and other unavoidable impurities; the sulfides of the non-quenched and tempered steel are spindle-shaped, with the number of sulfides with an aspect ratio ≤5 accounting for more than 90% of the total number of sulfides, and the number of hard inclusions with a size <15μm per square millimeter cross section not exceeding 600.

[0010] In terms of composition design, the aforementioned non-quenched and tempered steel, while optimizing the composition of Mn, S, etc., can balance the strength and toughness of the steel, combined with the synergistic effect of Ca and Ce in a specific low content range: During the research process, the inventors discovered that a certain number of spindle-shaped sulfide inclusions in the steel are beneficial to improving machinability, but it cannot contain large particles or a large number of hard alumina inclusions, otherwise it will cause tool breakage and tool wear. Ca is mainly used to coat oxides such as Al2O3, transforming them into low-melting-point calcium aluminates, in order to maintain high purity, improve machinability, and avoid causing nozzle nodules; Ce is a strong sulfide-forming element, which can combine with Mn and S to form fine CeS or Ce2S3 before Mn, serving as a heterogeneous nucleation core for MnS, making the most... The resulting MnS is finer, more uniform, and spindle-shaped. Compared to long, strip-shaped MnS inclusions, which deteriorate the transverse mechanical properties and cutting anisotropy of steel, spindle-shaped sulfides with an aspect ratio ≤ 5 have no sharp edges, which can avoid scratching the cutting edge of the tool and reduce the risk of tool chipping. This is beneficial for effectively improving the cutting performance of non-quenched and tempered steel. Moreover, the uniform distribution and lack of obvious directionality can reduce stress concentration and suppress anisotropy. The composite addition of Ca and Ce utilizes the synergistic effect of Ca-modified oxides and Ce refining sulfide nucleation. Under the premise of not adding too much, the sulfides are shaped into spindles at the lowest cost, while ensuring the castability of molten steel. This is to take into account high strength, high toughness, and excellent cutting performance, so as to meet the processing and service requirements of hot forged parts such as crankshafts.

[0011] To further improve the cleanliness and machinability of steel and reduce the risk of continuous casting nozzle blockage, inclusions can be further controlled. In the preferred technical solution, the non-quenched and tempered steel contains Class A fine inclusions ≤2.5 grade and no Class A coarse inclusions. Coarse Class A alumina inclusions can scratch the cutting tool and reduce machining accuracy, while excessive fine inclusions will affect the smoothness of cutting. This index can make the cutting process more stable. Class B nitride inclusions have high hardness and poor toughness. Excessive inclusions will become stress concentration sources, leading to a decrease in the toughness of the steel. It is preferred that Class B inclusions be ≤0.5 grade. Excessive Ds spherical oxide inclusions will reduce the density of the steel and affect its mechanical properties. It is preferred that Ds inclusions be ≤0.5 grade.

[0012] To further improve service performance while taking into account the strength and toughness of non-quenched and tempered steel, in the preferred technical solution, the tensile strength of the non-quenched and tempered steel is ≥1000MPa, the yield strength is ≥950MPa, and the elongation after fracture is ≥11%.

[0013] The second aspect of the present invention is to provide a method for manufacturing high-purity free-machining non-quenched and tempered steel, for manufacturing high-purity free-machining non-quenched and tempered steel as described in any of the above-mentioned methods, the manufacturing method comprising the following steps performed in sequence: converter or electric furnace smelting, LF furnace refining, VD or RH vacuum treatment, continuous casting, and rolling; before the LF furnace refining station exits, calcium wire is fed into the molten steel to perform calcium treatment; after the VD or RH vacuum treatment breaks the void, S is added and Ce is fed in; aluminum is not added during VD or RH vacuum treatment, and Ca treatment is strictly prohibited.

[0014] During their research, the inventors discovered that excessive Ca content, incorrect aluminum or Ca addition during the VD process, can lead to a surge in the number of hard inclusions, directly causing severe nozzle blockage and affecting Ce recovery and sulfide morphology control. Based on the composition design, the above method utilizes calcium treatment before refining in the LF furnace to modify Al2O3 into low-melting-point calcium aluminate, allowing it to fully float to the slag phase and avoiding the formation of large particles and numerous hard alumina inclusions. The strong deoxidizing effect of Ca prevents Ce from preferentially reacting with O to form Ce2O3 during subsequent vacuum treatment to break up the void, thus maintaining the recovery rate. Afterwards, "VD or RH vacuum treatment followed by S addition and Ce feeding" is employed to strictly control the void. The key process of "no aluminum addition and Ca treatment" fundamentally avoids the formation of large Al2O3 by aluminum addition after void breaking and the formation of high-melting-point CaS by calcium treatment, solving the industry problem of continuous casting nozzle blockage in high-purity aluminum-killed sulfur-containing steel and ensuring smooth process operation. On the other hand, it avoids the interference of Ca and S reaction with Ce and S reaction under calcium treatment after void breaking, ensuring Ce yield and Ce's core role in sulfide nucleation, allowing Ca and Ce to play a synergistic role in the low content range to obtain spindle-shaped sulfides. As a result, the strength, plasticity and toughness of the obtained free-machining non-quenched and tempered steel are improved compared with existing crankshaft steel grades, while the machinability is significantly improved.

[0015] To further promote steel deoxidation and reduce inclusions, slag adjustment during steelmaking in converters or electric furnaces can be further controlled. In a preferred technical solution, aluminum ingots, carbon powder, quicklime, and a cleaning agent are added during steelmaking in the converter or electric furnace. Aluminum ingots can react with O to generate Al2O3 for deep deoxidation. The total amount of aluminum ingots added is 0.48~1.68 kg / t steel, avoiding excessive Al from generating Class A coarse inclusions or insufficient Al from causing incomplete deoxidation. Carbon powder plays an auxiliary role in deoxidation and carbon replenishment. The amount of carbon powder added is 0.3~0.7 kg / t steel, which can reduce the oxidizability of molten steel and reduce aluminum ingot consumption. Quicklime can adjust the slag basicity and promote inclusion removal. The amount of quicklime added is 2~4 kg / t steel. The cleaning agent can enhance the flotation of inclusions, improve the cleanliness of molten steel and the efficiency of inclusion removal. The amount of cleaning agent added is 5~7 kg / t steel.

[0016] To further optimize deoxidation and control the formation of Class A coarse inclusions, the timing and amount of aluminum ingots added can be further optimized. In the preferred technical solution, during the steelmaking process in the converter or electric furnace, the aluminum ingots are added in no fewer than three batches to the impact zone of the molten steel. The impact zone is where the mixing of molten steel is most intense. Adding in batches allows the aluminum ingots to dissolve quickly and disperse evenly, avoiding excessive aluminum in certain areas or the formation of Class A coarse inclusions due to adding a large amount in a single batch. This also alleviates the intensity of the deoxidation reaction and reduces molten steel splashing. The final carbon content is controlled to be ≤0.20% before tapping the steel. The target temperature is ≥1600℃. The final carbon content directly reflects the oxidizing properties of the molten steel. The total amount of aluminum ingots added is determined based on the final carbon content of the converter: when the steel output per heat is 155~165t, when the final carbon content is <0.06%, the total amount of aluminum ingots added is 200~260kg / heat; when 0.06%≤final carbon content≤0.08%, the total amount of aluminum ingots added is 150~195kg / heat; when the final carbon content is >0.08%, the total amount of aluminum ingots added is 80~145kg / heat. This can avoid waste or insufficient deoxidation.

[0017] To further regulate the basicity and fluidity of LF furnace slag and improve its inclusion adsorption capacity, in the preferred technical solution, after the submerged arc of the LF furnace refining process is stabilized, lime and fluorite are added to adjust the slag fluidity. After the submerged arc is stabilized, the slag stratification is clear, and the material can be quickly dissolved and evenly dispersed. Lime can replenish CaO in the slag and enhance inclusion removal by adding alkali. The preferred amount of lime added is ≤1.94 kg / t steel to avoid excessive lime leading to increased slag viscosity. Fluorite is a low-melting-point flux that can optimize slag fluidity. The preferred amount of fluorite added is 0.03~0.06 kg / t steel to avoid excessive fluorite erosion of the LF furnace lining.

[0018] To further control slag fluidity and LF basicity, the amount of lime used can be further optimized. In the preferred technical solution, after the submerged arc refining in the LF furnace is stabilized, the carbon content directly reflects the oxidizing property of the molten steel. The amount of lime added is determined according to the carbon content: the amount of molten steel per furnace is 155~165t. When the carbon content is <0.06%, the amount of lime added is 150~300kg / furnace, and the cleanliness is ensured by supplementing alkali with a high amount of lime. When the carbon content is 0.06%≤carbon content≤0.08%, the amount of lime added is 60~145kg / furnace, and the slag fluidity is stabilized by adjusting the amount of lime used. When the carbon content is >0.08%, the amount of lime added is 0~50kg / furnace, and the excess slag phase and excessive basicity are avoided by using zero or low amounts of lime.

[0019] To further enhance deoxidation and improve inclusions, in the preferred technical solution, a diffusion deoxidizer is introduced during the refining and power-feeding stage of the LF furnace. During the power-feeding stage, the deoxidizer is more easily dispersed and dissolved, promoting the flotation of inclusions. The diffusion deoxidizer includes one or more of aluminum briquettes, calcium carbide, and composite silicon carbide. Aluminum briquettes, as a strong deoxidizer, can rapidly reduce oxygen activity. Preferably, the amount of aluminum briquettes used is ≤0.32 kg / t of steel to avoid excessive Al production and the generation of Class A coarse inclusions. Calcium carbide reacts with FeO and MnO in the molten steel, both assisting in deoxidation and generating CaO to increase slag basicity and enhance inclusion adsorption. The amount of calcium carbide used is ≤0.65 kg / t of steel to avoid... Excessive use can lead to boiling of molten steel, air entrainment causing secondary oxidation, or poor slag fluidity. Composite silicon carbide combines the deoxidizing effects of both Si and C, with a mild exothermic reaction. The preferred dosage of composite silicon carbide is 0.48~1.16 kg / t steel, and the total dosage of diffusion deoxidizer is ≥1.09 kg / t steel to avoid insufficient deoxidation. It is preferable to add diffusion deoxidizer in small amounts multiple times to improve deoxidation uniformity. In the early stage of deoxidation, calcium carbide and composite silicon carbide are used, which can be adapted to the highly oxidizing slag phase. In the middle and later stages of deoxidation, oxygen-composite silicon carbide is the main component, which can deeply deoxidize, avoid excessive alkalization of the slag phase, ensure fluidity, and connect with subsequent calcium treatment.

[0020] In the preferred technical solution, the aluminum content is adjusted to 0.040%~0.070% in the initial refining stage of the LF furnace, which can meet the initial strong deoxidation requirements, quickly reduce oxygen activity, and avoid excessive Al to produce Class A coarse inclusions; the basicity of the LF is controlled at 3.5~6, and the high basicity slag can efficiently adsorb fine inclusions such as Al2O3 remaining in the molten steel, reducing the number of hard inclusions in the steel.

[0021] In the preferred technical solution, 20-50 meters of calcium wire is fed into the LF furnace before refining, which can transform the high-melting-point Al2O3 inclusions in the steel into low-melting-point calcium aluminate, prevent nozzle nodule formation, and improve the purity of the molten steel. Then, a covering agent is added to protect the molten steel, isolate it from air to prevent secondary oxidation, reduce heat loss, and adsorb floating inclusions. The temperature of the molten steel leaving the station is 1600-1620℃, which can improve the calcium yield and the uniformity of the molten steel composition, and connect to the subsequent VD or RH vacuum treatment.

[0022] To further degas and promote the flotation of inclusions, in the preferred technical solution, the VD or RH vacuum treatment controls the vacuum degree to below 67 Pa for 10-15 minutes, which can deeply degas and purify the molten steel; the weak stirring bottom blowing argon flow rate is 30-50 NL / min, and the weak stirring time is 12-18 minutes, which can uniformly stir the molten steel, enhance degassing, avoid insufficient flow causing slow flotation of inclusions, and avoid excessive flow causing violent turbulence of molten steel and slag particles to be entrained in the molten steel; the soft blowing time before leaving the station is 15-20 minutes, and the ladle temperature is controlled at 1550-1560℃, which can further promote the flotation of inclusions and make the molten steel flow state tend to be stable.

[0023] To further avoid secondary contamination of molten steel, the continuous casting process reduces inclusions in the steel through protective casting. In a preferred technical solution, the tundish is covered with a double layer of calcium covering agent and carbonized rice husk during continuous casting. The calcium covering agent has a high melting point, low thermal conductivity, and high alkalinity, which can directly isolate the molten steel from the air, reduce the temperature loss of the molten steel in the tundish, and prevent secondary oxidation of the molten steel. The carbonized rice husk has a low thermal conductivity and good spreadability, which can enhance heat preservation, reduce heat loss, and prevent the molten steel from churning, thus reducing the risk of slag entrapment. To further control the continuous casting process, avoid nozzle nodules, and reduce internal defects in the billet, the superheat is preferably controlled at 20~35℃ and the casting speed is 0.80~0.95m / min during continuous casting.

[0024] To further improve the comprehensive performance of the material, the rolling process uses a high-temperature diffusion process to homogenize the microstructure, a large reduction to refine the grains, and a slow cooling process to improve the microstructure of the non-quenched and tempered steel. In the preferred technical solution, the preheating temperature during rolling is below 600℃ to avoid thermal stress cracking, the high-temperature temperature is 1200~1250℃, the high-temperature heating time is 80~150min, and the total heating time is 180~240min. The high-temperature diffusion process can promote the homogenization of austenite grains to achieve a uniform microstructure. The reduction rate is above 70%, and the grain size can be improved by large reduction to achieve a balance between strength and toughness.

[0025] To further improve the metallographic structure of the material, in the preferred technical solution, the slow cooling time after rolling is ≥24h, which can eliminate rolling internal stress and avoid cracks.

[0026] A third aspect of the present invention is to provide an application of a high-purity free-machining non-quenched and tempered steel, including using the high-purity free-machining non-quenched and tempered steel as described in any of the above claims as a material for manufacturing hot-forged parts such as crankshafts and rods that have high requirements for strength, toughness and machinability.

[0027] Compared with the prior art, the beneficial effects of the present invention are at least as follows:

[0028] (1) In view of the current situation that it is difficult for existing non-quenched and tempered steels to simultaneously achieve high strength, high toughness and excellent machinability, the non-quenched and tempered steel of the present invention is based on a medium carbon alloy structure. Through optimized composition design, especially the composite addition of Ca and Ce in a specific low content range, the synergistic effect of Ca modified oxides and Ce refining sulfide nucleation is utilized. Without sacrificing the strength, toughness and cleanliness of the steel, that is, at a high strength level of tensile strength Rm≥1000MPa, the hard inclusions with a size <15μm are <600 / mm. 2 With ultra-high cleanliness, through precise control of the spindle-shaped morphology of sulfides, the number of sulfides with an aspect ratio ≤5 accounts for more than 90% of the total number of sulfides, which can effectively improve the cutting performance. The chipping frequency can reach more than 11,000 pieces / time. It has overcome the technical barrier of balancing high strength and easy machinability, solved the industry problem, and is suitable for hot forged parts such as automobile crankshafts and rods that have high requirements for strength, toughness and machinability, and has good market application prospects.

[0029] (2) In view of the current situation that the existing non-quenched and tempered steel manufacturing methods have a narrow process window and are difficult to control, the manufacturing method of the present invention adopts the key process of feeding Ce after VD vacuum breaking and strictly prohibiting aluminum and calcium treatment, which fundamentally avoids the formation of high melting point CaS and large Al2O3, solves the industry problem of poor castability of high-purity aluminum killed sulfur-containing steel, ensures Ce yield and process smoothness, and achieves the spindle-shaped morphology and cleanliness of sulfides at the lowest cost. It can further improve the inclusions in the steel through converter smelting, LF furnace refining, VD vacuum treatment and continuous casting, to achieve A-class fine inclusions ≤2.5 grade, no A-class coarse inclusions, B-class inclusions ≤0.5 grade, and Ds-class inclusions ≤0.5 grade. Combined with rolling and slow cooling processes, non-quenched and tempered steel with excellent cutting performance and comprehensive mechanical properties is produced as a production route for high-grade crankshaft steel series with high quality requirements, ensuring stable and efficient production and having good industrial adaptability. Attached Figure Description

[0030] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which:

[0031] Figure 1 The image shows a metallographic photograph of type A sulfide inclusions in the non-quenched and tempered steel obtained in Example 1.

[0032] Figure 2 The image shows a metallographic photograph of the type A sulfide inclusions in the non-quenched and tempered steel obtained in Comparative Example 1.

[0033] Figure 3 The image shows a metallographic photograph of the Ds-type inclusions in the non-quenched and tempered steel obtained in Example 1.

[0034] Figure 4The image shows a metallographic photograph of the Ds-type inclusions in the non-quenched and tempered steel obtained in Comparative Example 3.

[0035] Figure 5 This is a statistical diagram of the inclusion size distribution of the non-quenched and tempered steel obtained in Example 1.

[0036] Figure 6 This is a statistical chart showing the inclusion size distribution of the non-quenched and tempered steel obtained in Comparative Example 1.

[0037] Figure 7 This is a statistical chart showing the size distribution of inclusions in the non-quenched and tempered steel obtained in Comparative Example 3.

[0038] Figure 8 This is a statistical chart showing the aspect ratio distribution of sulfides in the non-quenched and tempered steel obtained in Example 1.

[0039] Figure 9 This is a statistical chart showing the aspect ratio distribution of sulfides in the non-quenched and tempered steel obtained in Comparative Example 1.

[0040] Figure 10 This is a statistical chart showing the aspect ratio distribution of sulfides in the non-quenched and tempered steel obtained in Comparative Example 3.

[0041] Figure 11 These are comparative cutting state diagrams of the non-quenched and tempered steel obtained in Embodiment 1 and Comparative Example 3 of the present invention; wherein (a) is a cutting state diagram of the non-quenched and tempered steel obtained in Embodiment 1, and (b) is a cutting state diagram of the non-quenched and tempered steel obtained in Comparative Example 3.

[0042] Figure 12 These are comparative images of the wear of cutting tools after cutting non-quenched and tempered steel obtained in Embodiment 1, Comparative Example 1, and Comparative Example 3 of the present invention; wherein (a) is a photograph of the cutting tool after cutting non-quenched and tempered steel obtained in Embodiment 1, (b) is a photograph of the cutting tool after cutting non-quenched and tempered steel obtained in Comparative Example 1, and (c) is a photograph of the cutting tool after cutting non-quenched and tempered steel obtained in Comparative Example 3. Detailed Implementation

[0043] The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention. Unless otherwise specified, the process equipment or apparatus used in the following embodiments are all conventional equipment or apparatus in the art. Unless otherwise specified, the raw materials used in the embodiments of the present invention are commercially available. For example, the main components of the cleaning agent, by mass percentage, include CaO: 53.5%, SiO2: 23.5%, Al2O3: 34.3%, MgO: 8.5%; the mass percentage of SiC in the composite silicon carbide is ≥70%, and the mass percentage of CaO in the calcium covering agent is ≥50%. Unless otherwise specified, the technical means used in the embodiments of the present invention are all conventional means well known to those skilled in the art.

[0044] A preferred embodiment of the high-purity free-machining non-quenched and tempered steel of the present invention comprises the following chemical element mass percentages: C: 0.38%~0.41%, Si: 0.20%~0.30%, Mn: 0.60%~0.70%, P≤0.018%, S: 0.020%~0.028%, Cr: 0.15%~0.25%, V: 0.04%~0.05%, N: 0.007%~0.009%, Al: 0.02%~0.04%, Cu≤0.10%, Ca: 0.0003~0.0015%, Ce: 0.001~0.002%, with the balance being Fe and other unavoidable impurities.

[0045] The chemical element design basis for the above-mentioned non-quenched and tempered steel includes:

[0046] (1) Carbon: C has a significant impact on the strength and toughness of steel. The plasticity and toughness of steel decrease as the mass percentage of C increases. Therefore, the lower the mass percentage of C, the better the plasticity and toughness of steel. However, C is also an important means to ensure strength. Insufficient carbon content will lead to the inability to meet the strength requirements. Therefore, in the free-cutting non-quenched and tempered steel described in this invention, when the mass percentage of C is controlled at 0.38% to 0.41%, it can ensure high strength while the plasticity and toughness do not decrease significantly.

[0047] (2) Silicon: Si can improve the strength of steel. Silicon can improve the strength of steel through solid solution strengthening. However, excessive Si content will promote the formation of martensite structure, which is not good for toughness. Therefore, the mass percentage of Si in the free-cutting non-quenched and tempered steel described in this invention is controlled at 0.20% to 0.30%.

[0048] (3) Manganese: In addition to improving strength, Mn’s mass percentage can also help toughness within a certain range. When the mass percentage of Mn is low, the strength is insufficient, but when the mass percentage of Mn is too high, it is not conducive to toughness and the contribution to strength is also weak. Therefore, the mass percentage of Mn in the free-cutting non-quenched and tempered steel described in this invention is controlled at 0.60% to 0.70%.

[0049] (4) Sulfur: S is a hot brittle and machinable element. If the mass percentage of S is too low, its machinability cannot be reflected. However, as the sulfur content increases, the hot workability deteriorates. Generally speaking, for sulfur and sulfur composite free-cutting steels that are mainly for machinability, the sulfur content is generally between 0.20% and 0.40%, and the highest reaches 0.60%. For those that are mainly for mechanical properties, the sulfur content is between 0.04% and 0.13%, and they are used to make more important components, but the machinability is poor. In the free-cutting non-quenched and tempered steel described in this invention, the mass percentage of S is controlled within the range of 0.020% to 0.028%, and the machinability can be improved by controlling the morphology of inclusions.

[0050] (5) Vanadium: V is an important precipitation strengthening element. Vanadium produces precipitation strengthening effect by precipitating V(C,N). Without affecting the plasticity and toughness, the precipitates in ferrite and austenite greatly improve the strength of the material. If the V content is too low, the strengthening effect is not obvious. When the mass percentage of added V is higher than 0.15%, the cost is increased, but the effect on improving the performance of the steel is not significant. Therefore, in the free-cutting non-quenched and tempered steel described in this invention, the mass percentage of V is controlled at 0.04%~0.05%.

[0051] (6) Nitrogen: N and alloying elements such as V and Al can easily produce nitrides or nitrides, which refine the grains and improve the strength and toughness of steel through precipitation strengthening. However, if the mass percentage of N in steel is too high, void defects are likely to be generated. Based on the influence of N on the performance of the free-cutting non-quenched and tempered steel described in this invention, the mass percentage of N is limited to 0.007% to 0.009%.

[0052] (7) Aluminum: When Al is added to steel, it deoxidizes during the steelmaking process, just like Si, and the resulting composite deoxidation products can effectively improve the chips. On the other hand, the AlN particles formed by Al and N can effectively refine the grains and avoid thermal defects such as overheating that affect the material properties during high-temperature heating. To achieve the above effects, the mass percentage of aluminum should not be less than 0.002%. However, when the mass percentage of aluminum is high, it is easy to undergo secondary oxidation during the casting process. Therefore, the mass percentage of Al in the free-cutting non-quenched and tempered steel described in this invention is limited to between 0.02% and 0.04%.

[0053] (8) Calcium: Ca can improve anisotropy by controlling the morphology of sulfides, reduce the aspect ratio of MnS, and encapsulate oxides to improve the machinability of materials. However, when the mass percentage of calcium exceeds 0.0015%, large particle Ds inclusions are easily generated, and the cutting process will cause chipping. Therefore, in the free-cutting non-quenched and tempered steel described in this invention, the mass percentage of Ca is controlled between 0.0003% and 0.0015%.

[0054] (9) Cerium: As a rare earth element, the addition of Ce can change the morphology and distribution of inclusions, provide nucleation sites for MnS, form dispersed MnS, reduce the length of MnS, and transform the original polygonal Al2O3 and MnS composite inclusions into ellipsoidal or spherical rare earth composite inclusions containing Al and MnS, so as to achieve the spindle shape of sulfides. With the increase of Ce content, the mechanical properties of steel are also improved, including tensile strength, yield strength, elongation and hardness, etc. However, the addition of too much Ce increases the cost and affects the casting performance of molten steel. Therefore, in the free-cutting non-quenched and tempered steel described in this invention, the mass percentage of Ce is controlled between 0.001% and 0.002%.

[0055] The above-mentioned manufacturing method for high-purity, free-machining, non-quenched and tempered steel follows a process of converter or electric furnace smelting → LF furnace refining → VD or RH vacuum treatment → continuous casting → rolling → slow cooling. Specifically:

[0056] The converter or electric furnace smelting process is used to smelt steelmaking raw materials into molten steel based on the chemical composition of non-quenched and tempered steel. Aluminum ingots, carbon powder, quicklime, and a cleaning agent are added during tapping. The total amount of aluminum ingots added is 0.48~1.68 kg / t steel, carbon powder is 0.3~0.7 kg / t steel, quicklime is 2~4 kg / t steel, and the cleaning agent is 5~7 kg / t steel. These amounts can be calculated proportionally based on the actual amount of steel tapped. The final carbon content is controlled to be ≤0.20%, and the target tapping temperature is ≥1600℃. Preferably, during tapping from the converter or electric furnace, the following are added: Aluminum ingots are added to the impact zone of molten steel in no less than three batches to control the final carbon content. The total amount of aluminum ingots added is determined based on the final carbon content of the converter or electric furnace: the steel output per furnace is 155~165t. When the final carbon content is <0.06%, the total amount of aluminum ingots added is 200~260kg / furnace; when 0.06%≤final carbon content≤0.08%, the total amount of aluminum ingots added is 150~195kg / furnace; when the final carbon content is >0.08%, the total amount of aluminum ingots added is 80~145kg / furnace, in order to promote deoxidation of molten steel and reduce inclusions.

[0057] The LF furnace refining process is used to transfer molten steel smelted in a converter or electric furnace to the LF furnace for deoxidation and to improve the purity of the molten steel. Specifically, after the LF furnace refining process has stabilized in the submerged arc (i.e., after power is supplied for 3-5 minutes), lime and fluorite are added to adjust the slag fluidity. The amount of lime added is ≤1.94 kg / t of steel, and the amount of fluorite added is 0.03-0.06 kg / t of steel. Preferably, after the LF furnace refining process has stabilized in the submerged arc, the amount of lime added is determined according to the carbon content: the amount of molten steel per furnace is 155-165 t. When the carbon content is <0.06%, the amount of lime added is 150-300 kg / furnace; when 0.06% ≤ carbon content ≤0.08%, the amount of lime added is 60-145 kg / furnace; and when the carbon content is >0.08%, the amount of lime added is 0-50 kg / furnace.

[0058] During the refining and power supply stage of the LF furnace, a diffusion deoxidizer is introduced. The diffusion deoxidizer includes one or more of aluminum briquettes, calcium carbide, and composite silicon carbide. The amount of aluminum briquettes is ≤0.32 kg / t steel, the amount of calcium carbide is ≤0.65 kg / t steel, the amount of composite silicon carbide is 0.48~1.16 kg / t steel, and the total amount of diffusion deoxidizer is ≥1.09 kg / t steel. Preferably, the amount of molten steel per furnace is 155~165 t, the amount of aluminum briquettes is 0~50 kg / furnace, the amount of calcium carbide is 0~100 kg / furnace, and the amount of composite silicon carbide is 80~180 kg / furnace. For the LF furnace, the total amount of diffusion deoxidizer used is ≥180kg / furnace. It is preferred to add the diffusion deoxidizer in small amounts multiple times to improve the uniformity of deoxidation. In the early stage of deoxidation, calcium carbide and composite silicon carbide are used for deoxidation, while in the middle and later stages, oxygen composite silicon carbide is the main deoxidizer. In the initial stage of refining in the LF furnace, the aluminum content is adjusted to 0.040%~0.070%, and the LF basicity is controlled at 3.5~6. Before leaving the LF furnace, 20~50 meters of calcium wire is fed to treat the molten steel with calcium, and then a covering agent is added to protect the molten steel. The temperature of the molten steel leaving the station is 1600~1620℃.

[0059] The VD or RH vacuum treatment is used to send molten steel refined in the LF furnace to the VD or RH station for vacuum treatment, to control the morphology of inclusions and sulfides, and to homogenize the sulfides. Specifically: the vacuum degree is controlled below 67 Pa and maintained for 10-15 min, the bottom blowing argon flow rate is 30-50 NL / min with weak stirring, and the weak stirring time is 12-18 min. After breaking the vacuum, the composition is finely adjusted, S is added, and Ce is fed in. Aluminum is not added during VD or RH vacuum treatment, and Ca treatment is strictly prohibited to avoid the formation of high melting point CaS and large Al2O3 in the post-vacuum stage, to prevent subsequent nozzle blockage, to ensure the Ce recovery rate and its effect on improving the morphology of sulfides, to obtain spindle-shaped sulfides, the soft blowing time before leaving the station is 15-20 min, and the ladle temperature is controlled at 1550-1560℃.

[0060] The continuous casting process involves transferring molten steel, which has undergone VD or RH vacuum treatment, to the tundish of the continuous casting machine via a ladle. The molten steel is then used to produce billets through the continuous casting machine. The molten steel is protected during pouring to prevent secondary oxidation and reduce inclusions. The tundish uses a calcium covering agent, and carbonized rice husks on top of the calcium covering agent form a double layer of coverage. The continuous casting process strictly controls the stability of the liquid level in the continuous casting machine's crystallizer and tundish, controls the superheat to 20~35℃, and the casting speed to 0.80~0.95m / min. Electromagnetic stirring is used to reduce internal defects in the billet. The resulting billet is a 300mm thick continuous casting square or rectangular billet.

[0061] The rolling process involves heating the cast billet into a high-temperature steel billet in a heating furnace, and then rolling it through a rolling line. The rolling process uses a high-temperature diffusion process to homogenize the microstructure, a large reduction to refine the grains, and a slow cooling process to improve the microstructure of the material. Specifically, the heating furnace adopts a three-stage heating method during rolling: including a preheating stage, a first heating stage, a second heating stage, and a soaking stage. The second heating stage and the soaking stage are high-temperature stages. The temperature of the preheating stage is controlled below 600℃, the temperature of the high-temperature stage is 1200~1250℃, the heating time of the high-temperature stage is 80~150min, and the total heating time is 180~240min. The reduction rate of the rolling line is above 70%. After rolling, the billet undergoes slow cooling for ≥24h to obtain a high-cleanliness, free-machining, non-quenched and tempered steel product.

[0062] The chemical composition by mass percentage for each embodiment and comparative example is shown in Table 1 below, where Cu ≤ 0.10%, and the balance is Fe and other unavoidable impurities:

[0063] Table 1. Chemical composition of each example and comparative example

[0064]

[0065] Example 1 involves converter smelting and RH vacuum treatment; Example 2 involves converter smelting and VD vacuum treatment; Example 3 involves electric furnace smelting and VD vacuum treatment; Comparative Example 1 involves converter smelting and RH vacuum treatment, with no calcium treatment before refining in the LF furnace and no Ce feeding after void breaking; Comparative Example 2 involves converter smelting and VD vacuum treatment, with calcium treatment before refining in the LF furnace and no Ce feeding after void breaking; Comparative Example 3 involves electric furnace smelting and VD vacuum treatment, with no calcium treatment before refining in the LF furnace, and Ce feeding, calcium treatment, and aluminum replenishment after void breaking; The specific manufacturing method parameters for each example and comparative example are shown in Table 2 below:

[0066] Table 2. Manufacturing method parameters for each embodiment and comparative example

[0067]

[0068] The non-quenched and tempered steels manufactured according to the various embodiments and comparative examples were tested for tensile strength Rm, yield strength Rp0.2, and elongation after fracture A according to GB / T228.1 "Metallic materials, tensile testing - Part 1: Test method at room temperature". The inclusion levels were tested according to GB / T10561 "Determination of non-metallic inclusion content in steel - Standard rating chart microscopic inspection method", including Class A fine inclusions, Class A coarse inclusions, Class B fine inclusions, Class B coarse inclusions, and Class Ds inclusions. The number of hard inclusions per square millimeter of cross-section in each size range was tested. Each size range included <5μm, ≥5μm and <10μm, ≥10μm and <15μm, and ≥15μm and <30μm. The proportion of Class A sulfides and the morphology of sulfides were statistically analyzed based on microscopic observation. Each aspect ratio range included <3, 3~5, and >5.

[0069] Cutting tests were conducted on the non-quenched and tempered steels manufactured according to the various embodiments and comparative examples. The machinability was evaluated in the rough turning process of automobile crankshafts. The main test indicators and conditions are as follows: Test standard: GB / T16461 "Test of single-edge turning tool life" standard was followed; Workpiece: Crankshaft forging blank made of the non-quenched and tempered steel described in this invention; Equipment and tools: CNC crankshaft lathe was used, and the tool was a general grade CNMG120408 indexable carbide insert; Cutting parameters: Cutting speed V c =150~180m / min, feed rate f=0.2~0.3mm / r, depth of cut a p =2~3mm; Evaluation index: The number of crankshafts (pieces) processed when the tool flank wear reaches VB=0.3mm is taken as the tool life. At the same time, the frequency of chipping during the processing is recorded and the chip morphology is observed.

[0070] Based on the statistical analysis of the sprue during the continuous casting process, the performance test results of each embodiment and comparative example are shown in Table 3 below:

[0071] Table 3. Performance test results of different steels

[0072]

[0073] As can be seen from the results of Comparative Example 1, in the case of non-quenched and tempered steel containing neither Ca nor Ce, and where no calcium treatment is performed before the steel exits the LF furnace refining station in the preparation method, and no Ce treatment is fed in after the void is broken, such as Figure 2 As shown, the sulfide morphology in the steel is poor, exhibiting the elongated strip-shaped morphology characteristic of steel without Ca and Ce treatment, such as... Figure 9 As shown, sulfides with an aspect ratio ≤ 5 account for a small percentage of the total number of sulfides, resulting in the lowest machinability and relatively poor mechanical properties. Figure 6 As shown, the number of hard inclusions is significantly higher than in the embodiments of the present invention.

[0074] As can be seen from the results of Comparative Example 2, when the non-quenched and tempered steel contains Ca but not Ce, and calcium treatment is performed before the steel leaves the LF furnace for refining in the preparation method, and no Ce treatment is fed in after the void is broken, the mechanical properties of the non-quenched and tempered steel are better than those of Comparative Example 1, proving that Ca has a certain improving effect. However, it lacks the core role of Ce in sulfide nucleation, and the effect is far inferior to that of the present invention. Its cleanliness is between that of the present invention and Comparative Example 1.

[0075] As can be seen from the results of Comparative Example 3, in the case of non-quenched and tempered steel with excessive Ca content and no Ce, where the LF furnace refining process does not perform calcium treatment before exiting the station, but instead feeds Ce after void breaking and performs calcium treatment, and simultaneously performs aluminum supplementation, although Ce is added, the excessive Ca content and incorrect aluminum supplementation after void breaking in VD or RH vacuum treatment lead to the following: Figure 4 As shown, the level of Ds-type inclusions is significantly increased; as Figure 7 As shown, the number of hard inclusions surged, directly leading to severe clogging of the sprue and a significant decrease in plasticity, i.e., elongation; for example... Figure 10 As shown, the number of sulfides with an aspect ratio ≤ 5 accounts for a small percentage of the total number of sulfides; this, in turn, proves the necessity of strictly controlling the Ca content and "strictly prohibiting the addition of aluminum and calcium" in VD or RH vacuum treatment in order to achieve both high cleanliness and smooth production.

[0076] As can be seen from the comparison results of Examples 1-3 and Comparative Examples 1-3, the present invention, through optimized component design, especially the combined addition of Ca and Ce in a specific low content range, utilizes the synergistic effect of Ca-modified oxides and Ce refining sulfide nucleation, such as... Figure 1 As shown, the sulfide morphology is well controlled, exhibiting the typical morphology of spindle-shaped sulfides; as Figure 8 As shown, sulfides with an aspect ratio ≤ 5 account for more than 90% of the total number of sulfides; more importantly, as Figure 5 As shown, the number of hard inclusions with a size of <15μm on each square millimeter cross section does not exceed 600, which is consistent with the high cleanliness target set by this invention. This can avoid excessive hard inclusions leading to excessive surface roughness, and prevent hard inclusions with excessive size from causing tool breakage during cutting or becoming the starting point of fatigue cracks during service.

[0077] Depend on Figure 1 and Figure 2 As can be seen from the sulfide morphology comparison shown, this invention achieves precise control over the spindle-shaped morphology of sulfides in steel at the lowest cost. Figures 5-7 The comparison of inclusion size distribution shown quantitatively demonstrates the advantages of this invention in inclusion size distribution. Figures 8-10The comparison of the aspect ratio distribution of sulfides quantitatively demonstrates the advantages of the aspect ratio distribution of inclusions in this invention, showing that this invention simultaneously achieves precise control over the cleanliness of steel. As can be seen from the results of Examples 1 to 3, it can achieve a non-quenched and tempered steel with A-class fine inclusions ≤2.5 grade, no A-class coarse inclusions, B-class inclusions ≤0.5 grade, and Ds-class inclusions ≤0.5 grade, thus obtaining a non-quenched and tempered steel with high strength, high toughness, and excellent machinability.

[0078] Correspondingly, this manufacturing method employs a key process of feeding Ce after VD void breaking and strictly prohibits aluminum and calcium supplementation, fundamentally avoiding the formation of high-melting-point CaS and large Al2O3. Figure 3 and Figure 4 As shown in the comparison of Ds-type inclusions, the control effect of Ds-type inclusions is better; the results of Examples 1-3 show that the sprue condition is better, solving the industry problem of poor castability in continuous casting of high-purity aluminum-killed sulfur-containing steel, ensuring stable and efficient production; the obtained free-cutting non-quenched and tempered steel has improved strength, ductility and toughness compared with existing steel grades, and all mechanical properties meet or exceed the set targets, achieving tensile strength ≥1000MPa, yield strength ≥950MPa, and elongation after fracture ≥11%, while... Figure 11 The comparison of cutting states shown demonstrates that the presence of a certain number of spindle-shaped sulfide inclusions in the steel is beneficial for improving cutting performance. This allows the chips to break rapidly along the sulfide distribution direction, forming shorter chips and preventing long, thin sulfide inclusions from entangled in the tool. Figure 12 The comparison of the cutting tool photos after cutting non-quenched and tempered steel shows that it can avoid scratching the cutting edge of the tool, reduce the risk of micro-chipping of the tool, and significantly improve the cutting performance. As can be seen from the results of Examples 1 to 3, the chipping frequency is increased by more than 10 times, reaching more than 11,000 pieces / time. It has overcome the technical barrier of balancing high strength and easy machinability, and is particularly suitable for hot forged parts such as automobile crankshafts and rods that have high requirements for strength, toughness and machinability.

[0079] The detailed descriptions listed above are merely specific illustrations of feasible embodiments of the present invention and are not intended to limit the scope of protection of the present invention. All equivalent embodiments or modifications made without departing from the spirit of the present invention should be included within the scope of protection of the present invention.

Claims

1. A high-cleanliness free-cutting non-quenched and tempered steel, characterized in that, The chemical element mass percentage includes: C: 0.38%~0.41%, Si: 0.20%~0.30%, Mn: 0.60%~0.70%, P≤0.018%, S: 0.020%~0.028%, Cr: 0.15%~0.25%, V: 0.04%~0.05%, N: 0.007%~0.009%, Al: 0.02%~0.04%, Cu≤0.10%, Ca: 0.0003~0.0015%, Ce: 0.001~0.002%, the balance being Fe and other inevitable impurities; the sulfide of the non-quenched and tempered steel is spindle-shaped, the number of sulfides with a length-width ratio ≤5 accounts for more than 90% of the total number of sulfides, and the number of hard inclusions with a size <15 μm per square millimeter of cross section is not more than 600.

2. The high-cleanliness free-cutting non-quenched and tempered steel according to claim 1, characterized in that, The non-quenched and tempered steel has ≤2.5 grade A type fine inclusions, no A type coarse inclusions, ≤0.5 grade B type inclusions, and ≤0.5 grade Ds type inclusions; the tensile strength of the non-quenched and tempered steel is ≥1000 MPa, the yield strength is ≥950 MPa, and the elongation after fracture is ≥11%.

3. A method of manufacturing a high-cleanliness free-cutting non-quenched and tempered steel, characterized in that, The manufacturing method for manufacturing the high-cleanliness free-cutting non-quenched and tempered steel of claim 1 comprises the following steps performed in sequence: converter or electric furnace smelting, LF furnace refining, VD or RH vacuum treatment, continuous casting, and rolling; the LF furnace refining feeds in a calcium wire before the outstation to perform calcium treatment on the molten steel, and the VD or RH vacuum treatment feeds in Ce after breaking the vacuum and supplements S, the VD or RH vacuum treatment does not supplement aluminum, and Ca treatment is strictly prohibited.

4. The method of claim 3, wherein the high-cleanliness free-machining non-quenched and tempered steel is manufactured by the steps of: The converter or electric furnace smelting adds aluminum ingots, carbon powder, lime, and a clean promoting agent when tapping; the total addition amount of the aluminum ingots is 0.48~1.68 kg / t of steel, the addition amount of the carbon powder is 0.3~0.7 kg / t of steel, the addition amount of the lime is 2~4 kg / t of steel, and the addition amount of the clean promoting agent is 5~7 kg / t of steel. ​ 5. The method of claim 4, wherein the high-cleanliness free-machining non-quenched and tempered steel is manufactured by the steps of: The converter or electric furnace smelting divides the aluminum ingots into not less than 3 batches and adds them to the impact area of the molten steel when tapping, controls the end-point carbon content ≤0.20%, the tapping target temperature ≥1600℃, and the tapping amount of each furnace is 155~165 t; when the end-point carbon content <0.06%, the total addition amount of the aluminum ingots is 200~260 kg / furnace; when 0.06%≤the end-point carbon content≤0.08%, the total addition amount of the aluminum ingots is 150~195 kg / furnace; and when the end-point carbon content >0.08%, the total addition amount of the aluminum ingots is 80~145 kg / furnace. ​ 6. The method of claim 3, wherein the high-cleanliness free-machining non-quenched and tempered steel is manufactured by the steps of: After the LF furnace refining submerged arc is stabilized, lime and fluorite are added to adjust the flowability of the slag, the lime addition amount is ≤1.94 kg / t steel, the fluorite addition amount is 0.03-0.06 kg / t steel; the diffusion deoxidizer is put in during the power transmission stage, the diffusion deoxidizer includes one or more of aluminum beans, carbide, and composite silicon carbide, the aluminum bean dosage is ≤0.32 kg / t steel, the carbide dosage is ≤0.65 kg / t steel, the composite silicon carbide dosage is 0.48-1.16 kg / t steel, and the total diffusion deoxidizer dosage is ≥1.09 kg / t steel; the aluminum content is adjusted to 0.040%-0.070% at the beginning of refining, and the LF basicity is controlled at 3.5-6; 20-50 meters of calcium wire is fed before the station, and then a covering agent is added to protect the molten steel, and the station molten steel temperature is 1600-1620℃. ​ 7. The method of claim 6, wherein the high-cleanliness free-machining non-quenched and tempered steel is manufactured by the steps of: After the LF furnace refining submerged arc is stabilized, the molten steel amount per furnace is 155-165 t, when the carbon content is <0.06%, the lime addition amount is 150-300 kg / furnace; when 0.06%≤carbon content≤0.08%, the lime addition amount is 60-145 kg / furnace; when the carbon content is >0.08%, the lime addition amount is 0-50 kg / furnace. ​ 8. The method of claim 3, wherein the high-cleanliness free-machining non-quenched and tempered steel is manufactured by the steps of: The VD or RH vacuum treatment controls the vacuum degree to be below 67 Pa for 10-15 min, the weak stirring bottom argon flow is 30-50 NL / min, the weak stirring time is 12-18 min, the soft blowing time before the station is 15-20 min, and the ladle temperature is controlled at 1550-1560℃. ​ 9. The method of claim 3, wherein the high-cleanliness free-machining non-quenched and tempered steel is manufactured by the steps of: The tundish is covered with calcium covering agent and carbonized rice husk double-layer covering during continuous casting, the superheat is controlled at 20-35℃, and the casting speed is 0.80-0.95 m / min. ​ 10. The method of producing a high-cleanliness free-machining non-quenched and tempered steel according to any one of claims 3 to 9, characterized in that, The preheating section temperature is below 600℃ during rolling, the high-temperature section temperature is 1200-1250℃, the high-temperature section heating time is 80-150 min, the total heating time is 180-240 min, the reduction is above 70%, and the slow cooling time after rolling is ≥24 h.

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