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Improvement of smelting operation method under low iron consumption in 120t converter of Jiugang

1. Introduction

Steelmaking equipment of Jiugang Carbon Steel Sheet Plant: There are 3 top-bottom double blowing converters with a nominal capacity of 120 t, 3 120 t three-phase AC LF ladle refining furnaces, 1 120 tRH furnace, 2 conventional continuous casting machines, 2 thin slab continuous casting machines [1]. In the face of the serious shortage of molten iron in the company, the output of steel and materials is restricted. At present, due to the limitation of iron water resources, the steelmaking process of the carbon steel sheet factory has appeared in different degrees of the iron-equivalent phenomenon, the steel output is insufficient, and the energy consumption is high, which seriously affects The production rhythm reduces economic benefits and increases operating costs to a certain extent. How to increase steel output without increasing the amount of molten iron has become the primary consideration of the steelmaking system. For this reason, various factors such as converter charging system, smelting heat balance, ladle capacity, temperature system, product quality, and comprehensive cost are comprehensively considered. Under the premise, plan and implement the implementation plan of reducing iron and increasing steel, to achieve the purpose of reducing the consumption of molten iron and increasing the output of steel. The main method is to increase the consumption of scrap iron and steel, reduce the consumption of molten iron, reduce the tapping temperature of the converter, and increase the compensation time of the refining temperature to ensure that the exit temperature and the temperature of the upper continuous caster remain unchanged, and the entry temperature is not lower than the steel grade liquid. Phase line temperature shall prevail. Combined with this scheme, it is necessary to make necessary adjustments to the processing system.

 

2. Theoretical analysis of the improvement of converter process operation According to the analysis of smelting products:

the pouring temperature of continuous casting remains unchanged, and the temperature of the refining exit station remains unchanged, and the temperature of the refining entry station can be adjusted appropriately. The liquidus temperature of the brand is between 1 507 and 1 536 ℃. The entry temperature is not lower than 1 530 ℃ as the premise, and the entry temperature is 30 ℃ lower than the actual.

2.1 Analysis of converter loading system

The cooling effect analysis of converter materials is shown in Table

  1. The charging system is implemented according to the four schemes in Table
  2. The maximum temperature drop in the charging system in Table 2 is 87.5 °C, which is taken as 90 °C.

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When the iron consumption is 860 kg/t and the tapping temperature drops to 1 620 ℃, the calculated scrap consumption increases to 207 kg/t, and the temperature drop reaches ( 207-160) × 120 /1 000 × 15 = 84.6 ℃.

 

2.2 Analysis of slag-making system

Dephosphorization must be considered the main task of the converter. Due to the low iron consumption of converter smelting, the process temperature is low, and Lp (the distribution ratio of phosphorus in slag steel) is increased. Under these conditions, it is necessary for The slag basicity and the total slag amount to be adjusted down; the decrease in Ls (the distribution ratio of phosphorus in the slag steel) may cause the endpoint of the sulfur in molten steel to rise, and the molten steel with high end-point sulfur can be reassigned to the molten steel desulfurization task by removing sulfur from tapping slag and refining desulfurization; considering the large temperature change, it is necessary to reduce the use of raw dolomite, limestone and iron oxide balls in the smelting process. reduce the temperature drop caused by auxiliary materials; for the maintenance of converter conditions, when the amount of slag decreases and the basicity decreases, increase the amount of light-burning dolomite, appropriately increase the MgO content in the slag, and improve the protection of the furnace lining; specific to the operation At the same time, the slag retention operation mode can also be used to make slag and to perform temperature compensation at the same time.

 

2.3 Oxygen supply system

Due to the increase in the amount of scrap iron and steel consumed by the converter, the gun position should be appropriately increased in the early stage of converter smelting to avoid the phenomenon of oxygen lance leakage, and the change in the gun position in the middle and later stage is small; Increasing the content of carbon and silicon in the metal material decreases, the consumption of oxygen decreases slightly, and the flow rate of oxygen and argon remains unchanged.

 

2.4 Due to the adjustment of the charging system, the tapping temperature will inevitably lead to insufficient heat in the converter smelting, so it is necessary to analyze and adjust the temperature system. According to the analysis of smelting products: the pouring temperature of continuous casting remains unchanged, and the temperature of refining exit remains unchanged, but the temperature of refining entry can be appropriately lowered. The temperature limit for lowering is based on the liquidus temperature. between 1 507 and 1 536 ℃. The temperature drop in the tapping process is 75-85 ℃, the benchmark of tapping temperature: the upper limit temperature of the steel type tapping temperature is 1 536 ℃ +85 ℃ = 1 621 ℃, the lower limit is 1 536 ℃ + 75 ℃ = 1 61 ℃, the operation control target is as follows The tapping temperature of 1 610~1 630 ℃ is controlled. The converter tapping temperature drops by about 30 ℃ from 1 640-1 660 ℃, the converter tapping temperature is controlled at about 1 610-1 630 ℃, and the average value of the tapping temperature is expected to be about 1 620 ℃. The pit temperature is around 1 530 °C, which is about 30 °C lower than the original pit temperature.

2.5 Analysis of molten steel composition

(1) Due to the low-temperature smelting state, the dephosphorization effect during the smelting process is better than that of the high iron consumption operation, and the phosphorus return in the later stage of smelting is alleviated, especially the distribution ratio Lp of P increases significantly: lgLp = 22 350 /T – 21.876 + 5.6lg (CaO%) + 2.5lg (∑FeO%) (1) Under the condition that other conditions remain unchanged, the temperature decreases by 30 °C, and the Lp changes from 124 to 190, and the slag dephosphorization ability increases greatly. Therefore, the slag basicity and slag amount can be appropriately reduced.

(2) The lack of heat or the phenomenon of molten steel overblowing, resulting in a decrease in the carbon content at the endpoint.

(3) Due to the increase in the amount of pig iron blocks, the sulfur load of the metal material in the furnace increases. According to the distribution ratio of sulfur: Ls = 1.4+16 (nCaO+nMgO+nMnO-2nSiO2-4nP2O∑FeO-2nAi2O3) ⑵Slag The alkalinity R and the amount of slag decreased, CaO decreased, and Ls decreased at this time. Final S content in steel [S% steel] = ∑S charge/(W gold+Ls×W slag), where: ∑S charge – S amount brought in by all incoming charges; W gold – total molten steel (according to 118~ 120 t); W slag-total amount of final slag (12.5 t); Ls-S distribution coefficient in slag steel. The molten steel with high end-point sulfur can be redistributed to the molten steel desulfurization task by removing sulfur from the tapping slag and refining and desulfurizing.

 

2.6 Furnace Condition Maintenance

Under normal circumstances, the converter furnace condition is greatly affected by the erosion of molten steel overblowing and high-temperature tapping, and the probability of furnace leakage in serious cases increases. For smelting under the condition of low iron consumption, the favorable factor is that the tapping temperature is low, and the corrosion of the furnace lining is less; the disadvantage is that the low iron consumption is time-consuming. The odds go up. To avoid the above unfavorable situation, the over-blowing caused by insufficient heat should be avoided in the operation, and the MgO content in the slag should be appropriately increased. 3 Improvements in the operation method of the converter and the application effect The carbon in the molten iron increases by 0.2%, the heat increases, and the temperature increases by 30 ℃; the Si in the molten iron increases by 0.05%, the heat increases, and the temperature increases by 15 ℃ (Table 3).

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The molten iron decreased by 3.6 t, the scrap steel increased by 6.45 t (including the scrap added to the ladle), and the tapping temperature decreased by 21. 52 °C, the pit temperature decreased by 21 °C, and the tapping volume decreased by 0.1 t. The experimental data corresponded well with the theoretical analysis results (Table 4). However, the cost of iron and steel materials and the proportion of scrap iron and steel are greatly affected by the purchase price of raw materials.

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3.1 implementation of the oxygen supply system   

 

Since the automatic oxygen opening point is 2.66 m lower in the smelting process, the manual oxygen opening operation is adopted. After oxygen opening at 2.8 m, the lance is gradually lowered to avoid corrosion and leakage of the lance caused by large-scale scrap steel; The increase of carbon content and silicon content in molten iron by 0.2% and 0.05% increased oxygen consumption, but the actual oxygen consumption decreased by 110 m3, which corresponds to the situation of theoretical analysis. The oxygen flow and pressure remain normal.

 

3.2 slagging system and final slag composition

 

To avoid the aggravation of erosion to the furnace lining, under the condition of lack of heat and reduction of slag, appropriately increase the MgO content in the slag and improve the saturation of the slag, which is conducive to the protection of the converter. At the same time, due to the good dephosphorization effect in the low-temperature smelting process, the basicity of slag is reduced by 0.45, the change of slag making material reduces the heat by 13.4 ℃, and the change of Σ FeO is only 0.11% (Table 5 and Table 6).

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3.3 Changes in the composition of molten steel at the endpoint

The phosphorus in incoming molten steel has decreased, which is related to the low-temperature smelting state. The increase of sulfur content is related to the change of slag basicity and slag amount, which corresponds to the phosphorus distribution ratio Lp and sulfur distribution ratio Ls in the theoretical analysis; Due to the high sulfur content of molten iron and slag retention, the carbon and phosphorus contents were controlled within the standard range (Table 7).

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The phosphorus and manganese in the incoming molten steel decreased, corresponding to the end point, and the increase in sulfur content was related to the high sulfur content of the molten iron and the slag retention (Table 8).

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3.4 Condition of converter lining

During the smelting period of low iron consumption, the converter conditions remained stable, and the MgO content in the slag increased by 1.59%. Under the unfavorable conditions of reduced slag content, the favorable conditions of temperature reduction can be utilized to ensure the safe operation of converter conditions. Among them, the measurement records of the converter lining show that the temperature of the furnace shell is controlled below 300 ℃, and the average is about 280 ℃, which maintains the safe operation of the furnace lining.

4. Conclusion

(1) The converter takes measures to reduce the tapping temperature, which can effectively reduce the iron consumption and increase the intake of scrap steel. Under the condition of ensuring the heat demand in the smelting process, the iron consumption can be reduced to 830-960 kg/t.

(2) The composition and temperature of the slag in the smelting process can meet the production needs, and the safe operation of the furnace lining can be effectively guaranteed.

(3) Due to insufficient heat, the large size of scrap steel in the smelting process may cause the scrap steel to be difficult to melt and stick to the inner wall of the furnace.

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