Influence of high scrap ratio on converter steelmaking process and Its Countermeasures

　　Abstract: in view of the problems that affect the converter steelmaking process after increasing the scrap ratio and reducing the unit consumption of molten iron, the scrap size and structure, the converter slagging process, the use of converter temperature raising agent and the converter smelting control model are optimized to achieve the stability of converter production index under the condition of high scrap ratio and improve the production efficiency.

　　Key words: converter; Scrap ratio; Slagging; Warming agent

　　The vast majority of iron and steel production in China adopts the "blast furnace converter" long process production, and the main raw materials of the converter are molten iron and scrap steel. With the large consumption of high-grade iron ore resources and high-quality coking coal resources at home and abroad and the increasing pressure of energy conservation and emission reduction, the production cost of blast furnace molten iron has gradually increased [1]. The quantity and production of scrap steel in China are increasing year by year, and the price is gradually decreasing. The converter with high scrap ratio smelting can not only reduce the dependence on ore, but also reduce carbon emissions and converter slag, and improve the steelmaking capacity.

　　The general steelmaking plant of Angang Steel Co., Ltd. (hereinafter referred to as "the general steelmaking plant") began to adjust the structure of steel and iron materials in October 2020, increase the scrap ratio and reduce the unit consumption of molten iron. The operation mode changed from low scrap ratio to high scrap ratio. When the scrap ratio was increased from 10.27% to 14.65%, all indicators of converter smelting fell. Therefore, it is necessary to analyze the influence of high scrap ratio on converter steelmaking process, so as to formulate corresponding optimization measures to ensure the stability of converter production index and improve production efficiency under the condition of high scrap ratio.
 

　　1. Influence of high scrap ratio on converter steelmaking process

　　1.1 impact on converter operation

　　The operation with high scrap ratio will have an adverse effect on the blowing, ignition and slagging of the converter, thus affecting the control of the end-point carbon temperature of the converter.

　　(1) The melting of scrap steel in the converter molten pool should go through three processes: molten iron condensation on the surface of scrap steel, melting of condensation layer and carburization melting of scrap steel. When the scrap ratio is high, after the molten iron and scrap are loaded, the small plate scrap with large specific surface area can melt rapidly in the early smelting stage, and the bath temperature will drop sharply, making it difficult to start blowing and ignition in the converter. In particular, in order to control explosion discharge, the phenomenon of poor ignition is particularly prominent when low oxygen pressure ignition is used for converter dry dedusting.

　　(2) Due to the low bath temperature, the viscosity of the metal in the furnace increases in the early smelting stage, which reduces the heat and mass transfer speed of the bath, the heat transfer speed of the oxygen blowing reaction zone to other parts of the bath decreases, and the flux temperature rise speed is slow, especially the melting speed of the large, non plate and low-carbon scrap itself is slow. When its ratio is high, the temperature rise speed is further delayed, resulting in the difficulty of slag formation in the converter, and the dephosphorization rate in the early stage decreases [2]. In addition, when temperature raising agent is used for heating up, ferrosilicon is used in some heats, resulting in the increase of slag amount and the deterioration of slag condition, which affects the dephosphorization effect. The melting rate of scrap with different structures varies greatly, the temperature fluctuates greatly in the smelting process, and the dephosphorization rate is unstable, which makes it difficult to control the smelting end point, resulting in a decrease in the hit rate of end point carbon temperature and an increase in high temperature and peroxide furnace times.

　　1.2 influence on maintenance of converter lining

　　When the scrap ratio is high, the amount of scrap is increased, and the impact force on the lining surface is increased when scrap is added, especially limited by the capacity of the scrap tank. In order to improve the scrap ratio, the ratio of heavy scrap is high, which not only prolongs the impact time of scrap on the refractory materials in the large area of the converter, but also improves the mechanical strength. At the early stage of smelting, due to low bath temperature, slag formation is difficult, alkalinity is low, and furnace lining erosion is serious [3]. Due to the reduction of hot metal ratio, the total slag amount of converter smelting decreases, and the unstable operation results in the increase of abnormal furnace times. The FeO content in the slag is high, the effect of slag splashing on furnace protection becomes worse, and the slag splashing layer becomes thinner and unstable, which increases the difficulty of furnace lining maintenance.

　　1.3 effect on composition of molten steel

　　The high scrap ratio operation with temperature raising agent increases the sulfur recovery of converter, and the nitrogen content of molten steel at the end of smelting tends to increase. After the scrap ratio increases, the source of scrap changes, the ratio of purchased scrap increases, the sulfur content is unstable, and the sulfur recovery from converter smelting fluctuates greatly, which is unfavorable to the control of sulfur composition in RH production of low sulfur steel. In order to increase the heat source of converter, coke is mainly used as temperature raising agent. The sulfur content of coke is high, which further increases the sulfur recovery of converter. The melting rate of large, non plate and low-carbon scrap is slow, which affects the denitrification effect of the converter. At the same time, in order to reduce the use of temperature raising agent, the end carbon content of the converter tends to be controlled at the lower limit, resulting in an increase in the nitrogen content of molten steel at the end of smelting.
 

　　2. Measures taken

　　2.1 optimize scrap size and structure

　　In order to reduce the impact of scrap adding process on furnace lining, the size of scrap entering the furnace shall be reduced as much as possible. The general steelmaking plant strictly controls the size of self-produced billet head, billet tail, tundish residual steel, scrap billet and other scrap steel, and formulates different cutting standards according to different types of converters, increases cutting times and reduces the weight of single scrap steel. Control the types of purchased scrap steel, mainly crushed materials and pig iron with small size. At the same time, strictly control the scrap structure, control the maximum ratio of non plate and low-carbon scrap, shorten the melting time of scrap in the converter, and reduce the impact on converter denitrification. The proportion structure of scrap steel is about 40% of crushed material plus pig iron, no more than 20% of self-produced heavy billet head scrap, no more than 20% of non plate and low-carbon scrap, and about 20% of self-produced scrap steel in rolling mill, and the proportion shall be kept stable. After the optimization of scrap size and structure, it can not only ensure the full melting of scrap, but also control the high sulfur scrap ratio, so as to effectively control the sulfur recovery of converter smelting.

　　2.2 optimization of converter slagging process

　　Generally, the converter slag forming path can be divided into "iron slag forming path" and "calcium slag forming path" according to the FeO content in the slag. As shown in Figure 1, the purpose of the two slag forming paths is to form slag with appropriate alkalinity and certain fluidity, and ensure that the final slag meets the requirements of slag splashing and furnace protection.

　　It can be seen from Figure 1 that in the iron slag forming path, the FeO content in the slag system at O1, O2 and O3 points is about 40%, 35% and 25% respectively. The core of this path is to quickly increase the FeO content in the slag by optimizing the slag making and oxygen supply system of the converter, and to control the slag system components in the smelting process as O1 → O2 → O3, that is, the initial slag system in the early smelting stage quickly meets the component requirements at O1 point in Figure 1. During the smelting process, the slag components are controlled to decarbonize at O2 point, At the later stage of smelting, the composition of final slag shall be controlled near O3 point. In order to maximize the iron slag forming effect and improve the dephosphorization capacity of process slag, the actual slag composition in the smelting process should be controlled within the shadow area surrounded by slag forming line SL and path 2.

　　Under the process conditions of low scrap ratio, most steelmaking plants adopt the calcareous slag forming path. This path has high slag alkalinity and low FeO content in the smelting process, which is conducive to protecting the furnace lining, stabilizing the blowing process, easy to hit the end point and conducive to automatic control. However, the slag is easy to dry back, which is unfavorable to the smelting of low phosphorus steel. Smelting with high scrap ratio is characterized by low bath temperature in the early stage and difficult slagging. Therefore, adopting iron slagging path is more conducive to scrap melting and rapid slagging. However, this path is not conducive to the stability of the converting process, is not conducive to automatic control, and has higher requirements for process operation. With the increase of scrap ratio in the general steelmaking plant, the slag forming mode of converter is changed from calcium slag forming path to iron slag forming path.

　　During the actual production and operation, slagging agent shall be added in batches. The first batch of material is mainly magnesium flux, and the addition time shall be delayed for 1 ~ 2 min, which is conducive to the temperature rise of the molten pool and the rapid melting of scrap steel. Increasing MgO content in slag in early stage can effectively protect slag splashing layer, and increasing FeO content can promote slagging. Then, according to the slagging situation, slagging materials are added in 3 ~ 5 batches in the smelting process to gradually increase the CaO content in the slag.

　　In order to avoid slag drying back during operation, the total amount of each batch of materials shall be controlled according to the slagging conditions. The feeding process is mainly based on the high gun position operation. If necessary, iron slagging agent can be added to increase the FeO content in the slag. The slagging agent shall be completely added at two-thirds of the oxygen blowing time. In the later stage of smelting, the lance shall be lowered in time, the carbon drawing time at the smelting end point shall be greater than 3 min, the FeO content in the slag shall be reduced to make the final slag composition fall near the O3 point, and the MgO content in the slag shall be within the range of 7% ～ 9%, so as to reduce the loss of iron and steel materials and make the final slag meet the requirements of slag splashing and furnace protection.

　　2.3 optimize the use of temperature improver

　　After the converter increases the scrap ratio, the heat is insufficient, so it is necessary to add temperature raising agent to ensure the heat balance. In particular, the temperature of molten iron entering the general steelmaking plant is low, the transportation time of molten steel is long, the temperature drop is large, and the use of temperature raising agent is large. Therefore, it is necessary to standardize the use of temperature raising agent, so as to reduce the impact on smelting operation. At present, the solid fuels that can be added to the converter include calcium carbide, silicon carbide, ferrosilicon, coke, etc. The cost of calcium carbide is high and the melting speed in the converter is slow. There is unreacted calcium carbide in the final slag of the converter, which makes the heating efficiency unstable and limits its use. The cost of silicon carbide as temperature improver is high and SiO2 is generated by reaction, which increases the consumption of slag. The addition amount is not easy to be too large, and the use is limited. As a temperature increasing agent, ferrosilicon is easy to control its block size and stability, the charging operation is more simplified, the use flexibility is high, and the dosage is controllable, but its cost is high and SiO2 is generated by reaction, and the dosage is not easy to be too large. As a temperature raising agent, coke has low cost, fast melting speed and high heating efficiency, so it is widely used in various types of converters. However, when the amount of coke is large, the influence on the increase of converter sulfur recovery should be considered.

　　After industrial tests, the general steelmaking plant finally determined the converter temperature raising mode with coke as the main component and ferrosilicon as the auxiliary component. In the actual production process, the amount of coke is limited for RH refined low sulfur steel, the amount of ferrosilicon is increased, and the amount of sulfur recovery is controlled. In view of the low silicon content in molten iron, especially in smelting low phosphorus steel, the addition of ferrosilicon can be appropriately increased to strengthen the slag melting effect of converter and improve the dephosphorization rate. In order to maximize the heating efficiency of the temperature improver, the temperature improver should be added immediately after the converter is successfully blown and ignited. After the temperature improver is heated, it can accelerate the melting of scrap steel and rapidly heat up the molten pool, providing good thermodynamic conditions for the melting of the first batch of slag.

　　2.4 optimization of converter operation process

　　After the scrap ratio is increased, in order to improve the hit rate of the final carbon temperature of converter smelting, the converter operation process is optimized, and the following measures are adopted: first, reduce the tapping temperature. The tapping temperature was reduced by 14 ℃ by optimizing the laying process of ladle and tundish insulation layer, the application of large tapping hole in converter, the development and application of ladle positioning management system, and the optimization of ladle insulation process. The second is to optimize the converter smelting control model according to the iron slagging path. The key points include refining the cooling efficiency of each flux at different adding time and improving the accuracy of temperature prediction; Improve the control model of smelting lance position and oxygen flow rate to ensure the slagging effect and control accuracy of slag system components in the smelting process; According to the requirements of carbon temperature at the end point of different steel grades, the control model of smelting end point is formulated accordingly, with the emphasis on the position and time of carbon drawing gun; Refine the mixing process after bottom blowing according to the requirements of steel grade to further reduce the FeO content in slag.
 

　　3. Results achieved

　　Through the optimization and improvement of the above smelting process, the scrap ratio finally reached 17%, and the converter smelting indicators were effectively improved. See Table 1 for the comparison of process indicators of converter smelting a steel before and after optimization.

　　It can be seen from table 1 that after the process optimization, the unit consumption of scrap steel is increased by 43 kg/t, the scrap ratio is increased to 17%, and the utilization factor is increased by 3 t/ (nominal t · d). The four indexes of the final carbon temperature hit rate, the final phosphorus content, the average FeO content of the final slag and the ratio of abnormal composition heats were significantly improved. The end-point nitrogen content and sulfur content were reduced by 0.0004% and 0.0010% respectively. Sulfur recovery and nitrogen increase were effectively controlled. Although compared with the operation of low scrap steel, they were still controlled at a low level, meeting the standard requirements of low nitrogen steel and low sulfur steel.
 

　　4. Conclusion

　　In view of the adverse effects of reducing hot metal consumption and increasing scrap ratio on converter smelting in the steelmaking plant of Angang Co., Ltd., the converter steelmaking process is optimized, and the following conclusions are drawn:

　　(1) The mechanical impact of scrap on furnace lining can be reduced by controlling the size of scrap and reducing the unit weight of scrap. Controlling the structure of the incoming scrap and maintaining the stability of the ratio of various types of scrap, especially the ratio of high sulfur scrap, is not only conducive to the rapid melting of scrap, reduce nitrogen increase, but also stably control the sulfur recovery of the converter;

　　(2) Adopting the iron slag forming path can quickly increase the MgO and FeO contents in the slag, reduce the lining erosion, improve the slagging effect, and gradually increase the CaO content in the slag, so as to meet the dephosphorization requirements. The control model of converter smelting is optimized according to the iron slag forming path. The carbon temperature hit rate is increased by 1.31%, and the average FeO content of final slag is reduced by 2.69%, which meets the requirements of slag splashing for furnace protection;

　　(3) The converter temperature raising mode with coke as the main component and ferrosilicon as the auxiliary component is adopted. For RH refined low sulfur steel, the addition of coke is limited and the addition of ferrosilicon is increased; In view of the low silicon content in molten iron, especially in smelting low phosphorus steel, the addition of ferrosilicon should be properly increased to strengthen the slag melting effect of converter.