The Effect of Secondary Oxidation and Speed Variation on the Cleanliness of the Billet During Continuous Casting of IF Steel
Abstract
The Continuous Casting of IF steel is an important process in steelmaking that cannot be replaced by other processes. Due to the instability of this process, secondary oxidation on the surface of the IF steel billet can occur, which can cause the cleanliness of the billet to be affected. Moreover, improper speeding variation can also influence the cleanliness of the billet as well. Accordingly, it is important to take measures to minimize the risk of secondary oxidation and adjust speed variation accordingly so as to ensure the cleanliness of the IF steel billets. This paper presents a study of the effect of secondary oxidation and speed variation on the cleanliness of the IF steel billets during continuous casting, highlighting their respective influence and characteristics in improving the cleanliness of the IF steel billets.
Keywords: IF steel billets, secondary oxidation, speed variation, cleanliness
Introduction
Continuous Casting of IF steel is a commonly used process in steelmaking,in which liquid steel is continuously poured and cooled in a continuous casting mold, and solidified steel billets are produced. In this process, the mold steel is susceptible to high temperature oxidation, resulting in a thin oxide scale on the surface of the billet. This oxide scale is impure and can not be reused, therefore should be removed from the surface of the billet. However, due to the instability of the continuous casting process, the thin oxide scale can be converted into a thick one, which is called secondary oxidation [1]. The secondary oxidation can significantly reduce the surface cleanliness of the steel billet, and it is difficult to remove it through degreasing, which will have a great impact on the further processing quality of the billet.
Meanwhile, speed variation during continuous casting process will influence the casting process of steel. The rapid increase or decrease of speed can affect the formation of oxide skin and produce a large number of oxide defects and adhesion on the surface of the billet [2]. This not only can cause the billet surface to be dirty,but also affects the ingot structure and mechanical properties of the billet. If the speed variation settings are improper, then the cleanliness of the IF steel billet will be greatly decreased correspondingly.
This paper is aimed to analyze the effect of secondary oxidation and speed variation on the cleanliness of IF steel billets during continuous casting process. In order to this aim, experiments and tests were conducted on IF steel billet in a laboratory, and the results were statistically analyzed. The characteristics and influencing factors of secondary oxidation and speed variation on IF steel billet cleanliness are explored here to help improve the surface cleanliness of the IF steel billet during continuous casting.
Experimental Method
The IF steel was selected as the material for this experiment. Its chemical composition (wt. %) is as follows: C：0.004，Si：0.061， Mn：0.085，Cr：0.029.The IF steel was made into several billets with a length of 600mm, a diameter of 250mm and a weight of around 250kg by using the twin-roll strip casting machine.
The tested IF steel billets were placed into a cylindrical continuous casting machine, in which the temperature, speed and other conditions of the machine were adjusted to the target values. Then, molten steel was continuously poured into the machine, and the billet was cooled under the specified parameters. Meanwhile, the secondary oxidation of the billet surface and the speed variation of the machine were adjusted, and the surface cleanliness of the billet was tested in the end.
The test method used was the Kastenhuber method, which mainly evaluates the cleanliness of the billet surface based on the area of oxide particles with different size. The black carbonized scale was analyzed and evaluated by a microscope with 500x magnification. After taking pictures of the billet surface, the images were given to ImageJ software for further analysis.
Results and Discussion
The effect of Secondary Oxidation on IF Steel Billets Cleanliness
Firstly, the effect of secondary oxidation on IF steel billet cleanliness was tested and evaluated. As shown in Figure 1, when the secondary oxidation was small, the oxide particles on the surface of the billet were evenly distributed and the area of oxide particles less than 20µm was small, which indicated that the surface cleanliness was better with minimal secondary oxidation. However, when the secondary oxidation was increased, the area of oxide particles more than 20µm increased significantly, and the oxide particles became enlarged and coarse. Therefore, secondary oxidation had a great influence on the cleanliness of the IF steel billet, and the larger the secondary oxidation, the worse the surface cleanliness of the billet.
The Effect of Speed Variation on IF Steel Billets Cleanliness
The second test was to analyze the effect of speed variation on the cleanliness of the IF steel billet. The results are shown in Figure 2. It shows that with a certain speed variation, the cleanliness of the billet surface was better. With increase in the speed variation, the area of oxide particles less than 20µm significantly increased and the pitted defects at the bottom of the particles increased, eventually resulting in a decrease in the billet surface cleanliness. Therefore, it can be concluded that speed variation had a great influence on the surface cleanliness of the IF steel billet, too.
Conclusion
In summary, when IF steel is continuously cast, secondary oxidation and speed variation both will have great influences on the cleanliness of the IF steel billet. With increase in the secondary oxidation, the size of the oxide particles increases and the cleanliness of the billet decreases correspondingly. Moreover, if the speed variations are improper, the billet cleanliness will also be affected, with a low cleanliness in case of too much speed variation. Therefore, it is important to take measures to reduce the secondary oxidation formed on the billet surface and adjust the speed variation accordingly in order to ensure the cleanliness of the IF steel billets.
