Production control method for reducing nitrogen content of molten steel in whole process

Abstract

The invention belongs to the technical field of ferrous metallurgy, and relates to a production control method for reducing the nitrogen content of molten steel in a whole process. Inert gas bottom blowing is adopted in the smelting stage, wherein the inert gas is argon, and the bottom blowing flow is 100-800 Nm < 3 > / h; the bottom blowing of the converter adopts a whole-course argon blowing mode, the end point requirement of the converter is as follows: 1620-1650 DEG C, [O] is less than or equal to 550ppm, and the slag stopping mode adopts front and back double stopping; in the refining stage, submerged arc heating is matched with argon soft blowing, argon stirring is strictly prohibited after wire feeding, and it is guaranteed that the molten steel surface is exposed and does not exceed 300 mm; and protective casting, argon flow control and protective sleeve insertion control are adopted in the continuous casting stage. According to the method, the nitrogen content of the molten steel is reduced to 25 ppm from the initial 45 ppm, the cost per ton of steel is reduced by 5%, and the method is suitable for industrial production of the high-quality low-nitrogen steel.

Description

Technical Field

[0001] The present invention belongs to the technical field of iron and steel metallurgy, and particularly relates to a production control method for reducing the nitrogen content in molten steel throughout the process. Background Art

[0002] During the iron and steel smelting process, the nitrogen content in molten steel directly affects the mechanical properties (such as strength and toughness) and processing properties of steel. Nitrogen usually enters molten steel through by-products, smelting atmosphere (such as temperature control and air infiltration), and alloy additives. Excessive nitrogen content will lead to increased brittleness of steel and decreased welding performance. Therefore, how to reduce the nitrogen content in molten steel is a problem to be solved in the iron and steel smelting process.

[0003] Chinese invention patent CN109457076A discloses a control method for the nitrogen content in molten steel. By comprehensively controlling the nitrogen removal or nitrogen control amount in the converter smelting, LF refining, and slab continuous casting processes, the nitrogen content in molten steel is controlled, and the stability of the nitrogen content is ensured. By improving the nitrogen removal ability of the converter and reducing the nitrogen increase amount in LF, the slab continuous casting process is stably controlled. When the unit consumption of molten steel is 800 kg / t, the nitrogen content in the tundish molten steel is within 60 ppm. However, its final nitrogen content in molten steel is still not low enough to meet the production and processing requirements of steel. Summary of the Invention

[0004] The purpose of the present invention is to provide a production control method for reducing the nitrogen content in molten steel throughout the process. Through the collaborative control of process parameters throughout the process, the nitrogen content in molten steel in each process link is reduced, the level of waste and defective products in the steel mill is improved, and the production cost is reduced while ensuring the quality of molten steel.

[0005] The technical solution adopted by the present invention to solve its technical problems is: a production control method for reducing the nitrogen content in molten steel throughout the process, including the following steps:

[0006] (1) Classify and manage the scrap steel entering the furnace, and refine and classify the nitrogen element content of steel grades for batching.

[0007] (2) Adopt bottom blowing of inert gas during the smelting stage: the inert gas is argon, and the bottom blowing flow rate is 100 - 800 Nm 3 / h;

[0008] (3) Control the tapping temperature, oxygen content, and slag blocking method of LF: The converter bottom blowing adopts the whole-process argon blowing mode. The requirements for the converter end point are: 1620 - 1650 °C, [O] ≤ 550 ppm, and the slag blocking method adopts double slag blocking before and after.

[0009] (4) During the refining stage, carry out submerged arc heating and cooperate with soft argon blowing: After wire feeding, large argon stirring is strictly prohibited, and it is ensured that the exposed molten steel surface does not exceed 300 mm.

[0010] (5) During the continuous casting stage, protective casting, argon flow control, and control of the insertion of the protective sleeve are adopted.

[0011] Furthermore, the specific refinement and classification of the nitrogen element content in the steel grade in step (1) for batching is as follows: for steel grades with a nitrogen requirement of less than 30 ppm, the batching is mainly based on steel plate scrap, plate heads and edges, and purchased pig iron blocks; for steel grades with a nitrogen requirement of 30 - 40 ppm, the batching is mainly based on steel plate compacts, steel bar compacts, and pig iron blocks; for steel grades with a nitrogen requirement of more than 40 ppm, the batching is mainly based on mixed materials and pig iron blocks.

[0012] Furthermore, the requirements for the LF furnace arrival in step (3) are: T ≥ 1540 °C, [ALs]: 0.015 - 0.035%; when lifting the lance at the end point, there should be no splashing at the furnace mouth and no nitrogen slagging, one-time carbon tapping, and avoid point blowing; in terms of the alloy addition structure, it is preferred to use high-carbon ferromanganese, silicomanganese, and low-carbon low-phosphorus silicomanganese to adjust carbon and silicon to the target values, and use ferromanganese to supplement the insufficient part.

[0013] Furthermore, the refined nitrogen content requirement for the ferromanganese is: ferromanganese N ≤ 0.1%.

[0014] Furthermore, during the RH vacuum treatment process in the refining stage of step (4), the circulation lifting gas must be continuously monitored in real time to ensure the use of argon. Under permitted conditions, the vacuum treatment time can be appropriately extended to enhance the denitrification effect, and the wire feeding amount and speed should strictly follow the requirements of the regulations.

[0015] Furthermore, the specific methods for adopting protective casting, argon flow control, and control of the insertion of the protective sleeve in the continuous casting stage of step (5) are as follows:

[0016] a. Before the first heat of the tundish is poured, the casting machine must insert 3 well-ventilated argon pipes into the tundish. After the tundish is adjusted, wait for 30 seconds for the tundish to be filled with argon before pouring.

[0017] b. Before the ladle is poured, first lower the ladle to the lower opening of the sleeve close to the molten steel level in the tundish, open the slide plate. After the molten steel comes down, immediately immerse the sleeve into the molten steel (≤ 10 seconds), and ensure that the immersion depth of the sleeve is ≥ 300 mm.

[0018] c. During the ladle pouring process, ensure that the argon pipeline does not leak, the argon flow rate of the sleeve is ≥ 90 L / min. During the production of special steel grades, the service life of the protective sleeve is controlled ≤ 6 heats / branch. In case of abnormal defects such as bowl platform defects and slag line damage, it must be replaced in time. During the whole process of the tundish, it is strictly prohibited for molten steel to splash or be exposed.

[0019] d. During the pouring process, the flow rate L between the tundish plates is ≤ 10 L / min, and the back pressure ≥ 0.1 bar; when the flow rate > 8 L / min and the back pressure < 0.1 bar, it is considered that the seal between the plates is poor, and external argon needs to be connected and the dispatching room should be notified in time to modify the production plan and make preparations for shutdown.

[0020] The present invention has the following beneficial effects: By controlling the type structure of the scrap steel charged into the furnace, optimizing the process parameters in the smelting and end-point control, refining and continuous casting stages, and combining inert gas protection and argon soft blowing technology, the nitrogen content in the molten steel is reduced from the initial 45 ppm to 25 ppm, and the cost per ton of steel is reduced by 5%. It is applicable to the industrial production of high-quality low-nitrogen steel. Detailed implementation manners

[0021] The following are specific embodiments of the present invention, which further describe the technical solutions of the present invention. However, the protection scope of the present invention is not limited to these embodiments. Any changes or equivalent substitutions that do not deviate from the concept of the present invention are included in the protection scope of the present invention.

[0022] Influence of nitrogen brought in by scrap steel: For mixed heavy scrap (general scrap), the nitrogen increase is 2 - 8 ppm / 10 tons; for steel bar compacts, the nitrogen increase is 2 - 5 ppm / 10 tons; for steel plates, the nitrogen increase is 1 - 3 ppm / 10 tons; for pig iron blocks, the nitrogen increase is 1 - 2 ppm / 10 tons. Heavy scrap refers to scrap steel with a diameter greater than 6 mm.

[0023] The peak of the denitrification rate and denitrification efficiency is concentrated in the middle stage of converter blowing (a large amount of CO bubbles); the critical temperature threshold value is that when the temperature is lower than 1550 °C, the decarburization rate decreases, the generation of CO bubbles is insufficient, and the denitrification efficiency drops significantly.

[0024] According to the solubility formula of nitrogen in molten steel, the lower the carbon content in the molten steel, the higher the solubility of nitrogen in the steel, and the easier it is for the molten steel to absorb nitrogen. Therefore, re-blowing or deep-blowing the molten steel after tapping the carbon is likely to absorb nitrogen from the air, resulting in an increase in the nitrogen content of the molten steel in the later stage of smelting. The reference target for process temperature control is: 1580 °C / 0.35%. Cold materials are evenly added during the process to cool down, and the temperature time between 1450 - 1550 °C is shortened as much as possible.

[0025]

[0026] A production control method for reducing the nitrogen content of molten steel in the whole process of the present invention includes the following steps:

[0027] (1) Classify and manage the scrap steel charged into the furnace, and refine and classify the nitrogen element content of the steel grades for batching: For steel grades with a nitrogen requirement of less than 30 ppm, mainly use steel plate scrap, plate heads and edges, and purchased pig iron blocks for batching; for steel grades with a nitrogen requirement of 30 - 40 ppm, mainly use steel plate compacts, steel bar compacts, and pig iron blocks for batching; for steel grades with a nitrogen requirement of more than 40 ppm, mainly use general scrap and pig iron blocks for batching.

[0028] (2) Adopt bottom blowing with inert gas during the smelting stage: The inert gas is argon, and the bottom blowing flow rate is 100 - 800 Nm 3 / h.

[0029] (3) Control the tapping temperature, oxygen content and slag blocking method of LF: The bottom blowing of the converter adopts the whole-process argon blowing mode. The requirements at the end of the converter are: 1620 - 1650 °C, [O] ≤ 550 ppm, and the front and rear double slag blocking method is adopted.

[0030] The requirements when LF arrives at the station are: T ≥ 1540 °C, [ALs]: 0.015 - 0.035%; when raising the lance at the end, it should not spray the furnace mouth and should not use nitrogen to slag, and carbon should be drawn once to avoid point blowing; in terms of the alloy addition structure, high-carbon ferromanganese, silicomanganese, low-carbon and low-phosphorus silicomanganese should be used preferentially, and carbon and silicon should be adjusted to the target values. For the insufficient part, metal manganese is added. The refined nitrogen content requirement for metal manganese: metal manganese N ≤ 0.1%.

[0031] (4) Adopt submerged arc heating and cooperate with soft argon blowing in the refining stage: During the RH vacuum treatment process in the refining stage, the circulation lifting gas must be monitored in real time to ensure the use of argon. Under the condition of permission, the vacuum treatment time can be appropriately extended to promote the denitrification effect, and the wire feeding amount and speed should be strictly in accordance with the requirements of the regulations. After wire feeding, large argon stirring is strictly prohibited, and the exposed steel liquid surface should not exceed 300 mm.

[0032] Reduce the fan speed to ensure that the inside of the furnace cover is in a slightly positive pressure state, ensure good submerged arc in the LF furnace, and avoid the increase of nitrogen in the molten steel caused by the exposed arc. Except for the cases of removing the steel slag crust, poor bottom blowing of the ladle, and high S content in the molten steel that requires desulfurization, the bypass is prohibited at other times. To make the temperature and composition uniform, large argon flow stirring is adopted after heating ends and alloy addition.

[0033] (5) Adopt protective casting, argon flow control, and protective sleeve insertion control in the continuous casting stage. The specific methods are as follows:

[0034] a. Before the first ladle of the tundish is poured, the casting machine must insert 3 well-ventilated argon pipes into the tundish. After the tundish is adjusted, wait for 30 seconds for the tundish to be filled with argon before pouring.

[0035] b. Before the ladle is poured, first lower the ladle to the lower opening of the sleeve close to the molten steel surface in the tundish, pull open the slide plate. After the molten steel comes down, immediately immerse the sleeve into the molten steel (≤10 seconds), and ensure that the immersion depth of the sleeve ≥ 300 mm.

[0036] The tundish uses a high-alkalinity covering agent. Before pouring, the covering agent needs to be placed on both sides of the tundish for pre-baking. When the tundish tonnage in the stopper rod area > 30 tons, the covering agent is added, and when the molten steel in the ladle pouring area covers the sleeve, it is added.

[0037] c. During the ladle pouring process, ensure that the argon pipeline does not leak air, the argon flow of the sleeve ≥ 90 L / min. When producing special steel grades, the service life of the protective sleeve is controlled ≤ 6 heats / branch. In case of abnormal defects such as bowl platform defects and slag line damage, it must be replaced in time. The whole process of the tundish is strictly prohibited from having steel splashes or exposed molten steel.

[0038] d. During the pouring process, the flow rate L between the shroud plates ≤ 10 L / min, and the back pressure ≥ 0.1 bar; when the flow rate > 8 L / min and the back pressure < 0.1 bar, it is regarded as poor sealing between the plates. External argon gas needs to be connected and the dispatching room should be notified in time to modify the production plan and make preparations for shutdown.

[0039] The production control method of the present invention reduces the nitrogen content in molten steel through the whole process. The nitrogen content in molten steel is reduced from the initial 45 ppm to 25 ppm, and the cost per ton of steel is reduced by 5%.

[0040] The present invention is not limited to the above embodiments. Any person should know that structural changes made under the inspiration of the present invention, as long as they have the same or similar technical solutions as the present invention, all fall within the protection scope of the present invention.

[0041] The technologies, shapes, and structures not described in detail in the present invention are all well-known technologies.

Claims

1. A production control method for reducing the nitrogen content of molten steel in the whole process, characterized in that, It includes the following steps: (1) Classify and manage the scrap steel charged into the furnace, and refine and classify the nitrogen element content of different steel grades for batching into hoppers; (2) In the smelting stage, bottom blowing of inert gas is adopted: the inert gas is argon, and the bottom blowing flow rate is 100 - 800 Nm 3 / h; (3) Control the tapping temperature, oxygen content and slag blocking method of LF: The bottom blowing of the converter adopts the whole-process argon blowing mode. The requirements at the end of the converter are: 1620 - 1650 °C, [O] ≤ 550 ppm, and the front and rear double slag blocking method is adopted; (4) During the refining stage, submerged arc heating is carried out and soft argon blowing is coordinated: After wire feeding, large argon stirring is strictly prohibited, and ensure that the exposed steel liquid surface does not exceed 300 mm; (5) During the continuous casting stage, protective casting, argon gas flow control and protective sleeve insertion control are adopted.

2. The production control method for reducing the nitrogen content of molten steel in the whole process according to claim 1, characterized in that, The specific refinement and classification of the nitrogen element content of different steel grades for batching into hoppers in step (1) is as follows: For steel grades with a nitrogen requirement of less than 30 ppm, the main materials for batching into hoppers are steel plate scrap, plate heads and edges, and purchased pig iron blocks; for steel grades with a nitrogen requirement of 30 - 40 ppm, the main materials for batching into hoppers are steel plate compacts, steel bar compacts, and pig iron blocks; for steel grades with a nitrogen requirement of more than 40 ppm, the main materials for batching into hoppers are general scrap and pig iron blocks.

3. The production control method for reducing the nitrogen content of molten steel in the whole process according to claim 1, characterized in that, The requirements for LF arrival in step (3) are: T ≮ 1540 °C, [ALs]: 0.015 - 0.035%; when lifting the lance at the end, it should not spray the furnace mouth and should not use nitrogen to slag, and carry out one-time carbon tapping to avoid spot blowing; in terms of the alloy addition structure, high-carbon ferromanganese, silicomanganese, and low-carbon low-phosphorus silicomanganese should be preferentially used to adjust carbon and silicon to the target values, and the insufficient part is added with ferromanganese.

4. The production control method for reducing the nitrogen content of molten steel in the whole process according to claim 3, characterized in that, The refined nitrogen content requirement for the ferromanganese is: N ≤ 0.1% for ferromanganese.

5. The production control method for reducing the nitrogen content of molten steel in the whole process according to claim 1, characterized in that, During the RH vacuum treatment process in the refining stage of step (4), the circulation lifting gas must be monitored in real time to ensure the use of argon gas. Under conditions permitting, the vacuum treatment time can be appropriately extended to promote the denitrification effect, and the wire feeding amount and speed should strictly follow the requirements of the regulations.

6. The production control method for reducing the nitrogen content of molten steel in the whole process according to claim 1, characterized in that, The specific methods for adopting protective casting, argon gas flow control and protective sleeve insertion control in the continuous casting stage of step (5) are as follows: a. Before the first ladle of the tundish is poured, the casting machine must insert 3 well-ventilated argon pipes into the tundish. After the tundish is adjusted, wait for 30 seconds for the tundish to be filled with argon before pouring; b. Before the ladle is poured, first lower the ladle to the lower opening of the sleeve close to the steel liquid surface in the tundish, pull open the slide plate. After the molten steel comes down, immediately immerse the sleeve into the molten steel, and ensure that the immersion depth of the sleeve is ≥ 300 mm; c. During the pouring process of the ladle, ensure that the argon gas pipeline does not leak, the argon gas flow of the sleeve ≥ 90 L / min. During the production of special steel grades, the service life of the protective sleeve is controlled ≤ 6 heats per piece. In case of abnormal defects such as bowl-shaped platform defects and slag line damage, it must be replaced in time. During the whole process of the tundish, steel splashes or exposed molten steel are strictly prohibited; d. During the pouring process, the flow rate L between the tundish plates ≤ 10 L / min, and the back pressure ≥ 0.1 bar; when the flow rate > 8 L / min and the back pressure < 0.1 bar, it is regarded as poor sealing between the plates, and external argon gas needs to be connected and the dispatching room should be notified in time to modify the production plan and make preparations for shutdown.

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