Steelmaking Processes

Tellurium is a famous element as a material for compound solar-cell or thermoelectronic devices, but it has been used for a long time in the field of steelmaking as an alloying element in free machining steel In order to control the concentration of tellurium in steel, it is important to know its thermodynamic properties in molten steel. However, there was almost no report about them because it was very difficult to measure them due to its high vapor pressure.

In this study, we developed a method which positively utilizes the high vapor pressure of tellurium. Thermodynamic stability of tellurium in molten iron and the effect of some other alloying elements were measured quantitatively. From these data, we can discuss, for example, the effect of temperature or concentration of other elements on a vaporazation rate of tellurium from molten steel.

For steel making process, it is very important to control oxygen concentration low in molten iron in order to achieve the strength and toughness. In general, aluminum are put into molten iron to remove oxygen as alumina. However, some of alumina remain in the steel and will decrease the strength and toughness. Adding rare earth metal can potentially improve this problem. So thermodynamic properties including both rare earth oxide and alumina have been studied.

Steel making slag is one kind of slag and made from pig iron, scrap and so on at steel making process. In Japan, about fifteen million tons of steel making slag is produced every year and these have been used for building and construction supplies. Now, more value-added uses of steel making slag are studied and the way to control slag to be more useful state is also studied. In order to condense or concentrate components of slag by simple and low-cost process, possibility of the component separation is studied by controlling the cooling condition.

Phosphorous (P) is known as one of the major impurities of steels, but it is not removed in the secondary refining process which is called the final adjustment process of steels. Light rare earth elements have strong reducing abilities and they are thought to be able to change phosphorous into phosphides. Hence, the dephosphorization in the secondary refining process is expected. Sulphur is also one of the measure impurities. Unlike phosphorous, it is removed in the secondary refining process. Still its behavior when light rare earth elements are added is yet to be revealed. We are doing basic research to clarify the desulphurization and dephosphorization abilities of light rare earth elements.