Introduction
Steel is an essential material in our daily life and plays an important role in modern industry. It is not only an important foundation material of construction and engineering, but also an important structural material of automobiles, ships, aircraft, household appliances and other products. According to the statistics of China Iron and Steel Association, the output of steel in China in 2018 was 928 million tons, up 4.7% year-on-year. With the rapid development of modern construction, transportation and manufacturing industry, the demand for iron and steel market will continue to grow.
In the production process of steel, the raw material and the smelting process have a great influence on the quality and performance of steel. Among them, trace elements in molten steel play an important role in steel quality. The trace elements in molten steel originate from the raw materials used and the additives added in the process of steelmaking. The trace elements have an important effect on the physical, chemical, mechanical and other properties of steel, and their content should meet the requirements of steel quality standards.
The purpose of this study is to analyze the effect of trace elements in steel on continuous casting slabs. The contents of trace elements in the raw material and additives entering the steelmaking process, the influence of the trace elements on the quality of the continuous casting slabs and the countermeasures to reduce their influence are discussed.
Contents
1. Trace elements in steel and their components
Trace elements (referred to as alloy elements) refer to elements other than iron, carbon and manganese in steel, including elements such as chromium, molybdenum, cobalt, tungsten, vanadium, titanium, aluminum, silicon, etc. According to their composition and origin, they can be divided into two categories: inherent elements and added elements.
2. Effects of trace elements on continuous cast slabs
2.1 Inclusion
Inclusions are the main cause of micro defects in steel, which will reduce the mechanical properties of steel. Inclusions are formed by the combination of primary and secondary phases, depending on the chemical composition of the steel and the process conditions. The most important role of trace elements is to improve the quality of the slabs by promoting the formation of appropriate and stable inclusion morphology. Because different shapes of inclusions can affect their segregation speed, thus affecting the further hot rolling process and product quality of steel.
2.2 Network forming element
The network forming elements including Ti, Al, Nb and V, contribute to the strength of steel by forming a network structure in the ferrite matrix. In the continuous casting process, the solution of these alloying elements is limited, and the content of the alloy elements has a great influence on the shape and macrostructure of the inclusion clusters.
2.3 Sulfide controlling elements
Sulfide controlling elements mainly refer to Mn, Cr and Mo. These elements reduce the rate of sulfide segregation and improve the plasticity of steel. Through the combined effect of these elements, the desulfurization effect is improved in smelting, which will make the liquid steel smooth and improve the plasticity of the molten steel and the surface quality of the continuous casting slabs.
3. Nutritional regulation and control of trace elements
3.1 Selection of raw materials
Reasonable selection of raw materials is an effective way to control the trace elements in steel. Generally, mild steel scrap, low-phosphorus steel scrap, pig iron or pig iron alloy or ferrous alloy are selected. Low-phosphorus steel scrap and pig iron should be avoided. It is usually necessary to replace other deoxidation agents to reduce the effect of trace elements in pig iron on the steel quality.
3.2 Controlling the nutritional dose of alloying elements
According to the requirements of steel grade quality, controlling the dosage of alloying elements is an important measure to improve the quality of continuous casting slabs. Common deoxidation agents and alloying additives include ferrosilicon, aluminum-silicon alloy, pure aluminum, ferromanganese, ferrochrome, ferrovanadium, calcium silicon, calcium metal, calcium aluminate, etc. It is necessary to select the appropriate alloying elements for different grades of steel according to their chemical composition.
3.3 Adjustment of molten steel refining process
From the point of view of molten steel refining process, early deoxidation, prevention of air injection and comprehensive argumentation are the main measures to reduce trace element inclusion and segregation. Optimize the furnace temperature, residence time and stirring intensity to reduce the segregation of trace elements. At the same time, use slag foaming and chemical purification to weaken the effect of trace elements on steel quality as much as possible.
Conclusion
Trace elements are important agents affecting the quality of continuous casting slabs. In order to control the content of trace elements and improve the quality of continuous cast slabs, it is necessary to first select suitable raw materials for smelting, and then adjust the smelting process and dosage of alloying elements. In addition, reducing the volatility of trace elements, controlling the temperature of molten steel and refining degree, improving the comprehensive comprehensive deoxidation capacity, using slag foaming and chemical purification technology, etc. are also effective measures to improve the quality of continuous cast slabs.
