1. What are the characteristics of special steel continuous casting?
Compared with ordinary steel continuous casting, special steel continuous casting has the following characteristics:
(1) Special steel has more alloying elements, higher content, and a wide range of carbon content (0.02%~2.3%C). Affected by alloying elements, its solidification characteristics are greatly different from those of ordinary carbon steel.
(2) The purity, uniformity, gas content, macrostructure, inclusion morphology and other requirements of special steel are higher than those of ordinary carbon steel.
(3) Some special steels contain active elements such as Cr, V, Ti, Nb, and Al, which are very easy to react with oxygen and nitrogen to form high-melting point compounds, which brings certain harm to the quality of the ingot; some alloying elements react with carbon in steel to form carbides or carbonitrides, which are easy to produce high-temperature solid phase precipitation, which has an important influence on the thermoplasticity of steel.
(4) Due to the special cooling and solidification conditions of the continuous casting process, component segregation and structural inhomogeneity are more prominent.
2. In view of the characteristics of special steel solidification, what are the key technologies that need to be solved in special steel continuous casting?
(1) Improving the cleanliness of molten steel
The main measures to improve the cleanliness of molten steel are: 1) Low-aluminum clean steel technology, by using non-aluminium deoxidizers, the oxygen content in steel is significantly reduced compared with traditional aluminium deoxidation under the premise of reducing the proportion of residual aluminium and A1203 in oxide inclusions in steel; 2) Ladle soft argon blowing technology; 3) Ladle slag automatic detection and control technology; 4) Tundish metallurgical technology; 5) Optimization of tundish and crystallizer protective slag; 6) Crystallizer metallurgical technology.
(2) Improving the quality of ingots
The improvement of ingot quality mainly depends on the optimization of continuous casting equipment and processes. As far as the current equipment in my country is concerned, the main measure to improve the quality of special steel ingots is to adopt a combination of low superheat casting and electromagnetic stirring to expand the equiaxed crystal area and reduce the concentration of segregation and looseness. At the same time, according to the characteristics of different steel grades, appropriate protective slag, crystallizer design and cooling, secondary cooling and billet drawing system should be selected to prevent the occurrence of defects such as cracks.
In order to further improve the quality of ingots, some new technologies have emerged in recent years: 1) automatic control of liquid level during casting; 2) optimization of electromagnetic stirring process in crystallizer and the final stage of solidification; 3) light pressure reduction technology for large square billets; 4) technology of refining as-cast structure by using external field or nucleating agent; 5) technology of controlling solidification structure by compound crystallizer; 6) technology of controlling steel liquid flow in crystallizer, etc.
3. How to reasonably select the cross-section of continuous casting billets?
Since some low-multiple defects of ingots can be reduced or eliminated by a certain compression ratio, and a certain compression ratio is also a necessary measure to stabilize the performance of steel. In order to ensure the quality of large-size special steel bars, large ingot cross sections must be used, and measures such as initial rolling and opening of billets must be adopted.
For example, in the 1980s and 1990s, the cross sections of special steel continuous casting machines put into production in Japan were all above 300m㎡. However, since the segregation and porosity of small-section ingots are smaller than those of large-section ingots under the same conditions, and with the development of clean steel production technology and the improvement of continuous casting technology, the total amount of harmful impurities and inclusions in steel has been significantly reduced, the adoption of electromagnetic stirring technology has effectively reduced the central segregation, making it possible to produce small-size special steel from small-section ingots and pyrotechnical products. Thus, the EAF (BOF)-LF-VD large billet continuous casting and large bar continuous rolling production line and the EAF-LF-small billet continuous casting and small bar and wire continuous rolling production line with their own characteristics are formed, and the latter has more competitive advantages in the production of small-size materials.
Each steel enterprise can select the appropriate ingot section according to the characteristics of the steel specifications and rolling mills, forming a specialized production line for large billets to produce large-size materials and small billets to produce small-size and wire materials.
4. What are the keys to low superheat casting and the technologies that support it?
The keys to low superheat casting are: (1) controlling inclusions in steel to prevent nozzle abscesses during low superheat casting; (2) accurately controlling the temperature of molten steel in the tundish during continuous casting; (3) controlling the production rhythm of steelmaking and continuous casting.
Calcium treatment technology is commonly used to control the composition of inclusions in steel. However, when the sulfur content in steel is high, CaS inclusions are easily formed, leading to nozzle abscesses, and the point-shaped inclusions formed by calcium treatment are very harmful to some steels. The use of low-aluminum clean steel technology can achieve the dual goals of improving cleanliness and optimizing inclusions.
At present, the temperature of molten steel can be controlled within a very narrow range through secondary refining. In addition, the use of tundish insulation technology and inclusion control technology in steel has created conditions for low superheat casting. Studies have shown that the most effective means to control the temperature stability of molten steel in the tundish is to use a plasma heating device to supplement the tundish with stable and reliable neutral heat energy. In addition, water-cooled nozzles or conical non-consumption condensers on the meniscus of the molten steel can be used to reduce the superheat of the molten steel for auxiliary cooling.
5. What are the advantages of using continuous casting technology to produce special steel continuous casting billets?
Using continuous casting technology to produce special steel continuous casting billets can improve the yield rate of special steel and reduce the loss of alloy elements (special steel usually contains higher alloy elements, and some also contain precious alloy elements). Therefore, less cutting can reduce production costs. Since the quality of the billets produced by the continuous casting process is uniform because alloy steel contains more alloy elements, the continuous casting process produces alloy steel continuous casting billets. The inherent characteristics of uniform quality have been brought into play, which not only improves the quality of the billets, but also helps to improve the deep processing performance and use performance of alloy steel, and at the same time, the qualified rate of the products is increased.
One Response
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