JP6273947B2 - Desulfurization treatment method for molten steel - Google Patents

Description

  The present invention relates to a molten steel desulfurization treatment method performed during secondary refining treatment of molten steel, and more specifically, powdered CaO is added while stirring molten steel using a vacuum treatment apparatus such as RH. The present invention relates to a desulfurization treatment method for molten steel that is blown or sprayed.

  In steel materials, S (sulfur) is generally desired to have a low concentration in order to reduce the workability and mechanical properties of steel materials. Therefore, in addition to desulfurization treatment at the hot metal stage, desulfurization treatment is often performed at the molten steel stage when producing low-sulfur steel. For this reason, many ideas have been conventionally made to rationally perform the desulfurization treatment in both the hot metal stage and the molten steel stage.

As a method for obtaining a good desulfurization efficiency at desulfurization treatment of molten steel, as in the invention described in Patent Document 1, the use of the flux of CaO-CaF ₂ based as desulfurizing agent, the proper slag composition on the molten steel Typically, reducing the (FeO) + (MnO) concentration of slag on molten steel is typical. CaF ₂ promotes melting and melting of CaO and lowers the melting point of the produced slag to increase fluidity, and also improves desulfurization efficiency because of its high reactivity.

However, if a flux containing CaF ₂ is used, F will be contained in the slag after the desulfurization treatment. For example, when the slag is reused for roadbed materials, etc., F may be dissolved and adversely affect the surrounding environment. There is sex. Therefore, it is necessary to strictly manage the slag or to set a restriction when it is reused, and there is a problem that the load and cost of the slag processing are increased. In addition, CaF ₂ has an excellent effect of lowering the melting point of the generated slag and having high reactivity. Therefore, it has a characteristic that the refractory is easily melted, and the life and cost of the refractory are reduced. From the point of view, use is not preferable.

Therefore, in recent years, development of technology for desulfurization treatment without using CaF ₂ has been advanced. As such a technique, for example, a technique using a CaO—Al ₂ O ₃ -based flux for the purpose of improving the meltability of CaO to be added, as in the invention described in Patent Document 2, or described in Patent Document 3 As in the invention, a technique using a mixture of CaO and metallic Ca or Ca alloy as a desulfurizing agent is disclosed. However, when a CaO—Al ₂ O ₃ -based flux or a mixture of CaO and metallic Ca or Ca alloy is used as a desulfurization agent, there is a problem that the cost of the flux (desulfurization agent) increases.

Moreover, in order to obtain good desulfurization efficiency, the (FeO) + (MnO) concentration of the slag on molten steel must be lowered. However, if it does so, since melting | fusing point of slag will raise and fluidity | liquidity will fall, the bad influence which prevents a desulfurization efficiency improvement becomes a problem. Therefore, simply suspending the use of CaF ₂ which has the excellent effect of promoting the melt hatching of CaO and lowering the melting point of the produced slag to increase the fluidity does not solve the problem.

As a method for reducing the (FeO) + (MnO) concentration of the slag on molten steel, as described in Patent Document 1, a metal Al or Al-containing flux is introduced into a ladle when steel is output from a converter. Thus, a method of reforming slag on molten steel is common. However, in this method, if the (FeO) + (MnO) concentration of the slag on the molten steel is lowered, the deoxidation of the molten steel is accompanied, so that the nitrogen concentration [N] in the molten steel increases. Therefore, for example, it is difficult to apply to a low N steel in which [N] ≦ 30 ppm in the product is required. If the slag is reformed by adding Al or the like to the slag after the completion of the steel output, the increase in [N] in the molten steel is suppressed, but the cost for reforming the slag increases instead. In addition, P ₂ O ₅ contained in the slag is easily reduced by Al and easily returned to the molten steel. For example, an extremely low P steel in which [P] ≦ 0.010 mass% in the product is required. It is not easy to apply to desulfurization treatment.

JP-A-5-214424 JP 2011-236456 A Japanese Patent Application Laid-Open No. 2003-342631

The problem of the present invention is that, without using CaF ₂ which has been generally used conventionally, Al is added into the ladle at the time of steelmaking to deoxidize the molten steel, or to modify the slag on the molten steel. In addition, as in the inventions described in Patent Documents 2 and 3, a CaO—Al ₂ O ₃ based flux or a mixture of CaO and metal Ca or Ca alloy is not used as a desulfurization agent, and it is cheaper. It is to provide a reasonable molten steel desulfurization treatment means.

(1) After producing molten steel with a C content of 0.01 to 0.10% by mass using a converter, the steel output is completed without adding Al during the steel output from the converter to the ladle. The chemical composition of molten steel at the time was adjusted to C: 0.04 to 0.20%, Si: 0.1 to 0.5%, Mn: 0.1 to 1.5% in mass%,
Then, the molten steel which has a vacuum tank and a dip tube, and blows or blows CaO powder into the molten steel using a secondary smelting apparatus that immerses the dip tube in the molten steel in the ladle and refines it under reduced pressure. A desulfurization processing method of
The secondary refining using the secondary refining apparatus is started without adding Al continuously after the molten steel is discharged, and Ca is added to the molten steel at 0.05 to 1 ton of molten steel during the secondary refining. 0.10 kg was added, Al was added simultaneously with the Ca addition or immediately after the addition was completed, and sol. Adjust the Al concentration to 0.020-0.100 mass% ,
Furthermore, the secondary refining is continued, and a CaO powder is blown into the molten steel, or a CaO powder is sprayed onto the molten steel in the vacuum tank .

According to the present invention, without using CaF ₂ that has been generally used in the past, without adding deoxidation of the molten steel by adding Al into the ladle at the time of steelmaking, or modifying the slag on the molten steel Moreover, the molten steel desulfurization treatment can be performed reasonably at low cost without using a CaO—Al ₂ O ₃ -based flux or a mixture of CaO and metallic Ca or Ca alloy as a desulfurization agent.

  In the present invention, since it is not necessary to use Al or an Al alloy as a deoxidizing agent when steel is output from a converter, low nitrogen steel that requires [N] ≦ 30 ppm in the product or [P in the product] ] ≦ 0.010 mass% is particularly suitable for desulfurization treatment of ultra-low phosphorus steel.

FIG. 1 is a graph showing the distribution of inclusion composition in a molten steel sample taken from molten steel after desulfurization treatment with a quick lime powder injection rate of 2 kg / t in Test I. FIG. 2 is a graph showing, in comparison, the transition of the molten steel desulfurization rate while continuing CaO powder injection in Test I and the transition of the molten steel desulfurization rate while continuing CaO powder injection after adding Ca in Test II. FIG. 3 shows the composition change of inclusions in molten steel accompanying CaO powder injection in Test I, with the CaO and Al ₂ O ₃ mass concentration ratio (CaO / Al ₂ O ₃ ) on the horizontal axis for each CaO powder injection amount. Is a graph showing the S mass concentration of the inclusions on the vertical axis. FIG. 4 shows the composition change of inclusions in molten steel due to CaO powder injection in Test II, with the CaO and Al ₂ O ₃ mass concentration ratio (CaO / Al ₂ O ₃ ) on the horizontal axis for each CaO powder injection amount. Is a graph showing the S mass concentration of the inclusions on the vertical axis. FIG. 5 is a graph showing the composition of inclusions in the molten steel immediately after Ca blowing, with the horizontal axis representing the Ca concentration in terms of oxide and the vertical axis representing the S concentration. FIG. 6 is a graph showing the (FeO + MnO)% and the desulfurization reaction efficiency of the slag on the molten steel before and after the secondary refining with RH, investigated in Test I and Test II.

  A mode for carrying out the present invention will be described. In the following description, “%” related to chemical composition means “% by mass”.

  In the present invention, after manufacturing molten steel using a converter, Si and Mn are added without adding Al to the steel discharged from the converter to the ladle, and then a vacuum tank and a dip tube are provided. Then, using a secondary refining equipment that smelts the dip tube in the molten steel in the ladle and refines under reduced pressure, secondary refining under reduced pressure is started without adding Al, and Ca is added to the molten steel. At the same time or immediately after the addition of Ca is completed, Al is added, and then CaO powder is blown into the molten steel, or CaO powder is sprayed onto the molten steel in the vacuum tank. is there.

  More specifically, after producing molten steel of C: 0.01 to 0.10% using a converter, without adding Al during steel output from the converter to a ladle, ferrosilicon or An alloy iron containing Si or Mn such as ferromanganese is added, and a C source is also added if necessary, and the chemical composition of the molten steel at the time of completion of steel production is C: 0.04 to 0.20%, Si : 0.1 to 0.5%, Mn: 0.1 to 1.5%. By adjusting the chemical composition of the molten steel to the above range by adding ferrosilicon, ferromanganese, etc. to the steel output, the molten steel in the ladle is appropriately deoxidized without adding Al, and The slag on the molten steel in the pan is also deoxidized to the extent that it does not hinder the desulfurization treatment according to the present invention.

  After this, use a secondary refining equipment such as RH that has a vacuum tank and a dip tube, immerses the dip tube in the molten steel in the ladle and refines it under reduced pressure, and adjusts the components in the molten steel. When necessary, a small amount of Ca is added to the molten steel to deoxidize the molten steel, and at the same time or immediately after the addition of the Ca is completed, Al is added to the molten steel. Al: The Al concentration is adjusted in the range of 0.020 to 0.100% so as to meet the destination product specification of this generally molten steel. Then, without interrupting secondary refining, CaO is continuously added to the molten steel to desulfurize the molten steel.

  Therefore, the secondary refining apparatus for carrying out the present invention includes, for example, the apparatus (RH-PB) shown in FIG. 1 of Japanese Patent Laid-Open No. 2000-73116, and FIG. As in the apparatus (RH injection) shown in (a), a molten steel stirring mechanism for sucking and circulating the molten steel into the vacuum chamber, an apparatus for adding a Ca source to the molten steel being stirred, and a CaO source Or a device for blowing must be attached.

  One or two or more dip tubes may be immersed in the molten steel. The Ca source may be in the form of an alloy such as calcium silicon or calcium ferrite in addition to the metal Ca. The addition method is 0.05 to 0.10 kg per 1 ton of molten steel in the form of a wire in the form of a wire and is fed into the molten steel from the vicinity of the dip tube, or the maximum particle size is 1 mm or less in the form of powder and the inside of the vacuum chamber It may be added by spraying from the upper spray lance installed on the surface of the molten steel or by injecting from a lance immersed in the molten steel.

As the CaO source, quick lime containing 90% or more of CaO is usually used, but CaO—Al ₂ O ₃ type flux can also be used. However, the CaO source may be added by spraying the molten steel in the vacuum tank using an upper blowing lance, blowing it into the molten steel using an immersion lance, or blowing it into the molten steel from a tuyere provided on a dip tube. Anyway, it is necessary to carry out by blowing with a carrier gas such as Ar. Therefore, the CaO source needs to be powdery, and the maximum particle size is assumed to be 1 mm or less. However, considering the speed of dissolution in molten steel and the reaction with S, it can be said that the smaller the particle size, the more preferable. Specifically, it is preferable to use fine powder having a maximum particle size of 0.5 mm or less.

  The addition of the Ca source and the CaO source into the molten steel is performed according to the following procedure. As described above, the molten steel whose chemical composition was adjusted to C: 0.04 to 0.20%, Si: 0.1 to 0.5%, and Mn: 0.1 to 1.5% at the completion of steel production First, Ca is added using the secondary refining apparatus described above. The Ca mass is preferably 0.05 to 0.10 kg per ton of molten steel. Simultaneously with the addition of Ca or immediately after the completion of the addition of Ca, Al is sol. Al: 0.020 to 0.100% and added so as to be within the range of product specifications, and then a CaO source is supplied.

C, Si, Mn, sol. In order to adjust the four components of Al in order as described above, a small amount of 0.05 to 0.10 kg of Ca per 1 ton of molten steel is blown into or blown into the molten steel and reacted with oxygen in the molten steel to cause CaO. Then, Al is added and reacted with oxygen in the molten steel remaining after deoxidation by addition of Ca to produce Al ₂ O ₃ , which was previously produced in the molten steel. It is reacted with to produce a _CaO-Al 2 _{O 3,} further reacted with CaO is continuously supplied to _CaO-Al 2 _{O 3} which generated them, of increasing the reaction efficiency between the molten steel sulfur, the present invention This is important for implementing the basic technical idea of such desulfurization treatment.

  During secondary refining by adding Si and Mn, which are required in the specifications of the destination product, without adding Al when steel is output from the converter, and deoxidizing the molten steel and slag in the ladle. The situation in which oxygen in the slag obstructs the progress of the desulfurization reaction of the molten steel by adding Ca or adding CaO to the steel can be reasonably avoided.

In addition, by adding Si and Mn without adding Al and deoxidizing the molten steel, an appropriate amount of oxygen is present in the molten steel with almost no Al ₂ O _3, so a predetermined small amount CaO can be produced and dispersed in molten steel by addition of Ca.

In such a state, the sol. When Al is added so as to be 0.020 to 0.10%, the produced CaO previously dispersed in the molten steel reacts with Al ₂ O ₃ produced by newly adding Al. Thus, the CaO—Al ₂ O ₃ compound is generated before the supply of CaO.

By supplying fine powder of CaO by blowing or spraying under these conditions, the compound and the newly supplied CaO powder are prepared using the previously formed CaO-Al ₂ O ₃ compound as a nucleus. Are combined, and the CaO concentration in the CaO—Al ₂ O ₃ compound gradually increases, so that the effect of improving the desulfurization efficiency is achieved.

In the conventional molten steel desulfurization treatment, a CaO—CaF ₂ -based desulfurization flux is often used. This is because (1) CaO, which is a desulfurizing agent, has a high melting point and is difficult to melt alone, so that it is melted together with CaF ₂ and (2) S absorption capacity of slag on molten steel (sulfide capacity) This is considered to be due to the fact that desulfurization of molten steel was promoted by two effects of enhancing CaO and CaF ₂ in combination to promote desulfurization of the molten steel and suppressing sulfurization from the slag into the molten steel. However, the use of CaF ₂ -containing flux has the above-mentioned problems, and therefore, development of a desulfurization method using a flux not containing CaF ₂ has been advanced.

However, as described above, in the disclosed invention, a CaO—Al ₂ O ₃ -based desulfurization flux is prepared and used, or when Ca and CaO are used together and desulfurization with Ca is performed, it is expensive. Become. Moreover, since those inventions add desulfurization flux after fully deoxidizing using Al, there exists a problem that N will be absorbed in molten steel or P will return in molten steel from slag. .

In contrast, in the present invention, Al is not added before a predetermined small amount of Ca is added in the secondary refining apparatus. Therefore, the problem that N is absorbed in the molten steel or P returns from the slag is greatly reduced. Moreover, Ca is used not for the purpose of desulfurization by reacting with S in the molten steel but for the purpose of dispersing fine CaO in the molten steel by reacting with oxygen in the molten steel. The fine CaO subsequently the Al generates _Al 2 _{O 3} reacts with the remaining oxygen to be added, the product _Al 2 _{O 3} and reacts with lower _CaO-Al 2 _{O 3} system having a melting point relatively It becomes a nucleus that becomes an inclusion. This inclusion serves as a nucleus and reacts with the continuously supplied CaO powder to form a CaO—Al ₂ O ₃ -based inclusion having a high desulfurization capability. This inclusion reacts with S in the molten steel to desulfurize the molten steel. Like that.

Therefore, the added Ca does not become CaS but serves as a nucleus that generates CaO—Al ₂ O ₃ inclusions having a high desulfurization capability, and is controlled so as to absorb S. High desulfurization is achieved.

  The present invention as described above has been completed through confirmation by the following experiment.

When CaF ₂ is not used, there is a concern that the melting of CaO cannot be promoted as in (1) above. Regarding this concern, as in Test I, as in the case of the conventional invention, after deoxidizing the molten steel and slag by adding Al together with Si and Mn at the time of steel from the converter, the CaO powder is adjusted with RH and the like. After the deoxidation of the molten steel and slag without adding Al and adding Si and Mn at the time of steel leaving from the converter as test II in the present invention as a test II, the component is formed with RH. After adjustment and the like, Ca was added, Al was subsequently added, and then a test in which CaO powder was blown into the molten steel was conducted, and the properties of the inclusions in the molten steel were comparatively investigated.

  However, the amount of Al added at the time of steel production in Test I is within a range that does not affect the above-described N pickup and the like. The Al concentration was added within a range not exceeding 0.001% (about 0.5 kg / t).

Test I (conventional type)
For 400t of molten steel that was decarburized and blown to 0.03-0.08% in the molten steel in the converter, Si, Mn, and Al were added to the molten steel when steel was discharged from the converter to the ladle. The molten steel components at the time of completion of steel production were C: 0.04 to 0.20%, Si: 0.1 to 0.5%, Mn: 0.1 to 1.5%, sol. Al: 0.001% The following adjustments were made. Thereafter, molten steel reflux treatment was started using RH, and quick lime powder having a maximum particle size of 0.5 mm or less was passed through the immersion lance shown in FIG. 3 (A) of JP-A-62-296317. 1-4 kg was blown in the mass of CaO per ton.

Test II (Invention type)
For 400t of molten steel that was decarburized and blown to 0.03-0.08% in the molten steel in the converter, Al was not added to the molten steel at the time of steel removal from the converter to the ladle. Is added to deoxidize, and the molten steel components at the time of completion of steel production are changed to C: 0.04 to 0.20%, Si: 0.1 to 0.5%, Mn: 0.1 to 1.5% After adjustment, using RH, during the molten steel recirculation treatment, Ca is converted into calcium by calcium silicon through a dipping lance shown in FIG. 3 (A) of Japanese Patent Laid-Open No. 62-196317. At the same time as 0.05-0.12 kg was blown in, Al was added and sol. After Al was adjusted to 0.020 to 0.100%, quick lime powder having a maximum particle size of 0.5 mm or less was blown in an amount of 1 to 3 kg at a CaO mass per 1 ton of molten steel through the same immersion lance as that into which Ca was blown.

  These results are shown in FIGS.

  First, FIG. 1 is a graph showing the distribution of inclusion composition in a molten steel sample taken from molten steel after desulfurization treatment with a quick lime powder injection rate of 2 kg / t in Test I (conventional type). The “first rounded numbers” in this graph are the numbers of the molten steel samples collected from the first investigation target molten steel. In the second round, three molten steel samples were collected. Since all the results were the same, only the third sample was shown.

In this investigation result, since the components of the compound constituting the inclusion were basically CaO and Al ₂ O ₃ , the horizontal axis of the graph of FIG. 1 shows the binary system of CaO—Al ₂ O ₃ . The Al ₂ O ₃ concentration is shown, and the number of inclusions (N number) randomly found in each molten steel sample is shown on the vertical axis for each Al ₂ O ₃ concentration.

As a result, it was found that the inclusions in the molten steel after such desulfurization treatment were mainly CaO 35% and Al ₂ O ₃ 65%. Such inclusions correspond to the composition of Al ₂ O ₃ saturation concentration at 1600 ° C.

The origin of Al ₂ O ₃ can be considered to be generated by deoxidation. From this result, even if CaO is blown alone, it reacts with Al ₂ O ₃ present in the molten steel to produce a CaO—Al ₂ O ₃ compound. By doing so, it was found that many were sufficiently melted. When CaO is blown with a simple substance such as quicklime to increase the desulfurization efficiency, the melting point of the CaO is much higher than the molten steel temperature, so it is difficult to achieve the purpose with such a simple substance such as quicklime. Has been. Therefore, conventionally, CaF ₂ or a CaO—Al ₂ O ₃ type flux has been used.

However, if the blown CaO reacts with Al ₂ O ₃ present in the molten steel to produce a CaO—Al ₂ O ₃ compound, the minimum Al ₂ O ₃ necessary for the reaction is present. It is thought that it should have done. The minimum amount of Al ₂ O ₃ required is 60% CaO-40% Al ₂ O ₃ which becomes a liquid phase at the molten steel desulfurization temperature (about 1600 ° C.) and can keep the activity of CaO high. It is presumed that the amount that produces is appropriate.

  FIG. 2 is a graph showing, in comparison, the transition of the molten steel desulfurization rate while continuing CaO powder injection in Test I and the transition of the molten steel desulfurization rate while continuing CaO powder injection after adding Ca in Test II.

  The definition of desulfurization rate is as follows.

Desulfurization rate (%) = {(before CaO powder injection [S]%) − (after CaO powder injection [S]%)} / (before CaO powder injection [S]%) × 100
In this test I, the desulfurization rate increases as the amount of CaO powder added increases to 4 kg per ton of molten steel, so that it can be seen that molten steel desulfurization proceeds even when CaO powder is added alone.

However, since a large amount of Al ₂ O ₃ is present in the molten steel before the addition of CaO powder under these conditions, the composition of inclusions found in the molten steel is Al _{2 at the} initial stage of injection when the amount of CaO powder blown is small. remains near O _3, such inclusions are believed absorption in the molten steel S is small.

  On the other hand, in Test II, since the injection of CaO powder is started after a small amount of Ca is added, the transition of the molten steel desulfurization rate with the increase of the CaO powder addition amount at that time is shown. It was confirmed that the rate of improvement of the desulfurization rate after starting the injection of CaO powder was clearly higher than in the case of Test I in which no Ca was added.

In Test II, a small amount of Ca is added to the molten steel in the absence of Al ₂ O ₃ due to Al deoxidation in the molten steel to reduce the oxygen concentration in the molten steel and produce CaO. results Al is added to conform to the standards, small compared to the amount of Al ₂ O ₃ generated in molten steel test I, Al ₂ O ₃ to be generated are generated in the molten steel above CaO It is thought that it originates in reacting with to become a CaO—Al ₂ O ₃ compound. In addition, as described above with reference to FIG. 1, such inclusions are expected to be melted in the molten steel and are considered to be advantageous for reaction and absorption with S in the molten steel.

  Then, the change situation of the inclusion composition in molten steel with the increase in CaO blowing amount was investigated about each of Test I and Test II.

3 and 4 show the composition change of inclusions in molten steel accompanying CaO powder injection, and the mass concentration ratio (CaO / Al ₂ O ₃ ) of CaO and Al ₂ O ₃ on the horizontal axis according to the amount of CaO powder blown. The vertical axis represents the S mass concentration of the inclusions.

As shown in the graph of FIG. 3, the change in inclusion composition in Test I is such that CaO / Al ₂ O ₃ is small in the initial stage where the amount of CaO blown is small, and CaO / Al ₂ O close to pure Al ₂ O _{3. 3} was 0 to less than 1.0. Such inclusions are in a solid phase in the molten steel, and the S-absorbing ability is low, so the S-containing concentration in the inclusions was substantially 0%. However, when the CaO blowing amount exceeds 3 kg / t, some CaO / Al ₂ O ₃ exceeds 1.0 from about 1.0, and the inclusions are in a molten state and the S absorption capacity is also high. Since it increased, the S content concentration in the inclusions was also high.

On the other hand, as shown in the graph of FIG. 4, the change in the inclusion composition in Test II was observed when CaO / Al ₂ O ₃ was 1.0 to 2.0 from the time before the start of CaO blowing. were so, CaO additive Ca origin described above reacts with _Al 2 _{O 3,} it had formed a _CaO-Al 2 _{O 3} compound from the previous blowing CaO was confirmed. Thereafter, when the CaO blowing amount was increased, even when the CaO blowing amount was 1 kg / t, the CaO / Al ₂ O ₃ was recognized from near 0 to over 2.0, and the test I without initial Ca addition was performed. Unlike the inclusions, CaO—Al ₂ O ₃ compounds with CaO / Al ₂ O ₃ exceeding 1.0 were present from the initial stage of CaO blowing. Since such a compound is in a liquid phase in molten steel and has a high S absorption capacity, it was confirmed that S was absorbed in the inclusions.

As described above, according to the investigation results shown in the graphs of FIGS. 2 to 4, it is confirmed that the above-described inference is correct, that is, desulfurization efficiency is improved by adding Ca while suppressing the generation amount of Al ₂ O _{3 in the} initial stage. confirmed.

  By the way, since Ca is expensive, it is desirable to receive the maximum effect with the minimum necessary amount. Therefore, the necessary amount of Ca blown in the initial stage was examined. In general, ultra-low S steel contains [Mn] and [Si] to some extent, and even if Al is not used at the time of steel leaving the converter, Si or Mn alloy is added to some extent to adjust the components. Is often performed.

  Since the present invention is assumed to be applied under such conditions, the chemical composition of the molten steel at the time of completion of the steel output is determined by adding C during the steel output from the converter to the ladle, and C: The adjustment to 0.04 to 0.20%, Si: 0.1 to 0.5%, and Mn: 0.1 to 1.5% is specified as one of the matters specifying the invention. Therefore, Test I and Test II were performed under the same conditions, and Ca addition conditions and CaO blowing conditions were investigated.

Under this condition, the dissolved oxygen in the molten steel is about 30 ppm. Under these conditions, it is expected that the required amount of Ca addition for reducing the amount of oxygen in the molten steel and suppressing the amount of Al ₂ O ₃ produced by the addition of Al can be reduced to a point where there is no problem in terms of cost. Therefore, as shown in the graph of FIG. 2, the Ca addition amount is set to 0.05 kg per 1 ton of molten steel, and sol. As a result of injecting CaO powder after adding Al so as to have an Al concentration, it was confirmed that the desulfurization rate is higher than the conventional method from the initial stage of CaO blowing and that the desired Ca addition effect is exhibited. It was.

  It was found that the desulfurization rate per unit injection of CaO increased when the amount of Ca was increased, so that the desulfurization efficiency by injection CaO was improved, but in the molten steel immediately after Ca injection when the amount of Ca was 0.12 kg / t. From the inclusion composition, it was found that a large amount of CaS was produced. Since the present invention does not intend to cause a direct desulfurization reaction due to the addition of Ca, a large amount of CaS is not included in the technical idea of the present invention.

  FIG. 5 is a graph showing the composition of inclusions in the molten steel immediately after Ca blowing, with the horizontal axis representing the Ca concentration in terms of oxide and the vertical axis representing the S concentration.

As shown in the graph of FIG. 5, there was a tendency that the S concentration increased with a certain slope from the vicinity of over 40% as CaO. This is a composition in which 40% is a liquid phase at the molten steel temperature in the CaO—Al ₂ O ₃ system, and since the slope is close to the stoichiometric ratio of Ca and S, inclusions having a high concentration in terms of CaO Is likely to be CaO + CaS. Compared with 0.05 kg / t or 0.06 kg / t, the amount of Ca added increases because the amount of inclusions of high S-concentration inclusions immediately after blowing was higher when the amount of Ca was 0.12 kg / t. It was suggested that the Ca functions directly as a desulfurization agent. The use of Ca under such conditions has a very high load in terms of cost, so it can be said that an initial Ca amount of about 0.1 kg / t is sufficient.

The same effect can be obtained by deoxidizing with a large amount of Al or the like at the time of steel output to sufficiently generate deoxidized Al ₂ O ₃ in the steel output and promoting floating before the treatment in secondary refining. Although it is obtained, when deoxidizing with a large amount of Al at the time of steel production, there are problems as described in the background art section, and this method has an advantage of avoiding this.

When CaF ₂ is not used, as described above, the S absorption capacity (sulfide capacity) of the slag on the molten steel is reduced, and the S that once floats as an inclusion from the molten steel and enters the slag. There is concern about the increase in sulfurization that returns to molten steel. In general, it is effective to reduce the (FeO) + (MnO) concentration of slag on molten steel in order to suppress sulfurization. However, in this method, dissolved Ca reduces slag at the slag-metal interface, and slag It is also expected to play a role of reducing the (FeO) + (MnO) concentration.

  Therefore, FIG. 6 shows (FeO + MnO)% of the slag on the molten steel after CaO powder injection with RH and the desulfurization reaction efficiency, which were investigated in Test I and Test II. Here, the desulfurization reaction efficiency (K value) is defined as the following equation (i).

K value = ln {(before desulfurization treatment [S]% / after desulfurization treatment [S]%)} / CaO basic unit (kg / t) (i)
The K value is an index representing the desulfurization agent utilization efficiency at the time of desulfurization, and corresponds to the slope when [S] is taken as the powder basic unit on the horizontal axis and logarithm (ln) on the vertical axis. It can be said that the larger this value, the higher the desulfurization rate with the same amount of powder and the higher the efficiency.

As shown in the graph of FIG. 6, it was confirmed that the slag (FeO + MnO) concentration was lower than the desulfurization treatment in which only the quick lime powder was blown in Test I, and the desulfurization reaction efficiency (K value) was higher accordingly. It was done. In Test II with addition of Ca, the (FeO + MnO) concentration of the slag on the molten steel is low, which compensates for the adverse effect of reducing the S absorption capacity (sulfide capacity) of the slag by not using CaF ₂ as a desulfurization agent. Therefore, it is unlikely that sulfurization from slag occurs, and it is considered that a satisfactory desulfurization treatment can be realized only by blowing CaO as a desulfurization agent.

As an example of utilizing the metal Ca or Ca alloy, there is an example such as Patent Document 3 described above. In this case, the role of Ca is to reduce the oxygen activity in the molten steel. However, as described above, when the desulfurizing agent as a base is considered as CaO as in the present invention, The purpose is not to reduce the activity of CaO by excessive production of Al ₂ O ₃ . Therefore, in this case, it is more effective to blow metal Ca all at once in the initial stage. In addition, the effect of reducing the degree of oxidation of the pan slag as shown in the graph of FIG. From the above, it can be said that the amount of Ca required can be reduced by blowing Ca in the initial stage.

  Examples are shown below.

  400 t of molten steel subjected to desiliconization, dephosphorization and decarburization blowing in a converter was taken out into a ladle, and then molten steel was desulfurized by blowing powder during the treatment in the RH vacuum degassing apparatus. Deoxidation was performed with a Mn alloy and a Si alloy at the time of leaving the converter, and [Si] and [Mn] were set to 0.1% and 1.1%, respectively. In the secondary refining, calcium silicon containing 30% of metallic Ca in the initial stage of treatment is converted into Ca mass through the injection lance of the apparatus (RH injection) shown in FIG. 3 (A) of Japanese Patent Laid-Open No. Sho 62-196317. At the same time as 0.06 kg / t was blown, 0.5 kg / t of Al was charged and sol. The Al concentration was 0.033%. Immediately thereafter, 3 kg / t of CaO powder was blown through the injection lance with quick lime having a maximum particle size of 0.5 mm.

  As a result, [S] was reduced from 33 ppm to 14 ppm. The desulfurization reaction efficiency (K value) at this time was 0.29.

  On the other hand, when the same operation was carried out in the converter and CaO powder was blown in 4.3 kg / t without blowing calcium silicon in RH, [S] decreased only from 40 ppm to 18 ppm. The desulfurization reaction efficiency (K value) at this time was 0.19, which was lower than the result of the present invention in which Ca was added at the initial stage of the RH treatment.

Claims (1)

After manufacturing molten steel having a C content of 0.01 to 0.10% by mass using a converter, Al is not added during the steel output from the converter to the ladle, and the molten steel at the time of completion of steel output. The chemical composition is adjusted in mass% to C: 0.04 to 0.20%, Si: 0.1 to 0.5%, Mn: 0.1 to 1.5%,
Then, the molten steel which has a vacuum tank and a dip tube, and blows or blows CaO powder into the molten steel using a secondary smelting apparatus that immerses the dip tube in the molten steel in the ladle and refines it under reduced pressure. A desulfurization processing method of
The secondary refining using the secondary refining apparatus is started without adding Al continuously after the molten steel is discharged, and Ca is added to the molten steel at 0.05 to 1 ton of molten steel during the secondary refining. 0.10 kg was added, Al was added simultaneously with the Ca addition or immediately after the addition was completed, and sol. Adjust the Al concentration to 0.020-0.100 mass% ,
Furthermore, the secondary refining is continued, and a CaO powder is blown into the molten steel, or a CaO powder is sprayed onto the molten steel in the vacuum tank.

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