With the implementation of the steel production restriction policy, the shortage of molten iron has become more serious, and the amount of scrap steel has increased year by year. At the same time, with the large-scale 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. Therefore, how to maximize the steel production without increasing the iron production has become the primary consideration of the steelmaking system. Taking into account the factors such as converter loading, smelting heat balance, converter lining maintenance, product quality and comprehensive cost, a high scrap steel ratio smelting plan is planned to achieve the purpose of reducing production costs and increasing economic benefits.
Effect of high scrap ratio on converter smelting process
Effect of high scrap ratio on converter charging
A high scrap ratio means the maximum consumption of scrap steel. The scrap steel trough is basically used for adding scrap steel to the converter. First, different types of scrap steel are loaded into the scrap steel trough by matching, and the scrap steel trough is hoisted by the charging cross crane to add the scrap steel into the converter. With the continuous increase of the scrap steel ratio, the scrap steel trough of some enterprises cannot meet the requirements of single bucket loading, and the scrap steel trough is expanded, heightened and lengthened. Although the amount of scrap steel added has increased, the total tonnage of scrap steel + trough exceeds the design tonnage of the charging cross crane, which does not meet the regulatory requirements. Considering the high investment cost of transforming the crane, the crane has not been transformed, which limits the space for improving the scrap steel ratio; another part of the enterprises choose to add scrap steel with double buckets. Although this mode solves the problem of crane overloading, it increases the converter smelting cycle and seriously affects the converter production efficiency. Therefore, in order to fundamentally solve the problem of scrap steel entering the furnace under the high scrap steel mode, other ways of adding scrap steel besides directly feeding scrap steel into the converter need to be considered.
The impact of high scrap ratio on steel material consumption indicators
The scrap steel ratio is a key factor affecting the steel material consumption index. Through the high scrap steel ratio smelting practice of Yulin Iron and Steel’s 120t top and bottom double-blown converter, the recovery rates of different types of scrap steel are different. According to the proportion of scrap steel purchased by Yulin Iron and Steel, the scrap steel recovery rate is calculated to be about 87.27%. According to the molten iron composition and the total slag amount of the converter, the molten iron recovery rate is calculated to be about 91.85%. Because the molten iron recovery rate is greater than that of scrap steel, for every 1kg/t increase in scrap steel consumption, the steel material consumption index increases by about 0.0458kg/t.
Effect of high scrap ratio on converter smelting process
(1) The melting of scrap steel in the converter pool needs to go through three processes: surface molten iron condensation, condensation layer melting, and scrap steel carburization melting. According to the practice of 120t top and bottom double-blowing converter smelting in Yulin Steel, under the high scrap ratio mode, the scrap steel entering the converter is not less than 32t. In the early stage of blowing, small and medium-sized scrap steel and plate-shaped scrap steel with large surface area melt quickly. The temperature of the converter pool drops significantly in the early stage of blowing. The dust removal system of the Yulin Steel converter is dry dust removal. In order to control the explosion during opening and unloading, low oxygen pressure ignition is adopted. The phenomenon of failure to ignite within 50s of oxygen supply from the lower gun of the converter is quite frequent.
(2) Molten iron is added to the converter, and part of the scrap steel is melted. In the early stage of blowing, the temperature of the molten pool is low, the heat and mass transfer speed of the molten pool is slow, and the temperature of the molten pool rises slowly. In addition, other scrap steel melts quickly, which delays the heating rate of the converter. It is difficult to slag after the first batch of auxiliary materials are added to the converter, and the dephosphorization efficiency of the converter decreases in the early stage of blowing. Yulin Iron and Steel adopts the one-pot system of loading molten iron into steel. Affected by the turnover rate of the molten iron tank, the temperature of the molten iron in a single tank varies greatly. In the early stage of smelting, it is necessary to flexibly adjust the amount and timing of auxiliary materials added according to the process temperature. The temperature control during the smelting process is difficult. In the early stage of smelting, low-temperature slag overflow and unstable dephosphorization rate are prone to problems. Poor slag reduction in the middle stage is prone to dryness, resulting in a decrease in the final carbon temperature hit rate and an increase in the number of overoxidation furnaces.
Effect of high scrap ratio on converter lining
According to the practice of 120t top and bottom double-blown converter smelting in Yulin Steel, in order to increase the scrap steel ratio, the scrap steel material type is mainly heavy scrap steel above 6mm. During the process of scrap steel entering the furnace, the impact on the large lining of the front of the converter is large, causing certain damage to the large lining of the front of the converter. During the smelting process, the oxides and non-metallic impurities in the scrap steel have a certain erosion effect on the refractory materials of the converter. At the same time, the temperature of the molten pool is low in the early stage of smelting, slagging is difficult, the slag basicity is low, and the lining is seriously eroded. Due to the low calorific value of scrap steel, high scrap steel ratio loading cannot provide enough heat, the converter process temperature is not surplus, and the terminal temperature of some furnaces is insufficient, which does not meet the converter tapping temperature standard. Spot blowing is required for tapping. The carbon content at the end of the spot blowing furnace is low, the molten steel is highly oxidizable, the FeO content in the slag is high, the converter splashing slag protection effect is poor, and the difficulty of converter furnace condition maintenance increases.
Effect of high scrap ratio on molten steel quality
The scrap steel entering the converter is mainly purchased scrap steel. The types of purchased scrap steel are diverse and difficult to distinguish. The trace elements such as sulfur, chromium and copper in the scrap steel are unstable. High scrap steel ratio smelting increases the amount of sulfur returned in the converter. At the same time, the nitrogen content of the molten steel at the end tends to increase. High sulfur and chromium content at the converter end often occur, affecting the quality of the steel billet and the rolling performance of the steel.
