Method for controlling nitrogen content in molten steel

Abstract

The invention belongs to the technical field of steel casting production, and provides a deoxidation control method for a high-alloy steel casting, which comprises an EAF furnace: adopting an oxidation smelting process, and controlling residual elements P to be less than or equal to 0.028% and V to be less than or equal to 0.08%; in the first LF furnace, a nitrogen control measure is adopted, firstly, CaO, fluorite, a synthetic refining slag agent and Al particles are added, and thin slag is rapidly manufactured; then CaO, aluminum-calcium alloy balls, Al particles and the like are supplemented to adjust the alkalinity of the slag, and the thickness of a slag layer is increased; in the VOD furnace, an oxygen blowing reaction and a decarburization reaction are carried out at the same time; and in the second LF furnace, the measures of combining deoxidation with microalloying are adopted, and the components of the molten steel are accurately adjusted. By means of the method, the total content of nitrogen in the molten steel can be controlled to be 50 ppm or below, the nitrogen content of a finished casting blank is controlled to be 50-65 ppm or below, and the use requirements of customers are met.

Description

Technical Field

[0001] This invention belongs to the field of steel casting production technology and provides a method for controlling the nitrogen content in molten steel. Background Technology

[0002] During the smelting of cast steel parts, in the general EAF furnace and the second LF furnace refining process, when the electric arc is ignited during heating, the electric arc at high temperature will accelerate the dissociation of N2 in the air to form nitrogen ions. Under the action of the electric current, the nitrogen ions directly enter the molten steel. In the early stage of the electric furnace, the slag arc submersion effect is poor. At the same time, nitrogen from ferroalloys, scrap steel and slag will enter the molten steel along with the furnace charge. During the tapping process, the molten steel comes into contact with the air, resulting in a large amount of nitrogen absorption by the molten steel. According to production test statistics, the nitrogen content from EAF furnace tapping to LF furnace refining is basically between 30ppm and 50ppm. The average nitrogen increase during the EAF furnace tapping process is between 10ppm and 20ppm. In the early stages of refining in the LF furnace, although the nitrogen content in the molten steel is low, it increases by 10 to 20 ppm during subsequent refining. During the subsequent casting process, the molten steel inevitably absorbs nitrogen from the air, increasing by another 10 to 20 ppm. These processes result in a nitrogen content of 80 to 120 ppm in the finished steel castings, far exceeding the required nitrogen content. Furthermore, in carbon steel and low-alloy steel, nitrogen is mostly present as an impurity. Nitrogen causes strain aging in low-carbon steel, increasing strength and hardness while decreasing toughness and increasing notch sensitivity. Nitrogen is also a major cause of steel brittleness, and its harmfulness is far greater than that of phosphorus. In conclusion, when smelting steels requiring high impact toughness and high toughness, and with strict control over titanium nitride inclusions, the nitrogen content must be strictly controlled. Therefore, how to control nitrogen content during the smelting process to improve the performance of steel castings is a pressing issue. Summary of the Invention

[0003] To address the aforementioned technical problems, it is necessary to propose a method for controlling the nitrogen content in molten steel.

[0004] A method for controlling nitrogen content in molten steel, specifically comprising an EAF furnace + first LF furnace + VOD furnace + second LF furnace, including the following steps:

[0005] EAF furnace: Employs oxidation smelting process, controlling residual elements P≤0.028% and V≤0.08%;

[0006] First LF furnace: Nitrogen control measures are adopted. CaO, fluorite, synthetic refining slag agent and Al particles are added first to quickly produce thin slag; then CaO, aluminum-calcium alloy balls and Al particles are added to adjust the slag basicity and increase the slag layer thickness.

[0007] VOD furnace: adopts a process in which oxygen blowing reaction and decarburization reaction are carried out simultaneously;

[0008] The second LF furnace: adopts deoxidation combined with microalloying measures to precisely adjust the composition of the molten steel.

[0009] In one embodiment, in the EAF furnace step, the furnace charge is mainly composed of Class A scrap steel, with lime added at a rate of 30 kg / ton of steel to 60 kg / ton of steel, and carbon raiser at a rate of 1.0% to 1.5%.

[0010] In one embodiment, during the EAF furnace step, a submerged arc foam slag operation is adopted, in which 30 kg / t to 60 kg / t of lime, 2 kg / t to 4 kg / t of fluorite, and 1 kg / t to 3 kg / t of carbon powder are added to increase the thickness of the slag layer.

[0011] In one embodiment, during the EAF furnace step, the decarburization rate is controlled to be 0.3% to 0.6%.

[0012] In one embodiment, during the EAF furnace step, no deoxidizer is added during the tapping process, but a synthetic refining slag agent is added. When one-third of the steel is tapped, 3 to 5 kg of the synthetic refining slag agent per ton of steel is added at once.

[0013] In one embodiment, during the first LF furnace step, CaO, aluminum-calcium alloy balls, Al particles, etc. are added to adjust the slag basicity, controlling the slag basicity to 2.5 to 4.0 and the slag layer thickness to 150 mm to 200 mm.

[0014] In one embodiment, in the second LF furnace step, slag is formed using a combination of fluorite, quicklime, and synthetic refining slag agent.

[0015] In one embodiment, during the second LF furnace step, the amount of aluminum used is reduced during the smelting process, and the Al content is controlled to be between 0.01% and 0.03%.

[0016] In one embodiment, during the second LF furnace step, when bottom blowing argon, the bottom blowing flow rate is less than 60 NL / min before energizing for 3 minutes; from energizing for 4 to 10 minutes, the bottom blowing flow rate is 60 NL / min to 200 NL / min; from energizing for 10 to 15 minutes, the bottom blowing flow rate is 60 NL / min to 200 NL / min; and after energizing, the bottom blowing flow rate is 200 NL / min to 400 NL / min.

[0017] In one embodiment, during the second LF furnace step, after refining, the bottom-blown argon flow rate of the ladle is 20 NL / min to 60 NL / min.

[0018] In one embodiment, the chemical composition of the above-mentioned smelted cast steel part is as follows: C≤0.04%, Si≤1.0%, Mn≤1.0%, P≤0.028%, S≤0.012%, 11.50%≤Cr≤14.0%, 0.40%≤Mo≤1.0%, 3.8%≤Ni≤5.0%, N≤0.007%, H≤0.0003%, O≤0.007%, Cu≤0.5%, V≤0.08%.

[0019] The beneficial effects of this invention are:

[0020] The method for controlling nitrogen content in molten steel provided by this invention effectively reduces the nitrogen content at the end of electric arc furnace (EAF) smelting by improving the foaming properties of the EAF slag, increasing the decarburization of the molten steel, and using synthetic refining slag agents during EAF tapping to reduce nitrogen absorption in the molten steel. During LF refining, a low-nitrogen production mode is adopted, controlling smelting time, rapidly forming slag, and strengthening submerged arc operation to reduce the chance of contact with air during refining and thus reduce nitrogen absorption in the molten steel. During VOD vacuum treatment, measures such as ensuring sufficient decarburization, strengthening steel stirring, reducing vacuum level, and extending vacuum holding time significantly improve the vacuum denitrification rate. In the steelmaking process, nitrogen content control involves multiple key links and steps. By adopting appropriate control strategies in each process and optimizing the process, the total nitrogen content in the molten steel can be controlled below 50 ppm, and the nitrogen content in the finished billet can be controlled within 50–65 ppm, meeting customer requirements and solving production technical difficulties. Detailed Implementation

[0021] To facilitate understanding of the present invention, a more comprehensive description is provided below, along with preferred embodiments. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of the present invention.

[0022] This embodiment takes a ZG04Cr13Ni4Mo cast steel part as an example. The chemical composition of this material is: C≤0.04%, Si≤1.0%, Mn≤1.0%, P≤0.028%, S≤0.012%, 11.50%≤Cr≤14.0%, 0.40%≤Mo≤1.0%, 3.8%≤Ni≤5.0%, N≤0.007%, H≤0.0003%, O≤0.007%, Cu≤0.5%, V≤0.08%. The specific smelting process is EAF furnace + first LF furnace + VOD furnace + second LF furnace, specifically including the following steps:

[0023] Step 1

[0024] EAF furnace: Provides molten steel for rough refining, employing an oxidation smelting process. The furnace charge is primarily composed of Class A scrap steel, with residual elements P ≤ 0.028% and V ≤ 0.08%. Lime is added at a rate of 30 kg / ton to 60 kg / ton of steel, and carbon raiser is added at 1.0% to 1.5%.

[0025] Specifically, after the furnace charge is melted and cleared, samples are taken to analyze the C and P content, followed by a decarburization operation. During the oxygen blowing decarburization process, the furnace door is tilted to remove the oxide slag. After the molten steel temperature reaches ≥1620℃, oxygen blowing decarburization begins, ensuring a decarburization rate of 0.3% to 0.6%. At the end of the oxidation period, a double-pipe deep oxygen blowing method is used to increase the stirring force of the molten steel in the electric furnace, promote the carbon-oxygen reaction and generate a large number of CO bubbles. As the bubbles rise, they adsorb nitrogen elements, reducing the nitrogen content in the molten steel. After the oxygen blowing is completed, samples are taken to analyze the P content until P ≤ 0.005%. Then, the temperature is raised to 1600 to 1650℃ before tapping the steel.

[0026] The specific operation for controlling nitrogen content in the EAF furnace is as follows: The EAF furnace employs submerged arc frothing slag operation, with lime added at 30-60 kg / t, fluorite at 2-4 kg / t, and carbon powder at 1-3 kg / t. Increasing the slag layer thickness prolongs the residence time of CO gas within the slag, achieving the effect of submerged arc heating, reducing the dissociation of N2 from the air by the electric arc, decreasing the direct contact between the molten steel and air, and reducing nitrogen absorption. The decarburization rate in the EAF furnace is controlled at 0.3-0.6%, increasing the decarburization of the molten steel. At the end of the oxidation process in the EAF furnace, double-pipe deep-blowing oxygen is inserted into the molten steel to increase the stirring force of the steel in the electric furnace, promoting the carbon-oxygen reaction and generating a large number of CO bubbles. These rising CO bubbles act as small "vacuum chambers," capable of carrying away some dissolved CO from the steel. Nitrogen is used to reduce the nitrogen content in molten steel. During the EAF furnace tapping process, no deoxidizer is added; only 3-5 kg ​​of synthetic refining slag is added per ton of steel. When 1 / 3 of the steel has been tapped, the synthetic refining slag is added to the ladle all at once. After all the slag is added, the argon pressure should be appropriately increased. Argon agitation or molten steel flushing is used to quickly form a slag layer covering the molten steel. Due to the high content of surface-active elements sulfur and oxygen in the molten steel, the amount of nitrogen absorbed by the molten steel is reduced. During the EAF furnace tapping process, the tapping speed at the tapping port should be maintained above 7 t / min to reduce runoff and avoid prolonged contact between the molten steel and air. After the molten steel arrives at the LF station, a pre-deoxidation operation is performed, and gas samples are taken for gas analysis. The nitrogen content in the molten steel is 30-50 ppm, which is lower than the previous 70 ppm and below 50 ppm, showing a significant nitrogen control effect.

[0027] Step Two

[0028] The first LF furnace: The main purpose of molten steel entering the LF furnace is deoxidation, desulfurization, and alloying. Extensive experiments have shown that during the LF refining and alloying process, the addition of a large amount of alloy ferrochrome introduces nitrogen into the molten steel, and the nitrogen content increases with the increase of Cr content. At the end of the LF furnace refining process, the nitrogen content in the molten steel reaches as high as 150-200 ppm, which also has a certain impact on degassing during the VOD refining process. Therefore, it is necessary to take certain nitrogen control measures before the molten steel enters the VOD furnace. The specific operation is as follows: After the ladle arrives at the LF furnace, the oxygen activity in the molten steel is first measured using an oxygen analyzer. Then, depending on the situation, CaO + fluorite + synthetic refining slag agent and Al particles are added first to quickly create a thin slag, reducing nitrogen absorption during power transmission. After creating the thin slag, CaO, aluminum-calcium alloy balls, and Al particles are added again to adjust the slag basicity and increase the slag layer thickness. High-temperature desulfurization is carried out by controlling the slag basicity R between 2.5 and 4.0, and controlling the sulfur content of the molten steel to within the finished product specifications. The slag thickness is maintained at 150–200 mm to ensure its foaming properties, achieving the effect of submerged arc heating. During alloying, the argon gas flow rate is carefully controlled to prevent large-area exposure of the molten steel at the argon blowing port, which could lead to nitrogen absorption. When the molten steel temperature is suitable and the oxygen activity is low, samples are taken for chemical composition analysis. Based on the spectral results, Mo, Cr, Ni, and other non-oxidizing alloys are added to the lower limit of the specifications, and C is adjusted to 0.2–0.6%. Then, the molten steel temperature is adjusted to 1600–1640℃ before entering the VOD furnace.

[0029] Step 3

[0030] VOD furnace: Ensuring sufficient decarburization, strengthening steel molten agitation, maintaining appropriate temperature, and preventing post-refining gas absorption are key aspects of the entire VOD refining process. Denitrification in the VOD furnace involves the transfer of nitrogen from the steel to CO bubbles; the oxygen blowing reaction and decarburization reaction occur simultaneously. A higher initial carbon content is a major factor in achieving a nitrogen content of less than 50 ppm. Additionally, over-blowing should be avoided as it can lead to severe chromium oxidation, negatively impacting subsequent LF refining efficiency. The initial blowing temperature should be controlled between 1600 and 1620°C, the oxygen blowing vacuum at 6–8 kPa, and the argon flow rate appropriately increased to enhance the CO reaction rate, achieving rapid denitrification under strong agitation. After oxygen blowing, the vacuum should be further increased to below 67 Pa, while simultaneously increasing the argon flow rate and maintaining this position for at least 15 minutes. If conditions permit, the time can be extended appropriately to allow further reaction between C and O in the steel, achieving the effects of carbon reduction, gas removal, and inclusion removal. After breaking the vacuum, a small amount of CaO, fluorite, Al particles, and synthetic refining slag agent are added to form slag and cover the molten steel. The reduction time is maintained at ≥10 min, which can be extended to 15 min if conditions permit. Then the steel is tapped to the LF furnace for further refining. Argon blowing and stirring are strictly prohibited during the transfer of molten steel to prevent the molten steel from absorbing nitrogen.

[0031] Step Four

[0032] The second LF furnace: employs deoxidation combined with microalloying to precisely adjust the steel composition. After vacuum degassing in the VOD furnace, the nitrogen content in the molten steel is at its lowest, making it more susceptible to absorbing nitrogen from the air, thus increasing the N content in the steel. Furthermore, the high temperature of the molten steel after the VOD furnace accelerates the reaction between the steel and nitrogen, further intensifying nitrogen absorption. Therefore, the second LF furnace refining process is the most critical step in controlling nitrogen content.

[0033] The specific steps include: using a combination of fluorite, quicklime, and synthetic refining slag agent for slag formation. While ensuring the melting effect of the refining slag, approximately 0.8–1.2 kg / t of fluorite is added at the initial stage of smelting. Taking advantage of the rapid slag formation of fluorite, slag is quickly formed to cover the molten steel and prevent air absorption during power transmission. This is combined with the use of synthetic refining slag agent to rapidly form a CaO-SiO2-Al2O3 slag system. This slag system lowers the melting point of the refining slag, effectively acting as a slag-forming agent. Its foaming time is longer than the original slag formation scheme ("fluorite + lime" slag formation scheme), which is beneficial for stabilizing the slag system. No further addition of fluorite is required afterward.

[0034] To increase the basicity of the steel slag and adjust its viscosity, it is necessary to add more active lime. The purpose is to increase the slag basicity, adjust the steel slag viscosity, and control the basicity between 2.5 and 4, making it highly alkaline. Under strongly alkaline and reducing conditions, a silicon-calcium-manganese-rare-earth-barium composite deoxidizer and Al particles are added for deep deoxidation. Subsequently, FeSi and MnFe are added. Selecting appropriate refining slag composition and quantity in the early stages of refining can effectively reduce nitrogen absorption by the molten steel.

[0035] During the smelting process, the amount of aluminum used should be reduced, and the Al content should be controlled within the range of 0.01% to 0.03%. While using a large amount of aluminum, a strong deoxidizing element, can completely deoxidize the molten steel, the high aluminum content on the surface of the steel results in a lack of surface-active oxygen, making it prone to combining with nitrogen during subsequent energization. Furthermore, excessively high Al content in the molten steel also hinders nitrogen removal during argon blowing. Therefore, the amount of aluminum used during smelting should be reduced, and the Al content controlled within the range of 0.01% to 0.03%.

[0036] During bottom blowing of argon, the bottom blowing flow rate is less than 60 NL / min for the first 3 minutes after energization; 60-200 NL / min for 4-10 minutes after energization; 60-200 NL / min for 10-15 minutes after energization; and 200-400 NL / min after energization ends. Furthermore, in the second LF furnace melting step, a low argon supply intensity and small flow rate are used to correctly and reasonably control the ladle argon blowing flow rate, preventing the molten steel from contacting the atmosphere and causing oxygen absorption and nitrogen increase. Before energizing, the argon blowing flow rate is controlled at 100–300 NL / min to melt the surface slag-forming material and stabilize the arc-starting current. Three minutes before energizing, the bottom blowing flow rate is less than 60 NL / min to reduce the contact between the molten steel and the atmosphere before the slag is fully melted. From 4 to 10 minutes after energizing, the bottom blowing flow rate is controlled at 60–200 NL / min to increase the flow rate while maintaining arc stability, promoting steel agitation and improving mass transfer rate. After energizing, the bottom blowing flow rate is controlled at 200–400 NL / min to homogenize the temperature and composition of the molten steel, increase the collision and floating speed of inclusions in the steel, and improve the deoxidation and desulfurization efficiency of the molten steel. From 10 to 15 minutes after energizing, the ladle bottom blowing flow rate is controlled at 60–200 NL / min. During soft blowing after refining, the ladle bottom blowing flow rate is controlled at 20–60 NL / min, ideally enough to break the slag surface and allow it to float slightly, avoiding exposure of the molten steel to the atmosphere. In addition, the rising small argon bubbles act as a "vacuum chamber," removing gases from the steel. To minimize the amount of nitrogen added to the low-oxygen, low-sulfur molten steel during the later stages of refining, the entire refining time is controlled to be no more than 60 minutes.

[0037] It should be noted that during the second LF furnace smelting, SiCa wire should be used sparingly or not at all during the refining process. During the wire feeding process, calcium bubbles formed by calcium vaporization blow open the surface of the molten steel, causing the steel to come into contact with air and absorb nitrogen. Calcium is a reactive metal and easily reacts with nitrogen and oxygen in the air to form compounds, so feeding calcium wire also increases nitrogen content in the molten steel. If the molten steel is under low oxygen conditions, the nitrogen increase during calcium wire feeding is even more severe. Before tapping and pouring, the mold cavity should be purged with argon for at least 15 minutes, and argon blowing should be used during the pouring process to reduce nitrogen absorption by the molten steel.

[0038] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A method of controlling the nitrogen content in molten steel, characterized by, The method comprises the following steps: E AF furnace: using oxidation smelting process, controlling residual elements P≤0.028%, V≤0.08%; First LF furnace: using nitrogen control measures, first adding CaO, fluorite, synthetic refining slag agent, Al particles to quickly produce thin slag; then adding CaO, aluminum-calcium alloy balls, Al particles to adjust the slag basicity and increase the slag layer thickness; VOD furnace: using the process of oxygen blowing reaction and decarburization reaction at the same time; Second LF furnace: using deoxidation combined with micro-alloying measures to accurately adjust the composition of the molten steel.

2. The method of controlling nitrogen content in molten steel according to claim 1, characterized by, In the EAF furnace step, the submerged arc foam slag operation measures are adopted, 30kg / t to 60kg / t of lime, 2kg / t to 4kg / t of fluorite and 1kg / t to 3kg / t of carbon powder are added to increase the thickness of the slag layer.

3. The method of controlling nitrogen content in molten steel according to claim 1, characterized by, In the EAF furnace step, the decarburization amount is controlled to be 0.3% to 0.6%.

4. The method of controlling nitrogen content in molten steel according to claim 1, characterized by, In the EAF furnace step, no deoxidizer is added during the tapping process, and a synthetic refining slag agent is added, 3kg to 5kg of synthetic refining slag agent per ton of steel is added at one time when one-third of the steel is tapped.

5. The method of controlling nitrogen content in molten steel according to claim 1, characterized by, In the first LF furnace step, CaO, aluminum-calcium alloy balls and Al particles are added to adjust the slag basicity, and the slag basicity is controlled to be 2.5 to 4.0, and the slag layer thickness is controlled to be 150mm to 200mm.

6. The method of controlling nitrogen content in molten steel according to claim 1, characterized by, In the second LF furnace step, the fluorite, active lime and synthetic refining slag agent combination scheme is used for slag making.

7. The method of controlling nitrogen content in molten steel according to claim 1, characterized by, In the second LF furnace step, the amount of aluminum is reduced during smelting, and the Al content is controlled to be 0.01% to 0.03%.

8. The method of controlling nitrogen content in molten steel according to claim 1, characterized by, In the second LF furnace step, during the bottom argon blowing, the bottom blowing flow is less than 60NL / min before power-on for 3min; the bottom blowing flow is 60NL / min to 200NL / min during power-on for 4min to 10min; the bottom blowing flow is 60NL / min to 200NL / min during power-on for 10min to 15min; after power-off, the bottom blowing flow is 200NL / min to 400NL / min.

9. The method of controlling nitrogen content in molten steel according to claim 1, characterized by, In the second LF furnace step, after refining, the ladle bottom argon blowing flow is 20NL / min to 60NL / min.

10. The method of controlling nitrogen content in molten steel according to any one of claims 1 to 9, characterized by, The chemical composition of the smelted steel material is: C≤0.04%, Si≤1.0%, Mn≤1.0%, P≤0.028%, S≤0.012%, 11.50%≤Cr≤14.0%, 0.40%≤Mo≤1.0%, 3.8%≤Ni≤5.0%, N≤0.007%, H≤0.0003%, O≤0.007%, Cu≤0.5%, V≤0.08%.