Nonmetallic inclusions formed in high carbon chromium steel were classified into microscopic and macroscopic ones. The behavior of their formation was studied.
The results are summarized as follows:
1. During solidification of steel, oxides and sulfides which have been dissolved in melt as oxygen and sulfur, are almost precipitated, forming microscopic inclusions. Inclusions produced in melt float up and are excluded rapidly.
2. In solid steel, oxygen can not virtually be dissolved and remains as oxide. Consequently the amount of oxide inclusions increases approximately in proportion to the oxygen content in steel.
3. Although the amount of sulfide inclusions increases according to the total amount of sulfur, they can be partially dissolved in steel. Therefore the amount of sulfide inclusions is also dependent on the amount of sulfur dissolved in solid steel. Sulfur content in steel is very effective for the cleanliness of steel.
4. When microsegregation degree of steel is decreased, the amount of sulfur dissolved in solid steel is increased. Therefore the amount of sulfide inclusion is decreased.
5. When molten steel containing much oxygen is killed with aluminum, cloudlike inclusions are produced. Relation among these inclusions, various melting and casting conditions, and microsegregation is investigated.
It is shown that alumina type inclusions are also precipitated during solidification of steel, and that these inclusions produced in molten steel are separated rapidly. In spite of containing much aluminum, oxygen can be dissolved in molten steel super saturatedly.
6. The melting condition for occurrence of B-type inclusions in killed steel is experimentally examined. Addition of aluminum, enrichment of oxygen and holding of molten steel are needed for occurrence of B-type inclusion. These conditions may be attributed to oxidation of aluminum in molten steel.
7. By analyzing the type of length distribution of inclusions found in steel parts, it is suggested that macroscopic inclusion is not a large inclusion which has grown up accidentally, but another type inclusion is produced under a particular condition.
8. Experimentally, macroscopic inclusion can be intentionally produced by promoting air oxidation of molten steel in teeming process. Accordingly, teeming in argon atmosphere was effective for preventing the occurrence of macroscopic inclusions. Air oxidation of molten steel by carrying air bubbles down into the molten steel in mold has much effect on the occurrence of macroscopic inclusions.
9. It is shown that there is another unknown factor for occurrence of macroscopic inclusions appearing through a certain teeming process, accompanied with air oxidation of molten steel.
10. Large inclusions adjacent to pinhole and blowhole, and B-type inclusions are regarded as a factor influencing the occurrence of macroscopic inclusions in special cases.