A smelting process for improving the strength and stability of non-heat-treated steel

Abstract

This invention relates to the field of iron and steel metallurgy technology and discloses a smelting process for improving the strength stability of non-adjusted steel. The process includes step 1: converter smelting process, step 2: LF refining process, step 3: RH vacuum treatment process, step 4: continuous casting process, step 5: billet slow cooling and surface finishing process, and step 6: smelting process traceability and control. This smelting process for improving the strength stability of non-adjusted steel allows for precise control of key components and a significant reduction in strength fluctuations. Through precise control of the entire process in the converter and LF furnace, the core elements C, Si, Mn, Cr, S, and N are controlled within a narrow range, with a composition deviation of ≤±0.02%. This eliminates the strength differences caused by composition fluctuations at the source, and the tensile strength fluctuation of a single batch is stably controlled within ±15MPa.

Description

Technical Field

[0001] This invention relates to the field of iron and steel metallurgy technology, specifically to a smelting process method for improving the strength and stability of non-adjusted steel. Background Technology

[0002] Non-quenched and tempered steel is widely used in the manufacture of high-end parts such as automobile crankshafts and key structural components of engineering machinery. These parts have extremely high requirements for the strength stability of the steel. The industry standard requires that the tensile strength fluctuation of a single batch be ≤±20MPa and the strength difference between batches be ≤15MPa in order to ensure the uniformity of forging and the safety of long-term service.

[0003] In actual production, traditional non-quenched and tempered steel smelting processes suffer from core problems such as a lack of precise control over composition throughout the entire process and insufficient control over the purity of molten steel: coarse control of the converter's final composition and insufficient precision in the fine-tuning of alloys in the LF furnace lead to excessive fluctuations in key strengthening elements such as C, Mn, S, and N; insufficient RH vacuum degassing and inadequate control of continuous casting protection and homogenization result in high levels of residual gas and inclusions in the steel, leading to compositional segregation and uneven microstructure in the cast billet. Industry test data shows that non-quenched and tempered steel produced using traditional processes exhibits tensile strength fluctuations of ±35~50MPa, far exceeding the standard allowable range. This directly causes uneven hardness and poor fatigue performance in forgings, and even results in batch scrapping, becoming a key technical bottleneck restricting the high-end application of non-quenched and tempered steel. Summary of the Invention

[0004] (a) Technical problems to be solved To address the shortcomings of existing technologies, this invention provides a smelting process method for improving the strength stability of non-adjusted steel, which has the advantages of precise control of key components and significant narrowing of strength fluctuations, and solves the problems of lack of precise component control and insufficient control of molten steel purity.

[0005] (II) Technical Solution To achieve precise control of the aforementioned key components and significantly narrow the strength fluctuation, this invention provides the following technical solution: a smelting process method for improving the strength stability of non-adjusted steel, comprising step 1: converter smelting process, step 2: LF refining process, step 3: RH vacuum treatment process, step 4: continuous casting process, step 5: billet slow cooling and surface finishing process, and step 6: smelting process traceability control. The converter smelting process includes step 101: raw material screening and furnace feeding control, step 102: precise control of dephosphorization and decarburization, and step 103: slag blocking and steel tapping operation. Among them, step 2: LF refining process includes step 201: slag formation, desulfurization and slag system regulation, step 202: fine adjustment of key alloy components and step 203: calcium treatment and inclusion modification; Among them, step 3: RH vacuum treatment process includes step 301: high vacuum deep degassing, step 302: vacuum soft blowing and settling, and step 303: final composition verification; Step 4: Continuous casting process includes step 401: full-process protective pouring, step 402: electromagnetic stirring and light pressure control, and step 403: constant speed billet pulling and weak cooling water distribution; Step 5, the process of slow cooling and surface finishing of the billet, includes step 501: slow cooling treatment of the billet, step 502: peeling and finishing of the billet, and step 503: non-destructive testing of the billet.

[0006] Preferably, step 101 involves raw material screening and furnace feeding control. Operation process: Select high-purity pig iron with low phosphorus and low residual elements and clean scrap steel, mix them in proportion and put them into the furnace, and prevent high impurity and high copper and nickel scrap steel from being mixed in; test the phosphorus content of the molten iron before it is put into the furnace, and unqualified raw materials are refining other steel grades. Control parameters: Phosphorus content in molten iron ≤ 0.17%, residual copper and nickel content in scrap steel ≤ 0.05%.

[0007] Preferably, step 102 involves precise control of dephosphorization and decarbonization; Operation process: The converter blowing process involves slag formation in stages. In the early stage, the high-basicity slag is rapidly dephosphorized, and in the later stage, the carbon is controlled by stable blowing. The blowing endpoint is sampled and tested using a direct-reading spectrometer, and blowing is stopped immediately after the target is met. Control parameters: final carbon content ≥ 0.08%, final phosphorus content ≤ 0.010%, tapping temperature 1620~1670℃.

[0008] Preferably, step 103 is: slag blocking and steel tapping operation; Operation process: The steel is tapped using a sliding plate slag-blocking process to prevent slag from mixing into the ladle; basic deoxidizers are added simultaneously during the tapping process to avoid secondary oxidation of the molten steel; Control parameters: slag-blocking thickness ≤ 50mm, deoxidizer addition amount uniform and stable.

[0009] Preferably, step 201 involves slag formation, desulfurization, and slag system regulation. Operation process: After the LF furnace is filled with lime and synthetic slag, lime and synthetic slag are quickly added to make white slag and adjust the slag alkalinity. High-efficiency desulfurization is achieved in the early stage, and the slag properties are finely adjusted in the middle and later stages to ensure sulfur recovery. Argon gas is used for stirring throughout the process to ensure that the slag and steel react fully. Control parameters: refining slag basicity 3.0~5.0, white slag retention time ≥35min, sulfur content after desulfurization ≤0.005%; Step 202: Fine-tuning of key alloy composition; Operation process: After the white slag is formed, silicon, manganese and chromium alloys are added in sequence according to the target composition to precisely adjust the matrix elements; after the temperature reaches the standard, vanadium-nitrogen alloy is added and nitrogen-manganese wire is fed in to increase nitrogen; after breaking the void, sulfur wire is fed in to fine-tune the sulfur content; after each fine-tuning step, samples are taken for testing. Control parameters: carbon content 0.37~0.41%, silicon content 0.53~0.62%, manganese content 1.42~1.53%, chromium content 0.11~0.15%; sulfur content 0.048~0.064%, nitrogen content 0.0125~0.0185%; carbon equivalent 0.765~0.805%. Step 203: Calcium treatment and inclusion denaturation; Operation process: After the alloy is fine-tuned, calcium wire is fed in to modify the oxide and sulfide inclusions. After the calcium wire is fed in, let it stand for 5 to 8 minutes to ensure that the inclusions are fully floated and removed. Control parameters: Calcium wire feeding length 55~75m, after feeding the wire, gently blow and stir for 3~5 minutes.

[0010] Preferably, step 301 involves high-vacuum deep degassing; Operation process: The ladle is moved into the RH vacuum device, and a vacuum is drawn to the target negative pressure. The high vacuum state is maintained to remove hydrogen and oxygen gases. Argon gas is used for circulation and stirring during the vacuum process to improve the degassing efficiency. Control parameters: vacuum degree ≤60Pa, high vacuum holding time ≥10min, hydrogen content after treatment ≤0.0001% (1ppm). Step 302: Vacuuming followed by soft blowing and settling; Operation process: After breaking the vacuum, continue to blow argon gas to promote the aggregation and floating of residual fine inclusions; after the soft blowing is completed, let the molten steel stand still to prevent inclusions from entering the continuous casting process. Control parameters: Argon flow rate for soft blowing 4~10L / min, soft blowing time ≥18min, and settling time ≥12min.

[0011] Preferably, step 303 is: final component verification; Operation process: After standing, take a sample of molten steel to test the full composition and gas content. Only after confirming that carbon, silicon, manganese, sulfur, nitrogen and hydrogen all meet the standards can it be transferred to the continuous casting process.

[0012] Preferably, step 401 involves full-process protective casting; Operation process: Long nozzle + sealing gasket non-oxidizing casting is adopted, and low carbon covering agent is added to the tundish to prevent molten steel from being exposed throughout the process; the tundish liquid level is automatically controlled to avoid slag entrapment and composition segregation caused by liquid level fluctuations. Control parameters: tundish liquid level 650~900mm, liquid level fluctuation ≤±30mm, molten steel superheat 20~45℃; Step 402: Electromagnetic stirring and gentle pressure control; Operation process: Electromagnetic stirring is activated simultaneously in the crystallizer and at the end to optimize grain refinement; light pressure is applied at the end of the billet solidification to eliminate central porosity and segregation and ensure uniform microstructure. Control parameters: Crystallizer electromagnetic stirring 200~260A / 2.5~5.0Hz, end electromagnetic stirring 300~420A / 5.0~8.0Hz, light pressing amount 2.0~4.0mm; Step 403: Constant speed casting and weak cooling water supply; Operating procedure: Set a stable casting speed and cast billets at a constant speed throughout the process. Rapid increases or decreases in casting speed are strictly prohibited. A weak cooling system is adopted in the secondary cooling zone, and gradient water distribution is used to avoid rapid cooling and cracking of the billet. Control parameters: Billet pulling speed 0.45~0.60m / min, pulling speed fluctuation ≤±0.05m / min; secondary cooling water volume 0.15~0.22L / kg, water volume in each zone decreases in a gradient of 1:0.8:0.6.

[0013] Preferably, step 501 involves slow cooling of the cast billet; Operation process: After the billet is cut to length, it is smoothly transferred to the slow cooling pit and stacked in layers. It is covered with insulation cotton and sealed for slow cooling to avoid internal stress and uneven structure caused by rapid cooling. Control parameters: Initial temperature of billet entering slow cooling pit ≥450℃, slow cooling time ≥24h; Step 502: Slab peeling and finishing; Operation process: The entire surface of the billet is peeled off using a grinding wheel to remove original defects such as surface cracks, slag pits, and pinholes, thus preventing strength fluctuations caused by surface defects after rolling. Control parameters: Peeling depth 2.0~5.0mm, after peeling the surface is smooth without pits or cracks; Step 503: Non-destructive testing of the cast billet; Operation process: After peeling, the billet is subjected to infrared and ultrasonic testing to detect internal shrinkage cavities, cracks, inclusions and other defects. Unqualified billets are removed. Control parameters: Allowable diameter of internal defects ≤ φ0.3mm, defect length ≤ 20mm.

[0014] Preferably, step 6 involves traceability and control of the smelting process; Operation process: Record parameters of each process in the converter, LF, RH and continuous casting throughout the entire process, and bind the furnace number and billet batch number; when strength fluctuations or hardenability abnormalities occur, quickly trace the source and adjust the process to ensure consistent strength stability of batch products.

[0015] (III) Beneficial Effects Compared with the prior art, the present invention provides a smelting process for improving the strength stability of non-heat-treated steel, which has the following beneficial effects: 1. This non-adjusted steel strength stability improvement smelting process method has precise control of key components and significantly narrowed strength fluctuations. Through precise control of the entire process of converter and LF furnace, the core elements C, Si, Mn, Cr, S and N are controlled within a narrow range, with a composition deviation of ≤±0.02%. This eliminates the strength difference caused by composition fluctuations from the root, and the tensile strength fluctuation of a single batch is stably controlled within ±15MPa.

[0016] 2. This non-adjusted steel strength stability improvement smelting process method greatly improves the purity of molten steel, eliminates weak points in strength, reduces hydrogen content to below 1ppm through RH deep degassing, and achieves an inclusion removal rate of over 95% through LF calcium treatment and soft blowing, completely eliminating strength failure points induced by gas embrittlement and inclusions, and significantly improving the uniformity of the steel matrix.

[0017] 3. This non-adjusted steel strength stability improvement smelting process homogenizes the billet, stabilizes the microstructure and strength simultaneously, and uses electromagnetic stirring, light reduction and weak cooling water distribution in continuous casting to control the compositional segregation and central porosity of the billet, ensuring the microstructure uniformity meets the standard and avoiding local strength abnormalities caused by uneven microstructure.

[0018] 4. This non-adjustable steel strength stability improvement smelting process method, billet finishing and defect control, prevents subsequent strength abnormalities, billet slow cooling to eliminate internal stress, deep peeling and non-destructive testing to remove surface and internal defects, blocks strength fluctuations caused by defects from the billet source, and ensures the strength consistency of batch products.

[0019] 5. The smelting process for improving the strength stability of non-quenched and tempered steel is subject to closed-loop control throughout the entire process, ensuring that batch stability fully meets the standards. All process parameters are traceable and adjusted in a closed loop, achieving a strength difference of ≤10MPa between batches, which fully meets the stringent requirements of high-end components for the strength stability of non-quenched and tempered steel. Detailed Implementation

[0020] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0021] This solution provides a technical approach, specifically a smelting process for improving the strength stability of non-adjusted steel, comprising the following steps: Step 1: Converter smelting process; pre-control of basic components and impurities. Step 101: Raw material screening and furnace feeding control; Operation process: Select high-purity pig iron with low phosphorus and low residual elements and clean scrap steel, mix them in proportion and put them into the furnace, and prevent high impurity and high copper and nickel scrap steel from being mixed in; test the phosphorus content of the molten iron before it is put into the furnace, and unqualified raw materials are refining other steel grades. Control parameters: Phosphorus content in molten iron ≤ 0.17%, residual copper and nickel content in scrap steel ≤ 0.05%; Step 102: Precise control of dephosphorization and decarbonization; Operation process: The converter blowing process involves slag formation in stages. In the early stage, the high-basicity slag is rapidly dephosphorized, and in the later stage, the carbon is controlled by stable blowing. The blowing endpoint is sampled and tested using a direct-reading spectrometer, and blowing is stopped immediately after the target is met. Control parameters: final carbon content ≥ 0.08%, final phosphorus content ≤ 0.010%, tapping temperature 1620~1670℃; Step 103: Slag blocking and steel tapping operation; Operation process: The steel is tapped using a sliding plate slag-blocking process to prevent slag from mixing into the ladle; basic deoxidizers are added simultaneously during the tapping process to avoid secondary oxidation of the molten steel; Control parameters: slag-blocking thickness ≤ 50mm, deoxidizer addition amount uniform and stable; Step 2: LF refining process; precise control of composition and modification of inclusions. Step 201: Slag formation, desulfurization, and slag system regulation; Operation process: After the LF furnace is filled with lime and synthetic slag, lime and synthetic slag are quickly added to make white slag and adjust the slag alkalinity. High-efficiency desulfurization is achieved in the early stage, and the slag properties are finely adjusted in the middle and later stages to ensure sulfur recovery. Argon gas is used for stirring throughout the process to ensure that the slag and steel react fully. Control parameters: refining slag basicity 3.0~5.0, white slag retention time ≥35min, sulfur content after desulfurization ≤0.005%; Step 202: Fine-tuning of key alloy composition; Operation process: After the white slag is formed, silicon, manganese and chromium alloys are added in sequence according to the target composition to precisely adjust the matrix elements; after the temperature reaches the standard, vanadium-nitrogen alloy is added and nitrogen-manganese wire is fed in to increase nitrogen; after breaking the void, sulfur wire is fed in to fine-tune the sulfur content; after each fine-tuning step, samples are taken for testing. Control parameters: carbon content 0.37~0.41%, silicon content 0.53~0.62%, manganese content 1.42~1.53%, chromium content 0.11~0.15%; sulfur content 0.048~0.064%, nitrogen content 0.0125~0.0185%; carbon equivalent 0.765~0.805%. Step 203: Calcium treatment and inclusion denaturation; Operation process: After the alloy is fine-tuned, calcium wire is fed in to modify the oxide and sulfide inclusions. After the calcium wire is fed in, let it stand for 5 to 8 minutes to ensure that the inclusions are fully floated and removed. Control parameters: Calcium wire feeding length 55~75m, soft blowing and stirring for 3~5min after feeding; Step 3: RH vacuum treatment process; deep degassing and purity enhancement Step 301: High vacuum deep degassing; Operation process: The ladle is moved into the RH vacuum device, and a vacuum is drawn to the target negative pressure. The high vacuum state is maintained to remove hydrogen and oxygen gases. Argon gas is used for circulation and stirring during the vacuum process to improve the degassing efficiency. Control parameters: vacuum degree ≤60Pa, high vacuum holding time ≥10min, hydrogen content after treatment ≤0.0001% (1ppm). Step 302: Vacuuming followed by soft blowing and settling; Operation process: After breaking the vacuum, continue to blow argon gas to promote the aggregation and floating of residual fine inclusions; after the soft blowing is completed, let the molten steel stand still to prevent inclusions from entering the continuous casting process. Control parameters: Argon flow rate for soft blowing is 4~10L / min, soft blowing time is ≥18min, and settling time is ≥12min; Step 303: Final component verification; Operation process: After standing, take a sample of molten steel to test the full composition and gas content. Only after confirming that carbon, silicon, manganese, sulfur, nitrogen and hydrogen all meet the standards can it be transferred to the continuous casting process. Step 4: Continuous casting process; Homogenization and microstructure stabilization of the cast billet. Step 401: Full-process protective pouring; Operation process: Long nozzle + sealing gasket non-oxidizing casting is adopted, and low carbon covering agent is added to the tundish to prevent molten steel from being exposed throughout the process; the tundish liquid level is automatically controlled to avoid slag entrapment and composition segregation caused by liquid level fluctuations. Control parameters: tundish liquid level 650~900mm, liquid level fluctuation ≤±30mm, molten steel superheat 20~45℃; Step 402: Electromagnetic stirring and gentle pressure control; Operation process: Electromagnetic stirring is activated simultaneously in the crystallizer and at the end to optimize grain refinement; light pressure is applied at the end of the billet solidification to eliminate central porosity and segregation and ensure uniform microstructure. Control parameters: Crystallizer electromagnetic stirring 200~260A / 2.5~5.0Hz, end electromagnetic stirring 300~420A / 5.0~8.0Hz, light pressing amount 2.0~4.0mm; Step 403: Constant speed casting and weak cooling water supply; Operating procedure: Set a stable casting speed and cast billets at a constant speed throughout the process. Rapid increases or decreases in casting speed are strictly prohibited. A weak cooling system is adopted in the secondary cooling zone, and gradient water distribution is used to avoid rapid cooling and cracking of the billet. Control parameters: Billet pulling speed 0.45~0.60m / min, pulling speed fluctuation ≤±0.05m / min; secondary cooling water volume 0.15~0.22L / kg, water volume in each zone decreases in a gradient of 1:0.8:0.6; Step 5: Slow cooling and surface finishing of the billet; elimination of internal stress and surface defects. Step 501: Slow cooling treatment of the billet; Operation process: After the billet is cut to length, it is smoothly transferred to the slow cooling pit and stacked in layers. It is covered with insulation cotton and sealed for slow cooling to avoid internal stress and uneven structure caused by rapid cooling. Control parameters: Initial temperature of billet entering slow cooling pit ≥450℃, slow cooling time ≥24h; Step 502: Slab peeling and finishing; Operation process: The entire surface of the billet is peeled off using a grinding wheel to remove original defects such as surface cracks, slag pits, and pinholes, thus preventing strength fluctuations caused by surface defects after rolling. Control parameters: Peeling depth 2.0~5.0mm, after peeling the surface is smooth without pits or cracks; Step 503: Non-destructive testing of the cast billet; Operation process: After peeling, the billet is subjected to infrared and ultrasonic testing to detect internal shrinkage cavities, cracks, inclusions and other defects. Unqualified billets are removed. Control parameters: Allowable internal defect diameter ≤ φ0.3mm, defect length ≤ 20mm; Step 6: Traceability and control of the smelting process; Operation process: Record parameters of each process in the converter, LF, RH and continuous casting throughout the entire process, and bind the furnace number and billet batch number; when strength fluctuations or hardenability abnormalities occur, quickly trace the source and adjust the process to ensure consistent strength stability of batch products; Furthermore, the precise control of key components significantly narrows the strength fluctuation. Through precise control of the entire process of converter and LF furnace, the core elements C, Si, Mn, Cr, S and N are controlled within a narrow range, with a composition deviation of ≤±0.02%. This eliminates the strength difference caused by composition fluctuation from the root, and the tensile strength fluctuation of a single batch is stably controlled within ±15MPa. Furthermore, the purity of the molten steel is greatly improved, eliminating weak points in strength. RH deep degassing reduces the hydrogen content to below 1ppm, and LF calcium treatment and soft blowing make the inclusion removal rate reach more than 95%, completely eliminating gas embrittlement and strength failure points induced by inclusions, and significantly improving the uniformity of the steel matrix. Furthermore, the homogenization of the billet ensures that the microstructure and strength are simultaneously stabilized. The continuous casting electromagnetic stirring, light reduction and weak cooling water distribution work together to eliminate component segregation and central porosity in the billet, ensuring that the microstructure is uniform and avoiding local strength abnormalities caused by uneven microstructure. Furthermore, billet finishing and defect control prevent subsequent strength abnormalities. Slow cooling of billets eliminates internal stress, and deep peeling and non-destructive testing remove surface and internal defects. This blocks strength fluctuations caused by defects from the source of the billet and ensures the consistency of strength of batch products. Furthermore, the entire process is under closed-loop control, batch stability fully meets the standards, and all process parameters are traceable and adjusted in a closed loop, achieving a strength difference of ≤10MPa between batches, which fully meets the stringent requirements of high-end components for the strength stability of non-quenched and tempered steel. Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A smelting process for improving the strength and stability of non-heat-treated steel, comprising step 1: converter smelting process, step 2: LF refining process, step 3: RH vacuum treatment process, step 4: continuous casting process, step 5: billet slow cooling and surface finishing process, and step 6: smelting process traceability and control, characterized in that: Step 1: The converter smelting process includes step 101: raw material screening and furnace feeding control, step 102: precise control of dephosphorization and decarburization, and step 103: slag blocking and steel tapping operation; Among them, step 2: LF refining process includes step 201: slag formation, desulfurization and slag system regulation, step 202: fine adjustment of key alloy components and step 203: calcium treatment and inclusion modification; Among them, step 3: RH vacuum treatment process includes step 301: high vacuum deep degassing, step 302: vacuum soft blowing and settling, and step 303: final composition verification; Step 4: Continuous casting process includes step 401: full-process protective pouring, step 402: electromagnetic stirring and light pressure control, and step 403: constant speed billet pulling and weak cooling water distribution; Step 5, the process of slow cooling and surface finishing of the billet, includes step 501: slow cooling treatment of the billet, step 502: peeling and finishing of the billet, and step 503: non-destructive testing of the billet.

2. The smelting process method for improving the strength stability of non-adjusted steel according to claim 1, characterized in that: Step 101: Raw material screening and furnace feeding control; Operation process: Select high-purity pig iron with low phosphorus and low residual elements and clean scrap steel, mix them in proportion and put them into the furnace, and prevent high impurity and high copper and nickel scrap steel from being mixed in; test the phosphorus content of the molten iron before it is put into the furnace, and unqualified raw materials are refining other steel grades. Control parameters: Phosphorus content in molten iron ≤ 0.17%, residual copper and nickel content in scrap steel ≤ 0.05%.

3. The smelting process method for improving the strength stability of non-adjusted steel according to claim 1, characterized in that: Step 102: Precise control of dephosphorization and decarbonization; Operation process: The converter blowing process involves slag formation in stages. In the early stage, the high-basicity slag is rapidly dephosphorized, and in the later stage, the carbon is controlled by stable blowing. The blowing endpoint is sampled and tested using a direct-reading spectrometer, and blowing is stopped immediately after the target is met. Control parameters: final carbon content ≥ 0.08%, final phosphorus content ≤ 0.010%, tapping temperature 1620~1670℃.

4. The smelting process method for improving the strength stability of non-adjusted steel according to claim 1, characterized in that: Step 103: Slag blocking and steel tapping operation; Operation process: The steel is tapped using a sliding plate slag-blocking process to prevent slag from mixing into the ladle; basic deoxidizers are added simultaneously during the tapping process to prevent secondary oxidation of the molten steel; Control parameters: slag-blocking thickness ≤ 50mm, deoxidizer addition amount uniform and stable.

5. The smelting process method for improving the strength stability of non-adjusted steel according to claim 1, characterized in that: Step 201: Slag formation, desulfurization, and slag system regulation; Operation process: After the LF furnace is filled with lime and synthetic slag, lime and synthetic slag are quickly added to make white slag and adjust the slag alkalinity. High-efficiency desulfurization is achieved in the early stage, and the slag properties are finely adjusted in the middle and later stages to ensure sulfur recovery. Argon gas is used for stirring throughout the process to ensure that the slag and steel react fully. Control parameters: refining slag basicity 3.0~5.0, white slag retention time ≥35min, sulfur content after desulfurization ≤0.005%; Step 202: Fine-tuning of key alloy composition; Operation process: After the white slag is formed, silicon, manganese and chromium alloys are added in sequence according to the target composition to precisely adjust the matrix elements; after the temperature reaches the standard, vanadium-nitrogen alloy is added and nitrogen-manganese wire is fed in to increase nitrogen; after breaking the void, sulfur wire is fed in to fine-tune the sulfur content; after each fine-tuning step, samples are taken for testing. Control parameters: carbon content 0.37~0.41%, silicon content 0.53~0.62%, manganese content 1.42~1.53%, chromium content 0.11~0.15%; sulfur content 0.048~0.064%, nitrogen content 0.0125~0.0185%; carbon equivalent 0.765~0.805%. Step 203: Calcium treatment and inclusion denaturation; Operation process: After the alloy is fine-tuned, calcium wire is fed in to modify the oxide and sulfide inclusions. After the calcium wire is fed in, let it stand for 5 to 8 minutes to ensure that the inclusions are fully floated and removed. Control parameters: Calcium wire feeding length 55~75m, after feeding the wire, gently blow and stir for 3~5 minutes.

6. The smelting process method for improving the strength stability of non-adjusted steel according to claim 1, characterized in that: Step 301: High-vacuum deep degassing; Operation process: The ladle is moved into the RH vacuum device, and a vacuum is drawn to the target negative pressure. The high vacuum state is maintained to remove hydrogen and oxygen gases. Argon gas is used for circulation and stirring during the vacuum process to improve the degassing efficiency. Control parameters: vacuum degree ≤60Pa, high vacuum holding time ≥10min, hydrogen content after treatment ≤0.0001% (1ppm). Step 302: Vacuuming followed by soft blowing and settling; Operation process: After breaking the vacuum, continue to blow argon gas to promote the aggregation and floating of residual fine inclusions; after the soft blowing is completed, let the molten steel stand still to prevent inclusions from entering the continuous casting process. Control parameters: Argon flow rate for soft blowing 4~10L / min, soft blowing time ≥18min, and settling time ≥12min.

7. The smelting process method for improving the strength stability of non-adjusted steel according to claim 1, characterized in that: Step 303: Final component verification; Operation process: After standing, take a sample of molten steel to test the full composition and gas content. Only after confirming that carbon, silicon, manganese, sulfur, nitrogen and hydrogen all meet the standards can it be transferred to the continuous casting process.

8. The smelting process method for improving the strength stability of non-adjusted steel according to claim 1, characterized in that: Step 401: Full-process protective pouring; Operation process: Long nozzle + sealing gasket non-oxidizing casting is adopted, and low carbon covering agent is added to the tundish to prevent molten steel from being exposed throughout the process; the tundish liquid level is automatically controlled to avoid slag entrapment and composition segregation caused by liquid level fluctuations. Control parameters: tundish liquid level 650~900mm, liquid level fluctuation ≤±30mm, molten steel superheat 20~45℃; Step 402: Electromagnetic stirring and gentle pressure control; Operation process: Electromagnetic stirring is activated simultaneously in the crystallizer and at the end to optimize grain refinement; light pressure is applied at the end of the billet solidification to eliminate central porosity and segregation and ensure uniform microstructure. Control parameters: Crystallizer electromagnetic stirring 200~260A / 2.5~5.0Hz, end electromagnetic stirring 300~420A / 5.0~8.0Hz, light pressing amount 2.0~4.0mm; Step 403: Constant speed casting and weak cooling water supply; Operating procedure: Set a stable casting speed and cast billets at a constant speed throughout the process. Rapid increases or decreases in casting speed are strictly prohibited. A weak cooling regime is adopted in the secondary cooling zone, and gradient water distribution is used to avoid rapid cooling and cracking of the billet. Control parameters: Casting speed 0.45~0.60m / min, casting speed fluctuation ≤±0.05m / min; The secondary cooling water volume is 0.15~0.22L / kg, and the water volume in each zone decreases in a gradient of 1:0.8:0.

6.

9. The smelting process method for improving the strength stability of non-adjusted steel according to claim 1, characterized in that: Step 501: Slow cooling treatment of the billet; Operation process: After the billet is cut to length, it is smoothly transferred to the slow cooling pit and stacked in layers. It is covered with insulation cotton and sealed for slow cooling to avoid internal stress and uneven structure caused by rapid cooling. Control parameters: Initial temperature of billet entering slow cooling pit ≥450℃, slow cooling time ≥24h; Step 502: Slab peeling and finishing; Operation process: The entire surface of the billet is peeled off using a grinding wheel to remove original defects such as surface cracks, slag pits, and pinholes, thus preventing strength fluctuations caused by surface defects after rolling. Control parameters: Peeling depth 2.0~5.0mm, after peeling the surface is smooth without pits or cracks; Step 503: Non-destructive testing of the cast billet; Operation process: After peeling, the billet is subjected to infrared and ultrasonic testing to detect internal shrinkage cavities, cracks, inclusions and other defects. Unqualified billets are removed. Control parameters: Allowable diameter of internal defects ≤ φ0.3mm, defect length ≤ 20mm.

10. A smelting process for improving the strength stability of non-adjusted steel according to claim 1, characterized in that: Step 6: Traceability and control of the smelting process; Operation process: Record parameters of each process in the converter, LF, RH and continuous casting throughout the entire process, and bind the furnace number and billet batch number; when strength fluctuations or hardenability abnormalities occur, quickly trace the source and adjust the process to ensure consistent strength stability of batch products.