JPH0250165B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a decarburization/reduction refining method for chromium-containing steel. Decarburization refining methods for chromium-containing steel include the AOD method (Argon Oxygen Decarburization), in which oxygen or an inert gas such as oxygen and argon is injected from the bottom into molten steel in a refractory-lined refining furnace, and similar furnaces. The converter method involves blowing oxygen or an inert gas such as oxygen and argon into the molten steel from above or from the bottom of the furnace;
VOD that blows inert gas such as argon from below
(Vacuum Oxygen Decarburization), etc., but the biggest challenge with any of these methods is how to reduce the cost of refractories and inert gas. The details of the present invention will be explained below using the AOD method as an example. As shown in Figure 1, in the AOD method, a mixture of oxygen gas and inert gas is blown into the tuyere 3 attached to the bottom side wall of the converter 2 containing the molten steel 1, so the energy for stirring the molten steel is extremely large. . For this reason, the surface of the refractories that come into contact with the molten steel is violently washed by the high-temperature molten steel, so that the rate of wear and tear is extremely high.
Although the quality of refractories has improved in recent years and the operating method of the AOD method has made significant progress, it is still far from reaching the 1,000-cycle lifespan of ordinary steel converters. In order to extend the life of the furnace as much as possible, it is necessary to shorten the one-heat refining time as much as possible, and to shorten the period during which refractories are most likely to be eroded during the one-heat refining process. Figure 2 shows the brick erosion rate during refining of stainless steel, taking as an example the case where MgO-Cr ₂ O ₃ based bricks are used. Each plot in the figure is an average value for one furnace. There is a strong correlation between refining time and brick erosion rate. For example, when the brick erosion rate is 2.5mm/ch
So, if the time is reduced by 10%, the brick erosion rate will be approximately
It is estimated that it will be 1.9 mm/ch, and if a 600 mm long brick is used, the service life can be expected to be significantly improved from 240 times to more than 300 times. In addition, as a method to know the amount of refractories that have been eroded, for example, in AOD furnaces,
Since MgO-Cr ₂ O ₃ or MgO-CaO-based refractories are used, the slag in each process of refining is
All you have to do is find the amount of increase in MgO. Figure 3 shows the amount of refractory erosion in each refining process using this method. As is clear from this figure, the amount of MgO in the slag increases significantly during the reduction period and desulfurization period after the completion of decarburization, and a significant increase in the amount of refractory erosion is observed. Therefore, it is expected that the erosion of refractories will be significantly reduced if the reductive desulfurization period is shortened as much as possible, and at the same time, by shortening the reductive desulfurization period, it can be expected that the amount of inert gas blown into the refractory will be significantly reduced. Based on the above idea, the inventor has repeatedly conducted various experiments, and as a result, the reductive desulfurization period can be significantly shortened, the life of the furnace body can be significantly extended, the refractory cost can be significantly reduced, and the efficiency can be improved by shortening the reductive desulfurization time. We have developed a method for refining chromium-containing steel that allows for improved performance and a significant reduction in the amount of blown gas. In order to achieve this objective, the present inventor first added a reducing agent and a slag forming agent after the decarburization was completed, and at the same time, inert gas was blown into the steel, and the steel was tapped within 3 minutes after the addition of the reducing agent. The main subject of the invention was made and filed as Japanese Patent Application No. 56-120069 (see Japanese Patent Publication No. 63-54045), but the present invention aims to further improve the invention and further increase efficiency and reduce costs. It is something to do. In the decarburization and reduction refining of chromium-containing steel, the present invention involves adding 50% or more of the slag forming agent in the final stage of decarburization, adding the remaining slag forming agent and reducing agent after the decarburization is completed, and adding an inert gas. The gist is to inject steel into the steel and tap the steel within 3 minutes after adding the reducing agent. The present invention will be explained in detail below based on an example in which the present invention is applied to the AOD method. FIG. 4 shows an example in which the method of the present invention is applied to an AOD method in comparison with a conventional AOD refining method. Figure 4a shows the conventional AOD refining method, in which decarburization is performed in four stages starting from a charging carbon content of 1.8%. In the AOD method, in order to efficiently decarburize the carbon in steel without oxidizing chromium,
The ratio of oxygen and argon injected is changed depending on the carbon concentration in the steel. In the example shown in Fig. 4a, the first stage of decarburization is a range of carbon concentration in the steel of 1.8 to 0.5%, and the oxygen-argon ratio O ₂ /Ar = 4/1, and the second stage is a range of carbon concentration of 0.5 to 0.25%. O ₂ /Ar = 2/1, O ₂ /Ar in the range of 0.25 to 0.12% carbon concentration as the third stage
= 1/2, carbon concentration as the 4th stage (final stage of decarburization)
An example of decarburizing the range of 0.12 to 0.06% with O ₂ /Ar=1/3 is shown. In addition, after the decarburization was completed, a reducing agent and a slag-forming agent were added immediately while blowing inert gas to recover the chromium oxidized during the decarburization process and to perform desulfurization, and the mixture was sufficiently stirred to complete the reduction reaction. Later, if necessary, the slag is removed, desulfurized again, the composition is adjusted, and the steel is tapped. Note that during the decarburization period, oxygen reacts with Cr to generate Cr ₂ O ₃ , a part of which migrates into the slag. Since the temperature of the molten steel rises during this process, a coolant is added to prevent the temperature from becoming higher than necessary. Ordinary steel scraps and stainless steel scraps of the same type are used as coolants. In addition, since Si in the raw material oxidizes to become SiO ₂ , CaO is added to maintain appropriate slag basicity during the decarburization process.
Fe-Si is used as the reducing agent added during the reduction period, and CaO and CaF ₂ are used as the slag forming agent. In an example of such a conventional method, it takes about 45 minutes for the oxidation stage and 10 to 15 minutes for the reduction/desulfurization stage. Figure 4b shows the outline of the method of the previously filed Japanese Patent Application No. 120069/1986.The decarburization period is the same as the conventional method, but the reducing agent and slag are added during the reduction and desulfurization period. The steel is characterized by being tapped within 3 minutes from the start of addition of the additive. Generally, there is no clear evidence regarding the time required for reduction and desulfurization, but in the case of the AOD method,
Although a time of 5 minutes or more is required, the present inventor has confirmed that 3 minutes is sufficient for the reduction and desulfurization time in the furnace by using stirring during tapping. . In this way, by tapping the steel within 3 minutes after the completion of decarburization, the reductive desulfurization period, which conventionally required 10 to 15 minutes, can be significantly shortened. , it becomes possible to significantly improve production capacity. However, in this method, the steel is tapped after a short period of stirring after adding the slag-forming agent.
If moisture is brought in together with the slag-forming agents CaO and CaF ₂ , it will remain in the steel as hydrogen, so it is necessary to dry the slag-forming agent sufficiently beforehand. Slag forming agents are highly hygroscopic, making it difficult to control moisture at production sites. Figure 4c shows the method of the invention. That is, this is a method in which 50% or more of the slag forming agent to be added is divided into the final stage of decarburization, that is, the fourth stage. The slag forming agent added in the fourth stage is dried and dehydrated by the heat of the molten steel, so even if the reducing agent and the remaining slag forming agent are added after decarburization and the steel is tapped within 3 minutes, the slag forming agent is There is no problem with hydrogen remaining in the molten steel. However, it has been found that if the amount of slag-forming agent added in the final stage of decarburization is too large, the recovery rate of chromium decreases in short-time reduction as in the method of the present invention. The reason for this is that when a large amount of slag forming agent is added in the final stage of decarburization,
It was found that this is because the basicity of the slag at the end of decarburization becomes high, so the melting point of the slag becomes high, and the reduction rate after adding the reducing agent becomes slow. Figure 5 shows the relationship between slag basicity and chromium yield at the end of decarburization. From the same figure, the basicity of the slag at the end of decarburization is 5.
It can be seen that the recovery rate of chromium can be stabilized at a high level by setting the following conditions. FIG. 4d shows a preferred embodiment of the present invention based on these findings. That is, in the final stage of decarburization (fourth stage), only inert gas is blown in without oxygen gas, and at the same time, the reducing agent (Fe −
This method is characterized by adding Si). First, the decarburization method in which only inert gas is blown will be briefly explained. In the example shown in FIG. 4d, the amount of oxygen blown in the final stage (fourth stage) of decarburization is set to zero, and only inert gas is blown in to perform decarburization. Oxygen sources necessary for decarburization at this time include chromic acid (Cr ₂ O ₃ ), iron oxide ( FeO), manganese oxide (MnO), etc. are used.
These oxides are reduced by the carbon in the steel by vigorous stirring with inert gas injection. The chemical formula for this is as follows: (Cr ₂ O ₃ ) + 3 [C] → 3CO↑ + 2 [Cr]
...(1) (FeO) + [C] → CO↑+ [Fe] ... (2) (MnO) + [C] → CO↑ + [Mn] ... (3) During this reduction reaction, Fe− Adding Si (1), (2),
The following reaction is caused simultaneously with reaction (3). 2 (Cr ₂ O ₃ ) + 3 [Si] → 3 (SiO ₂ ) + 4Cr ... (4) 2 (FeO) + [Si] → (SiO ₂ ) + 2Fe ... (5) 2 (MnO) + [Si] → (SiO ₂ ) + 2Mn ...(6) (SiO ₂ ) generated by equations (4), (5), and (6) is removed from the final stage of decarburization (fourth stage, in this case, inert gas decarburization). period)
It has the effect of lowering the basicity of the slag-forming agent added to it, avoiding a highly basic state and facilitating reductive desulfurization. In addition, the melting point of the slag is lowered and its fluidity is increased, making stirring of the slag and molten steel more efficient, further improving the decarburization efficiency. Furthermore, as shown in Figure 4a, the method of the present invention is characterized in that all the slag is tapped in the ladle without being discharged before tapping, so the entire amount of slag must be stored in the ladle. No. For this purpose, it is desirable to reduce the amount of slag as much as possible. As a method for this, first of all, the amount of (Al ₂ O ₃ ) in the slag after reduction is 5 to 40%.
It is desirable to add a source of Al ₂ O ₃ (eg metallic aluminum). This results in CaO・SiO ₂・Al ₂ O ₃
(MgO)-based slag has significantly improved fluidity, and short-time stirring makes it possible to sufficiently reduce the slag, while reducing the amount of fluorite (CaF ₂ ) added to ensure fluidity. I can do it. When metal Al is added at the same time, a reduction reaction occurs as follows: (Cr ₂ O ₃ ) + 2 [Al] → (Al ₂ O ₃ ) + 2 [Cr] ... (7), while when metal Si is added, (Cr ₂ O ₃ ) + 3/2Si → 3/2 (SiO ₂ ) + 2 [Cr]...(
8) causes the reduction reaction. The amount of slag produced at this time is 1.5 moles of (SiO ₂ ) per 1 mole of (Al ₂ O ₃ ), and the amount of slag produced in the reduction with metallic aluminum is smaller than that in the reduction with metallic Si. By adding metal Al as described above, sufficient fluidity of the slag can be ensured and the amount of slag can be reduced, an example of which is shown in FIG. 4d. Next, it is desirable to minimize the oxidation of chromium during the decarburization stage, and for this purpose, as shown in Figure 4e, 0.25% of chromium is
to 0.18% C), it is desirable to perform decarburization by blowing only inert gas. By the end of the early stage of decarburization (second stage), there are already sufficient oxides (Cr ₂ O ₃ ) (FeO) (MnO) in the slag, so the The decarburization rate due to inert gas decarburization during this period is almost the same as that during normal O ₂ /Ar = 1/2 blowing, and it is possible to decarburize efficiently. By using this method, it is possible to reduce the amount of slag forming agent (CaO) that needs to be added in the final stage (fourth stage) of decarburization, and the basicity of slag at the end of decarburization can be controlled to 5 or less. It becomes easier to do so.
At the same time, it is also possible to reduce the amount of chromic acid to be reduced, and the reduction can be carried out sufficiently in a short time. In the example in Figure 4e, 0.18%C is used to avoid the temperature drop during inert gas decarburization in the middle stage of decarburization.
Oxygen blowing decarburization is performed from 0.12%C to 0.12%C using O ₂ /Ar=1/2. In this way, decarburizing by injecting only inert gas in the middle stage of decarburization is particularly suitable when refining extremely low carbon materials (for example, SUS304L with C≦0.03%). In other words, in the case of extremely low carbon materials, it is necessary to decarburize the carbon in the steel to 0.03% or less as shown in Figure 4f, and the carbon content in the steel must be decarburized to 0.06% or less as shown in Figure 4a. The amount of chromic acid generated in the final stage of decarburization increases significantly, the amount of slag forming agent that must be added in the final stage of decarburization is large, and the amount of chromic acid that must be reduced is large. It becomes difficult to sufficiently perform reductive desulfurization by short-time stirring, which is a feature of the present invention. In order to solve these problems, decarburization is carried out by injecting only inert gas in the middle stage of decarburization to minimize the amount of chromic acid produced, and Fe-Si is added in the final stage of decarburization to increase basicity. Control is desirable.
In addition, the (Al ₂ O ₃ )% in the slag after reduction is 5 to 40%.
% after decarburization. This method is illustrated in Figure 4g. Of course, if there is enough room in the molten steel temperature, argon decarburization can be carried out even further in the high coal region, and good results can be obtained. Each of the methods explained above in Figure 4 a to g is applied to SUS304
Examples applied to AOD refining of stainless steel are shown in Tables 1 and 2.

【table】

[Table] As described above, the present inventor provides a decarburization/refining method that can significantly shorten the reductive desulfurization period by controlling the basicity of slag after decarburization using various methods. It is something. Industrial application of the present invention brings about the following remarkable effects. In other words, per unit of refractory material
It is possible to improve the conventional method by 50%, efficiency by 15%, and gas consumption by 20%. It should be added that in either method, maintaining a device that can finely adjust the composition after AOD tapping makes it possible to carry out the present invention even more effectively and economically. .

[Brief explanation of the drawing]

Figure 1 is an explanatory diagram of the AOD method, and Figure 2 shows the degree of brick erosion during stainless steel refining with MgO−
Figure ₃ shows the amount of refractory erosion in the stainless steel smelting process. Figure ₄ shows an example of applying the present invention to the AOD method. In the diagram shown in comparison with the AOD refining method of
A diagram showing an overview of the method of No. 120069, c is a diagram showing the method of the present invention, d to g are diagrams showing other embodiments of the present invention, and Figure 5 is a diagram showing the slag basicity and chromium yield at the end of decarburization. FIG.

Claims (1)

[Claims] 1. In a refining method for chromium-containing steel that involves blowing oxygen or oxygen and an inert gas into molten steel in a refractory-lined refining furnace, 50% of the slag-forming agent is added during the final stage of decarburization.
% or more, and after the completion of decarburization, the remaining slag forming agent and reducing agent are added and inert gas is blown in, and the steel is tapped within 3 minutes after adding the reducing agent. Method. 2. The method for refining chromium-containing steel according to claim 1, characterized in that the basicity of the slag at the end of decarburization is set to 5 or less. 3. The chromium-containing steel according to claim 2, characterized in that in the final stage of decarburization, the molten steel is stirred by injecting only an inert gas without injecting oxygen gas, and a part of a reducing agent is added. Refining method. 4 After decarburization, Al ₂ O ₃ in the reduced slag becomes 5
4. The method for refining chromium-containing steel according to claim 3, characterized in that the Al ₂ O ₃ source is added so that the Al 2 O 3 source becomes 40%. 5. The method for refining chromium-containing steel according to claim 4, characterized in that the molten steel is stirred by injecting only an inert gas without injecting oxygen gas during the middle stage of decarburization.