STEEL-MAKING PROCESS IN CONVERTER

Abstract: A steel-making process in a converter is disclosed, which comprises using a carbon monoxide gas as an agitating gas supplied from a position lower than molten metal bath level.

Description: This invention relates to steel-making, In general, an oxygen top-blown converter (LD process) has been used for obtaining molten steel by decarburizing hot metal and simultaneously reducing impurity elements such as P, S and the like in the hot metal. In this type of converter, a bottom blowing process with an inert gas has recently become popular for the enhancement of the refining properties. It is known that the bottom blowing of the inert gas is largely effective for improving the agitation of the molten steel bath in the converter as well as for reducing impurities, increasing the iron yield, improving the control accuracy between the components and the temperature at the end of blowing, and the like. Heretofore, argon (Ar), nitrogen (N₂) or carbon dioxide (CO₂) has been used as an agitating gas. Ar gas is completely inert to molten steel, and so is particularly suitable for the above mentioned object but is expensive. On the other hand, N₂ gas is cheaper than Ar gas but dissolves into the molten steel so that the nitrogen concentration in the molten steel is increased during the blowing in the converter and this may cause problems with the quality of the steel. CO₂ gas is able to reduce the amount of top-blown O₂ gas by an amount corresponding to the decarburization reaction CO₂ + C →2CO occurring when the carbon concentration in the molten steel is high, and is relatively low in cost. Thus the use of CO₂ gas is economically advantageous. However, CO₂ gas is an oxidizing gas (and hence is different to Ar and N₂) so that the service life of the tuyere or porous plug used for blowing CO₂ gas into molten steel becomes shorter. Besides the above gases, it has been proposed to use an unburned exhaust gas recovered from the converter or a so-called LD gas as a cheap agitation gas. However, such an LD gas contains fairly great amounts of N₂ gas and CO₂ gas, which causes problems with the service life of the tuyeres and increases the nitrogen concentration in the molten steel as mentioned above so the use of LD gas is not yet practised. It is an object of the present invention to eliminate the aforementioned problems arising with the conventional agitating gases and to provide a steel-making process in a converter in which the tuyere used for the gas blowing and the porous bricks of the converter are subjected to reduced wear without causing problems with the cost and the quality of the molten steel. In Patent Abstracts of Japan, Vol 7, No. 97 (C-163)(1242) of 23rd April 1983 there is a reference to JP-A-58 22 315 which describes a steel refining process wherein carbon monoxide gas, generated during the maximum decarburization period, is drawn off from the inner mid portion of an oxygen converter and then blown back into the molten steel bath from the converter bottom. However the amount of carbon monoxide gas re-introduced into the bath in this way is very small and is insufficient to prolong the life of the inlet port for the gas. EP-A1-0107609 describes a process in which some of the carbon monoxide gas from the converter waste gas may, together with a carbonaceous material, be supplied to the bottom of the bath. However, the amount of carbon monoxide supplied is too small and the content of impurities is too large to prolong the life of the inlet port for the gas. According to the present invention, there is provided a steel making process in which oxygen gas is supplied to a bath of molten steel from a position higher than the molten steel bath level and an agitating gas comprising carbon monoxide is supplied to the bath from an inlet port at a position lower than the molten steel bath level to agitate the bath characterised in that the agitating gas is supplied in an amount of from 0.05 to 0.25 Nm³/min per ton of steel in the bath and in that the agitating gas contains not less than 80% by volume of carbon monoxide gas. In an embodiment of the invention the agitating gas contains not more than 10% by volume of hydrogen gas. In another embodiment of the invention, the agitating gas contains not more than 15% by volume of nitrogen gas. In a further embodiment of the invention, the agitating gas contains not more than 20% by volume, and preferably not more than 10% by volume of carbon dioxide gas. By using carbon monoxide, which is cheap and effective, to effectively agitate molten steel in accordance with the invention, the service life of the tuyeres can be improved without damaging the quality of the molten steel. For a better understanding of the invention and to show how the invention may be carried into effect, reference will now be made by way of example to the accompanying drawing which is a graph showing the influence of the CO concentration on the wearing loss rate of the tuyere. The inventors have made studies with respect to the agitating gas used in the steel-making process and have found that the durability of gas inlet ports such as tuyeres, porous plugs and the like can be considerably enhanced without causing problems with the quality of the molten steel by using cheap carbon monoxide gas as the agitating gas. In this connection, the invention will be described in detail with reference to the following experiment. First, a tuyere made of a stainless steel tube with an inner diameter of 3 mm and having a structure capable of blowing an inert gas such as CO gas or others was arranged in the bottom of a converter of 5 t capacity made from magnesia carbon bricks, and about 5 tons of hot metal (e.g. C=4.2%, Si=0.28%, Mn=0.36%) of 1,200-1,240°C was charged into the converter. Then, O₂ gas was supplied at a rate of 15 Nm³/min from a top-blowing lance, while an agitating gas was supplied from the bottom through the tuyere. After the blowing had been repeated until the molten steel was decarburized to C=0.03-0.07%, the wearing loss rate of the tuyere as a result of the blowing was measured to determine the state of wear of the tuyere and the bottom bricks surrounding it. In this case, the temperature of the molten steel was set to 1,630-1,670°C at the end of blowing. As the agitating gas, each of Ar gas, N₂ gas, CO₂ gas and CO gas was used at a rate of 0.05-0.25 Nm³/min per ton of hot metal. The experiment using each agitating gas was carried out with 10 charges and thereafter the average wearing loss rate was determined. It was found that the wearing loss rate was 1.1-2.5 mm/charge when using Ar gas and N₂ gas, 1.9-3.8 mm/charge when using CO₂ gas, and 0.4-1.6 mm/charge when using CO gas from which it is obvious that the use of CO gas is particularly effective for preventing the wearing loss of the tuyere. High-purity CO gas is usually produced by thermal decomposition of formic acid, while low-purity CO gas is produced by partial oxidation of asphalt or pitch, or the like. However, the former is undesirably expensive, while the latter exhibits a close relationship between purity and cost. Now, the inventors have made studies with respect to the acceptable limit on the concentrations of H₂ gas, CO₂ gas and N₂ gas contained as a main impurity in CO gas. As a result of experiments using CO gas with a purity of 90-100% obtained by adding H₂ gas in an amount of up to 10% to CO gas, it became apparent that such an addition of H₂ gas hardly affects the service life of the tuyere. However, when the concentration of H₂ gas in the CO gas exceeds 10%, the wearing loss rate of the tuyere increases and an increase in the H concentration of the molten steel is observed at the blowing end, so that the addition of more than 10% of H₂ gas becomes disadvantageous with regard to the quality of the molten steel. As to the concentration of CO₂ gas, experiments were carried out in the same manner as described above using CO gas with a purity of 70-100% obtained by adding CO₂ gas to CO gas. When the concentration of CO is not less than 80%, as shown in the drawing, it is apparent that the wearing loss rate of the tuyere is less than 0.9-2.4 mm/charge, which represents a degree of protection of the tuyere equal to or greater than that obtained using Ar or N₂ gas (wearing loss rate: 1.1-2.5 mm/charge). The effect is more improved at a CO concentration of not less than 90%. Further, the influence of N₂ concentration on the service life of tuyere was examined in the same manner as previously mentioned using CO gas with a purity of 50-100% obtained by adding N₂ gas to CO gas. From this it is apparent that there is a tendency for the wearing loss of the tuyere to increase as the CO concentration decreases, but the wearing loss was small as compared with the case where 100% N₂ gas is used. Further, an increase in the N concentration in the molten steel was observed at the blowing end together with an increase in the N₂ concentration in the CO gas which causes problem in regard to the quality of the steel product. In this connection, it has been confirmed that when the blowing amount of the agitating gas per ton of hot metal is within a range of 0.05-0.25 Nm³/min, it is necessary to limit the N₂ concentration in the CO gas to not more than 15% in order to restrict the N concentration in the molten steel at the blowing end to not more than 20 ppm which causes no problems with product quality. As described above, it is apparent that the use of an agitating gas consisting mainly of CO is excellent for preventing the wearing loss of the tuyere of the converter. This is considered to be due to the fact that the CO gas acts to prevent the oxidation of the bricks surrounding the tuyere by FeO since CO gas is not merely an inert gas but also a reducing gas as is well-known. Also, it is considered that a part of the CO gas dissolves into the molten steel as C and O according to the reaction of CO (g) → C + O produced in the vicinity of the tuyere, during which the tuyere is cooled because the reaction is endothermic. Although the invention has been mainly explained with respect to a steel-making process in a converter, it will be apparent that when the invention is applied to various ladle-refining processes in which a gas is blown into molten steel for accelerating the reaction, damage to the gas blowing tuyere or the porous plug can effectively be avoided. According to the invention, the durability of the gas blowing means can be considerably be enhanced by agitating the molten metal with CO gas of a purity which can be obtained relatively cheaply without adversely affecting the product quality.

Claims: A steel making process in which oxygen gas is supplied to a bath of molten steel from a position higher than the molten steel bath level and an agitating gas comprising carbon monoxide is supplied to the bath from an inlet port at a position lower than the molten steel bath level to agitate the bath characterised in that the agitating gas is supplied in an amount of from 0.05 to 0.25 Nm³/min per ton of steel in the bath and in that the agitating gas contains not less than 80% by volume of carbon monoxide gas. A process according to claim 1 wherein the agitating gas contains not more than 10% by volume of hydrogen gas. A process according to claim 1 or 2 wherein the agitating gas contains not more than 15% by volume of nitrogen gas. A process according to claim 1, 2 or 3 wherein the agitating gas contains not more than 20% by volume of carbon dioxide gas. A process according to claim 4 wherein the agitating gas contains not more than 10% by volume of carbon dioxide gas.

Inventors: FUJII, TETSUYA C/O TECHNICAL RESEARCH DIVISION, HABU, YASUHIRO C/O TECHNICAL RESEARCH DIVISION, IMAI, TAKUO MIZUSHIMA WORKS KAWASAKI STEEL CORP., NAKAMURA, HIDEO C/O TECHNICAL RESEARCH DIVISION, SAKURAYA, TOSHIKAZU C/O TECHNICAL RESEARCH DIV.
Applicants: KAWASAKI STEEL CORP
Application year: 1985
Main classification: C21C5
IPC codes: C21C5