Method for smelting titanium-containing stainless steel in VOD (Vacuum Oxygen Decarburization) furnace

Abstract

The invention discloses a method for smelting titanium-containing stainless steel through a VOD furnace. The method comprises the steps that S1, initial molten steel is provided; s2, the initial molten steel is subjected to temperature rise, slagging and desulfurization and primary adjustment of main element components in an LF furnace, no titanium alloy is added in the process, and LF furnace molten steel is obtained; s3, the molten steel in the LF furnace is sequentially subjected to oxygen blowing decarburization, reduction and vacuum degassing in a VOD furnace, deep denitrification and deoxidation are conducted on the molten steel, the end point nitrogen content is controlled within 80 ppm, the end point oxygen content is controlled within 20 ppm, and no titanium alloy is added in the process; and S4, the blocky titanium alloy is added into the steel ladle according to 85%-95% of the target titanium content, then a titanium wire is added according to the difference value between the titanium content and the target titanium content, and the titanium-containing stainless steel is obtained. The method has the characteristics that the titanium yield is high and stable, the molten steel purity is high, the titanium component control is accurate, and the process universality is high.

Description

Technical Field

[0001] This invention relates to a method for smelting stainless steel, and more particularly to a method for smelting titanium-containing stainless steel in a VOD furnace. Background Technology

[0002] Titanium (Ti), as a strong carbide-nitride forming element, is often added to stainless steels (such as 321 and 316Ti) or certain alloy steels. It combines with carbon and nitrogen to form stable TiC or TiN, thereby inhibiting the precipitation of chromium carbides and improving the steel's resistance to intergranular corrosion. However, titanium is also a highly reactive metal that oxidizes easily. In the high-temperature oxidizing environment of steelmaking, improving titanium yield and reducing titanium inclusions has been a long-standing technical challenge.

[0003] The traditional titanium-containing steel smelting process typically involves EAF (Extractable Acid Furnace) → AOD (Advanced Oxygen Deposition) → LF (Fuel Refining Furnace). During the AOD tapping process, a suitable amount of titanium alloy is added to the ladle to complete the initial titanium composition. Fine-tuning of the titanium composition is then performed in the LF furnace, and the steel is finally poured after LF refining. This method has significant drawbacks: 1. Low and unstable titanium yield: When using traditional smelting processes, the nitrogen content in the steel is relatively high due to the limitations of the AOD furnace equipment, typically exceeding 120 ppm (approximately 150 ± 30 ppm) after AOD refining. During the AOD tapping process, the molten steel undergoes secondary oxidation and nitrogen enrichment upon contact with air. When titanium alloy is added to the ladle at this time, some of the titanium will be oxidized and burned off, while some will react with the nitrogen in the steel, resulting in an extremely low titanium alloy yield (possibly below 70%).

[0004] 2. Poor purity of molten steel: titanium inclusion grade ≥ 3.0. Numerous fine inclusions (such as TiO2 and TiN) generated by the reaction of titanium with oxygen and nitrogen are difficult to float and remove from the molten steel, severely deteriorating its purity and affecting the surface quality and material properties of the steel.

[0005] 3. Poor accuracy in composition control: Due to fluctuations in yield, it is difficult to guarantee the accuracy of final titanium content control, which often exceeds the specification range. Summary of the Invention

[0006] The purpose of this invention is to provide a method for smelting titanium-containing stainless steel in a VOD furnace. This invention features high and stable titanium yield, high steel purity, precise control of titanium composition, and strong process versatility.

[0007] The technical solution of the present invention: A method for smelting titanium-containing stainless steel in a VOD furnace, comprising the following steps: S1.EAF electric arc furnace smelting: provides initial molten steel; S2.LF Furnace Processing: The initial molten steel is heated, slag-forming, desulfurized, and the main element composition is initially adjusted in the LF furnace without adding any titanium alloys, resulting in LF furnace molten steel; S3. VOD furnace refining: The molten steel from the LF furnace is sequentially subjected to oxygen blowing decarburization, reduction and vacuum degassing in the VOD furnace to deeply denitrify and deoxidize the molten steel, controlling the final nitrogen content to within 80ppm and the final oxygen content to within 20ppm, without adding any titanium alloys during the process. S4. Titanium alloying operation: First, add block titanium alloy to the ladle according to the target titanium content of 85%-95%, and then add titanium wire according to the difference between the target titanium content and the titanium alloy content to obtain titanium-containing stainless steel.

[0008] In the aforementioned method for smelting titanium-containing stainless steel in a VOD furnace, in step S2, desulfurization is performed to ensure that the sulfur content does not exceed 0.002%.

[0009] In the aforementioned method for smelting titanium-containing stainless steel in a VOD furnace, in step S2, the main element composition is adjusted to the upper-middle range of the target range of the finished product; in step S3, the main element composition is adjusted to the target range of the finished product.

[0010] In the aforementioned method for smelting titanium-containing stainless steel in a VOD furnace, in step S3, a high vacuum of ≤67Pa is maintained for 2-3 minutes during the vacuum period before oxygen blowing for decarburization, and then oxygen is blown in to utilize the carbon-oxygen reaction to make the C content in the molten steel ≤0.04%.

[0011] In the aforementioned method for smelting titanium-containing stainless steel in a VOD furnace, in step S3, during the vacuum degassing stage, a high vacuum of ≤67Pa is maintained for ≥20 minutes.

[0012] In the aforementioned method for smelting titanium-containing stainless steel in a VOD furnace, the nitrogen content in step S3 is controlled at 30-60 ppm.

[0013] In the aforementioned method for smelting titanium-containing stainless steel in a VOD furnace, in step S4, argon gas is blown from the bottom of the ladle for stirring, with an argon gas pressure of 0.1-0.5 MPa and a stirring time of 3-8 min.

[0014] In the aforementioned method for smelting titanium-containing stainless steel in a VOD furnace, in step S4, the feeding speed of the titanium wire is 2-3 m / s.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. Extremely high and stable titanium yield: Because titanium is added to molten steel with low nitrogen and low oxygen after VOD refining, the titanium alloying is performed post-processed, ensuring that the molten steel has favorable low oxygen and low nitrogen conditions before the titanium alloy is added, resulting in minimal titanium loss. This invention can stably increase the overall titanium yield to over 90%, and can reach up to 95%.

[0016] 2. Significantly improved steel purity: Titanium inclusion grade ≤ 1.5. Due to the addition of titanium after VOD refining, the formation of titanium-containing inclusions such as TiN and TiO2 is greatly reduced, and the total oxygen content and titanium inclusion level in the steel are significantly lowered.

[0017] 3. Precise composition control: The secondary alloying mode of "block titanium alloy + titanium wire feeding" is adopted, which realizes coarse and fine adjustment of titanium content. The composition control is highly accurate and can easily control the titanium content in the finished product within a very narrow range, with an error of no more than ±3% from the target value.

[0018] 4. Strong process versatility: This method is not only applicable to titanium-containing stainless steel, but also to other alloy steels with high titanium content requirements, such as nickel-based alloy steels like N08810 (Ti content: 0.15%-0.60%) and N08825 (Ti content: 0.60%-1.20%), and has wide applicability. Detailed Implementation

[0019] The present invention will be further described below with reference to embodiments, but these embodiments are not intended to limit the scope of the invention.

[0020] A method for smelting titanium-containing stainless steel using a VOD furnace, the core of which lies in optimizing the process flow (EAF→LF→VOD), placing the titanium alloying operation entirely after the VOD refining process. Specific steps include: S1.EAF electric furnace smelting: provides initial molten steel.

[0021] S2.LF Furnace Processing: The initial molten steel is heated, slag-forming, desulfurized, and the main element composition (such as Cr and Ni) is initially adjusted in the LF furnace. The sulfur content does not exceed 0.002%, and the main element composition is adjusted to the upper-middle range of the target range of the finished product. No titanium alloys are added during the process to obtain LF furnace molten steel.

[0022] S3. VOD Furnace Refining: Molten steel from the LF furnace undergoes sequential oxygen blowing decarburization, reduction, and vacuum degassing in the VOD furnace. Before oxygen blowing decarburization, a high vacuum of ≤67Pa is maintained for 2-3 minutes, followed by the introduction of oxygen. The carbon-oxygen reaction is used to reduce the carbon content in the molten steel to ≤0.04%. During the vacuum degassing stage, a high vacuum of ≤67Pa is maintained for ≥20 minutes to deeply denitrify and deoxidize the molten steel. The final nitrogen content is controlled below 80ppm, preferably between 30-60ppm, and the final oxygen content is controlled below 20ppm. The main element composition is adjusted to the target range of the finished product. No titanium alloys are added during this process, creating a pure environment of low nitrogen and low oxygen for subsequent titanium alloying.

[0023] S4. Titanium Alloying Operation: After all refining processes in the VOD furnace are completed, blocky titanium alloys are first added to the ladle to achieve a target titanium content of 85%-95%, which are then melted using the sensible heat of the molten steel. Then, for precise "endpoint" control, titanium wire is injected deep into the molten steel using a wire feeder at a speed of 2-3 m / s to fine-tune the titanium composition and replenish the remaining 5%-15% of the target titanium content. During this process, argon gas is blown from the bottom of the ladle for stirring to ensure uniform composition and temperature, resulting in titanium-containing stainless steel.

[0024] This invention postpones the titanium alloying process entirely, adding titanium after VOD refining. Furthermore, strict preconditions are set before alloying to ensure that the nitrogen content in the molten steel is ≤80ppm and the oxygen content is ≤20ppm. This significantly reduces the formation of titanium inclusions, resulting in a marked reduction in total oxygen content and inclusion levels in the steel. The titanium inclusion grade is ≤1.5, creating a favorable environment for improving the purity of the molten steel and minimizing titanium loss. The overall titanium yield is stably increased to over 90%.

[0025] This invention first uses block titanium alloy to roughly prepare the titanium content to 85%-95%, and then fine-tunes the titanium content by using titanium wire based on the sampling results. The two-step alloying operation of "rough preparation + fine adjustment" achieves economic efficiency and precise control of titanium content in the smelting of titanium-containing stainless steel.

[0026] Example 1: Taking the smelting of 316Ti stainless steel (target Ti value: 0.40%) as an example, the specific composition requirements are: C≤0.08%, Si≤1.00%, Mn≤2.00%, P≤0.045%, S≤0.030%, Cr: 16.0%-18.0%, Ni: 10.0%-14.0%, Mo: 2.00%-3.00%, Ti: 5(C+N)%-0.70%, N≤0.10%; A method for smelting 316Ti stainless steel using a VOD furnace includes the following steps: S1.EAF electric furnace smelting: Add stainless steel scrap and alloy, and add appropriate amounts of lime and fluorite to form slag; after the materials are completely melted, tap the steel at a temperature greater than 1600℃ to obtain the initial molten steel.

[0027] S2.LF Furnace Processing: Initial molten steel is poured into the ladle. Slag is first removed to ensure complete removal, then appropriate amounts of lime and fluorite are added to form slag. The molten steel undergoes slag adjustment in the LF furnace, and is heated to ≥1580℃. Desulfurization and main element composition (Cr, Ni, Mo) are performed to ensure sulfur content does not exceed 0.002% and main element composition is adjusted to the upper-middle range of the finished product target (Cr: 17.0%-17.45%, Ni: 11.00%-11.20%, Mo: 2.10%-2.20%). Before ladle hoisting, the steel temperature is raised to above 1650℃, ensuring C ≤0.35%, Si: 0.15%~0.30%, and other main elements (Cr, Ni, Mo) remain at the upper-middle range of the finished product target. Once the steel temperature reaches 1650℃-1700℃, the ladle is hoisted to the slag removal station, where the slag is removed completely, and then hoisted to the VOD station. No titanium iron is added throughout the process.

[0028] S3. VOD Furnace Refining: Molten steel undergoes sequential oxygen blowing decarburization, reduction, and vacuum degassing within the VOD furnace. Before oxygen blowing decarburization, a high vacuum of ≤67 Pa is maintained for 2-3 minutes. Oxygen is then blown in, utilizing the carbon-oxygen reaction to ensure the C content in the molten steel is ≤0.04%. After the carbon-oxygen reaction, the furnace is opened and samples are taken. The amounts of aluminum ingots and ferrosilicon to be added are calculated based on the sample composition. Then, appropriate amounts of lime and fluorite are added to form slag, followed by the addition of aluminum ingots and ferrosilicon for reduction deoxidation. Afterward, the furnace is closed, bottom-blown argon stirring is performed, and vacuum treatment is initiated, maintaining a high vacuum of ≤67 Pa for ≥20 minutes for deep denitrification and deoxidation. The final nitrogen content is controlled below 80 ppm, and the oxygen content below 20 ppm. A routine sample and a gas sample are taken, ensuring the N content is below 80 ppm. Based on the C and N composition, the amount of titanium alloy to be added is calculated, with minor adjustments made to other main elements according to the target range for the finished product.

[0029] S4. Titanium Alloying Operation: First, add block titanium alloy to the ladle for coarse adjustment. The block titanium alloy is added according to the target titanium content of 90%. A weak argon purge is applied to the bottom of the ladle at a pressure of 0.3 MPa. After stirring for 5 minutes, a sample is taken for analysis. Based on the difference between the sample result and the target titanium content (0.40%), the subsequent fine-tuning of the titanium wire feed rate is determined. Feeding the titanium wire at a speed of 2-3 m / s using a wire feeder ensures that the titanium wire penetrates the slag layer, improving the yield. This allows for precise fine-tuning of the titanium content to the target value of 0.40%, resulting in 316Ti stainless steel.

[0030] The overall titanium yield reached 92% throughout the process.

[0031] It should be understood that the above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them. Those skilled in the art can modify the technical solutions described in the above embodiments, or make equivalent substitutions for some of the technical features; and all such modifications and substitutions should fall within the protection scope of the appended claims of the present invention.

Claims

1. A method for smelting titanium-containing stainless steel in a VOD furnace, characterized in that: Includes the following steps: S1.EAF electric arc furnace smelting: provides initial molten steel; S2.LF Furnace Processing: The initial molten steel is heated, slag-forming, desulfurized, and the main element composition is initially adjusted in the LF furnace without adding any titanium alloys, resulting in LF furnace molten steel; S3. VOD furnace refining: The molten steel from the LF furnace is sequentially subjected to oxygen blowing decarburization, reduction and vacuum degassing in the VOD furnace to deeply denitrify and deoxidize the molten steel, controlling the final nitrogen content to within 80ppm and the final oxygen content to within 20ppm, without adding any titanium alloys during the process. S4. Titanium alloying operation: First, add block titanium alloy to the ladle according to the target titanium content of 85%-95%, and then add titanium wire according to the difference between the target titanium content and the titanium alloy content to obtain titanium-containing stainless steel.

2. The method for smelting titanium-containing stainless steel in a VOD furnace according to claim 1, characterized in that: In step S2, desulfurization is performed so that the sulfur content does not exceed 0.002%.

3. The method for smelting titanium-containing stainless steel in a VOD furnace according to claim 1, characterized in that: In step S2, the main element composition is adjusted to the upper-middle range of the target range of the finished product; in step S3, the main element composition is adjusted to the target range of the finished product.

4. The method for smelting titanium-containing stainless steel in a VOD furnace according to claim 1, characterized in that: In step S3, a high vacuum of ≤67Pa is maintained for 2-3 minutes during the vacuum period before oxygen blowing decarburization, and then oxygen is blown in to make the C content in the molten steel ≤0.04% by utilizing the carbon-oxygen reaction.

5. The method for smelting titanium-containing stainless steel in a VOD furnace according to claim 1, characterized in that: In step S3, during the vacuum degassing stage, a high vacuum of ≤67Pa is maintained for ≥20 minutes.

6. The method for smelting titanium-containing stainless steel in a VOD furnace according to claim 1, characterized in that: In step S3, the nitrogen content is controlled at 30-60 ppm.

7. The method for smelting titanium-containing stainless steel in a VOD furnace according to claim 1, characterized in that: In step S4, argon gas is blown from the bottom of the ladle for stirring. The argon gas pressure is 0.1-0.5 MPa, and the stirring time is 3-8 min.

8. The method for smelting titanium-containing stainless steel in a VOD furnace according to claim 1, characterized in that: In step S4, the feeding speed of the titanium wire is 2-3 m / s.