A method for on-line changing of protective slag for different steel grades

Abstract

The application discloses a method for on-line replacement of protective slag in different steel grades, and the method steps are as follows: (1) when different steel grades are cast in the same tundish, after casting of a previous ladle is completed, a next ladle is rotated to a casting position and waits, and a casting speed is reduced to 0.8-0.9 m / min; when the tonnage of the tundish is reduced to 40-50% of the tundish capacity, the next ladle starts casting, and the molten steel is poured into the tundish; (2) when the tonnage of the tundish is increased to 80-90% of the tundish capacity, the tonnage of the tundish is controlled to be 85-95% of the tundish capacity, the temperature of the tundish is measured, and the overheat degree of the molten steel is calculated according to the measured temperature of the tundish; (3) the casting speed is reduced from 0.8-0.9 m / min to 0.3-0.4 m / min, and the crystallizer slag removal operation is started in the process of reducing the casting speed; and (4) after the original protective slag is removed, the new protective slag is added. The method effectively guarantees the lubrication and heat transfer characteristics of the protective slag, and meanwhile, does not cause liquid level fluctuation and slag rolling of the crystallizer.

Description

Technical Field

[0001] This invention relates to a continuous casting method, and more particularly to a method for online replacement of protective slag when mixing different steel grades. Background Technology

[0002] To meet the current demands for efficient and low-cost production, the continuous casting process in steelmaking commonly employs mixed casting of different steel grades. This effectively reduces casting preparation time and refractory material consumption, while increasing steel yield. However, the mixing of dissimilar steel grades inevitably leads to the mixing of different compositions in the crystallizer. This can cause the protective slag's physicochemical properties to fail to meet requirements for lubrication and heat transfer, resulting in difficulties in smooth casting. Furthermore, if the steel composition differs significantly before and after mixing, different types of protective slag must be used to adapt to the different process requirements. Therefore, developing a method for online replacement of protective slag during mixed casting of dissimilar steel grades is crucial.

[0003] Patent application number 202110273638.2 discloses "a crystallizer protective slag replacement device and a crystallizer slag removal method". The crystallizer protective slag replacement device with this structure can quickly and accurately remove the protective slag on the surface of the molten steel in the crystallizer, avoiding the danger of workers working near the crystallizer and effectively preventing the harm to the human body from high temperature and radiation sources; at the same time, it reduces the labor intensity of workers, ensures the slag replacement effect, and effectively avoids poor slag removal effect due to worker fatigue and negligence, resulting in poor melting effect of protective slag and crystallizer sticking and steel leakage accidents.

[0004] Patent application number 202110283325.5 discloses "a mold flux replacement device and a mold flux replacement method". The mold flux replacement device can avoid bringing out too much powdery flux when scooping up agglomerated flux, thereby ensuring the continuous consumption of flux in the mold, which can meet the high requirements of continuous casting production and ensure the quality of steel billets.

[0005] The above methods provide protective slag replacement devices and methods, enabling rapid slag replacement and ensuring its effectiveness through equipment investment and modification, while reducing the labor intensity of employees. However, during the slag replacement process, factors such as the timing of removing the original protective slag and adding the new protective slag, as well as the steel temperature and casting speed, all affect the lubrication and heat transfer characteristics of the protective slag in the crystallizer, making it difficult for the protective slag to adapt to production needs. Therefore, all the above methods have technical shortcomings. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a method for online replacement of protective slag when mixing different steel grades, which can effectively ensure the lubrication, heat transfer and other properties of the protective slag.

[0007] To solve the above-mentioned technical problems, the method and steps adopted by the present invention are as follows: (1) When different steel grades are mixed in the same tundish, after the previous ladle is poured, the next ladle is rotated to the pouring position to wait, and the pulling speed is reduced to 0.8-0.9 m / min; when the tundish tonnage is reduced to 40%-50% of the tundish capacity, the next ladle is poured and the molten steel is injected into the tundish.

[0008] (2) After the tundish tonnage increases to 80% to 90% of the tundish capacity, control the tundish tonnage range to 85% to 95% of the tundish capacity, measure the tundish temperature, and calculate the superheat of the molten steel based on the measured tundish temperature.

[0009] (3) The pulling speed is reduced from 0.8-0.9 m / min to 0.3-0.4 m / min, and the slag removal operation of the crystallizer is started during the process of reducing the pulling speed;

[0010] (4) After the original protective slag has been removed, new protective slag is added;

[0011] (5) After the new protective slag is added, the pulling speed is increased from 0.3-0.4 m / min to 0.8-0.9 m / min and maintained;

[0012] (6) Increase the pulling speed to meet the target pulling speed required by the process.

[0013] Furthermore, the capacity of the intermediate package is 60 to 80 tons.

[0014] Furthermore, in step (5), the rate of change of the pulling speed is 0.2–0.3 m / min. 2 .

[0015] Furthermore, in step (5), the time is maintained for 3 to 5 minutes.

[0016] Further, in step (6), the thickness of the protective slag liquid layer in the crystallizer is measured at the position between the narrow side of the crystallizer and the immersion nozzle of the tundish; if the thickness of the liquid slag layer is ≥8mm, the pulling speed is increased to the target pulling speed that meets the process requirements; if the thickness of the liquid slag layer is <8mm, the pulling speed is not increased, and the current pulling speed is maintained for 1 to 3 minutes before being increased to the target pulling speed that meets the process requirements.

[0017] The beneficial effects of adopting the above technical solution are as follows: This invention, by controlling the steel temperature, casting speed, the timing of removing the original protective slag, and the addition of the new protective slag, completes the replacement of the protective slag in the crystallizer, effectively ensuring the lubrication and heat transfer characteristics of the protective slag, while preventing fluctuations in the crystallizer liquid level and slag entrapment, thus ensuring efficient and smooth continuous casting production. Furthermore, this invention is simple to operate, safe and reliable, requires no additional equipment investment, and has low cost. Detailed Implementation

[0018] The steps for online replacement of protective slag in this method of mixing dissimilar steel grades are as follows:

[0019] (1) The capacity of the tundish is 60 to 80 tons; when different steel grades are mixed in the same tundish, after the previous ladle is finished, the next ladle is rotated to the pouring position to wait, and the pulling speed is reduced to 0.8 to 0.9 m / min; when the tundish tonnage is reduced to 40% to 50% of the tundish capacity, the next ladle is started to pour, and the molten steel is injected into the tundish.

[0020] (2) After the tundish tonnage gradually increases to 80% to 90% of the tundish capacity, the tundish tonnage control is switched from manual to automatic control and the tundish temperature is measured. When the tundish tonnage is controlled automatically, the tundish tonnage control range is 85% to 95% of the tundish capacity. The superheat of the molten steel is calculated based on the measured tundish temperature. Superheat = measured tundish temperature - liquidus line of the next heat of steel. The superheat requirement is ≥20℃.

[0021] (3) The pulling speed is reduced from 0.8-0.9 m / min to 0.3-0.4 m / min, and the crystallizer slag removal operation is started during the process of reducing the pulling speed.

[0022] (4) After the original protective slag is removed, new protective slag is added.

[0023] (5) After the new protective slag is added, the pulling speed is increased from 0.3-0.4 m / min to 0.8-0.9 m / min and maintained for 3-5 min. The rate of change of the pulling speed during the increase is 0.2-0.3 m / min. 2 .

[0024] (6) Measure the thickness of the protective slag liquid layer in the crystallizer. The measurement position is the middle position between the narrow side of the crystallizer and the immersion nozzle of the tundish. The thickness of the liquid slag layer should be ≥8mm. If the thickness of the liquid slag layer is ≥8mm, the pulling speed can be increased to the target pulling speed that meets the process requirements. If the thickness of the liquid slag layer is <8mm, do not increase the pulling speed. Maintain the current pulling speed for 1 to 3 minutes and then increase it to the target pulling speed that meets the process requirements.

[0025] Example 1: The method for online replacement of protective slag in the mixed casting of dissimilar steel types is as follows.

[0026] In a certain casting cycle, the tundish capacity is 70 tons. Within the same tundish, steel grades HC280 / 440DP and BGH340 are mixed, using protective slag A and protective slag B respectively. The previous heat was HC280 / 440DP, and the next heat is BGH340. The online replacement of the protective slag follows these steps:

[0027] (1) After the current HC280 / 440DP ladle is poured, the next BGH340 ladle is rotated to the pouring position to wait, and the pouring speed is gradually reduced from the current pouring speed of 1.2m / min to 0.8m / min. When the tonnage of the tundish is reduced from 63 tons to 30 tons, the next BGH340 ladle is poured, the hydraulic cylinder of the ladle nozzle is opened to 100%, and the molten steel is poured into the tundish.

[0028] (2) When the tundish tonnage gradually increases to 60 tons, the tundish tonnage control is switched from manual to automatic control, and the tundish tonnage is stabilized between 60 and 65 tons. The tundish temperature is measured to be 1539℃, and the liquidus temperature of steel grade BGH340 is 1513℃. At this time, the superheat of molten steel = 1539℃ - 1513℃ = 26℃.

[0029] (3) The casting speed is gradually reduced from 0.8 m / min to 0.4 m / min, and the slag removal operation of the crystallizer is started during the speed reduction process. The powdery slag and molten slag blocks of protective slag A are removed from the vicinity of the tundish submerged entry nozzle towards the two narrow sides of the crystallizer using a slag removal shovel. After slag removal, there is basically no black powdery slag on the surface of the crystallizer, and the whole surface is in a red molten state with no exposed molten steel.

[0030] (4) After the protective slag A is removed, protective slag B is added.

[0031] (5) After adding protective slag B, the pulling speed is 0.2 m / min 2 The rate of change increased from 0.4 m / min to 0.8 m / min and remained at that rate for 3 min.

[0032] (6) The thickness of the liquid slag layer was measured at the midpoint between the narrow side of the crystallizer and the immersion nozzle of the tundish, and the pulling speed was increased from 0.8 m / min to the target pulling speed of 1.2 m / min.

[0033] After replacing the protective slag, the continuous casting process was stable, the heat flow curve of the crystallizer was stable, there was no sticking alarm for steel leakage prediction, and there were no slag inclusion defects after the corresponding billet was rolled.

[0034] Example 2: The method for online replacement of protective slag in the mixed casting of dissimilar steel types is as follows.

[0035] In a certain casting cycle, the tundish capacity is 70 tons. Within the same tundish, steel grades HX340LAD+Z and CR340 / 590DP are mixed, using protective slag C and protective slag D respectively. The previous heat was HX340LAD+Z, and the next heat is CR340 / 590DP. The online replacement of the protective slag follows these steps:

[0036] (1) After the current heat of HX340LAD+Z ladle is poured, the next heat of CR340 / 590DP ladle is rotated to the pouring position to wait, and the pouring speed is gradually reduced from the current pouring speed of 1.3m / min to 0.9m / min. When the tonnage of the tundish is reduced from 67 tons to 32 tons, the next heat of CR340 / 590DP ladle is poured, the hydraulic cylinder of the ladle nozzle is opened to 100%, and the molten steel is poured into the tundish.

[0037] (2) When the tundish tonnage gradually increases to 63 tons, the tundish tonnage control is switched from manual to automatic control, and the tundish tonnage stabilizes between 60 and 65 tons. The measured tundish temperature is 1545℃, and the liquidus temperature of the steel grade CR340 / 590DP is 1516℃. At this time, the superheat of the molten steel is 1545℃-1516℃=29℃.

[0038] (3) The casting speed is gradually reduced from 0.9 m / min to 0.3 m / min, and the slag removal operation of the crystallizer is started at the same time during the reduction of casting speed. The powdery slag and molten slag blocks of protective slag A are removed from the vicinity of the tundish submerged entry point to the two narrow sides of the crystallizer using a slag removal shovel. After slag removal, there is basically no black powdery slag on the slag surface of the crystallizer, and the whole surface is in a red molten state with no exposed molten steel.

[0039] (4) After the protective slag C is removed, protective slag D is added.

[0040] (5) After adding the protective slag D, the pulling speed is 0.2 m / min 2 The rate of change increased from 0.3 m / min to 0.9 m / min and remained there for 4 min.

[0041] (6) The thickness of the liquid slag layer was measured to be 5 mm at the midpoint between the narrow side of the crystallizer and the immersion nozzle of the tundish. Since the thickness of the liquid slag layer was <8 mm, after pouring for 2 minutes at a pouring speed of 0.9 m / min, the thickness of the liquid slag layer was measured again to be 10 mm, and the pouring speed was increased from 0.9 m / min to the target pouring speed of 1.3 m / min.

[0042] After replacing the protective slag, the continuous casting process was stable, the heat flow curve of the crystallizer was stable, there was no sticking alarm for steel leakage prediction, and there were no slag inclusion defects after the corresponding billet was rolled.

[0043] Example 3: The method for online replacement of protective slag in the mixed casting of dissimilar steel types is as follows.

[0044] In a certain casting cycle, the tundish capacity is 60 tons. Within the same tundish, steel grades BGH340 and HC280 / 440DP are mixed, using protective slag B and protective slag A respectively. The previous heat was BGH340, and the next heat is HC280 / 440DP. The online replacement of the protective slag follows these steps:

[0045] (1) After the current heat of BGH340 ladle is poured, the next heat of HC280 / 440DP ladle is rotated to the pouring position to wait, and the pouring speed is gradually reduced from the current pouring speed of 1.2m / min to 0.85m / min. When the tonnage of the tundish is reduced from 55 tons to 30 tons, the next heat of HC280 / 440DP ladle is poured, the hydraulic cylinder of the ladle nozzle is opened to 100%, and the molten steel is poured into the tundish.

[0046] (2) When the tundish tonnage gradually increases to 48 tons, the tundish tonnage control is switched from manual to automatic control, and the tundish tonnage is stabilized between 51 and 55 tons. The tundish temperature is measured according to the liquidus temperature of steel grade HC280 / 440DP. At this time, the superheat of the molten steel is 25℃.

[0047] (3) The casting speed is gradually reduced from 0.85 m / min to 0.35 m / min, and the slag removal operation of the crystallizer is started at the same time during the speed reduction. The powdery slag and molten slag blocks of the protective slag B are removed from the vicinity of the tundish submerged entry nozzle towards the two narrow sides of the crystallizer using a slag removal shovel. After slag removal, there is basically no black powdery slag on the surface of the crystallizer, and the whole surface is in a red molten state with no exposed molten steel.

[0048] (4) After the protective slag B is removed, protective slag A is added.

[0049] (5) After adding protective slag A, the pulling speed is 0.3 m / min 2 The rate of change increased from 0.35 m / min to 0.85 m / min and remained constant for 5 min.

[0050] (6) The thickness of the liquid slag layer was measured to be 6 mm at the midpoint between the narrow side of the crystallizer and the immersion nozzle of the tundish. Since the thickness of the liquid slag layer was <8 mm, after pouring for 1 min at a speed of 0.85 m / min, the thickness of the liquid slag layer was measured again to be 8 mm, and the pouring speed was increased from 0.85 m / min to the target pouring speed of 1.2 m / min.

[0051] After replacing the protective slag, the continuous casting process was stable, the heat flow curve of the crystallizer was stable, there was no sticking alarm for steel leakage prediction, and there were no slag inclusion defects after the corresponding billet was rolled.

[0052] Example 4: The method for online replacement of protective slag in the mixed casting of dissimilar steel types is described in detail below.

[0053] In a certain casting cycle, the tundish capacity is 80 tons. Within the same tundish, steel grades CR340 / 590DP and HX340LAD+Z are mixed, using protective slag D and protective slag C respectively. The previous heat was CR340 / 590DP, and the next heat is HX340LAD+Z. The online replacement of the protective slag follows these steps:

[0054] (1) After the current CR340 / 590DP ladle is poured, the next HX340LAD+Z ladle is rotated to the pouring position to wait, and the pouring speed is gradually reduced from the current pouring speed of 1.3m / min to 0.8m / min. When the tundish tonnage is reduced from 72 tons to 32 tons, the next HX340LAD+Z ladle is poured, the hydraulic cylinder of the ladle nozzle is opened to 100%, and the molten steel is poured into the tundish.

[0055] (2) When the tundish tonnage gradually increases to 68 tons, the tundish tonnage control is switched from manual to automatic control, and the tundish tonnage is stabilized between 70 and 76 tons. The tundish temperature is measured. According to the liquidus temperature of steel grade HX340LAD+Z, the superheat of the molten steel is 20℃ at this time.

[0056] (3) The casting speed is gradually reduced from 0.8 m / min to 0.4 m / min, and the slag removal operation of the crystallizer is started during the speed reduction process. The powdery slag and molten slag blocks of protective slag A are removed from the vicinity of the tundish submerged entry nozzle towards the two narrow sides of the crystallizer using a slag removal shovel. After slag removal, there is basically no black powdery slag on the surface of the crystallizer, and the whole surface is in a red molten state with no exposed molten steel.

[0057] (4) After the protective slag D is removed, protective slag C is added.

[0058] (5) After adding the protective slag C, the pulling speed is 0.25 m / min 2 The rate of change increased from 0.4 m / min to 0.8 m / min and remained there for 4 min.

[0059] (6) The thickness of the liquid slag layer was measured to be 5 mm at the midpoint between the narrow side of the crystallizer and the immersion nozzle of the tundish. Since the thickness of the liquid slag layer was <8 mm, after pouring for 3 minutes at a pouring speed of 0.8 m / min, the thickness of the liquid slag layer was measured again to be 9 mm, and the pouring speed was increased from 0.8 m / min to the target pouring speed of 1.3 m / min.

[0060] After replacing the protective slag, the continuous casting process was stable, the heat flow curve of the crystallizer was stable, there was no sticking alarm for steel leakage prediction, and there were no slag inclusion defects after the corresponding billet was rolled.

Claims

1. A method for online replacement of protective slag when casting dissimilar steel grades, characterized in that, The method and steps are as follows: (1) When different steel grades are mixed in the same tundish, after the previous heat is finished, the next heat is rotated to the pouring position and waited, and the casting speed is reduced to 0.8-0.9 m / min; when the tundish tonnage is reduced to 40%-50% of the tundish capacity, the next heat is poured and the molten steel is injected into the tundish. (2) After the tundish tonnage increases to 80% to 90% of the tundish capacity, control the tundish tonnage range to 85% to 95% of the tundish capacity, measure the tundish temperature, and calculate the superheat of the molten steel based on the measured tundish temperature. (3) The pulling speed is reduced from 0.8-0.9 m / min to 0.3-0.4 m / min, and the slag removal operation of the crystallizer is started during the process of reducing the pulling speed; (4) After the original protective slag has been removed, new protective slag is added; (5) After the new protective slag is added, the pulling speed is increased from 0.3-0.4 m / min to 0.8-0.9 m / min and maintained for 3-5 min, with a pulling speed change rate of 0.2-0.3 m / min. 2 ; (6) Increase the pulling speed to meet the target pulling speed of the process requirements, measure the thickness of the protective slag liquid layer in the crystallizer, and measure the position between the narrow side of the crystallizer and the immersion nozzle of the tundish; if the liquid slag layer thickness is ≥8mm, increase the pulling speed to meet the target pulling speed of the process requirements; if the liquid slag layer thickness is <8mm, do not increase the pulling speed, maintain the current pulling speed for 1 to 3 minutes and then increase it to meet the target pulling speed of the process requirements.

2. The method for online replacement of protective slag in mixed casting of dissimilar steel grades according to claim 1, characterized in that: The capacity of the intermediate tundish is 60 to 80 tons.