LF-VD process route for producing high-clean steel

Abstract

This invention discloses a method for producing high-purity steel using the LF-VD process. The process steps are as follows: When the molten steel is poured into the ladle after the initial smelting in the primary furnace, aluminum deoxidizer, alloy, lime, and synthetic slag are added simultaneously; 1.2±0.5 kg / t of lime is added in the early stage of refining to assist in deep desulfurization, composition fine-tuning, and slag surface deoxidation; the total refining time is 55-85 min; soft stirring is performed for 5-10 min before the end of refining; the molten steel undergoes VD vacuum treatment, maintained at a vacuum of ≤133 Pa for 20-30 min, while the bottom blowing stirring flow rate in the ladle is controlled to be 2.8-3.8 times the soft stirring flow rate in the later stage of LF refining; the molten steel is then allowed to stand and softly blown; finally, the molten steel is cast into shape. As can be seen from the method of this invention, by using low-melting-point synthetic slag for slag formation, maintaining a stable refining slag system during the refining process, and performing long-term strong stirring vacuum smelting using VD, high-purity aluminum-killed non-sulfur-containing carbon steel and alloy structural steel can be obtained, achieving a steel T.[O] = 4 × 10⁻⁶. ‑6 ~8×10 ‑6 The oxide inclusion level is stably controlled within 0.5.

Description

Technical Field

[0001] This invention relates to the field of aluminum-killed, sulfur-free carbon steel and alloy structural steel production, specifically to a high-cleanliness steel production method using the LF-VD process route for aluminum-killed, sulfur-free carbon steel and alloy structural steel. Background Technology

[0002] Carbon steel and alloy structural steel are widely used, with many grades being processed into crucial components for high-end manufacturing equipment, such as shafts, gears, flanges, hydraulic hoses, shock absorbers, wheel hubs, firing pins, piston sleeves, lead screws, steering columns, fasteners, and connectors. These products require more stringent cleanliness standards, necessitating vacuum treatment during smelting. Under current technology, steel products obtained using an RH furnace have an advantage in cleanliness compared to those obtained using a VD furnace. However, VD furnace treatment has not achieved the expected removal of inclusions, and the improvement in steel cleanliness control is not significant. Related literature also indicates that VD treatment increases the number and average size of large inclusions in the steel.

[0003] This makes it difficult for many steel companies equipped with VD furnaces to produce truly high-quality steel, and the VD furnace process route restricts the product development of steel companies. Therefore, how to produce high-quality steel using VD furnaces has significant and far-reaching implications for steel smelting. Summary of the Invention

[0004] The purpose of this invention is to improve the cleanliness of aluminum-killed non-sulfur-containing carbon steel and alloy structural steel produced under the existing LF-VD process route. The cleanliness of the steel can be controlled at the following level: T.[O] in the steel = 4 × 10⁻⁶. -6 ~8×10 -6 Non-metallic oxide inclusions should be tested according to Method A in GB / T10561, with a grade <1.0 and a maximum particle size of <15μm.

[0005] The technical solution adopted in this invention is: The LF-VD process is a high-cleanliness production method, using high-cleanliness steel as... w ([C])≥0.10%, w ([Si]): ≥0.10%, w ([Al]): 0.010%~0.050% aluminum-killed non-sulfur-containing carbon steel and alloy structural steel, including the following steps: 1) When pouring molten steel obtained from primary smelting into the ladle, aluminum, alloy, lime, and low-melting-point synthetic slag should be added sequentially. The total slag amount in the ladle at the end of tapping should be <2.0 kg / t, and the initial molten steel content should be... w ([Al]) = 0.015%~0.025%, primary slag is formed in the ladle; 2) The molten steel is refined in the LF process, with a total refining time of 55–85 minutes. During the first 10–15 minutes of refining, 1.2 ± 0.5 kg / t of lime is added to assist in further desulfurization. Silicon carbide, calcium carbide, and aluminum wire are used for slag surface deoxidation during the refining process. Aluminum wire is fed in to adjust the aluminum content in the steel to maintain a certain level. w ([Al]) = 0.020%~0.040%. For the last 5~10 minutes before the end of refining, set the ladle bottom blowing flow rate to 0.2~0.7 NL / (min•t) with weak stirring; 3) After LF refining, the ladle is processed in a VD furnace under a vacuum of ≤133 Pa for 20–30 minutes. The bottom blowing agitation flow rate of the ladle is set to 2.8–3.5 times the weak agitation flow rate for the first 5–10 minutes after LF refining. The soft blowing time after vacuum breaking is <35 minutes. 4) After the soft blowing is completed, the molten steel is cast into shape.

[0006] A further improvement to this invention is that, in step 1), the amount of lime is 5.0–6.0 kg / t, and the amount of synthetic slag is 2.9–3.3 kg / t. The mass percentage of the main components of the synthetic slag is as follows: w (CaO) = 25%~30%, w (SiO2) = 25%~35%, w (MgO) = 3%~4%, w (Al2O3) = 33%~38%.

[0007] A further improvement to this invention is that, in step 1), the main components of the primary smelting slag are... w (CaO) = 52%~55%, w (SiO2) = 10%~12% w (MgO) = 3.0%~4.5%, w (Al2O3) = 21%~25%, w (TFe)+ w (MnO) = 1.0%~2.0%.

[0008] A further improvement to this invention is that, in step 2), the refining process deoxidizes the slag surface. For the first 20 minutes of refining, silicon carbide, calcium carbide, and aluminum wire are used for diffusion deoxidation in a ratio of 3:0.5:0.5 to 3:1:1, with silicon carbide dosage ranging from 0.6 to 1.2 kg / t. After 20 minutes of refining, silicon carbide and aluminum wire are used for diffusion deoxidation in a ratio of 4:1 to 4:0.5. The total silicon carbide dosage is 1.7 to 2.3 kg / t.

[0009] A further improvement of this invention is that, in step 2), by adding lime, silicon carbide, calcium carbide, and aluminum wire strips to deoxidize the slag surface, the composition of the final refined slag is basically similar to that of the initial slag, and the main components of the final refined slag are... w (CaO) = 49%~53%, w (SiO2) = 11%~13% w (MgO) = 4.0%~5.5%, w (Al2O3) = 22-26%, w (TFe)+ w (MnO) < 0.5%.

[0010] The beneficial effects of this invention are as follows: First, this invention uses a lower melting point alloy slag and lime in a specific ratio to form a low melting point, highly fluid refining slag in the ladle. This facilitates rapid slag formation in the early stages of refining, desulfurization, and adsorption of small particulate inclusions formed during deoxidation in the steel.

[0011] Secondly, in the LF refining process, by adding lime and using silicon carbide and aluminum for slag surface deoxidation, the slag composition fluctuates little during the refining process, ensuring that the contents of CaO, SiO2, MgO and Al2O3 in the initial and final refining slag remain basically unchanged, and the slag is a low-melting-point, highly fluid refining slag that easily adsorbs inclusions from beginning to end.

[0012] Third, the present invention, by adding lime in the early stage of LF refining and using calcium carbide + silicon carbide slag surface deoxidation, can promote rapid and efficient deoxidation of slag surface, achieve efficient desulfurization in the early stage of refining, and at the same time achieve slag foaming and slag formation, and utilize electrode submerged arc heating.

[0013] Fourth, in the later stages of refining, only silicon carbide + aluminum wire is used for slag surface deoxidation. This ensures the relative stability of the slag composition and can both deoxidize the slag surface and avoid calcium gain in the molten steel during the steel-slag reaction due to slag surface deoxidation.

[0014] Fifth, in this invention, the bottom blowing stirring flow rate of the molten steel during the VD treatment process is set according to the flow rate of the weak stirring in the later stage of refining, which is convenient and reliable. It avoids the need to adjust the flow rate relatively randomly due to the need to observe the stirring situation during vacuuming, which would affect the degassing and steel slag reaction under vacuum.

[0015] Sixth, according to this invention, high-purity steel can be produced using the LF-VD process route, achieving a level comparable to that produced using the LF-RH process route, with T.[O] in the steel ranging from 4 to 8 × 10⁻⁶. -6 Non-metallic oxide inclusions should be tested according to Method A in GB / T10561, with a grade <1.0 and a maximum particle size of <15μm. Detailed Implementation

[0016] This invention patent provides a high-cleanliness steel production technology and method using the LF-VD process route.

[0017] The technical solution of the present invention will be described in detail below through specific embodiments.

[0018] The aluminum-killed non-sulfur-containing carbon steel and alloy structural steel of this invention were industrially produced in the steelmaking workshop of Jiangsu Shagang Group Huaigang Special Steel Co., Ltd. The specific process is as follows: Example 1 Gear steel SAE8620H is produced using the LF-VD process route. 1) When pouring the molten steel obtained from the primary smelting furnace into the ladle, aluminum, alloy, lime (6.0 kg / t), and low-melting-point synthetic slag (3.3 kg / t) are added sequentially. The total slag amount in the ladle after tapping is approximately 1.5 kg / t. Initial samples of the molten steel are taken after tapping. w ([Al]) = 0.015%, the main component of the primary slag formed in the ladle. w (CaO)=52.3%, w (SiO2) = 11.9%, w (MgO)=3.4%, w (Al2O3)=24.9%, w (TFe)+ w (MnO)=1.5%; 2) The molten steel is refined in the LF process for a total of 60 minutes. During the first 10-15 minutes of refining, 1.6 kg / t of lime is added to assist in further desulfurization. For the first 20 minutes of refining, silicon carbide, calcium carbide, and aluminum wire are used in a 3:1:1 ratio for diffusion deoxidation, with silicon carbide at 1.0 kg / t. After 20 minutes of refining, silicon carbide and aluminum wire are used in a 4:0.5 ratio for diffusion deoxidation, with a total silicon carbide usage of 2.1 kg / t. Aluminum wire is fed in to adjust the aluminum content in the steel to maintain a certain level. w ([Al]) = 0.025%~0.035%. The bottom-blowing gas flow rate in the ladle was 0.5 NL / (min·t) 5 minutes before the end of refining. The main components of the obtained refining slag... w (CaO)=52.4%, w (SiO2)=11.1%, w (MgO)=4.6%, w (Al2O3) = 25.1%, w (TFe)+ w (MnO)=0.32%.

[0019] 3) After LF refining, the ladle is processed in a VD furnace under a vacuum of ≤133 Pa for 25 minutes. The bottom blowing agitation flow rate of the ladle is set to 3.0 times the weak agitation flow rate 5-10 minutes before the end of LF refining. Soft blowing after vacuum breaking takes 25 minutes. 4) After the soft blowing is completed, the molten steel is cast into shape.

[0020] In the steel obtained through steps 1) to 4), T.[O] = 7.6 × 10⁻⁶. -6 Non-metallic oxide inclusions shall be tested according to Method A in GB / T10561, with a grade ≤0.5 and a maximum particle size of oxide inclusions <15μm.

[0021] Example 2 42CrMo steel is produced using the LF-VD process. 1) When pouring the molten steel obtained from the primary smelting furnace into the ladle, aluminum, alloy, lime (5.5 kg / t), and low-melting-point synthetic slag (3.1 kg / t) are added sequentially. The total slag amount in the ladle after tapping is approximately 1.5 kg / t. Initial samples of the molten steel are taken after tapping. w ([Al]) = 0.018%, the main component of the primary slag formed in the ladle. w (CaO)=53.6%, w (SiO2)=12.1%, w (MgO)=3.9%, w (Al2O3) = 23.2%, w (TFe)+ w (MnO) = 1.2%; 2) The molten steel is refined in the LF process for a total of 70 minutes. During the first 10-15 minutes of refining, 1.1 kg / t of lime is added to assist in further desulfurization. For the first 20 minutes of refining, silicon carbide, calcium carbide, and aluminum wire are used in a 3:1:1 ratio for diffusion deoxidation, with silicon carbide at 0.7 kg / t. After 20 minutes of refining, silicon carbide and aluminum wire are used in a 4:0.5 ratio for diffusion deoxidation, with a total silicon carbide usage of 2.0 kg / t. Aluminum wire is fed in to adjust the aluminum content in the steel to maintain a certain level. w ([Al]) = 0.025%~0.040%. The bottom-blowing gas flow rate in the ladle was 0.5 NL / (min·t) 8 minutes before the end of refining. The main components of the obtained refining slag... w (CaO)=53.9%, w (SiO2)=11.3%, w (MgO) = 4.7%, w (Al2O3)=24.9%, w (TFe)+ w (MnO)=0.21%.

[0022] 3) After LF refining, the ladle is processed in a VD furnace under a vacuum of ≤133 Pa for 30 minutes, and the bottom blowing agitation flow rate is set to 2.8 times the weak agitation flow rate in the 10 minutes before the end of LF refining. Soft blowing after vacuum breaking takes 25 minutes. 4) After the soft blowing is completed, the molten steel is cast into shape.

[0023] In the steel obtained through steps 1) to 4), T.[O] = 6.3 × 10⁻⁶. -6 Non-metallic oxide inclusions shall be tested according to Method A in GB / T10561, with a grade ≤0.5 and a maximum particle size of oxide inclusions <10μm.

[0024] Example 3 Flange rings are manufactured using 65Mn steel via the LF-VD process. 1) When pouring the molten steel obtained from the primary smelting furnace into the ladle, aluminum, alloys, lime (5.0 kg / t), and low-melting-point synthetic slag (2.9 kg / t) are added sequentially. The total slag amount in the ladle after tapping is approximately 1.5 kg / t. Initial samples of the molten steel are taken after tapping. w ([Al]) = 0.015%, the main component of the primary slag formed in the ladle. w (CaO)=55.0%, w (SiO2)=11.8%, w (MgO)=4.1%, w (Al2O3)=22.5%, w (TFe)+ w (MnO) = 1.3%; 2) The molten steel is refined in the LF process for a total of 65 minutes. During the first 15 minutes of refining, 1.1 kg / t of lime is added to assist in further desulfurization. For the first 20 minutes of refining, silicon carbide, calcium carbide, and aluminum wire are used in a 3:1:1 ratio for diffusion deoxidation, with silicon carbide at a dosage of 0.6 kg / t. After 20 minutes of refining, silicon carbide and aluminum wire are used in a 4:0.5 ratio for diffusion deoxidation, with a total silicon carbide dosage of 1.7 kg / t. Aluminum wire is fed in to adjust the aluminum content in the steel to maintain a certain level. w ([Al]) = 0.020%~0.030%. The bottom-blown gas flow rate in the ladle was 0.3 NL / (min·t) in the last 10 minutes before the end of refining. The main components of the obtained refining slag. w (CaO)=53%, w (SiO2) = 12.9%, w (MgO)=4.3%, w (Al2O3) = 22.1%, w (TFe)+ w (MnO)=0.17%.

[0025] 3) After LF refining, the ladle is processed in a VD furnace under a vacuum of ≤133 Pa for 25 minutes, and the bottom blowing agitation flow rate is set to 3.3 times the weak agitation flow rate in the 10 minutes before the end of LF refining. Soft blowing after vacuum breaking takes 20 minutes. 4) After the soft blowing is completed, the molten steel is cast into shape.

[0026] In the steel obtained through steps 1) to 4), T.[O] = 6.1 × 10⁻⁶. -6 Non-metallic oxide inclusions shall be tested according to Method A in GB / T10561, with a grade ≤0.5 and a maximum particle size of oxide inclusions <15μm.

Claims

1. A high-cleanliness steel production method using the LF-VD process, characterized by: Aluminum-killed non-sulfur-containing carbon steel and alloy structural steel with w([C])≥0.10%, w([Si])≥0.10%, and w([Al])0.010%~0.050%; The production process steps are as follows: 1) When the molten steel obtained from the primary smelting furnace is poured into the ladle, aluminum, alloy, lime, and low-melting-point synthetic slag are added in sequence. After tapping, the total amount of slag in the ladle is <2.0 kg / t, the initial w([Al]) of the molten steel is 0.015%~0.025%, and primary smelting furnace slag is formed in the ladle. 2) The molten steel is refined in LF (Leakage Refining), with a total refining time of 55-85 min. During the first 10-15 min of refining, 1.2 ± 0.5 kg / t of lime is added to assist in further desulfurization. Silicon carbide, calcium carbide, and aluminum wire are used for slag surface deoxidation during the refining process. Aluminum wire is fed in to adjust the aluminum content in the steel to maintain w([Al]) = 0.020%-0.040%. The bottom blowing flow rate of the ladle is set to 0.2-0.7 NL / (min•t) 5-10 min before the end of refining. In step 2), the refining process deoxidizes the slag surface. For the first 20 minutes of refining, silicon carbide, calcium carbide, and aluminum wire are used in a ratio of 3:0.5:0.5 to 3:1:1 for diffusion deoxidation, with silicon carbide dosage ranging from 0.6 to 1.2 kg / t. After 20 minutes of refining, silicon carbide and aluminum wire are used in a ratio of 4:1 to 4:0.5 for diffusion deoxidation, with a total silicon carbide dosage ranging from 1.7 to 2.3 kg / t. 3) After LF refining, the steel ladle is processed in a VD furnace. The vacuum degree is maintained at ≤133pa for 20-30 minutes. The bottom blowing stirring flow rate of the ladle is set to 2.8-3.5 times the soft stirring flow rate 5-10 minutes before the end of LF refining. The soft blowing time after breaking the vacuum is <35 minutes. 4) After the soft blowing process is completed, the molten steel is cast into shape. In step 1), the amount of lime is 5.0–6.0 kg / t, and the amount of synthetic slag is 2.9–3.3 kg / t. The main components of the synthetic slag by mass percentage are: w(CaO) = 25%–30%, w(SiO2) = 25%–35%, w(MgO) = 3%–4%, w(Al2O3) = 33%–38%. In step 1), the main components of the primary smelting slag are: w(CaO) = 52%–55%, w(SiO2) = 10%–12%, w(MgO) = 3.0%–4.5%, w(Al2O3) = 21%–25%, w(TFe) + w(MnO)=1.0%~2.0%. In step 2), by adding lime, silicon carbide, calcium carbide, and aluminum wire strip slag surface deoxidation, the composition of the final refined slag is basically similar to that of the initial slag. The main components of the final refined slag are w(CaO)=49%~53%, w(SiO2)=11%~13%, w(MgO)=4.0%~5.5%, w(Al2O3)=22%~26%, and w(TFe)+ w(MnO)<0.5%.