抄録
As a part of the study of special chemical elements in steel, we have investigated the effects of niobium upon the properties of steel. The following are the results of test;
1. Niobium added to steel refines remarkably the cast structure and austenite structure of steel. An adequate and yet minimum quantity of niobium necessary for the refining control of austenite--grains in steel is 0.03 to 004%.
2. With an addition of niobium, the coarsening temperature of austenite-grains will rise. For example, in the case of 0.03 to 004% niobium addition, the coarsening temperature rises by approximately 160°C and reaches 1050°C.
3.The deoxidation power of niobium is comparatively small and it does not show any substantial deoxidation effect upon the molten steel used for this test and other molten steels containing silicon.It is, however, considerably greater than manganese and accordingly, it is supposed that niobium may combine with oxygen which becomes super-saturated with a lowering of the temperature during the process of solidification of steel to precipitate nonmetallic inclusions consisting of various niobium oxides. But, being extremely minute and very small in quantity, such oxides do not exert a great influence on the cleanliness value of steel material as determined by JIS method. The affinity of niobium for carbon and nitrogen is so great that niobium may easily combine with these chemical elements to form carbides and nitrides.
4. The refining phenomenon of austenite-grains is due to the existence of niobium carbides and nitrides in steel material, and the elevation of the coarsening temperature of austenite-grains is the outcome of their growth being restrained by these compounds.
It is due to the existence of various oxides containing niobium precipitated in the molten steel or during the process of solidification that the cast structure of steel becomes refined.
5. By an addition of niobium, the yield point and Charpy impact value of steel material are remarkably improved and the ratio of yield point to tensile strength increases.
