A control method for reducing nitrogen content of molten steel in a low iron consumption mode of a converter

Abstract

This invention discloses a method for controlling the nitrogen content of molten steel in a converter with low iron consumption mode, belonging to the field of steel manufacturing technology. The method includes the following steps: controlling the molten iron inlet temperature to 1310-1330℃, controlling the silicon content in the molten iron to 0.30%-0.50%, controlling the molten iron charging ratio to 65-70%, and the scrap steel charging ratio to 30-35%; controlling the converter iron-to-steel ratio to 720-750 kg / t; after adding scrap steel into the furnace, shaking the furnace back and forth until the furnace mouth flame is normal, and baking the scrap steel; adding 50% of the slag in the first 4 minutes of the converter blowing process, and then adding the remaining slag in small amounts multiple times over 4-8 minutes, controlling the furnace temperature, and stirring the molten pool back and forth from high to low with an oxygen lance before the end point; using nitrogen-argon switching bottom blowing during the converter blowing process; adding ladle slag modifier before tapping, and then adding deoxidizer for alloying, while simultaneously performing ladle bottom blowing with argon. The control method of the present invention can control the nitrogen content of converter steel to within 40 ppm under the condition that the iron-to-steel ratio in the converter is as low as 720-750 kg / t.

Description

Technical Field

[0001] This invention belongs to the field of steel manufacturing technology, specifically relating to a method for controlling the nitrogen content of molten steel under a low iron consumption mode in a converter. Background Technology

[0002] When the price of scrap steel is lower than the price of molten iron, the lower the iron consumption per unit during steelmaking, the lower the production cost. Many steel mills reduce production costs by increasing the amount of scrap steel added during steelmaking to lower iron consumption per unit. However, as iron consumption per unit decreases, the ability to control nitrogen in the steelmaking process weakens, leading to an increase in nitrogen content. For most steel grades, nitrogen is a harmful element. Increased nitrogen content in steel reduces impact toughness and plasticity, and leads to age hardening. Nitrogen also significantly increases the ductile-brittle transition temperature of steel, causing low-temperature temper brittleness, and nitrides can also lead to hot brittleness.

[0003] Chinese patent CN109457076A discloses a method for controlling the nitrogen content in molten steel, including controlling the nitrogen content in the converter smelting, LF refining, and slab continuous casting processes respectively. This invention controls the nitrogen content in molten steel by comprehensively controlling the denitrification or nitrogen control amounts in the converter smelting, LF refining, and slab continuous casting processes, ensuring the stability of the nitrogen content. By improving the converter's denitrification capacity and reducing the nitrogen addition in the LF process, the nitrogen content in the tundish steel is stabilized and controlled to within 60 ppm when the molten steel consumption is 800 kg / t, meeting the steel performance requirements and effectively reducing production costs. However, this control method is complex, and when the molten steel consumption is 800 kg / t, the nitrogen content in the tundish steel is as high as 60 ppm or more, limiting the effectiveness of this method in reducing nitrogen content.

[0004] Therefore, there is an urgent need to provide a method that can effectively control the nitrogen content of molten steel under conditions of low iron consumption. Summary of the Invention

[0005] In view of the shortcomings of the prior art, one of the objectives of this invention is to provide a method for controlling the nitrogen content of molten steel in a converter with low iron consumption mode. This method can control the nitrogen content of molten steel at the converter endpoint to within 40 ppm under the condition that the iron-to-steel ratio in the converter is as low as 720-750 kg / t.

[0006] To achieve the above objectives, the specific technical solution of the present invention is as follows:

[0007] A method for controlling the nitrogen content of molten steel in a converter with low iron consumption mode includes the following steps:

[0008] The temperature of molten iron entering the furnace is controlled at 1310-1330℃, the silicon content in the molten iron is controlled at 0.30%-0.50%, the proportion of molten iron charged is controlled at 65-70%, and the proportion of scrap steel charged is controlled at 30-35%; the iron-to-steel ratio in the converter is controlled at 720-750 kg / t.

[0009] After adding scrap steel into the furnace, shake the furnace back and forth until the flame at the furnace opening is normal, and bake the scrap steel.

[0010] Add 50% of the slag in the first 4 minutes of the converter blowing process to increase the rate of temperature rise in the early stage of blowing and form a carbon-oxygen reaction in the furnace. Then add the remaining slag in small amounts several times over 4 to 8 minutes to control the furnace temperature. Before the end, use an oxygen lance to stir the molten pool from high to low to accelerate the melting of pig iron at the bottom and ensure that the carbon-oxygen reaction in the furnace continues.

[0011] During the converter blowing process, nitrogen and argon switching bottom blowing is adopted. Nitrogen bottom blowing is used from the start of blowing to the oxygen percentage of 30%, and argon bottom blowing is used after the oxygen percentage reaches 30%.

[0012] A ladle slag modifier is added in the early stage of tapping to form a protective layer on the surface of the molten steel, which isolates the molten steel in the ladle from contact with air. Then, deoxidizing and alloying elements are added to carry out deoxidation and alloying. At the same time, argon is blown into the ladle bottom before tapping.

[0013] This invention first bakes the scrap steel to burn off the oily substances in the scrap steel, and uses high-temperature combustion to form NO from the nitrogen-containing compounds in the scrap steel. x Gas extraction reduces the nitrogen content introduced by scrap steel. Under low iron-to-steel ratio conditions, the converter carbon-oxygen reaction weakens, the temperature is insufficient, the carbon-oxygen reaction time in the molten pool is short, and the discharged nitrogen content is low. This invention reduces the amount of slag added in the early stage of blowing to increase the rate of furnace temperature rise in the early stage, so as to form the carbon-oxygen reaction in the furnace as soon as possible. In the middle stage, multiple batches of small-volume charging are used to maintain the molten pool temperature and ensure the continuous occurrence of the carbon-oxygen reaction in the furnace. At the same time, before the end, the molten pool is stirred back and forth from high to low with an oxygen lance to ensure the continuous carbon-oxygen reaction in the furnace. This invention ensures the stable progress of the converter carbon-oxygen reaction through a series of measures. It uses CO gas generated by the carbon-oxygen reaction in the furnace to reduce the nitrogen partial pressure, so that nitrogen overflows from the steel and reduces the nitrogen content of the molten steel.

[0014] The control method of the present invention reduces the nitrogen content in molten steel through a series of measures such as baking scrap steel, controlling the carbon-oxygen reaction balance in the converter, improving the endpoint hit rate, and alloying the tapped steel. Under the condition of a low iron-to-steel ratio in a 720-750 kg / t converter, the nitrogen content in the molten steel in the converter is controlled to be below 40 ppm.

[0015] Preferably, the scrap steel comprises the following components by mass percentage: pig iron ≤ 20%, primary briquettes ≥ 60%, recycled scrap steel ≤ 20%, and tertiary briquettes ≤ 6%. This invention reduces the nitrogen content introduced by scrap steel and pig iron by adjusting the scrap steel composition and decreasing the amount of pig iron added.

[0016] Preferably, the proportion of molten iron added is 68.7%, and the proportion of scrap steel added is 31.3%.

[0017] Preferably, the final temperature of the converter blowing process is controlled at 1610–1630°C, and the final carbon content of the converter blowing process is controlled at 0.05%–0.08%.

[0018] Preferably, when the endpoint temperature is controlled below 1610℃ and the endpoint C content is below 0.05%, ferrosilicon heating agent is added and oxygen is supplied for ignition, and the number of ignitions is controlled to be no more than once.

[0019] Preferably, the argon bloom during bottom blowing of the ladle is 300-500 mm.

[0020] Compared with the prior art, the advantages of the present invention are:

[0021] This invention first bakes the scrap steel to burn off the oily substances in the scrap steel, and uses high-temperature combustion to form NO from the nitrogen-containing compounds in the scrap steel. x Gas extraction reduces the nitrogen content introduced by scrap steel. Under low iron-to-steel ratio conditions, the carbon-oxygen reaction in the converter weakens, the temperature is insufficient, the carbon-oxygen reaction time in the molten pool is short, and the discharged nitrogen content is low. This invention reduces the amount of slag added in the early stage of blowing to increase the rate of furnace temperature rise in the early stage, so as to form a carbon-oxygen reaction in the furnace as soon as possible. In the middle stage, multiple batches of small-volume charging are used to maintain the molten pool temperature and ensure the continuous occurrence of carbon-oxygen reaction in the furnace. At the same time, before the end, the molten pool is stirred back and forth from high to low with an oxygen lance to ensure the continuous carbon-oxygen reaction in the furnace. Through a series of measures, the carbon-oxygen reaction in the converter is kept balanced. The CO gas generated by the carbon-oxygen reaction in the furnace is used to reduce the nitrogen partial pressure, so that nitrogen overflows from the steel and reduces the nitrogen content of the molten steel.

[0022] The control method of this invention reduces nitrogen absorption and lowers nitrogen content by taking a series of measures, such as baking scrap steel, controlling the carbon-oxygen reaction balance in the converter, improving the endpoint hit rate, and alloying the tapped steel. Under the condition of a low iron-to-steel ratio in a 720-750 kg / t converter, the nitrogen content of the molten steel in the converter is controlled to be below 40 ppm. Detailed Implementation

[0023] The technical solution of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] Example 1

[0025] This embodiment provides a method for controlling the nitrogen content of molten steel in a converter with low iron consumption mode, including the following steps:

[0026] S1. Control the molten iron temperature at 1310℃, control the silicon content in the molten iron at 0.50%, control the molten iron charging ratio at 68.7%, and the scrap steel charging ratio at 31.3%; control the converter iron-to-steel ratio at 735 kg / t;

[0027] S2. Control the final temperature of converter blowing to 1610℃ and control the final carbon content of converter blowing to 0.05%;

[0028] S3. Add scrap steel to the furnace and rock the furnace back and forth (forward to an 80° tilt angle, backward to a 50° tilt angle) to bake the scrap steel and burn off the oily substances in it until there is no thick smoke from the flame at the furnace opening. Utilize the high temperature inside the furnace to burn off the oily substances containing C, H, and N compounds in the scrap steel to form NO. x Gas is removed to prevent it from entering the molten steel;

[0029] S4. Adjust the scrap steel composition: pig iron accounts for 15%, grade 1 briquettes or rebar briquettes account for 69%, internal recycled scrap steel accounts for 10%, and grade 3 briquettes or industrial baled blocks account for 6%.

[0030] S5. Add 50% of the slag in the first 4 minutes of the converter blowing process to increase the rate of temperature rise in the early stage of blowing and form carbon-oxygen reaction in the furnace. Then add the remaining slag in small amounts several times over 4 to 8 minutes to control the furnace temperature. Before the end, use an oxygen lance to stir the molten pool from high to low to accelerate the melting of pig iron at the bottom and ensure that the carbon-oxygen reaction in the furnace continues.

[0031] S6. During the converter blowing process, nitrogen and argon switching bottom blowing is adopted. Nitrogen bottom blowing is adopted from the start of blowing to the oxygen blowing percentage of 30%, and argon bottom blowing is adopted from the oxygen blowing percentage of 30% to the end of tapping.

[0032] S7. When the endpoint temperature is controlled below 1610℃ or the endpoint C content is below 0.05%, add ferrosilicon heating agent and then supply oxygen for ignition, controlling the number of ignitions to ≤1.

[0033] S8. Before tapping, ladle slag modifier (lime + fluorite + bauxite) is added to modify the ladle and form a protective layer on the surface of the molten steel to isolate the molten steel from the air. Then, deoxidizing and alloying elements are added to deoxidize and alloy the steel. At the same time, argon gas is blown into the ladle before tapping to fill the ladle with an argon atmosphere and reduce the contact between the molten steel and the high-temperature air in the ladle during the tapping process.

[0034] The control method in this embodiment reduces nitrogen content through a series of measures, such as baking scrap steel, controlling the carbon-oxygen reaction balance in the converter, improving the endpoint hit rate, reducing the number of blows, and alloying the tapped steel. After the above methods are used, the nitrogen content in the molten steel in the converter is below 40 ppm.

[0035] Example 2

[0036] This embodiment provides a method for controlling the nitrogen content of molten steel in a converter with low iron consumption mode, including the following steps:

[0037] S1. Control the temperature of molten iron entering the furnace to 1330℃, control the silicon content of molten iron to 0.30%, control the proportion of molten iron to be charged to 70%, and the proportion of scrap steel to be charged to 30%; control the iron-to-steel ratio of the converter to be 750 kg / t;

[0038] S2. Control the final temperature of converter blowing to 1630℃ and control the final carbon content of converter blowing to 0.08%;

[0039] S3. Add scrap steel to the furnace and rock the furnace back and forth (forward to an 80° tilt angle, backward to a 50° tilt angle) to bake the scrap steel and burn off the oily substances in it until there is no thick smoke from the flame at the furnace opening. Utilize the high temperature inside the furnace to burn off the oily substances containing C, H, and N compounds in the scrap steel to form NO. x Gas is removed to prevent it from entering the molten steel;

[0040] S4. Adjust the scrap steel composition: pig iron accounts for 10%, grade 1 briquettes or steel bar briquettes account for 75%, internal recycled scrap steel accounts for 10%, and grade 3 briquettes or industrial baled blocks account for 5%.

[0041] S5. Add 50% of the slag in the first 4 minutes of the converter blowing process to increase the rate of temperature rise in the early stage of blowing and form carbon-oxygen reaction in the furnace. Then add the remaining slag in small amounts several times over 4 to 8 minutes to control the furnace temperature. Before the end, use an oxygen lance to stir the molten pool from high to low to accelerate the melting of pig iron at the bottom and ensure that the carbon-oxygen reaction in the furnace continues.

[0042] S6. During the converter blowing process, nitrogen and argon switching bottom blowing is adopted. Nitrogen bottom blowing is adopted from the start of blowing to the oxygen blowing percentage of 30%, and argon bottom blowing is adopted from the oxygen blowing percentage of 30% to the end of tapping.

[0043] S7. When the endpoint temperature is controlled below 1610℃ or the endpoint C content is below 0.05%, add ferrosilicon heating agent and then supply oxygen for ignition, controlling the number of ignitions to ≤1.

[0044] S8. Before tapping, ladle slag modifier (lime + fluorite + bauxite) is added to modify the ladle and form a protective layer on the surface of the molten steel to isolate the molten steel from the air. Then, deoxidizing and alloying elements are added to deoxidize and alloy the steel. At the same time, argon gas is blown into the ladle before tapping to fill the ladle with an argon atmosphere and reduce the contact between the molten steel and the high-temperature air in the ladle during the tapping process.

[0045] After being controlled using the above methods, the nitrogen content in the converter molten steel is below 40 ppm.

[0046] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for controlling the reduction of nitrogen content in molten steel under low iron consumption mode in a converter, characterized in that, Includes the following steps: The temperature of molten iron entering the furnace is controlled at 1310-1330℃, the silicon content in the molten iron is controlled at 0.30%-0.50%, the proportion of molten iron charged is controlled at 65-70%, and the proportion of scrap steel charged is controlled at 30-35%; the iron-to-steel ratio in the converter is controlled at 720-750 kg / t. After adding scrap steel into the furnace, shake the furnace back and forth until the flame at the furnace opening is normal, and bake the scrap steel. Add 50% of the slag in the first 4 minutes of the converter blowing process to increase the rate of temperature rise in the early stage of blowing and form a carbon-oxygen reaction in the furnace. Then add the remaining slag in small amounts several times over 4 to 8 minutes to control the furnace temperature. Before the end, use an oxygen lance to stir the molten pool from high to low to accelerate the melting of pig iron at the bottom and ensure that the carbon-oxygen reaction in the furnace continues. During the converter blowing process, nitrogen and argon switching bottom blowing is adopted. Nitrogen bottom blowing is used from the start of blowing to the oxygen percentage of 30%, and argon bottom blowing is used after the oxygen percentage reaches 30%. A ladle slag modifier is added in the early stage of tapping to form a protective layer on the surface of the molten steel, which isolates the molten steel in the ladle from contact with air. Then, deoxidizing and alloying elements are added to carry out deoxidation and alloying. At the same time, argon is blown into the ladle bottom before tapping.

2. The method for controlling the reduction of nitrogen content in molten steel under low iron consumption mode in a converter according to claim 1, characterized in that, The scrap steel comprises the following components by mass percentage: pig iron ≤ 20%, primary briquettes ≥ 60%, recycled scrap steel ≤ 20%, and tertiary briquettes ≤ 6%.

3. The method for controlling the reduction of nitrogen content in molten steel under low iron consumption mode in a converter according to claim 1, characterized in that, The proportion of molten iron added is 68.7%, and the proportion of scrap steel added is 31.3%.

4. The method for controlling the reduction of nitrogen content in molten steel under low iron consumption mode in a converter according to claim 1, characterized in that, The final temperature of the converter blowing process is controlled at 1610–1630℃, and the final carbon content is controlled at 0.05%–0.08%.

5. The method for controlling the reduction of nitrogen content in molten steel under low iron consumption mode in a converter according to claim 4, characterized in that, When the final temperature is controlled below 1610℃ and the final C content is below 0.05%, add ferrosilicon heating agent and then supply oxygen for ignition, controlling the number of ignitions to no more than once.

6. The method for controlling the reduction of nitrogen content in molten steel under low iron consumption mode in a converter according to claim 1, characterized in that, During the bottom blowing of argon in the ladle, the argon sparks are 300-500 mm.