The purpose of controlling end-point carbon in electric furnaces is to reduce oxygen consumption, prevent oxidation of the molten pool, reduce the deoxidation burden of secondary refining after tapping in electric furnaces, shorten refining, do not add a large amount of carburizing agent, and reduce costs.
Keywords: electric furnace; terminal carbon; refining; cycle; cost
Overview
The electric furnace branch of a company was completed and put into operation in 2010. As of June 2023, it has been in operation for 14 years and has produced 1.2 million tons of steel. The main equipment is equipped with a molten iron pouring station, a molten iron slagging station, a scrap steel batching station, and an ultra-high power One electric arc furnace (nominal capacity 110t), two 120tLF furnaces (single station + double station), a double-station RH vacuum treatment device, two casting machines, including a 5-strand round billet continuous casting machine (Æ380 /Æ450/Æ500/Æ600/Æ700), one 5-strand billet continuous casting machine (250*250mm/380*450mm), two RHs and one VD furnace. At present, the aluminum consumption in the refining and smelting process is unstable and the purity of molten steel is poor. To this end, research on the carbon retention process of electric furnaces has been carried out since 2022, and the LF cycle is shortened to control the stability of the electric furnace and refining processes. Through the optimization of relevant processes and operations, it is conducive to improving the purity of molten steel, reducing aluminum losses, and improving product quality.
Scheme design
(1) According to the specific composition requirements of the steel type, appropriately increase the composition range of the designed LF arrival station and increase the amount of alloy added in the electric furnace tapping. Before adjustment: “The design target of elements such as C, Si, Mn and other elements at the station of LF is generally LF outbound internal control target -0.15%”, and the amount of LF alloy added is larger; after adjustment: according to “The target of elements such as Si, Mn and other elements arriving at the station is -0.15%” The LF outbound target – (0.05 to 0.10%), and the C component inbound target is LF outbound target – 0.10%” are used to improve the overall design of the LF inbound component range.
(2) Improve the electric furnace tapping and LF furnace slag making systems, and move part of the LF slag forward to the electric furnace for tapping. Before adjustment: the amount of lime added to the electric furnace tapping is 400kg, the refining slag is 300kg, and the total amount of LF lime and refining slag is about 1t; after adjustment: the amount of lime added to the electric furnace tapping is 600kg, the refining slag is 600kg, and the total amount of LF lime and refining slag is added About 500kg, ensure that the total amount of electric furnace tapping + LF slag remains unchanged before and after adjustment.
(3) Reduce the number of sampling times in the LF process. The previous “design of 4 samplings and 3 alloy adjustments” was adjusted to “3 samplings and 2 alloy adjustments”.
(4) Increase the end point C of the electric furnace. The previous “0.06~0.30%” (usually between 0.08-0.15%) is adjusted to “0.06~0.30%, internal control requirements: 0.20~0.30%”, and the aluminum and iron addition process is accordingly refined based on the endpoint C content. Optimize the order of adding alloy slag during the electric furnace tapping process, and adjust the alloy to be added before the slag to reduce the occurrence of alloy lumps.
Overall completion status
Figure 1 Completion status of LF refining cycle
According to statistics, after excluding the effects of online washing tanks, high alloy steel, accident heats, etc., the LF refining cycle dropped from 104.8 minutes before the research to 85 minutes, a decrease of 18.9%, completing the research target.
Process key indicator control
Electric furnace end point C control
It can be seen from Figure 2 that the C content at the end point of the electric furnace meets the process requirements of 0.06 to 0.30% in the work instructions. Among them, the end point C between 0.15 and 0.30% accounts for 94.75%. The proportion that meets the internal control between 0.20 and 0.30% is 70.44%. The end point C content of the electric furnace The control level has been greatly improved compared with before the adjustment. The higher end point C content is conducive to reducing the oxygen content of the electric furnace steel, creating good initial conditions for improving the purity of the refined steel. 2.2 LF alloy adjustment amount. Among the steel types that have optimized the alloy addition system, steel types with a larger number of production furnaces were selected for comparative analysis, namely LZ50, G2 and ER7. The changes in the alloy addition amount per ton of steel and the carburizer addition amount are as follows: As shown in Table 1-2.
| Table 1 Changes before and after adding amount of steel alloy per ton | ||||||||
| Steel type | Before adjustment (kg/t) | After adjustment (kg/t) | Before adjustment (kg/t) | After adjustment (kg/t) | ||||
| EAF alloy content | average | EAF alloy content | average | LF alloy content | average | LFalloy content | average | |
| LZ50 | 7.24-12.43 | 10.54 | 10.54-12.44 | 11.55 | 2.69-6.34 | 4.19 | 1.58-4.85 | 3.33 |
| G2 | 8.39-8.87 | 8.66 | 9.83-10.53 | 10.18 | 3.23-6.11 | 4.64 | 1.97-4.38 | 2.88 |
| ER7 | 10.96-11.56 | 11.27 | 11.43-12.75 | 12.29 | 3.36-5.45 | 4.64 | 2.03-4.30 | 3.24 |
| Table 2 Changes in the amount of carburizer added before and after | ||||||||
| Steel type | Before adjustment(kg) | After adjustment (kg) | Before adjustment(kg) | After adjustment (kg) | ||||
| EAF toner quantity | average | EAF toner quantity | average | LF toner quantity | average | LFtoner quantity | average | |
| LZ50 | 327-454 | 395 | 338-458 | 412 | 23-232 | 115 | 49-192 | 101 |
| G2 | 398-580 | 500 | 475-579 | 542 | 68-290 | 175 | 47-147 | 108 |
| ER7 | 226-393 | 327 | 273-406 | 343 | 38-209 | 133 | 61-155 | 145 |
As can be seen from Table 1-2, taking LZ50, G2 and ER7 steel types as examples, by optimizing the alloy addition system and increasing the LF arrival component requirement range, the tapping alloy amount increased, and the alloy addition amount per ton of LF steel was reduced by 0.86kg/t respectively. , 1.76kg/t, 1.4kg/t, the reduction ratios are 21%, 38%, and 30%. The addition amount of LF carburizer is reduced by 12%, 38%, and 14% respectively, and the effect is more obvious. 2.3 Desulfurization situation of 1/2 of the LF sample (1) During the test, there were no abnormal desulfurization or modification accidents of LF due to adjustments to the slagging process. (2) Taking LZ50, G2 and ER7 steel types as examples, the LF furnace sample 1S and desulfurization rate are reduced to varying degrees (Table 3).
| Table 3 Changes before and after desulfurization | ||||||||||||||||
| Steel type | Before adjustment | After adjustment | ||||||||||||||
| 1 kind S | average | 2 kinds of S | average | 1 sample desulfurization rate | average | 2sample desulfurization rate | average | 1 kind S | average | 2 kinds of S | average | 1 sample desulfurization rate | average | 2sample desulfurization rate | average | |
| LZ50 | 0.00 2- 0.01 8 | 0. 01 1 | 0.0 01- 0.0 14 | 0.003 | 20. 1- 87. 6 | 42. 6 | 29. 4- 92. 3 | 76. 4 | 0.00 2- 0.01 4 | 0. 00 9 | 0.001 – 0.008 | 0. 00 3 | 24. 3- 91. 6 | 48.1 | 36. 3- 92. 0 | 80.3 |
| G2 | 0.00 4- 0.0 8 | 0. 01 0 | 0.0 02- 0.0 15 | 0. 00 4 | 28. 1- 66. 7 | 55.2 | 31. 2- 91. 7 | 75 .1 | 0.00 2- 0.01 4 | 0. 00 7 | 0.001 – 0.008 | 0. 00 3 | 27. 0- 89. 4 | 6 7 . 6 | 52. 3- 91. 0 | |
| ER7 | 0.00 1- 0.01 7 | 0. 00 9 | 0.0 01- 0.0 12 | 0. 00 4 | 18. 3- 95. 9 | 54. 6 | 46. 8- 97. 9 | 81. 5 | 0.00 2- 0.01 5 | 0. 00 7 | 0.001 – 0.010 | 0. 005 | 23. 6- 94 .5 | g 61.6 | 47. 3- 97.1 | 8 2.8 |
As shown in Table 3, the average S of LZ50, G2 and ER7 steel LF sample 1 decreased by 0.002%, 0.003%, and 0.002% respectively, and the reduction ratios were 18%, 30%, and 22% respectively. The desulfurization rate of LF sample 1 increased. , mainly related to the fact that part of the LF slag material is moved forward and added during the tapping process, and the speed of white slag forming is accelerated. The 2S content and desulfurization rate of the LF furnace sample are basically close to those before adjustment. 2.4 Changes in the composition of LF slag samples In order to verify the impact of adjusting the slag addition system on the changes in the composition of LF slag, steel types of different composition categories were selected and the changes in the composition of LF slag samples before and after the adjustment were compared. The results are shown in Table 4-5.
| Table 4 Composition of LF slag sample of conventional high-aluminum wheel steel | |||||||||||||
| Steel type | Total amount of EAF+LF slag | Slag component | |||||||||||
| Lime/kg | Refining slag/kg | Aluminum iron (kg/t) | CAO | SIO2 | MNO | AL₂O₃ | TF E | MGO | AlkalinityR | CAO/AL203 | |||
| ER7、 ER8、 ER9-1 | After adjustment | minimum value | 851 | 584 | 2.0 | 51. 55 | 7.1 4 | 0.0 3 | 25.7 1 | 0. 08 | 2.8 7 | 4.7 | 1.7 |
| maximum value | 1067 | 822 | 2.7 | 58. 75 | 11. 76 | 0.1 6 | 33.0 8 | 0. 70 | 4.9 7 | 7.1 | 2.3 | ||
| average | 917 | 692 | 2.1 | 54. 03 | 10. 07 | 0.0 6 | 29.7 1 | 0. 36 | 4.3 9 | 5.8 | 1.9 | ||
| Before adjustments | minimum value | 827 | 663 | 1.9 | 47. 22 | 6.2 7 | 0.0 2 | 24.4 9 | 0. 06 | 3.2 2 | 3.7 | 1.5 | |
| maximum value | 1111 | 713 | 2.8 | 60. 03 | 14. 29 | 0.1 7 | 32.6 7 | 0. 73 | 8.9 6 | 8.3 | 2.2 | ||
| average | 906 | 689 | 2.2 | 55. 02 | 10. 47 | 0.0 7 | 29.5 9 | 0. 36 | 5.2 2 | 5.5 | 1.9 | ||
| Table 5 Conventional process low aluminum wheel steel LF slag sample composition | |||||||||||||
| Steel type | Total amount of EAF+LF slag | Slag component | |||||||||||
| Lime/kg | Refining slag/kg | Aluminum iron (kg/t) | CAO | SI0₂ | MN0 | AL203 | TFE | MGO | AlkalinityR | CAO/AL203 | |||
| CL60〔 K)、 G2 | After adjustment | minimum value | 789 | 876 | 2.1 | 49.65 | 6.93 | 0.05 | 26.12 | 0.2 | 4.02 | 3.7 | 1.5 |
| maximum value | 1046 | 901 | 2.2 | 54.42 | 13.3 | 0.11 | 34.62 | 0.47 | 5.62 | 7.7 | 2.1 | ||
| average | 929 | 888 | 2.14 | 53.45 | 11.62 | 0.08 | 28.59 | 0.35 | 4.6 | 4.8 | 1.9 | ||
| Before adjustments | minimum value | 773 | 853 | 2.1 | 44.5 | 6.43 | 0.02 | 22.6 | 0.1 | 2.71 | 3 | 1.5 | |
| maximum value | 1134 | 998 | 2.7 | 59.71 | 15.4 | 0.35 | 33.3 | 0.76 | 8.53 | 8.5 | 2.3 | ||
| average | 953 | 887 | 2.2 | 54.51 | 10.38 | 0.07 | 28.66 | 0.37 | 5.1 | 8.4 | 1.9 | ||
As can be seen from the above table, after optimizing the slag addition system, the key component indicators such as LF slag Al2O3, basicity R, CAO/Al2O3 are relatively stable under different slagging systems of high-aluminum, low-aluminum wheel steel and non-wheel steel using the current conventional process. Basically the same as before adjustment. 2.5 Process Al consumption Select different steel types to compare the total aluminum consumption of EAF+LF before and after process adjustment. The results are shown in Table 6.
| Table 6 Comparison of total aluminum consumption of EAF+LF | ||||||||
| Steel type | Experiment | Before adjustments | ||||||
| Treatment cycle/min | minimum value | maximum value | average | Treatment cycle/min | minimum value | maximum value | average | |
| CL60(K)、G2 | 89.9 | 1.37 | 1.55 | 1.44 | 114.58 | 1.37 | 1.85 | 1.53 |
| ER8/ ER9 | 92.5 | 1.60 | 2.05 | 1.81 | 116.13 | 1.51 | 2.25 | 1.91 |
| LZ50 | 75.1 | 1.58 | 2.40 | 1.82 | 90.60 | 1.31 | 2.41 | 1.31 |
As can be seen from Table 6, the total aluminum consumption of the above conventional steel types EAF+LF has decreased compared with before, among which the consumption of CL60(K), G2 low-aluminum wheel steel, ER8/ER9 medium-high aluminum wheel steel, and LZ50 axle series The total amount of aluminum was reduced by 0.09, 0.10, and 0.05 kg/t steel respectively. By optimizing slagging and controlling LF bottom blowing argon gas, the process aluminum consumption was further reduced.
Conclusion
1. Since the launch of the research, the process control of the electric furnace and refining process has been stable, the LF refining cycle has been shortened significantly, and the research goals have been completed.
2. The control of key process indicators such as electric furnace end point C, LF alloy adjustment amount, desulfurization rate, and slag sample composition is relatively stable. The amount of aluminum consumed during the smelting process has been reduced, indicating that optimization of relevant processes and operations can help improve the purity of molten steel and reduce aluminum losses.
3. Since the launch of the research, there have been no batch quality accidents or metallurgical quality abnormalities caused by relevant process adjustments. It is recommended to gradually solidify the relevant processes.