Blowholes - Slab defects

Blowholes - Slab defects

Figure 1: Blowholes.

Defect name: Blowhole
Record No.: 702
Type of defect (Internal/Surface): Internal, surface
Defect classification: Slab defects
Steel name: Steel
Steel composition in weight %: No data.
Note: For the study, we selected only the heats used to cast steel billets Ø250 mm and Ø270 mm, respectively. For a more complete analysis, we took into account all the steel grades (carbon steel, low alloy steel, alloy steel) used to produce these two types of steel billets. The steel that is going to be continuously cast is primarily intended to obtain semi-finished products for tubes (Ø180 mm, Ø250 mm, Ø270 mm and Ø310 mm) and bloom (240 x 270 mm), for subsequent re-rolling.

Blowholes (Fig. 1) are cavities in the outer surface or in the subcutaneous zone of the billet, located at few tenths of millimetres from the stand surface. They have a diameter of 3 mm and a length (depth) that can reach up to 25 mm. Usually, they contain CO, relatively low H2 and Ar, and they are often associated with inclusions. If they are superficial and/or few, they are grinded (not to exceed the allowed dimensional tolerance after grinding). They are caused by:
1.) insufficient steel deoxidation (presence of gases: hydrogen, nitrogen, oxygen);
2.) humidity of the casting powder;
3.) quality of the casting powder (% carbon, viscosity, basicity) - quantity and uniformity of its distribution;
4.) variation of the steel level in the mould, existence of moisture in the refractory lining of the tundish;
5.) the presence of argon entered in the mould during the injection of argon for filling the nozzle.

The measures to be taken to remedy these defects could be:
1.) sufficient deoxidation of steel by using dry materials and additives, protection of ladle and tundish;
2.) use of dry casting powder (and preheated, if possible);
3.) possibly choosing a casting powder compatible with the steel grade, temperature and casting speed (and, of course, a good correlation between the casting power quantity and the casting speed);
4.) controlling the steel level fluctuations in the mould, to prevent the steel to flow over the casting powder and to embed it, controlling the nozzle immersion depth, use of nozzles free of defects;
5.) avoiding the high casting temperatures;
6.) maintaining the argon debit below the critical value, to avoid the capture of argon bubbles by the meniscus and the development of slag foaming around the nozzle.
Reference: Not shown in this demonstration version.