JPH0361721B2 - - Google Patents

Description

[Detailed description of the invention]

Industrial Application Field The present invention provides a method for efficiently carrying out the desulfurization reaction while preventing erosion of the refractory when molten steel is desulfurized using a reaction tank lined with a basic refractory containing MgO. The present invention relates to a treatment method, and moreover, to a method for efficiently reducing inclusions in molten steel at the same time. Conventional technology Recently, research has focused on the hydrogen-induced cracking resistance of line pipe materials and the lamellar tear characteristics of marine structural materials.
Requirements regarding steel properties have become significantly more stringent. In order to meet these demands, it is essential to reduce the content of sulfur (hereinafter referred to as S) in molten steel as much as possible, and also to reduce the content of gas components such as nitrogen (hereinafter referred to as N) and hydrogen (hereinafter referred to as H). ), the amount of oxide inclusions must also be reduced. Molten iron desulfurization methods are broadly divided into molten iron desulfurization methods performed at the hot metal stage and molten steel desulfurization methods performed at the molten steel stage. It is necessary to combine the processing of both hot metal and molten steel. Conventionally, a well-known molten steel desulfurization method is an injection method in which a CaO-based composite mixture or a Ca alloy is injected into molten steel in a ladle together with a carrier gas. This method is characterized by desulfurization by a strong slag-metal reaction formed by strong stirring by injection. However, in desulfurization treatment that mainly involves a slag-metal reaction under strong stirring like this method,
During the treatment, there are problems such as rephosphorization from the dephosphorization slag, a decrease in the yield of addition of alloys such as Al, and a relatively large temperature drop. In addition, due to the turbulence on the molten steel surface caused by strong stirring, absorption of gaseous components - especially H, N, etc. - from the atmosphere and slag into the molten steel is unavoidable. In order to produce steel types that require simultaneous reduction of N and N, further degassing processes such as RH or DH are required. An increase in the amount of molten steel processed is unavoidable, such as the need for superheating, and furthermore, quality is often adversely affected. As a means to solve these problems, a treatment method has recently been developed in which a desulfurizing agent is blown into the molten steel together with a carrier gas while the molten steel is being lifted vertically into an empty tank, thereby simultaneously degassing and desulfurizing the molten steel. Among them, like JP-A-60-59011,
In a desulfurization method in which the desulfurization agent is added to complete the main desulfurization reaction in the bath without substantially stirring or fluidizing the slag on the bath surface, at least a weight concentration of at least % by weight with good slag property is added. , preferably 40%
By using a treatment agent that contains CaF ₂ and the remainder is CaO as a main component, it has extremely low sulfur and [N],
Molten steel with low [O] and [H] contents has been obtained with a low processing agent consumption rate. However, as a result of further research by the present inventors, it was found that the method of JP-A-60-59011 has the following problems. Problems to be Solved by the Invention (i) The processing agent used in JP-A-60-59011 has a high desulfurization ability. However, since the concentration of _CaF2 contained in the treatment agent is as high as 20% by weight or more,
Basic refractories containing MgO that are generally used as reaction vessels for molten steel, such as magnesia, magnesia carbon, magnesia chrome,
Promotes erosion of refractories made of dolomite, spinel, etc. alone or in combination. Therefore, when the frequency of melting ultra-low sulfur steel increases, the life of the reaction tank is shortened and the cost of refractories increases. (ii) When the desulfurization method of JP-A-60-59011 is applied to molten steel with a relatively large amount of inclusions, the treatment agent blown into the bath and the oxide-based inclusions in the bath aggregate and coalesce, Inclusion components such as Al ₂ O ₃ are absorbed into the processing agent. This reduces the desulfurization ability of the treatment agent, so
When obtaining ultra-low sulfur steel with [S]≦5ppm, the injection unit consumption of a treatment agent with a high CaF ₂ concentration becomes high. Therefore, as described above, the life of the reaction tank is shortened and the cost of refractories is increased. Furthermore, since this method does not substantially stir or flow the slag on the bath surface, it has the advantage of suppressing the entrainment of ladle slag and the atmosphere. however,
When a large amount of a treatment agent with a low melting point and a high CaF ₂ concentration is injected to melt ultra-low sulfur steel, the treatment agent with a high CaF ₂ concentration accumulates on the underside of the ladle slag, causing slag. The melting point of the slag drops significantly, and the top surface of the slag becomes molten. In such a case, the movement of oxygen in all the slag becomes easy, and therefore oxygen easily enters the molten steel from the slag. In addition, such molten slag is likely to be drawn into the bath by the vortices generated by the injection flow into the mold or tundish during the subsequent casting step, and can become new inclusions. Therefore, in order to efficiently perform both desulfurization and inclusion reduction,
It became necessary to develop a new method. Means for Solving the Problems The present invention solves the problems in the conventional molten steel deflow treatment described above, does not impair the [S] reduction ability of the treatment agent, and has a basic composition mainly composed of MgO. By preventing corrosion of the chemically resistant materials as much as possible,
The following means are used to economically and efficiently desulfurize molten steel. In other words, (1) When desulfurizing molten steel is performed in a reaction tank lined with a basic refractory containing MgO, the carrier gas blown into the bath floats in the bath and reaches the upper surface of the bath by reducing the pressure or It is maintained in an inert gas atmosphere and substantially free of slag, and is made of CaO, CaF ₂ , MgO, and other unavoidable components, using an inert gas as a carrier gas, with MgO being 10 to 60% by weight. ,
Weight ratio {CaF ₂ / (CaO + CaF ₂ )} x 100 = 20~
A first treatment agent consisting of 80% is blown into the bath, and before and/or after blown into the bath, a first treatment agent consisting of CaF ₂ and CaO as the main components and the remainder consisting of unavoidable components, and from the above CaF ₂ and CaO in the main component
A second treatment agent having a CaF ₂ concentration of 20% by weight or less,
A method for producing molten steel characterized by injecting an inert gas into the bath as a carrier gas, and (2) injecting it into the bath when desulfurizing molten steel in a reaction tank lined with a basic refractory containing MgO. The upper surface of the bath, where the carrier gas floats up and reaches the bath, is maintained at reduced pressure or in an inert gas atmosphere, and the inert gas is used as the carrier gas to maintain CaO, CaF ₂ , and
Consisting of MgO and other unavoidable components, MgO
10 to 60% by weight, weight ratio {CaF ₂ /(CaO+
_A first treatment agent consisting of 20% to 80% CaF ₂ A second treatment agent having a CaF ₂ concentration of 20% by weight or less in the main components consisting of CaF ₂ and CaO is blown into the bath using an inert gas as a carrier gas, and then the slag on the bath surface is blown into the bath. This is a method for treating molten steel, characterized in that an inert gas is blown into the molten steel without substantially stirring or flowing the molten steel. In the processing agent used in the present invention, impurities that are usually unavoidably contained include Al ₂ O ₃ and SiO ₂
etc., and the content of each is 3% or less in terms of weight concentration. Effect The present invention uses CaO, CaF ₂ ,
Consists of MgO and other unavoidable components,
MgO is 10-60% by weight, weight ratio is {CaF ₂ /(CaO
+CaF ₂ )}×100=20 to 80% of the desulfurization treatment agent is used. As a result, even if the desulfurization treatment agent is added to the molten steel that does not form slag on the bath surface melted in the MgO refractory container, approximately 60% of the slag is removed as shown in Figure 1A. High desulfurization rate can be obtained.
When the weight concentration of MgO is up to 60%, the desulfurization rate is only slightly lower than that without MgO. Moreover, in this range of MgO content, the deflow rate shown in JP-A-60-59011 is
_CaF2 concentration dependence is not impaired. That is, in the range of {CaF ₂ /(CaO+CaF ₂ )}×100≧40% in terms of weight ratio, a high desulfurization rate of 70% or more is shown when MgO is 60% by weight or less. Furthermore, MgO-Cr ₂ O ₃ -based refractory rods (10 mm diameter) were added to the treatment agent at a weight ratio of {CaF ₂ / (CaO + CaF ₂ )} x 100 = 20 to 80% while maintaining various MgO concentrations.
Figure 1(b) shows the maximum amount of erosion when soaked for 10 minutes. In the range of {CaF ₂ /(CaO+CaF ₂ )}×100=20 to 80%, if the weight concentration of MgO is 10% or more, the amount of erosion of the refractory becomes extremely small. In addition, in the present invention, as shown in FIG. 3, the carrier gas blown into the bath floats in the bath and reaches the upper surface of the bath, which is maintained at reduced pressure or in an inert gas atmosphere, and the slag is The above-mentioned desulfurization treatment agent is blown into the bath without being substantially present. Unlike the conventional stirring vessel for desulfurization shown in Fig. 8, this makes it possible to carry out the desulfurization reaction in an environment or atmosphere that is less affected by the slag on the top surface of the molten steel in the ladle, that is, in the molten steel bath. This eliminates the need for the strong direct reaction between molten steel, slag on the upper surface of the ladle, and slag-metal as in the conventional method, and this substantially eliminates rephosphorization during processing and rephosphorization of the processing ratio caused by slag flow and stirring. Not only is there no need for slag removal or slag reforming before treatment, which is carried out in conventional molten steel deflow methods, it is possible to significantly reduce treatment costs. In addition, since the desulfurization of the present invention completes _the main reaction in molten steel, the desulfurization of the present invention is highly efficient and reduces the basic unit of the desulfurization treatment agent, which is a concern due to the use of large amounts of CaF 2. In addition, there is no flow of the slag line, and as a result, it is reduced to the point that it is not a substantial problem. In addition, there is no need to use a basic ladle to promote desulfurization ability, and the cost of refractory materials for the ladle is greatly reduced. In order to stir and react in an environment where the molten steel is not easily affected by the slag on the upper surface of the molten steel in the ladle, as shown in FIG. Deterioration of cleanliness is less likely to occur. However, as in the case of ultra-low sulfur steel melting, where the basic unit of desulfurization treatment agent is high,
As a large amount of the desulfurization treatment agent with a high CaF ₂ concentration accumulates on the bottom surface of the slag above the bath surface, the melting point of the slag drops significantly, and when it melts to the top surface of the slag, as mentioned above, the entire slag Since the movement of oxygen in the slag becomes easier, oxygen easily enters the molten steel from the slag. In addition, such molten slag is likely to be drawn into the bath by the vortex generated by the injection flow into the mold or tundish in the subsequent casting step, and may become new inclusions. In addition, as mentioned above, when desulfurizing molten steel with relatively many inclusions, inclusions such as Al ₂ O ₃ aggregate in the desulfurization treatment agent blown into the bath, reducing the desulfurization ability of the desulfurization treatment agent. This results in a negative effect of a decrease in performance. In order to solve the above-mentioned various problems and to perform highly efficient desulfurization to the extremely low sulfur range while maintaining molten steel at a high level of cleanliness, the present invention uses the second treatment agent, i.e.
The main component is CaO−CaF ₂ , and the CaF ₂ concentration is expressed in weight% (%CaF ₂ )/{(%CaO) + (%CaF ₂ )}≦0.2
The process of injecting a treatment agent for reducing inclusions into the molten steel is carried out before and/or after the injection of the desulfurization treatment agent. By doing so, the amount of inclusions in the molten steel before and/or after the desulfurization treatment agent is injected can be efficiently reduced. In other words, Figure 2 shows the effect of the ratio of CaO and CaF ₂ in the treatment agent on the amount of inclusions absorbed into the treatment agent when CaO-CaF ₂ type treatment agent is injected into molten steel. An example of ₂ O ₃ inclusions will be shown. CaO−
For CaF ₂ -based treatment agents, (%CaF ₂ )/{(%
By setting CaO)+(% _CaF2 )}≦0.2, the amount of _{Al2O3 -} based inclusions absorbed into the processing agent can be extremely increased. In this way, once inclusions are absorbed by the treatment agent for inclusion reduction, they are formed into clusters that are difficult to float, and high melting point particles that exist in a non-spherical shape even in a static bath.
The Al ₂ O ₃ -based inclusions become molten and float to the surface as spherical inclusions that are easily observed in a static bath. In addition, FeO and FeO present in molten steel before Al deoxidation
-MnO-based inclusions and SiO ₂ -MnO-based inclusions, which have a low melting point and are observed in a spherical shape in a static bath, but have a high specific gravity and are difficult to float, have a low specific gravity. It aggregates with the treatment agent for inclusion reduction, reduces the specific gravity of inclusions, and promotes flotation. In the present invention, when the above-mentioned treatment agent is blown into the bath, the bath (molten steel) is stirred or fluidized without substantially entraining the slag on the bath surface into the bath.
By using a reduced pressure tank or an inert gas atmosphere tank in the circulation path of the reaction tank as shown in Figure 3, there is no slag on the upper surface of the bath where the blown carrier gas or floatation promoting gas floats in the bath and reaches it. The molten steel is blown into the above-mentioned molten steel under reduced pressure or an inert gas atmosphere. By doing so, it is possible to prevent the harmful effect of stirring the slag on the bath surface and generating new inclusions. Further, the injected treatment agent is interposed between the lower surface of the slag and the upper surface of the molten steel after the inclusions are absorbed. By doing this, the oxidizing slag layer on the bath surface is absorbed by the molten steel.
Blocks the slag-molten steel interface reaction that causes unnecessary oxidation of Al, Si, etc. Due to this action, the blocking of oxygen supply (oxygen intrusion) from the slag on the bath surface is achieved with a treatment agent that has an unprecedentedly low unit consumption, and the above-mentioned inclusion absorption effect of the treatment agent is achieved without any countervailing factors. It grows even higher. Due to the above-mentioned effects, the amount of clusters and non-spherical inclusions that normally exist in the bath becomes extremely small, leaving only spherical inclusions that float easily. These spherical inclusions are reduced to a level that hardly causes any trouble to the product by the time of casting. The present invention fully utilizes the effect of the treatment agent for reducing inclusions described above. Next, the case where the inclusion reduction treatment agent is injected before the desulfurization treatment agent is injected,
The effects of both when injected later will be explained in detail. _CaF2 concentration 20% by weight before adding desulfurization treatment agent
When the following CaO-CaF _2- based inclusion reduction treatment agent is added, the action of the inclusion reduction treatment agent with a CaF ₂ concentration of 20% by weight or less will reduce Al ₂ O ₃ in the bath before the desulfurization treatment agent is blown. The amount of inclusions can be reduced. Therefore, the absorption of inclusions such as Al ₂ O ₃ into the treatment agent, which is a factor that inhibits the desulfurization reaction that occurs when the desulfurization treatment agent is blown into the treatment agent, is reduced, thereby promoting the scouring effect of the desulfurization treatment agent. In addition, by injecting a treatment agent with a high melting point and a low CaF ₂ concentration and interposing it between the bottom surface of the slag and the top surface of the molten steel, the treatment agent with a high CaF ₂ concentration is deposited on the bottom surface of the slag on the bath surface. It is possible to prevent the melting point of the slag from lowering. This effect makes it possible to reduce the infiltration of oxygen from the slag into the molten steel even when a large amount of a treatment agent with a low melting point and high CaF ₂ concentration is injected. On the other hand, after adding the desulfurization treatment agent, by adding a CaO-CaF ₂ inclusion reduction treatment agent with a CaF ₂ concentration of 20% by weight or less, Al in the bath that cannot be absorbed by the desulfurization treatment agent with a high CaF ₂ concentration alone can be removed. ₂ O ₃ inclusions are removed from the second
Due to the effect of the treatment agent for reducing inclusions as shown in the figure, they can be almost completely eliminated. Further, the melting point is lowered by the desulfurization treatment agent, and an inclusion reduction treatment agent having a high melting point and a low CaF ₂ concentration can be interposed on the lower surface of the molten slag. Therefore, it is possible to reduce the entrainment of slag due to the swirling of the injection flow during the casting stage that occurs when only the desulfurization treatment agent is added, and therefore the generation of new inclusions can be suppressed. Due to the effects described above, the present invention provides excellent desulfurization effects and inclusion reduction effects, and can also reduce melting loss of refractories during desulfurization. As mentioned above, the effect of the CaO-CaF ₂ inclusion reduction treatment agent containing 20% by weight or less of CaF ₂ used in the present invention is to make inclusions that are difficult to float and separate into spheroids, or to reduce the density of inclusions into molten steel. The purpose is to make it difficult to get wet and to promote flotation and separation from the bath.
This effect works more effectively by blowing an inert gas into the molten steel after blowing in the treatment agent while substantially preventing the slag on the bath surface from being entrained. In other words, as a result of repeated studies by the present inventors, it has been found that when inert gas is blown into molten steel under conditions that do not substantially entrain slag on the bath surface, spherical inclusions, in particular, It was found that the reduction effect was remarkable. This is due to the promotion of agglomeration of spherical inclusions due to strong stirring when inert gas is blown in, and the fact that the apparent density of inclusions is reduced due to the adhesion of the blown gas to the surface and inside of spherical inclusions with a low melting point. This is because the levitation of the spherical inclusions is promoted by the reduction and other effects. The effect of this inert gas injection is small on Al ₂ O ₃ clusters. Therefore, spherical,
In order to reduce all inclusions such as Al ₂ O ₃ clusters, a CaO-CaF ₂ inclusion reduction treatment agent with a CaF ₂ concentration of 20% by weight or less is injected before and/or after the desulfurization treatment agent, and Al ₂ After the O ₃ clusters are absorbed into the treatment agent and the inclusions are spheroidized, it is necessary to carry out inert gas blowing. Alternatively, the inert gas may be injected after only the desulfurization treatment agent is injected. In this case, non-spherical Al ₂ O ₃ cluster inclusions that cannot be absorbed by the desulfurization treatment agent with a high CaF ₂ concentration remain, but the spherical inclusions are reduced by the effect of the inert gas blowing described above. Furthermore, the effect of reducing spherical inclusions by blowing inert gas can only be obtained under conditions where slag on the bath surface is not substantially entrained. For example, the 8th
In a reaction vessel as shown in the figure in which the slag on the bath surface is stirred, the slag on the bath surface is dragged in and the amount of inclusions in the bath increases, so it is difficult to obtain the effects of the present invention. Furthermore, a reaction tank (circulation path) for molten steel in the present invention
For this purpose, a reduced pressure tank or an inert gas atmosphere tank is used. This prevents oxygen from entering the molten steel from the atmosphere, which is effective in reducing inclusions. Further, since degassing and decarburization reactions proceed simultaneously, the concentrations of nitrogen, hydrogen, carbon, etc. are also reduced, which is preferable. Note that similar desulfurization and inclusion reduction effects can be obtained even if the above treatment agent is used in a reaction tank lined with a refractory that does not contain MgO. In addition, the processing agent may be a mixture of CaO, CaF ₂ , MgO, etc., a sintered product, or a pre-melt product. Examples Next, examples of the present invention are shown in Tables 1 to 10 and Table 3.
This will be explained in detail with reference to Figures A and B and Figures 4 to 7. Table 1 shows specific examples A to D of only the desulfurization treatment pattern, which is the main component of the present invention, and comparative examples E to K thereof. Table 4 shows examples of the overall configuration of the present invention, including: Reduction of inclusions - Desulfurization Reduction of inclusions - Desulfurization - Injection of inert gas only Desulfurization - Reduction of inclusions Desulfurization - Reduction of inclusions - Injection of inert gas only Examples CA to CD of processing agents and blowing conditions in processing patterns are shown. Table 7 shows other overall configuration examples of the present invention, including treatment agents and blowing conditions in the treatment pattern: inclusion reduction - desulfurization - inclusion reduction - inclusion reduction - desulfurization - inclusion reduction - inert gas only injection. implementation
Indicates AA, AB. Table 9 shows specific examples BB and BC of the treatment agent and blowing conditions in the treatment pattern of desulfurization and blowing only inert gas, as an example of a combination of the main structure of the present invention and other treatments. In Table 1, each specific example A to D uses processing equipment consisting of a ladle 1, a reaction tank (circulation path) 2, a blowing pipe 3, molten steel 7, and slag 8 shown in Figure 3 A and B. there was. The reaction tank 2 in Figure 3A is a vacuum degassing tank,
Reaction tank 2 in Figure 3B is a tank that communicates with the atmosphere through an inert gas floating in the molten steel, and the upper surface of the reaction tank 2 is kept in a sufficient inert gas atmosphere, and the slag is virtually non-existent. In the figure, 6 is a conduit, and 5 is a tapping port. Further, Comparative Examples E to J were treated using the equipment in Figures A and B and under the same treatment agent injection conditions as Examples A to D. The processing agents of Comparative Examples E, F, and G inevitably contained 2% by weight or less of AgO. Furthermore, in addition to Comparative Examples E, F, G, H, I, and J, K is a conventional example in which only degassing was performed without injecting any processing agent using the equipment shown in Figure 3 A. . The refractory material of the reaction tank is MgO.
It is a MgO−Cr ₂ O ₃ system containing 74% by weight, and other
SiO ₂ and Al ₂ O ₃ are inevitably included. This refractory is generally used in reaction vessels of molten steel processing equipment. Specific examples A, B, C, and D, Comparative example E,
For F, G, J, and K, the number of treatments was set to 100 in order to measure the amount of refractory erosion. The components of the molten steel used were: C: 0.08-0.1% by weight, Si: 0.18-0.22% by weight, Mn: 0.9-1.2% by weight, Al: 0.02-0.05% by weight.
, and remained within this range both before and after treatment. Table 2 shows an example of the results of the molten steel S concentration before and after treatment and the desulfurization rate of the present example and comparative example when the processing agent consumption rate was 2.5 to 3.0 kg/ton of molten steel. In addition, specific examples A to D and comparative examples E, F,
Table 3 shows the maximum amount of erosion of the refractory per treatment in G, J, and K of the conventional example, which were measured near the bath surface in the reaction tank after 100 treatments. As is clear from Table 2, specific examples A, B,
For C and D, the processing agent consumption rate is 2.5 to 3.0 kg/
ton of molten steel, desulfurization rate of 60% or more, reached molten steel S
A concentration of 12 ppm or less was obtained. In particular, in specific examples B, C, and D where the weight ratio is {CaF ₂ / (CaO + CaF ₂ )}×100≧40% or more, the desulfurization rate is 38% or more,
Extremely sulfurized steel with an ultimate S concentration of 3 to 5 ppm was obtained. In addition, as shown in Table 3, specific examples A, B,
For C and D, the amount of erosion of the refractory in the vacuum reaction tank per treatment was 0.4 to 0.7 mm, which was comparable to the conventional example K in which no treatment agent was injected. . On the other hand, as shown in Table 2, in Comparative Examples E, F, and G, both the desulfurization rate and the achieved S concentration were almost the same as the specific example. However, as is clear from Table 3, the comparative examples E, F, G, and J
In this case, the amount of refractory erosion per treatment was more than 1.1 mm, which resulted in the life of the refractory in the reaction tank being shortened to less than half compared to conventional K, which did not add a treatment agent. Ta. Furthermore, as shown in Table 2, the desulfurization rate is
Especially with comparative examples H and J, which were 55% or less,
In the conventional K without adding a treatment agent, the desulfurization rate is at most 10%, and the S concentration after treatment is 27 to 30 ppm.
It remained at that level. In the specific examples A to D explained above, the melting loss is the most severe.
This is an example using a reaction tank lined with a refractory containing 74% by weight of MgO, but the inventors have also
The same treatment as in this example was carried out using reaction vessels lined with refractories with MgO contents of 30% and 55% by weight, but the desulfurization rate of the inventive example exceeded 60%, and The wear rate was less than 0.3 mm per treatment, showing good results. Next, examples of the overall configuration of the present invention shown in Table 4 will be described. In Examples CA and CB, for the purpose of reducing inclusions, (%CaF ₂ )/{(%CaO) + (%CaF ₂ )}×
100=8 and 14% of CaO-CaF _2- based treatment agent is carried out in the treatment, and in the subsequent treatment, for the purpose of desulfurization, (%CaF ₂ )/{(%CaO) +
(% _CaF2 )}×100=45, and 51%, MgO from 10 to
A CaO-CaF ₂ -MgO-based treatment agent containing 20% by weight was injected. In addition, in the CC and CD of the example, (%CaF ₂ )/{(%CaO) + (%CaF ₂ )}×100=45,
and 50% CaO− containing 10–20 wt% MgO
After performing desulfurization treatment by injecting CaF ₂ −MgO-based treatment agent, in order to reduce inclusions in the subsequent treatment,
(%CaF ₂ )/{(%CaO) + (%CaF ₂ )}×100=8,
and 14% CaO-CaF ₂ type treatment agent was injected. Furthermore, in Examples CB and CD, processing,
After the completion of the process, Ar is supplied from the blowing pipe 3 at a flow rate of 1800Nl/min.
A treatment was performed in which gas was blown for 5 minutes. In each of the above examples, the composition of the processing agent was changed between treatments by changing the composition of CaO, CaF ₂ and MgO received in separate hoppers into each hopper according to the required blending ratio that changes according to changes over time in the processing. The experiment was carried out by continuously changing the blending ratio of CaO and CaF ₂ . The composition of the molten steel used in Examples CA, CB, CC, and CD is the same as in Examples A to D: C: 0.08 to
0.1% by weight, Si: 0.18-0.22% by weight, Mn: 0.9-1.2
% by weight, Al: 0.02 to 0.05% by weight, and remained within this range both before and after treatment. In addition, the refractory material of the circulation path 2 is the same as that used in specific examples A to D.
It is a MgO−Cr ₂ O ₃ system containing 74% by weight of MgO,
SiO ₂ and Al ₂ O ₃ are inevitably included as other components. Molten steel [S] concentration, desulfurization rate, and total oxygen concentration before and after treatment of CA, CB, CC, and CD in Examples
Examples are shown in Table 5. Furthermore, the frequency of detection of inclusions with a diameter of 25 μm or more after treatment is classified into spherical, non-spherical, and Al ₂ O ₃ clusters and is shown in FIG. Table 5, 5th
The figure shows specific examples B and C, comparative examples F and G.
An example of the results is also shown. Note that the inclusion detection frequency is a relative value when the total number of inclusions in Comparative Example G is set to 10. As is clear from Table 5, the CA of the example in which the inclusion reduction treatment agent was injected before the desulfurization treatment agent was injected,
and CD have a lower molten steel [S] concentration after treatment than both specific examples B and C and comparative examples F and G,
Desulfurization rate increased. Furthermore, as is clear from Table 5 and FIG. It was reduced in comparison. In particular, CC and CD of examples where the inclusion reduction treatment agent was injected after the desulfurization treatment agent was injected.
In this case, the frequency of detection of Al ₂ O ₃ clusters of 25 μm or more after treatment became zero, indicating a remarkable effect in reducing Al ₂ O ₃ -based inclusions. Furthermore, in CB and CD, in which Ar gas was blown into the bath after blowing the treatment agent, the frequency of detection of all inclusions was reduced. Next, after each of Examples CA, CB, CC, and CD was processed 50 times, the maximum amount of erosion of the refractory per treatment measured near the bath surface in the circulation path is shown in Table 6. . In all cases, the reduction was compared to the comparative examples E, F, G, and J shown in Table 3, and the high _CaF2
Erosion of the reactor refractories, which is a problem when injecting a high concentration treatment agent, has been reduced. The equipment shown in Figure 4 includes MgO hopper 58,
Hopper 50 for CaF ₂ , hopper 51 for CaO, molten steel sampling analyzer 52, processing pattern setting device 53, processing agent cutting feeder 54, 55, 5
9. Input the signal of the sampling analyzer 52,
The required blending ratio and addition timing of CaF ₂ , CaO, and MgO are calculated by constantly comparing with the setting processing pattern of the setting device 53, and the hopper 50, 5
It consists of an arithmetic command device 58 which issues a cutting command to the feeders 54, 55, 59 by subtracting the processing agent conveyance time to the blowing pipe 3 from 1,58. Next, the examples shown in Table 7 will be explained. The examples shown in Table 7 use the apparatus shown in FIG. 4, similar to Examples CA to CD. In Examples AA and AB,
Before and after injecting the desulfurization treatment agent, a treatment agent with CaF ₂ of 20% by weight or less is injected, and in Example AB, after injecting all the treatment agents, only Ar gas is blown at a flow rate of 2000 Nl/min from the blowing pipe 3 for 5 minutes. I did some processing to add it. The changes in the composition of the treatment agent from the first half to the second half of the treatment and between treatments in Examples AA and AB shown in Table 7 are similar to Examples CA to CD, in which CaF ₂ and CaO received in separate hoppers were CaO and CaF ₂ are cut out from each hopper according to the required blending ratio that changes over time.
The mixing ratio was continuously changed. The processing agent also contained 0.42% by weight of Al ₂ O ₃ , 3.04% by weight of SiO ₂ , and 0.43% by weight of MgO as inevitable components. In addition, in both Examples AA and AB, each treatment was started after deoxidation. The composition of the molten steel used was [C]: 0.08 to 0.15% by weight, [Si]: 0.15
~0.23% by weight, [Mn]: 0.92-1.30% by weight, [Al]:
It was 0.02 to 0.05% by weight, and the molten steel composition remained within this range both before and after treatment. Molten steel [S] concentration before and after treatment of Examples AA and AB,
and the total oxygen amount are also shown in Table 8. Furthermore, the frequency of detection of inclusions after treatment is shown in FIG. As is clear from Table 8, in Examples AA and AB, the molten steel [S] concentration after treatment was reduced to 2 ppm and the total oxygen amount was reduced to 7 to 9 ppm, and ultra-low sulfur high cleanliness steel was obtained. . In addition, as shown in FIG. 6, the frequency of detection of inclusions became extremely low. Especially in the example
With AB, a remarkable effect was obtained with an inclusion detection frequency of 1 or less. In addition, in both AA and AB of Examples, non-spherical shapes and Al ₂ O ₃ clusters were not detected. In the examples AA and AB described above, the lining of the reaction tank (circulation path) 2 shown in FIG.
A CrO ₃ -based refractory was used. In this case, the wear of the reaction tank lining is 0.6 mm or less per treatment,
It showed better results than the aforementioned Comparative Examples E to J. Next, the examples shown in Table 9 will be explained. Examples BB and BC, like Examples A to D, used the apparatus shown in Figure 3A, and after injecting only the desulfurization treatment agent, the slag on the bath surface was substantially not stirred or fluidized. , shows the case where molten steel is stirred with an inert gas. In Examples BB and BC, after a desulfurization treatment agent was blown in under the same treatment conditions as in Examples B and C, Ar gas was blown in from the blowing pipe 3 at a flow rate of 2500 Nl/min for 4 minutes. Table 10 shows the molten steel [S] concentration, desulfurization rate, and total oxygen amount before and after the BB and BC treatments in Examples, and FIG. 7 shows the total inclusion detection frequency after the treatments. The desulfurization rates in Examples BB and BC are shown in Table 2 and Table 5.
Although it is almost equivalent to Examples B and C shown in the table, the total amount of oxygen after treatment was reduced compared to Examples B and C. Moreover, as is clear from FIG. 7 and FIG. 5, in BB and BC of the example, specific example B
The frequency of detection of inclusions was lower than that of C and C. In particular, the effect of reducing spherical inclusions in Examples BB and BC was remarkable, but the Al ₂ O ₃ clusters were almost the same as in Examples B and C. Furthermore, in Examples BB and BC, similar to Specific Examples A to D, the refractory material of the reaction tank (circulation path) 2 was MgO-Cr ₂ O ₃ containing 74% by weight of MiO. After 50 treatments each of Examples BB and BC, the maximum amount of corrosion loss of the refractory per treatment measured near the bath surface in the reaction tank was 0.7 mm or less, and is shown in Table 3. It was significantly improved compared to Comparative Examples E to J.

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[Table] Effects of the invention As described above, the present invention provides CaO, CaF ₂ ,
and MgO and other unavoidable components,
MgO is 10 to 90% by weight, weight ratio is {CaF ₂ /(CaO
+CaF ₂ )} x 100 = 20 to 80%, so it does not impair the excellent desulfurization properties of CaO-CaF ₂ -based treatment agents that contain high concentrations of CaF ₂ , and
A problem occurred when using a treatment agent containing _CaF2 .
It is possible to prevent erosion of a reaction tank made of a refractory material containing MiO. Therefore, when molten steel is desulfurized continuously and in large quantities, an increase in the cost of refractories is advantageously prevented and the life of the reaction tank is not shortened, resulting in a significant reduction in manufacturing costs. Furthermore, in the present invention, the treatment agent is blown into the bath without substantially stirring the slag present on the molten steel in the ladle, and the main desulfurization reaction is completed in the molten steel. Desulfurization treatment by slag-metal reaction is no longer necessary, and ultra-low sulfur steel can be obtained with a small unit of desulfurization agent and in a short time.
Therefore, great effects such as reducing the temperature drop of molten steel, saving resources, and saving energy can be obtained. In addition, the present invention provides a method for preventing slag existing on the surface of molten steel from being drawn into the bath, and furthermore, in order to avoid an interfacial reaction between molten steel and slag, the carrier gas or floatation promoting gas blown into the bath reaches the bath. The weight concentration is (%CaF ₂ )/{(%CaO) + (%
CaF ₂ )}≦0.2 CaO-CaF _2- based inclusion reduction treatment agent is injected before and/or after the above-mentioned desulfurization treatment agent injection, so that inclusions in molten steel along with desulfurization will not cause any problems in product use. to a certain extent, it becomes possible to reduce it rapidly, reliably and economically. Furthermore, since it is possible to prevent a decrease in desulfurization ability due to the agglomeration of inclusions in the desulfurization treatment agent, highly efficient desulfurization is also possible. In the present invention, after blowing in all of the above-mentioned treatment agents, inert gas is blown in without substantially entraining the slag on the bath surface. The effect of promoting agglomeration and floating of objects can be greatly improved. Furthermore, the reaction environment of the present invention is RH or
Since a vacuum degassing tank such as DH or an inert gas atmosphere tank is used, it is possible to perform degassing while reducing inclusions. This makes it possible to omit processes through integration and combination, and as a result, the present invention has great industrial effects, such as a significant reduction in manufacturing costs and an improvement in manufacturing yield.

[Brief explanation of drawings]

Figure 1A shows the influence of the MgO concentration in the treatment agent on the desulfurization rate in the present invention by (%CaF ₂ )/
Figures shown by {(%CaO) + (%CaF ₂ )} (weight ratio),
Figure 1B shows the effect of the treatment agent on the amount of erosion of the basic refractory mainly composed of MgO in the present invention.
The effect of MgO concentration is (%CaF ₂ )/{(%CaO) +
(%CaF ₂ )} (weight ratio); Figure 2 is a diagram showing the relationship between the CaF ₂ concentration of the CaO-CaF ₂ -based treatment agent and the amount of Al ₂ O ₃ -based inclusions absorbed into the treatment agent in the present invention. , 3rd
Figures A and B show specific examples A to D and example BB,
BC, an elevational view of the equipment used in Comparative Examples E to J and Conventional Example K, etc.; Figure 4 is an elevational view of the equipment used in Examples CA to CD and
Fig. 5 is an explanatory diagram of the configuration of the apparatus using AA and AB, and shows the frequency of inclusion detection after processing in specific examples B and C, examples CA to CD, and comparative examples F and G. A diagram showing the inclusions divided into spherical and Al ₂ O ₃ clusters, Figure 6 is a diagram showing the frequency of inclusion detection after treatment in Examples AA and AB, and Figure 7 is an example
Figure 8 shows the detection frequency of inclusions after treatment in BB and BC, divided into spherical inclusions, non-spherical inclusions that are difficult to float, and Al ₂ O ₃ clusters. FIG. 2 is an elevational view of a reaction vessel in which desulfurization is performed by stirring slag. 1...Ladle, 2...Reaction tank (circulation path), 3...
Blowing pipe, 5...Taking pot opening, 6...Conduit, 7... Molten steel, 8...Slag, 11...Lid, 32...Exhaust system, 40...Pressure reduction device, 50, 51, 58...Hopper , 52... Analyzer, 53... Processing pattern setter, 54, 55, 59... Feeder, 5
6... Arithmetic/command device, 57... Setting signal.

Claims (1)

[Claims] 1. When desulfurizing molten steel is carried out in a reaction tank lined with a basic refractory containing MgO, a carrier gas blown into the bath floats in the bath and reaches the upper surface of the bath. CaO,
Consists of CaF ₂ , MgO, and other unavoidable components; MgO is 10 to 60% by weight, and the weight ratio is {CaF ₂ /
(CaO + CaF ₂ )} x 100 = 20 to 80% of the first treatment agent is blown into the bath, and before and/or after this is blown into the bath, CaF ₂ and CaO are the main components, and the remainder is unavoidable components. and the above CaF ₂ and
A method for treating molten steel, characterized in that a second treating agent having a CaF ₂ concentration in the main component consisting of CaO is 20% by weight or less is blown into the bath using an inert gas as a carrier gas. 2 When desulfurizing molten steel in a reaction tank lined with a basic refractory containing MgO, the upper surface of the bath, where the carrier gas blown into the bath floats up and reaches the bath, is under reduced pressure or in an inert gas atmosphere. CaO,
Consists of CaF ₂ , MgO, and other unavoidable components; MgO is 10 to 60% by weight, and the weight ratio is {CaF ₂ /
(CaO + CaF ₂ )} x 100 = 20 to 80% of the first treatment agent is blown into the bath, and before and/or after this is blown into the bath, CaF ₂ and CaO are the main components, and the remainder is unavoidable components. and the above CaF ₂ and
A second treatment agent having a _CaF2 concentration of 20% by weight or less in the main component consisting of CaO is blown into the bath using an inert gas as a carrier gas, and then the slag on the bath surface is substantially stirred or fluidized. without making you feel
A method for treating molten steel, which comprises blowing an inert gas into the molten steel.