A method for producing nitrogen-containing stainless steel by VOD

Abstract

The present application relates to a kind of VOD production nitrogen-containing stainless steel method, belong to VOD smelting technical field.The method includes: the whole VOD smelting process includes: ladle into station, vacuum extraction, oxygen blowing, into oxygen decarburization stage, when oxygen potential rapid decline approaches 0, again continue to blow oxygen 3~5min, then oxygen is stopped, into vacuum decarburization stage.Vacuum degree is reduced to limit vacuum, and is blown for about 20min, when oxygen potential is again rapidly reduced and approaches 0, vacuum decarburization stage ends, into reduction stage.Slagging agent, alloy and reducing agent etc. are added to molten steel, nitrogen is bottom-blown for 15~20min, then vacuum is broken, continue to bottom-blown nitrogen, according to the target requirement of tapping to determine the nitrogen blowing time, and the composition and temperature of molten steel are slightly adjusted, then temperature is measured and sampled, when reaching the target requirement, stop blowing, and ladle is lifted away.The method can reduce the amount of argon blowing, reduce the amount of nitriding alloy, simplify process flow, and reduce smelting cost.

Description

Technical Field

[0001] This invention belongs to the field of VOD smelting technology and relates to a method for producing nitrogen-containing stainless steel using VOD. Background Technology

[0002] Replacing nickel with nitrogen in stainless steel alloying has become an important trend in the development of new stainless steel varieties. In traditional processes, nitrogen alloying of molten steel is usually achieved by adding nitriding alloys to the molten steel after degassing and refining. However, this process has the following drawbacks: First, the preparation process of nitride alloys is lengthy and energy-intensive, and the market price is expensive. Adding large quantities will significantly increase the alloying cost of molten steel, weakening the cost advantage brought by replacing nickel with nitrogen.

[0003] Secondly, during the preparation of nitriding alloys, impurities such as carbon, silicon, phosphorus, sulfur, and metallic inclusions are inevitably carried. When added to molten steel, these impurities can cause secondary pollution, increasing the content of inclusions in the steel, reducing the cleanliness of the steel, and consequently affecting the corrosion resistance and mechanical properties of the steel, making it difficult to meet the quality requirements of high-cleanliness steel.

[0004] Vacuum oxygen decarburization (VOD) refining furnaces are key refining equipment in stainless steel smelting. The furnace bottom is equipped with a bottom-blowing system, which can flexibly switch the type and flow rate of bottom-blowing gas according to the smelting process to agitate the molten steel, promoting homogenization of its composition and temperature, and providing favorable kinetic conditions for decarburization and degassing reactions. If nitrogen can be directly blown into the molten steel through the bottom-blowing system to achieve nitrogen enrichment, the step of adding nitriding alloys can be eliminated.

[0005] Therefore, how to utilize the bottom blowing system of the VOD refining furnace to directly add nitrogen to the molten steel through bottom blowing nitrogen to achieve nitrogen alloying, while avoiding impurity contamination caused by nitriding alloys and improving the cleanliness of steel, effectively reducing production costs and realizing the production of high-cleanliness, low-carbon, nitrogen-containing stainless steel, is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0006] In view of this, the purpose of the present invention is to provide a method for producing nitrogen-containing stainless steel by VOD, which reduces the amount of argon blowing, reduces the amount of nitriding alloy added, simplifies the process flow, and reduces smelting costs.

[0007] To achieve the above objectives, the present invention provides the following technical solution: A method for producing nitrogen-containing stainless steel via VOD includes ladle loading and ladle unloading, and between ladle loading and ladle unloading, the method further includes the following steps: Oxygen blowing decarburization stage: reduce the vacuum level, lower the oxygen lance, start oxygen blowing, and stir with bottom blowing argon gas; Vacuum decarburization stage: Stop oxygen blowing, maintain ultimate vacuum, and stir with bottom blowing argon gas; Reduction stage: Maintain ultimate vacuum, add slag-forming agent, reducing agent and alloy, and bottom-blown nitrogen to increase nitrogen content in molten steel; Fine-tuning stage: break the vacuum, measure the temperature and take samples. Based on the sampling results, determine whether to continue bottom blowing nitrogen and adjust the composition and temperature of the molten steel.

[0008] Furthermore, when the ladle enters the furnace, the molten steel must have a C content of ≤0.5%, a Si content of ≤0.2%, and a temperature of 1600–1650°C to prevent splashing of molten steel during the decarburization stage.

[0009] Furthermore, during the oxygen blowing decarburization stage, when the vacuum level drops to 25 kPa or below, the lance position is lowered to begin oxygen blowing, with an oxygen flow rate of 900–1000 Nm³. 3 / h, bottom-blown argon gas stirring, argon gas flow rate is 2.5~3 NL / min / t.

[0010] Furthermore, during the oxygen blowing decarburization stage, the smelting process inside the furnace is monitored using the oxygen potential change curve. When the oxygen potential drops rapidly to near 0 mV, oxygen blowing continues for another 3 to 5 minutes before being stopped, and the process enters the vacuum decarburization stage.

[0011] Furthermore, in the vacuum decarburization stage, the vacuum level is reduced to the ultimate vacuum, and bottom-blown argon gas is used for stirring, with an argon gas flow rate of 4.5–5 NL / min / t.

[0012] Furthermore, during the vacuum decarburization stage, the oxygen potential increases again. When the oxygen potential decreases rapidly to near 0 mV, the vacuum decarburization stage ends, and the reduction stage begins.

[0013] Furthermore, during the reduction stage, slag-forming agents such as lime and fluorite are added to the molten steel, reducing agents such as SiFe or Al particles are added, and alloys such as ferrochrome, ferromanganese, and other required alloys are added. The entire reduction stage lasts for 15 to 20 minutes. During this period, nitrogen is bottom-blown at a flow rate of 2.5 to 3 NL / min / t to increase nitrogen in the molten steel.

[0014] Furthermore, after the reduction stage is completed, the vacuum is broken, nitrogen is continuously blown from the bottom, and the composition and temperature of the molten steel are finely adjusted. Then, temperature is measured and samples are taken. When the target requirements are met, the blowing is stopped, the ladle is lifted off the ground, and the smelting process is completed.

[0015] Furthermore, throughout the smelting process, a nitrogen content control model is used to monitor the nitrogen content in the molten steel in real time. This control model is established through the thermodynamics and kinetics of nitrogen absorption and denitrification, and the real-time nitrogen content in the molten steel is calculated by the change in the bottom-blown gas flow rate.

[0016] The beneficial effects of this invention are as follows: 1) Reduce alloying costs and improve the purity of molten steel. This invention directly blows nitrogen into the molten steel during the reduction and fine-tuning stages to complete nitrogen alloying. By replacing the nitriding alloy with bottom-blown nitrogen, the amount of nitriding alloy added is reduced. At the same time, it avoids secondary pollution of the molten steel by impurity elements carried by the nitriding alloy, thereby improving the purity of the molten steel and facilitating the production of high-purity low-carbon nitrogen-containing stainless steel.

[0017] 2) Reduce argon consumption and save smelting costs. In the reduction and fine-tuning stages, this invention uses nitrogen to replace part of the argon as the bottom-blowing stirring gas. While ensuring the stirring effect of the molten steel and promoting the homogenization of the composition and temperature of the molten steel, it significantly reduces the amount of argon blown, thereby reducing smelting costs.

[0018] 3) Simplified process flow and shortened smelting cycle. This invention completes the nitrogen addition of molten steel during the VOD refining process. The nitrogen content of the molten steel meets the target requirements before tapping, eliminating the need for subsequent nitrogen addition processes such as LF (Ladle Refining Furnace). This simplifies the operation process, reduces process costs, and shortens smelting time.

[0019] 4) Real-time monitoring and precise control of nitrogen content in molten steel. This invention employs a nitrogen content control model based on the thermodynamics and kinetics of nitrogen absorption and denitrification. The nitrogen content in the molten steel is calculated in real time by changing the flow rate of bottom-blown gas. The nitrogen addition process can be determined without repeatedly measuring temperature, taking samples, and waiting for test results, thus improving the accuracy of nitrogen content measurement and compositional stability in the tapped steel.

[0020] 5) Improved stability of the smelting process. By limiting the carbon content, silicon content, and temperature conditions of the molten steel entering the furnace, this invention effectively prevents splashing of molten steel during the decarburization stage, thereby improving the safety of the smelting process and the metal yield.

[0021] Other advantages, objectives, and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination, or may be learned from practice of the invention. The objectives and other advantages of the invention can be realized and obtained through the following description. Attached Figure Description

[0022] To make the objectives, technical solutions, and advantages of the present invention clearer, the preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, wherein: Figure 1 This is a flowchart of the method for producing nitrogen-containing stainless steel using VOD according to the present invention. Detailed Implementation

[0023] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments are only schematic representations of the basic concept of the present invention. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0024] The accompanying drawings are for illustrative purposes only and are schematic diagrams, not actual pictures. They should not be construed as limiting the invention. To better illustrate the embodiments of the invention, some parts in the drawings may be omitted, enlarged, or reduced, and do not represent the actual product dimensions. It is understandable to those skilled in the art that some well-known structures and their descriptions may be omitted in the drawings.

[0025] In the accompanying drawings of the embodiments of the present invention, the same or similar reference numerals correspond to the same or similar components. In the description of the present invention, it should be understood that if terms such as "upper," "lower," "left," "right," "front," and "rear" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, they are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, the terms used to describe positional relationships in the drawings are only for illustrative purposes and should not be construed as limiting the present invention. For those skilled in the art, the specific meaning of the above terms can be understood according to the specific circumstances.

[0026] Implementation 1: Please see Figure 1 This embodiment provides a method for producing nitrogen-containing stainless steel using VOD, specifically including the following steps: Step 1, Ladle entry: The composition of the molten steel in the ladle is C: 0.18%, Si: 0.08%, N: 0.047%, temperature 1640°C, and the empty space in the ladle is 1000mm.

[0027] Step 2, Oxygen blowing decarburization stage: When the vacuum level drops to 25 kPa, oxygen blowing begins at a flow rate of 900 Nm³. 3 / h, bottom blowing with argon gas for stirring, argon gas flow rate of 2.6 NL / min / t, oxygen blowing time of 19min.

[0028] Step 3, Vacuum Decarburization Stage: Reduce the vacuum level to the ultimate vacuum to enter the vacuum decarburization stage. Stir with bottom-blown argon gas at a flow rate of 4.5 NL / min / t for 20 minutes. When the oxygen potential drops rapidly again to near zero, the reduction stage begins.

[0029] Step 4, reduction stage: Add 5.8 kg / t of low-carbon lime, 3.6 kg / t of fluorite, 2.1 kg / t of ferrochrome, ferromanganese, ferrosilicon, and 4 kg / t of Al particles to the molten steel. Bottom-purge nitrogen gas at a flow rate of 2.8 NL / min / t for 17 min.

[0030] Step 5, Fine-tuning stage: After the reduction stage is completed, the vacuum is broken and nitrogen is continuously blown from the bottom at a flow rate of 4 NL / min / t for 10 minutes. Then, the temperature is measured and a sample is taken. The nitrogen content in the molten steel is 0.072%.

[0031] Step 6: Lift the ladle away.

[0032] Example 2: Please see Figure 1 This embodiment provides a method for producing nitrogen-containing stainless steel using VOD, specifically including the following steps: Step 1, Ladle entry: The composition of the molten steel in the ladle is C: 0.26%, Si: 0.10%, N: 0.059%, temperature 1618°C, and the empty space in the ladle is 1080mm.

[0033] Step 2, Oxygen blowing decarburization stage: When the vacuum level drops to 20 kPa, oxygen blowing begins at a flow rate of 1000 Nm³. 3 / h, bottom blowing argon gas stirring, argon gas flow rate is 3 NL / min / t, oxygen blowing time is 23min.

[0034] Step 3, Vacuum decarburization stage: Reduce the vacuum level to the ultimate vacuum and enter the vacuum decarburization stage. Stir with bottom-blown argon gas at a flow rate of 4.8 NL / min / t and maintain for 22 minutes before entering the reduction stage.

[0035] Step 4, reduction stage: Add 6 kg / t of low-carbon lime, 3.7 kg / t of fluorite, ferrochrome, ferromanganese, ferromolybdenum, 2.2 kg / t of ferrosilicon, and 3.8 kg / t of Al particles to the molten steel. Bottom-purge nitrogen gas at a flow rate of 3 NL / min / t for 20 min.

[0036] Step 5, Fine-tuning stage: After the reduction stage is completed, the vacuum is broken, the temperature is measured and a sample is taken. The nitrogen content of the molten steel is 0.033%, and there is no need to add nitrogen by bottom blowing argon gas.

[0037] Step 6: Lift the ladle away.

[0038] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for producing nitrogen-containing stainless steel via VOD, comprising ladle loading and ladle unloading, characterized in that, Between the arrival of the ladle at the station and the removal of the ladle, the method also includes the following steps: Oxygen blowing decarburization stage: reduce the vacuum level, lower the oxygen lance, start oxygen blowing, and stir with bottom blowing argon gas; Vacuum decarburization stage: Stop oxygen blowing, maintain ultimate vacuum, and stir with bottom blowing argon gas; Reduction stage: Maintain ultimate vacuum, add slag-forming agent, reducing agent and alloy, and bottom-blown nitrogen to increase nitrogen content in molten steel; Fine-tuning stage: break the vacuum, measure the temperature and take samples. Based on the sampling results, determine whether to continue bottom blowing nitrogen and adjust the composition and temperature of the molten steel.

2. The method for producing nitrogen-containing stainless steel using VOD according to claim 1, characterized in that, When the ladle enters the furnace, the molten steel should have a C content of ≤0.5%, a Si content of ≤0.2%, and a temperature of 1600–1650°C to prevent splashing of molten steel during the decarburization stage.

3. The method for producing nitrogen-containing stainless steel using VOD according to claim 1, characterized in that, During the oxygen blowing decarburization stage, when the vacuum level drops to 25 kPa or below, lower the lance position to begin oxygen blowing, with an oxygen flow rate of 900–1000 Nm³. 3 / h, bottom-blown argon gas stirring, argon gas flow rate is 2.5~3 NL / min / t.

4. The method for producing nitrogen-containing stainless steel using VOD according to claim 1 or 3, characterized in that, During the oxygen blowing decarburization stage, the smelting process in the furnace is monitored using the oxygen potential change curve. When the oxygen potential drops rapidly to close to 0 mV, oxygen blowing continues for another 3 to 5 minutes before stopping, and the process enters the vacuum decarburization stage.

5. The method for producing nitrogen-containing stainless steel using VOD according to claim 1, characterized in that, During the vacuum decarburization stage, the vacuum level is reduced to the ultimate vacuum, and bottom-blown argon gas is used for stirring, with an argon gas flow rate of 4.5–5 NL / min / t.

6. The method for producing nitrogen-containing stainless steel using VOD according to claim 1 or 5, characterized in that, During the vacuum decarburization stage, the oxygen potential increases again. When the oxygen potential decreases rapidly to near 0 mV, the vacuum decarburization stage ends and the reduction stage begins.

7. The method for producing nitrogen-containing stainless steel using VOD according to claim 1, characterized in that, During the reduction stage, slag-forming agents added to the molten steel include lime and fluorite, reducing agents include SiFe or Al particles, and alloys include ferrochrome, ferromanganese, or other required alloys. The entire reduction stage lasts for 15 to 20 minutes. During this period, nitrogen is bottom-blown at a flow rate of 2.5 to 3 NL / min / t to increase nitrogen in the molten steel.

8. The method for producing nitrogen-containing stainless steel using VOD according to claim 1, characterized in that, After the reduction stage is completed, the vacuum is broken, nitrogen is continuously blown from the bottom, and the composition and temperature of the molten steel are finely adjusted. Then, temperature is measured and samples are taken. When the target requirements are met, the blowing is stopped, the ladle is lifted off the ground, and the smelting process is completed.

9. The method for producing nitrogen-containing stainless steel using VOD according to claim 1, characterized in that, Throughout the smelting process, a nitrogen content control model is used to monitor the nitrogen content in the molten steel in real time. This control model is established through the thermodynamics and kinetics of nitrogen absorption and denitrification, and the real-time nitrogen content in the molten steel is calculated by the change in the bottom-blown gas flow rate.

Images