A continuous casting method for stably controlling the composition of tundish covering flux for bearing steel

Abstract

The present invention belongs to the technical field of steelmaking processes and relates to a continuous casting method for stably controlling the composition of tundish covering flux for bearing steel. By adding cryolite to the tundish covering flux, the melting point of the covering flux is reduced, and under the condition of not adding carbonized rice husk, the caking of the covering flux can be avoided. By using a double-layer alumina flow control sand without SiO2, Fe2O3, and Cr2O3 contents, the entry of oxidizing substances into the tundish covering flux is avoided. The composition of the upper layer of the flow control sand is mainly Al2O3 and CaF2, and the composition of the lower layer of the flow control sand is mainly Al2O3. By adopting the above method, the basicity of the covering flux can be maintained above 7.0 during the entire casting process of bearing steel, and the contents of Fe2O3 and Cr2O3 in the covering flux can be stably controlled within 0.5%. The oxidation of the molten steel by the covering flux is greatly weakened, the continuous casting heat number of bearing steel can be increased from the initial 6 heats to more than 13 heats, and the qualified rate of ultrasonic inspection after water immersion is significantly improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of steelmaking processes, and particularly relates to a continuous casting method for stably controlling the composition of tundish covering flux for bearing steel. Background Art

[0002] Bearing steel is generally produced by the process of "converter → LF refining → RH vacuum → continuous casting". In the LF refining process, aluminum deoxidation and high-alkalinity refining slag are usually used. Under such conditions, the inclusions in the steel are mainly magnesium-aluminum spinel, accompanied by a small amount of calcium aluminate. During the RH vacuum treatment process, spinel inclusions in the steel can be efficiently removed, and the remaining inclusions are mainly liquid calcium aluminate. However, during the continuous casting process, many high-melting-point magnesium-aluminum spinel inclusions are regenerated in the steel, resulting in the nozzle of the bearing steel becoming blocked, poor castability of the molten steel. At the same time, the random peeling of the nodule substances into the steel causes frequent fluctuations in the mold level, which is also the main reason why the casting performance of high-carbon chromium bearing steel has been a difficult problem in the industry for a long time.

[0003] Research results show that secondary oxidation of molten steel during the casting process is the main reason for the poor castability of bearing steel. Further, as the casting progresses, the composition of the covering flux above the tundish molten steel has changed greatly. Among them, the alkalinity of the covering flux gradually decreases, and the alkalinity has dropped from the initial 10.0 to less than 2.0 in the middle and late stages of casting. The FeO content in the covering flux is as high as 5-10%, and the Cr2O3 content is as high as 10-15%. This shows that the covering flux has evolved from the initial high-alkalinity and low-oxidation slag to a low-alkalinity and high-oxidation slag, which leads to inevitable secondary oxidation of the molten steel during the casting process.

[0004] In order to improve the castability of bearing steel molten steel, Chinese Patent Application No. CN202110693371.2 proposes using aluminum-carbon material for the ladle molten pool and bottom part, alumina-based dry material for the tundish, and integral aluminum-carbon material for the stopper rod, which can significantly increase the number of continuous casting heats of bearing steel. Chinese Patent Application No. CN202210110399.3 proposes controlling the aluminum content of bearing steel very low to improve the castability of molten steel. Although these methods are adopted, the castability of bearing steel molten steel can be improved on the original basis, but the problem of secondary oxidation in the tundish has not been solved, that is, the problem of bearing steel nodulation has not been fundamentally solved. Summary of the Invention

[0005] In order to solve the problem of the oxidation of molten steel by the covering flux during the casting process of bearing steel, based on the deficiencies existing in the current process of bearing steel, the present invention proposes a continuous casting method for stably controlling the composition of the tundish covering flux of bearing steel. By adding cryolite to the tundish covering flux, the melting point of the covering flux is reduced, and under the condition of not adding carbonized rice husk, the covering flux crusting can be avoided. By using a double-layer aluminous flow control sand without SiO2, Fe2O3, and Cr2O3 content, the entry of oxidizing substances into the tundish covering flux is avoided. The composition of the upper layer of the flow control sand is mainly Al2O3 and CaF2, and the composition of the lower layer of the flow control sand is mainly Al2O3. By adopting the above method, the basicity of the covering flux during the entire casting process of bearing steel can be maintained above 7.0, and the contents of Fe2O3 and Cr2O3 in the covering flux can be stably controlled within 1%. The oxidation of the molten steel by the covering flux is greatly weakened, and the number of continuous casting heats of bearing steel can be increased from the initial 6 heats to 13 heats, and the qualified rate of ultrasonic inspection is significantly improved.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is: a continuous casting method for stably controlling the composition of the tundish covering flux of bearing steel:

[0007] The mass percentage composition of the bearing steel is: C: 0.95 - 1.05%, Si: 0.15 - 0.30%, Mn: 0.25 - 0.40%, P: <0.020%, S: <0.015%, Al: 0.01 - 0.03%, Cr: 1.40 - 1.60, and the rest are iron and residual elements.

[0008] A continuous casting method for stably controlling the composition of the tundish covering flux of bearing steel, this process includes the following key points:

[0009] (1) Double-layer flow control sand is used for the initial pouring from the ladle. The Al2O3 content of the lower layer of the flow control sand is 95 - 100%, and the contents of SiO2, Fe2O3, and Cr2O3 are within 1%. The Al2O3 content of the upper layer of the flow control sand is 65 - 75%, the SiO2 content is 2 - 6%, the CaF2 content is 15 - 20%, and the contents of Fe2O3 and Cr2O3 are within 1%. In order to avoid the oxidation of the molten steel by the tundish covering flux and the newly formed high-melting-point MgO·Al2O3 spinel inclusions in the tundish molten steel, it is required that the contents of Fe2O3, Cr2O3, and MgO in the flow control sand are as low as possible.

[0010] (2) The addition amount of the lower layer of the flow control sand is 10 kg / heat, the addition amount of the upper layer of the flow control sand is 10 - 20 kg / heat, and the particle size of the flow control sand is controlled within 0.2 - 1.0 mm.

[0011] (3) When starting to pour a large ladle, when the molten steel in the tundish reaches 50 - 65% of the total mass of the molten steel in the tundish, add 2.0 - 2.5 kg / t of basic covering flux to each baking hole of the tundish (which has 3 baking holes and 1 impact zone), and then add 2.0 - 2.5 kg / t of basic covering flux to the impact zone; after adding the basic covering flux, add 0.2 - 0.4 kg / t of cryolite to each baking hole, and then add 0.4 - 0.7 kg / t of cryolite to the impact zone;

[0012] The addition amount of cryolite should not be too low, otherwise it will cause the melting point of the covering flux to remain relatively high, and the covering flux will still form a crust without using carbonized rice husk; if the addition amount of cryolite is on the high side, although the covering flux will not form a crust, due to the high F content in the covering flux, it is easy to erode the refractory of the tundish and reduce the service life of the tundish. The added covering flux is a conventional basic covering flux in the steelmaking industry, and its composition is CaO: 45 - 52%, SiO2: 2 - 5%, Al2O3: 35 - 45%, MgO: 2 - 5%, and the initial basicity of the covering flux is 9.0 - 10.0. The change in the basicity and composition of the covering flux from the 1st furnace to the 13th furnace of casting is very small, and the basicity is stable at 7.5 - 8.6; the contents of FeO and Cr2O3 in the covering flux can be stably controlled within 1%.

[0013] (4) During the tundish casting process, it is prohibited to add rice husk ash to the covering flux for heat preservation.

[0014] Through the research on the evolution law of the tundish covering flux composition by the present invention, it is found that the main factors affecting the tundish covering flux composition of bearing steel are carbonized rice husk and tapping sand. Among them, carbonized rice husk contains more than about 50% of silicon dioxide. Since the tundish molten steel temperature of bearing steel is only about 1480°C, which is the lowest among all steel grades, although the conventional calcium-aluminum-based covering flux has a low melting point, compared with such a low tundish molten steel temperature, carbonized rice husk must be added above the covering flux for heat preservation, otherwise the covering flux is very easy to form a crust. Once the covering flux forms a crust, it will affect the oscillation of the stopper rod, thus affecting continuous casting production. In addition, at present, most steel grades such as bearing steel use chromite tapping sand, and the tapping sand contains about 20 - 30% SiO2, 20 - 30% Fe2O3, 30 - 40% Cr2O3. After the ladle starts pouring, the tapping sand will directly flow into the tundish covering flux, further reducing the basicity of the covering flux, and at the same time, the contents of FeO and Cr2O3 in the covering flux increase significantly.

[0015] The progressive effects of the present invention are as follows: By adding cryolite to the covering agent, the melting point of the covering agent is reduced. Without adding carbonized rice husk, the covering agent can be prevented from crusting. By developing an aluminum double-layered flow-guiding sand, highly oxidizing substances are prevented from entering the covering agent, ultimately solving the problem of the covering agent oxidizing the molten steel. The number of continuous casting heats of the molten steel can be increased from the initial 6 heats to more than 13 heats. At the same time, the probability of nodular substances randomly falling into the steel is significantly reduced, resulting in a significant increase in the qualified rate of ultrasonic inspection of the steel. Description of the Drawings

[0016] Figure 1 It is the composition of the covering agent in the tundish impact zone during the casting process of Example 1;

[0017] Figure 2 It is the stopper rod curve during the casting process of Example 1;

[0018] Figure 3 It is the composition of the covering agent in the tundish impact zone during the casting process of Example 2;

[0019] Figure 4 It is the stopper rod curve during the casting process of Example 2;

[0020] Figure 5 It is the composition of the covering agent in the tundish impact zone during the casting process of Comparative Example 1;

[0021] Figure 6 It is the stopper rod curve during the casting process of Comparative Example 1.

[0022] Figure 7 It is the composition of the covering agent in the tundish impact zone during the casting process of Comparative Example 2;

[0023] Figure 8 It is the stopper rod curve during the casting process of Comparative Example 2.

[0024] Figure 9 It is the composition of the covering agent in the tundish impact zone during the casting process of Comparative Example 3;

[0025] Figure 10 It is the stopper rod curve during the casting process of Comparative Example 3. Detailed Embodiments

[0026] Example 1:

[0027] For 13 heats of bearing steel in 1 casting campaign. The tundish molten steel capacity is 45t. Before the ladle receives the molten steel, double-layered aluminum flow-guiding sand is added to the nozzle. First, 10 kg of the lower-layer flow-guiding sand is added, and then 20 kg of the upper-layer flow-guiding sand is added. By mass percentage, the Al2O3 content of the lower-layer flow-guiding sand is 97%, and the rest are impurities; the Al2O3 content of the upper-layer flow-guiding sand is 72%, the SiO2 content is 4%, the CaF2 content is 18%, and the rest are impurities.

[0028] After the first heat is tapped, when the tundish reaches 25t, first add 300kg of covering flux to the 3 baking holes in the casting area of the tundish, then add 100kg of covering flux to the impact area of the tundish. After the covering flux is added, add 40kg of cryolite to the 3 baking holes in the casting area and 20kg of cryolite to the impact area. No carbonized rice husk is added during the entire casting process.

[0029] During the entire casting process, the composition of the covering flux in the impact area of the tundish is as Figure 1 , from the first heat to the 13th heat, the basicity of the tundish covering flux always remains at a relatively high level, with the basicity controlled at 7.5 - 8.6, and the contents of FeO and Cr2O3 maintained within 1%.

[0030] The stopper rod curve during the casting process is as Figure 2 , no accretion occurs during the entire casting process, and the pourability of the molten steel is well controlled; the composition can still remain stable after more than 13 heats, but the tundish life reaches its end, and it actually ends after the 13th heat, which does not mean that the maximum number of heats for stable casting is 13.

[0031] Meanwhile, for each heat of the 13 heats of steel, 8 rolled products are taken for immersion flaw detection. The flaw detection qualification rate is 98.2%. Among them, only when all 8 rolled products of each heat of steel pass the flaw detection can it be considered that the heat of steel passes the flaw detection. The calculation method of the flaw detection qualification rate is the number of qualified heats / the total number of heats.

[0032] Example 2:

[0033] There are 13 heats of bearing steel in one casting campaign. The molten steel capacity of the tundish is 45t. Before the ladle receives the molten steel, add double - layer aluminous drainage sand to the tundish nozzle. First add 10kg of the lower - layer drainage sand, and then add 20kg of the upper - layer drainage sand. The composition of the double - layer aluminous drainage sand is the same as that in Example 1.

[0034] After the first heat is tapped, when the tundish reaches 30 tons, first add 300kg of covering flux to the 3 baking holes in the casting area of the tundish, then add 100kg of covering flux to the impact area of the tundish. After the covering flux is added, add 40kg of cryolite to the 3 baking holes in the casting area and 20kg of cryolite to the impact area. No carbonized rice husk is added during the entire casting process.

[0035] During the entire casting process, the composition of the covering flux in the impact area of the tundish is as Figure 3 , the basicity of the tundish covering flux always remains at a relatively high level, with the basicity controlled at 7.6 - 8.2, and the contents of FeO and Cr2O3 within 1%.

[0036] The stopper rod curve during the casting process is as Figure 4 , no accretion occurs during the entire casting process, and the pourability of the molten steel is well controlled. The composition can still remain stable after more than 13 heats, but the tundish life reaches its end, and it actually ends after the 13th heat, which does not mean that the maximum number of heats for stable casting is 13.

[0037] Meanwhile, 8 rolled products were taken from each of the 13 heats of steel for immersion flaw detection. The qualified rate of flaw detection was 96.5%. Only when all 8 rolled products of each heat of steel passed the flaw detection could the heat of steel be considered qualified for flaw detection. The calculation method of the qualified rate of flaw detection was the number of qualified heats / the total number of heats.

[0038] Comparative Example 1:

[0039] There were 6 heats of bearing steel in 1 casting campaign. The molten steel capacity of the tundish was 45 t. Before receiving molten steel, double-layer aluminous drainage sand was added to the nozzle of the ladle. First, 10 kg of the lower-layer drainage sand was added, and then 20 kg of the upper-layer drainage sand was added. The composition of the double-layer aluminous drainage sand was the same as that in Example 1.

[0040] After the first heat was tapped, when the tundish reached 25 tons, first 300 kg of covering agent was added to the 3 baking holes in the casting area of the tundish, then 100 kg of covering agent was added to the impact area of the tundish. After the covering agent was added, 30 kg of carbonized rice husk was added to the impact area for heat preservation, and 20 kg of carbonized rice husk was added to each of the 3 baking holes in the casting area for heat preservation. The principle of adding carbonized rice husk was that if red slag appeared, that is, the rice husk ash was consumed, carbonized rice husk ash was replenished to the covering agent. The addition amount of carbonized rice husk for 6 heats of steel was 480 kg.

[0041] During the whole casting process, the composition of the covering agent in the impact area of the tundish was as Figure 5 , and the basicity of the tundish covering agent gradually decreased. In the last heat of the casting campaign, the basicity had dropped to 2.9, but the contents of FeO and Cr2O3 were both controlled at a relatively low level, both within 1%.

[0042] The stopper curve during the casting process was as Figure 6 , the stopper rose during the casting process, the nozzle became encrusted, the pourability of the molten steel was relatively poor. 8 rolled products were taken from each heat of steel for immersion flaw detection. The qualified rate of flaw detection was 82.7%. Only when all 8 rolled products of each heat of steel passed the flaw detection could the heat of steel be considered qualified for flaw detection. The calculation method of the qualified rate of flaw detection was the number of qualified heats / the total number of heats.

[0043] Comparative Example 2:

[0044] There were 6 heats of bearing steel in 1 casting campaign. The molten steel capacity of the tundish was 45 t. Before receiving molten steel, conventional chromic drainage sand was added to the nozzle of the ladle, and the addition amount was 30 kg, in which the SiO2 content was 21%, the Al2O3 content was 11%, the Cr2O3 content was 37%, and the Fe2O3 content was 22%.

[0045] After the first heat was tapped, when the tundish reached 25 tons, first 300 kg of covering agent was added to the 3 baking holes in the casting area of the tundish, then 100 kg of covering agent was added to the impact area of the tundish. After the covering agent was added, 40 kg of cryolite was added to the 3 baking holes in the casting area in total, and 20 kg of cryolite was added to the impact area. No carbonized rice husk was added during the whole casting process.

[0046] During the entire casting process, the composition of the covering flux in the tundish impact zone is as follows Figure 7 , the basicity of the tundish covering flux gradually decreases. In the last furnace of the casting campaign, the basicity has dropped to 3.8. At the same time, the FeO content is between 2.8% and 7.8%, and the Cr2O3 content is between 4.8% and 10.2%.

[0047] The stopper rod curve during the casting process is as follows Figure 8 , the stopper rod rises during the casting process, the nozzle becomes nodular, and the pourability of the molten steel is poor. For each heat of steel, 8 rolled products are taken for immersion flaw detection. The flaw detection qualification rate is 76.5%. Among them, only when all 8 rolled products of each heat of steel pass the flaw detection can it be considered that the heat of steel passes the flaw detection. The calculation method of the flaw detection qualification rate is the number of qualified heats / the total number of heats.

[0048] Comparative Example 3:

[0049] 6 heats of bearing steel in one casting campaign. The molten steel capacity of the tundish is 45t. Before receiving the molten steel, 30kg of conventional chromite drain sand is added to the nozzle. The SiO2 content is 21%, the Al2O3 content is 11%, the Cr2O3 content is 37%, and the Fe2O3 content is 22%.

[0050] After the first furnace starts casting, when the tundish reaches 25 tons, first add 300kg of covering flux to the 3 baking holes in the tundish casting area, then add 100kg of covering flux to the tundish impact zone. After the covering flux is added, add 30kg of carbonized rice husk to the impact zone for heat preservation, and add 20kg of carbonized rice husk to each of the 3 baking holes in the casting area for heat preservation. The principle of adding carbonized rice husk is that if red slag appears, that is, the rice husk ash is consumed, then carbonized rice husk ash is supplemented to the covering flux. The carbonized rice husk addition amount for 6 heats of steel is 450kg.

[0051] During the entire casting process, the composition of the covering flux in the tundish impact zone is as follows Figure 9 , the basicity control of the tundish covering flux gradually decreases. In the last furnace of the casting campaign, the basicity has dropped to 1.3. At the same time, the FeO content is between 3.5% and 7.8%, and the Cr2O3 content is between 5.6% and 18.6%.

[0052] The stopper rod curve during the casting process is as follows Figure 10 , the stopper rod rises during the casting process, the nozzle becomes nodular, and the pourability of the molten steel is poor. For each heat of steel, 8 rolled products are taken for immersion flaw detection. The flaw detection qualification rate is 63.4%. Among them, only when all 8 rolled products of each heat of steel pass the flaw detection can it be considered that the heat of steel passes the flaw detection. The calculation method of the flaw detection qualification rate is the number of qualified heats / the total number of heats.

Claims

1. A continuous casting method for stably controlling the composition of tundish covering flux for bearing steel, characterized in that, It includes the following steps: In the continuous casting process of producing bearing steel, before the ladle receives molten steel, a double-layer alumina flow control sand is added to the tundish nozzle; the lower-layer flow control sand has an Al2O3 content of 95-100%, and the contents of SiO2, Fe2O3, and Cr2O3 are within 1%; the upper-layer flow control sand has an Al2O3 content of 65-75%, a SiO2 content of 2-6%, a CaF2 content of 15-20%, and the contents of Fe2O3 and Cr2O3 are within 1%. After the ladle starts pouring, when the molten steel in the tundish reaches 50-65% of the total molten steel mass that the tundish capacity can hold, 2.0-2.5 kg / t of basic covering agent is added to each baking hole of the tundish respectively, and then 2.0-2.5 kg / t of basic covering agent is added to the impact zone; after the basic covering agent is added, 0.2-0.4 kg / t of cryolite is added to each baking hole respectively, and then 0.4-0.7 kg / t of cryolite is added to the impact zone; no rice husk ash is added during the entire casting process; the basicity of the tundish covering agent is controlled at 7.5-8.6 during the entire continuous casting process.

2. The continuous casting method for stabilizing and controlling the composition of tundish covering flux for bearing steel according to claim 1, characterized in that: The tundish has a total of 3 baking holes and 1 impact zone.

3. The continuous casting method for stabilizing and controlling the composition of the tundish covering agent for bearing steel according to claim 1, characterized in that: The composition of the said bearing steel is: C: 0.95-1.05%, Si: 0.15-0.30%, Mn: 0.25-0.40%, P: <0.020%, S: <0.015%, Al: 0.01-0.03%, Cr: 1.40-1.60, and the rest are iron and residual elements.

4. The continuous casting method for stably controlling the composition of the tundish covering agent of bearing steel according to claim 1, the addition amount of the lower-layer flow control sand is 10 kg / furnace, the addition amount of the upper-layer flow control sand is 10-20 kg / furnace, and the particle size of the flow control sand is controlled at 0.2-1.0 mm.

5. The continuous casting method for stably controlling the composition of the tundish covering agent of bearing steel according to claim 1, the added covering agent is a conventional basic covering agent used in the steelmaking industry, and its composition is CaO: 45-52%, SiO2: 2-5%, Al2O3: 35-45%, MgO: 2-5%, and the initial basicity of the basic covering agent is 9.0-10.

0.

6. The continuous casting method for stably controlling the composition of the tundish covering agent of bearing steel according to claim 1, adding rice husk ash to the covering agent for heat preservation is prohibited during the tundish casting process.

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