The process that continuous casting process produces alloy steel billets can improve the yield of alloy steel and reduce the loss of alloy elements ( some alloys contain precious metal elements). Therefore, less cutting head can reduce the production cost, and because the quality of the casting billet produced by continuous casting process is uniform, it is conducive to improving the processing performance and service performance of alloy steel. Compared with ordinary carbon steel continuous casting, special steel continuous casting has the following characteristics:
(1) Special steel has more alloying elements, higher content, and a large range of carbon content (0.02%~2.3%C), and its solidification characteristics are very different from ordinary carbon steel.
(2) The purity, uniformity, gas content, low microstructure, inclusions and other requirements of special steel are higher than that of ordinary carbon steel.
(3) Some special steels contain Cr, V, Ti, Nb, Al and other active elements, which are easy to react with oxygen and nitrogen to produce high melting point compounds, affecting the quality of the billet; Some alloying elements form carbides or carbon nitride with carbon in steel, which is easy to produce high temperature solid phase precipitation, and has an important impact on the thermoplasticity of steel.
(4) Due to the special cooling and solidification conditions of the continuous casting process, the composition segregation and organizational inhomogeneity are more prominent.
(5) The special steel contains more active metal elements such as Al, Ti, Cr, Ni, Mn, etc., which is easy to combine with O and N to generate more complex inclusions such as A1203, Ti02, Ti(CN), (CR-A1)203, (MN-Ti)204, which makes the liquid steel poor fluidity and easy to plug the water outlet during the pouring process.
The main measures for continuous casting of special steel to improve the cleanliness of liquid steel are as follows: 1) Low aluminum clean steel technology, by using non-aluminum deoxidizer, under the premise of reducing the proportion of residual aluminum and oxide inclusion A1203 in steel, the oxygen content of steel is significantly reduced than that of traditional aluminum deoxidization; 2) Ladle soft argon blowing technology; 3) Automatic detection and control technology of ladle slag; 4) tundish metallurgy technology; 5) Optimization of tundish and mould protection slag; 6) Mould metallurgy technology.
The improvement of the quality of special steel billet mainly depends on the optimization of continuous casting equipment and technology. Under the current equipment and technology conditions, the method of combining low superheat casting and electromagnetic stirring is mainly adopted to expand the equiaxial crystal zone and reduce the concentration of segregation and loose. At the same time, according to the characteristics of different steel grades, suitable protection slag, mold design and cooling, secondary cooling and drawing system are selected to prevent cracks and other defects. The key to achieve low superheat casting is to control inclusions in steel and keep the water temperature of clad steel stable during continuous casting. In order to control the composition of inclusions in steel, calcium treatment technology is generally used, but when the content of sulfur in steel is high, CaS inclusions are easy to form and lead to nodule at the water inlet, and the spot inclusions formed by calcium treatment are very harmful to some steels. The low aluminum clean steel technology can achieve the dual purpose of improving the cleanliness and optimizing the inclusions. At present, the temperature of molten steel can be controlled within a very narrow range through secondary refining, and the adoption of tundish insulation technology and steel inclusion control technology creates conditions for low superheat casting. The research shows that the most effective means to control the temperature stability of molten steel in tundish is to use plasma heating device to supply stable and reliable neutral heat energy to tundish. In addition, to reduce the superheat of molten steel, it can also be considered to use water-cooled water nozzle or conical lossless condenser in the liquid steel meniscus for auxiliary cooling.
In recent years, some new technologies have been applied to improve the quality of casting billet to a certain extent, such as:
1) Automatic control of liquid level during casting;
2) Optimization of the electromagnetic stirring process of the mould and the final stage of solidification;
3) Light pressing technology of generous billet;
4) Using external field or nucleating agent to refine as-cast structure technology;
5) Compound mould control solidification structure technology; 6) mould liquid steel flow control technology.
