Analysis of w ［N］ change in converter molten steel
Nitrogen may exist in the steel as nitrogen atoms [N] in the free state or as nitrides in the bonded state. In actual production, nitrides are not formed at the steelmaking temperature, and nitrogen exists in the form of free nitrogen atoms in the molten steel.
The solubility of free nitrogen in steel follows Xihua’s law: 1 / 2N2 = ［N］ (1)
In the formula: w ［N］——mass fraction of nitrogen in molten steel;
fN — activity coefficient in nitrogen;
KN ———— equilibrium constant of reaction formula (1);
PN2 ———— Partial pressure of gas phase nitrogen in equilibrium with nitrogen in molten steel.
the solubility of nitrogen in steel increases with the increase of PN2, and the chemical composition and temperature of molten steel affect the solubility of nitrogen in molten steel through the effects of fN and KN.
For the converter process, it is to minimize the contact between the molten steel and the atmosphere. The dissolution of nitrogen in the molten steel is an endothermic reaction. As the temperature increases, the nitrogen partial pressure increases, and the solubility of nitrogen in the molten steel is also Higher
Converter smelting control w [N] process measures
Converter smelting control w [N] process measures
Load system optimization
Hot metal and scrap are the main raw materials for converter steelmaking. The ratio of the two directly affects the end point of the converter w [N]. Although w [N] in the molten iron is higher than w [N], the denitrification reaction in the converter The carbon generated by the carbon-oxygen reaction and the bottom-blown argon gas form a large number of bubbles, and the carbon content of the molten iron is significantly higher than that of the scrap steel. Therefore, increasing the molten iron ratio can effectively increase the carbon content in the molten pool, which is conducive to the large number of CO bubbles generated. It helps to eliminate [N] in the molten pool.
Therefore, through continuous adjustments in production, the ratio of molten iron loaded in smelting low-nitrogen steel was adjusted from the original 89% to about 93% now, increasing the ability of denitrification in the blowing process; at the same time, ensuring the smelting of low-nitrogen steel such as DC04 The composition and temperature of water. The molten iron [Si] is too low or too high, and steel types with strict nitrogen requirements are not smelted.
Optimization of bottom blowing mode
On the one hand, the double blowing technology can form small bubbles in the molten steel by means of bottom blowing gas, reducing the nitrogen partial pressure, and on the other hand, it can enhance the stirring force of the molten pool and help the bubbles in the molten steel to float. When smelting low nitrogen steel, the gas supply mode of bottom blowing is adjusted from the nitrogen and argon switching mode of ordinary steel types to the full argon blowing mode. In the early stage of blowing, a larger gas supply intensity is used to strengthen the agitation of the molten pool and promote the carbon-oxygen reaction; in the middle stage of the carbon-oxygen reaction, the generated CO bubbles are enough, and the bottom-blow gas supply can be appropriately reduced The strength can also achieve better denitrification effect; in the middle and late stages of the blowing, the carbon-oxygen reaction weakens, and then the bottom blowing gas supply intensity is appropriately increased, and the CO bubbles in the molten pool are timely supplemented by the bottom blowing argon to slow down the later converter smelting Nitrogen increase due to weakened carbon-oxygen reaction.
Optimization of slagging system
Reasonable addition of auxiliary materials can ensure good foam slag coverage on the molten steel surface. When adding the first batch of smelting low-nitrogen steel, properly mix the ore or iron balls to quickly melt the slag into a slag to cover the surface of the molten steel; it is not easy to concentrate too much in the subsequent feeding process to prevent the reaction from violently destroying the foamed slag This causes nitrogen absorption. When the slag is dried back, it is necessary to add ore and iron balls in time to increase the iron oxide in the slag, thereby alleviating the retrying phenomenon, and forming a uniform slag phase to cover the molten steel to avoid nitrogen absorption. Adding appropriate iron balls in the later stage of converter blowing can make the slag foam, reduce the nakedness in the fire zone, and avoid nitrogen increase, which is conducive to reducing the nitrogen content of the steel.
End point system optimization
The end point w ［C］ has a significant effect on the end point w ［N］ of molten steel. When the molten steel carbon is lowered, the carbon-oxygen reaction slows down, the CO partial pressure drops sharply, the pressure difference at the furnace mouth decreases, and air is easily drawn into the molten steel Absorb nitrogen. Under the condition that the bottom-blowing is effective, the carbon-oxygen reaction can proceed more thoroughly due to the argon stirring of the bottom-blowing, which delays the occurrence of nitrogen absorption in the molten steel, and the nitrogen absorption by the molten steel is obvious when the bottom-blowing fails. When smelting low-nitrogen steels, on the basis of ensuring the temperature and w [O] in the steel, it is not appropriate to control the end point w [C] of the converter too low, which can reduce the occurrence of nitrogen increase. During the point-blow process, w [C] in the molten pool is generally low, the CO partial pressure drops sharply, and the pressure difference at the furnace mouth decreases. Therefore, the point-blow process has a more serious impact on the nitrogen addition of molten steel. In the case of point blowing, the bottom blowing mode selects a larger gas supply intensity to suppress the nitrogen increase phenomenon due to insufficient CO bubbles; the point blowing time should not be greater than 1 min and the number of times should not be greater than 1 to minimize the increase in the point blowing process. The occurrence of nitrogen.
Optimization of deoxidation and alloying in tapping
In the process of converter tapping, molten steel is in direct contact with air. The length of tapping time has a direct relationship with w [N] in the molten steel. As the tapping time increases, w [N] in the molten steel also continues to increase. If the state of the tap hole is not good, the phenomenon of scattered flow will increase the contact area of the molten steel and air, which will increase the nitrogen increase of the molten steel. Therefore, when smelting low nitrogen steel, try to choose a converter with a good tap state for smelting. To ensure that the tapping does not diverge and reduce the nitrogen increase during tapping.
Deoxidizing alloying process
When smelting ultra-low carbon steel, on the basis of ensuring refining w [O], tapping adopts weak deoxidation or non-deoxidation. If deoxidation of converter steel requires deoxidation, weak coke is used. On the one hand, the deoxidation products will not pollute the molten steel, and on the other hand, the CO / CO2 bubbles generated by the deoxidation will stir the molten steel and take away some steel types. [N] To achieve the purpose of further nitrogen removal.
The effect after process optimization
Through the improvement of various technological systems, w [N] of molten steel after the furnace is effectively controlled, and w [N] in the finished steel is guaranteed. Figure 2 shows the comparison of nitrogen content in molten steel after converter tapping before and after process optimization. The average nitrogen content after tapping is 29. 4 × 10 -6 reduced to 20 after process optimization. 2 × 10-6, laying a good foundation for smelting low nitrogen steel.