What Is Semi-Steel?

A type of hypereutectoid cast steel with extremely high carbon content (1.4% to 2.4%). This type of high carbon cast steel has actually entered the category of cast iron, so it is commonly called semi-steel. For a long time, China's metallurgical workers have accumulated more experience on how to process semi-steel in high-quality, low-consumption, high-yield places in converters. Hot metal pretreatment has also received increasing attention, and it has undergone dephosphorization, desulfurization, and desiliconization. It also has the characteristics of semi-steel.

China's Inner Mongolia, Sichuan, Yunnan and other places contain extremely rich multi-element symbiosis iron ore resources. In order to make better use of these valuable resources, steel mills such as Panzhihua Iron and Steel and Maanshan Iron and Steel have adopted spraying vanadium and converter vanadium extraction processes. Extraction of metals such as vanadium and titanium. After the hot metal processed in this way, the content of silicon and manganese is small, and the carbon also has a certain amount of refining loss, so-called semi-steel. For a long time, China's metallurgical workers have accumulated more experience on how to process semi-steel in high-quality, low-consumption, high-yield places in converters. Hot metal pretreatment has also received increasing attention, and the "three-desulfurization" hot metal after dephosphorization, desulfurization, and desiliconization also has the characteristics of semi-steel ^[1]

General composition of semi-steel

Blast furnace hot metal. After pretreatment such as "vanadium extraction" or "triple desorption", it becomes a semi-steel. Vanadium, titanium silicon and other easily oxidizable elements are traced, manganese is reduced, and carbon is reduced by 0.5 to 1%.

Reduced heat sources for steelmaking

Compared with semi-steel and conventional hot metal, the heating elements carbon, silicon, and manganese burn more, and the chemical heat of converter smelting will be greatly lost. It has brought certain difficulties to the heating up of converter smelting, slag slag and eating more scrap steel. Taking the composition of molten iron and semi-steel as an example, the heat of chemical reaction in the smelting of semi-steel will be reduced by 20% compared to molten iron (calculated based on the secondary combustion rate of 10%).

Difficult to slag

In conventional molten iron smelting, slag is a multi-component system. In addition to CaO, SiO ₂ , and FeO, there are components such as MnO, Al ₂ O ₃ , and MgO, which make the slag form a complex multi-element system, which promotes the reduction of the slag melting point, and can form slag as soon as possible and slow down the re-drying in the medium term. occur. For semi-steel, if it is still smelted by conventional methods, the slag is basically CaO-FeO system. As a result, the melting point of the slag will be increased, the slag formation will be slow, the removal of phosphorus and sulfur will be difficult, and a series of unfavorable problems such as easy-to-stick guns and furnace mouth will be caused.

Conducive to less slag smelting

"Because the amount of impurities in molten iron has already been low, the burden on converter smelting has been greatly reduced. The complex smelting process of the converter has become a simple decarbonization and temperature rise, and it is necessary for the slag smelting of converters to be used. The converter is used. The "three-stroke" hot metal adopts a low-slag steelmaking process, and the lime consumption is reduced from the traditional 90kg / t steel to 40kg / t steel. The smelting time is shortened and the life of the converter village is improved.

Created conditions for the direct introduction of chloride into the furnace. Adding lean manganese ore in converter smelting improves the residual manganese at the end point, which can save manganese iron. Since there are many SiO ₂ and other impurities in lean manganese ore, if the Manganese ore. In order to maintain a certain alkalinity, a large amount of slag must be made, which brings various adverse effects on the process.

In the semi-steel smelting, since the Si content in the molten pool is already low, the Si content in the lean manganese ore will be beneficial to the slag. Therefore, adding manganese ore in semi-steel smelting not only facilitates slagging, but also creates conditions for improving the residual manganese at the end point. ^[2]

The traditional steelmaking process of the converter using ordinary molten iron, due to the removal of phosphorus and sulfur, was forced to adopt a large slag operation, which brought a series of adverse effects such as low metal yield, high slag consumption and long smelting time. The converter uses "three-off" hot metal for steelmaking, which greatly reduces the training load. With less slag smelting, the conditions for manganese ore direct feed into the furnace have become mature. Consumption of lime and other slag materials is reduced by about 50% after the "three-drop" molten iron is used for steelmaking. Consumption and blow loss of steel materials, oxygen and slag materials are also significantly reduced.

The optimal manganese content in low-silicon hot metal steelmaking is 0.5% to 0.7%, so semi-steel steelmaking must be supplemented with MnO to facilitate the rapid formation of initial slag, improve desulfurization, reduce metal splashing and thicken final slag . Manganese ore and waste manganese slag are used to make slag in the production of semi-steel steel. The added amount is 6 to 8 kg / t steel, and the final manganese oxide reaches 2% to 3%, and the desulfurization efficiency is increased by about 2%. Residual manganese in molten steel increased by 0.03% to 0.05%. Japan uses a double-blowing process. The amount of manganese ore added is 7 to 15 kg / t steel, and the recovery rate is 45% to 60%. And reduced process spatter ^[2]

Due to the continuous popularization and improvement of the hot metal pretreatment process and the reality of the associated elemental iron in China, more converter plants will use semi-steel steelmaking. Through unremitting efforts, the majority of metallurgical workers have figured out many valuable experiences in solving the two major problems of slag and temperature control in smelting of semi-steel. For example, using adiabatic ladle, adding carbon materials in the furnace, using double-flow oxygen lance blowing, multi-component slagging, etc. These are the hot metal pretreatment-converter re-blowing-out-of-furnace refining and optimization of smelting process , Do not set the foundation. In the future related semi-steel smelting. According to the semi-steel steelmaking experience that China has obtained and combined with the actual situation of the plant, it will be very necessary and beneficial to formulate a more complete and reasonable process system ^[2] .