Method for improving purity of molten steel by adjusting LF slag system

Abstract

The invention belongs to the technical field of steel smelting, and particularly discloses a method for improving the purity of molten steel by adjusting an LF (ladle furnace) slag system, which comprises the following steps of: firstly, injecting desulfurized molten iron into a converter for smelting until the total iron content in the slag is less than or equal to 1.5 percent; after tapping, lime, fluorite and a deoxidizing agent are added into a steel ladle and heated, and it is ensured that the alkalinity of top slag is 2.5-3.0; then adding a ferrosilicon alloy, a silicon manganese alloy and a high-carbon ferrochromium alloy into the molten steel to enable the components to reach designed components, then adding quartz sand into a steel ladle, and heating to reduce the alkalinity of refining slag; and finally, pure calcium wires are fed into the molten steel, and then the steel ladle is lifted to a continuous casting rotary table for standing and then transferred to a casting position for casting. In order to solve the problem that the dual requirements of molten steel desulfurization and inclusion plasticization control are difficult to consider in the prior art, the purposes of efficiently removing molten steel inclusions and purifying molten steel are achieved by adjusting the alkalinity of the steel slag in different stages of LF furnace refining and changing the alkalinity and components of the steel slag in the smelting process.

Description

Technical Field

[0001] This invention relates to the field of iron and steel smelting technology, specifically to a method for improving the purity of molten steel by adjusting the LF slag system. Background Technology

[0002] Steel purity is a core indicator for measuring steel quality. It mainly refers to the content, morphology, size, and distribution of gases (hydrogen, nitrogen, oxygen) and non-metallic inclusions (oxides, sulfides, nitrides, silicates, etc.) in the steel. Higher purity results in superior mechanical properties, processing performance, and corrosion resistance of the steel and the final steel product. This is especially crucial for high-end steel grades such as bearing steel, automotive panel steel, and pipeline steel.

[0003] In the metallurgical industry, 82B steel is a typical steel used for high-strength prestressed steel wire and strand. Due to its excellent mechanical and processing properties, it is widely used in key engineering fields such as railway tracks, highway bridges, nuclear power plants, high-rise buildings, and cement products. Because this type of steel needs to withstand alternating loads, corrosive environments, and extreme temperature changes for extended periods, stringent requirements are placed on its internal quality to meet the demands of these harsh and complex operating conditions. This requires not only strict control of chemical composition, such as the precise proportions of elements like carbon, manganese, and silicon, but also minimizing the content of harmful elements like sulfur, phosphorus, hydrogen, and nitrogen. In particular, the quantity, size, and type of non-metallic inclusions must be strictly controlled. This necessitates minimizing the number of inclusions during the smelting process to improve the purity of the molten steel. The main non-metallic oxide inclusions in 82B steel fall into three categories: CaO-Al2O3-SiO2, CaO-Al2O3-SiO2-MgO, and CaO-SiO2-MgO-Al2O3-CaS. Many factors influence the plastic deformation, size, and number of these inclusions. Among these, the liquidus temperature is the primary factor affecting the plastic deformation of non-metallic oxide inclusions, and this liquidus temperature is mainly influenced by their composition. When the liquidus temperature is below 1500℃, the inclusions exhibit good plasticity. Therefore, inclusions with high liquidus temperatures and high hardness, such as alumina inclusions, should be avoided. The composition of the inclusions is affected by the reaction with steel slag; therefore, adjusting the content of refining slag can alter the inclusion composition, thereby improving the inclusions' plasticity.

[0004] Under current technology, most LF furnace refining processes adopt a single slag system control strategy, which makes it difficult to simultaneously meet the dual requirements of steel desulfurization and inclusion plasticization control. Therefore, to solve the above problems, it is necessary to design a method to improve the purity of molten steel by adjusting the LF furnace slag system. By adjusting the quantity and type of slag at different stages of the LF furnace refining process, the production needs at different stages can be met. Furthermore, by utilizing the changes in the basicity and composition of the steel slag during the LF furnace refining process, the goal of efficiently removing inclusions and purifying the molten steel can be achieved. Summary of the Invention

[0005] The purpose of this invention is to provide a method for improving the purity of molten steel by adjusting the LF slag system, so as to solve the problem that the production of 82B steel in the prior art is affected by the inability to effectively remove inclusions in molten steel and optimize the morphology of inclusions.

[0006] To achieve the above objectives, the basic solution provided by this invention is: a method for improving the purity of molten steel by adjusting the LF slag system, comprising the following steps: S1: Converter smelting: First, the desulfurized molten iron is poured into the converter for smelting, so that the total iron content in the slag is ≤1.5%, and then the steel is tapped. During the tapping process, lime, silicon-manganese alloy and carbon are added to the molten steel in sequence, and argon gas is blown into the molten steel for stirring. S2: LF furnace refining: Then lime, fluorite and deoxidizer are added to the ladle in sequence, and the argon blowing rate is adjusted. At the same time, the molten steel is heated to ensure that the basicity of the top slag of the molten steel is 2.5-3.0. S3: Alloying Desulfurization: Under the condition of high basicity slag in molten steel, ferrosilicon alloy, ferrosilicon-manganese alloy and high carbon ferrochrome alloy are added to the molten steel in sequence, and the argon blowing volume is adjusted so that the content of C in the molten steel is 0.80%-0.83%, the content of Si is 0.18%-0.28%, the content of Mn is 0.7%-0.8%, the content of P is ≤0.02%, the content of S is ≤0.015%, and the content of Cr is 0.19%-0.24%. Then, quartz sand is added to the ladle and the molten steel is heated by electricity to reduce the basicity of the refining slag in the molten steel by 0.8-1.2. S4: Finally, pure calcium wire is fed into the molten steel. After the wire feeding is completed, the argon gas is adjusted to the soft blowing state and soft blowing begins. After the soft blowing time exceeds 10 minutes, the ladle is hoisted to the continuous casting turret using a metallurgical crane. The ladle is left to stand on the continuous casting turret for 5 minutes before being transferred to the casting position to begin pouring.

[0007] The beneficial effects of the present invention are as follows: (1) The present invention desulfurizes the high basicity slag in the early stage of LF furnace refining, thereby reducing the sulfur content in the molten steel, so that the total iron content in the slag is ≤1.5%. In the later stage of refining, the basicity of the slag is reduced and quartz sand is added for modification, which reduces the liquid phase temperature of inclusions in the molten steel, and makes the CaO-Al2O3-SiO2 system inclusions plastic, thereby breaking through the limitation of traditional single slag system that cannot take into account both desulfurization and inclusion control, and thus realizing staged slag system regulation and simultaneously achieving high efficiency. Desulfurization and inclusion plasticization; (2) By setting the argon flow rate according to different stages of LF furnace refining and controlling the diameter of the exposed surface of the molten steel, excessive stirring is avoided to prevent slag from being rolled in the molten steel, thereby improving the efficiency of inclusion adsorption and flotation in the molten steel; (3) The proportion of inclusions larger than 10μm in the molten steel is reduced by the above method, thereby improving the cleanliness of the molten steel, meeting the stringent requirements of 82B steel for inclusions, and ensuring the reliability of prestressed steel wire and steel strand in bridges, nuclear power plants and other application scenarios.

[0008] Option 2, which is the preferred option of the basic option, involves a tapping temperature of 1580℃-1600℃ in S1.

[0009] Option 3, which is the preferred option of the basic option, involves adding carbon powder and deoxidizer to the bottom of the ladle in sequence before tapping the steel in the converter. The carbon powder is added at a standard of 0.6 kg to 1.2 kg per ton of steel, and the deoxidizer is added at a standard of 0.6 kg per ton of steel. Adding carbon powder and deoxidizer before tapping can start deoxidation and form a reducing atmosphere during the tapping process, inhibiting secondary oxidation of molten steel and creating favorable conditions for subsequent LF furnace refining.

[0010] Option 4, which is the preferred option of the basic option, involves adding lime at 1.2 kg per ton of steel during the tapping process in S1, and adding silicon manganese alloy and carbon according to the composition of the molten steel when the converter stops blowing; this ensures that a primary slag with a certain alkalinity is formed before refining in the LF furnace, which is beneficial for subsequent rapid slag formation and desulfurization.

[0011] Option 5, an optimal choice from the basic option, involves adding 2.4-3.6 kg of lime, 0.8-1.2 kg of fluorite, and 0.6-0.9 kg of deoxidizer per ton of steel in S2. The amount of fluorite added is adjusted according to the viscosity of the top slag. The given range of additions provides precise batching guidance for on-site operations, ensuring the rapid and stable formation of a high-alkalinity slag system.

[0012] Option 6, the preferred option of the basic scheme, involves an argon gas injection rate of 15 Nm³ in S2. 3 / h-20Nm 3 / h, the diameter of the exposed surface of the molten steel is 300mm-450mm; this argon flow rate range can ensure sufficient stirring intensity, promote desulfurization of molten steel, control the fluctuation of the molten steel surface, and improve the heating efficiency of molten steel.

[0013] Option 7, which is the preferred option of the basic option, involves adding 0.6-0.9 kg of quartz sand per ton of steel in S3. It is clear that the amount of quartz sand added can accurately reduce the slag basicity from 2.5-3.0 by 0.8-1.2, thereby changing the composition of inclusions, lowering their melting point, and making them plastic.

[0014] Option 8, the preferred option of the basic scheme, involves adjusting the argon gas injection rate in S3 to 30 Nm³. 3 / h-40Nm 3 / h, so that the diameter of the exposed surface of the molten steel is 500mm-600mm; at this time, the argon flow rate is adjusted to strong blowing, which can accelerate the melting of alloys, the uniform distribution of components, and promote the melting of quartz sand and the reaction with refining slag, ensuring uniform refining slag modification, limiting the diameter of the exposed surface of the molten steel, and avoiding slag entrapment caused by excessive stirring.

[0015] Option 9 is the preferred option of the basic option. In S4, the feed rate of pure calcium wire is 0.6m-0.9m per ton of steel. Pure calcium wire can change the morphology and properties of inclusions, thereby improving the purity and performance of steel. Moreover, the modified inclusions are not easy to accumulate at the nozzle, ensuring the continuity and stability of the continuous casting process.

[0016] Option 10, this is the preferred option of the basic option, in which the argon gas blowing rate in S4 is adjusted to 3 Nm. 3 / h-6Nm 3 / h; at this time, reducing the argon flow rate can promote the full floating of fine inclusions to the slag layer for absorption, and ensure that the slag layer is not disturbed, preventing the top slag from being re-rolled into the molten steel and causing secondary pollution. Detailed Implementation

[0017] The present invention will be further described in detail below through specific embodiments: Example 1 A method for improving the purity of molten steel by adjusting the LF slag system includes the following steps: S1: Converter smelting: First, the desulfurized molten iron is poured into the converter for smelting, so that the total iron content in the slag is ≤1.5%. Then, the steel is tapped at a temperature of 1580℃-1600℃. Before tapping, carbon powder and deoxidizer are added to the bottom of the ladle in sequence. The carbon powder is added at a standard of 0.6kg-1.2kg per ton of steel, and the deoxidizer is added at a standard of 0.6kg per ton of steel. During the tapping process, lime, silicon manganese alloy and carbon are added to the molten steel in sequence, and argon gas is blown into the molten steel for stirring. The lime is added at a standard of 1.2kg per ton of steel, and the silicon manganese alloy and carbon are added according to the composition of the molten steel when the converter stops blowing. S2: LF furnace refining: Then, 631 kg of lime, 366 kg of fluorite, and 131 kg of deoxidizer are added sequentially to the ladle, and the argon blowing rate is adjusted to 15 Nm. 3 / h-20Nm 3 / h, the diameter of the exposed surface of the molten steel is 300mm-450mm, and the molten steel is heated at the same time to ensure that the basicity of the top slag of the molten steel is 2.5-3.0; S3: Alloying Desulfurization: Under the condition of high basicity slag in molten steel, 49 kg of ferrosilicon alloy, 301 kg of ferrosilicon-manganese alloy, and 100 kg of high-carbon ferrochrome alloy are added to the molten steel in sequence, and the argon blowing rate is adjusted to 30 Nm. 3 / h-40Nm 3 / h, the diameter of the exposed surface of the molten steel is 500mm-600mm, so that the content of C in the molten steel is 0.80%-0.83%, the content of Si is 0.18%-0.28%, the content of Mn is 0.7%-0.8%, the content of P is ≤0.02%, the content of S is ≤0.015%, and the content of Cr is 0.19%-0.24%. Then, quartz sand is added to the ladle at a standard of 0.6kg-0.9kg per ton of steel and the molten steel is heated by electricity, so that the basicity of the refining slag in the molten steel is reduced by 0.8-1.2. S4: Finally, feed pure calcium wire into the molten steel at a standard rate of 0.6m-0.9m per ton of steel. After feeding, adjust the argon gas blowing rate to 3Nm. 3 / h-6Nm 3 Soft blowing begins at / h. After 17 minutes of soft blowing, the ladle is lifted to the continuous casting turret using a metallurgical crane. The ladle is left to stand on the continuous casting turret for 5 minutes before being transferred to the casting position to begin pouring.

[0018] Example 2 A method for improving the purity of molten steel by adjusting the LF slag system, the steps of which are the same as those in Example 1, the difference being: S2: LF furnace refining: Then, 584 kg of lime, 314 kg of fluorite, and 134 kg of deoxidizer are added sequentially to the ladle, and the argon blowing rate is adjusted to 15 Nm. 3 / h-20Nm 3 / h, the diameter of the exposed surface of the molten steel is 300mm-450mm, and the molten steel is heated at the same time to ensure that the basicity of the top slag of the molten steel is 2.5-3.0; S3: Alloying Desulfurization: Under the condition of high basicity slag in molten steel, 126 kg of ferrosilicon alloy, 221 kg of ferrosilicon-manganese alloy, and 98 kg of high-carbon ferrochrome alloy are added to the molten steel in sequence, and the argon blowing rate is adjusted to 30 Nm. 3 / h-40Nm 3 / h, the diameter of the exposed surface of the molten steel is 500mm-600mm, so that the content of C in the molten steel is 0.80%-0.83%, the content of Si is 0.18%-0.28%, the content of Mn is 0.7%-0.8%, the content of P is ≤0.02%, the content of S is ≤0.015%, and the content of Cr is 0.19%-0.24%. Then, quartz sand is added to the ladle at a standard of 0.6kg-0.9kg per ton of steel and the molten steel is heated by electricity, so that the basicity of the refining slag in the molten steel is reduced by 0.8-1.2.

[0019] Example 3 A method for improving the purity of molten steel by adjusting the LF slag system, the steps of which are the same as those in Example 1, the difference being: S2: LF furnace refining: Then, 576 kg of lime, 284 kg of fluorite, and 171 kg of deoxidizer are added sequentially to the ladle, and the argon blowing rate is adjusted to 15 Nm. 3 / h-20Nm 3 / h, the diameter of the exposed surface of the molten steel is 300mm-450mm, and the molten steel is heated at the same time to ensure that the basicity of the top slag of the molten steel is 2.5-3.0; S3: Alloying Desulfurization: Under the condition of high basicity slag in molten steel, 107 kg of ferrosilicon alloy, 269 kg of ferrosilicon-manganese alloy, and 84 kg of high-carbon ferrochrome alloy are added to the molten steel in sequence, and the argon blowing rate is adjusted to 30 Nm. 3 / h-40Nm 3 / h, the diameter of the exposed surface of the molten steel is 500mm-600mm, so that the content of C in the molten steel is 0.80%-0.83%, the content of Si is 0.18%-0.28%, the content of Mn is 0.7%-0.8%, the content of P is ≤0.02%, the content of S is ≤0.015%, and the content of Cr is 0.19%-0.24%. Then, quartz sand is added to the ladle at a standard of 0.6kg-0.9kg per ton of steel and the molten steel is heated by electricity, so that the basicity of the refining slag in the molten steel is reduced by 0.8-1.2.

[0020] Table 1: Slag Sample Composition and Basicity Information at Each Stage of LF Furnace Refining CaO MgO Al2O3 SiO2 FeO MnO CaF2 R (alkalinity) Slag 59.230 2.654 4.911 21.450 0.832 0.049 10.874 2.76 Alloying 61.056 2.324 4.753 21.124 0.363 0.050 10.330 2.89 Alloying complete 60.399 2.315 4.793 21.282 0.976 0.065 10.170 2.84 Turn into trash 55.003 2.343 4.601 28.974 1.051 0.230 7.798 1.90 Soft blow start 53.785 2.543 4.655 29.493 1.619 0.262 7.643 1.82 Soft blow ends 53.870 2.722 4.778 28.816 2.319 0.266 7.229 1.87 Table 1 shows the changes in slag composition and basicity at each stage of the LF furnace refining process. Using the above steelmaking method, the slag basicity in the early stage of LF furnace refining is 2.76-2.84, which meets the design requirement of 2.5-3.0 for the slag basicity in the early stage of refining. In the later stage, after adding quartz sand to change the basicity, the slag basicity is between 1.82-1.9, which meets the design requirement of a slag basicity reduction of 0.8-1.2 after slag transformation.

[0021] Table 2: Information on the Quantity of Impurities in Finished Products Five groups of samples were randomly selected from rolled steel using the above method for inclusion quantity analysis. The results in Table 2 show that the number of inclusions in the five groups of samples all meet the standard of 82 steel. Table 2 also shows that the number of inclusions in molten steel is continuously reduced by the above steel smelting method, thus improving the purity of molten steel.

[0022] In summary, by using the above methods to control the slag basicity and slag composition in stages during the LF furnace refining process and optimizing the stirring intensity of argon gas at different stages, the plasticity of inclusions is improved while also facilitating their adsorption. This reduces the number of inclusions and the proportion of large-sized inclusions, significantly improving the purity of molten steel and enabling 82B steel to meet the requirements of harsh and complex operating conditions.

[0023] The above descriptions are merely embodiments of the present invention, and common knowledge regarding specific structures and characteristics is not elaborated upon here. It should be noted that those skilled in the art can make various modifications and improvements without departing from the structure of the present invention, and these should also be considered within the scope of protection of the present invention. These modifications and improvements will not affect the effectiveness of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A method for improving the purity of molten steel by adjusting the LF slag system, characterized in that, Includes the following steps: S1: Converter smelting: First, the desulfurized molten iron is poured into the converter for smelting, so that the total iron content in the slag is ≤1.5%, and then the steel is tapped. During the tapping process, lime, silicon-manganese alloy and carbon are added to the molten steel in sequence, and argon gas is blown into the molten steel for stirring. S2: LF furnace refining: Then lime, fluorite and deoxidizer are added to the ladle in sequence, and the argon blowing rate is adjusted. At the same time, the molten steel is heated to ensure that the basicity of the top slag of the molten steel is 2.5-3.

0. S3: Alloying Desulfurization: Under the condition of high basicity slag in molten steel, ferrosilicon alloy, ferrosilicon-manganese alloy and high carbon ferrochrome alloy are added to the molten steel in sequence, and the argon blowing volume is adjusted so that the content of C in the molten steel is 0.80%-0.83%, the content of Si is 0.18%-0.28%, the content of Mn is 0.7%-0.8%, the content of P is ≤0.02%, the content of S is ≤0.015%, and the content of Cr is 0.19%-0.24%. Then, quartz sand is added to the ladle and the molten steel is heated by electricity to reduce the basicity of the refining slag in the molten steel by 0.8-1.

2. S4: Finally, pure calcium wire is fed into the molten steel. After the wire feeding is completed, the argon gas is adjusted to the soft blowing state and soft blowing begins. After the soft blowing time exceeds 10 minutes, the ladle is hoisted to the continuous casting turret using a metallurgical crane. The ladle is left to stand on the continuous casting turret for 5 minutes before being transferred to the casting position to begin pouring.

2. The method for improving the purity of molten steel by adjusting the LF slag system according to claim 1, characterized in that, In S1, the tapping temperature is 1580℃-1600℃.

3. The method for improving the purity of molten steel by adjusting the LF slag system according to claim 1, characterized in that, In S1, carbon powder and deoxidizer are added sequentially to the bottom of the ladle before the steel is tapped from the converter. The carbon powder is added at a standard of 0.6 kg to 1.2 kg per ton of steel, and the deoxidizer is added at a standard of 0.6 kg per ton of steel.

4. The method for improving the purity of molten steel by adjusting the LF slag system according to claim 1, characterized in that, In S1, during the tapping process, lime is added at 1.2 kg per ton of steel, and silicon manganese alloy and carbon are added according to the composition of the molten steel when the converter stops blowing.

5. The method for improving the purity of molten steel by adjusting the LF slag system according to claim 1, characterized in that, In S2, the amount of lime added is 2.4kg-3.6kg per ton of steel, the amount of fluorite added is 0.8kg-1.2kg per ton of steel, and the amount of deoxidizer added is 0.6kg-0.9kg per ton of steel. The amount of fluorite added is adjusted according to the viscosity of the top slag.

6. The method for improving the purity of molten steel by adjusting the LF slag system according to claim 1, characterized in that, In S2, the argon gas blowing rate is 15 Nm. 3 / h-20Nm 3 / h, the diameter of the exposed molten steel surface is 300mm-450mm.

7. The method for improving the purity of molten steel by adjusting the LF slag system according to claim 1, characterized in that, In S3, the amount of quartz sand added is 0.6kg-0.9kg per ton of steel.

8. The method for improving the purity of molten steel by adjusting the LF slag system according to claim 1, characterized in that, In S3, the argon gas injection rate is adjusted to 30 Nm. 3 / h-40Nm 3 / h, so that the diameter of the exposed surface of molten steel is 500mm-600mm.

9. A method for improving the purity of molten steel by adjusting the LF slag system according to claim 1, characterized in that, In S4, the feed rate of pure calcium wire is 0.6m-0.9m per ton of steel.

10. A method for improving the purity of molten steel by adjusting the LF slag system according to claim 1, characterized in that, In S4, the argon gas injection rate is adjusted to 3 Nm. 3 / h-6Nm 3 / h.

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