The bottom ventilating element of converter is used to supply AR or N2 from the bottom of converter, which will produce high temperature and strong mixing effect during combined blowing, so the magnesia carbon brick with high temperature resistance, erosion resistance, wear resistance and thermal shock resistance is used.
The main physical and chemical indexes are:
Mg ≥ 76%, Al2O3 ≤ 0.4%, SiO2 ≤ 0.6%, C10% ~ 15%, bulk density ≥ 2.9g/cm3, normal temperature compressive strength (110 ℃, 24h) ≥ 30MPa, high temperature flexural strength (1500 ℃, 3H) ≥ 11mpa, apparent porosity ≤ 3%.
The reasonable selection of the structure and form design parameters of converter bottom blowing gas supply elements is the decisive factor for the success of combined blowing technology. According to the structure, the gas supply components for converter bottom blowing can be roughly divided into nozzle type gas supply components and brick type gas supply components.
1. Nozzle type air supply element:
Inspired by the oxygen lance nozzle used for converter top blowing, a series of nozzle type gas supply components have been developed successively: single tube double layer casing annular slot type (also divided into single annular slot type and double annular slot type).
The first use of single pipe air supply components. From the point of view of single stirring, the stirring gas is blown into the molten metal pool through a single tube, and the stirring effect is better.
However, due to the cooling effect of the gas, the refractory at the bottom of the furnace contacts with the molten steel, resulting in local cooling and congealing into an umbrella, which often leads to the sticking of the nozzle and the congealing and blocking of the molten steel. Then the double casing was developed on the basis of single pipe.
Oxygen is blown from the inner pipe of the double-layer casing nozzle, and protective gas is blown from the annular gap between the inner and outer pipes. The double casing nozzle can effectively avoid the blockage problem similar to the single pipe nozzle.
But it also has its own shortcomings:
Due to the large bubbles coming out of the inner tube, which produce a great counter impact force and seriously damage the refractory around the nozzle, and the limited range of gas adjustment, it is difficult to adjust it in a large range according to the smelting requirements, especially in the smelting of high carbon steel, for the steel with carbon content of 0.04% ~ 0.4%, it is required that the gas supply element has a gas adjustment of 0.05 ~ 0.1m3 / (min · T), which is difficult to reach the double-layer casing nozzle. In order to eliminate this structural defect, metallurgical workers have developed the annular slot nozzle.
The annular nozzle is divided into single annular nozzle and double annular nozzle
(1) The width of single circular seam pipe nozzle is generally 0.5-5.0mm, and the best control width is less than 2mm. The ratio of the maximum flow rate to the minimum flow rate is 3 ~ 5, which is suitable for the nozzle flow control. It can stably control the gas flow rate in a large range.
Different from the inner tube of double-layer casing, the inner cavity of the inner tube of the nozzle is filled with refractory material, and only the gas is blown through the circular seam to make the bubble smaller, so as to reduce the recoil force of the bubble. Therefore, the loss of refractories around the nozzle is reduced.
(2) Double annular slot type nozzle, which can blow different gases through two annular slots. If oxygen is blown into the inner ring seam, inert gas or cooling gas is blown into the outer ring seam. When operating at low flow rates, the pressure of the blown gas is stable.
The inner ring gap is blown into O2 to react with C in the molten pool to generate Co, i.e. O2 + 2C → 2CO. It can be seen from the reaction formula that the gas volume increases significantly, that is, the stirring force increases significantly. This structure is widely used in production because of its wide range of gas flow regulation (10 times of gas flow), good mixing effect and small erosion of nozzle.
As long as the concentricity of the double casing is maintained, the refractory life of the nozzle will be long, but it is very difficult to maintain the concentricity of the double casing in practice. Because of the uneven girth, it is difficult to stabilize the air flow, so the brick type air supply element was developed later.
2. Brick type air supply element:
The development route of brick type air supply element is the dispersion type brick joint combination type air supply element with multi pass. Due to the poor erosion and erosion resistance of dispersion type, low life and durability, and the brick joint combination type is easy to crack, at present, the converter ventilation element is mainly through pass type.
10-150 fine stainless steel pipes or heat-resistant steel pipes are set in the straight through brick, and the diameter of single pipe is 0.5-3mm. The stainless steel pipe heads are combined and inserted in the high-pressure box under the brick, with a centralized air supply device. The gas enters the steel water through the stainless steel pipe channel.
This structure has the following advantages:
(1) The gas flows in the metal tube with small resistance;
(2) The metal pipe is welded on the metal air chamber, with good air tightness;
(3) The metal pipe can strengthen the surrounding refractories, so that the air supply components are not easy to peel and crack;
(4) The gas flowing in the metal tube can cool and protect the refractory;
(5) Refractories protect the metal in direct contact with molten steel;
(6) The range of gas volume regulation is large, up to more than 10 times;
(7) The safety is good. Even in the extreme condition of gas cut-off, the molten steel will not be filled to the end, but solidified in the upper part of the air chamber.
The design idea of this kind of gas permeable element is to select the appropriate stainless steel pipe diameter and number of pipes, to meet the maximum and minimum mixing strength required by the process within the pressure range provided by the gas source, and to avoid the phenomenon of steel pouring back.
At present, the most typical gas supply elements used in converter bottom blowing are the fine metal pipe through hole type gas supply element and the double ring gap gas supply element with two-layer metal intermediate seal.