We prepared a niobium steel for glass lining use by utilizing outstanding: chemical and metallurgical properties of niobium on the basis of the results of its investigation as reported in our previous paper which was published as a part of our studies of special elements having effects on steel. As the result of our examination, we have found that as compared with titanium steel heretofore in use, niobium steel is free from difficulties in melting and accordingly it is possible to develope niobium steel having eminent properties as a material for glass lining use.
The following are the results of our study on niobium steel.
1. The surface of niobium steel ingot is much better than that of titanium steel and fatal surface defects such as sand marks, sand bites, slag bites, etc. are extremely few in it as compared with titanium steel. In the case of titanium steel, there is the tendency that surface defects will be remarkably increased with an increase of titanium content, but such a phenomenon can not be observed in the case of niobium steel. Accordinglly, it is possible to produce rolled or forged materials with less surface defects for glass lining by using niobium steel instead of titanium steel.
2. Titanium is easily oxidizable and in consequence, its recovery in molten steel is extremely low. It is, therefore, rather difficult to add an exactly constant quantity of titanium to molten steel. On the other hand, niobium is not so easily oxidizable as titanium and yet its recovery in molten steel is high;for example, it is more than 90% in killed steel and in most cases niobium can be added to molten steel in an almost perfectly constant quantity.
3. Most of oxide inclusions found in titanium steel are crystalline titanium oxides and such oxide inclusions have a tendency to increase with an increase of the quantity of titanium contained in steel. On the contrary, oxides found in niobium steel are glassy compounds composed of SiO2 and Al2O3, and niobium oxides are comparatively scarce.
4. With an increase of titanium addition the cleanliness of steel generally tends to be lost, but such a phenomenon can not happen in the case of niobium steel.
5. By adding titanium or niobium to steel, austenitic grain of steel is refined very much, but in either case of titanium or niobium, this grain tends to become coarsened when the quantity of titanium or niobium contained in steel is increased over a certain extent.
6. By adding titanium or niobium to steel, the point of transformation of steel has a tendency to rise and the coefficient of thermal expansion of steel has a tendency to fall. The coefficient of thermal expansion at 100 to 300°C is a little greater in niobium steel than in titanium steel.
7. As to the blister phenomenon in the surface of the glass coating of titanium or niobium steel material in its firing process, it has a tendency to decrease as the ratio of Ti/C or Nb/C in steel increases, and this tendency is very remarkable in niobium steel.
8. Regarding the adherence of the fire-coated glass to the surface of the test piece material of steel at normal temperature, it is the best when steel contains 0.25% titanium or 0.44 % niobium there in. It has, however, a tendency to become worse if the quantity of titanium or niobium contained in steel is increased over the aforesaid percentage.
9. Niobium steel is much better than titanium steel in respect of the thermal shock resistance at 200°C and 220°C of the glass coating on the surface of the test piece material of steel.