Nonmetallic Inclusions in Bulk Steel Ingots
In the production of steel alloys, nonmetallic inclusions can cause various production problems and limit the properties of finished alloys. These inclusions can form in steel during the melting process and through their accumulation during solidification. In order to improve the quality of steels and alloys and their respective performances, steel manufacturers must pay special attention to the nonmetallic inclusions that form in bulk steel ingots.
Nonmetallic inclusions can take several forms and can originate from both the raw materials and the equipment. By considering what type of inclusion is present and its morphology, the source of the inclusion can be identified. The most common inclusions in bulk steel ingots arise from the slag, raw materials, and refractory lining, as well as from oxidation-related events during processing. Oxide inclusions may arise from reaction between slag and metal, or from interaction between incoming oxygen and steel, forming such elements as manganese, calcium, and silicon oxides. During the melting process, slag particles are entrapped in the steel and become inclusions. Slag particles usually appear to be angular and are composed of flux and fluxing agents. Additionally, refractory materials such as silica and alumina can form inclusions during the melting process. Nonmetallic inclusions such as nitrogen and hydrogen can also be present in steel due to the subsequent reactions during solidification.
The presence of nonmetallic inclusions affects the properties of the steel alloy. Generally, the larger the inclusions, the more harmful the effect on steel performance. Smaller inclusions form less of a problem since they tend to dissolve or spheroidize quickly over time due to heat treatments or when mixed with additional components. Furthermore, inclusions may also reduce the hot-workability of steels and alloys, as well as their hot cracking resistance and their fatigue strength.
A number of methods have been developed to control nonmetallic inclusions in bulk steel ingots. Firstly, manufacturers of steel alloys must pay special attention to the atmospheric conditions that affect formation of inclusions. If the atmosphere has a high oxygen content, for example, oxidation of the metal is more likely to occur, resulting in a higher number of inclusions. It is also important to use high purity raw materials to reduce the presence of inclusions. Additionally, producers of steel can use flux to purify the steel during the initial melting process, thereby providing a more homogenous alloy.
Finally, stainless steel producers can apply various processes such as vacuum degassing, argon bubbling, and electrolysis to further reduce the presence of nonmetallic inclusions. Furthermore, steel can be subjected to mechanical stirring in the molten state to reduce segregation of inclusions and nonmetallic elements. Finally, by using an additional remelting process, inclusions can be further reduced and a more uniform alloy achieved.
In conclusion, it is important for steel manufacturers to pay attention to the presence of nonmetallic inclusions in bulk steel ingots. By monitoring the inclusions, understanding their origin and form, and controlling the atmosphere, temperature, and purification methods, steel manufacturers can improve the performance of steels and alloys, ensuring a higher quality product for their customers.
