A high-purity, high-strength, free-machining steel for commercial vehicle steering worm gears and its manufacturing method.

Abstract

This invention belongs to the field of alloy steel smelting technology, specifically disclosing a high-purity, high-strength, and easily machinable steel for commercial vehicle steering worm gears and its manufacturing method. The steel for commercial vehicle steering worm gears is composed of the following chemical composition by mass percentage: C: 0.61%~0.63%, Si: 0.20%~0.30%, Mn: 0.85%~0.90%, P≤0.020%, S: 0.025%~0.035%, Cr: 0.30%~0.35%, Cu≤0.15%, Ni≤0.30%, Mo≤0.05%, Al: 0.020%~0.035%, [H]≤0.00015%, [O]≤0.0012%, with the balance being Fe. The manufacturing method sequentially includes raw material selection, converter smelting, refining, RH vacuum degassing, inclusion modification and soft blowing, continuous casting, heating, rolling, and finishing. This invention features a rationally designed composition and advanced manufacturing process, resulting in higher strength, higher purity, and superior machinability compared to the 55 steel currently available for worm gears on the market. The application of this steel grade can effectively improve the strength, surface finish, and fatigue life of steering worm gears in commercial vehicles; this invention can significantly improve product quality and enhance market competitiveness.

Description

Technical Field

[0001] This invention belongs to the field of alloy steel smelting technology, and relates to the manufacture of worm gear steel, specifically to a high-purity, high-strength, free-machining commercial vehicle steering worm gear steel and its manufacturing method. Background Technology

[0002] The steering worm gear is an important component in automotive steering systems. A worm is a rod-shaped part with a pitch along its axis, which, in conjunction with a worm wheel via gears, forms a steering reduction mechanism, thereby enabling the vehicle's steering function. It is an important safety component. The quality of the worm gear determines the effectiveness of the vehicle's steering function.

[0003] Currently, domestically manufactured automotive steering worm gears typically use 55 steel. After heat treatment, 55 steel achieves a strength of 700-750 MPa, which is insufficient to meet the demands of the international market. Although domestic manufacturing processes for 55 steel have improved, the increased strength has resulted in poor machinability, higher processing difficulty, and a need to improve steel cleanliness. Existing domestic 55 steel can no longer meet the needs of modern automotive steering worm gears. There is an urgent need to develop a new, high-cleanliness, high-strength, and easily machinable steel for commercial vehicle steering worm gears with independent intellectual property rights, utilizing microalloying elements to improve steel performance.

[0004] The national standard "High-Quality Carbon Structural Steel" (GB / T 699-2015) specifies the following for 55 steel: C: 0.52%~0.60%, Si: 0.17%~0.37%, Mn: 0.50%~0.80%, P≤0.035%, S≤0.035%, Cu≤0.25%, Cr≤0.25%, Ni≤0.30%. General porosity, central porosity, ingot segregation, and central segregation should be ≤2.5 grade. Normalized properties: Rm≥645MPa, ReL≥380MPa; hot-rolled hardness≤255HBW. Non-metallic inclusions: Class A ≤2.5 grade, Class B ≤2.5 grade, Class C ≤2.5 grade, Class D ≤2.5 grade. Currently, the hot-rolled properties of 55 steel used in automotive steering worm gears are: Rm: 680MPa~750MPa, ReL: 400MPa~430MPa; hot-rolled hardness 210HBW~230HBW. Non-metallic inclusions are ≤2.5 for categories A, B, C, D, and Ds. However, this steel has low strength, makes internal through-hole drilling difficult, results in poor machined surface quality, and has low purity, affecting its safety in use. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a high-purity, high-strength, and easily machinable steel for steering worm gears in commercial vehicles. This steel exhibits higher machinability, strength, purity, and safety than the 55 steel specified in the "High-Quality Carbon Structural Steel" (GB / T 699-2015) standard. Another objective of the present invention is to provide a method for manufacturing this steering worm gear steel.

[0006] This invention is achieved through the following technical solution: A high-purity, high-strength, and easily machinable steel for commercial vehicle steering worm gears, comprising the following chemical composition by mass percentage: C: 0.61%~0.63%, Si: 0.20%~0.30%, Mn: 0.85%~0.90%, P≤0.020%, S: 0.025%~0.035%, Cr: 0.30%~0.35%, Cu≤0.15%, Ni≤0.30%, Mo≤0.05%, Al: 0.020%~0.035%, [H]≤0.00015%, [O]≤0.0012%, with the balance being Fe.

[0007] The rationale for limiting the chemical composition of the high-purity, high-strength, and easily machinable steering worm gear steel for commercial vehicles in this invention is as follows: C is the most effective element for improving the strength and hardenability of steel. However, as the C content increases, the plasticity and impact toughness of steel decrease significantly. C also increases the cold brittleness and aging sensitivity of steel. In order to improve the strength of steel for automotive worm gears, this invention increases the C content by 0.07% based on the existing 55 steel, and limits the C content to between 0.61% and 0.63%. Si acts as a reducing agent and deoxidizer in steel. Killed steel generally contains more than 0.17% Si. Si can dissolve in ferrite and austenite to improve the strength and hardness of steel. The addition of 0.20% to 0.30% Si in this invention mainly serves to deoxidize and improve yield strength. Mn can improve the strength, hardness, and hardenability of steel. Mn also has a solid solution strengthening effect, which can expand the austenite region, lower the transformation temperature of austenite to ferrite, and thus refine the ferrite grains, improving the strength and toughness of the steel. However, an Mn content of 0.90% easily leads to segregation, increasing the temper brittleness of the steel. Therefore, the Mn content is increased to 0.85%-0.90% in 55 steel. Phosphorus (P) is generally considered a harmful element in steel. P causes cold brittleness, reduces toughness, worsens weldability, and decreases ductility. Therefore, this invention strictly controls the P content to ≤0.020%. Sulfur (S) is a segregating element that reduces the ductility and toughness of steel, and also has an adverse effect on weldability and corrosion resistance. However, the steel grade of this invention requires deep hole machining, which demands very high machinability. Therefore, it is necessary to add more than 0.025% S to improve the machinability of the steel. Thus, the S content in this invention is strictly controlled: S: 0.025%-0.035%. Cr can significantly improve the strength, hardness, wear resistance, and hardenability of steel. However, excessive Cr content will increase the brittle transition temperature and temper brittleness of steel. This invention aims to improve the strength and surface hardening hardness of steel by controlling the Cr content between 0.30% and 0.35%. Cu is generally considered a harmful element in steel. When the Cu content exceeds 0.20% and the steel temperature exceeds 1100℃, oxidation and decarburization occur on the steel surface. This decarburization layer tends to accumulate at the grain boundaries, leading to copper embrittlement and surface cracking, which severely affects the quality of the steel. Therefore, this invention controls the Cu content in the smelting process to ≤0.15%.

[0008] Ni can improve the strength of steel while maintaining good plasticity and toughness. It can also lower the brittle transition temperature of steel and improve its low-temperature impact toughness. However, because nickel is a scarce resource and expensive, the Ni content in this steel grade will be controlled to ≤0.30%.

[0009] Mo is a highly hardenable element that has a solid solution strengthening effect on ferrite and can significantly improve the hardenability of steel. However, Mo is a precious metal, and this invention does not specifically add Mo, and controls the residual Mo to be ≤0.05%.

[0010] Al is a strong reducing element and is often used as a deoxidizer in steel. When the Al content is greater than 0.020%, it significantly refines the grain size of the steel as the Al content increases. However, excessively high Al content promotes graphitization in steel, reducing its high-temperature strength and toughness. To ensure the strength and toughness of the steel used in automotive worm gears, this invention limits the Al content to 0.020%~0.035%.

[0011] Excessive hydrogen content can cause hydrogen embrittlement in steel, resulting in white spots when the stress is below the steel's ultimate limit, which seriously affects the fatigue life of the steel. Therefore, this invention limits H to ≤ 0.00015%.

[0012] O (O) has little effect on the strength of steel at room temperature, but it significantly reduces the elongation and reduction of area. At lower temperatures and extremely low O content, the strength and plasticity of the material decrease sharply with increasing O content. Regarding impact performance, the maximum impact value gradually decreases with increasing O content, while the brittle transition temperature rises rapidly, and the range of the brittle transition temperature widens. Simultaneously, with increasing O content, the probability of oxide inclusions in the material increases significantly, thereby reducing the fatigue life of the material. This invention and its production process can stably control the O content below 0.0012%.

[0013] A further improvement to the present invention is as follows: A method for manufacturing high-purity, high-strength, and easily machinable steel for commercial vehicle steering worm gears, the method comprising, in sequence, raw material selection, converter smelting, refining, RH vacuum degassing, inclusion modification and soft blowing, continuous casting, heating, rolling, and finishing.

[0014] Furthermore, when selecting raw materials, molten iron with an S content of 0.015% to 0.035% is used for smelting, and the residual S in the molten iron is used to adjust the S element.

[0015] Furthermore, during the converter smelting process, the blowing is carried out in a top-and-bottom blown converter with a capacity of 90 tons or more, using 85% molten iron and 15% scrap steel as raw materials for initial refining; after 10-20 tons of steel are tapped, 90-110 kg of aluminum cake is added for strong deoxidation, followed by the addition of a low-nitrogen carbon raiser; after 30 tons of steel are tapped, 540 kg of ferromanganese, 270 kg of low-titanium high-carbon ferrochrome, 400 kg of ferromanganese, and 180 kg of ferrosilicon are added for initial composition adjustment, followed by the addition of synthetic refining slag, quartz sand, and lime; at the end of the tapping process, a sliding plate is used to block slag to ensure slag-free tapping and prevent molten steel from returning to phosphorus.

[0016] Furthermore, during the refining process, deoxidation, sulfur control, and alloying of molten steel are carried out in an LF furnace of 100t or more, with 150kg of lime added to adjust the slag; immediately after refining and slag formation, a small amount of low-titanium, high-carbon ferrochrome, ferromanganese, and ferrosilicon are added for alloy fine-tuning to ensure the second refining process; the refining time is controlled to be more than 50 minutes, with weak stirring throughout the refining process, deoxidizing and removing inclusions through long-term steel slag reaction, while preventing secondary oxidation of molten steel; the sulfur content in the refining ladle is controlled to be 0.023%~0.028%, and the feeding of sulfur into the refining line is strictly prohibited in the later stages of refining.

[0017] Furthermore, during the RH vacuum degassing process, the vacuum degassing treatment is carried out in a 100t dual-station RH furnace, and the vacuum degree is maintained below 80pa for ≥28min, while controlling the molten steel void H≤1.3ppm.

[0018] Furthermore, during the inclusion denaturation and soft blowing process, after RH vacuum degassing, a 50m~80m silicon-calcium wire is fed in immediately to denature the inclusions, while soft blowing is performed simultaneously for 20~35 minutes to ensure that the inclusions are fully floated and removed.

[0019] Furthermore, during continuous casting, a 210mm×210mm billet continuous casting machine is used for casting. Low-silicon tundish covering agent and high-carbon steel special crystallizer protective slag are used. An automatic baking and automatic addition device for the protective slag is adopted to ensure uniform and timely addition. The continuous casting of billets is carried out with full protection throughout the entire process. A two-stage electromagnetic stirring device of M-EMS+F-EMS is used to fully and uniformly organize the structure and improve the internal quality of the billet. A displacement mode of 2mm, 3mm, 3mm, and 2mm is used for light pressing to improve the internal quality, reduce central porosity, and reduce segregation.

[0020] Furthermore, during the heating process, after the cutting marks are removed from the continuously cast billet, a regenerative walking beam gas heater is used for heating. The billet heating adopts a fully automatic intelligent combustion system, with a heating temperature deviation of no more than ±5℃. The furnace gas temperature in the second heating stage is controlled at 1090℃~1120℃, the furnace gas temperature in the soaking stage is controlled at 1120℃~1150℃, and the heating time is controlled at 110min-130min.

[0021] Furthermore, during the rolling process, high-temperature rolling is adopted, with the initial rolling temperature controlled at 1020℃~1040℃, and rolling is carried out using an 18-stand continuous rolling mill + 4 heavy KOCKS rolling mills; And / or, during finishing, a process of straightening + shot blasting + mechanical chamfering + ultrasonic flaw detection + magnetic flux leakage flaw detection + steel grade identification using a steel sorting instrument is used to inspect and eliminate defects one by one to ensure the internal and external quality of the steel.

[0022] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention uses alloying elements such as C, Si, Mn and Cr for alloying, adds 0.020%~0.035% Al for microalloying, increases the C content to 0.61%~0.63%, the Mn content to 0.85%~0.90%, and adds 0.30%~0.35% Cr element, so that it has higher strength than 55 steel in the standard of "High-quality carbon structural steel" (GB / T 699-2015).

[0023] (2) The present invention controls the S content of steel to 0.025%-0.035% to improve the machinability of steel. It selects molten iron with an S content of 0.015%~0.035% for direct smelting. It uses the original S element in the molten iron and the S element brought in from the raw materials to adjust the S content of the finished product, controls the refining and desaturation, and then carries out sulfide modification to ensure that the sulfides are fine and dispersed, thereby improving the machinability of steel.

[0024] (3) The blowing process is carried out in a top and bottom blowing converter of 90t or above, with 85% molten iron and 15% scrap steel as raw materials for primary refining; 90kg to 110kg aluminum cake is added for strong deoxidation after 10t to 20t of steel tapping, followed by the addition of low nitrogen carbonizing agent; after 30 tons of steel tapping, 540kg silicon manganese, 270kg low titanium high carbon ferrochrome, 400kg ferromanganese and 180kg silicon ferrosilicon are added for initial composition adjustment, followed by the addition of synthetic refining slag, quartz sand and lime; at the end of the tapping period, a sliding plate is used to block slag to ensure slag-free steel tapping and avoid molten steel returning to P.

[0025] (4) The refining time is controlled at more than 50 minutes. The refining process is weakly stirred throughout. The steel slag reaction is deoxidized and impurities are removed through long-term reaction, while preventing secondary oxidation of molten steel.

[0026] (5) After RH vacuum degassing, feed in 50m~80m silicon-calcium wire immediately to denature the inclusions, and at the same time perform soft blowing. The soft blowing time is kept at 20~35 minutes to ensure that the inclusions are fully floated and removed.

[0027] (6) A 210mm×210mm billet continuous casting machine is used for casting. Low-silicon tundish covering agent and high-carbon steel special crystallizer protective slag are used. Automatic baking and automatic addition device of protective slag is used to ensure uniform and timely addition. Full protection casting is implemented to produce continuous casting billets. M-EMS+F-EMS two-stage electromagnetic stirring device is used to fully and uniformly organize and improve the internal quality of the billet. Light pressure is carried out in displacement mode of 2mm, 3mm, 3mm, 2mm to improve the internal quality, reduce central porosity and reduce segregation. (7) Heating: After the cutting head is removed from the continuous casting billet, it is heated by a regenerative walking gas heating furnace. The billet heating adopts a fully automatic intelligent combustion system. The heating temperature deviation does not exceed ±5℃. The furnace gas temperature in the second heating stage is controlled at 1090℃~1120℃, the furnace gas temperature in the soaking stage is controlled at 1120℃~1150℃, and the heating time is controlled at 110min-130min.

[0028] (8) High temperature rolling is adopted, with the initial rolling temperature controlled at 1020℃~1040℃, and rolling is carried out using 18 continuous rolling mills + 4 heavy KOCKS rolling mills.

[0029] (9) The steel is inspected and defect-removed one by one using a process of straightening + shot blasting + mechanical chamfering + ultrasonic testing + magnetic flux leakage testing + steel grade identification by steel sorting instrument to ensure the internal and external quality of the steel.

[0030] (10) The composition of this invention is reasonably designed and the process is advanced. Compared with the 55 steel for worm gears currently supplied on the market, it has higher strength, higher purity and better machinability. Class D inclusions have been stably controlled at ≤1.0 grade, Class B inclusions have been stably controlled at ≤0.5 grade, and Class C and Ds inclusions have not been detected. The process control level of this invention has reached the international advanced level. General porosity, central porosity, ingot segregation, and central segregation are ≤1.0 grade. Hot-rolled properties: Rm: 870-900MPa, Rel: 450-510MPa; hot-rolled hardness 245~255HBW, non-metallic inclusions A coarse ≤1.5 grade, A fine ≤2.5 grade, B coarse ≤0.5 grade, B fine ≤0.5 grade, C coarse ≤0.5 grade, C fine ≤0.5 grade, D coarse ≤0.5 grade, D fine ≤1.0 grade, Ds ≤0.5 grade. After quenching, the surface hardness is 64-66 HRC, and the tool life for internal through holes exceeds 20,000 cuts. The application of this steel grade can effectively improve the strength, surface finish, and fatigue life of steering worm gears in commercial vehicles; this invention can significantly improve product quality and enhance market competitiveness. Detailed Implementation

[0031] The present invention will now be described in detail with reference to specific embodiments.

[0032] Examples 1-3 The chemical composition comparison between the 55 steel currently used in China and that of this invention is shown in Table 1 below.

[0033] Table 1 Comparison of Chemical Composition (wt%) chemical composition C Si Mn P S Cr Cu Ni Al [H] [O] Mo Existing 55 steel 0.52-0.60 0.17-0.37 0.50-0.80 ≤0.035 ≤0.035 ≤0.25 ≤0.25 ≤0.30 - - - - This invention 0.61-0.63 0.20-0.306 0.85-0.90 ≤0.020 0.025-0.035 0.30-0.35 ≤0.15 ≤0.30 0.020-0.035 ≤0.00015 ≤0.0012 ≤0.05 This invention is prepared using the following production process: (1) Raw material selection: Use molten iron with an S content of 0.015%~0.035% for smelting, and use the residual S in the molten iron to adjust the S element.

[0034] (2) Converter smelting: The smelting is carried out in a top and bottom blown converter of 90t or above, with 85% molten iron + 15% scrap steel as raw materials for primary smelting; 90kg~110kg aluminum cake is added for strong deoxidation after 10t~20t of steel tapping, followed by the addition of low nitrogen carbon raiser. After 30 tons of steel tapping, 540kg silicon manganese, 270kg low titanium high carbon ferrochrome, 400kg ferromanganese, and 180kg silicon ferrosilicon are added for initial composition adjustment, followed by the addition of synthetic refining slag, quartz sand, and lime; at the end of the tapping period, a sliding plate is used to block slag to ensure slag-free tapping and avoid molten steel returning to P; (3) Refining: Deoxidize, control sulfur and alloy the molten steel in an LF furnace of 100t or more, add 150kg of lime to adjust the slag; immediately after refining the slag, add a small amount of low titanium high carbon ferrochrome, ferromanganese and ferrosilicon for alloy fine adjustment to ensure the second refining; control the refining time to more than 50min, and weakly stir throughout the refining process to deoxidize and remove inclusions through long-term steel slag reaction, while preventing secondary oxidation of molten steel; control the S content of the refining ladle to 0.023%~0.028%, and strictly prohibit feeding sulfur line in the later stage of refining; (4) RH vacuum degassing: Vacuum degassing is carried out in a 100t dual-station RH furnace, and the vacuum degree is kept below 80pa for ≥28min, and the vacuum level of molten steel is controlled to be ≤1.3ppm; (5) Inclusion denaturation and soft blowing: After RH vacuum degassing, feed in 50m~80m silicon-calcium wire immediately to denature the inclusions, and at the same time perform soft blowing. The soft blowing time is maintained at 20~35 minutes to ensure that the inclusions are fully floated and removed. (6) Continuous casting: A 210mm×210mm billet continuous casting machine is used for casting. Low-silicon tundish covering agent and high-carbon steel special crystallizer protective slag are used. Automatic baking and automatic addition device of protective slag is used to ensure uniform and timely addition. Full protection casting is implemented to produce continuous casting billets. M-EMS+F-EMS two-stage electromagnetic stirring device is used to fully and uniformly organize and improve the internal quality of the billet. Light pressure is carried out in displacement mode of 2mm, 3mm, 3mm, 2mm to improve the internal quality, reduce central porosity and reduce segregation. (7) Heating: After the cutting head is removed from the continuous casting billet, it is heated by a regenerative walking gas heating furnace. The billet heating adopts a fully automatic intelligent combustion system. The heating temperature deviation does not exceed ±5℃. The furnace gas temperature in the second heating stage is controlled at 1090℃~1120℃, the furnace gas temperature in the soaking stage is controlled at 1120℃~1150℃, and the heating time is controlled at 110min-130min.

[0035] (8) Rolling: High temperature rolling is adopted, with the initial rolling temperature controlled at 1020℃~1040℃. Rolling is carried out using 18 continuous rolling mills + 4 heavy KOCKS rolling mills.

[0036] (9) Finishing: The steel is inspected and defect-removed one by one using a process of straightening + shot blasting + mechanical chamfering + ultrasonic testing + magnetic flux leakage testing + steel grade identification by steel sorting instrument to ensure the internal and external quality of the steel.

[0037] For any process conditions not specified in the above preparation methods, please refer to conventional techniques in this field.

[0038] The chemical composition of the obtained steel for the worm gear of commercial vehicle steering gear is shown in Table 2. The gas content, mechanical properties, surface hardening hardness, low magnification structure and non-metallic inclusions are compared with the prior art in Tables 3, 4, 5 and 6.

[0039] Table 2. Steel composition (wt%) for worm gears

[0040] Table 3. Comparison of gas content of the present invention with that of the prior art.

[0041] Table 4 Comparison of the mechanical properties and surface hardening hardness of the present invention with those of existing technologies.

[0042] Table 5. Comparison of low-magnification tissue samples of the present invention with those of existing technologies.

[0043] Table 6 Comparison of the non-metallic inclusions of the present invention with existing technologies

[0044] In this invention, the D-type inclusions have been stably controlled at ≤1.0 level, the B-type inclusions have been stably controlled at 0 level, and the C-type and Ds inclusions were not detected. The process control level of this invention has reached the international advanced level.

[0045] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent transformations or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A high cleanliness, high strength, free-cutting steel for commercial vehicle steering gear worm, characterized in that, It is composed of the following chemical components by mass percentage: C: 0.61%~0.63%, Si: 0.20%~0.30%, Mn: 0.85%~0.90%, P≤0.020%, S: 0.025%~0.029%, Cr: 0.30%~0.35%, Cu≤0.15%, Ni≤0.30%, Mo≤0.05%, Al: 0.023%~0.035%, [H]≤0.00015%, [O]≤0.0012%, with the balance being Fe; The steel has the following properties: general porosity, central porosity, ingot segregation, and central segregation ≤ 1.0 grade; hot-rolled properties: Rm: 870-900MPa, Rel: 450-510MPa, hot-rolled hardness 245~255HBW, surface hardness after quenching 64~66HRC; non-metallic inclusions: A coarse ≤ 1.5 grade, A fine ≤ 2.5 grade, B coarse ≤ 0.5 grade, B fine ≤ 0.5 grade, D coarse ≤ 0.5 grade, D fine ≤ 1.0 grade, and C-type and Ds inclusions were not detected.

2. The method of manufacturing a high cleanliness, high strength, free-cutting commercial vehicle steering gear worm steel of claim 1, wherein, The manufacturing method includes, in sequence, raw material selection, converter smelting, refining, RH vacuum degassing, inclusion modification and soft blowing, continuous casting, heating, rolling and finishing; During the refining process, deoxidation, sulfur control, and alloying of molten steel are carried out in an LF furnace with a capacity of 100t or more, with 150kg of lime added to adjust the slag. Immediately after refining and slag formation, a small amount of low-titanium, high-carbon ferrochrome, ferromanganese, and ferrosilicon are added for alloy fine-tuning to ensure the second refining process is successful. The refining time is controlled to be more than 50 minutes, with weak stirring throughout the refining process. Deoxidation and inclusion removal are achieved through long-term steel slag reaction, while preventing secondary oxidation of the molten steel. The sulfur content in the refining ladle is controlled to be 0.023%~0.028%, and the feeding of sulfur into the ladle is strictly prohibited in the later stages of refining. During the inclusion denaturation and soft blowing process, after RH vacuum degassing, a 50m~80m silicon-calcium wire is fed in immediately to denature the inclusions, while soft blowing is performed simultaneously. The soft blowing time is maintained at 20~35 minutes to ensure that the inclusions are fully floated and removed. During continuous casting, a 210mm×210mm billet continuous casting machine is used for casting. Low-silicon tundish covering agent and high-carbon steel special crystallizer protective slag are used. An automatic baking and automatic addition device for the protective slag is used to ensure uniform and timely addition. The continuous casting of billets is carried out with full protection throughout the entire process. A two-stage electromagnetic stirring device of M-EMS+F-EMS is used to fully and uniformly organize the structure and improve the internal quality of the billet. A displacement mode of 2mm, 3mm, 3mm, 2mm is used for light pressing to improve the internal quality, reduce central porosity, and reduce segregation. During the heating process, after the cutting marks are removed from the continuously cast billet, a regenerative walking beam gas heater is used for heating. The billet heating adopts a fully automatic intelligent combustion system, and the heating temperature deviation does not exceed ±5℃. The furnace gas temperature in the second heating stage is controlled at 1090℃~1120℃, the furnace gas temperature in the soaking stage is controlled at 1120℃~1150℃, and the heating time is controlled at 110min-130min. During the rolling process, high-temperature rolling is adopted, with the initial rolling temperature controlled at 1020℃~1040℃, and rolling is carried out using an 18-stand continuous rolling mill + 4 heavy KOCKS rolling mills. During the finishing process, a series of steps are employed, including straightening, shot blasting, mechanical chamfering, ultrasonic testing, magnetic flux leakage testing, and steel grade identification using a steel sorting instrument, to inspect and eliminate defects in each piece, ensuring the internal and external quality of the steel.

3. A method of manufacturing a high cleanliness, high strength, free-cutting commercial vehicle steering gear worm steel according to claim 2, characterized in that: When selecting raw materials, molten iron with an S content of 0.015% to 0.035% is used for smelting.

4. A method of manufacturing a high cleanliness, high strength, free-cutting commercial vehicle steering gear worm steel as claimed in claim 2, characterized in that: During the converter smelting process, the blowing is carried out in a top-and-bottom blown converter with a capacity of 90 tons or more. The initial smelting is carried out using 85% molten iron and 15% scrap steel as raw materials. After tapping 10-20 tons of steel, 90-110 kg of aluminum cake is added for strong deoxidation, followed by the addition of a low-nitrogen carbon raiser. After tapping 30 tons of steel, 540 kg of ferromanganese silicon, 270 kg of low-titanium high-carbon ferrochrome, 400 kg of ferromanganese, and 180 kg of ferrosilicon are added for initial composition adjustment. Then, synthetic refining slag, quartz sand, and lime are added in sequence. At the end of the tapping process, a sliding plate is used to block slag to ensure slag-free tapping and prevent the molten steel from returning to phosphorus.

5. A method of manufacturing a high cleanliness, high strength, free machining commercial vehicle steering gear worm steel according to claim 2, characterized in that: During the RH vacuum degassing process, the vacuum degassing treatment is carried out in a 100t dual-station RH furnace, and the vacuum degree is maintained below 80pa for ≥28min, while controlling the molten steel void H≤1.3ppm.

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