Experimental Study On Refining High Nitrogen Steels By High-pressure And Bottom-blowing Nitrogen

The type of steel whish contains more than 0.08wt% nitrogen in a ferrite matrix or more than 0.4wt% nitrogen in an austenitic matrix is defined as high nitrogen steel. Compared with other general steels, the high nitrogen steels have the nature of high strength, good tenacity, uniform corrosion resistance and local corrosion resistance (corrosive pitting, corrosion seam). Because adding nitrogen in molten steels is very difficult, so high-pressure melting and powder metallurgy have successively been developed abroad. On improving of the smelting technic and equipment, adding synthetic slag and deoxy agent increases the contact surfaces between nitrogen gas and liquid steel in the melting process, and reduces the obstacles of oxygen and sulfur, so it enhances the absorption of nitrogen.High nitrogen can be produced in a short time by using the high-pressure and bottom-blowing nitrogen technique. This technic significantly increases the nitrogen content in steel. In melting process, considering all of the influencing factors on refining high nitrogen steels in techniques, pressure is the most significant factor, and the second is subaeraton flux, and the last is the time of blowing. Under the pressure of 0.5MPa and the subaeraton flux is 0.12m3/h,0.18 m3/h and 0.24 m3/h, and the apparent mass transfer coefficient of steel is 1.690×10-3m/min,3.201×10-3m/min and 5.765×10-3m/min in 1873K. The apparent mass transfer coefficient of steel increase as the flux of blowing from bottom and it is the primary factor. The effect on the increasing of nitrogen is much better in the first 20~25min under certain pressure. The quality of the high nitrogen steels becomes better though keeping the temperature 10min at the end of the melting process.The composition of top slag makes little effect on the increasing of nitrogen. Under the pressure of 1.0Mpa, the adding of Al can remove the free oxygen in steel, and promote the absorption of nitrogen. The composition of top slag affects the effectiveness of the deoxidization. The use of 10CaF2-80Cao-10A12O3 promotes the effect on Al'deoxidization.