Steelmaking method for controlling inclusions in cold heading steel

Abstract

The present application discloses a steelmaking method for controlling inclusions of cold upsetting steel, comprising: an electric furnace smelting step, smelting molten steel, adding aluminum iron in the early stage of tapping process, adding slagging material pre-melted slag and lime in the middle and late stage of tapping process, and adding aluminum particles after the end of tapping process; an LF ladle furnace refining step, supplementing lime to continue slagging at the beginning of refining, then using SiC to deoxidize slag surface and form foamed slag; a VD vacuum degassing step, controlling the flow of bottom blowing gas and high vacuum time, and supplementing the required composition according to the sampling composition after the end of VD vacuum degassing; and a pouring step, pouring the molten steel into a casting blank. The present application ensures that the granular Ds inclusions of cold upsetting steel are controlled at 1.0 level or below, i.e. Ds inclusion ≤27 μm.

Description

Technical Field

[0001] This invention relates to the field of iron and steel smelting technology, and more particularly to steelmaking technology for controlling inclusions. Background Technology

[0002] Cold heading steel is generally made of medium- and low-carbon high-quality carbon structural steel and low-alloy steel. It is a key raw material in the fastener industry, and its wire products are mainly used to manufacture fasteners such as bolts, nuts, and screws, as well as cold-headed parts. In recent years, my country's automotive, petrochemical, power, telecommunications, and construction industries have developed rapidly, and the demand for fasteners in these industries has also increased. High-quality, high-strength cold heading steel is in high demand. Lightweighting of automobiles requires weight reduction of its components, which necessitates the development of high-strength cold heading steel to meet the needs of automotive lightweighting. However, as the strength of cold heading steel increases, its sensitivity to inclusions also increases. The most important cause of cold heading cracking and cracking during automobile assembly is crack initiation from inclusions. Among non-metallic inclusions in cold heading steel, Class B and Class D (Ds) are the most harmful. It is generally believed that the larger the size of the inclusion and the closer it is to the steel surface, the greater the harm. Therefore, inclusion control in cold heading steel is receiving increasing attention within the industry.

[0003] Regarding inclusion control in cold heading steel, Chinese patent application number 201310431036.0 discloses a smelting process for aluminum-containing cold heading steel. This process includes smelting with low-silicon raw materials, rapid slag formation, deoxidation of the slag surface using calcium carbide and aluminum particles as deoxidizer in the refining furnace, and calcium feeding in the continuous casting ladle. This reduces the Si content in the steel while increasing the Al content, ensuring continuous casting of molten steel. This method primarily addresses the issue of ensuring stable casting after increasing the aluminum content of cold heading steel. However, this patent does not propose specific targets or related control methods for inclusion control. Chinese patent application number 201710197647.1 discloses a method for deoxidation and slag formation in the LF refining process for producing medium-carbon, low-silicon cold heading steel. The main method involves using a mixture of aluminum powder and carbon powder as a foaming deoxidizer in the LF furnace. This is achieved through small-batch, multi-batch addition to the slag surface via hopper vibration, maintaining a foaming and reducing atmosphere throughout the refining process. This deoxidation method effectively reduces slag oxidizability and prevents calcium buildup in the molten steel due to slag surface deoxidation, thus benefiting the control of oxygen content in the molten steel. However, this patent does not propose any methods for controlling inclusions. Besides the patent mentioned above, in actual cold heading steel production, most steel mills use calcium treatment. Calcium treatment easily generates liquid calcium aluminates at steelmaking temperatures. The aggregation and growth of liquid inclusions can easily form large-particle Ds inclusions.

[0004] To address the problems existing in the current technology, a new method for controlling inclusions in cold heading steel is needed, including the target of inclusion control and the specific production process. Summary of the Invention

[0005] This invention discloses a steelmaking method for controlling inclusions in cold heading steel. The control target for inclusions is: among inclusions with an equivalent diameter greater than 3 micrometers, by mass percentage, the proportion of Al2O3+MnS inclusions is greater than 90%, and the combined proportion of Al2O3-MgO-MnS inclusions, Al2O3-CaO-MnS inclusions, and Al2O3-CaO-MgO-MnS inclusions is less than 10%. The steelmaking method includes:

[0006] The electric arc furnace smelting process involves treating molten steel in an electric arc furnace, adding aluminum and iron in the early stages of the tapping process, adding slag-forming pre-melted slag and lime in the middle and later stages of the tapping process, and adding aluminum granules after the tapping process is completed.

[0007] The refining process of the LF ladle furnace begins with the addition of lime to continue slag formation, followed by the use of SiC for slag surface deoxidation and foam slag formation.

[0008] In the VD vacuum degassing step, the flow rate of the bottom-blown gas and the high vacuum time are controlled. After the VD vacuum degassing is completed, the required components are replenished according to the sampled components.

[0009] The casting process involves pouring molten steel into a billet.

[0010] According to one embodiment of the present invention, the control target for inclusions is: among inclusions with an equivalent diameter greater than 3 micrometers, by mass percentage: Al2O3+MnS inclusions account for more than 90%, and the combined proportion of Al2O3-MgO-MnS inclusions, Al2O3-CaO-MnS inclusions, and Al2O3-CaO-MgO-MnS inclusions is less than 10%.

[0011] According to one embodiment of the present invention, in the electric furnace smelting step, the tapping carbon is controlled at 0.08-0.2%. Aluminum iron is added during the first 1 / 3 of the tapping process at a ratio of 4-5 kg / t, and the size of the aluminum iron is controlled at 2-3 cm. Slag-forming pre-melted slag and lime are added during the 2 / 3 to 5 / 6 of the tapping process. After tapping, aluminum particles are added from the high-level silo of the electric furnace station at a ratio of 0.5-1 kg / t.

[0012] According to one embodiment of the present invention, in the refining step of the LF ladle furnace, lime is added at the beginning of refining to continue slag formation, and then SiC balls are added to deoxidize the slag surface and form foam slag. The addition ratio of SiC balls is 1-2 kg / t, and the final slag composition is controlled by mass percentage as follows: CaO 45-55%, MgO 5-10%, Al2O3 30-35%, SiO2 8-15%.

[0013] According to one embodiment of the present invention, in the LF ladle furnace refining step, the bottom-blown gas is Ar gas. In the first 10 min to 15 min of LF refining, the flow rate of the bottom-blown Ar gas is controlled at 200 to 500 L / min; thereafter, the flow rate of the bottom-blown Ar gas is controlled at 80 to 120 L / min.

[0014] According to one embodiment of the present invention, in the VD vacuum degassing step, the flow rate of the bottom-blown gas is controlled at 50-100 L / min throughout the process, the high vacuum time is greater than 15 min, and after the VD vacuum degassing is completed, aluminum wire is added according to the sample composition to make the aluminum content of the molten steel 0.015-0.03%, and sulfur wire is added to make the sulfur content of the molten steel 0.004-0.006%, and no calcium treatment is performed.

[0015] According to one embodiment of the present invention, in the casting step, the melting point of the ladle covering agent in the casting process of casting molten steel into a billet is controlled at 1350-1400°C.

[0016] The steelmaking method for controlling inclusions in cold heading steel according to the present invention is achieved through the control of inclusion composition and production process, ensuring that the particulate Ds inclusions in cold heading steel are controlled at level 1.0 or below, that is, Ds inclusions ≤27μm. At the same time, a MnS protective layer is precipitated around the inclusions, making the inclusions harmless as a whole. Attached Figure Description

[0017] Figure 1 A flowchart of the steelmaking method for controlling inclusions in cold heading steel according to the present invention is disclosed. Detailed Implementation

[0018] This method aims to propose a new method for controlling inclusions in cold heading steel, including the target of inclusion control and the specific production process. Its purpose is to control harmful particulate Ds inclusions in cold heading steel and achieve overall harmlessness of inclusions, providing technical guarantee for stable production and quality improvement of high-end cold heading steel.

[0019] The steelmaking method for controlling inclusions in cold heading steel according to the present invention has the following control target for inclusions: among inclusions with an equivalent diameter greater than 3 micrometers (μm), by mass percentage: Al2O3+MnS inclusions account for more than 90%, and the combined proportion of Al2O3-MgO-MnS inclusions, Al2O3-CaO-MnS inclusions, and Al2O3-CaO-MgO-MnS inclusions is less than 10%. This requirement is mainly because Al2O3+MnS inclusions are easily crushed during the rolling process of cold heading steel, and the MnS layer precipitated around them has low hardness, which can buffer the friction between the inclusions and the steel matrix during the rolling process, thereby ensuring that there are no crack sources around the inclusions during the rolling process. Because the production process inevitably involves the reaction of aluminum in molten steel with CaO in the slag, resulting in calcium inclusions, and the unavoidable reaction of aluminum with refractory materials to form magnesium inclusions, the combined proportion of Al2O3-MgO-MnS, Al2O3-CaO-MnS, and Al2O3-CaO-MgO-MnS inclusions must be less than 10%. Based on inclusion formation calculations, this proportion of less than 10% significantly reduces the risk of forming large-particle Ds inclusions. Furthermore, an MnS layer should precipitate around the latter three types of inclusions to mitigate their harmful effects. For ease of description, all percentages listed below are by mass.

[0020] To achieve the aforementioned control objectives, the present invention employs the following processing: Figure 1 A flowchart of the steelmaking method for controlling inclusions in cold heading steel according to the present invention is disclosed, with reference to... Figure 1 As shown, the steelmaking method for controlling inclusions in cold heading steel includes the following steps:

[0021] The electric arc furnace (EAF) smelting process involves treating molten steel in an EAF. Aluminum-iron is added in the early stages of tapping, and slag-forming pre-melted slag and lime are added in the middle and later stages of tapping. Aluminum granules are added after tapping. In one embodiment, the carbon content of the tapped steel is controlled at 0.08–0.2%. Aluminum-iron is added during the first 1 / 3 of the tapping process at a ratio of 4-5 kg / t, with the aluminum-iron size controlled at 2-3 cm. Slag-forming pre-melted slag and lime are added during 2 / 3–5 / 6 of the tapping process. After tapping, aluminum granules are added from the high-level hopper at the EAF station at a ratio of 0.5–1 kg / t.

[0022] The method of this invention requires a slightly higher carbon content in the tapped steel from the electric furnace. This aims to reduce the oxygen content in the tapped steel, thus providing better initial conditions for subsequent refining to reduce the total amount of inclusions. Controlling the tapped carbon content to 0.08-0.2% is relatively easy to achieve in production. During the first third of the tapping process, 4-5 kg / t of ferroaluminum is added, with the ferroaluminum size controlled at 2-3 cm. The purpose is to utilize the impact force generated when the molten steel flows into the ladle to rapidly deoxidize the aluminum. This operation is chosen to be completed in the first third of the timeframe because the ferroaluminum melting time is approximately 1-2 minutes. If added later, the ferroaluminum will not melt and will mix with the subsequently added slag-forming material, forming uncontrollable inclusions. The range of ferroaluminum addition ensures that the aluminum content in the deoxidized steel is controlled to be greater than 0.08%. The requirement for the ferroaluminum size to be controlled at 2-3 cm is primarily to ensure that it melts within 1-2 minutes. Adding slag-forming materials, including pre-melted slag and lime, between 2 / 3 and 5 / 6 of the tapping time still primarily utilizes the impact force of molten steel to accelerate slag formation. This timeframe is chosen because it's considered to add these materials after the aluminum and iron have melted; adding them too late would result in poor slag formation due to the impact of molten steel. After tapping, aluminum granules are added again from the high-level silo for slag surface deoxidation. This is because the slag is still highly oxidizing immediately after tapping, requiring further deoxidation with aluminum. Simultaneously, the slag doesn't have good fluidity at this stage, preventing aluminum from settling and reacting with the molten steel during the aluminum slag surface deoxidation process. If aluminum enters the molten steel, it can introduce calcium elements, forming calcium aluminate (Ds) inclusions. Adding from the high-level silo ensures that the aluminum granules are evenly distributed on the slag surface, preventing aluminum accumulation during deoxidation.

[0023] In the LF ladle furnace refining process, lime is added at the beginning of refining to continue slag formation. Then, SiC balls are added to deoxidize the slag surface and create foamy slag. The addition ratio of SiC balls is 1-2 kg / t. The final slag composition is controlled by mass percentage as follows: CaO 45-55%, MgO 5-10%, Al2O3 30-35%, SiO2 8-15%. In the LF ladle furnace refining process, Ar is used as the bottom-blown gas. During the first 10-15 minutes of LF refining, the Ar gas flow rate is controlled at 200-500 L / min; subsequently, the Ar gas flow rate is controlled at 80-120 L / min.

[0024] In the LF refining process, the method of this invention mainly continues to use SiC for slag surface deoxidation instead of aluminum slag surface deoxidation. The main reason is that SiC slag surface deoxidation generates CO2 bubbles, which acts as a foaming slag, beneficial for rapid slag melting and LF electric arc submerged arc operation. If aluminum slag surface deoxidation is used, the LF furnace slag already possesses good fluidity, and aluminum slag surface deoxidation would cause the aluminum liquid to sink and carry calcium into the molten steel, resulting in the formation of large-particle calcium aluminate (Ds) inclusions. The method of this invention fully considers the deoxidation and inclusion control in cold heading steel production. This slag composition (CaO 45-55%, MgO 5-10%, Al2O3 30-35%, SiO2 8-15%) has good deoxidation ability, and the low activity of CaO makes it less likely to cause calcium gain in the molten steel. Considering that bottom blowing in the LF furnace will form slag entrapment, in order to ensure uniform steel temperature, the bottom blowing Ar gas flow rate is ultimately set at 200-500 L / min for the first 10-15 minutes of refining, as bottom blowing is required to melt slag and alloy, and to complete the fine adjustment of cold heading steel alloy. After that, since there is no need to melt alloy and melt slag again, the bottom blowing Ar gas flow rate is controlled at 80-120 L / min. The above flow rate settings are all set according to the results obtained from the relevant water model.

[0025] In the VD vacuum degassing step, the flow rate of the bottom-blown gas and the high-vacuum time are controlled. After VD vacuum degassing, the required components are replenished according to the sampled composition. During the VD vacuum degassing step, the flow rate of the bottom-blown gas is controlled at 50-100 L / min throughout, and the high-vacuum time is greater than 15 min. After VD vacuum degassing, aluminum wire is added according to the sampled composition to make the aluminum content of the molten steel 0.015-0.03%, and sulfur wire is added to make the sulfur content of the molten steel 0.004-0.006%. Calcium treatment is not performed.

[0026] For the bottom-blowing gas flow control during the VD vacuum degassing process, the main consideration is that cold heading steel is made of wire rod, and the deoxidation requirements are not as strict as for bar steel. Therefore, the bottom-blowing gas flow rate during the VD vacuum degassing process should not be too high to suppress slag entrainment. After VD vacuum degassing, if the aluminum content of the molten steel does not meet the requirements, aluminum can be added. Since VD has already completed deoxidation, adding aluminum will not form large particles of harmful alumina inclusions. At the same time, sulfur is added after VD, controlling the sulfur content of the molten steel to 0.004-0.006%, mainly to form MnS inclusions encapsulated in Al2O3 and other oxide inclusions. The method of this invention does not require calcium feeding, mainly because the previous operations have already ensured that the inclusions are mainly alumina + MnS inclusions, which are not easy to clog the nozzle. Adding calcium would lead to the formation of large particles of calcium aluminate (Ds) inclusions.

[0027] The casting process involves pouring molten steel into billets or large square billets. During this process, the melting point of the tundish covering agent used in the casting of the billets is controlled between 1350 and 1400°C. Maintaining this melting point aims to prevent the covering agent from easily becoming entangled and forming slag-like inclusions during casting if its melting point is too low. Conversely, if the melting point is too high, it will not melt easily during production and will fail to form a liquid protective layer to prevent oxidation of the molten steel.

[0028] The steelmaking method for controlling inclusions in cold heading steel according to the present invention is implemented in a 150t-EAF-LF refining furnace-VD vacuum furnace-billet continuous casting production process. Specific embodiments are as follows:

[0029] Example 1

[0030] The electric arc furnace (EAF) is used for smelting cold heading steel. The steel grade is SCM435, with a carbon content of 0.35% and small amounts of Mo and Cr. When the carbon content reaches 0.15% during the electric arc furnace blowing process, the molten steel temperature reaches the tapping temperature of 1620℃. Oxygen blowing is then stopped, and the steel is tapped. Tapping involves opening the eccentric furnace bottom and pouring molten steel into a pre-prepared ladle. The tapping process takes 5 minutes. After 1.5 minutes of tapping... 700 kg of aluminum-iron was added through the vibrating hopper of the high-level silo. The aluminum content of the aluminum-iron was 40%, and the average diameter of the blocks was 2.5 cm. When the steel was tapped for 3.5 minutes, 0.8 t of lime and 1 t of pre-slag were added again through the vibrating hopper of the high-level silo. After the steel was tapped, the slag above the molten steel in the ladle was in a semi-molten state. At this time, 100 kg of aluminum particles were once again evenly sprinkled onto the slag surface from the high-level silo. The added particles were mainly used for deoxidation of the slag surface.

[0031] LF refining: After 5 minutes of energizing, 500 kg of lime is added to continue slag formation. After 10 minutes of energizing, the slag achieves good fluidity. 200 kg of SiC balls are added to deoxidize the slag surface and create foamy slag. Alloy and carbon powder are added according to the initial sample composition to ensure the steel composition meets requirements. The final LF slag composition (mass content) is controlled as follows: CaO 51%, MgO 6%, Al2O3 32%, SiO2 10%, with the remainder being 1% FeO. x Unpredictable substances such as MnO, K2O, and CaF2 are present. For the first 15 minutes of LF refining, the bottom-blown Ar gas flow rate is controlled at 300 L / min to complete fine adjustments to the cold-heading steel alloy. Subsequently, the bottom-blown Ar gas stirring flow rate is controlled at 100 L / min, and samples are taken based on temperature measurements. If the composition is within acceptable limits and the temperature is measured at 1610℃, LF refining is complete, with a final aluminum content of 0.04%. Regardless of the final aluminum content, no further aluminum is added to the LF.

[0032] Vacuum degassing (VD) was performed with a low-flow rate controlled at 90 L / min throughout the process, and a high vacuum time of 16 min. After VD treatment, the aluminum content was 0.01% according to the sample composition analysis. Subsequent feeding of aluminum wire brought the final aluminum content to 0.0156%, and a slight supplementation of sulfur wire was also performed to control the sulfur content of the molten steel at 0.0048%. No calcium treatment was required after VD treatment.

[0033] The large billet is poured into a cast billet, and the selected tundish covering agent has a melting point of 1380℃ during the pouring process.

[0034] The large billets were subsequently rolled into wire rods through primary rolling and wire rod production. Analysis of the inclusions within the wire rods revealed a maximum inclusion size of 18 μm, consisting of a composite inclusion of 78% Al₂O₃, 10% CaO, 5% MgO, and 7% MnS. The inclusion composition was as follows: Al₂O₃ + MnS inclusions accounted for 94%, while Al₂O₃-MgO-MnS, Al₂O₃-CaO-MnS, and Al₂O₃-CaO-MgO-MnS inclusions combined accounted for 6%. All inclusions were surrounded by a protective MnS layer. This stable inclusion control keeps the granular Ds inclusions in cold heading steel at grade 1.0 or below, i.e., Ds inclusions ≤27 μm. Simultaneously, the presence of a protective MnS layer around each inclusion renders the entire inclusion harmless.

[0035] Example 2

[0036] In the electric arc furnace (EAF) smelting station, a 150-ton AC EAF furnace with eccentric bottom tapping is used to smelt cold heading steel of grade 45K. This steel has a carbon content of 0.45%. When the carbon content of the molten steel reaches 0.2% during EAF blowing, the steel temperature reaches the tapping temperature of 1600℃. Oxygen blowing is stopped and the steel is tapped. The tapping process involves opening the eccentric bottom and pouring molten steel into a pre-prepared ladle. The tapping process takes 5 minutes. When the tapping time reaches 1.2 minutes, 750 kg of ferroaluminum is added through the vibrating hopper of the high-level hopper. The ferroaluminum contains 40% aluminum and has an average block diameter of 3 cm. When the tapping time reaches 3.8 minutes, 0.7 t of lime and 0.8 t of pre-slag are added again through the vibrating hopper of the high-level hopper. After the tapping is completed, the slag above the molten steel in the ladle is in a semi-molten state. At this time, 80 kg of aluminum particles are once again evenly sprinkled onto the slag surface from the high-level hopper. The added particles are mainly used for deoxidation of the slag surface.

[0037] LF refining: After 3 minutes of energizing, 450 kg of lime is added to continue slag formation. After 10 minutes of energizing, the slag achieves good fluidity. 150 kg of SiC balls are added to deoxidize the slag surface and create foamy slag. Alloy and carbon powder are added according to the initial sample composition to ensure the steel composition meets the requirements. The final LF slag composition (mass content) is controlled as follows: CaO 55%, MgO 7%, Al2O3 30%, SiO2 7%, with the remainder being 1% FeO. xUnpredictable substances such as MnO, K2O, and CaF2 were present. For the first 13 minutes of LF refining, the bottom-blown Ar gas flow rate was controlled at 250 L / min to complete fine adjustments to the cold-heading steel alloy. Subsequently, the bottom-blown Ar gas stirring flow rate was controlled at 80 L / min, and samples were taken based on temperature measurements. The composition was found to be within acceptable limits, and the temperature was measured at 1601℃. LF refining was then complete, with a final aluminum content of 0.05%. Regardless of the final aluminum content, no further aluminum was added to the LF.

[0038] Vacuum degassing (VD) was performed with a low-blowing flow rate controlled at 100 L / min throughout the process, and a high vacuum time of 16 min. After VD treatment, the aluminum content was 0.013% according to the sample composition analysis. Subsequent feeding of aluminum wire brought the final aluminum content to 0.02%, and a slight supplementation of sulfur wire was also performed to control the sulfur content of the molten steel at 0.0058%. No calcium treatment was required after VD treatment.

[0039] The large billet is poured into a cast billet, and the selected tundish covering agent has a melting point of 1380℃ during the pouring process.

[0040] The large billets were subsequently rolled into wire rods through primary rolling and wire rod production. Analysis of the inclusions within the wire rods revealed a maximum inclusion size of 15 μm, consisting of a composite inclusion of 68% Al₂O₃, 20% CaO, 5% MgO, and 7% MnS. The inclusion composition was as follows: Al₂O₃ + MnS inclusions accounted for 91%, while Al₂O₃-MgO-MnS, Al₂O₃-CaO-MnS, and Al₂O₃-CaO-MgO-MnS inclusions combined accounted for 9%. All inclusions were surrounded by a protective MnS layer. This stable inclusion control keeps the granular Ds inclusions in cold heading steel at level 1.0 or below, i.e., Ds inclusions ≤ 27 μm. Simultaneously, the presence of a protective MnS layer around each inclusion renders the inclusions harmless overall.

[0041] Example 3

[0042] In the electric arc furnace (EAF) smelting station, a 150-ton AC EAF furnace with eccentric bottom tapping is used to smelt cold heading steel, grade 10B21, with a carbon content of 0.2%. When the carbon content of the molten steel reaches 0.2% during EAF blowing, the steel temperature reaches the tapping temperature of 1600℃. Oxygen blowing is stopped and the steel is tapped. The tapping process involves opening the eccentric bottom and pouring molten steel into a pre-prepared ladle. The tapping process takes 5 minutes. When the tapping time reaches 1.6 minutes, 750 kg of ferroaluminum is added through the vibrating hopper of the high-level hopper. The ferroaluminum contains 40% aluminum and has an average block diameter of 2 cm. When the tapping time reaches 3.8 minutes, 0.8 t of lime and 0.9 t of pre-slag are added again through the vibrating hopper of the high-level hopper. After the tapping is completed, the slag above the molten steel in the ladle is in a semi-molten state. At this time, 140 kg of aluminum particles are evenly sprinkled onto the slag surface from the high-level hopper. The added particles are mainly used for deoxidation of the slag surface.

[0043] LF refining: After 4 minutes of energizing, 600 kg of lime is added to continue slag formation. After 10 minutes of energizing, the slag achieves good fluidity. 300 kg of SiC balls are added to deoxidize the slag surface and create foamy slag. Alloy and carbon powder are added according to the initial sample composition to ensure the steel composition meets requirements. The final LF slag composition (mass content) is controlled as follows: CaO 55%, MgO 5%, Al2O3 31%, SiO2 8%, with the remainder being 1% FeO. x Unpredictable substances such as MnO, K2O, and CaF2 were present. For the first 13 minutes of LF refining, the bottom-blown Ar gas flow rate was controlled at 500 L / min to complete fine adjustments to the cold-heading steel alloy. Subsequently, the bottom-blown Ar gas stirring flow rate was controlled at 120 L / min, and samples were taken based on temperature measurements. The composition was found to be within acceptable limits, and the temperature was measured at 1611℃. LF refining was then complete, with a final aluminum content of 0.04%. Regardless of the final aluminum content, no further aluminum was added to the LF.

[0044] Vacuum degassing (VD) was performed with a low-blowing flow rate controlled at 100 L / min throughout the process, and a high vacuum time of 16 min. After VD treatment, the aluminum content was 0.011% according to the sample composition analysis. Subsequent feeding of aluminum wire brought the final aluminum content to 0.025%, and a slight supplementation of sulfur wire was also performed to control the sulfur content of the molten steel at 0.004%. No calcium treatment was required after VD treatment.

[0045] The large billet is poured into a cast billet, and the selected tundish covering agent has a melting point of 1400℃ during the pouring process.

[0046] The large billets were subsequently rolled into wire rods through initial rolling and wire rod production. Analysis of the inclusions within the wire rods revealed a maximum inclusion size of 15 μm, consisting of a composite inclusion of 90% Al₂O₃, 2% CaO, 2% MgO, and 6% MnS. The inclusion composition was as follows: Al₂O₃ + MnS inclusions accounted for 95%, while Al₂O₃-MgO-MnS, Al₂O₃-CaO-MnS, and Al₂O₃-CaO-MgO-MnS inclusions combined accounted for 5%. All inclusions were surrounded by a protective MnS layer. This stable inclusion control kept the granular Ds inclusions in the cold heading steel at grade 1.0 or below, i.e., Ds inclusions ≤ 27 μm. Simultaneously, the presence of a protective MnS layer around each inclusion rendered the entire inclusion harmless.

[0047] The steelmaking method for controlling inclusions in cold heading steel according to the present invention is achieved through the control of inclusion composition and production process, ensuring that the particulate Ds inclusions in cold heading steel are controlled at level 1.0 or below, that is, Ds inclusions ≤27μm. At the same time, a MnS protective layer is precipitated around the inclusions, making the inclusions harmless as a whole.

Claims

1. A steelmaking method for controlling inclusions in cold heading steel, characterized in that, The control target for inclusions is as follows: among inclusions with an equivalent diameter greater than 3 micrometers, by mass percentage, the proportion of Al2O3+MnS inclusions is greater than 90%, and the combined proportion of Al2O3-MgO-MnS inclusions, Al2O3-CaO-MnS inclusions, and Al2O3-CaO-MgO-MnS inclusions is less than 10%. The steelmaking method includes: The electric arc furnace smelting process involves treating molten steel in an electric arc furnace, adding aluminum and iron in the early stages of the tapping process, adding slag-forming pre-melted slag and lime in the middle and later stages of the tapping process, and adding aluminum granules after the tapping process is completed. The refining process of the LF ladle furnace begins with the addition of lime to continue slag formation, followed by the use of SiC for slag surface deoxidation and foam slag formation. In the VD vacuum degassing step, the flow rate of the bottom-blown gas and the high vacuum time are controlled. After the VD vacuum degassing is completed, aluminum wire is added according to the sample composition to make the aluminum content of the molten steel 0.015~0.03%, and sulfur wire is added to make the sulfur content of the molten steel 0.004~0.006%. Calcium treatment is not performed. The casting process involves pouring molten steel into a billet.

2. The steelmaking method for controlling inclusions in cold heading steel as described in claim 1, characterized in that, In the electric furnace smelting process, the carbon content of the tapped steel is controlled at 0.08~0.2%. Aluminum iron is added during the first 1 / 3 of the tapping process at a ratio of 4-5 kg / t, and the size of the aluminum iron is controlled at 2-3 cm. During the 2 / 3 to 5 / 6 of the tapping process, slag-forming pre-melted slag and lime are added. After tapping, aluminum particles are added from the high-level hopper at the electric furnace station at a ratio of 0.5~1 kg / t.

3. The steelmaking method for controlling inclusions in cold heading steel as described in claim 1, characterized in that, In the refining step of the LF ladle furnace, lime is added at the beginning of refining to continue slag formation, and then SiC balls are added to deoxidize the slag surface and form foam slag. The addition ratio of SiC balls is 1~2 kg / t. The final slag composition is controlled by mass percentage as follows: CaO 45~55%, MgO 5~10%, Al2O3 30~35%, SiO2 8~15%.

4. The steelmaking method for controlling inclusions in cold-heading steel as described in claim 3, characterized in that, In the LF ladle furnace refining step, the bottom-blown gas is Ar gas. In the first 10 to 15 minutes of LF refining, the flow rate of the bottom-blown Ar gas is controlled at 200 to 500 L / min; thereafter, the flow rate of the bottom-blown Ar gas is controlled at 80 to 120 L / min.

5. The steelmaking method for controlling inclusions in cold heading steel as described in claim 1, characterized in that, In the VD vacuum degassing step, the flow rate of the bottom-blown gas is controlled at 50~100L / min throughout the process, and the high vacuum time is greater than 15min.

6. The steelmaking method for controlling inclusions in cold heading steel as described in claim 1, characterized in that, In the casting step, the melting point of the ladle covering agent used in the casting process of pouring molten steel into a billet is controlled at 1350~1400℃.

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