Blowholes - Slab defects

Blowholes - Slab defects

Figure 1: Blowholes formed on at beam blanks are inspected by scarfing.

Defect name: Blowholes
Record No.: 2703
Type of defect (Internal/Surface): Surface
Defect classification: Slab defects
Steel name: Steel
Steel composition in weight %: No data.
Note: It is well known that the presence of dissolved [O], [N], [H] in the liquid steel can cause blowholes formation in the continuous casting process. So, gas levels must be kept at a minimum in the steelmaking and continuous casting process. The aim of this work is to investigate the possible sources of these gases including hydrogen, nitrogen and oxygen, focusing on the preparing process that are in contact with the liquid steel during refining and casting. The tundish refractory lining, covering agents, slag and casting powders were analyzed. Effects of chemical composition and gas level on defects formation are also investigated. The results showed that not only the humidity content in those refractories using in tundish preparation is critical, but also improving the oxygen removing efficiency is important as well.

The appearance of blowholes in the steel occurred at Dragon Steel is required to speedily identify their origins, which were found in some heats near the surface and inside the continuously casting (CC) beam blanks with heavy sections, and are out of acceptable quality. From some reports, the solutes of [O], [N], [H] dissolved into liquid steel exceed the equilibrium solubility of gas and then precipitate along interdendritic liquid during the solidification. Some control methods to suppress blowholes formation are applied. The deoxidising, suppression of secondary oxidation, and reduce adequately the hydrogen pickup of ladle slag and the moisture in the refractory material. Blowholes were found in some heats near the subsurface and inside the beam blank after scarfing and some of them were easily visible to the naked eye, as shown in Fig.1.
Reference: Not shown in this demonstration version.